Azobenzene-siloxane hybrids with lamellar structures from bridge-type alkoxysilyl precursors

Lamellar azobenzene-siloxane hybrids were prepared by controlled hydrolysis and polycondensation of three types of precursors, where azobenzene is sandwiched by mono-, di- and triethoxysilyl groups using propylene linkers. All precursors underwent reversible and fast trans-cis isomerization upon UV/Vis irradiation in dilute solution. Upon hydrolysis of the triethoxysilylated precursor in a homogeneous solution under acidic conditions, precipitation occurred by self-assembly of hydrolyzed monomers into a lamellar structure. Although di- and mono-ethoxysilylated precursors produced less ordered products under identical conditions, highly ordered lamellar films were obtainable either by evaporation induced self-assembly of the hydrolyzed monomers or by solid-state reactions of precursor films. The degree of trans-cis isomerization of azobenzene moieties in the hybrid films was enhanced by decreasing the cross-linking degree of siloxane networks using precursors with less condensable alkoxy groups.