Conductivity of polysilane

Table 5.3 Conductivities of Polysilanes (S cm-1) Doped with Iodine or FeCl3a...

Figure 5.18 Schematic drawing of experiment used to determine hole conductivity of polysilanes. Reprinted by permission from M. Stolka et al., J. Polym. Sci., Polym. Chem. Ed,., 1987, 25, 823. Copyright 1988 John Wiley and Sons, Inc.

In this chapter, the first section will deal with the complex question of conformations of polysilanes. This will be followed by sections on the chromotropism of polysilanes, beginning with their UV thermochromism. Despite intensive study this phenomenon is still not well understood, and is the subject of important recent findings. Solvatochromism, ionochromism and other types of chromotropic behavior of polysilanes will then be considered. A final brief section will deal with the electrical conductivity of polysilanes, a topic which has not previously been reviewed. 

TABLE 1. Conductivities of polysilanes (Scm ) doped with iodine or Fecl3 ... 

Table 15.1. Conductivity of polysilanes doped with oxidizing agents...

The reaction of dichlorosilane with lithium metal in THE produces a high molecular-weight poly silane (13) as an air-sensitive orange solid. The conductivity of polysilane 13 as compressed disks has been measured. The values of the conductivity lie in the range of 10" to 10 S cm" for the pristine sample [17]. 

The electric conductivities of polysilanes are < 1 x 10 Qcm and it is almost an insulator, but, if they are doped with an electron acceptor such as AsFs, for example, the electric conductivity of [(Me2Si)o.5(PhMeSi)i.o]n AsF5 becomes 2 X 10 Ocm , and the electric conductivities are drastically increased as shown in Table 8.5 [49]. 

Table 8.5 Conductivity of polysilanes doped with oxidants [49]...

Dehydrogenative Coupling of Hydride Functional Silanes. The autocouphng of dihydridosilanes was first observed usiag Wilkinson s catalyst (128). A considerable effort has been undertaken to enhance catalyst turnover and iacrease the molecular weight of polysilane products (129) because the materials have commercial potential ia ceramic, photoresist, and conductive polymer technology. 

A rapidly increasing number of publications on polysilanes documents current interest in these polymers (JJ. Polysilanes are potentially applicable in microlithography as high resolution UV-resists (2J, imageable etch barriers ), or contrast enhancement layers (4). They have been successfully used as precursors to Si-C fibers (5J and ceramic reinforcing agents ((L). Polysilanes have also initiated polymerization of vinyl monomers (J ). Doping of polysilanes have increased their conductivity to the level of semiconductors (8). Very recently polysilanes were used as photoconductors (9) and non-linear optical materials (10b... 

Polysilanes as reducing agents. In a paper on the formation of conductive circuits by UV patterning of polysilane films, the inherent reducing ability of polysilanes to effect the reduction of a silver salt to silver metal was described.307 PMPS, PPHS [(PhSiH) ], and PDFIS were separately spin-coated on glass or silicon wafer substrates, and after drying... 

The optical and electronic functions of polysilanes owe to their delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) that are occupied by holes and conduction electrons, respectively. The polymer does not show high conductivity or optical nonlinearity if the electrons or holes are localized on a small part of the polymer chain. To elucidate the structure of HOMO and LUMO is therefore important for the molecular design of polysilanes as functional materials. 

Analyses of the electronic and electron spin resonance (ESR) spectra of the radical cation and anion of polysilanes make it possible to elucidate the structure of HOMO and LUMO, because an unpaired electron in the radical anion or cation occupies HOMO or LUMO, respectively. As schematically depicted in Fig. 10, the radical ions of polysilanes show absorption bands in UV and near-IR regions [29 31]. The former band corresponds to intraband transitions between valence and conduction bands. The latter band corresponds to transitions within the valence or the conduction band [32,33]. Because the near-... 

IR bands of polysilane radical anions and the location of conduction electrons. Except for poly(diarylsilanes), the conduction electron is confined either in the skeleton or in one of the pendant groups. 

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. 

It has been shown that the EL of polysilane-based LEDs is emitted near the interface between the polysilane and the electron injecting electrode, because of the strong unipolar (hole conductive) nature of polysilanes. Defect levels existing at the interface are considered to play an essential role in the emission of EL in the visible region,93 and have both positive and negative effects on the LED characteristics. The positive space charges generated by... 

Simple admixture of arylamines to polysilanes also serves to increase the conductivity upon iodine doping. Addition of 30 wt% of AWW. iV -tetrakisO-methylphenyl)-1,3-diaminobenzene to (PhSiMe)n increased the conductivity of this polymer to 3 x 10"4 Scm-1. 

The synthesis and characterization of polyacetylene (PA) provided new incentive for understanding 7r-electronic spectra, electron-phonon interactions and electronic correlations[l, 2, 14]. The electrical conductivity of chemically doped PA rivals that of metals. Families in Fig. 2 such as polydiacetylenes (PDAs), poly thiophenes (PTs), cr-conjugated polysilane (PSs) and polyparaphenylene vinylene (PPVs), among... 

An LCAO (linear combination of atomic orbitals) local-density functional approach was used to calculate the band structures of a series of polymer chain conformations unsubstituted polysilane in the all-trans conformation and in a 411 helical conformation, and all-trans poly(dimethylsilane). Calculated absorption spectra predict a highly anisotropic absorption for the all-trans conformation of polysilane, with the threshold absorption peak arising strictly from polarizations parallel to the chain axis. The absorption spectrum for the helical conformation is much more isotropic. Results for the dimethyl-substituted polysilane chain suggest that the states immediately surrounding the Fermi level retain their silicon-backbone a character upon alkyl-group substitution, although the band gap decreases by I eV because of contributions from alkyl substituent states both below the valence band and above the conduction band to the frontier states. 

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