Molecular Photoconductor with Simultaneously Photocontrollable Localized Spins

Abstract

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 (BPY(2+)) 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 (BPY(2+)/[Ni(dmit)(2)](-), nonmagnetic insulator) and a CT complex (BPY(2(1-δ)+)/[Ni(dmit)(2)]((1-δ)-) (δ ≈ 0.1), magnetic conductor) in the solid state.

Development of a Control Method for Conduction and Magnetism in Molecular Crystals

“…They are practically isostructural, though the volume of unit cell doubles in the BPY salt compared to that of the MV salt. 122,123 The crystal structure of the BPY salt is shown in Figure 41. 122 The length of a-axis doubles, yet basic molecular arrangement in the MV salt is identical with that in the BPY salt.…”

“…122,123 The crystal structure of the BPY salt is shown in Figure 41. 122 The length of a-axis doubles, yet basic molecular arrangement in the MV salt is identical with that in the BPY salt. Under dark conditions, the cations and the anions are 2+ and 1¹, respectively, and only the anions have a single unpaired electron (S = 1/2) each.…”

Dye-Sensitized Molecular Charge Transfer Complexes: Magnetic and Conduction Properties in the Photoexcited States of Ni(dmit)2 Salts Containing Photosensitive Dyes

“…This implies that CT transition occurs between cations and anions under UV-irradiation. As the CT transitions between two different components in solids produce net photocarriers and photoexcited spins at the same time [10][11][12]16], the observed photoresponse in ESR spectra accounts for the observed photoresponse in electrical behavior mentioned above. The g-values of a part of the spins on the N atoms (#3 (N) in the dark conditon in Table A2 were markedly enhanced compared with those of isolated spins on the bipyridine derivatives (g~2.00 for N atoms), indicating strong interaction with the spins having larger g-values such as those in heavy atoms and transition metals, i.e., indicating strong interaction between cations and anions.…”

“…Based on the discussion thus far, the overall relaxation time of photoexcited spins on the cations and anions is unusually prolonged to produce the observed photoresponse in ESR and conduction. Such prolonged relaxation times have been observed only in the CT complexes containing bipyridine derivatives with apparently mixed-stacking structures [10][11][12]. The mechanism can be related to the characteristic or advantage in this kind of molecular CT complexes such as CT interactions with photosensitive dyes, which would stabilize the photoexcited states.…”

“…In fact, such examples have been recently reported [10][11][12]16]. It has been demonstrated that metallic and paramagnetic properties can be realized in some CT complexes containing bipyridyl derivatives and π-acceptor molecules under UV irradiation.…”