Ultrafast structural dynamics of photoactive metal complexes in solar hydrogen generation

Abstract

Photoexcited states of metal complexes are precursors for many important photochemical processes in solution phase which lead to solar hydrogen generation. Therefore, knowing their structures with atomic resolution and sufficient time resolution is crucial in correlating structures with molecular properties. Using x-ray transient absorption (XTA) spectroscopy, transient metal oxidation states, coordination geometry, and atomic rearrangements during photochemical processes can be probed. Such an approach complements with ultrafast optical laser spectroscopy in obtaining kinetics and coherence information among different excited states as well as intra- and intermolecular energy/charge transfer processes associated with solar energy conversion. Excited state structures of transition metal complexes, such as metalloporphyrins in solution, created by photoexcitation have been studied by XTA combined with optical transient absorption spectroscopy. Direct evidences of photoinduced redox reactions and coordination geometry changes as well as electronic configurations of the metals can be observed. These experimental studies are combined with quantum mechanical calculations to rationalize the evolution of the ultrafast excited state pathways with electronic configuration changes that may be responsible for the reactivity of the molecules in solar hydrogen generation. Preliminary time-resolved X-ray absorption near edge structure (XANES) studies on Pt coated TiO₂ nanoparticles during photocatalysis show a significant potential impact of XTA in understanding solar hydrogen production.