yield of TPE‐4oM was 640‐fold higher than that of TPE‐4mM in solution.
In 2018, Tang et al. [57] studied a series of TPE derivatives with varying structural rigidities and AIE properties using ultrafast spectroscopy combined with quantum computation. They found that the stretch and twist of the central double bond in TPE unit upon photoexcitation were two dominant events that caused nonradiative decay.
Figure 3.29 shows the structures of TPE derivatives 18–23 in the order of increasing structural rigidity. While 18, 19, and 22 showed typical AIE activity, 20 displayed strong fluorescence in both solution and solid, but 21 and 23 have very low fluorescence quantum yields in both solutions and solids. These phenomena do not seem to match the prediction of the RIM mechanism because the fluorescence quantum yields of their solution should also gradually increase as their rigidity. However, compounds 22 and 23 with the most rigid structures have very low fluorescence quantum yields in solutions. In contrast, the phenyl rings of compound 20 are not hinged by intramolecular cyclization, but its Φf in solution reached an astonishing 60%. In this case, the exact mechanism that affects their fluorescent intensity in solution was worth a further study.
Firstly, they explored the geometry changes of 18–20 and 22 and 23 in THF solution from S0 to S1 using DFT calculation. The calculated results revealed that the absolute change of the phenyl torsion and double bond twisting in TPE derivatives decreased as the rigidity of the molecular structure increases upon excitation. In the excited state, the double bonds of TPE derivatives except for 23 showed a significant extension. Compared with the emission peaks in the film, the fluorescence emission spectra in dilute solutions displayed extra peaks, which were confirmed to be the emission peaks of the photocyclization product by experiments. The above results illustrated that both double bond twisting and phenyl torsion may be responsible for the nonemission of these TPE derivatives in solutions.
Then, they further constructed the 3D potential energy surface (PES) of 18 in solution (see Figure 3.30). Along the minimum energy path (MEP) of 18 in the ground state, the phenyl torsion increased from 50 to 90°, but the change of the twist of double bond (<9°) and potential energy (PE) (<7 kcal/mol) was slight, indicating that the torsion of the phenyl rings dominated the ground‐state dynamics in 18. In the excited sate, the stretch and torsion of the double bond resulted in the FC* geometry changing into minimum energy geometry (S1, minute) along the MEP. In this course, the twist of double bond was ~50°, which was accompanied by the phenyl torsion with an amplitude of less than 25°.
The ultrafast time‐resolved spectroscopy was employed to detect the geometry changes and photocyclized intermediates of 18–23 in excited state. For flexible molecules like 18, 19, and 21, the measurement demonstrated that the stretch of double bond occurred in the subpicosecond timescale (0.6–1.3 ps), and then the stretched double bond began to twist during 1.3–3.79 ps. After 3.79 ps, the photocyclization happened. For 20, due to the steric hindrance from the substituents at the o‐position, the rotation of the double bond and photocyclization were suppressed and made the decay of emission band much longer than 18 and 19. For 22 or 23 with a rigid structure, the formation of the photocyclized intermediate took place directly on the subpicosecond timescale so that the fluorescence was very weak.
Figure 3.29 TPE derivatives 18–23 with increased structural rigidity and their transformation upon UV irradiation (QY: fluorescence quantum yield).
Source: Reproduced with permission from Ref. [57]. Copyright 2018, Royal Society of Chemistry.
There is a competitive relationship between the two processes of photocyclization and intramolecular rotation. When TPE possessed a flexible structure, the rotations of phenyl rings prohibited the photocyclization between two adjacent phenyl rings and allowed the excited double bond to rotate. But when phenyl rings are hinged by ethylene bridge, the short distance of rings promoted the ultrafast formation of the photocyclization on the subpicosecond timescale, giving no opportunity for the C═C bond to rotate. Just like molecule 20, only after these two nonradiative processes were blocked at the same time, TPE derivatives could render strong emission.
Figure 3.30 The PES of 18 in the ground state and excited state as a function of the (quasi) C═C bond twisting and phenyl torsion dihedral angles. (a) Top view of the first excited‐state PES. (b) Top view of the ground‐state PES.
Source: Reproduced with permission from Ref. [57]. Copyright 2018, Royal Society of Chemistry.
In 2018, Sada et al. [58] disclosed the RDBR process of disubstituted TPE derivatives TPE‐2OMe and TPE‐2F through a combination of photochemical experiments and theoretical computations. As shown in Figure 3.31, E‐ or Z‐rich isomers exhibited EZI behavior after the solution was exposed to UV irradiation. Photostationary state approached in four hours under a deep‐ultraviolet (deep‐UV) lamp irradiation (6.2 mW/cm2) or in 48 hours under ambient light. Furthermore, the solution in dark conditions or the solid under a deep‐UV lamp did not show EZI phenomenon, revealing that the EZI process was triggered by UV irradiation and was suppressed in the aggregated state. However, no photocyclization was observed in 1H NMR measurements, indicating that isomerization was indeed contained in the fluorescence measurement process, rather than the photocyclization.
The more detailed process was simulated by calculating the steepest‐descent (SD) pathways in the S1 states for E‐ or Z‐isomer, starting from the FC structures. Along the SD pathways, the rotational motion around the central double bond leads to the perpendicular structure. As the change of the TPE structure, the S1 energy gradually decreased and the S0 energy increased accordingly. Eventually, the S1 and S0 energies came to the closest when the double bond twisted about 90°, suggesting the existence of a CI near that place (see Figure 3.32).
Figure 3.31 (a−c) Photoisomerization of TPE‐2OMe and TPE‐2F in chloroform (a) under deep‐UV lamp irradiation, (b) under ambient‐light irradiation, and (c) in the dark. (d) Photoisomerization of TPE‐2OMe and TPE‐2F in the solid state.
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