Electrochromic electrically conductive Cu₃(HHTP)₂ films with adaptation to diverse and low-concentration water

Sea water, the most abundant water resource on the Earth, is rich in Na+, K+, Mg2+, and Ca2+ and possesses excellent conductivity, making it a highly promising and renewable source of electrolyte. However, the confined and narrow crystal spacing of traditional electrochromic materials limits the diffusion of largely hydrated ions in diverse ion electrolytes. Metal-organic frameworks (MOFs) with large intrinsic nano to microporous structures can serve as a promising host for the fast transport of hydrated ions. In this study, triphenylene-based electrically conductive MOF films were prepared and they underwent a reversible blue-to-yellow electrochromic process in diverse aqueous solutions (NaCl, KCl, MgCl2 and CaCl2). Due to the film's heightened sensitivity and wide-ranging compatibility, the ultra-low concentration electrochromic behavior at 0.1 mM can be maintained in sodium chloride solution. The MOF films show 16.9% optical modulation range and 1.5 s/2.0 s coloration/bleaching time in seawater. As a result, sea water, tap water and distilled water can be evidently distinguished by film color. This study not only demonstrates the potential application of the material as a visual ion concentration detector using low-concentration water and tap water in daily life, but also holds promise for applications in the realm of smart windows.