Dicyclopentaannelated Hexa-peri-hexabenzocoronenes with a Singlet Biradical Ground State

Article abstract of DOI:10.1002/anie.202102932

Synthesis of two dicyclopentaannelated hexa-peri-hexabenzocoronene (PHBC) regioisomers was carried out, using nonplanar oligoaryl precursors with fluorenyl groups: mPHBC 8 with two pentagons in the “meta”-configuration was obtained as a stable molecule, while its structural isomer with the “para”-configuration, pPHBC 16, could be generated and characterized only in situ due to its high chemical reactivity. Both PHBCs exhibit low energy gaps, as reflected by UV-vis-NIR absorption and electrochemical measurements. They also show open-shell singlet ground states according to electron paramagnetic resonance (EPR) measurements and density functional theory (DFT) calculations. The use of fully benzenoid HBC as a bridging moiety leads to significant singlet biradical characters (y₀) of 0.72 and 0.96 for mPHBC 8 and pPHBC 16, respectively, due to the strong rearomatization tendency of the HBC π-system; these values are among the highest for planar carbon-centered biradical molecules. The incorporation of fully unsaturated pentagons strongly perturbs the aromaticity of the parent HBC and makes the constituted benzene rings less aromatic or antiaromatic. These results illustrate the high impact of cyclopentaannelation on the electronic structures of fully benzenoid polycyclic aromatic hydrocarbons (PAHs) and open up a new avenue towards open-shell PAHs with prominent singlet biradical characters.

Full text of DOI:10.1002/anie.202102932

Angewandte
Communications
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How to cite: Angew. Chem. Int. Ed. 2021, 60, 11300–11304
International Edition:
German Edition:
doi.org/10.1002/anie.202102932
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Polycyclic Aromatic Hydrocarbons Hot Paper
Dicyclopentaannelated Hexa-peri-hexabenzocoronenes with a Singlet
Biradical Ground State
Qiang Chen, Martin Baumgarten, Manfred Wagner, Yunbin Hu, Ian Cheng-Yi Hou,
Akimitsu Narita,* and Klaus Mꢀllen*
In memory of Professor Siegfried Hꢀnig
[
1]
Abstract: Synthesis of two dicyclopentaannelated hexa-peri-
hexabenzocoronene (PHBC) regioisomers was carried out,
using nonplanar oligoaryl precursors with fluorenyl groups:
mPHBC 8 with two pentagons in the “meta”-configuration
was obtained as a stable molecule, while its structural isomer
with the “para”-configuration, pPHBC 16, could be generated
and characterized only in situ due to its high chemical
reactivity. Both PHBCs exhibit low energy gaps, as reflected
by UV-vis-NIR absorption and electrochemical measurements.
They also show open-shell singlet ground states according to
electron paramagnetic resonance (EPR) measurements and
density functional theory (DFT) calculations. The use of fully
benzenoid HBC as a bridging moiety leads to significant
(opto)electronic and spintronic devices. In addition to
[2]
introducing extended zigzag edges, such as in anthenes,
[
3]
[1f,4]
periacences and zethrenes,
annelation of fully unsatu-
rated pentagons onto PAHs has proven to be another
effective way to induce an open-shell nature. To this end,
several types of cyclopentannelated PAHs (PPAHs) have
been constructed by fusing two indeno groups onto small
[
5]
[6]
[7]
PAHs, including naphthalene, anthracene, pyrene, per-
[8]
[9]
[10]
ylene, bischrysene and corannulene. Furthermore, the
addition of two methine bridges to the bay regions of PAHs
has also been carried out to construct PPAHs (Sche-
[
11]
me 1A). However, in all of these examples, the two fully
unsaturated pentagons (methine radical centers) are fused to
not-fully benzenoid PAHs, and most of them exhibit closed-
shell or open-shell ground states with relatively small singlet
singlet biradical characters (y ) of 0.72 and 0.96 for mPHBC 8
0
and pPHBC 16, respectively, due to the strong rearomatization
tendency of the HBC p-system; these values are among the
highest for planar carbon-centered biradical molecules. The
incorporation of fully unsaturated pentagons strongly perturbs
the aromaticity of the parent HBC and makes the constituted
benzene rings less aromatic or antiaromatic. These results
illustrate the high impact of cyclopentaannelation on the
electronic structures of fully benzenoid polycyclic aromatic
hydrocarbons (PAHs) and open up a new avenue towards
open-shell PAHs with prominent singlet biradical characters.
biradical characters (y < 0.7). The incorporation of two
0
unsaturated pentagons into fully benzenoid aromatic hydro-
carbons is expected to generate PPAHs with a prominent
open-shell character, since larger numbers of Clarꢀs aromatic
p-sextets exist in the open-shell forms than in the closed-shell
forms. This strategy, however, has rarely been implemented to
date, except for the indenofluorene family.
Hexa-peri-hexabenzocoronene (HBC) is a classic exam-
ple of a fully benzenoid PAH, which has also been seen as
a “superbenzene” due to its D6h symmetry and close relation
to benzene. In its ground state, HBC can be described as
incorporating seven separated benzene rings, according to
[
1a,12]
[
13]
P
olycyclic aromatic hydrocarbons (PAHs) with open-shell
electronic structures have drawn tremendous attention owing
to their unique optical, magnetic and electronic properties,
which make them promising candidates for use in organic
[14]
Clarꢀs aromatic p-sextet rule. The special electronic struc-
ture and high stability of HBC make it an attractive molecular
[
*] Dr. Q. Chen, Prof. Dr. M. Baumgarten, Dr. M. Wagner, Dr. Y. Hu,
Dr. I. C.-Y. Hou, Prof. Dr. A. Narita, Prof. Dr. K. Mꢀllen
Synthetic Chemistry, Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Dr. Y. Hu
Current address: College of Chemistry and Chemical Engineering,
Central South University
Changsha, Hunan 410083 (P. R. China)
E-mail: narita@mpip-mainz.mpg.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.202102932.
muellen@mpip-mainz.mpg.de
Prof. Dr. K. Mꢀllen
Institute of Physical Chemistry, Johannes Gutenberg-University
Duesbergweg 10–14, 55128 Mainz (Germany)
ꢁ
2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
Prof. Dr. A. Narita
Organic and Carbon Nanomaterials Unit, Okinawa Institute of
Science and Technology Graduate University
Okinawa 904-0495 (Japan)
Dr. Q. Chen
Current address: Department of Chemistry, University of Oxford,
Chemistry Research Laboratory
Oxford OX1 3TA (UK)
1
1300
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Scheme 1. A) Representative examples of cyclopentaannelated, not-
[
11b]
fully benzenoid PAHs, i.e., dicyclopentaannelated perylene (n=0)
and dicyclopentaannelated bisanthene (n=1);
[
11a]
and B) dicyclopen-
Scheme 2. Synthesis of mPHBC 8 (A) and pPHBC 16 (B). Reagents
taannelation on fully benzenoid HBC, giving access to mPHBC and
pPHBC and their resonance between the Kekulꢂ structure and open-
shell biradical form.
and conditions: a) (1) LDA, THF, ꢀ788C; (2) TMSCl, r.t., 5 h, 22%
yield; b) 4-tert-butylphenylboronic acid, Pd(dppf)Cl ·CH Cl , K PO ,
2
2
2
3
4
DMF, 1008C, 24 h, 77% yield; c) Br , DCM/MeOH, 258C, overnight,
2
9
1
1% yield; d) (Bpin) , Pd(dppf)Cl ·CH Cl , XPhos, K CO , DMF,
2 2 2 2 2 3
108C, 24 h, 55% yield; e) 4-bromo-9H-fluoren-9-one, Pd (dba) ·CHCl ,
2
3
3
model for studying how radicals delocalize their spin density
and interact with each other through large conjugated p-
systems. For example, Wuꢀs group investigated the synthesis
of HBC-quinone derivatives, which exhibited open-shell
XPhos, K CO , 1,4-dioxane/H O, 1008C, 24 h, 91% yield; f) (1) MesLi,
2
3
2
THF, ꢀ788C to r.t., overnight; (2) BF
·OEt , Et SiH, DCM, r.t., over-
3 2 3
night, 57% yield; g) DDQ, MSA, r.t., 1 h, then TFMSA, r.t., 1 h, 5%
yield; h) 4-tert-butylphenylboronic acid, Pd(PPh ) , K CO , 1,4-dioxane/
3
4
2
3
[
15]
H
2
O, 908C, 24 h, 68% yield; i) (1) t-BuLi, THF, ꢀ788C, 2.5 h; (2) 2-
biradical to multiradical features. In these molecules, the
radical centers (oxygen atoms) are not directly involved in the
p-conjugated backbone. In comparison, incorporation of the
radical centers into HBC could facilitate intramolecular
radical conjugation and provide promising model molecules
to investigate structure-biradical property relationships. One
question is how the regioisomerism influences the exchange
interaction between radicals, but this remains unanswered.
Herein, we report our synthetic approach towards two
pentagon-fused HBC regioisomers with two five-membered
rings located on the bay regions in the “meta”-configuration
isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, ꢀ788C to r.t., over-
night, 29% yield; j) 4-bromo-9H-fluoren-9-one, Pd (dba) ·CHCl ,
2
3
3
XPhos, K CO , 1,4-dioxane/H O, 1008C, 24 h, 79% yield; k) (1) MesLi,
2
3
2
THF, ꢀ788C to r.t., overnight; (2) BF ·OEt , Et SiH, DCM, r.t., 12 h,
3
2
3
66% yield; l) FeCl , DCM/MeNO , r.t., 0.5 h, 86% yield; m) n-BuLi,
3 2
THF, r.t.; n) I2, r.t. LDA=lithium diisopropylamide, TMS=trimethyl-
silyl, dppf=1,1’-bis(di-phenylphosphino)ferrocene, dba=dibenzylide-
neacetone, Mes=mesityl, DDQ=2,3-dichloro-5,6-dicyanobenzoqui-
none, MSA=methanesulfonic acid, TFMSA=trifluoromethanesulfonic
acid, Bpin=boronic acid pinacol ester, THF=tetrahydrofuran,
DMF=N,N-dimethylformamide, DCM=dichloromethane.
(mPHBC) and “para”-configuration (pPHBC) (Scheme 1B).
Three and five more Clarꢀs p-sextets are gained in the open-
shell biradical forms of mPHBC and pPHBC, respectively,
compared with their corresponding fully conjugated Kekulꢁ
structures, reflecting their preferences to exist as energetically
more favorable biradicals. The relevant features, such as the
low energy gap, open-shell character and antiaromaticity, are
comprehensively studied by UV-vis-NIR absorption and
electrochemical experiments, variable-temperature nuclear
magnetic resonance (NMR) and electron paramagnetic
resonance (EPR) measurements, and density functional
theory (DFT) calculations.
(v/v) mixture of dichloromethane and methanol gave dibro-
moterphenyl 4 in 91% yield. Next, Miyaura borylation on 4
afforded diborylated terphenyl 5 in 55% yield. After Suzuki
coupling with 4-bromo-9H-fluoren-9-one, the key intermedi-
ate diketone 6 was obtained in 91% yield. Subsequently, 6
was treated with mesityllithium (MesLi), followed by reduc-
tion with BF ·OEt /Et SiH to furnish bisfluorene 7 in 57%
3
2
3
yield over two steps. The cyclization of 7 was first attempted
with ferric chloride (FeCl ) through a commonly used method
3
[16]
for HBC synthesis, but only partial cyclization (three bonds
were closed) was suggested by matrix-assisted laser desorp-
tion/ionization time-of-flight mass spectrometry (MALDI-
TOF MS) analysis (see Figure S1). Increasing the reaction
To synthesize mPHBC 8 (Scheme 2A), we first prepared
2
,3-dibromo-1,4-bis(trimethylsilyl)benzene (2) from 1,2-
dibromobenzene (1) by lithiation with lithium diisopropyla-
mide (LDA), followed by nucleophilic substitution with
trimethylsilyl chloride in 22% yield. Then, twofold Suzuki
coupling of 2 with 4-tert-butylphenylboronic acid provided
time and equivalents of FeCl had no effect on the result.
3
Using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as
the oxidant in the presence of either methanesulfonic acid
[
17]
terphenyl 3 in 77% yield. Bromination of 3 with Br in a 1:1
(MSA) or trifluoromethanesulfonic acid (TFMSA)
gave
2
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complex reaction mixtures. Remarkably, using DDQ as an
oxidant in the presence of MSA followed by the addition of
TFMSA successfully provided the target compound mPHBC
8
(Figure S2), albeit in only 5% yield after removing the
major oxidized byproducts by silica gel chromatography.
Although high-quality crystals suitable for the single-crystal
X-ray analysis could not be obtained, mainly resulting in the
decomposition, the formation of 8 could be validated by
NMR (vide infra) in addition to the MALDI-TOF MS
analysis.
The synthesis of pPHBC 16 was attempted in a similar
way, starting from 1,4-dibromo-2,5-diiodobenzene (9) (Sche-
me 2B). After Suzuki coupling of 9 with 4-tert-butylphenyl-
boronic acid and borylation, terphenyl borate ester 11 was
synthesized in 29% yield. Then, Suzuki coupling with 4-
bromo-9H-fluoren-9-one provided diketone 12 in 79% yield,
which was treated with MesLi and reduced by BF ·OEt /
3
2
Et SiH to provide 13 in 66% yield. Different from the
3
synthesis of mPHBC 8, the treatment of 13 with DDQ as the
oxidant and MSA as the acid, followed by the addition of
TFMSA, resulted in messy reaction mixtures without assign-
able peaks in the MS spectrum. In this case, however, the use
of FeCl provided intermediate HBC 14 in 86% yield. Further
dehydrogenation/aromatization was attempted using differ-
3
1
Figure 1. A) Aromatic region of variable-temperature H NMR spectra
ent conditions, including the application of DDQ or p-
of mPHBC 8 (500 MHz, 273 K and 298 K in CD Cl , 313–373 K in
2
2
t
1
chloranil as an oxidant or BuOK/DMF, which have been
C
2
D
2
Cl
dianion 15 (300 MHz) and pPHBC 16 at 298 K and 213 K (500 MHz,
D ]THF). The height of the solvent residual peaks was normalized for
4
); B) aromatic region of H NMR spectra of HBC precursor 14,
[18]
proven effective for the aromatization of similar precursors.
All these conditions were unsuccessful, and only oxidized
i.e., oxygen containing) pPHBC could be detected by
[
8
comparison.
(
MALDI-TOF MS (see Figures S4,S5). Since pPHBC 16
could not be isolated, presumably due to its high reactivity,
we turned to an in situ generation and characterization of
pPHBC 16, employing a sequence of deprotonation with n-
BuLi and oxidation with iodine.
(vide infra for the further characterizations of the in situ
generated 16).
The UV-vis-NIR absorption spectra (Figure 2A) of
mPHBC 8 and in situ generated pPHBC 16 exhibit intense
As demonstrated in Figure 1A, the aromatic region of the
1
H NMR spectrum of mPHBC 8 measured in CD Cl at 273 K
2
2
clearly shows four doublet and three singlet peaks, which
could be fully assigned via two-dimensional NMR (see
Figures S30,S31). With increasing temperature, these sharp
peaks broadened, decreased in intensity and finally disap-
peared at 373 K. We ascribe this observation to the presence
of a thermally excited triplet state due to a small singlet-
triplet energy gap, as has been commonly observed in other
[
3a,9a,11a,19]
open-shell systems,
although partial thermal decom-
position of 8 cannot be excluded. As illustrated in Figure 1B,
1
the H NMR spectrum of HBC derivative 14 in [D ]THF
8
exhibits two sets of fully assignable peaks due to the existence
of its cis-/trans-isomers. After the addition of 2.0 equiv. of n-
BuLi to this solution, the peaks belonging to the methine
bridges disappeared, and the whole spectrum became simpler,
in agreement with the highly symmetric structure of the
resulting dianion 15. In the following step, 2.0 equiv. of I were
added to oxidize dianion 15, which caused the disappearance
of the sharp peaks. Even if the temperature was decreased to
2
Figure 2. A) UV-vis-NIR absorption of mPHBC 8 in DCM and in situ
generated pPHBC 16 in THF under an argon atmosphere with
ꢀ5
a concentration of 10 M; the inset shows the magnified long-wave-
length-absorption region. B) Cyclic voltammogram of mPHBC 8 in dry
o-dichlorobenzene with 0.1 M n-Bu NPF as an electrolyte at room
2
13 K, no well-resolved peak could be observed from the
NMR spectrum. After the in situ synthesis, MALDI-TOF MS
analysis showed oxidized species of pPHBC 16 without any
other side products, such as the addition of n-butyl or iodo
groups (see Figure S6), which supported the formation of 16
4
6
+
temperature with Fc/Fc as reference. C and D) Variable-temperature
EPR spectra of mPHBC 8 in the solid state and in situ generated
pPHBC 16 in a THF solution.
1
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absorption bands in the long wavelength region centered at
82 nm and 770 nm, respectively, together with weak tailing
6
into the 800–1200 nm region. The longest absorption wave-
lengths at approximately 1075 nm (8) and 1150 nm (16)
originate from the forbidden HOMO!LUMO transition
according to TD-DFT calculations (UB3LYP/6-31G(d, p),
HOMO!LUMO transitions of mPHBC 8 and pPHBC 16
are calculated at 1012 nm (f = 0.0761) and 1025 nm (f =
0
.0598), respectively). These features are starkly different
from the spectrum of pristine HBC with the main UV-vis
absorption wavelength below 400 nm (see Figure S7). The
half-life times of mPHBC 8 and pPHBC 16 in degassed
solutions in the dark under an inert atmosphere were
estimated to be 19 days and 7 h, respectively, from the
decays of their absorbance at 682 nm and 770 nm (see
Figures S9, S10, S14, S15). The cyclic voltammetry (CV) of
mPHBC 8 (Figure 2B) gave two reversible oxidation and
Figure 3. Spin density distribution of the triplet diradical state of
ox
A) mPHBC 8 and B) pPHBC 16, the numbers indicate spin densities
on these positions; XY NICS(1) maps calculated for C) mPHBC 8 and
D) pPHBC 16. GIAO shieldings (shown with negative values) were
calculated at the B3LYP/6-31G(d, p) level of theory 1 ꢃ above the
molecular plane.
reduction peaks, with half-wave potentials at E1 = 0.42 V
/2
red
and 0.79 Vand at E
= ꢀ0.88 Vand ꢀ1.16 V with reference
1
/2
+
to Fc/Fc . The HOMO and LUMO energy levels were
accordingly calculated to be ꢀ5.14 eV and ꢀ3.86 eV, respec-
tively. The low LUMO energy level indicates that mPHBC 8
is potentially a good electron acceptor. Treating mPHBC 8
with hydrazine solution results in the formation of a reduction
product containing two hydrogens more than mPHBC 8 (see
Figure S3), which has not been accessible by cyclodehydro-
genation of compound 7.
probing the gauge-independent atomic orbital (GIAO)
shielding 1 ꢂ above the molecular planes (Figure 3C,D).
The resulting XY nucleus-independent chemical shift (NICS)
(1) maps reveal that in mPHBC 8, the five-membered ring
zones are strongly deshielded (NICS(1) = 39), which also
causes deshielding of the adjacent two benzene rings (on top,
NICS(1) = 16) connecting these two five-membered rings,
while the other five benzene rings remain aromatic (NICS-
(1) = ꢀ9 to ꢀ22). This observation was experimentally
confirmed by NMR measurements showing high-field shifts
of the protons (d = 7.32 ppm, 6.89 ppm) on the benzene rings
fused to the five-membered rings (Figure 1A). The XY
NICS(1) map of pPHBC 16 displays a different pattern. The
deshielding effect is particularly large above two five-
membered rings (NICS(1) = 53) and noticeably weaker
above the other seven benzene rings (NICS(1) = ꢀ3 to 14),
indicating more localized antiaromaticity of pPHBC.
EPR measurements were performed on mPHBC 8 in the
solid state and in situ generated pPHBC 16 in a THF solution
at different temperatures (Figure 2C, D). Both mPHBC 8 and
pPHBC 16 exhibit unresolved broad EPR signals. The
intensity of the EPR signal increases at higher temperatures,
which reflects the existence of thermally accessible triplet
states for both 8 and 16. DFT calculations were conducted at
the B3LYP/6–31G(d, p) level of theory to further understand
the electronic properties of the PHBCs. Using the occupation
numbers of the spin-unrestricted Hartree–Fock natural orbi-
[
20]
tals, mPHBC 8 appears to have a moderate singlet biradical
character (y ) of 0.72, while pPHBC 16 possesses a much
0
larger y value of 0.96. This can be rationalized by drawing
0
resonance structures of PHBCs (Scheme 1B). It appears that
two more Clarꢀs p-sextets are destroyed for pPHBC 16 when
preserving the fully conjugated Kekulꢁ structure, which
makes the biradical form of 16 energetically more favorable.
Spin density distribution calculations indicate that unpaired
electrons are mainly delocalized on the rim of PHBCs, with
the methine bridges at the bay regions having the highest spin
densities (Figure 3A,B), while the spin densities of the
adjacent carbons are relatively small. In addition, the
calculations show that the singlet biradical forms of
mPHBC 8 and pPHBC 16 have lower energies than those
of both their closed-shell and open-shell triplet biradical
In summary, we designed two dicyclopenta-fused HBCs,
i.e., mPHBC 8 and pPHBC 16, and successfully obtained the
former and in situ generated the latter. The synthetic routes
involved the cyclodehydrogenation of adequately designed
precursors with one pair of preinstalled fluorenyl groups as
key steps. Both PHBC isomers exhibit remarkable singlet
biradical ground-state features, as proven by variable-temper-
ature NMR and EPR experiments, together with DFT
calculations. The most characteristic features of the PHBCs
are their low energy gaps, as indicated by the long wavelength
optical absorption and CV measurements, and the disruption
of the Kekulꢁ structures of the parent HBC. The prominent
singlet biradical character found for the PHBCs can presum-
ably be ascribed to the strong tendency of the HBC core to
locally sustain the fully benzenoid form, leading to partial
localization of spin over the methine carbons. This design
strategy may afford access to additional unique cyclopen-
taannelated PAHs based on other existing fully benzenoid
PAHs that are abundant in the literature. These cyclopen-
forms (see Table S1). The singlet-triplet energy gaps (DE
were calculated to be ꢀ1.80 kcalmol and ꢀ0.18 kcalmol
)
S-T
ꢀ1
ꢀ
1
for mPHBC 8 and pPHBC 16, respectively, indicating a singlet
biradical ground state of both molecules, which agrees with
the variable-temperature NMR and EPR measurements.
The effects of cyclopentaannelation on the aromaticity of
the HBC core in mPHBC 8 and pPHBC 16 were visualized by
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taannelated PAHs could find potential applications in opto-
electronics, spintronics and quantum information technology.
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[
Acknowledgements
We are grateful for the financial supported from the Max
Planck Society, the FLAG-ERA Grant OPERA by DFG
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T. Y. Gopalakrishna, Y. Han, Q. Jiang, J. Wu, J. Am. Chem. Soc.
The authors declare no conflict of interest.
Keywords: dicyclopentaannelation ·
2019, 141, 7266 – 7270.
hexa-peri-hexabenzocoronene · low energy gap ·
not-fully benzenoid PAH · open-shell biradical
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Manuscript received: February 26, 2021
Accepted manuscript online: March 22, 2021
Version of record online: April 7, 2021
1
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