Aromaticity Relocation in Perylene Derivatives upon Two-Electron Oxidation To Form Anthracene and Phenanthrene

Article abstract of DOI:10.1002/chem.201602188

We prepared perylene dications 1²⁺and 2²⁺by using “capped” perylene derivatives, and for the first time, successfully obtained single crystals of a perylene dication 1²⁺that enabled us to perform its structural analysis. We realized that the substituted aryl groups on perylene control the positions of positive charges, thus the remaining electronic system satisfies Clar's sextet rule toward the highest number of localized sextets. Experimental and theoretical evidence proved that Clar's aromatic π-sextet rule could be applied even for the dicationic perylenes in a very simple way.

Full text of DOI:10.1002/chem.201602188

A Journal of
Accepted Article
Title: Aromaticity Relocation in Perylene Derivatives upon Two-electron Oxidation
to Form Anthracene and Phenanthrene
Authors: Akinobu Matsumoto; Mitsuharu Suzuki; Hironobu Hayashi; Daiki Kuzu-
hara; Junpei Yuasa; Tsuyoshi Kawai; Naoki Aratani; Hiroko Yamada
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To be cited as: Chem. Eur. J. 10.1002/chem.201602188
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Aromaticity Relocation in Perylene Derivatives upon Two-electron
Oxidation to Form Anthracene and Phenanthrene
Akinobu Matsumoto,[a] Mitsuharu Suzuki,[a] Hironobu Hayashi,[a] Daiki Kuzuhara,[a] Junpei Yuasa,[a]
Tsuyoshi Kawai,[a] Naoki Aratani*[a] and Hiroko Yamada*[a]
Dedication
Abstract: We prepared perylene dications 12+ and 22+ by using
"capped" perylene derivatives, and successfully obtained the single
crystals of perylene dication 12+, for the first time, which enables us
to perform its structural analysis. We realized that the substituted
aryl groups on perylene control the positions of positive charges,
thus the remaining electronic system satisfies Clar's sextet rule
toward the highest number of localized sextets. Experimental and
theoretical evidences proved that Clar's aromatic -sextet rule could
be applied even for the dicationic perylenes in a very simple way.
sulfuric acid.[6]
The aromatic -sextets are defined as six -electrons localized
in a single benzene-like ring separated from adjacent rings by
formal C–C single bonds.[1] The concept makes it possible to
explain various physical and chemical properties of polycyclic
aromatic hydrocarbons (PAHs). Accordingly, if more than one
structure can be drawn for a given electronic system, those of
them, which have the highest number of localized sextets, are
the most stable structures. For instance, pyrene consists of four
fused benzene rings. Figure 1 shows X-ray and Kekulé
structures of pyrene and the structure with localized sextets
denoted as the circles within the rings. There is a good
correspondence with the experimental C–C bond lengths
observed in X-ray structure and the one predicted by the sextet
rule. From the single crystal X-ray structural analysis, it is
expected that the structure in Figure 1b is more stable relative to
the other resonance contributors at the ground state. In fact, the
lateral rings in pyrene are significantly more aromatic than two
inner benzene rings shown by the indicators of aromaticity,
harmonic oscillator model of aromaticity (HOMA)[2] and nucleus
independent chemical shift (NICS)[3] values (vise infra). Recently
we have reported synthesis and physical properties of the
Figure 1. a) X-ray crystal structure of pyrene with HOMA values (in bold),[7] b)
one of the Kekulé resonance structures of pyrene, c) Clar structure of pyrene.
The carbon designation is described in b).
We have reported that the sterically hindered pyrene could
be oxidized with SbCl5 to give the persistent pyrene dication.[8]
Thus, an excellent crop of dark blue colored crystals of 1,3,6,8-
tetrakis(4-mesityloxyphenyl)pyrene dication suitable for X-ray
studies were obtained (Figure 2).[7] To the best of our knowledge,
this is the first X-ray crystal structure of pyrene dication.
tetrakis-mesityloxy-substituted
tetrabenzoperipentacene,
in
which the local aromaticity absolutely depicted eight benzene
rings according to the sextet rule.[4]
Although PAHs are very common in chemistry, their isolated
cationic and dicationic species are practically unavailable in
normal experimental conditions due to their remarkable reactivity.
Cation radicals and dications are important intermediates in
organic synthesis, appearing in the molecular electronic and
optical materials for a wide range of applications.[5] As an
interesting example, Bettinger has discovered the fact that
dications of higher linear acenes are formed more easily in
Figure 2. The bond lengths (Å) and the calculated HOMA values of 1,3,6,8-
tetrakis(4-mesityloxyphenyl)pyrene dication.[7] The mesityl groups and counter
anions are omitted for clarity.
[a]
A. Matsumoto, Dr. M. Suzuki, Dr. H. Hayashi, Dr. D. Kuzuhara, Dr.
J. Yuasa, Prof. Dr. T. Kawai, Prof. Dr. N. Aratani, Prof. Dr. H.
Yamada
Graduate School of Materials Science
Nara Institute of Science and Technology (NAIST)
8916-5 Takayama-cho, Ikoma 630-0192 (Japan)
E-mail: aratani@ms.naist.jp, hyamada@ms.naist.jp
To check the aromaticity of the neutral and positively-
charged pyrenes, we performed HOMA value estimation on the
basis of crystal structures (Figures 2 and S17). By the electronic
system rearrangement upon the two-electron oxidation, the bond
Supporting information for this article is given via a link at the end of
the document.

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lengths at the inner benzenes become rather close to
naphthalene, giving higher HOMA values (Figure 2).[8] These
spectra are shown in Figure 3. After addition of 1 eq. of NOSbF6
to 1, the intense bands at 432 nm and 458 nm decreased,
whereas new bands in visible region at 743 and 595 nm and the
experimental results show
a
correlation with theoretical
calculations (Figure S14). The NICS is another aromaticity index. NIR region at 1002 nm arose. The absorption spectrum is the
However, the NICS values for charged systems tend to be high
due to the electron deficient situation (Figure S19).[9]
same as that of the electrochemically oxidized 1 at +0.52 V
(versus Fc/Fc+) (Figure S12). The calculated absorptions for 1•+
performed by time-dependent density functional theory (TD-
DFT) at the BLYP35/SVP (with CPCM solvent model
implementations in MeCN)[13] using the Gaussian 09 package[14]
roughly agree with the experimental data. The excitation at 879
nm region is responsible for the observed absorption of 1•+ at
1002 nm. Addition of 1 eq. of NOSbF6 to 2 made the intense
bands at 434 nm and 462 nm decreased, whereas new bands in
visible region at 714 nm and the NIR region at 969 nm arose
(Figure 3b). This absorption spectrum is also the same as that of
the electrochemically oxidized 2 at +0.56 V (versus Fc/Fc+)
(Figure S13). The predicted absorptions for 2•+ are in
accordance with the experimental data. The calculated
absorption at 818 nm is responsible for the observed absorption
of 2•+ at 969 nm and the one at 685 nm corresponds to the
experimental absorption at 714 nm.
The two-electron oxidation changes the electronic system of
pyrene into that of naphthalene ⎯ according to this finding, thus
we have quested for a double image of the other PAH as well.
Here, we select perylene as a particular example to highlight the
aromaticity relocation.
While perylene anions and dianions can be isolated in the
moderate experimental conditions,[10] the cationic and dicationic
counterparts are unstable due to their high reactivity.[11] The
oxidation of the unsubstituted perylene having an amine bridge
at a bay-area results in the formation of directly-linked dimers.[12]
Therefore, in order to investigate the isolated cationic species, it
is necessary to cap the most reactive positions by the
substituents. We have prepared 3,9- and 3,10-diarylperylenes 1
and 2 by the Suzuki–Miyaura cross-coupling between 4-
borylanisole and dibromoperylenes
3 and 4, respectively
(Scheme 1). These compounds have been fully characterized by
1H and 13C NMR, high-resolution MALDI-TOF mass
spectroscopy, and single crystal X-ray analysis (SI).[7]
Scheme 1. Synthesis of perylene derivatives 1 and 2. (a) Br2, benzene. (b) p-
MeOPhB(OH)2, K2CO3, Pd cat. Ar = p-methoxy-phenyl.
UV-vis absorption spectra of 1 and 2 in toluene are shown in
Figure 3. Compared to the pristine perylene (max = 439 nm), 1
and 2 exhibit red-shifted and broader absorption (max = 461 and
462 nm, respectively), suggesting the certain electronic
communication between the perylene and phenyl units.
Compounds 1 and 2 show reversible oxidative waves in
cyclic voltammetry at +0.41 and +0.79 V for 1, and +0.42 and
+0.78 V for 2 (versus Fc/Fc+, Figure S24). Compared to the first
oxidation potential of the pristine perylene (0.54 V vs Fc/Fc+),
those of compounds 1 and 2 shifted to lower potentials. The
reversible oxidation encouraged us to investigate the ability of 1
and 2 for the generation of the stable cationic and dicationic
species.
Figure 3. UV-vis-NIR absorption spectra of (a) 1 (black), 1•+ (red), and 12+
(blue) (4.4 × 10–6 M) and (b) 2 (black), 2•+ (red), and 22+ (blue) (1.8 × 10–6 M)
along with the calculated oscillator strength, formed under the oxidation with
NOSbF6 in CH2Cl2/CH3CN at room temperature.
The generation of the cationic and dicationic species of 1
and 2 was carried out with NOSbF6 (1 eq. and 2 eq.,
respectively) in CH2Cl2/CH3CN. The UV-vis-NIR absorption
Upon further addition of NOSbF6 (totally 2 eq.), 1•+ was
oxidized to form the dicationic species 12+, resulting in the
appearance of visible band at 631 nm (Figure 3a), and the

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disappearance of the ESR signals. These indicate that the
singlet state of 12+ is more stable than triplet state. For dication
12+, the simulated absorption agrees with the experimental data.
The excitation at 802 nm is responsible for the observed
absorption of 12+ at 631 nm. This excitation is associated with
from HOMO to LUMO with the oscillator strength of 1.39.
By the electronic system rearrangement upon the two-
electron oxidation, the bond lengths at C4-C5, C6-C6a, C10-C11,
and C12-C12a in 1 become longer and those at C3a-C4, C5-C6,
C6a-C6b, C6b-C12d, C9b-C12d, C9b-C10, C11-C12, and C12a-
C12b become shorter, being close to anthracene. Here, the
planes constructed from C1, C2, and C3, and C7, C8 and C9 in
12+ are twisted against the planar central anthracene core by 10°
(Figure S23). The bond length alternations from 1 to 12+ and the
twist of the lateral 3C units in 12+ indicate the large change of its
electronic system. HOMA values for 1 and 12+ on the basis of
crystal structures are well-correlated with the theoretical
calculations (Figure S18). Consequently the aromaticity of the
perylene (larger HOMA values) relocates from the (two)
naphthalene parts into the anthracene unit upon the two-electron
oxidation (bold in Figure 4 and Scheme 2).
We have tried to make single crystals of 12+. By using similar
conditions to the pyrene dication, dark blue crystals of
12+2(SbCl6)– from anhydrous dichloromethane and heptane
were obtained (Figure 4).[7] The crystal structure of 12+2(SbCl6)–
revealed that the counter anions are positioned above and
below the perylene plane. The dihedral angles of phenyl units in
12+ are 35. Compared to the neutral state, the phenyl groups
tilted about 26°. The quinoidal structure of methoxyphenyl
groups should contribute to stabilize dicationic state. Although
there are literatures on the charged perylenes,[10,11] this is the
first X-ray crystal structure of perylene dication, thus the first
experimental structural analysis of perylene dication.
Scheme 2. Aromaticity relocation of perylenes 1 and 2 upon the two-electron
oxidation.
Further addition of NOSbF6 (totally 2 eq.) to 2•+ resulted in
the formation of the dicationic species 22+. The appearance of
visible band at 700 nm (Figure 3b) and the disappearance of the
ESR signals indicate that the singlet state of 22+ is more stable
than triplet state. The ground state of dicationic species was also
estimated to be singlet by the DFT calculations. The excitation at
724 nm is responsible for the observed absorption of 22+ at 700
nm. This excitation is associated from HOMO to LUMO with the
oscillator strength of 1.71. In spite of many attempts, the crystals
of 22+ suitable for X-ray measurement could not be obtained so
far. Theoretical calculations for 22+ predict similar behavior to
12+; the bond lengths at C4-C5, C6-C6a, C10-C11, and C12-
C12a become longer and those at C3a-C4, C5-C6, C6a-C6b,
C6b-C12d, C9b-C12d, C9b-C10, C11-C12, and C12a-C12b
become shorter, being close to phenanthrene (Figure S16 and
Scheme 2).
Considering the resonance contributors of the perylene
dication, we realized that the substituted aryl groups control the
positions of positive charges, thus the remaining electronic
system is willing to satisfy Clar's sextet rule. Eventually the
Figure 4. Crystal structure of a) 12+2(SbCl6)–, and the bond lengths (Å) and
the calculated HOMA values of b) 1 and c) 12+2(SbCl6)–. Thermal ellipsoids
are scaled at 50% probability.

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pristine aromaticity relocates into another unit upon the two-
electron oxidation toward the highest number of localized
sextets, called Clar structure (Scheme 2).
characteristics vibronic absorption spectrum around 350 nm.
The aromaticity relocation would also occur in other PAHs.
Radical cations and dications are not only important
intermediates in organic transformations, but also crucial for a
wide range of materials because of their intriguing electronic and
optical properties. The characteristic absorption spectra of
charged PAHs can be seen as their fingerprints, making it
possible to identify the compound even in the interstellar
medium.[15] We are actively investigating larger PAHs upon
oxidation to explore the reactivity of the charged PAHs.
Finally, we have double-checked MO diagrams of 12+ in
detail in order to understand the apparent vibronic structure in
the absorption spectrum of 12+ at 387, 372, 358, and 346 nm
(Figure 3a). At a glance, this characteristic spectral shape is
recognizable to -* transition of anthracene (abs = 377, 358,
341, and 326 nm in CH2Cl2). Curiously, this absorption is
assigned to be from HOMO to LUMO+2 of 12+, and their orbital
coefficients involve those of HOMO and LUMO of anthracene
(dotted squares in Figure 5). These facts inspire us to research
the reactivity of perylene dications as the doubly positive-
charged anthracene. This is our next target.
Acknowledgements
This work was partly supported by Grants-in-Aid for Scientific
Research (Nos. 25288092, 26105004, 26288038, 16H02286
and 15H00876 'AnApple'), the Green Photonics Project in NAIST,
and the program for promoting the enhancement of research
universities in NAIST supported by MEXT. We thank Mr. S.
Katao, Mr. F. Asanoma, Ms. Y. Nishikawa, NAIST, for the X-ray
diffraction analysis, for EPR measurements, and for the mass
spectroscopy, respectively. We also thank Prof. Y. Misaki,
Ehime Univ. for fruitful discussion.
–2.55
LUMO+2
Anthracene
–4.09
LUMO+1
LUMO
HOMO
Keywords: aromaticity • oxidation • dication • bond length
alternation • perylene
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Figure 5. MO diagrams of perylene dication 12+ (singlet). Inset shows the
frontier orbital of anthracene
In summary, multiple experimental and theoretical evidences
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Entry for the Table of Contents (Please choose one layout)
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We prepared perylene dications 12+
A. Matsumoto, M. Suzuki, H. Hayashi,
D. Kuzuhara, J. Yuasa, T. Kawai, N.
Aratani,* and H. Yamada*
Ar
Ar
Ar
and 22+ by using "capped" perylene
derivatives, and successfully obtained
the single crystal of perylene dication
12+, for the first time. Experimental
and theoretical evidences proved that
Clar's aromatic -sextet rule can be
applied even for the dicationic
– 2e–
Page No. – Page No.
Aromaticity Relocation in Perylene
Derivatives upon Two-electron
Oxidation to Form Anthracene and
Phenanthrene
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
perylenes in a very simple way.
– 2e–