href="#fb3_img_img_8e49d4ba-50fd-5b0b-8f17-b401ff3f976e.jpg" alt="Schematic illustration of the structure of AIEgens 14 and 15."/>
Figure 3.24 The structure of AIEgens 14 and 15.
Figure 3.25 The synthesis of the emissive molecule 17.
3.2.4 Research of Theoretical Calculation on RDBR
The effect of restriction of the double bond rotation can be shown intuitively from the experimental phenomenon, but to understand in more details the behavior of the double bond in the excited state and the role it plays, more theoretical studies are needed. In this regard, quantum‐computational simulation and ultrafast time‐resolved spectroscopy are two major methods. The former can simulate the changes in molecular energy and structure during the fluorescence process, by comparing the energy barriers of different decay routes in the excited state to find where the nonradiative relaxation takes place. The latter can probe and resolve the excited‐state dynamics and reaction processes by monitoring the structural changes and the emergence of new species, finding nonradiative process [49, 50].
Through computational studies, detailed activities of the molecule in the excited state can be described, especially in single‐molecular behaviors, which is similar to the state of AIEgens in solution. In this part, an emerging theory called the restricted access to conical intersection (RACI) mechanism has been used for explaining the AIE effect of many AIEgens. Some conical intersections (CIs) are blamed for the weak emission of AIEgens in solution, such as π twist, photocyclization, ring puckering, excited‐state intramolecular proton transfer (ESIPT), bond stretch, and so on [51–53]. Recent computational studies have contributed to the clarification of the excited‐state decay process of TPE in the solution state, and the results indicated the presence of a CI in the ultrafast quenching process of TPE including photocyclization and/or π twist, rather than the propeller‐like rotation of the side phenyl groups. Despite an ongoing debate, there are reports revealing that the twist of the double bond is a typical CI for the deactivation of TPE. Excitation to the S1 state (HOMO → LUMO) causes an elongation of the double bond, which initiates the twisting dynamics. This motion stabilizes the first excited state (S1) and destabilizes the ground state (S0), ultimately causing the two states to become degenerate, which are referred to as CI (see Figure 3.26). The dynamic process explains subsequent E–Z photoisomerization and weak emission of isolated TPE molecules.
Figure 3.26 A brief illustration of the conical intersection (CI) process through the rotation of double bond in the excited state for TPE in solution.
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 (Φf.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 (Φf = 0.1%) and TPE‐4oM (Φf = 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
