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Journal ArticleDOI

Competing Pathways in the photo-Proton-Coupled Electron Transfer Reduction of fac-[Re(bpy)(CO)3(4,4′-bpy]+* by Hydroquinone

12 Jul 2011-Journal of Physical Chemistry Letters (American Chemical Society)-Vol. 2, Iss: 15, pp 1844-1848
TL;DR: In this article, the metal-to-ligand charge transfer (MLCT) excited state of fac-[ReI(bpy)(CO)3(4,4′-bpy)]+ is reductively quenched by 1,4-hydroquinone (H2Q) in CH3CN at 23 ± 2 °C by competing pathways to give a common electronproton-transfer intermediate.
Abstract: The emitting metal-to-ligand charge transfer (MLCT) excited state of fac-[ReI(bpy)(CO)3(4,4′-bpy)]+ (1) (bpy is 2,2′-bipyridine, 4,4′-bpy is 4,4′-bipyridine), [ReII(bpy–•)(CO)3(4,4′-bpy)]+*, is reductively quenched by 1,4-hydroquinone (H2Q) in CH3CN at 23 ± 2 °C by competing pathways to give a common electron–proton-transfer intermediate. In one pathway, electron transfer (ET) quenching occurs to give ReI(bpy–•)(CO)3(4,4′-bpy)]0 with k = (1.8 ± 0.2) × 109 M–1 s–1, followed by proton transfer from H2Q to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+. Protonation triggers intramolecular bpy•– → 4,4′-bpyH+ electron transfer. In the second pathway, preassociation occurs between the ground state and H2Q at high concentrations. Subsequent Re → bpy MLCT excitation of the adduct is followed by electron–proton transfer from H2Q in concert with intramolecular bpy•– → 4,4′-bpyH+ electron transfer to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+ with k = (1.0 ± 0.4) × 109 s–1 in 3:1 CH3CN/H2O.
Citations
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Journal ArticleDOI
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

Journal ArticleDOI
TL;DR: The field of excited-state proton-coupled electron transfer (PCET) with d6 metal complexes is reviewed in this paper, which aims to illustrate the usefulness of such complexes for mechanistic studies of excitedstate PCET.

67 citations

Journal ArticleDOI
TL;DR: In this article, electron transfer from phenol molecules to a photoexcited ruthenium(II) complex was investigated as a function of the para-substituent (R = OCH3, CH3, H, Cl, Br, CN) attached to the phenols.
Abstract: Electron transfer (ET) from phenol molecules to a photoexcited ruthenium(II) complex was investigated as a function of the para-substituent (R = OCH3, CH3, H, Cl, Br, CN) attached to the phenols. For phenols with electron-donating substituents (R = OCH3, CH3), the rate-determining excited-state deactivation process is ordinary ET. For all other phenols, significant kinetic isotope effects (KIEs) (ranging from 2.91 ± 0.18 for R = Br to 10.18 ± 0.64 for R = CN) are associated with emission quenching, and this is taken as indirect evidence for transfer of a phenolic proton to a peripheral nitrogen atom of a 2,2′-bipyrazine ligand in the course of an overall proton-coupled electron transfer (PCET) reaction. Possible PCET reaction mechanisms for the various phenol/ruthenium couples are discussed. While 4-cyanophenol likely reacts via concerted proton–electron transfer (CPET), a stepwise proton transfer–electron transfer mechanism cannot be excluded in the case of the phenols with R = Br, Cl, and H.

58 citations

Journal ArticleDOI
TL;DR: This study aimed to explore how a difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes.
Abstract: Two rhenium(I) tricarbonyl diimine complexes, one of them with a 2,2′-bipyrazine (bpz) and a pyridine (py) ligand in addition to the carbonyls ([Re(bpz)(CO)3(py)]+), and one tricarbonyl complex with a 2,2′-bipyridine (bpy) and a 1,4-pyrazine (pz) ligand ([Re(bpy)(CO)3(pz)]+) were synthesized, and their photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge transfer (MLCT) excitation occurs toward the protonatable bpz ligand in the [Re(bpz)(CO)3(py)]+ complex while in the [Re(bpy)(CO)3(pz)]+ complex the same type of excitation promotes an electron away from the protonatable pz ligand. This study aimed to explore how this difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes. Transient absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D kinetic isotope effects are observed i...

40 citations

Journal ArticleDOI
TL;DR: Data indicate control of excited-state lifetime via a pre-equilibrium between the (3)MLCT state that initiates H-bond dynamics with the solvent and the ( 3)MC state that serves as the principal pathway for nonradiative decay.
Abstract: Photophysics of the MLCT excited-state of [Ru(bpy)(tpy)(OH2)]2+ (1) and [Ru(bpy)(tpy)(OD2)]2+ (2) (bpy = 2,2′-bipyridine and tpy = 2,2′:6′,2″-terpyridine) have been investigated in room-temperature H2O and D2O using ultrafast transient pump-probe spectroscopy. An inverse isotope effect is observed in the ground-state recovery for the two complexes. These data indicate control of excited-state lifetime via a pre-equilibrium between the 3MLCT state that initiates H-bond dynamics with the solvent and the 3MC state that serves as the principal pathway for nonradiative decay.

30 citations

References
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Journal ArticleDOI
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

Journal ArticleDOI
TL;DR: In this paper, a critical review of conversion constants amongst various reference electrodes reported in the literature reveals that in most cases the comparisons of redox potential values are far from accurate, and therefore, caution should be exercised when one is comparing the redox properties of complexes measured in CH 3 CN solutions versus different reference electrodes.

1,212 citations

Journal ArticleDOI
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

Journal ArticleDOI
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

Journal ArticleDOI
TL;DR: An integrated sequence of light-driven reactions that exploit coupled electron-proton transfer (EPT) could be the key to water oxidation.
Abstract: All higher life forms use oxygen and respiration as their primary energy source. The oxygen comes from water by solar-energy conversion in photosynthetic membranes. In green plants, light absorption in photosystem II (PSII) drives electron-transfer activation of the oxygen-evolving complex (OEC). The mechanism of water oxidation by the OEC has long been a subject of great interest to biologists and chemists. With the availability of new molecular-level protein structures from X-ray crystallography and EXAFS, as well as the accumulated results from numerous experiments and theoretical studies, it is possible to suggest how water may be oxidized at the OEC. An integrated sequence of light-driven reactions that exploit coupled electron-proton transfer (EPT) could be the key to water oxidation. When these reactions are combined with long-range proton transfer (by sequential local proton transfers), it may be possible to view the OEC as an intricate structure that is "wired for protons".

472 citations