SOLVENT EFFECTS ON THE OXYGEN ORGANIC-MOLECULE CHARGE-TRANSFER ABSORPTION

Molecular oxygen and 1-methylnaphthalene form a ground-state ''contact complex'', which can absorb light to populate a charge-transfer (CT) state. The effect of solvent on this transition has been examined. The onset of this cooperative absorption Linearly shifts to a longer wavelength with an increase in the parameter (n(2) - 1)/(2n(2) + 1), where n is the solvent refractive index at optical frequencies. The data do not correlate with functions in which the static dielectric constant of the solvent is used. These results are consistent with (a) a ground-state complex that does not have substantial charge separation, and (b) a solute-solvent interaction in the Franck-Condon CT state that is most likely dominated by dipolar coupling terms. Recent studies indicate that the CT state lifetime is approximately the same as the time required for solvent orientational relaxation. As a consequence, envisioning the CT state principally as a Franck-Condon state is likely to be accurate for many common organic solvents. From this perspective, the spectroscopic data reported herein should be useful in attempts to estimate the relative energy of the CT state in a given solvent.