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A porous structure is also formed when molecule 2 is depos-
ited on Cu(111) surfaces. The network assembly is built from
what appear to be single, large, bright protrusions (pink filled
ovals in Figure 2d) with a size corresponding to an individual
molecule. All molecules are linked with four neighbours in an
X-shaped assembly geometry and separate two adjacent pores
(red empty circles in Figure 2d), each pore being surrounded
by six molecules. Every molecule shares two further smaller
pores of different size (blue and green empty circles in Fig-
ure 2d), each surrounded by three molecules. The resulting
hexagonal porous network has a rhombic unit cell with param-
eters a=b=(35.4Æ1.7) ꢁ and q=(60Æ4.8)8, which is signifi-
cantly larger than that observed on Au(111).
MD simulations of this second type of porous network
reveal that all the molecules adopt the same orientation with
respect to the substrate and interact with their neighbours
through different substituents. Consistent with the observa-
tions, three main types of voids characterise our calculated
stable network structure: 1) The smallest pores (green circles
in Figure 2 f), stemming from the interaction of strongly inter-
digitated (phenyl-4-phenylethynyl) groups of three neighbour-
ing molecules; 2) The intermediate size pores (blue circles in
Figure 2 f), also originating from the interaction of three mole-
cules, this time through their (phenyl-4-phenylethynyl) and
Mes groups; 3) The largest pores (red circles in Figure 2 f), de-
riving from alternating (phenyl-4-phenylethynyl) and Mes
groups of six contiguous molecules.
Figure 3. STM images of the assemblies formed by molecule 2 deposited on
Au(111) (left panels) and Cu(111) (right panels) surfaces held at 140 K. Large-
scale images (a and b) displaying molecular clusters of different sizes. Insets
in (a and b) show individual molecules. High-resolution images of c) ring-
and d) arc-like supramolecular architectures composed of six and four mo-
lecular units, respectively.
forces. All observations suggest extremely weak molecule–sub-
strate interactions caused by an effective separation of the
central molecular borazine core from the underlying metallic
surface. The mutual steric hindrance between alternating Ph
and Mes substituents in molecule 1, force the substituents to
adopt an almost perpendicular orientation with respect to the
borazine core. As a consequence, molecule 1 physisorbs on
both substrates, essentially not experiencing any in-plane
modulation of the adsorption potential.
Low coverage deposition experiments of molecule 2 were
also performed at 140 K on both substrates. These resulted in
the formation of small clusters, composed from two to a few
tens of molecules (Figure 3a and b). All clusters are relatively
stable over time and diffusing molecules were observed only
occasionally. This stability is more pronounced on Cu(111) than
on Au(111), suggesting stronger interactions for borazine 2
with the former substrate. On both surfaces, the isolated mole-
cules are visualised as three bright lobes alternated by three
dim spikes (insets of Figure 3a and b). Whereas the former fea-
tures clearly correspond to the Mes moieties, the latter can be
assigned to the ethynyl-phenyl peripheries, proving that the
molecules do not fragment during thermal sublimation. Larger
clusters are organised into ring-like architectures, similar in size
and structure to the ratchet motifs formed at RT on Au(111)
surfaces. This observation is particularly interesting for Cu(111)
surfaces (Figure 3b and d), as these low-temperature clusters
are denser and significantly different from those formed at RT
on the same substrate (compare with Figure 2 f). However, the
assemblies formed at 140 K appear to be metastable, as they
irreversibly transform into the RT porous honeycomb in Fig-
ure 2d and b when the sample is annealed at Tꢀ300 K.
Coming now to molecule 2, the interaction with the Au(111)
surface is also relatively weak and the supramolecular assem-
bly is again largely governed by intermolecular forces, with the
molecular packing being driven by the vdW interdigitation of
the aromatic rings. The resulting organisation can thus be seen
as a natural extension of the molecular assembly of borazine 1,
in which the longer substituents space apart neighbouring
[60,61]
molecules, yielding a porous molecular network.
A porous network is also formed by borazine 2 on Cu(111),
but with a significantly different structure associated with
a much lower degree of interdigitation. The relation between
these two supramolecular arrangements can be visualised by
considering the honeycomb assembly on Au(111) as being
formed by molecules oriented in an alternating anti-parallel
fashion (red and green triangles in Figure 4a), which allows
them to closely pack in a chiral hexagonal organisation. If
every second molecule in this structure was rotated by 608
and the neighbouring molecules were allowed to rearrange to
maximise vdW interactions (left-handed side in Figure 4b),
a new type of trimer would be formed that would constitute
the basic unit of the porous hexagonal lattice observed on
Cu(111) (right-handed side in Figure 4b).
Rationale
On both substrates, molecule 1 self-assembles in large and
highly ordered molecular islands with an extremely low density
of defects. The 2D suprastructure is characterised by an interdi-
gitated packing of Ph substituents that, independent of the
metal surface, is driven by intermolecular short-range vdW
It should be noticed that structure and size of the large
pores in this lattice (red circles in Figure 2d and f) are such
Chem. Eur. J. 2014, 20, 11856 – 11862
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