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A Series of Photoswitchable Azobenzene-Containing MetalOrganic Frameworks with Variable Adsorption Switching Effect Zhengbang Wang, Kai Müller, Michal Valášek, Sylvain Grosjean, Stefan Brase, Christof Wöll, Marcel Mayor, and Lars Heinke J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05843 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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The Journal of Physical Chemistry

A Series of Photoswitchable Azobenzene-Containing Metal-Organic Frameworks with Variable Adsorption Switching Effect Zhengbang Wang,1,2 Kai Müller,1 Michal Valášek,3 Sylvain Grosjean,4,5 Stefan Bräse,4,6 Christof Wöll,1 Marcel Mayor,3,7 Lars Heinke1,* 1 Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-HelmholtzPlatz 1, 76344 Eggenstein-Leopoldshafen, Germany, E-mail: [email protected] 2 Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, China. 3 Institute of Nanotechnology (INT), KIT, Hermann-von-Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany 4

Institute of Organic Chemistry (IOC), KIT, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany

5 Institute of Biological Interfaces 3 (IBG3), KIT, Hermann-von-Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany 6 Institute of Toxicology and Genetics (ITG), KIT, Hermann-von Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany 7

Department of Chemistry, University of Basel, St. Johannsring 19, CH-4056 Basel, Switzerland

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. While 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 5 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 probe molecule. For fast and reproducible measurements, thin well-defined MOF films, referred to as 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.

Introduction Integration of photoswitchable elements into molecular materials enable the remote-control over their chemical and physical properties and thus attract increasing attention.1-3 Examples of such smart, light-responsive materials are polymers,4-5 liquid crystal films6-7 and self-assembled monolayers,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 remotecontrollable crystalline materials. In particular, the incor-

poration of azobenzene, which can be switched from its nonpolar planar trans form to its polar bent cis form by UV light irradiation and back from cis to trans by irradiation with visible light or thermal relaxation,5, 12 Figure 1a, enables the remote control of various functions.13-14 On the 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 ref.17 and ref.22 or ref.18 and ref.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 prop-

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erties21, 24 of the guest molecules in the pores. Moreover, membranes of MOFs with azobenzene 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 While it was already found that the photo-induced azobenzene isomerization is sterically hindered when the pore size is too small,28-29 systematic investigations and guidelines for structural optimization 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 surfacemounted 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 XRD patterns was already found in previous studies.2223, 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.37Butanol was chosen as 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.

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, cis azobenzene undergoes isomerization to the thermodynamically stable trans form. b) Sketch of the series of photoswitchable MOF structures with the azobenzene side groups in the trans form. The azobenzene-free reference 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. 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 38-39 and improve the SURMOF growth. The SURMOF films were prepared in a step-by-step fashion by alternately im2


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The Journal of Physical Chemistry mersing 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 32, 39 the film thickness. The metal solutions were 1 mM ethanolic copper(II) acetate solutions. The linker solutions were AzoBiPyB 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 followed: 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 = 1,4benzenedicarboxylic acid (terephthalic acid) and dabco = 1,4diazabicyclo[2.2.2]octane. While BPDC, BDC and dabco were purchased from Sigma Aldrich and were used without further purification, the other linker molecules were custommade. The syntheses of AzoBiPyB, AzoBPDC and DMTPDC 21, 28 were previously described. The synthesis of AzoTPDC is described in the supporting information S1. The SURMOF samples were prepared in 60 synthesis cycles. The volumes of the MOF unit cells and the numbers of azobenzene moieties per pore are presented in table 1.

the LED (81 mW, radiant flux), we estimate a light intensity -2 of roughly 4 mW cm . Since the time constant for the transto-cis switching of the azobenzene-SURMOF-samples under irradiation with these conditions was determined as approx22 imately 8 min, it can be assumed that the maximum amounts of cis azobenzene were obtained in all samples after 20 min.

X-ray diffraction was used for structural investigation of the SURMOF samples. The X-ray diffractograms, 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 (IRRAS) 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. 34-35

The uptake amount was investigated using a QCM. The QCM cell was connected to the gas flow system with argon as 36 carrier gas. The pure argon gas flow was instantly switched to the gas flow enriched with the guest molecules, here 1butanol 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 cis-azobenzene SURMOF, the samples were irradiated with light of a wavelength of 365 nm from an UV LED for 20 min. Each QCM uptake experiment was performed 3 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

Figure 2: Infrared spectra of Cu2(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 -1 33, 40 band at about 720 cm (see ref. ) shows that approximately 64.5 % of azobenzene isomerizes to the cis state in (a) and 63.8 % cis in (b) and 63.0 % cis in (c). The large (unaf-1 -1 -1 fected) vibration bands, e.g. at 750 cm , 775 cm , 814 cm , -1 -1 22, 33 850 cm and 868 cm , result from the MOF scaffold. 3



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trans-cis-isomerization follows a rotation pathway. By infrared spectroscopy with 3 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, Figure 2. The maximum amounts of cis azobenzene are approximately 64±1%. This 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 azoben28 zene side groups, if not sterically hindered, is not affected by the pore size or by the position of the azobenzene.

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 3 smallest MOF unit cell, Cu2(AzoBPDC)2(dabco), is 2.1 nm , the unit cell volume of the largest MOF, 3 Cu2(DMTPDC)2(AzoBiPyB), amounts to 6.8 nm . 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 28 group. Based on a previous publication, we assume the

Cu2(BPDC)2 (dabco)

Cu2(DMTPDC)2 (AzoBiPyB)

Cu2(AzoTPDC)2 (dabco)

Cu2(BDC)2 (AzoBiPyB)

Cu2(AzoBPDC)2 (AzoBiPyB)

Cu2(AzoBPDC)2 (dabco)

unit cell volume 3 / nm

2.12

6.85

3.49

2.16

4.11

2.11

number of azobenzene moieties per pore

0

1

2

1

3

2

0

0.15

0.57

0.46

0.73

0.94

density of azobenzene per 3

nm

Table 1: The 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.

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).

4


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The Journal of Physical Chemistry

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 reference SURMOF before and after UV irradiation are essentially identical with deviations of less than 1.5%, which may also serve as 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 23 the guest molecule, which was confirmed by previous infrared spectroscopy study and supported by density-functional theory calculations. 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 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 and 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 to Figure 4. 21

Also noteworthy, unlike the uptake of butanediol, 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 path ways. Presumably, defects such 37 as surface barriers may also affect the uptake time constants. These defects are not affected by the trans-cis azobenzene switching and thus cancels out at the trans-cis comparison. High azobenzene densities can be realized with a large number of azobenzene containing linkers, such as in Cu2(AzoBPDC)2(AzoBiPyB) with 3 azobenzenes per unit cell, or by relatively small pore size, such as in Cu2(AzoBPDC)2(dabco) with 2 azobenzenes per unit cell.

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. 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, 20 the CO2 adsorption switching in Azo-MOF-5 seems already excellent due to the high azobenzene density. On the other hand, the effect of the CO2 adsorption switching in Azo-UiO24 68 might be further increased by increasing the azobenzene density, e.g. by decreasing the pore size, resulting in Azo29 UiO-66 or Azo-UiO-67. 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 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 azobenzenecontaining 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 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 19-20, 41 adsorption capacity of various gases, on the diffusion of 21, 24 guest molecules, on the molecular selectivity of mem22 branes as well as on the photoswitchable protonconduction follows similar optimization guidelines.

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Supporting information, i.e. synthetic details, XRD, infrared and uptake data, for this article is given via a link at the end of the document.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

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.

ACKNOWLEDGMENT We thank the Volkswagenstiftung, Helmholtz Research Program STN (Science and Technology of Nanosystems) and the DFG (SFB 1176, projects C4 and C6) for financial support.

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