One-electron oxidation of a hydrogen-bonded phenol occurs by concerted proton-coupled electron transfer

The hydrogen-bonded phenol 2-(aminodiphenylmethyl)-4,6-di-tert-butylphenol (HOAr-NH2) was prepared and oxidized in MeCN by a series of one-electron oxidants. The product is the phenoxyl radical in which the phenolic proton has transferred to the amine, *OAr-NH3+. The reaction of HOAr-NH2 and tris(p-tolyl)aminium ([N(tol)3]*+) to give *OAr-NH3+ + N(tol)3 has Keq = 2.0 +/- 0.5, follows second-order kinetics with k = (1.1 +/- 0.2) x 105 M-1 s-1 (DeltaG = 11 kcal mol-1), and has a primary isotope effect kH/kD = 2.4 +/- 0.4. Oxidation of HOAr-NH2 with [N(C6H4Br)3]*+ is faster, with k congruent with 4 x 107 M-1 s-1. The isotope effect, thermochemical arguments, and the dependence of the rate on driving force (DeltaDeltaG/DeltaDeltaG degrees = 0.53) all indicate that electron transfer from HOAr-NH2 must occur concerted with intramolecular proton transfer from the phenol to the amine (proton-coupled electron transfer, PCET). The data rule out stepwise paths that involve initial electron transfer to form the phenol radical cation *+HOAr-NH2 or that involve initial proton transfer to give the zwitterion -OAr-NH3+. The dependence of the electron-transfer rate constants on driving force can be fit with the adiabatic Marcus equation, yielding a large intrinsic barrier: lambda = 34 kcal mol-1 for reactions of HOAr-NH2 with NAr3*+.     

Calculation of vibronic couplings for phenoxyl/phenol and benzyl/toluene self-exchange reactions: implications for proton-coupled electron transfer mechanisms

The vibronic couplings for the phenoxyl/phenol and the benzyl/toluene self-exchange reactions are calculated with a semiclassical approach, in which all electrons and the transferring hydrogen nucleus are treated quantum mechanically. In this formulation, the vibronic coupling is the Hamiltonian matrix element between the reactant and product mixed electronic-proton vibrational wavefunctions. The magnitude of the vibronic coupling and its dependence on the proton donor-acceptor distance can significantly impact the rates and kinetic isotope effects, as well as the temperature dependences, of proton-coupled electron transfer reactions. Both of these self-exchange reactions are vibronically nonadiabatic with respect to a solvent environment at room temperature, but the proton tunneling is electronically nonadiabatic for the phenoxyl/phenol reaction and electronically adiabatic for the benzyl/toluene reaction. For the phenoxyl/phenol system, the electrons are unable to rearrange fast enough to follow the proton motion on the electronically adiabatic ground state, and the excited electronic state is involved in the reaction. For the benzyl/toluene system, the electrons can respond virtually instantaneously to the proton motion, and the proton moves on the electronically adiabatic ground state. For both systems, the vibronic coupling decreases exponentially with the proton donor-acceptor distance for the range of distances studied. When the transferring hydrogen is replaced with deuterium, the magnitude of the vibronic coupling decreases and the exponential decay with distance becomes faster. Previous studies designated the phenoxyl/phenol reaction as proton-coupled electron transfer and the benzyl/toluene reaction as hydrogen atom transfer. In addition to providing insights into the fundamental physical differences between these two types of reactions, the present analysis provides a new diagnostic for differentiating between the conventionally defined hydrogen atom transfer and proton-coupled electron transfer reactions.     

Energetics of direct and water-mediated proton-coupled electron transfer

Proton-coupled electron transfer (PCET) is an elementary chemical reaction crucial for biological oxidoreduction. We perform quantum chemical calculations to study the direct and water-mediated PCET between two stacked tyrosines, TyrO(?) + TyrOH → TyrOH + TyrO(?), to mimic a key step in the catalytic reaction of class Ia ribonucleotide reductase (RNR). The energy surfaces of electronic ground and excited states are separated by a large gap of ~20 kcal mol(-1), indicative of an electronically adiabatic transfer mechanism. In response to chemical substitutions of the proton donor, the energy of the transition state for direct PCET shifts by exactly half of the change in energetic driving force, resulting in a linear free energy relation with a Br?nsted slope of ?. In contrast, for water-mediated PCET, we observe integer Br?nsted slopes of 1 and 0 for proton acceptor and donor modifications, respectively. Our calculations suggest that the π-stacking of the tyrosine dimer in RNR results in strong electronic coupling and adiabatic PCET. Water participation in the PCET can be identified perturbatively in a Br?nsted analysis.     

Light harvesting in microscale metal-organic frameworks by energy migration and interfacial electron transfer quenching

Microscale metal-organic frameworks (MOFs) were synthesized from photoactive Ru(II)-bpy building blocks with strong visible light absorption and long-lived triplet metal-to-ligand charge transfer ((3)MLCT) excited states. These MOFs underwent efficient luminescence quenching in the presence of either oxidative or reductive quenchers. Up to 98% emission quenching was achieved with either an oxidative quencher (1,4-benzoquinone) or a reductive quencher (N,N,N',N'-tetramethylbenzidine), as a result of rapid energy migration over several hundred nanometers followed by efficient electron transfer quenching at the MOF/solution interface. The photoactive MOFs act as an excellent light-harvesting system by combining intraframework energy migration and interfacial electron transfer quenching.     

Visible-light-induced synthesis of phosphorylated N-heterocycles through proton-coupled electron transfer

正Phosphorus-containing organic compounds are important feedstock for the synthesis of value-added bioactive molecules. Therefore, the development of highly efficient synthetic methods for the construction of phosphorus-element bonds has drawn huge attention in the past decades [1].Particularly, the formation of P–C bonds from phosphoruscentered radicals has been demonstrated to be one of the most efficient and convenient strategies, which has been widely applied for the synthesis of organic phosphorus compounds in recent years.     

Theoretical analysis of proton relays in electrochemical proton-coupled electron transfer

The coupling of long-range electron transfer to proton transport over multiple sites plays a vital role in many biological and chemical processes. Recently the concerted proton-coupled electron transfer (PCET) reaction in a molecule with a hydrogen-bond relay inserted between the proton donor and acceptor sites was studied electrochemically. The standard rate constants and kinetic isotope effects (KIEs) were measured experimentally for this double proton transfer system and a related single proton transfer system. In the present paper, these systems are studied theoretically using vibronically nonadiabatic rate constant expressions for electrochemical PCET. Application of this approach to proton relays requires the calculation of multidimensional proton vibrational wave functions and the incorporation of multiple proton donor-acceptor motions. The decrease in proton donor-acceptor distances due to thermal fluctuations and the contributions from excited electron-proton vibronic states play important roles in these systems. The calculated KIEs and the ratio of the standard rate constants for the single and double proton transfer systems are in agreement with the experimental data. The calculations indicate that the standard PCET rate constant is lower for the double proton transfer system because of the smaller overlap integral between the ground state reduced and oxidized proton vibrational wave functions, resulting in greater contributions from excited electron-proton vibronic states with higher free energy barriers. The theory predicts that this rate constant may be increased by modifying the molecule in a manner that decreases the equilibrium proton donor-acceptor distances or alters the molecular thermal motions to facilitate the concurrent decrease of these distances. These insights may guide the design of more efficient catalysts for energy conversion devices.     

Theoretical and mechanistic aspects of proton-coupled electron transfer in electrochemistry

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Excited-state double proton transfer of 7-azaindole in water nanopools

The excited-state proton-transfer dynamics of 7-azaindole occurring in the water nanopools of reverse micelles has been investigated by measuring time-resolved fluorescence spectra and kinetics, as well as static absorption and emission spectra, with varying water content and isotope. 7-Azaindole molecules are found to exist in the bound-water regions of reverse micelles. The rate constant and the kinetic isotope effect of proton transfer are smaller than those in bulk water although both increase with the size of the water nanopool. The retardation of proton transfer in the bound regions is attributed to the increased free energy of prerequisite solvation to form a cyclically H-bonded 1:1 7-azaindole/water complex.     

Photoreaction in BLUF receptors: proton-coupled electron transfer in the flavin-Gln-Tyr system

Photoinduced electron transfer from tyrosine to the flavin chromophore is involved in activation of BLUF (sensor of blue light using FAD) photoreceptors. We studied the electron transfer (ET) coupled with proton-transfer (PT) reactions, by means of XMCQDPT2//CASSCF calculations on a molecular cluster model. By defining a minimum active space in the CASSCF calculations, we could compute the entire photoreaction pathway. We find that the crossing of the locally excited and ET states is located along the flavin bond-stretching coordinate. The ET state is stabilized by a proton transfer from the electron donor to the electron acceptor. We mapped two different PT pathways from tyrosine to flavin via the conserved glutamine. These reactions generate a tautomeric form of glutamine. Along the PT coordinates, we find geometries where the ET and the electronic ground states degenerate. At the state crossing structures, either formation of the ground state biradical intermediate or a relaxation back to the Franck-Condon minimum takes places. The computed relaxation pathways reveal that the hydrogen bonds involving glutamine in the chromophore-binding pocket control BLUF photoefficiency.     

Excited state electron transfer precedes proton transfer following irradiation of the hydrogen-bonded single water complex of 7-azaindole with UV light

High resolution electronic spectra of the single water complex of 7-azaindole (7AIW) and of a deuterated analog (7AIW-d(3)) have been recorded in a molecular beam, both in the absence and presence of an applied electric field. The obtained data include the rotational constants of both complexes in their ground (S(0)) and first excited (S(1)) electronic states, their S(1)-S(0) electronic transition moment and axis-tilting angles, and their permanent electric dipole moments (EDM's) in both electronic states. Analyses of these data show that the water molecule forms two hydrogen bonds with 7AI, a donor O-H···N(7) bond and an acceptor O···H-N(1) bond. The resulting structure has a small EDM in the S(0) state (μ = 0.54 D) but a greatly enhanced EDM in the S(1) state (μ = 3.97 D). We deduce from the EDM's of the component parts that 0.281 e(-) of charge is transferred from the acidic N(1)-H site to the basic N(7) site upon UV excitation of 7AIW, but that water-assisted proton transfer from N(1) to N(7) does not occur. A model of the resulting electrostatic interactions in the solute-solvent pair predicts a solvent-induced red-shift of 1260 cm(-1) which compares favorably to the experimental value of 1290 cm(-1).     

Electrochemical and homogeneous proton-coupled electron transfers: concerted pathways in the one-electron oxidation of a phenol coupled with an intramolecular amine-driven proton transfer

Proton-coupled electron transfers currently attract considerable attention in view of their likely involvement in many natural processes. Electrochemistry, through techniques such as cyclic voltammetry, is an efficient way of investigating the reaction mechanism of these reactions, and deciding whether proton and electron transfers are concerted or occur in a stepwise manner. The oxidation of an ortho-substituted 4,6-di (tert-butyl)-phenol in which the phenolic hydrogen atom is transferred during the reaction to the nitrogen atom of a nearby amine is taken as illustrative example. A careful analysis of the cyclic voltammetric responses obtained with this compound and its OD derivative allows, after estimation of the various thermodynamic parameters, ruling out the occurrence of the square scheme mechanism involving the proton-electron and electron-proton sequences. Simulation and comparison of the rate constant and H/D kinetic isotope effect with theoretical predictions show that the experimental value of the preexponential factor is ca. 1 order of magnitude larger than the theoretical value. Detailed calculations suggest that an electric field effect is responsible for this discrepancy.     

A unified perspective on the hydrogen atom transfer and proton-coupled electron transfer mechanisms in terms of topographic features of the ground and excited potential energy surfaces as exemplified by the reaction between phenol and radicals

The relation between the hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET) mechanisms is discussed and is illustrated by multiconfigurational electronic structure calculations on the ArOH + R(*) --> ArO(*) + RH reactions. The key topographic features of the Born-Oppenheimer potential energy surfaces that determine the predominant reaction mechanism are the conical intersection seam of the two lowest states and reaction saddle points located on the shoulders of this seam. The saddle point corresponds to a crossing of two interacting valence bond states corresponding to the reactant and product bonding patterns, and the conical intersection corresponds to the noninteracting intersection of the same two diabatic states. The locations of mechanistically relevant conical intersection structures and relevant saddle point structures are presented for the reactions between phenol and the N- and O-centered radicals, (*)NH2 and (*)OOCH3. Points on the conical intersection of the ground doublet D0 and first excited doublet D1 states are found to be in close geometric and energetic proximity to the reaction saddle points. In such systems, either the HAT mechanism or both the HAT mechanism and the proton-coupled electron transfer (PCET) mechanism can take place, depending on the relative energetic accessibility of the reaction saddle points and the D0/D1 conical intersection seams. The discussion shows how the two mechanisms are related and how they blend into each other along intermediate reaction paths. The recognition that the saddle point governing the HAT mechanism is on the shoulder of the conical intersection governing the PCET mechanism is used to provide a unified view of the competition between the two mechanisms (and the blending of the two mechanisms) in terms of the prominent and connected features of the potential energy surface, namely the saddle point and the conical intersection. The character of the dual mechanism may be understood in terms of the dominant valence bond configurations of the intersecting states, which are zero-order approximations to the diabatic states.     

Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols

The kinetics and mechanism of proton-coupled electron transfer (PCET) from a series of phenols to a laser flash generated [Ru(bpy)(3)](3+) oxidant in aqueous solution was investigated. The reaction followed a concerted electron-proton transfer mechanism (CEP), both for the substituted phenols with an intramolecular hydrogen bond to a carboxylate group and for those where the proton was directly transferred to water. Without internal hydrogen bonds the concerted mechanism gave a characteristic pH-dependent rate for the phenol form that followed a Marcus free energy dependence, first reported for an intramolecular PCET in Sj?din, M. et al. J. Am. Chem. Soc. 2000, 122, 3932-3962 and now demonstrated also for a bimolecular oxidation of unsubstituted phenol. With internal hydrogen bonds instead, the rate was no longer pH-dependent, because the proton was transferred to the carboxylate base. The results suggest that while a concerted reaction has a relatively high reorganization energy (lambda), this may be significantly reduced by the hydrogen bonds, allowing for a lower barrier reaction path. It is further suggested that this is a general mechanism by which proton-coupled electron transfer in radical enzymes and model complexes may be promoted by hydrogen bonding. This is different from, and possibly in addition to, the generally suggested effect of hydrogen bonds on PCET in enhancing the proton vibrational wave function overlap between the reactant and donor states. In addition we demonstrate how the mechanism for phenol oxidation changes from a stepwise electron transfer-proton transfer with a stronger oxidant to a CEP with a weaker oxidant, for the same series of phenols. The hydrogen bonded CEP reaction may thus allow for a low energy barrier path that can operate efficiently at low driving forces, which is ideal for PCET reactions in biological systems.     

Stereoselective fluorescence quenching by photoinduced electron transfer in naphthalene-amine dyads

Intramolecular chiral recognition in electron-transfer-induced fluorescence quenching has been observed for diastereomeric dyads composed of a naphthalene chromophore and an amine.     

Enhanced electron transfer by dendritic architecture: energy transfer and electron transfer in snowflake-shaped Zn porphyrin dendrimers

[structure: see text] Photoinduced electron transfer was observed for the snowflake-shaped dendrimer with the Zn porphyrin core and anthraquinonyl terminals. Comparison of the electron-transfer efficiency of the dendrimer with the linear analogues indicates the advantage of the dendritic structure for the electron-transfer process.     

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy)₃³⁺-derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (ΔG⁰_ET and ΔG⁰_PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG⁰_ET and ΔG⁰_PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron–proton transfer (CEPT) to an electron transfer limited (ETPT_lim) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPT_lim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis.     

Fluorescence quenching of alternant and non-alternant polycyclic hydrocarbons by electron transfer

Rate constants of fluorescence quenching by electron acceptors are greater for alternant than for non-alternant hydrocarbons with equal E_oo. The reverse is true for quenching by electron donors. This is consistent with the lowering of the π-orbital energies of non-alternant compared to alternant polycyclic hydrocarbons of equal E_oo.