Conducting polymers display a range of interesting properties, from electrical conduction to tunable optical absorption and mechanical flexibility, to name but a few. Their properties arise from positive charges (carbocations) on their conjugated backbone that are stabilised by counterions doped in the polymer matrix. In this research we report hydrolysis of these carbocations when poly(3,4-ethylenedioxy thiophene) is exposed to 1 mM aqueous salt solutions. Remarkably, two classes of anion interactions are revealed; anions that oxidise PEDOT via a doping process, and those that facilitate the SN1 hydrolysis of the carbocation to create hydroxylated PEDOT. A pKa of 6.4 for the conjugate acid of the anion approximately marks the transition between chemical oxidation and hydrolysis. PEDOT can be cycled between hydrolysis and oxidation by alternating exposure to different salt solutions. This has ramifications for using doped conducting polymers in aqueous environments (such as sensing, energy storage and biomedical devices).
Conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) are widely researched for application in electronic devices. Researcher’s look to exploit the ability of these polymers to conduct electrical charge. To induce conductivity, the polymers are doped with counterions; for PEDOT, this is typically done with poly(styrenesulfonate) or tosylate (Tos). The Tos anions inserted within the PEDOT nanostructure stabilize positive defects (holes) on the polymer’s conjugated backbone, which, in turn, facilitates electrical conduction. In this study, we use X-ray photoelectron spectroscopy to investigate the Tos doping of PEDOT within the outermost region (<15 nm) of electrochemically oxidized or reduced PEDOT:Tos nanoscale films. Computation of the predicted density of states from density functional theory studies is also conducted to aid in interpreting the ultraviolet photoelectron spectroscopy spectra. We observe that the doping of PEDOT:Tos is more complex than first thought, likely involving the nonionic triblock copolymer used during PEDOT’s oxidative polymerization. This hypothesis is corroborated by time-of-flight secondary-ion mass spectrometry measurements on the outer 2 nm of the oxidized and reduced PEDOT:Tos. The observation of complex and heightened doping near the surface opens opportunities for the deliberate surface engineering of PEDOT:Tos nanofilms in polymer electronic applications such as electrochemical transistors and electrical connections.
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