Modulating the self-assembly of rigid "clicked" dendrimers at the solid-liquid interface by tuning non-covalent interactions between side groups

Article abstract of DOI:10.1039/c1cc13099d

First generation poly(triazole-phenylene) dendrimers equipped with peripheral alkyl or carboxylic acid groups to engage in van der Waals and hydrogen-bonding interactions, respectively, assemble into distinct two-dimensional nano-structures at the solid-l

Full text of DOI:10.1039/c1cc13099d

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Cite this: Chem. Commun., 2011, 47, 10578–10580
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COMMUNICATION
Modulating the self-assembly of rigid ‘‘clicked’’ dendrimers at the
solid–liquid interface by tuning non-covalent interactions between side groupswz
Andrea Cadeddu,ya Artur Ciesielski,ya Tamer El Malah,yb Stefan Hecht*b and
Paolo Samorı*a
Received 26th May 2011, Accepted 12th August 2011
DOI: 10.1039/c1cc13099d
First generation poly(triazole-phenylene) dendrimers equipped with
peripheral alkyl or carboxylic acid groups to engage in van der
Waals and hydrogen-bonding interactions, respectively, assemble
into distinct two-dimensional nano-structures at the solid–liquid
interface as revealed by high resolution STM investigations.
dendrimers on surfaces has less frequently been explored. In
fact, hitherto dendrimers6 have been visualized by Scanning
Tunneling Microscopy (STM) only under ultrahigh vacuum,7
in air forming disordered aggregates,8 and at the solid–liquid
interface as non-planar structures.9
Here, we report for the first time a sub-molecularly resolved
STM study of the first generation of a novel type of dendrimer,
physisorbed at the solid–liquid interface into highly ordered mono-
layers. We have focussed our attention on five derivatives of rigid
and fully planar 1,3,5-tris(1-phenyl-1,2,3-triazol-4-yl)benzenes
(TPTB, Fig. 1), which constitute the first generation in a series of
new shape-persistent poly(triazole-phenylene) dendrimers.10
The linking triazole moieties are readily derived via
Cu-catalyzed 1,3-cycloadditions, i.e. the so called ‘‘click’’
reaction.11 The aim of this investigation was to explore the
symmetry mismatch between the graphite substrate and the
dendrimer scaffold, due to the presence of the five-membered
triazole moieties,12 in combination with different orientations
of the self-recognition groups attached in the periphery of the
TPTB. In particular, molecule 1, equipped with six carboxylic
acid moieties in the meta-positions of the peripheral phenyl
rings, was designed to promote the formation of C6-symmetric
self-assembled structures linked via multiple H-bonds, i.e. six
carboxylic acid dimers based on di-hapto O–Hꢀ ꢀ ꢀO hydrogen-
bonding. To cast light on the role of such intermolecular
The self-assembly of molecular components into larger supra-
molecular architectures is ubiquitous in nature and arguably
constitutes the most powerful method to fabricate functional
nanomaterials from the bottom up. In the confinement of a solid
substrate surface this approach can be exploited to generate
periodically ordered two-dimensional structures from suitably
designed molecular building blocks.1 Hence, self-assembly at
the interface enables the defined positioning of functional
entities with sub-nanometre precision over areas of microns
and thereby allows for fine tuning of numerous properties of
the resulting hybrid nanomaterials2 for technological applications,
in particular in electronics and optics.3 To allow for structure
formation under thermodynamic control, i.e. to warrant
equilibration, the use of weak yet multiple non-covalent interactions
has proven advantageous. In particular, the combination of van der
Waals and hydrogen-bonding interactions constitutes a successful
strategy to generate sophisticated two- and three-dimensional
architectures as these interactions enable reversibility, directionality,
specificity, and even cooperativity in the self-assembly process.4
Both aliphatic residues as well as hydrogen-bonding sites can be
attached to suitable molecular building scaffolds to provide either
linear, cyclic or branched arrangements. The latter gives rise to
dendritic architectures, which can be tailored to self-assemble
into a variety of super-structures in solution and in the bulk.5
However, due to their commonly rather flexible structure and
their typically three-dimensional shape the self-assembly of
a
´
´
ISIS/UMR CNRS 7006, Universite de Strasbourg, 8 allee Gaspard
Monge, 67000 Strasbourg, France. E-mail: samori@unistra.fr;
Web: http://www.nanochemistry.fr
b Department of Chemistry, Humboldt-Universitat zu Berlin,
¨
Brook-Taylor-Str. 2, 12489 Berlin, Germany.
E-mail: sh@chemie.hu-berlin.de; Web: http://www.hechtlab.de
w Electronic supplementary information (ESI) available: Synthesis,
characterization data, and experimental details. See DOI: 10.1039/
c1cc13099d
z This article is part of the ChemComm ‘Molecule-based surface
chemistry’ web themed issue.
y All three authors contributed equally to this work.
Fig. 1 Chemical formulae of the investigated TPTB derivatives.
c
10578 Chem. Commun., 2011, 47, 10578–10580
This journal is The Royal Society of Chemistry 2011

ESIw) where each molecule of 1 forms six R2 (8)4c homodimeric
2
hydrogen-bonding pattern, our study was extended to derivative
2, which exposes six n-hexadecylcarbonyloxy side chains in these
meta-positions. To finally explore the influence of the orientation
of the substituents on the peripheral phenyl rings, regioisomers
3–5, bearing three n-octadecyloxy side chains in their ortho-,
meta-, and para-positions of the peripheral phenyl moieties,
respectively, were investigated. In the case of compounds 2–5,
the van der Waals interactions between the alkyl side-chains
can be expected to play a key role in the self-assembly.
To allow for comparison, all experiments in this study were
carried out by applying a drop of solutions of 1–5 in 1-phenyl-
octane (concentrations up to 2 mM) to the basal plane of
freshly cleaved highly oriented pyrolytic graphite (HOPG)
surfaces. Studies of TPTB-based systems using different
solvents, 1,2,4-trichlorobenzene, tetradecane and 1-heptanoic
acid, did not produce any ordered monolayers. The unit cell
parameters of the various crystalline patterns were determined
via peak detection in the Fast Fourier Transform (FFT) on
STM images previously corrected for the thermal drift by
using the underlying graphite lattice as reference (see ESIw for
more details on the experimental procedures).
H-bonds, i.e. twelve strong O–Hꢀ ꢀ ꢀO hydrogen-bonds with
neighbouring molecules. Importantly, this 2D motif incorporates
two pore types, i.e. A and B, with inner voids of 1.5 nm2 and
0.65 nm2, respectively. The formation of hexameric pores A is a
direct consequence of the 1201 angle between the two
carboxylic functions. Due to the molecular geometry of 1 two
of the remaining carboxylic moieties interact via hydrogen-
bonds with adjacent molecules, forming the second type of
pores, i.e. dimeric pores B of smaller size (see ESIw for a more
detailed explanation and model). From the data it appears that the
formed hierarchical pore pattern is dictated by the formation of
carboxylic acid dimers via self-complementary hydrogen-bonding
interactions. The TPTB scaffold can adapt to the six-fold
symmetry of the HOPG by rotating the triazole moieties in one
direction only. The difference in contrast between the observed
molecules is the result of the Moire effect (see Fig. S4 in ESIw).
Exchanging the hydrogen-bonding interactions for weaker
van der Waals interactions leads to formation of rather
different self-assembled monolayers in the case of compound
2. The associated STM images (Fig. 2c and d) show a lamellar
structure featuring a nanoscale phase segregation between the
TPTB cores and the long alkyl side-chains. Within a lamella
(marked with a white arrow in Fig. 2c) the cores are arranged
in a ‘‘head-to-head’’ and ‘‘tail-to-tail’’ dimeric motif (Fig. 2d).
The entire supramolecular architecture is stabilized by the
interdigitation of alkyl side chains belonging to adjacent
molecules, adsorbed along one of the Miller–Bravais
symmetric axes of the underlying HOPG lattice (indicated in
the inset of Fig. 2c). For each molecule only four out of six
hexadecyl side chains have been detected in the STM image
(Fig. 2d). The alkyl side chains belonging to molecules of
adjacent lamellae are interdigitated, leading to an interlamellar
distance of (3.09 ꢁ 0.22) nm. However taking the unit cell
parameters and the size of the molecule as obtained by
Molecular Mechanics simulations into account, it is more
likely that the two remaining side chains are also physisorbed
on the HOPG surface but are not imaged due to their high
conformational dynamics on the time scale of the STM
experiments. By comparing structure formation of 1 and 2,
it seems that van der Waals interactions dominate the
self-assembly of 2 and lead to a change from a C6-symmetric
rosette pattern to a linear lamellar structure.
Hexa-acid 1 forms ordered physisorbed monolayers on
HOPG. The respective STM current images (Fig. 2a and b)
reveal a monocrystalline structure featuring a 2D porous
honeycomb-like motif, which topologically coincides with a
rhombitrihexagonal tiling. Each unit cell contains four
molecules of 1, physisorbed flat on the surface. The hundreds
of nm2 large crystalline domains were found to be stable over
tens of minutes. The supramolecular architecture is stabilized
by strong (O–Hꢀ ꢀ ꢀO) hydrogen-bonds (Fig. 2a and b and S3 in
To furthermore discern the effect of side-chain orientation on
self-assembly, high resolution STM images of regioisomers 3–5
were obtained (Fig. 3). The monolayer of ortho-substituted
compound 3 exhibits a crystalline structure with a motif
similar to the rhombitrihexagonal tiling observed for 1, but
incorporating only two molecules within the unit cell (Fig. 3b). It
is most likely that the supramolecular architecture is stabilized
by weak van der Waals interactions between adjacent molecules.
The proposed molecular packing model as well as the unit cell
parameters suggests that all n-octadecyloxy side chains of 3 are
back-folded into the supernatant solution, which can be
ascribed to insufficient van der Waals contacts between the
alkoxy chains of adjacent molecules 3. The two domains
observed in Fig. 3a exhibit the same packing motif, as proven
by their identical cell parameters. Importantly, highly ordered
self-assembled structures of 3 were observed only upon use of
Fig. 2 STM images of 1 (a, b) and 2 (c, d) at the solid–liquid interface:
(a) survey image, (b–d) small scale image. Images were recorded in (a, b, d)
current mode and (c) height mode. Unit cell parameters: (a, b) a = (3.12 ꢁ
0.2) nm, b = (5.51 ꢁ 0.2) nm, a = (87 ꢁ 2)1, A = (17.16 ꢁ 1.26) nm2,
Amol = (4.34 ꢁ 0.32) nm2, space group p6m; (c, d) a = b = (4.15 ꢁ 0.2)
nm, a = (45 ꢁ 3)1, A = (12.17 ꢁ 0.72) nm2, Amol = (6.09 ꢁ 0.31) nm2,
space group cm; tunneling parameters (a, b): average tunneling current (It)
= 18 pA, tip bias voltage (Vt) = 600 mV; (c, d): It = 15 pA, Vt = 300 mV.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10578–10580 10579

This work was financially supported by the EC Marie-Curie
ITN-SUPERIOR (PITN-GA-2009-238177), FP7 ONE-P large-
scale project no. 212311, the NanoSci-E+ project SENSORS, the
International Center for Frontier Research in Chemistry (FRC),
and the Fonds der chemischen Industrie. T.E.M. acknowledges
the Egyptian Government for providing a doctoral fellowship.
The authors thank Wacker AG, BASF AG, Bayer Industry
Services, and Sasol Germany for generous donations of chemicals.
Notes and references
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Fig. 3 STM current images of (a, b) 3, (c) 4 and (d) 5 recorded at the
solid–liquid interface. Unit cell parameters: (b) a = (3.68 ꢁ 0.2) nm, b =
(3.70 ꢁ 0.2) nm, a = (58 ꢁ 3)1, A = (11.58 ꢁ 0.67) nm2, Amol = (5.76 ꢁ
0.33) nm2, space group p6m. Tunneling parameters (a, b): It = 15 pA, Vt =
900 mV; (c) It = 15 pA, Vt = 860 mV; (d) It = 15 pA, Vt = 860 mV.
highly concentrated solutions (c = 2 mM). In strong contrast
to compound 3, its meta- and para-regioisomers 4 and 5,
respectively, form only unstable and poorly ordered lamellar
structures on HOPG, highlighting the high molecular mobility
on a timescale faster than the tip scan, thereby hindering high
resolution STM mapping. The interlamellar distances for the
assembly of molecules 4 and 5 amount to (2.98 ꢁ 0.18) nm and
(2.75 ꢁ 0.33) nm, respectively, providing evidence for a tight
molecular packing probably caused by interdigitation of alkyl
side chains belonging to molecules of adjacent lamellae.
In summary, we have performed a comparative STM study of
the self-assembly of various derivatives of the first generation of
new poly(triazole-phenylene) dendrimers at the HOPG–solution
interface. All molecules were found to physisorb in a flat
adsorption geometry on graphite forming 2D supramolecular
structures. Different crystalline nanopatterns were observed,
ranging from honeycomb-like networks for compounds 1 and 3
to various lamellar structures of different stability in the case of
derivatives 2, 4, and 5. Our results provide unambiguous evidence
that subtle modification in the substitution pattern of the TPTB
scaffold leads to pronounced effects on its 2D self-assembly at the
liquid–solid interface. Two factors seem to be responsible for
the variation in the observed structures and their stability: (i) due
to the presence of the triazole moieties, the TPTB core cannot
completely match the six-fold symmetry of the HOPG substrate,
i.e. both lattices are not commensurate, and thereby gives rise to an
inherent flexibility and thermodynamic instability, and (ii) as a
consequence the exact presentation of the alkyl side chains by the
TPTB scaffold governs the overall structure formation in the self-
assembly process. In general, the capacity of flat dendrimers to
pack into highly ordered supramolecular structures at the solid–li-
quid interface described herein could lead to the design of more
complex and multicomponent structures based on dendritic cores.5
10 The synthesis of related higher generation dendrimers will be
published separately.
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12 We have been exploring the 1,2,3-triazole moiety as a structure-
directing motif in a variety of systems: (a) R. M. Meudtner,
M. Ostermeier, R. Goddard, C. Limberg and S. Hecht,
Chem.–Eur. J., 2007, 13, 9834–9840; (b) R. M. Meudtner and
S. Hecht, Angew. Chem., Int. Ed., 2008, 47, 4926–4930;
(c) R. M. Meudtner and S. Hecht, Macromol. Rapid Commun.,
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S. Hecht and P. Samorı, Chem.–Eur. J., 2009, 15, 4788–4792;
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c
10580 Chem. Commun., 2011, 47, 10578–10580
This journal is The Royal Society of Chemistry 2011