2D supramolecular structures of a shape-persistent macrocycle and co-deposition with fullerene on HOPG

Article abstract of DOI:10.1021/ja060469f

Two 2D supramolecular structures of macrocycle 1 and 1/C₆₀ have been obtained on HOPG by self-assembly under ambient conditions and investigated by high-resolution STM. The monolayers of 1 are characterized by structures with perfect ordering o

Full text of DOI:10.1021/ja060469f

Published on Web 03/09/2006
2D Supramolecular Structures of a Shape-Persistent Macrocycle and
Co-deposition with Fullerene on HOPG
Ge-Bo Pan,† Xiao-Hong Cheng,‡ Sigurd Ho¨ger,*,‡ and Werner Freyland*,†
Institute of Physical Chemistry and Institute for Technical Chemistry and Polymer Chemistry, Karlsruhe UniVersity,
Kaiserstr. 12, 76128 Karlsruhe, Germany
Received January 20, 2006; E-mail: werner.freyland@chemie.uni-karlsruhe.de; hoeger@chemie.uni-karlsruhe.de
Self-organized systems have attracted considerable attention due
to their potential applications in nanotechnology as a “bottom-up”
approach for the construction of molecule-scale devices and
nanostructures.1 The organized objects can be molecular, macro-
molecular, supramolecular, and colloidal. Shape-persistent macro-
cycles are composed of a rigid molecular backbone and flexible
(functional) side groups, which allow tailoring of new templates
for self-assembly, and can be considered as promising building
blocks.2 By using nonspecific attractive interaction, one- and two-
dimensional (1D and 2D) organizations have been obtained during
the last years. On the other hand, highly ordered arrays of fullerenes
are of particular interest from both the scientific and technological
points of view and have been investigated on various substrates by
using different methods.3,4
Herein, we report the investigation of the 2D supramolecular
structures of the shape-persistent macrocycle 1 and 1/C60 (a two-
component system) on HOPG under ambient conditions. Highly
ordered arrays of 1 and 1/C60 have been fabricated from 1,2,4-
trichlorobenzene (TCB) by self-assembly and characterized by
scanning tunneling microscopy (STM). In brief, a drop of TCB
solution containing 1 (less than 1 mM) or 1/C60 (1:4) was directly
deposited onto the freshly cleaved surfaces of HOPG, dried in the
Ar flow for about 2 h, and then transferred into a home-built air-
tight chamber for STM measurement.
Figure 1a represents a large-scale STM image of the (long range)
ordered arrays of 1. The angle between different domain directions
is 120 ( 2°, corresponding to the hexagonal graphite symmetry.
The bright protrusion in the STM image is attributed to the
unsaturated backbone of the macrocycles, whereas the dark stripes
are occupied by the alkyl chains. This is the usual case that the
aromatic and conjugated parts of the molecules show a higher
Figure 1. Ordered arrays of 1 on HOPG under ambient conditions. (a)
tunneling probability than the aliphatic alkyl parts.5
Typical large-scale STM image. Vbias ) 0.35 V; It ) 0.6 nA. An oblique
A higher resolution STM image of 1 at the surface of the graphite
is shown in Figure 1b. The macrocycles are organized into rows
that are separated by the alkyl chains, which are interdigitated and
oriented along one of the main symmetry axes of underlying HOPG
lattice, maximizing their interaction with the substrate. The overall
shape of the contrast of an individual macrocycle is qualitatively
comparable with the electronic density of the highest occupied
molecular orbital (HOMO), shown as an inset in Figure 1b. The
four extraannular groups with oligo-alkyl side chains are also clearly
visible and indicated with the green circles. These data strongly
indicate that the rings are flat adsorbed at the graphite in order to
maximize the compound/substrate interaction.6 On the basis of the
analysis above, the parameters of an oblique unit cell were
determined as a ) 3.6 ( 0.2 nm, b ) 5.0 ( 0.2 nm, and γ )70 (
2°. These values agree well with the calculated molecular sizes of
1 by AM1 method (see Supporting Information). Figure 1c shows
unit cell is indicated in green. (b) Higher resolution STM image. Vbias
)
0.41 V; It ) 0.5 nA. The four extraannular groups are indicated with green
circles. The inset is the HOMO of the cyclic backbone of 1. (c) Proposed
structural model. The blue and red octagons indicate the two types of
nanoscale holes, A and B (for large-size STM images, see Supporting
Information).
a schematic model for the ordered arrays of 1. Note that only two
alkyl chains for each extraannular side group can be observed. The
remaining alkyl chains are either directed to the air or adsorbed in
a disordered fashion. Obviously, the structure contains the intrinsic
empty holes A (blue octagon, ∼1.4 nm) of the macrocycle as well
as a second empty area B (red octagon, ∼ 2.5 nm × 1.8 nm). At
present, it is not clear whether this empty space contains the
remaining alkyl chains or not.
In addition to the investigation of the pure compound 1, attempts
have been made to self-assemble the two-component system of 1
and C60. Fullerene-oligothiophene composites have attracted
considerable attention during the past decade due to their remarkable
electrooptical properties in organic solar cells.7 Figure 2a shows
† Institute of Physical Chemistry.
‡ Institute for Technical Chemistry and Polymer Chemistry.
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10.1021/ja060469f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
have not succeeded in observing the alkyl chains due to their low
contrast. Nevertheless, they should be located between the adjacent
molecular rows and are interdigitated according to the lattice
parameter.
In summary, two 2D supramolecular structures of macrocycle 1
and 1/C60 have been obtained on HOPG by self-assembly under
ambient conditions and have been investigated by high-resolution
STM. The monolayers of 1 are characterized by structures with
perfect ordering over relatively large area. In the case of 1/C60, the
size of the macrocycle 1 and the presence of two individual
bithiophene units per ring lead in the final superstructure to a 1:2
stoichiometry. The fullerenes are not trapped at the graphite surface
inside the macrocyclic holes but are located around the periphery
of the bithiophene units. This clearly shows that the donor-acceptor
interaction between C60 and the electron-rich units of the ring is
the dominant factor for the structure formation. The as-prepared
2D supramolecular structures at present are used as a template for
complex 3D structures.
Figure 2. Ordered arrays of 1/C60 on HOPG under ambient conditions.
(a) Typical large-scale STM image. Vbias ) 0.8 V; It ) 0.3 nA. (b) Higher
resolution STM image. Vbias ) 1.0 V; It ) 0.3 nA. Arrows I and II indicate
individual macrocycles. (c) Higher resolution STM image, revealing the
adsorption site of C60 on the macrocycle. Vbias ) 1.0 V; It ) 0.3 nA. (d)
Proposed structural model.
Acknowledgment. This work was supported by Center of
Functional Nanostructures at Karlsruhe University, DFG, and Fonds
der Chemischen Industrie. G.-B.P. thanks the Alexander von
Humboldt Foundation.
Supporting Information Available: Experimental methods, syn-
thesis of macrocycle 1, and large-size STM images. This material is
available free of charge via the Internet at http://pubs.acs.org.
an example of the large-scale STM image. An interesting feature
in the 1/C60 array are the well-ordered bright spots, which are
significantly different from the pure 1 array discussed before. Each
bright spot can be associated with an individual molecule of C60.
Note that these bright spots can be easily swept off during the
continuous scanning without changing the imaging parameters, such
as bias voltage and setpoint current. This last observation is
indicative of the weak bonding between the C60 and macrocycle 1.
Careful inspection shows that areas of ordered bright spots are
coexisting with small ordered domains of 1.
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Figure 2b and 2c shows higher resolution STM images acquired
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