Molecular Photoconductor with Simultaneously Photocontrollable Localized Spins

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 ∼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−δ)+/[Ni­(dmit)2](1−δ)– (δ ≈ 0.1), magnetic conductor) in the solid state.