Abstract
For di- and triphenylmethane dye molecules in liquid solution it is well-known that the fluorescence quantum yield varies drastically with the viscosity of the solvent. Torsional motions of the phenyl rings of the dye molecules are believed to control the fluorescence quantum yield, i.e., if the solvent viscosity permits phenyl-group twisting the fluorescence is weakened. However, recent theoretical studies of photoexcited TICT states in liquid solution, have emphasized the importance of a two-dimensional reaction coordinate approach [1]. In the latter, the dynamics in the excited state is determined not only by the reaction coordinate (e.g., twisting), but also by the solvation coordinate. In this paper, we present results of femtosecond fluorescence upconversion experiments of two diphenyl methane dyes (Figure 1), Michler’s ketone (MK) and its bridged analog 3,6-bis(dimethylamino)-10,10-dimethylanthrone (BMK). For the unbridged MK in alcoholic solution very fast fluorescence decay kinetics of only a few picoseconds is reported, whereas its bridged counterpart BMK has a lifetime at least one order of magnitude longer. The molecules are identified as prototype examples of the slow and fast solvation limits in the model of Kim and Hynes.
© 2002 Optical Society of America
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