Electrochromic functions of organic-metallic hybrid polymers

Organic-metallic hybrid polymers are synthesized by self-assembly of organic modules, which have several coordination sites such as bis-terpyridines, with metal ions such as iron, cobalt, or ruthenium. The hybrid polymers have a specific color based on the metal-to-ligand charge transfer (MLCT) absorption from the metal ions to the terpyridine parts of the organic module in the hybrid polymers. In the cyclic voltammetry measurement of the polymers, a reversible redox wave is observed according to the redox of the metal ions. The polymers have a high molecular weight of over 50,000 and the polymer film is easily formed by spin coating. Interestingly, a polymer film cast on an ITO electrode exhibits excellent electrochromic properties: the color of the film disappears when a higher potential than the redox potential of the metal ions is applied to the polymer film. On the other hand, the film returns to its original color when a reductive potential below the redox potential of the metal ions is applied. Various colors such as purple, blue, red, and orange can be expressed by changing the metal species and/or by modifying the organic modules. Moreover, it is confirmed by in situ electro-spectroscopic measurements that the electrochromic changes in the hybrid polymers show high stability for the operations that are repeated more than 4000 times. In addition, multicolor electrochromic changes appear on introducing two kinds of metal ions to the polymer. We also succeeded in fabricating an electrochromic solid-state device by using these polymers.