Viologen-Integrated MOF Thin Films: Reversible Electrochromic Behavior at Low Voltage

Viologens (N-mono- and N,N′-disubstituted-4,4′-bipyridiniums) are among the most extensively studied redox-active materials due to their reversible electrochemical reduction into deeply colored radical species, making them attractive for electrochromic applications. However, most research has focused on solution-based systems or amorphous polymer composites, which limits their integration into electronic devices. To expand their application range, we incorporated electrochemically active monosubstituted viologens into a crystalline solid-state framework as building blocks of a permanently porous metal–organic framework (MOF), {[Ni3F(cpb)3(bdc)1.5]·guests}n (Hcpb·Cl = 1-(4-carboxyphenyl)-4,4′-bipyridinium chloride; H2bdc = benzene-1,4-dicarboxylic acid). We demonstrate that this MOF can be grown as thin films on conductive substrates via a simple solvent-mediated process. The MOF’s high void space (>80%) enables efficient ionic mobility for reversible switching between transparent and colored states at a low driving voltage of 1.0 V. While electrochromic behavior in viologen-based MOFs has been rarely explored, this work represents one of the first examples of a permanently porous viologen MOF exhibiting robust and reversible electrochromism. The synergy between nanoporosity, which facilitates electrolyte penetration, and the redox-active ligand’s low-voltage response positions this material as a promising candidate for smart windows and electrochromic display technologies.