Sunlight-Driven Emission of Eu³⁺ and Tb³⁺ Nalidixate Complexes: Achieving Visible Red and Green Light in PMMA Films

In this work, a series of complexes with the general formula [RE₂(nal)₆(H₂O)₃]·9H₂O (RE³⁺: Eu, Gd, and Tb) was successfully synthesized. The crystal structure of the Tb³⁺ compound was determined by single-crystal X-ray diffraction, revealing a bimetallic arrangement with distinct coordination environments. Photophysical properties indicate that the Tb³⁺ complex shows efficient sensitization through the S₁ level of the nal ligand. The presence of an LMCT band in the Eu³⁺ complex does not result in significant luminescence quenching. This can be attributed to hydrogen-bonding interactions within its structure and to the nal ligand's high capacity as an indirect luminescence sensitizer. As a result, the Eu³⁺ complex exhibits a relatively high intrinsic quantum yield value (∼24%) for a highly hydrated system. The PMMA:(1%)-Eu³⁺ film shows a higher yield of highly luminescent films (54%) than the complex. In addition, the PMMA doped with Eu³⁺ and Tb³⁺ ions exhibits red and green emission under solar exposure. To the best of our knowledge, this behavior has not yet been reported in the scientific literature, considering the same ligand. These findings highlight the potential of these photonic materials as candidates for luminescent solar concentrator (LSC) devices.