Transparent-to-dark electrochromic behavior in naphthalene-diimide-based mesoporous MOF-74 analogs

Abstract

The deliberate insertion of redox-active naphthalene diimide ligands in 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 with the MOF-74 topology that display redox activity stemming from a naphthalene diimide salicylic acid (NDISA) ligand. We show that the new materials can be deposited as thin films on fluorinedoped tin oxide glass. The surprisingly different morphology 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.