Studies of solvent effects on rotational isomerization and rotational diffusion for dyes in solutions
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Nonradiative processes in p-dialkylaminobenzylidenemalononitrde (DABIMN) molecules were investigated in a series of normal alcohols with different polarities and viscosities. A solvent polarity parameter £"^(30) was introduced to account for the potential well change for DABIMN in different solvents. After correcting for the solvent polarity effect, the effect of solvent viscosity on nonradiative rate of DABIMN in solvents was found to be insignificant. The variation of the nonradiative rate of DABIMN in the alcohols is attributed mainly to polarity-induced free energy change and energy gap change rather than to viscosity change. The same model was then applied to rhodamine B dye which exhibits a similar rotational isomerization configuration to DABIMN dye immediately after excitation. The results for both dyes are consistent. The fact that the same lifetime was observed for rhodamine B under close to isopolarity conditions (prepared by adding different weight of poly(ethylene oxide) into methanol) further proves that there is no dependence of the nonradiative rate on solvent viscosity. A new method to assess the experimental uncertainties in acquiring and fitting fluorescence decays with a fast analog technique is described. With this model the uncertainties present can be determined totally from experimental data, and the reduced chi-square value is close to unity. Simulation work was done to estimate the resolution capability of the instrument and of the Marquardt algorithm in lifetime experiments. The fast analog technique was used to study the solvent viscosity dependence of the excited state rotational reorientation time of rhodamine B in alcohols. Our results can be attributed to solvent-attachment effects (due to hydrogen bonding) and to long-chain effects (due to stick and slip boundary conditions).