MODE-SPECIFIC VIBRATIONAL REORGANIZATION ENERGIES ACCOMPANYING PHOTOINDUCED ELECTRON-TRANSFER IN THE HEXAMETHYLBENZENE TETRACYANOETHYLENE CHARGE-TRANSFER COMPLEX

Resonance Raman excitation profiles have been measured across the broad, intense charge-transfer absorption band of the hexamethylbenzene/tetracyanoethylene complex in CCl4 solution. Nonresonant Raman spectra of the complex and its perdeuterated derivative in the solid state have also been obtained with excitation at 1064 nm. Absorption of light within the charge-transfer band results in electron transfer from hexamethylbenzene to tetracyanoethylene to form a radical-ion pair. The geometry change along each vibrational mode as a result of this photoinduced electron transfer and the accompanying internal reorganization energies have been determined through quantitative simulation of the absorption spectrum and absolute resonance Raman intensities using time-dependent wavepacket propagation methods. The largest individual contributions to the total measured reorganization energy of approximately 3500 cm-1 arise from the presumed donor-acceptor intermolecular stretching mode at 165 cm-1 and the C=C stretch of tetracyanoethylene at 1551 cm-1, with smaller contributions from other modes localized on both tetracyanoethylene and hexamethylbenzene. The importance of these results for understanding trends in the rates of nonphotochemical return electron transfer is discussed.