Molecular structural dynamics of photoactive transition metal complexes in solar energy conversion studied by ultrafast optical spectroscopy and LITR-XAS

Transition metal complexes play important roles in solar energy conversion due to their intense absorption within the solar spectrum, their capability of transferring electron and energy through the metal to ligand charge transfer (MLCT) or ligand to metal charge transfer (LMCT) excited state, and their capability of binding substrates through coordination geometry changes. Hence, structural dynamics of these complexes in their excited states need to be understood in order to gain knowledge of controlling solar energy conversion processes. We will present our recent studies on structural dynamics of two metal complexes using both ultrafast optical Spectroscopy and laser initiated time-resolved x-ray absorption Spectroscopy (LITR-XAS). In order to fully understand the roles of transition metal complexes in solar energy conversion, structural dynamics of these molecules in their excited states on ultrafast time scales are needed. We will discuss mainly two different transition metal complex systems, 1) ultrafast structural dynamics in the MLCT state of [Cu(I)(dmp)₂]⁺ in different solvents, and 2) excited state structural dynamics of metalloporphyrins in solution. In the first system, a structural dynamics study using ultrafast up-conversion revealed time scale of the internal structural changes in the MLCT state, which provides guidance in future ultrafast structural determination using fs x-ray sources, such as LCLS. In the second system, a few examples of excited state metalloporphyrin structures obtained by LITR-XAS will be discussed with potential applications in photocatalysis and solar energy conversion.