Two series of lanthanide complexes with 4-chlorophenylacetate ligand displaying luminescence and significant magnetocaloric effect

Abstract

Two series of lanthanide complexes based on 4-chlorophenylacetic acid (HCPA), dinuclear [Ln2(CPA)6(DMF)2(H2O)2] (Ln = Eu (1); Gd (2); Tb (3); Dy (4)) and one-dimensional (1D) linear [Ln2(CPA)6(H2O)]n (Ln = Eu (5); Gd (6); Tb (7); Dy (8)), were prepared, which are sensitive to the pH value. The dinuclear structure can be obtained when the pH value was controlled to be about 4.2, while the 1D structure can be formed when the pH value is ca. 5.1. The structure transformation of the dinuclear to 1D analogue at different pH values indicates that the CPA− ligand has a better affinity to replace the H2O/DMF molecule to bridge the neighboring Ln3+ ions to form the 1D structure at a higher pH value (∼5.1). The magnetic studies show that complexes 2 and 6 with isotropic Gd3+ ion display significant cryogenic magnetocaloric effect with a maximum −ΔSm value of 20.27 J kg−1 K−1 at 3 K and 7 T and 22.50 J kg−1 K−1 at 2 K and 7 T, respectively. Furthermore, the solid-state photophysical properties of 1/5 and 3/7 were studied and respectively exhibit strong characteristic emission peaks of Eu3+ and Tb3+ ions in the visible region, indicating that Eu- and Tb-based luminescences are sensitized effectively by the energy transfe from the ligand to the spin centers.