Coupled Electron Transfer Research Articles

Direct observation of transient species generated from protonation and deprotonation of the lowest triplet of p-nitrophenylphenol

Photo-induced proton coupled electron transfer (PCET) is essential in the biological, photosynthesis, catalysis and solar energy conversion processes. Recently, p-nitrophenylphenol (HO-Bp-NO2) has been used as a model compound to study the photo-induced PCET mechanism by using ultrafast spectroscopy. In transient absorption spectra both singlet and triplet states were observed to exhibit PCET behavior upon laser excitation of HO-Bp-NO2. When we focused on the PCET in the triplet state, a new sharp band attracted us. This band was recorded upon excitation of HO-Bp-NO2 in aprotic polar solvents, and has not been observed for p-nitrobiphenyl which is without hydroxyl substitution. In order to find out what the new band represents, acidic solutions were used as an additional proton donor considering the acidity of HO-Bp-NO2. With the help of results in strong (~10−1 mol/L) and weak (~10−4 mol/L) acidic solutions, the new band is identified as open shell singlet O-Bp-NO2H, which is generated through protonation of nitro O in 3HO-Bp-NO2 followed by deprotonation of hydroxyl. Kinetics analysis indicates that the formation of radical O-Bp-NO2 competes with O-Bp-NO2H in the way of concerted electron-proton transfer and/or proton followed electron transfers and is responsible for the low yield of O-Bp-NO2H. The results in the present work will make it clear how the 3HO-Bp-NO2 deactivates in aprotic polar solvents and provide a solid benchmark for the deeply studying the PCET mechanism in triplets of analogous aromatic nitro compounds.

A Dimeric Chromium(II) Pincer as an Electron Shuttle for N=N Bond Scission.

Reduction of the bis-pyrazolyl pyridine complex [CrL]2 with 4 KC8 , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K3 Cr(PhNNPh)3 ]. This has three κ2 PhNNPh2- ligands and K+ bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K2 CrL(PhNNPh)2 ], which has C2 symmetry except for the intimate ion pairing of two K+ ions to reduced azobenzene nitrogen atoms, and to pyrazolate and phenyl rings. The origin of the observed delivery of reducing equivalents to several, not to a single N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving a second electron, and is thus a general phenomenon. [CrL]2 alone is shown to be a two-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr2 L2 (BCC)], in which the reducing equivalents originate purely from CrII . An analogous study of the reaction of [CrL]2 with azobenzene yields [Cr2 L2 (PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr2 L2 unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K0 , have the ability to reduce the N=N bond. Further KC8 reduction of preformed Cr2 L2 (RNNR) inevitably gives products in which K+ stabilizes the charge in the increasingly electron-rich nitrogen atoms, in a phenomenon which mimics proton coupled electron transfer: K+ performs the role of H+ . A least-squares fit of the two singly reduced DFT structures shows that the only major change is a re-orientation of one of the two phenyl rings in order to avoid repulsion with potassium but to still allow interaction of that phenyl π system with K+ . This shows both the impact of K+ , being modest to nitrogen/chromium interactions, but nevertheless accommodating some π donation of phenyl to potassium. Finally, delivering increasing equivalents of KC8 leads to complete cleavage of the N=N bond, and both N bind to three CrII . The varied impacts of the K+ electrophile on NN multiple bond reduction is discussed.

Femtosecond time-resolved spectroscopic observation of long-lived charge separation in bimetallic sulfide/g-C3N4 for boosting photocatalytic H2 evolution

Copper-nickel sulfides could effectively suppress deep trapping states of active charge in carbon nitride. It also improves the efficiency of the shallow trapped electron transfer through CS bond for enhancing visible-light-driven photocatalytic hydrogen production with rates up to 752.8 μmol h−1 g−1, that is 470 times higher than that of pristine g-C3N4 (1.6 μmol h−1 g−1) so far. The kinetic coupling of electron transfer and long-lived charge separation (∼ 4896 ps) are systematically investigated by femtosecond time-resolved absorption spectroscopy (fs-TA). The TA signal of the composite is quenched by hole sacrificial agent, assigning to the effective hole extraction for high photocatalytic activity. Furthermore, a remarkable near-infrared-driven photocatalytic H2 evolution (0.32 μmol h−1 g−1, λ > 800 nm) was achieved due to the hole transfer from copper-nickel sulfide to the trap state of g-C3N4, indicating that the strong interaction between copper-nickel sulfide and g-C3N4 is favorable to charge transfer and long-lived charge separation states.

Exciton Isolation in Cross-Pentacene Architecture.

Null aggregates are an elusive, emergent class of molecular assembly categorized as spectroscopically uncoupled molecules. Orthogonally stacked chromophoric arrays are considered as a highlighted architecture for null aggregates. Herein, we unveil the null exciton character in a series of crystalline Greek cross (+)-assembly of 6,13-bisaryl-substituted pentacene derivatives. Quantum chemical computations suggest that the synergistic perpendicular orientation and significant interchromophoric separations realize negligible long-range Coulombic and short-range charge-transfer-mediated couplings in the null aggregate. The Greek cross (+)-orientation of pentacene dimers exhibits a selectively higher electron-transfer coupling with near-zero hole-transfer coupling and thereby contributes to the lowering of charge-transfer-mediated coupling even at shorter interchromophoric distances. Additional investigations on the nature of excitonic states of pentacene dimers proved that any deviation from a 90° cross-stacked orientation results in the emergence of delocalized Frenkel/mixed-Frenkel-CT character and the consequent loss of null exciton/monomer-like properties. The retention of exciton isolation even at a short-range coupling regime reassures the universality of null excitonic character in perpendicularly cross-stacked pentacene systems. The null-excitonic character was experimentally verified by the observation of similar spectral characteristics in the crystalline and monomeric solution state for 6,13-bisaryl-substituted pentacene derivatives. The partitioned influence of aryl and pentacene fragments on interchromophoric noncovalent interactions and photophysical properties, respectively, resulted in the emergence of pentacene centric Kasha's ideal null exciton, providing novel insights toward design strategies for cross-stacked chromophoric assemblies. Identifying the Greek cross (+)-stacked architecture-mediated null excitons with a charge-filtering phenomenon for the first time in the ever-versatile pentacene chromophoric systems can offer an extensive ground for the engineering of functional materials with advanced optoelectronic properties.

Proton coupled electron transfer mechanism for the design and construction of crystalline hybrid photochromic halometallates based on nonphotoactive polypyridine-derivative moieties

Photochromic materials have application in various areas such as anti-counterfeiting, display, switches, etc. Most of reported photochromic materials are supported by photoactive moieties such as diarylethenes, pyridinium, azo compounds. Compared with the broadly investigated photochromic materials driven by photoactive units, the generation of new photochromic materials based on nonphotoactive units is of paramount significance from the perspective of basic research and practical application. Based on the proton coupled electron transfer (PCET) mechanism, the introduction of conjugated triangular tripyridine-derivative unit TPT (2,4,6-tri(4-pyridyl)-1,3,5-triazine) and linear dipyridine-derivative unit BPYB (1,4-di(pyridine-4-yl)benzene) into the zincochloride system generates two hybrid halometallates with photochromic performance, [H2BPYB][ZnCl4]·H2O (1) and [H3TPT]2[Zn3Cl12]·4H2O (2). Both 1 and 2 feature isolated inorganic [ZnCl4]2− tetrahedra (as electron donors and H-bond acceptors) and protonated organic polypyridine derivatives units [H2BPYB]2+ and [H3TPT]3+ (as electron acceptors and H-bond donors). Experimental characterization, together with the structure-property relationship analysis, indicates that the resulting photocoloration and related photoresponsive properties of 1 and 2 is ascribable to the PCET mechanism.

Di-2-pyridylketone-N1-substituted thiosemicarbazone derivatives of copper(II): Biosafe antimicrobial potential and high anticancer activity against immortalized L6 rat skeletal muscle cells

The basic aim of this study pertains to developing antimicrobial or anticancer agents based on N, S-donor organic ligands bonded to metals. In the present investigation, di-2-pyridylketone-N1-substituted thiosemicarbazone (py2tscH-N1HR2, Chart 2) thio-ligands were reacted with copper(I) halides in organic solvents yielding copper(II) complexes of stoichiometry, [Cu(N,N,S-py2tsc-N1HR2)X] (X = I, R2 = H, 1; Me, 2; Et, 3; Ph, 4; X = Br, R2 = H, 5; Me, 6; Et, 7; Ph, 8; X = Cl, R2 = H, 9; Me, 10; Et, 11; Ph, 12); the formation of CuII probably occurs through a proton coupled electron transfer (PCET) process. Electron spin resonance, ultraviolet-visible spectroscopy and X-ray crystallography (2, 3, 5, 7, 11) supported a distorted square planar geometry of these complexes. Moderate to high antimicrobial activities of these complexes against methicillin resistant Staphylococcus aureus, Gram positive bacteria, Staphylococcus aureus and Gram negative bacteria, Klebsiella pneumoniae 1, Salmonella typhimurium 2 and one yeast Candida albicans were recorded. Complexes were found to be biosafe with 88–91% cellular viability. All complexes have shown high anticancer activity against the immortalized L6 rat skeletal muscle cell line with very low IC50 values.

Proton-dependent photocatalytic dehalogenation activities caused by oxygen vacancies of In2O3

Increasing number of evidences has proved that the oxygen vacancies played a key role in the photocatalytic reactions. However, traditional theories for oxygen vacancies to participate in the reactions mainly focused on its accompanying enhanced trapping capability of photo-induced electrons. In this work, by turning the amounts of oxygen vacancies in In2O3 photocatalysts, it was found that the photocatalytic dehalogenation of decabromodiphenyl ether and hexabromobenzene by In2O3 was also strongly dependent on the enhanced trapping of protons caused by the oxygen vacancies. Through a series of characterizations and measurements, the photo-induced electrons and protons were proved to be simultaneously trapped and confirmed to be determined by the concentration of oxygen vacancies. Moreover, the kinetic isotope studies investigated the proton coupled electron transfer (PCET) pathway in the photocatalytic dehalogenation reactions of In2O3. Protons transfer was demonstrated to be existed in the rate-determining step of the whole photocatalytic reaction. The enhanced concentration of oxygen vacancies resulted in the superior trapping capability of photo-induced electrons and protons, simultaneously promoted the transfer efficiency of protons and electrons, accelerated the separation of photo-reduced carriers and finally improved the photocatalytic activities of In2O3 in the PCET reactions.

Ultrathin CoTe nanoflakes electrode demonstrating low overpotential for overall water splitting

Hydrogen production by electrocatalytic water splitting is of supreme significance for revolving the energy crisis at hand, and developing highly efficient, robust and cost-effective electrocatalyst materials is a challenging task on the way. Herein, we present unique three-dimensional (3D) ultrathin cobalt telluride nanoflakes (CoTe-NF) as single electrocatalyst with exceptionally high bifunctional activity both for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The hydrothermally synthesized CoTe material exhibits nanoscale and highly porous structural morphology and characterized by various techniques. For OER, the CoTe 3D electrode demonstrates a low onset potential of 1.44 V vs RHE with Tafel slope value of 66 mV dec−1 indicating a proton coupled electron transfer (PCET) mechanism and well-balanced kinetics. On the other hand, the electrocatalyst also executes HER at low onset potential of 0.07 V vs RHE with a modest Tafel slope of 143 mV dec−1. The materials also showed excellent stability. This work may also reveal valuable insights for developing cost-effective, efficient and low overpotential single electrocatalyst executing overall water splitting using 3D architecture Co-chalcogenide precursors posing a great alternative to noble-metals derived materials.

Phenol Oxidation by a Nickel(III)-Fluoride Complex: Exploring the Influence of the Proton Accepting Ligand in PCET Oxidation.

In order to gain insight into the influence of the H+ -accepting terminal ligand in high-valent oxidant mediated proton coupled electron transfer (PCET) reactions, the reactivity of a high valent nickel-fluoride complex [NiIII (F)(L)] (2, L=N,N'-(2,6-dimethylphenyl)-2,6-pyridinecarboxamidate) with substituted phenols was explored. Analysis of kinetic data from these reactions (Evans-Polanyi, Hammett, and Marcus plots, and KIE measurements) and the formed products show that 2 reacted with electron rich phenols through a hydrogen atom transfer (HAT, or concerted PCET) mechanism and with electron poor phenols through a stepwise proton transfer/electron transfer (PT/ET) reaction mechanism. The analogous complexes [NiIII (Z)(L)] (Z=Cl, OCO2 H, O2 CCH3 , ONO2 ) reacted with all phenols through a HAT mechanism. We explore the reason for a change in mechanism with the highly basic fluoride ligand in 2. Complex 2 was also found to react one to two orders of magnitude faster than the corresponding analogous [NiIII (Z)(L)] complexes. This was ascribed to a high bond dissociation free energy value associated with H-F (135 kcal mol-1 ), which is postulated to be the product formed from PCET oxidation by 2 and is believed to be the driving force for the reaction. Our findings show that high-valent metal-fluoride complexes represent a class of highly reactive PCET oxidants.

Electrochemical response of a Ru(II) benzothiazolyl-2-pyridinecarbothioamide pincer towards carbon dioxide and transfer hydrogenation of aryl ketones in air

A ruthenium(II) complex of 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (pbcta), of the formula [Ru(pbcta)Cl2(dmf)] (1, where DMF = dimethyl formamide) was prepared from RuCl3•xH2O and pbcta in DMF at reflux under argon atmosphere. The identity of 1 was confirmed from its elemental analysis, ESI MS, and a series of spectroscopic measurements. Voltammetric measurements on 1 in DMF and DFT studies on the structure optimized in the gas phase revealed predominantly ligand based electron transfer processes under argon. In the presence of a proton source, proton coupled electron transfer to the ligand occurs. Under a carbon dioxide atmosphere, voltammetric studies revealed that 1 is inactive for CO2 reduction, and the redox responses observed in the presence of the proton source and/or CO2 are ligand based leading to reactions with the coordinated pbcta. Transfer hydrogenation (TH) of aryl ketones was efficiently carried out in 2-propanol using 1 at reflux. TH of the aryl ketone substrates proceeded in air with almost quantitative conversions at 0.2–1.0 mol% catalyst.

Exploring the binding pocket of quinone/inhibitors in mitochondrial respiratory complex I by chemical biology approaches.

ATP: adenosine triphosphate; BODIPY: boron dipyrromethene; complex I: proton-translocating NADH-quinone oxidoreductase; DIBO: dibenzocyclooctyne; EM: electron microscopy; FeS: iron-sulfur; FMN: flavin adenine mononucleotide; LDT: ligand-directed tosylate; NADH: nicotinamide adenine dinucleotide; ROS: reactive oxygen species; SMP: submitochondrial particle; TAMRA: 6-carboxy-N,N,N',N'-tetramethylrhodamine; THF: tetrahydrofuran; TMH: transmembrane helix.

Proton-Coupled Electron Transfer Catalyst: Heterogeneous Catalysis. Application to an Oxygen Evolution Catalyst

Proton coupled electron transfer (PCET) catalysts are investigated in the framework of cyclic voltammetry (CV). We analyze heterogeneous catalysts and provide a detailed kinetic analysis of the var...

On the Coupling of Electron Transfer to Proton Transfer at Electrified Interfaces.

Many electrochemical processes are governed by the transfer of protons to the surface, which can be coupled with electron transfer; this electron transfer is in general non-integer and unknown a priori, but is required to hold the potential constant. In this study, we employ a combination of surface spectroscopic techniques and grand-canonical electronic-structure calculations in order to rigorously understand the thermodynamics of this process. Specifically, we explore the protonation/deprotonation of 4-mercaptobenzoic acid as a function of the applied potential. Using grand-canonical electronic-structure calculations, we directly infer the coupled electron transfer, which we find to be on the order of 0.1 electron per proton; experimentally, we also access this quantity via the potential-dependence of the pKa. We show a striking agreement between the potential-dependence of the measured pKa and that calculated with electronic-structure calculations. We further employ a simple electrostatics-based model to show that this slope can equivalently be interpreted to provide information on the degree of coupled electron transfer or the potential change at the point of the charged species.

Modeling voltammetry curves for proton coupled electron transfer: The importance of nuclear quantum effects.

We investigate rates of proton-coupled electron transfer (PCET) in potential sweep experiments for a generalized Anderson-Holstein model with the inclusion of a quantized proton coordinate. To model this system, we utilize a quantum classical Liouville equation embedded inside of a classical master equation, which can be solved approximately with a recently developed algorithm combining diffusional effects and surface hopping between electronic states. We find that the addition of nuclear quantum effects through the proton coordinate can yield quantitatively (but not qualitatively) different IV curves under a potential sweep compared to electron transfer (ET). Additionally, we find that kinetic isotope effects give rise to a shift in the peak potential, but not the peak current, which would allow for quantification of whether an electrochemical ET event is proton-coupled or not. These findings suggest that it will be very difficult to completely understand coupled nuclear-electronic effects in electrochemical voltammetry experiments using only IV curves, and new experimental techniques will be needed to draw inferences about the nature of electrochemical PCET.

Uniform potential difference scheme to evaluate effective electronic couplings for superexchange electron transfer in donor-bridge-acceptor systems.

This article proposes an ab initio quantum chemical method to evaluate the effective electronic coupling that determines the rate of superexchange electron transfer in donor-bridge-acceptor (D-B-A) systems. The method utilizes the fragment charge difference to define electronic diabatic states and to apply an electrostatic potential in a form of a uniform potential difference that mimics solvation effects on the relative energies of the electronic states. The two-state generalized Mulliken-Hush method is used to obtain the effective electronic coupling as the nondiagonal element of the effective Hamiltonian that is derived based on the Green's function approach and the quasi-degenerate perturbation theory. A theoretical basis is provided for the dependence of the calculated effective electronic coupling on the applied potential and for how to find the optimal potential to give the desired effective electronic coupling that coincides with the result of the minimum energy splitting method. The method is applied to typical D-B-A molecules and gives the effective electronic couplings in reasonable agreement with the experimental estimates.

An electron density based analysis to establish the electronic adiabaticity of proton coupled electron transfer reactions.

Electrons and protons are the main actors in play in proton coupled electron transfer (PCET) reactions, which are fundamental in many biological (i.e., photosynthesis and enzymatic reactions) and electrochemical processes. The mechanism, energetics and kinetics of PCET reactions are strongly controlled by the coupling between the transferred electrons and protons. Concerted PCET reactions are classified according to the electronical adiabaticity degree of the process. To discriminate among different mechanisms, we propose a new analysis based on the use of electron density based indexes. We choose, as test case, the 3-Methylphenoxyl/phenol system in two different conformations to show how the proposed analysis is a suitable tool to discriminate between the different degree of adiabaticity of PCET processes. The very low computational cost of this procedure is extremely promising to analyze and provide evidences of PCET mechanisms ruling the reactivity of many biological and catalytic systems.

Electron transfer calculations between edge sharing octahedra in hematite, goethite, and annite

A key reaction underlying the charge transport in iron containing oxides, clays, micas is the Fe2+-Fe3+ exchange reaction between edge-sharing iron octahedra. These reactions facilitate conduction in these minerals by the thermally-activated hopping of small polarons across the lattice. Depending on the mineral and local charge state the small polaron can either encase an electron or hole. The probability for conduction of small polarons depends strongly on the height and adiabicity of the reaction barrier, with larger and more diabatic barriers yielding slow conduction associated with either weak coupling or a large prerequisite rearrangement of the lattice during charge transport. To model these reactions, a first principle electron transfer (ET) method was developed to model the small polaron hopping between the edge-sharing octahedra sites in hematite (e- polaron), goethite (e- polaron), and annite (h+ polaron) bulk structures. The ET method is based on electronic structure methods (i.e., plane-wave Density Functional Theory) capable of performing calculations with periodic cells and large size systems efficiently while at the same time being accurate enough to be used in the estimation of the electron-transfer coupling matrix element, VAB, and the electron transfer transmission factor, κel. The calculations confirmed the existence of small polarons in all three minerals, and the reactions were predicted to be strongly adiabatic. It was found that transfer of a hole in the octahedral layer of annite had an adiabatic barrier of 0.311 eV, and the transfer of an extra electron in hematite and goethite had adiabatic barriers of 0.242 eV and 0.232 eV respectively. The electronic coupling parameters,VAB, were found to be 0.188 eV, 0.196 eV, and 0.102 eV respectively for hematite, goethite, and annite. While similar bonding topologies pertain, the findings reveal the importance of subtle differences in local structure.

The Role of Fe Species on NiOOH in Oxygen Evolution Reactions

The Pourbaix diagram of Ni electrodes under reaction conditions presents several metastable NiOxHy phases and Fe doping enlarges the stability area of oxyhydroxo species. For the Ni-only phase, water adsorption and intercalation can significantly lower both the surface and interface energies, and even introduce “negative surface energy”. Thus, water can exfoliate layers, leading to Fe ion adsorption on inner layers, as demonstrated by ab initio molecular dynamics. These single atoms have been carefully speciated (i.e., initially prepared as Fe2+ and Fe3+) and proton coupled electron transfer between the H2O–Fe and lattice oxygen ions has been observed in all ab initio molecular dynamics simulations, which is attributed to the Fe incorporation, since no proton coupled electron transfer occurs under free water conditions. Furthermore, 15 possible oxygen evolution reaction mechanisms near Fe ions show that the main active species corresponds to the Ni2+, which is reduced from Ni3+ via H transfer when a Fe2+ iron adsorbs nearby, and the overpotential can be significantly reduced to 0.23 V.

Metal‐Ion Coupled Electron Transfer Kinetics in Intercalation‐Based Transition Metal Oxides

AbstractElectrochemical metal‐ion intercalation systems are acknowledged to be a critical energy storage technology. The kinetics of the intercalation processes in transition‐metal based oxides determine the practical characteristics of metal‐ion batteries, such as the energy density, power, and cyclability. With the emergence of post lithium‐ion batteries, such as sodium‐ion and potassium‐ion batteries, which function predominately in nonaqueous electrolytes of special formulation and exhibit quite varied material stability with regard to their surface chemistries and reactivity with electrolytes, the practical routes for the optimization of metal‐ion battery performance become essential. Electrochemical methods offer a variety of means to quantitatively study the diffusional, charge transfer, and phase transformation rates in complex systems, which are, however, rather rarely fully adopted by the metal‐ion battery community, which slows down the progress in rationalizing the rate‐controlling factors in complex intercalation systems. Herein, several practical approaches for diagnosing the origin of the rate limitations in intercalation materials based on phenomenological models are summarized, focusing on the specifics of charge transfer, diffusion, and nucleation phenomena in redox‐active solid electrodes. It is demonstrated that information regarding rate‐determining factors can be deduced from relatively simple analysis of experimental methods including cyclic voltammetry, chronoamperometry, and impedance spectroscopy.

Mechanistic insights into the crucial roles of Glu76 residue in nickel-dependent quercetin 2,4-dioxygenase for quercetin oxidative degradation

Combined QM/MM calculations and MD simulations are utilized to investigate the detailed mechanisms of reactions catalyzed by wild-type nickel-dependent quercetin 2,4-dioxygenase (Ni-QueD) and its Glu76Asp and Glu76Gln mutants. The conserved nickel-ligating Glu76 residue in the deprotonated form is found to be essential for initiating the catalytic reaction by proton coupled electron transfer process. The generated protonated Glu76 promotes the subsequent reaction by regulating hydrogen-bonding (H-bonding) interaction with the carbonyl groups of quercetin. Investigations of Glu76Gln and Glu76Asp mutants show that mutation of Glu76 suppresses such H-bonding interaction and results in the lower catalytic activity observed experimentally. Thus, our results reveal the critical role of Glu76 residue in steering the reactivity of Ni-QueD. This work is not only useful for better understanding the mechanisms of reactions catalyzed by other metal ion-dependent QueDs, but also provides insights into how enzymes achieve specific reactions by utilizing H-bonding interaction from the metal center-ligating residues.