Multidirectional color palette of electrochromic metal-organic frameworks

Electrochromic metal-organic frameworks (MOFs) combine advantages from both inorganic/organic electrochromic materials by enabling stable structures/performances as well as tunable functionalities. Their current design and synthesis, however, are inherently complicated, as they often involve amendments to the chromogenic components and/or MOF structures. Inspired by reticular chemistry, we herein demonstrate a multidirectional 'color palette' based on the colorful electrochromic behaviors across a total of 40 zirconium-based MOFs via systematically combining diverse naphthalene diimide (NDI)-based primary linkers (R-groups) and auxiliary linkers (X-groups), namely the NKM-908-R/NKM-906-R series (csq/scu topology) and their corresponding NKM-908-R-TPDC-X/NKM-906-R-TPDC-X derivatives. A new broad color gamut over these robust MOF thin films was showcased via systematical crystal engineering and was thoroughly investigated. The benefits include enhancing the primary set of 'colors' from altering peripheral R-groups without redesigning the NDI core, introducing different X-groups as the secondary sets of 'colors' and topology adjustments to incorporate more X-linkers for intensification of the latter. Together, such designable color-mixing/changing sequences successfully mimicked the use of the routine color palette with molecular-level precision. This work not only holds great potential for further extension, but also provides unique insights into the development of next-generation electrochromic devices.