in fluorescence quantum yield. As shown in the molecular structure of HPS, the central silole is connected with six phenyl rings by rotatable single bonds, which make HPS to show a great conformational flexibility in isolated phase. Due to the six phenyl rings around a tiny core, the HPS molecular structure is rather distorted. This torsion makes a great tension between adjacent phenyl rings, and it is difficult for the entire molecule to form a planar conformation. It can be seen from the crystal structure that HPS molecules possess a propeller‐like conformation and each phenyl rings exhibit different degrees of distortion compared with the silole core. This nonplanar structure makes it difficult for molecules to pack tightly. Therefore, no π−π stacking interaction is observed for HPS in the solid state, avoiding the ACQ effect. Meanwhile, the dynamical rotation of six phenyl rings is restricted in the solid state and the nonradiative channel is blocked. Therefore, in aggregates, high emission is accessible.
Figure 3.7 Structure of DPDSB and BDPVA and their cross‐dipole stacking.
Figure 3.8 Emission of CN‐MBE in the solid state by molecular coplanarization.
Source: Reproduced with permission from Ref [8]. Copyright 2002, American Chemical Society.
Figure 3.9 Molecular structure and crystal structure of TOP viewed (a) perpendicular to the molecular plane and (b) along the molecular plane; (c) the slipped π–π packing of the molecules.
Figure 3.10 (a) Fluorescence quantum yield of HPS vs water fraction in acetone/water.
Source: Reproduced with permission from Ref. [11]. Copyright 2003, American Chemical Society.
(b) Molecular conformation of HPS.
Source: Reproduced with permission from Ref. [12]. Copyright 2015, American Chemical Society.
(c) HPS molecule emission.
Source: Reproduced with permission from Ref. [15]. Copyright 2015, Royal Society of Chemistry.
However, it is found that some luminophores lack substituents that can rotate but display typical AIE phenomenon. For example, annulenylidene THBA (see Figure 3.11a), whose phenyl rings are fixed by a pair of ethylene tethers, still exhibits strong emission in aggregates but no emission in dilute solution [12, 31, 32]. Consequently, the RIV mechanism is raised. For this class of luminophores, intramolecular vibration instead of rotation is restricted in the aggregated state, which turns on the luminescence. Recently, Tang et al. reported cyclooctatetrathiophene (COTh) and its derivatives that are AIE active [33]. There are no rotatable unit in the molecular structure of COTh (see Figure 3.11b) in both ground and excited states. However, in solution, the intramolecular vibration of excited state could cause up‐down conformation inversion and result in a fluorescent quenching. Because of the RIV mechanism, COTh emits a strong green fluorescence in the solid state.
If an AIEgen includes both rotating and vibrating units, obviously, neither RIR nor RIV can explain its AIE effect alone. Therefore, RIM mechanism is used to explain AIEgens with more complex structures and emission units. Some examples of such AIEgens are shown in Figure 3.12 Those molecules are suitable for the RIM mechanism because of their multiform motions of isolated molecules [34–37]. RIM mechanism advocates that the fluorescent quenching of AIEgens in dilute solution results from various ways of intramolecular motions including twisting, deforming, flapping and probably motion of solvating molecules in addition to rotation and vibration. While it greatly broadens the scope of application of the AIEgens, it also poses new challenges for researchers. Because, under real conditions, in order to better guide the optimization of the structure of AIEgens, people need to know the exact factor of motions, rather than simple and general motions. Consequently, although the RIM mechanism is the most popular and applicable, a lot of detail contents need to be embodied and supplied.
Figure 3.11 (a) RIV mechanism of THBA.
Source: Reproduced with permission from Ref. [12]. Copyright 2014, WILEY‐VCH Verlag GmbH & Co. KGaA.
(b) RIV mechanism of COTh.
Source: Reproduced with permission from Ref. [33]. Copyright 2019, Springer Nature.
Figure 3.12 Examples of luminogens whose AIE activities are ascribed to the process of restriction of intramolecular motions (RIMs). (a) Molecular structure of PTZ‐BZP and DFT‐optimized geometry of a simplified structure with a methyl substituent.
Source: Reproduced with permission Ref. [34]. Copyright 2014, WILEY‐VCH Verlag GmbH & Co. KGaA.
(b) Molecular structure and single‐molecular structure of dP‐TCAQ.
Source: Reproduced with permission from Ref. [35]. Copyright 2013, Royal Society of Chemistry.
3.2.2 Investigation of RDBR AIE Mechanism by E/Z isomerization
It is known in the textbook of organic chemistry that a carbon–carbon double bond is unable to rotate in general conditions. However, under irradiation or at high temperature, the π‐bond of the double bond can undergo homolytic cleavage and the double bond can rotate. The most well‐known example is molecular photoswitches or even molecular motors in which the Z‐isomer and E‐one can be reversibly transformed into each other (E/Z isomerization, EZI) through double bond rotation under irradiation or heating [38, 39]. Among AIEgens, most of them
