Lamellar Bridged Silsesquioxanes: Self‐Assembly through a Combination of Hydrogen Bonding and Hydrophobic Interactions

Abstract

The synthesis of four bis(trialkoxysilylated) organic molecules capable of self-assembly--(EtO)3Si(CH2)3NHCONH-(CH2)n-NHCONH(CH2)3Si(OEt)3 (n = 9-12)--associating urea functional groups and alkylidene chains of variable length is described. These compounds behave as organogelators, forming supramolecular assemblies thanks to the intermolecular hydrogen bonding of urea groups. Whereas fluoride ion-catalysed hydrolysis in ethanol in the presence of a stoichiometric amount of water produced amorphous hybrids, acid-catalysed hydrolysis in an excess of water gave rise to the formation of crystalline lamellar hybrid materials through a self-organisation process. The structural features of these nanostructured organic/inorganic hybrids were analysed by several techniques: attenuated Fourier transformed infrared (ATR-FTIR), solid-state NMR spectroscopy (13C and 29Si), scanning and transmission electron microscopy (SEM and TEM) and powder X-ray diffraction (PXRD). The reaction conditions, the hydrophobic properties of the long alkylidene chains and the hydrogen-bonding properties of the urea groups are determining factors in the formation of these self-assembled nanostructured hybrid silicas.

Self-Structuring of Lamellar Bridged Silsesquioxanes with Long Side Spacers

“…The condensation degrees deduced (Table S1) are on the same order of magnitude as those reported previously for the lamellar L12 material 10,12,15 and more recently for the sponge-like L12D material. 17 These findings suggest that in the C 10 C n C 10 BSs, the siloxane framework is a 2D network composed of linear polymer siloxane linkages.…”

“…Chains with less than eight methylene groups led, however, to disordered materials, regardless of the reaction conditions employed. 12,14 The case of the dodecylene (n = 12)-based precursor P12 is worth mentioning in this context because it is an excellent example of easy tuning of the degree of order, dimension, and design of the final hybrid compound via judicious modification of the reaction conditions. The HCl-catalyzed synthesis performed in a large excess of water (molar ratio P12/H 2 O/HCl = 1:600:0.2) induced the formation of a crystalline lamellar structure (L12).…”

“…The HCl-catalyzed synthesis performed in a large excess of water (molar ratio P12/H 2 O/HCl = 1:600:0.2) induced the formation of a crystalline lamellar structure (L12). 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.…”

Photoluminescence of Bridged Silsesquioxanes Containing Urea or Urethane Groups with Nanostructures Generated by the Competition between the Rates of Self-Assembly of Organic Domains and the Inorganic Polycondensation

“…The broad N-H stretching vibration band at 3331 cm -1 is characteristic of hydrogen-bonded N-H groups of varying strength, with a partial contribution of H-bonded SiOH groups. 20,32 …”

“…A possible explanation of the fast structuring is via cooperative weak interactions between H-bonding of urea groups combined with the -stacking of the three aromatic rings present in the structure of the bridge. 17,18,20 For both U-1 and U-2 no harmonics were observed at higher values of q, indicating that both materials exhibit an amorphous character with a short-range ordered structure. The level of order, even if restricted to a short range, may be estimated from the width of the interference peak at half of its maximum, ¢q, that determines a correlation length (oe): 19 This distance corresponds to the periodicity in the stacking of lamella alternating inorganic and organic layers.…”