Optical ratiometric thermometry combining Raman and luminescence bands in PMMA/Eu^III complex films

Optical temperature sensing plays an essential role in monitoring a variety of chemical, physical, and biological processes. Among several techniques, Raman thermometry enables structural insight through vibrational modes, whereas luminescence thermometry offers high thermal sensitivity. Lanthanide(III) (Ln^III) coordination compounds are among the most extensively studied systems for the development of optical temperature probes, particularly within the framework of self-calibrated ratiometric approaches. However, achieving superior thermal sensitivity in ratiometric Raman-based systems remains a significant challenge. Furthermore, bright single-centered Ln^III luminescent probes typically exhibit limited thermal sensitivity when relying solely on ratiometric approaches. Herein, we report a hybrid optical thermometer that combines Raman and luminescence bands within a single Raman spectrum to calculate a ratiometric thermometric parameter (Δ = I_luminescence/I_Raman). This optical feature is achieved by incorporating the [Eu(nta)₃(phen)] (nta: 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butadione; phen: 1,10-Phenanthroline) complex into a poly(methyl methacrylate) (PMMA) matrix, fabricating transparent and homogeneous films that allow simultaneous detection of PMMA vibrations and Eu^III intraconfigurational 4f⁶ emissions in the same Raman spectrum. Raman measurements from 100 to 420 K reveal two distinct temperature-dependent regimes of the thermometric parameter: a Raman-dominated exponential response below 280 K (maximum relative thermal sensitivity, S_m, of 1.1 % K^-1 at 280 K), and a luminescence-driven response above 280 K (S_m of 5.2 % K^-1 at 420 K). The thermometric parameter remained stable over nine heating-cooling cycles (100-420 K), confirming its robust repeatability. Therefore, the ratiometric approach based on luminescence and Raman bands offers a versatile platform with operational flexibility for developing highly sensitive temperature probes based on Eu^III coordination compounds.