Elucidating Thermally Activated Luminescence Quenching in a EuIII β-Diketonate Complex 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 are affected during the fabrication of practical platforms remain sought after. Here, we investigate the temperature-dependent luminescence dynamics of the dinuclear complex [Eu 2 (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 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 5 D 0 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 (T 1 ￫ S 0 )( 7 F 1 ￫ 5 D 0 ) 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 clarify the mechanisms governing thermal quenching in the Eu III β-diketonate complex and demonstrate that polymer processing enables flexible thermometric platforms without significantly compromising performance.