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Communication
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Direct Visualization of Surface-Assisted Two-Dimensional Diyne
Polycyclotrimerization
Haitao Zhou,§,‡ Jianzhao Liu,∥,†,‡ Shixuan Du,*,§,‡ Lizhi Zhang,§ Geng Li,§ Yi Zhang,§
Ben Zhong Tang,*,∥ and Hong-Jun Gao*,§
§
Institute of Physics & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing, China
Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
∥
S Supporting Information
*
design guideline for the next-generation carbon-based functional materials. This, however, has been a challenging task. For
example, the exact conformation of regioisomers, i.e., 1,3,5- and
1,2,4-trisusbstituted benzene rings, in the polymerization
process remains unclear. Most of the past investigations have
relied on computational simulations but not direct experimental
evidence.5,6
In this work, we have combined scanning tunneling
microscope (STM) measurement with density functional
theory (DFT) calculation to study diyne cyclotrimerization
on the Au(111) surface. STM possesses submolecular
resolution and has been used to scrutinize the mechanisms of
a number of chemical reactions, including cyclodehydrogenation,7,8 alkane polymerization,9 and Ullmann coupling.10,11 We
have successfully resolved atomic structures of the intermediates and products of the surface-confined reaction, which show
distinct differences from those of the solution-based reactions
in terms of stereochemistry and regiostructure. The DFT
calculation of acetylene cyclotrimerization on a gold cluster has
revealed that the presence of gold can reduce reaction barrier,
thus leading to occurrence of the reaction at much lower
temperature, in comparison with those for the uncatalyzed
alkyne cyclotrimerization processes (∼400 °C).12
The experiments were performed by utilizing a commercial
Omicron low-temperature STM system with a base pressure
better than 1 × 10−10 mbar. The Au(111) surface was cleaned
by cycles of argon-ion sputtering and annealing. Molecules of
4,4′-diethynyl-1,1′-biphenyl (DEBP) were thermally evaporated at 30 °C, while the gold substrate was kept at room
temperature. Post-annealing at 100 °C could initiate the alkyne
polymerization. The samples were subsequently cooled down
to 5 K for STM imaging. The annealing temperature was kept
at not higher than 100 °C in an effort to avoid possible
interference or complications from potential thermolysis
processes such as thermal decomposition or degradation. To
support our experimental observations as well as reaction
mechanism, the DFT calculation was performed with
GAUSSIAN09 program13 using the M06 method.14 For Au,
the SDD basis set with effective core potential (ECP)15 was
used; for the rest of the atoms, the 6-31+G** basis set was
used. The central four gold atoms were fully relaxed, while the
other gold atoms were fixed.
ABSTRACT: Cyclotrimerization of alkynes to aromatics
represents a promising approach to two-dimensional
conjugated networks due to its single-reactant and atomeconomy attributes, in comparison with other multicomponent coupling reactions. However, the reaction
mechanism of alkyne cyclotrimerization has not yet been
well understood due to characterization challenges. In this
work, we take a surface reaction approach to study
fundamental polymerization mechanism by using a diyne
monomer named 4,4′-diethynyl-1,1′-biphenyl as a test bed.
We have succeeded in directly characterizing reactants,
intermediates, and their reaction products with the aid of
scanning tunneling microscope, which allows us to gain
mechanistic insights into the reaction pathways. By
combining with density functional theory calculation, our
result has revealed for the first time that the polycyclotrimerization is a two-step [2+2+2] cyclization reaction.
This work provides an in-depth understanding of
polycyclotrimerization process at the atomic level, offering
a new avenue to design and construct of single-atom-thick
conjugated networks.
C
haracterization of a chemical reaction at molecular level is
of great importance and can offer valuable mechanistic
insights into reaction pathways. Scanning probe microscope
(SPM) is a unique and powerful tool to directly “see” reactants,
products, and even intermediates in a reaction.1 Alkyne
polycyclotrimerization is a polymerization process developed
from ethyne cyclization2,3 and can be used to construct twodimensional (2D) conjugated polymer networks by using
planar aromatic diyne monomers on solid substrates,4 whose
topology can serve as an ideal model for studying miscellaneous
conjugated molecules. From chemistry viewpoints, the 2D
conjugated polymers by aromatic diyne polycyclotrimerization
on solid surfaces resemble the graphene structure possessing
single planar sheet of sp2-bonded carbon atoms. It has been
known that the o-, m-, and p-linkages between the benzene
rings formed by alkyne polycyclotrimerization can substantially
modify electronic communications of the resultant polyphenylenes, resulting in the conjugated polymers with tunable
properties such as electrical conductivity and light emission.
Deciphering the mechanism of alkyne cyclotrimerization at the
submolecular level is highly desirable, as it will not only
contribute to fundamental chemical science but also offer
© 2014 American Chemical Society
Received: February 7, 2014
Published: April 2, 2014
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dx.doi.org/10.1021/ja501308s | J. Am. Chem. Soc. 2014, 136, 5567−5570
Journal of the American Chemical Society
Communication
DEBP was chosen as a model diyne molecule in this study. It
was prepared according to our previously published experimental procedures.16 Previous studies have mainly focused on
the reactions in either gas or liquid phase in the presence of
catalysts at specific temperatures.2,17 In this work, we employ
STM to investigate diyne polycyclotrimerization on an Au(111)
substrate that can allow us to visualize reactions and determine
their reaction products unambiguously. Figure 1 illustrates
Figure 2. STM images of molecular layer before and after annealing.
(a) Initial deposition on Au(111) surface with about 0.5 ML coverage.
The inset shows a zoom-in image with several molecular models
superimposed to guide the eyes. (b) After annealing at 100 °C, the
diyne molecules are polymerized, generating networks with branch
and hexagon structures. (c) Magnified images clearly show two types
of molecular interconnection, which are highlighted by solid black and
dashed blue circles and denoted by I1 and I2, respectively. (d,e) Highresolution images (left) and molecular orbitals (right) of the two
patterns. I1 and I2 correspond to symmetric 1,3,5- and asymmetric
1,2,4-trisubstitutions, respectively. Scanning parameters: (a,b) Vs = 3.0
V, It = 0.05 nA; (c,e) Vs = 2.0 V, It = 0.1 nA; (d) Vs = 2.0 V, It = 0.3
nA.
Figure 1. Schematics of experimental procedure and proposed
polymeric mechanism. (Upper panel) DEBP monomers are thermally
deposited onto Au(111) surface. (Lower panel) Annealing causes
cyclotrimerization of the diyne molecules: three triple bonds are
cyclotrimerized into one benzene ring, with either 1,3,5- (blue) or
1,2,4- (red) trisubstitution.
fundamental building blocks, which are denoted as isomeric
configurations 1 (I1) and 2 (I2) and highlighted by solid and
dashed circles, respectively. Our images unambiguously
determine a three-fold symmetry for structure I1, while I2
shows a tree-fork shape with dihedron angles of 60°, 120°, and
180° (a straight line). These geometries are consistent with
those proposed in Figure 1, and we can thus confidently assign
I1 and I2 to 1,3,5- and 1,2,4-isomers, respectively.
These images unambiguously confirm the existence of the
regioisomeric structures in the polymerization process of the
diyne monomer. Statistical analysis of the STM images
indicates that the 1,3,5-structure (I1) predominates in the
polymeric products, and the ratio of I1 to I2 is about 5.6:1. As
compared with the previous solution polymerization of the
same monomer that gave a ratio of 1:2 for I1 to I2
configurations,2 our current observation suggests that the
1,2,4-isomeric structure (I2) has been significantly suppressed
in the 2D polycyclotrimerization process. This can be
understood by our theoretical simulation, which reveals that
the 1,3,5-isomer has a planar geometry, whereas the 1,2,4isomer is nonplanar in configuration. The steric hindrance from
the neighboring benzene ring renders it unfavorable to form the
1,2,4-isomeric structure on the 2D Au(111) surface.
Theoretic calculations were carried out in an effort to gain
further insights into the reaction mechanism. Because of the
large sizes of the DEBP molecule and the planar substrate, we
choose to simplify the system to a smaller one, i.e., acetylene on
a gold cluster with 14 gold atoms (Figure 3). We believe this
model simplification can address the essence of real reaction
pathway in our 2D polymerization, because it not only keeps
the main part of the DEBP molecule, i.e., the ethyne unit that
participates in the cyclotrimerization reaction but also considers
reaction scheme and possible structures of a polymeric product
of DEBP. Briefly, the diyne molecules thermally evaporated
onto the atomically flat Au(111) surface serve as building
blocks for ordered self-assembly structures. The annealing
process enables the DEBP molecules to migrate on the surface,
and three triple-bonds of the diyne monomers are cyclotrimerized into one benzene ring (blue or red ring in Figure 1).
Iteration of this process leads to the formation of 2D polymeric
network. It has been known that regioisomers, i.e., symmetric
1,3,5- and asymmetric 1,2,4-trisubstituted benzene rings, are
formed in this cyclotrimerization reaction, as denoted by the
blue and red rings in Figure 1, respectively.
Figure 2a shows a typical STM image of the DEBP molecules
self-assembled on the Au(111) surface with ∼0.5 ML
(monolayer) coverage. These molecules initially stay along
the fcc stripes and are then connected with each other across
the hcp stripes, resulting in the formation of uniform islands
(for images with different coverage, see Figure S1). Individual
molecules can be clearly distinguished in the image shown in
the inset, excluding the possibility of polymerization during the
thermal deposition process. Annealing at 100 °C for 30 min
after initial deposition causes significant changes in the
molecular arrangements, giving predominately branched and
hexagonal motifs, as shown in Figure 2b (occasionally
nonhexagon polygons are also observed with much less
probability, see Figure S2). High-resolution images given in
Figure 2c−e clearly reveal the existence of two types of
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dx.doi.org/10.1021/ja501308s | J. Am. Chem. Soc. 2014, 136, 5567−5570
Journal of the American Chemical Society
Communication
Figure 3. Potential energy surface of surface-supported cyclotrimerization to benzene on Au(111) with respect to acetylene on
Au(111) substrate.
Figure 4. STM characterization of the intermediates during the
reaction. (a,b) STM images with some intermediates of 1,3,5- and
1,2,4-isomers highlighted by solid and dashed arrows, respectively. (c,
d), Schematic drawings of molecular structures of the features in the
dotted squares in (a) and (b). The structures denoted by circles and
rectangles are the intermediates of 1,3,5- and 1,2,4-isomers,
respectively. The scanning parameters are (a) Vs = 2 V, It = 0.1 nA
for and (b) Vs = 2 V, It = 0.05 nA.
the role of the substrate. It is worth noting that while acetylene
trimerization has been studied previously by ab initio
calculations,18,19 most of the investigations were relevant to
homogeneous reactions, with few studies on the heterogeneous
reactions occurring on solid surfaces.20,21 Particularly, there is
no report on the reaction performed on the Au(111) surface.
Therefore, our calculation offers insights into molecule−
substrate coupling by explicitly computing reaction barriers.
Figure 3 shows the reaction pathway for the ethyne
trimerization. The trimerization can be completed in two
steps: First, two acetylene molecules are adsorbed on the
substrate with a triple bond length of 1.21 Å. Through a
transition state (TS1), in which the triple bonds (C1−C2 and
C3−C4) are elongated to 1.26 and 1.23 Å, an intermediate
(INT) with two endmost carbon atoms bound to the substrate
is formed. The original triple bonds are further elongated to
1.36 Å and the bond connecting them (C2−C3) is 1.45 Å in
length, similar to butadiene with double- and single-bond
lengths of 1.338 and 1.454 Å,22 respectively. The lengths of the
triple bonds in the intermediate are longer than those of the
double bonds in butadiene, which can be due to the fact that
the endmost carbon atoms are bound to their neighboring gold
atoms. The transition barrier for the first step is found to be
1.54 eV. In the second step, one more acetylene molecule
participates in the reaction. At the second transition state
(TS2), the C−C bond length of the third acetylene is elongated
to 1.25 Å, while the bond lengths of C1−C2 and C3−C4 are
further elongated to 1.38 and 1.39 Å, respectively. The C2−C3
bond, however, is reduced to 1.42 Å. After crossing a barrier of
1.72 eV, a hexagonal structure with the bond length of 1.39 and
1.40 Å (typical value for benzene) is formed. The distance
between the benzene ring and the gold surface is about 3.27 Å.
The transition barrier of the second step is 0.18 eV higher than
that of the first step, suggesting that the second step is the ratedetermining step. Importantly, this also implies that the
intermediate might be observable when the reaction proceeds
on the Au(111) substrate to support our proposed reaction
pathway.
Figure 4a,b shows some examples of STM images of the
dimerization intermediates during the 2D polymerization
reaction. The structure denoted by a solid arrow (Str1) has
an angle of ∼120°, while those marked by dashed arrows (Str2)
are straight lines. Considering the configuration of DEDP
molecule, the experimentally observed structures Str1 and Str2
can be assigned to the intermediates of the 1,3,5- and 1,2,4-
isomers, respectively. Figure 4c,d schematically presents
molecular arrangements of the structures shown in Figure
4a,b, respectively, for the intermediates of 1,3,5- and 1,2,4isomers.
Our atomically resolved capability achievable in experiment
can also provide an opportunity to evaluate the role of gold
adatom in the process of polymerization. The Au adatom has
been believed to be an excellent catalyst in the reactions such as
dithiol reactions on Au surfaces.23,24 In our experiment, we got
some clues that the gold adatom might have participated in the
diyne reaction. In Figure 2c, there are some bright protrusions
occasionally observable in the polymer network that are
highlighted by white solid arrows in the image. These
protrusions might be attributed to the gold adatoms according
to their sizes as well as the previous studies25,26 (details in
Figure S3). In the case of thiol reactions, the formation of RS−
Au−SR structure typically involves a huge number of gold
adatoms and significantly lifts the herringbone reconstruction
of Au(111) surface, leading to the change in the periodicity of
the soliton line from 6.3 nm (the value for a clean surface) to
7.5 nm.24 In our experiment, however, the periodicity remains
the same value (6.3 nm), suggesting that only few adatoms have
participated in the cyclotrimerization reaction, in consistency
qualitatively with our observation that the bright protrusions
are absent in most of the polymer structures. Moreover, the
participation of the Au adatom in the cyclization reaction can
decrease the critical polycyclotrimerization temperature to as
low as 50 °C (for details, see Figure S4).
In summary, we have observed that the DEBP molecules can
form 2D networks on the Au(111) substrate. The structural
changes in the reaction are directly visualized with the aid of
STM. Our results unambiguously confirm that the diyne
polycyclotrimerization on the Au(111) surface is a two-step
[2+2+2] cyclization reaction. For the first time, the
intermediate species are clearly captured. This is consistent
with our DFT calculations, which suggest that the second step
has a higher transition barrier and is the rate-determining step.
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Journal of the American Chemical Society
Communication
Moreover, we find out that the ethyne cyclotrimerization on the
Au(111) substrate has a significantly lower critical reaction
temperature than that in the homogeneous liquid media. In
addition to the previously reported dehydration method,27,28
our study offers an alternative way for constructing single-atomthick 2D conjugated networks, whose structure can be
analogous to the graphene, but whose properties are tunable
by decorating the diyne monomers with different functional
groups.
■
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ASSOCIATED CONTENT
S Supporting Information
*
STM images and details. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
hjgao@iphy.ac.cn; tangbenz@ust.hk; sxdu@iphy.ac.cn
Present Address
†
Department of Chemistry, The University of Chicago,
Chicago, Illinois, United States.
Author Contributions
‡
These authors contributed equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank Yuxue Li at Shanghai Institute of Organic Chemistry
for helpful suggestions and discussions. This work was
supported by the MOST (2011CB932700, 2011CB921702
and 2013CB834701), NSFC (51325204, 61390501), RGC
(HKUST2/CRF/10 and N_HKUST620/11) and SSC of
China.
■
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