Structure−Activity Correlations Among Iridium(III) Photosensitizers in a Robust Water-Reducing System

Abstract

A photocatalytic water-reducing system utilizing a bis-cyclometalated bipyridyl iridium(III) photosensitizer (PS) and a platinum or palladium heterogeneous catalyst was used to identify systematic property−activity correlations among a library of structural derivatives of [Ir(ppy)2(bpy)]+. A heterogeneous Pd catalyst proved to be more durable than its previously reported Pt-based counterpart, allowing for more reliable photosensitizer study. The deliberate steric and electronic variations of the ppy and bpy moieties resulted in a dramatic decrease of the degradation rates observed with selected photosensitizers when compared to the more substitution-labile [Ir(ppy)2(bpy)]+ parent compound. An improved photosensitizer structure with a Pd catalyst in a nonligating solvent exhibited a 150-fold increase in catalyst turnover numbers compared to the system using [Ir(ppy)2(bpy)]+ and a Pt catalyst. Furthermore, photocatalytic and photophysical studies at varied temperatures provided information on the rate-limiting step of the photocatalytic process, which is shown to be dependent on both the PS and the Pt or Pd catalytic species