Electrochromic Coordination Polymers

Abstract

This award account focuses on metallo-supramolecular polymers (MSPs), which are composed of metal ions and multitopic ligands, as a new branch of coordination polymers. Their high solubility in polar solvents, such as methanol or water and their amorphous nature in the solid state, enabled the formation of thin layers of MSPs with a uniform thickness on an electrode surface by spin- or spray-coating the polymer solution, resulting in reversible electrochromic (EC) properties triggered by the electrochemical redox reactions of the metal. The EC properties were characterized in terms of contrast, switching time, charge consumption, and coloration efficiency (η). Dimensionally controlled (one-, two-, and three-dimensional) MSPs were designed, synthesized, and evaluated for their EC properties, finding that the three-dimensional hyperbranched structures enhanced the η value up to 689 cm2/C. The introduction of multimetal species in MSPs led to multicolor EC function owing to the different redox potentials of the metals. An MSP with Fe(II), Ru(II), and Os(II) exhibited quad-color EC behavior. The insolubility of MSPs in weak polar solvents, such as acetonitrile or propylene carbonate, enabled the fabrication of EC devices (ECDs) by combining MSPs with electrolytes dissolved in such solvents. The selection of the appropriate counter material in ECDs successfully decreased the operation voltage down to 0.8 V. Furthermore, the fabrication of ECDs driven at 0.6 V was achieved by modifying the ligands in MSPs.