Proton-Coupled Electron and Energy Transfer in Molecular Triads

ConspectusElectrons and protons are the simplest particles in chemistry, and their transfers are among the most fundamental chemical reactions. It is increasingly recognized that these two particles often transfer in the same elementary kinetic step, resulting in the most common type of proton-coupled electron transfer (PCET). PCET has evolved from a curiosity to a major research field that is central to a broad range of processes in chemistry, biology, and materials science.PCET evolved from electron transfer, in both its experimental and theoretical origins. One wonders how the field would be different if it had been called electron-coupled proton transfer. This equivalent terminology illustrates that the proton is on equal footing to the electron, making PCET perhaps the simplest case where the quantum properties of both an electron and a nucleus need to be considered.The fundamental understanding of PCET in solution builds on the remarkably impactful theory of electron transfer (ET) developed by R. A. Marcus and others. At a basic level, ET theory is marked by a quadratic dependence of the reaction barrier on the reaction free energy (ΔG^⧧ on ΔG°), with normal and 'inverted' regions separated by a barrierless region (ΔG^⧧ = 0), plus an electronic coupling that determines the electron tunneling probability. The theory for PCET includes additional essential elements: the quantum mechanical treatment of the transferring proton(s) as tunneling particles, multiple channels corresponding to reactant and product electron-proton vibronic states, vibronic coupling rather than electronic coupling, and a distribution of proton donor-acceptor distances.Our recent studies of ultrafast intramolecular PCET in molecular triads were the first to demonstrate the corresponding free-energy dependence for PCET, including the inverted region. Inverted behavior was previously thought to be difficult to observe experimentally for PCET because it connects vibronic states rather than electronic states. Due to the more closely spaced vibronic state energy levels compared to electronic state energy levels, there is usually a nearly barrierless pair of reactant and product vibronic states that obviates the inverted region. For these molecular triads, however, the vibronic coupling is very small for the barrierless pair, allowing observation of the hallmark inverted region.While looking for ultrafast PCET, we discovered a new elementary chemical reaction that we denoted proton-coupled energy transfer (PCEnT). In PCEnT, proton transfer (PT) is coupled to electronic excitation energy transfer. As with PCET, PT is required for the reaction to be thermodynamically accessible. In our molecular triads, PT occurs within the phenol-pyridine acceptor unit, concerted with electron transfer to a photoexcited anthracene (PCET) or electronic excitation energy transfer from a photoexcited anthracene (PCEnT). The dominant reaction depends on the molecular substituents and reaction conditions. A theory for PCEnT with some of the same essential elements as PCET theory, along with some fundamental differences, has been developed and applied to a triad system.