Electrochemistry and electrochromic properties of phenanthroline-iron(II/III) complex films

New films of the iron complexes with bis((2-hydroxyphenyl)methylaminosulfonyl)bathophenanthroline(HPBP) and bis((2-aminophenyl)methylaminosulfonyl)bathophenanthroline(APBP) ligands are prepared on the electrode surfaces by electrochemical polymerization. The resulting film-coated electrode shows a well-defined reversible voltammogram corresponding to the redox reaction of the Fe(II/III) complexes and an electrochromic change from red(absorption maximum: 540 nm) to colorless. The response rate of the color change to a potential step was found to be correlated to the apparent diffusion coefficient(Dapp) for the homogeneous charge-transport process within the film. The Dapp values estimated are (3-4) × 10⁻⁹cm²s⁻¹ for the [Fe(APBP)₃] film and(1-2) × 10⁻⁸cm²s⁻² for the [Fe(HPBP)₃] film, respectively, by potential-step chronoamperometric and chronocoulometric methods. The result of electrochemical quartz crystal microbalance(EQCM) measurements⁴) and dependence of the formal potential of the metal complex of the Fe(II/III) redox couple with activity of the supporting electrolyte anion in NaClO₄ aqueous solution showed that anion, cation, and solvent move simultaneously across the polymer film/solution interface during the redox reaction. A piezoelectric admittance measurement⁴⁾ of the poly[Fe(APBP)₃] coated quartz crystal electrode showed that the viscosity of the film is affected by the oxidation state of iron.     

Redox Properties of Ferricyanide Ion on Layer‐by‐Layer Thin Film‐Coated Gold Electrodes; Effects of Type of Polycationic Components in the Film

The surface of a gold (Au) electrode was coated with layer‐by‐layer (LbL) thin films composed of poly(vinyl sulfate) (PVS) and different type of poly(amine)s including poly(allylamine) (PAH), poly(ethyleneimine) (PEI) and poly(diallyldimethylammonium chloride) (PDDA) and redox properties of ferricyanide ion ([Fe(CN)₆]^3?) on the LbL film‐coated Au electrodes were studied. The LbL film‐coated electrodes exhibited redox response to [Fe(CN)₆]^3? ion when the outermost surface of the LbL film was covered with the cationic poly(amine)s while virtually no response was observed on the LbL film‐coated electrodes whose outermost surface was covered with PVS due to an electrostatic repulsion between [Fe(CN)₆]^3? ion and the negatively‐charged PVS layer. The redox properties of [Fe(CN)₆]^3? ion on the LbL film‐coated electrodes significantly depended on the type of polycationic materials in the LbL film. The LbL film‐coated electrodes which had been immersed in the [Fe(CN)₆]^3? solution for 15 min exhibited redox response even in a [Fe(CN)₆]^3? ion‐free buffer solution, suggesting that [Fe(CN)₆]^3? ion is confined in the films. In the buffer solution, redox peaks were observed between +0.1 and 0.4 V depending on the type of polycations in the film. Thus, [Fe(CN)₆]^3? ion can be confined in the film and the redox potential is polycation‐dependent.     

Electrochemically Polymerized N,N-Dimethylaniline Films Containing Tris-(Bathophenanthroline Disulfonato)Iron(II/III) Complexes

The electropolymerization of N, N-dimethylaniline (DMA) was carried out in an aqueous CF₃COONa solution (pH 1.0) containing DMA in the presence of tris(bathophenanthroline disulfonato)iron(II), Fe(bphen)₃ ^4-. Poly(N, N-dimethylanilinium trifluoroacetate) (PDMA) film was formed on electrode surfaces and, at the same time, Fe(bphen)₃ ^4- ions were stably confined in the formed PDMA film by electrostatic interaction between them and the positively charged quaternary ammonium sites of the PDMA film. The PDMA-Fe(bphen)₃ ^4-/3- film thus prepared displayed well-defined reversible electroactivity and electrochromic properties ascribable to those of the Fe(bphen)₃ ^4-/3- couple confined in the film. The PDMA-Fe(bphen)₃ ^4- film is red, and the PDMA-Fe(bphen)₃ ^3- film is colorless. The response rate of the color change to a potential pulse was found to be correlated with the kinetic parameters characterizing the rate of the overall charge-transfer reaction at the PDMA-Fe(bphen)₃ ^4-/3- film-coated electrode, that is, the apparent diffusion coefficient (D _app) for the homogeneous charge-transport process within the film and the standard rate constant (k) of the heterogeneous electron-transfer reaction at the electrode/film interface. For the PDMA-Fe(bphen)₃ ^4-/3- film with larger k° and D _app values, the response rate of the color change was larger, Further, k°, D _app, and response rate depended on the concentration (C°) of the Fe(bphen)₃ ^4- (or Fe(bphen)₃ ^3-) confined in the PDMA film; and at a given film thickness, the lower C°, the higher were k°, D _app, and response rate. At a given C°, the thinner the film thickness, the greater was the response rate.     

The Effect of Adsorption of Ions of the Hexacyanoferrate(II)/(III) Redox Pair on Self-Assembly of Octanethiol at Its Adsorption from Aqueous Solutions on Gold Electrode

The effect of components of the redox pair K₃[Fe(CN)₆]/K₄[Fe(CN)₆] on the dynamics of formation of octanethiol (OT) monolayers from aqueous thiol-containing solutions of 0.1 М NaClO₄ is studied by cyclic voltammetry (CVA). The formation of OT monolayers is shown to depend on the presence of ions of hexacyanoferrate(II)/(III) in solution. Being added to solution, the components of the [Fe(CN)₆]^3–/4– redox pair sharply increase the time of formation of the insulating monolayer OT films and make them less stable. The destabilizing and inhibiting action of [Fe(CN)₆]^3–/4– ions becomes stronger as their concentration in solution increases. The adsorption activity of individual components of the redox pair is assessed. The strong and approximately equal adsorption activity of ions [Fe(CN)₆]^3– and [Fe(CN)₆]^4– on gold in the presence of octanethiol is observed. At the same time, OT and the hexacyanoferrate(II)/(III) ions can be placed in the following row: OT > [Fe(CN)6]^3– ≈ [Fe(CN)₆]^4–. Recommendations are given on how to eliminate the interfering action of the K₃[Fe(CN)₆]/K₄[Fe(CN)₆] redox-pair ions when studying the insulating properties of thiol monolayers on gold.     

Characterization of Anilinepentacyanoferrate (II) and (III)

The anilinepentacyanoferrate (II) complex has been characterized in aqueous solution. The complex exhibits a predominant ligand field transition at λ_max = 415 nm with ?_max = 494 M^?1 cm^?1. The corresponding Fe(III) complex displays a strong absorption at λ_max = 700nm(?_max = 1.61×10⁴ M^?1 sec^?1) which can be assigned as a ligand to metal charge transfer transition. The rate constants of formation and dissociation for the Fc(II) complex are (3.14±0.18)×10² M^?1W^?1 and 0.985±0.005 sec^?1, respectively, at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The cyclic voltammetry of the complex shows that a reversible redox process is observed with E_1/2 value of 0.51±0.01 V vs. NHE at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The kinetic study of the oxidation of the Fe(II) complex by ferricyanide ion yielded the rate constant of the reaction k_et = (1.43±0.04)x10 M sec^?1 at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C.     

Layer-by-layer thin film-coated electrodes for electrocatalytic determination of ascorbic acid

Multilayer thin films composed of poly(allylamine hydrochloride) (PAH) and carboxymethyl cellulose (CMC) have been prepared on the surface of a gold (Au) disk electrode by a layer-by-layer deposition of PAH and CMC and ferricyanide ions ([Fe(CN)₆]³⁻) were confined in the film. [Fe(CN)₆]³⁻ ions can be successfully confined in the films from weakly acidic or neutral [Fe(CN)₆]³⁻ solutions, while, in basic solution, [Fe(CN)₆]³⁻ ion was not confined. The [Fe(CN)₆]³⁻ ion-confined Au electrode showed clear redox peaks in the cyclic voltammogram around 0.35 V versus Ag/AgCl. The amounts of [Fe(CN)₆]³⁻ ions confined in the films depended on the thickness of the films or the number of layers in the LbL films. The [Fe(CN)₆]³⁻ ion-confined Au electrode was used for electrocatalytic determination of ascorbic acid in the concentration range of 1-50 mM.     

Electrochemical Properties of a Conducting Film Derived from Iron(II) Tris(diaminopolypyridyl) Complex in the S(IV) Oxoanions Reduction

A new conducting film derived from the complex [Fe (diaphen)₃]²⁺, (diaphen=5,6‐diamino‐1,10‐phenanthroline) was electropolymerized by cyclic voltammetry onto a glassy carbon electrode. Poly‐[Fe^II (diaphen)₃] was studied by cyclic voltammetry, SEM, UV‐vis and micro‐Raman spectroscopy. Poly‐[Fe^II (diaphen)₃] shows electrocatalytic activity in HSO₃^? reduction in an ethanol/water solution. Electrocatalysis is centered at the π ring of phenanthroline. Rotating disk electrode studies showed a 0.117 V/dec Tafel slope, suggesting an EC process where the electrochemical process is the determining step. The chemical step was studied by UV‐vis spectroelectrochemistry. Amperometric behavior showed a linear range between 47.5 µM to 417 µM and the LOD was 19.5 µM.     

Calorimetric investigations of luminescent films polycarbonate (PC) doped with europium complex [Eu(TTA)<Subscript>3</Subscript>(H<Subscript>2</Subscript>O)<Subscript>2</Subscript>]

Polymers doped with rare earth complexes are advantaged in film production for many applications in the luminescent field. In this luminescent polycarbonate (PC) films doped with diaquatris(thenoyltrifluoroacetonate)europium(III) complex [Eu(TTA)₃(H₂O)₂] were prepared and their calorimetric and luminescent properties in the solid state are reported. The thermal behavior was investigated by utilization of differential scanning calorimetry (DSC) and thermogravimetry (TG). Due of the addition of rare earth [Eu(TTA)₃(H₂O)₂] into PC matrix, changes were observed in the thermal behavior concerning the glass transition and thermal stability. Characteristic broadened narrow bands arising from the ⁵D₀ → ⁷F_J transitions (J = 4−0) of Eu³⁺ ion indicate the incorporation of the Eu³⁺ ions in the polymer. The luminescent films show enhancement emission intensity with an increase of rare earth concentration in polymeric matrix accompanied by decrease in thermal stability.     

Chemical Bonding in Homoleptic Carbonyl Cations [M{Fe(CO)5}2]+ (M=Cu,Ag, Au)

Syntheses of the copper and gold complexes [Cu{Fe(CO)₅}₂][SbF₆] and [Au{Fe(CO)₅}₂][HOB{3,5-(CF₃)₂C₆H₃}₃] containing the homoleptic carbonyl cations [M{Fe(CO)₅}₂]⁺ (M=Cu, Au) are reported. Structural data of the rare, trimetallic Cu₂Fe, Ag₂Fe and Au₂Fe complexes [Cu{Fe(CO)₅}₂][SbF₆], [Ag{Fe(CO)₅}₂][SbF₆] and [Au{Fe(CO)₅}₂][HOB{3,5-(CF₃)₂C₆H₃}₃] are also given. The silver and gold cations [M{Fe(CO)₅}₂]⁺ (M=Ag, Au) possess a nearly linear Fe-M-Fe’ moiety but the Fe-Cu-Fe’ in [Cu{Fe(CO)₅}₂][SbF₆] exhibits a significant bending angle of 147° due to the strong interaction with the [SbF₆]⁻ anion. The Fe(CO)₅ ligands adopt a distorted square-pyramidal geometry in the cations [M{Fe(CO)₅}₂]⁺, with the basal CO groups inclined towards M. The geometry optimization with DFT methods of the cations [M{Fe(CO)₅}₂]⁺ (M=Cu, Ag, Au) gives equilibrium structures with linear Fe-M-Fe’ fragments and D₂ symmetry for the copper and silver cations and D_4d symmetry for the gold cation. There is nearly free rotation of the Fe(CO)₅ ligands around the Fe-M-Fe’ axis. The calculated bond dissociation energies for the loss of both Fe(CO)₅ ligands from the cations [M{Fe(CO)₅}₂]⁺ show the order M=Au (D_e=137.2 kcal mol⁻¹)>Cu (D_e=109.0 kcal mol⁻¹)>Ag (D_e=92.4 kcal mol⁻¹). The QTAIM analysis shows bond paths and bond critical points for the M−Fe linkage but not between M and the CO ligands. The EDA-NOCV calculations suggest that the [Fe(CO)₅]→M⁺←[Fe(CO)₅] donation is significantly stronger than the [Fe(CO)₅]←M⁺→[Fe(CO)₅] backdonation. Inspection of the pairwise orbital interactions identifies four contributions for the charge donation of the Fe(CO)₅ ligands into the vacant (n)s and (n)p AOs of M⁺ and five components for the backdonation from the occupied (n-1)d AOs of M⁺ into vacant ligand orbitals.     

Non‐covalent Immobilization of Iron‐triazole (Fe(Htrz)3) Molecular Mediator in Mesoporous Silica Films for the Electrochemical Detection of Hydrogen Peroxide

Mesoporous silica thin films encapsulating a molecular iron‐triazole complex, Fe(Htrz)₃ (Htrz=1,2,4,‐1H‐triazole), have been generated by electrochemically assisted self‐assembly (EASA) on indium‐tin oxide (ITO) electrode. The obtained modified electrodes are characterized by well‐defined voltammetric signals corresponding to the Fe^II/III centers of the Fe(Htrz)₃ species immobilized into the films, indicating fast electron transfer processes and stable operational stability. This is due to the presence of a high density of redox probes in the material (1.6×10^?4 mol g^?1 Fe(Htrz)₃ in the mesoporous silica film) enabling efficient charge transport by electron hopping. The mesoporous films are uniformly deposited over the whole electrode surface and they are characterized by a thickness of 110 nm and a wormlike mesostructure directed by the template role played by Fe(Htrz)₃ species in the EASA process. These species are durably immobilized in the material (they are not removed by solvent extraction). The composite mesoporous material (denoted Fe(Htrz)₃@SiO₂) is then used for the electrocatalytic detection of hydrogen peroxide, which can be performed by amperometry at an applied potential of ?0.4 V versus Ag/AgCl and by flow injection analysis. The organic‐inorganic hybrid film electrode displays good sensitivity for H₂O₂ sensing over a dynamic range from 5 to 300 μM, with a detection limit estimated at 2 μM.     

Facile cation electro-insertion into layer-by-layer assembled iron phytate films

Molecular layer-by-layer assembly from pre-saturated aqueous solutions of Fe³⁺ and phytate is employed to build up iron phytate deposits on tin-doped indium oxide (ITO) electrodes. Globular films with approximately 1 nm growth per layer are observed by AFM imaging and sectioning. In electrochemical experiments the iron phytate films show well-defined voltammetric responses consistent with an immobilised Fe(III/II) redox system in aqueous (LiClO₄, NaClO₄, KClO₄, phosphate buffer) and in ethanolic (LiClO₄, NaClO₄, NBu₄ClO₄) electrolyte solutions. The Fe(III/II) redox system is reversible and cation insertion/expulsion occurs fast on the timescale of voltammetric experiments even for more bulky NBu₄⁺ cations and in ethanolic solution. Peak shape analysis and scan rate dependent midpoint potentials suggest structural changes accompanying the redox process and limiting propagation. Iron phytate is proposed as a versatile and essentially colourless cation electro-insertion material and as a potential energy storage material.     

Electrochemical and thermodynamic properties of hexacyanoferrate(II)/(III) redox system on multi-walled carbon nanotubes

Novel films consist of multi-walled carbon nanotubes (MWCNT) were fabricated by means of catalytic chemical vapor deposition (CVD) technique with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene (FeCp₂) as catalyst. The electrochemical and thermodynamic behavior of the ferrocyanide/ferricyanide, [Fe(CN)₆]^3−/4− redox couple on synthesized MWCNT-based films was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques at T = (278.15, 283.15, 293.15, and 303.15) K. The redox couple [Fe(CN)₆]^3−/4− behaves quasi-reversibly on fabricated MWCNT-based films and its reversibility is enhanced upon increasing temperature. Namely, the findings establish that with the rise in temperature the barrier for interfacial electron transfer decreases, leading, consequently, to an enhancement of the kinetics of the charge transfer process. According to thermodynamics the equilibrium of the redox process is shifted towards the formation of [Fe(CN)₆]³⁻ at elevated temperatures.     

Factors Determining the Charge Transfer Rate in Polymers Based on Fe(II), Ru(II), and Os(II) Complexes with 5-Chloro-1,10-Phenanthroline

The effect of ionic diffusion on parameters of redox electrical conductivity in poly-[M(5-Cl-phen)₃]^{2 +} (M = Fe, Ru, Os) was studied.     

Cooperative catalysis and critical decomposition distances in water oxidation by tris(ethylenediamine)ruthenium (III) complex confined in a Nafion membrane

The activity of tris(ethylenediamine)ruthenium (III) complex, [Ru(en)₃]³⁺, as a water oxidation catalyst was studied in a homogeneous aqueous solution and a heterogeneous Nafion (Nf) membrane. In the aqueous solution, the apparent catalytic activity (k_app (s⁻¹)) decreased monotonously with the concentration due to a bimolecular decomposition of the complex. The bimolecular decomposition of the complex was remarkably suppressed by incorporating it into a Nf membrane. An optimum complex concentration for k_app in the Nf membrane was exhibited, which was explained both by a cooperative catalysis and a bimolecular decomposition of the complex. The k_app in the Nf membrane was analyzed in terms of an intrinsic catalytic activity (k_O2 (s⁻¹)) of the complex, a cooperative catalysis distance (r_co (nm)) and a critical decomposition distance (r_d (nm)) between them based on intermolecular distance distribution to obtain the k_O2=8.5×10⁻⁵ s⁻¹, r_co=1.44 nm and r_d=1.07 nm. The results in the [Ru(en)₃]³⁺ system were compared with those obtained in the [Ru(NH₃)₆]³⁺ system.     

Potentiometric flow injection determination of manganese(II) by using a hexacyanoferrate(III)-hexacyanoferrate(II) potential buffer

A highly sensitive potentiometric flow injection analysis method for the determination of manganese(II), utilizing a redox reaction with hexacyanoferrate(III) in near neutral media containing ammonium citrate is described. The analytical method is based on the detection of the change in potential of a flow-through type redox electrode detector, resulting from the composition change of an [Fe(CN)₆]³⁻-[Fe(CN)₆]⁴⁻ potential buffer solution. A linear relationship between the potential change (peak height) and the concentration of manganese(II) was found. Manganese(II) in a wide concentration range from 10⁻⁴ to 10⁻⁷ M could be determined by appropriately altering the concentration of the potential buffer from 10⁻³ to 10⁻⁵ M. The lower detection limit of manganese(II) was determined to be 1×10⁻⁷ M. The sampling rate and relative standard deviation were 20 h⁻¹ and 1.9% (n=8) for 6×10⁻⁶ M manganese(II), respectively. The proposed method was successfully applied to the determination of manganese(II) in actual soil samples obtained from tea fields. Analytical results obtained by the proposed method were in good agreement with those obtained by an atomic absorption spectrophotometric method.     

Differential pulse voltammetric determination of P(V) following adsorptive accumulation of alpha-[PMo(12)O(40)](3-) on a polypyrrole-modified glassy carbon electrode

On the basis of the formation and pre-concentration of an α-Keggin-type [PMo₁₂O₄₀]³⁻ complex, a novel voltammetric method was developed for the determination of trace levels of P(V). The α-[PMo₁₂O₄₀]³⁻ complex was formed by heating a 5×10⁻⁴ M Mo(VI)-0.2 M HCl-40% (v/v) CH₃CN system containing a trace amount of P(V) at 70 °C for 30 min. During the electrochemical polymerization of pyrrole in the α-[PMo₁₂O₄₀]³⁻ solution, the α-[PMo₁₂O₄₀]³⁻ complex was accumulated into the polypyrrole film on a glassy carbon electrode. The differential pulse voltammetric peak current due to the α-[PMo₁₂O₄₀]³⁻ complex incorporated in the polypyrrole film was linearly dependent on the P(V) concentration in the range of 5×10⁻⁹-5×10⁻⁷ M; a detection limit of 2×10⁻⁹ M was achieved.     

Preparation and spectroscopic characterization of iron(II) and copper(II) complexes of a functionalized crown ether

Condensation between 4′-aminobenzo-15-crown-5- and 4-antipyrinecarboxaldehyde yielded the functionalized crown ether (L = 1,5-dimethyl-4-[(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-15-ylimino)methyl]-2-phenyl-1,2-dihydro-3H-pyrazol-3-one). A 1:1 (Na⁺:L) complex has been prepared. The reaction of Fe(II) and Cu(II) salts with L gave complexes of composition [Fe(L)Cl₂] and [Cu(L)₂Cl₂]. Heteronuclear complexes [Fe(L)Cl₂Na]ClO₄ and [Cu(L)₂Cl₂Na]ClO₄ have also been synthesized from the reactions of [Fe(L)Cl₂] and [Cu(L)₂Cl₂] with NaClO₄. The compounds have been characterized by microanalyses and spectroscopic methods.     

Investigation into Charge Transport Dynamics of [Os(bpy)2(picolinate)]Cl Nafion Films

This work examines the charge transport properties of redox films of the immobilized enzyme mediator [Os(bpy)₂(picolinate)]Cl ([Os(bpy)₂Pic]⁺). Chronoamperometry was used to calculate D_CT values (typically of the order of 1.5×10^?8 cm² s^?1) and the effect of %loading of redox material was determined for three electrolytes over a range of concentrations. The data obtained implies that charge transfer within the film occurs via a redox site diffusion mechanism. The concentration of redox sites was determined using an approximate film thickness of 7 μm, as determined from profilometry studies. Experiments were also performed using immobilized carbon nanotubes within the redox film in order to examine the impact of the increased surface area and conductivity. The presence of the carbon nanotubes had the effect of doubling the surface coverage values and enhancing D_CT^1/2 C_M values from 1.9×10^?9 to 16.1×10^?9 mol cm^?1 s^?1.     

Preparation of Thallium Hexacyanoferrate Film and Mixed‐Film Modified Electrodes with Cobalt(II) Hexacyanoferrate

Thallium hexacyanoferrate films have been prepared from various aqueous electrolyte solutions using consecutive cyclic voltammetry. The cyclic voltammograms recorded the direct deposition of thallium hexacyanoferrate films from the mixing of Tl³⁺ and [Fe(CN)₆]^3? ions from solutions of seven cations: Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, H⁺, and Tl⁺. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of the thallium hexacyanoferrate films. The thallium hexacyanoferrate film shows a single redox couple with a formal potential between +0.6 V and +1.2 V, and shows a cation effect (H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and Tl⁺). A mixed film and a two‐layered modified electrodes composed of a thallium hexacyanoferrate film with cobalt(II) hexacyanoferrate film were prepared.     

Effects of gamma radiation on the photoluminescence properties of polycarbonate matrices doped with terbium complex

Luminescent films containing terbium complex [Tb(acac)₃(H₂O)₃] (acac=acetylacetonate) doped into a polycarbonate (PC) matrix were prepared and irradiated at low-dose gamma radiation with ratio of 5 and 10 kGy. The PC polymer was doped with 5% (w/w) of the Tb³⁺ complex. The thermal behavior was investigated by utilization of differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). Changes in thermal stability due to the addition of doping agent into the polycarbonate matrix. Based on the emission spectra of PC:5% Tb(acac)₃ film were observed the characteristic bands arising from the ⁵D₄→⁷F_J transitions of Tb³⁺ ion (J=0–6), indicating the ability to obtain the luminescent films. Doped samples irradiated at low dose of gamma irradiation showed a decrease in luminescence intensity with increasing of the dose.