Janus-like 3D tectons: Self-assembled 2D arrays of functional units at a defined distance from the substrate

Article abstract of DOI:10.1002/anie.201008212

Janus faces: The Janus-like 3D tecton is a doubly functionalized molecular building block that consists of two different faces (see picture, gray: A and blue: B) linked by a rigid spacer. A is designed to act as a pedestal capable of guiding 2D self-assem

Full text of DOI:10.1002/anie.201008212

Communications
DOI: 10.1002/anie.201008212
Self-Assembly
Janus-Like 3D Tectons: Self-Assembled 2D Arrays of Functional Units
at a Defined Distance from the Substrate**
David Blꢀger, Fabrice Mathevet, David Kreher, Andrꢀ-Jean Attias,* Amandine Bocheux,
Sylvain Latil, Ludovic Douillard, Cꢀline Fiorini-Debuisschert, and Fabrice Charra*
Supramolecular-based architectures onto surfaces have been
widely investigated during the past two decades.[1] The most
commonly employed strategies to create 2D hierarchical
structures exploit noncovalent molecular interactions
between planar tectons. Various approaches have been
explored to generate highly ordered monocomponent struc-
tures and multicomponent assemblies.[2] In addition to the
control of molecule positioning that leads to complex nano-
patterns, the control of surface properties has also been
addressed more recently. In that respect, nanoporous net-
works are of particular interest. Owing to the host properties
of these networks towards organic guests, they allow for
addressing various interesting aspects such as molecular
dynamics, selection, recognition, and immobilization, hence
leading to functional surfaces.[3]
Besides these features confined in-plane, the demanding
forthcoming applications in nanotechnology require to create
out-of-plane functions and to be able to fully exploit the space
above the substrate. Accessing the third dimension is
mandatory for the progress of for example, molecular
electronics and photonics, in which the proximity of adsorbed
photoactive units with conducting substrates results in a fast
quenching of electronic excitations.[4] To avoid this drawback,
the decoupling between active molecular units and conduct-
ing substrates is currently achieved by lifting the molecule,
either by covering the surface with a uniform insulating
layer[5] or through the attachment of bulky chemical side
groups (e.g. tert-butyl groups) to the molecule, which act as
spacer legs that lead to the so-called molecular “landers”.[6]
However, these examples suffer from limited short-range in-
plane organizations. Several ways are explored to circumvent
this problem based on molecular self-assembly. They consist
in controlling interactions between 3D tectons, for example
with H-bonded molecular “landers”,[7] or in exploiting a self-
assembled insulating layer as template for an upper functional
layer.[8] In this context, the development of general and
versatile methods for the design of 3D tectons that are able to
self-assemble with long-range lateral order and to expose the
desired functionality at a defined distance from the surface,
remains an open challenge.
In a previous work[9] we demonstrated the 2D self-
assembly of building blocks with intermolecular noncovalent
bonding units (the so-called “clips”) on highly-oriented
pyrolytic graphite (HOPG) at the liquid–solid interface.
More precisely, we showed that the attachment of clips to
the lower-deck of multilayered [2.2]paracyclophane deriva-
tives leads to the self-assembly of building blocks that are
oriented face-on with the cyclophane rings that stack
perpendicularly to the substrate.[10]
Herein, we introduce the concept of Janus-like 3D
molecular tectons, which are doubly-functionalized building
blocks that expose two opposite faces (A and B) linked by a
rigid spacer. A is a pedestal designed for guiding 2D self-
assembly on the substrate (HOPG) and B is a functional
moiety. We show that the in-plane self-assembling ability of A
controls the positioning of the out-of-plane moiety B.
As a first example to illustrate this concept, we designed
and synthesized the 3D tecton 1 (Figure 1), which was
obtained as a racemic mixture (1,1’) because of the planar
chirality inherent to asymmetrically functionalized paracy-
clophanes.[11] The target compounds (1,1’) combine 1) a
pedestal that consists of two molecular clips able to self-
assemble on HOPG, 2) a [3.3]dithiaparacyclophane unit[12]
3.3 ꢀ in height that acts as a two-story linker, and 3) a
functional molecule, namely a distyrylbenzene fluorophore
(highlighted in blue in Figure 1). This building block is
designed to act as a Janus tecton, that is, it self-assembles in a
well-organized in-plane monolayer and distributes the func-
tional units according to a periodic pattern that parallels its
subjacent counterpart several ꢀ higher, thanks to the para-
cyclophane pillar. To confirm this behavior, STM experiments
were carried out in situ, that is, at the solid–liquid interface.
As a guide to the interpretation of STM images, DFT
modeling has been performed on this system.
[*] Dr. D. Blꢀger, Dr. F. Mathevet, Dr. D. Kreher, Prof. A.-J. Attias,
A. Bocheux
Laboratoire de Chimie des Polymꢁres-UMR 7610
Universitꢀ Pierre et Marie Curie
4 place Jussieu—case 185, 75252, Paris Cedex 05 (France)
E-mail: andre-jean.attias@upmc.fr
Dr. D. Blꢀger
Current address: Laboratory of Organic Chemistry and Functional
Materials, Department of Chemistry
Humboldt Universitꢂt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
A. Bocheux, Dr. S. Latil, Dr. L. Douillard, Dr. C. Fiorini-Debuisschert,
Dr. F. Charra
Laboratory for Organic Electronics and Nanophotonics
Service de Physique et Chimie des Surfaces et Interfaces, IRAMIS
CEA 91191 Cedex (France)
E-mail: fabrice.charra@cea.fr
[**] We thank the Agence Nationale de la Recherche (ANR) for generous
support through the NOMAD project. D.B. acknowledges the
Ministꢁre de la Recherche et de l’Ensignement Supꢀrieur (MESR)
for providing a doctoral fellowship.
The overall synthetic route to the Janus-like building
block (1,1’) is outlined in Scheme 1 (see the Supporting
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201008212.
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reduction to aldehydes, were allowed to react with the
phosphonate derivative 8 by a Horner–Wadsworth–Emmons
reaction. The racemate (1,1’) was used as such for the STM
experiments, that is, the two enantiomers were not separated.
Remarkably, this modular synthetic approach allows for
asymmetric substitution of the upper and lower decks, hence
giving rise to distinct faces A and B, and is therefore highly
versatile.
Figure 2 shows a short-scale STM image of the self-
assembly of (1,1’) on a HOPG surface at the interface with an
approximately 10À4 m solution in phenyloctane. One distinctly
Figure 1. Janus 3D tecton. a) Structure of the two-faced building block
lying on the substrate (alkyl chains are omitted for clarity), and
b) chemical structures of enantiomers 1 and 1’. The distyrylbenzene
upper level (B) is highlighted in blue.
Information for experimental details). The [3.3]dithiaparacy-
clophane derivative
4 was assembled by condensation
between the lower- and upper-deck precursors 2 and 3. On
one hand, the upper-deck of pillar 4 was functionalized by two
bromo atoms suitable for subsequent Suzuki coupling with
the commercial boronic ester 5. On the other hand, the lower
deck was functionalized by methylester groups which, after
Figure 2. STM image of the self-assembly of (1,1’) at the
HOPG–phenyloctane interface; parameters a=1.95Æ0.1 nm,
b=3.95Æ0.2 nm, and a=113Æ28; Vbias =À1.38 V and It =5.1 pA
(8.1ꢃ8.1 nm2). Two distinct orientations of the ellipsoid protrusions
are highlighted with blue ellipses. The scaled model of the molecular
assembly of the lower level A is superimposed on the STM image
(gray: carbon; red: oxygen; white: hydrogen). Distyrylbenzene upper
levels B are omitted for clarity.
observes the two levels of the Janus-like compound. The
bright four-armed patterns are characteristic of the lower
level (face A),[9,10] and can be attributed to their rigid
conjugated cores. These bright patterns alternate with
darker regions that correspond either to the alkyl chains
adsorbed in the Groszek geometry[13] or to uncovered parts of
the HOPG (neglecting the presence of the solvent). These
four-armed patterns are partially masked with brighter
ellipsoids. Such features are not visible with planar building
blocks that consist of face A exclusively,[14] and are thus to be
attributed to the distyrylbenzene upper level (face B). The
experimental lattice parameters (see Figure 2) are consistent
with our previous studies.[10] Therefore, the self-assembled
structures correspond to clipped molecules that form supra-
molecular chains (see model superimposed in Figure 2; only
the pedestal, that is, face A, is drawn, distyrylbenzene upper
levels B are omitted for clarity). Notice that this reconstruc-
tion breaks the axial symmetry of the graphite surface, so that
two mirror-symmetric domain orientations exist, denoted R
and L. In the following, only R domains are shown, R being
defined as the orientation depicted in Figures 2 and 3 (see
Scheme 1. Synthesis of 1, 1’. Reagents and conditions: a) KOH,
MeOH, CH2Cl2, RT, 5 h; b) trans-2-(3,5-dimethoxyphenyl)vinylboronic
acid pinacol ester 5, [Pd(PPh3)4] (0.05 equiv), toluene/EtOH/H2O
2:1:2, Na2CO3, 778C, 5 h; c) DIBAL-H, THF, 08C to RT, 2 h, then PCC,
CH2Cl2, RT, 2 h; d) 8, tBuOK, THF, 08C, 1 h. DIBAL-H=diisobutylalu-
minum hydride, PCC=pyridinium chlorochromate.
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Figure S1 in the Supporting Information for the molecules in
their L conformations).
the two distinct orientations of the bright ellipsoid protrusions
on the STM image. These two orientations are thus the
signature of the co-adsorption of 1 and 1’ in each single
domain. Quantitatively, in a domain with orientation R,
enantiomers are distributed according to the ratio 75 Æ 3% of
1 versus 25 Æ 3% of 1’ (in conformations denoted R1 and R1’
respectively, see Figure S1). By symmetry, the ratio is
reversed for L-oriented domains, R1 and R1’ becoming L1’
(mirror image of R1) and L1 (mirror image of R1’).
Of particular interest is the process that leads to the
formation of enantiomerically enriched domains. Although
each species integrates a domain of its own orientation (R or
L), the resulting domains are heterochiral (with the observed
3:1 ratio in favor of 1). This interesting aspect is a direct
consequence of the structure of the two-layered molecule.
Indeed, the lateral organization is dictated by face A alone,
whereas the chirality arises from face B. The formation of 2D
heterochiral domains were already reported for chiral[15] as
well as prochiral[16] molecules, even though homochiral
domains are more commonly observed.
A closer inspection of the ellipsoid protrusions (faces B)
reveals two distinct orientations of the longitudinal axis of the
ellipses (highlighted in blue in Figure 2) relative to the lattice
axes. Figure 3a shows an extended domain imaged at an
increased voltage and set-point current. With such acquisition
parameters, the lower level (A) is not visible anymore, each
molecule appears as a bright ellipsoid protrusion only.
Clearly, when discarding a few molecules that appear
darker, only two distinct orientations are observable inside
this specific domain. The proposed model of the self-assembly
is superimposed in Figure 3a. Figure 3b shows an enlarged
section, in which the two CPK models of the enantiomers are
superimposed on the different ellipsoid protrusions. The two
orientations of the distyrylbenzene upper levels B (in blue on
the CPK model) that correspond to 1 and 1’ fit quite well with
In order to gain insight into the observed distribution of 1
and 1’ according to a 3:1 ratio, full structural optimization of
the 3D tectons adsorbed onto graphite (0001) was per-
formed.[17] The optimized geometries of the two enantiomers
adsorbed in an R-oriented domain are represented in
Figure 4. The side views of the two enantiomers highlight
the significant distortion of the upper level of 1’ compared to
1, hence indicating a higher steric hindrance in the 1’
conformation when adsorbed in an R-oriented domain. The
optimization also indicates that the R1 configuration is more
favorable than the R1’ one (accordingly L1’ is more favorable
than L1), with a difference in adsorption energies as large as
DE = 298 meV (i.e. 6.9 kcalmolÀ1).
As expected from the observed 3:1 ratio between 1 and 1’,
the energy difference DE is in favor of 1. However, DE is
about one order of magnitude higher than kT, and therefore
indicates that the observed 3:1 ratio between enantiomers in a
single domain cannot result from a thermodynamic equilib-
rium but rather from a kinetic trap. Indeed, owing to the large
energy difference between configurations, it is unlikely that
the differentiation occurs concomitantly with adsorption from
the solution. Therefore, two R-like or L-like conformations
should already be differentiated in the solution. That is, each
enantiomer exists as a mixture of two conformers in the
supernatant solution (see Figure S1). The presence of such
conformers, which arises from hindered rotations about the
two clip–cyclophane single bonds in face A, has been reported
before for similar structures.[18]
If we admit the existence of two R-like (R1, R1’) and two
L-like (L1, L1’) conformations of each enantiomer 1 and 1’ in
solution, the most probable scenario is the following: the four
species can adsorb on the surface through their face A, each
isomer integrating a domain of its own orientation (R or L).
Once adsorbed, the replacement of a R1’ molecule by a more
favorable R1 (DE = 298 meV in favor of R1), and respectively
the replacement of L1 by L1’, would require a desorption–
adsorption process. However, the desorption energy of one
molecule is about 4.8 eV for R1, as evaluated by our DFT
calculations.[19] Such a high barrier prevents any desorption at
Figure 3. Insight into the self-assembly of enantiomers 1 and 1’ at the
HOPG–phenyloctane interface. a) Large-scale STM image; the scaled
model of the molecular assembly is superimposed on the STM picture
(only lower levels A are represented for clarity); the two distinct
orientations of the ellipsoids, relative to the two enantiomers 1 and 1’,
are highlighted in blue. Vbias =À1.56 V and It =27.0 pA
(49.3ꢃ49.3 nm2). b) Enlargement of an area of STM image 3a
(delimited by the blue dashed frame) together with the model of the
molecular self-assembly of 1 and 1’ (gray: lower level; blue: upper
level; red: oxygen; yellow: sulfur).
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estimated for molecular landers, 3.5–4.5 ꢀ, which already
permits a decoupling between molecular orbitals and the
substrate, sufficient to obtain the behavior of an electronic
nanowire.[21] Our estimated value is also larger than the
alumina insulating layer, approximately 5 ꢀ, which ensures a
suppression of Dexter electron-exchange quenching and thus
allows the luminescence of an excited fluorescent dye
immobilized over a metal substrate.[5b] In our case, the optical
gap of the fluorescent dye moiety is similar to that of the
pedestal, and their respective HOMO and LUMO orbital
levels are nearly aligned. Because of the resultant resonant
coupling, the pedestal does not play the role of an insulating
layer, which prevents the confinement of such an excited
state. However, the versatility of our synthetic strategy makes
the replacement of the current face B by a smaller bandgap
chromophore possible; the excited state should thus be
confined to the upper level. Ongoing research in our
laboratories is concerned with this aspect.
In conclusion, the concept of doubly-functionalized 3D
building blocks, the so-called Janus tectons, describes an
original bottom-up route for the positioning of functional
units above a conducting substrate, herein exemplified with
distyrylbenzene chromophores. As shown by STM studies, the
2D self-assembled pattern of face A on the surface controls
the organization of the superjacent chromophores (face B),
hence leading to a periodic array of functional units at a
defined distance from the substrate. The highly versatile
synthetic approach presented here allows, through modifica-
tion of face B, for the preparation of a wide range of 3D Janus
tectons that expose various functional units ranging from
fluorophores and photoswitches to molecular receptors and
noncovalent binding motifs for out-of- plane host–guest
systems. More generally, the bottom-up manufacturing of
periodic arrays of 3D functional entities opens new perspec-
tives towards active surfaces and interfaces.
Figure 4. Representations of the epitaxial network of a) 1 and b) 1’ on
HOPG, when adsorbed in an R-oriented domain, as studied by
ab initio calculations (gray: HOPG surface; blue: lower level; green:
upper level; red: oxygen; yellow: sulfur; white: hydrogen). In each
case, a side view is displayed on the panels to the right, including the
molecules (balls and sticks), and HOPG top layer (sticks only). The
molecular network primitive cell is drawn in white.
room temperature, and instead of beeing engaged in a
thermodynamic equilibrium that can usually take place at
the solid–liquid interface,[20] the molecules are kinetically
trapped on the surface. This result was confirmed during the
STM experiments, in which no changes in the self-assembly
could be detected over time (that is, all ellipsoids kept their
initial orientation). Finally, since one cannot expect signifi-
cant differences in the rate of adsorption of the different
species, the 3:1 ratio between R1 and R1’ (or between their
respective mirror images L1’ and L1) reflects the proportion
already present in solution as was obtained from synthesis.
Besides the 2D self-assembly properties, it is also possible
to determine the vertical position of the face B, one of the key
parameters in view of potential decoupling. As observed by
STM, the apparent height of the chromophore relative to
uncovered HOPG area is approximately 4.5 ꢀ (in the
conditions of Figure 2). However, because of the low
conductivity of the molecules, this value is only a lower
limit of the actual height. The molecular simulation reported
in Figure 4 indicates a larger height difference of approx-
imately 7 ꢀ. Consequently, our results indicate that the
vertical position of the functional unit is larger than the one
Experimental Section
STM images were acquired at room temperature with a homemade
digital system. The fast scan axis was kept perpendicular to the sample
slope. Images acquired simultaneously in both fast scan directions are
systematically recorded and compared. All images are corrected for
the drift of the instrument by combining two successive images with
downward and upward slow-scan directions. The solvent was phenyl-
octane (Aldrich, 98%), which avoids the coadsorption often observed
with linear alkanes. The substrate was HOPG (Goodfellow) and the
tips were mechanically formed from a 250 mm Pt–Ir wire (Pt80/Ir20,
Goodfellow). The monolayers were formed by immersing the STM
junction in a droplet (ca. 10 mL) of solution immediately after
cleaving the substrate and approaching the STM tip. The concen-
trations of the solutions were around 10À4 m and the solutions were
systematically heated prior to deposition on the HOPG surface.
Full structural optimization of the complete paracyclophane
molecules adsorbed onto graphite (0001) were performed using
ab initio calculations. The graphite surface was approximated by a
single layer; further details are given in the Supporting Information.
Received: December 27, 2010
Revised: April 26, 2011
Published online: June 10, 2011
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Communications
Joachim, F. Besenbacher, T. R. Linderoth, ACS Nano 2010, 4,
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Keywords: chirality · chromophores ·
scanning probe microscopy · self-assembly ·
supramolecular chemistry
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