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 molecules [12–17].

However, many AIEgens such as TPE, α‐cyano‐stilbene, and their derivatives, which are most extensively studied as AIE molecules, bear a central double bond [12–17]. Under irradiation, their double bond may undergo rotation due to the homolytic cleavage of the π‐bond at the excited state (see Figure 3.3). Direct and indirect evidence as well as theoretical calculation have demonstrated the restriction of the double bond rotation (RDBR), just like the restriction of the phenyl ring rotation, and play a key role on the AIE phenomena. Some researchers even consider that the RDBR mechanism has a major effect on the AIE effect. Therefore, RDBR is the important complement and enrichment of RIR mechanism. It further specifies the factors that affect the AIE phenomena and will be beneficial for exactly designing and improving novel AIE molecules. Therefore, we make a review on the RDBR mechanism of AIE phenomena in this chapter.