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 Φf 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 (kf) and nonradiative (knr) rate constants of 3, 4, and 7–10 (kf, knr (ns−1)) could be calculated. The kf and knr 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 knr vs. kf for 3, 4, and 7–10 were 3.20, 1.04, 31.5, 16.8, 3.48, and 51.5, respectively. The knr/kf 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 cis‐isomers. Two typical time components τ1 and τ2 from the dynamic decay process were obtained. By the global analysis, the first component τ1 representing the rise component of ESA and associated with the geometry relaxation from the Franck–Condon (FC) configuration was the negative amplitude. The second component τ2 represented the nonradiative internal conversion (IC) decay of the excited state. From time‐resolved spectra, an obviously growing process and the increase ending up in less than 20 ps were observed for the species at short wavelength, suggesting that the planar excited state could be transformed into the twisted excited state. Therefore, the double bond rotation at the excited state occurred for all the TPE dicycles.
However, there was an obvious difference of the double bond rotation between the cis‐ and gem‐isomers. As an index of the rotation, the component τ1 was 6 ps for gem‐10 and obviously shorter than 15 ps of cis‐9. This should be ascribed to the more freely rotating double bond in gem‐isomers than in cis‐isomers. At 21 and 14 ps, the rotation was accomplished because the maximum intensity of the absorption spectra of cis‐isomer 9 and gem‐isomer 10 at the excited state was reached, respectively. It was found that the absorption maximum wavelength of the gem‐isomer 10 was shortened by 15 nm compared with that of the cis‐isomer 9. Moreover, the area ratio of short‐wavelength band vs. long‐wavelength band was much larger for the gem‐isomer 10 than that for the cis‐isomer 9, further corroborating that the double bond of the gem‐isomer rotated more freely than that of the cis‐isomer. Therefore, the gem‐isomer showed lower fluorescence quantum yield than the cis‐one because of the freer double bond rotation at the excited state and more nonradiative decay (see Figure 3.19).
Recently, Zheng et al. [45] have exploited the RDBR mechanism to improve the sensitivity of DNA detection and enhance the chiroptical properties from TPE AIEgens. In this regard, cis‐TPE macrocycle diquaternary ammoniums 11 were designed and synthesized. As a comparison, gem‐isomers 12 were also prepared. As soon as the two ammonium arms at the cis‐position simultaneously hold on one DNA chain by electrostatic attraction, the formed cycle together with the original cycle will completely immobilize the double bond rotation at the excited state and arouse the enhancement of the AIE effect (see Figure 3.20).
Figure 3.19 (a) Femtosecond transient absorption spectra of 9 and 10 at the respective maximum intensity. (b) Diagrammatic sketch of the normal excited state (cis* and gem*) and twisted excited state (cis** and gem**) of the double bond.
Source: Reproduced with permission from Ref. [44]. Copyright 2018, American Chemical Society.
Figure 3.20 Structures of cis‐ and gem‐TPE macrocycle diquaternary ammoniums 11 and 12.
Due to the limitation of crown ether cycle, both cis‐11 and gem‐12 display weak emission in solution. However, while the quantum yield of gem‐isomer 12 was 1.5%, the cis‐TPE ammonium 11 had a Φf of 3.0%, which was a 2.0‐fold stronger than that of the gem‐one. This should be ascribed to the partial limitation of the double bond rotation in cis‐isomers but no restriction of the double bond rotation in the gem‐one. When cis‐TPE ammonium cis‐11a or cis‐11b was added to the solution of calf thymus DNA (CT‐DNA), strong new CD signals were induced. In addition to CD signals of DNA itself at short wavelengths, one bisignate band from about 370 nm (+) to 310 nm (−), which should be ascribed to the single‐handed propeller‐like conformation of the TPE unit, appeared. Conversely, gem‐isomers gem‐12a and gem‐12b showed weak CD signals
