Remote-controllable functional molecules can change their chemical or physical properties as a response to external stimuli . Since light is a fast, tunable, handy and usually non-destructive stimulus, photoswitchable molecules attract particular attention in biological and material science , , . The most extensively investigated photoswitchable molecule is azobenzene, which usually can undergo light-induced isomerization when it is in solution or when it is incorporated in polymers or dispersed on a surface , , . By irradiation with UV light, the thermodynamically stable trans azobenzene can be switched to the cis state. The cis azobenzene can isomerize back to the trans state either by irradiation with visible light or by thermal relaxation. The thermal relaxation process is crucial for the stability of the photoswitches, determining the life time of the cis state. The molecular environment of the azobenzene has a significant impact on the isomerization process and on the activation energy . As a result, the lifetime of the cis azobenzene can vary by many orders magnitude. On the other hand, pure azobenzene, C12H10N2, which has a melting point of about 70°C, shows no photoisomerization behavior in the solid, crystalline form. Thus, the experimental investigation of the isomerization of pure azobenzene presents a very challenging task.
Metal-organic frameworks, MOFs, are nanoporous, crystalline hybrid materials composed of metal nodes connected by organic linker molecules , , , . MOFs possess several unique properties such as a very high specific surface area, flexibility and variety. Thin well-defined MOF films can be prepared directly on the substrate surface by means of liquid-phase epitaxy in a layer-by-layer (lbl) fashion, see Figure 1a. These nanoporous thin films are often referred to as surface-mounted MOFs, SURMOFs , .
The unique properties of SURMOFs are briefly reviewed here: the layer-by-layer MOF film growth results in a precise control of the thickness, see Figure 1a . Controlled lateral patterning, e.g. by lithography , and hetero-epitaxy of multi-layer-MOF films, by exchanging the linker molecules and/or metal nodes during the synthesis , are possible. The crystalline growth direction of the MOF can be controlled by the substrate functionalization . Surface-sensitive spectroscopic techniques like X-ray photoelectron spectroscopy (XPS)  and infrared reflection–absorption spectroscopy (IRRAS) can be applied. A quartz crystal microbalance  can be used for quantifying the mass changes and investigating the diffusion of guest molecules in the nanopores , .
The incorporation of azobenzene as a pendant side to the MOF structures enables the preparation of photoswitchable materials , . It was demonstrated that SURMOFs with azobenzene side groups can be used for various purposes: as membranes which allow the remote-controlled switching of the separation performance , for the remote-controlled release of guest molecules from multi-layered nanoporous films  as well as for switching the adsorption capacity . Using fluorinated azobenzene side groups enables the switching of the MOF material properties with visible light only, avoiding UV light . As a result, the proton conduction properties of the guest molecules can be remote-controlled . The experimental data of the well-defined, crystalline model system allowed us to compare the results with theoretical data in detail. There, steric hindrance of the azobenzene side groups was identified in certain MOF structures . Recently, we used thin MOF films which possess virtually isolated azobenzene pendant groups to investigate the thermal cis-to-trans isomerization . Since the experiments were performed in ultrahigh vacuum, solvent effects were avoided and it serves as a valuable model system for the isomerization of isolated azobenzene molecules. For instance, Saalfrank and coworkers employed it as an experimental benchmark system. Their detailed calculations of the isomerization process were compared with experimental data, enabling a critical evaluation of the applicability of Eyring transition state theory .
In this article, we use thin MOF films with a pillared-layer Cu2(BDC)2(AzoBiPyB) structure, Figure 2 (AzoBiPyB: 4,4′-(2-(phenyldiazenyl)-1,4-phenylene)dipyridine) ; BDC: benzene-1,4-dicarboxylic acid). The azobenzene is connected by one phenyl ring to the framework, while the other phenyl ring points into the pore. The pore size is sufficiently large enough to enable the isomerization of the azobenzene. However, the azobenzene groups may interact with each other due to the small pore size. Here, we investigate the thermal cis-to-trans isomerization in a temperature range of 80–100°C in an argon gas atmosphere as well as in liquid butanediol.
The Cu2(BDC)2(AzoBiPyB) SURMOF, Figure 2a, was prepared on the LSPR substrate in the layer-by-layer fashion by subsequently exposing the sample to the solution of the metal nodes [copper(II) acetate in ethanol with a concentration of 1 mM; 15min] and to the solution of the organic linker molecules (AzoBiPyB and BDC in ethanol with a concentration of 0.2mM; 30min). In between, the sample was rinsed with pure ethanol for 2min. The sample was prepared in 100 synthesis cycles at a temperature of 50°C. Before the synthesis, the substrate was functionalized using an oxygen plasma. The crystallinity and the oriented growth of the SURMOF film can be seen from the X-ray diffractogram, Figure 2b. The X-ray diffractograms were recorded in θ–θ geometry with a Bruker D8 Advance diffractometer using CuKα radiation (0.154 nm wavelength).
Localized surface plasmon resonance (LSPR) is sensitive to the changes of the optical density in the close proximity to the sensing layer, i.e. close to the layer of the gold nanodiscs of the LSPR sensor . The setup is sketched in Figure 1b and c, more information and details of LSPR can be found in refs. , , , , . It was shown that LSPR is also sensitive to the cis-trans isomerization of azobenzene and allows their examination . Since the SURMOF is grown directly on the LSPR sensor, the LSPR signal is sensitive to changes of the optical density of the SURMOF. Here, an XNano device from Insplorion© was used to record the position (centroid) of the LSPR frequency. Previously, we used the XNano device to investigated multi-component uptake of dye molecules by transparent ,  MOF films .
The sample was placed in the LSPR cell and was equilibrated at temperatures of 80°C, 90°C and 100°C, which is the highest-possible temperature in the device. The azobenzene-SURMOF was irradiated in the closed cell by UV light of 365nm for approximately 10min, resulting in trans-to-cis-photoisomerization of the azobenzene groups. After the UV irradiation, the shift of the LSPR frequency, which measures the cis-trans isomerization , was recorded.
3 Results and discussion
Typical LSPR wavelength versus time curves are shown in Figure 3a. The data can be fitted with a mono-exponential decay function and the rate constant can be determined. By measuring the rate constants at different temperatures, i.e. 80°C, 90°C and 100°C, the activation energy can be calculated using an Arrhenius plot, Figure 3b.
The activation energies of the thermal relaxation of cis azobenzene side groups in the Cu2(BDC)2(AzoBiPyB) SURMOF in argon gas and liquid butanediol are shown in Figure 4. The activation energy in pure argon, 1.17±0.20eV, is very similar to the activation energy in ultrahigh vacuum, previously determined by infrared spectroscopy, 1.18±0.12eV. This finding was expected since the amount of argon molecules at room temperature and atmospheric pressure is extrapolated to be very small, meaning the pores are virtually empty . Please note that the azobenzene side groups in Cu2(BDC)2(AzoBiPyB) are not fully isolated and may interact with each other. For comparison, in Cu2(DMTPDC)2(AzoBiPyB) with large pores, where the azobenzene moieties are fully isolated, an activation energy for the thermal cis-to-trans isomerization of 1.09±0.09eV was previously determined . Unfortunately, the preparation of large pore Cu2(DMTPDC)2(AzoBiPyB) SURMOF on the LSPR substrates did not result in crystalline thin films, possibly due to the substrate roughness or functionalization.
The activation energy for the thermal relaxation in the liquid butanediol environment is 1.26±0.15eV, which is slightly larger than in the empty pores.
For determining the plasmon resonance frequency, the sample is irradiated with visible light of relatively low intensity, which, in principle, may also result in the cis-to-trans isomerization. Since the rate constants determined by plasmon resonance frequency corresponds to the rate constants determined by infrared spectroscopy, it can be assumed that the low-intensity-visible-light irradiation has no significant impact on the trans-cis isomerization in the considered temperature and, thus, rate-constant range.
The thermal cis-to-trans relaxation of azobenzene side groups in MOF thin films is investigated by employing LSPR. The energy barrier for the cis-to-trans-relaxation of the azobenzene side groups in Cu2(BDC)2(AzoBiPyB) in liquid butanediol, 1.26±0.15eV, is slightly larger than in argon, 1.17±0.2eV, which corresponds to the previously determined value in vacuum.
This setup represents a comparatively simple and inexpensive approach for measuring the thermal cis-to-trans isomerization of isolated azobenzene side groups in MOFs. In addition, it allows the investigation in different media such as gas, vapor or liquids. Although the precision of the data is slightly reduced in comparison to the previous IR experiments in UHV and the temperature range is limited, it is a valuable tool for investigating the isomerization in thin nanoporous films.
We are grateful for the support by the Volkswagenstiftung and the DFG (SFB 1176).
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About the article
Published Online: 2018-01-05
Published in Print: 2018-12-19