Thiazolo[5,4-d]thiazole conjugated viologens for hydrogel-type all-in-one electrochromic devices

Viologen-based organic electrochromic materials have been extensively studied due to their excellent electrochromic performance. Nevertheless, the dimerization of the radical cation state will destroy the reversibility of viologens and result in poor stability of the electrochromic device (ECD). Herein, two pi-extended viologen derivatives (Spr-MPTT and Sbu-MPTT) are synthesized by inserting a thiazolo[5,4-d]thiazole unit between bipyridine moieties and N-bisubstituted with alkyl sulfonate groups. All-in-one hydrogel ECDs are assembled using Spr-MPTT or Sbu-MPTT and ferrocene modified by a water-soluble group. The Spr-MPTT-based ECD exhibited alternating color change between yellow-green and violet, while the Sbu-MPTT-based ECD displayed alternating color change between yellow-green and purple at 1.0/0 V pulse voltage. The extended conjugation of the viologen chromophore confers these devices with visible-NIR absorption and good cyclic stability. Their optical contrasts at 535 nm are 54.85% and 60.17%, respectively, for Spr-MPTT or Sbu-MPTT based ECD. These results demonstrate their potential application in electrochromic hydrogel smart windows.