ABSORPTION, FLUORESCENCE, AND RESONANCE RAMAN-SPECTROSCOPY OF THE HEXAMETHYLBENZENE TETRACYANOETHYLENE CHARGE-TRANSFER COMPLEX - TOWARD A SELF-CONSISTENT MODEL

The weak, far-red fluorescence spectrum of the hexamethylbenzene/tetracyanoethylene charge-transfer complex has been measured in CCl4 and cyclohexane solvents. The total fluorescence quantum yield in CCl4 with 633 nm excitation is about 5 x 10(-5). The absorption spectrum, absolute resonance Raman excitation profiles, and fluorescence spectrum in CCl4 are simulated with a common model that explicitly includes the 11 most strongly Franck-Condon-active internal vibrations as Well as the solvent, treated as an overdamped Brownian oscillator. The fits to the data require a large (2450 cm(-1)) ''solvent'' reorganization energy, which may involve a combination of true solvent motions and very low-frequency intermolecular complex vibrations. The same model is used to calculate the nonphotochemical return electron-transfer rate and compared with previous measurements of the ion-pair lifetime. This represents the first time, to our knowledge, that all four pieces of data (absorption, fluorescence, Raman, and electron-transfer rate) have been simulated with a common model and compared with experimental results.