A Pedot by any other name: Tuning the electroactivity of dioxy-heterocycles for redox and solid state applications
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Redox active conjugated polymers are used in a variety of applications including electrochromism, charge storage, plasmonic resonance tuning, and electrochemical transistors. This thesis describes the development of processable, electron-rich dioxyheterocycle-based polymers for use in electrochromic and charge-storage applications. This thesis reports the development of soluble polymers that can be solution processed from organic or aqueous solutions (inks) and are redox active in organic or aqueous electrolytes with variable optical and redox properties through subtle differences in the repeat units and heteroatoms used. This thesis describes the development of processable, electron-rich dioxyheterocycle-based polymers for use in electrochromic and charge-storage applications. Materials with low onsets of oxidation and broad electroactivity over a large voltage window, as seen with electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT), are of interest for charge storage applications. Chapter 3 reports and discusses the copolymerization of alkoxy-functionalized 3,4-propylenedioxythiophenes (ProDOTs) with unfunctionalized 3,4-ethylenedioxythiophene (EDOT) in varying ratios using direct arylation to produce a series of solution processable polymers with highly tunable optical and electronic properties. Within this series, we have identified poly(ProDOT-alt-biEDOT) (PE2), a copolymer containing 67% EDOT compositionally, which combines the low oxidation potential, the redox behavior, and the deep-blue neutral color that are characteristic of PEDOT with the high solubility, exceptional electrochromic contrast, and color neutrality in the oxidized state characteristic of alkoxy-functionalized polyProDOTs. 3,4-Phenylenedioxythiophene (PheDOT) is a unit that is similar to EDOT in terms of planarity and steric interactions but has different electronic properties due to an attached phenyl ring that allows for electron delocalization away from the thiophene ring. This unit, and its use in soluble copolymers prepared via direct (hetero) arylation polymerization (DHAP), is discussed in Chapter 4. The notable C-H inactivity of the attached phenylene unit is compared to aromatic solvents used for DHAP and the redox stability of these copolymers are demonstrated. The first example of a solution processable dioxythiophene-alt-dioxyselenophene polymer, poly(ProDOT-alt-EDOS), prepared via DHAP is reported and its optical and electrochemical properties are compared to the all thiophene analog and other relevant dioxythiophene polymers. By substituting the sulfur atom for a selenium atom on one of the monomers in the repeat unit, a significant red-shift of both the neutral and polaronic absorbances results, as well as a reduction in the onset of oxidation compared to the all thiophene analog.Due to the redox and optical similarities of PE2 and PEDOT, the solid-state electrical conductivity of the chemically doped ProDOTxEDOTy series (discussed in Chapter 3) is studied in Chapter 6. Additional polymer structures are introduced to understand the structure property relationships and the conductivity of PE2 is optimized to ~250 S/cm. Because of this high electrical conductivity, an oxidized PE2 film was used as a transparent electrode for another electrochromic polymer to switch between purple and colorless states. Chapters 7 and 8 outline chemical defunctionalization methods that allows for the preparation of conjugated polymer films that can be processed from both aqueous and organic solvents and used as redox active films in either organic or aqueous-based electrochemical devices. The versatility and robustness of this solvent resistant polymer is shown in a series of aqueous electrolyte systems, as well as in biologically compatible electrolytes (human serum, sports drinks, etc.). To show the use of these materials in redox active devices; two applications, namely electrochromics and charge storage, were examined using environmentally benign salt water as the electrolyte. Supercapacitors using these films in NaCl/water remained operational after 175,000 charge/discharge cycles.