Spectroscopic and Kinetic Investigation of Ethyl Viologen Reduction in Novel Electrochromic Plastic Films

2010 In this work we present an absorbance investigation, mainly in the visible range, of two electrochromic devices (ECD), prepared with a viologen molecule and two different anodic substances (hydroquinone and ferrocene). The ethyl viologen has been used instead of other viologens because of its greater solubility into the selected plastic matrix. On the other hand the methyl viologen has been excluded due to its well-known high toxicity. Since, in view of practical applications, we are interested in the device behavior within the visible region, the ECD electrooptical properties were investigated with a spectrometer working in the 350-1200 nm range, which also extends into the NIR region. The measured spectra, as a function of time, have been registered while the coloration process was in progress during the application of a voltage supply across the film. The absorbance of each spectral transition was obtained by numerical integration and studied in its time evolution. The time evolution of the spectral data has been interpreted in the framework of a simple kinetic model. This model considers the two reactions involved in the chemical processes and the resulting kinetic equations have been numerically solved, without any approximation. The results of such analysis help to evaluate the assignment and correctness of various transitions. From the experimental data a value of 2.7 × 107 cm2 mol-1 for the specific absorption has been estimated, which is in agreement with the corresponding evaluation available in the literature. Moreover a coherent peak attribution, to cation and neutral species of the viologen, is proposed. No transition due to anodic compounds inserted in the film is present, since the spectra of the two films, differing only for the used anodic species, exhibit the same transitions.