Electrochromic Properties of Metallo-Supramolecular Polymers and the Display Device Application

Metallo-supramolecular polymers were synthesized by the complexation of transition metal ions including Fe(II), Ru(II), and Os(II) with ditopic organic ligands such as bisterpyridines. The obtained polymers were easily processed to the film state by spray-coating due to the amorphous nature of polymers. The polymers have specific color such as blue, red, and yellow, based on the metal-to-ligand charge transfer (MLCT) absorption in the complex moieties of polymer. The author and coworkers found reversible electrochromism in the polymer film, triggered by the electrochemical redox of the metal ions in the polymers. We confirmed high durability to the repeated color changes more than 100,000 times. Fe(II)-based metallo-supramolecular polymer (polyFeL1), which was prepared by the 1:1 complexation of Fe(II) ions and bis(terpyridyl)benzene (L1), exhibited high coloration efficiency (263.8 cm 2 /C). When tris(terpyridyl)benzene (L2) and Fe(II) ions were further added to a polyFeL1 solution, the linear polymer structure of polyFeL1 was hyperbranched. Interestingly, the hyperbranched structure has enhanced the coloration efficiency of the polymer film up to 383.4 cm 2 /C. We could introduce two metal species (Fe(II) and Ru(II)) in the polymer by simple mixing of Fe(II), Ru(II), and L1 at the molar ratio of 0.5:0.5:1. The obtained heterometallo-supramolecular polymer displayed multi-color electrochromism by changing the applied potential, because the redox potential of Fe(II)/(III) and Ru(II)/(III) are different. We succeeded in fabricating electrochromic devices using the polymers. Owing to the amorphous nature of polymers, flexible electrochromic sheet devices were also fabricated. The nonvolatile display devices with both high durability and abundant color variation will be attractive for wide applications containing smart windows, wearable IoT displays, and digital signages.