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

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 O–H⋯O, N–H⋯O, C–H⋯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.