Molecular Photoconductor
with Simultaneously Photocontrollable
Localized Spins
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Abstract
UV irradiation reversibly switches a new insulating and
nonmagnetic
molecular crystal, BPY[Ni(dmit)<sub>2</sub>]<sub>2</sub> (BPY = <i>N</i>,<i>N</i>′-ethylene-2,2′-bipyridinium;
Ni(dmit)<sub>2</sub> = bis(1,3-dithiole-2-thione-4,5-dithiolato)nickelate(III)),
into a magnetic conductor. This is possible because the bipyridyl
derivative cations (BPY<sup>2+</sup>) trigger a photochemical redox
reaction in the crystal to produce a change of ∼10% in the
filling of the Ni(dmit)<sub>2</sub> 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)<sub>2</sub> 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)<sub>2</sub> and the BPY,
respectively. This high photoconductivity can be explained by charge
transfer (CT) transitions between Ni(dmit)<sub>2</sub> 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 (BPY<sup>2+</sup>/[Ni(dmit)<sub>2</sub>]<sup>−</sup>, nonmagnetic insulator) and a CT complex (BPY<sup>2(1−δ)+</sup>/[Ni(dmit)<sub>2</sub>]<sup>(1−δ)–</sup> (δ ≈ 0.1), magnetic conductor) in the solid state
