Redox doping behaviour of poly(3,4-ethylenedithiothiophene) – The counterion effect

Poly(3,4-ethylenedithiothiophene) – PEDTT, an alkylene sulphur derivative of PEDOT, presents itself as an interesting polymer with a number of disparate redox and chromic properties compared to its close analogue – PEDOT. In this study we present the results of an investigation into the electrochemical doping process of PEDTT, using four different electrolyte solutions, differing in anion content of the chosen salt. The results show that the anion identity plays a key role in the redox reactions accompanying these processes in what could be interpreted as anion ionochromism. In situ UV–Vis spectroelectrochemical experiments reveal an intriguing double electrochromic transition of PEDTT films during their oxidative doping, going from golden-yellow through green to pomegranate – a quality not so common within the family of electroactive conjugated polymers. The evolution of each UV–Vis spectrum over a potential range indicates that different redox states of the polymer are responsible for the chromatic changes. In the reduction half-cycle, the dedoping process of PEDTT appears to follow a path dissimilar to the p-doping one, featuring only one, direct electrochromic transition of the film’s colour, bypassing the green state, and a distinct two-step bleaching process of doping-induced charge carrier bands. The observed electrochemical and spectral phenomena have been accredited to the specific redox behaviour of doping-induced radical cation and cationic defect states interacting with the dithioalkylene sulphur atom.

Redox doping process of poly(3,4-ethylenedithiothiophene) – PEDTT has been investigated in four different electrolyte anion environments using in situ UV–Vis-NIR spectroelectrochemistry. Changes of spectral bands of the polymer at different doping levels, disclosed complex interplay of the counterions and doping-induced charges, rendering PEDTT an ionochromic polymer. Thick PEDTT films were found to exhibit three chromatic states in the course of their p-doping, while in the subsequent dedoping process only two extreme states developed. Localisation of doping-induced charges of the polymer on the sulphur atoms of the 3,4-dithioethylene bridge, together with retention of polarons in the reduced polymer, has been put forward to explain the observed spectral signatures of PEDTT at different doping levels, and to provide a credible explanation of previously observed spectroelectrochemical behaviour of the polymer upon its doping.Figure optionsDownload high-quality image (125 K)Download as PowerPoint slideHighlights
► Redox doping behaviour of poly(3,4-ethylenedithiothiophene) – PEDTT.
► PEDTT exhibits three chromatic states in the course of its p-doping, while only two upon its dedoping.
► Doping-induced charges localise on the sulphur atoms of the 3,4-dithioethylene bridge.
► Retention of polarons is held accountable for the chromatic effects.
► Interaction of counterions with doping-induced charges, render PEDTT an ionochromic polymer.