Ultrafast Nonradiative Decay of Electronically Excited States of Malachite Green: Ab Initio Calculations

We have investigated the nonradiative deactivation process of malachite green in the singlet excited states, S₁ and S₂, 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 S₁ 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 S₁ and S₀ 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 S₁ potential energy surface. The decay mechanism from the S₂ state also involves the twisting motion of phenyl rings. In contrast to the excitation to the S₁ 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 S₁ state at the conical intersection between S₂ and S₁, and then it decays back to the ground state through the intersection between S₁ and S₀. In relaxation from both S₁ and S₂, large distortion of phenyl rings is required for the ultrafast nonradiative decay to the ground state.