A Yellow-Emitting Homoleptic Iridium(III) Complex Constructed from a Multifunctional Spiro Ligand for Highly Efficient Phosphorescent Organic Light-Emitting Diodes

Article abstract of DOI:10.1021/acs.inorgchem.7b01034

To suppress concentration quenching and to improve charge-carrier injection/transport in the emission layer (EML) of phosphorescent organic light-emitting diodes (PhOLEDs), a facial homoleptic iridium(III) complex emitter with amorphous characteristics was designed and prepared in one step from a multifunctional spiro ligand containing spiro[fluorene-9,9′-xanthene] (SFX) unit. Single-crystal X-ray analysis of the resulting fac-Ir(SFXpy)₃ complex revealed an enlarged Ir···Ir distance and negligible intermolecular π-π interactions between the spiro ligands. The emitter exhibits yellow emission and almost equal energy levels compared to the commercial phosphor iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C²′)acetylacetonate (PO-01). Dry-processed devices using a common host, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl, and the fac-Ir(SFXpy)₃ emitter at a doping concentration of 15 wt % exhibited a peak performance of 46.2 cd A^-1, 36.3 lm W^-1, and 12.1% for the current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE), respectively. Compared to control devices using PO-01 as the dopant, the fac-Ir(SFXpy)₃-based devices remained superior in the doping range between 8 and 15 wt %. The current densities went up with increasing doping concentration at the same driving voltage, while the roll-offs remain relatively low even at high doping levels. The superior performance of the new emitter-based devices was ascribed to key roles of the spiro ligand for suppressing aggregation and assisting charge-carrier injection/transport. Benefiting from the amorphous stability of the emitter, the wet-processed device also exhibited respectful CE, PE, and EQE of 32.2 cd A^-1, 22.1 lm W^-1, and 11.3%, respectively, while the EQE roll-off was as low as 1.7% at the luminance of 1000 cd m^-2. The three-dimensional geometry and binary-conjugation features render SFX the ideal multifunctional module for suppressing concentration quenching, facilitating charge-carrier injection/transport, and improving the amorphous stability of iridium(III)-based phosphorescent emitters.

Full text of DOI:10.1021/acs.inorgchem.7b01034

Article
pubs.acs.org/IC
A Yellow-Emitting Homoleptic Iridium(III) Complex Constructed from
a Multifunctional Spiro Ligand for Highly Efficient Phosphorescent
Organic Light-Emitting Diodes
Bao-Yi Ren,†,‡,§,⊥ Run-Da Guo,∥,⊥ Dao-Kun Zhong,†,⊥ Chang-Jin Ou,§ Gang Xiong,† Xiang-Hua Zhao,§
Ya-Guang Sun,*,† Matthew Jurow,‡ Jun Kang,# Yi Zhao,∥ Sheng-Biao Li,*,¶ Li-Xin You,†
Lin-Wang Wang,# Yi Liu,*,‡ and Wei Huang§
†Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, College of Applied Chemistry, Shenyang University of
Chemical Technology, Shenyang 110142, P. R. China
∥State Key Laboratory on Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, Changchun
130012, P. R. China
#
‡The Molecular Foundry and Materials Science Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley,
California 94720, United States
§Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergetic
Innovation Center for Advanced Materials, Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023,
P. R. China
¶College of Chemistry, Central China Normal University, Wuhan 430070, P. R. China
S
* Supporting Information
ABSTRACT: To suppress concentration quenching and to improve charge-carrier
injection/transport in the emission layer (EML) of phosphorescent organic light-
emitting diodes (PhOLEDs), a facial homoleptic iridium(III) complex emitter with
amorphous characteristics was designed and prepared in one step from a
multifunctional spiro ligand containing spiro[fluorene-9,9′-xanthene] (SFX) unit.
Single-crystal X-ray analysis of the resulting fac-Ir(SFXpy)3 complex revealed an
enlarged Ir···Ir distance and negligible intermolecular π−π interactions between the
spiro ligands. The emitter exhibits yellow emission and almost equal energy levels
compared to the commercial phosphor iridium(III) bis(4-phenylthieno[3,2-
c]pyridinato-N,C2′)acetylacetonate (PO-01). Dry-processed devices using a
common host, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl, and the fac-Ir(SFXpy)3 emitter
at a doping concentration of 15 wt % exhibited a peak performance of 46.2 cd A−1,
36.3 lm W−1, and 12.1% for the current efficiency (CE), power efficiency (PE), and
external quantum efficiency (EQE), respectively. Compared to control devices using PO-01 as the dopant, the fac-Ir(SFXpy)3-
based devices remained superior in the doping range between 8 and 15 wt %. The current densities went up with increasing
doping concentration at the same driving voltage, while the roll-offs remain relatively low even at high doping levels. The
superior performance of the new emitter-based devices was ascribed to key roles of the spiro ligand for suppressing aggregation
and assisting charge-carrier injection/transport. Benefiting from the amorphous stability of the emitter, the wet-processed device
also exhibited respectful CE, PE, and EQE of 32.2 cd A−1, 22.1 lm W−1, and 11.3%, respectively, while the EQE roll-off was as
low as 1.7% at the luminance of 1000 cd m−2. The three-dimensional geometry and binary-conjugation features render SFX the
ideal multifunctional module for suppressing concentration quenching, facilitating charge-carrier injection/transport, and
improving the amorphous stability of iridium(III)-based phosphorescent emitters.
excited state (S1) to the lowest triplet excited state (T1), can
reach unity because of the extraordinary heavy-atom effect of
the centric IrIII ion, thus laying the foundation for using
cyclometalated iridium(III) complexes as dopants for highly
efficient PhOLEDs.5−7 In the meantime, the intrinsic properties
and EL performance of iridium(III)-based emitters, e.g.,
INTRODUCTION
■
Phosphorescent organic light-emitting diodes (PhOLEDs)
based on cyclometalated iridium(III) complexes have attracted
enormous attention for their potential use in full-color flat-
panel displays and solid-state light sources because of their
ability to fully harvest both singlet (S) and triplet (T) excitons
by strong spin−orbital coupling.1−4 The high electrolumines-
cent (EL) efficiency of iridium(III)-based emitters, attributable
to their high intersystem-crossing yield from the lowest singlet
Received: April 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
emission color and efficiency, frontier molecular orbital (FMO)
energy levels, suppression of the concentration quenching, and
charge-carrier injecting/transporting ability, are closely depend-
ent on the molecular structures of organic ligands coordinated
to the IrIII ions. Until now, it still remains a crucial problem for
the PhOLEDs that the improvement of the device efficiency is
often impeded by serious concentration quenching and triplet−
triplet annihilation (TTA) because of strong intermolecular
interactions of excited molecules, especially at high doping
levels.8−10 Previously it has been demonstrated that the
concentration quenching and TTA of iridium(III)-based
phosphors can be relieved by the use of dendritic ligands
possessing highly branched molecular structures. An important
advantage of dendrimers is the presence of dynamic internal
cavities, in which IrIII ions can be comfortably embedded to give
more protected metal centers. However, such ligands also suffer
from laborious synthesis and troublesome purification.11−13
Another feasible and effective approach to restraining the
strong intermolecular interactions of iridium(III) emitters and
to improve their spectroscopic and morphological stabilities is
to introduce sterically hindered groups into the cyclometalated
ligand. This approach can break the planarity of the
cyclometalated ligand, while providing a dendrimer-like internal
cavity for protecting the IrIII centers, especially for homoleptic
iridium(III) complexes.14,15 For instance, Che and co-workers
initially demonstrated that self-quenching was significantly
reduced for the iridium(III) complex in solution, even at very
high concentration, because of the steric effect of the pinene
spacer in the homoleptic iridium(III)-based emitter.16 As a
result, a series of ligands with sterically hindered groups, such as
alkyl,17 trimethylsilyl,18 cycloalkenyl,19 phenyl,20,21 2,6-dime-
thylphenoxy,22 and diarylfluorene,23 have been used to
construct cyclometalated ligands for homoleptic or heteroleptic
iridium(III) complexes. In particular, Wong and co-workers
demonstrated that attaching arylamino units to the fluorene
ring of the cyclometalated ligand can effectively decrease
intermolecular interactions to afford iridium(III)-based phos-
phors with anti-TTA properties.24−26
cules.31 In addition, the SFX-based derivatives were proven to
be bipolar host materials with separated FMOs for high-
performance PhOLEDs.32 Hence, relying on the interrupted
and binary π systems of SFX, we anticipate that additional
functions can be incorporated into the SFX-based ligands other
than steric effects, including tuning the emission color, reducing
aggregation, and assisting charge-carrier injection/transport for
iridium(III)-based phosphorescent emitters.
Yellow emission is one important chromaticity component to
fulfill the requirement of a full-color display and to achieve the
high-performance two-color (blue, yellow) white PhOLEDs. A
series of yellow-emitting iridium(III) complexes have been
developed,33,34 yet a detailed investigation of yellow-emissive
iridium(III) complexes with SFX-based ligands remains absent.
It is even more desirable to render the yellow-emissive
iridium(III) emitters more compatible with both dry and wet
processes, which are important for the low-cost, large-area
manufacturing of high-performance devices.35
In this contribution, a yellow-emitting homoleptic iridium-
(III) complex with a facial configuration was conceived and
synthesized using SFXpy as the cyclometalated ligand. The
steric structural features and optoelectronic properties were
revealed on the basis of single-crystal X-ray crystallographic,
photophysical, and electrochemical investigations. The yellow-
emitting PhOLED devices were fabricated with different doping
concentrations of the iridium(III) complex by dry and wet
processes. As the doping concentration of the emitter was
increased while keeping the same driving voltage, the current
density increased accordingly, clearly indicating the iridium-
(III)-based emitter’s capability of assisting charge-carrier
injection/transport. The maximum device efficiency for dry-
processed devices is superior to that for the control device
fabricated using the commercial yellow emitter PO-01 as the
dopant via vacuum thermal deposition, and for wet-processed
devices, the efficiency is comparable to that of the control
device. At the same time, such devices displayed low roll-off at
high doping concentration, especially for the wet-processed
device W2 with an EQE roll-off as low as 1.7% at a luminance
of 1000 cd m−2.
The common characteristics of steric groups in cyclomalated
ligands are bulky, rigid, and nonelectroactive. An obvious
disadvantage of nonelectroactive bulky moieties with steric
hindrance effects, however, is the loss of luminance and
emission efficiency with suppression of the concentration
quenching.16 Hence, seeking multifunctionality beyond the
steric effect is essential to overcoming the predicament for
phosphorescent iridium(III) complexes.27 The spiro aromatic
skeleton has become a promising motif for constructing
multifunctional steric-hindrance-type ligands. For example,
the rigid spirobifluorene (SBF) with a three-dimensional
(3D) orthogonal configuration was successfully introduced
into the cyclometalated ligands, and the corresponding
phosphorescent iridium(III) complexes showed satisfactory
performances for PhOLEDs, attributable to the amorphous
character of the dopants and controlled host−guest or guest−
guest interactions.28,29 Benefiting from the convenient synthesis
of spiro[fluorene-9,9′-xanthene] (SFX),30 a heteronuclear
analogue to SBF, we designed and synthesized a pyridine-
appended bulky SFX ligand, 2-(spiro[fluorene-9,9′-xanthene])-
pyridine (SFXpy), for the phosphorescent platinum(II)
complex, and the corresponding PhOLED devices showed a
yellow emission peak at 548 nm, and no excimer emission or
Pt···Pt interaction was observed in its EL spectra because of
weakened intermolecular interactions of the emitter mole-
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The SFXpy ligand and
fac-Ir(SFXpy)3 complex were prepared following the proce-
dures described in Scheme 1. Using commercially available 2-
bromofluoene and phenol as starting materials, the spiro
skeleton of 2-BrSFX was facilely synthesized under acidic
conditions. The sequential protocols of palladium-catalyzed
Scheme 1. Synthesis of the fac-Ir(SFXpy)3 Complex
B
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Miyaura borylation and Suzuki coupling were employed to
afford the spiro ligand SFXpy (see the Supporting Information,
SI, for the single-crystal X-ray crystallographic data of SFXpy).
The final complex was synthesized by treating Ir(acac)3 with
SFXpy (3.4 equiv) in glycerol at 220 °C for 18 h. fac-
Ir(SFXpy)3 was obtained as a yellow powder after column
and N−Ir bond lengths of the fac-Ir(ppy)3 complex (2.042 and
2.084 Å, respectively),36 the average bond lengths of C−Ir and
N−Ir in fac-Ir(SFXpy)3 are slightly altered to 2.016 and 2.117
Å, respectively. The nearest distance between two IrIII centers
in the crystal of fac-Ir(SFXpy)3 is 10.996 Å, which is much
larger than the corresponding distance in fac-Ir(ppy)3 single
crystals (7.701 Å) and the typical Ir−Ir bond length (ca. 2.6−
3.1 Å) in multimetallic clusters, suggesting a lack of metal−
metal interactions.36−39 The noncoordinating xanthene moiety
of the ligand is nearly perpendicular to the plane of the
chelating fluorene−pyridine unit. This renders the molecule
highly sterically bulky, which is generally beneficial for reducing
the concentration quenching of the emission.40 The nearest
centroid−centroid distance between the fluorene and xanthene
rings is 4.28 Å (Figure 1b), which is longer than 3.8 Å, the
upper bound of the π−π-stacking distance in metal
complexes.41 The absence of intermolecular π−π interaction
in the solid state of fac-Ir(SFXpy)3 can thus be concluded,
which is usually responsible for perturbing the energy levels of
radiative transitions, altering the radiative lifetime (τr),
decreasing the radiative relaxation quantum yield, and even
generating metal-to-ligand−ligand charge-transfer transi-
tions.28,42
fac-Ir(SFXpy)3 exhibits good solubility in common organic
solvents, such as dichloromethane, ethyl acetate, chloroform,
and toluene. The good solubility of fac-Ir(SFXpy)3, derived
from the deaggregation effect of the spiro SFXpy ligands, is
favorable for solution-based device fabrication. Thermogravi-
metric analysis (TGA) of fac-Ir(SFXpy)3 indicates a degrada-
tion temperature of 356 °C at 5 wt % weight loss, confirming its
excellent thermal stability (see Table 2 and Figure S1a). On the
other hand, the lack of any endothermic/exothermic peaks in
the differential scanning calorimetry (DSC) curve indicates that
no crystallization and glass transformation occurs during the
heating/cooling cycle of fac-Ir(SFXpy)3 (Figure S1b). Such
thermal behavior is conducive for the formation of high-quality
thin films by vacuum deposition.
1
chromatographic purification in a yield of 35%. The H NMR
spectrum of the complex reveals that the three ligands
surrounding the IrIII ion are magnetically equivalent, in keeping
with the 3-fold molecular symmetry in a facial configuration. X-
ray diffraction analysis of single crystals of the fac-Ir(SFXpy)3
complex, obtained by the slow diffusion of hexane into its
dichloromethane solution, clearly confirms its facial config-
uration. Details of the crystallographic data are summarized in
Tables 1 and S1 and S2. As shown in Figure 1a, three SFXpy
Table 1. Crystallographic Data and Structure Refinement
Information for fac-Ir(SFXpy)3
identification code
empirical formula
fw
CCDC 1482495
C90H54IrN3O3
1417.38
cryst syst
space group
a (Å)
orthorhombic
Pbcn
27.9461(7)
21.9877(5)
25.6824(5)
15781.1(6)
8
b (Å)
c (Å)
volume (Å3)
Z
ρcalc (g cm−3
)
1.543
μ (mm−1
)
2.087
F(000)
7408.0
2θ range for data collection
(deg)
6.04−50.02
index ranges
−25 ≤ h ≤ 33, −26 ≤ k ≤ 25, −19 ≤ l ≤
30
reflns collected
indep reflns
44760
13877 [Rint = 0.0459, Rσ = 0.0583]
Photophysical Properties. Figure 2 shows the absorption
and photoluminescence (PL) spectrum of fac-Ir(SFXpy)3 at
room temperature and the PL spectra at 77 K. In the electronic
spectra, the high-intensity absorption bands below 300 nm can
be assigned to the spin-allowed ligand-centered 1π → π*
ligands are coordinated to the central IrIII ion to form a slightly
distorted octahedral configuration with a cis-C,N chelate
disposition, which is similar to the typical structural pattern
of the fac-Ir(ppy)3. Compared to the reported average C−Ir
Figure 1. Crystal structure of fac-Ir(SFXpy)3 with ellipsoids shown at the 40% probability level (a) and a representation of the crystal packing (b).
C
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Physical Properties for the fac-Ir(SFXpy)3 Complex
a
abs
a
b
b
b
c
d
complex
λ
(nm)
λPL (nm)
Φp (%)
τp (μs)
kr (s−1
)
Td (°C)
HOMO/LUMO
fac-Ir(SFXpy)3
278, 303, 318, 334, 400
542, 580
18
0.11
5.56 × 105
356
−5.06 (−4.52)/−2.75 (−2.58)
a
b
c
d
Measured in a CH2Cl2 solution (room temperature). Measured in degassed CH2Cl2; λex = 365 nm. Td = decomposition temperature. HOMO
was calculated according to the formula −[4.8 eV + Eox/red (vs EFc/Fc )]; the LUMO level was calculated from HOMO and Eogpt. The data in
+
parentheses are from theoretical calculations.
intersystem-crossing yield (ΦISC) is assumed to be 1.0 for
iridium complexes because of the strong heavy-atom effect.7,47
The relatively high radiative rate constants, combined with the
significantly shortened triplet lifetime, would be beneficial for
high performance in devices because the triplet excitons can
decay rapidly through the radiative pathway, while the TTA
could be restrained simultaneously.43
Electrochemical Properties and Theoretical Calcula-
tions. The electrochemical behavior of fac-Ir(SFXpy)3 was
investigated by cyclic voltammetry (CV) using a conventional
three-electrode setup and the ferrocene/ferrocenium redox
couple as an internal reference (Figure 3). The onset oxidation
Figure 2. UV−vis absorption and PL spectra of fac-Ir(SFXpy)3 in a
CH2Cl2 solution at 298 K. The inset is the PL spectrum in degassed 2-
methyltetrahydrofuran at 77 K.
intraligand charge-transfer transition. The absorption bands in
the range of 300−350 nm arise from spin-allowed metal-to-
ligand charge transfer (1MLCT) mixed with an interligand
charge transfer. The weaker and lower-energy bands above 350
3
nm are presumably from an admixture of the MLCT and the
3
intraligand π → π* transitions, originating from the strong
spin−orbit coupling of the fac-Ir(SFXpy)3 complex. According
to the absorption edge of fac-Ir(SFXpy)3 at 537 nm, the optical
energy gap (Eogpt) was calculated to be 2.31 eV.43 The emitter
shows intense PL emission in a CH2Cl2 solution at a
temperature of 298 K, with a strong emission peak located at
542 nm and a weak shoulder emission near 580 nm. Compared
Figure 3. Cyclic voltammogram of fac-Ir(SFXpy)3 in a chloroform
to the homoleptic analogue fac-Ir(ppy)3, which has a PL peak at
solution. The insets are the calculated HOMO and LUMO orbital
494 nm in an ethanol/methanol solution at 77 K, the
plots of the complex.
bathochromic shift in the PL spectra of fac-Ir(SFXpy)3 can
be attributed to the extended conjugation of the fluorene−
pyridine moiety coordinated to a centric IrIII ion.44,45 The
potential (Eox) is 0.26 V with respect to Fc/Fc+, which can be
triplet emission peak exhibits a large Stokes shift (ca.140 nm)
attributed to the metal-centered IrIII/IrIV oxidation couple.48
from the lowest-energy absorption peak. At 77 K, the PL
spectra of fac-Ir(SFXpy)3 undergoes a pronounced rigid-
ochromic shift and exhibits a well-resolved vibronically
structured line shape between two major peaks, indicating
Therefore, the highest occupied molecular orbital (HOMO)
level is determined to be −5.06 eV. Thelowest unoccupied
molecular orbital (LUMO) level of −2.75 eV is estimated from
the optical gap Eogpt. For the well-known fac-Ir(ppy)3, the
corresponding HOMO and LUMO levels are −5.1 and −2.6
eV, respectively.49 In comparison, fac-Ir(ppy)3 possesses
approximately the same HOMO level as that of fac-Ir(ppy)3,
while the LUMO is 0.15 eV lower in the more π-extended fac-
Ir(SFXpy)3. Interestingly, the HOMO and LUMO levels of fac-
Ir(SFXpy)3 are almost the same as those of the commercial
emitter PO-01 (−5.1 and −2.7 eV for the HOMO and LUMO
levels, respectively).50,51 For phosphorescent materials, the
close FMO levels between two dopants usually signify that they
possess similar charge-carrier injecting/transporting ability in
an emission layer (EML) under the same configuration of
PhOLED devices. Therefore, control devices using PO-01 as
the dopant were fabricated for benchmarking the EL perform-
ance and the charge-carrier injecting/transporting abilities (vide
infra).
3
3
that the mixing between the MLCT and π → π* levels is so
effective that a ligand-centered emission can clearly be observed
in the frozen matrix. The vibronic progression of ca. 1210 cm−1
(ν0−1) in the emission profile of fac-Ir(SFXpy)3 corresponds to
the aromatic stretching of the spiro ligand, which is diagnostic
of the substantial involvement of the ligand-centered π−π*
transition in PL.26 Therefore, it suggests that the PL emission
of the complex predominantly arises from the ligand-centered
3π → π* excited state.46 The PL quantum yield (PLQY; Φp),
measured in degassed CH2Cl2, is 0.18 for fac-Ir(SFXpy)3 with a
triplet lifetime (τp; Figure S2) of ca. 0.11 μs at 298 K.
Accordingly, τr of the triplet excited state, deduced from the
equation τr = τp/Φp, is 0.61 μs. The triplet radiative and
nonradiative rate constants, kr and knr, calculated from Φp and
τp using the expressions Φp = ΦISC[kr/(kr + knr)] and τp = (kr +
knr)−1, are 5.56 × 105 and 3.55 × 105 s−1, where the
D
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Configuration Diagrams of Dry- and Wet-Processed PhOLEDs Using fac-Ir(SFXpy)3 as the Emitter and Structural
Representations of Molecular Components
For a deeper understanding of the electrochemical behavior
and FMO distribution of fac-Ir(SFXpy)3, the 3D geometries
were calculated based on the crystallographic structure of the
molecule (computational details are described in the SI). The
HOMO and LUMO surfaces of fac-Ir(SFXpy)3 are plotted in
Figure 3. The orbital distributions suggest that the HOMO is
composed of a mixture of π orbitals of the coordinated
phenylpyridine moiety and IrIII d orbital (t2g), and the LUMO
is mainly distributed on the π orbital of the pyridine−fluorene
moiety of the SFXpy ligand. The calculated HOMO and
LUMO levels are −4.52 and −2.58 eV, respectively. Addition-
ally, there are no FMOs located on the xanthene moiety of the
ligand according to the calculated results, likely because of the
interrupted linkage between the fluorene and xanthene rings by
the spiro atom of SFX. Overall, the peripheral xanthene moiety
imposes a steric effect on fac-Ir(SFXpy)3, yet the latent
capability of xanthene for hole transport should not be
ignored.32 At the same time, shrinkage of the FMO energy
levels and extended π system in the fluorene−pyridine
conjugate should be favorable for charge-carrier injection/
transport in fac-Ir(SFXpy)3.
fac-Ir(SFXPy)3 (D1−D5, in which x = 5, 8, 12, 15, and 18,
respectively), where MnOx and Libpp served as hole- and
electron-injection layers, m-MTDATA and Bphen served as
hole- and electron-transporting layers, and Ir(ppz)3 was used as
the hole-transporting and electron-blocking material, respec-
tively.
The normalized EL spectra of these devices (Figure 4, at a
driving voltage of 4.5 V) all showed a maximum emission peak
located at 542 nm, together with a shoulder peak at 584 nm.
There are no undesirable emission peaks from the host of CBP
and the excimer of fac-Ir(SFXpy)3, even at high operation
Dry-Processed OLEDs. To evaluate fac-Ir(SFXpy)3 as the
phosphorescent emitter, PhOLED devices were fabricated with
the following configurations via vapor evaporation deposition:
MoOx (10 nm)/m-MTDATA (40 nm)/Ir(ppz)3 (10 nm)/
CBP:x wt % fac-Ir(SFXPy)3 (10 nm)/Bphen (40 nm)/Libpp
(3 nm)/Al (90 nm). Scheme 2 depicts the device configuration
diagram and molecular structures of the layer components.
Devices D1−D5 used a common host of 4,4′-bis(N-
carbazolyl)-1,1′-biphenyl (CBP) with varying doping levels of
Figure 4. Normalized and non-normalized (inset) EL spectra of
devices D1−D5 under a voltage of 4.5 V.
E
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
voltages (see Figure S3). The close resemblance between the
EL and PL spectra suggests that the sufficient energy transfer
from the host to the phosphor molecule in the EML is
minimally affected by the doping concentration. Moreover, the
non-normalized EL spectrum (inset in Figure 4) showed that
the EL emission intensity gradually increased along with rising
doping concentration. For devices D1−D5, the turn-on
voltages are all at 2.8 V (Figure 5). It is worth noting that
Figure 6. Efficiency curves of devices D1−D5.
remarkable that the devices show little self-quenching effect and
high efficiency even at a high doping ratio of 15 wt %.
Suppression of the concentration quenching can be attributed
to the steric effect induced by the xanthene moiety to prevent
clustering between the emitters. It is worth noting that the
efficiency roll-off is relatively low in the high doping
concentration range of 12−18 wt %. When the devices are
compared at a luminance of 1000 cd m−2, the PE of device D3
is 28.1 lm W−1, corresponding to a roll-off of 8.5% (the
maximum PE of D3 is 30.7 lm W−1), while device D1 with a
low doping level exhibits a PE of 18.7 lm W−1, corresponding
to a drastic roll-off of 32% (the maximum PE of D1 is 27.5 lm
W−1). Overall, the steric effect imposed by the xanthene unit in
the phosphor guest can considerably reduce the concentration
quenching, akin to previous reports using the uncoordinated
fluorene moiety of SBF as the spacer in the framework of a
phenylpyridine-type ligand.28
In order to further survey the versatile roles of the fac-
Ir(SFXpy)3 emitter, the EML thickness was varied in devices
D4 and D5 that have displayed high efficiency and low roll-off
at high doping concentrations (15 and 18 wt %). Two devices
with EML thickness of 20 nm, namely D4−2 and D5−2, were
fabricated to study the effect of thicker EML layer on charge-
carrier injection/transport and device performance (Figures 7
and 8). For comparison, a control device D-PO was fabricated
using 6 wt % of the standard dopant PO-01, at which
concentration peak efficiency was reported.53,54 For devices
D4-2 and D5-2, the turn-on voltages are both about 3.0 V,
which is slightly higher than that of devices D4 and D5 and
attributable to the nearly doubled thickness of the EML, while
the emission peak positions and CIE coordination still remain
unchanged (see Figure S4). Among all of these dry-processed
Figure 5. Current density (J)−voltage (V)−luminance (L) relation-
ships of devices D1−D5.
the low driving voltages of 3.9−4.4 V of D1−D5 at a luminance
of 1000 cd m−2 are favorable for the application of these devices
in portable equipment (Figure 5 and Table 3). Similar to the
doping concentration dependence trend observed for the EL
intensity, the current densities of the devices increase as the
doping concentration increases under the same driving voltage,
which clearly indicates that the incorporation of fac-Ir(SFXpy)3
facilitates both emission and charge-carrier injection/trans-
port.52
The influence of the doping concentration of fac-Ir(SFXpy)3
on the device efficiency, specifically on the current efficiency
(CE), power efficiency (PE), and external quantum efficiency
(EQE), is shown in Figure 6 and summarized in Table 3. As the
doping concentration changes from 5 to 8 wt %, the device
efficiency increases and then levels off within the doping range
between 8 and 15 wt %. The efficiency starts to drop upon a
further increase of the doping concentration to 18 wt %. The
device performances are maintained at high levels as 37.8−38.2
cd A−1, 30.7−33.5 lm W−1, and 9.5−10.2% for CE, PE, and
EQE, respectively, in the doping range of 8−15 wt %. It is quite
Table 3. Summary of the EL Data of PhOLEDs
a
b
b
b
a
device
concn (wt %) operation voltage (V) CE (cd A−1
)
PE (lm W−1
)
EQE (%)
1931 CIE coordinates
maximum luminance (cd m−2
)
D1
D2
D3
D4
D5
5
8
2.8/4.4
2.8/4.2
2.8/4.2
2.8/3.9
2.8/3.9
3.0/4.3
3.0/4.3
3.2/5.1
3.6/5.0
3.6/5.0
31.5/26.4
38.2/35.5
37.8/37.4
37.8/35.5
29.4/28.4
46.2/44.7
34.2/33.5
33.1/28.4
30.2/29.9
32.2/31.6
27.5/18.7
33.4/26.8
30.7/28.1
33.5/28.2
25.6/22.9
36.3/32.6
26.9/24.4
24.2/17.8
20.2/18.7
22.1/19.8
8.1/7.0
(0.41, 0.58)/(0.41, 0.58)
(0.41, 0.58)/(0.41, 0.58)
(0.42, 0.57)/(0.42, 0.57)
(0.42, 0.57)/(0.42, 0.57)
(0.42, 0.57)/(0.42, 0.57)
(0.42, 0.58)/(0.42, 0.58)
(0.42, 0.57)/(0.42, 0.57)
(0.47, 0.52)/(0.47, 0.52)
(0.43, 0.52)/(0.43, 0.52)
(0.40, 0.52)/(0.40, 0.52)
10018
15147
29254
18443
25170
21048
26788
14103
12860
13117
9.5/8.9
12
15
18
15
18
6
10.2/9.9
9.5/9.0
7.8/7.6
c
D4−2
D5−2
D-PO
12.1/11.6
8.7/8.5
c
c
9.5/8.0
c
c
W1
4
10.6/10.4
11.3/11.1
W2
8
a
b
c
In the order of at turn-on and at 1000 cd m−2. In the order of at maximum and at 1000 cd m−2. In the order of at maximum and at 6 V.
F
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
bulky emitter ensures excellent chemical compatibility with the
small-molecule host, which allows the use of a small-molecule
cohost matrix in the EML. The two wet-processed devices
exhibit the same EL features as the peaks at 540 and 583 nm
(Figure S5), which are also consistent with those of the dry-
processed devices (Table 3). The turn-on and operation
voltages at 1000 cd m−2 of the two devices are 3.6 and 5.0 V,
respectively. The current density of the devices increases as the
doping concentration is changed from 4 to 8 wt % (Figure 9).
Figure 7. J−V−L relationships of devices D4-2, D5-2, and D-PO.
Figure 9. J−V−L relationships of devices W1 and W2.
In spite of an EML thickness of 40 nm, the charge-carrier
injection/transport and energy transfer remain very efficient, as
indicated by the J−V−L curves. Both devices W1 and W2
exhibit high performances, with device W2 showing a relatively
superior efficiency (Figure 10 and Table 3). The slightly
Figure 8. Efficiency curves of devices D4-2, D5-2, and D-PO.
devices, device D4-2 exhibits the highest CE, PE, and EQE of
46.2 cd A−1, 36.3 lm W−2, and 12.1%, respectively. Additionally,
devices D4-2 and D5-2 possess lower roll-off compared to
devices D4 and D5, indicating the effective suppression of self-
quenching even at higher EML thickness. The reference device
D-PO exhibited inferior device characteristics (CE, 33.1 cd A−1;
PE, 24.2 lm W−2; EQE, 9.5%) and significant roll-offs of 14.2%,
26.5%, and 15.8% for the CE, PE, and EQE under a luminance
of 1000 cd m−2, respectively. In contrast to PO-01, fac-
Ir(SFXpy)3 has slightly shrunken FMO levels and extended π
orbitals, which may be favorable for a more balanced electron
and hole recombination in the EML.21 Together with the 3D
spiro geometry of the fac-Ir(SFXpy)3 emitter, the excellent
performance of the devices could be attributed to the
synergistic effects of the SFXpy ligand on charge-carrier
injection/transport and on the intermolecular interactions
between guest−guest/host−guest.
Wet-Processed OLEDs. The EL performance of fac-
Ir(SFXpy)3 was also tested in wet-processed PhOLEDs. The
devices W1 and W2 were fabricated in the following
configuration: ITO/PEDOT:PSS (20 nm)/EML (40 nm)/
TmPyPB (60 nm)/LiF (0.8 nm)/Al (100 nm) (Scheme 2b). In
the EML, the cohosts of TAPC and mCP were used in a weight
ratio of 30:70, and the doping concentrations of the emitter
were 4 and 8 wt % for devices W1 and W2, respectively. It
should be noted that, for wet-processed OLEDs, the emitters
are often doped into a polymer-based host matrix instead of a
small molecular host matrix to avoid phase separation and
ensure good film quality, which is often at the expense of the
device performance.14 In the case when fac-Ir(SFXpy)3 is
employed as the emitter, the good amorphous stability of this
Figure 10. Efficiency curves of devices W1 and W2.
decreased PE and the relatively high operation voltage
compared to the dry-processed devices may be due to the
thicker EML in wet-processed devices. However, the high
current efficiencies prove that the multifunctional feature of fac-
Ir(SFXpy)3 remains effective in promoting charge-carrier
injection/transport in the wet-processed devices. Remarkably,
under a luminance of 1000 cd m−2, the EQE roll-off of device
W2 is as low as 1.7%, which stands out from all of the reported
wet-processed yellow PhOLEDs.33 These results collectively
demonstrate that the SFX-based ligand endows the fac-
Ir(SFXpy)3 emitter with good solution processability while
suppressing the concentration quenching by evenly distributing
the IrIII dopant in the EML.
It can be found that the dry- and wet-processed devices show
different efficiency roll-off behaviors; the wet-processed devices
G
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
possess lower roll-off than the dry-processed devices, especially
at high luminance. As reported recently,55 the emitter
molecules tend to form networks in a vacuum-deposited
EML, even at doping concentrations as low as 5 wt %, and
percolated networks form through the entire EML as the
doping concentrations are slightly increased. Such networks of
emitter molecules in a vacuum-deposited EML, which is
different from the even distribution of emitter guests in a
solution-processed EML, would improve charge-carrier trans-
port, but it could also give rise to more nonradiative pathways
for the emissive state as well as a resultant loss of efficiency. In
our cases, the maximum efficiencies of dry-processed devices
stay on a high level in the doping range between 8 and 15 wt %,
and the improved charge-carrier transport was observed
simultaneously. This phenomenon suggests efficient blocking
of the nonradiative pathways using these sterically hindered
SFXpy ligands. The roll-offs of wet-processed devices are even
lower. For further elucidating such low roll-offs of wet-
processed devices, we measured the phosphorescent lifetimes
of spin-coating films that blend fac-Ir(SFXpy)3 into mCP at
doping concentrations of 5, 10, and 20 wt % (Figure 11). The
shows yellow emission (λmax = 542 and 580 nm, respectively)
with a significantly shortened τp value of about 0.11 μs and
FMO energy levels (HOMO = −5.06 eV; LUMO = −2.75 eV)
similar to those of the commercial emitter PO-01. In dry-
processed devices using the common CBP host material and
varying doping levels of fac-Ir(SFXpy)3, the devices with high
doping levels exhibit excellent device performance. The peak
efficiencies of vacuum-deposited devices are 46.2 cd A−1, 36.3
lm W−1, and 12.1% for CE, PE, and EQE, respectively, at a
doping concentration of 15 wt % with a 20-nm-thick EML. The
devices kept high efficiencies and low roll-offs within the
doping concentration range between 8 and 15 wt %.
Furthermore, the wet-processed devices also exhibited excellent
performance, with maximum efficiencies of 32.2 cd A−1, 22.1 lm
W−1, and 11.3% for CE, PE, and EQE, respectively, at a doping
concentration of 8 wt %, as well as a very low EQE roll-off of
1.7%. Compared with the control device using PO-01 as the
emitter, fac-Ir(SFXpy)3-based devices show a superior perform-
ance as yellow-emitting PhOLEDs. The EL performance
studies of both dry- and wet-processed devices indicate that
the SFXpy ligand endows fac-Ir(SFXpy)3 with good amorphous
stability and effective suppression of the concentration
quenching while facilitating charge-carrier injection/transport
in the devices. The research illustrates that SFX is a versatile
module for building multifunctional cyclometalated emitters,
whose unique structural features incorporating steric consid-
erations may open the door to a more informed search for
better-performing PhOLED materials.
EXPERIMENTAL SECTION
■
General Information. The solvents were dried using standard
procedures, and all other reagents were used as received from
commercial sources, unless otherwise stated. NMR spectra were
recorded on a Bruker Avance500 II. The 1H NMR chemical shifts (δ)
of the signals were given in ppm and referenced to residual protons in
the deuterated solvents: chloroform-d1 (7.26 ppm). Mass spectrometry
was recorded on an AB SCIEX TF4800 matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) spectrometer.
UV−vis−near-IR (NIR) spectra were recorded using a UV-3150
Plus UV−vis−NIR spectrophotometer. PL spectra were recorded
using a RF-5301PC spectrofluorophotometer. Absolute PLQYs
measured in CH2Cl2 were recorded on a FLS920 spectrometer with
a xenon light source (450 W) through the Edinburgh Instruments
integrating sphere. The integrating sphere is 150 mm in diameter and
has its inner surface coated with barium sulfate (BaSO4). The excited-
state lifetime data were analyzed using F900 software by minimizing
the reduced function (χ2) and visual inspection of the weighted
residuals. CV was performed using a 273A potentiostat (Princeton
Applied Research), wherein glassy carbon, platinum, and a silver wire
act as the working, counter, and pseudoreference electrode,
respectively. Samples were prepared in a CHCl3 solution with
tetrabutylammonium hexafluorophosphate (0.1 M) as the electrolyte
at a scan rate of 100 mV s−1, using a ferrocene/ferrocenium (Fc/Fc+)
redox couple as an internal standard. The HOMO and LUMO energy
levels (eV) of the two compounds are calculated according to the
Figure 11. Phosphorescent lifetime (excited at 365 nm) curves under
298 K for spin-coating films blended fac-Ir(SFXpy)3 with mCP at
doping concentrations of 5, 10, and 20 wt %.
corresponding lifetimes are 5.54, 5.55, and 5.74 μs, respectively.
Additionally, the lifetime of fac-Ir(SFXpy)3 in a neat film is 672
ns under the same measurement conditions (Figure S14). The
transient lifetime dependence on the average distance between
iridium(III) complexes (R) and the concentration-quenching
rate constant are shown to be dependent on R−6.56 Therefore,
the tiny change of the transient lifetimes of solid film samples
with different doping concentrations indicates that a spiro-
functionalized SFXpy ligand effectively weakened the inter-
molecular interaction of fac-Ir(SFXpy)3 emitters. These results
collectively demonstrate that the SFX-based ligand endows the
fac-Ir(SFXpy)3 emitter with good solution processability while
suppressing the concentration quenching by evenly distributing
the IrIII dopant in the EML.
+
formula −[4.8 eV + Eox/red (vs EFc/Fc )]. DSC analyses were performed
on a Shimadzu DSC-60A instrument. TGA was conducted on a
Shimadzu DTG-60H thermogravimetric analyzer under a heating rate
of 10 °C min−1 and a nitrogen flow rate of 50 cm3 min−1.
CONCLUSIONS
■
2-BrSFX. A mixture of 2-bromo-9H-fluoren-9-one (2.00 g, 7.70
mmol, 1 equiv), phenol (4.32 g, 46.3 mmol, 6 equiv), and
methanesulfonic acid (2.0 mL, 2.96 g, 30.8 mmol, 4 equiv) was
heated at 150 °C under nitrogen for 12 h. The reaction mixture was
then slowly added into water (25 mL) and extracted with
dichloromethane. The combined extracts were dried over MgSO4,
evaporated, and purified by column chromatography to afford the
In summary, a SFX-based ligand, SFXpy, was incorporated in a
robust homoleptic iridium(III) complex, fac-Ir(SFXpy)3,
following simple synthetic procedures. Single-crystal X-ray
analysis confirmed the facial configuration, elongated Ir···Ir
distance (no less than 11 Å), and negligible intermolecular π−π
interactions of adjacent ligands in the crystal. The complex
H
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
product as a colorless solid (2.45 g, 77%). 1H NMR (500 MHz,
CDCl3): δ 7.77 (d, J = 7.6 Hz, 1H), 7.66 (d, J = 8.1 Hz, 1H), 7.49 (dd,
J = 8.1 and 1.8 Hz, 1H), 7.38 (t, J = 7.5 Hz, 1H), 7.28 (d, J = 1.7 Hz,
1H), 7.25−7.14 (m, 6H), 6.80 (ddd, J = 8.2, 6.4, and 2.1 Hz, 2H), 6.39
(d, J = 7.5 Hz, 2H).
The effect of van der Waals interaction was included by using the D3
correction scheme of Grimme.59
OLED Fabrication and Measurements. Dry-Processed
OLEDs. All of the devices were fabricated onto prepatterned
indium−tin oxide (ITO) with a sheet resistance of 10 Ω □−1. The
substrates were cleaned sequentially with detergent, acetone, ethanol,
and deionized water under sonication, then dried in an oven, and
finally treated in an ultraviolet-ozone chamber. After organic
deposition, 100-nm-thick aluminum was covered directly on the
organic layer serving as the cathode. The cathode area defines an active
device area of 2 × 2 mm2 through a shadow mask. Current−voltage−
luminance (J−V−L) characteristics were measured with a PR650
spectrascan spectrometer and a Keithley 2400 programmable voltage−
current source. EQE was determined following a reported method.60
All of the measurements were carried out in the ambient environment
without encapsulation.
Wet-Processed OLEDs. A PEDOT:PSS layer was spin-coated
directly onto the precleaned ITO glass at a spin speed of 2000 rpm for
60 s and dried at 120 °C for 20 min under vacuum conditions. fac-
Ir(SFXpy)3 and the host material were dissolved separately in
chlorobenzene, from which different weight ratios of fac-Ir(SFXpy)3/
hosts were prepared. The emissive layer was spin-coated onto the top
of the PEDOT:PSS layer and annealed at 100 °C for 20 min to
remove residual solvents. TmPyPB, LiF, and the aluminum cathode
were vacuum-deposited in order under a pressure lower than 5 × 10−4
Pa. The thicknesses of the organic films were measured using a
spectroscopic ellipsometry (α-SE, J. A. Wollam Co. Inc.). The current
J−V−L characteristics were measured by a Keithley 2602 sourcemeter
with a calibrated silicon photodiode. The EL spectra of the devices
were analyzed with a PR655 spectrometer. All device characterizations
were carried out under ambient laboratory air at room temperature.
The emission area of the devices was 12 mm2.
2-BpinSFX. 2-BrSFX (1.50 g, 3.65 mmol, 1 equiv), bis(pinacolato)
diboron (1.02 g, 4.01 mmol, 1.1 equiv), 1,1′-bis-
(diphenylphosphinoferrocene) dichloropalladium(II) (148 mg 0.18
mmol, 0.05 equiv), potassium acetate (713 mg, 7.30 mmol, 2.0 equiv),
and dimethyl sulfoxide (30 mL) were placed in a flask, and the mixture
was stirred for 5 h under a nitrogen atmosphere at 80 °C. After the
reaction mixture was cooled to room temperature, water was added,
the reaction mixture was transferred to a separation funnel, and the
mixture was extracted with chloroform. The organic layer was
concentrated under reduced pressure, and the concentrate was
purified by silica gel column chromatography (hexanes/chloroform)
1
to afford 2-BpinSFX (0.98 g, 59%). H NMR (500 MHz, CDCl3): δ
7.89 (d, J = 7.6 Hz, 1H), 7.83 (d, J = 7.9 Hz, 2H), 7.60 (s, 1H), 7.37
(d, J = 7.5 Hz, 1H), 7.25−7.15 (m, 6H), 6.77 (t, J = 8.3 Hz, 2H), 6.39
(d, J = 7.9 Hz, 2H), 1.31 (s, 12H).
SFXpy. A mixture of 2-BpinSFX (0.98 g, 2.14 mmol, 1 equiv), 2-
bromopyridine (0.37 g, 2.35 mmol, 1.1 equiv), Pd(PPh3)4 (136 mg,
0.12 mmol, 0.05 equiv), K2CO3 (2.0 M aqueous solution, 2.5 mL), and
toluene (15 mL)/tetrahydrofuran (15 mL) was stirred at 90 °C for 24
h. After it was cooled to room temperature, 50 mL of CHCl3 was
added to the reaction mixture. The organic portion was separated and
washed with brine before drying over anhydrous MgSO4. The solvent
was evaporated off, and the solid residue was purified by column
chromatography on silica gel to afford SFXpy as a white solid (0.71 g,
1
81%). H NMR (500 MHz, CDCl3): δ 8.60 (d, J = 4.6 Hz, 1H), 8.10
(d, J = 8.0 Hz, 1H), 7.91 (d, J = 8.0 Hz, 1H), 7.83 (d, J = 7.6 Hz, 1H),
7.77 (s, 1H), 7.68−7.60 (m, 2H), 7.38 (t, J = 7.4 Hz, 1H), 7.24−7.13
(m, 7H), 6.77 (t, J = 7.4 Hz, 2H), 6.45 (d, J = 9.3 Hz, 2H). 13C NMR
(125 MHz, CDCl3): δ 151.47, 149.65, 136.74, 128.81, 128.58, 127.91,
127.12, 125.84, 124.70, 124.37, 123.47, 122.16, 120.77, 120.39, 116.93,
54.45.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01034.
fac-Ir(SFXpy)3. Ir(acac)3 (720 mg, 1.50 mmol) and 2-pyridinyl-
9,9′-spirobifluorene (2.04 g, 5.18 mmol) were mixed in glycerol (135
mL) under a nitrogen atmosphere. The reaction mixture was heated at
220 °C for 18 h, after which the mixture was cooled to room
temperature and water (150 mL) was added. The resulting mixture
was extracted with CH2Cl2 (3 × 100 mL), and the organic phase was
dried over MgSO4. Upon solvent removal under vacuum, the residue
was purified by column chromatography using CH2Cl2 as the eluent to
afford the title compound as an orange solid (761.8 mg, yield 35%).
1H NMR (500 Hz, CDCl3): δ 7.74 (d, J = 8.3 Hz, 3H), 7.61 (d, J = 5.2
Hz, 3H), 7.51−7.47 (m, 6H), 7.36 (d, J = 7.6 Hz, 3H), 7.24−7.21 (m,
12H), 7.17 (dd, J = 13.4 and 6.3 Hz, 6H), 7.10 (d, J = 6.2 Hz, 6H),
7.03−6.98 (m, 3H), 6.81 (dd, J = 13.6 and 7.0 Hz, 6H), 6.71 (t, J = 7.4
Hz, 3H), 6.46 (d, J = 7.8 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ
166.43, 161.87, 156.41, 151.70, 151.46, 148.00, 147.22, 144.48, 140.24,
135.82, 128.80, 128.29, 127.89, 126.74, 126.05, 125.35, 123.23, 121.79,
121.32, 120.68, 119.60, 116.69, 53.83. MS (MALDI-TOF): m/z
1417.230 ([M+]).
X-ray Crystallography. X-ray diffraction data were obtained from
a Bruker Smart-1000 CCD diffractometer using graphite-monochro-
mated Mo Kα (λ = 0.71073 Å) radiation at 295 K. The data were
collected using the ω/2θ scan mode and corrected for Lorentz and
polarization effects as well as absorption during data reduction using
SHELXL-97 software. CCDC 1482495 and 1522839 contain the
supplementary crystallographic data of fac-Ir(SFXpy)3 and SFXpy,
respectively. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
NMR, MS, TGA, and DSC characterization, crystal data
of SFXpy and fac-Ir(SFXpy)3, and the EL performance of
the devices (PDF)
Accession Codes
CCDC 1482495 and 1522839 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: sunyaguang@syuct.edu.cn.
*E-mail: ccnusbli@mail.ccnu.edu.cn.
*E-mail: yliu@lbl.gov.
ORCID
Bao-Yi Ren: 0000-0001-5879-3291
Ya-Guang Sun: 0000-0001-5850-0938
Yi Liu: 0000-0002-3954-6102
Wei Huang: 0000-0001-7004-6408
Computational Details. The calculations were performed using
the Vienna ab initio simulation package (VASP).57 The generalized
gradient approximation of Perdew−Burke−Ernzerhof58 was adopted
for the exchange-correlation functional. The energy cutoff for plane-
wave expansion was set to 400 eV. All atoms ware allowed relax until
the atomic Hellmann−Feynman forces were smaller than 0.02 eV Å−1.
Author Contributions
⊥These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
I
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(17) Kim, D. H.; Cho, N. S.; Oh, H.-Y.; Yang, J. H.; Jeon, W. S.; Park,
J. S.; Suh, M. C.; Kwon, J. H. Highly efficient red phosphorescent
dopants in organic light-emitting devices. Adv. Mater. 2011, 23, 2721−
2726.
(18) Jung, S. O.; Zhao, Q.; Park, J.-W.; Kim, S. O.; Kim, Y.-H.; Oh,
H.-Y.; Kim, J.; Kwon, S.-K.; Kang, Y. A green emitting iridium(III)
complex with narrow emission band and its application to
phosphorescence organic light-emitting diodes (OLEDs). Org.
Electron. 2009, 10, 1066−1073.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (Grant 21501122), and part of
the work was carried out as a user project in the Molecular
Foundry, a user facility supported through the Office of
Science, Office of Basic Energy Sciences, U.S. Department of
Energy, under Contract DE-AC02-05CH11231.
(19) Kang, D. M.; Kang, J.-W.; Park, J. W.; Jung, S. O.; Lee, S.-H.;
Park, H.-D.; Kim, Y.-H.; Shin, S. C.; Kim, J.-J.; Kwon, S.-K. Iridium
complexes with cyclometalated 2-cycloalkenyl-pyridine ligands as
highly efficient emitters for organic light-emitting diodes. Adv. Mater.
2008, 20, 2003−2007.
(20) Liu, Y.; Ye, K.; Fan, Y.; Song, W.; Wang, Y.; Hou, Z. Amidinate-
ligated iridium(III) bis(2-pyridyl)phenyl complex as an excellent
phosphorescent material for electroluminescence devices. Chem.
Commun. 2009, 3699−3701.
(21) Wong, W.-Y.; Ho, C.-L.; Gao, Z.-Q.; Mi, B.-X.; Chen, C.-H.;
Cheah, K.-W.; Lin, Z. Multifunctional iridium complexes based on
carbazole modules as highly efficient electrophosphors. Angew. Chem.,
Int. Ed. 2006, 45, 7800−7803.
(22) Tong, B.; Mei, Q.; Wang, S.; Fang, Y.; Meng, Y.; Wang, B.
Nearly 100% internal phosphorescence efficiency in a polymer light-
emitting diode using a new iridium complex phosphor. J. Mater. Chem.
2008, 18, 1636−1639.
(23) Zou, J.; Wu, H.; Lam, C.-S.; Wang, C.; Zhu, J.; Zhong, C.; Hu,
S.; Ho, C.-L.; Zhou, G.-J.; Wu, H.; Choy, W. C. H.; Peng, J.; Cao, Y.;
Wong, W.-Y. Simultaneous optimization of charge-carrier balance and
luminous efficacy in highly efficient white polymer light-emitting
devices. Adv. Mater. 2011, 23, 2976−2980.
(24) Wong, W. Y.; Zhou, G. J.; Yu, X. M.; Kwok, H. S.; Tang, B. Z.
Amorphous diphenylaminofluorene-functionalized iridium complexes
for high-efficiency electrophosphorescent light-emitting diodes. Adv.
Funct. Mater. 2006, 16, 838−846.
(25) Wu, H.; Zhou, G.; Zou, J.; Ho, C.-L.; Wong, W.-Y.; Yang, W.;
Peng, J.; Cao, Y. Efficient polymer white-light-emitting devices for
solid-state lighting. Adv. Mater. 2009, 21, 4181−4184.
REFERENCES
■
(1) Yang, X.; Zhou, G.; Wong, W.-Y. Functionalization of
phosphorescent emitters and their host materials by main-group
elements for phosphorescent organic light-emitting devices. Chem. Soc.
Rev. 2015, 44, 8484−8575.
(2) Wong, W.-Y.; Ho, C.-L. Heavy metal organometallic electro-
phosphors derived from multi-component chromophores. Coord.
Chem. Rev. 2009, 253, 1709−1758.
(3) Lee, J.; Chen, H.-F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.;
Thompson, M. E.; Forrest, S. R. Deep blue phosphorescent organic
light-emitting diodes with very high brightness and efficiency. Nat.
Mater. 2015, 15, 92−98.
(4) Chi, Y.; Chou, P.-T. Transition-metal phosphors with cyclo-
metalating ligands: fundamentals and applications. Chem. Soc. Rev.
2010, 39, 638−655.
(5) Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard,
W. A.; Thompson, M. E. Temperature Dependence of Blue
Phosphorescent Cyclometalated Ir(III) Complexes. J. Am. Chem. Soc.
2009, 131, 9813−9822.
(6) Ying, L.; Ho, C.-L.; Wu, H.; Cao, Y.; Wong, W.-Y. White Polymer
Light-Emitting Devices for Solid-State Lighting: Materials, Devices,
and Recent Progress. Adv. Mater. 2014, 26, 2459−2473.
(7) Tang, K.-C.; Liu, K. L.; Chen, I. C. Rapid intersystem crossing in
highly phosphorescent iridium complexes. Chem. Phys. Lett. 2004, 386,
437−441.
(8) Takayasu, S.; Suzuki, T.; Shinozaki, K. Intermolecular interactions
and aggregation of fac-tris(2-phenylpyridinato-C2,N)iridium(III) in
nonpolar solvents. Phys. Rev. B 2013, 117, 9449−9456.
(26) Zhou, G.-J.; Wong, W.-Y.; Yao, B.; Xie, Z.; Wang, L.
Multifunctional metallophosphors with anti-triplet-triplet annihilation
properties for solution-processable electroluminescent devices. J.
Mater. Chem. 2008, 18, 1799−1809.
(9) Wehrmeister, S.; Jager, L.; Wehlus, T.; Rausch, A. F.; Reusch, T.
̈
C. G.; Schmidt, T. D.; Brutting, W. Combined electrical and optical
analysis of the efficiency roll-off in Phosphorescent organic light-
emitting diodes. Phys. Rev. Appl. 2015, 3, 024008.
̈
(27) Zhou, G.-J.; Wang, X.-Z.; Wong, W.-Y.; Yu, X.-M.; Kwok, H.-S.;
Lin, Z. New platinum(II) complexes as triplet emitters for high-
efficiency monochromatic pure orange electroluminescent devices. J.
Organomet. Chem. 2007, 692, 3461−3473.
(28) Xie, L.-H.; Zhu, R.; Qian, Y.; Liu, R.-R.; Chen, S.-F.; Lin, J.;
Huang, W. Spiro-functionalized ligand with supramolecular steric
hindrance to control π-π interaction in the iridium complex for high-
performance electrophosphorescent devices. J. Phys. Chem. Lett. 2010,
1, 272−276.
(29) Yao, J. H.; Zhen, C.; Loh, K. P.; Chen, Z.-K. Novel iridium
complexes as high-efficiency yellow and red phosphorescent light
emitters for organic light-emitting diodes. Tetrahedron 2008, 64,
10814−10820.
(30) Xie, L.-H.; Liu, F.; Tang, C.; Hou, X.-Y.; Hua, Y.-R.; Fan, Q.-L.;
Huang, W. Unexpected one-pot method to synthesize spiro[fluorene-
9,9′-xanthene] building blocks for blue-light-emitting materials. Org.
Lett. 2006, 8, 2787−2790.
(31) Zhao, X.-H.; Xie, G.-H.; Liu, Z.-D.; Li, W.-J.; Yi, M.-d.; Xie, L.-
H.; Hu, C.-P.; Zhu, R.; Zhao, Q.; Zhao, Y.; Zhao, J.-F.; Qian, Y.;
Huang, W. A 3-dimensional spiro-functionalized platinum(II) complex
to suppress intermolecular π-π and Pt···Pt supramolecular interactions
for a high-performance electrophosphorescent device. Chem. Commun.
2012, 48, 3854−3856.
(32) Zhao, J.; Xie, G.-H.; Yin, C.-R.; Xie, L.-H.; Han, C.-M.; Chen,
R.-F.; Xu, H.; Yi, M.-D.; Deng, Z.-P.; Chen, S.-F.; Zhao, Y.; Liu, S.-Y.;
Huang, W. Harmonizing triplet level and ambipolar characteristics of
wide-gap phosphine oxide hosts toward highly efficient and low
(10) Baldo, M. A.; Adachi, C.; Forrest, S. R. Transient analysis of
organic electrophosphorescence. II. Transient analysis of triplet-triplet
annihilation. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62,
10967−10977.
(11) Zhou, G.; Wong, W.-Y.; Yao, B.; Xie, Z.; Wang, L.
Triphenylamine-dendronized pure red iridium phosphors with
superior OLED efficiency/color purity trade-offs. Angew. Chem., Int.
Ed. 2007, 46, 1149−1151.
(12) Xu, H.; Chen, R.; Sun, Q.; Lai, W.; Su, Q.; Huang, W.; Liu, X.
Recent progress in metal-organic complexes for optoelectronic
applications. Chem. Soc. Rev. 2014, 43, 3259−3302.
(13) Lo, S.-C.; Burn, P. L. Development of dendrimers: macro-
molecules for use in organic light-emitting diodes and solar cells.
Chem. Rev. 2007, 107, 1097−1116.
(14) Mroz, W.; Ragni, R.; Galeotti, F.; Mesto, E.; Botta, C.; Cola, L.
D.; Farinola, G. M.; Giovanella, U. Influence of electronic and steric
effects of substituted ligands coordinated to Ir(III) complexes on the
solution processed OLED properties. J. Mater. Chem. C 2015, 3,
7506−7512.
(15) You, Y.; Park, S. Y. Phosphorescent iridium(III) complexes:
toward high phosphorescence quantum efficiency through ligand
control. Dalton Trans. 2009, 1267−1282.
(16) Xie, H. Z.; Liu, M. W.; Wang, O. Y.; Zhang, X. H.; Lee, C. S.;
Hung, L. S.; Lee, S. T.; Teng, P. F.; Kwong, H. L.; Zheng, H.; Che, C.
M. Reduction of self-quenching effect in organic electrophosphor-
escence emitting devices via the use of sterically hindered spacers in
phosphorescence molecules. Adv. Mater. 2001, 13, 1245−1248.
J
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
driving voltage blue and green PHOLEDs: an effective strategy based
on spiro-systems. Chem. Mater. 2011, 23, 5331−5339.
(33) Fan, C.; Yang, C. Yellow/orange emissive heavy-metal
complexes as phosphors in monochromatic and white organic light-
emitting devices. Chem. Soc. Rev. 2014, 43, 6439−6469.
(34) Yang, X.; Xu, X.; Dang, J.-S.; Zhou, G.; Ho, C.-L.; Wong, W.-Y.
From mononuclear to dinuclear iridium(III) complex: effective tuning
of the optoelectronic characteristics for organic light-emitting diodes.
Inorg. Chem. 2016, 55, 1720−1727.
(48) Nazeeruddin, M. K.; Wegh, R. T.; Zhou, Z.; Klein, C.; Wang,
Q.; De Angelis, F.; Fantacci, S.; Gratzel, M. Efficient green-blue-light-
̈
emitting cationic iridium complex for light-emitting electrochemical
cells. Inorg. Chem. 2006, 45, 9245−9250.
(49) Kanno, H.; Holmes, R. J.; Sun, Y.; Kena-Cohen, S.; Forrest, S. R.
White stacked electrophosphorescent organic light-emitting devices
employing MoO3 as a charge-generation layer. Adv. Mater. 2006, 18,
339−342.
(50) Huang, S.-Y.; Meng, H.-F.; Huang, H.-L.; Chao, T.-C.; Tseng,
M.-R.; Chao, Y.-C.; Horng, S.-F. Uniform dispersion of triplet emitters
in multi-layer solution-processed organic light-emitting diodes. Synth.
Met. 2010, 160, 2393−2396.
(51) Wang, X.; Zhang, S.; Liu, Z.; Yue, S.; Zhang, Z.; Chen, Y.; Xie,
G.; Xue, Q.; Zhao, Y.; Liu, S. Hybrid white organic light-emitting
diodes with improved color stability and negligible efficiency roll-off
based on blue fluorescence and yellow phosphorescence. J. Lumin.
2013, 137, 59−63.
(52) Chen, X.; Liao, J.-L.; Liang, Y.; Ahmed, M. O.; Tseng, H.-E.;
Chen, S.-A. High-efficiency red-light emission from polyfluorenes
grafted with cyclometalated iridium complexes and charge transport
moiety. J. Am. Chem. Soc. 2003, 125, 636−637.
(53) Zhang, S. M.; Yue, S. Z.; Wu, Q. Y.; Zhang, Z. S.; Chen, Y.;
Wang, X. H.; Liu, Z. Y.; Xie, G. H.; Xue, Q.; Qu, D. L.; Zhao, Y.; Liu,
S. Y. Color stable multilayer all-phosphor white organic light-emitting
diodes with excellent color quality. Org. Electron. 2013, 14, 2014−
2022.
(35) Jou, J.-H.; Lin, Y.-X.; Peng, S.-H.; Li, C.-J.; Yang, Y.-M.; Chin,
C.-L.; Shyue, J.-J.; Sun, S.-S.; Lee, M.; Chen, C.-T.; Liu, M.-C.; Chen,
C.-C.; Chen, G.-Y.; Wu, J.-H.; Li, C.-H.; Sung, C.-F.; Lee, M.-J.; Hu, J.-
P. Highly efficient yellow organic light emitting diode with a novel
wet- and dry-process feasible iridium complex emitter. Adv. Funct.
Mater. 2014, 24, 555−562.
(36) Breu, J.; Stossel, P.; Schrader, S.; Starukhin, A.; Finkenzeller, W.
̈
J.; Yersin, H. Crystal structure of fac−Ir(ppy)3 and emission properties
under ambient conditions and at high pressure. Chem. Mater. 2005, 17,
1745−1752.
(37) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
Bau, R.; Thompson, M. E. Synthesis and characterization of
phosphorescent cyclometalated platinum complexes. Inorg. Chem.
2002, 41, 3055−3066.
(38) Coleman, A. W.; Eadie, D. T.; Stobart, S. R.; Zaworotko, M. J.;
Atwood, J. L. Pyrazolyl-bridged iridium dimers. 2. contrasting modes
of two-center oxidative addition to a bimetallic system and reductive
access to the starting complex: three key diiridium structures
representing short nonbonding and long and short bonding metal-
metal interactions. J. Am. Chem. Soc. 1982, 104, 922−923.
(39) Yang, X.; Feng, Z.; Zhao, J.; Dang, J.-S.; Liu, B.; Zhang, K.;
Zhou, G. Pyrimidine-based mononuclear and dinuclear iridium(III)
complexes for high performance organic light-emitting diodes. ACS
Appl. Mater. Interfaces 2016, 8, 33874−33887.
(54) Mu, Y.; Zhang, S.; Yue, S.; Wu, Q.; Zhao, Y. High efficiency
yellow organic light-emitting diodes with optimized barrier layers.
Solid-State Electron. 2015, 114, 87−89.
́
(55) Tonnele, C.; Stroet, M.; Caron, B.; Clulow, A. J.; Nagiri, R. C.
R.; Malde, A. K.; Burn, P. L.; Gentle, I. R.; Mark, A. E.; Powell, B. J.
Elucidating the spatial arrangement of emitter molecules in organic
light-emitting diode films. Angew. Chem. 2017, DOI: 10.1002/
ange.201610727.
(40) Mi, B. X.; Wang, P. F.; Gao, Z. Q.; Lee, C. S.; Lee, S. T.; Hong,
H. L.; Chen, X. M.; Wong, M. S.; Xia, P. F.; Cheah, K. W.; Chen, C.
(56) Kawamura, Y.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C.
Intermolecular interaction and a concentration-quenching mechanism
of phosphorescent Ir (III) complexes in a solid film. Phys. Rev. Lett.
2006, 96, 017404.
̂
H.; Huang, W. Strong luminescent iridium complexes with CNN
structure in ligands and their potential in efficient and thermally stable
phosphorescent OLEDs. Adv. Mater. 2009, 21, 339−343.
(41) Janiak, C. A critical account on π-π stacking in metal complexes
with aromatic nitrogen-containing ligands. J. Chem. Soc., Dalton Trans.
2000, 3885−3896.
(57) Kresse, G.; Furthmuller, J. Efficient iterative schemes for ab
̈
initio total-energy calculations using a plane-wave basis set. Phys. Rev.
B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186.
(58) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient
approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868.
(59) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and
accurate ab initio parametrization of density functional dispersion
correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010,
132, 154104.
(42) Huang, K.; Wu, H.; Shi, M.; Li, F.; Yi, T.; Huang, C. Reply to
comment on ’aggregation-induced phosphorescent emission (AIPE) of
iridium(III) complexes’: origin of the enhanced phosphorescence.
Chem. Commun. 2009, 1243−1245.
(43) Cui, L.-S.; Liu, Y.; Liu, X.-Y.; Jiang, Z.-Q.; Liao, L.-S. Design and
synthesis of pyrimidine-based Iridium(III) complexes with horizontal
orientation for orange and white phosphorescent OLEDs. ACS Appl.
Mater. Interfaces 2015, 7, 11007−11014.
(60) Meerheim, R.; Nitsche, R.; Leo, K. High-efficiency monochrome
organic light emitting diodes employing enhanced microcavities. Appl.
Phys. Lett. 2008, 93, 043310.
(44) Dedeian, K.; Djurovich, P. I.; Garces, F. O.; Carlson, G.; Watts,
R. J. A new synthetic route to the preparation of a series of strong
photoreducing agents: fac-tris-ortho-metalated complexes of iridium-
(III) with substituted 2-phenylpyridines. Inorg. Chem. 1991, 30, 1685−
1687.
(45) Ostrowski, J. C.; Robinson, M. R.; Heeger, A. J.; Bazan, G. C.
Amorphous iridium complexes for electrophosphorescent light
emitting devices. Chem. Commun. 2002, 784−785.
(46) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.;
Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada,
S.; Hoshino, M.; Ueno, K. Homoleptic cyclometalated iridium
complexes with highly efficient red phosphorescence and application
to organic light-emitting diode. J. Am. Chem. Soc. 2003, 125, 12971−
12979.
(47) Yu, X.-M.; Zhou, G.-J.; Lam, C.-S.; Wong, W.-Y.; Zhu, X.-L.;
Sun, J.-X.; Wong, M.; Kwok, H.-S. A yellow-emitting iridium complex
for use in phosphorescent multiple-emissive-layer white organic light-
emitting diodes with high color quality and efficiency. J. Organomet.
Chem. 2008, 693, 1518−1527.
K
DOI: 10.1021/acs.inorgchem.7b01034
Inorg. Chem. XXXX, XXX, XXX−XXX