Borosilicate-Based Framework: Synthesis, Single-Crystal Structure Study, and Physical Properties

Herein, we report the synthesis, crystal structure, and optical properties of a metal-free three-dimensional (3D) inorganic covalent framework ((H2en)[Si(B4O9)], named CityU-11, where H2en is the abbreviation for ethanediamine). With the assistance of a tiny amount of F– ions and the selection of SiO2 as Si sources, single crystals of CityU-11 can be successfully prepared under solvothermal conditions. The precise structure information on CityU-11 has been disclosed through both single-crystal X-ray diffraction (SCXRD) and low-dose high-resolution transmission electron microscopy (LD-HRTEM). The SCXRD results showed that CityU-11 crystallized in the noncentrosymmetric space group of Pnn2, while LD-HRTEM suggested that CityU-11 possessed almost the same interplanar distances of 0.6 nm for both (200) and (020) crystal planes, which finely matched with the double peaks of 2θ = 15° in the pattern of its powder X-ray diffraction (PXRD). CityU-11 also displayed an interesting optical property with a moderate birefringence of 0.0258@550 nm.