Solvent Effects on Vibronic Coupling in a Flexible Bichromophore: Electronic Localization and Energy Transfer induced by a Single Water Molecule

Size and conformation-specific ultraviolet and infrared spectra are used to probe the effects of binding a single water molecule on the close-lying excited states present in a model flexible bichromophore, 1,2-diphenoxyethane (DPOE). The water molecule binds to DPOE asymmetrically, thereby localizing the two electronically excited states on one or the other ring, producing a S1/S2 splitting of 190 cm–1. Electronic localization is reflected clearly in the OH stretch transitions in the excited states. Since the S2 origin is imbedded in vibronic levels of the S1 manifold, its OH stretch spectrum reflects the vibronic coupling between these levels, producing four OH stretch transitions that are a sum of contributions from S2-localized and S1-localized excited states. The single solvent water molecule thus plays multiple roles, localizing the electronic excitation in the bichromophore, inducing electronic energy transfer between the two rings, and reporting on the state mixing via its OH stretch absorptions.