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We experimentally discovered and theoretically analyzed a photochemical mechanism, which we term proton-coupled energy transfer (PCEnT). A series of anthracene-phenol-pyridine triads formed a local excited anthracene state after light excitation at a wavelength of ~400 nanometers (nm), which led to fluorescence around 550 nm from the phenol-pyridine unit. Direct excitation of phenol-pyridine would have required ~330-nm light, but the coupled proton transfer within the phenol-pyridine unit lowered its excited-state energy so that it could accept excitation energy from anthracene. Singlet-singlet energy transfer thus occurred despite the lack of spectral overlap between the anthracene fluorescence and the phenol-pyridine absorption. Moreover, theoretical calculations indicated negligible charge transfer between the anthracene and phenol-pyridine units. We construe PCEnT as an elementary reaction of possible relevance to biological systems and future photonic devices.

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We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XHn in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H2), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H2) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

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The multiredox reactivity of bioinorganic cofactors is often coupled to proton transfers. Here we investigate the structural, thermochemical, and electronic structure of ruthenium-amino/amido complexes with multi- proton-coupled electron transfer reactivity. The bis(amino)ruthenium(II) and bis(amido)ruthenium(IV) complexes [RuII(bpy)(en*)2]2+ (RuII-H0 ) and [RuIV(bpy)(en*-H2)2]2+ (RuIV-H2 ) interconvert reversibly with the transfer of 2e-/2H+ (bpy = 2,2'-bipyridine, en* = 2,3-diamino-2,3-dimethylbutane). X-ray structures allow correlations between the structural and electronic parameters, and the thermochemical data of the 2e-/2H+ multi-square grid scheme. Redox potentials, acidity constants and DFT calculations reveal potential intermediates implicated in 2e-/2H+ reactivity with organic reagents in non-protic solvents, which shows a strong inverted redox potential favouring 2e-/2H+ transfer. This is suggested to be an attractive system for potential one-step (concerted) transfer of 2e-and 2H+ due to the small changes of the pseudo-octahedral geometries and the absence of charge change, indicating a relatively small overall reorganization energy.

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Concerted proton-coupled electron transfer (PCET) in the Marcus inverted region was recently demonstrated (Science 2019, 364, 471-475). Understanding the requirements for such reactivity is fundamentally important and holds promise as a design principle for solar energy conversion systems. Herein, we investigate the solvent polarity and temperature dependence of photoinduced proton-coupled charge separation (CS) and charge recombination (CR) in anthracene-phenol-pyridine triads: 1 (10-(4-hydroxy-3-(4-methylpyridin-2-yl)benzyl)anthracene-9-carbonitrile) and 2 (10-(4-hydroxy-3-(4-methoxypyridin-2-yl)benzyl)anthracene-9-carbonitrile). Both the CS and CR rate constants increased with increasing polarity in acetonitrile:n-butyronitrile mixtures. The kinetics were semi-quantitatively analyzed where changes in dielectric and refractive index, and thus consequently changes in driving force (-ΔG°) and reorganization energy (λ), were accounted for. The results were further validated by fitting the temperature dependence, from 180 to 298 K, in n-butyronitrile. The analyses support previous computational work where transitions to proton vibrational excited states dominate the CR reaction with a distinct activation free energy (ΔG*CR ∼ 140 meV). However, the solvent continuum model fails to accurately describe the changes in ΔG° and λ with temperature via changes in dielectric constant and refractive index. Satisfactory modeling was obtained using the results of a molecular solvent model [J. Phys. Chem. B 1999, 103, 9130-9140], which predicts that λ decreases with temperature, opposite to that of the continuum model. To further assess the solvent polarity control in the inverted region, the reactions were studied in toluene. Nonpolar solvents decrease both ΔG°CR and λ, slowing CR into the nanosecond time regime for 2 in toluene at 298 K. This demonstrates how PCET in the inverted region may be controlled to potentially use proton-coupled CS states for efficient solar fuel production and photoredox catalysis.

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We report a combined photocatalytic and hydrogen atom transfer (HAT) approach for the light-mediated epimerization of readily accessible piperidines to provide the more stable diastereomer with high selectivity. The generality of the transformation was explored for a large variety of di- to tetrasubstituted piperidines with aryl, alkyl, and carboxylic acid derivatives at multiple different sites. Piperidines without substitution on nitrogen as well as N-alkyl and aryl derivatives were effective epimerization substrates. The observed diastereoselectivities correlate with the calculated relative stabilities of the isomers. Demonstration of reaction reversibility, luminescence quenching, deuterium labeling studies, and quantum yield measurements provide information about the mechanism.

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Electron transfer reactions slow down when they become very thermodynamically favorable, a counterintuitive interplay of kinetics and thermodynamics termed the inverted region in Marcus theory. Here we report inverted region behavior for proton-coupled electron transfer (PCET). Photochemical studies of anthracene-phenol-pyridine triads give rate constants for PCET charge recombination that are slower for the more thermodynamically favorable reactions. Photoexcitation forms an anthracene excited state that undergoes PCET to create a charge-separated state. The rate constants for return charge recombination show an inverted dependence on the driving force upon changing pyridine substituents and the solvent. Calculations using vibronically nonadiabatic PCET theory yield rate constants for simultaneous tunneling of the electron and proton that account for the results.

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Proton-coupled electron transfer (PCET) covers a wide range of reactions involving the transfer(s) of electrons and protons. The best-known PCET reaction, hydrogen atom transfer (HAT), has been studied in detail for more than a century. HAT is generally described as the concerted transfer of a hydrogen atom (H• ≡ H+ + e-) from one group to another, Y + H-X → Y-H + X, but a strict definition of HAT has been difficult to establish. Distinctions are more challenging when the transfer of "H•" involves e- and H+ that transfer to/from spatially distinct sites or even completely separate reagents (multiple-site concerted proton-electron transfer, MS-CPET). MS-CPET reactivity is increasingly proposed in biological and synthetic contexts, and some reactions typically described as HAT more resemble MS-CPET. Despite that HAT and MS-CPET reactions "look different," we argue here that these reactions lie on a reactivity continuum, and that they are governed by many of the same key parameters. This Account walks the reader across this PCET reactivity continuum, using a series of studies to show the strong similarities of reactions that move protons and electrons in seemingly different ways. To prepare for our stroll, we describe the thermochemical and kinetic frameworks for HAT and MS-CPET. The driving force for a solution HAT reaction is most easily discussed as the difference in the bond dissociation free energies (BDFEs) of the reactants and products. BDFEs can be analyzed as sums of electron and proton transfer steps and can therefore be obtained from p Ka and E° values. Even though MS-CPET reactions do not make and break H-X bonds in the same way as HAT, the same thermochemical description can be used with the introduction of an effective BDFE (BDFEeff). The BDFEeff of a reductant/acid pair is the free energy of that pair to form H•, which can be obtained from p Ka and E° values in an analogous fashion to a standard BDFE. When the PCET thermochemistry is known, HAT and PCET rate constants can be understood and often predicted using linear free energy relationships (the Brønsted catalysis law) and Marcus theory type approaches. After this background, we walk the reader through a continuum of PCET reactivity. Our journey begins with a study of metal-mediated HAT from hydrocarbon substrates to a metal-oxo complex and travels to the MS-CPET end of the reactivity spectrum, involving the transfer of H+ and e- from the hydroxylamine TEMPOH to two completely separate molecules. These examples, and those in between, are all analyzed within the same thermodynamic and kinetic framework. A description of the first examples of MS-CPET with C-H bonds uses the same framework and highlights the importance of hydrogen bonding and preorganization. The examples and analyses show that the reactions along the PCET continuum are more similar than they are different, and that attempts to divide these reactions into subcategories can obscure much of the essential chemistry. We hope that developing the many common features of these reactions will help experts and newcomers alike to explore exciting new territories in PCET reactivity.

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The transfer of protons and electrons is key to energy conversion and storage, from photosynthesis to fuel cells. Increased understanding and control of these processes are needed. A new anthracene-phenol-pyridine molecular triad was designed to undergo fast photoinduced multiple-site concerted proton-electron transfer (MS-CPET), with the phenol moiety transferring an electron to the photoexcited anthracene and a proton to the pyridine. Fluorescence quenching and transient absorption experiments in solutions and glasses show rapid MS-CPET (3.2 × 1010 s-1 at 298 K). From 5.5 to 90 K, the reaction rate and kinetic isotope effect (KIE) are independent of temperature, with zero Arrhenius activation energy. From 145 to 350 K, there are only slight changes with temperature. This MS-CPET reaction thus occurs by tunneling of both the proton and electron, in different directions. Since the reaction proceeds without significant thermal activation energy, the rate constant indicates the magnitude of the electron/proton double tunneling probability.

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A series of novel cyclometalated RuII complexes were investigated featuring the tridentate dqp ligand platform (dqp is 2,6-di(quinolin-8-yl)pyridine), in order to utilize the octahedral coordination mode around the Ru center to modulate the electrochemical and photophysical properties. The heteroleptic complexes feature C1 symmetry due to symmetry breaking by the peripheral five- or six-membered carbanionic chelate (phenyl, naphthyl, or anthracenyl units). The chelation mode is controlled by the steric effects and C-H activation selectivity of the ligand, which prompted the development of a general synthesis protocol. The optimized conditions to achieve high overall yields (55-75%) involve NaHCO3 as the base and an simplified purification protocol: i.e., facile chromatographic separation using commercially available amino-functionalized silica applying nonaqueous salt-free conditions to omit the necessity of counterion exchange. The structural, photophysical, and electrochemical properties were studied in depth, and the results were corroborated by density functional theory (DFT) calculations. Steady state and time-resolved spectroscopy revealed red-shifted absorption (up to 750 nm) and weak IR emission (800-1000 nm) combined with prolonged emission lifetimes (up to 20 ns) in comparison to classical tpy-based (tpy is 2,2':6',2″-terpyridine) complexes. An enhanced stability was observed by blocking the reactive positions of the carbanionic ligand framework, while the reactive positions may be exploited for further functionalization.

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The distance dependence of concerted proton-coupled electron transfer (PCET) reactions was probed in a series of three new compounds, where a phenol is covalently bridged by a 5, 6, or 7 membered carbocycle to the quinoline. The carbocycle bridge enforces the change in distance between the phenol oxygen (proton donor) and quinoline nitrogen (proton acceptor), dO···N, giving rise to values ranging from 2.567 to 2.8487 Å, and resulting in calculated proton tunneling distances, r0, that span 0.719 to 1.244 Å. Not only does this series significantly extend the range of distances that has been previously accessible for experimental distance dependent PCET studies of synthetic model compounds, but it also greatly improves the isolation of dO···N as a variable compared to earlier reports. Rates of PCET were determined by time-resolved optical spectroscopy with flash-quench generated [Ru(bpy)3]3+ and [Ru(dce)3]3+, where bpy = 2,2'-bipyridyl and dce = 4,4'-dicarboxyethylester-2,2'-bipyridyl. The rates increased as dO···N decreased, as can be expected from a static proton tunneling model. An exponential attenuation of the PCET rate constant was found: kPCET(d) = k0PCETexp[-ß(d - d0)], with ß âˆ¼ 10 Å-1. The observed kinetic isotope effect (KIE = kH/kD) ranged from 1.2 to 1.4, where the KIE was observed to decrease slightly with increasing dO···N. Both ß and KIE values are significantly smaller than what is predicted by a static proton tunneling model. We conclude that vibrational compression of the tunneling distances, as well as higher vibronic transitions, that contribute to concerted proton coupled electron transfer must also be considered.

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Bis-tridentate Ru(II) complexes based on the dqp scaffold (dqp is 2,6-di(quinolin-8-yl)pyridine) with multiple aryl substituents were explored to tailor the absorption and emission properties. A synthetic methodology was developed for the facile synthesis and purification of homo- and heteroleptic bis-tridentate Ru complexes. The effect of the aryl substituents in the para positions of the pyridine and quinoline subunits was detailed by X-ray crystallography, steady state and time-resolved spectroscopy, electrochemistry, and computational methods. The attachment of the aryl groups results in enhanced molar extinction coefficients with the largest effect in the pyridine position, whereas the quinoline substituent leads to red-shifted emission tailing into the NIR region (up to 800 nm). Notably, the excited state lifetimes remain in the microsecond time scale even in the presence of O2, whereas the emission quantum yields are slightly increased with respect to the parental complex [Ru(dqp)2](2+). The peripheral functional groups (Br, Me, OMe) have only a minor impact on the optical properties and are attractive to utilize such complexes as functional building blocks.

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The influence of H-bond geometry on the dynamics of excited state intramolecular proton transfer (ESIPT) and photoinduced tautomerization in a series of phenol-quinoline compounds is investigated. Control over the proton donor-acceptor distance (dDA ) and dihedral angle between the proton donor-acceptor subunits is achieved by introducing methylene backbone straps of increasing lengths to link the phenol and quinoline. We demonstrate that a long dDA correlates with a higher barrier for ESIPT, while a large dihedral angle opens highly efficient deactivation channels after ESIPT, preventing the formation of the fully relaxed tautomer photoproduct.

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A series of homoleptic bis(tridentate) [Ru(L)2](2+) (1, 3) and heteroleptic [Ru(L)(dqp)](2+) complexes (2, 4) [L = dqxp (1, 2) or dNinp (3, 4); dqxp = 2,6-di(quinoxalin-5-yl)pyridine, dNinp = 2,6-di(N-7-azaindol-1-yl)pyridine, dqp = 2,6-di(quinolin-8-yl)pyridine] was prepared and in the case of 2 and 4 structurally characterized. The presence of dqxp and dNinp in 1-4 result in anodically shifted oxidation potentials of the Ru(3+/2+) couple compared to that of the archetypical [Ru(dqp)2](2+) (5), most pronounced for [Ru(dqxp)2](2+) (1) with a shift of +470 mV. These experimental findings are corroborated by DFT calculations, which show contributions to the complexes' HOMOs by the polypyridine ligands, thereby stabilizing the HOMOs and impeding electron extraction. Complex 3 exhibits an unusual electronic absorption spectrum with its lowest energy maximum at 382 nm. TD-DFT calculations suggest that this high-energy transition is caused by a localization of the LUMO on the central pyridine fragments of the dNinp ligands in 3, leaving the lateral azaindole units merely spectator fragments. The opposite is the case in 1, where the LUMO experiences large stabilization by the lateral quinoxalines. Owing to the differences in LUMO energies, the complexes' reduction potentials differ by about 900 mV [E(1/2)(1(2+/1+)) = -1.17 V, E(c,p)(3(2+/1+)) = -2.06 V vs Fc(+/0)]. As complexes 1-4 exhibit similar excited state energies of around 1.80 V, the variations of the lateral heterocycles allow the tuning of the complexes' excited state oxidation strengths over a range of 900 mV. Complex 1 is the strongest excited state oxidant of the series, exceeding even [Ru(bpy)3](2+) by more than 200 mV. At room temperature, complex 3 is nonemissive, whereas complexes 1, 2, and 4 exhibit excited state lifetimes of 255, 120, and 1570 ns, respectively. The excited state lifetimes are thus somewhat shortened compared to that of 5 (3000 ns) but still acceptable to qualify the complexes as photosensitizers in light-induced charge-transfer schemes, especially for those that require high oxidative power.

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En esta comunicación se reporta la síntesis y caracterización de nuevos compuestos de rutenio(II) con ligantes ferrocenílicos y/o fosfínicos de fórmula general: [RuCl2(PP)(NN)], donde PP = 1,1'-bis (difenilfosfina)ferroceno (dppf) ó 1,2-bis (difenilfosfina)etano (dppe); NN = 3,3'- dicarboxi-2,2'-bipiridina (3,3'-dcbpy) ó 2,2'-bipiridina (bpy). El estudio teórico DFT de estos compuestos permitió racionalizar algunos resultados experimentales y a la vez dio indicios teóricos de que compuestos de Ru(II) con ligantes ferrocenílicos tienen ciertas propiedades para utilizarlos como fotosensibilizadores en celdas solares sensibilizadas mediante colorantes (CSSC). La caracterización de los ligantes y de los complejos de rutenio( II) se realizó por 1 H-RMN y 31P - RMN, UV-Vis, IR, voltametría cíclica y diferencial de pulso.

Novel ruthenium(II) complexes with ferrocenylic and/or phosphinic ligands of the type [RuCl2(PP)(NN)], with PP = 1,1'- bis(diphenylphosphino)ferrocene (dppf) or 1,2-dipheylphospinoethane (dppe) and NN = 3,3'-dicarboxyl- 2,2'-bipyridine (3,3'- dcbpy) or 2,2'-bipyridine (bpy) were synthesized and characterized. DFT studies of these compounds allowed to explain some experimental aspects, leading to a theoretical design of modified Ru(II) ferrocenylic complexes in order to be used as a dye for Photosensitized Solar Cells. The ligands and the complexes were characterized by 1H y 31P - NMR, UV-Vis, IR and Cyclic and differential pulse voltammetries.

Nesta comunicação reporta-se a síntese e caracterização de novos compostos de rutênio(II) com ligantes ferrocenílicos e/ou fosfínicos de fórmula geral: [RuCl2 (PP)(NN)], onde PP = 1,1'-bis (difenilfosfina) ferroceno (dppf) ou 1,2-bis(difenilfosfina) etano (dppe); NN = 3,3'-dicarboxi- 2,2'-bipiridina (3,3'-dcbpy) ou 2,2'-bipiridina (bpy). O estudo teórico DFT destes compostos permitia racionalizar alguns resultados experimentais e, ao mesmo tempo, deu indicações teóricas de que compostos de Ru(II) com ligantes ferrocenílicos têm umas propriedades para utilizar-se como fotosensivilizadores em Células Solares Sensibilizadas diante Colorantes (CSSC). A caracterização dos ligantes e dos complexos de rutênio(II) foi por 1H-RMN e 31P-RMN, UV-Vis, voltametria cíclica e diferencial de pulso.