Phosphorescent OLEDs assembled using Os(ii) phosphors and a bipolar host material consisting of both carbazole and dibenzophosphole oxide

Article abstract of DOI:10.1039/c2jm16674g

We report on the synthesis of a new series of Os(ii) complexes (1-3) functionalized with 2-pyridyl (or 2-isoquinolyl) pyrazole chelates, together with a new diphosphine, 1,2-bis(phospholano)benzene chelate (pp2b). The resulting Os(ii) complexes are fully characterized and their structural versus spectroscopic properties have been comprehended by absorption/emission together with computational approaches. The inherent electron richness, restricted rotational barrier and good steric hindrance of pp2b lead to the production of both orange and red phosphorescence with high quantum efficiency. For exploring these Os(ii) based OLEDs, we also synthesized a bipolar material 5-[4-(carbazo-9-yl)phenyl] dibenzophosphole-5-oxide (CzPhO), possessing both carbazole donor and dibenzophosphole oxide acceptor. Successful fabrication of OLEDs using complexes 1 and 3 as the dopant and either 4,4′-N,N′- dicarbazolebiphenyl (CBP) or CzPhO as host is reported. For comparison, the CBP and CzPhO devices with 1 as the emitter showed peak efficiencies EQE of 10.9%, η_L of 21.7 cd A^-1, and η_p of 11.9 lm W^-1, and EQE of 14.3%, η_L of 34.8 cd A^-1, and η_p of 45.2 lm W^-1, respectively. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm16674g

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PAPER
Phosphorescent OLEDs assembled using Os(II) phosphors and a bipolar host
material consisting of both carbazole and dibenzophosphole oxide†
a
a
a
a
a
Cheng-Huei Lin, Che-Wei Hsu, Jia-Ling Liao, Yi-Ming Cheng, Yun Chi, Tsung-Yi Lin,b
*
b
b
b
c
Min-Wen Chung, Pi-Tai Chou, Gene-Hsiang Lee, Chih-Hao Chang, Chin-Yao Shihc and Chi-Lung Hoc
*
*
Received 19th December 2011, Accepted 21st March 2012
DOI: 10.1039/c2jm16674g
We report on the synthesis of a new series of Os(II) complexes (1–3) functionalized with 2-pyridyl (or
2-isoquinolyl) pyrazole chelates, together with a new diphosphine, 1,2-bis(phospholano)benzene
chelate (pp2b). The resulting Os(II) complexes are fully characterized and their structural versus
spectroscopic properties have been comprehended by absorption/emission together with computational
approaches. The inherent electron richness, restricted rotational barrier and good steric hindrance of
pp2b lead to the production of both orange and red phosphorescence with high quantum efficiency. For
exploring these Os(II) based OLEDs, we also synthesized a bipolar material 5-[4-(carbazo-9-yl)phenyl]
dibenzophosphole-5-oxide (CzPhO), possessing both carbazole donor and dibenzophosphole oxide
acceptor. Successful fabrication of OLEDs using complexes 1 and 3 as the dopant and either
4,40-N,N0-dicarbazolebiphenyl (CBP) or CzPhO as host is reported. For comparison, the CBP and
CzPhO devices with 1 as the emitter showed peak efficiencies EQE of 10.9%, hL of 21.7 cd Aꢀ1, and
hp of 11.9 lm Wꢀ1, and EQE of 14.3%, hL of 34.8 cd Aꢀ1, and hp of 45.2 lm Wꢀ1, respectively.
temperature, therefore, giving better luminescent efficiency
Introduction
versus those without the phosphorous donor ligand.
Third-row Ir(III) and Os(II) metal complexes, both of which
With respect to the Os(II) phosphors, initially, the homoleptic
possess a d6-electron configuration, are often considered as the
polypyridyl Os(II) complexes such as [Os(bpy)3]2+ showed near-
most promising triplet emitters for organic light-emitting diodes
infrared emission with low efficiency at room temperature;4 they
(OLEDs).1 This is due to their excellent luminescent quantum
are thus not suitable in fabrication of OLED displays or solid-
yield and the greater feasibility of fine-tuning the emission
state luminaries. This is perhaps due to the fact that the Os(II)
wavelength across the whole visible spectrum, i.e. from deep blue
atom has lower oxidation potential and a smaller HOMO/
to saturated red. The commonly studied high efficiency Ir(III)
LUMO energy gap, resulting in the very red-shifted emission and
phosphors comprise at least one cyclometalating heteroaromatic
faster radiationless deactivation. In one of elegant approaches,
chelate, for which simple variation of the substituent on chelate
various diphosphine chelates were added to destabilize the
allows fine-tuning of the respective photophysical properties.2
respective metal-centered dd excited state as well as to blue shift
Another research direction is to assemble the emissive Ir(III)
the emission wavelength toward the visible region. The resulting
metal complexes via the introduction of either mono-dentate or
chelating phosphorous ancillaries.3 In this approach, the greater
structural modification gave orange and red-emitting Os(II)
phosphors with photoluminescent quantum efficiency as high as
ligand-field strength of the resulting metal-P dative bond then
45% in degassed solution.5
destabilized the metal-centered dd excited state, making thermal
For further investigating the potential of Os(II) phosphors, our
population to this deactivating state less accessible at room
group has focused on the utilization of a class of monoanionic
2-pyridyl azolate chelate in making the neutral Os(II) phosphors,
namely: [Os(fppz)2(PPhMe2)2] and [Os(fptz)2(PPh2Me)2].6
aDepartment of Chemistry, National Tsing Hua University, Hsinchu 30013,
Taiwan. E-mail: ychi@mx.nthu.edu.tw
Scheme 1 depicts the structural drawings of these representative
Os(II) phosphors. Phosphorescent OLEDs based on these emit-
bDepartment of Chemistry, National Taiwan University, Taipei 10617,
ters gave highly efficient saturated red emission (CIExy ¼ 0.64,
Taiwan. E-mail: chop@ntu.edu.tw
cDepartment of Photonics Engineering, Yuan Ze University, Chung-Li
32003, Taiwan. E-mail: chc@saturn.yzu.edu.tw
0.36), with low turn-on voltage of 2.5 V, and peak luminance
efficiency hL of 29.2 cd Aꢀ1, power efficiency hP of 25.2 lm Wꢀ1
,
and external quantum efficiency hext up to 18.0%.7 It is notable
that these Os(II) phosphors contain two phosphines located at the
mutual trans dispositions. Their mono-dentate nature might
allow facile ligand dissociation under stringent conditions, so
† Electronic supplementary information (ESI) available: X-ray
crystallographic data file (CIF) of the studied Os(II) complex 1 and the
bipolar host compound CzPhO. CCDC reference numbers 872662 and
872663. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2jm16674g
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fabrication of the OLEDs using the titled Os(II) complexes as the
dopant is reported. The details are elaborated as follows.
Experimental section
General procedures
All reactions were performed under argon atmosphere and
solvents were distilled from appropriate drying agents prior to
use. Commercially available reagents were used without further
purification unless otherwise stated. 1,2-Bis(phospholano)
benzene (pp2b) was synthesized according to literature proce-
dures,10 while 5-phenyldibenzophosphole was prepared by
combination of OPPh3 with phenyl lithium, followed by acidifi-
cation.11 All reactions were monitored using pre-coated TLC
plates 0.20 mm with fluorescent indicator UV254. Mass spectra
were obtained on a JEOL SX-102A instrument operating in
electron impact (EI) or fast atom bombardment (FAB) mode. 1H
and 13C NMR spectra were recorded on a Varian INOVA-500
instrument. Elemental analysis was carried out with a Heraeus
CHN–O Rapid Elementary Analyzer. Cyclic voltammetry (CV)
measurements were performed using a BAS 100 B/W electro-
chemical analyzer.
Scheme 1
that this class of samples is sensitive to e.g. the activated silica gel,
despite they are reasonably robust and stable in non-chlorinated
organic solvents and even in air over an extended period of time.
One way to overcome this drawback in stability is to employ
a chelating phosphine, such as 1,2-bis(diphenylphosphino)ethene
(dppee) and others. The isolated Os(II) phosphor, cf.
[Os(fppz)2(dppee)] (Scheme 2),8 indeed exhibits notable
improvement in stability against activated silica gel; unfortu-
nately, the emission is undesirably blue-shifted to the shorter
wavelength. Apparently, this is attributed to the electron with-
drawing character of dppee with respect to PPh2Me or PPhMe2
in the trans-Os(II) counterparts. The electron rich nature of
PPh2Me or PPhMe2, however, destabilizes the metal centered dp
(or t2g) orbitals, which causes the bathochromic shift observed
for the trans-dppee derivatives bearing identical 2-pyridyl azolate
chelates.
Synthesis of 1,2-bis(phospholano)benzene
A modified version of the literature procedure was followed. A
hexane solution of n-BuLi (2.5 M, 5.6 mL, 14.1 mmol) was first
slowly added into a 100 mL of THF solution of 1,2-bis(phos-
phino)benzene (10 wt.% in hexane, 1.0 g, 7.04 mmol) at 0 ꢁC.
After warming to RT, the solution was stirred for 1.5 h. This
solution was then cannulated to a solution containing tetra-
To regain the saturated red emission for Os(II) phosphors with
diphosphine ligand, we herein further proceed to develop a new
type of chelate, such as 1,2-bis(phospholano)benzene (pp2b). We
expect that the tetramethylene fragment on phosphole would
produce electron donating character equivalent to two methyl
groups as in PPhMe2. Moreover, the cyclic structure of phosp-
hole segment in pp2b and its reduced rotational degree of
freedom would also exert a large steric protection over the
adjacent 2-pyridyl azolate chromophores, which may inhibit the
unwanted environmental perturbation. This new diphosphine
was then employed for the preparation of emissive Os(II) metal
phosphors in attempts to enhance both stability and emission
efficiency. All of these Os(II) complexes were fully characterized
and their photophysical properties were comprehended by
absorption/emission and computational approaches to gain
insight into the relationship regarding structural versus spectro-
scopic properties. Amid improving the device performance,
a bipolar host material consisting of both carbazole and diben-
zophosphole oxide entities was synthesized, for which the
dibenzophosphole oxide represents a special class of phosphoryl
group critical to the organic electronics.9 As a result, successful
ꢁ
methylene sulfate (2.17 g, 14.3 mmol) in THF (10 mL) at 0 C
and the mixture was stirred for another 2 h at RT. After then,
a second solution of n-BuLi (5.6 mL, 14.1 mmol) was added at
0 ꢁC and was stirred overnight at RT. After then, a few drops of
MeOH were added to quench the reaction, and the mixture was
filtered through a pad of celite and washed by anhydrous diethyl
ether. The filtrate was concentrated under vacuum to get
1,2-bis(phospholano)benzene (pp2b) as a colorless oil (1.15g,
4.6 mmol, 65%).
1
Spectral data of pp2b: H NMR (500 MHz, C6D6, 294 K):
d 7.23–7.16 (m, 2H), 7.06–7.00 (m, 2H), 1.98–1.88 (m 8H), 1.72–
1.60 (br, 4H), 1.58–1.46 (br, 4H). 31P–{1H} NMR (202 MHz,
C6D6, 294 K): d ꢀ22.06 (s, 2P).
Preparation of 1
A mixture of Os3(CO)12 (150 mg, 0.165 mmol) and 3-tri-
fluoromethyl-5-(2-pyridyl) pyrazole (fppzH, 224 mg, 1.05 mmol)
in diethylene glycol monomethyl ether (DGME, 20 mL) was
heated to reflux for 24 h. After cooling to RT, the freshly
sublimed Me3NO (75 mg, 1.09 mmol) in DGME (10 mL) was
ꢁ
added dropwise into the mixture and heated for 1 h at 110 C.
After cooling to RT, pp2b (144 mg, 0.578 mmol) was added and
heated for another 24 h at 190 ꢁC. Finally, the solvent was
removed under vacuum and the residue was separated by column
chromatography over silica gel eluting with ethyl acetate and
hexane (1 : 1). Subsequent recrystallization from a mixture
of CH2Cl2 and methanol at RT gave orange crystalline solid
Scheme 2
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ꢁ
(1, 209 mg, 0.241 mmol, 48.8%). Single crystals of 1 were
was heated to 100 C for 12 h. After cooling to room tempera-
ture, the solvent was removed under vacuum and the residue was
extracted with CH2Cl2 (50 mL ꢃ 3).
obtained from a mixture of CH2Cl2 and hexane at RT.
1
Spectral data of 1: MS (FAB, 192Os): m/z 866 (M + 2)+; H
NMR (500 MHz, d6-acetone, 294 K): d 7.91 (d, J ¼ 8.5 Hz, 2H),
7.82–7.74 (m, 4H), 7.45 (br, 2H), 7.41 (d, J ¼ 5.5 Hz, 2H), 7.13
(s, 2H), 7.08 (t, J ¼ 6.5 Hz, 2H), 3.18–3.06 (m, 2H), 2.41–2.28
(m, 2H), 1.98–1.85 (m, 2H), 1.82ꢂ1.70 (m, 2H), 1.40–1.24
(m, 4H), 1.00–0.87 (m, 2H), 0.86–0.76 (m, 2H). 19F–{1H} NMR
(470 MHz, d6-acetone, 294 K): d ꢀ60.59 (s, 6F). 31P–{1H} NMR
(202 MHz, d6-acetone, 294 K): d 37.91 (s, 2P). Anal. Calcd. for
C32H30F6N6OsP2: N, 9.72; C, 44.44; H, 3.50. Found: N, 9.41; C,
43.97; H, 3.81.
Without further purification, the CH2Cl2 solution was treated
with 30% hydrogen peroxide (10 mL). After stirred overnight, the
solution was sequentially washed with water twice to remove
unreacted of H2O2, dried over MgSO4 and evaporated off the
solvent under reduced pressure. The crude solid was then purified
by silica gel column chromatography eluting with ethyl acetate
and CH2Cl2 (1 : 3) to give white solid (1.22 g, 2.77 mmol, 34%).
Single crystals of CzPhO were obtained by slow diffusion of
hexane into the concentrated CH2Cl2 solution at RT.
1
Selected crystal data of 1: C32H30F6N6OsP2; M ¼ 864.76;
Spectral data of CzPhO: MS (EI): m/z 441 (M)+; H NMR
ꢀ
monoclinic; space group ¼ P 21/n; a ¼ 12.3979(2) A, b ¼
(400 MHz, CDCl3): d 8.10 (d, J ¼ 7.6 Hz, 2H), 7.79–7.89 (m, 6H),
7.61–6.64 (m, 4H), 7.35–7.45 (m, 6H), 7.27 (t, J ¼ 7.2 Hz, 2H).
31P–{1H} NMR (202 MHz, CDCl3): d 34.22 (s, 1P). Anal. Calcd.
for C30H20NOP: N, 3.17; C, 81.62; H, 4.57. Found: N, 3.66; C,
81.47; H, 4.66.
ꢁ
ꢀ
ꢀ
19.2597(4) A, c ¼ 13.7144(3) A, b ¼ 94.1952(10) , V ¼
3265.95(11) A ; Z ¼ 4; rc ¼ 1.759 Mg mꢀ3; F(000) ¼ 1696; crystal
3
ꢀ
3
ꢀ
size ¼ 0.22 ꢃ 0.17 ꢃ 0.13 mm ; l(Mo-Ka) ¼ 0.71073 A; T ¼
150(2) K; m ¼ 4.070 mmꢀ1; 19709 reflections collected, 7448
independent reflections (Rint ¼ 0.0412), GOF ¼ 1.068, final
R1[I > 2s(I)] ¼ 0.0430 and wR2 (all data) ¼ 0.1173.
Selected crystal data of CzPhO: C30H20NOP; M ¼ 441.44;
ꢀ
orthorhombic; space group ¼ Pbca; a ¼ 8.1571(4) A, b ¼
3
ꢀ
ꢀ
ꢀ
16.5407(9) A, c ¼ 33.1095(18) A, V ¼ 4467.3(4) A ; Z ¼ 8; rc ¼
1.313 Mg mꢀ3; F(000) ¼ 1840; crystal size ¼ 0.50 ꢃ 0.18 ꢃ
Preparation of 2
3
ꢀ
0.13 mm ; l(Mo-Ka) ¼ 0.71073 A; T ¼ 150(2)K; m ¼
0.147 mmꢀ1; 32581 reflections collected, 5142 independent
reflections (Rint ¼ 0.0844), GOF ¼ 1.112, final R1[I > 2s(I)] ¼
0.0548 and wR2 (all data) ¼ 0.1413.
Similar to that reported for 1, dark-red crystalline solid of 2 was
obtained from sequential reaction of Os3(CO)12 with 3-tri-
fluoromethyl-5-(1-isoquinolyl) pyrazole (ifpzH), followed by
addition of pp2b. Yield: 32%.
1
Spectral data of 2: MS (FAB, 192Os): m/z 967 (M + 1)+; H
Photophysical measurement
NMR (400 MHz, d6-acetone, 294 K): d 8.96 (d, J ¼ 8.4 Hz, 2H),
7.89–7.66 (m, 10H), 7.54–7.34 (m, 6H), 3.30–3.20 (m, 2H), 2.50–
2.39 (m, 2H), 2.00–1.80 (m, 2H), 1.75–1.60 (m, 2H), 1.40–1.20
(m, 4H), 0.92–0.60 (m, 4H). 19F–{1H} NMR (470 MHz, d6-
acetone, 294 K): d ꢀ60.62 (s, 6F). 31P–{1H} NMR (202 MHz, d6-
Steady-state absorption, emission, and phosphorescence lifetime
measurements in solution and solid have been described in our
previous reports.12 To determine the photoluminescence
quantum yield, the samples were degassed by three freeze–pump–
thaw cycles. 4-(Dicyanomethylene)-2-methyl-6-(paradimethy-
laminostyryl)- 4H-pyran (DCM, lmax ¼ 615 nm, Exciton) in
methanol, with a quantum yield of ꢂ0.44, served as the standard
for measuring the quantum yield. Lifetime studies were per-
formed with an Edinburgh FL 900 photon-counting system using
a hydrogen lamp as the excitation source. Data were analyzed
using a nonlinear least-squares procedure in combination with an
iterative convolution method. The emission decays were fitted by
the sum of exponential functions with a temporal resolution of
ꢂ300 ps after the deconvolution of instrument response function.
acetone, 294 K):
d
38.22 (s, 2P). Anal. Calcd. for
C40H34F6N6OsP2: N, 8.71; C, 49.79; H, 3.55. Found: N, 8.35; C,
49.83; H, 3.80.
Preparation of 3
Similar to that reported for 1, red-orange crystalline solid of 3
was obtained from sequential reaction of Os3(CO)12 with 3-
phenyl-5-(2-pyridyl)-1,2,4-triazole (pptzH), followed by addition
of pp2b. Yield: 40%.
Spectral data of 3: MS (FAB, 192Os): m/z 882 (M-2)+; 1H NMR
(500 MHz, d6-acetone, 294 K): d 8.19 (d, J ¼ 7.5 Hz, 4H), 8.08
(d, J ¼ 7.5 Hz, 2H), 7.92–7.81 (m, 4H), 7.64 (br, 2H), 7.48
(br, 2H), 7.34 (t, J ¼ 7.5 Hz, 4H), 7.27 (t, J ¼ 7.5 Hz, 2H), 7.20
(t, J ¼ 6.0 Hz, 2H), 3.40–3.20 (m, 2H), 2.60–2.40 (m, 4H), 1.95–
1.70 (m, 2H), 1.55–1.30 (m, 4H), 1.20–0.80 (m, 4H). 31P–{1H}
NMR (202 MHz, d6-acetone, 294 K): d 36.66 (s, 2P). Anal.
Calcd. for C40H38N8OsP2: N, 12.69; C, 54.41; H, 4.34. Found: N,
12.43; C, 54.03; H, 4.83.
Computational methodology
Calculations on electronic singlet and triplet states of all titled
complexes were carried out using the density functional theory
(DFT) with B3LYP hybrid functional.13 Restricted and unre-
stricted formalisms were adopted in the singlet and triplet
geometry optimization, respectively. A ‘‘double-z’’ quality basis
set consisting of Hay and Wadt’s effective core potentials
(LANL2DZ)14 was employed for the Ir(III) metal atom, and
a 6-31G* basis set,15 for the rest of atoms. The relativistic
effective core potential (ECP) replaced the inner core electrons of
Os(II) metal atom, leaving only the outer core valence electrons
(5s25p65d6) to be concerned. Time-dependent DFT (TDDFT)
calculations using the B3LYP functional were then performed
based on the optimized structures at ground states.16 Moreover,
Synthesis of 5-[4-(carbazo-9-yl)phenyl]diphosphole-5-oxide
(CzPhO)
5-H-dibenzophosphole (1.5 g, 8.15 mmol), 9-(4-bromophenyl)
carbazole (2.63 g, 8.15 mmol), CuI (0.15 g, 0.8 mmol), 1,10-
phenanthroline (0.29 g, 1.6 mmol) and Cs2CO3 (5.31 g,
16.3 mmol) were suspended in dry toluene (30 mL). The mixture
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considering the solvation effect, the calculations were then
combined with an integral equation formalism-polarizable
continuum model (in dichloromethane), IEF-PCM, imple-
mented in Gaussian 03.17 Typically, 10 lower triplet and singlet
roots of the non-hermitian eigenvalue equations were obtained
to determine the vertical excitation energies. Oscillator strengths
were then deduced from the dipole transition matrix elements
(for singlet states only). All calculations were carried out using
Gaussian 03.18
is comparable with those of relevant phospholane derivatives
documented in literature.20
The synthesis of cis-substituted Os(II) complexes
[Os(fppz)2(pp2b)] (1) and [Os(ifpz)2(pp2b)] (2) follows the
routine protocol, in which the 2-pyridyl pyrazole (i.e. fppzH or
ifpzH) ligand was first treated with Os3(CO)12 in refluxing
diethylene glycol monomethyl ether (DGME) to generate the
dicarbonyl intermediate with formula [Os(fppz)2(CO)2] and
[Os(ifpz)2(CO)2].21 Without further purification, the intermediate
was immediately treated with freshly sublimed Me3NO to kick
off the carbonyl ancillaries, followed by addition of pp2b and
heating to complete the substitution. The phenyl triazolate
derivative with formula 3 was also synthesized in good yield
using the similar approach. Isolation of 3 offers a type of Os(II)
phosphor free from any fluorinated substituent, which may have
the advantage of avoiding the fluorine atom induced device
deterioration.22 Scheme 3 gives the structural drawing of these
Os(II) complexes.
Single crystal X-ray diffraction studies
Single crystal X-ray diffraction data were measured with
a Bruker SMART Apex CCD diffractometer using (Mo-Ka)
ꢀ
radiation (l ¼ 0.71073 A). The data collection was executed
using the SMART program. Cell refinement and data reduction
were performed with the SAINT program. The structure was
determined using the SHELXTL/PC program and refined using
full-matrix least squares.
Single crystal X-ray diffraction analysis on 1 was conducted to
reveal their basic coordination arrangement. Fig. 1 depicts the
molecular structure of 1, showing a mandated cis-P,P arrange-
ment and a relaxed P–Os–P bite angle of 84.45(6)ꢁ for the pp2b
chelate. The fppz chromophores adopt an eclipse orientation
with both pyrazolate nitrogen atoms N(2) and N(5) aligned trans
to each other, and with the pyridyl nitrogen atoms N(1) and N(4)
located trans to the phosphole groupings. The adoption of cis-
P,P arrangement in 1 has notably improved the Os–P bond
Fabrication of OLEDs
All purchased compounds were subject to temperature-gradient
sublimation under high vacuum before use. OLEDs were
fabricated on the indium-tin-oxide (ITO)-coated glass substrates
(#15 U) with multiple organic layers sandwiched between the
transparent bottom ITO anode and the top metal cathode. The
organic and metal layers were deposited by thermal evaporation
in a vacuum chamber with a base pressure of <10ꢀ6 torr.
Deposition rate of organic layers was kept at ꢂ0.1 nm sꢀ1. The
deposition system permitted the fabrication of a complete device
structure in a single pump-down without breaking vacuum. The
active area of the device was 2 ꢃ 2 mm2, as defined by the shadow
mask for cathode deposition. Current density–voltage–lumi-
nance (J–V–L) characterization of the devices was measured
using an Agilent 4156C semiconductor parameter analyzer
equipped with a calibrated Si-photodiode. EL spectra of devices
were collected by using an Ocean Optics spectrometer.
strength. This is confirmed by a reduction of averaged Os–P
ꢀ
distance from 2.36
A
in the trans-P,P complex
[Os(fppz)2(PPh2Me)2] to ꢂ2.25 A in 1,6d along with an increase of
ꢀ
ꢀ
the Os–N(py) bond distance to ꢂ2.15 A versus that of the trans
ꢀ
Os–N(pz) distance of ꢂ2.07 A within the same complexes. Both
phospholane rings of pp2b adopt a distorted asymmetric enve-
lope conformation with four out of five atoms (including phos-
phorus) being coplanar in the ring. Relevant conformation has
been confirmed for the homologous series of diphosphine ligands
(CH2)nP(CH2)3P(CH2)n, n ¼ 4–6, and the respective Pt(II) and
Rh(I) coordination complexes.23
With respective to the host material 5-[4-(carbazo-9-yl)phenyl]
dibenzophosphole oxide (CzPhO), later used in the device
fabrication, it was conveniently prepared from coupling of
9-(4-bromophenyl)carbazole and dibenzophosphole in presence
of Cu catalyst, followed by direct oxidation using aqueous H2O2
to afford the oxide CzPhO.24 Among the procedures, the
dibenzophosphole was in situ generated from 5-phenyl-
dibenzophosphole and represented the most critical step for
whole synthetic protocol shown in Scheme 4.
Results and discussion
Synthesis and characterization
The required phosphine chelate of Os(II) complexes, namely 1,2-
bis(phospholano)benzene (pp2b), was readily synthesized by the
one-pot treatment of commercially available 1,2-bis(phosphino)
benzene with n-BuLi, followed by addition of tetramethylene
sulfate and subsequent introduction of a second crop of n-BuLi
reagent for completing the phosphole formation.10 The advan-
tage of cyclic sulfate over seemingly comparable 1,4-electrophiles
such as 1,4-diiodobutane19 lies in that, upon its initial formation
of P–C bond, a terminal sulfate leaving group is produced. This
sulfate leaving group renders lower reactivity compared to the
starting sulfate reagent. Thus, neither competitive intermolecular
oligomerization nor intramolecular ring closure is expected prior
to the complete addition of tetramethylene sulfate. The addition
of second portion of n-BuLi allows full deprotonation of the
remaining PH functional group and hence facilitates the
formation of phosphacycles, giving the as-synthesized pp2b in
high yield. Its 31P NMR chemical shift (see Experimental section)
Scheme 3
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Fig. 1 ORTEP diagram of complex 1 with thermal ellipsoids shown
at 30% probability level; selected bond distances: Os–N(1) ¼ 2.148(5),
Os–N(2) ¼ 2.065(5), Os–N(4) ¼ 2.152(5), Os–N(5) ¼ 2.070(5), Os–P(1) ¼
Fig. 2 ORTEP diagram of CzPhO with thermal ellipsoids shown at 50%
probability level; bond distances: P(1)–O(1) ¼ 1.484(2), P(1)–C(1) ¼
1.803(2), P(1)–C(7) ¼ 1.805(2), P(1)–C(18) ¼ 1.797(2), N(1)–C(4) ¼
ꢀ
ꢀ
2.2528(16), and Os–P(2) ¼ 2.2462(16) A.
1.426(2), N(1)–C(19) ¼ 1.397(3) and N(1)–C(30) ¼ 1.396(3) A.
Characterization of CzPhO was conducted using routine
spectroscopic methods and single crystal X-ray diffraction. The
ORTEP diagram and critical bond distances are depicted in
Fig. 2. It is notable that dihedral angle for the carbazolyl and
dibenzophosphole (phosphoryl) unit to the central phenyl group
are measured to be 53.1ꢁ and 85.9ꢁ, respectively. The orthogonal
disposition of these aromatic units disrupted the p-conjugation
and ensured the greater excitation energy of this material.25
Other essential parameters, such as electrochemical, thermal
and spectroscopic data, and triplet energy gap are compiled in
Table 1 for scrutiny.
to the intra-ligand pp* transition mainly associated with
2-pyridyl (or 2-isoquinolyl) pyrazolate (or triazolate).
Upon excitation, as shown in Fig. 3, the titled complexes
showed bright orange to near-infrared emissions with peak
wavelength ranging from 618 nm to 725 nm. Table 2 summarizes
the observed lifetimes together with the calculated radiative (kr)
and non-radiative (knr) rate constants. First of all, the radiative
decay rate constants are derived to be in the range ꢂ105 sꢀ1
,
ascertaining their phosphorescent origin.26 As illustrated in
Fig. 3, complex 1 has the highest energy emission as compared to
the others. Its emission peak wavelength (618 nm) is only slightly
blue shifted with that (630 nm) of [Os(fppz)2(PPhMe2)2] and
almost superimposed with [Os(fppz)2(PPh2Me)2], both of which
possess the same pyrazolate chelates but with two monodentate
PPhMe2 or PPh2Me ligands placed at the trans positions. In stark
contrast, however, the emission quantum yield of 1 (0.73) in
degassed solution is as large as that of [Os(fppz)2(PPhMe2)2] and
[Os(fppz)2(PPh2Me)2] by 4.0 and 1.5 times, respectively.6d
Photophysical properties
UV/Vis spectra of the tilted complexes are illustrated in Fig. 3
and pertinent data are listed in Table 2. On the basis of previous
studies,6 it is unambiguous to assign the lowest-lying absorption
band with lower molar extinction coefficients (3) to the metal-to-
ligand (2-pyridyl (or 2-isoquinolyl) moiety) charge transfer
transition mixed, to certain extent, with the intra-ligand pp*
excitation. Subsequently, the higher lying absorption bands in
the region of 300–450 nm with 3 value >104 Mꢀ1cmꢀ1 are ascribed
Further confirmation is given by the derived non-radiative rate
constant of 1.9 ꢃ 105 sꢀ1 for complex 1, which is ꢂone order of
the magnitude less than that of the trans-P,P complex
[Os(fppz)2(PPh2Me)2].6 These results manifest the fact that
alteration of phosphine ligands from trans diaxial to cis-
arranged, chelating pp2b, modifies the structural properties but
keeps the electronic properties unchanged. The introduction of
pp2b greatly restricts the rotational degree of freedom of the
tetramethylene fragment, which avoids the possible rotational
quenching executed by the PPh2Me or PPhMe2 ligand,6
providing a proof of concept for the original design strategy.
Upon replacement of the CF3 substituted pyrazolate by phenyl
anchored triazolate, the resulting complex 3 exhibits a bath-
ochromic emission at ꢂ634 nm as compared to that of 1
(618 nm). These results could plausibly be rationalized by an
increase of the p-conjugation length, in part, due to the phenyl
Scheme
4 Synthetic conditions: (i) CuI, 1,10-phenanthroline and
Cs2CO3, (ii) H2O2.
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Table 1 Electrochemical, thermal and photophysical properties of CzPhO
abs. lmax (nm)
[3 ꢃ 10ꢀ3
FL, PL
(lmax, nm)
a
a
ox
E1/2 (V)
red
E1/2 (V)
]
HOMO (eV)b
LUMO (eV)c
ES (eV)d
ET (eV)e
Tg (ꢁC)f
105
Td (ꢁC)g
292 [33.9]
425, 458
0.98
ꢀ2.33
ꢀ5.78
ꢀ2.33
3.55
2.71
365
a
E1/2 (V) refers to [(Epa + Epc)/2] where Epa and Epc are the anodic and cathodic peak potentials referenced to the Fc+/Fc couple. The oxidation and
ox
reduction experiments were conducted in CH2Cl2 and THF solution, respectively. b Calculated from the oxidation potential E1/2
.
Calculated from the
c
d
e
red
reduction potential E1/2
phosphorescence spectrum in 2-methyltetrahydrofuran. Measured by DSC at 10 ꢁC minꢀ1
10 ꢁC minꢀ1
.
Calculated as Es ¼ |HOMO ꢀ LUMO | from the redox data. Highest energy peak (y0,0) of the low temperature (77 K)
f
g
.
Temperature with 5% mass loss measured by TGA at
.
Theoretical approach
Theoretical calculations based on density functional theory
(DFT) implemented in Gaussian 03 package were also performed
to gain more insight into the photophysical properties of the
titled complexes. Structural optimization of the singlet ground
state in the gas phase followed by the time dependent (TD) DFT
approach were carried out to provide a priori viewpoints about
the characteristics of the optical transitions for these complexes.
The spin density contours of the orbitals involved in the low
energy excitations as well as the results of TDDFT calculations
are depicted in Fig. 4 and tabulated in Table 3.
As illustrated in Fig. 4, the results of the frontier orbital
analyses for all titled complexes indicate that the electron density
contours for these complexes share almost similar patterns. In
brief, the highest occupied molecular orbitals (HOMO)
predominately locate at the pyrazolate/triazolate fragment and
the central metal dp orbitals, whereas the lowest unoccupied
molecular orbitals (LUMO) shift toward the pyridyl or iso-
quinolyl part of the heterocyclic chelates. The change in
compositions of frontier orbitals also suggests a relatively strong
metal-to-ligand charge transfer (MLCT) character. For example,
except for complex 3, the MLCT characters for the lowest lying
singlet and triplet transitions (listed in Table 3) show mostly as
high as 60%. This is plausible due to the low oxidation state of
Os(II) metal atom, to a small extent, together with electron
donating effect exerted by the pyrazolate fragments. The rela-
tively lower MLCT percentage in 3 could be rationalized by the
lift of the p-energy level on the phenyl substituted triazolate
moiety due to the elongation of the effective p-conjugation.
Careful examination of the energetics of the involved orbitals in
low-lying transition also points out several general viewpoints.
First, the replacement of the strong electron withdrawing
trifluoromethyl functional group with a phenyl group, which
seems plausibly to exert electron donating effect to the triazolate
Fig. 3 Absorption and normalized emission spectra of complexes 1–3
recorded in degassed CH2Cl2 solution at RT.
substitution, resulting in a lift of the HOMO p level as compared
to the CF3 substituted pyrazolate. Support of this viewpoint is
also given in the computational approach, in which the pp*
contribution to the lowest electronic transition is substantially
larger in 3 than that in 1 (vide infra). On the other hand, anchored
by more p-conjugated isoquinoline moiety (cf. pyridine in 1 and
3), complex 2 shows an emission peak red shifted to 725 nm,
with the emission profile extending to near infrared region
(ꢂ1000 nm). It is thus not surprising that the associated emission
quantum yield decreases to 10ꢀ2, together with large knr > 3.7 ꢃ
106 sꢀ1, simply due to the operation of energy gap law, which
summarizes the fact that quenching process for a deep-red
emitter with emission maximized at >700 nm is commonly
attributed to the thermal deactivation associated with overtones
of high-frequency vibrational modes.27
Table 2 Selected photophysical properties of Os(II) metal complexes 1–3 measured in degassed CH2Cl2 solution at RT
Complex
abs. lmax(nm) [3 ꢃ 10ꢀ3
]
em lmax (nm)
Q.E.a
s
obs (ms)
kr (ꢃ 105 sꢀ1
5.25
)
knr (ꢃ 105 sꢀ1
1.90
)
1
2
263[24.7], 290[30.5], 317[15.7],
345[13.0], 480[2.4]
310[32.6], 346[20.3], 361[20.8],
391[12.0], 423[11.4], 586[2.5]
295[48.6], 314[43.7], 486[2.4]
618
725
634
0.73, [0.74]
0.06
1.39
0.25
1.52
2.36
37.5
3
0.49, [0.57]
3.20
3.40
a
Data shown in square brackets is obtained for 4 wt.% doped thin films of CzPhO.
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Fig. 4 The electron density contours of the titled complexes pertinent to the lowest energy optical transitions on the basis of the structurally optimized
S1 state geometries.
moiety, influences the energetics of HOMOs. For instance, the
HOMO energetics of 3 is relatively higher than the rest of the
titled complexes (ꢀ4.76 vs.ꢂꢀ4.89 eV). Second, the isoquinolyl
fragment, owing to the longer effective p-conjugation length
than that of the pyridyl moiety, renders substantially lower
LUMOs in 2 than in both 1 and 3 (ꢀ1.94 eV (2) vs. ꢀ1.48 eV (1)
and ꢀ1.45 eV (3)). Combining these, the corresponding ener-
getics can thus be used to rationalize the relatively smaller energy
gaps in 2 than 1 and 3. Accordingly, the excitation energies of
either the singlet or triplet optical transitions for the titled
complexes reveal a trend in the order 1 > 3 > 2, being in good
agreement with the absorption and emission data shown in Fig. 3
and Table 2.
with both carbazole moiety as the electron donor (D) and
phosphoryl moiety as the electron acceptor (A). On the one
hand, the carbazole is known to be a promising hole-transporting
component, while its high triplet energy gap makes it efficient to
serve as host material for the phosphorescent dopants.28 On the
other hand, the inductive phosphine oxide moiety facilitates
electron injection and transport without significant lowering the
triplet energy gap.29
Consequently, the HOMO and LUMO of ꢀ5.78 eV and
ꢀ2.33 eV (versus vacuum) were estimated for CzPhO by the
cyclic voltammetry (CV) and the onset of the absorption spec-
trum, respectively. Differential scanning calorimetry (DSC) was
then utilized to examine the morphological properties. The glass
transition temperature (Tg) of 105 ꢁC was observed, showing
adequate rigidity and good solid state packing. In addition,
thermogravimetric analysis (TGA) was applied to study the
thermal stabilities, for which the decomposition temperature
(Td, i.e. temperature of 5% weight loss) of 365 ꢁC was recorded to
Bipolar behavior of CzPhO
To fully explore the potential of these Os(II) phosphors in device
fabrication, we developed a new bipolar host material, (CzPhO),
Table 3 The calculated energy levels and orbital transition analyses of the studied Os(II) complexes
States
T1
lcal
f
Assignments
MLCT (%)
65.47
1
2
3
498.4
HOMO / LUMO (89%)
HOMO-1 / LUMO + 1 (5%)
HOMO / LUMO + 1 (5%)
HOMO / LUMO (96%)
HOMO / LUMO (77%)
HOMO / LUMO + 1 (10%)
HOMO-1 / LUMO + 1 (7%)
HOMO / LUMO (96%)
HOMO / LUMO (72%)
HOMO-1 / LUMO + 1 (13%)
HOMO / LUMO + 1 (13%)
HOMO / LUMO (93%)
S1
T1
474.8
595.1
0.0083
0.0085
0.0087
64.20
62.8
S1
T1
560
526.2
65.3
46.18
S1
480.9
45.94
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show satisfactory thermal stability. Accordingly, CzPhO is
capable of forming homogeneous and stable amorphous films
upon thermal evaporation.
because its triplet gap of 2.56 eV suffices for efficient host-to-
guest energy transfer as well as for effective exciton confinement
in the emitting layer.31 Also under consideration is 2,20-bis(3-
(N,N-di-p-tolylamino)phenyl)biphenyl (3DTAPBP), which not
only possesses a higher hole mobility (ꢂ10ꢀ3 cm2 Vꢀ1 s) but also
a low LUMO (2.1 eV), and hence can be used to deter excessive
electron injection into the HTL.32 Furthermore, 3DTAPBP has
a wide triplet gap of about 2.68 eV, thereby enabling its
employment as the hole-transport or host material in red-emit-
ting OLEDs. From the viewpoint of electron transport materials,
3,30,5,50-tetra[(m-pyridyl)-phen-3-yl]biphenyl (BP4mPy) was
chosen as electron transport layer because of its adequate triplet
energy gap (ET ¼ 2.73 eV) and electron transport capabilities
(10ꢀ3–10ꢀ4 cm2 Vꢀ1 s).33 As a result, device A (CBP used as host)
and B (CzPhO used as host) were fabricated using a simple three-
layer architecture, namely: ITO/3DTAPBP (40 nm)/Host doped
with 4 wt.% of Os complex (30 nm)/BP4mPy (40 nm)/LiF
(0.8 nm)/Al (150 nm). Fig. 6 depicts the configurations and
structures of key components.
To investigate its bipolar nature, single carrier devices were
constructed and compared with those of proper ionization
potential (IP) and electron affinity (EA). Accordingly, the hole
mobility of 4,40-bis(carbazol-9-yl)biphenyl (CBP) and the elec-
tron mobility of 3-bis(9-carbazolyl)benzene (mCP) is about 10ꢀ3
cm2 Vsꢀ1 and 2 ꢃ 10ꢀ4 cm2 Vsꢀ1 at the applied field of 0.25 MV
cmꢀ1, and seems to be the suitable references for making intimate
comparison for mobility.30 Moreover, the HOMO energy levels
of CBP and CzPhO were estimated to be ꢀ5.8 and ꢀ5.78 eV,
respectively; while the LUMO levels of mCP and CzPhO were
evaluated to be about ꢀ2.4 and ꢀ2.33 eV, respectively.
The hole-only devices employ architecture as: ITO/MoO3
(1 nm)/TAPC (30 nm)/TCTA (10 nm)/CBP or CzPhO (100 nm)/
MoO3 (20nm)/Al (110 nm), for which MoO3 was used as hole
injection/electron blocking layer, while di-[4-(N,N-ditolyl-
amino)-phenyl]cyclohexane (TAPC) and 4,40,400-tris(carbazol-9-
yl)-triphenylamine (TCTA) were used as dual hole-transport
layers (TAPC/TCTA) for providing a smooth hole-injection into
the tested materials, by taking advantage of their gradual
increase of IPs. On the other hand, the electron-only devices were
composed of ITO/Al (20 nm)/mCP or CzPhO (100 nm)/TmPyPB
(10 nm)/TmPyPB doped with 10 wt.% of Cs2CO3 (10 nm)/Al
(110 nm), where 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB)
was used as the electron transport layer and the host material for
the n-type Cs2CO3 dopant. The J–V curves of the hole-only and
electron-only devices, shown in Fig. 5, indicate that the current
densities of hole-only device of CzPhO is similar to that of CBP,
demonstrating the excellent hole transport capability of CzPhO.
Again, the current densities of CzPhO-based electron-only device
are also higher than those of mCP. Consequently, CzPhO
possesses superior carrier transport properties for both positive
and negative charges.
The electroluminescence (EL) characteristics and the numeric
data are shown in Fig. 7 and Table 4. Both devices A and B
exhibited bright emission of dopants, supporting the presence of
effective energy transfer between the host and guest. No other
emission except for that of the dopant was observed, implying
that the carrier recombination had been effectively confined
within the EML, such that the exciton diffusion to the adjacent
layers has been avoided. As revealed in the efficiency curves,
devices A1 (1) and A2 (3) exhibited peak EL efficiencies of 10.9%
(21.7 cd Aꢀ1 and 11.9 lm Wꢀ1) and 10.1% (13.7 cd Aꢀ1 and
OLED fabrication
Complexes 1 and 3 were employed as the dopants for fabrication
of phosphorescent OLEDs due to their higher emission quantum
efficiencies. After we had taken into account the triplet energy
gaps of both 1 (ꢂ2.4 eV) and 3 (ꢂ2.3 eV), CzPhO (2.71 eV)
should be suitable for fabrication of standard three-layer
OLEDs. Moreover, for a fair comparison, another bipolar host
material 4,40-N,N0-dicarbazolebiphenyl (CBP) was also selected,
Fig. 5 Current density-voltage (J–V) characteristics of the hole- and
Fig. 6 The structural drawing of employed materials and OLED
electron-only devices.
architecture.
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Fig. 7 (a) The normalized EL spectra, (b) current density–voltage–luminance (J–V–L) characteristics, (c) external quantum efficiency versus luminance,
and (d) power efficiency versus luminance for the three devices A1, B1 and C1 that employed Os(II) complex 1 as dopant.
10.0 lm Wꢀ1) for the forward direction. The slightly inferior
efficiency of A2 was probably due to the lower photo-
luminescence quantum yield of dopant 3 (PLQY ꢂ 0.57).
voltage of B1 and B2 exhibited a significant drop to 2.5 and
2.6 V, respectively. We can thus conclude that the tailor-made
CzPhO indeed has great potential for employing as a host of
phosphorescent OLEDs. On the other hand, the slight differ-
ences on the EL spectra of devices were resulted from the optical
effects.
Better efficiencies were obtained when CzPhO was used as the
host in the emitting layer, cf. 14.3%, 34.8 cd Aꢀ1 and 45.2 lm Wꢀ1
for B1 and 9.9%, 15.7 cd Aꢀ1 and 20.5 lm Wꢀ1 for B2 (see Table
4). Considering the emission quantum yields of both 1 and 3,
such high external quantum efficiencies imply internal quantum
efficiency of nearly 100% in both devices. The superior carrier
balance was mainly the result of the adequate mobility of the host
material and energy barrier matching. Furthermore, as demon-
strated from the efficiency curves, the carrier balance was ach-
ieved in a range from 0.01 to 1 mA cmꢀ2 in both devices because
of the fact that bipolar carrier transport can prevent accumula-
tion of emissive excitons at the interface. In addition, comparing
the performances of devices A and B, the maximum luminance of
52,624 (B1) and 23,761 cd mꢀ2 (B2) were achieved through
applying relatively lower voltages. The results of the higher
current density and better carrier balance at higher voltages are
due to the great carrier transport property of CzPhO. In a sharp
contrast to the A1 and A2 with CBP as the host, the turn-on
For further optimizing the device performance, one of the key
criteria for phosphorescent OLEDs is the spectral overlaps
between the emission profile of host and the absorption of
dopant to invoke effective energy transfer.34 In addition, a host
with a wider triplet energy gap is indispensable for preventing the
reverse energy transfer from dopant to host. Once the phosphors
possess smaller triplet energy gaps, such as that of red-emitting
phosphors, choices for the host materials become versatile due to
their fewer requirements in matching the triplet energy, greatly
facilitating the device architecture.35 As a result, a third set of
devices C with the architecture: ITO/3DTAPBP (40 nm)/
3DTAPBP doped with 4 wt.% of Os complex (10 nm)/BP4mPy
doped with 4 wt.% of Os complex (20 nm)/BP4mPy (40 nm)/LiF
(0.8 nm)/Al (150 nm) were fabricated, the procedures of which
involve dispersing dopant into materials with opposite transport
Table 4 EL characteristics of red-emitting OLEDs with architectures of A, B and C
hext (%)
hl (cd Aꢀ1
)
hp (lm Wꢀ1
)
CIE 1931 coordinates
Von (V)
Max. L.
(cd mꢀ2) (V)
a
b
a
b
a
b
c
b
d
Dopant
A1
A2
B1
B2
C1
C2
1
3
1
3
1
3
10.9
10.1
14.3
9.9
12.9
10.7
10.9
10.1
14.1
9.6
12.6
8.7
21.7
13.7
34.8
15.7
27.9
18.5
21.7
13.7
34.2
15.3
27.5
15.0
11.9
10.0
45.2
20.5
21.8
19.3
9.0
6.1
27.8
10.9
16.1
7.4
4.3
4.0
2.5
2.6
3.6
3.5
19797 (21.6 V)
25167 (19.0 V)
52624 (14.2 V)
23761 (13.8 V)
14150 (14.4 V)
23733 (15.6 V)
(0.57, 0.43)
(0.61, 0.39)
(0.54, 0.46)
(0.60, 0.40)
(0.56, 0.44)
(0.59, 0.41)
(0.57, 0.43)
(0.61, 0.39)
(0.54, 0.46)
(0.59, 0.40)
(0.56, 0.44)
(0.59, 0.41)
a
c
d
Maximum efficiencies. b Data recorded at 102 cd mꢀ2
.
Turn-on voltage measured at 1 cd mꢀ2
.
Data recorded at 103 cd mꢀ2
.
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properties, such that the carrier recombination zone (RZ) can be
enlarged and stayed away from the cathode to avoid exciton
quenching. The resulting EL performances are also depicted in
Fig. 7 and Table 4 for a fair comparison.
Acknowledgements
This work was supported by the National Science Council of
Taiwan and the Ministry of Economic Affairs (100-EC-17-A-08-
S1-042). We are also grateful to the National Center for High-
Performance Computing for computer time and facilities.
Evidently, the associated turn-on voltages are significantly
lower due to the removal of inherent barriers among electron,
hole transport layers and even host material. As shown in Table
4, device C1 exhibited peak EL efficiencies of 12.9%, 27.9 cd Aꢀ1
and 21.8 lm Wꢀ1 for the forward direction. At the practical
brightness of 102 cd mꢀ2 (and 103 cd mꢀ2), the efficiencies were
recorded to be 12.6% (12.1%), 27.5 cd Aꢀ1 (26.4 cd Aꢀ1),
and 16.1 lm Wꢀ1 (11.3 lm Wꢀ1), respectively, while device C2
exhibited efficiencies of up to 10.7%, 18.5 cd Aꢀ1 and
19.3 lm Wꢀ1. At the practical brightness of 102 cd mꢀ2 (and
103 cd mꢀ2), the forward efficiencies also remained high at
around 8.7% (7.8%), 15.0 cd Aꢀ1 (13.5 cd Aꢀ1) and 7.4 lm Wꢀ1
(4.6 lm Wꢀ1), confirming their superiority versus those
employing CBP host. The increased carrier recombination
near the 3DTAPBP/BP4mPy interface might have contributed
to the balanced carrier transport. Furthermore, their wider
triplet energy gaps also wielded significant impacts on the
confinement of excitons and carriers. In addition, upon closer
inspection of the efficiency curves of all tested devices, one can
clearly observe similar trends of reduced roll-off at higher
brightness. This observation is in contrast to the triplet–triplet
annihilation (TTA) observed in typical phosphorescent OLEDs
with unipolar host material.36 As the mitigated TTA phenom-
enon was prevalent, the result indicates that the exciton
formation in C is not localized at the interface; instead, the
excitons may be spread over both electron and hole transport
layers. On the basis of this scenario, we propose that the
carriers not only hop onto the host but also enter directly
into the dopants. Furthermore, the carrier trapping in both
emitting layers also causes the relaxation of the excitons and
gathering in a narrow space.37 Overall, this simple double-layer
architecture incorporating 3DTAPBP and BP4mPy renders
satisfactory performances even under the conditions of high
current density.
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Conclusion
In summary, we present the design and synthesis of three rela-
tively stable Os(II) phosphors 1–3 functionalized with diphos-
phine chelate pp2b as well as a new bipolar material CzPhO with
both carbazole donor and dibenzophosphole oxide acceptor
unit. For better understanding their functional capability and
prospective, this class of Os(II) complexes are fully characterized
and their structural versus photophysical properties are com-
prehended by absorption/emission together with computational
approaches. Fabrication of OLEDs is then accomplished using
two Os(II) dopants 1 and 3 and on three distinctive device
architectures; namely: with either CBP or CzPhO as host mate-
rial for emissive layer, and those by simply dispersing the Os(II)
dopants into both hole and electron transporting materials.
Consequently, the CzPhO devices with 1 as the emitter reveal the
best peak efficiencies of EQE 14.3%, hL of 34.8 cd Aꢀ1, and hp of
45.2 lm Wꢀ1, demonstrating the great potential for both the Os(II)
dopant and the class of CzPhO bipolar host for application in
phosphorescent OLEDs.
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