Gadolinium(III) and Terbium(III) Complexes with an Asymmetric Phosphine Ligand—Synthesis, Structure, and Photoluminescent Properties

New complexes [GdL3(NO3)3] (I) and [TbL3(NO3)3] (II) with asymmetric phosphine oxide were synthesized and studied. The Gd(III) complex was characterized by X-ray diffraction (CIF file CCDC no. 2 523 426). The luminescence spectra of the obtained complexes were studied. Substitution of one phenyl ring in TPPO with a methyl group yields ligand phosphine oxide (L) L with a reduced triplet energy (~25 000 cm–1), which matches Latva’s optimal range for Tb(III) sensitization. Сomplex II exhibits bright green emission, confirming efficient energy transfer. Ligand L is thus proposed as an ancillary ligand for quenching solvent displacement in terbium diketonate and carboxylate complexes.