Two-Dimensional Chirality Transfer via On-Surface Reaction

Article abstract of DOI:10.1021/jacs.6b05597

Two-dimensional chirality transfer from self-assembled (SA) molecules to covalently bonded products was achieved via on-surface synthesis on Au(111) substrates by choosing 1,4-dibromo-2,5-didodecylbenzene (12DB) and 1,4-dibromo-2,5-ditridecylbenzene (13DB

Full text of DOI:10.1021/jacs.6b05597

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Article
Chirality Transfer via On-Surface Reaction
Haiming Zhang, Zhongmiao Gong, Kewei Sun, Ruomeng Duan,
Penghui Ji, Ling Li, Chen Li, Klaus Müllen, and Lifeng Chi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05597 • Publication Date (Web): 22 Aug 2016
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Chirality Transfer via On-Surface Reaction
Haiming Zhang,1 Zhongmiao Gong,1 Kewei Sun,1 Ruomeng Duan,2 Penghui Ji,1 Ling Li,1 Chen Li,2
Klaus Müllen,2,3 Lifeng Chi1*
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1. Institute of Functional Nano&Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional
Materials & Devices, Soochow University, 199 Ren’ai Road, Suzhou, 215123, Jiangsu, P. R. China
2. Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
3. Institute of Physical Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, D-55128 Mainz
Keywords: Chirality, molecular self-assembly, on-surface synthesis, scanning tunneling microscopy
ABSTRACT: Chirality transfer from self-assembled (SA) molecules to covalently bonded products was achieved via on-
surface synthesis on Au (111) substrate by choosing 1,4-dibromo-2,5-didodecylbenzene (12DB) and 1,4-dibromo-2,5-
ditridecylbenzene (13DB) as designed precursors. Scanning tunneling microscopy (STM) investigations reveal that their
aryl-aryl coupling reaction occurs by connecting the nearest neighboring precursors and thus preserving the SA lamellar
structure. The SA structures of 12(13)DB precursors determine the final structures of produced oligo-p-phenylenes (OPP)
on the surface. Pure homochiral domains (12DB) give rise to homochiral domains of OPP, whereas lamellae containing
mixed chiral geometry of the precursor (13DB) result in the formation of racemic lamellae of OPP.
1. Introduction
groups, e.g. halides,21,24-25 alkynes26,27 and boronic acids,28,29
can be activated to produce radicals on surfaces after the
dissociation of chemical bonds. Typically, the radicals will
connect with substrate atoms to form metal-organic hy-
brids as an intermediate state and finally give rise to
thermally stable 1D and 2D structures by reverting metal
atoms to the substrate.25,30,31 Up to date, considerable
achievements have been made in this area, as exemplified
by the synthesis of graphene nanoribbons with atomic
precision.32-34 However, little research has been directed
towards the synthesis of chiral structures on surfaces,
partially because the connection of radicals on surfaces
often proceeds in a random and uncontrollable fashion.
One work was reported by De Schryver et.al. that self-
assembled domains of alkylated diacetylenes can be par-
tially polymerized at liquid/substrate interface under the
exposure of UV lamp.35 In this case, the chirality of the
domains can be preserved after photopolymerization.
Because the reaction is limited to molecules with func-
tional group of diacetylene, a more applicable method is
required, specifically concerning the synthesis of func-
tional chiral materials, e.g. chiral graphene nanoribbons.
Taking into account the role of noncovalent interactions
in directing chiral self-assembly on surfaces, we report
here the successful chirality transfer from self-assembled
molecules to covalently connected oligomers via aryl-aryl
coupling reaction on Au (111) under the assistance of non-
covalent interactions. Chirality expression of produced
oligomers is determined by the packing structures of self-
assembled monolayers so that it can also be controlled
through adjusting self-assembled structures. The crucial
Chiral synthesis and separation are particularly im-
portant in pharmaceutical industry, because of the de-
mand of pure enantiomers.
1-3
Since solid surfaces are
essential for basic physical and chemical processes such
as adsorption, self-assembly, catalysis, on-surface chirality
has become a significant issue in this active research field.
Thanks to the invention of scanning tunneling microsco-
py (STM), on-surface chirality has been intensively ex-
plored at a molecular scale in the past decades.4-7 It has
been realized that the adsorption of molecules, regardless
of their chiral,8-10 prochiral11-14 or achiral structure,15-17 can
give rise to a chiral packing structure on surfaces. Alt-
hough most of the two-dimensional (2D) chiral structures
prepared on single crystal surfaces are overall 2D race-
mates, on-surface homochirality can be obtained by in-
troducing a small amount of guest chiral molecules to the
self-assembled monolayers (SAMs)18-19 or using a chiral
solvent at the liquid/substrate interface.20 However, since
the forces governing such an on-surface chiral packing are
mainly noncovalent, e.g. via hydrogen bonding, van der
Waals interactions as well as metal-organic coordination,
the low stability of the resulting chiral structures limits
their practical applications in chiral synthesis and separa-
tion.
Recent developments towards on-surface synthesis, on
the other hand, establish a way to fabricate thermally
stable 1D and 2D covalently connected structures on sur-
faces.21-23 Initiated by thermal annealing or ultraviolet
(UV) irradiation, precursor molecules with functional
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role of noncovalent adsorbate-substrate interactions in Alkyl chains of the 12DB molecule are found to adsorb
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chirality transfer from chiral SAMs to chiral oligomers
will be highlighted.
along [110] direction. The intermolecular distance be-
tween alkyl chains is 0.5nm which is consistent with the
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periodic distance of gold atoms along [112] direction.
2. Results and discussion
Since the adsorption of 12DB on the Au (111) surface
breaks the mirror plane (σh) of the molecule (C2h point
group with a mirror plane parallel to the benzene plane),
chiral packing structures are observed in the SAMs of
12DB, as depicted in Figure 1b. Two adjacent homochiral
domains, marked as ‘R’ and ‘S’ respectively, are connected
to form an angle of 30° between the long axis of each unit
cell. Detailed structures of each homochiral domain are
collected in high resolution STM images, as depicted in
Figure 1c. The enantiomeric packing of 12DB can be easily
recognized from atomic resolution STM images and is
more clearly displayed in a structural model (see Figure
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1d)
.
Figure 1. (a) Self-assembled monolayers of 1,4-dibromo-2,5-
didodecylbenzene (12DB) on the reconstructed Au(111). (b)
STM image of the chiral packing structures in the SAMs of
12DB. The dotted line highlights the boundary between two
homochiral domains of ‘R’ and ‘S’. (c) High resolution STM
image of the boundary between ‘R’ and ‘S’ domains, showing
detailed molecular packing of each domain. (d) A structural
model describing the packing of 12DB to form chiral struc-
tures on surface.
1,4-Dibromo-2,5-didodecylbenzene (12DB) and 1,4-
dibromo-2,5-ditridecylbenzene (13DB) are used as precur-
sor molecules based on the following considerations: two
alkyl chains can provide sufficient molecule-molecule
interaction to direct the self-assembly of molecules on Au
(111) surface and two bromo substituents offer potential
sites for homolytic radical formation and subsequent
carbon-carbon connections. The existence of alkyl chains
breaks the mirror plane (σv) of 1,4-dibromobenzene, lead-
ing to possible enantiomers upon adsorption of the pre-
cursors on surfaces. Different alkyl chains with even
(12DB) and odd (13DB) carbon numbers are adopted to
investigate the role of noncovalent interactions in chirali-
ty transfer of produced oligomers, i.e. oligo-p-phenylenes
(OPP). Shown in Figure 1a is a typical STM image of 12DB
SAMs prepared by evaporating 12DB on Au (111) substrate
held at room temperature (RT) under ultrahigh vacuum
conditions. 12DB molecules self-assemble on the Au (111)
surface to form a well-ordered lamellar structure, a char-
acteristic feature of the SAMs of alkane derivatives.12,17 The
unit cell parameters are a = 2.11 ± 0.02nm and b = 1.01 ±
0.02nm. Two bright bumps observed in the middle of the
molecule represent the position of bromo substituents.
Figure 2. Chirality transfer from 12DB molecules to oligo-
mers. (a) STM image of the sample annealed at 100°C, show-
ing the co-existence of intact 12DB molecules and “dimers”.
(b) STM image of homochiral islands of 12DB oligomers,
highlighted as ‘R’ and ‘S’ respectively. Detailed structures of
each homochiral island are collected in high resolution STM
image of ‘R’ (c) and ‘S’ (d) island.
To investigate the chirality transfer from molecular
packing of 12DB to the chains of OPP, the Au (111) sub-
strate covered by ca. 0.5 monolayer (ML) 12DB was an-
nealed at 100°C for breaking C-Br bonds and achieving
aryl-aryl coupling. The STM image (Figure 2a) depicts the
beginning of a series of such bond forming processes.
While the overall structure of the 12DB SAMs is main-
tained, e.g. unchanged distance between lamellae, the
appearance of dark spots (marked by white arrows in
Figure 2a) in the SAMs reflects the preliminary stage of
the coupling reaction. The origin of the dark spot can be
attributed to the formation of a new entity, which is com-
posed of four alkyl chains and one central core, as marked
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tained respectively at Vbias = -1.55V (b) and -1.35V (c) exhibit
by a dashed outline in Figure 2a. Since each 12DB loses
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remarkable difference of 12DB “trimer” and “tetramer”.
one bromo substituent and the distance (1.05 ± 0.02nm)
between two bright bumps of the central core is in line
with the distance between bromo substituents of a 12DB
dimer, we assign the new entity to the 12DB “dimer”, as
displayed in Figure 2a. It is therefore a reasonable as-
sumption that the dark spot is formed by squeezing out
adjacent 12DB molecules by connecting 12DB in the same
lamella. Further annealing of the sample at 120°C for 5
minutes leads to additional reactions within the SAMs on
Au (111), in which no single 12DB molecule is observable
anymore. Homochiral islands of 12DB oligomers, marked
as ‘R’ and ‘S’ respectively, can be clearly recognized from
the STM image, as shown in Figure 2b. The alkyl chains of
To further verify the covalently connected OPP struc-
tures, we used scanning tunneling spectroscopy (STS) to
investigate the different electronic structures of 12DB
“trimer” and “tetramer”. STS measurements of the oligo-
phenylenes have been performed on several samples with
various tips. Shown in Figure 3a is typical dI/dV spectra
recorded with the STM tip on top of the center of 12DB
“trimer” (black) and “tetramer” (red), as indicated by a
black and a red spot in the inset of Figure 3a. To ensure
the reliability of STS data, a spectrum on bare Au (111)
(gray) with the same tip is recorded as a reference before
each measurement. Both dI/dV curves are featureless over
the range measured in the empty states (0 < Vbias < 1),
whereas a remarkable difference in the filled states is
detectable. The resonant state of 12DB “trimer” (black
curve) at Vbias = -1.5V is 0.15V negative from that obtained
on top of 12DB “tetramer” (red curve, -1.35V). Such a dif-
ference in dI/dV curves is more evident in the spatial
distribution of electronic structures, which is recorded by
constant bias dI/dV mapping at a sample bias near the
resonant state of 12DB “trimer” (-1.55V, Figure 3b) and
“tetramer” (-1.35V, Figure 3c). The enhanced intensity is
only observed in the central part of oligophenylenes,
which is consistent with the fact that the frontier orbitals
of OPP are mainly distributed around the phenylene moi-
eties.36 The dI/dV intensity of 12DB “trimer” (highlighted
by a white rectangle) in the central part is significantly
weakened when the sample bias is shifted from -1.55V
(Figure 3b) to -1.35V (Figure 3c). The red-shift of the reso-
nant state from 12DB “trimer” to “tetramer” corresponds
well with previous experimental results that the highest
occupied state of OPP will decrease with increasing the
length of the phenylene moieties.36 STS measurements
can thus further confirm the structure of oligo-p-
phenylenes.
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12DB “oligomers” are adsorbed along the [110] direction,
similar with that of a single 12DB molecule adsorbed on
the Au (111). A closer view at each homochiral island re-
veals detailed structure of 12DB “oligomers”, as displayed
in Figure 2c and 2d. The SAMs of 12DB “oligomers” are
mainly composed of 12DB “tetramers”, but a few of “tri-
mers”, highlighted by a white rectangle, are still observa-
ble in the SAMs. Partial alkyl chains close to phenylene
moieties exhibit enhanced tunneling probability in STM
image, which can be attributed to the slight twisting of
adjacent phenylene moieties due to the steric effect. A
higher temperature annealing (ca. 160°C) can give rise to
longer oligomer chains on Au(111)(see supporting infor-
mation, Figure S1). However, because of the complicated
twisting structure of phenylene moieties in the longer
chains, chirality assignment of each chain becomes diffi-
cult from STM images.
In general, chirality transfer from self-assembled pre-
cursors to oligomers via on-surface synthesis is difficult to
achieve, as newly formed covalent bonds between mole-
cules can significantly decrease intermolecular distance
and thus determine the final structure of products. The
latter, however, is normally different from the original
molecular packing in SAMs. In consideration of the in-
termediate state during on-surface reactions, the struc-
tural similarity of SAMs and produced oligomers or poly-
mers becomes more difficult in practice. In the remarka-
ble case of chirality transfer of 12DB, molecules are fully
confined in the lamellae and can only be coupled with the
nearest neighboring molecules by squeezing out the next-
nearest neighboring molecules, as the rotation and the
flip of molecules are restricted by lateral interactions of
alkyl chains. With the assistance of parallel packed alkyl
chains, on-surface coupling reactions of 12DB can proceed
in a controllable way, thus allowing the chirality transfer
and also chiral amplification from the self-assembled
molecules to the linear oligomers.
Figure 3. STS measurement of the 12DB “oligomers”. (a)
Typical dI/dV spectra recorded on bare Au (111) (gray), 12DB
“trimer” (black) and “tetramer” (red). Both dI/dV spectra of
12DB “oligomers” are offset vertically for a better view. Spatial
distributions of electronic structure in dI/dV mappings ob-
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Figure 4. Self-assembled monolayers of 13DB on reconstruct-
To further demonstrate the correlation between struc-
tures of SAMs and oligomers, another precursor molecule
13DB was adopted. The SAMs of 13DB were prepared on
Au (111) surface held at RT temperature in the similar way
as for 12DB. Shown in Figure 4a is an STM image of the
13DB SAMs, in which parameters of the unit cell are a =
2.23 ± 0.02nm and b = 0.99 ± 0.02nm. While the lamellar
structure of 13DB is similar to that of 12DB, careful inspec-
tion indicates that no homochiral domains exist. In the ‘R’
dominated domain as displayed in Figure 4a, still some
13DB molecules are adsorbed with ‘S’ geometry (high-
lighted by white arrows) on the surface. As a result, no
distinct boundary between ‘R’ and ‘S’ domains appears in
the SAMs of 13DB. Instead, 13DB molecules adsorbed on
the Au (111) surface with mixed ‘R’ and ‘S’ geometry are
frequently observed, as demonstrated in Figure 4b. La-
mellae of 13DB adsorbed on the Au (111) with identical
geometry are marked by minus (-) at the bottom of the
image, whereas stars (*) mark the lamellae containing
both ‘R’ and ‘S’ geometry. Detailed packing of lamellae
with mixed ‘R’ and ‘S’ geometry is presented in Figure 4c.
As for adjacent 13DB with ‘R’ and ‘S’ geometry, 13DB mol-
ecules have been slightly tilted to fit the transition from
one enantiomer to the other. Considering the separate ‘R’
and ‘S’ domains in the 12DB SAMs (see Figure 1b), the
obvious difference in the 13DB SAMs can be attributed to
the so called ‘odd-even effect’, which is frequently ob-
served in the SAMs of molecules containing alkyl chains.12
Due to the terminal direction of methyl groups, molecules
with odd carbon number of alkyl chains tend to form
different structures in comparison to the corresponding
molecules with even carbon number within the alkyl
chains.12,17,37 Increasing alkyl chain length with only one
single methylene unit can lead to morphology switches
from a 2D racemate to a 2D conglomerate at a liq-
uid/substrate interface.12,37
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ed Au (111) surface. a) 13DB SAMs with dominant ‘R’ struc-
ture. Still some 13DB molecules adsorbed with ‘S’ geometry
are mixed in the ‘R’ domain, as highlighted by white arrows.
Unlike the SAMs of 12DB, homochiral domains can hardly be
observed in the 13DB SAMs. b) The 13DB SAMs with mixed
‘R’ and ‘S’ structures. A minus (-) at the bottom of the image
marks a row of 13DB molecules adsorbed with identical ge-
ometry and a star (*) marks a row of molecules adsorbed
with mixed ‘R’ and ‘S’ geometry. c) A structural model show-
ing detailed packing of lamellae with mixed ‘R’ and ‘S’ geom-
etry.
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To investigate the chirality transfer of the 13DB SAMs,
the Au (111) substrate covered by 13DB SAMs was annealed
at 120°C for on-surface coupling reactions. New packing
structures are observed in the SAMs of 13DB OPP (Figure
5a), showing a large scale domain with identical chirality.
A magnified STM image of the domain (see Figure 5b)
displays detailed packing of 13DB OPP. A staggered pack-
ing structure, in addition to lamellar structure, can be
distinguished from the SAMs of 13DB “oligomers”, which
has rarely been observed in the 2D packing of oligo-
phenylene obtained from 12DB. We attribute the change
in packing of 13DB OPP to the increased alkyl chain
length. The longer alkyl chain can slightly increase the
adsorption energy of 13DB (about 12.4kJ/mol),38 which, in
turn, affects the reaction of 13DB. A higher adsorption
energy of 13DB might impede the squeezing process in
adjacent lamellae, leading to the staggered packing of
13DB “oligomers”.
Figure 5. Chirality expression of oligomers from 13DB upon
annealing samples of 13DB SAMs at 120°C. a) Large scale STM
image of a homochiral domain of 13DB “oligomers”. b) A
closer view of the homochiral domain with ‘R’ geometry of
13DB “oligomers”. c) Separated lamellae of 13DB “oligomers”
showing homochiral and racemic geometry. d) A closer view
of a racemic lamella.
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Recalling that no homochiral domains are observed in
to form commensurate structures on reconstructed Au
(111) surfaces,17 we assume that the adsorption geometry of
OPP is essentially to optimize adsorbate-substrate inter-
actions on Au (111). It is therefore safe to claim that the
self-assembly of OPP should be directed by physical in-
teractions between OPP and Au (111) substrate rather than
the covalent bonds between phenylene moieties. Other-
wise, random distributions of OPP wires would be ex-
pected, similar to the on-surface synthesis of 1D struc-
tures.31-34 Accordingly, a schematic illustration of chirality
transfer via on surface reaction of 12DB is displayed in
Figure 6. Molecules marked in green color are considered
to desorb from the SAMs during parallel connection (red
arrows) of the nearest neighboring 12DB precursors, as
the newly formed C-C bonds can significantly decrease
the intermolecular distance of coupled precursors. Inter-
estingly, the intramolecular distance (0.49nm) between
alkyl chains of produced OPP equals the intermolecular
distance of alkyl chains in self-assembled lamellar struc-
tures, keeping the same physical interactions between
alkyl chains during the whole aryl-aryl coupling process.
Such an unchanged interaction could account for the
chirality transfer from the SAMs to the oligophenylene
products, because the basic lamellar structures are pre-
served during the aryl-aryl coupling process with the help
of alkyl chains.
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the 13DB SAMs, the ‘R’ domain of 13DB OPP should be
formed from ‘R’ dominated SAMs. We guess that a few
13DB molecules with ‘S’ geometry are squeezed out from
the ‘R’ dominated domain during the on-surface coupling
reaction. However, in domains with mixed ‘R’ and ‘S’ 13DB
lamellae, annealing of the SAMs at 120°C gives rise to the
formation of separated lamellae with either identical or
mixed chirality, as depicted in Figure 5c. The detailed
structure of a mixed lamellae can be seen in Figure 5d, in
which 13DB OPP with ‘R’ and ‘S’ adsorption geometry are
close-packed to form a blend lamella due to the symmetry
plane between ‘R’ and ‘S’ geometry. Since such a racemic
lamella has never been obtained from 12DB SAMs, its
formation reveals the structural correlation between the
SAMs and the produced OPP.
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3. Conclusion
In summary, we demonstrate that chirality can be
transferred from self-assembled 12(13)DB molecules to
oligo-p-phenylenes via on-surface synthesis. Alkyl chains
play a critical role in controlled coupling reaction of
12(13)DB precursors. Optimized adsorbate-substrate in-
teractions are observed during the whole process of the
coupling reaction, preserving the self-assembled lamellar
structures and allowing the chirality transfer. Structures
of the SAMs can determine the chirality expression of the
newly formed oligomers. The strategy developed here
sheds new light on the synthesis of stable, structure-
adjustable chiral surfaces from 2D molecular self-
assembly. Although produced OPP oligomers appear
chiral characteristics only when they are confined on
surfaces, the method can be readily applied to the synthe-
sis of 1D functional polymers with ‘planar chirality’, e.g.
chiral graphene nanoribbons. Furthermore, a homochiral
surface with enhanced stability can be expected in com-
bination of the techniques for the preparation of pure 2D
homochiral surfaces18-20 and the concept for chirality
transfer present in this work.
Figure 6. A schematic illustration of chirality transfer from
self-assembled 12DB to OPP4 via on surface synthesis. 12DB
precursors marked as green are considered to desorb from
self-assembled lamellae for the parallel aryl-aryl coupling of
adjacent molecules (indicated by red arrows).
Chirality transfer from self-assembled 12(13)DB precur-
sors to OPP via on-surface synthesis should proceed sev-
eral steps, e.g., breaking of active bonds, coupling of adja-
cent radicals, and the self-assembly of OPP. In these pro-
cesses, the formation and subsequent self-assembly of
OPP are critical in chirality transfer and magnification by
forming 2D homochiral domains. In the aryl-aryl coupling
of 12(13)DB precursors, only short OPPn (where n is the
number of phenylene moieties), e.g., OPP3 and OPP4, are
synthesized on Au (111) surfaces (ca.120°C), which facili-
tate the self-assembly process to form well-ordered 2D
homochiral domains on surfaces. Careful inspection of
the structure of 12(13)DB OPP reveals that alkyl chains of
ASSOCIATED CONTENT
ꢀ
OPP are adsorbed along the [110] direction (Figures 2b,
Supporting Information. Experimental details, STM image
of the sample annealed at 160°C, synthesis of 12(13) DB. This
material is available free of charge via the Internet at
http://pubs.acs.org.
3a,c, 5b,c). The intramolecular distance between alkyl
chains is measured to be 0.49 ± 0.01nm (averaged with
more than tens of STM images). The measured distance
(0.49nm) equals the periodic distance of gold atoms along
ꢀ
the [112] direction (0.5nm). Recalling that normal alkanes
AUTHOR INFORMATION
Corresponding Author
are adsorbed on gold atom troughs along <110> directions
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20. Tahara, K.; Yamaga, H.; Ghijsens, E.; Inukai, K.; Adisoejoso,
*Email: chilf@suda.edu.cn
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ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (NSFC, grant nos. 91227201 and
21527805). We also thank the Collaborative Innovation Cen-
ter of Suzhou Nano Science & Technology, and the Priority
Academic Program Development of Jiangsu Higher Educa-
tion Institutions. K.M. acknowledges support from the Jo-
hannes Gutenberg University via a Gutenberg Research Fel-
lowship.
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