New europium coordination polymers with efficient energy transfer from conjugated tetracarboxylate ligands to Eu~(3+) ion: Syntheses, structures, luminescence and magnetic properties

摘要

Two novel lanthanide coordination polymers, [Eu_2(EBTC)(DMF) _5(NO_3)_2]·DMF (1) and [Eu _2(BBTC)_(1.5)(CH_3OH)_2(H _2O)_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 ? for 1versus 4.275 and 5.360 ? for 2) between the neighboring Eu~(3+) ions connected by oxygen atoms of the tetracarboxylates.