Abstract
New luminescent lanthanide-carboxylate frameworks, formulated as [Ln(4)(m-BDC)(6)(H2O)(4)(DMF)]center dot(H2O)(2)center dot(DMF) (Ln = Eu 1; Gd 2; Tb 3) (m-H2BDC 1,3-benzenedicarboxylic acid) have been solvo(hydro) thermally synthesized. Single-crystal X-ray diffraction studies reveal that compounds 1-3 are isomorphous and each displays a layered structure constructed by helical lanthanide-carboxylate chains. DFT and TD-DFT calculations were conducted in detail to discuss the energy levels, including the HOMO, LUMO, singlet and triplet energies of the m-H2BDC ligand. The results indicate that the m-BDC2- dianion acts as an antenna chromophore that is able to efficiently absorb and transfer energy to the lanthanide ions. Compounds 1-3 all exhibit high thermostability and are stable up to 450 degrees C, as confirmed by the TGA and temperature-dependent PXRD experiments. Compounds 1 and 3 are strong luminescent materials. Compound 3 is among some of the best examples of luminescent lanthanide compounds, with highly efficient emission quantum yields of 81.40% at 77 K, 75.37% at room temperature and 56.64% after calcination at 450 degrees C, respectively. In addition, the energy transfer processes for compounds 1 and 3 have also been studied and illustrated in detail.