Elevating the triplet energy levels of dibenzofuran-based ambipolar phosphine oxide hosts for ultralow-voltage-driven efficient blue electrophosphorescence: From D-A to D-π-A systems

Article abstract of DOI:10.1002/chem.201203719

A series of donor (D)-π-acceptor (A)-type phosphine-oxide hosts (DBF_xPOPhCz_n), which were composed of phenylcarbazole, dibenzofuran (DBF), and diphenylphosphine-oxide (DPPO) moieties, were designed and synthesized. Phenyl π-spacer groups were inserted between the carbazolyl and DBF groups, which effectively weakened the charge transfer and triplet-excited-state extension. As the result, the first triplet energy levels (T₁) of DBF_xPOPhCz_n are elevated to about 3.0 eV, 0.1 eV higher than their D-A-type analogues. Nevertheless, the electrochemical analysis and DFT calculations demonstrated the ambipolar characteristics of DBF_xPOPhCz_n. The phenyl π spacers hardly influenced the frontier molecular orbital (FMO) energy levels and the carrier-transporting ability of the materials. Therefore, these D-π-A systems are endowed with higher T₁ states, as well as comparable electrical properties to D-A systems. Phosphorescent blue-light-emitting diodes (PHOLEDs) that were based on DBF_xPOPhCz_n not only inherited the ultralow driving voltages (2.4 V for onset, about 2.8 V at 200 cd m ^-2, and <3.4 V at 1000 cd m^-2) but also had much-improved efficiencies, including about 26 cd A^-1 for current efficiency, 30 Lm W^-1 for power efficiency, and 13 % for external quantum efficiency, which were more than twice the values of devices that are based on conventional unipolar host materials. This performance makes DBFDPOPhCz_n among the best hosts for ultralow-voltage-driven blue PHOLEDs reported so far. Copyright

Full text of DOI:10.1002/chem.201203719

FULL PAPER
DOI: 10.1002/chem.201203719
Elevating the Triplet Energy Levels of Dibenzofuran-Based Ambipolar
Phosphine Oxide Hosts for Ultralow-Voltage-Driven Efficient Blue
ꢀ
ꢀ ꢀ
D p A Systems
Electrophosphorescence: From D A to ACTHUNGTRENNNUG
Chunmiao Han,[a] Zhensong Zhang,[b] Hui Xu,*[a] Jing Li,[a] Yi Zhao,*[b]
Pengfei Yan,[a] and Shiyong Liu[b]
Abstract: A series of donor (D)–p–ac-
ceptor (A)-type phosphine-oxide hosts
(DBFxPOPhCzn), which were com-
culations demonstrated the ambipolar
characteristics of DBFxPOPhCzn. The
phenyl p spacers hardly influenced the
frontier molecular orbital (FMO)
energy levels and the carrier-transport-
ing ability of the materials. Therefore,
ting diodes (PHOLEDs) that were
based on DBFxPOPhCzn not only in-
herited the ultralow driving voltages
(2.4 V for onset, about 2.8 V at
200 cdmꢀ2, and <3.4 V at 1000 cdmꢀ2)
but also had much-improved efficien-
cies, including about 26 cdAꢀ1 for cur-
rent efficiency, 30 LmWꢀ1 for power
efficiency, and 13% for external quan-
tum efficiency, which were more than
twice the values of devices that are
based on conventional unipolar host
materials. This performance makes
DBFDPOPhCzn among the best hosts
for ultralow-voltage-driven blue PHO-
LEDs reported so far.
posed of phenylcarbazole, di
ACHTUNGTRNEbNUNG enzo-
ACHTUNGTRENNUNGfuran (DBF), and diphenylphosphine-
oxide (DPPO) moieties, were designed
and synthesized. Phenyl p-spacer
groups were inserted between the car-
bazolyl and DBF groups, which effec-
tively weakened the charge transfer
and triplet-excited-state extension. As
the result, the first triplet energy levels
(T1) of DBFxPOPhCzn are elevated to
about 3.0 eV, 0.1 eV higher than their
ꢀ ꢀ
these D p A systems are endowed
with higher T1 states, as well as compa-
ꢀ
rable electrical properties to D A sys-
tems. Phosphorescent blue-light-emit-
Keywords: donor–acceptor
sys-
tems electrophosphorescence
·
·
heterocycles · phosphorous · triplet
energy levels
ꢀ
D A-type analogues. Nevertheless, the
electrochemical analysis and DFT cal-
Introduction
phosphorescent dopants; 2) facilitating and balancing carrier
injection and transportation; and 3) transferring energy to
the dopants.[4] Therefore, an eligible host materials should
be both optically and electrically active.[5] However, for
blue-light-emitting PHOLEDs, the optical and electrical
properties of the hosts often contradict each other and
Phosphorescent organic light-emitting diodes (PHOLEDs)
have promising applications in the next generation of flat-
panel displays[1] and in solid-state lighting,[2] owing to their
advantages in terms of energy saving, such as their high in-
ternal quantum efficiencies (approaching 100%), in de
G
rarely support good optoelectronic performance simul
ACHTUNGTRENNUNG
ACHTUNGTRENNUNGta-
in which the host material is one of the determinants of
neously.[6] The origin of this problem is the similar effects of
device performance.[3] During electroluminescent (EL) pro-
these modification approaches on the singlet (S1) and triplet
excited energy levels (T1).[7] In this sense, it is rather difficult
to achieve exothermic host–guest energy transfer and im-
proved carrier-injecting/transporting ability at the same
time, because the former requires a high enough T1 state in
the hosts (approaching 3.0 eV for conventional blue phos-
ACHTUNGTRENNUNGcesses, the hosts in the emitting layers (EMLs) of doping-
type PHOLEDs have three functions: 1) Suppressing multi-
particle quenching effects through the uniform dispersion of
[a] C. Han,+ Dr. H. Xu, J. Li, 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] Z. Zhang,+ Prof. Y. Zhao, Prof. S. Liu
phor
[iridium
G
G
pyridinato-
N,C2)picolinate] (FIrpic)),[8] whereas the latter should origi-
nate from a relatively low S1 state in the hosts to make their
frontier molecular orbital (FMO) energy levels fit to those
of the adjacent carrier-transporting layers.[9] Obviously, the
selective modulation of the excited energy levels of a host is
the key point in facing this great challenge.[10]
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
Although some effective strategies, such as meso-,[11] insu-
lating,[8,12] and twisted linkages,[13] have been used to con-
struct hosts with high T1 states, these hosts still cannot pro-
vide enough electrical performance. Therefore, there are
only few FIrpic-based devices that have low driving voltages
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203719.
Chem. Eur. J. 2013, 19, 1385 – 1396
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1385

of less than 3 V at 100 cdmꢀ2.[9,12j,14] Recently, the success of
ambipolar hosts for blue electrophosphorescence has point-
ed the way towards highly efficient hosts with high T1 states
and balanced carrier-transporting ability.[9,11b,12g,l,13b,15] Ambi-
polar structures can support both hole and electron injection
and transportation through electron-donating and electron-
withdrawing groups, respectively.[16] Thus, the electrical
properties of a molecule can be conveniently tuned by incor-
porating different kinds and numbers of electrically active
DBF DPPO hybrids through inserting phenyl groups be-
ꢀ
tween the carbazolyl and DBF moieties as p spacers to re-
strict the direct CT (Scheme 1), namely (2-(4-(9H-carbazol-
9-yl)phenyl))-(6-diphenylphosphoryl)dibenzofuran
(DBFSPOPhCz), (2-(4-(9H-carbazol-9-yl)phenyl))-(4,6-bis-
AHCTUNGTREG(NNNU diphenylphosphoryl))dibenzofuran (DBFDPOPhCz), and
(2,8-bis(4-(9H-carbazol-9-yl)phenyl))-(4,6-bis(diphenylphos-
phoryl))dibenzofuran (DBFDPOPhCz2), collectively named
DBFxPOPhCzn. As expected, although the additional
phenyl groups increase the conjugation, the T1 states of
DBFxPOPhCzn were elevated to about 3.0 eV, owing to the
effective suppression of CT. Simultaneously, the ambipolar
characteristics of DBFxPOPhCzn endowed FIrpic-based de-
vices with extremely low driving voltages (2.4 V for onset,
<2.8 V for 200 cdmꢀ2, <3.4 V for 1000 cdmꢀ2), which were
the same as those of DBFxPOCzn-based devices, whilst the
efficiencies of devices that were based on DBFxPOPhCzn
were dramatically improved by more than 30%. Our investi-
gation not only exhibited the potential of DBFxPOPhCzn as
hosts for highly efficient low-voltage-driven PHOLEDs, but
also clarified the negative influence of the CT states and ex-
tended conjugation on the T1 states. This result can further
direct the controllable tuning of molecular optoelectronic
properties and the purposeful design of high-performance
host materials.
ꢀ
groups. Nevertheless, a strong donor–acceptor (D A) inter-
action remarkably lowers the T1 state, owing to the forma-
tion of charge-transfer (CT) states.[17] To solve this problem,
one of the main strategies involves blocking the intramolec-
ular interplay by insulating linkages, such as saturated Si[12g]
or C atoms.[12i] We also found that, on the basis of indirect
linkage through the C9 atom of fluorene, the interplay be-
tween hole-transporting carbazolyl and electron-transporting
diphenylphosphine-oxide (DPPO) groups can be effectively
suppressed to achieve ambipolar hosts with a preserved T1
state of 3.0 eV.[12l] Both high EL efficiencies and efficiency
stability were realized; however, the driving voltage at
100 cdmꢀ2 was over 3.5 V. This result implies that elec
ACHTUNGTRENNUNGcally inert saturated Si and C sites in the hosts are inferior
ACHTUNGTNERtNUNG ri-
in terms of carrier injection and transportation. Thus, we re-
cently constructed a series of dibenzofuran (DBF)-based ter-
nary hosts (DBFxPOCzn) through short-axis and meso link-
ACHTUNGTRENNUNG
ages (Scheme 1).[9] Inspiringly, these hosts endowed their
Results and Discussion
Design and synthesis: It is common sense that extended con-
jugation can result in a simultaneous decrease of the S1 and
T1 states, whilst the formation of low-energy CT states also
induces instability of the T1 state. Therefore, it seems that
both of these features should be avoided when designing
high-energy-gap hosts for blue electrophosphorescence.
However, if bipolar structures were involved in hosts to en-
hance their electrical properties, the suppression of the neg-
ative influence of their potential CT states on the T1 states
becomes a serious problem. One immediate thought is the
use of saturated linkers, such as aliphatic chains, to block
the interplay between the donors and acceptors. However,
the incorporation of inert insulated moieties would also de-
crease their electrical performance. Meier reported that
p spacers can also weaken the interaction between donors
and acceptors.[17] However, p spacers would also increase
the conjugation. Therefore, their introduction appears to
present both positives and negatives. In fact, the answer to
this quandary is to determine which of the CT excited state
and extended conjugation has the main effect on the T1
state. Obviously, the conclusion is significant for the pur-
poseful design of ternary high-energy-gap host materials.
Therefore, we designed three DBF-based ternary phos-
phine-oxide (PO) hosts (DBFxPOPhCzn) in which carbazol-
yl groups act as donors and DPPO-substituted DBF groups
act as acceptors with phenyl groups between them as p spac-
Scheme 1. Design strategy and a structural comparison of DBFxPOPhCzn
and DBFxPOCzn.
FIrpic-based PHOLEDs with extremely low driving voltages
of less than 2.8 V at 100 cdmꢀ2, presumably owing to the use
of active DBF groups instead of inert Si or C moieties to
bridge the carbazolyl and DPPO groups. However, the weak
CT between the directly linked DBF and carbazolyl groups
still lowered the T1 states (about 2.9 eV) and, consequently,
resulted in lower EL efficiencies.
Until now, the determinants of the excited-state and elec-
trical properties are still not very clear. The detailed influ-
ence of molecular structure and various effects on the pho-
toelectrical performance of the hosts are still one of the
most important issues in this field. Therefore, herein, we
ꢀ ꢀ
ers. Thus, DBFxPOPhCzn are D p A-type structures, com-
ꢀ
ꢀ
report the synthesis of another series of ternary carbazolyl
pared to D A-type DBFxPOCzn (Scheme 1).
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Chem. Eur. J. 2013, 19, 1385 – 1396

Ultralow-Voltage-Driven Blue Electrophosphorescence
FULL PAPER
Scheme 2. Synthesis of DBFxPOPhCzn; NBS=N-bromosuccinimide.
DBFxPOPhCzn was conveniently prepared from o-
DBFPPO[14b] or o-DBFDPO[18] through a simple two-step
procedure of bromination and Suzuki coupling, with a good
total yield about 50% (Scheme 2). Structural characteriza-
tion was established on the basis of MS, NMR spectroscopy,
and elemental analysis.
Thermal and morphological properties: The thermal stabili-
ty of DBFxPOPhCzn was investigated by thermogravimetric
analysis (TGA), which revealed a high temperature of de-
composition (Td >4508C; Figure 1 and Table 1). Along with
the extension of conjugation, the Td values of
DBFSPOPhCz, DBFDPOPhCz, and DBFDPOPhCz2 grad-
ually increased to 467, 516, and 5448C, respectively. Further-
more, compared with the corresponding DBFxPOCzn
series,[9] the incorporation of
Figure 1. TGA curves of DBFxPOPhCzn.
phenyl
spacers
into
DBFxPOPhCzn induced a re-
markable increase in the Td
value (10–308C). This increase
is attributed to the stronger
structural adjustability and the
decreased volatility of the latter
series. Their improved thermal
stability makes device fabrica-
tion through vacuum evapora-
tion more feasible. Differential
scanning calorimetry (DSC)
analysis showed that the melt-
Table 1. Physical properties of DBFxPOPhCzn.
[d]
Host
Absorption
[nm]
Emission
[nm]
S1[c]/T1
[eV]
Td/Tm/Tg
RMS
[nm]
HOMO/LUMO
[eV]
[oC]
DBFSPOPhCz 340, 328, 294, 315, 370[a]
3.89/2.98
3.54/2.97
467/253/
165
0.711
ꢀ5.31/ꢀ1.31[e]
273, 245[a]
ꢀ6.09/ꢀ2.84[f]
344, 330, 314, 387, 405,
298, 249[b]
427[b]
DBFDPOPhCz 341, 329, 309, 407[a]
516/311/
156
0.751
ꢀ5.36/ꢀ1.44[e]
ꢀ6.09/ꢀ2.87[f]
293, 274,
247[a]
345, 329, 313, 407[b]
298, 254[b]
DBFDPOPhCz2 340, 329, 293 410[a]
345, 328, 297, 410[b]
255[b]
3.49/2.97
544/323/
157
0.758
ꢀ5.39/ꢀ1.53[e]
ꢀ6.09/ꢀ2.93[f]
ing
points
(Tm)
of
DBFSPOPhCz,
DBFDPOPhCz,
[a] In CH2Cl2 (ꢁ10ꢀ6 molLꢀ1); [b] in a film; [c] estimated according to the absorption edges; [d] calculated ac-
cording to the 0–0 transitions in the phosphorescence spectra; [e] DFT-calculated results; [f] calculated accord-
ing to the onset voltages of the redox peaks.
and
DBFDPOPhCz2 were 253, 311,
and 3238C, respectively, where-
as their glass-transition temper-
atures (Tg) were 165, 156, and 1578C. Both the Tm and Tg
values of DBFxPOPhCzn were lower than those of their
parent molecule, o-DBFDPO, which should be attributed to
state morphologies (Figure 2). DBFSPOPhCz revealed local
aggregation in its SEM image, which could be ascribed to its
least-bulky moieties and the smallest molecular volume
among the DBFxPOPhCzn series. However, no aggregation
and crystallization were observed in the SEM images of the
bigger analogues, DBFDPOPhCz and DBFDPOPhCz2. This
result indicates that conjugated and sterically demanding
host materials are advantageous, not only in terms of electri-
ꢀ
decreased p p intermolecular interactions and the lower
molecular rigidity, owing to the presence of phenylcarbazole
moieties with a high degree of freedom, in DBFxPOPhCzn.
SEM and AFM images of vacuum-evaporated thin films
of DBFxPOPhCzn showed direct evidence of their solid-
Chem. Eur. J. 2013, 19, 1385 – 1396
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1387

H. Xu, Y. Zhao et al.
Figure 2. SEM (top) and AFM images (bottom) of vacuum-evaporated films of DBFxPOPhCzn.
cal performance, but also in terms of morphological stability.
Nevertheless, AFM analysis showed that all of the films
were rather smooth with root-mean-square (RMS) rough-
ness of around 0.7 nm, which confirms the excellent film-
forming ability of these host materials. These stable and
high-quality amorphous films are very important for improv-
ing device performance by restricting the formation of de-
fects in the EMLs or at the interfaces with adjacent layers.
Optical properties: The photophysical properties of
DBFxPOPhCzn were indicated from their UV/Vis and pho-
toluminescence (PL) spectra as dilute solutions in CH2Cl2
(1ꢁ10ꢀ6 molLꢀ1) and as thin films (Figure 3). Their absorp-
tion spectra can be divided into three main absorption
bands, at 320–350, 270–320, and 220–270 nm. The n!p*
transitions of the DBF and carbazolyl moieties overlap in
the range 320–350 nm, thereby revealing two peaks at about
329 and about 341 nm, respectively. Compared with its other
two analogues, this absorption band of DBFDPOPhCz2 be-
comes much stronger, in accord with its increased propor-
tion of carbazolyl groups. The distinct absorption peaks at
about 293 nm are attributed to the p!p* transition of the
carbazolyl groups, which are also superimposed with the
peaks that originate from the p!p* transition of the DBF
groups (273, 283, and 309 nm). The third absorption bands,
with peaks at about 250 nm, are ascribed to the p!p* tran-
sitions of the phenyl groups in the DPPO moieties. There-
Figure 3. Top: absorption and FL spectra of DBFxPOPhCzn in CH2Cl2 (ꢁ
10ꢀ6 molLꢀ1, filled shapes) and in films (empty shapes). Bottom: phos-
phorescence spectra in CH2Cl2 (ꢁ10ꢀ6 molLꢀ1) at 77 K after a decay of
300 ms.
fore, both the phenylcarbazole and DBF moieties provide
the main contributions to the molecular excited energy
levels. Furthermore, compared with the optical energy gaps
(which correspond to the S1 state) of o-DBFPPO (3.89 eV)
and o-DBFDPO (3.63 eV), the introduction of one phenyl-
carbazole group into DBFSPOPhCz (3.54 eV) and
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Ultralow-Voltage-Driven Blue Electrophosphorescence
FULL PAPER
DBFDPOPhCz (3.49 eV) results in a decrease in the S1
state of 0.35 and 0.14 eV, respectively, whilst the second
is almost constant, along with the extension of conjugation.[9]
ꢀ
This result means that, compared with D A-type structures,
ꢀ ꢀ
phenyl
N
D p A-type bipolar structures can indeed suppress the re-
ther decrease of 0.05 eV. Notably, the formation of a bipolar
structure can dramatically lower the S1 state by more than
0.14 eV. Then, each additional DPPO or phenylcarbazole
group gives rise to a further decrease of 0.05 eV. Thus,
through combining different kinds of carrier-transporting
moieties and tuning the ratio of these functional groups, we
can feasibly realize the controllable modulation of the S1
energy levels. The peak position and shape of all of these
bands in their absorption spectra in solution are completely
duction effect of CT on the T1 energy levels but, simultane-
ously, sacrifice the stability of the T1 excited states.
ꢀ ꢀ
DFT calculations: To determine the influence of the D p
A-type bipolar structure on their FMOs, density function
theory (DFT) calculations were conducted with the Gaussi-
an 03 package at the B3LYP level and the 6-31G* basis set
(Figure 4). The HOMOs of DBFSPOPhCz, DBFDPOPhCz,
preserved in the spectra in films, with negligible bath
chromic shifts (less than 5 nm). This result further demon-
strates the weak intermolecular interactions of
ACHTUNGERTNoNUNG -
ACHTUNGTRENNUNG
DBFxPOPhCzn in the solid state, as indicated in the AFM
and SEM images (Figure 2).
The
fluorescence
(FL)
emission
spectrum
of
DBFSPOPhCz at room temperature as a dilute solution in
CH2Cl2 consists of a duplicate peak at 350 and 370 nm. In
film, this emission peak is located at 403 nm, with a batho-
chromic shift of 33 nm, which should be induced by aggrega-
tion, as shown in its SEM image (Figure 2). The FL emission
spectra of DBFDPOPhCz and DBFDPOPhCz2 show main
peaks at 405 and 410 nm, respectively, which are red-shifted
by approximately 30 nm compared with that of
DBFSPOPhCz, mainly owing to their second DPPO groups.
From DBFDPOPhCz to DBFDPOPhCz2, the second
phenylACHTUNGTRENNUNGcarbazole group only induces a small red-shift of
5 nm. Nevertheless, it is noticeable that the solid-state emis-
sions of DBFDPOPhCz and DBFDPOPhCz2 are almost the
same as those in solution, which further confirms their sup-
pressed aggregation in their solid states, consistent with the
results of the morphological investigation. Furthermore, the
solid-state emissions of DBFxPOPhCzn are very similar in
shapes and range. Therefore, in their PHOLEDs, the singlet
energy transfer to phosphorescent dopants should be also
similar. The emissions of DBFxPOPhCzn in films have wide-
ranging overlap with the metal-to-ligand charge-transfer
(MLCT) absorption bands of conventional blue, green,
yellow, and red phosphors to support efficient Fçrest reso-
nance energy transfer (FRET). The phosphorescence (PH)
spectra of DBFxPOPhCzn at 77 K, after a delay of 300 ms,
were measured to determine their T1 energy levels through
an estimation from their u0,0 transitions, which were identi-
fied as the highest-energy bands. We showed that the com-
bined linkage strategy of meso- and short-axis substitution
endowed DBFxPOPhCzn with very similar T1 energy levels
(around 3.0 eV), which corresponded to about 415 nm
(Figure 3 and Table 1), as shown by DBFxPOCzn.[9] More
Figure 4. DFT calculations of the molecular configurations and molecu-
lar-orbital contours in the ground states and the spin-density distributions
in the T1 states of DBFxPOPhCzn.
and DBFDPOPhCz2 are ꢀ5.306, ꢀ5.360, and ꢀ5.360 eV, re-
spectively, 1.0 eV higher than those of o-DBFPPO and
o-DBFDPO. Moreover, the calculated LUMOs of
DBFSPOPhCz, DBFDPOPhCz, and DBFDPOPhCz2 are
ꢀ1.306, ꢀ1.442, and ꢀ1.534 eV, respectively, 0.1 eV lower
than those of o-DBFPPO[14b] and o-DBFDPO.[18] This result
shows that, owing to its electron-withdrawing effect, the
second DPPO group in DBFDPOPhCz induces a decrease
of 0.14 eV in the LUMO and a decrease of 0.054 eV in the
HOMO. Therefore, the HOMO–LUMO energy gap (Eg) of
DBFDPOPhCz is about 0.1 eV smaller than that of
DBFSPOPhCz.
The
second
phenylcarbazole
in
ꢀ ꢀ
significantly, the D p A structures of DBFxPOPhCzn ele-
vate the T1 level by more than 0.1 eV compared with the
ꢀ
DBFDPOPhCz2 also lowers the LUMO by about 0.1 eV;
however, it has no influence on the HOMO. As a result, the
Eg value of DBFDPOPhCz2 is decreased by a further
0.1 eV. Furthermore, the two symmetrical phenylcarbazoles
in DBFDPOPhCz2 mainly contribute to the HOMO and
HOMOꢀ1, respectively, which are almost degenerate, with
a very small energy difference (0.028 eV). Because their
D A-type DBFxPOCzn structures. Nevertheless, closer ob-
servation shows that the intensity of the u0,0 transition sharp-
ly decreases from DBFSPOPhCz to DBFDPOPhCz and
DBFDPOPhCz2. This situation is highly distinct from that
of DBFxPOCzn, in which the intensity of the u0,0 transitions
Chem. Eur. J. 2013, 19, 1385 – 1396
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1389

H. Xu, Y. Zhao et al.
symmetrical configuration and small intramolecular energy
gap are advantageous for carrier hopping between different
molecules, DBFDPOPhCz2 can be expected to show im-
proved hole-transporting ability. Owing to their higher
LUMOs and lower HOMOs, the Eg values of
DBFSPOPhCz, DBFDPOPhCz, and DBFDPOPhCz2 are
0.1, 0.2, and 0.3 eV larger than those of their corresponding
DBFxPOCzn analogues,[9] respectively. This result reflects
the fact that the phenyl p spacers can (at least partially)
weaken the CT between the carbazolyl and DBF groups.
Furthermore, the LUMO+1 levels of DBFxPOPhCzn are
mainly located on the DBF groups and on the phenyl
groups of the phenylcarbazoles. This result is different to
the situation of DBFxPOCzn, in that their LUMO+1 levels
are mainly constituted of DPPO groups. On the other hand,
the contribution of the DBF groups in DBFxPOPhCzn to
the HOMOs is smaller than in DBFxPOCzn. Therefore, the
phenyl p spacers effectively suppress the interplay between
the carbazolyl and DBF groups in DBFxPOPhCzn.[19]
The triplet states of DBFxPOPhCzn were also optimized
at the B3LYP level. The calculated T1 states are 3.10, 3.06,
and 3.17 eV, respectively, which are in accord with the re-
sults estimated from the PH spectra. The contours of the
spin-density distributions (SDD) of the triplet states indicate
that the T1 states are absolutely localized on either the DBF
or carbazolyl groups (see the Supporting Information, Fig-
ure S1). Therefore, the incorporation of phenyl p spacers
can restrain the CT between the carbazolyl and DBF
groups, without having a negative influence on the triplet
states.
shift to 1.49 and 2.41 V, respectively, its onset voltage is also
at 1.31 V. However, owing to its additional phenylcarbazole
group, DBFDPOPhCz2 has two neighboring peaks from the
carbazolyl groups at 1.66 and 1.81 V, which are in accord
with the DFT calculations, whereas its DBF-based peak is at
2.35 V. Nevertheless, according to the onset voltage, its
HOMO is also at ꢀ6.09 eV. Although DFT calculations
showed that the DBF groups in DBFxPOPhCzn made slight
contributions to the HOMOs, the same oxidation onset volt-
AHCTUNGTERNGUNaN ges of DBFxPOPhCzn directly demonstrated the limited in-
fluence of the DPPO and DBF groups on the HOMOs. The
DBFxPOPhCzn series all revealed similar reversible reduc-
tion peaks at around 2.2 V, which were ascribed to the DBF
groups. The peak for DBFSPOPhCz was at ꢀ2.25 V, with an
onset voltage of ꢀ1.94 V, which corresponded to a LUMO
of ꢀ2.84 eV. With one more DPPO group, DBFDPOPhCz
had a reduction peak at ꢀ2.08 V and an onset voltage of
ꢀ1.91 V, which corresponded to a LUMO of ꢀ2.87 eV.
Therefore, the second DPPO induced a small decrease in
the LUMO of only 0.03 eV. However, the reduction peak of
DBFDPOPhCz2 further shifted to ꢀ2.05 V, with an onset
voltage of ꢀ1.85 V, which corresponded to a LUMO of
ꢀ2.93 eV. This sequential lowering of the LUMO by 0.06 eV
should be ascribed to the extension of conjugation by bind-
ing one more phenyl group onto DBF. These CV data are
precisely consistent with those from the DFT calculations.
Next, we fabricated nominal single-carrier-transporting
devices with the configurations ITO/MoOx (2 nm)/4,4’,4’’-
tri(N-3-methylphenyl-N-phenylamino)triphenylamine
(m-
MTDATA):MoOx (15 wt.%, 30 nm)/m-MTDATA (10 nm)/
[tris(phenylpyrazole)iridium]
DBFxPOPhCzn (40 nm)/[Ir
(10 nm)/m-MTDATA:MoOx
(2 nm)/Al and Al/Cs2CO3 (1 nm)/4,7-diphenyl-1,10-phenan-
throline (BPhen) (40 nm)/DBFxPOPhCzn (40 nm)/Bphen
(40 nm)/Cs2CO3 (1 nm)/Al for hole-only and electron-only
carriers, respectively, to investigate the carrier-transporting
properties of DBFxPOPhCzn (Figure 6). The current density
(J) of a hole-only device that was based on DBFSPOPhCz
was lower than that of a hole-only device that was based on
([Ir
(ppz)3], 10 nm)/m-MTDATA
(15 wt.%, 30 nm)/MoOx
(ppz)3],
10 nm)/
Carrier-injection/transportation properties: Cyclic voltam-
metry was performed to investigate the redox behavior of
DBFxPOPhCzn (Figure 5). All of these compounds had two
sets of oxidation peaks, which corresponded to the carbazol-
yl and DBF moieties, respectively. For DBFSPOPhCz, the
first peak for the carbazolyl moiety was at 1.42 V, with an
onset voltage of 1.31 V, thus corresponding to a HOMO of
ꢀ6.09 eV, whereas the second group from DBF was at
2.28 V. Although the oxidation peaks of DBFDPOPhCz
ACHTUNGTRENNUNG
Figure 6. I–V characteristics of nominal hole-only (empty shapes) and
electron-only devices (filled shapes) based on DBFxPOPhCzn.
Figure 5. CV redox curves of DBFxPOPhCzn in CH2Cl2.
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Chem. Eur. J. 2013, 19, 1385 – 1396

Ultralow-Voltage-Driven Blue Electrophosphorescence
FULL PAPER
DBFDPOPhCz,
whereas
DBFDPOPhCz2 endowed its
hole-only device with the high-
est J values (at the same volt-
ACHTUNGTRENNUNGages). In our previous report,
we found that, for DBFxPOCzn,
the J values of their hole-only
devices were proportional to
the number of carbazolyl
groups and inversely propor-
tional to the number of DPPO
units.[9] Furthermore, a symmet-
rical configuration is advanta-
geous in carrier transportation.
However, owing to the greater
hindrance and higher number
of degrees of freedom in
phenylACHTUNGTRENNUNGcarbazoles, the effect of
molecular configuration be-
comes a determinant. There-
fore, the hole-only J values of
Scheme 3. Energy-level diagram of blue-, green-, yellow-, and red-light-emitting PHOLEDs and the molecular
structures of the materials that were used.
deACHTUNGTRENNUNGvices that were based on the
more-symmetrical DBFDPOPhCz structure were higher
than those based on DBFSPOPhCz. As the DFT calcula-
tions showed, although the DPPO groups enhanced the
electron-injection ability of the compound, the DBF groups
had strong electron-transporting ability. More peripheral
groups in DBFDPOPhCzn suppress the interaction between
the DBF groups. This suppression is the main reason that
DBFSPOPhCz endows its electron-only device with the
highest J value. Thus, when the pendant functional groups
have a strong steric effect, except for the inherent properties
of electrically active moieties for carrier transportation, the
molecular configuration also cannot be ignored.
bazol-9-yl)benzene (mCP) and 4,4’,4’’-tris(carbazol-9-yl)-tri-
phenylamine (TCTA) for comparison, respectively
(Scheme 3). All of these devices revealed a pure emission
from FIrpic (Figure 7a, inset), which confirmed the efficient
energy transfer from the hosts to the phosphor.
Inspiringly, DBFxPOPhCzn also endowed their devices
(BA–BC) with extremely low driving voltages (2.4 V for
onset, about 2.8 V at 200 cdmꢀ2, and <3.4 V at 1000 cdmꢀ2
;
Figure 7a and the Supporting Information, Table S1), which
were the same as those of DBFxPOCzn-based devices with
similar configurations.[9] This result further indicates the lim-
ited influence of the p spacer on the carrier-injecting and
ꢀ ꢀ
CV analysis and the single-carrier-only I–V characteristics
of DBFxPOPhCzn showed that it had similar HOMO (about
ꢀ6.1 eV) and LUMO levels (about ꢀ2.85 eV) and compara-
ble carrier-transporting abilities to DBFxPOCzn.[9] There-
fore, we believe that, although the incorporation of phenyl
p spacers restrains the intramolecular CT, the carrier-inject-
ing/transporting abilities and the ambipolar characteristics
of DBFxPOPhCzn are not weakened.
-transporting abilities of the D p A systems. Compared
with devices BD and BE, which were based on the conven-
tional mCP and TCTA structures, the driving voltages of de-
vices BA–BC were dramatically decreased (by 0.3 V for
onset, 0.4 V at 100 cdmꢀ2, and 0.5 V at 1000 cdmꢀ2). Devices
BA–BC have very similar driving voltages at a luminance of
less than 1000 cdmꢀ2, which is in accord with the closer
FMO energy levels of DBFxPOPhCzn. Nevertheless, the
J value of BA was much higher than those of BB and BC at
the same voltage. Although the hole-transporting ability of
DBFSPOPhCz is the weakest, because the minor carrier in
the prototype devices is electrons as the bottleneck of cur-
rent increases, the much stronger electron-transporting abili-
ty of DBFSPOPhCz gives rise to the remarkable enlarged
current in device BA. However, the luminance of devices
BB and BC was equivalent to—and even slightly higher
than—that of BA at voltages below 3.5 V, which should be
attributed to the relatively balanced carrier injection and
transportation in devices BB and BC. On the contrary, at
higher voltages, along with the rapid increase of exciton con-
centration, the luminance of devices BB and BC became
lower than that of BA. This result reflects the weaker triplet
exciton stability of DBFDPOPhCzn, owing to their larger
EL properties: The high T1 levels (approaching 3.0 eV) and
ambipolar characteristics of DBFxPOPhCzn inspired us to
fabricate their blue-light-emitting PHOLEDs with the con-
figuration ITO/MoOx (2 nm)/m-MTDATA:MoOx (15 wt.%,
30 nm)/m-MTDATA (10 nm)/[Ir
ACHTUNGTRENNUNG
(10 wt.%, 10 nm)/BPhen (40 nm)/Cs2CO3
(100 nm), where MoOx and Cs2CO3 served as the hole- and
electron-injecting layers, m-MTDATA and BPhen served as
hole- and electron-transporting layers (HTL and ETL), [Ir-
ACHTUNGTRENNUNG(ppz)3] was used as a hole-transporting and exciton/electron-
blocking material, and FIrpic was the blue-light-emitting
phosphor, respectively. Devices BA–BE were based on
DBFSPOPhCz, DBFDPOPhCz, and DBFDPOPhCz2, as
well as conventional blue phosphorescent hosts 1,3-bis(car-
Chem. Eur. J. 2013, 19, 1385 – 1396
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1391

H. Xu, Y. Zhao et al.
that of device BE. At 200 cdmꢀ2 (for displays), the C.E.
values of devices BB and BC decreased to 26.2 and
26.1 cdAꢀ1, which corresponded to E.Q.E. values of 13.0
and 12.6%, respectively, with a roll-off of less than 2%.
Their P.E. values also decreased to 27.4 and 29.3 LmWꢀ1,
with a roll-off of 9 and 6%, respectively. At 1000 cdmꢀ2 (for
indoor lighting), their efficiencies were further decreased to
22.8 and 23.1 cdAꢀ1, 19.9 and 21.3 LmWꢀ1, and 11.3 and
11.1%, which corresponded to roll-off values of 15 and
11% for C.E., 34 and 32% for P.E., and 14 and 12% for
E.Q.E., respectively. Notably, along with the increase in
brightness, the decrease in efficiency of devices BB and BC
was remarkably larger than that of devices BD and BE. The
unstable triplet excited states mainly origAHCUTNGRENUNGiAHCTUNRTGEGnNNUN ated from the ex-
tended conjugation and lower molecular rigidity of
DBFDPOPhCzn. Nevertheless, the efficiencies of devices
BB and BC were still much higher than those of devices BD
and BE at a luminance of less than 5000 cdmꢀ2. Further-
more, compared with DBFDPOCzn-based devices, the maxi-
mum efficiencies of devices BB and BC were elevated by at
less 20%. However, the efficiency stability of devices BB
ꢀ ꢀ
and BC was inferior. This result further indicates that D p
A systems with higher T1 levels can lead to a remarkable in-
crease in maximum EL efficiency, through suppressing the
CT effect, at a cost of weakening the excited-state stability
owing to the extension of conjugation and the decrease of
molecular rigidity. Nevertheless, because at practical lumi-
nance levels, DBFDPOPhCzn supported their blue-light-
emitting PHOLEDs with much higher efficiencies without
sacrificing driving voltages, devices BB and BC are among
the most efficient low-voltage-driven blue-light-emitting
PHOLEDs reported so far.
Figure 7. Brightness–current-density (J)–voltage curves (a), EL spectra
(inset), and efficiency–brightness curves (b) of blue-light-emitting PHO-
LEDs based on DBFxPOPhCzn.
conjugation and poorer structural rigidity, as shown in the
optical analysis.
Furthermore, we also fabricated green-, yellow-, and red-
light-emitting PHOLEDs based on DBFxPOPhCzn, namely
GA-GC, YA–YC, and RA–RC, respectively. The device
configurations were similar, except for the phosphors
ꢀ ꢀ
When we designed these D p A-type hosts, we expected
that their higher T1 levels could further improve the EL effi-
ciencies of their blue-light-emitting PHOLEDs. Although
the unbalanced carrier transportation in device BA remark-
ably decreased its efficiency, its maximum current efficiency
(C.E.) was 12.8 cdAꢀ1 at 183 cdmꢀ2, which corresponded to
a maximum external quantum efficiency (E.Q.E.) of 6.4%;
these values were slightly higher than those of device BE
but lower than those of device BD (Figure 7b and the Sup-
porting Information, Table S1). Owing to the much lower
driving voltages of device BA, its maximum power efficien-
cy (P.E.) was 14.3 LmWꢀ1, which was equivalent to that of
device BD. More significantly, with its much better carrier-
transporting performance, DBFDPOPhCzn dramatically in-
creased the efficiencies of their devices. The maximum C.E.
of devices BB and BC were 26.7 and 26.1 cdAꢀ1 at 60 and
261 cdmꢀ2, which corresponded to maximum E.Q.E. values
of 13.2 and 12.6%, respectively; these values were about
60% higher than those of device BD and twice of those of
device BE. Owing to the extremely low driving voltages, the
maximum P.E. of devices BB and BC were greatly improved
to 30.0 and 31.3 LmWꢀ1 at 60 and 85 cdmꢀ2, respectively,
which were two times that of device BD and three times
([tris(2-phenylpyridine)iridium] ([Ir
emitting devices, [bis(2-phenyl-6-fluorobenzothiozolato)iri-
dium(acetylacetonate)] ([Ir(F-bt)2acac]) for yellow-light-
emitting devices, and [bis(2-methyldibenzo-
[f,h]quinoxaline)iridium(acetylacetonate)] ([Ir(MDQ)2acac)]
ACHTUNGRTEN(NUNG ppy)3]) for green-light-
AHCTUNGTRENNUNG
G
G
ACHTUNGTRENNUNG
for red-light-emitting devices) and the doping concentration
(6 wt.%; Scheme 3). The efficient host–guest energy transfer
was demonstrated by the pure emissions from the dopants
in the EL spectra. All of the devices revealed improved I–V
characteristics. The driving voltages of devices GA–GC
were also as low as about 2.4 V for onset, about 2.8 V at
200 cdmꢀ2, and <3.4 V at 1000 cdmꢀ2, which were similar to
those of devices YA–YC (Figure 8a,c,e and the Supporting
Information, Table S1). Devices RA–RC had the higher
driving voltages (2.8 V for onset, 3.6–4.6 V at 200 cdmꢀ2).
The I–V characteristics of these devices were almost the
same as those of devices BA–BC. The strongest electron-
transporting ability of DBFSPOPhCz supported the highest
J values of devices GA, YA, and RA. For green- and
yellow-light-emitting devices, at high voltages, the luminance
of DBFDPOPhCzn-based devices was less than those based
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Ultralow-Voltage-Driven Blue Electrophosphorescence
FULL PAPER
Figure 8. Brightness-current-density–voltage curves (a, c, e), EL spectra (insets), and efficiency–brightness curves (b, d, f) of green- (a, b), yellow- (c, d),
and red-light-emitting PHOLEDs (e, f) based on DBFxPOPhCzn.
on DBFSPOPhCz. Owing to the much bigger excited
energy gap for energy transfer to red dopants, the unstable
excitons of DBFDPOPhCzn became much easier to quench.
Therefore, the luminance of devices RB and RC was re-
markably lower than that of device RA. On the other hand,
cies to those of GB, YA, and YB, respectively. Furthermore,
with the advantage of DBFSPOPhCz in terms of exciton
stability, the efficiency of device RA was much higher than
those of devices RB and RC.
The EL performance of these devices reflects the fact that
ꢀ ꢀ
[Ir
(ppy)3], [Ir
E
N
the higher T1 state in these D p A-type hosts is beneficial
the hole injection and transportation, which can modify the
unbalanced carrier-injecting/transporting in DBFSPOPhCz.
Therefore, devices GA and YA showed comparable efficien-
for their EL efficiencies. In addition, the p spacers hardly in-
fluence their I–V characteristics. Therefore, the efficiencies
of these devices can be remarkably improved with the pres-
Chem. Eur. J. 2013, 19, 1385 – 1396
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1393

H. Xu, Y. Zhao et al.
ervation of ultralow driving voltages as a feature of ambipo-
lar hosts, even though the extended conjugation and lower
Experimental Section
ꢀ ꢀ
Materials and instrumentation: All reagents and solvents for the synthe-
sis of the title compound were purchased from Aldrich or Acros and
used without further purification. (2-Bromo)-(4-diphenylphosphoryl)di-
benzofuran (DBFSPOBr), (8-bromo)-(4,6-bis(diphenylphosphoryl)) di-
benzofuran (DBFDPOBr), and (2,8-dibromo)-(4,6-bis(diphenylphosphor-
yl))dibenzofuran (DBFDPOBr2) were prepared according to our previ-
ous report.[9]
molecular rigidity of D p A system also weaken their effi-
ciency stability.
Conclusion
1H NMR spectroscopy was performed on a Varian Mercury plus 400NB
spectrometer relative to tetramethylsilane (TMS) as an internal standard.
Molecular mass was determined on a FINNIGAN LCQ mass spectrome-
ter (ESI) or on a MALDI-TOF MS. Elemental analysis was performed
on a Vario ELIII elemental analyzer. Absorption and PL emission spec-
tra of the target compound were measured on Shimadzu UV-3150 and
RF-5301PC spectrophotometers, respectively. Thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) were performed on
Shimadzu DSC-60A and DTG-60A thermal analyzers under a nitrogen
atmosphere at a heating rate of 108Cminꢀ1. Cyclic voltammetry (CV)
studies were conducted on an Eco Chemie B. V. AUTOLAB potentiostat
in a typical three-electrode cell with a platinum-sheet working electrode,
a platinum-wire counter electrode, and a silver/silver-nitrate (Ag/Ag+)
reference electrode. All electrochemical experiments were performed
under a nitrogen atmosphere at RT in CH2Cl2. Phosphorescence spectra
were measured in CH2Cl2 on an Edinburgh FPLS 920 fluorescence spec-
trophotometer at 77 K under cooling with liquid nitrogen and a delay of
300 ms by using the time-correlated single-photon-counting (TCSPC)
method with a microsecond pulsed xenon light source for lifetime meas-
urements in the range 10 ms–10 s; the synchronization photomultiplier for
signal collection and the multichannel scaling mode of the PCS900 fast
counter PC plug-in card were used for data processing. The thin films of
the PO compounds were prepared through vacuum evaporation onto
glass substrates under the same conditions for device fabrication. The
morphological characteristics of these films were measured by atomic
force microscopy (AFM) on an Agilent 5100 under the tapping mode
and by SEM on a HITACHI S-4800 (spraying, accelerating voltage:
5.0 kV, current: 10 mA, observed altitude: 8 mm).
ꢀ ꢀ
In conclusion, a series of D p A-type phosphine-oxide
hosts (DBFxPOPhCzn) with phenylcarbazole, DBF, and
DPPO moieties to afford high T1 and ambipolar characteris-
tics were designed and synthesized. The incorporation of
phenyl groups as p spacers increased the structural adjusta-
bility of these molecules and resulted in improved thermal
and morphological stability. Photophysical analysis showed
that the S1 states of DBFxPOPhCzn could be feasibly tuned
through adjusting the number and ratio of functional
groups. Significantly, phenyl p spacers effectively weakened
the CT between the carbazolyl and DBF moieties. There-
fore, the influence of CT on the excited states was restrain-
ꢀ
ed.
Compared
with
their
D A-type
analogues
(DBFxPOCzn), the T1 states of DBFxPOPhCzn (about
3.0 eV) are successfully elevated by about 0.1 eV. Further-
more, DFT calculations indicate that phenyl p spacers can
block the extension of the T1 excited state, such that the T1
states are fully localized on either the DBF or carbazolyl
groups. Indeed, both DFT calculations and CV analysis
demonstrated
the
ambipolar
characteristics
of
DBFxPOPhCzn for double-carrier injection. The I–V charac-
teristics of single-carrier-only devices that were based on
DBFxPOPhCzn further confirmed their double-carrier-trans-
porting ability. Although CT is suppressed by the p spacer,
the FMO energy levels and carrier-transporting performance
of DBFxPOPhCzn are close to those of DBFxPOCzn. There-
fore, through inserting phenyl p spacers, higher T1 states are
achieved without any negative influence on their ambipolar
characteristics. The FIrpic-based devices of DBFxPOPhCzn
had ultralow driving voltages (2.4 V for onset, <3.4 V at
1000 cdmꢀ2), which were similar with those for devices that
were based on DBFxPOCzn. Simultaneously, the elevation
of the T1 states of DBFxPOPhCzn further increased their ef-
ficiency by at least 20% compared with those of
Synthesis
(2-(4-(9H-Carbazol-9-yl)phenyl))-(6-diphenylphosphoryl)dibenzofuran
(DBFSPOPhCz): A 2m aqueous solution of NaOH (3 mL, 6 mmol) was
added to a stirring solution of DBFSPOBr (447 mg, 1 mmol), 9-(4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole
(553 mg, 1.5 mmol), [PdACHTNURTGNEUNG(PPh3)4] (116 mg, 0.1 mmol), and tetra-n-butylam-
monium bromide (TBAB, 32 mg, 0.1 mmol) in THF (10 mL). Then, the
reaction mixture was warmed to 908C and stirred for 24 h. The reaction
was quenched with an aqueous solution of NH4Cl (10 mL) and extracted
with CH2Cl2 (3ꢁ10 mL). The organic layer was dried with anhydride
Na2SO4. The solvent was removed in vacuo and the residue was purified
by column chromatography on silica gel (petroleum ether/EtOAc, 2:1–
0:1) to obtain the product as a white powder (415 mg, 68% yield).
1H NMR (TMS, CDCl3, 400 MHz): d=8.258 (d, J=7.6 Hz, 2H), 8.188
(d, J=7.6 Hz, 2H), 7.927–7.826 (m, 7H), 7.760 (dd, J=8.6, 1.8 Hz, 1H),
7.694 (d, J=8.4 Hz, 2H), 7.640–7.436 (m, 12H), 7.338 ppm (t, J=7.4 Hz,
2H); MS (LDI-TOF): m/z (%): 610 (100) [M]+; elemental analysis calcd
(%) for C42H28NO2P: C 82.74, H 4.63, N 2.30, O 5.25; found: C 82.91,
H 4.65, N 2.42, O 5.51.
ꢀ ꢀ
DBFxPOCzn. Thus, on the basis of D p A-type
DBFxPOPhCzn, highly efficient ultralow-voltage-driven
blue-light-emitting PHOLEDs were realized. The perform-
ance of the DBFxPOPhCzn devices also showed their poten-
tial for applications as universal hosts for portable full-color
ꢀ ꢀ
PHOLEDs. Nevertheless, we also noted that these D p A
systems suffered from the lower stability of their excited
states, owing to their extended conjugation and lower struc-
tural rigidity, which worsened the decrease in efficiency at
high brightness and J values. To solve this problem, more-
accurate configuration control, on the basis of rational
mixed linkages, is imperative and work towards this goal is
underway in our laboratory.
(2-(4-(9H-Carbazol-9-yl)phenyl))-(4,6-bis(diphenylphosphoryl))dibenzo-
furan (DBFDPOPhCz):
A
2m aqueous solution of NaOH (3 mL,
6 mmol) was added to
a stirring solution of DBFDPOBr (647 mg,
1 mmol),
9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-
(PPh3)4] (116 mg, 0.1 mmol), and
carbazole (553 mg, 1.5 mmol), [PdAHCNUTGTRENNUNG
TBAB (32 mg, 0.1 mmol) in THF (10 mL). Then, the reaction mixture
was warmed to 908C and stirred for 24 h. The reaction was quenched
with an aqueous solution of NH4Cl (10 mL) and extracted with CH2Cl2
(3ꢁ10 mL). The organic layer was dried with anhydride Na2SO4. The sol-
vent was removed in vacuo and the residue was purified by column chro-
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Chem. Eur. J. 2013, 19, 1385 – 1396

Ultralow-Voltage-Driven Blue Electrophosphorescence
FULL PAPER
matography on silica gel (petroleum ether/EtOAc, 2:1–0:1) to obtain the
product as a white powder (470 mg, 58% yield).
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1H NMR (TMS, CDCl3, 400 MHz): d=8.485 (t, J=1.6 Hz , 1H), 8.321
(q, J=13.6, 2.0 Hz, 1H), 8.271 (d, J=7.6 Hz, 1H), 8.175 (d, J=7.6 Hz,
2H), 7.882 (d, J=8.4 Hz, 2H), 7.809–7.607 (m, 11H), 7.548–7.446 (m,
9H), 7.445–7.359 (m, 8H), 7.360–7.300 ppm (m, 2H); MS (LDI-TOF): m/
z (%): 810 (100) [M]+; elemental analysis calcd (%) for C54H37NO3P2:
C 80.09, H 4.61, N 1.73, O 5.93; found: C 80.14, H 4.67, N 1.86, O 6.14.
ACHTUNGTRENNUNG(2,8-Bis(4-(9H-carbazol-9-yl)phenyl))-(4,6-bis(diphenylphosphoryl))di-
benzofuran (DBFDPOPhCz2): A 2m aqueous solution of NaOH (6 mL,
12 mmol) was added to a stirring solution of DBFDPOBr2 (726 mg,
1 mmol),
carbazole (1106 mg, 3 mmol), [PdAHCNUTGTRENNUNG
9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-
(PPh3)4] (232 mg, 0.2 mmol), and TBAB
(64 mg, 0.2 mmol) in THF (20 mL). Then, the reaction mixture was
warmed to 908C and stirred for 24 h. The reaction was quenched with an
aqueous solution of NH4Cl (10 mL) and extracted with CH2Cl2 (3ꢁ
10 mL). The organic layer was dried with anhydride Na2SO4. The solvent
was removed in vacuo and the residue was purified by column chroma-
tography on silica gel (petroleum ether/EtOAc, 2:1–0:1) to obtain the
product as a white powder (610 mg, 58% yield).
1H NMR (TMS, CDCl3, 400 MHz): d=8.584 (s, 2H), 8.232 (q, J=13.4,
1.8 Hz, 2H), 8.176 (d, J=7.6 Hz, 4H), 7.879 (d, J=8.4 Hz, 4H), 7.801–
7.670 (m, 12H), 7.573–7.386 (m, 20H), 7.371–7.298 ppm (m, 4H); MS
(LDI-TOF): m/z (%): 1051 (100) [M]+; elemental analysis calcd (%) for
C72H48N2O3P2: C 82.27, H 4.60, N 2.67, O 4.57; found: C 82.35, H 4.62,
N 2.79, O 4.65.
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ground states and triplet states of the molecules in vacuo were optimized
without any assistance of experimental data by using the restricted and
unrestricted formalism of Beck’s three-parameter hybrid-exchange func-
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(B3LYP)/6–31G(d), respectively. The fully optimized stationary points
were further characterized by harmonic vibrational frequency analysis to
ensure that “real” local minima had been found without any imaginary
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coated glass substrates were scrubbed and sonicated successively with
acetone, EtOH, and deionized water. All of the organic layers were ther-
mally deposited under vacuum (about 4.0ꢁ10ꢀ4 Pa) at a rate of 1–2 ꢂsꢀ1
and monitored in situ by the quartz oscillator. To reduce the ohmic loss,
a heavily p-doped layer with MoOx, considering the low doping efficiency
into an amorphous organic matrix with transition-metal-oxide-based ac-
ceptors, was directly deposited onto the ITO substrate for each sample.
After the deposition of LiF, the samples were transferred into a metal
chamber and suffered from a vacuum break owing to the change of the
shadow masks to determine the active area. Their current–voltage–lumi-
nance characteristics were measured on a PR650 spectrascan spectrum-
ACHTUNGTRENNUNGeter with a Keithley 2400 programmable voltage–current source. All of
the samples were measured directly after fabrication without encapsula-
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Acknowledgements
This work was financially supported by the National Key Basic Research
and Development Program of China (2010CB327701), the NSFC
(50903028, 61176020, and 60977024), the Key Project of the Ministry of
Education (212039), the New Century Talents Developing Program of
Heilongjiang Universities (1252-NCET-005), the Education Bureau of
Heilongjiang province (2010td03), and by Heilongjiang University
(2010hdtd-08).
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Received: October 17, 2012
Published online: December 11, 2012
1396
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Chem. Eur. J. 2013, 19, 1385 – 1396