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 S2 and S1 states using the fewest-switches surface-hopping scheme. In the S2 photodynamics, the system first relaxes to the S2 minimum, which immediately hops to the S1 state via an S2/S1 conical intersection. In the S1 state, 90% trajectories evolve into a structurally symmetric S1 minimum; the remaining ones proceed toward two propeller-like S1 minima. Two kinds of S1 minima then decay to the S0 state via the S1/S0 conical intersections. The S1 photodynamics is overall similar to the S1 excited-state dynamics as a result of the ultrafast S2 → S1 internal conversion in the S2 photodynamics, but the weights of the trajectories that decay to the S0 state via three different S1/S0 conical intersections are variational. Moreover, the S2 relaxation dynamics mainly happens in a concerted synchronous rotation of three phenyl rings. In comparison, in the S1 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 S2 and S1 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.