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 (glum) 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 portable UV lamp both in organic solvent and in water. Under a 365‐nm light from one fluorophotometer, the absorption spectrum of cis‐13 in CH2Cl2 had a gradual absorbance increase at 355 and 278 nm but a constant decrease at 315 nm with irradiation time, and showed an obvious isosbestic point at 336 and 302 nm, indicating the conversion from cis‐isomer to trans‐one. In CDCl3, after irradiation by a 365‐nm portable UV lamp for one hour, another set of signals appeared beside signals of cis‐13 in the 1H NMR spectrum. Especially, besides the signals of the aromatic proton near to the oxygen atom (6.71 ppm, double), benzyl methylene (4.82 ppm, single), and the ethylene proton close to the aromatic ring (4.09 ppm, triple), new peaks that were well separated and had the same shape and split with that of cis‐isomer appeared at a lower field. The integral area of these new peaks was all almost equal to that of cis‐13, suggesting a 50% conversion of cis‐isomer to trans‐one. In DMSO, the converted trans‐isomer (about 20%) by light could completely come back to cis‐isomer under heating at 180 °C. This result suggested that the double bond of the TPE derivatives was easy to rotate in the excited state.
Figure 3.22 Schematic diagram of the binding of TPE cycle diammoniums 11 (a) and 12 (b) to the DNA strand.
The TPE ammonium 13 was tested for the detection of fish sperm DNA. It was found that cis‐13 had a high sensitivity (139 pM) and high‐intensity ratio Imax/I0 (6.6), whereas a mixture of cis/trans‐13 about 1 : 1 displayed a low‐sensitivity (326 pM) and low‐intensity ratio Imax/I0 (4.5), indicating that the cis‐isomer was a much better DNA sensor than the trans‐one due to more restriction of intramolecular motion including the double bond rotation (see Figure 3.23).
There are more examples in which TPE cycle at the cis‐position could emit fluorescence in solution due to the restriction of double bond rotation. Rathore et al. synthesized a class of stilbenoprismands 14 via an intramolecular McMurry coupling reaction, which contained a TPE core‐bearing cycle at the cis‐side (see Figure 3.24 left) [46]. While the parent TPE showed no detectable emission, the solution of 14 showed significant emission under similar conditions although there were two substituted phenyl rings that could freely rotate. By the direct coordination of TPE tetraimidazolium salts with silver or gold, Hahn and Strassert [47] prepared TPE dicycles 15 on the cis‐side. The TPE dicycles 15 showed strong emission in solution but TPE tetraimidazolium salts emit no fluorescence, demonstrating the important role of the RDBR process on the fluorescence (see Figure 3.24 right).
Hu et al. [48] reported one TPE derivative dicycle 17, in which two pairs of the phenyl rings at the cis‐position were connected by two diacetylenes. While the no cyclized TPE reactant 16 emitted very weak fluorescence in solution, the emission of 17 is very strong even in dilute solution. This fluorescence enhancement after cyclization at the cis‐position should mainly come from the RDBR process (see Figure 3.25).
Figure 3.23 (a) The cis‐/trans‐isomerization of 13 under a 365‐nm light irradiation and heating. (b) Change in UV–vis spectrum of compounds cis‐13 in dichloromethane under an irradiation of 365‐nm light from fluorophotometer for different periods. [cis‐13] = 1.0 × 10−5 M. (c) The 1H NMR spectra of cis‐13 in CDCl3 before and after irradiation by
