Two new POM-based compounds modified by lanthanide-Schiff base complexes with interesting NLO properties

Two POM-based lanthanide derivatives, namely {triaqua[2,6-diacetylpyridine bis(semicarbazone)-κ⁵O,N,N',N'',O']terbium(III)}-μ-oxido-[tricosa-μ₂-oxido-dodecaoxido-μ₁₂-phosphato-dodecamolybdenum(VI)] pentahydrate (1), [Tb(C₁₁H₁₅N₇O₂)(H₂O)₃][PMo₁₂O₄₀]·5H₂O, and the dysprosium(III) analogue (2), have been isolated successfully by the reaction of Keggin-POM [PMo₁₂O₄₀]^3- (abbreviated as PMo₁₂), the Ln³⁺ ion and the Schiff base 2,6-diacetylpyridine bis(semicarbazone) (DAPSC) ligand under hydrothermal conditions. [Ln(DAPSC)(H₂O)₃][PMo₁₂O₄₀]·5H₂O is a PMo₁₂-supported cluster featuring a lanthanide-Schiff base complex [denoted Ln-L(Schiff base)]. Single-crystal X-ray diffraction analysis reveals that the Ln^III ion is in a distorted tricapped trigonal prismatic arrangement, coordinated by six O atoms and three N atoms. Two O atoms and three N atoms are provided by one DAPSC ligand, while the additional O atoms originate from a PMo₁₂ cluster and three water molecules. Hydrogen-bonding interactions between adjacent clusters form an interesting three-dimensional supramolecular structure. The identities of 1 and 2 were characterized by IR spectroscopy, thermogravimetric analysis and powder X-ray diffraction. Interestingly, both compounds possess excellent two-photon absorption (TPA) responses to the third-order nonlinear optics (NLO) (2264 GM for 1 and 941 GM for 2), suggesting that they have potential applications in the field of nonlinear optics (NLO). To our knowledge, 1 and 2 are the first POM-based Ln-L(Schiff base) complexes showing excellent two-photon responses. Meanwhile, the electrochemical properties of both compounds were studied in detail.