Transition-Metal Clusters and Complexes: Ab Initio Studies on Inducing Photoexcitation and Photocatalytic Activity.

Theoretical methods are indispensable in studying photocatalytic reaction pathways, the energy barriers of each elementary reaction step and in elucidating the active sites in catalysis. Transition metal sulfides of the 3d elements, M = Fe, Co, Ni, Cu, mimic natural reductase and carbon monoxide dehydrogenase enzymes. The exact environment of enzyme centers, which includes a protein surrounding the active site and coordinated ligands, cannot be achieved by synthesis. Careful modeling of ligands coordinated to the active site can predict whether photoactivation occurs and ways to enhance visible light absorption. For heavier transition metals, such as the f-block elements, the photophysical properties become more pronounced. From the computational results, supported by experiments, sulfur emerges as a key element in promoting photochemical and photophysical activity. Bridging sulfur in the clusters and complexes of the 3d elements (M2-S2) acts as an electron donor similar to coordinated carbonyl or nitrosyl ligands. Lanthanides also take advantage of the coordination of sulfur-containing ligands to fine-tune light absorption and light emission, producing lanthanide sulfide photocatalysts or high-intensity luminescent complexes. The excited-state energy transfer mechanism and factors responsible for the specific lanthanide emission are successfully predicted by combined DFT/TDDFT, multireference and semiempirical calculations in close correlation with the experiment.