Complementary Asymmetric Fluorinated Viologen and Conjugated Phenyl Viologen for Black Electrochromic Devices with High Cycling Stability

Black electrochromic devices (ECDs) that offer broadband visible-light absorption and a visually neutral dark state are highly attractive for smart windows, displays, and adaptive light-management systems. However, achieving high-quality black electrochromism with high optical contrast, fast switching, and long-term cycling stability remains challenging, primarily owing to the unbalanced spectral absorption and radical-cation dimerization issues inherent in conventional viologen-based systems. Herein, we address the limitations of conventional viologen-based electrochromic devices via a strategy integrating molecular design and spectral superposition engineering. A fluorinated benzyl viologen derivative, 1-(4-fluorobenzyl)–1 ′ -(3,5-bis(trifluoromethyl)benzyl)–4,4 ′ -bipyridinium di(tetrafluoroborate) (TFBV), is designed to suppress radical-cation aggregation and improve redox stability. To compensate for the weak absorption of TFBV in the 450–550 nm region, a cyanophenyl-substituted viologen (1,1 ′ -bis(4-cyanophenyl)–4,4 ′ -bipyridinium bis(tetrafluoroborate) (CPV)) with extended π-conjugation is introduced as a complementary chromophore, enabling red-shifted and balanced visible-light absorption. Benefiting from the synergistic absorption of TFBV and CPV, the optimized ECD exhibits a high bleached-state transmittance of 81.1%, a low colored-state transmittance of 5.2%, and a large optical contrast of 75.9% across the visible region. The device also shows rapid switching with coloration and bleaching times of 4.4 and 4.1 s, respectively, a high coloration efficiency of 218.65 cm 2 C –1 , and excellent cycling stability with 99.8% contrast retention after 10,000 cycles. Furthermore, a prototype smart window based on TFBV/CPV exhibits effective light and heat regulation. This work presents an effective molecular design strategy for stable, fast-response black ECDs and expands the potential of viologen-based materials for energy-saving smart windows.