Structure and optical properties of the planar silicon compounds polysilane and Wöhler siloxene

The two-dimensional silicon backbone structure of planar polysilane and Wöhler siloxene is responsible for their exciting luminescing properties. We have prepared single crystals of siloxene by a topotactic reaction from crystalline CaSi${}_2$. The chemical composition was determined as [Si${}_6$H${}_3$(OH)${}_3$]${}_n.$ The x-ray crystal structure analysis identifies the so-called Wöhler siloxene as 2D-poly[1,3,5-trihydroxocyclohexasilane]. Polysilane exhibits the same structural properties but with a chemical composition [Si${}_6$H${}_6$]${}_n.$ The optical properties (infrared transmission, photoluminescence, excitation spectroscopy) of these well-defined materials are presented. A heat treatment above $350°$C in vacuum of Wöhler siloxene results in a destruction of the planar ${}_{\infty }^2$ [Si${}^{-}$] structure by internal rearrangements, which is evidenced by the x-ray-diffraction pattern and characteristic changes in the optical spectra. The involvement of Wöhler siloxene in the optical properties of porous Si is critically reviewed.