Novel tri-carbazole modified fluorene host material for highly efficient solution-processed blue and green electrophosphorescent devices

Article abstract of DOI:10.1016/j.tet.2014.04.060

A series of novel solution-processable small-molecule host materials: 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz comprising a fluorene monomer as the rigid core and tri-carbazole as the periphery have been designed and synthesized, and their optical, electrochemical, and thermal properties have been fully characterized. The host materials exhibit high glass-transition temperatures (231-310 °C) and high triplet energy levels (2.61-2.73 eV). High-quality amorphous thin films can be obtained by spin-coating the host materials from solutions. It is found that the HOMO level of the host materials can be tuned by linking the tri-carbazole unit to the 2,7 positions of the fluorine core, resulting in appropriate HOMO energy levels (-5.36 to -5.23 eV) for improved hole-injection in the device. Solution-processed blue and green electrophosphorescent devices bases on the developed host materials exhibit high efficiencies of 21.2 and 34.8 cd A^-1, respectively.

Full text of DOI:10.1016/j.tet.2014.04.060

Tetrahedron 70 (2014) 3847e3853
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Novel tri-carbazole modified fluorene host material for highly
efficient solution-processed blue and green electrophosphorescent
devices
*
*
Jinan Tang, Yuanshen Chen, Lei Cong, Baoping Lin , Yueming Sun
School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel solution-processable small-molecule host materials: 2DPF-TCz, 2SBF-TCz, 27DPF-TCz,
and 27SBF-TCz comprising a fluorene monomer as the rigid core and tri-carbazole as the periphery have
been designed and synthesized, and their optical, electrochemical, and thermal properties have been
fully characterized. The host materials exhibit high glass-transition temperatures (231e310 ꢀC) and high
triplet energy levels (2.61e2.73 eV). High-quality amorphous thin films can be obtained by spin-coating
the host materials from solutions. It is found that the HOMO level of the host materials can be tuned by
linking the tri-carbazole unit to the 2,7 positions of the fluorine core, resulting in appropriate HOMO
energy levels (ꢁ5.36 to ꢁ5.23 eV) for improved hole-injection in the device. Solution-processed blue and
green electrophosphorescent devices bases on the developed host materials exhibit high efficiencies of
21.2 and 34.8 cd Aꢁ1, respectively.
Received 21 November 2013
Received in revised form 13 April 2014
Accepted 22 April 2014
Available online 29 April 2014
Keywords:
Carbazole
Fluorene
High triplet energy
Electrophosphorescence
Solution processed
Host
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
metal catalyst used in polymer synthesis), which can result in
dramatically negative influence on the device performance. In
Phosphorescent organic lighting-emitting diodes (PhOLEDs)
have attracted a great deal of attention due to their potential ap-
plications in full-color flat-panel displays and lighting sources.1e5
Phosphorescent emitters are considered to be superior over their
fluorescent counterparts because they can utilize both singlet and
triplet excitons, thus approaching 100% internal quantum effi-
ciency. In general, to achieve high performance, phosphorescent
emitters are doped in host materials to reduce the severe concen-
tration quenching. Recently, high-performance vacuum-deposited
PhOLEDs with multi-layer device structures have been
reported.6e20 However, these vacuum-deposited devices require
a complicated fabrication process and a high fabrication cost. In
contrast, solution-processed PhOLEDs offer an economically at-
tractive alternative to the fabrication of large area light-emitting
devices.
contrast to the polymer counterparts, solution-processable small-
molecule host materials are advantageous because they can be
conveniently synthesized and purified.25e32 A good host material
for solution processed PhOLEDs should possess the following fea-
tures: high triplet energy level, good charge-transport properties,
high solubility, and good film-forming property. In addition, suit-
able highest occupied/lowest unoccupied molecular orbital
(HOMO/LUMO) energy levels matching those of the adjacent layers
in the devices are required to reduce the hole/electron injection
barriers. In the search of high performance host materials for
PhOLEDs, carbazole and fluorene units have been widely used due
to their high triplet energy levels and good charge-transport
properties.33e43 In our previous work, we proposed to modify
4,40-bis(9-carbazolyl)-2,20-biphenyl (CBP) by the linking of two 3,6-
di-tert-butylcarbazole moieties into the 3,6 positions of the carba-
zole units to build a host material Cz-CBP.29 As a result, the new
host exhibits excellent thermal and morphological stability, but the
triplet energy of Cz-CBP is only 2.68 eV. In consideration of the high
triplet energy and the steric molecular configuration of spirobi-
fluorene and 9,9-diphenyl-fluorene, we utilized tri-carbazole as the
periphery and fluorene monomer as the rigid core to design a series
novel solution-processable small-molecule host materials,
3,300,6,600-tetra-tert-butyl-90-(9,9-diphenyl-9H-fluoren-2-yl)-90H-
To date, most of the solution-processed PhOLEDs have been
fabricated by physically blending phosphorescent emitters and
a polymeric host, such as PVK and PFO.21e24 However, the poly-
meric hosts have some intrinsic disadvantages, such as uncertainty
in molecular structures and chemical impurities (especially the
* Corresponding authors. Tel./fax: þ86 25 52090621; e-mail addresses: sun@seu.
edu.cn, 101011462@seu.edu.cn (Y. Sun).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.04.060

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J. Tang et al. / Tetrahedron 70 (2014) 3847e3853
9,30:60,900-tercarbazole (2DPF-TCz), 90-(9,90-spirobi[fluoren]-2-yl)-
3,300,6,600-tetra-tert-butyl-90H-9,30:60,900-tercarbazole
(2SBF-TCz),
90,90000-(9,9-diphenyl-9H-fluorene-2,7-diyl)bis(3,300,6,600-tetra-tert-
butyl-90H-9,30:60,900-tercarbazole)
(27DPF-TCz), and 2,7-
biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and iri-
dium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) or
tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3), was spin coated
onto the PEDOT:PSS layer from 1,2-dichloroethane solution and
annealed at 100 ꢀC for 30 min. The substrate was then transferred
into an evaporation chamber, where the TPBI was evaporated at an
bis(3,300,6,600-tetra-tert-butyl-90H-[9,30:60,900-tercarbazol]-90-yl)-
9,90-spirobi[fluorene] (27SBF-TCz). Due to the limit conjugation,
2DPF-TCz and 2SBF-TCz have relatively higher triplet energy than
the previous published material with CBP as the core structure. In
addition, because of the substitution of 3,6-position of the carba-
zole unit with electron-donor group, the HOMO energy level of
these hosts is raised and approach the work function of poly(3,4-
ethylenedioxythiophene) (PEDOT, ꢁ5.2 eV), which is believed to
facilitate the hole injecting. Furthermore, tert-butyl groups are in-
troduced into the molecules to improve the solubility of the ma-
terials to form high-quality thin films. The thermal, photophysical,
and electrochemical properties of the materials have been in-
vestigated. With the new host materials, the solution-processed
electrophosphorescent devices showed high performances, with
a luminous efficiency (LE) of 21.2 cd Aꢁ1 for blue PhOLEDs and
34.8 cd Aꢁ1 for green PhOLEDs. The high device performances in-
dicate that the tri-carbazole modified fluorene molecules are
promising host materials for solution-processed electro-
phosphorescent devices.
evaporation rate of 1e2 A/s under a pressure of 4ꢂ10ꢁ4 Pa and the
ꢀ
Cs2CO3/Al bilayer cathode was evaporated at evaporation rates of
ꢀ
0.2 and 10 A/s for Cs2CO3 and Al, respectively, under a pressure of
1ꢂ10ꢁ3 Pa. The currentevoltageebrightness characteristics of the
devices were characterized with a Keithley 4200 semiconductor
characterization system. The electroluminescent spectra were col-
lected with a Photo Research PR705 Spectrophotometer. All mea-
surements of the devices were carried out in ambient atmosphere
without encapsulations.
2.3. Materials
The synthesis of the compounds 3,300,6,600-tetra-tert-butyl-90H-
9,30:60,900-tercarbazole (TCz), 2-bromo-9,9-diphenyl-9H-fluorene,
2-bromo-9,90-spirobi[fluorene], 2,7-dibromo-9,9-diphenyl-9H-flu-
orene, and 2,7-dibromo-9,90-spirobi[fluorene] were prepared
according to published procedures.44,45
3,300,6,600-tetra-tert-butyl-90-(9,9-diphenyl-9H-fluoren-2-yl)-
90H-9,30:60,900-tercarbazole (2DPF-TCz), 90-(9,90-spirobi[fluoren]-2-
2. Experimental
yl)-3,300,6,600-tetra-tert-butyl-90H-9,30:60,900-tercarbazole
(2SBF-
TCz), 90,90000-(9,9-diphenyl-9H-fluorene-2,7-diyl)bis(3,300,6,600-tetra-
tert-butyl-90H-9,30:60,900-tercarbazole) (27DPF-TCz), and 2,7-
bis(3,300,6,600-tetra-tert-butyl-90H-[9,30:60,900-tercarbazol]-90-yl)-
9,900-spirobi[fluorene] (27SBF-TCz) were synthesized by a similar
procedure: A mixture of 2-bromo-9,9-diphenyl-9H-fluorene (2-
bromo-9,90-spirobi[fluorene], 2,7-dibromo-9,9-diphenyl-9H-fluo-
rene, and 2,7-dibromo-9,90-spirobi[fluorene]) (1.5 mmol), TCz
(1.6 mmol or 3.2 mmol), CuI (0.06 mmol), 18-crown-6 (0.06 mmol),
K2CO3 (4.5 mmol), and DMPU (5 mL) was heated at 180 ꢀC for 48 h.
After cooling, the mixture was treated with water and extracted
with diethyl ether. The organic extraction was dried over MgSO4
with removal of the volatiles. The residue was purified by column
chromatography on silica gel using petroleum ether/ethyl acetate
as an eluent.
2.1. General information
All reactants and solvents were purchased from commercial
sources and used as received. 1H NMR and 13C HMR spectra were
measured on a Bruker ARX300 NMR spectrometer with tetra-
methylsilane as the internal standard. Elemental analysis was
performed on an Elementar Vario EL CHN elemental analyzer.
Mass spectrometry was performed with a Thermo Electron Cor-
poration Finnigan LTQ mass spectrometer. Absorption spectra
were recorded with a UVevis spectrophotometer (Agilent 8453)
and PL spectra were recorded with a fluorospectrophotometer
(Jobin Yvon, FluoroMax-3). TGA was measured with a Netzsch
simultaneous thermal analyzer (STA) system (STA 409PC) under
a dry nitrogen gas flow at a heating rate of 10 ꢀC minꢁ1. Glass-
transition temperature was obtained by DSC at a heating rate
of 10 ꢀC minꢁ1 with a thermal analysis instrument (DSC 2910
modulated calorimeter). Cyclic voltammetry was performed on
a Princeton Applied Research potentiostat/galvanostat model 283
voltammetric analyzer in CH2Cl2 solutions (10ꢁ3 M) at a scan rate
of 100 mV sꢁ1 with a platinum plate as the working electrode,
a silver wire as the pseudo-reference electrode, and a platinum
wire as the counter electrode. The supporting electrolyte was
tetrabutylammonium hexafluorophosphate (0.1 M) and ferrocene
was used as the internal standard. The solutions were bubbled
with argon for 10 min before measurements. The film morphol-
ogy was measured with atomic force microscopy (AFM, Seiko
Instruments, SPA-400).
2.3.1. 2DPF-TCz. White solid. Yield 51%. 1H NMR (300 MHz, CDCl3,
d
): 8.20 (s, 5H), 8.15e7.70 (m, 5H), 7.60 (s, 4H), 7.50 (d, J¼7.5 Hz,
6H), 7.35e7.29 (m, 15H), 1.51 (s, 36H). 13C NMR (300 MHz, CDCl3,
): 153.5, 151.4, 145.4, 142.6, 139.8, 139.2, 130.9, 128.5, 128.3, 128.1,
d
127.9, 127.0, 126.5, 125.9, 124.0, 123.1, 121.5, 120.5, 116.2, 111.2,
109.2, 32.1. MS (MALDI-TOF) [m/z]: 1037.5 [Mþ]. Anal. Calcd for
C
77H71N3: C, 89.06; H, 6.89; N, 4.05. Found: C, 88.95; H, 6.90; N,
4.07.
2.3.2. 2SBF-TCz. White solid. Yield 56%. 1H NMR (300 MHz, CDCl3,
d
): 8.18 (s, 6H), 8.02 (d, J¼7.5 Hz, 1H), 7.88 (d, J¼7.5 Hz, 2H), 7.78
(d, J¼7.8 Hz, 1H), 7.53e7.39 (m, 11H), 7.31e7.19 (m, 8H), 7.11 (s,
1H), 6.96 (d, J¼7.5 Hz, 2H), 6.89 (d, J¼7.8 Hz, 1H), 1.50 (s, 36H). 13
C
2.2. Device fabrication and characterizations
NMR (300 MHz, CDCl3, d): 153.9, 151.8, 150.7, 145.1, 144.4, 144.1,
143.4, 142.8, 139.1, 133.4, 131.0, 130.7, 129.0, 128.5, 126.9, 126.5,
126.4, 126.1, 125.7, 125.3, 124.0, 122.9, 121.8, 118.8, 113.7, 111.7,
34.7. MS (MALDI-TOF) [m/z]: 1035.4 [Mþ]. Anal. Calcd for
In a general procedure, indiumetin oxide (ITO)-coated glass
substrates were carefully pre-cleaned and treated by UV ozone for
4 min. A 40 nm poly(3,4-ethylenedioxythiophene):poly(styrene-4-
sulfonate) (PEDOT:PSS) layer was spin coated onto the ITO sub-
strate and baked at 200 ꢀC for 10 min. The PEDOT:PSS-coated
substrates were then transferred into a nitrogen-filled glove box,
where the light-emitting layer, containing host, 1,3-bis[4-tert-
butylphenyl0-1,3,4-oxidiazolyl]phenylene (OXD-7) or 2-(4-
C
77H69N3: C, 89.23; H, 6.71; N, 4.05. Found: C, 89.10; H, 6.72; N,
4.04.
2.3.3. 27DPF-TCz. White solid. Yield 35%. 1H NMR (300 MHz, CDCl3,
d
): 8.28 (s, 4H), 8.21 (s, 10H), 7.94 (d, J¼5.2 Hz, 2H), 7.92 (d, J¼5.2 Hz,
2H), 7.70e7.60 (m, 8H), 7.52e7.36 (m, 26H), 1.51 (s, 72H). 13C NMR

J. Tang et al. / Tetrahedron 70 (2014) 3847e3853
3849
(300 MHz, CDCl3,
d
): 153.8, 144.9, 142.8, 140.1, 138.7, 137.0, 131.1,
126.5, 126.2, 126.1, 125.9, 124.3, 124.1, 123.0, 122.8, 118.0, 111.7, 34.6.
MS (MALDI-TOF) [m/z]: 1754.9 [Mþ]. Anal. Calcd for C129H122N6: C,
88.21; H, 7.00; N, 4.78. Found: C, 88.18; H, 6.93; N, 4.76.
129.3, 129.0, 128.7, 128.3, 128.1, 127.5, 126.5, 126.3, 126.1, 124.8,
124.2, 123.9, 123.6, 123.2, 122.0, 119.5, 116.4, 111.3, 109.2, 32.1. MS
(MALDI-TOF) [m/z]: 1756.8 [Mþ]. Anal. Calcd for C129H124N6: C,
88.11; H, 7.11; N, 4.78. Found: C, 88.05; H, 7.15; N, 4.75.
3. Results and discussion
2.3.4. 27SBF-TCz. White solid. Yield 34%. 1H NMR (300 MHz, CDCl3,
3.1. Synthesis of the materials
d
): 8.24 (s, 4H), 8.20 (s, 8H), 7.82 (d, J¼7.6 Hz, 6H), 7.65 (d, J¼7.5 Hz,
4H), 7.47e7.30 (m, 18H), 7.01 (d, J¼7.5 Hz, 6H), 6.95 (d, J¼7.5 Hz,
4H), 1.50 (s, 72H). 13C NMR (300 MHz, CDCl3,
): 153.7, 150.6, 144.3,
144.0, 143.2, 141.8, 138.2, 133.2, 131.1, 130.5, 128.9, 128.6, 126.8,
The synthetic routes for the tri-carbazole/fluorene-based ma-
terials (2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz) are shown
in Scheme 1. Preparation of the intermediates 3,300,6,600-tetra-tert-
d
Scheme 1. Synthetic routes of target materials.

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J. Tang et al. / Tetrahedron 70 (2014) 3847e3853
butyl-90H-9,30:60,900-tercarbazole (TCz) requires three steps. Tosy-
lation of carbazole and subsequent iodination afford 3,6-diiodo-9-
tosyl-9H-carbazole. Reaction of 3,6-diiodo-9-tosyl-9H-carbazole
with 3,6-di-tert-butyl-9H-carbazole, followed by deprotection of
the carbazole nitrogen, gives TCz. 2-Bromo-9,9-diphenyl-9H-fluo-
rene, 2-bromo-9,90-spirobi[fluorene], 2,7-dibromo-9,9-diphenyl-
9H-fluorene, and 2,7-dibromo-9,90-spirobi[fluorene] were pre-
pared according to the published procedures.44,45 Finally, the aro-
matic CeN coupling reactions of 2-bromo-9,9-diphenyl-9H-
periphery. Noticeably, the glass transition temperatures (Tg) of the
compounds fall in the range of 231e310 ꢀC, which are much higher
than those of commercially available hosts such as 1,3-bis(9-car-
bazolyl)benzene (mCP) (Tg¼60 ꢀC) and CBP (Tg¼62 ꢀC). The high Tg
values for these compounds should be ascribed to their high mo-
lecular weight and rigid structures, which can suppress the crys-
tallization of the compounds. The morphology of the thin films
prepared by spin coating a blend solution of the host materials,
FIrpic or Ir(mppy)3 onto PEDOT:PSS layer was investigated by AFM
measurements. As shown in Fig. 2, the films of 2DPF-TCz and 2SBF-
TCz doped with FIrpic are free of pinholes and quite smooth, with
small root-mean-square (rms) values of 0.52 and 0.49 nm, re-
spectively. In addition, the rms values of 27DPF-TCz and 27SBF-TCz
films doped with Ir(mppy)3 are 0.32 and 0.35 nm, respectively.
From the AFM measurements, no phase separation or any aggre-
gated domains were observed. These results indicate that the newly
developed host materials can form high quality thin films from
solution-processes.
fluorene,
2-bromo-9,90-spirobi[fluorene],
2,7-dibromo-9,9-
diphenyl-9H-fluorene and 2,7-dibromo-9,90-spirobi[fluorene]
with TCz leads to 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz
with moderate yields. All compounds show good solubility in
common organic solvents, and can be easily purified by the silica
column method and recrystallization, yielding pure white powders.
1H NMR, 13C NMR, matrix-assisted laser desorption ionization time-
of-flight (MALDI-TOF) mass spectrometry, and elemental analysis
confirmed the chemical structures of the materials.
3.2. Thermal analysis and film-forming properties
Table 1
The thermal, electrochemical, and photophysical data of 2DPF-TCz, 2SBF-TCz,
27DPF-TCz, and 27SBF-TCz
Fig. 1 shows thermal gravimetric analyses (TGA) and differential
scanning calorimetry (DSC) of the new compounds, and the de-
tailed data are listed in Table 1. As shown in Fig. 1, the thermal
decomposition temperatures (Td, corresponding to 5% weight loss)
of the compounds fall in the range of 459e502 ꢀC. The Td increases
with increasing the number of tri-carbazole moieties in the
Tg/Td [ꢀC] lmax,abs lem
lem
[nm]a [nm]b
ET[eV] HOMOc/LUMOd [eV]
[nm]a
2DPF-TCz
2SBF-TCz
27DPF-TCz 304/495
27SBF-TCz 310/502
231/459
249/474
327
326
349
350
400
402
409
410
455
454
474
472
2.72
2.73
2.61
2.63
ꢁ5.36/ꢁ1.94
ꢁ5.32/ꢁ1.93
ꢁ5.25/ꢁ1.96
ꢁ5.23/ꢁ1.97
a
Measured in CH2Cl2.
Measured at 77 K.
Deduced from cyclic voltammetry.
Estimated from the optical band gap together with HOMO values.
b
c
d
3.3. Electrochemical properties
Cyclic voltammetry (CV) measurement was performed to ex-
plore the electrochemical properties of the compounds (Fig. 3).
Upon the anodic sweep in dichloromethane, all compounds
showed similar three reversible oxidation processes. No reduction
process was detected. The excellent reversible oxidation properties
can be attributed to the introduction of two tert-butyl groups at the
C3 and C6 positions of carbazole. According to the onset potentials
in oxidation, we calculated the HOMO energy levels of 2DPF-TCz,
2SBF-TCz, 27DPF-TCz, and 27SBF-TCz using ferrocene as an internal
standard (ꢁ4.8 eV below vacuum),which are ꢁ5.36, ꢁ5.32, ꢁ5.25,
and ꢁ5.23 eV, respectively. The HOMO levels of these compounds
are higher than those of PVK (ꢁ5.8 eV) and mCP (ꢁ6.15 eV), and
approach the work function (ꢁ5.2 eV) of (PEDOT:PSS). The LUMO
energy levels of 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz are
estimated to be ꢁ1.94, ꢁ1.93, ꢁ1.96 and ꢁ1.97 eV, respectively, by
subtracting the HOMO levels with the optical band gaps obtained
from the absorption spectra.
3.4. Photophysical properties
Fig. 4 depicts the absorption, photoluminescence (PL), and
phosphorescence spectra of the compounds in CH2Cl2 and all the
data are summarized in Table 1. The compounds with the same tri-
carbazole unit exhibit almost identical spectral features. Obviously,
the absorption is basically determined by the tri-carbazole unit and
the attached substituents. 2DPF-TCz and 2SBF-TCz exhibit similar
absorption spectra with three absorption bands centered at
250e350 nm and the maximums band peaked at 326 and 327 nm
are attributed to the lowest
pep transition of the entire conju-
*
gated backbone. Compared to the absorption spectra of 2DPF-TCz
and 2SBF-TCz, the absorption spectra of 27DPF-TCz and 27SBF-TCz
Fig. 1. TGA traces (a) and DSC traces (b) of 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-
TCz recorded at a heating rate of 10 ꢀC minꢁ1
.

J. Tang et al. / Tetrahedron 70 (2014) 3847e3853
3851
Fig. 2. AFM topographic images of 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz.
Fig. 3. Oxidation part of the CV curves of 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-
TCz in CH2Cl2 solutions (10ꢁ3 M).
have similar features but show 22e24 nm red-shifts, with the ab-
sorption peaks at 349 and 350 nm. Upon UV excitation, the PL
spectra of these four compounds show different peaks in dilute
CH2Cl2 solutions. 2DPF-TCz and 2SBF-TCz exhibit structureless
emission with emission peaks at 400 and 402 nm, respectively,
while 27DPF-TCz and 27SBF-TCz show 8e9 nm red-shifts in PL
spectra compared to 2DPF-TCz and 2SBF-TCz. Their phosphores-
cence spectra were measured in a frozen CH2Cl2 matrix at 77 K. The
triplet energy (ET) of 2DPF-TCz and 2SBF-TCz are determined to be
2.72 and 2.73 eV, respectively, which are higher than those of the
commonly used sky-blue triplet emitter FIrpic. It is worth noting
that 27DPF-TCz and 27SBF-TCz show lower ET values of 2.61 and
2.63 eV, respectively, indicating that linking bulky and large-gap
moieties into the 2,7 positions of fluorine significantly decreases
the triplet energy of the whole molecules due to increased conju-
gation length. Hosts with high triplet energies are necessary pro-
vision for effective confinement of the triplet excitons on the guest
by prevention of back energy transfer from the guest to the host. To
further confirm their exciton confinement property, the transient
PL decays of 5 wt % FIrpic doped 2DPF-TCz, 2SBF-TCz, 27DPF-TCz,
and 27SBF-TCz films were measured. The 27DPF-TCz:FIrpic and
27SBF-TCz: FIrpic films exhibit extremely long decay component,
which can be attributed to the exothermic back energy transfer
from FIrpic to 27DPF-TCz and 27SBF-TCz, while 2DPF-TCz:FIrpic
and 2SBF-TCz: FIrpic films clearly exhibit monoexponential decays,
indicating that the triplet energy transfer from guest to host is
completely suppressed. Furthermore, the 27DPF-TCz:Ir(mppy)3 and
27SBF-TCz:Ir(mppy)3 (5%) films were investigated. The transient PL
decays of these films show monoexponential decay curves, in-
dicating that the triplet energy is well confined on Ir(mppy)3
molecules in the emissive layer. Based on these experimental re-
sults, the newly synthesized compounds with high triplet energy
Fig. 4. Normalized absorption and emission spectra (a), phosphorescence (77 K)
spectra (b), and transient photoluminescence decay curves (c) of 2DPF-TCz, 2SBF-TCz,
27DPF-TCz, and 27SBF-TCz.

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J. Tang et al. / Tetrahedron 70 (2014) 3847e3853
levels are expected to serve as high performance host materials for
FIrpic and Ir(mppy)3.
materials to facilitate the electron transport in the light-emitting
layer; FIrpic and Ir(mppy)3 with optimized concentrations of 10
and 8 wt %, respectively, were used as the triplet emitter; TPBI was
used as hole-blocking and exciton-confining layer; Cs2CO3 was
used as the electron-injection layer. The current densi-
tyevoltageeluminance (JeVeL) curves and key characteristics of
the devices were presented in Fig. 5 and Table 2. As shown in Table
2, the EL spectra of A-type devices are almost identical with CIE
coordinates of (0.17, 0.35) for 2DPF-TCz and (0.16, 0.32) for 2SBF-
TCz, respectively, corresponding to the sky-blue emission from
FIrpic. No additional emission from 2DPF-TCz or 2SBF-TCz was
observed, indicating efficient energy transfer from the host to
FIrpic. Since the similar molecular configuration, triplet energy, and
film-forming property, as illustrated in Fig. 5, the devices with these
two host materials (2DPF-TCz and 2SBF-TCz) showed the same
turn-on voltages of 5.1 V (voltage to reach 1 cd mꢁ2), the similar
maximum LE values of 21.2 and 19.8 cd Aꢁ1, respectively, and the
corresponding maximum external quantum efficiencies of 10.5 and
10.0%, respectively. The performances of the devices are far supe-
rior over those of mCP-based devices.26 As to 27DPF-TCz and
3.5. Electroluminescent properties
To evaluate these solution-processable compounds as host
materials in PhOLEDs, we fabricated a series of devices using FIrpic
and Ir(mppy)3 as the blue and green phosphorescent dopants, re-
spectively. The A-type devices with
a configuration of ITO/
PEDOT:PSS/2DPF-TCz or 2SBF-TCz:OXD-7(30 wt %):FIrpic(10 wt %)
/TPBI/Cs2CO3/Al and the B-type devices with a configuration of ITO/
PEDOT:PSS/27DPF-TCz or 27SBF-TCz:PBD(30 wt %):Ir(mppy)3
(8 wt %)/TPBI/Cs2CO3/Al have been fabricated by spin coating the
light emitting layer from solutions. The conducting polymer
PEDOT:PSS was used as the hole-injection layer; the electron-
transporting material OXD-7 or PBD were mixed into host
27SBF-TCz, due to the extend p-conjugation length, the ET of them
can only confine the green emission triplet emitter. Therefore, the
B-type devices showed the state-of-art device efficiency with the
maximum LE values of 27DPF-TCz and 27SBF-TCz based devices are
34.8 cd Aꢁ1 and 33.6 cd Aꢁ1, and the corresponding external
quantum efficiencies of 9.8 and 9.5%, respectively. In addition, the
turn-on voltages of 27DPF-TCz and 27SBF-TCz based devices are
4.6 and 4.7 V, respectively. No additional emission from the host
materials was observed, indicative of efficient energy transfer from
the hosts to Ir(mppy)3. Compared to the previous published CBP
core structure compound,29 the improved device performance with
the tri-carbazole modified fluorene as the host material can be
attributed to the excellent film forming property, high triplet en-
ergy, well matched energy levels with the PEDOT:PSS layer and
a balanced charge recombination in the devices.
Table 2
Summary of the characteristics of FIrpic and Ir(mppy)3 doped phosphorescent
devices
Von [V]a hc,maxb [cd Aꢁ1
]
hext,max [%] CIE [x,y]d
c
Device Host
A
2DPF-TCz
2SBF-TCz
27DPF-TCz 4.6
5.1
5.1
21.2
19.8
34.8
33.6
10.5
10.0
9.8
(0.17,0.35)
(0.16,0.32)
(0.29,0.61)
(0.32,0.60)
B
27SBF-TCz
4.7
9.5
a
b
c
Recorded at 1 cd mꢁ2
Maximum current efficiency.
Maximum external quantum efficiency.
Measured at 8 V.
.
d
4. Conclusions
We designed and synthesized a series of novel host materials:
2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz for solution-
processed PhOLEDs. By substitution at the 2 and (or) 7 positions
of fluorene with the rigid Tri-carbazole units, the new compounds
showed twisted configurations, which effectively suppress crys-
tallization of the compounds, and render excellent film formation
property and high triplet energy level for the compounds. Utilizing
these new compounds as hosts, we achieved highly efficient
solution-processed blue PhOLEDs with FIrpic as
a dopant
(21.2 cd Aꢁ1, 10.5%) and green PhOLEDs with Ir(mppy)3 as a dopant
(34.8 cd Aꢁ1, 9.8%). The high EL performance can be attributed to
the high triplet energy level, well matched energy levels with the
PEDOT:PSS layer and excellent film forming property of the new
host materials. The design principles for the host materials in this
Fig. 5. IeVeL characteristics (a), luminance efficiency versus current density plots (b),
and EL spectra (c) of 2DPF-TCz, 2SBF-TCz, 27DPF-TCz, and 27SBF-TCz.

J. Tang et al. / Tetrahedron 70 (2014) 3847e3853
3853
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 51103023 and 21173042) and the
National Basic Research Program of China (Grant No.
2013CB932900).
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