Lanthanide-based metal–peptide frameworks prepared by ionothermal method: Anion direct effect, DFT calculation and luminescence property

A series of new isomorphic lanthanide-based metal–peptide frameworks, [Ln(PODC)(H2O)2]Br (Ln = La(1), Nd(2), Eu(3), Tb(4), H2PODC = 2, 5-piperazinedione-1, 4-diacetic acid), were synthesized under the ionic liquid medium. Crystal structure measure results exhibit that they are new three-dimensional frameworks, where bromine anions of ionic liquid are enveloped in the pore. Interestingly, it is found that bromine anions can effectively counteract the effects of lanthanide ions (e.g. lanthanide contraction). Via the first-principles DFT calculation, it is obtained that bromine anions of ion liquid influence the electronic properties of PODC2 − ligand and thereby directly control the structure. Luminescence analysis demonstrates that compounds 3 and 4 have good fluorescence properties, where the emission peaks can be ascribed to the transitions of 5D0 → 7FJ (J = 0, 1, 2, 3, 4) for 3 and 5D4 → 7FJ (J = 6, 5, 4, 3) for 4.