Unexpected Nonresponsive Behavior of a Flexible Metal-Organic Framework under Conformational Changes of a Photoresponsive Guest Molecule.

In this article, we describe the synthesis, characterization, and optical properties of a photochromic-guest-incorporated metal-organic framework (MOF). The photochromic guest molecule, 2-phenylazopyridine (PAP), was introduced into a pre-synthesized porous crystalline host MOF, [Zn2(1,4-bdc)2(dabco)] n (1). The successful embedment of PAP has been confirmed by elemental analysis, powder X-ray diffraction measurements, IR spectroscopy, etc. The number of PAP molecules per unit cell of host was 1.0, as evidenced by elemental and thermogravimetric analyses of the host-guest composite, 1⊃PAP. The 1⊃PAP composite did not adsorb N2, revealed by the adsorption isotherm of 1⊃PAP, which indicates the pore blockage by the close contact of the host framework with the guest PAP in the trans form. The light-induced trans/cis isomerization with partial reversibility of the guest molecule (PAP) in this hybrid host-guest compound (1⊃PAP) has been investigated by detailed IR spectroscopy and UV-vis spectroscopy. The structural transformation from tetragonal in 1 to orthorhombic in 1⊃PAP exhibits dynamic nature of the framework upon inclusion of guest in the framework, which remarkably becomes nonresponsive with the photoirradiation of guest PAP, retaining its orthorhombic structure in the photoirradiated complex, 1⊃PAP(UV).

Introduction

Photoswitchable molecules show promising applications in a variety of applied fields, namely, molecular switches, bioimaging, switchable catalysis, optical memory, optical limiting devices, and molecular sensing because of their photoisomerized substantial conformational changes, which induce alteration in their functional properties.[1−7] Extensive investigations have been done on the inherent magnetic, electrochemical, catalytic, and biological properties of the metal complexes incorporating photoswitchable ligands.[8] Metal-organic frameworks (MOFs) have emerged as a potential interesting material in last 20 years because of their ultrahigh porosity, high surface area, wide applicability in methane storage,[9−12] harmful gas adsorption,[13,14] separation of gases,[14,15] and heterogeneous catalysis.[16] Later, solid photochromic metal-organic frameworks have also come up as an intense field of research.[17,18] Stock and co-workers have developed photoresponsive azo compounds, covalently bound to organic linker molecules as a side chain.[19−21] Encapsulation of drug molecules inside the host MOFs has also been studied by Férey et al.[22,23] The thermoresponsive study of a rigid nested MOF with encapsulating guests by introducing a size-matching ligand as bolt to lock the pores of MOF was investigated by Bu.[24,25] Studies also revealed that metal nanoparticles inside the host MOFs can be obtained through the decomposition of inserted precursor molecules into MOFs.[26,27] Furthermore, nanoscale titanium particles were prepared as a guest inside MOF-5 by the solvent-free adsorption method using titanium isopropoxide.[28] However, the embedment of larger functional molecules in MOFs is scarce. Among the photoswitchable molecules, mainly MOFs incorporating azobenzene (AB) have been studied intensely because it has well-defined photochromic behavior with clear absorption bands for trans/cis isomers in the UV–vis spectrum. Ruschewitz reported several MOFs, namely, MOF-5, MIL-68(Ga), MIL-68(In), and MIL-53(Al) loaded with azobenzene and studied light-induced trans/cis isomerization, which revealed that a different packing of azobenzene into the host is responsible for their differential photoisomerization behavior.[29] The influence of photoswitching properties of loaded guest spiropyran on MOF-5 was investigated to compare its switching properties in solution.[30] The photoresponsive surface area, pore volume, and CO2 uptake of spiropyran-incorporated MOF-808 were also studied.[31] Furthermore, the intricate structural investigation by polarized light microscopy shows the perfect alignment of photochrome diarylethene along the c axis of the host, DMOF-1.[32] The photoswitching and photoluminescence properties of azobenzene in nanoporous HKUST-1 MOF films were also examined using UV–vis or only visible light.[33,34] Moreover, Kitagawa et al. reported a reversible structural change of the host framework, observed with the photoswitching of azobenzene embedded into the flexible MOF, [Zn2(1,4-bdc)2(dabco)], leading to the drastic change in the gas adsorption properties.[35] Another important class of photochromic ligand is 2-(arylazo)pyridines, and their photoactive behaviors have been extensively studied in metal complexes.[36] The photoswitching properties were well exploited for the light-triggered fluorescence modulation of Zn–porphyrins.[37] In particular, Llebaria has shown the potential application of phenylazopyridines as photoisomerizable compounds to control biological functions with light.[38] Llebaria et al. have developed a series of phenylazopyridines with light-dependent activity as negative allosteric modulators of metabotropic glutamate receptor subtype 5 (mGlu5).[38] Hitherto, no materials have been reported encapsulated with 2-(phenylazo)pyridine (PAP). Hence, understanding of the photochromic nature of the 2-(phenylazo)pyridine (PAP) molecule inside a solid matrix is completely unknown.

For the first time, we have developed a host–guest matrix, where [Zn2(1,4-bdc)2(dabco)] (1) is used as a host and 2-(phenylazo)pyridine (PAP) as a guest in this work. [Zn2(1,4-bdc)2(dabco)] has been chosen as the host because of being colorless and thus minimally interfering in the optical investigation. More importantly, the pores and channels of 1 are suitable for PAP, which is observed from the crystal structure of 1 (vide infra). Again, we have explored the photoisomerization of PAP in solid MOF and investigated the influence of conformational transformation of PAP on the host hybrid MOF, 1. A striking difference was observed in the host framework (1) upon guest inclusion. Structural changes of the PAP molecule upon photoirradiation do not cause structural transformation of the crystalline host framework, which is a significant change in comparison with 1⊃AB (vide infra). This type of artificial guest-to-host structural flexibility/stability has a great potential in producing a dynamic switching of the optic, electric, and magnetic functions of the host–guest composites. Nevertheless, it can be an important platform for further investigations not only in gas storage but also as a sensor.

Results and Discussion

Synthesis and Characterization

The photosensitive guest ligand, 2-(phenylazo)pyridine, was synthesized by a combination of 2-aminopyridine and nitrosobenzene in basic medium (see the Experimental Section). The product was purified by silica gel chromatography with the pet ether (80%) and ethyl acetate (20%) solvent system and was characterized in detail by analytical and spectroscopic techniques. The porous metal-organic framework [Zn2(1,4-bdc)2(dabco)](1) (Scheme ) has been prepared using the solvothermal process in a Teflon-lined steel bomb containing Zn(NO3)2·6H2O, H2bdc, and dabco ligands in the N,N-dimethylformamide (DMF) solvent (see the Experimental Section). The solid framework of 1 was characterized for crystallinity, surface area, and morphology using X-ray diffraction (XRD), N2 adsorption/desorption, and scanning electron microscopic (SEM) studies, respectively.

Scheme 1

Schematic Three-Dimensional (3D) Structure of [Zn2(1,4-bdc)2(dabco)] (1)[39]

The single crystal of MOF 1 was successfully generated. The cell parameters and data exactly matched with those of the guest-free crystal structure reported by Kim et al. (Figure S1a).[39] The MOF 1 structure appears as a lower-symmetry tetragonal form with cell parameters a = b = 10.93(3) Å and c = 9.06(2) Å. It was observed that terephthalic acid ligands are linearly attached to the paddle–wheel Zn2 unit, resulting in a perfect two-dimensional square grid, which is linked with the dabco ligands to form a three-dimensional framework. Most importantly, the structure comprises a significant distance of ∼10 Å between two adjacent Zn2 paddle–wheel units in the layer including wide open channels ∼7.5 Å along the c axis (Figure S1b), which is an utmost requirement for the insertion of large functional molecule PAP. Notably, the length of azobenzene is ∼9.0 Å in its trans form, and after irradiation, it decreases to ∼5.5 Å in the cis isomer.[40,41] Hence, from geometric consideration, PAP should fit into the channels or pores of the MOF structure.

Trans-PAP has been loaded into the pores or channels of 1 at room temperature (see the Experimental Section). To confirm the complete insertion of PAP molecules into the pores of 1⊃PAP, we have studied the leaching experiment (Figure S2) where we have mixed 1⊃PAP with CHCl3 and kept it for 12 h. It was observed that compound 1⊃PAP is still orange, whereas the upper solvent is completely colorless. The upper solvent was monitored by UV spectroscopy, which certainly rules out the possibility of PAP on the surface of MOF 1. The IR measurement of the residue, 1⊃PAP, was performed to ensure the presence of PAP (vide infra). It is needless to say that the PAP ligand is red and highly soluble in CHCl3. This confirms the complete inclusion of PAP inside 1 i.e., 1⊃PAP, which becomes completely insoluble in CHCl3. Moreover, while investigating the back-switching photoisomerization process of free PAP, which was performed thermally at 60 °C for 40 h, it was observed that the transmittance intensity of obtained trans-PAP from cis-PAP was significantly lower compared to that of the original trans-PAP, whereas the transmittance intensity of 1⊃PAP does not alter under the same experiment conditions. This again attributes successful loading of PAP in channels or pores of MOF 1, not on the surface of MOF 1 (vide infra).

Figure S3 exhibits the Fourier transform infrared (FT-IR) spectra of MOF 1, free PAP ligand, and host–guest composite 1⊃PAP. It should be noted here that the azo N=N stretching vibration[42] of the free PAP ligand appears near 1424 cm–1, whereas the same vibration shifts to lower energy if the azo N=N is coordinated with Zn to form a complex.[43] The IR data of 1⊃PAP presents no peak in the range of 1380–1400 cm–1 although a strong signal is visible at 1422 cm–1 similar to that in the IR spectrum of the free PAP ligand because of the N=N stretching vibration. This also indicates the non-coordinating mode of the azo nitrogen of the PAP ligand. Furthermore, the tendency of axial coordination of pyridine nitrogen in 3- and 2-(phenylazo)pyridines to ZnTPP is substantially different, as reported by Otsuki et al.[37] The pyridine nitrogen of trans-2-(phenylazo)pyridine does not favor to coordinate with Zn in the ZnTPP complex because of appreciable hindrance coming from the phenylazo group at the 2 position, whereas the large association constant has been obtained between trans-MeO-3-Azo and ZnTPP.[37] This strongly suggests the active nature of PAP as a guest over the nonactive nature of the PAP ligand as a binding site in the present 1⊃PAP composite.

Elemental analysis and thermogravimetric analyses (TGA) have been performed to obtain the composition of PAP molecules per unit cell of MOF 1. The TGA data indicates that composite 1⊃PAP loses its guest molecule, PAP, in the temperature range of 150–250 °C and the decomposition of the resulting porous host framework starts after 300 °C (Figure S4). As evidenced by elemental analyses, the number of PAP ligand as a guest per unit cell of 1 was 1.0 (Tables S1 and S2), which agrees with the weight loss observed in TGA in the temperature range of 150–200 °C.

The successful introduction of the PAP molecule into host 1 was also confirmed by powder X-ray diffraction (PXRD) measurements. Significant changes in the reflection intensities in PXRD patterns have been observed after inclusion of PAP in 1 (Figure ). It should be noted here that the PXRD pattern of as-synthesized MOF 1 is consistent with the PXRD pattern of the guest-free sample reported by Kim et al.[39] PXRD measurements show a significant shifting of peaks to the higher 2θ values from 1 to 1⊃PAP. Moreover, the PXRD pattern of 1⊃PAP is similar to that of the composite reported with azobenzene in [Zn2(1,4-bdc)2(dabco)] (1), described by Kitagawa et al.[35] It was also consistent with the compound obtained after introducing benzene into 1.[39] As mentioned in the reported papers that the overall connectivity of the framework remains undisturbed but the tetragonal structure is transformed to an orthorhombic crystal framework, featuring dynamic behavior of MOF 1, the present 1⊃PAP system also exhibits a similar phenomenon, leading to shrinking upon guest inclusion with structural transformation from a square grid to an orthorhombic net.

Figure 1

PXRD patterns of (a) MOF 1 (black) and (b) 1⊃PAP (red).

Photoswitching Study

The photoswitching study of 1⊃PAP (Scheme ) has been carried out by UV–vis spectroscopy, IR spectroscopy, and 1H NMR measurements. Photoisomerization of guest trans-PAP to cis-PAP inside 1⊃PAP (Scheme ) has been achieved upon UV irradiation (380 nm), whereas the back-switching (cis-to-trans) was achieved thermally (heating sample at 60 °C). The data obtained were compared with the photoisomerization data of free PAP in the same conditions.

Scheme 2

Schematic Representation of trans-PAP to cis-PAP Geometrical Transformation inside the Orthorhombic Solid Framework of [Zn2(1,4-bdc)2(dabco)] (1)

The free PAP exhibits a strong absorbance at λ = 318 nm because of the π–π* transition and comparatively less intense absorbance band at λ = 454 nm because of the n–π* transition of the azo (N=N) group (Figure S5).[44] After UV irradiation, there is a significant decrease in the 320 nm absorption band in PAP(UV), confirming the transformation of trans-PAP to cis-PAP. The reversible back-switching was achieved by applying heat. It is denoted PAP(Heat). The absorbance at 320 nm again increases, proving the reversible nature of the photochromic behavior of the PAP ligand (Figure S5). Similar phenomena have been observed upon UV irradiation of 1⊃PAP in the solid state (Figure ).

Figure 2

UV–vis spectra of 1⊃PAP (black), 1⊃PAP(UV) (red), and 1⊃PAP(Heat) (green) and the switching cycle of 1⊃PAP (pink) in the inset.

The 325 nm absorption band decreases upon UV irradiation although the intensity has not been diminished fully upon prolonging UV irradiation, indicating the photostationary state of the PAP ligand inside the solid framework. This is termed as 1⊃PAP(UV). The ratio of cis/trans isomer is 28:72, which is less compared to the free PAP in the dimethyl sulfoxide (DMSO) solvent, giving isomerization to the cis isomer about 60%. This can be accounted as the restricted conformational transformation of the PAP ligand because of hindrance in the solid state, whereas free rotation is possible in the solution phase.

The back-switching was checked upon giving heat on 1⊃PAP(UV). The intensity of the 325 nm absorption band was regained but not fully, comprising partial reversibility of the photochromic process. The spectrum of 1⊃PAP(Heat) (green) in Figure has been recorded after heating 1⊃PAP(UV) at 60 °C for 10 h. The observed partial reversibility could be because of the photobleaching effect as the back process requires prolong heating or the steric hinderance of the switching process. Same observations were made with repeating this procedure. Partial reversibility was also reported previously for a metal-organic framework with a photoswitchable phenylazo group.[19] Again, according to Kitawaga’s report, more than 80% cis-azobenzene in 1⊃AB(UV) remained after 1 month if no external stimuli were used,[35] indicating that the geometrical conversion from cis-azo to trans-azo is a much slower process. This could be a reason for getting partial reversibility because it takes a longer time even with external stimuli. Also, in the present system, as no structural transformation of the host framework has been occurred (vide infra), it makes the back-switching process nonlabile. The photoswitching cycle has been also measured (Figure inset). It was observed that the photochromic process is switchable up to three cycles, after which the reversibility is significantly going down.

The photoswitching behavior of trans-PAP to cis-PAP in 1⊃PAP was also investigated by IR spectroscopy (see the Experimental Section for method). In the case of free PAP, after UV irradiation, a new peak emerged at 702 cm–1, which is because of the formation of cis-PAP, whereas trans-PAP has a characteristic peak at 690 cm–1 (Figure S6).

It can be noted here that the IR spectrum of azobenzene-inserted MOF 1 exhibits a peak at 690 cm–1 in its trans form, whereas upon irradiation, a new peak generated at 697 cm–1 for the cis isomer, reported by Kitagawa et al.[35] A similar observation was made for 1⊃PAP, where a peak at 702 cm–1 corresponding to the cis form of the PAP ligand was generated (Figure ). The new peak at 702 cm–1 was disappeared upon giving heat, which indicates reversibility of the photochromic process although it is partial (vide supra). The ratio of the cis/trans isomer is 26:74 calculated using IR spectroscopy, which is similar to the data obtained from UV–vis measurements, indicating the authenticity of the isomerization process inside the hybrid framework. The ratio of geometrical conversion was also studied by the 1H NMR study, which is consistent with the data obtained by UV–vis and IR spectroscopic methods (vide infra).

Figure 3

IR spectra of 1⊃PAP (black), 1⊃PAP(UV) (red), and 1⊃PAP(Heat) (blue).

The back-switching process was also attempted by applying light (500 nm). It shows a decrease in intensity of the 702 cm–1 peak, but less reversibility was obtained compared to that in the heat method in the same time interval, which indicates that the back process with light follows a slower kinetics than with heat (Figure S7). Moreover, the reversibility of 1⊃AB was also checked by treating 1⊃AB(UV) with heat.[35]

The photoswitching study can be performed up to three cycles by IR spectroscopic measurements similar to UV–vis study (Figure ).

Figure 4

Photoswitching cycle of 1⊃PAP.

Furthermore, the photochromic process has been investigated by the 1H NMR study. The guest molecules (PAP) were isolated from the composite material by dissolving them in a tetrasodium ethylenediaminetetraacetate (Na4EDTA) solution. The nine aromatic protons from trans-PAP appear in the range of 8.7–7.39 ppm (Figure ). However, the 1H NMR spectrum obtained after photoirradiation shows the signals from both the geometrical forms of trans-PAP and cis-PAP. The new peaks appearing at 8.37 and 6.6–7.2 ppm represent the cis isomer of PAP. The peak positions for both the isomers are consistent with the literature value.[36] The proton signals of isolated PAP from 1⊃PAP(Heat) show only the trans isomer, revealing the reversibility of the photochromic process, as suggested by other studies. The 1H NMR study gave a cis/trans isomer ratio of 25:75, which is consistent with the result obtained from UV and IR spectroscopy. Notably, in the case of the azobenzene composite with MOF 1, the obtained cis/trans isomer ratio was 38:62, indicating partial switching.[35]

Figure 5

1H NMR spectra of PAP (in CDCl3) isolated from (a) 1⊃PAP, (b) 1⊃PAP(UV), and (c) 1⊃PAP(Heat).

Characterization of 1⊃PAP(UV)

The MOF composite after UV irradiation, 1⊃PAP(UV), was characterized by PXRD measurements. PXRD data obtained for the 1⊃PAP(UV) composite shows no change in the pattern from its precursor (Figure ), 1⊃PAP, revealing the orthorhombic crystal form for 1⊃PAP(UV) after UV irradiation also. It is remarkable to state that the photoinduced conformational transformation of the guest PAP ligand is functional without disturbing the crystal lattice framework. Additionally, it should be noted here that [Zn2(1,4-bdc)2(dabco)] (1) was famous for its flexible nature,[39] which was also observed in the case of azobenzene-embedded MOF 1 where the rhombic form of 1⊃AB was distorted to form a tetragonal form for 1⊃AB(UV) upon UV irradiation while going from the trans form of azobenzene to the cis form of azobenzene.[35]

Figure 6

PXRD patterns of (a) 1⊃PAP, (b) 1⊃PAP(UV), and (c) 1⊃PAP(Heat).

Repetition of Kitagawa’s work has been carried out to investigate whether the framework shows flexibility with azobenzene (AB) photoisomerization under the same irradiation conditions used for PAP. The trans-to-cis photoisomerization of AB in the pores of 1 has been performed by IR and PXRD measurements. The PXRD measurements were carried out for 1⊃AB, 1⊃AB(UV), and 1⊃AB(Heat) (Figure S8), which show similar observation with the reported data,[35] revealing the transformation of the orthorhombic structure of 1⊃AB to the tetragonal form in 1⊃AB(UV) upon irradiation of azobenzene, confirming the flexible nature of the framework and authenticity of our experiment with 1⊃PAP. In the IR study, the native IR spectrum of 1⊃AB showed a peak at 690 cm–1 because of the trans isomer, and after UV irradiation, a new peak at 697 cm–1 was emerged because of the formation of the cis isomer of AB. The back-switching was also carried out by applying heat (Figure S9). The data is consistent with the result reported by Kitagawa et al., which indicates that the photoisomerization conditions used for the current work are feasible for PAP. The unexpected nonresponsive behavior of 1⊃PAP upon photoirradiation of PAP compared to that of MOF 1 with the inclusion of azobenzene may be because of the interaction of the pyridine-N atom of PAP with the paddle–wheel of the MOF, eventually replacing dabco at least partly. Also, it can be explicated that the attractive interaction between 1 and PAP is significantly weaker than the attractive interaction between 1 and trans-azobenzene.

The SEM images of 1⊃PAP and 1⊃PAP(UV) are shown in Figure . The morphologies of both the MOF composites 1⊃PAP and 1⊃PAP(UV) were well-defined rhombic crystals at 50 μm. Both show a very similar feature, which suggested that isomerization does not lead to a change in the morphology. The thermal treatment of 1⊃PAP(UV) denoted 1⊃PAP(Heat) to obtain trans-PAP back in 1 shows the same PXRD pattern, revealing the rhombic network of the MOF structure (Figure ).

Figure 7

SEM images of (a) 1⊃PAP and (b) 1⊃PAP(UV).

The electron beam diffractions of 1⊃PAP and 1⊃PAP(UV) were measured using a transmission electron microscope (TEM) (Figure ). In both the cases, the diffraction patterns were attributed to the orthorhombic host structure. Hence, indeed, there is no structural change in the host after UV irradiation.

Figure 8

TEM images for (a) 1⊃PAP and (b) 1⊃PAP(UV) and in the inset their diffraction patterns.

Again, to understand the unusual nonresponsive behavior of the host, nitrogen adsorption measurements were performed for both the composite materials. It was observed that 1⊃PAP does not adsorb N2, which was expected because the pores of the host molecules were blocked because of the close contact of the guest moiety with the host framework. After irradiating 1⊃PAP with UV light, the amount of adsorbed N2 increases to 20 mL g–1 (Figure ), which is significantly lower than that for 1⊃AB where the adsorption amount drastically increases to 45 mL g–1.[35] It was explained that the expansion of the host framework to the tetragonal form upon UV irradiation in 1⊃AB increases the microporosity of the material.[35] Hence, the slight increase in the amount of adsorbed N2 in the present system, 1⊃PAP(UV), can be attributed only to the conformational change in guest PAP (trans to cis) without any change in the host framework. To the best of our knowledge, this is the first example of the unusual inflexibility of a metal-organic framework by conformational changes of a photoresponsive guest molecule.

Figure 9

Adsorption isotherms of N2 at 77 K for (a) 1⊃PAP (pink) and (b) 1⊃PAP(UV) (purple).

Conclusions

In conclusion, we have synthesized, characterized, and investigated the photochromic behavior of 2-phenylazopyridine (PAP) embedded in metal-organic framework 1. The cis/trans isomerization of the PAP ligand occurs within the pores or nanochannels of MOF by irradiating the MOF composite with UV light, whereas back-switching has been successfully achieved thermally. Notably, the framework structure of [Zn2(1,4-bdc)2(dabco)] (1) does not show any structural transformation of the host with conformational changes of embedded photochromic guest molecule PAP, showing the unexpected nonresponsive behavior of the 1⊃PAP composite. Because of this surprising unperturbation, there is little change in nitrogen adsorption behavior going from 1⊃PAP to 1⊃PAP(UV). The present work highlights a simple strategy to control the guest-to-host structural transmission combined with induced structural inflexibility by guest photoisomerization using light as external stimuli.

Experimental Section

Instruments

The powder diffraction measurements were carried out using Lab XRD-6100 Shimadzu with Cu Kα1 radiation. FT-IR measurements were performed as KBr pellets on Spectrum Two FT-IR spectrometers by PerkinElmer. The UV–vis spectra of the solid and solution were recorded by a UV–vis spectrophotometer by Shimadzu, UV-1800 instrument. TGA analysis has been done using an STA 6000 Simultaneous Thermal Analyzer (PerkinElmer). The heating rate of the measurement was 10 °C min–1 in a N2 atmosphere. The elemental analyses of carbon, hydrogen, and nitrogen were carried out using a Vario MICRO cube Elementar. The surface area and pore size distribution were directly analyzed by nitrogen adsorption on the solid sample using ASAP 2020 V4.00(V4.00 H) (Micromeritics).

Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%), terephthalic acid (H2bdc, 98%), 1,4-diazabicyclo[2.2.2]octane (dabco), and dry N,N-dimethylformamide (DMF) were purchased from commercial suppliers and used without further purification.

Synthesis of [Zn2(1,4-bdc)2(dabco)] (1)

The synthesis of 1 was carried out according to the literature procedure.[39] Anal. Calcd for [Zn2(C8H4O4)2(C6H12N2)]; C, 45.61; H, 3.50; N, 4.90. Found: C, 45.67; H, 3.26; N, 4.57.

Synthesis of 2-(Phenylazo)pyridine (PAP)

The ligand, 2-(phenylazo)pyridine (PAP), has been synthesized by the method used in a previous paper.[45] Yield 0.957 g (87%). UV–vis:(DMSO) λmax = 318 nm and λ = 454. 1H NMR (400 MHz, CDCl3; δ, ppm): δ 8.74 (1H), 8.06 (2H), 7.91 (1H), 7.83 (1H), 7.55 (3H), 7.42 (1H). m.p. 32–34 °C.

Synthesis of 1⊃PAP

MOF 1 was washed three times with CHCl3 to exchange solvent. The solid was activated by heating at 120 °C under vacuum for 12 h. The material (1) was cooled down to room temperature (35 °C). MOF 1 (200 mg) was taken in a sealed two-necked round bottom flask and degassed by applying vacuum. PAP (200 mg) (liquid in nature at 35 °C) was added in activated 1 at 35 °C and was kept for 20 h. trans-PAP was adsorbed into the pores/nanochannels of 1. Excess PAP was removed by heating at 58 °C, where excess PAP was deposited on the side wall of the flask. The orange host–guest composite is termed as 1⊃PAP. The product, 1⊃PAP, has been characterized by various spectroscopic techniques (vide supra).

Leaching Experiment

The leaching experiment was performed using 0.010 g of the 1⊃PAP sample mixed with 5 mL of CHCl3 in a vial. It was mixed well and kept for settling down. It was observed that the 1⊃PAP composite was orange, but the upper solvent remained colorless (monitored by UV–vis spectroscopy).

Photoswitching Experiment

The photoswitching experiment was performed with a 150 W Xe lamp with a suitable optical filter (380 nm) to selectively irradiate the sample. The UV–vis spectra have been recorded using a UV–vis spectrophotometer (Shimadzu UV-1800 with inbuilt TCC-100 (thermoelectrically temperature controlled cell holder)). For the solid UV–vis method, a known amount of MOF 1 has been uniformly pasted in a circular disk form on the quartz slide, which was kept in the sample holder, and the base line spectrum has been recorded. The same amount of 1⊃PAP has been taken in the same manner, and the spectrum was recorded. The 1⊃PAP-pasted quartz slide has been irradiated by light (380 nm), and spectra (in situ) were recorded at certain intervals of time.

1 was used to calculate the cis/trans ratio from the UV–vis method.[46]For the IR method, base line has been recorded with the KBr pellet. A mixture of KBr and 1⊃PAP has been used for the preparation of pellet, which was used for the IR experiment. The pellet was irradiated with 380 nm light, and the spectra were recorded at certain time intervals. The back-switching process has been checked by applying light (500 nm) and thermally (60 °C). The ratio of cis isomer was also calculated from IR spectroscopy. 2 was used to calculate the cis/trans ratio from the IR method, where Tcis and Ttrans belong to the transmittance at 702 cm–1 (cis) and 690 cm–1 (trans), respectively.

1H NMR Study

The 1H NMR study was carried out using an Agilent Technologies (1H NMR 400 MHz) instrument. The guest PAP molecules were isolated by dissolving the composite material 1⊃PAP in a tetrasodium ethylenediaminetetraacetate (Na4EDTA) solution. The same method was followed for isolating the PAP molecule from 1⊃PAP(UV) and 1⊃PAP(Heat). The cis/trans ratio was calculated from the intensity change of a particular peak for one proton.[47]