Enhanced electrochromic properties of terpyridine-attached asymmetric viologen with high transmittance and switching stability

A terpyridine attached asymmetric viologen (TpyV) was synthesized, and its electrochromic properties in a single-layered electrochromic device (ECD) were demonstrated. The ECD showed a reversible color transition in the absence of anodic species, with a maximum transmittance of higher than 91%. Fast switching speeds to the colored (6.3 s) and bleached states (5.8 s) and enhanced kinetic stability of up to 360 cycles without transmittance loss were observed. The coloration efficiency (CE) was estimated to be 232.87 cm(2)/C, which is higher than those of the common viologen based ECDs. The relationship between the change in absorbance and the charge density (Q(d)) under various voltage biases showed a steadily increasing trend in Q(d) values with charge density, resulting in decreased CE values. The bulkiness of the counter anions showed a potent effect over the bleaching of the electroreduced state of TpyV. The optical memory of the TpyV ECD with BE4- as a counter anion lasted 30 min due to the weak charge transport. A flexible electrochromic device was also prepared by sandwiching all-in-one TpyV ion gel between ITO-PET substrates. The prepared device showed a reversible color transition in both flat and bent states. Current results suggest a promising approach in developing high-performance viologen electrochromes with high transmittance and switching stability.