The effects of the alkoxy side chain on the conformation of oligosilane have been studied for perpentoxypentasilane. The UV-absorption maxima shifted extremely to a shorter wavelength than that of peralkyloligosilane. Molecular dynamics and ab initio, MO calculations showed that the excitation energy of the twisted structure for peroxyloligosilane (a model of perpentoxypentasilane) was smaller than that of permethyloligosilane, because of the interaction between the n orbital of the oxygen atom and the sigma orbital of the Si-Si bond. The Coulomb repulsion energies between the oxygen atoms greatly affect the stability of the twisted conformation of the silicon backbone. (C) 2003 Elsevier Science B.V. All rights reserved.