Potential-Energy Surface and Dynamics Simulation of THBDBA: An Annulated Tetraphenylethene Derivative Combining Aggregation-Induced Emission and Switch Behavior

Electronic structure calculations and nonadiabatic dynamics simulations were carried out on the excited state decay mechanism of a typical aggregation-induced emission (AIE)-active molecule, 10,10 ',11,11 '-tetrahydro-5,5 '-bidibenzo[a,d][7]annulenylidene (THBDBA), which was experimentally found to be almost non-emissive in THF solution but highly luminescent as an aggregate. At the MS-CASPT2//CASSCF level, the decay path from S-1 leads to a conical intersection (CI) seam with the ground state (S-0) via a cyclization process of two neighboring phenyl groups. Nonadiabatic dynamics simulations of 367 surface hopping trajectories show that all the structures of hopping points resemble those of the S-1/S-0 CI seam with cyclized geometry. The final structures of trajectories hopping to S-0 belong to either Franck-Condon (FC) like structures (66.8 %) or cyclized conformers (33.2 %), which agrees well with the experimental result of 23 % cyclization product (Y. Cai et al. Chem. Sci., 2011, 2, 2029-2034). Our results also indicate that long-time irradiation leads to a photostationary state where the open and cyclized forms interconvert photochemically. The preference for photocyclization over double bond isomerization is due to the rigidity of the dihydrofulvene rings. From the point of view of AIE, the calculations suggest that THBDBA follows our restricted access to a CI (RACI) model because the photocyclization requires large displacements of the phenyl rings. These displacements must be blocked in the aggregate phase, inhibiting the decay and leading to luminescence. Our study contributes to understand the AIE and switch behavior of THBDBA and will be useful to support the design of new molecules with these features.