On-Surface Synthesis and Characterization of a Cycloarene: C108 Graphene Ring

Article abstract of DOI:10.1021/jacs.9b10151

The synthesis of cycloarenes in solution is challenging because of their low solubility and the often hindered cyclodehydrogenation reaction of their nonplanar precursors. Using an alternative on-surface synthesis protocol, we achieved an unprecedented double-stranded hexagonal cycloarene containing 108 sp² carbon atoms. Its synthesis is based on hierarchical Ullmann coupling and cyclodehydrogenation of a specially designed precursor on a Au(111) surface. The structure and other properties of the cycloarene are investigated by scanning tunneling microscopy/spectroscopy, atomic force microscopy, and density functional theory calculations.

Full text of DOI:10.1021/jacs.9b10151

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Article
On-Surface Synthesis and Characterization
of a Cycloarene: C108 Graphene Ring
Qitang Fan, Daniel Martin-Jimenez, Simon Werner, Daniel Ebeling, Tabea Koehler, Tobias
Vollgraff, Jörg Sundermeyer, Wolfgang Hieringer, Andre Schirmeisen, and J. Michael Gottfried
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b10151 • Publication Date (Web): 16 Dec 2019
Downloaded from pubs.acs.org on December 16, 2019
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On-Surface Synthesis and Characterization of a Cycloarene: C108 Graphene Ring
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Qitang Fan,1‡ Daniel Martin-Jimenez,2‡ Simon Werner,1‡ Daniel Ebeling,2* Tabea Koehler,1 Tobias
Vollgraff,1 Jörg Sundermeyer,1* Wolfgang Hieringer,3 André Schirmeisen,2 J. Michael Gottfried1*
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1Department of Chemistry and Material Sciences Center (WZMW), Philipps-Universität Marburg, Hans-Meerwein-
Straße, 35032 Marburg, Germany
2Institute of Applied Physics (IAP) and Center for Materials Research (LaMa), Justus-Liebig-Universität Gießen,
Heinrich-Buff-Ring 16, 35392 Gießen, Germany
3Theoretical Chemistry and Interdisciplinary Center for Molecular Materials (ICMM), Department of Chemistry and
Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
ABSTRACT
The synthesis of cycloarenes in solution is challenging because of their low solubility and the often
hindered cyclodehydrogenation reaction of their nonplanar precursors. Using an alternative on-surface
synthesis protocol, we achieved an unprecedented double-stranded hexagonal cycloarene containing 108
sp2 carbon atoms. Its synthesis is based on hierarchical Ullmann coupling and cyclodehydrogenation of a
specially designed precursor on a Au(111) surface. The structure and other properties of the cycloarene
are investigated by scanning tunneling microscopy/spectroscopy, atomic force microscopy, and density
functional theory calculations.
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INTRODUCTION
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Cycloarenes are polycyclic aromatic compounds with a central cavity surrounded by annulated benzene
rings.1 Only few cycloarenes have been synthesized in solution, including kekulene,2 septulene,3
cyclo[d,e,d,e,e,d,e,d,e,e]decakisbenzene,4 and octulene5 with a single strand of benzene rings. More
recently, a hexagonal C216 cycloarene bearing three strands of benzene rings has been synthesized.6
Solution-based cycloarene synthesis is challenging, because intramolecular cyclodehydrogenation
reactions of the nonplanar precursor macrocycles, providing the planar cycloarenes, is often
unsuccessful.1 Additionally, their isolation and characterization remains difficult due to low solubility.
Recently, a versatile alternative intramolecular cyclodehydrogenation reaction on metal surfaces has
been widely used to planarize twisted organic molecules to form fully fused nanographenes.7-16 The
advantage of cyclodehydrogenation on metal surfaces is its reduced barrier and higher selectivity,
compared to undesired intermolecular dehydrogenative coupling. In part, this is due to the release of
torsion strain in the adsorbed non-planar precursors.12-13, 17-18 This approach appears to be feasible for the
synthesis of cycloarenes, which in an alternative view can be reckoned as nanorings cut out of graphene.
Such carbon-based nanorings with delocalized π-electrons may serve as useful candidates for studying
properties of quantum ring systems, e.g., the Aharonov-Bohm effect19-21 and persistent ring currents
(PRCs).22-26 In particular, the much smaller sizes (diameter < 10 nm) of these graphene quantum rings,
compared to the mesoscopic metal rings in previous work (diameters around 0.6 – 2 µm),22, 27 can lead to
larger PRCs at a given low temperature or to PRCs persisting at higher temperatures.27
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Here, we report the first successful on-surface synthesis of a cycloarene. Specifically, the hexagonal
cycloarene C108 (3, Scheme 1) with two strands of annulated benzene rings, containing 108 sp2 carbons
was synthesized by Ullmann coupling and subsequent cyclodehydrogenation of 7,10-dibromo-1,4,11-
triphenyl-triphenylene (DBTT 1, Scheme 1) on a Au(111) surface. DBTT was designed to favor
cyclotrimerization, resulting in the nonplanar trimer macrocycle 2, over polymerization into the chain 4.
This selectivity is achieved by the weaker steric repulsion between the phenyl groups of two adjacent
triphenyl-triphenylene building blocks in the trimer macrocycle 2 than in the polymer chain 4, as
illustrated by the green and red ovals in Scheme 1. The identity of DBTT 1, synthesized by a catalytic
multistep metal-organic reaction protocol, was established as described in the supporting information (SI).
The structure and other properties of the C108 ring were analyzed by scanning tunneling
microscopy/spectroscopy (STM/STS), low-temperature atomic force microscopy (AFM) with CO-
functionalized tips, and DFT calculations.
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Scheme 1. Reactions of 7,10-dibromo-1,4,11-triphenyl-triphenylene (DBTT) 1. Ullmann coupling of 1 leads to
the formation of the trimer macrocycle 2 (sterically favored) and the polymer chain 3 (sterically unfavored).
The C108 graphene ring 3 forms via 2 by cyclodehydrogenation. Green and red ovals mark the weak and strong
steric repulsion between the phenyl groups, respectively.
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RESULTS AND DISCUSSION
In the first step, 0.8 monolayer (ML) of 1 was vapor deposited onto the Au(111) surface held at 300 K.
Based on previous work with other but similarly sized precursors,28 1 is expected to adsorb on Au(111) as
an intact molecule at 300 K. As shown by Figure 1a, 1 forms densely-packed ordered domains, which
consist of repeating motifs outlined by the red contours in Figure 1d. The distance between two bright
dots in a single motif (Figure 1d) is measured to be 8.4 ± 0.3 Å, in agreement with the distance of 8.5770(4)
Å between the centers of the two tilted phenyl groups (marked by orange spheres in Figure 1g) in the 3D
crystalline state of DBTT (see the XRD analysis in the part (d) of the SI). Therefore, the STM features are
attributed to intact DBTT molecules 1, as shown by the chemical structures in Figure 1g. Noteworthy, the
prochiral 1 becomes chiral in the two-dimensional (2D) confinement, forming two 2D enantiomers labeled
by “L” and “R” in Figure 1g.
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Figure 1. STM images of the structures formed from DBTT 1 at different temperatures. (a-c) Overview STM
images taken after deposition of a submonolayer of 1 onto Au(111) held at (a) 300 K, (b) 473 K and (c)
subsequent annealing to 593 K. The green and red arrows in (b) mark the linear-chain side product and an
ultra-bright protrusion. (d-f) show zoom-in images of the species in panels (a-c), respectively. Tunneling
parameters: (a) U = 1.7 V, I = 0.08 nA; (b) U = 2.4 V, I = 0.09 nA; (c),(f) U = -1.7 V, I = 0.16 nA; (d) U = 1.7 V, I =
0.11 nA; (e) U = 3.5 V, I = 0.13 nA. (g-i) The corresponding chemical structures of the species that are marked
with contour lines in panels (d-f). The orange spheres mark the tilted phenyl groups. The three triphenyl-
triphenylene units are colored differently for clarity. C-C bonds formed by Ullmann and cyclodehydrogenative
coupling are red and blue colored, respectively.
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To trigger the C-C bond formation via Ullmann coupling, 0.8 ML of 1 was vapor deposited onto the Au(111)
surface held at 473 K. In addition to the cyclization-favoring precursor design (Scheme 1), we employed
on-surface high dilution conditions29-30 to maximize the yield of macrocycles. Therefore, a low deposition
rate of 0.08 ML per minute (ML/min), was used during the coupling reaction. Figure 1b shows that an
ordered structure with a densely-packed hexagonal 2D lattice is formed by this procedure. Between the
ordered domains, ultra-bright protrusions (marked by red arrow) are occasionally observed at the domain
boundaries. The magnified STM image (Figure 1e) of a typical domain shows two types of species with
hexagonal shape, as outlined by the red and cyan dashed-line hexagons. The majority species (red
hexagon) is assigned to the trimer macrocycle formed by C-C coupling of three DBTT molecules 1 having
the same 2D handedness (here "L"), as shown by the chemical structure in Figure 1h (left part). This is
supported by the nine bright dots of the hexagon species, which are attributed to the nine tilted phenyl
groups at the periphery (Figure 1h left). Moreover, the distance between the two dots located at two
adjacent corners is measured to be 8.6 ± 0.3 Å, in line with the expected distance (8.8 ± 0.4 Å) between
two phenyl groups at two adjacent corners of the trimer macrocycle in Figure 1h (left part). The ultra-
bright protrusions at the domain boundaries are related to the minority species; one example is marked
by the cyan hexagonal contour. This minority species forms through C-C coupling of DBTT molecules 1
with different 2D handedness. In the example illustrated by the chemical structure in Figure 1h (right
part), this trimer macrocycle contains two “L” handed and one “R” handed triphenyl-triphenylene
moieties. This combination results in four phenyl groups located at one edge of the hexagon and only two
phenyl groups at another edge. The missing phenyl ring can clearly be seen in Figure 1e (cyan hexagon).
At the edge overloaded with four phenyl groups, steric hindrance leads to substantial out-of-plane
deformation and thus to the ultra-bright protrusion (Figure 1h, right). These “wrong” connections lead to
the formation of defective trimer macrocycles, which represent only a minority species, because the steric
repulsion between the colliding phenyl groups makes this product energetically and kinetically
unfavorable.
Subsequent annealing of the sample in Figure 1b to 593 K leads to the formation of hexagonal macrocycles
with reduced numbers (ranging from 3-6) of bright dots at their edges (Figure 1c), compared to the trimer
macrocycle 2 with nine bright dots (Figure 1b). This indicates that some of the tilted phenyl groups
undergo cyclodehydrogenation reactions, resulting in a flat geometry. For instance, the hexagonal
macrocycle outlined by the red dashed-line hexagon in Figure 1f has four bright dots (i.e., four tilted
phenyl groups) left. The other five formerly tilted phenyl groups have been flattened by
cyclodehydrogenation as shown by the chemical structure in Figure 1i (left part). Noteworthy, the partially
dehydrogenated defective macrocycles (containing a phenyl vacancy at one side) with four bright dots
have also been observed, as enclosed by the cyan dashed-line hexagon in Figure 1f. The corresponding
chemical structure is shown in Figure 1i (right part). Unexpectedly, the partially flattened defective
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macrocycle (Figure 1f, cyan hexagon) does not show the ultra-bright protrusion originating from the
crowding of four phenyl groups at the same edge (as in Figure 1e, cyan hexagon). This finding indicates
the loss of one of the four phenyl groups at the overloaded edge by C-C bond dissociation, in agreement
with previous work.31
Further annealing of the sample in Figure 1c to 653 K leads to a higher degree of the surface-catalyzed
cyclodehydrogenation of the trimer macrocycles 2, as shown by the overview and magnified STM images
in Figure 2a and 2b, respectively. Hexagonal macrocycles again represent the major products. Rarely, short
linear chains occur, as indicated by the green arrow in Figure 2a. As can be seen from Figure 2b, completely
flat hexagonal rings (yellow arrow) and defective hexagonal rings (red arrow) are formed. STM images
with submolecular resolution (Figure 2c) and AFM images (Figure 2e) of the intact hexagonal macrocycle
reveal its C108 cycloarene structure 3, consisting of 108 sp2 carbon atoms as shown by the chemical
structure in Figure 2g. Similarly, Figure 2d and 2f reveal that the defective hexagonal macrocycle is a C102
graphene ring (Figure 2h), which can be reckoned as the C108 cycloarene with one missing benzene ring.
The C102 graphene ring is therefore the final product of the reaction starting with the wrong connection
of the monomers, followed by the detachment of a phenyl ring during planarization, as described above.
Noteworthy, not all of the hexagonal macrocycles are completely flattened by cyclodehydrogenation at
653 K. Some of them still contain between one and three tilted phenyl groups. Further increase of the
annealing temperature to 693 K is sufficient to flatten all of the phenyl groups by completing the
intramolecular cyclodehydrogenation reaction. However, the high temperature also increases the
probability of intermolecular dehydrogenative coupling, which partly results in the formation of C-C bond
linked graphene rings as shown by Figure 3.
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Figure 2. STM and AFM images of the C108 cycloarene 3 and the defective C102 graphene ring. (a) Overview
and (b) zoom-in STM image taken after annealing the sample in Figure 1c to 653 K. The green arrow in (a) marks
the linear-chain side product. The yellow and red arrows in (b) mark the completely flattened C108 and C102
macrocycles, respectively. (c-h) Submolecularly resolved (c,d) STM, (e,f) AFM images (measured with CO-
functionalized tip) and (g,h) the derived chemical structures of the C108 and C102 graphene ring, respectively.
Tunneling parameters: (a),(b) U = 2.2 V, I = 20 pA; (c) U = 10 mV, I = 80 pA; (d) U = 10 mV, I = 90 pA. Tip heights
for AFM images: (e) z = -55 pm and (f) z = -50 pm from a STM set point of I = 10 pA with U = 100 mV on Au(111).
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Figure 3. (a) STM topography and (b) constant-height frequency shift AFM images of chains of connected
graphene rings obtained by annealing a submonolayer of DBTT molecules to 693 K. Tunneling parameters: (a)
U = 50 V, I = 10 pA. Tip height for the AFM image: (b) z = 125 pm with respect to a STM set point of I = 30 pA
and U = 7 mV above the Au(111) surface. In both images, the oscillation amplitude of the tuning fork sensor
was set to 52.5 pm.
The electronic structure of the adsorbed C108 cycloarene 3 has been studied using STS. The six colored
curves in Figure 4a show the voltage-dependent differential conductance spectra (dI/dV versus V)
recorded at six different positions of the graphene ring (Figure 4a, inset, marked by the correspondingly
colored dots). The gray curve is the reference dI/dV spectrum measured on the bare Au(111) surface. The
bold red and blue spectra show the most prominent peaks located at -1.15 V and +1.82 V, respectively.
These two peaks are attributed to the positive and negative ion resonances (PIR and NIR) related to the
highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) of the adsorbed C108
cycloarene 3. The constant height dI/dV maps of 3 have been measured at the energies of the PIR and NIR
peaks as shown by Figure 4b and 4c. These maps allow to visualize the spatial distribution of the electron
density states related to HOMO and LUMO, which correlate to some extent with the DFT-calculated
HOMO and LUMO shown in Figures S2 and S3.
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Figure 4. Electronic properties of the adsorbed C108 cycloarene 3. (a) dI/dV spectra acquired on 3 at the
positions marked by differently colored dots in the inset. (b),(c) Constant-height differential conductance
(dI/dV) maps taken at bias voltages of -1.15 V (b) and 1.82 V (c), corresponding to the positive and negative ion
resonances (PIR and NIR) related to HOMO and LUMO. Tunneling parameters: inset in (a) U = 100 mV, I = 10
pA; (b) U = -1.15 V and (c) U = 1.82 V. (d) DFT-calculated C-C bonds lengths and harmonic oscillator model of
aromaticity (HOMA) values of the C108 cycloarene, represented by gradient colors as indicated.
The C108 cycloarene 3 is an interesting case for aromaticity considerations, similar to the case of kekulene
in recent work.32 Accordingly, we have calculated the C-C bond lengths and derived the HOMA values
(Figure 4d) for each benzene moiety.33 A higher HOMA value is generally associated with higher degree of
-electron delocalization and increased aromatic stabilization. Benzene as the reference molecule has a
HOMA value of 1.0.33 Due to its D6h symmetry, C108 contains only five different types of benzene rings.
Three of them show high HOMA values of 0.882, 0.825, and 0.807 and thus experience substantial
aromatic stabilization. The other two benzene rings have HOMA values of only 0.509 and 0.400, indicating
less aromatic stabilization. These findings are in line with Clar’s sextet formula of this molecule (Figure
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S1a). Additional aromatic stabilization may arise from conjugation along the [54]annulene path (Figure
S1b, blue path) of the outer periphery (0.741), whereas the inner periphery (enclosing the central hole)
with its [18]annulene path (Figure S1b, red path) has a lower HOMA value of only 0.498.
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CONCLUSION
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In conclusion, we have reported the successful synthesis of a hexagonal C108 cycloarene on a Au(111)
surface by an eight-step sequence starting from phenanthren-9,10-dione: The first six steps towards
novel precursor DBTT 1 involve catalytic C-C coupling reactions in solution, the final two steps towards
title compound C108 involve catalytic Ullmann and cyclodehydrogenative on-surface C-C coupling
reactions. To optimize the macrocycle formation, the on-surface high-dilution principle is applied and the
DBTT precursor is specially designed to sterically favor cyclotrimerization over open-chain polymerization.
The structure and electronic properties of the C108 cycloarene have been characterized by STM, AFM,
and DFT. HOMA values derived from DFT-calculated bond lengths suggest that the aromatic system is well
described by Clar’s sextet formula.
METHODS
Synthesis: The synthetic details and characterization of 7,10-dibromo-1,4,11-triphenyl-triphenylene are
described in the Supporting Information part (d).
STM/AFM/STS: STM, AFM and STS data were acquired with a commercial low-temperature STM/AFM
(Scienta Omicron, Germany) at 5.2 K or a variable-temperature SPECS STM Arhus 150 STM and a base
pressure below 1 × 10−10 mbar. An external lock-in amplifier and phase-looked loop electronics (MFLI,
Zürich Instruments, Switzerland) was used to excite the Qplus tuning fork sensor.34 The AFM tip was
sharpened by indentations into the Au(111) surface and by voltage pulses. For the STM and AFM data, the
tip was functionalized with a single CO molecule.35 The Qplus sensor was operated in frequency
modulation mode at constant amplitude (52.5 and 70 pm). The resonance frequency was about 26.970
kHz and the quality factor was in a range between 17000 and 33000. AFM images were measured on
constant-height mode. The MFLI lock-in amplifier was also employed to acquire the STS data. dI/dV signal
was extracted by adding a sinusoidal bias signal (ac bias: amplitude = 20 mV, frequency = 700 Hz) between
the tip and the sample. STS spectra were measured with a metallic tip. Before acquiring the spectra, the
tip was located at the corresponding position above the molecule and the Au surface with a tunneling
setpoint of I =200 pA and U = -1.3 V.
The DBTT molecules were evaporated using a home-built evaporation device held at 517 K.36 The Au(111)
single crystal with an alignment of better than 0.1° relative to the nominal orientation were purchased
from MaTecK, Germany. The Au(111) crystal was cleaned by Ar+ sputtering ((E = 1.5 kV, I = 1.4 µA, p = 3 ×
10−6 mbar) and annealing (1000 K) cycles.
Computational details: Density-functional theory (DFT) calculations of the isolated, planar cycloarene
molecule were carried out in D6h point group symmetry using the PBE37 functional, augmented with the
D3(BJ)38-40 dispersion correction, and a TZVP41 basis set. All DFT calculations were performed with the
Turbomole program package, and images of molecular orbitals were generated using TmoleX18.42
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website.
Synthetic details and characterization of DBTT; illustrations of the conjugation pathways of the C108 ring;
calculated molecular orbitals and geometry of the cycloarene; X-ray crystallographic data of DBTT (CCDC
1946678).
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AUTHOR INFORMATION
Corresponding Author
* jsu@staff.uni-marburg.de
* daniel.ebeling@ap.physik.uni-giessen.de
* michael.gottfried@chemie.uni-marburg.de
Author Contributions
‡Q.F., D.M.J, and S.W. contributed equally to this work.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
Financial support by the Deutsche Forschungsgemeinschaft via the grants GO 1812/2-1, 223848855-SFB
1083, the EXC 315, EB 535/1-1, SCHI 619/13, and the GRK 2204 is gratefully acknowledged. The RRZE of
the University of Erlangen−Nürnberg is acknowledged for computational resources. Q.T.F. thanks the
Alexander von Humboldt-Foundation for a Research Fellowship for Postdoctoral Researchers.
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