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

      Zhao et al. [54] report results of the semiclassical simulation study of the excited‐state dynamics of photoisomerization of TPE. By monitoring the change of the length with time, the stretching vibrational mode of ethylenic bond in the excited state was examined. When TPE was excited by a femtosecond laser pulse, the central double bond was excited to stretch from the initial 1.37 to around 2.20 Å in 300 fs. Then, the twisting motion of the fully extended double bond was activated by the energy released from the relaxation of the stretching mode, until the central double bond formed a perpendicular formation and gave an ethylenic bond twisted about 90°. This process was completed in 600 fs, and this twisted structure remains approximately until about 4800 fs. At 4800 fs, a nonadiabatic transition to the electronic ground state occurred. The results of the simulation clearly showed that the ethylenic bond twisting takes place in the subpicosecond scale. This research first revealed the important influence of twisting of the ethylenic bond on the nonradiative decay of the photoexcited TPE at molecular levels through the employment of computational studies.

Corminboeuf et al. [55] descripted excited‐state dynamics of isolated TPE through trajectory surface hopping (TSH) simulations using linear response time‐dependent density functional theory (TD‐DFT) within the Tamm–Dancoff approximation (TDA) at the PBE0/def2‐SVP level. By analyzing motion trajectories, they found that the excited TPE undergoes an ultrafast CI to the ground state through the rotation of the excited double bond. As shown in Figure 3.27, with the rotation of the C═C bond, the energy of the initial excited state (S₁, red curve) continued to decrease, but the ground‐state (S₀, magenta curve) energy was increasing. After ~1 ps, the S₁ state became nearly degenerate with the ground state and eventually reached the CI between the S₁ and S₀ states. To efficiently reach the S₀/S₁, CIs were responsible for fluorescence quenching after TPE photoexcitation in solution. However, there were more trajectories (75%) that decayed to the ground state through photocyclization. The author also found that the two processes are incompatible. The phenyl rings were initially close to one another and cyclization dominated. As the twisting motion around the central double bond proceeded, the cyclization became inaccessible and another decay channel (ethylenic twist) opened.

Figure 3.27 The twist angle of the double bond (upper panel) and electronic‐state potential energies (lower panel) as a function of time for two representative trajectories showing the ethylenic twist process.

      Source: Reproduced with permission from Ref. [55]. Copyright 2016, Royal Society of Chemistry.

Figure 3.28 Molecular structures of TPE‐4mM and TPE‐4oM and their fluorescent quantum yields in THF (Φ_f.s) are shown below.

Thiel et al. [56] reported a calculation study of two TPE derivatives, TPE‐4mM and TPE‐4oM (see Figure 3.28 right) in the isolated gas‐phase state. There is a huge difference of fluorescence quantum yields between TPE‐4mM (Φ_f = 0.1%) and TPE‐4oM (Φ_f = 64%) in solution. They combined static electronic structural calculations (TD‐DFT, CASSCF, and MS‐CASPT2) and OM2/MRCI nonadiabatic dynamics simulations to explore the nonradiative excited‐state decay mechanisms of them. The computational results showed two pairs of minimum‐energy S₁/S₀ CI structures for both TPE‐4mM and TPE‐4oM. For TPE‐4mM, there was no barrier to reach the CI of photocyclization. The energy barrier for CI of the π twist was small (1.8 kcal/mol), indicating that the rotation of the double bond may also be blamed for the nonemission of TPE‐4mM in solution. But in contrast, for TPE‐4oM, the ortho‐methyl groups in TPE‐4oM effectively suppressed the rotation of both the phenyl rings and double bond. The energy barriers for the above two decay paths were non‐negligible barriers (6.2 and 8.4 kcal/mol, respectively), which prevented the nonradiative relaxation of TPE‐4oM. Consequently, the fluorescent quantum 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 Φ_f 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