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.
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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.
