Oligomers and cyclooligomers of rigid phenylene-ethynylene-butadiynylenes: Synthesis and self-assembled monolayers

Article abstract of DOI:10.1002/anie.201001625

Starting from the same bisacetylene, different reaction conditions (palladium or copper catalysis) selectively yielded cyclic or acyclic oligomers with n=2-6 (see picture for n=3) linked by freely rotating corner units. STM images of self-assembled monola

Full text of DOI:10.1002/anie.201001625

Angewandte
Chemie
DOI: 10.1002/anie.201001625
Self-Assembled Monolayers
Oligomers and Cyclooligomers of Rigid Phenylene–Ethynylene–
Butadiynylenes: Synthesis and Self-Assembled Monolayers**
Stefan-S. Jester,* Natalia Shabelina, Stephan M. Le Blanc, and Sigurd Hꢀger*
Nanoscale shape-persistent macrocycles have attracted
increasing attention because of their interesting structural,
optical, and electronic properties. Moreover, they are able to
build up highly organized supramolecular nanostructures in
one, two, and three dimensions.[1] Recently, special emphasis
has been given to self-assembled monolayers (SAMs) of
shape-persistent macrocycles on solid substrates.[2] The ring
sizes and distances can be adjusted by the building blocks, and
their interior and exterior can synthetically be addressed
independently. Together, this allows a surface functionaliza-
tion with atomic-level precision. In several cases, such
patterns could be used for the epitaxial deposition of
admolecules.[3]
The driving forces for the self-assembly are the molecule–
substrate and molecule–molecule interactions, which are
dominated by van der Waals forces. Although rigid macro-
cycles and rigid linear oligomers[4] have come into the focus of
recent scanning tunneling microscopy (STM) studies, a
systematic investigation of rigid rods connected by freely
jointed or freely rotating linkers[5] and their corresponding
closed-loop structures on a solid substrate have been rarely
reported, if at all.
In our own ongoing studies, we have prepared arylene–
ethynylene–butadiynylene macrocycles with a variety of
symmetries, sizes, and functionalities. In most cases, the
final ring closure towards our target structures is achieved by
an oxidative coupling of rigid bisacetylenes. This transforma-
tion typically proceeds by Glaser, Glaser–Eglinton, Hay, or
palladium-mediated homocoupling reactions, and has been
used numerous times for the formation of macrocyclic
structures.[6,7] Oxidative acetylene coupling reactions are
easy to perform and tolerate a wide variety of functional
groups. Notwithstanding, the actual reaction outcome
strongly depends not only on the catalyst/oxidant mixture
but also on the solvent and reaction temperature, and of
course on the specific substrate structure. Sometimes cyclic
and acyclic reaction products are formed simultaneously.[8] In
other cases, a catalyst discrimination between different
macrocyclic[9] or between acyclic and cyclic reaction products
was observed.[10]
Herein, we present a series of acyclic and cyclic phenyl-
ene–ethynylene–butadiynylene(PEB) oligomers prepared by
oxidative acetylene oligomerization. These oligomers are
based on the same constitutional repeating units (CRUs), in
which the rigid elements of the target structures are con-
nected by freely rotating linkers bearing pyridyl functions.[11]
Our study was motivated by the question as to whether these
oligomers can form stable SAMs at the solid/liquid interface
that can be investigated by STM. We wanted to determine the
behavior of oligomers of different length and compare
directly acyclic and cyclic structures, a topic that has not yet
been addressed.
The synthesis of the precursors (half-rings) 1a and 4a is
described in the Supporting Information. They were coupled
using CuCl/TMEDA (1:1)[12] as catalyst and base, air oxygen
as oxidant, and dichloromethane as solvent (Scheme 1,
left).[13] Analytical gel permeation chromatography (GPC)
of the crude product indicated the formation of dimers along
with higher oligomers (see Figure 2A(a)). Similarly, the
palladium-catalyzed oxidative acetylene coupling of 1a and
1b and of 4a and 4b using [Pd(PPh3)2Cl2] and CuI as catalysts,
I2 as oxidant, and iPr2NH as base in THF as solvent
(Scheme 1, right) produced again an oligomer mixture.
However, the peak molecular weights of the oligomers were
significantly lower (see Figure 2B(a)).
Recycling GPC (recGPC) allowed an efficient separation
and detailed analysis of the different products. From the
copper-catalyzed coupling of 4a, we separated acyclic oligo-
mers [5a]n from the dimer (n = 2) up to the hexamer (n = 6) in
yields between 15% and 4%;[14] from the palladium-catalyzed
coupling, cyclic oligomers [6a]n from the dimer (n = 2) to the
hexamer (n = 6) were obtained in yields between 19% and
2%.[15] From the copper-promoted coupling reaction of 1a,
we isolated the acyclic oligomers ([2a]n; n = 2–6) in yields
between 22% and 6%. Under palladium catalysis, both 1a
and 1b only gave cyclodimers [3a]2 and [3b]2; higher
oligomers show the presence of defects (Supporting Informa-
tion).[16–18] However, the pure acyclic and the cyclic oligomers
are slightly yellow and show a strong blue fluorescence in
solution.
With oligomers [5a]n and [6a]n (n = 2–6) now available,
we were able to systematically investigate not only the
adsorption behavior of defined, monodisperse oligomers of
different lengths (and compare them with the monomer, 4a),
but could also directly compare cyclic and acyclic compounds
of the same chain length. An STM image of the monomer
adsorbed at the interface of highly oriented pyrolytic graphite
(HOPG)/1,2,4-trichlorobenzene (TCB) is shown in Figure 1a,
[*] Dr. S.-S. Jester, Dr. N. Shabelina, S. M. Le Blanc, Prof. Dr. S. Hꢀger
Kekulꢁ-Institut fꢂr Organische Chemie und Biochemie
Rheinische Friedrich-Wilhelms-Universitꢃt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax: (+49)228-73-5662
E-mail: stefan.jester@uni-bonn.de
hoeger@uni-bonn.de
[**] Financial support by the DFG, the SFB 624, and the Volkswagen-
Stiftung is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001625.
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Scheme 1. Oxidative acetylene coupling of 1a/b and 4a/b. TMEDA=N,N,N’,N’-tetramethylethylenediamine, OEtHex=2-ethylhexyloxy.
unit cells can be indexed. In the case of packing errors and
domain boundaries of the S-shaped adsorbate rows, the
(ethynyl terminated) open ends of the acyclic oligomers can
be discerned (as marked by white arrows in Figure 2A(d–f)),
and individual molecules are distinguishable.[20] Alternatively,
one might also expect W-shaped adsorbate geometries (see
Supporting Information). However, such pattern motifs were
not observed. At present, we assume that the commensur-
ability of the alkyl chains with the HOPG substrate would
cause unfavorable strain in the backbone.
Whilst open-chain oligomers can easily adopt S-shaped
geometries and therefore pack closely on the HOPG/TCB
interface, the texturing behavior of the corresponding macro-
cycles is, apart from the shape-persistent compound [6a]2,
hardly predictable. [6a]2 forms rows of densely packed rings,
and the structure is indistinguishable from the O-shaped
polymorph of [5a]2. The higher macrocycles formed by an
even number of monomers, [6a]4 and [6a]6, adopt an
elongated collapsed conformation. Two of the meta-substi-
tuted corner segments form the short sides of a quasi-
rectangular structure, while the other two (four in [6a]6) are
integrated into the long sides of the latter. This allows a dense
packing of the compounds at the interface. Again, in all cases
the distance between the opposite long sides of the molecule
(1.3 nm) is indistinguishable within experimental error from
the intermolecular distance between two macrocycles
(1.3 nm). An open (less-dense) adsorbate conformation
(Supporting Information), in which the macrocycles have
large cavities, is in not observed in any of the cases. The
macrocycles formed by an uneven number of monomers
cannot assemble into comparably dense structures. Com-
pound [6a]3 forms mushroom-shaped adsorbates organized
into ordered double rows (for details see the Supporting
Information). However, for [6a]5 (though treated similarly),
no two-dimensional crystallization could be observed. The
molecules adopt a larger variety of conformations, although
Figure 1. a) STM image of 4a at the solid/liquid interface between
highly oriented pyrolytic graphite (HOPG) and 1,2,4-trichlorobenzene
(TCB). b) Molecular model of the SAM of 4a (see also Ref. [21]). The
unit cells of the O- and S-shaped polymorphs are shown in red.
and STM images of the acyclic and cyclic oligomers are shown
in Figure 2A,B(b–f). Bright and dark colors indicate local
high and low tunneling currents resulting from unsaturated
(backbone) and saturated (alkoxy side chain) hydrocarbon
segments, respectively.[19]
For the monomer 4a (Figure 1a) and the acyclic dimer
[5a]2 (Figure 2A(b)), O- and S-shaped polymorphs are
observed concurrently, while for the higher (n > 2) acyclic
oligomers, solely S-shaped geometries are adopted. The
backbone distances in lamellar alignments of linear oligo-
PE(B)s are determined by the alkyl/alkoxy substituent chain
lengths,[4] and the corner units of 4a and [5a]2–6 perfectly
match the expected distances for hexyloxy-substituted oligo-
PEBs.[4f] The intramolecular and intermolecular rod–rod
distances (1.3 nm) cannot be distinguished at the experimen-
tal resolution. This is also strongly supported by the models in
Figure 1b and 2A,B(b–f).
The acetylene ends of adjacent molecules point to each
other, and the kinks form densely packed structures. The
sinuous adsorbates interlock in such a way that rectangular
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Figure 2. a) Superposed analytical GPC elugrams of the product mixtures of [5a]n from the copper-catalyzed reaction (A) and [6a]n from the
palladium-catalyzed reaction (B) before (a) and after (c) preparative recGPC separation. Colors indicate the oligomers of different lengths:
n=2–6. b)–f) STM images of self-assembled monolayers of the separated oligomers at the solid(HOPG)/liquid(TCB) interface. For each STM
image, the corresponding molecular model is shown.[22] (Further details, such as image and unit cell dimensions, are given in Ref. [24]; additional
images are shown in the Supporting Information.)
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conformer (i) of [6a]5 is repeatedly observed (Figure 2B(e),
arrows), and more elongated shapes (for example (ii),
indicated by dashed boxes) are also found. Structure (i) can
be constructed from two mushroom-shaped structures [6a]3
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structures of the SAMs of the acyclic oligomers (freely
rotating chains) are determined by the kinks and alkoxy
substituents, and by the shape complementarity of the
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monomers follow a common conformation principle, trimers
construct a unique and unexpected molecular grid, and
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Although STM studies on shape-persistent macrocycles
are currently a topic of high interest, the exploration of higher
(and more flexible) oligomers is still in its infancy. Currently,
we investigate how general these observed structural motifs
are and whether compounds of analogous structures will
adopt similar conformations.
[5] Freely jointed chains are rigid segments of fixed length,
interconnected by linkers that allow variable valence angles
and rotation. In contrast, the hinges of freely rotating chains
enforce fixed angles between adjacent segments, whilst still
allowing free rotation with all torsion angles being equally likely.
After adsorption to the substrate, the rotation is restricted, and
the linked units may adopt cisoidal and transoidal conformation.
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[12] The ratio of TMEDA to CuCl is of high importance for the
reaction. With an excess of TMEDA, halogenation of the
acetylenes was observed (see Supporting Information); see also:
T. Hamada, X. Ye, S. S. Stahl, J. Am. Chem. Soc. 2008, 130, 833.
[13] Attempts to couple 1a or 4a under pseudo-high-dilution
conditions using CuCl/CuCl2 as catalyst/oxidant mixture and
pyridine as solvent were not successful, and only a dark brown
Received: March 18, 2010
Revised: May 4, 2010
Published online: July 20, 2010
Keywords: gel-permeation chromatography · Glaser coupling ·
.
macrocycles · scanning probe microscopy ·
self-assembled monolayers
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tarry material of unknown composition could be isolated. The
absence of cyclodimers was surprising, as the same coupling
conditions worked well in most of our acetylene coupling
reactions and gave (with a similar but smaller substrate) high to
nearly quantitative product yields; see: N. Shabelina, S. Klyat-
skaya, V. Enkelmann, S. Hꢀger, C. R. Chim. 2009, 12, 430. Opris
et al. also obtained cyclic products when bipyridyl containing
bis(acetylene)s were oxidatively coupled under copper catalysis:
D. M. Opris, A. Ossenbach, D. Lentz, A. D. Schlꢁter, Org. Lett.
2008, 10, 2091. Similarly, Kim et al. obtained rectangular shape-
persistent macrocycles under copper catalysis; see: J.-K. Kim, E.
Lee, M.-C. Kim, E. Sim, M. Lee, J. Am. Chem. Soc. 2009, 131,
17768.
[22] In Figure 1, Figure A2(b–f), and Figure 2B(b,d), the unit cell
vectors along the lamellar directions are oriented with 9 Æ 28
towards the main axis of the HOPG substrate (see Supporting
Information), indicating surface induced chirality of the mole-
cules. For the alkoxy substituents, we assume the common
commensurability of all-gauche-constituted alkyl chains on the
graphene surface lattice.[23] We conclude that the angle between
the rigid PEB rods and the alkyl chains must be 81 Æ 28 for the
considered systems [5a]n and [6a]n. Clearly, this angle is not a
generally constant value for all alkoxy-substituted rigid systems,
but will rather strongly depend on the concrete PEB backbone
sequence. However, as individual short alkoxy chains (such as
OC6H13 in this case) cannot be resolved by STM under the
applied conditions, their orientation shall not be further
discussed herein.
[14] 1b and 4b were not investigated under the conditions of the
copper-catalyzed reaction.
[15] Only the cyclodimer [6b]2 was identified when 4b was coupled
under palladium catalysis; higher oligomers were not charac-
terized.
[23] For SAMs of alkanes on HOPG, see for example: T. Yang, S.
Berber, J.-F. Liu, G. P. Miller, D. Tomꢇnek, J. Chem. Phys. 2008,
128, 124709, and references therein.
[16] Considering 1b and 4b with 2-ethylhexyloxy side chains,
solubility is clearly not responsible for the observed discrim-
ination between cyclic and acyclic products.
[24] The sizes of the STM images, tunneling parameters, applied
solutions, and annealing temperatures, and the dimensions of the
unit cells of the self-assembled monolayers of acyclic oligomers
[5a]2–6 are as follows: b) dimer [5a]2 (34.0 ꢆ 34.0 nm2, Vs =
À1.1 V, It = 150 pA, c = 10À5 molLÀ1, unit cell: a = 3.9 Æ 0.1 nm,
b = 2.7 Æ 0.1 nm, g = 68 Æ 28); c) trimer [5a]3 (32.4 ꢆ 32.4 nm2,
Vs = À1.35 V, It = 100 pA, c = 10À5 molLÀ1, annealed to 608C for
2 min, unit cell: a = 8.2 Æ 0.2 nm, b = 3.8 Æ 0.1 nm, g = 90 Æ 28);
d) tetramer [5a]4 (28.0 ꢆ 28.0 nm2, Vs = À1.1 V, It = 120 pA, c =
10À6 molLÀ1, annealed to 808C for 2 min, unit cell: a = 5.4 Æ
0.2 nm, b = 3.8 Æ 0.1 nm, g = 90 Æ 28); e) pentamer [5a]5 (24.3 ꢆ
24.3 nm2, Vs = À0.85 V, It = 11 pA, c = 10À5 molLÀ1, annealed to
808C for 2 min, unit cell: a = 13.5 Æ 0.2 nm, b = 3.8 Æ 0.1 nm, g =
90 Æ 28); f) hexamer [5a]6 (25.3 ꢆ 25.3 nm2, Vs = À0.3 V, It =
246 pA, c = 10À5 molLÀ1, annealed to 808C for 2 min, unit cell:
a = 8.0 Æ 0.2 nm, b = 3.8 Æ 0.1 nm, g = 90 Æ 28). The respective
parameter sets for cyclic oligomers [6a]2–6: b) dimer [6a]2 (20.9 ꢆ
20.9 nm2, Vs = À0.42 V, It = 14 pA, c = 10À5 m, unit cell: a = 3.8 Æ
0.1 nm, b = 2.7 Æ 0.1 nm, g = 69 Æ 28); c) trimer [6a]3 (30.8 ꢆ
30.8 nm2, Vs = À1.1 V, It = 5 pA, c = 10À6 molLÀ1, annealed to
808C for 2 min, unit cell: a = 10.1 Æ 0.2 nm, b = 4.7 Æ 0.1 nm, g =
44 Æ 28); d) tetramer [6a]4 (26.0 ꢆ 26.0 nm2, Vs = À1.43 V, It =
14 pA, c = 10À5 molLÀ1, annealed to 808C for 2 min, inset:
12.1 ꢆ 12.1 nm2, Vs = À1.20 V, It = 10 pA, unit cell: a = 7.6 Æ
0.2 nm, b = 2.7 Æ 0.1 nm, g = 79 Æ 38); e) pentamer [6a]5 (50 ꢆ
50 nm2, Vs = À0.87 V, It = 43 pA, c = 10À5 molLÀ1, annealed to
808C for 2 min, amorphous); f) hexamer [6a]6 (40 ꢆ 40 nm2, Vs =
À0.6 V, It = 14 pA, c = 10À5 molLÀ1, annealed to 808C for 2 min,
expected unit cell: a = 11.5 nm, b = 2.7 nm, g = 838). Note: a and
b denote the long and short unit cell vectors, respectively. By
definition, the unit cells for (sufficiently large) odd acyclic
oligomers [5a]n (n = 3, 5) are larger than for the respective even
oligomers (n = 4, 6). All images were calibrated in situ using the
HOPG substrate as reference grid (see the Supporting Informa-
tion).
[17] Proton NMR studies (see Supporting Information) showed that
all oligomers obtained from the copper-promoted coupling
reactions still contain ethynyl end groups, whereas all separated
fractions of the palladium-catalyzed coupling reactions did not
exhibit ethynyl protons. Furthermore, the peak patterns of the
aromatic protons in the spectra are different for both reactions.
HRMS data for [5a]2 and [6a]2 show the exact masses for the
proposed structures.
[18] The difference in conformational freedom between the acyclic
and cyclic oligomers is also reflected by their thermal behavior.
Although the acyclic dimers [2a]2 and [5a]2 have melting points
of 718C and 958C, the cyclic analogue [3a]2 melts at 2218C and
[6a]2 decomposes at > 3258C; see: H. A. Staab, H. Brꢂumling,
K. Schneider, Chem. Ber. 1968, 101, 879, and Ref. [10].
[19] For theoretical descriptions of the contrast mechanism in STM,
see for example: a) R. Lazzaroni, A. Calderone, J. L. Brꢅdas, J. P.
Rabe, J. Chem. Phys. 1997, 107, 99; b) P. Sautet, Chem. Rev. 1997,
97, 1097.
[20] As visualized by the inset of the molecular model in Fig-
ure 2A(b), the distance of the terminal ethynyl units is smaller
than the lateral changes of the tunneling current. Therefore, we
cannot clearly resolve the difference of the cyclic and acyclic
dimer, [5a]2 and [6a]2, by microscopy. Nevertheless, the presence
of a cyclic dimer [6a]2 as impurity in [5a]2 could be excluded by
GPC, HRMS, and 1H NMR analysis.[17] Furthermore, the
presence of two distinguishable O- and S-shaped polymorphs
of the dimer [14a]2 on HOPG is rather expected instead of
peculiar.
[21] STM image size, tunneling parameters, concentration, and unit
cell dimensions for the SAM of the monomer 4a on HOPG are:
24.1 ꢆ 24.1 nm2, Vs = À0.95 V, It = 6 pA, c = 10À4 molLÀ1
;
O-shaped polymorph: a = 3.8 Æ 0.1 nm, b = 2.7 Æ 0.1 nm, g =
68 Æ 28; S-shaped polymorph: a = 3.6 Æ 0.1 nm, b = 2.7 Æ
0.1 nm, g = 90 Æ 28.
Angew. Chem. Int. Ed. 2010, 49, 6101 –6105
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6105