Optically Controlled Molecular Metallofullerene Magnetism via an

Sep 3, 2018 - After ultraviolet (365 nm) irradiation, the isomerization of azobenzene groups in the AzoMOF was found to be able to modulate the spin ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 32607−32612

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Optically Controlled Molecular Metallofullerene Magnetism via an Azobenzene-Functionalized Metal−Organic Framework Haibing Meng,†,‡ Chong Zhao,†,‡ Mingzhe Nie,†,‡ Chunru Wang,*,† and Taishan Wang*,† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China

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ABSTRACT: Molecular magnets with optically controlled property have significant applications in data storage and quantum information processing. Herein, we report the optically controlled molecular magnetism of endohedral metallofullerenes, Sc3C2@C80 and DySc2N@C80, by incarcerating them into the pores of a photoswitchable azobenzenefunctionalized metal−organic framework (MOF) (AzoMOF). After ultraviolet (365 nm) irradiation, the isomerization of azobenzene groups in the AzoMOF was found to be able to modulate the spin relaxation of Sc3C2@C80 and also improve the single-molecule magnet behavior of DySc2N@C80. The photoisomerization of azobenzene side groups changes the host−guest interaction between metallofullerene and AzoMOF pores and endows them with the potential to modulate the magnetic properties with light. These findings offer an effective method to create smart molecular magnetic materials and also promote their applications in information recording, spintronics, and sensors. KEYWORDS: metallofullerene, MOF, magnetic property, azobenzene group, light control

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INTRODUCTION Molecular magnetic materials have potential applications in high-density data storage, quantum information processing, spintronics, and so forth.1−5 Magnetic endohedral metallofullerenes, such as Sc3C2@C80, DySc2N@C80, Y2@C79N, La@C82, and so forth, are a unique kind of molecular magnetic material because of their remarkable stability of encapsulated spin and spherical molecular structure.6−11 The magnetic properties of metallofullerenes can be manipulated due to their sensitive electron spin, which can respond to external stimuli such as cage modification, temperature, environment, local magnet, and so forth.8,10,12−17 Even so, it is essential to explore more applicable methods to modulate the spin states of metallofullerenes. Recently, nanomaterials based on photoresponsive units have attracted considerable interest because light has superior advantages in information recording and retrieving.18,19 It inspires us to explore controllable magnetism of metallofullerenes upon light illumination and promote their applications in information processing and storage. Metal−organic frameworks (MOFs) are three-dimensional porous crystals, constructed from metal clusters/ions (nodes) and organic ligands (linkers).20−22 One extraordinarily interesting property of MOFs is the availability of inner free space, which allows the incorporation of many molecules with a host−guest interaction.23,24 Moreover, the available free space in MOFs can be tuned by stimuli, resulting in the stimuli-responsivity of MOFs.25,26 Among the researched stimuli-responsive MOFs, MOF materials with a photoswitchable structure are very attractive.27−29 For example, © 2018 American Chemical Society

photo-responsive MOFs constructed by azobenzene building blocks show geometrical changes based on photoisomerization of azobenzene upon ultraviolet (UV) light irradiation.30−32 The photoisomerization of azobenzene groups also involves polar changes of the MOFs. All of these changes could largely influence the host−guest interaction between the guests and the MOFs. In view of this, we have investigated the optically controlled magnetism of metallofullerenes by incorporating them into the pores of photoresponsive MOFs, considering the susceptibility of the metallofullerene spin. Briefly, we report the controlled molecular magnetism by incarcerating metallofullerenes (Sc3C2@C80 and DySc2N@C80) into the pores of AzoMOF with azobenzene building groups (see Figure 1). Electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) magnetometry were executed to disclose the changed spin state of Sc3C2@C80 and the single-molecule magnet (SMM) behavior of DySc 2N@C80 within AzoMOF after UV (365 nm) irradiation, respectively.

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RESULTS AND DISCUSSION Magnetic metallofullerenes, Sc3C2@C8033 and DySc2N@C80,13 were synthesized by the Kräschmer−Huffman arc-discharge method and isolated by high-performance liquid chromatogReceived: July 4, 2018 Accepted: September 3, 2018 Published: September 3, 2018 32607

DOI: 10.1021/acsami.8b11098 ACS Appl. Mater. Interfaces 2018, 10, 32607−32612

Research Article

ACS Applied Materials & Interfaces

Figure 2. Optical images of AzoMOF (a) and Sc3C2@C80@AzoMOF (b). (c) PXRD patterns of AzoMOF and Sc3C2@C80@AzoMOF. The inset shows the changes in the characteristic peaks for AzoMOF after incarceration of Sc3C2@C80. (d) N2 adsorption isotherms of AzoMOF and Sc3C2@C80@AzoMOF at 77 K.

Figure 1. Schematic illustration of the synthesis of AzoMOF, with azobenzene units as building blocks, and construction of metallofullerene@AzoMOF. Metallofullerene (dark orange sphere) occupies the octahedral pore, which is highlighted in green color. The azobenzene building blocks are colored in green and orange (the nitrogen−nitrogen bonds are marked in purple), and some of them are omitted for clarity. The schematic structures of Sc3C2@C80 and DySc2N@C80 are shown at the bottom (blue, Sc; pink, N; gray, C; and orange, Dy).

stemming from Sc3C2@C80 distributes well in AzoMOF (Figure S2), indicating the filling of the AzoMOF pores with Sc3C2@C80 molecules. The PXRD patterns provide another demonstration for the incarceration of Sc3C2@C80 in AzoMOF. As revealed by the data in Figure 2c, the characteristic peaks of PXRD for Azo MOF are changed after encapsulation of Sc3C2@C80. To be specific, the characteristic peaks of AzoMOF decrease by 0.08° and 0.04° owing to encapsulated Sc3C2@C80, which illustrates the tiny structural deformation of AzoMOF. In addition, nitrogen sorption isotherms directly demonstrate the incorporation of Sc3C2@C80 into AzoMOF, as shown in Figure 2d. The surface area of Sc3C2@C80@AzoMOF is 300.59 m2/g, whereas that of pristine AzoMOF is 656.72 m2/g. From the above results, we can conclude that Sc3C2@C80 has been loaded as guest molecules into the pores of AzoMOF. Similar to the azobenzene building blocks in solution (Figure S7), irradiation of AzoMOF and metallofullerene@AzoMOF with UV light results in an obvious n−π* band enhancement, indicating the photoswitch of azobenzene units in AzoMOF between trans-form and cis-form; see Figure 3a,b; but for the reverse isomerization, they exhibit different changes. The reverse isomerization of UV-irradiated azobenzene building blocks in solution with visible light can induce increase of the azobenzene π−π* band and a decrease of the n−π* band, illustrating the reverse photoisomerization (cis-totrans) of azobenzene units (Figure S7). Similarly, the reverse isomerization of UV-irradiated AzoMOF with visible light results in an obvious n−π* band decrease, also indicating the reverse isomerization of azobenzene units from the cis-form to the trans-form in AzoMOF (Figure S8). However, after visiblelight treatment, the decrease of the n−π* band is invisible for UV-irradiated Sc3C2@C80@AzoMOF (Figure 6a), which illustrates that the reverse isomerization of azobenzene units from the cis-form to the trans-form is blocked for Sc3C2@ C80@AzoMOF.

raphy (HPLC), as shown in Figure S1. The photoswitchable Azo MOF was synthesized by the solvothermal method.31,32 The pioneering work has reported that AzoMOF is composed of small tetrahedral and large octahedral pores, which are connected through sharing an azobenzene ligand as long as 14 Å (Figure 1). It can be seen that the large octahedral pores in AzoMOF are big enough to accommodate Sc3C2@C80 and DySc2N@C80 (their sizes are about 8 Å, which were measured from their optimized structure by density functional theory calculation). Also, the aromatic frameworks of AzoMOF can facilitate the accommodation through π−π interactions. Experimentally, 5 mg of AzoMOF powder was immersed into toluene solution (1.5 mL) of metallofullerenes (0.1 mg) at room temperature until the solution became clear, indicating that the metallofullerenes were absorbed completely. The resultant powder became dark in color after accommodating Sc3C2@C80 molecules (Figure 2a,b), which illustrates the incarceration of Sc3C2@C80 within the AzoMOF. The obtained complexes were denoted as Sc3 C2@C80@AzoMOF and DySc2N@C80@AzoMOF. To further characterize the metallofullerene@AzoMOF complexes, we performed transmission electron microscopy (TEM), nitrogen sorption isotherms, and powder X-ray diffractions (PXRD) of Sc3C2@C80@AzoMOF. From the TEM data, it is clear that the grid lines of AzoMOF remain intact after absorbing Sc3C2@C80, and the scandium element 32608

DOI: 10.1021/acsami.8b11098 ACS Appl. Mater. Interfaces 2018, 10, 32607−32612

Research Article

ACS Applied Materials & Interfaces

Figure 3. Diffuse reflectance spectra of AzoMOF (a) and Sc3C2@ C80@AzoMOF (b) at 298 K upon irradiation with UV light for different times. Arrows clarify the trends of the spectral changes. (c) PXRD patterns of AzoMOF upon irradiation with UV light for different times. (d) N2 adsorption isotherms of AzoMOF in the pristine state (black curve), right after irradiation with UV for 30 min (red curve), and kept in the dark for 3 days after irradiation with UV for 30 min (blue curve).

The PXRD patterns of AzoMOF and Sc3C2@C80@AzoMOF upon UV irradiation stay changeless, illustrating the integrity of Azo MOF after UV irradiation (Figures 3c, S9, and S11). In Azo MOF, the cavities are surrounded by azobenzene units, and the photoisomerization of azobenzene units in AzoMOF leads to exquisite geometrical and dipole moment changes of frameworks. To be specific, the photoisomerization of azobenzene involves not only the distance change of the para-carbon atoms in an azobenzene unit from 9 Å in the transisomer to 5.5 Å in the cis-isomer but also the dipole moment change of the unit from 0 D in the trans-form to 3 D in the cisone.34 As a result, the trans-to-cis photoisomerization of azobenzene building blocks in AzoMOF promoted both steric effect (pore blockage) of the diffusion event and the dipole− quadrupole interaction between the pore surface and N2 molecules, resulting in a reduced N2 uptake of AzoMOF right after UV irradiation (Figure 3d). In addition, the decreased N2 uptake of AzoMOF right after UV irradiation can be maintained even if it has been kept in the dark for 3 days, confirming the stable cis-form of azobenzene groups in AzoMOF. EPR experiments were employed on Sc3C2@C80@AzoMOF to disclose the changed spin state of Sc3C2@C80 by light control, as shown in Figure 4. For Sc3C2@C80, it has an unpaired electron localized on the C2 unit, resulting in the paramagnetic property. For paramagnetic Sc3C2@C80, three equivalent Sc nuclei of Sc3C2@C80 couple with one unpaired electron, generating highly symmetric 22 lines in its EPR spectrum (Figure S3).36,37 To quickly realize the optically controlled magnetism of Sc3C2@C80, we synthesized a Sc3C2@ C80@AzoMOF film, as shown in Figure 4a. The AzoMOF film was synthesized on the functionalized gold surface.35 It can be clearly seen from Figure S10 that the PXRD spectrum of Azo MOF film is identical to that of AzoMOF powder, indicating

Figure 4. (a) SEM image of the AzoMOF film. The inset shows the cross-sectional image for the AzoMOF film. (b) EPR spectra of the Sc3C2@C80@AzoMOF film in the pristine state (black) and after irradiation with UV for 30 min (red). (c) Time-dependent EPR peaks intensity of the Sc3C2@C80@AzoMOF film upon UV irradiation. (d) Line widths of the EPR spectra for the Sc3C2@C80@AzoMOF film in the pristine state (black) and after irradiation with UV for 30 min (red) plotted against quantum number MI of the Sc3 nuclear magnetic moment. (e) First integral of the EPR spectra for the Sc3C2@ C80@AzoMOF film in the pristine state and after UV irradiation. (f) Line widths of EPR spectra for the Sc3C2@C80@AzoMOF film upon UV irradiation for different times obtained from the first integral of the EPR spectra.

its maintained crystalline structure of AzoMOF. The scanning electron microscopy (SEM) image showed a homogeneous MOF membrane with approximately 3 μm of thickness on top of the gold surface. Also, we performed EPR spectra of Azo MOF in the pristine state and in the UV-irradiated state to exclude the influence of the background (Figure S5). The resultant spectra of the AzoMOF film show no EPR signal. We also performed the time-dependent EPR spectra of the Sc3C2@ C80@AzoMOF film in the dark after irradiation with UV light for 5 min (Figure S4) to avoid the influence of charge transfer between them, and the unchanged EPR signal intensity indicates that charge transfer does not occur between Sc3C2@C80 and aromatic pores in AzoMOF along with EPR measurements. Notably, after the Sc3C2@C80@AzoMOF film was irradiated with UV light, the intensity of EPR signals decreases; see Figure 4b. To further illustrate the EPR changes, the signal− intensity curve of the EPR spectra (the maximum difference of positive values and negative values at the central part of the 32609

DOI: 10.1021/acsami.8b11098 ACS Appl. Mater. Interfaces 2018, 10, 32607−32612

Research Article

ACS Applied Materials & Interfaces EPR spectra) for the Sc3C2@C80@AzoMOF film upon UV irradiation at different times was plotted (Figure 4c). It can clearly be seen that the EPR signal intensity obviously decreases. Also, the line-broadening effect of the EPR spectra for the Sc3C2@C80@AzoMOF film upon UV irradiation can be observed (Figure 4d,f). For Sc3C2@C80 in AzoMOF, the line width of the peak-to-peak ΔHpp depends on the quantum number (mI) in the EPR spectra (Figure 4d) and can be expressed in the form ΔHpp = k 0 + k1mI + k 2mI 2 + k4mI 4

Figure 6. (a) Diffuse reflectance spectra of Sc3C2@C80@AzoMOF at 298 K upon irradiation with visible light for different times. Arrows clarify the trends of the spectral changes. Inset: Enlargement of the signal at 400−550 nm. (b) EPR spectra of the Sc3C2@C80@AzoMOF film after UV irradiation (black) and irradiation with visible light for 5 h after UV irradiation (red).

where all ki parameters depend on the rotational correlation time τc (τc = 4πηR3/3kT), which is originated from the Debye theory of rotational relaxation.17,38 In this formula, η is the viscosity of the solvent and R is the hydrodynamic radius of the molecule. The decreased EPR signal intensity and the linebroadening effect of the Sc3C2@C80@AzoMOF film upon UV irradiation can be ascribed to the changes of host−guest interaction between Sc3C2@C80 and AzoMOF. The photoresponsive azobenzene units in AzoMOF undergo an exquisite geometrical change triggered by UV light between trans- and cis-isomers, which provides not only steric but also polar (aromatic) effects on guest Sc3C2@C80 (Figure 5).

irradiation, hindering the cis-to-trans isomerization of azobenzene units with visible light or heat treatment. The recoverable magnetism of the Sc3C2@C80@AzoMOF film may be realized through constructing new MOFs with larger pores to lower the energy barrier of isomerization in the future. Still, the optically controlled magnetism of the Sc 3 C 2 @ C80@AzoMOF film can be realized through UV irradiation, which can also expand the applications of Sc3C2@C80 in the future. Moreover, the Sc3C2@C80 molecule can be regarded as a spin probe to sense the pore changes of AzoMOF. The strengthened host−guest interaction after photoisomerization of AzoMOF was also employed to investigate the SMM behavior of DySc2N@C80. We performed magnetic hysteresis and magnetic susceptibility measurements on DySc2N@C80@AzoMOF powder with a SQUID magnetometer (Figures 7 and S13). For comparison, we performed magnetic

Figure 5. Schematic representation of the effect of UV irradiation on the host−guest interaction between metallofullerene and AzoMOF.

Because of the susceptible spin of metallofullerene Sc3C2@C80, the photoswitchable geometrical and polar changes in AzoMOF could influence its paramagnetic properties through changing the π−π interaction between Sc3C2@C80 and the confined space of AzoMOF. After irradiation with UV light, the photoisomerization of AzoMOF leads to stronger host−guest interaction between AzoMOF and Sc3C2@C80 with restricted rotation of Sc3C2@C80, modulating the spin relaxation of Sc3C2@C80 finally. This confinement effect contributes to the increases in the viscosity and the rotational correlation time. As a result, the EPR signal intensity decreases and the line width broadens for the Sc3C2@C80@AzoMOF film upon UV irradiation. We further performed EPR measurements of the Sc3C2@ C80@AzoMOF film and tried to realize the recoverable paramagnetic properties through visible light or heat treatment (Figures 6 and S6). We treated the UV-irradiated Sc3C2@ C80@AzoMOF film with visible light (475 nm) for 5 h and measured its EPR signals (Figure 6b), which shows little changes. We also heated the UV-irradiated Sc 3 C 2 @ C80@AzoMOF film at 353 K for 8 h and then measured its EPR signals (Figure S6), which also shows little changes. The reason proposed is that there is stronger host−guest interaction between AzoMOF and Sc3C2@C80 after UV

Figure 7. (a) Hysteresis loops for DySc2N@C80 at 2 K. (b) Hysteresis loops for DySc2N@C80@AzoMOF in the pristine state (black) and after irradiation with UV for 30 min (red) at 2 K.

hysteresis and magnetic susceptibility measurements on DySc2N@C80 (Figures 7a and S12). We also performed magnetic measurements on AzoMOF before and after UV irradiation to exclude background contribution; see Figures S14 and S15. After exact subtraction of the background contribution of AzoMOF powders, the magnetic properties of DySc2N@C80@AzoMOF in the pristine state and after irradiation with UV light still exhibit changes (Figures 7b and S13). For pristine DySc2N@C80, its magnetic moment is originated from the DyIII ion, and it has a butterfly-shaped hysteresis loop at 2 K, and the sharp drop in the hysteresis loop at low fields can be ascribed to the quantum tunneling of magnetism (QTM) effect. 13 However, for DySc 2 N@ C80@AzoMOF, it exhibits distinctly different magnetization 32610

DOI: 10.1021/acsami.8b11098 ACS Appl. Mater. Interfaces 2018, 10, 32607−32612

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loops, and there is increased remanent magnetization at zero field (H = 0 T) at 2 K compared to pristine DySc2N@C80. Obviously, the QTM effect of DySc2N@C80 in the pores of Azo MOF is suppressed, which is originated from the crystalfield perturbation of AzoMOF. The strong host−guest interactions between DySc2N@C80 and AzoMOF generate crystal-field splitting. As a result, the magnetic ground states are split, and the quantum coherence of DySc2N@C80 in Azo MOF is influenced. After the DySc2N@C80@AzoMOF powder was irradiated with UV light for 30 min, it showed enhanced remanent magnetization at zero field (H = 0 T) at 2 K compared to that in the pristine state (Figure 7b). As discussed above, the photoisomerization of AzoMOF induces more strengthened host−guest interaction between DySc2N@C80 and AzoMOF, leading to the more suppressed QTM effect and influencing the response of hosted DySc2N@C80 to the external magnetic field. Compared with DySc2N@C80@AzoMOF in the pristine state, there is larger remanence at low field for DySc2N@ C80@AzoMOF after UV irradiation, indicating the improved memory of magnetization history in the zero field. As a result, the crystal-field perturbation from AzoMOF is enhanced and the ground states of DySc2N@C80 can be changed. In general, a noncontact control of SMM behavior for DySc2N@C80 was realized by incorporating it into photoresponsive AzoMOF. This photoresponsive SMM behavior of DySc 2 N@ C80@AzoMOF may endow it with the potential to record information with light.

Chunru Wang: 0000-0001-7984-6639 Taishan Wang: 0000-0003-1834-3610 Notes

The authors declare no competing financial interest.

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CONCLUSIONS In summary, we have realized optically controlled magnetic properties for Sc3C2@C80 and DySc2N@C80 by accommodating them into the pores of a photoresponsive AzoMOF. The trans/cis isomerization of azobenzene units in the AzoMOF upon UV irradiation triggers changes of steric geometries and dipole moments for AzoMOF pores. By using these changes, the host−guest interaction between magnetic metallofullerenes (Sc3C2@C80 and DySc2N@C80) and AzoMOF pores was altered efficiently, resulting in optically controlled magnetic properties. In addition, this kind of control on metallofullerenes is a noncontact approach, which is very different from the previous chemically modified method.8,10,14,16 These findings may offer an effective method to create smart magnetic materials based on metallofullerenes and would promote the applications of metallofullerenes in information recording, spintronics, and sensors. ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11098.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (61227902, 51672281, and 51472248) and the Beijing Natural Science Foundation (2162050). In particular, T.W. thanks the Youth Innovation Promotion Association of CAS (2015025).

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Materials, synthesis, HPLC data, matrix-assisted laser desorption ionization time-of-flight mass spectrometry data, EPR spectrum, PXRD data, and other characterizations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.W.). *E-mail: [email protected] (C.W.). 32611

DOI: 10.1021/acsami.8b11098 ACS Appl. Mater. Interfaces 2018, 10, 32607−32612

Research Article

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DOI: 10.1021/acsami.8b11098 ACS Appl. Mater. Interfaces 2018, 10, 32607−32612