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.