A covalently linked ruthenium tris(bipyridyl) complex and viologen have been chemisorbed at the surface of the constituent nanocrystals of a transparent nanostructured TiO 2 film supported on conducting glass. This resulting heterosupramolecular assembly, when incorporated as the working electrode in a sealed two-electrode cell, yields a device which can be written to using blue–green light, read using red light and erased by applying a voltage.