Solvatochromic Shifts Reconsidered: Field-Induced Mixing in the Nonlinear Region and Application to Indole

Abstract

In previous work we showed that the observed discrepancy between gas-phase Stark effect measurements of excited-state dipole moments and those measured by solvatochromic shifts could be explained by possible intervention of a nearby excited state. Field-induced mixing of the two excited states causes solvent shifts that are linear in the high-field limit, where most solvent field studies are carried out, but the excited-state dipole moment must be replaced by an effective dipole moment. This effective moment is a combination of the excited-state dipole moments of both states as well as the transition moment between the two states. This new moment may have effects both on the solvent shifts observed and on the intensities. In the low-field gas-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 shown to be field-dependent near an avoided cross...