Expanded Kekulenes

Article abstract of DOI:10.1021/jacs.1c06757

The synthesis of kekulene and its higher homologues is a challenging task in organic chemistry. The first successful synthesis and characterization of the parent kekulene were reported by Diederich and Staab in 1978. Herein, we report the facile preparati

Full text of DOI:10.1021/jacs.1c06757

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Expanded Kekulenes
Wei Fan, Yi Han, Xuhui Wang, Xudong Hou, and Jishan Wu*
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ABSTRACT: The synthesis of kekulene and its higher homologues is a challenging
task in organic chemistry. The first successful synthesis and characterization of the
parent kekulene were reported by Diederich and Staab in 1978. Herein, we report
the facile preparation of a series of edge-extended kekulenes by bismuth(III) triflate-
catalyzed cyclization of vinyl ethers from the properly designed macrocyclic
precursors. Their molecular structures were confirmed by X-ray crystallographic
analysis and NMR spectroscopy. Their size- and symmetry-dependent electronic
structures (frontier molecular orbitals, aromaticity) and physical properties (optical
and electrochemical) were investigated by various spectroscopic measurements,
assisted by theoretical calculations. Particularly, the acene-like units along each zigzag
edge demonstrate a dominant local aromatic character. Our studies provide an easy
synthetic strategy toward various fully fused carbon nanostructures and give some
insights into the electronic properties of cycloarenes.
ization of the parent kekulene.5 This work was considered as a
milestone in cycloarene chemistry because it not only provided
a general synthetic strategy for cycloarenes but also answered
the question regarding their electronic structure. X-ray
crystallographic analysis and NMR measurements revealed a
local aromatic character for the kekulene, which was further
confirmed by modern on-surface synthesis and a scanning
probe microscopy technique.6 In 1986, a contracted cycloarene
was synthesized by a similar strategy.7 The original synthetic
protocol for kekulene involved multistep tedious procedures,
and the final product has poor solubility, both limiting the
further development of expanded kekulenes. Therefore,
chemists started to look for new synthetic methods for
cycloarenes.8 In 2012, King et al. reported the synthesis of a
heptagonal homologue of kekulene, septulene, by intra-
molecular alkene metathesis.9 The molecule shows a similar
electronic structure to kekulene, with low solubility. In 2016,
INTRODUCTION
■
According to Staab and Diederich, “cycloarenes” were defined
as “polycyclic aromatic compounds in which, by a combination
of angular and linear annellations of benzene units, fully
annelated macrocyclic systems are present enclosing a cavity
into which carbon−hydrogen bonds point”.1 A typical example
of this family is a D6h symmetry cycloarene molecule, which
was named as “kekulene” in homage to Kekule (Figure 1).
2
́
́
Stepien et al. developed a fold-in synthetic strategy and
̨
obtained alkoxy chain-substituted kekulene and octulene; both
are soluble in common organic solvents.10 Meanwhile, other
related structures such as extended coronoids have been
synthesized either in solution or on the surface, which show
different electronic properties from the single-chain cyclo-
arenes.11 In 2020, our group developed an efficient synthetic
method for aryl-substituted kekulene and octulene through a
Figure 1. From kekulene to core-expanded kekulenes ([m,n]-
cycloarenes).
The early interest in cycloarenes was focused on their
electronic structure, that is, whether the π-electrons are
globally delocalized through the entire molecular skeleton or
only delocalized into individual benzene-type rings.3 This
question provoked experimental synthesis of cycloarenes,
particularly the D6h symmetric kekulene, which turned out to
be a very challenging task.4 In 1978, Diederich and Staab
reported the successful synthesis and structural character-
Received: June 30, 2021
Published: August 20, 2021
https://doi.org/10.1021/jacs.1c06757
J. Am. Chem. Soc. 2021, 143, 13908−13916
© 2021 American Chemical Society
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Scheme 1. (a) Synthetic Strategy toward Expanded Kekulenes; (b) Synthesis of the Key Building Blocks; and (c) Synthesis of
a
the Derivatives of Expanded Kekulenes
a
Reagents and conditions: (a) Pd(PPh3)4, K2CO3, toluene/ethanol/H2O, 90 °C; (b) Pd(OAc)2, K2CO3, DMF/H2O, 100 °C; (c) B2pin2,
[Ir(OMe)cod]2, dtbpy, cyclohexane, 80 °C; (d) Pd2(dba)3, DPEphos, K3PO4, toluene, reflux; (e) B2pin2, Pd(dppf)Cl2, KOAc, dioxane, 110 °C; (f)
Pd2(dba)3, [(tBu)3PH]BF4, NaHCO3, THF/H2O, 80 °C; (g) Bi(OTf)3, 1,2-dichloroethane, 90 °C.
bismuth(III) triflate-catalyzed cyclization of vinyl ethers.12 The
macrocyclic precursors can be easily synthesized, and the
Bi(OTf)3-mediated cyclization is very efficient, which was also
demonstrated by other related systems.13 We expected that
this method could be a suitable approach toward a series of
edge-extended kekulenes (Figure 1), which were only
theoretically discussed in the literature but experimentally
not available.14 Different from septulene and octulene, the
core-expanded kekulenes discussed here have six zigzag edges
like kekulene, but the number of linearly annelated benzenoid
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Figure 2. X-ray crystallographic structures of [3,4]CA-1. (a, b) Top view of the whole molecule and side view of the backbone; (c, d) overlap and
close contacts of two neighboring molecules and 3D packing structures; (e) selected bond lengths (in Å) from the X-ray structures.
rings increases. According to the nomenclature proposed by
Bergan et al.,15 they can be called [m,n]cycloarenes, where m
and n represent the number of annelated benzenoid rings on
the two types of zigzag edges, respectively (Figure 1). In this
article, we will report the synthesis and structural character-
ization of their derivatives. Their size- and symmetry-
dependent electronic structures and physical properties will
also be discussed.
coupling reactions between these building blocks under a
catalytic condition involving Pd2(dba)3, [(t-Bu)3PH]BF4, and
aqueous NaHCO3 (Scheme 1c). In the initial test, the
expanded [3,4]cycloarene derivatives [3,4]CA-1 and
[3,4]CA-2 with either 4-tert-butyphenyl or mesityl substituents
at different positions were prepared. Reactions between A and
D gave the intermediate 10 in 30% isolated yield, and
subsequent Bi(OTf)3-catalyzed cyclization in 1,2-dichloro-
ethane at 90 °C afforded [3,4]CA-1 as a yellow solid in 53%
yield. On the other hand, Suzuki couplings between B and C
(in 15% yield) followed by Bi(OTf)3-catalyzed cyclization (in
35% yield) gave [3,4]CA-2. Next, the further expanded
[4,4]cycloarene derivative [4,4]CA was synthesized as an
orange solid by a similar strategy starting from building blocks
B and D in an overall 10.2% (15% × 68%) yield. The
cycloarenes [3,5]CA and [4,5]CA, containing five linearly
annelated benzenoid rings, were obtained starting from
building blocks A/E and B/E in an overall 5.8% and 3.8%
yield, respectively. It is worth noting that in both cases the
reactions at the second step should be conducted in the dark
since the final products are light sensitive (vide infra).
Ground-State Geometry. Single crystals of [3,4]CA-1
were grown by slow diffusion of methanol into its solution in
tetrahydrofuran (THF).17 X-ray crystallographic analysis
reveals a slightly distorted backbone in the solid state (Figure
2a,b), which could be due to the strain induced by the
repulsion between the two neighboring aryl substituents. The
molecules are packed into a slipped 1D chain with regular
dislocation between the adjacent molecules due to intermo-
lecular steric repulsion between the aryl substituents (Figure
2c,d). Close π−π contacts with distances of 3.31 and 3.25 Å
are found. Bond length analysis shows that the bond lengths of
RESULTS AND DISCUSSION
■
Synthesis. The general synthetic strategy toward these
core-expanded kekulenes is shown in Scheme 1a. The
macrocyclic precursors (F) with different size arene units
and six vinyl ether groups can be synthesized by a Suzuki
coupling reaction between the building blocks A/B carrying
two bromine or triflate (OTf) groups and the building blocks
C/D/E carrying two pinacol borate (Bpin) groups. Then,
Bi(OTf)3-catalyzed cyclization of vinyl ethers is supposed to
give the desired cycloarenes (G) with either 3- or 6-fold
symmetry. The building blocks A and C were prepared
through a similar synthetic route previously reported by us
(Scheme 1b).12 Building block B, which contains a
naphthalene unit, was synthesized by Suzuki coupling between
116 and 2 (Scheme 1b). The second naphthalene-based
building block D was prepared by Suzuki coupling reaction
between 3 and 4, followed by Ir-catalyzed regioselective C−H
borylation. Similarly, the anthracene-based building block E
was obtained by Suzuki coupling between 6 and 7 followed by
regioselective Ir-catalyzed C−H borylation. The structure of E
was unambiguously confirmed by X-ray crystallographic
analysis.17 Then, three types of macrocyclic oligo(m-arylene)
intermediates were synthesized by combinational Suzuki
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Figure 3. Selected bond lengths (in Å) from the optimized structures, calculated NICS(1)zz values (the numbers in blue and pink color): (a)
[3,3]CA, (b) [3,4]CA-1, (c) [3,4]CA-2, (d) [4,4]CA, (e) [3,5]CA, (f) [4,5]CA. All substituents are omitted for clarity.
1
the six nearly equivalent bonds a along the outer periphery are
around 1.32−1.34 Å, close to that for typical olefins (∼1.35 Å)
(Figure 2e). The CC bonds b, k linking these “double bonds”
are much longer (1.44−1.46 Å), implying a typical C(sp2)−
C(sp2) single-bond character. Additionally, the CC bonds c, d,
e, f, g, h along ring A and the neighboring ring B had typical
bond lengths (1.37−1.44 Å) similar to that of an aromatic
naphthalene ring, whereas the bond lengths in ring C (1.38−
1.43 Å) are close to that of an aromatic benzene ring. In
addition, the bonds i and l linking these benzenoid rings
(1.46−1.47 Å) also exhibit typical C(sp2)−C(sp2) single-bond
character. All these suggest that [3,4]CA-1 has a dominant
local aromatic character with electrons on the backbone mainly
delocalized in three benzene rings and three naphthalene rings.
The geometries of all five molecules were optimized by
density functional theory (DFT) at the B3LYP/6-31G(d,p)
level of theory and compared with [3,3]CA (Figure 3 and
Figure S32 in the Supporting Information (SI)). The
calculations indicate that [3,4]CA-2, [3,5]CA, and [4,5]CA
possess an almost planar backbone, while [3,4]CA-1 and
[4,4]CA have a slightly distorted backbone due to the steric
repulsion between the two neighoring 4-tert-butylphenyl
substituents. The calculated bond lengths show that all the
fused CC bonds from the vinyl ethers are close to 1.36 Å and
similar to the typical olefins, and the bond lengths of center
benzene/naphthalene/anthracene units along each zigzag edge
resemble that of the respective aromatic rings. Therefore, the
π-electrons are mainly delocalized into the central individual
benzene, naphthalene, and anthracene rings rather than
globally delocalized.
Electronic Structure and Aromaticity. The H NMR
spectra show that both the inner- and outer-rim protons of the
five cycloarenes are deshielded and their resonances appear in
the low field (Figure 4a). All the NMR peaks were assigned by
2D ROESY and NOESY NMR techniques and gauge-
independent atomic orbital (GIAO) calculations (see Support-
1
ing Information). The H NMR spectra of [3,4]CA-1 and
[3,4]CA-2 that share the same backbone are similar to each
other. The inner protons of these two molecules are further
shifted to low field compared to those of [3,3]CA, presumably
owing to the additional deshielding effect from the
naphthalene ring which has one extra benzenoid ring. The
further expanded kekulene [4,4]CA can be regarded as a
super-kekulene. It exhibits certain aggregation in common
1
organic solvents, and its H NMR signals in CDCl2CDCl2 are
broadened at room temperature, while heating leads to
sharpening of the peaks (Figures S5 and S6 in SI). The
resonances are also slightly shifted to higher field when the
concentration is increased (Figure S7 in SI). When the sample
was heated to 363 K, the resonances for the inner-rim protons
a and b split into two peaks (Figure 4a). However, all of the
respective protons from [4,4]CA appear at higher field as
compared to [3,3]CA, [3,4]CA-1, and [3,4]CA-2. As to
[3,5]CA, the constitutional isomer of [4,4]CA, the inner-rim
protons (a−c) are deshielded into lower field, indicating a
different π-conjugation throughout the backbone. For
[4,5]CA, whose edges possess one extra benzenoid ring as
compared to [3,5]CA, the inner-rim protons (a−c) are less
deshielded.
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1
Figure 4. (a) H NMR spectra (aromatic region) of the five cycloarenes ([3,4]CA-1 in CD2Cl2, others in CDCl2CDCl2). The resonance
assignment refers to the structure and labeling in Scheme 1. Calculated ACID plots of (b) [3,4]CA-1, (c) [4,4]CA, (d) [3,5]CA, and (e) [4,5]CA
with an isovalue of 0.02; the arrows indicate the ring current flow.
Figure 5. (a) Calculated 2D ICSSzz (top) and 3D ICSS (bottom) maps: (a and e) for [3,4]CA-1; (b and f) for [4,4]CA; (c and g) for [3,5]CA; (d
and h) for [4,5]CA. The isovalue of 3D ICSS is 4.
In order to further understand the electronic structures and
magnetic shielding response of these cycloarenes, nucleus-
independent chemical shift (NICS) (Figure 3), anisotropy of
the induced current density (ACID) (Figure 4b−e), and 2D/
3D isochemical shielding surface (ICSS) calculations (Figure 5
and Figure S48 in SI) were conducted. Generally, the
calculated NICS(1)zz values (the numbers in pink color) for
the hexagons fused from vinyl ethers are much less negative
compared to those for the benzenoid rings between them
(numbers in blue color), indicating that they are only weakly
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aromatic. ACID maps also show clockwise (diatropic) ring
current circuits for those aromatic units. Nevertheless, weak
ring current flow over the CC double bonds at each corner is
also observed in all cases (see additional ACID maps with
different isovalues in Figures S43−S47 in SI), implying that the
π-electrons are also globally delocalized along the periphery to
some extent. The calculated 2D ICSS maps show a
magnetically deshielded chemical environment (negative
values) in the inner cavity and outside the periphery (Figure
5a). Three-dimensional ICSS maps reveal that the shielding
spheres mainly locate on the backbone of these expanded
kekulenes (Figure 5b). All these computational results together
with the experimental data suggest a dominant local aromatic
character, with π-electrons mainly confined in the individual
benzene/naphthalene/anthracene rings.
It is also found that the π-electron delocalization and
aromatic character vary slightly depending on the size and
symmetry, which is reflected by the different calculated
magnetic shielding tensor quantities and the experimentally
observed NMR chemical shifts. For example, the NICS(1)zz
value for the fused rings from vinyl ethers in [4,4]CA (−5.3
ppm) is less negative in comparison to [3,4]CA-1/[3,4]CA-2
(ca. −8 ppm) and [3,3]CA (−14 ppm) (Figure 3), indicating
a weaker π-conjugation between the neighboring arms. Two-
dimensional ICSS maps also clearly reveal a less deshielded
cavity in [4,4]CA than that of [3,4]CA-1/[3,4]CA-2 (Figure
5a), which well explains the observed high-field shift of the
resonances for the inner-rim protons in the NMR spectrum of
[4,4]CA (Figure 4a). Moving on to the [4,5]CA, the coupling
becomes even weaker, as indicated by the near-zero NICS(1)zz
values (ca. −2.5 ppm) for the bridging hexagons (Figure 3f)
and nearly broken ring current flow across the CC double
bond at the corner (Figure 4e). The π-electron delocalization
in [3,5]CA lies in between [3,4]CA-1/[3,4]CA-2 and
[4,5]CA as inferred from the calculated NICS(1)zz values
(ca. −7.0 ppm) of the bridging hexagons and the magnetic
shielding response in the cavity. As a consequence, the
chemical shifts for the inner-rim protons also locate in between
(Figure 4a). The trend observed in this series of expanded
kekulenes is consistent with the calculated NMR data (Figures
S49−S54 in SI).
Optical and Electrochemical Properties. All these
expanded kekulene compounds together with the kekulene
derivative [3,3]CA display well-resolved absorption spectra in
solution (Figure 6a, and the data are collected in Table 1). The
absorption maxima (λabs) of [3,4]CA-1 and [3,4]CA-2 are red-
shifted to 352 and 349 nm, respectively, compared with that of
[3,3]CA (λabs = 327 nm), which can be explained by the
extended π-conjugation. In addition, several weak shoulder
peaks at the 400−500 nm region are found for both, which can
be assigned to a combination of multiple HOMO−n →
LUMO+m (n,m = 0−2) electronic transitions according to the
time-dependent (TD) DFT calculations (see SI). It is worth
noting that these peaks are much stronger than the
corresponding peaks in [3,3]CA. Such change can be
explained by the decreased symmetry in the D3h symmetric
[3,4]cycloarene backbone in comparison to the D6h symmetry
kekulene. Calculations also show that the frontier HOMO and
LUMO coefficients of [3,4]CA-1 and [3,4]CA-2 are primarily
distributed along the edges with four annelated benzenoid
rings somewhat similar to the tetracene, while the HOMO and
LUMO in [3,3]CA are delocalized along the whole backbone
(Figure 7a,b). The absorption maximum of [4,4]CA was
Figure 6. (a) UV−vis absorption and (b) normalized fluorescence
spectra of [3,3]CA and the five cycloarenes measured in THF (1 ×
10−5 M) (except that [4,4]CA was measured in tetrachloroethane) at
room temperature. Inset in (a) shows the magnified spectra at the
long-wavelength region, and that in (b) displays the photos of five
cycloarenes under UV lamp irradiation (from left to right: [3,4]CA-1,
[3,4]CA-2, [4,4]CA, [3,5]CA, and [4,5]CA).
further shifted to 390 nm. The spectral structure resembles
that of [3,3]CA due to a similar 6-fold symmetry of the
backbone, with very weak shoulder bands beyond 500 nm. It
shows two nearly degenerated HOMOs and LUMOs, with
coefficients distributed at half of the molecule (Figure 7c).
[3,5]CA shares the same number of benzenoid rings in the
backbone as [4,4]CA but shows a quite different absorption
spectrum (Figure 7a). In addition to the major absorption at
375 nm, several vibrational peaks are clearly observed in the
450−600 nm region, with maxima at 482, 517, and 553 nm.
The band structure at this fingerprint region resembles the
absorption spectrum of pentacene.18 [3,5]CA also shows two
nearly degenerated HOMOs and LUMOs, with coefficients
distributed at half of the molecule. The profile at each edge
with five fused benzenoid rings is similar to that of the
pentacene (Figure 7d), which is consistent with the observed
pentacene-like band structure at the low-energy edge.19
According to TD-DFT calculations, this weak band is
originated from the HOMO/HOMO−1 → LUMO/LUMO
+1 electronic transitions (see SI). Compound [3,5]CA in THF
was much more stable when kept in the dark, but it slowly
decomposed under the ambient light irradiation in air with a
half-life of around 8 h, as monitored by the UV−vis
spectrometry (Figures S17, S18, S21, and S22 in SI).
Compound [4,5]CA exhibits a maximum absorption at 408
nm and a long tail with two weak shoulder peaks at 495 and
519 nm. The HOMO and LUMO coefficients are mainly
distributed among the three pentacene-like arms (Figure 7e).
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Table 1. Photophysical Properties and Energy Levels of the Cycloarenes
a
b
b
a
c
c
c
d
λabs [nm]
λonset [nm]
Egopt[eV]
λem [nm]
LUMO [eV]
HOMO [eV]
Egcal [eV]
ΦF
τF(ns)
[3,3]CA
[3,4]CA-1
[3,4]CA-2
[4,4]CA
[3,5]CA
[4,5]CA
327
352
349
390
375
408
406
496
484
480
568
524
3.05
2.50
2.56
2.58
2.18
2.37
460
554
538
534
561/619
559
−1.68
−2.16
−2.25
−1.94
−2.41
−2.40
−5.18
−4.68
−4.84
−4.92
−4.68
3.50
2.52
2.59
2.98
2.27
2.30
4.6%
11%
8%
5.5%
5.5%
4.0%
23
17
20.5
29.1
9.0
b
−4.70
16.5
a
b
λabs and λem were measured in THF (1.0 × 10−5 M); [4,4]CA in tetrachloroethane (1.0 × 10−5 M). Obtained from the edge of the absorption
c
d
spectra according to Egopt = (1240/λonset). Calculated at the B3LYP/6-31G(d,p) level. Fluorescence lifetimes of [3,3]CA (λexc = 340 nm, λprobe
460 nm) and the five cycloarenes (λexc = 365 nm, λprobe = 553, 554, 534, 561, and 559 nm, respectively).
=
Figure 7. (a) Calculated frontier molecular orbital profiles of the cycloarenes: (a) [3,3]CA, (b) [3,4]CA-1, (c) [4,4]CA, (d) [3,5]CA, and (e)
[4,5]CA.
The weak long-wavelength absorption band is originated from
a combination of HOMO-n → LUMO+m (n,m = 0, 1, 2)
electronic transitions according to the TD-DFT calculations
(see SI). Compound [4,5]CA in THF was also stable in the
dark, and it underwent slow decomposition under the ambient
air and light conditions, with a half-life of about 30 h (Figures
S19−S21 and S23 in SI). From the mass spectra (Figures S25
and S26 in SI), photochemical oxygenation reactions occur
when solutions of [3,5]CA and [4,5]CA were exposed to air
and ambient light over the time and followed the first-order
kinetic reaction process (Figures S22 and S23).20 Among all six
compounds, [3,5]CA has the lowest optical energy gap (2.18
eV) as roughly estimated from the onset of the lowest energy
absorption, which is also in agreement with the calculated
HOMO−LUMO energy gaps (Table 1).
23.0, 17.0, 20.5, 29.1, 9.0, and 16.5 ns, respectively (Table 1,
Figure S24 in SI). Cyclic voltammetry measurements of the
five expanded kekulenes in chlorobenzene generally revealed
multiple irreversible redox waves (Figures S27−S31 in SI),
which can be distinguished by differential pulse voltammetry.
[3,4]CA-1 and [3,4]CA-2 exhibit three similar oxidation peaks
with half-wave potentials (E1/2ox) at 1.0/1.28/1.76 V and 1.04/
1.27/1.78 V (vs Fc+/Fc), respectively. [4,4]CA shows just one
apparent oxidation wave with E1/2ox at around 0.9 V due to the
aggregation in solution. [3,5]CA also shows three oxidation
ox
peaks with E1/2 at around 0.83, 0.99, and 1.33 V, whereas
[4,5]CA displays only one broad oxidation peak with E1/2ox at
around 0.98 V owing to its low solubility.
The fluorescence (FL) spectra of the five cycloarenes are all
red-shifted compared to that of [3,3]CA, attributed to their
extended conjugated structures (Figure 6b). [3,4]CA-1 and
[3,4]CA-2 show the maximum emission (λem) peaks at 554
and 538 nm, respectively. [3,3]CA and [4,4]CA display similar
well-resolved emission spectra, probably originating from their
similar symmetry. Due to extended π-conjugation, the emission
maximum of [4,4]CA is red-shifted to 534 nm as compared to
[3,3]CA (λem = 460 nm). [3,5]CA exhibits two major
emission peaks at 561 and 619 nm, while [4,5]CA displays
an emission maximum at 559 nm. The absolute fluorescence
quantum yields of [3,3]CA, [3,4]CA-1, [3,4]CA-2, [4,4]CA,
[3,5]CA, and [4,5]CA in solution were determined to be
4.6%, 11.0%, 8.0%, 5.5%, 5.5%, and 4.0%, respectively, by using
the integrating sphere technique (Table 1). The relatively low
quantum yields may be related to their generally weak
absorption in the lower energy region. In addition, their
corresponding fluorescence lifetimes (τ) were measured as
CONCLUSION
■
In summary, five expanded kekulene molecules were
synthesized by a facile strategy involving macrocyclization by
the Suzuki coupling reaction followed by Bi(OTf)3-catalyzed
cyclization of vinyl ethers. Similar to the kekulene, the π-
electrons in these skeletons are mainly delocalized into the
individual benzene/naphthalene/anthracene units at the center
of each arm. They display size- and symmetry-dependent
electronic properties, which are reflected from their electronic
absorption spectra, fluorescence spectra, and NMR chemical
shift. With extension of the zigzag edges, the molecules show
an acene-like electronic structure and become more reactive,
implying the challenge to synthesize even more extended
kekulenes. The method could be applicable for the synthesis of
other belt-shaped molecules or large-size hexagonal nano-
graphenes, and related works are ongoing in our group.
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(5) Diederich, F.; Staab, H. A. Benzenoid versus Annulenoid
Aromaticity: Synthesis and Properties of Kekulene. Angew. Chem., Int.
Ed. Engl. 1978, 17, 372.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c06757.
̌
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E.; Pena, D.; Gross, L.; Pérez, D. Revisiting Kekulene: Synthesis and
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Synthetic procedures and characterization data, addi-
tional spectra, X-ray crystallographic data, and DFT
calculations details (PDF)
Accession Codes
CCDC 2081432−2081433 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(7) Funhoff, D. J. H.; Staab, A. Cyclo[d.e.d.e.e.d.e.d.e.e.]-
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AUTHOR INFORMATION
■
(b) Majewski, M. A.; Step̧ ien, M. Bowls, Hoops, and Saddles:
Synthetic Approaches to Curved Aromatic Molecules. Angew. Chem.,
Int. Ed. 2019, 58, 86.
Corresponding Author
Jishan Wu − Department of Chemistry, National University of
Singapore, Singapore 117543, Singapore; Joint School of
National University of Singapore and Tianjin University,
International Campus of Tianjin University, Binhai New
City, Fuzhou 350207, China; orcid.org/0000-0002-
8231-0437; Email: chmwuj@nus.edu.sg
(9) Kumar, B.; Viboh, R. L.; Bonifacio, M. C.; Thompson, W. B.;
Buttrick, J. C.; Westlake, B. C.; Kim, M.; Zoellner, R. W.; Varganov, S.
A.; Mörschel, P.; Teteruk, J.; Schmidt, M. U.; King, B. T. Septulene:
The Heptagonal Homologue of Kekulene. Angew. Chem., Int. Ed.
2012, 51, 12795.
́
(10) Majewski, M. A.; Hong, Y.; Lis, T.; Gregolinski, J.;
́
́
Chmielewski, P. J.; Cybinska, J.; Kim, D.; Step̧ ien, M. Octulene: A
Authors
Hyperbolic Molecular Belt that Binds Chloride Anions. Angew. Chem.,
Int. Ed. 2016, 55, 14072.
Wei Fan − Department of Chemistry, National University of
Singapore, Singapore 117543, Singapore
(11) (a) Beser, U.; Kastler, M.; Maghsoumi, A.; Wagner, M.;
Yi Han − Department of Chemistry, National University of
Singapore, Singapore 117543, Singapore
Xuhui Wang − Department of Chemistry, National University
of Singapore, Singapore 117543, Singapore
Xudong Hou − Department of Chemistry, National University
of Singapore, Singapore 117543, Singapore
Castiglioni, C.; Tommasini, M.; Narita, A.; Feng, X.; Mullen, K. A
̈
C216-Nanographene Molecule with Defined Cavity as Extended
Coronoid. J. Am. Chem. Soc. 2016, 138, 4322. (b) Hou, I. C.; Sun, Q.;
Eimre, K.; Giovannantonio, M. D.; Urgel, J. I.; Ruffieux, P.; Narita, A.;
Fasel, R.; Mullen, K. On-Surface Synthesis of Unsaturated Carbon
̈
Nanostructures with Regularly Fused Pentagon-Heptagon Pairs. J.
Am. Chem. Soc. 2020, 142, 10291. (c) Giovannantonio, M. D.; Yao,
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c06757
X.; Eimre, K.; Urgel, J. I.; Ruffieux, P.; Pignedoli, C. A.; Mullen, K.;
̈
Fasel, R.; Narita, A. Large-Cavity Coronoids with Different Inner and
Outer Edge Structures. J. Am. Chem. Soc. 2020, 142, 12046. (d) Fan,
Q.; Martin-Jimenez, D.; Werner, S.; Ebeling, D.; Koehler, T.;
Vollgraff, T.; Sundermeyer, J.; Hieringer, W.; Schirmeisen, A.;
Gottfried, J. M. On-Surface Synthesis and Characterization of a
Cycloarene: C108 Graphene Ring. J. Am. Chem. Soc. 2020, 142, 894.
(e) Hou, H.; Zhao, X.; Tang, C.; Ju, Y.; Deng, Z.; Wang, X.; Feng, L.;
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Lin, D.; Hou, X.; Narita, A.; Mullen, K.; Tan, Y. Synthesis and
̈
J.W. acknowledges financial support from an A*STAR AME
grant (A20E5c0089) and an NRF Investigatorship (NRF-
NRFI05-2019-0005). We also thank Prof. D. Jiang and Mr. K.
Geng for the support on the fluorescence measurement.
assembly of extended quintulene. Nat. Commun. 2020, 11, 3976.
(12) Fan, W.; Han, Y.; Dong, S.; Li, G.; Lu, X.; Wu, J. Facile
Synthesis of Aryl-Substituted Cycloarenes via Bismuth(III) Triflate-
Catalyzed Cyclization of Vinyl Ethers. CCS Chem. 2021, 2, 1445.
(13) (a) Yang, L.; Zhang, N.; Han, Y.; Zou, Y.; Qiao, Y.; Chang, D.;
Zhao, Y.; Lu, X.; Wu, J.; Liu, Y. A sulfur-containing hetero-octulene:
synthesis, host-guest properties, and transistor applications. Chem.
Commun. 2020, 56, 9990. (b) Su, J.; Fan, W.; Mutombo, P.; Peng, X.;
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