Modular synthesis and photophysical and electrochemical properties of 2,3,5,6-tetraaryl-substituted benzo[1,2-b:5,4-b′]difurans

Article abstract of DOI:10.1002/hc.20682

We developed a versatile synthesis of tetraaryl-substituted benzo[1,2-b:5,4-b′]difurans (m-BDFs) via a zinc-mediated intramolecular double cyclization reaction of 4,6-bis(phenylethynyl)-1,3-benzenediol, followed by a palladium-catalyzed cross-coupling rea

Full text of DOI:10.1002/hc.20682

Heteroatom Chemistry
Volume 22, Number 3/4, 2011
Modular Synthesis and Photophysical
and Electrochemical Properties of
2,3,5,6-Tetraaryl-Substituted
ꢀ
Benzo[1,2-b:5,4-b ]difurans
Hayato Tsuji,1 Chikahiko Mitsui,1 Yoshiharu Sato,2
and Eiichi Nakamura1,2
1Department of Chemistry, School of Science, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
2Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency,
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received 16 August 2010; revised 14 October 2010
INTRODUCTION
ABSTRACT: We developed a versatile synthesis of
tetraaryl-substituted benzo[1,2-b:5,4-bꢀ]difurans (m-
Benzofuran and congeners have recently re-
BDFs) via a zinc-mediated intramolecular dou-
ble cyclization reaction of 4,6-bis(phenylethynyl)-1,3-
benzenediol, followed by a palladium-catalyzed cross-
coupling reaction. In comparison with the corre-
sponding benzo[1,2-b:4,5-bꢀ]difuran (p-BDF) isomers
reported previously, the m-BDFs are slightly wider be-
tween HOMO and LUMO, as well as a lower energy
level of the former. These differences in properties be-
tween the structural isomers can be ascribed to the dif-
ference in the conjugation type. The compounds had
high charge carrier mobility up to 1 × 10−3 cm2/Vs,
which makes them attractive materials for organoelec-
ceived growing attention because of their useful
photophysical and electric properties [1,2,3,4,5].
We recently reported the synthesis of a variety
of tetraaryl-substituted benzo[1,2-b:4,5-bꢀ]difurans
(p-BDFs), their p-type semiconductor characteris-
tics and high hole mobility (up to 10−3 cm2/Vs
for amorphous film), and their versatility in or-
ganic light-emitting diodes [1]. These findings led
us to study the m-isomer benzo[1,2-b:5,4-bꢀ]difuran
(m-BDF) 4. In this article, we describe the prepara-
tion, photophysical, and electrochemical properties,
and carrier mobilities of m-BDF.
ꢁ
C
tronics applications. 2011 Wiley Periodicals, Inc. Het-
eroatom Chem 22:316–324, 2011; View this article online
at wileyonlinelibrary.com. DOI 10.1002/hc.20682
RESULTS AND DISCUSSION
Synthesis of m-BDF
The present synthesis of m-BDF relies on the zinc-
mediated cyclization of an ortho-alkynyl phenol into
3-zinciobenzofuran that we developed recently and
extended to the synthesis of a variety of heterocyclic
π-electronic compounds [6,7,8,9,10]. As shown in
Scheme 1, dialkynylbenzenediol 2, which is readily
available from dibromobenzenediol 1, was treated
sequentially with butyllithium and zinc chloride to
effect twofold intramolecular oxozincation of the
Dedicated to Professor Kin-ya Akiba on the occasion of his 75th
birthday.
Correspondence to: H. Tsuji; e-mail: tsuji@chem.s.u-tokyo.ac.jp.
E. Nakamura; e-mail: nakamura@chem.s.u-tokyo.ac.jp.
c
ꢀ
2011 Wiley Periodicals, Inc.
316

Modular Synthesis and Photophysical and Electrochemical Properties of 2,3,5,6-Tetraaryl-Substituted Benzo[1,2-b:5,4-bꢀ]difurans 317
1) BuLi / THF
0 °C to rt
Ph
Ph
Br
Br
2) ZnCl2
toluene
120 °C, 2 h
HO
OH
HO
OH
Ph
1
2
(D)
(D)
H
ClZn
ZnCl
H
H2O
(D2O)
Ph
Ph
Ph
O
O
O
O
FIGURE 1 ORTEP drawings of m-BDF derivatives (30%
probability for thermal ellipsoids). The solvent molecule and
crystallographically independent molecule are omitted for
clarity. (a) p-Me2N-C H4-substituted m-BDF 4c and (b) C6F5-
substituted m-BDF 46f.
4a (quant)
3
Ar
Ar
Ar–X
Ph
Ph
Pd2(dba)3•CHCl3
P(t-Bu)3
O
O
Ar = C6H5 (63%)
: 4b
C6H4-p-NMe2 (62%) : 4c
β-aromatic groups are twisted against the BDF ring
(twist angles of 34.83◦ and 43.26◦ for tetraphenyl-p-
BDF). On the other hand, in m-BDFs, the α-phenyl
groups lie almost in the plane of the BDF ring (9.48◦
and 11.40◦ for 4c and 1.29◦ and 4.33◦ for 4f) [11] and
the β-aryl rings are highly twisted (74.51◦ and 78.63◦
for 4c and 64.76◦ and 67.30◦ for 4f). This is likely the
result of steric hindrance among the aromatic rings
at α- and β-positions, and the hydrogen atom at the
5-position of the BDF core. Naturally, the effect of
the packing force cannot be discounted.
: 4d
: 4e
: 4f
C6H4-p-NPh2 (58%)
C6H4-p-Cz (65%)
C6F5 (20%)
F
F
O
O
F
Ph
Ph
F
F
F
when Ar = C6F5
F
Ph
Ph
F
O
O
F
F
5 (57%)
SCHEME 1
Photophysical Properties
triple bonds and provide quantitatively 3,5-dizincio-
m-BDF 3 as a stable intermediate that serves as a
synthetic module to obtain a variety of derivatives.
Upon quenching with water, this intermediate gave
3,5-protonated m-BDF 4a in 99% isolated yield.
The Negishi coupling of 3 with iodoben-
zene, p-bromo-N,N-dimethylaniline p-bromo-N,N-
diphenylaniline, and N-(p-bromophenyl)carbazol
afforded the corresponding 3,5-diarylated m-BDF
derivatives 4b–4e in 63%, 62%, 58%, and 65% iso-
lated yields, respectively. Interestingly, the reaction
with pentafluorophenyl bromide gave m-BDF dimer
5 in 57% isolated yield in preference to the desired
3,5-bis(pentafluorophenyl)-m-BDF 4f, which was ob-
tained in 20% yield. This is an indication that the
first pentafluorophenyl group at the 3-position has
a strong electron-withdrawing effect on the remote
organozinc moiety at the 5-position and promotes
the homocoupling reaction.
We next measured the ultraviolet (UV)–visible ab-
sorption and fluorescence spectra of m-BDFs 4a–4f
and found that these m-BDFs have a slightly wider
bandgap than the p-BDF analogue. The spectra are
shown in Fig. 2 and the data are summarized in
Table 1. The longest wavelengths of the absorp-
tion maxima of these compounds are 340–356 nm
and the absorption onset wavelengths (λonset) are
373–382 nm for the m-BDFs except dimethyl- and
diphenylanilino-m-BDFs 4c and 4d, which have a
significantly red-shifted onsets (λonset = 400 nm and
399 nm for 4c and 4d, respectively). Both absorp-
tion maxima and onset wavelengths of the m-BDFs
are slightly blue-shifted (860–1470 cm−1) compared
with the corresponding p-BDF derivatives.
To explain the absorption spectra, we analyzed
the structure–property relationships of p- and m-
BDFs as discussed below for α,αꢀ-diphenyl-BDFs.
One can consider two thought experiments in dis-
cussing the conjugation of two BDF isomers (Fig. 3):
(a) cyclic bridging of the m- and p-styrylstilbene sys-
tem with two oxygen atoms and (b) replacement
of two π-electrons of a C C bond with two non-
bonding electrons of an oxygen atom; that is, re-
placement of two double bonds in 2,7- and 2,6-
diphenylanthracene with two oxygen atoms. As the
former is cross-conjugation, the difference in the
Crystal Structure
We performed single-crystal X-ray crystallographic
analysis of β,βꢀ-bis(dimethylanilino)-m-BDF 4c and
β,βꢀ-bis(pentafluorophenyl)-m-BDF 4f, and found
interesting structural characteristics. The ORTEP
drawings of these molecules are shown in Fig. 1.
In p-BDFs that we previously reported, both α- and
Heteroatom Chemistry DOI 10.1002/hc

318 Tsuji et al.
(a)
4a
4b
4f
300
350
400
450
500
550
600
Wavelength (nm)
(b)
4c
4d
4e
300
350
400
450
500
550
600
Wavelength (nm)
FIGURE 2 UV–visible absorption and fluorescence spectra of m-BDFs measured in dichloromethane at room temperature.
(a) 4a, 4b, and 4f and (b) 4c–4e.
wavelengths of absorption maximum of the m− and
p-styrylstilbene isomers is rather large (Fig. 3a, cal-
culated absorption wavelengths: λmax = 316 nm and
386 nm, respectively, ꢀE = 0.71 eV) [12] whereas
that for diphenylanthracene isomers is small (calcu-
lated absorption wavelengths: λmax = 302 nm and 319
nm for 2,7- and 2,6-isomer, respectively, ꢀE = 0.22
eV) [13]. Considering the small difference in absorp-
tion wavelengths between m- and p-BDFs (calculated
absorption wavelengths: λmax = 346 nm and 367 nm
for m- and p-BDFs, respectively, ꢀE = 0.20 eV), the
electronic structure of α,αꢀ-diphenyl-BDFs may be
considered to be more similar to that of diphenylan-
thracene isomers than to that of styrylstilbenes.
The phenyl- and pentafluorophenyl-substituted
m-BDFs (4a, 4b, and 4f) have a series of sharp flu-
orescence bands with fine vibration structures be-
tween 1150 and 1200 cm−1. The Stokes shift was
very small (840 cm−1 for 4a, 1420 cm−1 for 4b, and
1290 cm−1 for 4f). The fluorescence quantum yields
were rather high (0.81–0.93) for these compounds.
On the other hand, the arylamino-m-BDFs 4c and
4d had a broad and structureless emission band ac-
companying a large Stokes shift (6260 cm−1 for 4c
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Modular Synthesis and Photophysical and Electrochemical Properties of 2,3,5,6-Tetraaryl-Substituted Benzo[1,2-b:5,4-bꢀ]difurans 319
TABLE 1 Summary of Photophysical and Electrochemical Properties of m-BDF derivatives 4a–4f
Compound
λaabs (nm)
λaonset (nm)
λbemis (nm)
Stokes Shift (cm−1
)
ꢀFc
Edox (V)
4a
4b
4c
4d
4e
4f
292, 341, 356
343, 357 (sh)
272, 299, 346, 359 (sh)
301, 343, 358 (sh)
342, 357 (sh)
373 (392)
378 (396)
400 (428)
399 (419)
382 (402)
373 (393)
367, 385
376 (sh), 393
463
455
402
371, 388
840
1420
6260
5950
3140
1290
0.92
0.93
0.67
0.73
0.74
0.81
0.82 (ir)
0.89 (r)
0.29 (r)
0.48 (r)
0.82 (qr)
1.18 (ir)
271, 297, 340, 354 (sh)
aAbsorption spectra measured in CH2Cl2 at room temperature. sh: shoulder.
bEmission spectra excited at 320–340 nm measured in CH2Cl2 at room temperature. sh: shoulder.
cDetermined employing the absolute method.
dFor 4b–4d: determined by the half-wave voltage of cyclic voltammetry (CV) measurement (vs. Fc/Fc+). For 4a, 4e, and 4f: determined by
differential pulse voltammetry (DPV) measurement (vs. Fc/Fc+). Both measurements were performed in dichloromethane (0.5 mM) using TBAP
(0.1 M) as an electrolyte. r: reversible, ir: irreversible, qr: quasi reversible.
FIGURE 3 Schematic representation of the effective conjugation lengths of styrylstilbenes, BDFs, and anthracene derivatives,
together with their absorption maximum wavelengths (calculated at the TD B3LYP/6–31G*//B3LYP/6–31G* level assuming
planarity of the conjugation system) and experimental data (shown in parentheses).
and 5950 cm−1 for 4d) and moderate quantum yield
(0.67 and 0.73 for compounds 4c and 4d, respec-
tively). We can ascribe this large shift to the well-
known charge-transfer character of the emission of
arylamines [14]. The carbazole-substituted deriva-
tive 4e also had a broad and structureless emission
band accompanying a moderate Stokes shift (3140
cm−1) and quantum yield (0.74), the magnitude of
which is between those of tetraphenyl derivative 4a
and arylamino derivatives 4c and 4d.
side as compared with diphenyl-m-BDF 4a (0.89 V
for 4b and 0.82 V for 4a). As expected, electron-
donating amino substituents caused a shift in the
first oxidation potential to the negative side (Eox
=
0.29 and 0.48 V for 4c and 4d), whereas an electron-
withdrawing pentafluorophenyl group caused a shift
to the positive side (Eox = 1.18 V for 4f), although
compound 4f failed to show reversibility of the first
oxidation wave for an unknown reason. These ox-
idation potentials shift approximately 0.1–0.15 eV
to the positive side as compared with the corre-
sponding p-BDF isomers (e.g., 0.74 for tetraphenyl-
p-BDF and 0.88 V for tetraphenyl-m-BDF), [1a] sug-
gesting increased stability of the m-isomers against
oxidation.
Electrochemical Properties
Cyclic voltammetry (CV) measurements presented
in Fig.
4 reveal that most of the tetraaryl-
substituted m-BDFs are electrochemically stable,
whereas diphenyl-m-BDF 4a is not. Their oxidation
potentials are summarized in Table 1. Tetraphenyl-
substituted m-BDF 4b had a reversible first oxida-
tion wave with a slight shift of Eox to the positive
Carrier Mobility
We performed carrier drift mobility measurements
for 4b, 4d, and 4e and found moderate to high
Heteroatom Chemistry DOI 10.1002/hc

320 Tsuji et al.
4a
4b
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
Potential (V) vs. Ferrocene
Potential (V) vs. Ferrocene
4c
4d
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
Potential (V) vs. Ferrocene
Potential (V) vs. Ferrocene
4e
4f
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
Potential (V) vs.Ferrocene
Potential (V) vs. Ferrocene
FIGURE 4 Cyclic voltammograms of m-BDFs 4a–4f measured in dichloromethane.
carrier mobilities of these compounds. The mo-
bilities were obtained by the time-of-flight (TOF)
method using the amorphous films prepared by vac-
uum deposition. Figure 5 plots the carrier mobility
against the applied electric field. Tetraphenyl-m-BDF
4b and diphenylanilino-m-BDF 4d had hole mobil-
ities of 5.3 × 10−5 and 2.5 × 10−4 cm2/Vs, respec-
tively, for an electric field (E) of 2.5 × 105 V/cm.
The carbazolyl-substituted derivative 4e showed am-
bipolarity like the corresponding p-isomer, [1b] hav-
ing hole and electron mobilities of μh = 1.7 × 10−3
cm2/Vs and μe = 1.4 × 10−3 cm2/Vs, respectively, at
E = 2.5 × 105 V/cm. We attribute this increase in
the hole mobility to a synergistic effect between the
BDF unit and the carbazole moiety. These carrier
mobilities are lower than those of the p-BDF ana-
logues (μh = 6.4 × 10−4 cm2/Vs for tetraphenyl-p-
BDF; μh = 2.8 × 10−3 cm2/Vs for diphenylanilino-p-
BDF; and μh = 3.7 × 10−3 cm2/Vs and μe = 4.4 × 10−3
cm2/Vs for carbazolyl-p-BDF) but still high enough
for applications in organic electronic devices. No-
tably, the amorphous films of m-BDFs used in the
TOF measurements did not crystallize in more than
half a year after fabrication, whereas the amorphous
film of tetraphenyl-p-BDF crystallized in less than 1
month upon standing on a shelf. We consider that
the stability of the amorphous state of the m−BDF is
due to the lower symmetry of this isomer.
CONCLUSION
We developed a versatile synthesis of tetraaryl-
substituted benzo[1,2-b:5,4-bꢀ]difurans (m-BDF) via
a zinc-mediated intramolecular double cyclization
reaction. Comparison between the p- and m-BDF
isomers revealed (a) similarity of the mode of the
conjugation in BDFs to that in substituted an-
thracenes rather than styrylstilbenes, (b) a wide-gap
and slightly lower oxidation potential of m-BDFs,
which should increase resistance to oxidation, and
Heteroatom Chemistry DOI 10.1002/hc

Modular Synthesis and Photophysical and Electrochemical Properties of 2,3,5,6-Tetraaryl-Substituted Benzo[1,2-b:5,4-bꢀ]difurans 321
10 -2
to afford the title compound (diastereomer mixture
A and B) as a white powder (353 mg, 81%). 1H NMR:
δ 1.60–1.74 (m, 6H for A and 6H for B, overlapped),
1.82–1.88 (m, 2H for A and 2H for B, overlapped),
1.94–1.97 (m, 2H for A and 2H for B, overlapped),
2.02–2.10 (m, 2H for A and 2H for B, overlapped),
3.58–3.63 (m, 2H for A and 2H for B, overlapped,
OCH2CH2) 3.82–3.85 (m, 2H for A, OCH2CH2),
3.87–3.92 (m, 2H for B, OCH2CH2), 5.42 (s, 2H for
B, OCHO), 5.51 (s, 2H for A, OCHO), 7.02 (s, 1H for
A, Ar), 7.09 (s, 1H for B, Ar), 7.66 (s, 1H for A and
1H for B, overlapped, Ar). 13C NMR of diastereomer
A: δ 18.3, 25.2, 30.2, 61.9, 97.5, 105.5, 106.6, 135.5,
153.7. 13C NMR of diastereomer B: δ 18.2, 25.2, 30.1,
61.8, 96.8, 104.9, 105.2, 135.6, 153.3. MS (APCI+):
435 (M+H). Anal. Calcd for C16H20Br2O4: C, 44.06;
H, 4.62. Found: C, 43.86; H, 4.60.
8
6
4
2
10 -3
8
6
4b (Hole)
4d (Hole)
4e (Hole)
4e (Electron)
4
2
10 -4
8
6
4
2
10 -5
300
400
500
E1/2 ([V/cm]
600
700
800
900
1/2
)
FIGURE 5 Carrier (hole and electron) drift mobilities of 4b,
4d, and 4e plotted against the square root of the electric field
measured employing the TOF method at room temperature.
2-[4,6-Bis(phenylethynyl)-3-(tetrahydro-2H-2-
pyranyloxy)phenoxy]tetrahydro-2H-pyran. Nitrogen
was bubbled through a solution of piperidine (36
mL) and triethylamine (100 mL) for 30 min. To this
solution was added 2-[4,6-dibromo-3-(tetrahydro-
2H-2-pyranyloxy)phenoxy]tetrahydro-2H-pyran
(8.67 g, 19.9 mmol) followed by CuI (380 mg, 1.99
mmol), PPh3 (1.05 g, 3.98 mmol), and Pd(PPh3)4
(2.31 g, 1.99 mmol). The mixture was stirred at
90◦C for 22 h. After cooling to ambient temperature,
additional CuI (76 mg, 0.40 mmol), PPh3 (210 mg,
0.80 mmol), and Pd(PPh3)4 (461 mg, 0.40 mmol)
were added and stirred at 90◦C for 18 h. The reaction
mixture was cooled to ambient temperature, and
piperidine and triethylamine were removed in
vacuo. After the reaction was quenched with water,
the organic layer was extracted with CHCl3, washed
with brine, and dried over MgSO4. After removal
of the solvent in vacuo, the crude mixture was
subjected to flash silica gel column chromatography
(hexane/ethyl acetate = 99/1 to 95/5) to afford the
title compound (8.27 g, 17.3 mmol, 87%) as a white
solid. 1H NMR: δ 1.60–1.64 (m, 2H for A and 2H for
B, overlapped), 1.70–1.74 (m, 4H for A and 4H for
B, overlapped), 1.86–1.91 (m, 2H for A and 2H for
B, overlapped), 2.00–2.10 (m, 2H for A and 2H for
B, overlapped), 2.13–2.18 (m, 2H for A and 2H for
B, overlapped), 3.61–3.65 (m, 2H for A and 2H for
B, overlapped, OCH2CH2), 3.94–4.03 (m, 2H for A
and 2H for B, OCH2CH2), 5.58 (s, 2H for A, OCHO),
5.64 (s, 2H for B, OCHO), 6.9 (s, 1H for B, Ar), 7.03
(s, 1H for A, Ar), 7.29–7.36 (m, 6H for A and 6H
for B, overlapped, Ph), 7.49–7.51 (m, 4H for A and
4H for B, overlapped, Ph), 7.64 (s, 1H for A and 1H
for B. overlapped, Ar). 13C NMR of diastereomer A:
δ 18.2, 25.2, 30.2, 61.7, 85.2, 92.2, 96.9, 104.4, 107.7,
123.9, 127.9, 128.3, 131.3, 136.8, 158.8. 13C NMR of
(c) high stability of the amorphous films. Applica-
tions of m-BDFs in organic electronic devices are
now in progress.
EXPERIMENTAL
All of the reactions dealing with air- or moisture-
sensitive compounds were carried out in a dry re-
action vessel under a positive pressure of nitrogen
or argon. Proton nuclear magnetic resonance (1H
NMR) and carbon nuclear magnetic resonance (13C
NMR) spectra were recorded on a JEOL ECA-500
1
(500 MHz for H and 125 MHz for 13C) NMR spec-
trometer using CDCl3 as a solvent at ambient tem-
perature unless otherwise noted. Tetramethylsilane
was used as an internal reference.
2-[4,6-Dibromo-3-(tetrahydro-2H-2-pyranyloxy)-
phenoxy]tetrahydro-2H-pyran. To
a
20
mL
two-necked flask were added 4,6-dibromo-1,3-
benzenediol (268 mg, 1.00 mmol) and 3,4-dihydro-
2H-pyran (DHP) (911 μL, 10.0 mmol). After stirring
at ambient temperature for 10 min, pyridinium
p-toluenesulfonate (PPTS) (5.0 mg, 0.020 mmol)
was added to the mixture. After stirring at ambient
temperature for 11 h, the reaction was quenched
with saturated sodium bicarbonate solution. The
organic layer was extracted twice with ethyl acetate,
washed with brine, and dried over Na2SO4. After
removal of the solvent in vacuo, the crude material
was subjected to flash column chromatography on
silica gel (hexane/toluene = 80/20 to toluene 100%)
Heteroatom Chemistry DOI 10.1002/hc

322 Tsuji et al.
diastereomer B: δ 18.1, 25.2, 30.0, 61.6, 85.2, 92.3,
96.2, 103.1, 107.3, 123.9, 127.8, 128.3, 131.3, 136.8,
158.5. MS (APCI+): 479 (M+H). Anal. Calcd for
C32H30O4: C, 80.31; H, 6.32. Found: C, 80.25; H, 6.28.
benzenediol (1.24 g, 4.00 mmol) in THF (4.0 mL),
a solution of n-butyllithium in hexane (4.82 mL,
1.66 mol/L, 8.00 mmol) at 0◦C was added. The re-
sulting mixture was allowed to warm to ambient
temperature and stirred for 30 min. A solution of
zinc chloride in THF (8.0 mL, 1.0 mol/L, 8.0 mmol)
and toluene (8.0 mL) was added to the reaction mix-
ture. The resulting pale yellow solution was heated
to 120◦C in a sealed tube and stirred for 3 h at
this temperature. After cooling to ambient temper-
ature, Pd2(dba)3·CHCl3 (414 mg, 0.40 mmol), P(t-
Bu)3 in toluene (1.6 mL, 1.0 mol/L, 1.6 mmol), and
then iodobenzene (1.07 mL, 9.60 mmol) were suc-
cessively added. After stirring at ambient tempera-
ture for 20 h, the reaction mixture was quenched
with a saturated ammonium chloride aqueous solu-
tion. The organic layer was extracted with CHCl3
and washed with H2O. After evaporating the sol-
vent in vacuo, crude material was purified by flash
column chromatography on silica gel employing a
gradient from hexane/chloroform = 95/5 to hex-
ane/chloroform = 70/30 as an eluent to give the ti-
tle compound. Final purification by high vacuum
(pressure under 20 mTorr) train sublimation at 230–
250◦C afforded the title compound as white powder
(1.16 g, 2.51 mmol, 63%). Mp: 222–223◦C. 1H NMR:
δ 7.26–7.33 (m, 6H, m−andp-Ph), 7.39 (dd, J = 7.5,
7.5 Hz, 2H, p-Ph), 7.42 (s, 1H, BDF), 7.45 (dd, J =
7.4, 7.4 Hz, 4H, m-Ph), 7.49 (d, J = 7.4 Hz, 4H, o-Ph),
4,6-Bis(phenylethynyl)-1,3-benzenediol(2). To a
solution of 2-[4,6- bis(phenylethynyl)-3-(tetrahydro-
2H - 2 - pyranyloxy)phenoxy]tetrahydro - 2H - pyran
(957 mg, 2.0 mmol) in 20 mL of CH2Cl2 was added
trifluoroacetic acid (TFA) (148 μL, 2.0 mmol) at
0◦C. The mixture was stirred at this temperature for
30 min and passed through a short pad of silica gel
employing CH2Cl2 as an eluent. After removing the
solvent in vacuo, the crude mixture was subjected
to flash silica gel column chromatography employ-
ing hexane/CH2Cl2/ethyl acetate = 90/5/5 as an elu-
ent to give the title compound (576 mg, 93%) as a
1
white powder. Mp: 124–125◦C. H NMR: δ6.00 (s,
2H, OH), 6.63 (s, 1H, Ar), 7.35–7.39 (m, 6H, m- and
p-Ph), 7.50–7.55 (m, 5H, o-Ph and Ar). 13C NMR: δ
82.0, 95.6, 101.4, 103.0, 122.3, 128.5, 128.8, 131.5,
134.6, 158.4. MS (APCI+): 311 (M+H). Anal. Calcd
for C22H14O2: C, 85.14; H, 4.55. Found: C, 85.03;
H, 4.66.
2,6-Diphenylbenzo[1,2-b:5,4-bꢀ]difuran (4a). To
a
solution
of
4,6-bis(phenylethynyl)-1,3-
benzenediol(150 mg, 0.483 mmol) in THF (0.5 mL)
was added a solution of n-butyllithium in hexane
(620 μL, 1.57 mol/L, 0.97 mmol) at 0◦C. The re-
sulting mixture was allowed to warm to ambient
temperature and stirred for 30 min. A solution of
zinc chloride in THF (0.97 mL, 1.0 mol/L, 0.97
mmol) and toluene (2.0 mL) was added. The yellow
solution was heated to 120◦C and stirred for 2 h
at this temperature. After cooling to ambient tem-
perature, the reaction mixture was quenched with
saturated ammonium chloride aqueous solution.
The crude material was filtrated and washed several
times with H2O and acetone to afford the title
compound (149 mg, 99%) as white powder. Mp:
7.65 (d, J = 7.4 Hz, 4H, o-Ph), 7.72 (s, 1H, BDF). 13
C
NMR: δ 93.9 (CH), 109.2 (CH), 117.4 (C), 126.8 (CH),
127.4 (C), 127.6 (CH), 128.2 (CH), 128.4 (CH), 129.1
(CH), 129.9 (CH), 130.7 (C), 132.9 (C), 150.8 (C),
152.8 (C). MS (APCI+): 463 (M+H). Anal. Calcd for
C34H22O2: C, 88.29; H, 4.79. Found: C, 88.46; H, 4.92.
3,5-Bis(4-N,N-dimethylaminophenyl)-2,6-diphe-
nylbenzo[1,2-b:5,4-bꢀ]difuran (4c). White powder.
Mp: 277–278◦C. 1H NMR: δ 3.03 (s, 12H, NMe2), 6.81
(d, J = 8.6 Hz, 4H, Ar), 7.26 (dd, J = 7.5, 7.5 Hz,
2H, Ar), 7.32 (dd, J = 7.5, 7.5 Hz, 4H, Ar), 7.36 (d,
J = 8.6 Hz, 4H, Ar), 7.50 (s, 1H, BDF), 7.67 (s, 1H,
BDF), 7.71 (d, J = 7.5 Hz, 4H, Ar). 13C NMR: δ 40.5,
93.6, 109.7, 112.9, 117.7, 120.4, 126.6, 127.7 (two
carbons are overlapped), 128.3, 130.5, 131.3, 149.8,
150.0, 152.8. HRMS (APCI+) calcd for C38H33N2O2
(M+H): 549.2542. Found: 549.2568.
1
272–273◦C. H NMR: δ 7.09 (s, 2H, furan), 7.36 (dd,
J = 7.5, 7.5 Hz, 2H, p-Ph), 7.46 (dd, J = 7.5, 7.5 Hz,
4H, m-Ph), 7.67 (s, 2H, BDF), 7.88 (d, J = 7.5 Hz,
4H, o-Ph). 13C NMR (60◦C): δ 94.3 (CH), 101.3 (CH),
110.8 (CH), 124.9 (CH), 126.4 (C), 128.4 (CH), 128.8,
(CH) 130.8 (C), 153.7 (C), 156.5 (C). MS (APCI+):
311 (M+H). Anal. Calcd for C22H14O2: C, 85.14; H,
4.55. Found: C, 84.95; H, 4.62.
3,5-Bis(4-N,N-diphenylaminophenyl)-2,6-diphe-
nylbenzo[1,2-b:5,4-bꢀ]difuran (4d). Pale yellow
Typical Procedure for Pd(0)-Catalyzed
Cross-Coupling of the Benzodifuranzinc
Intermediate with Aryl Halides
2, 3, 5, 6 - Tetraphenylbenzo[1, 2 - b:5, 4 - bꢀ]difuran
(4b). To a solution of 4,6-bis(phenylethynyl)-1,3-
1
powder. Mp: 241–242◦C. H NMR (CD2Cl2): δ 7.06
(dd, J = 7.5 Hz, 7.5 Hz, 4H, p-Ph), 7.12 (d, J =
8.6 Hz, 4H, phenylene), 7.17 (d, J = 7.5 Hz, 8H,
o-Ph), 7.29 (d, J = 8.6 Hz, 4H, phenylene), 7.29–7.39
(m, 14H, m- and p-Ph), 7.53 (s, 1H, BDF), 7.72
Heteroatom Chemistry DOI 10.1002/hc

Modular Synthesis and Photophysical and Electrochemical Properties of 2,3,5,6-Tetraaryl-Substituted Benzo[1,2-b:5,4-bꢀ]difurans 323
against Fc/Fc+. CV was measured with a scan rate
of 100 mV/s.
Crystallographic data reported in this
manuscript have been deposited with Cambridge
Crystallographic Data Centre as supplementary pub-
lication nos. CCDC-788664 (4c) and CCDC-788663
(4f). Copies of the data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge, CB2 1EZ, UK; fax: +44
1223 336033; or deposit@ccdc.cam.ac.uk).
(s, 1H, BDF), 7.72 (d, J = 7.5 Hz, 4H, o-Ph). 13C
NMR (CD2Cl2): δ 93.4 (CH), 109.3 (CH), 116.8 (C),
122.7 (CH), 122.9 (CH), 124.5 (CH), 125.6 (C), 126.4
(CH), 127.0 (C), 127.8 (CH), 128.1 (CH), 129.0 (CH),
130.1 (CH), 130.6 (C), 147.0 (C), 147.1 (C), 150.3
(C), 152.5 (C). MS (APCI+): 797 (M+H). Anal. Calcd
for C58H40N2O2: C, 87.41; H, 5.06; N, 3.52. Found: C,
87.45; H, 5.16; N, 3.39.
3,5-Bis[4-(9H-carbazolyl)phenyl]-2,6-diphenylbe-
nzo[1,2-b:5,4-bꢀ]difuran (4e). White powder. Mp:
1
312–313◦C. H NMR: δ 7.30 (dd, J = 8.0 Hz, 8.0
Hz, 4H, Ar), 7.37 (dd, J = 7.5 Hz, 7.5 Hz, 2H, Ar),
7.40–7.44 (m, 8H, Ar), 7.55 (d, J = 8.0 Hz, 4H, Ar),
7,71 (d, J = 8.0 Hz, 4H, Ar), 7,72 (s, 1H, BDF),
7.76 (d, J = 7.5 Hz, 4H, Ar), 7.80 (d, J = 8.0 Hz,
4H, Ar), 7.80 (s, 1H, BDF), 8.15 (d, J = 8.0 Hz, 4H,
Ar). 13C NMR (CDCl2CDCl2): δ 94.6, 109.2, 110.0,
116.6, 120.4, 120.5, 123.4, 126.3, 127.2, 127.3, 128.8,
130.5, 131.3, 131.8, 137.1, 140.5, 151.7, 153.0 (three
carbons are overlapped). MS (APCI+): 793 (M+H).
Anal. Calcd for C58H36N2O2: C, 87.86; H, 4.58; N,
3.53. Found: C, 88.03; H, 4.81; N, 3.39.
ACKNOWLEDGMENTS
The authors would like to thank MEXT (KAKENHI
for E.N., No. 22000008, H.T., No. 20685005) and the
Global COE Program for Chemistry Innovation. This
work was partly supported by the Strategic Promo-
tion of Innovative R&D, JST. CM thanks JSPS for
the Research Fellowship for Young Scientists (No.
21·9262).
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