Structure and third-order nonlinear optical properties of the two-dimensional CoIIcoordination polymer [Co(1,2-BIB)(PA)]n{1,2-BIB is 1,2-bis[(1H-imidazol-1-yl)methyl]benzene and H2PA is phthalic acid}

The design of nonlinear optical (NLO) materials with large NLO responses has been a challenge for materials scientists and chemists. Recently, organic polymers have received attention regarding their NLO properties and fast nonlinear response, and clusters involving delocalizeddπ–pπ systems and conjugateddπ–pπ systems are expected to be a new class of NLO materials. Metal–organic coordination polymers combine the advantages of the organic and inorganic species. Thus far, tuning the third-order NLO properties of these materials has been a significant challenge. A two-dimensional coordination polymer, namely poly[(μ-benzene-1,2-dicarboxylato-κ3O1,O1′:O3){μ-1,2-bis[(1H-imidazol-1-yl)methyl]benzene-κ2N3:N3′}cobalt(II)], [Co(C8H4O4)(C14H14N4)]n, was synthesized by hydrothermal methods. In the structure, the CoIIion is pentacoordinated by two N atoms from two different 1,2-bis[(1H-imidazol-1-yl)methyl]benzene (1,2-BIB) ligands and three O atoms from two symmetry-related benzene-1,2-dicarboxylate (isophthalate, PA) anions. The pentacoordinated CoIIions are linked by PA ligands to form a chain structure parallel to thecaxis. Adjacent chains are further connected by 1,2-BIB ligands to produce a two-dimensional layered structure. The compound exhibits strong third-order nonlinear absorption and nonlinear refraction effects as a thin film. The third-order susceptibility χ(3)is calculated to be 1.07 × 10−8esu, which is much larger than the values found for pure inorganic semiconductors and some polymers.