Self-assembled monolayers of clamped oligo(phenylene-ethynylene- butadiynylene)s

Article abstract of DOI:10.1039/c1cc12435h

Rigid rod oligo(phenylene-ethynylene-butadiynylene)s (oPEBs), "half-rings" of two rigid rods connected via a molecular clamp unit, and shape-persistent macrocycles (cyclic "half-ring dimers") are synthesized and their self-assembled monolayers (SAMs) are

Full text of DOI:10.1039/c1cc12435h

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COMMUNICATION
Self-assembled monolayers of clamped oligo(phenylene-ethynylene-
butadiynylene)swz
Stefan-S. Jester,* Daniela Schmitz, Friederike Eberhagen and Sigurd Hoger*
¨
Received 26th April 2011, Accepted 1st June 2011
DOI: 10.1039/c1cc12435h
Rigid rod oligo(phenylene-ethynylene-butadiynylene)s (oPEBs),
‘‘half-rings’’ of two rigid rods connected via a molecular clamp
unit, and shape-persistent macrocycles (cyclic ‘‘half-ring
dimers’’) are synthesized and their self-assembled monolayers
(SAMs) are investigated by scanning tunneling microscopy
(STM) at the interface of 1,2,4-trichlorobenzene (TCB)/highly
oriented pyrolytic graphite (HOPG). The results are important
for the design of molecular building blocks for two-dimensional
nanoscale architectures on solid surfaces.
description of the synthesis and full characterization of all new
compounds can be found in the SI.z
The rigid rods 1 and 2 are dimers and tetramers of pPE
trimers that are connected by butadiynylene units (diacetylene
units). Half-ring structures 3 and 4 can be viewed as pairs of
rigid rods that are connected via a quinquearylene clamp. The
ends of 1 and 2, and the rigid rods of 3 and 4 are terminated by
acetylene functions. Macrocycles 5 and 6 can be viewed as
cyclic dimers of two covalently connected half-rings 3 and 4,
respectively (Fig. 1).
1 forms two-dimensional crystalline arrays. The rigid PEB
backbones assemble into lamellar rows, so that the flexible
alkoxy substituents are adsorbed on the substrate and inter-
digitate intermolecularly defining the rod–rod distances. From
the high-resolution image shown in Fig. 2b, the unit cell (A) is
determined to be a = 4.5 ꢀ 0.2 nm, b = 1.4 ꢀ 0.1 nm, and
g(a,b) = 85 ꢀ 21 (cf. molecular model in Fig. 2c). The backbones
c are aligned along a. The alkoxy side chains are aligned along
the HOPG main axis directions (denoted as d1),6 and the back-
bone-side chain angle g(c,d1) is 95 ꢀ 21. The orientation of
the backbones in A0 (with the unit cell vectors a0 and b0), the
enantantiomers of A, is a result of identical direction of the
alkoxy side chains along d1 (as compared to A), and g(c0,d1) =
95 ꢀ 21, explaining the domain angle of g(b,b0) = 20 ꢀ 21 and
g(c,c0) = 170 ꢀ 21 (cf. Fig. 2c). Contrary, the domain angle
g(b,b0 0) = 601 is drawn back on rotation of the molecules (of A),
Shape-persistent molecules attract a high level of interest
because of their unique structural, electrical, and optical
properties, and as building blocks for functional nanostructures.
The intramolecular connection of two rigid rods with one or
two molecular clamp units leads to half-rings (that can be
described as freely rotating chains)1 or shape-persistent macro-
cycles, respectively,2 that form SAMs on solid substrates.3,4
The rigid backbones allow the tailored design of the adsorbate
structures, required to achieve predictable surface patterns.
SAMs of shape-persistent macrocycles can be applied
as templates for the epitaxial codeposition of admolecules,
additional guests that otherwise do not adsorb onto solid
surfaces.5
In a previous study, we have focused on cyclic and acyclic
oligomers of shape-persistent rigid rods, connected via
rotatable oligoarylene corner units.3 The alkoxy substituents
define the distances of lamellarly aligned, self-assembled
rigid rod units, and the quinquearylene clamps match the
width predetermined by the rod–rod distances of hexyloxy
substituted oPEB units. In the present work, we maintain the
intra- and intermolecular rod–rod spacing and extend the
length of the rigid rods systematically to yield structures
with an end-to-end distance of up to 12.4 nm. A detailed
Rheinische Friedrich-Wilhelms-Universitat Bonn,
¨
Kekule-Institut fur Organische Chemie und Biochemie,
¨
´
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany.
E-mail: S.-S. J.: stefan.jester@uni-bonn.de,
S. H.: hoeger@uni-bonn.de; Fax: 49 228 73-5662;
Tel: 49 228 73-3495
w This article is part of the ChemComm ‘Molecule-based surface
chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
details, synthesis, additional STM images. See DOI: 10.1039/
c1cc12435h
Fig. 1 Chemical structures of the linear rigid rods 1 and 2, the half-
rings 3 and 4, and the macrocycle 5 and 6.
c
8838 Chem. Commun., 2011, 47, 8838–8840
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intramolecular periodicity of the pPE trimer units, 2.3 nm. The
shifts reflect the kinetically driven nature of the adsorption
process for the longer rods 2 compared to the thermodynamically
favored pairwise interaction of adjacent rods.7 Nevertheless,
defined shift distances keep the energetically favorable
alkoxy chain interdigitation degree high. The indexed unit cell,
a = 9.2 ꢀ 0.2 nm, b = 1.4 ꢀ 0.1 nm, g(a,b) = 87 ꢀ 21 is
consistent with the molecular model proposed in Fig. 2f (right
side), and the alkoxy-backbone angles are—similar as observed
for 1—g(c,d) = 95 ꢀ 21.
STM images of the half-ring structures 3 and 4 at the
TCB/HOPG interface are shown in Fig. 3. Two polymorphs
are observed for 3 (Fig. 3a–d). In both cases, lamellar patterns
are observed where the molecular length axes are tilted with
respect to the lamellar normal. The corner unit of each
half-ring intercalates asymmetrically into the gaps provided
by two half-rings of the neighboring lamella, leading to gaps
(‘‘U-gaps’’, Fig. 3c and d) between the corner units. In
polymorph A, both acetylene-terminated ends of the half-rings
point toward each other to form O-shaped quasi-dimers. A unit
cell of aA = 6.0 ꢀ 0.1 nm, bA = 3.1 ꢀ 0.1 nm, g(aA,bA) =
101 ꢀ 21 is indexed, and the rigid rod parts of the half-rings
(pointing in direction cA) are aligned with g(bA,cA) = 114 ꢀ 21.
The alkoxy-backbone angle of 3 is g(cA,d) = 103 ꢀ 31
(assuming commensurable adsorption of the side chains along
the substrate main axis), significantly larger than for 1 and 2
(where alkoxy-backbone angles of 95 ꢀ 21 were observed). In
polymorph B, the half-rings assemble toward broken sinuous
lines, so that the acetylene-termini of each U-shaped molecule
point toward the ends of two (different) adjacent molecules.
Two of these ends are in contact with each other and determine
the packing distance along direction cB, while between the two
other ends the ‘‘I-gap’’ (Fig. 3d) is formed with dH-H = 1.2 nm.
A unit cell of aB = 6.4 ꢀ 0.1 nm, bB = 3.1 ꢀ 0.1 nm, and
g(aB,bB) = 90 ꢀ 21 is indexed. The alkoxy-backbone angle
g(cB,d) and the orientation of the rigid rod parts, g(bB,cB), are
the same as determined for polymorph A.
Fig. 2 STM images, molecular and schematic models of SAMs of
(a)–(c) 1, and (d)–(f) 2 at the TCB/HOPG interface. (a) Large-area
STM image of 1 (VS = ꢁ1.0 V, It = 80 pA, 100 ꢂ 100 nm2, c = 10ꢁ5 M);
(b) high-resolution image of 1 (VS = ꢁ0.86 V, It = 100 pA, 24.5 ꢂ
24.5 nm2, c = 10ꢁ5 M); (c) molecular model of pattern A and A0 of
1 as seen in (a) and (b). (d) Large-area STM image of 2 (VS = ꢁ1.0 V,
It = 20 pA, 86.6 ꢂ 86.6 nm2, c = 10ꢁ4 M, thermally annealed to 80 1C
for 30 s); (e) high-resolution image of 2 (VS = ꢁ1.0 V, It = 20 pA,
35.7 ꢂ 35.7 nm2, c = 10ꢁ4 M, thermally annealed to 80 1C for 30 s); (f)
schematic (left) and molecular (right) model of the pattern observed in
(e). The unit cells vectors, HOPG main axis directions, and backbone
directions are given in red, white (black) and blue color, respectively.
4 adsorbs—similarly to 3—into tilted lamellar patterns. The
ends of the rigid rods are aligned along a continuous line. A
unit cell of a = 11.9 ꢀ 0.3 nm, b = 2.8 ꢀ 0.1 nm and g(a,b) =
831 is indexed, and the rigid rod parts are aligned along c with
g(b,c) = 93 ꢀ 21. Assuming commensurate adsorption of the
alkoxy substituents along the HOPG main axis d, the
backbone-alkoxy-angle g(c,d) is 84 ꢀ 21. No other polymorph
is observed.
so that the alkoxy side chains of A0 0 are oriented along d2
(Fig. 2a). Conclusively, although the side chains cannot be
submolecularly resolved by STM (at r.t.), the alkoxy chain
directions and the alkoxy-backbone angles significantly determine
the molecular alignment with respect to the HOPG crystallo-
graphic directions.6
Macrocycles 5 assemble in tilted patterns (Fig. 4a). A unit
cell of a = 6.3 ꢀ 0.2 nm, b = 3.0 ꢀ 0.2 nm and g(a,b) = 79 ꢀ 21
is indexed, and the rigid rod parts are (on average) aligned
along c with g(c,b) = g(c,d) = 90 ꢀ 21. The molecular model
shown in Fig. 4b is based on an idealized structure of the
monomers with rigid rod units aligned in parallel and an
alkoxy-backbone-angle of 901. However, the molecular back-
bones appear to be significantly deformed, as compared with
the half-rings of similar rod length, 4 (Fig. 3a and b), originating
from higher sterical demand of the alkoxy chains (see molecular
model in Fig. 4c).
Rigid rods 2 are also aligned in parallel to form quasi-lamellar
assemblies (Fig. 2d). However, compared to 1, the lamellae of 2
do not form patterns with two-dimensional order, but appear to
be grown into random directions (Fig. 2d). This is a result of the
shift of the oligomers in direction of the lamella normal that
appears of accidental order and magnitude. At higher resolution
(Fig. 2e), a systematic stepping of the aforementioned parallel
shift of the oligomers within a lamella is discernable, as
schematically visualized in Fig. 2f. Each rod 2, consisting of four
butadiynylene-bridged pPE trimers, is shifted in parallel to the
rod direction so that the shift distances match multiples of the
Macrocycles 6 assemble—similarly to 5—in tilted patterns
of lamellarly aligned backbones (Fig. 4d). A unit cell of
a = 10.9 ꢀ 0.4 nm, b = 2.9 ꢀ 0.2 nm and g(a,b) = 83 ꢀ 21
c
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Fig. 4 (a) STM image of a SAM of 5 at the TCB/HOPG interface.
(VS = ꢁ1.0 V, It = 11 pA, 41.2 ꢂ 41.2 nm2, c = 10ꢁ5 M, thermally
annealed at 60 1C for 1 min). (b) Idealized molecular packing model.
(c) Molecular model of 5 after force field geometry optimization
(starting conformation as shown in (b)). (d) STM image of a SAM
of 6 at the TCB/HOPG interface (VS = ꢁ1.4 V, It = 10 pA,
65.2 ꢂ 65.2 nm2, c = 10ꢁ5 M, thermally annealed at 80 1C for
30 s). (e) Idealized molecular packing model. (f) Molecular model of 6
(obtained analogously to (c)). The unit cells vectors, HOPG main axis
directions, and backbone directions are given in red, white (black) and
blue color, respectively.
Fig. 3 (a), (b) STM images and (c), (d): Molecular models of a SAM
of 3 at the TCB/HOPG interface; (e), (f): STM image and molecular
model of a SAM of 4 at the TCB/HOPG interface. (a) Large-area
scan (VS = ꢁ1.0 V, It = 5 pA, 100 ꢂ 100 nm2, c = 10ꢁ5 M); (b)
high-resolution image (VS = ꢁ0.95 V, It = 10 pA, 27.6 ꢂ 27.6 nm2,
c = 10ꢁ5 M); (c) model of polymorph A of 3, (d) model of polymorph
B of 3. (e) Large-area scan (VS = ꢁ1.0 V, It = 20 pA, 36.6 ꢂ 36.6 nm2,
c = 5ꢂ10ꢁ5 M, thermally annealed at 60 1C for 30 s). The unit cells
vectors, HOPG main axis directions, and backbone directions are
given in red, white (black) and blue color, respectively.
Financial support by the DFG, the SFB624, the FCI, and
the VolkswagenStiftung is gratefully acknowledged.
Notes and references
1 S.-S. Jester, N. Shabelina, S. M. Le Blanc and S. Hoger, Angew.
¨
Chem., Int. Ed., 2010, 49, 6101.
2 (a) S. Hoger, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, 2685;
¨
(b) W. Zhang and J. S. Moore, Angew. Chem., Int. Ed., 2006,
45, 4416.
3 For recent publications about SAMs of shape-persistent linear
is indexed, and the rigid rods are (on average) aligned along c
with g(c,b) = 90 ꢀ 21 and g(c,d) = 84 ꢀ 21. The molecular
model shown in Fig. 4e is again based on an idealistic
alignment of the alkoxy substituents—as compared to the
force field geometry-optimized molecular model shown in
Fig. 4f. Again sterical requirements of the constrained alkoxy
substituents induce deformation of the molecular backbones
(Fig. 4f), leading to misalignment of the macrocycles in the
monolayer patterns. Most notably, the patterns shown in
Fig. 4a and d include backbones (indicated with white arrows)
that exhibit 601 bendings from the linear shape, an observation
that was not expected for a clamped rod system.8
oligomers see, e.g.: (a) D. Mossinger, S.-S. Jester, E. Sigmund,
¨
¨
U. Muller and S. Hoger, Macromolecules, 2009, 42, 7974;
¨
(b) Z. Mu, L. Shu, H. Fuchs, M. Mayor and L. Chi, Langmuir,
2011, 27, 1359.
4 For recent publications about SAMs of rigid macrocycles see, e.g.:
(a) S. Hoger, K. Bonrad, A. Mourran, U. Beginn and M. Moller,
¨
¨
J. Am. Chem. Soc., 2001, 123, 5651; (b) K. Tahara, C. A. Johnson II,
T. Fujita, M. Sonoda, F. C. De Schryver, S. De Feyter,
M. M. Haley and Y. Tobe, Langmuir, 2007, 23, 10190.
5 (a) J. Adisoejoso, K. Tahara, S. Okuhata, S. Lei, Y. Tobe and S. De
Feyter, Angew. Chem., Int. Ed., 2009, 48, 7353; (b) T. Chen,
G.-B. Pan, H. Wettach, M. Fritzsche, S. Hoger, L.-J. Wan,
¨
H.-B. Yang, B. H. Northrop and P. J. Stang, J. Am. Chem. Soc.,
2010, 132, 1328.
6 For SAMs of alkanes on HOPG see, e.g.: T. Yang, S. Berber,
The results prove that the investigated systems behave
as rigid rods of defined length—however with unexpected
systematic shifts in the lamellar packing of the longer rigid
rods. A connection of two rods via one or two oligoarylene
clamp units has yielded half-rings as well as macrocyclic ring
structures, respectively, that form also nanoscale patterns
on HOPG.
´
J.-F. Liu, G. P. Miller and D. Tomanek, J. Chem. Phys., 2008,
128, 124709, and references therein.
7 N. Severin, I. M. Sokolov, N. Miyashita, D. G. Kurtz and
J. P. Rabe, Macromolecules, 2007, 40, 5182.
8 S.-B. Lei, K. Deng, Y.-L. Yang, Q.-D. Zeng, C. Wang, Z. Ma,
P. Wang, Y. Zhou, Q.-L. Fan and W. Huang, Macromolecules,
2007, 40, 4552.
c
8840 Chem. Commun., 2011, 47, 8838–8840
This journal is The Royal Society of Chemistry 2011