Locking of helicity and shape complementarity in diarylethene dimers on graphite

Article abstract of DOI:10.1021/ja075917d

Upon deposition of an achiral solution of diarylethene 1 on the achiral surface of graphite, a highly ordered monolayer with large homochiral domains is formed by self-assembly. Scanning tunneling microscopy and molecular dynamics show that the building blocks forming this monolayer are chiral dimers. We suggest that the mechanism of formation of these chiral dimers is first, surface-induced atropoisomerism, which provides diarylethene 1 with a fixed helicity, and second, shape recognition between atropoisomers having similar helicities. Copyright

Full text of DOI:10.1021/ja075917d

Published on Web 12/21/2007
Locking of Helicity and Shape Complementarity in Diarylethene Dimers on
Graphite
Nathalie Katsonis,* Andrea Minoia, Tibor Kudernac, Toshiki Mutai, Hong Xu, Hiroshi Uji-i,
Roberto Lazzaroni,* Steven De Feyter,* and Ben L. Feringa*
Department of Organic and Molecular Inorganic Chemistry, Stratingh Institute, UniVersity of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands, Department of Chemistry, Laboratory of Photochemistry and
Spectroscopy and INPAC-Institute for Nanoscale Physics and Chemistry, Katholieke UniVersiteit LeuVen,
Celestijnenlaan 200-F, B-3001 LeuVen, Belgium, and Laboratory for Chemistry of NoVel Materials,
UniVersity of Mons-Hainaut, MaterianoVa, B-7000 Mons, Belgium
Received August 10, 2007; E-mail: B.L.Feringa@rug.nl; Steven.Defeyter@chem.kuleuven.be; Roberto@averell.umh.ac.be; N.H.Katsonis@rug.nl
Induction of helical chirality in achiral condensed phases is of
high importance in various fields, including liquid crystalline
technology.1 It is, however, still not well understood how helical
chirality is obtained, enhanced, modulated, or preserved in supra-
molecular systems.2 Interfacial assemblies provide model systems
to study these phenomena because of geometrical restrictions
introduced by 2D confinement.3 Scanning tunneling microscopy
Figure 1. Schematic representation of the interconversion occurring at room
temperature between P and M conformers of diarylethene 1 in solution.
The synthesis of 1 is described in the Supporting Information.
(STM) investigations of surface-induced chirality in organic mono-
layers of achiral compounds4 have shown that, at interfaces, the
formation of a chiral element arises either from molecule-substrate
interactions imposing a twist between the lattice of the physisorbed
molecules and the surface4a or from specific intermolecular interac-
tions such as H-bonding4b or metal/ligand complexation4c which
create supramolecular nanostructures.5 Recently, helical shape
recognition between enantiomers has also been evidenced for rigid
molecules such as helicenes6 or tetracenes.7 In these cases, the
physisorption of molecules with a rigid helicity, in vacuum and at
low temperature, followed by shape recognition led to the formation
of multimeric nanostructures.
In this contribution, we show that helical chirality emerges from
an achiral solution of a flexible diarylethene derivative because of
atropoisomerism on the surface.8 Atropoisomerism occurs because
Figure 2. (a) Enantiomorphic domains (λ and F) formed by diarylethene-
based dimers. The orientations of the main crystallographic axes of graphite
are schematically represented by white lines. The relevant reference axis
for the respective domains is indicated in blue: 52.5 × 27.3 nm2, 24 pA,
-322 mV. (b) High-resolution image and proposed packing model obtained
by force-field calculations: 9.55 × 6.30 nm2, 35 pA, 311 mV. (c) A dimer
on HOPG. The blue parts of the molecules are those directly adsorbed on
HOPG. The red ones correspond to the moieties which are not directly
adsorbed.
the rotational interconversion between two preferred conformers
is sterically hindered on the surface. Moreover, we show that shape
recognition occurs between atropoisomers having similar helicities
and leads to the formation of homochiral dimers.
In solution, diarylethenes such as molecule 1 (Figure 1) are free
to adopt a number of energetically similar conformations. In their
preferred conformations, the two methyl groups grafted on the
thiophene at the center of the conjugated unit are pointing in either
the same direction (parallel orientation)9 or in opposite directions
(antiparallel orientation). In the latter case, the overall conformation
of the molecule is helical, and depending on the sign of the helix,
the conformers are described as P or M isomers.10 However, in
solution and at room temperature (rt), the interconversion between
P and M helices is so fast that these forms cannot be distinguished.
Under these conditions, a solution of diarylethene 1 in 1-phenyl-
octane is consequently achiral.8
After application of a drop of solution of 1 in 1-phenyloctane
onto highly oriented pyrolytic graphite (HOPG), a well-ordered
monolayer is formed over a few hundreds of nanometers. Figure
2a shows an STM image in which two different domains of the
monolayer have a comparable orientation with respect to the
scanning direction. In each of these domains, STM highlights the
formation of (homochiral, vide infra) molecular dimers (see also
Figure 2b). These domains are not equivalent because the sigmoidal-
shaped dimers which constitute them are mirror images. Besides
the formation of locally chiral monolayers, it is remarkable that 1
forms chiral dimers on the surface. Surface-induced molecular
dimers arranged in rows have already been observed on HOPG,
on Au(111), and on MoS2.11
Within a dimer, the length of the bright contrast features (∼1.5
nm) corresponds to the length of an individual conjugated head
(∼1.65 nm). Alkyl moieties are visible under the aspect of straight
lines in the darker regions. The corresponding unit cell (a ) 1.7 (
0.1 nm, b ) 3.0 ( 0.1 nm, R ) 61 ( 3°) contains one dimer. The
angle between the unit cell vector a and main symmetry axes of
HOPG is +20 ( 3° (F) or -20 ( 3° (λ), depending on the chirality
of the domain, indicating that the self-assembly is surface-mediated.
The size of the structures observed in high-resolution STM
images (Figure 2b) rules out the possibility that the molecules
physisorb in the parallel conformation because the overall length
of 1 adsorbed in parallel orientation would be consistently smaller
than observed.9,12 Also, because of its folded geometry, the parallel
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10.1021/ja075917d CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
These conclusions on stability of the prochiral or chiral dimers
of 1 have been further confirmed by MD simulations including
solvent molecules. For this, the solvent was introduced in the model
by adding two layers of solvent molecules on the top of the surface
and by using the dielectric constant of 1-phenyloctane. The results
show that both dimers are stabilized by a few kcal/mol due to the
presence of the solvent molecules. However, since the stabilization
is comparable for both dimers, the PP dimer remains significantly
more stable than the PM dimer. For the latter, the kinetics of the
dissociation slows down, probably due to a cage effect of the solvent
molecules. Finally, an additional indication about the structure of
the dimer building block of the monolayer arises from the fact that
with PM dimers it is not possible to build a monolayer matching
the structure observed with STM. In contrast, the monolayer of PP
dimers as modeled by force-field simulations is in good agreement
with the experiment.
In conclusion, we have shown that chiral domains are formed at
the liquid/solid interface by achiral diarylethenes because of surface-
induced atropoisomerism. On the surface of HOPG, these molecules
with a locked helicity form dimers by shape complementarity.
Molecular dynamics show that these dimers of diarylethenes are
chiral dimers (PP and MM), rather than prochiral dimers (PM and
MP). These results add to our understanding of how chirality can
emerge in 2D systems.
Figure 3. Molecular dynamics trajectory showing the molecule-molecule
binding energy for two types of dimers (heterodimers PM and homodimers
PP) on HOPG, at room temperature, extracted from the total energy of the
system. Blue line: PM dimer. Red line: PP dimer. The curve shows that,
after 20 ps, the heterodimer dissociates, whereas the homodimer remains
stable.
Table 1. Average Molecule-Molecule Binding Energy and
Standard Deviation for the Two Dimers on Graphite, Based on the
MD Simulations Shown in Figure 3
dimer
binding energy (kcal/mol)
STD (kcal/mol)
PM
PP
-1.85
-9.27
2.21
1.45
conformer is unlikely to allow for stabilizing molecule-molecule
or molecule-substrate interactions. Moreover, as all experiments
are carried out in the dark, we exclude that 1 adsorbs in a
photoisomerized form. We therefore assume that 1 exclusively
physisorbs in its antiparallel conformation on HOPG.
Acknowledgment. This work has been supported by The
Netherlands Organization for Scientific Research (NWO-CW)
through a VENI grant (N.K.), the Marie Curie RTN CHEXTAN
(MRTN-CT-2004-512161), FNRS, the Fund of Scientific Research-
Flanders (FWO), and the Belgian Federal Science Policy Office
through IAP-6/27.
Dimer formation, as observed experimentally, suggests that an
energetically favorable interaction occurs between either prochiral
(PM/MP) or chiral (PP and MM) pairs of adsorbed molecules. In
order to understand the stereochemistry of these dimeric building
blocks, molecular dynamics (MD) have been performed for PP and
for PM dimers on the HOPG surface, at rt and in a dry situation,
that is, without solvent molecules.13 The calculations show a
dissociation of the prochiral dimer on the surface, while the chiral
dimer remains intact, due to favorable interactions between the
molecules and the surface; for example, the alkyl chains can be
fully adsorbed on graphite and among the molecules themselves.
In order to evaluate the strength of the interactions between the
molecules, the molecule-molecule binding energy for homodimers
and heterodimers on HOPG (Figure 3) has been extracted from
the total energy of the system (including the surface binding energy),
over a MD trajectory of 50 ps. From these curves, it appears that
on graphite the PP dimer is clearly more stable than its PM
counterpart, which dissociates after 20 ps. High-temperature simula-
tions at 400 K confirmed this trend. Table 1 summarizes these
results by showing the average binding energy and the standard
deviation for the two types of dimers. A detailed analysis of the
components of the binding energies of the dimers shows that the
main difference in their stability arises from weak van der Waals
interactions acting between the two helices. Therefore, we propose
that the difference of stability between the dimers is due to a better
locking between the conjugated heads of the molecules in the PP
(and MM) dimers rather than from a better adsorption of the alkyl
chains on the surface of HOPG. This means that both the molecular
locking, in terms of relative position of the helices (which determine
the shape of the dimer), and the binding energy depend on the
details of the molecular geometries. The fact that PP and MM
dimers are the most stable dimers added to the fact that the two
domains of Figure 2a are mirror images and form an angle which
is not 60° with respect to each other suggest that these domains
are formed by either PP or MM dimers.
Supporting Information Available: Synthesis and characterization
of 1, schematic representation of its parallel conformation, and
description of STM experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
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