Transparent-to-Dark Electrochromic Behavior in Naphthalene-Diimide-Based Mesoporous MOF-74 Analogs

The deliberate insertion of redox-active naphthalene diimide ligands into the versatile family of metal-organic frameworks known as MOF-74 (CPO-27) gives rise to a mesoporous electrochromic MOF that can be switched from transparent to dark, a desirable feature in electrochromic devices such as smart windows. Specifically, we report two new materials that have the MOF-74 topology and display redox activity stemming from a naphthalene diimide salicylic acid (NDISA) ligand. We show that these materials can be deposited as thin films on fluorine-doped tin oxide glass. The surprisingly different morphologies of MOF films obtained from Mg²⁺ and Ni²⁺ ions is likely controlled by the nucleation kinetics of Mg₂(NDISA) and Ni₂(NDISA), respectively. Both materials exhibit well-behaved quasi-reversible redox events associated with the [NDI]/[NDI]^·– and [NDI]^·–/[NDI]^2– redox couples, which are also responsible for the electrochromic switching.