6: H -Benzo[4,5]thieno[2,3- b] indole as a novel donor for efficient thermally activated delayed fluorescence emitters with EQEs over 20%

Article abstract of DOI:10.1039/c9tc04679h

In this report, we developed two novel indole-based electron donors, 6H-benzo[4,5]thieno[2,3-b]indole (synBTI) and 6H-benzofuro[2,3-b]indole (synBFI), for designing green thermally activated delayed fluorescence (TADF) emitters. Two TADF emitters, 2-(6H-b

Full text of DOI:10.1039/c9tc04679h

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Materials Chemistry C
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6H-Benzo[4,5]thieno[2,3-b]indole as a novel
donor for efficient thermally activated delayed
fluorescence emitters with EQEs over 20%†
Cite this: J. Mater. Chem. C, 2019,
7, 13912
Rajendra Kumar Konidena, Kyung Hyung Lee and Jun Yeob Lee
*
In this report, we developed two novel indole-based electron donors, 6H-benzo[4,5]thieno[2,3-b]indole
(synBTI) and 6H-benzofuro[2,3-b]indole (synBFI), for designing green thermally activated delayed
fluorescence (TADF) emitters. Two TADF emitters, 2-(6H-benzo[4,5]thieno[2,3-b]indol-6-yl)-5-(4,6-
diphenyl-1,3,5-triazin-2-yl)benzonitrile (BTITrz) and 2-(6H-benzofuro[2,3-b]indol-6-yl)-5-(4,6-diphenyl-
1,3,5-triazin-2-yl)benzonitrile (BFITrz), were synthesized by integrating the two new donors with an
aryltriazine acceptor. The effects of the heteroatoms in the donor units on the photophysical and
electroluminescence properties of the TADF emitters were investigated in detail. The photophysical
analysis revealed that BTITrz showed a relatively small singlet–triplet energy splitting, short delayed
fluorescence lifetime and high photoluminescence quantum yield (PLQY) compared to its congener
BFITrz. As a result, the organic light emitting diode fabricated using the BTITrz emitter exhibited superior
device performance with a maximum external quantum efficiency (EQE) of 20.7% and current efficiency
(CE) of 63.6 cd AÀ1. These results suggest that synBTI can function as a potential donor for designing effi-
cient TADF emitters.
Received 24th August 2019,
Accepted 16th October 2019
DOI: 10.1039/c9tc04679h
rsc.li/materials-c
electron acceptors such as cyano, aryltriazine, sulfone, phos-
phine oxide, oxadiazole, benzophenone, pyrazine, arylborane,
Introduction
Recently, metal-free thermally activated delayed fluorescence pyridine, and pyrimidine have been developed for the design of
(TADF) emitters have emerged as the most promising alterna- blue and green TADF emitters.1–3 However, potential electron
tive to traditional phosphorescent organic light-emitting diodes donors are mostly limited to carbazole and acridine derivatives.4
(PHOLEDs), which can reach 100% internal quantum efficiency Therefore, the development of efficient new electron donors for
(IQE), due to their ability to harvest non-emissive triplet exci- blue/green TADF emitters is highly desirable to expand the
tons through reverse inter-system crossing (RISC) by manipu- donor library.
lating the singlet–triplet energy splitting (DEST).1,2
Herein, we report two novel electron donors, viz., 6H-benzo-
The small DEST and efficient RISC are the two important [4,5]thieno[2,3-b]indole (synBTI) and 6H-benzofuro[2,3-b]indole
requirements for designing potential TADF emitters.1,2 (synBFI), featuring different heteroatoms (O or S) to develop
To achieve the small DEST, it is essential to minimize the green TADF emitters. 2-(6H-Benzo[4,5]thieno[2,3-b]indol-6-yl)-5-
spatial orbital overlap between the highest occupied molecular (4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (BTITrz) and 2-(6H-
orbital (HOMO) and the lowest unoccupied molecular orbital benzofuro[2,3-b]indol-6-yl)-5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzo-
(LUMO).1–3 In this regard, the development of donor–acceptor nitrile (BFITrz) were synthesized by combining the two donors
type bipolar molecular systems with or without p-conjugated synBTI and synBFI with an aryltriazine acceptor, respectively.
linkers has been exemplified as the most promising approach The effects of the heteroatoms in the donor scaffolds on the
to ensure the spatially separated HOMO and LUMO in the photophysical and electroluminescence properties of the TADF
donor and acceptor units.1–3 It has been demonstrated that the emitters were studied in detail. The BTITrz material showed a
choice of donor and acceptor units plays an important role in small DEST, a short delayed fluorescence lifetime (td) and a high
achieving superior device performances.1–3 So far, numerous photoluminescence quantum yield (PLQY) compared to BFITrz
due to the relatively strong donor character of the synBTI unit.
The potential of these materials as TADF emitters was tested in
multilayered OLED devices. The BTITrz-based device demon-
School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro,
Jangan-gu, Suwon, Gyeonggi 440-746, Korea. E-mail: leej17@skku.edu
strated excellent performance with a maximum EQE of 20.7%
and CE of 63.6 cd AÀ1
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9tc04679h
.
13912 | J. Mater. Chem. C, 2019, 7, 13912--13919
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Results and discussion
Molecular design and synthesis
on their model compounds using a B3LYP/6-31G(d,p) basis set.
The optimized geometries and their frontier molecular orbital
distributions (HOMO/LUMO) of the compounds are portrayed in
Fig. 1 and 2. The compounds showed largely distorted structures
between the donors and aryltrizine acceptor with a dihedral
angle of above 501 due to the severe steric crowding between the
donor and cyano acceptor on the phenyl linker. As a result, the
HOMOs of the compounds were mainly localized on the synBTI
or synBFI donor units, while the LUMOs were delocalized over
the aryltriazine acceptor and cyanophenyl bridge. As expected
from the FMO distribution, time dependent (TD)-DFT calculations
were revealed that the compounds had a small DEST of 0.16 eV
with a singlet energy (S1) of 2.56 eV and a triplet energy (T1) of
2.40 eV for BTITrz and DEST of 0.19 eV with S1 of 2.58 eV and T1
of 2.39 eV for BFITrz. These results indicate that the com-
pounds would trigger effective reverse intersystem crossing for
TADF emission.
Our molecular design was mainly intended to develop novel
heterocycle fused indole-based donors (synBTI and synBFI) for
blue/green TADF emitters. We focused on studying the effects
of the heteroatoms (O or S) in the donor scaffolds on the
photophysical and electroluminescence properties of the TADF
emitters. The employed synthetic procedures to synthesize the
target materials are schematized in Scheme 1. The synBTI and
synBFI donors were synthesized in a two step synthetic protocol
starting from the palladium-catalyzed Suzuki coupling reaction
between boronic acids of 1a and 1b with 2-bromonitrobenzene to
produce 2a and 2b, respectively.5 Subsequently, Cadogan ring
closing reactions were performed on intermediates 2a and 2b to
yield novel donors of synBTI and synBFI, respectively.6 Finally, the
synBTI and synBFI donors were reacted with the acceptor scaffold
5-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-fluorobenzonitrile under cesium
carbonate mediated N-arylation reaction conditions to offer the
target green TADF emitters BTITrz and BFITrz in excellent
yields after column purifications and vacuum train sublimation
procedures. The chemical structures of the compounds were
analyzed by nuclear magnetic resonance spectroscopy (1H and
13C NMR) and mass spectroscopy (MS) measurements.
Electrochemical and thermal properties
The redox behaviors of these compounds were investigated by
using cyclic voltammetry measurements in a dilute dichloro-
methane solution with 0.1 M tetrabutylammonium perchlorate
as a supporting electrolyte. The oxidation potentials of the
compounds were calibrated by using an Fc/Fc+ redox couple
and the pertinent data are listed in Table 1. The electron
donating strengths of the organic chromophores can be esti-
Theoretical calculations
To estimate the frontier molecular orbital (FMO) distributions mated based on their oxidation potentials. To understand the
and singlet–triplet energy splittings (DEST) of these compounds, donor strengths of these new donors, their oxidation potentials
density functional theoretical (DFT) calculations were performed were collected. Interestingly, synBTI showed a relatively small
Scheme 1 Synthetic approach of target materials.
Fig. 1 Optimized geometries of the compounds.
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Fig. 2 FMO distributions of the compounds calculated employing the B3LYP/6-31G(d,p) basis set of the Gaussian 09 program.
Table 1 Photophysical, electrochemical and thermal data of the compounds
b
labsa, nm
lem (nm),
HOMO/
ES/ET (eV) DEST (eV) LUMOe (eV) Eg (eV) tp/td (ns/ms) Fph/Fd /FPL
Compound (emax, MÀ1 cmÀ1 Â 103) (film)
Tonset (1C)
c
d
f
g
i
j
k
BTITrz
BFITrz
381 (12.8), 271 (107.8)
379 (9.3), 270 (83.3)
526
518
2.79/2.76
2.82/2.75
0.03
0.07
5.79/3.48
5.84/3.47
2.31
2.37
13.1/4.3
14.6/26.6
0.20/0.46/0.66 441
0.16/0.34/0.50 419
a
b
c
Collected in THF solution. Collected for 10 wt% doped films in DPEPO. Calculated from the onsets of fluorescence and phosphorescence
d
e
f
spectra. Calculated singlet–triplet energy gap. HOMO and LUMO estimated from the cyclic voltammograms. Electrochemical band gap.
g
h
i
Prompt (tp) and delayed (td) fluorescence lifetimes. Prompt fluorescence (Fp). Delayed (Fd) PLQY determined from the total PLQY and the
j
proportion of the integrated area of the individual components in the transient PL spectra to the total integrated area. The absolute PLQY of
the doped films in DPEPO measured using an integrating sphere in a nitrogen atmosphere at room temperature. The temperature corresponds
k
to 5% weight loss (Tonset).
oxidation potential (+0.86 eV) compared to its analogue synBFI Therefore, the compounds exhibited desirable thermal stability
(+0.92 eV), indicating a strong donor character of the former. for vapor deposited device fabrication.
This could be attributed to the low electronegativity of the
Photophysical properties
sulfur atom in synBTI compared to the oxygen atom in synBFI.
Consequently, BTITrz revealed a high oxidation propensity The photophysica1 properties of these compounds were investi-
when compared to BFITrz. The HOMO and LUMO energy gated by using ultraviolet-visible (UV-vis) absorption and photo-
levels of the compounds were calculated from their onset luminescence (PL) emission measurements. The electronic
potentials of the oxidation and reduction waves with reference absorption spectra of the compounds recorded in tetrahydro-
to ferrocene. The corresponding HOMO/LUMO energy levels furan (10À5 M) and emission spectra of 10 wt% doped films in a
of BTITrz and BFITrz were À5.79 eV/À3.48 eV and À5.84/ DPEPO host are presented in Fig. 4 and the relevant data are
À3.47, respectively.
listed in Table 1. The compounds mainly showed two indepen-
The thermal stability of these compounds was investigated dent absorption bands in the range of 250–400 nm. The short
using thermogravimetric analysis (TGA) and the TGA traces wavelength absorption peaks appearing below B300 nm are
are shown in Fig. 3 and the relevant data are listed in Table 1. assigned to the localized p–p* electronic transitions of the
Both BTITrz and BFITrz displayed excellent thermal stability, different aromatic conjugative units in the compounds. On the
with marked thermal decomposition temperatures (T5d) corres- other hand, the long wavelength absorption peak that appeared
ponding to 5% weight loss of 441 1C and 419 1C, respectively. above B350 nm is attributed to the intramolecular charge
13914 | J. Mater. Chem. C, 2019, 7, 13912--13919
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Fig. 3 TGA traces observed for the compounds.
Fig. 5 Phosphorescence spectra of the compounds recorded in THF
matrix at 77 K.
transfer (ICT) transition from the synBTI or synBFI donor to the
aryltriazine acceptor. Both compounds displayed a single emis-
sion peak, indicating that the fluorescence originated from
the charge transfer (CT) states as expected from their FMO
distribution. Furthermore, to understand the CT character of
the singlet excited state, solvatochromism study was performed
in different solvents by varying their Reichardt polarity index
from non-polar cyclohexane (CH) to polar dichloromethane
(MC). The compounds exhibited positive solvatochromism with
large degrees of bathochromic shift (Dlem) of 115 nm for BTITrz
and 105 nm for BFITrz, confirming the CT nature of the excited
singlet state.7 Interestingly, the compounds showed vibrational
features in non-polar CH and structureless spectra in the polar
solvent. The phosphorescence spectra of the compounds were
collected at low temperature in frozen THF matrix (Fig. 5). Both
compounds showed featureless emission profiles, indicating
that the triplet excited state has CT nature. The S1/T1 energies
of BTITrz and BFITrz estimated from their onset wavelengths of
the fluorescence and phosphorescence spectra were found to
be 2.79/2.76 eV and 2.82/2.75 eV, respectively. As a result, the
corresponding DESTs of BTITrz and BFITrz were calculated to
be 0.03 eV and 0.07 eV, respectively.
The TADF nature of these compounds was investigated by
transient PL measurements using 10 wt% emitter doped
DPEPO films at room temperature under vacuum and their
decay profiles are shown in Fig. 6. The compounds displayed
both prompt and delayed fluorescence components, confirming
the presence of TADF character in these compounds. The
prompt/delayed fluorescence lifetimes of BTITrz and BFITrz
were 13.1 ns/4.3 ms and 14.6 ns/26.6 ms, respectively. BTITrz
showed a short delayed fluorescence lifetime compared to
BFITrz due to the relatively small DEST with the aid of the strong
donor character of the synBTI unit. Moreover, the local triplet
energy of the synBTI donor close to the CT triplet energy of
BTITrz may assist the accelerated RISC (Fig. S2, ESI†). In the case
of BFITrz, the energy difference between local triplet energy and
CT triplet energy was relatively large, which could not improve
the RISC. Furthermore, the absolute PLQYs of the doped films
were analyzed by an integrating sphere method under a nitrogen
atmosphere and the PLQY values of BTITrz and BFITrz were
found to be 66% and 50%, respectively. The higher PLQY of
Fig. 6 Transient PL decay profiles of the 10 wt% emitter doped DPEPO
Fig. 4 Absorption and emission spectra of the compounds recorded in
films.
THF solution and 10 wt% doped DPEPO films, respectively.
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BTITrz than that of BFITrz is ascribed to the efficient T1 to S1 materials compared to the DPEPO host, the direct charge
RISC process of the former compound, judging from the high trapping in the dopant would be the dominating EL mecha-
PLQY of the delayed component (46%) of BTITrz compared to nism in these devices. In particular, the large HOMO gap
BFITrz (34%). Further, the RISC rate constants of BTITrz and between the mCP and DPEPO interrupted the hole injection
BFITrz estimated from their transient PL analysis and PLQY and transport, which is evident from the energy level diagram
data were found to be 4.92 Â 105 sÀ1 and 3.20 Â 105 sÀ1
,
(Fig. 7). Therefore, the dopants would play a key role of hole
respectively. These results suggest that BTITrz would work as trapping and transporting medium in the devices.8 As a result,
an efficient TADF emitter in OLED devices.
the J and L of the compounds increased with increasing doping
concentration. The EQE vs. L plots of the compounds at different
doping concentrations are shown in Fig. 8. The optimized device
Electroluminescence properties
The electroluminescence (EL) performance of these compounds with a doping concentration of 20 wt% showed a maximum
was investigated in vacuum processed multilayer OLED devices EQE of 20.7% and 12.0% for BTITrz and BFITrz, respectively.
by employing them as dopants in the DPEPO host. The adopted Interestingly, both compounds did not show any significant
OLED device configuration is ITO (50 nm)/PEDOT:PSS (60 nm)/ decrease in EQE according to the increase of the doping concen-
TAPC (20 nm)/mCP (10 nm)/EML (25 nm: x wt%)/TSPO1 (5 nm)/ tration from 20 wt% to 40 wt%, indicating that the concentration
TPBi (20 nm)/LiF (1.5)/Al (200 nm). The doping concentration of quenching effect is not evident in these materials. In general,
the emitters was varied from 10% to 40% to verify the doping the concentration quenching effect is mostly induced by the
concentration effect. Diphenylphosphine oxide-4-(triphenylsilyl)- distance between emitters because the probability of exciton
phenyl (TSPO1) and 1,3-bis(N-carbazolyl)benzene (mCP) served collision and quenching is increased at high doping concen-
as electron transporting type and hole transporting type exciton tration. However, it was not serious in the two emitters due to
blocking layers, respectively. Di-[4-(N,N-ditolyl-amino)phenyl]- the introduction of the two new donors, relieving strong inter-
cyclohexane (TAPC) and LiF used as hole transport and electron molecular interaction. This can also be indirectly confirmed by
injection layers, respectively. The emitting layers (EMLs) were the small red-shift of the EL spectrum according to the doping
BTITrz or BFTTrz (10, 20, 30, or 40 wt%) doped DPEPO layers. concentration. As shown in the EL spectra, the TADF emitters
Here, high triplet energy DPEPO was used as the host to confine showed green emission with CIE color coordinates in the
the excitons in the emitting layer. The device structure, energy range of 0.27 o x o 0.33 and 0.50 o y o 0.57 for BTITrz and
level alignment diagram and chemical structures of the organic 0.29 o x o 0.35 and 0.51 o y o 0.56 for BFITrz, depending on
materials used in the device are presented in Fig. 7. The current their doping concentration. The EL peak wavelengths of the
voltage ( J)–voltage (V)–luminance (L), and EQE versus L and EL compounds showed slight bathochromic shifts (B10–12 nm) by
spectra of the TADF devices according to the doping ratio are increasing the doping concentration from 10 wt% to 40 wt%.
depicted in Fig. 8.
The EL spectra of both the compounds displayed slight batho-
As shown in Fig. 8, the J and L of the devices increased chromic shifts according to the doping concentration due to the
gradually upon increasing the doping concentration from intermolecular interaction at the high doping concentration by
10 wt% to 40 wt%. This can be explained by the dopant assisted the shortened intermolecular distance. However, the degree of
charge injection and hopping mechanisms in the devices. red shift was quite small compared to other TADF emitters. The
Owing to the low-lying LUMO and shallow HOMO of these EL spectra were measured at 1000 cd mÀ2. Overall, BTITrz
Fig. 7 Device structure and energy level alignment of the materials.
13916 | J. Mater. Chem. C, 2019, 7, 13912--13919
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Fig. 8 I–V–L plots of BTITrz (a) and BFITrz (d), EQE vs. luminance plots of BTITrz (b) and BFITrz (f), and EL spectra of BTITrz (c) and BFITrz (g).
Table 2 EL properties of the compounds
EQE (%)
Dopant Concentration (wt%) Voltage (V) Max. @100 cd mÀ2 Max.
Current efficiency (cd AÀ1
)
Power efficiency (lm WÀ1
)
@100 cd mÀ2
Max
@100 cd mÀ2
CIE (x, y)
BTITrz
10
20
30
40
10
20
30
40
4.2
3.6
3.6
3.5
4.5
3.6
3.6
3.5
18.8
20.7
20.3
19.0
11.3
12.0
10.7
10.3
8.2
15.7
16.8
17.3
2.8
7.3
7.6
54.8
63.6
63.6
59.8
33.4
36.8
33.3
32.1
23.4
47.5
52.2
54.4
8.0
22.2
23.5
25.1
40.3
51.0
57.0
50.3
24.9
29.9
27.4
26.4
7.8
23.5
30.3
33.0
2.1
9.7
12.0
13.4
0.27, 0.50
0.30, 0.54
0.32, 0.56
0.33, 0.57
0.29, 0.51
0.32, 0.55
0.33, 0.56
0.35, 0.56
BFITrz
8.1
exhibited superior device performance with a maximum EQE of can function as the potential donor for developing efficient TADF
20.7%, current efficiency (CE) of 63.6 cd AÀ1 and power efficiency emitters. The device performances are summarized in Table 2 and
(PE) of 51.0 lm WÀ1 compared with the BFITrz, which showed are compared with other TADF devices in the ESI.†
a maximum EQE of 12.0%, CE of 36.8 cd AÀ1 and PE of
29.9 lm WÀ1. This is attributed to the high PLQY, short td and
efficient RISC of BTITrz compared to BFITrz. Although both
compounds showed efficiency roll-off at a high luminance of
Conclusions
1000 cd mÀ2, the BTITrz device displayed a relatively small In conclusion, we designed and synthesized two new green
efficiency roll-off (26%) compared to the BFITrz device (40%), TADF emitters named BTITrz and BFITrz, which were derived
which is ascribed to its short td. These results indicate that synBTI from the synBTI and synBFI donor units. The effects of the
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heteroatoms in the donor units on the photophysical and EL the crude product by column chromatography using dichloro-
properties of the TADF emitters were investigated in detail. methane (DCM) : hexane (1: 2) eluent yielded intermediate 2a as
1
BTITrz showed a relatively small DEST, high PLQY and short a colorless solid, yield: 5.0 g, 80%. H NMR (500 MHz, CDCl3,
delayed fluorescence lifetime compared to BFITrz. Further- d ppm): 7.99 (d, J = 8.0 Hz, 1H), 7.78 (s, 1H), 7.69–7.66 (m, 1H),
more, BTITrz exhibited superior performance with a maximum 7.60–7.54 (m, 2H), 7.39–7.35 (m, 2H), 7.28–7.25 (m, 1H). MS
EQE of 20.7% and CE of 63.6 cd AÀ1 compared to BFITrz, which (APCI) m/z 256.01 [(M + H)+].
demonstrated a maximum EQE of 12.0% and CE of 36.8 cd AÀ1
.
6H-Benzo[4,5]thieno[2,3-b]indole (synBTI). A mixture of 2a
We believe that synBTI can expand the donor library for (5.0 g, 22.2 mmol) and triphenylphosphine (17.2 g, 66 mmol) was
designing efficient TADF emitters.
suspended in 20 mL of o-dichloromethane and degassed for
15 min. The reaction mixture was kept at 150 1C for 6 h and the
progress of the reaction was monitored by TLC. After completion of
the reaction, the reaction mixture was extracted with DCM and the
organic layer was dried over anhydrous MgSO4. The removal of the
solvent under vacuum resulted in a black colored crude mixture.
The purification of the crude mixture by column chromatography
using DCM : hexane (1 : 1) eluent resulted in synBTI as a pale yellow
solid, yield: 4.0 g, 81%. 1H NMR (500 MHz, CDCl3, d ppm): 8.23 (s,
1H), 7.88 (d, J = 10.0 Hz, 1H), 7.82 (d, J = 10.0 Hz, 1H), 7.55 (d, J =
10.0 Hz, 1H), 7.44 (d, J = 10.0 Hz, 1H), 7.38–7.35 (m, 1H), 7.33–7.30
(m, 1H), 7.28–7.22 (m, 2H). MS (APCI) m/z 224.01 [(M + H)+].
2-(6H-Benzo[4,5]thieno[2,3-b]indol-6-yl)-5-(4,6-diphenyl-1,3,5-
triazin-2-yl)benzonitrile (BTITrz). A mixture of synBTI (1.0 g,
4.5 mmol), 5-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-fluorobenzonitrile
(1.5 g, 4.5 mmol) and cesium carbonate (4.3 g, 13.0 mmol) was
suspended in 20 mL of dimethylformamide in a pressure tube.
The reaction mixture was refluxed for 3 h. The progress of the
reaction was monitored by TLC analysis. After completion of
the reaction, extraction with DCM, drying over MgSO4 and the
evaporation of the solvent gave a yellow residue. The crude
reaction mixture was purified on column chromatography using
DCM : hexane (1 : 1) as an eluent to get BTITrz (2.0 g, 83% yield) as
Experimental
General methods
The chemicals and solvents purchased from commercial sup-
pliers were used as received. The materials were purified by a
column chromatography technique using 100–200 mesh silica
as a stationary phase. All reactions were performed under an
inert atmosphere. Freshly distilled solvents were used for the
1
chemical reactions and analytical measurements. The H and
13C nuclear magnetic spectral data (NMR) were recorded on a
500 MHz NMR spectrometer. The chemical shift values are
quoted against Me4Si (0.00 ppm). The absorption and emission
measurements were recorded on a UV-vis spectrophotometer at
room temperature. The absolute PLQYs of 10 wt% emitter
doped films in a DPEPO host were measured using a calibrated
integrating sphere connected to a spectrofluorimeter. Transient
PL measurements were collected on the 10 wt% emitter doped
DPEPO films. The thermal stability of the compounds was
estimated using a thermogravimetric analyzer (TGA) under a
N2 atmosphere at a heating rate of 10 1C minÀ1. Electrochemical
measurements were collected on a CHI electrochemical analyzer
with a conventional three electrode configuration containing a
glassy carbon working electrode, a platinum wire auxiliary
electrode and a nonaqueous Ag/AgNO3 acetonitrile reference
electrode. An Fc/Fc+ internal standard was used to calibrate the
peak potentials. Theoretical calculations were performed using
DFT computational methods on the Gaussian 09 programme
package.9
1
a yellow solid. H NMR (500 MHz, CDCl3, d ppm): 9.26 (s, 1H),
9.11–9.09 (m, 1H), 8.76 (d, J = 8.0 Hz, 4H), 8.15 (d, J = 8.0 Hz, 1H),
8.10 (d, J = 8.0 Hz, 1H), 7.91 (d, J = 8.0 Hz, 1H), 7.81 (d, J = 8.0 Hz,
J = 1H), 7.66–7.58 (m, 6H), 7.51 (d, J = 7.0 Hz, 1H), 7.42–7.30
(m, 4H). 13C NMR (CDCl3, d ppm): 172.30, 169.08, 143.81, 142.55,
141.91, 138.41, 136.91, 135.68, 134.80, 133.33, 132.70, 129.32,
129.05, 127.80, 125.65, 124.06, 123.37, 122.97, 122.41, 121.41,
120.68, 119.65, 116.16. MS (APCI) m/z 556.10 [(M + H)+].
3-(2-Nitrophenyl)benzofuran (2b). Intermediate 2b was pre-
pared from 1b (5.0 g, 30.8 mmol), 2-nitrobromobenzene (9.2 g,
Synthesis
The starting materials such as benzo[b]thiophen-3-ylboronic acid 46.2 mmol), tetrakis(triphenylphosphine)palladium(0) (1.7 g,
(1a), benzofuran-3-ylboronic acid (1b), and 2-nitrobromobenzene 1.54 mmol) and potassium carbonate (12.7 g, 92.4 mmol) using
were purchased from TCI Chemicals. The acceptor scaffold a similar procedure to that described above for 2a. Yellow solid.
5-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-fluorobenzonitrile was synthe- Yield, 5.4 g, 75%. 1H NMR (500 MHz, CDCl3, d ppm): 8.00 (d, J =
sized as per the given procedure in the literature.10
8.0 Hz, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.88–7.65 (m, 1H), 7.64–7.54
3-(2-Nitrophenyl)benzo[b]thiophene (2a). A mixture of 1a (m, 1H), 7.42–7.32 (m, 4H). MS (APCI) m/z 240.01 [(M + H)+].
(5.0 g, 28.0 mmol), 2-nitrobromobenzene (8.4 g, 42.1 mmol), 6H-Benzofuro[2,3-b]indole (synBFI). The donor synBFI was
tetrakis(triphenylphosphine)palladium(0) (1.6 g, 1.4 mmol) and prepared from 2b (5.0 g, 20.9 mmol) and triphenylphosphine
potassium carbonate (11.6 g, 84.0 mmol) was dissolved in a 1 : 3 (16.4 g, 62.7 mmol) using a similar procedure to that described
mixture of THF and water. The reaction mixture was refluxed above for synBTI. Colorless solid. Yield, 3.5 g, 81%. 1H NMR
for 24 h and the progress of the reaction was examined by thin (500 MHz, CDCl3, d ppm): 8.28 (s, 1H), 8.09 (d, J = 8.0 Hz, 1H),
layer chromatography (TLC). The reaction mixture was washed 8.0 (s, 1H), 7.79 (d, J = 7.0 Hz, 1H), 7.48–7.45 (m, 1H), 7.40 (d, J =
with dichloromethane and water several times. The organic layer 8.0 Hz, 1H), 7.28–7.26 (m, 3H). MS (APCI) m/z 208.00 [(M + H)+].
was dried over anhydrous MgSO4 and the removal of the solvent
2-(6H-Benzofuro[2,3-b]indol-6-yl)-5-(4,6-diphenyl-1,3,5-triazin-
under vacuum resulted in a colorless residue. The purification of 2-yl)benzonitrile (BFITrz). BFITrz was prepared from synBFI
13918 | J. Mater. Chem. C, 2019, 7, 13912--13919
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Paper
Journal of Materials Chemistry C
A. Hu, Z. Qin, S. Zhao, S. J. Su and B. Z. Tang, Adv. Funct. Mater.,
2017, 27, 1606458; (i) A. Maheshwaran, V. G. Sree, H. Y. Park,
H. Kim, S. H. Han, J. Y. Lee and S. H. Jin, Adv. Funct. Mater.,
2018, 28, 1802945.
(1.0 g, 4.8 mmol), 5-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-fluorobenzo-
nitrile (1.7 g, 4.8 mmol) and cesium carbonate (4.6 g, 14.4 mmol)
using a similar procedure to that described above for BTITrz.
Yellow solid. Yield, 1.8 g, 75%. 1H NMR (500 MHz, CDCl3, d ppm):
9.41 (s, 1H), 9.19 (s, 1H), 8.80 (s, 1H), 7.94–7.88 (m, 3H), 7.64–7.48
(m, 7H), 7.41–7.35 (m, 5H). 13C NMR (CDCl3, d ppm): 172.37,
135.53, 134.16, 133.31, 129.83, 128.73, 128.23, 124.45, 122.57,
120.11, 119.32, 112.30, 111.82. MS (APCI) m/z 540.13 [(M + H)+].
3 (a) R. Pashazadeh, P. Pander, A. Bucinskas, P. J. Skabara,
F. B. Dias and J. V. Grazulevicius, Chem. Commun., 2018,
54, 13857; (b) M. Yokoyama, K. Inada, Y. Tsuchiya,
H. Nakanotani and C. Adachi, Chem. Commun., 2018,
54, 8261; (c) T. Ohsawa, H. Sasabe, T. Watanabe, K. Nakao,
R. Komatsu, Y. Hayashi, Y. Hayasaka and J. Kido, Adv. Opt.
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X. Song, X. Yan, K. Ye, Y. Liu, G. Yang and Y. Wang, ACS Appl.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Basic Science Research Program
(Grant No. 2016M3A7B4909243 and 2018R1D1A1B07048498)
through the National Research Foundation (NRF) of Korea
funded by the Ministry of Science, ICT, and Future Planning.
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