Trans polysilane

FIGURE 5. The cr-symmetry orbitals of a long, all-trans polysilane chain showing backbone orbitals (left), orbitals of the SiH bonds (right), and the result of their mutual interaction (center). (Reprinted from Ref. 63.)... 

This dominant feature is essentially the same for both the unsubstituted and dimethyl-substituted all-trans polysilane chains, and an equivalent feature is found when a smaller basis set is used for the dimethyl-, diethyl-, and dipropyl-substituted poly silanes. For the helical conformation, however, along with the larger band gap in this conformation (Figure 3c), a pronounced shift of the direct-gap absorption peak to higher energy is observed, with a trend toward a less anisotropic absorption. 

The value of mh greatly depends on the helical angle 6, whereas that of m does not. The mh of gauche polysilane by DZ is 8mo, being 66 times heavier than the mh of the trans polysilane, 0.12/no. 

This finding is interpreted physically as follows. The valence band edge of the polysilane consists of pure 3p orbital of the silicon atom. The orbital interaction is maximum at the trans conformation because the orbital stands in a coplane. On the other hand, the two 3p orbital lobes in gauche polysilane are not arranged in a coplane. The interaction of these orbitals, therefore, is less than that for the trans polysilane. 

The coordinated silylenes in both the iron and the chromium compounds can be photolytically activated Photolysis of the complexes in the presence of triphenylphosphine gives the trans-silylene-phosphine complex, which in a second step is transformed into the trnns-bisphosphine compound by excess phosphine. If the silylenes are not trapped, polysilanes are isolated in almost quantitative... 

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. 

Figure 4. The INDO/S first singlet excitation energies of polysilanes, (SiH2)n full line, all trans broken line, all gauche.

The Si-Si bonds of the ladder polysilanes are cleaved by electrophiles.39,60,61 When 1 was allowed to react with an excess amount of PdCl2(PhCN)2, trans-1 and cis-1 were obtained in 36 and 15% yields, respectively (Scheme 10).39 Other chlorinated products were not formed, indicating the bridgehead Si-Si bond was selectively cleaved. The selectivity is rationalized by the coordination of the bridgehead Si-Si bond, in which... 

The reaction of 1 with hydrobromic acid gave quantitatively the cis adduct 14 (Scheme 11). The reaction of 1 with hydrochloric acid gave the cis adduct 15 in 79% yield and the trans adduct 16 in 18% yield. In these reactions, no other Si-Si bond cleavage products were obtained. The ladder polysilane 1 did not react with hydrofluoric acid. The reactions of 1,4-di-ter -butyl-2,2,3,3,5,5,6,6-octaisopropylbicyclo[2.2.0]hexasilane with hydrobromic acid and hydrochloric acid were attempted, but no reactions took place. This result is ascribed to steric hindrance by the tert-butyl groups on the bridgehead silicon atoms. 

Table III shows x values for other structurally regular substituted silane high polymers measured both at 1.064 and 1.907 /an. Examination of this data suggests relatively little difference between the polysilanes with nonplanar, yet regular structures and trans planar PDN6S which is included in the table for comparison. This result is a little surprising given that changes in backbone conformation can cause spectral absorption shifts of more than 60 nm.

On the other hand, the polysilanes are not strictly infinite one-dimensional chains. Several experimental studies (15.45.47) in solution have suggested that the polysilanes are trans or nearly trans planar only over 20-35 silicon atoms, restricted... 

There has been a great deal of interest in the UV-visible spectroscopy of the polygermanes, particularly in comparison with the analogous chains that have silicon or tin backbones.41,42 Both conventional and Raman spectroscopy have been employed. One interesting observation is that the symmetrically disubstituted polyfdi- n - a I k y I g e n nanes) exhibit thermochromic transitions at temperatures below those of their polysilane analogues. Another is the conclusion that in poly(di-n-hexylgermane) the side chains adopt trans-planar conformations as in the polysilane counterpart. The two chains are also similar in that both backbones can, under certain circumstances, also adopt planar zig-zag conformations. 

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