Remote chiral communication in coadsorber-induced enantioselective 2D supramolecular assembly at a liquid/solid interface

Article abstract of DOI:10.1002/anie.201410927

Remote chiral communication in 2D supramolecular assembly at a liquid/solid interface was investigated at the molecular level. The stereochemical information in a chiral coadsorber was transmitted over a flexible spacer with a length of up to five methylene groups to a 2D supramolecular assembly of achiral building blocks with the cooperation of specific hydrogen bonding between the chiral coadsorber and achiral building blocks and the confinement effect during 2D crystallization. When the position of the stereogenic center was changed with respect to the stereocontrolling moiety, an odd-even effect was found. A stereogenic center closer to the stereocontrolling moiety transmitted the stereochemical information to the 2D supramolecular assembly more reliably. This result is beneficial not only for mechanistic understanding of chiral communication in 2D supramolecular assembly on surfaces but also for the rational design of homochiral supramolecular assemblies on surfaces.

Full text of DOI:10.1002/anie.201410927

Angewandte
Chemie
DOI: 10.1002/anie.201410927
Surface Chirality
Remote Chiral Communication in Coadsorber-Induced
Enantioselective 2D Supramolecular Assembly at a Liquid/Solid
Interface**
Ting Chen, Shu-Ying Li, Dong Wang,* Man Yao, and Li-Jun Wan*
Abstract: Remote chiral communication in 2D supramolec-
ular assembly at a liquid/solid interface was investigated at the
molecular level. The stereochemical information in a chiral
coadsorber was transmitted over a flexible spacer with a length
of up to five methylene groups to a 2D supramolecular
assembly of achiral building blocks with the cooperation of
specific hydrogen bonding between the chiral coadsorber and
achiral building blocks and the confinement effect during 2D
crystallization. When the position of the stereogenic center was
changed with respect to the stereocontrolling moiety, an odd–
even effect was found. A stereogenic center closer to the
stereocontrolling moiety transmitted the stereochemical infor-
mation to the 2D supramolecular assembly more reliably. This
result is beneficial not only for mechanistic understanding of
chiral communication in 2D supramolecular assembly on
surfaces but also for the rational design of homochiral
supramolecular assemblies on surfaces.
duced by including one or more stereogenic center in the
molecule by direct covalent binding, as has been described for
[
1]
[2]
helical polymers, metal–organic ensembles, and 1D, 2D,
[
3]
and 3D supramolecular assemblies. In these cases, the
molecular structure, including the number of the stereogenic
center(s)and their positions, the spacer between the stereo-
genic center, and the stereocontrolling moiety, acts a major
factor for the communication of the stereochemical informa-
[
4]
tion. In the case of 1D helical polymers or supramolecular
organizations, for example, a stereogenic center linked
directly or through a rigid spacer to the stereocontrolling
[1]
moiety is preferred for efficient chiral communication.
Moreover, when the position of the stereogenic center
relative to the stereocontrolling moiety is changed, the
chiral features of the helical supramolecular architecture
[
3a,b,5]
follow an odd–even rule.
The reliability of the chiral
communication with respect to the molecular structure is
a result of a complex interrelation of steric, stereoelectronic,
[
4c]
C
hiral communication is common in many biological and
kinetic, and thermodynamic effects, and different results
[
5d,e]
chemical processes. For example, in stereoselective reactions,
a small amount of a chiral catalyst may favor the formation of
one enantiomer of the product. Similarly, in polymer synthesis
and supramolecular organization, the introduction of a ste-
reogenic center in the monomers or building blocks usually
leads to supramolecular architectures with a specific handed-
ness. In these processes, the generation of a specific chirality
in the product or the supramolecular architecture is closely
related to the efficient communication of the stereochemical
information. In general, stereochemical information is intro-
have been reported depending on the molecular system.
Recently, some studies showed that chiral guest molecules
or chiral solvents that can specifically interact through
noncovalent bonds with achiral hosts can efficiently transmit
their stereochemical information to the achiral building
blocks to enable asymmetric control in the supramolecular
[
6]
assembly of achiral molecules in solution. This approach
avoids the tedious asymmetric synthesis of chiral building
blocks and is considered to be a simple strategy to induce the
formation of homochiral supramolecular architectures. For
example, Schenning and co-workers found that the enantio-
selective supramolecular self-assembly of achiral oligo-
[
*] Dr. T. Chen, S.-Y. Li, Prof. D. Wang, Prof. L.-J. Wan
Key Laboratory of Molecular Nanostructure and Nanotechnology
Institute of Chemistry, Chinese Academy of Sciences and
Beijing National Laboratory for Molecular Sciences
Beijing 100190 (P.R.China)
(p-phenylenevinylene) (OPV) derivatives can be induced by
both chiral regulators (R- or S-citronellic acid) and chiral
[
6e,f]
solvents (S- or R-citronellol).
Moreover, the approach is
also applicable to 2D supramolecular assembly at a liquid/
[
7]
E-mail: wangd@iccas.ac.cn
solid interface. De Feyter and co-workers reported that the
introduction of a chiral solvent or a chiral auxiliary or
“handle” induced the formation of a supramolecular assem-
bly of achiral OPV derivatives with a preferred handed-
wanlijun@iccas.ac.cn
S.-Y. Li
University of the Chinese Academy of Sciences (China)
[7a–d]
Prof. M. Yao
ness.
We reported the induction of homochirality in
School of Materials Sciences and Engineering
Dalian University of Technology
Dalian 116024 (P.R.China)
enantiomorphous molecular networks as triggered by chiral
coabsorbers at a liquid/solid interface. In view of the great
[7e]
convenience of using a noncovalent interaction rather than
a covalent bond to bias the homochiral assembly, it is of great
interest to understand the chiral communication in such
chiral-guest-induced enantioselective 2D supramolecular
assembly at the liquid/solid interface. In particular, the
dependence of chiral transmission on the characteristics of
the noncovalent interactions, such as the specificity and the
[
**] This research is supported by the National Key Project on Basic
Research (grants 2011CB808701 and 2011CB932304), the National
Natural Science Foundation of China (grants 21233010, 21127901,
and 21373236), and the Strategic Priority Research Program of the
Chinese Academy of Sciences (Grant No. XDB12020100).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410927.
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Communications
strength, needs to be clarified. However, there is little
information available about remote chiral communication in
2
D supramolecular assembly at solid surfaces, even for
molecular systems in which the stereogenic center is cova-
lently introduced, although it has been studied extensively in
solution. Supramolecular assembly at the liquid/solid inter-
face is a confined 2D crystallization process. The effects of
some pivotal factors, such as specific noncovalent interac-
tions, the configuration and position of the stereogenic center,
the spacer, and the substrate are still unclear for chiral
communication at solid surfaces.
In this study, we investigated remote chiral communica-
tion through noncovalent interactions in enantioselective 2D
supramolecular assembly at a liquid/solid interface. Molec-
ular assemblies of 5-(benzyloxy)isophthalic acid (BIC) deriv-
atives, which form 2D enantiomorphous porous networks at
the liquid/solid interface when codeposited with alkyl alco-
[
7e]
hols, were studied as a model system (Figure 1).
It was
Figure 2. a) Typical STM image of the enantiomorphous networks of
BIC-C6 coassembled with 1-heptanol. b) High-resolution STM image of
the CCW domain. c) Proposed structural model of the CCW network.
d) Illustration of intermolecular interactions within the trimeric aggre-
gations. Tunneling conditions: a) I=0.600 nA, Vbias =0.900 V;
b) I=0.900 nA, Vbias =0.700 V.
The codeposition of BIC analogues with achiral
1
-heptanol and 1-octanol was first explored. The porous
network formed by BIC-C6 and 1-heptanol is composed of
windmill-shaped molecular trimers (Figure 2a). According to
the rotational direction of the molecular trimers (clockwise or
anticlockwise), two mirror domains, named the CW network
and the CCW network, respectively, are discerned in the
adlayer. Statistical analysis of more than 200 STM images
recorded at different positions suggests that the CW network
and the CCW network are present in the adlayer with nearly
the same probability, thus implying that the network is locally
chiral but globally racemic. In a high-resolution STM image of
the CCW network (Figure 2b), the solvent molecules
Figure 1. Chemical structures of BIC derivatives and alkyl alcohols. BIC
derivatives are abbreviated as BIC-Cn, in which n is the number of
carbon atoms in the alkyl chain. In the nomenclature of the coadsorb-
ers, the first two characters indicate the position and spatial config-
uration of the stereogenic center, and the last letter denotes the
backbone structure of the coadsorber, that is, H, C, and O represent
heptanol, citronellol, and octanol, respectively.
(denoted with yellow lines) were clearly resolved. Figure 2c
is a proposed structural model of the CCW network. The
intermolecular hydrogen bonding between 1-heptanol and
BIC-C6 is illustrated in Figure 2d. The configuration of the
hydroxy group exactly fits the chiral hydrogen-bonding
pockets of the BIC trimers to form a 10-membered ring
stabilized by three OÀH···O hydrogen bonds (outlined by the
found that a stereogenic center could transmit its stereo-
chemical information to the achiral BIC building blocks and
further to the 2D nanoporous network even when it was
separated from the stereocontrolling BIC moieties by five
methylene groups and a hydrogen-bonding group (ÀOH). The
orange circles). A high-resolution STM image and proposed
structural model of the CW network are shown in Figure S1 of
the Supporting Information. We also investigated the co-
assembly of BIC-C7 or BIC-C8 with achiral 1-heptanol and 1-
octanol. Similar porous networks were formed by BIC-C7 or
BIC-C8 with 1-octanol, except that the periodicity increased
from 4.6 to 4.8 and 5.0 nm, respectively (see Figure S2 in the
Supporting Information). The length of the coadsorber is
crucial for the formation of the coassembly: The codeposition
of BIC-C7 or BIC-C8 with 1-heptanol led to the disappear-
ance of the porous networks. This result is ascribed to the size
handedness of the 2D nanoporous networks followed an odd–
even alternation rule when the position of the stereogenic
center was changed with respect to the hydroxy end group.
Moreover, the reliability of the remote chiral communication
increased when the stereogenic center was shifted closer to
the hydroxy group. Molecular mechanistic (MM) simulations
provide quantitative understanding of the remote chiral
communication.
2
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mismatch between the length of 1-heptanol and the vacant
space enclosed by neighboring BIC-C7 (or BIC-C8) trimers.
We next examined the codeposition of BIC derivatives
with chiral alcohols. We found a similar size-matching effect
between the BIC derivatives and the chiral alcohols. Never-
theless, whenever a nanoporous network was possible, all
analyzed domains in the coassembly possessed the same
handedness. As shown in Table 1, homochiral induction not
Dilute solutions of a chiral alcohol in nonselective achiral
1-heptanol as the solvent were used to study the chiral-
transmission efficiency. For chiral alcohols 1S-H and 5S-H,
the coverage of the CCW network on the surface increased
nonlinearly with the concentration of the chiral alcohol even
if only a small amount of the chiral alcohol was present in the
solution (Figure 3). All of the networks were CCW when the
Table 1: Results of the supramolecular assembly of BIC analogues
[
a]
codeposited with pure achiral and chiral alcohols.
BIC-C6
BIC-C7
BIC-C8
1
1
1
2
3
5
6
1
3
-heptanol
-octanol
S-H
S-H
S-C
S-H
S-O
R-H
R-C
CW, CCW
CW, CCW
CCW
CW
–
–
CW, CCW
–
–
–
–
CW
–
CW, CCW
–
–
CCW
–
CW
–
–
CCW
CCW
CW
–
–
CW
[a] CW and CCW indicate that the CW network and the CCW network are
formed in the coassembly, respectively. A dash indicates that no porous
network was present in the coassembly.
Figure 3. Correlation of the coverage of CCW networks on the surface
with the concentration of the coadsorber 1S-H or 5S-H as a solution in
achiral 1-heptanol. The coverage percentage of CCW networks was
deduced on the basis of the number of domains.
only exists for 1S-H and 1R-H, which have a stereogenic
center adjacent to the hydroxy end group, but is also possible
with a chiral coadsorber that has a stereogenic center far away
from the hydroxy end. In the coassembly of BIC analogues
with 6S-O, for example, a stereogenic center separated by six
covalent bonds from the hydroxy end group resulted in
a preference for one porous network with a specific handed-
ness, thus implying efficient remote chiral communication in
the 2D supramolecular assembly at the liquid/solid interface.
Typical STM images of the homochiral networks of BIC-C6
and 1S-H or 1R-H are shown in Figure S3 of the Supporting
Information. Although chiral-solvent-mediated asymmetric
molecular assembly at a liquid/solid interface has been
concentration of the chiral coadsorber reached 20%. How-
ever, in the lower concentration range (< 20%), the coverage
of the CCW network with 1S-H was higher than that with 5S-
H at the same concentration of the chiral coadsorber. For
example, when 5% of the chiral coadsorber was present in the
solution, (80 Æ 2)% of the networks were CCW with 1S-H,
whereas only (68 Æ 5)% of the networks were CCW with 5S-
H. As the CW and CCW networks are present in nearly equal
amounts when coassembled with the diluent achiral 1-
heptanol, we propose that the excess of the CCW network
formed on the surface when a chiral coadsorber is present in
the solution reflects the reliability of the chiral communica-
tion during 2D supramolecular assembly. Such a dilution
experiment has not been performed for the chiral-solvent-
[
7a,b,f,g]
reported previously,
the role of chiral alcohols in the
present system can be unambiguously identified as that of
a coadsorber on the basis of the high-resolution STM
[
7e]
image.
[
7a,b,f,g]
The handedness of the 2D supramolecular assembly is
closely dependent on the absolute configuration and the
position of the stereogenic center. Stereogenic centers at the
same position with mirror absolute configurations induce
opposite handedness in the 2D assembly. If the absolute
configuration is kept the same but the stereogenic center is
moved from one atom to the next along the alkyl backbone,
the handedness of the resulting network follows an odd–even
alternation rule. A special case is the coassembly of BIC-C6
and 6S-O, which gives a CCW network rather than the
expected CW network, thus implying a violation of the odd–
even alternation rule. It was inferred that the adsorbed
conformation of the stereogenic center is inverted as com-
pared to that of other chiral coadsorbers owing to steric
hindrance (see Figure S4 for further analysis and details of
MM simulation).
induced asymmetric assembly process.
The efficient
chiral communication in the presence of a small amount of
a chiral alcohol further validated the hypothesis that the
coadsorber molecules, rather than a chiral environment
imposed by a chiral solvent, were important for the induction
[
7e]
of homochirality in the present study.
To further verify the difference in the chiral-communica-
tion efficiency for stereogenic centers at different positions,
a mixture of 1R-H and 5S-H (1:1, v/v), which induce CW and
CCW assembly, respectively, was used as the solvent and
codeposited with BIC-C6. Of the resulting networks, 79% of
were CW and the remaining 21% were CCW, which
correspond to coassembly with 1R-H and 5S-H, respectively.
This result confirms that the stereochemical information of
a stereogenic center at the 1-position can be more efficiently
transmitted to the 2D supramolecular assembly at the liquid/
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À1
solid interface than that of a stereogenic center at the 5-
position.
Table 2: Adsorption energy (in kcalmol ) of the alcohols as coabsorbers
[
a]
in the CW and CCW BIC-C6 trimeric chiral units.
There is a long-standing argument about the induction
mechanism of supramolecular homochirality. One viewpoint
is that the domain boundary plays an important role in the
Adsorption
energy
1-Heptanol 1S-H
2S-H
3S-H
4S-H
5S-H
ECW
257.53 267.86 257.81 265.34 260.61
268.01 257.60 266.06 258.94 265.55
[
8]
263.89
chiral induction. The formation of a homochiral surface with
a large superstructure of the major enantiomer and a disor-
ganized minor enantiomer can maximize entropy and is thus
ECCW
DECW–CCW
[
b]
0
0
10.48
4.12
10.26
3.97
8.25
2.17
6.4
1.45
4.94
1.66
[
c]
DEchiral–achiral
[
8a]
thermodynamically favored.
On the other hand, the
[a] The chiral structural unit of the coassembly of BIC-C6 and 1S-H, 2S-H,
asymmetric interaction of disordered molecules with the
building unit of the homochiral assembly at the domain
boundary can also induce homochirality. We explored the
dependence of the domain size on the concentration of the
chiral coadsorber in the solution and found that the domain
size only relates to the concentration of BIC derivatives. For
or 5S-H is drawn according to the STM images. The analogous chiral
units composed of BIC-C6 and 3S-H or 4S-H are proposed by analogy.
[b] DECW–CCW is the difference in adsorption energy between the CW and
CCW units (absolute value). [c] DEchiral–achiral is the difference in adsorp-
tion energy between the energetically favored enantiomer of the chiral
alcohol in a coassembly with BIC and the achiral 1-heptanol in
a coassembly with BIC.
[
8b]
example, when the concentration of BIC-C6 was kept at 4 ꢀ
À4
1
0
m for the dilution measurements, the domain size is found
2
to be about 400–900 nm , independent of the concentration of
the chiral coadsorber. Moreover, the binding of disorganized
molecules at the domain boundaries was not observed in the
present study. Thus, it is supposed that the role of the domain
boundary in the chiral induction is limited. Another perspec-
tive is that the chirality of the supramolecular assembly is
the homochirality of the surface even when diluted by an
achiral nonselective solvent in the dilution experiment.
Moreover, when the stereogenic center is moved away from
the hydroxy end of the coadsorber, the difference in
adsorption energy between the CW and CCW chiral units
decreases gradually. At the same time, the difference in
adsorption energy between the energetically favored chiral
units of BIC-C6 with the chiral coadsorber and achiral 1-
heptanol also decreases gradually. This result suggests that
a stereogenic center closer to the hydroxy end group has
a stronger effect on the adsorption of the chiral unit and
induces a clearer preference for one of two mirror config-
urations. The adsorption energy of individual chiral alcohols
does not change much with the position of the stereogenic
center. This result is in sharp contrast to that obtained for the
chiral coassemblies (Table 2). A plausible reason for the
difference is that 2D coassembly at the liquid/solid interface is
a confined process. The adsorption of the coadsorber is not
only affected by the substrate but also constrained by the
neighboring molecules. The violation of the odd–even alter-
nation rule in the coassembly of BIC-C6 and 6S-O is an
example of such a confinement effect.
[
7]
determined by the conformational preference. We used MM
simulation to understand the contribution of the conforma-
tional preference to the chiral communication. We simulated
the adsorption energy of the alcohols as coabsorbers of the
CWand CCW BIC-C6 spiral aggregations (Figure 4, Table 2),
which are the essential chiral structural units of the networks.
The adsorption energy of achiral 1-heptanol coadsorbed with
the CW or CCW BIC trimer is comparable, thus suggesting
there is no preference for the CW or CCW chiral unit. When
a stereogenic center is present in the coadsorber, a conforma-
tional preference appears: The chiral unit with the methyl
branch oriented away from the surface is energetically
favored (see Figure S5). The adsorption energy of the
energetically favored conformation is higher than that of
the chiral unit composed of BIC-C6 and 1-heptanol, which is
consistent with the ability of the chiral coadsorber to control
Overall, it is believed that in the present system, the
stereochemical information is communicated through con-
formational preference during supramolecular assembly. We
found that the degree to which the stereochemical informa-
tion could be communicated through conformational prefer-
ence was significantly affected by the spacer between the
[
1a–d,3b,9]
stereogenic center and the interaction site.
A rigid
spacer facilitates efficient long-range transmission of stereo-
chemical information in 1D helical polymers or organizations.
In our system, although the spacer, that is, the alkyl backbone
between the stereogenic center and the hydroxy group, is
generally believed to be flexible, the conformational prefer-
ence originating from the stereogenic center can be trans-
mitted over this long flexible spacer to determine the
handedness of the 2D assembly. It is proposed that the
cooperation of noncovalent interactions during the 2D
crystallization plays an important role in the efficient
remote chiral communication.
Figure 4. Illustration of the remote chiral transmission in the coassem-
bly of BIC derivatives with 1S-H and 2S-H. The hydrogen atoms on the
alkyl backbones of 1S-H and 2S-H are not shown for clarity. The BIC
aggregations are simplified to clearly reveal the chiral hydrogen-
bonding pockets.
Figure 4 illustrates the remote chiral transmission in the
coassembly of BIC derivatives with 1S-H and 2S-H. Owing to
4
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the steric effect, the stereogenic center has the same
conformation in all chiral coadsorbers adsorbed on the
surface; that is, the methyl group points towards the solution.
Meanwhile, 2D crystallization of the alkyl chains on graphite
surface, as induced by van der Waals interactions, makes the
cation. The chirality of the supramolecular assembly follows
an odd–even alternation rule with respect to the position of
the stereogenic center owing to the alternation of the spatial
configuration of the stereogenic center when the coadsorber
is adsorbed on a surface. This result provides mechanistic
insight at the molecular level into the transmission of
stereochemical information during supramolecular assembly
on a 2D surface. The tunable reliability of the chiral
communication within the supramolecular assembly may
benefit the rational design of supramolecular chiral structures
and materials.
[
10]
flexible alkyl chains act as rigid spacers.
Therefore, the
conformational preference of the stereogenic center can be
transferred over a long distance to determine the orientation
of the hydroxy group. Subsequently, the hydroxy group with
a specific orientation selectively interacts with BIC trimers of
a specific handedness to form the exact geometric comple-
ment and the unique hydrogen-bonded ring. The conforma-
tional preference is further transmitted through the non-
covalent bonding to the BIC trimers. For coadsorbers with
stereogenic centers that have the same absolute configuration
but at neighboring positions, for example, 1S-H and 2S-H, the
configuration of the hydroxy group is opposite, and BIC
trimers of opposite handedness are formed. As a result, the
handedness of the 2D supramolecular assembly follows an
odd-even alternation rule when the stereogenic center moves
from one position to the next along the alkyl chain. This effect
is similar to the Sol-Rel, Sed-Rod alternation rules developed
for cholesteric liquid crystals and odd–even effects later
described for polymers and a variety of self-assembled
Received: November 11, 2014
Revised: January 20, 2015
Published online: && &&, &&&&
Keywords: chiral communication · interfaces ·
.
scanning tunneling microscopy · supramolecular assembly ·
surface chirality
[
1] a) D. B. Amabilino, E. Ramos, J.-L. Serrano, T. Sierra, J.
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coassembly of the diluent, that is, 1-heptanol, which forms
enantiomorphous networks with BIC derivatives, cannot be
excluded. The adsorption energy of the energetically favored
chiral unit in 5S-H is only a little higher than that of achiral 1-
heptanol. Therefore, 1-heptanol may readily coassemble with
BIC-C6, thus leading to a decreased enantiomeric excess in
the adlayer.
In conclusion, communication of the stereochemical
information in chiral alcohols induced 2D asymmetric supra-
molecular assembly of achiral BIC derivatives at a liquid/solid
interface was demonstrated. The cooperation of noncovalent
interactions, including the hydrogen-bonded rings connecting
BIC derivatives and the coadsorber and the van der Waals
interactions during 2D crystallization, plays an important role
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www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Surface Chirality
Getting the message across: The trans-
mission of stereochemical information
through a long flexible spacer of up to five
methylene groups and noncovalent
hydrogen bonds was observed in 2D
supramolecular assembly at a liquid/
solid interface. The confinement effect
during 2D crystallization and specific
noncovalent interactions between the
chiral coadsorber and the achiral building
blocks play an important role in efficient
chiral communication.
T. Chen, S.-Y. Li, D. Wang,* M. Yao,
L.-J. Wan*
&&&& — &&&&
Remote Chiral Communication in
Coadsorber-Induced Enantioselective 2D
Supramolecular Assembly at a Liquid/
Solid Interface
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!