  Time-resolved X-ray absorption spectroscopy has been utilized to monitor the
bimolecular electron transfer in a photocatalytic water splitting system for
the first time. This has been possible by uniting the local probe and element
specific character of X-ray transitions with insights from high-level ab initio
calculations. The specific target has been a heteroleptic [Ir$^{\rm
III}$(ppy)$_2$(bpy)]$^+$ photosensitizer, in combination with triethylamine as
a sacrificial reductant and Fe$_3$(CO)$_{12}$ as a water reduction catalyst.
The relevant molecular transitions have been characterized via high-resolution
Ir L-edge X-ray absorption spectroscopy on the picosecond time scale. The
present findings enhance our understanding of functionally relevant bimolecular
electron transfer reactions and thus will pave the road to rational
optimization of photocatalytic performance.
