Celebration of the 20th anniversary of aggregation-induced emission with research highlights from Royal Society of Chemistry journals

The concept of aggregation-induced emission (AIE) was first coined in 2001 by Tang et al (Chem. Commun., 2001, 1740–1741, DOI: 10.1039/B105159H). Unlike conventional aggregation-caused quenching (ACQ) dyes, AIE luminogens (AIEgens) show weak or negligible emission in dilute solution but emit intensely in an aggregated state. Since then, many research groups from different disciplines have worked in AIE research. This virtual themed issue of Journal of Material Chemistry C features a collection of the AIE papers published in a variety of the Royal Society of Chemistry's journals over the past 20 years involving working mechanisms of AIE, new AIE systems, and the exploration of their optical, electronic, magnetic, chemical, energy and biomedical applications. We hope this themed issue will attract a broad audience range to the area of AIE research.

In the field looking at AIE working mechanisms, Prof. Lluís Blancafort and Prof. Quansong Li described how the low frequency twisting motion around the fulvene exocyclic double bond and bond stretch in the dibenzofulvene ring were responsible for the nonradiative decay of these AIEgens in solution (Chem. Commun., 2013, 49, 5966–5968, DOI: 10.1039/C3CC41730A). One year later, Prof. J. Axel Zeitler, Prof. Ben Zhong Tang and Prof. Emma Pickwell-MacPherson et al. collaboratively reported terahertz time-domain-spectroscopy of tetraphenylethene (TPE) to support the restriction of intramolecular rotation (RIR) hypothesis for the mechanism of AIE (Mater. Horiz., 2014, 1, 251–258, DOI: 10.1039/C3MH00078H). Prof. Luis M. Campos et al. presented a series of difluorenylidene dihydroacene compounds, which AIE characteristics originate from the restriction of the “flapping” vibrational mode of the molecules in the solid state (Chem. Sci., 2019, 10, 10733–10739, DOI: 10.1039/C9SC04096J).

Many new AIE systems have been reported and developed, including metal clusters, non-conjugated organic luminogens and room temperature phosphorescence (RTP) systems. In view of metal clusters, Prof. Jianping Xie et al. developed Au(I)/Ag(I)-thiolate motifs on the nanocluster surface, thus generating strong luminescence via AIE (Nanoscale, 2014, 6, 157–161, DOI: 10.1039/C3NR04490D). Prof. Peng Zhang et al. elucidated the gold-thiolate model structure of highly luminescent nanoclusters stabilized by bovine serum albumin (Chem. Sci., 2018, 9, 2782–2790, DOI: 10.1039/C7SC05086K). For non-conjugated organic fluorophores, Prof. Prasun K. Mandal et al. reported carbon dots based on aggregated hydroxymethylfurfural derivatives demonstrating clusteroluminescence characteristics with an orange-red emission (Phys. Chem. Chem. Phys., 2016, 18, 28274–28280, DOI: 10.1039/C6CP05321A). Prof. Huiliang Wang et al. also discussed clusteroluminescence and molecular weight-dependent fluorescence properties in poly(maleic anhydride-alt-vinyl pyrrolidone) copolymers (J. Mater. Chem. C, 2017, 5, 8082–8090, DOI: 10.1039/C7TC02381B). Efficient pure organic RTP materials have drawn considerable attention recently. Prof. Elena Cariati et al. reviewed organic RTP materials where intermolecular interactions are mainly focused on halogen bonding and π–π type interactions (J. Mater. Chem. C, 2018, 6, 4603–4626, DOI: 10.1039/C8TC01007B).

In the field of supramolecular chemistry, Prof. Ognjen Š. Miljanić et al. reported trigonal fluorinated pyrazoles with triazine-centered compounds assembled into porous molecular crystals that showed AIE (Chem. Commun., 2017, 53, 10022–10025, DOI: 10.1039/C7CC03814C). Prof. Takeharu Haino et al. reported a luminescent micelle that is prepared through the self-assembly of an amphiphilic, neutral Pt(II) complex with isoxazole moieties in THF/water on account of its AIE property (Chem. Commun., 2020, 56, 1137–1140, DOI: 10.1039/C9CC07819C).

In view of the molecular design of AIEgens, Prof. Youngmi Kim et al. designed new AIEgens based on meso-trifluoromethyl BODIPY via J-aggregation (Chem. Sci., 2014, 5, 751–755, DOI: 10.1039/C3SC52495G). Prof. Gen-ichi Konishi et al. reported para-substituted bis(piperidyl)anthracenes that exhibit AIE properties (J. Mater. Chem. C, 2015, 3, 5940–5950, DOI: 10.1039/C5TC00946D). Prof. Kazuo Tanaka and Prof. Yoshiki Chujo et al. controlled the luminescence properties of organoboron complexes between ACQ and AIE with or without a chemical bond at a single site in the pyridinoiminate skeleton (Mater. Chem. Front., 2017, 1, 1573–1579, DOI: 10.1039/C7QM00076F). Prof. Rachel K. O’Reilly et al. developed a new family of thiophenol substituted aminomaleimide-based AIEgens with tunable fluorescent properties (Chem. Commun., 2018, 54, 3339–3342, DOI: 10.1039/C8CC00772A). Prof. Artur J. Moro designed and synthesized a new naphthyridine-ethynyl–gold(I) complex capable of exhibiting AIE due to aurophilic interactions (Dalton Trans., 2020, 49, 171–178, DOI: 10.1039/C9DT04162A).

Research into AIE-active chiral functional materials is a popular field in AIE. Prof. Kam Sing Wong and Prof. Ben Zhong Tang et al. fabricated a film of a chiral silole derivative with mannose units as the side chains obtained in microfluidic channels, which emits right-handed circularly polarized luminescence (CPL) with an extraordinarily high g_lum of −0.32 (Chem. Sci., 2012, 3, 2737–2747, DOI: 10.1039/C2SC20382K). Prof. Xiaoyu Cao presented a series of chiral face-rotating sandwich structures based on TPE moiety through restricting the phenyl flipping of TPE (Chem. Sci., 2018, 9, 8814–8818, DOI: 10.1039/C8SC03404D). Prof. Na Zhao, Prof. Xiaoyan Zheng and Prof. Nan Li et al. constructed a series of chiral AIEgens by inserting various bridged alkyl chains into the hydroxyl groups of 3,3′-dicyanomethylene functionalized (R)-[1,10-binaphthalene]-2,2′-diol, which exhibited tunable solid-state CPL with the maximum g_lum value of 10⁻² (Mater. Chem. Front., 2019, 3, 1613–1618, DOI: 10.1039/C9QM00292H). Very recently, we summarized the recent research progress in CPL based on AIEgens (J. Mater. Chem. C, 2020, DOI: 10.1039/C9TC07022B).

In the field of photophysical study, Prof. Fabrizia Negri, Prof. Marc Gingras and Prof. Paola Ceroni et al. constructed an AIEgen consisting of a hexathiol–benzene core and peripheral tolyl substituents, which exhibited phosphorescence properties when aggregated (J. Mater. Chem. C, 2013, 1, 2717–2724, DOI: 10.1039/C3TC00878A). Prof. Bin Liu et al. developed an effective new strategy to fine-tune the singlet–triplet energy gap in AIEgens for ¹O₂ generation, which showed great promise in image-guided cancer therapy (Chem. Sci., 2015, 6, 5824–5830, DOI: 10.1039/C5SC01733E). Prof. Zujin Zhao and Prof. Ben Zhong Tang et al. reported a kind of interesting photophysical process of aggregation-induced delayed fluorescence (AIDF) from twist AIEgens with a donor–acceptor framework (J. Mater. Chem. C, 2016, 4, 3705–3708, DOI: 10.1039/C5TC03588K). Prof. Dongmao Zhang et al. utilized polarized resonance synchronous spectroscopy as a powerful tool for studying the kinetics and optical properties of AIE (J. Mater. Chem. C, 2019, 7, 12086–12094, DOI: 10.1039/C9TC04106K).

AIEgens present a new class of luminescent functional materials. They have proven their utility in various diverse fields of research. Bioimaging and theranostics are the most attractive fields. Prof. Deqing Zhang and Prof. Bin Liu reported an AIE light-up probe for selective recognition and image-guided photodynamic killing of Gram-positive bacteria (Chem. Commun., 2015, 51, 12490–12493, DOI: 10.1039/C5CC03807C). Prof. Ju Mei and Prof. Jianli Hua et al. synthesized a red AIEgen with a large two-photon absorption cross-section and application to in vivo two-photon fluorescence imaging of blood vessels (Mater. Chem. Front., 2017, 1, 1396–1405, DOI: 10.1039/C7QM00024C). Prof. Ben Zhong Tang et al. developed an AIE theranostic system that can target cancer cells over normal cells and kill cancer cells through photodynamic therapy (PDT) (Chem. Sci., 2017, 8, 1822–1830, DOI: 10.1039/C6SC04947H). Prof. Nitish V. Thakor and Prof. Bin Liu et al. fabricated highly emissive AIE dots by co-encapsulating an AIEgen and a semiconducting polymer for glioblastoma margins-guided photothermal therapy (PTT) (Mater. Horiz., 2019, 6, 311–317, DOI: 10.1039/C8MH00946E). Prof. Yang-Hsiang Chan et al. realized in vitro specific cellular imaging as well as in vivo tumor targeting in mice using narrow-band NIR fluorescent semiconducting polymer AIE dots (Chem. Sci., 2019, 10, 198–207, DOI: 10.1039/C8SC03510E). Prof. Xuanjun Zhang and Prof. Leilei Tian et al. developed a new DNA nanoprobe based on a Y-shape and pyrene-modified DNA self-assembly for microRNA imaging in live cells (Chem. Commun., 2020, 56, 1501–1504, DOI: 10.1039/C9CC08093G).

AIEgens also found many applications in optoelectronics. For example, Prof. Juozas V. Grazulevicius et al. synthesized blue/sky-blue AIDF emitters for non-doped and doped organic light-emitting diodes (OLEDs) with maximum external quantum efficiencies of 6.6% and 16.3% (J. Mater. Chem. C, 2018, 6, 13179–13189, DOI: 10.1039/C8TC04867C). Prof. Wallace W. H. Wong et al. utilized donor and acceptor AIEgens as energy-transfer pairs that can maximize light-harvesting as well as reduce reabsorption simultaneously (Mater. Chem. Front., 2018, 2, 615–619, DOI: 10.1039/C7QM00598A).

In the field of chemical sensors, Prof. Jin Ouyang et al. reported a dual-emission fluorescent sensor based on AIE-Au nanoclusters for the detection of mercury and melamine (Nanoscale, 2015, 7, 8457–8465, DOI: 10.1039/C5NR00554J). Prof. Loredana Valenzano, Prof. Jianbo Wang, Prof. Marina Tanasova, Prof. Fen-Tair Luo and Prof. Haiying Liu et al. prepared an AIE cyanine-based fluorescent cassette which offered sensitive imaging of pH changes in live cells via through-bond energy transfer from TPE to cyanine (Chem. Commun., 2018, 54, 1133–1136, DOI: 10.1039/C7CC08986D). Prof. F. Christopher Pigge and Prof. Moustafa T. Gabr reported a fluorescent turn-on probe for cyanide anion detection based on an AIE active cobalt(II) complex (Dalton Trans., 2018, 47, 2079–2085, DOI: 10.1039/C7DT04242F). Prof. Hongwei Hou and Prof. Kai Li et al. reported a novel o-phthalimide-based AIEgen of 2,3-diphenylquinoxaline-6,7-dicarboimide for multifunctional sensing applications, such as pH, UV and Hg sensors (Mater. Chem. Front., 2019, 3, 50–56, DOI: 10.1039/C8QM00454D).

AIE also demonstrates great applicability in the fields of mechano-chromism/luminescence, stimuli-responsive materials, polymer chemistry, etc. Prof. Cassandra L. Fraser et al. developed a halide-substituted difluoroboton β-diketonate dye with mechanochromic luminescence properties (J. Mater. Chem. C, 2015, 3, 352–363, DOI: 10.1039/C4TC02268H). Prof. Zhen Li demonstrated that the special molecular packing of AIEgen 1,1,2,2-tetrakis(4-methoxyphenyl)ethane, in the P2₁(c) crystal accounts for its efficient mechanoluminescence performance (Mater. Horiz., 2016, 3, 220–225, DOI: 10.1039/C6MH00025H). Prof. Pakkirisamy Thilagar et al. reported two multifunctional AIEgens – 10-(dimesitylboryl)phenothiazine and 10-(bis(2,6-dimethylphenyl)boryl)phenothiazine – with triboluminescence and mechanofluorochromism properties, and temperature sensing characteristic (Chem. Commun., 2017, 53, 3641–3644, DOI: 10.1039/C6CC09717K). Prof. Dongxia Zhu, Prof. Zhongmin Su and Prof. Martin R. Bryce et al. designed and synthesized a new AIE-active neutral Ir(III) complex which exhibited piezochromic luminescence properties (J. Mater. Chem. C, 2019, 7, 10876–10880, DOI: 10.1039/C9TC03646F). Prof. Cassandra L. Fraser et al. reported difluoroboron β-diketonate AIEgens, which exhibited solid-state switchable luminescence under an external stimulus (Mater. Chem. Front., 2017, 1, 158–166, DOI: 10.1039/C6QM00008H). Prof. Maciej Kopeć reported a low molecular weight (M_n < 10k) bulk polyacrylonitrile as well as thin film (d < 15 nm), surface-grafted polymer brushes with AIE properties (Polym. Chem., 2020, 11, 669–674, DOI: 10.1039/C9PY01213C).

The publications highlighted in this virtual themed issue focus on the progress in AIE systems over the past 20 years. We sincerely hope that this special issue will attract a broad range of readers to the area of AIE research. We sincerely thank the Royal Society of Chemistry editorial teams for their great support to make this themed issue possible.

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