3.2.3 Investigating of RDBR AIE Mechanism by Immobilization of TPE Propeller‐like Conformation
In addition to the observation of the EZI process that can disclose RDBR mechanism, immobilization of TPE propeller‐like conformation, especially cyclization of TPE at cis‐position, can be used to explore the RDBR process. After bridging between two phenyl rings of TPE at the cis‐position, the double bond will be unable to freely rotate due to the restriction of the bridge chain. But the phenyl rings can still freely rotate. Therefore, the effect of RDBR on the fluorescence will be clearly observed.
In 2016, Zheng’s group [43] found an ideal route for the synthesis of cis‐TPE dicycle in which the double bond rotation can be blocked at the excited state (see Figure 3.15). By intramolecular nucleophilic substitution of 2 with 1,4‐bis(bromomethyl)benzene, cis‐TPE dicycle tetraldehyde 3 could be obtained in a 43% yield. The formation of cis‐TPE dicycles between two phenyl groups at the cis‐position instead of those at the gem‐position was ascribed to the length of 1,4‐benzenedimethyl tether that was more compatible with the distance between two cis‐phenyl rings than that between two gem‐phenyl ones. With this key intermediate in hand, even TPE tetracycle 6 whose propeller‐like conformation was completely immobilized was obtained.
Noticeably, while the TPE derivatives 1 and 2 bearing no intramolecular cycle did not emit fluorescence in solution, cis‐TPE dicycles 3–5 displayed a strong emission in solution. In THF, Φf of 2 was 0% but that of 3 and 4 was 24 and 49%, respectively. Due to the bridge, p‐phenylenedimethyl units were short and rigid, and the propeller‐like conformation of the TPE dicycle should have been fixed. However, the enantiomers obtained by chiral high‐pressure liquid chromatography (HPLC) rapidly racemized in solution at room temperature. This result indicated that the phenyl rings of the cis‐TPE dicycles was still able to rotate although the rotation had a little restriction. In addition, the formaldehyde groups of cis‐TPE dicycle 3 reacted with chiral α‐methylbenzylamine to form imine, and positive circular dichroism (CD) signals could be induced by (R)‐α‐methylbenzylamine but negative CD signals were aroused by (S)‐α‐methylbenzylamine. The CD signals should result from a single‐handed propeller‐like conformation induced by the enantiomer of the chiral amine because neither individual 3 nor amine enantiomers could show such signals. This meant that the propeller‐like conformation of 3 could be transformed from the left‐handed helical direction to the right‐handed helical direction, demonstrating that the phenyl could freely rotate. Consequently, it could be inferred that about 25–50% Φf should come from the RDBR process (see Figure 3.16).
As expected, the resolved enantiomers from TPE tetracycle 6 were stable due to complete immobilization of its propeller‐like conformation. The racemate of 6 had Φf up to 97%, and both of the two enantiomers had a quantitative Φf up to 100% due to restriction of not only double bond rotation but also phenyl ring rotation. Compared with the corresponding TPE dicycle 4, TPE tetracycle 6 showed a twofold increase in fluorescence intensity, demonstrating that RDBR and RIR play equal key roles on the AIE effect.
In addition, the two enantiomers of 6, M‐6 (left‐handed helical propeller‐like configuration) and P‐6 (left‐handed helical one), emitted strong circularly polarized luminescence (CPL) signals in THF with a dissymmetric factor (glum) of +3.1 × 10−3 for M‐6 and −3.3 × 10−3 for P‐6, indicating that the propeller‐like conformation of TPE was maintained even at the excited state. Moreover, the |glum| of CPL was similar with the gabs of absorbance (gabs = 2(Δε/ε)) of 2.4 × 10−3 for M‐6 and P‐6, indicating little conformational change between the ground state and excited state. This further confirmed that no double bond rotation occurred for this TPE tetracycle (see Figure 3.17).
Figure 3.16 CD spectra of a mixture of TPE dicycle 3 and enantiomer of α‐methylbenzylamine 7 in the presence of acetic acid in 1,2‐dichloroethane.
Figure 3.17 The crystal structures of M‐6 (a) and P‐6 (b); (c) photos of 3, 4, and 6 in THF solution under a 365‐nm UV light and (d) CPL spectra of M‐6 and P‐6 in THF (1.0 × 10−3 M).
Source: Reproduced with permission from Ref. [43]. Copyright 2016, American Chemical Society.
In order to further disclose the important contribution of the RDBR process to the AIE effect, gem‐TPE dicycles 7 and 8 and even the typical isomers cis‐ and gem‐TPE dicycles 9 and 10 were designed and synthesized in Zheng’s group (see Figure 3.18) [44]. Their configuration had been confirmed by the crystal structure. By comparing the fluorescence intensity of these isomers, the effect of groups, atoms, and the bridge chains on the fluorescence could be excluded. Therefore, more direct and more exact RDBR evidence could be furnished.
Figure 3.18 (a) Structures of TPE dicycle isomers 7–10 (left) and photos of their solution in THF (1.0 × 10−4 M) under a 365‐nm UV light (middle). (b) The change of 1H NMR spectra of 7 in CDCl3 with temperature.
Source: Reproduced with permission from Ref. [44]. Copyright 2018, American Chemical Society.
As expected, while cis‐TPE dicycles 3, 4, and 9 emitted a strong fluorescence, the gem‐TPE dicycles 7, 8,
