Photoreduction of water sensitised by Rose Bengal

Abstract

Rose Bengal was used to sensitise the photoreduction of water in aqueous ethanol solution (5 % EtOH v/v; pH 4.4)via both a reductive and an oxidative quenching mechanism, using EDTA and methyl viologen, respectively, as quenchers of its triplet state. In the presence of EDTA (which acts as both tiplet quencher and sacrificial electron donor) the photoreduction of water sensitised by Rose Bengal was inefficient for a number of reasons, including a low triplet quenching rate constant (8 × 10³ dm³ mol^–1 s^–1) and a rapid disproportionation reaction involving the protonated semi-reduced dye radicals. In the presence of methyl viologen (triplet quencher) and EDTA (sacrificial electron donor) the photoreduction of water sensitised by Rose Bengal was inefficient due to a low cage-escape yield (ϕ≈ 5 × 10^–2) following the primary electron-transfer step, a fast back reaction (k 1 × 10⁸ dm³ mol^–1 s^–1) and formation of a complex between the dye and methyl viologen (K= 6400 ± 300 dm³ mol^–1). The complexed form of Rose Bengal appeared able to photosensitise, inefficiently, the photoreduction of methyl viologen in the presence of EDTA, without exhibiting problems of dye fade.