Metallopolymer Films of a Manganese(II) Chelate with Hexadentate Ligand for High-Performance Electrochromism

To achieve high-performance electrochromism, one of the most critical challenge is how to attain a transparent state. For this purpose, we elaborately designed a diethylenetriamine-derived hexadentate nitrogen chelator functionalized with triphenylamine groups, which coordinates with manganese(II) ion to form an octahedral manganese(II) chelate. As d-d transition of the half-filled d⁵ manganese(II) center is forbidden, this manganese(II) chelate is colorless. Through electropolymerization of triphenylamine groups, we successfully fabricated a transparent metallopolymer film with manganese(II) chelates as repeating units. Under applied potentials, the initially transparent metallopolymer film undergoes stepwise oxidation at both manganese(II) and triphenylamine centers, so that the colorless film turns brown-yellow at 0.9 V and further blue at 1.3 V. The solid-state electrochromic device exhibits tricolor electrochromism from colorless to brownish yellow and blue at the voltages of 1.7 and 2.4 V, respectively. The coloring/bleaching time, color contrast, and electrochromic efficiency are 1.4 s/0.4 s, 40%, and 337.5 cm² C^-1 between colorless and brownish yellow, whereas they become 2.2 s/0.4 s, 73%, and 397.1 cm² C^-1 between colorless and blue. The device exhibits excellent stability with up to 10 000 cycles for manganese-based electrochromism.