J. C. Scott

Preparation and characterization of neutral and oxidized polypyrrole films

Abstract Oxidized and neutral films of polypyrrole have been prepared electrochemically in the absence of oxygen and water. The neutral films are insulating and can be readily oxidized by chemical oxidizing agents to give films of greater conductivity than can be achieved by electrochemical oxidation. Optical spectroscopy provides evidence for the similarity of the polymeric carbonium ion produced by both types of oxidation. NMR studies are consistent with the α,α’ bonding in these polymers; they also show the expected downfield shifts relative to the neutral polymer on both chemical and electrochemical oxidation. ESR studies of both the electrochemically oxidized and the neutral polymer suggest the presence of highly mobile spins.

The evidence for bipolarons in pyrrole polymers

Abstract We present experimental data and theoretocal studies which support the hypothesis that bipolarons are the charge carriers responsible for the conductivity of polypyrrole. A consistent model emerges from consideration of earlier transport, ESR, photoelectron spectroscopy and optical absorption results.

Comparative ESR study of three (BEDT-TTF): ReO4 salts: An organic superconductor, a peierls metal and a semiconductor

Abstract We present electron spin resonance results on three different conducting phases of the organic conductor based on the planar molecule BEDT-TTF ( bis (ethylenedithio)tetrathiafulvalene) and the anion ReO 4 in different stoichiometries. Both the paramagnetic susceptibility χ as well as the ESR-linewidth Δ H pp may be used to classify the magnetic behaviour in terms of known behaviour of other organic metals. They indicate that the 2:1 phase, which under pressure becomes superconducting, has an anion-ordering assisted metal-insulator transition near 80 K and that the 3:2 phase has a Peierls transition at 88 K, below which two ESR lines are resolved. A third phase, which includes solvent in the crystal lattice, shows no evidence of a transition.

Synthesis of Biethylenedithiolylene-Tetrathiafulvalene Donors (BEDT-TTF) and Electrochemical Preparation of their Charge Transfer Complexes

Abstract Various synthetic procedures for preparing BEDT-TTF are critically examined. Starting from readily available thiapendione, BEDT-TTF can be made in a high yield, simple, two-step synthesis. Electrochemical crystal growth of charge transfer complexes of BEDT-TTF typically involves the formation of multiple phases and stoichiometries. In the case of perrhenate (ReO- 4) at least five different complexes have been identified. Controlled preparation of the superconducting phase (BEDT-TTF2ReO4) remains an elusive goal.

Evolution of Polaron States Into Bipolarons In Polypyrrole

Abstract The kinetics of polaron recombination into bipolarons in polypyrroie have been studied using optical absorption spectroscopy on films doped in situ in an electrochemical cell. ESR studies confirm that the metastable states possess spin, while the stable states do not. We also report the development of a new electrochemical technique which indicates charge is stored at two localized energy levels in the gap. The optical and ESR data both reveal a slow evolution, consistent with polaron diffusion rates limited by the mobility of the accompanying counterion.

Infrared and Raman Study of the Anion-Donor Interactions in Tetrahedral Anion (TMTSF)2 X and (Tmttf)2 X Salts

Abstract Some spectral features that can be used to probe the anion-donor interactions have been identified in the infrared absorption spectra of the title compounds with X=Reo− 4, BF− 4, and ClO− 4. The sudden increase of vibronic, absorption intensities observed at the anion ordering (AO) transitions in the Reo− 4 and BF− 4 salts of TMTSF and TMTTF shows that the ordering is accompanied by a distortion of the conducting chains. An anomalously large splitting is observed for these salts at low temperature in the 1500-1650 cm−1 region where the ring C=C stretching modes are expected to occur. However a single resonance is observed for the central C=C stretching mode in the Raman spectra. Analysis of the spectroscopic data point to a strong intramolecular polarization of the electronic charge on the TMTSF or TMTTF units induced by the interaction with the anions. The localization of carriers along the chains is either much less than predicted by the theory or its effects are averaged by rapid random motion...

Photothermal deflection spectroscopy : a sensitive absorption technique for organic thin films

Abstract An important property of an organic film is its absorption spectrum. However, because of the short optical path length in a thin film, only very strong electronic transitions can be measured by standard spectrometric techniques. With the sensitive photothermal deflection approach it is possible to measure the absorption from transitions which can be as much as four orders of magnitude weaker than the main electronic transitions. We have applied photothermal deflection spectroscopy to a variety of organic thin films with the thickness ranging from sub-micron to hundreds of microns. Several small molecule and polymer binding matrix systems have been studied. Some results are presented and compared with standard UV-Vis spectra.

The Chemistry of Polyacetylene Doping.

Abstract Natural abundance 13C NMR experiments using cross polarization and magic angle spinning provide a unique method for monitoring the doping of polyacetylene. In contrast with previous authors we find that the doping of cis-polyacetylene with AsF5 leads, even at low doping levels, to a new signal characteristic of the doped regions of the polymer. Similar measurements on trans-polyacetylene indicate that the doping of the trans isomer proceeds considerably more uniformly than that of the cis isomer. Comparisons with appropriate model compounds suggest that the observed shift of the 13C NMR signal on doping is primarily a chemical shift rather than a Knight shift. Finally we discuss recent results on the nature of proton acid doping of polyacetylene, a process which shows remarkable similarities to oxidative doping.

The Properties of the Series of Organic Conductors (TMTSF)2X: Spin Density Waves and Charge Density Waves

The series of one-dimensional organic conductors (TMTSF)2X, X = PF6-, AsF6-, NO3-, BF4-, ReO4-, ClO4- and others has shown a remarkable richness of interesting properties. Most members of the series become superconductors under modest pressures ranging from ambient to 15 kbar at temperatures between 0.9 and 1.6 K. Metal-insulator transitions occur in the majority of compounds in the family but the nature hereof and the transition temperature is widely different among the individual members: The hexafluorides seem to be very much alike and they all probably undergo antiferromagnetic phase transitions into spin density wave states near 15 K. By contrast the ReO4- compound undergoes a Peierls type of transition near 180 K. (TMTSF)2ClO4 does not undergo a complete metal-insulator transition and remains highly conducting down to 1.5 K at which temperature it becomes superconducting. However, an anomaly is found in the spin susceptibility of (TMTSF)2ClO4 near 6 K, so the existence of spin density waves in this material is not yet ruled out. Experimental results reported here include electron spin resonance, nuclear magnetic resonance, microwave conductivity, microwave dielectric constant and static magnetic susceptibility of various members of the family of organic conductors (TMTSF)2X.

Filamentary Superconducting-Normal-Metal Composites

Abstract We describe the properties of a composite material consisting of superconducting filaments separated by normal metal barriers. The coupling between superconducting elements, and hence the diamagnetic response, is by proximity effect diffusion through the normal material and increases as the temperature is lowered. This behavior is contrasted with that of Josephson tunnelling in anisotropic structures which has been shown to lead to a decoupling transition. The diamagnetism and critical field behavior of (SN)X, TaSe3, and NbSe3 are compared with the predictions of the model.