High thermal stability 3, 6-fluorene-carbazole-dendrimers as host materials for efficient solution-processed blue phosphorescent devices

Article abstract of DOI:10.1016/j.tet.2013.04.004

A novel series of solution-processable 3,6-disubstituted-fluorene-carbazole based host materials 36FCzG1 and 36FCzG2 are designed and synthesized. Owing to the highly asymmetry tetrahedral configuration, these hosts exhibit high glass transition temperatures (T_g) (161 and 162 C, respectively), high triplet energy levels (2.80 and 2.80 eV, respectively), excellent film forming capabilities, and chemical miscibility. Phosphorescent organic lighting-emitting diodes (OLEDs), which base on these host materials doped with the guests of iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) by spin coating, possesses a low turn-on voltage of 4.0 V, a maximum efficiency of 18.5 cd/A (8.1 lm/W), and a maximum external quantum efficiency of 10.3%. These results show that the devices are among the excellent solution processable blue phosphorescent OLEDs based on dendrimers. Furthermore, a novel way is developed to construct solution processable small molecules based on 3,6-disubstituted fluorene and carbazole dendrimers combined in a highly rigid configuration.

Full text of DOI:10.1016/j.tet.2013.04.004

Tetrahedron 69 (2013) 4169e4175
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
High thermal stability 3, 6-fluorene-carbazole-dendrimers as host
materials for efficient solution-processed blue phosphorescent
devices
,
a
b
Yinbo Qian a , Feng Cao , Wenping Guo
*
a School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR China
b Wuhan National Laboratory for Optoelectronics, Wuhan 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
novel series of solution-processable 3,6-disubstituted-fluorene-carbazole based host materials
Received 26 November 2012
Received in revised form 23 March 2013
Accepted 2 April 2013
36FCzG1 and 36FCzG2 are designed and synthesized. Owing to the highly asymmetry tetrahedral con-
figuration, these hosts exhibit high glass transition temperatures (Tg) (161 and 162 ꢀC, respectively), high
triplet energy levels (2.80 and 2.80 eV, respectively), excellent film forming capabilities, and chemical
miscibility. Phosphorescent organic lighting-emitting diodes (OLEDs), which base on these host materials
doped with the guests of iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) by spin coat-
ing, possesses a low turn-on voltage of 4.0 V, a maximum efficiency of 18.5 cd/A (8.1 lm/W), and
a maximum external quantum efficiency of 10.3%. These results show that the devices are among the
excellent solution processable blue phosphorescent OLEDs based on dendrimers. Furthermore, a novel
way is developed to construct solution processable small molecules based on 3,6-disubstituted fluorene
and carbazole dendrimers combined in a highly rigid configuration.
Available online 6 April 2013
Keywords:
PhOLED
Dendrimer
Solution-processable
Carbazole
Fluorene
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
PhOLEDs, the hosts would possess the following properties:
(i) A high triplet energy gap (>2.70 eV), which prevents the reverse
Organic light-emitting diodes (OLEDs) have attracted much at-
tention as it has been recognized as the next-generation high-
quality full color displays.1e5 The efficiencies of OLEDs have been
improved dramatically because of the development of efficient
phosphorescent hosts & dopants containing transition metals that
can harvest both singlet and triplet excitons for emission, providing
the opportunity to realize internal quantum efficiency reach to
100% theoretically.6e10 To date, most of the efficient PhOLEDs have
been fabricated through vacuum thermal evaporation in multilayer
configurations.11e14 But it requires complex technological processes
and a large amount of organic materials are wasted, leading to
relatively high fabrication costs, at the same time, pixilation limits
large-size scalabilities, and high-resolution applications.15 Solution
processes offers an attractive alternative to vacuum deposition
techniques, mainly due to their better compatibility with low-cost
production techniques and large substrates. Compared with suc-
cess of developing the red and green PhOLEDs fabricated by solu-
tion processing,16,17 high performance solution-processed blue
PhOLEDs are still scarce, owing to the lack of appropriate host
materials. Generally, for high efficient solution-processed blue
energy transfer from the dopant to the host; (ii) Suitable highest
occupied/lowest unoccupied molecular orbital (HOMO/LUMO) en-
ergy levels matching those of the adjacent layers are required to
reduce the operational voltage; (iii) Good solubility and film for-
mation ability.
Recently, solution-processed blue PhOLEDs using small organic
molecules and conjugated polymers as host materials have been
reported.18e22 For polymer host materials, impurities, especially
the metal catalyst used in polymer synthesis would dramatically
influence the performance of OLEDs. In contrast to the polymer
hosts, the small molecule hosts can be easily synthesized and
purified, but the configuration isn’t rigid enough so that the ma-
terial would be recrystallized easily. Dendrimer hosts combine the
advantages of both the polymers and the small molecules. As far as
we know, almost all of them incorporates a solubilizing group
such as an alkyl or alkoxy chain to overcome their poor solubilities
and film-forming abilities. It is well known that alkyl or alkoxy
groups are electrically insulating, and introducing such groups
into a molecule to affect its conductivity. Even worse, by attaching
alkyl or alkoxy chains to a molecule, its glass-transition temper-
ature (Tg) drops rapidly, which reduces morphological stability,
especially when the molecule is small.23,24 Thus it is very difficult
for dendrimers to possess both morphological stability and
* Corresponding author. E-mail address: purse@smail.hust.edu.cn (Y. Qian).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.04.004

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Y. Qian et al. / Tetrahedron 69 (2013) 4169e4175
solution processability, especially for blue phosphorescent host
materials whose effective conjugation length must be very short
to encompass the phosphor emitter. Kakimoto et al. reported that
solution-processed green PhOLEDs hosted by triphenylamine/
benzimidazole hybrids, and a maximum current efficiency of
27.3 cd/A was realized by using fac-tris(2-phenylpyridine)iridium
(Ir(ppy)3) as a guest, but this host material couldn’t be used as the
blue phosphorescent for the low triplet energy level.25 Usluer
et al. designed
a series of compounds including the 2,7-
disubstituted fluorene derivatives and carbazole dendrimers,
a maximum current efficiency of 7.7 cd/A was harvested by using
the Alq3 as emitter layer and these compounds as hole transport
layer (HTL).26 Qiu et al. also reported a series of carbazole den-
drimer hosts, a maximum current efficiency of 11.5 cd/A was
achieved based on the deep-blue phosphorescent emitter FIr6, but
the electron transporting material OXD-7 was also added for the
double hosts.27
As we all know, 2,7-disubstituted fluorene is often used as
common groups in the fluorescence emitters for both high quan-
tum yield and excellent film formation ability, but it isn’t suitable
for blue PhOLED materials for the low triplet energy levels. Re-
cently, 3,6-substituted fluorene derivatives were reported as the
promising blue hosts for polymer PhOLEDs.28 However, blue PhO-
LEDs using 3,6-substituted fluorene derivatives as dendrimer
host materials have not been explored. In this study, we design
and synthesis a series of 3,6-substituted fluorene derivatives in-
cluding carbazole dendrimers 9,90-(9-phenyl-9-(9-phenyl-9H-car-
bazol-3-yl)-9H-fluorene-3,6-diyl)bis(3,6-di-tert-butyl-9H-carbazo-
le) (36FCzG1), 9,90-(9-phenyl-9-(9-phenyl-9H-carbazol-3-yl)-9H-
fluorene-3,6-diyl)bis(3,6-bis(3,6-di-tert-butyl-9H-fluoren-9-yl)-9H
-carbazole) (36FCzG2), using a 3,6-substituted fluorene derivatives
as the rigid core with two carbazole dendrimer groups linked to the
core part through the 3,6-positions. Additionally, tert-butyl groups
are introduced into the molecular structure to ensure good solu-
bility and to form high-quality films. As a result, the newly syn-
thesized compounds possess three important characteristics: (I)
relative high triplet energy levels (2.80 eV) because of the non-
conjugated linkage; (II) appropriate HOMO energy levels (ꢁ5.21 to
ꢁ5.36 eV), thereby facilitating the transfer of holes from poly(3,4-
ethyle-nedioxy-thiophene): poly(styrene-4-sulfonate) (PEDOT:
PSS) to the emitting layer; (III) the high thermal stability of forming
stable amorphous thin films as a result of the highly twisted con-
figuration of the molecules. In our work, with the newly synthe-
sized hosts and the blue phosphorescent emitter FIrpic, the device
performance reaches a maximum efficiency value of 18.5 cd/A. This
series of molecules exhibits excellent thermal and morphological
stability, good film-forming ability, and solubility making them
very promising candidates for optoelectronics.
Scheme 1. Synthetic routes toward the compounds 36FCzG1 and 36FCzG2.
All compounds were purified using the silica gel column
method, producing very pure powders. 1H, 13C NMR, mass spec-
trometry, and elemental analysis were employed to confirm the
chemical structures of above mentioned compounds as described
in the Experimental section. This result matches the 3D model of
the two compounds optimized by the Amsterdam Density Func-
tional 2009.01 (ADF2009.01) program, which is to be discussed in
the following section.
3. Thermal analysis
For a better insight into the structureeproperty relationship,
thermal properties were measured. Fig. 1 shows high thermal sta-
bility as determined by thermogravimetric analysis (TGA), and the
decomposition temperature (Td), corresponding to 5%-weight-loss,
is 464 and 461 ꢀC for the compounds 36FCzG1 and 36FCzG2, re-
spectively. The Tg explored by differential scanning calorimetry
(DSC) reached 161 ꢀC for 36FCzG1, and 162 ꢀC for 36FCzG2, com-
pared with N,N0-di(naphthalen-1-yl)-N,N0-diphenylbiphenyl-4,40-
diamine (NPB) (Tg¼98 ꢀC), a commonly used material in OLEDs, our
compounds show greatly improved thermal resistance.30 It is ob-
viously related to their increased molecular sizes by the in-
troduction of tert-butyl groups in their structures.
100
80
36FCzG1
36FCzG2
2. Results and discussion
2.1. Synthesis and characterization
161 oC
162 oC
The synthetic routes and chemical structures of 36FCzG1 and
36FCzG2, are shown in Scheme 1. The carbazole dendrimers G1 and
G2, and 3,6-dibromo-9H-fluoren-9-one were synthesized accord-
ing to a literature method.28,29 3,6-Dibromo-9-phenyl-9H-fluoren-
9-ol (1) was synthesized by using Grignard Reagent with a high
yield (70%). The following reaction of the compound 1 and N-
phenylcarbazole losses a H2O molecule through the strong acid to
form 3-(3,6-dibromo-9-phenyl-9H-fluoren-9-yl)-9-phenyl-9H-car-
bazole (2), and the final products are prepared by the classic Ull-
mann reaction of the compound 2 and the compounds G1 and G2 in
the presence of a catalytic amount of copper(I) iodide and 18-
crown-6 in 1,3-dimethyltetra-hydropyrimidin-2(1H)-one (DMPU)
with high yield 85% and 88%, respectively.
60
40
100 Temperature200oC
( )
50
150
250
100
200
300
400
500
600
700
Wavelength (nm)
Fig. 1. TGA traces of 36FCzG1 and 36FCzG2 recorded at a heating rate of 10 ꢀC/min.
Inset: DSC measurement recorded at a heating rate of 10 ꢀC/min.

Y. Qian et al. / Tetrahedron 69 (2013) 4169e4175
4171
A high Td makes it possible to endure high temperatures ac-
companied by thermal vacuum sublimation during device fabri-
cation or further purification by gradient sublimation. All the
compounds, which show no glass transition within the experi-
mental temperature range, displayed intrinsic amorphous proper-
ties. It is a very desirable feature for photoelectronic applications.
Furthermore, atomic force microscopy (AFM) was used to in-
vestigate the morphology of films prepared from the compounds
doped with FIrpic, through spin-coating a chlorobenzene solution
onto the PEDOT:PSS layer. As shown in Fig. 2, the surface of the
compounds doped with 20 wt % FIrpic, are free of pinholes and
quite smooth, and the root-mean-square (rms) values range from
0.55 to 0.44 nm. No phase separation or any aggregated domains
were observed. However, the films prepared from mCP doped with
FIrpic onto the PEDOT:PSS layer exhibits a kind of aggregation, or
phase segregation. The results demonstrate that these new com-
pounds can form uniform amorphous films upon solution-
processing, which is highly important in improving the efficiency
and lifetime of OLEDs.
be ꢁ5.80 and ꢁ5.82 eV, for 36FCzG1 and 36FCzG2, respectively,
according to the corresponding onset potentials of 1.10 and 1.12 eV,
respectively. The HOMO energy levels are clearly raised and ap-
proach the work function of PEDOT (ꢁ5.2 eV). These results reveal
that the introduction of two carbazole groups linked through the
3,6 positions of the fluorene unit can lead to the reduction of the
hole injection barrier, thereby facilitating the injection of positive
charge carriers. All of these results together with absorption spectra
were used and then the LUMO energy levels are obtained (Table 1).
3.2. Theoretical calculations
To gain insight into the relationship of the compound 36FCzG1
at the molecular level, the geometrical and electronic properties of
the compounds were studied using density functional theory (DFT)
calculations. The geometries of these new compounds in Fig. 4
showed that the carbazole dendrimers and phenyl units are sig-
nificantly twisted with the fluorene core, resulting in a non-planar
structure in each molecule. The calculated HOMO and LUMO levels
are ꢁ5.40 and ꢁ2.13 eV, respectively, which are in good agreement
with the experimental results. These geometrical characteristics
can effectively prevent intermolecular interactions between p-
systems and thus suppress molecular recrystallization and limit the
extent of conjugation between the central core and the branches.
As a result, it improves the morphological stability of thin film and
keeps the triplet energy gap at a very high level in these molecules.
Calculated HOMO and LUMO density maps of the compounds are
also included in Fig. 4. The LUMO levels of all the compounds are
localized predominantly on the fluorene core, while the HOMO
levels are distributed over the electron-rich tert-butyl-carbazole
fragment. The calculated values of the energy levels are in agree-
ment with the results measured by electrochemical CV.
Fig. 2. AFM topographic images of 36FCzG1 (left) and 36FCzG2 (right) with 20 wt %
FIrpic in thin solid films (40 nm thick).
3.1. Electrochemical analysis
4. Photophysical properties
The electrochemical properties of the compounds were studied
in solution through cyclic voltammetry (CV) using supporting
electrolyte tetrabutylammoniumhexafluorophosphate (TBAPF6)
(Fig. 3). During the anodic scan in dichloromethane, 36FCzG1, and
36FCzG2 showed reversible oxidation behavior, which could be
attributed to the introduction of two tert-butyl groups at the 3,6-
positions of carbazole, suggesting that the compounds can be sta-
ble upon accepting holes in OLEDs. As revealed in the literature,31 it
is important to block the active sites of carbazole derivatives when
the compounds transport positive charge carriers in devices.
Fig.
5 exhibits the room-temperature absorption, photo-
luminescent (PL) spectra of the compounds in CH2Cl2 and the
phosphorescent spectra of the compounds in 2-methyltetra-
hydrofuran and all the results are summarized in Table 1.
The absorption spectra of the two compounds exhibit similar
patterns with three absorption bands centered at 250e350 nm,
which are nearly identical to those of the unsubstituted carbazole
monomer and thus can be attributed to the nep* and pep* tran-
sitions33 upon UV excitation, the PL spectra lost vibronic structure
and the emission peaks red-shifted to 393e398 nm with respect to
the carbazole monomer emission. Due to the steric effect of the two
tert-butyl groups at the 3,6-positions of the carbazole, the peaks of
absorption and emission of 36FCzG2 are shifted to longer wave-
length, relative to the compound 36FCzG1. The emission of
36FCzG2 has the shoulder peak at about 360 nm, which would be
attributed to the second generation carbazole dendrimer groups.
The highest energy 0e0 phosphorescent emission was used to
calculate the triplet energy level, giving a value of 2.80 eV for
36FCzG1, 2.80 eV for 36FCzG1, higher than the values of the com-
monly used triplet blue-emitter FIrpic (2.62 eV).34 Using host ma-
terials that possess high triplet energies is a provision for effective
confinement of the triplet excitons on the guest, while it prevents
back energy transfer between the host and dopant molecules.
Based on the results of the measurements, the newly synthesized
compounds with high triplet energy levels are expected to serve as
appropriate hosts for FIrpic.
4
36FCzG1
3
36FCzG2
2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential (V vs Ag/Ag+)
Fig. 3. Cyclic voltammograms of 36FCzG1 and 36FCzG2 in dilute dichloromethane
solution.
5. Electroluminescent devices
The HOMO energy levels were obtained according to the fol-
lowing equation: HOMO¼ꢁ(4.7þEoox) eV,32 using Ag/Agþ as
a reference anode (ꢁ4.7 eV in a vacuum). These were calculated to
In view of the high triplet energy (ET¼2.80 eV) of 36FCzG1
and 36FCzG2, their applications as host material for blue

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Y. Qian et al. / Tetrahedron 69 (2013) 4169e4175
Table 1
The thermal, electrochemical, and photophysical data of 36FCzG1 and 36FCzG2
Tg/Td (ꢀC)
lmax,abs (nm)a
lem (nm)a
lem (nm)b
ET (eV)
HOMO/LUMOc (eV)
HOMO/LUMOd (eV)
36FCzG1
36FCzG2
161/464
162/461
347, 296
348, 296
393
398
443
443
2.80
2.80
5.80/2.32
5.82/2.38
ꢁ5.40/ꢁ2.13
d/d
a
Measured in CH2Cl2.
Measured in 2-MeTHF glass at 77 K.
Deduced from cyclic voltammetry.
b
c
d
Estimated from the optical band gap together with HOMO values.
Fig. 4. Optimized geometries and calculated HOMO and LUMO density pictures for 36FCzG1 according to DFT calculations at the B3LYP/6-31* level.
(a)
electrophosphorescent devices were investigated by a simple
three-layer configuration: indium tin oxide (ITO)/poly(3,4-
ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS)
(40 nm)/Host: FIrpic (20 wt %) (40 nm)/TmPyPB (40 nm)/LiF (1 nm)/
Al (100 nm). Where the conducting polymer PEDOT:PSS was used
as the hole injection layer; FIrpic with an optimized concentration
of 20 wt % was used as the emitter; Both TPBI and TmPyPB were
used as the electron transporting layer and the hole- and exciton-
confining layer; and LiF was used as the electron injection layer.
Fig. 6 presents the current densityevoltageseluminance (JeVeL)
characteristics and curves of efficiency versus luminance of the
devices. According to the JeV characteristic curves of the devices,
300
200
100
0
4
3
2
1
0
10
10
10
10
10
36FCzG1
36FCzG2
6
8
10
12
14
Voltage (V)
(b)
2
1
0
2
10
10
10
10
10
10
(a)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
36FCzG1
1
36FCzG2 0.8
0.6
0.4
0.2
0.0
36FCzG1
36FCzG2
0
-1
10
-2
-1
10
10
4
-1
0
1
2
3
10
10
10
10
10
10
250 300 350 400 450 500 550
2
Luminance (Cd/m )
(c)
Wavelength (nm)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
36FCzG1
36FCzG2
36FCzG1
36FCzG2
400
500
600
700
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 5. (a) Normalized absorption and emission spectra of 36FCzG1 and 36FCzG2 in
dilute dichloromethane solution; (b) the normalized phosphorescence spectra of
36FCzG1 and 36FCzG2 in a frozen 2-methyltetrahydrofuran matrix at 77 K.
Fig. 6. (a) Current efficiencyevoltageeluminance (JeVeL) curves of the two devices;
(b) Luminanceecurrent efficiency and luminanceepower efficiency characteristics; (c)
EL spectra of the two devices.

Y. Qian et al. / Tetrahedron 69 (2013) 4169e4175
4173
the introduction of two tert-butyl groups in the molecules didn’t
2
4
6
LUMO
2.9
LiF/Al
2.6
lead to a decrease in current density, the turn-on voltages (corre-
sponding to 1 cd/m2) of the devices are similar with the previously
reported values based on solution processed blue electro-
phosphorescent devices. The turn-on voltage indicates charge in-
jection, transport, and combination isn’t efficiently, most likely due
to the HOMO energy levels of these hosts aren’t well matched with
the PEDOT:PSS and thereby the hole injection of the device is dif-
ficult. The devices based on 36FCzG1 and 36FCzG2 show a maxi-
mum luminance (Lmax) of 6302 cd/m2 (12.6 V) and 7626 cd/m2
2.0
2.1
3.0
3.3
S
S
1
c
2
i
P
B
:
G
p
G
T
P
a
r
a
I
y
O
4.7
C
C
D
P
F
E
m
P
ITO
T
5.5
5.7
5.5
5.2
(12.4 V), a maximum efficiency (hc max, hp,max) of 18.5 cd/A, 8.1 lm/
,
W and 16.1 cd/A, 7.8 lm/W, corresponding to a maximum external
quantum efficiency (hext,max) of 10.3% and 9.7%, respectively. The
performances of the devices are far superior to those of the cor-
responding mCP based devices (Lmax of 8900 cd/m2, hc,max of 6.2 cd/
A and hext,max of 3.1%) in previously reported.19 Table 2 summarizes
the performance of the devices based on hosts 36FCzG1 and
36FCzG2. As shown in Table 2, this devices performance is also
consistent with the view that the introduction of tert-butyl groups
into the host materials can reduce the intermolecular interactions
and suppress tripletetriplet annihilation in the devices.35
6.7 HOMO
Fig. 7. Energy level diagram of HOMO and LUMO levels (relative to vacuum level) for
materials investigated in this work.
molecular recrystallization and limit the extent of conjugation,
and exhibited excellent film formation ability, high triplet energy
level and high thermal stability. Utilizing these new compounds
as hosts, the solution-processed blue phosphorescence OLEDs
with FIrpic as a dopant show normal turn-on voltage of 4.0 V,
a maximum current efficiency of 18.5 cd/A, a maximum efficiency
of 8.1 lm/W and a maximum external quantum efficiency 10.3%.
This can be attributed to the high triplet energy level and ex-
cellent film forming ability. The performances of the devices are
far superior to those of the corresponding mCP-based devices,
which is outstanding for a solution-processed blue phosphores-
cent OLED. This work suggests that highly efficient solution
processed blue phosphorescence OLEDs can be achieved in the
design of such host materials with high thermal stability, high
triplet energy, excellent film-forming ability, and morphological
property.
Table 2
Summary of the characteristics of FIrpic doped blue phosphorescent devices
Host
Von
Lmax
hc,max
(cd/A)c
hp,max
(lm/W)d
hext,max
(%)e
CIE (x,y)f
(V)a
(cd/m2)b
36FCzG1
36FCzG2
5.8
5.5
6302 (12.6 V)
7626 (12.4 V)
18.5
16.1
8.1
7.8
10.3
9.7
(0.15,0.31)
(0.15,0.31)
a
Recorded at 1 cd/m2.
Maximum luminance.
Maximum current efficiency.
Maximum powder efficiency.
Maximum external quantum efficiency.
b
c
d
e
f
Measured at 8 V.
7. Experimental
The electroluminescence (EL) spectra of devices are almost
7.1. General information
identical with the Commission Internationale de L’Eclairage (CIE)
coordinates of (0.15,0.31), corresponding to the emission of FIrpic,
which was comparable to the literature value reported by Kido’s
group,36 with CIE coordinates of (0.16,0.32). When the driving
voltage was increased to 12 V, the CIE coordinates of the emission
color remained almost unchanged. Furthermore, no additional
emission coming from the host materials was observed, indicative
of efficient energy transfer from the host to FIrpic. The obtained
results are among the best for solution processed blue PhOLEDs
based on carbazole dendrimer molecular host materials. The high
performance could be attributed to the excellent film forming
ability and the high triplet energy of host materials and balanced
charge recombination in the devices. For the compounds 36FCzG1
and 36FCzG2, the turn-on voltages of the devices were determined
by the LUMO energy barriers between the TmPyPB and the host
36FCzG1 or 36FCzG2 (Fig. 7). The HOMO levels of the TmPyPB are
enough to block the exciton diffusion, the LUMO level barriers
between the 36FCzG2 and TmPyPB are larger than that of the
36FCzG2, leading to the device hosted by the 36FCzG2 shows the
lower turn-on voltage.
1H NMR and 13C NMR spectra were measured on a Bruker-
AF301 AT 400 MHz spectrometer. Elemental analyses of carbon,
hydrogen, and nitrogen were performed on an Elementar (Vario
Micro cube) analyzer. Mass spectra were carried out on an Agilent
(1100 LC/MSD Trap) using ACPI ionization and the final products
were characterized by the MALDI-TOF. UVevis absorption spectra
were recorded on a Shimadzu UVevis-NIR Spectrophotometer
(UV-3600). PL spectra were recorded on Edinburgh instruments
(FLSP920 spectrometers). Differential scanning calorimetry (DSC)
was performed on a PE Instruments DSC 2920 unit at a heating rate
of 10 ꢀC/min from 20 to 250 ꢀC under nitrogen. The glass transition
temperature (Tg) was determined from the second heating scan.
Thermogravimetric analysis (TGA) was undertaken with a Per-
kineElmer Instruments (Pyris1 TGA). The thermal stability of the
samples under a nitrogen atmosphere was determined by mea-
suring their weight loss while heating at a rate of 10 ꢀC/min from
30 to 700 ꢀC. Cyclic voltammetry measurements were carried out
in a conventional three electrode cell using a Pt button working
electrode of 2 mm in diameter, a platinum wire counter electrode,
and an Ag/AgNO3 (0.1 M) reference electrode on a computer-
controlled EG&G Potentiostat/Galvanostat model 283 at room
temperature. Reductions CV of all compounds were performed in
6. Conclusions
In summary, we have designed and synthesized a new series
of host materials 36FCzG1 and 36FCzG2 for solution processed
blue phosphorescent organic light-emitting devices. By sub-
stitution at the 3,6- and 9,9-positions of fluorene with the rigid
tert-butylcarbazole dendrimer units, the novel compounds
showed highly twisted configurations, which effectively suppress
dichloromethane containing 0.1 M tetrabutylammoniumhexa-
fluorophosphate (Bu4NPF6) as the supporting electrolyte. The onset
potential was determined from the intersection of two tangents
drawn at the rising and background current of the cyclic voltam-
mogram. The film surface morphology was measured with AFM
(Seiko Instruments, SPA-400).

4174
Y. Qian et al. / Tetrahedron 69 (2013) 4169e4175
7.2. Quantum chemical calculations
(m, 5H), 7.40 (dd, J¼8.1, 1.8 Hz, 2H), 7.77 (d, J¼1.8 Hz, 2H). APCIþMS
(m/z): calcd for C19H12Br2O, 413.9; found, 415.2.
The geometrical and electronic properties were performed with
the Amsterdam Density Functional (ADF) 2009.01 program pack-
age. The calculation was optimized by means of the B3LYP (Becke
three parameters hybrid functional with LeeeYangePerdew
correlation functionals)37 with the 6e31G(d) atomic basis set.
7.4.2. 3-(3,6-Dibromo-9-phenyl-9H-fluoren-9-yl)-9-phenyl-9H-car-
bazole (2). To 0.8 g 2 and 0.577 g 9-penyl carbazole dissolved in
CH2Cl2 was added 0.5 mL CF3COOH, then the mixture was stirred at
room temperature for 14 h. Ice cold NaHCO3 aqueous was added,
and the product was extracted with CH2Cl2. Yield: 70%.
Then the electronic structures were calculated at
s-HCTHhyb/
6e311þþG(d, p) level. Molecular orbitals were visualized using
ADFview.
7.4.3. 36FCzG1. White solid. Yield: 65%. 1H NMR: (CDCl3,
400 MHz):
7.75e7.73 (d, J¼8.0 Hz, 2H), 7.61e7.53 (m, 6H), 7.49e7.35 (m, 19H),
1.45e1.44 (m, 36H). 13C NMR: (CDCl3, 100 MHz):
(ppm) 150.66,
d
(ppm) 8.13e8.06 (m, 6H), 7.95e7.94 (d, J¼4.0 Hz, 2H),
7.3. Device fabrication and performance measurements
d
All the devices were fabricated by spin-coating process. The ITO
146.16, 142.91, 141.28, 141.17, 139.97, 139.23, 137.94, 137.65, 137.04,
129.88, 128.53, 128.34, 127.57, 127.48, 127.01, 126.64, 126.50, 126.12,
123.64, 123.45, 123.28, 123.23, 120.42, 119.99, 119.54, 118.49, 116.23,
109.87, 109.35, 65.50, 34.72, 32.00. MS (MALDI-TOF): calcd for
C77H71N3: 1037.5642, found, 1037.5654. Anal. Calcd for C77H71N3: C,
89.06; H, 6.89; N, 4.05. Found: C, 89.14; H, 6.78; N 4.08.
substrate with a sheet resistance of 20
U/, was cleaned with the
cleaner and deionized water under the ultrasound for 15 min, re-
spectively. Then the ITO was dried in an oven for 3 h. Finally, the ITO
was treated with UV-ozone for 5 min. The device structure of the
blue PhoLEDs was ITO/PEDOT:PSS (40 nm)/host: FIrpic (20 wt %)
(40 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm). A 40 nm
PEDOT:PSS (Baytron P VP CH 8000) aqueous solution was spin
coated onto the ITO substrate and baked at 120 ꢀC for 10 min to
remove the residual water. The substrates were then taken into
a nitrogen glove box, where the emitting layers were spin coated
onto the PEDOT:PSS layer from 1,2-dichloroethane solution and
annealed at 80 ꢀC for 30 min. The substrate was then transferred
into an evaporation chamber, where 40 nm TmPyPB were evapo-
7.4.4. 36FCzG2. White solid. Yield: 60%. 1H NMR: (CDCl3,
400 MHz):
d (ppm) 8.45e8.38 (m, 1H), 8.35e8.28 (m, 1H),
8.24e8.22 (m, 2H), 8.16e8.06 (m, 11H), 8.05e7.72 (m, 7H),
7.65e7.52 (m, 11H), 7.48e7.31 (m, 26H), 1.46e1.45 (m, 72H). 13C
NMR: (CDCl3, 100 MHz):
d (ppm) 153.10, 150.62, 147.22, 146.26,
142.91, 142.80, 141.28, 141.17, 139.97, 139.25, 138.10, 137.94, 137.65,
137.04, 129.89, 129.84, 129.27, 129.24, 128.53, 128.35, 128.34, 127.57,
127.48, 127.01, 126.64, 126.50, 126.12, 125.53, 123.64, 123.52, 123.45,
123.28, 123.23, 123.21, 123.09, 120.42, 119.99, 119.54, 118.49, 117.94,
116.23, 116.18, 116.10, 114.42, 109.98, 109.87, 109.34, 65.55, 34.73,
32.04. MS (MALDI-TOF): calcd for C144H131N7: 1923.0500, found,
1923.0547. Anal. Calcd for C77H71N3: C, 88.04; H, 6.86; N, 5.10.
Found: C, 88.15; H, 6.74; N 5.11.
ꢀ
rated at an evaporation rate of 1e2 A/s under a pressure of
4ꢂ10ꢁ4 Pa and the LiF/Al bilayer cathode was evaporated at evap-
ꢀ
oration rates of 0.2 and 10 A/s for LiF and Al, respectively, under
a pressure of 1ꢂ10ꢁ3 Pa. The active area of the device was 9 mm2.
The EL spectrums, luminance, CIE coordinates, and the cur-
rentevoltageeluminance-efficiency characteristics of the devices
were measured with a rapid scan system using a Photo Research
PR655 spectrophotometer and a Keithley 2400 digital source. All
the date of EL characteristics were measured at room temperature
under an ambient atmosphere.
Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (NSFC, grant no. 61008050 and no. 41006019). Also
thank the Analytical and Testing Centre Huazhong University of
Science and Technology for measurements.
7.4. Materials
Compounds N-phenylcarbazole,38 compounds G1, G2,29 and the
compound 3,6-dibromo-9H-fluoren-9-one28 were prepared
according to published procedures. 9,90-(9-Phenyl-9-(9-phenyl-
9H-carbazol-3-yl)-9H-fluorene-3,6-diyl)bis(3,6-di-tert-butyl-9H-
carba-zole) (36FCzG1), 9,90-(9-phenyl-9-(9-phenyl-9H-carbazol-3-
yl)-9H-fluorene-3,6-diyl) bis(3,6-bis (3,6-di-tert-butyl-9H-fluoren-
9-yl)-9H-carbazole) (36FCzG2) were synthesized by the same
procedure: A mixture of 3-(3,6-dibromo-9-phenyl-9H-fluoren-9-
yl)-9-phenyl-9H-carbazole (1.5 mmol), 3,6-di-tert-butyl-carbazole
(G1) (or 3,300,6,600-tetra-tert-butyl-90H-9,30:60,900-tercarbazole (G2))
(3.2 mmol), CuI (0.10 mmol), K2CO3 (6.0 mmol), 18-Crown-6
(0.10 mmol), and DMPU (5.0 mL) was heated at 170 ꢀC for 24 h. After
cooling, the mixture was treated with water and extracted with
dichloromethane. The organic extract was dried over anhydrous
MgSO4 with removal of the volatiles. The residue was purified by
column chromatography on silica gel using hexaneedichloro-
methane as the eluent.
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