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AIEgens: Synthesis and Applications

       Ming Chen1, Anjun Qin3, and Ben Zhong Tang2,3,4

       1College of Chemistry and Materials Science, Jinan University, Guangzhou, China

       2Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China

       3State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, Center for Aggregation‐Induced Emission, South China University of Technology, Guangzhou, China

       4Shenzhen Institute of Aggregate Science and Technology, School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong, China

      Luminescent materials have been widely applied in display, illumination, and information transfer, etc [1–3]. However, in real‐word applications, they are most employed in the aggregate state (e.g. solid‐state‐emissive thin film in organic light‐emitting diodes (OLEDs) and nanoimaging materials) [4–8]. The traditional chromophores show extremely strong emissions in the solution but exhibit almost quenched emission behavior upon aggregation. For such a reason, scientists are in enthusiastic pursuit of highly efficient luminescent materials in the solid state. One would consider it to be better if the aggregation could be utilized to play a positive instead of a negative role in enhancing luminescence. This idea had come true until 2001 when Tang had a beautiful encounter with aggregation‐induced emission (AIE) [9–12]. AIE luminogens (AIEgens) possess the luminescence behavior opposite to the traditional luminogens as their twisted and flexible molecular conformation allows them to dissipate the excited‐state energy nonradiatively by molecular motion in the solution, while such motion is suppressed in the aggregate state to open up the radiation channel [13–16]. Thus, the problem of aggregation‐caused quenching (ACQ) effect perplexed in traditional dyes has been overcome thoroughly by AIE, and more than 10 000 works aiming at mechanism study, molecular design, and functionality exploitation have been published based on this hot topic.

Schematic illustration of molecular structures of AIEgens of tetraphenylethene (TPE), triphenylethene, tetraphenyl-1,4-butadiene (TPBD), distyrylanthracene (DSA), hexaphenylsilole (HPS), pentaphenylpyrrole (PentaPP), phenyl-substituted oxidized benzothiophene (DP-BTO), and tetraphenylpyrazine (TPP).

      By contrast, no central double bond exists in the heterocycle‐based AIEgens, making them free of such trouble. The typical heterocycle‐based AIEgens are hexaphenylsilole (HPS), pentaphenylpyrrole (PentaPP), phenyl‐substituted oxidized benzothiophene (DP‐BTO), and so on Chart 1.1 [24–30]. The introduction of heteroatoms in these AIEgens obviously endows them with different electronic properties. For example, in HPS, the interaction of σ* orbital of the silicon atom and π* orbital of the carbon atom enables it to possess low‐lying LUMO energy level. It imparts HPS with high electron affinity, which can act as an electron‐accepting unit in molecular design and increase the electron‐transporting property as material [31]. On the other hand, the orbitals of the nitrogen atom in PentaPP is sp2‐hybridized, while the lone pair electrons occupy the p orbital, which arrays parallelly with the p orbitals from adjacent carbon atoms. It remarkably increases the pπ interaction and makes the central pyrrole ring electron‐rich. Some electron‐donating and hole‐transporting properties of PentaPP are thus obtained in molecular and material designs [32]. However, although these heterocycle‐based AIEgens show good photo‐ and thermal stabilities, their chemical stability should be improved. For example, the silole ring in HPS is easy to decompose under basic atmosphere. Besides, their synthesis is always tedious, the reaction condition is rigorous, and the purification is difficult. Thus, it is urgent to develop new AIEgens in combination with the advantages from the above hydrocarbon and heterocycle ones.

      With the development in the last five years, more and more works with TPP as motif have emerged. Firstly, besides the conventional methods, the new methodologies in the preparation of TPP and its derivatives have been established. By using the new catalytic systems, rather high yields (>98%) are obtained in the preparation. A recent work shows that TPP can even be prepared efficiently by direct heating of the starting materials in the solid phase. Secondly, many luminescent applications (e.g. highly efficient OLEDs,

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