Undoped State Research Articles

Electrochromism and electrochemical characterization of soluble poly[3-alkylhetero(arylene)s

Abstract Soluble conducting poly[3-alkyhetero(arylene)s] have been prepared electrochemically from 3-alkylheteroarylene monomers. Optical conditions for the electrochemical polymerization were in acetonitrile and tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) as the solvent and the electrolyte, respectively. Two oxidation potential values ( E ox ) were exhibited in the ranges 0.75–1.12 V and 1.86–2.02 V versus Ag/AgCl, respectively. On the other hand, the polymer oxidation potentials ( E pox ) in monomer-free solution were 1.34–1.42 V versus Ag/AgCl. The electrochemical data of these compounds are compared with those of poly(3-methylthiophene), polythiophene and their derivatives prepared under the same polymerization conditions. The conductivities of the electrochemically grown films were measured using thefour-probe technique before and after exposure to doping agents. The values for the conductivities (σ) of the “as-grown” films were in the order of ≈ 10 −2 –10 ω −1 cm −1 . The infrared (IR) and nuclear magnetic resonance (NMR) spectra of these polymers indicated that the bonding of the monomer units are mainly 2,5 linkages. Solubility tests of these polymers are also given. Thin films formed under constant current density of 1 mA cm −2 (galvanostatically) or cast by evaporation of the solvent on optically transparent electrodes (OTE) displayed electrochromic properties. The film colors are dark blue and brick red in the oxidized (doped) and in the neutral (undoped) states, respectively. The formal potential ( E 0 ′) and n values of the polymer films are reported. The E 0 ′ values of the films in the monomer-free solution were 0.73–0.96 V versus Ag/AgCl. These values were compared to the ( E prmpox ) values from the cyclic voltammetric technique for the same polymer films. The fluorescence spectra of the polymer solutions in different solvents showed an intense broad emission bond at ≈ 410–660 nm.

Isochronal Annealing Study of Positron Trapping Centres in a Polymer Matrix

A positron annihilation study of poly(2,6-dimethyl-1,4-phenylene oxide) under isochronal annealing treatment is carried out both in the undoped state and in the 0.2% iodine-doped state. All the measured lifetime spectra are resolved into 3 components. From these lifetime values and their resp. intensities, trapping rates of positrons in the ordered regions and of positronium in the disordered regions of the polymer matrix are calcd. following the kinetic equations of A. V. Goldanskii et al. (1987). The obsd. variation of trapping rates as well as orthopositronium lifetime and intensity as a function of annealing temp. reflect a glass-rubbery transition at 145° in this polymer both in undoped and doped states. [on SciFinder(R)]

Organic-Inorganic Junctions formed on Porous Silicon: Isolation of a Surface Configuration primary to the Luminescence Mechanism.

ABSTRACTOrganic-inorganic junctions were formed between porous silicon and various conjugated conducting polymers, poly(3-methylthiophene) and polypyrrole. Schottky type barriers were observed between the conducting polymers in their doped state and p and n-type porous silicon. In their undoped state the conducting polymers behave like p-type semiconductors. Consistent with this, ohmic contacts were observed between undoped conducting polymers and p-type porous silicon while rectifying behavior typical of a p-n junction was observed for conducting polymers deposited onto n-type porous silicon. During characterization of the porous silicon substrate, an investigation of the surface chemistry revealed a strong correspondence between solution pH and the luminescence intensity of porous silicon. Surface titration experiments were performed on p and n-type porous silicon and the results indicate that a monoprotic surface acid with a pKa between 3–4 is a primary component in the luminescence mechanism of porous silicon.

Intercalation of copper into bismuth telluride

The intercalation and leaching of copper into and out of bismuth telluride was investigated using aqueous and solid state electrochemical methods. It was found that copper intercalates up to a concentration of 3 mg/g bismuth telluride. Copper may also react with bismuth telluride to form copper telluride (Cu2Te). Boiling hydrochloric acid leaches, or controlled potential electrochemical leaches, were effective at removing intercalated copper from bismuth telluride, returning the bismuth telluride to its original, undoped state. E.m.f. measurements using a solid, copper-ion conducting electrolyte resulted in copper activities of between 0.000 73 and 0.044 for intercalated bismuth telluride. An effective copper diffusion coefficient of 4×10−5 cm2s−1 was measured for a sintered polycrystalline bismuth telluride pellet at 483 K.

Random-phase-approximation approach to collective modes around inhomogeneous Hartree-Fock states: One-dimensional doped Hubbard model.

All the linear excitations are calculated in the random-phase approximation around a doped inhomogeneous Hartree-Fock state as well as the undoped uniform antiferromagnetic state in the one-dimensional Hubbard model. Upon doping, shape modes appear, localized around an introduced hole, as well as some shake-off'' branches related to combinations of local and extended Hartree-Fock orbitals. These modes are in addition to spin wave and ultragap branches in the pure antiferromagnetic state. Some characteristics of the shape modes are similar to bare processes in the {ital t}-{ital t}{prime}-{ital J} model derived from the Hubbard model at strong coupling. Spectral weights in different susceptibility channels are calculated and shown to have peaks in low-energy regions, arising from specific shape modes.

Electronic structure of aromatic conjugated polymers: Influence of ring torsions on the electronic properties of polyanilines and polyparaphenylene sulfide in the undoped and doped states

Electronic structure of aromatic conjugated polymers: Influence of ring torsions on the electronic properties of polyanilines and polyparaphenylene sulfide in the undoped and doped states

Electrosyntheses, Spectroelectrochemical, Electrochemical, and Chronovoltabsorptometric Properties of Family of Poly (Aromatic Amines), Novel Processible Conducting Polymers: I . Poly(Benzidines)

Direct electropolymerization of a family of aromatic amines yields novel, stable conducting polymers with good conductivity and ca. 8 weight/weight percent (0.3M) or higher solubility in doped and undoped states in organic media. The polymers are electroactive in solution and recastable as films with spectroelectrochemical and electrochemical activity identical to virgin films. Detailed properties are reported in the present communication for two of the polymers, poly(N,N′‐diphenyl benzidine) and poly(benzidine), including solution electrochemistry, film properties of virgin vs. recast films, spectroelectrochemical properties which show broad‐band absorption across the visible spectral region and clear development of bipolaron bands, and charging capacities. Chronocoulometry, chronoamperometry, and chronovoltabsorptometry (optical transmission at specific wavelength as function of time and applied potential) show reversible behavior for films of poly(N,N′‐diphenyl benzidine) over several thousand cycles for pulses as short as 0.1 s, but polymer degradation on extended cycling for poly(benzidine). Polymer structural elucidation indicates a combination of N‐ andpara‐linkages for poly(N,N′‐diphenyl benzidine).

Electronic structure of poly(p‐phenylene sulfide)

AbstractThe electronic structure of poly(p‐phenylene sulfide) (PPS) was studied through steady state photocurrent spectra and temperature dependence of dark conductivity for undoped film, by changes in the in situ optical absorption and ESR spectra under photoirradiation, and optical absorption spectra for doped film. The charge carriers in the undoped state are electronic since the photoconductivity is large and the kink point in the temperature dependence of dark conductivity is higher than the glass transition temperature. A sharp peak near the absorption edge in the photocurrent spectrum of undoped PPS is caused by the diffusion of photocarriers in a sheet with high surface recombination under conditions of low applied field. The in situ optical absorption and ESR spectra of SO3‐doped PPS were similar to those of di‐p‐tolyl sulfide (a monomer unit of PPS) and the formation of sulfur‐centered radical cations was suggested rather than bipolaron states. The stable conduction of SO3‐doped PPS after exposure to atmosphere was analyzed by the classical Lorentz model and the existence of nearly free carriers within subchains was suggested. Their conductivity and mobility were estimated to be 2 S/cm and 3.6 cm2/Vs, respectively. Inter‐chain conduction appears to be very low.

Photoelectron spectroscopy of a conducting polymer polyazulene

Electrochemically synthesized conducting polyazulene (PAz) is investigated by means of ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). There is a shift of the onset positions in the UPS of conductive doped state and less conductive undoped state, which relates to the change in the band gap of the polymer. The UPS of PAz differs from that of azulene free molecules. This suggests the conformation of the polymer is coplanar and the interaction between the rings is large enough to produce new levels. The C 1s XPS of undoped PAz consists of two peak components and a satellite peak. Each component becomes wider upon doping.

Degradation studies of doped and dedoped poly(3-octyl thiophene)

The conductivity of poly(3-alkylthiophenes) has been found to decrease by thermal dedoping. The doped state of the conducting polymer is unstable because the doped state has a higher energy than the dedoped or the undoped state. The conductivity decreases exponentially with time and the change can be fitted to an Arrhenius type equation. The change of conductivity is accompanied by a degeneration of the polymer.

The eletronic structure of emeraldine doped in situ from HCl in the gas phase as studied by photoelectron spectroscopy

The chemical and electronic structure of spin-coated films of polyaniline in the emeraldine state, treated in-situ with HCl in the gas phase, have been studied for the first time by means of X-ray Photoelectron Spectroscopy (XPS) and Ultra-violet Photoelectron Spectroscopy (UPS). Ultra thin films, of about several hundreds of Ångströms thickness, were spin-coated from a solution of chemically prepared emeraldine in NMP, onto substrates of aluminum or silicon. The salt forms were prepared both by (a) “wet” doping in 1M HCl and (b) the in-situ treatment of the films with HCl in the gas phase. With XPS three different types of chlorine have been observed, two ionic and one covalent. With UPS the valence band spectra of the doped and undoped state, are studied.

Electrochemical and electrical properties of poly(hexachlorobuta-1,3-diene), an electrochemically graphitized film

Electroreduction of hexachlorobuta-1,3-diene in Bu4NBF4–CH3CN forms an electroactive black film, PHCBD, with a graphitic structure on a glassy carbon electrode. The electrochemical and electrical properties of PHCBD were studied by cyclic voltammetry and a thin-layer twin-electrode cell technique, E0′=–0.4 V vs. Ag/Ag+, and an irreversible oxidation at Ep, a= 1.5 V. The former reaction is a reductive doping accompanied location inter-calation and the latter is an oxidative doping accompanied by anion intercalation. The conductivity of the PHCBD film is 10–7–10–6 S cm–1 in the undoped state and 4 × 10–4 S cm–1 upon reductive or oxidative doping.

Comparative study on poly(3-alkylpyrroles)

Polypyrrole(PPY), poly(3-methylpyrrole)(PMPY) and poly(3-octylpyrrole) (POCPY) were prepared under the same electrochemical polymerization conditions and their properties were comparatively studied. Electrical conductivity increased in the order POCPY < PMPY < PPY. Absorption spectra measuredin situ showed that while peak energy of the interband transition increased in the order PPY < PMPY < POCPY, the absorption edge of PPY was located at almost equal energy as that of PMPY and POCPY. Stability, indicated by decrease in the ratio of conductivities after storage to the initial conductivity, was primarily dependent on the length of alkyl chains, secondly on dopants and associated with spectral deterioration. While the initial conductivity was strongly dependent on temperature and current density during electrochemical polymerization, the stability was virtually independent of these conditions. The stability increased in the order: POCPY < PMPY < PPY. Spectral change during storage in air was dependent on the chemical structure of polymers but almost independent of dopants. The spectral change indicated PPY to be in nearly initial states after 10 days and POCPY to be in almost completely undoped states after 200 days. In cyclic voltammograms, PPY and POCPY showed one anodic peak and PMPY two anodic peaks. Their peak potentials were associated with conjugation length.

The characterization of doped and undoped states of conducting polypyrrole, poly‐3‐methylthiophene and poly(3‐thiopheneacetic acid) thin films on a platinum surface by specular‐reflectance Fourier transform infrared spectroscopy

AbstractAn improved specular‐reflectance Fourier transform infrared spectroscopy (SRFTIRS) is applied in the characterization of several kinds of conducting polymer, which were synthesized electrochemically and coated on platinum electrodes. This technique allows the use of the same electrode as that used in electrochemical experiments as the optical reflectance substrate and requires no further sampling. Satisfactory spectra were obtained for a variety of film thicknesses. Both the conducting and insulating states of the polymer and the anion and cation dopants can be identified clearly. The technique is fast, very versatile and convenient. Furthermore, the results obtained indicate its potential application in the analysis and characterization of other conducting polymer films and/or other coatings on metal substrates.

Electrochemical Polymerization of N-Trimethylsilylpyrrole and Other Substituted Pyrroles Which Contain Si or Ge Atom(s) in Their Monomer Structures

Abstract N-Trimethylsilylpyrrole and three new compounds; N-trimethylgermylpyrrole, N-chloromethyldimethylsilylpyrrole, and N-pentamethyldisilanylpyrrole, were synthesized. These four monomers were electrochemically polymerized and four new organic conducting polymers were obtained. Resulting polymers had good film producibility and conductivity as high as that for polypyrrole. They showed three distinguishable colors at different potentials: blue in the doped state, yellow in the undoped state, and reddish color in between.

Momentum dependence of photoemission spectra for high- Tc superconductors

This paper discusses the validity of the band picture in high- T c superconductors on the basis of rigorus spectroscopic results for a Cu 4O 8 cluster. The calculated single particle spectra at low energies are qualitatively consistent with the band model. However, a large energy gap appears in the undoped ground state where a four-spin interaction is found to be important.

Structural control of conjugation in functionalized polythiophenes

A comparative analysis of the electrochemical and optical properties of 3-substituted poly- (thiophenes) has been carried out on four polymer structures, poly(3-methylthiophene) (PMT), poly(3- isoamylthiophene) (PiAT), poly(3-nonylthiophene) (PNT), and poly(3-(3,6-dioxyheptyl)thiophene) (PDHT). The spectroelectrochemical properties of these polymers have been investigated by cyclic voltammetry (CV), derivative cyclic voltammetric absorptometry (DCVA), and spectroelectrochemistry. CV shows that the symmetry of the anodic and cathodic waves increases from PMT to PDHT; furthermore in this latter case, a second redox system appears in the CV. The DCVA responses of the polymers indicate that the oxidation potential decreases in the order PiAT > PMT > PNT > PDHT whereas the increasing symme- try of the curves suggests an increasing electrochemical reversibility. UV-visible absorption spectra have been recorded in situ at various applied potentials. In the undoped state, the absorption maximum of the polymers shifts bathochromically from PiAT to PDHT and the absorption spectrum of PDHT exhibits several defined structures resembling those previously observed at low temperature on poly(3- alkylthiophenes). Upon electrochemical doping, the spectroscopic behavior of PiAT, PNT, and PDHT differs from that of PMT by a more intense absorption at 1.5 eV at the intermediate doping level and by a more pronounced narrowing of the band gap with the appearance of metallic-like behavior at high applied potential. These effects that are particularly evident in PDHT are interpreted by an extension of the effective mean conjugation length resulting from the stabilization of more planar conformations.

Characterization of metal/organic molecule and metal/polymer interfaces by NEXAFS spectroscopy

NEXAFS spectroscopy is a technique well suited to probing metal/organic molecule or metal/polymer interfaces. Two systems, related to the Pt/organic conducting polymer and Ni/polyacrylonitrile interfaces, are discussed in detail.For poly-3-alkylthiophene (and selenophene) thin films electrochemically deposited onto Pt, the C K-edge characteristics reveal that (i) the polymeric chain unit is composed of the same architecture as the monomer and (ii) the doping proceeds via a narrowing of the band gap with the appearance of metallic-like behaviour. The same tendencies are observed by varying the structure of the monomer, the nature of the dopant and the film thickness, which evidences a similar conduction mechanism for these five-membered polyheterocycles. The NEXAFS spectra reveal that the polymeric chains are well ordered on the metallic surface: in its undoped state, the layers lie flat on Pt with strong interactions between the electron states of the metal and the antibonding π* band of the polymer; when doped, the NEXAFS characteristics exhibit changes as a function of thickness. The first layer lies flat on the Pt surface while the other ones are randomly oriented, due to the intercalation of anions during the oxidation process. Finally, the orientation of the polymeric chain switches from a ‘lying-down’ to an ‘on-edge’ configuration when a long alkyl chain is added on the ring, this alkyl chain is oriented perpendicular to the Pt surface.Polyacrylonitrile (PAN) thin films electrochemically deposited on Ni have been studied by NEXAFS as a function of the film thickness and annealing treatment. For 20 A thick films, the polymer chains are oriented perpendicular to the surface with the CN groups parallel to it. Below a few A, no polymerization occurs, but molecules are adsorbed perpendicular to the surface. Annealing at 300 °C results in the loss of the majority of the N content of the film in contrast to the admitted mechanism for bulk PAN.

Electrochemical Polymerization of N-Trimethylsilylpyrrole and Characterization of the Resulting Conducting Polymer

Abstract A new conducting polymer was synthesized by electrochemical polymerization of N-trimethylsilylpyrrole. The polymer had good film producibility. The as grown film, which was in the doped state, showed conductivity of 5 × 101 S cm−1. The film showed three colors depending on the doping level, blue in the doped state, yellow in the undoped state, and reddish color in between.

Optical, magnetic resonance and conductivity studies of the ladder polymer BBL

We report on the optical, magnetic and electric properties of the ladder polymer BBL. Our optical studies indicate a gap of approximately 1.8eV in the neutral undoped state and the presence of charged defects which increase in number upon doping. Electron spin resonance studies suggest that these defects are polarons. The resistivity is very anisotropic and could be fit to Mott's model of variable range hopping. The anisotropy is attributed to anisotropic localization lengths ( ξ ‖, ξ ⊥) of the localized states.