Poly silanes helical conformation

In this study, we investigated a set of model polysilane chain systems that illustrate the basic physics and chemistry of some optical properties of these materials. In particular, we looked at the band structure for unsubstituted polysilane in an all-trans conformation, as well as in a 4/1 helical conformation with four silicon atoms contained in one translational repeat unit. In addition, we compared results for the dimethyl-substituted polysilane in an dl -trans conformation with the results for the unsubstituted poly silane. 

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 local-density functional approach was used to compare the band structures of the sW-trans conformation of unsubstituted polysilane with a 4/1 helical conformation and with an dll-trans conformation of dimethyl-substituted poly silane. In line with previous theoretical studies, the electronic wave functions in the vicinity of the Fermi level are primarily silicon-back-bone states, with the major effect of methyl substitution being a decrease in the gap. The predicted absorption spectra for the dll-trans conformations of unsubstituted and dimethyl-substituted polysilane are similar for nearthreshold absorption. Given this similarity, we believe that the shift in energy and strong anisotropy of threshold absorption that we predict for the two extremes of the dll-trans conformation and the dll-gauche model will also occur in alkyl-substituted systems, which are currently under investigation. 

Reversible control over the helix sense in polysilanes was achieved in the case of poly(diaryl-silanes)128 as well as in (co)polymers of ((S)-3,7-dimethyloctyl)(3-methylbutyl)silane,129 which both showed helix reversal upon heating. For the latter polymer (Figure 12a), it was calculated that the potential curve has a double-well ( W j shape (Figure 12b) with a slight preference for the M-helix over the P-helix. CD spectroscopy indeed revealed that above the transition temperature the ordered (low-entropy) M-helical conformation becomes less stable than the entropically more favored P-helical state (Figure 12c,d). 

In conclusion, the phase behavior of symmetrically di-n-alkyl-substituted poly(silane)s and poly(silylenemethylene)s is similar both classes of polymers form the same type of mesophase, although it cannot be obtained in pure form for the poly(silylenemethylene) studied. The mobility in the mesophase, characterized by the quadmpole splitting, appears to depend strongly on the chemical structure of the backbone. For different poly(silane)s, the mobility in the crystalline phase is obviously influenced by the conformation of the backbone zW-trans vs. 73-helical) and therefore depends on the length of the side-chains. Application of pressure to poly(silane)s with 73-helical backbones leads to the formation of high-pressure crystalline modifications with dA -trans backbone conformation. The pVT studies made it possible to define the precise conditions for the pressure-induced phase transitions. 

Conformational calculations are carried out on poly(di-n-hexylsilanes). The most significant finding from the energy calculations is that the a -trans conformation is not the lowest energy structure for the symmetrically alkyl-substituted silane polymers. A helical structure is preferred for the isolated molecule. 

Farmer et al. evaluated the conformations and dynamics of poly(di- -hexylsilane). " - " The lowest energy conformer for a polymer containing eight silicon atoms was a helical arrangement with 30° torsional angles in the silane backbone. The authors also monitored different backbone and side chain torsions during dynamics simulations and concluded that the conformation present in the crystalline solid is controlled by intermolecular effects. 

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