On-Surface Dehydro-Diels-Alder Reaction of Dibromo-bis(phenylethynyl)benzene

Article abstract of DOI:10.1021/jacs.9b11755

On-surface synthesis under ultrahigh vacuum conditions is a powerful tool to achieve molecular structures that cannot be accessed via traditional wet chemistry. Nevertheless, only a very limited number of chemical reactions out of the wide variety known f

Full text of DOI:10.1021/jacs.9b11755

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Communication
On-surface dehydro-Diels-Alder reaction
of dibromo-bis(phenylethynyl)benzene
Marco Di Giovannantonio, Ashok Keerthi, José I. Urgel, Martin Baumgarten,
Xinliang Feng, Pascal Ruffieux, Akimitsu Narita, Roman Fasel, and Klaus Müllen
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Jan 2020
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Journal of the American Chemical Society
On-surface dehydro-Diels-Alder reaction of dibromo-
bis(phenylethynyl)benzene
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Marco Di Giovannantonio, Ashok Keerthi,
José I. Urgel, Martin Baumgarten, Xinliang
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1,6,*
2,*
Feng, Pascal Ruffieux, Akimitsu Narita, Roman Fasel, Klaus Müllen
1Empa, Swiss Federal Laboratories for Materials Science and Technology, nanotech@surfaces Laboratory, 8600
Dübendorf, Switzerland
2Max Planck Institute for Polymer Research, 55128 Mainz, Germany
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3Department of Chemistry, The University of Manchester, Manchester M13 9PL, UK
4Center for Advancing Electronics Dresden, Department of Chemistry and Food Chemistry, Technische Universität
Dresden, 01062, Dresden, Germany
5Okinawa Institute of Science and Technology Graduate University, Okinawa 904-0495, Japan
6Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland
Supporting Information Placeholder
six-membered rings.11,12 An alternative version of the Diels-
Alder reaction is the dehydro-Diels-Alder (DDA) reaction
where one or both double bonds in the diene component are
ABSTRACT: On-surface synthesis under ultrahigh vacuum
conditions is a powerful tool to achieve molecular structures
which cannot be accessed via traditional wet chemistry.
Nevertheless, only a very limited number of chemical reac-
tions out of the wide variety known from solution chemistry
have been reported to proceed readily on atomically flat
substrates. Cycloadditions are a class of reactions that are
13,14
replaced by a triple bond (Scheme S1).
Another distinction
of the DDA reaction from the former is that, in most of the
studied cases, it proceeds via multiple steps by formation of
cyclic allenes and migration of hydrogens.
2
particularly important in the synthesis of sp -hybridized
As a complement to conventional synthetic organic chem-
istry, on-surface synthesis on metal substrates under ultra-
high vacuum conditions has recently emerged as a strong
method to achieve novel PAHs that have not been obtained
in solution, including e.g. unstable higher acenes, up to un-
carbon-based nanostructures. Here, we report on a specific
type of [4+2] cycloaddition, namely a dehydro-Diels-Alder
(
DDA) reaction, performed between bis(phenylethynyl)-
benzene precursors on Au(111). Unlike a Diels-Alder reaction,
DDA exploits ethynyl groups to achieve the formation of an
extra six-membered ring. Despite its extensive use in solu-
tion chemistry for more than a century, this reaction has
never been reported to occur on surfaces. The specific choice
of our precursor molecule has led to the successful synthesis
of benzo- and naphtho-fused tetracene and heptacene prod-
ucts bearing styryl groups, as confirmed by scanning tunnel-
ing microscopy and noncontact atomic force microscopy.
The two products arise from DDA dimerization and trimeri-
zation of the precursor molecules, respectively, and their
observation opens perspectives to use DDA reactions as a
novel on-surface synthesis tool.
1
,10,15
decacene.
Several chemical reactions traditionally used in
solution chemistry have been proven to successfully proceed
1
6–20
also on surfaces.
One of the most utilized reactions is the
21,22
Ullmann-like coupling,
or more generally aryl-aryl cou-
pling, which afforded various conjugated polymers as well as
two-dimensional covalent organic frameworks on surfaces.
This reaction was also combined with intramolecular cy-
clodehydrogenation to yield atomically precise graphene
nanoribbons with varying widths and edge structures, reveal-
ing highly intriguing structure-dependent electronic proper-
23–26
ties.
Scheme 1. On-surface reaction of 1,4-dibromo-2,5-
bis(phenylethynyl)benzene (1) and obtained dimer
and trimer products.
Polycyclic aromatic hydrocarbons (PAHs) are disc-type
conjugated molecules consisting of fused benzene rings,
which occur in various shapes and sizes, including cor-
onenes, acenes, and larger PAHs that can be regarded as
1
–3
nanographenes. These materials possess intriguing proper-
ties that make them appealing for applications in organic
4
5
electronics, as well as for more fundamental studies. The
preparation of PAHs is typically carried out through “planar-
ization” of polyphenylene precursors by photochemical or
oxidative cyclodehydrogenation or via metal-catalysed or
6
–9
oxidative annulation of tailor-made building blocks.
Cy-
cloaddition reactions are widely used in the synthesis of large
10
PAHs. In this regard, a specific class of [4+2] cycloaddition,
namely Diels-Alder reaction where cycloaddition occurs
between diene and dienophile, has served as an extremely
versatile synthetic protocol to achieve the formation of extra
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To study the coupling of precursor 1, we deposited it onto
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an Au(111) surface held at room temperature (RT) in ultra-
high vacuum (UHV). Figure 1a shows an overview STM image
of the resulting sample, which consists of intact molecules
self-assembled into chains. Halogen-hydrogen interactions
are most likely stabilizing this assembly (see Figure S1) which
remains unaltered upon annealing up to 100 ºC. This observa-
tion is in agreement with previous studies where the C-Br
bond was found to be stable up to this temperature on
3
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Au(111). After annealing of the sample to 150 ºC, a more
disordered assembly of the molecular units appears from the
corresponding STM image (Figure 1b). However, some regu-
lar moieties are also visible (Figure 1f). The presence of round
protrusions between the molecular units suggests the pres-
ence of an organometallic structure, as sketched in Figure 1j
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7,38
and S1.
Indeed, the measured C-Au distance of 2.4 ± 0.2 Å
agrees with previously reported values for organometallic
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9,40
bonding.
The first step toward the expected aryl-aryl
22
coupling is therefore almost complete at this temperature
(
some of the bromine atoms are still covalently attached to
the precursors, red arrows in Figure 1f).
Annealing the sample to 200 ºC produces further modifica-
tions in the molecular assemblies (Figure 1c,g). The overall
appearance of the surface is characterized by large, ill-
defined structures, probably originating from aggregation as
well as non-selective coupling reactions (e.g. CH activation
and lateral fusion) leading to ill-defined oligomers. The for-
mation of the targeted polymer illustrated in Scheme 1, po-
tentially leading to 11-AGNRs, appears to be prevented by
steric hindrance between hydrogens (highlighted in red in
Scheme 1) and/or unexpected reactions involving the triple
bonds that might be activated on the gold surface at 200 °C
(vide infra). Nevertheless, we also observe molecular objects
which can be assigned to dimers and trimers formed from
precursor 1. They possess one or two out-of-plane phenyl
rings (green arrows in Figure 1g,k) which are tilted due to the
steric hindrance against the planar aromatic cores.
Figure 1. (a-d) STM images of the Au(111) surface after depo-
sition of precursor 1 at RT followed by repeated annealing
steps at the indicated temperatures. (e-h) Magnified STM
images at different reaction steps. (i-l) Molecular schemes of
the structures observed in the STM images. Scanning param-
eters: (a,e) I
t
=70 pA, V
b
=0.2 V; (b) I
t
=30 pA, V
b
=0.3 V; (c,g)
=0.1 V.
I
t
=30 pA, V =1 V; (d) I
b
t
=10 pA, V
b
=1 V; (f,h) I
t
=70 pA, V
b
Ullmann-like coupling reactions obviously also encounter
limitations in achieving more complex architectures. For
instance, only single C-C bonds can be created, while some-
times the formation of an extra ring would be desirable with-
27,28
out having to refer to cyclodehydrogenation.
Hence,
cycloaddition reactions have appeared as a valid on-surface
chemistry tool to expand the spectrum of attainable chemical
29–34
structures.
Despite its extensive use in synthetic organic chemistry,
the DDA reaction has not been investigated for on-surface
processes. During our attempts to synthesize eleven atoms
wide armchair GNRs (11-AGNRs) on Au(111) from 1,4-
35
dibromo-2,5-bis(phenylethynyl)benzene (1) as a precursor,
we have observed the formation of dimer- and trimer-like
molecules instead of 11-AGNRs (Scheme 1). Here, we demon-
strate the dimerization and trimerization of 1, forming ben-
zo[fg]naphtho[1,2,3-op]tetracene and acenaphtho[4,3,2,1-
a1b1c1d1]dibenzo[fg,uv]naphtho[3,2,1-lm]heptacene, respec-
tively, through unprecedented on-surface DDA reactions
Figure 2. STM images of dimer (a) and trimer (d) products
observed after annealing precursor 1 to 300 °C on Au(111).
b,e) Laplace-filtered images of constant-height frequency-
shift nc-AFM images performed with CO-functionalized tip.
See Figure S2 for the raw data. (c,f) Chemical structures of
(
(
Scheme 1). The precise chemical structures of the dimer and
trimer products are revealed by high-resolution scanning
tunneling microscopy (STM) and noncontact atomic force
microscopy (nc-AFM). We discuss a plausible mechanism for
the formation of the observed products via surface-assisted
DDA cycloadditions.
the observed products. Scanning parameters: (a) I
t
=70 pA,
V
b
=0.1 V; (d) I =100 pA, V =–0.02 V.
t
b
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Scheme 2. Proposed reaction pathway on Au(111), based on the observed intermediates and final products. Pre-
cursor 1 adsorbs intactly on the substrate held at RT and up to 100 °C. (a) Annealing of the surface to 150 °C
yields organometallic structures (T1). (b) Subsequent heating to 200 °C activates a dehydro-Diels-Alder cycload-
dition reaction toward the trimer T2. (c) At 300 °C, sequential cyclodehydrogenation and reduction of ethynyl to
vinyl take place to form T3 and T4.
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Some of the molecules reach a (nearly) planar confor-
mation after annealing the sample to 300 ºC (Figure 1d). We
identify two molecular products that are selectively formed
with apparently well-defined chemical structures (Figure 1h).
Bond-resolved nc-AFM imaging unambiguously elucidates
their structures to consist of 44 and 66 carbon atoms, having
a benzo- and naphtho-fused tetracene and dibenzo- and
dinaphtho-fused heptacene backbones, respectively, which
extends laterally with styryl pendent groups (Figure 2). These
two products originate from the covalent coupling of two
and three monomers, respectively, as sketched with different
colors in Figure 2c,f.
moieties (see Figure S3 for details). Finally, the steric hin-
drance between neighboring hydrogens at the cove-edges
can result in the occasional formation of five-member rings
via intramolecular cyclodehydrogenation, leading to e.g.
acenaphtho[4,3,2,1-a1b1c1d1]dibenzo[fg,uv]naphtho[3,2,1-
lm]heptacene T4 (experimentally imaged).
4
3
Products larger than trimers appear as non-regular by STM
imaging. We ascribe them to lateral fusion via non-selective
CH activation. Here, the DDA reaction between activated
precursors 1 can afford products up to trimers leading to an
heptacene backbone. This behavior is tentatively attributed
to the necessity of simultaneous coupling of the radical posi-
tions and cycloadditions involving the ethynyl groups, which
makes coupling of more than three monomers impossible,
thus yielding only dimers and trimers.
Statistical analysis of ~350 molecules on the surface re-
vealed that 9% of them form dimeric products along with
12% of trimeric products. The remaining 79% of the mono-
mers form side products and larger assemblies, which we
could not clearly identify because of their increased non-
planarity. However, we tentatively ascribe the side products
to a non-complete DDA reaction or to different reactions
involving ethynyl groups of two or more monomers. On the
other hand, the larger assemblies could be aggregates of the
products as well as covalently bonded oligomers/polymers
originating from non-selective intermolecular couplings. The
out-of-plane parts of the tetracene and heptacene cores (blue
arrows in Figure 2) are due to steric repulsion between
neighboring hydrogens. In contrast, a five-membered ring is
observed on the opposite side of the heptacene backbone
In conclusion, we have reported DDA reactions between
bis(phenylethynyl)benzene precursors on an Au(111) surface.
Detailed structural characterization of the products at differ-
ent temperatures reveals the presence of gold-mediated
organometallic assemblies as reaction intermediates. The
adsorbates subsequently undergo DDA coupling at 200 ºC,
and are finally converted into planar structures at 300 ºC.
Heptacene and tetracene core structures are observed on the
surface, which is most likely because a simultaneous activa-
tion to the radical and the triple bond is required for this on-
surface reaction. Our findings demonstrate that the DDA
reaction occurs also in two-dimensional space, thus provid-
ing an additional tool to the field of on-surface synthesis.
(
red arrows in Figure 2). In this case, the steric overlap be-
tween hydrogens at a cove-edge position promoted surface-
4
1
assisted cyclodehydrogenation, as previously reported.
ASSOCIATED CONTENT
Supporting Information
Sequential reaction steps leading to the formation of the
observed trimer are proposed in Scheme 2 (see Scheme S3 for
the dimer). After debromination of 1 on Au(111), organome-
tallic structures are observed, forming short chains on the
surface (T1). Upon further annealing, covalent coupling of
three activated monomers takes place with two-fold [4+2]
cycloadditions for each trimer, which we ascribe to a DDA
reaction towards dibenzo[a,h]naphtho[2,3-c]pentaphene T2.
Annealing the sample at 300 ºC furnishes diben-
zo[a1b1,lm]dinaphtho[1,2,3-fg:1',2',3'-uv]heptacene T3 via the
cyclodehydrogenation of out-of-plane phenyls and the reduc-
The Supporting Information is available free of charge on the
ACS Publications website. Experimental methods; additional
reaction schemes and experimental data (PDF).
AUTHOR INFORMATION
Corresponding Author
*
*
roman.fasel@empa.ch
muellen@mpip-mainz.mpg.de
4
2
tion of ethynyl groups at the core. We speculate that the
availability of atomic hydrogen on the Au surface might have
led to reduction of the pendent ethynyl groups into vinyl
Author Contributions
#These authors contributed equally.
3
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Notes
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The authors declare no competing financial interests.
(
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Di Giovannantonio, M.; Contini, G. Reversibility and Interme-
This work was supported by the Swiss National Science
Foundation under Grant No. 200020_182015, the European
Union’s Horizon 2020 research and innovation program
under grant agreement number 785219 (Graphene Flagship
Core 2), the Office of Naval Research (N00014-18-1-2708), and
the Max Planck Society. AK thanks the Ramsay Memorial
Fellowships Trust for the award of fellowship. Lukas Rotach
is acknowledged for excellent technical support during the
experiments.
diate Steps as Key Tools for the Growth of Extended Ordered Polymers
via On-Surface Synthesis. J. Phys. Condens. Matter 2018, 30, 093001.
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Commun. 2017, 53 (56), 7872–7885.
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