When electrons and protons get excited

Abstract

The coupling of electron and proton transfer reactions is central to a broad range of biological and chemical processes, including photosynthesis, respiration, and various solar energy devices. Elucidation of the fundamental physical principles underlying proton-coupled electron transfer (PCET) reactions (1⇓⇓⇓–5), in which both electrons and protons are transferred, is critical for a complete understanding of these energy conversion processes. This level of understanding is particularly important for the development of alternative renewable energy sources. The detailed mechanistic study of PCET reactions is challenging because of the complexity of the biological and chemical systems associated with such reactions. As illustrated by a study in PNAS (6), however, the application of modern spectroscopic techniques to relatively simple model systems provides an unprecedented level of mechanistic detail about PCET reactions. This pioneering work lays the foundation for future experimental and theoretical investigations to further unravel the intricacies of coupled electron and proton motions. The report by Westlake et al. (6) in PNAS centers on the PCET reactions induced by optical excitation of hydrogen-bonded dyes. One of the model systems studied is a phenol molecule hydrogen-bonded to an amine base, as depicted in Fig. 1 A . Optical excitation of this system induces intramolecular charge transfer (ICT), which can be viewed as an electron transfer from the oxygen to the nitro group within the phenol molecule. This shifting of the electron density away from the oxygen decreases the strength of the O-H bond and leads to proton transfer from the oxygen to the nitrogen in the base. The experiments implicate two different mechanisms for this PCET process. The first mechanism is sequential, where the intramolecular electron transfer is followed by proton transfer. The second mechanism is a concerted electron–proton transfer (EPT) mechanism in which the electron and proton transfer reactions … [↵][1]1E-mail: shs{at}chem.psu.edu. [1]: #xref-corresp-1-1