Synthesis, Structure, and Spectroscopic Properties of Perhexyloligosilanes

A series of linear perhexyloligosilanes (Hex(SiHex 2 ) n Hex, n = 3−10) was prepared by Wurtz coupling of a mixture of trihexylchlorosilane and dihexyldichlorosilane. The σ→σ* absorption maxima red-shift with increasing chain length, which is similar to what was observed for a series of permethyloligosilanes. However, the temperature dependence of the spectra is significantly different between these two series. The origin of the blue shift with decrease in temperature observed for short-chain oligosilanes ( n = 3−5) is ascribed to the parabolic dependence of the transition energies on the SiSiSi bond angle. The red shift of the absorption maxima observed for long-chain oligosilanes ( n = 6−10) probably results from the ordering of the chain to the all pseudo-trans (extended) form at low temperatures. The long-chain perhexyloligosilanes show stronger temperature dependence similar to that of perhexylpolysilane polymers, suggesting that the barriers of the transition from the folded to extended chains are much larger than those for permethyloligosilanes, due to the steric effects of the side-chain alkyl substituents. The emission spectral features of perhexyloligosilanes are essentially similar to those of permethyloligosilanes studied by Michl et al. A significant difference between these two series of oligosilanes is also observed on the temperature dependence of the emission spectra.