Mechanochemical synthesis, luminescent and magnetic properties of lanthanide benzene-1,4-dicarboxylate coordination polymers (Ln0.5Gd0.5)2 (1,4-BDC)3(H2O)4; Ln = Sm, Eu, Tb

Abstract

Mechanochemical reactions of benzene-1,4-dicarboxylate (BDC²⁻) and lanthanide carbonates, Ln₂(CO₃)₃·xH₂O (Ln = Sm, Eu, Gd, Tb) yield phase pure lanthanide coordination polymers, (Ln_0.5Gd_0.5)₂(1,4-BDC)₃(H₂O)₄ with Ln = Sm, Eu, Tb, which are isostructural with Tb₂(1,4-BDC)₃(H₂O)₄ as confirmed by powder X-ray diffraction and vibrational spectroscopy. Upon excitation with UV light all three compounds display strong emissions, characteristic for the respective optically active lanthanide ion, namely, red for Eu³⁺, green for Tb³⁺ and orange-red for Sm³⁺. In case of the Tb³⁺-containing compound, the energy difference between the triplet energy level of benzene-1,4-dicarboxylate ligand (BDC²⁻) allows for the most efficient BDC²⁻–Tb³⁺ energy transfer. As a consequence, an intense green luminescence with rather long lifetime (0.81 ms) and high quantum yield (22%) is observed after allowed excitation of the BDC²⁻ ligand. The compounds are paramagnetic with no onset of long range magnetic ordering down to liquid He temperatures.