Luminescent coatings from bipyridine-based bridged silsesquioxanes containing Eu³⁺and Tb³⁺ salts

The sol-gel fabrication of bridged silsesquioxane thin films containing trivalent lanthanide ions (Ln³⁺) has been achieved from a 4,4'-diureido-2,2'-bipyridine bridged organosilane (P4) in the presence of Eu³⁺or Tb³⁺salts. Crack- and defect-free thin films (~50 nm) of optical quality were successfully deposited on glass substrates by spin-coating after optimizing the spinning rate and solvent system. Ellipsometry investigations revealed a significant decrease in the refractive index of the films with ageing of the corresponding precursor sols. In contrast, the photoluminescence properties of the films are independent of precursor ageing time, although significant differences are observed with respect to the corresponding bulk materials. Compared to the bulk materials, the films exhibit a significant blue-shift of the excitation spectra and an increase of the excited state lifetime. The optical conversion efficiencies (ɲopt) of the F4-Eu and F4-Tb-based collectors were also determined and potential applications of the coatings in such areas as luminescent solar concentrators are discussed.