Gianni Zotti

Cyclic potential sweep electropolymerization of aniline: The role of anions in the polymerization mechanism

Abstract The cyclic potential sweep (CPS) method was applied to aniline electropolymerization in several strong acids (H 2 SO 4 , HNO 3 , HCl, HBF 4 , HClO 4 and CF 3 COOH). It is mainly the type of anion that determines the morphology of polyaniline (PANI) deposits, promoting either a compact (BF − 4 , ClO 4 − and CF 3 COO − , class 1 anions) or an open structure (SO 4 2− , NO 3 − and Cl − , class 2 anions), as suggested by the linear (class 1) or quadratic (class 2) dependence of deposition charges on number of cycles, scan rate and concentration of anilinium and anion, as well as by SEM analysis. From the potential dependence of deposition currents, PANI growth onto PANI is suggested to occur via adsorption of anilinium-anion ionic couples onto fully oxidized (pernigraniline) sites of PANI.

Electrosynthesis and properties of ring-substituted polyanilines☆

Abstract Electrochemical polymerization of ring-substituted anilines, with electron donor and acceptor substituents in the 2- and 3-positions, was investigated in 2.0 M sulfuric acid. Polymerization is mostly effective with sigma- and pi-donor substituents, the latter in the 2-position. Polymeric deposits were obtained from 2- and 3-methylaniline (toluidines), 2-methoxyaniline (anisidine) and 2-chloroaniline, and were characterized by cyclic voltammetry, elemental analysis, ir spectroscopy, uv—vis spectroelectrochemistry and electrical conductivity. Polytoluidine and polyanisidine are reversibly oxidized in two steps as polyaniline, whereas polycholoroaniline is oxidized in a single one-electron process. Polymerization was found to be more effective on polyaniline-modified electrodes.

Chemical and electrochemical synthesis and characterization of polydiphenylamine and poly-N-methylaniline

Abstract Chemical and electrochemical oxidation of diphenylamine and N -methylaniline in acid produces polyaniline-like polymers, which can be characterized by elemental analysis, cyclic voltammetry, IR, UV-visible and mass spectroscopy. Diphenylamine undergoes para C-C coupling to oligomers ( n = 3-4), while N -methylaniline couples C-N at the para position, attaining a high degree of polymerization. The conductivity of polydiphenylamine (3 × 10 −2 Ω −1 cm −1 ) increases when polymerization is performed in acetonitrile (0.5 Ω −1 cm −1 ), in which n increases to 10–11. Poly- N -methylaniline has a conductivity of 1 × lO −3 Ω −1 cm −1 . The polymers are oxidized reversibly from the yellow (leuco) to the green form (emeraldine) and finally to the blue (pernigraniline) form in two steps, each involving 0.5 electrons per monomeric unit. With increasing chain length and conductivity, the electrochemical stability of the emeraldine form of polydiphenylamine also increases.

Conducting polymers from anodic coupling of some regiochemically defined dialkoxy-substituted thiophene oligomers

Abstract Electrochemical polymerization of thiophene oligomers ( n = 2−4) bearing pentoxy groups in regiochemically defined positions (3 or 4 of the terminal thiophene rings) was performed in acetonitrile. Cyclic voltammetry and UV-Vis spectroscopy showed that the degree of polymerization decreases as the monomer length increases and this result is accounted for by the corresponding decrease of the coupling rate constant, measured by cyclic voltammetry for the less reactive 3-substituted series. Compared with alkyl substitution, alkoxy substitution induces a smaller bandgap and lowers the oxidation potential, therefore giving the polymer a higher stability of the doped state. However, no variation in the effective conjugation length was found when alkoxy groups were present in head-to-head or tail-to-tail linkages. It is suggested that the presence of the oxygen atom not only minimizes the steric hindrance but also plays a role in planarizing the polymer chain.

Electrodeposition of polythiophene, polypyrrole and polyaniline by the cyclic potential sweep method

The cyclic potential sweep (CPS) technique was applied to deposition of electroactive polymers and tested on polythiophene (PT) and polypyrrole (PP), both in acetonitrile, and polyaniline (PANI) in sulfuric acid. The relationship between deposition charge Qi and the CPS parameters (number of cycles N, scan rate v and switching potential Eλ) has the simple form Qi ~ (N/v)n exp(aEλ), with n =1 for PT and PP and n = 2 for PANI. The dependence on monomer concentration is analogously square for PANI and linear for PT; for PP, the dependence is linear only in the presence of water, which acts as a scavenger of electrogenerated protons. The growth mechanism is reflected by n (over a substrate of constant area for n = 1 and expanding linearly in time for n = 2). The CPS relationships are explained satisfactorily by chronoamperometric results.

Efficient π electrons delocalization in α,α′‐dimethyl end‐capped oligothiophenes: A vibrational spectroscopic study

α,α′‐dimethyl substituted oligothiophenes with increasing chain length (from the dimer up to the hexamer) were recently synthesized by chemical methods. In this paper we have investigated the vibrational Fourier transform‐IR and Fourier transform‐Raman spectra of solid α,α′‐dimethyl substituted oligothiophenes in the neutral state. The data are consistent with the existence of a chain‐length dependent π electron delocalization: a large frequency dispersion with conjugation length is observed for some Raman and infrared active vibrational modes, particularly at the high‐energy side of the aromatic C=C stretching region. Vibrational assignments are proposed for the main vibrational features in the whole spectral range. This vibrational spectroscopic analysis of the solid samples thus becomes a tool for deriving information on the structure of these neutral materials in solution and in the doped state.

Monolayers and multilayers of conjugated polymers as nanosized electronic components.

Conjugated polymers (CPs) are interesting materials for preparing devices based on nanoscopic molecular architectures because they exhibit electrical, electronic, magnetic, and optical properties similar to those of metals or semiconductors while maintaining the flexibility and ease of processing of polymers. The production of well-defined mono- and multilayers of CPs on electrodes with nanometer-scale, one-dimensional resolution remains, however, an important challenge. In this Account, we describe the preparation and conductive properties of nanometer-sized CP molecular structures formed on electrode surfaces--namely, self-assembled monolayer (SAM), brush-type, and self-assembled multilayer CPs--and in combination with gold nanoparticles (AuNPs). We have electrochemically polymerized SAMs of carboxyalkyl-functionalized terthiophenes aligned either perpendicular or parallel to the electrode surface. Anodic coupling of various pyrrole- and thiophene-based monomers in solution with the oligothiophene-based SAMs produced brush-like films. Microcontact printing of these SAMs produced patterns that, after heterocoupling, exhibited large height enhancements, as measured using atomic force microscopy (AFM). We have employed layer-by-layer self-assembly of water-soluble polythiophene-based polyelectrolytes to form self-assembled multilayers. The combination of isostructural polycationic and polyanionic polythiophenes produced layers of chains aligned parallel to the substrate plane. These stable, robust, and dense layers formed with high regularity on the preformed monolayers, with minimal interchain penetration. Infrared reflection/adsorption spectroscopy and X-ray diffraction analyses revealed unprecedented degrees of order. Deposition of soluble polypyrroles produced molecular layers that, when analyzed using a gold-coated AFM tip, formed gold-polymer-gold junctions that were either ohmic or rectifying, depending of the layer sequence. We also describe the electronic conduction of model alpha,omega-capped sexithiophenes featuring a range of electron donor/acceptor units and lengths of additional conjugation. The sexithiophene cores exhibit redox-type conductivity, developing at the neutral/cation and cation/dication levels with values depending the nature of the substitution and the redox system. Extending the conjugation beyond the sexithiophene frame introduces further oxidation processes displaying enhanced conductivity. Finally, we discuss the ability of CP-based monolayers to coordinate AuNPs. Although thiophene- and pyrrole-based oligomers aggregate toluene-soluble AuNPs, alkyl substitution inhibits the aggregation process through steric restraint. Consequently, we investigated the interactions between AuNPs and polypyrrole or polythiophene monolayers, including those formed from star-shaped molecules. The hindered aggregation provided by alkyl substituents allowed us to adsorb thiol-functionalized oligothiophenes and oligopyrroles directly onto preformed AuNPs. Novel materials incorporating AuNPs of the same size but bearing different conjugated ends or bridges have great promise for applications in electrocatalysis, electroanalysis, and organic electronics.

Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena

Abstract Irreversible changes occurring in the electrochemical reduction of some polythiophenes in acetonitrile have been investigated by cyclic voltammetry (CV), UV-Vis and IR spectroscopy and the electrochemical quartz crystal microbalance (EQCM). Reduction in the presence of alkali metal cations (Li + , Na + , K + and Cs + ) does not result in n-doping but in hydrogenation of the alkene moieties. The promotion of double-bond saturation by alkali metal cations following the order Li + , Na + > K + > Cs + is attributed to their charge-pinning action which decreases as the ion radius is increased. Reversible reduction in tetralkylammonium electrolyte (n-doping) is accompanied by production of hydroxide ions within the polymer which are thereby inserted by subsequent oxidation. The resulting quinone-like groups are responsible for the CV pre-peaks usually observed in conducting polymers after n-doping and previously assigned to trapped charges.

Steric and electronic effects in methyl and methoxy substituted polyanilines

Various methyl and methoxy monosubstituted and disubstituted anilines were electropolymerized in acidic conditions. From UV—visible absorption and electrochemical measurements, it has been found that methyl substituted anilines give polymers which are less planar than the methoxy derivatives. This has been explained in terms of more pronounced steric effects of the methyl group than those induced by the methoxy group. However, alkoxy monosubstituted anilines exhibit lower conductivities than those obtained with the alkyl derivatives. This has been explained by a less regular structure for poly(alkoxyanilines). Indeed, as shown by the cyclic voltammetry experiments, the selectivity of the head-to-tail couplings is reduced by the incorporation of a methoxy group at the ortho position. However, more regular materials have been obtained with 2,5-disubstituted polyanilines. For instance, poly(2,5-di-methoxyaniline) exhibits a high conjugation length and a good conductivity (ca. 0.3 S cm−1).

Structure-property relationships in polyaniline derivatives

Abstract Different alkyl and alkoxy-substituted polyanilines were electropolymerized in acidic conditions. The steric and electronic effects of the substituents on the electrical and optical properties of the resulting polymers, were principally investigated. It has been found that alkyl-substituted anilines give less planar polymers than the alkoxy derivatives. However, alkoxy mono-substituted anilines exhibit lower conductivities than those obtained with the alkyl derivatives, because the selectivity of the head-to-tail couplings is reduced by the incorporation of a methoxy group at the ortho position. Nevertheless, more regular and processable materials have been obtained with poly(2,5-dialkoxyanilines): for instance, poly(2,5-dimethoxyaniline) exhibits a high conjugation length and a good conductivity (ca. 0.3 S/cm).