Polysilane polymers conductivity

As the low temperature synthesis yields a more homogenous polymer with respect to polydispersity and molecular weight, toluene can be exchanged by lower boiling solvents such as, e.g. THE Solvent effects, due to polarity and stabilization of the active species in the polymerization, are marginal compared to the temperature effect. The results of a Wurtz-type polysilane synthesis conducted at ambient temperature are given in Table 1, illustrating the low yields and PDI [19]. 

Interest in polysilane polymers has been reawakened because they have appeared as new potential industrial raw materials for production of conducting and semi-conducting electronic devices, photomemories, photoresists, UV-absorbing and thermochromic materials, radical photoinitiators for polymerization, precursors for silicon carbide ceramics and fibers, organic glasses and m cal drugs [7-12]. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

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 are cr-conjugated polymers composed of Si-Si skeletons and organic pendant groups. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong cr conjugation, they have rather narrow band gaps of less than 4 eV [24,25] and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Recently they have attracted much attention because of their potential utility as one-dimensional conductors, nonlinear optical materials, and electroluminescent materials [26-28]. 

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. 

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. 

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.

Doping with iodine, on the other hand, gave lower conductivities (ca 10-6 to 10-8 Scm-1, see Table 1), but which could be correlated with the ionization potentials of the polymers (lower ionization potentials giving higher conductivity). This is consistent with simple electron transfer from the polysilane to the iodine, with formation of delocalized holes along the polysilane chain. 

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. 

Silicon-based polymers form a dimensional hierarchy from disilanes, to crystal silicon, and through polysilanes, ladder polymers, siloxenes, polysilane alloys, clusters, and amorphous silicons and include unsaturated systems, such as polysilenes, hexasilabenzenes, and so on. Their properties depend basically on the network dimensions and can vary from conducting (metallic) and semiconducting to insulating. 

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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