Structure-photoluminescence relationship in Eu(III) beta-diketonate-based organic-inorganic hybrids. Influence of the synthesis method: carboxylic acid solvolysis versus conventional hydrolysis

Organic-inorganic hybrids incorporating Eu(nta)3(.)bpy (where nta and bpy stand for 1-(2-naphthyl)- 4,4,4-trifluoro-1,3-butanedionate and 2,2'-bipyridine, respectively) were prepared either by acetic acid solvolysis or a conventional hydrolysis sol - gel route. The host framework of these materials, classed as di-ureasil, consists of a siliceous network grafted, through urea cross-linkages, to both ends of poly(ethylene oxide) chains. The resulting Eu(III)-based di-ureasils were investigated by small angle X-ray scattering, X-ray diffraction, Fourier transform mid-infrared spectroscopy, Si-29 and C-13 nuclear magnetic resonance, and photoluminescence spectroscopy, with particular attention paid to the effect of the adopted synthesis strategy on the relationship between structure and emission properties. The dimensions and the degree of condensation of the siloxane nanodomains depend noticeably on the synthesis route and the overall emission quantum yield decreases from 15 (conventional hydrolysis) to 6% (solvolysis route). The broad white-light emission typical of the di-ureasil host was not detected here suggesting, therefore, the activation of energy transfer channels between the hybrid host's emitting centres and the Eu(III) ions. As the first coordination shell of Eu(III) is essentially independent of the synthesis method employed, the significant decrease in the emission quantum yield for the di-ureasil prepared by acetic acid solvolysis might be explained by the interaction between the hybrid emitting centres and the nta ligand levels, favouring a larger non-radiative transition probability.