Electronic structure calculations and nonadiabatic dynamics simulations on reversible photochromic mechanism of spirooxazine

In this work, the combined electronic structure calculations (OM2/MRCI and MS-CASPT2//CASSCF) and Tully's fewest-switches surface hopping dynamics simulations (OM2/MRCI) have been conducted to investigate the photoinduced ring-opening, relaxation, and subsequent isomerization of spirooxazine. The OM2/MRCI and MS-CASPT2//CASSCF methods share very similar structures of S0 and S1 minima and S1/S0 conical intersections, and also provide comparable relative energies at the same time. The relevant reaction paths indicate that two bonded conical intersections and two unbonded ones will play very important roles in the nonadiabatic transitions. In addition, the S1 state nonadiabatic dynamics simulations reveal two effective and competitive S1 → S0 decay pathways via the bonded and unbonded conical intersections. And the C–O bond dissociation could take place either in the S1 state before hopping or in the S0 state after nonadiabatic transitions, which will finally generate three types of ground-state products, i.e. the closed-loop spirooxazine, a variety of merocyanine intermediates, and a small amount of hydrogen-transferred merocyanine isomers. Besides, the out-of-plane hydrogen (HOOP) oscillations could be an efficient and ultrafast inactivation channel for the excited-state relaxation processes. The currently simulated results provide important mechanistic insights into spirooxazine-based photochromic materials.