Photophysics of Auramine-O: electronic structure calculations and nonadiabatic dynamics simulations

Diphenylmethane dyes are very useful photoinduced molecular rotors; however, their photophysical mechanisms are still elusive until now. In this work, we adopted combined static electronic structure calculations (MS-CASPT2//CASSCF) and trajectory-based surface-hopping dynamics simulations (OM2/MRCI) to study the S1 excited-state relaxation mechanism of a representative diphenylmethane dye Auramine-O. On the basis of the optimized S1 minima and the computed emission bands, we have for the first time assigned experimentally proposed three transient states (i.e. S1-LE, S1-I1 or S1-I2, and S1-II). Mechanistically, upon irradiation to the S1 state, the system first relaxes to the locally excited S1 minimum (S1-LE). Starting from this point, there exist two kinds of relaxation paths to S1-II. In the sequential path, the system first evolves into S1-I1 or S1-I2 and then runs into S1-II; in the concerted one, the system, bypassing S1-I1 and S1-I2, directly runs into S1-II. In addition, the system can decay to the S0 state in the vicinity of three S1/S0 conical intersections i.e. S1S0-I1, S1S0-I2, and S1S0-II. In the S1 dynamic simulations, 54% trajectories decay to the S0 state via S1S0-II; the remaining trajectories are de-excited to the S0 state via S1S0-I1 (11%) and S1S0-I2 (35%). Our present theoretical investigation does not support the experimentally proposed S1 excited-state hypothesis that the intramolecular rotation of the two dimethyl groups around the C-N bond is responsible for the rapid decay of the emission band at about 500 nm; instead, it should be heavily interrelated with the rotation of the two dimethylanilino groups. Finally, this work provides important mechanistic insights into similar diphenylmethane dyes.