Formation and decay of dual fluorescences in p-dimethylaminobenzonitrile

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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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