Synthesis and mesomorphism of 1,10-dialkylpermethyl-decasilanes

1,10-dialkylpermethyldecasilanes, R[Si(CH3)(2)](10)R where R = C2H5 (1b), C 3H7 (1c), and C4H9 (1d), were prepared from 1,6-dichloropermethylhexasilane in a three-step reaction. Decasilanes 1b-d show the enantiotropic mesophas e in the range of 50-105, 22-87, and 23-69 degrees C, respectively. The mol ecular alignment for the decasilanes in the mesophase were investigated by X-ray diffraction method. These oligosilane molecules are hexagonally order ed to form a multilayer interdigitated structure. The alkyl and silicon cha ins of these oligosilanes adopt almost all-trans conformations and are perp endicular to the layers. These decasilanes show absorption bands with maxim a at 293-294 nm in the mesophase. These absorption maxima are red-shifted b y 12-14 nm, comparing with those in the liquid state and in solution.