Masaaki Abe

Piezofluorochromism (PFC) is a phenomenon that fluorescence color changes reversibly in response to isotropic pressure. Under extremely high pressure at the order of GPa, organic molecular crystals may undergo unusual changes of electronic structures due to extraordinal condition of the molecular and packing structures. Namely, PFC is one of the fundamental methods to explore extraordinal electronic structures that cannot be achieved under ambient conditions via changes of visible properties, and thus has attracted much attention in terms of some applications. In this review, we introduce the recent progress in PFC of organic molecular crystals.

A silane coupling-based procedure for decoration of an insulator surface containing abundant hydroxy groups by constructing redox-active self-assembled monolayers (SAMs) is described. A newly synthesized ferrocene (Fc) derivative containing a triethoxysilyl group designated FcSi was immobilized on SiO2/Si by a simple operation that involved immersing the substrate in a toluene solution of the Fc silane coupling reagent and then rinsing the resulting substrate. X-ray photoelectron spectroscopy (XPS) measurements confirmed that the Fc group was immobilized on SiO2/Si in the Fe(II) state. Cyclic voltammetry measurements showed that the Fc groups were electrically insulated from the Si electrode by the SiO2 layer. The FcSi on SiO2/Si structures were found to serve as a good scaffold for formation of organic semiconductor thin films by vacuum thermal evaporation of C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), which is well-known as an organic field-effect transistor (OFET) material. The X-ray diffraction profile indicated that the conventional standing-up conformation of the C8-BTBT molecules perpendicular to the substrates was maintained in the thin films formed on FcSi@SiO2/Si. Further vacuum thermal evaporation of Au provided an FcSi-based OFET structure with good transfer characteristics. The FcSi-based OFET showed pronounced source-drain current hysteresis between the forward and backward scans. The degree of this hysteresis was varied reversibly via gate bias manipulation, which was presumably accompanied by trapping and detrapping of hole carriers at the Fc-decorated SiO2 surface. These findings provide new insights into application of redox-active SAMs to nonvolatile OFET memories while also creating new interfaces through junctions with functional thin films, in which the underlying redox-active SAMs play supporting roles.

Recent progress in the redox chemistry of multimetallic transition-metal complexes and clusters is reviewed. Special attention is paid to high-nuclearity complexes wherein redox-active sites (e.g., ferrocenyl pendants or Ru centers) are assembled in a close proximity with a well-defined geometry via the covalent or coordination bonds. A special emphasis is placed on literature survey for paddlewheel diruthenium(II,III) complexes coordinated by axial or equatorial ferrocenyl ligands, polyferrocenyl compounds in the form of wire, star, and ring, ferrocenyl-based dendrimers, and ligand-mediated molecular triangles and squares. Finally, future prospects of these mixed-valent systems are briefly discussed.