Molecular switches on metal surfaces typically show very little photoreactivity. Using scanning tunneling microscopy, we show that the ring- opening–ring-closing switch nitrospiropyran thermally and optically isomerizes to the open merocyanine form on a Bi(110) surface. Irradiation by blue light of a monolayer of spiropyran molecules leads to mixed domains of the two isomers. At large illumination intensities a photostationary state is established, indicating the bidirectional ring-opening and ring-closing reaction of these molecules on the bismuth surface. The enhanced photoactivity contrasts with the case of adsorption on other metal surfaces, probably due to the low density of states at the Fermi level of the semimetallic Bi(110) surface.
