An Approach to Dendritic Oligosilanes: Controlling the Conformation through Ring Formation

The synthesis, solid-state structure, and UV spectroscopic properties of the first-generation single-core oligosilane dendrimer MeSi[SiMe2Si(SiMe3)2Me]2[SiMe2Si(SiMe3)3] (5) and the tethered analogue
(CH2)4[Si(SiMe3)2SiMe2]2Si[SiMe2Si(SiMe3)3]Me (4) are reported. UV spectroscopic measurements in
solution and in the solid state (diffuse reflectance) show the absorption maximum of tetramethylene-tethered 4 to be shifted to wavelengths longer than those of 5, the dendrimers MeSi[SiMe2Si(SiMe3)3]3
(6) and MeSi[SiMe2Si(SiMe3)2Me]3 (7), and even that of the linear heptasilane Si7Me16. This can be
explained by the results of the X-ray analysis of 4 and 5, which reveal for 4 the presence of an A−D−A−D conformer, which is optimal for σ-conjugation, whereas in 5 only D−D−O−D conformers have
been found.