Substituent Effects on the Electrochemical Properties of Pyrroles and Small Oligopyrroles (original) (raw)
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The Journal of Chemical Physics, 2008
Polypyrrole is a conjugated polymer prototype of conducting polymers. The energetically preferred spatial conformation of n-pyrrole oligomers ͑n =1-24͒ in both the reduced and oxidized phases is obtained and analyzed in this paper within the hybrid density functional theory. Binding energies, gap energies, radius of gyration, end-to-end distance, and vibrational frequencies are reported as functions of oligomer length. Reduced n-pyrrole are bent chains for all sizes showing a dramatic departure from planarity. Vibrational spectra of n-pyrrole oligomers indicate the presence of two fairly size-insensitive frequency regions, which increase in intensity with increasing oligomer size. Several oxidation levels were analyzed for n-pyrrole through the distribution of the carbon-carbon bond orders and single/double bond lengths. It is shown that the oxidation level is directly related to the way positive charge localizes along the n-pyrrole oligomer chain. If charge/n Ͻ 1 / 3, the oligomers are bent and charge is delocalized; if charge/n ജ 1 / 3, the oligomers are planar and charge notoriously localizes in n/charge regions along the backbone. Calculations with electronegative dopants show that charge localizes in the neighborhood of the dopant. It is demonstrated that one localized state in the gap between the highest occupied and lowest-unoccupied states appears for every +2e in the oxidation level. The band structure of infinite reduced polypyrrole gives a band gap energy in excellent agreement with experiment. The evolution of the band gap and the charge-localized band as a function of polypyrrole oxidation level is reported.
Electrochemical study of electroactive reagent retention in overoxidised polypyrrole films
Analytica Chimica Acta, 1999
The analytical performance of polypyrrole (PPy) films containing immobilised chelating reagents in stripping voltammetrytype applications is unsatisfactory in terms of the detection limits obtainable. This may be attributed to the large background currents due to the presence of a conducting PPy material on the working electrode surface. Overoxidation of PPy films is known to destroy their electronic conductivity. An electrochemical study of chelating reagent retention in overoxidised PPy (OPPy) has been undertaken in order to ascertain the retention, or otherwise, of such reagents in the OPPy film. Two electroactive reagents, 3-(2-pyridyl)-5,6-diphenyl-4,4 -disulfonate-1,2,4-triazine (PDTDS 2− ) and anthraquinone-2,6-disulfonate (AQDS 2− ), have been studied. These reagents were detected in the conducting (as grown) PPy film, exhibiting expected pH and sweep rate dependencies. The molar ratios of the pyrrole monomer-to-reagents in such films [11.8 (±1.9) : 1 for the PDTDS 2− system and 6.9 (±1.1) : 1 for the AQDS 2− system] were close to that expected for a PPy film with a 0.25 oxidation level per pyrrole moiety doped with di-anions. The reduction currents for the redox-active agents in the films after overoxidation were much decreased compared to prior to overoxidation. The molar ratios of pyrrole monomer-to-reagent in the OPPy films were not constant; rather, the amount of reagent detectable, via integration of the voltammetric peak, remained constant with increasing amounts of deposited pyrrole. This can be due to: (i) trapping of the anions in polymer regions inaccessible to the electrode surface so that they become immobile and cannot diffuse through the polymer for detection at the underlying electrode; (ii) the electroinactivity of OPPy, meaning that no redox species are available in the film to shuttle charge to/from the trapped agent at locations remote from the electrode surface; or (iii) expulsion of the anions from the film.
Electrochemical oxidation of pyrrole derivatives in alcoholic medium
Collection of Czechoslovak Chemical Communications, 1989
Electrochemical oxidation of pyrrole (I), 1-methylpyrrole (II), 1,2,5-trimethylpyrrole (III), methyl 1-methyl-2-pyrrolecarboxylate (IV) and diethyl 3,5-dimethyl-2,4-pyrroledicarboxylate (V) has been studied. An advantageous method of preparation of polypyrroles (PP) and conductive PVC-PP composites has been elaborated, permitting a 20 fold starting concentration of the monomer. Electrooxidation of II in methanol leads either to 5,5-dimethoxy-1-methyl-3-pyrrolin-2-one (VII) or 1-methyl-2,2,5,5-tetramethoxy-3-pyrroline (VI), their ratio depending on water content in the alcohol used. Oxidation of IV affords the analogous 5-carbomethoxy-5-methoxy-1-methyl-3-pyrrolin-2-one (X), oxidation of III and V leads to products of substitution at the methyl groups.
Oxidative coupling and polymerization of pyrroles
Electrochimica Acta, 2005
The electrochemical oxidation of 2,4-dimethyl-3-ethylpyrrole in acetonitrile has been studied using cyclic voltammetry, constant current coulometry, preparative electrolyses and ab initio calculations. The product analysis after the preparative electrolyses was carried out by HPLC combined with UV-vis and electrospray ionization MS detection. The aim of the work was to address some of the unresolved problems in the oxidative oligomerization and polymerization of alkylpyrroles. The title compound was chosen as a model for studies of pyrroles that are more basic than the solvent-supporting electrolyte system and for that reason are forced to serve as the base accepting the protons released during the coupling steps. The voltammograms obtained by cyclic voltammetry at a substrate concentration of 2 mM and voltage scan rates between 0.02 and 2 V s −1 showed a characteristic trace-crossing phenomenon that could be demonstrated by digital simulation to be related to that fact that the deprotonations of the initially formed dimer dication are slow with second order rate constants in the range 10 3 -10 4 M −1 s −1 . The relative stability of the different tautomers of the protonated pyrrole monomer and the corresponding 2,2 -dimer was determined by ab initio calculations at the RHF 6-31G(d) level. The studies also included investigations of the effects resulting from addition of a non-nucleophilic base, 2,6-di-tertbutylpyridine, to the voltammetry solutions. The major product observed after preparative electrolyses was a trimer the structure of which is proposed to include a central 2H-pyrrole unit. Since 2H-pyrroles are stronger bases than the corresponding 1H-pyrroles, the trimer is effectively protected against further oxidation by protonation. Two other trimers were observed as minor or trace products as well as a 1H,2H-dimer and several tetramers, also in trace amounts. In addition to the dimer, the trimers and the tetramers, a number of other minor products could be detected. These could all be traced back to the nucleophilic attack by residual water on the radical cations or dications of the 2,2 -dimer and the trimers. The results obtained by constant current coulometry are in agreement with the formation of a 2H-pyrrole based trimer as the major product.
Electrochemical studies of fused-pyrrole systems
Synthetic Metals, 1987
An electrochemical study of fused pyrroles, i.e., 1,4-dihydropyrrolo-[3,2-b] pyrrole (PP), 3,6-di-t-butyl-l,4-dihydropyrrolo[ 3,2-b]pyrrole (BPP) and dimethyl 1,4-dihydro-3,6-di-t-butyl-pyrrolo [ 3,2-b] pyrrole-1,4-dicarboxylate (BMPP), was carried out. PP could be polymerized electrochemically on In-Sn oxide conducting glass (ITO), basal-plane pyrolytic graphite, Pt or Au electrodes, while BPP and BMPP could not be electropolymerized. The polymeric film (PPP) prepared from PP was electroactive in both acetonitrile and aqueous solutions. The morphology of PPP films was found to depend on the supporting electrolytes used. The conductivity of the film was 5 × 10-s S cm-1 just after its preparation and decreased gradually when exposed to air. The film thickness (¢) was linearly related to the amount (Q) of charge passed in the polymerization, with Q in the range 5 to 150 mC cm-2 (slope of the ¢ versus Q plot: 4 pm C-1 cm2). The PPP film was found to undergo degradation via electrochemical oxidation at the electrode and chemical oxidation by oxygen in air. Furthermore, the physical and electrochemical properties are compared with those of polypyrroles, polymers of other fused heterocycles, etc.
Polymer, 2006
This work reports a theoretical investigation about the structural and electronic properties of polymers constituted by pyrrole and Nhydroxymethylpyrrole in both neutral and p-doped states. Ab initio quantum mechanical calculations were performed on neutral and positively charged oligomers to evaluate the bond length alternation pattern in the p-system, the molecular conformation, the p-p* transition energies and the ionization potential. Results, which have been extrapolated to infinite polymer chains, allow analyze the influence of N-hydroxyalkylation of polypyrrole on these properties.
The Journal of Physical Chemistry B, 1997
A simple model of polymeric relaxation, associated with the electrochemical switching of polypyrrole films between their reduced (insulating) and oxidized (electronically conducting) states, offers a reasonably precise description of the form of chronoamperograms obtained after previous subjection of the polymer film to cathodic potentials (which control the compactness of the neutral polymer) for long periods of time. The opening of the structure driven by the anodic potential is not uniform: nucleation of conducting zones inside the neutral polymer and their overlap at long times of anodic polarization are taken into account in the model. Diffusion of counterions from the solution across the oxidized zones is also included. The definition of a relaxation time (depending on both cathodic and anodic overpotentials and on temperature), and the inclusion of nucleation and diffusion processes, allows a theoretical simulation of chronoamperograms, in good agreement with experimental results from potential steps performed by changing the anodic and cathodic limits or at different temperatures.
Electrochemical properties of the polypyrrole films doped with benzenesulfonate
Synthetic Metals, 2007
The electrochemical redox behavior of the polypyrrole films doped with benzenesulfonate was investigated by cyclic voltammetry, the surface morphology of the films was characterized by AFM, and the interactions between pyrrole oligomers and the benzenesulfonate anion were modeled with quantum chemical methods. It is the first systematic study of the redox properties of this interesting system, somewhat of a model system with two complementing interactions (electrostatic and aromatic stacking). The influence of the electrodeposition charge and current density on the properties of the polymer film is explored. The voltammetric measurements show that redoxactivity of the polypyrrole films doped with benzenesulfonate anions is quite high, and markedly depends on the thickness of the film. Experiments with bilayered films show that electrode surface has an important but limited impact on the formation of organized compact structures.
Polypyrrole oxidation: Kinetic coefficients, activation energy and conformational energy
Synthetic Metals, 2009
Polypyrrole films after reduction for a constant time at rising cathodic potentials were used as initial states for subsequent oxidation. The potentiostatic oxidation occurs, at intermediate times, under chemical kinetic control. Rate coefficients (k), activation energies (E a) and reaction orders were determined from experimental results. Otherwise than most of the electrochemical reactions, k and E a change with the reduction potential. The obtained experimental results follow the evolution predicted by the Electrochemically Stimulated Conformational Relaxation model, which states that rising reduced, shrinked and conformational packed states of polymeric conformations are obtained by cathodic prepolarizations at increasing cathodic potentials. Slower oxidation rates, requiring higher activation energies, were obtained when rising packed conformations were used as initial states for the oxidation. The activation energy includes two components: the constant chemical activation energy and the conformational packing energy that increases linearly with the reduction potential. So, k and E a quantify the conformational packing state of reduced polymeric chains, opening new possibilities for the interaction between Electrochemistry and Polymer Science.