Excited-State Decay Paths of Ortho-Terphenyl: Electronic Structure Calculations and Nonadiabatic Dynamics Simulations

Abstract

Photoexcitation of ortho-terphenyl (OTP) to cyclize into 4a,4b-dihydrotripheneylene (DHT) is regarded as the first step in the synthesis of triphenylene. Surprisingly, the nonadiabatic cyclization mechanisms of this textbook reaction have not been examined explicitly until now. Herein we focus on the photoinduced dynamics of OTP by using static electronic structure calculations (MS-CASPT2//CASSCF) and trajectory-based surface-hopping dynamics simulations (OM2/MRCI). In terms of the calculated results, we shed light on two distinct relaxation pathways from the $S_1$ state and also the mechanism of photocyclization. The pyramidalization of central benzene ring and the subsequent $S_1$ → $S_0$ transition via the S1S0-OTP intersection region is the predominant route (PATH I, ~ 81.5%); while the relaxation from the $S_1$ state to the $S_0$ state through the S1S0-DHT intersection region is the minor pathway and can be determined to be the key step for the formation of DHT conformer in the ground state (PATH II, ~ 11.3%). In the simulated period of 3.0 ps, the final recovery yield of $S_0$ OTP conformer is predicted to be 92.8% due to thermally activated ring reopening of DHT conformer; that is, the intermediate DHT is not so stable in the ground state. In addition, two relaxation time constants (538.7 fs and 5.1 ps) are predicted for the $S_1$ dynamics of OTP with the faster process assigned to the nonreactive deactivation (PATH I) and the slower one assigned to the ring closure reaction (PATH II), which are well consistent with the experimental values of 700 fs and 3.0 ps in the solutions of tetrahydrofuran (THF). Finally, the present work can provide important photocyclization mechanistic insights for OTP system and also the similar ortho-arenes.