Introduction

Metal-organic frameworks (MOFs) are types of porous crystalline materials with ordered and repeating structures composed of metal ions/clusters and organic ligands through periodic connections1,2. Their unique advantages have allowed MOFs to be used in many cutting-edge fields such as gas adsorption/separation, smart sensors, energy catalysis, and biomedicine3,4,5,6,7,8,9. Among them, stimuli responsive MOFs, which can change their physicochemical properties in response to external stimuli (including temperature, force, light and electricity, etc.), are ideal candidates for designing smart functional materials10,11. In particular, electrochromic MOFs undergo stable and reversible color changes when stimulated by an applied electric field, which are attractive in being employed to antiglare rear-view mirrors, smart windows, optical displays, and other abundant applications12,13,14,15,16,17,18,19. MOFs are no doubt acknowledged as one of the hotspots in the research of electrochromic materials20,21,22,23. Due to its highly ordered porous structure, the entire framework of electrochromic MOF could be under direct contact with the electrolyte. Meanwhile, the diffusion of ions in the electrolyte and the transfer of electrons during the process can be greatly improved, thereby facilitating efficient electrochromic conversion24. In addition, the outstanding tunability and designability of MOF structures allow for various electrochromic behaviors, and various redox units have been used to construct electrochromic MOFs. Dincă's group pioneering the preparation of electrochromic MOF thin films in 2013 through solvothermal reaction of Zn2+ ions with naphthalene diimide (NDI) containing ligands25. By grafting different functional groups onto the ligands, they were able to further modulate the color changes of these MOFs. In 2016, Dincă et al. constructed two mesoporous electrochromic MOF-74 analog MOFs films inserted with naphthalenedicarboximide which showed fast and reversible color transitions21. It was also discovered that the design of mesoporous MOFs facilitates the detachment and embedding of ions in the electrolyte, promoting the efficient electron transfer during electrochromism. Recently, Ott et al. designed MOFs using mixed linkers respectively contain PMDI and NDI cores that leverage differences in electron affinity, and sophisticatedly revealed the mechanism of redox hopping from combining the spectroscopic signatures under different valence states26. Meanwhile, they also fabricated three dipyrazole-terminated XDI-MOFs, among which Zn-PDI@FTO had a record high coloration efficiency of 941 cm2 C−1 27. Wang et al. developed the MOF-74-type materials with perylenetetracarboxylic dianhydride (PDI) units introduced, achieving desirable electrochromic properties with excellent electrochemical stability23. Diring et al. constructed PDI-containing Zn-PDI-Cl and Zn-PDI-SO2 presenting different color variations28. In addition to NDI- and PDI-based ligands, Farha and Cai et al. introduced pyrene-containing linkers for the construction of NU-901 and hydrogen-bonded organic framework PFC-1 electrochromic films, respectively29,30. Similarly, ligands featuring viologen, triphenylene or triphenylamine have also been used to design MOF films with diverse electrochromic properties31,32,33. Additionally, Lee and Cai et al. achieved desirable electrochromic properties and large-scale preparation of electrochromic devices by designing metal coordination polymers with metals as redox centers34,35. However, the current methodology of altering the electrochromic properties of MOFs can only be achieved via the synthesis from scratch, resulting in a time-consuming design cycle with high financial investment, and a low success rate. Therefore, it is urgently needed to develop easy, fast and efficacious methods for modulating the electrochromic properties of specific MOFs.

Zr-MOFs are known for their high stability, structural designability, and ease of modification36. Since 2008, various types of Zr-MOFs with a variety of excellent properties have been reported37,38,39,40,41,42,43,44, making them a potential research platform for achieving the simple regulation of the electrochromic performance of MOFs. On the other hand, linker installation has proven to be an effective and convenient post-synthetic modification strategy to precisely tune the structures and properties of Zr MOFs38,45,46,47,48. Herein, we synthesized a NDI-based electrochromic zirconium-organic framework with coordination unsaturated pockets inside the structure, where the successful installation of TPDC-X (TPDC = 2’,3’,5’,6’-substitued phenyl-4,4”-dicarboxylate, X represents different substituent groups derived on TPDC) linkers with different substituents is demonstrated. This linker installation further enables the precise regulation of two kinds of electrochromic behaviors for the MOFs, that are deepening the colors and generating new colors/shifting modes. These results indicated that the regulation of electrochromic behaviors for MOFs was simply achieved by linker installation, which further proved the practicability of this strategy.

Results

Design, synthesis and structure description of redox active MOFs with coordination unsaturated pockets

To construct electrochromic MOFs, it is necessary to introduce redox active centers. NDI is a classic redox active group, and several electrochromic MOFs based on NDI active centers have been investigated20,21,49,50. On the other hand, the size of the coordinatively unsaturated open pockets of the MOFs highly depends on the length of the ligand. Our research group previously constructed the H4TPTB ligand on the basis of H4TPCB through elongation of such tetratopic ligand (Supplementary Fig. 1), thereby expanding these open pockets and achieving broader applications51,52. Therefore, for facilitating the introduction of diverse redox centers, further elongating the length of the ligand on the main architecture to obtain pockets that can accommodate larger ligands is worthwhile yet challenging. Consequently, we designed and synthesized a tetratopic carboxylic ligand H4NDTB (5’,5”“-(1,3,6,8-tetraoxo-1,3,6,8-tetrahydrobenzo[lmn][3,8]phenanthroline-2,7-diyl)bis(2’-methoxy-[1,1’:3’,1”-terphenyl]-4,4”-dicarboxylic acid)) containing NDI group (Fig. 1a and Supplementary Fig. 2), and predicted that it could self-assemble with Zr ions to obtain a csq topological Zr-MOF with open pockets similar to PCN-608 and PCN-808.

Fig. 1: Construction of NKM-908.
figure 1

a The tetratopic ligand H4NDTB in NKM-908. b The 8-connected Zr6 cluster in NKM-908. c Crystal structure of NKM-908 viewed along the c axis. For clarity, the hydrogen atoms were omitted. d The csq topology network of NKM-908. e PXRD patterns of NKM-908. f N2 sorption isotherms of NKM-908 under 77 K (The inset shows the corresponding pore size distribution).

Full size image

As a result, NKM-908 was synthesized by the solvothermal reaction of H4NDTB and ZrCl4 in N,N-dimethylformamide (DMF) using benzoic acid as modulating reagents (Fig. 1, Supplementary Figs. 4-7). Single crystal X-ray diffraction experiments confirmed that NKM-908 crystallized in the hexagonal space group P6/mmm, which is the same as PCN-608 and PCN-808 series. In NKM-908, each Zr6O4(OH)8(H2O)4 cluster coordinates with eight deprotonated NDTB4- fragments, while the remaining four unsaturated coordination sites are occupied by terminal H2O/-OH− groups. Noteworthy, the relatively weak interactions between these four sites and H2O/-OH− groups makes the latter easy to be removed, providing the possibility for the installation of linear carboxylate ligands in these zirconium unsaturated coordination sites. Meanwhile, each deprotonated NDTB4- fragment is coordinated to four Zr6O4(OH)8(H2O)4 clusters. In addition, the central NDI unit is perpendicular to the phenyl groups on both sides, while the phenyl groups on both sides are close to coplanar. The resulting symmetry of such ligand is C2v, and further leading to a three-dimensional (3D) framework with a csq topology. A hexagonal one dimensional (1D) open channel and a triangular 1D channel were formed in NKM-908.

Powder X-ray diffraction (PXRD) patterns of NKM-908 (Fig. 1e) show that the characteristic diffraction peaks at low angles attributed to the pores of freshly synthesized sample are not detected due to the presence of solvent. After activation, the characteristic peaks match well with the simulated patterns, indicating the phase purity of the obtained NKM-908. Typical type IV nitrogen adsorption isotherm was observed for NKM-908 under 77 K (Fig. 1f)53, and the Brunauer–Emmett–Teller (BET) specific surface area of NKM-908 was calculated to be 2924 m2 g−1. Additionally, as shown in the inset (Fig. 1f), the pore size distribution of NKM-908 can be obtained using the NLDFT method, and demonstrated that two types of pores are distributed in NKM-908. Those are the triangular-shaped micropores of about 1.2 nm in diameter and hexagonal shaped mesopores of about 2.7 nm in diameter (Supplementary Figs. 8 and 9). The pore size distribution of NKM-908 was also simulated via Zeo + + showing distributions at 2.85, 0.97, and 1.38 nm, which is consistent with the pore size calculated by the NLDFT method (Supplementary Fig. 10). Scanning electron microscopy (SEM) images display that NKM-908 is a rod-shaped crystal with a size distribution around 7 μm, which is similar to that of PCN-608 and PCN-808 (Supplementary Fig. 11).

Thin film fabrication and analysis

To investigate the electrochromic behavior of NKM-908, NKM-908 thin film was grown onto the ITO (indium tin oxide) glass using solvothermal method and obtain a nearly colorless, well adhered thin film on ITO electrode (Fig. 2a, inset). Notably, the in-situ deposition growth of MOFs on ITO electrode can greatly improve the interaction between the material-support interface and prevent the exfoliation of MOFs as well as ensuring the rapid charge transfer, which plays a positive role in the detachment and embedding of electrolyte ions during electrochromic process. The PXRD patterns (Fig. 2a) confirms that the phase purity of the in-situ grown NKM-908 film is consistent with that of the powder, with only minor differences in the intensity and width of the peaks. Figure 2b and Supplementary Fig. 12 show the SEM images of NKM-908 film at different magnifications, in which the surface of the ITO glass is uniformly and densely covered with 6 μm-thick rod-shaped MOFs.

Fig. 2: Characterizations of NKM-908 thin film.
figure 2

a PXRD patterns of NKM-908 powder and thin film (The inset shows the optical image of NKM-908 thin film on ITO electrode). b SEM image of NKM-908 thin film. Scale bars, a 10 mm, b 25 μm.

Full size image

Electrochromic property of NKM-908 thin film

In order to evaluate the electrochromic performance, as prepared NKM-908 thin film was directly used as the working electrode, and electrochemical measurements were taken in 0.1 M [(nBu)4N]PF6/DMF solution using a standard three-electrode setup. Figure 3a shows the cyclic voltammetry (CV) curve of NKM-908 thin film between 0 and −1.4 V, with two distinct pairs of redox peaks appearing around −0.51 V/−0.43 V and −0.9 V/−0.73 V, respectively. Meanwhile, the nearly colorless NKM-908 thin film in the electrolyte shows two reversible color changes (light yellow and then green) under the potential change during the CV scanning process (Fig. 3c). This confirms that the NKM-908 thin film can realize the de-embedding of the electrolyte cations. Simultaneously, the Faraday current formed in the potential interval of the electrode electrochemical reaction is absolutely attributed to the NDTB4- linker due to the d0 electronic structure of the Zr(IV) metal cluster secondary building block and its redox inactive feature20. This point is corroborated by the CV tests with pure ligands. The CV curve of H4NDTB dissolved in DMF containing 0.1 M [(nBu)4N]PF6 exhibits two sets of redox peaks (Supplementary Fig. 13), which correspond to the two forms of the NDI moiety (NDI → NDI•− →NDI2−) (Supplementary Fig. 14), and are basically consistent with the redox peaks of NKM-908 thin film. The slight displacement present in the CV curve of the thin film compared to that of the ligand solution may be attributed to the difference in intermolecular dynamics with the liquid after the formation of the coordination framework.

Fig. 3: Electrochromic property of NKM-908 thin film.
figure 3

a CV curve for NKM-908 thin film in 0.1 M [(nBu)4N]PF6/DMF solution at 50 mV s−1 purged with N2 b In-situ UV-vis absorption spectra for NKM-908 thin film in 0.1 M [(nBu)4N]PF6/DMF solution. c Optical images of NKM-908 thin film at various applied potentials (0 V, −0.6 V, and −1.4 V vs. Ag/AgCl) during CV scanning. Scale bars, 3 mm.

Full size image

To gain further insight into the mechanism of color change at different potentials, in situ UV-vis absorption spectrum was used to monitor the real-time changes of the thin films (Fig. 3b). The characteristic π-π* absorption bands of NDI groups at 364 nm and 382 nm were observed in the UV-vis spectra of the NKM-908 thin film in the absence of an applied electric field. During the process of applying the potential to −0.6 V, these two peaks gradually decrease, while new absorption peaks appear and enhance at 475 nm, 606 nm, 706 nm, and 786 nm, which correspond to the formation of the NDI•- species, and leading to the first color change into light yellow (Fig. 3c).

When the NDI•- species continue to be reduced into NDI2-, the absorption peak at 475 nm decreases, while new absorption peaks appear at 398 nm, 422 nm, 575 nm and 625 nm, which increase with the potential rising to −1.4 V. This changing process in the spectra corresponds to the shift towards green color in the thin film. Subsequently, the absorbance curves at 0 V and −1.4 V were converted to transmittance curves to obtain the optical contrast (ΔT) of 39.7% at 422 nm for NKM-908 thin film (Supplementary Fig. 15). Furthermore, the electrochromism of the NKM-908 thin film is reversible, and the color returns to bleached state when the applied oxidation potential returns to 0 V. Besides the optical contrast, cyclic stability is another highlighted concern for electrochromic materials in practical applications. As shown in Supplementary Figs. 16 and 17, the absorbance has not significantly decreased after 10 cycles of alternating potentials of 0 V and −1.4 V at 422 nm, respectively, confirming that the NKM-908 film has great stability and reversibility. After the cyclic stability experiment, the PXRD pattern of NKM-908 thin film still matched well with its initial pattern (Supplementary Figs. 18a). Meanwhile, the SEM image of NKM-908 thin film after cycling tests also show that its original morphology was retained (Supplementary Figs. 18b), further indicating its high stability.

The response time is the time when the optical transmittance reaches 90% between the original state and the colored state. The NKM-908 thin film exhibits a coloring time of 49 s (tc) and a bleaching time of 35 s (tb) (Supplementary Fig. 19). According to (1)19, the coloring efficiency (CE) of NKM-908 at 422 nm was calculated to be 158.0 cm2 C−1.

$${{{\rm{CE}}}}({{{\rm{\lambda }}}})=\frac{\Delta {{{\rm{OD}}}}}{{{{\rm{Q}}}}}$$
(1)

where ΔOD = log[Tb/Tc], where Tb and Tc are the transmittances at bleached and colored states, OD is optical density, and Q is the injected/ejected charge density (C cm−2).

Tuning the structure of MOFs by linker installation

Although multicolor transformation and electrochromic reversibility have been achieved for NKM-908, electrochromic behavior regulation and performance optimization of the redox active MOF remain challenging. There is a high potential of showing different behaviors in color changes while MOFs containing multiple redox active centers. Therefore, linker installation was performed to regulate the electrochromic properties of NKM-908. The size of the pockets formed between two neighboring Zr6 clusters along the c-axis in NKM-908 is roughly 15.6 Å, which is longer than those in the previously reported PCN-608 and PCN-808 series. In our previous work, it has been demonstrated that a linear BDC ligand with a length of 6.9 Å can be fitted into PCN-608 with a pocket of 8.2 Å, and a linear BPDC ligand with a length of 11.2 Å can be fitted into PCN-808 with a pocket of 11.7 Å. Based on this, we reasonably infer that TPDC-X could be installed into NKM-908. Cleavages had emerged on the large crystals of NKM-908 after linker installation, thus the current state-of-the-art 3D microelectron diffraction technique was used to determine the post-installation structure, taking NKM-908-TPDC-4F as a representative. Single crystal data of NKM-908-TPDC-4F obtained by 3D electron diffraction experiments confirmed that TPDC-4F was successfully installed into NKM-908 (Fig. 4a, b, e, f, Supplementary Figs. 20 and 21). In particular, the open pockets of NKM-908 along the c-axis are completely occupied by TPDC-4F, replacing the terminal coordinated OH−/H2O. As a result, the Zr6 clusters in NKM-908 were shifted from 8- to 10-connection mode, and the structure is changed from (4,8)-c csq topology to (4,10)-c topology (Fig. 4c, d and Supplementary Fig. 22). Importantly, such linker installation method guarantees a crystallographically ordered insertion of functional groups into the NKM-908 architecture, as well as preventing the formation of other by-products.

Fig. 4: TPDC-X Linker installation in NKM-908 (taking TPDC-4F as a representative).
figure 4

a Enlarged coordination unsaturated pockets in NKM-908 viewed along a axis. b Crystal structure of NKM-908-TPDC-4F viewed along a axis. Color scheme: black, C; red, O; light blue, Zr; green, F. For clarity, the hydrogen atoms were omitted. c The 4,8-c csq net of NKM-908. d The 4,10-c net of NKM-908-TPDC-X. Electron diffraction (ED) characterization of NKM-908-TPDC-4F observed on a Rigaku XtaLAB Synergy-ED electron diffractometer: e the selected grain, f example of the corresponding diffraction image. Scale bar, e 2 μm. g N2 sorption isotherms of NKM-908 and NKM-908-TPDC-X under 77 K. h PXRD patterns of NKM-908 and NKM-908-TPDC-X thin films.

Full size image

In order to investigate the effect of installing linkers with different functional groups on NKM-908, N2 adsorption isotherms and PXRD patterns were measured to reveal changes in the pore environment and phase purity. The N2 adsorption isotherms after linker installation show typical type IV adsorption behavior for all MOFs and decrease in N2 adsorption capacity (Fig. 4g)53. The main reason for the decrease in N2 uptake capacity is that the installed linkers occupy the void space in the MOFs cavity, which reduces the porosity of the MOFs. Based on the N2 adsorption data, the calculated BET specific surface areas are listed in Supplementary Table 1. Additionally, as shown in Supplementary Fig. 23, the pore size distributions of MOFs after linker installation were obtained using the NLDFT method, which were consistent with that of NKM-908, with a predominant distribution of triangular-shaped micropores of approximately 1.2 nm and hexagonal-shaped mesopores of approximately 2.5–3 nm. The pore size distribution of NKM-908-TPDC-4F at 2.67, 2.74, 0.9, and 0.7 nm was simulated using Zeo + +, which is similar to the pore size distribution calculated by the NLDFT method (Supplementary Fig. 24). Supplementary Fig. 25 presents the PXRD patterns of activated NKM-908-TPDC-X after installation of different linkers. The measured data are in well agreement with the simulation results, revealing the phase purity of the MOFs before and after installation of the bitopic linkers. 1H NMR spectra of the digested samples are shown in Supplementary Figs. 26–31 and Supplementary Table 2 to further explore the linker ratios after linker installation. Subsequently, the linker installation strategy was extended to NKM-908 thin film. The visible light absorption ability of NKM-908 and NKM-908-TPDC-X were probed by solid-state UV-vis spectroscopy (Supplementary Fig. 32). After installing the TPDC-X linker, the obtained MOFs show significantly different absorption intensities, while NKM-908 exhibits a wider and stronger absorption signal. The band gaps of NKM-908 and NKM-908-TPDC-X were calculated and listed in Supplementary Fig. 33 and Supplementary Table 3. A series of ITO electrodes, named NKM-908-TPDC-X thin films, were obtained by immersing the NKM-908 thin films obliquely into 20 mL vials containing DMF solutions of TPDC-X ligands for 1 day at 100 °C (Supplementary Fig. 34). The PXRD patterns (Fig. 4h) affirm that the phase purity of all the thin films after linker installation. The SEM images show that the thickness and morphology of as-prepared NKM-908-TPDC-X thin films are basically the same as that of the original NKM-908 thin film (Supplementary Figs. 35–40). The above results confirm the successful installation of TPDC-X linkers into NKM-908 under relative mild conditions, which further demonstrating the effectiveness of the linker installation strategy.

Electrochromic property regulating by linker installation

After the successful installation of TPDC-X linkers into NKM-908 thin films, the electrochromic behaviors were recorded separately. Firstly, three redox inactive bitopic carboxylate ligands TPDC-2OMe, TPDC-4Me and TPDC-4F were selected to be installed into the coordination unsaturated Zr sites of NKM-908. The optical photos of the as-synthesized thin films all show slight color changes comparing to the original film, which may realize potential regulation of electrochromic performance. Unsurprisingly, the electrochromic behaviors of NKM-908-TPDC-2OMe, NKM-908-TPDC-4Me and NKM-908-TPDC-4F thin films are basically identical to that of NKM-908. No differences are found in the CV curves (Fig. 5a, d, g, insets show the installed TPDC-X linkers), and two sets of redox peaks are detected at −0.51 V/0.43 V and −0.9 V/0.73 V, respectively. Meanwhile, their color changed from light yellow to dark yellow and then to green from 0 V to −0.6 V to −1.4 V (Fig. 5c, f, i). However, the darkening of the film after linker installation results in the darker colors than NKM-908 during all stages of the electrochromic process. In situ UV-vis spectra reveal the characteristic absorption peaks of the electrochromic groups NDI, NDI•- and NDI2- (Fig. 5b, e, h). Plots of transmittance change versus wavelength were obtained and evaluated for three different MOF films at 0 V and −1.4 V (Supplementary Fig. 41). Thin films of NKM-908-TPDC-2OMe, NKM-908-TPDC-4F and NKM-908-TPDC-4Me show transmittance changes of 49.1%, 48.4% and 49.6% at 422 nm, respectively. The values of transmittance changes are all higher than that of NKM-908 thin film (39.7%), which confirms that the modulation of electrochromic performance can be achieved by linker installation. Additionally, all three thin film electrodes were tested for cycling stability, after 10 cycles experiments through double-potential-step chronoamperometry (0 and −1.4 V), NKM-908-TPDC-2OMe thin film, NKM-908-TPDC-4F thin film and NKM-908-TPDC-4Me thin film attenuated by 4.16%, 4.31%, and 7.31%, respectively (Supplementary Figs. 42–47). The electrodes were characterized after cycling tests, the PXRD patterns and SEM images ascertain that the phase purity and electrode morphology are well retained (Supplementary Figs. 48), further demonstrating the excellent stability and reversibility of these three electrodes. The tc and tb at 422 nm for NKM-908-TPDC-2OMe thin film, NKM-908-TPDC-4F thin film and NKM-908-TPDC-4Me thin film were calculated to be 50/36 s, 45/49 s and 51/50 s, respectively, which became slower than the NKM-908 thin film (Supplementary Figs. 49–51). The CE values at 422 nm were calculated to be 88.0, 136.0 and 138.5 cm2 C−1, respectively.

Fig. 5: Electrochromic properties of NKM-908-TPDC-2OMe, NKM-908-TPDC-4F and NKM-908-TPDC-4Me thin films.
figure 5

a, d, g CV curves for NKM-908-TPDC-2OMe, NKM-908-TPDC-4F and NKM-908-TPDC-4Me thin films in 0.1 M [(nBu)4N]PF6/DMF solution at 50 mV s−1 purged with N2 (The inset shows the installed TPDC-X ligands). b, e, h In-situ UV-vis absorption spectra of NKM-908-TPDC-2OMe, NKM-908-TPDC-4F and NKM-908-TPDC-4Me thin films in 0.1 M [(nBu)4N]PF6/DMF solution. c, f, i Optical images of NKM-908-TPDC-2OMe, NKM-908-TPDC-4F and NKM-908-TPDC-4Me thin films at various applied potentials (0 V, −0.6 V, and −1.4 V vs. Ag/AgCl) during CV scanning. Scale bars, 3 mm.

Full size image

Considering that the electrochromic mechanism of NDI groups is based on the change of free radicals, we further designed and synthesized three redox active TPDC ligands (TPDC-TDA, TPDC-AN, and TPDC-Py) with thiadiazole (TDA), anthracene (AN), and pyridine (Py) functional units, which can also achieve transitions to free radical forms. After in-situ installation of TPDC-TDA, TPDC-AN, and TPDC Py linkers into NKM-908 thin films, optical images show that only NKM-908-TPDC-TDA thin film presents a light-yellow color, and both the other two thin films show similar color to that of NKM-908. Unlike the CV curves of the NKM-908 thin film, the NKM-908-TPDC-TDA thin film exhibits two sets of redox peaks with different peak positions, situated at −0.9 V/−0.73 V and −1.42 V/−1.3 V (Fig. 6a, inset shows the installed TPDC-TDA ligand). The former set of peaks corresponds to the oxidation and reduction process of the NDI unit, while the latter group corresponds to the oxidation and reduction process of TDA components, which can be determined by the CV test of TPDC-TDA ligand in the solution phase (Supplementary Fig. 52). Strikingly, during the CV process, the NKM-908-TPDC-TDA thin film exhibits a reversible color transition from light yellow to deep yellow, then to purple, and finally to green (Fig. 6c), followed by a full return to light yellow when the potential dropped to 0 V. The synchronously monitored UV-vis spectra are shown in Fig. 6b, where a gradual increase of the absorption band at 384 nm can be observed when the potential was applied from 0 V to −1.1 V. This observation coincides with the characteristic π-π* absorption band of the NDI group, which corresponds to the gradual color change from light yellow to deep yellow. Interestingly, when increasing the potential from −1.6 V to −2 V, a new absorption peak at the wavelength of 520 nm appeared at −1.6 V, and gradually weakened with increasing the potential. This new peak belongs to the purple state during the color change process, which is attributes to the one-electron reduction process of the TDA moiety, and the possible mechanism of the redox process is shown in Supplementary Fig. 53. When further applying the potential from −2 V to −2.3 V, the absorption peak at 520 nm gradually weakened and disappeared. Meanwhile, new absorption peaks at 398 nm, 422 nm, 575 nm and 625 nm are generated, which are ascribed to NDI2- and thus the thin film turns to green. The characteristic peak of NDI reduction was not shown at 475 nm, which probably due to the restricted accessibility of charge-balancing counterions. The transmittance versus wavelength plot of NKM-908-TPDC-TDA thin film reveals that the ΔT is 53.0% at 850 nm (Supplementary Fig. 54a). In addition, the cycling stability of the NKM-908-TPDC-TDA thin film was assessed. The optical contrast of the NKM-908-TPDC-TDA was attenuated by 1.82% (Supplementary Figs. 55 and 56) after 10 cycles of experiments with the dual-potential-step chronoamperometry (0 V and −2.2 V). Moreover, the PXRD patterns and SEM images after the cycling stability tests demonstrate that the phase purity and morphology are retained (Supplementary Fig. 57), which further affirms that the NKM-908-TPDC-TDA thin film also possesses good reversibility and stability. The tc and tb of NKM-908-TPDC-TDA thin film at 520 nm was calculated to be 110/106 s, which is slower than the NKM-908 electrode (Supplementary Fig. 58). The CE value at 520 nm was calculated as 33.9 cm2 C−1.

Fig. 6: Electrochromic properties of NKM-908-TPDC-TDA, NKM-908-TPDC-AN and NKM-908-TPDC-Py thin films.
figure 6

a, d, g CV curves for NKM-908-TPDC-TDA, NKM-908-TPDC-AN and NKM-908-TPDC-Py thin films in 0.1 M [(nBu)4N]PF6/DMF solution at 50 mV s−1 purged with N2 (The inset shows the installed TPDC-X ligands). b, e, h In-situ UV-vis absorption spectra of NKM-908-TPDC-TDA, NKM-908-TPDC-AN and NKM-908-TPDC-Py thin films in 0.1 M [(nBu)4N]PF6/DMF solution. c, f, i Optical images of NKM-908-TPDC-TDA, NKM-908-TPDC-AN and NKM-908-TPDC-Py thin films at various applied potentials (0 V, −1.1 V, and −2.2 V vs. Ag/AgCl) during CV scanning. Scale bars, 3 mm.

Full size image

Two sets of redox peaks at −0.91 V/−0.77 V and −1.80 V/−1.97 V can be clearly observed in the CV curve of the NKM-908-TPDC-AN thin film, which are attributed to the redox of the NDI moiety as well as the redox process of anthracene, respectively (Fig. 6d and Supplementary Fig. 59). Surprisingly, during the CV scanning process, the color of the film changed from nearly colorless to blue and then to light green, further demonstrating the successful regulation of the electrochromic performances. In situ UV-vis absorption spectra were utilized to explore the color changes during the electrochromic process (Fig. 6e). Absorption peaks are detected at 356 nm, 374 nm, and 395 nm without applying potential, which due to the characteristic absorption peaks of anthracene in TPDC-AN and the characteristic π-π* absorption band of NDI. As the potential increased to −1.1 V, the absorptions at the above three positions are gradually enhanced, which were ascribed to the characteristic absorption peaks when anthracene undergoes redox reaction, and the possible mechanism of the discoloration is shown in Supplementary Fig. 60. When the potential further changed to −2.2 V, the thin film turns into light green, and new absorption peaks appear at 625 nm, 575 nm, and 422 nm, indicating the formation of NDI2-. The transmittance versus wavelength plot of the NKM-908-TPDC-AN film unveils that the value of ΔT is 39.5% at 422 nm (Supplementary Fig. 54b). Thereafter, the cyclicity of the NKM-908-TPDC-AN thin film was evaluated. A 10 cycles dual-potential-step chronoamperometry (0 V and −2.2 V) test demonstrated that the optical contrast of the NKM-908-TPDC-AN was attenuated by 6.87% (Supplementary Figs. 61 and 62). The PXRD pattern of NKM-908-TPDC-AN thin film shows the retention of high crystallinity after cyclicity tests (Supplementary Fig. 57c), and the SEM images of NKM-908-TPDC-AN thin film also agree well with its initial morphology (Supplementary Fig. 57 d). The tc and tb of NKM-908-TPDC-AN thin film at 422 nm was calculated to be 107/102 s, which is also slower than the NKM-908 thin film (Supplementary Fig. 63). The CE value of NKM-908-TPDC-AN thin film at 422 nm is calculated as 34.4 cm2 C−1.

Furthermore, similar color modulation of electrochromism was also achieved through the installation of TPDC-Py linker. Figure 6g shows the CV curve of the NKM-908-TPDC-Py thin film, that two sets of redox peaks are clearly observed (−0.9 V/−0.73 V and −2.16 V/−2.04 V). The former set of peaks corresponds to the redox peaks of the NDI unit, and the latter set of peaks attribute to the redox peaks of TPDC-Py, which is in consist to the CV curve of pure TPDC-Py in solution (Supplementary Fig. 64). Meanwhile, the color of the thin film changed from nearly colorless to deep yellow to deep pink at 0 V, −1.1 V and −2.2 V during the CV scanning process (Fig. 6i). In situ UV-vis adsorption experiments were tested to probe the electrochromic mechanism (Fig. 6h). No absorption peaks were detected at 0 V for NKM-908-TPDC-Py thin film. As the potential gradually increased to -1.1 V, new absorption peaks at 355 nm and 378 nm emerged with gradually increasing intensities along the process, which correspond to the deep yellow state in electrochromism. During the following potential change from −1.1 V to −2.2 V, new absorption peaks at 448 nm, 488 nm and 810 nm generated, which correspond to the deep pink state in electrochromism. The proposed electrochromic mechanism is shown in Supplementary Fig. 65. The plot of transmittance versus wavelength shows that the value of ΔT for NKM-908-TPDC-Py thin film is 68.0% at 850 nm (Supplementary Fig. 54c). The cyclic stability was explored, NKM-908-TPDC-Py thin film attenuated by 2.36% after 10 cycles of experiments with the two-potential-step chronoamperometry (0 V and −2.2 V) (Supplementary Figs. 66 and 67). Combining with the neglectable changes to PXRD patterns and SEM images after cyclicity test (Supplementary Fig. 57e,f), the considerable stability and reversibility of NKM-908-TPDC-Py thin film are strongly demonstrated. The tc and tb of NKM-908-TPDC-Py thin film at 488 nm was calculated to be 110/86 s, which is also slower than the NKM-908 thin film (Supplementary Fig. 68). The CE value of NKM-908-TPDC-Py thin film at 488 nm is calculated as 41.6 cm2 C−1.

Discussion

In conclusion, by further elongating the tetratopic ligand, we synthesized a csq-topological redox active Zr MOF, NKM-908, containing NDI units. The fabricated NKM-908 thin film exhibits reversible electrochromic properties with color changing from an essentially colorless state to light yellow and then to green. Such electrochromic behavior can be further precisely regulated by successfully installing a series of TPDC-X linkers with different substituents into the coordination unsaturated pocket of NKM-908. The installation of redox inactive linkers TPDC-4Me, TPDC-4F and TPDC-2OMe deepens the colors of the corresponding thin films, while new color changes emerge after the installation of redox active linkers TPDC-TDA, TPDC-AN, and TPDC-Py. Our work demonstrates an effective approach of regulating electrochromism in specific MOFs, and provide a quick and convenient platform of screening desired electrochromic behaviors without synthesizing MOFs from scratch. This study sheds light on application-oriented, efficient and convenient construction and property modulation of electrochromic materials.

Methods

Materials and Instrumentation

All reagents and solvents used in the synthesis studies were commercially available and were used without further purification. Ligands were synthesized by the procedures in Supplementary Figs. 2 and 3. 1H NMR and 13C NMR spectra were recorded using Bruker 400 MHz NMR spectrometer, and referenced to resonances of the residual protons in the deuterated solvents. Multiplicities are recorded as: s = singlet, d = doublet, t = triplet, dd = doublet of doublets, br = broad singlet and m = multiplet. High resolution mass spectrometry was performed using Agilent 6545Liquid chromatography quadrupole time of flight mass spectrometer. Gas adsorption measurements were performed on a Micromeritics ASAP 2020 system. Single crystal X-ray diffraction experiments were performed on a Bruker D8-Venture diffractometer equipped with a copper sealed tube (λ = 1.54178 Å). Electron diffraction experiments were performed using a Rigaku XtaLAB Synergy-ED diffractometer equipped with a HyPix-ED detector optimized for operation in the electron diffraction experimental setup and a 200 kV electron source at a wavelength of 0.025 Å. Powder X-ray diffraction (PXRD) experiments were performed on a Bruker D8-Focus Bragg-Brentano X-ray powder diffractometer equipped with a copper-sealed tube (λ = 1.54178 Å) at 40 kV and 40 mA. The calculated PXRD patterns were produced using the Mercury software and single crystal reflection data. The SEM images were examined on the FEI Nova Nano 230 scanning electron microscope. The energy of the electron beam was 15 and 20 keV, respectively. A KU-T6PC UV spectrophotometer (Nanjing KENFAN Electronic Technology Co. Ltd.) was taken for detect UV-vis absorbance.

Synthesis of NKM-908

ZrCl4 (20 mg), H4NDTB (10 mg), benzoic acid (225 mg), and DMF (3 mL) were charged in a Pyrex vial. The mixture was heated in a 120 °C oven for 72 h. After cooling down to room temperature, the pale-yellow crystals of NKM-908 were harvested (yield: 65 %).

Synthesis of NKM-908-TPDC-X

NKM-908 (100 mg), TPDC-X (200 mg) and DMF (20 mL) were charged in a Pyrex vial. The mixture was heated in a 100 °C oven for 24 h. After cooling down to room temperature, the crystals of NKM-908-TPDC-X were harvested (yield: 97 %).

MOF thin film fabrication

First, the ITO glass was cut into rectangle form (60 × 7 mm2), which were subsequently cleaned by successive ultrasonic cleaning in soapy water, ethanol, and acetone for 15 min, respectively. H4NDTB (90.8 mg, 0.094 mmol), ZrOCl2·8H2O (99.9 mg, 0.31 mmol) and benzoic acid (1.275 g) were dissolved in anhydrous DMF (10 mL) and anhydrous methanol (1 mL) in a 20 ml vial. The pre-cleaned ITO substrate was submerged in the solution obliquely with the conductive side facing down. The above operations were carried out at room temperature and pressure. The reaction solution was then heated to 120 °C in a convection oven for 3 days. After cooling down to room temperature, all adherent solids on the non-conductive side were wiped off using a DMF-soaked cotton swab, followed by rinsing the MOF film with dry DMF and drying with a stream of nitrogen.

Single-crystal X-ray diffraction crystallography

Single crystal of NKM-908 was taken directly from the mother liquid without further treatment, transferred to oil, and mounted onto a loop for single crystal X-ray data collection. Single crystal X-ray diffraction experiments were carried on a Bruker Smart Apex diffractometer equipped with a Cu-Kα sealed-tube X-ray source (λ = 1.54178 Å, graphite monochromated) under 100 K by using a ω scan mode. The data frames were recorded using the program APEX3 and processed using the program SAINT routine within APEX3. The data were corrected for absorption and beam corrections based on the multi-scan technique as implemented in SADABS. The structures were solved by direct methods using SHELXS and refined by full-matrix least-squares on F2 using SHELXL software. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms on the aromatic rings were located at geometrically calculated positions and refined by riding. However, the hydrogen atoms for the coordinated molecules cannot be found from the residual electron density peaks and the attempt of theoretical addition was not done. The free solvent molecules are highly disordered in MOFs, and attempts to locate and refine the solvent peaks were unsuccessful. The diffused electron densities resulting from these solvent molecules were removed using the SQUEEZE routine of PLATON; structures were then refined again using the data generated.

Three-dimensional micro electron diffraction crystallography

Electron diffraction data of crystalline particles of NKM-908-TPDC-4F were collected at room temperature using a Rigaku XtaLAB Synergy-ED, HyPix-ED, electron source at 200 keV diffractometer. The sample powder was dispersed in an ethanol solution and sonicated for 5 min, then dropped onto a copper grid, and the collection of 3D electron diffraction data began after the surface of the copper grid was completely dry. Data processing was done using CrysAlisPro, all structures were solved via intrinsic phasing methods using ShelXT and refined with ShelXL within the Olex2 graphical user interface.

N2 adsorption isotherms

N2 adsorption measurements were performed using a Micromeritics ASAP 2020 surface area and pore size analyzer. Before sorption experiments, as-synthesized samples were washed with DMF several times to remove unreacted starting ligands and inorganic species. Afterwards, the crystals were carefully decanted and washed with DMF and acetone several times. Then the samples were activated under vacuum at 80 °C for 10 h. Low-pressure N2 adsorption isotherms were measured at 77 K in a liquid nitrogen bath. The specific surface areas were determined using the Brunauer–Emmett–Teller model from the N2 sorption data.

1H NMR spectroscopy

For 1H NMR spectroscopy, the activated samples (around 5 mg) in 4 mL vials were digested with one drop of D2SO4-d2. About 0.5 mL DMSO-d6 was added to the vial and the mixture was sonicated for 5 min before the upper clear solution was collected for NMR measurement.

Electrochromic tests

Cyclic voltammetry (CV) curves were recorded by a CHI760E potentiostat. All measurements were performed using a standard three-electrode system. All measurements were performed in 0.1 M [(nBu)4N]PF6/DMF purged with nitrogen for 30 min. The working electrode is NKM-908 or NKM-908-TPDC-X thin film ITO electrode, the reference electrode is Ag/AgCl, and the platinum mesh electrode is used as the counter electrode. The CV test of the ligand was performed using a glassy carbon electrode (electrode area: 0.07068 cm2), with a solution of 1 mM H4NDTB containing 0.1 M [(nBu)4N]PF6 in DMF. UV-vis spectroscopy test: A 10 mm path length quartz cuvette was used to test the UV-vis spectroscopy of the thin film electrode, with platinum wire as the counter electrode, Ag/AgCl as the reference electrode, and 0.1 M [(nBu)4N]PF6/DMF as the electrolyte.