20 Years of Aggregation‐Induced Emission Research

Aggregation-induced emission (AIE) is a concept coined in 2001 for explaining a photophysical phenomenon, in which molecular aggregates display stronger fluorescence than isolated molecules owing to the restriction of intramolecular motion. Molecular science focuses on studying material structure at the single-molecule level. Beyond single molecules, AIE research has validated the emergence of many new behaviors and functions in molecular aggregates, illuminating all applications involving aggregate science in optical, biomedical, electronic and energy fields. The structure–property relationships established for aggregates inject new vitality to materials science. Owing to its fundamental importance and practical applications, AIE has attracted many researchers with different academic and technological backgrounds to this fascinating wonderland. According to statistic from Web of Science (using the keyword of “aggregation-induced emission”), the total number of publications and citations has reached up to 7722 and 209,354, respectively, contributed by over 4,500 research groups in more than 80 countries. This special issue is devoted to a collection of significant advances in AIE material design and their applications in energy, electronic, and biomedical fields. Tang and co-workers[1] have contributed a review article on how bioconjugation could help realize AIE luminogens (AIEgens) with good water solubility, specific targeting ability and wide functionality. This special issue also consists of four progress reports. Yang and co-workers[2] summarized novel aggregation-induced delayed fluorescence (AIDF) luminophores for time-resolved luminescence imaging and sensing in vitro and in vivo. Yin and co-workers[3] overviewed the recent development of metallacycle/metallacage-cored fluorescent supramolecular with AIE attribute. Yang and co-workers[4] summarized the recent advances in the design and preparation of artificial AIE-active light-harvesting systems. Zang and co-workers[5] presented the recent progress in constructing enantiomerically pure chiral coinage metal clusters in the aggregate state. An efficient molecular design strategy is crucial for exploring new AIE materials. Klymchenko and co-workers[6] developed a strategy for converting poorly emissive ionic dyes in polymeric nanoparticles into AIE-active using bulky counterions. Ajayaghosh and co-workers[7] studied the self-assembly induced modulation of p-phenyleneethynylene with different alkoxy chains, while Cao and co-workers[8] reported a series of shape-controllable and AIE-active supramolecular organic frameworks constructed hierarchically from host-guest complexation. Besides, Kim and co-workers[9] developed an analyte-directed formation of J-aggregates featured with the AIE attribute. The understanding of AIE mechanism has stimulated the useful applications. Based on the molecular motion dependent emission characteristics of AIEgens, Thilagar and co-workers[10] realized the detection of local viscosity and temperature. Gao and co-workers[11] reported a positively charged AIEgen for imaging-guided precise treatment of bacterial infection and cancers. In addition, Zhao and co-workers[12] developed a series of AIEgens with excited-state intramolecular proton transfer (ESIPT) property, attaining specific lipid droplets and gram-positive bacteria imaging. Cai and co-workers[13] reported a class of foldamers with tetraphenylethylene as sidechains, which is circularly polarizable with relatively large luminescence dissymmetry factor. AIEgens have also found applications in electronic devices. Choi and co-workers[14] reported a non-doped solution-processable AIDF emitter, exhibiting an optimized external quantum efficiency (EQE) and CIE color coordinates of 9.90% and (0.17, 0.07), respectively. Wang and co-workers[15] developed a blue AIEgen via constructing low-lying locally excited state and high-lying charge-transfer state for effective triplet-to-singlet conversion, resulting in a better electroluminescence performance in nondoped blue organic light-emitting diodes (OLEDs) with a high EQE of 4.6% and negligible efficiency roll-off. Zhang and co-workers[16] reported white LEDs from nonconjugated polymer microspheres with a high color rendering index of up to 95. Seki and co-workers[17] coupled aggregation-induced emission enhancement and ESIPT into a room temperature nematic liquid crystalline phase, allowing the orientation of the molecular dipoles in response to external electric fields. In summary, we hope this special issue sparks further inspirations in the development of new AIE systems and their applications. We believe it will not only draw new researchers into this promising field but also inspires veteran researchers to drive AIE to a higher stage. We thank all contributing authors for their outstanding contributions and all the reviewers for their dedicated work. We would specially thank Dr. Jos Lenders and the editorial team of Advanced Optical Materials for their support in preparing this special issue. Ryan T. K. Kwok received his B.Sc. in Chemistry and Ph.D. in Nano Science and Technology from Hong Kong University of Science & Technology (HKUST) in 2009 and 2013, respectively. He worked as a Research Associate in Professor Ben Zhong Tang's research group from 2013 to 2017. He has been appointed as Research Assistant Professor of the Department of Chemistry and Junior Fellow of HKUST Jockey Club Institute for Advanced Study since 2017. His research focuses on the development of functional AIE materials and exploration of their applications in biological imaging and sensing. He has been listed by Clarivate as Highly Cited Researcher 2019 in the subject of cross-field. Bin Liu is currently Provost's Chair Professor and Head of the Department of Chemical and Biomolecular Engineering, National University of Singapore (NUS). She received her Ph.D. from NUS and conducted her postdoctoral research at the University of California, Santa Barbara, USA. She joined NUS as an Assistant Professor in 2005 and was promoted to full Professor in 2016. Her research focuses on the development of conjugated polymers and organic nanomaterials and exploration of their applications in energy and biomedical applications. She has been listed by Thomson Reuters as Highly Cited Researcher in Materials Science. She is now serving as Deputy Editor of ACS Materials Letters. Ben Zhong Tang is the Stephen K. C. Cheong Professor of Science, Chair Professor of Chemistry, and Chair Professor of Chemical and Biological Engineering at HKUST, Hong Kong. He received his Ph.D. from the Kyoto University, Japan, and conducted his postdoctoral work at the University of Toronto, Canada. His research interests include the exploration of new AIE materials, new luminescent processes, fluorescent bioprobes, and new polymerization reactions. He has published >1,500 papers. His publications have been cited >99,900 times, with an h-index of 145. He has been listed by Thomson Reuters as Highly Cited Researcher in both areas of Chemistry and Materials Science. He is now serving as Editor-in-Chief of Materials Chemistry Frontiers.