Isomeric effects of poly-viologens on electrochromic performance and applications in low-power electrochemical devices

Electrochromic (EC) behaviors of poly-viologens are tuned by selecting positions (ortho (o), meta (m), and para (p)) of the benzene rings linked to 4,4′-bipyridine. All-in-one type EC devices (ECDs) are conveniently fabricated by adding poly-viologens to highly conductive solid-state polymeric gel electrolytes. The maximum transmittance contrast (ΔT) of the o-poly-viologen based ECD is much lower than that of the other isomers, and its bleaching is not reversible, whereas a large ΔT and high stability are achieved with the m- and p-poly-viologens. Considering the high solubility of the m-poly-viologen in ion gels, it is anticipated to have a larger hydrodynamic size, leading to low diffusivity (∼1.1 × 10⁻¹² cm²/s for cationic and ∼2.7 × 10⁻¹² cm²/s for anionic current). As a result, the m-poly-viologen ECD shows a superior memory effect and long-lasting colored state without voltage supply. This feature allows us to demonstrate a low-power ECD based on the m-poly-viologen, which only requires ∼3.54 μW/cm² to maintain the transmittance below 30%. The results indicate that adjusting the isomeric molecular structure of poly-viologen EC chromophores is an effective way to control their EC performance.