Reversible four-color electrochromism triggered by the electrochemical multi-step redox of Cr-based metallo-supramolecular polymers

Four color electrochromism (yellow, magenta, blue, and navy) has been achieved in Cr(iii)-based metallo-supramolecular polymers (polyCr), which were synthesized by 1 : 1 complexation of Cr ions and 1,4-di[[2,2′:6′,2′′-terpyridin]-4′-yl]benzene (L). The polymer structure was determined by X-ray absorption fine structure (XAFS) measurement and X-ray photoelectron spectroscopy (XPS). The molecular weight of polyCr was calculated as 3.2 × 10⁷ Da using right angle light scattering (RALS). The EXAFS fitting indicated that the bond distances of Cr-N are 2.020 Å and 2.208 Å. A film of polyCr shows multi-color electrochromism (EC) or absorption: A sharp peak at 380 nm at 0 V vs. Ag/Ag⁺ (yellow), a sharp peak at 510 nm and a broad peak at 800 nm at-0.6 V vs. Ag/Ag⁺ (magenta), a broad peak at 610 nm and between 700-900 nm at-1.2 V vs. Ag/Ag⁺ (blue), a broad peak between 450-900 nm at-1.8 V vs. Ag/Ag⁺ (navy). The transmittances change (ΔT), the switching times for coloring and bleaching (T_c, T_b) and the coloration efficiency (η_c, η_b): [ΔT, (T_c, T_b), (η_c, η_b)] were [39.2%, (5.56 s, 1.39 s), (169 cm² C^-1, 230 cm² C^-1)] at 510 nm between-0.6 and 0.2 V vs. Ag/Ag⁺, [67.0%, (6.93 s, 2.52 s), (138 cm² C^-1, 172 cm² C^-1)] at 610 nm between-1.2 and 0.2 V vs. Ag/Ag⁺, [86.1%, (6.80 s, 3.03 s), (167 cm² C^-1, 134 cm² C^-1)] at 780 nm between-1.8 and 0.2 V vs. Ag/Ag⁺, respectively, during the cycles. The durability experiment indicates that polyCr shows an EC property for at least 100 cycles.