Exploring Lanthanide Luminescence in Metal-Organic Frameworks:  Synthesis, Structure, and Guest-Sensitized Luminescence of a Mixed Europium/Terbium-Adipate Framework and a Terbium-Adipate Framework

Two lanthanide-organic frameworks were synthesized via hydrothermal methods. Compound 1 ((Eu,Tb)(C6H8O4)3(H2O)2.(C10H8N2), orthorhombic, Pbcn, a = 21.925(2) A, b = 7.6493(7) A, c = 19.6691(15) A, alpha = beta = gamma = 90 degrees, Z = 4) takes advantage of the similar ionic radii of the lanthanide elements to induce a mixed-lanthanide composition. Compound 2 (Tb2(C6H8O4)3(H2O)2.(C10H8N2), orthorhombic, Pbcn, a = 21.866(3) A, b = 7.6101(10) A, c = 19.646(3) A, alpha = beta = gamma = 90 degrees, Z = 8) is the terbium-only analogue of compound 1. Solid-state measurements of their luminescence behavior demonstrate that the neutral guest molecule (4,4'-dipyridyl) residing in the extraframework channels is successful in sensitizing lanthanide ion emission. In compound 1, columinescence occurs, and both lanthanide ions show emission. Additionally, quantum yield and lifetime measurements support the premise that the Tb3+ center is also acting to sensitize the Eu3+, effectively enhancing Eu3+ emission.