Reversible color changes induced by photosensitized charge separation in partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complex and viologen films

Abstract

The partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complex and viologen films (RuQPImC n VC 16 films: n=4 and 6) have been prepared, and the reversible color change reaction in these films has been investigated. The metallopolymers consist of bis(2,2′-bipyridine)ruthenium(II) complex coordinated with two imidazolyl residues and the viologen having a long alkyl group linked covalently to the imidazolyl residue through the alkyl spacer on the partially quaternized poly(1-vinylimidazole). Upon visible light irradiation ( λ>440 nm) of these metallopolymer films in air, the color changed from orange to blue, and reverted back to orange upon standing in the dark. The color change reaction was reversible with the responses being more rapid from the orange to blue relative to the opposite cycle. UV–vis absorption spectra showed that the blue color arose from the viologen radicals, and the absorption spectra of the ruthenium(II) complex residue did not change during light irradiation, indicating that the viologen radicals were generated through the photosensitized charge separation reaction. FTIR and UV spectra suggested that the adsorbed water in these polymer films through hydrogen bonding and electrostatic attraction acted as an electron donor for the ruthenium(III) complex residue, and the quaternized imidazolium residues acted as the scavenger molecules for one-electron oxidized water.