Self-host homoleptic green iridium dendrimers based on diphenylamine dendrons for highly efficient single-layer PhOLEDs

Article abstract of DOI:10.1039/c3tc32024c

The fabrication of electroluminescent devices that combine high device performance with simple device configuration remains an attractive challenge due to their low cost and simple fabrication processes. In this paper, a new series of electrophosphorescent small molecule iridium(iii) complexes with diphenylamine-based dendrons of good solubility have been designed. The relationships between their dendritic structures and their photophysical, electrochemical, and electrophosphorescent performances have been systematically investigated. With second-generation dendrons, the photoluminescence quantum yields of the neat film of the dendrimers are almost seven times higher than that of their prototype G0 (Ir(LG0)₃, LG0 = 1-methyl-2-phenyl-1H- benzimidazole), and three times that of the first-generation dendron G1 (Ir(LG1)₃, LG1 = 4-(1-methyl-1H-benzimidazol-2-yl)-N,N- diphenylbenzenamine). High-quality films of the dendrimers G2 (Ir(LG2) ₃, LG2 = 1-methyl-2-[4-bis[4-(diphenylamino)phenyl]-aminophenyl]-1H- benzimidazole) and G2Cz (Ir(LG2Cz)₃, LG2Cz = 1-methyl-2-[4-bis[4-(9- carbazolyl)phenyl]-aminophenyl]-1H-benzimidazole) have been fabricated by spin-coating, producing highly efficient, non-doped phosphorescent organic light-emitting diodes (PhOLEDs). With a device structure of indium tin oxide/poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonic acid)/neat dendrimer/Cs₂CO₃/Al, maximum luminous efficiencies of 14.02 cd A^-1 and 18.35 cd A^-1 have been realized, exhibiting ultrahigh luminous efficiency for single-layer self-host green PhOLEDs. The excellent performances are due to the flower bouquet-shaped iridium dendrimers, which may improve the electron injection and result in greater balance between electron and hole fluxes by the exposure of electron-deficient moieties. The molecular design reported here provides a simple and effective approach to balance charge injection/transporting capacities and develops highly efficient non-doped phosphors suitable for low-cost single-layer device technologies.

Full text of DOI:10.1039/c3tc32024c

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Materials Chemistry C
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PAPER
Self-host homoleptic green iridium dendrimers
based on diphenylamine dendrons for highly
efficient single-layer PhOLEDs†
a
Cite this: J. Mater. Chem. C, 2014, 2,
1104
Wenwen Tian,a Chang Yi,a Bo Song,a Qi Qi,ab Wei Jiang, Yingping Zheng,a
*
a
ab
*
Zhengjian Qi and Yueming Sun
The fabrication of electroluminescent devices that combine high device performance with simple device
configuration remains an attractive challenge due to their low cost and simple fabrication processes. In
this paper,
a new series of electrophosphorescent small molecule iridium(III) complexes with
diphenylamine-based dendrons of good solubility have been designed. The relationships between their
dendritic structures and their photophysical, electrochemical, and electrophosphorescent performances
have been systematically investigated. With second-generation dendrons, the photoluminescence
quantum yields of the neat film of the dendrimers are almost seven times higher than that of their
prototype G0 (Ir(LG0)3, LG0 ¼ 1-methyl-2-phenyl-1H-benzimidazole), and three times that of the first-
generation dendron G1 (Ir(LG1)3, LG1 ¼ 4-(1-methyl-1H-benzimidazol-2-yl)-N,N-diphenylbenzenamine).
High-quality films of the dendrimers G2 (Ir(LG2)3, LG2 ¼ 1-methyl-2-[4-bis[4-(diphenylamino)phenyl]-
aminophenyl]-1H-benzimidazole) and G2Cz (Ir(LG2Cz)3, LG2Cz ¼ 1-methyl-2-[4-bis[4-(9-carbazolyl)
phenyl]-aminophenyl]-1H-benzimidazole) have been fabricated by spin-coating, producing highly
efficient, non-doped phosphorescent organic light-emitting diodes (PhOLEDs). With a device structure
of indium tin oxide/poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonic acid)/neat dendrimer/
Cs2CO3/Al, maximum luminous efficiencies of 14.02 cd Aꢀ1 and 18.35 cd Aꢀ1 have been realized,
exhibiting ultrahigh luminous efficiency for single-layer self-host green PhOLEDs. The excellent
performances are due to the flower bouquet-shaped iridium dendrimers, which may improve the
electron injection and result in greater balance between electron and hole fluxes by the exposure of
electron-deficient moieties. The molecular design reported here provides a simple and effective
approach to balance charge injection/transporting capacities and develops highly efficient non-doped
phosphors suitable for low-cost single-layer device technologies.
Received 16th October 2013
Accepted 9th November 2013
DOI: 10.1039/c3tc32024c
www.rsc.org/MaterialsC
are more promising with respect to the reduction of fabrication
cost as well as the realization of large-area displays. Particu-
Introduction
larly, as the intermiscibility of the interfaces between the upper
and lower layers is considered to be a serious problem in
multilayered solution-processed devices, it would be more
advantageous if both carrier-transporting materials and emis-
sive guests could be integrated into only one active layer.16–20 In
view of this, most single-layer devices have been developed
containing both hole-transporting and electron-transporting
materials as dopants, such as poly(N-vinylcarbazole) (PVK) and
Phosphorescent organic light-emitting diodes (PhOLEDs) that
contain transition-metal complexes, especially iridium(III)
complexes, have attracted much attention in recent years
because they can fully utilize both singlet and triplet excitons
to realize a theoretical internal quantum efficiency of 100%.1–11
However, most of the high performance PhOLEDs have
multilayered device structures fabricated via sequential
vacuum deposition of small molecules with different func-
tions, which is time-consuming and costly.12–15 In contrast to
thermal evaporation methods, solution-processed techniques
2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD)
used in the literature.19,21–24 Dispersion of the Ir3+ complex in
these host materials not only contributes to charge transport,
but also separates the phosphors and avoids self-quenching.
Although this technique is usually effective, these doping
systems were inferior in phase stability, processability, and
repeatability. To overcome these problems, non-doped devices
should be fabricated, where the phosphor is utilized as
the emitting layer independently.25–32 Therefore, it is highly
aSchool of Chemistry and Chemical Engineering, Southeast University, Nanjing,
Jiangsu, 211189, P. R. China. E-mail: 101011462@seu.edu.cn; sun@seu.edu.cn
bState Key Lab of Silicon Materials, Zhejiang University, Hangzhou, Zhejiang, 310027,
P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc32024c
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desirable to develop such self-host phosphors for single-layer comparison. The CzDPA dendritic Ir(III) complex G2Cz was also
devices. designed to study the effect of increased molecular rigidity as
Dendrimers are three-dimensional macromolecules which well as maintaining good solubility. The devices from G2 and
possess the advantages of the well-dened structures of small G2Cz showed maximum luminance efficiencies of 14.02 cd Aꢀ1
molecules together with the good solution processibility of and 18.35 cd Aꢀ1 for single-layer self-host devices respectively.
polymers.33 Moreover, the periphery dendrons can effectively To the best of our knowledge, the EL efficiencies enabled by G2
reduce the strong concentration quenching resulting from the and G2Cz are the highest ever reported for single-layer self-host
interactions of emitting cores. Recently, Burn and co-workers green emissive devices fabricated by spin-coating. This work
rst developed a class of phosphorescent dendrimers composed should encourage further extensive investigations into the
of an Ir(III) core, meta-bonded phenylene dendrons, and 2-eth- importance of the location of the substituents on the ligands of
ylhexyloxy surface groups for good solubility.34,35 However, this phosphorescent complexes regarding device characteristics and
usually causes signicant reduction in charge mobility.36,37 performance.
Thus, to achieve better charge transport, carbazole-based and
triphenylamine-based dendrons are introduced to iridium
Experimental
General information
complexes.38–48 In addition, bipolar iridium dendrimers have
been developed by introducing electron-rich and electron-de-
cient moieties to the molecules.49–51 Unfortunately, despite good
performances in multilayered PhOLEDs with these iridium
dendrimers as emissive materials, poor performances have
been observed for single-layered ones.38,49 The low cost and
simple fabrication of electrophosphorescent devices that
combine high device performance with simple device congu-
ration remains an attractive challenge.
1H-NMR spectra were measured on a BRUKER AMX 300- or
500 MHz instrument with tetramethylsilane as the internal
standard. Molecular masses were determined by electrospray
ionization-mass spectrometry (ESI-MS) using a FINNIGAN LCQ
instrument, or matrix-assisted laser desorption-ionization time-
of-ight mass spectrometry (MALDI-TOF-MS) using a BRUKER
DALTONICS instrument, with a-cyano-hydroxycinnamic acid as
a matrix. Elemental analyses were performed on a Vario EL III
elemental analyzer. Absorption and photoluminescence emis-
sion spectra of the target compound were measured using a
SHIMADZU UV-2450 spectrophotometer and a HORIBA FLUO-
ROMAX-4 spectrophotometer, respectively. The solution PL
quantum efficiency was measured by a relative method using
(ppy)2Ir(acac) (Fp ¼ 0.34 in 2-methyltetrahydrofuran) as the
standard in degassed DMSO solutions. The lm PL quantum
efficiency was measured with an integrating sphere under an
excitation wavelength of 370 nm. Phosphorescence spectra at
77 K were measured in a dichloromethane solvent. Phospho-
rescence lifetime measurements were performed on a HORIBA
FM-4P-TCSPC uorescence spectrometer, with a pulsed 390 nm
NANOLED source in degassed DMSO solutions. Thermogravi-
metric analysis (TGA) was performed on a NETZSCH STA449-F3
thermal analyzer under nitrogen atmosphere at a heating rate of
10 ꢁC minꢀ1. Cyclic voltammetry (CV) was performed on a
CHI750C voltammetric analyzer in CH2Cl2 solutions at a scan
rate of 100 mV sꢀ1 with a glassy carbon rod as the working
electrode, a silver wire as the pseudo-reference electrode, and a
platinum wire as the counter electrode. The supporting elec-
trolyte was tetrabutylammonium perchlorate (0.1 M) and
ferrocene was selected to determine the potential of the silver
wire electrode (0.14 V vs. saturated calomel electrode (SCE)).
The solutions were bubbled with a constant nitrogen ow for
15 min before measurements. X-ray diffraction (XRD) was
performed on a BRUKER D8-DISCOVER diffractometer using
Cu Ka radiation.
Hence, herein, in searching for electrophosphorescent
iridium complexes with good balanced electron and hole
transporting capacities for single-layer devices, we report the
synthesis and electroluminescent properties of new, green
emitting iridium(III) complexes with diphenylamine-based
dendritic ligands up to the second generation. The main
advantage of this design can be summarized in several points.
Firstly, the high triplet energy of these dendrons can prevent
back energy transfer from the emissive Ir core to peripheral
dendrons.43,52 Secondly, the complex is surrounded by a
branched shell to prevent self-aggregation or concentration
quenching of the emissive core in the solid state. Thirdly, the
diphenylamine-based dendrons, which display stronger elec-
tron donating abilities than their triphenylamine-based and the
carbazole-based counterparts, are ideal electron donors with low
aggregation tendency, high carrier mobility, and high thermal
and photochemical stability.53 Additionally, the dendritic
diphenylamine compounds have better solubility in common
organic solvents relative to carbazole-based dendrons as a result
of increased exibility.54 Finally, by introducing the dendrons
into the para-site of the phenyl segment of the 2-phenyl-
benzimidazole ligand, ower bouquet-shaped iridium den-
drimers have been obtained, which place the benzimidazole
moieties on the surface rather than trapped in the center of the
molecule. Since the electron clouds of the LUMO levels of den-
drimers are largely located on the phenylbenzimidazole moie-
ties according to theoretical calculations,40 the electron injection
(EI) characteristics could be greatly improved because of the
reduced barrier for the injection of electrons. Coupled with the
good hole injection properties afforded by the dendrons, good
EI characteristics can ensure excellent EL performance.55
Quantum chemical calculations
In this work, a dendritic phenyl benzimidazole ligand LG2 The geometric optimization of the ground state of the
and its encapsulated homoleptic Ir(III) complex G2 have been complexes was carried out using the Becke–Lee–Yang–Parr
designed and synthesized, along with G0 and G1 for composite exchange correlation functional (B3LYP) method.
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“Double-x” quality basis sets were employed for the C, H and N d6): 8.02 (d, J ¼ 9.0 Hz, 2H), 7.53 (s, 2H), 7.29 (t, J ¼ 8.0 Hz, 8H),
(6-31G) and the Ir (LANL2DZ).56 An effective core potential (ECP) 7.15–6.98 (m, 24H).
replaces the inner core electrons of Ir leaving the outer core
(5s)2(5p)6 electrons and the (5d)6 valence electrons of Ir(III). All
2-[4-Bis[4-(9H-carbazol-9-yl)phenyl]aminophenyl]-1H-
of the calculations were accomplished with the Gaussian 03
soware package in vacuo without any constrained symmetry.
The molecular orbitals were visualized using Gaussview.
benzimidazole (8)
This compound was prepared according to the procedure for
the synthesis of (3), with a yield of 47%. 1H NMR (300 MHz,
DMSO-d6): 12.83 (s, 1H), 8.27–8.20 (m, 6H), 7.66 (d, J ¼ 8.1 Hz,
5H), 7.52–7.41 (m, 15H), 7.32–7.29 (m, 4H), 7.20–7.19 (m, 2H).
Device fabrication and performance measurements
In a general procedure, indium tin oxide (ITO)-coated glass
substrates were precleaned carefully and treated by UV ozone
for 4 min. A 40 nm poly(3,4-ethylenedioxythiophene) doped
with poly(styrene-4-sulfonate) (PEDOT:PSS) aqueous solution
was spin coated onto the ITO substrate and baked at 210 ꢁC for
10 min. The substrates were then taken into a nitrogen glove
box, where the emitting layer (100 nm) was spin coated onto the
PEDOT:PSS layer from 1,2-dichloroethane solution and
4-(1-Methyl-1H-benzimidazol-2-yl)-N,N-diphenylbenzenamine
(LG1)
4-(1H-Benzimidazol-2-yl)-N,N-diphenylbenzenamine (3.61 g,
10.0 mmol) was dissolved in DMF (80 mL) containing potas-
sium carbonate (3.45 g, 25.0 mmol). The mixture was cooled in
an ice bath while iodomethane (1.70 g, 12.0 mmol) was added in
a dropwise manner. The reaction was allowed to return to room
temperature and stirred for an additional 12 h at which time the
reaction mixture was ltered to remove the potassium
carbonate. The DMF was removed under reduced pressure and
the residue in 100 mL chloroform was washed with water and
dried over anhydrous sodium sulfate. Aer the solvent had been
removed, the residue was puried by column chromatography
on silica gel with petroleum/ethyl acetate (5 : 1) as the eluent to
ꢁ
annealed at 120 C for 30 min. The substrate was then trans-
ferred into an evaporation chamber, where the Cs2CO3/Al
bilayer cathode was evaporated at evaporation rates of 0.2 and
10 A sꢀ1 for Cs2CO3 and Al, respectively, under a pressure of 1 ꢂ
˚
10ꢀ3 Pa. The current–voltage–brightness characteristics of the
devices were characterized with a Keithley 4200 semiconductor
characterization system. The electroluminescent spectra were
collected with a Photo Research PR705 Spectrophotometer. All
measurements of the devices were carried out in an ambient
atmosphere without further encapsulation.
1
give the product in a yield of 72% (2.70 g). H NMR (300 MHz,
DMSO-d6): 7.76 (d, J ¼ 8.7 Hz, 2H), 7.63 (d, J ¼ 7.8 Hz, 1H), 7.57
(d, J ¼ 7.8 Hz, 1H), 7.38 (t, J ¼ 7.8 Hz, 4H), 7.26–7.13 (m, 8H),
7.05 (d, J ¼ 8.7 Hz, 2H), 3.88 (s, 3H). MS (ESI): m/z 376.2 [M + H]+.
Materials
1-Methyl-2-[4-bis[4-(diphenylamino)phenyl]aminophenyl]-1H-
benzimidazole (LG2)
All reagents were used as purchased without further purication.
All manipulations involving air-sensitive reagents were per-
formed under a dry nitrogen atmosphere. 1-Methyl-2-phenyl-1H-
benzimidazole (LG0),57 4-(diphenylamino)benzaldehyde (2),58
4-[bis[4-(diphenylamino)phenyl] amino]benzaldehyde (5),59 and
4-[bis[4-(9H-carbazol-9-yl)phenyl]amino]benzaldehyde (6)59 were
prepared according to literature procedures.
This compound was prepared according to the procedure for
1
the synthesis of LG1, with a yield of 76%. H NMR (300 MHz,
DMSO-d6): 7.73 (d, J ¼ 8.1 Hz, 2H), 7.61 (d, J ¼ 7.8 Hz, 1H), 7.55
(d, J ¼ 7.5 Hz, 1H), 7.32–7.17 (m, 11H), 7.12–6.99 (m, 21H), 3.86
(s, 3H). MS (ESI): m/z 710.4 [M + H]+.
4-(1H-Benzimidazol-2-yl)-N,N-diphenylbenzenamine (3)
1-Methyl-2-[4-bis[4-(9H-carbazol-9-yl)phenyl]aminophenyl]-
1H-benzimidazole (LG2Cz)
A
mixture of 1,2-diaminobenzene (1.30 g, 12.0 mmol),
4-(diphenylamino)benzaldehyde (2.73 g, 10.0 mmol), ammo-
nium chloride (0.54 g, 10.0 mmol), and DMF (40 mL) was heated
at 80 ꢁC for 12 h. Aer cooling to room temperature, the mixture
was poured into water for extraction with ethyl acetate. The
organic extracts were washed with water and dried over anhy-
drous sodium sulfate. Aer the solvent had been removed, the
residue was puried by column chromatography on silica gel
with petroleum/ethyl acetate (5 : 1) as the eluent to give the
This compound was prepared according to the procedure for
the synthesis of LG1, with a yield of 84%. H NMR (300 MHz,
DMSO-d6): 8.26 (d, J ¼ 7.5 Hz, 4H), 7.93 (d, J ¼ 8.7 Hz, 2H), 7.69–
7.61 (m, 6H), 7.55–7.41 (m, 14H), 7.33–7.27 (m, 6H), 3.95 (s, 3H).
MS (ESI): m/z 706.3 [M + H]+.
1
Dendrimer G0
A mixture of 1-methyl-2-phenyl-1H-benzimidazole (1.1 mmol),
iridium chloride trihydrate (0.17 g, 0.5 mmol), 2-ethoxyethanol
(10 mL), and water (3 mL) was reuxed under nitrogen for 24 h.
The precipitate was collected by ltration, washed with water
and ethanol, and then vacuum dried. A mixture of the chloro-
bridged dimer, 1-methyl-2-phenyl-1H-benzimidazole (0.6
mmol), and K2CO3 (0.35 g, 2.5 mmol) in glycerol (30 mL) was
1
product in a yield of 42%. H NMR (500 MHz, DMSO-d6): 8.03
(d, J ¼ 8.6 Hz, 2H), 7.53 (s, 2H), 7.36 (t, J ¼ 7.8 Hz, 4H), 7.16–7.12
(m, 8H), 7.03 (d, J ¼ 8.6 Hz, 2H).
2-[4-Bis[4-(diphenylamino)phenyl]aminophenyl]-1H-
benzimidazole (7)
This compound was prepared according to the procedure for heated to 190 ꢁC for 24 h under nitrogen. Aer cooling to room
the synthesis of (3), in a yield of 40%. 1H NMR (500 MHz, DMSO- temperature, the mixture was poured into water for extraction
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with dichloromethane. The organic extracts were washed with in
a
single-layer device, 2-phenylbenzimidazole is para-
water and dried over anhydrous sodium sulfate. Aer the substituted on the phenyl segment by the dendrons, so as
solvent had been removed, the residue was puried by column to utilize the hole-transporting properties of diphenyla-
chromatography on silica gel with petroleum/dichloromethane mine groups and the electron-transporting properties of
1
(2 : 3) as the eluent to give the product in a yield of 17%. H benzimidazole.
NMR (300 MHz, DMSO-d6): 7.85 (d, J ¼ 7.8 Hz, 3H), 7.69 (d, J ¼
The benzimidazole (BI) derivatives were synthesized by the
8.1 Hz, 3H), 7.18–7.12 (m, 3H), 6.83–6.78 (m, 3H), 6.71–6.65 (m, condensation of benzaldehyde derivatives and o-phenylenedi-
9H), 6.01 (d, J ¼ 7.8 Hz, 3H), 4.26 (s, 9H). MALDI-TOF-MS (m/z): amine in the presence of a mild catalyst, sodium metabisulte
calcd. for C42H33N6Ir: 813.98, found: 813.22. Anal. calcd (%) for or ammonium chloride. Then the ligands were conveniently
C42H33N6Ir: C, 61.97; H, 4.09; N, 10.32. Found: C, 62.02; H, 4.23; prepared by methylation of the BI derivatives (Scheme 1). The
N, 10.24.
Ir(III) complexes were prepared according to the following well-
established, two-step approach60 (Scheme 2): the chloride-
bridged dimers were rst formed by treating IrCl3$3H2O with
excessive ligands in a mixed solvent of 2-ethoxyethanol and
water. Then the crude dimers reacted with excessive ligands to
convert to the facial (fac) isomer at 190 ꢁC in glycerol. The
complexes were puried through column chromatography.
Notably, higher yields of G1, G2, and G2Cz compared to G0 were
obtained under similar conditions due to better solubility.
Therefore, the gram-scale preparation of these dendrimers
became much easier. The structures of the complexes were
veried using 1H-NMR, MALDI-TOF-MS, and elemental
analysis.
Dendrimer G1
This compound was prepared according to the procedure for
the synthesis of G0, with a yield of 35%. H NMR (300 MHz,
1
DMSO-d6): 7.66 (d, J ¼ 8.1 Hz, 3H), 7.50 (d, J ¼ 8.7 Hz, 3H), 7.20–
7.10 (m, 15H), 6.93 (t, J ¼ 7.5 Hz, 6H), 6.74–6.66 (m, 15H), 6.56
(d, J ¼ 2.1 Hz, 3H), 6.06 (t, J ¼ 8.4 Hz, 6H), 4.19 (s, 9H). MALDI-
TOF-MS (m/z): calcd. for C78H60N9Ir: 1315.62, found: 1314.00.
Anal. calcd (%) for C78H60N9Ir: C, 71.21; H, 4.60; N, 9.58. Found:
C, 71.18; H, 4.64; N, 9.56.
Dendrimer G2
This compound was prepared according to the procedure for
1
Thermal and phase properties
the synthesis of G0, with a yield of 28%. H NMR (500 MHz,
DMSO-d6): 7.58 (d, J ¼ 8.0 Hz, 3H), 7.46 (d, J ¼ 8.5 Hz, 3H), 7.14–
7.08 (m, 27H), 6.92 (t, J ¼ 7.5 Hz, 15H), 6.84 (d, J ¼ 8.0 Hz, 21H),
6.70 (d, J ¼ 8.5 Hz, 15H), 6.63 (br, 15H), 6.30 (d, J ¼ 8.5 Hz, 3H),
6.22 (d, J ¼ 8.0 Hz, 3H), 3.95 (s, 9H). MALDI-TOF-MS (m/z): calcd
for C150H114N15Ir: 2318.88, found: 2317.21. Anal. calcd (%) for
The decomposition temperatures of the complexes were deter-
mined from the thermogravimetric analysis (TGA) curves under
a nitrogen atmosphere. As shown in Fig. 1, the iridium
complexes were very stable under a N2 atmosphere and no
signicant weight loss was seen below 350 ꢁC. The temperatures
of decomposition (Td: corresponding to 5% weight loss) of G0
C
150H114N15Ir: C, 77.69; H, 4.95; N, 9.06. Found: C, 77.51; H,
4.81; N, 9.14.
ꢁ
ꢁ
and G1 are 456 C and 436 C, while the Td values of G2 and
ꢁ
ꢁ
G2Cz increase to 571 C and 609 C, respectively. It was found
that the decomposition of G0 started at 360 ꢁC, and at 610 ꢁC the
weight loss ratio was 25%, which corresponds to the loss of one
ligand (M ¼ 207) in G0. It means that one of the ligands in the
complex decomposed with the corresponding coordination
bond breaking in the temperature range mentioned above.
However, the corresponding temperature ranges of G1, G2, and
Dendrimer G2Cz
This compound was prepared according to the procedure for
1
the synthesis of G0, with a yield of 27%. H NMR (300 MHz,
DMSO-d6): 8.13 (m, 12H), 7.74 (d, J ¼ 9.0 Hz, 3H), 7.63 (d, J ¼ 8.4
Hz, 3H), 7.37 (d, J ¼ 8.7 Hz, 12H), 7.22–7.09 (m, 54H), 6.75 (t, J ¼
7.5 Hz, 3H), 6.68 (d, J ¼ 8.7 Hz, 3H), 6.21 (d, J ¼ 8.1 Hz, 3H), 4.09
(s, 9H). MALDI-TOF-MS (m/z): calcd. for C150H102N15Ir: 2306.79,
found: 2305.79. Anal. calcd (%) for C150H102N15Ir: C, 78.10; H,
4.46; N, 9.11. Found: C, 77.87; H, 4.30; N, 9.03.
ꢁ
G2Cz, with the same weight loss of 207, are 351–678 C, 386–
771 ꢁC, and 397–782 ꢁC, respectively. It indicates that the
rupture of the coordination bond can be retarded in the second
generation dendrimers, which is obviously related to their
increased molecular sizes.
Results and discussion
Molecular design and synthesis
In order to elucidate the phase status of the complexes in
the solid, X-ray diffraction (XRD) analyses of G0–G2Cz were
Herein, we chose a phenyl benzimidazole ligand homoleptic performed. As shown in Fig. 2, the four strongest sharp peaks
Ir(III) complex as the chromophore, and diphenylamine-based with 2q angles at 8ꢁ, 9ꢁ, 12ꢁ, and 21ꢁ proved the existence of
dendrons as encapsulating groups in order to separate the crystallization in G0, and the size of the crystal evaluated by the
phosphorescent centers. Methyl iodide was reacted with the Scherrer equation (D ¼ 0.89 l/b cos q) is 61 nm. The crystalli-
N–H group of benzimidazole, which not only avoids interfer- zation of G1 was observed by the sharp peaks at 7ꢁ, 8ꢁ, and 9ꢁ
ence during the following coordination process, but is also which also exist in G0. The size of the crystal is about 81 nm.
1
convenient to characterize the complexes by H-NMR spectra. Since crystallization is the process of three-dimensional
Moreover, there is no need to additionally introduce exible regular packing of molecules, and strong intermolecular
surface groups owing to the good solubility of the branches. interactions are always present to hold the molecules in
Considering the importance of the balance of charge transport the correct positions in the crystals, it indicates that the
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Scheme 1 Synthetic procedure of the ligands.
Scheme 2 (a) Synthesis of the dendrimers with G1 as an example. (b) Molecular structures of the dendrimers G0, G1, G2 and G2Cz.
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corresponds to the phenyl moiety. For LG1, LG2, and LG2Cz,
apart from the absorption peaks originating from phenyl
benzimidazole, the peaks at 341 nm and 346 nm are attributed
to the triphenylamine moieties and those at 261 nm and 330 nm
correspond to carbazole moieties. The notable red-shi
(169 nm) in the PL spectra from LG0 to LG2 could be attributed
Fig. 1 TGA curves of the complexes recorded under a nitrogen
atmosphere.
Fig. 2 XRD spectra of the complexes.
rst-generation dendron is not large enough to completely
avoid the aggregation of the Ir(III) cores. In contrast, only broad
and weak peaks were found in the XRD spectra of G2 and G2Cz.
Both of them are amorphous. It is clear that the highly
branched structures of the molecules can remarkably decrease
the crystallization tendency and stabilize the amorphous glass
state of the materials. In addition, the weak intermolecular
interaction and aggregation of G2 and G2Cz might have strong
effects on the reduction of concentration quenching and T–T
annihilation, which are necessary for high-performance
PhOLEDs.
Photophysical properties
UV/vis and PL spectra of the ligands and the corresponding
complexes were measured in their dilute CH2Cl2 solutions
(Fig. 3). The absorption peak at 292 nm of LG0 is attributed to
Fig. 3 (a) The absorption and PL spectra of the ligands in CH2Cl2
solution. (b) The absorption and PL spectra of the complexes in CH2Cl2
solution. (c) The PL spectra of G2 in dilute CH2Cl2 solution, neat film,
the benzimidazole moieties, and the other peak at 228 nm and CH2Cl2 glass.
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to the increased delocalization caused by a great number of radiative decay rates (Kr) and non-radiative decay rates (Knr)
diphenylamine substitutions. were calculated and listed in Table 1. For complexes G0, G1 and
Compared with the absorption spectra of the ligands, in the G2Cz, the Knr decreases as the molecular rigidity increases. The
absorption spectra of the complexes, the strong absorption deviation from this trend for G2 is presumably ascribed to the
bands below 360 nm closely resemble those of the free ligands. rotational oscillation of the peripheral diphenylamine substit-
These short-wavelength absorptions are assigned to spin- uents in solutions, resulting in an additional ultrafast decay
allowed ligand-centered p–p* transitions. The weak absorption process.
shoulders in the range 360–500 nm are attributed to the metal-
to-ligand charge-transfer (MLCT) transitions. As seen in
Fig. 3(b), the complexes in dichloromethane exhibit emission
Electrochemical properties
spectra with a similar shape and a red-shi of about 15 nm from To demonstrate the contribution of the diphenylamine-based
G0 to G2, which is a smaller red-shi range compared with that dendrons to the carrier injection in the complexes, cyclic vol-
of the ligands from LG0 to LG2 (169 nm). It indicates that as the tammetry (CV) analysis was performed (Fig. 4). The highest
molecular size of the complexes increases from G0 to G2 and occupied molecular orbital (HOMO) energy levels of the den-
G2Cz, the dendritic substitutions are highly twisted to the drimers were determined from the onset of the oxidation
emissive center as a result of the enhanced steric hindrance potentials according to the equation reported by de Leeuw
onset/SCE
effect, which limits the extent of conjugation between the et al.,66 EHOMO ¼ ꢀ(E
+ 4.4) eV, and the lowest unoc-
oxy
central core and the branches. This means that connecting the cupied molecular orbital (LUMO) levels were deduced from the
diphenylamine dendrons to the core G0 at the para-position of HOMO energy levels and energy gaps determined by the onset
the phenyl does not obviously change the optical properties, of absorption (Table 1). Upon the anodic sweep, G0, G1, G2, and
and therefore the dendrons mainly act as an encapsulation layer G2Cz exhibit one, two, three and two reversible oxidation
to separate the emissive cores from each other. The predomi- processes, respectively. No reduction signal was detected for any
nant ligand-centered 3p–p* (3LC p–p*) character, other than of the dendrimers during the cathodic scan. The relatively low
the 3MLCT character of the emissive excited states, is conrmed oxidation potentials (Eonset) of the complexes are ascribed to the
by the vibronically structured emission spectra.61,62 Further- mixture of the oxidation of Ir d orbitals and ligands. The HOMO
more, the emissions of these complexes in spin-coated lms energy levels determined from Eonset for G1 (ꢀ5.01 eV) and G2
show smaller red-shis than those in solution, which also (ꢀ5.00 eV) are notably higher than that of G0 (ꢀ5.15 eV). These
shows their weak self-aggregation in the solid. At 77 K in CH2Cl2 data show that incorporation of the diphenylamine-based
glass, the complexes all show more structured emission spectra, dendrons in G1 and G2 has a great impact on the electro-
which exhibit small blue-shis (2–3 nm) with respect to their chemical process and can improve their hole-injection (HI)
emission spectra at room temperature, further conrming the properties. By contrast, the HOMO energy level of G2Cz
predominant 3LC p–p* character of the emissive excited decreases to ꢀ5.25 eV, which is lower than that of G2 by 0.25 eV,
states.63–65 The hypsochromic shis are due to solvent reorga- reecting the fact that the outer carbazole units are electron-
nization in a uid solution, which are signicantly impeded in a withdrawing through an inductive or p-polarization effect.
rigid matrix at 77 K. Fig. 3(c) shows the PL spectra of G2 in dilute
CH2Cl2 solution, neat lm, and CH2Cl2 glass as an example.
Compared with the prototype G0, dendrimers G1, G2, and
G2Cz exhibit extra oxidation waves at a higher potential. The
Both the solution and lm PL quantum yield (Fp) of the second set of reversible oxidation for G1 begins at 0.89 V, which
dendrimers were measured and the data are listed in Table 1. is reasonably attributed to the oxidation of the diphenylamine
The dendrimer G2 exhibits a neat lm Fp of 28%, which is units, and is consistent with the results reported by Lee et al.67
almost identical to that of G2Cz, 7 times of that of G0, and 3 For G2, extra oxidation waves at 0.75 V and 1.02 V are observed,
times that of G1, indicative of a signicantly reduced interac- which are respectively attributed to the oxidation of the inner
tion between emissive Ir(III) cores with increasing generation of diphenylamine units as well as the outer ones. The inner ones
the dendrimers. The solution Fp of G1 (52%) is slightly higher are reversible, with the onset lower than that of G1 by 0.14 V,
than that of G0 (49%), but for G2 the Fp decreases to 43%, implying that the inner layer of G2 is more electron-rich due to
which shows a different trend compared to that in lm. Inter- the presence of p-donating outer diphenylamine substituents.
estingly, the solution Fp of G2Cz increases to 60% aer intro- On the other hand, G2Cz exhibits two oxidation waves besides
ducing the carbazole end-capped diphenylamine (CzDPA) the rst reversible oxidation peak: one irreversible peak (0.97 V)
dendron to G0. The large difference between the Fp of G2 and corresponds to the oxidation of the inner diphenylamineunits,
G2Cz in organic solvent may be a result of their difference in while the other reversible peak (1.22 V) is relevant to the
rotational freedom.59 The rotational freedom of G2Cz is lower oxidation of the outer carbazole dendrons. However, the onset
than that of G2, for the molecular rigidity is enhanced with a potential of the former peak is higher than that of G1 by 0.08 V,
substitution of carbazole end-capped units for diphenylamine supporting the assumption that the outer carbazole dendrons
units, which helps to reduce non-radiative decay processes. The are electron-withdrawing, as elaborated in earlier sections.
rotational freedom of both G2Cz and G2 are reduced rapidly in Therefore, both G2 and G2Cz possess oxidation potential
the lm, and their lm Fp values are very close to each other.
gradients such that the outer layer is electron-poor and the
The lifetimes (s) of these complexes were measured in inner layer is electron-rich. It is believed that the intramolecular
degassed DMSO solution at room temperature, and their potential gaps facilitate the intramolecular charge transfer.68
1110 | J. Mater. Chem. C, 2014, 2, 1104–1115
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Table 1 The photophysical, electrochemical, and thermal data of the complexes
sb [ms] Kr (ꢂ105) Knr (ꢂ105) lemd [nm]/T1 [eV] HOMO/LUMOe [eV] Td [ꢁC]
a
Complex labs [nm] (log 3)
lem [nm] Fp
G0
227(4.7), 242(4.7), 298(4.6), 312(4.6),
508a/
512c
0.49b/
0.64 7.7
0.81 6.4
0.93 4.6
0.91 6.6
8.0
5.9
6.2
4.4
506/2.45
514/2.41
522/2.38
513/2.42
ꢀ5.15/ꢀ2.74
ꢀ5.01/ꢀ2.64
ꢀ5.00/ꢀ2.64
ꢀ5.25/ꢀ2.88
456
436
571
609
372(4.1), 410(3.8), 455(3.5), 483(3.1)
233(4.8), 317(4.7), 359(4.8), 397(4.7),
450(4.0), 500(2.7)
229(5.0), 307(5.0), 357(5.0), 409(4.8),
455(4.2), 506(2.6)
0.04c
0.52b/
0.09c
0.43b/
0.28c
0.60b/
0.27c
G1
517a/
526c
G2
523a/
527c
G2Cz
235(5.5), 294(5.1), 343(5.1), 399(4.8),
453(4.0), 501(2.8)
515a/
526c
a
b
In CH2Cl2 solution (1 ꢂ 10ꢀ6 M). 3 denotes the molar extinction coefficients. In degassed DMSO solutions. The Fp was measured versus
c
d
(ppy)2Ir(acac) (Fp ¼ 0.34 in 2-methyltetrahydrofuran). s ¼ 1/(Kr + Knr) and Fp ¼ Kr/(Kr + Knr). Neat lm data measured at 298 K. In CH2Cl2
glass at 77 K. e The HOMO and LUMO energies were determined from cyclic voltammetry and absorption data.
the size of the molecule increases. Furthermore, the benz-
imidazole moieties concentrate on one side of the octahedral
structure and the dendrons point in the opposite direction.
Consequently, a ower bouquet-shaped structure is formed in
the dendrimer.
The electron density contours of the three lowest unoccupied
(LUMO to LUMO + 2) and the three highest occupied molecular
orbitals (HOMO to HOMO + 2) for G0–G2Cz are presented.
Notably, the outer diphenylamine and carbazole dendrons in
G2 and G2Cz participate in the formation of the HOMOs, while
the LUMOs are located on the surface of the molecules, which
may lead to the increased hole and electron mobility when they
act as the self-host materials. The balanced charge injection/
transporting capacities and the weak intermolecular interac-
tions are essential for developing highly efficient non-
doped phosphors suitable for low-cost single-layer device
technologies.
Fig. 4 CV curves of the complexes (versus a silver wire pseudo-
reference electrode). Inset: CV curves of ferrocene measured using
Electroluminescent properties
Inspired by the amorphous states and excellent PL and elec-
trochemical properties of G2 and G2Cz, the host-free devices A
and B with the conguration of ITO/PEDOT:PSS/neat den-
drimer/Cs2CO3/Al have been fabricated by spin-coating, where
PEDOT:PSS was used as the hole-injection layer and Cs2CO3 was
used as the electron-injection layer. The single-layer devices A
and B were based on G2 and G2Cz respectively. Fig. 5 presents
the current density–voltage–luminance (J–V–L) characteristics
and curves of luminance efficiency versus current density of the
SCE or Ag as a reference electrode.
Thus, more oxidation states might be anticipated to make the
two second-generation dendrimers superior to G1 and G0 in
hole injection and transporting.
Theoretical calculation
Density functional theory (DFT) calculations have also been devices, and the main device data are collected in Table 2. The
performed to characterize the three-dimensional geometries turn-on voltage of device A (4.0 V) is lower than that of B (4.3 V)
and the frontier molecular orbitals of the complexes by due to the reduced energy barrier between the anode and
employing the Gaussian 03 package (Fig. S1, ESI†). The diphe- emission layer of the former (Fig. 6). The HOMO and LUMO
nylamine-based dendrons are signicantly twisted against the levels of G2 were embedded between the HOMO of PEDOT:PSS
2-phenylbenzimidazole (PBI), resulting from the steric (ꢀ5.20 eV) and the work function of Cs2CO3/Al (ꢀ2.20 eV). Thus,
hindrance effect caused by introducing the dendrons into the it behaved as an effective trapping site for holes and electrons,
para-site of the phenyl segment of the ligands. These geomet- which were easily injected from the two electrodes and
rical characteristics can effectively prevent intermolecular combined in the iridium phosphor. The device A achieves a
interactions between emitting cores, thus, suppress molecular maximum current efficiency (hc,max) of 14.02 cd Aꢀ1 and a
recrystallization and limit the extent of conjugation between the maximum external quantum efficiency (hext,max) of 4.67%
central core and branches, which improves the morphological photons per electron at the brightness of 147.94 cd mꢀ2; even at
stability of the thin lm and maintains the optical properties as a high brightness of 1000 cd mꢀ2 or 10 000 cd mꢀ2, hc is still as
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Fig. 6 Schematic energy-level diagram of the devices A and B.
high as 12.86 cd Aꢀ1 or 8.04 cd Aꢀ1. The small values of effi-
ciency roll-off should be attributed to the sufficiently depressed
concentration quenching of the emissive core and the good
charge-transporting ability of the branching units. In contrast, a
similar device fabricated from G2Cz shows a better perfor-
mance; it exhibits higher performances with hc,max of 18.35 cd
Aꢀ1, hext,max of 6.12%, and smaller values of efficiency roll-off
than device A because of the increased hole-injection barrier
and reduced electron-injection barrier to eliminate exciton
quenching at the cathode interface.
We note that the EL efficiencies enabled by G2 and G2Cz are
the highest ever reported for single-layer self-host green emis-
sive devices fabricated by spin-coating (which is observed in
Ding's bipolar dendrimers with hc,max of 5.5 cd Aꢀ1, hext,max of
1.6%).49 The high performances indicate an efficient and
balanced charge injection, transport, and combination besides
the sufficiently depressed concentration quenching and T–T
annihilation. With the aim of achieving a large size molecular
phosphor used in host-free spin-coated devices, highly sterically
hindered dendrons such as triphenylamine-based and carba-
zole-based dendrons are widely used. However, the perfor-
mance of the single-layer device is very poor relative to that of
the multilayer device, which may result from the excellent hole-
transporting properties, but poor electron-injection and elec-
tron-transport capabilities of these dendrons.38 The high
performance of our single-layer device without introducing any
additional electron-transport dendrons is profound. The den-
drons were introduced at the para-position of the phenyl
instead of the N-position in phenyl benzimidazole.41,42,49 This
not only reduces the intermolecular interaction efficiently due
to steric effects, but it also positions benzimidazole moieties on
the surface rather than being trapped in the center of the
molecule. Fig. 7 shows the difference between the molecular
structures as well as single-layer self-host device performances
of G2Cz and G2 (Ding et al.).38 It also presents the different
Fig. 5 (a) J–V–L characteristics and (b) luminance efficiency versus
current density plots and (c) EL spectra for the devices from G2 and
G2Cz.
Table 2 Device performances of the electrophosphorescent OLEDs
hc,maxb [cd Aꢀ1
]
hext,max [%]
hcd [cd Aꢀ1
]
hextd [%]
leme [nm]
CIEe (x,y)
a
c
Von [V]
G2
G2Cz
4.0
4.3
14.02
18.35
4.67
6.12
12.86, 8.04
16.52, 13.11
4.29, 2.68
5.51, 4.37
526
524
0.37, 0.58
0.38, 0.58
a
b
Recorded at 1 cd mꢀ2
.
Maximum current efficiency. c Maximum external quantum efficiency. d Data were measured at 1000 cd mꢀ2 and 10 000
cd mꢀ2 respectively. Data were measured at 8 V.
e
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three-dimensional structures caused by the substitution in commercial applications. The second generation dendrimers
different positions. The former is ower bouquet-shaped, while G2 and G2Cz have improved thermal stability compared with
the latter is shaped like a ball cactus. Since the electron clouds the rst generation dendrimer G1. XRD analysis proved that
of the LUMO levels of dendrimers were largely located on the the diphenylamine-based dendrons have a strong effect on
benzimidazole moieties, the electron injection/transporting in reducing the aggregation. Both G2 and G2Cz are amorphous,
devices A and B were improved greatly. In addition, although and they all exhibit much better PL properties in lm than G0
the electron-decient moieties (oxadizole dendrons) have been and G1. Furthermore, the greater number of oxidation states
adopted in literature,49,50 the performance is still very poor due of G2 and G2Cz might be anticipated to make it superior to G1
to the less efficacious electron-transport as the distance from and G0 in hole injection and transporting. By the exposure of
the outer electron-decient dendrons to the emissive core electron-decient moieties on the surface of the dendrimers
increases. Therefore, this indicates that the current molecular for balancing electron and hole uxes, G2 and G2Cz show the
design is very promising for the development of solution- best performances recorded so far for single-layer self-
processable, single-layer self-host devices through simpler host green PhOLEDs. Importantly, these materials should
molecular structures.
provide a promising strategy to develop highly efficient non-
doped phosphors suitable for low-coat single-layer device
technologies.
Conclusions
In this work, we have designed and synthesized a new series of
homoleptic iridium(III) complexes G0, G1, G2, and G2Cz for
Acknowledgements
green phosphorescent organic light-emitting devices. Our We are grateful for grants from the National Basic Research
investigation showed that the introduction of diphenylamine- Program China (2013CB932902), National Natural Science
based dendrons is a feasible approach to realize high-effi- Foundation of China (21173042, 51103023), and the Science
ciency small molecule phosphorescent materials in which the and Technology Support Program (Industry) Project of Jiangsu
excellent solubility and high steric hindrance are utilized to Province (BE 2013118). We also thank the support of the Key
approach solution-processing for self-host low-cost large-area Laboratory of Novel Thin Film Solar Cells (KF201112) and the
Fig. 7 The comparison of the molecular structure as well as single-layer self-host device performance between G2Cz and G2 (Ding et al.).38 The
phenyl benzimidazole moieties of the complexes are highlighted with red circles.
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