CHARGE SEPARATION IN DONOR CHROMOPHORE ACCEPTOR COMPLEXES - INVERTED REGION BEHAVIOR IN REVERSE ELECTRON-TRANSFER REACTIONS

Time-resolved absorption studies which elucidate intramolecular electron transfer rates have been performed on a series of covalently linked Ru(bipyridine)3-donor-acceptor complexes. In these complexes the electron donor is a phenothiazine moiety linked to a bipyridine by a (-CH2-)4 chain, and the electron acceptor is an N,N'-diquaternary-2,2'-bipyridinium moiety, linked to a bipyridine by a (-CH2-)2 chain. Excitation to the Ru(bipyridine)3 metal-to-ligand charge transfer (MLCT) state leads to a long-lived charge-separated state which is shown to form by oxidative quenching of the Ru(bpy)3(2+) moiety followed by phenothiazine-to-ruthenium electron transfer. A wavelength dependent excitation into a charge transfer state results in very rapid formation of the charge-separated state. The magnitude of the fast component in the transient absorption serves as an internal standard for determining reverse electron transfer rates as a function of the energetic driving force. Marcus inverted behavior is observed and analyzed in terms of high frequency electron acceptor vibrations.