CHARACTERIZATION, POLYMERIZATION, AND INVESTIGATION OF CHEMICALLY-MODIFIED THIOPHENES

Abstract

The design, synthesis and characterization of a novel monomer consisting of the fusion of an electrochemically polymerizable thiophene (3,4-ethylene dioxythiophene, EDOT) and a phospholipid plasma membrane component (1,2 dipalmitoyl-sn-glycero-3-phosphate (16:0 phospholipid)) is described. This new molecule, EDOT-phospholipid, was electrochemically polymerized into the corresponding polymer PEDOT-phospholipid. It was found that the monomer exhibited residual liquid crystalline order when heated above its melting point at 75.8 °C, with crystalline properties being recoverable upon cooling to form negatively aligned spherulites. PEDOT-phospholipid was electrochemically polymerized from a 50:50 THF:water mixed solvent system at +1.61V to produce an iridescent thin film that reduced the impedance of the coated electrode by more than one order of magnitude at low frequencies (~0.1 Hz). The addition of the hydrophobic EDOT molecule onto the head of the phospholipid evidently disrupted the amphiphilicity of the phospholipid, leading to a decreased solubility in many solvents, and preventing the molecule from being cultured with living cells to a reasonable degree. These results confirmed the ability to create phospholipid-incorporating conductive polymer thin films that reduce the impedance of metal electrodes at low frequencies. Optical photothermal infrared response (O-PTIR) spectroscopy is an emerging technique of particular interest for examining the local chemistry and structure of organic molecular and polymer materials. Here, we used O-PTIR to examine the 3,4-ethylenedioxythiophene (EDOT) and maleimide-functionalized (EDOT-MA) monomers, and also thin films (~100 nm) of their corresponding polymers (PEDOT and PEDOT-MA) electrochemically deposited on interdigitated (5 μm) gold electrodes. The O-PTIR technique provided high resolution (~1 m) information about the chemical structure, including the ability to map local variations in the composition of the MA side groups. Certain limitations were found, particularly in samples that were strongly optically absorbing. In particular, Raman analysis proved to be more difficult than IR analysis on these conjugated polymer systems. The polymerization of PEDOT in aqueous solutions with concentrated organic acids has been previously documented, but the detailed mechanism and progression of the reaction over time is still poorly understood. In this work, time-resolved UV/Vis assays, Raman spectroscopy, and NMR measurements were used to provide new insights into this reaction. EDOT monomer, EDOT dimer, and EDOT trimer were all subjected to formic acid - catalyzed polymerization, and the spectra were examined as a function of time at room temperature. The samples showed immediate color changes in solution, forming vibrant oranges and reds. Raman spectroscopy showed increasing peak clusters at 1440 cm-1 and 2876 cm-1 showing progressive polymerization over the course of six months. EDOT monomer solutions developed UV/Vis peaks consistent with EDOT dimer at 430 nm and consistent with EDOT trimer at 503 nm and 536 nm. All three showed increased absorbances at 640+ nm as the reaction progressed, consistent with the formation of PEDOT oligomers and polymers. Inert EDOT monomer NMR showed thiophene hydrogens at δ = 6.30 ppm while EDOT dimer and trimer showed these at δ = 6.25 ppm, with all EDOT oligomers showing methylene hydrogens between δ = 4.35 ppm - δ = 4.18 ppm. In formic acid, all thiophene hydrogen peaks disappeared and the evolution of the methylene hydrogen peaks at δ = 4.57 ppm and δ = 4.11 ppm for EDOT dimer and trimer show the creation of EDOT hexamer. The disappearance of the thiophene hydrogens as well as the reversible UV/Vis absorbance bands for EDOT dimer and EDOT trimer reveal the formation of stable activated EDOT oligomers when exposed to high concentrations of formic acid.