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

Abstract

The bridged silsesquioxane precursor (EtO)3Si(CH2)3NH(CO)NH-(CH2)12-NH(CO)NH(CH2)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 EuCl3·6H2O. Supramolecular self-assembly of the growing structure relies primarily on the establishment of strong and ordered hydrogen bonding interactions. In the case of the Eu3+-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 Eu3+ 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 Eu3+ concentrations, Eu3+-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 Eu3+ intra-4f6 transitions. The photoluminescence features (5D0→7F0 energy, 5D0 quantum efficiency, number of coordinated water molecules, and experimental intensity parameters) as a function of the Eu3+ 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.

Self-Structuring of Lamellar Bridged Silsesquioxanes with Long Side Spacers

“…The emission spectra of selected C 10 C n C 10 samples are practically independent of the length of the central alkylene spacer and similar to those reported for hybrid materials containing propylene side spacers, such as the nonstructured BS A12, 16 the diureasils, 59,60 the diurethanesils, 61 the diamidosils, 63 and the structured lamellar bilayer monoamidosils. 62 The emission is ascribed to the overlap of two distinct components that occur within the urea, urethane, and amide cross-linkages in the case of the diureasils, diurethanesils, and amidosils, respectively, and within the siliceous nanodomains.…”

“…Similarly to the situation found for the emission data, these excitation spectra are identical to those monitored for the A12 16 hybrid. The high-and low-energy side were attributed to the preferential excitation of the siliceous-and urea-related emission 59,60 Apart from an increase in the relative intensity of the high-wavelength side of the emission spectra, those acquired at 12 K ( Figure S9) resemble those measured at room temperature ( Figure 10).…”

“…10,12À14 In contrast, the addition of ammonium fluoride (NH 4 F), ethanol (EtOH), and a stoichiometric amount of water (molar ratio P12/H 2 O/EtOH/NH 4 F = 1:6:60:0.01) resulted in an amorphous material with either granular-like morphology (A12) 12 or an irregular microsphere shape with a rough surface composed of plates. 16 Recently, we reported that lanthanide ions may play a structuring role under the same reaction conditions employed to produce L12 and A12. 15,16 When the synthetic procedure used to prepare L12 was adopted and Eu 3+ ions were incorporated (introduced as europium chloride (EuCl 3 )) (molar ratio P12/H 2 O/HCl/EuCl 3 = 1:600:0.2:x, where x = 0.05, 0.95, and 2.9), a photoluminescent lamellar BS (Eu@L12) was formed.…”

“…16 Recently, we reported that lanthanide ions may play a structuring role under the same reaction conditions employed to produce L12 and A12. 15,16 When the synthetic procedure used to prepare L12 was adopted and Eu 3+ ions were incorporated (introduced as europium chloride (EuCl 3 )) (molar ratio P12/H 2 O/HCl/EuCl 3 = 1:600:0.2:x, where x = 0.05, 0.95, and 2.9), a photoluminescent lamellar BS (Eu@L12) was formed. 15 The combination of NH 4 16 The influence of the type of solvent was emphasized more recently.…”

“…15,16 When the synthetic procedure used to prepare L12 was adopted and Eu 3+ ions were incorporated (introduced as europium chloride (EuCl 3 )) (molar ratio P12/H 2 O/HCl/EuCl 3 = 1:600:0.2:x, where x = 0.05, 0.95, and 2.9), a photoluminescent lamellar BS (Eu@L12) was formed. 15 The combination of NH 4 16 The influence of the type of solvent was emphasized more recently. 17 When the synthetic route employed to prepare L12 was combined with the incorporation of a large excess of dimethylsulfoxide (DMSO) (molar ratio P12/H 2 O/DMSO/ HCl = 1:527:388:0.175), a material with a microsphere-like shape that mimics sea sponges was produced (L12D).…”