Defining Direct Orbital Pathways for Intermolecular Electron Transfer Using Sensitized Semiconducting Surfaces.

High-performance electronic materials and redox catalysts often rely on fast rates of intermolecular electron transfer (IET). Maximizing IET rates requires strong electronic coupling (HDA) between the electron donor and acceptor, yet universal structure-property relationships governing HDA in outer-sphere IET reactions have yet to be developed. For ground-state IET reactions, HDA is reasonably approximated by the extent of overlap between the frontier donor and acceptor orbitals involved in the electron-transfer reaction. Intermolecular interactions that encourage overlap between these orbitals, thereby creating a direct orbital pathway for IET, have a strong impact on HDA and, by extension, the IET rates. In this Forum Article, we present a set of intuitive molecular design strategies employing this direct orbital pathway principle to maximize HDA for IET reactions. We highlight how the careful design of redox-active molecules anchored to solid semiconducting substrates provides a powerful experimental platform for elucidating how electronic structure and specific intermolecular interactions affect IET reactions.