A Trimer of Ultrafast Nanomotors
COMMUNICATION
that the molecule adsorbs ex-
clusively through one side,
which is flat and allows all aro-
matic parts to interact with the
surface
(Figure 3d).
The
methyl and methoxy groups
are protruding out of the plane
of the molecule on the oppo-
site side. Based on these exper-
imental observations, a tenta-
tive packing model is suggest-
ed (Figure 3c). On this model,
the methyl groups that are
pointing out from the surface
are shown in green. Since they
have the highest physical
height, it is expected that the
STM signal from the aromatic
parts surrounding the empty
voids is negligible compared
with the contribution of these
methyl groups. Therefore, the
apparent size of the voids is
larger than the empty spaces
Figure 3. a) STM image of the self-assembled monolayer of enantiomerically pure (S)-(M)-(S’)-(M’)-(S’’)-(M’’)-
5 on HOPG; 18.2ꢁ18.2 nm2, VT =739 mV, iT =21pA. The unit cell is indicated by a red rhombus (a=(2.88Æ
0.1) nm, b=(2.97Æ0.2) nm, a=(66.3Æ4)8). The main crystallographic directions of the underlying HOPG lat-
tice are marked by the three red lines. b) Corresponding symmetrised correlation-averaged image of the STM
image (5.88ꢁ5.88 nm2). A superlattice of brighter spots is evidenced in red with this colour code. c) Schematic
representation of the proposed packing model. The superlattice of brighter spots is shown in green. It is likely
to be related to the contrast of the methyl groups pointing out of the plane. d) Side view of (S)-(M)-(S’)-(M’)-
(S’’)-(M’’)-5 after geometry optimisation in the presence of an HOPG model surface with methyl and methoxy
groups pointing out of the plane.
(Figure 3). STM evidences that (S)-(M)-(S’)-(M’)-(S’’)-(M’’)-
5 forms a honeycomb structure reminiscent of what has
been reported for other large aromatic C3 symmetrical mol-
ecules.[29–31] Each hexagonal pattern of the honeycomb struc-
ture is formed by six spots with a triangular shape (see red
triangles on Figure 3a). Each spot with a triangular shape is
composed of three bright spots. The dimensions of one
bright spot corresponds to the calculated dimensions of a
single motor unit; therefore, we conclude that a single trian-
gle composed of three bright spots corresponds to one mo-
lecular motor trimer 5. The central phenyl core is not visible
and within a single molecule the three motor units have a
different shape and contrast (Figure 3b).[32] This might be a
consequence of different conformations of motor parts
within a trimer molecule or a consequence of their physi-
sorption on different binding sites of HOPG.[33] Upon close
inspection of the hexagonal pattern, it appears that the edge
of a triangle representing a molecule is not pointing toward
the centre of a hexagon, thus forming a chiral array charac-
terised by a clockwise rotational symmetry (Figure 3a,b).[34]
Analysis of the orientation of the molecular pattern reveals
that it forms an average angle of q=(15.6Æ5)8 with respect
to the underlying graphite lattice (the main axis of the
HOPG lattice are schematically represented in Figure 3a).
Due to the variation of the contrast and shape of individual
motor units this angle varies for the triangles forming one
hexagon, which induces a large standard deviation. The
average angle is similar to the angle (188) between the arms
of 1,3,5-triethynylbenzene-based scaffold and the axis be-
tween the centre of the fluorenyl moiety and the central
benzene ring. Consequently, we can reasonably infer that
the triethynylbenzene-based scaffold follows the symmetry
and orientation of the underlying graphite. We also assume
of the model: the diameter of the apparent voids on the
STM image corresponds to the diameter of the voids on the
model as delimited by the methyl groups. Due to the specif-
ic packing, the asymmetry of the trimer is expressed on two
levels: 1) at the molecular level, as a result of the orienta-
tion of each triangular-shaped molecule with respect to
graphite, determined by the chirality of the molecule, and 2)
at the supramolecular level, as a result of the clockwise or
anticlockwise rotational symmetry of the hexagonal pattern.
By using enantiomerically pure (S)-(M)-(S’)-(M’)-(S’’)-(M’’)-
5, only domains with clockwise orientation have been ob-
served.
It is remarkable that large spots are uncovered by 5 in the
centre of the hexagon, which is thermodynamically not fav-
oured because of the low surface coverage. Such empty
voids were recently observed in systems in which strong and
highly directional intermolecular interactions are present.[35]
In the case of 5, only weak van der Waals interactions can
be established between molecules. When a mixture of dia-
stereoisomers of 5 is deposited on graphite, a self-assembled
monolayer is also formed, but with a drastically different
structure (see Figure S3 in the Supporting Information).
Therefore, the formation of a “low” density packing con-
taining empty voids is attributed to chiral self-recognition
between the helically shaped motor units.[36,37]
In conclusion, we have developed a new ultrafast molecu-
lar motor that contains an aryl bromide moiety for its incor-
poration into more complex systems by Sonogashira cou-
pling. Motor 4 is, in principle, capable of 3.3ꢁ103 rotations
per second at room temperature.[38] Molecular motor trimer
5 was found to form organised arrays of molecules by self-
assembly at the phenyloctane/HOPG interface. The mole-
cules pack in a honeycomb lattice. In the monolayer the
Chem. Eur. J. 2009, 15, 2768 – 2772
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