Short-axis substitution approach selectively optimizes electrical properties of dibenzothiophene-based phosphine oxide hosts

Article abstract of DOI:10.1021/ja308273y

Two dibenzothiophene (DBT)-based phosphine oxide hosts, named 4-diphenylphosphoryl dibenzothiophene (DBTSPO) and 4,6-bis(diphenylphosphoryl) dibenzothiophene (DBTDPO), were prepared by short-axis substitution with the aim to selectively adjust electrical

Full text of DOI:10.1021/ja308273y

Article
pubs.acs.org/JACS
Short-Axis Substitution Approach Selectively Optimizes Electrical
Properties of Dibenzothiophene-Based Phosphine Oxide Hosts
Chunmiao Han,†,‡ Zhensong Zhang,§,‡ Hui Xu,*,† Shouzhen Yue,§ Jing Li,† Pingrui Yan,§
Zhaopeng Deng,† Yi Zhao,*,§ Pengfei Yan,† and Shiyong Liu§
†Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Heilongjiang University, 74 Xuefu Road, Harbin
150080, P.R. China
§State Key Laboratory on Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, 2699 Qianjin
Street, Changchun 130012, P.R. China
S
* Supporting Information
ABSTRACT: Two dibenzothiophene (DBT)-based phos-
phine oxide hosts, named 4-diphenylphosphoryl dibenzothio-
phene (DBTSPO) and 4,6-bis(diphenylphosphoryl) dibenzo-
thiophene (DBTDPO), were prepared by short-axis sub-
stitution with the aim to selectively adjust electrical properties.
The combined effects of short-axis substitution and the
involvement of electron-donating S atom in conjugation
effectively suppress the influence of electron-withdrawing
diphenylphosphine oxide (DPPO) moieties on the frontier
molecular orbitals and the optical properties. Therefore,
DBTSPO and DBTDPO have the nearly same hole injection
ability and the excited energy levels, while more electron-
transporting DPPOs and the symmetrical configuration endow DBTDPO with enhanced electron-injecting/transporting ability.
As the result, on the basis of this short-axis substitution effect, the selective adjustment of electrical properties was successfully
realized. With the high first triplet energy level (T1) of 2.90 eV, the suitable energy levels of the highest occupied molecular
orbital and the lowest unoccupied molecular orbital of −6.05 and −2.50 eV and the improved carrier-transporting ability,
DBTDPO supported its blue- and white-emitting phosphorescent organic light-emitting diodes as the best low-voltage-driving
devices reported so far with the lowest driving voltages of 2.4 V for onset and <3.2 V at 1000 cd m−2 (for indoor lighting)
accompanied with the high efficiencies of >30 lm W−1 and excellent efficiency stability.
adjust the integral performance characteristics through
purposeful and organizable modifications one by one. In this
way, the uncontrollable interruptions between the approaches
due to their overlapping effects can be avoided.
1. INTRODUCTION
After decades of development, organic optoelectronic materials
have demonstrated their great potential in a number of
applications, such as organic photovoltaics,1 organic light-
emitting diodes (OLEDs),2−5 and sensors.6 In recent years, the
rising and urgent demand for high performances for
commercial applications has driven persistent efforts to
construct integrated systems with comprehensive and modified
photoelectronic properties.7−12 As a result, the contradictory
between optical and electrical properties becomes more and
more incisive. The origin of this issue is the common
dependence of these properties (e.g., excited energy levels
and charge injection ability) on molecular characteristics,
especially the molecular energy levels. In this case, tuning
one property often induces the simultaneous change of another
one, which in turn induces an uncontrollable adjustment of
material properties.13−23 Therefore, the selective modulation of
optoelectronic characteristics is still one of the biggest
challenges in the field of high-performance organic optoelec-
tronic materials. Nevertheless, we believe that modulation of a
single property through one approach is more important than
that of multiple properties, because it is much more operable to
As a typical example, the contradiction between optical and
electrical properties of host materials in blue-emitting
phosphorescent organic light-emitting diodes (PHO-
LEDs)24−28 is the sensitivity of both the singlet and triplet
excited energy levels (S1 and T1) and the carrier injection/
transporting ability to the functional substitution.29 Active
functional groups often simultaneously change the optical,
carrier injection, and carrier-transporting performance of the
materials.30−33 In this situation, the high T1 for exothermic
energy transfer, suitable frontier molecular orbital (FMO)
energy levels for carrier injection, and optimal molecular
configuration and components for carrier transportation are
hard to realize at the same time.34 Therefore, although efforts
have been undertaken to organize the optoelectronic properties
of the hosts by integration of functional groups, only a few have
Received: August 20, 2012
Published: October 29, 2012
© 2012 American Chemical Society
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achieved high electroluminescence (EL) efficiencies, such as
external quantum efficiency (EQE) >10% and power efficiency
(PE) >20 lm W−1, accompanied by low driving voltages (e.g.,
<3 V for onset and <4V at 100 cd m−2 for portable
display).11,35−44 Thus, controllably modulating the optoelec-
tronic properties of hosts is still one of the most significant
issues in the development of efficient low-voltage driving
PHOLEDs.
With the goal of alleviating the contradiction between optical
and electrical properties and selective adjustment of specific
properties, we constructed several ambipolar ternary fluorene-
based hosts through indirect linkage to preserve the high T1 of
3.0 eV.37 Recently, we further designed a series of carbazole−
phosphine oxide (PO) hybrids with dibenzofuran (DBF)
bridges, whose FMOs and carrier injection/transporting
abilities were continuously modified without influencing T1.
The extremely low driving voltages of 2.4 V for onset and 2.8 V
at 100 cd m−2 were realized owing to the much improved hole
injection/transportation by carbazolyls; however, the cost was
an efficiency loss of 25−50%, compared with their parent
molecules 4-diphenylphosphoryl dibenzofuran (DBFSPO)39,40
and 4,6-bis(diphenylphosphoryl) dibenzofuran (DBFDPO).45
Density functional theory (DFT) calculation showed that the
electrostatic potential (ESP) distribution on DBF is not
uniform due to the strong electronegativity of the O atom
(Scheme 1), which enhances the stabilization ability of DBF
tion to achieve highly efficient full-color PHOLEDs. An
interesting effect of short-axis substitution on optoelectronic
properties was discovered. As a result, we successfully and
selectively improved the electron-injecting/transporting abil-
ities with negligible influence on the optical properties and the
highest occupied molecular orbital (HOMO) energy levels.
Compared with DBFDPO-based devices, DBTDPO endows
its blue- and white-emitting devices with the much higher
efficiencies of ∼15% for EQE and >30 lm W−1 for PE, while
their driving voltages are dramatically reduced to 2.4 V for
onset, <2.8 V for 100 cd m−2, and <3.2 V for 1000 cd m−2.
These ultralow operation voltages are even lower than the
corresponding data for DBF-based donor−acceptor-type
ambipolar PO hosts46 and are the lowest reported so far
among prototype blue- and white-emitting PHOLEDs.29,47 The
comprehensive EL performance indicates DBTDPO to be one
of the best hosts reported so far for low-voltage driving of
highly efficient PHOLEDs and demonstrates huge potential for
use in portable energy-saving displays and lighting driven by
low-voltage power sources.
2. RESULTS AND DISCUSSIONS
2.1. Design and Synthesis. It is known that although
DPPO hardly changes excited energy levels owing to the
insulating linkage of PO, its electron-withdrawing character-
istic results in a strong inductive effect. Commonly, for long-
axis-substituted PO hosts, when the DPPO moieties in the
molecules are increased, the HOMO and the lowest
unoccupied molecular orbital (LUMO) energy levels are
remarkably reduced by >0.2 eV at the same time.48−52
However, our previous work indicated that short-axis
substitution can reduce the influence of DPPO moieties on
the HOMO.39,45 Nevertheless, when the chromophore, such as
DBF, is readily electron-transporting, short-axis substitution
can still induce unipolarity of the molecules (Scheme 1).
Therefore, in order to selectively tune a specific property of the
host, the first step is to find a suitable chromophore with self-
adaptability to modification.
Scheme 1. ESP Distributions and Dipole Moments of
DBTSPO and DBTDPO in Comparison to Their DBF-
Based Analogues
It is recognized that DBT has a much stronger aromaticity
than DBF owing to its much lower electronegativity and the
involvement of an S atom in the conjugation. This is revealed
by the uniform distribution of positive charge on the whole ring
in ESP of DBT (Scheme 1). With the short-axis-substituted
DPPOs, these two molecules can be efficiently polarized with
the negative charge located on O atoms and positive charges on
DBT and phenyls. In this case, the second DPPO, linked in the
same direction, can further accumulate polarization of the
disubstituted derivative. Therefore, DBTDPO has a bigger μ,
4.34 D, than DBTSPO, 4.15 D. In addition, substitution at the
ortho-position may amplify the effects of the S atom by
enhancing the electronic communication between the P and S
atoms. This reflects the stability of the properties of short-axis-
substituted DBT-PO derivatives and implies the potential for
selective modulation of some specific properties.
DBTSPO and DBTDPO were conveniently prepared
through a three-step procedure of regioselective lithiation,
phosphorization, and oxidation, with moderate total yields of
∼60% and ∼30%, respectively (Scheme 2). The molecular
structures of DBTSPO and DBTDPO were confirmed by
single-crystal X-ray diffraction (XRD) analysis (Figure 1).
Adjacent DBTSPO molecules were connected in a 1D chain
structure through intermolecular C−H···π interactions (3.826
Å) along the a axis, as shown in Figure SI1a. It is noticed that
when an electron is injected. After substitution with
diphenylphosphine oxide (DPPO) moieties, the negative
charge is still located on this O atom, as well as O atoms in
DPPOs. In this case, the preference of a DBF core for electron
capture would be amplified to make the molecule more
unipolar. Therefore, symmetrical substitution with two DPPOs
instead reduces the dipole moment (μ) of DBFDPO to 3.00 D,
compared with 4.25 D for unsymmetrical DBFSPO. This might
be the main reason for the much higher driving voltages and
reduced EL efficiencies of DBFDPO-based PHOLEDs.
In this contribution, we report two dibenzothiophene
(DBT)-based PO hosts, namely 4-diphenylphosphoryl diben-
zothiophene (DBTSPO) and 4,6-bis(diphenylphosphoryl)
dibenzothiophene (DBTDPO), based on short-axis substitu-
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2.2. DFT Simulation and Electrical Properties. DFT
calculation shows that the DPPO moiety in DBFSPO and
DBFDPO can remarkably reduce the LUMO by ∼0.25 eV per
DPPO (Figures 2 and SI4). However, their HOMOs also
accordingly decrease by 0.12 eV. Obviously, the enhancement
of electron injection ability of DBFSPO and DBFDPO is based
on sacrificing partial hole injection ability, similar to some
carbazole-based PO hosts.50,58 Due to the strong electro-
negativity of the O atom in DBF, the HOMOs of the DBF-
based molecules are localized on phenyls without the
contribution of O atoms. This limited distribution decreases
the HOMO energy levels. In contrast, the S atom in DBT
contributes remarkably to the HOMO, so that the extensive
conjugation results in its significantly higher HOMO energy
level. The energy gap between the HOMOs of DBF and DBT
reaches 0.19 eV (Figure 2). Nevertheless, DFT calculation of
2,8-substituted DBT-DPPO analogues (named 2-DBTSPO
and 2,8-DBTDPO) showed that the para-substitution can still
induce the simultaneous decrease of the LUMO and the
HOMO of DBT cores (Figure SI5). As a result, the energy gap
between FMOs of DBT, 2-DBTSPO and 2,8-DBTDPO, is
nearly constant. The case is similar with DBF-based molecules.
Inspiringly, as we expected, on the basis of short-axis
substitution, DBTSPO and DBTDPO reveal excellent ability
to adapt to the modifications. Even though 4-C and 6-C have
remarkable contributions to the HOMOs, the substitution and
the number of DPPO moieties have almost no influence on the
HOMOs of DBTSPO and DBTDPO. With the small
contributions by the S atoms, the LUMOs of DBTSPO and
DBTDPO are reduced by the inductive effect of PO by 0.19
and 0.27 eV, respectively. Therefore, the electron injection
ability of DBTSPO and DBTDPO is improved without
weakening their hole injection ability. The energy gaps of
FMOs between DBTSPO/DBFSPO and DBTDPO/
DBFDPO are ∼−0.1 eV for the LUMO and >0.4 eV for the
HOMO, respectively, which reflects the comparable electron
injection ability and much stronger hole injection ability of
DBTSPO and DBTDPO. The smaller energy gap of
DBTDPO than that of DBTSPO is also in accord with the
results based on their dipole moments (Scheme 1). To figure
out this unusual controllable modulation of molecular orbital
energy levels, we further performed DFT calculations on meta-
substituted DBT-DPPO analogues, named 3-DBTSPO and
3,7-DBTDPO (Figure SI5). Although the contributions of 3-C
and 7-C to the FMOs of the molecules are almost negligible,
the LUMOs and HOMOs of 3,7-substituted analogues are still
much lower than those of DBT. As a result, only 4,6-
substitution can suppress the influence of DPPO on the
HOMOs. Obviously, this effect is related to the substitution
Scheme 2. Synthetic Route to DBTSPO and DBTDPO
Figure 1. Single-crystal structures of DBTSPO (a) and DBTDPO (b).
there is no π−π interaction with DBT in adjacent molecules.
Furthermore, for DBTDPO, no intermolecular interactions are
recognized except for van der Waals force (Figure SI1b).
Therefore, DPPO moieties in the short-axis direction effectively
suppress strong interactions, such as π−π stacking. This is
beneficial in that it reduces multiparticle quenching effects.53−56
As the bridges formed between adjacent molecules, the effects
of DPPOs on the intermolecular communications were
amplified. The symmetrical configuration further endows
DBTDPO with enhanced thermal and morphological stability.
Its decomposition temperature (Td = 442 °C) and glass
transition temperature (Tg = 232 °C) are outstanding among
the small molecular hosts (Figure SI2 and Table 1). The latter
is improved by 126 °C compared with its 2,8-substituted
analogue PO15,57 owing to the more compact and rigid
structure of the short-axis-substituted configuration. Atom force
microscopy (AFM) and scanning electron microscopy (SEM)
analyses of the vacuum-evaporated thin film of DBTDPO
indicated a uniform amorphous state with root-mean-square
roughness as small as 0.41 nm (Figure SI3). Obviously, the
symmetrical and compact configuration of DBTDPO is
beneficial to intermolecular arrangement, which induces the
high film quality for nanometer-scale devices.
Table 1. Physical Properties of DBTSPO and DBTDPO
a
a
h
absorption peak (nm)
emission peak (nm)
S1/T1 (eV)
lifetime (ns)
PLQY (%)
HOMO/LUMO (eV)
Td /Tm/Tg (°C)
DBTSPO
a
a
a
c
d
f
332, 294, 284, 266, 238, 228
363
3.54 /2.98
7.42
45.8
88.9
−5.741/−1.116
−6.05/−2.41
342/226/−
b
b
e
e
g
297, 286, 253, 229
364
4.63 /3.10
DBTDPO
a
c
d
f
340, 292, 288, 268, 251, 238
363
3.46 /2.90
6.59
−5.741/ −1.197
442/289/232
b
b
e
e
g
343, 301, 291, 268, 256, 249, 230
364
4.55 /3.35
−6.05/−2.50
a
b
c
d
e
In CH2Cl2, 1 × 10−6 mol L−1. In film. Estimated from the absorption edge. Estimated from the first triplet transition peak. Results of DFT
f
g
h
calculation. According to DFT calculation. Calculated with the onset potentials of the redox peaks in the cyclic voltammogram. At 5% weight loss.
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Figure 2. Energy level scheme of FMOs of DBF and DBT derivatives according to DFT calculation.
position and the hetero atoms in the chromophore rings. Aside
from the weaker electronegativity, the S atom in DBT has 3p
lone-pair electrons with much larger scope than that of 2p lone-
pair electrons of the O atom in DBF. In this sense, the potential
electron communications between the S atom and ortho-
substituted DPPO moieties in DBTSPO and DBTDPO might
be more effective. Closer observation of the LUMO electron
cloud distributions indicates that, compared with DBT, the
contribution of the S atom in DBTSPO to the LUMO is much
reduced, and the S atom in DBTDPO is hardly involved in the
LUMO. This situation is just the reverse of that of their para-
and meta-substituted analogues, for which the contributions of
their S atoms to the LUMO are almost unchanged after DPPO
modification (Figure SI5). Since the distances between S and P
atoms in DBTSPO and DBTDPO are ∼3.4 Å, much longer
than the common PS bond length of ∼1.95 Å, the electron
communication between them cannot form substantive PS
bonds. It might be a kind of long-range interaction similar to
back-bonding,59 which strongly polarizes S atoms and
consequently results in their non-involvement in LUMO.
Comparison of single-crystal structure data of DBTSPO,
DBTDPO, DBFSPO,39 and DBFDPO45 gives direct evidence.
The lengths of C−P bonds in DBFSPO and DBFDPO are
1.79−1.81 Å, similar to the common C−P bond length of PO
compounds reported previously.59 However, for DBTSPO and
DBTDPO, the lengths of C−P bonds adjacent to S atoms
increase by 0.001−0.01 Å. Since the C−P bond length in
known phosphine sulfide compounds is 1.81−1.82 Å,59 the
elongation of C−P in DBTSPO and DBTDPO further directs
the influence of 3p lone-pair electrons of the S atom on the P
atom, similar to back-bonding effect. For further comparison,
DFT calculations of DBT analogues with two different
electron-withdrawing substituents, F atom and nitro group,
without back-bonding effect were also performed (Figure SI6).
The remarkable reduction of the HOMOs in these molecules is
almost unrelated to the substitution position. Therefore, we
think at least one of the main reasons for this selective FMO
tuning is the electronic communication between S and P atoms
on the basis of short-axis substitution. This is a brand-new
effect, named the short-axis substitution effect, discovered for
controllable tuning of molecular orbitals, which can further
guide the design of the third and later row elements involved in
functional conjugated systems. DFT calculation indicates that
the ortho-substituted DPPO hardly influences T1 states of
DBTSPO and DBTDPO, at ∼3.1 eV, which is ascribed to the
predominant contribution of DBT to T1 states as shown by the
localized spin densities of their T1 states on DBTs (Figure SI7).
The electrochemical analysis presented direct experimental
evidence for the short-axis substitution effect (Figure 3). The
cyclic voltammograms of DBTSPO and DBTDPO showed
irreversible oxidation peaks with exactly the same turn-on
voltage of 1.65 V, corresponding to a HOMO energy level of
−6.05 eV, while the turn-on voltages of the irreversible
oxidation peaks of the DBF-based analogues were 1.77 and 1.89
Figure 3. Cyclic voltammograms of DBTSPO and DBTDPO. The
measurement was performed in tetrahydrofuran and dichloromethane,
respectively, with tetrabutylammonium hexafluorophosphate as
electrolyte at room temperature and a rate of 100 mV s−1.
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eV, corresponding to HOMO energy levels of −6.17 and −6.29
eV, respectively. DBTSPO, DBFSPO, and DBFDPO have
irreversible reduction peaks with turn-on voltages of −1.99,
−1.99, and −1.75 V, corresponding to LUMO energy levels of
−2.41, −2.41, and −2.65 eV, respectively. DBTDPO showed a
reversible reduction peak with the turn-on voltage of −1.90 V,
corresponding to a LUMO energy level of −2.50 eV. These
results are exactly in accord with DFT calculation. The
HOMOs of DBTSPO and DBTDPO are 0.12 and 0.24 eV
higher than those of DBFSPO and DBFDPO, respectively.
However, DBTSPO and DBFSPO have the same LUMO
energy levels, and the LUMO energy levels of DBTDPO and
DBFDPO are similar. The second DPPO moiety has no
influence on the HOMO of DBTDPO but decreases its
LUMO. Therefore, the improved electron injection ability is
achieved with preservation of original hole injection ability.
We further investigated the carrier-transporting ability of
these materials through their nominal hole-only and electron-
only devices (Figure 4). The hole-only and electron-only
originating from their contributions to unoccupied molecular
orbitals (LUMO+1 for DBTSPO; LUMO+1 and LUMO+2 for
DBTDPO). The more and symmetrically substituted DPPO
moieties in DBTDPO provide much more efficient electron
transport than that in DBTSPO. The electron-transporting
characteristic of DPPOs is also the main reason for the
predominant electron transport in both DBTSPO and
DBTDPO. It is noticeable that, compared with DBFSPO,
the enhancement of electron-transporting ability of DBFDPO
is based on the much more remarkable reduction in its hole-
transporting ability. DBTDPO is distinctly superior to
DBFDPO in carrier transportation. Although the hole-
transporting ability of DBTDPO is weaker than that of
DBTSPO, the superiority of the former in electron transport is
more conspicuous. DBTSPO exhibited the more balanced
carrier-transporting ability; however, among these four PO
hosts, DBTDPO exhibits the strongest electron-transporting
ability, accompanied by moderate hole-transporting ability.
Obviously, the advantages of DBTDPO in carrier injection and
transportation establish a solid basis for their use in low-voltage
driving devices.
2.3. Optical Properties. The absorption spectra of
DBTSPO and DBTDPO (in dilute CH2Cl2 solutions (1 ×
10−6 mol L−1)) are nearly the same (Figure 5 and Table 1),
Figure 4. I−V characteristic curves of single-carrier-transporting
devices based on DBTSPO and DBTDPO.
devices were fabricated with the structures of ITO|MoOx (2
nm)|m-MTDATA:MoOx (15 wt%, 30 nm)|m-MTDATA (10
nm)|Ir(ppz)3 (10 nm)|PO Host (30 nm)|Ir(ppz)3 (10 nm)|m-
MTDATA (10 nm)|m-MTDATA:MoOx (15 wt%, 30 nm)|
MoO3 (2 nm)|Al and ITO|Cs2CO3 (1 nm)|BPhen (40 nm)|PO
Host (30 nm)|BPhen (40 nm)|Cs2CO3 (1 nm)|Al, respectively,
where MoOx and Cs2CO3 served as hole- and electron-injecting
layers, m-MTDATA is 4,4′,4″-tris(N-3-methylphenyl-N-
phenylamino)triphenylamine as the hole-transporting layer,
Ir(ppz)3 is tris(phenylpyrazole)iridium as hole-transporting/
electron-blocking layer, and BPhen is 4,7-diphenyl-1,10-
phenanthroline as electron-transporting/hole-blocking layer.
The electron-only current densities (J) of DBTSPO and
DBTDPO were much bigger than their hole-only J, which
indicated predominant electron transport in DBTSPO and
DBTDPO. It is also noticed that the electron-transporting
ability of DBTDPO is stronger than that of DBTSPO by an
order of magnitude, while for the hole-transporting ability the
situation is precisely the opposite. According to DFT
calculations, both holes and electrons are readily injected in
DBT cores, which are separated by DPPO moieties. Therefore,
DBTSPO is superior in hole transportation with the smaller
interference by its less pendent DPPO moiety. However,
DPPOs can facilitate intermolecular electron transport
Figure 5. Absorption, fluorescence, and phosphorescence (inset)
spectra of DBTSPO and DBTDPO in dichloromethane (1 × 10−6
mol L−1). The excitation wavelength was 250 and 280 nm for
DBTSPO and DBTDPO, respectively. Absorption and fluorescence
spectra were measured at room temperature. Phosphorescence spectra
were recorded at 77 K with a delay of 300 μs.
composed of four bands at 315−350 nm (n→π* transition of
DBT), 275−315 nm (π→π* transition of DBT), 235−270 nm
(π→π* transition from DPPO to DBT), and 225−235 nm
(π→π* transition of DPPO). The larger absorption cross
section of DBTDPO induces its bigger extinction coefficient. S1
values of DBTSPO and DBTDPO are estimated as 3.54 and
3.46 eV according to their absorption edges at 350 and 358 nm,
respectively. Their close S1 values are in accord with their
similar HOMO−LUMO energy gaps by DFT calculation
(Figure 2). The fluorescent emissions of DBTSPO and
DBTDPO (1 × 10−6 mol L−1 in CH2Cl2 at room temperature)
are also nearly the same in shape and emission peak (at 363
nm), which are ascribed to the radiative π*→n transition of
DBT cores. Furthermore, no bathochromic shift was observed
in their emission spectra in various solvents with different
polarities (Figure SI8). This indicates the suppressed charge
transfer between DBT cores and DPPOs through the short-axis
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Figure 6. Brightness−current density (J)−voltage curves (a,c) and efficiency−brightness curves (b,d) of blue- and white-emitting PHOLEDs based
on DBTSPO and DBTDPO.
substitution. The absorption and emission spectra of DBTSPO
and DBTDPO in film are similar with their spectra in solution
(Figure SI9). The preserved fine structures of absorption bands,
the stable emission peaks at 364 nm, and the narrow emissions
with full widths at half-maximum of <55 nm revealed their weak
intermolecular interaction. The time-resolved phosphorescence
spectra of DBTSPO and DBTDPO in CH2Cl2 glass at 77 K
were measured to determine their T1 values. Their 0−0
transition bands are at 416 and 428 nm, corresponding to T1 =
2.98 and 2.90 eV, respectively, which are high enough for the
exothermic energy transfer to blue phosphors.
It is obvious that the short-axis substitution effect selectively
suppresses the influence of DPPOs on the optoelectronic
properties of DBTSPO and DBTDPO. Therefore, they exhibit
similar HOMO energy levels and excited energy levels.
Furthermore, the distinctions in molecular symmetry and
number of electron-transporting DPPOs result in different
carrier-transporting abilities of DBTSPO and DBTDPO. In
this sense, we successfully and selectively enhanced the
electron-injecting/transporting ability of DBTSPO and
DBTDPO with only small influences on other optoelectronic
properties. DBTSPO and DBTDPO actually provide a perfect
platform to investigate the influence of carrier-injecting/
transporting ability of the hosts on the EL performance of
PHOLEDs.
2.4. EL Performance of PHOLEDs. To clarify the effect of
selective modification of electrical properties on improving EL
performance, the blue-emitting devices BA and BB were
fabricated with the general configuration of ITO|MoOx (2 nm)|
m-MTDATA:MoOx (15 wt%, 30 nm)|m-MTDATA (10 nm)|
Ir(ppz)3 (10 nm)|Host:Dopants (10 nm, 10%)|BPhen (40
nm)|Cs2CO3 (1 nm)|Al, in which bis[2-(4,6-difluorophenyl)-
pyridinato-N,C2]picolinate iridium(III) (FIrpic) was used as
blue-emitting dopant. DBTSPO and DBTDPO served as host
in BA and BB, respectively (Scheme SI1). The most common
hosts for PHOLEDs, namely tris(4-(9H-carbazol-9-yl)phenyl)-
amine and N,N-dicarbazolyl-3,5-benzene, were also used to
fabricate devices BC and BD for comparison. The driving
voltages of BA were 2.6 V for onset, <3.0 V (100 cd m−2), <3.4
V (1000 cd m−2), and <5.0 V (10 000 cd m−2), respectively,
which were lower than those of BC and BD (2.7 V for onset,
∼3.2 V at 100 cd m−2, and ∼3.8 V at 1000 cd m−2) (Figure 6a).
Inspiringly, the most electron-transporting DBTDPO endowed
BB with the extremely low driving voltages of 2.4 V for onset, <
2.8 V for 100 m−2 (333 cd m−2), < 3.2 V for 1000 cd m−2 (1819
cd m−2), and <4.4 V for 10 000 cd m−2 (10 800 cd m−2),
respectively. To the best of our knowledge, these are the lowest
driving voltages reported so far for the blue-emitting
PHOLEDs with the prototype device configurations.29
Furthermore, the driving voltage at 1000 cd m−2 of BB was
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dramatically reduced by ∼1 V compared with similar devices of
DBFSPO (Table SI1).
configuration (except for their dual EMLs) of Host:FIrpic
(10 wt%, 4 nm)|Host:iridium(III) bis(4-phenylthieno[3,2-
c]pyridinato-N,C2) acetylacetonate (PO-01) (6 wt%, 6 nm),
respectively. The white emission was achieved through the
mixture of complementary blue (from FIrpic) and yellow (from
PO-01) lights. The ultralow driving voltages were also inherited
for WC (Figure 6c and Table SI1). The luminance >1000 cd
m−2 for indoor lighting and 10 000 cd m−2 for outdoor lighting
was realized at as low as 3.0 (1294 cd m−2) and 4.2 V (12 010
cd m−2) with small current density (J) of 3.6 and 45.3 mA
cm−2, which can be easily afforded by commercial lithium ion
batteries (4.2 V, 1200 mAh). Therefore, WC is suitable and
feasible for use in portable light sources, such as tactical
spotlights, signal lamps, and cap-lamps. It can also be integrated
in portable digital devices, e.g., mobile phones, as backlight for
active matrix displays and emergency lighting. The driving
voltages of WA were ∼0.2 V higher than those of WC at the
same luminance, due to the inferior electron injection and
transportation in DBTSPO. Nevertheless, WA can still be
competent for portable applications.
Because of the predominant contribution of yellow in
complementary-color white light, the better fitness of DBTSPO
to yellow phosphors endowed WA with slightly higher
maximum CE and EQE (39.6 cd A−1 and 13.9%) than those
of WC (37.6 cd A−1 and 13.0%), even though the lower driving
voltage supported a higher maximum PE of WC (43.5 lm W−1)
than that of WA (41.2 lm W−1) (Figure 6d and Table SI1).
Both WA and WC showed such extremely high efficiency
stability that at 1000 cd m−2 the efficiency roll-offs of WA were
only 1% CE, 7% PE, and 1% EQE, respectively, which were less
than half those of DBFSPO-based WPHOLEDs. At 5000 cd
m−2, the roll-offs of WA were still well controlled,with 12% CE,
30% PE, and 12% EQE. The efficiency roll-offs of WC were
slightly bigger: 3, 12, and 3% at 1000 cd m−2 and 16, 37, and
16% at 5000 cd m−2 in the order of CE, PE, and EQE.
Compared with DBFSPO-based WPHOLEDs, PEs of WA and
WC were improved by 15%, accompanied by their reduced roll-
offs. Through additional luminance enhancement with micro-
lens outcoupling, their PE at 1000 cd m−2 can approach 80 lm
W−1. Therefore, DBTSPO and DBTDPO endowed their
single-host WPHOLEDs with high performance comparable to
the best reports of multihost WPHOLEDs.60
Along with the ultralow driving voltages, high and stable
efficiencies were also achieved by BA and BB. The maximum
efficiencies of BA reached 21.9 cd A−1 for current efficiency
(CE), 22.9 lm W−1 for PE, and 10.9% for EQE, which were
much higher than those of BC and BD (Figure 6b). BB realized
the largest efficiencies among these blue-emitting devices, its
maximum efficiencies being as high as 28.8 cd A−1, 33.1 lm
W−1, and 14.3%, which make BB favorable among the most
efficient blue-emitting devices reported so far.29,47 The ultralow
driving voltages of BA and BB made their PE bigger than CE,
which is unusual for blue PHOLEDs and reveals their
advantage in energy conservation. BA and BB also exhibited
the stable efficiencies, in that at 1000 cd m−2 their efficiency
roll-offs were only 9.6% CE, 22.1% PE, and 9.2% EQE for BA
and 5.2% CE, 13.6% PE ,and 4.9% EQE for BB. It is noticed
that BB showed much better performance than BA owing to
the stronger electron-transporting ability and preserved hole-
injecting/transporting ability of DBTDPO. The suppressed
influences of DPPO on optoelectronic properties by the unique
short-axis substitution effect make the EL performance of
DBTSPO and DBTDPO opposite to that of their DBF-based
analogues (Table SI1). The efficiencies of BB were similar to
those of DBFSPO-based devices, while BB was superior with
much lower operating voltages. This result directly demon-
strates the superiority of selective modulation in accurate
optimization of optoelectronic properties.
We also fabricated the green (G)-, yellow (Y)-, and red (R)-
emitting PHOLEDs with the general configuration based on
tris(2-phenylpyridine) iridium(III) (Ir(ppy)3), bis(2-
phenylbenzothiazolato)(acetylacetonate) iridium(III) (Ir-
(bt)2acac), and bis(2-methyldibenzo[f,h]quinoxaline)-
(acetylacetonate) iridium(III) (Ir(MDQ)2acac), respectively,
with the corresponding concentrations of 8, 6, and 10%. GA,
YA, and RA used DBTSPO as host, while GB, YB, and RB
were based on DBTDPO (Scheme SI1). DBTDPO endowed
its devices with the extremely low driving voltages of ≤2.6 V for
onset and <3.2 V at 100 cd m−2 for display, which were
generally 0.2 V lower than those of DBTSPO-based devices
(Figure SI11 and Table SI2). However, at voltages >4.0 V, the
brightness of GA and YA gradually exceeded that of GB and
YB. This implied worse quenching in these DBTDPO-based
devices. As distinct evidence, for the G- and Y-emitting devices,
DBTSPO supported higher maximum efficiencies and
efficiency stabilities than DBTDPO. In addition, although the
maximum efficiencies of RB were higher than those of RA, the
former still revealed much sharper efficiency reduction at high
luminance. Since the optical properties of these two DBT-
based PO hosts are similar, the worse emission quenching in
EMLs of small energy-gap phosphors-doped DBTDPO should
be attributed to the mismatch of carrier-injecting/transporting
ability between the host and dopants. The extremely strong
electron transport provided by DBTDPO induced excessive
accumulation of excitons at the interfaces between electron-
blocking Ir(ppz)3 and EMLs, which worsened the multiparticle
quenching effects.55,56 Nevertheless, both DBTSPO and
DBTDPO realized the high-performance green, yellow, and
red PHOLEDs with low driving voltages and high efficiencies.
The excellent monochromic PHOLEDs inspired us to
further investigate the performance of DBTSPO and
DBTDPO as the single host for white-emitting PHOLEDs
(WPHOLEDs), namely WA and WC with the general
The EL spectrum of WA at 1000 cd m−2 showed warm white
emission with a correlated color temperature (CCT) of 4138 K,
while the more remarkable contribution of the blue portion in
the EL spectrum of WC resulted in a higher CCT of 4983 K
(Figure SI12). The different EL spectra of WA and WC should
be attributed to the variation of exciton recombination zones.
The yellow emission in these devices was generated through
two channels: direct recombination in the PO-01 layer and
exciton diffusion from the FIrpic layer. Since the thickness of
the FIrpic layer is only 4 nm to support efficient energy transfer
to the PO-01 layer, the stronger blue emission of WC implied
that its recombination zone should be definitely located in the
FIrpic layer. To figure out this issue, we fabricated two other
devices, WB and WD, of DBTSPO and DBTDPO with the
position-interchanged dual EMLs of Host:PO-01 (6 wt%, 4
nm)|Host:FIrpic (10 wt%, 6 nm). Only yellow emissions from
PO-01 can be recognized in EL spectra of WD (Figure SI12).
The absence of blue emission clearly indicated that the strong
electron-transporting ability of DBTDPO limited the exiton
recombination to a narrow zone of <4 nm at the interface
between the Ir(ppz)3 layer and EMLs. In contrast, weak but
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distinct blue emissions were observed in EL spectra of WB. In
comparison with WA, the smaller blue contribution in EL
spectra of WB points out that their exciton recombination
zones were at the interface between two EMLs and majorly
located in the one adjacent to the Ir(ppz)3 layer. This should be
due to the more balanced carrier-injecting/transporting ability
of DBTSPO. Therefore, WA and WB were superior in wide
emission zones to suppress the concentration quenching effect
and multiparticle annihilation processes. This should be the
main reason for the slightly higher efficiencies and efficiency
stability of WA. We also noticed that the brightness and
efficiencies of WA were much higher than those of BA. This
should be due to the efficient utilization of excessive excitons in
FIrpic layers through exciton diffusion. To demonstrate it, we
fabricated another WA-type device but with equal thicknesses
of the dual EMLs of 5 nm. The proportion of blue emission
became much bigger. The maximum luminance was reduced to
<30 000 cd m−2, accompanied by the smaller efficiency of 32 cd
A−1. This is obviously ascribed to the 1 nm longer exciton
diffusion distance from the exciton recombination zone to the
PO-01 layer. The longer exciton diffusion distance for WC
should be another main reason for its smaller efficiencies and
bigger roll-off than those of WA.
The well-controlled electrical properties of DBTSPO and
DBTDPO were dramatically presented through their high-
performance PHOLEDs. The investigation clearly reflects the
direct relationships between the carrier-injecting/transporting
ability of the hosts and the driving voltages, efficiencies, and
roll-offs of the devices. Therefore, the purposeful and selective
modulation of optoelectronic properties can rationally improve
the corresponding performance of the devices. Table SI1 makes
the comparison between EL performance of several represen-
tative hosts for blue- and white-emitting PHOLEDs. It is
noteworthy that DBTSPO and DBTDPO not only supported
their devices with the lowest driving voltages but also achieved
favorable efficiencies. Owing to their advantages in reducing
operation voltage, their WPHOLEDs were outstanding in
energy conservation, with PE >40 lm W−1. The state-of-the-art
performance makes DBTSPO and DBTDPO among the best
hosts for blue- and white-emitting PHOLEDs reported so far.29
voltage-driving PHOLEDs. The ultralow operation voltages,
high efficiencies, and improved efficiency stability pave the way
for the practical applications of these devices in portable
displays and lighting. Furthermore, the short-axis substitution
effect would guide subsequent investigations and designs, since
similar effects may exist in other conjugation systems involving
third and later row elements. Related work is ongoing in our
labatory.
4. EXPERIMENTAL SECTION
4.1. Materials and Instruments. All the reagents and solvents
used for synthesis were purchased from Aldrich and Acros companies
and used without further purification.
1H NMR spectra were recorded using a Varian Mercury plus 400NB
spectrometer relative to tetramethylsilane (TMS) as internal standard.
Molecular masses were determined by a Finnigan LCQ electrospray
ionization mass spectrometry (ESI-MS), or a MALDI-TOF-MS.
Elemental analyses were performed on a Vario EL III elemental
analyzer. Absorption and photoluminescence (PL) emission spectra of
the target compound were measured using a Shimadzu UV-3150
spectrophotometer and a Shimadzu RF-5301PC spectrophotometer,
respectively. Thermogravimetric analysis and differential scanning
calorimetry were performed on Shimadzu DSC-60A and DTG-60A
thermal analyzers under nitrogen atmosphere at a heating rate of 10
°C min−1. Cyclic voltammetry studies were conducted using 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 carried out under a nitrogen
atmosphere at room temperature in CH2Cl2 for oxidation and
tetrahydrofuran (THF) for reduction. Phosphorescence spectra were
measured in dichloromethane (DCM) using an Edinburgh FPLS 920
fluorescence spectrophotometer at 77 K cooling by liquid nitrogen
with a delay of 300 μs using the time-correlated single photon
counting method with a microsecond pulsed xenon light source for 10
μs−10 s lifetime measurement, the synchronization photomultiplier
for signal collection and the multichannel scaling mode of the PCS900
fast counter PC plug-in card for data processing. The crystal suitable
for single-crystal XRD analysis was obtained through the slow
evaporation of the ethanol solution of the compound at room
temperature. All diffraction data were collected at 295 K on a Rigaku
RAXIS-RAPID diffractometer with graphite monochromatized Mo Kα
(λ = 0.71073 Å) radiation in ω scan mode. All structures were solved
by direct method and difference Fourier syntheses. Non-hydrogen
atoms were refined by full-matrix least-squares techniques on F2 with
anisotropic thermal parameters. The hydrogen atoms attached to
carbons were placed in calculated positions with C−H = 0.93 Å and
U(H) = 1.2Ueq(C) in the riding model approximation. All calculations
were carried out with the SHELXL97 program. The thin films of the
PO compounds were prepared through vacuum evaporation on glass
substrates under the same condition of device fabrication. The
morphological characteristics of these films were measured with an
Agilent 5100 atom force microscope under the tapping mode and a
HITACHI S-4800 scanning electron microscope (spraying; accelerat-
ing voltage, 5.0 kV; current, 10 mA; and observed altitude, 8 mm).
4.2. Synthesis. 4-Diphenylphosphoryl Dibenzothiophene
(DBTSPO). At −78 °C, a solution of 4.0 mL of n-butyllithium (2.5
M in n-hexane, 10 mmol) was added dropwise to a stirred solution of
1.8426 g of DBT (10 mmol) and 1.51 mL of N,N,N′,N′-
tetramethylethenyldiamine (10 mmol) in 50 mL of ethyl ether. After
all the n-butyllithium was added, the cooling bath was removed, and
the reaction mixture was warmed to room temperature. After 16 h, the
mixture was cooled to −78 °C, and a solution of 1.79 mL of
chlorodiphenylphosphine (10 mmol) was added dropwise. The
cooling bath was removed, and the reaction mixture was stirred for
another 16 h. The reaction was quenched by 10 mL of water, and
extracted by DCM (3 × 30 mL). The organic layer was dried with
anhydride Na2SO4. The solvent was removed in vacuo. The residue
was dissolved by 50 mL of DCM, 3 mL of hydrogen peroxide (30%)
3. CONCLUSIONS
We selectively and effectively suppressed the negative influence
of electron-withdrawing DPPO groups on the optoelectronic
properties of two DBT-based PO hosts, DBTSPO and
DBTDPO, with a unique short-axis substitution effect.
DBTSPO and DBTDPO reveal almost the same hole injection
ability and optical properties with similar HOMO energy levels
and excited energy levels. However, differences in the molecular
symmetry and the number of electron-transporting DPPO
groups result in the distinct electron-injecting/transporting
abilities of DBTSPO and DBTDPO. This controllable
modulation endowed DBTDPO with the comprehensive
properties of stronger electron-injecting/transporting ability
without interference in hole injection/transportation and T1.
Therefore, selective modification of electron injection and
transportation without changing other optoelectronic proper-
ties was successfully realized. The corresponding blue and white
PHOLEDs based on DBTDPO showed excellent performance,
including driving voltages of <2.8 V for 100 cd m−2 and 3.2 V
for 1000 cd m−2 and maximum efficiencies of 33.1 and 43.5 lm
W−1 for blue- and white-emitting devices, respectively, which is
the best result reported so far among the prototype low-
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was added, and the reaction mixture was stirred for 4 h. The reaction
was extracted by sodium hydrogen sulfite-saturated solution. The
organic layer was dried with anhydride Na2SO4. The solvent was
removed in vacuo. The residue was purified with flash chromatograph.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Key Basic
Research and Development Program of China
(2010CB327701), NSFC (50903028, 61176020 and
60977024), Key Project of Ministry of Education (212039),
New Century Talents Developing Program of Heilongjiang
Province (1252-NCET-005), Education Bureau of Heilongjiang
Province (10td03), and the Supporting Program of Highlevel
Talents of HLJU (2010hdtd08).
1
Yield: 2.3 g of white powder (60%). H NMR (400 MHz, CDCl3,
TMS): δ (ppm) 8.335 (d, J = 7.2 Hz, 1H, DBT-H); 8.231−8.157 (m,
1H, DBT-H); 7.849−7.720 (m, 5H); 7.591 (t, J = 7.0 Hz, 2H);
7.547−7.431 (m, 8H); LDI-TOF: m/z (%): 384 (100) M+. Elemental
analysis (%) for C24H17OPS: C 74.98, H 4.46, O 4.16, S 8.34; found C
75.06, H 4.42, O 4.27, S 8.54.
4,6-Bis(diphenylphosphoryl) Dibenzothiophene (DBTDPO). This
compound was prepared according to the procedure described for
DBTSPO, but from 1.8426 g of DBT (10 mmol) with 3 equiv of n-
butyllithium and chlorodiphenylphosphine. Yield: 1.75 g of white
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ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files for single crystals; spin density distribution contours;
thermal and morphological analysis results; DFT calculation
results of DBT analogues; absorption and PL spectra of
DBTSPO and DBTDPO in film and various solvents; PL
decay of DBTSPO and DBTDPO in DCM; device energy level
diagram; EL spectra; EL performance of green-, yellow-, and
red-emitting devices; and efficiency−J curves of the devices.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
hxu@hlju.edu.cn; zhao_yi@jlu.edu.cn
Author Contributions
‡C.H. and Z.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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