Eu³⁺-Assisted Short-Range Ordering of Photoluminescent Bridged Silsesquioxanes

The bridged silsesquioxane precursor (EtO) 3 Si(CH 2 ) 3 NH(C═O)NH-(CH 2 ) 12 -NH(C═O)NH(CH 2 ) 3 Si(OEt) 3, combining polymerizable silylated groups, urea functionalities, and alkyl chains, undergoes fluoride (F − )-catalyzed sol−gel reactions in the presence or absence of EuCl 3 ·6H 2 O. Supramolecular self-assembly of the growing structure relies primarily on the establishment of strong and ordered hydrogen bonding interactions. In the case of the Eu 3+ -containing hybrids the lanthanide ions play a totally unparallel dual-role acting simultaneously as structure directing agents and structural probes to sense locally morphological alterations. In the early stages of the synthesis, while a fraction of the Eu 3+ ions promotes the formation of a unidirectional urea−urea hydrogen-bonded array, the remaining ions coordinate to silanol (Si−OH) groups inhibiting the growth of the siloxane network and yielding small anisotropic flakes (∼200 nm long). These are subsequently assembled on the micrometer scale in a brick-like tile-to-tile arrangement that ends up with the formation of fibers or twisted bundles (3.0−4.0 μm long and 0.5−1.0 μm wide). At higher Eu 3+ concentrations, Eu 3+ -based ionic interfiber cross-links hinder the solvent flow and force adjacent fibers to adopt a bow-tie form (3.0−4.0 μm long and 3.0 μm wide at the tips). The hybrids are room temperature multiwavelength emitters because of the convolution of the hybrids’ intrinsic emission and the Eu 3+ intra-4f 6 transitions. The photoluminescence features ( 5 D 0 → 7 F 0 energy, 5 D 0 quantum efficiency, number of coordinated water molecules, and experimental intensity parameters) as a function of the Eu 3+ content and acidic- and F − -catalyzed conditions used in the synthesis are compared to address the effect of the morphology in the photoluminescence features of the hybrid materials.