Higher Acenes by On-Surface Dehydrogenation: From Heptacene to Undecacene

Article abstract of DOI:10.1002/anie.201802040

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on-surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on-surface annealing. The structure of the generated acenes has been visualized by high-resolution non-contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

Full text of DOI:10.1002/anie.201802040

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Higher Acenes by On-Surface Dehydrogenation: From
Heptacene to Undecacene
Authors: Antonio M. Echavarren, Rafal Zuzak, Ruth Dorel, Marek
Kolmer, Marek Szymonski, and Szymon Godlewski
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802040
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10.1002/anie.201802040
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Higher Acenes by On-Surface Dehydrogenation: From Heptacene
to Undecacene
Rafal Zuzak,[a],+ Ruth Dorel,[b],+ Marek Kolmer,[a] Marek Szymonski,[a] Szymon Godlewski,*[a] and Antonio
M. Echavarren*[b,c]
Abstract: A unified approach to the synthesis of the series of higher
acenes up to previously unreported undecacene has been developed
through the on-surface dehydrogenation of partially saturated
precursors. These molecules could be converted into the parent
acenes by both atomic manipulation with the tip of a scanning
tunneling and atomic force microscope (STM/AFM) as well as by on-
surface annealing. The structure of the generated acenes has been
visualized by high resolution non-contact AFM imaging and the
evolution of the transport gap with the increase of the number of fused
benzene rings has been determined on the basis of scanning
tunneling spectroscopy (STS) measurements.
than 10 annulated rings,[ 7 ] yielding interesting magnetic and
electronic properties and therefore making their synthesis and
characterization quite appealing.
A common strategy to circumvent the intrinsic instability of higher
acenes is the use of substituents directly attached to the aromatic
core,[8] which has allowed the preparation of several stabilized
derivatives of higher acenes up to nonacene.[9] In contrast, the
synthesis and study of the non-functionalized hydrocarbons have
10
]
constituted an enormous challenge until recently.[
The
photodecarbonylation of stable α-(diketo)precursors in solid
matrices of noble gases allowed the generation of higher acenes
]
]
up to nonacene.[ 11 Furthermore, both hexacene[ 12 and
heptacene[13] could also be isolated in bulk by thermolysis of the
corresponding carbonyl-bridged adduct and dimers, respectively.
Acenes are a family of polycyclic aromatic hydrocarbons (PAHs)
constituted by fused benzene rings disposed in a straight linear
arrangement. They have captivated scientists for years on
account of their unique properties,[1] which make them attractive
for use in molecular electronics.[2] Furthermore, acenes can be
also regarded as the narrowest graphene nanoribbons with
zigzag edges, and therefore have potential applications in
spintronics[3] and plasmonics.[4] The properties of acenes strongly
depend on their size, thus expecting for the largest members of
the series higher charge carrier mobilities, lower reorganization
energies, and smaller HOMO-LUMO (highest occupied molecular
The surface-assisted synthesis performed in ultra-high vacuum
(UHV) conditions constitutes a powerful complementary tool to
solution chemistry, thus enabling the preparation and stabilization
of intrinsically unstable compounds. This approach pioneered in
2000 by Hla and coworkers[14] proved to be successful in the
atomically precise generation of a range of covalent molecular
architectures,[15] and more recently in the synthesis of new elusive
compounds.[16] Furthermore, the development of new scanning
tunneling and atomic force microscopy (STM/AFM) methods has
not only allowed for detailed mapping of the molecule electronic
structure,[17] but more importantly made the detailed structural
imaging feasible providing an unprecedented resolution and
precision.[18] The approach based on UHV deposition of stable
5
]
orbital
–
lowest unoccupied molecular orbital) gaps.[2c,
Nonetheless, the reactivity and therefore the instability of acenes
also increases as the number of fused rings grows, which can be
easily understood invoking the Clar’s aromatic sextet rule.[6] On
the other hand, theoretical calculations predict that the
contribution of the open-shell singlet state to the electronic ground
state configuration shall rapidly grow for acenes containing more
derivatives on
a
crystalline substrate and subsequent
transformation into target molecules has been successfully
applied for synthesis of some members of the acene series. A
pioneer example was described in 2013 for the synthesis of
pentacene from tetrathienoanthracene on a Ni(111) surface.[19] In
[a]
[b]
R. Zuzak,+ Dr. M. Kolmer, Prof. M. Szymonski, Dr. S. Godlewski
Centre for Nanometer-Scale Science and Advanced Materials,
NANOSAM, Faculty of Physics, Astronomy and Applied Computer
Science, Jagiellonian University, Łojasiewicza 11, PL 30-348 Kraków,
Poland
20
]
2017 both heptacene[
and heptacene organometallic
complexes[ 21 ] were generated from α-diketone precursors on
Ag(111) and Au(111), respectively. Furthermore, epoxyacenes
have been used over the last two years to generate tetracene on
Cu(111),[22] and hexacene[23] and decacene[24] on Au(111).
E-mail: szymon.godlewski@uj.edu.pl
Dr.
R.
Dorel,+
Prof.
A.
M.
Echavarren
Institute of Chemical Research of Catalonia (ICIQ), Barcelona
Institute of Science and Technology
Hydroacenes – partially saturated acenes – were already
described as “hydrogen-protected acenes” more than one decade
ago.[25] However, their use as direct precursors of the conjugated
systems has been typically circumscribed to the preparation of
acenes with up to five linearly fused benzene rings. We recently
developed a general method for the synthesis of hydroacenes
based on the gold(I)-catalyzed cyclization of the aryl-tethered 1,7-
enynes,[26] and successfully used tetrahydrononacene, one of the
resulting partially saturated molecules, as a stable precursor for
Av.
Països
Catalans
16,
43007
Tarragona,
Spain
E-mail: aechavarren@iciq.es
[c]
[+]
Prof. A. M. Echavarren
Departament de Química Orgànica i Analítica, Universitat Rovira i
Virgil
C/ Marcel·lí Domingo s/n, 43007 Tarragona, Spain
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201802040
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the synthesis and study of the properties of nonacene on a
Au(111) surface.[27] On the basis of these results, we anticipated
the development of a unified approach for the preparation and
study of higher acenes on metallic surfaces, which could be
extended for the synthesis of yet unknown even higher acenes.
Herein we report the on-surface synthesis of the complete series
from heptacene (2) up to previously unknown undecacene (6)
(1e) exhibit a solubility comparable to their kinked counterparts
1d’ and 1e’ in a range or organic solvents, and consequently
could not be isolated as single isomers. Therefore, mixtures of
1d/1d’ and 1e/1e’ were used in this study for the on-surface
synthesis of the corresponding acenes.[28]
I
through
the
dehydrogenation
of
the
corresponding
-
+ SbF6
+
tBu
tBu
tetrahydroacene precursors 1 (Scheme 1). In order to confirm the
generation of intrinsically unstable higher acenes, high resolution
nc-AFM imaging with a CO functionalized tip was applied,[18a]
which provided doubtless structural and chemical identification of
the synthesized compounds. The generation of the acene series
was complemented by scanning tunneling spectroscopy (STS)
measurements, which revealed the resonances associated with
HOMO and LUMO orbitals and therefore allowed us to quantify
the transport electronic gap evolution upon extension of the
molecule board for acenes physisorbed on Au(111).
n
P
Au NCMe
I
7a: n=0
7b: n=1
7c: n=2
OMe
A
Sonogashira PdCl2(PPh3)2
coupling CuI, Et3N
OMe
n
n’
OMe
n’
8
H
H
H
H
H
Au(I)-catalyzed
A, CH2Cl2
cyclization
n
n’
H
H
H
Acene precursors 1
1’
+
1
on-surface
dehydrogenation
n
Au(111)
Scheme 2. Synthesis of tetrhydroacenes 1.
Heptacene (2): n=n’=0
Octacene (3): n=1, n’=0
With the tetrahydroacenes 1a-b and 1d-e in hand, we next turned
our attention to their application as precursors of the acene series
by on-surface dehydrogenation following a procedure related to
the one we previously developed for the preparation of nonacene
from 1c.[27] In general, parent acenes could be generated either
thermally by annealing of the sample to 520 K for 10 minutes or
by the application of the microscope tip.[28] The latter process
could be performed locally with the tip apex located over the non-
aromatic ring bearing two methylene groups. Increasing the
applied bias voltage results in a dehydrogenation process
accompanied with the aromatization and planarization of the
molecular fragment subjected to manipulation. The threshold
voltage for the dehydrogenation depends on the actual structure
of the molecule.[27] However, we found that raising the bias
voltage to 2.5 V was sufficient to carry out the dehydrogenation of
all the investigated hydroacene compounds 1. Furthermore, the
tip-induced dehydrogenation process could also be achieved in a
slightly different manner with the tip scanning across the surface
at elevated bias voltage, which was found to be a competitive
alternative to the thermal treatment and gave rise to the
dehydrogenation of the majority of the physisorbed molecules.
Nonetheless, the results herein presented mainly focus on the
thermal treatment protocol due to its high efficiency in the
simultaneous dehydrogenation of an array of precursors. The tip-
induced approach was also tested to confirm its applicability and
is demonstrated for the synthesis of undecacene.[28]
Nonacene (4): n=n’=1 [ref. 27]
Decacene (5): n=2, n’=1
Undecacene (6): n=2, n’=2
Scheme 1. Unified approach to higher acenes by on-surface dehydrogenation
of hydroacenes.
The synthesis of the tetrahydroacene precursors 1 was based on
the double gold(I)-catalyzed cyclization of dienynes 8,[26] which
were assembled through the double Sonogashira cross-coupling
between 1,4-diiodobenzene and terminal alkynes 7 (Scheme
2).[28] The double cyclization of 8 may give rise to the desired
acene precursors 1 and their kinked isomers 1’ depending on the
regioselectivity of the second cyclization. In the case of the
cyclization of 8a (n =n’ = 0), 8b (n = 1, n’ = 0), and 8c (n = n’ = 1),
the linear cyclization products 1a-c – precursors of hepta- (2),
octa- (3), and nonacene (4), respectively – can be easily
separated from 1a-c’ due to their differences in solubility. In
contrast, tetrahydrodecacene (1d) and tetrahydroundecacene
Figure 1 illustrates the on-surface generation of higher acenes.
Submonolayer coverage of 1a, 1b and 1d/1d’ was sublimed onto
the Au(111) surface. Figure 1a shows the typical STM overview
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with mostly intact tetrahydroacene species, which are recognized
in empty state images by their characteristic appearance with two
pronounced lobes associated with the presence of four methylene
groups per molecule. These lobes are also clearly recognizable
in the STM image profiles shown in Figure 1c. The precursors are
preferentially located in the corners of the surface herringbone
reconstruction similarly to the previously reported pentacene[29]
and nonacene[27] molecules. With the increase of coverage, the
molecules tend to follow the reconstruction pattern settling
themselves mostly in-between the brighter appearing rows of the
surface reconstruction. In STM images precursors 1a, 1b and 1d
could be easily discerned. Whereas intrinsically in all cases the
separation of the two non-aromatic rings is identical and equals
three benzene units located in-between, compound 1a appears
as the shortest with symmetrically located two bright lobes (Figure
1a, violet rectangle). In contrast, precursor 1b appears slightly
longer due to the extension of one terminus from a single benzene
ring to the naphthalene moiety (Figure 1a, green rectangle). This
is also evidenced by the elbow present in the left hand side of the
green cross section presented in Figure 1c. Compound 1d is
found as the longest precursor with clearly visible extended
aromatic fragments observed at both termini of the molecule in
the topographic image (Figure 1a, blue rectangle). This could also
be noted in the blue cross section in Figure 1c. It is noticeable that
apart from acene precursors 1a-b,d, kinked species that we
assign to 1d’ could also be visualized. A small amount of partially
or completely dehydrogenated species (dihydroacenes and
acenes, respectively) could also be found at this stage, as
exemplified by the molecules in the white and red contours in
Figure 1a. After annealing compounds 1 were converted into
acenes as documented in the STM image shown in Figure 1b,
which clearly indicates the disappearance of the bright lobes
attributed to the methylene moieties as previously observed for
nonacene.[27] Vanishing of lobes could also be noted in the purple,
grey and yellow profiles visualized in Figure 1c. Moreover, the
behavior of the molecules during STM imaging differs between
saturated precursors and parent acenes. Whereas some
precursors 1 tend to relocate over the surface as indicated by
white arrows in Figure 1a, a fraction of acenes could be
characterized by a frustrated movement indicated by yellow
arrows in Figure 1b and recorded as a noisy appearance of the
termini, in agreement with the situation previously reported for
nonacene molecules.[27] At first glance one can note by comparing
Figures 1a and 1b that all molecules shown in Figure 1b have
been converted into the corresponding conjugated acenes upon
annealing, thus indicating that the dehydrogenation process is
highly efficient, leading practically to almost 100% conversion rate.
Figure 1. On-surface generation of higher acenes; (a-b) empty state STM
images of the Au(111) surface partially covered with (a) hydrogen protected
acene precursors 1a, 1b, and 1d, and kinked isomer 1d’, and (b) heptacene (2),
octacene (3), decacene (5) and kinked decacene isomer (5’). Precursor
molecules are easily discernible by the presence of two pronounced lobes
corresponding to two non-aromatic rings each containing two methylene groups,
exemplary precursors are marked by rectangles: 1a – violet, 1b – green, 1d –
blue. Parent acenes in (b) are indicated by rectangles: 2 – purple, 3 – grey, 5 –
yellow. Dashed lines in (a) and (b) indicate directions of profile lines shown in
(c); after deposition a small fraction of molecules could be found already partially
(white contour) or very rarely completely dehydrogenated (red rectangle). White
and yellow arrows in (a) and (b) indicate the typical displacement of precursors
and frustrated motion of acenes during STM imaging, respectively. Tunneling
current 30 pA, bias voltage + 2.0 V; (c) profile lines along precursor molecules
and parent acenes clearly showing a difference in their STM appearance and
evidencing the presence of two pronounced lobes in the topographies of the
precursors; (d-h) Laplace filtered constant height, frequency shift nc-AFM
images of 2 (d), 3 (e), 4 (f), 5 (g), and 5’ (h) generated by annealing, scale bar:
5 Å; (i) structural scheme of 5’.
Since simple STM imaging was not sufficient to discern the details
of the atomic structure of adsorbates, we applied high resolution
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nc-AFM measurements with the probe tip functionalized with a
CO molecule in order to doubtlessly prove formation of the series
of parent acenes (Figure 1d-g). Hence, submolecularly resolved
nc-AFM images provided the ultimate insight into the chemical
structure of the generated molecules. Detailed inspection of
constant height frequency shift images clearly indicates the
presence of linearly fused benzene rings with single hydrogen
termination of carbon atoms located in the elbows of the zig-zag
edges, thus confirming formation of unsubstituted acenes.
Additionally, we demonstrate that the bent species could be
ascribed to the presence of kinked decacene isomer (5’) with
clearly discernible pentacene and tetracene moieties annulated at
the angle of 120˚ by the central benzene ring (Figure 1h-i).
both approaches, on-surface annealing and tip induced molecule
manipulation.[28] Figure 2b shows the sample after annealing at
520 K for 10 minutes with clear disappearance of the previously
observed pronounced lobes attributed to the methylene moieties
of 1e and 1e’, thus indicating that the initial molecules have been
dehydrogenated. The high resolution STM image displayed in
Figure 2c clearly indicates the presence of eleven lobes
corresponding to the eleven linearly fused benzene rings of
undecacene (6). In order to provide the ultimate confirmation of
the generation of undecacene we performed high resolution nc-
AFM imaging (Figure 2d). For the sake of completeness, the
structure of the kinked undecacene isomer 6’ is shown in Figure
2e.
The nc-AFM images plainly unveil the planarity of the entire acene
series, which strongly supports relatively weak van-der-Waals
type interactions with the substrate without covalent linkage. This
finding is further supported by the high mobility of the molecules
on the surface, which allows for efficient planar displacement
even at moderate bias voltages and low tunneling currents.[28] This
behavior observed on Au(111) is in contrast to the recently
reported increased interaction of the central part of heptacene
molecules with the Ag(111) surface, which results in a clearly
discernible distortion from the planar configuration.[20] The lack of
covalent bonding with the Au(111) surface makes perspectives
for spatial electronic mapping of nearly undistorted molecular
orbitals,[27] but it also makes detailed imaging more challenging
due to frequent unintentional tip-induced displacements of the
molecules. We found that the immobilization of decacene
molecules was particularly difficult to achieve, which may arise
from their increased length compared to smaller homologs
hindering from location between the elevated surface
reconstruction rows. The above-mentioned location in-between
the surface pattern has been observed preferentially for shorter
acenes. Therefore, the high resolution nc-AFM image of a
decacene molecule was recorded when the organic compound
was immobilized in the vicinity of the surface step, as clearly
visible in the upper part of Figure 1f.
Figure 2. On-surface generation of undecacene (6); empty state STM images
of the Au(111) surface (a) with the mixture of undecacene precursors 1e and
1e‘ and (b) subsequently thermally generated parent 6 and kinked undecacene
isomer 6’. Tunneling current: (a,b) 30 pA, (c) 150 pA, bias voltage + 2.0 V; (c)
high resolution filled state STM image of 6 with clearly visible eleven lobes along
the molecule, tunneling current 30 pA, bias voltage -1.0 V; (d-e) present Laplace
filtered constant height, frequency shift nc-AFM images of 6 (d) and 6’ (e); scale
bar: 5 Å.
After the successful preparation of the above described acene
series, we next embarked on the synthesis of previously
unexplored undecacene, which constitutes the longest acene
described to date, starting from a mixture of 1e/1e’. The situation
occurred to be remarkably different compared to the previously
conducted experiments for shorter acenes. We immediately noted
The synthesis of the whole acene series provided a unique
chance to analyze the dependence of the STS measured
transport gap on the number of fused benzene rings. Figure 3
shows the results obtained in our study, including data previously
reported for pentacene[29] and hexacene.[23] A simple and often
used empirical equation links linearly the gap with the inverse
number of fused benzene rings.[31] It shall be expected that for
longer acenes the exponential decay of the gap with the number
of annulated repeating units should better describe the trend and
the saturation value.[32] However, in the analyzed range of only a
few (5-10) fused benzene units, as indicated in Figure 3a, both
inverse proportionality (red line) and the exponential decay (blue
line) could be well-fitted. In contrast, undecacene shows clear
deviation toward larger gap values, which points towards the
saturation of further band gap lowering for higher acenes. This
trend is even better visualized in Figure 3b, where the gap is
drawn as a function of inversed number of annulated rings and
clearly exhibits deviation from linearity for the longest acenes.
These findings could be rationalized by the anticipated increased
the presence of
a
considerable amount of kinked
tetrahydroundecacene isomers 1e’ along with a significantly lower
amount of linear 1e molecules. In addition, smaller fragments
were also located preferentially in the surface reconstruction
pattern elbows, known for being more reactive. We attribute the
presence of the latter compounds to the deposition of the
fragmented undecacene precursors 1e,[28] which is reasonable if
we take into account the decrease in stability as the number of
annulated rings increases.[30] Similarly, the decreased ratio of
linear precursors to kinked ones could be rationalized considering
the higher stability of the latter. Figure 2a illustrates the Au(111)
sample after molecule deposition with clearly visible 1e and 1e’
species. In order to generate undecacene molecules we applied
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10.1002/anie.201802040
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COMMUNICATION
contribution of the open-shell configuration to the overall
electronic configuration, which according to recent theoretical
studies are particularly important for acenes containing more than
10 rings.[7] We note here that the absolute gap values are affected
by screening effects, which are known to induce
renormalization.[33]
synthetized on a Au(111) surface through the dehydrogenation of
tetrahydroacene precursors. The conjugated systems could be
accessed both by surface annealing and tip-induced
dehydrogenation. Unambiguous proof of their formation was
obtained by high resolution nc-AFM imaging. Furthermore, the
evolution of the transport gap on the surface with the acene length
was established on the basis of STS measurements. This work
therefore showcases the versatility of hydroacenes as hydrogen-
protected precursors of elusive higher acenes when combined
with the on-surface synthesis toolbox, which gives future
perspective for the study of even higher members of this intriguing
family of PAHs.
Acknowledgements
This research was supported by the National Science Center,
Poland (2017/26/E/ST3/00855), Agencia Estatal de Investigación
(CTQ2016-75960-P MINECO/AEI/FEDER, UE), AEI-Severo
Ochoa Excellence Accreditation 2014-2018, SEV-2013-0319),
the European Research Council (Advanced Grant No. 321066),
Figure 3. STS measured transport gap for long acenes on Au(111); (a)
dependence of the gap on the number of fused benzene rings with the inverse
proportionality fit (red curve) and exponential decay (blue curve) for pentacene-
decacene data. Values recorded for kinked isomers are displayed by orange
triangles[28]; (b) transport gap drawn as a function of the inversed number of
rings with the linear fit for pentacene-decacene data; gap values for
pentacene[29] and hexacene[23] are taken from the literature.
the AGAUR ((2017 SGR 1257), and CERCA Program
/
Generalitat de Catalunya. R.Z. acknowledges support received
from the National Science Center, Poland (2017/24/T/ST5/
00262).
Keywords: acenes • undecacene • hydroacenes • on-surface
In summary, the complete series of higher acenes from
heptacene up to previously unreported undecacene has been
synthesis • dehydrogenation
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Entry for the Table of Contents
H
H
H
H
on-surface
dehydrogenation
Au(111)
n
n’
n
n’
H
H
H
H
q
q
q
Higher acenes from heptacene to undecacene
High resolution nc-AFM structural characterization
STS electronic characterization
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