Ultrafast Photoinduced Interfacial Proton Coupled Electron Transfer from CdSe Quantum Dots to 4,4′-Bipyridine

Pyridine and derivatives have been reported as efficient and selective catalysts for the electrochemical and photoelectrochemical reduction of CO₂ to methanol. Although the catalytic mechanism remains a subject of considerable recent debate, most proposed models involve interfacial proton coupled electron transfer (PCET) to electrode-bound catalysts. We report a combined experimental and theoretical study of the photoreduction of 4,4′-bipyridium (bPYD) using CdSe quantum dots (QDs) as a model system for interfacial PCET. We observed ultrafast photoinduced PCET from CdSe QDs to form doubly protonated [bPYDH₂]^+• radical cations at low pH (4-6). Through studies of the dependence of PCET rate on isotopic substitution, pH and bPYD concentration, the radical formation mechanism was identified to be a sequential interfacial electron and proton transfer (ET/PT) process with a rate-limiting pH independent electron transfer rate constant, k_int, of 1.05 ± 0.13 × 10¹⁰ s^-1 between a QD and an adsorbed singly protonated [bPYDH]⁺. Theoretical studies of the adsorption of [bPYDH]⁺ and methylviologen on QD surfaces revealed important effects of hydrogen bonding with the capping ligand (3-mercaptopropionic acid) on binding geometry and interfacial PCET. In the presence of sacrificial electron donors, this system was shown to be capable of generating [bPYDH₂]^+• radical cations under continuous illumination at 405 nm with a steady-state photoreduction quantum yield of 1.1 ± 0.1% at pH 4. The mechanism of bPYD photoreduction reported in this work may provide useful insights into the catalytic roles of pyridine and pyridine derivatives in the electrochemical and photoelectrochemical reduction of CO₂.