Role of Preaggregation in Single-Molecule Photoredox Catalysis

We investigate the role of preaggregation among photocatalyst and substrate in the paradigmatic molecular dye rhodamine 6G by means of fluorescence correlation experiments and quantum-mechanical modeling. By varying the substrate concentration, we experimentally confirm earlier conclusions that consecutive photoelectron transfer between rhodamine 6G and a model substrate molecule is not diffusion limited, raising puzzling questions regarding the mechanisms underlying its established photocatalytic activity. By theoretically exploring alternative explanations for ultrafast excited-state charge transfer, it is indeed found that preaggregation between photocatalyst and substrate is required to reach charge-transfer rates similar to those experimentally observed. Electrostatic as well as dispersive interactions are found to be most important for the molecular attraction involved in preaggregation. We show that tuning these contributions by chemical design alters the binding energies of photocatalyst-substrate assemblies. This suggests that reaction rates can be adjusted by adapting the composition of the species involved in preaggregation, which appears as an appealing concept in photocatalysis.