Studies of hybrid organic-inorganic [2] and [3]rotaxanes bound to Au surfaces

Article abstract of DOI:10.1039/c3cc38699f

Hybrid organic-inorganic [2]- and [3]rotaxanes have been synthesised, and their ability to bind to Au surfaces studied; the length of the tethering group is found to control how the supramolecular assembly binds to the surface and we find that [2]rotaxane

Full text of DOI:10.1039/c3cc38699f

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Studies of hybrid organic–inorganic [2] and
[3]rotaxanes bound to Au surfaces†
Harapriya Rath,a Grigore A. Timco,a Valdis Corradini,b Alberto Ghirri,c
Umberto del Pennino,b Antonio Fernandez,a Robin G. Pritchard,a
Christopher A. Muryn,a Marco Affronte*b and Richard E. P. Winpenny*a
Cite this: Chem. Commun., 2013,
49, 3404
Received 4th December 2012,
Accepted 6th March 2013
DOI: 10.1039/c3cc38699f
www.rsc.org/chemcomm
Hybrid organic–inorganic [2]- and [3]rotaxanes have been synthe-
sised, and their ability to bind to Au surfaces studied; the length of
the tethering group is found to control how the supramolecular
assembly binds to the surface and we find that [2]rotaxanes show
improved stability over previous studies of simple inorganic rings.
We have made five [2]rotaxanes (ArQC6H4):
[(MeSArCH2)NH2C2H4Ph][Cr7NiF8(O2CtBu)16] 1
[(MeSArCH2)2NH2][Cr7NiF8(O2CtBu)16] 2
[(MeSArCH2)NH2C2H4Py][Cr7NiF8(O2CtBu)16] 3
[(MeSArC2H4)NH2CH2Ph][Cr7NiF8(O2CtBu)16] 4
[MeC(O)S(CH2)6NH2CH2Ph][Cr7NiF8(O2CtBu)16] 5
and three related [3]rotaxanes: [(MeSArCH2)NH2(CH2)6]2-
[Cr7NiF8(O2CtBu)16]2 6
Many proposed applications of molecular nanomagnets involve
supporting these species on surfaces. There have been beautiful
results reported using XMCD studies of {Fe4} on Au surfaces
showing retention of single molecule magnetism,1 and a report
of a terbium phthalocyanine molecules forming part of molecular
[(MeSArCH2)NH2(CH2)12NH2(CH2Py)][Cr7NiF8(O2CtBu)16]2 7
{[(MeSArCH2)NH2C2H4Py][Cr7NiF8(O2CtBu)16]}2[Cu(O2CtBu)2]2 8
The crystal structures of compounds 1–4, 6 and 8 have been
spin valves and transistors,2 and where the blocking temperature determined; 2, 3, 6 and 8 are shown in Fig. 1. The secondary amine
for relaxation of the magnetisation can be raised above 100 K by threads were designed with a methylene spacer between the ammo-
binding to a Ni ferromagnet.3 Recently we have shown we can form nium site and the bulky aromatic stoppers such as 4-methylthiophenyl
an organised monolayer of {Cr7Ni} anti-ferromagnetic rings on an at each end of the axle. This creates sufficient space to accommodate
Au surface by sublimation.4 For applications it would be advanta- the heterometallic {Cr7Ni} rings. Both NMR and electrospray mass
geous if more complex molecules could be attached to surfaces. spectrometry confirm these are rotaxanes, not pseudo-rotaxanes (see
Here we attach hybrid organic–inorganic rotaxanes5,6 to surfaces, ESI†). To construct the [3]rotaxane 6 we used a C12 chain between the
inspired by some of the beautiful surface chemistry shown pre- two ammonium sites as this allows two rings to be bound. The threads
viously for catenanes bound to surfaces.7
were made via a Schiff base condensation followed by reduction (see
We have designed and synthesized interlocked hybrid ESI†).9 The [2]rotaxane 3 was designed with a pyridyl-head group so that
organic–inorganic systems with functionalized axles where it could be used to bind to a further metal site. Reaction of 3 with
the organic axles are stoppered by 4-methylthio phenyl and/or [Cu(O2CtBu)2]2 then leads to a [3]rotaxane 8, where the dimetallic
4-pyridyl groups contain templating ammonium group(s) copper complex forms part of the organic thread (Fig. 1d).
around which the inorganic heterometallic ring(s) are grown.
Previous studies10–12 have shown that S-functionalised {Cr7Ni}
The rings contain one Ni2+ and seven Cr3+ centres arranged in derivatives can be deposited from liquid phase. The S-terminated
an octagon, held together by fluoride and carboxylates.8
amine assures strong grafting of the ring to surface, but also leads
to the formation of an S-containing self-assembled monolayer
(SAM). At best we achieved around 30% {Cr7Ni} and about 30%
S-containing SAM coverage.10 The {Cr7Ni} rings deposited retain
their chemical composition and show magnetic behaviour similar
to bulk materials. Here we study rotaxanes as they present severe
challenges for liquid deposition onto Au(111); we wish to establish
the limits of this approach.
The rotaxanes were attached to the Au(111) surface from a
10À3 M CH2Cl2 solution. X-ray photoelectron spectroscopy (XPS)
was performed to demonstrate that the rotaxanes were intact, and
to study the binding mode of the ligands. The results show that the
a School of Chemistry and Photon Science Institute, The University of Manchester,
Oxford Road, M13 9PL, Manchester, UK.
E-mail: richard.winpenny@manchester.ac.uk
b Institute Nanoscience-CNR, S3 Centre and Dipartimento di Fisica,
Universita di Modena e Reggio Emilia, Via Campi 213/A, 41125 Modena, Italy
c Institute Nanoscience-CNR, S3 Centre and Dipartimento di Fisica,
Universita di Milano, Via Celoria 16, 20133 Milano, Italy
† Electronic supplementary information (ESI) available: Experimental details and
crystallographic data for 1–4, 6 and 8. CCDC 907149–907152, 912256 and 923150.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cc38699f
c
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Table 1 XPS data for compounds 1–8
Ratios of XPS peaks
Surface coverage/%
F-1s/Cr-2p
[1.14]
Cr-2p/
Ni-2p[7] Cr-2p
S-2p/
S1/S2 MeS-SAM Cr7Ni rotax
1
2
3
4
5
6
7
8
1.17 Æ 0.05 6.5 Æ 2
1.15 Æ 0.05 5 Æ 2 70 Æ 5 [2] 9 : 1
1.15 Æ 0.05 5.5 Æ 2 10 Æ 5 [1] 4 : 1
1.17 Æ 0.05 6 Æ 2 6 Æ 5 [1] 7 : 3
5 Æ 5 [1] 4 : 1
15 Æ 5
95 Æ 5
10 Æ 5
20 Æ 5
50 Æ 5
10 Æ 5
20 Æ 5
20 Æ 5
35 Æ 5
15 Æ 5
12 Æ 5
50 Æ 5
60 Æ 5
38 Æ 5
50 Æ 5
10 Æ 5
1.18 Æ 0.05 6.5 Æ 2 10 Æ 5 [1] 4 : 1
1.12 Æ 0.05
1.16 Æ 0.05
1.10 Æ 0.05
5 Æ 2
7 Æ 2
6 Æ 5 [2] 1 : 9
5 Æ 5 [1] 4 : 1
6 Æ 2 20 Æ 5 [2] 4 : 1
Fig. 2 XPS measurements: S-2p core level best fits for MLs of 1 (a), 6 (b) and 7
(c). In the right panel, a sketch of the surface-binding.
parameters: spin–orbit splitting equal to 1.2 eV, branching ratio
equal to 0.5, Lorentzian width equal to 574 meV and Gaussian
width equal to 2400 meV for all components. For all rotaxanes, two
components are observed at 162.0 eV (S1 peak) and 163.1 eV (S2).
S1 can be assigned to sulfur species after C–S bond cleavage bound
to Au, while S2 is due to intact MeSPh groups containing S atoms
that are not bound to Au.
Fig. 1 The structures of compounds (a) 2, (b) 3, (c) 6 and (d) 8 in the crystal.
Me-groups of pivalates and H-atoms removed for clarity. Colours: Cr, Ni, green; F,
yellow; O, red; N, light blue; S, orange; C, grey; Cu, blue.
The number of S–Me endgroups within the organic thread of
the rotaxanes strongly influence their interaction with the surface.
Cr-2p, F-1s and Ni-2p core level line-shapes measured on sub- In [2]rotaxanes with a single S–Me such as 1, 3 and 4, there will be
monolayers (MLs) (see ESI†) are very similar to corresponding data rotaxanes where the S to methyl bound has broken, leading to a
measured on multilayers that were drop cast from saturated strong Au–S bond tethering the rotaxanes to the surface, in
solutions onto HOPG. The ratios of the peaks match the stoichio- addition to some surface bound –SMe groups (see Fig. 2a). In 2,
metric values (reported in brackets) calculated for the chemical a [2]rotaxane with two S–Me groups, the S1 peak is very intense,
formulae for these elements, indicating that the molecular core for which shows that S–C cleavage is important for 2.
compounds 1–8 is largely maintained (Table 1). Conversely, the
For 6, a [3]rotaxane with two S–Me, the S2 peak is dominant
S-2p/Cr-2p ratio indicates in many cases an excess of sulfur in the (Table 1), suggesting that in this topology the sulfur of the
MLs. This is probably due to formation of an S-containing ML, organic thread prefer not to bind to the surface (see Fig. 2b). On
possibly formed by loss of –SMe from the organic thread.
the contrary for 7 where only one S–Me is present, the S1 peak is
The nature of the S-bonding has been investigated by dominant, suggesting that the [3]rotaxane preferably grafts to
evaluating the different contributions to the S-2p core level the surface through an Au–S bond (see Fig. 2c).
(Fig. 2 and Table 1). The S-2p fitting procedure has been performed
We calculated the surface coverage of the sulfur-bound
using spin–orbit split doublets (Voigt functions), with the following species by taking the ratio of the S-2p with the Au-4f peak; this
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side might suggest that the structure of 6, with two {Cr7Ni} rings, is
retained after the deposition on Au(111) (Fig. 1c). In agreement with
XPS analysis, the grafting of the [3]rotaxane thus preferentially
occurs with the longest side of the complex lying flat on the surface.
These results show that [2]- and [3]rotaxanes deposit from
solution while preserving the core of the structure. The S-containing
ligands can detach themselves and form SAMs, but also assure
strong attachment of the rotaxanes. For [2]rotaxanes the best results
for 1 and 4, show that the use of [2]rotaxanes increases the coverage
of {Cr7Ni} molecules and decreases the coverage of S-containing
SAMS compared with previous studies.10
Fig. 3 STM images on Au(111) of: (a) 1, (b) 4 and (c) 5. Scan area is 40 Â
40 nm2. (d–f) Height profiles along lines shown in panels (a–c).
For the [3]rotaxanes the configuration of the molecule on
the surface is determined by the interplay of the Van der Waals
interaction between the {Cr7Ni} rings and the surface. In 6 the
bulky structure leads to the two S–Me groups at the ends of
the axle floating above the surface. Conversely, in the case of 7,
the presence of a single S–Me end group at just one end of the
axle, leads to convincing evidence of an Au–S bond. The results
suggest that not only can we bind these hybrid rotaxanes to
surfaces, but also that we should be able to chose the orienta-
tion of the rotaxane to the surface by choice of binding groups.
We thank the Royal Society for a Newton International
Fellowship (to HR) and a Wolfson Merit Award (to REPW). This
work was supported by the EPSRC(UK) and the EC through
MolSPINQIP and through a Marie Curie IEF to AFL.
Fig. 4 (a) STM image of [3]rotaxane 6 on Au(111). (b) Line profiles taken along
the two orthogonal directions shown in panel (a). Scan area is 30 Â 30 nm2.
suggests that for 2 we have a complete sulfur-ML, while for the
other compounds the coverage is below 50% (Table 1). By
comparing the Au-4f peak with the Cr-2p, and taking into
account the gold signal attenuation due to the overlayer, we
have also derived the coverage of the {Cr7Ni} rotaxanes on the
surface (Table 1). This varies substantially, with the highest
coverage for 4, 5 and 7, which approaches 50–60%.
Sub-monolayers of rotaxanes 1–8 on Au(111) have been investi-
gated by means of scanning tunnelling microscopy (STM). For
[2]rotaxanes 1, 4 and 5 stable imaging conditions were obtained at
low tunnelling current (20 pA) and high bias voltage (2 V) (Fig. 3).
Line profiles show structures in close agreement to those expected.4
The three [2]rotaxanes give different coverage: for 1 and 5, (Fig. 3a
and c respectively) there are homogeneous distributions of clusters
separated by a few nm. The coverage is much higher for the latter,
suggesting that the long thread containing a C6 chain in 5 is more
effective for the grafting of the cluster on the surface.
The STM images for 4 (Fig. 3b) differ significantly from those
of 1 and 5. Here molecules pack together in dense 2D-domains
that alternate with regions of low coverage (see ESI†). Rotaxanes
1 and 4 differ only in the length of the thread, i.e. in 1 the SÁ Á ÁN
distance is 0.69 nm in 1 and 0.82 nm in 4 in the crystal
structures. Such small structural modifications have significant
influence on the deposition process.
We also investigated the deposition of [3]rotaxanes such as 6 on
Au(111) (see ESI†) by STM (Fig. 4). Here we observe the presence of
slightly elongated clusters with fairly reproducible shapes. In most
cases the line profiles have FWHM of 3.1 nm as the shortest side,
and 3.8 nm as the longest one (Fig. 3d) which is close to the size of 6
determined by crystallography (1.7 nm and 2.8 nm, respectively).
The presence of a double peak in the height profile along the longest
Notes and references
1 (a) M. Mannini, F. Pineider, C. Danieli, F. Totti, L. Sorace, P. Sainctavit,
M.-A. Arrio, E. Otero, L. Joly, J. C. Cezar, A. Cornia and R. Sessoli, Nature,
2010, 468, 417; (b) M. Mannini, F. Pineider, P. Sainctavit, C. Danieli,
E. Otero, C. Sciancalepore, A. M. Talarico, M.-A. Arrio, A. Cornia,
D. Gatteschi and R. Sessoli, Nat. Mater., 2009, 8, 194.
2 (a) R. Vincent, S. Klyatskaya, M. Ruben, W. Wernsdorfer and F. Balestro,
Nature, 2012, 488, 357; (b) M. Urdampilleta, J.-P. Cleuziou, S. Klyatskaya,
M. Ruben and W. Wernsdorfer, Nat. Mater., 2011, 10, 502; (c) A. Candini,
S. Klyatskaya, M. Ruben, W. Wernsdorfer and M. Affronte, Nano Lett.,
2011, 11, 2634.
3 A. L. Rizzini, C. Krull, T. Balashov, J. J. Kavich, A. Mugarza,
P. S. Miedema, P. K. Thakur, V. Sessi, S. Klyatskaya, M. Ruben,
S. Stepanow and P. Gambardella, Phys. Rev. Lett., 2011, 107, 177205.
4 (a) A. Ghirri, V. Corradini, V. Bellini, R. Biagi, U. del Pennino, V. De Renzi,
J. Cezar, C. Muryn, G. A. Timco, R. E. P. Winpenny and M. Affronte,
ACS Nano, 2011, 5, 7090; (b) V. Corradini, A. Ghirri, E. Garlatti, R. Biagi,
V. De Renzi, U. del Pennino, V. Bellini, S. Carretta, P. Santini, G. Timco,
R. E. P. Winpenny and M. Affronte, Adv. Funct. Mater., 2012, 22, 3706.
5 C.-F. Lee, D. A. Leigh, R. G. Pritchard, D. Schultz, S. J. Teat,
G. A. Timco and R. E. P. Winpenny, Nature, 2009, 458, 314.
6 B. Ballesteros, T. B. Faust, C.-F. Lee, D. A. Leigh, C. A. Muryn,
R. G. Pritchard, D. Schultz, S. J. Teat, G. A. Timco and R. E. P.
Winpenny, J. Am. Chem. Soc., 2010, 132, 15435.
7 V. Balzani, A. Credi and M. Venturi, ChemPhysChem, 2008, 9, 202.
8 (a) M. Affronte, S. Carretta, G. A. Timco and R. E. P. Winpenny,
Chem. Commun., 2007, 1789; (b) G. A. Timco, T. B. Faust, F. Tuna
and R. E. P. Winpenny, Chem. Soc. Rev., 2011, 40, 3067.
9 B.-L. Fei, W.-Y. Sun, K.-B. Yu and W.-X. Tang, J. Chem. Soc., Dalton
Trans., 2000, 805.
10 V. Corradini, A. Ghirri, U. del Pennino, R. Biagi, V. A. Milway,
G. Timco, F. Tuna, R. E. P. Winpenny and M. Affronte, Dalton
Trans., 2010, 39, 4928.
11 V. Corradini, F. Moro, R. Biagi, V. De Renzi, U. del Pennino, V. Bellini,
S. Carretta, P. Santini, V. A. Milway, G. Timco, R. E. P. Winpenny and
M. Affronte, Phys. Rev. B: Condens. Matter Mater. Phys., 2009,
79, 144419.
12 V. Corradini, R. Biagi, U. del Pennino, V. De Renzi, A. Gambardella,
M. Affronte, C. A. Muryn, G. A. Timco and R. E. P. Winpenny, Inorg.
Chem., 2007, 46, 4937.
c
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