Enhanced Photocatalytic Proton-Coupled Electron Transfer by Ligand Design in a Zr Coordination Cage

Developing excited state proton and electron donors is a promising area of research that merges the benefits of proton-coupled electron transfer (PCET) with the use of light as renewable energy input. Based on the demonstrated PCET reactivity of Zr coordination cages for reductive photocatalysis, here we synthetize and characterize a new cage with enhanced photocatalytic activity. The new design targets the extended biphenyl-4,4-dicarboxylate linker with an amino group in the meta position relative to the carboxylate. Our results show that these aspects are key to increase the stability and reduction power of the excited state, features that are typically tuned by inductive effects. As a result, the new Zr-cage promotes significantly faster PCET reactions than the previous related platform, resulting in higher chemical and quantum yields. We further showcase how the solvent can impact the photophysical properties and the PCET reaction rates depending on the cage structure. These results highlight the factors that influence excited state PCET reactivity and complement similar efforts made in the realm of H₂ evolution.