We investigate the role of preaggregation among photocatalystandsubstrate in the paradigmatic molecular dye rhodamine 6G by meansof fluorescence correlation experiments and quantum-mechanical modeling.By varying the substrate concentration, we experimentally confirmearlier conclusions that consecutive photoelectron transfer betweenrhodamine 6G and a model substrate molecule is not diffusion limited,raising puzzling questions regarding the mechanisms underlying itsestablished photocatalytic activity. By theoretically exploring alternativeexplanations for ultrafast excited-state charge transfer, it is indeedfound that preaggregation between photocatalyst and substrate is requiredto reach charge-transfer rates similar to those experimentally observed.Electrostatic as well as dispersive interactions are found to be mostimportant for the molecular attraction involved in preaggregation.We show that tuning these contributions by chemical design altersthe binding energies of photocatalyst-substrate assemblies.This suggests that reaction rates can be adjusted by adapting thecomposition of the species involved in preaggregation, which appearsas an appealing concept in photocatalysis.