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3 Aggregation‐induced Emission from the Restriction of Double Bond Rotation at the Excited State
Ming Hu and Yan-Song Zheng
Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, China
3.1 Introduction
In the whole history of human being, mankind is being dependent on light, from natural sunlight, flame light, incandescent lamp, to light‐emitting diode (LED). While light has become an integral part of human civilization, it is far away from fully understanding the light. Luminescence, a class of light emitted by luminophoric molecules and called as “cold” light unlike sunlight and torch light, not only can light up macroscopic space but can also help us to see the microscopic species such as cell and protein. Therefore, luminescence is a cutting‐edge research field in chemistry, materials, and biology. Countless chemists are committed to the design and preparation of new luminescence materials and their applicational development and mechanistic disclosure and have made a huge contribution to the progress of the scientific community.
During the course of luminescence research over the past decades, people have found that there is a deleterious photophysical phenomenon called concentration quenching (CQ): luminescence of solution becomes dimming and even quenching when concentration is increasing. It has gradually been identified that the quenching of luminescence is caused by the aggregation of luminescence molecules, referred to as “aggregation‐caused quenching” (ACQ). Aromatic hydrocarbon luminophores, due to their high modifiability and diversity, have always been the most studied luminescence materials. However, the ACQ effect is especially obvious in aromatic hydrocarbons and their derivatives [1]. Taking fluorescein as an example (see Figure 3.1, upper panel) [2], the dilute solution of fluorescein in water has a bright green emission but the fluorescence of the solution gradually diminishes and eventually disappears after acetone fraction is increased and aggregates are formed. This ACQ effect is ascribed to the close π–π stacking interactions between fluorescein molecules in aggregated state due to planar structure of the polycyclic aromatic hydrocarbon. While these planar luminophores are beneficial for application in solution, they suffer from severe limitation in applications that must be carried out in aggregated state, such as biosensor [3] and organic light‐emitting diode (OLED) [4], which are usually used in solid state.
Figure 3.1 Fluorescence photographs of solutions and suspensions of (upper panel) fluorescein (15 μM) in water/acetone mixtures with different fractions of acetone (fa) and (lower panel) hexaphenylsilole (HPS; 20 μM) in THF/water mixtures with different fractions of water (fw).
Source: Reproduced with permission from Ref. [2]. Copyright 2014, WILEY‐VCH Verlag GmbH & Co. KGaA.
Figure 3.2 Representative AIEgens, including pentaphenylsilole, tetraphenylethylene (TPE), aryl‐o‐carborane, 1‐cyano‐1,2‐bis(4′‐methylbiphenyl)ethylene (CN‐MBE), and diphenyl dibenzofulvene (DPDBF).
Many efforts have been made to mitigate the effects of the ACQ, but few have succeeded until 2001 [5]. In this year, Tang et al. discovered an uncommon luminophore, 1‐methyl‐1,2,3,4,5‐pentaphenylsilole, which conducted no emission in dilute solution but emitted strong fluorescence in aggregated state. They coined a new phenomenon as aggregation‐induced emission (AIE), which is opposite to the ACQ phenomenon. Since then, a wide variety of molecules such as hexaphenylsilole (HPS) [6], tetraphenylethylene (TPE) [7], 1‐cyano‐1,2‐bis (4'‐methylbiphenyl)ethylene (CN‐MBE) [8], and aryl‐o‐carborane [9] (see Figure 3.2) have been discovered to have AIE properties. These AIE luminogenes are also shortened as AIEgens afterwards. Taking HPS as an example, it is nonemissive when its molecules are dissolved in a good solvent like THF, but its fluorescence turns on and becomes very strong after poor solvent like water was added and a suspension appeared (see Figure 3.1, lower panel). In contrary to the planar structure of the conventional luminophores, AIEgens often have a nonplanar and twisted structure, bearing crowded and anticoplanar aromatic rings. Due to the discovery of this new system and providing a better platform to study fluorescence in aggregated states, many research groups have been enthusiastically working on the practical application and decryption of AIE effect in the past two decades. A large number of magnificent achievements have been reached in various fields, ranging from solid emitter, bioimaging, chemosensing, optoelectronics to stimuli‐responsive systems [10–23].
To better guide the discovery and practical application of novel AIE molecules, an insight into the detailed luminescence process is highly needed. Up to date, although there are a large number of AIE mechanisms that have been put forward, the most popular and the most successful mechanism is the mechanism of restriction of intramolecular rotation (RIR) [12]. By this mechanism, it is known that the substituted groups such as aromatic rings of the AIEgens are able to freely rotate in solution so that the excited energy is released by the nonemissive rotation process and no or weak fluorescence light is emitted. In the aggregated state, the rotation of the substituted groups is restricted and the nonemissive rotation process is blocked. Therefore, the luminophore turns on its luminescence light and even displays strong emission. Later, RIR mechanism incorporates the process of the restriction of intramolecular vibrations (RIVs) and is developed into a mechanism of restriction of intramolecular motions (RIMs), which is applicable for wider AIE
