Molecular engineering of π-extended viologens consisting of Oxadiazoles-based bridges for highly stable electrochromic devices

Four π-extended viologen derivatives (Spr-PO/Sbu-PO/OHpr-PO/ OHet-PO) were synthesized by introducing oxadiazole group and side chain modification. Compared with viologen with bipyridines as the chromophore, these POs possessed extended conjugation lengths and optimized photoelectron conversion efficiency. The CMC gel electrochromic devices based on them could realize the color transformation from colorless to yellow-green under the working voltage of 1.1 V. Furthermore, due to different side chain groups, the diffusion rates of POs in the gel changed significantly. Compounds based on hydroxyl side chain groups showed higher response rates (<10 s) in the CMC gel system, while sulfonate side chain modified POs showed slower response characteristics (>10 s). All POs showed excellent stability, the optical contrasts were almost unchanged after continuous working for 100 cycles. This study provided an important method for obtaining efficient and stable electrochromic devices.