Investigation of solvent isotope effects on Raman and fluorescence intensity of LDS750 in CH3OH and CD3OD

The resonance Raman spectra of LDS750 in methanol-h(4) and methanol-d(4) ha ve been determined at excitation wavelengths on the blue side of the absorp tion maximum. In contrast to our previous studies of the solvatochromic mol ecule betaine-30, Raman intensities of LDS750 are observed to be independen t of solvent isotopomer. The fluorescence quantum yield, however, was signi ficantly larger in methanol-clr than in methanol-h(4) The results suggest t hat the dephasing dynamics that determine the resonance Raman intensities a re dominated by intramolecular relaxation rather than solvent motion. Excit ed-state population relaxation, on the other hand, is perturbed by solvent isotope and the more intense emission in deuterated solvent is a consequenc e of less favorable Franck-Condon factors for radiationless decay. The radi ative lifetime calculated from the quantum yield and fluorescence lifetime is considerably longer than that found using the Strickler-Berg equation, s uggesting that the transition moment in the relaxed geometry is reduced fro m that in the Franck-Condon geometry. Our conclusions concerning excited st ate dynamics in LDS750 are compared with those obtained from time-domain me asurements on this dye.