A new phosphine oxide host based on ortho-disubstituted dibenzofuran for efficient electrophosphorescence: Towards high triplet state excited levels and excellent thermal, morphological and efficiency stability

Article abstract of DOI:10.1002/chem.201100695

An efficient host for blue and green electrophosphorescence, 4,6-bis(diphenylphosphoryl)dibenzofuran (o-DBFDPO), with the structure of a short-axis-substituted dibenzofuran was designed and synthesised. It appears that the greater density of the diphenylp

Full text of DOI:10.1002/chem.201100695

FULL PAPER
DOI: 10.1002/chem.201100695
A New Phosphine Oxide Host based on ortho-Disubstituted Dibenzofuran
for Efficient Electrophosphorescence: Towards High Triplet State Excited
Levels and Excellent Thermal, Morphological and Efficiency Stability
Chunmiao Han,[a] Guohua Xie,[b] Jing Li,[a] Zhensong Zhang,[b] Hui Xu,*[a]
Zhaopeng Deng,[a] Yi Zhao,*[b] Pengfei Yan,[a] and Shiyong Liu[b]
Abstract: An efficient host for blue
and green electrophosphorescence, 4,6-
bis(diphenylphosphoryl)dibenzofuran
(o-DBFDPO), with the structure of a
short-axis-substituted dibenzofuran was
designed and synthesised. It appears
that the greater density of the diphe-
nylphosphine oxide (DPPO) moieties
in the short-axis substitution configura-
tion effectively restrains the intermo-
lecular interactions, because only very
weak p–p stacking interactions could
be observed, with a centroid-to-cent-
roid distance of 3.960 ꢀ. The improved
thermal stability of o-DBFDPO was
corroborated by its very high glass
transition temperature (Tg) of 1918C,
which is the result of the symmetric
disubstitution structure. Photophysical
investigation showed o-DBFDPO to
be superior to the monosubstituted de-
rivative, with a longer lifetime (1.95 ns)
and a higher photoluminescent quan-
tum efficiency (61%). The lower first
singlet state excited level (3.63 eV) of
o-DBFDPO demonstrates the stronger
polarisation effect attributable to the
greater number of DPPO moieties. Si-
multaneously, an extremely high first
triplet state excited level (T1) of
3.16 eV is observed, demonstrating the
tiny influence of short-axis substitution
on T1. The improved carrier injection
ability, which contributed to low driv-
ing voltages of blue- and green-emit-
ting phosphorescent organic light-emit-
ting diodes (PHOLEDs), was further
confirmed by Gaussian calculation.
Furthermore, the better thermal and
morphological
properties
of
o-
DBFDPO and the matched frontier
molecular orbital (FMO) levels in the
devices significantly reduced efficiency
roll-offs. Efficient blue and green elec-
trophosphorescence based on the o-
DBFDPO host was demonstrated.
Keywords: electrophosphores-
cence · molecular devices · molecu-
lar electronics · phosphine oxide ·
short-axis substitution
Introduction
of triplet excitons and the stronger intermolecular interac-
tion between phosphors, however, triplet–triplet annihilation
(TTA) and concentration quenching become worse in neat
films of the electrophosphorescent materials and conse-
quently result in significant efficiency roll-offs in PHO-
LEDs.[3] A basic strategy widely used to restrain these ef-
fects is to separate the phosphorescent units and to reduce
their intermolecular interaction by dispersing them in ma-
trixes.[4] With this as a goal, integrated host–guest systems
(such as electrophosphorescent polymers[5–9] and dendrim-
ers[10–15]) and doping/blending systems[16,17] have been devel-
oped to achieve high-efficiency PHOLEDs. Although it is
expected that integrated systems should be advantageous in
terms of phase stability, most efficient PHOLEDs are still
based on doping/blending systems, in which the emission
layers consist of the charge-transporting host material
doped/blended with phosphorescent guests.[16] Through this
technology the electroluminescent (EL) performance of
green- and red-emitting PHOLEDs have approached the re-
quirements for commercial applications. Nevertheless, stable
and efficient blue-emitting PHOLEDs remain a significant
challenge.[4]
Phosphorescent organic light-emitting diodes (PHOLEDs)
are believed to be the most promising candidates for highly
energy-efficient flat-panel displays and the next generation
of solid-state lighting because of the ideal characteristics of
electrophosphorescence, such as the internal quantum effi-
ciency, approaching 100%,[1,2] resulting from the efficient
utilisation of triplet excitons. Because of the longer lifetimes
[a] C. Han, J. Li, Dr. H. Xu, Z. Deng, Prof. P. Yan
Key Laboratory of Functional Inorganic Material Chemistry
Ministry of Education, Heilongjiang University
74 Xuefu Road, Harbin 150080 (P.R. China)
Fax : (+86)451-86608042
E-mail: hxu@hlju.edu.cn
[b] G. Xie, Z. Zhang, Prof. Y. Zhao, Prof. S. Liu
State Key Laboratory on Integrated Optoelectronics
College of Electronics Science and Engineering
Jilin University, 2699 Qianjin Street
Changchun 130012 (P. R. China)
E-mail: zhao_yi@jlu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100695.
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The most famous host is 4,4’-bis(9H-carbazol-9-yl)biphe-
O has been utilised to form some phosphine oxide (PO)
hosts for blue electrophosphorescence.[30–45] It has been es-
tablished that the P=O moieties not only serve as insulating
linkages but also modify the electrical properties of the mol-
ecules as a kind of electron-withdrawing group.[33] PO hosts
are thus believed to represent both good carrier injection/
transporting capabilities and high T1 values. Nevertheless, it
is notable that nearly all of the PO hosts reported so far
have been produced through substitution with P=O moieties
along the long axes of the chromophores, which also influen-
ces the T1 values and limits the complicated modification of
PO hosts. Our group recently reported a series of high-per-
formance PO hosts with high T1 values and relatively bal-
anced carrier injection/transporting capabilities.[42,43] These
showed that the single ortho-linkage structure is superior for
retention of high T1 values and for polarising the chromo-
phore.[43] The devices exhibited low driving voltages (turn-
on voltages of about 3 V) and high efficiencies (external
quantum efficiencies (EQEs) more than 10%). Although
the ortho-linkage structure exhibited some advantages, there
are still some important points that remain unclear, such as
the relationship between the number of P=O moieties and
the carrier injection/transporting capability, the effects of
unsymmetrical or symmetrical structures on the morphologi-
cal and optical properties and so on.
ACHTUNGTRENNUNG
In continuation of this work, here we report another effi-
cient host for blue and green electrophosphorescence, based
on the structure of a dibenzofuran (DBF) doubly substituted
with diphenylphosphine oxide (DPPO) groups and named
4,6-bis(diphenylphosphoryl)dibenzofuran
(o-DBFDPO,
Scheme 1). The structural difference between o-DBFDPO
and o-DBFPPO (from our previous work) is only that of
double or single substitution. However, it was shown that
the symmetrical structure and the larger size of the o-
DBFDPO molecule produced very different thermal and
morphological properties, whereas the T1 values of the two
compounds were nearly the same, demonstrating both the
enhancing effects of double substitution on the thermal and
The requirements for an efficient host for blue electro-
phosphorescence are a high T1 value for the exothermic
energy transfer to blue-emitting phosphors and suitable
frontier energy levels facilitating carrier injection and stabil-
ity. However, it is very difficult to achieve hosts both with
high T1 values and with balanced carrier injection and trans-
porting properties. Further-
more, for practical applications
the electrical performance, such
as the driving voltage, which di-
rectly determines the power ef-
ficiency, is also an important in-
dicator.
The design and synthesis of
high-efficiency
blue-emitting
PHOLEDs with excellent elec-
trical performance are there-
fore still significant challenges.
To improve carrier injection ca-
pabilities, Kido, Su et al. uti-
lised electron-transporting pyri-
dine moieties instead of phenyls
to construct highly efficient
hosts.[26–29] Recently, a special
insulating linkage based on P= Scheme 1. Short-axis substitution structure design strategy.
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New Phosphine Oxide Host for Efficient Electrophosphorescence
FULL PAPER
phase stabilities and the tiny influence of short-axis substitu-
tion on the triplet state excited levels. The influence of the
greater number of DPPO moieties in o-DBFDPO and the
effect of different substitution positions on the frontier mo-
lecular orbitals (FMOs) were also investigated by Gaussian
simulation. The relationship between the number of P=O
groups and the electron-injection ability and the effect of
short-axis substitution were established. As a result of the
high T1 value of 3.16 eV and the excellent thermal and mor-
phological stabilities of o-DBFDPO, efficient blue and
green electroluminescence of PHOLEDs based on the com-
pound were achieved.
Scheme 2. Synthetic procedure for o-DBFDPO.
The molecular structure was further confirmed by single-
crystal X-ray diffraction analysis (Figure 1a). Weak aromatic
p–p stacking interactions between adjacent o-DBFDPO
molecules, giving rise to a dimer with a centroid-to-centroid
distance of 3.960 ꢀ, can be observed as shown in Figure 1b.
Weak p–p interaction of this kind contributes to the stability
of the solid films. Nevertheless, no further p–p stacking or
hydrogen-bonding interactions exist between the dimers.
This is far different from the case of the corresponding
para-linked DBF phosphine oxide derivative (p-DBFDPO),
in which much closer p–p interactions (3.385 ꢀ) between
the DBF bridges are observed in the solid state accompa-
Results and Discussion
Design and synthesis: For most reported aryl PO hosts, the
substitution of P=O moieties is along the long axis of the
chromophore group, which can include dibenzofuran, diben-
zothiophene, carbazole or fluorene, because bromination is
feasible at their 2,8-, 3,6- or 2,7-positions, determining the
final substitution positions of the P=O moieties, such as in
p-DBFDPO in Scheme 1. Although it is believed that the
DPPO moiety, as a kind of insulating group, does not influ-
ence the excited energy levels of the molecules, these long-
axis-substitution configurations still have an effect on reduc-
ing T1 values and weakening the polarising effect of the
electron-withdrawing DPPO, as already shown in our previ-
ous work[43] and discussed in depth in this paper. An effec-
tive strategy to solve this problem is to substitute the chro-
mophore groups along their short axes (Scheme 1). Configu-
rations of this kind not only completely retain high T1
values of the chromophore groups, but also amplify the po-
larisation effect of the P=O moieties because the electron-
withdrawing effects of the P=O moieties in the long-axis-
substitution configuration are cumulative but partly cancel
each other out. Short-axis substitution is therefore outstand-
ing in offering two advantages for the P=O moieties:
1) preservation of high T1 values and 2) polarisation of mol-
ecules to enhance the carrier-transporting capabilities. In
our previous work we reported the asymmetric PO host o-
DBFPPO (Scheme 1) with the structure of a monosubstitut-
ed DBF. Both because of its unsymmetrical structure and
because of its small molecular volume, its thermal stability
is insufficient (Tg <1008C). To form a symmetrical structure,
o-DBFDPO, based on double substitution of DPPO moiet-
ies at the 4- and 6-positions of DBF, was designed. Im-
proved thermal and morphological stability can be expected.
On the other hand, the influence of the presence of more
DPPOs on the excited energy levels and the carrier-trans-
porting properties of the whole molecule, as well as the rela-
tionship between them, were also focused on.
ꢁ
ꢁ
nied by two close P O—H C interactions (all C–O distan-
ces ꢀ3.4 ꢀ) with the DBF ring on adjacent molecules. The
o-DBFDPO was conveniently prepared through a three-
step procedure involving regioselective lithiation, phosphori-
sation and oxidation, in a good total yield of over 60%
(Scheme 2). Structure characterisation was achieved by mass
spectrometry, NMR spectroscopy and elemental analysis.
Figure 1. a) Crystal structure of o-DBFDPO, and b) intermolecular inter-
action.
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H. Xu, Y. Zhao et al.
density of DPPO moieties in the short-axis-substitution con-
figuration thus effectively restrains the intermolecular inter-
actions, which is beneficial for reduction of concentration
quenching and TTA.
of DBF. Another absorption peak at 252 nm is ascribed to
the p!p* transitions of the phenyl groups in the DPPO
moieties. It indicates that DBF serves as the chromophore
group and determines the excited state energy levels of the
whole molecule. In films, all of the bands show bathochro-
mic shifts. Nevertheless, the main peaks can be recognised,
such as 254 nm for p!p* transitions of phenyl groups and
274, 300 and 311 nm for p!p* transition of DBF, which
also implies weak intermolecular interaction in o-DBFDPO
in the solid state.
The main emission peak of o-DBFDPO in CH2Cl2 solu-
tion is at 328 nm. A weaker shoulder peak at 376 nm attrib-
uted to the dimer is also observed. In film, because of the
enhanced intermolecular interaction, the emission peak of
the dimers at 374 nm has become stronger than that of the
monomer. It is shown that the PL emissions of o-DBFDPO
both in solution and in film at room temperature range from
300 to 500 nm, completely overlapping with the metal-to-
ligand charge transfer (MLCT) absorption peaks of the
Thermal properties: The thermal properties of o-DBFDPO
were investigated by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC). Its temperature of
decomposition (Td) is as high as 3648C (Figure S1 in the
Supporting Information), which is an improvement of 708C
relative to o-DBFPPO and makes device fabrication
through vacuum evaporation more feasible. DSC analysis
shows that the melting point (Tm) of o-DBFDPO is 3568C
(Figure S1, inset in the Supporting Information), which is
nearly 1708C higher than that of o-DBFDPO and attributa-
ble to the enhanced p–p intermolecular interaction. Signifi-
cantly, the glass transition temperature (Tg) of o-DBFDPO
is as high as 1918C, about 100 and 908C higher than those
of o-DBFPPO[43] and p-DBFDPO,[33] respectively, and is
outstanding among the small molecular hosts. Obviously, the
symmetrical structure of o-DBFDPO greatly improves the
thermal performance. The compact structure resulting from
the short-axis substitution amplifies the strong steric effect
of neighbouring DPPO moieties, which restrains the struc-
ture adjustment of the whole molecule and results in the
high Tg value. It has been shown that the symmetrical struc-
ture is superior to the asymmetrical structure in thermal and
morphological stability, so potential phase separation
through thermal effects in the film of o-DBFDPO during
device operation can be effectively suppressed. This is very
important for improving device lifetime.
blue-emitting phosphor iridium
pyridinato-N,C2]picolinate (FIrpic) and the green-emitting
phosphor iridium(III) bis(2-phenylpyridinato-C2,N)acetyla-
cetonate, Ir(ppy)2acac. At the same time, the PL lifetime of
ACHUTNGTRENNUG(III) bisACHTUTGNREN[NUGN 4,6-(difluorophenyl)
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
o-DBFDPO is 1.95 ns (Figure S2 in the Supporting Informa-
tion), which is longer than that of o-DBFPPO (1.57 ns, Fig-
ure S3 in the Supporting Information). A more stable excit-
ed state is superior in facilitating energy transfer to the
guests. Furthermore, the PL quantum yield (PL QY) of o-
DBFDPO is 61%, which is 14% higher than that of o-
DBFPPO. A higher PL QY usually corresponds to less non-
radiative loss of excited energy, so efficient energy transfer
between o-DBFDPO and phosphorescent guests can be ex-
pected. An essential condition for a suitable host for a blue-
emitting PHOLED is a high T1 value of more than 3.0 eV.
The phosphorescence spectrum of o-DBFDPO at 77 K was
measured to determine an extremely high T1 value of
3.16 eV, estimated from the u0,0 transition identified as the
highest-energy band (393 nm). As hypothesised, the T1 of o-
DBFDPO is as the same as that of o-DBFPPO.[43] This
strongly indicates that the short-axis substitution hardly in-
fluences the triplet state excited levels of the whole mole-
cules. Simultaneously, it is notable that the first singlet
energy level (S1) of o-DBFDPO estimated from its absorp-
tion edge is only 3.63 eV, so the energy gap between S1 and
T1 (DEST) of o-DBFDPO is only 0.47 eV. A small DEST
value has a positive effect on facilitating the carrier injection
and transport in a PHOLED.[26] The unique characteristics
of o-DBFDPO imply that the short-axis substitution can ef-
fectively preserve the T1 value of the chromophore and si-
multaneously reduce S1, which can be attributed to the en-
hanced insulating structure and polarisation effects of the
P=O moieties.
Optical properties: UV/Vis and photoluminescence (PL)
spectra of o-DBFDPO in dilute CH2Cl2 solutions (1ꢂ
10ꢁ6 molLꢁ1) and in films were measured to investigate its
photophysical properties (Figure 2). The weakest duplicate
absorption peaks at 322 and 336 nm are attributed to the
n!p* transition of DBF, whereas the three stronger peaks
at 274, 289 and 306 nm originate from the p!p* transition
Theoretical calculation: To understand the nature of the op-
tical properties of o-DBFDPO and the influence of short-
axis substitution on FMOs, DFT calculations were conduct-
ed with the aid of the Gaussian 03 package at the B3LYP6–
Figure 2. UV/Vis and PL spectra of o-DBFDPO. UV of DBFDPO in
CH2Cl2 (c); UV of DBFDPO in film (b), FL of DBFDPO in
~
&
*
CH2Cl2 ( ), PH of DBFDPO at 77 K ( ), FL of DBFDPO in film ( ).
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New Phosphine Oxide Host for Efficient Electrophosphorescence
FULL PAPER
31G* level. The corresponding calculation on p-DBFDPO
was also performed for comparison. The calculated highest
occupied molecular orbitals (HOMOs) of o-DBFDPO and
p-DBFDPO are ꢁ6.258 and ꢁ6.340 eV, respectively, where-
as the calculated lowest unoccupied molecular orbital
(LUMOs) of o-DBFDPO and p-DBFDPO are ꢁ1.333 and
ꢁ1.279 eV, respectively (Figure 3). The HOMO–LUMO
moieties in o-DBFDPO. Nevertheless, the Eg of o-
DBFDPO is 0.025 eV smaller than that of o-DBFPPO,
whereas the Eg values of p-DBFDPO and p-DBFPPO are
nearly the same. This also shows the stronger polarisation
effect of short-axis substitution. Furthermore, the differen-
ces between LUMOꢁ1/LUMO and HOMO/HOMO+1 in
o-DBFDPO are 0.265 and 0.082 eV, respectively, much
smaller than those in o-DBFDPO (0.462 and 0.109 eV, re-
spectively). It is believed that the small intramolecular
energy gaps are advantageous for carrier hopping between
different molecules, and consequently improve carrier trans-
port.[46] The contour plots of the FMOs are presented in
Figure 3. It is shown that because of their symmetric struc-
tures the electron cloud density distributions of FMOs of o-
DBFDPO are, on average, on DBF or phenyls, unlike in o-
DBFPPO. Nevertheless, it is notable that all of FMOs of o-
DBFDPO include a contribution from DBF, whereas the
electron clouds of the LUMOꢁ1 and the LUMO of o-
DBFDPO are localised at DPPO moieties and DBF, respec-
tively and its HOMO and HOMO+1 are thoroughly local-
ised on DBF. Because of the small difference between
LUMO and LUMOꢁ1 of o-DBFDPO, hole- and electron-
injection into DPPOs and DBF, respectively, can be expect-
ed, which should facilitate the balanced carrier injection in
the host matrix.
EL properties: Inspired by the improved thermal and opti-
cal properties of o-DBFDPO, we fabricated blue- and
green-emitting PHOLEDs based on o-DBFDPO to investi-
Figure 3. Calculation results (Gaussian) for o-DBFDPO and p-
DBFDPO.
gate its role as
a host material. Devices A and B
energy gap (Eg) in o-DBFDPO is 4.925 eV, which is 0.14 eV
smaller than that in p-DBFDPO. With the lower LUMO
and higher HOMO, o-DBFDPO can be expected to be
more effective in double-carrier injection than p-DBFDPO.
It is also noticed that in relation to the FMOs of o-
DBFPPO, both the HOMO and the LUMO of o-DBFDPO
are significantly reduced by about 0.1 eV, which should be
attributed to the more strongly electron-withdrawing P=O
(Scheme 3), which are blue-emitting, with the compositions
ITO/MoOx (2 nm)/4,4’,4’’-tris(N-3-methylphenyl-N-phenyla-
mino)triphenylamine
(m-MTDATA):MoOx
(15 wt.%,
30 nm)/m-MTDATA
(10 nm)/tris(phenylpyrazole)iridium
(IrACTHNUTRGNE(UGN ppz)3, 10 nm)/FIrpic:o-DBFDPO (10%, y nm)/4,7-di-
phenyl-1,10-phenanthroline (BPhen) (50ꢁy nm)/LiF (1 nm)/
Al (y=10 nm for A and 20 nm for B) were fabricated.
MoOx and LiF served here as hole- and electron-injecting
Scheme 3. Structures of the materials in the devices and the energy level diagram.
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H. Xu, Y. Zhao et al.
layers, m-MTDATA and BPhen as hole- and electron-trans-
porting layers (HTL and ETL), and Ir(ppz)3 was used as
hole-transporting and electron-blocking material. The
green-emitting devices C and D were designed with the
same configuration except for the different guest, Ir-
same luminance (Figure 5). This is consistent with the con-
clusion that a wider emission zone should increase the rate
of recombination and reduce the concentration quenching.
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG(ppy)2acac, and the different doping concentration of 6%.
The thicknesses of the emitting layers (EMLs) of devices C
and D were 10 and 20 nm, respectively.
Devices A and B had lower turn-on voltages of 3.2 V
(Figure 4). A practical luminance of 1000 cdmꢁ2 was ach-
ieved at low driving voltages of less than 4.8 V (1465 cdmꢁ2
&
*
Figure 5. Efficiency curves of devices A ( ) and B ( ).
The maximum CEs of A and B were 18.1 cdAꢁ1 at
209 cdmꢁ2 and 19.5 cdAꢁ1 at 194 cdmꢁ2, respectively. The
CEs of
A and B
were 16.2 cdAꢁ1 (1037 cdmꢁ2) and
17.9 cdAꢁ1 (961 cdmꢁ2), respectively. The corresponding ef-
ficiency roll-offs were 10.5 and 9.2%. The maximum power
efficiencies (PEs) of A and B were achieved as 14.2 and
15.3 LmWꢁ1 at the same luminance, corresponding to the
maximum external quantum efficiencies (EQEs) of 9.3 and
10.0%. The EQE roll-offs of A (at 1037 cdmꢁ2) and B (at
961 cdmꢁ2) were 9.7 and 8.7%, respectively. Although
except for their EQEs the maximum efficiencies of PHO-
LEDs based on p-DBFDPO are higher than those based on
o-DBFDPO, those data are obtained at very low J values
(about tens of microamps per square cm). At high lumi-
nance and J, the PEs and EQEs of A and B were even
higher than those of PHOLEDs based on p-DBFDPO. Sig-
nificantly the efficiency roll-offs (at 800 cdmꢁ2) of PHO-
LEDs based on p-DBFDPO are notably higher than those
of A and B (22% for CE, 52% for PE and 18% for EQE,
double those of A and B). This significant difference should
have its origin in the different natures of the hosts. From the
single-crystal and thermal analysis, o-DBFDPO has stronger
morphological stability and displays much weaker intermo-
lecular interaction than p-DBFDPO. This facilitates the uni-
form dispersion of the dopants in o-DBFDPO and keeps
the compositions of the films stable at high voltage and J
values. The weaker intermolecular interaction between hosts
and host/guest results in restraint of multiparticle annihila-
tion processes, such as triplet–triplet annihilation
(TTA),[47,48] triplet-polaron quenching (TPQ)[49] and colli-
sion-induced exciton dissociation.[50] Furthermore, the small
barrier to electron injection reduced the driving voltages of
A and B, and to some degree restrained the field-assisted
exciton dissociation. The better thermal and morphological
properties of o-DBFDPO and the well-matched FMO levels
of EMLs with those of the carrier-transporting layers are
&
Figure 4. Brightness/current density/voltage curves of devices A ( ) and
*
B ( ).
for A and 1357 cdmꢁ2 for B at 4.8 V). The operating voltag-
es of A and B were much lower than those of FIrpic-based
PHOLEDs with p-DBFDPO as the host, for which
800 cdcmꢁ2 can be achieved at 5.4 V.[33] This also demon-
strates the stronger carrier injection ability of o-DBFDPO
relative to p-DBFDPO, which can be attributed to the
stronger polarisation effect of short-axis substitution and
further confirms the conclusions of optical and theoretical
calculation analysis. It was shown that devices A and B have
almost identical brightness/voltage (B/V) characteristics.
The thicker EML did not induce an increase in the driving
voltage. This implies excellent carrier injection in the EMLs,
which can be attributed to the small barrier of 0.1 eV for
electron injection and the comparable electron transport ca-
pabilities of o-DBFDPO and Bphen (Scheme 3). Neverthe-
less, device A exhibited better current density/voltage (J/V)
characteristics. Because the barrier between IrACTHUNRGTNEUNG(ppz)3 and o-
DBFDPO is as high as 1.1 eV, the hole injection in EMLs is
through filled-assistant carrier capture by FIrpic. Because
few candidates show superior electron mobility to Bphen,
the inferior J/V characteristics of device B suggest intrinsic
electron-dominating characteristic of our devices with heavi-
ly p-doping structure and somewhat inferior electron mobili-
ty of o-DBFDPO to Bphen, the electron mobility of which
is among the highest values reported.[51]
Because of the lower J value and similar brightness of B,
its current efficiency (CE) was higher than that of A at the
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New Phosphine Oxide Host for Efficient Electrophosphorescence
FULL PAPER
therefore some of the main reasons for the remarkably re-
duced efficiency roll-offs of A and B. Their excellent effi-
ciency stabilities at high luminance also reflects the good
loading capabilities of the whole devices for higher voltages
and higher currents, which are directly related to the operat-
ing lifetime. Although it is believed that a narrow EML
might reduce efficiency because of the relatively high exci-
ton concentration, it was found that the differences in effi-
ciencies between A and B were small. Here, a wide EML
seems not to play a major role in improving the efficiency.[48]
Some other mechanisms are believed to contribute to the
evolution of the efficiencies.
The EL spectra of A and B both consisted only of the
emission bands from FIrpic: a main peak at 472 nm and a
shoulder peak at 500 nm (Figure 6). The intensity ratio of
&
Figure 7. Brightness/current density/voltage curves of devices C ( ) and
*
D ( ).
&
*
Figure 6. EL spectra of devices A ( ) and B ( ).
&
*
Figure 8. Efficiency curves of devices C ( ) and D ( ).
the main peak and shoulder peak was 1:0.7 for both A and
B, whereas for the p-DBFDPO-based devices it was almost
process, such as vibration relaxation. Nevertheless, the effi-
ciency stabilities of C and D were outstanding among devi-
ces based on hosts with high T1 values. Similarly to those of
A and B, the EL spectra of C and D were also very stable,
with the main peak at 520 nm much stronger than the
shoulder peak at 548 nm (Figure 9).
1:1.[33] Because the hole-injection barrier between Ir
ACTHNUTRGEN(UNG ppz)3
and o-DBFDPO is as high as 1.1 eV, holes were directly in-
jected into EML and were trapped at the dopant sites. The
stable EL spectra therefore also imply the balanced distribu-
tion of the carrier in the EMLs of A and B.
The green-emitting devices C and D were also fabricated
to demonstrate the potential of o-DBFDPO as a universal
host material. Devices C and D had the same low turn-on
voltage of 3.2 V (Figure 7). Practical luminance around
1000 cdmꢁ2 was achieved at 4.8 V for C and at 5.0 V for D.
The low operating voltages were also attributable to barrier-
free injection of electrons in o-DBFDPO and efficient hole-
The EL performances of the devices are listed in Table 1.
It is shown that no matter which dopant is used, o-
trapping in IrACHTUNGTRENNUNG(ppy)2acac. The maximum CEs and PEs of C
and D were 24.8 and 24.6 cdAꢁ1 and 18.1 and 18.0 LmWꢁ1,
accompanied by maximum EQEs of 6.91 and 6.87%, respec-
tively (Figure 8). Significantly, C and D also showed excel-
lent efficiency stability. The efficiency roll-offs of C and D
were only 2.5 and 4.9% for CE, 12.7 and 18.3% for PE and
2.9 and 8.3% for EQE. The moderate efficiencies of C and
D are ascribed mainly to the extremely high T1 value, close
to 3.2 eV, of o-DBFDPO, because the huge triplet energy
gap of 0.77 eV between o-DBFDPO and IrACHTUGNRTEN(NUGN ppy)2acac in-
*
volves some nonradiative transitions in the energy-transfer
Figure 9. EL spectra of devices C (&) and D ( ).
Chem. Eur. J. 2011, 17, 8947 – 8956
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www.chemeurj.org
8953

H. Xu, Y. Zhao et al.
Table 1. EL performances of devices A–D.
[b]
[b]
Device
Voltage [V][a]
CE [cdAꢁ1
]
PE [LmWꢁ1
]
EQE. [%][b]
Efficiency roll-off [%][c]
CIE [x, y][d]
A
B
C
D
3.2, 4.0, 4.6
3.2, 4.0, 4.6
3.2, 4.0, 4.8
3.2, 4.2, 5.0
18.11, 16.23, 7.37
19.49, 17.89, 8.44
24.84, 24.21, 17.94
24.61, 23.38, 17.22
14.42, 11.08, 3.74
15.31, 12.22, 4.02
18.08, 15.85, 9.39
18.00, 14.69, 8.73
9.33, 8.37, 3.80
10.00, 9.14, 4.31
6.91, 6.73, 4.99
6.87, 6.30, 4.50
10.38, 23.16, 10.29
8.21, 20.18, 8.60
2.54, 11.99, 2.60
5.00, 18.39, 8.30
0.157, 0.307
0.158, 0.310
0.313, 0.624
0.314, 0.622
[a] In the order of at about 1, 100 and 1000 cdmꢁ2. [b] In the order of maximum, at 1000 and 5000 cdmꢁ2. [c] At 1000 cdmꢁ2 in the order of CE, PE and
EQE [d] At 1000 cdmꢁ2
.
Experimental Section
DBFDPO is superior in reducing annihilation effects and
improving carrier injection/transport. These advantages are
the result of the fine structure design of short-axis substitu-
tion and the resulting physical properties.
Materials and instruments: All reagents and solvents used for the synthe-
sis of the title compound were purchased from the Aldrich and Acros
companies and used without further purification.
1H NMR spectra were recorded with a Varian Mercury plus 400NB spec-
trometer relative to tetramethylsilane (TMS) as internal standard. Molec-
ular masses were determined with a Finnigan LCQ Electro-Spraying Ion-
isation-Mass Spectrometer (ESI-MS) or a MALDI-TOF MS. Elemental
analyses were performed with a Vario EL III elemental analyzer. Ab-
sorption and PL emission spectra of the target compound were measured
with a SHIMADZU UV-3150 spectrophotometer and a SHIMADZU
RF-5301PC spectrophotometer, respectively. Thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) were performed with
Shimadzu DSC-60A and DTG-60A thermal analysers under nitrogen at
a heating rate of 108Cminꢁ1. Cyclic voltammetric (CV) studies were con-
ducted with an Eco Chemie B. V. AUTOLAB potentiostation with a typ-
ical three-electrode cell with a platinum sheet working electrode, a plati-
num wire counter electrode and a silver/silver nitrate (Ag/Ag+) reference
electrode. All electrochemical experiments were carried out under nitro-
gen at room temperature in acetonitrile. Phosphorescence spectra were
measured in dichloromethane with an Edinburgh FPLS 920 fluorescence
spectrophotometer at 77 K with liquid nitrogen cooling. The crystal suita-
ble for single-crystal X-ray diffraction analysis was obtained through the
slow evaporation of an ethanol solution of the compound at room tem-
perature. All diffraction data were collected at 295 K with a Rigaku
Raxis-Rapid diffractometer and graphite monochromatised Mo-Ka (l=
0.71073 ꢀ) radiation in w scan mode. All structures were solved by direct
methods and difference Fourier syntheses. Non-hydrogen atoms were re-
fined by full-matrix least-squares techniques on F2 with anisotropic ther-
mal parameters. The hydrogen atoms attached to carbons were placed in
Conclusion
An efficient host material, o-DBFDPO, for blue and green
PHOLEDs was designed and synthesised to validate an ef-
fective strategy named short-axis substitution. It appears
that, relative to p-DBFDPO, with its long-axis substitution
structure, the amplified steric effect induced by the ortho
linkage significantly improves the thermal and morphologi-
cal performance of o-DBFDPO. A high Tg value and
weaker intermolecular interaction are observed. At the
same time, o-DBFDPO is also superior to the unsymmetri-
cal o-DBFPPO in thermal stability, with only weak p–p in-
teraction introduced, demonstrating the advantage of sym-
metric structures in film stability. The optical investigations
showed that short-axis-substituted DPPO groups do not in-
fluence the triplet state excited level of the chromophore,
because the T1 values of o-DBFDPO and of o-DBFPPO
are equivalent. Furthermore, the DEST value of o-DBFDPO
is 0.27 eV lower than that of o-DBFPPO, which strongly
corroborates the enhanced polarisation effect ascribed to
short-axis substitution structure. The stronger polarisation of
DBF in o-DBFDPO was further verified by Gaussian calcu-
lations, which indicated a lower LUMO and a higher
HOMO in o-DBFDPO than in p-DBFDPO and the partial-
ly separated electron clouds of the FMOs of o-DBFDPO.
The improved EL performance from o-DBFDPO-based
blue- and green-emitting PHOLEDs, such as low driving
voltages and stable efficiencies, reflects the advantages of
short-axis substitution: 1) an effectively polarising chromo-
phore to improve carrier injection/transport and conse-
quently to reduce the driving voltages, 2) a high T1 value to
facilitate exothermic energy transfer to the dopants and
3) reduction of intermolecular interaction to form a uniform
film and to restrain annihilation effects. These results indi-
cate that short-axis substitution is an effective strategy for
achieving high-performance hosts for electrophosphores-
cence. However, the hole injection and transporting capabil-
ities of the PO hosts are also only sufficient, depending on
the polarisation effects of the P=O moieties. To solve this
problem, further multi-functionalisation modification based
on short-axis substitution is imperative, and is already in
progress in our laboratory.
ꢁ
calculated positions with C H=0.93 ꢀ and U (H)=1.2Ueq (C) in the
riding model approximation. All calculations were carried out by use of
the SHELXL97 program.
Synthesis
4,6-Bis(diphenylphosphoryl)dibenzofuran (DBFDPO): A solution of n-
butyllithium (2.4m in n-hexane, 12.5 mL, 30 mmol) was added dropwise
at ꢁ788C to a stirred solution of dibenzofuran (1.6819 g, 10 mmol) and
N,N,N’,N’-tetramethylethylenediamine (TMEDA, 4.57 mL, 30 mmol) in
diethyl ether (50 mL). After all the n-butyllithium had been added, the
cooling bath was removed, and the reaction mixture was allowed to
warm to room temperature. After 16 h, the mixture was cooled to
ꢁ788C, and a solution of chlorodiphenylphosphine (5.92 mL, 33 mmol)
in diethyl ether was added dropwise. The cooling bath was removed, and
the reaction mixture was stirred for another 16 h. The reaction was
quenched with water (10 mL), and the mixture was extracted with di-
chloromethane (3ꢂ30 mL). The organic layer was dried with anhydrous
Na2SO4. The solvent was removed in vacuo. The residue was dissolved in
dichloromethane (50 mL). Hydrogen peroxide (30%, 3 mL) was added,
and the reaction mixture was stirred for 4 h. The reaction was extracted
with saturated sodium hydrogensulfite solution. The organic layer was
dried with anhydrous Na2SO4. The solvent was removed in vacuo. Yield:
3.4 g of white powder (60%). 1H NMR (400 MHz, CDCl3, TMS): d=
8.185 (d, J=7.6 Hz, 2H), 7.782 (dd, J=7.4, 13 Hz, 2H), 7.691–7.590 (m,
8H), 7.520–7.432 (m, 6H), 7.401–7.307 ppm (m, 8H); LDI-TOF: m/z
(%): 568 (100) [M]+; elemental analysis calcd (%) for C36H26O3P2: C
76.05, H 4.61, O 8.44; found C 76.09, H 4.62, O 8.56.
8954
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New Phosphine Oxide Host for Efficient Electrophosphorescence
FULL PAPER
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Acknowledgements
C. Han and G. Xie contributed equally to this work. This work was finan-
cially supported by the National Key Basic Research and Development
Program of China under Grant No. 2010CB327701, the NSFC under
Grants 50903028 and 60977024, the Natural Science Foundation of Hei-
longjiang province under Grant QC08C10, the Education Bureau of Hei-
longjiang province under Grant 2010td03, and Heilongjiang University
under Grant 2010 hdtd-08. C.H. thanks the 2010 Student Innovation
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Published online: June 28, 2011
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