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

Abstract

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.

Polysilane Dendrimers

“…The synthesis of irregular single-core dendritic polysilanes was motivated by efforts to compare structural and spectroscopic properties of dendrimers with the same silicon chain length, but with different substitution patterns [45]. The preparation of irregular dendrimers requires a stepwise assembly of strategic silicon-silicon bonds using differently structured metalated wings to be attached to the central core unit.…”

“…In all three dendrimers, however, the heptasilane chains with largest dihedral angles are fairly similar to each other, which correlate with the similar UV spectroscopic data of 24, 40 and 25. The strong impact of the silicon chain conformation on the energy of the UV absorption maxima of this class of single-core dendrimers is best demonstrated by comparing dendrimer 40 with conformationally constrained dendrimer 44; both have longest heptasilane chains and the same total number of silicon atoms in the molecule [45]. Assuming that the absorption maximum originates from silicon chains with largest dihedral angles, 44 is red-shifted by 8 nm as a result of a conformational change from a ADOD conformer in 40 to a ADAD conformer in 44.…”

Charge doping effect on σ-π conjugated copolymers

“…They also exhibit other intriguing properties such as photoconductivity, nonlinear optical response, and thermochromism [20,21]. Homopolymers such as polysilane [22][23][24][25][26][27][28][29][30] (PSi) and polythiophene [31][32][33][34][35] have been widely studied from both theoretical and experimental points of view. It is well understood that their interesting properties arise from pure -and -conjugations, respectively.…”

Cyclic and Bicyclic Methylpolysilanes and Some Oligosilanylene-Bridged Derivatives

“…In more recent times several attempts were made to control the conformational properties of polysilanes. The applied strategies range from the introduction of chiral side-chains to the incorporation of the polysilane into a rigid framework, , and from the inclusion of alkylated polysilanes into cyclodextrines to the use of pentacoordination of silicon atoms . Work from our group , and others featured the use of bulky end-groups such as the tris(trimethylsilyl)silyl unit in order to align methylated polysilane chains.…”