Reduction of p-terphenyl,p-quaterphenyl and p-sexiphenyl using alkali metal in liquid ammonia: Process and characterization of the reduced compounds

Oligomers of PPP (p-terphenyl, p-quaterphenyl and p-sexiphenyl) were doped using Na or K to promote a reduction. The used method, with liquid ammonia resulted in a solid state doped material without insertion of solvent. The characterization with XPS and Raman spectroscopy showed the formation of at least one phase of the doped oligomer, which contains dianions (bipolarons).     

E.P.R. study of potassium-doped poly(p-perfluorophenylene): A conducting polymer with non-degenerate ground states

Samples of poly(p-perfluorophenylene) heavily doped with potassium (K-FPPP) give rise to a conductivity that is slightly larger than that of the analogously doped poly(p-phenylene), K-PPP, but the unpaired spin concentration is decreased by an order of magnitude. Even so, the measured e.p.r. susceptibilities have a temperature dependence that suggests equilibrium between separated polaron defects and the ground state bipolarons, similar to that observed for K-PPP.     

Electrochemical preparation and properties of poly(3-methoxy-2,5-thiophenediyl) and poly(3-methylthio-2,5-thiophenediyl)

Soluble and conductive poly(3-methylthio-2,5-thiophenediyl) and poly(3-methoxy-2,5-thiophenediyl) films were prepared by the electrochemical polymerization of 3-(methylthio)thiophene (MTT) and 3-methoxy-thiophene (MOT), respectively. The polymers were identified from their i.r. spectra and found to be soluble in chloroform, propylene carbonate (PC), dimethyl sulfoxide (DMSO) and 1-methyl-2-pyrrolidinone (NMP). When doped, they were soluble in PC, DMSO and NMP. Cast films could be obtained from their solutions. The degrees of polymerization of poly-MTT and poly-MOT were 7.4 and 79, respectively. The vis-near i.r. specta of either polymer solution had characteristic peaks due to bipolarons. The doped poly-MTT solution became undoped after four days in air. The doped poly-MOT solution was stable for a few months, and the conductivity of the pressed pellet of doped poly-MOT did not change for as long as 11 months. With regard to differences in the absorption spectra between the films and solutions of the undoped state, the films had bands at longer wave-length than the solutions. The shifts may possibly have been due to internal rotation about single bonds, probably occurring more freely in solution than in films. Solid-state samples retained the coplanar configuration. In cyclic voltammograms, the oxidation potential of poly-MOT was lower than 0 V versus Ag/Ag⁺. It is thus evident that the doped state of poly-MOT is stable in air.     

The reversible hydroxide doping of polypyrrole. Delay of polaron and mobile bipolaron injection in polypyrrole by H bonding of hydroxide anions

The reversible hydroxide-anion doping of polypyrrole (PPy) has been investigated by FT-IR, cyclic voltammetry, in situ conductivity and ESR techniques. FT-IR confirmed that the hydroxide anion substitutes the counteranion reversibly in doped PPy. The dopant hydroxide anion produces a strong negative shift of the oxidation potential of PPy and doubling of the reversible charge. No similar effects are displayed in N-substituted PPys. In situ conductivity shows that the material becomes conducting (up to 2 S cm⁻¹) only after about 50% of the doping charge is passed. Similarly, the ESR signal is absent during the initial stages of charging, developing just before the onset of conductivity. The results indicate that the hydroxide anion stabilizes the positive charges in PPy by NH bonding. The interaction is so strong that fixed bipolarons are formed without the intermediate formation of polarons. Polarons are produced only at higher charges with subsequent injection of mobile bipolarons and setting in of conductivity.     

Solitons or not in polyacetylene (and does it matter?)

The results of a series of experiments are described which demonstrate that solitons are the important excitations in trans-(CH)_x and that the properties of these non-linear excitations can be directly studied through measurements on trans-(CH)_x samples, either during photoexcitation of after doping. The importance of these concepts in the more general context of conducting polymers is addressed. Although the two-fold degenerate ground state of trans-(CH)_x is quite special, the relevant concepts can be generalized to confined soliton pairs (bipolarons) and applied to a wide variety of conjugated polymers in which the ground state degeneracy is not present.     

SO3-doped poly(P-phenylene sulphide) II: Motion of electrons studied by the frequency dependence of proton N.M.R. T1−1

The frequency dependence of proton n.m.r. T₁⁻¹ was exploited to investigate the dynamic aspect of the electronic state in poly(P-phenylene sulphide) (PPS) doped heavily with SO₃. The doping was shown to be inhomogeneous even in the most heavily doped sample prepared with liquid SO₃, as evidenced by a non-exponential recovery of the proton magnetization. T₁⁻¹ has been described well by the inverse-square law dependence upon frequency with a negative intercept, being interpreted in terms of a diffusive motion of electrons (or holes) in the form of polarons, but not of bipolarons, which are delocalized over several sites along a doping-induced one-dimensional conjugated chain.     

The FeCl3-doped poly(3-alkyithiophenes) in solid state

The FeCl₃-doped three poly(3-alkylthiophenes) (P3ATs) in solid state, i.e. poly(3-octylthiophene) (P3OT), poly(3-dodecylthiophene) (P3ODT) and poly(3-octadecylthiophene) (P3ODT), were investigated in this paper. In X-ray diffraction results, there are obvious variations of the interlayer and interlayer spacings in the layered structures of P3ATs. In addition, it is found that some orientations of the side-chain groups occur after the doping process. The infrared spectra have also shown the microstructural changes arising from the readjustments of the polymer chains due to the intervention of the dopant. The presence of dopant leads to the formation of bipolarons and polarons at the same time. The conductivity measurements reveal that the conductivity decreases with the increase of the length of side-chain group. We have also observed the relaxation behaviors in the conductivities of the doped polymers.     

Electron localization studies of alkoxy polyanilines

Substituted polyanilines, namely poly(o-methoxyaniline), poly(o-ethoxyaniline), and an equimolar blend of polyaniline and poly(o-methoxyaniline) were synthesized at various oxidant/monomer molar ratios with slow and fast addition of the oxidant. Electrical conductivity, magnetic susceptibility and electron spin resonance studies are reported for these polymers doped with the following acids: hydrochloric, para-toluene sulfonic, sulfosalicylic and trifluoroacetic. Results on thin films of camphorsulfonic acid doped poly(o-methoxyaniline) and polyaniline blends with poly(o-methoxyaniline) are also reported. A systematic study of these polymers shows that the conduction is due to quasi-one-dimensional variable range hopping. A Pauli-like susceptibility is seen in all the polymers indicating the existence of ordered regions. The electron localization length is found to be much larger in poly(o-alkoxyanilines) compared with the corresponding poly(o-alkylanilines). We propose that this effect arises from the electron-donating ability of the alkoxy group in contrast to the electron-withdrawing nature of the alkyl group. We also find that samples prepared with larger amounts of the oxidant are more disordered.     

Conformation and electronic structure of poly(3,4-diisopropylidenecyclobutene), a cross-conjugated conductive polymer

It has been reported recently that the title polymer, 1, synthesized by a novel olefin-metathesis route, can be oxidatively doped to yield materials with moderate conductivities. Theoretical studies reveal that, due to steric crowding, polymer 1 cannot achieve a planar, fully-conjugated structure in either its undoped or doped states. Rather, the structure consists of essentially orthogonal tetramethylhexatriene units. Such a structure is incompatible with conventional conduction mechanisms involving polarons or bipolarons. It is proposed that, instead, conduction involves intra- and/or inter-chain charge transfer.     

Optical absorption from polarons and bipolarons in polythiophene

We present here a theoretical calculation, within the one-electron Hückel model, of the optical absorption due to polarons and bipolarons in doped polythiophene. The results are used to describe the absorption spectra at thermal equilibrium, when the two kinds of excitation are present. Comparison with experimental data obtained in the case of doping shows that the Coulomb effects must be taken into account and the binding energy to the dopant is estimated to be about 0.14 eV. Although the bipolaron still remains the lowest-lying charged excitation, the contribution of Coulomb effects is shown to increase the proportion of polarons at thermal equilibrium.     

An in situ ESR study on poly(3-methylthiophene): charge transport due to polarons and bipolarons before the evolution of metallic conduction

In situ ESR measurements with ClO₄⁻-doped poly(3-methylthiophene) (PMT) are carefully performed over a wide range of doping level from 0.02% to 23% in order to correlate a drastic increase of carrier mobilities by doping with the nature of charged species in the π-conjugated polymer. Either or both of a Gaussian and Lorentzian components with different g-factors are observed, depending on the doping level. The broad Gaussian signal ascribable to structural defects in the polymer remains almost unchanged at various doping stages while the Lorentzian signal varies greatly with the doping level. From spin concentrations for the Lorentzian signal, molar fractions of polarons and diamagnetic species (bipolarons) are evaluated as a function of doping level. Combination of the molar fractions and the mobility data obtained earlier demonstrates that the conduction process in the lightly doped PMT film (0.02%–1%) can be expressed by independent driftings of polarons and bipolarons under electric field, whose mobilities are 2×10⁻⁵ and 10⁻³ cm² V⁻¹ s⁻¹, respectively. At higher doping levels of 1%–23%, conductivity is much more enhanced than expected from this model and the line width of the Lorentzian signal increases from 1.2 to 7.5 G, suggesting the evolution of a metallic conduction.     

Does photogeneration produce bipolarons in poly (para-phenylene vinylene)?

Extensive photoinduced absorption (PA) and photoconductivity (PC) measurements have been carried out on poly (para-phenylene vinylene) (PPV) and its derivatives to determine which excitations are present in different time frames after exposure to light. Picosecond PA experiments have led to the conclusion that the large majority of photons above the absorption edge create long-lived (ns) polaron pairs, positive and negative polarons bound on adjacent chains by Coulomb attraction. We suggest that the steady-state PA that has been attributed to bipolarons in undoped samples is also due to polaron pairs. The main evidence for this is the coincidence of the observed steady-state visible and near-infrared PA with that of the corresponding ps PA due to polaron pairs, and the fact that the frequencies of the observed infrared-active vibrational modes (IRAV) require strong pinning. The long tail of the PC that has been attributed to the slow motion of bipolarons in the bulk is more reasonably explained by the effect of traps, for example, thermally and photochemically created carbonyls. The increase in PA and PC observed for photon frequencies higher than about 1/2 eV above the absorption edge, that has been variously attributed to generation of bipolarons and of free electrons in a continuum of energy levels, we assert is due to free polarons. At such photon energies some electrons and holes may be able to escape their Coulomb attraction and avoid being trapped into either polaron pair or exciton states. We conclude that there is no evidence that photogeneration results in a significant number of bipolarons in PPV and its derivatives, apart from samples with particular impurities or defects that foster bipolaron creation.     

An e.s.r. study of isomerization and doping in Durham polyacetylene

Electron spin resonance has been used to study the transformation reaction of a soluble precursor polymer into all-trans polyacetylene. A spin signal (ΔH = 750 ± 300 μT) that develops during the reaction is found to be associated primarily with cis-trans isomerization rather than with the initial elimination reaction, which produces cis-rich polymer. The effects of doping with AsF₅, I₂ and Li on the e.s.r. spectra are quite diverse, and give information on the spin dynamics and the nature of the spin/charge carriers present in the doped polymers. Some of the results contrast with those found for the conventional Shirakawa polyacetylene, although both polymers can be doped to very high conductivities.     

A new route to synthesize high degree polythiophene in a room temperature melt medium

Conducting polymers suffer from folds and kinks because of random nucleation and solvation of a free radical cation to yield a cross linked/disordered polymer and therefore a solvent free electrochemical polymerization in a room temperature melt medium is adopted to yield a high degree polymer with high electronic conductivity. Electropolymerization of thiophene was performed on platinum/ITO substrates using cyclic voltametry or galvenostatic mode in chloroaluminate room temperature melt medium to obtain a reddish brown free standing film which can be peeled off from the electrode surface after a minimum of 10 cycles. The conductivity was found to be around 10² S/cm. The degree of polymerization was calculated to be around 44 from IR studies. A layered structure supportive for high degree of polymerization was witnessed from potential step technique. From UV spectra the charge carriers were found to be bipolarons. The morphology of the film was found to be crystalline from SEM and XRD studies. Capacitative impedance properties for doped samples were interpreted from impedance spectroscopy.     

Spin polarized injection and transport in organic polymers

Organic spintronics has been an attractive topic since the experiment in which injection of spin polarized electrons from ferromagnetic metals (FM) into organic polymers was reported. By considering both the spin polarons and spinless bipolarons as carriers in organic polymers, we studied the spin injection into organic polymers from both an electrocircuit model and the diffusion theory separately. It was found that a high current polarization could be obtained by adjusting the proportion of the polarons over the carriers. Effect of conductivity matching of the FM layer and the polymer layer on the spin injection was discussed.     

Role of polarons in the antiferromagnetic behaviour of poly (3-dodecylthiophene)

A mathematical model was suggested and tested to elucidate the antiferromagnetic behaviour of temperature dependence of the magnetic susceptibility of poly(3-dodecylthiophene) (PDDT) which takes advantage of the physical properties of polarons and bipolarons. It has been found that the responsibility for the antiferromagnetic behaviour of temperature dependence of the magnetic susceptibility of PDDT is the conversion of polarons to bipolarons, rather than the presence of several crystalline sublattices of the polymer with different orientations of magnetic moments. The antiferromagnetic character is caused by paramagnetic polarons, the number of which decreases with lowering temperature. The proposed model was verified, with good results also for other conductive polymers exhibiting antiferromagnetic properties known from the literature.     

Electrical properties of Cu(II)-N,N′-dimethyldithiooxamide polymers

Cu(II)-dithiooxamide polymers were prepared by a direct deprotonation of the ligand with a Cu(II) salt. The polymers had a chain length of about 13 units. The undoped polymers showed low electrical conductivity with an Arrhenius-type temperature dependence. Doping of Cu(II)-N,N′-dimethyldithiooxamide was performed by iodine in the vapour phase and by FeCl₃ in solution. In both cases, the oxidative doping resulted in highly increased conductivities. A thermopower measurement indicated p-type conduction. The infrared spectra of the undoped material and the one doped with FeCl₃ were completely identical, indicating that charge transfer from the ligand to the dopant is small. The conductivity curves of the doped polymer did not show an Arrhenius-type behaviour. In order to find an explanation for the conduction mechanism different models were tested. The best fit of the experimental conductivity data of the FeCl₃-doped polymers was obtained with the theoretical prediction of a variable-range hopping model.     

A study of thermal,optical and electrical properties of new branched triphenylamine-based polyazomethines

A series of branched aromatic polyazomethines have been obtained by high temperature solution polycondensation of 4,4′,4″-triformyltriphenylamine with 3,3′-dimethoxybenzidine with different feed molar ratio. For three polymers additional condensation of chain end groups with monofunctional monomers such as 4-formyltriphenylamine or 2-naphthylamine was carried out. Moreover, two model compounds were prepared and investigated for comparison with branched polymers. The structures of polymers and models were characterized by means FTIR, ¹H, ¹³C NMR spectroscopy, elemental analysis and gel permeation chromatography (GPC). UV–vis properties of the thin films of the polymers and compounds were investigated on the glass substrate. Eg of the branched polymers was found about 2.47 eV. UV–vis and FTIR spectroscopy for iodine doped compounds were investigated. Doping decreased the value of Eg of the branched polyazomethines to about 1.71 eV. Refractive index (n) for branched polyazomethines was found about 1.97, while for the doped compounds was a little higher (～2.48). Absorption (UV–vis) properties of the doped with iodine branched imines were investigated additionally after heating in different temperatures from 50 to 200 °C. Intensity of photoluminescence of branched imines in relation to 9,10-diphenylanthracene was found in the range 0.2–1.0% and 2.7–43.7% in dependence on the excitation wavelengths. Current–voltage (I–V) measurements were performed on ITO/TiO₂/polymer/Al, ITO/polymer/Alq₃/Al and ITO/TiO₂/polymer/Alq₃/Al devices in the dark and during irradiation with light (under illumination 1000 W/m²). The sol–gel technique was applied to prepared TiO₂ layer. TiO₂ layers and devices were investigated by Atomic Force Microscopy (AFM). Moreover, properties of these branched polymers were compared with the linear polyazomethine based on 3,3′-dimethoxybenzidine and 4,4′-diformyltriphenylamine.     

Humidity sensitive properties of substituted polyacetylenes

Thin film resistive humidity sensors have been prepared based on a series of homogeneously doped soluble substituted polyacetylenes synthesized using palladium acetylide complex catalyst. The humidity sensitive properties of the polymers (poly(propiolic acid) (PPA); poly(propiolic acid-co-ethynylbenzene) (PA-co-EB); poly(propiolic acid-co-p-diethynylbenzene) (PA-co-DEB); poly(propiolic acid-co-propargyl alcohol) (PA-co-OHP)) doped with FeCl₃, H₂SO₄ and HClO₄ have been investigated and compared. The chemical structures of polymers and the doping agents have great influence on the sensing properties of the sensors. A sensor based on PA-co-OHP doped with HClO₄ shows the best response. Its logarithm of impedance varies linearly with relative humidity (RH) for four orders of magnitude (10⁷–10³ Ω) over a wide range of 30–95%RH, and the response time is <6 and <15 s for absorption and desorption, respectively. Furthermore, the effect of temperature on the sensing behaviour of the polymers has also been described.     

Electrical and optical properties of molecularly doped conducting polymers

Novel electrical and optical characteristics have been observed in conducting polymers doped with molecular dopants such as fullerenes (C₆₀, C₇₀ etc.), photochromic dyes and another (guest) conducting polymer. Highly effective photo-induced charge transfer results in various effects such as photoluminescence quenching, photoconductivity enhancement, electroluminescence quenching and persistent photoconductivity which have been observed in fullerene-doped conducting polymers. Unique photo-sensitive characteristics have also been found in conducting polymer/ fullerene systems. Photoluminescence and photoconductivity in conducting polymers doped with photochromic dyes have been changed dramatically by photo-induced isomerization of photochromic dyes and memory effects in both dark conductivity and photoconductivity have been observed. We also present here some results on conducting polymer-conducting polymer mixtures and discuss general features of such composites. These characteristics can be explained by the asymmetry of the relative electronic energy states of conducting polymer and molecular dopants on account of self-trapping effects. Superconductivity evolved upon alkali metal doping of C₆₀-conducting polymer composites, and has been confirmed by low-field microwave absorption (LFMA) and SQUID magnetometry measurements. The origin of superconducting phases in this doubly doped conducting polymer is discussed.