Resonance Raman study of solvent dynamics in electron transfer. II. Betaine-30 in CH₃OH and CD₃OD Available to Purchase

Raman excitation profiles for twelve vibrational modes of the solvatochromic dye betaine-30 have been measured in CH₃OH and $CD3$OD solutions at wavelengths that span the $S0→S1$ charge transfer transition. Though the absorption spectra of the dye are the same in the protonated and deuterated forms of the solvent, Raman cross-sections for all modes were found to be generally lower in the CH₃OH solution than in$CD3OD$⁠. The time-dependent theory of Heller was applied to model the absorption and Raman profiles, and both mono- and bi-exponential solvent relaxation were considered in order to account for solvent induced electronic dephasing. The two models lead to different physical pictures for the relevant solvent dynamics, but in either case the amplitude of solvent dephasing is reduced in deuterated compared to protonated methanol. The effect is interpreted in terms of stronger solvent–solute and solvent–solvent hydrogen bonding in deuterated methanol solution. Comparing to previous results for betaine-30 Raman cross-sections in acetonitrile [Y. Zong and J. L. McHale, J. Chem. Phys. 106, 4963 (1997)], it is concluded that slower solvent dynamics and perturbations to the electronic structure of betaine-30 lead to larger Raman intensities in methanol. The data suggest that solvent dynamics depend strongly on the electronic state of betaine-30.