Spectroscopic studies of supramolecular organization in bridged silsesquioxanes via Self-Assembly through Hydrogen Bonding

Nanostructuring and control over morphology in hybrid solids are of great interest for the design of polyfunctional materials. For appropriately designed bridged silsesquioxanes, the bridging organic unit is able to self-direct the structure of the solid network independently of any external structure-directing agent. Indeed anisotropic organization due to interactions between organic substructure units was demonstrated with urea functional groups capable of self-assembly via H-bonding (1). We previously showed that H-bonding interactions between precursor molecules during the hydrolysis–condensation of trialkoxysilanes can deeply influence kinetic parameters of the gelation and modify the texture and morphology of the resulting silsesquioxane (2).Considering that the bridging unit plays a key role on controlling the arrangement of the material over a long-range, we developed new hybrid precursors (fig 1) liable to exhibit modulated hydrogen-bonding properties via organic substructures with either two urea groups (UU) or two thiourea groups (TT). It is expected that the strength of the H bonding decreases from urea to thiourea links.We here report our studies on the influence of H-bonding strength on the self-organization properties of the organic substructures and corresponding hybrid solids using vibrational spectroscopies (Raman, Far and Middle Infrared spectroscopies coupled with DFT calculations). Far infrared domain probing directly the intermolecular H-bonding vibrations is shown to be very sensitive to long range arrangement of organic substructure. Besides, as amide vibrations are very sensitive to intermolecular H bonding strength and dispersion, temperature dependence studies of the vibrational dynamic of the internal modes, likely to give insights into the supramolecular interactions, were performed. Additionally, in-situ high pressure infrared studies of intra- and intermolecular interactions demonstrate the role of supramolecular interactions on the mechanical response of hybrid materials towards compression.Following these studies, we propose a mechanistic model for the self-assembling process in H-bonded bridged silsesquioxanes