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

Abstract

This study concerns development of a non-destructive method to control conduction and magnetism of molecular solids such as single crystals of charge-transfer complexes. The method is named “optical doping”, where appropriate irradiation is utilized under ambient conditions. Owing to this feature, it can be applied to a wide range of substances while measuring the properties during the control. In addition, the method adds unique conduction and magnetic properties to common insulators. Unlike other doping methods, optical doping only affects the properties and/or structures of the irradiated part of a sample while leaving the rest of the sample unchanged. There are two patterns in the optical doping. Irreversible optical doping produces junction-structures on the single molecular crystals, which exhibit characteristic behavior of semiconductor devices such as diodes and varistors. Reversible optical doping produces “giant photoconductors” and “photomagnetic conductors” by realizing unprecedented metallic photoconduction. In the latter case, localized spins are also excited to produce a Kondo system, where carriers and localized spins interact with each other. Not only the control of conduction and magnetism, the optical doping has realized the observation of physical properties in molecular crystals hardly observed under any thermodynamic condition.

Light-Induced Control of the Spin Distribution on Cu–Dithiolene Complexes: A Correlated Ab Initio Study

“…Our results show the presence of a set of excited states accessible by UV–Vis irradiation, with different nature than the ground state, which can explain the differences observed in the EPR spectra under irradiation. This study provides an alternative interpretation of the reported experimental data of Noma et al [22,23,24] and insights on the physics effects governing the photocontrol of the spin distribution in this compound. The relevance of this study is not just constrained to this particular system, but can also help in interpreting the properties of related metal–dithiolene compounds.…”

“…Recently, a similar optical doping has been reported for a family of three hybrid salts based on the [Cu(dmit) 2 ] −2 complex, synthesized, and characterized by Noma et al [22,23,24]. In these cases, there is not a net charge transfer between cations and anions, but the UV irradiation produces reversible changes in the spin distributions of the anions that manifest in the signals of the EPR spectra, different in the dark and under UV-irradiated conditions.…”

“…Once the UV irradiation ceases, the EPR spectra recover their original shape, confirming the reversibility of the process. There then exists a direct and reversible optical control of the spin distribution [22,23,24].…”

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.…”

“…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.…”