Molecular rotors based on styryl dyes. Viscosity dependence of rotation of molecular fragments

Three donor-acceptor styryl dyes R-Het + -CH=CH-C 6 H 4 -NR′R″ClO 4 − (Het is pyridyl) were synthesized. Their spectral-luminescence behavior allows their assignment to a class of molecular rotors. The influence of the viscosity, polarity, and temperature of the medium on their absorption and fluorescence properties was studied. A pronounced enhancement of dye fluorescence accompanied by a short-wavelength shift of the fluorescence maximum is observed with an increase in the viscosity of the medium and temperature decrease. The measurements of the fluorescence spectra of the dyes in the poly(methyl methacrylate) films at 293 and 77 K confirmed the effect of the medium viscosity on the rotation ability of molecular fragments upon photoexcitation. According to the quantum chemical calculations, in excited molecules of the styryl dyes in nonpolar solvents, molecular fragments rotate mainly about the central ethylene bond . In polar solvents, the rotation barriers around the ordinary bonds of the ethylene fragment decrease.