Synthesis, structural characterization and photoluminescence properties of mononuclear Eu3+, Gd3+ and Tb3+ complexes derived from cis-(±)-2,4,5-tris(pyridin-2-yl)-imidazoline as ligand

Abstract

Three mononuclear lanthanide complexes of compositions [Eu(L)(NO3)3(MeOH)]⋅0.5MeOH⋅Et2O (1), [Gd(L)(NO3)3(MeOH)]⋅0.5MeOH⋅Et2O (2) and [Tb(L)(NO3)3(H2O)]⋅H2O⋅0.5MeOH⋅Et2O (3) with L = cis-(±)-2,4,5-tris(pyridin-2-yl)-imidazoline were successfully synthesized and characterized by elemental and TG-DSC analysis, single-crystal X-ray crystallography and FT-IR, UV–Vis and fluorescence spectroscopy. Complexes 1–3 are isostructural with ten-coordinate Ln3+ ions embedded in distorted bicapped square-antiprismatic polyhedra formed by three nitrogen atoms of ligand L, a solvent molecule (MeOH or H2O) and three nitrate anions bound in the bidentate mode. In the solid state, complexes 1–3 are linked via OH⋯O, NH⋯O, CH⋯O and π−π interactions. Complex 3 exhibits bright green luminescence (CIE 0.28, 0.56) with a quantum yield of 58% due to the favorable energy difference between the ligand triplet state (3π−π∗) and the excited state of Tb3+[ΔE (3ππ∗ − 5D4) = 3129 cm−1], rendering this compound a promising candidate for the fabrication of luminescent materials. On the contrary, for the Eu3+-polypyridyl complex 1, red emission (CIE 0.41, 0.23) and a relatively low luminescence efficiency are noted (quantum yield 21%), which is attributed to poor matching of the ligand triplet state and the excited emissive states of the metal ion [ΔE (3ππ∗ − 5D0) = 6269 cm−1]. For 1 and 3, the lifetimes of the excited states are 960 and 1700 μs, respectively.