Polyacetylene photoconductivity

The excellent agreement between the TSC and P1A results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the P1A probes only the carrier density, there seems to be no dominant influence of temperature on the carrier mobility. This was also found in other conjugated polymers like /ra/ry-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped earners) and photo-induced absorption probe the same charged entity [36, 37J. 

Photoconduction and absorption spectra of trans-polyacetylene are presented in Fig. 16 [104],... 

These and other results on absorption and luminescence were the reason why the soliton model was proposed for the photoconduction process in trans-polyacetylene [103-110],... 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

Chance and Baughman24 studied photoconductivity in poly[2,4-hexadiyn-l, 6-diol bis(p-toluene sulfonate)]25. Simple polyacetylenes such as this one are unique... 

When a new polymer has been synthesized and its structure and properties have been clarified, then it becomes an interesting problem to develop functions of the polymer. As has been stated above, the electrical conductivity of substituted polyacetylenes is much lower than that of polyacetylene, and therefore their application to electric and electronic fields might be restricted. The photoconductive behavior of poly(phenylacetylene) has been reported 103). 

A number of other unsaturated poiyhydrocarbons have practical applications. These include poiy(phenyl acetylene) and poly((E,E)-[6.2]paracyclophane-1,5-diene), which have been studied as photoconducting polymers. The thermal decomposition of polyacetylenes and of poly((E,E)-[6.2]paracyclophane-1,5-diene) generates fragments summarized in Table 7.1.8 [19]. 

Keeping all these things in mind, it is intriguing to address the following two questions. (1) Why do all-frans-/3-carotene crystals give rise to photoconductivity that extends to the near-infrared region (2) Is it possible to find metastable states in dA -trans- -carotene crystals that corresponds to the solitons in fran -polyacetylene In order to answer these questions, we have applied photoinduced and time-resolved absorption spectroscopies to the aW-trans-/3-carotene single crystals (Hashimoto et al., 1998). 

Photoconductivity has been observed in rawj -polyacetylene, but found to be absent in cw-polymer d39 other hand, photoluminescence has been observed in the cis- but... 

Typical functions of substituted poly acetylenes are based on their (i) high gas permeability and (ii) electronic and photonic properties. The former originates from the rigid main chain and bulky substituents. Though electrical insulators, substituted polyacetylenes are more or less conjugated polymers, and this feature has been utilized to develop their electronic and photonic functions such as photoconductivity, electrochromism, optical nonlinearity and ferromagnetism. 

In this paper we report on the photoconductivity of fully oriented Durham/Graz-polyacetylene and of fully converted, highly oriented polyphenylenevinylene (PPV) films. Though the two materials are different in several aspects - e.g. polyacetylene is adegenerate ground state system whereas PPV is the non-degenerate one, which has important consequences for the kind of possible excitations - they both show similar behaviour, when looking at the transient photocurrent response. 

For the ECPs with degenerated ground state, solitons are responsible for the charge transport. Such a polymeric system is polyacetylene. The major problem on the preparation of polyacetylene/Cgjj nanocomposite is the insolubility of polyacetylene. This has led to the discovery of soluble polyacetylene derivatives such as poly(o-trimethylsilylphenylacetylene) (PTMSiPA) [28]. Small amoimts of doping in PTMSiPA is found to enhance the photoconductivity of pristine PTMSiPA [29]. 

Relative quantum efficiency of the polyacetylene photoelectro-chemical cell. The open circles represent data taken with light incident through the highly eolored electrolyte. The data corrected for light absorption by the electrolyte (x), and photoconductivity response normalized to the photoelectrochemical cell... 

It is well known that in polyacetylene the PA spectrum consists of two absorption peaks, the so-called low-energy (LE) peak at 0.45 eV and the high-energy (HE) peak at about 1.4 eV, where the LE peak is attributed to charged excitations (S S pairs), while the HE peak is due to neutral soliton pairs (S S°) [161,128] (see section 6.3.1). Thus, with respect to photoconductivity the LE peak is of particular interest for comparison with our experimental data. 

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