Electrochromic devices based on surface-confined Prussian blue or Ruthenium purple and aqueous solution-phase di-n-heptyl viologen

Prototype electrochromic devices (ECDs) based on anodically-colouring thin-film Prussian blue (PB) or Ruthenium purple (RP) and cathodically-colouring aqueous solution-phase di-n-heptyl viologen are described. The initial (‘off’) state of each ECD is set with the PB (or RP) in the colourless reduced form and the di-n-heptyl viologen as the colourless di-cation. Switching the ECDs to the coloured state (‘on’), forms on oxidation the coloured mixed-valence PB (or RP), with simultaneous reduction of the di-n-heptyl viologen di-cation to form the purplish-red di-n-heptyl viologen radical cation dimer salt as a thin film. The overall perceived reversible colour changes were colourless to deep blue/purple for the PB/di-n-heptyl viologen ECDs and colourless to pinkish-purple for the RP/di-n-heptyl viologen ECDs. Using the Commission Internationale de l'Eclairage (CIE) system of colorimetry, the colour stimuli of the ECDs were calculated from in situ visible region spectra recorded under electrochemical control, the depth of colour being controlled by the di-n-heptyl viologen concentration. For the coloured states of the PB/di-n-heptyl viologen ECDs, the CIELAB 1976 colour space coordinates for a D55 illuminant were L⁎=60, a*=22 and b*=−47, and L*=39, a*=47 and b*=−55, respectively for 5 and 10 mmol dm−3 di-n-heptyl viologen solution concentrations. For the RP/di-n-heptyl viologen ECDs, the coordinates were L*=70, a*=31 and b*=−27, and L*=63, a*=44 and b*=−34, respectively for 5 and 10 mmol dm−3 di-n-heptyl viologen solution concentrations. L* quantifies the lightness, with +a*, −a*, +b* and −b* respectively giving the red, green, yellow and blue directions away from the achromatic point (0, 0).