Professor Aron Huckaba of the University of Kentucky

“Improving Light Absorption in Re (I) Complexes Using a Donor-pi bridge-acceptor Design Paradigm"

Some of the most powerful synthetic methods to be developed recently involve the use of photoredox chemistry through light absorbing metal complexes. The light absorbers can either act as a photosensitizer or photocatalyst. Typically, the amount of light absorbed and utilized by these complexes compared to the whole visible light spectrum is not large. This is especially true for Re (I) complexes, where typically only light <500 nm is utilized. If the photons used to realize catalytic transformations are solar photons, then the molecule’s absorption cross section of the solar spectrum should be increased to increase the catalyst quantum efficiency. The two manners in which light absorption can be improved is in the molecular extinction coefficient and in the energy of the lowest energy catalytically implicated light absorption. To improve the molar extinction coefficient, we envisioned utilizing a donor-p-acceptor architecture for the ligand. To lower the energy of a catalytically useful excited state, we will utilize complexes that can utilize photons from either Metal-to-ligand charge transfer (MLCT) transition or intraligand charge transfer (ILCT) transitions. The metal is chelated to a 2-(azolyl)pyridine p-bridge, with various donor and/or acceptor motifs. In this presentation, the impact of ligand structure on metal complex light absorption will be discussed, as will the impact on molecular energy levels, and electro- and photocatalytic performance. We observed in our first studies into D-p-A Re (I) metal complexes that we could greatly improve, in both absorption energy and extinction coefficient, the light absorption of the Re (I) complexes.