Electrochromic Polymers Processed from Environmentally Benign Solvents

The ability to process conjugated polymers from environmentally benign solvents is essential for making organic electronics commercially viable by reducing costs and enhancing safety in the printing and processing environment. To enhance the solubility of typically alkyl-functionalized redox-active and electrochromic polymers, poly­(3,4-propylene­dioxythiophenes) with ester-functionalized side chains were synthesized via direct arylation polymerization, resulting in polymers that are processable from 2-methyl­tetrahydrofuran, ethyl acetate, and propyl acetate. Optical and atomic force microscopy results of spray-processed films indicate that topological features, such as film roughness, can be manipulated  via the vapor pressure of the processing solvent. The solvent choice affects the resulting onset of absorbance and relative intensities of vibronic features, which translates into distinctly observable and quantifiable color differences. While  the color is sensitive to the casting solvent, the redox properties and onset of oxidation are fairly independent of the processing medium. Most notably, electrochromic properties, such as contrast and switching times, are not drastically affected by the casting solvent or underlying morphologies. Independent of casting solvent, each polymer exhibited a transmittance change greater than 70% at λ_max in the oxidized state with switching speeds of ∼2 s for 2 cm² films in organic electrolytes. This work highlights the synthetic tailorability of the poly­(3,4-propylene­dioxythiophene) family of materials to introduce functional groups that improve processability without sacrificing electrochromic performance.