Enhancement effects of l-tyrosine esters on photosensitized charge separation using ruthenium(II) complex- and viologen-containing polymers

Abstract

Ruthenium(II) complex- and viologen-containing polymers, which consist of the bis(2,2′-bipyridine) ruthenium(II) complex coordinated with imidazolyl residues on poly(1-vinylimidazole) (RuVPIm) and partially quaternized poly(1-vinylimidazole) (RuVQPIm) and viologen covalently linking these polymers through a hexylene spacer, have been synthesized. These polymers have two kinds of ruthenium(II) complex residues, the 557Ru(II) complex compartmentalized by the viologen residues and polymer backbone and a 640Ru(II) complex exposed to the bulk solution, which show emission maxima at 557 and 640 nm, respectively. The influence of l-tyrosine esters with hexyl (C 6Tyr), octyl (C 8Tyr), and dodecyl (C 12Tyr) groups on the luminescence properties and photosensitized charge separation has been investigated in methanol. The luminescence intensities of these polymers decreased with the addition of the l-tyrosine esters, indicating that the quenching of the ruthenium(II) complex residues increased. The initial rates of the viologen radical formation was enhanced by the addition of the l-tyrosine esters. These results were attributed to a mediated effect of the l-tyrosine esters which acted as a mediator for photoinduced electron transfer from the photoexcited ruthenium(II) complex to viologen residue. Furthermore, the l-tyrosine esters undergoing interaction with a solvophobic domain where 557Ru(II) complex residues were compartmentalized acted as the mediator for only the quenching reaction, while they undergo an interaction with the polymer backbone mediating both the quenching reaction and the back reaction in the photosensitized charge separation.