DOI: 10.1002/chem.201303585
On-Surface Magnetometry: The Evaluation of Superexchange Coupling Constants in Surface-Wired Single-Molecule Magnets Erik Tancini,[a] Matteo Mannini,[b] Philippe Sainctavit,[c, d] Edwige Otero,[d] Roberta Sessoli,[b] and Andrea Cornia*[a]
High-spin molecules exhibiting a memory effect (singlemolecule magnets, SMMs) are of continuing interest in the field of molecular spintronics, with the ultimate goal of probing and controlling their directionally bistable magnetic moment by electric currents.[1] Scanning probe methods offer the simplest solution to electrically address individual molecules deposited on a metal surface.[2] However, examples of SMMs that remain structurally and functionally stable on surface are so far extremely limited, as shown by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements. According to these techniques, which provide monolayer sensitivity along with element and valence specificity, SMMs belonging to the important Mn6[3a] and Mn12[3b–d] families are prone to structural or redox changes when interfaced with metal substrates. The investigation of surface-supported bis(phthalocyaninato)terbiumACHTUNGRE(III) complexes has further evidenced a strong reduction or even the complete suppression of magnetic hysteresis in otherwise intact molecules.[4] As a very welcome result, sulfur-functionalized tetraironACHTUNGRE(III) clusters (Fe4) with a metal-centered triangular geometry (Figure 1 a) still display slow magnetic relaxation when deposited on a gold film[5a,b] or linked to gold nanoparticles.[5c] To prove that their ferrimagnetic spin structure persists on surface, isostructural complexes with a central Cr3 + ion (Fe3Cr) (Figure 1 b) have been designed,[6] and the antiparallel alignment of Fe3 + (s = 5/2) and Cr3 + (s = 3/2) spins clearly visualized by XMCD.[6b]
[a] Dr. E. Tancini, Prof. Dr. A. Cornia Dipartimento di Scienze Chimiche e Geologiche & UdR INSTM Universit di Modena e Reggio Emilia via G. Campi 183, 41125 Modena (Italy) Fax: (+ 39) 059-373543 E-mail: [email protected] [b] Dr. M. Mannini, Prof. Dr. R. Sessoli Dipartimento di Chimica Ugo Schiff & UdR INSTM Universit di Firenze, 50019 Sesto Fiorentino (Italy) [c] Dr. Ph. Sainctavit IMPMC-CNRS UMR 7590, Universit Pierre et Marie Curie 75252 Paris Cedex 5 (France) [d] Dr. Ph. Sainctavit, Dr. E. Otero Synchrotron SOLEIL, LOrme des Merisiers, Saint Aubin BP48 91192 Gif-sur-Yvette Cedex (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303585.
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Figure 1. Structure of the metal/oxygen core in a) Fe4 and b) Fe3Cr species, with arrows depicting the spin arrangement in the ground state with total spin S. c) Molecular structure of 1. H atoms and disorder on alkyl chains have been omitted for clarity. Color code: large grey spheres = Fe3 + (s = 5/2), white sphere = Cr3 + (s = 3/2), small grey spheres = O, medium grey spheres = S, black spheres = C.
In this work, we have determined the strength of the Fe Cr antiferromagnetic coupling in a monolayer of Fe3Cr species deposited on gold. As in traditional magnetometry, this corresponds to evaluating the energy of excited spin states and requires temperature (T)-dependent studies. Because the magnetic polarization (and the XMCD signal) of paramagnets decreases rapidly with heating, few molecular materials were studied in this way as bulk phases.[7a,b] Even fewer were investigated as monolayers, and only in a very limited T range.[7c–e] Complex [Fe3Cr(L)2ACHTUNGRE(dpm)6] (1) was chosen for our study, because its tetraironACHTUNGRE(III) analogue 2 is one of the few SMMs that proved functionally active when deposited on gold.[5b] Here, Hdpm is dipivaloylmethane and H3L is 7-(acetylthio)2,2-bis(hydroxymethyl)heptan-1-ol, a thioacetyl-terminated tripodal ligand containing a pentamethylene spacer. Compound 1 was obtained as a crystalline material with a Fe/Cr ratio of 3.54, requiring residual Fe4 species to be present in the crystal lattice.[6a,b] Single-crystal structural analysis of 1 (Figure 1 c; Figures S1 and S2 and Tables S1 and S2 in Supporting Informa-
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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COMMUNICATION tion) showed that MO distances for the central ion are shorter than in 2 (1.949(3)–1.956(3) vs. 1.968(3)–1.989(3) ), thereby suggesting a Cr-centered structure.[6] The magnetic properties recorded on a microcrystalline sample support this view (Figure S3 in Supporting Information). The maximum cMT value (19.8 emu K mol1 at 5.5 K) is close to the Curie constant for a S = 6 ground state (21.0 emu K mol1), but significantly larger than expected for both Fe4 (S = 5) and potential iron-centered Fe3Cr (S = 4) species (15.0 and 10.0 emu K mol1, respectively). Assuming that all Cr-containing species are Cr-centered,[6a,b] the experimental Fe/Cr ratio indicates a 0.118 molar fraction of Fe4 complexes, the known magnetic contribution of which[5b] was then subtracted from the data. The corrected cMT product reaches 20.5 emu K mol1 at 5.5 K, a value fully consistent with an S = 6 ground state. For a quantitative data analysis we used a Heisenberg plus Zeeman Hamiltonian [Eq. (1)], which assumes threefold symmetry and includes both FeCr (J) and FeFe (J’) interactions: ^ ¼Jð^sFe1 ^sCr þ ^sFe2 ^sCr þ ^sFe20 ^sCr Þþ H J 0 ð^sFe1 ^sFe2 þ ^sFe1 ^sFe20 þ ^sFe2 ^sFe20 Þ þ gm0 mB HSi^si,z
ð1Þ
Herein, sˆi are the spin operators for the individual metal centers (atom labels in Figure 1 b) and H is the magnetic field, which is applied along z. The best-fit parameters so obtained are J = 15.25(13) and J’ = 0.40(4) cm1 with g = 2.002(2). To allow for a direct comparison with XMCD results, an alternative fit was carried out by fixing J’ to zero and applying no correction for Fe4 species, which gives J = 13.99(4) cm1 and g = 1.9705(5). Isothermal magnetization data below 5 K confirm the presence of a S = 6 ground state with an axial zero-field splitting parameter D = 0.18 cm1 (Figure S3 in Supporting Information).[6] By virtue of its easy axis anisotropy and high-spin ground state, 1 behaves as a SMM. Frequency- and T-dependent signals are in fact observed in the out-of-phase component of AC magnetic susceptibility in zero field. Detailed measurements were performed in 1 kOe applied field in order to slow down magnetic relaxation (Figure S4 in Supporting Information). Fitting of T-dependent relaxation times to the Arrhenius law (Figure S5 in Supporting Information) gives Ueff/kB = 10.9(2) K and t0 = 8.1(9) 108 s as activation parameters (in the analysis we neglected the contribution from Fe4 species, which are present in small amounts and have a lower susceptibility than the dominant Fe3Cr complexes). To test the potential of T-dependent XMCD magnetometry, we first investigated a drop-cast thick film of 1 as a bulk reference sample. XAS and XMCD spectra (Figure 2) were recorded at the Fe and Cr L2,3 edges at T = 1.8 K and H = 65 kOe in order to maximize the magnetic response of the sample. The observed XAS spectral shapes are in accordance with the presence of high-spin Fe3 + and Cr3 + centers pseudo-octahedrally coordinated by oxygen donors.[6b] The negative XMCD signal at the Fe L3 edge and the positive one at the Cr L3 edge already provide direct indication that
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Figure 2. XAS and XMCD spectra of a bulk sample of 1 at Fe (top) and Cr (bottom) L2,3 edges, recorded at 1.8 K and 65 kOe. The XMCD signal is expressed as percentage of the maximum in the L3 average spectrum, (s + + s)/2. Here s + and s are the cross sections with X-ray photon helicity parallel and antiparallel to the applied magnetic field, respectively.
Fe3 + and Cr3 + ions have opposite magnetic polarizations, namely parallel (for Fe3 + ) and antiparallel (for Cr3 + ) to the applied field.[6b] Maximum XMCD amplitudes are reached at 709.1 (Fe L3 edge) and at 577.5 eV (Cr L3 edge), at which the XMCD signal amounts to 89.6 and 76.1 %, respectively. The signal intensity decreases considerably at higher T (Figure S6 in Supporting Information), but the spectra maintain the same shape, meaning that all the involved electronic transitions are similarly affected by the thermal population of the excited spin states. Although the magnetic moment of 3d elements can in principle be derived from XMCD sum rules at L2,3 edges,[8] ligand field multiplet calculations showed that the local magnetic moments in these systems are accurately measured by XMCD amplitudes.[6b] Hence, in Figure 3 we present the T dependence of the XMCD signal at 709.1 and 577.5 eV multiplied by T (XMCD vs. T plots are in Figure S7 in Supporting Information). The peak in both plots directly confirms the occurrence of superexchange interactions accompanied by low-T saturation effects, as in cMT versus T curves obtained by traditional magnetometry at constant field. Interestingly, the responses at the two edges are not only different in sign, but also not proportional to each other. In fact, XMCD probes the local polarizations at metal sites, which sum up to give the overall molecular polarization but are not necessarily proportional to it.[7a] Using the scheme for superexchange interactions in Equation (1) and setting J’ = 0 to avoid over-
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Figure 3. XMCD response at 709.1 (Fe) and 577.5 (Cr) eV in bulk (top) and monolayer (bottom) samples of 1, along with best-fit curves. Scale factors were adjusted to bring the magnetic polarization at Cr and Fe sites on the same scale as the corresponding XMCD signal. Error bars have been omitted when smaller than the symbol size.
parametrization, we could satisfactorily fit the data in Figure 3 and estimate J = 14.3(7) cm1 (the percentage of Fe4 species was neglected). Note that even such a crude model correctly predicts the observed asymmetric response at Cr and Fe centers. Rewardingly, the estimated J value is within experimental error from that found using traditional methods. A self-assembled monolayer was then prepared by incubating a flame-annealed AuACHTUNGRE(111) substrate in a dichloromethane solution of 1 for 18 h. Over this timeframe, 1 remains intact in solution as shown by 1H NMR spectroscopy (Figure S8 in Supporting Information). XAS and XMCD spectra of the monolayer were recorded from 10 to 300 K in a field of 50 kOe (Figure S9 in Supporting Information). Because of the substantially reduced amount of absorbing material, the signal-to-noise ratio is lower than in the bulk reference. The Cr edge is particularly affected, since Cr amounts to less than 25 % of the total metal content. Nonetheless, the spectra show the same fine structure and a comparable amplitude with respect to the bulk phase, with opposite polarizations at the Fe and Cr L3 edges (Figure 4). Thus, antiferromagnetic FeCr interactions persist when 1 is adsorbed on gold.[6b] Most important, the XMCD signal of both ions exhibits a clear T dependence (Figure 3 and Figure S10 in Supporting Information), which closely parallels that found in our bulk sample. Fitting of the data to Equation (1) (again setting J’ = 0 and neglecting Fe4 species) gave J = 14.7(7) cm1. It is apparent that intramolecular superexchange interactions remain the same, within experimental error, as those found in the bulk phase. These results clearly
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Figure 4. XAS and XMCD spectra of a monolayer sample of 1 at Fe (top) and Cr (bottom) L2,3 edges, recorded at 10 K and 50 kOe. See caption to Figure 2 for definitions.
suggest that the surface environment does not significantly alter the electronic structure of the metallic core in terms not only of sign but also of strength of superexchange coupling interactions. Notice that with these J values the polarization at Cr should change sign around 180 K, as the applied field reverses the orientation of the magnetic moment on the central ion from field-antiparallel to field-parallel. At this T, however, the XMCD signal at Cr L2,3 edges is already far too small to be measured with the required precision in both drop cast and monolayer samples. In conclusion, the magnetic response of a monolayer sample of 1 has been probed by XMCD over a wide T range. FeCr superexchange interactions, which largely determine the pattern of spin states, remain the same as in bulk samples. We argue that the structure of the metal core is not significantly altered by interaction with the surface, a rare situation among complex molecular architectures based on coordination bonds.[7c–e] The rigid structure imposed by the two tripodal ligands and a weak electronic coupling with the substrate due to the bulky dpm groups are likely reasons for the behavior of these tetrametallic SMMs, which conform to the requirements for fundamental studies in single-molecule spintronics.
Experimental Section The procedure reported for the synthesis of a longer-chain analogue[6b] gave 1 in 55 % yield (see Supporting Information). 1H NMR (200 MHz, CD2Cl2, 25 8C, TMS): d = 11.0 (s, 108 H; tBu), 3.1 (s, 4 H; CH2-S),
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2.37 ppm (s, 6 H; CH3CO); elemental analysis calcd (%) for C88H152O20S2Fe3.118Cr0.882 : C 58.26, H 8.44, S 3.53, Fe 9.60, Cr 2.53; found: C 57.96, H 8.26, S 3.51, Fe 9.68, Cr 2.55. Crystal data for 1: C88H152CrFe3O20S2, Mr = 1813.77, crystal size: 0.40 0.28 0.12 mm3, monoclinic, C2/c, a = 24.8901(12), b = 16.2593(8), c = 24.9576(10) , b = 96.632(2)8, V = 10 032.6(8) 3, Z = 4, 1calcd = 1.201 g cm3, m = 0.633 mm1, FACHTUNGRE(000) = 3896, 2qmax = 48.028, T = 140(2) K, l = 0.71073 , collected/ unique reflections 66 821/7857 (Rint = 0.0555), data/restraints/parameters 7857/318/626, GOF on F2 = 1.085, (D1)max,min = 0.375, 0.356 e 3, wR2 = 0.2118 (all data), R1 = 0.0640 [I > 2s(I)]. CCDC-960083 (1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. A drop-cast thick film of 1 on a flame-annealed AuACHTUNGRE(111) substrate was prepared from a 2 mm solution of 1 in dichloromethane and measured at the DEIMOS beamline of synchrotron SOLEIL (Gif-sur-Yvette, France). All operations were carried out in a glove-box with residual H2O and O2 levels both below 1 ppm. The sample was then transferred to the cold finger of the cryostat by means of a load lock directly connected to the ultra-high vacuum line. A monolayer sample of 1 was analogously prepared by soaking a flame-annealed AuACHTUNGRE(111) substrate in a 2 mm solution of 1 in dichloromethane and leaving it undisturbed in the dark for 18 h. The substrate was then repeatedly washed with pure dichloromethane to remove physisorbed molecules before measurement at the ID08 “Dragon” beamline of the European Synchrotron Radiation Facility (Grenoble, France). All XAS/XMCD experiments were carried out in the Total Electron Yield (TEY) to achieve surface sensitivity.
[3]
[4]
[5]
Acknowledgements This work was financially supported by European Union through ERANET project “NanoSci-ERA: Nanoscience in European Research Area” SMMTRANS, by European Research Council through the Advanced Grant MolNanoMas (grant no. 267746), and by Italian MIUR through a PRIN2008 project. We are grateful to Prof. Larry Falvello (Instituto de Ciencia de Materiales de Arag n, Facultad de Ciencias, Universidad de Zaragoza-C.S.I.C.) for advice on crystal structure refinement. We are glad to acknowledge the instrumental development by Bernard Muller and Lo c Joly on the DEIMOS beamline. The DEIMOS glovebox was provided by IMPMC-UMR7590 through ANR-07-BLANC0275.
[6]
[7]
Keywords: chromium · iron · single-molecule magnets · surface analysis · X-ray spectroscopy
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Received: September 10, 2013 Published online: November 6, 2013
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On-Surface Magnetometry: The Evaluation of Superexchange Coupling Constants in Surface-Wired Single-Molecule Magnets Erik Tancini,[a] Matteo Mannini,[b] Philippe Sainctavit,[c, d] Edwige Otero,[d] Roberta Sessoli,[b] and Andrea Cornia*[a]
High-spin molecules exhibiting a memory effect (singlemolecule magnets, SMMs) are of continuing interest in the field of molecular spintronics, with the ultimate goal of probing and controlling their directionally bistable magnetic moment by electric currents.[1] Scanning probe methods offer the simplest solution to electrically address individual molecules deposited on a metal surface.[2] However, examples of SMMs that remain structurally and functionally stable on surface are so far extremely limited, as shown by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements. According to these techniques, which provide monolayer sensitivity along with element and valence specificity, SMMs belonging to the important Mn6[3a] and Mn12[3b–d] families are prone to structural or redox changes when interfaced with metal substrates. The investigation of surface-supported bis(phthalocyaninato)terbiumACHTUNGRE(III) complexes has further evidenced a strong reduction or even the complete suppression of magnetic hysteresis in otherwise intact molecules.[4] As a very welcome result, sulfur-functionalized tetraironACHTUNGRE(III) clusters (Fe4) with a metal-centered triangular geometry (Figure 1 a) still display slow magnetic relaxation when deposited on a gold film[5a,b] or linked to gold nanoparticles.[5c] To prove that their ferrimagnetic spin structure persists on surface, isostructural complexes with a central Cr3 + ion (Fe3Cr) (Figure 1 b) have been designed,[6] and the antiparallel alignment of Fe3 + (s = 5/2) and Cr3 + (s = 3/2) spins clearly visualized by XMCD.[6b]
[a] Dr. E. Tancini, Prof. Dr. A. Cornia Dipartimento di Scienze Chimiche e Geologiche & UdR INSTM Universit di Modena e Reggio Emilia via G. Campi 183, 41125 Modena (Italy) Fax: (+ 39) 059-373543 E-mail: [email protected] [b] Dr. M. Mannini, Prof. Dr. R. Sessoli Dipartimento di Chimica Ugo Schiff & UdR INSTM Universit di Firenze, 50019 Sesto Fiorentino (Italy) [c] Dr. Ph. Sainctavit IMPMC-CNRS UMR 7590, Universit Pierre et Marie Curie 75252 Paris Cedex 5 (France) [d] Dr. Ph. Sainctavit, Dr. E. Otero Synchrotron SOLEIL, LOrme des Merisiers, Saint Aubin BP48 91192 Gif-sur-Yvette Cedex (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303585.
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Figure 1. Structure of the metal/oxygen core in a) Fe4 and b) Fe3Cr species, with arrows depicting the spin arrangement in the ground state with total spin S. c) Molecular structure of 1. H atoms and disorder on alkyl chains have been omitted for clarity. Color code: large grey spheres = Fe3 + (s = 5/2), white sphere = Cr3 + (s = 3/2), small grey spheres = O, medium grey spheres = S, black spheres = C.
In this work, we have determined the strength of the Fe Cr antiferromagnetic coupling in a monolayer of Fe3Cr species deposited on gold. As in traditional magnetometry, this corresponds to evaluating the energy of excited spin states and requires temperature (T)-dependent studies. Because the magnetic polarization (and the XMCD signal) of paramagnets decreases rapidly with heating, few molecular materials were studied in this way as bulk phases.[7a,b] Even fewer were investigated as monolayers, and only in a very limited T range.[7c–e] Complex [Fe3Cr(L)2ACHTUNGRE(dpm)6] (1) was chosen for our study, because its tetraironACHTUNGRE(III) analogue 2 is one of the few SMMs that proved functionally active when deposited on gold.[5b] Here, Hdpm is dipivaloylmethane and H3L is 7-(acetylthio)2,2-bis(hydroxymethyl)heptan-1-ol, a thioacetyl-terminated tripodal ligand containing a pentamethylene spacer. Compound 1 was obtained as a crystalline material with a Fe/Cr ratio of 3.54, requiring residual Fe4 species to be present in the crystal lattice.[6a,b] Single-crystal structural analysis of 1 (Figure 1 c; Figures S1 and S2 and Tables S1 and S2 in Supporting Informa-
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16902 – 16905
COMMUNICATION tion) showed that MO distances for the central ion are shorter than in 2 (1.949(3)–1.956(3) vs. 1.968(3)–1.989(3) ), thereby suggesting a Cr-centered structure.[6] The magnetic properties recorded on a microcrystalline sample support this view (Figure S3 in Supporting Information). The maximum cMT value (19.8 emu K mol1 at 5.5 K) is close to the Curie constant for a S = 6 ground state (21.0 emu K mol1), but significantly larger than expected for both Fe4 (S = 5) and potential iron-centered Fe3Cr (S = 4) species (15.0 and 10.0 emu K mol1, respectively). Assuming that all Cr-containing species are Cr-centered,[6a,b] the experimental Fe/Cr ratio indicates a 0.118 molar fraction of Fe4 complexes, the known magnetic contribution of which[5b] was then subtracted from the data. The corrected cMT product reaches 20.5 emu K mol1 at 5.5 K, a value fully consistent with an S = 6 ground state. For a quantitative data analysis we used a Heisenberg plus Zeeman Hamiltonian [Eq. (1)], which assumes threefold symmetry and includes both FeCr (J) and FeFe (J’) interactions: ^ ¼Jð^sFe1 ^sCr þ ^sFe2 ^sCr þ ^sFe20 ^sCr Þþ H J 0 ð^sFe1 ^sFe2 þ ^sFe1 ^sFe20 þ ^sFe2 ^sFe20 Þ þ gm0 mB HSi^si,z
ð1Þ
Herein, sˆi are the spin operators for the individual metal centers (atom labels in Figure 1 b) and H is the magnetic field, which is applied along z. The best-fit parameters so obtained are J = 15.25(13) and J’ = 0.40(4) cm1 with g = 2.002(2). To allow for a direct comparison with XMCD results, an alternative fit was carried out by fixing J’ to zero and applying no correction for Fe4 species, which gives J = 13.99(4) cm1 and g = 1.9705(5). Isothermal magnetization data below 5 K confirm the presence of a S = 6 ground state with an axial zero-field splitting parameter D = 0.18 cm1 (Figure S3 in Supporting Information).[6] By virtue of its easy axis anisotropy and high-spin ground state, 1 behaves as a SMM. Frequency- and T-dependent signals are in fact observed in the out-of-phase component of AC magnetic susceptibility in zero field. Detailed measurements were performed in 1 kOe applied field in order to slow down magnetic relaxation (Figure S4 in Supporting Information). Fitting of T-dependent relaxation times to the Arrhenius law (Figure S5 in Supporting Information) gives Ueff/kB = 10.9(2) K and t0 = 8.1(9) 108 s as activation parameters (in the analysis we neglected the contribution from Fe4 species, which are present in small amounts and have a lower susceptibility than the dominant Fe3Cr complexes). To test the potential of T-dependent XMCD magnetometry, we first investigated a drop-cast thick film of 1 as a bulk reference sample. XAS and XMCD spectra (Figure 2) were recorded at the Fe and Cr L2,3 edges at T = 1.8 K and H = 65 kOe in order to maximize the magnetic response of the sample. The observed XAS spectral shapes are in accordance with the presence of high-spin Fe3 + and Cr3 + centers pseudo-octahedrally coordinated by oxygen donors.[6b] The negative XMCD signal at the Fe L3 edge and the positive one at the Cr L3 edge already provide direct indication that
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Figure 2. XAS and XMCD spectra of a bulk sample of 1 at Fe (top) and Cr (bottom) L2,3 edges, recorded at 1.8 K and 65 kOe. The XMCD signal is expressed as percentage of the maximum in the L3 average spectrum, (s + + s)/2. Here s + and s are the cross sections with X-ray photon helicity parallel and antiparallel to the applied magnetic field, respectively.
Fe3 + and Cr3 + ions have opposite magnetic polarizations, namely parallel (for Fe3 + ) and antiparallel (for Cr3 + ) to the applied field.[6b] Maximum XMCD amplitudes are reached at 709.1 (Fe L3 edge) and at 577.5 eV (Cr L3 edge), at which the XMCD signal amounts to 89.6 and 76.1 %, respectively. The signal intensity decreases considerably at higher T (Figure S6 in Supporting Information), but the spectra maintain the same shape, meaning that all the involved electronic transitions are similarly affected by the thermal population of the excited spin states. Although the magnetic moment of 3d elements can in principle be derived from XMCD sum rules at L2,3 edges,[8] ligand field multiplet calculations showed that the local magnetic moments in these systems are accurately measured by XMCD amplitudes.[6b] Hence, in Figure 3 we present the T dependence of the XMCD signal at 709.1 and 577.5 eV multiplied by T (XMCD vs. T plots are in Figure S7 in Supporting Information). The peak in both plots directly confirms the occurrence of superexchange interactions accompanied by low-T saturation effects, as in cMT versus T curves obtained by traditional magnetometry at constant field. Interestingly, the responses at the two edges are not only different in sign, but also not proportional to each other. In fact, XMCD probes the local polarizations at metal sites, which sum up to give the overall molecular polarization but are not necessarily proportional to it.[7a] Using the scheme for superexchange interactions in Equation (1) and setting J’ = 0 to avoid over-
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Figure 3. XMCD response at 709.1 (Fe) and 577.5 (Cr) eV in bulk (top) and monolayer (bottom) samples of 1, along with best-fit curves. Scale factors were adjusted to bring the magnetic polarization at Cr and Fe sites on the same scale as the corresponding XMCD signal. Error bars have been omitted when smaller than the symbol size.
parametrization, we could satisfactorily fit the data in Figure 3 and estimate J = 14.3(7) cm1 (the percentage of Fe4 species was neglected). Note that even such a crude model correctly predicts the observed asymmetric response at Cr and Fe centers. Rewardingly, the estimated J value is within experimental error from that found using traditional methods. A self-assembled monolayer was then prepared by incubating a flame-annealed AuACHTUNGRE(111) substrate in a dichloromethane solution of 1 for 18 h. Over this timeframe, 1 remains intact in solution as shown by 1H NMR spectroscopy (Figure S8 in Supporting Information). XAS and XMCD spectra of the monolayer were recorded from 10 to 300 K in a field of 50 kOe (Figure S9 in Supporting Information). Because of the substantially reduced amount of absorbing material, the signal-to-noise ratio is lower than in the bulk reference. The Cr edge is particularly affected, since Cr amounts to less than 25 % of the total metal content. Nonetheless, the spectra show the same fine structure and a comparable amplitude with respect to the bulk phase, with opposite polarizations at the Fe and Cr L3 edges (Figure 4). Thus, antiferromagnetic FeCr interactions persist when 1 is adsorbed on gold.[6b] Most important, the XMCD signal of both ions exhibits a clear T dependence (Figure 3 and Figure S10 in Supporting Information), which closely parallels that found in our bulk sample. Fitting of the data to Equation (1) (again setting J’ = 0 and neglecting Fe4 species) gave J = 14.7(7) cm1. It is apparent that intramolecular superexchange interactions remain the same, within experimental error, as those found in the bulk phase. These results clearly
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Figure 4. XAS and XMCD spectra of a monolayer sample of 1 at Fe (top) and Cr (bottom) L2,3 edges, recorded at 10 K and 50 kOe. See caption to Figure 2 for definitions.
suggest that the surface environment does not significantly alter the electronic structure of the metallic core in terms not only of sign but also of strength of superexchange coupling interactions. Notice that with these J values the polarization at Cr should change sign around 180 K, as the applied field reverses the orientation of the magnetic moment on the central ion from field-antiparallel to field-parallel. At this T, however, the XMCD signal at Cr L2,3 edges is already far too small to be measured with the required precision in both drop cast and monolayer samples. In conclusion, the magnetic response of a monolayer sample of 1 has been probed by XMCD over a wide T range. FeCr superexchange interactions, which largely determine the pattern of spin states, remain the same as in bulk samples. We argue that the structure of the metal core is not significantly altered by interaction with the surface, a rare situation among complex molecular architectures based on coordination bonds.[7c–e] The rigid structure imposed by the two tripodal ligands and a weak electronic coupling with the substrate due to the bulky dpm groups are likely reasons for the behavior of these tetrametallic SMMs, which conform to the requirements for fundamental studies in single-molecule spintronics.
Experimental Section The procedure reported for the synthesis of a longer-chain analogue[6b] gave 1 in 55 % yield (see Supporting Information). 1H NMR (200 MHz, CD2Cl2, 25 8C, TMS): d = 11.0 (s, 108 H; tBu), 3.1 (s, 4 H; CH2-S),
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On-Surface Magnetometry
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2.37 ppm (s, 6 H; CH3CO); elemental analysis calcd (%) for C88H152O20S2Fe3.118Cr0.882 : C 58.26, H 8.44, S 3.53, Fe 9.60, Cr 2.53; found: C 57.96, H 8.26, S 3.51, Fe 9.68, Cr 2.55. Crystal data for 1: C88H152CrFe3O20S2, Mr = 1813.77, crystal size: 0.40 0.28 0.12 mm3, monoclinic, C2/c, a = 24.8901(12), b = 16.2593(8), c = 24.9576(10) , b = 96.632(2)8, V = 10 032.6(8) 3, Z = 4, 1calcd = 1.201 g cm3, m = 0.633 mm1, FACHTUNGRE(000) = 3896, 2qmax = 48.028, T = 140(2) K, l = 0.71073 , collected/ unique reflections 66 821/7857 (Rint = 0.0555), data/restraints/parameters 7857/318/626, GOF on F2 = 1.085, (D1)max,min = 0.375, 0.356 e 3, wR2 = 0.2118 (all data), R1 = 0.0640 [I > 2s(I)]. CCDC-960083 (1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. A drop-cast thick film of 1 on a flame-annealed AuACHTUNGRE(111) substrate was prepared from a 2 mm solution of 1 in dichloromethane and measured at the DEIMOS beamline of synchrotron SOLEIL (Gif-sur-Yvette, France). All operations were carried out in a glove-box with residual H2O and O2 levels both below 1 ppm. The sample was then transferred to the cold finger of the cryostat by means of a load lock directly connected to the ultra-high vacuum line. A monolayer sample of 1 was analogously prepared by soaking a flame-annealed AuACHTUNGRE(111) substrate in a 2 mm solution of 1 in dichloromethane and leaving it undisturbed in the dark for 18 h. The substrate was then repeatedly washed with pure dichloromethane to remove physisorbed molecules before measurement at the ID08 “Dragon” beamline of the European Synchrotron Radiation Facility (Grenoble, France). All XAS/XMCD experiments were carried out in the Total Electron Yield (TEY) to achieve surface sensitivity.
[3]
[4]
[5]
Acknowledgements This work was financially supported by European Union through ERANET project “NanoSci-ERA: Nanoscience in European Research Area” SMMTRANS, by European Research Council through the Advanced Grant MolNanoMas (grant no. 267746), and by Italian MIUR through a PRIN2008 project. We are grateful to Prof. Larry Falvello (Instituto de Ciencia de Materiales de Arag n, Facultad de Ciencias, Universidad de Zaragoza-C.S.I.C.) for advice on crystal structure refinement. We are glad to acknowledge the instrumental development by Bernard Muller and Lo c Joly on the DEIMOS beamline. The DEIMOS glovebox was provided by IMPMC-UMR7590 through ANR-07-BLANC0275.
[6]
[7]
Keywords: chromium · iron · single-molecule magnets · surface analysis · X-ray spectroscopy
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Received: September 10, 2013 Published online: November 6, 2013
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