Low-voltage, high-performance viologen derivatives for dual-band optical regulation electrochromic devices

Viologen derivatives were designed and synthesized by tuning the energy‐level structures and spatial configurations. • CNFBZPV exhibits the lowest reduction potential and good cycling durability. • A colorless transparent-to-black electrochromic device was fabricated. • ECD exhibits excellent dual-band optical regulation performance. Viologen molecules have emerged as one of the most promising classes of electrochromic materials owing to their highly transparent bleached state and large optical contrast. However, the high redox potential and irreversible dimerization of viologen radical cations formed during the electrochemical reduction process limit their practical applications. Herein, a series of viologen derivatives were designed and synthesized by incorporating benzene-bridged units, substituted benzyl groups, and electron-withdrawing substituents. The synergistic interactions among these functional groups and structural moieties can effectively modulate the energy-level distribution and molecular spatial configuration of the viologen derivatives. It is intriguing that CNFBZPV bearing cyano and fluoro substituents exhibits a lower LUMO energy level (−3.32 eV) and more positive redox potentials (−0.75 V/-0.56 V), along with greater steric hindrance and a larger dihedral angle, which collectively contribute to favorable redox reaction kinetics. Moreover, an electrochromic device (ECD 4 ) fabricated with CNFBZPV and ferrocene achieves a high optical contrast of 80% in both the visible and near-infrared regions, along with excellent coloration efficiency (233.99 cm 2  C −1 ) and cycling stability, retaining 98% of its initial optical contrast after 3000 consecutive cycles. In addition, a colorless-to-black electrochromic device (ECD 5 ) was assembled with two cathode coloring materials CNFBZPV and 1-butyl-1′-phenyl-[4,4′-bipyridine]-1,1′-diium bis(hexafluorophosphate) (HBV), and one anode coloring material TMTPA, which enables broad optical modulation within a low operating voltage (0.5 V ∼ -1.8 V), combining a highly transparent bleached state (nearly 100%) with a black state featuring low transmittance (less than 15% in the visible region and approximately 20% in the near-infrared region). On this basis, ECD 6 with optimized thickness of the electrochromic layer and the encapsulation process exhibits excellent photothermal regulation capability as well as faster electrochromic switching speed (<5 s) and improved cycling stability, which can retain nearly 60% of its optical contrast after 3000 cycles, which demonstrates great application potential in electrochromic sunglasses and smart windows. Download: Download high-res image (376KB) Download: Download full-size image