A series of metal-organic frameworks based on alkaline earth metal ions (Ca, Sr and Ba) and 1,3,5benzenetribenzoic acid (BTB) have been synthesized and characterized. These are [H 2 NIJCH 3 ) 2 ]ijCa 7 IJBTB) 5 - 3). All the structures are three-dimensional in nature with different secondary building units. Compound 1 contains one-dimensional Ca-O-Ca zigzag chains while compound 2 contains Sr 5 O 28 pentameric clusters and compound 3 contains one-dimensional Ba-O-Ba chains. Both compounds 1 and 3 form (3,6)-net connectivity while compound 2 forms (3,12)-net connectivity. Optical band gap energy measurements show that compound 1 (2.65 eV) has low band gap energy compared to 2 (3.22 eV) and 3 (3.32 eV). This variation in band gap energy may be due to the difference in structural arrangement. Compound 3 crystallizes in a non-centrosymmetric space group (Pna2 1 ), which belongs to the polar point group C 2v . This compound displays a strong SHG response and good ferroelectric and piezoelectric properties.
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