Defective Nanographenes Containing Seven-Five-Seven (7-5-7)-Membered Rings

Article abstract of DOI:10.1021/jacs.0c12116

Defects have been observed in graphene and are expected to play a key role in its optical, electronic, and magnetic properties. However, because most of the studies focused on the structural characterization, the implications of topological defects on the physicochemical properties of graphene remain poorly understood. Here, we demonstrate a bottom-up synthesis of three novel nanographenes (1-3) with well-defined defects in which seven-five-seven (7-5-7)-membered rings were introduced to their sp2 carbon frameworks. From the X-ray crystallographic analysis, compound 1 adopts a nearly planar structure. Compound 2, with an additional five-membered ring compared to 1, possesses a slightly saddle-shaped geometry. Compound 3, which can be regarded as the "head-to-head"fusion of 1 with two bonds, features two saddles connected together. The resultant defective nanographenes 1-3 were well-investigated by UV-vis absorption, cyclic voltammetry, and time-resolved absorption spectra and further corroborated by density functional theory (DFT) calculations. Detailed experimental and theoretical investigations elucidate that these three nanographenes 1-3 exhibit an anti-aromatic character in their ground states and display a high stability under ambient conditions, which contrast with the reported unstable biradicaloid nanographenes that contain heptagons. Our work reported herein offers insights into the understanding of structure-related properties and enables the control of the electronic structures of expanded nanographenes with atomically precise defects.

Full text of DOI:10.1021/jacs.0c12116

pubs.acs.org/JACS
Article
Defective Nanographenes Containing Seven-Five-Seven (7−5−7)-
Membered Rings
Yiyang Fei, Yubin Fu, Xueqin Bai, Lili Du, Zichao Li, Hartmut Komber, Kam-Hung Low,
Shengqiang Zhou, David Lee Phillips, Xinliang Feng, and Junzhi Liu*
Cite This: J. Am. Chem. Soc. 2021, 143, 2353−2360
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Defects have been observed in graphene and are expected to play
a key role in its optical, electronic, and magnetic properties. However, because
most of the studies focused on the structural characterization, the implications of
topological defects on the physicochemical properties of graphene remain poorly
understood. Here, we demonstrate a bottom-up synthesis of three novel
nanographenes (1−3) with well-defined defects in which seven-five-seven (7−
5−7)-membered rings were introduced to their sp2 carbon frameworks. From the
X-ray crystallographic analysis, compound 1 adopts a nearly planar structure.
Compound 2, with an additional five-membered ring compared to 1, possesses a
slightly saddle-shaped geometry. Compound 3, which can be regarded as the
“head-to-head” fusion of 1 with two bonds, features two saddles connected
together. The resultant defective nanographenes 1−3 were well-investigated by
UV−vis absorption, cyclic voltammetry, and time-resolved absorption spectra and further corroborated by density functional theory
(DFT) calculations. Detailed experimental and theoretical investigations elucidate that these three nanographenes 1−3 exhibit an
anti-aromatic character in their ground states and display a high stability under ambient conditions, which contrast with the reported
unstable biradicaloid nanographenes that contain heptagons. Our work reported herein offers insights into the understanding of
structure-related properties and enables the control of the electronic structures of expanded nanographenes with atomically precise
defects.
INTRODUCTION
■
Graphene is an atomically thick sheet composed of a hexagonal
network of sp2-hybridized carbon atoms with exceptional
electronic and optical properties and is also a credible starting
point of materials for new disruptive technologies across a wide
range of fields.1−4 However, graphene is a semimetal without a
bandgap and cannot be directly used for electronics. This
makes finding a way to open the bandgap for graphene-based
nanoelectronics crucial.5 The most prominent way is to locally
confine the electrons in what renders stripes of graphene, so-
called graphene nanoribbons (GNRs), as semiconducting
materials.6−9 Another pathway to influence the electronic
structure of graphene is the introduction of imperfections or
Figure 1. (a) Azulene unit. (b) Stone−Wales defect. (c) Recent
defects in the basal plane of the graphene.10 Theoretical
examples of nanographenes with an open-shell character containing
5−7-membered rings.17,20 (d) The defective nanographenes with 7−
5−7-membered rings reported in this work.
calculations have described that the topological defects in
graphene significantly affect its optical, electronic, and
magnetic properties.11 The typical stable graphene defects,
including atom dislocation (pentagon−heptagon pair, Figure
1a), Stone−Wales (SW) defects (point defects, Figure 1b),
vacancies (single and multiple), and one-dimensional defects
(line defects), have been predicted and also experimentally
observed.10 From the viewpoint of energetic stability, the
pentagon−heptagon pair (5−7 rings) is one of the reasonable
defect models that can be generated in a graphene layer due to
Received: November 21, 2020
Published: January 27, 2021
https://dx.doi.org/10.1021/jacs.0c12116
J. Am. Chem. Soc. 2021, 143, 2353−2360
© 2021 American Chemical Society
2353

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Scheme 1. Schematic Illustration of the Synthesis of 1, 2, and 3
Figure 2. X-ray crystallographic molecular structures of (a) 1 and (b) 2. The hydrogen atoms are omitted for clarity. Side view of (c) 1 and (d) 2 as
well as the bond lengths of the embedded 7−5−7-membered rings (the substituents in 1 and 2 are removed for clarity). (e and f) Crystal packing
of 1 and 2, respectively. Orange represents the M-enantiomer, and light green represents the P-enantiomer.
2354
https://dx.doi.org/10.1021/jacs.0c12116
J. Am. Chem. Soc. 2021, 143, 2353−2360

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
an atom dislocation (Figure 1a).12,13 Generally, the topological
defects either can be introduced to graphene or related 2D
materials by electron irradiation or are formed during a high-
temperature growth.14,15 Despite the growing number of
experimental observations, the defects in graphene are still not
well understood, especially in regard to evaluating and studying
the effect of topological defects on the optical, electronic, and
magnetic properties.16 Therefore, the investigation of the
chemical and physical properties that arise from the defined
defects plays an important role in the defect engineering of
graphene, which can provide insight into the understanding of
the structure−property relationship of graphene.
Recently, processes embedding pentagon−heptagon pairs
(an azulene unit) into nanographenes (also called polycyclic
aromatic hydrocarbons, PAHs) have been developed. For
instance, we reported the synthesis of open-shell nano-
graphenes containing a pair of azulene units both in a solution
and on a surface (Figure 1c).17 Contorted PAHs embedding
two azulene cores were synthesized by Mastalerz et al. through
cyclopent- and cycloheptannulations.18 A nonbenzenoid PAH
with two pentagons and two heptagons with high hole-mobility
was reported by Zhang and coauthors.19 Konishi and Yasuda et
al. reported an open-shell nonalternant isomer of bisanthene
that contained a heptalene structure (Figure 1c).20 Chi et al.
demonstrated the synthesis of an azulene-embedded nano-
graphene due to the unexpected naphthalene-to-azulene
rearrangement during the Scholl reaction.21 Very recently,
helical nanographenes containing azulene units were success-
fully and independently synthesized by us22 and Takasu et
al.23. Despite this achieved progress, the synthesis of a
nonalternant topology in graphene nanostructures,24−26 such
as with the seven-five-seven (7−5−7)-membered ring motifs
(Figure 1d), remains elusive due to the lack of a facile synthetic
approach27−30 and the unexpected rearrangements.21,31,32
Herein, for the first time we demonstrate an efficient
synthetic strategy toward three unprecedented defective
nanographenes containing 7−5−7-membered ring motifs
(1−3, Figure 1d and Scheme 1). Compared with 1, there is
an additional five-membered ring in 2, and compound 3 with
two pairs of 7−5−7-membered rings can be regarded as a
“head-to-head” fusion of 1 at the zigzag edge. Single-crystal X-
ray analysis unequivocally reveals the unique nonalternant
topologies embedded in 1−3. Compound 1 adopts a nearly
planar structure, while 2 possesses a slightly saddle-shaped
geometry, with torsion angles of 20.6° and 21.4°, respectively
(Figure 2). Compound 3 adopts a different geometry
compared to its monomer 1 in which the subunits “1” feature
the saddle-shaped geometry, resulting in two saddles
connected together (Figure 3). Interestingly, ring A in 1
(Figure 2a) possesses a pronounced bond alternation for the p-
quinodimethane framework28 (Figure 2c), leading to its
antiaromatic character in contrast to the aromatic dibenzo-
corannulene with 6−5−6-membered rings (Figure 1d).
However, ring A in 2 (Figure 2b) does not feature the bond
alternation of the p-quinodimethane unit as exhibited in 1,
suggesting its open-shell biradical character (Scheme S1).33−35
Indeed, from the density functional theory (DFT) calculations
compounds 1 and 3 display a closed-shell (antiaromatic)
feature (see below), while 2 has a moderate singlet biradical
character (y0 = 0.29, UHF method) with a large singlet−triplet
energy gap (ΔES‑T = −15.1 kcal/mol, UCAM-B3LYP/6-
31G(d)). However, all the experimental results, including
temperature-dependent NMR, electron paramagnetic reso-
Figure 3. X-ray crystallographic molecular structure of 3. The
hydrogen atoms are omitted for clarity. (a) Top view and (b) side
view of 3 (the substituents are removed for clarity). (c) The bond
lengths of the embedded 7−5−7-membered rings. (d) Crystal
packing of 3.
nance (EPR), and superconducting quantum interfering device
(SQUID) measurements, suggest that it is more reasonable to
describe 2 as a closed-shell antiaromatic compound in its
ground state. Therefore, the resultant antiaromatic nano-
graphenes 1−3 exhibit high stabilities over months; this feature
stands in contrast with the reported unstable open-shell
nanographenes containing 5−7-membered rings (Figure
1c).17,20 Our work reported herein provides a new line to
the synthesis of air-stable antiaromatic nanographenes with
nonalternant topologies and enables the control of their
electronic structures.
RESULTS AND DISCUSSION
■
Synthesis of 1, 2, and 3. The synthetic routes toward
compounds 1−3 are descried in Scheme 1. First, the key
building block 1,6-dibromo-3,4-dichloro-7,10-dimethylfluoran-
thene (8) was synthesized from the starting material 1,2-
dihydroacenaphthylene (4) in four steps through chlorination,
bromination, oxidation, and a Diels−Alder reaction.36 Then,
compound 9 was obtained via the Suzuki coupling of 8 and (4-
(tert-butyl)phenyl)boronic acid in a 23% yield. The methyl
groups in 9 were transformed into dialdehydes by bromination,
esterification, hydrolysis, and oxidation to afford compound 10
with a yield of 47%. Dihydro-precursor 11 was synthesized by
the treatment of 10 with mesitylmagnesium bromide
(MesMgBr), followed by Friedel−Crafts cyclization. The
dehydrogenation of 11 using 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) provided the dark yellow compound
2355
https://dx.doi.org/10.1021/jacs.0c12116
J. Am. Chem. Soc. 2021, 143, 2353−2360

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Figure 4. Calculated isomerization process of (a) 1, (b) 2, and (c) 3. The relative Gibbs free energy (kcal/mol) was calculated at the B3LYP/6-
31G(d) level. All calculations were corrected for dispersion (D3BJ).
1 with a yield of 90% in three steps. On the other hand,
compounds 12 and 16 were achieved through the selective
Suzuki coupling of 8 and (4-(tert-butyl)phenyl)boronic acid.
Subsequently, the Pd-involved Heck reaction of 12 gave 13
(quantitative). Following a similar synthetic strategy, com-
pound 2 was successfully synthesized as dark red solid with a
yield of 92% in three steps. In addition, the key precursor
dialdehyde 17 was synthesized in a 33% yield from compound
16. Finally, dark red compound 3 containing two pairs of 7−
5−7-membered rings was achieved through the Yamamoto
coupling of 19 in a 73% yield. The chemical structures of 1, 2
and 3 were confirmed by high-resolution mass spectrometry
(HR-MS), NMR analysis (both see the Supporting Informa-
tion), and X-ray single-crystal analysis (vide infra).
Structural Characterization. The single crystals of 1 and
2 were each grown by slow evaporation from an n-hexane/
dichloromethane solution, which could distinctly unveil their
structural uniqueness with the presence of 7−5−7-motifs
through the X-ray single-crystal analysis. Compound 1 adopts a
nearly geometric plane surface (Figure 2a) in which the
heptagonal ring B induces the adjacent benzene ring out of
planarity with a dihedral angle of 21.9° (Figure 2c), whereas
the other seven-membered ring C features a planar structure.
Accordingly, there are two different conformations in the solid
state of 1, that is, enantiomers M and P coexist with a ratio of
1:1 in the crystal (Figure 2e). In comparison to 1, compound 2
with an additional five-membered ring displays a slightly
saddle-shaped geometry, where the heptagonal rings B and C
arise from the steric hindrance between their adjacent benzene
rings E and F and G and H with the dihedral angles of 21.6°
and 20.4°, respectively (Figure 2b and d). In the packing
pattern, compound 2 crystallizes along the c-axis of the unit cell
with a pair of enantiomers (M and P) to establish columnar
superstructures with a minimum interplanar space of 3.28 Å
(Figure 2f).
its monomer 1. The subunits “1” in 3 feature the saddle-shaped
geometries, resulting in two saddles connected together in 3
(Figure 3b). Accordingly, the dihedral angles of rings E and F
and G and H in 3 are 23.0° and 28.4° (Figure 3a), respectively,
which are larger than those in 2. Due to the presence of the
sterically hindered mesityl and tert-butyl groups in 3, no
intermolecular π−π stacking interaction can be formed
between the adjacent molecules (Figure 3d). In addition, the
X-ray analysis discloses the detailed bonding information on 1,
2, and 3 (Figures 2c and d and 3c). The bond lengths in 1
reveal that the six-membered ring A (Figure 2c) has two short
C−C bonds (1.346(4) and 1.376(3) Å) and four long C−C
bonds (1.437(3)−1.463(3) Å), manifesting a pronounced
bond alternation for the p-quinodimethane skeleton in 128,29 as
indicated by the antiaromaticity that appeared in the B−A−C
framework (7−6−7 rings, Figure 2). For 2, the intermediate
value (1.407(8) Å, Figure 2d) of bond a is substantially longer
than that of 1 (1.370(3) Å), indicating the contribution of the
open-shell resonance structure in the ground state of 2 (2−1 in
Scheme S1);37−40 nevertheless, the bond length of a in 2 is
shorter than that of the open-shell indeno[2,1-b]fluorene41
(1.437 Å, y0 = 0.68, Figure S64), coinciding with the relatively
smaller open-shell character of 2 (y0 = 0.29). However, the
temperature-dependent NMR (Figure S18), EPR, and SQUID
(Figure S63) measurements suggest that it is more reasonable
to describe 2 as a closed-shell antiaromatic compound in its
ground state. Interestingly, the bond length of b at the junction
(ring I) in 3 is 1.462(4) Å, suggesting its single-bond character
(Figure 3a). The bond lengths of the six-membered ring A in 3
are comparable with those in 1 (Figure 3c).
Theoretical Characterization. Since there are different
conformations in the solid state, DFT calculations for the
relative energies of 1, 2, and 3 as well as the transition states
(TS), were performed at the B3LYP/6-31G(d) level of theory
to evaluate the isomerization process of the different
enantiomers (Figure 4). The dispersion correction was
conducted using Grimme’s D3 version with the BJ damping
function.42 The thermodynamic stabilities of the M- and P-
enantiomers for 1 are similar in terms of their Gibbs free
Moreover, the single crystals of 3 were obtained from an n-
hexane/dichloroethane solution. Compound 3, which can be
regarded as a “head-to-head” fusion of 1 at the rings F and G
with two bonds (Figure 3a), adopts a different geometry than
2356
https://dx.doi.org/10.1021/jacs.0c12116
J. Am. Chem. Soc. 2021, 143, 2353−2360

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Figure 5. NICS(1)zz‑avg values (calculated at the GIAO-B3LYP/6-31+G(2d,p) level of theory) and calculated ACID plots of (a and b) 1, (c and d)
2, (e and f) 3 , and (g and h) dibenzo-corannulene. The isovalue is 0.05, and the diamagnetic (clockwise) and paramagnetic (counterclockwise)
ring currents under the magnetic field parallel to the z-axis are highlighted by red and blue arrows, respectively.
energies (ΔG). DFT calculations show that compound 1 can
undergo an M-to-P conversion through a flat transition state
(Figure 4a). The free-activation energy of 1 was estimated to
be 3.08 kcal/mol, which is significantly higher than that
without a dispersion correction (2.82 kcal/mol, Figure S72)
and substantially smaller than that of dibenzocorannulene with
a 6−5−6-membered ring (8.10 kcal/mol, Figure S73),
indicating a fast isomerization process even at room temper-
ature. This result is also in line with the variable-temperature
NMR measurements (Figure S6) and the fact that both M- and
P-enantiomers were experimentally observed in the solid state
of 1 (Figure 2e). Therefore, two enantiomers of racemic 1
always exist together, and it is not possible to separate them for
further investigations. Similar to compound 1, the thermody-
namical stabilities of the M- and P-isomers for 2 are almost the
same (Figure 4b). The energy barrier for the isomerization
process in 2 was calculated to be 2.69 kcal/mol based on the
dispersion-corrected model. This is comparable to that without
a dispersion correction (2.76 kcal/mol, Figure S72) and lower
than that of 1 (3.08 kcal/mol). For 3, DFT calculations with
the dispersion-corrected model predict the activation energies
of 1.17 and 2.97 kcal/mol for the M,M-to-P,M and P,M-to-P,P
isomerization processes (Figure 4c), respectively, which are
lower than that of compound 1 (3.08 kcal/mol).
Furthermore, to evaluate the aromaticity of each of the rings
in these three defective nanographenes 1−3, nucleus-
independent chemical shift (NICS) calculations were
performed at the GIAO-B3LYP/6-311+G(2d,p) level of
theory. The obtained NICS(1)zz values of 1 are 13.5 and 9.4
for the heptagonal rings (B, C) and hexagonal ring A (Figure
5a), respectively, which is consistent with the X-ray structural
analysis and demonstrates their antiaromatic feature; mean-
while, large negative values were found at the peripheral
benzene rings E (−15.7) and F (−20.7). Accordingly, the
paratropic character on ring A and the heptagons in 1
magnetically suppresses the aromatic character of the
peripheral hexagons E and F. Therefore, the 7−5−7 ring-
embedded nanographene 1 exhibits an antiaromatic nature
compared to the aromatic dibenzo-corannulene with a 6−5−6
motif (Figure 5g). Compound 3 containing dual 7−5−7 ring
motifs features a similar behavior to that of 1 (Figure 5e). For
instance, the values of rings A and B in 3 are comparable with
those of 1, while the ring G in 3 displays a large positive value
(21.4). Moreover, the values of rings A and B in 2 are similar
to those of 1 (Figure 5c); however, the central five-membered
ring D (−1.2) in 2 seems to behave as a nonaromatic ring in
which the value is noticeably higher than that of in 1 (−14.1)
(Figure 5a). This difference in 1 and 2 also suggests the
possibility of an open-shell character for 2. To further support
the local aromaticity of the 7−5−7 embedded nanographenes
1−3, the anisotropy of the induced current density (ACID)
analysis was evaluated (Figure 5b, d, and f, respectively).
Continuous counterclockwise paratropic ring currents appear
around the 7−6−7 units (rings B−A−C) in the ACID plots of
1, 2, and 3. However, diamagnetic ring currents are found in
other six-membered rings. These results indicate that the 7−
6−7 rings (rings B−A−C) in 1−3 display an antiaromatic
character, which is consistent with the results of the NICS
calculations.
Optical and Electrochemical Properties. Compounds 1,
2, and 3 have excellent solubilities in common organic solvents
such as dichloromethane (DCM), tetrahydrofuran (THF),
toluene, etc. The UV−vis absorption spectra of 1−3 in DCM
solutions are shown in Figure 6a. Compounds 1 and 2 display
similarly shaped UV−vis absorption patterns. For instance,
there is one major absorption peak at 310 nm and another
absorption peak centered at 520 nm with two shoulder peaks
at 490 and 455 nm for 1. Compound 2 exhibits a
bathochromic shift by 49 nm relative to that of 1, with an
absorption maximum at 569 nm and two shoulder peaks at 530
2357
https://dx.doi.org/10.1021/jacs.0c12116
J. Am. Chem. Soc. 2021, 143, 2353−2360

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
reduction waves. The corresponding electrochemical energy
gaps for 1, 2, and 3 were calculated to be 2.24, 1.86, and 1.66
eV, respectively.
Time-Resolved Absorption Spectroscopy. Femtosec-
ond transient absorption spectra with 400 nm excitation were
recorded in THF to study the excited-state photophysical
dynamics of defective nanographenes 1, 2, and 3. As shown in
Figure 7, the ground-state bleaching (GSB) signals were
Figure 6. (a) UV−vis absorption spectra of 1, 2, and 3 in DCM (1 ×
10−5 mol/L). The insert shows photos of their solutions. (b) Cyclic
voltammetry of 1, 2, and 3 in THF containing 0.1 M nBu4NPF6 at a
scan rate of 50 mV/s.
and 492 nm. The maximum absorption peak of 3, which can be
regarded as the dimerization of 1 with a “head-to-head” fusion,
displays a red-shift by 10 nm, with a broad absorption shoulder
at 582 nm compared to 1 due to the π-expanded conjugation.
The calculated UV−vis spectra of compounds 1−3 are also
displayed in Figures S74−S76 and Tables S1−S3, respectively,
which are consistent with the experimental results. Accord-
ingly, the optical energy gaps for 1, 2, and 3 were determined
to be 2.21, 2.06, and 1.96 eV, respectively, from the onset of
their UV−vis absorption spectra. Interestingly, the DCM
solutions of compounds 1−3 were nonemissive when excited
with UV light, which is also in agreement with their
antiaromatic character. Furthermore, time-dependent UV−vis
measurements were performed under ambient conditions to
investigate the stability of 1, 2, and 3. There were no significant
changes in the absorption curves for 1−3 within two months
(Figure S39). The electrochemical behaviors of 1, 2, and 3
were investigated by cyclic voltammetry (CV) in THF (Figure
6b). Compound 1 shows one reversible oxidation wave with a
half-wave potential (Eo1/x2) at 0.92 V (vs Fc+/Fc) and one
reversible reduction wave with a half-wave potential (Er1e/2) at
−1.32 V (vs Fc+/Fc). However, compound 2 manifests two
reversible oxidation waves with half-wave potentials (Eo1/x2) at
0.76 and 1.15 V and one reversible reduction wave with a half-
wave potential (Er1e/2) at −1.10 V (vs Fc+/Fc). For compound
3, there are two reversible oxidation waves (Eo1/x2) and two
reversible reduction waves (Er1e/2) with half-wave potentials at
0.98 and 1.01 and −0.68 and − 0.91 V, respectively. Thus, the
HOMO/LUMO energy levels were estimated to be −5.61/−
3.37, − 5.45/−3.59, and −5.67/−4.01 eV for compounds 1, 2,
and 3, respectively, based on the first reversible oxidation/
Figure 7. Femtosecond time-resolved absorption (fs-TA) spectra of
compounds (a) 1, (b) 2, and (c) 3 in THF at a 400 nm laser pulse
excitation.
observed for compounds 1−3, which is consistent with their
UV−vis spectra. Unlike the GSB, the excited-state absorption
(ESA) spectra were explicitly different for compounds 1−3. As
shown in Figure 7a, the compound 1 exhibits two main ESA
bands at 440 and 630 nm with a shoulder at 690 nm, which
can be assigned to the singlet excited states of compound 1 due
to its short lifetime (15 ps, Figure S62a) and nonfluorescent
behavior. Unlike compound 1, only one ESA band at 450 nm
with an absorption tail band extending up to 700 nm was
2358
https://dx.doi.org/10.1021/jacs.0c12116
J. Am. Chem. Soc. 2021, 143, 2353−2360

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Authors
observed for compound 2, as shown in Figure 7b. This band
can also be assigned to the singlet excited states of compound
2 with a lifetime of 12 ps (Figure S62b). Considering the
different ESA pattern, one is able to conclude that the ESA
spectra for both compounds 1 and 2 have contributions from
the different electronic configurations of their singlet excited
states. Furthermore, the fs-TA spectra of compound 3 in
Figure 7c exhibit only one broad ESA band at 680 nm, which is
red-shifted by 50 nm compared to that of compound 1 and
suggests that an extended conjugation contributes to the
overall singlet-states stabilization in compound 3. A short
lifetime within 10 ps was also observed for the singlet excited
states of compound 3 in Figure S62c. Therefore, those
differences in the fs-TA spectra for compounds 1−3 are likely
due to their different molecular topologies, suggesting that
their singlet excited states have different electronic config-
urations.
Yiyang Fei − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The University of Hong
Kong, 999077 Hong Kong, P. R. China
Yubin Fu − Centre for Advancing Electronics Dresden
(CFAED), Faculty of Chemistry and Food Chemistry,
Technische Universität Dresden, 01062 Dresden, Germany;
orcid.org/0000-0002-2613-394X
Xueqin Bai − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The University of Hong
Kong, 999077 Hong Kong, P. R. China
Lili Du − Department of Chemistry and State Key Laboratory
of Synthetic Chemistry, The University of Hong Kong,
999077 Hong Kong, P. R. China
Zichao Li − Helmholtz-Zentrum Dresden-Rossendorf, Institute
of Ion Beam Physics and Materials Research, 01328 Dresden,
Germany
Hartmut Komber − Leibniz-Institut für Polymerforschung
Dresden e. V., 01069 Dresden, Germany; orcid.org/0000-
0001-6176-6737
Kam-Hung Low − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The University of Hong
Kong, 999077 Hong Kong, P. R. China; orcid.org/0000-
0003-3292-8893
Shengqiang Zhou − Helmholtz-Zentrum Dresden-Rossendorf,
Institute of Ion Beam Physics and Materials Research, 01328
Dresden, Germany; orcid.org/0000-0002-4885-799X
David Lee Phillips − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The University of Hong
Kong, 999077 Hong Kong, P. R. China
Xinliang Feng − Centre for Advancing Electronics Dresden
(CFAED), Faculty of Chemistry and Food Chemistry,
Technische Universität Dresden, 01062 Dresden, Germany;
orcid.org/0000-0003-3885-2703
CONCLUSIONS
■
In summary, we have demonstrated the synthesis of three
unprecedented nanographenes with seven-five-seven (7−5−
7)-membered rings. The crystallographic characterizations
unambiguously show that 1 adopts a nearly planar structure,
while compound 2, with an additional five-membered ring
compared to 1, manifests a slightly saddle-shaped geometry.
Interestingly, the subunits “1” in compound 3, which can be
regarded as the “head-to-head” fusion of 1 with two bonds,
feature the saddle-shaped geometries. The structural analyses
combined with the DFT calculations demonstrated that
compounds 1, 2, and 3 represent a new type of air-stable
and fully conjugated antiaromatic nanographenes containing
7−5−7-membered rings. More importantly, the synthetic
strategy in regard to the defect engineering in nanographenes
by building 7−5−7-membered rings can be applied to the
construction of other air-stable and π-expanded nanographenes
with antiaromatic or open-shell structures.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12116
Funding
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c12116.
■
This work was supported by the Hong Kong Research Grants
Council (HKU 27301720). J.L. is grateful for the startup
funding from The University of Hong Kong and the funding
support from ITC to the SKL.
sı
Synthetic procedures and characterization data for all
new compounds, general experimental methods, addi-
tional NMR and HR-MS spectroscopic data and spectra,
DFT calculation details, and crystallographic data (PDF)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the UGC funding administered by The University of
Hong Kong (HKU) for supporting the time-of-flight mass
spectrometry facilities under the Support for Interdisciplinary
Research in Chemical Science, and the HKU University
Development Fund for funding the X-ray diffractometer
facilities. Y. Fu thanks the Center for Information Services
and High Performance Computing (ZIH) at TU Dresden for
the generous allocations of computational resources.
Accession Codes
CCDC 2033693 for compound 1, CCDC 2010758 for
compound 2, and CCDC 2042159 for compound 3 contain
the supplementary crystallographic 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.
REFERENCES
■
(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.;
Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-
dimensional gas of massless Dirac fermions in graphene. Nature 2005,
438, 197−200.
(2) Du, X.; Skachko, I.; Duerr, F.; Luican, A.; Andrei, E. Y.
Fractional quantum Hall effect and insulating phase of Dirac electrons
in graphene. Nature 2009, 462, 192−195.
AUTHOR INFORMATION
Corresponding Author
Junzhi Liu − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The University of Hong
Kong, 999077 Hong Kong, P. R. China; orcid.org/0000-
0001-7146-0942; Email: juliu@hku.hk
■
2359
https://dx.doi.org/10.1021/jacs.0c12116
J. Am. Chem. Soc. 2021, 143, 2353−2360

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(3) Narita, A.; Wang, X. Y.; Feng, X.; Mu
̈
llen, K. New advances in
(24) Yamamoto, K.; Ie, Y.; Tohnai, N.; Kakiuchi, F.; Aso, Y.
Antiaromatic character of cycloheptatriene-bis-annelated indenofluor-
ene framework mainly originated from heptafulvene segment. Sci. Rep.
2018, 8, 17663.
nanographene chemistry. Chem. Soc. Rev. 2015, 44, 6616−6643.
(4) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater.
2007, 6, 183−191.
(25) Pigulski, B.; Shoyama, K.; Wurthner, F. NIR−Absorbing π−
̈
(5) Weiss, N. O.; Zhou, H.; Liao, L.; Liu, Y.; Jiang, S.; Huang, Y.;
Duan, X. Graphene: An Emerging Electronic Material. Adv. Mater.
2012, 24, 5782−5825.
Extended Azulene: Non−Alternant Isomer of Terrylene Bisimide.
Angew. Chem., Int. Ed. 2020, 59, 15908−15912.
(26) Kirschbaum, T.; Rominger, F.; Mastalerz, M. A Chiral
Polycyclic Aromatic Hydrocarbon Monkey Saddle. Angew. Chem.,
Int. Ed. 2020, 59, 270−274.
(6) Narita, A.; Feng, X.; Mullen, K. Bottom-up synthesis of
̈
chemically precise graphene nanoribbons. Chem. Rec. 2015, 15, 295−
309.
(27) Zhu, C.; Shoyama, K.; Wurthner, F. Conformation and
̈
(7) Talirz, L.; Ruffieux, P.; Fasel, R. On-Surface Synthesis of
Atomically Precise Graphene Nanoribbons. Adv. Mater. 2016, 28,
6222−6231.
Aromaticity Switching in a Curved Non−Alternant sp2 Carbon
Scaffold. Angew. Chem., Int. Ed. 2020, 59, 21505−21509.
(28) Yang, X.; Liu, D.; Miao, Q. Heptagon-Embedded Pentacene:
Synthesis, Structures and Thin Film Transistors of Dibenzo[d,d’]-
benzo[1,2-a:4,5-a’]dicycloheptenes. Angew. Chem., Int. Ed. 2014, 53,
6786−6790.
(8) Narita, A.; Chen, Z.; Chen, Q.; Mullen, K. Solution and on-
̈
surface synthesis of structurally defined graphene nanoribbons as a
new family of semiconductors. Chem. Sci. 2019, 10, 964−975.
(9) Jolly, A.; Miao, D.; Daigle, M.; Morin, J.-F. Emerging Bottom-Up
Strategies for the Synthesis of Graphene Nanoribbons and Related
Structures. Angew. Chem., Int. Ed. 2020, 59, 4624−4633.
(10) Banhart, F.; Kotakoski, J.; Krasheninnikov, A. V. Structural
Defects in Graphene. ACS Nano 2011, 5, 26−41.
(29) Konishi, A.; Kubo, T. Benzenoid Quinodimethanes. Top. Curr.
Chem. 2017, 375, 83.
(30) Konishi, A.; Yasuda, M. Breathing New Life into Nonalternant
Hydrocarbon Chemistry: Syntheses and Properties of Polycyclic
Hydrocarbons Containing Azulene, Pentalene, and Heptalene Frame-
works. Chem. Lett. 2021, 50, 195−212.
(11) Yazyev, O. V.; Louie, S. G. Electronic transport in
polycrystalline graphene. Nat. Mater. 2010, 9, 806−809.
(12) Warner, J. H.; Margine, E. R.; Mukai, M.; Robertson, A. W.;
Giustino, F.; Kirkland, A. I. Dislocation-Driven Deformations in
Graphene. Science 2012, 337, 209−212.
(13) Stone, A. J.; Wales, D. J. Chem. Phys. Lett. 1986, 128, 501−503.
(14) Hashimoto, A.; Suenaga, K.; Gloter, A.; Urita, K.; Iijima, S.
Direct evidence for atomic defects in graphene layers. Nature 2004,
430, 870−873.
(15) Huang, P. Y.; Ruiz-Vargas, C. S.; van der Zande, A. M.;
Whitney, W. S.; Levendorf, M. P.; Kevek, J. W.; Garg, S.; Alden, J. S.;
Hustedt, C. J.; Zhu, Y.; Park, J.; McEuen, P. L.; Muller, D. A. Grains
and Grain Boundaries in Single-Layer Graphene Atomic Patchwork
Quilts. Nature 2011, 469, 389−392.
(16) Rasool, H. I.; Ophus, C.; Zettl, A. Atomic Defects in Two
Dimensional Materials. Adv. Mater. 2015, 27, 5771−5777.
(17) Liu, J.; Mishra, S.; Pignedoli, C. A.; Passerone, D.; Urgel, J. I.;
Fabrizio, A.; Lohr, T. G.; Ma, J.; Komber, H.; Baumgarten, M.;
(31) Das, S.; Wu, J. Toward Singlet-Triplet Bistable Nonalternant
́
Kekule Hydrocarbons: Azulene-to-Naphthalene Rearrangement. Org.
Lett. 2015, 17, 5854−5857.
(32) Shiotari, A.; Nakae, T.; Iwata, K.; Mori, S.; Okujima, T.; Uno,
H.; Sakaguchi, H.; Sugimoto, Y. Strain-induced skeletal rearrangement
of a polycyclic aromatic hydrocarbon on a copper surface. Nat.
Commun. 2017, 8, 16089.
(33) Frederickson, C. K.; Rose, B. D.; Haley, M. M. Explorations of
the Indenofluorenes and Expanded Quinoidal Analogues. Acc. Chem.
Res. 2017, 50, 977−987.
(34) Rudebusch, G. E.; Zafra, J. L.; Jorner, K.; Fukuda, K.; Marshall,
J. L.; Arrechea-Marcos, I.; Espejo, G. L.; Ponce Ortiz, R.; Gomez-
Garcia, C. J.; Zakharov, L. N.; Nakano, M.; Ottosson, H.; Casado, J.;
Haley, M. M. Diindeno-fusion of an anthracene as a design strategy
for stable organic biradicals. Nat. Chem. 2016, 8, 753−759.
(35) Ma, J.; Zhang, K.; Schellhammer, K. S.; Fu, Y.; Komber, H.; Xu,
C.; Popov, A. A.; Hennersdorf, F.; Weigand, J. J.; Zhou, S.; Pisula, W.;
Ortmann, F.; Berger, R.; Liu, J.; Feng, X. Wave-shaped polycyclic
hydrocarbons with controlled aromaticity. Chem. Sci. 2019, 10, 4025−
4031.
Corminboeuf, C.; Berger, R.; Ruffieux, P.; Mullen, K.; Fasel, R.; Feng,
̈
X. Open-Shell Nonbenzenoid Nanographenes Containing Two Pairs
of Pentagonal and Heptagonal Rings. J. Am. Chem. Soc. 2019, 141,
12011−12021.
(36) Seiders, T. J.; Elliott, E. L.; Grube, G. H.; Siegel, J. S. Synthesis
of Corannulene and Alkyl Derivatives of Corannulene. J. Am. Chem.
Soc. 1999, 121, 7804−7813.
(18) Yang, X.; Rominger, F.; Mastalerz, M. Contorted Polycyclic
Aromatic Hydrocarbons with Two Embedded Azulene Units. Angew.
Chem., Int. Ed. 2019, 58, 17577−17582.
(37) Kubo, T. Recent Progress in Quinoidal Singlet Biradical
Molecules. Chem. Lett. 2015, 44, 111−122.
(19) Zhang, X.-S.; Huang, Y.-Y.; Zhang, J.; Meng, W.; Peng, Q.;
Kong, R.; Xiao, Z.; Liu, J.; Huang, M.; Yi, Y.; Chen, L.; Fan, Q.; Lin,
G.; Liu, Z.; Zhang, G.; Jiang, L.; Zhang, D. Dicyclohepta[ijkl,uvwx]-
rubicene with Two Pentagons and Two Heptagons as a Stable and
Planar Non−benzenoid Nanographene. Angew. Chem., Int. Ed. 2020,
59, 3529−3533.
́
(38) Zeng, Z.; Shi, X.; Chi, C.; Lopez Navarrete, J. T.; Casado, J.;
Wu, J. Pro-aromatic and anti-aromatic π-conjugated molecules: an
irresistible wish to be diradicals. Chem. Soc. Rev. 2015, 44, 6578−
6596.
(39) Dressler, J. J.; Haley, M. M. Learning how to fine-tune diradical
properties by structure refinement. J. Phys. Org. Chem. 2020, 33,
No. 4114.
(40) Fix, A. G.; Deal, P. E.; Vonnegut, C. L.; Rose, B. D.; Zakharov,
L. N.; Haley, M. M. Indeno[2,1-c]fluorene: A New Electron-
Accepting Scaffold for Organic Electronics. Org. Lett. 2013, 15,
1362−1365.
(41) Shimizu, A.; Kishi, R.; Nakano, M.; Shiomi, D.; Sato, K.; Takui,
T.; Hisaki, I.; Miyata, M.; Tobe, Y. Indeno[2,1-b]fluorene: A 20-π-
Electron Hydrocarbon with Very Low-Energy Light Absorption.
Angew. Chem., Int. Ed. 2013, 52, 6076−6079.
(42) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the damping
function in dispersion corrected density functional theory. J. Comput.
Chem. 2011, 32, 1456−1465.
(20) Konishi, A.; Horii, K.; Shiomi, D.; Sato, K.; Takui, T.; Yasuda,
M. Open-Shell and Antiaromatic Character Induced by the Highly
Symmetric Geometry of the Planar Heptalene Structure: Synthesis
and Characterization of a Nonalternant Isomer of Bisanthene. J. Am.
Chem. Soc. 2019, 141, 10165−10170.
(21) Han, Y.; Xue, Z.; Li, G.; Gu, Y.; Ni, Y.; Dong, S.; Chi, C.
Formation of Azulene-Embedded Nanographene: Naphthalene to
Azulene Rearrangement During the Scholl Reaction. Angew. Chem.,
Int. Ed. 2020, 59, 9026−9031.
(22) Ma, J.; Fu, Y.; Dmitrieva, E.; Liu, F.; Komber, H.; Hennersdorf,
F.; Popov, A. A.; Weigand, J. J.; Liu, J.; Feng, X. Helical
Nanographenes Containing an Azulene Unit: Synthesis, Crystal
Structures, and Properties. Angew. Chem., Int. Ed. 2020, 59, 5637−
5642.
(23) Ogawa, N.; Yamaoka, Y.; Takikawa, H.; Yamada, K.; Takasu, K.
Helical Nanographenes Embedded with Contiguous Azulene Units. J.
Am. Chem. Soc. 2020, 142, 13322−13327.
2360
https://dx.doi.org/10.1021/jacs.0c12116
J. Am. Chem. Soc. 2021, 143, 2353−2360