Single-Molecule Magnet Monolayers on Gold - American Chemical

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Tetrairon(III) Single-Molecule Magnet monolayers on gold: insights from ToF-SIMS and isotopic labeling Pasquale Totaro, Lorenzo Poggini, Annaick Favre, Matteo Mannini, Philippe Sainctavit, A Cornia, Agnese Magnani, and Roberta Sessoli Langmuir, Just Accepted Manuscript • DOI: 10.1021/la500846a • Publication Date (Web): 07 Jul 2014 Downloaded from http://pubs.acs.org on July 11, 2014

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Tetrairon(III) Single-Molecule Magnet monolayers on gold: insights from ToF-SIMS and isotopic labeling Pasquale Totaro, a, ¥ Lorenzo Poggini,a, ¥ Annaick Favre,a Matteo Mannini,a* Philippe Sainctavit,b Andrea Cornia,c Agnese Magnani,d Roberta Sessoli.a a

Laboratory of Molecular Magnetism (LaMM), Dipartimento di Chimica ‘Ugo Schiff’,

Università degli Studi di Firenze & INSTM RU of Firenze, via della Lastruccia 3-13, 50019 Sesto Fiorentino (FI), Italy; E-mail: [email protected]. b

IMPMC-CNRS, Université Pierre et Marie Curie, 4 place Jussieu 75252 Paris Cedex 05,

France. c

Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio

Emilia & INSTM RU of Modena and Reggio Emilia, via G. Campi 183, 41125 Modena, Italy. d

Dipartimento di Biotecnologie, Chimica e Farmacia, Università degli Studi di Siena & INSTM

RU of Siena, Via A. Moro 2, 53100 Siena, Italy. ¥

These authors contributed equally to the manuscript.

KEYWORDS Self Assembled Monolayer, ToF-SIMS, Single Molecule Magnet

ACS Paragon Plus Environment

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ABSTRACT: To work as magnetic components in molecular electronics and spintronics, singlemolecule magnets (SMMs) must be reliably interfaced with metals. The organization on gold of a Fe4 SMM carrying two acetyl-protected thiol groups has been studied by exploiting the surface sensitivity of time of flight secondary ion mass spectrometry (ToF-SIMS), additionally powered by the use of an isotopic labeling strategy. Deposition from millimolar dichloromethane solutions results in a higher surface coverage and better packed monolayers as compared with previous protocols based on more diluted solutions. Fe4 complexes are chemically tethered to the surface via a single Au-S bond while they still contain an intact SAc group. INTRODUCTION Molecular systems hold great promise to drive technological advances in electronics owing to their chemical versatility, light weight and close to perfect monodispersity. Such a long term perspective encompasses bulk molecular materials for plastic electronics and spintronics, molecular arrays at solid interfaces, and devices containing single molecules as active components.1,2 The organization of molecules on a target substrate is most simply achieved by thermal evaporation, which works well for volatile, thermally robust molecules. In alternative, properly-functionalized molecules in solution can be made to self-assemble spontaneously at a solid interface as a monolayer held together by molecule-substrate and molecule-molecule interactions. This soft and cost-effective deposition method3 has been widely applied to singlemolecule magnets (SMMs), a class of molecular compounds which behave as nanomagnets at low temperature4 and are of interest for spintronics and data storage.5,6 The structure of SMMs is based on coordination bonds and is usually too fragile to withstand thermal evaporation. Deposition from solution is more suited, but growing evidence has accumulated that the


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electronic structure and functionality of SMMs is often compromised by interaction with a surface. Tetrairon(III) complexes (Fe4 hereafter)7 represent a welcome exception.

Figure 1. a) Two schematic views of the investigated Fe4 derivative 1-d6 with the yellow arrows representing the arrangement of the iron(III) spins in the ground S = 5 state. b) Synthetic procedure for the selective deuteration of the ligand H3L.

The propeller-like structure of their magnetic core (see Fig. 1a) is well protected by two tripodal ligands and monolayer deposits of sulfur-functionalized Fe4 derivatives on Au(111) retain the magnetic properties of the bulk material.8,9 Since free thiols are not compatible with the 3+ oxidation state of the metal ions, these studies have relied on the direct grafting of thioacetyl groups,8–11 without the addition of exogenous deprotecting agents (normally, aqueous acids or bases)12–16 In fact, the formation of a covalent Au-S bond via spontaneous cleavage of the CH3C(O)-S linkage at the surface is known to compete with the adsorption of intact thioacetyls,12,17,18 in accordance with theoretical predictions.19 Such a direct grafting protocol12,20–22 affords lower-quality monolayers17,23 as compared with the corresponding thiols.


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However, it can be applied to molecular systems, like Fe4 complexes,8–11 which are chemically unstable in the presence of most deprotecting agents. The first Fe4 derivative shown to retain its magnetic properties on gold was [Fe4(L)2(dpm)6] (1), where Hdpm is dipivaloylmethane and H3L is 1-(acetylthio)-2,2bis(hydroxymethyl)undecan-1-ol, a thioacetyl-terminated tripodal ligand.8,11 The long alkyl spacers in this molecule (Fig. 1a) allow different grafting modes depending on whether one or both terminal groups are involved. Furthermore, attachment to the surface may occur either through covalent Au-S bonds (following deacetylation) or via noncovalent AuS(Ac) interactions. As long as the occurrence of free-standing SH groups is excluded, five different grafting modes can be envisaged, as schematized in Figure 2.

Figure 2. Possible grafting modes for complex 1-d6 depending on the loss of two (a), one (b,c) or zero acetyl groups (d,e) with the formation of one (c,e) or two (a,b,d) interacting sites per molecule.


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The occurrence of both standing-up (Fig. 2c,e) and lying-down arrangements (Fig. 2a,b,d) was invoked to explain the powder-like magnetic response8 and structural isotropy9 of monolayers prepared from submillimolar solutions of 1 in dichloromethane.10 To exclude the presence of overlayers or aggregates on the surface, such samples were studied by Time-ofFlight Secondary Ion Mass Spectrometry (ToF-SIMS). ToF-SIMS is a mass spectrometry technique based on a soft ionization process induced by a primary ion bombardment and is particularly well suited for the detection of complex systems.10,24–26 Because only the topmost layers of the sample can be efficiently ionized and detected, it has surface specificity and has been widely employed in the investigation of self-assembled monolayers,18,27 rarely associated with isotopic labeling.25,28 In these experiments, monolayers and microcrystalline deposits were found to exhibit significantly different fragmentation patterns.19 For instance, in the positive-ion spectra both samples exhibit signals from [M-dpm]+ and [M-2dpm]+ ions. However, species [M-dpm-Ac]+ and [M-2dpm-Ac]+ are detected in the monolayer only, as are molecular ion clusters with Au. Moreover, bulk samples feature a prominent [M]- peak which is not detected in the monolayers. Instead, it is replaced by a signal at slightly higher m/z assigned to a partially-oxidized species carrying a terminal sulfonate group, [M-Ac+3O]-. Such observations confirm that the monolayer samples contain mainly surface-bound molecules featuring either one (Fig. 2b,c) or two (Fig. 2d,e) intact thioacetyls. We have now found that monolayers of 1 on gold exhibit a much simpler composition and structure when deposition conditions are changed and more concentrated solutions are used. For an unambiguous interpretation of ToF-SIMS spectra we have prepared and investigated both standard and isotopically-enriched samples featuring d3-acetyls.


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EXPERIMENTAL SECTION The selective deuteration of H3L on acetyl groups was achieved through the preparation of thiolacetic-d3-acid-d and its conversion to thiolacetic-d3-acid by hydrogen-deuterium exchange in water (Fig. 1b). Then, reaction of CD3COSH with 2,2-bis(hydroxymethyl)-10-undecen-1-ol afforded tripodal ligand H3L-d3 (Fig. 1b), which was treated with [Fe4(OMe)6(dpm)6] to give [Fe4(L-d3)2(dpm)6] (1-d6) (Fig. 1a).11 Details on the synthetic procedures are given in the Supporting Information (SI). Monolayers of 1 and 1-d6 were prepared by immersing flame-annealed Au(111)/mica substrates in a 2 mM dichloromethane solution of the complex for 20 hours (0.3 mM solutions were used in the earlier protocol8,10). The gold slabs were then washed several times in the same pure solvent and dried under a nitrogen stream. Bulk reference samples were prepared as 0.1 m thick films by drop casting 50 µL of a 2 mM dichloromethane solution of the complexes on similar gold substrates. All sample preparations were carried out under dry nitrogen atmosphere in a portable glove-box. X-ray absorption spectroscopy, performed at the X11-MA SIM beamline of the SLS synchrotron (Switzerland) has been used to cross-check the intactness of the molecular system8 and to evaluate the influence of solution concentration on surface coverage (see SI for details). ToF-SIMS analysis was carried out with a TRIFT III time-of flight secondary ion mass spectrometer (Physical Electronics, Chanhassen, MN, USA) equipped with a gold liquid-metal primary ion source (see SI for details). Positive- and negative-ion spectra were acquired with a pulsed Au+ primary ion beam, by rastering the ion beam over a 100 µm  100 µm sample area.


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RESULTS AND DISCUSSION Unsurprisingly, XAS investigations reveal a significant increase, ca 40% of the absorption at the Fe L3 edge (see SI) for samples deposited from 2 mM solution as compared with those obtained from 0.3 mM solution, while keeping the same incubation time, indicating that the former feature a higher surface coverage. However, pronounced differences in chemical composition and structure emerge clearly from ToF-SIMS studies. Figure 3 shows the positive-ion mass spectra of monolayers and bulk reference samples of both 1 and 1-d6 in the region m/z 13502100, where signals attributable to large and representative ion fragments are expected. In Table 1 we present the assignment of the positive-ion spectra that contain the most relevant peaks in this class of compounds.10 In agreement with previous reports on Fe4-type molecules10 the molecular ion peaks [M]+ expected at m/z 1929 and 1935 for 1 and 1-d6, respectively, are very weak in the bulk reference samples (Fig. 3a,b). The most intense peaks are due to ion fragments which have lost one dpm ligand [M-dpm]+, occurring at m/z 1746 and 1752 for the standard and deuterated samples, respectively; the observed shift of +6 m/z units confirms the success of the deuteration protocol and the proposed assignment. Signals of medium intensity assigned to [M-2dpm]+ ions are also found in both systems.


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Figure 3. Positive-ion ToF-SIMS spectra of a) bulk 1-d6; b) bulk 1; c) monolayer of 1-d6 prepared from 2mM solution; d) monolayer of 1 prepared from 2mM solution in the region from m/z 1350 to 2100 (left) and 1730 to 1910 (right). M' and M'' label intact 1 and 1-d6, respectively; lines above the experimental data represent the theoretical isotopic distribution of the most significant peaks. Orange asterisks are used to mark peaks coming from the gold substrate (see SI).

Moving on to the monolayer deposits (Fig. 3 c,d), we can notice that the aforementioned signals from [M]+, [M-dpm]+ and [M-2dpm]+ ions are very weak or absent. The spectra are dominated by species that have lost one (or two) dpm ligand(s) and only one Ac or SAc group. In particular prominent peaks are present at m/z 1900 (1) and 1903 (1-d6) which are undetectable or very weak in the bulk references. The shift of +3 m/z units upon deuteration proves that the associated fragments contain one intact thioacetyl group and supports the assignment of the above peaks to [M-dpm-Ac+Au]+.


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Table 1. Most relevant peaks in the positive-ion ToF-SIMS spectra recorded on monolayers of 1 and 1-d6 and on the corresponding bulk phases. Monolayer†

Bulk m/z 1

1-d6

1

1-d6 1

1-d6

M+

1929 1935 vw vw

-

-

[M – dpm – Ac + Au]+

1900 1903 vw -

s

s

[M – dpm]+

1746 1752 s

s

vw

-

[M – dpm – Ac]+

1700 1703 w

w

s

s

[M – dpm – SAc]+

1668 1671 vw w

m

m

[M – 2dpm]+

1563 1569 m

m

-

-

[M – 2dpm – Ac]+

1520 1523 -

-

w

w

[M – 2dpm – SAc]+

1487 1490 w

w

vw

vw

†

Monolayers prepared from 2mM solution in CH2Cl2. Intensity of peaks has been indicated as strong (s), medium (m) weak (w) and very weak (vw).

These findings suggest that, when deposition is carried out from more concentrated solution (2 mM vs 0.3 mM) while keeping the same incubation time, all molecules loose one acetyl protection and covalently interact with the surface through a single S-Au bond. As no significant peak has been observed at the same m/z value for monolayers of 1 and 1-d6 it seems unlikely that Fe4 complexes may be tethered to the Au substrate through two Au-S covalent bonds. In these conditions, two grafting modes are however still possible (Fig 2b,c) depending on the involvement of the intact SAc group in non-covalent interactions with the substrate.


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Figure 4. Negative-ion ToF-SIMS spectra of a) bulk 1-d6; b) bulk 1; c) monolayer of 1-d6 prepared from 2mM solution; d-e) monolayers of 1 (prepared from 2mM and 0.3mM solution, respectively) in the region from m/z 1910 to 1950. M' and M'' label intact 1 and 1-d6, respectively; lines above the experimental data represent the theoretical isotopic distribution of the most significant peaks. Equally interesting is the comparison of negative-ion spectra. Figure 4 presents the spectral region of the [M]- molecular ion for the bulk references and for the monolayers. While the [M]peak is well evident in bulk samples of both 1 and 1-d6 at m/z 1929 and 1935, respectively (Fig. 4a,b), the monolayers prepared from a 2 mM solution display no visible signal in this region (Fig. 4c,d). In stark contrast to this, the monolayer of 1 obtained from a 0.3 mM solution features a prominent peak at m/z 1934 (Fig. 4e), attributed to a monosulfonate derivative [M-Ac+3O]-. No peak indicative of a disulfonate derivative (expected at m/z 1939) is however observed.


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Simple thiols were reported to undergo a similar oxidation process at their gold-bound sulfur atom when assembled in a non densely packed monolayer.29,30 Thus, the fragment containing a single sulfonate group is likely to arise from the oxidation of molecules grafted as in Figure 2b,c. Deposition from more concentrated solution may limit such oxidation processes by increasing surface coverage and favoring a better packing.

OUTLOOK The combination of selective isotopic labeling with the surface sensitivity of ToF-SIMS has been here used to evidence the effect of the grafting protocol on the composition and structure of Fe4 monolayers deposited on gold from dichloromethane solution. Within the limitations of ToFSIMS technique, all detected signals suggest that deposition from millimolar solutions results in a single bonding scheme, in which molecules are chemically tethered to the surface through one Au-S bond while the second SAc group remains intact. Combined XAS and ToF-SIMS data also suggest that an increased concentration favors a higher surface coverage and the formation of more densely-packed monolayers. Such a detailed information on the grafting mode, which is critical in the transport properties of single-molecule electronic devices,6 would have been hardly accessible by employing spectroscopic or topographic tools. From these results ToF-SIMS emerges as a unique mass spectrometry tool to aid the realization of nanostructured materials containing complex and fragile functional molecules. ASSOCIATED CONTENT Supporting Information Available: Synthetic details, 1H-NMR spectra of thiolacetic-d3 acid-d, thiolacetic-d3 acid and H3L-d3, ToF-SIMS experimental details and additional ToF-SIMS and


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XAS characterization data for bulk 1 and monolayers of 1 prepared from 0.3 and 2 mM solution. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. MM, AC and RS designed the experiments. PT, AF and AC synthesized the molecular componds. LP, MM, AM performed the ToF-SIMS experiments, LP, PT and MM performed the SIMS data analysis, MM and PS performed the XAS characterization and analysis. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by European Research Council through the Advanced Grant MolNanoMas (grant no.267746), and by Italian MIUR through FIRB projects RBFR10OAI0 and RBAP117RWN, SLS synchrotron staff, B. Mueller, J. Loic and J.P. Kappler of the IPCMSCNRS are fully acknowledged for the support before and during XAS experiments. REFERENCES

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Pasquale Totaro, Lorenzo Poggini, Annaick Favre, Matteo Mannini,* Philippe Sainctavit, Andrea Cornia, Agnese Magnani, Roberta Sessoli Langmuir 2014, xx, XXXX Tetrairon(III) Single-Molecule Magnet monolayers on gold: insights from ToF-SIMS and isotopic labeling

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The organization on gold of a Fe4 singlemolecule magnet carrying two acetylprotected thiol groups has been studied by exploiting the surface sensitivity of time of flight secondary ion mass spectrometry, additionally powered by the use of an isotopic labeling strategy. Monolayers deposited from millimolar dichloromethane solutions comprise Fe4 molecules chemically tethered to the surface via a single Au-S bond while still containing an intact SAc group.


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Single-Molecule Magnet Monolayers on Gold - American Chemical

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