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

Article abstract of DOI:10.1021/jacs.9b04080

A nonbenzenoid hydrocarbon, difluoreno-[1,9,8-alkj:1′,9′,8′-gfed]heptalene 1, is synthesized. Experimental and theoretical investigations demonstrate that the planar and symmetric heptalene core within 1 effectively induces the antiaromatic and open-shell character. These properties are not shared by bisanthene 2, a benzenoid isomer of 1.

Full text of DOI:10.1021/jacs.9b04080

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Communication
Open-shell and Antiaromatic Character Induced by the Highly
Symmetric Geometry of the Planar Heptalene Structure: Synthesis
and Characterization of a Non-alternant Isomer of Bisanthene
Akihito Konishi, Koki Horii, Daisuke Shiomi, Kazunobu Sato, Takeji Takui, and Makoto Yasuda
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b04080 • Publication Date (Web): 27 May 2019
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Open-shell and Antiaromatic Character Induced by the Highly Sym-
metric Geometry of the Planar Heptalene Structure: Synthesis and
Characterization of a Non-alternant Isomer of Bisanthene
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Akihito Konishi,*,†,‡ Koki Horii,† Daisuke Shiomi,§ Kazunobu Sato,§ Takeji Takui,§ and Makoto Ya-
suda*,†
†Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-
0871, Japan
‡Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita,
Osaka 565-0871, Japan
§Department of Chemistry and Molecular Materials Science, Graduate School of Science, Osaka City University, 3-3-138
Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
Supporting Information Placeholder
heptalene core may fix the geometry into a plane. The planar hep-
ABSTRACT: A non-benzenoid hydrocarbon, difluoreno[1,9,8-
talene skeleton, which corresponds to a transition structure of the
ring inversion and/or the -bond shift (Figure 1B), provides ac-
cess to its 12 antiaromatic character.18 As a model molecule, we
designed difluoreno[1,9,8-alkj:1',9',8'-gfed]heptalene 1, which is a
non-alternant isomer of bisanthene 2 that incorporates a planar-
ized heptalene skeleton (Figure 1C). Bisanthene19,20 is a ben-
zenoid polycyclic aromatic hydrocarbon (PAH) and the smallest
member of the peri-acene series.21 Although bisanthene possesses
a singlet closed-shell ground state with a local aromatic character,
larger homologs, teranthene22 and peri-tetracene,23,24 are singlet
open-shell compounds. For peri-tetracene, its non-alternant iso-
mer with two azulene cores was successfully synthesized on a
Au(111) surface, demonstrating the appearance of an open-shell
character and a reduction in the HOMO–LUMO energy gap.3g
alkj:1',9',8'-gfed]heptalene 1, is synthesized. Experimental and
theoretical investigations demonstrate that the planar and symmet-
ric heptalene core within 1 effectively induces the antiaromatic
and open-shell character. These properties are not shared by
bisanthene 2, a benzenoid isomer of 1.
Non-alternant polycyclic hydrocarbons1 have experienced a
remarkable renaissance thanks to their unique electronic configu-
rations and molecular orbital characteristics, which are never
shared by alternant systems.2 -Extended non-alternant polycyclic
hydrocarbons such as azulene3-, pentalene4- and indacene5-based
molecules have recently fueled rich insights into the electronic
properties of (anti)aromaticity6 and/or open-shell character.7 The
topology of the -electron network is a primary concern that de-
termines the ground state electronic structures of polycyclic hy-
drocarbons.8–15 The transformation of naphthalene into azulene is
a thought-provoking example of the topological difference in -
conjugation with the same number of -electrons (Figure 1A).
The replacement of the hexagons in naphthalene with a heptagon
and a pentagon is associated with structural changes. These
changes reduce the molecular symmetry from D2h to C2v, resulting
in unsymmetrically distributed frontier orbitals and the unusual
physical or chemical properties of azulene. Many related studies
have illustrated that incorporating a non-alternant scaffold into an
alternant system is a powerful strategy to impact the electronic
properties of polycyclic hydrocarbons.16,17 However, reducing the
molecular symmetry upon the transformation of an alternant to a
non-alternant system is rarely considered despite the fact that it
triggers large electronic perturbations.
We are interested in the crossover of the ground state from the
aromatic closed-shell state to the antiaromatic open-shell state in
an isomeric pair of alternant and non-alternant hydrocarbons with
the same molecular symmetry. We selected a 12-heptalene scaf-
fold as an antiaromatic moiety. Pristine heptalene is a twisted
molecule with a non-aromatic character, but ring fusions into the
Figure 1. Molecular structures of (A) azulene and naphthalene,
(B) dynamic processes of heptalene, and (C) difluoreno[1,9,8-
alkj:1',9',8'-gfed]heptalene 1 and bisanthene 2. Mes = 2,4,6-
trimethylphenyl.
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Page 2 of 6
subsequent treatment with
a copper powder gave 9,9'-
bifluorenylidene derivative 5.29 Cyclization by treating 5 with
heated trifluoromethanesulfonic acid quantitatively yielded dione
6 with two heptagons. Nucleophilic attack on dione 6 with aryl-
magnesium bromides or (triisopropylsilyl)ethynyl lithiate afforded
diols 7. The reductive dearomatization of 7 with SnCl2 afforded a
complicated mixture, presumably due to the lack of two benzene
rings in 1 from 7. Alternatively, treating diols 7 with a tetra-
fluoroboric acid diethyl ether complex in CH2Cl2 immediately
produced dication salts 1ꞏ2BF4 as green to purple solids, which
are the two-electron oxidized species of target molecules 1. Spec-
troscopic measurements and X-ray crystallographic analysis con-
firmed all dications 1ꞏ2BF4 and the molecular geometry of the 4-
tert-butyl-m-xylyl derivative 1cꞏ2BF4, respectively (Figure S2).
Two-electron reduction of 12+ with decamethylferrocene fur-
nished the mesityl derivative 1b and the 4-tert-butyl-m-xylyl de-
rivative 1c as greenish black solids. Although reductions of the
other dications 1d–g2+ were attempted, they gave unidentified
mixtures, presumably due to the inadequate kinetic protection of
the reactive site under the reaction conditions. A CH2Cl2 solution
of 1b gradually decomposed with a half-life of three days upon
exposure to air under room light and temperature (Figure S13).
1
2
3
4
5
6
7
8
Figure 2. Resonance structures of 1a. Hexagonal rings in gray
denote benzenoid rings. Bold lines indicate the 4n-conjugated
circuit that contributes significantly to the canonical structure.
9
Our target molecule 1 serves as an acceptable non-alternant
isomer of 2. The D2h-symmetric scaffold of 1 eliminates the re-
duction of the molecular symmetry in the non-alternant system,
allowing the effect of the incorporated odd-membered rings to be
directly assessed. The existence of the two o-quinoidal subunits in
1 realizes the conversion of the aromatic/closed-shell character of
2 into the antiaromatic/open-shell character (1a-A in Figure 2).
The central heptalene unit of 1 should induce a paratropic charac-
ter as a 12-electron system (1a-B). In contrast to the reported
heptalene derivatives,25 the planar geometry given by the highly
fused structure and the formal delocalization of the o-quinoidal
substructures should enhance the local antiaromatic character.
Furthermore, the recovery of the benzenoid character of the o-
quinoidal hexagons should attain the singlet open-shell state of 1,
pushing out the two unpaired electrons on the head carbon of the
heptagons (1a-C).
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Scheme 1. Synthesis of 1a
O
O
O
O
OMe
OH
CrO3
MeOH
CH3COOH/H2O
100 °C, 2 h
70%
H2SO4
80 °C, 12 h
90%
3
4
Figure 3. (A) Ortep drawing of 1b with at the 50% probability
level, and (B) bond lengths (mean values) in the main core rings
of 1b.
1) SOCl2, DMF,
90 °C, 20 h
CF3SO3H
O
OMe
O
O
MeO
O
2) Cu, xylene
120 °C, 15 h
94% (2 steps)
90 °C, 3 h
quant.
Careful recrystallization from a chloroform/chlorobenzene so-
lution in a degassed sealed tube gave a single crystal of 1b suita-
ble for X-ray crystallographic analysis (Figure 3). X-ray crystal-
lographic analysis illustrated the main core of 1b assumes a planar
structure and the two mesityl groups form a large dihedral angle
(ca. 85°) with the main core (Figure S3). The main core of 1b
assumes an approximate D2h symmetry, and the observed bond
lengths of the main core of 1b agree well with those of the theo-
retically optimized D2h model rather than C2h model of 1a (Figure
S14). When comparing the bond length alternation (BLA) of the
heptagons in 1b with that of 1c2+, the antiaromatic nature of 1b is
obvious. The dication 1c2+ bearing tropylium substructures exhib-
its less BLA on the heptagons (1.423(3)–1.454(3) Å; Figure S4).
On the other hand, the degree of the BLA of the heptagons in 1b
increases (1.428(2)–1.470(2) Å). Although the reported heptalene
derivatives exhibits twisted geometries to avoid the instability of
the 4n-electron delocalization,30 the highly planar and symmetric
structure of 1b, which are realized by the surrounding ring-fusions,
should enhance the contribution of the 4n-system. The NICS(1)
values for the heptagons estimated at the GIAO-(U)B3LYP/6-
311+G* level also supported this idea, indicating a large positive
value of +11.2 for 1a and a negative value of –6.70 for 1a2+ (Fig-
ure S16). The paratropic character on the heptagons for 1a should
magnetically suppress the aromatic character of the surrounding
hexagon (denoted by A in Figure 3; NICS(1); –0.84), which is
also estimated in -extended pentalenes.31 The pentagons of 1
seems to behave as non-aromatic rings (NICS(1)/HOMA: –3.90/–
0.032). The calculated electrostatic potential map indicated that
the contribution of the polarized structure like azulene is negligi-
ble (Figure S15A).
(E/Z)
6
5
1) RMgBr or RLi
THF
HBF4·Et2O
CH2Cl2
R
OH
R
2BF4
R
R
2) H+ aq.
HO
12+
7
b
c
d
e
25%
b
c
d
71%
:45% :40%
f
g
:quant. :60%
f
g
:
:
:
:
e
:
89%
:
:
:
23% 35% 37%
qunat.
quant.
F
F
F
F
F
b
c
d
f
:
:
:
Cp*2Fe
CH3CN
R
R
tBu
OMe
SiiPr3
e
:
g
:
:
1
b
c
:78%
:
86%
a DMF = N,N-dimethylformamide, Cp* Fe = decamethylferro-
2
cene.
Scheme 1 depicts the synthetic route for the derivatives of 1.
The less aromatic character of 1 should impede the progress of
conventional reactions26 such as dehydrogenation or dehydration
to construct a fully conjugated system. Thus, we designed a route
using an aromatic dication species 12+ as a key precursor. Accord-
ing to Harper’s procedure,27 oxidation of fluoranthene afforded
florenone derivative 3. Esterification of the carboxylic group of 3
yielded 4.28 Then dichlorination of the carbonyl group of 4 and
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The observed high symmetric structure of 1b reflects the con-
tribution of the singlet open-shell character to the ground state
electronic configuration. The biradical resonance contribution is
found in a contraction of the a bonds due to their double-bond
character. The length of the a bond in 1b is 1.373(2) Å, which is
considerably shorter than the length of a typical C(sp2)–C(sp2)
single bond (1.467 Å). Additionally, the peripheral four hexagons
sustain a high degree of the benzenoid character. The harmonic
oscillator model of aromaticity (HOMA) analysis32 indicated a
larger value for the hexagons (ring A; +0.80). Furthermore, these
geometric features are consistent with the degree of the diradical
character (y) estimated at the CASSCF(2,2)/6-31G* level, which
indicates a large LUMO occupation number of 0.72 for 1a com-
pared to only 0.065 for 2a. The spin densities are mainly distrib-
uted to the head carbons on the heptagons (Figures S15B and C).
Judging from these experimental and theoretical assessments, the
dominant resonance structure of 1 should be drawn as 1a-C (Fig-
ure 2), which possesses a certain degree of antiaromatic character
derived from 1a-B.
1
2
3
4
5
6
7
8
9
Figure 5. UV/vis/NIR absorption spectra of 1b (red) and 2b
(black) in CH2Cl2. Inset shows a magnified view.
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The open-shell and antiraomtic nature of 1 are featured by the
small HOMO−LUMO energy gap. The cyclic voltammogram of
ox
1b showed three reversible and one irreversible redox wave (E2
= +0.30 V, E1ox = –0.34 V, E1red = –1.56 V, and E2red, pc = –2.01 V
vs Fc/Fc+, Figure 4). The electrochemical HOMO−LUMO gap of
1.22 eV, which was estimated from the difference between the
first oxidation and reduction potentials, is 0.46 eV smaller than
that of 2b.35 The electronic absorption spectrum of 1b in CH2Cl2
gave the lowest-energy band (max) at 934 nm ( = 4579 M–1cm–1),
which is significantly red-shifted compared with those of 2b (max
= 686 nm, = 67300 M–1cm–1; Figure 5). Notably, the absorption
intensity (i.e. molar absorption coefficient) of the lowest-energy
band of 1b is less than one-tenth of that of 2b. The DFT calcula-
tion of 1a at the UB3LYP/6-311+G* level indicates that the em-
bedded odd-membered rings give rise to the reconfiguration of the
frontier orbitals of 2b, and consequently lead to the pseudo de-
generation between the LUMO and LUMO+1 level (Figure S17).
The time-dependent (TD)-DFT calculations on 1a suggest that the
band at 934 nm of 1b is ascribed to the partially allowed
HOMO→LUMO+1 transition ( = 935 nm, f = 0.021, see Figure
S17 and Table S3). These orbitals associated with the transition
show the disjoint feature36 with / spins localized at the two
head carbons on the heptagons. Thus, the observed weak and
lowest-energy absorption of 1b should be derived from the open-
shell character of 1.
The determined physical properties of 1b support the open-
shell character. Superconducting quantum interference device
(SQUID) measurements were conducted for the microcrystalline
sample of 1b. The measurement showed an increasing susceptibil-
ity above 150 K (Figure S6). From the Bleaney−Bowers fitting of
the observed increase, the value of the singlet−triplet energy gap
(∆ES–T) is –1080 K (–2.15 kcal/mol), which is reasonable with
regard to the theoretical estimation of –1370 K (–2.72 kcal/mol)
calculated using the UB3LYP-D3/6-311G* method for 1a. The
experimentally determined ∆ES–T of 1b is smaller than those of
the teranthene (–3.82 kcal/mol)22 and peri-tetracene (–2.5
kcal/mol)24 derivatives, even though 1b is the smallest -
conjugated system among them. This unusual trend of 1 may be
due to the destabilization of the singlet ground state induced by
4n-delocalization. The small ∆ES–T indicates that 1b is easily
activated to the triplet state, even at low temperature. Actually, an
1H NMR signal of the main core of 1b in THF-d8 was not ob-
served, even at –100 °C (Figure S9).33,34 The ESR measurements
of 1b clearly displayed signals typical of a triplet species. The
glassy toluene sample of 1b gave a splitting pattern of Ms = ±1
signal and a forbidden Ms = ±2 half-field signal at 170–115 K
(Figure S7). The signal intensity decreases upon cooling, which
leads to the singlet ground state in 1b.
In conclusion, we successfully synthesized and characterized 1,
the non-alternant isomer of bisanthene 2. 1 displays an antiaro-
matic character from the central heptalene core and possesses
singlet biradical features, which are not observed in bisanthene 2.
The highly planar and symmetric geometry of 1 realizes unusual
electronic properties in an unexpectedly small -conjugated sys-
tem. The present study sheds light on the importance of topologi-
cally design for non-alternant polycyclic hydrocarbons. Further
studies on related systems as well as the development of the facile
synthetic methods are ongoing in our group.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS
Publications
website
at
DOI:
XXXXXXX.
Additional experimental, spectroscopic, and calculation data
(PDF)
Crystallographic data for 7b (CIF: CCDC 1906920)
Crystallographic data for 1c2+ (CIF: CCDC 1906919)
Crystallographic data for 1b (CIF: CCDC 1906918)
Figure 4. Cyclic voltammograms of 1b (red) and 2b (black) (V vs
Fc/Fc+, in 0.1 M nBu4NClO4/CH2Cl2, scan rate = 100 mV/s, room
temperature). Small signal around –0.5 V of 1b was caused by
inseparable impurities (see also Figure S10B).
AUTHOR INFORMATION
Corresponding Author
*a-koni@chem.eng.osaka-u.ac.jp
*yasuda@chem.eng.osaka-u.ac.jp
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Sugisaki, K.; Sato, K.; Takui, T.; Yasuda, M. Synthesis and Characteriza-
tion of Dibenzo[a,f]Pentalene: Harmonization of the Antiaromatic and
Singlet Biradical Character. J. Am. Chem. Soc. 2017, 139, 15284.
(5) For selected recent reviews and reports: (a) Frederickson, C. K.;
Rose, B. D.; Haley, M. M. Explorations of the Indenofluorenes and Ex-
panded Quinoidal Analogues. Acc. Chem. Res. 2017, 50, 977. (b) Dressler,
J. J.; Teraoka, M.; Espejo, G. L.; Kishi, R.; Takamuku, S.; Gómez-García,
C. J.; Zakharov, L. N.; Nakano, M.; Casado, J.; Haley, M. M. Thiophene
and Its Sulfur Inhibit Indenoindenodibenzothiophene Diradicals from
Low-Energy Lying Thermal Triplets. Nat. Chem. 2018, 10, 1134. (c)
Dressler, J. J.; Zhou, Z.; Marshall, J. L.; Kishi, R.; Takamuku, S.; Wei, Z.;
Spisak, S. N.; Nakano, M.; Petrukhina, M. A.; Haley, M. M. Synthesis of
the Unknown Indeno[1,2-a]Fluorene Regioisomer: Crystallographic Char-
acterization of Its Dianion. Angew. Chem. Int. Ed. 2017, 56, 15363. (d)
Shimizu, A.; Tobe, Y. Indeno[2,1-a]Fluorene: An Air-Stable Ortho-
Quinodimethane Derivative. Angew. Chem. Int. Ed. 2011, 50, 6906. (e)
Liu, J.; Ma, J.; Zhang, K.; Ravat, P.; Machata, P.; Avdoshenko, S.; Hen-
nersdorf, F.; Komber, H.; Pisula, W.; Weigand, J. J.; Popov, A. A.; Berger,
R.; Müllen, K.; Feng, X. π-Extended and Curved Antiaromatic Polycyclic
Hydrocarbons. J. Am. Chem. Soc. 2017, 139, 7513. (f) 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. (g)
Sbargoud, K.; Mamada, M.; Marrot, J.; Tokito, S.; Yassar, A.; Frigoli, M.
Diindeno[1,2-b:2',1'-n]Perylene: A Closed Shell Related Chichibabin’s
Hydrocarbon, the Synthesis, Molecular Packing, Electronic and Charge
Transport Properties. Chem. Sci. 2015, 6, 3402. (h) Ma, J.; Liu, J.; Baum-
garten, M.; Fu, Y.; Tan, Y.-Z.; Schellhammer, K. S.; Ortmann, F.; Cuni-
berti, G.; Komber, H.; Berger, R.; Müllen, K.; Feng, X. A Stable Saddle-
Shaped Polycyclic Hydrocarbon with an Open-Shell Singlet Ground State.
Angew. Chem. Int. Ed. 2017, 56, 3280. (i) Melidonie, J.; Liu, J.; Fu, Y.;
Weigand, J. J.; Berger, R.; Feng, X. Pyrene-Fused s-Indacene. J. Org.
Chem. 2018, 83, 6633. (j) Hsieh, Y.; Wu, C.-F.; Chen, Y.-T.; Fang, C.;
Wang, C.; Li, C.; Chen, L.; Cheng, M.; Chueh, C.; Chou, P.; Wu, Y. 5,14-
Diaryldiindeno[2,1-f:1',2'-j]Picene: A New Stable [7]Helicene with a
Partial Biradical Character. J. Am. Chem. Soc. 2018, 140, 14357. (k)
Majewski, M. A.; Chmielewski, P. J.; Chien, A.; Hong, Y.; Lis, T.; Wit-
wicki, M.; Kim, D.; Zimmerman, P. M.; Stępień, M. 5,10-
Dimesityldiindeno[1,2-a:2',1'-i]Phenanthrene: A Stable Biradicaloid De-
rived from Chichibabin’s Hydrocarbon. Chem. Sci. 2019, 10, 3413. (l) 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. (m) Lu, R.-Q.; Wu,
S.; Yang, L.-L.; Gao, W.-B.; Qu, H.; Wang, X.-Y.; Chen, J.-B.; Tang, C.;
Shi, H.-Y.; Cao, X.-Y. Stable Diindeno-Fused Corannulene Regioisomers
with Open-Shell Singlet Ground States and Large Diradical Characters.
Angew. Chem. Int. Ed. 2019, DOI: 10.1002/anie.201902028.
(6) (a) Breslow, R. Antiaromaticity. Acc. Chem. Res. 1973, 6, 393. (b)
Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. I. A. Aromaticity and Anti-
aromaticity: Electronic and Structural Aspects; Wiley-Interscience publi-
cation; J. Wiley & Sons, 1994. (c) Frederickson, C. K.; Zakharov, L. N.;
Haley, M. M. Modulating Paratropicity Strength in Diareno-Fused Anti-
aromatics. J. Am. Chem. Soc. 2016, 138, 16827. (d) Majzik, Z.; Pavliček,
N.; Vilas-Varela, M.; Pérez, D.; Moll, N.; Guitián, E.; Meyer, G.; Peña,
D.; Gross, L. Studying an Antiaromatic Polycyclic Hydrocarbon Adsorbed
on Different Surfaces. Nat. Commun. 2018, 9, 1198.
(7) (a) Abe, M. Diradicals. Chem. Rev. 2013, 113, 7011. (b) Y. Gopala-
krishna, T.; Zeng, W.; Lu, X.; Wu, J. From Open-Shell Singlet Diradica-
loids to Polyradicaloids. Chem. Commun. 2018, 54, 2186. (c) Zeng, Z.;
Shi, X.; Chi, C.; López 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. (e) Kubo, T. Recent Progress
in Quinoidal Singlet Biradical Molecules. Chem. Lett. 2015, 44, 111. (f)
Nakano, M. Open-Shell-Character-Based Molecular Design Principles:
Applications to Nonlinear Optics and Singlet Fission. Chem. Rec. 2017,
17, 27. (g) Sun, Z.; Wu, J. Open-Shell Polycyclic Aromatic Hydrocarbons.
J. Mater. Chem. 2012, 22, 4151.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by JSPS KAKENHI (Grants Number
JP15H05848 in Middle Molecular Strategy, JP18H01977,
JP18K19079 and JP18K14201). A.K. would like to thank the
Tonen General Sekiyu Research & Development Encouragement
Foundation and Iketani Science and Technology Foundation for
financial supports. We thank Dr. N. Kanehisa (Osaka University)
for the valuable advice regarding X-ray crystallography. We also
thank Prof. S. Kuwabata and Dr. T. Uematsu for the assistance
with electronic absorption measurements. Finally, we recognize
Dr. K. Inoue, (the Analytical Instrumentation Facility, Graduate
School of Engineering, Osaka University) for the assistance with
VT-NMR spectroscopy.
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REFERENCES
(1) (a) Hafner, K. Structure and Aromatic Character of Non-Benzenoid
Cyclically Conjugated Systems. Angew. Chem. Int. Ed. Engl. 1964, 3, 165.
(b) Nozoe, T. Topics in Nonbenzenoid Aromatic Chemistry; Wiley, 1973.
(c) Hafner, K. New Aspects of the Chemistry of Nonbenzenoid Polycyclic
Conjugated π-Electron Systems. Pure Appl. Chem. 1982, 54, 939. (d)
Tobe, Y. Non-Alternant Non-Benzenoid Aromatic Compounds: Past,
Present, and Future. Chem. Rec. 2015, 15, 86. (e) Tobe, Y. Quinodime-
thanes Incorporated in Non-Benzenoid Aromatic or Antiaromatic Frame-
works. Top. Curr. Chem. 2018, 376, 12.
(2) Gleiter, R.; Haberhauer, G.; Hoffmann, R. Aromaticity and Other
Conjugation Effects; Wiley-VCH, 2012.
(3) For selected reports: (a) Fukazawa, Y.; Aoyagi, M.; Itô, S. Naph-
tho[1,8-ab:4,5-a'b']Diazulene, the First Nonalternant Isomer of Dibenzop-
yrene. Tetrahedron Lett. 1981, 22, 3879. (b) Nakasuji, K.; Todo, E.; Mu-
rata, I. Azuleno[4,5,6-cd] Phenalene; A Novel Nonalternant Isomer of
Benzo[a] Pyrene. Angew. Chem. Int. Ed. Engl. 1977, 16, 784. (c) Todo,
E.; Yamamoto, K.; Murata, I. BENZO[4,5]CYCLOHEPT[1,2,3-
bc]ACENAPHTHYLENE
AND
BENZO[a]NAPHTH[3,4,4a,5-
cde]AZULENE. NONALTERNANT ISOMERS OF BENZO[a]PYRENE.
Chem. Lett. 1979, 8, 537. (d) Hibi, D.; Kitabayashi, K.; Fujita, K.; Takeda,
T.; Tobe, Y. Diindenopyrenes: Extended 1,6- and 1,8-
Pyrenoquinodimethanes with Singlet Diradical Characters. J. Org. Chem.
2016, 81, 3735. (e) Uehara, K.; Mei, P.; Murayama, T.; Tani, F.; Hayashi,
H.; Suzuki, M.; Aratani, N.; Yamada, H. An Anomalous Antiaromaticity
That Arises from the Cycloheptatrienyl Anion Equivalent. Eur. J. Org.
Chem. 2018, 2018, 4508. (f) Yamamoto, K.; Ie, Y.; Tohnai, N.; Kakiuchi,
F.; Aso, Y. Antiaromatic Character of Cycloheptatriene-Bis-Annelated
Indenofluorene Framework Mainly Originated from Heptafulvene Seg-
ment. Sci. Rep. 2018, 8, 17663. (g) Mishra, S.; Lohr, T. G.; Pignedoli, C.
A.; Liu, J.; Berger, R.; Urgel, J. I.; Müllen, K.; Feng, X.; Ruffieux, P.;
Fasel, R. Tailoring Bond Topologies in Open-Shell Graphene Nanostruc-
tures. ACS Nano 2018, 12, 11917.
(4) For selected recent reviews and reports: (a) Hopf, H. Pentalenes-
From Highly Reactive Antiaromatics to Substrates for Material Science.
Angew. Chem. Int. Ed. 2013, 52, 12224. (b) Saito, M. Synthesis and Reac-
tions of Dibenzo[a,e]Pentalenes. Symmetry 2010, 2, 950. (c) Kawase, T.;
Nishida, J. π-Extended Pentalenes: The Revival of the Old Compound
from New Standpoints. Chem. Rec. 2015, 15, 1045. (d) Levi, Z. U.; Tilley,
T. D. Synthesis and Electronic Properties of Extended, Fused-Ring Aro-
matic Systems Containing Multiple Pentalene Units. J. Am. Chem. Soc.
2010, 132, 11012. (e) Cao, J.; London, G.; Dumele, O.; Rekowski, M. v.
W.; Trapp, N.; Ruhlmann, L.; Boudon, C.; Stanger, A.; Diederich, F. The
Impact of Antiaromatic Subunits in [4n+2] π-Systems: Bispentalenes with
[4n+2] π-Electron Perimeters and Antiaromatic Character. J. Am. Chem.
Soc. 2015, 137, 7178. (f) Oshima, H.; Fukazawa, A.; Yamaguchi, S. Facile
Synthesis of Polycyclic Pentalenes with Enhanced Hückel Antiaromaticity.
Angew. Chem. Int. Ed. 2017, 56, 3270. (g) Sekine, K.; Schulmeister, J.;
Paulus, F.; Goetz, K. P.; Rominger, F.; Rudolph, M.; Zaumseil, J.; Hashmi,
A. S. K. Gold-Catalyzed Facile Synthesis and Crystal Structures of Ben-
zene-/Naphthalene-Based Bispentalenes as Organic Semiconductors.
Chem. Eur. J. 2019, 25, 216. (h) Konishi, A.; Okada, Y.; Nakano, M.;
(8) Clar, E. Polycyclic Hydrocarbons: Volume 1 and 2; Academic Press
Inc, 1964.
(9) Randić, M. Aromaticity of Polycyclic Conjugated Hydrocarbons.
Chem. Rev. 2003, 103, 3449.
(10) Wu, J.; Pisula, W.; Müllen, K. Graphenes as Potential Material for
Electronics. Chem. Rev. 2007, 107, 718.
ACS Paragon Plus Environment

Page 5 of 6
Journal of the American Chemical Society
(11) Morita, Y.; Suzuki, S.; Sato, K.; Takui, T. Synthetic Organic Spin
Chemistry for Structurally Well-Defined Open-Shell Graphene Fragments.
Nat. Chem. 2011, 3, 197.
K.; Feng, X. Toward Full Zigzag-Edged Nanographenes: peri-Tetracene
and Its Corresponding Circumanthracene. J. Am. Chem. Soc. 2018, 140,
6240.
1
2
3
4
5
6
7
8
9
(12) Hu, P.; Wu, J. Modern Zethrene Chemistry. Can. J. Chem. 2017,
95, 223.
(13) Narita, A.; Wang, X.-Y.; Feng, X.; Müllen, K. New Advances in
Nanographene Chemistry. Chem. Soc. Rev. 2015, 44, 6616.
(14) Narita, A.; Chen, Z.; Chen, Q.; Müllen, K. Solution and On-
Surface Synthesis of Structurally Defined Graphene Nanoribbons as a
New Family of Semiconductors. Chem. Sci. 2019, 10, 964.
(15) Konishi, A.; Kubo, T. Benzenoid Quinodimethanes. Top. Curr.
Chem. 2017, 375, 83.
(24) Ni, Y.; Gopalakrishna, T. Y.; Phan, H.; Herng, T. S.; Wu, S.; Han,
Y.; Ding, J.; Wu, J. A Peri-Tetracene Diradicaloid: Synthesis and Proper-
ties. Angew. Chem. Int. Ed. 2018, 57, 9697.
(25) (a) Dauben, H. J.; Bertelli, D. J. HEPTALENE. J. Am. Chem. Soc.
1961, 83, 4659. (b) Vogel, E.; Königshofen, H.; Wassen, J.; Müllen, K.;
Oth, J. F. M. A Heptalene Synthesis from 1,6-Methano[10]Annulene;
Evidence for a Fast π-Bond Shift. Angew. Chem. Int. Ed. Engl. 1974, 13,
732. (c) Briquet, A. A. S.; Uebelhart, P.; Hansen, H.-J. Double-Bond
Shifts in [4n]Annulenes as a New Principle for Molecular Switches: First
Results with Dimethyl Heptalene-1,2- and -4,5-Dicarboxylates. Helv.
Chim. Acta 1996, 79, 2282.
(26) Iyoda, M.; Tanake, S.; Nishioka, K.; Oda, M. Synthesis and Prop-
erties of Butatrienes Containing a Bisdehydro[13]Annulene System as
Terminal Groups. Tetrahedron Lett. 1983, 24, 2861.
(27) George, S. R. D.; Scott, L. T.; Harper, J. B. Synthesis of 1-
Substituted Fluorenones. Polycycl. Aromat. Compd. 2016, 36, 697.
(28) Hojo, M.; Ichi, T.; Shibato, K. Synthesis and Structure of a New
Stable Carbocation Stabilized by Two Neighboring Sulfur Atoms. Dime-
thyl[9,9-Bis(Methylthio)-1-Fluorenyl]Carbenium Ion. J. Org. Chem. 1985,
50, 1478.
(29) Wawzonek, S.; Henry, J. P. Studies on the Possibility of Resolving
Dicarboxy-Δ9,9 ' -Bifluorenes. J. Org. Chem. 1953, 18, 1461.
(30) (a) Lindner, H. J.; Kitschke, B. Crystal and Molecular Structure of
Dimethyl 1,2-Heptalenedicarboxylate. Angew. Chem. Int. Ed. Engl. 1976,
15, 106. (b) Stegemann, J.; Lindner, H. J. The Crystal and Molecular
Structure of Dimethyl-3,8-Heptalene-Dicarboxylate. Tetrahedron Lett.
1977, 18, 2515. (c) Oshima, H.; Fukazawa, A.; Sasamori, T.; Yamaguchi,
S. A Nonaromatic Thiophene-Fused Heptalene and Its Aromatic Dianion.
Angew. Chem. Int. Ed. 2015, 54, 7636.
(31) Konishi, A.; Okada, Y.; Kishi, R.; Nakano, M.; Yasuda, M. En-
hancement of Antiaromatic Character via Additional Benzoannulation into
Dibenzo[a,f]Pentalene: Syntheses and Properties of Benzo[a]Naphtho[2,1-
f]Pentalene and Dinaphtho[2,1-a,f]Pentalene. J. Am. Chem. Soc. 2019, 141,
560.
(32) (a) Kruszewski, J.; Krygowski, T. M. Definition of Aromaticity
Basing on the Harmonic Oscillator Model. Tetrahedron Lett. 1972, 13,
3839. (b) Krygowski, T. M. Crystallographic Studies of Inter- and Intra-
molecular Interactions Reflected in Aromatic Character of π-Electron
Systems. J. Chem. Inf. Model. 1993, 33, 70.
(33) Konishi, A.; Hirao, Y.; Matsumoto, K.; Kurata, H.; Kishi, R.;
Shigeta, Y.; Nakano, M.; Tokunaga, K.; Kamada, K.; Kubo, T. Synthesis
and Characterization of Quarteranthene: Elucidating the Characteristics of
the Edge State of Graphene Nanoribbons at the Molecular Level. J. Am.
Chem. Soc. 2013, 135, 1430.
(16) (a) Hafner, K.; Fleischer, R.; Fritz, K. Pentaleno[2,1,6-
def]Heptalene– a Non-Benzenoid Isomer of Pyrene. Angew. Chem. Int. Ed.
Engl. 1965, 4, 69. (b) Anderson, A. G.; MacDonald, A. A.; Montana, A. F.
Dicyclopenta[ef,kl]Heptalene (Azupyrene). J. Am. Chem. Soc. 1968, 90,
2993. (c) Reel, H.; Vogel, E. Dicyclohepta[cd,gh]Pentalene?A New Py-
rene Isomer. Angew. Chem. Int. Ed. Engl. 1972, 11, 1013. (d) Reid, D. H.;
Stafford, W. H.; Ward, J. P. The Fine Structure of Azulene. Part I. Cyclo-
Hepta[bc]Acenaphthylene and Cyclohepta[def]Fluorene. J. Chem. Soc.
1955, 84, 1193. (e) Gardner, P. D.; Wulfman, C. E.; Osborn, C. L. Cyclo-
hepta[klm]Benz[e]Indene. Further Considerations on the Stability of
Complex Polynuclear Systems. J. Am. Chem. Soc. 1958, 80, 143. (f)
Boekelheide, V.; Langeland, W. E.; Liu, C.-T. A Study of the Synthesis
and Some Properties of Acepleiadiene. J. Am. Chem. Soc. 1951, 73, 2432.
(17) (a) Yang, X.; Shi, X.; Aratani, N.; Gonçalves, T. P.; Huang, K.-
W.; Yamada, H.; Chi, C.; Miao, Q. Benzo[4,5]Cyclohepta[1,2-b]Fluorene:
An Isomeric Motif for Pentacene Containing Linearly Fused Five-, Six-
and Seven-Membered Rings. Chem. Sci. 2016, 7, 6176. (b) Dickens, T.
K.; Mallion, R. B. Topological Ring-Currents and Bond-Currents in the
Altan-[r,s]-Coronenes. Chem. Commun. 2015, 51, 1819. (c) Rizzo, D. J.;
Veber, G.; Cao, T.; Bronner, C.; Chen, T.; Zhao, F.; Rodriguez, H.; Louie,
S. G.; Crommie, M. F.; Fischer, F. R. Topological Band Engineering of
Graphene Nanoribbons. Nature 2018, 560, 204. (d) Wang, Q.; Gopala-
krishna, T. Y.; Phan, H.; Herng, T. S.; Dong, S.; Ding, J.; Chi, C. Cyclo-
penta Ring Fused Bisanthene and Its Charged Species with Open-Shell
Singlet Diradical Character and Global Aromaticity/Anti-Aromaticity.
Angew. Chem. Int. Ed. 2017, 56, 11415. (e) Jiang, Q.; Tao, T.; Phan, H.;
Han, Y.; Gopalakrishna, T. Y.; Herng, T. S.; Li, G.; Yuan, L.; Ding, J.;
Chi, C. Diazuleno-s-Indacene Diradicaloids: Syntheses, Properties, and
Local (Anti)Aromaticity Shift from Neutral to Dicationic State. Angew.
Chem. Int. Ed. 2018, 57, 16737.
(18) (a) Toyota, A.; Kataoka, M.; Koseki, S. Energy Component Anal-
ysis of the Pseudo Jahn-Teller Effect in Pentalene and Heptalene. Chem.
Lett. 1992, 21, 791. (b) Toyota, A.; Koseki, S. Energy Component Analy-
sis of the Pseudo-Jahn−Teller Effect in the Bicyclic Nonalternant Hydro-
carbons: The Pentalenoid and Heptalenoid Systems. J. Phys. Chem. 1996,
100, 2100. (c) Kozuch, S. Heavy Atom Tunneling in the Automerization
of Pentalene and Other Antiaromatic Systems. RSC Adv. 2014, 4, 21650.
(d) Hafner, K.; Knaup, G. L.; Lindner, H. J. Syntheses and Dynamic Be-
havior of Chiral Heptalenes. Bull. Chem. Soc. Jpn. 1988, 61, 155.
(19) Scholl, R.; Meyer, K. Der Blaue Aromatische Grundkohlen-
wasserstoff Des Meso-Naphtho-Dianthrons Und Seine Überführung
Durch Maleinsäure-Anhydrid in Anthro-Dianthren. Ber. Dtsch. Chem. Ges.
(A B Series) 1934, 67, 1236.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(34) (a) Miyoshi, H.; Miki, M.; Hirano, S.; Shimizu, A.; Kishi, R.; Fu-
kuda, K.; Shiomi, D.; Sato, K.; Takui, T.; Hisaki, I.; Nakano, M.; Tobe, Y.
Fluoreno[2,3-b]Fluorene vs Indeno[2,1-b]Fluorene: Unusual Relationship
between the Number of π Electrons and Excitation Energy in m-
Quinodimethane-Type Singlet Diradicaloids. J. Org. Chem. 2017, 82,
1380. (b) Zeng, W.; Gopalakrishna, T. Y.; Phan, H.; Tanaka, T.; Herng, T.
S.; Ding, J.; Osuka, A.; Wu, J. Superoctazethrene: An Open-Shell Gra-
phene-like Molecule Possessing Large Diradical Character but Still with
Reasonable Stability. J. Am. Chem. Soc. 2018, 140, 14054.
(35) Konishi, A.; Hirao, Y.; Matsumoto, K.; Kurata, H.; Kubo, T. Fac-
ile Synthesis and Lateral π-Expansion of Bisanthenes. Chem. Lett. 2013,
42, 592.
(36) Bendikov, M.; Duong, H. M.; Starkey, K.; Houk, K. N.; Carter, E.
A.; Wudl, F. Oligoacenes: Theoretical Prediction of Open-Shell Singlet
Diradical Ground States. J. Am. Chem. Soc. 2004, 126, 7416.
(20) Clar, E. Synthesen von Benzologen Des Perylens Und Bisanthens.
(Aromatische Kohlenwasserstoffe, XLVIII. Mitteil.). (Mitbearbeitet von H.
Frömmel.). Chem. Ber. 1949, 82, 46.
(21) (a) Jiang, D.; Sumpter, B. G.; Dai, S. First Principles Study of
Magnetism in Nanographenes. J. Chem. Phys. 2007, 127, 124703. (b) Hod,
O.; Barone, V.; Scuseria, G. E. Half-Metallic Graphene Nanodots: A
Comprehensive First-Principles Theoretical Study. Phys. Rev. B 2008, 77,
1. (c) Moscardó, F.; San-Fabián, E. On the Existence of a Spin-Polarized
State in the n-Periacene Molecules. Chem. Phys. Lett. 2009, 480, 26. (d)
Plasser, F.; Pašalić, H.; Gerzabek, M. H.; Libisch, F.; Reiter, R.;
Burgdörfer, J.; Müller, T.; Shepard, R.; Lischka, H. The Multiradical
Character of One- and Two-Dimensional Graphene Nanoribbons. Angew.
Chem. Int. Ed. 2013, 52, 2581.
(22) Konishi, A.; Hirao, Y.; Nakano, M.; Shimizu, A.; Botek, E.;
Champagne, B.; Shiomi, D.; Sato, K.; Takui, T.; Matsumoto, K.; Kurata,
H.; Kubo, T. Synthesis and Characterization of Teranthene: A Singlet
Biradical Polycyclic Aromatic Hydrocarbon Having Kekulé Structures. J.
Am. Chem. Soc. 2010, 132, 11021.
(23) Ajayakumar, M. R.; Fu, Y.; Ma, J.; Hennersdorf, F.; Komber, H.;
Weigand, J. J.; Alfonsov, A.; Popov, A. A.; Berger, R.; Liu, J.; Müllen,
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