New europium coordination polymers with efficient energy transfer from conjugated tetracarboxylate ligands to Eu³⁺ ion: Syntheses, structures, luminescence and magnetic properties

Two novel lanthanide coordination polymers, [Eu₂(EBTC)(DMF) ₅(NO₃)₂]·DMF (1) and [Eu ₂(BBTC)_1.5(CH₃OH)₂(H ₂O)₂]·7DMF·HNO₃ (2) (EBTC ^4- = 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylate; BBTC^4- = 1,1′-butadiynebenzene-3,3′,5,5′- tetracarboxylate), were successfully synthesized from conjugated ligands of EBTC^4- and BBTC^4-. Although the two tetracarboxylate ligands have similar structures, their different rigidity/flexibility results in quite different networks upon complexation. Complex 1 has a two-dimensional (2-D) layered structure with two crystallographically independent Eu ³⁺ ions, one in a distorted monocapped square-antiprism and the other in a distorted square-antiprism coordination geometry. Complex 2 exhibits a three-dimensional (3-D) porous framework, with one type of Eu³⁺ in a distorted square-antiprism and the other in a trigondodecahedron environment. Both 1 and 2 emit the intensely red characteristic luminescence of Eu ³⁺ ion at room temperature, with a long lifetime of up to 1.3 and 0.7 ms, respectively, during which the ligand emission of EBTC^4-/ BBTC^4- was quenched by the Eu³⁺ ion, indicating the existence of efficient energy transfer between the conjugated ligand of EBTC^4-/BBTC^4- and the Eu³⁺ ion. Thus, both EBTC^4- and BBTC^4- are ideal ligands with an "antenna" effect for the Eu³⁺ ion. The two complexes show the single-ion magnetic behaviors of Eu³⁺ with strong spin-orbit coupling interactions even if there are shorter distances (5.714 Å for 1versus 4.275 and 5.360 Å for 2) between the neighboring Eu³⁺ ions connected by oxygen atoms of the tetracarboxylates.