New Insight into the Photoisomerization Process of the Salicylidene Methylamine under Vacuum

The deactivation process of salicylidene methylamine in the gas phase has been explored using static calculations (CASSCF, CASPT2, and CC2) and on-the-fly surface hopping dynamics simulations (CASSCF). Five minimum energy conical intersections (MECIs) were located upon the geometry optimization calculations. One corresponds to the excited state intramolecular proton transfer (ESIPT) process, and the remaining four arise from CN bond rotational motion. Our calculation results found that the molecule prefers to decay to the ground state through the four rotational motion related MECIs rather than the ESIPT related one. This mechanistic scenario is verified by the energy profiles connecting the Franck–Condon point and the MECIs at CASSCF, CASPT2, and CC2 levels. Our proposed new decay mechanism can explain the previous experimental findings of femtosecond pump–probe photoionization spectroscopy and can provide additional guidance to the rational design of photochemically switchable molecules.