Abstract <p>This work presents novel diphenylphosphoryl-substituted 1,3-diketone derivatives and demonstrates their potential as ligands for the design of Eu³⁺-containing nanoparticles with reproducible thermally dependent photoluminescence. Structural analysis of the synthesized lanthanide complexes in acetonitrile medium using NMR, IR spectroscopy, and computational modeling revealed weak coordination of the phosphoryl group to the central Ln³⁺ ion. Nevertheless, the presence of this group in the ligand molecule enhances the sensitization efficiency of Eu³⁺ luminescence due to a shielding effect. This effect was confirmed by comparing the quantum yields of Eu³⁺ complexes with diphenylphosphoryl-substituted 1,3-diketone ligands to their structural analogs lacking the phosphoryl group. The shielding of the lanthanide coordination sphere by the diphenylphosphoryl substituent promotes more efficient incorporation of the complexes into polystyrene sulfonate nanoparticles. The resulting polyelectrolyte colloids exhibit chemically stable luminescent properties in the presence of biologically relevant amino acids and phosphates. Additionally, they demonstrate reproducible thermally dependent luminescence in the biologically relevant temperature range of 25–50°C, with a sensitivity of 1.73–1.76%/°C.</p>