Defects, poly acetylene

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Figure 4.8-5 Raman spectra of poly(acetylene), degraded by exposure to ambient conditions. The numbers refer to the exposure time in hours, according to Knoll and Kuzmany, 1984 (a), and to the conductivity of the samples after iodine doping with different concentrations of defects, characterized by the ratio R of the satellite peak intensity to the primary- peak intensity of the C=C stretching mode, according to Schaefer-Siebert et al. 1987 (b).

Figure 1,17. Electronic energy levels associated with various types of defect structures found in doped poly acetylene.

It has already been observed that spin-charge inversion is one of the remarkable features of a conjugational defect in poly acetylene charged defects are spinless neutral defects carry a magnetic moment. This inversion is related to fractionized charges, which also occur in elementary-particle physics and in the fractionized quantum Hall effect [107]. Spin-charge inversion is a symmetry property of the system, it makes use of the quasi-particle concept, but non-dispersive motion is not essential in this respect. 

An important progress (concerning (CH) properties) occurred by a comparison of the various types of poly(acetylene) [23] and revealed some astonishing correlations conductivity was directly proportional to crystallinity and inversely proportional to the number of sp orbitals. This discovery was the key to the production of new poly(acetylene) types with fewer defects and greater stability. Another important advance was the modification of the polymerization conditions, e.g., using silicone oil or other viscous media. For instance, (CH) can be polymerized at room temperature to yield a new (CH) poly(acetylene) of at least the same quality as the standard (CH) obtained at... 

Pristine poly(acetylene) in either cis- or trans-form exhibits optical spectra typical for a semiconductor. The absorption peak of the cis-polymer is structured with maxima at 2.1, 2.3 and 2.4 eV, the absorption coefficient is about 4 10 cm" The structureless and very broad absorption of the trans-polymer peaks at 1.9 eV with an absorption coefficient of 3 10 cm . The large halfwidth of the peak is generally explained as being due to the rather broad chain length distribution in addition, the small crystallite size and crystal defects may contribute to the broadening. A discussion is still ungoing whether the peak is linked to a direct interband transition or whether it is of an excitonic character.2-4, 7,32... 

Fig. 1-1 Schematic representation of 7r-conjugation in the CP poly(acetylene). Left Basic schematic. Rieht 3-dimensional, including defects.

Sketch the structure of trans-poly acetylene containing a carbanion (the organic equivalent to an electron ) and a carbocation (hole) as separate sohtons, not as an exciton pair in to configurations (i) with both defects on one side of the chain and (ii) with the defects on opposite sides of the chain. Briefly explain why the structure is more stable with defects on opposite sides of the chain. 

Numerous articles have appeared on poly(arylene vinylene) systems, acetylenes and related polymers. The temperature dependence of the photoluminescence from poly(p-phenylene vinylene) (PPPV) has been examined in terms of its influence on diffusion rates while a magnetic field has been found to enhance luminescence intensity. A series of poly(alkoxyphenylenes) have been synthesised with high luminescence quantum yields whereas vapour deposited poly(p-phenylene vinylene) has been found to have an unidentifiable defect structure when annealed at high temperatures. ... 

---