Revisiting Acepleiadylene: Two-Step Synthesis and π-Extension toward Nonbenzenoid Nanographene

Article abstract of DOI:10.1021/jacs.1c01826

Acepleiadylene (APD), a nonbenzenoid nonalternant isomer of pyrene, exhibits different electronic properties from pyrene, but has been rarely studied since its first synthesis in 1956, probably due to the difficulties in synthesis and further derivatization. In this work, we revisited this long-known compound and developed a new two-step synthetic route to efficiently access APD on the gram scale. Theoretical and experimental characterizations elucidated the unique properties of APD as compared with its benzenoid isomer pyrene, particularly revealing its dipolar structure with a narrow optical gap. The functionalization of APD was demonstrated for the first time, providing doubly brominated APD as a key precursor for further π-extension. As a proof of concept, a π-extended APD and a cyclotrimer nanographene (C48H24) were constructed, opening up new avenues to nonbenzenoid nanographenes with low HOMO-LUMO gaps.

Full text of DOI:10.1021/jacs.1c01826

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Communication
Revisiting Acepleiadylene: Two-Step Synthesis and π‑Extension
toward Nonbenzenoid Nanographene
̈
Pengcai Liu, Xing-Yu Chen, Jiawen Cao, Lukas Ruppenthal, J. Michael Gottfried, Klaus Mullen,
and Xiao-Ye Wang*
Cite This: J. Am. Chem. Soc. 2021, 143, 5314−5318
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ABSTRACT: Acepleiadylene (APD), a nonbenzenoid nonalternant isomer of pyrene, exhibits different electronic properties from
pyrene, but has been rarely studied since its first synthesis in 1956, probably due to the difficulties in synthesis and further
derivatization. In this work, we revisited this long-known compound and developed a new two-step synthetic route to efficiently
access APD on the gram scale. Theoretical and experimental characterizations elucidated the unique properties of APD as compared
with its benzenoid isomer pyrene, particularly revealing its dipolar structure with a narrow optical gap. The functionalization of APD
was demonstrated for the first time, providing doubly brominated APD as a key precursor for further π-extension. As a proof of
concept, a π-extended APD and a cyclotrimer nanographene (C48H24) were constructed, opening up new avenues to nonbenzenoid
nanographenes with low HOMO−LUMO gaps.
As an isomer of pyrene, cyclohept[fg]acenaphthylene, which
olycyclic aromatic hydrocarbons (PAHs) have attracted
Pintense research interest in chemistry, physics, and is known as acepleiadylene (APD), exhibits a deep red color
materials science in recent decades.1−3 On one hand, they
have played a key role in the development of organic
semiconductors for optoelectronic and biological applica-
tions;4−7 on the other hand, they have served as structurally
defined nanographenes with sizes above 1 nm.8−10 In
particular, nonbenzenoid PAHs have recently sparked special
interest due to their unique electronic and magnetic proper-
ties.11−24 One classical nonbenzenoid PAH is the deep blue
azulene, which exhibits a dipole moment of 1.08 D (Figure
1).25 Azulene has been widely employed as a building block of
π-extended conjugated molecules,26−28 oligomers and poly-
mers,29−31 graphene nanoribbons,32 and metal−organic frame-
works.33 These materials have shown great potential in
optoelectronics and energy technologies.34−37
with a narrow HOMO−LUMO gap and a dipole moment of
0.49 D (Figure 1).38 APD was first synthesized by Boekelheide
et al. in 195639 and has attracted considerable theoretical and
experimental interest in terms of its aromaticity, chemical
reactivity, and electron-accepting capability.39−43 However,
compared with azulene, APD has been significantly less
investigated, especially after 1980s. Despite the intriguing
properties of APD, its further π-extension has never been
reported. The original synthesis by Boekelheide et al. required
six steps, including a harsh Friedel−Crafts reaction in molten
sodium aluminum chloride,44 to afford APD in a total yield of
2.4% (Scheme 1a). The somewhat inconvenient synthetic
method might have hampered the broad interest in utilizing
APD.
Herein, we develop a gram-scale synthesis of APD, which
requires only two steps from commercially available 5,6-
dibromoacenaphthene, giving an overall yield of 44% (Scheme
1b). We demonstrate further functionalization and π-extension
of APD, providing new opportunities for employing APD in π-
conjugated materials and nonbenzenoid nanographenes with
low HOMO−LUMO gaps.
The two protocols are illustrated in detail in Scheme 1. The
original route by Boekelheide et al. started from a Friedel−
Crafts reaction between acenaphthene (1) and succinic
anhydride to afford β-(3-acenaphthoyl)-propionic acid (2a),
Received: February 16, 2021
Published: March 30, 2021
Figure 1. Comparison of naphthalene and azulene, as well as pyrene
and acepleiadylene (APD). The experimental dipole moments were
measured by the Halverstadt−Kumler method.25,38 Note that a dipole
moment of 0.796
0.014 D was determined for azulene by the
microwave spectroscopy.45 The DFT-calculated dipole moments were
obtained at the B3LYP/6-311++G(d,p) level.
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a
Scheme 1. Synthetic Routes to Acepleiadylene (APD)
Figure 2. (a,d) Calculated ESP maps of APD (a) and pyrene (d). The
molecular surface uses an iso-surface with an electron density of 0.001
e/Bohr3. (b,e) Calculated ACID plots (contribution from π electrons
only) of APD (b) and pyrene (e). The magnetic field is perpendicular
to the XY plane and points out through the paper. The red arrows
indicate the clockwise (diamagnetic) current flow. (c,f) Calculated
2D-ICSS maps of APD (c) and pyrene (f). The images are mapped at
1 Å above the XY planes.
a
Reagents and conditions: (i) Succinic anhydride, AlCl3, nitro-
benzene, 0 °C; (ii) MeOH, conc. H2SO4, heating; (iii) NaCl, AlCl3,
150 °C; (iv) LiAlH4, benzene, reflux; (v) HCl, EtOH, rt; (vi) Pd/C,
mesitylene, reflux; (vii) 1,3-butadiene, 9-BBN, THF, 25 °C, 12 h;
then Pd(OAc)2, SPhos, K3PO4, THF/H2O, 70 °C, 2 h; (viii) DDQ, t-
BuOK, toluene, 80 °C.
(Figure 2d), stems from the tendency of both the five-
membered ring and the seven-membered ring to gain
aromaticity. The aromatic character of APD is evidenced by
the ACID plots, which display the same clockwise ring current
flow as pyrene (Figure 2b,e) and thus the same global
aromaticity. The 2D-ICSS map of APD reveals a stronger
shielding effect on the five-membered ring side, different from
the feature of pyrene (Figure 2c,f).
The UV−vis absorption spectrum of APD displays an
absorption band from 350 to 600 nm, which is responsible for
its red color (see Figure S3 in the Supporting Information).
The optical gap of APD is calculated from its absorption onset
(565 nm) to be 2.19 eV, significantly lower than that of pyrene
(3.59 eV) (Table 1). The emission maximum of APD is at 560
while a regioisomeric byproduct, that is, β-(1-acenaphthoyl)-
propionic acid (2b), could not be avoided and thus
complicated the purification.44 An esterification of 2a was
necessary for the second Friedel−Crafts reaction, which was
undertaken in molten sodium aluminum chloride. After
reduction of the diketone and an elimination reaction,
compound 6 was dehydrogenated under Pd/C to afford
APD. The whole synthesis required six steps with an overall
yield of 2.4%. In our synthesis, we employed a double Suzuki-
type coupling with 5,6-dibromoacenaphthene (7) to construct
a new seven-membered ring, furnishing compound 8 in 80%
yield. After extensive screening of dehydrogenation conditions,
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and t-
BuOK provided APD with an isolated yield of 55%, whereas
DDQ alone yielded less than 10%, together with partially
dehydrogenated and chlorinated byproducts (for more details,
see the Supporting Information). The structure of APD was
proven by 1H and 13C NMR spectroscopies and high-
resolution mass spectrometry (HRMS). The advantages of
our gram-scale synthetic route in terms of fewer steps, milder
conditions, and a higher overall yield lay the ground for further
investigations of APD.
To compare the electronic structure of APD and its
benzenoid isomer pyrene, we calculated (i) the electrostatic
potential (ESP), (ii) the anisotropy of the induced current
density (ACID), and (iii) the two-dimensional iso-chemical
shielding surface (2D-ICSS). Consistent with the charge-
separated resonance structure of APD (Figure 1), the ESP map
of APD (Figure 2a) indicates that the negative charge is mainly
distributed on the five-membered ring, whereas the seven-
membered ring is slightly positive. The charge-separated
electronic structure of APD, as opposed to that of pyrene
Table 1. Optical Energy Gap, as well as the HOMO and
a
LUMO Levels of APD and Pyrene
Compd Egopt (eV) LUMOCV (eV) HOMOCV (eV) HOMOUPS (eV)
APD
pyrene
2.19
3.59
−2.80
−2.31
−5.38
−5.63
−5.53
−5.85
a
Definitions: Egopt is the optical energy gap derived from the lowest-
energy absorption onset in the absorption spectrum; the LUMOCV
and HOMOCV energy levels are calculated based on the onset
potentials of the first reductive and oxidative waves, respectively; the
HOMOUPS energy levels are calculated according to the first energy
maxima below the vacuum level.
nm, which is largely red-shifted compared with that of pyrene
(374 nm) (Figure S5). As calculated based on cyclic
voltammograms (CVs), APD exhibits a lower LUMO energy
level (−2.80 eV) and a higher HOMO energy level (−5.38 eV)
than pyrene (LUMO: −2.31 eV; HOMO: −5.63 eV),
indicating that APD has a narrower HOMO−LUMO gap
(Table 1). Furthermore, according to the ultraviolet photo-
electron spectroscopy (UPS) characterization (Figure S10),
APD in the thin-film state displays a HOMO level of −5.53 eV,
higher than the HOMO of pyrene (−5.85 eV), yielding a
difference of 0.32 eV.
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To demonstrate the potential of APD as a building block for
π-extended PAHs and larger nanographenes, we first
investigated the bromination of APD as a starting point for
further derivatizations (Scheme 2). The 2-fold bromination of
a
Scheme 2. π-Extension of APD toward Nanographenes
1
Figure 3. (a) Partial H NMR spectra of 12 and 9, with a mark
indicating the residual solvent CHCl3. (b) HRMS spectrum of 12
showing the experimental and simulated isotopic distributions. (c)
The UV−vis absorption spectrum of 12 in THF. (d) Comparison of
the energy levels of 12 and a literature-reported reference compound
13 as calculated by DFT at the B3LYP/6-311++G(d,p) level.
a
Reagents and conditions: (i) NBS, THF, 25 °C; (ii) phenylboronic
acid, Pd(OAc)2, SPhos, K3PO4, THF/H2O, 70 °C; (iii) DDQ, TfOH,
DCM −78 °C; (iv) Ni(cod)2, cod, bipy, dioxane, 120 °C.
APD was achieved with N-bromosuccinimide (NBS) in a
dilute THF solution (5 mM), occurring at the 7- and 8-
positions of APD on the five-membered ring, which exhibits a
higher electron density (Figure 2a). With the doubly
brominated APD (9) as a key precursor for further π-
extension, we performed an efficient Suzuki coupling to attach
two phenyl rings and a subsequent cyclodehydrogenation
reaction to furnish compound 11 in 52% yield. In comparison
with the parent APD, the π-extended PAH 11 exhibits red-
shifted absorption and emission maxima, as well as a smaller
HOMO−LUMO gap as indicated by both the CV data and
DFT calculations (see the Supporting Information). These
findings indicate the crucial influence of π-extension on the
properties of APD derivatives.
than that of APD (2.19 eV). Furthermore, compound 12
displays a significantly red-shifted emission maximum (702
nm) relative to APD (560 nm), indicating the profound effect
of π-extension. In comparison with a reported star-shaped
benzenoid nanographene 13 (C54H30) with the same number
of fused rings (Figure 3d),48 the absorption feature of
nanographene 12 is remarkable, as nanographene 13 mainly
absorbs photons in the UV region.48 DFT calculations indicate
that nanographene 12 possesses a narrower HOMO−LUMO
gap. These results demonstrate the great potential of
employing APD as a building block for nonbenzenoid
nanographenes with low HOMO−LUMO gaps.
In summary, we have developed an efficient synthetic
method for the gram-scale synthesis of APD in two steps with
an overall yield of 44%. Theoretical and experimental studies
on the properties of APD in comparison with its benzenoid
isomer pyrene have indicated that APD exhibits similar global
aromaticity as pyrene, but possesses an electronic structure
with a charge-separation feature and a significantly narrower
HOMO−LUMO gap. The functionalization and π-extension
of APD have been demonstrated for the first time, particularly
achieving a star-shaped nonbenzenoid nanographene (C48H24)
with a low HOMO−LUMO gap. These results have placed
APD in a privileged position for the development of novel
PAHs, π-conjugated polymers, and graphene nanoribbons with
intriguing properties. This work would direct renewed interest
in APD chemistry and set a new starting point for utilizing
APD derivatives in future optoelectronic and energy-related
applications.
Inspired by the recent accomplishments in threefold
symmetric nanographenes,46 we conducted a cyclotrimeriza-
tion reaction of compound 9 under Yamamoto conditions,47
achieving a star-shaped nonbenzenoid nanographene 12
(C48H24) in 69% yield (Scheme 2). Although the character-
izations of such a poorly soluble nanographene were
challenging, its chemical structure could be convincingly
1
verified by H NMR and HRMS. As depicted in Figure 3a,
nanographene 12 displays four sets of aromatic proton signals.
Compared with the signals of precursor 9, proton a of the
cyclotrimer is downfield shifted because of the additional
deshielding effect from the adjacent aromatic rings, whereas
protons b, c, and d are upfield shifted owing to the reduced
aromaticity of APD moiety, as indicated by the nucleus-
independent chemical shift (NICS) calculations (Figure S11).
The shift of proton signals corroborates the success of
cyclotrimerization. The HRMS of 12 shows a molecular ion
signal at m/z = 600.1872 Da, in good agreement with the
expected molecular mass of 600.1878 Da for C48H24. The
experimental isotopic distribution also matches well with the
simulated pattern, clearly proving the chemical identity of
nanographene 12 (Figure 3b). The UV−vis absorption
spectrum of 12 reveals an absorption onset of 628 nm (Figure
3c), corresponding to an optical gap of 1.98 eV, which is lower
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c01826.
■
sı
Synthetic procedure, experimental details, photophysical
and electrochemical characterizations, UPS data, the-
oretical calculations (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
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Xiao-Ye Wang − State Key Laboratory of Elemento-Organic
Chemistry, College of Chemistry, Nankai University, Tianjin
300071, China; orcid.org/0000-0003-3540-0277;
Email: xiaoye.wang@nankai.edu.cn
Authors
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Pengcai Liu − State Key Laboratory of Elemento-Organic
Chemistry, College of Chemistry, Nankai University, Tianjin
300071, China
Xing-Yu Chen − State Key Laboratory of Elemento-Organic
Chemistry, College of Chemistry, Nankai University, Tianjin
300071, China
Jiawen Cao − State Key Laboratory of Elemento-Organic
Chemistry, College of Chemistry, Nankai University, Tianjin
300071, China
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Aromatic Hydrocarbons with Two Embedded Azulene Units. Angew.
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(16) Lombardi, F.; Lodi, A.; Ma, J.; Liu, J.; Slota, M.; Narita, A.;
Lukas Ruppenthal − Department of Chemistry, Philipps
University Marburg, 35032 Marburg, Germany
J. Michael Gottfried − Department of Chemistry, Philipps
University Marburg, 35032 Marburg, Germany;
orcid.org/0000-0001-5579-2568
Myers, W. K.; Mullen, K.; Feng, X.; Bogani, L. Quantum Units from
̈
the Topological Engineering of Molecular Graphenoids. Science 2019,
366, 1107−1110.
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Formation of Azulene-Embedded Nanographene: Naphthalene to
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Klaus Mullen − Max Planck Institute for Polymer Research,
̈
55128 Mainz, Germany; orcid.org/0000-0001-6630-
8786
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c01826
(19) Urieta-Mora, J.; Krug, M.; Alex, W.; Perles, J.; Fernández, I.;
Molina-Ontoria, A.; Guldi, D. M.; Martín, N. Homo and Hetero
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Notes
The authors declare no competing financial interest.
(20) Zhu, C.; Shoyama, K.; Wurthner, F. Conformation and
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Aromaticity Switching in a Curved Non-Alternant sp2 Carbon
Scaffold. Angew. Chem., Int. Ed. 2020, 59, 21505−21509.
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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]-
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ACKNOWLEDGMENTS
■
We are grateful to the financial support from the National
Natural Science Foundation of China (Nos. 21901128 and
22071120), the Fundamental Research Funds for the Central
Universities, and the German Science Foundation
(223848855-SFB1083). This work is dedicated to the memory
of Prof. Virgil Boekelheide and Prof. Klaus Hafner.
(22) Mishra, S.; Beyer, D.; Berger, R.; Liu, J.; Gröning, O.; Urgel, J.
I.; Mullen, K.; Ruffieux, P.; Feng, X.; Fasel, R. Topological Defect-
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