Combination of single-molecule magnet behaviour and luminescence properties in a new series of lanthanide complexes with tris(pyrazolyl)borate and oligo(β-diketonate) ligands

Abstract

A series of tris(pyrazolyl)borate mono-, di- and trinuclear complexes, [Tp₂Ln]_nX (Ln = Eu, Tb, Gd, Dy, X^n- = various mono-, bis- and tris(β-diketonates) has been prepared. The Tb³⁺ and Dy³⁺ complexes are luminescent single molecular magnets (SMM) and exhibit luminescence quantum efficiencies up to 73% for the Tb³⁺ and 4.4% for the Dy³⁺ compounds. Similar Eu³⁺ complexes display bright emission only at lower temperatures. The Dy³⁺ and Tb³⁺ complexes possess SMM behavior in a non-zero dc field at low temperatures, while the polynuclear Dy³⁺ complexes also show slow magnetic relaxation even in zero dc field up to 8 K. U_eff-values determined from dynamic magnetic measurements were up to 31 and 6 cm^-1 for the Dy³⁺ and Tb³⁺ complexes, respectively. It was found that within a series of Dy³⁺ and Tb³⁺ compounds, U_eff and luminescence quantum yields decreased with increasing nuclearity of the compounds and a shortening of the intramolecular Ln-Ln distance. Δ_Orbach-values estimated from low-temperature luminescence spectra were significantly higher than those obtained from ac magnetic data, which may be due to involvement of additional processes in the relaxation mechanism (quantum tunneling, Raman, direct) reducing the energy barrier. Some of the Tb³⁺-compounds also display metal-centred electroluminescence, giving them potential as emitting layers in LEDs.