An initial review (PCET1) on proton-coupled electron transfer (PCET) by Huynh and Meyer appeared in Chemical Reviews in 2007. 1 This is a perennial review, a follow up on the original. It was intended for the special Chemical Reviews edition on Proton Coupled Electron Transfer that appeared in December, 2010 (Volume 110, Issue 12 Pages 6937-710). The reader is referred to it with articles on electrochemical approaches to studying PCET by Costentin and coworkers, 2 theory of electron proton transfer reactions by Hammes-Schiffer and coworkers, 3 proton-coupled electron flow in proteins and enzymes by Gray and coworkers, 4 and the thermochemistry of proton-coupled electron transfer by Mayer and coworkers. 5 Coverage for the current review is intended to be broad, covering all aspects of the topic comprehensively with literature coverage overlapping with the later references in PCET1 until late 2010. There is a growing understanding of the importance of PCET in chemistry and biology and its implications for catalysis and energy conversion. This has led to a series of informative reviews that have appeared since 2007. They include: "The possible role of Proton-coupled electron Transfer (PCET) in Water oxidation by Photosystem II" by Meyer and coworkers in 2007, 6 "Theoretical studies of proton-coupled electron transfer: Models and concepts relevant to bioenergetics" by Hammes-Schiffer and coworkers in 2008, 7 "Electrochemical Approach to the Mechanistic Study of Proton-Coupled Electron Transfer" by Costentin in 2008, 8 "Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems" by Nocera and Reece in 2009, 9 and "Integrating Proton-Coupled Electron Transfer and Excited States" by Meyer and coworkers in 2010. 10

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The simultaneous, concerted transfer of electrons and protonselectron-proton transfer (EPT)-is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4′-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H þ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated þ H─B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.electron transfer | proton-coupled electron transfer P roton-coupled electron transfer (PCET), in which electrons and protons are both transferred, is at the heart of many energy conversion processes in chemistry and biology (1-6). PCET reactions can occur by sequential two-step transfers (e.g., electron transfer followed by proton transfer, ET-PT, or proton transfer followed by electron transfer, PT-ET) or by concerted electron-proton transfer (EPT) (1, 2). EPT pathways are important in avoiding high-energy intermediates, playing an integral role in photosynthesis and respiration, for example.Photo-driven EPT (photo-EPT), with electron and proton transfers occurring simultaneously during the optical excitation process, would appear to be ruled out on fundamental grounds, because electronic excitation occurs rapidly on the time scale for nuclear motions, including proton transfer. Using a combination of femtosecond pump-probe and coherent Raman techniques, we have observed simultaneous electron-proton transfer induced by intramolecular charge transfer (ICT) excitation in two different hydrogen-bonded adducts formed between an organic dye (A─O─H) and an external base (:B). One is formed between a para-nitrophenyl-phenol and an amine base, and the other between a coumarin derivative and an imidazole base (Fig. 1).The shift in electron density away from the hydroxyl group to the intramolecular ...

We used femtosecond transient absorption (TA) spectroscopy to examine the excited state dynamics of singlewalled carbon nanotube (SWNT) bundles embedded in polymer matrices. The SWNTs were excited by a femtosecond pump pulse centered at either 1800, 900, or 550 nm and probed using a white-light continuum extending from 400 to 750 nm. We observed a structured TA spectrum consisting of a series of narrow induced transmission (IT) and induced absorption (IA) bands. The TA spectrum, which is independent of excitation wavelength, appeared on a time scale shorter than our instrument response (200 fs) and persisted for up to 100 ps. TA spectra obtained at a series of pump-probe delay times provided a window through which to monitor the exciton dynamics. We observed three distinct spectral signatures in the time-dependent data: (1) the decay of a broad photobleach, (2) the biphasic decay of narrow IT and IA features, and (3) a dynamical spectral shift of IA bands. These processes were attributed to plasmon relaxation, electron-hole recombination, and lattice relaxation associated with exciton self-trapping, respectively. Analysis of the transient spectrum suggested that it arose from a nonlinear optical response of the SWNT, where excitons produced by the pump pulse modified the transition frequencies of subsequent carrier excitations. The result was a series of IT bands (bleaches) at the ground state absorption frequencies, and associated with each was a corresponding red-shifted absorption band. These induced absorptions were attributed to the formation of biexcitons, fourparticle excitations that are produced through the sequential excitation of two closely spaced electron-hole pairs.