Structural and photophysical insights into lanthanide complexes with a sterically hindered phenanthroline ligand

A new phenanthroline-based ligand, 2-(2,6-diisopropylphenoxy)-1,10-phenanthroline was synthesized and characterized by NMR, IR, UV–vis, elemental analysis, and single-crystal X-ray diffraction. The ortho-substituent imposes a distorted, sterically hindered coordination environment. Complexes with Eu 3+ , Sm 3+ , Tb 3+ , and Dy 3+ were prepared in polar solvents. Eu 3+ and Sm 3+ complexes show strong f–f emission in solution and in the solid state, with long lifetimes and high quantum yields. The Tb 3+ and Dy 3+ analogues emit only in solution or at 77 K; no solid-state emission is observed at room temperature. These differences do not correlate with the expected energy match between the ligand triplet level and lanthanide emissive states. Structural data indicate that geometric distortion may suppress energy transfer in the terbium and dysprosium complexes. Fluorescence titration suggests 1:2 stoichiometry with positive cooperativity. The Tb 3+ and Dy 3+ complexes also show antibacterial activity against selected Gram-positive strains.