2D assembly of metallacycles on HOPG by shape-persistent macrocycle templates

Article abstract of DOI:10.1021/ja907220f

The synthesis and scanning tunneling microscopy (STM) investigations of shape-persistent arylene-ethynylene-butadiynylene macrocycles along with their codeposites with metallacycles are reported. 2D ordered arrays of macrocycles and macrocycle/metallacycle architectures (1:1) have been obtained on HOPG by self-assembly under ambient conditions. It is found that the ordered macrocycle array acts as a template for the deposition of the adlayer molecules. For each underlying macrocycle, one metallacycle has been detected. The unit-cell data of both, the macrocycles and their codeposites, show that the structural information of the macrocycle layer is perfectly transformed to the guest molecules. A rather unexpected observation is that the present compound could not be coadsorbed with C60, indicating that only a minor change in the structure of the macrocycle has a dramatic effect on the ability of the monolayer to bind additional guest molecules.

Full text of DOI:10.1021/ja907220f

Published on Web 01/07/2010
2D Assembly of Metallacycles on HOPG by Shape-Persistent
Macrocycle Templates
Ting Chen,† Ge-Bo Pan,*,‡ Henning Wettach,§ Martin Fritzsche,§ Sigurd Ho¨ger,*,§
Li-Jun Wan,*,† Hai-Bo Yang,|| Brian H. Northrop,|| and Peter J. Stang||
Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125,
P. R. China, Kekule´-Institut fu¨r Organische Chemie und Biochemie, Rheinische
Friedrich-Wilhelms-UniVersita¨t Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, and
Department of Chemistry, UniVersity of Utah, 315 South 1400 East, Salt Lake City, Utah 84112
Received August 26, 2009; E-mail: gbpan2008@sinano.ac.cn; hoeger@uni-bonn.de; wanlijun@iccas.ac.cn
Abstract: The synthesis and scanning tunneling microscopy (STM) investigations of shape-persistent
arylene-ethynylene-butadiynylene macrocycles along with their codeposites with metallacycles are reported.
2D ordered arrays of macrocycles and macrocycle/metallacycle architectures (1:1) have been obtained on
HOPG by self-assembly under ambient conditions. It is found that the ordered macrocycle array acts as a
template for the deposition of the adlayer molecules. For each underlying macrocycle, one metallacycle
has been detected. The unit-cell data of both, the macrocycles and their codeposites, show that the structural
information of the macrocycle layer is perfectly transformed to the guest molecules. A rather unexpected
observation is that the present compound could not be coadsorbed with C60, indicating that only a minor
change in the structure of the macrocycle has a dramatic effect on the ability of the monolayer to bind
additional guest molecules.
Introduction
template for the assembly of guest molecules with complemen-
tary size, electron affinity, or functionalization.5 Although the
Self-assembly as an important way to create ordered nano-
scale structures and molecule-based devices has attracted
considerable attention in materials science.1 However, this
approach toward complex molecular structures on solid surfaces
is still a great challenge and requires not only the assembly of
a single molecule but the (predictable) formation of multicom-
ponent assemblies. Therefore, the investigation of codeposits
of two or more molecular compounds on solid surfaces is
currently a highly attractive field of research, mostly performed
by scanning tunneling microscopy (STM).2,3 Shape-persistent
macrocycles are promising building blocks in that context. Their
backbone can be electron-deficient or electron-rich, and their
interior as well as their exterior can be functionalized indepen-
dently.4 This allows for utilizing the macrocycle pattern as a
coadsorption of rigid macrocycles with fullerenes and flat
organic molecules has recently been described by us and other
groups, the formation of adlayers of metallacycles onto rigid
macrocycle templates has not been described so far.6,7 In this
work, we report the synthesis and STM investigations of shape-
persistent arylene-ethynylene-butadiynylene macrocycles 1 and
2 along with their codeposites with the metallacycles 3 and 4.8
Results and Discussion
Synthesis. Scheme 1 displays the synthesis of the shape-
persistent macrocycles 1 and 2 (experimental details are given
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Bai, C.-L. Angew. Chem. 2003, 115, 2853. (b) Surin, M.; Samori, P.
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† Institute of Chemistry, Chinese Academy of Sciences.
‡ Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy
of Sciences.
§ Rheinische Friedrich-Wilhelms-Universita¨t Bonn.
|| University of Utah.
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1328 J. AM. CHEM. SOC. 2010, 132, 1328–1333
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2D Assembly of Metallacycles on HOPG
A R T I C L E S
Scheme 1. Synthesis of 1 and 2a
a (a) [Pd(PPh3)4], Na2CO3, toluene, EtOH, H2O, reflux, 8 h; then 70 °C, 16 h, 83%; (b) 1.) FeCl3, CH2Cl2, CH3NO2, rt, 40 min, 2.) MeOH, rt, 1 h, 19%;
(c) BBr3, CH2Cl2, -78 °C to rt, 100%; (d) (S)-(+)-1-bromo-2-methylbutane, K2CO3, KI, DMF, 65 °C, 2 d, 60%; (e) n-BuLi, TMEDA, I2, THF, -78 °C to
rt, 12: 90%, 12a: 87%; (f) [PdCl2(PPh3)2], CuI, PPh3, THF, piperidine, rt, 2 d, 14: 93%, 14a: 97%; (g) TBAF, THF, rt, 2 h, 15: 91%,15a: 80%; (h) CuCl,
CuCl2, pyridine, 40 °C, 96 h, 1: 20%, 2: 3%.
in the Supporting Information), which are based on the
phenylene-ethynylene-butadiynylene backbone. Bithiophene
5 was previously described in the literature and prepared
accordingly.9 For the synthesis of the bithiophene 11 an
alternative approach was worked out in which the (chiral) side
groups were attached to the condensed heterocyclic core at the
last step of the reaction sequence. Suzuki-coupling of 1,2-
dibromo-4,5-dimethoxy benzene (6) with commercial available
3-thiophene boronic acid (7) and subsequent Fe(III) mediated
thiophene-thiophene coupling lead to the dimethoxy compound
9 that was demethylated with boron tribromide to give the diol
10. Alkylation with the corresponding alkyl bromide gave 11.
This reaction sequence allows a side group variation at a later
stage of the synthesis and guarantees a high synthetic flexibility.
Pd-catalyzed Sonogashira-Hagihara coupling of 5 (11) with the
monoprotected bisacetylene 13 and subsequent fluoride induced
deprotection of the silyl groups generated the bisacetylenic “half
rings” 15a and 15b, respectively. For the macrocycle synthesis,
the corresponding “half rings” were oxidatively dimerized under
pseudo high-dilution conditions by adding a dilute solution of
the corresponding bisacetylene in pyridine over four days to a
slurry of CuCl/CuCl2 in the same solvent.10 Purification of the
macrocylces was achieved by column chromatography on silica
gel and subsequent recycling GPC (see the Supporting Informa-
tion).
STM Investigations. Figure 1 shows the molecular structures
of 1-4 and their corresponding space-filling models with
calculated molecular sizes. 1 and 2 contain (electron-rich)
polycyclic bithiophene units in the backbone. Highly ordered
arrays of the macrocycles on HOPG have been fabricated by
self-assembly and characterized by STM. In brief, for the
macrocycle arrays, a drop of a macrocycle solution in 1-phe-
nyloctane was deposited onto the freshly cleaved surface of
HOPG and then directly used for STM measurements.
Figure 2a represents a typical STM image of long-range-
ordered monolayers of 1. Different domains usually cross each
other at an angle of 60 or 120°. The bright protrusions in the
STM image are attributed to the unsaturated backbone of the
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A R T I C L E S
Chen et al.
Figure 1. Molecular structures of 1-4 and space-filling models with calculated sizes.
macrocycles, whereas the dark stripes are occupied by the alkyl
chains. The macrocycles are organized into rows that are
separated by the alkyl chains, which are interdigitated and
aligned with underlying graphite lattice to maximize the
intermolecular interactions (Figure 2b). The overall shape of
the contrast of an individual macrocycle is qualitatively
comparable with the electronic density of the frontier orbitals
of 1 (see the Supporting Information). The four extraannular
groups with oligo-alkyl side chains are also visible and
highlighted with green circles. These data strongly indicate that
the macrocycles are flat adsorbed at the graphite.5b The
parameters of an oblique unit cell were determined as a ) 4.5
( 0.1 nm, b ) 5.0 ( 0.1 nm, and R ) 72 ( 1° (Figure 2c) and
agree well with the calculated molecular size of 1 (Figure 1).
Figure 2c shows a tentatively proposed structural model for the
ordered monolayer of 1.
(5) (a) Kromer, J.; Rios-Carreras, I.; Fuhrmann, G.; Musch, C.; Wunderlin,
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At higher macrocycle concentration, the formation of bilayers
can be observed. Figure 3a displays two different types of
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Soc. 2008, 130, 6666. (f) Lei, S.; Tahara, K.; Feng, X.; Furukawa, S.;
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2D Assembly of Metallacycles on HOPG
A R T I C L E S
Figure 3. Ordered bilayers of 1/1 on HOPG at a concentration of ca. 5 ×
10-5 M. (a) Typical STM image. Vbias ) 1.23 V; It ) 201 pA. (b) A
composite STM image, in which the tunneling parameters have been
changed at the dashed white line, showing the underlying and upper layers
simultaneously. The scanning direction was from top to bottom. Upper
region: Vbias ) 1.2 V; It ) 201 pA. Lower region: Vbias ) 600 mV; It ) 201
pA. (c) A section analysis along the white line in a is displayed in c.
Figure 2. Ordered monolayers of 1 on HOPG at a concentration of ca.
0.5 × 10-5 M. (a) Large-scale STM image. Vbias ) 764 mV; It ) 506 pA.
(b) Higher-resolution STM image. Vbias ) 408 mV; It ) 694 pA. The four
extraannular groups are indicated with green circles. (c) Proposed structural
model. An oblique unit cell is indicated in black.
that the first monolayer of 1 is stable enough to act as a template.
In addition, some macrocycle cores, indicated with the green
arrows, appear brighter at random places in the image, though
always close to the molecular row. The number of such brighter
spots increases a little with increasing macrocycle concentration.
This effect can be attributed to the adsorption of more than two
macrocycle cores on top of each other.
appearance of the macrocycles with the cross-section analysis.
This might be either due to the different adsorption registry or
the formation of a bilayer 1/1.11 Once a monolayer is formed,
it can act as a template for the epitaxial adsorption of an
additional layer of macrocycles. Figure 3b shows a composite
STM image, in which the underlying and upper layers are shown
simultaneously. Apparently, the macrocycle molecules are
located on top of the first monolayer. The alkyl chains for the
second layer are not resolved. Nevertheless, they should be
located between the adjacent molecular rows and are interdigi-
tated according to the lattice parameter. Similar phenomena have
also been observed in the self-assembly of alkyl-substituted
tetrathiafulvalene.12 It has to be noted that only small ordered
domains could be found for the bilayer structures in the present
experiments. Within experimental errors, the parameters of an
oblique unit cell of 1/1 are identical to those of 1, indicating
Because 1 and 2 contain (electron-rich) polycyclic bithiophene
units in their backbone, the macrocycle pattern at the HOPG
surface is an ideal platform to act as template for the deposition
of (electron deficient) guest molecules. After recording the
ordered monolayer of 1 (Figure 2a) at the 1-phenyloctane/HOPG
interface, a drop (∼0.5 µL) of a solution of the square
metallacycle 3 in 1-phenyloctane was directly added. Figure
4a shows a typical large-scale STM image recorded about 10
min after adding the solution. The well-ordered bright spots are
an interesting feature of the array, a significant difference from
the array of 1 discussed before. They result either from a bi- or
multilayer structure of 1 or from an epitaxialy grown com-
mensurable adlayer of 3 on a monolayer of 1 (1/3).13,14 The
latter is most likely, because first only the ordered monolayers
of 1 have been observed before adding the solution of metal-
lacycle 3. Second, only the monolayer structures of 1 could be
recorded when other guests such as C60, triphenylene, perylene,
coronene, or TCNQ were added by using the same experimental
procedure as for 3. And finally, only small-ordered domains
with sporadically distributed brighter spots could be observed
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(11) The term 1/1 describes a monolayer of 1 on a substrate (here HOPG)
that acts as template for the adsorption of a second monolayer, in this
specific case again of 1, so that a double layer is formed.
(12) (a) Lu, J.; Zeng, Q.-D.; Wang, C.; Wan, L.-J.; Bai, C.-L. Chem. Lett.
2003, 32, 856. (b) Abdel-Mottaleb, M. M. S.; Gomar-Nadal, E.; Surin,
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(13) Accordingly, the term 1/3 describes a monolayer of 1 on a substrate
(here HOPG) that acts as template for the adsorption of a second
monolayer, in this specific case of 3.
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Chen et al.
Figure 4. Ordered arrays of 1/3 on HOPG. (a) Large scale STM image.
Vbias ) 1.30 V; It ) 195 pA. (b) High resolution STM image. Vbias ) 1.38
V; It ) 189 pA. (c) Proposed structural model. Each red square stands for
one metallacycle.
Figure 6. Ordered monolayers of 2 on HOPG. (a) Large-scale STM image.
Vbias ) 578 mV; It ) 683 pA. (b) Higher resolution STM image. Vbias
)
529 mV; It ) 717 pA. The inset is a close-up image of one macrocycle
molecule. (c) Proposed structural model. An oblique unit cell is indicated
in black.
respectively. Within experimental errors, the data of an oblique
unit cell, containing either one square 3 or one rectangle 4, are
identical to those of pure 1. These observations indicate that
the structural information of the macrocycle layer is perfectly
transformed to the guest molecules and that the first monolayer
of 1 is stable enough to act as a template for the adsorption of
metallacycles 3 and 4. It should be noted that unlike a previous
study on the macrocycle/fullerene system (1: 2 stoichiometrty),6a
the formation of bicomponent architectures of 1/3 and 1/4 has
led to a 1:1 stoichiometrty.
Figure 5. Ordered arrays of 1/4 on HOPG. (a) Typical STM image of
1/4. Vbias ) 1.17 V; It ) 201 pA. The red arrows indicate the underlying
macrocycle molecules 1. (b) Proposed structural model. The blue rectangles
indicate the position of 4 at the template 1.
To explore the generality of the formation of the macrocycle/
metallacycles bicomponent structures, the aforementioned ex-
perimental procedures have also been applied for the deposition
of 2, 2/3, and 2/4. Figure 6a represents a large-scale STM image
of the ordered monolayer of 2.
for the second layers of 1 although different (even high)
concentrations of 1 have been investigated. This is not the case
for the ordered structure of 1 laced with a solution of 3 that
can form large-ordered domains. Therefore, we strongly assume
that the above observations demonstrate the formation of a
bicomponent architecture of 1/3.
Figure 4b is a higher-resolution STM image and reveals
details of the internal molecular structure and orientation of the
composite structure. The underlying macrocycles are indicated
with red arrows. It is clear that the metallacycles are not included
in the intrinsic molecular void of 1, but located above the
macrocycles to form the second layer. Additionally, for each
macrocycle, only one square metallacycle can be detected. This
is reasonable because the intraannular cavity of 1 is not large
enough to accommodate a metallacycle (Figure 1).
Similar assembling behavior has been observed for the
metallacycle 4 on the ordered monolayer of 1. Figure 5a shows
a typical STM image of an ordered bilayer of 1/4. Each bright
spot in the image is associated with an individual molecule of
4 on an underlying layer of 1 and the bright spot is located
above the center of macrocycle 1. Figures 4c and 5b show
schematic models for the ordered bilayers of 1/3 and 1/4,
Similar to the macrocycle 1, the bright protrusions in the STM
image are attributed to the unsaturated backbone of the
macrocycles, whereas the dark stripes are occupied by the alkyl
chains. The macrocycles are organized into rows that are
separated by the alkyl chains which are interdigitated and aligned
with the underlying graphite lattice to maximize the intermo-
lecular interaction (Figure 6b). The macrocycles are flat adsorbed
at the graphite in order to maximize the molecule-substrate
interaction. Figure 6c is a tentatively proposed structural model
for the ordered monolayer of 2. The parameters of an oblique
unit cell were determined as a ) 3.9 ( 0.1 nm, b ) 5.0 ( 0.1
nm, and R ) 78 ( 2°. These data are slightly different from
those of 1. This can be attributed to the different groups, n-octyl
versus 2-methylbutyl, at the polycyclic bithiophene units of the
two macrocycles (Figure 1). Moreover, the existence of the
methylbutyl groups can hinder the formation of a bilayer 2/2.
In the present study, only sporadically distributed brighter spots
have been observed for the bilayer 2/2, even at higher macro-
cycle concentrations. This is possibly because the presence of
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2D Assembly of Metallacycles on HOPG
A R T I C L E S
the formation of this superstructure is not the interaction of the
metallacycles with the uncovered HOPG surface inside the rings.
At present, the driving force for the formation of ordered
structures of 1/3, 1/4, and 2/3, 2/4, respectively, is not
understood yet and will be the object of further investigations.
One might speculate that the electron-rich units of the bithiophene
macrocycle can electronically favorably interact with electron-
deficient parts of the metallacycles as well as the diffusion
barrier formed by the primary layers of macrocycle 1 and 2.
Conclusions
Figure 7. Typical STM images of (a) 2/3 and (b) 2/4 on HOPG. (a) Vbias
) 1.30 V; It ) 195 pA. (b) Vbias ) 1.38 V; It ) 189 pA. The unit-cell
parameters of both 2/3 and 2/4 (a ) 3.9 ( 0.1 nm, b ) 5.0 ( 0.1 nm, and
R ) 78 ( 2°) are the same as those for 2 within experimental errors.
2D ordered arrays of shape-persistent macrocycle and mac-
rocycle/metallacycles (1:1) have been obtained on HOPG by
self-assembly under ambient conditions. The ordered macrocycle
array acts as a template for the epitaxial deposition of the adlayer
molecules. For each underlying macrocycle, one metallacycle
molecule has been detected and the structural information of
the macrocycle layer is perfectly transformed to the guest
molecules. The driving force can be that the electron rich units
of the bithiophene macrocycle electronically favorably interact
with the electron deficient parts of the metallacycles as well as
the diffusion barrier formed by the primary layer of macrocycles.
A rather unexpected observation is that the present compound
could not be coadsorbed with C60, indicating that only a minor
change in the structure of the macrocycle has a dramatic effect
on the ability of the monolayer to bind additional guest
molecules. At present, the as-prepared 2D-supramolecular
structures are used as a template for complex 3D structures.
the branched 2-methylbutyl groups reduces the π-π interaction
between the macrocycle cores compared to the linear n-octyl
groups.
After recording the ordered monolayer of 2 at the 1-phenyl-
octane/HOPG interface, a drop (∼0.5 µL) of a solution of either
square metallacycle 3 or rectangular metallacycle 4 in 1-phe-
nyloctane was directly added. Figure 7 show typical STM
images of 2/3 and 2/4 recorded about 10 min after adding the
solution. Again, the well-ordered bright spots are an interesting
feature of the array, a significant difference from the array of
pure 2. As discussed above, the macrocycles 2 themselves could
not form large ordered domains of a bilayer structure. These
bilayer structures are thus from a bicomponent architecture of
2/3 and 2/4, respectively. As for the composite structures of 1,
for each macrocycle 2, only one metallacycle, either square 3
or rectangle 4, can be detected. In all cases, the bithiophene
macrocycles determine the distance between adjacent coad-
sorbed metallacycles, indicating that the bithiophene macro-
cycles play an important role in determining the lattice constants
of the ordered bicomponent architectures of 2/3 and 2/4.
Although the metallacycles 3 and 4 are nonplanar, their
deposition and codeposition on solid surfaces has been reported
recently.7b As far as pure metallacycles 3 and 4 on HOPG are
concerned, only disordered layers have been observed (see the
Supporting Information). Therefore, and because of the sizes
of the macrocycles and the metallacycles, the driving force for
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (20821003, 20873160, and
20821120291), the National Basic Research Program of China
(2010CB934100), the Chinese Academy of Sciences, the Deutsche
Forschungsgemeinschaft, the SFB 624, and the NSF (CHE-
0820955) at Utah.
Supporting Information Available: Additional STM, synthe-
sis, characterization, and molecular modeling of 1 and 2. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA907220F
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J. AM. CHEM. SOC. VOL. 132, NO. 4, 2010 1333