Photophysics of Polymers 9780841214392, 9780841212053, 0-8412-1439-5

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Photophysics of Polymers

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS SYMPOSIUM SERIES 358

Photophysics of Polymers Charles E. Hoyle,

EDITOR

University of Southern Mississippi

John M. Torkelson,

EDITOR

Northwestern University

Developed from a symposium sponsored by the Division of Polymer Chemistry, Inc., at the 192nd Meeting of the American Chemical Society, Anaheim, California, September 7-12, 1986

American Chemical Society, Washington, DC 1987

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Library of Congress Cataloging-in-Publication Data Photophysics of polymers/Charles E. Hoyle, John M . Torkelson, editors "Developed from a symposium sponsored by the Division of Polymer Chemistry at the 192nd Meeting of the American Chemical Society, Anaheim, California, September 7-12, 1986." p. cm.—(ACS symposium series, ISSN 0097-6156; 358) Bibliography: p. Includes index. ISBN 0-8412-1439-5 1. Polymers and polymerization—Congresses 2. Photochemistry—Congresses. I. Hoyle, Charles E., 1948. II. Torkelson, John M. III. American Chemical Society. Division of Polymer Chemistry. IV. American Chemical Society. Meeting (192nd: 1986: Anaheim, Calif.) V. Series. QD381.8.P47 1987 547.7'0455-dcl9

87-27307 CIP

Copyright © 1987 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, M A 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS Symposium Series M. Joan Comstock, Series Editor 1987 Advisory Board Harvey W. Blanch

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University of California—Berkeley

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W H Norton

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In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Foreword The

A C S S Y M P O S I U M S E R I E S was founded in 1974 to provide a

m e d i u m for p u b l i s h i n g s y m p o s i a q u i c k l y in b o o k form. T h e format o f the Series parallels that o f the c o n t i n u i n g A D V A N C E S IN C H E M I S T R Y S E R I E papers are not typese

reproduce

y

by the authors in camera-ready form. Papers are reviewed under the supervision o f the E d i t o r s w i t h the assistance o f the Series A d v i s o r y B o a r d and are selected to maintain the integrity o f the s y m p o s i a ; h o w e v e r , v e r b a t i m reproductions o f p r e v i o u s l y pub­ lished papers are not accepted. B o t h reviews and reports o f research are acceptable, because s y m p o s i a m a y e m b r a c e both types o f presentation.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Preface SIGNIFICANT Q U E S T I O N S A R E A R I S I N G as the field o f p o l y m e r science continues to develop. M a n y o f these questions can be addressed only at the most fundamental levels. P o l y m e r photophysics has e v o l v e d over the past two decades, p r o v i d i n g the p o l y m e r scientific c o m m u n i t y w i t h a v i a b l e tool for p r o b i n g p o l y m e r structure on a m o l e c u l a r scale. H e r e i n lies its true value. B y investigating photophysical phenomena o f p o l y m e r systems, we can develop an accurate picture o f h o w polymers exis T h e chapters in this engaged in basic and applied p o l y m e r research with a clear understanding of the current status o f p o l y m e r photophysics. T h e concepts developed in this v o l u m e s h o u l d stimulate others to a p p l y p h o t o p h y s i c a l techniques in their o w n research. We thank the authors for their outstanding response. T h e cooperation of each contributor has made assembling this book an unexpected pleasure. We also thank P a t r i c i a L i n t o n and Susan Barge for their efforts above and beyond the call o f duty. I ( C E H ) thank Fred L e w i s and James G u i l l e t for introducing me to the w o r l d o f photochemistry and photophysics. I also thank my family for their patience

and

understanding

in a l l o w i n g

p r e p a r i n g this b o o k . T h i s demonstration

me

to

spend

precious

time

o f love by K a r e n , A b b i e , and

A u s t i n makes this book invaluable. A

special a c k n o w l e d g m e n t

is extended

to the Petroleum

Research

F u n d , administered by the A m e r i c a n C h e m i c a l Society, and to the A C S D i v i s i o n o f P o l y m e r C h e m i s t r y , Inc., for their generous support. T h e i r financial contributions a l l o w e d several foreign contributors to participate in the A C S meeting upon w h i c h this book is based. C H A R L E S E. H O Y L E

University of Southern Mississippi Hattiesburg, MS 39406-0076 J O H N M. T O R K E L S O N

Northwestern University Evanston, IL 60201 August 4, 1987 XI

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 1

Overview of Polymer Photophysics Charles E. Hoyle Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406-0076

In order to write a complete review of polymer photophysics one would need a series o aspects of this continuall number of excellent collections and reviews (1-17) on polymer photophysics to which the reader is referred. The book by Guillet (1) is p a r t i c u l a r l y instructive as an overview of polymer photophysics and for that matter to the whole field of polymer photochemistry and is highly recommended to anyone who wishes to gain a rapid, but thorough introduction to the area. This introductory chapter is designed to introduce the reader to the current status of polymer photophysics. By analogy with small molecule photophysics, luminescence can be c l a s s i f i e d as fluorescence or phosphorescence depending on whether emission occurs from a singlet state or a triplet state, respectively. Polymer photophysics, both in its historical development as well as its current practice, can be divided into relatively few categories: excimer formation, luminescence anisotropy, luminescence quenching, luminescent probes and excited state energy migration. After introducing each of these basic categories, this chapter is concluded by a short analysis of the future of polymer photophysics. The fundamental and/or applied aspects of polymer photophysics will be noted where appropriate in each section. Excimer Formation Excimers are excited state complexes which consist of two identical species, one of which i s in the excited state prior to complexation (See Scheme I ) . The subject has been thoroughly reviewed for polymers in a recent a r t i c l e by Semerak and Frank ( 4 ) . Briefly (Scheme I ) , an excited monomer species M* combines with an identical ground state molecule M to produce an excimer E*. Both excited species M* and E* may undergo the normal processes f o r deactivation of excited states, i . e . , non-radiative decay, radiative decay, or product formation.

0097-6156/87/0358-0002$06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1. HOYLE

3

Overview of Polymer Photophysics

krjMtM!

M

^

*

Ε

kMD

2M +hiT

Μ+ηυ'

heat • product

• heat + product Scheme

I

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

4

In the publications on excimer formation in polymers to date, the vast majority have concentrated on homopolymers or copolymers having pendant aromatic chromophores such as phenyl or naphthyl groups. Polymers and copolymers based on Ί - v i n y l n a p h t h a l e n e , s t y r e n e , 2 - v i nyl naphthalene and N-vi nylcarbazole have probably received the most attention while polymers based on vinyl toluene, acenaphthalene, vinylpyrene, 2-naphthylmethacrylate, and a number of other monomers have also been studied, but to a l e s s e r e x t e n t . Excimer formation in such polymer systems i s especially favorable when the interacting species are "nearest neighbors" pendant to the polymer backbone and separated by three carbon atoms. However, excimers have also been reported for copolymer systems where the interactive chromophores are separated by a larger number of atoms. Theories dealing with the photophysics of excimer formation and decay i n v o l v i n g the " i s o l a t e d monomer" and "energy migration" concepts have been developed i n order to e x p l a i n the complex f l u o r e s c e n c e decay curve Application of these fluorescenc of i n t e r e s t as w i l l be demonstrated in chapters throughout t h i s book.

Lumi nescence An i sotropy If a randomly d i s t r i b u t e d ensemble of anisotropic fluorescent chromophores absorbs l i g h t which i s l i n e a r l y p o l a r i z e d , the resultant fluorescence w i l l retain, to a degree, the polarization of the exciting l i g h t source. The "depolarization" of the emission i s dependent on s e v e r a l f a c t o r s i n c l u d i n g the inherent degree of anistropy of the fluorescent chromophore, the degree of energy migration, and the rotation of the chromophore during i t s excited state l i f e t i m e . From s t e a d y - s t a t e and t r a n s i e n t emission a n i s o t r o p i c measurements, r o t a t i o n a l r e l a x a t i o n times can be deduced. Depending on the location of the anisotropic chromophore in the polymer, attached as a pendant group or incorporated as an integral part of the polymer backbone, the rotational relaxation times r e f l e c t either main chain or side group rotational properties. Since rotational relaxation times are d i r e c t l y related to rotational d i f f u s i o n c o e f f i c i e n t s , luminescence a n i s o t r o p i c measurements provide information on conformational states, chain dynamics and microviscosity which might otherwise be d i f f i c u l t to acquire. In g e n e r a l , luminescence a n i s o t r o p i c measurements are extremely important in evaluating c r i t i c a l properties of both synthetic and natural polymers. Examples are presented throughout t h i s book.

Luminescence Quenching Luminescence quenching involves deactivation of either an excited singlet (fluorescence quenching) or t r i p l e t state (phosphorescence quenching) by long range or short range interaction with a quencher molecule. The quenching process e f f i c i e n c y i s determined by a variety of factors which include both orientational and interactive (electron transfer, dipole-dipole, e t c . ) parameters. In general, quenching can be described as a process in which two participating species (the excited state molecule and the quencher) i n t e r a c t either by a diffusion ( c o l l i s i o n ) controlled (Stern-Volmer kinetics)

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1. HOYLE

Overview of Polymer Photophysics

5

or a s t a t i c mechanism (Perrin kinetics) (see Réf. 1 for d e t a i l s ) . In polymer systems, i t i s possible for the excited state chromophore and the quencher to exist in several combinations. The following are representative of the combinations one might expect to encounter in quenching experiments involving polymers: (a) The luminescent species i s attached to the polymer and the quencher i s a small molecule. (b) The luminescent species i s detached from the polymer and the quencher i s bound to the polymer. (c) Both the luminescent species attached to the same polymer.

and the quencher are

(d) The luminescent species and the quencher are attached to different polyme Depending on the particular combination, quenching studies generate detailed data concerning energy migration in polymers, degree of interpénétration of polymer chains, local quencher concentration, diffusion of small molecules in polymer networks, and phase changes.

Excited State Energy Migration Upon absorption of a photon of l i g h t by a particular chromophore on a polymer chain, one option of the resulting excited state species is to transfer i t s energy to an equivalent neighboring group in the ground state. This second species may then transfer i t s energy to another group. The probability of the transfer process occurring (versus deactivation) i s dictated by, among other f a c t o r s , i t s proximity to an equivalent neighbor in the ground state and the orientation of the two species involved in the energy transfer step. The sequential transfer of excited state energy from one chromophore to the next can result in energy migration over a large number of e q u i v a l e n t groups. T h i s energy migration phenomenon has been compared to the antenna effect in photosynthesis (]_). Theories to describe singlet and t r i p l e t energy migration in polymer systems have been developed and are included in several chapters in the book.

Luminescent Probes One of the basic goals in polymer science i s to identify the nature of the local environmental domains ( v i s c o s i t y , hydrophobicity, e t c . ) domains w i t h i n a polymer s o l i d or s o l u t i o n . T h i s may be accomplished by using a variety of photophysical techniques a l l of which involve mixing or covalently attaching a small amount of a luminescent molecule (probe) into a polymer system. The probe molecule i s designed such that one or more of i t s photophysical properties i s d i r e c t l y dependent on some aspect of i t s environment. For example, both the f l u o r e s c e n c e l i f e t i m e and the r e l a t i v e i n t e n s i t i e s of the vibrational structure of pyrene are altered when exposed to an aqueous, as opposed to a hydrophobic, medium. Polyelectrolytes such as p o l y ( a c r y l i c a c i d ) , under the proper

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

β

PHOTOPHYSICS OF POLYMERS conditions of pH and added e l e c t r o l y t e , have hydrophobic domains which are capable of s o l u b i l i z i n g pyrene. The net result i s pyrene molecules which behave photophysically as though they are in a hydrophobic environment, even though the primary medium i s water. The techniques u t i l i z e d to study hydrophobic domains in polymer solutions are derived, in part, from the large body of l i t e r a t u r e dealing with luminescent probes in mi c e l l a r solutions (1J_). Other examples of the use of luminescent molecules to probe m i c r o environments in polymer systems are l i s t e d below. (a) Molecules with twisted excited states sensitive probes of polymer microviscosity.

can act as

(b) Highly anisotropic luminescent molecules can be used as p r o b e s t o i n v e s t i g a t e o r i e n t a t i o n a l e f f e c t s i n stretched films o f i b e r s (c) Bichromophori excimer formation i s v i s c o s i t y dependent can be used to d e t e c t the l o c a l m i c r o v i s c o s i t y of polymer solutions. Such probes can also be used to monitor v i s c o s i t y changes which occur during polymerization. (d) Luminescent chromophores whose e x c i t e d state properties are solvent p o l a r i t y or v i s c o s i t y dependent can be i n c o r p o r a t e d as p a r t o f t h e p o l y m e r during polymerization and thereby act as a direct probe of the local environment. (An anisotropic probe bound to the polymer has already been discussed in an e a r l i e r section as one example of this phenomenon). (e) Small c h a r g e d m o l e c u l e s which change t h e i r luminescent p r o p e r t i e s upon c o m p l e x a t i o n with polyelectrolytes in aqueous media can be used to probe the effect of polymer conformation on binding. One can c e r t a i n l y imagine other uses for luminescent probe molecules (either attached or free) in addition to those l i s t e d . This area i s advancing rapidly and examples are given in individual chapters in the book.

Conclusions and Future As the contents of this book w i l l attest, the f i e l d of polymer photophysics continues to expand. The f i r s t decade of research in polymer photophysics was primarily, but c e r t a i n l y not e n t i r e l y , d i r e c t e d toward c h a r a c t e r i z a t i o n of the b a s i c e x c i t e d s t a t e properties of macromolecules. During the past ten years there has been an i n t e n s i f i e d e f f o r t to employ photophysics to solve basic questions about the nature of polymer systems. There i s c e r t a i n l y no reason to suspect anything other than an i n c r e a s i n g use of polymer photophysics as a fundamental tool to investigate phenomena such as blending, microviscosity, hydrophobic domains, polymer conformational structure, d i f f u s i o n , and polymerization. Polymer photophysics appears to have evolved from an academic c u r i o s i t y into

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1. HOYLE

Overview of Polymer Photophysics

a useful d i s c i p l i n e capable of solving problems in the polymer f i e l d . It i s a r t i c l e s presented in t h i s book w i l l appreciation f o r polymer photophysics contributing to the future development

7

both fundamental and applied hoped that the c o l l e c t i o n of furnish the reader with an and the potential i t has f o r of polymer science.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Guillet, J. E., Polymer Photophysics and Photochemistry, Cambridge University Press, Cambridge (1985). Ε. V. Anufrieva and Y. Y. Gotlib, Adv. Polym. Sci., 40, 1 (1981). K. P. Ghiggino, A. J. Roberts, and D. Phillips, Adv. Polym. Sci., 40 69 (1981). S. N. Semerak and C. W. Frank, Adv. Polym. Sci., 54, 33 (1984). Polymer Photophysics York, NY (1985). Photophysical and Photochemical Tools in Polymer Science, ed., M. Winnik, Reidel, Dordrecht, Netherlands (1987). Photochemistry and Photophysics of Polymers, eds., N. S. Allen and W. Schnabel, Applied Science, Ltd., London (1984). D. A. Holden and J. E. Guillet, in Developments in Polymer Photochemistry-1, ed., N. S. Allen, (1980), Applied Science, Ltd., London. E. D. Owen, in Developments in Polymer Photochemistry-1, ed. N. S. Allen, (1980), Applied Science, Ltd., London. A. C. Somersall and J. E. Guillet, J. Macromol. Sci., Rev. Macromol. Chem. C, 13, 135. J. Kerry Thomas, The Chemistry of Excitation at Interfaces, ACS Monograph 181, American Chemical Society, Washington, DC (1984). H. Morawetz, Science, 203, 405 (1979). Y. Nishijima, J. Polym. Sci., Part C, 31 353 (1970). Y. Nishijima, Progr. Polym. Sci. Jpn., 6, 199 (1973). Y. Nishijima, J. Macromol. Sci., Phys., 8, 407 (1973). Ann. Ν. Y. Acad. Sci., eds., H. Morawetz and I. Z. Steinberg, 366 (1981). I. Soutar in Developments in Polymer Photochemistry, ed., N. S. Allen, 3 (4), Applied Science, Ltd., London (1982).

RECEIVED August 4, 1987

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 2

Study of Complex Polymer Materials Fluorescence Quenching

Techniques

Mitchell A. Winnik Department of Chemistry and Erindale College, University of Toronto, Toronto M5S 1A1, Canada

Fluorescence quenching techniques (1) provide a battery of useful tools fo polymer systems. On of these techniques is to the study of interfaces and interphases in polymer systems. Many polymer materials contain polymer-polymer interfaces. These include polymer blends, interpenetrating networks, core-shell polymer c o l l o i d s , and polymer micelles. The properties of these materials depend, one believes, on the nature of the interface and on factors which operate within very short distances (50A - 100Â) of the interface. These are the dimensions o f polymer molecules, which means that a proper understanding of the performance of these materials requires understanding of the interface at the molecular level. Among the tools that permit one to obtain molecular information about interfaces [e.g., x-ray and neutron scattering, s o l i d state nmr (2)], fluorescence quenching methods (3) o f f e r some important advantages. They are s e n s i t i v e . The equipment i s r e a d i l y a v a i l a b l e and r e l a t i v e l y inexpensive. There i s scope and v e r s a t i l i t y to those methods. There are many sources i n the l i t e r a t u r e one can turn to for ideas f o r new experiments to study systems composed of synthetic polymers, because of the wide-spread applications of fluorescence techniques i n the b i o l o g i c a l sciences (4). This chapter provides a b r i e f introduction to some applications of fluorescence quenching to study interfaces i n polymer systems. STRATEGY Mechanism. Fluorescence quenching i s defined operationally as any interaction between a species F* i n an e l e c t r o n i c a l l y excited state and another species Q which leads to a decrease i n the fluorescence i n t e n s i t y or fluorescence decay time of F*. This d e f i n i t i o n encompasses a l l quenching mechanisms.

0097-6156/87/0358-0008$06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Fluorescence Quenching Techniques

2. WINNIK

9

A l l fluorescence quenching processes between non-diffusing pairs of excited states and quencher molecules are characterized by rate constants k (r) which are a function of the distance r between q them. Orientation may also be important. The s e n s i t i v i t y of each quenching process to distance and orientation depends upon the quenching mechanism. Energy transfer by dipole coupling (5,6) (Fôrster energy transfer) can occur over distances up to 100A. Electron transfer (6) can occur over 15À to 20A. Quenching by paramagnetic species requires o r b i t a l overlap and i s a short range process (6). A convenient way of c l a s s i f y i n g these processes i s to define a parameter R which represents the distance at which the quenching rate (for randomly oriented F/Q pairs) equals the unquenched decay rate of F*. R i s a measure of the span of the quenching process. The value of R w i l l depend upon the d e t a i l s of the quenching mechanism and the p a r t i c u l a s e l e c t i o n of R ranges c o l l e c t e d i n Table I. 0

0

0

0

Table I.

Bimolecular Excited State Quenching Processes effective

interaction

0

distance '**

mechanism 1.

Energy transfer by dipole coupling electron exchange reabsorption Electron transfer Exciplex formation Excimer formation Non-emissive self-quenching Heavy atom e f f e c t Chemical bond formation

2. 3. 4. 5. 6. 7.

10A to 100A 4A to 15A as f a r as emission reaches 4A to 25A 4A to 15A ca. 4A 4A to 15A ca. 4A ca. 2A to 4A

a

The minimum interaction distance i s a r b i t r a r i l y taken to be except where new chemical bonds are formed.

4A

^Each p a i r of chromophores, f o r each i n t e r a c t i o n mechanisms, has i t s own c h a r a c t e r i s t i c distance R at which the i n t e r a c t i o n rate equals the decay rate of the unquenched excited state. These values are estimates of the range of R for randomly oriented non-diffusing pairs of species. p

0

The experimenter chooses the quenching process and F/Q p a i r according to the distance scale he or she wishes to explore. For example, i n the study of simple polymer blends, energy transfer (here R = 22A) was much more s e n s i t i v e than exciplex formation (R = ca. 7 to 10A) at detecting small amounts of chain interpénétration at the interface (7,8). 0

0

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

10

Synthesis. Chemical synthesis i s an e s s e n t i a l part of any application of fluorescence quenching to the study of interfaces. Three quite d i f f e r e n t approaches have been described in the l i t e r a t u r e . F i r s t , one can label polymer-1 with F and polymer-2 with Q. In a blend of the two polymers F* and Q interact only at the interface between the polymers. Second, polymer-2 may f o r t u i t o u s l y quench fluorescence of a dye F attached to polymer-1. For example, poly(vinylmethyl ether) [PVME] quenches anthracene fluorescence (9). This lucky observation has been used i n conjunction with anthracene-labelled polystyrene [PS-Ant] to map out the phase diagram of mixtures of PS with PVME and to study spinodal decompostion k i n e t i c s (9). F i n a l l y , one can label e i t h e r polymer-1 or polymer-2 with F, and choose as a quencher a small molecule which i s p r e f e r e n t i a l l y soluble i n one of the polymers. Examples of t h i s approach are described below. This approach works best when one of the polymers i s i n the glassy state and the other i s rubbery Span. The span of a quenchin sensed by the experiment. It represents the resolution of the measurement. There are i n fact two quite d i f f e r e n t distance scales involved i n fluorescence quenching experiments. One i s determined by R . In experiments i n which d i f f u s i o n i s unimportant, R i s the only important distance scale. I f d i f f u s i o n of F or Q occurs over a distance comparable to or larger than R on a time scale of the l i f e t i m e of F*, the span w i l l be somewhat larger that R . In some experiments, mixing or demixing occurs on a much longer time scale. Here, where one studies changes i n fluorescence i n t e n s i t y (I) over a period of minutes or hours, the relevant distance scale of the experiment i s much larger. For demixing of PS-Ant and PVMF (9), the span i s larger than the dimensions of i n d i v i d u a l macromolecules and probably r e f l e c t s the s i z e of the i n i t i a l l y formed phase domains. In sorption experiments of a small molecule Q into a thin f i l m or small p a r t i c l e l a b e l l e d with F, the span i s the f i l m thickness or p a r t i c l e radius (10). 0

0

0

0

EXAMPLES The examples which follow are chosen from my laboratory. I do not wish to imply that they are the most i n t e r e s t i n g or most important experiments which use fluorescence quenching applications to rather complicated materials. They represent a prototype of the kinds of experiments and the kinds of materials one would study i n an i n d u s t r i a l laboratory. The materials we studied are non-aqueous dispersions of polymer p a r t i c l e s . C o l l o i d a l s t a b i l i t y of these p a r t i c l e s i n hydrocarbon solvents i s conferred by a surface covering of a highly swollen polymer (the s t a b i l i z e r ) on a second polymer, insoluble in the medium (the core polymer), which comprises 90*X of the material (11). These p a r t i c l e s are prepared by dispersion polymerization : polymerization of a monomer soluble in the medium to y i e l d an insoluble polymer, c a r r i e d out i n the presence of a soluble polymer which becomes the s t a b i l i z e r . In the examples discussed here, the core polymer i s formed by free r a d i c a l polymerization. Hydrogen abstraction from the soluble polymer present in the reaction medium

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2. WINNIK

11

Fluorescence Quenching Techniques

leads to formation o f a graft copolymer. In t h i s way we have prepared poly(methyl methacrylate) [P^MÀ] p a r t i c l e s s t a b i l i z e d by polyisobutylene [PIB] (12,13) and p o l y ( v i n y l acetate) [PVAc] p a r t i c l e s s t a b i l i z e d by poly(2-ethylhexyl methacrylate) [PEHMA] (14,15). PW1A P a r t i c l e s . ΡIB-stabilized PMMA p a r t i c l e s were prepared containing naphthalene [N] groups covalently attached to the PMMA chains. This was effected quite simply by adding 1-naphthylmethyl methacrylate to the MMA polymerization step of the p a r t i c l e synthesis. From r e a c t i v i t y r a t i o s , one knows that the Ν groups are randomly d i s t r i b u t e d along the P^f!A chains. The p a r t i c l e s were p u r i f i e d by repeated centrifugation, replacement o f the supernatant serum with fresh solvent (isooctane) and redispersion. A fluorescence spectrum of the dispersion was t y p i c a l of that o f a 1-alkyl-naphthalene. Chemical analysi indicated particl composition IB/NWA/N o When small amount 1 χ 10" M] were added to p a r t i c l e dispersions, energy transfer from N* to Ant could be observed (cf. Figure 1)(12). At f i r s t we thought t h i s occurred only from Ν groups near the p a r t i c l e s surface. When the concentration [Ant] was altered, the fluorescence decay time of the N* also changed (12,13). Light penetrates into these 2 μια diameter p a r t i c l e s to a depth equal to at least i t s wavelength (ca. 3000Â). I f the l i f e t i m e o f a l l the Ν groups i s decreased by Ant added outside the p a r t i c l e , the Ant groups must have diffused into the p a r t i c l e i n t e r i o r during sample preparation. This curious observation i s at odds with a s o l i d core structure for the p a r t i c l e . One can estimate the t r a n s l a t i o n a l d i f f u s i o n c o e f f i c i e n t o f Ant i n PWA (/2

(2)

where Θ(t) i s the angle (the t r a n s i t i o n moment i n FAD) rotates during time t , the brackets mean an ensemble average. The time i n t e r v a l t during which the evolution of I ^ C t ) can_^>g recordgd i s d i r e c t l y related to the fluorescence l i f e t i m e (10 to 10 s ) ; e x p e r i m e n t a l l y M2(t) can be obtained u n t i l approximately 10 s (100 ns). It should be pointed out that FAD i s v i r t u a l l y unique i n i t s a b i l i t y to obtain the OACF. A detailed study of the FAD of anthracene labeled polybutadiene i n a matrix of Diene 45 NF has been performed i n the temperature range 240K-353K ( 2 ). At 335.7K, the FAD curve shown i n Figure 3 i s p a r t i c u l a r l y suitable f o r comparison with various molecular models of l o c a l chain dynamics. Indeed, at t h i s temperature the decay i s not too f a s t , i n such a way that an accurate comparison with the models can be made over the f u l l time window available, but nevertheless the FAD curve goes close to zero. I t appears that the i s o t r o p i c model (single exponential f o r the OACF) does not f i t the data and that OACF expressions s p e c i f i c a l l y proposed f o r polymer dynamics (for a review of them, see Ref. _3 and _5 ) are required. The s p e c i f i c behavior of polymer chains i s due to the chain connectivity requirement and the description of l o c a l dynamics requires consideration of the following features: - "elementary motions" with a c h a r a c t e r i s t i c time, τ^ι which diffuse along the chain sequence and lead to a non-exponential short time term i n the OACF. - a damping or a truncation of t h i s d i f f u s i o n , which yields i n the OACF an exponential loss, with a c h a r a c t e r i s t i c time τ. (τ > τ ) For bulk polybutadiene, the best f i t , shown i n Figure 2r, i s reached i n using the OACF expression proposed by HALL and HELFAND ( 6 ): M (t) 2

where I

= e x p ( - t / T ) exp( - tl τ )I (t/T ) 2

χ

Q

1

(3)

represents the modified Bessel function of order 0.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

50

100

200

300

400

CHANNEL

Figure 3. Comparison of the best f i t obtained from the Hall-Helfand expression reconvoluted by the measured instrumental function (excitation pulse) (continuous l i n e ) with the experimental anisotropy (dots) of labeled polybutadiene at 335.7K. The excitation pulse i s plotted as a dash-dot l i n e (arbitrary scaled). The upper graph represents the weighted residuals.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

51

Local Dynamics in Polybutadienes

5. MONNERIE ET AL.

It i s worth noting that t h i s expression also gives the best f i t for FAD curves of labeled polystyrene i n solution ( 5_ )· In bulk polybutadiene, the r a t i o ^2^ \ ^ Y s i g n i f i c a n t l y with temperature and remains rather high (=30). This implies that the processes responsible for the damping i n polybutadiene are slow compared to the d i f f u s i v e ones. The temperature dependence of has been studied i n the range 240K- 353K. I t i s interesting to compare t h i s dependence with the prediction of the well-known William, Landel, Ferry time-temperature superposition equation ( 7. ) which can be written as : τ

log τ

cj/ ( T - T J

= Cte +

Τ

Too

=

T

g

o e s

n o t

v a r

(4)

- c:

where Tg i s the glass-rubbe 45 NF) and C ^ , C 2 are phenomenological parameters taken from low frequency v i s c o e l a s t i c measurements (For Diene 45 NF, ϋ = 11.2 and C = 60.5K ( ]_ ) ) . In Figure 4, the c o r r e l a t i o n time i s plotted versus Ι/ζΤ-Τ^) and the dash-line corresponds to the W.L.F. equation. The experimental data satisfactory f i t a linear dependence with a slope f a i r l y close to the predicted one. This means that the l o c a l reorientation processes observed by FAD are involved i n the glass-rubber t r a n s i t i o n phenomenon. It should be pointed out that a comparison of the FAD curves of labeled polybutadiene to the ones obtained for free 9,10 ( d i a l k y l ) anthracene probes i n a Diene 45 NF matrix led to the conclusion ( 8 ) that the l o c a l motions observed by the FAD experiments performed on labeled polybutadiene involve about 6 butadiene units. 8

8

g

χ

2

CARBON 13 N.M.R. 13 The l o c a l dynamics of polymer chains can be studied by C NMR through measurements of J^e s p i n - l a t t i c e magnetic relaxation time, T^. The spin of a given ^C relaxes by dipolar relaxation mechanism with the bonded Η so that the corresponding T^ i s related to the reorientation motion of the involved CH internuclear vector through the following expression ( 9. ) : 9 J. = 1 4

IT

2

2

Ύ Ύ _9_H 10

1[ r CH

J ( u )

H

ω

C

j

+

3

j(

w

) C

+

6

J ( ( i )

+ uO]

(5)

oo

J (ω) = /

ο

M„(t) e " I

i u ) t

dt

where h i s the Planck c o n s t a n t , and γ„ are the gyromagnetic r a t i o s of C and ''H respecγjvely and ^ are the Larmor resonance frequencies of C and ^H, J(o)) i s the spectral density at the frequency ω of the OACF of the CH motion, and r „ i s the length of the CH bond. r

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

Θ Diene 45 NF

Θ UR A N

4

5

6

7

8

10 /(T-TOO) 3

Figure 4 . - Logarithmic plot of the correlation time τ- vs ( T-T r . a/ determined from FAD f o r labeled polybutadiene i n a Diene 45 NF matrix. ^ b/ τ determined from C T measurements f o r polyButadiene Uran. Dashed l i n e s correspond to Equation 4 using i n each case the c o e f f i c i e n t s appropriate to the considered matrix. 1

m

1

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

5.

MONNERIE ET AL.

53

Local Dynamics in Poly butadienes A J

The main interest of C NMR spectroscopy i s that i t allows one to investigate the l o c a l motions performed by the various groups of the polymer chain. Thus, i t y i e l d s more detailed information than other techniques. On the other hand, to derive the correlation times of the motions from the T, measurements, i t i s necessary to use an "a p r i o r i " expression 01 the OACF, M2(t). In the case of polymer melts at temperatures higher than (Tg+60K), the segmental mobility i s high enough to get narrow resonance l i n e s i n the NMR spectrum and thus to perform the T. measurements on the same spectrometers which are commonly used for high resolution NMR i n l i q u i d s . Polybutadiene Uran has been studied at 25.15 MHz and 62.5 MHz i n the temperature range 224 Κ - 358 Κ. The nT, values obtained for CH and 0Η« groups of Uran, wh^re η = 1 tor CH and η = 2 for CH are shown as a function of 10 /T i n Figures 5 and 6 On the same figures are plotted the best f i t s obtained by usin c l e a r l y appears that th minimum are much higher than those expected from t h i s OACF expression. I t should be noticed that the comparison would be even worse with the single exponential OACF corresponding to an i s o t r o p i c motional model. Secondly i n the whole temperature range, the r a t i o T (CH)/T (CH ) i s d i f f e r e n t from the expected value of 2, i t s value l i e s around 1.5 at both observation frequencies. These two discrepancies can be accounted for by considering, i n addition to the damped d i f f u s i o n of elementary motions along the chain sequence (which leads to the Hall-Helfand expression for the OACF), either a Brownian motion of the CH bonds inside a cone of half angle Θ, or a s p e c i f i c anisotropic motion of the c i s 1,4 sequences of the polybutadiene chain ( £ ). In both cases, the r e s u l t i n g OACF can be written as : 2>

1

1

2

M ( t ) = (1 - a) exp( -

exp( - t / T ^ I ^ t / ^ )

2

+

a exp(

-

t/τ

) exp(

-

t/τ

)I (t/T

}

Q

χ

)

(6)

where the parameter a describes the contribution of the anisotropic motion with correlation time τ . The best f i t s (shown on Figures 5 and 6) are obtained with τ /τ > 150, τ /τ > 600 and a(CH) = 0.27, a(CH" ) = 0Î46? These values indicate that the additional anisotropic motion (characterized by τ ) i s very fast as compared to the elementary segmental motions (described by T- ). In the explored temperature range, τ should be snorter than J§ s at 224K and 10 s at §58K. Recent C T, measurements performed on various bulk polymers with low Tg, such as polyisoprene, polyisobutylene, polyvinylmethylether and polypropyleneoxide, have shown that the experimental value of T^ at the minimum i s always much higher than the prediction from Hall-Helfand expression. Thus, t h i s discrepancy cannot be assigned to a s p e c i f i c motional anisotropy of the c i s 1,4 sequences of the polybutadiene chain. I t seems more l i k e l y to assign i t to a fast anisotropic motion of the CH bonds inside a cone. In the case of polybutadiene, the corresponding half angle of the cones should be 26° and 36° for CH and CH 2

?

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

54

PHOTOPHYSICS OF POLYMERS

H

« 4.5

« 4

35

« 3

ô-

~~

10 /Τ 3

13 Figure 5. Temperature dependence of C T^ at 25.15 MHz and 62.5 MHz for CH of Uran polybutadiene. Dots are experimental data. The dash-dot l i n e s correspond to the Hall-Helfand f i t (Equation 3). The dashed l i n e s are the best f i t obtained by using Equation 6. nT, (s)

4.5

4

3.5

3

10 /τ 3

13 Figure 6. Temperature dependence of C T^ at 25.15 MHz and 62.5 MHz for C ^ of Uran polybutadiene (same representation as i n Figure 5 ) .

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

5.

MONNERIE ET AL.

Local Dynamics in Poly butadienes

55

respectively. This result i s quite satisfactory, the constraints due to the size and the r i g i d i t y of the double bond decreasing the amplitude of the l i b r a t i o n motion of the CH bonds r e l a t i v e to that of the CH groups which are linked to the chain backbone through single bonds only. Another interesting feature deals with the high value obtained for the ratioT2/T^. This means that i n high c i s content polybutadiene, the damping of the d i f f u s i o n of the elementary motions along the chain sequence i s very weak. The correlation^time derived from T^ values of Uran i s plotted vs (T-T ) on Figure 4. A satisfactory agreement i s found with the preïiction of Equation 4, using the appropriate c o e f f i c i e n t s ( T = 101K, C j = 11.35, C = 59.6K ( J_ ) ) . Thus, the elementary motions of the polybutadiene chain which are investigated by C NMR are involved i n the glass-rubber t r a n s i t i o n phenomenon 2

8

8

œ

COMPARISON OF THE CORRELATIO The spectroscopic techniques which have been used to investigate the segmental motions of bulk polybutadiene i n a temperature range f a r enough from the glass-rubber t r a n s i t i o n show that these motions are involved i n the glass-rubber t r a n s i t i o n phenomenon. Thus, i t i s interesting to go further i n comparing the absolute values of the obtained correlation times. At a given (T-T ) ^ i n t e r v a l , for example 192.3K corresponding to a value of 5.2 for lO^/CT-TJ, the correlation time of the anthracene l a b e l i n the middle of the chain i s 6.5x10 s. At the same temperature the value of , ^ corresponding to the_çjementary motions responsible ror C relaxation i s 4.4x10 s. Thus, there i s about 2 decades of difference i n the correlation times. It i s worthnoting that such a difference i n correlation time values does not originate from the difference of microstructure between Diene 45 NF and Uran. Indeed, preliminary measurements have shown that for the same (T-T^) i n t e r v a l , similar T^ values are found for bot^polymers. °° As the C T^ measurements do not imply any labeling, the corresponding correlation time has to be considered as the actual c h a r a c t e r i s t i c time of the elementary motions of the polybutadiene chain. The longer correlation time observed with FAD for anthracene labeled polybutadiene might originate either from an i n e r t i a l e f f e c t of the anthracene group or, more l i k e l y , from the larger volume which i s required to rotate s i g n i f i c a n t l y the anthracene t r a n s i t i o n moment lying along the l o c a l chain axis, compared to the volume which i s involved i n the conformational changes leading to C s p i n - l a t t i c e relaxation. Such a statement i s consistent with the result ( 8 ) that about 6 monomer units are involved i n the motions observed by FAD for labeled polybutadiene. œ

τ

CONCLUSION In t h i s paper we have investigated the segmental motions of bulk polybutadiene, i n a temperature range h i g ^ r than (Tg + 60K), by using fluorescence anisotropy decay, and C spin-lattice magnetic relaxation time, T . 1

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

56

PHOTOPHYSICS OF POLYMERS

The FAD technique has shown that the orientation autocorrelation function of l o c a l motions i n a polymer chain does not correspond to the single exponential associated with i s o t r o p i c rotation. On the other hand, the Hall-Helfand expression which accounts for the chain connectivity requirement leads to a very satisfactory agreement. Thus the segmental dynamics of polymer melts should be described by elementary motions undergoing a damped d i f f u s i o n along the chain sequence, i n a similar way to that i s found i n polymer solutions. This indicates that the same types of conformational changes are involved i n both cases, the slowing down of these jumps i n polymer melts, compared to the case of polymer solutions, arises from the intermolecular c o n s t r a i n ^ . In addition to these elementary motions, the C T^ measurements have shown that a much faster anisotropic motion occurs, corresponding to a l i b r a t i o n motion of the CH bonds inside a cone. Recent C T^ measurements ( 10 ) performed on bulk polybutadiene containin shown a lower mobility units. This result i l l u s t r a t e s the main interest of C studies, allowing one to reach more detailed information on the dynamics of each group i n the polymer chain and thus yielding a way of setting up a relationship between the chemical structure and the dynamic behavior of bulk polymers. Another important result deals with the temperature dependence of the correlation times of the elementary motions, which agrees f a i r l y well with the prediction of the William, Landel, Ferry equation, using the phenomenological c o e f f i c i e n t s obtained from low frequency v i s c o e l a s t i c measurements. T M s means that the elementary motions which are observed by FAD and C T^ are involved i n the glass-rubber t r a n s i t i o n phenomenon. In t h i s paper, the considered spectroscopic techniques have been applied to bulk polybutadiene. Similar studies on other polymer chains are under progress.

LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Valeur, B.; Monnerie, L. J . Polym. Sci. , Polym. Phys. Ed. , 1976, 14 , 11. Viovy, J.L.; Monnerie, L . ; Merola, F. Macromolecules , 1985, 18 , 1130. Brochon, J.C. In Protein Dynamics and Energy Transduction ; Shin'ishi Ishiwata,Ed.; Taniguchi Foundation: Japan, 1980. Monnerie, L. In Static and Dynamic Properties of the Polymeric Solid State ; Pethrick,R.A.; Richards, R.W., Ed.; NASI Series: Reidel Publ., 1982. Viovy, J.L.; Monnerie, L . ; Brochon, J.C. Macromolecules , 1983, 16 , 1845. Hall, C.K.; Helfand, E. J.Chem.Phys. , 1982, 77 , 3275. Ferry,J.D. Viscoelastic Properties of Polymers , 3rd ed.; Wiley: New-York, 1980. Viovy, J.L.; Frank, C.W.; Monnerie, L. Macromolecules , 1985, 18 , 2606. Gronski,W. Makromol. Chem. , 1977, 178 , 2949. Dejean de la Batie, R. Dr.Sc. Thesis, Université Pierre et Marie Curie, Paris, 1986.

RECEIVED March 13, 1987 In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 6

Excluded Volume Effects on Polymer Cyclization Mitchell A. Winnik Department of Chemistry and Erindale College, University of Toronto, Toronto M5S 1A1, Canada

Recent theory suggests that cyclization phenomena (1) are much more sensitiv other properties o fluorescence quenching processes i n molecules contain­ ing appropriate end groups permit one to study both the dynamics and thermodynamics of end-to-end cycliza­ tion. As a consequence, the sensitivity of polymer cyclization to excluded volume can be examined. Excluded volume effects (2) i n polymers are defined as those e f f e c t s which come about through the s t e r i c interaction of monomer units which are remotely positioned along the chain contour. Each individual interaction has only a small e f f e c t , but, because there can be many such interactions i n a long polymer, excluded volume e f f e c t s become very large. One consequence of excluded volume i s to expand the polymer c o i l dimensions over that predicted from simple random walk models. The "unperturbed" values of the root-mean-squared end-to-end distance Rp° and radius of gyration R^ 0

for ideal (random walk) chains expand (to R„ and R~) i n the presence J?

b

of excluded volume. In solution, excluded volume e f f e c t s on a polymer can be suppressed by decreasing the q u a l i t y of the solvent f o r the polymer. Shrinkage o f the mean dimensions occur. At various individual 2 combinations of solvent and temperature, R., , as measured from l i g h t b

2 or neutron scattering, exactly equals (RQ°) · In these "theta-solvents," other properties o f polymers are expected to follow ideal behaviour. Recent theory suggests that c y c l i z a t i o n e q u i l i b r i a are doubly s e n s i t i v e to excluded volume e f f e c t s (3). Not only does chain expansion increase the mean chain end separation, but a second factor due to p a i r correlations also acts to decrease the p r o b a b i l i t y o f the chain ends being i n proximity: The two chain 0097-6156/87/0358-0057$06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

58

PHOTOPHYSICS OF POLYMERS

ends cannot, obviously, occupy the same space. In addition, the adjacent segments on the two chain ends also i n t e r f e r e with each other. Their positions are correlated with those of the chain ends. These e f f e c t s sum up i n such a way as to make i t very d i f f i c u l t f o r the chain ends even to get near one another. In poor solvents, unfavorable solvent-polymer interactions lead to net a t t r a c t i o n between the chain ends and t h e i r adjacent segments. This tends to overcome excluded volume repulsion. At the theta-point, these two factors should be exactly i n balance. The chain should recover ideal behaviour. Ideal behaviour f o r chain conformation i s represented by a Gaussian d i s t r i b u t i o n W(r) of end-to-end distances. Excluded volume e f f e c t s give a much d i f f e r e n t end distance d i s t r i b u t i o n function, Figure 1, with the biggest differences operating on chains which happen to have t h e i r chain ends i n proximity (4). C y c l i z a t i o n p r o b a b i l i t y depends upon the r a d i a l d i s t r i b u t i o n function 2

W(0) = lim 47ir W(r) =

4ΤΓΓ [3/2nR ] 2

2

3/2

(1)

r-K>

-3/2 For ideal chains W(0) should decrease as chain length Ν . For very long chains experiencing f u l l excluded volume, the exponent i s predicted to take the value -1.92. Measurements of c y c l i z a t i o n e q u i l i b r i a should allow these predictions to be tested. The theory of c y c l i z a t i o n dynamics i s less advanced. The prediction of the c l a s s i c treatment of Wilemski and Fixman (5) suggests that the D -1.5 d i f f u s i o n - c o n t r o l l e d c y c l i z a t i o n rate constant k^ ~ Ν i n the absence of excluded volume. The following sections of t h i s chapter examine experimental r e s u l t s about excluded volume e f f e c t s on experimental values of the c y c l i z a t i o n equilibrium constant Κ and cy the rate constant f o r end-to-end c y c l i z a t i o n k^ (6). The Polymer.

The focus here i s on the c y c l i z a t i o n behaviour of —6 polystyrene i n d i l u t e solution (ca. 2 χ 10 M) as studied through measurements of intramolecular excimer formation i n 1, (7) and exciplex formation i n 2 (8). (CH ) C0 CH CH -(CHCH ) (CHgCH)^-CHgCHgOgC(CHg), fFS^ '"2'3"2"2"2 T 2'm 2? 'n v

ο

2

3

2

2

2

V

2

m

v

o

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

WINNIK

Excluded

Volume Effects on Cyclization

Figure 1. A plot o f the end-distance d i s t r i b u t i o n function W(0) vs. the r a t i o of the end separation divided by R for a Q

Gaussian chain and for self-avoiding walk [SAW, f u l l excluded volume], following Ref. 4.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

60

CH CH- (- C H Ç H - ) - C H C H 0 C (C H ) 3

2

οι

X

Ζι

ο

2

2

2

2

3
— PC

N D : YAG

4 0 3 nm EXCITATION x 588nm PROBE

SAMPLE

SUM XTAL

POL

2x XTAL

DL

/ H D L

2

V A R I A B L E PROBE DELAY

Figure 1. Q-switched, mode-locked Nd:YAG laser with two synchronously pumped dye lasers: PC = Pockels' c e l l ; POL = polarizer with escape window; DL1, DL2 = cavity dumped dye lasers; PMT = photomultiplier tube. (Reproduced from Ref. 7. Copyright 1986 American Chemical Society.)

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

7.

WALDOW E T AL.

71

Time-Resolved Optical Spectroscopy

Nd:YAG laser synchronously pumps two dye lasers. One of the dye lasers (DL1) i s operated at a wavelength of 649 nm. A cavitydumped single pulse from this dye laser i s frequency summed with a single pulse of the YAG fundamental (1064 nm) to produce the excitation wavelength of 403 nm. This pulse i s beamsplit to form the grating. A second dye laser (DL2) provides the probe pulse (588 nm) which i s sent down an optical delayline before entering the sample at the Bragg angle relative to the grating formed by the excitation beams. Varying the position of a r e t r o r e f l e c t o r on the optical delayline varies the timing between the excitation and probe pulses and thus provides the time base for the experiment. The diffracted probe beam i s detected by a photomultiplier and a l o c k - i n amplifier. A microcomputer controls the data acquisition by varying the probe polarization and monitoring the d i f f r a c t e d signal strength and the delayline position. The sample temperature during any given measurement constant t ± 0.1°C Experimental Technique picosecon grating techniqu has been extensively u t i l i z e d to study electronic excitation transfer(13_5_1£), rotational reorientation of small molecules(lQ), and acoustic phenomena(15). On longer time scales, the same concept is u t i l i z e d in "Forced Rayleigh scattering experiments to measure translational diffusion(16). In a transient grating experiment, optical interference between two crossed laser pulses creates a s p a t i a l l y periodic intensity pattern in an absorbing sample. This results in a spatial grating of excited states which then d i f f r a c t s a third (probe) beam brought into the sample at some l a t e r time. The two observable experimental quantities are the intensity of the diffracted signal for the probe beam polarized p a r a l l e l (T||(t)) and perpendicular (Tx(t)) to the polarization of the excitation beams. Figure 2 shows T||(t) and Tj_(t) for anthracene-labeled polyisoprene in dilute hexane solution. The sharp r i s i n g edge in the data indicates the time when the excitation pulse enters the sample. At times very soon after t h i s , there i s a large difference between the diffracted signal for the two different probe polarizations. Τ π("t) i s larger than Tj_(t) since a larger number of excited state transition dipoles are oriented p a r a l l e l to the excitation pulse p o l a r i z a t i o n . As molecular motions of the polymer occur, the excited state transition dipoles randomize their orientations, and the difference between T y i t ) and Tj_(t) goes to zero. Both curves continue to decay due to the excited state l i f e t i m e (~ 8 nsec). Quantitatively, the shapes of T||(t) and Tj_(t) are given in terms of the orientation autocorrelation function CF(t) and the excited state decay function K(t) as follows: T„(t) =

{K(t)(l+2r(t))}

Ti(t) =

{K(t)(l-r(t))}

(1)

2

2 In these equations, r ( t ) is the time-dependent anisotropy which i s proportional to CF(t) as

(2) function,

r ( t ) = r(0)CF(t)

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

(3)

72

PHOTOPHYSICS OF POLYMERS

S

0

1

1

2

1

3

1

4

1

i

5

6

1

7

1

r

8

Nanoseconds Figure 2. Transient grating decays for 9,lO-bis(methylene)anthracene labeled polyisoprene in dilute hexane solution. Τ and Τι are the d i f f r a c t i o n e f f i c i e n c i e s of the grating for the probe beam polarized p a r a l l e l and perpendicular to the excitation beams (see Equations 1 and 2). The two curves are i n i t i a l l y d i f ­ ferent because the excitation beams create an anisotropic orientational d i s t r i b u t i o n of excited state transition dipoles. As backbone motions occur, the transition dipoles randomize and the two curves coalesce. Both curves eventually decay due to the excited state l i f e t i m e . The structure of the anthracene-labeled polyisoprene i s also displayed, with the position of the tran­ s i t i o n dipole indicated by a double arrow. (Reproduced from Ref. 7. Copyright 1986 American Chemical Society.) K

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Time-Resolved Optical Spectroscopy

where r(0) i s the fundamental anisotropy for the transition being observed. r ( t ) can be obtained from the transient grating signals by the following manipulation: r(t) =

/TjjTt)

-

STJt) _ _ _

(4)

vTTTt) + 2VT]Tt) The experimental anisotropy contains information about molecular motion, but i s independent of the excited state l i f e t i m e . Equation 4 indicates that the orientation autocorrelation function can be obtained d i r e c t l y and unambiguously (within the m u l t i p l i c a t i v e constant r(0)) from the results of a transient grating experiment. By setting the time delay between the excitation and probe beams to a given value and then alternating the polarization of the probe beam, the experimenta obtained using Equation experimental anisotropy i s obtained at a r e l a t i v e l y small number of time points but with quite good precision at each point. The results of such an experiment on a dilute solution of labeled polyisoprene in hexane are shown in Figure 3. The smooth curve running through the data points i s the best f i t theoretical curve using the Hall-Helfand model. In general, the f i t t i n g of the anisotropy involved three adjustable parameters: r ( 0 ) , and the two parameters in each correlation function. The f i n i t e lengths of the excitation and probe pulses were accounted for in the f i t t i n g procedure. Further details on data a c q u i s i t i o n , error analysis, and curve f i t t i n g are presented in Ref. 7. Materials. A polyisoprene prepolymer was prepared by anionic polymerization in benzene at room temperature. The prepolymer was coupled with 9,10-bis(bromomethyl) anthracene to form a labeled polymer of twice the molecular weight of the prepolymer. Fractionation of the resulting products allowed polyisoprene chains containing exactly one anthracene chromophore in the chain center to be isolated. The labeled polymer was M = 10,800 and M /M = 1.16. The structure of the labeled chain i s shown as an inset in Figure 2 (see below). The chain microstructure was determined to be 39% c i s - 1 , 4 , 36% trans-1,4, and 25% v i n y l - 3 , 4 . A l l solvents were used as received and showed negligible absorption in the wavelength region of i n t e r e s t . The hexane and cyclohexane were spectrophotometry grade (99+%) while the 2-pentanone was 97%. Solvent v i s c o s i t i e s were obtained from Ref. 17. n

w

n

Label E f f e c t . In order to assess at least p a r t i a l l y the e f f e c t of the label on the chain dynamics, we also performed measurements on dilute solutions of 9,10-dimethyl anthracene. The reorientation time for the free dye in cyclohexane was -10 psec, 50 times faster than the time scale for motion of the labeled chain in cyclohexane. Hence we conclude that the observed correlation functions are not dominated by the hydrodynamic interaction of the chromophore i t s e l f with the solvent, but can be attributed to the polymer chain motions.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

74

PHOTOPHYSICS OF POLYMERS

1 Hli ι 1 '''t Τ '

t

\

1

î

1

α ο

c
- · «ο ·»->

C

i-

— C O fO (D -r31 CD >

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Time-Resolved Optical Spectroscopy

the correlation function in 2-pentanone. For the sake of c l a r i t y , we have chosen to f i t this data to only one model. The model of Hall and He!fand was chosen because i t provided a good f i t to data from a l l three solvents. In addition, the model i s easily understood in physical terms(11). This model considers a two state system involving both correlated and isolated conformational transitions. Correlated transitions allow small internal sections of chains to change their conformational states without requiring reorientation of the chain ends. Isolated conformational transitions involve some sort of translation or reorientation of larger sections of the polymer chain. In this model, τχ and τ are time constants for correlated and isolated t r a n s i t i o n s , respectively (see Table I ) . Figure 5 shows time dependent anisotropics for dilute solutions of labeled polyisoprene in 2pentanone at various temperatures. Also shown are the best f i t s to the anisotropics using the Hall-Helfand model The best f i t parameters are given in shape of the correlatio temperatures studied i s the same as the shape of the correlation functions observed in hexane and cyclohexane. We can analyze the temperature dependence of the local segmental dynamics in terms of the Kramers picture of diffusion over a barrier(18,19). Presumably, more than one fundamental process contributes to the local motion observed in these experiments. In this case, we can write an expression for the rate w-| of the i process which contributes to the reorientation of the chromophore's transition dipole, in terms of the viscosity n and the activation energy Ε ·: 2

t

n

Ί

w « - exp (-E /kT) i n i

(6)

The fact that the observed correlation functions do not change shape as a function of temperature implies that the r e l a t i v e contributions of these various fundamental processes do not change s i g n i f i c a n t l y with temperature. Hence we can replace Equation 6 with w « - exp (-E/kT) η

(7)

where w i s inversely proportional to the time scale of the correlation function decay and Ε represents an average activation energy for the local segmental motions being observed. If we l e t τ represent the time required for the correlation function to reach 1/e of i t s original value, then a plot of 1η(τ/η) versus 1/T w i l l y i e l d E. Figure 6 displays this plot, with the slope giving Ε = 7.4 = 0.7 kJ/mole. (Of course, other procedures for picking a c h a r a c t e r i s t i c τ would y i e l d the same activation energy.) To the best of our knowledge, other measurements of the activation energy for the local segmental dynamics of polyisoprene have not been reported. Our estimate of the activation energy i s in reasonable accord with the 8.3 kJ/mole torsional barrier for propene(20). The above treatment i m p l i c i t l y assumes that the local segmental dynamics are not affected by changes in global chain

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

78

Nanoseconds

Figure 4. Time-dependent anisotropy for anthracene-labeled polyisoprene in dilute cyclohexane solution. The smooth curve through the data i s the best f i t to the Bendler-Yaris model(T]=210 ps, X2=2750 ps, and r(0)=0.243).

Figure 5. Time-dependent anisotropics for labeled polyisoprene chains in dilute 2-pentanone solutions. The smooth curves through the data points are the best f i t s to the Hall-Helfand model for 22.8 °C, -8.6 °C, and -26.5 °C (bottom to top). The data at 35.1 °C i s omitted for c l a r i t y . Semilog plots of the best f i t correlation functions are shown in the inset. Note that a l l the correlation functions are quite non-exponential.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

WALDOW E T AL.

Time-Resolved Optical Spectroscopy

7.2T

7.1 7.0 6.9 l- 6.8

z 6.7

6.5

6

-v

1000/T(K)

Figure 6. Arrhenius plot for dilute solutions of labeled polyisoprene in 2-pentanone. The activation energy calculated from the slope of the best f i t line is 7.4 kJ/mole. On the ver t i c a l scale, τ represents the 1/e point of the best f i t correla tion functions using the Hall-Helfand model. The data points represent results of independent experiments. The units for τ and n are ps and centipoise, respectively.

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PHOTOPHYSICS OF POLYMERS

dimensions that may result from temperature changes. This assumption i s expected to be valid for the chains used in this study because of their low molecular weight. Experiments currently in progress are examining the molecular weight dependence of the local segmental dynamics of polyisoprene. Comparison to Previous Work Hatada, et al. have reported C-13 NMR experiments on a dilute solution of polyisoprene in methylene chloride(21). It is d i f f i c u l t to make a rigorous comparison with these measurements for two reasons: 1) The NMR measurements were made at a single frequency and temperature. Hence no information about the shape of the correlation function is available. 2) The NMR experiment senses reorientation of a C-H vector which i s perpendicular to the chain backbone. Our optical experiments measure the correlation function for a vector p a r a l l e make at least a rough compariso the Brownian dynamics simulations of Weber and He1fand(22). These simulations for a polyethylene-like chain compared the correlation functions for vectors both p a r a l l e l and perpendicular to the chain backbone. Their results indicate that the correlation function for the perpendicular bond decays - 4 times faster than the correlation function for the p a r a l l e l bond. If we assume that the correlation function for a C-H vector in polyisoprene has the shape obtained in the computer simulation, and further assume that the relationship between p a r a l l e l and perpendicular vectors revealed by the simulations is v a l i d for polyisoprene, a comparison can be made between the optical and NMR experiments. Following the above procedure, we used the NMR data of Hatada, et al. to estimate the correlation function for a bond in the chain backbone. The resulting estimate was - 2.5 times faster than results obtained in the optical experiments. The difference represents either the crudeness of the method used for comparison, or the perturbation of the chain dynamics caused by the presence of the label in the optical experiments. In the future, we w i l l perform measurements with both techniques on systems where the two sets of results can be more easily correlated. Viovy, Monnerie, and Brochon have performed fluorescence anisotropy decay measurements on the nanosecond time scale on d i l u t e solutions of anthracene-labeled p o l y s t y r e n e . In contrast to our results on labeled polyisoprene, Viovy, et al. reported that t h e i r Generalized Diffusion and Loss model (see Table I) f i t their results better than the Hall-Helfand or Bendler-Yaris models. This conclusion i s similar to that recently reached by Sasaki, Yamamoto, and Nishijima(3) after performing fluorescence measurements on anthracene-labeled poly(methyl methacrylate). These differences in the observed correlation function shapes could be taken either to r e f l e c t the non-universal character of local motions, or to indicate a s i g n i f i c a n t difference between chains of moderate f l e x i b i l i t y and high f l e x i b i l i t y . Further investigations w i l l shed l i g h t on this point. In spite of the difference in the shape of the observed correlation functions, i t i s interesting to compare the timescale

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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81

of the decays in the polystyrene and polyisoprene systems. At the same v i s c o s i t y , our results for labeled polyisoprene indicate motions occuring - 9.5 times faster than for the labeled polystyrene investigated by Viovy, et al. This i s in very good agreement with results from C-13 NMR(21,23) which under comparable conditions give a ratio of Ti(polyisoprene)/Ti(polystyrene) of - 10. This comparison indicates that the hydrodynamic interaction of the label with the solvent does not dominate the observed dynamics. Concluding Remarks In this paper, we have shown the u t i l i t y of time-resolved optical techniques for the investigation of local segmental motions in polymer chains on a sub-nanosecond time scale. Detailed information about chain motions is contained in the time dependence of the orientation autocorrelatio The constant shape of th at different temperatures implies that the same mechanisms are involved in local motions under a l l conditions investigated. In terms of the Hall-Helfand model, the ratio of correlated to uncorrected transitions i s constant. Analysis of the temperature dependence of the labeled polyisoprene y i e l d s an activation energy of 7.4 kJ/mole for local segmental motions. In experiments currently in progress, the techniques used in this paper are being applied to the observation of local polymer dynamics in concentrated solutions and in the bulk. Measurements can be made on time scales as long as several t r i p l e t lifetimes (~ 100 msec) because the transient grating technique u t i l i z e s absorption and not fluorescence. This long time window w i l l allow the investigation of local motions in the bulk as a function of temperature from the rubbery state to the glass t r a n s i t i o n . Acknowledgments This work was supported in part by the Graduate School of the University of Wisconsin, the Shell Foundation (Faculty Career I n i t i a t i o n Grant), Research Corporation, Dow Chemical USA, and the National Science Foundation (DMR-8513271). In addition, acknowledgment i s made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Mr. H. Tominaga and Mr. N. Ota at Toyohashi University of Technology for their help in the preparation and characterization of the polymer.

Literature Cited 1. 2. 3.

Ricka, J.; Amsler, K.; Binkert, Th. Biopolymers 1983, 22, 1301. Phillips, D. Polymer Photophysics: Luminescence, Energy Migration, and Molecular Motion in Synthetic Polymers; Chapman and Hall: London, 1985. Sasaki, T.; Yamamoto, M.; Nishijima, Y. Makromol. Chem., Rapid Commun. 1986, 7, 345.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

82

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

PHOTOPHYSICS OF POLYMERS

Viovy, J.L.; Monnerie, L.; Brochon, J.C. Macromolecules 1983, 16, 1845. Viovy, J.L.; Frank, C.W.; Monnerie, L. Macromolecules 1985, 18, 2606; Viovy, J . L . ; Monnerie, L.; Merola, F. Macromolecules 1985, 18, 1130. Viovy, J.L. Monnerie, L. Polymer 1986, 27, 181. Hyde, P.D.; Waldow, D.A.; Ediger, M.D.; Kitano, T.; Ito, K. Macromolecules 1986, 19, 2533. Biddle, D. Nordstrom, T. Arkiv Kemi 1970, 32, 359. Valeur, Β. Monnerie, L. J. Polym. Sci. 1976, 14, 11. Moog, R.S.; Ediger, M.D.; Boxer, S.G.; Fayer, M.D. J. Phys. Chem. 1982, 86, 4694. Hall, C.K. Helfand, E. J. Chem. Phys. 1982, 77, 3275. Bendler, J.T. Yaris, R. Macromolecules 1978, 11, 650. Ediger, M.D.; Domingue, R. P.; Fayer, M.D. J. Chem. Phys. 1984, 80, 1246. Fayer, M.D. Ann. Rev Nelson, K.A.; Miller Phys. 1982, 53, 1144. Nemoto,N.; Landry, M.R.; Noh, I.; Yu, H. Polym. Comm. 1984, 15, 141. International Critical Tables; McGraw-Hill: New York, 1930, VII, 211. Kramers, H.A. Physica 1940, 7, 284. Helfand, E. J. Chem. Phys. 1971, 54, 4651. Flory, P.J. Statistical Mechanics of Chain Molecules; Wiley Interscience: New York, 1969; p.52. Hatada, K.; Kitayama, T.; Terawaki, Y.; Tanaka,Y.; Sato,H. Polym. Bull. 1980, 2, 791. Weber, T.A. Helfand, E. J. Phys. Chem. 1983, 87, 2881. Inoue, Y. Konno, T. Polymer J. 1976, 8, 457.

RECEIVED April 27, 1987

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 8

Phosphorescence Decay and Dynamics in Polymer Solids Kazuyuki Horie Institute of Interdisciplinary Research, Faculty of Engineering, University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153, Japan

Non-exponential phosphorescence decay is frequently observed for various aromatic chromophores molecularly dispersed in polyme mechanisms for non-exponentia and a dynamic quenching mechanism by polymer matrices including the effect of a time-dependent transient term in the rate coefficient is discussed in some detail. The biphotonic t r i p l e t - t r i p l e t annihilation mechanism is also introduced for the non-exponential decay under high-intensity and/or repeated laser irradiation. The i n f l u e n c e o f m o l e c u l a r s t r u c t u r e and motion o f polymer m a t r i c e s on t h e p h o t o p h y s i c a l and photochemical p r o c e s s e s o f m o l e c u l a r l y d i s p e r s e d chromophores i s a t o p i c o f i n c r e a s i n g i n t e r e s t . There a r e s e v e r a l reasons f o r t h i s a c t i v i t y . Polymer m a t r i c e s have been c o n s i d e r e d as c o n v e n i e n t media f o r s p e c t r o s c o p i c i n v e s t i g a t i o n s o f e x c i t e d t r i p l e t s t a t e s over wide temperature ranges, and c o n v e r s e l y , the use o f p h o t o p h y s i c a l l y d e t e c t a b l e probes a l l o w s t h e i n v e s t i g a t i o n o f t h e s t r u c t u r e o f polymer m a t r i c e s and o f p h o t o p h y s i c a l t r a n s i t i o n s connected w i t h changes i n the m o b i l i t y o f c e r t a i n s t r u c t u r a l u n i t s . Another reason f o r such s t u d i e s i s connected w i t h the p r a c t i c a l i n t e r e s t s c o n c e r n i n g t h e r e a c t i v i t y o f l o w - m o l e c u l a r w e i g h t compounds embedded i n polymer m a t r i c e s i n photomemory and p h o t o s e n s i t i v e polymer systems, and o f t h e r e a c t i v i t y o f a d d i t i v e s admixed t o polymers as s t a b i l i z e r s a g a i n s t p h o t o d e g r a d a t i o n and thermal d e g r a d a t i o n . The main d i f f e r e n c e between s o l i d - s t a t e r e a c t i o n s and those i n s o l u t i o n i s t h a t o f freedom o f m o l e c u l a r motion (1-3) due t o r e s t r i c t i o n o f m o b i l i t y o f r e a c t a n t s i n s o l i d s . Another i m p o r t a n t f e a t u r e i s the heterogeneous p r o g r e s s o f r e a c t i o n s (3,4) f r e q u e n t l y observed i n s o l i d s t a t e s due t o the m i c r o s c o p i c a l l y heterogeneous s t a t e s o f a g g r e g a t i o n o r f r e e volume d i s t r i b u t i o n o f t h e r e a c t i o n media. I n t h e case o f p o l y ( m e t h y l m e t h a c r y l a t e ) (PMMA), which i s an o r g a n i c g l a s s and i s u s u a l l y r e g a r d e d as an i n e r t m a t r i x f o r p h o t o p h y s i c a l and photochemical p r o c e s s e s , a marked d e v i a t i o n from

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84

PHOTOPHYSICS OF POLYMERS

the e x p o n e n t i a l decay o f benzophenone phosphorescence has been o b s e r v e d ( 5 , 6 ) . A r r h e n i u s p l o t s o f phosphorescence i n t e n s i t y ( 7 ) , l i f e t i m e and d e p o l a r i z a t i o n (5,6,8,9) o f v a r i o u s chromophores i n polymer m a t r i c e s showed b r e a k s a t temperatures c o r r e s p o n d i n g t o t h e g l a s s t r a n s i t i o n (T ) and s u b g l a s s t r a n s i t i o n s (Τ , , T_ , Τ ) o f t h e m a t r i x polymers. I n the p r e s e n t paper, we d i s c u s s m a i n l y the n o n - e x p o n e n t i a l decay o f phosphorescence and i t s o r i g i n i n polymer m a t r i c e s . The e f f e c t o f m u l t i - p h o t o n p r o c e s s e s on the decay c u r v e s i s a l s o discussed. D e v i a t i o n s o f Phosphorescence Decay from E x p o n e n t i a l i t y Solids

i n Polymer

The g e n e r a l p h o t o p h y s i c a l b e h a v i o r o f e x c i t e d t r i p l e t s t a t e s i s i n v a r i o u s t e x t b o o k s (10,11) The main p r o c e s s e s f o r a chromophore A, a r e the f o l l o w i n g : 3 * A

*

A + h

V

phosphorescence (k .j.)

(D

p

nonradiative

deactivation

(2)

(kjj)

3 * A + Β

A +

3 * Β

triplet

energy t r a n s f e r

(3)

3 * A + A

A +

3 * A

triplet

energy m i g r a t i o n

(4)

3 * 3 * A + A

1* > A + A

triplet-triplet

(5)

annihilation

T r i p l e t - t r i p l e t energy t r a n s f e r was f i r s t c l e a r l y demonstrated by T e r e n i n and Ermolaev (12,13) who showed the phosphorescence o f naphthalene ( a c c e p t o r , B) r e s u l t a n t upon e x c i t a t i o n o f benzophenone (donor, A) f o l l o w e d by t r i p l e t energy t r a n s f e r from A t o Β i n r i g i d s o l u t i o n a t 77 K. T r i p l e t energy t r a n s f e r r e q u i r e s m o l e c u l a r o r b i t a l o v e r l a p between the donor and a c c e p t o r , and the t r a n s f e r e f f i c i e n c y depends on the energy gap between the energy l e v e l s o f the e x c i t e d t r i p l e t s t a t e s , E , o f the donor and a c c e p t o r . Such energy t r a n s f e r due t o the e l e c t r o n exchange i n t e r a c t i o n was t h e o r i z e d by Dexter (14), a f t e r whom the mechanism i s named. T

Stern-Volmer Model. When the chromophores a r e s u f f i c i e n t l y m o b i l e as i n f l u i d s o l u t i o n , the b i m o l e c u l a r quenching p r o c e s s o f A by a quenching m o l e c u l e , B, i n c l u d i n g the t r i p l e t energy t r a n s f e r and some c o l l i s i o n a l quenching, w i l l r e s u l t i n the s i n g l e e x p o n e n t i a l 3 * A + Β

>

A + Β

t r i p l e t quenching

(k^)

(6)

phosphorescence decay o f the donor chromophore, A. Thus, t h e phosphorescence decay p r o f i l e , l ( t ) , i s e x p r e s s e d by eq (7) I ( t ) = Cexp(-t/x) = C e x p [ - t ( l / x ^ + k ^ [ B ] ) ]

(7)

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

8.

HORIE

Phosphorescence Decay and Dynamics

85

where C i s a c o n s t a n t , and τ i s a s i n g l e l i f e t i m e , w h i c h i s r e l a t e d to t h e phosphorescence l i f e t i m e i n the absence o f quencher, = l / ( k p + k-rrj,), quenching r a t e c o n s t a n t , k , and the quencher c o n c e n t r a t i o n , [ B ] . The r e l a t i v e phosphorescence y i e l d , I / I , d e f i n e d as t h e r a t i o o f t h e y i e l d s i n t h e presence o f a c c e p t o r t o t h a t i n i t s absence, has a s o - c a l l e d Stern-Volmer c o n c e n t r a t i o n dependence ( 1 5 ) . τ

t

ο

n

e

T

I/I

ο

= 1/(1 + k τ [Β]) qο

(8)

P e r r i n Model. A t the o t h e r extreme, P e r r i n (16) c o n s i d e r e d t h e case where t h e donor and a c c e p t o r m o l e c u l e s a r e immobile, and energy t r a n s f e r o c c u r s i n s t a n t a n e o u s l y when t h e two m o l e c u l e s l i e w i t h i n a c r i t i c a l t r a n s f e r d i s t a n c e , R , and does n o t o c c u r a t a l l a t l a r g e intermolecular separations. The decay f u n c t i o n f o r donor phosphorescence i s g i v e n by I(t)

= 1

I(t)

= exp [-(t/τ ) - ( C / C ) ]

(9) n

O

D

(t>0)

O

where C i s t h e a c c e p t o r c o n c e n t r a t i o n , C i s a parameter c a l l e d t h e c r i t i c a l t r a n s f e r c o n c e n t r a t i o n (which i s d e f i n e d as t h e r e c i p r o c a l of t h e s p h e r i c a l volume o f t h e r a d i u s R ) . The r e l a t i v e y i e l d o f donor phosphorescence i s g i v e n by eq (10? f o r t h i s c a s e . I / I = exp(-C /C ) ο b o

(10)

D

I n o k u t i - H i r a y a m a Model. The P e r r i n model i s t o o s i m p l i f i e d , a l t h o u g h i t i s c o n v e n i e n t f o r p r a c t i c a l use. The s t a t i c t r i p l e t t r i p l e t energy t r a n s f e r between immobile chromophores d i s p e r s e d i n s o l i d s can be w e l l d e s c r i b e d by t h e I n o k u t i - H i r a y a m a t h e o r y (17). Based on t h e Dexter mechanism, w i t h a d i s t a n c e - d e p e n d e n t r a t e c o e f f i c i e n t f o r t r i p l e t energy t r a n s f e r , a n o n - e x p o n e n t i a l decay f u n c t i o n f o r donor phosphorescence i n a r i g i d s o l u t i o n was d e r i v e d as I(t)

= exp[-(t/x ) - r" (C /C )G(e t/x )] ο B o ο 3

T

n

where γ i s r e l a t e d t o D e x t e r ' s Ύ y

eh

q u a n t i t i e s by

= 2R IL ο

(12)

= 2ïïk /h'F (E)£ (E)dE Α ρ 2

ο

(11)

A

0

(13)

G(z) = Unz) +1.732(lnz) +5.934(lnz)+5.445 3

2

(14)

Mataga et al. ( 1 8 ) s t u d i e d energy t r a n s f e r from t h e e x c i t e d t r i p l e t of benzophenone t o naphthalene by l a s e r f l a s h p h o t o l y s i s a t 77K and showed t h a t t h e n o n - e x p o n e n t i a l decay c u r v e s o f t h e benzophenone t r i p l e t obey t h e I n o k u t i - H i r a y a m a e q u a t i o n ( e q ( 1 1 ) ) . I n o k u t i and Hirayama (17) themselves compared t h e d a t a on t r i p l e t - t r i p l e t t r a n s ­ f e r between c e r t a i n a r o m a t i c m o l e c u l e s o b t a i n e d by T e r e n i n and Ermolaev w i t h eq (11) and r e p o r t e d t h a t a good f i t was found w i t h an a p p r o p r i a t e c h o i c e o f t h e parameters C^and γ.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Thus, i t i s thought t h a t the n o n - e x p o n e n t i a l phosphorescence decay o f donor i n the presence o f an a c c e p t o r i n an i m m o b i l i z e d system i s w e l l e x p l a i n e d by the I n o k u t i - H i r a y a m a model based on t h e s t a t i c t r i p l e t - t r i p l e t energy t r a n s f e r mechanism. T h i s model e x p e c t s a s i n g l e - e x p o n e n t i a l decay f o r the phosphorescence o f a chromophore i n t h e absence o f a c c e p t o r m o l e c u l e s . However, t h e phosphorescence decays o f o r g a n i c m o l e c u l e s m o l e c u l a r l y d i s p e r s e d i n polymer m a t r i c e s a r e known t o be n o n - e x p o n e n t i a l i n some cases even i n the absence o f o t h e r a d d i t i v e s . C o n s e q u e n t l y , o t h e r r e a s o n s s h o u l d be c o n s i d e r e d f o r such d e v i a t i o n s from e x p o n e n t i a l i t y . The d e v i a t i o n from a s i n g l e e x p o n e n t i a l c u r v e f o r the phosphor­ escence decay o f some chromophores d i s s o l v e d i n p l a s t i c s was f i r s t noted by O s t e r et al.(19) N o n e x p o n e n t i a l decay c u r v e s were o b t a i n e d f o r n a p h t h a l e n e and t r i p h e n y l e n e phosphorescence i n p o l y ( m e t h y l m e t h a c r y l a t e ) (PMMA) a t room temperature ( 2 0 ) , w h i l e e x p o n e n t i a l decays a t room temperature were o b s e r v e d i n PMMA f o r anthracene t r i p l e t (21) escence ( 2 0 ) . Graves et al. dependence o f phosphorescence parameters f o r a number o f a r o m a t i c hydrocarbons i n PMMA from 77 t o 400K and suggested the e x i s t e n c e o f i n t e r m o l e c u l a r t h e r m a l l y a s s i s t e d energy t r a n s f e r from t h e chromophore t o the h o s t p l a s t i c i n the h i g h e r temperature r e g i o n . E l - S a y e d et al. (20) i n t e r p r e t e d the n o n - e x p o n e n t i a l decay p r o f i l e s o f n a p h t h a l e n e and t r i p h e n y l e n e inPMMA a t room temperature i n terms o f a t r i p l e t - t r i p l e t a n n i h i l a t i o n mechanism. The d e c r e a s e of the e x t e n t o f n o n - e x p o n e n t i a l b e h a v i o r w i t h d e c r e a s i n g e x c i t a t i o n i n t e n s i t y and the o b s e r v a t i o n o f d e l a y e d f l u o r e s c e n c e were the bases of t h e i n t e r p r e t a t i o n . They suggested t h a t the n o n - e x p o n e n t i a l decays were o b s e r v e d f o r m o l e c u l e s h a v i n g f i r s t - o r d e r l i f e t i m e s o f the o r d e r o f s e v e r a l seconds. However, coronene w i t h a t r i p l e t l i f e t i m e o f 8.5s gave an e x p o n e n t i a l decay. They a l s o suggested r a t h e r h i g h c o n c e n t r a t i o n s o f the e x c i t e d t r i p l e t s t a t e o f t h e chromophore based on the d i f f u s i o n - c o n t r o l l e d t r i p l e t - t r i p l e t a n n i h i l a t i o n mechanism. L a t e r , J a s s i m et al. (24) proposed a n o t h e r type o f t r i p l e t - t r i p l e t a n n i h i l a t i o n mechanism f o r the none x p o n e n t i a l phosphorescence decay c o n s i s t i n g o f energy t r a n s f e r from the chromophore t o the m a t r i x polymer, t r i p l e t energy m i g r a t i o n through the m a t r i x polymer, and t r i p l e t - t r i p l e t a n n i h i l a t i o n between the chromophore t r i p l e t and the polymer t r i p l e t . H o r i e and M i t a (5) measured the phosphorescence decay o f benzo­ phenone i n PMMA o v e r a wide temperature range (80 t o 433K). N o n - e x p o n e n t i a l decays were o b s e r v e d f o r temperatures between T ( o n s e t o f e s t e r s i d e group r o t a t i o n o f the m a t r i x polymer) and Τ ( g l a s s t r a n s i t i o n t e m p e r a t u r e ) , and the decay p r o f i l e was ^ independent o f t h e i n t e n s i t y o f the e x c i t a t i o n l a s e r p u l s e over a 100 t i m e s change o f t h e i n t e n s i t y ( 2 5 ) . Thus, the n o n - e x p o n e n t i a l decay was a t t r i b u t e d t o a s i n g l e photon p r o c e s s comprised o f i n t e r m o l e c u l a r dynamic quenching o f the benzophenone t r i p l e t by e s t e r groups i n the s i d e c h a i n o f PMMA. D e t a i l e d d i s c u s s i o n o f t h e dynamic quenching mechanism w i l l be g i v e n i n the n e x t s e c t i o n . I t i s noteworthy t h a t the t r i p l e t decay c u r v e s a t room temperature o f v a r i o u s chromophores i n PMMA o b s e r v e d i n t h e l i t e r a t u r e can be d i v i d e d a p p r o x i m a t e l y i n t o two groups a c c o r d i n g t o the Ε o f the chromophores as i s shown i n T a b l e I . N o n - s i n g l e R

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

8. HORIE

87

Phosphorescence Decay and Dynamics

e x p o n e n t i a l phosphorescence decay has been o b s e r v e d f o r chromophores w i t h an E^, s m a l l e r than t h a t o f PMMA (297 t o 301 k j ) ( 2 3 , 2 4 ) , w h i l e s i n g l e - e x p o n e n t i a l phosphorescence o r T-T a b s o r p t i o n decay i s o b t a i n e d f o r chromophores w i t h lower t r i p l e t energy l e v e l s . These r e s u l t s a l s o s u p p o r t t h e o c c u r r e n c e o f dynamic quenching f o r e x c i t e d - t r i p l e t - s t a t e chromophores by m a t r i x PMMA due t o an endot h e r m i c t r i p l e t energy t r a n s f e r mechanism. R e c e n t l y , R i c h e r t and B a e s s l e r ( 3 7 ) r e g a r d e d t h e n o n - e x p o n e n t i a l decay o f benzophenone as a d i s p e r s i v e t r i p l e t t r a n s p o r t phenomenon t o t r a p s i t e s , and approximated i t by a s t r e t c h e d e x p o n e n t i a l f i t ( I n I ( t ) + t/τ = - C ( t / t ) ) w i t h a time dependent d i s p e r s i o n ο ο parameter α. α

Phosphorescence Decay o f Benzophenone i n Polymer

Solids

T y p i c a l decay c u r v e s o f benzophenon (BP) phosphorescenc (analyzed a t 450 nm) a t v a r i o u s temperature nitrogen laser pulse a phosphorescence i n t e n s i t y , l ( t ) , d e c r e a s e s as a s i n g l e e x p o n e n t i a l below t h e temperature c o r r e s p o n d i n g t o the e s t e r s i d e - g r o u p r o t a t i o n (Τβ = - 3 0 ° C f o r PMMA). D e v i a t i o n s from a s i n g l e - e x p o n e n t i a l decay a r e o b s e r v e d f o r T>T^, w h i c h i n c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e , but t h e d e v i a t i o n becomes l e s s marked above t h e g l a s s t r a n s i t i o n temperature, Τ , o f t h e m a t r i x polymer and d i s a p p e a r s a t 1 5 0 ° C . A s i m i l a r t e m p l r a t u r e dependence o f t h e decay p r o f i l e was a l s o o b s e r v e d f o r benzophenone phosphorescence i n o t h e r a c r y l i c polymers (28) and i n p o l y s t y r e n e (PS) and p o l y c a r b o n a t e (PC) ( 2 9 ) . T r a n s i e n t s p e c t r a showed t h a t t h e whole c o u r s e o f t h e decay c u r v e i s due t o benzophenone t r i p l e t ( 6 ) , and t h e i r r a d i a t i o n i n t e n s i t y independence shown i n F i g . 2 e x c l u d e d t h e o c c u r r e n c e o f T-T a n n i h i l a t i o n under t h e p r e s e n t e x p e r i m e n t a l c o n d i t i o n s ( 2 5 ) . S i n c e t h e d e v i a t i o n i s n o t o b s e r v e d a t temperatures below T^ o f each a c r y l i c polymer o r Τγ o f p o l y s t y r e n e and p o l y c a r b o n a t e , t h i s quenching i s n o t suggested t o be o f a s t a t i c c h a r a c t e r l i k e t h e I n o k u t i - H i r a y a m a type mentioned i n t h e p r e v i o u s s e c t i o n . Instead, i t i s suggested t o be o f a dynamic c h a r a c t e r due t o c o l l i s i o n between the f u n c t i o n a l groups. The o c c u r r e n c e o f quenching was a s c e r t a i n e d by comparison w i t h t h e model quenching r a t e c o n s t a n t of benzophenone t r i p l e t by methyl a c e t a t e i n a c e t o n i t r i l e s o l u t i o n (3.9x10 M" S " a t 30°C) ( 2 8 ) . Quenching o f benzophenone t r i p l e t by p h e n y l o r phenylene g r o u j s j n p o l y s t y r e n e o r p o l y c a r b o n a t e has a l s o been s t u d i e d (1.2x10 M s~ f o r p o l y s t y r e n e i n benzene a t 30°C) ( 3 0 ) . These o r d e r s o f magnitude f o r t h e quenching r a t e c o n s t a n t s i n n o n v i s c o u s s o l u t i o n a r e r e a s o n a b l e f o r an u p h i l l - t y p e endothermic t r i p l e t energy t r a n s f e r mechanism. As t h e dynamic quenching f o r a phosphorophore g i v e s s i n g l e e x p o n e n t i a l decay i n n o n v i s c o u s s o l u t i o n ( S t e r n - V o l m e r model), i t i s n e c e s s a r y t o c o n s i d e r why t h e dynamic quenching i n polymer s o l i d s e s p e c i a l l y below T^ r e s u l t s i n a n o n - e x p o n e n t i a l decay p r o f i l e . K i n e t i c s f o r N o n - e x p o n e n t i a l Decay Due t o Dynamic Quenching The decay p r o c e s s o f t r i p l e t benzophenone, following:

(6,29)

3 BP, i s g i v e n by t h e

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

88

PHOTOPHYSICS OF POLYMERS Table I. Type of T r i p l e t Decay Curves and T r i p l e t Energies, E^, for Various Chromophores Dispersed i n PMMA at Room Temperature ( 2 7 )

chromophore

type of t r i p l e t decay

fluorene benzophenone triphenylene phenanthrene naphthalene

non-single-exponential non-single-exponential non-single-exponential s ingle-exponential non-single-exponential sing1e-exponentia1 single-exponential single-exponential single-exponential s ing1e-exponential

coronene benz i l pyrene anthracene

ν kj/mol 284 283 278 260 255 228 227 202 179

20

40

60

ms(é)

0

3

16

2ί

ms(ti

Ù

2 1

f

6

msfd

0 0

.

β

WO 16 rime

ref 20 5,6 20,24 24 20 24 20 26 22 21

60 VS(d) 2i

μ*(β)

F i g u r e 1. S e m i l o g a r i t h m i c decay curves o f benzophenone phospho­ rescence i n PMMA e x c i t e d by 10-ns n i t r o g e n l a s e r p u l s e a t 337 nm. Temperature and symbols f o r time s c a l e s a r e g i v e n b e s i d e t h e curves. (Reproduced from Reference 6. C o p y r i g h t 1984 American Chemical S o c i e t y .

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

HOME

Phosphorescence Decay and Dynamics

A

Ο

10

2 0 Time

3.0

( m sec )

F i g u r e 2. Phosphorescence decay p r o f i l e s o f BP i n PMMA a t 20°C. Excitation intensity: (1) 4.0 χ 1 0 , (2) 6.4 χ 1 0 , (3) 2.8 χ 10*2 p h o t o n / p u l s e . (Reproduced w i t h p e r m i s s i o n from Reference 25. C o p y r i g h t 1984, Pergammon.) 1 5

1 3

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

90

PHOTOPHYSICS OF POLYMERS

k 3

* BP

° >

3 * BP

+ [Q]

BP

(15)

k q

»

BP + [Q]

(16)

where k = kp^, + k^p i s the r a t e c o n s t a n t f o r spontaneous d e a c t i v a t i o n of benzophenone t r i p l e t , [Q] i s the c o n c e n t r a t i o n of e s t e r , p h e n y l , o r phenylene group i n the m a t r i x polymer. The b i m o l e c u l a r r a t e c o e f f i c i e n t , k , i s g i v e n by eq (17) i n c l u d i n g b o t h the d i f f u s i o n and c h e m i S a l s t e p s ( 3 0 ) : Q

,

4ÏÏRDN

q

=

L

1 + 4ïïRDN/k

,

1

1

+

R

-j

(17)

( l + 4ïïRDN/k)(πΟΤ)^

where D i s the sum of d i f f u s i o i n benzophenone and f o r by s i d e - c h a i n r o t a t i o n and l o c a l segmental m o t i o n o f the polymer c h a i n , R i s r e a c t i o n r a d i u s between the two groups, k i s the i n t r i n s i c ( c h e m i c a l ) r a t e c o n s t a n t t h a t would p e r t a i n i f the e q u i l i b r i u m c o n c e n t r a t i o n of the quenching groups were m a i n t a i n e d , and Ν i s Avogadro's number d i v i d e d by 10 . When k i s c o n t r o l l e d by the d i f f u s i o n p r o c e s s o f the two groups, eq (17? i s reduced t o eq ( 1 8 ) : k

q

= 4*RDN(1 + R/(ïïDt)^ = A + B / t *

(18)

with A = 4nRDN Β = 4R

2

^ (xD) N a

Thus, the r a t e c o e f f i c i e n t k i n c l u d e s a time-dependent term t h a t i s i m p o r t a n t a t the v e r y e a r l y Stage of r e a c t i o n where the s t e a d y - s t a t e d i f f u s i v e f l u x of the quenching group i s not y e t a t t a i n e d . As the decay r a t e of benzophenone t r i p l e t i s g i v e n by eq (19) 3

- d [ B P * ] / d t = (k

3

ο

+

k [Q])[ BP*] q %

3

= ( k + A[Q] + B [ Q ] t " ) ] B P * ] ο

(19)

A

we get 3

3

%

[ BP*] = [ B P * ] e x p [ - ( k + A [ Q ] ) t - 2B[Q]t ] o

(20)

o

3 * f o r t h e ^ c o n c e n t r a t i o n of benzophenone t r i p l e t , [ BP J , a t time t , where [ BP ] i s the i n i t i a l c o n c e n t r a t i o n o f benzophenone t ^ i p ^ e t . The phosphorescence i n t e n s i t y , l ( t ) , i s p r o p o r t i o n a l t o kp^L BP ]» so we get f i n a l l y r

In I ( t )

= -(k

+ A [ Q ] ) t - 2 B [ Q ] t * + In I

« -(t/τ) - C ( t / x )

%

+ In I ο

n

Q

(21)

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

8.

HORIE

91

Phosphorescence Decay and Dynamics

where l/τ = k + A[Q] = k + 4 RDN[Q] ο ο Β

= CT-*/(2[Q])

= 4R (ÏÏD)^N 2

(22) ( 2 3 )

The c u r v e f i t t i n g f o r the phosphorescence decay c u r v e s w i t h eq ( 2 1 ) g i v e s the v a l u e s o f r e c i p r o c a l l i f e t i m e l/τ = k + A[Q] and a parameter B, w h i c h a r e shown i n F i g . 3 f o r the c a s e s o f PMMA and p o l y s t y r e n e . The breaks r e f l e c t the changes i n the m o l e c u l a r motions a t Τ , Τ (PMMA) o r Τ (PS, PC) c o r r e s p o n d i n g t o t h e l o c a l mode r l l a x a t i o n o f the main c h a i n , and Τ (PMMA) o r Τ (PS, PC) c o r r e s p o n d i n g t o the o n s e t o f r o t a t i o n ^ o f the s i d e - 2 h a i n e s t e r o r phenyl group o r the m a i n - c h a i n phenylene group. A r r h e n i u s p l o t o f 1 / f o r benzophenone i n p o l y ( m e t h y l a c r y l a t e ) (PMA) ( 6 ) showed another break a t 40°C (abov Τ f PMA) w h i c h c o r r e s p o n d t an a c t i v a t i o n - c o n t r o l l e f o r r e a c t i n g c a r b o n y l group Β a t each temperature a l s o showed b r e a k s a t each t r a n s i t i o n temperature, as e x e m p l i f i e d i n F i g . 4 f o r the c a s e s o f PMMA, p o l y s t y r e n e and p o l y c a r b o n a t e . I t s h o u l d be noted t h a t D i n F i g . 4 i s d e f i n e d as t h e t r a n s l a t i o n a l d i f f u s i o n c o e f f i c i e n t f o r t h e r e a c t i n g f u n c t i o n a l groups but n o t f o r the m o l e c u l e . The d i f f u s i o n p r o c e s s a t temperatures below Τ would be caused by r o t a t i o n o f the benzophenone m o l e c u l e and segmental motion w i t h i n a few monomer u n i t s o f m a t r i x polymers. N e v e r t h e l e s s , the v a l u e s o f D i n ^ t h e s e polymers a t 100°C a r e compared t o the v a l u e o f D = 5.6x10" cm /s (32) f o r mass d i f f u s i o n o f e t h y l b e n z e n e i n p o l y s t y r e n e a t 30°C. The r e a c t i o n r a d i u s , R, amounts t o 3-5 À. The t r a n s i t i o n temperatures o f the m a t r i x polymers m o n i t o r e d by phosphorescence decay o f benzophenone i n c l u d i n g the case i n p o l y ( v i n y l a l c o h o l ) a r e summarized i n T a b l e I I . The α t r a n s i t i o n f o r PMMA and o t h e r a c r y l i c polymers and β t r a n s i t i o n f o r p o l y s t y r e n e , p o l y c a r b o n a t e , and p o l y ( v i n y l a l c o h o l ) a r e a t t r i b u t e d t o a l o c a l mode r e l a x a t i o n of the main c h a i n . The phosphorescence probe t e c h n i q u e i s e f f e c t i v e f o r the d e t e c t i o n o f t h i s s u b - g l a s s t r a n s i t i o n as w e l l as o t h e r r o t a t i o n mode t r a n s i t i o n s ( T o r Τ ) o f polymer m a t r i c e s . β γ Hydrogen A b s t r a c t i o n o f Benzophenone T r i p l e t i n P o l y ( v i n y l a l c o h o l ) t

τ

0

The phosphorescence o f BP (0.Π,) i n p o l y ( v i n y l a l c o h o l ) (PVA) f i l m (250 m i c r o n t h i c k n e s s ) e x c i t e d by a 10-ns n i t r o g e n l a s e r p u l s e a t 337 nm decays e x p o n e n t i a l l y f o r Τ < Τ (-100°C) o r Τ > Τ (85°C), but d e v i a t e s from s i n g l e e x p o n e n t i a l f o r ^ T < Τ < Τ . The S e v i a t i o n was a t t r i b u t e d t o the d i f f u s i o n - c o n t r o l l e d ^ h y d r o g e n ^ a b s t r a c t i o n r e a c t i o n between benzophenone t r i p l e t and the PVA m a t r i x ( 3 3 ) . The o c c u r r e n c e o f n o n - e x p o n e n t i a l decays i n p o l y ( v i n y l a l c o h o l ) where t h e r e i s no p o s s i b i l i t y o f t r i p l e t energy m i g r a t i o n i s an a d d i t i o n a l p r o o f o f the absence o f the T-T a n n i h i l a t i o n mechanism i n the phosphorescence decay o f benzophenone i n polymer m a t r i c e s . The d i f f u s i o n - c o n t r o l l e d r a t e c o e f f i c i e n t f o r hydrogen a b s t r a c t i o n , k , o f benzophenone t r i p l e t f o r PVA and t h e phosphorescence i n t e n s i t y , l ( t ) , were g i v e n by eqs (24) and ( 2 5 ) , i n a s i m i l a r manner t o the cases o f p h y s i c a l quenching o f t h e benzophenone phosphorescence by m a t r i x polymers.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOPHYSICS OF POLYMERS

1/T (ΐο- κ-')

Figure 3. Temperature dependence of reciprocal l i f e t i m e , l / τ , (Ο, Δ ) and contribution of non-exponential term, Β, (Φ,Α) f o r benzophenone phosphorescence i n PMMA ( 0 #) polystyrene a

200 100 π ι * ι ι ι

ι ι—ι—ι—ι—ι

4

1

Γ

n

d

-100 ι

5

i

n

»

6

Î/TCIO^K" ) 1

Figure 4. Arrhenius p l o t s of d i f f u s i o n c o e f f i c i e n t , D, of interacting groups f o r dynamic quenching of benzophenone t r i p l e t by phenyl, phenylene, and ester groups i n polystyrene ( #,•) , polycarbonate ( Ο , Δ ) , and PMMA ( • ) , respectively.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

8.

Phosphorescence Decay and

HORIE

k

2

a

93

Dynamics

= 4ÏÏRDN(1 + R / ( D t ) ) = A + B / t

2

(24)

ïï

In I ( t ) = -(k

ο

+ A[PVA])t-2B[PVA]t^

+ In I

ο

(25)

The curve f i t t i n g for the experimental phosphorescence decay with eq (25) gives the values of l/τ = k + A[PVA] and B. The breaks at Τ = 85°C and Τ = 30°C and the appearance of Β at Τ = -100°C c f e a r l y r e f l e c t the change i n molecular motions of matrix poly(vinyl alcohol). In order to know the net quantum y i e l d for the benzophenone disappearance, δ(-ΒΡ), by hydrogen abstraction i n poly(vinyl alcohol), the change i n UV spectra of benzophenone at 256 nm i n the poly(vinyl alcohol) f i l m was followed during continuous i r r a d i a t i o n of 365 nm UV l i g h t . The Φ(-BP) given i n Table III i s very small for Τ < Τ , suggesting the predominant occurrence of backward reaction ^ ° cage for the temperatur spectra and l i f e t i m e of kety Τ >120°C (34). The supposed reaction scheme for the photochemistry of benzophenone i n poly(vinyl alcohol) including the backward cage reaction i s summarized i n F i g . 5. β

Phosphorescence Decay of Benzophenone under Multi-photon Conditions Salmassi and Schnabel (35) measured the decays of phosphorescence and t r i p l e t absorption of benzophenone i n PMMA and polystyrene under the high intensity i r r a d i a t i o n of 347 nm frequency-doubled ruby laser gingle pulse. The i n i t i a l t r i p l e t concentration amounted up to^6xl0 mol/1 i n comparison with the value of less than 6x10 mol/1 for the case (6,25) i n the preceding sections with the nitrogen laser pulse. In PMMA a single t r i p l e t decay mode following f i r s t - o r d e r kinetics was observed at Τ 410K. Two d i s t i n c t modes of t r i p l e t decay were observed i n the intermediate temperature range: a fast f i r s t - o r d e r process, the l i f e t i m e (ca. 4ys at 295K) being independent of the i n i t i a l t r i p l e t concentration, [ T ] , and a slow second-order process, the f i r s t h a l f l i f e t i m e being p r o p o r t i o n a l to [T] " . S i m i l a r l y , two d i s t i n c t modes of t r i p l e t decay were also o B t a i n e d with polystyrene m a t r i c e s for 180K 350 nm) a n d t o r e t u r n t o t h e i n i t i a l v a l u e i n 30 h i n t h e d a r k a t 20°C. T h e s l o w r e c o v e r y o f t h e v i s c o s i t y i n t h e d a r k was a c c e l e r a t e d b y v i s i b l e l i g h t irradiation(λ >470 n m ) . On a l t e r n a t e i r r a d i a t i o n o f u l t r a v i o l e t and v i s i b l e l i g h t , t h e v i s c o s i t y r e v e r s i b l y c h a n g e d b y a s much a s 6 0 % . M e c h a n i s m ( 2 ) e m p l o y s a n e l e c t r o s t a t i c f o r c e o f r e p u l s i o n among photogenerated charges as a d r i v i n g force f o r a conformational change. T r i p h e n y l m e t h a n e l e u c o d e r i v a t i v e s a r e t h e most c o n v e n i e n t chromophores t o produce p o s i t i v e charges i n t h e pendant groups o f p o l y m e r s , because t h e quantum y i e l d o f t h e p h o t o d i s s o c i a t i o n i s r e a s o n a b l y h i g h ( Φ>0.2) a n d t h e p h o t o g e n e r a t e d p o s i t i v e c h a r g e s have a r a t h e r l o n g l i f e t i m e ( x ^ m i n ) . Upon u l t r a v i o l e t i r r a d i a t i o n , the chromophore d i s s o c i a t e s i n t o an i o n p a i r w i t h p r o d u c t i o n o f an intensely colored triphenylmethyl cation. The c a t i o n r e c o m b i n e s thermally with the counter ion(18); #

2

(2)

X

T r i p h e n y l m e t h a n e l e u c o h y d r o x i d e r e s i d u e s were i n t r o d u c e d i n t o t h e p e n d a n t g r o u p s b y c o p o l y m e r i z i n g t h e v i n y l d e r i v a t i v e ( 2 , X= OH, R= C H = C H ) w i t h N , N - d i m e t h y l a c r y l a m i d e ( 1 2 ) . I n t h e d a r k b e f o r e i r r a d i a ­ t i o n , a methanol s o l u t i o n c o n t a i n i n g t h e copolymer e x h i b i t e d a p a l e green c o l o r . Upon u l t r a v i o l e t i r r a d i a t i o n ( λ>270 n m ) , t h e s o l u t i o n 2

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became d e e p g r e e n , w h i c h c o l o r d i s a p p e a r e d s l o w l y i n t h e d a r k w i t h a h a l f l i f e t i m e o f 3.3 m i n ( F i g u r e 2 A ) . The a p p e a r a n c e o f a d e e p g r e e n c o l o r means t h a t t h e p e n d a n t t r i p h e n y l m e t h y l l e u c o h y d r o x i d e residues d i s s o c i a t e i n t o t r i p h e n y l m e t h y l c a t i o n s and h y d r o x i d e i o n s . The photogenerated p o s i t i v e charges recombine w i t h the d i s s o c i a t e d hydroxide ions t o reproduce the c o l o r l e s s leuco form. Concurrently w i t h the c o l o r a t i o n , the reduced v i s c o s i t y of the s o l u t i o n , r| p/c, s h o w e d a r e m a r k a b l e i n c r e a s e f r o m 0.55 t o 1.6 d l / g a s d e p i c t e d i n F i g u r e 2B. A f t e r r e m o v a l o f t h e l i g h t , ^gp/c r e t u r n e d t o t h e i n i t i a l v a l u e w i t h a h a l f - l i f e t i m e o f 3.1 m i n . The v i s c o s i t y c h a n g e i n d i c a t e s t h a t t h e polymer c h a i n expands upon u l t r a v i o l e t i r r a d i a t i o n and s h r i n k s i n t h e d a r k . The g o o d c o r r e l a t i o n b e t w e e n t h e v i s c o s i t y c h a n g e a n d t h e a b s o r p t i o n i n t e n s i t y a t 620 nm i m p l i e s t h a t e x p a n s i o n o f t h e p o l y m e r c o n f o r m a t i o n i s i n d u c e d by the e l e c t r o s t a i c r e p u l s i v e f o r c e s among t h e p e n d a n t t r i p h e n y l m e t h y l cations. S

The c o n c e n t r a t i o n d e p e n d e n c s i o n mechanism. In the was l i n e a r ; t h e r e d u c e d v i s c o s i t y d e c r e a s e d w i t h d e c r e a s i n g t h e c o n c e n t r a t i o n of the polymer. During u l t r a v i o l e t i r r a d i a t i o n , t h i s d e p e n d e n c e showed an a n o m a l o u s b e h a v i o r ; flgp/c s t e e p l y i n c r e a s e d a t low polymer c o n c e n t r a t i o n . The v i s c o s i t y d u r i n g p h o t o i r r a d i a t i o n was 4 t i m e s l a r g e r t h a n t h e v i s c o s i t y i n t h e d a r k a t 0 . 0 4 g / d l . At low polymer c o n c e n t r a t i o n , s c r e e n i n g of the e l e c t r o s t a t i c p o t e n t i a l by t h e c o u n t e r i o n s b e c o m e s weak a n d c o n s e q u e n t l y t h e i n c r e a s e o f the r e p u l s i v e f o r c e s of the p o s i t i v e charges along the polymer chain expands the dimension of the c h a i n . The p h o t o - e f f e c t due t o t h e e l e c t r o s t a t i c f o r c e s i s much l a r g e r t h a n t h e e f f e c t o b s e r v e d f o r p o l y amides having azobenzene r e s i d u e s i n the backbone. I t i s worthwhile t o n o t e t h a t t h e p h o t o s t i m u l a t e d i n c r e a s e o f r|sp/c was s t r o n g l y s u p p r e s s e d by t h e p r e s e n c e o f s a l t ( 1 0 M LiBr). The r a t i o o f t h e s p e c i f i c v i s c o s i t y d u r i n g p h o t o i r r a d i a t i o n t o t h a t i n t h e d a r k , Hp/^d t increased with i n c r e a s i n g content of triphenylmethane l e u c o h y d r o x i d e r e s i d u e s i n t h e p e n d a n t g r o u p s and r e a c h e d a maximum o f 3.3 a t 0.18 m o l e f r a c t i o n . A b o v e t h e content,the r a t i o d e c r e a s e d , t h e d e c r e a s e b e i n g due t o t h e l o w s o l u b i l i t y o f t h e residues i n methanol. The c o n c e p t t o a d o p t t h e e l e c t r o s t a t i c r e p u l s i v e f o r c e a s a d r i v i n g f o r c e f o r a photostimulated expansion of the polymer chain i s u s e f u l and w i d e l y a p p l i c a b l e t o o t h e r p o l y m e r s y s t e m s . Polys t y r e n e and p o l y a c r y l a m i d e h a v i n g p e n d a n t l e u c o h y d r o x i d e and l e u c o c y a n i d e groups were found t o change t h e i r s o l u t i o n v i s c o s i t y i n m e t h y l e n e c h l o r i d e and i n w a t e r , r e s p e c t i v e l y . Photostimulated

D i l a t i o n of Polymer Gels

- Macro

Level

I t i s i n f e r r e d f r o m t h e a b o v e r e s u l t s on t h e c o n f o r m a t i o n a l changes i n s o l u t i o n t h a t the e l e c t r o s t a t i c f o r c e of r e p u l s i o n b e t w e e n p h o t o g e n e r a t e d c h a r g e s , m e c h a n i s m ( 2 ) , i s a more e f f e c t i v e d r i v i n g f o r c e f o r c o n f o r m a t i o n a l changes than t r a n s - c i s g e o m e t r i c a l i s o m e r i z a t i o n of unsaturated l i n k a g e s , mechanism(1). In attempting t o a m p l i f y t h e l a r g e c o n f o r m a t i o n a l c h a n g e s due t o t h e e l e c t r o s t a t i c r e p u l s i v e f o r c e s i n s o l u t i o n at the molecular l e v e l to the v i s i b l e m a c r o l e v e l , we i n t r o d u c e d t h e m e c h a n i s m ( 2 ) i n t o t h e g e l p h a s e . A c r y l a m i d e g e l s ( 3 ) c o n t a i n i n g a s m a l l amount o f t r i p h e n y l -

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

IRIE

111

Light-Induced Conformational Changes

F i g u r e 1. S c h e m a t i c i l l u s t r a t i o n o f p h o t o s t i m u l a t e d t i o n a l chages o f polymer c h a i n s .

x

OA

^

Dark

Dark

conforma­

A

^0.2

0

•
2 7 0 n m ) , t h e g e l q u i c k l y b e n t i n 1 m i n . The g e l e n d m o v e d t o t h e d i r e c t i o n of a p o s i t i v e electrode(Figure 6B). During the bending motion, the center o f g r a v i t y o f the g e l remained a t t h e i n i t i a l p o s i t i o n . T r a n s l a t i o n a l movement o f t h e e n t i r e g e l t o t h e n e g a t i v e e l e c t r o d e was n o t o b s e r v e d . By c h a n g i n g t h e p o l a r i t y o f t h e e l e c t r i c f i e l d , t h e g e l a g a i n becomes s t r a i g h t a n d t h e n bends t o t h e a n o t h e r d i r e c t i o n ( F i g u r e 6 C ) . T h e r e s p o n s e t i m e o f t h e g e l s h a p e c h a n g e was around 2 min. A f t e r s w i t c h i n g o f f the l i g h t , the g e l slowly returned to t h e i n i t i a l s t r a i g h t shape i n t h e e l e c t r i c f i e l d . The r e s u l t suggests t h a t p h o t o d i s s o c i a t i o n of t h e l e u c o c y a n i d e groups i n t h e g e l i s indispensable t o the g e l bending motion. In o r d e r t o d e t e r m i n e q u a n t i t a t i v e l y t h e response time o f t h e m o t i o n , one end o f t h e r o d - s h a p e d l fixed t th wall d th moving d i s t a n c e o f t h e o t h e was m e a s u r e d a s a f u n c t i o p h o t o s t i m u l a t e d b e n d i n g m o t i o n o f t h e g e l ( 2 6 mm i n l e n g t h a n d 2 mm i n s e c t i o n d i a m e t e r ) i n an e l e c t r i c f i e l d ( 1 0 V/cm). The f r e e e n d m o v e d t o w a r d t h e p o s i t i v e e l e c t r o d e w i t h a i n i t i a l s p e e d o f 0.40 mm/ sec. By c h a n g i n g t h e p o l a r i t y o f t h e e l e c t r i c f i e l d , t h e e n d m o v e d to another d i r e c t i o n . Upon s w i t c h i n g o f f t h e e l e c t r i c f i e l d , t h e b e n t g e l r e t u r n e d t o t h e i n i t i a l p o s i t i o n w i t h a s p e e d o f 0.075 mm/ sec. The b e n d i n g r a t e d e p e n d s o n t h e a p p l i e d f i e l d . Upon i n c r e a s i n g the f i e l d , t h e r e s p o n s e t i m e i n c r e a s e s i n p r o p o r t i o n t o t h e a p p l i e d field. A l t h o u g h t h e b e n d i n g r a t e became v e r y s l o w , t h e b e n d i n g m o t i o n was o b s e r v e d i n a v e r y weak f i e l d o f 1.25 V/cm. In t h i s case, e f f e c t i v e v o l t a g e a p p l i e d t o t h e g e l was o n l y 0.25 V. I n t h e a b o v e e x p e r i m e n t s , d e i o n i z e d w a t e r was u s e d a s t h e external solution. As d e s c r i b e d i n t h e p r e v i o u s s e c t i o n , t h e addition of s a l t s t o the solution decreased the photostimulated d i l a t i o n of the gels. I f the bending motion i n the e l e c t r i c f i e l d was d u e t o t h e o s m o t i c p r e s s u r e m e c h a n i s m , t h e a d d i t i o n o f s a l t w o u l d a l s o s u p p r e s s t h e m o t i o n . T h i s i s n o t t h e c a s e . On t h e c o n t r a r y , t h e r e s p o n s e t i m e o f t h e b e n d i n g m o t i o n was a c c e l e r a t e d b y t h e addition of salts t o the external solution. The b e n d i n g r a t e i n t h e s o l u t i o n c o n t a i n i n g 2 χ 1 0 ~ 3 m o l e / l N a C l was 1.5 mm/sec, w h i c h i s 4 time f a s t e r than t h e r a t e i n t h e absence o f NaCl. The r e s u l t i n d i ­ c a t e s t h a t t h e b e n d i n g m o t i o n i n t h e e l e c t r i c f i e l d i s n o t due t o t h e o s m o t i c p r e s s u r e mechanism. When a s o l u t i o n c o n t a i n i n g s a l t s i s u s e d , i t i s d i f f i c u l t t o examine p u r e e l e c t r i c f i e l d e f f e c t w i t h o u t b e i n g d i s t u r b e d by t h e e l e c t r o c h e m i c a l r e a c t i o n s on t h e e l e c t r o d e s . The r e a c t i o n s on t h e e l e c t r o d e s p r o d u c e pH g r a d i e n t i n t h e s o l u t i o n . Although the leuco­ c y a n i d e g e l s a r e r a t h e r i n s e n s i t i v e t o t h e pH c h a n g e , t h e c o r r e l a t i o n b e t w e e n t h e b e n d i n g m o t i o n a n d t h e pH c h a n g e was e x a m i n e d b y a d d i n g a pH i n d i c a t o r , p h e n o l r e d , i n t o t h e e x t e r n a l s o l u t i o n . The b e n d i n g m o t i o n was f o u n d t o b e much f a s t e r t h a n t h e c o l o r c h a n g e o n t h e e l e c t r o d e . The r e s u l t s u g g e s t s t h a t t h e r a p i d b e n d i n g m o t i o n i s i n d e p e n d e n t o f t h e pH c h a n g e . T h i s was f u r t h e r c o n f i r m e d b y a p p l y i n g a l t e r n a t i n g e l e c t r i c f i e l d , a s shown i n F i g u r e 8. The r o d s h a p e d g e l , one e n d o f w h i c h i s f i x e d on t h e w a l l , v i b r a t e s i n r e s p o n s e t o t h e a l t e r n a t i n g e l e c t r i c f i e l d o f 0.5 H z . u n d e r u l t r a v i o l e t i r r a d i a t i o n .

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F i g u r e 7. P h o t o s t i m u l a t e d b e n d i n g m o t i o n o f a r o d s h a p e d a c r y l a m i d e g e l (26 mm i n l e n g t h a n d 2 mm i n s e c t i o n d i a m e t e r ) h a v i n g 3.1 m o l e % t r i p h e n y l m e t h a n e l e u c o c y a n i d e g r o u p s i n a n e l e c t r i c f i e l d (10 V/cm) i n w a t e r . T h e e l e c t r i c f i e l d was r e m o v e d a f t e r 120 s e c . ( R e p r o d u c e d f r o m R e f . 26. C o p y r i g h t 1986 A m e r i c a n C h e m i c a l S o c i e t y . )

F i g u r e 8. P h o t o s t i m u l a t e d v i b r a t i n a l m o t i o n o f a r o d s h a p e d a c r y l a m i d e g e l h a v i n g 3.1 m o l e % t r i p h e n y l m e t h a n e leucocyanide g r o u p s i n a n a l t e r n a t i n g e l e c t r i c f i e l d ( 0 . 5 H z , ± 8V/cm) i n water. I n t h i s c a s e , t h e pH o f t h e s o l u t i o n r e m a i n e d i n t h e n e u t r a l v a l u e (around 6.5). The b e n d i n g b e h a v i o r o f t h e g e l s u g g e s t s i n h o m o g e n e o u s e x p a n s i o n of t h eg e l i nt h ee l e c t r i c f i e l d . The n e g a t i v e e l e c t r o d e s i d e o f t h e g e l expands l a r g e r than t h e o t h e r s i d e . Since thee l e c t r i c f i e l d i s a p p l i e d p e r p e n d i c u l a r t o t h e g e l a x i s , m o b i l e n e g a t i v e i o n s , CN~", a r e a t t r a c t e d t o t h ep o s i t i v e electrode side i n theg e l . Consequently, excess p o s i t i v e charges a r e l e f t on t h e o t h e r s i d e . Internal r e p u l s i v e f o r c e between t h e p o s i t i v e charges, which a r e f i x e d i n t h e gel network, i sconsidered t o cause t h e expansion o f t h e negative electrode side o f t h eg e l .

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

IRIE

Light-Induced Conformational Changes

Other Properties

of Photoresponsive

121

Polymers

Because o f l i m i t a t i o n o f t h i s chapter, o n l y a few p r o p e r t i e s o f photoresponsive polymers are described. Table I I includes several other properties so f a r reported(27-50). A l l o f these p h y s i c a l and c h e m i c a l p r o p e r t i e s a r e found t o be c o n t r o l l e d r e v e r s i b l y by p h o t o i r r a d i a t i o n . I t i s now g e n e r a l l y a c c e p t e d t h a t p h o t o c h r o m i c r e a c t i o n s are u s e f u l as a t o o l t o photo-control the p r o p e r t i e s o f s y n t h e t i c p o l y m e r s . The p h o t o r e s p o n s i v e p o l y m e r s have p o t e n t i a l a p p l i c a t i o n s f o r many p h o t o a c t i v e d e v i c e s , s u c h a s s e n s o r s , s w i t c h e s , m e m o r i e s , photo-mechanical transducers and so on. Table I I .

Photocontrol o f P h y s i c a l and Chemical P r o p e r t i e s o f Polymer S o l u t i o n s and S o l i d s

Solution V i s c o s i t y (3-12,27,28) pH-value (5,29) S o l u b i l i t y (30-32) M e t a l Ion C a p t u r e (7,34)

Membrane P o t e n t i a l ( 3 5 ) W e t t a b i l i t y (36) Shape (19,20,37-45) S o l - G e l T r a n s i t i o n (46,47) Tg ( 4 8 ) C o m p a t i b i l i t y o f Polymer Blends (49) Absorptive A b i l i t y (50)

Acknowledgments A c k n o w l e d g m e n t i s made t o t h e D o n o r o f t h e P e t r o l e u m R e s e a r c h F u n d , administered by the American Chemical S o c i e t y , f o r p a r t i a l support o f this activity.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Morawetz, H. Macromolecules in Solution; 2nd ed., Wiley: New York, 1975; p 344. Irie, M. In Molecular Models of Photoresponsiveness; Montagnoli, G.;Erlanger, B. F., Ed.; Plenum: New York, 1983;p 291. Irie, M. In Photophysical and photochemical Tools in Polymer Science; Winnik, M.A., Ed.,; Reidel: Dordrecht, 1986; p 269 Irie, M.; Hayashi, K. J. Macromol. Sci. Chem. 1979, A13, 511. Irie, M.; Hirano, K.; Hashimoto, S.; Hayashi, K. Macromolecules 1981, 14, 262. Blair, H.S.; Pogue, H.I.; Riordan, J.E. Polymer, 1980, 21, 1195. Kumar, G.S.; DePra, P.; Neckers, D.C. Macromolecules, 1984, 17, 2463.. Zimmermann, E.K.; Stille, J.K. Macromolecules, 1985, 18, 321. Irie, M.; Menju, Α.; Hayashi, K. Macromolecules, 1979, 12, 1176. Menju, Α.; Hayashi, K.; Irie, M. Macromolecules, 1980, 14, 755. Irie, M.; Hayashi, K.; Menju, A. Polymer Photochem. 1981, 1, 233. Irie, M.; Hosoda, M. Makromol. Chem. Rapid Commun. 1985, 6, 533. Merian, E. Text. Res. J . 1966, 36, 612.

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14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

PHOTOPHYSICS OF POLYMERS

Smets, G. Adv. Polym. Sci. 1983, 50, 18. Matejka, L . ; Dusek, K.; Ilavsky, M. Polymer Bull. 1979, 1, 659. Matejka, L . ; Ilavsky, M.; Dusek, K.; Wichterle, O. Polymer 1981, 22, 1511. de Lange, J.J.; Robertson J.M.; Woodward, J . Proc. R. Soc. 1939, A171, 398.: Mampson, G.C.; Robertson, J.M. J. Chem. Soc. 1942, 409. Herz, M. L. J. Am. Chem. Soc. 1975, 97, 6777. Irie, M.; Kungwatchakun, D. Makromol. Chem. Rapid Commun. 1984, 5, 829. Irie, M.; Kungwatchakun, D. Macromolecules 1986, 19, 2476. Tanaka, T.; Fillmore, D.J. J. Chem. Phys. 1979, 70, 1214. Glignon, J.; Scallan, A. M. J. Appl. Polym. Sci. 1980, 25, 2829. Ricka, J.; Tanaka, T. Macromolecules 1984, 17, 2916. Tanaka, T.; Nishio, J.; Sun, S-T.; Ueno-Nishio, S. Science 1982, 218, 467. Osada, Y.; Hasebe, M Irie, M. Macromolecule Irie, M.; Schnabel, W. Makromol. Chem. Rapid Commun. 1984, 5, 413. Irie, M.; Schnabel, W. Macromolecules 1986, 19, 2846. Irie, M. J. Am. Chem. Soc. 1983, 105, 2078. Irie, M.; Tanaka, H. Macromolecules 1983, 16, 210. Irie, M.; Iwanaga, T.; Taniguchi, Y. Macromolecules 1985. 18, 2418. Irie, M.; Schnabel, W. Macromolecules 1985, 18, 394. Kumar, G. S.; DePra, P.; Neckers, D. C. Macromolecules, 1984, 17, 1912. Shinkai, S.; Kinda, H.; Ishihara, M.; Manabe, O. J . Poly. Sci. Chem. 1983, 21, 3525. Irie, M.; Menju, Α.; Hayashi, K. Nippon Kagaku Kaishi 1984, 227. Ishihara, K.; Hamada, N.; Kato, S.; Shinohara, I. J . Polym. Sci. Chem. 1983, 21, 1551. Agolini, F.; Gay, F.P. Macromolecules 1970, 3, 349. Van der Veen, G.; Prins, W. Nature, Phys. Sci. 1971, 230, 70. Smets, G.; Evans, G. Pure Appl. Chem. Suppl. Macromol. Chem. 1973, 8, 357. Eisenbach, C. D. Polymer 1980, 21, 1175. Osada, Y.; Katsumura, E.; Inoue, K. Makromol. Chem. Rapid Commun. 1981, 2, 411. Blair, H.S.; Pogue, H.I. Polymer, 1982, 23, 779. Ishihara, K.; Hamada, N.; Kato, S.; Shinohara, I. J . Polym. Sci. Chem. 1984, 22, 121. Aviram, A. Macromolecules 1978, 1, 1275. Kohjiya, M.; Hashimoto, T.; Yamashita, S.; Irie, M. Chem. Lett. 1985, 1479. Irie, M.; Iga, R. Makromol. Chem. Rapid Commun. 1985, 6, 403. Irie, M.; Iga, R. Macromolecules 1986, 19, 2480. Irie, M.; Mohri, M.; Hayashi, K. Polym. Prep. Jpn. 1985, 34, 716. Irie, M.; Iga, R. Makromol. Chem. Rapid Commun. 1986, 7, 751. Okamoto, Y; Sakamoto, H; Hatada, K; Irie, M. Chem. Lett. 1986, 983.

RECEIVED March 13, 1987

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 11

Luminescence Studies of Molecular Motion in Poly(n-butyl acrylate) J. Toynbee and I. Soutar Chemistry Department, Heriot-Watt University, Riccarton, Currie, Edinburgh, EH14 4AS, Scotland

Relaxation processe have been studied and probe techniques. A wide range of effective test frequencies have been accessed through the employment of both singlet and triplet excited states as reporter molecules. Segmental relaxation of the polymer i n the region of the glass transition has been studied by emission anisotropy measurements on samples labelled with acenaphthylene and 1-vinylnaphthalene respectively. In addition probe interrogation of the matrix micro-viscosity has revealed an involvement of the polymer matrix in the photophysical behaviour of the triplet state of naphthyl chromophores and information germane to speculation over the existence of triplet excimers of naphthalene is discussed. The a p p l i c a t i o n o f luminescence t e c h n i q u e s t o t h e study o f macrom o l e c u l a r b e h a v i o u r has enjoyed an enormous growth r a t e i n the l a s t decade. The a t t r a c t i o n o f such methods l i e s i n the degrees o f b o t h s p e c i f i c i t y and s e l e c t i v i t y a f f o r d e d t o t h e i n v e s t i g a t o r . Cons e q u e n t l y the polymer may be doped o r l a b e l l e d a t s u f f i c i e n t l y l o w c o n c e n t r a t i o n l e v e l s o f luminophore as t o induce m i n i m a l p e r t u r b a t i o n of t h e system. P o l a r i z e d p h o t o s e l e c t i o n t e c h n i q u e s o f f e r p a r t i c u l a r a t t r a c t i o n i n the study o f r e l a x a t i o n phenomena b o t h i n s o l u t i o n and s o l i d s t a t e s . I n p r i n c i p l e , astute l a b e l l i n g can allow e l u c i d a t i o n of the m o l e c u l a r mechanisms r e s p o n s i b l e f o r the m a c r o s c o p i c r e l a x a t i o n s e x h i b i t e d by the macromolecular system. In addition, l u m i n e s c e n t probes c a n address the m i c r o v i s c o s i t y o f t h e i r e n v i r o n ment. The use o f b o t h s i n g l e t and t r i p l e t s t a t e s as m o l e c u l a r r e p o r t e r s p e r m i t s access t o an e x t r e m e l y wide range o f e f f e c t i v e t e s t frequencies. F l u o r e s c e n c e d e p o l a r i z a t i o n t e c h n i q u e s have been employed e x t e n s i v e l y t o study the r e l a x a t i o n b e h a v i o u r o f macromolec u l e s i n s o l u t i o n ( c f . r e f e r e n c e s (1 - 3) and r e f e r e n c e s t h e r e i n ) . Such a n i s o t r o p y measurements have a l s o been used, t o a much l e s s e r e x t e n t , t o study h i g h frequency r e l a x a t i o n p r o c e s s e s i n the p o l y m e r i c 0097-6156/87/0358-0123$06.00/0 © 1987 American Chemical Society

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s o l i d s t a t e . (3 - 6) Phosphorescence d e p o l a r i z a t i o n , on the o t h e r hand, o f f e r s i n t e r e s t i n g p o s s i b i l i t i e s t o study t h e macromolecular s o l i d s t a t e a t much l o n g e r time s c a l e s upon which g r e a t e r r e s o l u t i o n of the v a r i o u s mechanisms which c o n s t i t u t e the o v e r a l l v i s c o e l a s t i c or d i e l e c t r i c r e l a x a t i o n s p e c t r a might be a n t i c i p a t e d . W h i l s t the p o t e n t i a l of phosphorescence p o l a r i z a t i o n measurements has been r e c o g n i z e d f o r many y e a r s v e r y few s u c c e s s f u l a p p l i c a t i o n s o f t h e t e c h n i q u e have been r e p o r t e d (9 - 11) f o r e m p i r i c a l r e a s o n s . I n t h e c u r r e n t paper we extend p r e v i o u s work on p o l y ( m e t h y l a c r y l a t e ) (9 - 10) and p o l y ( m e t h y l m e t h a c r y l a t e ) (9 - 11) i n which the phosphorescence d e p o l a r i z a t i o n t e c h n i q u e was shown t o p r o v i d e d a t a which were c o n s i s t e n t w i t h r e p o r t e d d i e l e c t r i c and m e c h a n i c a l r e l a x a t i o n e x p e r i m e n t s , t o t h e study o f t h e m o l e c u l a r b e h a v i o u r o f p o l y ( n - b u t y l m e t h a c r y l a t e ) . T h i s polymer, w h i l s t o f t e c h n o l o g i c a l a p p l i c a t i o n , has r e c e i v e d much l e s s e r a t t e n t i o n u s i n g c o n v e n t i o n a l dynamic r e l a x a t i o n t e c h n i q u e s than has been devoted t o PMA and PMMA In a d d i t i o n , f l u o r e s c e n c employed i n an attempt t the h i g h e r frequency b e h a v i o u r o f the polymer. In a d d i t i o n , luminescence i n t e n s i t y and l i f e t i m e d a t a o b t a i n e d not o n l y from t h e l a b e l l e d polymer b u t a l s o . f r o m e m i s s i o n of d i s p e r s e d n a p h t h a l e n e , acenaphthene and 1 , 1 - d i n a p h t h y l - l , 3 - p r o p a n e (DNP) a r e b r i e f l y d i s c u s s e d . These measurements have p r o v i d e d i n f o r ­ m a t i o n n o t o n l y o f r e l e v a n c e t o the r e l a x a t i o n b e h a v i o u r of t h e polymer m a t r i x b u t a l s o t o the p h o t o p h y s i c s which occur t h e r e i n and i n t r a m o l e c u l a r l y w i t h i n the DNP. Experimental M a t e r i a l s . Acenaphthene ( A c ) , acenaphthylene (ACE) and naphthalene (Np) ( A l d r i c h ) were p u r i f i e d by m u l t i p l e r e c r y s t a l l i z a t i o n from e t h a n o l f o l l o w e d by m u l t i p l e s u b l i m a t i o n under h i g h vacuum. 1 - V i n y l n a p h t h a l e n e (1-VN) was p r e p a r e d by d e h y d r a t i o n of methy11 - n a p h t h y l c a r b i n o l ( K o c h - L i g h t ) w i t h 10% p o t a s s i u m hydrogen s u l p h a t e and p u r i f i e d by f r a c t i o n a l d i s t i l l a t i o n under reduced p r e s s u r e . 1 , 1 - d i n a p h t h y l - l , 3 - p r o p a n e (DNP) was p r e p a r e d a c c o r d i n g t o the method o f Chandross and Dempster ( 1 2 ) . η-Butyl a c r y l a t e was f r e e d of i n h i b i t o r , vacuum degassed, p r e p o l y m e r i z e d and p u r i f i e d by h i g h vacuum f r a c t i o n a l d i s t i l l a t i o n immediately p r i o r t o use. P o l y ( n - b u t y l a c r y l a t e ) (PBA) was p r e p a r e d by f r e e r a d i c a l p o l y ­ m e r i z a t i o n (AIBN) under h i g h vacuum t o l e s s than 10% c o n v e r s i o n a t 60%C. L a b e l l e d PBA was o b t a i n e d by c o p o l y m e r i z a t i o n w i t h c a. 0.2 mole % 1-VN o r ACE t o produce polymers P/VN and P/ACE r e s p e c t i v e l y . The polymers were p u r i f i e d by m u l t i p l e r e p r e c i p i t a t i o n from benzene i n t o c o l d methanol o r 40/60 p e t r o l e u m e t h e r . Molar^masses o f t h e r e s u l t a n t polymers e s t i m a t e d by gpc exceeded 10 and comprized mononodal d i s t r i b u t i o n s . Sample P r e p a r a t i o n . Polymer samples f o r luminescence i n v e s t i g a t i o n were produced as t h i n polymer f i l m s by s o l v e n t c a s t i n g from CH«C1 ?

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

11.

TOYNBEE AND SOUTAR

Luminescence Studies of Molecular Motion

125

i n quartz c e l l s . R e s i d u a l s o l v e n t was removed under h i g h vacuum a t 100°C f o r 5-10 days p r i o r t o s e a l i n g . Instrumentation. Steady s t a t e luminescence measurements were made u s i n g a P e r k i n - E l m e r MPF-3L s p e c t r o f l u o r i m e t e r . Samples were c o n t a i n e d i n an o p t i c a l dewar and temperature v a r i a t i o n a c h i e v e d u s i n g a f l o w o f p r e - h e a t e d o r c o o l e d d r y n i t r o g e n gas. Phosphor­ escence measurements were made u s i n g a r o t a t i n g - c a n phosphoroscope. F l u o r e s c e n c e l i f e t i m e s and t i m e - r e s o l v e d s p e c t r o s c o p i c measure­ ments were o b t a i n e d u s i n g an E d i n b u r g h I n s t r u m e n t s 199 spectrometer u s i n g the t i m e - c o r r e l a t e d s i n g l e photon c o u n t i n g technique u s i n g e x c i t a t i o n from a c o a x i a l f l a s h lamp w i t h hydrogen as the d i s c h a r g e medium. Temperature c o n t r o l was a c h i e v e d u s i n g an Oxford I n s t r u m e n t s C r y o s t a t . The 199 spectrometer was a l s o m o d i f i e d t o a c c o m p l i s h phosphorescence decay a n a l y s i s u s i n g a m i c r o s e c o n d f l a s h l a m p . The e x p e r i m e n t a l arrangemen R e s u l t s and D i s c u s s i o n Phosphorescence P o l a r i z a t i o n Measurements. The degree of p o l a r i z ­ a t i o n was determined as a f u n c t i o n o f temperature f o r the systems P/ACE, P/VN and P-Np ( d i s p e r s e d naphthalene p r o b e ) . The e x c i t a t i o n wavelength was 310 nm and t h a t o f a n a l y s i s , 490 nm. The onset o f d e p o l a r i z a t i o n was observed a t ca. 210, 195 and 180K f o r P/ACE, P/VN and P-Np r e s p e c t i v e l y . The onset temperature f o r P/ACE i s i n good agreement w i t h t h a t of the c o n v e n t i o n a l v a l u e f o r Tg, c o n s i s t e n t w i t h the low frequency e q u i v a l e n c e o f t h e t i m e - s c a l e imposed by the phosphorescent l a b e l a t t h i s temperature i n the PBA m a t r i x . The p o l a r i z a t i o n d a t a were transformed i n t o apparent r e l a x a t i o n t i m e s , 1^, v i a the P e r r i n e q u a t i o n (P

_ 1

- 1/3) = ( P

_ o

1

- 1/3)(1 + 3 τ / τ ) θ

(1)

Γ

where τ r e p r e s e n t s the l i f e t i m e of the e l e c t r o n i c a l l y e x c i t e d s t a t e and Ρ i s the i n t r i n s i c p o l a r i z a t i o n , e s t i m a t e d i n t h i s i n s t a n c e as t h a t a t t a i n e d a t low temperatures (77K). The r e s u l t a n t d a t a a r e p l o t t e d i n A r r h e n i u s form i n F i g u r e 1 and the r e s u l t a n t v a l u e s f o r the a c t i v a t i o n e n e r g i e s o f the r e l a x a t i o n processes are l i s t e d i n Table I . Table I . System P/ACE

Temperature Dependence of R e l a x a t i o n Times E / k J mol"" a 98 ± 5

P/VN

ca. 100 44 ± 5

P-Np

100 ± 5 33 ± 5

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1

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PHOTOPHYSICS OF POLYMERS

F i g u r e 1. Temperature dependence of r e l a x a t i o n times d e r i v e d from phosphorescence d e p o l a r i z a t i o n d a t a .

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

11.

T O Y N B E E AND S O U T A R

Luminescence Studies of Molecular Motion 127

The mode o f attachment o f the l a b e l s t o the polymer c h a i n i s d e p i c t e d below.

P/ACE

Ρ/1-VN

The acenaphthylene l a b e l , b e i n g i n c a p a b l e o f m o t i o n independent o f the polymer c h a i n , i s expected t o m o n i t o r segmental r e l a x a t i o n o f t h e macromolecule. The r e l a x a t i o n d a t a adhere w e l l t o an A r r h e n i u s r e l a t i o n s h i p over the r e l a t i v e l y r e s t r i c t e d f r e q u e n c y - t e m p e r a t u r e range sampled. T h i s b e h a v i o u r i s s i m i l a r t o t h a t observed f o r t h e PMA r e l a x a t i o n as determined by phosphorescence d e p o l a r i z a t i o n ( 9 ^ _ 1 0 ) but c o n t r a s t s w i t h t h a t o f PMMA i n which backbone m o t i o n o f l e s s e r a c t i v a t i o n energy was s e n s i b l I t i s apparent t h a t the bone s u b s t i t u e n t s a r e i m p o r t a n t i n d e t e r m i n a t i o n o f the e x t e n t t o which backbone segments e x p e r i e n c e m o b i l i t y as the g l a s s t r a n s i t i o n i s approached. I n t h i s r e s p e c t the i n f l u e n c e o f t a c t i c i t y i n 1 , 1 - d i s u b s t i t u t e d a l k y l m e t h a c r y l a t e polymers r e q u i r e s i n v e s t i g a t i o n . The g r e a t e r degree o f freedom enjoyed by the 1-naphthyl l a b e l i n P/VN i s most l i k e l y a s c r i b a b l e t o motion independent o f the polymer c h a i n about the bond o f attachment. S i n c e the v e c t o r s f o r a b s o r p t i o n and e m i s s i o n a r e l o c a t e d w i t h i n d i f f e r e n t p l a n e s w i t h i n the m o l e c u l a r framework f o r t r i p l e t e m i s s i o n , such motions c o n s t i t u t e a mechanism of enhanced d e p o l a r i z a t i o n f o r the P/VN system. I n the case o f PMMA i t has been shown (11) t h a t independent m o t i o n o f a 1-VN l a b e l o c c u r s i n the v i n c i n i t y o f the r e l a x a t i o n o f the polymer but i s c h a r a c t e r ­ i z e d by an apparent a c t i v a t i o n energy i n f e r i o r t o t h a t sensed by an e s t e r l a b e l o r as a f f o r d e d by d i e l e c t r i c and dynamic r e l a x a t i o n d a t a f o r the 3 - p r o c e s s . Consequently w h i l s t the onset o f m o t i o n might be c o n s i s t e n t w i t h an i n c r e a s e d degree o f f r e e volume r e l e a s e d by t h e 3-mechanism, the a c t i v a t i o n energy f o r n a p h t h y l group r e o r i e n t a t i o n w i t h i n the l i f e t i m e o f the e x c i t e d s t a t e s s h o u l d not be equated w i t h t h a t o f the β-process i t s e l f i n PBA. M o t i o n o f the d i s p e r s e d n a t h t h a l e n e t r i p l e t probe i n P-Np i s e v i d e n t a t 180K. By analogy w i t h p r e v i o u s o b s e r v a t i o n s o f the PMMA s y s t e m ( l l ) such m o b i l i t y might be concomitant w i t h the o n s e t o f the 3 - r e l a x a t i o n . I t i s apparent from dynamic m e c h a n i c a l d a t a a t 1Hz t h a t r e s o l u t i o n o f the α and 3 r e l a x a t i o n s i s d i f f i c u l t ( 1 4 ) . I t may be t h a t the use o f m o l e c u l a r probes and l a b e l s such as 1-VN ( a s d e s c r i b e d above) o f f e r s a more s e n s i t i v e s e n s o r y e x p l o r a t i o n o f t h e m a t r i x than i s a f f o r d e d by m a c r o s c o p i c t e c h n i q u e s . I t i s a l s o p o s s i b l e t h a t the degrees o f freedom f o r m o t i o n imbued i n the m a t r i x do not r e s u l t from the m o t i o n o f e s t e r groups as g e n e r a l l y i n f e r r e d i n PMA and PMMA f o r the 3 - r e l a x a t i o n but t h a t c o n f o r m a t i o n a l changes w i t h i n the η-butyl s u b s t i t u e n t i t s e l f , c o n s t i t u t e s u f f i c i e n t r e l e a s e of f r e e volume w i t h i n the m a t r i x t o a l l o w r o t a t i o n a l m o b i l i t y o f b o t h the 1-VN l a b e l and the d i s p e r s e d naphthalene probe. I t i s apparent t h a t o u r concepts o f the r e l a x a t i o n a l p r o c e s s e s i n w e l l s t u d i e d systems such as the p o l y ( n - a l k y l a c r y l a t e s ) and p o l y ( n - a l k y l m e t h a c r y l a t e s ) w a r r a n t f u r t h e r s t u d y and t h a t

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOPHYSICS OF POLYMERS

luminescence probe and l a b e l l i n g t e c h n i q u e s o f f e r unique o p p o r t u n ­ i t i e s f o r the study of the l e s s u n d e r s t o o d systems which c o n s t i t u t e the m a j o r i t y of polymer s o l i d s . Phosphorescence I n t e n s i t y Measurements. The i n t e n s i t y of phosphorescence e m i s s i o n from the system P/ACE, P/VN and Ρ/Νρ (λ = 310 nm; λ = 490 nm) was m o n i t o r e d as a f u n c t i o n of temperature. C o n s i d e r the p h o t o p h y s i c a l k i n e t i c scheme: 1

M + hv

M

*

a ->

X

M

+

%

-> 1 M

(2.1)

I

a

[V:

(2.2)

[ V ]

(2.3)

k [ M*]

(2.5)

k

(2.6)

k

F

M

k

I

M

*

3

-+

M* 3

->

M*

\ +

hv

T

p T

hi

I

T

[ V ]

i n which the r a t e c o n s t a n t s k j ^ and k ^ r e f e r to a l l r a d i a t i o n l e s s d e a c t i v a t i o n s t e p s of the e x c i t e d s i n g l e t and t r i p l e t s t a t e s r e s p e c t i v e l y and can be expanded to accommodate b i m o l e c u l a r quenching; k and k a r e the r a d i a t i v e r a t e c o e f f i c i e n t s a s s o c i a t e d w i t h f l u o r ­ escence and phosphorescence r e s p e c t i v e l y . The quantum y i e l d of phosphorescence, φ^, may be w r i t t e n as F T

p T

Φ

ρ

=

*TM

C k

W

/ ( k

M

+

k

IT

) 3

(3)

where φ i s the quantum y i e l d of i n t e r s y s t e m c r o s s i n g . The i n t e n s i t y of phosphorescence, I p , i s p r o p o r t i o n a l t o the quantum y i e l d o f phosphorescence. S e p a r a t i o n of the nonr a d i a t i v e r a t e c o e f f i c i e n t kjj i n t o temperature-independent and t h e r m a l l y a c t i v a t e d components f u r n i s h e s the e x p r e s s i o n (15,16). 1/1

- 1/1 Ρ

= Β exp(-E/RT)

(4)

ο

where I i s the phosphorescence i n t e n s i t y a t a b s o l u t e z e r o and Ε i s the " a c t i v a t i o n energy". S o m e r s a l l ^ e t al(16) have shown t h a t p l o t s of l n ( I ) as a f u n c t i o n of Τ a r e u s e f u l i n e x h i b i t i o n of " t r a n s i t i o n t e m p e r a t u r e s " i n s o l i d p o l y m e r s . The l i m i t a t i o n s of such r e p r e s e n t a t i o n s i n e v a l u a t i o n of " a c t i v a t i o n e n e r g i e s " has been d i s c u s s e d ( 1 0 , 1 1 ) . I n the c u r r e n t work, d i s c o n t i n u i t i e s i n such p l o t s o c c u r r e d a t two " t r a n s i t i o n t e m p e r a t u r e s " f o r each of the systems P/ACE and P/VN and Ρ - Np as shown i n F i g u r e 2. I n the case of the l a b e l l e d systems, P/ACE, and P/VN, the h i g h e r temperature d i s c o n t i n u i t y o c c u r s a t c a . 215K i n good agreement w i t h t h a t of ca. 218K f u r n i s h e d by d i e l e c t r i c and m e c h a n i c a l r e l a x a t i o n s t u d i e s a t l H z ( 1 7 ) f o r the a - t r a n s i t i o n ( g l a s s t r a n s i t i o n ) of the polymer. The onset temperature f o r t h i s t r a n s i t i o n as sensed by the n a p h t h a l e n e probe i n Ρ - Np i s 200K. I t i s l i k e l y t h a t " m o l e c u l a r p r o b i n g " of the microenvironment Q

p

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

11.

Luminescence Studies of Molecular Motion 129

T O Y N B E E AND S O U T A R

a c c u r a t e l y r e f l e c t s the g l a s s - r u b b e r t r a n s i t i o n o f the polymer i n t h i s i n s t a n c e as a consequence o f the i n f l u e n c e o f polymer m o b i l i t y upon r a d i a t i v e and r a d i a t i o n l e s s d e a c t i v a t i o n pathways o f the e x c i t e d t r i p l e t s t a t e . Indeed, t h e c o r r e l a t i o n between " t r u e " t r a n s i t i o n temperatures and phosphorescence i n t e n s i t y changes i s s t r o n g e r t h e g r e a t e r the c o n c e n t r a t i o n o f b i m o l e c u l a r l y quenching ground s t a t e s p e c i e s (such as oxygen)(10,11) P l o t s o f l n ( I - I ) , where I i s approximated i n t h i s s t u d y as the i n t e n s i t y o f ° phosphorescence a t 77K, p r o v i d e apparent e n e r g i e s o f a c t i v a t i o n o f the p h o t o p h y s i c a l d e a c t i v a t i o n o f the t r i p l e t naphthalene s p e c i e s , which are i n broad agreement f o r a l l o f the systems s t u d i e d . _^ A lower temperature d i s c o n t i n u i t y i n the l n l p v s . Τ plots i s observed f o r a l l o f the systems s t u d i e d i s apparent a t c a . 140K. I n v i e w o f the d a t a f u r n i s h e d by p o l a r i z a t i o n d a t a ( v i d e supra) and of the problems a s s o c i a t e d i n dynamic m e c h a n i c a l r e l a x a t i o n w i t h s e p a r a t i o n o f the a - an suggested t h a t t h i s " t r a n s i t i o n d i s c u s s e d below. A c a v e a t must be sounded as t o the use o f i n t e n s i t y ( o r l i f e ­ time) d a t a a c c r u e d through the use o f t r i p l e t probes i n s e n s i n g p o l y m e r i c t r a n s i t i o n s i n t h i s manner. The p r o b e / l a b e l d a t a a r e complementary b u t v i c a r i o u s i n the n a t u r e o f t h e i n f o r m a t i o n r e l a y e d to the o b s e r v e r . Apparent e n e r g i e s o f a c t i v a t i o n through the use o f e q u a t i o n (4) of c a . 20 and 14 k J mol~~^ r e s p e c t i v e l y were r e c o r d e d f o r the upper and lower t r a n s i t i o n temperature t r a n s i t i o n s apparent from 1^ d a t a . Q

P

Phosphorescence L i f e t i m e Data The l i f e t i m e , ! , o f t h e t r i p l e t s t a t e i s r e l a t e d t o the r a t e c o e f f i c i e n t s o f r a d i a t i v e ( k ) and n o n r a d i a t i v e ( k ) decay by the r e l a t i o n p T

I T

τ"

1

- k

k

p T +

I T

(5)

S e p a r a t i o n o f the r a d i a t i o n l e s s d e c a y - r a t e c o e f f i c i e n t i n t o temper­ a t u r e dependent and independent components y i e l d s t h e e x p r e s s i o n τ"

where τ ent o f

-1 k

= k^ .

1

_

1

- τ = C(exp - E/RT) (6) ° + k' ,k* b e i n g the temperature independent compon-

I T

A c t i v a t i o n energy d a t a were a f f o r d e d by e q u a t i o n (6) t r a n s l a t e d i n t o A r r h e n i u s form. P l o t s o f 1 η τ as a f u n c t i o n o f T~~* y i e l d e d apparent t r a n s i t i o n temperatures i h agreement w i t h those a f f o r d e d by i n t e n s i t y d a t a (as r e p o r t e d above). A c t i v a t i o n e n e r g i e s i n the h i g h and low temperature regimes were not i n g e n e r a l a c c o r d w i t h those r e g i s t e r e d u s i n g i n t e n s i t y d a t a b e i n g c a . 50 and 3 k J mol~* r e s p e c t ­ ively. These d a t a , t a k e n a t f a c e v a l u e , might be i n t e r p r e t e d as b e i n g i n d i c a t i v e o f a temperature dependence o f by comparison o f e q u a t i o n s (3) and ( 5 ) . However, t h e photophysics of the polymer systems a r e much more complex than h i t h e r t o s u p p o s e d ( l O ^ l l ) as d i s c u s s e d below. I n p a r t i c u l a r , the decays o f phosphorescence i n t e n s i t y a r e n o t e x p o n e n t i a l , and t h e l i f e t i m e d a t a used i n e q u a t i o n (6) f o r the e s t i m a t i o n of e n e r g i e s o f a c t i v a t i o n a r e those ρ

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

130

PHOTOPHYSICS OF POLYMERS

o b t a i n e d from the " t a i l s ' O f the decays. S i m i l a r methods were adopted f o r the e s t i m a t i o n o f τ v a l u e s i n the e v a l u a t i o n o f r e l a x a t i o n d a t a v i a equation (1). I t i s apparent t h a t the use o f phosphorescence d a t a i n the study of p o l y m e r i c t r a n s i t i o n s i s f r a u g h t w i t h c o m p l i c a t i o n s and the use o f s t e a d y - s t a t e i n t e n s i t y d a t a and low t i m e - r e s o l u t i o n decay d a t a , i n p a r t i c u l a r , a r e o f l i m i t e d a p p l i c a t i o n t o the study o f r e l a x a t i o n phenomena. Nature o f the Lower Temperature T r a n s i t i o n . The complex none x p o n e n t i a l phosphorescence d e c a y s , apparent under the h i g h e r time r e s o l u t i o n a f f o r d e d by use o f the m o d i f i e d 199 s p e c t r o m e t e r are a consequence o f i n t e r j e c t i o n by the polymer m a t r i x i n the p h o t o p h y s i c s e x p e r i e n c e d by the chromophore. N o n - e x p o n e n t i a l decays o f t r i p l e t n a p h t h a l e n e (and o t h e r chromophore) e m i s s i o n s have been observed i n PMMA. H o r i e et al.(18-20) a s c r i b e such e f f e c t s t o dynamic interm o l e c u l a r quenching o f MacCallum et al(21-23) i n v o k polymer f o l l o w i n g quenching o f the t r i p l e t s t a t e o f the n a p h t h a l e n e . I n the PBA m a t r i x , e x p o n e n t i a l decays were n o t observed a t any temperature i n excess of 140K. I n a d d i t i o n , d e l a y e d f l u o r e s c e n c e was o b s e r v a b l e from the n a p h t h y l chromophores a t h i g h e r photon f l u x e s . C o n s i d e r i n g the low c o n c e n t r a t i o n s o f chromophore and the f a c t t h a t f o r the l a b e l l e d systems, d i f f u s i v e motion o f the n a p h t h y l s p e c i e s i s p r o h i b i t e d , these d a t a a r e s u p p o r t i v e o f the MacCallum mechanism. These d a t a s h a l l be p u b l i s h e d more f u l l y e l s e w h e r e . F l u o r e s c e n c e Measurements. D e p o l a r i z a t i o n becomes apparent a t ca270K. The i n i t i a l steady i n c r e a s e o f P~^ w i t h temperature a t temperatures i n excess of 270K i s f o l l o w e d by a d r a m a t i c enhancement a t temperatures s u p e r i o r t o c a . 380K. F i g u r e 3 d i s p l a y s the d a t a f o r the P/ACE system f o l l o w i n g t r a n s f o r m a t i o n i n t o r e l a x a t i o n times v i a e q u a t i o n ( 1 ) . The c u r v e s f o r P/VN, P/ACE and P - N p may be d i v i d e d i n t o two p s e u d o - l i n e a r regimes y i e l d i n g a c t i v a t i o n e n e r g i e s of ca. 60 and 25 k J mol"* i n the h i g h e r and lower temperature regions r e s p e c t i v e l y . I t i s apparent t h a t ; (i) the d i s p e r s e d naphthalene probe senses the same r e l a x a t i o n and e x p e r i e n c e s a s i m i l a r temperature dependence f o r decay o f i t s photos e l e c t e d o r i e n t a t i o n t o those sensed by t h e l a b e l l e d systems. T h i s i s somewhat s u r p r i z i n g c o n s i d e r i n g t h e s m a l l molar volume o f the probe. J a r r y and Monnerie(4) have i n d i c a t e d t h a t the dynamic b e h a v i o u r o f a p a r t i c u l a r f l u o r e s c e n t probe depends upon i t s s i z e r e l a t i v e t o t h a t o f the k i n e t i c segments o f the polymer i n v o l v e d i n the t r a n s i t i o n . S i m i l a r correspondence t o probe s i z e have been r e p o r t e d u s i n g paramagnetic probe i n t e r r o g a t i o n s o f polymer matrices(24). (ii) i n v o k i n g the onset o f segmental m o t i o n , c o n s i s t e n t w i t h t h e o c c u r r e n c e o f the α r e l a x a t i o n , i n e x p l a n a t i o n o f the d e p o l a r i z a t i o n s observed a t 270K, t h i s temperature i s e l e v a t e d some 50K compared t o t h a t o b t a i n e d f o r the g l a s s t r a n s i t i o n measured a t low f r e q u e n c i e s . T h i s magnitude of time-temperature s h i f t would be as g e n e r a l l y expected f o r the s h i f t o f some e i g h t decades o f f r e q u e n c y t o t h e f l u o r e s c e n c e time base. A g a i n , the l a c k o f e v i d e n c e f o r an independ­ e n t l y r e s o l v e d 3 p r o c e s s on these time s c a l e s i s t o be e x p e c t e d .

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

11.

T O Y N B E E AND SOUTAR

Luminescence Studies of Molecular Motion 131

F i g u r e 3. Temperature dependence o f r e l a x a t i o n times d e r i v e d from f l u o r e s c e n c e d e p o l a r i z a t i o n d a t a f o r the P/ACE system.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

132

PHOTOPHYSICS OF POLYMERS

C o n s e q u e n t l y , u s i n g t h i s i n t e r p r e t a t i o n , the h i g h e r temperature t r a n s i t i o n e v i d e n t a t c_a. 380K might be a s c r i b e d t o the o c c u r r e n c e o f a l i q u i d - l i q u i d t r a n s i t i o n as the k i n e t i c segment l e n g t h i n v o l v e d i n the r e l a x a t i o n s o f the m e l t a l t e r s . A l t e r n a t i v e l y the c u r v a t u r e o f the p l o t may m e r e l y show t h a t the r e l a x a t i o n d a t a a r e p o o r l y des­ c r i b e d , even over t h i s l i m i t e d f r e q u e n c y - t e m p e r a t u r e range, by an Arrhenius functional representation. (iii) the " a c t i v a t i o n e n e r g i e s " o b t a i n e d from the A r r h e n i u s p l o t s are low r e l a t i v e t o low f r e q u e n c y phosphorescence p r o b i n g . W h i l s t comparable d i e l e c t r i c and dynamic r e l a x a t i o n d a t a a r e not a v a i l a b l e f o r PBA over a broad f r e q u e n c y range, the s h i f t t o lower apparent a c t i v a t i o n energy as f r e q u e n c y i n c r e a s e s i s g e n e r a l l y a n t i c i p a t e d for α relaxations. A p l i c a t i o n o f I n t r a m o l e c u l a r Excimer F o r m a t i o n t o the Study o f PBA R e l a x a t i o n . DNP was d i s p e r s e d i n PBA and i t s p h o t o p h y s i c a l b e h a v i o u r compared t o the P-Ac an Fluorescence studie i n f o r m a t i o n r e g a r d i n g the m i c r o v i s c o s i t y o f the m a t r i x . U n u s u a l l y the probe seems t o r e f l e c t t h e low f r e q u e n c y g l a s s t r a n s i t i o n and c a u t i o n must be a p p l i e d i n i n t e r p r e t i n g p h o t o p h y s i c a l d a t a o b t a i n e d w i t h such probes i n terms o f polymer r e l a x a t i o n mechanisms. More i n t e r e s t i n g l y , t r i p l e t s t a t e s t u d i e s have produced date which expand o u r e x p e r i e n c e o f i n t r a m o l e c u l a r t r i p l e t s t a t e i n t e r ­ a c t i o n s and a r e r e l e v a n t t o the i s s u e r e g a r d i n g the e x i s t e n c e o f t r i p l e t excimer s p e c i e s i n naphthalene compounds(25-30). The d a t a may be summarized as f o l l o w s : (a) A t l o w t e m p e r a t u r e s , A + M k^ = k^ + k2 and kg = k$ + kg Scheme 1 0097-6156/87/0358-0186$06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

187

Intramolecular Excimer Formation

DESCHRYVERETAL.

>x

A i s t h e l o c a l l y e x c i t e d s t a t e (LE) and (AM)* i s t h e e x c i t e d s t a t e complex ( E ) . The f l u o r e s c e n c e spectrum c o n s i s t s o f t h e e m i s s i o n o f L E , hv', a t h i g h e r e n e r g i e s and o f t h e complex E, which e m i t s a t lower e n e r g i e s , h v ,due t o t h e s t a b i l i s a t i o n and r e p u l s i o n energy terms In case o f a δ e x c i t a t i o n t h e time dependence o f t h e e m i s s i o n can be d e s c r i b e d by t h e f o l l o w i n g e q u a t i o n s " 1

k

I (t)» I LE

a

b

l [(β - X ) e x p ( - 3 t ) + ( X - β )βχρ(-β ί)]

s

2

1

ι

eq.l

2

&z - h k k [M] 5

ι

χ

κΜ

T

3

e x

abs

I

(

P "

"

e x

(

P "P2

t ) ]

e
L£., and o f t h e c o m p l e x ^ g , a n d t h e r a t i o t h e r e o f can be f o r m u l a t e d as follows: >98%) i n t h e TG c o n f o r m a t i o n . The excimer f o r m i n g s t e p i s one r o t a t i o n around one bond t o form t h e TT c o n f o r m a t i o n i n w h i c h t h e two chromophores o v e r l a p e x t e n s i v e l y . ( f i g . 2 ) c

c

Q

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

D E S C H R Y V E R E T AL.

TG

Intramolecular Excimer

Formation

TT

F i g . z . S p a c e f i l l i n g models r e p r e s e n t i n g t h e TG ground s t a t e and TT* e x c i t e d s t a t e . T h e a c t u a l excimer s t r u c t u r e must have t h e two pyrene groups f a r t h e r a p a r t f o r s t e r i c reasons.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

189

PHOTOPHYSICS OF POLYMERS

190

Scheme 2 hv TG TG*

> TG* > TT*

Important parameters o f the p h o t o p h y s i c s i n t a b l e 1.

o f 1 a-c are assembled

T a b l e 1 K i n e t i c and thermodynamic parameters i n t h e excimer of l a - c i n i s o o c t a n e

formation

c

parameter

l

1 a

FWMH cm" e m i s s i o n max. nm E kJmol" k 10 s~ k g ^ a t 20°C i n ns ΔΗ° k J m o l " AS° J K " m o l "

c

a

l

c

b

l

c

c

340

a

1

3

1 0

l b

3

1

1

a)excimer

1

16 310 37 -20±3 -17±5

10 10 22 --

17 60 145 -16 -42

e m i s s i o n b ) p r e e x p o n e t i a l term

S u b s t i t u t i o n o f t h e c e n t r a l methylene group by an oxygen no l o n g e r p e r m i t s t h e u s e o f t h e above mentioned nmr method t o o b t a i n i n f o r m a t i o n on t h e c o n f o r m a t i o n a l d i s t r i b u t i o n f o r t h e meso d i a stereoisomer. An a n a l y s i s o f t h e luminescence b e h a v i o r o f I Q C ^ - a s i n g l e e x p o n e n t i a l decay a t -30 °C i n i s o o c t a n e when a n a l y s e d i n t h e l o c a l l y e x c i t e d s t a t e , an a c t i v a t i o n energy f o r excimer f o r m a t i o n o f 17,6 k J m o l " i n i s o o c t a n e , t h e presence o f o n l y one excimer w i t h a k g o f 85 ns and a FWHM o f 3800 cm" - suggests a TG ground s t a t e conformation. These r e s u l t s a l l o w t h e c o n c l u s i o n t h a t i n a b i c h r o m o p h o r i c m o l e c u l e l i n k e d by t h r e e c a r b o n - c a r b o n bonds o r i n t h e e t h e r analog w i t h a w e l l d e f i n e d c o n f o r m a t i o n o f t h e c h a i n , a symmetric sub­ s t i t u t i o n a t t h e chromophore and i n absence o f i n t e r a c t i o n s i n t h e ground s t a t e excimer f o r m a t i o n c a n be d e s c r i b e d by a i n t r a m o l e c u l a r v e r s i o m o f t h e k i n e t i c scheme 1. Nmr a n a l y s i s o f l e^^ i n d i c a t e d t h a t t h e meso isomer i n a l k a n e s o l v e n t s a t room temperature has m a i n l y (>95%) a TG c h a i n c o n f o r ­ m a t i o n w h i l e a t lower t e m p e r a t u r e s o n l y t h e TG c o n f o r m a t i o n i s p r e s e n t . N e v e r t h e l e s s t h e time p r o f i l e o f t h e f l u o r e s c e n c e a t -50^C can n o t be a n a l y s e d as a s i n g l e e x p o n e n t i a l b u t good f i t s a r e o b t a i n e d upon a n a l y s i s o f t h e l o c a l l y e x c i t e d s t a t e as a sum o f two exponentials. Spectral information c l e a r l y indicates that a t t h i s t e m p e r a t u r e no back d i s s o c i a t i o n o f t h e complex o c c u r s . ^ A s u b s t a n t i a l s h i f t o f t h e excimer e m i s s i o n maximum , a broadening o f t h e f u l l w i d t h a t medium h e i g h t (FWMH) o f t h e excimer e m i s s i o n and t h e a n a l y s i s o f t h e f l u o r e s c e n c e decay a c r o s s t h e excimer band 1

1

_ 1

1

c

1

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

Intramolecular Excimer Formation

DESCHRYVERETAL.

c l e a r l y e s t a b l i s h e d t h e presence o f two e x c i m e r s . were o b t a i n e d f o r l e . ( t a b l e 2)

191

Analogous

results

Q

T a b l e 2 E x c i t e d s t a t e p r o p e r t i e s o f l e and l e i n i s o o c t a n e Q

Q

parameter a

max. 298 K nm λ max. 183 K nm FWMH(298 K ) cm" FWMHU83 K ) cm" α / α a t 450 nm c^/c^ β^ i n n s a t 450 nm 32~* i n ns at450 nm n ns a t 520 n &C\ iit ns a t 520 n 82'* iinn ns λ

a

a

1

a

1

b

χ

2

a

t

5

1

2

0

n

m

c

^'foJlO

472 492 4800 4400 1.5 15 160 80 161

21 2 12

19 7 8

1

1

€

E ( k J mol" ) *3 p > s - ')5^ E p (kJ mol" )* 3 i f o

1

0

1

1

3

480 500 4300 3800 1.44 8.5 80 47 80

Q

a) o f t h e excimer e m i s s i o n b) r a t i o o f t h e p r e e x p o n e n t i a l terms o f t h e two e x p o n e n t i a l decay f u n c t i o n d e s c r i b i n g t h e t i m e p r o f i l e o f t h e excimer e m i s s i o n measured a t 298 Κ i n i s o o c t a n e . c ) β exponential terms o f t h e decay f u n c t i o n . S i n c e t h e amount o f bac& d i s s o c i a t i o n a t t h i s temperature i s v e r y s m a l l t h e y can be s e t e q u a l t o k g . d ) p r e e x p o n e t i a l term o f t h e r a t e c o n s t a n t f o r f o r m a t i o n o f t h e l o n g l i v e d excimer.e) a c t i v a t i o n energy o f t h e r a t e c o n s t a n t f o r f o r m a t i o n of t h e l o n g l i v e d e x c i m e r . f ) p r e e x p o n e n t i a l term o f t h e r a t e c o n s t a n t f o r f o r m a t i o n o f t h e s h o r t l i v e d excimer.g) a c t i v a t i o n energy o f t h e r a t e constant f o r formation o f t h e s h o r t l i v e d excimer. χ 2

_ 1

The f o r m a t i o n o f two e x c i m e r s s t a r t i n g from one c h a i n con­ f o r m a t i o n was r e l a t e d t o t h e presence o f d i f f e r e n t rotamers o f t h e non s y m m e t r i c a l l y s u b s t i t u t e d 1 - p y r e n y l group i n t h e TG c o n f o r m a t i o n ( f i g . 3 ) . One o f t h e e x c i m e r s formed - t h e l o n g e r l i v e d one d e r i v e d from t h e T^G^ conformer - has a k g " v a l u e a t room temperature c l o s e t o t h e one o b s e r v e d f o r r e s p e c t i v e l y meso l c and meso l c and e m i t s a t l o n g e r w a v e l e n g t h s . A t low temperature i t i s t h e main c o n t r i b u t o r t o t h e excimer e m i s s i o n . T r a n s i e n t p i c o s e c o n d a b o r p t i o n s p e c t r o ­ s c o p y ^ o f l c and l e s u b s t a n t i a t e s t h i s i n t e r p r e t a t i o n . S i n c e a TT c o n f o r m a t i o n o f l c r e s u l t s i n a f u l l o v e r l a p o f t h e two pyrene group a s i m i l a r s p a t i a l arrangement can be suggested f o r t h i s l o n g l i v e d excimer. The second excimer formed from o t h e r r o t a m e r ( s ) w i l l have o n l y p a r t i a l o v e r l a p o f t h e two chromophores. An u n e q u i v o c a l assignment o f t h e decay parameters t o t h e d i f f e r e n t rotamers f o r m i n g t h e two d i f f e r e n t e x c i m e r s c o u l d be made by t h e a n a l y s i s o f t h e decay of t h e l o c a l l y e x c i t e d s t a t e and t h e excimer decay a t 205 Κ where t h e l o n g l i v e d excimer i s t h e s o l e c o n t r i b u t o r and t h e l o c a l l y e x c i t e d s t a t e can be c o r r e l a t e d w i t h t h e growing i n o f t h i s e x c i m e r . This s u g g e s t s t h a t t h e r a t e o f r o t a t i o n o f t h e chromophore s h o u l d be s m a l l e r than t h e r a t e o f excimer f o r m a t i o n . B r o a d e n i n g o f t h e nmr s i g n a l o f t h e pyrene group a t 185K i n d i c a t e s slow r o t a t i o n o f t h i s 1

c

Q

1

Q

Q

Q

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

192

F i g . 3 . S c h e m a t i c r e p r e s e n t a t i o n o f two p o s s i b l e ground s t a t e c o n f o r m a t i o n s l e a d i n g t o two d i f f e r e n t e x c i m e r s of

l

c

c'

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

Intramolecular Excimer Formation

DESCHRYVERETAL.

193

group a t t h e nmr time s c a l e and hence on t h e time s c a l e o f t h e p h o t o p h y s i c a l experiment. The p a r t i a l l y o v e r l a p p i n g excimer has a lower a c t i v a t i o n energy o f f o r m a t i o n due t o t h e f a c t t h a t a s t a b i l i s i n g i n t e r a c t i o n between t h e two pyrene groups s t a r t s e a r l i e r a l o n g t h e r e a c t i o n c o o r d i n a t e decreasing the activation barrier. The a c t i v a t i o n energy o f f o r m a t i o n o f t h e f u l l o v e r l a p excimer i s i n t h e non s y m e t r i c a l l y s u b s t i t u t e d compounds l e and l e , w i t h i n e x p e r i m e n t a l e r r o r , comp a r a b l e t o t h e a c t i v a t i o n e n e r g i e s observed f o r r e s p e c t i v e l y l c and c

e

c

These r e s u l t s i n d i c a t e t h a t t h e c o m p l e x i t y o f t h e f l u o r e s c e n c e decay o f meso l e and meso l e , where t h e pyrene i s non s y m m e t r i c a l l y s u b s t i t u t e d , i s n o t due t o a c o n f o r m a t i o n a l d i s t r i b u t i o n o f t h e c h a i n but t o r o t a t i o n a l isomerism around t h e carbon carbon bond l i n k i n g t h e chromophore t o t h e c h a i n backbone p r o v i d i n g t h e p o s s i b i l i t y t o form more t h a n one excimer i n t h e s e systems c

The Racemic

Q

Diastereoisomer

What a r e t h e k i n e t i c consequences i f t h e chromophores a r e l i n k e d by a c h a i n f o r w h i c h more t h a n one c h a i n c o n f o r m a t i o n contributes substantially? T h i s a s p e c t can be c o n s i d e r e d by a study o f t h e racemic d i a s t e r e o - i s o m e r s o f l c ' T h e racemic d i a s t e r e i s o m e r s l c a r e systems i n w h i c h t h e c h a i n i s p r e s e n t i n d i f f e r e n t conformations namely t h e TT and t h e GG conformers ( f i g 4 ) . I n both d e u t e r a t e d cyclohexane and d e u t e r a t e d c h l o r o f o r m , a t room temperature, t h e TT c o n f o r m a t i o n o f racemic l c is predominant : 80 % TT and 73 % TT r e s p e c t i v e l y . The observed s o l v e n t e f f e c t on t h e c o n f o r m a t i o n a l d i s t r i b u t i o n i s s i m i l a r but s m a l l e r than found f o r ( l e ) ^ . These r e s u l t s agree q u i t e w e l l w i t h t h o s e o b t a i n e d on s i m i l a r compounds^» 10»17,19 ^ I n t h e NMR d a t a o f t h e racemic d i a s t e r e o i s o m e r s o f l c and l c one i m p o r t a n t d i f f e r e n c e shows up. The p o s i t i o n o f t h e s i n g l e t absorption signal of oij t h e pyrene r i n g i s s h i f t e d s i g n i f i c a n t l y t o h i g h e r Ô v a l u e s f o r l c compared w i t h l c . The p o s i t i o n o f t h e HJL p r o t o n s i n t h e ^H-NMR spectrum i s a f u n c t i o n o f t h e c o n t r i - b u t i o n o f t h e TT c o n f o r m a t i o n s i n c e t h e s e p r o t o n s a r e s h i e l d e d by t h e mutual ring current effect o f t h e pyrene chromophores i n t h e TT conformation. I t can t h e r e f o r e be concluded t h a t t h e e q u i l i b r i u m between TT and GG i n t h e case o f l c i s s h i f t e d more toward t h e GG c o n f o r m a t i o n compared t o l c . T h i s c a n be e x p l a i n e d by comparing t h e bond l e n g t h s o f t h e C-0 bond and t h e C-C bond i n t h e r e s p e c t i v e compounds. The s h o r t e r e t h e r band induces a s t r o n g e r 1,4 i n t e r a c t i o n between t h e methine hydrogens and t h e pyrene chromophores i n t h e e t h e r compound c a u s i n g a s h i f t o f t h e c o n f o r m a t i o n a l e q u i l i b r i u m from TT t o GG. The steady s t a t e f l u o r e s c e n c e s p e c t r a o f l c and l c have an excimer f l u o r e s c e n c e band t h a t i s c o m p l e t e l y superimposable at a l l t e m p e r a t u r e s i n v e s t i g a t e d w i t h t h a t o f t h e i r r e s p e c t i v e meso d i a s t e r e o i s o m e r s s u g g e s t i n g an i d e n t i c a l g e o m e t r i c a l s t r u c t u r e o f t h e excimer and hence a l s o t h e p r e s e n c e o f o n l y one excimer. The f l u o r e s c e n c e decay c u r v e s o f l c and l c m o n i t o r e d a t 377 nm and 500 nm c a n be d e s c r i b e d by t h e same decay laws used f o r t h e r e s p e c t i v e meso d i a s t e r e o i s o m e r s . x

x

1

5

c

Q

c

Q

Q

c

Q

c

(

c

c

f

Q

Q

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

194

PHOTOPHYSICS OF POLYMERS

Fig.Λ.Space f i l l i n g r e p r e s e n t a t i o n o f t h e extended ( C 5 ) and f o l d e d ( C y ) c o n f o r m a t i o n o f 2 a e . I n t h e f o l d e d c o n f o r m a t i o n t h e pyrene group a r e d e p i c t e d i n t h e excimer geometry.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

Intramolecular Excimer Formation

DESCHRYVERETAL.

195

The f l u o r e s c e n c e decays o f t h e l o c a l l y e x c i t e d s t a t e can be a n a l y s e d as a two e x p o n e n t i a l decay f u n c t i o n i n t h e temperature domain between 298 Κ and 233 Κ . Below t h i s temperature t h e decays a r e one e x p o n e n t i a l . Furthermore e x c e l l e n t agreement between t h e decay parameters measured i n t h e l o c a l l y e x c i t e d and excimer r e g i o n of t h e e m i s s i o n spectrum i s o b t a i n e d . The m o l e c u l a r dynamics o f t h e racemic d i a s t e r e o i s o m e r upon e x c i t a t i o n a r e however more c o m p l i c a t e d than t h a t o f t h e meso diastereoisomers due t o t h e presence o f two ground state conformations. S i n c e t h e decay laws used f o r meso d i a s t e r e i s o m e r s can a l s o be a p p l i e d t o t h e racemic d i a s t e r e o i s o m e r , a l l t h e r a t e c o n s t a n t s d e s c r i b i n g t h e e q u i l i b r i u m between t h e ground s t a t e c o n f o r m a t i o n s must be l a r g e compared t o t h e r a t e c o n s t a n t o f excimer formation. The q u e s t i o n however still remains from which c o n f o r m a t i o n does excimer f o r m a t i o n t a k e p l a c e . I t i s not l i k e l y t h a t i t o c c u r s d i r e c t l y from t h e GG c o n f o r m a t i o n T h i s would mean a s i m u l t a n e o u s r o t a t i o n aroun r e l a t i v e l y high a c t i v a t i o f o r m a t i o n t a k e s p l a c e from t h e TT c o n f o r m a t i o n . This p o s s i b i l i t y i s d e p i c t e d i n scheme 3. A t h i r d r o u t e c o u l d be excimer f o r m a t i o n s t a r t i n g from t h e TG conformation. T h i s i n t e r m e d i a t e c o n f o r m a t i o n between t h e TT and GG c o n f o r m a t i o n has however a r e l a t i v e l y h i g h energy c o n t e n t owing t o an u n f a v o u r a b l e i n t e r a c t i o n between t h e m e t h y l group and t h e pyrene moiety. Upon t a k i n g i n t o account t h i s p r e e q u i l i b r i u m t h e r a t e c o n s t a n t s and a c t i v a t i o n b a r r i e r s o f excimer f o r m a t i o n can be r e w r i t t e n i f scheme 3 i s c o n s i d e r e d u s i n g t h e f o l l o w i n g e q u a t i o n s : k

obs

=

f

k

e (

TT 3 K

I"TT

l

1 0

K

1 2 eq 12

=

1 + K

L

+ κ κ χ

2

Ki= k / k .

a

eq 13

K = k. /k

b

eq 14

a

2

b

ΔΗ° E

obs

=

E

2

+ α+Κ )ΔΗ° 2

1 e c

3 +

l

1 5

1 + K + κ κ In these equations k and E a r e t h e observed r a t e c o n s t a n t and a c t i v a t i o n b a r r i e r o f excimer f o r m a t i o n . The f r a c t i o n s o f t h e TT or TG c o n f o r m a t i o n s a t a g i v e n temperature a r e r e p r e s e n t e d by f-p^ and TG" I t c o u l d be s h o w n t h a t s i n c e t h e f r a c t i o n o f t h e TT c o n f o r ­ m a t i o n i s c o n s i d e r a b l e and decreases w i t h i n c r e a s i n g temperature Scheme 3 s h o u l d l e a d t o a n e g a t i v e d e v i a t i o n from l i n e a r i t y i n t h e A r r h e n i u s p l o t o f t h e r a t e c o n s t a n t o f excimer f o r m a t i o n . T h i s c o u l d be e x p e r i m e n t a l l y v e r i f i e d p r o v i n g t h e v a l i d i t y o f scheme 3 The k i n e t i c and thermodynamic d a t a o f l c and l c a r e sum­ m a r i z e d i n t a b l e 3. I f the linear part i n the Arrhenius p l o t i s l

χ

o b s

2

o b s

f

1 S

13

c

Q

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

196

PHOTOPHYSICS OF POLYMERS

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

Intramolecular Excimer Formation

DESCHRYVERETAL.

197

e x t r a p o l a t e d t o h i g h e r t e m p e r a t u r e s , an e s t i m a t e o f f ^ can be made. T h i s assumes t h a t t h e e x t r a p o l a t e d r a t e c o n s t a n t s a t a l l g i v e n t e m p e r a t u r e s a r e e q u a l t o t h e v a l u e o f k . The v a r i a t i o n o f f ^ w i t h temperature i n t h e h i g h temperature r e g i o n (303 K-343 K) can t h e n be used t o determine Δ Η and A S f o r the conformational e q u i l i b r i u m betwee TT and GG o f t h e r a c e m i c d i a s t e r e o i s o m e r . I n t h e case o f l c t h e r e s p e c t i v e v a l u e s a r e 12,2 k J m o l " and 35 Jmol K " . In the case of l c these values are 5 k J mol" and 12 J m o l ^ K " respectively. The c o n f o r m a t i o n a l d i s t r i b u t i o n between TT and GG can t h e n be c a l c u l a t e d and e q u a l s 70 % TT/ 30% GG f o r l c ' and 61 % TT/ 39 % GG f o r l c a t room t e m p e r a t u r e . 3

σ

Q

0

L

1

1

1

1

Q

c

Q

Table 3. K i n e t i c and thermodynamic parameters l c and l c i n i s o octane ( a ) : p r e - e x p o n e n t i a l f a c t o r i n t h e A r r h e n i u s e q u a t i o n ; (b) determined i n t h e l i n e a r p o r t i o n o f t h e A r r h e n i u s p l o t a t low temperature (243 K-203 K) c

,

X

1

E ( k J m o l " ) (b) °obs ~ ) ) o b s

K

E

( s

( a

χ

1

1

C

Ί1

40 (± 1 3

8)

3 (± 2 ) x l 0

-14 (± 7) -47 (± 12)

0

1

U

5.0 (± D x i O

ΔΗ ( k J mol" ) ASoUmol" K" )

0

22 (± 2) 3.3 (±0.3)χ10

34 (± 5)

1

( s " ) (a)

5

X

8

( k J mol" )

5

C

20 ( ± 2 ) · (±0.2)xlO

1

k°

C

Q

1 4

-18 (± 10) -57 (± 30)

T h e r e s u l t s o b t a i n e d f o r t h e r a c e m i c d i a s t e r e o i s o m e r s l c ' and l c i n d i c a t e t h a t even when t h e r a t e o f c o n f o r m a t i o n a l change between t h e d i f f e r e n t c h a i n c o n f o r m a t i o n s i s much f a s t e r t h a n t h e r a t e o f excimer f o r m a t i o n , r e s u l t i n g i n f l u o r e s c e n c e decays s i m i l a r t o t h o s e o f t h e meso d i a s t e r e o i s o m e r s i t i s p o s s i b l e t o e x t r a c t some i n f o r m a t i o n on t h e c o n f o r m a t i o n a l d i s t r i b u t i o n o f t h e c h a i n from t h e obtained data. t

c

Q

BIS(PYRENYLALANINE)PEPTIDES By c h o o s i n g another l i n k between t h e two chromophpores i t i s p o s s i b l e t o slow down t h e r a t e o f c o n f o r m a t i o n a l i n t e r c h a n g e as t o be i n t h e same o r d e r o f magnitude as t h e i n v e r s e o f t h e f l u o r e s c e n c e l i f e t i m e o f t h e chromophore. T h i s was r e a l i z e d i n b i s ( p y r e n y l a l a n i n e ) p e p t i d e s o f t h e g e n e r a l s t r u c t u r e 2. CH CO-NH CHCO-NH CHCO-XCH 3

A

B

3

CH2

1

i

2 i f H^,Hg i s r e p l a c e d by a methylgroup 2 ( M E o r ME ) R = 1-pyrenyl = a R = 2-pyrenyl = b a

b

Structure 2

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

198

PHOTOPHYSICS OF POLYMERS

N - a c e t y i - b i s ( l - p y r e n y l ) a l a n i n e m e t h y l e s t e r 2a c a n occur i n two d i a s t e r e o i s o r a e r i c forms- t h r e o ( t ) and e r y t h r o ( e ) - w h i c h c o u l d be s e p a r a t e d by c h r o m a t o g r a p h y ^ . i i n e r t solvents,not capable of f o r m i n g hydrogen bonds,the r a t i o o f excimer e m i s s i o n over e m i s s i o n from t h e l o c a l l y e x c i t e d s t a t e i s s u b s t a n t i a l l y h i g h e r t h a n i n s o l v e n t s which can accept hydrogen bonds. The d e c r e a s e o f t h e emission ratio c a n be c o r r e l a t e d w i t h the T a f t basicity parameter.. These o b s e r v a t i o n s and t h e f a c t t h a t 3ae always has a h i g h e r e f f i c i e n c y o f excimer f o r m a t i o n than 3 a t were e x p l a i n e d by a c o n s e c u t i v e k i n e t i c scheme i n v o l v i n g t h e l o c a l l y e x c i t e d s t a t e o f pyrene i n two d i f f e r e n t c o n f o r m a t i o n s o f t h e p e p t i d e c h a i n : an extended c h a i n c o n f o r m a t i o n , C 5 , w i t h a l t e r n a t e s i d e c h a i n s ( f i g . 4 ) and a f o l d e d c o n f o r m a t i o n , C y , w i t h quasi p a r a l l e l side chains(fig.4) and supported by an i n t r a m o l e c u l a r hydrogen bond between the carbony1 f u n c t i o n o f the a c e t y l p r o t e c t i n g group and the amine f u n c t i o n o f t h e second p y r e n y l a l a n i n e r e s i d u e Only t h e Cy c o n f o r m a t i o groups t o a p a r t i a l l l i f e t i m e o f the e x c i t e d pyrene moeiety. T h i s p o i n t c o u l d be proven by t h e s u b s t i t u t i o n o f t h e hydrogen i n v o l v e d i n t h e hydrogen bond formation i n t h e Cy c o n f o r m a t i o n by a m e t h y l g r o u p 2(ME^)a r e s u l t i n g i n t h e d i s s a p e a r e n c e o f t h e excimer band i n t h e e m i s s i o n spectrum. The f l u o r e s c e n c e decay o f t h e l o c a l l y e x c i t e d s t a t e a t temper a t u r e s were t h e excimer does not d i s o c i a t e back c o u l d be a n a l y s e d as a sum o f two e x p o n e n t i a l s - * .That t h i s i s n o t due t o r o a t i o n a l i s o m e r i s m o f t h e 1-pyrenyl group c o u l d be proven by t h e a n a l y s i s o f 2b w h i c h showed analogous behavior.The a n a l y s i s o f t h e f l u o r e s c e n c e decay a c c o r d i n g t o scheme 4 p e r m i t s t h e d e t e r m i n a t i o n o f the r a t i o o f 2

n

2 1

2 2

2 2

2

c /c . 7

5

Scheme 4. j^fol

^ex

-fol -ex I n i n e r t s o l v e n t s the m o l e c u l e i s s t a b i l i s e d by f o l d i n g l e a d i n g to a h i g h Cy p o p u l a t i o n and hence a more i n t e n s e excimer e m i s s i o n than i n hydrogen bonding s o l v e n t s t h a t s h i f t the c o n f o r m a t i o n a l d i s t r i b u t i o n i n the ground s t a t e more t o t h e C5 conformer .When compaied i n t h e same s o l v e n t t h e t h r e o d i a s t e r e o i s o m e r has a lower f o l d e d p o p u l a t i o n then t h e e r y t h r o i n p a r t due t o i n c r e a s e d s t e r i c hinder a n c e and i n p a r t due t o t h e absence o f a s t a b i l i s i n g N-H pyrene interaction The s o l v e n t induced s h i f t o f the c o n f o r m a t i o n a l e q u i l i b r i u m was c o n f i r m e d , i n t o l u e n e a t -20°C a r a t i o o f 3 and 0.75 was c a l c u l a t e d f o r 2ae and 2 a t r e s p e c t i v e l y w h i l e i n e t h y l a c e t a t e these values d e c r e a s e t o 0.8 and 0.4. k

k

CONCLUSIONS I n t h i s c o n t r i b u t i o n we attempt t o e v a l u a t e t h e i n f l u e n c e o f c o n f o r m a t i o n a l and c o n f i g u r a t i o n a l a s p e c t s ,both on excimer f o r m a t i o n r a t e s

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

D E S C H R Y V E R E T AL.

Intramolecular Excimer Formation

199

and on excimer p r o p e r t i e s . F r o m t h e r e s u l t s p r e s e n t e d , we c o n - c l u d e that the difference i n photophysical b e h a v i o r o f two d i f f e r e n t c o n f i g u r a t i o n s c a n be r e l a t e d t o t h e c o n f o r m a t i o n a l d i s t r i b u t i o n o f each d i a s t e r e o i s o m e r . I f o n l y one c o n f o r m a t i o n i s p r e s e n t complex f l u o r e s c e n c e decay s t i l l c a n a r i s e by t h e non s y m m e t r i c a l s u b s t i t u t i o n o f t h e chromophore l i n k e d t o t h e c h a i n . I f more t h e n one c h a i n c o n f o r m a t i o n i s p r e s e n t w i l l t h i s l e a d t o complexity i n the fluorescence decay o n l y i f t h e r a t e o f conf o r m a t i o n a l change i s comparable t o t h e r a t e o f excimer f o r m a t i o n a in the dipeptides. I n absence o f t h i s c o m p l i c a t i o n i t i s however n e c e s s a r y t o i n t e r p r e t t h e o b t a i n e d r e s u l t s t a k i n g i n t o account t h e f a s t p r e e q u i l i b r i u m between t h e d i f f e r e n t conformers. The n a t u r e o f t h e excimer formed and i t s p r o p e r t i e s w i l l f o r a g i v e n chromophore depend on t h e d i a s t e r e o i s o m e r . The b i n d i n g energy of t h e excimer w i l l be a f f e c t e d by s t a b i l i s i n g o r d e s t a b e l i s i n g e f f e c t s of the chain. Furthermore i f r o t a t i o n a l isomerism o f t h e chromophore i s p o s s i b l slow compared t o excime e x c i t e d s t a t e complex i s p o s s i b l e and depends on t h e r e s p e c t i v e s t a b i l i s a t i o n o f t h e d i f f e t r e n t complexes.The r e l a t i v e c o n t r i b u t i o n of each a t a g i v e n t e m p e r a t u r e w i l l depend on t h e r o t a t i o n a l d i s t r i b u t i o n and t h e r e s p e c t i v e r a t e o f excimer f o r m a t i o n

Acknowledgments The a u t h o r s w i s h t o thank t h e B e l g i a n N a t i o n a l S c i e n c e F o u n d a t i o n f o r c o n t i n o u s s u p p o r t t o t h e laboratory.Dr.M.VdA i s a Reseach A s s o c i a t e of t h e NFWO.Dr.Boens i s thanked f o r h i s e s s e n t i a l c o n t r i b u t i o n t o t h e development o f t h e s o f t w a r e used i n t h e a n a l y s i s o f t h e f l u o r e s c e n c e decays o b t a i n e d by t i m e c o r r e l a t e d s i n g l e photon c o u n t i n g . REFERENCES

(1) Förster,T. and Kaspar,K.,Z.Phys.Chem.(N.F.),1,275,(1954). (2) a. Azumi,T. and McGlynn,S.P.,J.Chem.Phys.,39,1186(1963) b. Azumi,T. and McGlynn,S.P.,J.Chem.Phys.,41,3131(1964) c. Azumi,T.,Armstrong,Α.Τ.,and McGlynn,S.P. J.Chem.Phys,41, 839,(1964) d. Förster,Τ.,Pure and Appl.Chem.,4,121(1962) e. Murell,J.N., and Tanaka,J.,J.Mol.Phys.,7,363,(1964) (3) a. Azumi,T. and Azumi,H.,Bull.Chem.Soc.Jpn.,39,2317,(1966) b. Chandra,A.K. and Lim,E.C.,J.phys.Chem.,48,2589(1968) c. Post,M.F.M.,Langelaar,J. and Van Voorst ,J.D.W., Chem.Phys.,15,445(1976) d. Padma Malar,E.J. and Chandra,Α.Κ., Theor.Chim.Acta, 55,153(1980) (4) a. Döller,E. and Förster,.,Z.Phys.Chem.(N.F.),34,132(1962) b.BirksJ.Β.,"Photophyics of aromatic molecules",ch.7, Wiley- Interscience,London,1970 (5) Stevens,B. and Ban,M.I.,Trans.Far.Soc.II ,60,1515(9164)

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

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(6) a.ref.5b p.309 b.O'Connor,D. and Ware,W.R.,J.Am.Chem.Soc.,101,121,(1979) (7) Palmans,J.P.,Van der Auweraer,M.Swinnen,A.M.and De Schryver,F.,J.Am.Chem.Soc.,106,7721(1984) (8) Longworth,J.W. and Bovey,F.A.,Biopolymers ,4,1115(1965) (9) Bovey,F.A.,Hood III,F.P.,Anderson,E.W. and Snyder,L.C., J.Chem.Phys.,42,3900(1965) (16)S. Ito,M. Yamamoto, Y. Nishijima, Bull.Chem.Soc.Jap 55,(1982) 363. (10) Jasse,B.,Lety,A. and Monnerie,L.,J.Mol.Struct., 18,413(1973) (11) Froelich,B.,Noel,C.,Jasse,B.and Monnerie,L., Chem.Phys.Lett.,44,159(1976) (12) a.Gorin,S. and Monnerie,L.,J.Chim.Phys. Physicochim. Biol.,67,869(1970) b. Yoon,D.Y.,Sundararajan,P.R d Flory,P.J. Macromolecules,8,776(1975 c. McMahon,P.E. and Tincher,W.C.,J.Mol.Spectrosc., 15,180(1965) (13)a.De Schryver,F.C.,Vandendriessche,J., Toppet,S., Demeyer,K. and Boens,N. Macromolecules,15,406(1982) b.Vandendriessche,J.,Palmans,P.,Toppet,S.,Boens,N,De Schryver,F.C. and Masuhara,H.,J.Am.Chem.Soc. 106,8057(1984) (14) Monnerie,L.,Bokobza,L.,De Schryver,F.C.,Moens,L.,Van derAuweraer,M. and Boens,N.,Macromolecules,15,64(1982) (15) Collart,P.,Toppet,S.,and De Schryver,F.C.,Macromolecules in press (16) Collart,P.,Toppet,S.,Zhou,Q.F.,,Boens,N. and De Schryver,F.C.,Macromolecules,18,1026(1985) (17) Collart,P.Demeyer,K.,Toppet,S. and De Schryver,F.C., Macromolecules,16,1390(1983) (18) a.Masuhara,H.,Tanaka,J.A.,Mataga,N.,De Schryver,F.C. and Collart,P.,Polym.J.,15,915(1983) b.Masuhara,H.,Tamai,T.,Mataga,N.,Collart,P. and De Schryver,F.C.,submitted (19) Ito,S.,Yamamoto,M. and Nishijima,Y.,Bull.Chem.Soc.Jpn., 55,363(1982) (20) Goedeweeck,R. and De Schryver,F.C.,Photochem.Photobiol 39,515(1984) (21) Mortimer,J.K.,Abbaud,J.L.,Abraham,M.H. and Taft,R.W., J.Org.Chem.,48,2877(1983) (22) Goedeweeck,R.,Ruttens,F.,Lopez Arbelao,F. and De Schryver,F.C.,submitted (23) Goedeweeck,R.,Van der Auweraer,M. and De Schryver,F.C., J.Am.Chem.Soc.,107,2334(1985) RECEIVED September 14, 1987

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 17

Photophysics of 1,5-Naphthalene Diisocyanate-Based Polyurethanes Charles E. Hoyle and Kyu-Jun Kim Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406-0076

Using both steady-stat decay spectroscopy, e formatio o intramolecula excimers in dilute solution of a naphthalene diisocyanate based polyurethane is identified. Investigation of an appropriate model compound leads to the conclusion that hydrogen bonding is a key factor in stabilizing excimers formed from naphthyl carbamates. While the decay kinetics of the model naphthyl carbamate are described by a typical Birks excimer scheme involving a single excited species in dynamic equilibrium with the excimer, the polymer decay kinetics can only be adequately interpreted by an "isolated monomer" scheme involving both an interactive (excimer forming) and non-interactive (isolated monomer) excited naphthyl carbamate moiety. The extent of excimer formation is dependent on the ability of the solvent to solvate the polyurethane, i.e., excimer formation is increased in poor solvents. In addition, excimer formation in solid polyurethane films is quite high where hydrogen bonding, as identified by the shift in the carbonyl stretching frequency, is prevalent. Excimers, which are simply excited state complexes formed from e q u i v a l e n t chemical species, one of which i s excited p r i o r t o complexation, were f i r s t r e p o r t e d i n s m a l l molecule systems. However, during the past 20 years one of the more vigorous research areas i n photophysics has been the investigation of excimers formed i n polymers (1). In most of the cases reported to date, the excimer studies i n polymer systems have been conducted on polymers bearing pendant aromatic chromophores. Only a few papers have been published on excimers formed from polymers with the two species p a r t i c i p a t i n g i n excimer formation spaced a t r e l a t i v e l y large

0097-6156/87/0358-0201 $06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

202

i n t e r v a l s from each other along the backbone (2-5). Herein, we report on intramolecular excimers between naphthyl carbamate groups spaced p e r i o d i c a l l y i n polyurethanes made from 1,5-naphthalene diisocyanate. I t i s found that even i n very d i l u t e solutions, well below the concentrations required for intermolecuiar interaction, excimer emission can be quite strong, depending on the nature of the s o l v a t i n g system employed. The d r i v i n g mechanism for excimer formation of the naphthyl carbamate groups i s shown to be based, at least i n part, on hydrogen bonding. EXPERIMENTAL

SECTION

Materials

Dichloromethane, dimethylformamide (DMF), and benzene were obtained from Burdick and Jackson and used without further p u r i f i c a t i o n 2,3-Butanediol and 1-butano Propyl benzene (Aldrich was used. Equipment

Emission spectra and absorption spectra were recorded on a PerkinElmer 650-10S Fluorescence Spectrophotometer and a Perkin-Elmer 320 UV Spectrophotometer, respectively. Fluorescence decay data were obtained on a single-photon-counting apparatus from Photochemical Research Associates. The samples were bubbled with nitrogen f o r the s t e a d y - s t a t e f l u o r e s c e n c e s p e c t r a and the fluorescence decay measurements. In some cases, front face spectra were taken. The data were analyzed by a software package from PRA cased on the i t e r a t i v e convolution method. C NMR spectra were obtained on a JEOL FX90Q, and FTIR spectra were recorded on a Nicolet 5DX. The elemental analyses were conducted by M-K-W Laboratories of Phoenix, AZ. 1 3

Synthesis of

Model Compounds

1,5-Naphthalene diisocyanate ( NDI ). To a s t i r r i n g s o l u t i o n of pdioxane (50 mL) containing the i,5-diaminonaphthalene (Fluka, 5.1 g) was added trichioromethyl chloroformate (Fluka, 17 g) i n p-dioxane (15 mL) through an addition funnel under a nitrogen stream. A white p r e c i p i t a t e was immediately observed. After addition, the temperature was increased to reflux. The forming HC1 was removed by passing through water. After 1 hour, the s o l u t i o n turned clear and was a l l o w e d t o react for another 3 hours. The p-dioxane was evaporated under reduced pressure and the r e s u l t i n g s o l i d was vacuum sublimed twice to give c o l o r l e s s c r y s t a l s i n 71% y i e l d : πτρ> 126128°C ( l i t . mp 129.5-131°C) ; IR 3022, 2300, 1600, 1500 cm"""; C NMR 130.9, 128.7, 127.7, 124.2, 122.2 ppm (benzene); A n a l . 12 6 2°2 - ' · < · ' · ex

/

e

m

= 330

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

17.

HOYLEANDKIM

Naphthalene Diisocyanate-Based Polyurethanes

207

( r ) and 15.3 ns ( τ ) . By monitoring at 480 nm in the excimer region and f i t t i n g the long-lived portion of the decay curve to a s i n g l e exponential decay function, a l i f e t i m e of 15.4 ns (Table I) was recorded. Comparison of the s h o r t - l i v e d component (0.33 n s — τ2 ) of the monomer decay i n the concentrated s o l u t i o n with the l i f e t i m e (3.53 n s — r ) i n the d i l u t e s o l u t i o n leads to the conclusion that the monomer emission i n the concentrated s o l u t i o n i s s i g n i f i c a n t l y quenched. Both the large decrease of the fluorescence l i f e t i m e of the monomer naphthyl carbamate and the presence of the long-lived component i n the monomer emission region suggests that the monomer e x i s t s i n dynamic equilibrium with the excimer as represented by the c l a s s i c a l scheme (Scheme I) for excimer k i n e t i c s . The value for k j ^ has a f i n i t e value which makes the d i s s o c i a t i o n of the excimer E* into i t s component species M and M* a v i a b l e process. At t h i s point, i t i s appropriate to consider the factors which may enter into s t a b i l i z a t i o n of the excimer i n the ?NC solutions F i r s t , i t i s worth notin monitored by the recordin monomer region, occurs at concentrations of ?NC as l i t t l e as 0.005 M i n organic solvents. However, i t i s o n l y above 0.5 M that appreciable baiId up of excimer emission i s recorded. It has long been recognized that polyurethanes, which contain carbamate chromophores, are c h a r a c t e r i z e d by hydrogen bonding between the hydrogen attached to the nitrogen of the carbamate group and the carbamate carbonyi (or an ether group i f present). This contributes to the unique physical properties of polyurethanes. Unsurprising, the PNC model system a l s o shows appreciable hydrogen bonding which i s p r e v a l e n t a t h i g h e r c o n c e n t r a t i o n s between carbamate chromophores. This i s i l l u s t r a t e d i n Figure 4 by the IR s p e c t r a of ?NC recorded at two concentrations. In the d i l u t e solution (0.2 M) the IR shows a s i n g l e peak i n the N-H stretching region. This peak at 3416 cm""- r e s u l t s from a non-bonded (or free) N-H stretching i n the carbamate moiety. At the higher concentration (1.5 M) a new band due to a hydrogen-bonded N-H stretching appears at 3320 cm""*. I t i s i n t h i s concentration region that the higher degree of excimer formation i s recorded i n Figure 2. Thus, i t i s reasonable to assume that hydrogen bonding between the PNC molecules contributes to the s t a b i l i z a t i o n of the excimer. In support of t h i s supposition, propyl N-methyl N-(1-naphthyl) carbamate (PNMNC) with a methyl group substituted on the nitrogen carbamate was synthesized. The methyl group p r o h i b i t s hydrogen bonding. Figure 5 shows that l i t t l e or no excimer emission i s recorded f o r PNMNC up to concentrations of 2.2 M i n dichloromethane. Only at concentrations above 3 M can any excimer emission be observed. Consequently, i t can be concluded that hydrogen bonding i s indeed an important factor i n the excimer formation of ?NC. 2

3

1

NÎCH )C0 Pr 3

2

PNMNC

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

208

Scheme I

k

M-

D M

[Ml

M k

MD

Μ+ηυ'

2Μ+ηυ"

+ heat + product

k

M

=

non-radiative excited

^DM

=

r

a

t

e

^MD

=

r

a

t

=

non

rate

constant

for

for

excimer

formation

for

dissociation

between

M and

.

e

constant

component kg

radiative

PNC monomer M * .

constant

M

plus

+ heat + product

radiative

species plus

of

excimer

Ε

M and M * .

radiative

rate

constant

for

excimer E * . M =

ground

s t a t e PNC.

M

=

excited

E*

=

PNC e x c i m e r .

into

s t a t e PNC.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

17.

H O Y L E AND KIM

Naphthalene Diisoeyanate-Based

Polyurethane*

ω ο c (Ό


' lg" Ru(bpy) U iio nm), 2 χ 10 M C PN ^excitation ' 2 χ 10 M pyrene fluorescence ^excitation ^ f i ° °f P i - aqueous solutions of PMA. -5 Relative fluorescence of 2 χ 10 M Au Ο (λexcitation 388 nm) i nintensity PAA. +

2+

3

e x c i

3 4 0

3

B.

4

n m

0

n m

i

a

a

o

n

s

n

+

4

d

a

u n c t

n

H

n

+

Fluorescence spectra of C PN i n aqueous solutions of PMA at pH 4, 5, 6 and 7 tt 356> nm. 4

excitation

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

32. CHU AND THOMAS

Interaction of Cationic Species

443

C^PN cause a l o c a l clustering of the probes thus enhancing s t a t i c excimer formation. If excimers are formed by migration together of two pyrene molecules, then excimer emission intensity w i l l increase with time, as exhibited by pyrene i n micelles.(46) However, i f pyrene i s stacked or clustered together on the assembly then the excimer emission at 480 nm w i l l be observed immediately after the laser pulse.(47) Figure 6A i l l u s t r a t e s an immediate formation and accelerated decay of excimer i n 1 χ 10~ M C PN i n aqueous solution of PMA at pH 6, showing that C^PN i s stacked or clustered i n PMA. The f i r s t order decay curves of, ln(Intensity of excimer C PN fluorescence) vs. time i n aqueous solutions of PMA at pH 6, 7 and 8 are given i n Figure 6B. The lifetime of the excimer i s larger at pH 6 than at pH 7 or pH 8. The longer l i f e t i m e of the excimer at pH 6 confirms that the environment of the probe at pH 6 i s less polar than at pH 7-8 and that at pH 6 PMA i s not f u l l y open or extended but s t i l l exist 4

+

4

+

+

4

Conformational Transition of PMA Induced by C T A B . Aqueous solutions of PMA at pH 8, i n the absence and i n the presence of C J Q T A B were examined by transmission electron microscopy. No p a r t i c l e s were observed i n simple aqueous solutions of PMA at pH 8 either v i a the p o s i t i v e l y charged or the negatively charged stains (uranyl acetate or phospho-tungstic a c i d ) . However, i n the presence of C, T A B (8 χ 10~ M), spherical p a r t i c l e s of about 350 Â - 500 Â diameter can be c l e a r l y observed v i a the above process, the electron micrograph of aggregates of PMA - C-JQTAB i s shown i n Figure 7. This indicates that the PMA polymer chain i s extended and stretched at pH 8 as shown i n e a r l i e r studies(1-17), while addition of cationic surfactants such as, C T A B r e f o l d the polymer chain to form spherical p a r t i c l e s . Photophysical studies both steady state and pulsed, of pyrene and i t s positive and negatively charged derivatives,(48) confirm that a conformational t r a n s i t i o n of PMA i s induced by C TAB. The stretched PMA chain at pH 8 collapses on addition of the cationic surfactants. N

3

Q

1 Q

n

C r i t i c a l Aggregate Concentration - the CAC. Figure 8 shows a sharp increase both i n I and Ιβ/Ι^ of pyrene fluorescence over a narrow range of C^QTAB concentration, i n aqueous solution of PMA at pH 8. I t i s noted that the r a t i o of I / I i n PMA - C ^ T A B solutions reaches a steady value (~ 0.70) at a C^QTAB concentration, which i s c a l l e d a C r i t i c a l Aggregate Concentration, CAC. At the CAC, the hydrophobic aggregates of Cj T A B and PMA are formed which host hydrophobic molecules such as pyrene. The CAC corresponds to the midpoint of the t r a n s i t i o n i n a plot of I of pyrene fluorescence versus [ C T A B ] . The decay rate constants sharply decrease at a C J Q T A B concentration 3 χ 10~ M, which i s i n good agreement with the CAC determined v i a fluorescence r a t i o of I 3 / I 1 measurement. This indicates that techniques that have been successfully used for the CMC measurements i n micellar systems,(49) can also be used to investigate the aggregates of PMA and C T A B . The i n i t i a l addition of C T A B (< CAC), causes a decrease i n I , the pyrene fluorescence i n t e n s i t y and an increase i n the decay rate constants, due to r

3

1

Q

r

1 Q

3

1 Q

1 0

r

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

Β

6.0 1.0

0

JΕ 4.0 ι—ι

Α

I

I

1 4

6 ρΗ

\

10

8

u

\

\

1Λ

•

(C PN1M 4

\, 2 Χ 1 0 "

4

/ ' » /' »

/Α

2.0

1X1 Ο*

4

0.0 10 ΡΗ

Dependence of the r a t i o of r e l a t i v e i n t e n s i t y of excimer to monomer fluorescence, I / I , on the pH of aqueous solutions of PMA. A. Various concentrations of C P N B. 2 χ 10" M C P N . e

m

+

4

6

+

n

Tieecns)

TieecnS) A. B.

+

Observed decay ( I vs. t) of the C P N excimer fluorescence i n aqueous solutions of PMA, at pH 6. F i r s t order decay (In I vs. t.) of the C P N excimer fluorescence i n aqueous solutions of PMA, a t pH 6, 7 and 8. r

4

+

r

4

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

32. CHU AND THOMAS

7.

Interaction of Cationic Species

445

Electron micrograph of the PMA-C TAB aggregates, [ C - Q T A B ] = 8 χ 10" M. PMA of Molecular weight = 1.1 χ 10 . (The length of the inserted bar i s equal to 1000 Â ) . 1Q

3

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

446

PHOTOPHYSICS OF POLYMERS

quenching by free bromide ions. These data also show that pyrene i s not associated with PMA or s o l u b i l i z e d i n PMA at pH 8, but s o l u b i l i z e d i n the water phase. However, at Q T A B concentrations above the CAC, pyrene i s p r e f e r e n t i a l l y s o l u b i l i z e d i n hydrophobic regions formed by the aggregates of PMA - C-JQTAB. Here i t i s protected from quenching by bromide i n the aqueous phase and I increases sharply. Figure 9 represents a p l o t of log(CAC) versus the number of carbon atoms of a cationic surfactant molecule, C T A B and log(CMC) vs. η i s also plotted for the sake of comparison. The figure shows that each CAC i s markedly lower than the CMC of the corresponding surfactant. This i s p a r t i c u l a r l y true f o r C , Q T A B , C C T A B , where the CACs are one to two orders of magnitude lower than the CMC! Similar experiments i n C ^ T A B - PMA d i d not lead to an increase i n I and I3/I.J up to surfactant concentrations of 0.5 M, and, a short chain quaternary ammonium s a l t tetrabutyl ammonium iodide TBI, does not a f f e c the C Q T A B surfactant i above experiments tend to indicate that the interaction between PMA and c a t i o n i c surfactants i s chainlength dependent. The shorter (n < 8) the chainlength of C T A B , the weaker the i n t e r a c t i o n between C T A B and PMA, giving r i s e to a larger CAC. The photophysical data show sharp changes over a narrow range of surfactant concentration, CAC. I t i s suggested that a cooperative process takes place consisting of a c o i l i n g of polymer assisted by reduced charge density and the hydrophobic interactions of the surfactant chains bound to PMA. The CAC also depends on the PMA concentrations following the stoichiometric relationship: r

N

T A B

a

n

d

1 2

1 6

r

N

N

-3

CAC (10

M) = 0.7

+

2.2 χ [PMA]

(1)

(0.1 g/L < [PMA] < 2 g/L) The data show that i n surfactant systems the polymer acts as an important component i n aggregate formation, rather than behaving as an i n e r t additive. Nature of the Aggregates. The aggregates are hydrophobic, loose structures with some residual surface charge from the anionic polymer. The fluorescence decay rate constants k of pyrene i n aggregates of PMA and C TAB, are i d e n t i c a l to corresponding rate constants the k i n miceliar alkyltrimethylammonium chloride, C TAC. I t i s concluded that the i n t e r i o r of aggregates of PMA C TAB i s hydrophobic and similar to that of a micelle. Several p o s i t i v e l y charged pyrene derivatives, such as C^PN and CJ-JPN"*", and negatively charged derivatives, such as 1pyrenebutyric acid (PYC3H7COOH), 1-pyrenedecanoic acid (PyCgH^gCOOH) provide further evidence about the hydrophobic structure of the PMA - C T A B aggregates. A l l fluorescence decay rate constants k of the pyrene derivatives, i n PMA - C Q T A B aggregates are closer or s i m i l a r to those found i n hexanol or i n PMA at pH 3 . The longer the carbon chain of the probe, the closer the agreement of k i n hexanol and i n the aggregates. This i s reasonable from the point of view of the hydrophobic effects involved. The quenching data show that the surface of the PMA A

n

m

n

n

1 Q

Q

1

Q

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

32. CHU AND THOMAS

Interaction of Cationic Species

V

B

0.1

( 0.4 0

4 8 3 CCiol.io M*

0.9>-

\ 0.7

T k k

ι

A

x —5 i0.2

0.5

0.3

L_

1,

>

[C ] ,10" M 3

1 0

Relative fluorescence intensity, I (o) and intensity r a t i o , I 3 / I 1 (Δ) of pyrene as functions of concentrations of C TAB, [ C , Q ] , i n aqueous solutions of PMA at pH 8 , [pyrene] = 2 χ 1 0 ~ M. Relationship between I of pyrene fluorescence and [CIQ] water. (Reproduced from Ref. 4 8 . Copyright 1 9 8 6 American Chemical Society.) R

1Q

6

B.

R

i

n

10

12

14

16

n in C T A B n

9.

Plot of Log(CAC) vs. n i n C TAB (A) and p l o t of log(CMC) vs. n i n C TAB(#). (Reproduced from Ref. 4 8 . Copyright 1 9 8 6 American Chemical Society.) n

n

American Chemical Society Library 1155 16th SHoyle, t , N.W. In Photophysics of Polymers; C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987. Washington, D.C 20036

447

448

PHOTOPHYSICS OF POLYMERS

C TAB aggregates has l i t t l e charge and that the k i n e t i c data are similar to those found for non-ionic micelles. Analysis of the quenching data of pyrene fluorescence by 1dodecylpyridinium chloride, DPC v i a pulsed laser studies confirms the Poisson d i s t r i b u t i o n of DPC amongst the aggregates. Figure 10A shows the excellent f i t of the Poisson kinetics to the time dependent quenching of Pyrene fluorescence i n deaerated aqueous solution of PMA at pH 8, contained 8 χ 10"" M C TAB and quencher 2 χ 10"^ M DPC. The Poisson equation used i s 1Q

3

1Q

I = I

Q

{Exp(-k t) - n[1-Exp(-k t)]} Q

(2)

g

where η i s the average number of DPC quencher molecules s o l u b i l i z e d in each aggregate, k^ and k^ are the f i r s t order rate constants f o r the decay of pyrene i n the absence and i n the presence of quencher^ respectively. In this figure k = 2 χ 10^ s" k = 2.0 χ 10 s~" calcd ° · · Figur fluorescence versus tim χ 10~ M C TAB (pH 8), with various concentrations of quencher DPC (0-1.4 χ 10 M). These are t y p i c a l plots of quenching data according to a Poisson distribution.(50,51) The aggregation numbers of C TAB calculated by the relationship n

=

2 7

3

1Q

4

1Q

[C TAB]/N = [DPC]/n _ _ 10 calcd.

(3)

in

correspond well to the results from steady state experiments (N = 105 ± 10).(48) The above experimental data confirm that the model for aggregation of PMA - C Q T A B i s not v i a l o c a l small and random clusters, but v i a discrete structures,that are much larger than pure C T A B micelles (Ν ~ 36).(52) It i s concluded that the aggregates of PMA - C Q T A B are large structures consisting of about one hundred C - J Q T A B molecules and one c o i l e d polymer chain. The i n t e r i o r of the aggregate has hydro­ phobic domain that i s similar to that of a micelle. However, the bromide ions are only i n bulk aqueous phase, and not close to the surface of aggregate. The degree of p o l a r i z a t i o n of 2-methylanthracene fluorescence i n PMA - C Q T A B aggregate i s four f o l d smaller than i n PMA compact c o i l at pH 2, two f o l d smaller than i n C J Q T A B micelle, indicating that the aggregate i s a much looser structure than a compact PMA c o i l , or a C T A B micelle. The data of electron microscopy show that the aggregates are spherical p a r t i c l e s which are about 350-500 Â, rather than h e l i x forms. PMA samples of molecular weight 3.9 χ 10 , 1.6 χ 10 and 6.4 χ 10^ emphasize the e f f e c t of molecular weight of PMA on the aggregates of PMA-C TAB. The curves of ^/I-j f o r 2 χ 10" M pyrene fluorescence i n aqueous solutions of PMA at pH 8 show a sharp increase i n above PMA samples at pH 8, indicating that i n a l l cases the aggregates of PMA - C TAB are formed above the CAC. The aggregation numbers of C TAB i n each aggregate, measured by analysis of the quenching data of pyrene fluorescence by DPC, are 90 ~ 100 surfactant molecules, and also contain a portion of the c o i l e d PMA chain. 1

1 Q

1

1

1 Q

4

1Q

1Q

1Q

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

32. CHU AND THOMAS

Θ.ΘΙ

8

10.

A.

Interaction of Cationic Species

•

.

288

400

680 800 Time CnS)

449

.

1000

Poisson quenching f i t of the time-dependent fluorescence decay of pyrene i n deaerated PMA solutions at pH 8, containing 8 χ 10" M C TAB and quencher 2 χ 10~ M DPC. The smooth one i s from computer f i t t i n g and the other one i s r e a l data. In(Intensity) of pyrene fluorescence vs. time i n deaerated PMA solutions containing 8 x 1 0 ~ " M Cj Q T A B at pH 8, with various concentrations of quencher DPC, (10" M): (a) 0; (b) 2.0; (c) 4.0; (d) 6.0; (e) 8.0; (f) 10.0; (g) 12.0; (h) 14.0. 3

1 Q

5

B.

3

5

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

450

PHOTOPHYSICS OF POLYMERS

CONCLUSIONS This study shows that fluorescence probing techniques are useful and powerful tools for investigation of conformational transitions of polyelectrolytes as induced by c a t i o n i c surfactants, pH or other means. Studies on the interaction of cationic probes with polyelectrolytes provide useful information on the intermediates that l i e between A states and Β states. I t i s concluded that the conformational t r a n s i t i o n induced by pH i s a progressive process over several pH units. Studies on the interaction of cationic surfactants with PMA at pH 8 show that the aggregates formed are large loose structures, while the i n t e r i o r of the aggregate has a hydrophobicity that i s similar to that of a micelle. Acknowledgment. We thank the National Science Foundation for support of this work via Grant CHE-01226-02

Literature Cited. 1. Katchalsky, Α.; Eisenberg, H. J. Polym. Sci., 6, 145, 1951. 2. Silberberg, Α.; Eliassaf, J.; Katchalsky, A. J. Polym. Sci., 7, 393, 1951. 3. Mandel, M.; Stadhouder, M. G. Makromol. Chem., 80, 141, 1964. 4. Anufrieva, Ε. V.; Birshtein, T. M. J. Polym. Sci. C, 16, 3519 1968. 5. Arnold, R. J. Coll. Sci., 12, 549, 1957. 6. Koenig, J. L.; Angood, A. C.; Semen, J.; Lando, J. B. J. Am. Chem.Soc.,91, 7250, 1969. 7. Crescenzi, V.; Quadrifoglio, F.; Delben, F. J. Polym. Sci., A 2, 10, 347, 1972. 8. Delben, F.; Crescenzi, V.; Quadrilfoglio, F. Eur. Polym. J., 8, 933, 1972. 9. Daoust, H.; Thanh, H. L; Ferland, P.; St-cyn, D. Can. J.Chem. 63, 1568, 1985. 10. Kern, Ε. E.; Anderson, D. K. J. Polym. Sci. A-1, 6, 2765, 1968. 11. Suzuki, K.; Taniguchi, Y. J. Polym. Sci. A-2, 8, 1679, 1970. 12. Kay, P. J.; Kelly, D. P.; Milgate, G. I.; Treloar, F. E. Makromol. Chemie., 177, 885, 1976. 13. Okamoto, H.; Wada, Y. J. Polym. Sci. Polym. Phys., 12, 2413, 1974. 14. Jager, J.; Engberts, J. B. F. N. J. Org. Chem., 50, 1474, 1985; J. Am. Chem.Soc.,106, 3331, 1984. 15. Moan, M.; Wolff, C.; Cotton, J. P.; Ober, R. J. Polym. Sci. Polym. Symp., 61, 1, 1977; Polym., 16, 781, 1975. 16. Irie, M. Makromol. Chem. Rapid Commun., 5, 413, 1984. 17. Bednar, B.; Marowetz, H.; Shafer, J. A. Macromolecules, 18, 1940, 1985. 18. Stork, W. J. H.; Van Boxsel, J. A. M.; DeGoeij, A. F. P. M.; Haseth, P. L. De.; Mandel, M. Biophys. Chem., 2, 127, 1974; Madel, M.; Stork, W. H. J. Biophys. Chem., 2, 137, 1974. 19. Anufrieva, Ε. V.; Birshtein, T. M.; Nekrasova, T. N.; Ptitsyn, O. B.; Sheveleva, T. V. J. Polym. Sci. C., 16, 3519, 1968. 20. Oster, G. J. Polym. Sci., 16, 235, 1955. 21. Baud, C. Eur. Polym. J., 13, 897, 1977.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

32. CHU AND THOMAS 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

Interaction of Cationic Species

Erny, B.; Muller, G. J. Polym. Sci. Polym. Chem., 17, 4011, 1979. Wang, Y.; Morawetz, H. Macromolecules, 19, 1925, 1986. Stork, W. H. J.; Hasseth, P. L. de.; Schippers, W. B.; Körmeling, C. M.; Mandel, M. J. Phys. Chem., 77, 1772, 1973. Stork, W. H. J.; Hasseth, P. L. de.; Lippits, G. J. M.; Madel, M. J. Phys. Chem., 77, 1778, 1973. Muller, G.; Fenyo, J. C. J. Polym. Sci. Polym. Chem., 16, 77, 1978. Snare, M. J.; Tan, K. L.; Treloar, F. E. J. Macro. Sci. Chem. Α., 17(2) 189, 1982. Chu, D. Y.; Thomas, J. K. Macromolecules, 17, 2142, 1984. Tokiwa, F.; Tsujii, K. Bull. Chem. Soc. Jpn., 46, 2684, 1973. Schwuger, M. J. J. Colloid & Interf. Sci., 43, 491, 1983. Shirahama, K. J. Colloid & Polym. Sci., 252, 978, 1974. Cabane, B. J. Phys Chem. 81 1639 1977 Shirahama, K.; Tohdo Sci., 86, 282, 1982 Turro, N. J.; Baretz, B. H.; Kuo, P. L. Macromolecules, 17, 1321, 1984. Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem., 89, 41, 1985. Kresheck, G. C.; Hargraves, W. J. Colloid & Interf. Sci., 95, 453, 1983; 105, 589, 1985. Dubin, P. L.; Davis, D. D. Macromolecules, 17, 1294, 1984. Leung, P. S.; Goddard, E. D. etc., Colloids Surf., 13, 47 & 63, 1985. Hayakawa, K.; Kwak, C. T. J. Phys. Chem., 86, 3866, 1982. Abuin, E. B.; Scaiano, J. C. J. Am. Chem.Soc.,106, 6274, 1984. Hayakawa, K.; Santerre, J. P.; Kwak, C. T. Macromolecules, 16, 1642, 1983. Chu, D. Y.; Thomas, J. L. J. Phys. Chem., 89, 4065, 1985. Cuniberti, C.; Perico, A. Eur. Polym. J., 13, 369, 1977; 16, 887,1980; Ann. N.Y. Acad. Sci., 366, 35, 1981. Winnik, M. A.l Redpeth, A. E. C.; Paton, K.; Danhelka, J. Polymer, 25, 91, 1984. Tazuke, S.; Ooki, H.; Sato, K.; Macromolecules, 15, 400, 1982 Suzuki, Y.; Tazuke, S. Macromolecules, 14, 1742, 1981. Thomas, J. K. Chem. Rev., 80, 283, 1980; Thomas, J. K. The Chemistry of Excitation at Interfaces, ACS Monograph, No. 181. Washington, D. C., 1984. DellaGardia, R. D.; Thomas, J. K. J. Phys. Chem., 87, 3550, 1983. Chu, D. Y.; Thomas, J. K. J. Am. Chem.Soc.,108, 6270, 1986. Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem.Soc.,99, 2039, 1977. Atik, S. S.; Singer, L. A. Chem. Phys. Lett., 59, 519, 1978. Hashimoto, S.; Thomas, J. K. J. Colloid & Interf. Sci., 102, 152, 1984. Tabtar, H. V. J. Colloid & Interf. Sci., 14, 115, 1959.

RECEIVED April 27, 1987

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 33

Fluorescence Monitoring of Viscosity and Chemical Changes During Polymerization 1

1

1

1

F. W. Wang , R. E. Lowry , W . J . Pummer , B. M . Fanconi , and En-Shinn Wu 2

1

Polymers Division, National Bureau of Standards, Gaithersburg, MD 20899 Department of Physics

2

Three approaches using fluorescent dyes dissolved in epoxy resins were used to determine the viscosity changes during the curing process. First, the intensity of excimer fluorescence from a dye which forms an intramolecular excimer was measured to determine the viscosity changes. In another approach, we used a dye whose fluorescence intensity increases with the increase in the local viscosity, and a second dye whose fluorescence intensity is insensitive to the local viscosity. The ratio of the fluorescence intensities of the two dyes was measured to monitor the cure of epoxy resins. In a third approach, we measured the diffusion coefficient of a fluorescent dye by a photobleaching technique to monitor the curing process. Finally, we used a fluorescence technique to monitor the formation of a polyimide polymer from poly(amide acid). The manufacture of polymer matrix composites involves complex chemical and physical changes that must be adequately controlled to produce desirable products. Monitoring techniques and models to correlate monitoring data to improve processing are therefore key aspects to increasing production rates and product quality. Fluorescence techniques are particularly useful to monitor the change in local viscosity because they are sensitive and can be easily adopted to in-situ, nondestructive monitoring. In a previous paper(1), we described an excimer-fluorescence technique to monitor the polymerization of methyl methacrylate. We show here an application of the excimer-fluorescence technique to monitor the cure of epoxy resins. In addition, we describe the cure monitoring of epoxy resins with the use of two fluorescent dyes, a dye whose fluorescence intensity increases with local viscosity, and another dye which serves as an internal standard with nearly constant fluorescence intensity. This second technique is similar to the ones used by Loutfy(2,3) and by Levy(4). However, to the best of 0097-6156/87/0358-0454$06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

33. WANG ET AL.

Viscosity and Chemical Changes

455

our knowledge, t h e r e has been no p r e v i o u s r e p o r t of the a p p l i c a t i o n of a v i s c o s i t y i n s e n s i t i v e i n t e r n a l s t a n d a r d . S i n c e the use of an i n t e r n a l s t a n d a r d e l i m i n a t e s the d i f f i c u l t i e s i n v o l v e d i n making a b s o l u t e measurements, i t i s a s i g n i f i c a n t s t e p f o r w a r d i n the a p p l i c a t i o n of f l u o r e s c e n c e s p e c t r o s c o p y t o cure m o n i t o r i n g i n the f a c t o r y environment. Furthermore, we r e p o r t a t h i r d t e c h n i q u e t h a t u t i l i z e s the measurement of the d i f f u s i o n c o e f f i c i e n t of a p h o t o b l e a c h a b l e probe t o monitor the cure of an epoxy r e s i n . F i n a l l y , we d e s c r i b e a f l u o r e s c e n c e method t o monitor the f o r m a t i o n of a p o l y i m i d e polymer. EXPERIMENTA L MONOMERS. Amine hardener, 4 , 4 ' - m e t h y l e n e - b i s - ( c y c l o h e x y l a m i n e ) (PACM), was d i s t i l l e d under reduced p r e s s u r e and s t o r e d under dry argon. I t was melted under dry argon b e f o r e v.se Epoxy r e s i n d i g l y c i d y l e t h e r of b i s p h e n c weight of a p p r o x i m a t e l purification. SYNTHESIS 0F_ POLYIMIDE. The p o l y i m i d e was prepared from 2,2b i s ( 3 , 4 - d i c a r b o x y p h e n y l ) h e x a f l u o r o p r o p a n e d i a n h y d r i d e (American Hoechst 6 F ) and 2 , 2 - b i s [ 4 ( 4 - a m i n o p h e n o x y ) p h e n y l ] h e x a f l u o r o p r o p a n e (Morton T h i o k o l 4BDAF). Both 6F and 4BDAF are s o l u b l e i n dry glyme at 25°C. 4BDAF (0.5g, 9.6xlC" mol) was d i s s o l v e d i n 3 ml of d r y glyme at 25°C w i t h s t i r r i n g i n a 25 ml g l a s s - s t o p p e r e d f l a s k . When d i s s o l u t i o n was completed ( u s u a l l y w i t h i n 3 m i n u t e s ) , 0.42g ( 9 . 6 x l 0 " mol) of s o l i d 6F i n s m a l l p o r t i o n s {O.lg each) was added t o the s o l u t i o n of 4BDAF at 25°C. W i t h i n 5 minutes a f t e r the 6F was added, s t i r r i n g was impeded by the i n c r e a s e d s o l u t i o n v i s c o s i t y . A f t e r .15 minutes of manually s w i r l i n g the c o n t e n t s , the s o l u t i o n v i s c o s i t y decreased s u f f i c i e n t l y t o a l l o w normal s t i r r i n g t o proceed. The r e a c t i o n was stopped a f t e r 44 hours at 25°C. T h i s s o l u t i o n (26% s o l i d s ) of the p o l y ( a m i d e a c i d ) i n glyme was used t o prepare f i l m s f o r f l u o r e s c e n c e s p e c t r o s c o p y as d e s c r i b e d below. A few drops of the p o l y ( a m i d e a c i d ) s o l u t i o n were spread on a c l e a n q u a r t z s l i d e by drawing a wedge of the s o l u t i o n beneath another c l e a n s l i d e . Room temperature s o l v e n t e v a p o r a t i o n and a l l heat t r e a t m e n t s were c a r r i e d out i n a i r . The f i l m was cured f o r 0.5 hour at each of seven temperatures r a n g i n g from 80° t o 350°C, w i t h oven warmup and c o o l i n g down times of up t o 0.5 hour each. F r o n t s u r f a c e f l u o r e s c e n c e from the same f i l m r e g i o n was measured at room temperature a f t e r each heat t r e a t m e n t . F i l m t h i c k n e s s averaged 13pm. 1

4

4

1

C e r t a i n commercial m a t e r i a l s and equipment are i d e n t i f i e d i n t h i s paper t o s p e c i f y a d e q u a t e l y the e x p e r i m e n t a l p r o c e d u r e . In no case does such i d e n t i f i c a t i o n i m p l y recommendation or endorsement by the N a t i o n a l Bureau of S t a n d a r d s , nor does i t i m p l y n e c e s s a r i l y the best a v a i l a b l e f o r the purpose.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

456

FiyP.?ESCENT._^ An e x c i m e r - f o r m i n g probe, l , 3 - b i s - ( l pyrene)propane (8PP), was o b t a i n e d from a commercial source and used w i t h o u t f u r t h e r p u r i f i c a t i o n . A v i s c o s i t y - s e n s i t i v e probe, 1( 4 - d i m e t h y l a m i n o p h e n y l ) - 6 - p h e n y l - l , 3, 5-hexatric?ne (DKA-DPH) , was d i s s o l v e d i n DGEBA by r o t a t i n g the r e s i n at 45°C f o r s e v e r a l h o u r s . An i n t e r n a l s t a n d a r d , 9 , 1 0 - d i p h e n y l a n t h r a c e n e (DPA), at a c o n c e n t r a t i o n of 2 x l 0 ~ mol/1 was added t o the r e s i n - h a r d e n e r mixture. 5

?MÇTION_.,ÇEL!L. The r e a c t i o n c e l l was made of a s i l i c o n e rubber gasket sandwiched between two pyrex cover s l i d e s . The c e l l was pressed a g a i n s t t h e f l a t f a c e of a c y l i n d r i c a l h e a t e r , which was p r o p o r t i o n a l l y c o n t r o l l e d t o w i t h i n 1°C. Two s m a l l h o l e s were bored near t h e top of the rubber gasket f o r i n s e r t i n g a thermocouple and f o r f i l l i n g the c e l l . c

P

y. :S_>ÎONITORING. I n a r e s i n c o n t a i n i n g a prob i n t o a stoppered s y r i n g e kept at 40°C. The amine hardener was then r a p i d l y mixed w i t h the epoxy r e s i n t o form a m i x t u r e c o n t a i n i n g the same e q u i v a l e n t s of the hardener and the r e s i n . F i n a l l y , the r e s i n m i x t u r e was i n j e c t e d i n t o the r e a c t i o n c e l l which had been heated to 60°C. Cure m o n i t o r i n g w i t h the probe BPP was c a r r i e d out i n t h e manner p r e v i o u s l y d e s c r i b e d i n Reference 1. The sample was i r r a d i a t e d a t 345 nm and the monomer and excimer f l u o r e s c e n c e i n t e n s i t i e s at 377 nm and 488 nm were measured as a f u n c t i o n of time. When DMA-DPH was used as a probe and DPA was used as an i n t e r n a l s t a n d a r d , they were e x c i t e d at 420 nm and 345 nm, respectively. FLUORESCENT The d i f f u s i o n c o e f f i c i e n t of t h e photobleachabïe probe, 1 , 1 - d i h e x y l - 3 , 3 , 3 ' , 3 ' t e t r a m e t h y l i n d o c a r b o c y a n i n e p e r c h l o r a t e [ D i I C e ( 3 ) ] , was determined by t h e FRAP method(5,6,7) d e s c r i b e d below. The c o n c e n t r a t i o n of the probe i n the m i x t u r e c o n t a i n i n g t h e same e q u i v a l e n t s of t h e epoxy r e s i n and t h e amine hardener was 2 x l 0 ~ mol/1. The r e s i n m i x t u r e , sandwiched between a microscope s l i d e and a cover s l i p , was p l a c e d on a stage t h e r m o s t a t i c a l l y c o n t r o l l e d t o w i t h i n 0.5°C. The t h i c k n e s s of t h e r e s i n m i x t u r e was about 5pm. I n a FRAP e x p e r i m e n t ( 6 ) , a s m a l l a r e a of the sample i s i l l u m i n a t e d w i t h a weak beam of e x c i t i n g l i g h t ( m o n i t o r i n g beam). The f l u o r e s c e n c e from t h i s area i s r e c o r d e d as F. At a p r e d e t e r m i n e d time, t = 0, the sample i s momentarily i l l u m i n a t e d w i t h a s t r o n g l a s e r beam ( b l e a c h i n g beam) t o cause an i r r e v e r s i b l e b l e a c h i n g of f l u o r o p h o r e . F o l l o w i n g the b l e a c h i n g , the f l u o r e s c e n c e i s again m o n i t o r e d by t h e m o n i t o r i n g beam. The f l u o r e s c e n c e , F ( t ) , i s i n i t i a l l y v e r y weak, but g r a d u a l l y i n c r e a s e s as the f r e s h f l u o r e s c e n t m o l e c u l e s d i f f u s e i n t o t h e bleached a r e a , and e v e n t u a l l y r e c o v e r s t o i t s o r i g i n a l i n t e n s i t y . From t h e r a t e of t h e r e c o v e r y of f l u o r e s c e n c e i n t e n s i t y , t h e d i f f u s i o n c o e f f i c i e n t can be determined. 1

5

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

33. WANG ET AL.

Viscosity and Chemical Changes

457

RESULTS AND DISCUSSION F i g u r e 1 g i v e s t h e change i n t h e i n t e n s i t y r a t i o FM/FD as a f u n c t i o n of cure time f o r 1 , 3 - b i s - ( 1 - p y r e n e ) p r o p a n e (BPP) d i s s o l v e d i n a m i x t u r e c o n t a i n i n g t h e same e q u i v a l e n t s of t h e epoxy r e s i n and the amine h a r d e n e r . Here, FM and Fn a r e , r e s p e c t i v e l y , t h e f l u o r e s c e n c e i n t e n s i t y of t h e monomer a t 377 nm and t h a t o f t h e excimer a t 488 nm. As c r o s s - l i n k i n g proceeded, t h e v i s c o s i t y i n c r e a s e d owing t o t h e growth i n m o l e c u l a r weight and t h i s l e d t o an i n c r e a s e i n t h e i n t e n s i t y r a t i o . A f t e r 45 minutes of t h e c u r e , t h e r e was a s m a l l decrease i n t h e i n t e n s i t y r a t i o due t o p h o t o d e g r a d a t i o n of t h e probe. The c o m p l i c a t i o n due t o p h o t o d e g r a d a t i o n can be e l i m i n a t e d by r e d u c i n g t h e exposure o f t h e r e s i n t o t h e e x c i t i n g UV r a d i a t i o n . The use o f an o p t i c a l m u l t i c h a n n e l a n a l y z e r i s one approach t o reduce exposure t i m e s . Owing t o t h e l a c k f sensitivit f t h BPP prob t th l o n g e r cure t i m e s , we hav v i s c o s i t y - s e n s i t i v e prob and e m i s s i o n s p e c t r a of t h e v i s c o s i t y s e n s i t i v e dye DMA-DPH i n d i g l y c i d y l e t h e r of b i s p h e n o l A (DGEBA). The e x c i t a t i o n and t h e e m i s s i o n s p e c t r a of t h e i n t e r n a l s t a n d a r d DPA a r e s i m i l a r t o t h e ones p u b l i s h e d by Berlman(8). F i g u r e 3 g i v e s t h e f l u o r e s c e n c e i n t e n s i t y , Fp, o f DMA-DPH a t 480 nm (the upper curve) and t h e f l u o r e s c e n c e i n t e n s i t y , F R , o f DPA at 415 nm ( t h e lower curve) as a f u n c t i o n of cure time f o r a m i x t u r e c o n t a i n i n g t h e same e q u i v a l e n t s of t h e epoxy r e s i n and t h e amine h a r d e n e r . The f l u o r e s c e n c e i n t e n s i t y a t t h e frequency o f t h e DMA-DPH e m i s s i o n i n c r e a s e d f i v e f o l d w h i l e t h a t a t t h e DPA peak emission increased only s l i g h t l y . (The apparent i n c r e a s e i n t h e DPA f l u o r e s c e n c e was m a i n l y due t o t h e i n c r e a s e i n t h e f l u o r e s c e n c e of i m p u r i t i e s i n t h e epoxy r e s i n . At t h e e x c i t a t i o n wavelength o f 345 nm, t h e f l u o r e s c e n c e i n t e n s i t y of t h e r e s i n m i x t u r e alone a t 415nm i n c r e a s e d by about 80%. However, t h i s i n c r e a s e c o n t r i b u t e d to t h e much s m a l l e r apparent i n c r e a s e i n t h e DPA f l u o r e s c e n c e because a t t h e b e g i n n i n g o f t h e c u r e , t h e f l u o r e s c e n c e i n t e n s i t y o f DPA was f o u r times l a r g e r than t h a t of t h e r e s i n alone.) I n F i g u r e 4, we show t h e i n t e n s i t y r a t i o F P / F R as a f u n c t i o n o f cure t i m e . The i n t e n s i t y r a t i o i n c r e a s e d s t e a d i l y w i t h cure t i m e , r e a c h i n g a p l a t e a u v a l u e a t 70 minutes. T h i s i n t e n s i t y r a t i o i s not s e n s i t i v e to t h e inhomogeneity or t h e d e f o r m a t i o n of t h e sample. We can t h e r e f o r e use t h i s r a t i o t o monitor t h e cure of samples which shrink during polymerization or contain r e i n f o r c i n g f i b e r s or particles. F i g u r e 5 shows t h e l o g a r i t h m o f t h e t r a n s l a t i o n a l d i f f u s i o n c o e f f i c i e n t o f t h e probe D i l C e ( 3 ) as a f u n c t i o n of cure time when the r e s i n was cured at 45°C, 60°C, and 75°C. I n a l l c a s e s , t h e t r a n s l a t i o n a l d i f f u s i o n c o e f f i c i e n t increased s l i g h t l y at the b e g i n n i n g of t h e cure when t h e sample temperature was r a i s e d from 22°C t o t h e cure temperature because of t h e decrease i n t h e r e s i n v i s c o s i t y w i t h t h e i n c r e a s e i n t e m p e r a t u r e . As c r o s s - l i n k i n g proceeded, t h e v i s c o s i t y i n c r e a s e d owing t o t h e growth i n m o l e c u l a r w e i g h t , and t h i s was r e f l e c t e d by t h e decrease i n t h e t r a n s l a t i o n a l diffusion coefficient.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

458

ο I

ι

0

1 Cure Time, Minutes

F i g u r e 1. R a t i o of monomer f l u o r e s e n c e i n t e n s i t y a t 377nm ( F M ) to excimer f l u o r e s c e n c e i n t e n s i t y at. 488nm (Fo) f o r 1 , 3 - b i s (1-pyrene)propane i n epoxy as a f u n c t i o n of cure time. The e x c i t a t i o n wavelength was 345 nm.

350

410

470

530

590

650

Wavelength (nm)

F i g u r e 2. E x c i t a t i o n and e m i s s i o n s p e c t r a of l - ( 4 dimethylaminophenyl)-6-phenyl-l,3,5-hexatriene(DMA-DPH) i n d i g l y c i d y l e t h e r of b i s p h e n o l A (DGEBA). The e x c i t a t i o n and e m i s s i o n wavelengths were 405 nm and 498 nm, r e s p e c t i v e l y .

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

33.

Viscosity and Chemical Changes

WANG ET AL.

—ι

1

1

"^-#-1·

jSr=\? > ^

7500 cm ) w h i l e t h e f l u o r e s c e n c e i s narrow (

T

32

μ

31

h

30

h

ABSORPTION ΜΑΧΙΜΑ M HEXANE IN TOLUENE

Λ d ~

σ c ω 29 h ζ tu 28

h

70

110

150

190

230

270

310

TEMPERATURE K e

F i g u r e H. Thermochromism

i n p o l y ( n - p r o p y l methyl

silylene),

1.02

110

150

190

230

270

310

TEMPERATURE °K

F i g u r e 5.

T h i n f i l m thermochromism f o r s e v e r a l p o l y s i l y l e n e s .

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Emission Spectra of Polysilylenes

35. HARRAH AND ZEIGLER

1.00 ι — ι — ι — ι

1—ι—ι—ι

J—ι—ι

340

38

340

380

420

«

1 1—ι

480

1—ι—ι

•—•—Γ

500

540

WAVELENGTH (nm)

340

370

400

430

460

490

520

WAVELENGTH (nm)

F i g u r e 6.

Laser e x c i t e d d e l a y e d e m i s s i o n from A. P o l y ( n - p r o p y l m e t h y l s i l y l e n e ) B. P o l y ( n - h e x y l m e t h y l s i l y l e n e ) C. P o l y ( n - d o d e c y l m e t h y l s i l y l e n e )

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

491

492

PHOTOPHYSICS OF POLYMERS

phosphorescence i n t e n s i t y p l a c e s t h e s e y i e l d s a t l e s s than 10 . No phosphorescence i s d e t e c t e d i n t h e s e f i l m s a t t e m p e r a t u r e s above 250°K. Even though the t r i p l e t appears l o c a l i z e d , a t l e a s t on a v i b r a ­ t i o n a l t i m e s c a l e , t h e apparent d e l a y e d f l u o r e s c e n c e does i n d i c a t e s u b s t a n t i a l m o b i l i t y on the time s c a l e o f phosphorescence decay. We h a v e e x a m i n e d t h e s e d e c a y s a n d f o u n d them t o be q u i t e n o n l i n e a r . W i t h the e x c e p t i o n o f the 3-naphthyl polymer, t h e f i r s t h a l f l i v e s are about 1-2 χ 10 seconds. F i g u r e 7 shows the phosphorescence s p e c t r a d u r i n g t h e 0.01 t o 0.1 t i m e p e r i o d s f o r η-propyl m e t h y l and n - h e x y l m e t h y l f i l m s . Comparison w i t h F i g u r e 6 c l e a r l y shows a s h i f t t o l o n g e r wavelengths and a l o s s i n v i b r a t i o n a l s t r u c t u r e . A g a i n , as i n the e a r l i e r time p e r i o d , the s p e c t r a o f t h e s e p o l y m e r s a r e s i m i l a r . The d e l a y e d f l u o r e s c e n c e i s a b s e n t and t h i s o b s e r v a t i o n t o g e t h e r w i t h the r e d s h i f t suggest t r a p p i n g a duced s c i s s i o n s i t e s . T a b l phosphorescence o f t h i n f i l m s s t u d i e d . F i g u r e 8 i s t h e p h o s p h o r e s c e n c e spectrum t a k e n from a g l a s s y s o l u t i o n o f p o l y ( n - p r o p y l methyl s i l y l e n e ) i n methyl c y c l o p e n t a n e a t 89°K. T h i s e m i s s i o n i s s i m i l a r i n w i d t h t o the f i l m e m i s s i o n , a s are t h e s o l u t i o n s p e c t r a o f t h e o t h e r p o l y m e r s . Again delayed f l u o r e s c e n c e i s e v i d e n t but the sharp v i b r a t i o n a l f i n e s t r u c t u r e i s l o s t . The s o l u t i o n and f i l m s p e c t r a a r e n o t e x p e c t e d t o be com­ p a r a b l e s i n c e t h e y r e p r e s e n t c o n f o r m a t i o n a l e q u i l i b r i a ( a t room t e m p e r a t u r e f o r f i l m a n d t h e Tg o f 3-methy1 p e n t a n e f o r t h e solutions). The p h o s p h o r e s c e n c e s p e c t r a o f t h e two a r y l p o l y s i l y l e n e s s t u d i e d a r e shown i n F i g u r e 9 and t h e i r f l u o r e s c e n c e a t room tem­ p e r a t u r e i n F i g u r e 10. A l t h o u g h phosphorescence quantum y i e l d s f o r t h e s e two polymers were not measured, e s t i m a t e s based on comparison w i t h the a l k y l i n t e n s i t i e s i n d i c a t e t h a t t h e s e p o l y m e r s e m i t w i t h s u b s t a n t i a l l y g r e a t e r y i e l d . Todesco and Kamat (21_) have measured the phosphorescence y i e l d o f a c o p o l y m e r o f α-naphthyl m e t h y l a n d d i m e t h y l s i l y l e n e u n i t s t o be 0.39. Our n a p h t h y l polymer g i v e s c l e a r l y the most i n t e n s e phosphorescence and p r o b a b l y has a quantum y i e l d near the 0.39 v a l u e f o r the copolymer. We e s t i m a t e the p h e n y l polymer y i e l d t o be * 1/10 o f the n a p h t h y l polymer. The f l u o r e s c e n c e o f the p h e n y l polymer i s s i m i l a r i n shape t o the f l u o r e s c e n c e from the a l k y l p o l y m e r s a n d t h e s i m i l a r s h a p e of t h e p h o s p h o r e s c e n c e spectrum, as w e l l , s u g g e s t s t h a t the o r i g i n s o f the e l e c t r o n i c spectrum a r e a l s o much t h e same. The a p p a r e n t i n ­ c r e a s e d quantum y i e l d f o r p h o s p h o r e s c e n c e i n p o l y ( p h e n y l methyl s i l y l e n e ) p r o b a b l y r e f l e c t s a m i x i n g o f the r i n g e l e c t r o n i c l e v e l s w i t h the l e v e l s o f the c h a i n . Both the f l u o r e s c e n c e and phosphores­ cence o f the n a p h t h y l d e r i v a t i v e a r e s u b s t a n t i a l l y a l t e r e d r e l a t i v e t o t h e p h e n y l polymer. F l u o r e s c e n c e resembles t h a t o f p o l y ( 8 - v i n y l n a p h t h a l e n e ) ( 17,29) w h i c h i s a t t r i b u t e d t o e x c i m e r e m i s s i o n . P h o s p h o r e s c e n c e i s s i m i l a r t o naphthalene i t s e l f . These o b s e r v a ­ t i o n s suggest t h a t the replacement o f an a l k y l w i t h p h e n y l m o i e t y d o e s n o t change t h e b a s i c n a t u r e o f the e l e c t r o n i c s t a t e but may i n c o r p o r a t e some ir c h a r a c t e r . Upon a n a p h t h y l s u b s t i t u t i o n both t h e f l u o r e s c e n c e and phosphorescence become p r i m a r i l y π-ττ l i k e .

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Table I.

493

Emission Spectra of Polysilylenes

35. HARRAH AND ZEIGLER

Solution and Film Fluorescence Data f o r Polysilylenes S o l u t i o n (Room Temp.) ~ F i l m (Low Temp^)

w i d t h (cm ) X w i d t h (cm )

Polymer S u b s t i t u e n t s

A

p h e n y l , methyl p - a n i s y l , methyl 3 - n a p h t h y l , methyl η-propyl, methyl n - h e x y l , methyl n - d o d e c y l , methyl d i n-hexyl β p h e n e t h y l , methyl Co _ i - p r o p y l , m e t h y l ; η-propyl, methyl

353 362 437 341

.08-.25 0.2-0.5 .76

342 338

.42 .14

340

.69

f

f

f

+

353

706

345

447

1275

371

472

1143

342

592

1043 1220 4167

+ e x c i t a t i o n wavelength dependent * highly c r y s t a l l i n e

Table I I . Polysilylene Phosphorescence Origins and Vibrational Splittings Vibrational Splittings Polymer S u b s t i t u e n t s λ Origin p h e n y l , methyl β-naphthyl, methyl η-propyl, methyl n - h e x y l , methyl n-dodecyl, methyl di n-hexyl co _ i - p r o p y l , m e t h y l ; η-propyl, methyl

390 467 380 380 380 375 380

1667 1450 722/1667 717/1694 801/1589 848/1613 894

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

494

350

390

430

470

510

WAVELENGTH (nm)

350

390

430

470

510

WAVELENGTH (nm)

F i g u r e 7. L a s e r e x c i t e d d e l a y e d e m i s s i o n a t times g r e a t e r than 0.01 seconds from A. P o l y ( n - p r o p y l methyl s i l y l e n e ) B. P o l y ( n - h e x y l m e t h y l s i l y l e n e )

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

35. HARRAH AND ZEIGLER

Emission Spectra of Polysilylenes

ω ο ; o.eo ζ LU ο w LU

K 0.40 Ο Ζ CL

(0 Ο 0.20

F i g u r e 9.

L a s e r e x c i t e d d e l a y e d e m i s s i o n from A. P o l y ( p h e n y l methyl s i l y l e n e ) B. P o l y (£-naphthyl methyl s i l y l e n e )

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

495

496

PHOTOPHYSICS OF POLYMERS

WAVELENGTH (nm) Β 0.8 ι ι

ι ι ι

ι ι ι

ι ι ι

ι

ι

» ι ,ι,.ν. '

I

1

I

1

1

1

I

1

Γ

280 300 320 340 360 380 400 420 440 460 480 500 520 540

WAVELENGTH (nm)

F i g u r e 10. Room temperature a b s o r p t i o n a n d e m i s s i o n s p e c t r a i n hexane A. P o l y ( p h e n y l methyl s i l y l e n e ) B. P o l y U - n a p h t h y l methyl s i l y l e n e )

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

35. HARRAH AND ZEIGLER

Emission Spectra of Polysilylenes

497

Summary We h a v e examined t h e e m i s s i o n s p e c t r a o f a v a r i e t y o f p o l y s i l y l e n e s as t h i n f i l m s a n d s o l u t i o n s . The s o l u t i o n f l u o r e s c e n c e t h e r ­ mochromism p r o v i d e s e v i d e n c e t o s u p p o r t t h e r o t a t i o n a l i s o m e r i c s t a t e model used t o i n t e r p r e t t h e a b s o r p t i o n s p e c t r u m . The s t r u c ­ t u r e d c h a r a c t e r and l o w y i e l d o f p h o s p h o r e s c e n c e i n t h e a l k y l p o l y s i l y l e n e s suggest t h a t t h e t r i p l e t i s t h e immediate p r e c u r s o r t o photochemical s c i s s i o n . The change i n c h a r a c t e r o f both f l u o r e s ­ cence and phosphorescence on p r o g r e s s i n g from phenyl t o n a p h t h y l i n the a r y l s e r i e s i n d i c a t e s ^that the t r a n s i t i o n s i n t h e n a p h t h y l polymers a r e p r i n c i p a l l y ττ-π . Acknowledgment T h i s work performed a t Sandia N a t i o n a l L a b o r a t o r i e s was supported by the U.S. Department o f Energy under c o n t r a c t number DE-AC04-76DP00789

Literature Cited 1.

2. 3. 4. 5. 6.

7. 8. 9.

10. 11.

12.

(a) Zeigler, J. M. Polymer Preprints 1986. 27(1), 109. (b) Zeigler, J. M.; Harrah, L. A.; and Johnson, A. W. Polymer Preprints 1987, 28(1), 0000. (c) West, R. J. Organometal. Chem. 1986, 99, 300. Zeigler, J. M.; Harrah, L. Α.; and Johnson, A. W. SPIE Advances in Resist Process and Technology II 1985, 539, 166. Johnson, A. W.; Zeigler, J. M.; and Harrah, L. A. SPIE Optical Microlithography IV 1985. Hofer, D.C.;Miller, R. D.; and Wilson, C. G. SPIE Advances in Resist Technology 1984, 469, 16. Hofer, D.C.;Miller, R. D.; Wilson, C. G.; and Neureuther, A. SPIE Advances in Resist Technology 1984, 469, 108. (a) Miller, R. D.; Hofer, D.C.;Wilson, C. G.; Trefonas, P., III; and West, R. Polymer Preprints 1984, 25, 307. (b) Miller, R. D.; Hofer, D.C.;Wilson, C. G.; Trefonas, P., III ACS Symposium Series 1984, No. 266, 293. Kepler, R. G.; Zeigler, J. M.; Harrah, L. Α.; and Kurtz, S. R. Phys. Rev. B, in press; Kepler, R. G.; Zeigler, J. M.; Harrah, L. A.; and Kurtz, S. R. Bull. Am. Phys. Soc. 1983, 28, 362. Harrah, L. A. and Zeigler, J. M. J. Poly. Sci., Poly. Lett. Ed. 1985, 23, 209. (a) Harrah, L. A. and Zeigler, J. M. Proc. 1984 International Congress of Pacific Basin Societies, 1984, Honolulu, Hawaii; Abstract 09F0016. (b) Trefonas, P., III; Damewood,J. R., Jr.; West, R.; and Miller, R. D. Organometallics 1985, 4, 1318. Harrah, L. A. and Zeigler, J. M. Bull. Am. Phys. Soc. 1985, 30, 540. (a) Schweizer, K. S. Chem. Phys. Lett. 1986, 125, 118. (b) Schweizer, K. S. Polymer Preprints 1986, 27, 354, (c) Schweizer, K. S. J. Chem. Phys. 1986, 85, 1156. (d) Schweizer, K. S. J. Chem. Phys., 85 1986, 1176. Miller, R. D.; Hofer, D.C.;Rabolt, J. F.; and Fickes, G. Ν. J. Am. Chem. Soc. 1985, 107, 2172.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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13.

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

(a) Lovinger, A. J.; Schilling, F. C.; Bovey, F. Α.; and Zeigler, J. M. Macromolecules 1986, 19, 2663. (b) Schilling, F. C.; Bovey, F. Α.; Lovinger, A. J.; and Zeigler, J. M. Macormolecules 1986, 19, 2657. (c) Damewood, J. R., Jr. Macromolecules 1985, 18, 1795. Zeigler, J. M.; Adolf, D. B.; and Harrah, L. A. Proc. 20th Organosilicon Symp., 1986, Abs. #P-2.25, Tarrytown, NY Boberski, W. G.; and Allred, A. L. J. Organometal. Chem. 1975, 88, 65. Harrah, L. A. and Zeigler, J. M. Bull. Am. Phys. Soc. 1983, 28(3), 362. Harrah, L. A. and Zeigler, J. M. Macromolecules, in press. Pitt, C. G. and Bock, H. J. Chem.Soc.,Chem. Comm. 1972, 29. Pitt, C. G.; Carey, R. N.; and Toren, E.C.,Jr. J. Am. Chem. Soc. 1972, 94, 3806. Kagawa, T.; Fujino M.; Takeda K.; and Matsumoto N Solid State Comm. 1986 Todesco R. V. and Johnson G. E. and McGrane, Κ. M. Polymer Preprints 1986, 27, 352. Frank, C. W. and Harrah, L. A. J. Chem. Phys. 1974, 61, 1526. Renschler, C. L. and Harrah, L. A. Anal. Chem. 1985, 55, 798. Berlman, I. B. "Handbook of Fluorescence Spectra of Aromatic Molecules," 2nd Ed., Academic: New York, 1971. Levinson, N. J. Soc. Ind. Appl. Math. 1962, 10, 442. Harrah, L. A. and Zeigler, J. M. J. Poly. Sci., Poly. Lett. Ed., in press. Becker, R. S. "Theory and Interpretation of Fluorescence and Phosphorescence," Wiley Interscience, New York, 1969. Takeda, K. and Matsumoto, N. Phys. Rev. B. 1984, 30, 5871. Bigelow, R. W. Chem. Phys. Lett. 1986, 126, 63.

RECEIVED March 13, 1987

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 36

Spectroscopic and Photophysical Properties of Poly(organosilylenes) G. E. Johnson and Κ. M . McGrane Xerox Corporation, Webster Research Center, Webster, NY 14580 The results of an experimental study concerning the nature of the excited electronic states of poly(organosilylenes) clas f polymer i which the backbone consist atoms, are presented compariso absorptio spectra of dilute solutions of alkyl and phenyl substituted silicon backbone polymers at room temperature and 77°K reveals pendant group dependent thermochromic effects which are attributed to temperature dependent conformational changes. The very narrow bandwidth fluorescence spectrum of these materials at 77°K i s shown to be consistent with the formation of an exciton band in which electronic excitation i s delocalized. Based on these results, as well as photoselection or polarized luminescence measurements, a model i s developed which describes individual chains of these sigma bonded silicon backbone polymers as consisting of a distribution of variable length all-trans sequences each with its own effective conjugation length. Energy migration along the polymer backbone i s found to occur by a mechanism in which energy i s transferred from shorter to longer sequences.

Linear macromolecules i n which the main chain i s composed of covalently bonded silicon atoms constitute a class of materials which has recently attracted a great deal of renewed attention (1,2). This resurgence of interest has been stimulated in large part by advances i n synthetic methods. High molecular weight materials with a variety of alkyl and/or aryl pendant groups are now available which can be cast into films or spun into fibers, are formable and in general are tractable, (3-6) a feature which distinguishes them from the f i r s t intractable silicon-based polymers synthesized over sixty years ago (7). These materials have been found to possess a number of remarkable properties which have led to a variety of technological 0097-6156/87/0358-0499$06.00/0 © 1987 American Chemical Society

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PHOTOPHYSICS OF POLYMERS a p p l i c a t i o n s (8). Perhaps t h e most n o t a b l e p r o p e r t y o f t h e o r g a n o s i l i c o n polymers i s t h e i r sensitivity to ultraviolet l i g h t ; consequently considerable e f f o r t s a r e c u r r e n t l y being devoted t o t h e development o f t h e s e m a t e r i a l s a s d r y o r s e l f developable r e s i s t s f o r p h o t o l i t h o g r a p h i c a p p l i c a t i o n s by a number o f i n d u s t r i a l l a b o r a t o r i e s (9~12). Polymers w i t h a s i l i c o n backbone have a l s o been found t o t r a n s p o r t p h o t o i n j e c t e d or photogenerated positive charge carriers with remarkably high mobilities (>10~ cm " v o l t " "sec" ) and thus constitute a n o v e l and p o t e n t i a l l y i m p o r t a n t c l a s s o f t r a n s p o r t m a t e r i a l s (13-15). I n a d d i t i o n t o t h e i n t e r e s t t h i s c l a s s o f m a t e r i a l s has a t t r a c t e d r e c e n t l y r e g a r d i n g t h e i r development f o r t e c h n o l o g i c a l applications they have, f o r a number o f y e a r s , r e c e i v e d c o n s i d e r a b l e a t t e n t i o n from a more b a s i c s c i e n t i f i c v i e w p o i n t (l). S t r u c t u r a l l y t h e l i n e a r p o l y s i l y l e n e s a r e analogs o f the saturated alkanes at l e a s t t o the experimenta l y i n g electronic t r a n s i t i o n i s strongly red-shifted into the e a s i l y a c c e s s i b l e near u l t r a v i o l e t r e g i o n o f t h e spectrum ( l 6 ) . Thus whereas b o t h t h e carbon and s i l i c o n c a t e n a t e s e x h i b i t a s h i f t o f t h e i r lowest l y i n g e l e c t r o n i c t r a n s i t i o n s t o longer wavelengths w i t h i n c r e a s i n g c h a i n l e n g t h , t h e a l k a n e a b s o r p t i o n remains c o n f i n e d t o t h e f a r u l t r a v i o l e t w h i l e t h a t o f the s i l i c o n c a t e n a t e s appears t o approach a s y m p t o t i c a l l y , a l i m i t i n g v a l u e a t wavelengths l o n g e r t h a n 300nm ( l 6 , 1 7 ) . I n essence, one has a t hand a c l a s s o f m a t e r i a l s w h i c h a r e sigma bonded l i k e t h e a l k a n e s y e t which a r e amenable t o d e t a i l e d i n v e s t i g a t i o n u t i l i z i n g more c o n v e n t i o n a l e x p e r i m e n t a l t e c h n i q u e s than t h e vacuum u l t r a v i o l e t methods g e n e r a l l y r e q u i r e d f o r t h e a l k a n e s and o t h e r s a t u r a t e d hydrocarbons. In many r e s p e c t s t h e o r g a n o s i l i c o n polymers exhibit p r o p e r t i e s w h i c h a r e more c h a r a c t e r i s t i c o f an u n s a t u r a t e d a r o m a t i c t h a n o f a c o m p l e t e l y s a t u r a t e d sigma bonded compound. F o r example t h e y e x h i b i t r e m a r k a b l y l o w i o n i z a t i o n p o t e n t i a l s w i t h v a l u e s a p p r o x i m a t e l y 3 eV l o w e r t h a n an a l k a n e o f comparable l e n g t h (18). Because o f t h e l o w i o n i z a t i o n energy o f t h e sigma e l e c t r o n s , t h e o r g a n o s i l i c o n c a t e n a t e s have been found t o form charge t r a n s f e r complexes w i t h c e r t a i n s t r o n g o r g a n i c e l e c t r o n i c a c c e p t o r s and t h u s constitute one o f t h e few examples o f a σ e l e c t r o n donorπ a c c e p t o r complex (19,20). A number o f c y c l i c p o l y s i l a n e s have been found t o undergo o n e - e l e c t r o n o x i d a t i o n and r e d u c t i o n t o form r a d i c a l c a t i o n s and a n i o n s (18,19,21,22). E l e c t r o n s p i n resonance s t u d i e s o f t h e r a d i c a l i o n s have shown t h e d e l o c a l i z e d n a t u r e o f the u n p a i r e d e l e c t r o n s i n b o t h t h e c a t i o n and a n i o n (1,21,22). While i t i s o n l y r e c e n t l y t h a t h i g h molecular weight s i l i c o n backbone p o l y m e r s have become a v a i l a b l e f o r d e t a i l e d s t u d y , compounds containing the s i l i c o n - s i l i c o n bond, including o l i g o m e r i c p o l y s i l y l e n e s , have a t t r a c t e d c o n s i d e r a b l e i n t e r e s t f o r many y e a r s . Thus t h e r e e x i s t s a number o f papers o f fundamental importance with particular relevance t o the polysilylenes. Paramount among t h e s e a r e t h e c o n t r i b u t i o n s o f P i t t and coworkers on t h e o p t i c a l s p e c t r o s c o p y (18,23,24)

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36.

Properties of Poly(organosilylenes)

JOHNSON AND McGRANE

501

and t h a t b y Bock and coworkers on t h e p h o t o e l e c t r o n s p e c t r o s c o p y of p e r m e t h y l a t e d l i n e a r and c y c l i c s i l a n e s and t h e homologous s e r i e s o f l i n e a r s i l a n e s (25-27). A most u s e f u l s u r v e y o f t h e f i e l d i n g e n e r a l and t h e group I V c a t e n a t e s i n p a r t i c u l a r as i t e x i s t e d up t o 1977 i s c o n t a i n e d i n t h e book t i t l e d "Homoatomic R i n g s , Chains and Macromolecules o f Main-Group Elements" (28). A more r e c e n t r e v i e w devoted t o s i l i c o n backbone polymers a l o n e has a l s o appeared (29). I n t h i s r e p o r t , t h e r e s u l t s o f an i n v e s t i g a t i o n which probes t h e n a t u r e o f t h e e x c i t e d e l e c t r o n i c s t a t e s o f h i g h molecular weight p o l y ( o r g a n o s i l y l e n e s ) i s presented. A model i s d e v e l o p e d , based on a v a r i e t y o f e x p e r i m e n t a l o b s e r v a t i o n s , which describe individual polymer chains i n terms o f a d i s t r i b u t i o n o f v a r i a b l e l e n g t h sequences o f monomer u n i t s i n an a l l - t r a n s c o n f o r m a t i o n . The f l u o r e s c e n c e emanating from these units i s exciton-like suggesting that energy i s d e l o c a l i z e d on a v e r y EXPERIMENTAL METHODS MATERIALS. A l l the s i l i c o n backbone polymers investigated here were s y n t h e s i z e d in-house b y a Wurtz t y p e r e d u c t i v e condensation o f t h e a p p r o p r i a t e l y s u b s t i t u t e d dichlorosilane w i t h h i g h l y d i s p e r s e d m o l t e n sodium m e t a l . Because o f t h e r a p i d p h o t o d e g r a d a t i o n e x p e r i e n c e d b y t h e p o l y m e r s on exposure t o u l t r a v i o l e t l i g h t , t h e r e a c t i o n s were c a r r i e d o u t i n l o w level yellow l i g h t . The polymers were o b t a i n e d , f o l l o w i n g a p p r o p r i a t e work up o f t h e r e a c t i o n m i x t u r e , b y p r e c i p i t a t i o n from s o l u t i o n b y a n o n s o l v e n t . The polymer i s f i l t e r e d , washed, r e p r e c i p i t a t e d and f i l t e r e d and f i n a l l y d r i e d i n vacuum. The molecular weight and m o l e c u l a r w e i g h t distribution of the p o l y m e r s was d e t e r m i n e d b y g e l p e r m e a t i o n chromatography and a r e based on p o l y s t y r e n e s t a n d a r d s . The p o l y m e r s , a l o n g w i t h molecular weight data f o r those materials that were c h a r a c t e r i z e d , are l i s t e d i n Table I .

T a b l e I . M o l e c u l a r Weights o f t h e P o l y ( o r g a n o s i l y l e n e s ) M " w

Polymer poly(methylphenylsilylene) (PMPS)

a

M

η

M /M w n

223,230 3,^00

2.8

M70

532,500

237,^00

2.2k

Vf,300

12,970

3.65

623,950

1Λ3

poly(methylcyclohexylsilylene (PMHS) poly(methyl n - o c t y l s i l y l e n e (PMOS) poly(methyl n-propylsilylene (PMPrS) a. Bimodal

distribution

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PHOTOPHYSICS OF POLYMERS

Solvents u t i l i z e d i n t h i s i n v e s t i g a t i o n , along w i t h t h e i r source and method o f p u r i f i c a t i o n , a r e l i s t e d below. Benzene; J.T. Baker C h e m i c a l Co. P h i l l i p s b u r g , N J ; Baker A n a l y z e d ' r e a g e n t grade o r B u r d i c k and J a c k s o n L a b o r a t o r i e s , Inc., Muskegan, M i c h i g a n . " D i s t i l l e d i n G l a s s " g r a d e - b o t h used as r e c e i v e d . 2-methylbutane (isopentane); A l d r i c h Chemical Company, Inc., Milwaukee, W i s . , s p e c t r o p h o t o m e t r i c g r a d e , G o l d L a b e l , used a s r e c e i v e d . 2- m e t h y l t e t r a h y d r o f u r a n (2MTHF); A l d r i c h C h e m i c a l Company, Inc., Milwaukee, W i s . ; s t a b i l i z e d w i t h 1% BHT. The s o l v e n t was f r e e d o f s t a b i l i z e r b y s h a k i n g i n a s e p a r a t o r y f u n n e l w i t h ~1 Ν NaOH u n t i l t h e aqueous phase remained clear. The s t a b i l i z e r f r e e 2MTHF was d r i e d o v e r m o l e c u l a r s i e v e s ( D a v i s o n M-518, Type hA) o v e r n i g h t and t h e n d i s t i l l e d . The m i d d l e t h i r d of t h e d i s t i l l e d 2MTHF was c o l l e c t e d and s t o r e d o v e r m o l e c u l a r sieves. The p u r i f i e stain-free amorphous e m i s s i o n a t t h e e x c i t i n g w a v e l e n g t h s , used i n t h e s e e x p e r i m e n t s . 3- methylpentane (3MP); Aldrich Chemical Company, Inc., Milwaukee, W i s . The s o l v e n t was shaken i n a s e p a r a t o r y f u n n e l w i t h s u l f u r i c a c i d t o remove p o s s i b l e a r o m a t i c i m p u r i t i e s . After r e m o v a l o f t h e s u l f u r i c a c i d l a y e r t h e 3MP was shaken w i t h d i l u t e NaOH s o l u t i o n , s e p a r a t e d and d r i e d o v e r m o l e c u l a r s i e v e s ( D a v i s o n M-518 Type UA). The 3MP was t h e n d i s t i l l e d and s t o r e d o v e r m o l e c u l a r s i e v e s f o r u s e i n a b s o r p t i o n and TT°K a b s o r p t i o n and e m i s s i o n s p e c t r o s c o p i c measurements. 1

INSTRUMENTATION. A b s o r p t i o n s p e c t r a were measured on a Cary 1TD spectrophotometer. F o r room temperature spectra, the s o l u t i o n s were c o n t a i n e d i n 1 cm p a t h l e n g t h H e l l m a f u s e d q u a r t z cells. S p e c t r a a t TT°K were r e c o r d e d on r i g i d g l a s s s o l u t i o n s c o n t a i n e d i n 1 cm p a t h l e n g t h s p e c t r o s i l grade q u a r t z t u b e s mounted i n a s m a l l f u s e d q u a r t z dewar w h i c h , when c a r e f u l l y c l e a n e d and c a r e e x e r c i z e d t o p r e v e n t i c e p a r t i c l e s f r o m e n t e r i n g the dewar, m a i n t a i n e d t h e l i q u i d n i t r o g e n i n a c o m p l e t e l y b u b b l e f r e e c o n d i t i o n t h r o u g h o u t t h e c o u r s e o f a measurement. These p r e c a u t i o n s y i e l d e d v e r y c l e a n s p e c t r a , f r e e from t h e n o i s e g e n e r a l l y i n t r o d u c e d by l i g h t s c a t t e r i n g from t h e bubbles o f b o i l i n g l i q u i d nitrogen. A stream o f d r y n i t r o g e n gas d i r e c t e d i n t o t h e C a r y 1TD sample compartment p r e v e n t e d t h e f o r m a t i o n of c o n d e n s a t i o n on t h e dewar. The l o w t e m p e r a t u r e spectra were r e c o r d e d a g a i n s t 1 cm o f s o l v e n t a t room t e m p e r a t u r e i n the r e f e r e n c e compartment o f t h e Cary 1TD. E m i s s i o n s p e c t r a were measured on a f l u o r o m e t e r c o n s t r u c t e d from t h e f o l l o w i n g components. E x c i t a t i o n was p r o v i d e d b y a 200-w Hg-Xe lamp ( O r i e l ) mounted on t h e e n t r a n c e s l i t o f a 0.25m Bausch and Lomb g r a t i n g monochromator. E m i s s i o n was viewed a t r i g h t a n g l e t o t h e e x c i t a t i o n w i t h a RCA C3103 + photomultiplier tube housed i n a t h e r m o e l e c t r i c a l l y c o o l e d P a c i f i c P r e c i s i o n I n s t r u m e n t s Model 3^07 PMT h o u s i n g . The PMT was mounted on t h e e x i t s l i t o f a McPherson Model 218, 0.3m, f / 5 . 3 s c a n n i n g monochromator. The i n s t r u m e n t g r a t i n g i

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503

was b l a z e d f o r 500nm w i t h 1200 grooves/mm g i v i n g a r e c i p r o c a l l i n e a r d i s p e r s i o n o f 2.65nm/mm. The p h o t o m u l t i p l i e r p h o t o c u r r e n t was measured w i t h a K e i t h l e y 6l0B e l e c t r o m e t e r and t h e o u t p u t m o n i t o r e d w i t h a S o l t e c Model 3312 r e c o r d e r . Optics f o r c o l l i m a t i n g and f o c u s i n g t h e e x c i t a t i o n onto t h e sample a s w e l l as f o r c o l l e c t i n g and p r o p e r l y f o c u s i n g t h e e m i s s i o n onto the entrance s l i t o f t h e v i e w i n g monochromator were o f f u s e d quartz. When p o l a r i z a t i o n o f e m i s s i o n s p e c t r a were determined using t h e method o f p h o t o s e l e c t i o n a q u a r t z Glan-Thomson p o l a r i z i n g p r i s m ( K a r l Lambrecht) was i n s e r t e d i n t h e e x c i t i n g and v i e w i n g o p t i c a l p a t h s . F l u o r e s c e n c e l i f e t i m e s were measured by t h e method o f time c o r r e l a t e d s i n g l e photon c o u n t i n g on a system c o n s t r u c t e d w i t h O r t e c , I n c . components. RESULTS AND DISCUSSION P r e s e n t a t i o n and d i s c u s s i o proceed i n the f o l l o w i n o f t h e p o l y s i l y l e n e s w i l l be d e s c r i b e d and a comparative a n a l y s i s o f t h e s p e c t r a i n room temperature f l u i d s o l v e n t media and r i g i d l o w temperature g l a s s e s a t TT°K made. T h i s w i l l be f o l l o w e d b y a d e s c r i p t i o n o f t h e r a t h e r remarkable e m i s s i o n p r o p e r t i e s o f these m a t e r i a l s w i t h emphasis on r e s u l t s o b t a i n e d a t TT°K. Included as p a r t o f t h e e m i s s i o n spectroscopic properties are the results of photoselection or polarization o f e m i s s i o n measurements o b t a i n e d i n a r i g i d g l a s s a t TT°K. Based on these r e s u l t s a model i s d e v e l o p e d w h i c h d e s c r i b e s i n d i v i d u a l c h a i n s o f these s i l i c o n polymers i n terms o f a d i s t r i b u t i o n o f a l l - t r a n s sequences w i t h v a r i a b l e e f f e c t i v e conjugation lengths. DILUTE SOLUTION ABSORPTION SPECTRA. Figure 1 shows t h e a b s o r p t i o n s p e c t r a o f a d i l u t e s o l u t i o n o f PMPS i n 2MTHF a t room temperature and a t TT°K. A t room temperature t h e spectrum i s c h a r a c t e r i z e d b y a s t r u c t u r e l e s s band with a maximum absorbance, a t 33Tnm w i t h an e x t i n c t i o n ^ c o e f f i c i e n t , € , at λ (max) o f 8.7x10 l i t e r - m o l e r . u . cm . Extinction c o e f f i c i e n t s a r e c a l c u l a t e d based on t h e m o l e c u l a r w e i g h t o f the silylene repeat unit ( r . u . ) which i n t h e case o f m e t h y l p h e n y l s i l y l e n e , f o r example, i s 120g/mole. The s t r o n g band a t λ > 300nm i s f o l l o w e d b y a second, weaker s t r u c t u r e l e s s band w i t h a peak i n t e n s i t y a t 270nm. A b s o r p t i o n b y t h e 2MTHF solvent prevents t h e observation o f higher l y i n g absorption bands. C o o l i n g t h e s o l u t i o n t o 77°K t o form t h e r i g i d g l a s s y s t a t e l e a d s t o changes i n t h e a b s o r p t i o n spectrum, most n o t a b l e o f w h i c h i s t h e i n c r e a s e i n i n t e n s i t y o f t h e l o w e s t energy a b s o r p t i o n band. The band a l s o e x h i b i t s a s l i g h t r e d s h i f t t o y i e l d a λ (max) o f 339nm w h i l e e x p e r i e n c i n g a decrease i r i band w i d t h . The e x t i n c t i o n c o e f f i c i e n t a t λ (max) i s 1.27x10 l i t e r - m o l e r . u . 'cm . a v a l u e n e a r l y 50% g r e a t e r t h a n t h a t a t room t e m p e r a t u r e . The same s e r i e s o f measurements was c a r r i e d o u t on d i l u t e s o l u t i o n s o f PMHS i n 2MTHF, however, s i n c e t h i s m a t e r i a l was

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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not c h a r a c t e r i z e d as t o i t s m o l e c u l a r weight and MWD and e x h i b i t e d l i m i t e d s o l u b i l i t y i n 2MTHF, no attempt was made t o determine q u a n t i t a t i v e v a l u e s o f e x t i n c t i o n c o e f f i c i e n t s . The f o l l o w i n g q u a l i t a t i v e o b s e r v a t i o n s a r e n o t e d . T h i s polymer e x h i b i t s a s t r u c t u r e l e s s l o w energy band w i t h a λ (max) a t 320nm f o l l o w e d b y a weak s h o u l d e r on t h e h i g h energy s i d e . C o o l i n g t o TT°K sharpens t h e l o w energy band slightly b u t t h e λ (max) remains a t 32'0nm. The band does n o t show t h e i n c r e a s e i n i n t e n s i t y e x h i b i t e d b y PMPS and, i n f a c t , t h e absorbance a t λ (max) i s s l i g h t l y d i m i n i s h e d a t 77°K compared t o room temperature. The most d r a m a t i c b e h a v i o r i s e x h i b i t e d b y PMOS and PMPrS. F i g u r e 2 shows t h e a b s o r p t i o n s p e c t r a o f d i l u t e s o l u t i o n s o f PMPrS i n 3MP a t room temperature and TT°K. I n f l u i d 3MP a t room temperature PMPrS exhibits a moderately intense, s t r u c t u r e l e s s band _ y i t h λ (max) a t 306nm and e =6.03x10-* l i t e r - m o l e r . u . 'cm glassy__jState a t 77° 2T^0cm t o t h e r e d ( λ (max)=33^nm), sharpens d r a m a t i c a l l y , and becomes s i g n i f i c a n t l y more i n t e n s e w i t h e i n c r e a s i n g t o 1.88x10 l i t e r - m o l e r . u . *cm . A t room temperature t h e f u l l w i d t h a t h a l f maximum (FWHM) o f t h e l o w e s t l y i n g band i s 5240cm w h i l e a t 77°K t h e FWHM decreases t o 1250cm . S i m i l a r f e a t u r e s a r e e x h i b i t e d b y PMOS. Based upon the absorption spectra o f the four poly(organosilylenes) i n v e s t i g a t e d here i t appears t h a t t h e b a s i c n a t u r e o f t h e l o w e s t e x c i t e d e l e c t r o n i c s t a t e remains r e l a t i v e l y unperturbed by a v a r i a t i o n i n t h e nature o f t h e pendant groups a t t a c h e d t o t h e s i l i c o n backbone. There a r e o b v i o u s l y s p e c t r a l s h i f t s s i n c e the (max) o f t h e l o w e s t l y i n g a b s o r p t i o n band i n t h e η-propyl and n - o c t y l d e r i v a t i v e s each o c c u r s a t 306nm w h i l e t h a t o f t h e c y c l o h e x y l o c c u r s a t 320nm and the phenyl a t 337nm, however, t h e s p e c t r a l bandshape remains practically unchanged. The lowest lying t r a n s i t i o n appears t o be a p r o p e r t y o f t h e s i l i c o n backbone and has i n f a c t been assigned as a t r a n s i t i o n from the h i g h e s t occupied d e l o c a l i z e d silicon molecular o r b i t a l t o e i t h e r t h e d e l o c a l i z e d a n t i b o n d i n g sigma o r b i t a l ( σ*) o r s i l i c o n 3d π type o r b i t a l (23-27). I t s h o u l d be noted h e r e , however, t h a t R o b i n has p r e s e n t e d p e r s u a s i v e arguments a g a i n s t any s i g n i f i c a n t 3dw o r b i t a l p a r t i c i p a t i o n i n t h e l o w e s t energy e l e c t r o n i c e x c i t a t i o n o f t h e s i l i c o n c a t e n a t e s ( 3 0 ) . The most d r a m a t i c e f f e c t i s t h e l a r g e r e d s h i f t and n a r r o w i n g o f t h e l o w e s t a b s o r p t i o n band o f t h e p o l y ( m e t h y l n - a l k y l s i l y l e n e s ) on c o o l i n g t o 77°K. T h i s thermochromic e f f e c t has been a t t r i b u t e d t o a c o n f o r m a t i o n a l change o f t h e polymer (31-33). I n t h i s r e g a r d i t s h o u l d be r e c a l l e d , a s noted e a r l i e r , t h a t the p o s i t i o n o f t h e lowest l y i n g t r a n s i t i o n i n t h i s c l a s s o f m a t e r i a l s i s dependent on m o l e c u l a r w e i g h t . T h i s was p r e d i c t e d e a r l y on t h r o u g h s i m p l e HMO c a l c u l a t i o n s (l6), c o r r o b o r a t e d by measurements on o l i g o m e r i c o r g a n o s i l a n e s and r e c e n t l y extended t o h i g h molecular weight p o l y ( a l k y l s i l y l e n e s ) by Trefonas, e t . a l . (17). The X(max) o f t h e l o w e s t l y i n g band o f t h e p o l y ( a l k y l s i l y l e n e s ) i n c r e a s e s p r o g r e s s i v e l y w i t h an i n c r e a s e

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Properties of Poly(organosilylenes)

JOHNSON AND McGRANE

300

280

320

340

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WAVELENGTH-nm

F i g u r e 1. Dilute solution absorption spectra of poly(methylp h e n y l s i l y l e n e ) i n 2 - m e t h y l t e t r a h y d r o f u r a n a t room t e m p e r a t u r e and 77 K.

3

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vu 220

240

260

280

300

320

340

WAVELENGTH-nm

F i g u r e 2. D i l u t e s o l u t i o n a b s o r p t i o n s p e c t r a o f p o l y ( m e t h y l n - p r o p y l s i l y l e n e ) i n 3-methylpentane a t room t e m p e r a t u r e and 77 K.

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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506

PHOTOPHYSICS OF POLYMERS i n t h e c h a i n l e n g t h and a c h i e v e s a l i m i t i n g v a l u e as the number o f monomer u n i t s approaches hO (17). S i m i l a r molecular weight effects have been reported by Harrah and Z e i g l e r f o r poly(methylphenylsilylene) (3*0. The m a t e r i a l s i n v e s t i g a t e d here exceed t h i s DP and hence, s h o u l d be i n the m o l e c u l a r w e i g h t regime when t h e λ (max) has a r r i v e d a t i t s l i m i t i n g v a l u e , a t l e a s t i n f l u i d s o l u t i o n . The f a c t t h a t t h e p o l y (methyl n - a l k y s i l y l e n e s ) i n v e s t i g a t e d here e x p e r i e n c e a significant r e d s h i f t and s p e c t r a l n a r r o w i n g upon c o o l i n g t o 77°K i s thus a consequence o f a t r a n s i t i o n t o a more t h e r m o d y n a m i c a l l y s t a b l e conformational s t a t e w i t h decreasing temperature. In contrast, the two polymers w i t h t h e p h e n y l and c y c l o h e x y l pendant groups do n o t e x h i b i t t h i s e f f e c t and t h e i r s p e c t r a remain r e l a t i v e l y invariant with a decrease i n temperature t o 77°K. This a p p a r e n t l y i s a consequence o f t h e s t e r i c c o n s t r a i n t s imposed by these r e l a t i v e l y b u l k y s u b s t i t u e n t s A recent i n v e s t i g a t i o n by C o t t s (35) suggest as somewhat expanded solution. Measurements o f t h e r a d i u s o f g y r a t i o n over a range o f t e m p e r a t u r e s i n d i c a t e d v e r y l i t t l e change (35) and t h i s appears c o n s i s t e n t w i t h West and M i l l e r ' s s u g g e s t i o n (33) t h a t the thermochromism i s due t o an i n c r e a s e i n p o p u l a t i o n o f t h e t r a n s c o n f o r m a t i o n i n the s i l i c o n backbone upon c o o l i n g . EMISSION SPECTRA. Emission s p e c t r a were measured f o r each o f t h e s i l i c o n polymers b o t h a t room temperature and i n r i g i d g l a s s m a t r i c e s a t 77°K. Room temperature measurements i n f l u i d s o l v e n t w i l l n o t be r e p o r t e d s i n c e t h e r e s u l t s a r e s u b j e c t to considerable v a r i a t i o n . T h i s i s a consequence o f t h e v e r y rapid photodegradation these materials experience under u l t r a v i o l e t e x c i t a t i o n . C o n t r a r y t o the e x t r e m e l y time dependent f l u o r e s c e n c e s i g n a l s observed i n f l u i d s o l u t i o n , i n r i g i d o r g a n i c g l a s s e s a t 77°K t h e p h o t o c h e m i c a l d e g r a d a t i o n i s virtually e l i m i n a t e d and f l u o r e s c e n c e s p e c t r a can be r e c o r d e d simply and r e p r o d u c i b l y . F i g u r e 3 shows t h e e m i s s i o n spectrum o f a d i l u t e s o l u t i o n o f PMPS i n 2MTHF a t 77°K a l o n g w i t h the l o w e s t energy a b s o r p t i o n band. The spectrum i s c h a r a c t e r i z e d b y a v e r y sharp f l u o r e s c e n c e band w i t h a λ (max) a t 3^+7.7nm f o l l o w e d by a weaker, v e r y b r o a d s t r u c t u r e l e s s e m i s s i o n band w h i c h extends out beyond 500nm. The f l u o r e s c e n c e i s i n t e n s e a n d , a l t h o u g h an a b s o l u t e quantum y i e l d d e t e r m i n a t i o n was n o t made, based on t h e l e v e l o f e m i s s i o n o f t h i s m a t e r i a l compared t o o t h e r compounds o f known quantum y i e l d whose e m i s s i o n s p e c t r a have been measured on t h e same e x p e r i m e n t a l s e t - u p , t h e f l u o r e s c e n c e quantum y i e l d o f PMPS i n d i l u t e 2MTHF s o l u t i o n a t 77°K i s e s t i m a t e d t o be on t h e o r d e r o f 0.6-0.7. The f l u o r e s c e n c e l i f e t i m e was found t o be on t h e o r d e r o f 1 nsec o r l e s s . ( i t i s not p o s s i b l e t o give a p r e c i s e value since the fluorescence decay was t o o f a s t t o be r e s o l v e d on t h e time c o r r e l a t e d s i n g l e photon c o u n t i n g system w i t h the e x i s t i n g l i g h t p u i s e r . However, subsequent t o s u b m i s s i o n o f t h i s m a n u s c r i p t an a r t i c l e has appeared reporting the fluorescence lifetime of p o l y ( d i - n - h e x y l s i l y l e n e ) t o be 130 psec (36) ). Clearly the f l u o r e s c e n c e quantum y i e l d and l i f e t i m e v a l u e s a r e i n d i c a t i v e

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

36.

JOHNSON AND McGRANE

Properties of Poly(organosilyienes)

507

o f a h i g h l y a l l o w e d t r a n s i t i o n . Most s t r i k i n g i s t h e sharpness of this band compared t o the l o w e s t l y i n g a b s o r p t i o n band. B e h a v i o r o f t h i s type s i g n i f i e s t h a t the e l e c t r o n i c e x c i t a t i o n i s d e l o c a l i z e d a l o n g t h e polymer c h a i n and c a n be d e s c r i b e d as a m o b i l e F r e n k e l e x c i t o n (37,38). The absence o f s i g n i f i c a n t v i b r a t i o n a l s t r u c t u r e s i g n i f i e s t h a t t h e Coulombic i n t e r a c t i o n between e l e c t r o n i c t r a n s i t i o n d i p o l e moments on n e i g h b o r i n g s i l i c o n - s i l i c o n bonds i s s t r o n g and t h a t t h e r a t e o f energy migration o r d e r e a l i z a t i o n i s fast r e l a t i v e t o the t y p i c a l p e r i o d o f n u c l e a r v i b r a t i o n s (39). F i g u r e k p r e s e n t s t h e e m i s s i o n spectrum o f a d i l u t e s o l u t i o n o f PMPrS i n 3ΜΡ a t 77°K. As was t h e case w i t h t h e p h e n y l d e r i v a t i v e t h e emission spectrum i s characterized by a very sharp f l u o r e s c e n c e band peaking slightly t o the red o f t h e λ (max) o f t h e l o w e s t l y i n g a b s o r p t i o n band. PMPrS a l s o d i s p l a y s a b r o a d u n s t r u c t u r e d luminescence band e x t e n d i n g w e l l beyon in t h e case o f PMPr magnitude l e s s i n t e n s e a t i t s λ (max) (kYJrm) t h a n the sharp f l u o r e s c e n c e band. The i n t e n s i t y o f t h i s l o w energy e m i s s i o n band i s even l e s s i n t h e case o f PMOS and, i s v i r t u a l l y i d e n t i c a l t o t h a t o f PMPrS w i t h r e s p e c t t o i t s s p e c t r a l l o c a t i o n and band shape. These p o l y ( o r g a n o s i l y l e n e s ) thus e x h i b i t a v a r i e t y o f luminescence p r o p e r t i e s w h i c h a r e most u n u s u a l and i n f a c t are considerably different t h a n those e x h i b i t e d by t h e i r s t r u c t u r a l a n a l o g s , the s a t u r a t e d a l k a n e s . L i p s k y and coworkers were the f i r s t t o observe fluorescence from saturated hydrocarbons (4θ) and e s t a b l i s h e d , t h r o u g h a s e r i e s o f e x t e n s i v e i n v e s t i g a t i o n s , c o r r e l a t i o n s between m o l e c u l a r s t r u c t u r e and the emission characteristics (hi). F o r t h e n-alkanes t h e f l u o r e s c e n c e was b r o a d , u n s t r u c t u r e d , and v i r t u a l l y independent o f the degree o f c a t e n a t i o n ( a t l e a s t from n=5 t o n=17). The fluorescence λ(max) d i s p l a y e d an u n u s u a l l y l a r g e S t o k e s s h i f t . An i n c r e a s e i n t h e degree o f c a t e n a t i o n l e d t o an i n c r e a s e i n t h e f l u o r e s c e n c e quantum y i e l d and ^ l i f e t i m e . Typically f l u o r e s c e n c e quantum y i e l d s a r e l o w (ω

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