Solid-State Electrochemistry of Organic Macrocycles and Mofs; Application to Organic Electronics and Energy Storage

Solid-state electrochemistry is a powerful tool to control the electronic properties of organic materials through charge/carrier injections. In contrast to the numerous studies on organic polymers, its application to small molecules has been limited, because most of them become soluble into solvents after redox reactions. In this perspective, the organic macrocycles and MOFs are promising materials due to their structural robustness. We will discuss the following three topics

1. Electric-double-layer field-effect transistors of organic macrocycles. Electric-double-layers (EDLs), formed at solid-electrolyte interfaces, induce extremely large electric fields. This results in a high charge carrier accumulation in the solid, much more effectively than solid dielectric materials. We investigated the EDL thin-film transisotrs of porphyrazines and octathio[8]circulene (sulflower) with gate dielectrics of ionic liquids, which have attracted much attention due to their wide electrochemical windows, low vapor pressures, etc. We revealed the factors, controlling the mobility and threshold voltage in these types of FETs.

2. Electrochemical doping to nano-porous single crystals of Li-phthalocyanine (LiPc). The crystal structure of a neutral π radical, LiPc in the x from, consists of 1D stacking chains of LiPc and nano-channels parallel to them. We found that the LiPc crystals were electrochemically oxidized in an acetonitrile solution of tetrabutyl ammonium chloride (TBA·Cl), exhibiting a crystal-to-crystal transformation to LiPc·Cl_x (0<x<0.5). The crystal structures of the LiPc·Cl_x series were successfully solved and refined by the X-ray single-crystal analysis, and the obtained structures clearly indicated the presence of the Cl⁻ions in the 1D channels and a systematic structural change in the π stacking chain of LiPc.

3. Synthesis and solid-state electrochemistry of a redox-active MOF, Cu(2,7-AQDC), toward high-capacity lithium battery. We synthesized a microporous redox-active MOF, Cu(2,7-AQDC) (2,7-H2AQDC=2,7-anthraquinonedicarboxylic acid), and utilized it as a cathode active material in lithium batteries. With a voltage window of 4.0-1.7 V, both metal clusters and anthraquinone groups in the ligands were found to exhibit reversible redox activity. By controlling the voltage window of operation, extremely high recyclability of batteries was achieved.