Abstract
We have studied by the methods of picosecond spectroscopy the fluorescence risetimes of the S1→S0 and S2→S0 emissions of p-dimethylaminobenzonitrile in several polar solvents of increasing viscosity at 25 and −78°C to determine the relaxation dynamics controlling their kinetics. Vibronic relaxation resulting from interaction with the solvent as a dissipative medium appears to precede the S1→S0 fluorescence. An earlier hypothesis that the S2→S0 emission follows a modified Debye theory of viscosity-controlled solvent reorganization is not completely supported by our data, though solvent viscosity does appear to influence the relaxation dynamics. Available data do not distinguish between intramolecular − N(CH3)2 rotation and nonviscosity limited solvent reorgainization as leading to S2→S0 emission, whose decay lifetime was determined to be ∼ 1 nsec.
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