Photoluminescence of Eu(iii)-doped lamellar bridged silsesquioxanes self-templated through a hydrogen bonding array

Abstract

An organic molecule, (EtO)₃Si(CH₂)₃NH(CO)NH–(CH₂)₁₂–NH(CO)NH(CH₂)₃Si(OEt)₃, combining urea functionalities and a 12-carbon alkyl chain, underwent sol–gel reactions in the presence of an acid catalyst, a large excess of water and variable amounts of EuCl₃, giving rise to the formation of crystalline hybrids through a self-directed organization process in which hydrogen bonding interactions play a key role. Three photoluminescent lamellar Eu³⁺-doped silsesquioxane hybrids incorporating 1.01, 1.83 and 15.04% Eu³⁺ were prepared and fully characterized. The lamellar hybrids are room temperature multi-wavelength emitters due to the convolution of the hybrid's emitting centres and the Eu³⁺ intra-4f⁶ transitions. Their emission colour can be easily tuned along the Commission Internationale d'Eclairage chromaticity diagram from the bluish-purple region to the pink area, either by changing the excitation wavelength or the Eu³⁺ amount. The Eu³⁺ first coordination shell is very similar for the three photoluminescent lamellar Eu³⁺-doped silsesquioxane hybrids incorporating oxygen atoms of the carbonyl group, Cl atoms and water molecules, despite the minor variations found in the ⁵D₀ quantum efficiency (16.3–19.2), number of water molecules (1.7–2.0) and experimental intensity Ω₂ parameter (8.9–10.1 × 10⁻²⁰ cm²) with the increase of Eu³⁺ content.