Large-Cavity Coronoids with Different Inner and Outer Edge Structures

Article abstract of DOI:10.1021/jacs.0c05268

Coronoids, polycyclic aromatic hydrocarbons with geometrically defined cavities, are promising model structures of porous graphene. Here, we report the on-surface synthesis of C168 and C140 coronoids, referred to as [6]- and [5]coronoid, respectively, usi

Full text of DOI:10.1021/jacs.0c05268

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Communication
Large-cavity coronoids with different inner and outer edge structures
Marco Di Giovannantonio, Xuelin Yao, Kristjan Eimre, José I. Urgel, Pascal
Ruffieux, Carlo A. Pignedoli, Klaus Müllen, Roman Fasel, and Akimitsu Narita
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05268 • Publication Date (Web): 26 Jun 2020
Downloaded from pubs.acs.org on June 27, 2020
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Large-cavity coronoids with different inner and outer edge struc-
tures
Marco Di Giovannantonio,1,*,‡ Xuelin Yao,2,‡ Kristjan Eimre,1,‡ José I. Urgel,1 Pascal Ruffieux,1 Carlo
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A. Pignedoli,1,* Klaus Müllen,2,3,* Roman Fasel,1,5 Akimitsu Narita2,4,*
1Empa, Swiss Federal Laboratories for Materials Science and Technology, nanotech@surfaces Laboratory, 8600 Dübendorf,
Switzerland
2Max Planck Institute for Polymer Research, 55128 Mainz, Germany
3Institute of Physical Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
4Organic and Carbon Nanomaterials Unit, Okinawa Institute of Science and Technology Graduate University, Okinawa 904-
0495, Japan
5Department of Chemistry and Biochemistry, University of Bern, 3012 Bern, Switzerland
Supporting Information Placeholder
molecule with 216 sp2 carbon atoms distributed in more than one
ABSTRACT: Coronoids - polycyclic aromatic hydrocarbons
layer of circularly fused benzene rings (Figure 1). The C216 coro-
noid can also be regarded as a nanographene with a cavity arising
upon removal of the central benzene ring from a C222 nanogra-
phene (Figure 1). Detailed characterization of C216 was compro-
mised by its vanishing solubility, which also limited further syn-
thetic studies of large coronoids. Very recently, Tan and coworkers
reported a synthesis of mesityl-substituted C108 coronoid, which
formed molecular bilayers.12
with geometrically defined cavities - are promising model struc-
tures of porous graphene. Here, we report the on-surface synthesis
of C168 and C140 coronoids, referred to as [6]- and [5]-coronoid,
respectively, using 5,9-dibromo-14-phenylbenzo[m]tetraphene as
the precursor. These coronoids entail large cavities (> 1 nm) with
inner zigzag edges, distinct from their outer armchair edges. While
[6]-coronoid is planar, [5]-coronoid is not. Low-temperature scan-
ning tunneling microscopy/spectroscopy and noncontact atomic
force microscopy unveil structural and electronic properties in ac-
cordance with those obtained from density functional theory calcu-
lations. Detailed analysis of ring current effects identifies the rings
with the highest aromaticity of these coronoids, whose pattern
matches their Clar structure. The pores of the obtained coronoids
offer intriguing possibilities of further functionalization, toward ad-
vanced host-guest applications.
Coronoids are macrocyclic conjugated hydrocarbons formed by
circularly fused benzenoid rings, featuring a cavity.1–3 Kekulene is
a representative coronoid with a single layer of benzene rings
around the cavity, belonging to the family of cycloarenes. The first
synthesis of kekulene in 19784 provided fundamental information
about the nature of macrocyclic conjugation and aromaticity, point-
ing out that kekulene is better represented by Clar’s model rather
than the so-called Kekulé structure. Other examples of cycloarenes
are cyclo[d,e,d,e,e,d,e,d,e,e]decakisbenzene5 – predicted to have
intriguing electronic and magnetic properties6–9 – septulene10 and
octulene.11 In 2016, we reported a C216 coronoid1 – a macrocyclic
Figure 1. From nanographenes to coronoids. Clar sextets are filled
in light blue. See Figure S3 for pore size determination.
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Figure 2. On-surface synthesis and characterization of [6]- and [5]-coronoids. (a) Synthetic scheme. (b,f) High-resolution STM images. (c,g)
nc-AFM images acquired with a CO-functionalized tip. (d,h) Simulated nc-AFM images of the DFT-optimized structures shown in panels
(e,i). (j) 3D view of tip height (z) dependent nc-AFM images of [5]-coronoid. (k) Frequency shift (Δf) vs Δz curves at different molecular
sites (the colors of the curves correspond to those of the dots and the dashed lines in panels g and j, respectively). Scanning parameters: (b)
Vb = ‒20 mV, It = 100 pA; (c) Δz = +150 Å with respect to STM set point: Vb = ‒5 mV, It = 100 pA; (f) Vb = ‒20 mV, It = 100 pA; (g) Δz =
+170 Å with respect to STM set point: Vb = ‒5 mV, It = 100 pA; (j) Δz with respect to STM set point (Vb = ‒5 mV, It = 100 pA) is indicated
for each layer.
On the other hand, on-surface chemistry offers an alternative
method to synthesize nanographenes and to explore their structure
and electronic properties via state-of-the-art scanning probe mi-
croscopy and spectroscopy.13–15 Recently, kekulene was studied us-
ing noncontact atomic force microscopy (nc-AFM), which corrob-
orated its structure in accordance with Clar’s model.16 Addition-
ally, the on-surface synthesis of a C108 coronoid (Figure 1) was
achieved,17 which represents a smaller homologue of the C216 co-
ronoid. However, to date no other coronoids have been synthesized
on surfaces.
supplemented by density functional theory (DFT) calculations sup-
porting the experimental results and shedding light on the aroma-
ticity of [5]- and [6]-coronoids.
For the synthesis of [6]-coronoid we have designed the U-shaped
precursor 1, having a benzo[m]tetraphene core with a pre-installed
zigzag edge as well as two bromo groups carefully positioned to
allow for the macrocyclization on surface (see SI for the synthesis
of characterizations of 1). When 1 was deposited onto a Au(111)
surface held at 200 °C, the homolytic cleavage of C-Br bonds read-
ily took place, initiating aryl-aryl coupling.24 Temperature increase
to 380 °C promoted the cyclodehydrogenation of the obtained
nanostructures.25–27 The final products of this sequence consisted
of various chain-like structures, as well as some macrocyclic mol-
ecules (Figure S1) including highly symmetrical ones (Figure 2b,f).
The yields of [5]- and [6]-coronoids could be increased by applying
high-dilution conditions,28 with the highest yields of 30% and 6%,
respectively, when the deposition rate of 1 onto Au(111) was set at
0.23 monolayer/hour (see Figure S2 for further details). The higher
yield of [5]-coronoid is most likely due to the ability of pentameric
intermediates to undergo intramolecular head-to-tail coupling be-
fore the incorporation of the sixth building block, prevented by the
high-dilution conditions.
Coronoids can also serve as models for graphene with nanoscale
cavities, namely nanoporous graphene, which, depending on the
size and structure of the pores, has intriguing electronic and mag-
netic properties.18–20 Nanoporous graphene with zigzag-edge cavi-
ties stands out21 since it is related to nanographenes and graphene
nanoribbons with zigzag edges.22,23 However, all reported coro-
noids have inner and outer armchair edges, and coronoids hosting
a cavity with zigzag edges have never been synthesized.
In this work, we targeted an exemplary coronoid (C168) with
outer armchair edges and a large pore (1.45 nm, C-C distance) fea-
turing inner zigzag edges. Conceptually, this molecule can be ob-
tained via removal of a kekulene from the aforementioned C216
coronoid (Figure 1). Using 5,9-dibromo-14-phenylbenzo[m]tetra-
phene (1) as a precursor, we achieved a thermally activated on-sur-
face synthesis of the C168 coronoid through dehalogenative aryl-
aryl coupling and subsequent cyclodehydrogenation on Au(111)
(Figure 2a). C168 is obtained by the coupling of six molecules 1,
and is hereafter called [6]-coronoid. The on-surface synthesis also
led to the formation of a coronoid consisting of five molecules 1,
namely C140 ([5]-coronoid), which adopts a strain-driven non-pla-
nar conformation. The on-surface structural and electronic charac-
terization of these coronoids was performed by scanning tunneling
microscopy/spectroscopy (STM/STS) and nc-AFM, which were
To assess the exact chemical structure of the observed macrocy-
cles, we acquired constant-height frequency-shift nc-AFM images
using a CO-functionalized tip,29 which clearly revealed the for-
mation of [6]- and [5]-coronoid (Figure 2c,g). [6]-Coronoid is pla-
nar on the Au(111) surface and displays the expected armchair/zig-
zag configurations of its outer/inner edges, as further confirmed by
the excellent agreement of the experimental results with simulated
nc-AFM images (Figure 2d) based on the DFT-optimized structural
model (Figure 2e). The size of the inner cavity is measured to be
1.40±0.05 nm (C-C distance of two opposing zigzag edges), in ac-
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cordance with the DFT-calculated value of 1.45 nm (see SI for de-
the “bridges” between the fused precursor units) exhibit positive
NICS value (16 ppm, Figure 4a,d) reminiscent of perylene.35 In this
regard, the [6]- and [5]-coronoids can be well represented by alter-
natingly fused perylene and pyrene constituents (Figure S6). To
shed further light on the observed aromaticity pattern, we per-
formed a harmonic oscillator model of aromaticity (HOMA) anal-
ysis (Figure 4b,e), which qualitatively matches the NICSzz(1) pat-
terns of [6]- and [5]-coronoid. Further, we studied the induced cur-
rents due to an applied magnetic field by analyzing the anisotropy
of the induced current density (ACID) (Figure 4c,f). The valence
electrons are delocalized throughout the [6]- and [5]-coronoids.
Each phenanthro[2,3,4,5-pqrab]perylene substructure displays a
macroscopic diatropic current on its outer edge (red arrows in Fig-
ure 4c,f). Within these substructures, the most aromatic sextets ex-
hibit local diatropic currents.
tails). The [5]-coronoid also reveals an armchair/zigzag configura-
tion of outer/inner edges (Figure 2f,g), while the size of the inner
cavity is reduced to 1.10±0.05 nm (diameter of the inscribed circle
through the central carbon atoms of the zigzag edges measured
from the nc-AFM data). Again, this value agrees with the pore size
of 1.09 nm estimated from the DFT-optimized geometry of the [5]-
coronoid on Au(111) (Figure 2i). For the [5]-coronoid, two out of
five inner edges appear as brighter/higher features in both the STM
and nc-AFM images (Figure 2f,g). Such a non-planar structure is
in line with a partial flattening of the more distorted gas-phase ge-
ometry of [5]-coronoid (Figure S3b) upon adsorption. For quanti-
fication, we acquired z-dependent nc-AFM images (Figure 2j) and
recorded the frequency shift (Δf) of the CO-functionalized tip while
it approached the molecule to a specific location (Figure 2k). This
allowed to extract a height difference between the highest (blue)
and lowest (red) part of the molecule of 180±10 pm, in good agree-
ment with a DFT-calculated value of 170 pm (determined from the
optimized geometry in Figure 2i). This height difference is substan-
tially smaller than the value calculated for the [5]-coronoid in gas
phase (360 pm, Figure S3b), which reflects the effect of van der
Waals interactions with the Au(111) surface upon flattening the
molecule.
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To characterize the electronic properties of the [5]- and [6]-co-
ronoids, we performed constant-height differential conductance
dI/dV measurements and constant-current dI/dV mappings at the
detected positive (PIR) and negative ion resonances (NIR) (Figure
3a-j). Gas-phase orbital calculations (Figure 3k) show for [6]- and
[5]-coronoid, respectively, 6 and 5 close-lying frontier orbitals both
for occupied and unoccupied levels. This is reflected in DFT cal-
culations of the molecules on Au(111), where both the HOMO and
LUMO are hybridized with the respective close-lying molecular or-
bitals into singular frontier ion resonances (see Figure S4 for de-
tails) that can be assigned to the experimentally measured PIR and
NIR signatures. Hence, the experimental HOMO-LUMO gaps of
the [6]- and [5]-coronoids on Au(111) are 2.2 and 2.5 eV, respec-
tively. DFT calculations predicted the gas-phase HOMO-LUMO
gaps of the [5]- and [6]-coronoid, C216 and C222 to be 2.30, 2.25,
2.15 and 1.77 eV, respectively (Figure 3k), highlighting the trend
of increasing gap with increasing pore size and decreasing number
of carbon atoms.
The [6]-coronoid could potentially constitute a building block
for extended nanoporous graphene with larger cavities than those
reported so far,20,30–32 and thus be of interest in the framework of
graphene antidot lattices.33 We calculated the electronic properties
of such hypothetical periodic layer via tight binding (TB) simula-
tions (see Figure S5 for details), which reveal a band gap of 0.63
eV (between frontier bands localized at each cavity rim), consider-
ably reduced compared to the [6]-coronoid HOMO-LUMO gap
(TB) of 1.51 eV.
Figure 3. Differential conductance measurements and DFT sim-
ulations of [6]- and [5]-coronoids. (a,f) dI/dV spectra. (b,c,g,h)
dI/dV maps. (d,e,i,j) DFT-calculated local density of states at the
frontier PDOS peaks (see Figure S4) of the two systems on Au(111)
at the PBE level of theory. (k) Gas phase electronic characterization
of the four indicated systems at the B3LYP/6-311+G** level of
theory. The red and blue dots represent the occupied and unoccu-
pied states, respectively The HOMO and LUMO wave functions
are displayed. For the [5]-coronoid, the LUMO is doubly quasi-de-
generate and both orbitals are shown.
Carbon macrocycles are known to have peculiar aromaticity
properties, leading to intriguing concepts, such as superaromatic-
ity,7,34 and represent an ideal playground for studying fundamental
aspects of the π-electron clouds. To display the aromaticity pattern
of the [6]- and [5]-coronoid, we performed nucleus-independent
chemical shift (NICS) calculations at 1 Å above the center of each
carbon ring (NICSzz(1), Figure 4a,d). The results qualitatively
match between similar locations of [6]- and [5]-coronoids, indicat-
ing that the non-planarity of the latter does not significantly affect
its aromaticity. Moreover, the NICSzz(1) patterns resemble the Clar
structure of these coronoids (compare Figures 1 and 4), with each
Clar sextet showing strong aromatic character (from ‒22 ppm to ‒
27 ppm), similar to benzene (‒29 ppm). The hexagons connecting
the aromatic phenanthro[2,3,4,5-pqrab]perylene substructures (at
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*E-mail: marco.digiovannantonio@empa.ch
*E-mail: carlo.pignedoli@empa.ch
*E-mail: muellen@mpip-mainz.mpg.de
*E-mail: narita@mpip-mainz.mpg.de
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Author Contributions
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‡These authors contributed equally.
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Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
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This work was supported by the Swiss National Science Founda-
tion under Grant No. 200020_182015, the NCCR MARVEL
funded by the Swiss National Science Foundation (51NF40-
182892), the Max Planck Society, the European Union’s Horizon
2020 research and innovation program under grant agreement num-
ber 785219 (Graphene Flagship Core 2), and the Office of Naval
Research BRC Program. Computational support from the Swiss
Supercomputing Center (CSCS) under project ID s904 is gratefully
acknowledged. We are thankful to E. Paenurk and R. Gershoni-
Poranne for fruitful discussion on aromaticity calculations and Lu-
kas Rotach (Empa) for his excellent technical support during the
experiments.
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