A rapid screening of Ru(II) photosensitizers

Abstract

When samples containing a Ru(II)-based photosensitizer such as Ru(bpy)₃²⁺ (bpy is 2,2′-bipyridine), methyl viologen (MV²⁺) and the sacrificial reductant triethanolamine (TEOA) are exposed to white light, the blue colour of the methyl viologen cation radical (MV^•+) develops by oxidative quenching of the sensitizer's excited state. This occurs even if the samples are not degassed, that is in spite of the known quenching of MV^•+ by O₂. In degassed media, however, a second pathway of MV^•+ formation occurs, known to involve the reduction of MV²⁺ by the oxidized and deprotonated form of TEOA (H₋₁TEOA^•) that arises from its function as a sacrificial reductant. This second pathway masks the effect of the photosensitization and disallows comparative analyses, but is suppressed by O₂. The second pathway also interferes in aqueous media, as the O₂ content becomes diffusion-limited. In order to comparatively assess the utility of Ru(II) photosensitizers, a method is described that uses non-degassed samples in CH₃CN under continuous irradiation with spectrophotometric monitoring of MV^•+. This requires little sample preparation and simple equipment, and is applicable to sensitizers with widely varying excited state lifetimes. A kinetic model that takes account of the aerobic quenching furnishes rate constants relating to MV^•+ formation and (non-aerobic) quenching that are reproducible, consistent with mechanistic expectations and concordant with the sensitizer excited state lifetime. The rate constants can then be used to compare sensitizers subject to the same oxidative quenching.