Cis Polyacetylene Research Articles

Detection of electronic excited states in conjugated polymers by picosecond transient strain spectroscopy.

We describe a new type of spectroscopy based on picosecond transient strain in absorption photomodulation that can be used to detect and identify both allowed and forbidden optical transitions in solid thin films. We have applied the new spectroscopy to a variety of conducting polymer films such as polythiophene, trans and cis polyacetylene, and poly(diethynyl-silane), in which we measured energy levels of various excitonic states with odd and even symmetry that are, respectively, allowed and forbidden in the optical absorption.

Polymerization of acetylenes with rare earth coordination catalysts

AbstractThe polymerization of acetylene and its derivatives by rare earth coordination catalysts and the characterization of the polymers so obtained in our laboratory are reviewed.Because of the metallic conductivity possessed by doped polyacetylene and the unique properties such as conductivity (semiconductivity), paramagnetism, migration and transfer of energy and chemical reactivity and complex formation ability often shown by acetylenic polymers, which seem promising as specialty polymers, there has been considerable interest in the polymers of acetylene and its derivatives. A wide variety of catalyst systems have been developed for the polymerization of acetylenes. But there has been no information concerning the use of rare earth compounds as catalysts in the polymerization of acetylene and its derivatives. We for the first time in 1981 have succeeded in the polymerization of acetylene with rare earth coordination catalysts, which in turn is a development based upon earlier work on the diene polymerization using rare earth coordination catalysts(Ref. 1). Using rare earth catalysts, acetylene can be polymerized conveniently into high cis polyacetylene films with metallic sheen at room temperature and phenylacetylene can also be polymerized into high molecular weight, high cis polyphenylacetylene films at ambient temperature. Thus new varieties of polyacetylenes have been developed and a novel family of coordination catalysts consisting of a rare earth compound plus trialkyl aluminum for the polymerization of acetylenes has been exploited. This article reviews our studies on the polymerization of acetylene and its derivatives with rare earth coordination catalysts and on the characterization of the polyacetylenes prepared.

Polarizabilities of trans and cis polyacetylene and interactions among chains in crystalline polyacetylene

In this paper, we use the semi-empirical Hartree–Fock method introduced within the finite electric field perturbation approach, to calculate the molecular and π-system polarizabilities of trans and cis polyene H(-CH=CH-)nH (n = 3, 4, 5, …, 18). The result is that for a given n, the order of both molecular and π-system polarizabilities of polyene is trans > cis. This result is consistent with the order of experimental values of trans and cis hexatriene. When the n increases from 3 to 18, the contribution of π-system polarizability to the molecular polarizability increases from 45 to 70%. Using these results, we have extrapolated the polarizabilities of both trans and cis polyacetylene. The order is trans > cis. These results and crystal parameters lead to an estimate of the dispersion energies among polyacetylene chains. The result is also Edisp(trans) > Edisp(cis). Keywords: polarizability, polyacetylene, cis/trans, CNDO, dispersion energy.

Polarized resonant Raman scattering of c i s polyacetylene

Polarized resonance Raman measurements of highly oriented films of cis polyacetylene have been carried out. The intensities of the three Raman fundamentals in the four different scattering configurations have been measured and corrected for the strong anisotropy of the optical properties. The experimental data are indicative of an almost perfect alignment of the cis polyenic chains along the stretching direction. Preliminary measurements of the depolarization ratios carried out on isotropic samples in the resonance region appear to be inconsistent with the view that the visible band is dominated by a single transition polarized along the chain axis.

Electronic structures of carbon diselenide

The first-principles, density functional, full potential, LMTO method for helical polymers is applied on various forms of polymeric CSe 2, C 2Se, and CSeH and on molecular CSe 2. Some of the earlier proposed forms are found to be unstable and the existence of other structures is predicted. Special emphasis is put on a C 2Se compound with similarities with cis polyacetylene. It is found that it has two valence and one conduction π band close to the Fermi level, and there exists two (meta-) stable configurations. It should therefore be of interest as a conjugated polymer. An alternative form of a polymeric CSe chain is found. However, by considering interactions between two such chains and between one chain and hydrogen atoms it is demonstrated that it is reactive. Also this compound is believed to be interesting as a synthetic metal.

Polarized Raman spectra of polyacetylenes and polythiophenes

The results of an extensive study of the polarized resonant Raman scattering from cis polyacetylene and from polythiophene are presented. Depolarization ratios measured on isotropic samples appear to be inconsistent with the view that the visible absorption band is dominated by a single transition polarized along the chain axis.

Mechanism of isomerization of nascent molecular Cis polyacetylene in course of Ziegler-Natta polymerization of acetylene

We have studied the cis/trans isomeric compositions relative to two series of pristine polyacetylene films synthetized at different temperatures (−78°C to 150°C, with (Al/Ti) = Const) and at different (Al/Ti)-ratios (1 t0 4, with T = Const). It is shown, that isomeric compositions result from a first order chemical process with Aτ = 37 ± 9 and with a surprisingly small activation energy E a = (2.3 ± 0.5) kcal/molePA. This is about ten to twenty times less than E as characterizing the thermal isomerisation of PA Cis contained in PA free standing films. A model is proposed of a first order chemical process based on the presence in the Z-N medium of an equilibrium of two active species of catalyst. Such equilibrium is both temperature and (Al/Ti) controled.

Model calculations for cis polyacetylene

A model Hamiltonian for describing cis polyacetylene is developed and examined. Compared with the familiar Su-Schrieffer-Heeger Hamiltonian it includes in the π electron tight-binding part next-nearest neighbour interactions as well as on-site terms and two different types of nearest neighbour hopping integrals. The δ part is generalized to a fourth order polynomial in the configuration coordinates in order to be able to vary the dimerization energies. Various choices of the parameters have been examined and for each set we have optimized geometries for ground states and singly charged solitons and polarons. For solitons and polarons gap states show up only for certain values of the hopping integrals. Although the lattice distortions depend strongly on the dimerization energies the induced electron densities do not.

Lattice Dynamical Calculations of Trans and Cis Polyacetylene

Abstract Lattice dynamical calculations of the lattice modes for both trans and cis polyacetylene were performed. Using rigid body approximation for the chain we obtained all 8 lattice frequencies and modes of motion for both trans and cis polyacetylene. The frequencies concerned with Raman active lattice modes are presented for the first time. We also performed far infrared transmission measurements of undoped trans and cis polyacetylene and confirmed the results obtained by Bechtold et al. Comparison of the lattice frequencies obtained by lattice dynamical calculations with the results obtained by far infrared measurement and inelastic neutron scattering is discussed.

Durham poly acetylene: preparation and properties of the unoriented material

Polyacetylene prepared by the Shirakawa route is a mat of crystalline fibrills, which are capable of being oxidized or reduced to a highly conducting state. Polyacetylene can also be produced from a soluble precursor polymer by a thermally driven transformation reaction (the ‘Durham’ route) as a solid, coherent film. This transformation consists of the elimination of a substituted benzene unit followed by the isomerization of the largely cis polyacetylene produced to a disordered trans isomer. We have studied the effect of different transformation conditions and from these studies it is possible to gain an insight into these processes. Techniques such as caloremetry, infrared, resonance Raman and optical spectroscopy, e.s.r., X-ray diffraction and electrical conductivity have all been used to probe the transformation and the product. The resultant information gives a consistent picture of the overall transformation processes detailed above but also shows an ordering associated with higher transformation temperatures. A thorough knowledge of these processes enables the orientation of the precursor to produce highly crystalline material.

Self-Consistent Electronic Structures of Polyacetylene. The π Electrons

The electronic structures of undimerized and dimerized trans and cis polyacetylene have been calculated by means of the first principles Linear-Muffin-Tin-Orbital method. The π electronic states, which are supposed to be particular important to many of the physical properties of polyacetylene, are investigated in detail. In this connection it is examined to which extent model Hamiltonians of the type used by Su, Schrieffer, and Heeger can be applied.

Synthesis of high cis polyacetylene with coordination catalysts consisting of rare earth naphtenate

High cis polyacetylene films with silvery metallic appearance were prepared by coordination catalysts, comprising of naphthenates of all fifteen rare earth elements, and trialkyl aluminum at room temperature. The resultant polyacetylene (PA)was characterized by infrared spectrophotometry, electron spin resonance, X-ray diffraction, scanning and transmission electron micrography, differential scanning calorimetry and electrical resistivity measurements. The enthalpy and apparent activation energy of thermal cis-trans isomerization of 98% cis PA were 1.7–1.8 kcal/mol and 19.0 kcal/mol respectively. Rare earth PA shows higher thermal and antioxidization stability than does PA with other catalysts.

Optical Modes of Cis and Trans Polyacetylene

Abstract Partially oriented (CH)X and (CD)X films were obtained by rolling in the cis and trans form. This orientation is evidenced by X ray and polarized infrared spectroscopy. A quantitative analysis of these spectra leads to a separation between the purely parallel and purely perpendicular contributions to absorption. A check with group theoretical predictions allows then a complete assignment of the observed peaks to normal modes of the chain.

Resonance Raman Studies of the Elimination and Transformation Reaction of Durham Polyacetylene

Abstract The elimination and transformation reaction of. Durham polyacetylene has been monitored “by resonance Raman spectroscopy. The resonance Raman and photoluminescence spectrum from short cis segments, present in the pre-cursor polymer, has been recorded. A distribution of cis polyacetylene sequence lengths has been observed from Raman measurements made at different time intervals as the reaction proceeds, using λg 647-1nm. The μ2 values recorded gradually decrease as the reaction proceeds the limiting value of 1472 cm−1 is significantly less than the 1450–1460 cm−1 values reported for the Shirakawa polymer.

Far infrared spectra of the lattice modes in trans and cis polyacetylene

Abstract Far infrared transmission spectra of undoped cis and trans polyacetylene are reported in the frequency range from 30 cm-1 to 450 cm-1. In both modifications two lines are found in the vicinity of 100 cm-1, which are attributed to lattice modes. Several additional lines are observed in cis polyacetylene.

New scandium coordination catalysts for the cis polymerization of acetylene

The catalytic activity of scandium has not been widely studied. This article reports the first study of a new kind of Ziegler-Natta catalyst, the combination of a scandium compound and aluminum trialkyl, for the polymerization of acetylene. Some characteristics and kinetic aspects of acetylene polymerization by scandium catalysts and the results of the characterization of polyacetylene so obtained are described. High cis polyacetylene films (95% cis) with silvery metallic sheen and high crystallinity (70%) were obtained at ambient temperature (30–0 °C). The electrical conductivity of iodine doped polyacetylene reached hundreds ohm −1 cm −1.

Far infrared optical properties of MoCl 5 doped CIS polyacetylene

Far infrared transmission spectra of MoCl 5 homogeneously doped polyacetylene are reported. They show that in this spectral range the optical index is frequency independent but increases strongly with increasing doping level. IR conductivity is compared to d.c. conductivity.

Dynamics of CIS polyacetylene

We present a dynamical study of cis polyacetylene. Using short range inter atomic forces we have calculated the vibration frequencies of the perfect chain and of the chain with several defects. Comparison with experimental results allows an iterpretation of observed IR and Raman spectra of the as grown material and of the material during cis trans isomerization.

Modification of (CH) x chains during thermal cis-trans isomerization

It is shown that the isomerization of cis polyacetylene at 195°C for long periods is accompanied by structural modifications. Infra-red and pyrolysis gas-chromatography data suggest that the aromatization process occurs even at this temperature.

Infrared photoluminescence in polyacetylene

More extensive photoluminescence measurements of polyacetylene reveal a broad new emission band between 1.2 and 1.6 eV in samples with various isomeric contents. The peak energy and the intensity of this low energy luminescence decrease as the cis fraction decreases, but the band is still present in fully converted trans samples. These characteristics suggest that the infrared emission either is due to perturbed fragments of cis polyacetylene or is unquenched 1Ag luminescence from segments of trans (CH) x.