Photosensitized charge separation using water-insoluble polymer-bound ruthenium(ii) complex films

Abstract

Water-insoluble ruthenium(II) complex-containing polymer films, in which bis(2,2′-bipyridine) complexes are coordinated with the imidazolyl residues on partially quaternized poly(1-vinylimidazole) (RuQPIm), have been prepared and characterized by spectroscopic and thermal measurements. The photosensitized charge separation in the sacrificial system, consisting of RuQPIm film, 1,1′-dimethyl-4,4′-bipyridinium (MV²⁺) or 1,1′-dioctyl-4,4′-bipyridinium (C₈V²⁺) dications, and triethanolamine (TEOA) has been investigated in aqueous solution. Here, RuQPIm film, MV²⁺ and C₈V²⁺, and TEOA act as a solid polymer photosensitizer, electron acceptors, and a sacrificial donor, respectively. The formation of viologen radical (MV⁺ or C₈V⁺) was observed on light irradiation (λ > 440 nm) of the aqueous solution. With MV²⁺ as an acceptor, the rate of MV⁺ formation decreased with increasing degree of quaternization. This suggested that the rate was controlled by at least three factors: (1) electrostatic repulsion, (2) π–π interaction between the heteroaromatic moieties of polymer and MV²⁺ and (3) steric hindrance of the alkyl side chain. On the other hand, the rate of C₈V⁺ formation was large, and dependence of the rate on the degree of quaternization differed significantly from that of the MV²⁺ system. This indicated that the C₈V²⁺ species had a further interaction in addition to the three described above, e.g. a hydrophobic interaction between alkyl chains in the C₈V²⁺ and the polymer. Further, the results obtained from the photosensitized charge separation in aqueous solution containing 1 M NaCl and in CH₃CN–H₂O suggested that the surface morphology of the polymer film was also a very important factor.