Substrate-Mediated C-C and C-H Coupling after Dehalogenation

Article abstract of DOI:10.1021/jacs.6b10936

Intermolecular C-C coupling after cleavage of C-X (mostly, X = Br or I) bonds has been extensively studied for facilitating the synthesis of polymeric nanostructures. However, the accidental appearance of C-H coupling at the terminal carbon atoms would li

Full text of DOI:10.1021/jacs.6b10936

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Article
Substrate Mediated C-C and C-H Coupling after Dehalogenation
Huihui Kong, Sha Yang, Hongying Gao, Alexander Timmer, Jonathan P Hill, Oscar Díaz
Arado, Harry Mönig, Xinyan Huang, Qin Tang, Qingmin Ji, Wei Liu, and Harald Fuchs
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b10936 • Publication Date (Web): 10 Feb 2017
Downloaded from http://pubs.acs.org on February 10, 2017
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Substrate Mediated C-C and C-H Coupling after Dehalogenation
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Huihui Kong1, Sha Yang2, Hongying Gao1,3,4*, Alexander Timmer3,4, Jonathan P. Hill5, Oscar Díaz
Arado3,4, Harry Mönig3,4, Xinyan Huang1, Qin Tang1, Qingmin Ji1, Wei Liu2*, Harald Fuchs1,3,4
*
1 Herbert Gleiter Institute of Nanoscience, and 2 School of Materials Science and Engineering, Nanjing University of Science
and Technology, Xiaolingwei 200, Nanjing 210094, Jiangsu, P. R. China
3 Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, and 4 Center for Nano-
technology (CeNTech), Heisenbergstrasse 11, 48149 Münster, Germany
5Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba,
Ibaraki 305-0044, Japan
Supporting Information
ABSTRACT: Intermolecular C-C coupling after cleavage of C-X (mostly, X = Br or I) bonds has been extensively studied for
facilitating the synthesis of polymeric nanostructures. However, the accidental appearance of C-H coupling at the terminal carbon
atoms would limit the successive extension of covalent polymers. To our knowledge, the selective C-H coupling after dehalogena-
tion has not so far been reported, which may illuminate another interesting field of chemical synthesis on surfaces besides in-situ
fabrication of polymers, i.e., synthesis of novel organic molecules. By combining STM imaging, XPS analysis and DFT calcula-
tions, we have achieved predominant C-C coupling on Au(111) and more interestingly selective C-H coupling on Ag(111), which
in turn leads to selective synthesis of polymeric chains or new organic molecules.
1. INTRODUCTION
unique C-H coupling which leads to the effective and efficient
synthesis of pure organic molecules.
Surface-assisted synthesis has received wide attention due
to its potential for the creation of novel functional organic
molecules and precise construction of well-defined robust
architectures with the prospect of nanomaterials and
nanodevices.1-3 For such studies, scanning tunneling micros-
copy (STM) in combination with X-ray photoemission spec-
troscopy (XPS) have proven to be an excellent toolkit which
allows a direct, real-space topographic identification and de-
tailed analysis of the chemical information of both reactants
and products.1,4-7 In recent years, a series of on-surface inter-
molecular reactions have been mainly focused on generating
distinct nanostructures, including polymeric chains and porous
organic networks.3-17 Besides the in-situ fabrication of poly-
mers, another interesting field of chemical synthesis, i.e., the
synthesis of small organic molecules via on-surface reaction
with specific regioselectivity, has been less studied, which
have yet exhibited great potential for the synthesis of novel
molecules with excellent selectivity and purity.1,18-23
Scheme 1 Schematic illustration of the controllable C-C or
C-H coupling of (R)-(-)-6,6'-dibromo-1,1'-bi-2-naphthol
(DN) molecule on different surfaces.
As a well-known model system for surface-assisted synthe-
sis, intermolecular C-C coupling after cleavage of C-X (usual-
ly, X = Br or I) bonds has been extensively studied. Such
reactions facilitate the synthesis of polymeric nanostructures,
especially various graphene nanoribbons with specific edges
and latitudes,24-27 while the accidental appearance of C-H
coupling at the terminal carbon atoms would limit the succes-
sive extension of covalent polymers and facilitate the for-
mation of small oligomers. To our knowledge, selective C-H
coupling after dehalogenation has not yet been reported.
Therefore, it is of great interest to study the possibility for
To conduct the selective C-H coupling and synthesis of or-
ganic molecules, the strategy in this study is to design a pre-
cursor molecule with multiple chemically active sites (includ-
ing, among others, C-X functional groups). The purpose of
such a strategy is that the by-products from other chemical
reaction processes could provide an efficient hydrogen source
for the C-H coupling after dehalogenation. According to pre-
vious investigations, both C-H28 (original C-H bonds in pre-
cursor molecules) and O-H bonds29,30 can be cleaved on metal
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surfaces to generate hydrogen atoms. Therefore, these should
Lifshitz-Zaremba-Kohn theory35,36 for the nonlocal Coulomb
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be able to serve as potential hydrogen sources. Unfortunately,
C-Br bonds exhibit smaller binding energies than C-H bonds
and are thus more easily activated with intermolecular C-C
coupling often occurring before breakage of C-H bonds.24-26
This strongly indicates that C-H functional groups are not a
good choice as hydrogen source. Therefore, in this study we
introduce O-H functional groups as the hydrogen donors for
C-H coupling and thus selected (R)-(-)-6,6'-dibromo-1,1'-bi-2-
naphthol molecule (DN) molecule (see Scheme 1) as a precur-
sor of the reaction. For such a molecule, C-Br groups are
prerequisite to study the C-H coupling after dehalogenation,
while O-H groups could dehydrate or dehydrogenate to pro-
vide hydrogen atoms for C-H coupling. By the combination of
high resolution STM imaging and state-of-the-art density
functional theory (DFT) calculations and XPS analysis, we
have systematically studied on-surface reactions of DN mole-
cules on different surfaces (Scheme 1). C-C coupling is pre-
dominantly achieved on Au(111) after dehalogenation, where
polymeric chains are synthesized. More interestingly, selective
C-H coupling is successfully achieved on Ag(111) after
dehalogenation, which subsequently leads to the synthesis of
organic molecular monomers. The key to make this unique C-
H coupling successful appears to be the simultaneous dehalo-
genation reaction and the formation of furan, which may make
the hydrogen atoms produced during its formation directly
diffuse to the carbon radicals where the hydrogen transfer
takes place.
screening within the bulk. The atomic zeroth-order regular
approximation37 was used to treat relativistic effects for Cu
atoms. The “tight” settings including the “tier2” standard basis
set in the FHI-aims code were used for light elements (H, C,
and O) and “tier1” for Au, Ag, and Br. A convergence criteri-
on of 0.02 eV/Å for the maximum final force was used for
structural relaxations. Also convergence criteria of 10-5 elec-
trons per unit volume for the charge density and 10-4 eV for the
total energy of the system were utilized for all computations.
The Ag(111) and Au(111) surfaces were modeled by 3-layer
slabs. The bottom metal layer was constrained whereas the
two uppermost two metal layers and molecules were allowed
to fully relax during the geometry relaxations. Different slabs
were separated by more than 40 Å vacuum to eliminate the
interaction between periodic images. The STM simulations
were carried out within the Vienna ab initio simulation pack-
age (VASP) code38 at a bias of +0.5 V.
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Synthesis. Molecules 1 and 3 were prepared following
literature methods39-41 and purified by column chromatography
(SiO2/n-hexane) followed by recrystallization from chloro-
form/methanol and tetrahydrofuran, respectively.
Molecule 1: 1H-NMR (d8-THF, 313 K, 300 MHz): δ =
3
4
3
9.65 (dd, 2H, J = 9.3 Hz, J = 1.8 Hz), 9.73 (d, 2H, J = 8.7
3
4
Hz), 9.80 (d, 2H, J = 9.3 Hz), 10.13 (d, 2H, J = 1.8 Hz),
10.80 (d, 2H, 3J = 9.3 Hz) ppm; 13C-NMR (d8-THF, 313 K, 75
MHz): δ = 113.71, 118.02, 119.01, 127.0, 128.02, 129.45,
131.56, 132.94, 154.77 ppm; MALDI-TOF-MS (dithranol):
calc’d for C20H10Br2O: 425.91; found m/z = 424.88 [(M – H)+].
2. METHODS
Molecule 3: 1H-NMR (CDCl3, 298 K, 300 MHz): δ = 7.57 (t,
2H, 3J = 7.5 Hz), 7.73 (t, 2H, 3J = 7.2 Hz), 7.80 (d, 2H, 3J = 8.7
Hz), 7.93 (d, 2H, 3J = 8.7 Hz), 8.04 (d, 2H, 3J = 7.8 Hz), 9.13
(d, 2H, 3J = 8.4 Hz) ppm; 13C-NMR (CDCl3, 298 K, 75 MHz):
δ = 112.69, 119.40, 124.37, 125.60, 126.15, 128.29, 128.61,
129.46, 131.21, 154.33 ppm; MALDI-TOF-MS (dithranol):
calc’d for C20H12O: 268.09; found m/z = 267.67 [M+].
STM, XPS and mass spectroscopy. STM experiments
were performed in a UHV chamber (base pressure 1 × 10-10
mbar) equipped with the low-temperature Omicron or JT-STM
held at 77 K, a molecular evaporator and other standard facili-
ties for sample preparation. XPS experiments were carried out
in a UHV chamber, which is directly connected to the Omi-
cron STM. The Au(111) and Ag(111) substrate were prepared
by several cycles of 1.5 keV Ar+ sputtering followed by an-
nealing at temperatures of 770 K and 700 K for 15 min, result-
ing in clean and flat terraces separated by monatomic steps.
The DN molecules (purchased from Tokyo Chemical Industry
Co., Ltd. purity >98%) were loaded into a quartz crucible in
the molecular evaporator. After a thorough degassing, the DN
molecules were deposited onto the clean surfaces held at room
temperature. Most of the STM images were obtained with a
tunneling current of 0.05 nA and sample bias of 500 mV,
except where indicated otherwise. The typical width of the
terraces obtained for both Au(111) and Ag(111) surfaces was
around 100 nm.
3. RESULTS AND DISCUSSION
Heat-induced dehydration or dehydrogenation reac-
tion of DN molecules on Au(111). Before deposition of the
DN molecules on specific substrates, we first conducted mass
spectrometry experiments to verify that the molecules remain
intact during the thermal sublimation. As expected, there was
no structural degradation in the DN molecules before they
reached the substrate (see Supporting Information, Figure S1).
Deposition of DN molecules onto Au(111) held at room tem-
perature lead to the formation of well-ordered self-assembled
structures (Figure 1a). From the high resolution STM image
(Figure 1b) we find that such nanostructures are composed of
tetramers as building blocks, where each molecule is imaged
as an ellipsoidal bright protrusion. XPS analysis further re-
veals that both the C-Br bonds and C-OH bonds remain intact
after deposition (Figure 4a and b). DFT calculations are em-
ployed to unravel the molecular configurations. We found that
the DN molecules adopt non-planar adsorption configurations
due to the intramolecular steric hindrance (Figure 1c). Fur-
thermore, a careful inspection reveals that the herringbone
reconstruction of Au(111) underneath the self-assembled
nanostructures is neither modified nor lifted, indicating weak
interactions between the DN molecules and the Au(111) sur-
face.
Mass spectrometry measurements were carried out on an
EPIC 1000 mass spectrometer (Hiden Analytical) operated in
its standard mode. Such a device is directly connected to the
Omicron STM system, allowing an in-situ analysis of the
sublimation process.
DFT calculations. The DFT calculations were carried
out in the numeric atom-centered basis set all-electron code
FHI-aims,31 together with the Perdew-Burke-Ernzerhof (PBE)
exchange-correlation functional.32 The PBE + vdWsurf meth-
od33 was employed to account for the van der Waals (vdW)
interactions and collective response effects. The PBE + vdW-
surf
method extends pairwise vdW approaches to modeling of
adsorbates on surfaces by a synergetic combination of the PBE
+ vdW method34 for intermolecular interactions with the
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calculations including the Au(111) substrate have been per-
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formed. In this case, multiple single-molecule products are
taken into account and relaxed on the substrate. After a com-
parison with high resolution STM images, we find that the
energetically favorable models of molecules 1 and 2 (see
Scheme 1), as well as the simulated STM images, are in good
agreement with the two experimentally observed newly-
synthesized species in both morphologies and dimensions, as
shown in Figure 1f and g (also shown in Figure S3). Further-
more, molecules 1 and 2 adopt an approximately flat adsorp-
tion configuration rather than non-planar alternatives (Figure
1h and i), which is consistent with their smaller apparent
heights as compared to the DN molecules on Au(111) prior to
the annealing. Based on the previous analysis, we confirm that
the chemical structures of the “V”-shaped and parallelogram-
shaped molecules can be attributed to molecules 1 and 2,
which are synthesized by intramolecular dehydration and
dehydrogenation reactions of the DN molecules, respectively.
Such an assignment could be further verified by XPS analysis,
which shows C-OH groups have completely transformed into
a furan group at 380 K.
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Figure 1. (a,b) Overview and high resolution STM images of the
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DN molecules on Au(111); (c) Top and side view models of the
DN molecules adsorbed on Au(111). (d,e) Overview and high-
resolution STM images showing two differing species after an-
nealing at 380 K; (f) STM image of a “V”-shaped molecule (left)
and the simulated STM image (right), as well as the overlaid
optimized model; (g) STM image of parallelogram-shaped mole-
cule (left) and the corresponding simulated STM image (right) as
well as the overlaid optimized model. (h,i) Top and side view
model of the “V”- and parallelogram-shaped molecules on
Au(111).
Figure 2. (a) Overview STM image showing irregular islands on
Au(111) after annealing at 480 K. (b) High-resolution STM image
showing zigzag polymeric chains obtained by C-C coupling after
dehalogenation. (c) The same STM image as (b) with the overlaid
proposed model. (d-f) High resolution STM images (upper panel)
and the proposed models (lower panel) showing diverse coupling
constitutions.
After annealing the DN/Au(111) sample at 380 K for 30
minutes, structural transformation from ordered self-
assembled nanostructures to irregular disordered islands is
clearly recognizable (Figure 1d). The high resolution STM
image shown in Figure 1e reveals that single-molecule config-
urations have been significantly changed compared with the
state before anneal. In this case two different predominant
species are identified. They are depicted by a blue and green
hand drawn contours (Figure 1e), which are imaged as “V”
and parallelogram-shaped molecules, respectively (also see
Figure 1f and g). Furthermore, statistical analysis reveals that
the ratio between molecule 1 and molecule 2 is around 10 : 7.
Moreover, the apparent height difference between these two
species and the ordered nanostructures in Figure 1a is about
2.5 Å (see Figure S2). These distinct changes in both apparent
height and molecular geometry induced by the thermal treat-
ment strongly imply that intramolecular chemical reactions
should have occurred to the DN molecule, resulting in the
formation of two new species.
Further heat-induced C-C coupling after dehalogena-
tion on Au(111). To study the possibility of C-C coupling or
C-H coupling occurring after a dehalogenation process in
molecules 1 and 2, further thermal treatment was performed.
After annealing the sample at 480 K for 30 minutes, we ob-
served that triangle-like islands are formed (Figure 2a). Care-
ful inspection reveals that the herringbone reconstruction of
Au(111) is modified, which can be attributed to the adsorption
of dissociated bromine atoms on Au(111), as it has been pre-
viously reported.42 Such dissociated bromine atoms are imaged
as dim spots in the corresponding high resolution STM image
(Figure 2b), corroborating the dehalogenation of molecules 1
and 2. This fact is also verified by the XPS by a pronounced
chemical shift of the Br 3d peaks. The cleavage of C-Br bonds
further triggers the formation of a number of covalently-linked
To obtain a deeper insight into the chemical structures of
these two newly-synthesized species in Figure 1f and g, DFT
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zigzag chains with random directions and lengths, which are
models nor the simulated STM images are in agreement with
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separated from each other by bromine atoms.
the STM contrast (Figure S7). This is further supported by the
XPS data for the C 1s region of DN molecules on Ag(111),
where a distinct spectroscopic signature supporting a C-Ag
bond formation was not observed. On the other hand, we built
the model based on molecule 3 (see Scheme 1) where the
carbon radicals are passivated by hydrogen atoms. Surprising-
ly, the optimized model and the corresponding simulated STM
image of molecule 3 show very good agreement with the ex-
perimental STM images, as illustrated in Figure 3b and Figure
S8. The simulated STM image of the nanostructure based on
molecule 3 (Figure S9) also show good agreement with that in
Figure 3a. Furthermore, the chemical structure of molecule 3
could reasonably lead to the formation of the dimer structures
in Figure 3c, where two “U”-shaped molecules are interlinked
by two directional C-H…O hydrogen ( Figure 3d).
To account for the formation mechanism of the covalently-
linked zigzag chains, we perform detailed analyses on various
high resolution STM images (Figure 2d-f). Such procedure
reveals that these zigzag chains are likely synthesized by in-
termolecular C-C coupling of the dehalogenated molecules 1
or 2 at the dehalogenated sites, including molecular homo-
coupling and hetero-coupling, as demonstrated by the pro-
posed model. These C-C coupling modes are further verified
by the good agreement between the simulated and experi-
mental STM images of covalent dimers (Figure S4). The high
resolution STM image including a proposed model superim-
posed in Figure 2c illustrates the diversity of constitutions
after C-C coupling. These covalent chains are found to grow
in limited lengths, and their terminations are passivated by
accidental C-H coupling, as evidenced by the good agreements
of the experimental STM images of minor monomers and
dimers with scaled optimized models as well as the simulated
STM images (Figure S4). It should be nevertheless noted that
the observed C-H coupling is less favored, with intermolecular
C-C coupling after dehalogenation still acting as the predomi-
nant coupling type.
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Heat-induced C-H coupling of DN molecules after
dehalogenation on Ag(111). In the next step, we explore the
controllable selection of C-H coupling by using the Ag(111)
substrate to slightly modulate the reaction energy barrier for
both dehydration (or dehydrogenation) and dehalogenation
reaction steps. After deposition of DN molecules onto
Ag(111) at RT, we observed two self-assembled nanostruc-
tures (Figure S5). After annealing this sample at 380 K for 30
minutes, two types of well-ordered nanostructures are mainly
observed (Figure 3a and c). All the molecules in both
nanostructures exist in the form of single units with identical
“U”-shaped molecular configurations. However, the “U”-
shaped molecular monomers in Figure 3a are separated from
each other by dim spots which are also in this case attributed
to bromine atoms, while in Figure 3c two of these molecules
are likely linked by non-covalent bonds to form dimers.
Figure 3. (a,c) STM images showing two ordered nanostructures
after deposition of DN molecule on Ag(111) with post annealing
at 380 K. (b) Close-up STM image (upper panel) and the simulat-
ed one (lower panel) as well as the overlaid optimized model
showing the “U”-shaped molecular topography in (a); (d) Close-
up STM image (upper panel) and the simulated one (lower panel)
as well as the overlaid optimized model showing hydrogen-
bonded dimer in (c) formed by two “U”-shaped molecules. (e)
STM image showing nanostructures after annealing at 480 K. (f)
Close-up STM image (upper panel) and the simulated one (lower
panel) as well as overlaid optimized model showing the covalent-
ly-linked dimer in (e).
To gain insight into the chemical structures of the “U”-
shaped molecules, DFT calculations have been performed.
First, we considered the case where the chemical structure of
the “U”-shaped molecules matched the one from molecules 1
or 2, and subsequently built the corresponding models on
Ag(111). By comparing the simulated STM images of mole-
cules 1 and 2 (Figure S6) to the STM contrast of the “U”-
shaped molecule, we find that neither of them correspond with
the experimentally observed molecular configuration. There-
fore, we ruled out these two candidates.
To further support our assumption on assigning the “U”-
shaped molecule to molecule 3, we have also ex-situ synthe-
sized molecule 3 and deposited it on Ag(111). High resolution
STM images (Figure S10) revealed that both the size and
single–molecule configuration of the ex-situ synthesized mol-
ecule 3 are in good agreement with those of the “U”-shaped
molecule on Ag(111). In view of such theoretical and experi-
mental evidence, we confirm that the “U”-shaped molecules
can be attributed to molecule 3. Interestingly, the assignment
of “U”-shaped molecule to molecule 3 strongly implies that
we have effectively inhibited the intermolecular C-C coupling
and successfully achieved the selective C-H coupling after
dehalogenation by deposition of DN molecules on Ag(111)
with post anneal at 380 K. Moreover, such newly-formed C-H
bonds remain intact upon further annealing to 480 K for 30
minutes (see Figure 3e). In this case, the C-H bonds close to
oxygen atoms are activated, and finally molecule 4 (see
Scheme 1) are synthesized, as verified by the good agreement
between the experimentally morphology observed by STM
Alternatively, a detailed inspection revealed that the “U”-
shaped molecule looks similar to molecule 1 but with shorter
wings. Therefore, we believe that the “U”-shaped molecules
could be assigned either to a dehalogenated molecule 1 with
radicals at the tails or to molecule 3. It is important to note that
both possibilities could result in the shorter wings visible in
the STM images. Therefore, we built up two models based on
dehalogenated molecule 1 with carbon radicals at its tail (Fig-
ure S7) and molecule 3 (see Figure S8 and Figure 3b), respec-
tively. After relaxation, we find that the dehalogenated mole-
cule 1 with carbon radicals strongly interacts with the Ag(111)
surface by forming C-Ag bonds and thus resulting in a bent
adsorption configuration. In this case, neither the optimized
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and the optimized model and simulated STM image as shown
in Figure 3f (also shown in Figure S11).
After deposition of DN on Au(111), XPS quantification of
the DN/Au(111) sample yields a surface elemental composi-
tion of 84 At% (carbon), 8 At% (oxygen) and 8 At% (bro-
mine), which is consistent with the stoichiometry of intact DN
molecules on the substrate. The spin-orbit split Br 3d doublet
corresponding to the RT deposited DN molecules shows two
peaks at binding energies of Eb(Br 3d 5/2)=69.4 eV and Eb(Br
3d 3/2)=70.4 eV which are in good agreement with values
commonly associated with C-Br bonds43-45 and further verify
our STM results shown in Figure 1a. After annealing at 380 K
the intensity of the Br 3d line is significantly reduced, while
the position of the two peaks remains at the same location.
After further annealing to 480K, the contribution at a higher
binding energy is now absent, and sparse additional intensity
appearing around Eb ≈ 67.9 eV is observed. This is commonly
attributed to Br atoms adsorbed on Au(111).44,46 Thus, it is
reasonable to conclude that the complete debromination of DN
molecules occurred at around 480 K and that subsequent de-
sorption of Br atoms through formation of Br2 occurs although
some Br atoms remain physisorbed on Au(111).
To further understand the unexpected C-H coupling of DN
molecules on Ag(111), we have ex-situ synthesized molecule
1 and subsequently performed a control experiment on the
Ag(111) surface. Since molecule 1 is the product of DN mole-
cules after dehydration, we were interested in avoiding the
influence of by-products on the C-C or C-H coupling. After
depositing molecule 1 on Ag(111) and annealing the sample at
380 K for 30 minutes, we found that C-Br bonds are cleaved
as expected. It was surprising to note that C-C or C-Ag cou-
pling rather than C-H coupling occurs after dehalogenation. In
this case, organometallic chains or five-member rings are
synthesized (Figure S12). Since the only difference between
molecule 1 and DN molecule is the availability of by-products
on Ag(111), we reach the conclusion that these are the source
of the hydrogen atoms that play important roles on the selec-
tive C-H coupling of DN molecules and the selective for-
mation of molecule 3.
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The corresponding O 1s spectra depicted in Figure 4b also
shows a distinct variation as a result of the stepwise thermal
treatment. The spectrum corresponding to the as deposited DN
molecules can be fitted with a single peak at Eb[OH](O 1s) =
532.7 eV. Such a contribution can be assigned to the oxygen
in the hydroxyl group of the naphthol units (C-OH).47-49 After
thermal annealing above 380 K, we observe that the peak is
shifted by ∆Eb= 0.4 eV to Eb [C-O-C](O 1s) = 533.1 eV, and that
the full width at half maximum (FWHM) remains unchanged.
The position of O 1s peak appears at a slightly lower binding
energy than previously reported values for furan on Ag(110).
Moreover, such a position is quite similar to values reported
for thin films of polyfuran or adsorption of furan with sub-
monolayer coverage on other substrates.50-54 Consequently, we
ascribe this spectroscopic observation to the formation of a
cyclic enol ether, i.e., furan, which is consistent with the STM
results where molecules 1 or 2 are observed. After annealing
at 480 K we do not observe a shift of the position of the O 1s
peak despite its decreased intensity. This fact indicates that no
further chemical reactions occurred to the oxygenated func-
tional group, while the decreased intensity points toward a
decrease of the surface coverage. The analysis and shape of
the C 1s spectrum are also consistent with a dehalogenation
and C-C coupling reaction (Figure S13).
On the other hand, the XPS data for the DN molecules on
Ag(111) is depicted in Figure 4c and d. The Br 3d spectrum
corresponding to the self-assembly of DN molecules on this
substrate (Figure 4c) shows a spin-orbit split doublet at
70.2/71.2 eV. This feature strongly indicates that the C-Br
bonds remain intact after the on-surface adsorption. After
thermal annealing at 380 K, a doublet with nearly the same
intensity appears at 69.0/68.0 eV, while the signal at the pre-
vious position where the peaks were observed is now absent.
This indicates a complete C-Br bond cleavage of DN mole-
cules on Ag(111) at 380 K and subsequent chemisorption of
bromine.55 Successive annealing at 480 K leads to a slight
decrease of the Br 3d signal but no significant peak shift is
observed.
Figure 4 XP spectra and analysis of the Br 3d (a) and O 1s (b)
peaks of DN molecules adsorbed on Au(111), as well as Br 3d (c)
and O 1s (d) peaks on Ag(111). The corresponding annealing
temperature for each sample preparation is indicated.
XPS analysis on the reaction processes of DN molecules
on Au(111) and Ag(111). To analyze the reaction process of
DN molecules on Au(111) and Ag(111), and more importantly,
unravel the role of OH groups on the C-H coupling of DN
molecules on Ag(111), additional core-level XPS experiments
have been carried out. The sample temperature dependent
evolution of the Br 3d and O 1s XPS core-level spectra of DN
on Au(111) and Ag(111) are shown in Figure 4.
The O 1s region depicted in Figure 4d shows that next to the
hydroxyl-related component at Eb[OH](O 1s)=532.6 eV, a sec-
ond peak at Eb[O-](O 1s)=530.5 eV is already visible at RT on
Ag(111). The second peak at a lower binding energy could be
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assigned to the O atom in a phenolate (C–O−) group indicating
cules. According to our XPS experiments, the key to facilitate
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a partial dehydrogenation of the DN molecule already occurs
upon adsorption.7,47,49 In combination with the DFT calculated
model shown in Figure S5, it is likely that the OH groups
closer to the surface may be dehydrogenated, probably due to
the possible non-planar adsorption configurations of the DN
molecules. In the proposed reaction mechanism, the hydrogen
atoms of both OH groups are designated to terminate the car-
bon radicals after cleavage of C-Br bonds. It should be noted
that a similar substrate-mediated hydrogen transfer has also
been proposed for a hydroxyphenyl-substituted porphyrin.56
Furthermore, though atomic hydrogen can undergo re-
combinative desorption from a bare Ag(111) substrate, it has
been clearly demonstrated that the specific interaction between
the surface hydrogen and phenoxy entity retains the hydrogen
atom far above the normal desorption temperature in the case
of phenol adsorbed on Al(111).57,58 Such a specific molecule-
substrate interaction could also stabilize the hydrogen atom in
our case.
the unpredictable C-H coupling seems to be the simultaneous
dehalogenation reaction and formation of furan rings so that
the hydrogen atoms produced during the formation of furan
rings can directly transfer to the carbon radicals formed due to
dehalogenation. These findings extend the on-surface reac-
tions toolbox for the bottom-up fabrication of organic nano-
materials, allowing the precise synthesis of small organic
molecules by the combination of their rational design and the
selection of substrates.
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ASSOCIATED CONTENT
Supporting Information. The Supporting Information is availa-
ble free of charge on the ACS Publications website.
AUTHOR INFORMATION
Corresponding Author
gaoh@uni-muenster.de, weiliu@njust.edu.cn, fuchsh@uni-
muenster.de.
After annealing at 380 K the peak at higher binding energy
shifts to Eb[C-O-C](O 1s) = 533.7 eV. Such a behavior is in very
good agreement with values reported for furan adsorbed on
Ag(110) and polyfuran films.50,54 Therefore, we ascribe this
peak to the oxygen atoms in the furan units after their for-
mation. This strongly implies that dehalogenation reaction of
DN molecules occurred simultaneously with the formation of
the furan ring. Subsequently, the hydrogen atoms (those gen-
erated from the thermal induced dissociation of the OH groups
as well as those generated already after adsorption) are likely
transferred to the carbon radical and form molecule 3. The
behavior of the second O 1s peak located at a lower binding
energy is initially less unambiguous, especially since the
FWHM is now significantly broader. In this case the peak
shifts to a lower binding energy of Eb(O 1s) = 530.1 eV and
after further annealing at 480 K back up to Eb(O 1s) = 530.3
eV. However, the O 1s peak at Eb[C-O-C](O 1s) = 533.7 eV does
not change its position. This strongly indicates that several
oxygen species contribute to the spectroscopic signal at lower
binding energies. It is likely that the shape of this peak origi-
nates from a coexistence of another DN species and oxygen
atoms adsorbed on the silver substrate, which are known to
contribute to the photoelectron intensity at Eb ≈530.0 eV.59
By another DN species we refer, for example, to the adsorp-
tion configurations depicted in Figure S5, where two C-
O- groups are bound to substrate and thus the formation of
furan ring is inhibited. The shift back to slightly higher bind-
ing energies could then be the result of the change in the ratio
between such two components. At the higher annealing tem-
perature of 480 K, oxygen atoms might desorb from the
Ag(111) surface, while the phenolate related signal remains
unchanged. For a more detailed analysis (including the C 1s
peak) see the supporting information (Figure S14). Finally, it
is important to note that our XPS results strongly support that
the final dehydrogenation reaction occurred simultaneously
with the formation of the furan ring at 380 K, resulting in the
formation of molecule 3 on Ag(111).
ACKNOWLEDGMENT
The authors acknowledge the financial supports from the Re-
search Fund for Postdoctoral Program of Jiangsu Province
(AD41606), Science and Technology Program of Jiangsu Prov-
ince (BK20151484 and AD41572). We also thank the Deutsche
Forschungsgemeinschaft (SFB 858 and TRR 61) for financial
support.
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