EFFECT OF THE SOLVENT DENSITY AND SPECIES ON THE BACK-ELECTRON TRANSFER RATE IN THE HEXAMETHYLBENZENE TETRACYANOETHYLENE CHARGE-TRANSFER COMPLEX/

The back-electron transfer (b-ET) process in the hexamethylbenzene/tet racyanoethylene charge-transfer complex was studied by the transient a bsorption spectroscopy in several fluids (ethane, nitrous oxide, carbo n dioxide, and trifluoromethane) from the critical density to twice th at of it at 323.2 K. The b-ET rate was determined by the decay rate of the excited state absorption. The b-ET rate increased with the increa se of solvent density in carbon dioxide and nitrous oxide. The b-ET ra te also increased in the order of ethane, nitrous oxide, carbon dioxid e, and trifluoromethane, compared at the similar reduced density divid ed by the solvent critical density. Based on the formulation by Marcus and Jortner, the reaction free energy and the solvent reorganization energy were estimated from the change of the absorption spectrum relat ive to the gaseous phase spectrum, simply by assuming that the intramo lecular reorganization energy does not depend on the solvent density a nd the species. The reaction free energy and the solvent reorganizatio n energy in fluids obtained in this way were almost linearly correlate d, and the density dependence was larger in the lower density region. The b-ET rates in various conditions showed a good correlation with th e reaction free energy estimated from the spectral simulation. The b-E T rate showed a significant isotope effect by perdeuteriation of hexam ethylbenzene as is predicted by the fluorescence quantum yield [K. Kul inowski et al., J. Phys. Chem. 99, 17715 (1995)], which could not be r eproduced by this model. (C) 1998 American Institute of Physics. [S002 1-9606(98)52604-9].