Distibazolium dyes are investigated by steady-state and time-resolved fluorescence techniques in a series of low- and high-viscosity polar solvents and in a silica sol-gel matrix. In all solvents and the sol-gel matrix, an interplay of photoinduced switching between different cis-trans isomers and solvation dynamics is observed. Even in a viscous solution (glycerol) and in silica gel, cis-trans isomerization is solvent-controlled. Whereas in glycerol the solvation results in a time-dependent fluorescence Stokes shift, the solvation-induced spectral heterogeneity in silica gel is mostly static, possibly due to a close proximity of dye molecules to the silica surfaces of the nanopores. Compared to low-viscosity solvents, where the cis-trans isomerization process takes place with a solvent-dependent rate on the timescale of about 120–150 ps, it slows down to about 1100–1400 ps in glycerol and about 1500 ps in a sol-gel matrix. Additionally, fluorescence kinetics of the dyes in the sol-gel reveals the presence of a range of different “frozen-in” conformers exhibiting a broad spectrum of lifetimes from 20 to 300 ps.
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Vol. 81 • No. 4