Relative Oxidation Potentials Research Articles

CdSe nanoplatelets as photocatalyst for organic photochemistry: Synthesis of amino acid derivatives by decarboxylative alkylation of aldimines

Colloidal semiconductor two dimensional CdSe nanoplatelets (NPLs) have been widely applied in photophysics, while rarely used in photocatalytic reactions. Here we show that CdSe NPLs act as photocatalyst in visible light-driven radical alkylation of aldimines, with 2,2′-bipyridine (BPY) as an additive. The photocatalytic reaction gave amino-acid derivatives in reliably good yields with various alkyl radical precursors. Furthermore, CdSe NPLs recycled from the reactions remain active and can be reused. The structural and photoelectric properties of fresh and recycled NPLs were characterized using XRD, XPS, TEM, EDS, TRPL, DRS and electrochemical analysis, revealing that BPY can bind to the surface defects of NPLs and adjust the relative oxidation and reduction potentials. As the synthesis of CdSe NPLs is low-cost, convenient and available at industrial scales, the results suggest NPLs could serve as a practical choice of photocatalyst for organic photochemistry.

Redox leveling of the Kok cycle of photosystem II established by water ligand binding to the oxygen evolving complex.

Photosystem II (PSII) produces dioxygen at the oxygen-evolving complex (OEC) by catalyzing light-driven water oxidation. The OEC Mn4CaO5 cluster cycles through a series of redox state intermediates Si (i = 0-4) along the Kok cycle, as the tyrosine Z radical oxidizes the cluster and accumulates multiple oxidizing equivalents. All S-state transitions occur within a narrow range of redox potentials and involve alternating oxidation and deprotonation steps. Here, we address the origin of the redox leveling mechanism as determined by the structure of hydrogen bonding surrounding the OEC and binding of water ligands to the OECs. We focus on the S2 to S3 transition to investigate the relative oxidation potential of the tyrosine radical as compared to the oxidation potential of high valent Mn centers regulated by water binding. Free energy calculations at the DFT QM/MM level of theory show that water binding lowers the free energy change for the S2/S3 transition by 13.7 kcal/mol, demonstrating the regulatory role of water binding on the reaction mechanism. The dynamics of water binding to the Mn is further analyzed by running molecular dynamics simulations where the interaction potential of water ligands to the Mn centers is parametrized at the DFT level. Our analysis demonstrates the capabilities of our simulations for studies of substrate water ligand exchange to the OEC that enables direct comparisons to time-resolved mass spectroscopic measurements.

The role of halide oxidation in perovskite halide phase separation

<h2>Summary</h2> Halide perovskites display full solid solubility for Br:I and Cl:Br compositions at equilibrium, yet initially homogeneous distributions often partition into Br- and I-rich (Cl- and Br-) regions under non-equilibrium conditions (e.g., illumination), imposing a major obstacle to operationally stable bandgap tunability critical for applications such as tandem photovoltaics and light emitting diodes. Halide oxidation plays a major role in the physical demixing of alloyed halide compositions in perovskite films; this step appears to initiate halide segregation, ultimately directed by several underlying thermodynamic and kinetic driving forces operating in both concert and competition. Notably, the potential energy needed to oxidize the halide, determined by their relative oxidation potentials (I⁻ < Br⁻ < Cl⁻), can be supplied by applied voltage or illumination biases; segregation will be dictated by the most easily oxidized species leading to concentration gradients in oxidized halide products. In this perspective, we critically analyze the behaviors of halide segregation predicted by such a model capable of rationalizing a wide variety of reported observations with only photoelectrochemical halide oxidation as a commonality. The fundamental and interdisciplinary concepts invoked here clarify the role of photoelectrochemistry in halide segregation, linking illumination- and voltage-induced phenomena, and suggest that these instabilities are rooted at an atomic orbital level because of coordination environment. The path to overcome these instabilities should be paved through highly collaborative efforts, especially between material engineers and inorganic chemists to carefully manipulate dynamic disorder, macro-, and microstrains.

Pt-Pd-doped NiO nanoparticle decorated at single-wall carbon nanotubes: An excellent, powerful electrocatalyst for the fabrication of An electrochemical sensor to determine nalbuphine in the presence of tramadol as two opioid analgesic drugs

In this study, a Pt-Pd-doped NiO nanoparticle decorated at the surface of single-wall carbon nanotubes (Pt-Pd/NiO-NPs/SWCNTs) was synthesized using a simple chemical precipitation method and characterized by XRD, TEM, and EDS methods. The results confirmed that Pt-Pd/NiO-NPs/SWCNTs were synthesized with good purity and at the nanoscale size. Moreover, a highly sensitive electroanalytical sensor was fabricated by incorporating synthesized Pt-Pd/NiO-NPs/SWCNT nanocomposites into a carbon paste electrode (CPE) in the presence of 1-ethyl-3-methylimidazolium methanesulfonate (EMICH3SO3−) as binder. The Pt-Pd/NiO-NPs/SWCNTs/EMICH3SO3−/CPE showed a powerful electro-catalytic activity for electro-oxidation of nalbuphine, and the results confirmed that the oxidation of nalbuphine was improved 6.34 times and relative oxidation potential was decreased about 110 mV compared to unmodified electrodes. The Pt-Pd/NiO-NPs/SWCNTs/EMICH3SO3−/CPE also showed good catalytic activity for the determination of nalbuphine in the presence of tramadol and the oxidation potential of these opioid analgesic drugs separated with ΔE =460 mV. In the final step, the Pt-Pd/NiO-NPs/SWCNTs/EMICH3SO3−/CPE was used to determine nalbuphine with a detection limit of 0.9 nM and tramadol with a detection limit of 50.0 nM in drug samples. The results confirmed the powerful and interesting ability of the sensor in the analysis of a real sample.

Ferrocene-Containing Tin(IV) Complexes Based on o-Benzoquinone and o-Iminobenzoquinone Ligands. Synthesis, Molecular Structure, and Electrochemical Properties.

The addition of different substituted o-benzoquinones and o-iminobenzoquinones to tin(II) bis(o-iminophenolates) of the types (Fc-IP)2SnII and (Fc-4,6-IP)2SnII (where Fc-IP is anion 2-(ferrocenylmethyleneamino)phenolate [Fc-C(H)═N(C6H4)O-] and Fc-4,6-IP is anion 2-(ferrocenylmethyleneamino)-4,6-di-tert-butylphenolate [Fc-C(H)═N(4,6-tBu-C6H2)O-]) in tetrahydrofuran leads to the oxidation of Sn(II) to Sn(IV) with formation of the corresponding tin(IV) catecholates (Fc-4,6-IP)2SnIV(3,6-Cat) (1), (Fc-IP)2SnIV(3,6-Cat) (2), (Fc-4,6-IP)2SnIV(4-Cl-3,6-Cat) (3), (Fc-IP)2SnIV(4-Cl-3,6-Cat) (4), (Fc-4,6-IP)2SnIV(4,5-Cl2-3,6-Cat) (5), and (Fc-IP)2SnIV(4,5-Cl2-3,6-Cat) (6) or the o-amidophenolates (Fc-4,6-IP)2SnIV(AP-Me) (7), (Fc-IP)2SnIV(AP-iPr) (8), and (Fc-4,6-IP)2SnIV(AP-iPr) (9). Here ligands 3,6-Cat, 4-Cl-3,6-Cat, and 4,5-Cl2-3,6-Cat are dianions 3,6-di-tert-butyl-, 4-chloro-3,6-di-tert-butyl-, and 4,5-dichloro-3,6-di-tert-butylcatecholates, respectively, and AP-Me and AP-iPr are dianions 4,6-di-tert-butyl-N-(2,6-dimethylphenyl)-o-amidophenolate and 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-amidophenolate, respectively. Complexes 1-9 have been characterized in detail by IR spectroscopy, cyclic voltammetry, and 1H, 13C, and 119Sn NMR spectroscopy. The molecular structures of tin(IV) complexes 5, 7, and 9 in the crystalline state were determined by single-crystal X-ray diffraction analysis. Complexes demonstrate a series of successive oxidations involving alternately catecholato/o-amidophenolato centers and ferrocenyl moieties. The relative oxidation potentials of these redox centers depend on the acceptor properties of the redox-active chelating O,O' or O,N ligand. An increase in the acceptor properties of redox-active o-quinonato-type ligands leads to an increase in the oxidation potentials of redox ligands as well as the following oxidation of ferrocenyl group(s). In two series of complexes, (Fc-4,6-L)2SnL' and (Fc-L)2SnL', where L' is AP-iPr, AP-Me, 3,6-Cat, 4-Cl-3,6-Cat, and 4,5-Cl2-3,6-Cat, a more pronounced convergence of the oxidation potentials of the redox-active o-quinonato ligand and ferrocenyl group occurs in the series (Fc-L)2SnL'.

Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics

Ultraviolet photoelectron spectroscopy (UPS) and cyclic voltammetry (CV) are employed to measure energy levels for charge transport in organic semiconductor films. A series of classical molecules/polymers used in organic bulk heterojunction solar cells are deposited on platinum substrates/electrodes to form thin films and a linear relationship of vertical ionization potential (IP) measured by UPS and relative oxidation potential (Eox) obtained by CV is found, with a slope equal to unity. The intercept varies with the different reference redox couples and repeated potential sweep numbers during experiment processes. The relationship provides for an easy conversion of values obtained by the two techniques and correlates well with device parameters. The precision in the CV‐derived IP values is not sufficient, however, to enable precise design of energy level alignment at heterojunction and the approach does not improve upon the current “best practice” for obtaining donor ionization potential–acceptor electron affinity gaps at heterojunctions.

The Oxygen Reduction Reaction Rate of Metallic Nanoparticles during Catalyzed Oxidation

This work reports the oxygen reduction reaction (ORR) kinetics of metal nanoparticle catalysts between 500 and 600 °C at low oxygen partial pressures. Ex situ and in situ TEM measurements demonstrate catalyzed nanowire growth initially follows linear kinetics; characteristic of being ORR rate limited. The ORR rates of Ag, Au, Cu, Ni, Pd, Rh and Pt measured at 600 °C form a volcano plot versus relative oxidation potential. Cu nanoparticles produce the maximum ORR rate under these conditions.

Synthesis, Properties, and Light-Emitting Electrochemical Cell (LEEC) Device Fabrication of Cationic Ir(III) Complexes Bearing Electron-Withdrawing Groups on the Cyclometallating Ligands.

The structure–property relationship study of a series of cationic Ir(III) complexes in the form of [Ir(C^N)2(dtBubpy)]PF6 [where dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine and C^N = cyclometallating ligand bearing an electron-withdrawing group (EWG) at C4 of the phenyl substituent, i.e., −CF3 (1), −OCF3 (2), −SCF3 (3), −SO2CF3 (4)] has been investigated. The physical and optoelectronic properties of the four complexes were comprehensively characterized, including by X-ray diffraction analysis. All the complexes exhibit quasireversible dtBubpy-based reductions from −1.29 to −1.34 V (vs SCE). The oxidation processes are likewise quasireversible (metal + C^N ligand) and are between 1.54 and 1.72 V (vs SCE). The relative oxidation potentials follow a general trend associated with the Hammett parameter (σ) of the EWGs. Surprisingly, complex 4 bearing the strongest EWG does not adhere to the expected Hammett behavior and was found to exhibit red-shifted absorption and emission maxima. Nevertheless, the concept of introducing EWGs was found to be generally useful in blue-shifting the emission maxima of the complexes (λem = 484–545 nm) compared to that of the prototype complex [Ir(ppy)2(dtBubpy)]PF6 (where ppy = 2-phenylpyridinato) (λem = 591 nm). The complexes were found to be bright emitters in solution at room temperature (ΦPL = 45–66%) with microsecond excited-state lifetimes (τe = 1.14–4.28 μs). The photophysical properties along with density functional theory (DFT) calculations suggest that the emission of these complexes originates from mixed contributions from ligand-centered (LC) transitions and mixed metal-to-ligand and ligand-to-ligand charge transfer (LLCT/MLCT) transitions, depending on the EWG. In complexes 1, 3, and 4 the 3LC character is prominent over the mixed 3CT character, while in complex 2, the mixed 3CT character is much more pronounced, as demonstrated by DFT calculations and the observed positive solvatochromism effect. Due to the quasireversible nature of the oxidation and reduction waves, fabrication of light-emitting electrochemical cells (LEECs) using these complexes as emitters was possible with the LEECs showing moderate efficiencies.

Accurate oxidation potentials of 40 benzene and biphenyl derivatives with heteroatom substituents.

The redox equilibrium method was used to determine accurate oxidation potentials in acetonitrile for 40 heteroatom-substituted compounds. These include methoxy-substituted benzenes and biphenyls, aromatic amines, and substituted acetanilides. The redox equilibrium method allowed oxidation potentials to be determined with high precision (≤ ±6 mV). Whereas most of the relative oxidation potentials follow well-established chemical trends, interestingly, the oxidation potentials of substituted N-methylacetanilides were found to be higher than those of the corresponding acetanilides. Density functional theory calculations provided insight into the origin of these surprising results in terms of the preferred conformations of the amides versus their cation radicals.

Relative Photooxidation and Photoreduction Activities of the {100}, {101}, and {001} Surfaces of Anatase TiO2

The photoredox ability of the TiO2 {100}, {101}, and {001} surfaces is investigated by examining the trapping energies, trapping sites, and relative oxidation and reduction potentials of simulated photogenerated holes and electrons in the form of more realistic polaronic states on the basis of density functional electronic structure calculations. Our results enable us to re-estimate their relative photooxidation ({100} > {101} > {001}) and photoreduction ({100} > {101} > {001}) activities, which rectify the conventional understanding. The dual functions of the surface under coordinated atoms acting as active adsorption sites for adsorbates and hindering the population of electrons to the outermost surface layer are identified, and the specific surface geometric structures also play an important role in trapping holes and electrons through the ease of lattice distortion. In addition, we attribute the commonly low photocatalytic performance of the {101} surface to the large and similar trapping energies and adjacent trapping sites for electrons and holes, which result in high electron-hole recombination rates. However, the large difference in trapping energies for electrons and holes on different surfaces allows us to spatially gather electrons and holes on different surfaces by artificially designing the exposing facets of nanocrystals without resorting to the energy band potential difference between surfaces, thus expanding the ideas to improve the photocatalytic properties of materials through the regulation of crystal facets. Our present work can provide a helpful message for the design of more reactive photocatalytic TiO2 nanocrystals and the fabrication of other reactive photocatalysts.

Measured and Calculated Oxidation Potentials of 1-X-12-Y-CB11Me10– Anions

Cyclic voltammetry of 31 icosahedral carborane anions 1-X-12-Y-CB(11)Me(10)(-) at a Pt electrode in liquid SO(2) revealed a completely reversible one-electron oxidation even at low scan rates, except for the anions with Y = I, which are oxidized irreversibly up to a scan rate of 5.0 V/s, and the anion with X = COOH and Y = H, whose oxidation is irreversible at scan rates below 1.0 V/s. Relative reversible oxidation potentials agree well with RI-B3LYP/TZVPP,COSMO and significantly less well with RI-BP86/TZVPP,COSMO or RI-HF/TZVPP,COSMO calculated adiabatic electron detachment energies. Correlations with HOMO energies of the anions are nearly as good, even though the oxidized forms are subject to considerable Jahn-Teller distortion. Except for the anion with X = F and Y = Me, the oxidation potentials vary linearly with substituent σ(p) Hammett constants. The slopes (reaction constants) are ~0.31 and ~0.55 V for positions 1 and 12, respectively.

Different types of phenolic units in lignins

The influence of cross-linking and branching on the number of interconnections between lignin units and the number of end groups (phenolic and non-phenolic) in the lignin molecules is discussed. Branching results in an increased number of end groups. It appears from an evaluation of the literature that p-hydroxyphenylpropane units are phenolic to a larger extent than guaiacylpropane units and that such units in turn are phenolic to a larger extent than syringylpropane units. It is proposed that this is related to the relative oxidation potentials of the lignin units. Guaiacylpropane units C-substituted in the 6-position are phenolic to a large extent. Alternative explanations for this are presented.

Calculation of the relative acidities and oxidation potentials of para-substituted phenols. A model for α-tocopherol in solution

Relative acidities (Delta pK(a)) of phenols and oxidation potentials (Delta E(ox)) of the phenoxide anions have been calculated for nine para-substituted phenols using density functional theory. Solvent effects were incorporated using the conductor-like polarisable continuum method. Using the calculated Delta pK(a) and Delta E(ox) values in a thermodynamic cycle, the DeltaBDE (bond dissociation enthalpy) of the phenols were also determined with all values calculated to within 1.5 kcal mol(-1) of experiment. The Delta pK(a) and Delta E(ox) values were calculated for 6-hydroxy-2,2,5,7,8-pentamethylchroman (HPMC), a model for alpha-tocopherol for which there are no known experimental values. The acidity of this compound is raised by 2.4 pK(a) units and lowered by -0.79 V relative to phenol with a calculated Delta BDE of -14.9 kcal mol(-1). There is a negative correlation (r(2) = 0.86) between the Delta pK(a) and the Delta BDE values. A stronger and positive correlation is found between the Delta E(ox) (r(2) = 0.98) and the Delta BDE values. Using these correlations it is uncovered that hydrogen abstraction of phenols, as measured by the Delta BDE, is driven by electron transfer rather than by proton transfer.

One-Electron Oxidation of DNA Oligomers That Lack Guanine: Reaction and Strand Cleavage at Remote Thymines by Long-Distance Radical Cation Hopping

The anthraquinone (AQ) photosensitized one-electron oxidation of DNA introduces a radical cation (electron "hole") that migrates through the duplex by hopping. The radical cation normally is trapped irreversibly by reaction at guanine. We constructed AQ-linked DNA oligomers composed exclusively of A/T base pairs. Their irradiation led to reaction and strand cleavage primarily at thymines. Long-distance radical cation hopping to distant thymines was demonstrated by the distance dependence of the process and by experiments with DNA oligomers that contain a single remote GG step. The reaction of the radical cation at thymine was shown to involve its 5-methyl group by the replacement of selected thymines with uracils. These findings show that the reactivity of radical cations in DNA cannot be explained simply by exclusive reliance on the relative oxidation potential of the nucleobases. Instead, the site of reaction is determined in accord with the Curtin-Hammett principle for reactive species in rapid equilibrium.

Kinetic Determinations of Accurate Relative Oxidation Potentials of Amines with Reactive Radical Cations†

Accurate oxidation potentials for organic compounds are critical for the evaluation of thermodynamic and kinetic properties of their radical cations. Except when using a specialized apparatus, electrochemical oxidation of molecules with reactive radical cations is usually an irreversible process, providing peak potentials, E(p), rather than thermodynamically meaningful oxidation potentials, E(ox). In a previous study on amines with radical cations that underwent rapid decarboxylation, we estimated E(ox) by correcting the E(p) from cyclic voltammetry with rate constants for decarboxylation obtained using laser flash photolysis. Here we use redox equilibration experiments to determine accurate relative oxidation potentials for the same amines. We also describe an extension of these experiments to show how relative oxidation potentials can be obtained in the absence of equilibrium, from a complete kinetic analysis of the reversible redox kinetics. The results provide support for the previous cyclic voltammetry/laser flash photolysis method for determining oxidation potentials.

Relationship between the ionization and oxidation potentials of molecular organic semiconductors

A relationship between the energy of the highest occupied molecular orbital (HOMO) and the oxidation potential of molecular organic semiconductors is presented. Approximating molecules as dipoles consisting of a positively charged ion core surrounded by an electron cloud, the HOMO energy (EHOMO) is calculated as that required to separate these opposite charges in a neutral organic thin film. Furthermore, an analysis of image charge forces on spherical molecules positioned near a conductive plane formed by the electrode in an electrochemical cell is shown to explain the observed linear relationship between EHOMO and the oxidation potential. The EHOMOs of a number of organic semiconductors commonly employed in thin film electronic devices were determined by ultraviolet photoemission spectroscopy, and compared to the relative oxidation potential (VCV) measured using pulsed cyclic voltammetry, leading to the relationship EHOMO = � (1.4 ± 0.1) · (qVCV) � (4.6 ± 0.08) eV, consistent with theoretical predictions.

Photosensitized Diels—‐Alder Reactions of N‐Arylimines: Synthesis of Tetrahydroquinoline Derivatives.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Determination of the oxidation potentials of organic benzene derivatives: theory and experiment

We have calculated the IP, Δ G e, and E ox values for 10 mono-substituted benzene molecules and compared them with experimental values obtained by linear sweep voltammetry. The E ox values were evaluated using the density functional method and thermodynamic cycles. The relative oxidation potentials are in close agreement with experimental values, while the UB3LYP/6-31+G(d) approach shows the absolute E ox values to be lower by about 0.9 V. Consideration of bulk solvent effects is important to fully describe the experimental variation in E ox. The HOMO, NBO, and Wiberg bond index were analyzed to investigate the changes when moving from neutral to cationic molecules.

Photosensitized Diels–Alder reactions of N-arylimines: synthesis of tetrahydroquinoline derivatives

Irradiation ( λ>345 nm) of catalytic amounts of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) in a CH 2Cl 2 solution of N-aryl imines and α-methylstyrene or styrene produced the corresponding [4+2] cycloaddition products, tetrahydroquinoline derivatives, in good yield. The reaction was controlled by the relative oxidation potentials of the dienophile and the diene.

Protein Protection by Antioxidants: Development of a Convenient Assay and Structure-Activity Relationships of Natural Polyphenols

In this work, a convenient test of antioxidant activity was developed, with BChE-contaminated HSA as the target of AAPH-induced oxidation and its esterase activity as the marker of protein integrity or degradation. The method is relatively simple, of low cost, and convenient to use. Its application to natural polyphenols showed that quercetin (1), verbascoside (2), chlorogenic acid (3), caffeic acid (4), 1,3,6,7-tetrahydroxyxanthone (5), and mangiferin (6), are good antioxidants (IC50<9 μM). 1,5-Dihydroxy-3-methoxyxanthone (7), flemichin D (8), and cordigone (9) showed modest activities (ca. 50 μM<IC50<350 μM), whereas danthrone (10) was inactive. Complementary experiments with two of the more active antioxidants, namely quercetin (1) and chlorogenic acid (3) showed that both antioxidants were better radical scavengers than chain-breaking antioxidants. The relative adiabatic oxidation potential (ΔHox), the relative H-bond dissociation energy (ΔHabs), and the first oxidation potential measured by cyclic voltammetry were found to be related to the radical-scavenging activity of these antioxidants.