Electron- and energy-transfer processes in a photocatalytic system based on an Ir(III)-photosensitizer and an iron catalyst

The reaction pathways of bis-(2-phenylpyridinato-)(2,2′-bipyridine) iridium(III)hexafluorophosphate [Ir(ppy)₂(bpy)]PF₆ within a photocatalytic water reduction system for hydrogen generation based on an iron-catalyst were investigated by employing time-resolved photoluminescence spectroscopy and time-dependent density functional theory. Electron transfer (ET) from the sacrificial reagent to the photoexcited Ir complex has a surprisingly low probability of 0.4% per collision. Hence, this step limits the efficiency of the overall system. The calculations show that ET takes place only for specific encounter geometries. At the same time, the presence of the iron-catalyst represents an energy loss channel due to a triplet-triplet energy transfer of Dexter type. This loss channel is kept small by the employed concentration ratios, thus favoring the reductive ET necessary for the water reduction. The elucidated reaction mechanisms underline the further need to improve the sun lights energy pathway to the catalyst to increase the efficiency of the photocatalytic system.