Fast synthesis of iridium(iii) complexes with sulfur-containing ancillary ligand for high-performance green OLEDs with EQE exceeding 31%

Article abstract of DOI:10.1039/c9tc01397k

Three green iridium(iii) cyclometalated complexes, namely, (4tfmppy)₂Ir(dipdtc), (24btfmppy)₂Ir(dipdtc), and (TN₄T)₂Ir(dipdtc), with the sulfur-containing ancillary ligand N,N-diisopropyldithiocarbamate (dipdtc)

Full text of DOI:10.1039/c9tc01397k

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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ARTICLE
Fast synthesis of iridium(III) complexes with sulfur containing ancillary
ligand for high-performance green OLEDs with EQE over 31%
Cite this: DOI:
10.1039/x0xx00000x
Guang-Zhao Lu,1,2# Zhen-Long Tu,1,# Liang Liu,1 Wen-Wei Zhang,1,* You-Xuan Zheng1*
Three green iridium(III) cyclometalated complexes, (4tfmppy)2Ir(dipdtc), (24btfmppy)2Ir(dipdtc) and
(TN4T)2Ir(dipdtc), with the sulfur containing ancillary ligand of N,N-diisopropyldithiocarbamate
(dipdtc) were synthesized for fifteen minutes at room temperature. The emission colours (peak = 497 -
534 nm) and photoluminescence quantum efficiencies (P = 71.6% - 94.2%) can be effectively
regulated by introducing different main ligands of 2-(4-(trifluoromethyl)phenyl)pyridine (4tfmppy),
2-(2,4-bis(trifluoromethyl)phenyl)pyridine (24btfmppy) and 2',6'-bis(trifluoromethyl)-2,4'-bipyridine
(TN4T). Employing these complexes as emitters, the organic light emitting diodes (OLEDs) with
double-emitting layers were fabricated. Particularly, the device based on complex (TN4T)2Ir(dipdtc)
showed superior performances with a maximum current efficiency of 86.59 cd A-1 and a maximum
external quantum efficiency of 31.24% with low efficiency roll-off. These results suggest that the
Ir(III) complexes with the four-membered ring Ir-S-C-S backbone containing N,N-
diisopropyldithiocarbamate have potential application in OLEDs.
Received 00th January 2019,
Accepted 00th January 2019
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
However, four-membered ring structures of Ir(III) complexes
with sulfur atoms in the ancillary ligands are rarely studied in
Highly efficient phosphorescent complexes have been widely
employed to fabricate organic light-emitting diodes (OLEDs)
because of their unique optoelectronic properties including high
quantum yield and excellent color tunability.1 Especially, the
iridium(III) complexes are the most important emitters, and the
demand of phosphorescent materials during the OLEDs
industrialization is particularly large.2 In order to speed up the
process of OLEDs industrialization, cost reduction is also
important. But for the production of most Ir(III) complexes, the
last reaction process is always to reflux the [(C^N)2Ir(µ-Cl)]2
chloride-bridged dimmer with cyclometalated or ancillary ligands
at high temperature for a long time, which would inevitably
increase the manpower and cost.3 For example, when the
traditional ancillary ligands such as 2-pyridinecarboxylic acid
(pic) and acetylacetone (acac) and so on are applied, the
respective iridium complexes are usually prepared under the
conditions of the long time over 12 h and high temperature over
110 oC. It is important, but difficult to find suitable ligands which
can form phosphorescent complexes at room temperature
efficiently to reduce the cost of OLEDs industrialization
significantly.4
OLEDs,6 and there is no new progress in the research of the
Ir(III) complexes containing the four-membered ring based on Ir-
S-C-S backbone. Furthermore, the Hard-Soft-Acid-Base (HSAB)
theory is widely used to judge the stability of the metal
complexes and also to explain the reaction mechanism.7 The
central theme of this theory is that when all other factors are the
same, the "soft" acid with the "soft" alkali and the "hard" acid
with the "hard" alkali react more quickly and form strong bonds.
Based on this theory, the central iridium atoms and sulfur ions
can be classified as soft acids and bases, respectively, thus the
bonding speed of complexes is fast and the formed compounds
are stable. From our previous study, it is found that the two bulky
trifluoromethyl substituents in the main ligand TN4T can cause
large steric hindrance effect and greatly inhibit the molecular
packing, which can increase the sublimation yield, inhibit the
concentration
quenching
and
finally
enhance
the
phosphorescence quantum yield. Besides, the iridium complexes
using TN4T as main ligand will have many nitrogen heterocycles,
which are beneficial to the enhancement of the electron mobility
and thus the OLED performance of the respective complexes.
In this paper, three Ir(III) complexes (4tfmppy)2Ir(dipdtc),
(24btfmppy)2Ir(dipdtc) and (TN4T)2Ir(dipdtc) were developed in
view of above consideration using the electron-deficient 2-(4-
For the previously reported Ir(III) complexes, ancillary ligands
such as acac or pic with central iridium atom mainly formed six
or five-membered ring structures. But the Ir(III) complexes
containing four-membered ring structures make the angle formed
by the two coordinated atoms of the ancillary ligand and the
iridium atom much reduced, suggesting the four-membered
metallocycles would possess greater coordination strain energy.5
(trifluoromethyl)phenyl)pyridine
(4tfmppy),
2-(2,4-
bis(trifluoromethyl)phenyl)pyridine (24btfmppy) and 2',6'-
bis(trifluoromethyl)-2,4'-bipyridine (TN4T) as main ligands and
the single monoanionic N,N-diisopropyldithiocarbamate (dipdtc)
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with electron-donating group diisopropylamine as the ancillary Experimental Section
ligand. All complexes have the unique four-membered Ir-S-C-S
backbone built by the metal center and the ancillary ligands,
Preparation of compounds
which were synthesized at room temperature in 15 minutes with
As shown in Fig. 1, the main ligands 24btfmppy, TN4T and the
exceedingly outstanding characteristics.8 Firstly, the coordination ancillary ligand of N,N-diisopropyldithiocarbamate (dipdtc) were
capability of sulfur atom with iridium atom is strong, resulting in prepared according to the reported publications.10 Three
high reaction yield and molecular stability, which is helpful to complexes were all prepared in fifteen minutes at room
boost the sublimation yield of Ir(III) complexes. From our temperature by the reaction of corresponding chloride-bridged
recently publication9 it can be concluded that as the S and Ir dimmers [(C^N)2Ir(µ-Cl)]2 with N,N-diisopropyl dithiocarbamate
approach to each other, the energy of the system gradually with the reaction yields and the sublimation yields all above 90%,
decreases until the equilibrium S-Ir bond length is reached and respectively, owing to the strong coordination capability of sulfur
further reduction of the S-Ir distance leads to a sharp energy atom with iridium atom. All compounds were fully characterized
increase. Hence, the formation of the S-Ir coordination bond does by 1H NMR spectrometry, high-resolved mass (HR-MS)
not own the transition state of the conventional sense, and it is not spectrometry, elemental analysis and the crystal structures further
necessary to overcome the corresponding energy barrier. The confirmed the identity of the complexes.
calculated free energy changes ΔG are far less than zero, which
Lithium N,N-diisopropyldithiocarbamate (Li-dipdtc). The
further demonstrate that formation of the Ir-S bond is diisopropyl amine (40 mmol) was added to a stirring slurry of
thermodynamically favorable. Secondly, the stable dithiolate LiOH (2.0 g, 48 mmol) in water (3 ml) and CS2 (3.6 g, 48 mmol)
compounds can reduce the work function of the emitter and the in benzene (10 ml). An exothermic reaction ensued, followed by
threshold electric field, therefore reducing the turn-on voltage of formation of yellow to orange precipitates. After 1 h, petroleum
the device.8 Finally, due to the nitrogen frame of the main ligand, ether was added and the reaction mixture was filtered. The solid
these complexes would have bipolar properties, which are was washed well with petroleum ether and dried in vacuo to give
beneficial for their device performances.9
as white to pale yellow solid, which was directly used in the next
procedure without further purification.
CF3
CF3
CF3
CF3
(a) LiOH, CS2, benzene-H2O, rt, 2h;
(b) IrCl3, EtOCH2CH2OH-H2O,115°C,12h;
S
H
Cl
Cl
S
N
a
(c) Li-dipdtc, EtOCH2CH2OH, 115°C, 6h;
N
b
c
N
Ir
Ir
Ir
S
Li
(d) Pd(PPh3)4, Na2CO3, THF-H2O, 70°C, 12h;
N
N
S
N
N
2
2
(e) n-BuLi, B(OCH3)3, -78°C, 24h;
Li-dipdtc
2
(4tfmppy)2Ir(dipdtc)
CF3
CF3
CF3
CF3
CF3
Br
Cl
S
CF3
b
c
d
F3C
F3C
F3C
N
Ir
N
Ir
Ir
Cl
N
F3C
S
N
N
N
2
B(OH)2
2
2
(24btfmppy)2Ir(dipdtc)
F3C
Ir
N
CF3
F3C
N
CF3
F3C
N
CF3
F3C
N
CF3
Br
S
S
Cl
Cl
F3C
N
CF3
F3C
N
CF3
c
e
d
N
b
N
Ir
Ir
N
N
N
N
B(OH)2
2
2
2
(TN4T)2Ir(dipdtc)
Fig. 1 The synthetic routes of ligands and three Ir(III) complexes.
Synthesis of Ir(III) complexes. The IrCl3 (0.64 g, 2.14 mmol)
(4tfmppy)2Ir(dipdtc) (yield: 91.0%): 1H NMR (400 MHz,
and 2.4 equivalent of three main ligands (5.14 mmol) were added CDCl3) δ 9.71 (d, J = 5.84 Hz, 2H), 7.95 (d, J = 7.77 Hz, 2H),
in a 2-ethoxyethanol and water mixture, which was heated at 110 7.84 (t, J = 8.53 Hz, 2H), 7.64 (d, J = 8.14 Hz, 2H), 7.35 (t, J =
oC for 16 h. After cooling to room temperature and the addition 6.61 Hz, 2H), 7.04 (d, J = 8.11 Hz, 2H), 6.52 (s, 2H), 1.32 (d, J =
of water, precipitated yellow-green powder was filtered and 56.30 Hz, 14H). HR-MS, m/z: calcd for C31H28N3F6S2Ir,
reacted with N,N-diisopropyldithiocarbamate without further 813.1258 [M]; found 814.1338 [M+H]+.. Anal. Calcd for
purification for 15 minutes at room temperature. Then, the C31H28N3F6S2Ir: C, 45.80; H, 3.47; N, 5.17. Found: C, 45.90; H,
solution was concentrated and the resulting residue was purified 3.60; N, 5.40%.
1
by silica gel column chromatography (CH2Cl2/petroleum ether
1:2 (v/v)), and then vacuum sublimated giving yellow crystals.
(24btfmppy)2Ir(dipdtc) (yield: 93.2%): H NMR (400 MHz,
CDCl3) δ 9.87 (dd, J = 1.15, 5.83 Hz, 2H), 8.40 (d, J = 8.47 Hz,
2 | J. Name., 2019, 00, 1-3
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ARTICLE
2H), 7.94-7.90 (m, 2H), 7.45-7.41 (m, 4H), 6.56 (s, 2H), 1.34 (d,
To fabricate stable OLEDs, the thermal stability of the emitters
J = 57.03 Hz, 14H). HR-MS, m/z: calcd for C33H26N3F12S2Ir, is also very important. From the DSC curves in Fig. S1 it can be
949.1006 [M]; found 950.1090 [M+H]+. Anal. Calcd for observed
C31H24N5F12S2Ir: C, 41.77; H, 2.76; N, 4.43. Found: C, 41.80; H, (4tfmppy)2Ir(dipdtc),
2.72; N, 4.39%.
the
melting
points
(24btfmppy)2Ir(dipdtc)
of
the
complexes
and
(TN4T)2Ir(dipdtc) are 323, 253 and 364 oC, respectively.
(TN4T)2Ir(dipdtc) (yield: 92.3%): 1H NMR (400 MHz, CDCl3) Observed from the TG curves (Fig. S2), three phosphors exhibit
δ 9.63 (d, J = 5.86 Hz, 2H), 8.13 (d, J = 7.80 Hz, 2H), 7.99 (s, the thermal decomposition temperatures (Td, corresponding to 5%
o
o
o
2H), 7.94 (t, J = 7.80 Hz, 2H), 7.33 (t, J = 6.71 Hz, 2H), 1.25 (d, weight loss) of 334 C, 290 C and 366 C, respectively (see
J = 6.42 Hz, 14H). HR-MS, m/z: calcd for C31H24N5F12S2Ir, Table 1). The high melting points and decomposition
951.0911 [M]; found 952.0991 [M+H]+. Anal. Calcd for temperatures of these complexes guarantee their stability during
C31H24N5F12S2Ir: C, 39.16; H, 2.54; N, 7.37. Found: C, 39.20; H, the device application.
2.72; N, 7.50%.
Electrochemical property and theoretical calculation
Results and discussion
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of the
dopants are important for the design of the OLEDs. In order to
determine their HOMO/LUMO levels, the electrochemical
properties of (4tfmppy)2Ir(dipdtc), (24btfmppy)2Ir(dipdtc) and
(TN4T)2Ir(dipdtc) were investigated by cyclic voltammetry in
deaerated CH2Cl2, relative to an internal ferrocenium/ferrocene
reference (Fc+/Fc) (Fig. S3). According to the equation of EHOMO
= − (Eox – EFc/Fc+ + 4.8) eV, the HOMO energy levels (EHOMO) of
three complexes were obtained. Based on the EHOMO and optical
X-ray crystallography and thermal stability of compounds
Single crystals of (4tfmppy)2Ir(dipdtc), (24btfmppy)2Ir(dipdtc)
and (TN4T)2Ir(dipdtc) were obtained by vacuum sublimation. Fig.
2 shows their Oak Ridge thermal ellipsoidal plot (ORETP)
diagrams given by X-ray analysis. Selected parameters of the
molecular structures and tables of atomic coordinates were
collected in the Table S1 and Table S2. As shown in Fig. 2, all
complexes have distorted octahedral coordination geometry
around iridium center by three chelating ligands, in which the N-
Ir-N angles for these complexes are 172.2°, 171.8° and 175.0°,
respectively. The four-membered Ir-S-C-S backbones could lead
to significantly more acute S-Ir-S bite angles (71.4°, 72.3° and
71.6°) than seen for the larger five-membered or six-membered
heterocycles based on such as pic, acac group and their
derivatives.11 The Ir-C bond lengths are 2.010-2.081 Å and the Ir-
N bond lengths are 2.029-2.060 Å, which are slightly shorter than
the Ir-S bonds (2.419-2.455 Å). The Ir-C and Ir-N bond lengths
are similar to those reported in other mononuclear complexes
with (CˆN)2Ir fragment.5 The similarity between C3-S1 (1.728-
1.746 Å) and C3-S2 (1.719-1.746 Å) in (4tfmppy)2Ir(dipdtc),
(24btfmppy)2Ir(dipdtc) and (TN4T)2Ir(dipdtc), indicates that -1
charge of N,N-diisopropyldithiocarbamate is delocalized over
both sulfur atoms. The two coordinated S atoms of ancillary
ligand reside in the equatorial plane trans to the metalated C
atoms of the three main ligands.
band gap calculated by the equation of ∆Ebandgap= 1240/λabs-onset
,
the lowest unoccupied molecular orbital (LUMO) energy levels
were also obtained. From Table 1 it can be observed that their
HOMO and LUMO levels are in the range of -5.25 - -5.76 eV and
-2.84 - -3.27 eV, respectively.
LUMOs
-1.77 eV
-2.08 eV
-2.12 eV
3.68
H-L energy gap
3.59
3.85
-5.45 eV
-5.67 eV
-5.97 eV
HOMOs
(4tfmppy)2Ir(dipdtc)
(24btfmppy)2Ir(dipdtc)
(TN4T)2Ir(dipdtc)
Fig. 3 Molecular orbital diagram for the HOMOs/LUMOs and their
energy levels of (4tfmppy)2Ir(dipdtc), (24btfmppy)2Ir(dipdtc) and
(TN4T)2Ir(dipdtc) calculated in CH2Cl2.
To determine the HOMO and LUMO distributions of the Ir(III)
molecules, the density functional theory (DFT) calculations were
also performed and the details of the theoretical calculations are
included in the SI. As shown in Fig. 3, the HOMO orbitals of the
Fig. 2 Oka Ridge Thermal Ellipsoidal plot (ORTEP) diagrams of
(4tfmppy)2Ir(dipdtc) (left, CCDC No. 1832358), (24btfmppy)2Ir(dipdtc)
(middle, CCDC No. 1832332) and (TN4T)2Ir(dipdtc) (right, CCDC No.
1832370) complexes with the atom-numbering schemes. Hydrogen atoms
are omitted for clarity. Ellipsoids are drawn at 50% probability level.
(4tfmppy)2Ir(dipdtc),
(24btfmppy)2Ir(dipdtc)
and
(TN4T)2Ir(dipdtc) complexes mostly located on the main ligands
(47.90%, 38.95% and 31.53%, respectively) together with d-
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orbitals of iridium atom (48.11%, 48.54% and 45.11%, reveals a peak emission at 513 nm (Fig. 4(b)), complex
respectively) with small portion of the N,N- (24btfmppy)2Ir(dipdtc) exhibits a 21 nm bathochromic shift due
diisopropyldithiocarbamate (8.79%, 12.51% and 23.37%, to
respectively). A larger scale distribution of electron clouds on the bis(trifluoromethyl)phenyl)pyridine.
a
replacement
by
the
main
ligand
2-(2,4-
However,
complex
iridium atom indicates an efficient MLCT of the phosphorescent (TN4T)2Ir(dipdtc) displays a 16 nm hypsochromic shift owing to
complexes, crucial to the high PL quantum efficiency. The replacement by the main ligand 2',6'-bis(trifluoromethyl)-2,4'-
LUMOs are mostly distributed over the π* orbitals of the main bipyridine. The obvious changes of emission wavelength coincide
ligands (92.83%, 94.61% and 94.04%, respectively) and to a with the large difference of energy gaps by the electrochemical
small extent on Ir d orbitals (3.94%, 3.42% and 2.88%, analysis and the theoretical calculations. The emission peak of the
respectively) and ancillary ligand (3.23%, 1.97% and 3.08%, three complexes at 77 K becomes narrower and the emission
respectively). As the main ligands change, the changed calculated shoulders become more, which are blue-shifted about 10 nm
energy gaps (3.68, 3.59 and 3.85 eV, respectively) correlate well relative to the ones measured at room temperature (Fig. S4). The
with the electrochemical results (Table S3).
PL quantum yields of the three Ir(III) complexes were measured
as 71.6%, 76.5% and 94.2%, respectively, and the lifetimes are
in the range of microseconds (Fig. S5, Fig. S6). And the
Photophysical property
Fig.
4
shows the ultraviolet-visible absorption and relatively short lifetimes are key to the device performance in
spectra of (4tfmppy)2Ir(dipdtc), which the abundant excitons produced instantaneously by
photoluminescence
(24btfmppy)2Ir(dipdtc) and (TN4T)2Ir(dipdtc) complexes at room electrical excitation can effectively decay.
temperature in CH2Cl2, and their photophysical data are listed in
(4tfmppy)2Ir(dipdtc)
(24btfmppy)2Ir(dipdtc)
(TN4T)2Ir(dipdtc)
(b)
(a)
(4tfmppy)2Ir(dipdtc)
(24btfmppy)2Ir(dipdtc)
(TN4T)2Ir(dipdtc)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Table 1. The intense absorption bands below 350 nm arise from
the spin-allowed intraligand π→π* transitions, and the relatively
weak bands in the visible region (350-530 nm) can be assigned to
the mixed MLCT and MLCT (metal-to-ligand charge-transfer)
states, or LLCT (ligand-to-ligand charge-transfer) transition
through strong spin-orbit coupling of iridium atom.12 With the
altering of the main ligands, the maximum emitting wavelength
of the complexes are tuned from 497 nm to 534 nm. Compared
1
3
200
300
400
500
600
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 4 The UV-vis absorption (a) and emission (b) spectra of complexes
(4tfmppy)2Ir(dipdtc), (24btfmppy)2Ir(dipdtc) and (TN4T)2Ir(dipdtc) in
degassed dichloromethane (5  10-5 M) at room temperature.
with
complex
(4tfmppy)2Ir(dipdtc)
using
2-(4-
(trifluoromethyl)phenyl)pyridine as the main ligand, which
Table 1. Photophysical data of three Ir(III) complexes.
a
b
c
c
d
Tm
Td
λabs
λem
λem
Φ e

ET
HOMO/LUMOg
(eV)
Energy Gap
(eV)
Complex
(°C)
(°C)
(nm)
(nm)
(nm)
(%)
(s)
(eV)
(4tfmppy)2Ir(dipdtc)
(24btfmppy)2Ir(dipdtc)
(TN4T)2Ir(dipdtc)
323
254
364
334
290
366
235/281/373
237/281/397
236/275/365
513
534
497
507
529
479
71.6
76.5
94.2
1.38c/0.74f
1.69c/1.47f
1.42c/1.03f
2.44
2.34
2.58
-5.25/-2.84
-5.47/-3.16
-5.76/-3.27
2.41
2.31
2.49
c
e
a: Melting temperature; b: Decomposition temperature; : measured in degassed CH2Cl2; d: measured at 77 K in degassed CH2Cl2; : Φ: emission
quantum yields were calculated with fac-Ir(ppy)3 standard in degassed CH2Cl2 solution (P = 0.4); f: measured in 5 wt% doped TCTA films; g: HOMO
(eV) = -(Eox-E1/2,Fc)-4.8, LUMO (eV) = HOMO+Ebandgap
.
OLEDs performance
(1 nm)
/
Al (100 nm) with (4tfmppy)2Ir(dipdtc),
(24btfmppy)2Ir(dipdtc) and (TN4T)2Ir(dipdtc) as dopants are
named as D1, D2 and D3, respectively. As shown in Fig. 5, the
materials of HAT-CN and LiF served as hole- and electron-
injecting interface modified materials, respectively. The bipolar
material 2,6DCzPPy and the hole-transporting material TCTA
were chosen as host materials, respectively. The stepwise
changed HOMO energy levels of TAPC (-5.5 eV), TCTA (-5.7
eV) and 2,6DCzPPy (-6.1 eV) are beneficial for the hole
injection and transport. Similarly, it is also beneficial for the
injection and transport of electrons owing to the gradual
To illustrate the electroluminescent properties of these
complexes, the OLEDs using these complexes as dopants with
double emissive layers were fabricated. The device
configuration consisting of ITO / HAT-CN (dipyrazino[2,3-
f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile,
5 nm) /
TAPC (di-[4-(N,N-ditolylamino)phenyl]cyclohexane, 50 nm) /
Ir(III) complexes (x wt%) : TCTA (4,4',4''-tris(carbazol-9-
yl)triphenylamine, 8 nm) / Ir(III) complexes (x wt%) :
2,6DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine, 8 nm) /
TmPyPB (1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene, 50 nm) / LiF
4 | J. Name., 2019, 00, 1-3
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changed LUMO energy levels of TmPyPB (-2.7 eV),
2,6DCzPPy (-2.6 eV) and TCTA (-2.4 eV). Additionally, the
triplet states (2.34, 2.44 and 2.58 eV, respectively) of the three
iridium complexes (24btfmppy)2Ir(dipdtc), (4tfmppy)2Ir(dipdtc)
and (TN4T)2Ir(dipdtc), respectively are all within the ones of
2,6DCzPPy, which is helpful to inhibit the energy reversion
from the guest to the host. Thus, holes and electrons will be
distributed in more balanced emissive layers and the exciton
recombination zone is expected to be broadened. Furthermore,
the TmPyPB HOMO energy level is 0.6 eV lower than that of
2,6DCzPPy and the TAPC LUMO energy level is 0.6 eV higher
than that of TCTA, which will result in exciton (hole-electron
pair) well conformed within emissive layers and the triplet
exciton quenching avoided effectively.
efficiency (ηp) versus luminance, and EQE as a function of
luminance characteristics of the devices are displayed in Fig. 6(a)-
(d), and the parameters of these devices are summarized in
Table 2.Fig. 6(a) shows the normalized EL spectra of the
devices measured at the voltage of 5 V with peaks at about 510,
534 and 494 nm for devices D1, D2 and D3, respectively, which
are very close to the PL spectra of three Ir(III) complexes,
indicating that the EL emission of the devices originates from
the triplet excited states of the phosphors. Due to the same
ancillary ligand and similar molecular structures, the device
performances of these materials mainly depend on their
photophysical and electrochemical properties.
Respectively, the device D1 (P of (4tfmppy)2Ir(dipdtc) is
71.6%) at a doping concentration of 8 wt% shows good EL
performances with the maximum luminance (Lmax) above 33466
cd m-2, the peak current efficiency (ηc,max) of 86.81 cd A-1 and
the peak power efficiency (ηp,max) of 59.85 lm W-1 and a
maximum external quantum efficiency (EQEmax) of 25.17%
with CIE coordinates of (0.253, 0.634). Relatively, the device
D2 (P of (24btfmppy)2Ir(dipdtc) is 76.5%) at a doping
concentration of 8 wt% displayed lower characteristics with a
Lmax of 38403 cd m-2, a ηc,max of 66.91 cd A-1, a ηp,max of 40.23
lm W-1 and an EQEmax of 17.30%. Device D3 based on
(TN4T)2Ir(dipdtc) (P of (TN4T)2Ir(dipdtc) is 94.2%) at a
-1.8eV
-2.4 eV
-2.6 eV
-2.7 eV
N
N
-4.3 eV
LiF/Al
TAPC
CN
NC
N
CN
N
-5.1 eV
ITO
N
N
N
N
N
NC
NC
-5.5 eV
N
N
2,6DCzPPy
CN
-5.5 eV
HAT-CN
-5.7 eV
-6.1 eV
N
N
N
-6.7 eV
N
N
N
doping concentration of
6 wt% showed the best EL
TCTA
-9.5 eV
TmPyPB
N
performances with a high brightness of 38812 cd m-2 and the
ηc,max, EQEmax and ηp,max are up to 86.59 cd A-1, 31.24% and
52.72 lm W-1, respectively. The EQE of 31.24% is among the
highest results ever reported in green Ir(III) complexes.13
Furthermore, the device efficiency roll-off is not serious. As for
device D3, the ηc and EQE can still be obtained as 79.21 cd A-1
and 28.53%, respectively, when the luminance reaches to 10000
cd m-2.
Fig. 5 Energy level diagram of HOMO and LUMO levels (relative to
vacuum level) for materials investigated in this work and their
molecular structures.
All devices were optimized and the details of optimizing the
doping concentration of Ir(III) emitters are included in the SI.
The electroluminescence (EL) spectra, current density (J)-
voltage (V)-luminance (L), current efficiency (ηc) and power
35
30
25
20
15
10
5
104
150
100
50
D1
D2
D3
D1
D2
D3
(d)
(c)
(a)
(b)
D1
D2
D3
1.0
0.8
0.6
0.4
0.2
0.0
100
80
60
40
20
0
100
50
0
103
102
101
100
D1
D2
D3
D1
D2
D3
D1
D2
D3
0
0
0
5000
10000
15000
20000
0
5000
10000
15000
)
20000
400 450 500 550 600 650 700
Wavelength (nm)
2
4
6
8
10
12
Voltage (V)
Luminance (cd m-2
Luminance (cd m-2
)
Fig. 6 Device characteristics: (a) EL spectra, (b) J-V-L, (c) c-L-p and (d) EQE-L curves of D1-D3.
Table 2. EL performances of the devices D1-D3.
a)
b)
Vturn-on
Lmax
(cd m-2)
ηc,max
(cd A-1)
ηext,max
ηp,max
(lm W-1)
ηcb)
(cd A-1)
ηext
(%)
CIEc)
(x, y)
Device
(V)
(%)
D1
D2
D3
3.1
4.2
3.6
33466
38403
38812
86.81
66.91
86.59
25.17
17.30
31.24
59.85
40.23
52.72
86.18
57.58
79.21
24.92
14.88
28.53
(0.253, 0.634)
(0.346, 0.615)
(0.191, 0.533)
a) Applied voltage recorded at a luminance of 1 cd m-2; b) Recorded at 10000 cd m-2; c) Measured at driving voltage of 5 V.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2019, 00, 1-3 | 5

View Article Online
DOI: 10.1039/C9TC01397K
Journal of Materials Chemistry C
Page 6 of 8
J. Mater. Chem. C
RSCPublishing
ARTICLE
In theory, the superior device properties of D3 may result
from the following facts. Firstly, nitrogen heterocycle TN4T was
used as main ligand and dithiocarbamate derivative with large
hindered spacers were applied as the ancillary ligands for stable
Ir(III) complexes, showing high PL efficiencies and bipolar
properties, which may contribute to a wider recombination area
and a more balanced distribution for the excitons and help to the
reduced the TTA, TPA and self-quenching effects. Secondly,
the stable dithiolate compounds can reduce the work function of
the emitter and reduce the threshold electric field, therefore
reducing the turn-on voltage and increase the device efficiency.8
Finally, double emissive layers would improve and balance
charge-injection/transporting properties of the devices.14
distribution, the TG curves, the DSC spectra, the cyclic voltammograms,
the emission spectra at 77 K, the selected lifetime curves of iridium
complexes. Current efficiency versus luminance of devices with
different doped concentrations.
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Conclusions
In conclusion, three Ir(III) complexes (4tfmppy)2Ir(dipdtc),
(24btfmppy)2Ir(dipdtc) and (TN4T)2Ir(dipdtc) containing the
unique four-membered ring Ir-S-C-S backbone were rapidly
prepared with high photoluminescence quantum efficiency
yields up to 94.2% in CH2Cl2 at room temperature. The double-
emitting-layer devices using the emitter (TN4T)2Ir(dipdtc)
displayed exceedingly good performances with a ηc,max of 86.59
cd A-1 and an EQEmax of 31.24%. Even at the brightness of
10000 cd m-2, the ηc and EQE can still be obtained as 79.21 cd
A-1 and 28.53%, respectively. This study provides a useful route
to design rapid synthesis of high-performance Ir(III) complex
with the four-membered ring Ir-S-C-S structure, which is key to
the indrustrial production of OLEDs.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51773088).
Notes and references
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Shenzhen Key Laboratory of Polymer Science and Technology,
College of Materials Science and Engineering, Shenzhen University
Shenzhen 518060, P.R. China, e-mail: gzhlu@szu.edu.cn
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DOI: 10.1039/C9TC01397K
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Journal of Materials Chemistry C
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Journal of Materials Chemistry C
Page 8 of 8
View Article Online
DOI: 10.1039/C9TC01397K
Three green iridium(III) complexes were fast synthesized at room temperature with
sulfur containing ligand, and their OLEDs show high performances with the EQEmax of
31.24% and low efficiency roll-off.