Structural dynamics of photoactive metal complexes in solar energy conversion

The photoexcited states of metal complexes are precursors for many important photochemical processes in solution phase for solar energy conversion. Therefore, knowing their structures with atomic resolution and sufficient time resolution is crucial in correlating structures with molecular properties. Combining ultrafast optical and x-ray transient absorption spectroscopy, the structural dynamics of several important metal complexes can be obtained, including their transient metal oxidation states, coordination geometry, and atomic rearrangements during photochemical processes. Excited state structures of transition metal complexes, such as metalloporphyrins and platinum(II) complexes in solution, created by photoexcitation have been studied. Direct evidences of photoinduced redox reactions and coordination geometry changes as well as electronic configurations of the metals have been obtained. The combined x-ray and laser spectroscopic studies also reveal and energy levels of d orbitals for metalloporphyrin, an unexpected light induced ultrafast internal molecular motion triggered in a copper(I) complex. These experimental studies are combined with time-dependent density functional theory (TDDFT) 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. These studies will have a great impact in fundamental understanding of photochemical and photophysical processes in solar energy conversion, such as solar fuel production.