Granville G. Miller

Electrical and optical properties of highly conducting charge-transfer complexes of poly(p-phenylene)

Abstract Electrical conductivity, Hall effectm and spectroscopic measurements have been made on AsF 5 -doped poly( p -phenylene). Doping increases the conductivity of the parent polymer by as much as 14 orders of magnitude to values as high as 5 × 10 4 S/m. Hall effect measurements indicate p-type conduction with a Hall mobility approaching 10 −4 m 2 /V s for doping levels between 0.24 and 0.42 moles of AsF 5 per mole of monomer. Doping with an electron donor, K, has increased the conductivity to about 10 3 S/m for a doping level of 0.57 moles of K per mole of monomer. Using this conductivity value, with the assumption of total charge transfer from the donor, suggests a drift mobility for electrons which is significantly less than that for holes. The assumption of an intercalant structure analogous to that of polyacetylene and graphite leads to the conclusion that the presently achieved AsF 5 -doping levels in poly( p -phenylene) correspond to a compound which is not wholly stage 1.

Solid‐state synthesis of highly conducting polyphenylene from crystalline oligomers

Paraphenylene oligomers (biphenyl, p‐terphenyl, p‐quaterphenyl, p‐quinquephenyl, p‐sexiphenyl) form electrically conducting complexes with AsF5. Prolonged exposure to AsF5 causes a polymerization of these p‐phenylene oligomers to give highly conducting charge‐transfer complexes of poly(p‐phenylene). Conductivities as high as 50 S/cm have been measured. Powders, thin films, and single crystals of p‐phenylene oligomers have been reacted with AsF5. The undoped oligomers and the doped, compensated, and annealed products have been investigated by means of x‐ray diffraction, and UV‐visible and IR transmission spectroscopy. The x‐ray diffraction studies give evidence for a change in lattice spacings to those characteristic of the crystalline polymer. The spectroscopic measurements during AsF5 doping reveal shifts in absorption bands in the UV and the IR to those characteristic of poly(p‐phenylene). Paraoligophenylenes have also been reacted with elemental potassium in THF solution with trace amounts of naphthale...

The structure of metallic complexes of polyacetylene with alkali metals

The crystal structures of sodium, potassium, rubidium, and cesium doped polyacetylene have been determined using crystal packing and x‐ray diffraction analyses. Each of these metallic complexes is tetragonal, with the polyacetylene chains forming a host lattice in which the alkali metal ions are present in channels. Lithium appears to be too small to stabilize the channel structure and an amorphous structure is observed. Predicted unit cell parameters and x‐ray diffraction intensities are in agreement with observed values. Similarities with the alkali metal doped graphite suggest that hybridization between carbon pz orbitals and metal s orbitals occurs. Such hybridization is expected to result in a high conductivity component normal to the chain direction. On the other hand, direct overlap between polymer chains appears small, since alkali metal columns separate polymer chains. Compositions calculated for the channel structures (from meridional diffraction spacings, the intensity of equatorial diffraction...

Processible, environmentally stable, highly conductive forms of polythiophene

Abstract As electronic materials, the most promising synthetic semiconductors and metals for practical large scale applications are those which can combine convenient processibility with high environmental stability, good mechanical properties, and high conductivity. Missing from the desired combination of properties for the majority of interesting conductive polymers is convenient processibility. A series of solution processible poly(3-alkylthiophenes) are described which form highly conductive, environmentally stable complexes with electron acceptors. These materials are quite unusual in that, in addition to their attractive properties, conductivities are generally high and surprisingly insensitive to the length of the alkyl substituents.

Staging in polyacetylene-iodine conductors

Evidence is presented for the existence of highly conducting polyacetylene complexes with structures related to high‐stage graphite, as well as structures related to first‐stage graphite. X‐ray diffraction measurements on polyacetylene–iodine complexes indicate equatorial lines at 7.7–8.0 and 13.8–14.3 A. The shorter spacing arises in part from a structure in which iodine‐rich planes alternate with planes of polyacetylene chains. The longer spacing, which disappears upon atmospheric exposure, is consistent with a structure analogous to third‐stage graphite in which dopant‐rich planes are separated by three close‐packed planes of polyacetylene chains. The third‐stage complex can be viewed as a perturbation of the structure of undoped polyacetylene, with the region between dopant layers consisting essentially of a one unit cell thickness of the parent polymer structure. Packing calculations for this model, in which a linear column of anions (I3− and/or I5−) displaces either every chain or every other chain ...

Highly conducting, soluble, and environmentally-stable poly(3-alkylthiophenes)

Homopolymers and copolymers of 3-alkylthiophenes incorporating alkyl groups equal to or greater than butyl in size can be handled in solution and form highly conducting, environmentally-stable compositions on doping.

EPR study of polarons in a conducting polymer with nondegenerate ground states: Alkali metal complexes of poly (p‐phenylene) and phenylene oligomers

EPR measurements are used to characterize electronic states relevant for carrier transport in alkali metal doped poly(p‐phenylene), PPP, fully deuterated poly(p‐phenylene), DPPP, and phenylene oligomers. Observed spin concentrations per carbon are at least one decade higher than the Curie spin concentration for Na‐doped polyacetylene. The number of these spins, which likely corresponds to polarons (mobile radical anions), is much less than the amount of alkali metal dopant, suggesting that much of the charge on the polymer chains is in bipolarons (spinless dianions). Relevant to the interaction between spins on the polymer chain and the metal cations, the observed g values are close to the free electron value and do not substantially vary with the donor dopant, temperature, or the molecular weight of the phenylene chain. Although the spin‐orbit effect on g values is small, room temperature linewidth tends to increase with increasing atomic number of dopant—suggesting some interaction, albeit a smaller mag...

Structure of polyacetylene–iodine complexes

We confirm the existence of a 15 A period in iodine‐doped polyacetylene and provide a new interpretation for this key feature as part of a general model for structural changes during iodine doping. The observed diffraction intensities for different samples suggest the existence of structures with two different types of dopant‐containing layers: layers obtained by complete replacement of polyacetylene chains by iodine columns (F layers) and layers obtained by replacement of every other polyacetylene chain by an iodine column (P layers). The F layers in the heavily doped complex alternate with dopant‐free layers of polyacetylene chains (U layers), corresponding to a (U F) n stacking sequence. The phase obtained at a lower dopant concentration, which provides the 15 A spacing, is attributed to a (U P U F) n stacking sequence. At still lower dopant concentrations, one obtains a (U P) n stacking sequence. This model, along with published Raman, Mossbauer, and photoelectron spectroscopy data, suggests that the ratio of I− 5 to I− 3 increases in going from P layers to F layers. Intense and monotonically decreasing, diffuse x‐ray scattering suggests that vacancies of size ∼3 A are present, probably in iodine columns. A diffuse reflection at 3.1 A, observed in all iodine‐doped samples, is due to an average iodine–iodine distance in disordered columnar arrays. On the other hand, ordered arrays of iodine columns in oriented samples give rise to sharp meridional reflections. All ten observed reflections (down to 1.17 A) in one sample could be indexed based on a 33.8 A repeat corresponding to (–I− 3–I− 5–I− 3–) n arrays. The observed diffraction pattern was calculated from this model without using any freely adjustable parameters.

Thermal enhancement of the electrical conductivities of alkali metal-doped polyacetylene complexes

Abstract Alkali metal-doped polyacetylene exhibits higher thermal stability than either the parent polymer or acceptor-doped complexes. For the Na, K, Rb and Cs complexes in which a channel structure is indicated, this stability appears to be related to the packing arrangement, which interferes with interchain reactions. In contrast with the case for acceptor doping, the conductivities of the alkali metal-doped cis -polyacetylenes are lower than for correspondingly doped trans -polyacetylenes. However, thermal annealing of the K-doped or Rb-doped polyacetylene (obtained using the naphthalide in THF) results in dramatic enhancement of conductivity for the doped cis -polymer, but not for the doped trans -polymer. After about a six-fold increase in room-temperature conductivity, the conductivity of K-doped polyacetylene is stable for many hours at 200 °C. Cis -polyacetylene films doped in the same way with other alkali metals do not show this enhancement. Significant changes in unit cell parameters, ESR linewidths, electrical anisotropy and contact resistance also occur during the annealing of K-doped cis -polyacetylene. Results presented suggest that thermal annealing of the K-doped polyacetylene eliminates residual conformational defects in partially chemically-isomerized polymer chains, thereby increasing crystalline perfection, increasing the effective conjugation length and increasing the hybridization of alkali metal and carbon orbitals. These changes probably result in the observed conductivity enhancement, the dramatic increase in ESR linewidth (6 – 12 G to 40 – 60 G) and the decrease in unit cell parameter.

Structural study of conducting polymers in solution

Abstract The aim of the present work is to investigate the conformational change of conjugated chains upon doping by small angle neutron scattering. Polymer solutions such as n-butylthiophene (C4HS(CH2)3CH3)n doped with NOSbF6 can provide such a goal. For the neutral chains, results show that, at room temperature, even for the lowest polymer concentration studied (cp = 0.1 mg/cc), a positive interchain interaction occurs. At higher temperature (T = 65°C), an isolated chain behavior can be measured allowing to measure the statistical length b = 55 A and the extension of the lateral groups L t = 12 A . With a polymer concentrationsas low as cp = 0.5 mg/cc and a dopant concentration equivalent to one dopant molecule per monomer unit, the chain conformation is modeled by a rod structure, b > 850 A . At the same dopant concentration, but with cp = 2.4 mg/cc, the scattering function show a q−2 behavior. In any case, no correlation hole is observed, which differs markedly from the usual behavior observed in polyelectrolyte solutions.