Solid-state studies polysilanes

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

Very recently, however, two papers were published by the group of Fujiki which report successful solid-state CD studies of chiral polysilanes. In the first, a helix-coil transition was described for film samples of poly[(A)-3,7-dimethyloctyl- -propylsilylene)], 113.327 This polymer has a relatively low glass transition temperature, T, which was considered critical for the observation of a helix-helix transition in the solid state, since helical inversion would be precluded if the inversion temperature, Tc, were below Ts as the segmental motion of the chain,... 

A second means of enhancing sensitivity is to increase the concentration of the sample in solution. Thus, 2D-INADEQUATE studies of 29 Si-29 Si correlations on polysilanes at natural 29Si abundance at high concentrations (1 -2 M) was reported12. Sophisticated solid-state NMR studies (see Section V) have also been performed, including two-dimensional... 

In recent years the literature has furnished a wealth of information concerning chemical structures from solid-state 29Si NMR studies of crystalline and noncrystalline silicates and aluminosilicate, polysiloxanes, polysilanes and other organosilicon compounds. 

With this new information in mind, an interpretation of the thermochromic behavior of polysilanes in solution can be outlined. We can begin with two calibration points (1) The polymers ( -butyl2Si) and ( -pentyl2Si) absorb at 315 nm both in the solid state and in solution both polymers as solids are known to have an all-D conformation, with co 154°. (2) The well-studied polymer ( -hexyl2 Si) , in its low-temperature phase, absorbs at 375 nm and has a (nearly) all-A conformation. Values of A,nax between 315 and 375 nm should correspond to intermediate values of the average torsional angle, ox... 

In the solid state, polysilanes present a far more complicated picture than in solution. Many different phases may be possible for a single polymer, and the transformations between them may be slow, so that the conformation and spectroscopic behavior often depends on the thermal history of the sample. In partial compensation, additional techniques for studying the polysilanes become available for the solids, especially X-ray scattering. Nevertheless the conformations and structures of solid polysilanes are not well understood. 

In addition, mechanistic studies of the photochemical reactions are necessary to determine whether similar processes occur in the solid state. Polymer chain scission is usually the predominant process in the solid state, although cross-linking reactions become more important in the presence of pendant unsaturation. However, little is known about the nature of the intermediates produced in the solid state. Information of this type is important, because most of the applications of polysilane derivatives require the materials as solid films. 

Crystalline and amorphous silicons, which are currently investigated in the field of solid-state physics, are still considered as unrelated to polysilanes and related macromolecules, which are studied in the field of organosilicon chemistry. A new idea proposed in this chapter is that these materials are related and can be understood in terms of the dimensional hierarchy of silicon-backbone materials. The electronic structures of one-dimensional polymers (polysilanes) are discussed. The effects of side groups and conformations were calculated theoretically and are discussed in the light of such experimental data as UV absorption, photoluminescence, and UV photospectroscopy (UPS) measurements. Finally, future directions in the development of silicon-based polymers are indicated on the basis of some novel efforts to extend silicon-based polymers to high-dimensional polymers, one-dimensional superlattices, and metallic polymers with alternating double bonds. 

Silicon-backbone materials include silane oligomers, polysilanes, silicon clusters, and amorphous and crystalline silicons. These materials have been investigated independently in two different fields. Crystalline and amorphous silicon are studied in the field of solid-state physics (i), whereas polysilanes and related molecules are studied in the field of organosilicon chemistry (2). Crystalline silicon (c-Si) and amorphous hydrogenated silicon (a-Si H) are well known as two of the most useful semiconductors for electronic and optical devices. Polysilanes have been investigated for application as SiC ceramic binders (3) and photoresists (4). The methods of synthesizing... 

The helical structure of polychloral was proposed by Vogl in 198028 and was demonstrated by Ute, Hatada, and Vogl via a detailed conformational analysis of chloral oligomers.29 As an example of a helical polymer with an inorganic backbone, polysilanes bearing a chiral side chain were synthesized and their conformational aspects were studied. A helical conformation with an excess screw sense for this class of polymers in solution was found in 1994 independently by Fujiki30a and by Moller.300 Matyjaszewski had pointed out such a conformation for chiral polysilanes in the solid state in 1992.30c... 

Polysilanes are a unique class of polymers in which the o--electrons are delocalized entirely along the sp -bonded silicon backbone, causing their electronic absorption properties to be strongly dependent on the conformation of silicon backbones [2]. This property has created much interest in the structure of these polymers in the solid state [3]. In spite of the usefulness of solid-state NMR, there are few systematic studies on the Si CP/MAS NMR of polysilanes [4-6]. Most recently, it has been demonstrated that the VT Si CP/MAS NMR experiment is very useful to study the conformational features of polysilanes in the solid state [7j. Measurements of the Si CP/MAS NMR spectra of poly (methylphenylsilane) (PMPS), in the solid state over a wide range of temperatures, are performed and the conformation... 

Solid-state multinuclear NMR experiments have successfully provided very useful information about inorganic-polymer conformations. The relationship between the solid-state conformations and NMR chemical shifts is of widespread interest. In this work, the structural behaviors of several substituted polysilanes in inorganic polymers were studied by means of Si CP/MAS... 

It was shown that VT Si CP/MAS NMR chemical shifts (S), Si-relax-ation times (Ti) and calculated Si-shielding constants (cr) are very useful in studying the solid-state order-disorder transitions in the polysilanes. 

Recently, high resolution solution and solid state 29Si NMR studies have been conducted on the polysilanes (RR Si)w124 127. It is obvious that this technique will be a powerful method for determining the microstructure of these polymers. Figure 12 shows a comparison between the 29SiNMR spectrum of poly(methyl-n-hexylsilane) and poly(di-n-hexylsilane)124. In the former case the effects of tacticity can be seen in the fine structure of the spectrum whereas in the latter case for which this is impossible only a single peak is observed. 

Electronic behavior of polysilanes in the solid state can be even more complex. The best-studied examples are (Hexyl2Si) (16) and (Pentyl2Si) (17). Compound 16 undergoes a reversible change, endothermic by 4.0 kcalmol-, at 42 °C96. Samples heated above 50 °C show Amax = 316 nm, much like that in solution, but in the low-temperature form the absorption band is shifted to 372 nm. Crystallization of the alkane side-chains is believed... 

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