Luminescence tuning of MOFs vialigand to metal and metal to metal energy transfer by co-doping of 2∞[Gd2Cl6(bipy)3]·2bipy with europium and terbium

Abstract

The series of anhydrous lanthanide chlorides LnCl₃, Ln = Pr–Tb, and 4,4′-bipyridine (bipy) constitute isotypic MOFs of the formula ²_∞[Ln₂Cl₆(bipy)₃]·2bipy. The europium and terbium containing compounds both exhibit luminescence of the referring trivalent lanthanide ions, giving a red luminescence for Eu³⁺ and a green luminescence for Tb³⁺ triggered by an efficient antenna effect of the 4,4′-bipyridine linkers. Mixing of different lanthanides in one MOF structure was undertaken to investigate the potential of this MOF system for colour tuning of the luminescence. Based on the gadolinium containing compound, co-doping with different amounts of europium and terbium proves successful and yields solid solutions of the formula ²_∞[Gd_2−x−yEu_xTb_yCl₆(bipy)₃]·2bipy (1–8), 0 ≤ x, y ≤ 0.5. The series of MOFs exhibits the opportunity of tuning the emission colour in-between green and red. Depending on the atomic ratio Gd:Eu:Tb, the yellow region was covered for the first time for an oxygen/carboxylate-free MOF system. In addition to a ligand to metal energy transfer (LMET) from the lowest ligand-centered triplet state of 4,4′-bipyridine, a metal to metal energy transfer (MMET) between 4f-levels from Tb³⁺ to Eu³⁺ is as well vital for the emission colour. However, no involvement of Gd³⁺ in energy transfers is observed rendering it a suitable host lattice ion and connectivity centre for diluting the other two rare earth ions in the solid state. The materials retain their luminescence during activation of the MOFs for microporosity.