Luminescent PMMA Systems Doped with Rare-Earth Nalidixates for Multicolor Emission Under UV and Sunlight

Trivalent rare-earth ions (RE³⁺) exhibit a peculiar electronic structure in which the 4f orbitals are shielded by the outer 5s and 5p orbitals, minimizing the influence of the chemical environment on their spectroscopic properties. Consequently, they display narrow emission bands and long luminescence lifetimes associated with 4f–4f transitions, although these transitions are Laporte-forbidden, leading to low molar absorptivity. To overcome this limitation, organic chromophore ligands can act as antennas, absorbing incident radiation and transferring energy to the excited states of RE³⁺ ions. In this work, the nalidixate ligand (nal⁻) was employed in the complexes [RE(nal)₃(bipy)] (RE³⁺ = Eu, Gd, Tb), showing efficient sensitization through the triplet state (19,800 cm⁻¹) for Eu³⁺ and the singlet state (26,000 cm⁻¹) for Tb³⁺. Incorporation of these complexes into PMMA matrices at different Eu³⁺/Tb³⁺ ratios yielded multicolored luminescent devices excitable by UV radiation (365 nm) and sunlight.