A fluorescence study of the excited-state dynamics of boron dipyrrin molecular rotors

Noticeable viscosity dependence has been revealed for fluorescence spectra of three phenyl derivatives of boron dipyrrin in aqueous glycerol solutions. This dependence is less pronounced for the fluorescence lifetimes. Such behavior is characteristic of molecular rotors used as media microviscosity sensors. A significant growth of the radiative deactivation constant is observed in the range 298-150 K. Quantumchemical calculation of the model fluorophore support the assumption on barrier-free relaxation of the excited state from the pretwisted to the flattened conformation. The spectral-kinetic parameters of fluorophores have been determined, and viscosity graduation curves are presented.