The optical absorbance spectra of electron-rich pi-conjugated oligomers and polymers can be tuned to yield electrochromic materials of all colors that can be switched to highly transmissive forms as desired for absorptive/reflective (display type) and absorptive/transmissive (window type) devices. This presentation will focus on the structural design and synthesis of fully conjugated polymers that complete the color palette of materials needed for subtractive color mixing via solution and device-based methods. Colorimetric tuning (using the L*a*b* color space) using a combination of electron rich and poor units, in conjunction with employing subtle changes in steric strain or relaxation, allows for the enhancement of the neutral form color vibrancy, and the transmissivity of the oxidized forms. Alternating polymers based on repeat units of 3,4-ethylenedioxythiophene (EDOT), 3,4-propylenedioxythiophene (ProDOT), and an acyclic dioxythiophene (AcDOT) in varying combinations were synthesized using direct (hetero)arylation polymerization. These polymers exhibit changes in contrast no less than 70% at the λmax with the exception of AcDOT-EDOT. The highly transmissive oxidized state color values for ProDOT2-EDOT2 and AcDOT2-ProDOT are 92, -3, -3 and 91, -2, -1, respectively. We demonstrate that Donor-Acceptor (D-A) polymers exhibit a broad dual-band absorption spectrum, where the peak at shorter wavelengths is attributed to the π-to-π* (extent of conjugation) excitation and the peak at longer wavelengths is attributed to the donor-acceptor charge transfer (D-A CT) interaction. Upon electrochromic switching, the materials in this family have an integrated contrast across the visible (380-780 nm, Δ%Tint) that is larger than 45%, with the highest achieved being ~52%, a substantial improvement over our earlier developed ECP-Black (Δ%Tint = ~32%). Minimizing torsional strain and increasing the equivalents of a bis-EDOT benzothiadiazole (EBE) unit reduces color saturation, thus, giving more muted colors with no observable loss in contrast where ProDOT-AcDOT0.65/EBE0.35 gave the highest integrated contrast. Two-component blends of ProDOT-AcDOT0.65/EBE0.35 with the all donor polymer ProDOT2-EDOT were cast into films giving more aesthetically pleasing black-to-transmissive electrochromes while maintaining high contrast at ~51%.
Turning to discrete length oligomers, cation-radical and dication states provide more localized NIR light absorption characteristics when compared to the broad “bipolaron” spectra of fully conjugated polymers. Acrylate coating and telechelic alcohols for polyester, polycarbonate, and polyurethane formation methodologies provide a separate route to color control in charge-neutral species, and highly transmissive oxidized states.