This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

The quasi-two-dimensional molecular semimetallic conductor α-STF2I3 is isostructural with α-ET2I3. The latter possesses a unique band structure showing a zero-gap state with Dirac cones under high ...

The physical properties of molecular crystals are governed by the frontier orbitals of molecules. A molecular orbital, which is formed by superposing the atomic orbitals of constituent elements, has complicated degrees of freedom in the crystal because of the influence of electron correlation and crystal field. Therefore, in general, it is difficult to experimentally observe the whole picture of a frontier orbital. Here, we introduce a new method called “core differential Fourier synthesis” (CDFS) using synchrotron X-ray diffraction to observe the valence electron density in materials. By observing the valence electrons occupied in molecular orbitals, the orbital state can be directly determined in a real space. In this study, we applied the CDFS method to molecular materials such as diamond, C₆₀ fullerene, (MV)I₂, and (TMTTF)₂X. Our results not only demonstrate the typical orbital states in some materials, but also provide a new method for studying intramolecular degrees of freedom.

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 ... Utilizing charge-transfer transitions, the conduction and magnetism of molecular crystals can be finely controlled with spatial resolution. Irreversible control has produced junction structures exhibiting the functions of semiconductor devices. Reversible control has produced “photomagnetic conductors” and “giant photoconductivity.” These optical treatments have realized unusual electronic states, structures, and physical properties, previously difficult to realize in any kind of material.