Shift In Reduction Potential Research Articles

Quantifying effects of second-sphere cationic groups on redox properties of dimolybdenum quadruple bonds.

A series of four dimolybdenum paddlewheel complexes supported by anionic N,N-dimethylglycinate (DMG) or zwitterionic N,N,N-trimethylglycine (TMG) ligands was synthesised to examine the effects of charged groups in the second coordination sphere on redox properties of MoMo bonds. An average shift in reduction potential of +35 mV per cationically charged group was measured, which is approximately half of what would be expected for an analogous mononuclear complex.

Mechanism of CO2 Capture and Release on Redox-Active Organic Electrodes

The mechanism of faradaic electro-swing for CO2 capture/release on a redox-active organic electrode is studied from the point of view of realizing a reversible process for CO2 separation. First, the cyclic voltammograms (CVs) of two redox-active organic monomers, anthraquinone (AQ) and 2,1,3-benzothiadiazole (BTZ), were measured under a CO2 atmosphere. The waveforms of CVs of the two redox-active organic monomers are altered under a CO2 atmosphere relative to a N2 atmosphere. There is a change in the number of redox waves from two to one for AQ and a change from reversible to irreversible waves for BTZ. To further understand the mechanisms of CO2 capture/release on redox-active organic compounds, redox-active polymer electrodes coated with polyanthraquinone (PAQ) and polybenzothiadiazole (PBTZ) were investigated using a spectroscopic analysis known as in situ attenuated total reflectance–surface-enhanced infrared absorption spectroscopy (ATR–SEIRAS) as well as density functional theory (DFT) calculations. The CVs measured with two redox-active polymer electrodes have more positive shifts of reduction potentials for PAQ and PBTZ under a CO2 atmosphere than under an N2 atmosphere, as measured versus Ag/AgNO3: −1.8 and −1.4 V → −1.4 and −0.9 V for PAQ and −2.5 V → −2.1 and −1.9 V for PBTZ. In addition, the oxidation current on the PBTZ electrode disappears only under a CO2 atmosphere. DFT calculations indicate that the positive shifts of reduction potentials in the two electrodes under CO2 conditions are due to an exergonic adsorption reaction of CO2 onto the redox-active organic compounds. To clarify the reaction behavior between CO2 and the redox-active organic electrode, an ATR–SEIRAS spectroscopic analysis was performed. Infrared peaks are observed at 2200 and 2100 cm–1 for PAQ and PBTZ electrodes, respectively, under a CO2 atmosphere, which have been confirmed by measurements under a 13CO2 atmosphere to have adsorbed CO2. The wavenumbers corresponding to the CO2 molecules adsorbed on the two electrodes are different. These findings indicate that one electron reduction and CO2 adsorption for each redox-active organic compound are occurring simultaneously. The calculated adsorption energy of CO2 for two redox-active organic electrodes indicates that the adsorption energies of two CO2 molecules for AQ and BTZ are −7.3 and −36.3 kJ/mol. The larger adsorption energy in BTZ than that in AQ is clearly related to the disappearance of the oxidation current. However, both adsorption energies indicating adsorption of CO2 onto organic units are less than the covalent bond energies of common organic compounds, indicating that such weak adsorptions are suitable for the faradaic electro-swing of CO2 capture/release on the redox-active organic units.

Effective tailoring of the MoS2 layer number on the surface of CdS nanorods for boosting hydrogen production rate.

Hydrogen fuel plays a ubiquitous role in empowering the sustainable green energy economy. As an eco-friendly production method for hydrogen, photo-assisted water splitting is accepted to be the most reliable. However, the fabrication of stable and efficient photocatalysts is challenging. To overcome this difficulty, here we present a novel and inexpensive oxidant-promoted ultrasonic-assisted liquid phase layer exfoliation technique to fabricate a CdS/H-MoS2 nano hybrid. The newly fabricated CdS/H-MoS2 shows a hydrogen evolution rate of 162.4 mmol g-1h-1, which is 16 times higher compared to that of CdS/Pt and 67 times higher compared to that of bare CdS. Theoretical results clearly demonstrate a built-in electrostatic potential in the heterostructure junction, and that a shift in water reduction potential plays a key role in the enhancement of hydrogen production rate. We believe that the proposed experimental strategies and theoretical studies will open up a new avenue to develop new photocatalysts with high hydrogen evolution efficiency.

An RNA aptamer that shifts the reduction potential of metabolic cofactors

The discovery of ribozymes has inspired exploration of RNA’s potential to serve as primordial catalysts in a hypothesized RNA world. Modern oxidoreductase enzymes employ differential binding between reduced and oxidized forms of redox cofactors to alter cofactor reduction potential and enhance the enzyme’s catalytic capabilities. The utility of differential affinity has been underexplored as a chemical strategy for RNA. Here we show an RNA aptamer that preferentially binds oxidized forms of flavin over reduced forms and markedly shifts flavin reduction potential by −40 mV, similar to shifts for oxidoreductases. Nuclear magnetic resonance structural analysis revealed π–π and donor atom–π interactions between the aptamer and flavin that cause unfavorable contacts with the electron-rich reduced form, suggesting a mechanism by which the local environment of the RNA-binding pocket drives the observed shift in cofactor reduction potential. It seems likely that primordial RNAs could have used similar strategies in RNA world metabolisms.

Resolving Halide Ion Stabilization through Kinetically Competitive Electron Transfers.

Stabilization of ions and radicals often determines reaction kinetics and thermodynamics, but experimental determination of the stabilization magnitude remains difficult, especially when the species is short-lived. Herein, a competitive kinetic approach to quantify the stabilization of a halide ion toward oxidation imparted by specific stabilizing groups relative to a solvated halide ion is reported. This approach provides the increase in the formal reduction potential, ΔE°′(Χ•/–), where X = Br and I, that results from the noncovalent interaction with stabilizing groups. The [Ir(dF-(CF3)-ppy)2(tmam)]3+ photocatalyst features a dicationic ligand tmam [4,4′-bis[(trimethylamino)methyl]-2,2′-bipyridine]2+ that is shown by 1H NMR spectroscopy to associate a single halide ion, Keq = 7 × 104 M–1 (Br–) and Keq = 1 × 104 M–1 (I–). Light excitation of the photocatalyst in halide-containing acetonitrile solutions results in competitive quenching by the stabilized halide and the more easily oxidized diffusing halide ion. Marcus theory is used to relate the rate constants to the electron-transfer driving forces for oxidation of the stabilized and unstabilized halide, the difference of which provides the increase in reduction potentials of ΔE°′(Br•/–) = 150 ± 24 meV and ΔE°′(I•/–) = 67 ± 13 meV. The data reveal that Keq is a poor indicator of these reduction potential shifts. Furthermore, the historic and widely used assumption that Coulombic interactions alone are responsible for stabilization must be reconsidered, at least for polarizable halogens.

Electrochemical Investigation and Molecular Docking Techniques on the Interaction of Acridinedione Dyes with Water-Soluble Nonfluorophoric Simple Amino Acids.

Electrochemical studies of resorcinol-based acridinedione (AD) dyes with nonfluorophoric simple amino acids, glycine, alanine, and valine, were carried out in water. AD probes are classified into photoinduced electron transfer (PET) and non-PET-based dyes, wherein the electrochemical properties and photophysical and photochemical behavior vary significantly based on the nature of substituent groups and the nature of the solute. The oxidation potential of PET dye (ADR1) to that of non-PET-based dye (ADR2) differs significantly such that the addition of amino acids results in a shift of the oxidation peak to a less positive potential and the reduction peak to a lesser negative potential. The extent of shift of oxidation and reduction potential in PET dye is more pronounced than that of non-PET dye on the addition of valine rather than glycine. The variation in the shift is attributed to the presence of an electron-donating moiety (OCH3) group in the ninth position of ADR1 dye. Consequently, the quenching of fluorescence is observed in ADR2 with non fluorophoric amino acids that are authenticated by the shift of the anodic and cathodic peaks toward a lesser positive potential. Molecular docking (MD) studies of PET and non-PET dye with amino acids portray that neither hydrophobic interactions nor electrostatic or weak interactions such as van der Waals and pi–pi interactions govern the electrochemical nature of dye on the addition of amino acids. Furthermore, the formation of a conventional hydrogen bond between dye and amino acid is established from MD studies. The existence of dye–water–amino acid competitive hydrogen-bonding interactions is presumably well-oriented throughout the aqueous phase as observed through photophysical studies which support our electrochemical investigation.

Synthesis and redox properties of heterobimetallic Re(bpyCrown-M)(CO)3Cl complexes, where M = Na+, K+, Ca2+, and Ba2+

The operational electrochemical potential of a molecular electrocatalyst is typically pinned to the reduction potential of the parent complex. The predominant strategy for minimizing the overpotential is to modify the ligand with electron withdrawing functional groups to anodically shift the reduction potential. However, these changes often result in decreased catalytic rates. Re(bpy)(CO)3Cl is a well-known electrocatalyst for selective CO2 reduction to CO. In this work, the synthesis of Re(bpyCrown-M)(CO)3Cl is described, where bpyCrown-M is a bipyridine ligand containing a cyclic ether cavity, and M is a Na+, K+, Ca2+, or Ba2+ ion encapsulated within that cavity. Addition of these cations results in anodic shifts in reduction potential, and the magnitude of the shifts correlate with overall cationic charge. However, electrolysis under catalytic conditions for CO2 reduction results in precipitation of carbonate salts, diminishing overall catalytic activity.

Promotion and Tuning of the Electrochemical Reduction of Hetero‐ and Homobimetallic Zinc Complexes**

AbstractCompounds containing multiple metals attract significant interest due to the useful redox and reactivity properties of such species. Here, the electrochemical properties of a family of macrocyclic complexes that feature a zinc(II) center paired with a secondary redox‐inactive metal cation in heterobimetallic (Na+, Ca2+, Nd3+, Y3+) motifs or homobimetallic (Zn2+) motifs have been investigated. The new complexes were prepared via a divergent strategy, isolated, and structurally characterized with single‐crystal X‐ray diffraction (XRD) analysis. XRD results show that the structures of the complexes are modulated by the identity of the incorporated secondary metal ions. Cyclic voltammetry data reveal that ligand‐centered reduction is promoted in the bimetallic complexes and that the paired metal ions synergistically influence the redox properties of the complexes. Similar to prior work from our group and others, the bimetallic complexes containing stronger Lewis acids undergo more significant reduction potential shifts; contrasting with prior work on complexes containing redox‐active metals, however, the zinc(II) complexes studied here display faster electron transfer (as judged by lower reorganization energies, λ) when incorporating di‐ or tri‐valent Lewis acids in contrast to monovalent (and more weakly acidic) sodium. The quantified trends in these data offer insights that could help distinguish metal‐ versus ligand‐centered reduction of bimetallic complexes.

Synthesis, Spectral and Electrochemical Studies of Phenothiazine Appended ‟Push-Pull” a3b Porphyrins and Their Utilization in Nonlinear Optics

Porphyrins are ubiquitous in nature [1] and playing a vital role in biological and material applications [2]. Particularly, ‘push-pull’ porphyrins are used in solar cells, nonlinear optics (NLO), toxic ion-sensing and catalysis [3]. Herein, we are reporting the facile synthesis of phenothiazine appended A3B porphyrins. The free base porphyrin, 5,10,15-tris(3-(N-ethylphenothiazinyl)-20-(4-nitrophenyl)porphyrin, H2A3B (1) and their metal Zn(II), Cu(II) and Ni(II) complexes were synthesized [4]. The phenothiazine appended A3B porphyrins exhibited red-shifted absorption and emission spectral features. We have also studied the electrochemical properties of all the synthesized porphyrins. ZnA3B (1a) porphyrin has shown cathodic shift for the first oxidation potential and marginal shift in reduction potential as compared to ZnTPP (Figure 1). The calculated ground state dipole moments of were in the range of 9.64-8.59D as compared to MTPPs (0.052-0.0013 D) and hence they are the potential candidates for NLO application. Ultrafast nonlinear optical (NLO) studies revealed strong two-photon absorption coefficients and cross-sections for these molecules. Further, femtosecond transient absorption studies demonstrated several ultrafast processes from various excited states in these push-pull porphyrin, useful for identifying the processes relevant for optical switching applications. Figure 1: Molecular structures of synthesized porphyrins and their comparative UV-visible spectra in CH2Cl2 and Cyclic voltammograms in CH2Cl2 using TBAPF6 as supporting electrolyte at 298 K. Reference Shao, S.; Rajendiran, V.; Lovell, J. F., Chem. Rev., 2017. 379, 99-120.D. R.; Summerville, D. A.; Basolo, F. Chem. Rev. 1979, 79, 139-179.(a) Prakash, K.; Manchanda, S.; Sudhakar, V.; Sharma, N.; Sankar, M.; Krishnamoorthy, K. Dyes Pigm. 2017, 147, 56-66. (b) Yadav, P.; Kumar, R.; Saxena, A.; Butcher, R. J.: Sankar, M. J. Inorg. Chem. 2017, 26, 3269-3274. (c) Kalnoor, B. S.; Bisht, P. B.; Jena, K. C.; Velkannan, V.; Bhyrappa, P. J. Phys. Chem. A. 2013, 117, 8216-8221. (d) Kumar, R.; Sudhakar, V.; Prakash, K.; Krishnamoorthy, K.; Sankar, M. ACS Appl. Energy Mater. 2018, 1, 2793–2801. (e) Rathi, P.; Ekta, Kumar, S.; Banerjee, D.; Soma V. R.; Sankar, M. Dalton Trans. 2020, 49, 3198-3208.(a) Routhmund et al., J. Am. Chem. Soc, 1936, 58, 625-626. (b) Adler et al., Org. Chem., 1967, 34, 476-479. Figure 1

Electrochemistry of uranium in molten LiCl-LiF

This work is focused on the electrochemical behaviour of uranium in molten eutectic LiCl–LiF (70–30 mol%) in the 550–650 °C temperature range. On tungsten electrode, U(III) ions were reduced in one step to U metal exchanging 3 electrons and oxidized also in one step to U(IV) exchanging one electron. Both systems were studied by cyclic and square wave voltammetries and chronopotentiometry. The reduction and oxidation mechanisms of U(III) ions were found to be diffusion controlled processes. The diffusion coefficient of U(III) was measured at different temperatures, and it followed an Arrhenius “type law”. Apparent standard potentials were measured in chloride-fluoride (LiCl-LiF eutectic) and pure chloride media (LiC-KCl eutectic). The addition of fluoride ions into a chloride salt leads to the formation of more stable complexes reduced at more negative potentials. This shift of reduction potentials is more pronounced for the U(IV)/U(III) transition (~500 mV) than for the U(III)/U system (~100 mV).

Mechanistic Insight into the Turn‐Off Sensing of Nitroaromatic Compounds Employing Functionalized Polyaniline

AbstractConducting Polymers (CPs), in recent times have contributed significantly in the detection of High Energy Materials possessing nitro functional groups through fluorescence quenching studies. Camphor sulphuric acid doped polyaniline (C‐PANI) is studied with a view to obtain easily processable PANI without π‐stacking for the detection of Picric acid(PA) and p‐nitro toluene(pNT) in trace levels. The quenching constant and limit of detection were found to be 2.236×10−6 M and 6.14×10−7 M for PA and 1.9447×10−7 M and 3.57×10−7 M for pNT, respectively showing a very good sensitivity in detection. Resonance Raman spectroscopy (RRS) showed the involvement of the bipolaronic and benzenoid group in the charge transfer complex formed between the conducting polymer and the analyte. The photoluminescence studies indicated a predominant PET mechanism. The electrochemical studies employing cyclic voltammetry gave higher band gap on comparison with absorption studies. The positive and negative shift in oxidation and reduction potential of polymer‐analyte was observed and collaborated with the fluorescence studies to understand the possible fluorescence quenching mechanism from electrochemical approach.

Direct Detection and Discrimination of Nucleotide Polymorphisms Using Anthraquinone Labeled DNA Probes

A novel electrochemical detection approach using DNA probes labeled with Anthraquinone (AQ) as a reporter moiety has been successfully exploited as a method for the direct detection of DNA targets. This assay uses simple voltammetry techniques (Differential Pulse Voltammetry) to exploit the unique responsiveness of AQ to its chemical environments within oxygenated aqueous buffers, providing a specific detection mechanism as a result of DNA hybridization. This measurement is based on a cathodic shift of the reduction potential of the AQ tag and the concurrent reduction in peak current upon DNA binding. The further utility of this approach for discrimination of closely related DNA targets is demonstrated using DNA strands specific to B. anthracis and closely related bacillus species. DNA targets were designed to the rpoB gene incorporating nucleotide polymorphisms associated with different bacillus species. This assay was used to demonstrate that the shift in reduction potential is directly related to the homology of the target DNA. The discriminatory mechanism is dependent on the presence of oxygen in the measurement buffer and is strongly linked to the position of the nucleotide polymorphisms; with homology at the terminus carrying the AQ functionalised nucleotide critical to achieving accurate discrimination. This understanding of assay design was used to demonstrate an optimized assay capable of discriminating between Yersinia pestis (the causative agent of plague) and closely related species based on the groEL gene. This method is attractive as it can not only detect DNA binding, but can also discriminate between multiple Single Nucleotide Polymorphisms (SNPs) within that DNA without the need for any additional reagents, reporters, or processes such as melting of DNA strands. This indicates that this approach may have great potential to be exploited within novel biosensors for detection and diagnosis of infectious disease in future Point of Care (PoC) devices.

The Effect of Guest Metal Ions on the Reduction Potentials of Uranium(VI) Complexes: Experimental and Theoretical Investigations.

Two mononuclear uranyl complexes, [UO2 L1 ] (1) and [UO2 L2 ]⋅0.5 CH3 CN⋅0.25 CH3 OH (2), have been synthesized from two multidentate N3 O4 donor ligands, N,N'-bis(5-methoxysalicylidene)diethylenetriamine (H2 L1 ) and N,N'-bis(3-methoxysalicylidene)diethylenetriamine (H2 L2 ), respectively, and have been structurally characterized. Both complexes 1 and 2 showed a reversible UVI /UV couple at -1.571 and -1.519 V, respectively, in cyclic voltammetry. The reduction potential of the UVI /UV couple shifted towards more positive potential on addition of Li+ , Na+ , K+ , and Ag+ metal ions to acetonitrile solutions of complex 2, and the resulting potential was correlated with the Lewis acidity of the metal ions and was also justified by theoretical DFT calculations. No such shift in reduction potential was observed for complex 1. All four bimetallic products, [UO2 L2 Li0.5 ](ClO4 )0.5 (3), [UO2 L2 Na(ClO4 )]2 (4), [UO2 L2 Ag(NO3 )(H2 O)] (5), and [(UO2 L2 )2 K(H2 O)2 ]PF6 (6), formed on addition of the Li+ , Na+ , Ag+ , and K+ metal ions, respectively, to acetonitrile solutions of complex 2, were isolated in the solid state and structurally characterized by single-crystal X-ray diffraction. In all the species, the inner N3 O2 donor set of the ligand encompasses the equatorial plane of the uranyl ion and the outer open compartment with O2 O'2 donor sites hosts the second metal ion.

A Heme Propionate Staples the Structure of Cytochrome c for Methionine Ligation to the Heme Iron.

Ligand-switch reactions at the heme iron are common in biological systems, but their mechanisms and the features of the polypeptide fold that support dual ligation are not well understood. In cytochrome c (cyt c), two low-stability loops (Ω-loop C and Ω-loop D) are connected by the heme propionate HP6. At alkaline pH, the native Met80 ligand from Ω-loop D switches to a Lys residue from the same loop. Deprotonation of an as yet unknown group triggers the alkaline transition. We have created the two cyt c variants T49V/K79G and T78V/K79G with altered connections of these two loops to HP6. Electronic absorption, NMR, and EPR studies demonstrate that at pH 7.4 ferric forms of these variants are Lys-ligated, whereas ferrous forms maintain the native Met80 ligation. Measurements of protein stability, cyclic voltammetry, pH-jump and gated electron-transfer kinetics have revealed that these Thr to Val substitutions greatly affect the alkaline transition in both ferric and ferrous proteins. The substitutions modify the stability of the Met-ligated species and reduction potentials of the heme iron. The kinetics of ligand-switch processes are also altered, and analyses of these effects implicate redox-dependent differences in metal-ligand interactions and the role of the protein dynamics, including cross-talk between the two Ω-loops. With the two destabilized variants, it is possible to map energy levels for the Met- and Lys-ligated species in both ferric and ferrous proteins and assess the role of the protein scaffold in redox-dependent preferences for these two ligands. The estimated shift in the heme iron reduction potential upon deprotonation of the "trigger" group is consistent with those associated with deprotonation of an HP, suggesting that HP6, on its own or as a part of a hydrogen-bonded cluster, is a likely "trigger" for the Met to Lys ligand switch.

Electrochemical approach to Fischer carbene complexes

Abstract Although the chemistry of Fischer carbene complexes is recently a growing direction of research, a review summarizing their electrochemical investigations is still missing. In the present, text evaluation and comparison of experimental data allow insight on how the structural changes of Fischer carbene complexes influence their redox properties. It is evident that the respective reduction and oxidation potentials depend on multiple parameters. To shed light on this problem, the available data were systematized in several series, where effects of individual structural changes were evaluated as shifts of oxidation and reduction potentials with respect to a reference compound. The changes in electron delocalization caused by different substitutions were rationalized using quantum chemical calculations.

Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn-OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II.

We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of FeIII/II3MnII-OH2 clusters leads to internal reorganization resulting in formal oxidation at Mn to generate FeIII/II3MnIII-OH. 57Fe Mössbauer spectroscopy reveals that oxidation state changes (three for FeIII/II3Mn-OH2 and four for FeIII/II3Mn-OH clusters) occur exclusively at the Fe centers; the Mn center is formally MnII when water is bound and MnIII when hydroxide is bound. Experimentally determined p Ka (17.4) of the [FeIII2FeIIMnII-OH2] cluster and the reduction potentials of the [Fe3Mn-OH2] and [Fe3Mn-OH] clusters were used to analyze the O-H bond dissociation enthalpies (BDEO-H) for multiple cluster oxidation states. BDEO-H increases from 69 to 78 and 85 kcal/mol for the [FeIIIFeII2MnII-OH2], [FeIII2FeIIMnII-OH2], and [FeIII3MnII-OH2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe3Mn-OH x] system was obtained by constructing a potential-p Ka diagram; the shift in reduction potentials of the [Fe3Mn-OH x] clusters in the presence of different bases supports the BDEO-H values reported for the [Fe3Mn-OH2] clusters. A lower limit of the p Ka for the hydroxide ligand of the [Fe3Mn-OH] clusters was estimated for two oxidation states. These data suggest BDEO-H values for the [FeIII2FeIIMnIII-OH] and [FeIII3MnIII-OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe3Mn═O] in this and related multinuclear systems.

Modulation of the Electrochemical Reactivity of Solubilized Redox Active Polymers via Polyelectrolyte Dynamics.

Redox active polymers (RAPs) are electrochemically versatile materials that find key applications in energy storage, sensing, and surface modification. In spite of the ubiquity of RAP-modified electrodes, a critical knowledge gap exists in the understanding of the electrochemistry of soluble RAPs and their relation to polyelectrolyte dynamics. Here, we explore for the first time the intersection between polyelectrolyte behavior and the electrochemical response that highly soluble and highly substituted RAPs with viologen, ferrocene, and nitrostyrene moieties elicit at electrodes. This comprehensive study of RAP electrolytes over several orders of magnitude in concentration and ionic strength reveals distinct signatures consistent with surface confined, colloidal, and bulk-like electrochemical behavior. These differences emerge across polyelectrolyte packing regimes and are strongly modulated by changes in RAP coil size and electrostatic interactions with the electrode. We found that, unlike monomer motifs, simple changes in the ionic strength caused variations over 1 order of magnitude in the current measured at the electrode. In addition, the thermodynamics of adsorbed RAP films were also affected, giving rise to standard reduction potential shifts leading to redox kinetic effects as a result of the mediating nature of the RAP film in equilibrium with the solution. Full electrochemical characterization via transient and steady-state techniques, including the use of ultramicroelectrodes and the rotating disk electrode, were correlated to dynamic light scattering, ellipsometry, and viscometric analysis. These methods helped elucidate the relationship between electrochemical behavior and RAP coil size, film thickness, and polyelectrolyte packing regime. This study underscores the role of electrostatics in modulating the reactivity of redox polyelectrolytes.

Pyridinium-Functionalized Pyromellitic Diimides with Stabilized Radical Anion States.

In this work, we report the stabilization of the reduced states of pyromellitic diimide by charge-balancing the imide radical anions with cationic pyridinium groups attached to the aromatic core. This structural modification is confirmed by single-crystal X-ray diffraction analysis. Characterization by (spectro)electrochemical experiments and computations reveal that the addition of cationic groups to an already electron-deficient ring system results in up to +0.57 V shifts in reduction potentials, largely as a consequence of charge screening and lowest unoccupied molecular orbital-lowering effects. This formal charge-balancing approach to stabilizing the reduced states of electron-deficient pyromellitic diimides will facilitate their incorporation into spin-based optoelectronic materials and devices.

Mechanism and kinetics of electrocarboxylation of aromatic ketones in ionic liquid

The electrochemical carboxylation of a range of substituted benzophenones was studied in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (IL, ([Bmpy][NTf2])). As expected, the aromatic carbonyls exhibited electrochemical reversibility for the first reduction to the radical anion at potentials which were a function of the sum of their Hammett substituent constants (Σσ). However, in the presence of CO2, the electrochemical reversibility was lost and positive shifts in reduction potentials were observed which were indicative of post-electron transfer chemical reaction which has been attributable to the nucleophilic radical anion/CO2 coupling reaction.Analysis of the positive potential shift a function of sweep rate (ν) indicated that the mechanism is either ECE or DISP1, or mixed ECE/DISP1. Also from the potential shift with ν, an apparent rate constant (kapp), and a pseudo-first order rate constant (k1), for the coupling reaction was determined and compared to molecular solvent where the rate is over two orders of magnitude lower in IL compared to dimethylformamide (DMF). The low polarity of the IL compared to DMF appears to be the cause of slow kinetics. Finally, plots of kapp vs. Σσ were strictly linear indicating that IL does not preferentially interact with any of the electrogenerated radical anions thus implying that the electrocarboxylation reaction may be a useful probe of IL environments and structure on radical anion reactions.

Carbon Nanotube/Graphite Mixtures Used as Cavity Cell Macroelectrodes and their Unexpected Behavior Towards Benzyl Bromide Electroreduction

AbstractPowder macroelectrodes were prepared by mixing carbon nanotubes (CNT) and graphite in different ratios, and evaluated as electron transfer medium for the benzyl bromide electroreduction. 1,2‐Diphenylethane (DPE) (1‐electron process) was favored, instead of toluene (2‐electron process), for all CNT:graphite mass ratios, while pure graphite or CNT favored toluene as major product. The 3:7 CNT:graphite mass ratio gave the best result, furnishing 70 % yield of DPE. Cyclic voltammetry showed a cathodic shift for the benzyl bromide reduction potential by using CNT:graphite electrode composition, favoring the 1‐electron process products during the electrolyses, and corroborating the results observed.