Single crystal biphenyl end-capped furan-incorporated oligomers: Influence of unusual packing structure on carrier mobility and luminescence

Article abstract of DOI:10.1039/c3tc30220b

We report the synthesis and characterization of two new furan-based biphenyl end-capped oligomers, 2-([1,1′-biphenyl]-4-yl)-5-(5-([1,1′- biphenyl]-4-yl)thiophen-2-yl)furan (BPFT) and 5,5′-di([1,1′- biphenyl]-4-yl)-2,2′-bifuran (BP2F) as candidate semiconductors for organic light-emitting field effect transistors (OLETs). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed the high thermostability of these furan-based semiconductors. X-Ray crystallography of single crystals grown by physical vapor transfer (PVT) method revealed a complicated herringbone packing of BPFT stacking with unusual flat and bent structures, which is different from that of BP2F and the bithiophene-based analogue 5,5′-di([1,1′-biphenyl]-4-yl)-2,2′-bithiophene (BP2T). BPFT single crystal showed a higher absolute quantum yield (51%) compared to that of BP2F and BP2T. Density Functional Theory (DFT) calculations showed that the different excitation energies between flat and bent structures led to the asymmetric transition dipoles in dark state of BPFT H-aggregates, which explains the highest PLQY of BPFT single crystal. Single crystal FET based on BPFT showed an ambipolar characteristic with high hole and electron mobilities, while single crystal FET based on BP2F exhibited p-type characteristic with a high hole mobility. Light emission was observed from the single-crystal FET based on BPFT. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3tc30220b

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Materials Chemistry C
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PAPER
Single crystal biphenyl end-capped furan-incorporated
oligomers: influence of unusual packing structure on
carrier mobility and luminescence†
Cite this: J. Mater. Chem. C, 2013, 1,
4163
a
a
a
Kazuaki Oniwa, Thangavel Kanagasekaran, Tienan Jin, Md. Akhtaruzzaman,b
*
Yoshinori Yamamoto,ac Hiroyuki Tamura,a Ikutaro Hamada,a Hidekazu Shimotani,d
Naoki Asao, Susumu Ikeda and Katsumi Tanigaki
a
a
a
*
*
We report the synthesis and characterization of two new furan-based biphenyl end-capped oligomers, 2-
([1,10-biphenyl]-4-yl)-5-(5-([1,10-biphenyl]-4-yl)thiophen-2-yl)furan (BPFT) and 5,50-di([1,10-biphenyl]-4-yl)-
2,20-bifuran (BP2F) as candidate semiconductors for organic light-emitting field effect transistors (OLETs).
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed the high
thermostability of these furan-based semiconductors. X-Ray crystallography of single crystals grown by
physical vapor transfer (PVT) method revealed a complicated herringbone packing of BPFT stacking with
unusual flat and bent structures, which is different from that of BP2F and the bithiophene-based
analogue 5,50-di([1,10-biphenyl]-4-yl)-2,20-bithiophene (BP2T). BPFT single crystal showed
a higher
absolute quantum yield (51%) compared to that of BP2F and BP2T. Density Functional Theory (DFT)
calculations showed that the different excitation energies between flat and bent structures led to the
asymmetric transition dipoles in dark state of BPFT H-aggregates, which explains the highest PLQY of
BPFT single crystal. Single crystal FET based on BPFT showed an ambipolar characteristic with high hole
and electron mobilities, while single crystal FET based on BP2F exhibited p-type characteristic with a
high hole mobility. Light emission was observed from the single-crystal FET based on BPFT.
Received 2nd February 2013
Accepted 26th April 2013
DOI: 10.1039/c3tc30220b
www.rsc.org/MaterialsC
characteristic of those materials such as oligofurans, limited
the electron injection to generate exciton results weak emission
intensity,2c these observations implied the potential application
opportunities of the furan-based semiconductors in organic
light-emitting eld effect transistors (OLETs).
Introduction
Furan-based organic semiconductors are of increasing interest
in organic electronics,1–4 even though they have attracted far
less attention than the well-known thiophene analogues due to
the lower stability of furan in oxidative conditions.5 Recent
advances on furan-based organic semiconductors in device
applications including organic eld-effect transistors (OFETs),2
organic light-emitting diode (OLEDs),3 and organic photovol-
taics (OPVs),4 demonstrated their sufficient stability, high
carrier mobility and more efficient uorescent property than
thiophene analogues. Although the p-type eld-effect
OLETs are one of the most promising multifunctional devices
that combine both switching properties of transistors and the
emission capability of LEDs in a simple device.6–9 OLETs require
the materials to possess both high carrier mobility and high
photoluminescence quantum yields (PLQYs) to realize the
potential applications in active matrix full color display and
electrically driven organic laser.6b However, in most cases,
organic semiconductors with high carrier mobility obtained from
the strong intermolecular p–p stacking lead to luminescence
quenching in the solid state due to the singlet ssion or exciton
quenching. For examples, rubrene, tetracene, and pentacene
exhibited superior FET mobility but they have low or no emis-
sion,10 while the PLQYs are vice versa.11 Nevertheless, most
recently the maximized charge mobility and the absolute PLQY
have been achieved by Perepichka and co-workers using 2-(4-
hexylphenylvinyl)anthracene (HPVAnt) as an OLET material in
the crystalline state.8 An alternative candidate for OLET materials
is thiophene/phenylene co-oligomers, which possess high lumi-
nescence efficiency and ambipolar FET behavior.9 For example,
aWPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University,
Sendai, 980-8577, Japan. E-mail: tjin@m.tohoku.ac.jp; sikeda@m.tohoku.ac.jp;
tanigaki@sspns.phys.tohoku.ac.jp; Fax: +81-22-217-5979; Tel: +81-22-217-6177
bDepartment of Chemistry, Faculty of Science, University of Malaya, 50603, Kuala
Lumpur, Malaysia
cState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian,
116012, P. R. China
dGraduate School of Science, Department of Physics, Tohoku University, Sendai, 980-
8577, Japan
† Electronic supplementary information (ESI) available: Thin lm OFET
characterizations, theoretical calculation data, and crystal data. CCDC 922141
and 922142. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3tc30220b
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recently we reported that FETs based on 5,50-di([1,10-biphenyl]-4-
yl)-2,20-bithiophene (BP2T) single crystal showed high ambipolar
carrier mobility and strong edge emission.9b However, in the
preliminary measurements, we observed that BP2T single crystal
has a moderate PLQY of 38% (vide infra). Considering that,
materials containing oxygen atom which has a smaller van der
Waals radius than sulfur atom may lead to closer packing
structures, and in terms of efficient uorescent property of oli-
gofurans,1 we envisioned that the partial or full replacement of
sulfur to oxygen in BP2T will supply the elevated materials with
both increased carrier mobility and luminescence efficiency. We
report here on the synthesis, characterization, implementation in
OFETs, and light-emitting behavior of two new biphenyl end-
capped furan-incorporated oligomers based on 2-([1,10-biphenyl]-
4-yl)-5-(5-([1,10-biphenyl]-4-yl)thiophen-2-yl)furan (BPFT) and 5,50-
di([1,10-biphenyl]-4-yl)-2,20-bifuran (BP2F) (Scheme 1). Interest-
ingly, the new BPFT single crystal containing thienylfuran moiety
showed two different at and bent structures which stack with
each other in a herringbone manner. OFETs based on BPFT show
an ambipolar characteristic with a higher absolute PLQY of 51%
than that of BP2T and BP2F in the crystalline state.
Fig. 1 DSC (a) and TGA (b) of BP2T, BPFT and BP2F.
(321 ꢀC) and BP2T (370 ꢀC). The thermogravimetric analysis
(TGA) revealed that a 10% weight was lost at 445, 413, and
398 ꢀC for BP2T, BPFT, and BP2F, respectively. The DSC and
TGA results revealed the high thermal stability of the furan-
based new materials BPFT and BP2F (Fig. 1).
Single crystal structures
Single crystals of the three materials were grown by a physical
vapor transport (PVT) method under argon gas atmosphere.
Single crystals of BP2T and BPFT were obtained as a lamella
shape, whereas BP2F showed a needle-like single crystal.12 The
crystal packing structures and conformations were determined
by means of X-ray crystallography. As shown in Fig. 2a and b,
Results and discussion
Synthesis and characterization
As shown in Scheme 1, BPFT was synthesized by dibromination BP2F in single crystals stack in a slipped herringbone manner
of 2- and 20-positions of 2-(20-thieno)furan followed by the with a completely at structure, and BPFT shows a herringbone
double Suzuki–Miyaura coupling with biphenyl boronic acid in structure similar to that of BP2T with the long axes of molecules
high yield. BP2F was prepared through Suzuki–Miyaura oriented in parallel with the herringbone angle of 59ꢀ. It is
coupling of 2-furanyl boronic acid with 4-bromo-1,10-biphenyl interesting that BPFT exhibited a more complicated herring-
followed by copper-mediated homo-coupling of the lithiated bone packing arrangement than BP2T and BP2F due to its
resulting 2-[(1,10-biphenyl)-4-yl]furan. These three materials unusual two molecular geometries, at and bent structures.
were puried by thermal sublimation under reduced pressure BPFT displayed an alternate at–bent packing along the z axis
for 2 times due to their poor solubility in common organic and at–at as well as bent–bent parallel packing along x axis
solvents. The structures of BPFT and BP2F were determined by (Fig. 2e and f). As a result, the intermolecular distances (d1 vs.
elemental analysis and X-ray crystallography. Among the d4, d2 vs. d5, d3 vs. d6) between those adjacent molecules in BPFT
differential scanning calorimetry (DSC) measurements, BPFT from both the thiophene side and the furan side are totally
ꢀ
showed the lowest melting points of 297 C compared to BP2F different; the shortest and the longest distances were shown to
Scheme 1 Synthesis of BPFT and BP2F as well as the structure of BP2T. Reaction conditions: (i) NBS, CH2Cl2, rt, ultrasound, 5 min. (ii) 1,10-Biphenyl-4-yl boronic acid,
Pd(PtBu3)2, KF, THF, rt. (iii) Furan-2-yl boronic acid, Pd(PtBu3)2, KF, THF, rt. (iv) n-BuLi, CuCl2, THF, ꢁ78 ꢀC to rt.
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Fig. 2 Packing structures of (a) slipped herringbone packing with flat structure of BP2F, (b) herringbone packing with flat and bent structures of BPFT, and intermolecular
distances of (c) BP2T, (d) BP2F, (e) BPFT from thiophene side, (f) BPFT from furan side. Hydrogen atoms are omitted for clarity. Red: oxygen atom, yellow: sulfur atom.
˚
Table 1 Summarized intermolecular distance (A)
BPFT
BP2T
S-sidea
O-sideb
BP2F
d1
d2
d3
d4
d5
d6
3.69
3.71
3.84
—
—
—
3.64
3.60
3.88
3.60
3.54
3.91
3.54
3.62
3.65
3.74
3.79
3.61
3.63
3.60
3.64
—
—
—
a
Thiophene side of BPFT. b Furan side of BPFT.
Table 2 Summarized optical properties
CHCl3 solution
Single crystal
labs
c
(nm)
lema (nm)
Ffb (%)
lem (nm)
Ffb (%)
BP2T
BPFT
BP2F
390
393
386
456, 481
448, 474
431, 455
25
40
80
587
549
516
38
51
37
a
b
Excitation wavelength is 390 nm for all compounds. Absolute
quantum yield determined by integrating sphere system. Ff :
c
photoluminescence quantum yield. Excitation wavelength: 420 nm
for BP2T, 380 nm for BPFT, 350 nm for BP2F.
˚
˚
be 3.54 A and 3.91 A, respectively (Table 1). These clear differ-
ences between BPFT and BP2F or BP2T in crystal structures
might be ascribed to the asymmetric structure of BPFT which
has the different ring size of thiophene and furan. The average
Fig. 3 Optical properties of BP2T, BPFT, and BP2F. (a) Absorption (solid line) and
fluorescent (dot line) spectra in CHCl3 and (b) photoluminescence spectra in the
crystalline state.
˚
intermolecular distance of 3.67 A in BPFT and the shortest
Optical property
˚
˚
distance of 3.60 A in BP2F are shorter than in BP2T (3.69 A)
(Fig. 2c and d, Table 1),13 indicating the tighter packing struc- Optical properties of the three materials were characterized in
tures of BPFT and BP2F than that of BP2T. This result is chloroform solution and in crystalline state, respectively, as
consistent with the trend in oligofurans and oligothiophenes. summarized in Table 2. As shown in Fig. 3a, the UV/vis spectra
We expected that the packing mode differences between BPFT, of the three compounds in chloroform showed almost similar
BP2F, and BP2T in crystalline state may exhibit distinct opto- absorption maxima (lmax) in the range of 386–393 nm. In
electronic and FET properties.
contrast, the clear blue-shis of emission maxima (lem) of BPFT
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Table 3 OFET performances of BP2T, BPFT, and BP2F in single crystalsa
mh
Vth (V)
me
Vth (V)
BP2T
BPFT
BP2F
0.08
0.27
0.32
ꢁ60
ꢁ45
ꢁ7
0.011
0.013
—
92
94
—
b
b
a
b
PMMA was used as insulator on SiO2 substrate. Not observed. Vth
threshold voltage; mh: hole mobility; me: electron mobility; Ion/Ioff
current on/off ratios.
:
:
and BP2F were observed compared to BP2T. The smaller stokes
shi in the furan-based molecules indicates that BPFT and
BP2F structures are more rigid than BP2T due to their more
quinoid structural characteristics. PLQY (Ff) for BP2F in solu-
tion is very high (80%) whereas the lower values were observed
for BPFT (40%) and BP2T (25%), suggested the signicant furan
effect on photoluminescence property. This quantum yield
enhancement can be explained by the decreased heavy atom
effect from sulfur to oxygen atom and the more molecular
rigidity of BP2F and BPFT relative to BP2T.
The uorescence spectra of the three materials in the crys-
talline state showed a signicant red-shi compared to that in
solution, implying that the energy gap of band structures
between the valence and conduction band caused by single
crystals with a highly ordered molecular packing, is smaller
than the HOMO–LUMO gap in the single molecule (Table 2 and
Fig. 3b). In addition, the blue shi of BPFT and BP2F compared
to BP2T explains their stronger p–p interaction in the single
crystals. The absolute PLQYs in crystalline state were measured
in an integrating sphere. The higher PLQY of 51% was observed
for single crystal BPFT than that for BP2F (37%) and BP2T (38%)
(Table 1). Time dependent density functional theory (TD-DFT)
calculations using B3LYP/6-31G++(d,p) as an exchange-correla-
tion functional and basis set showed that the energies of singlet
exciton (S1) are 2.63 eV for BP2T, 2.69 eV for BPFT, and 2.77 eV
for BP2F, respectively (Table S2†). The energies of the two
triplets (T1) of the three compounds were calculated to be in the
range of 1.66–1.85 eV, which are unfavorable for generating
singlet ssion (S1 < 2T1).8,10 We also evaluated excitation energy
and transition dipole moment by TDDFT from their aggregates
(Table S3†).14 Although the opposite transition dipole cancels
out in the dark state of the BP2T H-aggregates which takes
ordinary herringbone packing, the transition dipole does not
completely cancel out in the dark state of the BPFT H-aggregates
consisting of the bent and at molecules. This is due to the
exciton population in BPFT the at molecules tend to be larger
than at the bent molecules because of the lower excitation
energy of at molecules than that of bent molecules; 2.93 eV for
at BPFT and 2.98 eV for bent BPFT (Table S4†).14 This calcu-
lation may explain the reason why the PLQY of BPFT crystal is
Fig. 4 Output (a) and transfer (b) characteristics of BPFT-based SC-FETs, output
(c) and transfer (d) characteristics of BP2F-based SC-FETs.
higher than that of BP2T, even though the transition dipole BP2F in crystalline state as mentioned in Fig. 3. These results
moments of the BP2T and BPFT single molecules are similar. In indicate that a slight modication from bithiophene to thienyl-
addition, despite the existence of bifuranyl moiety, the PLQY of furan or bifuran skeleton in a molecule leads to a dramatic
single crystal BP2F showed a similar uorescent efficiency to difference in the optical properties due to the varied packing
that of BP2T, which is ascribed to the strong p–p interaction of arrangement and the decreased heavy atom effect.
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Fig. 4a might be attributed to the time-dependent electron-
current decay as a result of the high applied gate voltage. The
similar phenomenon was also observed in other ambipolar SC-
FETs, such as BP3T.9a In the case of SC-FET based on BP2T
(Fig. S2†), the similar phenomenon might be exhibited if the
drain voltages are more than 140 V. The difference between
BPFT and BP2T should be related to the lower threshold
voltage (Vth) of BPFT than that of BP2T for holes (45 V vs. 60 V,
Table 3). SC-FETs based on BP2F exhibited only p-type char-
acteristic; the hole mobility increased up to 0.32 cm2 Vꢁ1 sꢁ1
due to the closed packing structure of BP2F single crystal
(Fig. 4c and d). The charge transfer integrals (t) among HOMOs
and LUMOs were calculated on the basis of the crystal struc-
tures to evaluate the experimental carrier mobility (Table
S6†).15 The maximum t values among HOMOs for BP2T, BPFT,
and BP2F in dimers were showed to be 45.0, 40.5, and 59.4
meV, while the maximum t values among LUMOs were 127.4,
94.9, and 66.7 meV, respectively. BP2F bearing bifuran unit
exhibited the highest transfer integrals among HOMOs, sug-
gesting its highest hole mobility which is in agreement with
the experimental result. Furthermore, the higher t values of
BP2T and BPFT than that of BP2F among LUMOs indicated
their ambipolar behavior. Moreover, the ratio of maximum and
minimum t values among HOMOs for BPFT is about 30 (40.5
meV vs. 1.4 meV), while the ratio is about 4 for BP2T (45.0 meV
vs. 12.8 meV), suggesting the much higher anisotropy for BPFT
than that for BP2T. The replacement of thiophene to furan in
these biphenyl end-capped oligomers did not show signicant
reorganization energy differences; the almost similar reorga-
nization energies for hole (0.21–0.22 eV) and electron (0.20–
0.21 eV) formation in the three materials were calculated
(Table S5†).
Fig. 5 Energy diagram of frontier orbitals calculated by DFT at the B3LYP/6-
31G++(d,p).
Fig. 6 Light emitting behavior of BPFT-based SC-FET.
OFET performances
FET properties based on these compounds were investigated
in thin lms and single crystals in bottom-gate and top-
contact conguration. The thin lm transistors (TFTs) were
fabricated by vacuum deposition of organic semiconductors
on the surface of PMMA pre-treated SiO2 substrate with gold as
source and drain electrodes (Fig. S1†). TFTs based on the
three compounds showed only p-type behavior, and the
slightly increased hole mobility was observed in BPFT
(0.031 cm2 Vꢁ1 sꢁ1) and BP2F (0.047 cm2 Vꢁ1 sꢁ1) compared to
that in BP2T (0.023 cm2 Vꢁ1 sꢁ1) under similar conditions
(Table S7†).
Single crystal FETs (SC-FETs) were also fabricated using Au
and Ca as the source and drain electrodes. Single crystals
grown by PVT method were electrostatically laminated on
PMMA pre-treated SiO2 substrate. The single crystal BPFT
behaves as an ambipolar semiconductor as shown in the
output and transfer characteristic curves in Fig. 4a, b and S3.†
The hole and electron mobility were extracted to be 0.27 cm2
Vꢁ1 sꢁ1 and 0.013 cm2 Vꢁ1 sꢁ1, respectively, by analysing the
saturation region (Table 3). The hole mobility of BPFT device
was higher than that in BP2T device (0.08 cm2 Vꢁ1 sꢁ1 for hole,
Fig. S2†) and the electron mobilities of both devices were
almost similar. The threshold voltages (Vth) of BPFT device in p
and n modes were deduced to be ꢁ45 V and 94 V. It should be
noted that, the steep increase in n-channel drain current at
high drain voltage indicates the onset of hole transport as
shown in Fig. 4a. However, the analogous electron current is
almost invisible in p-channel drain current because of the
much lower electron current than the hole current. Moreover,
the unusual shape of the electron current in n-channel
compared to the p-channel in the output characteristic of
HOMO and LUMO energy calculations
Theoretical calculations revealed that both HOMO and LUMO
levels of BPFT and BP2F, as well as their band gaps increased
by the introduction of furan ring (Fig. 5). The increased band
gaps are ascribed to the much increased LUMO levels of BPFT
and BP2F due to the wave function of LUMO localized on
hetero atoms. As a result, the introduction of furan moiety
enhanced the p-type FET characteristic due to the decreased
hole injection barrier between gold electrode (work function ¼
5.1 eV) and HOMO, as well as the increased electron injection
barrier between calcium electrode (work function ¼ 2.9 eV)
and LUMO.
Light emission SC-FETs
The emission was observed from the SC-FET based on BPFT
single crystal. Fig. 6 shows a series of light-emitting FET device
top images with various drain voltage (Vd) between 70 V and 120
V at a xed gate voltage (140 V). The clear light emission
appeared from the edge of single crystal BPFT at low Vd regime.
The intensity of the emission increased with increasing Vd. We
note that the similar light-emitting behavior of SC-FET based on
BP2F was not observed.
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Materials
Conclusion
The commercially available compounds were used as received.
The BP2T was purchased from Kanto Chemical Co., Inc., and 2-
(2-thienyl)furan (1) was purchased from Aldrich. Single crystals
of BP2T, BPFT, and BP2F were grown by PVT method, and the
structures of BPFT and BP2F were determined by elemental
analysis and X-ray crystallography.
In conclusion, we have disclosed that a new series of fur-
an-based BPFT and BP2F showed distinct optoelectronic and
FET properties relative to the thiophene analogue BP2T
material. The PLQY of BPFT crystal is higher than that of BP2T
and BP2F due to the unusual packing arrangement of BPFT
consisting of at and bent structures, even though BPFT has a
more dense packing structure than that of BP2T. Our DFT
calculations showed that the different excitation energy
between at and bent structures led to the asymmetric tran-
sition dipoles in dark state of BPFT H-aggregates, which
explains the highest PLQY of BPFT single crystal. OFET based
on BPFT exhibited an ambipolar characteristic, while
BP2F showed a unipolar behavior with a high hole mobility.
The bright light emitting FET was observed upon the applied
drain voltage demonstrating the good electroluminescent effi-
ciency of BPFT single crystal. This observation provides a
promising strategy to develop new OLET materials which
possess both high photoluminescence efficiency and charge
mobility.
Synthesis of 2-bromo-5-(5-bromothiophen-2-yl)furan (2)
A mixture of 2-(thiophen-2-yl)furan (1) (1 mmol, 150 mg) and N-
bromosuccinimide (2 mmol, 365 mg) in CH2Cl2 (15 mL) was
subjected to ultrasonic irradiation for 3 min at rt. Reaction
mixture was puried by orisil column chromatography
(eluent : hexane), giving the corresponding 2-bromo-5-(5-bro-
mothiophen-2-yl)furan (2) in 98% (300 mg) yield as a yellow oil.
1H NMR (400 MHz, CDCl3): d 6.97 (d, J ¼ 4.4 Hz, 1H), 6.96 (d, J ¼
4.4 Hz, 1H), 6.39 (d, J ¼ 3.2 Hz, 1H), 6.34 (d, J ¼ 3.2 Hz, 1H); 13
C
NMR (100 MHz, CDCl3): d 150.09, 133.90, 130.44, 122.88,
121.38, 113.39, 111.63, 107.60.
Synthesis of BPFT
To a mixture of 4-biphenylboronic acid (4 equiv., 2.61 g), Pd[P(t-
Bu3)]2 (2 mol%, 40 mg), KF (1.5 g) was added the 2-bromo-5-(5-
bromothiophen-2-yl)furan (2) (3.3 mmol, 1.02 g) in THF (20 mL)
solution under N2 atmosphere. The reaction mixture was stirred
for 12 h at rt. To the reaction mixture was added MeOH (10 mL)
and the resulting residue was ltered by washing with water and
MeOH. The residue was further puried by sublimation at 300
ꢀC (yellow solid; 1.30 g, 87%). Anal. calcd for C32H22OS: C 84.55,
H 4.88, O 3.52, S 7.05; found: C 84.60, H 5.09, S 7.04%.
Experimental
General information
1H NMR and 13C NMR spectra were recorded on JEOL JNM AL
400 (400 MHz) spectrometers. H NMR spectra are reported as
follows: chemical shi in ppm (d) relative to the chemical shi
of CDCl3 at 7.26 ppm, integration, multiplicities (s ¼ singlet,
d ¼ doublet, t ¼ triplet, q ¼ quartet, m ¼ multiplet and br ¼
broadened), and coupling constants (Hz). 13C NMR
spectra were recorded on JEOL JNM AL 400 (100.5 MHz)
1
spectrometers with complete proton decoupling, and chemical Synthesis of 2-([1,10-biphenyl]-4-yl)furan (4)
shi reported in ppm (d) relative to the central line of triplet
4-Bromobiphenyl (3) (4 mmol, 932 mg), 2-furanboronic acid (1.2
for CDCl3 at 77 ppm. UV/vis absorption spectra were
recorded on a JASCO V-650DS spectrometer. Fluorescence
spectra were recorded on a HITACHI F-7000 spectrophotom-
eter and absolute uorescence quantum yields were measured
by a photon-counting method using an integration sphere on a
Hamamatsu Photons C9920-02 spectrometer. Elemental anal-
yses were measured on J-SCIENCE Lab JM-10 and YANAKO
HNS-ah/HSU-20 in Research and Analytical Center for Giant
equiv., 560 mg), Pd[P(t-Bu3)]2 (2 mol%, 41 mg) and KF (900 mg)
were dissolved in THF (10 mL) under nitrogen atmosphere. The
mixture was stirred for 12 h at rt. The resulting mixture was
poured into water and extracted by CH2Cl2. Aer concentration,
the residue was puried by silica gel chromatography, giving
the corresponding 2-([1,10-biphenyl]-4-yl)furan (4) in 89% (780
mg) yield as a white solid. 1H NMR (400 MHz, CDCl3): d 7.75 (d,
2H), 7.62 (d, 4H), 7.33–7.50 (m, 3H), 7.32–7.37 (m, 1H), 6.69 (d,
1H), 6.42–6.40 (m, 1H); 13C NMR (100 MHz, CDCl3): d 153.62,
142.05, 140.48, 139.85, 129.75, 128.72, 128.71, 127.25, 126.81,
124.09, 111.67, 105.08.
Molecules, Tohoku University. DSC was measured by
a
RIGAKU DSC8230 using N2 atmosphere at a scan rate of 10 K
minꢁ1. TGA was measured by a RIGAKU TAG8120. X-Ray
analysis was carried out at ꢁ180 ꢀC with a Rigaku VariMax with
RAPID diffraction by using graphite monochromated Cu-Ka
radiation. The structures were solved by direct method.
Synthesis of BP2F
Column chromatography was carried out employing silica gel To a THF (45 mL) solution of 2-([1,10-biphenyl]-4-yl)furan (4) (3
60 N (spherical, neutral, 40–100 mm, KANTO Chemical Co.). mmol, 670 mg) was added n-BuLi (1.57 M in n-hexane, 3 mmol,
Analytical thin-layer chromatography (TLC) was performed on 1.92 mL) at ꢁ78 ꢀC. Aer stirring for 0.5 h, the reaction mixture
ꢀ
0.2 mm precoated plate Kieselgel 60 F254 (Merck). Single was warmed gradually to ꢁ40 C. CuCl2 (3 mmol, 420 mg) was
crystals of BP2T, BPFT, and BP2F were grown by a physical added, and the mixture kept for 1 h at this temperature followed
vapor transport (PVT) method under argon gas atmosphere by warming to rt. Aer stirring at rt for 5 h, the mixture was
using a separation temperature controller (AMF-9P-III, ASAHI ltered and the resulting residue was washed by water and
RIKA).
MeOH. Aer sublimation at 310 ꢀC, BP2F was obtained as a
4168 | J. Mater. Chem. C, 2013, 1, 4163–4170
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Journal of Materials Chemistry C
yellow-green solid in 39% yield (260 mg). Anal. calcd for
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C
32H22O2: C 87.65, H 5.06, O 7.30; found: C 87.93, H 5.22%.
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P. M. Beaujuge and J. M. Frechet, J. Am. Chem. Soc., 2011,
BP2T, BPFT, and BP2F single crystals were grown by physical
vapor transport in a stream of argon gas with a purity of
99.9999%. Pure powders of BP2T, BPFT, and BP2F (1–5 mg)
were placed at the end of the tube on aluminum foil. Aer
heating for 12 h, the good lm-like crystals of BP2T and BPFT
were obtained. The BP2F single crystal showed a needle-like
shape. A highly doped silicon wafer with a 200 nm thermally
grown SiO2 layer was covered with a 60 nm thick lm of PMMA
by spin coating using toluene as solvent. The resulting lm was
annealed at 90 ꢀC for 50 h in the glove box lled with argon gas.
A thin crystal was laminated on this PMMA pretreated SiO2
substrate. The top contact symmetric and asymmetric elec-
trodes were realized by thermally evaporating gold and calcium
metals through a shadow mask on top of the crystals. Electrical
and optical characterizations were performed in the glove box
under an inert Ar atmosphere by using a semiconductor
parameter analyzer (Agilent Technology B1500A) and a CCD
camera through an optical microscope. The observed light
emission images were captured with a CCD camera.
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Acknowledgements
This research was supported by the World Premier Interna-
tional Research Center Initiative (WPI), MEXT, Japan. We thank
Prof. N. Kobayashi and Dr S. Shimizu at Tohoku University,
Prof. T. Minato at Kyoto University, Prof. T. Matsuo at Kinki
University, Dr T. Otani at Waseda University for their help with
uorescence measurements, Prof. N. Teramae and Prof.
S. Nishizawa at Tohoku University for their help with thermal
analysis measurements, and Dr E. Kwon at Tohoku University
for X-ray single crystal analysis. Some of the calculations were
performed using the supercomputing resources at Cyberscience
Center, Tohoku University.
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Journal of Materials Chemistry C
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4170 | J. Mater. Chem. C, 2013, 1, 4163–4170
This journal is ª The Royal Society of Chemistry 2013