RESONANCE RAMAN-STUDY OF SOLVENT DYNAMICS IN ELECTRON-TRANSFER .2. BETAINE-30 IN CH3OH AND CD3OD

Raman excitation profiles for twelve vibrational modes of the solvatoc hromic dye betaine-30 have been measured in CH3OH and CD,OD solutions at wavelengths that span the S-0-->S-1 charge transfer transition. Tho ugh the absorption spectra of the dye are the same in the protonated a nd 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 absorpti on and Raman profiles, and both mono-and bi-exponential solvent relaxa tion were considered in order to account for solvent induced electroni c dephasing. The two models lead to different physical pictures for th e relevant solvent dynamics, but in either case the amplitude of solve nt dephasing is reduced in deuterated compared to protonated methanol. The effect is interpreted in terms of stronger solvent-solute and sol vent-solvent hydrogen bonding in deuterated methanol solution. Compari ng to previous results for betaine-30 Raman cross-sections in acetonit rile [Y. Zong and J. L. McHale, J. Chem. Phys. 106, 4963 (1997)], it i s concluded that slower solvent dynamics and perturbations to the elec tronic structure of betaine-30 lend to larger Raman intensities in met hanol. The data suggest that solvent dynamics depend strongly on the e lectronic state of betaine-30. (C) 1997 American Institute of Physics.