H. Eckhardt

Structure‐property predictions for new planar forms of carbon: Layered phases containing sp2 and sp atoms

Structure, thermodynamics, and electronic properties are predicted for a new low energy phase of carbon which contains planar sheets equally occupied by sp2 and sp carbon atoms. The isolated planar sheets have the same planar symmetry as do the layers in graphite (p6m) and can be formally viewed as resulting from the replacement of one‐third of the carbon–carbon bonds in graphite by –C 3/4 C– linkages. This material, called graphyne, is predicted to have a crystalline state formation energy of 12.4 kcal/mol carbon, which appears to be much lower than for any carbon phase which contains acetylenic groups as a major structural component. Based on the major structural reorganization required for graphitization and the observed high temperature stability of known model compounds, high temperature stability is predicted for graphyne. While graphyne will have similar mechanical properties as graphite, it is predicted to be a large bandgap semiconductor (Eg=1.2 eV) rather than a metal or semimetal. Based on this...

The electronic and electrochemical properties of poly(phenylene vinylenes) and poly(thienylene vinylenes): An experimental and theoretical study

The electronic and electrochemical properties of poly(p‐phenylene vinylene), poly(thienylene vinylene), and their derivatives with electron donating moieties such as methyl, methoxy, and ethoxy are studied using the newly developed electrochemical potential spectroscopy (ECPS) and optical spectroscopy. It is shown that electrochemically derived band gaps agree well with band gap values obtained from optical measurements. Substitution with electron donating groups substantially lowers the ionization potentials and band gaps. A similar effect can be attributed to the incorporation of a vinylene linkage between rings of the polymer backbone. Our results imply that through a proper choice of substituents and backbone structure one can adjust the electrochemical potentials over a wide range as well as red shift the absorption edge of these polymers. In the case of the alkoxythienylene vinylenes the absorption edge is shifted through the visible range of the spectrum into the near infrared (NIR) yielding polyme...

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...

Conducting complexes of polyphenylene sulfides

Poly(p‐phenylene) sulfide, poly(m‐phenylene) sulfide, and the newly synthesized polymer poly(thio‐2,8‐dibenzothiophenediyl) have been treated with strong electron acceptors (AsF5, SbF5) to form conducting complexes with p‐type electronic conductivities up to 3 S/cm. Near IR to UV absorption spectra and temperature‐dependent conductivity measurements suggest a localization of charge carriers even at high doping levels. Elemental analysis and IR spectroscopy demonstrate that heavy exposure to AsF5 causes substantial changes in the backbone structure of these polymers. The dopant appears to predominantly induce the formation of carbon–carbon bonds bridging the sulfur linkages to form thiophene rings. This chemical modification enhances the conductivity of the complex and, in the case of poly(m‐phenylene), is shown to be an actual prerequisite for achieving high conductivity.

The structure of a novel polymeric metal: Acceptor-doped polyaniline

Abstract Key structural features of acceptor-doped polyaniline (polyemeraldine salt) are deduced from the crystal structures for the ClO 4 − and the BF 4 − dication salts of H(C 6 H 4 NH) 4 C 6 H 5 , radical cation salts of shorter oligomers and neutral H 5 C 6 NC 6 H 4 NC 6 H 5 . Contrary to the previous conclusion that the rings in acceptor-doped polyaniline are largely benzoid, strong quinoid distortions are observed for all rings except terminal phenyls. These distortions for the radical cation and dication molecules agree in relative magnitude with the predictions of valence bond theory, amount to about 50% of that for the quinoid ring in the above diimine, and imply that radical cations are not completely localized on the nitrogens. The corresponding partial multiple-bond character of the nitrogen-ring bonds, indicated by observed bond lengths, drives the rings to within a dihedral angle of about +15 and −15° (alternately) with respect to a planar, zigzag chain made by the nitrogen atoms. Major distortion of the NCC bond angles as a consequence of the resulting steric interference is observed. Bond lengths derived for acceptor-doped polyaniline using the oligomer data are consistent with those derived theoretically for either a polaron lattice or a disordered polaron or bipolaron lattice.

Effects of substituent‐induced strain on the electronic structure of polydiacetylenes

The valence effective Hamiltonian technique (VEH), and modified neglect of differential overlap (MNDO) calculations are used to study the influence of strain induced by side chains on the geometry of polydiacetylene backbones and the resulting polymer band structure, band gap, and ionization potential. Simulations of strain effects on the polymer backbone yield variations in optical properties which are similar to those observed experimentally during thermochromic phase changes in urethane‐substituted polydiacetylenes. Our results suggest that these changes in optical properties are related to strain at points of substituent attachment and not to fundamental changes in the backbone geometry such as an acetylenic‐to‐butatrienic transformation.

Vibrational studies of polyparaphenylene-vinylene (PPV)

Abstract In this paper, we present a complete vibrational study of polyparaphenylene-vinylene (PPV) using Resonance Raman Scattering (RRS) and Infrared spectroscopy. A dynamical model based on valence force-field calculations has been built in order to assign the observed Raman and IR vibrational modes, and trans-stilbene served as a model compound to establish a good set of force constants for the neutral polymer. Upon doping with FeCl 3 , the Raman spectra exhibit rather strong modifications. In order to obtain a good agreement with experimental data of doped PPV, a new set of force constants is needed which supports the existence of a quinoid-like structure along the PPV chains.

Bi‐Sr‐Ca‐Cu‐O and Pb‐Bi‐Sr‐Ca‐Cu‐O superconductor films via an electrodeposition process

A novel electrochemical process has been developed for the formation of superconducting films. Using this process, superconducting films of Bi2Sr2Ca1Cu2O8 and (Pb,Bi)2Sr2Ca1Cu2O8 have been formed. The process consists of simultaneously depositing the metallic constituents of the superconductor from a single electrolyte, and thermally oxidizing the resulting precursor film to form the superconducting phase. Application of −4 to −5 V vs Ag/Ag+ to a conductive cathode substrate, immersed in an electrolyte containing salts of all of the metals, reduces the metal cations causing them to deposit on the cathode as a metallic film precursor. Precursor films having desired stoichiometries were obtained by regulating the electrolyte bath composition.

Chiral Metals: Synthesis and Properties of A New Class of Conducting Polymers

Abstract A new class of organic conductors, namely chiral conducting polymers, were prepared by three approaches. The synthesis of complexes incorporating either chiral polymer backbones, chiral dopants or chiral solvating ligands (for dopant ions) are described. Chiral complexes with conductivities as high as 60 S/cm have been obtained, but resulting conductivities are strongly dependent upon the structural nature of the complexes.

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.