Cationic Redox Species Research Articles

Electrochemical, Spectroelectrochemical, and Electrocatalytic Dioxygen Reducing Properties of Peripheral Tetra‐2,6‐dimethoxyphenoxy Substituted Phthalocyanines

AbstractBy using of peripherally tetrakis‐2,6‐dimethoxyphenoxy substituted Co(II), Fe(III), and Mn(III) phthalocyanines was observed to extend the redox richness of the phthalocyanine ring with the addition of reversible metal‐based electron transfer couples to the phthalocyanine ring‐based electron transfer processes in solution. The emergence of highly steady anionic and cationic redox species with clear spectral and colour changes both in solution and in the film during in situ spectroelectrochemical and in situ electrocolorimetric measurements pointed out their applicability as electrochromic materials. Furthermore, the phthalocyanine complexes demonstrated strong interaction with dioxygen and thus, high electrocatalytic enforcement for its reduction.

Surface-Charge Effects on Voltammetry in Carbon Nanocavities.

Ion transport controlled by electrostatic interactions is an important phenomenon in biological and artificial membranes, channels, and nanopores. Here, we employ carbon-coated nanopipets (CNPs) for studying permselective electrochemistry in a conductive nanopore. A significant accumulation (up to 2000-fold) of cationic redox species and anion depletion inside a CNP by diffuse-layer and surface-charge effects in a solution of low ionic strength were observed as well as the shift of the voltammetric midpeak potential. Finite-element simulations of electrostatic effects on CNP voltammograms show permselective ion transport in a single conducting nanopore and semiquantitatively explain our experimental data. The reported results are potentially useful for improving sensitivity and selectivity of CNP sensors for ionic analytes.

Novel 1,2,4-triazole substituted metallo-phthalocyanines: Synthesis, characterization and investigation of electrochemical and spectroelectrochemical properties

The phthalonitrile compound (3) and peripherally tetra substituted zinc(II) (4), nickel (II) (5), iron (II) (6), cobalt (II) (7) and copper (II) (8) phthalocyanines were synthesized for the first time in this study. The structural characterization of these novel compounds were performed with FT-IR, 1H NMR, 13C NMR, UV–Vis and mass spectroscopy. The redox properties of the complexes were investigated by cyclic voltammetry, square wave voltammetry, controlled-potential coulometry and in-situ spectroelectrochemistry. The effects of redox active and inactive metal centers on the electrochemistry of 4-2-(4-methyl-5-phenyl-4H-1,2,4-triazol-3-ylthio)ethoxy substitute phthalocyanine were examined. The color changes associated with the redox processes and electro-generated anionic and cationic redox species were suggested their applicability in the fields of the electrochemical technologies.

Electrostatic Ion Enrichment in an Ultrathin-Layer Cell with a Critical Dimension between 5 and 20 nm.

Electrostatic interactions play an essential role in many analytical applications including molecular sensing and transport studies using nanopores and separation of charged species. Here, we report the voltammetric quantification of electrostatic ion enrichment in a 5-20 nm thin electrochemical cell. A simple lithographic micro/nanofabrication process was used to create ultrathin-layer cells (UTLCs) with a critical dimension (i.e., cell thickness) as small as 5 nm. The voltammetric response of a UTLC was found to be largely dominated by the electrostatic interaction between charges on the cell walls and the redox species. We show that the ultrasmall cell dimension yielded a 100-300-fold enrichment for cationic redox species. An interesting surface adsorption effect was also demonstrated.

Redox-Switchable New Phthalocyanines Containing Hydrazono-Thiazole-Carboxylate Fragments: Synthesis, Electrochemical, Spectroelectrochemical and Electrocolorimetric Investigation.

New phthalonitrile compound with Schiff base, carbothioamide and thiazole moieties as substituents and its corresponding metal-free and metallophthalocyanines (Zn(II), Ni(II), Co(II), and Cu(II)) were synthesized and characterized for the first time. The solubilities of these novel phthalocyanines were high in organic solvents. The redox properties of the compounds have been researched by cyclic voltammetry, square wave voltammetry, controlled-potential coulometry and in situ spectroelectrochemistry in dimethylsulfoxide. The compounds displayed metal and/or phthalocyanine ring-based, generally reversible or quasi-reversible reduction and oxidation processes. The effect of aggregation on the redox behavior of these complexes was also discussed. In general, decreased intensity and broadening in the main Q absorption band and the appearance of a new blue-shifted band confirmed the presence of H-type aggregates in the solutions of the complexes 4, 6 and 8 in DMSO/TBAP. The color changes associated with the redox processes and electrogenerated anionic and cationic redox species were also recorded with in situ electrocolorimetric measurements. In situ UV-vis spectral and associated color changes monitored during the reduction processes of the complexes suggested their applicability in the fields of the electrochemical technologies.

Anionic redox chemistry in Na-rich Na 2 Ru 1−y Sn y O 3 positive electrode material for Na-ion batteries

The synthesis and Na- electrochemical activity of Na-rich layered Na2Ru1−ySnyO3 compounds is reported. Like their Li-analogue, Na2Ru1−ySnyO3 shows capacities that exceed theoretical capacity calculated from the cationic redox species. The high capacity was found, by means of XPS analysis, to be associated to the accumulation of both cationic (Ru4+/Ru5+) and anionic (O2−/O2n−) redox processes. The structural evolutions during cycling have been followed and found to be associated with the cation disordering and loss of crystallinity on cycling.

Electrochemical, Electrocatalytic Dioxygen Reducing and Dielectric Relaxation Properties of Non-Peripheral Tetra-2,3-dihydro-1H-inden-5-yloxy Substituted Phthalocyanines

Novel metal-free and metallophthalocyanines were synthesized by cyclotetramerization of 3-(2,3-dihydro-1H-inden-5-yloxy)phthalonitrile and characterized. The redox properties of these compounds were investigated by cyclic voltammetry, square-wave voltammetry, controlled-potential coulometry and in situ spectroelectrochemistry in dimethylsulfoxide and dichloromethane. The compounds displayed metal and/or phthalocyanine ring-based reduction and oxidation processes. The color changes associated with these redox processes and electrogenerated anionic and cationic redox species were recorded with in situ electrocolorimetric measurements. The electrocatalytic performances of the compounds toward oxygen reduction reaction having vital importance for fuel cell applications were determined by hydrodynamic rotating disk and bipotentiostatic rotating ring-disk voltammetry measurements. The Vulcan XC-72/Nf/FePc modified glassy carbon electrode showed much higher catalytic performance toward oxygen reduction than those of the other modified ones. Furthermore, the field dependence of dielectric response and the conductivity of the samples were measured as a function of frequency and temperature. Dielectric relaxation behaviors of the phthalocyanines were also investigated. The ITO/ZnPc/Au sample showed the highest value of ɛ′(ω) as compared to the other samples. The analysis of the real and imaginary parts of the dielectric permittivity with frequency was carried out assuming a distribution of relaxation times as well as the scaling behavior of the dielectric loss spectra.

Synthesis and characterization of terminalalkynyl-substituted unsymmetrical zinc phthalocyanine conjugated with well-defined polymers

Synthesis of unsymmetrically terminalalkynyl substituted zinc (II) phthalocyanine (ZnPc) through an efficient statistical condensation reaction with the unprotected phthalonitrile (2) is reported for the first time. Isolated ZnPc bearing one terminalalkynyl group is a good precursor for Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) reaction which is classified under the term “click chemistry”. This was approved by the successful click reaction between ZnPc and azide end-functional well-defined polymers obtained by atom transfer radical polymerization (ATRP), and consecutive azidation of corresponding bromo end-functional polymers. The click-reaction efficiencies for the formation of Pc-end functional poly(tert-butyl acrylate) (PtBA), and polystyrene (PS) have been found to be 75, 93% respectively. Functionalization of ZnPc with PS increased the chemical stability of the complex but decreased the electrochemical reversibility during redox reactions. In-situ electrocolorimetric measurements of the complexes allowed the quantification of color coordinates of the each electrogenerated anionic and cationic redox species.

Titanyl phthalocyanines: Electrochemical and spectroelectrochemical characterizations and electrochemical metal ion sensor applications of Langmuir films

Electrochemical and spectroelectrochemical characterizations of titanyl phthalocyanines, which were peripheral (α-TiOPc) and nonperipheral (β-TiOPc) tetra and octa substituted (octa-TiOPc) with 3,4-(methylendioxy)-phenoxy moieties were performed to determine their possible applications in different electrochemical technologies. Voltammetric and in situ spectroelectrochemical measurements illustrate that position and number of 3,4-(methylendioxy)-phenoxy substituents affect the redox behavior of the complexes. While α-TiOPc gives up to four reduction processes, β-TiOPc and octa-TiOPc gives up to three reduction processes. While octa-TiOPc gives metal–metal–ring–ring reductions, α-TiOPc and β-TiOPc complexes give metal–ring–ring–metal reduction mechanism. Solvent of the media also alter the electrochemical behavior of the complexes. In situ electrocolorimetric measurements shows distinct color differences among the electrogenerated anionic and cationic redox species, which indicates their possible applications in display technologies. Multiple and reversible, diffusion controlled and multi-electron redox reaction indicate possible usage of the complexes especially in electrosensing applications. The complexes sense Ag+ and Pd2+ ions in the solution and in the solid states.

Effects of peripheral and nonperipheral substitution to the spectroscopic, electrochemical and spectroelectrochemical properties of metallophthalocyanines

The syntheses, spectroscopic and electrochemical properties of metallo-phthalocyanine complexes, tetra- and octa-substituted with 3,4-(dimethoxyphenylthio) moieties at non-peripheral and peripheral positions were studied. These complexes were characterized by FT-IR, mass spectroscopy, electronic spectroscopy and electrochemical methods. 3,4-(Dimethoxyphenylthio) moieties increase the solubility of the complexes in various polar and nonpolar organic solvents and prevent the aggregation (in the same solvents) within a wide concentration range. Electronic absorption spectroscopy revealed that the presence of electron donating sulphur on synthesized complexes shift the Q-band of the complexes to the higher wavelengths. Redox active metal centers, CoII, FeII and MnIIIOAc into the phthalocyanine core extend the redox worthy of the complexes due to the reversible metal-based reduction and oxidation couples of the metal centers. In situ electrocolorimetric measurements of the complexes allow quantification of color coordinates of each electrogenerated anionic and cationic redox species. Electrochemical and in situ spectroelectrochemical analysis indicate that cobalt and manganese phthalocyanines catalyze the molecular oxygen in aprotic media.

Electrochemical and spectroelectrochemical characterization of newly synthesized manganese, cobalt, iron and copper phthalocyanines

Manganese, cobalt, iron, and copper phthalocyanines, tetra-substituted with 3,4-(dimethoxyphenylthio) moieties at peripheral positions, were synthesized and characterized by FT-IR, mass spectroscopy, and electronic spectroscopy. Electrochemical and spectroelectrochemical measurements exhibit that while copper phthalocyanine gives only ring-based electron transfer reactions, incorporating redox active metal centers, CoII, FeIIIAc, and MnIIIAc, into the phthalocyanine core extends the redox richness of the phthalocyanine ring with the metal-based reduction and oxidation couples of the metal centers in addition to the common phthalocyanine ring-based electron transfer processes. In situ electrocolorimetric measurements of the complexes allow quantification of color coordinates of the each electrogenerated anionic and cationic redox species. Presence of O2 in the electrolyte system influences the redox couples of the complexes due to the interaction between O2 and metallophthalocyanines having redox active metal center.

Synthesis, electrochemical and spectroelectrochemical properties of peripherally tetra-imidazole substituted metal free and metallophthalocyanines

The synthesis and characterization of novel organosoluble peripherally tetra imidazole substituted metal-free (4), zinc(II) (5), nickel(II) (6), cobalt(II) (7), lead(II) (8) and oxo-titanium(IV) (9) phthalocyanines are described for the first time in this study. Electrochemical and spectroelectrochemical measurements exhibit that while copper phthalocyanine gives only ring-based electron transfer reactions, incorporating redox active metal centers, Co(II), and Ti(IV)OPc, into the phthalocyanine core extends the redox richness of the phthalocyanine ring with the metal-based reduction and oxidation couples of the metal centers in addition to the common phthalocyanine ring-based electron transfer processes. In-situ electrocolorimetric measurements of the complexes allow quantification of color coordinates of the each electrogenerated anionic and cationic redox species.

Investigation of the anion uptake properties of cathodically electropolymerized poly(4-vinylpyridine) membranes

The cathodic electropolymerization conditions for poly(4-vinylpyridine) and the uptake characteristics of anions were evaluated with respect to their application to electrochemical sensing. The membranes were deposited from acidified acetonitrile solutions to minimize hydrogen gas evolution, which is prevalent in aqueous media at the applied potential range. Mid-infrared spectroscopic studies resulted in quantitative data relating the amount of monomer protonated with respect to the added acid, while 1H nuclear magnetic resonance spectroscopy provided insight into the resonance form preferentially adopted by the protonated monomer. The presence of defects in poly(4-VP) films was indirectly measured by monitoring the obstructed diffusion of cationic redox species through the film, and by correlating these electrochemical studies with mid-infrared spectroscopic analysis it became possible to determine the protonated-to-unprotonated monomer ratio yielding uniformly coated electrodes. Finally, the diffusion of Fe(CN)64− through poly(4-VP)-coated electrodes was evaluated as a function of pH, resulting in the conclusion that, unlike similarly deposited poly(2-VP) membranes, poly(4-VP) does not possess the loading capacity required for pre-concentration of anions.

PH- and Ionic Strength-Controlled Cation Permselectivity in Amine-Modified Nanoporous Opal Films

The influence of pH and ionic strength on permselective transport in nanoporous opal films prepared from 440 nm silica spheres was investigated by cyclic voltammetry in aqueous and acetonitrile solutions. Three-layer opal films were deposited from a 1.5 wt % colloidal solution of silica spheres onto 25-microm-diameter Pt microdisk electrodes shrouded in glass. The films were chemically modified by immersing them in a dry acetonitrile solution of 3-aminopropyl triethoxysilane. When the surface amino groups of the modified opal films are protonated and there is little or no supporting electrolyte present in solution, the flux of cationic redox species through the opal membrane is blocked because of electrostatic repulsion. The permselectivity is pH-dependent and can be modulated by adjusting the Debye screening length within the nanopores of the opal by changing the ionic strength of the contacting solution.

Chemically Modified Opals as Thin Permselective Nanoporous Membranes

Thin-film opals comprising three layers of 440 nm diameter SiO2 spheres were assembled on Pt electrodes and modified with amino groups on the silica surface. Diffusion of anionic, cationic, and neutral redox species through the opals was studied by cyclic voltammetry. The chemically modified opal membranes demonstrate high molecular throughput and, at low pH, selectively block transport of a cationic redox species relative to that of anionic and neutral redox species. This permselective behavior is attributed to the electrostatic interactions that are enhanced by the tortuous pathway within the opal and by the high surface area of the chemically modified spheres.

Potential shifts at electrodes coated with ion-exchange polymeric films

Voltammetry at electrodes modified with ion-exchange polymers, named “ion exchange voltammetry”, has been recently developed for characterizing and determining quantitatively ionic electroactive analytes preconcentrated at the electrode surface. Like for other voltammetric techniques, characterization is based on the position of the response on the potential scale, but an appreciable difference is frequently observed between the formal half-wave potential for redox couples incorporated within ion-exchange polymeric films and those for the same redox couples in solution as measured at bare electrodes. Such a difference has been rationalized here by a generalized equation, inferred from a suitable elaboration of the Nernst equation, whose validity has been tested by a thorough investigation performed at glassy carbon electrodes modified with either cationic (Nafion) or anionic (Tosflex) polymeric films. With this purpose, the effect of both charge and concentration of the analyte and of the loading counterion, this last introduced as the cation or anion of the supporting electrolyte, of the ion-exchange selectivity coefficients of the redox partners and of their stoichiometric coefficients, as well as of the number of electrons involved in the charge transfer has been evaluated. The results obtained agree quite well with theoretical expectations and indicate that the potential shifts found are mainly conditioned by both charge and concentration of the counterion from the supporting electrolyte and by the ratio of the ion-exchange equilibrium constants for the two redox partners involved. Other parameters considered have no influence on the potential shift or lead to negligible effects, provided that the quantities of the redox partners incorporated within the ion-exchange coating represents less than 5% of the film capacity. Again in agreement with theoretical expectations, positive shifts are found for increasing supporting electrolyte concentrations when cationic redox species incorporated within cationic films are involved, while the opposite effect is found for anionic redox species incorporated within anionic films.

Perfluoro anion-exchange polymeric films on glassy carbon electrodes

Solutions of the perfluoro anion-exchange membrane TosHex® in a solvent mixture composed of methanol + isopropanol + water (1:1:1) were prepared and applied in coating glassy carbon electrodes. The evaporated films were used to accumulate the Fe(CN) 3− 4− 6 redox couple on the electrode surface. The magnitude of the electrochemical response of the loaded films is comparable with that for Nafion® incorporated cationic redox species. The multicharged Fe(CN) 3− 4− 6 couple accumulated in Tosflex® film causes an ion cross-linking of the polymeric backbone, thus decreasing ion transport in the film substantially.