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Novel color-reinforcing electrochromic device based on surface-confined ruthenium purple and solution-phase methyl viologen
journal contributionposted on 11.01.2013, 09:39 by Roger J. Mortimer, Thomas S. Varley
A novel color-reinforcing electrochromic device (ECD) is described in which the anode and cathode reactions simultaneously exhibit reversible colorless to intense purple changes. Under coloration, the mixed-valence iron(III) hexacyanoruthenate(II) chromophore is formed on oxidation of iron(II) hexacyanoruthenate(II), with simultaneous reduction of the methyl viologen dication to form a mixture of the radical cation monomer/dimer. Using the CIE (Commission Internationale de l’Eclairage) system of colorimetry, the color stimulus of such ECDs and the changes that take place on reversibly switching between the colored and colorless states have been calculated from in situ visible spectra recorded under electrochemical control. On ECD color switching, with the excellent color-matching between the two purple states, sharp and reversible changes in the hue and saturation occur, as shown by the minimal hysteresis of the track of the CIE 1931 xy chromaticity coordinates. Extrapolation of the xy track to the color locus gave a complementary wavelength (λc) of 565 (±5) nm in close agreement with values obtained for the individual electrochromic materials. The concentration of the solution-phase methyl viologen and its diffusion to the cathode controlled both the proportion of surface-confined Ruthenium purple (RP) that is switched to the intense purple form and the overall ECD changes. For the ECDs’ “on” states, the CIELAB 1976 color space coordinates were L* = 86, a* = 9, and b* = −15, and L* = 79, a* = 15, and b* = −22, respectively, for 5 and 10 mmol dm–3 methyl viologen solution concentrations. CIELAB 1976 color space coordinates showed that the ECDs were fully transparent and colorless in the “off” states, with L* = 100, a* = 0, and b* = 0. Switching times, as estimated for 95% of the total absorbance change, were 4 and 10 s respectively, for coloration and bleaching for the 5 mmol dm–3 methyl viologen ECD, and 8 and 16 s for the 10 mmol dm–3 methyl viologen ECD.