Excited-state quenching by proton-coupled electron transfer
12 May 2007-Journal of the American Chemical Society (American Chemical Society)-Vol. 129, Iss: 22, pp 6968-6969
TL;DR: The protonated, reduced complex [Ru(bpy)2(bpzH•)]2+ functions as a H-atom reductant toward quinone or benzaldehyde with potential implications for net photochemistry and energy conversion.
Abstract: The emitting metal-to-ligand charge transfer (MLCT) excited state of [Ru(bpy)2(bpz)]2+ (bpy is 2,2‘-bipyridine; bpz is 2,2‘-bipyrazine) is reductively quenched by hydroquinone (H2Q) by proton-coupled electron transfer (PCET), most likely by concerted electron−proton transfer (EPT). The identity of the transient products ([Ru(bpy)2(bpzH•)]2+ and HQ•) and the kinetics of their formation and disappearance have been established by steady-state emission and time-resolved emission, absorption, and EPR measurements. The protonated, reduced complex [Ru(bpy)2(bpzH•)]2+ functions as a H-atom reductant toward quinone or benzaldehyde with potential implications for net photochemistry and energy conversion.
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TL;DR: Proton-coupled electron transfer is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues and several are reviewed.
Abstract: ▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...
2,182 citations
TL;DR: The analysis of such stepwise mechanisms both in aprotic media and in water is reviewed, with particular recent emphasis on electrochemical and theoretical approaches to proton-coupled electron transfer processes.
Abstract: The coupling between electron and proton transfers has a long experimental and theoretical history in chemistry and biochemistry. To take just one example, the fact that acceptance of an electron triggers the addition of an acid or the removal of a base and vice versa for oxidations towers over all understanding of organic electrochemistry. Protoncoupled electron transfer (PCET) reactions also play a critical role in a wide range of biological processes, including enzyme reactions, photosynthesis, and respiration. A recent impressive review describes PCET reactions and phenomena. PCET is employed here as a general term for reactions in which both an electron and a proton are transferred, either in two separate steps or in a single step. Reactions in which the electron and proton transfer between the same donor and acceptor, that is, hydrogen atom transfer, are, of course, not considered here because we consider electrochemical PCET reactions in which electrons are flowing into or from an electrode while protons are transferred between acid and base. Molecular electrochemistry, through nondestructive techniques such as cyclic voltammetry, has proved to be very useful in characterizing electron transfers and deciphering mechanisms in which chemical reactions are associated with electron transfer. Therefore, it has been a convenient tool for the mechanistic study of reactions in which electron transfer is coupled to proton transfer, that is, in which an electron leaves or enters an electrode while a proton is transferred from or to the redox species. Until recently, PCET has been mostly thought of as stepwise electron and proton transfer (ET-PT or PT-ET). We thus review in an initial section (section 2) the analysis of such stepwise mechanisms both in aprotic media and in water. In aprotic media, * E-mail address: cyrille.costentin@univ-paris-diderot.fr. Cyrille Costentin was born in Normandy, France, in 1972. He received his undergraduate education at Ecole Normale Superieure (Cachan, France) and pursued his graduate studies under the guidance of Prof. Jean-Michel Saveant and Dr. Philippe Hapiot at the University of ParisDiderot (Paris 7), where he received his Ph.D. in 2000. After a year as a postdoctoral fellow at the University of Rochester, working with Prof. J. P. Dinnocenzo, he joined the faculty at the University of Paris-Diderot as an associate professor. He was promoted to professor in 2007. His interests include mechanisms and reactivity in electron transfer chemistry with particular recent emphasis on electrochemical and theoretical approaches to proton-coupled electron transfer processes. Chem. Rev. 2008, 108, 2145–2179 2145
363 citations
TL;DR: In this article, a photoelectrochemical synthesis cell (DS-PEC) is proposed for coupled, light driven oxidation and reduction in artificial photosynthesis, where photolysis of organic charge transfer excited states with H-bonded bases or in metal-to-ligand charge transfer (MLCT) excited states in pre-associated assemblies with H bonded electron transfer donors or acceptors.
149 citations
145 citations
TL;DR: Recent studies of excited-state PCET with d(6) metal complexes with central question whether concerted proton-electron transfer (CPET) can compete kinetically with sequential electron and proton transfer steps are described.
Abstract: Proton-coupled electron transfer (PCET) plays a crucial role in many enzymatic reactions and is relevant for a variety of processes including water oxidation, nitrogen fixation, and carbon dioxide reduction. Much of the research on PCET has focused on transfers between molecules in their electronic ground states, but increasingly researchers are investigating PCET between photoexcited reactants. This Account describes recent studies of excited-state PCET with d6 metal complexes emphasizing work performed in my laboratory.Upon photoexcitation, some complexes release an electron and a proton to benzoquinone reaction partners. Others act as combined electron-proton acceptors in the presence of phenols. As a result, we can investigate photoinduced PCET involving electron and proton transfer in a given direction, a process that resembles hydrogen-atom transfer (HAT). In other studies, the photoexcited metal complexes merely serve as electron donors or electron acceptors because the proton donating and acceptin...
132 citations
References
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TL;DR: Life on earth is almost entirely solar-powered, with carbohydrate acting as a source of high-energy electrons and dioxygen providing a lower-energy destination for these electrons.
Abstract: Life on earth is almost entirely solar-powered. We can get some idea of the enormous quantity of energy received from the sun by noting that during daylight hours, the sun provides several thousand times more power to the surface of the U.S.A. than is produced by all of the nation’s electrical power stations. 1,2 Around 50% of the radiation that reaches the earth’s surface, roughly the visible region, is of a frequency useful to photosynthetic organisms. Oxygenic photosynthetic organisms convert this radiation into chemical energy, in the form of carbohydrate and dioxygen, at an optimal efficiency of something like 25%. 3 These products together sustain the rest of aerobic life, with carbohydrate acting as a source of high-energy electrons and dioxygen providing a lower-energy destination for these electrons. The overall equation of oxygenic photosynthesis is given in eq 1, where (CH2O) represents carbohydrate:
1,367 citations
TL;DR: The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electronTransfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion.
Abstract: The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.
916 citations
TL;DR: Intrinsic barriers for PCET can be comparable to or larger than those for ET, and many PCET/HAT rate constants are predicted well by the Marcus cross relation.
Abstract: Proton-coupled electron transfer (PCET) reactions involve the concerted transfer of an electron and a proton. Such reactions play an important role in many areas of chemistry and biology. Concerted PCET is thermochemically more favorable than the first step in competing consecutive processes involving stepwise electron transfer (ET) and proton transfer (PT), often by >=1 eV. PCET reactions of the form X-H + Y X + H-Y can be termed hydrogen atom transfer (HAT). Another PCET class involves outersphere electron transfer concerted with deprotonation by another reagent, Y+ + XH-B Y + X-HB+. Many PCET/HAT rate constants are predicted well by the Marcus cross relation. The cross-relation calculation uses rate constants for self-exchange reactions to provide information on intrinsic barriers. Intrinsic barriers for PCET can be comparable to or larger than those for ET. These properties are discussed in light of recent theoretical treatments of PCET.
705 citations
TL;DR: This theoretical framework allows predictions of rates, mechanisms, and kinetic isotope effects for proton-coupled electron transfer reactions.
Abstract: This Account presents a theoretical formulation for proton-coupled electron transfer reactions. The active electrons and transferring protons are treated quantum mechanically, and the free energy surfaces are obtained as functions of collective solvent coordinates corresponding to the proton and electron transfer reactions. Rate expressions have been derived in the relevant limits, and methodology for including the dynamical effects of the solvent and protein has been developed. This theoretical framework allows predictions of rates, mechanisms, and kinetic isotope effects for proton-coupled electron transfer reactions.
405 citations
TL;DR: The physical structure and energetics of PSII are reviewed and a metalloradical enzyme mechanism for the water-oxidation process it catalyzes is discussed, which is based on the specifics of the chemistry in which O2 participates.
Abstract: Dioxygen is thermodynamically hot but kinetically cool, which makes it an ideal reagent for maximizing biological free energy production and for carrying out difficult chemical transformations in enzyme active sites.1 The widespread use of dioxygen in biological catalysis has led to an enzyme classification scheme s monooxygenases, dioxygenases, oxidases s that is based on the specifics of the chemistry in which O2 participates. Examples of the remarkable utility of dioxygen in biology abound and include its use in maximizing ATP production in aerobic respiration, in C-H bond activation in the P450 enzymes and methane monoxygenases, and in the degradation of important biomaterials such as lignin. Although nature has devised a multitude of mechanisms by which to activate dioxygen for useful chemistry, only one system, Photosystem II (PSII) in plants and algae, has evolved that has the capacity to lift water out of its thermodynamic well to generate dioxygen. This singular development provided photosynthetic organisms with an abundant and ubiquitous substrate for growth and diversification. The molecular mechanism by which PSII is able to strip hydrogen atoms from water and release O2 as waste is coming into view. In this article, we review the physical structure and energetics of PSII. We then discuss and analyze a metalloradical enzyme mechanism for the water-oxidation process it catalyzes.
372 citations