An in situ colorimetric measurement study of electrochromism in the di-n-heptyl viologen system

An in situ colorimetric method, based on the CIE (Commission Internationale de l'Eclairage) system of colorimetry, has been applied to the study of the electrochromic N,N ́-bis(n-heptyl)-4,4 ́-bipyridylium (‘di-n-heptyl viologen’) system in aqueous solution on transmissive ITO/glass substrates. On electrochemical reduction of the di-n-heptyl viologen di-cation, the purple di-n-heptyl viologen radical cation salt deposits as a film and the changes in hue and saturation have been tracked using CIE 1931 xy chromaticity coordinates. The CIELAB 1976 colour space coordinates of the purple di-n-heptyl viologen radical cation salt were L* = 76, a* = 33 and b* = –20, with a complementary wavelength of 548 nm. A sharp decrease in luminance was found on formation of the di-n-heptyl viologen radical cation salt. Colour coordinates for the reverse (oxidation) direction plots show hysteresis, implying that specific choice of colour values depends on both the potential applied and from which direction the potential is changed.