NMF effect on photosensitized charge separation using partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes

Abstract

The solvent effects on the photosensitized charge separation using partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complexes (RuQPIm), in which bis(2,2′-bipyridine)ruthenium(II) complexes are coordinated to the imidazolyl residues on the poly(1-vinylimidazole) partially quaternized by 1-bromohexadecane and the degree of quaternization is 19 (RuQPIm-19) and 44 (RuQPIm-44) molar percentage, have been investigated in methanol and methanol–NMF (NMF: N-methylformamide). These systems consist of RuQPIm-19 and RuQPIm-44 as photosensitizers, 1,1′-dimethyl-4,4′-bipyridinium dication (MV 2+) and 1,1′-didodecyl-4,4′-bipyridinium dication (C 12V 2+) as electron acceptors, and triethanolamine (TEOA) as a sacrificial donor. The addition of NMF mainly affect the forward and back reactions in the charge separation reactions. In RuQPIm-19 systems, the reaction proceeds through an interactive process, which is through the viologen having π–π interaction for MV 2+ and van der Waals interaction for C 12V 2+. In contrast, the reaction proceeds through a direct process by MV 2+ having no interaction and through an interactive process by C 12V 2+ undergoing van der Waals interaction with the polymer for RuQPIm-44 systems. For MV 2+, the rates of MV ·+ formation increased, although the quenching efficiency decreased with increasing NMF content for RuQPIm-19 and RuQPIm-44. These results are attributed to stabilization of MV ·+ species by π–π interaction for RuQPIm-19 and steric repulsion between MV ·+ species and RuQPIm-44; namely, the restriction of the back reactions by these effects. In contrast, for C 12V 2+, the rates of C 12V ·+ formation decreased and the quenching efficiency increased with the addition of NMF. These are attributed to that the addition of NMF increases the van der Waals interaction of the C 12V 2+ with these polymers and the diffusion of the C 12V ·+ species into the bulk solution; namely, the back reaction is accelerated. Furthermore, it is suggested that the conformational changes in these polymers contribute to the charge separation reaction.