PHOTOCONTROLLED AGGREGATION OF COLLOIDAL SILICA

Spirobenzopyran having a decamethylene spacer has been chemisorbed on the surface of colloidal silica particles of diameter 150 nm. The sili ca particles covered with the photochromic units displayed normal phot ochromism when they were dispersed in organic solvents having refracti ve indices matched with that of the silica. The fact that normal photo chromism was observed even in highly polar ethylene glycol, in which a low-molecular-weight spiropyran shows reversed photochromism, indicat es that the highly hydrophilic character of the silica surface is conv erted to a relatively hydrophobic one owing to the long hydrocarbon ch ain. When the photochromic particles were dispersed in carbon tetrachl oride, the UV irradiation induced flocculation, causing sedimentation of the colloidal silica. Subsequent visible light irradiation recovere d the dispersed state, showing the photocontrol of the dispersibility and offering a novel photoresponsive system.