Photocontrolled dispersibility of colloidal silica by surface adsorption of a calix[4]resorcinarene having azobenzene groups

A novel calix[4]resorcinarene (4-AzCRA) having fourp-butylazobenzene residues attached to the upper rim of the cyclic framework has been prepared and subjected to adsorption on the surface of colloidal silica particles. The trans–cis photoisomerization of 4-AzCRA went almost to completion without being affected by the adsorption on the surface. UV irradiation of a homogeneous dispersion of surface-modified silica particles in cyclohexane enhanced the sedimentation owing to the increase in surface polarity, leading to the formation of porous precipitates. Subsequent visible light irradiation regenerated a dispersed state, displaying photocontrol of the reversible alternation between dispersed and aggregated states of the silica particles.