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

Abstract

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.

Diffusional Electron Transport Coupled to Thermodynamically Driven Electron Transfers in Redox-Conductive Multivariate Metal–Organic Frameworks

“…All of the above assignments are assisted by spectroelectrochemistry measurements on monolinker Zn(NDI) and Zn(PMDI) thin films (Figures S16−S19). 21 Importantly, the stepwise transformation of different electronic states in the cathodic scan can be perfectly reversed during the anodic back scan (Figure S15), demonstrating that the mixed-linker MOF can be reversibly switched between up to five distinct redox states by modulating the applied potential. For an easy comparison of these redox states, their representative spectroscopic signatures are summarized in Figure 4a.…”

“…To construct MOFs with two electrochemically distinct linkers, bis-pyrazole-terminated diimides of PMDI and NDI (PMDI = pyromellitic diimide bis-pyrazoles, NDI = naphthalene diimide bis-pyrazoles; Figure a) with comparable length and molecular topology were synthesized (see the Supporting Information (SI) for more details of both linkers, 1 H NMR data in Figures S1 and S2, absorption data in Figures S3 and S4, and cyclic voltammetry data in Figures S5 and S6). Their equivalent molecular size and linkage chemistry are important for making mixed-linker MOFs with varying linker stoichiometry and should also result in the statistical distribution of the linkers within the materials . To facilitate the fundamental redox hopping studies, high-quality mixed-linker MOF thin films as well as monolinker reference electrodes were prepared on conductive FTO substrates by varying the feeding ratio of NDI/PMDI linkers (100:0, 80:20, 50:50, 20:80, and 0:100) during the solvothermal synthesis (see Experimental Section for details). , The resulting linker ratio in the mixed-linker MOFs was determined by integration of the corresponding redox waves in slow scan rate cyclic voltammetry (CV) experiments (Figures S10–S12, with more discussions below).…”

“… Their equivalent molecular size and linkage chemistry are important for making mixed-linker MOFs with varying linker stoichiometry and should also result in the statistical distribution of the linkers within the materials . To facilitate the fundamental redox hopping studies, high-quality mixed-linker MOF thin films as well as monolinker reference electrodes were prepared on conductive FTO substrates by varying the feeding ratio of NDI/PMDI linkers (100:0, 80:20, 50:50, 20:80, and 0:100) during the solvothermal synthesis (see Experimental Section for details). , The resulting linker ratio in the mixed-linker MOFs was determined by integration of the corresponding redox waves in slow scan rate cyclic voltammetry (CV) experiments (Figures S10–S12, with more discussions below). As envisaged, an excellent correlation between the feeding ratio of the linkers and their incorporation in the MOF is observed (Figure S13), and the chemical composition of the mixed-linker MOF is thus well-controlled in a bottom-up manner.…”

“…Among the redox-conductive MOFs, Zn­(NDI) (NDI = naphthalene diimide bis-pyrazolate) has been extensively studied, mainly in the context of electrochromism and fundamental redox hopping behaviors. − This MOF consists of infinite chains of Zn 2+ ions as redox-inert secondary building units and redox-active NDI linkers, which are electronically isolated. Consequently, electrons propagate through the MOF by hopping between neighboring NDI linkers, a process that is coupled to the diffusion migration of charge balancing counterions .…”

Biphenyl Dicarboxylic-Based Ni-IRMOF-74 Film for Fast-Switching and High-Stability Electrochromism

“…Moreover, the increase from 8 to 14 π-electrons in the Dhtp and Dhbdc ligands can potentially regulate electronic transport properties and carrier lifetimes, affecting the response speed and stability during the electrochromic process. This variation also influences the absorption characteristics of the MOF material within the visible light range . Synthesizing Ni-IRMOF-74 with Dhbdc as a ligand enables the attainment of a larger specific surface area and pore size along with improved electronic transport properties, adjusted carrier lifetimes, and enhanced absorption characteristics within the visible light range, showcasing greater potential in electrochromic materials.…”