Homogeneous Electron Transfer Reactions Research Articles

Comparison of Au Nanoparticle/Poly(9-vinylcarbazole) Thin-Film Electrogeneration at 3 Distinct Liquid/Liquid Interfaces: Water/1,2-Dichloroethane, /α,α,α-Trifluorotoluene, Or/Ionic Liquid.

Metal nanoparticle (NP) incorporated conductive polymer films are attractive for their mechanical stability for biomedical applications and as heterogeneous electrocatalysis materials. Novel approaches to generate these materials with tunable properties are still being sought. Herein, the interface between two immiscible electrolyte solutions (ITIES) has been employed as a molecularly sharp and reproducible platform for simultaneous Au NP and poly(9-vinylcarbazole) generation. Three interfaces have been compared, including between water|1,2-dichloroethane (w|DCE), water|α,α,α-trifluorotoluene (w|TFT), and water|ionic liquid (w|IL). In this case the IL was P8888TB (tetraoctylphosphonium tetrakis(pentafluorophenyl)borate). 9-Vinylcarbazole (VC) can polymerize via two routes, either propagating through the vinyl substituent or the aryl rings. The former gives rise to a white semiconducting polymer with a wide bandgap, while the latter produces a green, conducting polymer. External potential control through voltammetric cycling was found to generate the film more rapidly favoring heterogeneous electron transfer with formation of the green poly(VC) variant at the ITIES. This was a free-standing film that could be easily removed from the interface. In the absence of external control, white polymer crystals formed within the oil phase spontaneously likely via AuCl4- w → o transfer followed by a homogeneous electron transfer reaction mechanism. Scanning electrochemical microscopy probe approach curve experiments were used to quantify the electroactivity of the film and are complemented by direct conductivity measurements.

Intense annihilation electrogenerated chemiluminescence from tris(2-phenylpyridinato)iridium(III) with organic radical cations and anions

The electrogenerated chemiluminescence (ECL) of 0.2 mM tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3) in acetonitrile with a potential step method was dramatically enhanced by the electrochemically generated radical cation of 10-methylphenothiazine (MePT), thianthrene (TH), and dibenzo-1,4-dioxin (DD) or the radical anion of benzophenone (BP) and benzonitrile (BN). In the case of using the radical cation of phenothiazine (PT), no ECL was seen. Among the binary systems containing 0.2 mM Ir(ppy)3 and 10 mM parent molecule of the radical ions, intense annihilation ECL of Ir(ppy)3 was observed from a solution containing BN; the ECL intensity was about 250 times greater than that of the single system of 0.2 mM Ir(ppy)3. Among the cases using the radical cations, the ECL intensity with TH was the largest (about 90 times higher than that of the single system). The mechanism for enhancing the ECL is discussed based on the redox potential, energy level of the lowest triplet excited (T1) state, and electron-donating and −accepting characteristics. The electron transfer reaction between the reduced form of Ir(ppy)3 (Ir(ppy)3•−) and the radical cations can produce the T1 state of Ir(ppy)3 (3Ir(ppy)3*) and that of MePT, TH, and DD, whereas the electron transfer reaction between the oxidized form of Ir(ppy)3 (Ir(ppy)3•+) and the radical anions can produce 3Ir(ppy)3* but not the T1 state of BN or BP. The lower ECL intensity using the radical cations relative to that of using BN suggests that the triplet energy transfer from the parent molecule of the radical cations to Ir(ppy)3 is not efficient. It is also shown that the homogeneous electron transfer reaction between Ir(ppy)3 and the electrochemically generated radical cation of TH and radical anion of BN to produce Ir(ppy)3•+ and Ir(ppy)3•−, respectively, which facilitates an ion annihilation reaction between Ir(ppy)3•− and Ir(ppy)3•+, is also responsible for the intense ECL.

Homogeneous electron-transfer reaction between anionic species of anthraquinone derivatives and molecular oxygen in acetonitrile solutions: Electrochemical properties of disperse red 60

In this work, the voltammetric behaviour of disperse red 60 (DR60) in acetonitrile solutions in the presence of water and dioxygen (O2) is reported for the first time. DR60 can be reduced in two one-electron steps to form first the anion radical and then the dianion. The first reduction process of DR60 is affected by the water content of the acetonitrile, with digital simulations of the cyclic voltammetric waves supporting interactions between the radical anion and water molecules. Additionally, the presence of O2 modifies the first voltammetric process of DR60, suggesting the occurrence of homogeneous electron transfer reaction between anion radicals of DR60 and O2. Furthermore, the water content of the acetonitrile also affects the reduction wave of O2, with digital simulations supporting the occurrence of two association steps with the superoxide intermediate. The homogeneous electron transfer reaction between the quinone and oxygen species was also examined by reacting bulk electrolyzed solutions containing the anions of DR60 and O2 with neutral species of O2 and DR60, respectively, and recording the variation in the colour of the electrolyzed solutions. An equilibrium constant value was obtained from the difference between the formal reduction potentials of DR60 and O2 (−0.9955 and −0.9770, respectively, both vs. Ag in CH3CN with 0.5 M n-Bu4NPF6), indicating the direction of the homogenous electron transfer reaction (Keq = 2.013). Similarly, to confirm the occurrence of the homogeneous electron transfer reaction between O2 and another quinone based molecule, cyclic voltammetry and bulk electrolysis experiments were carried out on 9,10-anthraquinone (AQ), and its equilibrium constant value was also obtained (Keq = 4.392).

Behavior of Ionic Liquids Around Charged Metal Complexes: Investigation of Homogeneous Electron Transfer Reactions Between Metal Complexes in Ionic Liquids

The second-order electron transfer reaction between the photo-excited triplet state of [Zn(TPP)]* (TPP = 5,10,15,20-tetraphenylporphyrin) and [Co(sep)]3+ (sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane) was investigated in three ionic liquids (ILs, 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide with R = butyl, pentyl, and hexyl) and in acetonitrile. Results of electrochemical and kinetic measurements indicated that ILs dissociate in the vicinity of charged metal complexes and at electrodes, although the dissociated anionic and cationic components of the ILs seem to exist as pairs around the metal complexes. Second-order rate constants for the electron transfer reaction are 1.88 × 109, 3.65 × 107, 2.63 × 107, and 2.01 × 107 kg·mol−1·s−1 in acetonitrile and in the butyl, pentyl and hexyl ILs, respectively, at 298 K, after correction of the contribution of diffusion. The average slope of the plot of the logarithmic second-order rate constants observed in acetonitrile and ILs against the logarithmic viscosity of each solvent was − 0.84. However, the slope of the same plot was much steeper (− 4.1) when data for only the three ILs were used. Detailed analyses of the experimental results on the basis of the Latner–Levin cross relation and the Marcus theory lead to the conclusion that the solvent properties such as the dielectric constant and refractive index around the polarized/charged transition states are different from those for the bulk ILs: observed self-exchange rate constants did not exhibit the Pekar factor dependence when dielectric constants and refractive indices for bulk ILs are used.

Homogeneous Electron Transfer Reactions of Electrochemically Generated Species in Electrogenerated Chemiluminescence

Homogeneous Electron Transfer Reactions of Electrochemically Generated Species in Electrogenerated Chemiluminescence

Heterogeneous versus homogeneous electron transfer reactions at liquid–liquid interfaces: The wrong question?

The exact mechanism of the electron transfer reactions at liquid–liquid interfaces still remains a source of interrogation. The purpose of this paper is to revisit this topic using a finite element simulation approach to analyze cyclic voltammograms for some previously published systems. Also, we compare the voltammograms obtained in the absence or presence of an adsorbed gold nanoparticle film. The current results indicate that the electron transfer between ferrocene in the organic phase and hexacyanoferrate(III) in the aqueous phase takes place by a potential independent homogeneous reaction in the aqueous phase, while the observed potential dependence stems from that of the concomitant ion transfer reaction of ferrocenium. In the presence of the interfacial gold nanofilm the electron transfer takes place by a bipolar mechanism where the electrons are shuttled through the metallic nanofilm.

Computational Analysis of Solvent Effects on Electron Transfer in Organic Radical Battery Cathode Materials

Organic polymers and molecules containing stable radical groups are currently of interest in a variety of electronic and energy storage applications. However, in many of these systems very little is understood about the fundamental mechanisms by which charge is transported. This includes electron transport, ionic migration as well as molecular rearrangements and solvent reorganization. This work focuses on utilizing modeling, specifically large scale molecular dynamics simulations and electronic structure calculations to understand charge transport at a molecular level for the prototypical nitroxide radical containing polymer poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA). Transport in PTMA films occurs homogeneously between the radical-containing groups, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). Considering the Marcus formalism, that the rate of transfer depends on several properties of the molecular system: the energy of an electron at each TEMPO site; the electronic coupling between pairs of TEMPO sites; and the molecular reorganization energy associated with the homogeneous electron transfer reaction. Furthermore we performed simulations to evaluate the electron transfer process in a pure PTMA film and a PTMA film swelled by acetonitrile, similar to what would happen in solution. This was accomplished by quantum mechanical calculations to find the site energy of each TEMPO group in the film and then compute the electronic density of states and the related distribution of charge-transfer reaction enthalpies. In addition, pairs of TEMPO groups were extracted and the inter- and intra-chain electronic coupling was also calculated. Utilizing this data, we found that based on the inter-TEMPO coupling values and the distances between sites the electronic-coupling-weighted radial distribution function to predict the effective electron transfer lengths as a function of solvent concentration. Finally, the implications of these results for how transport occurs within other organic radical polymer materials will be discussed in greater detail.

Electron-transfer studies of a peroxide dianion.

A peroxide dianion (O2(2-)) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)⊂mBDCA-5t-H6](2-), stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)⊂mBDCA-5t-H6](2-) with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O2(2-) occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O2(2-) within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O2)⊂mBDCA-5t-H6](2-) as a chemical reagent.

Modeling of Magnetic Enhancement of Homogeneous Electron Transfer Reactions

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Homogeneous Electron-Transfer Reaction between Electrochemically Generated Ferrocenium Ions and Amine-Containing Compounds

The homogeneous catalytic oxidation of propylamine (PrA), diethylamine (DEA), pyrrolidine (Pyr), and triethylamine (TEA) has been investigated for the first time in the presence of electrochemically generated ferrocenium ions as the catalyst. Mechanistic details for this electrocatalytic process have been investigated by cyclic voltammetry and other electrochemical techniques. A one-electron oxidative process was observed for all amines. Deviation from this mechanism was only observed during controlled-potential bulk electrolysis of ferrocene in the presence of high concentrations of propylamine, where problems with electrode fouling and catalyst deactivation processes were encountered. The catalytic efficiency and the catalytic oxidation rate constants were estimated and found to follow the order TEA > Pyr > DEA > PrA. Interestingly, the catalytic reaction was not observed when ferrocene was replaced by decamethylferrocene. This observation was analyzed in terms of thermodynamic and structural effects, a...

The Hydrogen-Bonded Dianion of Vitamin K1 Produced in Aqueous–Organic Solutions Exists in Equilibrium with Its Hydrogen-Bonded Semiquinone Anion Radical

When the quinone, vitamin K1 (VK1), is electrochemically reduced in aqueous-acetonitrile solutions (CH3CN with 7.22 M H2O), it undergoes a two-electron reduction to form the dianion that is hydrogen-bonded with water [VK1(H2O)y(2–)]. EPR and voltammetry experiments have shown that the persistent existence of the semiquinone anion radical (also hydrogen-bonded with water) [VK1(H2O)x(–•)] in aqueous or organic–aqueous solutions is a result of VK1(H2O)y(2–) undergoing a net homogeneous electron transfer reaction (comproportionation) with VK1, and not via direct one-electron reduction of VK1. When 1 mM solutions of VK1 were electrochemically reduced by two electrons in aqueous-acetonitrile solutions, quantitative EPR experiments indicated that the amount of VK1(H2O)x(–•) produced was up to approximately 35% of all the reduced species. In situ electrochemical ATR-FTIR experiments on sequentially one- and two-electron bulk reduced solutions of VK1 (showing strong absorbances at 1664, 1598, and 1298 cm(–1)) in CH3CN containing <0.05 M H2O led to the detection of VK1(–•) with strong absorbances at 1710, 1703, 1593, 1559, 1492, and 1466 cm(–1) and VK1(H2O)y(2–) with strong absorbances at 1372 and 1342 cm(–1).

Evaluation of the Chemical Reactions from Two Electrogenerated Species in Picoliter Volumes by Scanning Electrochemical Microscopy

The volume created by the positioning of two scanning electrochemical microscope (SECM) probes (tip and substrate) at a micrometric distance defines a "picoliter beaker" where homogeneous electron-transfer reactions are studied. The SECM is used to concurrently electrogenerate in situ two reactive species and to evaluate the possibility of detecting their reactivity. Two reaction cases are studied: the first, called the "reversible case", occurs when the electrochemically generated species at the substrate electrode can also react at the tip to yield the same product as the reaction in the gap. The second case, named the "irreversible case", occurs when the electrochemically generated species at the substrate are not able to react at the tip. Digital simulations are performed and compared to experimental studies. These show that an unusual compensation between collection and feedback effects render the analysis inapplicable in the "reversible case". The "irreversible case" is shown experimentally.

Photoinduced homogeneous proton-coupled electron transfer: Model study of isotope effects on reaction dynamics

A model Hamiltonian for photoinduced homogeneous proton-coupled electron transfer reactions is presented, and the equations of motion for the reduced density matrix elements in an electron-proton vibronic basis are derived. This formalism enables a detailed analysis of the proton vibrational dynamics, as well as the dynamics of the electronic state populations, following photoexcitation. The application of this theory to model systems provides insight into the fundamental physical principles underlying these types of processes. The initial nonequilibrium state is prepared by vertical photoexcitation from the ground electronic state to a coherent vibrational mixture in the donor electronic state. This nonstationary state relaxes to the equilibrium distributions in the donor and acceptor electronic states via dynamical processes arising from nonadiabatic transitions between the donor and acceptor vibronic states concurrent with energy dissipation to the bath. During the initial stage, when the proton vibrational population in the donor state is distributed among higher vibrational states and the donor proton wavepacket is oscillating with large amplitude, the electronic state population dynamics exhibits virtually no hydrogen/deuterium isotope effect. After vibrational relaxation, when the proton vibrational population in the donor state becomes concentrated in the lower vibrational states and the donor proton wavepacket becomes more localized near the minimum of the donor potential, a significant hydrogen/deuterium isotope effect on the electronic state population dynamics is exhibited. These model system calculations lead to experimentally testable predictions about the qualitative behavior of these isotope effects.

Blue bottle light: lecture demonstrations of homogeneous and heterogeneous photo-induced electron transfer reactions

The classic, non-photochemical blue bottle experiment involves the reaction of methylene blue (MB) with deprotonated glucose, to form a bleached form of the dye, leuco-methylene blue (LMB), and subsequent colour recovery by shaking with air. This reaction is a popular demonstrator of key principles in kinetics and reaction mechanisms. Here it is modified so as to highlight features of homogenous and heterogeneous photoinduced electron transfer (PET) (Pure Appl. Chem., 2007, 79, 293-465) reactions, i.e.blue bottle light experiments. The homogeneous blue bottle light experiment uses methylene blue, MB, as the photo-sensitizer and triethanolamine as the sacrificial electron donor. Visible light irradiation of this system leads to its rapid bleaching, followed by the ready restoration of its original colour upon shaking away from the light source. The heterogeneous blue bottle light experiment uses titania as the photo-sensitizer, MB as a redox indicator and glucose as the sacrificial electron donor. UVA light irradiation of this system leads to the rapid bleaching of the MB and the gradual restoration of its original colour with shaking and standing. The latter 'dark' step can be made facile and more demonstrator-friendly by using platinised titania particles. These two photochemical versions of the blue bottle experiment are used to explore the factors which underpin homogeneous and heterogeneous PET reactions and provide useful demonstrations of homogeneous and heterogeneous photochemistry.

Self-Exchange Reaction of [Ni(mnt)2]1−,2− in Nonaqueous Solutions

The rate constant, k, for the homogeneous electron transfer (self-exchange) reaction between the diamagnetic bis(maleonitriledithiolato)nickel dianion, [Ni(mnt) 2] (2-), and the paramagnetic monoanion, [Ni(mnt) 2] (1-), has been determined in acetone and nitromethane (CH 3NO 2) using (13)C NMR line widths at 22 degrees C (mnt = 1,2-S 2C 2(CN) 2). The values of k (2.91 x 10 (6) M (-1) s (-1) in acetone, 5.78 x 10 (6) M (-1) s (-1) in CH 3NO 2) are faster than those for the electron transfer reactions of other Ni(III,II) couples; the structures of [Ni(mnt) 2] (1-) and [Ni(mnt) 2] (2-) allow for a favorable overlap that lowers the free energy of activation. The values of k are consistent with the predictions of Marcus theory. In addition to k, the spin-lattice relaxation time, T 1e, of [Ni(mnt) 2] (1-) is obtained from the NMR line width analysis; the values are consistent with those predicted by spin relaxation theory.

Theory of ac-voltammetric current of electron transfer reaction at liquid∣liquid interface

Theoretical equations of ac-voltammetric current or the equivalent admittance and/or impedance of the electron transfer reaction between a redox couple of A species, A RED and A OX, in organic (O) phase and another redox couple of B species, B OX and B RED, in aqueous or water (W) phase at the O∣W interface: A RED(O) + B OX(W) = A OX(O) + B RED(W) are derived when the electron transfer reaction is dc-voltammetric-reversible. The theoretical equations are discussed with reference to the mechanisms by which the electron transfer reaction may take place at the interface, that is, either by the heterogeneous electron transfer reaction mechanism at the interface or by the mechanism in which the homogeneous electron transfer reaction is associated with the ion transfer across the interface.

Voltammetric study of the interfacial electron transfer between bis(phthalocyaninato)lutetium in 1,2-dichloroethane and hexacyanoferrate in water

The electron transfer reaction between bis(phthalocyaninato)lutetium(III) ([Lu(pc) 2] 0) in 1,2-dichloroethane (1,2-DCE) and hexacyanoferrate(II/III) redox couple ([Fe(CN) 6] 4−/3−) in water (W) at the interface was studied by the use of normal pulse voltammetry and ac-voltammetry. The ac-voltammetric results have reconfirmed that the electron transfer reaction between [Lu(pc) 2] in 1,2-DCE and [Fe(CN) 6] 4−/3− in W takes place by way of the heterogeneous electron transfer at the interface, but not by the homogeneous electron transfer reaction associated with the ion transfer of redox species across the interface. The kinetic parameters of the heterogeneous electron transfer reaction at the interface were determined.

Cyclic Voltammetry of the Electron Transfer Reaction between Bis(cyclopentadienyl)iron in 1,2-Dichloroethane and Hexacyanoferrate in Water

The electron transfer (ET) reaction between bis(cyclopentadienyl)iron(II) ([Fe(II)(C(5)H(5))2]) in 1,2-dichloroethane (1,2-DCE) and hexacyanoferrate redox couple ([Fe(II/III)(CN)6](4-/3-)) in water (W) at the interface has been studied by using cyclic voltammetry. The voltammetric results can be explained well by a theoretical equation for the so-called IT-mechanism, in which a homogeneous ET reaction between [Fe(C(5)H(5))2] (partially distributed from 1,2-DCE) and [Fe(CN)6](3-) takes place in the W phase and the resultant [Fe(C(5)H(5))2]+ ion is responsible for current passage across the interface. The forward rate constant of the homogeneous ET reaction, [Fe(C(5)H(5))2] + [Fe(CN)6](3-) = [Fe(C(5)H(5))2]+ + [Fe(CN)6](4-) in W phase, k(f)(IT), was determined to be (2.9 +/- 2.2)x 10(10) M(-1) s(-1), which was in good agreement with k(f)(IT) = (3.2 +/- 2.0)x 10(10) M(-1) s(-1), which had been determined by using normal-pulse voltammetry.

The Micro-Optical Ring Electrode. 3: Transient Photocurrent Studies of Photophysical−Electrochemical and Photophysical−Chemical−Electrochemical Systems

The micro-optical ring electrode (MORE) is a photoelectrochemical device based on a ring microelectrode that uses the insulating material interior to the ring electrode as a light guide. In this paper, we describe the preparation and characterization of very thin ring MOREs with (ring inner radius)/(ring outer radius) > 0.99. Theoretically, we derive asymptotic analytical expressions for the time dependence of the diffusion-limited transient light-on photocurrent generated by two general types of photoelectrochemical systems: (a) the PE (photophysical-electrochemical) system, wherein the photoexcited species itself is directly detected on the ring; (b) the PCE (photophysical-chemical-electrochemical) system, wherein the photoexcited species undergoes a homogeneous electron transfer reaction prior to electrochemical detection. Experimentally, we establish that it is possible to use such MOREs to study the wavelength dependence of photocurrents derived from photoelectrochemically active systems, such as the Ru(bipy)3 2+/Fe3+ PCE system, demonstrating the potential utility of the MORE as a selective electroanalytical probe. We also use our expressions for the time dependence of photocurrents at the MORE to derive values for the photoelectrochemical kinetic parameters of this system, including the rate coefficient for the back reaction of photogenerated Ru(bipy)3 3+ (0.115 s(-1)) and the quantum efficiency for the primary redox products, Ru(bipy)3 3+ and Fe2+, escaping cage recombination, phi(CE) = 0.099.

Voltammetric study of the interfacial electron transfer between bis(cyclopentadienyl)iron in 1,2-dichloroethane and in nitrobenzene and hexacyanoferrate in water

Abstract The electron transfer (ET) reaction between bis(cyclopentadienyl)iron(II) ([Fe II (C 5 H 5 ) 2 ]) in 1,2-dichloroethane (1,2-DCE) and in nitrobenzene (NB) and hexacyanoferrate redox couple ([Fe II/III (CN) 6 ] 4−/3− ) in water (W) at the interface has been studied by use of normal-pulse voltammetry. The voltammetric results indicate that the ET reaction between [Fe II (C 5 H 5 ) 2 ] in O (O = 1,2-DCE or NB) and [Fe II/III (CN) 6 ] 4−/3− in W takes place not by way of the heterogeneous ET at O|W interface but by the mechanism in which a homogeneous ET reaction between [Fe II (C 5 H 5 ) 2 ] (partially distributed from O) and [Fe III (CN) 6 ] 3− takes place in W phase and the resultant [Fe III (C 5 H 5 ) 2 ] + ion is responsible for the current passage across the interface. From the limiting current, the forward rate constants of the homogeneous ET reaction: [Fe(C 5 H 5 ) 2 ] + [Fe(CN) 6 ] 3− = [Fe(C 5 H 5 ) 2 ] + + [Fe(CN) 6 ] 4− in W phase, k f IT , were determined to be (3.2 ± 2.0) × 10 10 M −1 s −1 and (2.0 ± 1.3) × 10 10 M −1 s −1 with the 1,2-DCE|W (1.5 M Li 2 SO 4 ) and NB|W (1.5 M Li 2 SO 4 ) interfaces, respectively. These values are in the order of the rate constant of diffusion-controlled bimolecular reaction in solution. Also, the formal (standard) potentials of the transfer of [Fe(C 5 H 5 ) 2 ] + ion at the interface, Δ O W ϕ Fc + 0 , were determined to be −0.015 ± 0.014 V and −0.087 ± 0.015 V with the 1,2-DCE|W (1.5 M Li 2 SO 4 ) and NB|W (1.5 M Li 2 SO 4 ) interfaces, respectively.