Giant molecular spoked wheels in giant voids: Two-dimensional molecular self-assembly goes big

Article abstract of DOI:10.1039/b806444j

We present here the formation of giant pores in surface-confined molecular networks of a triangular-shaped dehydrobenzo-[12]annulene derivative: the diameter of the pores reaches over 7 nm and the giant pores are used as templates to accommodate a giant molecular spoked wheel, which allows us to observe rotational and adsorption-desorption dynamics of single guest molecules. The Royal Society of Chemistry.

Full text of DOI:10.1039/b806444j

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Giant molecular spoked wheels in giant voids: two-dimensional molecular
self-assembly goes bigw
b
c
a
Kazukuni Tahara,ab Shenbin Lei, Dennis Mossinger, Hiroyoshi Kozuma,
¨
a
b
b
c
¨
Koji Inukai, Mark Van der Auweraer, Frans C. De Schryver, Sigurd Hoger,*
Yoshito Tobe*a and Steven De Feyter*b
Received (in Cambridge, UK) 16th April 2008, Accepted 16th May 2008
First published as an Advance Article on the web 27th June 2008
DOI: 10.1039/b806444j
We present here the formation of giant pores in surface-confined
molecular networks of triangular-shaped dehydrobenzo-
Moreover the giant pores can be used as a template to
accommodate a rigid molecular spoked wheel based on
phenylene–ethynylene subunits with a diameter of ca. 7 nm
as a guest.11
a
[12]annulene derivative: the diameter of the pores reaches over
7 nm and the giant pores are used as templates to accommodate
a giant molecular spoked wheel, which allows us to observe
rotational and adsorption–desorption dynamics of single guest
molecules.
The choice of alkylated DBA derivatives as building blocks
for a host matrix is motivated by their proven performance in
forming porous honeycomb networks with size-tunable hexa-
gonal voids. Under proper concentration conditions the size of
Engineering of nanometre-scale two-dimensional (2D) mole-
cular architectures, so-called 2D molecular networks, by sur-
face-confined self-assembly receives considerable attention due
to its relevance in nanotechnology.1 This approach turns out
to be successful not only at the level of monolayer formation
of the molecular building blocks themselves2 but also by 2D
host–guest chemistry via binding events between the supra-
molecular template layer and guest molecules.3 In the latter
case the guest molecules could be ‘‘isolated’’, therefore favor-
ing single molecule studies.4
Despite these successes, it is challenging to control 2D
molecular self-assembly with a targeted periodicity around
10 nm by using large molecular building blocks.5–7 Non-
balanced intermolecular and too strong molecule–substrate
interactions may lead to disordered patterns,8 or multilayer
formation.9 In addition, the formation of supramolecular
networks with giant pores is thermodynamically unfavored
because of the low surface coverage of the molecular networks.
As a method to overcome this obstacle, we have shown that it
was possible to form a porous molecular network with giant
voids of diameter up to 5.4 nm based on the self-assembly of
icosyloxy (OC20H41) substituted dehydrobenzo[12]annulene
(DBA) 1 (Fig. 1a) by high dilution of the adsorbate
concentration.10c However, the upper size limit of such voids,
i.e. pores on the graphite surface, has not been explored yet.
a Division of Frontier Materials Science, Graduate School of
Engineering Science, Osaka University, Toyonaka, Osaka 560-8531,
Japan. E-mail: tobe@chem.es.osaka-u.ac.jp
b Department of Chemistry, Division of Molecular and Nanomaterials,
Laboratory of Photochemistry and Spectroscopy, INPAC-Institute
for Nanoscale Physics and Chemistry, Katholieke Universiteit
Leuven (KULeuven), Celestijnenlaan 200 F, B-3001 Leuven,
Fig. 1 (a) Molecular structures of DBAs 1, 2 and 3 and molecular
spoked wheel 4 (R = OC13H33). (b, c) STM images of the self-
Belgium. E-mail: Steven.DeFeyter@chem.kuleuven.be
´
c
Kekule-Institut fur Organische Chemie und Biochemie, Universitat
¨
¨
assembly of 2 at the TCB–graphite interface (Iset = 46 pA, Vbias
=
Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany. E-mail:
hoeger@uni-bonn.de
w Electronic supplementary information (ESI) available: Synthesis of
3 and additional STM images. See DOI: 10.1039/b806444j
ꢀ0.60 V). (d) Tentative network model of the honeycomb arrange-
ment of 2. The red and blue arrows indicate the corner to corner
distance (7.5 nm) and edge to edge distance (7.0 nm), respectively.
ꢁc
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the voids is defined by the alkyl chain length.10b,c In this
context, we designed DBA 2 with triacontyloxy chains
(OC30H61, the alkyl chain length is 3.9 nm) in order to trap
individual molecular wheel molecules 4. Modeling studies
predict that DBA 2 could form a honeycomb pattern with a
periodicity of 8.4 nm and voids reaching a diameter of more
than 7 nm. (7.5 nm corner-to-corner, 7.0 nm edge-to-edge,
Fig. 1d). Such voids should be ideal to trap the molecular
wheel 4, given the size and shape complementarity. Our
synthetic approach to DBA 2 is to introduce the substituents
after the construction of the DBA p-core to avoid synthetic
obstacles (See ESIw). For this purpose, tert-butyldimethylsilyl-
protected hexahydroxy-DBA 3 was synthesized as a precursor
of 2. The six-fold alkylation reaction between anions gener-
ated in situ by treating CsF with 3, and 1-bromotriacontane
gave 2 in 48% yield.12z
Scanning tunnelling microscopy (STM) observations of
physisorbed self-assembled monolayers formed by DBA 2 at
the 1,2,4-trichlorobenzene (TCB)–graphite interface show that
non-porous structures are favored (Fig. S1 in ESIw) except for
the lowest concentrations probed (5.0 ꢂ 10ꢀ6 mol Lꢀ1), where
DBA 2 shows a porous honeycomb network. The bright spots
correspond to the conjugated core of the DBAs and the darker
features connecting them are attributed to the interdigitated
alkyl chains (Fig. 1b and c).10,13 The featureless hexagonal
bright regions with a size of about 7 nm correspond to the
empty pores of the 2D network. In addition, broader dark
stripes correspond to the densely packed linear structure of
DBA 2. Exclusive 2D porous network formation was not
achieved, thereby revealing the size limit of the voids that
can be constructed at the liquid–solid interface based on
interdigitation of the alkoxy groups of DBAs.
Fig. 2 (a, b) STM images of the assembly of a mixture of DBA 2 and
molecular spoked wheel 4 at the TCB–graphite interface (Iset = 48 pA.
V
bias = ꢀ0.42 V for (a) and Iset = 46 pA. Vbias = ꢀ0.50 V for (b)). (c)
Network model of 2D host–guest architecture composed of DBA 2
(blue) and molecular spoked wheel 4 (turquoise). The blue arrows
indicate the location of immobilized spoked wheel 4 and the red
arrows point to pores with featureless bright spots.
In a next step, the 2D host–guest properties of this template
layer versus the targeted molecular wheel guest 4 were inves-
tigated. Upon dropcasting a mixture of 2 and 4 in a 10 : 1
molar ratio in TCB on graphite, in some voids seven bright
spots were observed (Fig. 2a and b: blue arrows). These bright
spots are attributed to the phenylene parts in the molecular
center and the six corners of the spoked wheel guest 4, which is
in line with its size and symmetry. The images of the wheel
inside the voids correspond well with the images of the bare
molecular wheel on graphite.11 Note that the bright spots at
the rim—the phenylene parts—appear to point to the corners
of the pore. This agrees with the predictions of the molecular
model (Fig. 2c) and is a result of the shape and size comple-
mentarity between the pore and the guest. In addition to
empty voids, interestingly, some of the voids also appear
bright though lacking any clear feature, as indicated by the
red arrows in Fig. 2a and S2 (See ESIw). We attribute this
particular (lack of) contrast to the (rotational) mobility of the
spoked wheel 4 within these flexible voids.10d,4 The 2D porous
template clearly affects the corrugation of the surface. Only
upon its formation, isolated molecular spoked wheels and also
their rotational dynamics can be visualized. Despite its size,
the interaction with the graphite substrate is weak and at the
liquid–solid interface, isolated single molecules are mobile
without the template. This is also confirmed by time-depen-
dent experiments indicating desorption–adsorption events of
the wheel molecules (Fig. S3 in ESIw). At present we attribute
this to the fact that the molecular wheel contains also alkyl
chains at its spokes which weaken the adsorption strength.
Note that smaller guest molecules were not immobilized in the
voids, nor was the spoked wheel 4 hosted in voids formed by
DBA 1.
As targeted, we demonstrated the formation of 2D mole-
cular patterns with huge voids, the diameter of which reaches
over 7 nm, at the liquid–solid interface. This pore size seems
also to be the practical limit of concentration controlled
polymorph selection for this kind of alkylated DBA
derivatives,10c as far as the formation of extended porous 2D
networks on graphite is concerned. As the structural proper-
ties of the host network were tuned to the symmetry and size
of the molecular wheel guest, individual guest molecules could
be trapped and their dynamics revealed. The size-tunability of
the voids, based upon the proper selection of the alkyl chain
length, is a powerful approach to isolate guests, reaching the
10 nm scale. As these networks are fragile, efforts to stabilize
them are in progress.
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan, the Belgian Federal Science
ꢁc
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ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 3897–3899 | 3899