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

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.