Delocalized silicon: Spectroscopy reveals more

Silicon frameworks

The sigma electrons in polycyclic oligosilanes are delocalised across the Si-Si bonds of a three-dimensional silicon framework. Such frameworks are often present in crystalline and amorphous silicon and related silicon clusters, such as the persilastaffanes now investigated using NMR spectroscopy.

Chemists Takeaki Iwamoto, Daisuke Tsushima, Eunsang Kwon, Shintaro Ishida and Hiroyuki Isobe of the Graduate School of Science at Tohoku University, in Aoba-ku, Sendai, Japan, explain that despite research directed at the synthesis and investigation of polycyclic oligosilanes very few studies have homed in on catenated polycyclic oligosilanes (essentially oligomers of polycyclic oligosilanes). Such materials would be expected to display strong interactions between polycyclic oligosilane cages.

Making a rod

One particular group of compounds, that have highly symmetrical rod-like structures, the persila[n]staffanes represent a potentially fascinating group in this context. They possess bicyclo[1.1.1]pentasilane units catenated at their bridgehead positions and as such have been predicted to have small electronic band gaps because sigma electrons can delocalize along the silicon cages giving them potential electronic applications.

The all-carbon staffanes were synthesised in 1988 by the polymerisation of propellane by Piotr Kaszynski and Josef Michl and such rigid rod molecules have been the focus of would-be nanotechnologists ever since. Silicon-containing versions of these compounds have been attempted but the Japanese team is apparently the first to publish stable concatenated analogues of the earlier all-carbon compounds.

To make their rods, the team designed a novel bicyclo[1.1.1]pentasilane (persila[1]staffane) as a building block for the persila[n]staffanes. They were able to obtain air-stable, colourless crystals with a yield of 70% of this unit, the structure of which they confirmed by NMR spectroscopy, X-ray crystallography and mass spectrometry. They then carried out stepwise catenation of the basic unit to construct the silane rods. NMR revealed the products to have highly symmetric structures in solution with both proton and carbon-13 spectra revealing singlets for the trimethylsilyl groups. Quite intriguingly, the UV/Vis absorption spectra displayed two intense absorption bands that are red shifted and grow with increasing number of bicyclopentasilane cages.

Nano connections

"Persila[n]staffanes may be fascinating rodlike silicon molecules as linear connectors for novel silicon-based finely defined materials," the team concludes.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.