Viologens and extended viologen derivatives with mono- and di-hexyl substituents for highly stable all-in-one ECDs and solar cell powered large-area ECDs

Viologen-based electrochromic devices (ECDs) have attracted considerable attention due to their potential applications in smart windows. Herein, two viologen derivatives with N-substituted monohexyl (MHEV) and dihexyl (DHEV), and two extended viologen derivatives with a thiophene unit inserted between the bipyridine moieties and N-substituted monohexyl (m-MHEV) and dihexyl (m-DHEV) were synthesized. All-in-one ECDs were assembled using these materials. m-DHEV and m-MHEV-based ECDs exhibited red and dark orange colors, while DHEV and MHEV-based ECDs exhibited blue and magenta colors. The extended conjugation of the chromophore by the thiophene unit endowed the devices with visible-NIR absorption and excellent cyclic stability. After 10000 cycles, ECDs based on m-DHEV and m-MHEV retained 94.2% and 95.0% of their initial optical contrasts, respectively. A binary chromophore mixture of DHEV and m-DHEV was used to fabricate a uniform large-area ECD (8 cm × 8 cm) with strong sunlight absorption ability. The device maintained high optical contrast (80.2% of initial state) after 2000 cycles. Furthermore, a solar cell powered ECD (8 cm × 8 cm) with negligible loss of optical contrast after 100 cycles of coloring and bleaching under sunlight was developed, demonstrating the application potential of self-powered smart windows using these materials.