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.