Abstract
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).
Highlights
Conducting polymers display a range of interesting properties, from electrical conduction to tunable optical absorption and mechanical flexibility, to name but a few
To observe how the stable carbocations interact with water, PEDOT: Tos prepared by vapour phase polymerisation (VPP) was electrochemically reduced (ER) and exposed to 1 mM aqueous solutions of sodium nitrate (NO3−), sodium chlorate (ClO3−), sodium bicarbonate (HCO3−), sodium sulfite (SO32−), sodium carbonate (CO32−), sodium phosphate (PO43−), and sodium hydroxide (OH−)
Khan et al reported that PEDOT:Tos exposed to OH− leads to the ion exchange of Tos− with OH− and a commensurate decrease in electrical performance, in polymer chain ordering and chain oxidation level[26]
Summary
Conducting polymers display a range of interesting properties, from electrical conduction to tunable optical absorption and mechanical flexibility, to name but a few. In the work of Ritchie et al they provided a strong argument to support the case that general base-catalysed hydrolysis proceeds via a concerted step; as shown in Fig. 1b; the water bonds to the carbocation and is deprotonated in one step ( the intermediate hydronium ion is not observed)[24] They state that “...general base catalysis is characteristic of the reactions of water with any cation of measurable stability in aqueous solution.”. To observe how the stable carbocations interact with water, PEDOT: Tos prepared by vapour phase polymerisation (VPP) was electrochemically reduced (ER) and exposed to 1 mM aqueous solutions of sodium nitrate (NO3−), sodium chlorate (ClO3−), sodium bicarbonate (HCO3−), sodium sulfite (SO32−), sodium carbonate (CO32−), sodium phosphate (PO43−), and sodium hydroxide (OH−) These anions were initially chosen for their diverse properties of geometry, size, charge, and solubility (Fig. 2f). This reaction is facilitated (or not) by the anions present in solution
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