A Para-Substituted 2-Phenoxy-1,10-Phenanthroline Ligand for Lanthanide Sensitization: Asymmetric Coordination and Enhanced Emission from Eu 3+ , Tb 3+ , Sm 3+ and Dy 3+ Complexes

A para-substituted 1,10-phenanthroline ligand, 2-(4-methylphenoxy)-1,10-phenanthroline (L24), was synthesized and structurally characterized. Complexes with Eu 3+ , Tb 3+ , Sm 3+ , and Dy 3+ were obtained in a 2:1 ligand-to-metal ratio and analyzed using single-crystal x-ray diffraction, photoluminescence spectroscopy, and TD-DFT calculations. Coordination via the phenanthroline nitrogen atoms, combined with steric asymmetry from the para-methylphenoxy group, induces low-symmetry environments favorable for electric-dipole transitions. Excited-state lifetimes reached 2.12 ms (Eu 3+ ) and 1.12 ms (Tb 3+ ), with quantum yields of 42% and 68%, respectively. The triplet-state energy of L24 (22,741 cm −1 ) aligns well with emissive levels of Eu 3+ and Tb 3+ , consistent with Latva’s criterion. Fluorescence titrations indicated positively cooperative complexation, with association constants ranging from 0.60 to 1.67. Stark splitting and high 5 D 0 → 7 F 2 / 7 F 1 intensity ratios (R 2 = 6.25) confirm the asymmetric coordination field. The para-methylphenoxy substituent appears sufficient to lower coordination symmetry and strengthen electric-dipole transitions, offering a controlled route to enhance photoluminescence in Eu 3+ and Tb 3+ complexes.