Relax: A Sterically Relaxed Donor-Acceptor Approach for Color Tuning in Broadly Absorbing, High Contrast Electrochromic Polymers

A family of five electrochromic polymers (ECPs) based on 3,4-dioxythiophenes (XDOTs) (where X = ethylene (E) and propylene (Pro)) or a 3,4-diacyclic-substituted dioxythiophene (AcDOT, Ac) as main chain donors coupled in a random fashion with the sterically relaxed (i.e., low torsional energy barriers to planarity) donor-acceptor-donor unit BDOT-BTD-EDOT (EBE, BTD = 2,1,3-benzothiadiazole) were synthesized using direct heteroarylation polymerization conditions. Designed as broadly light absorbing materials to improve overall contrast, ECPs were solution spray-cast into thin. films, and their electrochromic properties were measured and compared against the previously reported ECP-Black (random copolymer composed of ProDOT and BTD). These new polymers exhibit enhanced integrated contrasts across the visible region (380-780 nm, Delta%T-int) larger than 45%, the highest achieved being similar to 52%, a substantial improvement over ECP-Black (Delta%T-int = similar to 32%). Increasing torsional strain of the main chain donor units moves the short wavelength peak to higher energy, allowing hue control. Increasing the composition of the EBE monomer in random copolymers yields more level and uniform absorption across the visible, reducing hue saturation and giving more muted colors relative to the normally vibrantly colored ECPs with no observable loss in contrast. Pro-Ac-0.65/EBE0.35 gave the highest integrated electrochromic contrast with color values (L*a*b* where L* indicates lightness, a* defines green and red, and b* defines blue and yellow hues, for negative and positive values, respectively); indicating improved color neutrality (45, 5, 3) when compared to ECP-Black (47, 3, -14). Two-component solution blends of Pro-Ac-0.65/EBE0.35 with the previously reported all donor polymer ProDOT(2)-EDOT in differing weight ratios were prepared and cast into films giving more aesthetically pleasing black-to-transmissive electrochromes, while maintaining a high integrated contrast at similar to 51%. The use of EBE demonstrates the synthetic capability to improve the contrast of broadly absorbing ECPs for black or dark-to-transmissive applications in electrochromic window and display-type devices.