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

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journal contribution
posted on 2011-08-18, 00:00 authored by Akira Nakayama, Tetsuya Taketsugu
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.