Size-dependent electron transfer from atomically defined nanographenes to metal oxide nanoparticles

Atomically defined nanographenes (NGs) feature size-dependent energy gaps induced by, and tuneable through, quantum confinement. Their energy-tunability and robustness make NGs appealing candidates as active elements in sensitized geometries, where NGs functionalize a metal oxide (MO) film with large-area-to-volume ratio. Despite the prominent relevance of NG/MO interfaces for developing novel architectures for solar energy conversion, to date, little information is available regarding the fundamentals of electron transfer (ET) processes taking place from NG donors to MO acceptors. Here, we analyze the interplay between the size of atomically precise NGs and ET dynamics at NG/MO interfaces. We observe that as the size of NG decreases, ET from the NG donating state to the MO acceptor state speeds up. This dependence can be rationalized from variations in the donor-to-acceptor interfacial overpotential as the NG size (HOMO-LUMO gap) is reduced (increased), and can be rationalized within the framework of Marcus ET theory.