Electron Transfer from Photoexcited Naphthalene Diimide Radical Anion to Electrocatalytically Active Re(bpy)(CO)₃Cl in a Molecular Triad

Abstract

Electron donor–acceptor photosensitizers having long charge separation lifetimes and high-reducing potentials that can be easily appended to thermodynamically difficult to reduce catalysts hold great promise for driving CO2 reduction. This study presents a new molecular triad utilizing a naphthalene diimide radical anion (NDI•–) donor chromophore appended to a 9,10-diphenylanthracene (DPA) acceptor, which is in turn linked to Re(bpy)(CO)3Cl. The NDI•– chromophore is readily generated by mild chemical or electrochemical reduction, absorbs at wavelengths as long as 800 nm, and has an excited state oxidation potential (−2.1 V vs SCE), which rivals or exceeds those of metalorganic and organometallic chromophores. Photoexcitation of NDI•– to *NDI•– is followed by ultrafast reduction of DPA to DPA•–, which then rapidly reduces the metal complex. The overall quantum yield for reduction of Re(bpy)(CO)3Cl is approximately 90% using visible light. The overall time constant for the forward electron transfer to reduce the metal complex is τ = 14.5 ps, while the time constant for back-electron transfer is τ = 24.5 ns. Under typical electrocatalytic conditions, the molecular triad demonstrates electrochemical reduction of CO2.