Investigation of the third-order nonlinear optical properties of a cobalt(II) complex constructed from 4,4′-oxydibenzoic acid and 4,4′-(1<i>H</i>-pyrazole-3,5-diyl)dipyridine

A novel mononuclear cobalt(II) complex, diaquabis[4-(4-carboxyphenoxy)<!?up><!?tlsb><!?down>benzoato-κO]bis{4-[3-(pyridin-4-yl)-1H-pyrazol-5-yl]pyridine-κN}cobalt(II), [Co(C₁₄H₉O₅)₂(C₁₃H₁₀N₄)₂(H₂O)₂] or [Co(Hoba)₂(bpp)₂(H₂O)₂] [H₂oba 4,4'-oxydibenzoic acid and bpp is 4,4'-(1H-pyrazole-3,5-diyl)bipyridine], was successfully synthesized via a hydrothermal approach. In this compound, the central Co^II atom exhibits a hexacoordinate octahedral geometry. The complex molecules are interconnected through hydrogen-bonding interactions, constructing a three-dimensional supramolecular framework. UV-Vis absorption spectroscopy revealed two distinct absorption bands at 274 and 367 nm. The third-order nonlinear optical properties were investigated using Z-scan measurements. The results demonstrated pronounced negative refraction behaviour with a nonlinear refractive index n₂ of -1.47 × 10^-12 m² W^-1 and a third-order susceptibility χ⁽³⁾ of 5.62 × 10^-7 esu. These remarkable optical characteristics suggest that this cobalt(II) complex is a potential candidate for advanced photonic applications demanding high-performance third-order nonlinear optical materials.