Chemical doping of EDOT azomethine derivatives: insight into the oxidative and hydrolytic stability

Abstract

A series of EDOT (3,4-ethylenedioxythiophene) containing azomethines were prepared for investigating their opto-electronic properties. These properties were compared to those of their thiophene azomethine counterparts and it was found that incorporating the EDOT moiety resulted in a 30 nm bathochromic shift in the absorbance. Meanwhile, the oxidation potential (E_pa) could be reduced by 100 mV by incorporating the electron rich moiety. Cyclic voltammetry revealed a one-electron oxidation process, resulting in a radical cation. This intermediate was stable when the azomethines contained amines in the 2,2′-positions, evidenced by the reversible oxidation in cyclic voltammetry. In contrast, the radical cation was irreversible when the 2,2′-positions were unsubstituted. It was found that the resulting radical cation was coupled by standard anodic polymerisation to form a polymer that was physisorbed onto the ITO electrode. The resulting polymer was mauve in colour with a λ_max of 515 nm and a degree of polymerisation of ca. 5. This was spectroscopically determined relative to an EDOT polyazomethine derivative and a soluble thiophene polyazomethine. The stability of the EDOT azomethine derivatives towards electrochemical and chemical oxidation was also spectroscopically investigated. It was found that the resulting radical cation exhibited a ca. 100 nm bathochromic shift in absorbance relative to the neutral form and reversible colour switching between the neutral and oxidized states was possible. Chemical doping with FeCl₃ generated a stable dication. High contrast colours between the neutral and oxidized states of the azomethines were observed. Multiple oxidation/neutralisation cycles were possible without detectable colour deterioration, demonstrating the chemical robustness of the conjugated azomethines towards oxidative decomposition and hydrolysis.