Tuning the painter's palette: subtle steric effects on spectra and colour in conjugated electrochromic polymers

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2015-01, Vol.3 (13), p.3211-3218
• Main Authors: Kerszulis, J. A., Johnson, K. E., Kuepfert, M., Khoshabo, D., Dyer, A. L., Reynolds, J. R.
• Format: Article
• Language: English
• Subjects: Color; Colour; Electrochromism; Energy gaps (solid state); Polymers; Strain; Tuning
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/C4TC02685C

A series of vibrantly coloured π-conjugated electrochromic polymers (ECPs) were designed and synthesized with the goal of extracting structure–property relationships from subtle changes in steric strain or relaxation. These are soluble all donor, electron rich, alternating polymers based on repeat units of 3,4-ethylenedioxythiophene ( EDOT ), 3,4-propylenedioxythiophene ( ProDOT ), and an acyclic dioxythiophene ( AcDOT ) in varying combinations to tune steric interactions and the subsequent optical absorption for fine colour control. Two families of polymers were formed where ProDOT2–EDOT , ProDOT–EDOT , and ProDOT2–EDOT2 constitute new shades of blues while AcDOT2–ProDOT , AcDOT–ProDOT , and AcDOT–EDOT yield new hues of magentas with the homopolymers of ProDOT and AcDOT and the copolymer AcDOT2–EDOT2 serving as comparisons. The polymers were synthesized using direct (hetero)arylation polymerization. Examinations of the optoelectronic properties via cyclic voltammetry, spectroelectrochemistry, and colorimetry show that by subtly varying the level of steric relaxation or strain in the form of EDOT or AcDOT content, lower or higher energy absorption transitions are produced respectively. This increase in relaxation or strain allows more short or long wavelength light to transmit, giving new shades of blues or magentas respectively. Since these are all donating polymers, they exhibit changes in contrast no less than 70% at the λ max with the exception of AcDOT–EDOT . The most desirable electrochromic properties were achieved with ProDOT2–EDOT2 and AcDOT2–ProDOT , with band gaps ( E g ) and neutral state L * a * b * colour values (− a * and + a * correspond to green and red and − b * and + b * correspond to blue and yellow respectively and L * represents the lightness) of 1.74 eV, 37, 12, −63 and 2.01 eV, 56, 59, −16 respectively. The highly transmissive oxidized state colour values for ProDOT2–EDOT2 and AcDOT2–ProDOT are 92, −3, −3 and 91, −2, −1 respectively. These structure–property relationships grant a greater ability to tune light absorption across the visible, with colour properties similar to ECPs made through other methods without using donor–acceptor effects. This all donor steric tuning method leads to considerably higher levels of transparency when the polymers are fully oxidized.