Interactions between Eu3+ ions in inorganic–organic hybrid materials

The optical properties of two-dimensional lanthanide dicarboxylates EuBDC or Eu2(H2O)2(O2C–C6H4–CO2)3 and EuCDC (denoted also MIL94) or Eu2(H2O)4(O2C–C6H10–CO2)3.2H2O are reported. The structures are built up from dimers of corner-sharing polyhedra and 1,3-benzenedicarboxylate (BDC) for EuBDC and from dimers of edge-sharing polyhedra and 1,3-benzenedicarboxylate (CDC) for EuCDC. The high Eu3+ concentration and the weak luminescence quenching allow the study of Eu3+ interactions. Anti-Stokes spectra from 5D1 are observed with excitation in 5D0. These results are very unusual for Eu3+ ions and reflect strong interactions between ions within a dimer. Excitation spectrum of the Eu3+ luminescence strongly differs in both compounds in the UV range. In case of EuBDC, an efficient sensitization of the luminescence due to the ligand is observed between 250 and 350 nm while only 4f–4f transitions are recorded on the Eu3+ excitation spectrum in EuCDC. The efficiency of the sensitization of the rare earth by the host is discussed by taking into account the geometrical arrangement and the electronic delocalization of the ligands.