Elucidating thermally activated luminescence quenching in EuIII β-diketonate complexes as crystals or polymeric films for application in thermometry

Abstract

Luminescence thermometry based on Eu^III β-diketonate complexes has been investigated to monitor temperature at the submicrometric scale. However, a detailed understanding of the mechanisms underlying thermally induced luminescence quenching and how these mechanisms are affected during the fabrication of practical platforms remains lacking. Here, we investigate the temperature-dependent luminescence dynamics of the dinuclear complex [Eu₂(bpm)(nta)₆] (nta: 1-(2-naphthoyl)-3,3,3-trifluoroacetone; bpm: 2,2′-bipyrimidine) in the crystalline state and after dispersion in poly(methyl methacrylate) (PMMA) films. Structural analysis confirms that the two Eu^III centers are bridged by bpm ligands and that the coordination environment is filled by three bidentate nta⁻ ligands. Upon incorporation into PMMA, FTIR and Judd–Ofelt analyses imply subtle conformational changes in the Eu^III coordination environment. The temperature dependence of the ⁵D₀ level lifetime shows a sigmoidal decrease with increasing temperature owing to thermally activated non-radiative processes. Intramolecular energy transfer modelling demonstrates that thermally assisted back energy transfer through the (T₁ → S₀)(⁷F₁ → ⁵D₀) pathway is the dominant quenching mechanism, while multiphonon relaxation plays a minor role. The activation energy for quenching decreases from 4985 cm⁻¹ in the crystal to 3410 cm⁻¹ in the PMMA:1% film, shifting the operational range to lower temperatures. Despite the reduced activation barrier, the PMMA film retains high relative thermal sensitivity (2.79% K⁻¹ at 380 K), comparable to that of the crystalline material (3.12% K⁻¹ at 450 K). These results shed light on the mechanisms governing thermal quenching in Eu^III β-diketonate complexes and demonstrate that polymer processing enables flexible thermometric platforms without significantly compromising performance.