A magenta polypyrrole derivatised with Methyl Red azo dye: synthesis and spectroelectrochemical characterisation

Abstract

A pyrrole derivative bearing 2-(4-dimethylaminophenylazo)benzoic acid, also known as Methyl Red (MR), was prepared by a simple synthetic route, and electropolymerised onto ITO/glass electrodes in (C4H9)4NBF4/CH3CN in presence of boron trifluoride diethyl etherate (BFEE). Films of polypyrrole (PPy) and PPy doped with MR (PPy/MR) were also deposited onto ITO/glass in order to compare their electrochromic properties with the films of PPy derivatised with MR. Cyclic voltammogram of the poly[3-(N-pyrrolyl)propyl 2-(4-dimethylaminophenylazo)benzoate] (PMRPy) film displayed a redox pair with anodic peak potential (Epa) at ca. 0.53V and cathodic peak potential (Epc) at 0.25V vs. Ag/Ag+, corresponding to the polymer p-doping, whilst the PPy/MR film shows capacitive behaviour with a redox pair in the cathodic region (Epa=−0.36V and Epc=−0.62V), similar to the PPy film (Epa=−0.10V, and Epc=−0.15V), and an anodic wave in the same potential range of that for PMRPy film. The electrochromic properties of the PMRPy film, such as chromatic contrast (Δ%T=34.2%), switching time (τ=10s) and stability (Δ%T=15% at the 100th cycle), were enhanced relative to the PPy/MR and PPy films. However, the colour of the PMRPy film changed from yellow (-0.8V) to magenta (E=1.0V) in the first cycle and became light magenta at −0.8V in the subsequent cycles. PMRPy films were also investigated in phosphate buffer solution (PBS, 2.0≤pH≤9.0) and after exposure to HCl vapour, in which the colour varied from magenta at pH=2.0 to yellow at pH=9.0. Such properties are interesting for application in pH sensors.