Impacting the balance between CO₂ and proton reduction by control over aggregation in a model π-conjugated...

In this study, we investigated the mechanistic factors that govern the selective photocatalytic reduction of CO₂ over protons within a simple π-conjugated N-heterocycle, proflavine. Diluted conditions, where aggregates of 65 nm are formed, favored the selective CO₂ photo-reduction, while the gradual transition to concentrated conditions enabled photo-reductive H₂ generation. Proton reduction is coupled to larger aggregates, in which an alternative photo-relaxation pathway is active. We used transient absorption spectroscopy to corroborate that at low proflavine concentrations the presence of an electron donor triggers the one-electron reduced proflavine to perform the direct CO₂ reduction. At high proflavine concentrations, protonation of the one-electron reduced proflavine was favored due to a positive shift in basicity in larger aggregates with sizes over 1 µm. In turn, H₂ abstraction began with a pair of one-electron reduced, protonated, intermediates. Our study demonstrates an effective approach to limiting water reduction, a key challenge in advanced metal-free organic photocatalysis.