Novel family of periodic mesoporous organosilicas containing azobenzene within the pore walls

Abstract

Three new bis-silylated azobenzenes, differently substituted, having terminal triethoxysilyl groups attached directly to the para position of the phenyl rings without linker, namely bis(4-triethoxysilyl)azobenzene, bis(2,6-dimethyl-4-triethoxysilyl)azobenzene and bis(2,6-diisopropyl-4-triethoxysilyl)azobenzene, have been successfully synthesized by Lithium-Iodine exchange from its diiodoazobenzene-derivatives, and used to prepare three novel azobenzene-bridged periodic mesoporous organosilica materials (Azb-PMO-R, R= H, Me and iPr). The synthesis of these materials from 100% of bis-silylated azobenzenes precursors was carried out under basic conditions using octadecyltrimethylammonium chloride (C18TMACl) as structure-directing agent. The successful synthesis was verified by N2 sorption measurements, solid state MAS-NMR spectroscopy, and IR spectroscopy. Materials with pore sizes between 3.7 and 4.6 nm and specific surface areas in the range of 400–700 m2/g were obtained. UV–Visible spectroscopy shows that the prepared PMOs are formed by rigid combination of isomers cis and trans of azobenzene moieties, which do not exhibit a regular photochemical trans-to-cis isomerization of the azo group upon alternating UV–Visible irradiation. However, these materials probably exhibit a dynamic and local bending which could be useful for reversible gas adsorption and release with light.