Ultrafast nonradiative decay of electronically excited states of malachite green: Ab initio calculations

摘要

We have investigated the nonradiative deactivation process of malachite green in the singlet excited states, S_1 and S_2, by high-level ab initio quantum chemical calculations using the CASPT2//CASCF approach. The deactivation pathways connecting the Franck-Condon region and conical intersection regions are identified. The initial population in the S_1 state is on a flat surface and the relaxation involves a rotation of phenyl rings, which leads the molecule to reach the conical intersection between the S_1 and S_0 states, where it efficiently decays back to the ground state. There exists a small barrier connecting the Franck-Condon and conical intersection regions on the S_1 potential energy surface. The decay mechanism from the S_2 state also involves the twisting motion of phenyl rings. In contrast to the excitation to the S _1 state, the initial population is on a downhill ramp potential and the barrierless relaxation through the rotation of substituted phenyl rings is expected. During the course of relaxation, the molecule switches to the S _1 state at the conical intersection between S_2 and S _1, and then it decays back to the ground state through the intersection between S_1 and S_0. In relaxation from both S_1 and S_2, large distortion of phenyl rings is required for the ultrafast nonradiative decay to the ground state.