Constructing Alternated Heterobimetallic [Fe(II)/Os(II)] Supramolecular Polymers with Diverse Solubility for Facile Fabrication of Voltage-Tunable Multicolor Electrochromic Devices

Abstract

Metallo-supramolecular polymers (MSPs)-based electrochromic devices (ECDs) have drawn much attention because of their variable colors and attractive electrochromic (EC) properties. However, fabrication of voltage-tunable multicolor ECDs using single MSP is yet hard to realize. We anticipated alternate introduction of two different redox active metal ions in an MSP combined with the adjustment of counter anions could be a solution to fabricate multicolor ECDs. The heterometals will induce color variability upon voltage alteration and counter anions will help to tune the solubility of MSP in different solvents. In an attempt to fulfil this target, we have synthesized four heterobimetallic supramolecular polymers (HBPs) having different counter anions (BF4-, Cl-, PF6-, and OAc-), in which Fe(II) and Os(II) are alternately complexed by two terpyridine units. To apply as EC material, the HBPs should be soluble in methanol and insoluble in acetonitrile for the preparation of EC film as well as ECDs. However, among the HBPs, only HBP-OAc is found to meet this requirement. The EC behaviors of the spray coating film of HBP-OAc on an indium tin oxide (ITO) coated glass substrate are investigated in terms of maximum transmittance contrast, coloration voltage, response time, coloration efficiency, and operational stability, which exhibits reversible multicolor electrochromism (the initial purple color of the film is changed to violet followed by greenish-yellow) upon alteration of the voltage from 0.0 V, to 0.7 V [required to oxidize Os(II) ion], and to 1.0 V [required to oxidize Fe(II) ion]. The film is also integrated into a laminated ECD using lithium-based gel electrolyte. Finally, as a proof-of-concept, a prototype voltage-tunable multicolor EC display (6 cm x 2.5 cm) is fabricated using a designed image containing flower, leaves, and flower pot, which exhibits six different types of multicolor image upon application of tunable voltages.