Efficient yellow-green organic light-emitting diodes based on sublimable cationic iridium complexes

Article abstract of DOI:10.1016/j.dyepig.2016.02.031

Three new yellow-emitting cationic iridium complexes have been prepared, and the photophysical and electrochemical properties have been investigated. These cationic iridium complexes have excellent thermal stability and can be thermally vacuum-sublimed as

Full text of DOI:10.1016/j.dyepig.2016.02.031

Accepted Manuscript
Efficient Yellow-Green Organic Light-Emitting Diodes Based on Sublimable Cationic
Iridium Complexes
Fuli Zhang, Yuqiao Guan, Shimin Wang, Suzhi Li, Fuqiang Zhang, Yafei Feng,
Shufen Chen, Guangxiu Cao, Bin Zhai
PII:
S0143-7208(16)30049-3
10.1016/j.dyepig.2016.02.031
DYPI 5121
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 12 November 2015
Revised Date: 12 February 2016
Accepted Date: 26 February 2016
Please cite this article as: Zhang F, Guan Y, Wang S, Li S, Zhang F, Feng Y, Chen S, Cao G, Zhai B,
Efficient Yellow-Green Organic Light-Emitting Diodes Based on Sublimable Cationic Iridium Complexes,
Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.02.031.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Efficient Yellow-Green Organic Light-Emitting Diodes Based on Sublimable
Cationic Iridium Complexes
a, 1
b, 1
c
a
a
Fuli Zhang , Yuqiao Guan , Shimin Wang , Suzhi Li , Fuqiang Zhang , Yafei
b
b,
a
a,
Feng , Shufen Chen *, Guangxiu Cao , Bin Zhai *
a
College of Chemistry and Chemical Engineering, Shangqiu Normal University,
Shangqiu 476000, P. R. China.
b
Key Laboratory for Organic Electronics and Information Displays and Institute of
Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing
2
10023, P. R. China.
c
Department of Materials and Chemical Engineering, Henan Institute of Engineering,
Zhengzhou 450007, P. R. China.
Corresponding author. Tel: +86-370-2594336.
*
E-mail: iamsfchen@njupt.edu.cn (Shufen Chen) ; zhaibin_1978@163.com (Bin Zhai).
1
Both authors contributed equally to this work.
Abstract
Three new yellow-emitting cationic iridium complexes have been prepared, and the
photophysical and electrochemical properties have been investigated. These cationic
iridium complexes have excellent thermal stability and can be thermally vacuum-
sublimed as phosphorescent dyes for the fabrication of efficient organic light-emitting
diodes. The devices based on these complexes give yellow-green electroluminescences
with Commission Internationale de L'Eclairage coordinates of (0.38, 0.58), (0.37, 0.58),
(0.38, 0.58). The maximum current efficiencies of devices based on these complexes are
1

ACCEPTED MANUSCRIPT
-1
2
0.2, 23.7 and 21.5 cd A that corresponds to maximum external quantum efficiencies
of 6.3 %, 6.8 % and 6.5 %, respectively.
Keywords: Organic light-emitting diode; Sublimable; Cationic iridium complex;
Yellow-green light emission; 4,5-Diazafluorene; Bis-carbazole
1
. Introduction
In the recent years, phosphorescent cyclometalated iridium(III) complexes have
attracted more and more interest in the field of optoelectronics due to their high
quantum efficiency, short lifetime of triplet excited states and broad range of emission
colors [1]. Iridium(III) complexes can be divided into two classes: (i) the neutral
complexes with molecular formula [Ir(C^N) ] [2-4] or [Ir(C^N) (L^X)] (C^N and L^X
3
2
stand for cyclometalated ligand and anionic ancillary ligand, respectively) [5-8] and (ii)
+
cationic iridium(III) complexes [Ir(C^N) (N^N)] containing neutral bidentate ancillary
2
ligands [9]. The former class of iridium(III) complexes have high quantum yields and
their emission wavelengths and excited-state lifetimes can be tuned by changing the
cyclometalated ligands. Most of the OLEDs based on these complexes were fabricated
by vacuum deposition due to their good volatility. As for the latter case, the ionic
character makes them quite soluble in polar solvents, which enable solution process to
form the film for solid-state light-emitting electrochemical cells (LECs) containing only
a single emissive layer [10]. LECs offer several advantages such as very simple device
architectures, being independent of the thickness of the active layer and being used with
air-stable electrodes. However, these devices suffer from a slow response and severe
excited-state quenching in the active layers compared with conventional OLEDs [11-14].
2

ACCEPTED MANUSCRIPT
Therefore, they are indeed difficult to be used in full-color flexible panel displays which
require fast switching. Such drawbacks in LECs can be significantly suppressed by
doping the phosphors in the polymer host materials as the emissive layer as well as
using an extra functional layer in solution-processed OLEDs. Consequently, by using
cationic iridium(III) complexes as emissive phosphors, blue, red, green, yellow and
near-infrared OLEDs have been realized [15-20]. Most of these kind of devices
commonly exhibit low luminance and efficiency due to the inferior compatibility of
cationic iridium complex guest with hydrophobic polymer hosts, which could hinder the
development of their widespread applications. It is expected that the performance of
OLEDs based on cationic iridium complexes can be significantly relieved as the dopants
are deposited by a vacuum-evaporable method. However, the intrinsic ionic nature of
cationic iridium complex can result in their poor volatility and severe thermal
degradation during vacuum evaporation. The volatility of cationic iridium complexes
can be improved by enhancing molecular structural rigidity. Wong et al. reported
OLEDs with sublimable cationic iridium complexes with rigid pyridinylfluorene
derivatives as dopants, which gave yellow light emission with a maximum luminance
−
1
efficiency of 20 cd A and a quantum efficiency of 7 % [19]. As far as we know, other
than the foregoing accounts there were no other reports on OLEDs based on cationic
iridium dopants by thermally vacuum-sublimed method.
In this work, three novel cationic iridium complexes were synthesized by using 2-
phenylpyridine as the main ligand and bis-carbazole substituted 4,5-diazafluorene
derivatives as the ancillary ligand. The bipolar ancillary ligands comprise two electron-
donating carbazole units and an electron-accepting 4,5-diazafluorene group, and were
even used as hosts for highly efficient blue phosphorescent OLEDs [21]. Such cationic
3

ACCEPTED MANUSCRIPT
iridium complexes have excellent thermal stability and can be thermally evaporated
under vacuum without significant decomposition in phosphorescent OLED fabrication.
2
. Experimental
2
.1. General experiments
All reactants and solvents were used as received, and were purified or dried by
standard methods when required. All reactions were monitored using precoated thin-
1
layer-chromatography plates (0.20 mm with fluorescent indicator UV254). H NMR
spectra were recorded on a JEOL JNMECA600 NMR spectrometer with
tetramethylsilane [Si(CH ) ] as the internal standard. Mass spectrometry (MS) was
3
4
performed with an Esquire-LC_00136 mass spectrometer. Elemental analysis for
carbon, hydrogen, and nitrogen was determined on an Exeter Analytical CE-440
elemental analyzer. The dimeric iridium(III) intermediate [(ppy) Ir(ꢀ-Cl)] and diimine
2
2
ligands ECAF (9,9-bis(9-ethylcarbazol-3-yl)-4,5-diazafluorene), EHCAF (9,9-bis(9-
ethylhexylcarbazol-3-yl)-4,5-diazafluorene) and PCAF (9,9-bis(9-phenylcarbazaol-3-
yl)-4,5-diazafluorene) were prepared according to the literature procedure [21, 22].
Absorption spectra were recorded with a UV–vis spectrophotometer (Agilent 8453) and
photoluminescent (PL) spectra were recorded with a fluorospectrophotometer (Jobin
Yvon, FluoroMax-3). The excited state lifetimes of the complexes were measured on a
transient spectrofluorimeter (Edinburgh Instruments, FLSP920) with a time-correlated
single-photon counting technique. Absolute PL quantum efficiency at room temperature
were measured with an absolute photoluminescence quantum yield measurement system
(Hamamatsu, C9920-02). Cyclic voltammetry was performed on a Princeton Applied
Research potentiostat/galvanostat model 283 voltammetric analyzer in DMF solutions
-3
-1
(
1 × 10 M) at a scan rate of 100 mV s with a platinum plate as the working electrode,
4

ACCEPTED MANUSCRIPT
a silver wire as the pseudo-reference electrode, and a platinum wire as the counter
electrode. The supporting electrolyte was n-Bu NPF (0.1 M) and ferrocene was
4
6
selected as the internal standard. The solutions were bubbled with argon for 10 min
before measurements.
2
.2. Fabrication and characterization of OLEDs
Indium-tin-oxide (ITO) substrates with sheet resistance of 10 ꢁ sq were sufficiently
-1
cleaned using chemical and UV-ozone methods before the deposition of the organic
layers. The poly-(3,4-ethylenedioxythiophene:poly-(styrene sulfonate) (PEDOT:PSS)
layer was spin-coated onto the ITO substrate in air and baked at 120 °C for 30 min. The
transporting material of TCTA (4,4’,4’’-tri(9-carbazoyl)triphenylamine) film (15 nm)
was then deposited on the hole-injecting layer. The phosphor (10 wt%) and CBP (4,4'-
bis(9H-carbazol-9-yl)biphenyl) host were co-evaporated to form 30 nm emitting layer
from two separate sources. Then, TmPyPb (1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene) (50
nm), LiF (0.5 nm), and Al (100 nm) were evaporated, respectively. The organic layers
-4
-1
were laminated in the above sequence under 6 × 10 Pa at a rate of 0.5 – 1 Å s
without breaking vacuum between each vacuum-deposition process. The layer thickness
was monitored in situ using a quartz crystal oscillator. The luminance–voltage–current
density characteristics were measured with a PR655 spectrometer and a Keithley 2400
programmable voltage-current source.The current efficiency and external quantum
efficiency characteristics were calculated from electrical parameters, electroluminescent
(
EL) spectra and luminance. All measurements of the devices were carried out in
ambient atmosphere without further encapsulations.
.3. Syntheses
2
5

ACCEPTED MANUSCRIPT
The ancillary ligands ECAF, EHCAF and PCAF were synthesized from a reported
procedure [21]. All reactions were performed under argon. Solvents were carefully
dried and distilled from appropriate drying agents prior to use.
Synthesis and Structural Characterization of [Ir(ppy) (ECAF)]PF (1): The
2
6
dichloro-bridged diiridium complex [(ppy) Ir(ꢀ-Cl) Ir(ppy) ] (0.35 g, 0.33 mmol) and
2
2
2
ECAF ancillary ligand (0.42 g, 0.76 mmol) were dissolved in 2-methoxyethanol (10
mL). The mixture was then refluxed at 150 °C for 15 h under an argon atmosphere to
form a clear yellow solution. After cooling to room temperature, an aqueous solution of
NH PF (0.87 g in 20 mL deionized water) was slowly added into the reaction mixture
4
6
under stirring, resulting in a yellow suspension. The suspension was then filtrated and
the resulting precipitate was washed with deionized water and dried under vacuum at 70
°
C for 12 h. The crude product was purified by column chromatography on silica gel
(
(
200–300 mesh) with CH Cl /acetone (10:1) as the eluent, yielding a yellow powder
2
2
−
1
0.62 g, 0.52 mmol). Yield: 79%. mp: > 300 °C. IR (KBr, cm ): 1564, 1236, 1121, 844,
1
7
9
7
2
7
7
6
6
51, 618, 559. H NMR (600 MHz, CDCl , δ): 8.10 (d, J = 7.8 Hz, 2H), 7.88 (dd, J =
3
.6 Hz and 9.0 Hz, 4H), 7.84 (s, 2H), 7.79 (d, J = 5.4 Hz, 2H), 7.75 (t, J = 7.8 Hz, 2H),
.70 (d, J = 5.4 Hz, 2H), 7.64 (d, J = 7.8 Hz, 2H), 7.50–7.47 (m, 2H), 7.43 (t, J = 7.8 Hz,
H), 7.38 (dd, J = 8.4 Hz and 7.8 Hz, 4H), 7.31–7.29 (m, 2H), 7.16 (t, J = 7.2 Hz, 2H),
.08 (dd, J = 7.2 Hz and 6.0 Hz, 2H), 6.99 (dd, J = 7.8 Hz and 7.2 Hz, 2H), 6.90 (dd, J =
.8 Hz and 7.2 Hz, 2H), 6.42 (d, J = 7.8 Hz, 2H), 4.34–4.30 (m, 4H), 1.39 (t, J = 7.2 Hz,
+
H). MS (MALDI-TOF) [m/z]: 1055.3 (M – PF ) . Anal. found: C 61.10, H 3.87, N
6
.95. Anal. Calcd for C H N PF Ir: C 61.04, H 3.86, N 7.00.
6
1
46
6
6
Synthesis and Structural Characterization of [Ir(ppy) (EHCAF)](PF ) (2): The
2
6
synthesis of complex 2 is similar to that of complex 1, except that ECAF was replaced
6

ACCEPTED MANUSCRIPT
−
1
with EHCAF, yielding a yellow powder. Yield: 81%. mp: > 300 °C. IR (KBr, cm ):
1
1
559, 1463, 1116, 838, 753, 619, 555. H NMR (600 MHz, CDCl , δ): 8.15 (d, J = 7.8
3
Hz, 2H), 7.91–7.87 (m, 4H), 7.81–7.73 (m, 8H), 7.65 (d, J = 7.8 Hz, 2H), 7.55–7.53 (m,
2
7
6
1
H), 7.44 (dd, J = 8.4 Hz and 7.2 Hz, 2H), 7.38–7.33 (m, 6H), 7.18 (dd, J = 8.4 Hz and
.2 Hz, 2H), 7.06 (dd, J = 7.2 Hz and 5.4 Hz, 2H), 7.01 (dd, J = 7.8 Hz and 7.2 Hz, 2H),
.92 (dd, J = 7.8 Hz and 7.2 Hz, 2H), 6.43 (d, J = 7.8 Hz, 2H), 4.17–4.09 (m, 4H), 2.05–
.98 (m, 2H), 1.39–1.24 (m, 16H), 0.91–0.88 (m, 6H), 0.85–0.82 (m, 6H). MS
+
(
MALDI-TOF) [m/z]: 1223.5 (M – PF ) . Anal. found: C 64.03, H 5.19, N 6.10. Anal.
6
Calcd for C H N PF Ir: C 64.07, H 5.16, N 6.14.
7
3
70
6
6
Synthesis and Structural Characterization of [Ir(ppy) (PCAF)](PF ) (3): The
2
6
synthesis of complex 3 is similar to that of complex 1, except that ECAF was replaced
−
1
with PCAF, yielding a yellow powder. Yield: 81%. mp: > 300 °C. IR (KBr, cm ): 1565,
1
1
454, 1115, 845, 757, 616, 557. H NMR (600 MHz, CDCl , δ): 8.15 (d, J = 7.8 Hz,
3
2
H), 7.96 (d, J = 7.8 Hz, 2H), 7.90–7.87 (m, 4H), 7.79 (d, J = 5.4 Hz, 2H), 7.75–7.72
(m, 4H), 7.64 (d, J = 7.8 Hz, 2H), 7.59–7.54 (m, 6H), 7.51 (d, J = 7.2 Hz, 4H), 7.45 (dd,
J = 7.8 Hz and 7.2 Hz, 2H), 7.40–7.35 (m, 6H), 7.29–7.27 (m, 2H), 7.22–7.23 (m, 2H),
7
.04–6.98 (m, 4H), 6.92–6.89 (m, 2H), 6.42 (d, J = 7.8 Hz, 2H). MS (MALDI-TOF)
+
[
m/z]: 1151.3 (M – PF ) . Anal. found: C 63.89, H 3.54, N 6.52. Anal. Calcd for
6
C H N PF Ir: C 63.93, H 3.58, N 6.48.
69
46
6
6
2
.4. Quantum Chemical Calculations
Density functional theory (DFT) calculations using the B3LYP functional were
performed. “Double-ξ” quality basis sets were employed for the C, H and N (6-31G**)
23] and the Ir (LANL2DZ) [24]. An effective core potential (ECP) replaces the inner-
[
2
6
6
core electrons of Ir leaving the outer core 5s 5p electrons and the 5d valence electrons
7

ACCEPTED MANUSCRIPT
of Ir(III). The solvent effect of CH Cl was taken into consideration using a conductor-
2
2
like polarizable continuum model (C-PCM). All of the calculations were performed
with Gaussian 03 software pakage [25].
3
. Results and discussion
3
.1. Synthesis and Structural Characterization
Scheme 1 shows the synthesis of the three cationic iridium complexes. The ancillary
ligands ECAF, EHCAF and PCAF were easily synthesized from 4,5-diazafluoren-9-one
and carbazole derivatives by using Eaton’s reagent as the catalyst and condensing agent
[
21]. Complexes 1, 2 and 3 were then routinely synthesized with yields of 79%, 81%
and 81%, respectively, by reacting dimeric iridium(III) intermediate [(ppy) Ir(ꢀ-
2
Cl) Ir(ppy) ] with ancillary ligands, followed by a counter ion-exchange reaction from
2
2
−
−
1
Cl to PF [26]. The three complexes were structurally characterized by H NMR (Fig.
6
S1−S3 in the SI), ESI (electron spray ionization) mass spectroscopy (Fig. S4−S6 in the
SI) and elemental analysis.
3
.2. Thermal stability
The thermal stability of emitters is very important for efficient OLEDs. The
decomposition temperatures of complexes 1, 2 and 3 were determined from the
thermogravimetric analysis (TGA) curves under a nitrogen atmosphere. As shown in
Fig. 1, these iridium complexes were very stable under a N atmosphere and no
2
o
significant weight loss was seen below 300 C. Presumably, because they have two
3
+
bulky carbazoles and a robust 4,5-diazafluorene core that completely surround the Ir
ions, weaker interactions between adjacent iridium molecules tend to increase the
thermal stability of these ionic complexes. From the TGA curves it can be observed that
o
o
the decomposition temperature (5% loss of weight) are 365 C for 1, 320 C for 2 and
8

ACCEPTED MANUSCRIPT
o
4
10 C for 3, respectively, indicating that the complexes are suitable for application in
OLEDs by vacuum-deposition method.
.3. Photophysical Properties
The absorption and PL spectra of complexes 1, 2 and 3 in CH Cl (5 × 10 M) are
3
-5
2
2
shown in Fig. 2, and detailed photophysical characterizations are collected in Table 1.
The strong and broad absorption bands in the ultra-violet up to around 360 nm are
1
attributed to spin-allowed intraligand LC (π→π*) transition of cyclometalated main
ligands and ancillary ligands. The relatively weak absorption bands from 360 nm
1
extending to the visible region are assigned to excitations to MLCT (metal-to-ligand
1
3
3
charge-transfer), LLCT (ligand-to-ligand charge-transfer), MLCT, LLCT, and ligand-
3
3
3
3
centered (LC) π→π* transitions. The spin-forbidden MLCT, LLCT, and LC π→π*
1
transitions gain considerable intensity by mixing with the higher-lying MLCT
transition through spin-orbit coupling (SOC) endowed by the heavy iridium atom [22,
2
7]. No distinct difference is observed in absorption profiles for the complexes 1, 2 and
3
, indicating the similar electronic and vibrational structures of the ground states (S0)
and the first excited states (S1).
As depicted in Fig. 2, complexes 1, 2 and 3 exihibit yellow light emission (peaks at
ca. 580 nm) in degassed CH Cl solutions at room temperature. Compared with their PL
2
2
spectra in solutions, the PL spectra of complexes 1, 2 and 3 in neat films are slightly
blue-shifted. In contrast, these complexes emit efficient yellow-green light with peaks
at 540, 536 and 540 nm, respectively, in 5% (by weight) doped poly(methyl
methacrylate) (PMMA) films. The large hypsochromic shift originates from the change
of the local environment of the complex, as previously observed for the similar cationic
iridium complexes [28, 29]. Generally, for mixed-ligand cationic iridium complexes,
9

ACCEPTED MANUSCRIPT
3
three excited states contribute to the observed light emission, that is, LC π→π*,
3
3
MLCT, and LLCT [30-32]. At room temperature, the three complexes display broad
and almost featureless emission spectra in CH Cl solutions and in films, which
2
2
3
indicates that the emissive excited states have predominantly characters of MLCT or
3
3
LLCT other than LC π→π*, as the latter always show vibronically structured emission
spectra [30, 32]. At 77 K in 2-MeTHF matrix, the emission spectra of the complexes 1,
2
and 3 are largely blueshifted (with peaks at 502, 503 and 504 nm, respectively) and
3
structured, which indicates that, at low temperatures, considerable LC π→π* character
develops in the emissive excited states of the complexes [30, 32, 33].
In degassed CH Cl solutions, complexes 1, 2 and 3 give photoluminescent quantum
2
2
yields (PLQY, Φ) of 0.32, 0.28 and 0.33, respectively, which are slightly larger than
that of yellow light-emitting iridium complex [Ir(ppy) (dtb-bpy)]PF (0.24 in degassed
2
6
CH CN solutions) [34]. The excited-state lifetimes (τ) of complexes 1, 2 and 3 in
3
degassed CH Cl solutions (0.62, 0.61 and 0.60 ꢀs, respectively) are similar to that of
2
2
[
Ir(ppy) (dtb-bpy)]PF (0.557 ꢀs in degassed CH CN solutions) [34]. The lack of
2 6 3
vibronic structure in the phosphorescence spectra and the relatively short calculated
radiative lifetimes (τrad = τ/Φ) indicate that these complexes emit from the charge-
transfer excited state [33, 35].
Unlike previously reported [26], the PLQY values of 1, 2 and 3 in thin films are 0.32,
0
.37 and 0.48, respectively, and are slightly larger than those in CH Cl solutions. This
2 2
could be attributed to the steric hindrance of the ancillary ligands and molecules can be
well dispersed in the neat films. In sharp contrast to the PLQYs in solutions and neat
films, the doped films of complexes 1，2 and 3 in PMMA exhibit significantly better
PLQYs of 0.74, 0.72 and 0.78, respectively, accompanied by a remarkable increase of
1
0

ACCEPTED MANUSCRIPT
excited state lifetimes. This enhancement in quantum yield and excited state lifetime
plausibly result from the great restriction of rotations and vibrations in rigid polymer
[
12].
3
.4. Electrochemical Properties and Theoretical Calculations
The electrochemical behavior of these iridium complexes was measured in
deoxygenated N,N-Dimethylformamide (DMF) solutions containing 0.1 M tetra(n-
butyl)ammonium hexafluorophosphate (n-Bu NPF ) as the supporting electrolyte by
4
6
-1
cyclic voltammetry (Fig. 3). During the anodic scan at the rate of 100 mV s , all of
ox
these complexes show the same first oxidation (Eonset ) wave at about 0.90 V, which is
almost identical to those of the ppy-based (ppy = 2-phenylpyridine) cationic iridium
complexes [15, 16, 26]. It is confirmed that the oxidation occurred mainly at the iridium
ox
metal center. On the basis of the onset potential of Eonset and the absorption edge data,
the energy levels of the highest occupied molecular orbitals (HOMOs) and lowest
unoccupied molecular orbitals (LUMOs) can be estimated from the empirical formulae:
ox
EHOMO = –e(Eonset + 4.4) and ELUMO = EHOMO + E (where E is the optical energy gap
g
g
obtained from the absorption threshold) [36, 37]. The HOMO (–5.3 eV for complexes 1,
2
and 3) and LUMO (–2.9 eV for complexes 1, 2 and 3) levels of these iridium
ox
complexes are very similar to each other due to their Eonset and E locating in the same
g
positions.
DFT calculations at the B3LYP/(6-31G** +LANL2DZ) level were carried out to gain
more insight into the photophysical and electrochemical properties of complexes 1, 2
and 3. Fig.4 displays the atomic orbital composition of HOMOs and LUMOs for
complexes 1, 2 and 3. As expected, the HOMO orbitals of all complexes were
composed of a mixture of Ir dπ orbitals and phenyl π orbitals distributed among the
1
1

ACCEPTED MANUSCRIPT
cyclometalated ligands, which are similar to those of other ppy-based iridium
complexes [16, 26]. However, their LUMO orbitals were mainly dominated by the
bipyridine units of the ancillary ligands ECAF, EHCAF and PCAF. This result, together
with the electrochemical properties, refers that the introduction of the substituents in the
ancillary ligands have no signicant effects on the frontier molecular orbitals of these
complexes.
3
.5. Electroluminescent Properties of OLED Devices
In order to evaluate the electroluminescent properties of these cationic iridium
complexes, complexes 1, 2 and 3 were used as doped emitters to fabricate OLEDs with
the structure of ITO/PEDOT:PSS (40 nm)/TCTA(15 nm)/emitting layer (30
nm)/TmPyPb (50 nm)/LiF (0.5 nm)/Al (100 nm), in which PEDOT:PSS acts as hole-
injecting layer, TCTA acts as hole-transporter, CBP acts as host, TmPyPb acts as
electron-transporter, and LiF/Al as the electron-injection layer and cathode (Fig. 5). The
optimized Ir(III) emitters with 10 wt% doped concentration in host material were used
as the emitting layer. The device architecture, as well as molecular structure of the
materials used are shown in Fig. 5.
Fig. 6 depicts the EL spectra of the OLEDs based on complexes 1, 2 and 3. Devices
based on complexes 1, 2 and 3 emit yellow-green light with CIE coordinates of (0.38,
0
.58), (0.37, 0.58), (0.38, 0.58), respectively. In the EL spectra, the emission from the
host CBP disappears completely, indicating that the complete energy and/or charge
transfer occurs from the host to the phosphors upon electrical excitation. In addition, the
emission wavelength and profile of EL spectra of devices closely resemble those of PL
spectra in PMMA film, indicating that phosphors 1, 2 and 3 were not decomposed
during the vacuum thermal evaporation and electroluminescence originated from their
1
2

ACCEPTED MANUSCRIPT
triplet excited states.
Fig. 7 shows the luminance–voltage–current density and current efficiency–current
density–external quantum efficiency characteristics of the devices. Detailed electrical
characteristics of the devices are summarized in Table 2. A maximum luminance (Lmax
)
−2
−1
of 8588 cd m , a maximum luminance efficiency (ηc) of 20.2 cd A and an external
−
2
quantum efficienty ( e
η
) of 6.3 % for complex 1, a Lmax of 11850 cd m , a maximum ηc
−
1
−2
of 23.7 cd A and a ηe of 6.8 % for complex 2, and a Lmax of 10340 cd m , a
−
1
maximum ηc of 21.5 cd A and a ηe of 6.5 % for complex 3 are achieved. As depicted
in Fig. 7, the EL efficiencies of devices decrease gradually with a further increase of
luminance. This phenomenon is the so-called EL efficiency roll-off and can be
attributed to the triplet–triplet annihilation [38] originating from the long lifetime of
triplet excited state and triplet–polaron quenching [39]. The EL efficiency roll-off
effects in these devices are very mild, which should be attributed to the good carrier
transport properties of the phosphorescent emitters with carbazole and 4,5-diazafluorene
groups. The results suggest these complexes are good yellow-green phosphors and have
potential applications in OLEDs. Ongoing novel cationic iridium(III) complexes with
excellent thermal stability and corresponding device studies are currently in progress.
4
. Conclusion
In conclusion, three novel cationic iridium complexes, [Ir(ppy) (ECAF)]PF (1),
2
6
[
Ir(ppy) (EHCAF)]PF (2) and [Ir(ppy) (PCAF)]PF (3), have been synthesized and
2 6 2 6
fully characterized. Due to the excellent thermal stability, yellow-green OLEDs were
fabricated by a vacuum-deposition method. Devices based on 1, 2 and 3 exibit the
-1
maximum current efficiencies of 20.2, 23.7 and 21.5 cd A and maximum external
1
3

ACCEPTED MANUSCRIPT
quantum efficiencies of 6.3 %, 6.8 % and 6.5 % with the CIE coordinates of (0.38, 0.58),
(0.37, 0.58), (0.38, 0.58), respectively.
Acknowledgements
We would like to thank the the Ministry of Science and Technology (973 project,
Grant No. 2015CB932202) and National Natural Science Foundation of China (Grant
Nos. 21371114, 21271125, 21201116, 61405054, 61274065, 21501117, 21571123), the
Program for Science and Technology Innovation Talents in Universities of Henan
Province (2012HASTIT031) and the Key Scientific Research Projects in University
of Henan Province (Grant No. 15A150023) for financial support. We also thank Prof.
Haishun Wu of Shangxi Normal University for the thoretical calculations.
Supplementary material
1
H NMR and ESI-MS spectra of the synthesized complexes were presented.
This material is available free of charge via the Internet.
References
[
1] Chi Y, Chou P-T. Transition-metal phosphors with cyclometalating ligands:
fundamentals and applications. Chem Soc Rev 2010;39(2):638-55.
2] Tsuboyama A, Iwawaki H, Furugori M, Mukaide T, Kamatani J, Igawa S, et al.
[
Homoleptic cyclometalated iridium complexes with highly efficient red
phosphorescence and application to organic light-emitting diode. J Am Chem Soc
2
003;125(42):12971-9.
1
4

ACCEPTED MANUSCRIPT
[
3] Sajoto T, Djurovich PI, Tamayo A, Yousufuddin M, Bau R, Thompson ME, et al.
Blue and near-UV phosphorescence from iridium complexes with cyclometalated
pyrazolyl or N-heterocyclic carbene ligands. Inorg Chem 2005;44(22):7992-8003.
[
4] Sasabe H, Takamatsu J-i, Motoyama T, Watanabe S, Wagenblast G, Langer N, et al.
High-Efficiency Blue and White Organic Light-Emitting Devices Incorporating a Blue
Iridium Carbene Complex. Adv Mater 2010;22(44):5003-7.
[
5] Adachi C, Kwong RC, Djurovich P, Adamovich V, Baldo MA, Thompson ME, et
al. Endothermic energy transfer: A mechanism for generating very efficient high-energy
phosphorescent emission in organic materials. Appl Phys Lett 2001;79(13):2082-4.
[
6] Avilov I, Minoofar P, Cornil J, De Cola L. Influence of substituents on the energy
and nature of the lowest excited states of heteroleptic phosphorescent Ir(III) complexes:
A joint theoretical and experimental study. J Am Chem Soc 2007;129(26):8247-58.
[
7] Zhu Y-C, Zhou L, Li H-Y, Xu Q-L, Teng M-Y, Zheng Y-X, et al. Highly Efficient
Green and Blue-Green Phosphorescent OLEDs Based on Iridium Complexes with the
Tetraphenylimidodiphosphinate Ligand. Adv Mater 2011;23(35):4041-6.
[
8] Li H-Y, Li T-Y, Teng M-Y, Xu Q-L, Zhang S, Jin Y-M, et al. Syntheses,
photoluminescence and electroluminescence of four heteroleptic iridium complexes
with 2-(5-phenyl-1,3,4-oxadiazol-2-yl)-phenol derivatives as ancillary ligands. J Mater
Chem C 2014;2(6):1116-24.
[
9] Costa RD, Orti E, Bolink HJ, Monti F, Accorsi G, Armaroli N. Luminescent Ionic
Transition-Metal Complexes for Light-Emitting Electrochemical Cells. Angew Chem
Int Ed 2012;51(33):8178-211.
[
10] Hu T, He L, Duan L, Qiu Y. Solid-state light-emitting electrochemical cells based
on ionic iridium(iii) complexes. J Mater Chem 2012;22(10):4206-15.
1
5

ACCEPTED MANUSCRIPT
[
11] Slinker JD, Rivnay J, Moskowitz JS, Parker JB, Bernhard S, Abruna HD, et al.
Electroluminescent devices from ionic transition metal complexes. J Mater Chem
007;17(29):2976-88.
12] He L, Duan L, Qiao J, Zhang D, Wang L, Qiu Y. Enhanced stability of blue-green
2
[
light-emitting electrochemical cells based on a cationic iridium complex with 2-(1-
phenyl-1H-pyrazol-3-yl)pyridine as the ancillary ligand. Chem Commun
2
011;47(22):6467-9.
[
13] Costa RD, Orti E, Bolink HJ, Graber S, Housecroft CE, Constable EC.
Intramolecular pi-Stacking in a Phenylpyrazole-Based Iridium Complex and Its Use in
Light-Emitting Electrochemical Cells. J Am Chem Soc 2010;132(17):5978-80.
[
14] Tordera D, Meier S, Lenes M, Costa RD, Orti E, Sarfert W, et al. Simple, Fast,
Bright, and Stable Light Sources. Adv Mater 2012;24(7):897-900.
15] He L, Duan L, Qiao J, Zhang D, Dong G, Wang L, et al. Efficient solution-
[
processed electrophosphorescent devices using ionic iridium complexes as the dopants.
Org Electron 2009;10(1):152-7.
[
16] Zhang F, Duan L, Qiao J, Dong G, Wang L, Qiu Y. Solution-processed blue-green
organic light-emitting diodes based on cationic iridium complexes with 1-pyridyl-3-
methylimidazolin-2-ylidene-C,C-2 as the ancillary ligand. Org Electron
012;13(7):1277-88.
17] He L, Duan L, Qiao J, Zhang D, Wang L, Qiu Y. Highly efficient solution-
'
2
[
processed blue-green to red and white light-emitting diodes using cationic iridium
complexes as dopants. Org Electron 2010;11(7):1185-91.
1
6

ACCEPTED MANUSCRIPT
[
18] Tao R, Qiao J, Zhang G, Duan L, Wang L, Qiu Y. Efficient Near-Infrared-Emitting
Cationic Iridium Complexes as Dopants for OLEDs with Small Efficiency Roll-off. J
Phys Chem C 2012;116(21):11658-64.
[
19] Wong W-Y, Zhou G-J, Yu X-M, Kwok H-S, Lin Z. Efficient organic light-emitting
diodes based on sublimable charged iridium phosphorescent emitters. Adv Funct Mater
007;17(2):315-23.
20] Tang H, Li Y, Zhao B, Yang W, Wu H, Cao Y. Two novel orange cationic
2
[
iridium(III) complexes with multifunctional ancillary ligands used for polymer light-
emitting diodes. Organ Electron 2012;13(12):3211-9.
[
21] Zheng C-J, Ye J, Lo M-F, Fung M-K, Ou X-M, Zhang X-H, et al. New Ambipolar
Hosts Based on Carbazole and 4,5-Diazafluorene Units for Highly Efficient Blue
Phosphorescent OLEDs with Low Efficiency Roll-Off. Chem Mater 2012;24(4):643-50.
[
22] Sprouse S, King KA, Spellane PJ, Watts RJ. Photophysical effects of metal-carbon
sigma. bonds in ortho-metalated complexes of iridium(III) and rhodium(III). J Am
Chem Soc 1984;106(22):6647-53.
23] Francl MM, Pietro WJ, Hehre WJ, Binkley JS, Gordon MS, DeFrees DJ, et al. Self
consistent molecular orbital methods. XXIII. A polarization‐type basis set for
second‐row elements. J Chem Phys 1982;77(7):3654-65.
24] Hay PJ, Wadt WR. Ab initio effective core potentials for molecular calculations.
Potentials for K to Au including the outermost core orbitals. J Chem Phys
985;82(1):299-310.
25] Gaussian 03, Revision B05, Gaussian, Inc, Wallingford, CT 2004.
.
[
‐
[
1
[
1
7

ACCEPTED MANUSCRIPT
[
26] He L, Duan L, Qiao J, Wang R, Wei P, Wang L, et al. Blue-emitting cationic
iridium complexes with 2-(1H-pyrazol-1-yl)pyridine as the ancillary ligand for efficient
light-emitting electrochemical cells. Adv Funct Mater 2008;18(14):2123-31.
[
27] Colombo MG, Brunold TC, Riedener T, Guedel HU, Fortsch M, Buergi H-B.
Facial tris cyclometalated rhodium(3+) and iridium(3+) complexes: their synthesis,
structure, and optical spectroscopic properties. Inorg Chem 1994;33(3):545-50.
[
28] He L, Duan L, Qiao J, Dong G, Wang L, Qiu Y. Highly Efficient Blue-Green and
White Light-Emitting Electrochemical Cells Based on a Cationic Iridium Complex with
a Bulky Side Group. Chem Mater 2010;22(11):3535-42.
[
29] Bolink HJ, Cappelli L, Cheylan S, Coronado E, Costa RD, Lardies N, et al. Origin
of the large spectral shift in electroluminescence in a blue light emitting cationic
iridium(iii) complex. J Mater Chem 2007;17(48):5032-41.
[
30] Colombo MG, Guedel HU. Synthesis and high-resolution optical spectroscopy of
bis[2-(2-thienyl)pyridinato-C3,N'](2,2'-bipyridine)iridium(III). Inorg Chem
993;32(14):3081-7.
31] Colombo MG, Hauser A, Guedel HU. Evidence for strong mixing between the LC
1
[
and MLCT excited states in bis(2-phenylpyridinato-C2,N')(2,2'-bipyridine)iridium(III).
Inorg Chem 1993;32(14):3088-92.
[
32] Lowry MS, Hudson WR, Pascal RA, Bernhard S. Accelerated Luminophore
Discovery through Combinatorial Synthesis. J Am Chem Soc 2004;126(43):14129-35.
33] Tamayo AB, Garon S, Sajoto T, Djurovich PI, Tsyba IM, Bau R, et al. Cationic
[
Bis-cyclometalated Iridium(III) Diimine Complexes and Their Use in Efficient Blue,
Green, and Red Electroluminescent Devices. Inorg Chem 2005;44(24):8723-32.
1
8

ACCEPTED MANUSCRIPT
[
34] Slinker JD, Gorodetsky AA, Lowry MS, Wang J, Parker S, Rohl R, et al. Efficient
Yellow Electroluminescence from a Single Layer of a Cyclometalated Iridium
Complex. J Am Chem Soc 2004;126(9):2763-7.
[
35] Zhao Q, Liu S, Shi M, Li F, Jing H, Yi T, et al. Tuning Photophysical and
Electrochemical Properties of Cationic Iridium(III) Complex Salts with Imidazolyl
Substituents by Proton and Anions. Organometallics 2007;26(24):5922-30.
[
36] Burrows PE, Shen Z, Bulovic V, McCarty DM, Forrest SR, Cronin JA, et al.
Relationship between electroluminescence and current transport in organic
heterojunction light‐emitting devices. J Appl Phys 1996;79(10):7991-8006.
[
37] Liang B, Wang L, Xu YH, Shi HH, Cao Y. High-Efficiency Red Phosphorescent
Iridium Dendrimers with Charge- Transporting Dendrons: Synthesis and
Electroluminescent Properties. Adv Funct Mater 2007;17(17):3580-9.
[
38] Baldo MA, Adachi C, Forrest SR. Transient analysis of organic
electrophosphorescence. II. Transient analysis of triplet-triplet annihilation. Phys Rev B
000;62:10967.
39] Reineke S, Walzer K, Leo K. Triplet-exciton quenching in organic phosphorescent
light-emitting diodes with Ir-based emitters. Phys Rev B 2007;75(12):125328.
2
[
1
9

ACCEPTED MANUSCRIPT
Captions for Schemes and Figures
Scheme 1. The synthetic route of iridium complexes 1–3.
Fig. 1. The TGA curves of complexes 1–3.
Fig. 2. Absorption and emission spectra of complexes 1 (a), 2 (b) and 3 (c).
-3
Fig. 3. Cyclic voltammograms of complexes 1, 2 and 3 in DMF solutions (10 M).
+
Potentials were recorded versus Fc /Fc.
Fig. 4. Molecular orbital surfaces of cationic iridium complexes. (a) LUMO orbital of
complex 1. (b) HOMO orbital of complex 1. (c) LUMO orbital of complex 2. (d)
HOMO orbital of complex 2. (e) LUMO orbital of complex 3. (f) HOMO orbital of
complex 4. All the MO surfaces correspond to an isocontour value of |Ψ| = 0.02.
Fig. 5. Device architecture and chemical structures of the materials used in these
OLEDs.
Fig. 6. EL spectra of the OLEDs based on complexes 1, 2 and 3 with 10 wt% doped
concentration in host material.
2
0

ACCEPTED MANUSCRIPT
Fig. 7. The current density–voltage–luminance (a) and current efficiency–current
density–external quantum efficiency characteristics (b) of the devices based on
complexes 1, 2 and 3.
Captions for Tables
Table 1 Photophysical characteristics of complexes 1, 2 and 3.
Table 2 Electrical characteristics of devices based on complexes 1, 2 and 3.
2
1

ACCEPTED MANUSCRIPT
Emission at
Emission at room temperature
[e]
7
7 K
[
a]
λabs [nm]
PMMA
film
λ[nm]
Φem [τ [ꢀs]]
Solution
λ [nm]
[
b]
CH
2
Cl
2
5 wt% in
Neat
[d]
λ[nm]
τ[ꢀs]
[c]
[b]
[c]
PMMA
Film
0
.32
0.74
[0.93]
0.32
[0.54]
1
2
3
239/288/410
239/289/411
242/290/409
580
540
502
503
504
4.26
4.13
4.34
[
[
[
0.62]
0
.28
0.72
[0.96]
0.37
[0.58]
580
582
536
0.61]
0
.33
0.78
[0.92]
0.48
[0.56]
540
0.60]
[a]
-5
[b]
[c]
In CH Cl
2 2
solutions (1 × 10 M).
In degassed CH
2
Cl
2
solutions. Doped films were made on quartz substrates with
[d]
[e]
thicknesses of about 100 nm. Neat films were made on quartz substrates with thicknesses of about 100 nm. In 2-MeTHF
matrix at 77 K.

ACCEPTED MANUSCRIPT
-2 [a]
-1 [a]
-1 [a]
[a]
Devices
D1
Vturn-on (V)
9.2
Lmax /cd m
ηc /cd A
ηp /lm W
ηe /%
6.3
λem /nm
540
CIE (x, y)
(0.38, 0.58)
(0.37, 0.58)
(0.38, 0.58)
8588
11850
10340
20.2
4.87
D2
8.4
23.7
5.32
6.8
540
D3
8.2
21.5
5.63
6.5
540
[a]
Maximum values of the devices.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT