Poly silanes electronic structure

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. 

The polysilanes represent another class of polymers that has been extensively studied for nonlinear optical applications [62- ]. These polymers have a molecular structure (R,—Si—R2) that is a long catenated a-bonded silicon backbone with two side groups R, and R2, usually carbon based, attached to each Si atom in the backbone chain. They are soluble in most hydrocarbon solvents and thin films of excellent optical quality can be fabricated with conventional spinning and coating techniques. In spite of the o--bonded nature of the backbone, the poly silanes show extensive electronic delocalization, resulting in strong transitions for excitations polarized parallel to the backbone [65]. They are unique in that they are transparent through the visible to the infrared in contrast to the 7r-electron polymers. 

Although the introduction of halogen-containing substituents into linear polysilanes have been reported,(55, 41, 45) further modification of structure and electronic properties of Aese polysilanes have not been extensively studied. We have recently reported that free-radical chlorination of poly(phenylsilane) with CCI4 provides a facile synthesis of poly(chlorophenylsilane), a versatile synthon for the preparation of a variety of functionalized poly(phenylsilane).(59) TTiis new synthetic capability has allowed us to investigate the influence of polymer substituent on the electronic properties of poly silanes. 

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