Aggregation-induced emission : materials and applications v1 9780841231559, 0841231559, 9780841231573, 0841231575

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Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.fw001

Aggregation-Induced Emission: Materials and Applications Volume 1

Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.fw001

Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

ACS SYMPOSIUM SERIES 1226

Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.fw001

Aggregation-Induced Emission: Materials and Applications Volume 1 Michiya Fujiki, Editor Nara Institute of Science and Technology Nara, Japan

Bin Liu, Editor National University of Singapore Singapore

Ben Zhong Tang, Editor Hong Kong University of Science and Technology Kowloon, Hong Kong

American Chemical Society, Washington, DC Distributed in print by Oxford University Press

Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.fw001

Library of Congress Cataloging-in-Publication Data Names: Fujiki, Michiya, editor. | Liu, Bin, 1974- editor. | Tang, Ben Zhong, editor. Title: Aggregation-induced emission : materials and applications Volume 1 / Michiya Fujiki, editor, Nara Institute of Science and Technology, Nara, Japan, Bin Liu, editor, National University of Singapore, Singapore, Ben Zhong Tang, editor, Hong Kong University of Science and Technology, Kowloon, Hong Kong. Description: Washington, DC : American Chemical Society, [2016]- | Series: ACS symposium series ; 1226 | Includes bibliographical references and index. Identifiers: LCCN 2016039915 (print) | LCCN 2016040743 (ebook) | ISBN 9780841231566 (v. 1) | ISBN 9780841231559 (ebook) Subjects: LCSH: Luminescence. | Aggregation (Chemistry) | Photoemission. Classification: LCC QC476.5 .A35 2016 (print) | LCC QC476.5 (ebook) | DDC 541/.35--dc23 LC record available at https://lccn.loc.gov/2016039915

The paper used in this publication meets the minimum requirements of American National Standard for Information Sciences—Permanence of Paper for Printed Library Materials, ANSI Z39.48n1984. Copyright © 2016 American Chemical Society Distributed in print by Oxford University Press All Rights Reserved. Reprographic copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Act is allowed for internal use only, provided that a per-chapter fee of $40.25 plus $0.75 per page is paid to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. Republication or reproduction for sale of pages in this book is permitted only under license from ACS. Direct these and other permission requests to ACS Copyright Office, Publications Division, 1155 16th Street, N.W., Washington, DC 20036. 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 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Foreword The ACS Symposium Series was first published in 1974 to provide a mechanism for publishing symposia quickly in book form. The purpose of the series is to publish timely, comprehensive books developed from the ACS sponsored symposia based on current scientific research. Occasionally, books are developed from symposia sponsored by other organizations when the topic is of keen interest to the chemistry audience. Before agreeing to publish a book, the proposed table of contents is reviewed for appropriate and comprehensive coverage and for interest to the audience. Some papers may be excluded to better focus the book; others may be added to provide comprehensiveness. When appropriate, overview or introductory chapters are added. Drafts of chapters are peer-reviewed prior to final acceptance or rejection, and manuscripts are prepared in camera-ready format. As a rule, only original research papers and original review papers are included in the volumes. Verbatim reproductions of previous published papers are not accepted.

ACS Books Department

Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.pr001

Preface In December 2015, the AIE community gathered in Honolulu, Hawaii, USA and enjoyed an engaging symposium entitled, “Aggregation-Induced Emission: Materials and Applications,” co-organized by Bin Liu, Ben Zhong Tang, and myself. This symposium offered a unique opportunity to bring together distinguished experts from different areas to share their exciting results and the latest developments in the field, which formed the foundation of this two-volume e-book published by the American Chemical Society (ACS). Aggregation-induced emission (AIE) stands for an intriguing phenomenon in which a series of non-emissive molecules in solutions are induced to emit strongly in the aggregate or solid state. The concept of AIE was first coined by Ben in 2001, when he and his co-workers serendipitously discovered that 1-methyl-1,2,3,4,5-pentaphenylsilole was almost non-emissive in ethanol solution but became extremely bright in water-ethanol mixtures. This seminal paper appeared in Chemical Communications (2001, 1740), which was immediately highlighted by Chemical and Engineering News (2001, 79, 29) of the ACS as an unusual phenomenon that is opposite to the behavior of traditional luminophores. Over the past 15 years, AIE has grown into a research field with high visibility and broad impact across both science and technology. The e-books of Aggregation-Induced Emission: Materials and Applications summarize the recent advances in AIE research, ranging from fundamentals, such as design, synthesis, and optical properties of AIE-active molecules, to mechanism studies supported by modeling and experimental investigations, and further to promising applications in the fields of energy, environment, and biology. Because of the large amount of excellent research, the contents have to be divided into two volumes, with Volume 1 focusing on materials and Volume 2 placing greater emphasis on applications. The topics covered in Volume 1 include: New mechanisms and theoretical understanding of AIE phenomena; Vibration-induced emission; The art of restriction of molecular rotation; Domino synthesis of AIE molecules; Small molecule AIE systems; and Mechanochromic AIE materials. The topics covered in Volume 2 include: AIE polymers; AIE-induced chirogenesis; Room-temperature phosphorescent AIE molecules; Liquid crystalline AIE molecules; AIE materials for energy devices; New chemo- and biosensors with AIE molecules; Cell structure and function imaging with AIE molecules; and AIE materials in drug delivery and therapy. These e-books offer readers an excellent perspective of the significant progress recently made in the field of AIE research. They are essential for chemists, physicists, materials scientists, and biologists who work on optic and ix Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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photonic materials and their applications. The book also provides an easy entry to researchers who are new to the field. Each volume should satisfy your curiosity and may stimulate new ideas in your own future work. I cannot end this preface without expressing my gratitude to all the people who have made contributions to these e-books. We thank all the authors and reviewers for their dedicated work and also the editorial team from ACS, especially Bob Hauserman, Jack Nestor, Elizabeth Hernandez, and Arlene Furman, whose efforts and support made these e-books possible.

Michiya Fujiki, Ph.D. Nara Institute of Science and Technology Graduate School of Materials Science Nara 6300192 Japan

Bin Liu, Ph.D. Department of Chemical and Biomolecular Engineering 4 Engineering Drive 4 National University of Singapore Singapore, 117576

Ben Zhong Tang, Ph.D. Hong Kong University of Science and Technology Chemistry Clear Water Bay, Kowloon Hong Kong

x Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Chapter 1

Introduction

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Bin Liu*,1,2 and Michiya Fujiki*,3 1Department of Chemical and Biomolecular Engineering, 4 Engineering Drive 4, National University of Singapore, Singapore 117585 2Institute of Materials Research and Engineering, 3 Research Link, 117602 Singapore 3Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma-Nara, 630-0101 Japan *E-mails: [email protected] (B.L.); [email protected] (M.F.)

The development of luminescent materials is of critical importance to human life. The recent discovery of aggregation-induced emission (AIE) has brought forth revolutionary changes in the role aggregation plays and has inspired research interest in AIE fluorogens (AIEgens) and their potential applications. Due to their extraordinary photophysical properties, AIEgens have been explored in a wide range of applications, including biosensing and therapeutics, optoelectronic and green energy devices, environment monitoring, and many more to come. The content of this book covers a broad range of AIE-related topics, e.g., fundamental understanding of AIE mechanism, sophisticated molecular designs, photophysical properties of AIEgens, elaborate functions, and the latest high-tech applications. A thorough knowledge and understanding of AIE should thus provide new ideas for researchers in the fields of materials science and engineering.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch001

The development of luminescent materials is of critical importance to human life. This is exemplified by the Nobel Prizes awarded to works on the development of green fluorescent proteins (2008) and super-resolved fluorescent microscopy (2014). So far, a great number of luminophores have been developed that are highly luminescent in diluted solutions. Their light emissions, however, are partially or even completely quenched upon molecular aggregation. Such a phenomenon of aggregation-caused quenching (ACQ) has been documented for more than half a century, since Förster’s discovery in 1954. As organic molecules naturally aggregate in solid state and aqueous media, the ACQ effect leads to low sensitivity in sensory systems and poor performance of optoelectronic devices. Although various approaches have been introduced to minimize the ACQ effect, limited success has been realized without creating new problems. The development of a luminogenic system in which aggregation plays a constructive role in the light emission process will bring forth a revolution both conceptually and technically. Aggregation-induced emission (AIE) is an intriguing photophysical process in which non-emissive molecules in solutions are caused to emit strongly in the aggregate or solid state. The luminogens with AIE attribute are called AIEgens. In sharp contrast to ACQ molecules, AIEgens emit more brightly in the useful aggregate state than the solution state. Since the concept of AIE was coined in 2001 by Ben Zhong Tang et al., it has changed the way people think and has brought forth a revolution in fluorescent materials. The mechanism of restriction of restricting intramolecular motion was proved in the following years through both experiments and modeling. This discovery is of great scientific value, as a new theorem needs to be established in order to understand this abnormal phenomenon and to change the way people think about the role of aggregation in the light emission process of a luminophore. Today, the AIE research has spread through many research domains, such as functional materials, energy, biomedical, and environmental sectors. The natural AIE process has a widespread influence in the world and far-reaching implications for the future. The luminescence behaviors of AIEgens could easily change in response to external stimuli or environmental variations, such as mechanical force, temperature, pH, fumes (vapor), light, solvent polarity, electric fields, and so on and so forth. Of particular significance are the triboluminescent AIEgens. Some triboluminescent AIEgens are non-emissive in crystalline state, but intense visible light appears in the presence of stress even without UV illumination. Such properties are of general interest to many researchers, particularly mechanical engineers who work on load-bearing structures or nano/micro machineries and are eager to find a convenient method to monitor the system stresses/strains. The last decade has seen significant progress made in the exploration of real applications for AIEgens in the fields of energy, healthcare, and environmental monitoring. As AIE-active light-emitting liquid crystals can polarize light and emit bright luminescence, they have been used directly for light-emitting liquid-crystal displays (LCDs), which eliminate backlight with a simplified device configuration, offering increased brightness, better contrast, and higher efficiency with reduced energy consumption as compared to traditional LCDs. In addition, various AIEgens with tunable emission colors, that reveal luminescence quantum 2 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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yields up to unity in the solid state, have been designed and synthesized for highly efficient light-emitting diodes. These technologies have attracted substantial interest from material scientists and electrical engineers, among others. To fully utilize the bright luminescence of AIE aggregates, AIE dots and nanoparticles have been successfully fabricated to bring AIEgens into aqueous media in order to offer a useful tool for researchers in the life science field. The AIE dots are super bright, non-blinking, and have high photostability. This, together with tunable size and color, as well as excellent biocompatibility, makes them ideal cell trackers for understanding stem cell therapy and monitoring of cellular processes. The value of the AIE dots is further enhanced by its capability in multi-photon fluorescence imaging and high resolution bioimaging. The ability to directly visualize cellular events in living mouse brains will help neuroscientists study brain functions and understand brain diseases. Of equal importance are the AIE light-up probes, which have been successfully developed for biological sensing and environmental monitoring. The unique non-emissive AIEgens, that are regarded as latent luminescent probes, allow the development of various light-up probes for specific analyte detection in real-time and at any place. Vivid colors have been observed for various analytes, such as bacteria, gases, solvent vapors, and different metal ions in water or ecosystems. The ability to clearly visualize analytes in real time with the naked eye is expected to make a broad impact on human life and well-being, which will help mankind become aware of and protect the environment for a better tomorrow. From the material development point of view, it is truly amazing that the integration of AIEgens into traditional ACQ fluorophores is able to transform the ACQ fluorophores into new AIEgens with unique optical properties. In addition, hierarchically self-organized AIEgens with helps of various organic and inorganic building blocks generate multifaceted luminogens with high quantum efficiency and tunable colors in the solid state. Rational design of hybrid structures will lead to the generation of advanced functional materials, particularly light emitters, with even greater potential in optoelectronic devices, chemical sensors, biological probes, and other technologies not yet anticipated. Their technological applications are vast in scope, limited only by the imagination. Hundreds of laboratories around the world are now performing AIE research, as evidenced by the exponentially increasing number of publications and citations (e.g., 4,701 in 2012, 6,558 in 2013, 11,324 in 2014, and 17,286 in 2015) on this theme. AIE was ranked third in research fronts for chemistry and materials science by Thomson Reuters in 2013, and second in 2015. In recognizing the increasing importance of AIE, several international conferences have been held in recent years that attracted many scientists from different countries to participate. The symposium on AIE in Pacifichem is of the highest significance in this area and represents the latest developments in the field of AIE research. It brings together distinguished experts from different areas to share their exciting and interesting results. Due to rapid developments in the field, this ACS symposium book represents a timely collection of novel results. This book includes the latest work done in AIE: the design, synthesis, and photophysical behaviors of AIE-active luminogens, the experimental and theoretic understanding of AIE mechanisms, as well as the exploration of high-tech applications of AIEgens. 3 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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This book is expected to be a valuable reference to readers who are working or planning to be involved in AIE research. We hope that this book will serve as a catalyst to stimulate new ideas and inspire more researchers as well as industries to work on and expand the field of AIE research.

4 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Chapter 2

New Mechanistic Insights into the AIE Phenomenon Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch002

Zikai He,1,2 Engui Zhao,1,2 Jacky W. Y. Lam,1,2 and Ben Zhong Tang*,1,2 1Department

of Chemistry, Division of Life Science, State Key Laboratory of Molecular Neuroscience, Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China 2HKUST Shenzhen Research Institute, No. 9 Yuexing First RD, South Area, Hi-tech Park Nanshan, Shenzhen 518057, China *E-mail: [email protected]

Luminescent materials with characteristics of AIE have drawn extensive attention. AIE now becomes not only a phenomenon but also a synonym of a class of novel functional materials. Deciphering of the underlying mechanisms is of great importance to fundamental understanding, luminogens explorations, and advanced applications. In this chapter, we conduct an in-depth mechanistic discussion on this special photophysical process focusing on the recently proposed mechanism of the restriction of intramolecular motion. We derived that the structural rigidification of a flexible luminogen is the intrinsic cause of AIE effect, which may serve as a rationale design principle for novel AIE systems.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Introduction Luminescent materials with characteristics of aggregation-induced emission (AIE) have drawn extensive attention since the debut of the AIE concept. In 2001, we reported an abnormal photophysical property of the silole systems (1). A series of silole derivatives (i.e. HPS) were then found to be nonluminescent in dilute solutions but became highly emissive when aggregated. Since the nonemissive silole molecules were induced to emit by aggregate formation, this novel phenomenon was termed as “aggregation-induced emission”. AIE now becomes not only a phenomenon but also a synonym of a class of functional materials (2–4). Deciphering of the underlying mechanisms of the AIE phenomenon is of great importance to fundamental understanding, luminogens explorations, and advanced practical applications (5). Theoretically, an excited luminogen molecule can decay through the photophysical and/or photochemical pathways (6). The photophysical one includes nonradiative and radiative processes. The photochemical one results a chemical reaction. In solution, the excited AIE luminogens (AIEgens) decay mainly through nonradiative photophysical or photochemical processes. In aggregated states, they decay mainly through radiative photophysical process. The collective effects give the unique AIE properties. Therefore, the investigations on the AIE mechanism should focus on finding out the detailed decay processes that account for these photoinduced behaviors. Numerous efforts have been continuously devoted to deciphering the AIE working principle. A number of possible mechanisms have been put forward, including conformational planarization, J-aggregate formation, E/Z isomerization, the restriction of twisted intramolecular charge transfer, as well as the excited-state intramolecular proton transfer, but none of them can be fully supported by the experimental data or perfectly applicable to all the AIE systems (7). With great and persistent efforts, the restriction of intramolecular rotation (RIR) process has been proposed to be the main mechanistic picture for the AIE effect by our group (8). However, some newly emerging AIE systems that are absent from multiple rotors, bring about some ambiguous issues to the RIR mechanism. As is well known, rotation and vibration are the two main modes of molecular motion accompanied by energy consumption of excited state. We proposed that the AIE effect of these rotor-absence luminogens maybe originate from the restriction of intramolecular vibrations (RIV). We integrated the RIR with the RIV as the restriction of intramolecular motion (RIM) as a more comprehensive AIE mechanism (Figure 1) (7). Derived from RIM, here we realized that the structural rigidification of a flexible luminogen is the intrinsic principle for AIE systems. Besides, we revisited the E/Z isomerization and photocyclization decay pathway, which may serve as considerable nonradiative photochemical processes during the excited states relaxation of AIEgens.

6 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 1. (Upper panel) Propeller-shaped AIEgens governed the restriction of intramolecular rotations. (Lower panel) Shell-like AIEgens working under the restriction of intramolecular vibrations. Adapted with permission from reference (7). Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

Restriction of Intramolecular Rotations (RIR) The RIR mechanism was proposed from sysmmetrical investigation of the prototypical AIEgens, namely hexaphenylsilole (1, HPS) and tetraphenylethylene (2, TPE). The external control experiments, internal structural modifications as well as theoretical calculations verified our RIR hypothesis. The RIR mechanism is the mostly widely used for explanation and exploration of novel AIE systems. After careful examination of the structure of HPS, we can find that the silole core is linked to six phenyl rings through single bonds, which makes the molecule conformational flexible (Figure 2A). As revealed by its single crystal structure, HPS molecule takes a propeller-like conformation with the large torsion angles between the peripheral phenyl rings and the central silole core (Figure 2B) (8). The flexible propeller-like structure will rationally explain its photoluminescence (PL) behavior.

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Figure 2. (A, C) Molecular structures and (B, D) single crystal conformations of HPS and TPE. Figure 3 demonstrates the AIE behaviors of HPS. HPS is soluble in many common organic solvents, such as THF, chloroform, acetonitrile and acetone, less soluble in methanol, and insoluble in water. Thus, water is used as a nonsolvent to induce aggregation of HPS molecules in a water-miscible solvent system. As shown in Figure 3A, the dilute solution of HPS in acetone is nonemissive, with a negligible fluorescence quantum yield (ΦF ~ 0.1%). Upon increasing water fractions (fw), the ΦF shows insignificant change before fw reaches 50 vol %, but starts to rise swiftly afterwards. At fw = 90 vol %, the ΦF is boosted to 22%, which is 220-fold higher than that of the acetone solution (8). The RIR mechanism is therefore proposed to explain the AIE effect of HPS. In solution, its multiple phenyl rotors can dynamically rotate against the silole stator via the C-C single-bonds, which serves as a nonradiative decay pathway for the excited states. In aggregate, such rotations are suppressed due to the physical constraints from around molecules. The nonradiative pathway is thus blocked, leading that the radiative channel becomes the dominant decay pathway. As media with high viscosity can slow down the intramolecular rotations, AIEgens in higher viscous media should exhibit stronger emission. Considering that the viscosity of glycerol (934 cp at 25 °C) is three orders of magnitude higher than that of methanol (0.544 cp at 25 °C), the PL of HPS were then measured in glycerol/methanol mixtures. With gradually increasing the viscosity of the solvent 8 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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mixture, the PL peak intensity of HPS linearly increases on the semilog scale with glycerol fractions (fG) in the range of 0–50 vol % at room temperature (Figure 3B). The fluorescence enhancement in this fG region should be primarily attributed to the viscosity effect, because the HPS molecules are soluble in these mixtures. In the glycerol/methanol mixtures with fG larger than 50 vol %, the PL intensity increases sharply, which is due to the formation of HPS nanoaggregates in solvent mixture with low solvating power.

Figure 3. (A) Plots of fluorescence quantum yield of HPS vs. water fraction in acetone/water mixtures and (B) its PL peak intensity vs. glycerol fraction in glycerol/methanol mixtures; [HPS] = 10 μM. Reprinted with permission from reference (8). Copyright 2003 American Chemical Society.

The external control experiments strongly support the RIR mechanism and prove that the silole emission can be modulated through physical and engineering manipulation (9, 10). Similarly, the structure modification can also serve as internal control experiments to examine the RIR mechanism through steric (11) and conjugation (12) modulation. For example, bulky isopropyl (i-Pr) groups are attached to different peripheral phenyl rings of HPS yielding 3-5 (Figure 4). All these three siloles are fluorescent in solutions, with the increase in solution ΦF in the order of 5 > 4 > 3, which is consistent with the difference in their rotational barriers caused by the steric hindrance from adjacent substituents. Such high rotation barriers of 3-5 will lead to structural rigidification, which plays a decisive role in making them more emissive in solutions than HPS (11). Similar to the silole systems, a great deal of work has been done with TPE derivatives, aiming at proving the RIR mechanism. As shown in Figure 2, TPE have the four phenyl rings linked to ethylene through single bonds, enjoying a flexible configuration (Figure 2C). As revealed by its single crystal structure, TPE molecule also takes a propeller-like conformation (Figure 2D). A variety of experiments have been carefully designed for RIR mechanism verification, including host-guest inclusion (13), steric effect (14), conjugation effect (15), intermolecular coordination (16), covalent bonding (17), and metal-organic framework locking (18), etc. 9 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 4. (A) Chemical structures and fluorescence photographs and (B) PL spectra of solutions of siloles 3–5 in acetone (10 µM). Adapted with permission from reference (11). Copyright 2005 American Chemical Society. For example, TPE is tethered with α-, β- or γ-cyclodextrins (CDs) through ester bonds between CDs and monocarboxylic acid-substituted TPEs to afford CDs-substituted TPE adducts. Among the three derivatives, TPE-α-CD (6) shows highest fluorescence intensity in solution as compared to others because α-CD has the smallest cavity size. In the small α-CD cavity, the intramolecular rotations of the phenyl rings and diphenylmethylene units become more restricted (Figure 5A), which accounts for its intensified emission (13). Multiple methyl groups are attached to the o-positions of the phenyl rings in TPE, giving the sterically crowded TPE derivative, TPE-TM (7). In THF solution, 7 shows a bright emission with a ΦF of 64.0% (Figure 5B). The four o-methyl groups increase the bulkiness and rotational barriers of the phenyl groups, efficiently suppressing nonradiative decay due to the restricted rotational freedom (14). When two diphenyl groups are attached to the o-positions of the phenyl rings in TPE, the resulted folded luminogen (Z)-o-TPE-BBP (8) is also emissive in a dilute THF solution with a ΦF of 45.0%. As the rotations of aryl rings are restricted due to the intramolecular through-space π-interactions and steric effect, the nonradiative decay rate of the excited states is decreased (Figure 5C) (15). Tetrakis(bisurea)-decorated TPE (9) (16) and tetra(4-pyridylphenyl)ethylene (10) (17) are weakly emissive in solutions, but exhibit “turn-on” fluorescence after addition of sulfate anion and Hg2+ cation, respectively (Figure 5D and 5E). The intramolecular rotations of TPE should be curbed by formation of coordination complexes. When tetrakis(4-carboxyphenyl)ethylene (11), a TPE derivative decorated with four carboxylic acid groups, was used as the ligand to construct MOFs, researchers can generate various luminescent MOFs (18). Anchoring AIEgens by metal ions within a robust matrix is hence supposed to be an effective method to restrict the intramolecular motion (Figure 5F). Electromagnetic radiation at frequency of tetrahertz (10 (12) Hz, 4.1 meV) is low enough in energy to probe low-frequency intermolecular interactions and some low energy intramolecular motion. It is sensitive to the relaxation dynamics in condensed matter. Terahertz time-domain spectroscopy (THz-TDS) has been applied to provide directly experimental support to the RIR mechanism. The measurement verifies that the phenyl ring rotations of TPE occur at the THz 10 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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frequencies with the higher overall absorption at the high temperature than the low temperature (19). The phenyl ring rotations is found play the key role in deactivating the excited state of TPE at room temperature.

Figure 5. (A) Chemical structure of TPE-α-CD. Reprinted with permission from reference (13). Copyright 2013 Royal Society of Chemistry. (B) Plots of I/I0 of TPE and TPE-TM (7) vs. water fractions in THF/water mixtures (10 μM), where I0 and I are the PL intensities in THF solution and a THF/water mixture, respectively. Inset: fluorescence photographs of TPE and 7 in THF solutions. Reprinted with permission from reference (14). Copyright 2014 Royal Society of Chemistry. (C) PL spectra of (Z)-o-TPE-BBP (8) in THF/water mixtures. Inset: Photographs of (Z)-o-TPE-BBP in THF/water mixtures (fw = 0, 90%) taken 11 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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under the illumination of a UV lamp. Reprinted with permission from reference (15). Copyright 2013 Royal Society of Chemistry. (D, E) Schematic illustrations of coordination-induced restriction of intramolecular rotations based on luminogens 9 (reprinted with permission from reference (16). Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA) and 10 (adapted with permission from reference (17). Copyright 2012 Royal Society of Chemistry.). (F) Representative example of metal-organic frameworks constructed by using carboxylic acid TPE derivatives (11). Adapted with permission from reference (18). Copyright 2011 American Chemical Society.

E/Z Isomerization Different from HPS, the situation of TPE becomes complicated by the fact that its central ethylenic double bond can be broken by UV irradiation. TPE could undergo another rotation after being excited, the rotation of the opened double bond. As a result, there is an issue of concern of the AIE mechanism to TPE involving E/Z isomerization process. What is the relationship between isomerization and emission processes of a TPE-based AIEgen? Herein, we summarize the recent understandings on these issues and give the conclusive answer that E/Z isomerization is involved in deactivation process, but play a minor role. It is the subsequent result of the double bond rotation rather than the reason for AIE effect. To investigate the role of E/Z isomerization in the PL process of TPE based AIEgens, a TPE derivative with polar substituents was synthesized and separated into E and Z isomers (Figure 6A) (20). After elaborate experiments, the E/Z isomerization process was followed by tracing the photoirradiation-induced changes in the chemical shifts of the E and Z isomers under “normal” PL spectrum measurement conditions by proton NMR. The E/Z isomerization process did occur with a considerable amount (~15% in 5 min). To further discriminate effect of E/Z isomerization and RIR process, another luminogen (TPE-Fl) was synthesized by linking TPE and fluorescein (Fl) units together. Its emission behaviors were investigated under UV (330 nm) and visible (480 nm) light irradiations that were capable and incapable of breaking ethylenic double bond of the TPE unit, respectively. Two wavelength excitation experiments revealed that E/Z isomerization was involved, but played a minor role in the luminescence quenching process of TPE-Fl (Figure 6B). RIR was confirmed to play a predominant role. On the other hand, we found that the major step of the E/Z isomerization was also an intramolecular rotation of the diphenylmethylene units. The rotations occur around newly formed single bond after UV irradiation and accounts for one of the nonradiative decays of the TPE units. The results here offer a more comprehensive picture of emission behavior in TPE-based AIEgens, filling up the gap in the mechanistic study. Particularly, it settles the concern and extends the content of RIR mechanism.

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Figure 6. (A) Chemical structures of (E/Z)-TPE-FM and TPE-Fl. (B) Changes in the Z ratio of TPE-FM with irradiation time (measured for twice). (C) Relative PL quantum yields of TPE-Fl in solution measured at different excitation wavelengths using Fl as reference. Adapted with permission from reference (20). Copyright 2016 Royal Society of Chemistry.

Restriction of Intramolecular Vibrations (RIV) Recently, some newly emerging AIE systems without multiple rotors, such as the nonplanar THBA, cannot be explained perfectly by the RIR mechanism. THBA has no rotators, as its phenyl rings are locked by ethane tethers. However, it exhibits typical AIE behavior: nonemissive in solution but highly luminescent as aggregates (Figure 7) (21). As is well known, rotation and vibration are the two main modes of molecular motion which can consume excited state energy. We proposed that the AIE effect of THBA may be mainly originated from the restriction of intramolecular vibrations (RIV). The phenyl rings could be viewed as vibration parts which are connected by a flexible heptagon bridge. Upon aggregation, the substantial intramolecular vibrations are restricted to block nonradiative decay pathway. Theoretical investigations verify that the intramolecular vibrations of the fixed phenyl rings are the key energy consumption modes of the excited states. As shown in Figure 7C, the isolated THBA molecules have six normal modes that consume significant amounts of excited state energy. In comparison, the clustered THBA molecules have only three normal modes consuming less amounts of excited state energy. In the cluster, a decrease in the number of vibrational normal modes and a loss of ~30% in the energy consumption of excited-state lead confirm that RIV leads THBA to radiatively decay after forming cluster. 13 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 7. (A) PL spectra of THBA (15) in THF/water mixtures with different water fractions (fw) and (B) change in PL intensity of 15 with water fraction ([15] = 20 μM). Plots of reorganization energy vs. normal mode wavenumbers for excited states of (C) molecular and (D) clustered species of 15. Adapted with permission from reference (21). Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 8. Chemical structures of COT containing AIEgens 16 and 17.

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The RIV hypothesis was also proved in other AIE systems, such as AIEgens bearing dibenzocyclooctatetraene (COT) moieties, 16 (22) and 17 (23), which are reported by Iyoda et al. and Yamaguchi et al., respectively (Figure 8). They are nonemissive in solutions but become fluorescent in crystals. In the solution, the flexible COT cores can undergo vibrational inversion among the various molecular conformations. Such intramolecular motion can dissipate the excited state energy through nonradiative decay pathways. In the crystals, these conformational motion are restricted by the intermolecular interactions. The excited states mainly undergo the radiative decay, resulting in emissive aggregates governed by the RIV mechanism.

Restriction of Intramolecular Motion (RIM) Now we know both the intramolecular rotations and vibrations can dissipate the excited state energy to make solution states nonemissive. Restriction of these intramolecular motion can block the nonradiative decay pathway, leading to AIE effect. Therefore, we would like to combine the RIR and RIV as restriction of the intramolecular motion (RIM). A series of AIE-active luminogens from 18 to 21 containing both vibratable cores and rotatable peripheries are shown in Figure 9. Since both RIR and RIV are involved in such systems, the luminogens are AIE active as expected, which can be explained using the principle of RIM. On the one hand, the nonplanar butterfly-like AIEgens contain bendable cores, such as phenothiazine in 18 (24), 11,11,12,12tetracyano-9,10-anthraquinodimethane in 19 (25), pentacenequinodi-methane in 20 (26) and 21 (27), respectively. On the other hand, these bendable cores are decorated with various rotatable groups. As a result, there are two nonradiative channels dissipating the excited state energy: (i) the intramolecular vibrational motion and (ii) the intramolecular rotational motion. With addition of poor solvent into their solution, the molecules must form aggregates. The rotations of the aryl groups and the vibrations of the bendable cores are restrained by a variety of intramolecular interactions and constrained surroundings in solid states. Thereby, RIM turns on the emission of these molecules. The RIM should be a general AIE mechanism. The principle of RIM will greatly extend the scope of AIE research, because it not only provides new insights into the photophysical fundamentals but also opens up new avenues to the explorations of new AIEgen systems.

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Figure 9. Examples of luminogens whose AIE activities are ascribed to the process of restriction of intramolecular motion (RIM). The structures are optimized by Chem 3D.

Restriction of Intramolecular Photocyclization As an alternative mechanism, the deactivation of TPE from its first excited states through the intramolecular photocyclization pathway is rarely considered. It is quite intriguing, considering its existence as the intermediate in the photo-oxidative reaction of stilbene and TPE (28). Recently, theoretical chemists from Switzerland reported that 75% of the trajectories (45/60) proceed through photocyclization (Figure 10). In comparison, only 5% of the trajectories (3/60) proceed through deactivation channel of ethylenic twist during their theoretical simulation (29). The remaining trajectories (12/60) persist in the excited states without change through the time length of the simulation. In detail, the TPE system, after excitation to the S1 state, evolves adiabatically on the same potential energy surface. Potential energies of S0/S1/S2/S3 are shown in magenta/red/blue/green curves, respectively, while the actual electronic state is indicated in black curve. All the energies are relative to the initial (0 fs) S0 energy. The phenyl ring torsions can bring the S1 and S0 close together, leading the system 16 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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to the conical intersection (Figure 10A). The nonradiative relaxation channel to the ground state results in various photoproducts, depending upon the precise conical intersection topology encountered by the trajectory.

Figure 10. (A) Relevant geometrical parameters (upper panel) and electronic state potential energies (lower panel) as a function of time for the photocyclization process. The time evolution of photocyclization is described by the relevant C-C distance. (B) Time evolution of the Θ twist angle for 60 trajectories. The trajectories are computed at the PBE0/def2-SVP level. Adapted with permission from reference (29). Copyright 2016 Royal Society of Chemistry. Although the photodynamical cycle of TPE is rather complicated, there is little doubt that photocyclization plays a key role. Figure 10B shows a time evolution of the twist angle Θ for the ensemble of trajectories. Red/blue/green lines represent molecules in S1/S2/S3 state, respectively, whereas S1/S0 crossing points are indicated by black dots. The cyclization dynamics can be easily distinguished from the ethylenic twist. The phenyl rings are initially close to one another and cyclization dominates. So restricting the torsional motion would greatly block the nonradiative decay and promote the radiative pathways. These findings will be of considerable value for the interpretation of the TPE-based AIE systems and extending the scope of RIM mechanism.

Conclusion The RIM mechanism now can be the unification of the RIR, RIV and RIP mechanisms. The new RIM mechanism with broader contents provides the simple, fundamental and comprehensive AIE mechanisms to work together for explanation and creation of AIE family. Intrinsically, the intramolecular motion described here boosts the nonradiative decay rates, arising from the flexible isolated molecular structures. Upon aggregation, such intramolecular motion is restricted, contributing to the enhanced structural rigidification and dramatically decreased nonradiative decay rates. Thus, the radiatve relaxation channels become favorable. Generally, in an AIE system, the flexible structures 17 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

weaken molecular rigidity and promote intramolecular motion to accelerate the nonradiative decay. The solutions become poorly emissive. The aggregation induces the structure rigidification and blocks the nonradiative decay channels, making intense fluorescence. Therefore, molecular rigidity of a flexible structure is the key factor of an AIE system.

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12. Chen, B.; Nie, H.; Lu, P.; Zhou, J.; Qin, A.; Qiu, H.; Zhao, Z.; Tang, B. Z. Conjugation versus Rotation: Good Conjugation Weakens the AggregationInduced Emission Effect of Siloles. Chem. Commun. 2014, 50, 4500–4503. 13. Liang, G. D.; Lam, J. W. Y.; Qin, W.; Li, J.; Xie, N.; Tang, B. Z. Molecular Luminogens Based on Restriction of Intramolecular Motions Through Host–Guest Inclusion for Cell Imaging. Chem. Commun. 2014, 50, 1725–1727. 14. Zhang, G.-F.; Chen, Z.-Q.; Aldred, M. P.; Hu, Z.; Chen, T.; Huang, Z.; Meng, X.; Zhu, M.-Q. Direct Validation of the Restriction of Intramolecular Rotation Hypothesis via the Synthesis of Novel ortho-Methyl Substituted Tetraphenylethenes and Their Application in Cell Imaging. Chem. Commun. 2014, 50, 12058–12060. 15. Zhao, Z.; He, B.; Nie, H.; Chen, B.; Lu, P.; Qin, A.; Tang, B. Z. Stereoselective Synthesis of Folded Luminogens with Arene–Arene Stacking Interactions and Aggregation-Enhanced Emission. Chem. Commun. 2014, 50, 1131–1133. 16. Zhao, J.; Yang, D.; Zhao, Y.; Yang, X.-J.; Wang, Y.-Y.; Wu, B. Anion-Coordination-Induced Turn-On Fluorescence of an OligoureaFunctionalized Tetraphenylethene in a Wide Concentration Range. Angew. Chem., Int. Ed. 2014, 53, 6632–6636. 17. Huang, G.; Zhang, G.; Zhang, D. Turn-On of the Fluorescence of Tetra(4pyridylphenyl)ethylene by the Synergistic Interactions of Mercury(II) Cation and Hydrogen Sulfate Anion. Chem. Commun. 2012, 48, 7504–7506. 18. Shustova, N. B.; McCarthy, B. D.; Dinca, M. Turn-On Fluorescence in Tetraphenylethylene-Based Metal–Organic Frameworks: An Alternative to Aggregation-Induced Emission. J. Am. Chem. Soc. 2011, 133, 20126–20129. 19. Parrott, E. P. J.; Tan, N. Y.; Hu, R.; Zeitler, J. A.; Tang, B. Z.; PickwellMacPherson, E. Direct Evidence to Support the Restriction of Intramolecular Rotation Hypothesis for the Mechanism of Aggregation-Induced Emission: Temperature Resolved Terahertz Spectra of Tetraphenylethene. Mater. Horiz. 2014, 1, 251–258. 20. Yang, Z.; Qin, W.; Leung, N. L.; Arseneault, M.; Lam, J. W. Y.; Liang, G.; Sung, H. H. Y.; Williams, I. D.; Tang, B. Z. A mechanistic study of AIE processes of TPE luminogens: intramolecular rotation vs. configurational isomerization. J. Mater. Chem. C. 2016, 4, 99–107. 21. Leung, N. L.; Xie, N.; Yuan, W.; Liu, Y.; Wu, Q.; Peng, Q.; Miao, Q.; Lam, J. W.; Tang, B. Z. Restriction of Intramolecular Motions: The General Mechanism behind Aggregation-Induced Emission. Chem. Eur. J. 2014, 20, 15349–15353. 22. Nishiuchi, T.; Tanaka, K.; Kuwatani, Y.; Sung, J.; Nishinaga, T.; Kim, D.; Iyoda, M. Solvent-Induced Crystalline-State Emission and Multichromism of a Bent π-Surface System Composed of Dibenzocyclooctatetraene Units. Chem. Eur. J. 2013, 19, 4110–4116. 23. Yuan, C.; Saito, S.; Camacho, C.; Kowalczyk, T.; Irle, S.; Yamaguchi, S. Hybridization of a Flexible Cyclooctatetraene Core and Rigid Aceneimide 19 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Chapter 3

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Vibration Induced Emission (VIE): An Intrinsic Fluorescence Tuning Mechanism of N,N′Disubstituted-dihydribenzo[a,c]phenazines Wei Chen,1 Zhi Lin,2 Jianhua Su,1 and He Tian*,1 1Key

Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, P. R. China 2College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China *E-mail: [email protected]

In pursuit of design and modification of photophysical properties on molecular level, N,N′-disubstituteddihydrodibenzo[a,c]phenazines were demonstrated as a vivid example of elaborate manipulation via ‘vibration induced emission’ (VIE), which is coined for this particular change in configuration and planarity motion. Significant alternation was observed upon environmental change in polarity, viscosity and temperature, including dual fluorescence and large Stokes shift, etc. In-depth investigations were utilized such as temperature dependent steady-state spectroscopy, nanosecond time-resolved spectroscopy, femtosecond dynamics and computational simulation of the reaction energy surfaces, which manifest this phenomenon as a novel mechanism attributed to intrinsic switch-ability of these molecules. In light of this case, the concept of VIE can be introduced as a universal criteria for facile control of the photophysical pattern and hopefully be extended to versatile applications in photoelectric and biomedical disciplines.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Introduction Due to the prosperity of luminescent materials in field of photo-active devices and biosensors, fluorescence mechanisms have always been regarded as the utmost doctrines in development of applicable molecules (1). Anomalous luminescence phenomena of some specific molecules, including large Stokes shift and dual fluorescence (2), can be potentially exploited to unveil the theoretical excited-state dynamics of fluorophores, thus receiving considerable interest. Currently, several mechanisms that lead to large Stokes shift, have been proposed and acknowledged, such as excimer/exciplex formation, charge transfer (CT), excited-state intramolecular proton transfer (ESIPT) (3), and Förster/ fluorescence resonance energy transfer (FRET) (4), facilitating the rational design of opto-functional molecules. Herein, we envisioned a novel mechanism of photochemical process with a plain nomenclature of ‘Vibration Induced Emission’, which indicates an intrinsic response of particular molecules upon environmental variations (Scheme 1). Conventional photochemical or photophysical processes usually involve the transfer of electrons, protons or overall energy. Excimer/exciplex formation (5, 6) and charge transfer (CT) processes (7, 8), for instance, include the intermolecular and intramolecular transfer of electrons and as a result of intramolecular interactions, can produce intramolecular charge transfer (ICT) state. Park et al. reported a donor-acceptor cocrystal in a loosely packed manner, showing a strong red-shifted luminescence based on intermolecular charge transfer (6). With an analogous functionality, TICT (twisted intramolecular charge transfer) appears with perpendicularly organized electron donor and acceptor (9, 10), leading to electronic decoupling of the overall molecule. Tang et. al. designed BODIPY derivatives which has remarkable solvatochromism phenomena as the molecules changed from locally excited states to TICT states upon increase of solvent polarity (11). ESIPT, on the other hand, involves tautomerization in a unimolecular basis, which leads to strong redistribution of the electronic density (12–15). Chou et al. reported 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), of which a seven-member-ring hydrogen bond validates the intramolecular proton transfer and a large Stokes shift upon tautomerization (16). FRET occurs through nonradiative dipole-dipole coupling between two light-sensitive chromophores (17–20). In this sense, Würthner et. al. reported a system where polymerized vesicles loaded with water-soluble perylene diimide were designed for sensitive pH response and its fluorescence color changes covered the whole visible range (21). Nevertheless, there are still several chromophores with large Stokes shift that might not attribute to mechanisms mentioned above. One typical scaffold is the biphenyl derivatives (22, 23), where a geometrical planarization occurs upon excitation. Recently, Würthner et. al. observed a type of zwitterionic perylene bisimide-entered radical with precedented stability and a distorted structure was obtained via structural analysis (24, 25). Similarly, anomalously large Stokes shift was also observed with several polycyclic aromatic molecules in the bent-to-planar motion (26, 27). Yet these structural vibrations were rarely studied intensively. 22 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Scheme 1. Diagrammatic sketch of fluorescent mechanisms: (A) Twisted Intramolecular Charge Transfer (TICT); (B) Excited-state Intramolecular Proton Transfer (ESIPT); (C) our newly coined Vibration Induced Emission (VIE). (Original mechanism diagram.) Recently, we have observed an intriguing phenomenon from the N,N′-disubsitituted-dihydrophenazines derivatives (Figure 1) upon investigation on their hole-transporting properties (28) with the basic scaffold of N,N′-dimethylphenazine (DMP). The solutions of these molecules are colorless in appearance and yet emit pronounced red fluorescence. For N,N′-dimethyl-9,14-dihydrodibenzo[a,c]phenazine (DMAC), N,N′-diphenyl-dihydrodibenzo[a,c]phenazines (DPAC), and N′-phenyl-N′-fluorenyl-dihydrodibenzo[a,c]phenazines (FlPAC), the absorption maxima locate at ~ 350 nm, while the peak wavelengths of fluorescence locate at ~ 600 nm and hardly change with the solvent, giving an anomalously large Stokes shift calculated to be > 11000 cm-1 (Figure 2). These unique photophysical behaviors can be rationalized by excited-state configuration transformations induced by vibration, and we named this new mechanism as Vibration Induced Emission (VIE) (29–32). What needs to be emphasized is that VIE turned out to be one of the intrinsic fluorescence property of the molecules, which is irrelevant with non-radiative energy dissipations caused by rotational motion mechanisms. 23 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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It is not, to be precise, limited to enhanced fluorescence emission, but instead refers to dynamic changes in intrinsic photophysical properties upon vibrational alternation.

Figure 1. (A) Structures of DMP, DMAC, DPAC and FlPAC; (B) Photographic images of color (emission) and sol-gel phase transition of DPAC doped in thermosensitive organogel under visible and 365 nm UV light. (Modified with permission from ref. (36). Copyright © 2015 American Chemical Society.)

Vibration Induced Emission (VIE) The behavior of compound FlPAC appeared to be a perfect interpretation of this new mechanism. Careful examination of FlPAC showed a typical dual emission phenomenon: a dominant (anomalous) emission band maximized at ~610 nm, while a weak but non-negligible (normal) emission band located around the blue region, which was identical with the excitation spectra (370 nm). The blue emission band showed pronounced Stokes shift as a result of solvatochromism caused by charge transfer, which red shifted from cyclohexane to acetonitrile by 2730 cm-1. In contrast to the normal emission, the red emission was insensitive to the solvent polarity. Thus, the probability of TICT mechanism as an explanation of this anomalous emission could be entirely eliminated. However, as is reported in the literature (33), the absorption maximum of dihydrophenazines DMP located at ~ 343 nm, while the emission band maximized at ~ 470 nm. The large Stokes shift up to 8000 cm-1 can attribute to symmetrical inhibition of S0→S1 transition (34). Schuster et. al. subsequently reported compound DMAC and invoked a similar symmetry rule to explain the unique photophysical properties of DMAC (35). According to this theory, a compound like FlPAC whose C2 symmetry has been broken down should not show anomalous red emission. However, this inference was decidedly inconsistent with the experimental data. 24 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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In addition, symmetry rule should not be dependent on temperature and viscosity, while the photophysical properties of DMAC were dramatically varied with the external environment based on our experiments. Therefore, the explanation of forbidden transition was not applicable to rationalize the anomalous Stokes shift of DMAC and FlPAC.

Figure 2. (A) Illustrative scheme of dihydrophenazine DPAC for VIE mechanism; (B) Typical emission spectrum of DPAC, indicating a remarkable Stokes shift from solid to dispersed solution. (Modified with permission from ref. (29). Copyright © 2015 Royal Society of Chemistry.) With further analysis of the single crystals of dihydrophenazines (36), we have found that all molecules were bent along the N1−N2 axis, which were more like saddle-shaped (V-shaped) structures rather than planar structures of phenazines (34). We then reasonably suspected that the vibration of two aryl rings along N1-N2 axis resulted in the excited-state configuration transformation from bent to planar state, inducing the red emission of dihydrophenazines. In solid state, the vibration was restricted due to the physical constraint, which blocked the planarization of the electronic structure in excited state, giving the result of a normal blue emission. In solution, however, the molecule structure was actively free and could vibrate to a co-plane in the excited state, inducing the anomalous red emission. Yamaguchi et al. reported a series of planarized 9-phenylanthracene derivatives, and the results showed that the planar and rigid structure definitely caused red-shifted and intense emission due to the effective π–conjuagation (37). To verify our conjecture, we then attempted to control the vibration of dihydrophenazines through changing temperature and viscosity. The results indicated that the optical properties of dihydrophenazines were 25 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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indeed temperature/viscosity dependent. With the decrease of temperature and the increase of viscosity, the blue emission intensity was gradually enhanced accompanied by the gradual reduction of red emission, which results from the hindrance of large-amplitude internal vibration. Moreover, the VIE mechanism can also be easily demonstrated by addition of a poor solvent (water) to a good solvent (THF). Results revealed that the dual emission of compound DPAC could be finely tuned by the ratio of water, caused by the restriction of structural vibration in excited state. In order to further validate the new mechanism, we performed a comprehensive study in cooperation with Prof. Chou and his coworkers, including nanosecond time-resolved spectroscopy, femtosecond dynamics and computational simulation of the reaction energy surfaces. With the in-depth research, a sequential, three-step kinetics (Figure 3) was established (36). The first stage was formation of initial charge transfer state (R*) with the solvent effect; The second stage is structural relaxation to local minimum state (I*) owing to the steric hindrance of N,N′-disubsituted side chain, which can be classified as an intermediate; The ultimate stage is structural vibration to the final planarization state (P*), with the elongation of the π-delocalization over the benzo[a,c]phenazines moiety. The anomalous photophysics of saddle-shaped dihydrophenazines were derived from the combination of the three excited states. Density functional Theory (DFT) calculations for FlPAC revealed that the bent angle of the two aryl rings along the N1−N2 axis underwent a transition from 136°(R*) to 133°(I) and ultimately vibrated to be 160°(P*), which is a thermodynamically stable structure (8, 38). In light of these confirmative phenomena and demonstrations, VIE mechanism proposed for vibration induced configurational change of dihydrophenazine is confirmed.

Figure 3. Three-step kinetic mechanism of compound FlPAC. (Reprinted with permission from ref. (36). Copyright © 2015 American Chemical Society.) 26 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

In merit of the tremendous manipulation potency of these dihydrophenazine molecules, a full-color display is plausible for further applications in luminescent materials. It is noteworthy that these anomalous photophysics properties including dual fluorescence, large Stokes shift and environmental responsive luminescence are not constrained to N,N′-disubstituted dihydrophenazines or simple C-C (C-N) single-bond rotation (37). Instead, any scaffold affording a controllable planar structure with electrons shared in the heterocyclic system has an intrinsic probability for VIE fluorescence mechanism, which can definitely be exploited as a design criteria for future functional fluorescent molecules.

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Technological Applications In principle, the VIE mechanism can be utilized to develop versatile fluorescent materials through the hindrance or control of the vibration. Currently available molecules such as N,N′-disubstituted dihydrophenazine and its derivatives is applicable in various fields including white-light materials, environmental responsive indicators and photoelectronic devices. In reality, there are still tremendous possibilities of applications beyond our imagination. Herein, we demonstrated several aspects of utilities in optoelectronic and sensory systems based on N,N′-disubstituted dihydrophenazines.

Monomolecular White-Light-Emitting Materials High-efficiency white-light-emitting materials have received great attention for their prospective applications in display devices. The strategies followed so far to obtain white emission have relied most on the combination of three (blue, green and red) or two (blue and orange) complementary colors from different fluorophores (27, 28). In comparison with multimolecular white materials, monomolecular white light generators show obvious advantages, such as better stability, better color reproducibility and simpler fabrication procedures, which have become a current research hot spot (39–41). However, the field is hindered because it remains a challenge to excavate applicable monomolecular white light materials with standard white light illumination. As N,N′-disubstituted dihydrophenazines have the privilege of dual fluorescence properties, these small molecules thus qualify themselves as candidates for novel monomolecular white light emitting materials, which is further manifested by experimental data. As is mentioned above, N,N′-disubstituted dihydrophenazines showed two emission bands in solution, located at ~ 460 nm (blue) and ~ 600 nm (reddish orange) respectively and can be tuned for white light on basis of VIE mechanism. A series of novel VIE fluorophores based on dihydrophenazines have been designed and synthesized (21, 42). Experimentally, DPAC and the blue-emitting fluorophore which serves as an energy donor and acceptor, respectively, were ligated via a non-conjugated six-member ring to form M1 (Figure 4), which both performed dual fluorescence (29). The emissions were easily tuned from red to blue through the control of solvatochromism of the blue emission, which is proved as a result of normal ICT effect. Following the VIE mechanism as a 27 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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guiding principle, vibrational modes of the molecule can be adjusted for dual fluorescent manipulation. By addition of good solvent H2O to poor solvent THF, a near-white light emission can be achieved in an appropriate ratio. For compound M1, the solution gave a near white emission (CIE 0.28, 0.27) with the water fraction of 90 %. The color-tunable luminescence ascribed to the restriction of molecular vibration, generating from the aggregation of molecules in poor solvent. Moreover, a white light could also be obtained via control of the intramolecular energy transfer (IET) efficiency. With the increase of solvent polarity (from cyclohexane to acetonitrile), the IET efficiency was reduced due to the decrease of overlapping area between energy donor emission and energy acceptor absorption, resulting in the enhancement of blue emission accompanied with the weakening of red emission. The combination of IET and VIE effect leaded to a close white emission of M1 in acetonitrile (CIE 0.34, 0.36).

Figure 4. Structure of compound M1.

Environmental Sensors Environmental monitoring plays an irreplaceable role in the industrial production and exploration of the nature, where the most frequently measured physics parameters are viscosity and temperature (42). Although conventional measures do exist for viscosity and temperature monitoring, fluorescent sensors draw considerable attention due to the high sensitivity, simplicity, low-cost of implementation and the flexibility in signal readout. On the basis of the VIE mechanism, a change of the external environment is expected to influence the molecular vibration of N,N′-disubstituted dihydrophenazines, resulting in a viscosity/temperature-dependent phenomenon, the ratio of the dual emission intensity in particular. This expectation has been primarily demonstrated by using n-butanol with a relatively high viscosity (2.948 cP/ 293 K), as the media to finely tune the ratiometric emission. Upon temperature decrease, the viscosity increases gradually, causing the red emission to decrease and the blue emission to increase. Prominent luminescence response can thus validate its potential utilization of N,N′-disubstituted dihydrophenazines as viscometers or thermometers. 28 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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In addition, Chen et. al. reported a new approach to monitor cryogenic temperature by use of compound FlPAC in MeTHF (2-methyl-tetrahydrofuran) solution as a soluble thermometer (43). On basis of VIE mechanism, the vibration mode of the excitation state can be inhibited when temperature is lowered down, which is unachievable through other mechanism. Luminescence of the solution varied from blue via magenta to orange, with the temperature increasing from 138 K to 343 K. It was noteworthy that the thermometer performed extremely high sensitivity at low temperature, which reached as high as 19.4 % per K at 138 K. The study not only provides a successful example of a ratiometric fluorescent thermometer, but also demonstrates a practical application of VIE mechanism.

Hole-Transporting Organic Light-Emitting Materials With its advantages in light weight, abundant variations and easy processibility for industrial promotion, organic photovoltalic materials have become an optimal candidate for the substitution of conventional inorganic materials. Hole-transporting materials, which serves as a decisive component in the OLED devices, can be modulated to increase the stability of the overall devices as well as manipulate the energy level of the light-emitting layer and the electrode. In fact, the in-depth investigation of the fluorescence originates in the process of performance perfection of optoelectronic materials. As the electron-donating groups in the hole-transporting materials appears to be electron abundant, sharing the electrons in a planar conjugated scaffold, an intrinsic fluorescence change can be achieved if the planar configuration could be disrupted. N,N′-disubstituted dihydrophenazines, in this case, possess satisfying hole-transport mobility as they usually include electron donors for stronger electropositivity. We therefore designed and synthesized a series of novel N,N′-disubstituted dihydrophenazine derivatives, with HOMO level between 2.83 eV and 5.08 eV, perfectly matching the HOMO level of the anode and light-emitting materials (29). Electronic devices based on compound b has a maximum luminance intensity up to 17437 cd/m2 as well as high current efficiency and density. It can thus be envisioned that analogous molecules can be applied to hole-transporting materials and most importantly, a bi-directional application, where more electron-abundant heterocyclic molecules utilized as hole-transporting materials can be further considered for vibrational motion manipulation, would definitely flourish the two fields, forming a circulated backflow that would propel the design and application of both vibration induced fluorescence molecules and optoelectronic materials.

Summary and Perspectives In conclusion, a series of N,N′-disubstituted dihydrophenazines have been intensively studied on their distinct photophysical properties, such as dual emissions, anomalously large Stokes shift and environmental sensitivity. With the full verification of steady-state spectroscopy, nanosecond time-resolved spectroscopy, femto-picosecond time-resolved spectroscopy and computational simulation, we proposed an elaborately coined mechanism Vibration Induced 29 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Emission (VIE) in order to emphasize the intrinsic phenomenon of our designed molecules. The most distinct feature of this particular mechanism is that it can address a dynamic emission change in color and a very large Stokes shift, which are unachievable through other mechanism. Also, this mechanism is experimentally distinguishable from others because this kind of phenomena can be demonstrated in dispersed system such as solutions. This new mechanism not only enriches the basic fluorescence theories in photoelectric materials, but also provides a new approach to obtain monomolecular emitters with color-tunable fluorescence and environmental-sensitive sponsors. As a primitive example of these photophysical phenomena, N,N′-disubstituted dihydrophenazine and its derivatives are proved be potentially applicable in various organic functional materials. Apart from monomolecular white emitters and thermometers reported, utilization in biomedicine, new energy resources and information materials could also be realized by molecular modification. For applications in biosensors and cell imaging, water solubility of dihydrophenazines need to be largely improved, which could be solved by introducing water soluble groups, such as crown ethers, amino acids and quaternary ammonium salts. It is also feasible to affect the structural vibration by molecular self-assembly, giving a great potential for N,N′-disubstituted dihydrophenazines to apply in molecular machines. It is worth noting that the highly-rich and redox-active properties of dihydrophenazines could add versatility of this kind of molecules and can therefore be utilized as good electron donors in dye-sensitized solar cells (DSSCs), as well as hole-transport materials in organic light emitting diodes. Finally, we envision that as a key supplement of luminescent mechanism, VIE would be a gold mine for researchers to dig over and be regarded as a brand-new concept of molecule design for optoelectronic materials.

Acknowledgments We thank those people with contributions to this research: Academician Benzhong Tang in Hong Kong University of Science and Technology, Prof. Pi-Tai Chou and his group in National Taiwan University, Prof. Hongbing Fu and his group in Institute of Chemistry Chinese Academy of Sciences, and Dr. Zhiyun Zhang, Dr. Wei Huang in our lab.

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Chapter 4

Theoretical Insights into the Mechanism of AIE

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Qian Peng*,1 and Zhigang Shuai*,2 1Beijing

National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, 100190 Beijing, China 2Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, 100084 Beijing, China *E-mails: [email protected] (Q.P.), [email protected] (Z.S.)

AIE has attracted considerable attentions. Better understanding of the mechanism of AIE can help to develop novel AIEgens and exploit novel applications. In this chapter, we have disclosed the microscopic mechanism of AIE by systematically investigating the steric hindrance effect, temperature effect, and aggregation effect on the fluorescence quantum efficiency through theoretical computations. Then we have proposed the plausible ways to probe the mechanism through establishing the relationship between unmeasurable geometrical reorganization energy and experimentally measurable emission and resonance Raman spectroscopy signals.

1. Introduction Traditionally, the investigations of molecular fluorescence are mostly carried out in solution phase. High fluorescence quantum efficiency is closely related to the extent of π-conjugation (1, 2). However, the large planar π-conjugation groups always form over strong intermolecular π-π interactions which would cause fluorescence quenching in concentrated solution or aggregate (1, 2). For a long time, numerous endeavors have been made to recover the solid-state fluorescence by using a variety of complicated physical and chemical methods, which have obtained limited successes (3–5). Since Tang et al. proposed the concept of aggregation-induced emission (AIE) in 2001 (6), a new possibility has been proffered in making highly efficient solid-state luminescent materials. The species of organic luminescent compounds have been unprecedentedly expanded, and both the “bright” and “dark” lumophores can now be chosen as the objects © 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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of study. AIE has become one of the research hot spot in the field of organic luminescence. After considerable efforts in the past decades, hundreds of excellent AIEgens have been synthesized and applied for biological probes, chemical sensor and optoelectronic devices and so on (7). For the further development, it is a desirable but challenging task to reveal the mechanism of AIE at the level of first-principles calculations. Experimentally, considerable attempts have been invested to reveal the mechanism by means of lowering temperature, increasing the viscosity of solution, tuning the molecular stacking through substitution and controlling the aggregation degree and so on. Whereupon, several possible AIE mechanism have been claimed, such as the restriction of intramolecular rotational/vibrational motions (8), J-aggregation formation (9), excimer formation (10), twisting intramolecular charge transfer (11), hydrogen-bonding assistance (12) and so on. Theoretically, intrinsic characteristics have been investigated at various microscopic levels. Motivated by revealing the AIE mechanism, Shuai et al. have developed a vibration correlation function formalism for quantitative evaluations of the radiative and non-radiative decay rates for molecules and aggregate (13). This chapter will largely explore how this method can lead to the computational understanding of AIE at the first-principles level. Li, Hayashi and Lin found the geometrical changes are suppressed due to the molecular packing, which results in small Huang-Rhys factors and low nonradiative decay rate in the solid phase (14). Li and Blancafort adopted a conical intersection model to explain the AIE of diphenyldibenzofulvene (15). The aim of this chapter is to propose the mechanism of AIE through systematically analyzing the effects of steric hindrance, temperature and aggregation on the radiative and nonradiative decay rates at the level of first-principles. And then the mechanism is further confirmed by establishing the relationship between geometrical reorganization energy and experimentally measurable spectral Stokes shift and intensity of resonance Raman spectrum.

2. Theoretical Methodology and Procedure The decay processes for the excited states (Jablonski diagram) are shown in Figure 1. The radiative decay rate constant (fluorescence) from S1 to S0 is denoted as kF, the internal conversion rate constant from S1 to S0 as kIC, intersystem crossing rate constant from S1 to T1 as kISC, the radiative and nonradiative decay rate constants from T1 to S0 as kP and knr, respectively. It should be noted that the higher triplet states and the inverse intersystem crossing from T1 to S1 are not shown in Figure 1 because they are not involved in the AIEgens under investigation here. From Figure 1, the fluorescence quantum efficiency . The intersystem crossing rate is is very slow owing to extremely small spin-orbit coupling and relatively large energy gap between S1 and T1. Hence, kISC can be neglected for the AIEgens involved in the chapter.

36 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 1. Jablonski diagram with straight arrows denoting radiative processes and wavy arrows representing nonradiative ones.

2.1. Spontaneous Radiative Decay Rate In the framework of quantum electrodynamics, the spontaneous photon radiative decay rate per molecule and per unit frequency (so-called the spontaneous emission spectrum) can be expressed as Eq.(1) based on Fermi golden rule and Born-Oppenheimer approximation (16).

Here c is the velocity of light in vacuum.

is the electric

transition dipole moment between the final (f) and initial (i) electronic states and

, which is dependent on the molecular vibrational normal coordinate

Q in principle.

and

is the vibrational wavefucntion of the i

and f electronic states, respectively. is the Boltzmann distribution of the vibration manifolds νi in the initial electronic state. For the strongly dipole-allowed transition, the zero-order term μ0 is dominant and other high-order terms are neglected. represents the adiabatic excitation energy of the two states including the electronic and vibrational states. Applying the Fourier transformation to the delta-functions,

37 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

The analytical path integral formalism for emission spectrum can be obtained as

where

is

the

partition

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of

the

initial

state

manifold.

is the thermal vibration correlation function and . kB is the Boltzmann

(TVCF) with constant.

function

and

represent the multidimensional harmonic oscillator

Hamiltonian. For the k-th normal mode, they are

and

for the initial and final electronic states, respectively. Here,

and

are the k-th mass-weighted nuclear normal momentum operator

and normal coordinate operator. The eigenvalue of Hk is . By using multidimensional Gaussian integrations in the path integral framework, the TVCF can be easily solved analytically (17). More detailed derivations can be found in Ref. (18). The radiative decay rate is the integration of the spontaneous emission spectrum over the whole range,

The radiative decay rate is mainly determined by two factors, electric transition dipole moment and transition energy. The line-shape of the emission spectrum is determined by the overlap between the vibration states of the two electronic states, so-called Franck-Condon factor. In addition, the absorption coefficient of the absorption spectrum has the similar form with the emission one, see Ref. (18).

2.2. Nonradiative Decay Rate Different from the radiative decay process, the internal conversion is caused by the nuclear kinetic energy perturbation and under the Fermi Golden rule framework, its rate reads (19),

where

. 38 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Applying the Condon approximation and Fourier transform, Eq. (5) turns into the solvable TCVF form as

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with the nonadiabatic electronic coupling

and

the TVCF . The analytical solution and detailed derivations can be seen in Ref. (20). Eq. (6) tells us that the nonradiative internal conversion rate is mainly ruled by three factors. They are non-adiabatic electronic coupling, transition energy and the vibronic relaxation. 2.3. Computational Procedures and Details The detailed computational procedures are given in Figure 2. Firstly, the electronic structure calculations are performed to obtain the optimized geometries, harmonic normal modes, excitation energies and electric transition dipole moments at both the electronic ground and excited states, and non-adiabatic electronic coupling and spin-orbit coupling constants between the two electronic states by using quantum chemistry softwares. For the solution phase, the solvent environment can be described using the polarizable continuum model (PCM). For the aggregate, the environment is mimicked through the electrostatic inter-molecular interaction using the combination of quantum mechanics and molecular mechanics (QM/MM) approach. In view of the nature of localized excitation in the solid-state AIE molecules, it is appropriate to build a computational model by setting the central molecule as the QM part and the surrounding ones as the MM part for an enough large cluster (radius more than 50 Å) cut from the crystal structure. Because the number of heavy atoms in a normal AIEgen is in general more than fifty, the density functional theory (DFT)/time-dependent DFT (TDDFT) is an appropriate choice to balance accuracy and computational cost. There are quantum chemistry software programs such as GAUSSIAN 03 (21), TURBOMOLE 6.5 (22), QCHEM (23), NWCHEM (24), and ORCA (25) all possess the modules for DFT and TDDFT for the QM calculation. Our QM/MM calculations are carried out by using ChemShell 3.4 package (26) interfacing TURBOMOLE for QM and DL-POLY (27) for MM part. General Amber Force Field (GAFF) and the electrostatic embedding scheme are adopted to treat MM part and the QM/MM interactions, respectively. Based on the optimized geometrical Cartesian coordinates and normal modes, the displacement vector Di(f) between the two electronic state potential energy parabolas can be obtained by , where 4x is the displacement in Cartesian coordinates and the Li(f) is the mass-weighted normal modes of the initial (final) state. The correlation and difference between two electronic state potential energy parabolas is expressed as 39 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

for the

k-th normal mode. Here, S is called Duschinsky rotation matrix (DRM) (28), which measures the mixing degree between different normal modes of the initial

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and final electronic states and is calculated by . Further combining the electronic transition dipole moment, non-adiabatic coupling or spin-orbit coupling constants, the spectra and different decay rates could be obtained according to the different modules selected in our home-built MOMAP program (29). The MOMAP program has a good interface with the popular quantum chemistry software mentioned above and can be downloaded freely. So far, it has been downloaded more than 1600 times by researchers from all over the world.

Figure 2. Structure of MOMAP: the computational procedures of molecular photophysical properties.

3. The Mechanism of AIE AIEgens always emit weakly in well solute phase while become strongly fluorescent in aggregate phase. This indicates that the radiative or non-radiative decay rate determining the quantum efficiency would be sensitive to different environments. It has been the center of controversy over whether AIE is caused by the enhancement in radiative decay or reduction in non-radiative decay, or the synergetic effect of both of them. Another question is about the role of intramolecular motions with respect to intermolecular interaction. In the following, we systematically study the effects of intramolecular steric hindrance, temperature and aggregation states on the radiative and non-radiative decay rates for the typical AIEgens. Through analyzing the key factors governing radiative and non-radiative decays, it is expected to figure out the mechanism of AIE and establish the structure-property relationships. 40 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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3.1. Steric Hindrance Effect Silacyclopentadines (siloles) are the first and typical AIEgens, which exhibit high luminescence quantum efficiency in the solid phase relative to that in solution at room temperature (30). In order to look into the influence of the intramolecular motions on luminescence, a set of silole regioisomers were synthesized by attaching alkyl groups to the periphery rings to tune the molecular steric hindrance and their luminescence quantum efficiencies were measured (31). As found, the bulky alkyl groups dramatically boost the fluorescence efficiency. For instance, the iPr-HPS with isopropyl groups linking to 3,4-position phenyl rings of 1,1,2,3,4,5-hexaphenylsilole show much stronger fluorescence with efficiency of 83% in acetone compared with the parent HPS of 0.3% in cyclohexane. Here, we comparatively investigate the radiative and non-radiative decay rates of the two compounds with and without iPr-substitution (iPr-DMTPS and DMTPS) through quantitatively calculations and clearly reveal luminescence mechanism at the microscopic level (32, 33).

Figure 3. The optimized molecular structures and frontier orbitals of DMTPS and iPr-DMTPS. Both DMTPS and iPr-DMTPS are symmetrical to some extent with equal dihedral angles between the central silacycle and peripheric phenyl rings at 2, 5positons or 3, 4-postions (seen in Figure 3). It also is obvious that iPr-DMTPS is much spaciously crowed and the iPr-substituted phenyl rings twisted to larger angles of 65.5° than the corresponding unsubstituted ones of 58.0° in DMTPS. As a result, more space is released along the direction of C2-C5 in the plane of silole core and the torsional dihedral angles of the phenyl rings at 2,5-positions relative to silacyle decrease from ca. 48.5° to 21.4°. This coplanarity favors the molecular electric dipole transition. Because the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) mainly distribute in the central silacycle and phenyl rings at 2, 5-positions for the silole derivatives seen in Figure 41 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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3. And the increase of the conjugation degree along the direction can induce larger transition dipole moment and assist the radiative decay process. As expected, the calculated radiative decay rate of iPr-DMTPS is 7.4×108 s−1, larger than 1.2×108 s−1 of DMTPS.

Figure 4. The reorganization energies versus the normal modes in DMTPS (a) and iPr-DMTPS (b). Contrary to the radiative decay rate, the calculated nonradiative decay rate of iPr-DMTPS is significantly smaller than that of DMTPS under the framework of the linear coupling harmonic model. From the rate theory presented in section 2, it is known that the non-adiabatic electronic coupling and vibrational relaxation are the two key factors to determine the nonradiative decay rate. The latter is more sensitive to the modification of the molecular geometrical structure than the former. Because the vibrational relaxation is characterized via the excited state reorganization energy which is directly determined by the degree of modification in geometrical structure between the excited and for the k-th normal ground states through the formula of mode. Therefore, the reorganization energy represents the ability to accept the excessive excited-state energy of the intramolecular motions and the normal modes with large reorganization energy are regarded as the main nonradiative decay channels. Figure 4 compares the reorganization energy of DMTPS and iPr-DMTPS. The total reorganization energy of DMTPS is 7550 cm-1, much larger than 3821cm-1 of iPr-DMTPS. There are four low-frequency ( 40 cm−1) in the low frequency region when going from solution to aggregate. More importantly, the RRS intensities are gravely reduced in the low frequency region while the ones are almost unchanged in the high frequency region for HPDMCb in aggregate relative to solution. These well characterize the nature of the molecular vibration change in the excited-state decay process in different environment. As expected, the RRS signals perfectly match the reorganization energy for all the normal modes. For instance, in solution the low-frequency modes of 24 cm−1, 51 cm−1 and 78 cm−1 have strong RRS signals and large reorganization energy while only the normal mode of 70 cm−1 in aggregation displays significant RRS signal and 53 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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reorganization energy. However, the high-frequency vibrational motions are unaffected by the environment with similar RRS and reorganization energy no matter in solution or aggregate. As assigned, the low-frequency normal modes are related to the out-of-plane rotational/twistable motions of the peripheral phenyl rings, which are always spurred in solution but are clogged owing to the surrounding space restriction and intermolecular interaction in the solid phase. Consequently, the nonradiative decay outweighs the radiative decay for the excited-state AIEgens with active intramolecular motions in solution and the weak even undetectable fluorescence. But in aggregate, the radiative decay process dominates the nonradiative decay one for AIEgens with restricted intramolecular motions leading to strong fluorescence. Thus, we reveal the mechanism of AIE by RRS through theoretical computations.

Figure 16. The resonance Raman spectroscopy (σ(ω)/ω) and the reorganization energy (λ) for HPDMCb in solution and aggregate. Reprinted with permission from Ref. (52). © 2015 by the American Chemical Society.

5. Summary and Outlook In this chapter, we have disclosed the mechanism of AIE at the molecular level through theoretical and computational studies. Firstly, we have presented the general radiative and nonradiative decay rate formalisms by taking into account the difference between the electronic excited-state and ground-state potential energy surfaces at the level of harmonic oscillator, and integrated them into our home-built MOMAP program package. The rate calculations require molecular parameters informations resulted from standard quantum chemistry packages such as the equilibrium structure coordinates, frequencies, normal mode matrixes of the ground and excited states, excitation energy, transition dipole moment, non-adiabatic coupling and spin-orbit coupling etc. In the calculations, the solvent effect is considered by using the polarizable continuum model (PCM) and the aggregation effect is mimicked through the electrostatic molecular interaction by using the combination of quantum mechanics and molecular mechanics (QM/MM) approach. 54 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Then, it is quantitatively investigated about the steric hindrance effect, temperature effect, and the aggregation effect on the radiative and nonradiative decay rates. Through the detailed comparison and analysis, it is found that for the AIEgens under examination, (i) the radiative decay rate is insensitive to internal steric hindrance, and external temperature and molecular packing as long as the extent of molecular conjugation of lumophore group is not changed substantially; (ii) in contrast, the nonradiative decay rate is extremely sensitive to the environment change. Because the geometry relaxation ability controls the nonradiative decay to a large extent and the decisive normal modes relate to the rotatable/twistable intramolecular motions with low frequency. In solution, the molecular rotatable/twistable motions are always spurred at room temperature and make nonradiative decay rate outweigh the radiative decay rate, which leads to the molecule non-emissive. While they are clogged at low temperature or by the steric hindrance and the surrounding space restriction and intermolecular interaction in the solid phase, which slows the nonradiative decay. As a result, the radiative decay starts to dominate and the strong fluorescence is observed. So far, the mechanism of AIE has been unmasked at the molecular level. Finally, through establishing the relationship between the geometrical reorganization energy and the experimentally measured spectroscopy signals, the plausible ways to probe the microscopic mechanism are proposed. According to the Franck-Condon principle, the maximum peak of the optical spectrum always appears at the vertical transition point and the Stokes shift can be regarded as the total reorganization energy in the ground and excited state potential energy surfaces. The decrease of Stokes shift induced by the decreasing reorganization energy causes the aggregation-induced blue-shift emission when going from solution to aggregate, which is a typical spectroscopy character of the AIEgens with RIR mechanism. Furthermore, the ratio of the RRS intensity to the frequency (σ(ω)/ω) is proportional to the reorganization energy of every vibration mode of a molecule. The nature of RRS intensity σ(ω)/ω exactly reflects the change character of the reorganization energy for the AIEgen moving from solution to aggregate. And the mechanism of AIE is confirmed by the RRS signals through theoretical computations, which is expected to be proved by future experiments. Accurately describing excited state structure and decay processes is still a longstanding challenge for both computational chemistry and physics because both the electron-electron correlation and electron-phonon coupling are required to be involved (56). At present, there is no ready-made tool to provide the luminescent properties for all kinds of molecules in the gas phase, let alone, the intermolecular interactions that bring greater complication to depict the excited state decay in the aggregate phase. In the chapter, the QM/MM approach only applies for dealing with molecular crystal. The molecular dynamics should be combined for irregular amorphous aggregates. MM polarization by the electron-density change of the QM molecule in the excited state has not been taken into consideration. Moreover, in the light of only considering one QM molecule in the QM/MM computational model, the effect of the intermolecular charge transfer or exciton interaction on AIE has not been considered. All these are being actively pursued.

55 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Acknowledgments This work is supported by the National 973 Program through Grant Nos. 2013CB834703 and 2015CB655002 and the National Natural Science Foundation of China through Grant Nos. 21290190, 21473214, and 91233105. Contributions from the following collaborators are greatly acknowledged: Shiwei Yin, Qunyan, Wu, Tian Zhang, Yujun Xie.

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Chapter 5

Aggregation-Induced Emission Materials: The Art of Conjugation and Rotation Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch005

Jie Yang, Jing Huang, QianQian Li, and Zhen Li* Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China *E-mail: [email protected] or [email protected]

Since the AIE phenomenon was first named by Tang’s group in 2001, more and more attention has been attracted for its great promising applications in the opto-electronic fields. In order to fully excavate the potential of AIE characteristic, the mechanism and applications should be both explored in details. In this chapter, we mainly focus on the restriction of intramolecular rotation (RIR) mechanism through the combination of experimental results and theoretical calculations, and the applications in OLEDs including some strategies to develop blue AIE luminogens, AIE host , and AIE PLEDs.

1. Introduction Organic luminogens with strong solid-state emission have attracted much attention due to their huge applications in the fundamental fields as biological probes, chemical sensors and particularly organic light-emitting diodes (OLEDs) (1, 2). However, the traditional organic luminogens with planar conformations, often suffer from the notorious aggregation-caused quenching (ACQ) effect or aggregation-induced red-shifted emission by the strong intermolecular π-π stacking which has been documented for more than half a century since Förster’s discovery of the concentration quenching effect in 1954, badly impeding their practical applications (3). In complete contrast to ACQ effect, the luminogens with aggregation-induced emission (AIE) characteristic might enjoy many advantages for the high performance of opto-electronic devices.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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The AIE phenomenon was first reported by Tang’s group in 2001: some luminogens exhibit weak or even non-emission in solution, but much enhanced luminescence in the aggregate state, such as in solid state or nanoparticles, which was termed as aggregation-induced emission (AIE) or aggregation-enhanced emission (AEE) (4). As shown in Figure 1, 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS), the first reported AIE luminogen, shows non-emission in dilute solution but turns to be highly-emissive in aggregate state (5). When acting as the emitter layer in OLED, it exhibits an excellent performance with the current efficiency and external quantum efficiency up to 20.0 cd A-1 and 8%, respectively, much higher than that of traditional light-emitting materials and showing great advantages for its AIE property. Inspired by the promising application in highly efficient electroluminescent devices and other related fields, more and more attention has been paid to the AIE research, especially the inherent mechanism and practical applications (6).

Figure 1. The propeller-shaped luminogen of 1-methyl-1,2,3,4,5pentaphenylsilole (MPPS) is non-emissive in dilute solution but turns to be highly-emissive for the restricted intramolecular rotation (RIR) effect in aggregate state.

2. Mechanism 2.1. Evidences: RIR Is Main Mechanism for AIE Effect Systematic studies have suggested that the restriction of intramolecular motion (RIM) is the main cause of the AIE effect including the restriction of intramolecular rotation (RIR) and the restriction of intramolecular vibration (RIV) (7). In this chapter, we mainly focus on the RIR mechanism. Fundamental physics teaches us that any molecular motions consume energy. As shown in Figure 2, tetraphenylethene (TPE) is constructed by four phenyl rings surrounding an ethylene group with a propeller-shaped conformation, which allows the four phenyl rings to rotate freely in the solution state, and provides a relaxation channel for the excited state to decay with non-emission. Once aggregated, the intramolecular rotation is restricted due to physical constraints, thus, the energy is consumed from the radiative transition, namely fluorescence. To check the validity of RIR mechanism, a series of external control tests have been conducted. 62 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 2 shows the piezochromism, viscochromism and thermochromism in detail. We may find that aggregation, high pressure, high viscosity, low temperature all could restrict the intramolecular rotation and realize the enhanced emission, which powerfully proves the accuracy of RIR as the main mechanism for AIE effect (3).

Figure 2. The propeller-shaped luminogen of tetraphenylethene (TPE) is non-emissive in dilute solution but turns to be highly-emissive for the restricted intramolecular rotation (RIR) effect through aggregation, high pressure, high viscosity, low temperature and structural modification.

Except for the external control tests, the structural modification as internal control tests also was conducted. Hexaphenylsilole (HPS), an analog of the first example of AIE molecules, is considered to be a typical AIE luminogen (Figure 3). It is non-emissive in dilute solution as the free rotation of the six peripheral phenyl rings. However, when the stereo-hindrance groups of isopropyl ones were introduced to yield HPS3,4, it shows strong green emission in solution. For example, in its dilute acetone solution, the quantum yield of HPS3,4 was found to be as high as 83%, 2-3 orders of magnitude higher than those of the ‘normal’ siloles (0.031-0.51%, with ~0.1% being most typical) (8). Thus, the minor structural change results in big differences from AIE to AEE behavior, as a result of the steric-hindrance effect from isopropyl groups, confirming the RIR mechanism of the AIE effect. Similar phenomenon was also observed in TPE and its derivatives (Figure 3): TPE is nearly non-emissive in solution, while TPE-TM gives a strong blue emission in THF solution with quantum yield up to 64% (9). The only difference is the absence or presence of methyl groups, which could make the rotation of the phenyl rotors much more difficult for TMTPE even in solution. Hence, the effective restriction of the rotation for the phenyl rotors would lead to the strong emission in solution, as powerfully proved by the cases of HPS3,4 and TMTPE. 63 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 3. HPS and TPE are typical AIE luminogens showing non-emission in solution. However, when stereo-hindrance groups such as isopropyl and methyl groups were introduced to get HPS3,4 and TMTPE, they show strong emissions in solutions as well as in aggregates.

2.2. Relationship between Rotors and AIE Effect The aforementioned studies have clearly demonstrated RIR as the main mechanism for the AIE effect. Then, one question arose, if once the bonded rotors exist in a propeller-shaped luminogen, could the AIE characteristic be obtained? The TPE system is simple, but can we develop even simpler AIE systems? Taking these into consideration, a pretty simple system was developed based on polyphenylbenzenes, in which one or two or three phenyl rings as rotors were linked to the central benzene core to yield compounds 1, 2 and 3 (Figure 4) (10). The results answer the questions perfectly: all of them are AIE-active. In dilute acetone solutions, compounds 1–3 are non-emissive. However, when large amounts of water were added to their acetone solutions, they tended to aggregate and gave strong emissions with the enhancement higher than 150 times from solution to aggregation states. What’s more, when increasing solvent viscosity and/or decreasing solution temperature, their PL emission could also be enhanced as a result of the restricted rotation of the peripheral phenyl rings, further proving that the RIR process is indeed involved in the AIE system. Then another propeller-shaped system was developed also based on benzene core, in comparison to compound 1-3, the only difference is that the peripheral phenyl rotors were replaced by carbazolyl units (Figure 5). The different rotors also led to different luminescent properties. For compounds 4-6, they are all emissive in the solution state and exhibited enhanced emissions in aggregation, with the typical AEE effect (9). However, their fluorescent quantum yields (ФF) in solutions decrease with the increasing number of the carbazolyl units from 4 (47.4%) to 5 (30.1%) to 6 (16.2%), indicating that with more carbazolyl units, more energy would be consumed from the intramolecular rotation, leading to lower ФF values. This phenomenon suggests that the RIR effect also exists in AEE system. 64 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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The changes of the ФF values are easy to be understood and confirm the RIR mechanism in another viewpoint. In order to repeat the above phenomenon and further confirm the RIR mechanism, we synthesized another system based on the carbazolyl core in which mono-rotor or dual-rotors were introduced (Figure 6) (11). For compounds 7, 9, and 11 with mono-rotor including the phenyl or biphenyl or carbazolyl rotor, they all show relatively higher ФF values in solution compared to those of compounds 8, 10, 12 with dual-rotors. These results also proved that more rotors would consume more energy from the intramolecular rotation and lead to lower ФF values. So this kind of phenomenon could be repeated and the RIR effect is widely existent in these propeller-shaped luminogens, confirming the RIR effect as the main mechanism for AIE and AEE characteristics.

Figure 4. Chemical structures of polyphenylbenzenes 1-3 (AIE).

Figure 5. Chemical structures of compound 4-6 (AEE).

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Figure 6. Examples of carbazole-based luminogens 7-12 and their fluorescent quantum yields (ФF) in solutions.

2.3. Relationship between Rotational Energy Barrier and AIE Effect In the above-mentioned systems, we could see that different kinds of rotors would lead to different luminescent properties from AIE to AEE effect. In order to further study the correlation between the structure and AIE or AEE properties, we designed another system based on the pyrene and TPE moieties: Py-4MethylTPE and Py-4mTPE, in which the pyrene unit act as the core and meta-tetraphenylethene or methyl-tetraphenylethene groups as rotors (Figure 7) (12). As shown in Figure 7b, the small structural changes result in big differences from AIE to AEE behavior. In order to reveal the correlation in theory, DFT/TD-DFT calculations were carried out on Py-4MethylTPE and Py-4mTPE by using the B3LYP/6-31G* basis set. We calculated the rotational energy barriers in the ground state for isolated Py-4MethylTPE and Py-4mTPE, and found that Py-4mTPE is more rigid than Py-4MethylTPE owing to its better conjugation, which leads to the transition from AIE to AEE and confirms the RIR as the main mechanism for AIE effect again.

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Figure 7. (a) The structures of Py-4MethylTPE and Py-4mTPE; (b) Plots of fluorescence quantum yields of Py-4MethylTPE and Py-4mTPE determined in THF/H2O solutions using 9,10-diphenylanthracene (Ф = 90 % in cyclohexane) as standard versus the fraction of water (fw). Insets: Photographs of Py-4MethylTPE and Py-4mTPE in THF/water mixtures (fw = 0 and 90 or 99 %) taken under the illumination of a 365 nm UV lamp; (C) the rotational energy barriers for isolated Py-4MethylTPE and Py-4mTPE in the ground state.

3. Application in OLEDs OLEDs have attracted increasing attention because of their huge potential in the applications as new display devices and solid state lighting. At the same time, AIE materials with enhanced solid state emissions might be good candidates as OLED emitters. Because of its grand AIE characteristic and simple structure, TPE unit was widely used to construct AIE materials ranging from deep blue to near-IR emissions (13). Particularly, through decorating traditional ACQ luminogens with TPE units, lots of highly efficient AIE molecules were obtained. Triphenylamine (TPA) and its derivatives are well-known for their high hole mobility, but showing typical ACQ effect, which much impedes their device 67 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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performances. Tang et. al decorated TPA dimer (DTPA) with four TPE units to yield 4TPEDTPA (Figure 8) (14). The resultant compound shows splendid AIE effect and excellent OLED performance. The device based on 4TPEDTPA gives a maximum current efficiency up to 8.0 cd A-1 even without the hole-transporting layer. Pyrene, another famous ACQ unit, was also decorated by four TPE moieties to obtain an efficient AEE molecule of TTPEPy, which shows a better performance with the current efficiency and external quantum efficiency up to 12.3 cd A-1 and 4.95%, respectively, closely approaching the theoretical limit for a singlet OLED (5%) (15). However, for these luminogens, their EL emissions are nearly out of the blue region owing to their extended conjugation. For three primary colors, green and red materials have been well developed, while efficient and stable blue materials are still scarce as a result of their intrinsic wide bandgap.

Figure 8. Examples of transition from the ACQ to AIE (AEE)—4TPEDTPA and TTPEPy and their OLED performances (OLED configurations-ITO/4TPEDTPA (30 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al and ITO/NPB (60 nm)/TTPEPy (26 nm)/TPBi (20 nm)/LiF (1 nm)/Al).

3.1. Some Strategies To Realize Blue Emission of AIE Luminogens TPE itself can emit deep blue emission (445 nm), but the corresponding OLED efficiency is not so good, just 0.45 cd A-1. Once some aromatic rings are linked to TPE moieties, the resultant AIE luminogens could possess higher device efficiencies, but the emissions would be red-shifted to outside of the blue region. So on the one hand, the introduction of additional aromatic rings could improve the OLED efficiency, meanwhile, on the other hand, the bonded aromatic moieties would extend the π-conjugation, leading to a red-shifted emission. Thus, to achieve pure blue or even deep blue emission, it is required to restrict the π-conjugation between TPE and the introduced aromatic rings. Thanks to the great effort of scientists, five main strategies are developed to control the intramolecular 68 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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conjugation and realize the blue emission: the crystallization-induced blue-shifted emission, the modification of the linkage mode, making a twisted conformation, the interruption of the conjugation, and the reduction of the conjugation unit. Figure 9 shows the basic models for these five strategies, by applying these, some good blue AIE luminogens have been designed (16).

Figure 9. Some models to control the conjugation and these would make much contribution to realize the blue emission of AIE materials.

3.1.1. Crystallization-Induced Blue-Shifted Emission For traditional luminogens, they often suffer from crystallization-induced quenched or red-shifted emission due to the close π-π stacking in their crystalline states. On the contrary, AIE materials were found to possess bluer and enhanced emission in the crystalline states compared to their amorphous types for their twisted conformations. Dong et al. have done a lot of work on this topic, however, seldom use this unique property to construct blue OLEDs (17, 18). In 2012, Li et al developed a blue AIE system based on a benzene core and TPE as peripheral rotors by utilizing crystallization-induced blue-shifted emission. As shown in Figure 10, Ph2TPE and Ph3TPE with peripheral TPE units show bluer emissions than PhTPE (19). It seems strange that Ph-2TPE and Ph-3TPE possess extended conjugation moieties and bluer emissions simultaneously, but it is actually reasonable. With the introduction of TPE moieties, the substituted benzene cores become more and more crowded and liable to form crystalline states, leading to the blue-shifted emission. Among them, Ph2TPE shows the bluest EL emission at 457 nm with a current efficiency up to 2.3 cd A-1 at CIE coordinates (0.16, 0.15). 69 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 10. Chemical structures of PhTPE, Ph2TPE and Ph3TPE and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/MoO3 (10 nm)/NPB (80 nm)/PhTPE or Ph2TPE or Ph3TPE (20 or 30 nm)/TPBi (30 nm)/LiF (1 nm)/Al).

3.1.2. To Modify the Linkage Mode Among the AIE luminogens, TPE can emit deep blue emission (445 nm) with the maximum current efficiency of only 0.45 cd A-1. When two TPE units were linked together to give BTPE, its efficiency could increase to 7.26 cd A-1, however the EL emission was red-shifted to sky blue (488 nm) owing to the extended conjugation. Basic organic chemistry tells us that ortho, meta, and para positions possess different conjugation effects. Thus, is it possible to develop blue AIE system by utilizing this strategy to control the intramolecular conjugation? In 2013, Li et al synthesized four BTPE derivatives, mTPE–pTPE, oTPE–pTPE, mTPE–mTPE, and oTPE–mTPE, by modifying the linkage mode (Figure 11) (20). Unlike their analogous BTPE (488 nm), these luminogens all show deep blue emissions from 435 nm to 459 nm. Among them, mTPE-pTPE exhibits the best performance with a current efficiency of 2.8 cd A-1 at CIE coordinates of (0.16, 0.16). As for oTPE-mTPE with bluest emission (435 nm), its OLED performance is inferior (1.8 cd A-1) owing to the much twisted conformation. So a balance should be adjusted between high efficiency and effective conjugation length.

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Figure 11. Chemical structures of BTPE, mTPE–pTPE, mTPE–mTPE, oTPE–mTPE, and oTPE–pTPE and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/MoO3 (10 nm)/NPB (60 nm)/mTPE–pTPE or oTPE–pTPE or mTPE–mTPE or oTPE–mTPE (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al).

3.1.3. To Make Twisted Conformation It is noteworthy that to make twisted conformation is another efficient approach to restrict the conjugation in basic organic chemistry. So also from BTPE, we designed another blue AIE system: Methyl-BTPE, Isopro-BTPE, Ph-BTPE and Cz-BTPE by the introduction of additional resistance groups between two TPE units (21). In order to investigate the structure-property relationship in theory, DFT/TD-DFT calculations were carried out on these four BTPE derivatives by using the B3LYP/6-31G* basis set. In Figure 12, the dihedral angles between the adjacent phenyl blades of the two TPE units are 88.6°, 84.5°, 57.4° and 50.0° for Methyl-BTPE, Isopro-BTPE, Ph-BTPE and Cz-BTPE respectively, much more twisted than that of BTPE (35.5°). Their EL emissions also follow the change trend of dihedral angles: with the increase of dihedral angles, their emissions were blue-shifted from 488 nm (BTPE) to 479 nm (Cz-BTPE) to 467 nm (Ph-BTPE) to 451 nm (Methyl-BTPE and Isopro-BTPE). Among them, Cz-BTPE exhibits the highest current efficiency of 3.7 cd A-1 at CIE coordinates of (0.17, 0.26). 71 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 12. Chemical structures of Methyl-BTPE, Isopro-BTPE, Ph-BTPE, and Cz-BTPE and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/MoO3 (10 nm)/NPB (60 nm)/Methyl-BTPE or Isopro-BTPE or Ph-BTPE or Cz-BTPE (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al).

3.1.4. Combination of Modifying the Linkage Mode and Making Twisted Conformation In the abovementioned examples, we have gotten some blue AIE materials through modifying the linkage mode or making twisted conformation. However, the highest current efficiency is only 3.7 cd A-1, leaving much room for the further improvement. So how about the combination of these two strategies? Could we develop some blue AIE materials with higher efficiency and realize the “one plus one larger than two” effect? In 2014, our group designed a blue AIE system based on TPA and TPE derivatives: TPA–3mTPE, TPA–3MethylTPE, MethylTPA–3pTPE, MethylTPA–3mTPE and MethylTPA–3MethylTPE (Figure 13), which were derived from the AIE molecule of 3TPETPA, reported by Tang et al. in 2010 (22). In the system, methyl groups were introduced to make a more twisted conformation and the linkage modes were tuned from para to meta to control the intramolecular conjugation. Their EL emissions are all in the blue region ranging from 459 to 480 nm for the restricted conjugation totally different from that of 3TPETPA (493, 511 nm). Among them, MethylTPA-3pTPE gives the best performance with the current efficiency and external quantum efficiency up to 8.03 cd A-1 and 3.99% respectively at CIE coordinates of (0.17, 0.28). MethylTPA-3mTPE shows the bluest emission (459 nm) while 72 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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retaining a comparable efficiency of 2.58 cd A-1 at CIE coordinates of (0.17, 0.19) owing to the combination of modifying the linkage mode and making twisted conformation. What’s more, they all show good hole-transporting ability inherited from the constructing block of TPA groups. When the hole-transporting layer was eliminated with the OLED configurations of ITO/MoO3 (10 nm)/ emitters (75 nm)/TPBi (35 nm)/LiF (1 nm)/Al, they could still retain similar performances. For example, the device based on MethylTPA-3pTPE exhibits better EL performance with the current efficiency and external quantum efficiency up to 6.51 cd A-1 and 3.39% respectively at CIE coordinates of (0.18, 0.25), which is comparable with those obtained from the standard device. Thus, through the combination of modifying the linkage mode and making twisted conformation, efficient blue AIE materials were obtained and “one plus one larger than two” effect was successfully realized.

Figure 13. Chemical structures of 3TPETPA, TPA–3mTPE, TPA–3MethylTPE, MethylTPA–3pTPE, MethylTPA–3mTPE and MethylTPA–3MethylTPE and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/NPB (60 nm)/3TPETPA (20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al and ITO/MoO3(10 nm)/NPB (60 nm)/TPA–3mTPE or TPA–3MethylTPA or MethylTPA–3pTPE or MethylTPA–3mTPE or MethylTPA–3MethylTPE (15 nm)/TPBi (35 nm)/LiF (1 nm)/Al). 73 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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3.1.5. To Interrupt the Conjugation It is easy to understand that the introduction of some appropriate less or even unconjugated moieties to interrupt the conjugation would be another effective approach to avoid the extension of conjugation and restrict strong intramolecular charge transfer, then lead to blue-shifted emission. In 2014, Li et al designed another blue AIE system based on TPA and TPE through a unconjugated fluorene core: TPE-pTPA and TPE-mTPA, while TPE-2pTPA and TPE-2mTPA are for extension (Figure 14) (23). Due to the presence of the sp3-hybridized carbon atom, the moieties on the 9,9’-positions are almost perpendicular, so the conjugations between TPE and TPA are effectively restricted. TPE-pTPA and TPE-mTPA both show deep blue emission at about 450 nm, totally different from that of TPATPE (492 nm) (24). Among the four blue AIE luminogens, TPE-pTPA gives the best performance with a current efficiency up to 3.37 cd A-1 at CIE coordinates of (0.16, 0.16), indicating that the introduction of fluorene groups could restrict the conjugation effectively as well as retain the EL efficiency for its unconjugated conformation and high carrier mobility.

Figure 14. Chemical structures of TPE–pTPA, TPE–mTPA, TPE–2pTPA, TPE–2mTPA, and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/PEDOT:PSS (30 nm)/NPB (30 nm)/TPE–pTPA or TPE–mTPA or TPE–2pTPA or TPE–2mTPA (10–30 nm)/TPBi (10 nm)/Alq3 (30 nm)/Ca:Ag). 74 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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3.1.6. To Reduce the Conjugation Unit

Figure 15. Chemical structures of DPA–PPB, Cz–PPB, DPA–TTP–CN, Cz–PPB–CN, mDPA–PPB–CN, mCz–PPB–CN, Cz–TPB–CN and BmPyPb (host) and the EL performances of the corresponding blue OLEDs (OLED configurations-ITO/MoO3 (10 nm)/NPB (60 nm)/DPA–PPB or Cz–PPB or Cz–PPB–CN or mDPA–PPB–CN or mCz–PPB–CN or Cz–TPB–CN or DPA–TTP–CN (nondoped 1) (30 nm)/TPBi (30 nm)/LiF (1 nm)/Al and ITO/MoO3 (10 nm)/NPB (60 nm)/mCP (10 nm)/DPA–TTP–CN (nondoped 2) (30 nm)/TPBi (30 nm)/LiF (1 nm)/Al and ITO/MoO3 (10 nm)/NPB (40 nm)/mCP (10 nm)/DPA–TTP–CN (nondoped 3) (15 nm)/TPBi (30 nm)/LiF (1 nm)/Al).

Because of the intrinsic conjugation length of TPE and silole, it is very difficult to utilize them as building blocks to develop deep blue AIE materials with CIE values (x≤0.15, y≤0.10) (25). Thus, some other AIE building blocks with efficient emission but low conjugation are in great demand. Pentaphenylbenzene (PPB), which has been proven to be AEE active, might be a good candidate. PPB shows weak conjugation and purple emission at about 355 nm, which violet-shifts by as much as 90 nm in comparison with TPE (445 nm) (26). So our group utilized pentaphenylbenzene as a platform to design a series of efficient deep blue emitters: DPA–PPB, Cz–PPB, DPA–TTP–CN, Cz–PPB–CN, mDPA–PPB–CN, mCz–PPB–CN, and Cz–TPB–CN (Figure 15) (27). Due to the AEE property of 75 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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PPB, these luminogens exhibit enhanced emission in the deep blue or blue-violet region, ranging from 420 to 451 nm. Among them, DPA-TTP-CN shows the best performance, and the nondoped OLED device with the configuration of ITO/MoO3 (10 nm)/NPB (60 nm)/mCP (10 nm)/DPA–TTP–CN (30 nm)/TPBi (30 nm)/LiF (1 nm)/Al gives a maximum current efficiency up to 2.0 cd A-1 at CIE coordinates of (0.15, 0.08). Furthermore, DPA–TTP–CN also displayed promising application as guest materials for its enhanced emission in the solid state. Its corresponding OLED device with a configuration of ITO/MoO3 (10 nm)/NPB (60 nm)/mCP (10 nm)/ BmPyPb:50%DPATTP-CN (20 nm)/BmPyPb (10 nm)/TPBi (30 nm)/LiF (1 nm)/ Al exhibits a better performance with current efficiency and external quantum efficiency up to 4.51 cd A-1 and 3.98% respectively at CIE coordinates of (0.16, 0.11). 3.2. AIE Host in OLEDs It is well known that for AIE luminogens, the restriction of the intramolecular rotation in solid state could block the nonradiative path and contribute much to the efficient emission, which is almost the same as one of the basic requirements for a good host materials in OLEDs. So could we utilize AIE materials as the host to achieve good device performance, not the general role of emitting layer? Because of the splendid AIE effect, easy functionalization and wide bandgap, we chose TPE as the build block to construct AIE host materials. However, our experimental results reveal that TPE is hole-dominated with the hole mobility about 10-4 cm2 V-1 s-1 and electron mobility about 10-5 cm2 V-1 s-1, which could not achieve the balance of the hole and electron injection and meet the demand of host materials. So oxadiazole, a well-known electron-transporting moiety, was chosen as the co-block to build new AIE host materials—Oxa-pTPE and Oxa-mTPE (Figure 16) (28). Oxa-pTPE and Oxa-mTPE both show bipolar transporting characteristics, owing to the hole-transporting ability of TPE and the electron-transporting ability of oxadiazole. Their carrier mobilities were measured by the time-of-flight (TOF) transient photocurrent technique and all in the same order of magnitude (about 10-4 cm2 V-1 s-1), showing the promising application as host materials. When fabricated in OLED devices as host materials, they both exhibited outstanding performances with a current efficiency up to 9.79 cd A-1 at CIE coordinates of (0.15, 0.34) for Oxa-pTPE and 9.82 cd A-1 at CIE coordinates of (0.15, 0.33) for Oxa-mTPE, respectively, by reasons of their bipolar transporting and specific AIE feature. Inspired by the abovementioned results, Wang et al. developed another AIE host system based on TPE and phosphine oxide (PO) of TPEDPO, TPEPO, DTPEPO and TTPEPO, in which TPE is responsible for the hole transporting property and phosphine oxide for the electron transporting property (Figure 17) (29). By adjusting the proportion of TPE and PO moieties, the electronic nature of these molecules could be successfully tuned from n-type (TPEDPO) to ambipolar (TPEPO) and then to p-type (TTPEPO). Among them, TPEPO gave the best performance as host material with a maximum current efficiency of 9.7 cd A-1 at CIE coordinates of (0.15, 0.35), owing to its more balanced carrier transporting 76 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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ability and specific AIE feature. Thus, this successful research confirms bipolar AIE luminogens as good candidate for host materials again.

Figure 16. Chemical structures of Oxa-pTPE, Oxa-mTPE and BUBD-1 (guest) and the performances of the corresponding blue OLEDs (OLED configurations-ITO/NPB (10 nm)/Oxa-pTPE:6% BUBD-1 or Oxa-mTPE:3% BUBD-1 (40 nm)/TPBi (10 nm)/Alq3 (20 nm)/Al) and the Electron and hole mobilities versus E1/2 for TPE, Oxa-pTPE and Oxa-mTPE.

Figure 17. Chemical structures of TPEDPO, TPEPO, DTPEPO and TTPEPO and the performances of the corresponding blue OLEDs (OLED configurations-ITO/NPB (10 nm)/TPEDPO or TPEPO or DTPEPO or TTPEPO: 5%BUBD-1 (40 nm)/TPBi (10 nm)/LiF (1 nm)/Al) and the Electron and hole mobilities versus E1/2 for TPEDPO, TPEPO, DTPEPO and TTPEPO. 77 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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3.3. AIE PLED Apart from the small AIE molecules, AIE polymers also show great potential in the application of polymer OLEDs (PLED) for their good solubility, efficient solid emission, good film-forming ability and low-cost fabrication of spin coating (30). Among the AIE polymers, hyperbranched polymers are of special interest because their easy synthetic accessibility, typically by one-pot syntheses, which allows their production in large quantities and their application on an industrial scale. However, the conjugated hyperbranched polymers with AIE characteristics are still seldom reported, especially in the application of PLEDs. Carbazole, a well-known hole-transporting unit, has been widely used to build OLED materials (31). Li et al designed a hyperbranched polymer of HP-TPE-Cz through the combination of multiple carbazole and TPE and the integration of the hole-transporting ability and AIE effect in the hyperbranched polymer, thus achieved the good PLED performance (32). For comparison, its analog linear polymer, LP-TPE-Cz, was also prepared. Thanks to the AIE effect of TPE units, the two polymers showed AEE characteristic. Then the PLED devices were fabricated through spin coating in which HP-TPE-Cz and LP-TPE-Cz as emitting layers (Figure 18). The hyperbranched polymer HP-TPE-Cz showed much better EL performance with a current efficiency of 2.13 cd A-1 at 508 nm, while LP-TPE-Cz was just 1.04 cd A-1. This abnormal outstanding EL performance of HP-TPE-Cz might be derived from its specific AEE characteristics, as well as the excellent hole-transporting property of its multiple carbazole-based core. In addition, the PLED result based on HP-TPE-Cz represents one of the highest values reported so far for conjugated hyperbranched polymers. At the same time, another hyperbranched polymers system based on fluorene, carbazole and TPE was developed (HP-Flu and HP-Cz) through an ‘A2+B4’ approach using an one-pot Suzuki polycondensation reaction (Figure 19) (33). As expected, they both show AEE characteristic. However, their PL emission peaks are red-shifted to about 530 nm due to the good conjugation of TPE and fluorene or carbazole moieties. When fabricated as PLED, HP-Flu exhibited a better performance with a current efficiency of 1.15 cd A-1 at 508 nm. Although this efficiency is higher than most other normal conjugated hyperbranched polymers, its EL emission is outside the blue region, not so satisfactory. As mentioned above, unlike their green and red congeners, efficient and stable blue materials are still scarce as a result of their intrinsic wide bandgap. Thus, in order to get blue AIE conjugated hyperbranched polymers, we utilized the strategy of modifying the linkage mode, just as in the small AIE materials, to develop another AEE conjugated hyperbranched polymers system of HP-mFlu and HP-mCz, in which TPE and fluorene or carbazole moieties were linked through the meta-position. Because of the strong AIE effect of TPE, they both show AEE characteristic. More excitedly, these two conjugated hyperbranched polymers exhibited sky blue PL emission about 470 nm owing to the meta-linkage mode of TPE, showing promising application in the blue PLED. This again confirmed the powerful control of intramolecular conjugation by simply changing the linkage mode.

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Figure 18. Chemical structures and PLED performances of HP-TPE-Cz and LP-TPE-Cz (PLED configurations-ITO/PEDOT:PSS (25 nm)/Poly-TPD (25 nm)/HP-TPE-Cz or LP-TPE-Cz (32 nm)/TPBi (35 nm)/Cs2CO3 (8 nm):Ag).

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Figure 19. Chemical structures of HP-Flu, HP-Cz, HP-mFlu and HP-mCz and their PL emissions.

4. Conclusion For any research topic, the mechanism and application are probably the two most important parameters just as that the driving force of scientific research mainly originates from the pursuit of truth and the solving approaches to practical problems. It is easily to be understood that the important practical applications could apparently affect our daily life, while the inherent mechanism is liable to be ignored, although with the clear mechanism, all the potentials of the materials can be well optimized and fully utilized. Since AIE phenomenon was firstly reported by Tang’s group in 2001, great attention has been paid on its mechanism and applications. Experimental results and theoretical calculations have been incorporated to explore the AIE mechanism. Based on the RIR and other mechanisms, many basic AIE building blocks have been designed. Also, the AIE materials for OLEDs have been well developed ranging from deep blue to near-IR, especially with TPE as building blocks. Particularly, five main strategies are developed to control the intramolecular conjugation and realize the blue emission. It is believed that more and more other excellent AIE materials would be developed by applying these strategies, to achieve better performance. In conclusion, although AIE is a newborn research topic, it has exhibited attractive research significance and great applications in many fields. We are enthusiastically looking forward to new advancements in this exciting area. 80 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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27. Zhan, X.; Sun, N.; Wu, Z.; Tu, J.; Yuan, L.; Tang, X.; Xie, Y.; Peng, Q.; Dong, Y.; Li, Q.; Ma, D.; Li, Z. Polyphenylbenzene as a platform for deepblue OLEDs: Aggregation enhanced emission and high external quantum efficiency of 3.98%. Chem. Mater. 2015, 27, 1847–1854. 28. Huang, J.; Yang, X.; Li, X.; Chen, P.; Tang, R.; Li, F.; Lu, P.; Ma, Y.; Wang, L.; Qin, J.; Li, Q.; Li, Z. Bipolar AIE-active luminogens comprised of an oxadiazole core and terminal TPE moieties as a new type of host for doped electroluminescence. Chem. Commun. 2012, 48, 9586–9588. 29. Mu, G.; Zhang, W.; Xu, P.; Wang, H.; Wang, Y.; Wang, L.; Zhuang, S.; Zhu, X. Constructing new n-Type, ambipolar, and p‑type aggregation-induced blue luminogens by gradually tuning the proportion of tetrahphenylethene and diphenylphophine oxide. J. Phys. Chem. C 2014, 118, 8610–8616. 30. Hu, R.; Leung, N.; Tang, B. AIE macromolecules: Syntheses, structures and functionalities. Chem. Soc. Rev. 2014, 43, 4494–4562. 31. Zhao, Z.; Chan, C.; Chen, S.; Deng, C.; Lam, J.; Jim, C.; Hong, Y.; Lu, P.; Chang, Z.; Chen, X.; Lu, P.; Kwok, H.; Qiu, H.; Tang, B. Using tetraphenylethene and carbazole to create efficient luminophores with aggregation-induced emission, high thermal stability, and good hole-transporting property. J. Mater. Chem. 2012, 22, 4527–4534. 32. Wu, W.; Ye, S.; Huang, L.; Xiao, L.; Fu, Y.; Huang, Q.; Yu, G.; Liu, Y.; Qin, J.; Li, Q.; Li, Z. A conjugated hyperbranched polymer constructed from carbazole and tetraphenylethylene moieties: convenient synthesis through one-pot “A2+B4” Suzuki polymerization, aggregation-induced enhanced emission, and application as explosive chemosensors and PLEDs. J. Mater. Chem. 2012, 22, 6374–6382. 33. Wu, W.; Ye, S.; Yu, G.; Liu, Y.; Qin, J.; Li, Z. Polymers with aggregation-induced emission: Synthesis through one-pot “A2+B4” polymerization and application as explosive chemsensors and PLEDs. Macromol. Rapid Commun. 2012, 33, 164–171.

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Chapter 6

Multicomponent and Domino Syntheses of AIE Chromophores Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch006

Thomas J. J. Müller* Institut für Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany *E-mail: [email protected]

Multicomponent and domino reactions are unique one-pot methodologies that enable the de novo formation of fluoro-phores in chromogenic reaction sequences. Particularly, by insertion-alkynylation or by alkynylation metal catalysis promises a catalytic entry into rapid, efficient and elegant formations of old and novel fluorogenic scaffolds. This chromophore concept also opened avenues to peculiar highly polar or highly polarizable solid state luminophores that show the characteristics of aggregation induced emission. The same synthetic concept also enables the diversity-oriented synthesis of solution and solid state blue-emissive molecular chromophores.

1. Introduction The quest for novel syntheses of functional π-electron systems (1) as constituting functional entities in molecular electronics (2, 3), in photo-electronic applications (4–10), in particular as organic light-emitting diodes (OLEDs) (11–14), dye-sensitized solar cells (DSSCs) (15, 16), and organic field effect transistors (OFETs) (17–19), and in sensing units in bio or environmental analytics (20–24) has become an increasingly important task for synthetic chemistry. In particular, an efficient and efficacious access to functional chromophores with specific photophysical properties remains a paramount challenge for organic and materials chemists. Ultimately and most advantageously, diversity-oriented syntheses (25–33) of functional molecules should occur in a one-pot fashion © 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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by conceptually concatenating fundamental organic reactions (34–38). With this respect, multicomponent (34, 39–51) and domino reactions (35, 36, 52–54) represent elegant solutions to this quest and these conceptual approaches have become attractive approaches to biologically active molecules (55–58). By definition, in domino processes two or more bonds are formed without isolation of intermediates starting from one, two or more substrates, whereas in multicomponent processes more than two starting materials lead to the product that contains most of the employed atoms (34–51). Multicomponent reactions (MCR) represent a reactivity based concept (59), which can be conducted according to three different scenarios. For domino-type MCR all reagents, catalysts, solvents etc. must be present from the very beginning of the process. In sequential MCR the subsequent well-defined order of addition of components from step to step occurs, however, maintaining the reaction conditions identical. The last category is the consecutive MCR where the components are added stepwise and where conditions can be altered from step to step. Inevitably, all three scenarios promise high structural and functional diversity and for investigation of functional molecules with the immense explorative potential of MCR they have become a powerful synthetic tool. Inspired by our methodological work on consecutive MCR-syntheses of heterocycles initiated by Pd/Cu-catalyzed alkynylation (60–65) we have launched a program to illustrate diversity-oriented syntheses of chromophores by multicomponent and domino reactions one and half decades ago (32, 33). For the development of novel one-pot syntheses of fluorophores we have been following the idea that the MCR or domino process could act as the chromogenic event, i.e. the fluorophore of interest is formed by virtue of the one-pot process. Therefore, we named this concept chromophore approach (Scheme 1).

Scheme 1. Diversity-oriented fluorophore formation by an MCR or domino process based chromophore approach. (see color insert) In the vast field of luminescence, which has actively been developed over many decades, there are aspects that either remained challenging or even became known just recently. For instance, luminophores displaying large Stokes shifts and emission at short wavelength, i.e. blue luminescence, are particularly requested in OLED technologies (11–14, 66, 67). A hot topical field is the induction of emission and eventually also the enhancement of fluorescence upon aggregation. Aggregation induced emission (AIE), a phenomenon coined by Tang’s group (68–70), and aggregation induced enhanced emission (AIEE) first reported by Park’s group (71), have become particularly attractive and stimulating for all aspects of luminophore research. In this synopsis our contribution in the synthetic advancement of blue-emitters and AIE chromophores based upon 86 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

one-pot transformations initiated by modified Sonogashira alkynylation (72–74) are highlighted and summarized in a flashlight fashion.

2. Solid-State Emissive Chromophores by Domino Insertion-Coupling Sequences

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After the discovery of the domino Sonogashira coupling-propargyl alcohol enone isomerization in 1999 (Scheme 2) (61, 74–76), we sought for an extension of this unusual detour of the Sonogashira alkynylation towards Negishi-type cyclic carbopalladation (77) towards 3-substituted benzo-furanones and indolones.

Scheme 2. Mechanistic rationale of the domino Sonogashira coupling-propargyl alcohol enone isomerization. As seminal examples the 3-propenylidene benzofuranone 1 and the 3-propynylidene indolone 2 were obtained in the sense of an insertioncoupling(-isomerization), however interestingly, with E-configuration of the alkenylidene/alkynylidene moiety (Scheme 3) (78). This proof of principle paved the way to novel domino sequences that were elaborated in different directions, some of them also towards novel fluorophores (vide infra). Also the newly formed extended Michael system of indolone 2 encouraged us later to scout new consecutive three-component syntheses to push-pull 3-amino alkenylidene indolones (vide infra). All this prompted us to revisit the unusual stereodivergent propynylidene formation from a methodological perspective. Variation of both the 3-substituted propynoyl ortho-iodo anilides 3 and alkynes 4 gave rise to the formation of alkynylidene indolones 5 in moderate to excellent yield, however, with varying E/Z selectivities (Scheme 4) (79). Quantum chemical modelling indicates a stereomutation on the stage of the vinyl-Pd-species that form upon insertion, depending on the nature of the terminal propynoyl substituent and the alkynyl reaction partner. A postcoupling treatment of the isolated stereochemically enriched enynes 5 with amine base excludes the a posteriori equilibration of the stereoisomers. 87 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Scheme 3. First examples of domino insertion-coupling(-isomerization) syntheses of E-configured alkenylidene benzofuranone 1 and alkynylidene indolone 2.

Scheme 4. Domino insertion-coupling synthesis of alkynylidene indolones 5.

Investigation of the electronic properties of alkynylidene indolones 5 revealed longest wave length absorption maxima in dichloromethane appearing in a range between 351 and 485 nm with extinction coefficients of 11300-75400 L mol-1 cm-1. Most remarkably, none of these chromophores luminesces in solution, however, all N-substituted derivatives (R1 ≠ H) display quite intense emission in the solid state upon irradiation with UV light (λexc = 365 nm). Depending on the substitution pattern the emission maxima in the solid state are found between 533 and 635 nm (Figure 1), where the most pronounced red shift can be identified for p-aminophenyl substituted derivatives, i.e. for push-pull substitution. This finding additionally supports the highly polar character of the vibrationally relaxed excited state of the alkynylidene indolone fluorophores. 88 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 1. Normalized absoprtion (solid lines) and emission spectra (dashed lines) of phenyl alkynylidene indolone 5a (left) and p-N,N-dibutylphenyl alkynylidene indolone 5b (right) (recorded at T = 298 K, λexc = λabs). Performing the domino insertion-coupling reaction with 3-substituted propynoyl ortho-iodo anilides 3 or 3-substituted propynoyl ortho-iodo phenolates 6 with 1-(hetero)aryl propargyl allylethers 7 furnished spiro-indolones 8 or spiro-benzofurans 9 quite efficiently in the sense of a insertion-couplingisomerization-Diels-Alder sequence (Scheme 5) (8, 80).

Scheme 5. Domino insertion-coupling-isomerization-Diels-Alder synthesis of spiro-indolones 8 or spiro-benzofurans 9. Based upon the product analysis this unusual hetero-domino process can be rationalized as follows. Oxidative addition of the substrates 3 or 6 with the palladium(0) complex furnishes after insertion of the tethered alkynoyl moiety a benzoanellated species 10 that readily undergoes alkynylation with the in situ formed copper acetylide 11 to give after reductive elimination the alkynylidene 89 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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indolone or benzofuranone 12. The electron withdrawing lactame/lactone functionality leads to a remote activation of the propargylic position in 12 to undergo a base catalyzed propargyl-allene isomerization furnishing the allene derivative 13. This vinyl allene with electron deficient substitution is particularly suited for a concluding Diels-Alder reaction with inverse electron demand. The tethered allyl ether on the allenyl system sets the stage for the intramolecular (4+2)-cycloaddition furnishing the spiro-indolones 8 or spiro-benzofurans 9. All spiro-compounds share structurally rigidified trans-cis-configured 1,3-butadienes with (hetero)aryl substitution representing the longest wavelength absorbing chromophore units with intense absorption bands between 327 to 398 nm. Upon excitation the model trans,cis-1,4-diphenyl butadiene does not luminesce, but rather returns to the electronic ground state by a conformational twisting and efficient internal conversion (81). However, all spirocyclic 1-(hetero)aryl-4-(hetero)aryl butadienes 8 and 9 display intense luminescence with large Stokes shifts in solution (4300 to 9600 cm-1, emission maxima range from 433 to 545 nm) and in the solid state (Figure 2). Time-correlated single photon counting (TCSPC) measurements in solution give luminescence lifetimes between 0.26 and 4.97 ns. While the structural nature of spiro-indolones 8 and spiro-benzofuranones 9 only affects absorption maxima to a minor extent, dimethyl substitution considerably increases the fluorescence quantum yield for spiro-benzofuranones 9, but only marginally for spiro-indolones 8. However, the dimethyl substitution has no influence on the emission wavelength of spiro-indolones 8.

Figure 2. Representative blue (8a), green (8b) and orange (8c) solid state fluorescent spiro-indolones 8 (λexc = 370 nm). (see color insert) The synthetic concept was extended to the preparation of several luminescent bichromophores 14 (82). Besides the fluorogenic 1,4-diaryl substituted trans-cis-configured 1,3-butadiene moiety the N-dansyl fragment was introduced at the diversity position of R1, additionally anthryl substitution at propargylic position furnished a further type of bichromophore (Figure 3). In comparison to the spiro-cyclic E,Z-fixed 1,4-diaryl butadiene displaying relatively sharp 90 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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absorption maxima at 350 nm, N-dansyl derivatives absorb at 360 nm. The emission spectra reveal two maxima, an intense one at 510 nm und a blue shifted shoulder at 470 nm. As a consequence N-dansyl derivatives apparently luminesce with turquoise luminescence, whereas the spiro-fixed E,Z-fixed 1,4-diphenyl butadiene chromophore emits intensive blue light (Figure 4) (80).

Figure 3. Selected N-dansyl- and anthryl-substituted spiro-indolones 14 with bichromophore emission characteristics (determined in CH2Cl2 at T = 298 K; a[Lcm-1mol-1]; bDetermined with quinine sulfate as a standard (0.1 m H2SO4), Φf = 0.54; cStokes shift Δṽ = ṽmax,abs - ṽmax,em; dDetermined with coumarin 153 as a standard in ethanol, Φf = 0.38).

Figure 4. Comparison of a blue fluorescent spiro-indolone (left), a turquoise emissive N-dansyl-spiro-indolone (center), and a yellow luminescent anthryl-substituted spiro-indolone (right) (λexc = 370 nm). (see color insert) 91 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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A comparison of the fluorescence quantum yields Φf supports an intramolecular interaction between the 1,4-diaryl butadiene chromophore and the dansyl chromophore in the excited state and deactivation by partial intramolecular energy transfer is very likely due to sufficient spectral overlap. Particularly interesting are anthryl-substituted spiro-indolone bichromophores that show a significant deviation of the absorption and emission behavior of the other series. Three distinct maxima and a shoulder can be clearly assigned to spiro-fixed E,Z-fixed 1,4-diphenyl butadiene and anthracene typical chromophores in the absorption spectra. However, the emission spectra of anthryl derivatives do not reveal anthracene typical behavior but rather emission from an expanded π-system (Figure 5). Indeed two emission maxima between 525 and 560 nm can be identified and upon eyesight the solutions are yellow luminescent.

Figure 5. Normalized absorption (solid line) and emission (dashed line) spectra of the anthryl-spiro-indolone bichromophore 14c (recorded in CH2Cl2 at T = 298 K).

3. Solid-State Emissive Chromophores by Multicomponent Insertion-Coupling-Addition Sequences The alkynylidene indolones 5 accessible by domino insertion-coupling sequence represent expanded Michael-type systems and the mild reaction conditions of their generation set the stage for devising consecutive threecomponent syntheses as already successfully demonstrated for ynones and amines to give enaminones in the sense of a one-pot coupling-addition reaction (83). Upon reaction of 3-substituted propynoyl ortho-iodo anilides 3 and alkynes 4 under the conditions of the insertion-coupling sequence the intermediary formed alkynylidene indolones 5 were directly reacted with primary and secondary amines 15 to furnish 4-aminopropenylidene indolones 16 in good to excellent yields (Scheme 6) (84). Most remarkably, the Michael-type addition proceeds in a highly stereoconvergent fashion, presumably via the intermediacy of zwitterion 17, which allows a thermodynamic equilibration due to delocalization of the enolate finally establishing the observed E,E-configuration of newly formed 92 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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conjugated diene. The class of 4-aminopropenylidene indolones 16 formed by the chromogenic three-component process can be considered as push-pull butadienes on the basis of the indolone scaffold. Interestingly, all representatives display intense orange to red luminescence in the solid state (crystal, amorphous or film); surprisingly, in solution the same chromophores are nonemissive (Figure 6).

Scheme 6. Three-component insertion-coupling-addition synthesis of 4-aminopropenylidene indolones 16. A closer inspection of the X-ray structure of the representative compound 16b reveals that the essentially coplanarily arranged push-pull chromophore is additionally stabilized by a parallel π-stacking alignment of the aryl substituents with interplanar centroid distances of 3.6 Å (Figure 7). The moderate positive absorption solvochromicity in solution underlines a relatively low polar electronic ground state with some charge-transfer of S0-S1-transition character (85). However, in the solid state spectra redshifted absorption maxima (492-502 nm) indicate J-aggregation (86) of the push-pull chromophores. Most remarkably, all dyes 16 display intense orange red fluorescence with large Stokes shifts (Δṽ ~2600 cm-1) and sharp emission maxima between 622 and 644 nm. This peculiar effect of the appearance of narrow red shifted aggregation induced emission bands (68–70) additionally rationalizes by Davydov splitting in the solid state (87, 88). Later this concept was extended with L-amino acid esters 18 as amino component to give film luminescent indolone merocyanines 19 containing L-amino acid esters as donors (Scheme 7) (89). The occurrence of mixtures of diastereomers as a result of the Michael-type addition of L-alanine ethyl ester and L-leucine methyl ester prompted us to study the energetics of the chromogenic event by computational methods. Assuming a stepwise Michael addition (see also Scheme 6) of the L-alanine methyl ester 18a the elusive allenol intermediate 20a should be formed from the zwitterion 17a (Figure 8). According to PM3 93 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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computations with the implemented solvation energy model SM5.4/P the final, irreversible 1,5-H-shift represents the thermodynamic driving force in the sense of an allenol-enone tautomerism. Although the isomer E,Z-19a is slightly more stable than the isomer E,E-19a the computed transition state energy difference ΔΔG‡(TS20a–E,E-19a – TS20–E,Z-19a) suggests that the formation of product E,E-19a proceeds by kinetic control.

Figure 6. Normalized absorption (solid line) and emission spectra (dashed line) of a drop-casted film of 4-aminopropenylidene indolone 16a.

Figure 7. X-ray structure analysis of 4-aminopropenylidene indolone 16b with a selected angle, torsional angle and interplanar distance (the grey shaded plane on the right indicates the chromophore plane). 94 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Scheme 7. Three-component insertion-coupling-addition synthesis of 4-amino acid ester substituted prop-3-enylidene indolones 19.

Figure 8. Computed reaction profile for the terminal 1,5-H-shift of the elusive allenol intermediate 20a to form the diastereomers E,E-19a and E,Z-19a. The 4-amino acid substituted prop-3-enylidene indolones 19 are also nonemissive in solution as found for the derivatives 16, however, they display intense orange to red luminescence in amorphous films with sharp emission bands in a range from 579 to 631 nm with Stokes shifts range between 4600 and 5600 cm–1. A further extension of the consecutive three-component insertion-couplingaddition sequence to the synthesis of triene push-pull systems was achieved by employing enamines, such as Fischer’s base (21), as nucleophiles after the formation of the alkynylidene indolone intermediate (90). Most remarkably, a mechanistic dichotomy was observed to proceed with excellent selectivity furnishing 1-styryleth-2-enylidene indolones 22 in good to excellent yields as violet solids with a metallic luster for electron rich 3-substituted propynoyl ortho-iodo anilides 3 (R1 = Me) (Scheme 8). However, for electron deficient 3-substituted propynoyl ortho-iodo anilides 3 (R1 = Tos) under the same conditions selectively 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylidene indolones 23 were formed in good to excellent yields as bluish black solids with a metallic luster (Scheme 9). 95 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Scheme 8. Three-component insertion-coupling-addition synthesis of 1-styryleth-2-enylidene indolones 22.

Scheme 9. Three-component insertion-coupling-addition synthesis 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylidene indolones 23.

This mechanistic bifurcation was studied computationally and the inspection of the Mulliken charge on the relevant zwitterionic intermediate 24 indicated that the methyl substitution leads to a less stable enolate part that rapid collapses with the iminium moiety in the sense of a 1,4-dipolar cyclization to give the cyclobutene 25. Conrotatory ring opening directly leads to the 1-styryleth-2-enylidene indolone 22. Conversely, tosyl substitution ensures a higher stability and, therefore, a longer persistence to allow for a 1,7-proton transfer to the allenol 26, which tautomerizes to the extended conjugated triene 23 via 1,5-sigmatropic H-shift (Scheme 10). The photophysical characteristics of 1-styryleth-2-enylidene indolones 22 are similar to the 4-aminopropenylidene indolones 16 and 19, showing deep red colored solutions and and film formation with intense, broad unstructured absorption bands between 510 and 522 nm (dichloromethane) and between 519 and 532 nm (films), respectively, as a consequence of J-aggregation (86). As a consequence aggregation induced emission is indicated by deep red luminescence of both amorphous films and dyes in the solid state with sharp bands appearing between 644 and 665 nm. As for the related indolones 16 and 19, the emission of the indolones 22 is completely quenched in solution. 96 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Scheme 10. Mechanistic rationale of the dichotomy furnishing the 1-styryleth-2-enylidene indolones 22 and 4-(1,3,3-trimethylindolin-2-ylidene)but2-en-1-ylidene indolones 23. Likewise 4-(1,3,3-trimethylindolin-2-ylidene)but-2-en-1-ylidene indo-lones 23 are dark blue to black solids with broad unstructured longest wave length absorption bands in solution in a range from 577 to 597 nm with high molar extinction coefficients. J-aggregation red shifts the absorption bands of the films to 599-623 nm. Upon eyesight the films of the merocyanines 23 did not display emission, in accordance with the energy gap law (91).

4. Solid-State Emissive Push-Pull Chromophores by Multicomponent Coupling-Addition Sequences Sonogashira coupling of acid chlorides and terminal alkynes under modified conditions (72, 73), i.e. employing only the stoichiometrically necessary equivalent of tertiary amine for scavenging the hydrochloric acid, opened new avenues to consecutive multicomponent syntheses of heterocycles as a consequence of the mild reaction conditions of this versatile entry to alkynones (60–65). Encouraged by Michael and Michael type additions of amines to ynones (83) and the previously discussed intermediate alkynylidene indolones (84, 89) and the addition of Fischer’s base to alkynylidene indolones (90) we conceived the Michael addition of Fischer’s base and in situ generated S,N-ketene acetals as a novel, efficient way of accessing push-pull chromophores in a one-pot fashion (92). The stepwise nature of the Michael type addition of enamines to ynones additionally opens mechanistic dichotomies imposed by steric and electronic effects (vide supra for alkynylidene indolone). Indeed, we observed three different scenarios of merocyanine formation depending on the ynone in conjunction with the employed nucleophile. 97 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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In the consecutive three-component coupling-addition sequence of aroyl chlorides 27 and (trimethylsilyl)acetylene (28) and Fischer’s base (21) or the in situ formed S,N-ketene acetal from 29 push-pull butadienes 30 were generated with moderate to good yield (Scheme 11). However, upon changing the alkyne coupling partner to aryl acetylenes 31 the resulting ynones open bifurcating pathways to the addition products. With dimethyl benzothiazolium iodide (29), which generates a reactive S,N-ketene acetal upon deprotonation, push-pull butadienes 32 were formed in moderate to good yield (Scheme 12), whereas with Fischer’s base (21) 2-styryl substituted push-pull ethylenes 33 are formed in good to excellent yield (Scheme 13). The mechanististic rationalization is similar to the related alkynylidene indolones, however, in these cases the stability of the zwitterionic intermediate is governed by stabilization of the iminium species, i.e. the S,N-ketene acetal forms a more stable iminium ion than Fischer’s base.

Scheme 11. Three-component coupling-addition synthesis of push-pull butadienes 30.

Scheme 12. Three-component coupling-addition synthesis of push-pull butadienes 32.

Scheme 13. Three-component coupling-addition synthesis of 2-styryl substituted push-pull ethylenes 33. 98 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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All three types of push-pull chromophores reveal interesting photophysical properties. The merocyanines 30 are luminescent in solution and the closer inspection of the solvent polarity effects reveals positive absorption and emission solvatochromism of merocyanine 30a according to Reichardt’s ET(30) (93) and Lippert-Mataga (94, 95) plots (Figure 9). The empirical linear free-enthalpy relationship (LFER) indicates a lower dipole moment for the electronic ground state and a minor charge transfer character of the excited Franck-Condon state upon S0-S1 transition (96). Supported by the Lippert-Mataga analysis the vibrationally relaxed S1 state possesses a high dipole moment with enhanced charge-transfer character. A small shift in solvent polarity from methyl cyclohexane to benzonitrile causes a red shift of 860 cm-1 in the emission.

Figure 9. Absorption and emission solvochromicity of of merocyanine 30a (top: plot of the longest wave length absorption (black) and shortest wave length emission (red) maxima against Reichardt’s solvatochromicity parameters ET(30) (r2abs = 0.93, r2em = 0.97); bottom: Lippert-Mataga plot of the Stokes shifts against polarity parameters (r2abs = 0.90). 99 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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While merocyanines 30 are luminescent both in solution and in the solid state, the push-pull butadienes 32 and the 2-styryl substituted push-pull ethylenes 33 are nonluminescent in solution. However, all three types of merocyanines reveal pronounced luminescence in the crystalline solid-state (Figure 10). In addition, push-pull butadienes 30 and 32 possess film forming properties and also in the amorphous solid state red-shifted absorption bands and intense sharp even further red-shifted emission maxima can be detected (Figure 11).

Figure 10. Solid state (left) and solution (in dichloromethane, right) luminescence of merocyanines 30a, 32a, and 33a (λexc = 366 nm). (see color insert)

Figure 11. Normalized absorption (black) and emission spectra (red) of a drop-casted film of merocyanine 32a and its emission of crystalline powder (λexc = 366 nm). (see color insert) Both H-aggregate (97) and J-aggregate formation (98, 99) of merocyanines have been described by UV/vis-spectroscopic studies. The X-ray structure analysis of compound 32b (Figure 12), crystallizing in the monoclinic space group C2/c, reveals further insight into the observed J-aggregation of the merocyanine molecules due to the anti-parallel alignment and self-organization by π-π-stacking of the molecules and thereby enhancing the polar microenvironment. 100 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 12. Crystal packing of closest parallel molecules of compound 32b (d = 3.52 Å within dimer, and d = 3.44 Å between dimers) in a stack (top) and in the lattice (bottom).

5. Blue Emissive Heterocyclic Chromophores by Multicomponent Coupling-Addition-Cyclocondensation Sequences For OLED applications red and green emissive fluorophores are predominantly reported (100, 101), since charge-transfer excited-states typically exhibit a narrower band gap than normal locally excited states. Inevitably blue, non- charge-transfer emitters usually possess a wider band gap (102–104). Therefore, novel diversity-oriented accesses to molecular blue emitters are highly desirable. Based upon the catalytic generation of alkynones as an entry to consecutive multicomponent syntheses of heterocycles (60–65) we have accessed several classes of blue emissive systems. 101 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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5.1. Three- and Four-Component Syntheses of Blue Emissive Pyrazoles by Coupling-Addition-Cyclocondensation Sequences Pyrazoles and imidazoles have received considerable interest due to unique electronic and optical properties and their use as optical brighteners, as UV stabilizers, and as constituents in photoinduced electron transfer systems (105–114). Based on the concept of catalytic generation of alkynones as an entry to consecutive multicomponent syntheses of heterocycles (60–65) we disclosed an efficient regioselective three-component synthesis of highly fluorescent 1,3,5-substituted pyrazoles 37 from acid chlorides 34, terminal alkynes 35, and hydrazines 36 (Scheme 14) (115). Absorption (λmax,abs between 260 and 385 nm) and emission (λmax,em between 320 and 380 nm) properties of pyrazoles 37 are strongly affected by substitution pattern. Most derivatives are intensely blue to green fluorescent with fluorescence quantum yields Φf between 11.0 (Figure 5). When the pH value of the solution increases from 7.0 to 11.0, the red emission at 565 nm reduces and the blue-green emission at 515 nm increases. The ratio of I515 to I565 shows adverse pH responses with I565, demonstrating a ratiometric detection of pH value by visual sensing of emission color change.

Figure 4. Chemical structures of AIE-active salicylaldehyde azine derivatives 9-12.

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Figure 5. (A) Photographs of 10c in B-R buffer solution with different pH values (top panel: under daylight; bottom panel: under 360 nm UV irradiation). (B) PL spectra and (C) I565 and I515/I565 versus pH value of 10c in buffer solution with different pH values. Adapted with permission from ref (16). Copyright 2015 Royal Society of Chemistry. When the ESIPT process is blocked by chemical modification of the hydroxyl group, the salicylaldehyde azine dyes only show weak blue emission; however, upon deprotection of the hydroxyl group, ESIPT process is rebooted by forming the intramolecular hydrogen bonds, which generates strong fluorescence in the aggregated state. Through this design principle, multi-target light-up fluorescence probe can be developed. For example, Liu et al. has synthesized SA dyes 12a-c specially designed for the response of multiple targets including palladium cation, perborate anion, and UV irradiation (17). Upon addition of Pd(PPh3)4 into the aqueous solution of 12a, Pd0-catalyzed Tsuji-Trost reaction takes place which cleaves the allyl group and release the hydroxyl group to afford emission from ESIPT process; the sensing of perborate is realized by selective deprotection of aryl acetates in 12b under mild conditions to regenerate the hydroxyl group; the UV sensing is realized by photo-cleavage of the well-developed photocleavable protecting nitrobenzyl group of alcohols and amines in 12c. The protected salicylaldehyde azine dyes generally show weak blue emission when ESIPT process is inhibited, after deprotection, ESIPT process regenerate and result in strong yellow emission at about 550 nm. Tong et al. also use similar strategy to develop a fluorescent probe 12d for the turn-on detection of cysteine (Cys) (18). 200 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

The salicylaldehyde azine is decorated with one acryloyl group on one of the hydroxyl group, which is a well-known selective reaction group of Cys. After addition of Cys, the acrylate group is hydrolyzed and generate the intramolecular hydrogen bonding, enabling the ESIPT process and turning on the emission.

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2.3. AIE-Active Heterocyclic Compounds Nowadays, aggregation-induced emission has been proved to be a generally applicable phenomenon to many known compounds, even without classical propeller-shaped structures. In particular, several heterocyclic compounds have demonstrated to be AIE-active recently (Figure 6). For example, Guo and Wong et al. have reported the AIE property of a long known compound, 2,2′:5′,2′′-terthiophene-5-carbaldehyde 13. In dilute THF solution, 13 emitted at 479 nm with a ΦF of 7.59%; when 70% water was added into the THF solution, the ΦF value was increased to 57.96% with a significant red-shift of the emission maximum of about 60 nm. Such bathochromic shift is attributed to typical twisted intramolecular charge transfer caused by the increased solvent polarity (19). It is suggested that the ordered nanoscale aggregates of 13 formed in THF/water mixtures are responsible for its AIE feature. Yang and Ma et al. have synthesized a new AIE heterocyclic 14 through facile one-pot tandem reaction. In THF solution, compound 14 emits at 400 nm with a ΦF value of 9.0%; when the fraction of water is raised, the fluorescence is first quenched, then regenerated a new emission peak at 450 nm and shown enhanced emission, as the molecules began to form sub-micron particles after the water fraction increased to 90% (20). This heterocyclic can undergo a ring-opening reaction in the presence of thiol nuclephile to enable selective cysteine and glutathione detection. Qian, Yi, and Huang et al. reported an AIE-active phenylbenzoxazole-based compound 15 with the locally excited state emission located at 343-351 nm and a twisted intramolecular charge transfer emission located at 470-525 nm, depending on the solvent polarity. The THF solution of 15 shows almost no emission (ΦF = 0.17%) upon UV irradiation, but exhibits strong blue emission in powder with a ΦF value of 8 ± 2% (21). The AIE feature of 15 is reported to arise from an emissive quasi-TICT excited state. The intramolecular rotation is more restricted with the increased order of molecular arrangements, which results in stronger quasi-TICT* emission. An AIE-active self-assembled organogel prepared from 5-(4-nonylphenyl)7-azaindole 16 was reported by García-Frutos et al. (22). The monomeric species of 16 in dilute cyclohexane solution possess an emission maximum at 349 nm. Increasing the solution concentration to 0.01 M can cause complete fluorescence quenching. When gel was formed in the concentrate solution at room temperature through the hydrophobic interaction of the long alkyl chain and the dual hydrogen bonds, blue emission was observed, red shifted compared with that of the dilute solution of 16. This is attributed to the formation of complexes with a more coplanar conformation which resulted the AIE effect. Similarly, Lin and Zhang et al. reported a rational design of organogelator 17, consisting of a coplanar nitrophenylfuran moiety, enabling π-π stacking interaction, in which the nitrophenyl group serves as a chromophore and the phenylfuran group serves as a 201 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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fluorophore to achieve dual-channel response (23). The aroylhydrazone linker is designed as the F- binding sites as well as the hydrogen bond self-assemble sites; a 3,4,5-tris(hexadecyloxy)phenyl tail is designed for good solubility in organic solvent as well as strong hydrophobic interaction which enables easy gelation and relative stable gelator. The hydrophobic interaction, π-π stacking interaction, and hydrogen bonding work together at elevated temperature to form stable organogels in various solutions at low critical gelation concentrations, accompanying with fluorescence enhancement at 524 nm. When F- was added into the organogel, the fluorescence was quenched and the appearance of the organogel turns from yellow to dark red, while the system remain to be an organogel. The emission and yellow color can be recovered by adding H+ into the gel.

Figure 6. Chemical structures of AIE-active heterocyclic compounds 13-20.

Qin and Tang et al. recently reported a new kind of AIEgen named tetraphenylpyrazine 18 which generally emit in the wavelength range of 390-460 nm. The tetraphenylpyrazine dyes possess good thermal stability, facile preparation, and tunable emission color by easy modulation of the structure (24). Chen and Xu et al. developed a new and efficient strategy, utilizing Cu/Pd-catalyzed isomerization/insertion/oxidative coupling cascade reaction of cyclopropene with internal alkynes to afford a large variety of cis-tetrasubstituted olefins as the single stereoisomer (25). These tetraarylethenes are proved to be AIE-active. For example, compound 19 emits at 500 nm with ΦF values of 0.47% and 9.69% in THF solution and solid state, respectively. Squarine or squarylium dyes with a unique four-member hydrocarbon ring are extensively studied as a red emissive compounds, however, their solid-state light emission has barely 202 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

reported. Ohsedo et al. have synthesized a series of asymmetric squarylium dyes such as compound 20 (26). The compound is non-emissive in DMF solution but show strong emission at 534 nm with a ΦF value of 36%.

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2.4. AIE-Active Boron-Containing Compounds Besides heterocyclics containing common heteroatoms such as N, O, and S, organoboron compounds have also attracted much attention as efficient luminescent dyes with high aggregated state emission. One typical example is the previously reported AIEgen-modified BODIPY derivatives (27, 28). Recently, scientists have paid more attention of boron ketoiminate units with the boron atom coordinated by a nitrogen atom, an oxygen atom, and two fluorine atoms. Tanaka and Chujo et al. have done many work related to highly emissive organoboron compounds (Figure 7). For instance, they have reported a variety of boron ketoiminates 21a-e with a bithiophene bridge to connect two ketoiminate units and different substitution groups on the terminal phenyl ring (29). These compounds possess emission maxima located in the range of 617-671 nm with solid state emission quantum yield of up to 48%. The emission intensity of the boron ketoiminates are generally enhanced in the solid states compared with the solution states. Mechanofluorochromic effect is generally observed for 21a-e, which suggests a phase transition between the crystalline and amorphous states. The shift of emission wavelength before and after the mechanical stimuli is highly dependent on the substituents: the bulky substituents lead to a bathochromic-shift after grinding while the small substituents are opposite. As an important analogues of BODIPYs, pyridine-based organoboron compounds has proved to possess high fluorescence quantum yield and high stability in solution. In particular, pyridine-based organoborons with unsymmetrical structures generally possess moderate ΦF values in the solid state (30). Wang, Liu and He et al. have reported two pyridine-ketoiminate-boron-based luminophores 22a-b with propeller-shaped structures and AIE feature. Two fused rings were formed by the coordination of boron with the pyridyl nitrogen atom and the ketoiminate oxygen atom, two periphery phenyl rings were decorated to provide intramolecular rotation and hence enable AIE effect. 22a-b emit faintly in solvents with low viscosity and their emission could be enhanced by increasing the solvent viscosity or aggregation. In the solid state, 22a-b possess narrow emission bands with high quantum yields of 53% and 46%, respectively, probably due to the weak intermolecular interactions such as C-H···F and C-H···π which fixing the molecular conformations and restricting the intramolecular motions. Similarly, Shankarling et al. reported a series of new keto-enol tautomeric benzoxazolyl and benzothiazolyl-1,2-diaryl β-ketoiminate based organoboron complexes 23a-b (31). They barely emit in THF solution but show enhanced emission efficiency in the THF/water mixtures with 90% water content, and their emission maxima are 505 nm (23a) and 531 nm (23b).

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Figure 7. Chemical structures of AIE-active boron-containing compounds 21-24. Furthermore, an AIE-active o-carborane 24, a polyhedral boron cluster compound with two adjacent carbon atoms in the cluster cage, is designed and synthesized as a stimuli-responsive compound by Morisaki and Chujo et al. (32) Anthracene was selected as the π-bridge to connect two o-carborane cages. The assemblies of the anthracene compounds can further tune the luminescence of the solid state. The THF solution, the THF/water mixture with 99% water content, and the crystal formed in CHCl3 of 24 show emission maxima at 648, 643, and 627 nm, respectively, with their emission efficiencies to be 80 nm), compared with 56, implies that the 2,2-dicyanovinyl group plays an important role in realizing the strong emission-colour response towards grinding (Scheme 8). A series of dicyanomethylenated acridone derivatives 57-60 (Scheme 9) are synthesized by Wang et al. (49). They are highly luminescent in crystalline state but non-emissive in amorphous state, showing crystallization induced emission (CIE) behavior. The molecular packing of 57-59 in crystals is easily regulated by modifying the length of alkyl chains, resulting in the tunable emission colors from green to red. This report presents a mechano-responsive emission on−off switching system with various emission colors (560 to 707 nm).

Scheme 9. Molecular structures of compounds 57-66. 2.3. Tetraphenylethylene Derivatives Tetraphenylethylene (TPE) derivatives are hotly investigated in recent years for their AIE properties. The recent studies show that some TPE derivatives possess solid state MFC performances (50). Some simple AIE molecules with high crystallinty, such as phenyl-substituted tetraphenylethylene (51) and TPE (52), are considered as no MFC due to their high crystalinity. Their crystal structures are too fragile under shearing force to be detected by their color changes or their crystalline structures recover too fast to be observed. However, the high-pressure studies on TPE using diamond anvil cell technique with associated spectroscopic measurements reveal that TPE shows MFC based on its conformation planarization. The mechanism of conformation planarization has been confirmed by Zou et al. (53). During the compression process, the λem gradually red shifted from 448 to 467 nm at 5.3 GPa and eventually to 488 nm at 10 GPa. Xu and Tian’s group (54) investigated the MFC and polymorphism-dependent emission of one TPE derivative 62 (Scheme 9). It was found that the covalently linking dimethylamino groups into TPE brought the intermolecular interactions 228 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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(such as C−H···πand C−H···Ninteractions), and these relative “soft interactions” are easily broken after grinding or under pressure. Then the resulting change of packing patterns or intramolecular conformation finally makes MFC be realized. Chi and Xu’s group (55) reported a novel TPE derivative (63, Scheme 9) with AIE and CIE activity (ΦPL up to 85%). 63 has an exceptionally large two-photon absorption cross section of 5548 GM, and exhibits striking multi-stimuli-responsive single- and two-photon fluorescence switching with excellent reversibility in the solid state (from 469 to 513 nm after grinding). Tang et al. (56) reported that 1,1,2,2-tetrakis(4-ethynylphenyl) ethane (61, Scheme 9) is AIE-active and MFC (from 477 to 505 nm after grinding). Carbazole and TPA-substituted ethenes (64, 65 and 66, Scheme 9) with high solid-state ΦPL (up to 97.6%) are also synthesized by Tang et al. (57). They exhibit MFC properties: their emissions can be repeatedly switched between blue and green colors by simple grinding–fuming and grinding–heating processes (from 455, 454 and 429 nm to 465, 490 and 500 nm after grinding, respectively). Tang et al. (58) continued to characterize a series of luminogens (67, 68, 69 and 70, Scheme 9) comprised of TPE plus spirobifluorene or 9,9-diphenylfluorene. Reversible MFC feature is observed from their solids (for example, from 445 to 503 nm for 67 after grinding).

Scheme 10. Molecular structures of compounds 67-72. Two novel AIE compounds (71 and 72, Scheme 10) derived from TPE and gallic acid were reported by Chi and Xu’s group (59). Both of them possessed mesomorphic properties and exhibited the thermal-induced mesomorphic transition from metastable to stable phases accompanied by a change of the luminescent color. Compound 71 had no MFC property, however, a significant red shift of about 20 nm (from 452 to 472 nm) was observed in 72 after pressing. During synthesis of near-planar aromatic hydrocarbons by the twisted TPEbased oligomers, Wang et al. (60) found that some of the intermediate molecules (73-78, Scheme 10) possessed the MFC properties. After a careful examination, three of the compounds (73, 76, and 77) exhibited obvious MFC behavior with spectral shifts of 50, 40 and 44 nm, respectively.

Scheme 11. Molecular structures of compounds 73-82. Zhang et al. (61) reported a TPE-based phosphine (79), which was used as a ligand to synthesize metal organic framework (MOF). The ligand shows AIE and MFC (from 468 to 499 nm after grinding). 229 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Tang et al. (62) presented the synthesis of three butterfly-like derivatives, 80, 81 and 82 (Scheme 11), with different substituents on the periphery phenyl rings. Compound 82 shows a multicolor luminescence switching between three colors (blue crystal 82-b, green cystal 82-g, and yellow amorphous solid 82-am). Similarly, 81 exhibits reversible MFC between blue crystal (425 nm) and yellow amorphous solid (535 nm). The loose molecular packing with noncovalent intermolecular interactions, the extent of conformational twisting, and the packing density of the luminogens, as well as freedom of intermolecular motion in the excited state, are stemmed for their reversible polymorphism dependent emission behaviors. Zhu et al. (63) reported a new series of geminal-substituted tetraarylethene (TAE) chromophores (83-91, Scheme 12) with AIE properties, which were probed with respect to steric and electronic effects. In comparison to the solvent-free 85 crystal, the solvated 85 with embedded methanol or dichloromethane leads to some non-negligible conformational and packing alterations, which accounts for its distinct fluorescence properties. As an example, they investigated the MFC property of 86 (from 455 to 480 nm after grinding).

Scheme 12. Molecular structures of compounds 83-91. Misra et al. (64) attached TPE unit on the pyrazabole and explored its AIE and MFC properties. Compound 92 (Scheme 13) exhibits strong blue colored emission upon aggregation, and highly reversible MFC feature (from 453 to 497 nm after grinding).

Scheme 13. Molecular structures of compounds 92-99. Three luminogens based on N-phenylcarbazol-substituted TAE 93, 94 and 95 (Scheme 13), were synthesized by Dai et al. (65). All of the luminogens show AIE characteristics with high solid-state ΦPL of up to 83%. Only 95 reveals obvious MFC property: from 441 to 505 nm after grinding. This proved that by introducing a methoxy group into one of the phenyl rings at the para position, MFC materials can be easily obtained. Yuan et al. (66) reported that rational bridging of four TPA units by an ethylene group affords 96 (Scheme 13) with AIE characteristics (ΦPL up to unity) and reversible MFC property (from 501 to 530 nm after grinding). Three new D−π−A−π−D type quinoxalines modified with TPEs 97, 98 and 99 (Scheme 13) were studied by Lu et al. (67). They show obvious MFC properties: from 466 and 491 nm to 500 and 507 nm after grinding, respectively. 230 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Ma et al. (68) reported a tricolored switchable MFC single crystal 100 (Scheme 14), which was synthesized by combining two different luminophores, a TPE unit and a rhodamine B (RhB) moiety together. The obtained single crystal of 100 switched sequentially from deep-blue (441 nm) to green (468 nm) and to a reddish (576 nm) color. The reddish color could be returned to bluish-green at around 465 nm by heating the colored powder at 150 °C for ten minutes. However, the bluish-green powder could not be fully returned to the original deep-blue at 441 nm either by heating or by solvent treatment. Accordingly, compound 101 (Scheme 14) without a boron atom in the structure was an amorphous powder and could not be cultured to a single crystal. More importantly, 101 exhibited only two-color switching from green to reddish upon grinding. It is certain that the boron is critical in the crystallization of 100 with original deep-blue color, which turns the mechanochromic fluorescent emission from two colors to three colors. Chi and Xu’s group (69) developed an AIE-active luminophore 102 (Scheme 14) with remarkable four-colored switching based on the mechano- and protonation-deprotonation control. TPE substituted phenanthroimidazoles 103 and 104 (Scheme 14) were synthesized by Misra et al. (70). They show reversible MFC behaviors with contrast colors between sky-blue and yellow green (from 460 and 450 nm to 509 and 508 nm after grinding, respectively).

Scheme 14. Molecular structures of compounds 92-99. Tang et al. (71) reported a TPE derivative substituted with the electron-acceptor 1,3-indandione (IND) group. The targeted IND-TPE (105, Scheme 14) solids show an evident reversible MFC process in multiple grinding–thermal annealing and grinding–solvent–fuming cycles (from 515 to 570 nm after grinding).

Scheme 15. Molecular structures of compounds 106-113. Shan et al. (72) characterized three pyridine-azole-based AIE materials modified by TPE unit, i.e. 106, 107 and 108 (Scheme 15). Their crystalline aggregates exhibit effective MFC properties with high contrast in both emission color and intensity: from 429 to 460 nm for 106, from 446 to 464 nm for 107 and from 430 to 455 nm for 108 after grinding. Importantly, directly visualized, the ground samples show much stronger emission than those of the as-synthesized and annealed ones. 231 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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The D–A type benzothiazole (BT) substituted TPEs (BT-TPEs) 109-111 (Scheme 15) were prepaired by Misra et al. (73) and the results showed that their photophysical, AIE and MFC properties are dependent on the linkage between the BT and the TPE unit (ortho, meta, and para). The meta isomer 110 shows the highest grinding induced spectral shift (51 nm, from 432 to 483 nm) whereas the ortho isomer 109 shows the lowest spectral shift (9 nm, from 478 to 487 nm). Misra et al. (74) synthesized pyrene-based solid state emitters 112 and 113 (Scheme 15) by substituting the TPE and TPAN units on pyrenoimidazole, respectively. 112 and 113 exhibit strong solid-state fluorescence and drastic reversible MFC between blue and green (from 461 to 499 nm and from 473 to 510 nm, respectively). Misra et al. (75) synthesized TPE substituted unsymmetrical D–A benzothiadiazoles (BTDs) 114, and 115 (Scheme 16). The results show that the cyano-group containing BTD 115 exhibits reversible MFC behavior between green (526 nm) and yellow (~565 nm, ground form), whereas the 114 do not show MFC. Tao et al. (76) developed a series of AIE-active fluorenyl-containing tetra-substituted ethylenes (116-118, Scheme 16). The emission color before and after grinding demonstrates high contrast: for 116, the PL blue-shifted about 58 nm to 517 nm from 459 nm of the crystals; for 117 and 118 the fluorescence red-shifted 34 nm from 473 to 507 nm, and 52 nm from 464 to 516 nm, respectively. Different substituents proved to have a clear effect on the optical and MFC behaviors. Tao et al. (77) continued to investigate their relationship between the molecular conformations and the MFC behavior, especially the role of mechano-stimuli on the thermal annealing crystallization process. Through paired comparisons, they disclosed that the mechano-stimuli could not only destroy the crystallinity of crystalline materials but also bring a significant effect on the amorphous-to-crystalline transition of amorphous materials. That is, only when an amorphous material undergoes mechano-stimuli can it crystallize by thermal annealing to recover its emission. To clearly understand the solid-state amorphous to crystalline transformation, Tao et al. (78) developed an in situ and realtime imaging procedure to record the interface evolution in a solid-state crystallization of molecular amorphous particles. The details disclosed in this observation will deepen the understanding for a series of solid-state crystallization.

Scheme 16. Molecular structures of compounds 114-120. Tang et al. (79) reported their attempt to enhance the robustness of the MFC feature by introducing more ionic species into an AIE-active molecule, 120, which is a homolog of 119 (Scheme 16). This compound demonstrates typical AIEE and MFC behaviors. But the transition between the highly efficient yellow emission (~560 nm) of the crystalline and the moderate red emission (605 nm) of the amorphous 120 solid becomes irreversible by simple treatments of thermal annealing and/or solvent vapor fuming, whereas the ground sample can recover 232 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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its emission only by recrystallization from dissolving the amorphous solid in suitable solvents. The emission peak exhibits a trend of a monotonous blue-shift in grinding–fuming cycles. These behaviours are evidently distinct from the phenomena observed for 119 and other uncharged TPE derivatives. Zhang et al. (80) proposed a new single-arm extension strategy on traditional TPE and successfully developed a new series (121, Scheme 17) of full-color (from ~450 to ~740 nm) tunable MFC materials. These materials exhibit efficient solidstate emission (ΦPL > 10%) and high MFC contrast (wavelength shift from ~50 to ~100 nm) (Figure 1).

Scheme 17. Molecular structures of compounds 121.

Figure 1. (a) The relationship between the MFC contrast (emission peak shift) and molecular long-to-short axis ratio of TPE and its group I derivatives. (b) The relationship between the MFC contrast (emission peak shift) and the molecular dipole moment of group II TPE derivatives (black dots), the molecular long-to-short axis ratio is also indicated (red dots). Adapted with permission from ref. (80). Copyright 2015 The Royal Society of Chemistry. Dong et al. (81) changed the substituted groups and obtained three compounds derived from the TABD (122) molecule, 123, 124, and 125 for the systematic and comparative study of the structural effect on MFC performance (Scheme 18). All of these TABD derivatives are found to possess AIEE features and MFC properties. The results show that the MFC performance (spectal shift) follows the sequence of 125 > 123 > 124. This order can be attributed to the distinctions in the molecular polarity of the three compounds, as indicated by exploration of their solvatochromic properties and through theoretical calculations. Based on AIE-active TPE, a group of diethylamino (DEA) functionalized analogues, i.e. 126, 127 and 128 (Scheme 18) were prepared by Yuan et al. (82). Ground 126 and 127 solids demonstrate rapid self-recovery without any exental 233 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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treatment within a few minutes or even several seconds, respectively. It is found that the ground part with dim yellow (490 nm) light emission mostly returns to the original cyan light (481) in 2-3 min, and is fully restored in 5 min. The slower self-recovery rate of 127 compared to 126 should be ascribed to its stronger dipole–dipole interactions, which slow down the molecular motions. When further aromatic building block with stronger electron-accepting capacity is introduced, 128 shows MFC (from 588 to 598 nm after grinding) without self-restoration within a short time.

Scheme 18. Molecular structures of compounds 122-128. Wang et al. (83) reported two rigid snowflake-shaped luminophores 129 and 130 (Scheme 19) based on six TPE units as peripheries and benzene as a core group. The emission spectrum of the ground 129 powder exhibited a large red-shift of 30 nm (from 467 to 497 nm), revealing that 129 has MFC behavior. However, the MFC behavior did not appear when 130 was ground. These results, combined with its less defined PXRD patterns of the pristine and ground state, can be probably explained by the conformation of 130, which has already undergone planarization and compaction in its pristine powder state due to the extended structures, in comparison with the more twisted conformation of 129.

Scheme 19. Molecular structures of compounds 129-138. Bhosale et al. (84) described a rigid star-shaped luminogen (131, Scheme 19) of cyclohexanehexone bearing six TPE moieties, which exhibited strong AIE activity and reversible MFC behaviour (from 469 to 500 nm after grinding). Zhu et al. (85) reported that the bisanthracene modified dibenzofulvene (132) exhibits efficient MFC properties with the emission reversibly altered between 536 nm (ΦPL = 63%) and 620 nm (ΦPL = 11%). With respects to 133 and 134, both of them exhibit MFC with changes in the λem of about 20 nm (Scheme 19). Zhou et al. (86) constructed a metal−organic framework (MOF) denoted as PCN-128W, starting from chromophoric TPE-based linker (135, Scheme 19) and zirconium salt. PCN-128W exhibits interesting MCF behavior, the color reversibly changes from white to yellow and so does the emission from blue to green (470 to 538 nm). The process is fully reversible by treating PCN-128Y with trifluoroacetic acid (TFAA) in DMF at elevated temperature (Figure 2). It 234 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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indicates that PCN-128W can be considered as a microscissor lift. This work illustrates a very rare example of reversible 3D mechanofluorochromic MOF.

Figure 2. Simplified schematic diagram illustrating the reversible motion of the microscissor lift. Reproduced with permission from ref. (86). Copyright 2015 American Chemical Society. To study the effects of donor and acceptor substitutions, 136-138 (Scheme 19) containing multiple AIE units were synthesized by Tang et al. (87). Luminogen 136 film displays efficient green fluorescence (494 nm, ΦPL = 100%), evident AIE characteristic (αAIE = 154), and reversible MFC (from 472 to 505 nm after grinding). Replacing two phenyls by two cyano (A) groups derives 137, whose film shows efficient orange fluorescence (575 nm, ΦPL = 100%) and evident AIE feature (αAIE = 13). The MFC behavior of 137 (from 541 to 563 nm after grinding) is reversible. Further decoration of 137 with N,N-diethyamino (D) groups results in 138. Due to the cooperative effects of D and A groups, 138 shows dramatic red-shifted emission (713 nm), and reversible MFC behavior. Three tetraphenylvinyl-capped ethane derivatives with 0, 1, and 2 cyano groups at the ethane moiety 139, 140, and 141 (Scheme 20), respectively, were synthesized and characterized by Chi and Xu’s group (88). The results indicate that the compounds possess reversible MFC properties. The introduction of cyano groups to the molecular structures significantly enhanced their MFC activity.

Scheme 20. Molecular structures of compounds 139-151. 2.4. Other Typical AIE Luminogens Recent years, some AIE luminogens have been reported but they do not contain any common AIE units, such as diarylvinylanthracene, TPE, triphenylethylene, cyanoethylene or silole structure. Moreover, some of them have MFC properties. Nakano et al. (89) have reported that 142 (Scheme 20) exhibited MFC (from 465 to 490 nm after grinding). However, the amorphous state of 142 was not so 235 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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stable due to rapid re-crystallization and hence the fluorescent color returned to the original soon after grinding. It is expected that the increase in glass-transition temperature (Tg) of the material makes the amorphous state more stable to prevent crystallization at room temperature. Therefore, Nakano and Mizuguchi (90) focused on 143 (Scheme 20), whose Tg is (86 °C) higher than that of 142 (8 °C). By grinding the crystalline sample, the light blue emission (473 nm) of 143 was changed to greenish yellow (495 nm), and was quite stable at room temperature as intended. In the present study, Nakano et al. (91) found that 142 and 143 were AIE active. In addition, the fluorescence of the resulting 142 particles was found to change in the suspension through vigorous stirring upon heating. Zhou et al. (92) synthesized a new family of TPA-based Schiff bases (144151, Scheme 20) that exhibit different AIE or ACQ behavior in THF/water and as solids. Compounds 146, 147, 149, and 151 show good AIE characteristics due to the existence of J-aggregates or multiple intra- and inter-molecular interactions restricting the intra-molecular vibration and rotation. In addition, emission colors change from 576 to 583 nm and from 520 to 551 nm, respectively, for 147 and 149, after grinding. Liu et al. (93) reported a series of novel, simple, and colorful Salen ligands. Most of the Salen ligands have no MFC and AIE properties. However, 152 (Scheme 21) is an MFC material showing turn-on strong green fluorescence.

Scheme 21. Molecular structures of compounds 152-163. Anthony et al. (94) reported that aryl-ether amine based simple Schiff base molecules (153-157, Scheme 21) showed AIEE effect in the solid state and rare stimuli responsive fluorescence off–on switching. The grinding of 153 resulted in irreversible fluorescence blue shift from greenish-yellow (520 nm, ΦPL = 40%) to green (508 nm, ΦPL = 43%). Heating or solvent exposure did not result in any fluorescence reversibility. Interestingly, the grinding of 154-157 led to the quenching of the solid state fluorescence and heating/solvent exposure produced clear bright fluorescence. It is noted that the turn-on fluorescence of 154-157 was slightly blue shifted compared to the initial solids. Zhou et al. (95) reported three new anthryl Schiff base derivatives containing a similar molecular structure. Among these, 158 displayed an AIE feature, 159 exhibited AIEE active, while 160 showed ACQ behavior. 159 exhibited an MFC characteristic with a 15 nm spectral blue-shifted after grinding (Scheme 21). Taking into account the balance of steric constraints, hydrogen bonding, and π–π stacking interactions, Han et al. (96) synthesized a trigonal azobenzene derivative (161, Scheme 21), a new AIEE-active azobenzene chromophore, in which three phenyl rings are connected to a central 1,3,5-trihydroxybenzene core via azo groups. The compound can self-assembly form red fluorescent 236 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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1D structures. The enhanced red fluorescence of the fibrous structures can be switched off by pressing, rubbing, or annealing. These findings may be applicable to the development of stimuli-responsive luminescent materials that range from optoelectronic devices and sensors to fluorescent polarizers combined with unidirectional molecular arrangements in polymeric objects. Liu et al. (97) studied solid emission properties of 162 (Scheme 21). After grinding, the solid powder of 162 with strong yellow emission (543 nm ΦPL = 31.5%) was converted to a green luminescent powder (530 nm, ΦPL = 20.2 %).

Figure 3. Influence of the ultrasonic power on the morphology and fluorescence color of 163 suspensions in the THF-H2O mixtures (90% content of water) with a frequency of 40 kHz. (a) Non-ultrasonic; (b) 80 W; (c) 120 W; (d) 160 W; (e) 200 W. Reproduced with permission from ref. (98). Copyright 2014 The Royal Society of Chemistry.

Compared with the reported external stimuli, ultrasonication has overriding advantages such as high energy efficiency and quantitative controlling effects. Thus, ultrasound is likely to become a convenient, highly efficient and controllable external stimulus applied in MFC materials. Zhang and Xu’s group (98) reported a novel ultrasonic-sensitive MFC AIE-compound (163, Scheme 21). The fluorescent properties of the 163 suspensions were greatly affected by the ultrasonic treatment and extremely sensitive to its power, which show remarkable blue-shifting and enhanced emission. The aggregation morphologies were found to be greatly affected by the ultrasonic treatment and extremely sensitive to its power (Figure 3). In other words, the luminescent properties are tunable through controlling of the molecular packing mode. Naka et al. (99) reported an AIE-active maleimide luminogen 164 (Scheme 22) with MFC. The yellow emission (553 nm) of 164 crystal changed to green emission (525 nm) after grinding, whereas the emission colors of 165-168 (Scheme 22) were not changed by grinding. Naka et al. (100) continued to report AIE-activoty N-alkyl aminomaleimide luminogens with various kinds of N-alkyl groups. The luminogens exhibited different emission behaviors depending on the chemical structure of the N-alkyl group. Furthermore, it was found that propyl-substitued luminogen 169c (Scheme 22) displayed MFC (from 502 to 489 nm after grinding). 237 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Scheme 22. Molecular structures of compounds 164-169.

Cheng et al. (101) synthesized three D−π−A−π−A indene-1,3dionemethylene-1,4-dihydropyridine derivatives (170-172, Scheme 23) with TPA end groups. These target compounds with highly twisted conformations showed AIEE properties. It is found that they show MFC properties (170: from 610 to 681 nm, Δλ = 70 nm; 171: from 624 to 688 nm, Δλ = 64 nm; 172: from 638 to 683 nm, Δλ = 45 nm). A fluorescence color change similar to the MFC property could also be achieved by a simple dissolution−desolvation process in different solvent systems, which could be considered a solvent-induced emission change.

Scheme 23. Molecular structures of compounds 170-175.

Yan et al. (102) reported a mechano-induced and solvent stimuli-responsive luminescent change by the assembly of a typical AIE molecule, niflumic acid (173, Scheme 23), into the interlayer region of Zn−allayered double hydroxides (LDHs) with heptanesulfonate (HPS) as a cointercalation guest. The 5%-173-HPS/LDH sample exhibits the most obvious MFC with a 16 nm blue-shift (from 439 to 423 nm) with increase in the intensity after grinding, while the pristine 173 solid shows little to no MFC behavior. Wang et al. (103) reported that AIE fluorenone derivatives 174 and 175 (Scheme 23) display reversible stimuli-responsive solid-state luminescence switching. 174 transforms between red (601 nm) and yellow (551 nm, crystals) under the stimuli of temperature, pressure, or solvent vapor. Similarly, 175 exhibits MFC behavior with luminescence switching between orange (571 nm) and yellow (557 nm). Single-crystal structures indicate that the variable solid-state luminescence is also attributed to the formation of different excimers in different solid phases (Figure 4). Additionally, the stimuli-responsive reversible phase transformations of 174 and 175 involve a structural transition between π−π stacking-directed packing and hydrogen bond-directed packing which result in a metastable solid/crystalline state luminescence system. 238 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 4. Schematic diagram for the excimer formation and emission of molecule 174. Reproduced with permission from ref. (103). Copyright 2014 American Chemical Society. Reversible MFC is known for difluoroboron β-diketonates (BF2bdks) (104). Fraser et al. (105) continued to report the MFC for the methoxy-substituted dinaphthoylmethane (176, Scheme 24) ligand even without coordination to boron. In the as spun state, films of 176 showed faint blue emission (475 nm, ΦPL = 3.3%). After annealed at 140 °C, emission spectra showed a broad peak at 440 nm (ΦPL = 3.6%). Some fine structure emerged in the blue-shifted portion (385–480 nm) of the spectrum, however a shoulder peak was observed near 500 nm. In contrast, the smeared state of 176 was blue-green in color (503 nm, ΦPL = 10.6%), which was broader than observed for as spun and thermally annealed states.

Scheme 24. Molecular structures of compounds 176-184. Feng et al. (106) reported a new dual-boron-cored luminogen (177, Scheme 24) ligated with a nitrogen-containing multidentate ligand and four bulky phenyl rings. The unique molecular structure endows this BN-containing luminogen with rich photophysical properties. The sterically congested structure of compound 177 which plays a key role in its AIE activity, may render it responsive to mechanical stimuli (from ~501 to ~521 nm after grinding). Based on this consideration that the luminescent colors of anthracene derivatives in the solid state can be modified by varying their assemblies, Chujo et al. (107) synthesized an o-carborane-based anthracene (178, Scheme 24), in which o-carboranes are substituted at the 9- and 10-positions of anthracene. Its single crystals, with incorporated solvent molecules, were obtained from the CHCl3, CH2Cl2, and C6H6 solutions. Scratching the crystals dramatically decreases the ΦPL value. For example, the ΦPL value of 178·CH2Cl2 changed from 0.66 to 0.08. In addition, the PL and excitation spectra for the scratched solid were bathochromically shifted (for 178·CH2Cl2 from 594 to 640 nm). Lu et al. (108) synthesized three carbazole-based terephthalate derivatives (179, 180 and 181, Scheme 24), in which carbazole and ethoxylcarbonyl groups are used as electron-donating and -accepting moieties, respectively. Application 239 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

of mechanical grinding to the crystals resulted in red-shifts of emissive wavelength of 179 and 180 with a spectral shift of 25 and 15 nm, respectively. However, no MFC behavior was found for the 181 crystals.

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2.5. AIE Complex In 2011, Chi and Xu’s group (109) had published a zinc ion complex which is the first mechano-responsive AIE complex. Three multifunctional cationic iridium(III)-based materials with AIE and MFC behavior have been synthesized by Su et al. (110). All complexes contain the same cyclometalated ligand with functionalized ancillary ligands. Complexes 182 and 183 (Scheme 24) undergo remarkable and reversible MLC in the solid state (from 461 and 462 nm to 482 and 478 nm after grinding, respectively). While complex 184 (Scheme 24), an amorphous material, which only displays AIE activity and on MLC property. More importantly, with the merits of reversible MLC and AIE properties of 183, the rare multi-channel color change and temperature-dependent emission behavior of the iridium(III) complex have been observed.

Scheme 25. Molecular structures of compounds 185-189.

Zhu et al. (111) described two new dinuclear cationic Ir(III) complexes, 185 and 186 (Scheme 25) with Schiff base bridging ligands. The results demonstrate that both complexes 185 and 186 are AIE-active and simultaneously show MLC. Grinding both 185 and 186 on quartz plates induced a red-shift of the emission by ca. 20 nm to 635 and 648 nm from 612 and 627 nm, respectively. Liu et al. (112) reported a series of diisocyano-based dinuclear gold(I) complexes differing only in the bridge linking the two (identical) arms. The gold(I) complexes 187, 188 and 189 (Scheme 25) all exhibit AIE characteristics and MLC behavior: their phosphorescence properties show reversible switchable off–on green luminescence (from 485 nm, ΦPL = 1% to 500 nm, ΦPL = 67.5%). Liu et al. (113) synthesized three trinuclear gold(I) complexes, which exhibit AIE characteristics and show irreversible off–on green luminescence in response to mechanical grinding. Upon grinding of solid power 190, 191 and 192 (Scheme 26), a new emission band at 496 nm was observed and the corresponding emission was converted into strong green luminescence. The changing of weak multiple intermolecular C–H···For π-π interactions, or the formation of aurophilic interactions are possibly responsible for their MLC phenomena. 240 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Scheme 26. Molecular structures of compounds 190-195.

Liu et al. (114) synthesized a series of constitutional isomers containing dinuclear gold(I) units. Complexes 193-195 (Scheme 26) exhibited significant AIE phenomena. The ortho-isomer 193 exhibited reversible MLC (from 430 to 502 nm after grinding), whereas meta-isomer 194 showed switchable mechanical force-induced luminescence enhancement behavior. No MLC behavior was observed for para-isomer 195. Liu et al. (115) continued to report two gold(I) complexes 196 and 197 (Scheme 27) with AIE active. The solid-state luminescence of complexes 196 and 197 can be significantly increased by grinding. Liu et al. (116) reported another dinuclear gold(I) complex with a fluorene-based skeleton (198, Scheme 27). Complex 198 is AIE-active and exhibits reversible MLC: from two emission bands at 490 and 523 nm to 559 nm after grinding. In addition, it shows crystallization-induced emission enhancement behavior. Šket et al. (117) reported a BF2 complex (199, Scheme 27), a molecule with two methoxy groups in one of the phenyl rings at meta positions. Compound 199 exists as two polymorphs having different mutual orientations of the two methoxy groups: in polymorph A away from each other (termed anti), while in polymorph B one methoxy group is oriented toward the other (syn−anti). It was observed that solid A but not solid B exhibited MLC and a striking CIEE effect. Solid A emitted strongly in the crystalline phase (490 nm) but only faintly in the amorphous phase (526 nm). The well ground powder of solid A on dropwise treatment with solvent (CH2Cl2) or on heating (thermochromism) reverted back to the initial blue emissive crystal state, as well as partially returning to the original emission color spontaneously at room temperature (chronochromism).

Scheme 27. Molecular structures of compounds 196-205.

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Chujo et al. (118) synthesized a variety of boron ketoiminates (200-205, Scheme 27) with AIE and MLC behavior to investigate the effect of the substituents on the optical properties by altering the end groups in the compounds.

3. Mechano-Switching Based on Emission Strength

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Tang et al. (119) reported a new AIEE-active luminogen, 206 (Scheme 28), a diaminomaleonitrile-functionalized Schiff base. Its crystal are nonemissive. The defect areas of the crystal, however, are highly emissive at 563 nm. Interestingly, the pressure caused it turn-on is very small (about 0.1 Newton).

Scheme 28. Molecular structures of compounds 206-211.

Zhang et al. (120) reported a bis(2′-hydroxychalcone)beryllium complex 207 (Scheme 28) that displays yellow fluorescence (557 nm; ΦPL = 0.10) in solution. Notably, the solution of this complex produces a non-emissive amorphous thin film (ACQ effect; fluorescent “OFF” state) but brightly emissive crystalline powders (AIEE-active; fluorescent “ON” state) with deep red (678 nm; ΦPL = 0.27) or near infrared (700 nm; ΦPL = 0.20) emission colors. The fluorescent “ON” and “OFF” states can be smoothly transformed into each other by simple mechanical grinding and solvent fuming. Zhang et al. (121) reported another class of beryllium complexes 208-211 (Scheme 28), which display CIE and exhibit morphology-dependent dark and bright red/NIR fluorescence. They show bright red/near-infrared (NIR) emission in the crystalline form (λem: 635–700 nm; ΦPL: 27%–40%) and faint emission in the amorphous state. Their emission can be smoothly switched “ON” and “OFF” by simple grinding/ solvent annealing processes. The two ligands of each complex are almost planar and perpendicularly fused by a beryllium atom with a dihedral angle of about 90°. The produced cross-shape structure facilitates molecules to arrange in a “#” manner with high molecular rigidity as well as packing stability confirmed by crystal structure analyses of 207 and 210 (Figure 5). The “#”-typed molecular packing feature may effectively restrict the photoinduced molecular distortions and eliminate the intermolecular π-electron interactions which is beneficial to the emission. After grinding, the molecules transfer into a random packing mode in which the intermolecular π–π interactions may dominate and the molecules can rotate freely, resulting in a fluorescence “OFF” state.

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Figure 5. Molecular packing modes in the ground and fumed samples. Adapted with permission from ref. (121). Copyright 2014 The Royal Society of Chemistry.

Zhang et al. (122) continued to report a novel family of organoboron compounds 212-215 (Scheme 29) with bright NIR emissions in the crystalline state. They show CIE property and a morphology-dependent emission “ON” and “OFF” feature. The ΦPL of compound 213 powder (34%) is very high for the NIR emissive luminogen with an emission peak beyond 730 nm. Compounds 213-215 show similar solid-state NIR fluorescence “ON/OFF” switching, reflecting the generality of this elegant luminescent behavior. The Ifumed/Iground ratios of emission intensity are 12, 13, and 15 for 213, 214, and 215, respectively.

Scheme 29. Molecular structures of compounds 212-216.

Xu and Tian’s group (123) systematically studied the intriguing turn-on and color-tuned luminescence of the molecular crystals of acridonyl (AD)-tetraphenylethene (TPE) (216, Scheme 28) in response to mechanical grinding and hydrostatic compression. The almost orthogonal conformation between TPE and AD fully separates the electronic distribution and inhibits the ICT process, leading to the emission from LE state in the D-phase of the molecular crystals. The twisted conformation can be changed by the force perturbation when the molecule is under the mechanical stimuli, resulting in an overlap of the frontier orbitals between donor and acceptor and the formation of ICT state. Thus, the switching of excited state characteristics by the mechanical stimuli induced the change in luminescence from the non-emission D-phase to the bright cyan emission B-phase. The concept of the mechanical switching of the excited state will inspire the development of a new class of MLC materials with high-contrast ratio, and further provide an important insight into the solid state luminescent properties of the twisted D−A molecules.

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4. Mechanochromism Based on Dual-Emission The switching range of emission of organic MLC materials is seriously impeded by only one kind of emission (either a fluorescent or phosphorescent peak) in the spectrum of single organic compounds. Chi and Xu’s group (124) presented a design strategy for pure organic compounds with excellent room-temperature fluorescent–phosphorescent dual-emission (rFPDE) properties, which combines the effective factors of dipenylsulfone group, crystalline state, and heavy atom effect. The phosphorescent peak of luminogen 217 is very weak, however, after introduction of iodine atom to obtain luminogen 218, its phosphorescent peak becomes very strong (Scheme 30). Following the principle of color mixing, myriad emission colors with a wide range from orange to purple and across white zone in a straight line in the chromaticity diagram of the Commission Internationale de l’Eclairage (CIE) can be obtained by simply mechanical grinding the compound. The change of emission colors is realized by the ratio of fluorescent and phosphorescent peaks, where phosphorescent peak is strongly dependent on the crystallinity which is affected by the grinding treatment (Figure 6). The unique properties could be concentrated on a pure organic compound through this design strategy, which provides a new efficient channel for the discovery of efficient mechano- responsive organic materials. Chi and Xu’s group (125) reported a novel white-light-emitting organic molecule, which consists of carbazolyl- and phenothiazinyl-substituted benzophenone (220, Scheme 30) and exhibits aggregation-induced emissiondelayed fluorescence (AIE-DF) and MFC properties. The CIE color coordinates of 220 were directly measured with a non-doped powder, which presented white-emission coordinates (0.33, 0.33) at 244 K to 252 K and (0.35, 0.35) at 298 K. The asymmetric donor–acceptor–donor′ (D−A−D′) type of 220 exhibits an accurate inherited relationship from dicarbazolyl-substituted benzophenone (219, D−A−D, Scheme 30) and diphenothiazinyl-substituted benzophenone (221, D′−A−D′, Scheme 30). By purposefully selecting the two parent molecules, that is, 219 (blue) and 221 (yellow), the white-light emission of 220 can be achieved in a single molecule. This finding provides a feasible molecular strategy to design new AIE-DF white-light-emitting organic molecules. The MFC properties of 220 were studied by PL and PXRD. The results indicated that the original 220 showed a dual emission of 456 and 554 nm and the crystalline component was gradually obliterated with increasing grinding time, whereas the corresponding PL intensity of blue emission simultaneously declined. The blue emission of 456 nm was ascribed to a crystallographic state and the yellow emission of 554 nm was attributed to an amorphous state.

Scheme 30. Molecular structures of compounds 217-221. 244 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 6. (A) PL spectra of luminogen 218 at different ground time. Excitation wavelength: 350 nm. (B) Corresponding CIE chromaticity coordinates in CIE-1931 chromaticity diagram of the 218 samples at different ground time. Adapted with permission from ref. (124). Copyright 2015Wiley-VCH.

Based on the above molecular design strategy through the molecular heredity principle for white-light emission molecules, Chi and Xu’s group (126) obtained another white-light emission molecule 224 (Scheme 31) with high contrast MFC and thermally activated delayed fluorescence (TADF). The symmetric compounds 222 and 223 (Scheme 31) are the parent molecules. 222 is a luminogen with impressive deep blue emission, whereas 223 has been reported to yield greenish-yellow TADF (127). The results reveal that both of 222 and 223 are MFC luminophores. Their offspring, 224, exhibits remarkable and linearly tunable MFC and bright white-light emission with TADF by fully inheriting the photophysical properties of the parent molecules 222 and 223. The deep blue and the yellow dual-emission of 224 can be assigned to two independent radiative decays of the excited 1CT states for the carbazole and phenothiazine moieties, respectively. In addition, it is proposed that the mechanism of luminochromism for 224 driven by the mechanical force correlates with the conformational planarization of the phenylcarbazole moiety. Such unusual observations have once again demonstrated that creating asymmetric molecules following the principle of molecular heredity holds promise as a strategy for the development of high performance functional materials.

Scheme 31. Molecular structures of compounds 222-227.

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5. Mechanoluminescence Organic materials exhibiting mechanoluminescence (ML) are promising for usage in displays, lighting and sensing. However, the mechanism for ML generation remains unclear, and the light-emitting performance of organic ML materials in the solid state has been severely limited by an ACQ effect. Chi and Xu’s group (128) reported two strongly photoluminescent polymorphs (i.e., Cg and Cb) with distinctly different ML activities based on a TPE derivative 225 (Scheme 31). As an AIE emitter, 225 perfectly surmounted the ACQ, making the resultant block-like crystals in the Cg phase exhibit brilliant green ML under daylight at room temperature (Figure 7). The ML-inactive prism-like crystals Cb can also have their ML turned on by transitioning toward Cg with the aid of dichloromethane vapor. Moreover, the Cg polymorph shows ML and mechanochromism (Cg from 498 to 523 nm, Cb from 476 nm to 523 nm, after grinding) simultaneously and respectively without and with UV irradiation under a force stimulus. It was point out that the different ML and mechanochromism behaviors of Cg and Cb are originated from the difference in conformation, electron distribution, dipole moment and energy level. Chi and Xu’s group (127) reported a series of diphenylsulfone derivativeds end-capped with carbazole and/or phenothiazine. It was found the molecules containing phenothiazine moiety exhibit AIE-active. Luminogen 226 is a normal TADF molecule, however, luminogens 223 and 227 are AIE-TADF molecules (Scheme 31). The asymmetry molecule (227) shows a high photoluminescence quantum yield of 93.3%, which is the highest quantum yield for long-lifetime non-doped emitters. Simultaneously, the compound with asymmetric molecular structure exhibited strong ML without pretreatment in the solid state, thus exploiting a design and synthetic strategy to integrate the features of TADF, AIE, and ML into one compound.

Figure 7. (a) The image of capital letters ‘AITL’ shown through ML of 225 in the dark under the pressure stimulus at room temperature. (b) ML images of 225 in the dark (left) and under daylight (right) at room temperature. (c) Writable mechanochromic fluorescence of 225 demonstrated by capital letters ‘PAIE’ generated with a metal rod. Reproduced with permission from ref. (128). Copyright 2015 The Royal Society of Chemistry. 246 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

6. Other Systems

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6.1. Doping System To develop such mechano-responsive luminescent materials, incorporating fluorophores with mesogens, which have good self-assembly ability, via covalent bonds has been proven to be an effective strategy. While synthesis of these fluorescent molecules is usually difficult and tedious, physically doping a small amount of fluorophores into mesomorphic media can be a more convenient way to prepare MLC materials. Chen et al. (129) reported a mechano-responsive luminescent system composed of a crown ether derivative (molecule 228) doped with AIE luminogen 229 (Scheme 32). This mixture exhibits luminescent intensity response upon mechanical stimuli, which takes advantage of the metastable melt to stable crystalline phase transition of 228 by shearing. In comparison to the usual way of incorporating fluorophores with mesogens via covalent bonds and the method of taking advantage of assemblies or chemical reaction of fluorescent molecules, this investigation focuses on the change in local “viscosity” around fluorophores doped in a mesomorphic matrix. This study provides a facile way for developing mechano-responsive luminescent materials with processable and reproducible advantage.

Scheme 32. Molecular structures of compounds 228-230.

6.2. Mechano-Memory Chromism Hu et al. (130) chose 1,6-hexamethylene diisocyanate (HDI) and 1,4-butanediol (BDO) as diisocyanate and chain extender to form hard segment (HSC=25%). The TPE-diol (230, Scheme 32) was directly connected to the polymer backbone (other than by physically mix) to increase the luminogen/matrix compatibility and the reliability of the performance in case the solvent extraction of the TPE unit is out of polymer film. This material is made of shape memory polyurethane with TPE units (0.1 wt%) covalently connected to the soft-segments (PCL, Mw = 54000). The material displays biocompatibility, shape fixity of 88–93%, and almost 100% shape recovery as well as reversible mechanochromic, solvatochromic, and thermochromic shape memory effect (Figure 8). 247 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Figure 8. A model to illustrate the molecular mechanism during, stretch-recovery process (a), heating-cooling process (b), solvent-dry process (c), and solvent induced shape recovery process (d). Reproduced with permission from ref. (130). Copyright 2014 Wiley-VCH.

7. Conclusion Lots of new mechano-responsive AIE luminogens have been designed and synthesized after the general feature of mechano-responsive emission was recognized for most AIE luminogens in 2011. In this chapter, we will briefly review the latest progress after 2012 on such rapidly developing field due to the limitation. The emission variation of these fuctional materials respond to external forces are comprehensive and systemic summaried. And the emphasis is focused on the relationship between the molecular structure and mechano-responsive luminescence property. We hope such short chapter may provide a clear panorama of these novel functional materials for different people and a guidance on design mechano-responsive AIE luminogens with various characteristic.

Acknowledgments This work was financial supported from NSFC (51473185), 863 Program (SS2015AA031701), Science and Technology Planning Project of Guangdong (2015B090913003 and 2015B090915003), the Fundamental Research Funds for the Central Universities and CSC.

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Editors’ Biographies

Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ot001

Michiya Fujiki Michiya Fujiki has been a Professor at the Graduate School of Materials Science, Nara Institute of Science and Technology in Japan since 2002. Previously, he worked for the research and development division of Nippon Telegraph and Telephone (NTT) Corporation from 1978 to 2002, and also served as a principal investigator of the JST-CREST program, directed by Emeritus Prof. Hideki Sakurai (Tohoku University), from 1998 to 2003. Prof. Fujiki is interested in chiroptics of σ-/π-conjugated polymers, oligomers, molecules, and atoms in the ground and photoexcited states. He has over 300 publications and an h-index of 48.

Bin Liu Bin Liu is the Dean’s Chair Professor in the Department of Chemical and Biomolecular Engineering, National University of Singapore. Her research focuses on the development of organic nanomaterials with explorations on their sensing, imaging, and device applications. She has over 260 publications and an h-index of 58. Bin Liu has received many prestigious awards and was named among The World’s Most Influential Minds by Thomson Reuters. Dr. Liu is a Fellow of the Royal Society of Chemistry and serves as an Associate Editor of Polymer Chemistry.

Ben Zhong Tang Ben Zhong Tang is Chair Professor in the Department of Chemistry and the Division of Biomedical Engineering at the Hong Kong University of Science & Technology. He is interested in polymer chemistry, materials science, and biomedical engineering. He has published more than 800 papers with an h-index of 96. He has been listed as one of the most cited researchers in the areas of both chemistry and materials science. Dr. Tang was elected to the Chinese Academy of Sciences and the Royal Society of Chemistry. He is now serving as Editor-in-Chief of Materials Chemistry Frontiers.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Subject Index

Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ix002

A AIE active heteroarylethylenes, 175 discussion, 178 CH3CN/H2O mixtures, AIE behavior, 182f CH3CN/H2O mixtures, fluoresence spectra, 181f, 183f CH3CN/H2O mixtures demonstrating AIE effect, fluorescence spectra, 186f dipyridyldiaryl- and tetrapyridylethylenes, synthesis, 184s fluorescence properties, initial assessments, 180 heteroarylethylenes via double Suzuki coupling, synthesis, 182s linked TPVP salts, structure, 188f McMurry coupling, advantages and drawbacks, 179s mono- and dicationic N-alkylpyridinium tetraarylethylenes, 184f monomeric analogues, luminescent properties, 187 pyridinium salt, fluorescence spectra, 185f TPVP derivatives, synthesis, 179s introduction, 176 one or more heteroaromatic core rings, tetraarylethylenes, 178f tetraphenylethylene (TPE) and some pyridine-substituted TPEs, 177f AIE phenomenon, new mechanistic insights, 5 E/Z isomerization, 12 (E/Z)-TPE-FM and TPE-Fl, chemical structures, 13f intramolecular motion (RIM), restriction, 15 luminogens, examples, 16f intramolecular photocyclization, restriction, 16 relevant geometrical parameters, 17f intramolecular rotations (RIR), restriction, 7 chemical structures and fluorescence photographs, 10f HPS and TPE, molecular structures and single crystal conformations, 8f

HPS vs. water fraction, plots of fluorescence quantum yield, 9f TPE-α-CD, chemical structure, 11f intramolecular vibrations (RIV), restriction, 13 COT containing AIEgens, chemical structures, 14f THBA, PL spectra, 14f restriction of intramolecular rotations, propeller-shaped AIEgens, 7f

C Conjugation and rotation, art introduction, 61 1-methyl-1,2,3,4,5- pentaphenylsilole (MPPS), propeller-shaped luminogen, 62f mechanism AIE effect, evidences, 62 carbazole-based luminogens 7-12 and their fluorescent quantum yields, examples, 66f compound 4-6, chemical structures, 65f polyphenylbenzenes 1-3, chemical structures, 65f Py-4MethylTPE and Py-4mTPE, structures, 67f rotational energy barrier and AIE effect, relationship, 66 rotors and AIE effect, relationship, 64 tetraphenylethene (TPE), propeller-shaped luminogen, 63f typical AIE luminogens, HPS and TPE, 64f OLEDs, application, 67 AIE materials, blue emission, 69f AIE PLED, 78 blue emission of AIE luminogens, some strategies, 68 BTPE, mTPE–pTPE, mTPE–mTPE, chemical structures, 71f crystallization-induced blue-shifted emission, 69 DPA–PPB, Cz–PPB, DPA–TTP–CN, chemical structures, 75f

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HP-Flu, HP-Cz, HP-mFlu and HP-mCz and their PL emissions, chemical structures, 80f HP-TPE-Cz and LP-TPE-Cz, chemical structures and PLED performances, 79f methyl-BTPE, Isopro-BTPE, Ph-BTPE, chemical structures, 72f OLEDs, AIE host, 76 Oxa-pTPE, Oxa-mTPE and BUBD-1, chemical structures, 77f PhTPE, Ph2TPE and Ph3TPE, chemical structures, 70f TPEDPO, TPEPO, DTPEPO and TTPEPO, chemical structures, 77f TPE–pTPA, TPE–mTPA, TPE–2pTPA, chemical structures, 74f 3TPETPA, TPA–3mTPE, TPA–3MethylTPE, chemical structures, 73f transition from the ACQ to AIE, examples, 68f

D 9,10-Distrylanthracene derivatives, 113 AIE luminogens based on 9,10distyrylanthracene, applications, 119 bioimaging, 129 BDSA and the photographs, chemical structure, 132f FFSNPs, scheme, 133f micelles, possible schematic drawing, 130f specific cellular imaging, performance, 131 9,10-distyrylanthracene, AIE luminogens, 114 DSA, synthetic routes and molecular structures of small molecules, 116s macromolecules, 115 fluorescent probes and sensing, 125 DSA, quaternary ammonium salt, 127 Pb2+ sensing mechanism, schematic illustration, 126f selective fluorescent aptasensor based on 9/GO probe, schematic description, 128f macromolecules, 117 macromolecules based on DSA, molecular structures, 118s small molecules, 116

solid state emitter, 119 BDPVA under UV light, single crystal, 120f ordered one-dimensional (1D) nanostructures, illustration, 121f stimuli-responses, 122 molecular aggregation states, stacking modes and corresponding emission colors, 123f 15 powder under different stimuli, emission switch, 124f TPE-An, molecular structure, 124f

L Luminescent organoboron element-blocks, 157 conclusion, 172 conventional fluorescent dyes to AIE-active molecules, transformation, 159 aggregation-induced blue-shifted emission, plausible mechanism, 163f AIE-active conjugated polymers composed of boron diiminates, optical properties, 168f AIE-active conjugated polymers composed of boron ketoiminates, chemical structures and optical properties, 166f boron diiminates, optical properties, 167f boron ketoiminates and plausible mechanism, optical properties, 162f film-type sensors, schematic illustration, 169f organoboron complexes, chemical structures, 160f organoboron complexes, comparison of optical properties, 161f protein sensors, schematic illustration, 165f shrinking, chromism, 164 solid-state emissive polymers, optical properties, 166f o-carborane-based AIE-active materials, 170 chemical structures and intensity enhancements, 171f stimuli-responsive emissive hydrogels, schematic model, 171f

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M Mechanism of AIE AIE, mechanism, 40 aggregation effect, 46 BFTPS, chemical structure, 46f BFTPS, reorganization energy versus the normal modes, 49f BFTPS in the gas and solid phases, frontier orbitals, 48f calculated nonradiative decay rate, 47 DMTPS and iPr-DMTPS, optimized molecular structures and frontier orbitals, 41f Duschinsky rotation matrix, contour map, 45f non-radiative decay rates of TPBD and DCPP, temperature dependences, 44f phenyls, selected bond lengths and torsional angles, 48t reorganization energies versus the normal modes, 42f steric hindrance effect, 41 temperature effect, 43 torsional angle of the phenyl ring, rotational energy barriers, 43f total reorganization energy from bond length, contributions, 45f TPBD and DCPP, reorganization energy λ versus the normal mode, 44f AIE systems, spectroscopic character AIEgens, blue-shifted emission spectra, 50 DSA, DCDPP and TPBD, molecular structures, 50f optical spectra based on the potential energy surfaces, theoretical model, 51f reorganization energy and the selected internal coordinates, 52f resonance Raman spectroscopy, 54f resonance Raman spectrum, 53 zero point energy for DSA, DCDPP and TPBD, calculated spectral properties, 52t geometrical reorganization energy and the experimentally measured spectroscopy signals, relationship, 55 theoretical methodology and procedure, 36 computational procedures and details, 39 MOMAP, structure, 40f

nonradiative decay rate, 38 radiative processes, Jablonski diagram with straight arrows, 37f spontaneous radiative decay rate, 37 Mechano-responsive AIE luminogens AIE luminogens compounds 1-6, molecular structures, 223s compounds 7-10, molecular structures, 224s compounds 11-15, molecular structures, 224s compounds 16-22, molecular structures, 225s compounds 23-30, molecular structures, 225s compounds 31-37, molecular structures, 226s compounds 38-46, molecular structures, 226s compounds 48-56, molecular structures, 227s compounds 57-66, molecular structures, 228s compounds 67-72, molecular structures, 229s compounds 73-82, molecular structures, 229s compounds 83-91, molecular structures, 230s compounds 92-99, molecular structures, 230s compounds 100-105, molecular structures, 231s compounds 106-113, molecular structures, 231s compounds 114-120, molecular structures, 232s compounds 121, molecular structures, 233s compounds 122-128, molecular structures, 234s compounds 129-138, molecular structures, 234s compounds 139-151, molecular structures, 235s compounds 152-163, molecular structures, 236s compounds 164-169, molecular structures, 238s compounds 170-175, molecular structures, 238s compounds 176-184, molecular structures, 239s compounds 185-189, molecular structures, 240s

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compounds 190-195, molecular structures, 241s compounds 196-205, molecular structures, 241s diarylvinylanthracene derivatives, 223 excimer formation and emission of molecule 174, schematic diagram, 239f MFC contrast and molecular long-to-short axis ratio of TPE, relationship, 233f reversible motion of the microscissor lift, simplified schematic diagram, 235f ultrasonic power on the morphology and fluorescence, influence, 237f conclusion, 248 dual-emission, mechanochromism, 244 capital letters ‘AITL’ shown through ML, image, 246f compounds 217-221, molecular structures, 244s compounds 222-227, molecular structures, 245s luminogen, PL spectra, 245f mechanoluminescence, 246 emission strength, mechano-switching, 242 compounds 206-211, molecular structures, 242s compounds 212-216, molecular structures, 243s ground and fumed samples, molecular packing modes, 243f introduction, 221 other systems, 247 compounds 228-230, molecular structures, 247s mechano-memory chromism, 247 molecular mechanism, model to illustrate, 248f Multicomponent and domino syntheses, AIE domino insertion-coupling sequences, solid state-emissive chromophores, 87 alkynylidene indolones, domino insertion-coupling synthesis, 88s anthryl-spiro-indolone bichromophore, 92f blue fluorescent spiro-indolone, comparison, 91f domino insertion-coupling(isomerization) syntheses, first examples, 88s domino Sonogashira coupling-propargyl alcohol

enone isomerization, mechanistic rationale, 87s phenyl alkynylidene indolone, normalized absoprtion and emission spectra, 89f selected N-dansyl- and anthryl-substituted spiro-indolones, 91f solid state fluorescent spiro-indolones, 90f spiro-indolones or spiro-benzofurans, domino insertion-couplingisomerization-Diels-Alder synthesis, 89s introduction, 85 diversity-oriented fluorophore formation, 86s multicomponent coupling-additioncyclocondensation sequences, blue emissive heterocyclic chromophores, 101 conclusion, 107 consecutive pseudo-five-component reactions, oligothiophenes, 106s 2,4-disubstituted thiophenes, threecomponent coupling-Fiesselmann synthesis, 105s 5-(3-indolyl)oxazole, blue luminescence, 105f 5-(3-indolyl)oxazoles, threecomponent microwave assisted coupling-cycloisomerization, 104s quinquethiophene, normalized absorption and emission spectra, 107f quinquethiophene, solution and solid state luminescence, 106f 1,3,4,5-substituted pyrazoles, biarylsubstitution, 103s 1,3,4,5-substituted pyrazoles, synthesis, 103s 1,3,5-substituted pyrazoles, synthesis, 102s multicomponent insertion-couplingaddition sequences, solid state-emissive chromophores, 92 4-amino acid ester substituted prop-3-enylidene indolones, three-component insertioncoupling-addition synthesis, 95s 4-aminopropenylidene indolone, drop-casted film, 94f 4-aminopropenylidene indolone, x-ray structure analysis, 94f

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long-term cell tracing images, 214f THF/water mixtures with different water contents, PL spectra, 215f introduction, 193 classical AIE compounds, chemical structures, 194f new mechanistic insight, AIE compounds, 207 AIE compounds, chemical structures, 208f AIE compounds without conventional fluorophore or extensive conjugated structure, chemical structures, 210f AIE polymers, chemical structures, 211f AIE small molecules without conventional fluorophore, 209 new structural design, AIE compounds, 195 AIE-active boron-containing compounds, 203 AIE-active boron-containing compounds, chemical structures, 204f AIE-active heterocyclic compounds, 201 AIE-active heterocyclic compounds, chemical structures, 202f AIE-active metal complexes, chemical structures, 205f AIE-active salicylaldehyde azine derivatives, chemical structures, 199f B-R buffer solution with different pH values, photographs, 200f DMF/water mixtures, fluorescence images, 206f highly emissive platinum metallacage, 197 tetragonal metallacages, chemical structures, 198f TPE-based symmetric compounds, chemical structures, 196f

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4-aminopropenylidene indolones, three-component insertion-coupling-addition synthesis, 93s ichotomy furnishing the 1-styryleth-2-enylidene indolones, mechanistic rationale, 97s 1-styryleth-2-enylidene indolones, three-component insertion-coupling-addition synthesis, 96s terminal 1,5-H-shift of the elusive allenol, computed reaction profile, 95f 4-(1,3,3-trimethylindolin-2ylidene)but-2-en-1-ylidene indolones, three-component insertion-coupling-addition synthesis, 96s solid state-emissive push-pull chromophores, 97 closest parallel molecules of compound, crystal packing, 101f drop-casted film of merocyanine, normalized absorption and emission spectra, 100f merocyanine, absorption and emission solvochromicity, 99f merocyanine, solid state and solution, 100f push-pull butadienes 30, three-component coupling-addition synthesis, 98s push-pull butadienes 32, three-component coupling-addition synthesis, 98s 2-styryl substituted push-pull ethylenes 33, three-component coupling-addition synthesis, 98s

N New AIE structural motifs AIE compounds with unique luminescence properties, 212 AIE compounds with unique luminescence behaviors, chemical structures, 215f different phases, mechanoluminescence spectra, 216f electron donor/acceptor-containing AIE compounds, chemical structures, 213f

S Silole-based cyclosiloxanes, 137 AIE properties, 146 10-3 mg/mL phenyl-substituted silafluorene compound, PL emission spectra, 149f 10-3 mg/mL silafluorene compound, PL emission spectra, 149f

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10-3 mg/mL tetraphenylsilole compound, fluorescence emission spectra, 148f three different silole-based fluorophores in solid state, normalized fluorescence emission, 147f three different silole-based fluorophores in THF solution, normalized fluorescence emission, 147f three different silole-based fluorophores in THF solution, normalized UV absorption, 146f design crystal unit cell, three different typical packing interactions, 144f silole-based cyclosiloxanes, crystal structures, 141s silole-based cyclosiloxanes, photoluminescent properties, 143t silole-based cyclosiloxanes, 29Si NMR, 142t silole-based cyclosiloxanes, synthetic route, 140s silole-based fluorophores, HOMO and LUMO diagram, 145f synthesis and characterization, 139 introduction, 138 silole-based cyclosiloxanes, applications, 150

AIE-active tetraphenylsilole-based cyclosiloxanes, 151f Escherichia coli stained AIE-active tetraphenylsilole-based cyclosiloxanes, fluorescence image, 151f

V Vibration induced emission (VIE), 21 environmental sensors, 28 hole-transporting organic light-emitting materials, 29 introduction, 22 DMP, DMAC, DPAC and FlPAC, structures, 24f fluorescent mechanisms, diagrammatic sketch, 23s monomolecular white-light emitting materials, 27 compound M1, structure, 28f technological applications, 27 VIE, 24 compound FlPAC, three-step kinetic mechanism, 26f dihydrophenazine DPAC for VIE mechanism, illustrative scheme, 25f

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