ROLE OF STRUCTURAL FLEXIBILITY IN THE FLUORESCENCE AND PHOTOCHROMISM OF SALICYLIDENEANILINE - THE GENERAL SCHEME OF THE PHOTOTRANSFORMATIONS

The mechanism of light-induced transformation in the salicylideneanili ne molecule was studied by semiempirical PM3 calculations. The structu res and energies of the minima and saddle points (transition states) o n the S-0, S-1 and T-1 potential energy hypersurfaces (PESs) were obta ined, together with the gradient lines on the PESs. The structure-ener gy scheme was compared with the experimental findings. According to th e results obtained, the following principle processes are observed: fa st S, excited state intramolecular proton transfer (ESIPT), followed b y typical ESIPT fluorescence; the formation of two S, twisted intramol ecular charge transfer (TICT) structures which quench the ESIPT fluore scence; the diabatic formation of two ground state metastable coloured ''post-TICT'' structures responsible for photochromism. (C) 1997 Else vier Science S.A.