On-surface synthesis of planar acenesviaregioselective aryl-aryl coupling

Article abstract of DOI:10.1039/d0cc01043j

The reaction of 2,2'-dibromo-biphenyl on a Ag(111) surface leads to the formation of planar acenes with a high-regioselectivity rather than nonplanar saddle-shaped tetraphenylene as the typical product in solution chemistry. The regioselective aryl-aryl coupling reaction is attributed to the hydrogen repulsion between the reactants on the confined two-dimensional surface.

Full text of DOI:10.1039/d0cc01043j

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DOI: 10.1039/D0CC01043J
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On-surface synthesis of planar acenes via regioselective aryl-aryl
coupling
a
a
b
a
a
a
a
Lin Feng, Tao Wang, Hongxing Jia, Jianmin Huang, Dong Han, Wenzhao Zhang, Honghe Ding,
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
Qian Xu, Pingwu Du, and Junfa Zhu*
DOI: 10.1039/x0xx00000x
The reaction of 2,2’-Dibromo-Biphenyl on a Ag(111) surface leads surface can reduce the molecular degrees of freedom, thus
to the formation of planar acenes with a high-regioselectivity significantly simplifying the reaction pathways and improving
1
7
rather than nonplanar saddle-shaped tetraphenylene as the typical the reaction selectivity. Therefore, on-surface synthesis
product in solution chemistry. The regioselective aryl-aryl coupling typically facilitates the formation of planar structures, providing
reaction is attributed to the hydrogen repulsion between the an unprecedented opportunity to synthesize planar PAHs. In
reactants on the confined two-dimensional surface.
contrast, the PAHs obtained from in-solution synthesis are
often nonplanar in the three-dimensional (3D) reaction
Polycyclic aromatic hydrocarbons (PAHs) refer to two or more
fused aromatic rings without substituents or heteroatoms,
which display exotic optical, electronic, and magnetic
properties.1 Attracted by their potential to become next-
generation functional materials using in organic thin-film
transistors, organic photovoltaics, nonlinear optics, spintronics,
and energy storage devices, the synthesis of PAHs has become
2, 23, 24
environments.
For the already known synthesis of PAHs
on surfaces, the planarization of products is often assisted by
17
the intramolecular cyclodehydrogenation. Moreover, some
-3
PAHs have been synthesized on surfaces via intermolecular aryl-
25-27
aryl coupling subsequent to dehydrogenation
dehalogenation.
and
For instance, Fasel et al. successfully
performed three PAHs via intermolecular aryl-aryl coupling of
21, 28
3
-5
one of the research hotspots for many years. Great efforts
have been made to synthesize PAHs, and multiple synthetic
21
2
,3-dibromotetracene molecules on Ag(111).
For the
intermolecular coupling, though no direct experimental
6
, 7
pathways have been proposed. In particular, an important
method to extend polycyclic aromatics is to add C fragments to
n
small PAHs through the aromatization of the newly formed C
6
7
rings via C-C coupling. However, the synthesis of PAHs in
solution is sometimes hampered by side reactions and tedious
separation of mixtures, resulting in low yield of target
products.5
, 8-11
To more efficiently synthesize PAHs, various strategies have
1
2-14
been adopted such as transition-metal catalysis,
and drastic
conditions.6
, 7, 10
In addition, the recent developed on-surface
chemistry also provides an effective route for engineering
shape-specific PAHs.1
5-22
Compared with the organic reactions
in solution, the two-dimensional (2D) confinement effect of a
a. National Synchrotron Radiation Laboratory, Department of Chemical Physics and
Key Laboratory of Surface and Interface Chemistry and Energy Catalysis of Anhui
Higher Education Institutes, University of Science and Technology of China, Hefei,
2
30029, China. E-mail: jfzhu@ustc.edu.cn
b.
Hefei National Laboratory of Physical Science at the Microscale, CAS Key
Laboratory of Materials for Energy Conversion, Department of Materials Science
and Engineering, iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials), University of Science and Technology of China, 96 Jinzhai Road, Hefei,
Scheme 1 Chemical structure of the precursor 2,2’-Dibromo-
Biphenyl (DBBP) and its different dimerization products, the
nonplanar saddle-shaped tetraphenylene that was
synthesized in solution, and the planar rhombic
dibenzo[e,l]pyrene formed on the metal surface in this work.
2
30026, P. R. China. E-mail: dupingwu@ustc.edu.cn
†
Electronic Supplementary Information (ESI) available: Synthetic procedure and
related STM images. See DOI: 10.1039/x0xx00000x
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Journal Name
evidence until now, it is expected that the 2D confinement of a particular, each dimer is symmetrically surroundVieewdArbtiycleeOinglihnet
surface could steer the reaction pathway, leading to the relatively dark dots, which refer to thDeOeI:i1g0h.1t03p9e/rDi0pChCe0ra10l4B3Jr
formation of completely different PAHs products with those in adatoms detached from the precursor molecules. This implies
solution. This would be valuable for the exploration of novel that the ordered molecular phase on the surface is stabilized by
low-dimensional PAHs which are difficult obtained in solution Br…H-ph hydrogen bonds.
synthesis.
Further annealing the sample at 450 K results in the
Herein, we report that the reaction pathways of a precursor
molecule are completely different in solution vs. on surface. The
planar acenes are selectively synthesized via intermolecular
aryl-aryl coupling on a metal surface, whereas the non-palanar
products were obtained, as illustrated in scheme 1. Deposition
of 2,2’-Dibromo-Biphenyl (DBBP, synthetic procedure: see Fig.
S1) precursor molecules onto the Ag(111) surface at room
temperature followed by annealing to 450 K leads to the
formation of planar dibenzo[e,l]pyrene (DB[e,l]P) via
hierarchical
intermolecular
aryl-aryl
coupling
and
cyclodehydrigenation, in sharp contrast with the nonplanar
2
9
saddle-shaped tetraphenylene products in solution. It is
demonstrated that the reaction mechanisms are different
between the solution-based synthesis and the present on-
surface synthesis: in the former case, the tetraphenylene is
obtained via a double Suzuki reaction or double Ullmann
reaction,8
, 30, 31
whereas in the latter case the DB[e,l]P is formed
by debrominative coupling and the subsequent combination of
3
2
debrominative and dehydrogenative coupling. The precise
regioselective aryl-aryl coupling can be attributed to the surface
Fig. 2 (a) Overview STM image taken after annealing the
sample at 450 K. (b) High-resolution STM image as well as the
confinement which reduces the flexibility of the molecules. Our superimposed with corresponding molecular models. (c) The
investigation shows that 2D template effect on surface plays a illustration of the regioselective C-C coupling and ortho-sites
C-H bond activation. Tunneling parameters: (a) T
t
= - 0.15 mA,
significant role for both intramolecular and intermolecular
reactions on surface. Scanning tunnelling microscopy (STM) and
synchrotron radiation photoemission spectroscopy (SRPES)
reveal the structural evolution of the molecules at an atomic
level.
Deposition of DBBP onto the Ag(111) surface at room
temperature leads to the formation of highly ordered dimer
arrays on the terraces of Ag(111), as revealed by STM image
shown in Fig. 1a. The orientation of molecules in each array
V
t
= - 1.90 V; (b) T
t
= - 0.14 mA, V = - 2.90 V. Color code: Ag, blue;
t
C, grey; H, white; Br, brown.
disappearance of organometallic (OM) dimer array phase. The
Ag(111) surface is populated with ordered planar rhombic
molecules (Fig. 2a and 2b). Here, we discuss two possible
reaction pathways to obtain a PAH: 1) a double Ullmann
coupling reaction would occur and tetraphenylene with eight-
membered ring would be constructed (molecule 3 shown in
(
[
marked by green line) is the same, rotating 15° with respect to
1-10] direction of the substrate. These molecules are arranged
29
Scheme 1); 2) a rhombic molecule (molecule 2 shown in
Scheme 1) could be synthesized by debrominative coupling and
the subsequent combination of debrominative and
dehydrogenative coupling. Because of the planar configuration
of final product shown in Fig. 2b the product cannot be
tetraphenylene. Thus, we consider to attribute this final
product to DB[e,l]P. It is found that the rhombic and planar
molecular model of DB[e,l]P match perfectly with the obtained
structure. In addition, the measured molecular length (9.9±0.4
Å) and width (7.2±0.4 Å) are in close agreement with widths of
the reported counterparts in armchair 9-Graphene nanoribbons
into a rhombic unit cell, whose parameters are a=b=13±0.4 Å
and α=135°. At this temperature, DBBP monomers undergo
complete debromination (as confirmed by the XPS results
shown in Fig. S2), and two resultant bi-radicals are linked by one
Ag atom forming a bow-shaped dimer (Fig. 1b). The bright
protrusion of the interstitial silver atoms can be seen more
clearly under a higher sample bias (Fig. S3). The bow-shaped
dimer has been demonstrated in the previous work to be
stabilized by four-fold C-Ag-C bonds, as seen in the
corresponding superimposed molecular model (Fig. 2b). In
3
3
34
35
(
GNRs) (9.67 Å) and zigzag 4-Zigzag GNRs (7.1 Å) ,
respectively. Hence, we propose that the OM dimers undergo
the second reaction pathway, generating the final product,
DB[e,l]P. On the Ag(111) surface, the three DB[e,l]P molecules
within the molecular unit cell have three different orientations.
Interestingly, the surrounding Br atoms also orient in three
directions (as guided by blue arrows in Fig. 3b), rotating by ~30°
with respect to the three high-symmetry directions of Ag(111).
Thus, the ordered DB[e,l]P molecular phase should be stabilized
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Fig. 1 (a) A large-scale STM image taken after deposition of
DBBP onto Ag(111) surface at RT. (b) Zoom-in STM image of
(
a) superimposed with corresponding molecular model.
Tunneling parameters: (a, b) T
t t
= - 0.18 mA, V = - 1.90 V. Color
Please do not adjust margins
code: Ag, blue; C, grey; H, white; Br, brown.

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by the Br···H bond interactions. Note that the intensity of C 1s and cross-coupling occur to complete the formatioVniewoAfrttihclee Ofinnlinael
declines obviously from 300 K to 450 K in SRPES, which can be product DB[e,l]P. The whole reaction proDcOeIs:s10is.10a3s9si/sDt0eCdCb0y10t4h3eJ
attributed to the partial desorption of molecules (see Fig. S2).
2D surface confinement which can reduce molecular flexibility
Probing deeply into the reaction process, the most possible and facilitate the planarization of products. In comparison, for
mechanism is shown in Fig. 2c. Initially, OM dimers undergo the the case in solution the molecules have high degree of freedom
breakage of four-fold C-Ag-C bonds, forming the two biradicals (flipping, rotating and diffusing), which minimizes the steric
with four equivalent activated sites (previous C-Br sites), as hindrance. Thus, the steric hindrance between H atoms of
marked by dashed black circles (number: 1-4). Subsequently, neighbouring biradical monomers is not an issue. Therefore, it
the two diagonal active sites (either 1 and 4 or 2 and 3) undergo is not surprising that the tetraphenylene, which is sterically
regioselective C-C coupling reaction, while another two active hindered on surface, dominates in solution. In short, the precise
sites remain unchanged. Here we take 1 and 4 active sites as regioselective aryl-aryl coupling can be attributed to the 2D
example for the Ullmann C-C coupling, shown as step 1 in Fig. confinement on the surface, which reduces the flexibility of the
2
c. After the C-C coupling at 1 and 4 sites, the two ortho C-H molecules.
bonds (5 and 8 sites) of newly generated C-C bond are activated
at elevated temperature (450 K). This is reasonable because the
Conclusions
C-H activation below 450 K has already been reported in prior
works.3
2, 36, 37
Finally, as shown in step 2, the two new active
In summary, we demonstrate that reaction pathway of
intermolecular C-C coupling of 2,2’-Dibromo-Biphenyl on a
Ag(111) surface is different from that in solution. The planar
dibenzo[e,l]pyrene is obtained with high selectivity on the
Ag(111) surface via aryl-aryl coupling rather than nonplanar
saddle-shaped tetraphenylene formed via double Ullmann
coupling. The highly regioselective aryl-aryl coupling can be
attributed to the 2D confinement of the surface which brings
steric hindrance between monomers. Our study demonstrates
that PAHs can be prepared differently from those in solution via
on-surface synthesis, especially planar molecules.
sites stemming from C-H activation form two C-C covalent
bonds with adjacent active sites (two remaining active sites
after step 1) to give the final product DB[e,l]P. Note that in step
1
the C-C coupling takes place between two active sites formed
by C-Br cleavage, whereas in step 2 the C-C coupling involves
the active sites originating from both C-H and C-Br activation
(cross-coupling). In addition, the proposed intermediate
between OM and product DB[e,l]P is not observed
experimentally, suggesting that the cleavage of C-Ag-C in OM is
the rate-determining step in the whole reaction steps. This
means once the C-Ag-C breakage happens, step 1 and 2 with
lower reaction barrier can take place immediately, leading to
the formation of the final products in a short time.
This work was financially supported by the National Natural
Science Foundation of China (21773222, 51772285, 21473170)
the National Basic Research Program of China
(
(
2017YFA0403402) and the DNL Cooperation Fund, CAS
DNL180201).
Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/D0CC01043J
TOC
The reaction of 2,2'-Dibromo-Biphenyl (DBBP) on the surface leads to the generation of planar
dibenzo[e,l]pyrene, in contrast to the nonplanar saddle-shaped tetraphenylene formed in
solution.