Fluorenobenzofuran as the core structure of high triplet energy host materials for green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c2jm30473b

Thermally stable high triplet energy host materials based on a fluorenobenzofuran (BFF) core were synthesized and the device performances of green phosphorescent organic light-emitting diodes were investigated. BFF was designed as the electron transport type core with improved thermal stability. Two host materials based on the BFF core showed a high glass transition temperature over 140 °C and high triplet energy above 2.70 eV. They were used as the host materials for green phosphorescent organic light-emitting diodes and a high quantum efficiency close to 20% was achieved. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm30473b

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PAPER
Fluorenobenzofuran as the core structure of high triplet energy host materials
for green phosphorescent organic light-emitting diodes
*
Soon Ok Jeon and Jun Yeob Lee
Received 24th January 2012, Accepted 20th March 2012
DOI: 10.1039/c2jm30473b
Thermally stable high triplet energy host materials based on a fluorenobenzofuran (BFF) core were
synthesized and the device performances of green phosphorescent organic light-emitting diodes were
investigated. BFF was designed as the electron transport type core with improved thermal stability.
Two host materials based on the BFF core showed a high glass transition temperature over 140 ꢀC and
high triplet energy above 2.70 eV. They were used as the host materials for green phosphorescent
organic light-emitting diodes and a high quantum efficiency close to 20% was achieved.
a new core structure for bipolar host materials of PHOLEDs.
Introduction
The BFF was effectively synthesized by the ring closing reaction
of the phenyl modified dibenzofuran and was modified with
a hole transport type carbazole unit. It was demonstrated that
the BFF based host materials showed a high triplet energy of
2.70 eV, high glass transition temperature above 140 ꢀC, bipolar
charge transport properties and over 19.0% external quantum
efficiency in green PHOLEDs.
It is important to improve the quantum efficiency and lifetime of
phosphorescent organic light-emitting diodes (PHOLEDs) by
developing new emitting materials such as host and dopant
materials. In particular, the host materials are critical to the
device performances of PHOLEDs and various host materials
have been synthesized.1–16
There are several requirements for the host materials in
PHOLEDs, which include high triplet energy, high glass transi-
tion temperature, and highest occupied molecular orbital
(HOMO)/lowest unoccupied molecular orbital (LUMO) levels
for hole and electron injection. Several classes of materials have
been reported and one of the most popular core structures to
meet the requirements of host materials for PHOLEDs was flu-
orene.17–26 The fluorene core structure shows a high triplet energy
of 2.90 eV and weak electron transport properties. In addition, it
can be chemically modified at 9,90-position or 2,7-position of
fluorene to tailor the physical and thermal properties with ease.
Various functional groups such as diphenylamine,18 diphenyl-
phosphine oxide19 and carbazole17 were introduced in the
molecular structure to control the charge transport properties of
the fluorene based organic compounds. However, the simple
chemical modification of the fluorene core could not improve the
glass transition temperature and it was difficult to obtain bipolar
charge transport properties due to weak electron transport
properties of the fluorene core. Therefore, a new core structure
with better electron transport properties and thermal stability is
required to develop bipolar host materials with high glass tran-
sition temperature.
Experimental section
Synthesis
The synthetic scheme of the BFF based compounds is described
in Scheme 1.
Synthesis of 4-(2-bromophenyl)dibenzo[b,d]furan (1). 4-Diben-
zofuranylboronic acid (5.00 g, 23.58 mmol), 1,2-dibromobenzene
(5.05 g, 21.43 mmol) and tetrakis(triphenylphosphine)palladium
(1.23 g, 1.07 mmol) were dissolved in anhydrous tetrahydrofuran
(THF) under nitrogen atmosphere. The reaction mixture was
stirred for 1 h. To the reaction mixture was added a solution of
potassium carbonate (2 M, 50 mL) dropwise slowly. The reaction
mixture was refluxed for 12 h at 90 ꢀC under nitrogen
In this work, a thermally stable high triplet energy core
structure, benzofuran modified flourene (BFF), was developed as
Department of Polymer Science and Engineering, Dankook University,
126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701, Republic of
Korea. E-mail: leej17@dankook.ac.kr; Fax: +82 31 8005 3585; Tel: +82
31 8005 3585
Scheme 1 Synthetic scheme of BFF1 and BFF2.
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atmosphere. After being cooled to ambient temperature, the
reaction mixture was extracted with dichloromethane and water.
The organic layer was evaporated with a rotary evaporator. The
residue was subjected to column chromatography (silica gel) by
n-hexane and a white powdery product was obtained.
18-crown-6 (0.11 g, 0.41 mmol) and potassium carbonate (2.28 g,
16.48 mmol) were dissolved in anhydrous o-dichlorobenzene
under nitrogꢀen atmosphere. The reaction mixture was stirred for
12 h at 120 C. The mixture was diluted with dichloromethane
and washed with distilled water (100 mL) three times. The
organic layer was dried over anhydrous MgSO4 and evaporated
in vacuo to give the crude product, which was purified by column
chromatography by n-hexane–dichloromethane (4 : 1). The final
white powdery product was oꢀbtained in 68% yield.
1
Yield 86%. H NMR (500 MHz, CDCl3): d 7.99 (d, 2H, J ¼
6 Hz), 7.77 (d, 1H, J ¼ 8.0 Hz), 7.53–7.32 (m, 8H). 13C NMR
(125 MHz, CDCl3) d 156.4, 137.8, 133.8, 132.7, 131.6, 130.2,
129.1, 128.1, 126.9, 125.6, 124.6, 123.9, 123.5, 122.5, 121.5, 120.4,
112.6, 111.6. MS (FAB) m/z 323 [(M + H)+]. Anal. calcd for
C18H11BrO: C, 66.89; H, 3.43. Found: C, 66.88; H, 3.44%.
1
BFF1 Yield 68%. Tg 146.9 C. H NMR (500 MHz, CDCl3):
d 8.33 (d, 1H, J ¼ 7.6 Hz), 8.10–7.93 (m, 6H), 7.73 (t, 2H, J ¼
7.9 Hz), 7.61–7.36 (m, 6H), 7.30–7.13 (m, 6H), 6.94–6.79 (m, 4H).
13C-NMR (125 MHz, CDCl3) d 170.6, 156.2, 150.1, 148.3, 147.6,
140.4, 139.9, 138.8, 128.2, 127.2, 125.8, 124.7, 124.0, 123.6, 122.7,
121.7, 121.1, 120.5, 120.2, 119.8, 118.9, 118.7, 117.3, 112.0, 109.6,
108.5, 66.9. MS (FAB) m/z 571 [(M + H)+]. Anal. calcd for
C43H24NO: C, 90.34; H, 4.41; N, 2.45. Found: C, 90.26; H, 4.56;
N, 2.47%.
Synthesis of 2-bromospiro[fluorene-9,70-fluoreno[4,3-b]benzo-
furan] (2). 4-(2-Bromophenyl)dibenzo[b,d]furan (1) (3.00 g,
9.28 mmol) was dissolved in 30 mL of anhydrous tetrahydro-
furan under argon and cooled to ꢁ78 ꢀC and n-butyllithium
(2.5 M in hexanes, 4.83 mL) was added dropwise slowly. Stirring
was continued for 2 h at ꢁ78 ꢀC, followed by the addition of
a solution of 2-bromofluorenone (3.12 g, 12.06 mmol) in THF
(40 mL) under an argon atmosphere. The resulting mixture was
gradually warmed to ambient temperature and quenched by
adding saturated, aqueous NaHCO3 (100 mL). The mixture was
extracted with dichloromethane. The combined organic layers
were dried over magnesium sulfate, filtered, and evaporated
Synthesis of 9-(4-(7-phenyl-7H-fluoreno[4,3-b]benzofuran-7-yl)
phenyl)-9H-carbazole (BFF2). This compound was prepared by
a procedure similar to that of BFF1 except that 7-(4-bromo-
phenyl)-7-phenyl-7H-fluoreno[4,3-b]benzofuran (3) was used as
the reactant instead of 2-bromospiro[fluorene-9,7’-fluoreno[4,3-
b]benzofuran] (2). Purification by column chromatography using
n-hexane–dichloromethane (4 : 1) gave a white powder.
BFF2 Yield 68%. 1H NMR (500 MHz, CDCl3): d 8.33 (d, 1H,
J ¼ 7.6 Hz), 8.10–7.93 (m, 6H), 7.73 (t, 2H, J ¼ 7.9 Hz), 7.61–
7.36 (m, 7H), 7.30–7.13 (m, 6H), 6.94–6.79 (m, 4H). 13C NMR
(125 MHz, CDCl3): d 157.2, 151.5, 150.9, 145.6, 145.1, 140.9,
138.1, 136.6, 130.4, 129.2, 128.9, 127.9, 127.5, 126.6, 125.5, 124.3,
123.6, 121.4, 121.1, 120.5, 119.9, 119.6, 112.7, 111.5, 110.6, 109.5,
67.7. MS (FAB) m/z 573 [(M + H)+]. Anal. calcd for C43H27NO:
C, 90.03; H, 4.74; N, 2.44. Found: C, 90.02; H, 4.73; N, 2.34%.
under reduced pressure.
A yellow powdery product was
obtained. The crude residue was placed in another two-necked
flask and was dissolved in acetic acid (40 mL). A catalytic
amount of aqueous HCl (5 mol%, 12 N) was then added and the
whole solution was refluxed for 12 h. After cooling to ambient
temperature, purification by silica gel chromatography using
dichloromethane–n-hexane (1 : 4) gave a white powder.
1
Yield 69%. H NMR (500 MHz, CDCl3): d 8.38 (d, 1H, J ¼
8.0 Hz), 7.95 (d, 1H, J ¼ 8.0 Hz), 7.89–7.82 (m, 1H), 7.77–7.67
(m, 3H), 7.51–7.35 (m, 5H), 7.19–7.08 (m, 2H), 6.87–6.69 (m,
4H). 13C NMR (125 MHz, CDCl3) d 156.8, 150.6, 148.3, 147.9,
147.6, 140.7, 139.3, 131.0, 128.3, 127.8, 127.1, 124.6, 124.2, 123.8,
123.3, 123.0, 121.5, 121.3, 120.5, 120.1, 119.8, 118.4, 111.9, 66.61.
MS (FAB) m/z 485 [(M + H)+]. Anal. calcd for C31H17BrO: C,
76.71; H, 3.53. Found: C, 75.83; H, 3.46%.
Device preparation and measurements
The device structure of green PHOLEDs was indium tin oxide
(ITO,
amino)-phenyl]-biphenyl-4,40-diamine (DNTPD, 60 nm)/4,40-
(cyclohexane-1,1-diyl)bis(N-phenyl-N-p-tolylaniline) (TAPC,
50
nm)/N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-tolyl-
Synthesis of 7-(4-bromophenyl)-7-phenyl-7H-fluoreno[4,3-b]
benzofuran (3). This compound was prepared by a procedure
similar to that of (2) except 4-bromobenzophenone that was used
as the reactant instead of 2-bromofluorenone. The product was
purified by column chromatography using dichloromethane–n-
hexane (1 : 4) which gave a white powder.
30 nm)/BFF1 or BFF2:tris(2-phenylpyridine) iridium (Ir(ppy)3)
(30 nm, 3%)/diphenylphosphine oxide-4-(triphenylsilyl)phenyl
(TSPO1, 25 nm)/LiF (1 nm)/Al (200 nm). A hole only device with
a device structure of ITO (50 nm)/DNTPD (60 nm)/TAPC
(30 nm)/BFF1 or BFF2 (30 nm)/Al and an electron only device
with a device structure of ITO (50 nm)/Ca (5 nm)/BFF1 or BFF2
(30 nm)/TSPO1 (25 nm)/LiF (1 nm)/Al (200 nm) were also
fabricated. The device performances of green PHOLEDs were
measured with a Keithley 2400 source measurement unit and
CS1000 spectroradiometer after encapsulating the device with
a glass lid and CaO getter.
The 1H and 13C nuclear magnetic resonance (NMR) was
recorded on a Varian 500 (500 MHz) spectrometer. The photo-
luminescence (PL) spectra were recorded on a fluorescence
spectrophotometer (HITACHI, F-7000) and the ultraviolet-
visible (UV-Vis) spectra were obtained using a UV-Vis spectro-
photometer (Shimadzu, UV-2501PC). The differential scanning
calorimeter (DSC) measurements were performed on a Mettler
1
Yield 77%. H NMR (500 MHz, CDCl3): d 8.33 (d, 1H, J ¼
8.0 Hz), 7.96 (d, 1H, J ¼ 8.0 Hz), 7.85 (d, 1H, J ¼ 8.0 Hz), 7.70 (d,
1H, J ¼ 8.0 Hz), 7.53–7.33 (m, 13H), 7.13–7.09 (m, 2H). 13C
NMR (125 MHz, CDCl3) d 156.3, 150.3, 149.8, 144.4, 137.2,
131.3, 130.3, 129.9, 129.0, 128.3, 127.7, 127.0, 125.7, 124.7, 123.7,
123.0, 122.4, 122.0, 120.4, 119.5, 118.7, 111.9, 110.8, 65.4. MS
(FAB) m/z 487 [(M + H)+]. Anal. calcd for C31H19BrO: C, 76.39;
H, 3.93. Found: C, 76.42; H, 3.94%.
Synthesis of 9-(spiro[fluorene-9,70-fluoreno[4,3-b]benzofuran]-2-
yl)-9H-carbazole (BFF1). 2-Bromospiro[fluorene-9,70-fluoreno
[4,3-b]benzofuran] (2) (2.00 g, 4.12 mmol), 9-carbazole (0.83 g,
4.94 mmol), copper powder (0.52 g, 8.24 mmol), dibenzo
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DSC 822e under nitrogen at a heating rate of 10 C minꢁ1. The
the Lee–Yang–Parr functional (B3LYP) with 6-31G* basis sets
were used for the simulation.27 Fig. 1 shows the HOMO and
LUMO distribution of BFF1 and BFF2. The HOMO
and LUMO distribution of BFF1 and BFF2 was quite similar.
The HOMO was localized on the 9-phenylcarbazole unit, while
the LUMO was dispersed over the BFF core. As can be expected
from the molecular structure, the HOMO and LUMO were
distributed over the electron donating 9-phenylcarbazole unit
and electron withdrawing BFF core unit, respectively. This
indicates that the HOMO of BFF1 and BFF2 is decided by the
carbazole, while the LUMO is determined by the BFF core.
Photophysical properties of BFF1 and BFF2 were measured
using UV-Vis and PL spectrometers. Fig. 2 represents the UV-
Vis and PL spectra of BFF1 and BFF2. Both host materials
showed similar UV-Vis and PL spectra although slight red-shift
of the spectra was observed in BFF1. The UV-Vis absorption
peaks at 293 nm, 323 nm and 340 nm are assigned to p–p*
transition of phenylcarbazole, while peaks at 276 nm and 308 nm
correspond to p–p* transition of the BFF core. The conjugation
of the phenylcarbazole is extended through the fluorene unit in
BFF1, slightly shifting the UV-Vis absorption peak to long
wavelength. Bandgaps of the BFF1 and BFF2 were calculated
from the edge of the UV-Vis absorption spectra, which were
3.53 eV and 3.55 eV, respectively. The extended conjugation of
the BFF1 slightly reduced the bandgap from 3.55 eV to 3.53 eV.
The PL emission was also shifted by 4 nm due to the extended
conjugation between fluorene and carbazole in BFF1. There was
little solvatochromic effect in both BFF1 and BFF2, indicating
weak charge transfer in host materials. Low temperature PL
measurement was also carried out at 77 K in liquid nitrogen to
ꢀ
low and high resolution mass spectra were recorded using
a JEOL, JMS-AX505WA spectrometer in FAB mode. The
energy levels were measured by cyclic voltammetry (CV).
Results and discussion
Two BFF based host materials, BFF1 and BFF2, were developed
as thermally stable bipolar host materials for balanced hole and
electron density in the emitting layer. The BFF core was designed
as the electron transport type core structure with high triplet
energy and high rigidity for high glass transition temperature.
The benzofuran unit withdraws electrons due to the oxygen with
high electronegativity, making the fluorene electron deficient.
Therefore, the BFF core can function as the electron transport
type core structure. In addition to the electron transport prop-
erties, the BFF may improve the glass transition temperature
because the molecular motion of the benzofuran unit is restricted
by the fluorene unit. The BFF core may have high triplet energy
as the phenyl unit in the fluorene is attached to the ortho-position
of the oxygen in the benzofuran and meta-position of the phenyl
in the benzofuran. The extension of the conjugation is limited by
the ortho- and meta-connection between the fluorene and
benzofuran, resulting in wide bandgap and high triplet energy.
As the BFF core was synthesized as the electron deficient core, it
was modified with a hole transport type carbazole unit to obtain
bipolar charge transport properties.
The BFF core could be synthesized by the coupling reaction
between 4-dibenzofuranylboronic acid and 1,2-dibromobenzene
followed by the ring closing reaction with 4-bromobenzophe-
none or 2-bromofluorenone. The synthetic yield of the bromi-
nated BFF cores was in the range from 69% to 77%. The
brominated intermediates were reacted with carbazole using
copper powder as the catalyst. The synthetic yield of BFF1 and
BFF2 from brominated BFF cores was 68% for both
compounds.
Molecular simulation of BFF1 and BFF2 was carried out to
study the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) distribution of
the host materials. A suite of the Gaussian 03 program and the
nonlocal density functional of Becke’s 3-parameters employing
Fig. 2 UV-Vis, solution PL and low temperature PL spectra of (a) BFF1
and (b) BFF2. UV-Vis and solution PL spectra were measured in tetra-
hydrofuran. Low temperature PL was obtained in liquid nitrogen.
Fig. 1 HOMO and LUMO distribution of BFF1 and BFF2.
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measure the triplet energy of the host materials. The triplet
energies were calculated from the first emission peak of low
temperature PL spectra, which were 2.70 eV and 2.71 eV for
BFF1 and BFF2, respectively. The triplet energies of the host
materials were high enough for energy transfer to green emitting
Ir(ppy)3 with a triplet energy of 2.4 eV.
Thermal transition of host materials was analyzed using DSC
at a heating rate of 10 ꢀC minꢁ1 in nitrogen atmosphere. No
melting transition was observed in BFF1 and BFF2 up to 400 ꢀC
and only glass transition was detected in BFF1. The glass tran-
sition temperature of the BFF1 was 147 ꢀC, which is higher than
that of common spirobifluorene based host materials.18,19
Therefore, the BFF core was effective to improve the glass
transition temperature of host materials. The high glass transi-
tion temperature of BFF1 is due to the rigidity of the BFF core
structure.
Fig. 4 Current density–voltage curves of hole only and electron only
devices of BFF1 and BFF2.
Green PHOLEDs doped with Ir(ppy)3 were fabricated to
evaluate BFF1 and BFF2 as the host materials. The doping
concentration of Ir(ppy)3 was 3%. Fig. 5 shows the current
density–voltage–luminance curves of green PHOLEDs with
BFF1 and BFF2 host materials. The current density of the BFF1
device was higher than that of the BFF2 device over all the
voltage range measured. Considering the hole only and electron
only device data, the high current density of the BFF1 device is
due to high electron density of the BFF1 emitting layer. Although
the hole current density of BFF1 was lower than that of BFF2,
the high electron current density increased the total current
density of the BFF1 device. The same trend was observed in the
luminance–voltage curves of green PHOLEDs.
The HOMOs of BFF1 and BFF2 were obtained by CV
measurement, which were ꢁ6.06 eV and ꢁ6.09 eV from the onset
of the oxidation peak (Fig. 3). Two host materials showed similar
HOMO levels because the HOMO is dispersed over the phe-
nylcarbazole unit in both compounds. The slight change of the
HOMO level is due to the different conjugation of the phenyl-
carbazole unit. The LUMO was calculated from the HOMO and
bandgap. The LUMOs of BFF1 and BFF2 were ꢁ2.53 eV and
ꢁ2.54 eV, respectively. As the LUMO was localized on the same
BFF core in the two host materials, there was little difference of
LUMO level.
Hole only and electron only devices of BFF1 and BFF2 were
fabricated to compare the charge transport properties of BFF1
and BFF2. Current density–voltage curves of hole only and
electron only devices are shown in Fig. 4. The current density of
the hole only device was similar in BFF1 and BFF2, while that of
the electron only device was high in BFF1. Comparing the hole
and electron current densities of BFF1, the hole current density
was similar to electron current density, indicating bipolar charge
transport properties of BFF1. However, the hole current density
of BFF2 was much higher than the electron current density of
BFF2. Although both BFF1 and BFF2 were designed as the
bipolar host materials, BFF1 was more suitable as the bipolar
host material. The high electron current density of BFF1 may be
due to the fluorene unit connected to the carbazole unit. As
fluorene is a weak electron transport unit, the additional fluorene
unit attached to the carbazole of BFF1 facilitates electron
transport of BFF1 compared with BFF2 without additional flu-
orene unit.
Quantum efficiency–luminance curves of green PHOLEDs
with BFF1 and BFF2 host materials are shown in Fig. 6. The
external quantum efficiency of the BFF1 device was higher than
that of the BFF2 device over all the luminance range. The
maximum external quantum efficiency of the BFF1 device was
19.7% and the quantum efficiency at 1000 cd mꢁ2 was 19.5%. In
particular, the quantum efficiency at 5000 cd mꢁ2 was also 18.3%.
There was less than 10% decrease of quantum efficiency up to
5000 cd mꢁ2. However, the quantum efficiency of the BFF2
device was greatly reduced at high luminance. The maximum
quantum efficiency of the BFF2 device was 18.6%, but the
quantum efficiency at 5000 cd mꢁ2 was only 11.5%. The differ-
ence of quantum efficiency between BFF1 and BFF2 devices can
be explained by the hole and electron current densities in the
Fig. 5 Current density–voltage–luminance curves of BFF1 and BFF2
Fig. 3 Oxidation CV curves of BFF1 and BFF2.
10540 | J. Mater. Chem., 2012, 22, 10537–10541
green PHOLEDs. Ir(ppy)3 was doped at a doping concentration of 3%.
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BFF2 18.6
19.5
17.3
(0.32,0.61) 60.5
(0.33,0.61) 57.3
38.8
32.2
a
b
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emitting layer. As shown in the hole and electron only device
data, BFF1 showed similar hole and electron density. The
balanced charge density in the emitting layer enhances the
quantum efficiency of the BFF1 device and maintains the charge
balance up to high luminance. However, the charge balance is
disrupted in the BFF2 device because of imbalance of charge
density. Especially, the hole current density is much higher than
the electron current density at high voltage, resulting in signifi-
cant reduction of quantum efficiency at high luminance. All
device performances of BFF1 and BFF2 are summarized in
Table 1.
25 T. Hwu, T. Tsai, W. Hung, S. Chang, Y. Chi, M. Chen, C. Wu,
K. Wong and L. Chi, Chem. Commun., 2008, 4956.
26 Y. Liao, C. Lin, Y. Liu, K. Wong, W. Hung and W. Chen, Chem.
Commun., 2007, 1831.
BFF1 and BFF2 were evaluated as host materials for blue
PHOLEDs, but the quantum efficiency was very low due to poor
energy transfer. A quantum efficiency of 9.2% was obtained in
red PHOLEDs.
27 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, T. Vreven, Jr,
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V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
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J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,
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J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
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Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
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Conclusions
In conclusion, thermally stable high triplet energy host materials,
BFF1 and BFF2, were synthesized using a fluorenobenzofuran
core and were effectively used as the bipolar host materials for
green PHOLEDs. BFF1 showed a high glass transition temper-
ature above 140 ꢀC and a high triplet energy of 2.70 eV. In
addition, a high external quantum efficiency of 19.7% was
obtained from the green PHOLED with a BFF1 host and less
than 10% decrease of quantum efficiency was achieved up to
5000 cd mꢁ2. Therefore, the new fluorenobenzofuran can be
useful as the core structure of thermally stable high triplet energy
host and charge transport materials.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 10537–10541 | 10541