A Different Photochromic Mechanism of Spirooxadiazine: Electronic Structure Calculations and Nonadiabatic Dynamics Simulations

Abstract

Previously, we conducted electronic structure calculations and surface hopping dynamics to investigate the reversible photochromic mechanism of spirooxazine (SO). Herein, a very similar calculation strategy was employed to simulate the excited-state ring-opening, relaxation, and subsequent isomerization of spirooxadiazine (SOD). Interestingly, we discovered a significantly different photochromic behavior compared to SO. Firstly, the bright state of SOD is the S2(ππ*) state, while the S1 state is a weakly absorbing state with a nπ* character. This is in stark contrast to SO, of which the S1 state is a ππ* state. Secondly, we identified a new excited-state decay channel along the CN dissociation within the benzoxadiazine ring, in addition to the closed-loop and CO cleavage relaxation pathways. Thirdly, the out-of-plane hydrogen (HOOP) oscillations were no longer operative in the relaxation processes, as the CH group was replaced by the N atom in SOD. Finally, several nonadiabatic trajectories will lead to the ‘N2 release’ product in the ground state, which might result in deactivation of SOD and affect the light fatigue resistance.

Keywords: Photochromism, Spirooxadiazine, Ring-opening reaction, Nonadiabatic dynamics simulations