Photocurable allyl viologens exhibiting RGB-to-black electrochromic switching for versatile heat-shielding capability

Abstract

This article presents the electrochromic behaviors of three photocurable allyl viologens: diallyl-dipyridyl-ethylene bridged viologen (DA-DPyEV), diallyl-tetrapyridyl phenyl bridged viologen (DA-TPyPhV), and diallyl viologen (DAV), which were designed to achieve RGB colorations. The viologen-based ion gel was utilized to prepare single-layered electrochromic devices (ECDs), thus overcoming the leakage issues commonly associated with liquid electrolytes. The individual ECDs displayed outstanding electrochromic performance, including high transmittance differences and cyclic stability, forming a wide color gamut. Additionally, we fabricated transmissive-to-black ECDs by combining the three viologens in appropriate ratios. The black coloration was achieved through subtractive color mixing theory and showed fast switching speed and high coloration efficiency. Furthermore, we manufactured a flexible large-area ECD with a working area of 10 cm × 10 cm using PET-ITO electrodes. The prepared device exhibited excellent electrochromic performance, showing stable and repeated switching behavior over extended periods. Due to substantial absorption in broad spectral regions, the flexible ECDs demonstrated excellent heat-shielding capabilities, significantly reducing indoor temperatures. Overall, the current findings suggest the potential of photocurable allyl viologens for producing flexible color-changing film devices that generate various visible colors while blocking outdoor heat flux.