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

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 Φ_f 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 from the TPE unit and did not form a new bisignate band. Probably because of more flexibility of the TPE unit in the gem‐isomer than in the cis‐one, DNA was unable to induce the stable single‐handed propeller‐like conformation.

Outstandingly, strong CPL emission was observed in the drop‐cast film from a mixture of cis‐11a and cis‐11b with CT‐DNA in water, while the gem‐isomer–CT‐DNA film emitted no CPL signals. The CPL dissymmetric factor (g_lum) of 0.0028 and 0.016 for cis‐11a and cis‐11b, respectively, was much larger than that from the mixture of DNA with other AIEgens. Even in solution, strong CPL light was emitted from a mixture of cis‐isomer with CT‐DNA in water but no CPL signals were found for the mixture of gem‐isomers with CT‐DNA. With fish sperm DNA (FS‐DNA), a similar result was obtained. Considering the structure of the cis‐isomers, the CPL enhancement should result from the more RDBR process of cis‐isomers in which the formed cycle in situ together with the original cycle in the cis‐position firmly restricts the double bond rotation not only at the ground state but also at the excited state (see Figure 3.21).

Given that the obvious interaction of the TPE diammoniums with DNA, they should be excellent sensor for the detection of DNA. It was truly that the fluorescence of TPE macrocycle diammoniums in water was increased when FS‐DNA was added into the solution at 1.0 × 10⁻⁶ M. But the solution from a mixture of cis‐isomer with FS‐DNA showed stronger fluorescence than that from the corresponding gem‐isomer and FS‐DNA. The fluorescence intensity was linearly increased in the range of DNA concentration less than 1.0 × 10⁻⁸ M. As a result, the detection limit for DNA analysis was obtained. It was found that the detection limits were 123, 74, 496, and 235 pM for 11a, 11b, 12a, and 12b, respectively. The cis‐isomers had always much lower detection limit than the gem‐ones. And the detection limitation of 74 pM from cis‐isomer is among the best results from AIE DNA sensors. The higher sensitivity of cis‐isomers than that of gem‐isomers should also come from the RDBR mechanism. As shown in Figure 3.22, the restriction of a double bond of cis‐isomers upon binding to DNA chain enhanced the AIE effect. Therefore, the sensitivity was significantly increased.

Figure 3.21 CPL spectra of a drop‐cast film (a) and solution (b) from a mixture of cis‐TPE isomers and gem‐TPE isomers with CT‐DNA in water.

When the cis‐TPE macrocycle diammonium was synthesized using octaethylene glycol as a bridge, the resultant crown ether cycle is large enough to allow the EZI. As shown in Figure 3.23, the as‐prepared cis‐13 could be converted into trans‐13 under an irradiation of a 365‐nm