Solvatochromic shifts reconsidered: Field-induced mixing in the nonlinear region and application to indole

In previous work we showed that the observed discrepancy between gas-phase Stark effect measurements of excited-state dipole moments and those measure d by solvatochromic shifts could be explained by possible intervention of a nearby excited state. Field-induced mixing of the two excited states cause s solvent shifts that are linear in the high-field limit, where most solven t field studies are carried out, but the excited-state dipole moment must b e replaced by an effective dipole moment. This effective moment is a combin ation of the excited-state dipole moments of both states as well as the tra nsition moment between the two states. This new moment may have effects bot h on the solvent shifts observed and on the intensities. In the low-field g as-phase limit, the shifts are also linear but caused by the simple excited -state dipole moment. In the intermediate field region, the solvent shifts are expected to be decidedly nonlinear. The effective dipole moment is show n to be field-dependent near an avoided crossing of the two excited states, and this results in nonlinear behavior of the solvent reaction field. By i dentification of the onset of nonlinearity or, even better, by the fitting of the solvent shift observed to predicted expressions, the individual cont ributions to the effective dipole moment can be unraveled, We show how this theory can be applied to observed solvent shifts in the molecule indole, w hich has been observed throughout the nonlinear region. Consistent dipolar parameters that reconcile gas-phase and solution-phase results are obtained .