The proton-transfer reaction in a model aromatic Schiff base, salicylidene methylamine (SMA), in the ground and in the lowest electronically-excited singlet states, is theoretically analyzed with the aid of second-order approximate coupled-cluster model CC2, time-dependent density functional theory (TD-DFT) using the Becke, three-parameter Lee–Yang–Parr (B3LYP) functional, and complete active space perturbation theory CASPT2 electronic structure methods. Computed vertical-absorption spectra for the stable ground-state isomers of SMA fully confirm the photochromism of SMA. The potential-energy profiles of the ground and the lowest excited singlet state are calculated and four photophysically relevant isomeric forms of SMA; α, β, γ, and δ are discussed. The calculations indicate two S1/S0 conical intersections which provide non-adiabatic gates for a radiationless decay to the ground state. The photophysical scheme which emerges from the theoretical study is related to recent experimental results obtained for SMA and its derivatives in the low-temperature argon matrices (J. Grzegorzek, A. Filarowski, Z. Mielke, Phys. Chem. Chem. Phys. 2011, 13, 16596–16605). Our results suggest that aromatic Schiff bases are potential candidates for optically driven molecular switches.
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