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Cite This: J. Phys. Chem. C 2018, 122, 19044−19050

Series of Photoswitchable Azobenzene-Containing Metal−Organic Frameworks with Variable Adsorption Switching Effect Zhengbang Wang,†,⊥ Kai Müller,† Michal Valaś ě k,‡ Sylvain Grosjean,§,# Stefan Bräse,∥,# Christof Wöll,† Marcel Mayor,‡,∇ and Lars Heinke*,† †

Institute of Functional Interfaces (IFG), ‡Institute of Nanotechnology (INT), §Institute of Biological Interfaces 3 (IBG3), and Institute of Toxicology and Genetics (ITG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ⊥ Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, China # Institute of Organic Chemistry (IOC), KIT, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany ∇ Department of Chemistry, University of Basel, St. Johannsring 19, CH-4056 Basel, Switzerland

J. Phys. Chem. C 2018.122:19044-19050. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 09/01/18. For personal use only.

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S Supporting Information *

ABSTRACT: Nanoporous metal−organic frameworks (MOFs) equipped with light-responsive azobenzene pendant groups present a novel family of smart materials, enabling advanced applications like switchable guest adsorption, membranes with tunable molecular separation factors, and photoswitchable proton conduction. Although it is obvious that for small pore sizes, steric constraints may prohibit azobenzene switching, guidelines for optimizing the MOF architecture to achieve large switching effects have not yet been established. Here, a series of five different photoswitchable azobenzene-containing pillared-layer MOF structures is presented. The switching effect is quantified by the light-induced increase of the uptake amount of butanol as the probe molecule. For fast and reproducible measurements, thin well-defined MOF films, referred to as surface-mounted MOFs (SURMOFs), were used in combination with a quartz crystal microbalance. Although the series comprises similar MOF structures, the magnitude of the switching effect considerably differs, here by a factor of 5. The uptake data show that, rather than the pore size or the number of azobenzene molecules per pore, the density of azobenzene per pore volume is crucial. The finding that a large switching effect is reached for a high density of azobenzene moieties per MOF unit cell provides the basis for further applications of photoswitchable MOFs and SURMOFs.

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INTRODUCTION Integration of photoswitchable elements into molecular materials enables the remote control over their chemical and physical properties and thus attracts increasing attention.1−3 Examples of such smart, light-responsive materials are polymers,4,5 liquid crystal films,6,7 and self-assembled monolayers (SAMs),8,9 for which properties like wettability, mechanical strength, or work function can be switched by light irradiation. Recently, a new class of materials has been demonstrated to be well suited for the incorporation of photoswitchable molecules, metal−organic frameworks (MOFs). These nanoporous, crystalline hybrid materials are assembled from metal or metal/oxo nodes, which are connected by organic linker molecules.10,11 Additional functionality can be incorporated into MOFs by incorporating photoswitchable molecules, thus yielding remote-controllable crystalline materials. In particular, the incorporation of azobenzene, which can be switched from its nonpolar planar trans form to its polar bent cis form by UV light irradiation and © 2018 American Chemical Society

back from cis to trans by irradiation with visible light or thermal relaxation,5,12 as shown in Figure 1a, enables the remote control of various functions.13,14 On one hand, the embedment of the photoswitches in the MOF pores is a straightforward realization, allowing moderate switching effects.15−18 On the other hand, the chemical attachment of the photoswitches to the MOF scaffold, for instance as pendant side groups, enables the control over the position and distribution of the smart moieties, typically resulting in larger switching effects than by simple embedment.19−21 (For instance, compare refs 17 and 22 or refs 18 and 23.) In addition to switching the color of the material, MOF structures with azobenzene side or pendant groups were used to photoswitch the adsorption capacities19,20 as well as the diffusion properties21,24 of the guest molecules in the pores. Moreover, membranes of MOFs with azobenzene Received: June 19, 2018 Revised: July 18, 2018 Published: July 19, 2018 19044

DOI: 10.1021/acs.jpcc.8b05843 J. Phys. Chem. C 2018, 122, 19044−19050

Article

The Journal of Physical Chemistry C

mization resulting in an increase of the trans−cis switching effect were not yet presented. In this work, a series of photoswitchable MOFs is presented and the light-induced trans−cis photoswitching effect is studied and optimized. For this purpose, we use thin, well-defined MOF films, also referred to as surface-mounted MOFs (SURMOFs) prepared by employing a layer-by-layer process.30−32 Five different pillared-layer MOF structures, which are composed of copper(II)-acetate-paddle-wheel nodes connected by ditopic carboxylic acid linkers and ditopic nitrogen-terminated linkers possessing azobenzene side groups, were presented and investigated; see Figure 1b and Table 1. For comparison, one MOF structure without azobenzene groups was also examined. A high stability of the scaffold of the azobenzene-containing MOF films without significant changes of the X-ray diffraction (XRD) patterns was already found in previous studies.22,23,28,33 Here, we focused on quantifying and optimizing the effect of trans−cis-azobenzene switching on the adsorption capacity of the MOF. The uptake amount was measured with a quartz crystal microbalance (QCM),34,35 allowing a straightforward quantification of the adsorption and diffusion properties of the guest molecules in thin MOF films.36 The QCM experiments were performed in a closed cell directly after the SURMOF synthesis, minimizing the exposure to potentially harmful conditions, like humid air, which may cause diffusion-hindering defects in MOFs.37 Butanol was chosen as a probe molecule due to its size and polar OH group, which is crucial for the adsorption-capacity switching effect.23 By comparing the trans−cis change of the uptake amount of the probe molecule by the azobenzene-containing MOF film, it was found that, despite the fact that the MOF structures are very similar, the switching effect varies by a factor of 5. Moreover, a correlation between the switching effect and the density of azobenzene moieties in the MOF structure was unveiled, giving a guidance to increase the switching impact.

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EXPERIMENTAL SECTION The pillared-layer SURMOF films were fabricated on goldcoated QCM sensors, which were functionalized with an 11mercapto-1-undecanol self-assembled monolayer to direct and improve the SURMOF growth.38,39 The SURMOF films were prepared in a step-by-step fashion by alternately immersing the substrate into the solutions of the metal nodes and into the solutions of the organic linker molecules. In between, the samples were rinsed with pure ethanol. This layer-by-layer process typically results in a linear increase of the film thickness.32,39 The metal solutions were 1 mM ethanolic copper(II) acetate solutions. The linker solutions were AzoBPDC and dabco, AzoTPDC and dabco, DMTPDC and AzoBiPyB, AzoBPDC and AzoBiPyB, BDC and AzoBiPyB, as well as BPDC and dabco, respectively, with concentrations of 0.1 mM each. The abbreviations are as follows: AzoBiPyB = (E)-2-(phenyldiazenyl)-1,4-bis(4-pyridyl)benzene, AzoTPDC = (E)-2′-(phenyldiazenyl)-[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylic acid, AzoBPDC = (E)-2-(phenyldiazenyl)-[1,1′biphenyl]-4,4′-dicarboxylic acid, DMTPDC = 2,2″-dimethyl[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylic acid, BPDC = [1,1′biphenyl]-4,4′-dicarboxylic acid, BDC = 1,4-benzenedicarboxylic acid (terephthalic acid), and dabco = 1,4diazabicyclo[2.2.2]octane. BPDC, BDC, and dabco were purchased from Sigma-Aldrich and were used without further purification, whereas the other linker molecules were custom-

Figure 1. (a) Isomerization of azobenzene. By irradiation with UV light, azobenzene in the nonpolar, planar trans form undergoes isomerization to the nonplanar cis form with a dipole moment of 3 D. By thermal relaxation or by irradiation with visible light, cisazobenzene undergoes isomerization to the thermodynamically stabletrans form. (b) Sketch of the series of photoswitchable MOF structures with the azobenzene side groups in the trans form. The azobenzene-free reference surface-mounted MOF (SURMOF) structure (bottom right) is separated by dotted lines. The name of the pillared-layer structure gives the molecular components of the MOFs; see Experimental Section.

side groups were used to control the membrane permeance and the separation factor in a dynamic, remote-controlled fashion.22,25 The remote control of the proton-conduction properties in nanoporous materials is also possible with these smart materials.26 So far, more than a dozen different MOF structures with azobenzene side groups have been published, summarized in a recent review.27 Although it was already found that the photoinduced azobenzene isomerization is sterically hindered when the pore size is too small,28,29 systematic investigations and guidelines for structural opti19045


Article

Cu2(AzoBPDC)2(dabco)

2.11 2 0.94 4.11 3 0.73

Cu2(AzoBPDC)2(AzoBiPyB)

3.49 2 0.57 6.85 1 0.15

Cu2(BDC)2(AzoBiPyB) Cu2(AzoTPDC)2(dabco) Cu2(DMTPDC)2(AzoBiPyB)

2.16 1 0.46

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RESULTS AND DISCUSSION The series of photoswitchable MOF films comprises similar structures with different pore sizes and different amounts of azobenzenes per pore; see Table 1. While the volume of the smallest MOF unit cell, Cu2(AzoBPDC)2(dabco), is 2.1 nm3, the unit cell volume of the largest MOF, Cu2(DMTPDC)2(AzoBiPyB), amounts to 6.8 nm3. On the other hand, the amount of azobenzene varies between 1 moiety and 3 moieties per pore. UV irradiation of the sample results in trans-to-cis isomerization of the azobenzene side group. On the basis of a previous publication,28 we assume the trans−cisisomerization follows a rotation pathway. By infrared spectroscopy with three exemplary SURMOF structures of different pore sizes with the azobenzene side group connected to the layer linker and/or to the pillar linker, the switching yield was determined, as shown in Figure 2. The maximum amounts of cis-azobenzene are approximately 64 ± 1%. The error is the standard deviation of the results, see Figure 2; systematic errors of the IR measurements may add. This

2.12 0 0

Cu2(BPDC)2(dabco)

made. The syntheses of AzoBiPyB, AzoBPDC, and DMTPDC were previously described.21,28 The synthesis of AzoTPDC is described in Supporting Information Section S1. The SURMOF samples were prepared in 60 synthesis cycles. The volumes of the MOF unit cells and the number of azobenzene moieties per pore are presented in Table 1. X-ray diffraction was used for structural investigation of the SURMOF samples. The X-ray diffractograms (Section S2) show that all samples were crystalline with the targeted MOF structure. The XRD pattern is unaffected by the trans−cis switching of the azobenzene side groups; see Figure S3. The vibrational spectra were obtained by infrared reflection absorption spectroscopy using a Bruker Vertex 80 spectrometer with a grazing incident reflection angle of 80° relative to the surface normal. Reference measurements were done with a perdeuterated hexadecanethiol-SAM on a gold surface. The uptake amount was investigated using a QCM.34,35 The QCM cell was connected to the gas flow system with argon as the carrier gas.36 The pure argon gas flow was instantly switched to the gas flow enriched with the guest molecules, here, 1-butanol vapor, resulting in the guest-molecule uptake by the MOF thin film. The uptake amount by the MOF was quantified by the QCM in a straightforward fashion by measuring the frequency shift and thus determining the mass changes of the sample. A sketch of the setup is shown in Figure S4. Before each uptake experiment, the sample was activated in pure argon flow at 60 °C for 12 h. In addition to emptying the pore space, the activation also results in a thermal relaxation of the sample to the trans state. Prior to the uptake by the cisazobenzene SURMOF, the samples were irradiated with light of wavelength 365 nm from a UV LED for 20 min. Each QCM uptake experiment was performed three times; the average values with the standard deviations are presented. During the QCM experiments, the samples remain in the closed cell under argon atmosphere, preventing the exposure to (humid) air. From the distance between the light-fiber and the sample (approximately 5 cm) and the optical power of the LED (81 mW, radiant flux), we estimate a light intensity of roughly 4 mW cm−2. Since the time constant for the trans-to-cis switching of the azobenzene SURMOF samples under irradiation with these conditions was determined as approximately 8 min,22 it can be assumed that the maximum amounts of cis-azobenzene were obtained in all samples after 20 min.

unit cell volume/nm3 number of azobenzene moieties per pore density of azobenzene per nm3

Table 1. Series of Photoswitchable Azobenzene-Containing MOFs with Volume of the MOF Unit Cell, the Number, and the Density of Azobenzene Moieties per Pore in the Defect-Free Structure


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reference SURMOF before and after UV irradiation are essentially identical with deviations of less than 1.5%, which may also serve as a reference for the accuracy of the uptake measurements. On the other hand, the uptake amounts of all azobenzene-containing SURMOFs are clearly increased upon trans-to-cis isomerization by UV irradiation. The increase of the uptake amount is a result of the attractive interaction between the polar cis-azobenzene and the polar OH group of the guest molecule,23 which was confirmed by a previous infrared spectroscopy study and supported by density functional theory calculations.26 Interestingly, although the MOF structures are similar, the switching effect substantially differs. The switching effect in Cu2(BDC)2(AzoBiPyB) is only 5%, but the butanol uptake amount in Cu2(AzoBPDC)2(dabco) increases by approximately 27% upon trans-to-cis switching of the azobenzene. In Figure 4, the switching effect, meaning the increase of butanol uptake upon trans-to-cis isomerization of the azobenzene side groups, is plotted as a function of the density of azobenzene groups per volume. Unlike plots of the switching effect versus the pore size or versus the number of azobenzene moieties per pore (Figure S5a,b), Figure 4 shows a clear trend that the switching effect increases by increasing the density of azobenzene moieties per volume. It should be noted that a precise mathematical relation for the trend line based on a physical model has not yet been derived and remains a future task. Small deviations from the general trend line are presumably caused by detailed features of the MOF structures in combination with details of the molecular interaction (in addition to the dipole−dipole interaction with cis-azobenzene), resulting in different orientations and positions of the guest molecules in the pores. Please note that the plot of the switching effect versus the density of azobenzene per free pore volume (Figure S5c) does not show an improved correlation compared with Figure 4. Also noteworthily, unlike the uptake of butanediol,21 the diffusion coefficient and thus the rate constant during the butanol uptake is not significantly influenced by the trans−cis isomerization of the azobenzene SURMOF. The different time-scales of the uptake (see Figure 3) are a result of the different diffusion coefficients due to the different pore sizes and different diffusion pathways. Presumably, defects such as surface barriers37 may also affect the uptake time constants. These defects are not affected by the trans−cis-azobenzene switching and thus cancel out at the trans−ciscomparison. High azobenzene densities can be realized with a large number of azobenzene-containing linkers, such as in Cu2(AzoBPDC)2(AzoBiPyB) with three azobenzenes per unit cell, or by relatively small pore size, such as in Cu2(AzoBPDC)2(dabco) with two azobenzenes per unit cell. The trend line or guidance also allows evaluation of whether the switching effect of published azobenzene-containing MOFs can be further improved. For example, on one hand, the CO2 adsorption switching in Azo-MOF-520 seems already excellent due to the high azobenzene density. On the other hand, the effect of the CO2 adsorption switching in Azo-UiO-6824 might be further increased by increasing the azobenzene density, e.g., by decreasing the pore size, resulting in Azo-UiO-6629 or AzoUiO-67.

Figure 2. Infrared spectra of Cu 2 (BDC) 2 (AzoBiPyB) (a), Cu2(AzoBPDC)2(dabco) (b), and Cu2(AzoBPDC)2(AzoBiPyB) (c) SURMOFs in the pristine trans state (black) and after UV irradiation with light of 365 nm for 20 min (red). The inset shows a magnification. The analysis of the trans-azobenzene band at about 720 cm−1 (see refs 33, 40) shows that approximately 64.5% of azobenzene isomerizes to the cisstate in (a), 63.8% cis in (b), and 63.0% cis in (c). The large (unaffected) vibration bands, e.g., at 750, 775, 814, 850, and 868 cm−1, result from the MOF scaffold.22,33

photostationary state was obtained for all investigated SURMOFs, where the azobenzene groups are connected to the dicarboxylate (layer) linker or/and to the dipyridyl (pillar) linker. This verifies the assumption that the isomerization of the azobenzene side groups, if not sterically hindered,28 is not affected by the pore size or by the position of the azobenzene. Typical uptake curves of butanol by the five azobenzenecontaining SURMOFs as well as by the non-photoswitchable reference SURMOF are shown in Figure 3. The uptakes by the

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CONCLUSIONS The effect of light-induced trans−cis switching on the uptake of guest molecules in a series of five different MOF thin films with 19047


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Figure 3. Butanol uptake by the series of azobenzene-SURMOFs shown in Figure 1a (and in Table 1) in the trans state (black) and in the cis state (red).

unit cell. This finding may be used as guidance to further improve the switching effect and to further explore the potential of photoswitchable nanoporous materials, such as MOFs and COFs, which goes beyond the switching of the adsorption amount of vapors. It can be foreseen that the improvement of the switching effect on the adsorption capacity of various gases,19,20,41 on the diffusion of guest molecules,21,24 on the molecular selectivity of membranes,22 as well as on the photoswitchable proton conduction follows similar optimization guidelines.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05843.

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Synthetic details, XRD, infrared, and uptake data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Figure 4. Switching effect, i.e., ratio of the butanol uptake amounts by the azobenzene-SURMOFs in the cis and in the trans state mcis:mtrans, versus the density of azobenzene moieties per volume. The dotted line represents an estimation of the trend line.

ORCID

Michal Valásě k: 0000-0001-9382-6327 Stefan Bräse: 0000-0003-4845-3191 Christof Wöll: 0000-0003-1078-3304 Marcel Mayor: 0000-0002-8094-7813 Lars Heinke: 0000-0002-1439-9695

azobenzene side groups was quantified by using a quartz crystal microbalance. In all azobenzene-MOFs, the adsorption capacities of the probe molecule, butanol, were found to increase upon trans-to-cis-azobenzene isomerization. Despite the fact that the series of azobenzene-containing MOFs possess similar structures, the switching impact substantially differs. The data revealed a trend that a large switching effect is reached for a high density of azobenzene moieties per MOF

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Volkswagenstiftung, Helmholtz Research Program STN (Science and Technology of Nanosystems) and 19048


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The Journal of Physical Chemistry C the DFG (SFB 1176, projects C4 and C6) for financial support.

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ABBREVIATIONS MOF, metal−organic framework; AzoBiPyB, (E)-2-(phenyldiazenyl)-1,4-bis(4-pyridyl)benzene; AzoTPDC, (E)-2′-(phenyldiazenyl)-[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylic acid; AzoBPDC, (E)-2-(phenyldiazenyl)-[1,1′-biphenyl]-4,4′-dicarboxylic acid; DMTPDC, 2,2″-dimethyl-[1,1′:4′,1″-terphenyl]4,4″-dicarboxylic acid; BPDC, [1,1′-biphenyl]-4,4′-dicarboxylic acid; BDC, 1,4-benzenedicarboxylic acid (terephthalic acid); dabco, 1,4-diazabicyclo[2.2.2]octane

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