Sharp luminescent violet- and blue-emitting stable (Eu3+ → Eu2+: PPA):SiO2 sonogel hybrid glasses: Synthesis, structural and overall photophysical characterizations

Abstract

Nanostructured Eu2O3:Polyphenylacetylene (Eu2O3:PPA) complexes prepared at different lanthanide content were synthesized as active ligand–to–metal charge–transfer (LMCT) systems. These phosphors were embedded into mesoporous a–SiO2 sonogel hosting networks at different dopant ratios to obtain luminescent (Eu2O3:PPA):SiO2 monolith glasses. The electroactive sonochemical medium of the colloidal sol–phase activates massive Eu3+→Eu2+ self–reduction processes along the polycondensation reaction of the doped emulsions, giving rise to sharp violet (407 nm) and blue (470 nm) PL–emission bands (FWHM ≤18 nm). The unusual violet–lines arise from the 4f65d1→4f7 transitions of Eu2+ ions to produce intense 6P7/2 → 8S7/2 emissions, while the blue–lines are ascribed to overlapping 5D2→7Fj (j = 0, 1) transitions of remanent Eu3+ ions assisted by LMCT and interionic energy transfers. The CIE coordinates of these composites lay within the light violet/blue region and the bandgap energy was reduced up to 2.16 eV for increasing Eu2O3 concentration. Our experimental approach shows that the Eu3+→Eu2+ process occurs at room temperature material processing, without the use of reducing atmospheres, stimulated UV photoreduction, or post–doping functionalization. Results suggest that guest–host redox interactions and Eu/SiO2 surface–assisted mechanisms are responsible for accomplishing a homogeneous self–reduction and the structuring of steady Eu2+–silicates. Extensive morphological, structural, linear and NLO spectroscopic characterizations, CIE–chromaticity, and energy bandgap evaluations, were performed to explore these novel nanostructured composites' physicochemical and photophysical properties, pointing out potential applications in tunable phosphor devices.