Photoswitchable Zirconium MOF: Comparison
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Please note this is a comparison between Version 1 by Vera V. Butova and Version 2 by Camila Xu.

UiO-66-NH2 is a metal–organic framework (MOF), which is constructed of zirconium and amino-terephthalate ions. Modification of MOFs with photochromic compounds allows managing their gas capacity and directing sorption-desorption processes. Photochromic molecules are able to reverse their configuration under UV‐light irradiation affecting available pore volume. The modification of UiO‐66‐NH2 with diarylethene molecules (DAE, 4‐(5‐Methoxy‐1,2‐dimethyl‐1H‐indol‐3‐yl)‐3‐(2,5‐dimethylthiophen‐3‐yl)‐4‐furan‐2,5‐dione) results in the formation of new photoswitchable material for light-driven H2 storage. Most of the DAE molecules inside of the UiO‐66‐pores had an open conformation after synthesis. However, the equilibrium was able to be shifted further toward an open conformation using visible light irradiation with a wavelength of 520 nm. Conversely, UV‐light with a wavelength of 450 nm initiated the transformation of the photoresponsive moieties inside of the pores to a closed modification. We have shown that this transformation could be used to stimulate hydrogen adsorption–desorption processes. Specifically, visible light irradiation increased the H2 capacity of modified MOF, while UV‐light decreased it. A similar hybrid material with DAE moieties in the UiO‐66 scaffold was applied for hydrogen storage for the first time. Additionally, the obtained results are promising for smart H2 storage that is able to be managed via light stimuli.

  • metal–organic frameworks
  • secondary building units
  • UiO-66

1. Introduction

Metal–organic frameworks (MOFs) are porous materials that have recently attracted a great deal of attention do their high potential for use in molecule design [1][2]. These materials are constructed from inorganic clusters and organic molecules. The first ones tended to be designated as secondary building units (SBUs). They often contained one metal ion or a group of ions coordinated by oxygen or nitrogen from organic components, which are also known as linkers. Both SBUs and linkers define the geometry and functionality of the resulting framework. The introduction of functional groups via linker molecules results in the pore surface being decorated with them. Because of this, they are able to manipulate the properties of a material in the desired way. 
For modern devices, it is essential to control the material properties via external stimuli. Light is to be considered a preferred type of stimuli because it can be controlled precisely in terms of both wavelength and radiation time[3]. In this way, imparting photoresponsive moieties to MOFs allows their fundamental properties to be managed. Diarylethenes (DAEs) are one of the most promising due to their fatigue-resistant and thermally irreversible performance [4]. These molecules are able to reverse their configuration from that of a colorless open isomer to a colored closed form under UV-light irradiation. During this transformation, chemical bonds are formed between two heterocyclic thiophene groups. Visible light initiates a reverse process [5]. Generally, DAE molecules can be incorporated into the MOF scaffold in two ways: as parts of linker molecules or as guests in the pores. The photochromic process in DAE molecules affects the available pore volume and the energy transfer process, so it was applied to control CO2 adsorption [6][7], gas separation [8][9][10], to manage the fluorescence of porphyrin [11][12] in previous studies. 
UiO-66 is constructed from zirconium ions and terephthalate linkers. Each SBU in UiO-66 contains six Zr4+ ions and bridge oxygen atoms and is able to be represented with a Zr6O4(OH)4 formula unit (Figure 1). Each SBU is coordinated with 12 linker molecules. High connectivity and strong covalent bonds result in the high stability of the UiO-66 scaffold. 
Polymers 13 04052 g001 550
Figure 1.
Schematic representation of the UiO-66 structure according to crystallographic data COD ID 4512072. (
a
) Dark green polyhedral represents the Zr-O cluster; gray spheres represent carbon atoms, dark red represents for oxygen. (
b
) Schematic representation of Zr
6
O
4
(OH)
4
SBU. Both oxygen positions are provided: µ
3
-O (dark red) and µ
3
-OH (orange). Oxygen atoms can alternate to occupy each of them The picture was prepared in Diamond, version 4.6.5.

2. Synthesis

UiO‐66‐NH2 was synthesized according to a previously reported technique [13]. The compound 4‐(5‐Methoxy‐1,2‐dimethyl‐1H‐indol‐3‐yl)‐3‐(2,5‐dimethylthiophen‐3‐yl)‐4‐furan‐2,5‐dione (DAE) was obtained according to previously published procedures [14]. The modification of UiO‐66‐NH2 was performed according to the following scheme (Figure 2).  

Figure 2. Scheme of UiO‐66‐NH2 functionalization.

The modification of UiO‐66‐NH2 was performed according to previously reported methods, with some modifications that were made based on the specific objective of obtaining diarylethenes with pyrrole‐2,5‐dione bridging fragments [35–37]. A solution containing 0.36 mmol of DAE in isopropyl alcohol (30 mL) was treated with UiO‐66‐NH2 (0.06 mmol) and 4‐dimethylaminopyridine (DMAP) (1 mg). The reaction mixture was refluxed for 10 h and cooled, and the solvent was removed by means of distillation. The resulting precipitate was washed with CH2Cl2 until no staining was detected. The obtained sample was designated as DAE‐UiO‐66.

3. Results

Both of the UiO‐66‐NH2 and DAEUiO‐66 samples had cubic symmetry with the Fm‐3m (225) space group. The lattice constants increased from 20.794 Å (UiO‐66‐NH2) to 20.836 Å (DAE‐UiO‐66). This could indicate stress in the framework of the DAE‐UiO‐66 sample due to the pores becoming filled with relatively photochromic molecules. The DAE‐UiO‐66 sample consisted of octahedral crystals that were 60–90 nm in size (Figure 3). In good agreement with the XRD data, we observed that the DAE‐UiO‐66 sample demonstrates lower porosity compared to the initial sample (Table 1).

Figure 3.

(a) XRD profiles of the UiO‐66‐NH

2

(black) and DAE‐UiO‐66 (brown) samples. Intensities are normalized and shifted along the y‐axis for better representation. (b) SEM image of DAR‐UiO‐66 sample.

Table 1.

Some properties of the synthesized samples UiO‐66‐NH

2

and DAE‐UiO‐66. SSA stands for the specific surface

area. 

Sampes' designation Unit cell parameter a=b=c, Å SSA, m2/g DAE content from TGA, mol.% DAE content from XRF, mol.%
UiO‐66‐NH2 20.794(5) 1111 - -
DAE‐UiO‐66 20.836(6) 398 9 8

 Analyzing joint data of TGA and XRF, we proposed the following chemical composition of DAE‐UiO‐66: 

Zr6O4(OH)4(C8H5O4N)5.2(C8H6O4NCl)0.3(C29H23O7N3S)0.5. One octahedral pore of UiO-66-NH2 contains one DAE molecule. 

We have traced the hydrogen capacity for the DAE‐UiO‐66 sample that was consistently irradiated with visible and UV light. The isotherms are presented in Figure 4a. After irradiation with visible light (wavelength 520 nm), the as‐synthesized sample adsorbed 0.58 weight% of hydrogen at 77 K. This value is slightly higher than the H2 capacity for the as‐synthesized sample (0.55 weight%). Irradiation with UV‐light (450 nm) reduced the hydrogen capacity. These changes are reversible because the same trend was reproduced after further irradiations with visible and UV light (Figure 4b). 

Figure 4.

(a) Isotherms of hydrogen adsorption (77 K) for DAE‐UiO‐66 sample irradiated with visible light with a wavelength of 520 nm (green plot) and UV‐light with a wavelength of 450 nm (violet plot). (b) Diagram of changing in H2 capacity during sequential irradiation with visible light with a wavelength of 520 nm (green columns) and UV‐light with a wavelength of 450 nm (violet columns).

4. Discussion

According to the experimental data, we propose that the DAE molecules undergo the following transformations inside of the pores (Figure 5). After the synthesis, most of the DAE molecules are in the “open” modification, as confirmed by UV–Vis spectra. After irradiation with visible light, this equilibrium strongly shifts to the left, resulting in the open form being the dominant DAE modification. The open form increased hydrogen capacity and resulted in pronounced changes in the IR spectrum. Under UV‐irradiation, the DAE molecules underwent a “closed” modification, which was determined according to

UV–Vis spectroscopy. Additionally, this resulted in the DAE‐UiO‐66 sample having a lower H2 capacity.

Figure 5. Scheme of DAE transformations inside DAE‐UiO‐66 pores under UV‐light (hν1) and visible light (hν2).

5. Conclusions

In summary, we modified UiO‐66‐NH2 with the photoactive DAE molecule 4‐(5‐Methoxy‐1,2‐dimethyl‐1H‐indol‐3‐yl)‐3‐(2,5‐dimethylthiophen‐3‐yl)‐4‐furan‐2,5‐dione. The obtained material was highly crystalline and was isostructural to the initial UiO‐66‐NH2 scaffold. An increase in the lattice constants was caused by stress due to the molecules in the pores. The obtained material, DAE‐UiO‐66, demonstrated a preserved porous structure with SSA of almost 400 m2/g. According to a complex analysis of the IR‐spectra, UV–Vis spectra, and hydrogen adsorption isotherms, we can conclude that the as‐modified sample contained a mixture of two forms of DAE‐molecules—”open” and “closed”. The equilibrium was shifted to the “open” modification from the very beginning. However, some changes could be observed after the irradiation of DAE‐UiO‐66 with a visible light source, initiating the transformation of the DAE fillers in the pores to the open conformation. This increased the hydrogen capacity. We measured the hydrogen capacity under UV/Vis irradiation and observed a reversible process that was able to be managed via light stimuli. Specifically, visible light increases the H2 capacity, while UV‐irradiation decreases it. We suppose that this result is an important step in the development of smart hydrogen storage with light‐induced safe H2‐desorption.

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