Pristine Trans-polyacetylene Research Articles

Relationships between the Raman Excitation Photon Energies and Its Wavenumbers in Doped trans-Polyacetylene

The resonance Raman spectra of trans-polyacetylene films doped heavily with electron donor (Na) and acceptor (HClO4) have been measured with excitation wavelengths between 488- and 1320-nm, and the relationships between the Raman excitation photon energies (2.54-0.94 eV) and its wavenumbers were discussed. We found the linear dependence of the Raman shifts with the exchanges of excitation photon energies. In particular, the Raman wavenumbers in the C=C stretching <TEX>$(V_1$</TEX> band) showed a dramatic decrease with the increase in Raman excitation photon energies. In the case of acceptor doping, its change is larger than that of donor doping. The observed wavenumber (1255-1267 <TEX>$cm^{-1}$</TEX>) of the <TEX>$V_2$</TEX> band (CC stretch) of Na-doped form is lower than that of the corresponding band (1290-1292 <TEX>$cm^{-1}$</TEX>) of its pristine trans-polyacetylene, whereas the contrary is the case for the HClO4 doped form (1295-1300 <TEX>$cm^{-1}$</TEX>). The origin of doping-induced Raman bands is discussed in terms of negative and positive polarons.

The 1320-nm excited FT-Raman spectra of Na-doped trans-polyacetylene

The FT-Raman spectra of trans-polyacetylene doped with Na at various concentrations were observed with the 1320-nm laser line. Doping-induced Raman bands for lightly doped trans-polyacetylene were observed for the first time. The observed wavenumbers of the ν 1 and ν 4 bands (C=C and C=C stretches) of lightly doped trans-polyacetylene are lower than those of the corresponding bands of pristine trans-polyacetylene, whereas the contrary is the case for heavily doped trans-polyacetylene. The origins of doping-induced Raman bands are discussed in terms of solitons and polarons.

Study of anisotropic spin dynamics in pristine trans-polyacetylene by means of 2 mm EPR spectroscopy

Spin-lattice and spin-spin relaxation times of paramagnetic centres in initial and partly oriented trans-polyacetylene have been measured by a saturation method at 2 mm waveband EPR in the 90–300 K temperature range. Temperature dependences of the intrachain diffusion and interchain hopping rates of the paramagnetic centres in the latter samples are evaluated. Spin dynamics in the oriented sample has been shown to depend on its orientation in a magnetic field and is in good agreement with Kivelson's theory of charge transport in trans-polyacetylene via intersoliton electron hopping. Temperature evaluation of the neutral soliton width has been investigated.

Spin dynamics study in FSO 3−-doped polyacetylene conversion from metallic state to neutral soliton by degradation

A frequency dependence of NMR T 1 and ESR line width were investigated to study dynamics of spin carrier in polyacetylene heavily doped with the acceptor ion FSO 3 −. Two kinds of spin species were found: Pauli-like and Curie-like spins. The observed NMR T 1 was concluded to be governed by the Curie-like spin but not by the Pauli-like one from a conductivity consideration. The dynamics of the Curie-like spin was found to be similar to that found for the neutral soliton in the pristine trans-polyacetylene. Furthermore, we found a gradual change of the spin species from the Pauli like spin to the Curie like one with time going by, where the sample was sealed into a glass tube. Finally, it was concluded that the present degradation should convert the heavily doped metallic state to the neutral solitons.

Spin-lattice relaxation of the solitonic defect in isotopically substituted trans-polyacetylene

We have measured the spin-lattice relaxation rates, R{sub 1}, for the spin of the electron defect in pristine trans-polyacetylene (t-PA) as a function of temperature for the isotopic forms (CD){sub x}, ({sup 13}CD){sub x}, (CH){sub x}, and ({sup 13}CH){sub x}. The data have been compared with the soliton-phonon-based dynamics model of Maki and the localized metal-phonon based dynamics model of Robinson et al. We concluded that the latter model is preferred; it exactly explains the R{sub 1} data for (CD){sub x} and ({sup 13}CD){sub x} t-PA. We find that the R{sub 1} data of the proton-containing forms of t-PA, (CH){sub x} and ({sup 13}CH){sub x}, are complicated by the presence of spin 1/2 nuclei, which are strongly coupled to each other, and which provide an additional dynamic path for defect-spin relaxation.

The physical meaning of the conjugation length in polymers

Raman scattering experiments were performed on pristine trans-polyacetylene, on polymer samples where defects had been accomodated systematically along the chains, and on polydiacetylene. The results show that the dispersion of the satellite line strongly depends on the concentration of the defects. A calculation showed the sp 3 carbons to be the most effective interruptions of the conjugation. Other physical and chemical properties like electrochemical doping, linewidth of e.s.r. signals and conductivity after doping are strongly related to the dispersion.

Characterization of neutral soliton dynamics in pristine trans-polyacetylene by means of anisotropic ESR T1 and line width

Abstract Anisotropy patterns for ESR T1 and line width at 300 K and at 50, 500 MHz and X-band were reported. Phases and frequency dependences of all patterns were understood by rapid 1-D diffusion of neutral solitons, except for line width of t−(CH)x, which could be reasonably interpreted by the trapping effect. This result means that in analysis on the neutral soliton dynamics, effects by trapping should be taken into account.

Evidence for soliton-phonon interaction in trans-polyacetylene: temperature and frequency dependence of electron spin-lattice relaxation data

The temperature, frequency, and isotope dependence of the electron spin-lattice relaxation rate R1 for the paramagnetic defects in pristine trans-polyacetylene is analyzed in terms of a model in which the electron-electron dipolar interaction between spins is modulated by one-dimensional diffusion induced by phonon scattering of the mobile spins. At room temperature, this model predicts a T/sup 1/2/ temperature dependence of R1; at low temperatures, depending upon assumptions about the phonon spectrum estimates on the temperature dependence of R1 range from TS to T/sup 5/2/. The frequency dependence of R1 is theoretically predicted to be described by a omega/sup -1/2/ dependence which is found to accurately fit the experimental data obtained over the frequency range 35 MHz to 40 GHz. The interpretation of electron spin-lattice relaxation data is consistent with ENDOR, electron phase memory relaxation data, EPR line-width data, and nuclear relaxation data. The implications of this analysis are considered for various solution descriptions of the paramagnetic defect. 42 references, 8 figures.

The Temperature Dependence of Electron Spin-Lattice Relaxation Data in Trans-Polyacetylene and the Evidence for a Soliton-Phonon Interaction

Abstract The temperature and frequency dependence of the electron spin-lattice relaxation rate Rj for the paramagnetic defects in pristine transpolyacetylene is analyzed in terms of a model in which the electron-electron dipolar interaction between spins is modulated by one-dimensional diffusion induced by phonon scattering of the spins. According to this model, at room temperatures R1 has a T1/2 temperature dependence; at low temperatures R1 has a T2 or T5/2 temperature dependence. R1 is described by a ω−1/2 frequency dependence. The model shows the symmetry between moving and stationary solitons scattered by phonons. The soliton wave function obtained from experimental ENDOR data is used to obtain analytical results. We discuss the possible temperature and frequency limitations of this model.

Frequency dependence of electron spin-lattice relaxation rate at 5–450 MHz in pristine trans-polyacetylene --- new evidence of one dimensional diffusive motion of electron spin (neutral soliton) ---

Electron spin-lattice relaxation rate T l −1 has been measured at low frequencies 5–450 MHz in pristine trans-polyacetylene. Frequency dependence of T l −1 gave dimensionality of diffusive motion of electron spins (neutral soliton). Relaxation mechanism was identified as dipolar and hyperfine origin based on an electron spin concentration dependence of T l −1.

Spin and charge excitations in polyacetylene

Abstract Spin and charge excitations in pristine and AsF5-doped polyacetylene have been investigated to resolve the issue of whether or not topological solitons have been observed. We present experimental results for both materials suggesting that the simple model from which the soliton-based interpretations have been derived is inapplicable. In pristine trans-polyacetylene, both protonated and deuterated, conclusions are deduced from spin-echo and continuous-wave EPR about the spin diffusion rate, its temperature dependence and anisotropy that contradict predictions of the soliton theory. In AsF5-doped cis-polyacetyelene, we conclude from magnetic and transport measurements that the doping is very inhomogeneous, the doped and undoped parts being largely distinct with the doped regions metallic. The conductivity changes previously interpreted as arising from a transition wthin the doped region are attributed to a change in the topology of these regions, i. e. the crossing of a percolation threshold to a r...

The structure of polyconjugated macromolecules

Recent experimental results concerning the crystal structure and morphology of nascent cis-poly-acetylene and pristine trans-polyacetylene are discussed. The unit cell data of cis-PA and two different modifications of trans-PA are reported. The chlorination and the hydrogenation of PA both of which give soluble derivatives under certain experimental circumstances are critically discussed. PA prepared by the Shirakawa technique has a lamellar morphology with some evidence for chain folding and not a fibirillar texture as sometimes assumed. The synthesis and spectral properties of polyenylanions and -cations are reported as a new class of polyconjugated macromolecules. The so-called doping of PA to produce highly conducting phases is explained as a chemical reaction giving rise to intermolecular CT-interaction. Finally, some new experiments concerning the solution properties of polydiacetylenes and the question of effective conjugation length in polyunsaturated polymers are discussed.