Transient Charge Transfer Absorption Bands as Probes of Ion-Pairing Dynamics and Energetics

Abstract

Very large time-dependent spectral blue shifts (up to over 7400 cm[sup [minus]1]) induced by ion pairing were observed for long-lived photoinduced charge-separated states of two probe molecules: 4-amino-4[prime]-nitrobiphenyl ([rho]-ANBP) and 4-amino-4[prime]-nitroterphenyl ([rho]-ANTP). These shifts are analogous to the well-known time-dependent fluorescence shifts (TDFS) of transient CT states, which are attributed to nonequilibrium dipolar solvation in polar media as well as to ionic atmosphere relaxation in solutions of electrolytes. It is demonstrated that the dynamic spectral shifts of transient CT absorption bands can be utilized as sensitive probes of ion-pairing processes and that they offer significant advantages over the traditional fluorescent probes. The long lifetime of charge-separated excited triplet states (e.g., > 3 [mu]s for [rho]-ANTP) allows time-resolved study of electrolyte dynamics in highly dilute solutions ( 100 mM salt) and limit the approach to highly polar solvents. 12 refs., 5 figs., 1 tab.