PHOTOINDUCED ELECTRON-TRANSFER IN AMINO-ACID ASSEMBLIES

The preparation and photophysical characterization of a series of redox-active lysines and related model compounds based on polypyridyl ruthenium complexes are described. Donor-chromophore-acceptor triad 1, [PTZpn-Lys(RuIIb2m)2+-NH-prPQ2+](PF6-)4 (see below), was prepared by assembly of a modified ruthenium bipyridyl chromophore (Ru(II)b2m, where b = 2,2'-bipyridine, m = 4'-methyl-2,2'-bipyridyl-4'-carbonyl), an electron donor (phenothiazine, PTZ), and an electron acceptor (paraquat, PQ2+) on a lysine (Lys) scaffold utilizing amide bonds. This derivatized amino acid exhibited efficient (>95%) quenching of the ruthenium metal-to-ligand charge-transfer (MLCT) excited state upon irradiation with a 420-nm laser pulse in CH3CN. The resulting redox-separated state, [(PTZpn.+)-Lys(RuIIb2m)2+-NH-(prPQ.+)], stored 1.17 eV and lived for 108 ns (k = 9.26 x 10(6) s-1) as observed by transient absorption spectroscopy. Also studied was a series of related model systems that included model chromophores, simple chromophore-quencher dyads linked by amide bonds, and chromophore-quencher dyads based on lysine. An account of the kinetic behavior of these systems including triad 1 and a discussion of factors that influence the lifetime of the redox-separated states, their efficiency of formation, and their energy storage ability are presented.