Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

Aggregation-Induced Emission: Together We Shine, United We Soar!

"United we stand, divided we fall."--Aesop. Aggregation-induced emission (AIE) refers to a photophysical phenomenon shown by a group of luminogenic materials that are non-emissive when they are dissolved in good solvents as molecules but become highly luminescent when they are clustered in poor solvents or solid state as aggregates. In this Review we summarize the recent progresses made in the area of AIE research. We conduct mechanistic analyses of the AIE processes, unify the restriction of intramolecular motions (RIM) as the main cause for the AIE effects, and derive RIM-based molecular engineering strategies for the design of new AIE luminogens (AIEgens). Typical examples of the newly developed AIEgens and their high-tech applications as optoelectronic materials, chemical sensors and biomedical probes are presented and discussed.

Aggregation-Induced Emission: The Whole Is More Brilliant than the Parts

Solid state emitters based on excited state intramolecular proton transfer (ESIPT) have been attracting considerable interest since the past few years in the field of optoelectronic devices because of their desirable unique photophysical properties. The photophysical properties of the solid state ESIPT fluorophores determine their possible applicability in functional materials. Less fluorescence quantum efficiencies and short fluorescence lifetime in the solid state are the shortcomings of the existing ESIPT solid state emitters. Designing of ESIPT chromophores with high fluorescence quantum efficiencies and a long fluorescence lifetime in the solid state is a challenging issue because of the unclear mechanism of the solid state emitters in the excited state. Reported design strategies, detailed photophysical properties, and their applications will help in assisting researchers to overcome existing challenges in designing novel solid state ESIPT fluorophores for promising applications. This review highlights recently developed solid state ESIPT emitters with focus on molecular design strategies and their photophysical properties, reported in the last five years.

Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters

It has been difficult to decipher the mechanistic issue whether E/Z isomerization is involved in the aggregation-induced emission (AIE) process of a tetraphenylethene (TPE) derivative, due to the difficulty in the synthesis of its pure E and Z conformers. In this work, pure stereoisomers of a TPE derivative named 1,2-bis{4-[1-(6-phenoxyhexyl)-4-(1,2,3-triazol)yl]phenyl}-1,2-diphenylethene (BPHTATPE) are successfully synthesized. Both isomers show remarkable AIE effect (αAIE ≥ 322) and high fluorescence quantum yield in the solid state (ΦF 100%). The conformers readily undergo E/Z isomerization upon exposure to a powerful UV light and treatment at a high temperature (>200 °C). Such conformational change, however, is not observed under normal fluorescence spectrum measurement conditions, excluding the involvement of the E/Z isomerization in the AIE process of the TPE-based luminogen. The molecules of (E)-BPHTATPE self-organize into ordered one-dimensional nanostructures such as microfibers and nanorods that...

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Click Synthesis, Aggregation-Induced Emission, E/Z Isomerization, Self-Organization, and Multiple Chromisms of Pure Stereoisomers of a Tetraphenylethene-Cored Luminogen

Aggregation-induced emission (AIE) is a unique and significant photophysical phenomenon that differs greatly from the commonly acknowledged aggregation-caused emission quenching observed for many π-conjugated planar chromophores. The mechanistic decipherment of the AIE phenomenon is of high importance for the advance of new AIE systems and exploitation of their potential applications. Propeller-like 2,3,4,5-tetraphenylsiloles are archetypal AIE-active luminogens, and have been adopted as a core part in the design of numerous luminescent materials with diverse functionalities. In this review article, we elucidate the impacts of substituents on the AIE activity and shed light on the structure–property relationship of siloles, with the aim of promoting the judicious design of AIE-active functional materials in the future. Recent representative advances of new silole-based functional materials and their potential applications are reviewed as well.

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Aggregation-induced emission of siloles

An anthracene carboxamide derivative of the excited-state intramolecular proton-transfer compound of 2-(2′-hydroxyphenyl)benzothiazole has been newly developed to produce the prominent characteristics of aggregation-induced enhanced emission (AIEE) with a high solid-state fluorescence quantum efficiency of 78.1%. Compared with our previously reported phenyl carboxamide derivatives, a small tailoring of the molecular structure was found to result in a big difference in the dominant factor of the AIEE mechanism. In the phenyl carboxamide derivatives, the dominant factor of the AIEE mechanism is the restriction of the twisted intramolecular charge transfer (TICT) of the enol excited state, regardless of their different aggregation modes. In the anthracene carboxamide derivative, N-(3-(benzo[d]thiazol-2-yl)-4-hydroxyphenyl) anthracene-9-carboxamide, the AIEE characteristics are not dependent on the restriction of TICT, but mainly attributed to the cooperative effects of J-aggregation and the restriction of the cis–trans tautomerization in the keto excited state. A specific N⋯π interaction was found to be the main driving force for this J-aggregation, as revealed by the single crystal analysis. The AIEE mechanism of this anthracene carboxamide derivative was studied in detail through photophysical investigations and theoretical calculations. On the basis of its AIEE characteristics, a stable non-doped organic light-emitting diode was achieved, with high color purity and a remarkably low efficiency roll-off.

A small change in molecular structure, a big difference in the AIEE mechanism

This study developed a new solid-state, highly emissive phenylbenzoxazole-based organic compound, N-(4-(benzo[d]oxazol-2-yl)phenyl)-4-tert-butylbenzamide, which exhibited a distinct aggregation-induced enhanced emission. The solid fluorescence efficiency of the newly developed compound was 50.3%, whereas that in THF solution was only 0.22%. The single-crystal analyses revealed that a specific three-dimensional #-shaped cross stacking between molecules was observed in the solid/aggregated state, driven by specific C–H···π interaction and various hydrogen bonds. The expansion of the cross-dipole stacking into the three-dimensional network was believed to be the dominant factor for the emission enhancement in the solids/aggregates, with respect to the assistant effect of the photoinduced twisted intramolecular charge transfer restriction.

More than Restriction of Twisted Intramolecular Charge Transfer: Three-Dimensional Expanded #-Shaped Cross-Molecular Packing for Emission Enhancement in Aggregates

A new spiro[fluorene-9,9′-xanthene]-based host material possessing no conventional hole- and electron-transporting units for efficient and low voltage blue PHOLED via simple two-step synthesis

White-light nano particle preparation method

The invention relates to a preparation method of an optical switch molecular compound based on diarylethene Schiff base. The preparation method comprises the following two reaction steps: (1) synthesizing an aldehyde compound based on diarylethene with the photochromic property in a corresponding solvent through processes such as chlorination of 2-methyl thiophene, acylation and reaction of McMurry, and carrying out purification and separation to obtain an intermediate product with high purity; and (2) reacting arylamine with the aldehyde compound for about 2-48 hours at 0-150 DEG C, generating precipitates in the solution after reaction is ended, and filtering and washing the precipitates to obtain the novel optical switch molecular compound based on the diarylethene Schiff base. The preparation method provided by the invention is simple and convenient in separation, purification and operation, high in yield, low in cost and environment-friendly and has a mass production value; and the reaction is non-irritating and non-corrosive. The structure general formula of the novel optical switch molecular compound based on the diarylethene Schiff base is as shown in the specification, wherein R in the formula I is an arylamine group.

Yuezhi Zhao

Papers

A small change in molecular structure, a big difference in the AIEE mechanism

More than Restriction of Twisted Intramolecular Charge Transfer: Three-Dimensional Expanded #-Shaped Cross-Molecular Packing for Emission Enhancement in Aggregates

A new spiro[fluorene-9,9′-xanthene]-based host material possessing no conventional hole- and electron-transporting units for efficient and low voltage blue PHOLED via simple two-step synthesis

White-light nano particle preparation method

Preparation method of optical switch molecular compound based on diarylethene Schiff base