Photodynamics of Schiff Base Salicylideneaniline: Trajectory Surface-Hopping Simulations

We report a computational study on the photochemistry of the prototypical aromatic Schiff base salicylideneaniline in the gas phase using static electronic structure calculations (TDDFT, OM2/MRCI) and surface-hopping dynamics simulations (OM2/MRCI). Upon photoexcitation of the most stable cis-enol tautomer into the bright S₁ state, we find an ultrafast excited-state proton transfer that is complete within tens of femtoseconds, without any CN double bond isomerization. The internal conversion of the resulting S₁ cis-keto species is initiated by an out-of-plane motion around the C–C single bond, which guides the molecule toward a conical intersection that provides an efficient deactivation channel to the ground state. We propose that the ease of this C–C single bond rotation regulates fluorescence quenching and photocoloration in condensed-phase environments. In line with previous work, we find the S₁ cis-keto conformer to be responsible for fluorescence, especially in rigid surroundings. The S₀ cis-keto species is a transient photoproduct, while the stable S₀ trans-keto photoproduct is responsible for photochromism. The trajectory calculations yield roughly equal amounts of the S₀ cis-enol and trans-keto photoproducts. Methodologically, full-dimensional nonadiabatic dynamics simulations are found necessary to capture the preferences among competitive channels and to gain detailed mechanistic insight into Schiff base photochemistry