Orange and Red to Transmissive Electrochromic Polymers Based on Electron-Rich Dioxythiophenes

As the color palette of available solution processable electrochromic polymers expands, there has remained the need for red, orange, and yellow to transmissive switching materials. Here we report on the synthesis and characterization of two such polymers, the orange to transmissive switching (poly{3,4-di(2-ethylhexyloxy)thiophene}) electrochromic polymer-orange (ECP-orange) and the red to transmissive switching processable polymer (poly{3,4-di(2-ethylhexyloxy)thiophene-co-3,4-di(methoxy)thiophene}) electrochromic polymer-red (ECP-red). The ECP-orange has a bandgap of 2.04 eV, an absorption λmax centered at 483 nm, and an E 1/2 of 0.37 V versus Ag/Ag+. The electrochromic contrast is 48% T at 483 nm with a time to reach 95% of the full optical contrast of 5.3 s for a film that has an absorbance of 0.98 au at λmax. Because of steric relaxations from the random copolymerization of a branched dialkoxy-substituted thiophene with a dimethoxy-substituted thiophene, the red to transmissive switching ECP-red has a bandgap of 2.00 eV, a λmax red-shifted by 42 to 525 nm, and an E 1/2 decreased to 0.21 V versus Ag/Ag+. Additionally, the red polymer has a higher contrast of 60% T and a shorter time to reach 95% of the full optical contrast of 2.3 s. These two reported polymers allow the field of electrochromics to come closer to a full set of fully solution processable materials that yield films whose optical absorption covers the full visible spectrum while switching to a highly transmissive oxidized state as needed for full color displays.