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.
Figure 3.3 The rotation of a double bond at the excited state results in no fluorescence of TPE in solution, but in solid state, the rotation is blocked and strong emission turns on.
3.2 AIE Phenomena and Applications from RDBR Mechanism
3.2.1 Evolvement and Development of AIE Mechanisms
Since AIE concept is coined for the first time by Tang’s group in 2001, a large number of AIE molecules are reported. Accompanying the appearance of the AIE molecules in massive quantities, different kinds of AIE mechanisms have also been suggested. In general, when a planar luminophore forms an excimer after irradiation, the excited energy of one molecule will be transferred to other one and nonradiative decay occurs. Therefore, the fluorescence of the luminophore will be quenched in solid state. However, in 2007, Tao et al. [24] found that fluorenone derivatives 2,7‐dip‐tolyl‐fluorenone (DTFO) and 2,7‐bis(4‐(tert‐butylthio) phenyl)‐fluorenone (DSFO) could form excimers but displayed AIE effect. Due to the formation of a dimer through hydrogen bonds in the crystal state, DSFO could give an excimer without arrangement adjustment after the dimer was excited. The excimer could decay back to the dimer, and the nonradiative decay pathways that existed in common excimers were greatly reduced, offering a strongly enhanced luminescence in the solid state (see Figure 3.4).
In 2004, Park et al. [25] reported that 1‐cyano‐trans‐1,2‐bis‐(3′,5′‐bis‐trifluoromethyl‐biphenyl) ethylene (CN‐TFMBE) could strongly gelatinate 1,2‐dichloroethane. The resultant gel emitted a strong fluorescence but the solution showed no fluorescence. They ascribed the AIE effect to the J‐aggregation of CN‐TFMBE molecules in solid state because a stronger red‐shift emission was observed in a mixed solvent of THF/H2O 1 : 4 compared with the solution in THF. Meanwhile, the absorption band of the suspension in THF/H2O 1 : 4 also displayed an obvious red shift with that of solution. But later, Ma et al. [26] found that the similar α‐cyano‐stilbene derivative CN‐DPDSB did not form compact π–π stacking in the crystal structure due to large cyano and phenyl groups. Instead, strong intermolecular cyano nitrogen–hydrogen interactions (2.43 and 2.56 Å) restricted the rotation of a double bond and a phenyl group in the solid state, causing its AIE effect (see Figure 3.5).
Figure 3.4 Structure of DTFO, DSFO, and excimer of DSFO.
Baumgartner et al. [27] synthesized a series of amphiphilic phosphonium compounds including DTP, which could form liquid crystalline and soft crystal phases in the solid state, emitting fluorescence 40 times stronger than its solution. It was found that DTP formed a folded conformation in solution in which the electron‐rich trialkoxybenzyl group was close to the electron‐deficient phospholium‐containing fused scaffold. In this closed structure, the photoinduced electron transfer (PET) process combined with phenyl rotation led to weak emission. In contrast, the solid DTP possessed an opened conformation with the trialkoxybenzyl ring pointed away from the main fused scaffold and the PET process disappeared. Therefore, DTP emitted a stronger fluorescence (see Figure 3.6).
Figure 3.5 Structure of CN‐TFMBE and CN‐DPDSB.
Figure 3.6 Structure of DTP and its PET process.
By cross‐dipole stacking instead of H‐aggregation, stilbene derivatives trans‐DPDSB without a cyano substituent could emit a strong blue fluorescence in the crystal state. It is considered that the cross‐dipole stacking of the trans‐DPDSB molecules can increase a finite transition dipole moment between the ground state and the lowest excited state of the clusters, promoting the light‐emission properties [28]. Tian et al. [29] also found that a butterfly‐like molecule 9,10‐bis(2,2‐diphenylvinyl) anthracene (BDPVA) formed cross‐dipole stacking in the crystal state and emitted a stronger fluorescence in solid than in solution (see Figure 3.7).
In addition, molecular planarization is also one of possible AIE mechanisms. Park et al. [8] ever reported that 1‐cyano‐trans‐1,2‐bis‐(4′‐methylbiphenyl)‐ethylene (CN‐MBE) possessed a twisted structure so that it emitted no fluorescence in solution. However, in solid state, the twisted structure was stretched out to give a more coplanar structure, which was helpful for its J‐aggregation and showed a strong emission (see Figure 3.8).
Very specially, Zheng et al. [30] recently found that a totally planar molecule 5,7,12,14‐tetraoxapentacene (TOP) also displays the typical AIE effect. In highly polar solvent, TOP had no emission due to the solvation role but emitted a strong fluorescence in solid state because of the largely slipped π–π stacking. Although this molecule is planar, oxygen atoms on the different molecular planes prevented cofacial π–π stacking that can arouse ACQ result because of the repulsive force between oxygen atoms (see Figure 3.9).
Although these above AIE mechanisms can well explain the emission or emission enhancement of some special luminophores in solid state, they are not applicable for more extensive and typical AIEgens such as TPE and phenylsilole derivatives until the RIR mechanism was put forward by Tang et al. [12–17]. The RIR mechanism was first proposed to explain the AIE effect of silole derivatives such as HPS (see Figure 3.10). HPS is a typical AIE molecule. When HPS molecules are dissolved in a good solvent at low concentrations, fluorescent quantum yield of the solution is extremely low (Φf ∼ 0.1%). However, the emission is dramatically enhanced after the fraction (fw) of poor solvent such that water reaches ∼50 vol % and reaches a maximum of ∼22% at fw
