Branched polysilanes

Dendrimers are structurally defined branched polymeric molecules and can form good X-ray quality crystals. This is particularly useful in the correlation of structure and properties. Two main synthetic approaches have been used divergent, which is appropriate for smaller dendrimers, and convergent, which is better for larger dendrimers. Branched polysilanes with a structurally defined branched core were described in 1994, but the growth from the core was not controlled and non-uniform structures were obtained.357... 

Most linear polysilanes show bright resonant luminescence in the UV region but network and branched polysilanes show luminescence in the visible... 

The branched polysilanes show a broad emission peak around 450 nm, where the large red shift in the fluorescence spectra is due to the influence of aryl substituents and the introduction of branched points. For (opto)electronic applications, the UV irradiation and thermal stability are crucial to device stability. Branched polysilanes have better thermostability and are more resistant to UV irradiation than are linear polysilanes. 

Baney and co-workers (iO) used the known Si-Si and Si-Cl bond redistribution of methylchlorodisilanes to prepare poly(methylchlorosilane)s. Pyrolysis of these branched polysilane polymers gives nearly stoichiometric amounts of silicon carbide (equation 6). 

In this study we aimed to evaluate the relationship between the central Si-Si bond length of the branched polysilanes a and the kinetic stability of corresponding silyl radicals b. 

Summary Functional first generation dendrimers were synthesized starting from methyltrichlorosilane. Successive treatment with the lithium derivatives of dimethyl-phenylsilane and methyldiphenylsilane and conversion of the phenyl groups into bromides gave a stepwise construction of a branched polysilane. 

An empirical relationship has also been developed by Hahn118 for the silicon chemical shift of linear and branched polysilanes (Table 26) ... 

Polysilanes have been the first class of precursors used to prepare silicon carbide ceramics. In all cases, on pyrolysis, polysilanes are converted into polycarbosilanes by a Kumada rearrangement prior to the formation of an amorphous silicon carbide network. Several synthetic routes including dehydro-polymerization, ring-opening polymerization of strained cyclosilanes, polymerization of masked disilenes, or base catalyzed disproportionation reactions of disilanes have been described to prepare linear or branched polysilanes but despite its drawbacks the Wurtz-coupling route remains the method applied most, especially to prepare linear polysilanes. 

The Wurtz-coupling reaction of diorganodichlorosilanes is still the method applied most to synthesize non-branched polysilanes [30]. 

Besides the Wurtz-coupling reaction, other synthetic routes towards non-branched polysilanes have been reported, which overcome the drawbacks of polymodal molar weight distribution and low yield of the high molar mass fraction. However, so far the application of these routes has been very limited. The anionic ring opening polymerization of the strained 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane leads to poly(methylphenylsilane) in high yield but the starting cyclotetrasilane has to be prepared in three steps from diphenyldichlorosilane via octaphenylcyclotetrasilane [84] ... 

In recent years, considerable attention has been directed toward a study of the more highly catenated linear and cyclic polysilanes (I, 2, 14, 18). An interesting facet of this recent work involves the preparation of branched polysilanes such as those illustrated below. 

The preparation of a variety of higher polysilanes using silylmetallic reagents has been described (4, 5, 14). More recently, silylmetallics have been successfully employed in the synthesis of branched polysilanes. Treatment of silicochloroform with either triphenyl- or dimethylphenyl-silyllithium gives the corresponding tris derivatives. 

On the basis of theoretical calculations, the highdimensional silicon framework results in a smaller bandgap than that of the low-dimensional one [19,20]. In order to reduce the bandgap of the polysilanes, tiie synthesis of branched polysilanes, poly(organosilynes), has been attempted. 

Wagner H, Baumgartner J, Miiller T, Marschner C (2009) Shuttling germanium atoms into branched polysilanes. J Am Chem Soc 131 5022... 

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