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

• Published in: Journal of the American Chemical Society Vol. 138; no. 3; pp. 884 - 892
• Main Authors: Chen, Jinquan, Wu, Kaifeng, Rudshteyn, Benjamin, Jia, Yanyan, Ding, Wendu, Xie, Zhao-Xiong, Batista, Victor S, Lian, Tianquan
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 27.01.2016
• Subjects: 3-mercaptopropionic acid; adsorption; carbon dioxide; catalysts; cations; electrochemistry; electron transfer; geometry; hydrogen bonding; ligands; lighting; methanol; pyridines; quantum dots
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.5b10354

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.