Handbook of Aggregation-Induced Emission, Volume 1. Группа авторов

The racemate of 6 had Φ_f up to 97%, and both of the two enantiomers had a quantitative Φ_f 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 (g_lum) 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 |g_lum| of CPL was similar with the g_abs of absorbance (g_abs = 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, and 10 had no emission in THF under an irradiation of a 365‐nm UV light. The fluorescence quantum yields of the cis‐TPE dicycles 3, 4, and 9 were 24, 49, and 22%, while those of the gem‐TPE dicycles 7, 8, and 10 were 3.1, 5.7, and 1.9%, respectively, in THF at 25 °C. Compared with the gem‐isomer 10, the cis‐isomer 9 displayed an increase of fluorescence intensity by 11.6‐fold. From the ¹H NMR of the gem‐TPE dicycle 7 at different temperatures, the phenyl ring of the TPE unit was fixed and failed to rotate due to linkage of the bridge at the m‐position of the phenyl rings. However, the bridging unit 1,4‐benzenedioxymethyl was rapidly rotating at 25 °C because the proton peaks of both the 1,4‐benzenedioxy and methylene unit were very wide and flat. When the temperature was lowered to 0 °C, these proton resonance signals became sharp and well resolved, indicating that the free rotation of these bridge units was also restricted. However, the Φ_f of 7 was only increased to 9.0% at 0 °C and was still much less than that of cis‐isomers. Compared with the cis‐isomer, the fluorescent decrease of the gem‐isomer in solution only came from free rotation of the double bond at the excited state after all other intramolecular rotations had been restricted (see Figure 3.18 right).

      Time‐resolved fluorescence decay of TPE dicycles 3, 4, and 7–10 disclosed that the fluorescence lifetime was in the range of 6.9–14 ns in THF and 13–19 ns in suspension for these TPE dicycles. In suspension, their fluorescence lifetime was always larger than that in solution. This demonstrated that there was further restriction of intramolecular rotation in solid state. By making use of the fluorescence quantum yields and lifetime values, the radiative (k_f) and nonradiative (k_nr) rate constants of 3, 4, and 7–10 (k_f, k_nr (ns⁻¹)) could be calculated. The k_f and k_nr were 0.035 and 0.112 for 3, 0.051 and 0.053 for 4, 0.004 and 0.126 for 7, 0.004 and 0.067 for 8, 0.023 and 0.080 for 9, and 0.002 and 0.103 for 10. And ratios of k_nr vs. k_f for 3, 4, and 7–10 were 3.20, 1.04, 31.5, 16.8, 3.48, and 51.5, respectively. The k_nr/k_f ratio from gem‐isomers was always much larger than that from cis‐isomers, demonstrating a more nonradiative process from gem‐isomers. This nonradiative process should be mainly ascribed to the double bond rotation.

      If the double bond rotated at the excited state, one intermediate state in which sp²‐hybridized orbital planes of two carbons are vertical instead of coplanar should exist. This twisted state of the double bond should be able to be observed by femtosecond transient absorption spectra because of the decreased conjugation with one another. It was true that two excited‐state absorption (ESA) bands, which were located at <460 and >600 nm, respectively, were observed. The former should come from a twisted excited double bond and the latter came from a planar double bond at the excited state, corroborating the occurrence of the double bond rotation. Surprisingly, the two ESA bands existed both in gem‐isomers and in