Switchable electrochromic devices based on disubstituted bipyridinium derivatives

Abstract

Gel switchable electrochromic devices based on disubstituted bipyridinium derivatives have been developed. The type of substituent from the 4,4′-bipyridinium cation and counter-ions can influence the reversibility and switching of colours of these electrochromic systems. Three switching colours (yellow, blue and red or violet) can be obtained for iodide salts due to dimer formation. In the case of bistrifluoromethanesulfonylimide [NTf₂] salts only two reversible colour switches (transparent and blue) were observed. Complementary electrochromic performance including chromatic contrast (ΔT%); transition times between the bleached and coloured states; colouration efficiencies (CE), colour coordinates (CIE L*a*b*) and preliminary cycling stability studies of the devices have been performed. The higher coloration efficiency is observed in the case of [(C₁₀)₂bpy][NTf₂]₂ (186.9 cm² C⁻¹), which is similar to other bipyridinium systems already reported in the literature.