Synthesis, photophysical, electrochemical and electroluminescence studies of red emitting phosphorescent Ir(III) heteroleptic complexes

Article abstract of DOI:10.1007/s12039-017-1350-y

Abstract: Five heteroleptic, cyclometalated (C^∧N) Iridium(III) complexes of acetylacetone (acac) and 1-phenyl-isoquinoline (piq) derivatives, Ir(acac)(piq)₂, Ir(acac)(2,4-difluoro-piq)₂, Ir(acac)(4-trifluoromethyl-piq)₂, Ir(acac)(4-N,N-dimethyl-piq)₂, Ir(acac)(4-acetyl-piq)₂, were synthesized and characterized. The (C ^∧N ) ₂Ir(acac) complexes in toluene showed phosphorescence (λ _max= 598 nm to 658 nm) with quantum yields (0.1 to 0.32) and microsecond lifetimes (0.43 to 1.9 μ s). The complexes were non-luminescent in thin films due to self-quenching but luminescent when lightly doped (5%) in a host organic material, 4,4^′-Bis(N-carbazolyl)-1,1^′-biphenyl (CBP). The HOMO levels determined using cyclic voltammetric oxidation potentials were in the range ?5.48 to ?5.80 eV. Electroluminescence properties and performance of the Ir complexes doped in CBP (active layer) were studied in a multilayer (ITO/F4TCNQ/TPD/doped CBP/BCP/LiF/Al) organic light emitting device (OLED). The electroluminescense (EL) spectra of the device matched with the phosphorescent spectra of the Ir complexes. The turn-on voltage at ～ 4.5 V, maximum brightness of 7600 cd/m ² and current efficiency of ～ 7.0 cd/A at a brightness of ～ 100 cd/m ² indicate that these are promising OLED materials. GRAPHICAL ABSTRACT: Synopsis. Heteroleptic, cyclometalated (C^∧N) Iridium(III) complexes of acetylacetone (acac) and 1-phenyl-isoquinoline were synthesized and their photophysical, electrochemical and electroluminescence properties were studied. The OLED of Ir complex as emitting material showed turn-on voltage at ～ 4.5 V, maximum brightness of 7600 cd/m ² and current efficiency of ～ 7.0 cd/A at a brightness of ～ 100 cd/m ².[Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-017-1350-y

J. Chem. Sci.
© Indian Academy of Sciences
DOI 10.1007/s12039-017-1350-y
REGULAR ARTICLE
Synthesis, photophysical, electrochemical and electroluminescence
studies of red emitting phosphorescent Ir(III) heteroleptic
complexes
FARMAN ALIa,b,∗, PABITRA K NAYAKa,c, N PERIASAMYa,d and NEERAJ AGARWALa,e,∗
aDepartment of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba,
Mumbai 400 005, India
bDepartment of Applied Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India
cClarendon Laboratory, University of Oxford, Oxford OX1 3PU, UK
d303, Golf Manor, Wind Tunnel Road, Bengaluru 560 017, India
eUM-DAE, Centre for Excellence in Basic Sciences, Health Centre Building, Kalina Campus, Santacruz (E),
Mumbai 400 098, India
E-mail: farmanali.ac@amu.ac.in; na@cbs.ac.in
MS received 21 March 2017; revised 14 June 2017; accepted 3 July 2017
Abstract. Five heteroleptic, cyclometalated (C∧N) Iridium(III) complexes of acetylacetone (acac) and
1-phenyl-isoquinoline (piq) derivatives, Ir(acac)(piq)2, Ir(acac)(2,4-difluoro-piq)2, Ir(acac)(4-trifluoromethyl-
piq)2, Ir(acac)(4-N,N-dimethyl-piq)2, Ir(acac)(4-acetyl-piq)2, were synthesized and characterized. The
(C∧N) Ir(acac) complexes in toluene showed phosphorescence (
= 598 nm to 658 nm) with quantum
λ
max
2
yields (0.1 to 0.32) and microsecond lifetimes (0.43 to 1.9 μs). The complexes were non-luminescent in thin
films due to self-quenching but luminescent when lightly doped (5%) in a host organic material, 4,4ꢀ-Bis(N-
carbazolyl)-1,1ꢀ-biphenyl (CBP). The HOMO levels determined using cyclic voltammetric oxidation potentials
were in the range −5.48 to −5.80 eV. Electroluminescence properties and performance of the Ir complexes doped
in CBP (active layer) were studied in a multilayer (ITO/F4TCNQ/TPD/doped CBP/BCP/LiF/Al) organic light
emitting device (OLED). The electroluminescense (EL) spectra of the device matched with the phosphorescent
spectra of the Ir complexes. The turn-on voltage at ∼4.5 V, maximum brightness of 7600 cd/m2 and current
efficiency of ∼7.0 cd/A at a brightness of ∼100 cd/m2 indicate that these are promising OLED materials.
Keywords. 1-Phenyl-isoquinoline; Suzuki-Miyaura coupling; Iridium(III)complex; phosphorescence; red
emitter; electroluminescence.
1. Introduction
transition. As a result, the radiative rate of phospho-
rescence from triplet to singlet is enhanced; resulting
in decreased lifetime of the triplet state and enhanced
luminescence quantum yield at room temperature.6 In
small molecule based OLEDs, recombination of elec-
trons and holes produces singlet and triplet excitons in
1:3 ratio. The triplet excitons in most organic materi-
als do not contribute to emission and go waste.7 Highly
phosphorescent molecules like Ir complexes are useful
to harvest both singlet and triplet excitons and increase
the electroluminescence efficiency.8 In spite of these
advantages, an Ir complex is rarely used as an active
emitting layer because of self-quenching. The Ir com-
plex is optimally doped in an active small molecule
host material such that triplets of the host molecule are
Luminescent, homoleptic and heteroleptic Ir(III) com-
plexes consisting of one or more cyclometalating (C∧N)
ligands are an important class of materials used in
electro-phosphorescent organic light emitting devices
(OLEDs).1–5 The luminescence of the heavy metal com-
plex is due to phosphorescence from the triplet state.
The normally forbidden transition between Ground and
Triplet state becomes permissible due to spin-orbit cou-
pling strongly induced by the heavy metal. Often, the
extinction coefficient (oscillator strength) for singlet-
triplet transition becomes comparable to singlet-singlet
*
For correspondence
Electronic supplementary material: The online version of this article (doi:10.1007/s12039-017-1350-y) contains supplementary material,
which is available to authorized users.

Farman Ali et al.
harvested and self-quenching is avoided. Doped electro- Deca Electrospray quadrupole ion trap mass spectrometer
(Thermo Electron Co., Hemel Hempstead, Herts, UK) or on
MALDI-TOF (Micromass Tof Spec 2E). Photoluminescence
was measured using SPEX Fluorolog 1681. Cyclic voltam-
metry was done using CH Instruments 600C.
phosphorescent devices using Ir complexes have yielded
nearly 100% internal efficiency.9 Nearly 16% exter-
nal efficiency has been reported in white light emitting
devices using doped electro-phosphorescent material.10
The emission colour of an Ir Complex is tuneable in
different ways like changing C∧N ligand structure in a
homoleptic complex and by changing ancillary ligand
in a hetroleptic complex.11–14 A number of studies have
revealed a wealth of information on the photophysical,
electrochemical, and electroluminescence properties of
a large number of Ir complexes, apart from valuable
information on synthetic routes and theoretical studies
on these molecules.15–17 Among the many cyclometa-
lating ligands, it is shown that Ir complex of 1-phenyl
isoquinoline (piq) has superior performance in elec-
2.2 Synthesis
2.2a General method for the synthesis of ligands:
Compounds 6–10 (see Figure 1 for the synthetic scheme and
structuresofmolecules1–10)weresynthesizedbytheSuzuki-
Miyaura coupling.23,24 In a screw cap reaction vial charged
with 1-chloroisoquinoline (1.0 mmol), aryl boronic acid (1.2
mmol), K2CO3 (5.0 mmol) and Pd(PPh3)4 (0.05 mmol), 1,2-
dimethoxyethane (DME) (3.0 mL) and water (3.0 mL) were
added and the reaction mixture was refluxed under nitrogen
atmosphere for 20 h. On completion of the reaction, water was
troluminescence. For example, PL quantum yield of added and the organic phase was separated. The aqueous layer
Ir(piq)3 was 0.32 in toluene18 and in OLEDs using
was washed thrice with ethyl acetate. All the organic layers
Ir(piq)3 as dopant, EL brightness of 11,000 cd/m2 has
were combined and washed with water, brine and dried over
been reported.19 The power efficiency and EQE are
anhydrous sodium sulphate. The crude product was purified
by column chromatography using hexanes and ethyl acetate
of 8.0 lm/W and 10.3% at a brightness of 100 cd/m2
as eluent. Pure compounds 6–10 were obtained as solid in
73–92% yield.
have been reported.20 Similarly, good performance was
observed in heteroleptic complex of Ir(piq)2X.21
2.2.1a 6: Yield: 0.19 g (92%); M.p.: 95◦C.1H NMR (500
MHz, CDCl3): δ ppm 7.54 (m, 4H), 7.69 (m, 4H), 7.89 (d, 1H,
Another way of tuning emission colour of an Ir
complex is by introducing electron donating and with-
drawing group in the cyclometalating ligand.22 A few
derivatives of piq have been synthesized and PL and EL
properties studied. In this work, we report the synthe-
sis of five heteroleptic Ir complexes. The photophysical
and electrochemical properties of these complexes are
compared with Ir(piq)2(acac). Electroluminescence of
one Ir complex(2) is also reported.
J = 8 Hz), 8.11 (d, 1H, J = 8.5 Hz), 8.61 (d, 1H, J = 5.5 Hz).
13C NMR (125 MHz, CDCl3) δ ppm 119.81, 120.00, 126.7,
126.88, 127.12, 127.52, 127.64, 128.32, 128.54, 129.79,
129.99, 136.84, 142.13, 142.28. ES-MS (m/z) for C15H11N
Calcd. 205.26; Obsd. 206.22 (M+1)+.
2.2.1b 7: Yield: 0.19 g (79%); M.p.: 80◦C. 1H NMR (500
MHz, CDCl3): δ ppm 7.0 (m, 2H), 7.57 (m, 2H), 7.71 (m, 2H),
7.78 (m,1H), 7.9 (d, 1H, J = 8.5 Hz), 8.63 (d, 1H, J = 5.5
Hz). 13C NMR (125 MHz, CDCl3) δ ppm 103.96, 104.17,
104.38, 111.66, 111.83, 120.8, 126.93, 126.98, 127.52,
130.29, 132.79, 132.82, 132.86, 142.32, 142.35. ES-MS (m/z)
for C15H9F2N Calcd. 241.24; Obsd. 242.26 (M+1)+.
2. Experimental
2.1 Materials and measurements
2.2.1c 8: Yield: 0.20 g (73%); M.p.: 120◦C. 1H NMR (500
MHz, CDCl3) δ ppm 7.57 (t, 1H, J = 7.0 Hz), 7.71 (m, 2H),
7.82 (m, 4H), 7.92 (d, 1H, J = 8.0 Hz), 8.02 (d, 1H, J = 8.5
Hz), 8.63 (d, 1H, J = 5.0 Hz).13C NMR (125 MHz, CDCl3) δ
ppm 28.81, 120.56, 125.31, 125.34, 126.53, 126.91, 127.18,
127.59, 130.25, 130.28, 136.84, 142.24, 142.28, 143.09,
159.13. ES-MS (m/z) for C16H10F3N Calcd. 273.26; Obsd.
274.27 (M+1)+.
All the solvents, Na2CO3 and K2CO3 were obtained from
SD Fine Chemicals (India). IrCl3, 1-chloroisoqinoline, acety-
lacetone and 2-ethoxyethanol were purchased from Sigma-
Aldrich and were used as received. F4TCNQ, BCP, CBP,
TPD, Pd(PPh3)4 and aryl boronic acids were purchased from
Sigma-Aldrich. Reactions that require anhydrous conditions
were carried out under an inert atmosphere of nitrogen. The
progress of the reaction was monitored using Silica Gel TLC
plates by UV detection (254 and 365 nm). Silica gel (100–
2.2.1d 9: Yield: 0.22 g (83%); M.p.: 101◦C. 1H NMR (500
200 mesh) was used for column chromatography. Melting MHz, CDCl2) δ ppm 3.04 (s, 6H), 6.87 (d, 2H, J = 8.5
points (uncorrected) were measured using a capillary melt- Hz), 7.52 (m, 1H), 7.56 (d, 1H, J = 6.0 Hz), 7.65–7.67 (m,
ing point apparatus. 1H and 13C NMR spectra were recorded 3H), 7.85 (d, 1H, J = 8.0 Hz), 8.25 (d, 1H, J = 8.0 Hz),
using Bruker spectrometer with working frequency 500 MHz 8.6 (d, 1H, J = 6.0 Hz).13C NMR (125 MHz, CDCl3) δ
for 1H NMR and 125 MHz for 13C NMR. For 1H NMR and ppm 40.19, 111.67, 115.03, 115.88, 118.76, 126.60, 126.77,
13C NMR, the chemical shifts were referenced to CH2Cl2 126.84, 127.23, 127.69, 127.76, 129.69, 131.06, 137.01,
or CHCl3 present as an impurity in the deuterated solvent. 142.07, 150.82, 160.70. MALDI-TOF (m/z) for C17H16N2
Mass spectra were measured using a Thermo Finnigan LCQ Calcd. 248.33; Obsd. 248.43 (M+).

Synthesis, photophysical, electrochemical and electroluminescence studies
Figure 1. Synthetic route to prepare the Ir(III) complexes, 1–5.
1
2.2.1e 10: Yield: 0.20 g (80%); M.p.: 100◦C. H NMR
2.2.2a Characterization data for 1: Yield: 0.33 g (95%).1H
(500 MHz, CDCl3) δ ppm 2.70 (s, 3H, CH3), 7.56 (t, 1H, NMR (500 MHz, CDCl3) δ ppm 1.25 (s, 6H, CH3), 5.20 (s,
J = 8.0 Hz), 7.7 (m, 2H), 7.82 (d, 2H, J = 8.0 Hz), 7.91 1H, CH), 6.38 (d, 2H, J = 7.0 Hz), 6.65 (t, 2H, J = 7.5 Hz),
(d, 1H, J = 8.5 Hz), 8.04 (d, 1H, J = 8.5 Hz), 8.13 (d, 2H, 6.89 (t, 2H, J = 7.0 Hz), 7.47 (d, 2H, J = 6.5 Hz), 7.71 (m,
J = 8.5 Hz), 8.63 (d, 1H, J = 5.5 Hz). 13C NMR (125 MHz, 4H), 7.94 (m, 2H), 8.20 (d, 2H, J = 8.0 Hz), 8.45 (d, 2H, J =
CDCl3) δ ppm 26.76, 107.5, 120.46, 120.51, 126.95, 127.12, 6.5 Hz), 8.98 (m, 2H). MALDI-TOF (m/z) for C35H27N2O2Ir
127.51, 128.32, 128.38, 130.19, 136.83, 136.93, 142.23, Calcd. 699.83; Obsd. 699.94 (M+), 601.10 (M+-acac).
142.29, 144.14, 159.42, 197.97. ES-MS (m/z) for C17H13NO
Calcd. 248.30; Obsd. 248.26 (M+).
2.2.2b Characterization data for 2: Yield: 0.36 g (93%).1H
NMR (500 MHz, CDCl3) δ ppm 1.76 (s, 6H, CH3), 5.22 (s,
1H, CH), 5.73 (d, 2H, J = 8.5 Hz), 6.41 (t, 2H, J = 10.0
Hz), 7.59 (d, 2H, J = 6.5 Hz), 7.66 (t, 2H, J = 7.5 Hz),
7.74 (t, 2H, J = 8.0 Hz), 7.91 (d, 2H, J = 8.0 Hz), 8.32
(d, 2H, J = 6.0 Hz), 8.41 (m, 2H). MALDI-TOF (m/z) for
C35H23N2O2F4Ir Calcd. 771.79; Obsd. 772.30 (M+), 673.34
(M+-acac).
2.2b General method for the synthesis of Ir(III) het-
eroleptic complexes: One of the ligands (6–10) (2.0
mmol), IrCl3 (1.0 mmol) were dissolved in 2-ethoxyethanol
(2.0 mL) and water (1.0 mL). The reaction mixture was
refluxed at 120◦C for 16 h. The solvent was evaporated under
reduced pressure, and the solid residue was washed three
times with methanol. The solid product (μ-chloro bridged
dimer) was dried in vacuum. The μ-chloro dimer (1.0 mmol),
acetylacetone (2.2 mmol), Na2CO3 (10.0 mmol) and 2-
ethoxyethanol (3.0 mL) were mixed and refluxed for 16 h
at 120◦C. The solvent was evaporated under vacuum and the
resulted solid residue was washed three times with methanol
to afford pure 1–5 as coloured solid in 32–98% yields.
2.2.2c Characterization data for 3: Yield: 0.13 g (32%). 1H
NMR (500 MHz, CDCl3) δ ppm 1.8 (s, 6H), 6.57 (s, 2H), 7.21
(d, 2H, J = 8.5 Hz), 7.69 (d, 2H, J = 6.5 Hz), 7.84 (m, 4H),
8.1 (m, 2H), 8.37 (d, 2H, J = 8.5 Hz), 8.48 (d, 2H, J = 6.5
Hz), 8.99 (m, 2H). MALDI-TOF (m/z) for C37H25N2O2 F6Ir
Calcd. 835.83; Obsd.836. 19 (M+), 737.30 (M+-acac).

Farman Ali et al.
2.2.2d Characterization data for 4: Yield: 0.32 g (82%). (1.0 mmol), aryl boronic acid (1.2 mmol), K2CO3 (5.0
1H NMR (500 MHz, CDCl3 δ ppm 1.58 (s, 6H), 1.84 (s, 6H),
2.60 (s, 6H), 5.55 (s, 1H), 6.36–6.37 (m, 2H), 7.35 (m, 2H),
DME/water under nitrogen atmosphere for 20 h. On
7.67 (m, 4H), 7.90 (d, 2H, J = 7.5 Hz), 8.04 (m, 4H), 8.36 (d,
mmol) and Pd(PPh3)4 (0.05 mmol) were refluxed in
completion of the reaction, the crude product was puri-
fied by column chromatography to get pure compounds
6–10 as solid in 73–92% yield. To synthesize the Iridium
complexes, one of the ligands 6–10 and IrCl3 (2:1 molar
2H, J = 6.0 Hz), 8.87 (d, 2H, J = 8.5 Hz). MALDI-TOF
(m/z) for C39H37N4O2Ir Calcd. 785.97; Obsd.785.40 (M+),
686.62 (M+-acac).
ratio) were refluxed in 2-ethoxyethanol (2.0 mL) and
2.2.2e Characterization data for 5: Yield: 0.40 g (98%).
1H NMR (500 MHz, CDCl3) δ ppm 1.78 (s, 6H, CH3), 2.10 water (1.0 mL) mixture for 16 h.19 On evaporation of the
(s, 6H, CH3), 5.25 (s, 1H, CH), 6.86 (s, 2H), 7.47 (m, 2H),
7.62 (d, 2H, J = 6.0 Hz), 7.77 (m, 4H), 7.99 (m, 2H), 8.26
(d, 2H, J = 8.5 Hz), 8.52 (d, 2H, J = 6.0 Hz), 8.96 (m, 2H).
MALDI-TOF (m/z) for C39H31N2O4Ir Calcd. 783.90; Obsd.
783.76 (Mn+), 684.84 (M+-acac).
solvents and washing the solid residue with methanol,
μ-chloro bridged dimer was obtained as solid. The μ-
chloro dimer (1.0 mmol), acetylacetone (2.2 mmol),
Na2CO3 were refluxed in 2-ethoxyethanol for 16 h. The
solvent was evaporated under vacuum and the resulted
solid residue was washed three times with methanol to
afford pure 1–5 as coloured solid in 32–98% yields.
The obtained solids were characterized by NMR, mass
methods.
2.3 OLED fabrication and device measurements
Multilayer structure OLED devices, namely, ITO/F4TCNQ/
TPD/CBP +Ir Complex/BCP/LiF/Al, were fabricated on
pre-patterned, pre-cleaned ITO-coated glass substrates. The
substrate was patterned using standard lithography and then
cleaned by mechanical scrubbing with detergent, followed by
ultrasonic cleaning in hot water. It was then blow dried using
dry N2 gas and was finally UV exposed for 10 min. Organic
layers were deposited one after another by thermal evapo-
ration of organic materials kept in separate quartz crucibles
in a high vacuum (∼10−6 torr) chamber. The doped layer of
CBP with Ir complex (5% wt/wt) was deposited by simul-
taneously evaporating the CBP and Ir complex from their
respective boats. The deposition rates were controlled with
two independent quartz crystals near the respective sources
and one near the substrate to monitor the film thickness. LiF
and Al layers were deposited without breaking the vacuum.
The thermal evaporation system was fitted with five evapora-
tion sources. In one experiment, five different layers can be
evaporated without breaking the vacuum. A thick layer of LiF
(100 nm) was deposited on top of Al cathode as capping layer.
After fabrication, the devices were transferred to another vac-
uum apparatus fitted with probes for measurements, which
resulted in a brief exposure of the device to air. The current
(I), voltage (V) and EL (L) characteristics of the OLEDs were
measuredinvacuumusingaKeithley617programmableelec-
trometer. EL intensity (cd/m2) measurement was done using
a large area calibrated silicon photodiode and fixed geome-
try of device and detector. EL spectra were measured using
a monochromator and Photomultiplier Tube (S-20 response)
operated at 720 V.
3.2 Absorption and emission studies
Absorptionspectraof1–5indichloromethaneareshown
in Figure 2. The emission spectra of 1–5 excited at the
absorption peak are shown in Figure 3. The excitation
Figure 2. Absorption spectra of 1–5 reco
rded in dichloromethane.
1.0
0.5
0.0
1
2
3
4
5
3. Results and Discussion
3.1 Synthesis
600
700
Wavelength (nm)
Iridium complexes 1–5 were synthesized as shown
in Figure 1. The ligands 6–10 were synthesized by
theSuzuki-Miyauracoupling.23,24 1-Chloroisoquinoline
Figure 3. Peak normalized emission spec
tra of Ir(III) complexes 1–5 in toluene.

Synthesis, photophysical, electrochemical and electroluminescence studies
spectra (not shown) were similar to the absorption
spectra. The optical spectra of Ir complexes have
been studied experimentally and theoretically.11–16 The
absorption spectra in the UV-Visible region consist sin-
glet and triplet transitions of the type π −π* and MLCT.
The normally forbidden triplet transition is strongly
allowed to such an extent that the extinction coefficients
3
3
of π − π* and MLCT are comparable to the corre-
sponding singlet transitions in some complexes.15 The
absorption spectra are thus broad and multiple peaks
are commonly observed. Spectra of π − π* transi-
tions may have vibrational structures characteristic of
the ligand whereas these are absent in MLCT transi-
tions. The absorption spectra of 1–5 have the above
common features. The spectral peaks were identified
from the derivative spectra and the peak wavelengths
and extinction coefficients are summarized in Table 1.
The absorption spectrum of 4 (N,N-dimethyl derivative)
is almost featureless and different from others whereas
its emission spectrum was similar to that of others. The
excitation spectrum was also similarly featureless indi-
cating that the unusual absorption spectrum of 4 is not
due to aggregates or impurities.
Ir complexes are weakly luminescent in aerated solu-
tion because of oxygen quenching of the long-lived
(microseconds) triplet emitting state. Therefore, N2 sat-
urated dilute solutions of 1–5 in toluene were used to
record the emission spectra (Figure 3). Phosphorescence
3
spectra originating from MLCT are generally broad
and featureless in comparison to emission from 3π −π*
state.11,15 Emission spectra of 1–5 were broad and fea-
tureless which indicate the 3MLCT as the lowest excited
state in these complexes. Though absorption and exci-
tation spectra of 4 are different from those of other
molecules, the emission spectra of all the molecules
were similar except for the shift in the luminescence
maxima. The parent molecule 1 has an emission peak
at 622 nm, in 2 and 3 they are blue shifted by 24 and 3
nm, respectively, and in 4 and 5 they are red shifted by
18 and 36 nm, respectively.
The Ir(III) complexes are almost non-luminescent as
solid thin films due to self-quenching. However, they
are highly luminescent when doped lightly in 4,4-N,N’-
dicarbazole-biphenyl (CBP) as the host material. Thin
films of 1–5 were prepared on quartz plate using 5%
(w/w) Ir(III) complexes in CBP by spin coating method.
Figure 4 shows the emission spectra recorded by excit-
ing CBP at 295 nm. The emission spectra of the thin
films were identical to the spectra obtained during direct
excitationofIrcomplexesattheircorrespondingabsorp-
tion peaks (see Figure 3) which indicate that efficient
energy transfer occurs from the excited state of host
CBP to the dopant Ir complex.

Farman Ali et al.
1.0
0.5
0.0
Fc/Fc+
1
2
3
4
5
4.0
0.0
5/5+
-4.0
0.0
0.5
Voltage (V)
1.0
1.5
600
700
Wavelength (nm)
Figure 5. Cyclic voltammogram
of 5 recorded in dichloromethane
Figure 4. Peaknormalizedemissionspec
tra of Ir(III) complexes 1–5 in thin films.
at
a scan rate of 100 mV/s
using tetrabutylammoniumhexaflu-
orophosphate (TBAHFP) as sup-
porting electrolyte; Ferrocene (Fc) is
used as internal reference molecule;
arrows show the sweep directions.
φ
The phosphorescence quantum yields ( Phos) of the
Ir(III) complexes in toluene were obtained using rho-
damine 6G as a standard.27 The quantum yields varied
φ
from 0.1 to 0.32 for 1 to 5. Phos of 2 and 3 were higher
as compared to 4 and 5.
The Ir(III) complexes 1–5 showed single exponential
photoluminescence decay in dry toluene with lifetime
varying from 0.43 to 1.9 μs. These values are compara-
ble to other heteroleptic Ir complexes.18
The radiative and non-radiative rate constants were
calculated by using the following equations, assuming
φ
φ
intersystem crossing yield,
= 1. The value of
,
isc
Phos
kr, knr listed in Table 1 are comparable to the values
reported for other Ir complexes.18
Figure 6. Energy level diagram of molecules used in mul-
tilayer OLED, ITO/F4TCNQ/TPD/CBP+2/BCP/LIF/Al.
φ
φ τ
)
kr =
/(
(1)
(2)
Phos
isc
τ
knr = (1/ ) − kr
3.4 Electroluminescence properties using Ir(III)
3.3 Electrochemical properties
complex as dopant molecules
Cyclic voltammetric redox potentials provide insight The electroluminescent properties and performance
into the HOMO and LUMO levels and stability of of 2 were studied in a multilayer OLED structure,
the radical anion/cation. Cyclic voltammetry of the ITO/F4TCNQ(20Å)/TPD(500Å)/CBP+2(400Å)/BCP
molecules (∼1 mM) were done in CH2Cl2 using (60Å)/LiF(15Å)/Al. F4TCNQ, TPD, BCP, LiF and Al
tetrabutyl ammoniumhexafluorophosphate (0.1 M) as were successively deposited onto the patterned ITO. The
supporting electrolyte using Ag/AgNO3 as a refer- device area was 2 mm2. The HOMO-LUMO energy lev-
ence electrode and ferrocene as an internal reference els of all the materials used in the device are shown in
molecule. Ir(III) complexes 1–5 showed characteristics Figure 6 in the same order as arranged in the device.
of reversible oxidation process in cyclic voltammetry The dashed line shows the energy levels of the Ir com-
(Figure 5 and Figure S11 in Supplementary Informa- plex 2, which is the dopant in CBP. The functions of the
tion). The peak potentials were determined with respect various layers are as follows. F4TCNQ facilitates hole
to ferrocene as an internal standard. The HOMO levels injection from ITO into TPD. BCP and LiF facilitate
of 1–5 were determined from the corresponding oxi- electron injection and electron transport to the active
dation peak potentials using the reported equations28–30 layer of CBP doped with 2. In addition, BCP serves as
and the values are summarized in Table 1. The HOMO a hole blocking layer and TPD as an electron blocking
levels of these Ir(III) complexes lie in the range of -5.48 layer. Thus, electron-hole recombination is expected to
to -5.80 eV.
occur in the CBP.

Synthesis, photophysical, electrochemical and electroluminescence studies
4. Conclusions
20
10
0
PL spectrum
EL spectrum
1.0
0.5
0.0
AseriesofnewIr(III)complexesof1-phenylisoquinoline
derivatives were synthesized and characterized. Photo-
physical studies of these complexes indicated emission
in the orange to the deep red region. Electrochemical
properties were used to determine the HOMO levels.
The energy levels and emission quantum yields of these
Ir complexes suggest that these molecules can serve as
red emitting dopants in OLEDs. The OLED prepared
using one of the molecules showed high brightness and
current efficiency.
600
700
Wavelength (nm)
0
4
8
Voltage (V)
Figure 7. Current-Voltage characteristics
ofthedeviceITO/F4TCNQ/TPD/CBP+2/BC
P/LIF/Al. The inset shows the electrolu-
minescence spectrum of the device and
photoluminescence spectrum of Ir com-
plex 2.
Supplementary information (SI)
1H-NMR and 13C-NMR spectra (Figures S1-S7), MA
LDI mass spectra (Figures S9 and S10), PL decays (Fig-
ure S8), and cyclic voltammograms of the Ir complexes
(Figure S11) are available at www.ias.ac.in/chemsci.
14
a
1000
7
500
Acknowledgements
We thank the national facility for high field NMR at TIFR
for NMR spectra. We also gratefully thank Prof. K. L.
Narasimhan and Mr. Meghan Patankar for their help in the
fabrication, measurements and interpretation of OLEDs. F.A.
thanks DST for INSPIRE fellowship.
0
8
0
3
6
9
8000
b
4
0
4000
0
6
8
10
Voltage (V)
12
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Figure 8. Light intensity (cd/m2) and
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BCP/LIF/Al. (a) at low applied voltage; (b)
at high applied voltage.
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