UV irradiation reversibly switches a new insulating and nonmagnetic molecular crystal, BPY[Ni(dmit)(2)](2) (BPY = N,N'-ethylene-2,2'-bipyridinium; Ni(dmit)(2) = bis(1,3-dithiole-2-thione-4,5-dithiolato)nickelate(III)), into a magnetic conductor. This is possible because the bipyridyl derivative cations (BPY2+) trigger a photochemical redox reaction in the crystal to produce a change of similar to 10% in the filling of the Ni(dmit)(2) valence band, leaving localized spins on the BPY themselves. In the dark, almost all of the BPY molecules are closed shell cations, and most of the Ni(dmit)(2). radical anions form spin singlet pairs; thus, this material is a diamagnetic semiconductor. Under UV irradiation, a photocurrent is observed, which enhances the conductivity by 1 order of magnitude Electron-spin resonance. measurements indicate that the 'UV:. irradiation reversibly generates. carriers and localized spins on the Ni(dmit)(2). and the BPY, respectively: This high photoconductivity can be explained by charge transfer (CT) transitions between Ni(dmit)(2) and BPY in the UV region. In other words, the photoconduction and "photomagnetism" can be described as reversible optical control of the electronic states between an ionic salt (BPY2+/[Ni(dmit)(2)](-), nonmagnetic insulator) and a CT complex (BPY2(1-delta)+/[Ni(dmit)(2)]((1-delta)-) (delta approximate to 0.1), magnetic conductor) in the solid state.