Role of structural flexibility in the fluorescence and photochromism of salicylideneaniline: the general scheme of the phototransformations

Abstract

The mechanism of light-induced transformation in the salicylideneaniline molecule was studied by semiempirical PM3 calculations. The structures and energies of the minima and saddle points (transition states) on the S 0, S 1 and T 1 potential energy hypersurfaces (PESs) were obtained, together with the gradient lines on the PESs. The structure-energy scheme was compared with the experimental findings. According to the results obtained, the following principle processes are observed: fast S 1 excited state intramolecular proton transfer (ESIPT), followed by typical ESIPT fluorescence; the formation of two S 1 twisted intramolecular charge transfer (TICT) structures which quench the ESIPT fluorescence; the diabatic formation of two ground state metastable coloured “post-TICT” structures responsible for photochromism.