Photoinduced synthesis and ordering of lamellar n-alkylsiloxane films

Abstract

Self-assembled organosilica films exhibiting various lamellar structures were prepared without solvent and watervia the fast photoacid-catalyzed sol–gel process of n-alkyltrimethoxysilanes (C_nTMS, n = 8, 10, 12, 16 and 18). This facile photochemical route towards nanostructured hybrid films relies on superacid photocatalyst generated by the UV photolysis of a soluble lipophilic iodonium salt: (C₁₂H₂₅)₂Φ₂I⁺ SbF₆⁻. A main emphasis has been on discussing the effect of the alkyl chain length on the level of ordering of the layered mesostructure (short- or long-range order), the alkyl chain packing arrangement (bilayered or interdigitated) and its conformational order (gauche and trans). X-Ray diffraction and electron microscopy (TEM and SEM) proved the high periodicity achieved with the long chain alkylsilanes (n = 16 and 18). Real-time Fourier transform infrared spectroscopy (RT-FTIR) was also instrumental in giving a unique insight into the sol–gel reaction kinetics and the progressive conformational ordering of the alkyl chains during the photoinduced self-assembly. Further, the combination of different solid-state NMR techniques (²⁹Si and ¹³C) shed light on the siloxy layer microstructure and the conformational structure of the alkyl chain respectively.