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

Abstract

Two novel lanthanide coordination polymers, [Eu(2)(EBTC)(DMF)(5)(NO(3))(2)]·DMF (1) and [Eu(2)(BBTC)(1.5)(CH(3)OH)(2)(H(2)O)(2)]·7DMF·HNO(3) (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 Eu(3+) 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 Eu(3+) ion, indicating the existence of efficient energy transfer between the conjugated ligand of EBTC(4-)/BBTC(4-) and the Eu(3+) ion. Thus, both EBTC(4-) and BBTC(4-) are ideal ligands with an "antenna" effect for the Eu(3+) ion. The two complexes show the single-ion magnetic behaviors of Eu(3+) with strong spin-orbit coupling interactions even if there are shorter distances (5.714 Å for 1 versus 4.275 and 5.360 Å for 2) between the neighboring Eu(3+) ions connected by oxygen atoms of the tetracarboxylates.