Spectroscopic and Theoretical Tools Unravel the Thermally–Stabilized Behavior of Eu3+-Based Complex Incorporated in Sustainable Urethanesil Film

Despite significant ongoing efforts to develop luminescent rare-earth β-diketonate complexes, achieving thermal stability remains a persistent challenge. In this study, we present a thermally stable organic-inorganic hybrid (OIH) compound, SiCO-[Eu(tta)3(H2O)2], where tta=thenoyltrifluoroacetonate, Si=3-(triethoxysilyl)propyl isocyanate, and CO=castor oil. Spectroscopic analysis reveals that while [Eu(tta)3(H2O)2] in its powder form undergoes irreversible photoluminescence quenching at 60 °C and embedding it in the SiCO polymer preserves its luminescence even after being annealed up to 180 °C. Notably, the hybrid film maintains stable emission properties after multiple heating-cooling cycles (29–70 °C) and exhibits reversible emission behavior. This finding is attributed to polymer-complex interactions and/or the replacement of water molecules with polymer coordination, resulting in a more rigid environment in the Eu3+ coordination sphere. The thermal dependence of intramolecular energy transfer (IET) indicates that the decrease in Eu3+ luminescence is linked to faster depopulation of the emitting level, driven by non-radiative relaxation and back-energy transfer. This outcome enabled us to strategize approaches to mitigate luminescence quenching in OIH while providing valuable insights into the photophysical properties of these compounds, thus offering a guide towards how we can boost the capabilities of these materials.