Europium oxide (Sigma, 99.99%), 4,4,4-trifluoro-1-(thiophen-2-yl)butane-1,3-dione (tta, Sigma, ≥98%), acetanilide (Sigma, 99%), sodium hydroxide (NaOH, Neon, ≥98%), hydrochloric acid (HCl, Sigma, 37%), ethanol (Neon, ≥99%), methanol (Neon, ≥99%), hexane (Neon, ≥99%), tetrahydrofuran (THF, Neon, ≥99%), 3-(Triethoxysilyl)propyl isocyanate (ICPTES, Aber, 95%), castor oil (VWR Chemicals, GPR grade), and chloroform (Neon, ≥99%) were purchased and used without further purification. All the following syntheses were carried out under standard atmospheric conditions.

The ligand 2-phenylimidazo(4,5-f) (1,10)phenanthroline (PIB) was first synthesized following a procedure described elsewhere (Li et al., 2014) and then used with the ligand tta to obtain the luminescent [Eu (tta)₃(PIB)] complex. Briefly, 0.15 g of tta and 0.07 g of PIB (3:1 M ratio) were added to a beaker containing 10 mL of methanol and 5 mL of THF. A methanolic NaOH solution was added dropwise until the pH reached 6. The resulting mixture was stirred at 50 °C for 30 min and subsequently transferred to a reaction flask containing 2.1 mL of a 0.1 mol L⁻¹ europium chloride solution (prepared by acid dissolution of europium oxide in HCl). The reaction mixture was then refluxed at 85 °C for 4 h. After completion, the solution was cooled to room temperature, and the volatile components were removed, yielding an orange solid. The crude product was dissolved in chloroform and reprecipitated with hexane, followed by filtration and washing four times with 10 mL portions of cold hexane. The final product was dried in a desiccator, resulting in a pale-yellow solid with a yield of 70%. The complex was characterized as described in the reference (Silva et al., 2024b). Elemental analysis of [Eu (tta)₃(PIB)] (molecular weight = 1,111.82 g mol⁻¹) gave the following values (found/calculated): C, 46.55% (46.45%); H, 2.18% (2.18%); N, 5.64% (5.04%). Standard acetanilide (found/calculated): C, 71.13% (71.09%); H, 6.42% (6.71%); N, 11.37% (10.36%).

The polymer precursor was synthesized following the methodology reported in (de Freitas et al., 2023). To prepare the SiCO precursor, castor oil was mixed with ICPTES in a 1:3 M ratio and stirred at 82 °C for 24 h. After synthesis, the SiCO precursor was stored in a plastic container at 10 °C for further use.

For the fabrication of the SiCO polymeric film, 0.750 g of the SiCO precursor was mixed with 5 mL of ethanol and 0.1 mL of 0.1 mol L⁻¹ HCl in a beaker. The mixture was stirred for 2 h, then cast onto a plastic Petri dish using a drop-casting procedure. The film was dried at 40 °C for 48 h in a ventilated oven. To prepare the films containing the [Eu (tta)₃(PIB)] complex, the same procedure was followed, adding 1.9 mg of the complex to 0.748 g of SiCO to obtain a concentration of 0.25% by weight. A pristine SiCO film and complex-containing films at 0.25, 0.50, 1.00, and 3.00% by weight were obtained and labeled as SiCO, SiCO-0.25Eu, SiCO-0.50Eu, SiCO-1.00Eu, and SiCO-3.00Eu, respectively.

Fourier transform infrared vibrational (FTIR) spectra were collected on a Vertex 70 spectrometer (Bruker) equipped with a DLATGS detector, acquiring 64 scans at 1 cm⁻¹ resolution over the 400–4,000 cm^-1 range using a diamond attenuated total reflection (ATR) crystal. Elemental CHN analyses were obtained using a FlashEA 1,112 analyzer (Thermo Scientific) calibrated with BBOT, sulfanilamide, and cystine standards at 950 °C. Ultraviolet–visible (UV-Vis) absorption spectra of ethanolic solutions (1 μmol L⁻¹) were recorded at room temperature using a Cary 60 spectrophotometer (Agilent Technologies). Transmittance spectra were collected at 298 K with a Lambda 950 dual-beam spectrophotometer (PerkinElmer) over 250–800 nm with a 1.0 nm resolution. Photoluminescence spectra of the SiCO-0.25Eu film in the solid state were measured between 13 and 312 K using a Fluorolog-3 FL3-122 spectrofluorometer (Horiba Jobin Yvon) equipped with a R928P photomultiplier (Hamamatsu) and a 450 W Xenon short-arc lamp (UXL-450S-O, Ushio Inc.) for steady-state excitation. Emission intensity decay profiles were recorded in the same equipment using a 150 W Xenon flash lamp (FX-1102, Excelitas Technologies) with 0.15 J per flash. The obtained decay curves were adjusted to a monoexponential decay function I t = I 0 + A ⁡ exp − t / τ , where I(t) is the intensity as a function of the time t, I ₀ is the intensity background, is the intensity amplitude, and τ is the emission lifetime (Thor et al., 2024). Although it is common practice to use the standard error of the fitted value as the uncertainty of τ, the resulting values are lower than the actual temporal resolution of the equipment. In this sense, we have used herein the time increment (0.05 m) as the uncertainty of τ to better reflect the equipment’s temporal accuracy. Temperature-dependent photoluminescence spectra and decay curves were measured in a closed-cycle helium cryostat under vacuum (4 × 10⁻⁴ Pa), controlled by a Lakeshore 331 temperature controller and monitored with a DT-470-SD silicon diode sensor, with accuracies of ±0.5 K (12–30 K), ±0.25 K (30–60 K), and ±0.15 K (60–340 K).

The relationship between temperature and the luminescent behavior of the SiCO-0.25Eu film was determined by establishing an intensity ratio between the integrated emission bands from the PIB ligand (I _PIB, 400–575 nm) and Eu^{3+ 5}D₀→⁷F₂ transition (I _Eu, 604–640 nm) taken from the emission spectra at each temperature, resulting in the thermometric parameter Δ = I _PIB/I _Eu. After the baseline subtraction, all spectra were converted from wavelength to energy (cm⁻¹) using the Jacobian transformation (Mooney and Kambhampati, 2013), where the intensity ratio as a function of the temperature was adjusted to a Boltzmann-type sigmoidal function. The relative thermal sensitivity S r = 1 Δ d Δ d T was used to quantify the performance of the thermometer’s responsiveness to temperature variations, where d Δ / d T indicates the first derivative of Δ (Brites et al., 2023). The uncertainty in temperature δ T = 1 S r δ Δ Δ represents the smallest temperature change detectable by Δ. The uncertainties of the integrated emission intensities I _PIB and I _Eu were determined by propagating the baseline noise through the numerical integration procedure. The noise amplitude was estimated as the root-mean-square fluctuation of the Jacobian-corrected intensity in the 714–735 nm spectral region (where there is no emission from the ligand nor from Eu³⁺). Because the trapezoidal integration corresponds to a weighted linear sum of the spectral intensities, the variance of the integrated area was calculated as the sum of the squared trapezoidal weights multiplied by the noise variance, explicitly accounting for the non-uniform energy spacing of the energy-converted spectra. The resulting standard deviation represents the statistical uncertainty associated with each integrated emission intensity. The uncertainty in Δ (δΔ) was then calculated by propagating the uncertainties of the integrated areas.

To investigate the molecular-level interactions between the complex and the polymer matrix, FTIR spectra were collected for the [Eu (tta)₃(PIB)] complex, the pristine SiCO film, and the SiCO-3.00Eu film (Figure 1A). The film with a higher complex concentration was intentionally selected to resolve its characteristic vibrational bands more clearly (Rozhkov et al., 2013; Silverstein et al., 2005), which tend to overlap with SiCO vibrational modes at lower concentration. The distinct features of the complex remain detectable in the composite film without noticeable shifts, indicating that its structural integrity is preserved within the polymer network. The full FTIR spectrum of the [Eu (tta)₃(PIB)] complex and the assignment of the vibrational modes can be seen in Supplementary Figure S1 and Supplementary Table S1, in the Supplementary Material.

Complementary optical characterization was performed on the [Eu (tta)₃(PIB)]-containing SiCO films, as shown in Figure 1B. The pure SiCO film displays two main absorptions at 272 and 320 nm (Figure 1B), corresponding to the π→π^* and n→π^*, respectively, from the CO chains and the C=O and C=N groups within the SiCO structure (de Freitas et al., 2023). The complex-containing films exhibit enhanced absorption in the 275–450 nm region relative to pure SiCO (Figure 1B), which accounts for the characteristic absorptions from the SiCO host matrix plus the absorption coming from the π→π^* transition in the tta and PIB ligands (Arauzo et al., 2025; Josephine Kanimozhi and Alexander, 2017). A weak absorption tail in the 380–450 nm range, responsible for the film’s faint yellowish hue, is attributed to ligand-centered transitions in the coordinated complex or to partial π–π stacking interactions involving aromatic rings (Mutti et al., 2021). Such interactions subtly alter the electronic environment of the ligands, producing small spectral shifts while preserving the overall optical response of the material. In the end, the obtained films present a transparency higher than 85% in the 450–750 nm range.

The excitation spectrum of the [Eu (tta)₃(PIB)]-containing SiCO films, monitored at the ⁵D₀→⁷F₂ transition of Eu³⁺ (Figure 2A) exhibits broad bands assigned to the S _n←S ₀ transitions of the tta and PIB ligands (Arauzo et al., 2025). The low intensity of sharp f–f lines (or absence, for samples SiCO-0.25Eu and SiCO-0.50Eu), characteristic of direct Eu³⁺ excitation, indicates that the population of the ⁵D₀ level occurs predominantly through ligand absorption followed by ligand-to-metal energy transfer via the antenna effect (Binnemans, 2015). A weak feature near 400 nm is also observed and is attributed to π–π stacking interactions, consistent with tail absorption seen in the transmittance spectra (Rozhkov et al., 2013).

Upon 254 nm excitation, the emission spectra of the films (Figure 2B) show the characteristic narrow bands of Eu³⁺ related to the ⁵D₀→⁷F_0–4 set of transitions superimposed on a broad band attributed to emissions from the SiCO matrix and the coordinated ligands (de Freitas et al., 2023; Gao et al., 2017). Derivatives of 1,10-phenanthroline, as the PIB ligand, are known to display intense visible fluorescence, with peak positions dependent on their molecular framework (Bing et al., 2004; Josephine Kanimozhi and Alexander, 2017). Although such a band is absent in the powdered complex, incorporation into the polymer matrix modifies the local chemical environment and the orbital overlap within the ligand system. These effects can reduce the efficiency of ligand-to-metal energy transfer, resulting in the residual broad band ligand-centered emission observed in the 425–575 nm region (Dandekar et al., 2018; Yang et al., 2016).

To gain further insight into the Eu³⁺ local environment, the Judd-Ofelt intensity parameters Ω₂ and Ω₄ were calculated for the complex in both powdered form and when embedded in the SiCO matrix, as presented in Table 1. The Ω₂ parameter is commonly associated with the degree of asymmetry and covalency at the coordination site, reflecting the polarizability of the Eu³⁺ surroundings, whereas Ω₄ is more sensitive to the rigidity and vibrational properties of the host, as well as to subtle changes in Eu–ligand bonding (Bünzli, 2015; Silverstein et al., 2005). Upon incorporation into the polymer matrix, both parameters exhibit variations, evidencing a clear modification of the Eu³⁺ coordination environment. In particular, the moderate increase in Ω₂ indicates that the metal center experiences a more distorted and polarizable local field, consistent with a reduced symmetry imposed by the SiCO polymer framework. In contrast, the more pronounced enhancement of Ω₄ suggests a strengthening of the ligand field and a concomitant increase in environmental rigidity, likely arising from constrained Eu–ligand vibrational modes and subtle reorganization of coordination interactions induced by the SiCO matrix. This indicates that, despite the molecular integrity of the [Eu (tta)₃(PIB)] complex being preserved after incorporation into the SiCO polymer, as shown by the FTIR results (Figure 2A), the polymeric surrounding environment plays a role in the complex coordination interactions.

Among the different films incorporating the [Eu (tta)₃(PIB)] complex, the SiCO-0.25Eu sample exhibits the highest relative contribution from the PIB-centered emission. Importantly, this formulation provides well-resolved emission from both the ligand and Eu³⁺ while requiring the lowest complex loading, making it an optimal platform to investigate temperature-dependent luminescence. Consequently, this sample was selected for thermal studies over the 13–312 K range (Figure 3A). As the temperature increases, the Eu³⁺ emission associated with the ⁵D₀→⁷F₂ transition (I _Eu) undergoes an exponential quenching, while the ligand-centered emission (I _PIB) displays a sigmoidal intensity decrease (Figure 3B). This contrasting behavior reflects the different deactivation pathways governing each emitting center. In particular, the strong thermal quenching of the Eu³⁺ emission is consistent with the progressive activation of non-radiative relaxation channels at higher temperatures. This interpretation is further supported by the systematic shortening of the Eu^{3+ 5}D₀ excited-state lifetime with increasing temperature (Figures 3C,D), which directly contributes to the overall emission intensity reduction of the material.

Based on these complementary thermal responses, the ratio between the integrated areas of the ligand-centered emission band and the Eu^{3+ 5}D₀→⁷F₂ emission was defined as the thermometric parameter Δ = I _PIB/I _Eu, indicating the relationship between the temperature-induced luminescent changes. The resulting calibration curve of Δ is shown in Figure 4A and reveals a well-defined, temperature-dependent luminescent response that follows a Boltzmann-like sigmoidal trend. It is important to note that above 282 K, I _Eu undergoes more pronounced thermal quenching than I _PIB (Figure 3B), leading to an inversion in the behavior of Δ, which begins to increase with increasing temperature. Under these conditions, the thermometric response becomes ambiguous, as a single Δ value may correspond to multiple temperatures. Similarly, the Δ values are pretty much the same between 13 and 42 K because I _Eu and I _PIB are nearly constant in this temperature range, as also observed in Figure 3B. As a result, the SiCO-0.25Eu sample works reliably and unambiguously as a luminescent thermometer within the 42–282 K temperature range.

Based on the temperature dependence of Δ, the relative thermal sensitivity S _r was calculated, reaching a maximum value of S _r = 1.31% K⁻¹ at 189 K, as shown in Figure 4B. This sensitivity confirms that the system is well-suited for luminescence thermometry, as values equal to or exceeding 1.00% K⁻¹ are generally regarded as adequate for practical sensing applications (Cheng et al., 2023; Gálico et al., 2019). Furthermore, the material maintains relatively high S _r values over a broad range of temperature, from 150 to 225 K, which highlights its robustness and applicability across an extended operational range rather than at a single optimal point. In addition to sensitivity, the temperature uncertainty (δT) was evaluated to assess the precision of the thermometric readout, with the results summarized in Figure 4C. The minimum value of δT = 0.43 K occurs at 173 K, indicating a high degree of accuracy in temperature determination within this regime. This low uncertainty, combined with the sensitivity across a wide temperature interval, further reinforces the suitability of the system for reliable luminescence-based thermometric investigations.

The stability of the SiCO-0.25Eu film was also assessed through thermal cycling experiments. After three consecutive heating-cooling cycles, no significant changes in the luminescent response were observed (Figure 4D). This result demonstrates that the [Eu (tta)₃(PIB)] complex remains structurally and photophysically stable within the SiCO matrix and does not undergo detectable degradation upon repeated thermal stress. Consequently, the obtained film exhibits sufficient robustness for repeated use, preserving its thermometric performance over multiple cycles. For a broader perspective, Table 2 summarizes representative luminescent complexes embedded in polymer matrices reported in the literature, together with their corresponding relative thermal sensitivities and operational temperature ranges. This comparison places the present system within the current state of the art and highlights its competitive performance among polymer-based luminescent thermometers.