Surface-Hopping Dynamics Simulations of Malachite Green: A Triphenylmethane Dye

Malachite green is a typical triphenylmethane dye widely used in fundamental and industrial research; however, its excited-state relaxation dynamics remains elusive. In this work we simulate its photodynamics from the S-2 and S-1 states using the fewest-switches surface-hopping scheme. In the S-2 photodynamics, the system first relaxes to the S-2 minimum, which immediately hops to the S-1 state via an S-2/S-1 conical intersection. In the S-1 state, 90% trajectories evolve into a structurally symmetric S-1 minimum; the remaining ones proceed toward two propeller-like S-1 minima. Two kinds of S-1 minima then decay to the S-0 state via the S-1/S-0 conical intersections. The S-1 photodynamics is overall similar to the S-1 excited-state dynamics as a result of the ultrafast S-2 -> S-1 internal conversion in the S-2 photodynamics, but the weights of the trajectories that decay to the S-0 state via three different S-1/S-0 conical intersections are variational. Moreover, the S-2 relaxation dynamics mainly happens in a concerted synchronous rotation of three phenyl rings. In comparison, in the S-1 relaxation dynamics, the rotations of two aminophenyl rings can proceed in the same and opposite directions. In certain trajectories, only the rotation of an aminophenyl ring is active. On the basis of the results, the S-2 and S-1 excited-state lifetimes of malachite green in vacuo are calculated to be 424 fs and 1.2 ps, respectively. The present work provides important mechanistic insights for similar triphenylmethane dyes.