Highly-efficient solution-processed deep-red organic light-emitting diodes based on heteroleptic Ir(III) complexes with effective heterocyclic Schiff base as ancillary ligand

Article abstract of DOI:10.1016/j.orgel.2020.105885

Three new deep-red heteroleptic phosphorescent iridium(III) complexes Ir(piq)₂(L₁), Ir(piq)₂(L₂), and Ir(piq)₂(L₃), comprising cyclometalated ligand 1-pheynylisoquionoloine(piq) and heterocyclic Schiff base ancillary ligands 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol(L₁), 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol (L₂), and 4-(4-methoxyphenyl)imino-methyl-3-methyl-1-phenyl-1H-pyrazol-5-ol (L₃) have been designed, synthesized, and characterized. All the compounds emit deep red emission with λ_max values in the spectral range of 602–620 nm, high quantum yield 0.44 to 0.52 and short excited state lifetime τ (0.51–0.55 μs) due to dominant strong field ligands, resulting an efficient triplet metal-ligand charge transfer (³MLCT) excited state. Time dependent density functional theory (TD-DFT) calculations and electrochemical measurements of the compounds strongly support their genuine deep red phosphorescent emission. The combination of ancillary and cyclometalated ligands significantly influence the molecular orbitals of Ir(III) complex, leading to clearly distinct electron density distributions of the LUMO and HOMO. The compounds show good thermal stability and quantum yield, these characteristics making them an ideal candidate to exploit in phosphorescent organic light emitting diodes (PhOLEDs). Highly-efficient PhOLEDs were developed by using Ir(piq)₂(L₁), Ir(piq)₂(L₂), and Ir(piq)₂(L₃) in solution process as deep red emitters and device composed of Ir(piq)₂(L₃) exhibited an excellent external quantum efficiency of 14.9percent and current efficiency of 10.8 cd/A with the stable CIE coordinates of (0.67,0.33).

Full text of DOI:10.1016/j.orgel.2020.105885

Journal Pre-proof
Highly-efficient solution-processed deep-red organic light-emitting diodes based on
heteroleptic Ir(III) complexes with effective heterocyclic Schiff base as ancillary ligand
Meha J. Prajapati, Rohit Ashok Kumar Yadav, Sujith Sudheendran Swayamprabha,
Deepak Kumar Dubey, Jaydip D. Solanki, Jwo-Huei Jou, Kiran R. Surati
PII:
S1566-1199(20)30271-8
DOI:
https://doi.org/10.1016/j.orgel.2020.105885
ORGELE 105885
Reference:
To appear in: Organic Electronics
Received Date: 13 April 2020
Revised Date: 21 June 2020
Accepted Date: 21 June 2020
Please cite this article as: M.J. Prajapati, R.A. Kumar Yadav, S.S. Swayamprabha, D.K. Dubey, J.D.
Solanki, J.-H. Jou, K.R. Surati, Highly-efficient solution-processed deep-red organic light-emitting diodes
based on heteroleptic Ir(III) complexes with effective heterocyclic Schiff base as ancillary ligand, Organic
Electronics (2020), doi: https://doi.org/10.1016/j.orgel.2020.105885.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. 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.
© 2020 Published by Elsevier B.V.

H3C(H2C)3O
H
CH3
N
Ir
Al
N
O
N
N
2
V
LiF
(0.66, 0.34)
TPBi
CBP:Emitter
PEDOT:PSS
ITO
(0.67, 0.33)
Glass substrate
H3CO
H
CH3
N
Ir
N
O
N
N
2
(0.67, 0.33)

Highly-Efficient Solution-Processed Deep-Red Organic Light-Emitting Diodes
Based on Heteroleptic Ir(III) Complexes with Effective Heterocyclic Schiff
Base as Ancillary Ligand
Meha J. Prajapatia,§, Rohit Ashok Kumar Yadavb,§, Sujith Sudheendran Swayamprabhab, Deepak
Kumar Dubeyb, Jaydip D. Solankia Jwo-Huei Joub* and Kiran R. Suratia*
,
aDepartment of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388120, Anand,
Gujarat, India
bDepartment of Materials Science and Engineering, National Tsing Hua University, Hsinchu
30013, Taiwan, Republic of China
*Corresponding authors: jjou@mx.nthu.edu.tw and kiransurati@yahoo.co.in
§: Both authors contributed equally to this work.
Abstract
Three new deep-red heteroleptic phosphorescent iridium(III) complexes Ir(piq)2(L1), Ir(piq)2(L2),
and Ir(piq)2(L3), comprising cyclometalated ligand 1-pheynylisoquionoloine(piq) and
heterocyclic Schiff base ancillary ligands 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-
pyrazol-5-ol(L1), 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol (L2), and 4-(4-
methoxyphenyl)imino-methyl-3-methyl-1-phenyl-1H-pyrazol-5-ol (L3) have been designed,
synthesized, and characterized. All the compounds emit deep red emission with λmax values in
the spectral range of 602-620 nm, high quantum yield 0.44 to 0.52 and short excited state
lifetime τ (0.51-0.55 µs) due to dominant strong field ligands, resulting an efficient triplet metal-
ligand charge transfer (3MLCT) excited state. Time dependent density functional theory (TD-
DFT) calculations and electrochemical measurements of the compounds strongly support their

genuine deep red phosphorescent emission. The combination of ancillary and cyclometalated
ligands significantly influence the molecular orbitals of Ir(III) complex, leading to clearly
distinct electron density distributions of the LUMO and HOMO. The compounds show good
thermal stability and quantum yield, these characteristics making them an ideal candidate to
exploit in phosphorescent organic light emitting diodes (PhOLEDs). Highly-efficient PhOLEDs
were developed by using Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3) in solution process as deep
red emitters and device composed of Ir(piq)2(L3) exhibited an excellent external quantum
efficiency of 14.9% and current efficiency of 10.8 cd/A with the stable CIE coordinates of
(0.67,0.33).
Keywords: Heteroleptic iridium(III) complex, deep red, solution process, PhOLED.
Introduction
Phosphorescent organic light emitting diodes (PhOLEDs) have drawn potential attention in last
few decades as most promising alternatives in next generation display and solid-state lighting
technology [1-7]. Compared to the conventional well established liquid crystal display (LCD)
and light emitting diodes (LED) technology, the advantage of OLEDs includes high speed vide
rate, wide visual angle, high contrast, and cold resistance [8]. Moreover, OLEDs technology also
capable of flexible/transparent devices and extremely thin light screens.
The OLED technology has been unfolding from single heterojunction structure into a double
heterojunction structure, taking advantage of the severance of the carrier transport and light
emitting layers into two territories. Besides these multi-heterojunction architectures, it facilitates
more confine the charge carriers and improves the performance [9-10]. To achieve high
performance OLEDs development of various materials including electrodes, electron/hole

transport, host, and emitters are of the key issues drawing extensive academic-industrial interest.
Among all these compounds for OLEDs, Ir(III) based compounds are consider to be the most
promising candidates for the facile chemical modification, tunable photophysical characteristics
and superior stability.
A huge research has been carried out aimed to develop Ir(III) complexes for different
applications in OLEDs. However, development of an efficient deep red phosphorescent device is
a formidable challenge as it experienced the energy gap law and luminous flux drop in the
saturated red region [11-16]. Good efficiency of deep red phosphorescent OLED (PhOLEDs)
with the maximum luminescence in the region 640-680 nm and the Commission International de
L’Eclairage (CIE) coordinates of x ≥ 0.70 and y ≤0.30 are rare so far [17-20]. Therefore,
developing novel high-performance red phosphorescent emitters through an effective molecular
design strategy will revamp the color purity, luminescence quantum yield, and lifetime, hence
better OLED efficiency devices. Numerous attempts have been reported with the derivatives of
phenyl quinoline or phenyl iso-quinolone as cyclometalated ligands and other ancillary ligands
such as acetylacetonate (acac), picolinate(pic), and N,N′-diisopropylbenzamidinate (dipba) etc.
However, these compounds suffer from the limitations that lower the luminescence quantum
yield, excited lifetime, and device efficiency [21-33].
In the present study, we have designed and synthesized three new heteroleptic Ir(III) complexes,
namely Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3). The photophysical and electrochemical
characteristics were studied and reveals that the synthesized compounds possess significant
potential towards deep red emission along with good luminescence quantum yield. The
compounds emit deep red emission with λmax values in the spectral range of 602-620 nm, high
quantum yield 0.44 to 0.52 and short excited state lifetime (0.51-0.55 µs) due to dominant strong

field ligands, resulting an efficient triplet metal-ligand charge transfer (3MLCT) excited state.
The pyrazolone based ligand offers several important features to the compounds such as
enhancing luminescence quantum yield due to strong field effect of the ligands as well as good
film forming properties [10, 34-35]. Density functional theory (DFT), time dependent DFT (TD-
DFT) calculations, and electrochemical measurements of the compounds strongly support their
genuine deep red phosphorescent emission. The compounds show good thermal stability and
high glass transition temperature. These Ir(III) compounds employed as dopant for the deep red
PhOLEDs, a maximum EQE of 14.9% with CIE coordinate of (0.67, 0.33) was recorded for the
Ir(piq)2(L3) dopant. All the devices display very stable electroluminescent emission, low turn-on
and high luminance.
Results and discussion
All synthesized ancillary compounds were stable at room temperature, soluble in common
organic solvents and further characterized through different spectroscopic techniques. The results
were in good agreement with the proposed structures.
Structural description
The synthesis route of ancillary ligand L1, L2, and L3 are shown in Scheme 1. Intermediate 1-
phenyl-4-formyl pyrazolone derivatives were synthesized according to our previous reports
[36,37]. The ligands were characterized by 1H-NMR, APT and ESI‐MS. The 1H-NMR spectra of
the ligands showed the characteristics of an enolic proton at δ 11.34-11.40, which disappeared
during complexation with the Ir(III) metal ion [36,38]. FT‐IR spectra of the complexes show
band between 1504-1535 cm-1 which is assigned to aromatic C=C bond. Strong bands observed

in the range of 1612-1620 cm-1 is due to C=N azomethine group. The lower bands between 500-
800 cm-1 is due to the Ir-O and Ir-N [39]. The mass spectra of complexes L1, L2, and L3 showed
the characteristic molecular ion signals at 949, 877.4 and 907 m/z, respectively (Fig. S1-S11). X-
ray structure of ligands determines and its crystallographic data are summarized in (Table S1-S2
and Fig. S12).
Scheme 1: Synthesis of ancillary ligands and Ir(III) complexes.
Photophysical properties
Figure 1 displays the UV-Vis absorption spectra of the Ir(III) compounds measured at room
temperature in dichloromethane (DCM) solution. The absorption spectra can be considered as
composed of two parts, the broad and intense absorption bands observed below 350 nm closely
1
resembled that of cyclometalated and ancillary ligand that are assigned to spin allowed π- π*
transition [40-42]. These transitions are also known as ligand centered (LC) transitions [25]. The
peaks at around 350-420 nm could be as a result of the spin allowed inter ligand charge transfer
(1ILCT) between the cyclometalated and ancillary ligands. However, the weaker absorption band

observed above 450-500 nm could be assigned to the spin allowed transitions metal-to-ligand
charge transfer band (1MLCT), while spin forbidden (3MLCT) transition bands appeared at 550-
650 nm [43-44].
1.0
Ir(piq)2(L1)
Ir(piq)2(L2)
Ir(piq)2(L3)
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
Wavelength (nm)
Figure 1: Electronic absorption spectra of the compounds in DCM.
The emission spectra of the Ir(III) compounds at room temperature were carried out in DCM
solution and solid-state by using integrating sphere are shown in Figure 2. All complexes display
deep-red luminescence peak in the range of 602-621 nm, data are summarized in Table 1. The
compound displays deep red PL emission ranging 601-603 nm in the DCM solution. The
complexes show good quantum yield in the range of 0.44-0.52 with reference to Ir(ppy)3.
Ir(piq)2(L1)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Ir(piq)2(L1)
Ir(piq)2(L2)
Ir(piq)2(L3)
(b)
(a)
Ir(piq)2(L2)
Ir(piq)2(L3)
500
550
600
650
700
570
600
630
660
Wavelength (nm)
Wavelength (nm)
Figure 2: PL spectra of compounds in (a) DCM and (b) solid state.

Our rational design complexes show superior quantum yield as compared with the homoleptic
and heteroleptic iridium(III) complexes such as Ir(piq)3 (0.26) and Ir(piq)2(acac) (0.20) at 620
and 622 nm, respectively [14,45]. Our compounds show excellent ΦPL as compared with other
reported complexes [15,18,46]. Due to such limitations, this homoleptic and heteroleptic
complexes do not show significant properties and device performance too. Our designed
heteroleptic complexes are superior in terms of ΦPL and lifetime in the range of 0.50-0.55 µs,
which may be due to the superior excited MLCT characters over the other complexes. Here the
increasing MLCT character subsequently enhances the spin-orbit coupling (SCO) and leads to
larger radiative rate kr as summarized in Table 1. These complexes also show good solid state
luminescence in the range of 618-621 nm. All these complexes show a stronger bathochromic
shift and broad emission spectrum in solid state than those discerned in the solution state, which
may be due to presence of the packed molecules interact and the formation of excimer in solid
state [47-48]. It is believed that intermolecular interactions arising from π-π stacking in the thin-
film or solid-state have always led to small red shifts in the emission spectra. In the solid state,
the quantum yields of complexes were calculated using integrating sphere method and were
found to be 0.39, 0.48 and 0.49 respectively. All the emitters show nearly same emission in
toluene, chloroform and acetone with a shift of 2 nm [49]. Representative spectra of complex L1,
L2, and L3 are shown in Fig. S13-S15. It is interesting to note optical bandgap of complex
Ir(piq)2(L3) is higher than that of other counterparts, which may be attributed to presence of
relatively less bulky group (-OCH3) and lower possibilities of steric hindrance in the Schiff base
units in complex, Ir(piq)2(L3).

Table 1: Photophysical, electrochemical and thermal properties of iridium compounds.
e
l
a
b
c
d
i
j
HOMOg LUMOh Td
ΦPL
ΦPL
(Solid state)
0.39
Lifetime
τ (μs)
0.51
f
Eonset
[eV]
Eg
krk
knr
Compounds λPL
λPL
λabs
λonset
[nm] [nm] [nm] [nm]
[eV]
1.99
2.00
1.97
[eV]
-5.59
-5.62
-5.60
[eV]
-3.60
-3.62
-3.63
[°C]
(107 s-1) (106 s-1)
0.496
0.522
0.503
602
602
603
618
621
620
340
335
336
623
618
629
310 0.44
292 0.51
318 0.52
0.862
0.927
0.981
1.098
0.890
0.905
Ir(piq)2(L1)
Ir(piq)2(L2)
Ir(piq)2(L3)
0.48
0.55
0.49
0.53
[a] measured in dichloromethane
[b] measured in solid state
[c] measured in dichloromethane at concentration 5×10-6mol/lit
[d] estimated from absorption spectra
[e] estimated from absorption onset from UV-visible
[f] estimated from cyclic voltammetry
[g] estimated from E1/2oxd and Eonset obtained from cyclic voltammetry
[h] estimated using HOMO and Eg
[i] estimated from TGA (5% weight loss)
[j] in dichloromethane
[k] the radiative kr = ΦPL τ-1
[l]the non-radiative knr = (1- ΦPL) τ-1

Electrochemical properties
Cyclic voltammetry (CV) was conducted to study the electrochemical characteristics of the
iridium compounds and carried out by three-electrode system on CHI660E electrochemical
workstation (Ocean Optics, USA). The measurements were carried out at room temperature in
DCM solutions with tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The
cyclic voltammograms of calibration compound ferrocene obtained with reference to Ag/Ag+
electrode is shown in Figure 3. The cyclic voltammograms are obeying quasi reversible nature
and by using onset oxidation potential values which are ranging from -0.359 to 0.564 V, the
HOMO and LUMO energy levels of the compounds are calculated and summarized in Table 1.
Ir(piq)2(L1)
4
2
Ir(piq)2(L2)
Ir(piq)2(L3)
0
-2
-1.0
-0.5
0.0
0.5
1.0
Potential (V) vs. non aqueous Ag/Ag+
Figure 3: Cyclic voltammograms of compounds.
Thermal properties
The thermal behavior of the compounds was investigated by the thermogravimetric analysis
(TGA) under nitrogen atmosphere with a heating rate of 10 °C/min from room temperature to
500 °C as shown in Figure 4. The characteristic provides information such as thermal stability,
glass transition temperature and melting assay of the materials. Such parameter is crucial in
terms of fabrication of device using materials which are essential for the fabrication of OLED by

a vapor deposition method [50]. All the iridium complexes exhibit good thermal stability (Td) in
the range of 292-318 °C. Differential scanning calorimetry (DSC) analysis was performed from
30 to 250 ºC at a heating rate of 10 ºC/min under the nitrogen atmosphere (Fig. S16). As
indicated by the DSC curves, no obvious glass transition temperatures were observed which
suggests that these Ir(III) complexes are highly amorphous. These complexes showed excellent
thermal stabilities high enough for OLED device fabrication [51-53].
100
80
60
40
Ir(piq)2(L1)
20
Ir(piq)2(L2)
Ir(piq)2(L3)
0
0
100
200
300
400
500
Temperature (oC)
Figure 4: TGA curves of synthesized iridium(III) complexes.
Theoretical calculation
The density functional theory (DFT) and time dependent DFT (TD-DFT) calculations of
complexes were performed using the Becke’s three parameters and Lee-Yang-Parr (B3LYP)
hybrid exchange-correlation method with LANL2DZ basis set for the iridium atom and 6-31G*
for the other atoms. To more understand about the spatial distribution of the frontier molecular
orbitals, the contour plots of LUMO + 2 to HOMO - 2 of iridium complexes in Figure5. All
calculations are performed in Spartan’18 Software. As shown in Figure 5, HOMO of L1, L2, and
L3 are mainly localized on the pyrazolone ring of ancillary ligand and iridium center. Meanwhile,
the LUMO of all complexes are localized on the two cyclometalated ligands, along with a minor

contribution of iridium center. The other heteroleptic and homoleptic complexes were also
carried out with the same basic set to establish the more understanding regarding the behavior of
these ligands and their contribution in the formation of FMOs (Table S3-S5). The ancillary
ligands acac and pic of complex Ir(SQ)2(acac), Ir(CTBT)2(acac), Ir(MTPQ)2(pic), and
Ir(BTP)2(acac) (Table S6) are not contributing in the formation of frontier molecular orbitals
(FMOs), whereas the designed ancillary ligands L1-L3 in complex are largely effects on the FMOs
and subsequently enhances the radiative process by tuning the band-gap which is also support by
the device data shows in Table S3.
Figure 5: Frontier molecular orbitals (FMOs) of the iridium complexes.
Device electroluminescent characteristics
Inspired by the excellent photophysical characteristics and comprehensive abilities of the
emitters, we have evaluated the potential application of the iridium compounds as dopant in the
solution processed PhOLED with CBP host. Single emissive layer electroluminescent (EL)

devices with a structure ITO/ poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS; 35 nm)/ CBP: wt% emitter (22 nm)/ 1,3,5-tris(N-phenylbenzimidazol-2-yl)-
x
benzene (TPBi; 40 nm)/ LiF (1.5 nm)/ Al (150 nm) were fabricated by solution processing.
Herein, ITO acts as an anode, PEDOT:PSS serves as the hole injection/transporting layer
(HIL/HTL), TPBi serve as an electron transporting/hole blocking layer (HTL/HBL), LiF acts as
the electron injecting layer (EIL), and Al as a cathode. In order to achieve high performance
PhOLEDs, different doping concentration of the Ir(III) emitters (x = 5.0, 7.5, 10, and 12.5 wt%)
in the host CBP matrix were deployed. The studied device structure and molecular energy level
alignment between the layers are displayed in Figure 6.
Figure 6: (a) Schematic device structure and (b) energy level diagram of the studied
solution processed deep red PhOLEDs.
The current density-voltage-luminance, efficiency characteristics and normalized EL spectra of
the deep red PhOLED are shown in Figure 7 and data are summarized in Table 2. The
Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3) compounds showed deep red electroluminescence
emission having maxima spectral λmax value at 616, 617 and 620 nm with CIE coordinates of

(0.66, 0.34), (0.67, 0.33) and (0.67, 0.33), respectively. Devices exhibit very stable EL emission
spectra without any effect on the CIE coordinates when increasing the dopant concentration. The
main peak in the EL spectra of the devices was well consistent with the corresponding
photoluminescence spectra of the films. A shoulder peak was observed in the lower energy
region of EL spectra around 660 nm and became gradually drop with increasing dopant
concentration, which may be seeded by the excimer formation in solid state.
Furthermore, the red shifted EL peak with an increasing doping concentration was also
confirmed. No emission from the CBP host and adjacent charge transporting layers were
observed which indicates an efficient energy transfer mechanism from host to dopant and
effective confinement of charges. The doping concentration strongly influences the device
performance. Table S7 and Figures S17-S19 summarizes the effects of doping concentration on
power efficiency (PE), current efficiency (CE), external quantum efficiency (EQE), CIE
coordinates, and maximum luminance.
A deep red emitting PhOLED devices with stable CIE coordinates of (0.67, 0.33), a maximum
CE of 10.8 cd/A and a maximum EQE of 14.9% was achieved for the Ir(piq)2(L3) (10 wt%)
based device featured the highest performance. This is the highest EQE among the simple design
solution processed Ir(III) based PhOLEDs (Table S8). The EQE is approximately 1.5 and 2 times
higher than that of the device based on Ir(piq)2(L1) and Ir(piq)2(L2), respectively. For Ir(piq)2(L1)
and Ir(piq)2(L2), a decent maximum EQE of 9.6% and 7.3% was realized, respectively. The
Ir(piq)2(L3) based device showed an increased EQEs from 7.1% to 10.4% as the doping
concentration increased from 5.0 wt% to 10.0 wt%, respectively. As the concentration of the
dopant is increased to 12.5wt% the EQE dropped to 8.0%, this decline in EQE might cause from
the doping concentration quenching and triplet-triplet annihilation process. The Ir(piq)2(L3)

based device showed an EQE of 11.7% and 9.9% at 100 and 1000 cd/m2, respectively. Herein, it
is interesting to note that the devices do not suffer from the efficiency roll-off, which is a major
issue faced in PhOLEDs. The surface morphologies of the thin films were studied using AFM in
trapping mode. The root mean square values of the films were recorded 0.67 nm and 0.65 nm for
7.5 wt% and 10 wt% of Ir(piq)2(L3) dopant with CBP host matrix, respectively. The AFM
images display a uniform and homogenous distribution of the film surface illustrated in Fig. S20.
Remarkably, no major changes have been observed in the emission spectra of molecules by
introducing the alkoxy group in the Schiff base units. However, molecular orbital energy level
and quantum yield of the compounds has been slightly altered. The quantum yield of the
compounds and EQE of the reported devices are quite good when compared with the complexes
those reported in the literature. The observed 14.9% EQE for the OLED device based on
complex Ir(piq)2(L3) may also be attributed to the presence of less bulky (-OCH3) group in the
Schiff base units.
It is also noticeable that observed PE and CE of the resultants OLED devices lower than that of
EQE. The reason beside this may be high PLQY value of emitters, efficient energy transfer
between host and guest molecules, generation of radiative exciton in the sufficient amount, and
balanced charge carrier in the recombination zone. The PE of OLED is inversionally
proportional to driving voltage of the device, a hole enriched device leads to low driving voltage
and high PE. However, the EQE of an OLED device depends on many factors and can be
determined by the optical out-coupling factor (ƞop), photoluminescence quantum yield (PLQY),
exciton generation (ƞr), and carrier balance factor (γ).

Figure 7. (a) Current density-voltage, (b) luminance-voltage, (c) current efficiency-
luminance, and (d) EL spectra plots of the solution processed deep red PhOLEDs.
Table 2. Electroluminescence performances of the deep red solution processed PhOLEDs.
Dopant
Dopant conc.
PEmax
(lmW-1)
6.2
CEmax
(cdA-1)
9.9
EQEmax
(%)
CIE
(wt%)
7.5
Coordinates
(0.66, 0.34)
(0.67, 0.33)
(0.67, 0.33)
Ir(piq)2(L1)
Ir(piq)2(L2)
Ir(piq)2(L3)
9.6
10
3.3
7.1
7.3
10
4.8
10.8
14.9
Conclusion
In summary three new heteroleptic deep red iridium(III) phosphorescent compounds were
designed with cyclometalated ligand (piq) and heterocyclic Schiff bases as ancillary ligands. All
the compounds are well characterized by spectroscopy techniques. The photophysical properties

of the complexes show excellent ΦPL 0.44 to 0.52 and short excited-state lifetime
τ (0.51-0.55
µs) due to dominant strong field ligands, L1-L3 resulting in efficient triplet metal-ligand charge
transfer (3MLCT) existed state. These results further support by the spin forbidden 3MLCT bands
550-650 nm. The DFT and TD-DFT calculations strongly support the involvement of ancillary
ligands in formation of FMOs and the resulting complexes offer radiative process with band gap
fine tuning. These complexes show excellent thermal stability and high glass transition
temperature. Highly efficient solution processed PhOLEDs were developed by using Ir(piq)2(L1),
Ir(piq)2(L2), and Ir(piq)2(L3) as deep red emitters. Device composed of Ir(piq)2(L3) exhibited an
excellent external quantum efficiency of 14.9% and current efficiency of 10.8 cd/A with the
stable CIE coordinates of (0.67,0.33).
Acknowledgments
Authors express sincere thanks to Science and Engineering Research Board (SERB), New Delhi,
India (Ref. No. SB/EMEQ-204/2014), scheme of Young Scientist &Technologist (SYST)(Ref.
No.SP/YO/008/2014(G)), Department of Science and Technology (DST), New Delhi under the
Technology System Group (TSG) program (Ref. No. DST/TSG/PT/2011/194-G)for the financial
support. Authors are thankful to the Head, Department of Chemistry, Sardar Patel Universityfor
providing basic infrastructural and instrumental facility, Saurashtra University, Rajkot for
instrumental facility. Special thanks to Dr. S. S. Soni and his group for cyclic voltammetry
measurements. MJPand JDS acknowledges financial support as INSPIRE fellowship
(DST/INSPIRE Fellowship/2015/IF-150296) and SYST(Ref. No.SP/YO/008/2014(G))from the
DST.

Conflicts of interest
There are no conflicts to declare.
References
[1] Li, T.-Y.; Wu, J.; Wu, Z.-G.; Zheng, Y.-X.; Zuo, J.-L.; Pan, Y. (2018). Rational design of
phosphorescent iridium(III) complexes for emission color tunability and their applications in
OLEDs. Coord. Chem. Rev. 374, 55.
[2] Wang, L., Zhao, Z., Wei, C., Wei, H., Liu, Z., Bian, Z., Huang, C. (2019). Review on the
electroluminescence study of lanthanide complexes, Adv. Opt. Mater. 7, 1801256.
[3] Liu, C.-F., Liu, X., Lai, W.-Y., Huang, W. (2018). Organic light-emitting field-effect
transistors: device geometries and fabrication techniques. Adv. Mater. 30, 1802466.
[4] Li, G, Congrave, D.G., Zhu, D., Su, Z., Bryce, M.R. (2018).Recent advances in luminescent
dinucleariridium(III) complexes and their application in organic electroluminescent devices.
Polyhedron. 140, 146.
[5] Jou, J.-H., Sahoo, S., Dubey, D.K., Yadav, R.A.K., Swayamprabha, S. S., Chavhan, S. D.
(2018) Molecule-based monochromatic and polychromatic OLEDs with wet-process
feasibility.J. Mater. Chem. C. 6, 11492.
[6] Lee, S. M., Kwon, J. H., Kwon, S., Choi, K. C. (2017). A review of flexible oleds toward
highly durable unusual displays. IEEE Trans. Electron Devices 64, 1922.
[7] Ou, Q., Tang, J. (2016). Recent advances in light manipulation for organic light-emitting
diodes, Materials China 35, 606.

[8] Liu, X., Yao, B., Zhang, Z., Zhao, X., Zhang, B., Wong, W. Y., Cheng, Y.,Xie, Z. (2016).
Power-efficient solution-processed red organic light-emitting diodes based on an exciplex
host and a novel phosphorescent iridium complex. J. Mater. Chem. C. 4, 5787.
[9] Fu, H., Cheng, Y.-M., Chou, P.-T., Chi, Y. (2011). Feeling blue? blue phosphors for OLEDs.
Mater. Today 14, 472.
[10] Pereira, L. (2012). Organic light emitting diodes: the use of rare earth and transition metals.
PanStandford Publisher, USA.
[11] Caspar, J. V., Meyer, T. J. (1983). Application of the energy gap law to nonradiative,
excited-state decay. J. Phys. Chem. 87, 952.
[12] Cummings, S. D.; Eisenberg, R. (1996). Tuning the excited-state properties of
platinum(ii)diiminedithiolate complexes. J. Am. Chem. Soc. 118, 1949.
[13] Chen, C. T. (2004). Evolution of red organic light-emitting diodes: materials and
devices.Chem. Mater.16, 4389.
[14] Su, Y. J., Huang, H. L., Li, C. L.,Chien, C. H., Tao, Y. T., Chou, P. T., Datta, S., Liu, R. S.
(2003). Highly efficient red electrophosphorescent devices based on iridium isoquinoline
complexes: remarkable external quantum efficiency over a wide range of current. Adv.
Mater.15, 884.
[15] Li, C. L., Su, Y. J., Tao, Y. T., Chou, P. T., Chien, C. H., Cheng, C. C., Liu, R. S. (2005).
Yellowand red electrophosphors based on linkage isomers of phenylisoquinolinyliridium
complexes: distinct differences in photophysical and electroluminescence properties. Adv.
Funct. Mater.15, 387.
[16] Fan, C. H., Sun, P.,Su, T. H., Cheng, C. H. (2011). Host and dopant materials for idealized
deep-red organic electrophosphorescence devices. Adv. Mater.23, 2981.

[17] Song, Y. H., Yeh, S. J., Chen, C. T., Chi, Y., Liu, C. S., Yu, J. K., Hu, Y. H., Chou, P. T.,
Peng, S. M., Lee, G. H. (2004). Bright and efficient, non-doped, phosphorescent organic red-
light- emitting diodes. Adv. Funct. Mater.14, 1221.
[18] Sree, V. G., Cho, W., Shin, S., Lee, T., Gal, Y. S., Song, M.,Jin, S. H. (2017). Highly
efficient solution-processed deep-red emitting heteroleptic thiophene-phenylquinoline based
Ir(iii) complexes for phosphorescent organic light-emitting diodes. Dyes and Pigments139,
779.
[19] Bae, H. J., Chung, J., Kim, H., Park, J., Lee, K. M., Koh, T. W., Lee, Y. S., Yoo, S., Do, Y.,
Lee, M. H. (2014). Deep red phosphorescence of cyclometalated iridium complexes by o-
carborane substitution. Inorg. Chem.53, 128.
[20] You, Y., An, C. G., Kim, J. J., Park, S. Y. (2007). A deep red phosphorescent Ir(iii) complex
foruse in polymer light-emitting diodes: role of the arylsilyl substituents. J. Org. Chem. 72,
6241.
[21] Kim, D.H., Cho, N.S., Oh, H.-Y., Yang, J. H., Jeon, W. S., Park, J. S., Suh, M. C., Kwon, J.
H. (2011). Highly efficient red phosphorescent dopants in organic light-emitting devices.
Adv. Mater.23, 2721.
[22] Su, T.-H., Fan, C.-H., Ou-Yang, Y.-H., Hsu, L.-C., Cheng, C.-H. (2013). Highly efficient
deep-red organic electrophosphorescent devices with excellent operational stability using bis
(indoloquinoxalinyl) derivatives as the host materials. J. Mater. Chem. C.1, 5084.
[23] Ho, C. L., Wong, W. Y., Gao, Z. Q., Chen, C. H., Cheah, K. W., Yao, B.,Xie, Z. Y., Wang,
Q., Ma, D. G., Wang, L. X., Yu, X. M., Kwok, H. S., Lin, Z.Y. (2008). Red-light-emitting
iridium
complexes
with
hole-transporting
9-arylcarbazole
moieties
for

electrophosphorescence efficiency/color purity trade-off optimization. Adv. Funct. Mater.18,
319.
[24] Niu, Z.-G., Yan, L.-P.,Wu, L.,Chen, G.-Y.,Sun, W.,Liang, X.,Zheng, Y.-X.,Li, G.-N., Zuo,
J.-L. (2019). Iridium(III) complexes adopting thienylpyridine derivatives for yellow-to-deep
red OLEDs with low efficiency roll-off.Dyes and Pigments162, 863.
[25] Li, Q., Shi, C., Huang, M.,Wei, X.,Yan, H.,Yang, C.,Yuan, A.(2019). B- and N-embedded
color-tunable phosphorescent iridium complexes and B-N Lewis adducts with intriguing
structural and optical changes.Chem. Sci. 10, 3257.
[26] Lu, K.-Y., Chou, H.-H., Hsieh, C.-H., Yang, Y.H. O., Tsai, H.- R., Tsai, H. Y., Hsu, L. C.,
Chen, C.-Y., Chen, I. C., Cheng, C.-H. (2011). Wide-range color tuning of iridium
̂
biscarbene complexes from blue to red by different N N ligands: an alternative route for
adjusting the emission colors. Adv. Mater.23, 4933.
[27] Li, G., Li, P., Zhuang, X., Ye, K., Liu, Y., Wang, Y. (2017) Rational design and
characterization of heteroleptic phosphorescent complexes for highly efficient deep-red
organic light-emitting devices. ACS Appl. Mater. Interfaces9, 11749.
[28] Lai, P.-N., Teets. T. S. (2019). Ancillary ligand effects on redemitting cyclometalated iridium
complexes. Chem. Eur. J. 25, 6026.
[29] Chao, Y., Huang, S., Chen, C., Chang, Y., Meng, H., Yen, F., Lin, I., Zan, H., Horng, S.
(2011). Highly efficient solution-processed red organic light-emitting diodes with long-side-
chained triplet emitter. Synth. Met. 161, 148.
[30] Kim, H. U., Jang, J.-H., Xu, F., Lee, J. Y., Hwang, D.-H. (2016). A new iridium(III) complex
as a deep-red phosphorescent emitter in organic light-emitting diodes. J. Nanosci.
Nanotechnol. 16, 2773.

[31] Cho, W., Reddy, S.S., Kim,J., Cho, Y.-R., Jin, S.-H. (2018). All solution-processed red
organic light-emitting diode based on a new thermally cross-linked heterolepticIr(III)
complex. J. Mater. Chem. C. 6, 11714.
[32] Sree, V. G., Park, H.-Y., Gal, Y.-S., Lee, J. W., Jin, S.-H. (2018). Highly efficient deep-red
emitting methyl substituted thiophenylquinoline based Ir(III) complexes for solution-
processed organic light-emitting diodes. Mol. Cryst. Liq. Cryst.660, 1.
[33] Chang, Y.-F., Yang, L.-S., Su, H.-T., Liao, S.-Y., Niu, M.-C., Mahesh, K. P. O., Chen, C.-H,
Chao, Y.-C., Meng, H.-F., Huang, H.-L., Chao, T.-C., Tseng, M.-R., Luo, C.-W., Zan, H.-
W.,Horng, S.-F. (2019). Allsolutionprocessed red and orangered organic lightemitting diodes
with highefficiencies: the effect of mixedhost emissive layers and thermal annealing
treatment. ChemPlusChem.84, 1375.
[34] Kido, J., Hayase, H., Hongawa, K., Nagai, K., Okuyama, K. (1994) Bright red
light emitting organic electroluminescent devices having a europium complex as an emitter.
Appl. Phys. Lett. 65, 2124.
[35] Binnemans, K. (2005) Rare-earth beta-diketonates. In Handbook of the Physics and
Chemistry of Rare Earths;Gschneidner, K.A., Jr., Bünzli, J.-C.G., Pecharsky, V.K., Eds.;
North Holland: Amsterdam, Holland,Elsevier, Volume 35, pp. 107–272. DOI:
https://doi.org/10.1016/S0168-1273(05)35003-3.
[36] Kumar, S., Surati, K.R., Lawrence, R., Vamja, A.C., Yakunin, S., Kovalenko, M. V., Santos,
E. J. G., Shih, C.-J. (2017). Design and synthesis of heterolepticIridium(III) phosphors for
efficient organic light-emitting devices. Inorg. Chem.56, 15304.

[37] Surati, K.R., Thaker, B.T. (2010) Synthesis, spectral, crystallography and thermal
investigations of novel Schiff base complexes of manganese(III) derived from heterocyclic β-
diketone with aromatic and aliphatic diamine. Spectrochim. Acta Part A75, 235.
[38] Prajapati M. J., Surati, K.R., (2019). Aggregation induced emission (AIE) properties of
heterolepticIr(III) complexes with non-chromophoric ligand effect:synthesis, characterization
and photophysical investigations.ChemistrySelect4, 8263.
[39] Nakamoto. K. (1978). Infrared and raman spectra of inorganic and coordination compounds.
3rd Edn., John Wiley and Sons, New York.
[40] H. Yersin, H. (2007). Highly efficiency OLEDs with phosphorescent materials.Wiley-VCH
Verlag GmbH & Co. KGaA.
[41] Lamansky, S., Djurovich, P., Murphy, D., Abdel-Razzaq, F.,Kwong, R.,Tsyba, I., Bortz, M.,
Mui, B., Bau, R., Thompson, M.E. (2001) Synthesis and characterization of phosphorescent
cyclometalated iridium complexes. Inorg. Chem. 40, 1704.
[42] Colombo, M.G., Hauser, A., Gudel, H.U. (1994).Competition between ligand centered and
charge transfer lowest excited states in bis cyclometalated Rh3+ and Ir3+ complexes. T.
Curr. Chem.171, 143.
[43] Kim, H.U., Jang, H.J., Choi, W., Kim, M., Park, S., Park, T., Lee, J.Y., Bejoymohandas, K.S.
(2019). Ancillary ligand-assisted robust deep-red emission in iridium(III) complexes for solution-
processable phosphorescent OLEDs. J. Mater. Chem. C.7, 4143.
[44] Lai, P.-N., Brysacz, C. H., Alam, M. K., Ayoub, N. A., Gray, T. G., Bao, J., Teets, T. S.
(2018). Highly efficient red-emitting biscyclometalated iridium complexes. J. Am. Chem.
Soc. 140, 10198.

[45] Tsuboyama, A., Iwawaki, H., Furugori, M., Mukaide, T., Kamatani, J., Igawa, S., Moriyama,
T., Miura, S., Takiguchi, T., Okada, S., Hoshino, M. (2003). Homoleptic cyclometalated
iridium complexes with highly efficient red phosphorescence and application to organic
light-emitting diode. J. Am. Chem. Soc. 25, 12971.
[46] Zhou, G., Wong, W.-Y., Yao, B., Xie, Z., Wang, L. (2007). Triphenylamine-dendronized
pure red iridium phosphors with superior OLED efficiency/color purity trade-offs. Angew.
Chem., Int. Ed. 46, 1149.
[47] Thaker, B. T., Surati, K. R., Oswal, S., Jadeja, R. N., Gupta, V. K. (2007). Synthesis,
spectral, thermal and crystallographic investigations on oxovanadium(IV) and
manganese(III) complexes derived from heterocyclic β-diketone and 2-amino ethanol.
Structural Chem.18, 295.
[48] Thakur, D., Dubey, D. K., Yadav, R., Venkateswarulu, M., Banik, S., Jou, J. H., Ghosh, S. A
(2020) Way to Clinch High Triplet Energy with Admirable Current and Power Efficiency
Devoiding Outcoupling Mode for RG Phosphorescent OLED. J. Mater. Chem. C 8 228-239.
[49] Huang, S.F., Sun, H.Z., Shan, G.G., Wu, Y., Zhang, M., Su, Z.M. (2016). Towards an
efficient blue emission cationic Ir(III) complex with azole-type ancillary ligands: a joint
theoretical and experimental study. New J. Chem.40, 4635.
[50] Schols, S. (2011). Device architecture and materials for organic light emitting
devices.Springer.
[51] Haines, P.J. (1995). Thermal method of analysis principle, application and problems.Blackie
academic & professional publisher, UK.
[52] Cao, X., Hu, J., Tao, Y., Yuan, W., Jin, J., Ma, X., Zhang, X., Huang, W. (2017). Alkyl
effects on the optoelectronic properties of bicarbazole/cyanobenzene hybrid host materials:

Double delayed fluorescent host/dopant systems in solution-processed OLEDs. Dyes and
Pigments 136, 543.
[53] Kim, H. U., Sohn, S., Choi, W., Kim, M., Ryu, S. U., Park, T., Jung, S., Bejoymohandas, K.
S. (2018). Substituents engineered deep-red to near-infrared phosphorescence from tris-
heteroleptic iridium(III) complexes for solution processable red-NIR organic light-emitting
diodes. Journal of Materials Chemistry C 6(39), 10640.

Highlights
•
•
•
Novel deep-red heteroleptic phosphorescent iridium(III) complexes is designed and
synthesized.
High thermal stabilities, excellent photophysical and electroluminescent features of the
complexes.
The solution-processed deep-red device shows CIE of (0.67, 0.33) and EQE of 14.9%.

Declaration of interests
☐ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐ The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: