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 ͑OH͒ 3 ͔ n . The x-ray crystal structure analysis identifies the so-called Wöhler siloxene as 2D-poly͓1,3,5-trihydroxocyclohexasilane͔. Polysilane exhibits the same structural properties but with a chemical composition ͓Si 6 H 6 ͔ n . The optical properties ͑infrared transmission, photoluminescence, excitation spectroscopy͒ 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 ϱ 2 ͓Si Ϫ ͔ 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.

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