Direct Imaging of Isolated Single Molecule Magnets in Metal-Organic

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Direct Imaging of Isolated Single Molecule Magnets in Metal-Organic Frameworks Darpandeep Aulakh, Lingmei Liu, Juby R. Varghese, Haomiao Xie, Timur Islamoglu, Kyle Duell, Chung-Wei Kung, Chia-En Hsiung, Yuxin Zhang, Riki J Drout, Omar K. Farha, Kim R. Dunbar, Yu Han, and Mario Wriedt J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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Journal of the American Chemical Society

Direct Imaging of Isolated Single Molecule Magnets in Metal-Organic Frameworks Darpandeep Aulakh,†,|| Lingmei Liu,‡,|| Juby R. Varghese,† Haomiao Xie,§ Timur Islamoglu,ξ Kyle Duell,† Chung-Wei Kung,ξ Chia-En Hsiung,‡ Yuxin Zhang,ǂ Riki J. Drout,ξ Omar K. Farha,ξ Kim R. Dunbar,§ Yu Han‡ and Mario Wriedt*,† †

Department of Chemistry & Biomolecular Science, Clarkson University, Potsdam, New York

13699, United States of America ‡

Advanced Membranes and Porous Materials Center, Physical Sciences and Engineering

Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia §

Department of Chemistry, Texas A&M University, College Station, Texas 77845, United States

of America ξ

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois

60208, United States of America ǂ

Multi-Scale Porous Materials Center, Institute of Advanced Interdisciplinary Studies & College

of Materials Science and Engineering, Chongqing University, Chongqing 400044, P. R. China ||

D.A. and L.L contributed equally to this work.

* Tel.: +1(315) 268-2355 Fax: +1(315) 268-6610 Email: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT Practical applications involving the magnetic bistability of single-molecule magnets (SMMs) for next-generation computer technologies require nanostructuring, organization and protection of nanoscale materials in two- or three-dimensional networks, to enable read-and-write processes. Owing to their porous nature and structural long-range order, metal-organic frameworks (MOFs) have been proposed as hosts to facilitate these efforts. Although probing the channels of MOF composites using indirect methods is well established, the use of direct methods to elucidate fundamental structural information is still lacking. Herein we report the direct imaging of SMMs encapsulated in a mesoporous MOF matrix using high-resolution transmission electron microscopy (HRTEM). These images deliver, for the first time, direct and unambiguous evidence to support the adsorption of molecular guests within the porous host. Bulk magnetic measurements further support the successful nanostructuring of SMMs. The preparation of the first magnetic composite thin films of this kind furthers the development of molecular spintronics.

INTRODUCTION SMMs are discrete molecular magnets of nanoscale proportions1-5 with potential applications in molecular spintronics as units for ultra-high-density information storage and quantum computing.6-12 Characteristic features of these molecular nanomagnets are that they exhibit slow relaxation of their magnetization below a threshold temperature and display magnetic hysteresis of purely molecular origin due to the combination of axial magnetic anisotropy and spin which leads to an energy barrier to reversing the direction of the magnetic moment. The resulting magnetic bistability allows for each molecule to behave as a single-domain magnetic nanoparticle


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which, unlike traditional bulk ferromagnets, would allow the design of data storage materials exceeding Kryder’s law which predicts the doubling of data storage densities every 13 months. Although the design of advanced molecular magnets has been a subject of intense research over the past several decades, their organization into 2D or 3D architectures, where each individual molecule can be used as a magnetic bit of information for read-and-write processes, remains a fundamental challenge in the field. For example, the exploration of lithographic techniques and self-assembly depositions on 2D support materials aimed at isolating nanostructures of SMMs led to magnetic composites that exhibited only structural short-range order of magnetic domains with mostly diminished magnetic properties.13-16 In recent landmark studies we closed this gap by demonstrating that mesoporous MOFs can be used to template the long-range nanostructuring of SMMs.17, 18 Key features of MOFs such as their crystalline nature, high thermal and chemical stability combined with the structural tunability of their pore size and topology render them perfect candidates to encapsulate SMMs. We demonstrated that the resulting novel SMM@MOF composite materials exhibit improved stabilities with the preservation of magnetic bistability while being embedded in a long-range porous matrix. In this context, structural characterization of these SMM nanostructures hosted in porous substrates has been established based solely on indirect analysis methods including a combination of specific surface area and pore volume analysis, Xray diffraction and thermal analyses.19 Incontrovertible evidence of SMM isolation through direct real space imaging techniques, such as HRTEM, however, has not been accessible due to the electron-beam sensitivity of the host-guest composites. This gap in structural analysis has been recently filled by Han et al. who reported the atomic-resolution imaging of MOFs using lowelectron dose HRTEM.20-22 This breakthrough study inspired us to employ the same technique to extract previously inaccessible direct morphological and structural information of SMM@MOF



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composites. To the best of our knowledge, this is the first report, in real space, of direct visualization of guest molecules encapsulated in the pores of a MOF. Specifically we designed a new SMM@MOF composite composed of the SMM [Mn12O12(O2CCH3)16(OH2)4] or Mn12Ac23, 24 and the MOF [Zr6(μ3–OH)8(OH)8(tbapy)2] (tbapy = 3,6,8-tetrakis(p-benzoic acid) pyrene) or NU100025 (Figure 1). Remarkable features such as: small size (1.6 nm ø) (Figure S1), high chemical stability under given experimental conditions, and a reasonably high blocking temperature of 3 K characterize the archetype SMM Mn12Ac as a perfect guest molecule, whereas the host framework NU-1000 is distinctive with its mesoporous nature of one-dimensional hexagonal channel pores (∼ 3 nm ø) and outstanding thermal and chemical stability. The successful bulk synthesis of this nanostructured magnetic composite along with its thorough structural and magnetic characterization are described. Impressively, these findings represent the molecular-resolution direct imaging at the host-guest interface showing the isolation of SMM molecules while their magnetic bistability is preserved under given nanoconfinement conditions. In addition, we present the first evidence of the successful fabrication of thin-film magnetic composites.


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Figure 1. Crystal structures of NU-1000 and Mn12Ac with the schematic representation of their host-guest arrangement. The nanostructuring of Mn12Ac guest molecules is represented in a realistic size relationship with respect to the NU-1000 host framework. Structures are shown in a space filling style, hydrogen atoms are removed for the sake of clarity. Color scheme: Mn, yellow; Zr, cyan; C, gray; and O, red.

RESULTS AND DISCUSSION Synthesis and compositional characterization. Mn12Ac@NU-1000 was prepared under mild solvent-mediated conditions by soaking crystals of NU-1000 in a concentrated acetonitrile solution of Mn12Ac (NPT, stirring for 12 h) followed by filtration and several acetonitrile washings of the crystals to remove any residual Mn12Ac molecules not incorporated within NU-1000. Subsequent characterization including inductively coupled plasma-optical emission spectroscopy (ICP-OES), energy-dispersive X-ray spectroscopy (EDX), nuclear magnetic resonance spectroscopy (NMR), powder X-ray diffraction (PXRD), physisorption (BET surface area and DFT pore size distribution), and thermal analyses (TGA and DSC) provided fundamental insight into the chemical composition and physical nature of the composite.


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Figure 2. Compositional and structural characterizations of Mn12Ac@NU-1000. (a) EDX spectrum of Mn12Ac@NU-1000. (b) PXRD patterns of NU-1000 (red) and Mn12Ac@NU-1000 (black). (c) N2 adsorption (filled symbols) and desorption (open symbols) isotherms of NU-1000 (red) and Mn12Ac@NU-1000 (black) collected at 77 K. (d) DFT pore size distributions by incremental pore volume of NU1000 (red) and Mn12Ac@NU-1000 (black). (e) TGA and (f) DSC curves of Mn12Ac (blue), NU-1000 (red), and Mn12Ac@NU-1000 (black). 6 ACS Paragon Plus Environment

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ICP-OES analysis of bulk Mn12Ac@NU-1000 yielded an average Mn:Zr atomic ratio of 0.68 (Table S1), corresponding to a 33.9 mol% or 26.3 wt% loading capacity of Mn12Ac. This corresponds to 0.34 Mn12Ac molecules per formula unit of NU-1000. Similar values of 36.2 mol% or 0.36 SMM per MOF were found by EDX analyses (Figure S6 and S7B, Table S2). Although both values are similar, the results from ICP analysis are more representative of the bulk composition of Mn12Ac@NU-1000. This qualitative assessment is further supported by NMR measurements of digested Mn12Ac@NU-1000 revealing the exact number of acetate molecules expected for 34 mol% of Mn12Ac per NU-1000 (Figure S8). The compositional stability of Mn12Ac after loading into NU-1000 can be also confirmed from this NMR experiment since exactly 16 acetate molecules were found per Mn12Ac cluster. Interestingly, the found loading capacity is significantly higher than previously reported molar loading capacities of porous media supported composites which usually range from 1.0 to 3.1 mol%.19 In addition, elemental mapping revealed an even distribution of Mn12Ac throughout the NU-1000 crystals (Figure S7). The structural integrity of the host scaffold upon composite formation was confirmed by the absence of any additional Bragg reflections in the PXRD pattern of Mn12Ac@NU-1000 vs. NU-1000 (Figure 2b). Notably, the peak intensity of the (100) reflection in Mn12Ac@NU-1000 has significantly decreased compared to NU-1000 which is attributed to a progressive phase cancellation phenomenon associated with the introduction of scattering material into the mesopore, therefore presenting the first evidence that Mn12Ac is encapsulated in NU-1000 rather than crystallizing on or adhering to its surface. The N2 adsorption-desorption experiments of NU-1000 and Mn12Ac@NU-1000 were performed to corroborate the aforementioned findings. Consistent type IV isotherms were observed with one well-defined step at intermediate partial pressures (~ p/p0 = 0.25) which is



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attributed to the capillary condensation of N2 inside the mesopores while the absence of hysteresis loops, as well as their sharp curvature, confirms the presence of unimodal pore size distributions. A comparison of isotherms of Mn12Ac@NU-1000 vs. NU-1000 reveals a slight shift of the inflection point to lower pressure values along with a significant decrease in N2 uptake from 948 cm3 g-1 in NU-1000 to 608 cm3 g-1 in Mn12Ac@NU-1000 at 1 p/p0 which is consistent with SMM molecules occupying space in mesopores (Figure 2c). Subsequent BET surface area calculations combined with DFT pore-size distributions resulted in specific surface areas and total pore volumes of 2130 m2 g-1 and 1.22 cm3 g-1 for NU-1000 vs. 1400 m2 g-1 and 0.82 cm3 g-1 for Mn12Ac@NU-1000. These data correspond to an experimental drop in pore volume of 32% which is in reasonable agreement with an expected drop of 27% calculated from crystallographic parameters of 34 mol% Mn12Ac incorporated into NU-1000. It is important to note that both pore regimes, the micropore and mesopore volumes, are impacted upon loading of Mn12Ac, although the adsorption of Mn12Ac can only occur in the mesopore of NU-1000 due to the large size of Mn12Ac (1.6 nm) (Figure 2d). This observation is not surprising as the partial decomposition of Mn12Ac and its small-molecule decomposition products are adsorbed in situ into the micropores during the process of thermal activation for adsorption analysis (1·10-6 bar, 150 °C, 24 hours). The small difference in pore volume drop experimental vs. expected of 5% can be attributed to this decomposition process. These findings further corroborate the inclusion of Mn12Ac into the pores of NU-1000. The thermal properties of Mn12Ac@NU-1000 were probed by TGA and DSC measurements (Figures 2e, f). The TGA data reveal a gradual mass loss of 13.8% upon heating Mn12Ac@NU-1000 to 350 °C which can be assigned to a combination of solvent removal and thermal decomposition of the adsorbed SMM clusters in addition to the dehydration of the MOF


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node. The quantification of this decomposition process can be understood by acknowledging that a 52.0% mass loss is observed upon heating the pristine SMM to 350 °C, which directly translates to a calculated mass loss of 12.6% for the decomposition of Mn12Ac in NU-1000 considering the 26.3 wt% loading capacity from ICP analysis. The variance between experimental and expected values, 13.8% - 12.6% = 1.2%, is attributed to the removal of solvent molecules as evidenced by the broad endothermic event in the 40 - 110 °C range of the DSC curve (Figure 2f). The most distinct feature of the thermal analysis remains the shift of the exothermic event in the DSC curve to higher temperatures comparing Mn12Ac (~210 °C) vs. Mn12Ac@NU-1000 (~250 °C). This event is attributed to the thermal decomposition of Mn12Ac, which indicates that the confinement of Mn12Ac within the nanoscopic cavities of NU-1000 significantly enhances the thermal stability of SMM molecules and that the overall framework stability remains unchanged. High-resolution transmission electron microscopy. A low-dose HRTEM technique was employed to directly probe the locations of Mn12Ac in NU-1000. Given the rod-like crystal morphology of NU-1000, ultramicrotomy was used to thinly slice the crystals (~70 nm) in order to facilitate the orientation of the structure along the [001] direction, i.e., the direction of onedimensional channels. A typical HRTEM image of Mn12Ac@NU-1000 acquired along the [001] direction is presented in Figure 3, which displays the ordered arrangement of the primary hexagonal channels and smaller trigonal channels of NU-1000. Notably, discrete particles with uniform sizes of ~2 nm are clearly identified by the image contrast throughout the observed area, as indicated by the arrows in Figure 3a. The enlarged images show that these particles reside precisely in the primary hexagonal channels, indicating the successful encapsulation of Mn12Ac in the pores of NU-1000 (Figures 3b, 3c). EDX spectroscopy detected apparent signals of element Mn in the imaged area (Figure S9). Due to the vertical distribution of Mn12Ac clusters in the



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HRTEM specimen (~ 70 nm thick) along the projection direction, all encapsulated clusters could not be observed at the same time, which indicates that the actual concentration of Mn12Ac in NU1000 might be much higher than the concentration observed in a single image (see Section S6 in SI for additional details). To confirm this, we intentionally prolonged the exposure time to damage the MOF structure, which resulted in shrinkage of the specimen and thus a variation in the height of Mn12Ac along the one-dimensional channeled pore.

2 nm

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Figure 3. Structural characterization of Mn12Ac@NU-1000 by HRTEM. (a) HRTEM image and electron diffraction pattern (inset) of Mn12Ac@NU-1000 acquired along the [001] zone axis of NU-1000. Arrows indicate the observed particles that correspond to Mn12Ac. (b) and (c) Enlarged images of the highlighted areas in areas 1 and 2 respectively, showing that the clusters of Mn12Ac


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are precisely encapsulated and perfectly fitted in the hexagonal channels of NU-1000. Images were processed by using a Gaussian filter to enhance the signal to noise ratio.

As expected, many discrete particles (Mn12Ac clusters) that were not observed in the initial HRTEM image due to improper focus conditions emerged in the imaged area with prolonged exposure (Figure 4a: right vs. left). To further confirm that the observed 2 nm particles are encapsulated Mn12Ac clusters, a side-by-side imaging comparison of NU-1000 and Mn12Ac@NU1000 is shown in Figure 4. The results show that regardless of the exposure time, no discrete particles were observed in the pristine sample of NU-1000 with intact or damaged structure (Figure 4b), thus precluding the possibility that the observed particles are made of Zr originating from the degradation of NU-1000.



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Figure 4. Structural changes upon decomposition of NU-1000 and Mn12Ac@NU-1000 monitored by HRTEM. Comparison of (a) Mn12Ac@NU-1000 and (b) pristine NU-1000. In both (a) and (b) the left images were acquired with short exposure times to maintain the crystalline structure of NU-1000 in order to visualize the ordered arrangement of the channels which are very sensitive to the electron beam; the right image was acquired for exactly the same area but with prolonged exposure time that resulted in the partial damage of the structure of NU-1000. The insets are enlarged images of the highlighted areas, showing hexagonal channels encapsulating Mn12Ac in NU-1000 versus empty hexagonal channels in pristine NU-1000. 12 ACS Paragon Plus Environment

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Magnetic studies. The magnetization dynamics of Mn12Ac@NU-1000 were investigated extensively using variable-frequency and variable-temperature ac susceptibility measurements. The data were collected under a zero dc field over a frequency range of 1-1500 Hz which revealed significant frequency dependence of the out-of-phase ac magnetic susceptibility (χ″) signal, the maxima of which shift to higher temperatures with increasing frequencies (Figure 5). This behavior is typical of the slow relaxation of the magnetization known for Mn12Ac, and is in accord with the conclusion that the unique magnetic properties of Mn12Ac are preserved during the incorporation process into NU-1000. Notably, additional less-intense and frequency-dependent χ″ peaks at lower temperatures (below 3 K) were observed which are attributed to the presence of a known fast-relaxing isomer of Mn12Ac in which the Jahn-Teller elongation axis of one of the Mn3+ centers is tilted 90° with respect to the normal slow-relaxing Mn12Ac species (Figure 5b).26 Interestingly, when compared to bulk Mn12Ac (Figure S10) this isomer is more prominent in the Mn12Ac@NU-1000 composite, which may originate from a change in the chemical environment of Mn12Ac upon nano-confinement in NU-1000. Similar Jahn-Teller isomerization has been reported for Mn12Ac embedded in polymer matrices,27,

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carbon nanotubes,29 and pressure-

induced environments.30 Fitting the corresponding temperature and frequency of the observed χ″ maxima to the Arrhenius law, τ = τ0 exp(ΔE/kBT), where τ is the relaxation time of magnetization, T is the temperature, ΔE is the activation energy, and kB is the Boltzmann constant, resulted in an energy barrier of ΔE/kB = 69(1) K and a pre-exponential factor τ0 = 2.0(4)·10-8 s for the relaxation processes (Figure S11). These values compare favorably to the values of 70 K and 1.2·10-8 s for bulk Mn12Ac. Additionally, the temperature dependence of dc and the real-part of the ac magnetic susceptibility ' were compared to rule out the interpretation that the observed SMM behavior is from a small fraction of the sample. It was found that χ'T gradually converges to χdcT as frequency



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decreases, thus suggesting that the entire paramagnetic species in Mn12Ac@NU-1000 contributes to the slow relaxation of magnetization behavior (Figure S13). Additional magnetic characterization via variable-temperature dc analysis χT also revealed behavior characteristic of SMMs (Figure S12). It should be noted however, that χT values of Mn12Ac@NU-1000 with 15 emu K mol-1 at room temperature (RT) and 17.5 emu K mol-1 at low temperature (LT) deviate from the literature and expected values of bulk Mn12Ac. For reference, reported χT values of Mn12Ac were 19.4 (RT) and 55.6 emu K mol-1 (LT) while values of 31.5 (RT) and 55 emu K mol-1 (LT) are expected for species with spin ground states of S = 10.31 The deviation at RT is compatible but lower than the expected value in agreement with antiferromagnetic coupling, whereas the discrepancy at LT can be assigned to the nanoconfinement of Mn12Ac inside the cavities of NU-1000 resulting in different magnetic moment orientations of Mn12Ac. The orientation problem could be trivial at RT due to thermal motion, but at LT, the magnetic moment axis could be frozen and anisotropic. It is known that χ’T values at LT are very different when parallel (~143 emu K mol-1) or perpendicular (~10 emu K mol-1) to the crystallographic c axis of a Mn12Ac single crystal.32 The LT value of 17.5 emu K mol-1 for Mn12Ac@NU-1000 fits well within this range reported for single crystals, and is also similar to the LT value of ~24 emu K mol-1 reported for [email protected] Field-dependent magnetization measurements also revealed behavior characteristic of SMMs (Figure 5c). The magnetic hysteresis at 1.8 K measures a sizeable coercivity of 0.8 T, remanence of 2 µB and saturation of 5 µB at 2 T, while a narrowing of the hysteresis loop at low fields is consistent with quantum tunneling of the magnetization. Although these characteristics of hysteresis are lower than those reported for bulk Mn12Ac,31, 33 they fit well within the range reported for similar nano-confined Mn12Ac-based composites, such as Mn12Ac@EDTrA40,34


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Mn12Ac@SBA(60),35 Mn12Ac@MCM-41,36 Mn12Ac@SBA-15,37 and Mn12Ac@CYCU-3. Specifically, a lower coercivity value for Mn12Ac@NU-1000 with respect to bulk Mn12Ac confirms a shorter spin relaxation time for the confined Mn12Ac inside NU-1000 which is also in agreement with the slightly lower energy barrier of Mn12Ac@NU-1000 vs. bulk Mn12Ac. In summary, the above detailed magnetic characterization demonstrates incontrovertibly that the general SMM behavior of Mn12Ac inside NU-1000 remains largely unaffected by the nanoscale confinement while the quantification of important magnetic parameters reveals the frozen orientation of magnetic moments at low temperature.

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Figure 5. Magnetic properties of Mn12Ac@NU-1000. (a) Frequency dependence of the out-ofphase ac magnetic susceptibility data at different temperatures. (b) Temperature dependence of the out-of-phase ac magnetic susceptibility at different ac frequencies. (c) Field dependent hysteresis of the magnetization.

Thin film synthesis and characterization. Among the various existing approaches to the nanostructuring of SMMs, grafting them onto surfaces is the method that is most prominently employed. These surface composites are expected to represent the next step toward device fabrication. Surfaces of gold, silicon and graphite have been thoroughly explored as suitable substrates in this regard.15, 16 Their short-range order, lack of stability and difficulty in processing, however, create impediments to further progress in this field. Therefore, as emphasized above, SMM@MOFs composites represent excellent candidates for overcoming these challenges. Since thin film growth is a key feature in enabling the expansion of MOF applications38, 39 we herein report the first study on the design and synthesis of a thin film of SMM@MOFs composite guided by our previous work on bulk composites.14, 17, 18 The availability of mesoporous MOF thin-films significantly limits progress in this field, but NU-1000 represents the first MOF of a mesoporous nature available as thin films, featuring free-standing single-crystalline sub-micrometer rods of 93%. The nonspin flip cross-sections, (R+) and (R-), are sensitive to the nuclear scattering length density depth profile, and their difference is related to the magnetic depth profile for the projection of the magnetization parallel to the applied field. The spin asymmetry, SA = (R+ – R–) / (R+ + R–), is extremely sensitive to the magnetic depth profile and was calculated from the raw data.

Magnetic measurements. Magnetic measurements were carried out on a Quantum Design MPMS XL SQUID magnetometer over the temperature range of 1.8-300K. AC magnetic susceptibility data were collected with an oscillating measuring field of 5 Oe in the frequency range of 1-1500 Hz. All magnetic data were scaled per mole of Mn12Ac incorporated in NU-1000 (33.9 mol%) while accounting for the diamagnetic contribution from Pascal’s constants of MOF, SMM, residual solvent (5.5 acetonitrile molecules per 0.34Mn12Ac@NU-1000) and sample holder.

ASSOCIATED CONTENT Supplementary information is available in the online version of the paper. This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS M.W. gratefully acknowledge Clarkson University for their generous start-up funding. We also acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work and we thank Julie Borchers for designing and performing PNR experiments. The magnetic measurements were conducted in the Department of Chemistry SQUID Facility at Texas A&M University with a magnetometer obtained by a grant from the National Science Foundation (CHE-9974899). The magnetic work in this study was performed by the K.R.D. group and was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-02ER45999. Y.H. thanks the support from King Abdullah University of Science and Technology through center competitive fund (FCC/1/197234), and O.K.F. gratefully acknowledges support from the Defense Threat Reduction Agency (HDTRA1-18-1-0003).


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Graphical Abstract



Crystal structures of NU-1000 and Mn12Ac with the schematic representation of their host-guest arrangement. The nanostructuring of Mn12Ac guest molecules is represented in a realistic size relationship with respect to the NU-1000 host framework. Structures are shown in a space filling style, hydrogen atoms are removed for the sake of clarity. Color scheme: Mn, yellow; Zr, cyan; C, gray; and O, red. 997x491mm (96 x 96 DPI)

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Compositional and structural characterizations of Mn12Ac@NU-1000. (a) EDX spectrum of Mn12Ac@NU1000. (b) PXRD patterns of NU-1000 (red) and Mn12Ac@NU-1000 (black). (c) N2 adsorption (filled symbols) and desorption (open symbols) isotherms of NU-1000 (red) and Mn12Ac@NU-1000 (black) collected at 77 K. (d) DFT pore size distributions by incremental pore volume of NU-1000 (red) and Mn12Ac@NU-1000 (black). (e) TGA and (f) DSC curves of Mn12Ac (blue), NU-1000 (red), and Mn12Ac@NU-1000 (black). 1346x1193mm (96 x 96 DPI)



Structural characterization of Mn12Ac@NU-1000 by HRTEM. (a) HRTEM image and electron diffraction pattern (inset) of Mn12Ac@NU-1000 acquired along the [001] zone axis of NU-1000. Arrows indicate the observed particles that correspond to Mn12Ac. (b) and (c) Enlarged images of the highlighted areas in areas 1 and 2 respectively, showing that the clusters of Mn12Ac are precisely encapsulated and perfectly fitted in the hexagonal channels of NU-1000. Images were processed by using a Gaussian filter to enhance the signal to noise ratio. 935x603mm (96 x 96 DPI)


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Structural changes upon decomposition of NU-1000 and Mn12Ac@NU-1000 monitored by HRTEM. Comparison of (a) Mn12Ac@NU-1000 and (b) pristine NU-1000. In both (a) and (b) the left images were acquired with short exposure times to maintain the crystalline structure of NU-1000 in order to visualize the ordered arrangement of the channels which are very sensitive to the electron beam; the right image was acquired for exactly the same area but with prolonged exposure time that resulted in the partial damage of the structure of NU-1000. The insets are enlarged images of the highlighted areas, showing hexagonal channels encapsulating Mn12Ac in NU-1000 versus empty hexagonal channels in pristine NU-1000. 1175x1111mm (96 x 96 DPI)



Magnetic properties of Mn12Ac@NU-1000. (a) Frequency dependence of the out-of-phase ac magnetic susceptibility data at different temperatures. (b) Temperature dependence of the out-of-phase ac magnetic susceptibility at different ac frequencies. (c) Field dependent hysteresis of the magnetization. 1346x1193mm (96 x 96 DPI)


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PXRD characterization of thin films. Change in relative reflection intensities observed comparing thin films of NU-1000 (black) vs. Mn12Ac@NU-1000 (red). 269x207mm (300 x 300 DPI)



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Direct Imaging of Isolated Single Molecule Magnets in Metal-Organic

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