Preparation and Properties of a Polycyclic p-Quinodimethane with Two Oxygen Bridges and its Radical Cation in Comparison with the Isomeric o-Quinodimethane

Article abstract of DOI:10.1002/asia.201801269

We prepared an electron-rich p-quinodimethane compound, 2,9-dibromo-7,14-dimesityl-5,12-dioxapentacene (1). The quinoidal character was clarified by bond-length alternation in the central ring. Clear vibrational structures in both absorption and emission

Full text of DOI:10.1002/asia.201801269

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Preparation and Properties of a Polycyclic p-Quinodimethane
with Two Oxygen Bridges and its Radical Cation in Comparison
with the Isomeric o-Quinodimethane
Authors: Masatoshi Kozaki, Chihiro Sato, Shuichi Suzuki, and Keiji
Okada
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To be cited as: Chem. Asian J. 10.1002/asia.201801269
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10.1002/asia.201801269
Chemistry - An Asian Journal
FULL PAPER
Preparation and Properties of a Polycyclic p-Quinodimethane
with Two Oxygen Bridges and its Radical Cation in Comparison
with the Isomeric o-Quinodimethane
Chihiro Sato,[a] Shuichi Suzuki,[a,b] Keiji Okada,[a,c] and Masatoshi Kozaki*,[a,c]
Dedication ((optional))
Abstract: We prepared an electron-rich p-quinodimethane
compound, 2,9-dibromo-7,14-dimesityl-5,12-dioxapentacene (1).
The quinoidal character was clarified by bond-length alternation in
the central ring. Clear vibrational structures in both absorption and
emission bands and small Stokes shift indicate the rigid
dioxapentacene framework. The reversible redox waves of 1 were
observed at +0.14 and +0.69 V vs Fc+/ Fc, respectively. Radical
cation salt 1 •SbF6 was successfully isolated and characterized by
means of X-ray structural analysis and ESR and UV-vis-near
infrared absorption spectroscopies. The molecular structure and
electronic properties of the neutral and radical cation species were
compared to those of the isomeric o-quinodimethane.
pQDM derivatives with ambipolar (electron and hole)
transporting abilities have been described.5 These reports
demonstrate that quinodimethane systems are promising
materials for organic electronics.
+
−
Figure 1. Formation of aromatic rings thhrough redox processes of p-quinoidal
compounds (X = O, NR, CR2).
Introduction
Radical cationic species are known to play an important role
as positive-charge carriers in p-type semiconducting layers.
Structure and properties of a pQDM-based radical cation should
provide novel information about a hole mobility of a pQDM-
based organic semiconductor. The generation and preparation
of pQDM-based radical cations in solution have been previously
reported;6 however, to the best of our knowledge, there is no
example of their isolation.
Quinoidal conjugated systems have intrinsic amphoteric redox
ability because they can form aromatic rings through both
oxidation and reduction processes (Figure 1).1 Absorption and
emission bands of the quinoidal conjugated systems generally
appear in longer wavelength regions compared to the
corresponding non-quinoidal neutral aromatic systems. Because
of these unique properties, quinoidal compounds have been
widely used as organic semiconducting, magnetic, and optical
materials.2 Interestingly, the most widely used quinoidal systems
are strong electron acceptors such as 7,7,8,8-tetracyano-p-
quinodimethane (TCNQ).3 On the other hand, a fewer quinoidal
compounds with strong electron-donating ability have been
investigated. Recently, some polycyclic compounds with p-
quinodimethane (pQDM) frameworks were reported as p-type
(hole-transporting) semiconducting materials.4 In addition, a few
In this paper, we investigated syntheses, structures, and
properties of a pQDM system, 2,9-dibromo-7,14-dimesityl-5,12-
dioxapentacene (1) and its radical cation salt 1•+•SbF6 . We
−
compared their properties with those of the electron-rich o-
quinodimethane
(oQDM)
system,
5,8-dioxapentaphene
derivatives 2 and 2•+•SbF6 , which were studied in our previous
paper.7 Comparison of fundamental properties between pQDM
and oQDM in the related system has rarely been reported.
–
[a]
Dr. C. Sato, Dr. S. Suzuki, Prof. Dr. K. Okada, Prof. Dr. M. Kozaki
Graduate School of Science
Osaka City University
3–3–138, Sugimoto, Sumiyoshi-ku
Osaka 558–8585 (Japan)
E-mail: kozaki@sci.osaka-cu.ac.jp
Dr. S. Suzuki
Present address: Graduate School of Engineering Science
Osaka University
Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Prof. Dr. K. Okada, Prof. Dr. M. Kozaki
Osaka City University Advanced Research Institute for Natural
Science and Technology (OCARINA)
3–3–138, Sugimoto, Sumiyoshi-ku
Osaka 558–8585 (Japan)
[b]
[c]
Results and Discussion
Synthesis
of
2,9-dibromo-7,14-dimesityl-5,12-
dioxapentacene
E-mail: okadak@sci.osaka-cu.ac.jp
The pentacene-like pQDM system 1 was constructed by a
procedure outlined in Scheme 1. The starting 2,5-
dibromoterephthalic acid, prepared by a reported method,8 was
Supporting information for this article is given via a link at the end of
the document.
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converted into the corresponding acid chloride 3 using thionyl
both molecules A and B, clear bond-length alternations around
the central ring C were observed (Table 1), indicating that 1 is a
pQDM homologue. The crystal structure of 1 displays a
decrease in the extent of bond-length alternations compared to
that of oQDM system 2 (Table S2), as evidenced by the
calculated HOMA (harmonic oscillator model of aromaticity)
index.9 The HOMA values of the central ring C were estimated to
be –0.202 for 2 and 0.308 (molecule A) and 0.335 (molecule B)
for of 1. These results suggest that the fused ring system in 1
had a more delocalized -conjugation as compared to that in 2.
Judging from bond lengths, oQDA 2 has a larger bond-length
alternation effect than pQDA 1. In order to obtain insight whether
the larger bond-length alternation for 2 is due to twisted structure
because of the aryl groups or not, we examined the optimization
of the structure of the parent compounds 7 and 8, obtained by
DFT calculations at the B3LYP/6-31G** level (Table 2). Both 7
and 8 have planar fused cyclic frameworks in the optimized
structures. Relatively small difference in bond lengths was
obtained between 7 and 8. The HOMA value of compound 8
changes to positive although it is still smaller than that of 7
(Table 2). These results suggest that the large differences in the
bond-length alternation effect and HOMA values between 1 and
2 are mainly due to the twisted structure of 2.
chloride. Next, the Friedel–Crafts reaction of
3 with 4-
bromoanisole was performed using aluminum chloride under
heating conditions, and dibenzoylbenzene 4 was obtained in
77% yield over two steps. The linearly fused framework was
constructed by the intramolecular Ullmann-type coupling
reaction using picolinic acid as a ligand, which afforded dione 5
in 65% yield. Finally, 5 was treated with mesityl Grignard
reagent and then tin(II) chloride to obtain pQDM system 1 as a
deep red solid in 40% yield.
O
O
SOCl2
Br
O
Br
O
DMF (cat.)
HO
Br
Cl
90 oC, 1 h
OH
Cl
Br
Br
3
1)
Br
O
OH
OMe
Br
AlCl3
2) 
CH2Cl2
0 oC to rt, 18 h
Br
Br
HO
reflux, 6 h
O
4
77% (in 2 steps)
O
O
picolinicacid
Br
O
O
CuI, K3PO4
DMSO
120 oC, 24 h
Br
5
65%
MgBr•LiCl
SnCl2
1
THF
60 ºC, 12 h
CH2Cl2
RT, 12 h
40%
Scheme 1. Synthetic route of 1.
Molecular structure
The detailed structure of 1 was determined by X-ray structural
analysis (Figure 2 and Table S1). Recrystallization by slow
diffusion of acetonitrile into the CH2Cl2 solution of 1 gave
suitable crystals for X-ray analysis. Two crystallographically
independent molecules (A and B) were observed in the unit cell,
each having an inversion center. The dihedral angles of the
mesityl groups were almost perpendicular to the backbone (89°).
Whereas molecule B is almost planar (177.10° between rings A–
C), molecule A showed a slightly bent -system (174.29°). The
differences are a result of packing effects because they were not
Figure 2. Crystal structures of 1: a) in the unit cell along b axis and b) side
views for molecule A and c) B. Hydrrogen atoms are omitted for clarity.
Thermal ellipsoids set at 50%. Gray double-headed arrows and values
indicate the dihedral angles between ringgs A and C.
seen in
a
DFT-optimized structure (Figure S4). This
consideration is further supported by intermolecular Br···H
(aromatic H in a mesityl group) interactions (3.02 Å) shorter than
the sum of the van der Waals radii in molecule B (Figure S1). In
Figure 3. Fused ring systems of a) 1 andd b) 2. Italic symbols denote assigned
names of the selected bonds and ring poositions.
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Table 1. The selected bond lengths[a] and HOMA values of 1 in the crystal
state.
structureless broad absorption band and quenching of the
radiative relaxation.12
Bonds[b]
a
b
c
d
e
HOMA[c]
0.308
1
0.8
0.6
molecule A 1.366(6) 1.453(4) 1.356(6) 1.441(5) 1.448(5)
molecule B 1.376(6) 1.439(6) 1.352(6) 1.450(7) 1.447(7)
0.335
[a] In Å unit. [b] The crystal structures and assignment of bond names are
shown in Figures 2 and 3, respectively. [c] Estimated using the bond lengths
of b, c, and d.
0.4

Table 2. Calculated values[a] of the selected bond lengths for 1, 2, 7, and 8.
0.2
0
300
400
500
600
700
800
Wavelength / nm
Figure 4. UV−vis absorption (black solidd line) and fluorescence (black broken
line, ex = 487 nm) spectra of 1 and UV-vvis absorption spectrum of 2 (red solid
line) in CH2Cl2.
Bonds
1[c]
a
b
c
d(d’)
e
HOMA[b]
0.386
1.383
1.451
1.362
1.438
1.451
1.439
(1.488)
1.437
2[c]
1.378
1.373
1.467
1.453
1.353
1.361
1.476
1.436
-0.183
0.368
7[c]
1.433
(1.478)
8[c]
1.370
1.451
1.358
1.439
0.147
[a] In Å unit. [b] for the central ring. [c] All geometries were optimized by DFT
calculation at the B3LYP/6-31G** level.
Electronic properties of 1
The UV–vis absorption spectrum of 1 in CH2Cl2 exhibited an
intense and sharp absorption band (max = 523 nm, max = 1.06 ×
105 M–1 cm–1) in the 400–550 nm region with a defined
vibrational structure (Figure 4). Form the peak separation values
in the absorption band of 1 (peak = 523, 487, and 456 nm), the
vibrational energy was estimated to be ca. 1400 cm–1, which can
be attributed to stretching vibrations of olefinic and/or aromatic
C=C bonds (C=C = 1400–1600 cm–1 in S1 states).10 In addition, 1
produced a sharp fluorescence band in the 500–650 nm region
(max = 534 nm), which resembles a mirror image of the lowest
energy absorption band. These photophysical properties of 1 are
drastically different from those of oQDM system 2, which
showed a structureless broad absorption and no emission
(Figure 4).7 The small Stokes shift (∆0–0 = 394 cm–1) for 1
suggests a rigid -backbone during the excitation and relaxation
processes. In addition, the TD-DFT calculations for 1 revealed
that the direction of the transition dipole moment of the S0→S1
excitation (calcd = 509 nm, Table S3) was parallel to the C=C
bonds in the pQDM moiety (Figure 5e). These results suggest
that the S0→S1 excitation of 1 can strongly couple with the
asymmetric stretching vibrations of the pQDM moiety. On the
other hand, the -backbone of 2 is twisted and flexible due to its
flipping motion. In addition, the transition dipole moment of the
S0→S1 excitation (calcd = 642 nm) of 2 was orthogonal to the C2
axis of the oQDM skeleton (Figure 5f).11 Presumably, these
structural and physical properties of 2 are responsible for the
Figure 5. Calculated structures and orbiitals a) HOMO and b) LUMO of 1, and
c) HOMO and d) LUMO of 2 (isovalue = 00.035) optimized at the DFT B3LYP/6-
31G** level. Theoretical transition dipolee moments of the S0→S1 excitations of
e) 1 and f) 2 were calculated by TDD-DFT at the B3LYP/6-31G** level.
Hydrogen atoms are omitted for clarity.
The cyclic voltammogram of 1 in CH2Cl2 (Figure 6) showed
two reversible redox waves at +0.14 V and +0.69 V (vs Fc+/Fc)
and one irreversible redox wave (–2.13 V as a peak potential).
Thus, the cationic species of 1 are electrochemically stable. In
fact, the spectral change of 1 under the application of an
external potential of +0.24 V (vs Fc+/Fc) revealed a quantitative
generation of the corresponding radical cationic species 1•+
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(Figure S2). The identification of 1•+ was supported by a
chemical oxidation (vide infra) and spectral assignment was
carried out using TD-DFT calculations (Table S4). The reduction
potentials for 1+/1 (+0.14 V) and 12+/1+ (+0.69 V) had a higher
voltage than the corresponding potentials of 2 (+0.07 V and
+0.42 V, respectively), because of the delocalization of the
HOMO of 1 in the planar -framework (Figure 5). On the other
hand, the gap between the potentials of 1+/1 and 12+/1+ (∆E =
0.55 V), which is involved by the onsite Coulomb repulsion
between the two positive charges, was larger than that of 2 (∆E
= 0.35 V) in spite of the delocalized nature of the HOMO of 1.
These results can be explained by the stability of the dicationic
species (Figure 7). After the two-electron oxidation, 12+ has a
contribution of the pentacene-like electronic structure 1B,4e
whereas 22+ can form a pentaphene-like electronic structure 2B.
According to Clar’s aromatic sextet theory,13 pentacene has only
one sextet ring whereas the pentaphene skeleton has two sextet
rings, indicating that the electronic structure of pentaphene is
more stable. Thus, the pentaphene-like resonance hybrid
stabilizes 22+ and facilitates the second oxidation.
DFT calculations of 1 and 2 show the localization of both the
HOMOs and the LUMOs on the fused-ring systems (Figure 5),
indicating insignificant effect of the mesityl groups on the
electrochemical and optical properties of the QDM systems due
to the large dihedral angles between the mesitiyl units and the
fused-ring systems.
Properties and crystal structure of radical cation salt
1•+•SbF6
−
The chemical oxidation of 1 was carried out using tris(4-
bromophenyl)aminium hexafluoroantimonate (TBPA•SbF6) as an
oxidant (Scheme 2), and the desired radical cation salt 1•+•SbF6
−
was isolated as a dark red solid in 98% yield by reprecipitation
from the CH2Cl2 solution using ether. The UV−vis−near infrared
(NIR) spectrum of 1•+•SbF6 in CH2Cl2 (Figure 8) is consistent
−
with that observed during the electrochemical oxidation (Figure
S2). Moreover, 1•+•SbF6 showed negligible spectral change
−
after 1 day under aerated conditions at room temperature,
indicating the high stability of 1•+•SbF6 (Figure S3). The lowest-
−
energy absorption band in the 700–1000 nm region, which
mainly ascribed to the SOMO−LUMO transition (Figure S2 and
Table S4), showed a clear vibrational structure (peak = 958, 835,
and 737 nm) similar to 1. The vibrational energies in the S1 state
were estimated to be 1538 and 1592 cm–1, suggesting that the
electronic structure of 1•+•SbF6− has a quasi-pQDM nature.10
Figure 6. Cyclic voltammogram of 1 in CH2Cl2 containing 0.1 M nBu4NPF6 as
a supporting electrolyte at a scan rate of 100 mV s−1
.
−
Scheme 2. Synthesis of 1•+•SbF6
.
Figure 8. UV−vis−NIR absorption spectrrum of 1•+•SbF6− in CH2Cl2.
−
The electron-spin resonance (ESR) spectrum of 1•+•SbF6 in
degassed CH2Cl2 at room temperature showed multiple split
lines with g = 2.0033 (Figure 9) and is drastically different from
that of oQDM system 2•+•SbF6 , in which the spin densities are
–
Figure 7. Resonance hybrids of a) 12+ and b) 22+
.
mainly localized on the central ring.7 The observed ESR
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spectrum was reproduced by spectral simulation with the
following hyperfine coupling constants for the four sets of 1H
nuclei (I = 1/2): |a1H| (×2) = 2.49 × 10-4 cm-1 (0.266 mT), |a1H|
(×2) = 2.10 × 10-4 cm-1 (0.225 mT), |a1H| (×2) = 1.22 × 10-4 cm-1
(0.131 mT), and |a1H| (×4) = 4.30 × 10-5 cm-1 (0.046 mT).14 With
the aid of the theoretical calculations of 1•+, the former three can
inversion center and a form similar to that of the corresponding
neutral species, i.e., a slightly bent (175.48°) -framework and
87°-twisting mesityl groups. Reduced bond-length alternations
around the central ring C were observed (Table 3), which is
consistent with the NIR absorption band with the well-resolved
vibrational structure. The bond lengths e, b, and d of 1•+ are
shorter than the neutral molecule, which reflects the antibonding
character of these bonds in HOMO of the neutral molecule
(Figure 5a). The HOMA value of the central ring C was
estimated to be 0.668, clearly indicating delocalization of the -
conjugated system of 1•+ and thus confirming the results of the
ESR study.
1
be assigned to the two equivalent H nuclei at the 6,13-, 3,10-,
and 1,8-positions in the 5,12-dioxapentacene moiety,
1
respectively (Figure 10). The remaining four H nuclei can be
attributed to one equivalent 1H nuclei in the four o-methyl groups
of the mesityl moieties, because of the hindered rotation of the
mesityl methyl groups in the ESR time scale. These results
indicate that the electron spin of pQDM system 1•+ is delocalized
over the entire -conjugated system whereas in 2•+ the spin
density is localized on the oQDM moiety.7
Figure 9. a) Observed and b) simulated ESR spectra of 1•+•SbF6− in CH2Cl2 at
room temperature. Parameters for simulation: g = 2.0033, |a1H| (×2) = 2.49 ×
10-4 cm-1 (0.266 mT), |a1H| (×2) = 2.10 × 10-4 cm-1 (0.225 mT), |a1H| (×2) = 1.22
× 10-4 cm-1 (0.131 mT), and |a1H| (×4) = 4.30 × 10-5 cm-1 (0.046 mT).
Figure 11. Crystal structures of 1•+•SbF66−: a) Top view and b) side view along
short axis of 1•+. Hydrogen atoms are ommitted for clarity. Thermal ellipsoids set
at 50%. In b), the counter anion and crrystal solvents are omitted for clarity.
The gray double-headed arrow and thhe value indicate the dihedral angle
between rings A and C (Figure 3).
−
Table 3. The selected bond lengths[a] annd HOMA values of 1•+•SbF6
.
Bonds[b]
a
b
c
d
e
HOMA[c]
0.668
−
1•+•SbF6 1.386(4) 1.432(4) 1.358((4) 1.420(4) 1.414(4)
[a] In Å unit. [b] Assignment of bonnd names are shown in Figure 3.
[c] Estimated using the bond lengths of bb, c, and d.
Conclusions
Figure 10. a) Spin density map (isovalue = 0.002, yellow positive spin, green:
negative spin) and b) spin density values of 1•+ optimized at the DFT
UB3LYP/6-31G** level. Hydrogen atoms are omitted for clarity.
An electron-rich pQDM system, 2,9-dibromo-7,14-dimesityl-5,12-
dioxapentacene, was prepared and its structure and properties
were compared to the isomeric oQDM system. Single crystal X-
ray analysis revealed smaller bond-length alternation in the
pQDM system than that in the twisted oQDM system. While the
oQDM system showed broad absorption bands and no
fluorescence, the absorption and fluorescence bands with well-
resolved vibrational structures were observed for the pQDM
system. Additionally, the radical cationic species of the pQDM
−
Moreover, the X-ray structural analysis of 1•+•SbF6 was
successfully performed (Figure 11 and Table S1).
Recrystallization by slow diffusion of ether into the CH2Cl2
solution of 1•+•SbF6 gave suitable crystals for X-ray analysis, in
−
which the observed 1•+•SbF6 contained two CH2Cl2 molecules
−
as crystal solvent. The crystal structure of 1•+•SbF6 has an
−
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(100 MHz, CDCl3):  = 197.6, 162.4, 141.6, 140.8, 134.8, 132.8, 120.9,
119.7, 118.5, 111.2 ppm; IR (KBr) mmax = 3147 (br), 3086, 1762, 1735,
1678, 1630, 1607, 1568, 1463, 1348, 1285, 1231, 1207, 1154, 1065, 945,
893, 827, 733, 645, 521 cm–1; MS (DART-TOF–): m/z: 630.9, 632.9,
634.9 [M–H−]; elemental analysis calcd (%) for C20H10Br4O4: C 37.89, H
1.59; found: C 38.16; H 1.63.
system was prepared by the chemical oxidation and successfully
isolated. The ESR spectrum of the pQDM radical cation
exhibited the well delocalized spin density pattern, although the
much simpler localized pattern was observed for the oQDM
radical cation. These results make a sharp contrast between the
oQDM and pQDM systems and provide useful guidelines for
material applications.
2,9-Dibromo-7,14-dihydro-5,12-dioxapentacene-7,14-dione (5)
Picolinic acid (118 mg, 0.96 mmol), CuI (91 mg, 0.48 mmol), and K3PO4
(1.36 g, 6.41 mmol) were placed in a 50 mL two-neck round bottom flask
with DMSO (16 mL). The mixture was degassed by a N2-bubbling for 5
min and heated at 50 °C for 30 min. To this suspension after cooling to
RT, 4 (1.01 g, 1.60 mmol) was added using DMSO (8 mL) as a rinse
solvent. The mixture was heated at 120 °C for 24 h under a N2
atmosphere. After cooling to RT, the mixture was poured into an excess
amount of 4 M aq HCl. The resulting precipitate was collected by filtration,
and the residue was washed with EtOH. The obtained crude product was
completely dissolved in 18 M H2SO4, and then the deep-red solution was
added dropwise into ice-cooled water. The generated precipitate was
collected by filtration, and the residue was washed with water and EtOH
to obtain 5 (490 mg, 65%) as a brownish yellow powder. m.p. > 300 °C;
1H NMR (400 MHz, D2SO4):  = 8.65 (s, 2H), 8.11 (d, J = 1.8 Hz, 2H),
7.81 (dd, J = 9.3 and 1.8 Hz, 2H), 7.35 ppm (d, J = 9.3 Hz, 2H); 13C NMR
(100 MHz, D2SO4):  = 177.5, 159.7, 150.9, 149.2, 127.5, 122.7, 121.4,
121.0, 118.1, 114.7 ppm; IR (KBr) maax = 3086, 1664, 1602, 1463, 1447,
1339, 1269, 1205, 1127, 913, 823, 787, 662, 531, 418 cm–1; MS (DART-
TOF+): m/z: 471.0, 472.0, 474.0 [M + H+]; elemental analysis calcd (%)
for C20H8Br2O4: C 50.88, H 1.71; found: C 50.72, H 1.84.
Experimental Section
Methods
Melting points were taken on a Yanaco MP J-3 and were uncorrected. 1H
and 13C NMR spectra were recorded on a Bruker Avance III 400
spectrometer. Chemical shifts were reported in parts per million
downfield for tetramethylsilane as an internal (for CDCl3) and an external
(for sulfuric acid-d2, using the value in CDCl3) standard or relative to
solvent signal (CHCl3:  = 7.26 for 1H NMR and  = 77.0 ppm for 13C
NMR) as internal standards, and all coupling constants are reported in
Hz (Multiplicity: s = singlet; d = doublet). ESR spectrum of 1•+•SbF6− was
recorded on a Bruker ELEXSYS E500 spectrometer. IR spectra were
recorded on a JASCO FT/IR-4600 spectrometer. DART-TOF and ESI-
TOF mass spectra were recorded on a JEOL AccuTOF LC-plus JMS-
T100LP spectrometer. Elemental analyses were obtained from the
Analytical Centre in Osaka City University, using a Fisons EA1108 and a
J-Science JM10. X-ray data were collected by a Rigaku Saturn 724 CCD
system with graphite monochromated Mo-Kα radiation. Absorption
spectra were recorded on a Shimadzu UV-2550 spectrometer and a
JASCO V670 spectrometer. Merck Silica gel 60 (63–200 mesh) or Kanto
Chemical Silica gel 60 (100–200 mesh) were used for column
chromatography. The progress of reactions was monitored using thin-
2,9-Dibromo-7,14-dimesityl-5,12-dioxapentacene (1)
To prepare mesityl Grignard reagent, in a 10 mL two-necked round
bottom flask under a N2 atmosphere, the solution of 2-bromomesitylene
(1.19 g, 6.00 mmol) in THF (3 mL) was added slowly to the suspension of
anhydrous LiCl (212 mg, 5.00 mmol) and Mg turnings (120 mg, 5.00
layer chromatography using Merck TLC silica gel 60 F254
commercially available compounds were reagent grade and used without
further purification. Dehydrate DMSO was purchased. THF was dried and
. All
distilled over sodium. CH2Cl2 was dried and distilled over calcium hydride. mmol) in THF (3 mL) at 40 °C, and the mixture was stirred for 1 h.
2,5-Dibromoterephthalic acid was prepared by the reported method.8
In another 30 mL two-necked round bottom flask under a N2 atmosphere,
1,4-Dibromo-2,5-di(5-bromo-2-hydroxybenzoyl)benzene (4)
5 (472 mg, 1.00 mmol) was placed with THF (10 mL) at 60 °C. To the
suspension of 5 was added dropwise slowly the prepared mesityl
Grignard reagent, and then the mixture was stirred for 12 h. After cooling
to RT, 4 M aq HCl was added to quench the reaction. The aqueous layer
was extracted with CH2Cl2, and the combined organic layer was dried
over Na2SO4. A removal of the solvent under reduced pressure gave the
crude mesitylated diol.
2,5-Dibromoterephthalic acid (4.56 g, 15.0 mmol) and one drop of DMF
were placed in a 300 mL two-necked round bottom flask with thionyl
chloride (65 mL). The suspension was refluxed at 90 °C for 1 h under a
N2 atmosphere. After cooling to RT, an excess amount of thionyl chloride
was removed under reduced pressure to obtain the corresponding acid
chloride 3 as a pale-brown crude solid. This crude compound was used
in the next step without further purification.
The obtained crude diol was dissolved in CH2Cl2 (60 mL), and the
mixture was degassed by a N2-bubblling for 5 min. After the degassing,
one drop of 4 M aq HCl and anhydrous SnCl2 (1.14 g, 6.00 mmol) were
added, and then the mixture was stiirred at RT for 12 h. The resulting
mixture was passed through Celite®® to remove the excess amount of
SnCl2, and then the filtrate was concentrated under reduced pressure.
The residue was purified by silica gel column chromatography using n-
hexane–CH2Cl2 (1:1 v/v) as an eluent, and then the obtained product was
washed with EtOH to obtain 1 (274 mg, 40%) as a deep red solid. Single
crystals of 1 suitable for X-ray analysis were obtained by a liquid diffusion
of CH3CN with a solution of 1 in CH2Cl2. m.p. 255 °C (decomp); 1H NMR
(400 MHz, CDCl3):  = 7.06 (dd, J = 8.6 and 2.3 Hz, 2H), 7.00 (s, 4H),
6.63 (d, J = 8.6 Hz, 2H), 6.48 (d, J = 2.3 Hz, 2H), 5.41 (s, 2H), 2.08 (s,
12H), 1.86 ppm (s, 6H); 13C NMR (100 MHz, CDCl3):  = 152.2, 150.8,
137.9, 137.0, 130.5, 129.8, 128.7, 128.1, 126.0, 125.8, 124.6, 116.5,
116.3, 100.0, 21.2, 19.7 ppm; IR (KBr) max = 2969, 2916, 2855, 1577,
After refilling the reaction equipment with N2, the obtained crude 3 was
dissolved in CH2Cl2 (100 mL), and the mixture was cooled at 0 °C. To
this solution was added finely-grounded AlCl3 by a portion. The resulting
mixture was stirred at 0 °C for 30 min and then warmed up to RT. After
stirring for 30 min at RT, the mixture was cooled again to 0 °C, and then
a solution of 4-bromoanisole (11.2 g, 60.0 mmol) in CH2Cl2 (50 mL) was
added slowly. The resulting mixture was allowed to warm up to RT. After
stirring for 18 h at RT, the mixture was refluxed at 90 °C for additional 6 h.
After cooling to RT, the mixture was poured into an excess amount of 4
M aq HCl cooled at 0 °C. The resulting precipitate was collected by
filtration, and the residue was washed with water and EtOH to obtain 4
(7.34 g, 77%) as a pale yellow powder. m.p. 276–277 °C; 1H NMR (400
MHz, CDCl3):  = 11.63 (s, 2H), 7.66 (dd, J = 8.9 and 2.4 Hz, 2H), 7.63 (s,
2H), 7.35 (d, J = 2.4 Hz, 2H), 7.03 ppm (d, J = 8.9 Hz, 2H); 13C NMR
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1555, 1468, 1407, 1377, 1302, 1260, 1212, 1190, 1165, 1072, 966, 914,
849, 808, 761, 664, 581 cm–1; MS (DART-TOF+): m/z: 677.2, 679.2,
681.2 [M + H+]; elemental analysis calcd (%) for C38H30Br2O2: C 67.27, H
4.46; found: C 67.50, H, 4.66.
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–
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in CH2Cl2 (3 mL). To this solution was added a solution of tris(4-
bromophenyl)aminium hexafluoroantimonate (TBPA•SbF6, 22.0 mg,
0.0307 mmol) in CH2Cl2 at RT, and the mixture was stirred for 1 h. The
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obtain 1•+•SbF6 (28.8 mg obtained as 1•+•SbF6−•(CH2Cl2)0.2, 98%) as a
−
dark-red solid. m.p. 250 °C (decomp); IR (KBr) max = 3082, 2973, 2919,
2859, 1609, 1553, 1482, 1434, 1390, 1333, 1275, 1173, 1139, 1067, 970,
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H 3.29; found: C 49.11, H 3.51.
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butylammonium hexafluorophosphate as a supporting electrolyte. The
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cell equipped with a fine mesh platinum as a working electrode, a
platinum wire as a counter electrode, and a SCE reference electrode.
The external potential was applied using a Bi-Potentiostat ALS/DY2323
for the electrochemical oxidation of 1 in CH2Cl2 containing 0.1 M tetra-n-
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Acknowledgements
This work was partially supported by Grant-in-Aid for Scientific
Research from JSPS KAKENHI (No. JP17K05790 for M.K. and
K.O. and No. JP26288041 and No. JP15H00956 for K.O.) and
financial support Grant-in-Aid for Scientific Research on
Innovative Areas “π-System Figuration: Control of Electron and
Structural Dynamism for Innovative Functions” from MEXT (No.
26102005 to S.S.). C.S. thanks the financial support by the
Sasakawa Scientific Research Grant from The Japan Science
Society.
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Keywords: EPR spectroscopy • Fused-ring systems •
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[14] ESR spectral simulation was carriied out with WINEPR Simfonia.
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Entry for the Table of Contents (Please choose one layout)
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Chihiro Sato, Shuichi Suzuki, Keiji
Okada, and Masatoshi Kozaki*
Page No. – Page No.
Preparation and Properties of a
Polycyclic p-Quinodimethane with
Two Oxygen Bridges and its Radical
Cation in Comparison with the
Isomeric o-Quinodimethane
An electron-rich p-quinodimethane system, 2,9-dibromo-7,14-dimesityl-5,12-
dioxapentacene, was prepared and oxidized to form a stable radical cation.
Molecular structure, photophysical properties of both neutral and cationic species
were investigated and compared with the corresponding isomeric o-quinodimethane
system.
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