Poly silanes alkyl polysilane

An important property of the alkyl poly silanes is their electronic absorption at relatively low energy. This was particularly surprising because such molecules lack tt, d ot lone pair electrons they were the first substances containing only bonding cr-electrons to show such long-wavelength absorptions. The unusual electronic properties of the polysilanes have sparked considerable interest in the chemophysical properties of these molecules. The UV spectra of cyclic polysUanes follow a quite different pattern . 

Photoablation of Copolymers, Other workers have also investigated the phenomenon of polysilane self-development. Zeigler and co-workers (13) have studied the self-development of a number of polysilane homo- and copolymers and found that self-development eflSciencies increase with the size of substituents. They also suggested that the material removal process for alkyl-substituted poly silanes at low fluences (<50 mj/cm per pulse) is predominantly photochemical rather than photothermal. By using a 1 1 copolymer, poly(methyl-n-propylsilane-co-isopropylmethylsilane), images were generated by excimer laser exposure at 248 nm. 

The next question is how side-chain substitutions and skeleton conformations affect the band structures of polysilanes. Side chains provide two interesting effects (7) band gap reduction caused by substitution of larger alkyl side chains and skeleton-side chain interaction (i.e., (j-tt mixing) in aryl poly silanes. This interaction was confirmed by UV photo spectroscopy (UPS) (8-9) and photoabsorption and luminescence measurements (iO, 11). Skeleton conformations are related to thermochromism (12-17). The ab initio... 

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. 

Because of the low solubility of most poly silanes in combination with expandable processing, that is, purification including the above mentioned hydrolysis steps, product yields and purity are often unsatisfactory if obtained by reductive coupling of chlorosilanes. A further negative aspect of this approach is the limited functional group tolerance. Accordingly, only the synthesis of polysilanes with inert pendant groups such as alkyl or aryl can be adequately achieved. 

Semi-empirical understanding of PM-transition characteristics and main chain stiffness of rod-like polysilanes thus leads to the new idea that minute structural modification of the achiral and chiral alkyl side groups could critically modify the transition characteristics, as indeed demonstrated in a series of 16-based copolymers, poly ((S)-3,7-dimethyloctyl-3-methylbutylsilane)-co-((S)-3,7-dimethyloctyl-2-methylpropylsilane) (22) and poly ((S)-3,7-di-methyloctyl-3-methylbutylsilane)-co-((P)-3,7-dimethyloctyl-2-methylpropyl-silane) (23) (see Scheme 5). 

The decomposition chemistry of most polysilanes above 300 °C is quite complex and involves restructuring of the polymer backbone as well as substantial cleavage of the side groups. Generdly a poly(alkyl)silane must go through a carbosilane intermediate prior to formation of the SiC network (Kumada rearrangement) [125] ... 

In order to produce polysilanes with controlled optical properties, the substituents have to be picked up properly thus, poly(alkyl(methoxyphenyl)silane) homo and copolymers are reported by Nakashima and Fujiki [40], who found that poly(alkyl(alkoxyphenyl)silane) with methyl, ethyl and n-hexyl moieties showed a typical absorption peak around 340-360 nm atUibuted to the delocalized cr-conjugation. 

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