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

The two-dimensional silicon backbone structure of planar polysilane and W\"ohler siloxene is responsible for their exciting luminescing properties. We have prepared single crystals of siloxene by a topotactic reaction from crystalline CaSi₂. The chemical composition was determined as [Si₆H₃(OH)₃]ₙ. The x-ray crystal structure analysis identifies the so-called W\"ohler siloxene as 2D-poly[1,3,5-trihydroxocyclohexasilane]. Polysilane exhibits the same structural properties but with a chemical composition [Si₆H₆]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 W\"ohler siloxene results in a destruction of the planar _∞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 W\"ohler siloxene in the optical properties of porous Si is critically reviewed.