Synthesis of new bipolar materials based on diphenylphosphine oxide and triphenylamine units: Efficient host for deep-blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.tet.2012.09.051

This paper reports the synthesis and physical properties of a series of bipolar host materials, using of a hole-transporting triphenylamine (TPA) monomer as a core incorporated with different numbers of diphenylphosphine oxide (PO) as electron-transporting moieties, 4-(diphenylphosphoryl)-N,N- diphenylaniline (DDPA), 4-(diphenylphosphoryl)-N-(4-(diphenylphosphoryl)phenyl)- N-phenylaniline (DDPP), and tris(4-(diphenylphosphoryl)phenyl)amine (TDPA), for solution-processed deep-blue phosphorescent organic light-emitting devices (PhOLEDs). With the increasing numbers of PO units, the glass-transition temperature of those compounds rise gradually. Moreover, the newly synthesized compounds all possess high triplet energies, which can prevent back energy transfer between the host and dopant molecules, and are expected to serve as appropriate hosts for iridium(III) tris(3,5-difluoro-4-cyanophenyl)pyridinato-N, C′ (FCNIrpic). The solution-processed devices using DDPP and TDPA as the hosts for the phosphorescence emitter FCNIrpic showed the maximum luminance efficiencies of 9.7 and 6.6 cd A^-1, respectively. The efficiency of TDPA based device shows nearly three times higher than the value of commonly used host material 1,3-bis(9-carbazolyl)benzene (mCP) with the same structure, which is outstanding with respect to other works related to the solution-processed deep-blue PhOLEDs based on small-molecule hosts.

Full text of DOI:10.1016/j.tet.2012.09.051

Tetrahedron 68 (2012) 9672e9678
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of new bipolar materials based on diphenylphosphine oxide and
triphenylamine units: efficient host for deep-blue phosphorescent organic
light-emitting diodes
,
b,
Wei Jiang a, Wen Yang a, Xinxin Ban a, Bin Huang a, Yunqian Dai a, Yueming Sun a , Lian Duan
,
*
*
Yong Qiu b
a School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, PR China
b Department of Chemistry, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2012
Received in revised form 25 August 2012
Accepted 11 September 2012
Available online 16 September 2012
This paper reports the synthesis and physical properties of a series of bipolar host materials, using of
a hole-transporting triphenylamine (TPA) monomer as a core incorporated with different numbers of
diphenylphosphine oxide (PO) as electron-transporting moieties, 4-(diphenylphosphoryl)-N,N-diphe-
nylaniline (DDPA), 4-(diphenylphosphoryl)-N-(4-(diphenylphosphoryl)phenyl)-N-phenylaniline (DDPP),
and tris(4-(diphenylphosphoryl)phenyl)amine (TDPA), for solution-processed deep-blue phosphorescent
organic light-emitting devices (PhOLEDs). With the increasing numbers of PO units, the glass-transition
temperature of those compounds rise gradually. Moreover, the newly synthesized compounds all possess
high triplet energies, which can prevent back energy transfer between the host and dopant molecules,
and are expected to serve as appropriate hosts for iridium(III) tris(3,5-difluoro-4-cyanophenyl)pyr-
idinato-N,C0 (FCNIrpic). The solution-processed devices using DDPP and TDPA as the hosts for the
Keywords:
Triphenylamine
Bipolar
Solution processed
High triplet energy
Host
phosphorescence emitter FCNIrpic showed the maximum luminance efficiencies of 9.7 and 6.6 cd Aꢀ1
,
Phosphorescence
respectively. The efficiency of TDPA based device shows nearly three times higher than the value of
commonly used host material 1,3-bis(9-carbazolyl)benzene (mCP) with the same structure, which is
outstanding with respect to other works related to the solution-processed deep-blue PhOLEDs based on
small-molecule hosts.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
luminous efficiency (LE) of 32.1 cd Aꢀ1.7 Yang et al. reported highly
efficient red PhOLEDs based on a solution-processable bipolar host
Phosphorescent organic lighting-emitting diodes (PhOLEDs)
have attracted a great deal of attention because they can harvest
singlet and triplet excitons for emission, theoretically yielding 100%
internal quantum efficiency, which show great applicable in full-
color flat-panel displays and solid-state lighting sources.1e5 Re-
cently, in order to simplify the fabrication process of PhOLEDs and
reduce the cost of larger-area displays, much research effort has
been devoted to develop highly efficient PhOLEDs with the emit-
ting layer prepared by solution-processing techniques. For exam-
ple, Doh et al. used small-molecule mixed host in the solution-
processed blue-PhOLEDs and reported high external quantum ef-
ficiency (EQE) of 14.6%.6 Zhao et al. has recently reported green
PhOLEDs using a solution-processed polyethyleneoxide (PEO) hole
blocking layer and a Cs2CO3/Al cathode, which gave a maximum
with a maximum LE value of 13.3 cd Aꢀ1 8
. While red, green, and
sky-blue PhOLEDs fabricated by solution processing have been
developed with obvious progress, high performance solution-
processed deep-blue PhOLEDs are still scarce, owing to lack of the
appropriate host materials. As an effective host for deep-blue
PhOLEDs, its triplet energy requires to be larger than that of the
phosphorescent emitters such as FCNIrpic (2.74 eV) and iridium(III)
bis(40,60-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate
(FIr6) (2.72 eV) to prevent the reverse energy transfer from the
dopant.9,10
During the search for host materials for solution-processed
PhOLEDs, a great deal of attention has been focused on polymer-
based materials, owing to their early access to solution-
processing strategy.11,12 However, the impurities in polymer mate-
rials could result in exciton quenching and device failure. In con-
trast to polymer counterparts, small-molecule materials are
advantageous as they have defined molecular structure, definite
molecular weight, high purity, and easy purification.13e19
* Corresponding authors. Tel./fax: þ86 25 52090621; e-mail addresses:
101011462@seu.edu.cn (Y. Sun), duanl@tsinghua.edu.cn (L. Duan).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.09.051

W. Jiang et al. / Tetrahedron 68 (2012) 9672e9678
9673
Nowadays, bipolar hosts have potential advantages arising from
more balanced injection and transport of holes and electrons,
which may result in a broad distribution of recombination region
within the emitting layer and a low efficiency roll-off of the
devices.20e28 One of approach to the design of bipolar host mate-
rials is to combine reported hole-transporting and electron-
transporting functional units. Triphenylamine (TPA) and diphe-
nylphosphine oxide (PO) have been regarded as promising moieties
for organic semiconductor materials owing to their hole and elec-
spectrometer with tetramethylsilane as the internal standard. Ele-
mental analysis was performed on an Elementar Vario EL CHN el-
emental analyzer. Mass spectrometry was performed with
a Thermo Electron Corporation Finnigan LTQ mass spectrometer.
Absorption spectra were recorded with an UVevis spectropho-
tometer (Shimadzu UV-2450) and PL spectra were recorded with
a fluorospectrophotometer (Jobin Yvon, FluoroMax-3). TGA was
recorded with a Netzsch simultaneous thermal analyzer (STA)
system (STA 409PC) under a dry nitrogen gas flow at a heating rate
of 10 ꢁC minꢀ1. Glass-transition temperatures were recorded by
DSC at a heating rate of 10 ꢁC minꢀ1 with a thermal analysis in-
strument (DSC 2910 modulated calorimeter). Cyclic voltammetry
was performed on a Princeton Applied Research potentiostat/gal-
vanostat model 283 voltammetric analyzer in CH2Cl2 solutions
(10ꢀ3 M) at a scan rate of 100 mV sꢀ1 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 selected as the internal standard. The solutions
were bubbled with argon for 10 min before measurements. The film
surface morphology was measured with AFM (Seiko Instruments,
SPA-400).
tron transport properties.29e35 Furthermore, the
p conjugation of
the molecules was interrupted by the insertion of P]O moieties,
which resulted in high triplet energy for molecules. Padmaperuma
et al. reported highly efficient blue PhOLEDs with an bipolar host
material 4-(diphenylphosphoryl)-N,N-diphenylaniline (DDPA) (see
Scheme 1), which gave a maximum EQE of 17.1%.36 However, the
DDPA suffers from poor thermal stability with a low glass-
transition temperatures (Tg¼51 ꢁC) (see Fig. 1b) and may result in
phase segregation and aggregation in the device.
The bipolar host materials containing carbazole and PO units are
nowadays widely investigated. However, few reported work has
focused on TPA-based PO bipolar host material. In this paper, we
synthesized two small-molecule based host materials, 4-(diphe-
nylphosphoryl)-N-(4-(diphenylphosphoryl)phenyl)-N-phenylani-
line (DDPP) and tris(4-(diphenylphosphoryl)phenyl)amine (TDPA),
using a hole-transporting TPA monomer as a core incorporated
with different numbers of PO as electron-transporting moieties.
The influence of the PO moieties on the thermal stability, electro-
chemical properties, photophysical behaviors, and EL performances
of those bipolar host materials are investigated.
2.2. Device fabrication and performance measurements
In a general procedure, indium-tin oxide (ITO)-coated glass
substrates were pre-cleaned carefully and treated by UV ozone for
4 min.
A 40 nm poly(3,4-ethylenedioxythiophene)doped with
poly(styrene-4-sulfonate) (PEDOT:PSS) aqueous solution was spin
N
DDPA
N
P
O
I
I
O
P
N
O
DDPP
CuI, Proline
P
P
N
+
H
Cs2CO3, Toluene
O
I
I
O
P
N
N
TDPA
I
I
O
P
P
O
Scheme 1. Synthetic routes toward DDPA, DDPP, and TDPA.
2. Experimental
coated onto the ITO substrate and baked at 200 ꢁC for 10 min. The
substrates were then taken into a nitrogen glove box, where
FCNIrpic-doped host layer was spin coated onto the PEDOT:PSS
layer from 1,2-dichloroethane solution and annealed at 80 ꢁC for
30 min. The substrates were then transferred into an evaporation
chamber, where the TPBI was evaporated at an evaporation rate of
1e2 A/s under a pressure of 4ꢂ10ꢀ4 Pa and the Cs2CO3/Al bilayer
2.1. General information
All reactants and solvents, unless otherwise stated, were pur-
chased from commercial sources and used as received. 1H NMR and
13C HMR spectra were measured on a Bruker ARX300 NMR

9674
W. Jiang et al. / Tetrahedron 68 (2012) 9672e9678
phenyl)-phenylamine (1.0 mmol), diphenylphosphine oxide
(2.4 mmol), CuI (0.20 mmol), proline (0.20 mmol), and Cs2CO3
(3.0 mmol) was added to toluene solution (20 mL). The mixture was
heated under reflux for 24 h. After the reaction finished, the re-
action mixture was extracted with dichloromethane and washed
with water. The organic layer was dried by anhydrous MgSO4 and
filtered. The product was isolated by silica gel column chromatog-
raphy using methanol/ethyl acetate (1:10) as eluent to afford an
oyster white solid (0.29 g, 45.1%). 1H NMR (300 MHz, CDCl3, TMS)
7.69 (d, J¼6.9 Hz, 4H), 7.65 (d, J¼6.9 Hz, 4H), 7.53e7.40 (m, 16H),
): 133.4,
d:
7.32 (t, J¼7.2 Hz, 2H), 7.13e7.07 (m, 7H); C NMR (CDCl3,
d
133.3, 132.1, 132.0, 131.9, 129.9, 128.5, 128.4, 126.7, 125.5, 122.6,
122.5; MS (MALDI-TOF) [m/z]: calcd for C42H33NO2P2, 645.7; found,
645.3. Anal. Calcd for C42H33NO2P2: C, 78.13; H, 5.15; N 2.17. Found:
C, 78.40; H, 4.99; N 2.25.
2.3.3. Synthesis of tris(4-(diphenylphosphoryl)phenyl)amine (TDPA). A
mixture of tris-(4-iodo-phenyl)-amine (1.0 mmol), diphenylphos-
phine oxide (3.6 mmol), CuI (0.30 mmol), proline (0.30 mmol), and
Cs2CO3 (4.5 mmol) was added to toluene solution (20 mL). The
mixture was heated under reflux for 36 h. After the reaction finished,
the reaction mixture was extracted with dichloromethane and
washed with water. The organic layer was dried by anhydrous
MgSO4 and filtered. The product was isolated by silica gel column
chromatography using methanol/ethyl acetate (1:5) as eluent to af-
ford an oyster white solid (0.30 g, 35.5%). 1H NMR (300 MHz, CDCl3,
TMS)
16H), 7.31e7.26 (m, 6H), 7.14 (t, J¼7.2 Hz, 8H), 7.02e6.98 (m, 4H); C
NMR (CDCl3, ): 133.8, 133.7, 133.2, 132.1, 132.0, 128.7, 128.5, 127.7,
d: 7.71 (d, J¼8.2 Hz, 4H), 7.67 (t, J¼6.9 Hz, 4H), 7.60e7.37 (m,
d
124.1, 124.0; MS (MALDI-TOF) [m/z]: calcd for C54H42NO3P3, 845.8;
found, 845.4. Anal. Calcd for C54H42NO3P3: C, 80.88; H, 5.43; N 3.14.
Found: C, 80.66; H, 5.32; N 3.25.
Fig. 1. (a) TGA traces of DDPA, DDPP, and TDPA recorded at
a
heatiꢀn1g rate of
10 ꢁC minꢀ1. (b) DSC measurement recorded at a heating rate of 10 ꢁC min
.
3. Results and discussion
cathode was evaporated at evaporation rates of 0.2 and 10 A/s for
Cs2CO3 and Al, respectively, under a pressure of 1ꢂ10ꢀ3 Pa. The
currentevoltageebrightness characteristics of the devices were
characterized with Keithley 4200 semiconductor characterization
system. The electroluminescent spectra were collected with
a Photo Research PR705 Spectrophotometer. All measurements of
the devices were carried out in ambient atmosphere without fur-
ther encapsulations.
3.1. Synthesis and morphological stability
Generally, the PO derivatives can be synthesized by coupling
reactions between lithiated intermediates and diphenyl-
chlorophosphine followed by oxidation with hydrogen peroxide. In
this paper, the target compounds were synthesized by the com-
monly Ullmann coupling reaction. Scheme 1 illustrates the syn-
thetic procedures for DDPA, DDPP, and TDPA. Initially, the starting
(4-iodo-phenyl)-diphenylamine, bis-(4-iodo-phenyl)-phenylamine
and tris-(4-iodo-phenyl)-amine were synthesized according to
methods previously described and then reacted with diphenyl-
phosphine oxide led to DDPA, DDPP, and TDPA in the presence of
CuI/proline with moderate yields, respectively. As shown in
Experimental section, the yield decreases with increasing numbers
of PO. The target compounds were purified by silica column
method and recrystallization, producing very pure powders. H
NMR, C NMR, mass spectrometry, and elemental analysis were
employed to confirm the chemical structures of above-mentioned
compounds as described in Experimental section. The thermal
and morphological stability of DDPA, DDPP, and TDPA were also
investigated by thermal gravimetric analyses (TGA) and differential
scanning calorimetry (DSC) (Fig. 1). These compounds possess
thermal decomposition temperature (Td, corresponding to 5%
weight loss) ranging from 275 to 486 ꢁC. They are all readily formed
a glass from their melts upon fast cooling, and the glass-transition
temperature (Tg) range from 51 to 121 ꢁC. It was found that the Tg
and Td of those compounds rose gradually with the increasing
numbers of PO units. The Tg of DDPP and TDPA is much higher than
commonly used host materials such as 4,40-bis(9-carbazolyl)-2,20-
biphenyl (CBP) (62 ꢁC) and mCP (55 ꢁC). The observed low Tg of
DDPA demonstrates the high tendency to crystallize in the operated
2.3. Materials
2.3.1. Synthesis
of
4-(diphenylphosphoryl)-N,N-diphenylaniline
(DDPA). A mixture of (4-iodo-phenyl)-diphenylamine (1.0 mmol),
diphenylphosphine oxide (1.2 mmol), CuI (0.10 mmol), proline
(0.10 mmol), and Cs2CO3 (1.5 mmol) was added to toluene solution
(20 mL). The mixture was heated under reflux for 24 h. After the
reaction finished, the reaction mixture was extracted with
dichloromethane and washed with water. The organic layer was
dried by anhydrous MgSO4 and filtered. The product was isolated
by silica gel column chromatography using methanol/ethyl acetate
(1:20) as eluent to afford an oyster white solid (0.25 g, 56.2%). 1H
NMR (300 MHz, CDCl3, TMS)
J¼6.9 Hz, 2H), 7.57e7.37 (m, 9H), 7.05e6.96 (m, 2H), 7.31 (t,
): 133.2, 133.1,
d: 7.71 (d, J¼6.9 Hz, 2H), 7.67 (d,
J¼7.2 Hz, 4H), 7.14e7.09 (m, 5H); C NMR (CDCl3,
d
132.0, 131.7, 129.6, 128.4, 128.3, 125.8, 125.68, 124.4, 120.3, 120.2;
(MALDI-TOF) [m/z]: calcd for C30H24NOP, 445.5; found, 445.1. Anal.
Calcd for C30H24NOP: C, 80.88; H, 5.43; N 3.14. Found: C, 80.66; H,
5.32; N 3.25.
2.3.2. Synthesis of 4-(diphenylphosphoryl)-N-(4-(diphenylphos-
phoryl)phenyl)-N-phenylaniline (DDPP). A mixture of bis-(4-iodo-

W. Jiang et al. / Tetrahedron 68 (2012) 9672e9678
9675
devices. Compared to DDPA, DDPP and TDPA show significantly
3.3. Electrochemical analysis
rises of the Td and Tg values by introducing additional numbers of
PO groups. The better thermal performance of DDPP and TDPA is
attributed to the tetrahedral geometry of PO, which effectively
hinders the molecule packing and crystallization. The Atomic force
microscopy (AFM) results indicate the excellent thermal and mor-
phological stability of these compounds (Supplementary data
Fig. S1). Therefore, DDPP and TDPA are advantageous in the film
morphology stability, which will suppress the phase separation
during device operation.
The electrochemical properties of DDPA, DDPP, and TDPA were
studied in solution using cyclic voltammetry (CV) using tetrabu-
tylammonium hexafluorophosphate(TBAPF6) as the supporting
electrolyte and ferrocene as the internal standard. As shown in
Fig. 3, during the anodic scan in dichloromethane, DDPA, DDPP, and
TDPA exhibited similar quasi-reversible oxidation process, which
can be assigned to the oxidation of electron-donating TPA moiety,
with onset potentials of 0.86, 1.05, and 1.25 V, respectively. No re-
duction waves were detected. On the basis of the onset potentials
for oxidation, we estimated the HOMO energy of DDPA, DDPP, and
TDPA to be ꢀ5.30, ꢀ5.49, and ꢀ5.69 eV. These results reveal that
the introduction of PO groups to the TPA core can lead to the raising
of HOMO levels, which are in agreement with the DFT calculations
results. These together with absorption spectra were then used to
obtain the LUMO energy levels.
3.2. Theoretical calculations
To further understand the relationship of the electronic prop-
erties at molecular level, density functional theory (DFT) calcula-
tions were carried out using B3LYP hybrid functional theory with 6-
31G basis sets. As can be seen from Fig. 2, the calculated geome-
*
tries of DDPA, DDPP, and TDPA show that the tetrahedral geometry
of PO units is significantly twisted against to the TPA core, resulting
in a non-planar structure in each molecule. These geometrical
characteristics are beneficial to the amorphous of the organic
compounds and, thus, suppress molecular recrystallization, which
are in agreement with the TGA and DSC results. Furthermore, the
tetrahedral geometry of PO moieties interrupted the extent of
conjugation between the central core and branches, which keeps
the triplet energy gap at a high level of these molecules. Their
triplet energies are calculated from the difference of their ground
state and triplet excited state energies. The calculated triplet en-
ergies
D
E (T1ꢀS0) of TPA, DDPA, DDPP, and TDPA are 3.18, 3.03, 2.89,
and 2.85 eV, and it further proves that insertion of PO moieties as
the branches in combination of TPA leads to desirable high triplet
energy, which are high enough to serve as hosts for blue triplet
emitters.
Fig. 3. The CV curves of DDPA, DDPP, and TDPA in CH2Cl2 solutions (10ꢀ3 M).
3.4. Photophysical properties
Fig. 4 depicts the UVevis absorption and photoluminescent
(PL) spectra of DDPA, DDPP, and TDPA in dilute CH2Cl2 solutions.
The absorption spectra of three PO compounds are similar, which
show peaks at around 230 nm could be assigned to the ne
transitions of TPA moiety, while the longer-wavelength absorp-
tions from 323 to 339 nm could be attributed to transitions
p
*
pep
*
from TPA moiety to PO moiety. Compared to DDPA, the absorp-
tion maxima of DDPP and TDPA are red-shifted by 16 and 12 nm,
respectively. The PL emission of DDPA, DDPP, and TDPA exhibits
peaks at 408, 388, and 379 nm, respectively. The hypochromatic
shift of the PL spectrum is clearly found by introducing PO
moiety, and the more PO moieties present in the TPA core, the
larger hypochromatic shift obtained. This phenomenon may be
attributed to the symmetrical geometry of DDPP and TDPA, which
lead to the decreasing of the magnitude of the change in the
Fig. 2. Optimized geometries and calculated HOMO and LUMO density maps for DDPA,
DDPP, and TDPA according to DFT calculations at B3LYP/6-31 level.
*
Calculated HOMO and LUMO density maps of DDPA, DDPP, and
TDPA are also included in Fig. 2. Compared with TPA core
(HOMO¼ꢀ4.94 eV), the HOMO levels of DDPA, DDPP, and TDPA
increase (from ꢀ5.16 to ꢀ5.46 eV) with the increasing numbers of
PO units, while the LUMO levels move down (from ꢀ0.80 to
ꢀ1.16 eV). As a result, the HOMOeLUMO energy gaps become
narrow (from 4.25 to 4.36 eV). HOMOs of all compounds are mainly
located at the hole-transporting TPA moieties. In contrast, their
dipole moment from the ground state to the excited state (Dm
)
and weaken the intramolecular charge transfer (ICT) effect. The
band gap was calculated from the edge of the UVevis absorption
peak, giving values of 3.49, 3.41, and 3.43 eV for DDPA, DDPP, and
TDPA, respectively. The highest energy 0e0 phosphorescent
emission was used to calculate the triplet energy level, giving
a value of 2.78 eV for DDPA, 2.80 eV for DDPP, and 2.85 eV for
TDPA, higher than the values of the commonly used triplet blue-
emitter FIrpic (2.62 eV) and FCNIrpic (2.74 eV). Inconsistent with
LUMOs mostly correspond to the lowest p-orbital of the periphery
triphenyl phosphine oxide moiety. It is interesting to note that all
molecules show significant separation in the spatial distributions
between HOMO and LUMO at their different moieties, which is
believed to facilitate the balance carrier injecting/transporting.25
the calculation results,
the smallest one among these hosts, and larger
D
E (T1-S0) of DDPA with one PO moiety is
E (T1ꢀS0) is
D

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W. Jiang et al. / Tetrahedron 68 (2012) 9672e9678
Table 1
Photophysical properties of the hosts in various solvents
Solvents
D
f
lmax,abs (nm)
lem (nm)
DDPA
DDPP
TDPA
DDPA
DDPP
TDPA
n-Hexane
THF
CH2Cl2
ACN
0.001
0.207
0.218
0.306
314
316
323
318
334
336
339
336
330
333
335
332
363
400
409
423
369
384
388
400
368
374
379
384
solvents. Such results indicated that the molecular dipole moments
of these compounds are different from each other. To obtain more
information about the change in the dipole moment upon excita-
tion, we used the LipperteMataga equation Eq. 1,37 which
expressed the Stokes shift as a function of the solvent polarity
parameter
Df(
3 ,n).
2
Dm2
hca3
D
f ð ; nÞ
3
ya
ꢀ
ya
¼
D
f ð
3
; nÞ þ C ¼ 10070Dm2
þ C
(1)
a3
where
D
f( 3 ,n) is calculated from Eq. 2:
ꢀ
ꢁ
3
ꢀ 1
n2 ꢀ 1
D
f ð
3 ; nÞ ¼
ꢀ
(2)
2n2 þ 1
2
3
þ 1
In these relations, na
nf represents the Stokes shift (in cmꢀ1),
ꢀ
where ya and yf are the spectral positions of the absorption maxi-
mum and solvent-equilibrated fluorescent maximum, respectively.
Dm is given by me mg, which is the magnitude of the change in the
ꢀ
dipole moment from the ground state to the excited state. The other
terms are: h corresponds to the Planck’s constant (6.6ꢂ10ꢀ34 J s), c
is the velocity of light in the vacuum (3.0ꢂ108 m sꢀ1) and a is the
Onsager cavity radius (in meter), respectively. Onsager cavity radius
was estimated from the optimized distance between the two far-
thest atoms in the direction of charge separation within the mol-
ecule.
Df is the orientation polarizability parameter of the solvent
where n is the refractive index of the medium and
3
is the static
dielectric constant of the solvent (both at room temperature). The
LipperteMataga plot for DDPA, DDPP, and TDPA is shown in Fig. 5.
From the slope of this plot, the difference of the dipole moment
between the excited state and the ground state is estimated to be
15.2, 10.65, and 8.1 D for DDPA, DDPP, and TDPA, respectively. It was
found that the Dm values of those compounds decrease gradually
with the increasing numbers of PO units. The calculated Dm of DDPA
is much larger than DDPP and TDPA, which is typical for photoin-
duced intramolecular charge transfer (ICT) processes. It was re-
ported that EL and consequently the performance of OLEDs are
strongly affected by the molecular dipole moment. The
Fig. 4. Normalized absorption (a), emission (b), and phosphorescence (77 K) spectra
(c) of DDPA, DDPP, and TDPA in dilute solutions.
achieved with introducing more PO moieties to TPA core. Using
host materials that possess high triplet energies is a provision for
effective confinement of the triplet excitons on the guest and,
consequently, prevention of back energy transfer between the
host and dopant molecules. Based on the results of the mea-
surements, the newly synthesized compounds with high triplet
energy levels are expected to serve as appropriate hosts for
FCNIrpic.
To gain more insight fluorescence spectra of the systems were
recorded in various solvents of different polarity. As shown in
Table 1, a red-shift is observed for solutions with solvents of in-
creasing polarity. As a change of the solvent from n-hexane to
acetonitrile (ACN), the maximum emission wavelengths were red-
shifted from 363 to 423 nm, 369 to 400 nm, and 368 to 384 nm for
DDPA, DDPP, and TDPA, respectively. The large Stokes shift for DDPA
in polar solvents as compared to non-polar solvents indicating
a considerable energetic stabilization of the excited state in polar
Fig. 5. LipperteMataga plot of DDPA, DDPP, and TDPA in various solutions.

W. Jiang et al. / Tetrahedron 68 (2012) 9672e9678
9677
intramolecular and intermolecular electric field arising from the
molecular dipoles is a detrimental factor that should be avoided in
the molecular design of high performance OLEDs.
DDPP and TDPA, devices showed the turn-on voltage (corresponding
to 1 cd mꢀ2) of 6.0 and 6.2 V, respectively. The device based on TDPA
shows better performance with the maximum LE value of 9.7 cd Aꢀ1
,
and the maximum EQE of 5.7%, while the maximum luminance ef-
ficiencies of DDPP based device was 6.6 cd Aꢀ1, and the corre-
sponding external quantum efficiency was 3.9%. The efficiency of
TDPA based solution-processed deep-blue PhOLEDs shows nearly
three times higher than the values of mCP with the maximum LE
value of 3.6 cd Aꢀ1 and EQE of 2.0% for the same device,17 which was
comparable to the literature value reported by Yang’s group,19 with
the maximum LE value of 12.5 cd Aꢀ1. The excellent performance is
probably due to the excellent thermal stability of the DDPP and TDPA,
which significantly enhances the capability of forming stable amor-
phous thin films. In addition, the bipolar charge transport nature of
DDPP and TDPA that increases carrier balance in the emitting layer
may also contribute to the high performance. As shown in Fig. 7b, the
electroluminescence (EL) spectra of the devices are almost identical
with the CIE coordinates of (0.18, 0.24), corresponding to the emis-
sion of FCNIrpic. Furthermore, no additional emission coming from
host materials was observed, indicative of efficient energy transfer
from the host to FCNIrpic.
3.5. Electroluminescent properties
The high triplet energy for those new compounds makes it a good
candidate as solution-processed hosts for deep-blue phosphorescent
emitters such as FCNIrpic. To investigate the function of those com-
pounds in deep-blue PhOLEDs, we fabricated double-layerdeep-blue
electrophosphorescent devices with DDPP and TDPA as hosts and
10 wt % FCNIrpic as the dopant. The device with the configuration
ITO/PEDOT:PSS/Host:FCNIrpic(10 wt %)//TPBi/Cs2CO3/Al has been
fabricated by spin-coating. The material structures and the energy-
level diagrams of the device are shown in Fig. 6. We used conduct-
ing polymer PEDOT:PSS as a hole-injection layer, TPBi as an electron-
transporting layer, and Cs2CO3 as the electron-injection layer. The
current densityevoltageeluminance (IeVeL) and luminous effi-
ciencyeluminance (LEeL) characteristics are shownin Fig. 7. By using
4. Conclusions
In summary, we have designed and synthesized two new bi-
polar host materials DDPP and TDPA for solution-processed deep-
blue PhOLEDs. Owing to their unique structural feature, different
numbers of PO units substituted TPA derivatives exhibit the ex-
cellent thermal stability and provide high triplet energy. In addi-
tion, it was found that the Dm values of those compounds decrease
gradually with the increasing numbers of PO units, which is ben-
eficial to improve device performance. Utilizing those compounds
as host materials, the solution-processed deep-blue phosphores-
cence OLEDs with FCNIrpic as a dopant show a maximum current
efficiency of 9.7 cd Aꢀ1, which is outstanding with respect to other
works based on small-molecule hosts.
Fig. 6. Structures of the materials in the devices and the energy-level diagrams.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (grant nos. 51103023 and 21173042).
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.09.051.
References and notes
1. Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
2. Forrest, S. R. Nature 2004, 428, 911.
3. Wu, C. C.; Lin, Y. T.; Wong, K. T.; Chen, R. T.; Chien, Y. Y. Adv. Mater. 2004, 16, 61.
4. Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591.
5. Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lueesem, B.; Leo, K.
€
Nature 2009, 459, 234.
6. Doh, Y. J.; Park, J. S.; Jeon, W. S.; Pode, R.; Kwon, J. H. Org. Electron. 2012, 13, 586.
7. Zhao, Y.; Duan, L.; Zhang, D. Q.; Hou, L. D.; Qiao, J.; Wang, L. D.; Qiu, Y. Appl. Phys.
Lett. 2012, 100, 088304.
8. Zhu, M. R.; Ye, T. L.; He, X.; Cao, X. S.; Zhong, C.; Ma, D. G.; Qin, J. G.; Yang, C. L. J.
Mater. Chem. 2011, 21, 9326.
9. Yook, K. S.; Lee, J. Y. Org. Electron. 2011, 21, 1711.
10. Holmes, R. J.; D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.; Thompson, M. E.
Appl. Phys. Lett. 2003, 83, 3818.
11. Liu, J.; Pei, Q. B. Macromolecules 2010, 43, 9608.
12. Liu, J.; Li, L.; Pei, Q. B. Macromolecules 2011, 44, 2451.
€
13. Ding, J. Q.; Zhang, B. H.; Lu, J. H.; Xie, Z. Y.; Wang, L. X.; Jing, X. B.; Wang, F. Adv.
Mater. 2009, 21, 4983.
14. Ge, Z. Y.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.; Miyamoto, H.; Kajita, T.;
Kakimoto, M. A. Adv. Funct. Mater. 2008, 18, 584.
15. Hou, L. D.; Duan, L.; Qiao, J.; Li, W.; Zhang, D. Q.; Qiu, Y. Appl. Phys. Lett. 2008, 92,
263301.
Fig. 7. Comparison of device characteristics (a) luminanceevoltageecurrent density,
(b) luminance efficiencyecurrent density, inset: EL spectrum.

9678
W. Jiang et al. / Tetrahedron 68 (2012) 9672e9678
16. Jiang, W.; Duan, L.; Qiao, J.; Wang, L. D.; Zhang, D. Q.; Qiu, Y. J. Mater. Chem.
2010, 20, 6131.
17. Jiang, W.; Duan, L.; Qiao, J.; Wang, L. D.; Zhang, D. Q.; Qiu, Y. J. Mater. Chem. 2011,
27. Duan, L.; Qiao, J.; Sun, Y. D.; Qiu, Y. Adv. Mater. 2011, 23, 1137.
28. Zeng, L. C.; Lee, T. Y. H.; Merkel, P. B.; Chen, S. H. J. Mater. Chem. 2009, 19, 8772.
29. Padmaperuma, A. B.; Sapochak, L. S.; Burrows, P. E. Chem. Mater. 2006, 18,
2389.
21, 4918.
18. Ye, S.; Liu, Y.; Chen, J.; Lu, K.; Wu, W.; Du, C.; Liu, Y.; Wu, T.; Shuai, Z.; Yu, G. Adv.
Mater. 2010, 22, 4167.
30. Burrows, P. E.; Padmaperuma, A. B.; Sapochak, L. S.; Djurovich, P.; Thompson,
M. E. Appl. Phys. Lett. 2006, 88, 183503.
19. Gong, S. L.; Fu, Q.; Wang, Q.; Yang, C. L.; Zhong, C.; Qin, J. G.; Ma, D. G. Adv.
Mater. 2011, 23, 4956.
31. Hsu, F. M.; Chien, C. H.; Shu, C. F.; Lai, C. H.; Hsieh, C. C.; Wang, K. W.; Chou, P. T.
Adv. Funct. Mater. 2009, 19, 2834.
20. Jeong, S. H.; Lee, Y. J. Org. Electron. 2012, 13, 1141.
21. Sasabe, H.; Seino, Y.; Kimura, M.; Kido, J. Chem. Mater. 2012, 24, 1404.
22. Su, S. J.; Cai, C.; Kido, J. J. Mater. Chem. 2012, 22, 3447.
23. Kim, D.; Coropceanu, V.; Bredas, J. L. J. Am. Chem. Soc. 2011, 133, 17895.
24. Fukagawa, H.; Irisa, S.; Hanashima, H.; Shimizu, T.; Tokito, S.; Yokoyama, N.;
Fujikake, H. Org. Electron. 2011, 12, 1638.
32. Sapochak, L. S.; Padmaperuma, A. B.; Cai, X.; Male, J. L.; Burrows, P. E. J. Phys.
Chem. C. 2008, 112, 7989.
33. Chou, H. H.; Cheng, C. H. Adv. Mater. 2010, 22, 2468.
34. Jiang, W.; Duan, L.; Qiao, J.; Wang, L. D.; Qiu, Y. Org. Lett. 2011, 13, 3146.
35. Ge, Z. Y.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.; Miyanoto, H.; Kajita, T.;
Kakimoto, M. A. Chem. Mater. 2008, 20, 2532.
25. Chaskar, A.; Chen, H. F.; Wong, K. T. Adv. Mater. 2011, 23, 3876.
26. Rothmann, M. M.; Fuchs, E.; Schidknecht, C.; Langer, N.; Lennartz, C.; Munster,
I.; Strohriegl, P. Org. Electron. 2011, 12, 1192.
36. Polikarpov, E.; Swensen, J. S.; Chopra, N.; So, F.; Padmaperuma, A. B. Appl. Phys.
Lett. 2009, 94, 223304.
37. Sumalekshmy, S.; Gopidas, K. P. J. Phys. Chem. B 2004, 108, 3705.