Reading the rainbow: tailoring the properties of electrochromic polymers
Kerszulis, Justin Adam
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The completion of the color palette has yielded a family of electrochromic polymers (ECPs) each able to absorb in unique regions across the visible spectrum. Synthetically, by varying the electronic content of phenylene type moieties coupled with the donor 3,4-propylenedioxythiophene (ProDOT), high band gaps can be achieved absorbing short wavelength light, yielding a family of yellow-to-transmissive electrochromic polymers. Using the synthetic approach to tune specific absorptions in a discrete region of the visible spectrum, a family of electrochromic polymers that possess sharpened or broadened absorption spectra relative to electrochromic materials previously produced has been developed. By varying the steric hindrance of dioxythiophenes along a conjugated backbone, new hues of magenta and blue have been achieved. Through progressively adding more steric hindrance and twisting the polymer backbone, the absorbance of a polymer can be pushed towards shorter wavelengths, allowing more red light and less blue light to pass through a film. This unequal passing of long and short wavelengths reduces the overall purple color that is normally exhibited by a previous magenta ECP, thereby giving brighter, truer magenta colored materials. By reducing steric hindrance and relaxing the polymer backbone, the opposite can be achieved: pushing the absorbance of a polymer to longer wavelengths allows more blue and less red light to transmit. These polymers also exhibit highly transmissive oxidized states that are attainable at low potentials. In the quest to achieve black (or dark as defined by low L*) to transmissive ECPs with suitable contrast for window or eyewear applications, a relaxed donor-acceptor architecture has been explored. These materials give broad neutral state absorptions with a %Tint (380-780 nm) > 50 %, bringing these materials closer to realization.