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, [Eu-2(EBTC)(DMF)(5)(NO3)(2)]center dot DMF (1) and [Eu-2(BBTC)(1.5)(CH3OH)(2)(H2O)(2)]center dot 7DMF center dot HNO3 (2) (EBTC4- = 1,1'-ethynebenzene-3,3', 5,5'-tetracarboxylate; BBTC4- = 1,1'-butadiynebenzene-3,3', 5,5'-tetracarboxylate), were successfully synthesized from conjugated ligands of EBTC4- and BBTC4-. 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 Eu3+ 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 Eu3+ ion at room temperature, with a long lifetime of up to 1.3 and 0.7 ms, respectively, during which the ligand emission of EBTC4-/BBTC4- was quenched by the Eu3+ ion, indicating the existence of efficient energy transfer between the conjugated ligand of EBTC4-/BBTC4- and the Eu3+ ion. Thus, both EBTC4- and BBTC4- 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 angstrom for 1 versus 4.275 and 5.360 angstrom for 2) between the neighboring Eu3+ ions connected by oxygen atoms of the tetracarboxylates.