Solid state polysilanes

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 previous papers [7, 17, 20], it has been demonstrated that VT Si CP/MAS NMR spectroscopy is a powerful means for characterizing the structures of solid-state polysilanes and other solid polymers [1, 30]. In this work, high-resolution Si NMR spectra and spin-lattice relaxation times, Ti, of CPTMPS/DMS in the solid state over a wide range of temperatures were measured and the conformational behavior and molecular motion of the main chain discussed. To discuss the experimental results in more detail, calculation of the Si shielding constants of the main Si atoms by means of FPT CNDO/2 MO framework was attempted. 

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. 

Finally, concerning dialkylpolysilanes, it is interesting to note that the solid-state (film) UV spectral profile of the thermochromism exhibited by 49,157 shown in Figure 28, almost exactly matches that of the solvatochromism (see Figure 16 above). This indicates that the before and after conformations are essentially the same and that the reduction of temperature or addition of HFIP are responsible for similar conformational changes in the polymer an abrupt straightening of the polysilane backbone. 

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,... 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

In an effort to increase the efficiency of scission, particularly in the solid-state where chain repair is most competitive, a number of external additives were auditioned. Since both the Xmax and the absorptivity of polysilane derivatives are functions of molecular weight, splitting the main polymer chain results in a rapid bleaching of the original absorption. This unusual feature can be used as a qualitative, diagnostic test of scission efficiency which greatly facilitates the testing of additives. 

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. 

Many polysilanes, both in the solid state and in solution, display a striking thermo-chromism—the absorption wavelength depends upon temperature. Before discussing this and the other chromotropic effects of polysilanes, it will be necessary to outline the modem theory of rotational conformations, described in the following section. 

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... 

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