Photoinduced hydrogen generation using polymer photosensitizers

Abstract

Photoinduced hydrogen generation using polymer photosensitizers, in which ruthenium(II) complexes are coordinated to imidazolyl residues on partially quaternized poly(1-vinylimidazole) (RuQPIm), were investigated in a methanol/water mixture (9:1 v/v). This system consists of photosensitizers RuQPIms having various degrees of quaternization, electron relays (acceptors) such as methylviologen (MV2+) and laurylviologen (C12V2+), triethanolamine as sacrificial donor (TEOA), and the platinum complex bis(2,2′-bipyridine)platinum(II) ([Pt(bpy)2]2+). Hydrogen generation was observed with light irradiation (λ > 440 nm) in argon atmosphere. The maximum volume of generated hydrogen decreased with increasing degree of quaternization. This was attributed to that the rate of formation of viologen radical species, corresponding to the efficiency of charge separation, is decreased by the increase in the electrostatic repulsion between the viologen species and the polymer backbone and steric hindrance of alkyl side chains. On the other hand, the volume of generated hydrogen for C12V2+ systems is less than that for the MV2+ systems, although the efficiency of photosensitized charge separation for C12V2+ systems is larger than that for MV2+ systems. This is the reason why the C12V2+ species have a weak van der Waals interaction between the alkyl side chains on the polymer backbone and alkyl group in the viologen. These results indicated that the diffusion of viologen radicals which are formed by electron transfer from the photoexcited ruthenium(II) complex to viologen in the polymer domain into the bulk solution is an important factor.