Effect of Electronic Coupling on Electron Transfer Rates from Photoexcited Naphthalenediimide Radical Anion to Re(bpy)(CO)₃X

The electron transfer rate between a donor and an acceptor depends on the free energy change for the reaction as well as differences in structure and electronic coupling between the initial and final states. Selective excitation of a naphthalenediimide radical anion (NDI^•-) covalently linked at the 4-, 5-, or 6-positions of the bipyridine (bpy) in the Re(bpy)(CO)₃X (X = Cl or pyridine) carbon dioxide reduction catalyst results in electron transfer from ²NDI^•- to Re(bpy)(CO)₃X to form Re(bpy^•-)(CO)₃X, the first intermediate in the photocatalytic reduction of CO₂. Femtosecond UV/vis, near-IR, and mid-IR spectroscopy on these constitutional isomers and a set of appropriate reference molecules show that systematically varying the electronic coupling as well as the reaction free energy increases the lifetime of Re(bpy^•-)(CO)₃X by an order of magnitude when the NDI chromophore is attached to the 6-position of bpy. NMR and X-ray structural studies along with computational modeling are used to identify the conformation of Re(6-NDI-bpy)(CO)₃X responsible for these favorable changes. Extending the lifetime of the reduced complex in the covalent photosensitizer-catalyst assembly is a critical requirement for the photocatalytic CO₂ reduction and artificial photosynthesis.