Electrochromism in Isoreticular Metal-Organic Framework Thin Films with Record High Coloration Efficiency

The power of isoreticular chemistry has been widely exploited to engineer metal-organic frameworks (MOFs) with fascinating molecular sieving and storage properties but is underexplored for designing MOFs with tunable optoelectronic properties. Herein, three dipyrazole-terminated XDIs (X = PM (pyromellitic), N (naphthalene), or P (perylene); DI = diimide) with different lengths and electronic properties are prepared and employed as linkers for the construction of an isoreticular series of Zn-XDI MOFs with distinct electrochromism. The MOFs are grown on fluorine-doped tin oxide (FTO) as high-quality crystalline thin films and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Due to the constituting electronically isolated XDI linkers, each member of the isoreticular thin film series exhibits two reversible one-electron redox events, each at a distinct electrochemical potential. The orientation of the MOFs as thin films as well as their isoreticular nature results in identical cation-coupled electron hopping transport rates in all three materials, as demonstrated by comparable apparent electron diffusion coefficients, D_e^app. Upon electrochemical reduction to either the [XDI]^•- or [XDI]^2- state, each MOF undergoes characteristic changes in its optical properties as a function of linker length and redox state of the linker. Operando spectroelectrochemistry measurements reveal that Zn-PDI@FTO (PDI = perylene diimide) thin films exhibit a record high coloration efficiency of 941 cm² C^-1 at 746 nm, which is attributed to the maximized Faradaic transformations at each electronically isolated PDI unit. The electrochromic response of the thin film is retained to more than 99% over 100 reduction-oxidation cycles, demonstrating the applicability of the presented materials.