Synthesis, structure and band gap energy of a series of thermostable alkaline earth metal based metal–organic frameworks

Abstract

Three new compounds based on alkaline earth element ions and 4,4′-oxy-bis(benzoic acid) (OBA) were synthesized and characterized. Compound 1, [Ca(H₂O)(OBA)], consists of a Ca₂O₁₀ dimer, which is connected through OBA anions to form the three-dimensional structure. While in compound 2, [Sr(H₂O)(OBA)], SrO₇ units are connected to form one-dimensional chains, which are in-turn connected by OBA anions to form the three-dimensional architecture. In compound 3, [Ba₂O₃(OBA)₂], BaO₈ and BaO₉ units are connected to form the 6-membered two-dimensional sheet structure. These two-dimensional layers are pillared by OBA anions to complete the three-dimensional connectivity. Variable temperature powder X-ray diffraction and thermogravimetric analysis studies show that all these structures are thermally stable up to 400 °C. We have measured the optical band gap energies and results show that the values decrease slightly from 1 to 3. This variation in band gap energy may be due to the different architectures, which affect the states of conduction and valence bands of the materials.