Concerning medium polarity effects on the photophysics andphotochemistry of TICT-forming dyes

Mechanism and dynamics of the excited state relaxation in compounds able to form TICT states were investigated from a theoretical point of view with special reference to the dielectric solvent effects. The study included 4-dimethylamino-4’-cyanostilbene (DCS) and Michler’s Hydrol Blue (MHB), which are thought to involve emitting and non-emitting TICT states, respectively. It was found that in DCS the formation ofTICT states, by twisting either the dimethylamino or the dimethylanilino groups, may in principle be induced by the solvent polarity but requires extreme conditions. InMHB, both calculations and new fluorescence quantum-yield measurements in a variety of solvents indicated that the TICT-state formation is essentially controlled by the solvent viscosity.