Optically Controlled Molecular Metallofullerene Magnetism via an

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Optically Controlled Molecular Metallofullerene Magnetism via Azobenzene-Functionalized MOF Haibing Meng, Chong Zhao, Mingzhe Nie, Chunru Wang, and Taishan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11098 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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ACS Applied Materials & Interfaces

Optically Controlled Molecular Metallofullerene Magnetism via Azobenzene-Functionalized MOF Haibing Meng1,2, Chong Zhao1,2, Mingzhe Nie1,2, Chunru Wang1,* and Taishan Wang1,* 1

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Molecular

Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. 2

University of Chinese Academy of Sciences, Beijing 100049, China.

*E-mail: [email protected]; [email protected] ABSTRACT: The 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 photo-switchable azobenzene-functionalized MOF (AzoMOF). After UV (365 nm) irradiation, the isomerization of azobenzene groups in

Azo

MOF

was found to be able to modulate the spin relaxation of Sc3C2@C80, and also improve the singlemolecule magnet behavior of DySc2N@C80. The photo-isomerization of azobenzene side groups changes the host-guest interaction between metallofullerene and

Azo

MOF pores, and endows

them with 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.

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KEYWORDS: Metallofullerene, MOF, magnetic property, azobenzene group, light control INTRODUCTION Molecular magnetic materials have potential applications in high density data storage, quantum

information

processing

(QIP),

spintronics,

etc.1-5

Magnetic

endohedral

metallofullerenes, such as Sc3C2@C80, DySc2N@C80, Y2@C79N, La@C82, etc., are a unique kind of molecular magnetic material due to their remarkable stability of encapsulated spin and spherical molecular structure.6-11 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, etc.8, 10, 12-17 Even so, it is essential to explore more applicable methods to modulate the spin states of metallofullerenes. Recently, nano-materials based on photo-responsive 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 3D porous crystals, constructing 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 host-guest interaction.23-24 Moreover, the available free space in MOFs is able to be tuned by stimuli, resulting in the stimuli-responsivity of MOFs.25-26 Among researched stimuli-responsive MOFs, MOF materials with photo-switchable structure are very attractive.27-29 For example, photo-responsive MOFs constructed by azobenzene building blocks show geometrical changes based on photo-isomerization of azobenzene upon ultraviolet (UV) light irradiation.30-32 The photo-isomerization of azobenzene groups also involves polar changes of the


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MOFs. All of these changes could largely influence the host-guest interaction between the guests and MOFs.

Figure 1. Schematic illustration of the synthesis of

Azo

MOF 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 of them are marked in purple) and some of them are omitted for clarity. The schematic structures of Sc3C2@C80, DySc2N@C80 are shown at the bottom (blue, Sc; pink, N; gray, C; orange, Dy). In view of this, we have investigated the optically controlled magnetism of metallofullerenes by incorporating them into the pores of photo-responsive 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


Azo

MOF with azobenzene

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building groups (see Figure 1). Electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) magnetometer were executed to disclose the changed spin state of Sc3C2@C80 and single-molecule magnet (SMM) behavior of DySc2N@C80 within AzoMOF after UV (365 nm) irradiation, respectively. RESULTS AND DISCUSSION Magnetic metallofullerenes Sc3C2@C8033 and DySc2N@C8013 were synthesized by KräschmerHuffman arc-discharge method and isolated by high performance liquid chromatography (HPLC), as shown in Figure S1. The photo-switchable

Azo

MOF was synthesized by 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 azobenzene ligand as long as 14 Å (Figure 1). It can be seen that the large octahedral pores in

Azo

MOF are big enough to

accommodate Sc3C2@C80 and DySc2N@C80 (their sizes are about 8 Å, which were measured from their optimized structure by DFT calculation). Also, the aromatic frameworks of

Azo

can facilitate the accommodation through π-π interaction. Experimentally, 5 mg

Azo

MOF MOF

powders were 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 resulted powder becomes dark in color after accommodating Sc3C2@C80 molecules (Figure 2a and 2b), which illustrates the incarceration of Sc3C2@C80 within

Azo

MOF.

The obtained complexes were denoted as Sc3C2@C80@AzoMOF and DySc2N@C80@AzoMOF, respectively. 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


Azo

MOF remain

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intact after absorbing Sc3C2@C80, and the scandium element stemming from Sc3C2@C80 distributes well in

Azo

MOF (Figure S2), indicating the filling of the

Azo

MOF pores with

Sc3C2@C80 molecules. The PXRD patterns provide another demonstration for the incarceration of Sc3C2@C80 in PXRD for

Azo

MOF. As revealed by the data in Figure 2c, the characteristic peaks of

Azo

MOF are changed after encapsulation of Sc3C2@C80. To be specific, the

characteristic peaks of

Azo

MOF decrease by 0.08° and 0.04° respectively owing to encapsulated

Sc3C2@C80, which illustrates tiny structural deformation of

Azo

MOF. In addition, nitrogen

sorption isotherms directly demonstrate the incorporation of Sc3C2@C80 into

Azo

MOF, as shown

in Figure 2d. The surface area of Sc3C2@C80@AzoMOF is 300.59 m2/g while that of pristine Azo

MOF is 656.72 m2/g. From above results, we can conclude that Sc3C2@C80 has been loaded as

guest molecules into the pores of AzoMOF.

Figure 2. Optical images of

Azo

MOF (a) and Sc3C2@C80@AzoMOF (b). (c) PXRD patterns of

Azo

MOF and Sc3C2@C80@AzoMOF. The inset shows the changes of characteristic peaks for


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Azo

Azo

MOF after incarceration of Sc3C2@C80. (d) N2 adsorption isotherms of

MOF and

Sc3C2@C80@AzoMOF at 77 K. Similar to the azobenzene building blocks in solution (Figure S7), irradiation of

Azo

MOF and

metallofullerene@AzoMOF with UV light results in an obvious n-π* band enhancement, indicating the photo-switch of azobenzene units in AzoMOF between trans-form and cis-form, see Figure 3a and 3b. But for the reverse isomerization, they exhibit different changes. The reverse isomerization of UV-radiated 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 photo-isomerization (cis-to-trans) of azobenzene units (Figure S7). Similarly, the reverse isomerization of UV-radiated AzoMOF with visible light results in an obvious n-π* band decrease, also indicating the reverse isomerization of azobenzene units from cis-form to trans-form in Azo

MOF (Figure S8). However, after visible light treatment, the decrease of n-π* band is invisible

for UV-radiated Sc3C2@C80@AzoMOF (Figure 6a), which illustrates that the reverse isomerization

of

azobenzene

units

from

cis-form

to

trans-form

is

blocked

for

Sc3C2@C80@AzoMOF. The PXRD patterns of AzoMOF and Sc3C2@C80@AzoMOF upon UV irradiation stay changeless, illustrating the integrity of AzoMOF after UV irradiation (Figure 3c, S9 and S11). In AzoMOF, the cavities are surrounded by azobenzene units, and the photo-isomerization of azobenzene units in Azo

MOF leads to exquisite geometrical and dipole moment changes of frameworks. To be

specific, the photo-isomerization of azobenzene involves not only the distance change of the para-carbon atoms in an azobenzene unit from 9 Å in the trans-isomer 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 cis-one.34 As a result, the trans-to-cis photo-isomerization of azobenzene building blocks in


Azo

MOF

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promoted both steric effect (pore blockage) of the diffusion event and the dipole-quadrupole interaction between the pore surface and N2 molecules, resulting in reduced N2 uptake of Azo

MOF right after UV irradiation (Figure 3d). In addition, the decreased N2 uptake of

Azo

MOF

right after UV irradiation can be maintained even if it has been kept in the dark for three days, confirming the stable cis-form of azobenzene groups in AzoMOF.

Figure 3. Diffuse reflectance spectra of AzoMOF (a) and Sc3C2@C80@AzoMOF (b) at 298 K upon irradiation with UV light for different time. Arrows clarify the trends of the spectral changes. (c) PXRD patterns of isotherms of

Azo

MOF upon irradiation with UV light for different time. (d) N2 adsorption

Azo

MOF in pristine state (black curve) and right after irridiated with UV for 30

minutes (red curve) and kept in the dark for 3 days after irridiated with UV for 30 minutes (blue curve). Electron paramagnetic resonance (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 paramagnetic


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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 Sc3C2@C80@AzoMOF film, as shown in Figure 4a. The

Azo

MOF film was synthesized on the

functionalized gold surface.35 It can be clearly seen from the Figure S10 that the PXRD spectrum of

Azo

MOF film is identical to that of

structure of

Azo

MOF powder, indicating its maintained crystalline

Azo

MOF. The scanning electron microscope (SEM) image showed a homogeneous

MOF membrane with approximately 3 µm of thickness on the top of gold surface. Also, we performed EPR spectra of

Azo

MOF in pristine state and UV-irradiated state to exclude the

influence of background (Figure S5). The resultant spectra of AzoMOF film show no EPR signal. We also performed the time-dependent EPR spectra of Sc3C2@C80@AzoMOF film in dark after irradiated with UV light for 5 minutes (Figure S4) to avoid the influence of charge transfer between them, and the unchanged EPR signal intensity indicates that charge transfer doesn’t occur between Sc3C2@C80 and aromatic pores in AzoMOF along with EPR measurements.


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


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Notably, after 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 EPR spectra (the maximum difference of positive values and negative values at the central part of EPR spectra) for Sc3C2@C80@AzoMOF film upon UV irradiation at different time was plotted (Figure 4c). It can clearly be seen that the EPR signal intensity obviously decreases. Also, the line broadening effect of EPR spectra for Sc3C2@C80@AzoMOF film upon UV irradiation can be observed (Figure 4d and 4f). For Sc3C2@C80 in

Azo

MOF, the line width of the peak-to-peak

depends on the quantum number ( ) in the EPR spectra (Figure 4d) and can be expressed

as the form k k k m k

where all the parameters depend on the rotational correlation time ( 4 /3 ), which is originated from the Debye theory of rotational relaxation.17, 38 In this formula, is the viscosity of the solvent and is the hydrodynamic radius of the molecule. The decreased EPR signal intensity and line broadening effect of Sc3C2@C80@AzoMOF film upon UV irradiation can be ascribed to the changes of host-guest interaction between Sc3C2@C80 and AzoMOF.

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


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The photo-responsive azobenzene units in

Azo

MOF 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). Due to the susceptible spin of metallofullerene Sc3C2@C80, the photo-switchable geometrical and polar changes in

Azo

MOF

could influence its paramagnetic properties through changing the π-π interaction between Sc3C2@C80 and the confined space of isomerization of

Azo

MOF. After irradiation with UV light, the photo-

Azo

MOF leads to stronger host-guest interaction between

Azo

MOF 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 viscosity and rotational correlation time. As a result, the EPR signal intensity decreases and the line width broadens for Sc3C2@C80@AzoMOF film upon UV irradiation.

Figure 6. (a) Diffuse reflectance spectra of Sc3C2@C80@AzoMOF at 298 K upon irradiation with visible light for different time. Arrows clarify the trends of the spectral changes. Inset: enlargement of the signal at 400-550 nm. (b) EPR spectra of Sc3C2@C80@AzoMOF film after UV irradiation (black) and irradiated with visible light for 5 h after UV irradiation (red). We further performed the EPR measurements of Sc3C2@C80@AzoMOF film and tried to realize the recoverable paramagnetic properties through visible light or heat treatment (Figure 6 and S6). We treated the UV-radiated Sc3C2@C80@AzoMOF film with visible light (475 nm) for 5 h and


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measured its EPR signals (Figure 6b), which shows little changes. We also heated the UVradiated Sc3C2@C80@AzoMOF film at 353 K for 8 h and then measured its EPR signals (Figure S6), which also shows little changes. The reason is proposed that there is stronger host-guest interaction between

Azo

MOF and Sc3C2@C80 after UV-irradiation, hindering the cis-to-trans

isomerization of azobenzene units with visible light or heat treatment. The recoverable magnetism of 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 Sc3C2@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 photo-isomerization of

Azo

MOF was also

employed to investigate the single-molecule magnet behavior of DySc2N@C80. We performed magnetic hysteresis and magnetic susceptibility measurements on DySc2N@C80@AzoMOF powder with SQUID magnetometer (Figure 7 and S13). For comparison, we performed magnetic hysteresis and magnetic susceptibility measurements on DySc2N@C80 (Figure 7a and S12). We also performed magnetic measurements on

Azo

MOF before and after UV irradiation to exclude

background contribution, see Figure S14 and S15. After exact subtraction of the background contribution of

Azo

MOF powders, the magnetic properties of DySc2N@C80@AzoMOF in pristine

state and after irradiation with UV light still exhibit changes (Figure 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 DySc2N@C80@AzoMOF, it exhibits distinctly different magnetization loops and there is an increased remanent


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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 AzoMOF is suppressed, which is originated from the crystal-field perturbation of and

Azo

MOF. The strong host-guest interactions between DySc2N@C80

Azo

MOF generate crystal-field splitting. As a result, the magnetic ground states are split and

the quantum coherence of DySc2N@C80 in AzoMOF is influenced.

Figure 7. (a) Hysteresis loops for DySc2N@C80 at 2 K. (b) Hysteresis loops for DySc2N@C80@AzoMOF in pristine state (black) and after irradiated with UV for 30 min (red) at 2 K. After DySc2N@C80@AzoMOF powder was irradiated with UV light for 30 min, it shows enhanced remanent magnetization at zero field (H = 0 T) at 2 K compared to it in pristine state (Figure 7b). As discussed above, the photo-isomerization of AzoMOF induces more strengthened host-guest interaction between DySc2N@C80 and

Azo

MOF，leading to the more suppressed

QTM effect and influencing the response of hosted DySc2N@C80 to external magnetic field. Comparing with DySc2N@C80@AzoMOF in pristine state, there is larger remanence at low field for DySc2N@C80@AzoMOF after UV irradiation, indicating the improved memory of magnetization history in zero field. As a result, the crystal-field perturbation from

Azo

MOF is

enhanced and the ground states of DySc2N@C80 can be changed. In general, a noncontact control


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of single-molecule magnet behavior for DySc2N@C80 was realized by incorporating it into photo-responsive

Azo

MOF. This photo-responsive SMM behavior of DySc2N@C80@AzoMOF

may endow it with potential to record information with light. CONCLUSION In summary, we have realized optically controlled magnetic properties for Sc3C2@C80 and DySc2N@C80 by accommodating them into the pores of photo-responsive AzoMOF. The trans/cis isomerization of azobenzene units in geometries and dipole moments for

Azo

MOF upon UV irradiation triggers changes of steric

Azo

MOF pores. By using of these changes, the host-guest

interaction between magnetic metallofullerenes (Sc3C2@C80 and DySc2N@C80) and

Azo

MOF

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 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 Supporting Information. Materials, synthesis, HPLC data, MALDI-TOF MS data, EPR spectrum, PXRD data and other characterizations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author *E-mail: [email protected]; [email protected] Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (61227902, 51672281, 51472248), Beijing Natural Science Foundation (2162050). T. Wang particularly thanks the Youth Innovation Promotion Association of CAS (2015025). REFERENCES (1) Affronte, M.; Casson, I.; Evangelisti, M.; Candini, A.; Carretta, S.; Muryn, C. A.; Teat, S. J.; Timco, G. A.; Wernsdorfer, W.; Winpenny, R. E. Linking rings through diamines and clusters: exploring synthetic methods for making magnetic quantum gates. Angew. Chem. Int. Ed. 2005, 44, 6496-6500. (2) Bogani, L.; Wernsdorfer, W. Molecular spintronics using single-molecule magnets. Nat. Mater. 2008, 7, 179. (3) Zadrozny, J. M.; Gallagher, A. T.; Harris, T. D.; Freedman, D. E. A Porous Array of Clock Qubits. J. Am. Chem. Soc. 2017, 139, 7089-7094. (4) Milios, C. J.; Vinslava, A.; Wernsdorfer, W.; Moggach, S.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. A record anisotropy barrier for a single-molecule magnet. J. Am. Chem. Soc. 2007, 129, 2754-2755. (5) Deb, A.; Boron, T. T., 3rd; Itou, M.; Sakurai, Y.; Mallah, T.; Pecoraro, V. L.; Penner-Hahn, J. E. Understanding spin structure in metallacrown single-molecule magnets using magnetic compton scattering. J. Am. Chem. Soc. 2014, 136, 4889-4892.


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Table of Contents We report the optically controlled molecular magnetism of metallofullerenes by incarcerating them into the pores of photo-switchable azobenzene-functionalized MOF.


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Optically Controlled Molecular Metallofullerene Magnetism via an

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