Self-assembling through H bonds in urea and thiourea based bridged silsesquioxanes
Résumé
Associations of organic and inorganic molecules involving non covalent and covalent interactions participate to a wide range of structures found in nature. Bridged silsesquioxanes (BS) can be designed with organic units selected to drive self-organization into a solid network. Therefore new hybrids functional materials with controlled morphologies can be synthesized. In particular urea groups, thanks to their ability to self-assemble though H bonds, have been incorporated in BS within the organic subunits (figure 1, UU). Depending on kinetics of the sol gel reaction, hybrid materials bridged by urea groups exhibiting a variety of textures and morphologies were synthetized. Considering the bridging group as a key element for the control of the assembly formed hybrids materials where thiourea groups have also been synthesized (Figure 1, UU or TT). As a matter of fact thiourea groups are also known to link them self via H bonds, but with geometry and strengths that can differ from urea links.
Vibrational spectroscopy is used here to explore the self-organization mechanisms involved in the formation of urea bridged and thiourea bridged silsequioxanes. The influence of H-bonding strength and the self-organization properties of the urea and thiourea bridged organic substructures are explored through infra-red spectroscopy coupled with DFT calculations. Some particular vibrations such as amides and thio-amides vibrations are often considered as signatures of H bond strength. The thermal dependence dynamics of such signatures is studied as an indicator of the bond properties. Finally, in-situ high pressure vibrational measurements demonstrate the role of supramolecular interactions on the mechanical response of hybrid materials to high pressure.