Electroactive films

The electrochemistry of a polymer-modified electrode is determined by a combination of thermodynamics and the kinetics of charge-transfer and transport processes. Thermodynamic aspects are highlighted by cyclic voltammetry, while kinetic aspects are best studied by other methods. These methods will be introduced here, with the emphasis on how they are used to measure the rates of electron and ion transport in conducting polymer films. Charge transport in electroactive films in general has recently been reviewed elsewhere.9,11... 

Furthermore, it has been demonstrated that the successful electrocatalytic reduction of C02 with [Ru(bpy)2(CO)2]2+ in aqueous MeCN is mainly due to the formation of a polymeric electroactive film, which occurs during the reduction of the complex.91 This film is composed of an open cluster polymer [Ru(bpy)(CO)2]ra (Scheme 6) based upon extended Ru°—Ru° bonds. Electropolymerization of [Ru(bpy)2(CO)2]2+ results from the overall addition of two electrons per mole of [Ru(bpy)2(CO)2]2+ and is associated with the decoordination of one bpy ligand (Equation (33)). 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

A very important electrochemical phenomenon, which is not well understood, is the so-called memory effect. This means that the charging/discharging response of a conducting polymer film depends on the history of previous electrochemical events. Thus, the first voltammetric cycle obtained after the electroactive film has been held in its neutral state differs markedly in shape and peak position from subsequent ones [126]. Obviously, the waiting time in the neutral state of the system is the main factor determining the extent of a relaxation process. During this waiting time, which extends over several decades of time (1-10 s), the polymer slowly relaxes into an equilibrium state. (Fig. 13) After relaxation, the first oxidation wave of the polymer appears at more... 

Ferrocenyl dendrimers also afford electroactive films on indium tin oxide (ITO) electrodes in the same manner as described above. UV-visible spectroelectro-chemical measurements of this modified electrodes on oxidation show changes characteristic for the formation of fenocenium cations. Thus, Figure 8 shows the UV-visible absorption spectrum of a film of 2 electrodeposited on a transparent ITO electrode, which exhibits a strong band at 260 nm and a weak absorption band centered at 600 nm, which agree with those observed for the cationic dendrimer [2 KPF j ]g in solution described above. 

Numerous bisthiols have been observed to form spontaneously multilayers on gold and silver on the basis of the oxidative formation of disulfides.15-27 Nonetheless, most of these compounds lack electroactive character, with few notable exceptions.23,27 In principle, the introduction of redox centers at the core of these molecules and their subsequent assembly into multilayers can be exploited to generate electroactive films. The concentration of redox centers within the resulting electrode coatings, as well as their thickness, can be significantly larger than those possible with electroactive thiols such as 1-4 (Fig. 7.1).11 14 In addition, the transition from electroactive monolayers to electroactive multilayers can translate into a significant enhancement in stability and a much more effective protection of the electrode surface. 

To assess the electron transport properties of our electroactive films, we applied a potential step from 0 to —0.65 V versus Ag/AgCl to an electrode coated with... 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

Voltammetry provides a powerful insight into the effect of the applied potential on the surface coverage, the free energy of adsorption, and the associated kinetics for electroactive films that form on electrode surfaces by irreversible adsorption. 

The use of surfactant films which resemble lipid bilayers and multilayers represents an important approach to preparing electroactive films of protein molecules. 

Important advantages are gained if proteins are immobilized at the electrode surface, hence giving an electroactive film that is ideally of monolayer coverage. This approach has been termed protein film voltammetry and one of its major advantages is that microscopic quantities of the protein are required and thermodynamic and kinetic information can be obtained with higher accuracy and resolution. 

Shi et al. [70] were the first to demonstrate the use of an air and moisture stable ionic liquid, [C4mim][PF,s], for the electrochemical synthesis of poly(thiophene), grown onto a platinum working electrode by potentiodynamic, constant potential or constant current techniques. The use of growth potentials between 1.7 and 1.9 V (vs. Ag/AgCl) reportedly gave smooth, blue-green electroactive films, whereas potentials above 2 V resulted in film destruction by overoxidation. 

Endres et al. [82] have demonstrated the suitability of an air- and water-stable ionic liquid for the electropolymerization of benzene. This synthesis is normally restricted to media such as concentrated sulfuric acid, liquid SO2 or liquid HF as the solution must be completely anhydrous. The ionic liquid used, l-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, can be dried to below 3 ppm water, and this ionic liquid is also exceptionally stable, particularly in the anodic regime. Using this ionic liquid, poly(para-phenylene) was successfully deposited onto platinum as a coherent, electroactive film. Electrochemical quartz crystal microbalance techniques were also used to study the deposition and redox behavior of the polymer from this ionic liquid (Section 7.4.1) [83]. 

Generation of polyoxyethyl-vinylogous tetrathiafulvalene electroactive films in the electropolymerization reaction of substituted dithiafulvenes was described <2007MI677>. 

The vinyl derivative 4-methyl-4 -vinyl-bpy is valuable for chemical electrode modification because its ruthenium and osmium complexes can be polymerized to generate electroactive films with variable properties.86 This unsymmetric ligand was prepared in 35% overall yield from 4,4 -dimethyl bipyridine by first lithiating one methyl group and quenching the anion with (chloromethyl)methyl ether, then reacting with potassium t-butoxide to effect elimination.87,88... 

Surface chemistry, in general, is an area in which the ability to selectively modify the chemical and physical properties of an interface is highly desirable. The synthetic chemistry of surfaces is now in a developing stage, particularly with respect to the attachment of electroactive redox sites to metal or semiconductor surfaces (L-3). Single component and bilayer (4) electroactive films have been a field of intense research activity since their applications are apparent in catalysis, solar energy conversion, directed charge transfer, electrochromic devices, and trace analysis. 

Deposition of a non-electroactive film on the surface of an electrode blocks the electron transfer from solution-based ions to the electrode. The efficiency of such blocking depends on the permeability of the film and the nature and density of defects, and heterogeneous electron transfer is routinely used to address these problems366. Capacitance measurements of the blocked electrodes also give valuable information about the thickness and integrity of the monolayer. These applications are described in Section II.D. 

Otero L, Silber JJ and Sereno L (1991) Electrooxidation of p-carotene in chlorinated solvents. The formation of an electroactive film on gold electrodes. J Electroanal Chem319 415-422... 

The chief problem posed by solid voltammetrlc electrodes Is the lack of reproducibility arlelng from the eeneor losses by blocking, poisoning or fouling. This problem Is completely overcome by automation. The conditioning of the electrode surface, Its coverage with an electroactive film and Its cleanup can be carried out In a more or less automated fashion. 

Direct registration of DNA hybridization advanced a lot with the use of electroactive films of conductive polymers as electrodes. Conductive polymers, which consist of conjugated backbones that are easily oxidized or reduced (doped) with a concomitant increase or decrease in conductivity, with each polymer having its own redox characteristics. 

Investigated examples include the determination of the spatial distribution of a polymer, solvent and mobile species in poly( -toluidine) [983, 984] and polybithiophene [989] films, film swelling and solvent content in electroactive films containing transition metal complexes [988, 985], postdeposition modified electroactive polymers [986] and organic adsorbate layers [987]. The method allows also the investigation of buried interfaces in bilayer systems of various polymers [988]. 

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