TRANSIENT CHARGE-TRANSFER ABSORPTION-BANDS AS PROBES OF ION-FAIRING DYNAMICS AND ENERGETICS

Very large time-dependent spectral blue shifts (up to over 7400 cm(-1) ) induced by ion pairing were observed for long-lived photoinduced cha rge-separated states of two probe molecules: 4-amino-4'-nitrobiphenyl (p-ANBP) and 4-amino-4'-nitroterphenyl (p-ANTP). These shifts are anal ogous to the well-known time-dependent fluorescence shifts (TDFS) of t ransient CT states, which are attributed to nonequilibrium dipolar sol vation in polar media as well as to ionic atmosphere relaxation in sol utions of electrolytes. It is demonstrated that the dynamic spectral s hifts of transient CT absorption bands can be utilized as sensitive pr obes of ion-pairing processes and that they offer significant advantag es over the traditional fluorescent probes. The long lifetime of charg e-separated excited triplet states (e.g., >3 mu s for p-ANTP) allows t ime-resolved study of electrolyte dynamics in highly dilute solutions (<0.1 mM salt) and extends spectroscopic investigations of ion-pairing processes to weakly polar media. The short-lived fluorescent probes ( e.g., coumarin, tau(f) approximate to 3 ns) necessitate the use of con centrated solutions (>100 mM salt) and limit the approach to highly po lar solvents.