Engineering the Thermometric Response of Dinuclear Eu^III Complexes via Terminal Ligand-Induced Nonradiative Processes

Luminescent temperature probes are powerful tools for monitoring various physical and chemical processes. Eu^III complexes are particularly attractive due to their bright luminescence and temperature sensitivity of the ⁵D₀ excited-level lifetime. Since this thermal response is linked to nonradiative deactivation pathways, controlling these processes is key to tune thermometric behavior. Herein, we investigate how different terminal ligands modulate nonradiative deactivation and, consequently, the thermal luminescence response of dinuclear Eu^III complexes. As proof-of-concept, 1,3-diphenyl-1,3-propanedionate (dbm^-) or 4,4,4-trifluoro-1-phenyl-1,3-butanedionate (btfa^-) were employed as terminal ligands and 2,2'-bipyrimidine (bpm) as the bridge ligand to synthesize [Eu₂(bpm)-(dbm)₆] (1) and [Eu₂(bpm)-(btfa)₆] (2) complexes. The nature of the β-diketone impacts the crystal system, coordination geometry, and electronic structure. 1 presents coordination polyhedra described by a distorted D_2d point group while 2 displays a distorted D_4d coordination environment with more packed structure due to prominent F···F and H-bonding interactions. Both complexes present bright luminescence upon ultraviolet excitation, assigned to Eu^III 5D₀ → ⁷F_0-4 transitions. However, the greater number of C-H bonds in dbm^- and less compact structure in 1 promote faster nonradiative decay and a lower activation energy for thermal quenching of luminescence. The terminal ligand also influences the S₁ and T₁ state energies and thus the intermolecular energy transfer as well as back energy transfer (BET) processes. Consequently, the thermometric performance using the ⁵D₀ excited-level lifetime as the thermometric parameter is tuned by nonradiative contributions induced by the terminal ligand: 1 operates between 270 and 420 K, while 2 works from 300 to 440 K, with maximum relative sensitivities of 3.4% K^-1 (370 K) and 3.6% K^-1 (410 K), respectively. These findings demonstrate how ligand scaffolds enable fine-tuning of structure-property relationships for temperature-responsive luminescent materials.