Endohedral metallofullerenes in self-assembled monolayers

Article abstract of DOI:10.1039/b915170b

A method has been developed for the attachment of a dithiolane group to endohedral metallofullerenes via a 1,3-dipolar cycloaddition reaction. This sulfur-containing functional group serves as an anchor, enabling efficient immobilisation of endohedral ful

Full text of DOI:10.1039/b915170b

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Endohedral metallofullerenes in self-assembled monolayersw
Maria del Carmen Gimenez-Lopez,a Jules A. Gardener,b Adam Q. Shaw,b
Agnieszka Iwasiewicz-Wabnig,b Kyriakos Porfyrakis,b Claire Balmer,b
b
Geraldine Dantelle,z Maria Hadjipanayi,c Alison Crossley,b Neil R. Champness,a
Martin R. Castell,b G. Andrew D. Briggsb and Andrei N. Khlobystov*a
Received 27th July 2009, Accepted 6th October 2009
First published as an Advance Article on the web 11th November 2009
DOI: 10.1039/b915170b
A method has been developed for the attachment of a dithiolane group to endohedral
metallofullerenes via a 1,3-dipolar cycloaddition reaction. This sulfur-containing functional group
serves as an anchor, enabling efficient immobilisation of endohedral fullerenes on Au(111)
surfaces at room temperature, directly from the solution phase. The functionalised fullerenes form
disordered monolayers that exhibit no long-range ordering, which is attributed to both the strong
bonding of the dithiolane anchor to the surface and to the conformational flexibility of the
functional group. Endohedral fullerenes Er3N@C80 and Sc3N@C80 have been used as models for
functionalisation and subsequent surface deposition. Their chemical reactivity towards dithiolane
functionalisation and their surface behaviour have been compared to that of C60. The endohedral
fullerenes appear to be significantly less reactive towards the functionalisation than C60, however
they bind in a similar manner to a gold surface as their dithiolane terminated C60 counterparts.
The optical activity of Er3N@C80 molecules is preserved after attachment of the functional group.
We report a splitting of the endohedral Er3+ emission lines due to the reduction in symmetry of
the functionalised fullerene cage, as compared to the highly symmetrical icosahedral C80 cage of
pristine Er3N@C80.
cage serves as a ‘‘nanocontainer’’ which facilitates the
Introduction
incorporation of individual endohedral atoms within supra-
molecular architectures, such as 1D molecular chains in
carbon nanotubes2 or 2D molecular arrays on surfaces.3
However, fullerene cages tend to have relatively isotropic
exteriors owing to their spheroidal shapes, and so precise
control of their positions and orientations can be difficult to
achieve. An attractive approach for solving this problem is
through chemical functionalisation of fullerene cages. This
would allow for control over the orientation of the molecules
via well-defined chemical bonding or highly directional
non-covalent interactions. For example, endohedral fullerenes
functionalised with an appropriate chemical group could be
able to form spontaneous molecular monolayers on surfaces
or molecular chains inside carbon nanotubes, within which the
distance between the endohedral atoms X could be precisely
controlled through the chemical functionality of fullerene
cage.4,5
Carbon is a unique element that can create hollow, polyhedral
cages called fullerenes. The void space within the fullerene
cage can be occupied by a small heteroatom, such as N, P, He
or a metal atom, or by a small metallic cluster.1 Encapsulation
of endohedral heteroatoms within fullerenes is a difficult task.
These so-called ‘‘endohedral fullerenes’’ X@CN (where X is an
atom or cluster incarcerated in the fullerene, and N is a
number of carbon atoms in the fullerene cage) are usually
formed in extremely low yields and require extensive purification,
which hinders investigations of their properties and integration
within functional materials.
The endohedral species in X@CN often possess useful
magnetic or optical properties1 that could potentially be
utilised in nano-electronic devices.2 In this case, the fullerene
a School of Chemistry, University of Nottingham, UK NG7 2RD.
E-mail: andrei.khlobystov@nottingham.ac.uk
To demonstrate this principle, we have selected the most
abundant type of endohedral fullerenes–trimetallic nitride
templated endohedral metallofullerenes (TNT EMFs). These
molecules comprise three endohedral metal atoms arranged in
a triangular fashion around a nitrogen atom incarcerated in a
C80 cage (Fig. 1). It is generally accepted that TNT EMFs
M3N@C80 are stabilized by six electrons transferred from the
trimetallic nitride (M3N) cluster to the icosahedral (Ih) C80
carbon cage, resulting in a closed-shell electronic structure
b Department of Materials, University of Oxford, Oxford,
UK OX1 3PH
c Department of Physics, University of Oxford, Parks Road, Oxford,
UK OX1 3PU
w Electronic supplementary information (ESI) available: Synthesis, 1H
and 13C NMR spectra of 3; MALDI-TOF mass spectrum of the crude
mixture of 1; heteronuclear multiple quantum correlation (HMQC)
spectrum of 1; 1H and 13C NMR spectra of 4; MALDI-TOF mass
spectrum of 4; HPLC chromatogram of 4; reactivities of C70 and C78
inthereaction of 1,3-dipolar cycloaddition. See DOI: 10.1039/
b915170b
described as [M3N]6+@[C80]6ꢀ 6–8
The transparency of the
.
z Present address: Laboratoire de Photonique Quantique et
Moleculaire-ENS Cachan-61 Avenue du President Wilson-F-94 235
Cachan Cedex, France.
[C80]6ꢀ cage for wavelengths longer that 1 mm, allows direct
excitation of endohedral metal atoms, such as Er3+, whose 4f
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Fig. 1 Cycloaddition of azomethine ylides to TNT EMFs and C60. Endohedral metal atoms (black circles) are arranged in a triangular fashion
around an endohedral nitrogen atom (grey circle).
electronic transitions occur in the near-IR range, important
for telecommunications. Future technological applications of
TNT EMFs may require the fabrication of well-ordered
arrays of these fullerenes on surfaces. We demonstrate that
a dithiolane group, which has a strong affinity for metal
surfaces,9 can be efficiently attached to Sc3N@C80 and
Er3N@C80. We investigate the effects of the functional group
on the optical properties of Er3N@C80 and the attachment of
functionalised TNT EMFs to a gold surface.
the synthesis of pyrrolidinofullerene derivate N-methyl-2-
(4-(liponyloxy)benzyl)-Sc3N@C80 (1), which was formed from
Sc3N@C80, dithiolane aldehyde 3 and sarcosine at 110 1C
(Fig. 1). The reactivity of the endohedral fullerene Sc3N@C80
towards 1,3-dipolar cycloaddition appears to be significantly
lower than that of C60, and is comparable to that of higher
fullerenes of similar size. However, Sc3N@C80 does not
entirely support the trend (Fig. 2b), showing a slightly higher
reactivity than a smaller fullerene C78, which may be related to
the presence of the endohedral group Sc3N inside C80. The
effects of Sc3N on the reactivity of the fullerene cage are
difficult to evaluate directly, as the empty C80-Ih fullerene does
not exist. Mono-adduct 1 was isolated with a 34% yield after
purification by column chromatography. Negative mode
MALDI-TOF mass spectrum of pure 1 (Fig. 3b) shows a
pronounced Mꢀ peak at 1447.14 m/z and only a small degree
of fragmentation leading to Sc3N@C80 at 1109.72 m/z.
Results and discussion
Synthesis and characterization of dithiolane functionalized
M3N@C80 (M = Sc, Er)
Synthesis and purification. We have explored the reactivity
of TNT EMFs towards dithiolane functionalisation using the
reaction of 1,3-dipolar cycloaddition of azomethine ylides
(Fig. 1). The cycloaddition reactions are commonly used
for functionalisation of fullerene cages, as they often yield iso-
merically pure products with high efficiency. However, one of
the main drawbacks of cycloaddition reactions is the problem
of addition of multiple functional groups to a fullerene cage.
In order to address this problem, the reaction progress was
carefully monitored by thin layer chromatography (TLC) and
matrix-assisted laser desorption/ionization time-of-flight mass
spectrometry (MALDI-TOF MS, see Supporting Information
file). Fullerene C60 appears to react smoothly with the dithiolane
aldehyde 3 at 110 1C, forming a fullerene functionalised with
one dithiolane group C60R (where R is a functional group),
and subsequent products of bis-C60(R)2, tris-C60(R)3 and
tetra-functionalised C60(R)4 fullerene (Fig. 2a). The reactivities
of C70 and a higher fullerene C78 (Supporting Information file)
appear to be significantly lower than C60 (Fig. 2b), which can
be attributed to the fact that the proportion of reactive
carbon–carbon bonds within the fullerene structure becomes
lower as the fullerene cage becomes larger.10
TNT EMFs with the same structure of the fullerene cage are
expected to have similar chemical reactivity. We have therefore
used the optimised conditions found for the dithiolane addition
reaction with Sc3N@C80 for functionalisation of Er3N@C80
without further modification. Mono-functionalised Er3N@C802
was isolated in a 28% yield following the same procedure
as for fullerene 1. The product 2 was observed clearly in
MALDI-TOF mass spectrum (negative ionisation mode) as
a Mꢀ peak at 1815.53 m/z (Fig. 4a). It is interesting that the
fullerene 2 undergoes a more extensive fragmentation in the
mass spectrometer, using the same conditions, than fullerene 1;
the peaks at 1682.51, 1658.45 and 1522.79 m/z emerge as a
result the breakage of dithiolane functional group. The molecular
peak of the mono-functionalised 2 is approximately twice as
broad as that of 1. This is due to the wider isotopic distribution
of erbium as compared to scandium, which is in a good
agreement with the calculated distributions. High performance
liquid chromatography (HPLC) through a 5PYE column
(Nacalai Tesque) with toluene as eluent (flow rate 7 ml minꢀ1
)
showed a retention time of 3.25 min for 2 (Fig. 4b) that is
significantly shorter than the retention time observed for
pristine Er3N@C80 under the same conditions (10.93 min,
Fig. 4c). The dithiolane containing functional group attached
to the carbon cage is quite polar and somewhat hydrophilic.
This is expected to reduce the retention time in a reversed
phase HPLC column. Moreover, the symmetry of the C80 cage
is broken by the functional group, which could induce a
Since the cycloaddition takes place on the timescale of
hours, the number of dithiolane groups appended to the fullerene
cage can be conveniently controlled by the duration of the
reaction. If the rate of formation of the mono-functionalised
product is known, the yield of this product can be optimised
by quenching the reaction before the bis-functionalised fullerene
is formed in significant quantities. We adopted this strategy for
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Fig. 2 (a) Evolution of mono- (E), bis- (’), tris- (K) and tetra-functionalised (m) fullerenes in the reaction of 1,3-dipolar cycloaddition of
P
dithiolane aldehyde 3 with C60 as a function of time. Irel is a relative conversion rate calculated as Irel (%) = [Iadduct/( Iadduct(i) + IC60)] ꢂ 100
measured by MALDI-TOF mass spectrometry. (b) Time required for bis-functionalised fullerene CN(R)2 to emerge in the reaction of 1,3-dipolar
cycloaddition measured for different fullerenes.
Fig. 3 (a) Evolution of the mono- and bis-functionalised Sc3N@C80 at 110 1C as a function of the reaction time. (b) MALDI-TOF mass spectrum
of purified 1 (inset: isotopic distribution pattern confirming the composition of monoadduct 1).
substantial dipole moment on the cage. The purity of the
product 2 was estimated to be at least 98% by HPLC. The
absence of secondary peaks after 5 cycles of HPLC in a
recycling mode confirm that 2 is formed as a single isomer.
coupled with the presence of only one set of pyrrolidine carbon
atoms, suggest the formation of a product where the pyrrolidine
ring is attached to a [5,6]-bond of the C80 cage, (Fig. 5), similar
to other examples of 1,3-dipolar cycloaddition reported for
TNT EMFs.11–18 In the case of C60, where the cycloaddition
reactions are known to take place at a [6,6]-bond, the mono-
functionalised N-methyl-2-(4-(liponyloxy)benzyl)-[6,6]-C60 (4)
has a much smaller difference between the chemical shifts of
the geminal hydrogen atoms Ha and Hb of the pyrrolidine ring
(Dd = 0.7 ppm). This is because the Ha and Hb of the
[6,6]-adduct 4 occupy more similar positions to those of the
[5,6]-adduct 1 (Fig. 5).
NMR. The five-member pyrrolidine ring is directly attached
to the carbon cage (Fig. 5), rendering analysis of 1H and
13C NMR spectra of the pyrrolidine group very important for
understanding the molecular structures of the functionalised
1
metallofullerenes.11–18 The H NMR spectrum of 1 indicates
the presence of a single regioisomer. The signals for the
pyrrolidine ring geminal protons Ha and Hb in 1 are separated
by 1.3 ppm and appear as two doublets at d = 4.38 and 3.08 ppm
(J = 9.7 Hz). The remaining pyrrolidine ring proton Hc gives a
singlet peak at d = 3.76 ppm, while for the N-methyl group
Hd atoms a singlet is observed at d = 3.15 ppm. The hetero-
nuclear multiple quantum correlation (HMQC) spectrum
(ESI)w enables correlation of the chemical shifts of the carbon
atoms with corresponding hydrogen atoms attached to them,
as summarised in Table 1. The relatively large difference in the
chemical shifts of the geminal hydrogen atoms Ha and Hb,
Raman spectroscopy. The Raman spectrum of 2 under a
532 nm laser excitation (Fig. 6) shows several peaks in the
range of 450–750 cmꢀ1, typical for the CS-SC moiety
(Table 2). The frequencies of these modes are known to vary
substantially depending on the precise structure of a ‘CS–SC’-
containing compound,19 making a complete peak assignment
difficult in this case. Nevertheless, a well-defined peak observed
at 498 cmꢀ1 is within the known range of the S–S bond
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Fig. 4 MALDI-TOF mass spectrum (a) and recycling-HPLC trace (b) of the purified product 2. The retention time of 2 is 3.25 min. After 5 cycles
no other peaks were detected. The retention time of 2 is significantly shorter than that of pristine Er3N@C80, under the same conditions (c).
A further proof of successful Er3N@C80 functionalisation is
provided by Raman peaks at B3000 cmꢀ1 (see inset in Fig. 6),
indicating the presence of C–H bonds associated with the
pyrrolidine, the dithiolane groups and the linking alkyl chain
(–CH2–)4.
Photoluminescence of dithiolane functionalized
Er3N@C80 (2) in solution
In order to gain an insight into the influence of the functional
group on the inherent properties of the incarcerated species,
comparative photoluminescence measurements of Er3N@C80
or 2 in solution have been performed. Optical excitation of the
fullerene cage leads to population in the Er3+4I13/2 state via
a series of incoherent non-radiative relaxation processes.20
4
Subsequent radiative relaxation occurs to the I15/2 manifold.
The emitted photons are of wavelengths beyond the window in
which the fullerene cage is absorbing, thus permitting their
detection. The photoluminescence spectra of Er3N@C80 and 2
(Fig. 7) comprise a series of sharp lines corresponding to the
4
emission from the lowest sublevel of I13/2, the only sublevel
4
populated at 5 K, down to the different sublevels of the I15/2
Fig. 5 Structural diagrams of pyrrolidine ring attached to [5,6]-bond
ground state. Those sublevels arising from the crystal field
splitting of the I13/2 and I15/2 Er3+ states are relatively
sensitive to the Er3+ local environment in comparison to other
sublevels.21 The emission lines of Er3N@C80 are mono-
component in nature (Fig. 7), whereas those of 2 are further
4
4
of Sc3N@C80 (a) and [6,6]-bond of C60 (b).
vibration at 470–530 cmꢀ1. The Raman peaks at 653 cmꢀ1 and
659 cmꢀ1 (Fig. 6) can be attributed to the C–S bonds of 2.
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Table 1 Chemical shifts of H and C atoms of pyrrolidine rings in Sc3N@C80 (1) and C60 (4)
Part of pyrrolidine ring
Chemical shifts in 1/ppm
Chemical shifts in 4/ppm
CH2 group
CH group
CH3 group
Ha 4.4, Hb 3.1, C1 72.5
Hc 3.8, C2 85.0
Ha 5.0, Hb 4.3, C1 70.1
Hc 4.9, C2 83.0
Hd 3.1, C3 41.4
Hd 2.8, C3 40.0
Fig. 6 Raman spectrum of 2 under 532 nm laser excitation, confirm-
ing the functionalisation. Raman shifts in the ranges typical for CS–SC
(main plot), and C–H (inset) bond associated vibrations are shown.
Fig.
7
Photoluminescence spectra of
2
(solid) and Er3N@C80
(dashed) in CS2 solutions at 5 K under 532 nm excitation. The inset
shows the most intense emission line at B1520 nm in greater detail,
highlighting the splitting of 2.
Table 2 Comparison of Raman peak positions in the range of
450–750 cmꢀ1 for 2 and compounds containing similar CS–SC
groups19 (peaks associated with S–S bond are highlighted in bold)
across the sample surface. The apparent diameters of these
features range from 1.1 nm to 2.6 nm. Typically, the diameters
of unfunctionalised TNT EMFs in close packed arrays are
B1.15 nm.23,24 However, in our case the molecules are not
densely packed and so tip convolution is likely to broaden
their appearance. In addition, the functional group is confor-
mationally flexible and so the fullerene cages are likely to move
due to interactions between the STM tip and the fullerenes.
This movement will significantly affect the measured fullerene
diameter and increase the range of measured values. Our
observations are consistent with these expectations, and therefore
we attribute each spherical feature in Fig. 8(a) and (b) as a
carbon cage of functionalised Er3N@C80.
2/cmꢀ1 D,L-6,8-thioctic acid amide/cmꢀ1 D,L-6,8-thioctic acid/cmꢀ1
498
546
578
653
659
720
496
504
533
585
664
675
708
456
B501
511
559
634
682
split. In Er3N@C80, all Er3+ of the Er3 cluster occupy
equivalent positions inside the C80 cage.22 In the case of 2,
the functional group reduces the C80 cage symmetry and
therefore perturbs the local environment of each Er3+ inside
the cage. Although the presence of the functional group does
not quench the luminescence of endohedral atoms, it appears
to make the endohedral metal atoms in the trigonal Er3N
cluster non-equivalent, which is manifested in the observed
splitting of their emission peaks.
Large areas of the surface are reasonably uniformly
covered with a monolayer or sub-monolayer of 2 (Fig. 8).
Unfunctionalised TNT EMFs deposited on Au-substrates
from the solution phase under similar conditions show much
less uniform coverage of the substrate, typically aggregating
and forming multilayered islands on Au(111). As expected,
the dithiolane group attached to the fullerene cage increases
the affinity of fullerene to the metal surface and facilitates the
formation of a fullerene monolayer. However, no long-range
ordering of the molecules has been observed for monolayers of
functionalised fullerenes. We attribute this observation to the
strong bonding between the dithiolane group and the Au(111)
surface. It is likely that two S–Au bonds per each molecule of 2
largely immobilise the functionalised fullerenes and so they
remain at or close to their initial adsorption sites, preventing
the surface migration of the molecules and formation of an
ordered array. However, isolated patches comprising close
Deposition of ditholane functionalised Er3N@C80 (2) onto a
gold surface
Deposition method. To evaluate the affinity of the
functionalised TNT EMFs to metal surfaces and their ability
to form thin films, we have studied the deposition of 2 and 4
on a gold surface by immersing freshly prepared Au/mica
substrates in dilute solutions of fullerenes.
Surface topography. We have assessed the topography of
the dithiolane functionalised Er3N@C802 self-assembled
monolayers on Au(111) using scanning tunnelling microscopy
(STM) (Fig. 8(a) and (b)). Spherical features are observed
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Fig. 8 Scanning tunnelling microscopy images of dithiolane functionalised Er3N@C802 on Au(111). Spherical features are observed and these are
attributed to the fullerene cages of the molecules. (a) A large scan area image (70 ꢂ 70 nm) showing that the layer is almost complete. (b) A more
detailed image (40 ꢂ 40 nm) in which the positions of individual molecules can be identified. A small close-packed cluster is highlighted by a white
square and is shown in the inset.
packed fullerenes are occasionally seen illustrating the
possibility of short-range molecular ordering (Fig. 8b, inset).
Bonding to the surface. To explore further the mechanisms
of the surface bonding for functionalised fullerenes, we have
taken advantage of the high surface sensitivity and detailed
chemical information that are offered by X-ray photoelectron
spectroscopy (XPS). In general, sulfur-terminated functionalised
fullerenes can bind to an Au(111) surface via either the
functionalised group or the fullerene cage, leading to a range
of possible geometries.25 XPS can be used to determine the
orientation of the dithiolane functionalised fullerene mono-
layers with respect to the Au(111) surface. In particular, the
S2p peaks provide excellent indicators of whether or not the
functional group is bound to Au.
High resolution XPS spectra of functionalised fullerenes 2
and 4 show a good agreement in lineshape of the core level S2p
peaks (Fig. 9), suggesting that the S atoms of dithiolane group
Fig. 9 S2p core level peak in XPS spectra of self-assembled mono-
have similar chemical environment in both cases (the tail
layers of dithiolane functionalised Er3N@C80 (2) and dithiolane
towards higher energies for fullerene 2 is due to the onset of
functionalised C60 (4) on Au(111). The peak intensity is offset for
the Er 4d peak). Spectral fitting shows the presence of S2p3/2
clarity. Gaussian peak fits are included.
(S2p1/2) peaks at 161.9 eV (163.1 eV) and 161.7 eV (162.9 eV)
for the dithiolane functionalised Er3N@C80 and C60 mono-
layers, respectively. These values are in good agreement with
the known spectral lines for S-atoms covalently bound to
Au,25 and unequivocally demonstrate that the dithiolane
group forms two S–Au bonds with the surface (Fig. 10).
This binding mode provides a very efficient interaction of
functionalised fullerenes with the surface, that is expected to
be twice as strong as the standard Au–thiol interaction
typically utilised for self-assembled monolayers. This confirms
that the lack of long-range order in the monolayers of 2 and 4
is related to the strong interaction of the dithiolane group with
the metal surface.
Fig. 10 Schematic representation of bonding of the dithiolane group
Surface photoluminescence of dithiolane functionalized
to the Au-surface.
Er3N@C80 (2) on Au(111)
that in solution (Fig. 11). An excellent agreement between
the peak positions is observed, verifying that the optical
The photoluminescence spectrum of dithiolane functionalised
Er3N@C80 deposited on Au(111) surface was compared to
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functionalised Er3N@C80 retains its optical properties within
a monolayer assembled on the surface. The methodology for
chemical functionalisation and surface deposition of EMFs
described in this study can be further extended to electron spin
active fullerenes, and in the long term could enable incorporation
of endohedral fullerenes in functional electronic devices,
harnessing their unique physicochemical properties for future
technological applications.
Experimental
Sample preparation
Raw erbium and scandium TNTs were supplied by Luna
Innovations. Erbium TNTs were further purified by high
performance liquid chromatography (HPLC) using a 5PYE
column (Nacalai Tesque). Only one isomer of Er3N@C80 was
detected. All other reagents and solvents were purchased from
Aldrich and were used without further purification. All reac-
tions were carried out under an argon atmosphere. Elemental
analyses (C, H, N) were performed by the Elemental Analysis
Service of London Metropolitan University. Infrared spectra
were measured as either KBr discs or in solution on a Nicolet
Fig. 11 Photoluminescence of 2 deposited on a Au(111) surface
under a 800 nm laser excitation (black). The spectrum of 2 in CS2
as shown in Fig. 7 is included for comparison (green). A typical
fluorescence map (50 ꢂ 50 mm) of 2 on Au(111) is shown in the inset.
functionality and chemical integrity of this fullerene have been
retained. Variations in the relative intensities of the peaks are
observed (this is particularly apparent for the B1519 nm
emission line). These differences could arise from the preferential
molecular orientation with respect to the substrate imposed by
the Au–S bonds. We note that spatial mapping has highlighted
inhomogeneities in the surface coverage, with photoluminescence
signal only obtained from isolated regions of approximately
10 mm in diameter (an example of which is shown in the inset
of Fig. 11). These patches are likely to arise from the regions of
higher surface concentration of 2, possibly attracted to defect
sites of the Au film.
Avatar 380 FT-IR spectrometer over the range 400–4000 cmꢀ1
.
1H and 13C NMR spectra were obtained on a Bruker DPX300,
400, and AV(III)500 spectrometers. Coupling constants (J) are
denoted in Hz, and chemical shifts (d), in ppm. Multiplicities
are denoted as follows: s = singlet, d = doublet, m = multiplet.
Mass spectrometry was carried out on a Bruker Ultraflex III
MALDI-TOF spectrometer using DCTB as matrix (355 nm)
and on a Bruker MicroTOF with electrospray ionization
(ESI). Analytical thin-layer chromatography (TLC) was
performed using aluminium-coated Merck Kieselgel 60 F254
plates.
Dithiolane aldehyde precursor. 4-(Liponyloxy)benzaldehyde
3 was synthesized according with the procedure shown in the
Supporting Information.
Conclusions
Endohedral metallofullerenes have been functionalised with
sulfur-containing groups for the first time and their inter-
actions with gold surfaces have been explored. We have
developed a method of attachment of dithiolane groups to
Er3N@C80 and Sc3N@C80 TNT fullerenes, and have isolated
isomerically pure, mono-functionalised endohedral fullerenes.
The presence of the sulfur-containing group on TNT fullerenes
has a significant effect on the interactions of these fullerenes
with gold surfaces. The functionalised fullerenes deposited
from solution give a more complete coverage of Au(111)
surfaces than unfunctionalised TNT fullerenes, due to strong
bonding between the dithiolane group and the metal
surface. The effects of the exohedral functional group on the
photoluminescence properties of the endohedral atoms have
been demonstrated for the first time using Er3N@C80 as a
model. The addition of chemical functionality to the highly
symmetrical icosahedral C80 cage lowers the symmetry of the
fullerene and results in splitting the major PL peaks of the
endohedral Er-atoms. This may give a potential mechanism
for controlling the functional properties of EMFs via the
exohedral chemical functionality. The exohedral functionality
does not quench the luminescence of the EMFs, so that the
N-Methyl-2-(4-(liponyloxy)benzyl)-Sc3N@C80 fulleropyrrol-
idine (1). 0.8 mg of Sc3N@C80 (7.6 ꢂ 10ꢀ4 mmol), 1.1 mg of
sarcosine (0.013 mmol) and 13.0 mg of 4-(liponyloxy)-
benzaldehyde (0.04 mmol) were dissolved in 15 mL of dry
toluene in a 50 mL two-neck Schlenk flask equipped with a
magnetic stirrer under argon. The mixture was heated and
stirred for 270 min at 110 1C using an oil bath. After cooling to
room temperature, the solvent was evaporated under reduced
pressure until the volume was approximately 10 mL The
reaction mixture was purified using silica gel column and
toluene as eluent (Rf 0.13). After evaporation, 0.45 mg of a
black powder was obtained (34% yield). MALDI-MS 1447.14
m/z [M]ꢀ.
N-methyl-2-(4-(liponyloxy)benzyl)-Er3N@C80 fulleropyrrol-
idine (2). Following the procedure for the synthesis of 1,
1.8 mg of Er3N@C80 (1.2 ꢂ 10ꢀ3 mmol), 1.6 mg of sarcosine
(0.018 mmol) and 18.5 mg of 4-(liponyloxy)benzaldehyde
(0.06 mmol) were refluxed. The crude mixture was purified
by column chromatography (silica, toluene) (Rf 0.14) to give 2
(0.55 mg) as a dark solid in 28% yield. MALDI-MS
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1815.53 m/z [M]ꢀ. HPLC (5PYE column, 10 mm ꢂ 250 mm,
7 mL minꢀ1 toluene, l = 312 nm): 3.25 min.
using
a
confocal microscope with an ꢂ100 objective
(Mitutoyo, 0.5 NA), which was mounted on a piezoelectric
XYZ-stage 1 nm resolution. The samples were placed inside a
continuous-flow liquid He microscope cryostat (Janis ST-500)
in order to do the measurements at 5 K. The excitation was
performed using a 800 nm Ti:Sapphire laser (Spectra-physics
Mai-Tai). The detection was carried out through a mono-
chromator (1200 lines/nm grating) equipped with the same
InGaAs detector.
N-methyl-2-(4-(liponyloxy)benzyl)-[6,6]-C60 fulleropyrrolidine
(4).
A solution of 4-(liponyloxy)benzaldehyde (51.6 mg,
0.16 mmol) dissolved in dry toluene (5 mL) was added dropwise
with stirring to a solution of C60 (100 mg, 0.14 mmol) and
sarcosine (61.8 mg, 0.7 mmol) in dry toluene (35 mL). The
resultant solution was refluxed at 110 1C under argon for 14 h.
After cooling to room temperature, the solvent was evaporated,
and the crude mixture was purified by column chromatography
(silica, toluene) (Rf 0.22). Further purification was accomplished
by subsequent precipitation with methanol to give 3 as a brown
Surface deposition and characterisation
Deposition of functionalised fullerenes on Au-subsrtates.
Au(111) films (thickness 150 nm) grown on mica were used
for the surface studies. These were prepared by flame annealing
1
solid in 37% yield. H NMR {400 MHz, CDCl3, 300 K} dH
7.17 (m, 4H, aromatic H), 5.03 (d, J = 9.3 Hz, 1H, –CH2
pyrrolidine), 4.99 (s, 1H, –CH pyrrolidine), 4.33 (d, J = 9.3 Hz,
1H, –CH2 pyrrolidine), 3.57 (m, 1H, –CH), 3.20–3.10 (m, 2H,
–CH2), 2.59 (t, J = 7.0 Hz, 2H, –CH2– alkyl chain), 2.49
(m, 1H, –CH2), 1.96 (m, 1H, –CH2), 1.76 (m, 2H, –CH2– alkyl
chain), 1.61 (m, 2H, –CH2– alkyl chain) ppm. 13C NMR
{500 MHz, CDCl3, 300 K} dC 170.70, 150.21, 148.41, 147.37,
146.63, 146.45, 146.37, 146.30, 146.22, 146.10, 146.0, 145.81,
145.34, 144.70, 144.47, 143.23, 142.72, 142.24, 142.32,142.14,
82.91, 70.10, 68.95, 68.41, 40.04, 34.02, 33.40, 28.15, 26.60,
24.75 ppm. MALDI-MS 1057.61 m/z [M]ꢀ. Elemental analysis
found (expected)%: C 87.32 (87.32), H 2.16 (2.28), N 1.28
(1.32). IR (KBr disk) 2922 m (–C–H alkyl chain), 1740 m
the substrate,
a procedure well-known to produce the
characteristic herringbone reconstruction of this surface. Thin
film samples were prepared by immersing freshly prepared
Au/mica substrates in dilute solutions of dithiolane
functionalised C604 and Er3N@C802 in toluene and drying
in a nitrogen gas stream.
STM. STM images were obtained from an ultra high
vacuum (UHV) JEOL JSTM4500S scanning tunnelling
microscope (base pressure B10ꢀ9 mbar). The samples were
prepared using the aforementioned ex situ procedure, immediately
before transferring to the UHV chamber. Images were taken at
room temperature at +2.3 V (bias applied to sample) and
0.2 nA using electrochemically etched tungsten tips.
(–COO–), 1458
m
(CQC (s) phenyl group), 1384
(–O–CO–CH2–), 1458 m (CQC (b) phenyl group). UV–vis
(CHCl3) lmax: 431.66, 700. HPLC (SiO2 FORTIS HILIC (5m)
column, 250 mm ꢂ 21 mm, 5 mL minꢀ1 3% ethylacetate in
toluene, l = 254 nm): 3.15 min (ESI).z
XPS. X-ray photoelectron spectroscopy measurements were
performed using a VG Michrotech CLAM 4 MCD analyser
system with a pass energy of 20 eV and slit size of 5 mm.
X-rays were provided from a Mg Ka X-ray source operated at
200 W (base pressure B10ꢀ10 mbar), whilst data was obtained
using SPECTRA version 8 operating system. A Shirley back-
ground subtraction was applied to all peaks prior to fitting.
Binding energy scales were referenced using standard values
for the Au4f7/2 (84.0 eV), Au4f5/2 (87.7 eV) and C1s (284.5 eV)
peaks of gold and fullerenes, respectively. S2p1/2 and S2p3/2
peak fitting was performed whilst constraining the binding
energy separation and relative peak abundances to values
expected from spin–orbit coupling (1.2 eV and 1 : 2, respectively).
Raman spectroscopy
Functionalised fullerenes were dissolved in CS2. The solution
was drop coated on to a glass optical slide and dried in air. The
Raman spectra were collected using a Horiba Jobin-Yvon Lab
Aramis Confocal Raman Microscope in backscattering
geometry, with an x100SLW objective (300 mm working
distance). Measurements were performed at room temperature,
under 532 nm excitation (solid state laser), on relatively thick
(multi-layered film) sample areas.
Photoluminescence measurements
Acknowledgements
PL in solution. Functionalised fullerenes were dissolved in
CS2. The solution was placed in a quartz tube, degassed and
sealed. For comparison, an Er3N@C80 solution was prepared
in a similar manner. The concentration of each solution was
not determined but was kept relatively low to avoid any
clustering. Photoluminescence measurements were performed
under a 532 nm excitation (15 mW), at 5 K using a He Oxford
Instruments CF204 continuous flow cryostat. The detection
was done through a monochromator (600 lines/nm grating)
equipped with an InGaAs array detector.
This work was supported by the Engineering and Physical
Sciences Research Council (EPSRC grant EP/D048761/01),
the European Science Foundation (ESF), and the Royal
Society. Raw samples of Er3N@C80 and Sc3N@C80 were
supplied by Luna Innovations, Blacksburg, VA, USA.
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