Follow the Yellow Brick Road: Structural Optimization of Vibrant Yellow-to-Transmissive Electrochromic Conjugated Polymers

Abstract

A series of conjugated polymers were designed and synthesized to extract structure–property relationships with the goal of yielding yellow-to-transmissive switching electrochromes. The polymers are based on repeat units of propylenedioxythiophene (ProDOT) in alternation with a variety of arylenes including 1,4-phenylene (ProDOT-Ph), 2,7-fluorene (ProDOT-Fl), 2,7-carbazole (ProDOT-Cbz), 2,5-dimethoxy-1,4-phenylene (ProDOT-Ph(MeO)2), and 2,7-pyrene (ProDOT-Py). Additionally, a random copolymer containing ProDOT and two different arylene units was produced: ProDOT-phenylene-ProDOT-dimethoxyphenylene (R-ProDOT-Ph/Ph(MeO)2) and two polymers with a ProDOT dimer in alternation with pyrene and phenylene composed ProDOT2-pyrene (ProDOT2-Py) and ProDOT2-phenylene (ProDOT2-Ph), respectively. The polymers were synthesized using Suzuki polycondensation. Examinations of the optoelectronic properties via UV–vis–NIR spectroscopy, differential pulse voltammetry, and spectroelectrochemistry show that varying the electron richness of the polymer by utilizing more electron rich arylenes, dimers of ProDOT, or less electron rich arylenes, the oxidation potential could be decreased or increased, respectively, ranging from 270 to 650 mV. Through subtle C–H ortho interactions from the arylene unit, yellow neutral state colors were maintained with transmissive or near-transmissive oxidized states. Colorimetry utilizing L*a*b*, where a*b* values correlate to the chroma or saturation of a color (note: −a* and +a* correspond to green and red and −b* and +b* correspond to blue and yellow, respectively) and L* represents the lightness, was used to show the maintenance of yellow colors in the neutral states. Herein, the yellow polymers had L* values above 84.0, a* values ranging from −11.6 to 24.8, and b* values greater than 47.6. In the oxidized states, the most transmissive forms had L* values above 70.0, a* values ranging from −2.1 to 2.0, and b* values ranging from −6.8 to −0.1. These structure–property relationships grant access to conjugated polymers with high energy absorbance in the visible, while allowing variability in redox potentials, providing a deeper understanding in yielding yellow-to-transmissive electrochromic polymers.