Synthesis and crystal structure of a nanometer-scale dendritic polysilane

A triply convergent synthesis has produced a dendritic polysilane with 13 silicons in the longest chain: [(Me3Si)(3)SiSiMe2SiMe2Si(SiMe3)(2)SiMe2](3)SiMe. The structure was proved by assignment of each H-1, C-13, and Si-29 resonance and by solution of the X-ray structure. The molecule represents a polysilane dendrimer that has achieved an approximately spherical shape. The roughly perpendicular distances between antipodal terminal methyls are 1.74, 1.70, and 1.65 nm, describing an approximate sphere with nanometer dimensions. The Si-Si-Si-Si dihedral angles indicate mixtures of gauche, orthogonal, and anti conformations. The longest all-anti segment is AAA, a hexasilane moiety. Despite the presence of numerous twists from gauche and orthogonal conformations, the molecule exhibits two ultraviolet maxima at 260 and 283 nm, with extinction coefficients respectively of 1.80 x 10(5) and 1.16 x 105. The wavelengths are comparable to that exhibited by linear polysilanes of similar length, but the extinction coefficients of the dendrimer are an order of magnitude higher. Thus electronic effects in dendrimers parallel those in linear polysilanes but possess greater intensity.