Pyrrole-fused azacoronene family: The influence of replacement with dialkoxybenzenes on the optical and electronic properties in neutral and oxidized states

Article abstract of DOI:10.1021/ja402371f

A novel pyrrole-fused azacoronene family was synthesized via oxidative cyclodehydrogenation of the corresponding hexaarylbenzenes as the key step, and the crystal structures of tetraazacoronene 3b and triazacoronene 4a were elucidated. The photophysical p

Full text of DOI:10.1021/ja402371f

Article
pubs.acs.org/JACS
Pyrrole-Fused Azacoronene Family: The Influence of Replacement
with Dialkoxybenzenes on the Optical and Electronic Properties in
Neutral and Oxidized States
Masayoshi Takase,*,† Tomoyuki Narita,† Wataru Fujita,† Motoko S. Asano,† Tohru Nishinaga,*,†
Hiroaki Benten,‡ Kenji Yoza,§ and Klaus Mullen*,∥
̈
†Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397,
Japan
‡Department of Polymer Chemistry, Graduate School of Science and Engineering, Kyoto University, Katsura, Nishikyo, Kyoto
615-8510, Japan
§Bruker AXS, Yokohama, Kanagawa 221-0022, Japan
∥Max-Planck-Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany
S
* Supporting Information
ABSTRACT: A novel pyrrole-fused azacoronene family was
synthesized via oxidative cyclodehydrogenation of the
corresponding hexaarylbenzenes as the key step, and the
crystal structures of tetraazacoronene 3b and triazacoronene
4a were elucidated. The photophysical properties for neutral
compounds 1−4 were investigated using steady-state UV−vis
absorption/emission spectroscopy and time-resolved spectros-
copy (emission spectra and lifetime measurements) at both room temperature and 77 K. The observation of both fluorescence
and phosphorescence allowed us to estimate the small S1−T1 energy gap (ΔES−T) to be 0.35 eV (1a), 0.26 eV (2a), and 0.36 eV
(4a). Similar to the case of previously reported hexapyrrolohexaazacoronene 1 (HPHAC), electrochemical oxidation revealed up
to four reversible oxidation processes for all of the new compounds. The charge and spin delocalization properties of the series of
azacoronene π-systems were examined using UV−vis−NIR absorption, ESR, and NMR spectroscopies for the chemically
generated radical cations and dications. Combined with the theoretical calculations, the experimental results clearly demonstrated
that the replacement of pyrrole rings with dialkoxybenzene plays a critical role in the electronic communication, where resonance
structures significantly contribute to the thermodynamic stability of the cationic charges/spins and determine the spin
multiplicities. For HPHAC 1 and pentaazacoronene 2, the overall aromaticity predicted for closed-shell dications 12+ and 22+ was
primarily based on the theoretical calculations, and the open-shell singlet biradical or triplet character was anticipated for
tetraazacoronene 32+ and triazacoronene 42+ with the aid of theoretical calculations. These polycyclic aromatic hydrocarbons
(PAHs) represent the first series of nitrogen-containing PAHs that can be multiply oxidized.
materials with well-defined structures typically have advanta-
geous synthetic routes and/or chemical stabilities compared to
recently developed PAHs that contain heteroatoms such as
boron, silicon, and phosphorus.12 As examples of N-doped
PAHs, coronene-,13−16 acene-,17−20 triphenylene-,21,22 and
other PAH-based π-systems23,24 have been synthesized. The
presence of imino nitrogens increases the overall electron-
accepting properties compared to hydrocarbon- and other
heteroatom-containing analogues. Therefore, it is well-known
that all-carbon triphenylenes are p-type and hexaazatripheny-
lenes are n-type semiconductors.22 In addition, metal-ion
complexation15,21a,d and alkylation/arylation11g,24,25 can func-
tionalize N-doped PAHs.
INTRODUCTION
■
Enormous interest exists in the development of π-electron
materials because they exhibit electronic, optoelectronic, and
magnetic properties that are suitable for their potential
application in molecular electronics.1−3 Compared to π-
conjugated polymers, polycyclic aromatic hydrocarbons
(PAHs) exhibit molecular structures and molecular weights
that provide advantages in investigations of their fundamental
structure−property relationships.1b,3 From these points of view,
several approaches, such as the peripheral modification of
known PAHs,4 the reorganization of fusion patterns,5−7 and the
introduction of curvature,8,9 have been adopted to produce
novel π-electron materials.
The introduction of heteroatoms, such as nitrogen, into
PAHs strongly affects the electronic structures, as has been
observed in the case of N-doped carbon materials.10 Similar to
PAHs that incorporate sulfur,11 nitrogen-containing π-electron
Received: March 6, 2013
© XXXX American Chemical Society
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Chart 1. Pyrrole-Fused Azacoronene Family Investigated in This Paper
a
Scheme 1. Synthesis of Pyrrole-Fused Azacoronenes 2a,b−4a,b and Partially Fused 5
a
Reagents and conditions: (i) (3,5-dialkoxyphenyl)boronic acid, Pd(PPh3)4, Aliquat 336, in toluene and H2O; (ii) 4-(tert-butylphenyl)boronic acid,
Pd(PPh3)4, Aliquat 336, in toluene and H2O; (iii) FeCl3 in CH2Cl2 and CH3NO2.
In contrast to the above-mentioned N-doped π-systems that
possess electron-accepting pyridine-type nitrogens, pyrrole and
its derivatives contain electron-donating nitrogen atoms.
Because this nitrogen offers two electrons to the five-membered
ring, pyrrole is an aromatic and electron-rich compound. Some
biological materials, pigments,26 and conductive polymers
based on the pyrrole-containing π-system27 have been
investigated, but few variations are known compared to the
number of variations in the pyridine-type π-system. Synthetic
studies on large pyrrole-containing π-systems are rather limited
to the porphyrin and phthalocyanine derivatives.28 Therefore,
we recently demonstrated that cyclodehydrogenation of
hexapyrrolobenzene effectively yielded a fused structure (i.e.,
hexapyrrolohexaazacoronene 1 (HPHAC)),16 where six
pyrroles were circularly connected (Chart 1). The crystal
structure of dication 12+ clearly revealed that the quinoidal
structures of the fused-pyrrole moiety played a critical role in
stabilizing the oxidation state.
mental interest as model systems for charge-transfer and
charge-conductive phenomena.27a,b,d,e,30,31 However, the π-
systems that include five- and six-membered rings within the
same PAHs have never been investigated systematically. The
marriage of these aromatics should influence their redox
properties and the electronic nature of their oxidation states as
well as the photophysical properties of their neutral states.
Herein, we report chemical features of the series of nitrogen-
containing PAHs that possess small S1−T1 energy gaps and can
be multiply oxidized with different spin multiplicities in their
dication states.
RESULTS AND DISCUSSION
■
Synthesis. In a manner similar to that used for a
hydrocarbon analogue (i.e., hexa-peri-hexabenzocoronene
(HBC)),32 the synthesis of a series of new pyrrole-fused
azacoronenes 2−4 was conducted following our previously
published procedure,16 where oxidative cyclodehydrogenation
is the key step (Scholl reaction).32,33 As shown in Scheme 1,
the hexaaryl precursors 9−12 were prepared by selective
nucleophilic aromatic substitutions (SNAr) of the correspond-
ing fluoroarenes with β-diarylpyrrole to yield 6−834 followed by
successive Suzuki−Miyaura coupling. Final planarization with
FeCl3 afforded targets 2−4 in good yields (41−86%). Notably,
the 3,5-dialkoxyphenyl group appears to be required for
In the present study, we have designed and synthesized a
series of pyrrole-fused azacoronenes 2−4 in which some of the
pyrrole rings of HPHAC are replaced with dialkoxybenzenes.
The nature of the charge distribution, the delocalization, and
the charge carriers is one of the most important subjects in
conjugated polymers.29 Therefore, linear conjugated oligomers
with mixed-valence or charged states have attracted funda-
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Figure 1. Crystal structures of (a) 1a,16 (b) 3b, (c) 4a, and (d) 12b. Thermal ellipsoids are at 50% probability. Solvent molecules, H atoms, and
peripheral alkyl and aryl groups are omitted for clarity.
a
a
Table 1. Selected Mean Bond Lengths (Å) and Inner Angles (deg) for 1a, 3b, 4a, and 12b from the Crystal Structures
Cα−Cβ(a)
Cα−Cβ(b)
N−Cα(c)
N−Cα(d)
C
Ph′−CPh′(e)
Ph′−CPh′(f)
Cβ−Cβ
Cα−Cα
N−CPh′
C
Cα−CPh
CPh−CPh′
CPh−CPh
∠Py
∠Ph
b
1a
1.438(1)
1.390(1)
1.398(3)
1.396(4)
1.389(3)
1.362(2)
1.395(1)
1.387(3)
1.401(6)
1.395(2)
1.401(2)
1.483(1)
1.366(1)
1.371(1)
1.399(3)
1.373(5)
1.396(3)
1.400(2)
−
−
−
113.5(1)
−
3b
1.438(4)
1.447(5)
1.380(3)
1.478(4)
1.430(6)
1.434(3)
112.6(4)
122.8(4)
4a
1.436(4)
1.435(5)
−
−
1.381(3)
1.426(3)
1.464(3)
1.448(1)
1.488(3)
1.416(3)
1.384(3)
111.5(2)
107.8(1)
123.4(3)
120.4(1)
12b
−
a
The values were averaged for C6v (1a), D2h (3b), D3h (4a), and C3 (12b) symmetry. The estimated standard deviations (esd) of mean values were
b
calculated from the experimental esd values using the following equation: δ(l) = 1/(∑(1/δi2))1/2. Reference 16.
complete planarization of the azacoronene core. When Scholl
reaction was performed with 10, which possesses a 4-tert-
butylphenyl group instead of a 3,5-dialkoxyphenyl group,
partially fused 5 was obtained as the main product (91%)
even though the completely fused structure was also detected
by matrix-assisted laser desorption ionization (MALDI) mass
spectrometry. Insight into this different reactivity was obtained
from the molecular orbitals near the highly occupied molecular
orbital (HOMO). For 9c, the HOMO (−5.56 eV) is localized
on all of the peripheral pyrrole and dialkoxyphenyl groups;
however, the HOMO (−5.51 eV) of 10 shows localization of
the MO on the pyrrole rings primarily at the meta- and para-
positions from the 4-alkylphenyl group. This result indicates a
low probability of the localization of radical cations required to
lead to complete planarization. A MO similar to that of 9c
appears at HOMO−3 (−5.58 eV) for 10 (Figure S2 in
interactions were observed (Figure S5 in SI). Table 1 shows the
selected geometrical parameters of 1a, 3b, 4a, and 12b. Briefly,
in association with the planarization, the bond lengths of the
peripheral moieties of the original pyrrole (Py) (i.e., Cα−
Cβ(a,b)) and the phenyl (Ph) group (i.e., CPh−CPh) increase,
whereas the bond lengths between the central benzene (Ph′)
and the peripheral aryl groups (i.e., N−CPh′ and CPh−CPh′
)
tend to decrease with little changes in the central benzene unit
(i.e., CPh′−CPh′(e,f)). The inner mean angles of the original aryl
groups become wider compared to those of 12b.
Photophysical Properties in the Neutral States. The
UV−visible absorption and emission spectra of 1a−4a in
CH2Cl2 are shown in Figure 2, and their optical properties are
summarized in Table 2. As shown in the insets of Figure 2, the
small molar extinction coefficients (ε for the 0−0 transition in
the range of 1400−2300 mol−1 L cm−1) indicate that the
transitions from the ground to the excited singlet state (S0 →
S1) are partially allowed for 1a−3a, which is typical phenomena
for disc-shaped molecules with high symmetry.5a,35 A similar
trend was also observed for 4a; however, the S0 → S1 transition
is allowed with a 10-fold larger ε than those of 1a−3a. The
solutions were all light-yellow in color but showed colorful
emissions from red to green as the number of pyrrole rings
decreased from 1 to 4 (Figure 2, insertion). The fluorescence
quantum yields (ΦF) were ∼2% for 1a, 2a, and 4a and 0.1% for
3a. The small kf ≈ 1 × 106 s−1 calculated from the ΦF and the
fluorescence lifetimes (τF) of 1a−3a are well correlated to the
partially allowed S0 → S1 transitions observed in their
absorption spectra (Table 2).
1
Supporting Information). The H NMR resonances of the
phenyl ring between the two alkoxy groups shifted downfield
(i.e., Δδ = 0.43 (2a), 0.41 (3a), and 0.18 (4a) ppm,
respectively), which reflects the complete planarization and
the effective π-electron conjugation. As revealed in the previous
π-system 1 (HPHAC),16 the azacoronenes 2a−4a were also
thermally and photochemically stable (Figures S3 and S13 in
SI).
X-ray Crystal Structures. Single-crystal X-ray structure
determinations were performed on 3b, 4a, and 12b (Figure 1).
The planarized azacoronene structures of 3b and 4a were
confirmed by X-ray crystallography, whereas a C3-symmetric
propeller structure was observed for 12b. The π-systems of 3b
and 4a were only slightly strained due to the bulky substitutions
on the pyrrole and benzene rings (Figure S4 in SI), and no π−π
To investigate the triplet state properties of the azacoronene
family, emission spectra and lifetimes were measured in 2-
C
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ΔES−T should play an critical role in the efficient formation of
the triplet state in the azacoronenes38 because they lack heavy
atoms and involve no clear n−π* transition around the S1 states
(Figures S10 and S11 in SI).39
The relative phosphorescence intensity and the phosphor-
escence lifetime (τp) varied drastically for 1a−4a. The
noticeable phosphorescence and the long τp (1100 ms) from
4a at 77 K can be explained by the reduced T1 → S0
intersystem crossing due to its large ET because the ET
estimated from the phosphorescence 0−0 transition decreased
from 2.25 eV (4a) to 2.03 eV (2a) and 1.95 eV (1a). The lack
of phosphorescence of 3a may be due to a smaller ET, which
results in a larger T1 → S0 intersystem crossing rate compared
to that of 1a.
Electrochemistry. To investigate the redox properties,
cyclic voltammograms (CV) were measured in CH2Cl2 (Figure
4, Table 3). Similar to HPHAC 1a, whose CV exhibited three
reversible oxidation peaks,16 partially substituted 2a−4a
exhibited up to four reversible oxidation peaks under the
measurement conditions (Figure 4). Differential pulse
voltammetry experiments confirmed that each wave corre-
sponded to the transfer of a single electron (Figure S14 in SI).
Notably, although the first two peaks for all of compounds have
similar oxidation potentials, the differences between the second
and third peaks (ΔEox3−ox2) are smaller in the order of 1a > 2a
> 3a ≅ 4a, which is consistent with the trend observed in the
DFT results (Figure 5). On the basis of the structural
differences, the introduction of dialkoxybenzenes instead of
pyrrole rings decreases onsite Coulombic repulsion in the step
from dication to trication; however, it does not change their
first two oxidation potentials. Therefore, an investigation of the
electronic properties in these oxidation states was then
performed with chemically generated radical cations and
dications.
Chemical Oxidations and Crystal Structure of 4b2+.
Chemical oxidation was conducted with SbCl5 in CH2Cl2 at
room temperature. Figure 6 shows the absorption spectra of the
radical cation and dication states of 1a and 2b−4b.40 When the
oxidant was added incrementally to the solutions, the spectra
changed dramatically with isosbestic points, which strongly
supports the sequential oxidation of the π-systems from a
neutral to a radical cation (M•+) and then to a dication (M2+)
(Figure S15 in SI). Therefore, neither disproportionation from
two radical cation species into neutral and dication species nor
π-dimer formation between two radical cation species30e,g,31e,h
nor follow-up chemical reactions, including decomposition,
were observed under the measurement conditions.
In the course of the chemical oxidation of a series of
compounds, single crystals suitable for the X-ray single-crystal
Figure 2. Absorption (solid line) and emission (dotted line) spectra of
azacoronenes 1a−4a in CH2Cl2 at room temperature. Insets show
enlargements of the cutoff region of the absorption spectra and
photographs of the solutions under irradiation with UV light (254
nm).
structure analyses were obtained for 42+ in the 4b2+(SbCl6 )2
−
form. The presence of two counteranions clearly identified the
dication state, and similar to the neutral structure of 4a, the
azacoronene moiety was nearly planar (Figure 7). Notably, a
comparison of the mean bond lengths near the pyrrole units of
neutral 4a and dication 4b2+ indicates a contribution from the
quinoidal structures of 42+, which was previously reported for
methyltetrahydrofuran (MTHF) rigid glass at 77 K. Figure 3
shows the prompt emission spectra of 1a−4a in MTHF at both
298 and 77 K and phosphorescence spectra at 77 K. Here, the
fluorescence spectra at 77 K can be obtained by subtracting the
corresponding phosphorescence fraction from the prompt
emission spectra. Notably, the S1−T1 energy gaps (ΔES−T),
which is less than 0.36 eV, were smaller than those of
condensed PAHs, including HBC (0.5 eV),36 chrysene (0.9
eV), and naphthalene (1.5 eV) (Figure S9 in SI).37 The small
16
−
−
12+(SbCl6 )2 (for 4b2+(SbCl6 )2, N−Cα 1.389(3), Cα−Cβ
1.413(3), Cβ−Cβ 1.412(5), and Cα−CPh 1.438(3) Å). In
addition, the mean bond length of O−CPh (1.336(3) Å) for the
dication decreased compared to that of the neutral form
(1.358(2) Å). Therefore, the resonance structuresthe so-
called benzenoid and quinoid structurescontribute signifi-
cantly to the stabilization of the cationic charges where
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a
Table 2. Optical Parameters of 1a−4a
absorption
emission
b
c
b
c
e
f
λmax
ES
λmax
(nm)
ET
τF
kf
kn + kisc
τP
Stokes shifts
(cm−1
ΔES−T
d
(nm)
ε
(eV)
(eV)
ΦF
(ns)
(106 s−1
)
(107 s−1
)
(ms)
)
(eV)
g
h
h
h
h
1a
2a
3a
4a
rt
521
517
504
500
528
518
451
456
1429
2.30
571
0.015
0.018
0.001
0.017
16
0.91
6.0
1531
1370
403
0.35
0.26
−
h
h
h
h
77 K
557
1.95
2.03
−
39
100
250
−
g
h
h
h
h
rt
1584
2322
2.29
2.18
2.61
548
15
1.2
6.6
77 K
540
23
81
g
h
h
h
h
rt
602
1.1
2.5
6.3
6.6
0.88
88
2740
2176
1854
1693
77 K
597
g
h
h
h
h
rt
15670
493 (sh)
525
2.7
16
0.36
77 K
2.25
1100
a
ΦF, quantum yield of fluorescence; τF, lifetime of fluorescence; kf, radiative rate constant; kn, nonradiative decay rate constant; kisc, intersystem
b
c
d
crossing rate constant. The 0−0 transition wavelength. Calculated from the 0−0 transition wavelength. Determined by comparison with quinine
sulfate in 0.5 M H2SO4 (Φ = 0.51). kf = ΦF/τF. kn + kisc = (1/τF) − kf. Measured in CH2Cl2. Measured in MTHF.
e
f
g
h
Figure 3. Prompt emission spectra of 1a−4a in MTHF at 298 K
(dotted colored line) and 77 K (solid colored line) and
phosphorescence spectra at 77 K (black solid line). The phosphor-
escence fractions were collected from the time domain from 15.0 to
15.1 ms after excitation.
Figure 4. Cyclic voltammograms of 1a−4a in CH2Cl2 (1.0 mM).
Conditions: concentration, 1.0 mM; supporting electrolyte, 0.1 M n-
Bu4NClO4; working electrode, Pt; reference electrode, Ag/AgNO3;
counterelectrode, Pt wire; scan rate, 100 mV/s, potentials, vs Fc/Fc+.
heteroatoms with large electronegativities, such as nitrogen and
oxygen, support the localization of cations.
Table 3. Oxidation Potentials (Eox) of 1a−4a in CH2Cl2
Electronic Properties in the Oxidized States. To
investigate the electronic properties of the cationic species,
electron spin resonance (ESR) spectra were measured for the
radical cations of 1a and 2b−4b with the solutions prepared
under the above-mentioned conditions. Broad but flat single-
line signals were observed for all of the azacoronenes, which
indicates the presence of delocalized radical cations (g =
2.0023−2.0034 and ΔH = 0.24−0.30 mT, Figure S16 in SI). In
contrast, the dications of 1a and 2b−4b were all ESR-silent at
room temperature, and thus NMR spectra were measured to
examine the closed- or open-shell nature. In the presence of
Fe(C5H4Ac)2·AgBF4,41 1.0 mM CD2Cl2 solutions of all of the
dications were prepared with a 1.0 M CD3CN solution of
NOSbF6, and the absorption spectra were monitored. As
previously reported for 1a2+,16 sharp 1H NMR spectrum of 2b2+
was obtained with low-field shifts at room temperature (Figure
S17 in SI). These ESR and NMR results clearly demonstrated
the closed-shell nature of 2b2+, which is consistent with the
results for 1a2+. The 1H NMR spectra of 3b2+ and 4b2+ showed
severe broadening in the aromatic region under the same
conditions in the range of 233−298 K.42 Therefore, super-
ox1
ox2
ox3
ox4
E
1/2
(V)
E
(V)
E
(V)
E
(V)
1/2
ΔEox3−ox2 (V)
1/2
1/2
a
1a
2a
3a
4a
0.04
0.23
1.23
1.80 (p.a.)
1.17
1.00
0.66
0.33
0.31
0.03
0.00
0.00
0.22
0.21
0.25
0.88
0.54
0.56
0.76
0.77
a
Quasi-reversible. Potential was estimated by differential pulse
voltammetry (DPV).
conducting quantum interference device (SQUID) measure-
ments were performed for the powder sample of
4b2+(SbCl6 )2, which resulted in nearly zero magnetic
−
susceptibilities at 100−300 K (Figure S18 in SI).42 On the
basis of the ESR, NMR, and SQUID results, the triazacoronene
dication 4b2+ is supposed to be an open-shell singlet
biradical.7,43 The DFT calculations of 4b2+ with the crystal
structure geometry also support the experimental results.
Therefore, a comparison of the energies for the three possible
configurations revealed that the singlet biradical state was most
stable compared to the other closed-shell and triplet states
(Table 4), which will be discussed in the next section.44
E
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Figure 5. Molecular orbital diagrams of 1c−4c calculated at the B3LYP/6-31G(d) level of theory.
Figure 6. Absorption spectra of the radical cations and dications of 1a and 2b−4b (1.0 mM) generated by the presence of 1.5 and 3.0 equiv of SbCl5
in CH2Cl2 at room temperature.
−
Figure 7. (a) Crystal structure of 4b2+(SbCl6 )2. Thermal ellipsoids
are at 50% probability. Solvent molecules and H atoms are omitted for
clarity. (b) Possible resonance forms of the partial triazacoronene
neutral 4 and dication 42+ with the mean bond lengths. The estimated
standard deviations (esd) were calculated from the experimental esd
values using the following equation: δ(l) = 1/(Σ(1/δi2))1/2
.
Figure 8. NICS(n) values (NICS(1)zz) of the dications 1c2+ and 2c2+
at various points. The calculations were performed at GIAO/HF/6-
311+G(d,p)//B3LYP/6-31G(d) level of theory. Values of n = +2 to
−2 are distances in Å from the ring centers of each ring. The
optimized structures of 1c2+ and 2c2+ are slightly bent with symmetries
of C6v and Cs, respectively (Figure S19 in SI).
For the experimentally confirmed closed-shell dications 1a2+
and 2a2+, nucleus-independent chemical shift (NICS) values45
were evaluated to study the aromaticity using DFT calculations.
The total MO contributions to the zz (i.e., out-of-plane)
component of the NICS tensor (NICS(1)zz) are also depicted
(Figure 8).46 The NICS results for 1c2+ showed large negative
values for all of the calculated points. Interestingly, the values
F
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were distinct in the concave area. Furthermore, the largest
absolute values were obtained at the center of the azacoronene
and became smaller toward the periphery (from A to C ring),
which suggests that the overall aromatic character for 1c2+
resulted from the quinoidal resonance structures of the fused-
pyrrole moiety.16 A similar trend was also observed for 2c2+ but
with much smaller absolute values. In analogy to 1c2+, the
center points above and below the A ring had relatively large
absolute values, and those in the outer dialkoxybenzene (i.e.,
the H ring) were negligibly small due to the quinoidal structure.
The introduction of dialkoxybenzene thus appears to prohibit
the cyclic π-conjugation in the dication states, resulting in
smaller NICS values, which, in turn, indicates weaker aromatic
character of 2c2+ compared to that of 1c2+. These aromatic
characteristics most likely support the discriminatively large
differences (i.e., onsite Coulombic repulsion) between the
second and third oxidation potentials (ΔEox3−ox2) of 1a and 2a
described above. Once azacoronenes are oxidized to the
dication states, aromatic stabilization of the π-system is
obtained by overall bond alternation (i.e., resonance), which
leads to difficulty in the third oxidation steps.
Figure 9. Spin-density maps of SOMO and molecular orbitals for the
α- and β-spins of (a) 3c2+ and (b) 4b2+ in the singlet biradical states.
Isovalues are 0.0004 e Å−3 for the spin densities and 0.02 e Å−3 for the
MOs. The geometry of 4b2+ was obtained from the single-crystal
structures.
DFT Consideration of the Spin Multiplicity of the
Dications. To shed some light on the spin multiplicity of the
dications, especially of the NMR- and ESR-silent 3b2+ and 4b2+
dications, theoretical calculations of the energies for the three
possible configurations were conducted using DFT: a restricted
wave function with a closed-shell configuration (R), an
unrestricted wave function with an open-shell singlet
configuration (singlet biradical: U), and an unrestricted wave
function with an open-shell triplet configuration (T). Table 4
Triazacoronene dication 4c2+ exhibited a stable triplet state
compared to the closed-shell singlet and singlet biradical states
by ∼2.3 kcal mol−1, even though the experimental character-
izations and the single-point calculations based on the crystal
structure geometry of 4b2+ revealed that the singlet biradical
was the most stable state. According to the DFT results for
4c2+, the bond lengths and the inner angles of the peripheral
aryl moieties are averaged in a symmetric fashion of D3h, C2v,
and Cs for R, U, and T, respectively. The spin-density maps and
the MOs for 4b2+ with the crystal structure geometry possess a
symmetry plane as demonstrated in Figure 9b, which indicates
the comparative similarity to the symmetries of the calculated
open-shell states (C2v or Cs) than the D3h of the closed-shell
state. In general, open-shell singlet biradical character appears
at an intermediate interaction strength of two spins. In this
context, the closed-shell nature of the pyrrole-rich 12+ and 22+
can be concluded to be caused by the quinoidal resonance
structure of the fused-pyrrole moiety. The replacement of the
pyrrole ring with dialkoxybenzene tends to prohibit the spin−
spin interaction, which leads to the singlet biradical and triplet
characteristics of azacoronenes 32+ and 42+.47
Table 4. Relative Energies in kcal mol−1 for the Dication of
1c−4c and 4b in the Spin-Restricted Singlet (R), Spin-
Unrestricted Singlet (U), and Triplet (T) States at the
B3LYP/6-31G(d) level
ΔE(R−U)
ΔE(T−U)
ΔE(T−R)
1c2+
2c2+
3c2+
4c2+
4b2+
0.04
0.00
3.56
0.01
3.10
16.44
6.13
16.39
6.13
0.15
−3.40
−2.25
1.65
−2.24
4.75
a
a
The geometry was obtained from the single-crystal structures.
SUMMARY AND CONCLUSION
summarizes the relative energies of these three states. Although
the energy differences of the closed-shell and open-shell singlet
(ΔER−U = ΔERB3LYP − ΔEUB3LYP) of 1c2+ and 2c2+ were nearly
equal, those of the open-shell triplet and closed-shell singlet
(ΔET−R) favored the stabilities of their closed-shell singlets,
which were consistent with the experimental results that
indicated the closed-shell nature of 1a2+ and 2a2+. For 3c2+, the
open-shell singlet biradical and triplet states were in agreement
with the NMR results and were evaluated to be stable
compared to the closed-shell singlet by 3.6 and 3.4 kcal
mol−1, respectively. A comparison between the open-shell
triplet and singlet (ΔET−U) indicated that the singlet biradical
state of 3c2+ had a slightly lower energy than the triplet state.
The ⟨S2⟩ value of 0.59 for the singlet state also supports the
biradical character of 3c2+. Notably, the spin-density maps of
the singly occupied molecular orbital (SOMO) and the MOs
for the α- and β-spins were disjointed by the dialkoxybenzenes
(Figure 9a and Figure S20 in SI). Similar phenomena for the
singlet biradical states have been previously reported.7,31g,43
■
We have demonstrated the comprehensive synthesis and
characterization of a series of pyrrole-fused azacoronenes,
which differs in the size and symmetry of their π-systems.
Similar to the case of the hydrocarbon analogue (i.e., HBC),
the synthesis of the planarized disc structures was conducted on
the basis of the oxidative cyclodehydrogenation reactions of the
corresponding hexaarylbenzenes. The introduction of two
alkoxy groups at the meta-positions on the benzene ring was
found to effectively promote the key coupling reaction between
not only the pyrrole−pyrrole rings but also the pyrrole−
benzene rings, which provided an opportunity to study the
optical and electronic properties in their neutral and oxidized
states. Reflecting their different π-conjugation system, the
stepwise substitution of the pyrrole with dialkoxybenzene rings
drastically altered the optical properties. Their small ΔES−T
values (<0.36 eV) may be of fundamental importance in the
area of molecular photonics because thermally activated T1 →
S1 intersystem crossing can be possible without use of high
G
dx.doi.org/10.1021/ja402371f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Chem. 1998, 63, 8632. (c) Aratani, N.; Osuka, A.; Kim, Y. H.; Jeong,
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excitation power density or the coherent laser for triplet−triplet
annihilation (TTA) process.48
Although the fusion of the pyrrole and dialkoxybenzene rings
does not induce discriminative differences in the spin
delocalization nature for their radical cation species, the
electronic properties of the dication species differ substantially.
Hexaazacoronene 12+ and pentaazacoronene 22+ clearly
exhibited a closed-shell nature, whereas tetraazacoronene 32+
and triazacoronene 42+ exhibited an open-shell nature. For the
pyrrole-rich compounds, higher oxidation states (more than
trication) became difficult to achieve due to the resonance
structures of 12+ and 22+. From these observations, we
concluded that fusion of the pyrrole rings facilitates charge/
spin delocalization, which is consistent with the results of the
polypyrrole study: in other words, the introduction of
dialkoxybenzenes instead of pyrrole rings within the pyrrole-
fused azacoronene family tends to prohibit the charge/spin
interaction in their dication states. Therefore, replacement of
the pyrrole rings with dialkoxybenzenes induces the higher
oxidation states and contributes to the appearance of “weakly
interacting” spins with open-shell character. In line with our
strategy to fuse hybrid aromatics, the further functionalization
of polycyclic aromatic hydrocarbons (PAH) would be beneficial
for molecular electronics and spintronics with well-defined
structures.
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ASSOCIATED CONTENT
* Supporting Information
■
S
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Experimental procedures and the characterization data for all
new compounds; Figure S2−S20, atomic coordinations of the
optimized structures of 1c−4c in neutral and dication states
(B3LYP/6-31G(d)), and X-ray data for 3b, 4a, 12b, and
(d) Wang, Z.; Dotz, F.; Enkelmann, V.; Mullen, K. Angew. Chem., Int.
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4b2+(SbCl6 )2 (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
mtakase@tmu.ac.jp (M.T.); nishinaga-tohru@tmu.ac.jp (T.N.);
muellen@mpip-mainz.mpg.de (K.M.).
■
Notes
The authors declare no competing financial interest.
(10) (a) Zhi, L.; Gorelik, T.; Friedlein, R.; Wu, J.; Kolb, U.; Salaneck,
W. R.; Mullen, K. Small 2005, 1, 798. (b) Gong, K.; Du, F.; Xia, Z.;
̈
ACKNOWLEDGMENTS
Durstock, M.; Dai, L. Science 2009, 323, 760. (c) Wei, D.; Liu, Y.;
Wang, Y.; Zhang, H.; Huang, L.; Yu, G. Nano Lett. 2009, 9, 1752.
(d) Wang, X.; Li, X.; Zhang, L.; Yoon, Y.; Waber, P. K.; Wang, H.;
Guo, J.; Dai, H. Science 2011, 324, 768. (e) Wang, Y.; Wang, X.;
Antonietti, M. Angew. Chem., Int. Ed. 2011, 50, 2.
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Lett. 2004, 6, 3325. (b) Miyasaka, M.; Rajca, A.; Pink, M.; Rajca, S. J.
Am. Chem. Soc. 2005, 127, 13806. (c) Chernichenko, K. Y.; Sumerin,
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■
This work is supported by a Grant-in-Aid for Scientific
Research (No. 22350021 and No. 23750045) from MEXT,
Japan, and a research grant from the Noguchi Institute and the
Asahi Glass Foundation. We thank Prof. Kotohiro Nomura
(Tokyo Metropolitan University) for valuable discussions and
Prof. Shinzaburo Ito (Kyoto University) for photophysical
measurements.
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(39) The theoretical calculations performed using the time-
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