Surface-assisted organic synthesis of hyperbenzene nanotroughs

Article abstract of DOI:10.1002/anie.201300610

A hexagonal macrocycle consisting of 18 phenylene units (hyperbenzene) was synthesized on a Cu(111) surface in ultrahigh vacuum by Ullmann coupling of six 4,4′′-dibromo-m-terphenyl molecules. The large diameter of 21.3 A and the ability to assemble in arrays makes hyperbenzene an interesting candidate for a nanotrough that could enclose metallic, semiconducting, or molecular quantum dots. Copyright

Full text of DOI:10.1002/anie.201300610

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Angewandte
Communications
DOI: 10.1002/anie.201300610
Nanotrough Arrays
Surface-Assisted Organic Synthesis of Hyperbenzene Nanotroughs**
Qitang Fan, Cici Wang, Yong Han, Junfa Zhu,* Wolfgang Hieringer, Julian Kuttner,
Gerhard Hilt, and J. Michael Gottfried*
The surface-assisted synthesis of organic molecules and
nanostructures is a promising bottom-up approach towards
functional surfaces for applications in catalysis, sensor sys-
tems, and organic electronics. The major challenge of this
approach is that most established CÀC coupling reactions
grown on surfaces defect-free over large areas, because it
represents the global energy minimum and thus can be
synthesized under conditions at which the CÀC bond for-
mation becomes reversible, in particular at high temper-
[4,18–21]
atures.
require solvents and thus cannot be performed on solid
surfaces under ultrahigh vacuum (UHV) conditions. One of
In the light of this fundamental problem, we pursue herein
a hierarchic strategy and first synthesize, directly on a copper-
(111) surface, CÀC-bonded molecular subunits, which then
[
1]
[2]
the few exceptions is the Ullmann reaction, which
achieves CÀC coupling between haloarene molecules by
assemble to form van der Waals bonded arrays. This approach
takes into account the fact that small units with fewer degrees
of freedom can structurally more easily be controlled than
large units, which becomes an important point when bond
formation is irreversible. The synthesis of small molecules on
surfaces using the Ullmann reaction is possible, as has been
means of metallic Cu. First reported in 1901, it is one of the
oldest heterogeneous reactions of organic chemistry and has
recently been employed, in a modified form, in attempts to
prepare one- and two-dimensional (1D/2D) polymers on
[
3–10]
metal single-crystal surfaces.
particular have attracted great interest for the nanostructur-
ing of functional surfaces.
2D covalent networks in
[
22–24]
shown for biphenyl.
The reaction steps can even be
[
11–14]
The synthesis of 2D networks
controlled by manipulation with a scanning tunneling micro-
[
25]
was especially successful for systems in which the monomers
are linked by relatively weak bonds, such as coordinative or
sope (STM). However, attempts to synthesize large cyclic
[
26]
molecules on surfaces have not yet been successful; only
sexiphenylene was observed as a minor byproduct in the
[
15,16]
hydrogen bonds.
which allows the system to reach a local energy minimum by
healing” of initial defects in the long-range order of the
Formation of these bonds is reversible,
[
6]
synthesis of oligophenylene chains.
“
Herein we used 4,4’’-dibromo-1,1’:3’,1’’-terphenyl (1; 4,4’’-
dibromo-m-terphenyl, DMTP; Scheme 1) as a precursor
monomer and Cu(111) as the substrate for the surface-
assisted synthesis. A (111) surface was chosen because its
hexagonal symmetry was shown to stabilize angles of 1208 in
network. In contrast, formation of CÀC bonds is usually not
reversible and thus defects will persist. For this reason,
covalent 2D networks grown from molecular precursors often
contain a wide range of local binding motifs, that is, many
[
11,13,17]
[27]
defects.
A notable exception is graphene, which can be
the adsorbate lattice owing to template effects. Physical
vapor deposition of DMTP onto the clean Cu(111) surface
held at 300 K leads to cleavage of the BrÀC bonds, as
[
*] Q. T. Fan, C. C. Wang, Y. Han, Prof. Dr. J. F. Zhu
National Synchrotron Radiation Laboratory
University of Science and Technology of China
Hefei 230029 (P.R. China)
confirmed by X-ray photoelectron spectroscopy (XPS; see
the Supporting Information, Figure S1) and in agreement
[28]
with previous work. The STM images displayed in Figure 1
show that the m-terphenylene fragments form elongated
islands consisting of zigzag chains, which have preferential
orientations relative to the high-symmetry directions of the
Cu(111) substrate (see the Figure caption for details). The
lattice constant along the chains is 26.5 ꢀ (Figure 1b). This is
larger than expected for direct CÀC linkage (which would
E-mail: jfzhu@ustc.edu.cn
J. Kuttner, Prof. Dr. G. Hilt, Prof. Dr. J. M. Gottfried
Fachbereich Chemie, Philipps-Universitꢀt Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
E-mail: michael.gottfried@chemie.uni-marburg.de
Priv.-Doz. Dr. W. Hieringer
Lehrstuhl fꢁr Theoretische Chemie
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
lead to a lattice constant of 21.8 ꢀ) and thus suggests that the
m-terphenylene fragment are linked by Cu atoms, forming the
1
D coordination polymer 2 (see Scheme 1). This conclusion is
[
**] J.F.Z. thanks the National Natural Science Foundation of China
Grant No.21173200), the Specialized Research Fund for the
Doctoral Program of Higher Education of Ministry of Education
Grant No. 20113402110029), and the National Basic Research
(
supported by the density functional theory (DFT) calcula-
tions described below, which predict a lattice constant along
the chains of 26.5 ꢀ for the C-Cu-C bonded coordination
polymer, and agrees with a previous publication about 4,4’’-
(
Program of China (2010CB923302). W.H. thanks the Deutsche
Forschungsgemeinschaft and the Cluster of Excellence “Engineer-
ing of Advanced Materials” granted to the University of Erlangen–
Nꢁrnberg. J.M.G. thanks the Chinese Academy of Sciences for
a Visiting Professorship for senior international scientists (Grant
No. 2011T2J33).
[28]
dibromo-p-terphenyl on Cu(111), in which linear coordi-
nation polymer chains with C-Cu-C bonds were reported. The
apparent height in STM varies along the chains, and the
terphenyl units (located at the bends; large maxima in
Figure 1g) can be distinguished from the C-Cu-C bridges (at
the straight parts of the chain; small maxima in Figure 1g).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300610.
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Scheme 1. Surface-assisted Ullmann coupling reaction of 4,4’’-
dibromo-m-terphenyl 1 forming a) the coordination polymer 2 at 300 K
and b) hyperbenzene molecules 3 at 550 K on Cu(111).
The extreme aspect ratio of the islands (up to 1:25) shows
that chain growth is much more likely than the start of a new
chain. The assembly of the individual chains to islands is
apparently driven by van der Waals forces (as opposed to
covalent or coordinative bonds), as indicated by the large
distance of 9.7 ꢀ between the chains (measured perpendic-
ular to the straight parts of the zigzag chains). Owing to the
1
208 bend in molecular subunits, the chains can back-fold at
Figure 1. Coordination polymers and oligomers with C-Cu-C bonds:
the end of an island, such that islands often consist of only
a single chain or very few chains, or that a chain continues
from one island to another island with a different orientation
a) Constant-current STM image after deposition of DMTP 1 onto
Cu(111) at 300 K at low coverage; tunneling parameters U=À3.6 V,
I=0.02 nA. The islands consist of chains of the coordination polymer
2
, which are oriented at angles of Æ58 relative to a set of substrate
(
Figure 1a,c). Occasionally, isolated hexagons with a diameter
¯
high-symmetry directions ([110] and equivalent), as marked with green
arrows. b) Higher-resolution image of the shaded area in (a) with
overlaid molecular model and unit cell, U=À3.6 V, I=0.01 nA. c) The
coordination polymer 2 at higher precursor coverage, U=À2.75 V,
I=0.08 nA. Inset: Magnified view of a hexagonal cyclic coordination
hexamer superimposed with a molecular model. d) After deposition of
of 25.2 ꢀ were observed, which, by size considerations, consist
of six m-terphenylene units linked by Cu atoms (Figure 1c,
inset). Deposition of DMTP onto Cu(111) held at 440 K
(
Figure 1d) also yields islands of the 1D coordination
polymer that are larger than those obtained at 300 K and
can exceed 200 nm in length and width (Supporting Informa-
tion, Figure S2), with defect-free areas larger than 30 nm ꢁ
1
onto Cu(111) held at 440 K, which leads to larger islands of the
coordination polymer 2; U=À3.6 V, I=0.02 nA. e) Side view of
a structure from periodic DFT calculations, illustrating the non-planar
geometry of the chains. f) DFT-calculated STM image for the tunneling
parameters in (b). g) Apparent height profile along the blue zigzag line
in (b) from left to right.
6
0 nm. However, also at this temperature no CÀC bond
formation is observed.
Completely different structures are formed when the
Cu(111) sample is held at 550 K during deposition of DMTP:
As shown in Figure 2, ordered arrays of hexagonal rings with
a diameter of 21.3 ꢀ are obtained besides chain-like and other
structures. Size comparison shows that each hexagonal ring is
built up of six m-terphenylene fragments (one at each corner)
and thus contains 18 phenylene groups. The macrocycle can
therefore be referred to as cyclo-octadecaphenylene. Moti-
vated by its hexagonal structure and in analogy to the much
smaller coronene, which is also known as “superbenzene”, the
new molecule will be denoted “hyperbenzene” in the follow-
ing.
Formation of one hyperbenzene molecule requires for-
mation of six new CÀC bonds. In line with this, the variation of
the apparent height along the ring is small compared to the
case of the coordination oligomers (compare the height
profile in Figure 2e with Figure 1g). Furthermore, the corner-
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residual thermal drift during the STM imaging. The distance
between the edges of two adjacent hyperbenzene molecules is
9
.3 ꢀ; this distance is almost identical to the distance between
the coordination polymer chains in Figure 1, which shows that
the assembly is mainly driven by van der Waals interactions
and that there are no CÀC bonds between the rings.
No rings smaller than hyperbenzene were observed.
Apparently, the combination of a rigid molecular precursor
with a substrate of hexagonal symmetry, which favors the 1208
angle, leads to a certain selectivity of the reaction with respect
to the ring systems formed. The template influence of the
surface is also evident from the preferential orientation of the
hyperbenzene islands relative to the high-symmetry direc-
tions of the substrate (Figure 2a).
We note that the surface-assisted synthesis reported
herein is presently the only feasible method to prepare
adsorbed hyperbenzene arrays, because vapor deposition of
hyperbenzene (which has not been synthesized in solution
yet) on a surface would not be possible owing to the high
À1
molecular weight of 1369.7 gmol based on the nominal
formula C108H . Apart from deposition from solution (which
72
is likely to result in contaminated layers), electrospray ion-
beam deposition, which is a complex and rarely applied
[
29]
technique, would be a possible approach.
Apart from the arrays of hyperbenzene molecules, small
areas with short zigzag chains are observed after deposition of
DMTP at 550 K (Figure 2b). Compared to the zigzag
coordination polymer in Figure 1, the unit cell is contracted
along the chains (from 26.5 ꢀ to 21.8 ꢀ) and the bend-to-
bend distance is reduced from 15.1 ꢀ to 12.5 ꢀ, a value very
close to the corner-to-corner distance in the hyperbenzene
rings (12.3 ꢀ), while the distance between the chains remains
unchanged. Furthermore, the brightness along the chains is
now almost uniform. These changes indicate that the terphe-
nylene units are connected by CÀC bonds; that is, oligophe-
Figure 2. Hyperbenzene: a) Overview constant-current STM image
obtained at 300 K after deposition of DMTP 1 onto Cu(111) held at
nylene chains are formed.
5
50 K. The hexagonal rings 3 (hyperbenzene) show a preferential
While most of the nanotroughs shown in Figure 2 appear
to be empty, occasionally circular-shaped objects can be
observed inside the cavities. Most likely this is a molecule or
molecular fragment that rotates on a time scale faster than
imaging, illustrating that the nanotroughs can indeed enclose
molecular objects.
orientation relative to the high-symmetry directions of the substrate.
b) Magnified view of a small section with oligophenylene chains.
c) Magnified view of a hyperbenzene 3 island with several defects and
inclusions in the central cavities. Molecular models and a unit cell are
overlaid. Tunneling parameters: U=À3.6 V, I=0.01 nA. d) DFT-calcu-
lated STM images for U=À3.6 V and I=0.01 nA with overlaid model
of the relaxed computed structure. e) Apparent height profile along the
perimeter of a hyperbenzene molecule, as marked by the blue hexagon
in the inset. Note that the height axis is identical to that in Figure 1g
to allow direct comparison.
To obtain more detailed insight into the structural
properties of the described nanoobjects, we have performed
periodic dispersion-corrected density-functional calculations
on the coordination polymer 2 and the hyperbenzene
molecule 3, both adsorbed on a Cu(111) slab (see the
Experimental Section for details). The adsorption structure
and simulated STM images are shown in Figure 1e,f and 2d.
The polymer 2 adopts a buckled, non-planar geometry on the
Cu(111) surface (Figure 1e), with the Cu atoms in the
polymer in close proximity to the Cu(111) surface, and the
central phenylene ring of the terphenylene units at larger
distance from the surface than the other two rings; in total,
the height varies by 0.9 ꢀ between the C atom with the largest
value of the z coordinate and the coordinated Cu atom, which
has the lowest value of the z coordinate. This peculiar
structure of 2 on Cu(111) has an impact on its STM image.
In both the experimental and simulated STM (Figure 1b,f),
to-corner distance is only 12.3 ꢀ (12.9 ꢀ according to DFT
calculations; see below). This is almost 2 ꢀ less than the bend-
to-bend distance in the coordination polymer chains (STM:
1
5.1 ꢀ, DFT: 15.3 ꢀ). Furthermore, the ring diameter is
almost 4 ꢀ smaller than the diameter of the cyclic coordina-
tion hexamer (Figure 1c, inset). These contractions and their
agreement with the results of DFT calculations (see below)
provide additional evidence that the fragments are now
connected by CÀC bonds instead of C-Cu-C bonds.
Hyperbenzene forms a hexagonal lattice on Cu(111); the
unit cell dimensions are shown in Figure 2c. The small
deviations from the ideal unit cell geometry are due to
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the bends of the coordination polymer, which can be
identified as the central rings of the terphenylene units,
appear as bright protrusions, reflecting their elevation from
the surface. The linking Cu atoms in the polymer, on the other
hand, are imaged in the STM as rather weak spots, reflecting
their large distance from the tip. The experimental variation
of the apparent height is smaller (ca. 0.2 ꢀ; see Figure 1g)
than the theoretical value, which is attributed to super-
imposed electronic effects, especially to the more efficient
tunneling through the coordinated metal centers, which
makes them appear more elevated than they are. Such effects
have previously been observed for organometallic com-
[
[
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Ullmann reaction. As possible precursors to the formation of
hyperbenzene, zigzag-shaped and cyclic coordination oligo-
and polymers with C-Cu-C bonds have been observed. Each
hyperbenzene molecule contains six precursor molecules and
thus requires the formation of six new CÀC bonds. The
[
1
hyperbenzene molecules aggregate to ordered arrays with
a hexagonal unit cell, a process that is driven by van der Waals
interactions. Owing to the large diameter of 21.3 ꢀ, the
nanotroughs could enclose a wide range of quantum dots,
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The
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[
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Received: January 23, 2013
Published online: March 19, 2013
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Keywords: hyperbenzene · macrocycles ·
[
.
scanning probe microscopy · self-assembled monolayers ·
Ullmann reaction
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