Two series of reactant's ratio-dependent lanthanide organic frameworks derived from nicotinic acid N-oxide and oxalate: synthesis, crystal structures and luminescence properties

Two series of lanthanide(III)-organic frameworks with the molecular formula [Ln(2)(NNO)(2)(OX)(2)(H2O)(4)](n) (Ln = Eu 1, Tb 2, Sm3, Dy 4, Gd 5) and [Ln(2)(NNO)(4)(OX)(H2O)(2)] n (Ln = Eu 6, Tb 7, Sm8, Dy 9, Gd 10) were synthesized successfully under the same hydrothermal conditions with nicotinic N-oxide (HNNO) and oxalic acid (H2OX) as the mixed ligands merely through varying the molar ratio of the reactants. The compounds were characterized by IR, elemental analysis, UV, TG-DTA and powder X-ray diffraction (XRD). X-ray single-crystal diffraction analyses of compounds 1 and 7 selected as representatives and powder XRD analysis of the compounds revealed that both the series of compounds feature three-dimensional (3-D) open frameworks, and crystallize in the triclinic P (1) over bar space group while with different unit cell parameters. In compound 1, pairs of Eu3+ ions and pairs of NNO- ligands connect with each other alternately to form a 1-D infinite Eu-NNO double chain, the adjacent 1-D double-chains are then joined together through OX2- ligands leading to a 2D layer, the 2-D layers are further 'pillared' by OX2- ligands resulting in a 3-D framework. In compound 7, the 1-D Tb-NNO infinite chain and its 2-D layer are formed in an almost similar fashion to that in compound 1. The difference between the structures of the two compounds 1 and 7 is that the adjacent 2-D layers in compound 7 are further connected by NNO- ligands resulting in a 3-D framework. The photoluminescence properties and energy transfer mechanism of the compounds were studied systematically. The energy level of the lowest triplet states of the HNNO ligand (23 148 cm(-1)) was determined based on the phosphorescence spectrum of compound 5 at 77 K. The D-5(0) (Eu3+) and D-5(4) (Tb3+) emission lifetimes are 0.46 ms, 0.83 ms, 0.69 ms and 0.89 ms and overall quantum yields are 1.03%, 3.29%, 2.58% and 3.78% for the compounds 1, 2, 6 and 7, respectively.