Photoinduced hydrogen generation from water-insoluble polymer photosensitizer films

Abstract

Photoinduced hydrogen generation from functional polymer films with photosensitizing ability, in which ruthenium(II) complexes are coordinated to imidazolyl residues on partially quaternized poly(1-vinylimidazole) (RuQPIm), have been investigated in aqueous solution. This system consists of RuQPIm free-standing films having various degrees of quaternization, electron relays such as 1,1′-dimethyl-4,4′-bipyridinium (MV 2+) and 1,1′-dioctyl-4,4′-bipyridinium (C 8V 2+) dications, the sacrificial donor triethanolamine (TEOA), and bis(2,2′-bipyridine)platinum(II) ([Pt(pby) 2] 2+) as a catalyst. In MV 2+, hydrogen generation was observed with light irradiation (λ > 440 nm) under an argon atmosphere. The maximum volume of hydrogen generated decreased with increasing degree of quaternization. This was attributed to a decrease in the efficiency of photosensitized charge separation induced by increasing electrostatic repulsion, being supported by the fact that the volume of hydrogen generated was increased by the addition of sodium chloride (NaCl). For C 8V 2+ the volume of hydrogen generated was smaller than that for MV 2+, although the efficiency of charge separation was highly larger than that for MV 2+. Since the C 8V 2+ species interacted with the polymer film through hydrophobic interaction, the C 8V + species formed by electron transfer from the photoexcited ruthenium(II) complex to C 8V 2+ hardly diffused into the bulk, leading to inhibition of the reaction between C 8V + and the positively charged platinum catalyst. Therefore, it was clear that the diffusion of viologen radical species into the bulk is an important factor for the hydrogen generation reaction. Further, it was found that these polymer films have excellent stability for the present reaction.