All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising
only C, H, and O atoms, is investigated on a Au(111) substrate using scanning
tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are
switched to a number of states, three of which carry a localized spin as
evidenced by conductance spectroscopy in high magnetic fields. The spin of a
single molecule may be reversibly switched on and off without affecting its
neighbors. We suggest that ReA on Au is readily converted to a radical by the
abstraction of an electron.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
