Organic light-emitting materials based on iridium(III) complexes bearing phenanthroimidazole ligands

Article abstract of DOI:10.1002/poc.3292

We present the elegant synthesis and the photophysical and electroluminescent properties of a series of cyclometalated iridium(III) complexes [Ir(PPI)₂(pic), PPI: 1,2-diphenyl-1H-phenanthro[9,10-d] imidazole; pic: picolinic acid]. The Ir(PPI)₂(pic) complexes showed characteristic phosphorescence with an emission range of 556-579 nm and a high quantum efficiency with microsecond lifetimes. The strongly allowed phosphorescence in these complexes is the result of significant spin-orbit coupling of the Ir center. All bis(PPI) derivatives exhibit intense triplet metal-to-ligand charge transfer (MLCT) photoluminescence in the fluid solutions at room temperature. The impact of different solvents, substituents on the phenanthroimidazole ligands and complex concentrations upon their emissive behavior have been examined and demonstrate that their emission energies can be systematically modified. Weak bands located at longer wavelength have been assigned to the ¹MLCT ← S₀ and ³MLCT ← S₀ transitions of iridium complexes. Application of the ³MLCT excited state of the [Ir(PPI)₂(pic)] materials in organic light-emitting devices are described.

Full text of DOI:10.1002/poc.3292

Research Article
Received: 11 October 2013,
Accepted: 12 January 2014,
Published online in Wiley Online Library: 7 March 2014
(wileyonlinelibrary.com) DOI: 10.1002/poc.3292
Organic light-emitting materials based on iridium
(III) complexes bearing phenanthroimidazole
ligands
Jayaraman Jayabharathia*, Ramalingam Sathishkumara and
Venugopal Thanikachalama
We present the elegant synthesis and the photophysical and electroluminescent properties of a series of cyclometalated
iridium(III) complexes [Ir(PPI)2(pic), PPI: 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole; pic: picolinic acid]. The Ir(PPI)2(pic)
complexes showed characteristic phosphorescence with an emission range of 556–579 nm and a high quantum efficiency with
microsecond lifetimes. The strongly allowed phosphorescence in these complexes is the result of significant spin–orbit
coupling of the Ir center. All bis(PPI) derivatives exhibit intense triplet metal-to-ligand charge transfer (MLCT) photoluminescence
in the fluid solutions at room temperature. The impact of different solvents, substituents on the phenanthroimidazole ligands
and complex concentrations upon their emissive behavior have been examined and demonstrate that their emission
1
energies can be systematically modified. Weak bands located at longer wavelength have been assigned to the MLCT ← S0
and 3MLCT ← S0 transitions of iridium complexes. Application of the 3MLCT excited state of the [Ir(PPI)2(pic)] materials in organic
light-emitting devices are described. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: charge transfer; electroluminescence; iridium complex; phosphorescence
high-efficiency OLEDs.[16] Our interest in the development of
highly efficient phosphors for application in OLEDs has prompted
INTRODUCTION
us to synthesize iridium complexes and investigate their
photophysical and electroluminescent characteristics.
Cyclometalated iridium(III) complexes have attracted considerable
attention in material research due to their outstanding performance
in organic light-emitting diodes (OLED).[1–4] The strong spin–orbit
coupling induced by a heavy-metal ion promotes an efficient
intersystem crossing (ISC) between the singlet and the triplet excited
state manifold. Therefore, both singlet and triplet excitons can be
harnessed, and then strong electroluminescence with an internal ef-
ficiency theoretically approaching to 100% can be achieved.[5–8]
Host materials in phosphorescent organic light-emitting diodes
(PhOLEDs) have drawn intensive attention owing to their capability
to prevent triplet–triplet annihilation and concentration quenching
effect.[9–11] The Ir(III) complexes reported generally contain two
cyclometalated ligands and a bidentate, monoanionic ancillary
ligand, or with three cyclometalated ligands. Both the luminescent
efficiency and emission colors of Ir(III) complexes can be tuned by
introduction of substituents with different electronic effects or
variations of the conjugation system on ligands.[12–16]
The energy gap of the synthesized complex has been tuned by
incorporating the substituents in the ligand to obtain the desired
emission. Thus, manipulation of the skeletal arrangement as well
as the substituent groups of the cyclometalating ligand may
represent a promising venue for the development of highly phos-
phorescent Ir(III) complexes. Although a wide range of Ir(III) mate-
rials have been reported, the number of highly phosphorescent
imidazole-based cyclometalated Ir(III) complexes is still rare.[17,18]
1,3,5-Tris(N-phenylbenz-imidazole-2-yl)-benzene, a commonly used
electron transporter and hole blocker, is a derivative of
benzoimidazole compounds. Therefore, benzoimidazole(bi)-
based cyclometalated iridium complexes may have good elec-
tron transporting ability, which is highly desirable in designing
Herein, we discuss a series of iridium(III) bis(PPI) complexes with
picolinic acid (pic) as ancillary ligands (Scheme 1). By varying the
substituents, we have been able to fine-tune their 3MLCT excited-
state properties. These complexes are strong emitters in the fluid
solutions, and their electroluminescent (EL) properties have been
studied in order to evaluate their suitability as OLED materials.
EXPERIMENTAL
Optical measurements and composition analysis
The ultraviolet–visible (UV–vis) spectra of the phosphorescent iridium
complexes were measured on UV–vis spectrophotometer (Perkin Elmer
Lambda 35) and corrected for background absorption due to solvent.
Photoluminescence (PL) spectra were recorded on a (Perkin Elmer
LS55) fluorescence spectrometer. NMR spectra were recorded on Bruker
400-MHz NMR spectrometer. The mass spectra of the samples were
obtained using a Thermo Fischer LC-Mass spectrometer in fast atom
bombardment mode. Cyclic voltammetry analysis was performed by
using CHI 630A potentiostat electrochemical analyzer. Measurements of
oxidation and reduction were undertaken using 0.1 M tetra(n-butyl)
ammonium-hexafluorophosphate as the supporting electrolyte, at scan
* Correspondence to: Dr. J. Jayabharathi, Professor of Chemistry, Department of
Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India.
E-mail: jtchalam2005@yahoo.co.in
a J. Jayabharathi, R. Sathishkumar, V. Thanikachalam
Department of Chemistry, Annamalai University, Annamalainagar 608 002,
Tamilnadu, India
J. Phys. Org. Chem. 2014, 27 504–511
Copyright © 2014 John Wiley & Sons, Ltd.

LIGHT-EMITTING IRIDIUM (III) COMPLEXES BEARING PHENANTHROIMIDAZOLE LIGANDS
Scheme 1. Synthetic route of iridium complexes 1–4
rate of 0.1 VSꢀ1. The potentials were measured against an Ag/Ag+ (0.01M
General procedure for the synthesis of iridium complexes
AgNO3) reference electrode using ferrocene/ferrocenium (CP2Fe/CP2Fe+)
as the internal standard. The onset potentials were determined from the
intersection of two tangents drawn at the rising current and background
current of the cyclic voltammogram. All calculations were performed using
density functional theory (DFT) as implemented in the with Guassian-03
program using the Becke3-Lee-Yang-Parr (B3LYP) functional supplemented
with the standard 6-31G (d, p) basis set.[19]
A mixture of phenanthrene-9, 10-dione (40 mmol), ammonium acetate
(30mmol), 4-fluorobenzaldehyde (30mmol) and aniline (30 mmol) was
refluxed in ethanol at 80°C in the presence of indium (III) fluoride (InF3) as
Lewis acid catalyst for 30 min which yields the phenanthroimidazole (PPI)
derivatives. The phenanthroimidazole-based cyclometalated iridium com-
plexes have been synthesized via Nonoyama route[20] (Scheme 1).
Iridium(III)bis(2-(4-fluorophenyl)-1-phenyl-1H-phenanthro [9,10-d]
imidazolato-N,C2′) (picolinate), [Ir(fppi)2 (pic)], (1)
Fabrication of OLEDs
The EL devices based on the iridium (III) complexes were fabricated by
vacuum deposition of the materials at 5 × 10ꢀ6 Torr onto a clean glass
precoated with a layer of indium tin oxide (ITO) as the substrate. The
glass was cleaned by sonication successively in a detergent solution,
acetone, methanol and deionized water before use. Organic layers
were deposited onto the substrate at a rate of 0.1nmsꢀ1. LiF and Alq3 were
thermally evaporated onto the surface of organic layer. The thickness of the
organic materials and the cathode layers was controlled using a quartz
crystal thickness monitor. A series of devices (I, II, III and IV) configuration
ITO/NPB (30 nm)/iridium complex: CBP (7%) (30 nm)/BCP (10 nm)/Alq
(40nm)/Mg:Ag, was fabricated, and measurements of current, voltage
and light intensity were made simultaneously using a Keithley 2400
sourcemeter. The EL spectra and luminance of the devices were carried
out in ambient atmosphere without further encapsulations.
Yield: 88%. 1H NMR (400 MHz, CDCl3): δ 9.16 (d, 1H, J = 8.0 Hz), 8.61 (dd,
2H, J = 8.4 Hz), 8.50 (d, 1H, J = 8.0 Hz), 8.45 (d, 1H, J = 8.8 Hz), 8.39 (d, 1H,
J = 7.6 Hz), 8.27 (d, 1H, J = 6.4 Hz), 8.08 (d, 1H, J = 8.0 Hz), 7.87–7.94 (m,
6H), 7.78 (t, 1H, J = 7.2 Hz), 7.71 (d, 1H, J = 7.6 Hz), 7.66 (d, 1H, J = 6.8 Hz),
7.61 (t, 1H, J = 8.0 Hz), 8.45 (d, 1H, J = 8.8 Hz), 7.46–7.53 (m, 5H), 7.39 (t,
1H, J = 6.0 Hz), 8.45 (d, 1H, J = 8.8 Hz), 7.18–7.25 (m, 4H), 7.12 (d, 1H,
J = 8.0 Hz), 7.06 (d, 1H, J = 8.4 Hz), 7.03 (d, 1H, J = 8.4 Hz), 6.69 (t, 1H,
J = 7.2 Hz), 6.63 (d, 1H, J = 9.2 Hz), 6.32–6.44 (m, 4H). 13C NMR (100 MHz,
CDCl3): δ 108.29, 108.53, 108.68, 108.91, 120.26, 120.50, 121.84, 121.98,
122.13, 122.39, 122.89, 123.39, 123.56, 123.99, 124.14, 124.40, 125.29,
125.44, 125.77, 125.65, 125.72, 126.09, 126.19, 126.46, 126.68, 126.76,
126.78, 127.00, 127.40, 127.51, 128.47, 128.53, 128.74, 128.84, 129.23,
129.72, 129.94, 130.92, 131.09, 131.22, 131.46, 131.54, 131.70, 132.56,
J. Phys. Org. Chem. 2014, 27 504–511
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J. JAYABHARATHI, R. SATHISHKUMAR AND V. THANIKACHALAM
133.33, 134.25, 137.49, 137.78, 146.24, 149.90, 150.66, 150.72, 152.75,
160.69, 160.80, 161.02, 162.12, 163.20, 163.54, 172.28. Anal. calcd. for
3H), 4.08 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 55.85, 55.94, 107.73, 107.86,
108.28, 108.50, 109.82, 115.79, 115.90, 116.17, 116.28, 116.59, 122.10,
122.94, 122.96, 123.54, 123.94, 124.11, 124.34, 125.20, 125.38, 125.50,
125.69, 126.70, 126.83, 126.93, 127.02, 127.35, 127.68, 128.23, 128.43,
128.72, 129.53, 129.69, 129.83, 130.06, 130.14, 130.24, 130.97, 137.44,
146.24, 150.02, 150.73, 152.87, 159.19, 161.07, 161.21, 161.44. Anal. calcd.
for C62H40F2IrN5O4: C, 64.80; H, 3.51; N, 6.09. Found: C, 64.77; H, 3.60; N,
6.15. MS: m/z 1149.53 [M+].
C
60H36F2IrN5O2: C, 66.16; H, 3.33; N, 6.43. Found: C, 66.67; H, 3.59; N,
6.54. MS: m/z 1089.73 [M+].
Iridium(III)bis(2-(4-fluorophenyl)-1-p-tolyl-1H-phenanthro [9,10-d]
imidazolato-N,C2′) (picolinate), [Ir(ftpi)2(pic)], (2)
Yield: 90%. 1H NMR (400 MHz, CDCl3): δ 9.16 (d, 1H, J = 8.0 Hz), 8.60 (dd,
2H, J = 7.6 Hz), 8.49 (d, 1H, J = 8.0 Hz), 8.45 (d, 1H, J = 8.4 Hz), 8.26 (d, 1H,
J = 6.6 Hz), 8.23 (d, 1H, J = 8.4 Hz), 7.45–7.73 (m, 13H), 7.20–7.38 (m, 6H),
7.12 (d, 1H, J = 8.0 Hz), 7.01 (d, 1H, J = 9.2 Hz), 6.70 (t, 1H, J = 7.2 Hz), 6.62
(d, 1H, J = 9.6 Hz), 6.44–6.49 (m, 3H), 6.36 (t, 1H, J = 8.4 Hz), 2.68 (s, 3H),
2.71 (s, 3H). 13C NMR (100 MHz, DMSO): δ 21.74, 21.80, 108.22, 108.45,
108.62, 108.85, 120.31, 120.56, 121.75, 122.10, 122.49, 122.93, 122.99,
123.33, 123.50, 123.93, 124.09, 124.35, 125.21, 125.36, 125.50, 125.70,
125.79, 126.13, 126.22, 126.41, 126.73, 126.82, 126.96, 127.01, 127.36,
127.57, 128.15, 128.44, 128.70, 128.84, 129.19, 129.55, 129.67, 131.49,
131.80, 131.87, 132.12, 132.65, 133.25, 134.20, 135.05, 135.07, 137.44,
141.28, 141.78, 146.24, 149.92, 150.58, 150.65, 152.74, 160.66, 160.86,
162.17, 163.16, 163.52, 172.28. Anal. calcd. for C62H40F2IrN5O2: C, 66.65;
H, 3.61; N, 6.27. Found: C, 66.78; H, 3.68; N, 6.32. MS: m/z 1117.60 [M+].
2-(4-fluorophenyl)-1-(3,5-dimethylphenyl)-1H-phenanthro [9,10-d]
imidazolato-N,C2′) (picolinate), [Ir(fdmppi)2(pic)], (4)
Yield: 91%. 1H NMR (400 MHz, DMSO): δ 9.17 (d, 1H, J= 8.3 Hz), 8.61 (dd, 2H,
J= 8.4 Hz), 8.49 (d, 1H, J= 8.3 Hz), 8.44 (d, 1H, J= 9.6 Hz), 8.25 (d, 1H,
J= 7.2 Hz), 8.21 (d, 1H, J= 8.1 Hz), 7.42–7.73 (m, 13H), 7.36 (t, 1H, J=7.2Hz),
7.18–7.27 (m, 5H), 7.12 (d, 1H, J= 8.4 Hz), 7.00 (d, 1H, J= 8.0 Hz), 6.70 (t, 1H,
J= 7.2 Hz), 6.62 (d, 1H, J= 8.0 Hz), 6.43–6.51 (m, 3H), 6.36 (t, 1H, J=8.1Hz),
2.68 (s, 6H), 2.71 (s, 6H). 13C NMR (100 MHz, DMSO): δ 21.71, 21.76, 29.72,
29.87, 108.21, 108.44, 108.61, 108.85, 120.31, 120.56, 121.93, 122.01, 122.49,
122.92, 123.01, 123.33, 123.50, 123.92, 124.08, 124.34, 125.20, 125.35, 125.49,
125.69, 126.40, 126.73, 126.80, 126.90, 126.99, 127.36, 127.57, 128.14, 128.43,
128.70, 128.83, 129.18, 129.67, 131.47, 131.82, 131.86, 132.10, 133.25, 134.19,
135.05, 135.07, 137.42, 160.65, 160.87, 162.18, 163.19, 172.25. Anal. calcd. for
C64H44F2IrN5O2: C, 67.12; H, 3.87; N, 6.11. Found: C, 67.14; H, 3.84; N, 6.15.
MS: m/z 1145.47 [M+].
Iridium(III)bis(2-(4-fluorophenyl)-1-(4-methoxyphenyl)-1H-phenanthro
[9,10-d]imida zolato-N,C2′) (picolinate), [Ir(fmppi)2(pic)], (3)
Yield: 86%. 1H NMR (400 MHz, CDCl3): δ 9.15 (d, 1H, J = 7.6 Hz), 8.64 (d, 1H,
J = 8.0 Hz), 8.59 (d, 1H, J = 8.0 Hz), 8.45 (d, 1H, J = 7.6 Hz), 8.26 (d, 2H,
J = 8.4 Hz), 8.06 (d, 1H, J = 8.0 Hz), 7.75 (d, 1H, J = 8.0 Hz),7.44–7.59 (m,
8H), 7.17–7.40 (m, 11H), 6.99 (d, 1H, J = 9.6 Hz), 6.70 (t, 1H, J = 7.6 Hz),
6.62 (d, 1H, J = 9.6 Hz), 6.48–6.53 (m, 3H), 6.39 (t, 1H, J = 8.4 Hz), 4.06 (s,
RESULTS AND DISCUSSION
Absorption and emission spectra
The absorption and emission spectra of complexes 1–4 in
dichloromethane at room temperature are shown in Fig. 1. The
absorption bands show three kinds of bands. The intense band
observed around 302 nm in the ultraviolet part of the spectrum
can be assigned to the spin-allowed ligand-centered (π–π*) tran-
sitions[21] which correlate with the transition observed for free
PPI and somewhat weaker bands also observed in the lower part
of energy. The absorption in the range of 385–401 nm corre-
sponds to spin-allowed S° → 1MLCT transition. The broad band
appeared at in the range of 422–436 nm can be assigned to
S° → 3MLCT transition and gains intensity by mixing with
1
the higher lying MLCT transition through the spin–orbit cou-
pling of iridium(III). This mixing is strong enough in this com-
3
plex that the formally spin forbidden MLCT has an extinction
coefficient that is almost equal to the spin-allowed 1MLCT
transition (Fig. 2).[17,22] Both singlet MLCT (1MLCT) and triplet
MLCT (3MLCT) bands are typically observed for these
Figure 1. Absorption and emission spectra of 1–4 in CH2Cl2
Figure 2. Spin–orbit coupling of heavy-metal facilitated triplet emission
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Copyright © 2014 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2014, 27 504–511

LIGHT-EMITTING IRIDIUM (III) COMPLEXES BEARING PHENANTHROIMIDAZOLE LIGANDS
complexes in all solvents. In order for these iridium(III) complexes
to be useful as phosphor EL devices, strong spin–orbit coupling
must be present to efficiently mix the singlet and triplet excited
states. Clear evidences for mixing of the singlet and triplet excited
states are seen in the absorption of these complexes.
Electronic transition theory
The time correlated single photon counting (TCSPC) results fit to
single exponentials decay (Fig. 3); DAS6 software was used for
the fit and the χ2 values are less than 1.2. The lifetime measure-
ments of all iridium complexes were made, and the signal was
measured at the emission wavelength of individual compound.
The absolute PL quantum yields were measured by comparing
emission intensities (integrated areas) of a standard sample
(Coumarin 46) and the unknown sample[26,27] using the formula
Mixing of excited states (LC and MLCT)
Phosphorescence of mononuclear metal complexes originates
from the ligand-centered excited state (LC), metal-centered
excited state and MLCT excited state. For the cyclometalated
iridium complexes, the wave function of the excited triplet state
ΦT, responsible for the phosphorescence, is expressed as,
ꢀ
ꢁꢀ ꢁꢀ
Astd
ꢁ
ηunk
ηstd
Iunk
Istd
Ф
unk = Фstd
2 where, Фunk is the quantum yield of
Aunk
the sample, Фstd is the quantum yield of the standard, Iunk and
Istd are the integrated emission intensities of the sample and
the standard, respectively. Aunk, and Astd are the absorbances
of the sample and the standard at the excitation wavelength,
respectively. ηunk and ηstd are the indexes of refraction of the
sample and standard solutions.[23] The radiative and non-
radiative decay of the excited state of iridium complexes has
been obtained using the quantum yields and lifetimes and is
listed in Table 1. Moreover, radiative lifetime of these complexes
falls in the range of 0.59–0.92 μs. The formula employed to
calculate the radiative (kr) and non-radiative (knr) rate con-
stants is Φp = ΦISC {KT/(kr + knr)}; kr = Φp/τ; knr = (1/τ) ꢀ (Φp/τ);
τ = (kr + knr)ꢀ1, where, here, ΦISC is the intersystem-crossing
yield. For the iridium complexes, ΦISC is safely assumed to be
1.0 because of the strong spin–orbit interaction caused by
heavy atom effects of iridium.[28] kr and knr are the radiative
and non-radiative deactivation; τf is the lifetime of the T1 ex-
cited state. Perusal of the radiative and non-radiative rate con-
stants shows that in most of the cases the radiative emission is
predominant over non-radiative transitions.
ΦT ¼ aΦTðπ ꢀ π*Þ þ bΦT ðMLCTÞ
(1)
where “a” and “b” are the normalized coefficient, ΦT (π–π*) and
3
ΦT (MLCT) are the wave function of (π–π*) and 3(MLCT) excited
states, respectively. For these iridium complexes, the wave func-
tion of the triplet state (ΦT) responsible for the phosphorescence
and Eqn (1) implies that the excited triplet state of these iridium
complexes is a mixture of ΦT (π–π*) and ΦT (MLCT).[23] The triplet
state is attributed to dominantly 3π–π* excited state when a > b
and dominantly 3MLCT excited state when b > a.
The photoluminescence spectra of these complexes obtained
at 298 K show a significant broad shape. According to our previ-
ous studies,[24–26] phosphorescence spectra from the ligand-
3
centered π–π* state display vibronic progressions, while those
3
from the MLCT state are broad in shape. Complexes 1, 3 and
3
4 have excited state with large contribution of MLCT whereas
3
complex 2 have excited state with large contribution of π–π*.
Effect of substituent on tuning wavelength
Substituent effect of the d orbital (t2g) stabilizes iridium(III)
through the carbon atom-iridium bonding, and this identifies
with the inductive effect of the substituents. Therefore, the
highest occupied molecular orbital (HOMO) stability and the
emission energy gap are controlled by the nature and num-
ber of substituent and its inductive influence on the aromatic
ring. The photophysical study of these complexes demon-
strates that the electron withdrawing substituents increase
the absorption and emission energies of complexes by
stabilizing the HOMO level. Besides increasing the emission
energy, the lower HOMO energies decrease the energy sepa-
1
3
ration between the MLCT and LC states, which in turn mod-
ified the excited-state properties of the iridium complexes.
The complexes 1–4 undergo
a reversible one-electron
oxidation wave of 0.91 V vs Fc+/Fc[29] in CH2Cl2, which can
be attributed to metal-centered IrIII/IrIV oxidation process.
As revealed previously by electrochemical studies and
Figure 3. Lifetime spectra of 1–4 in CH2Cl2
Table 1. Absorption (λabs, nm), emission (λemi, nm), fluorescence quantum yield (Ф), lifetime (τ, μs), radiative rate constant (kr,
106 sꢀ1), nonradiative rate constant (knr, 106 sꢀ1) and electrochemical behavior of 1–4
1/2
oxi
Complex
λabs
λemi
Φ
τ
kr
knr
Eonset
E
(V)
HOMO (eV)
LUMO (eV)
Eg
1
2
3
4
298, 385, 422
303, 397, 434
299, 389, 426
308, 401, 436
556
572
567
579
0.56
0.54
0.58
0.59
0.59
0.68
0.78
0.92
0.95
0.79
0.74
0.64
0.74
0.68
0.54
0.45
452
441
443
457
0.81
0.83
0.79
0.75
5.61
5.63
5.59
5.55
2.87
2.82
2.79
2.84
2.74
2.81
2.80
2.71
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J. JAYABHARATHI, R. SATHISHKUMAR AND V. THANIKACHALAM
theoretical calculations, the oxidation occurred mainly at the
Ir metal cationic site, together with a minor contribution
from the cyclometalated phenyl fragment.[30] Variations of
cyclometalated ligands, including replacement of pyridine
by other heteroaromatic rings and incorporation of substitu-
ents on the rings, were shown to affect the oxidation poten-
tial of the iridium ion in congeners of (ppy)2Ir (acac).[31]
Accordingly, replacing the benzoimidazole fragment with a
phenanthro[9,10-d]imidazolyl moiety would significantly
affect the electrochemical property of this complexes.
However, the oxidization potential of these complexes 1–4
appears to be only slightly higher (10 mV) than that of Ir(bi)
2(acac), showing that the metal localized HOMOs are at very
similar energies. According to the equation Eg = ELUMO
EHOMO = 1239/λ onset (eV) and EHOMO = ꢀEOX ꢀ 4.80 eV,[32]
the HOMO and LUMO levels of complexes 1–4 were
calculated to be ꢀ5.61, ꢀ5.63, ꢀ5.59, ꢀ5.55 and ꢀ2.87,
ꢀ2.82, ꢀ2.79, ꢀ2.84 eV respectively. This is consistent with
the reported electrochemical studies and theoretical calcula-
tions that the one-electron oxidation of such d6 complexes
would mainly occur at the metal site, together with a minor
contribution from the surrounding chelates.[30] The 3D or-
bitals of HOMO and lowest unoccupied molecular orbital
(LUMO) of complex 1 are shown in Fig. 4. The calculated en-
ergies of the HOMO and LUMO are given in Table 1. The irid-
ium complexes HOMO and LUMO energies were calculated
based on the HOMO energies and the lowest-energy absorp-
tion edges of the absorption spectra.[33] From the energy gap
values, it was concluded that all the reported dopant (1–4)
are green emitters.
Description of the structure of 1
The selected bond lengths and bond angles of 1 are
presented in Table 2, and the optimization has been obtained
using density functional theory (DFT/B3LYP/6-31 (d,p)). This
complex exhibits an octahedral geometry around metal
iridium and prefers cis-C,C and trans-N,N chelate disposition
instead of trans-C,C and trans-N,N chelate. Electron-rich phenyl
rings normally exhibit very strong influence and trans effect.
Therefore, the trans-C,C arrangement is expected to be
thermodynamically higher in energy and kinetically more
labile. This well-known phenomenon has been referred
to as “transphobia”.[34] The Ir–C bonds of the complex 1, i.e.
Ir–Cav = 2.038 Å is shorter than Ir–N bonds, i.e. Ir–Nav = 2.060 Å.
The Ir–O bond length [2.138 Å] is longer than the mean Ir–O
bond length (2.088 Å) reported[35], and these observations
reflect the trans influence of the phenyl groups. All other bond
lengths and bond angles are analogous to the similar type of
complexes (Fig. 5).
Thermal studies
The thermal properties of the iridium complexes 1–4 have
been investigated by differential scanning calorimetry (DSC)
and thermal gravimetric analyses (TGA) under nitrogen atmo-
sphere and displayed in Fig. 6. TGA and DSC measurements
have shown that iridium complexes 1–4 are a highly thermally
stable material. Thermal decomposition temperature (Td5),
defined as the temperature at which the material showed a
5% weight loss, has been measured to be 378, 383, 369 and
386 °C for compounds 1, 2, 3 and 4, respectively. Its glass
transition temperature (Tg) is in the range of 115 to 129 °C.
Figure 4. HOMO–LUMO orbital pictures of 1
Figure 5. Optimized structure of 1
Table 2. Selected bond lengths (Å) and bond angles (°) of 1 by DFT/B3LYP/6-31 (d,p)
Bond lengths (Å)
DFT/B3LYP/6-31 (d,p) (Å)
Bond angles (°)
DFT/B3LYP/6-31 (d,p) (°)
Ir(1)–C(2)
Ir(1)–C(9)
C(31)–N(29))
C(13)–F(86)
C(7)–F(85)
C(37)–O(39)
2.0075
2.0208
1.4146
1.4084
1.4071
1.3226
C(2)–Ir(1)–C(9)
C(3)–C(7)–F(85)
C(10)–C(13)–F(86)
N(34)–C(32)–N(36)
O(39)–C(37)–O(38)
C(37)–C(19)–N(18)
92.58
117.71
117.84
109.98
126.54
117.25
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J. Phys. Org. Chem. 2014, 27 504–511

LIGHT-EMITTING IRIDIUM (III) COMPLEXES BEARING PHENANTHROIMIDAZOLE LIGANDS
Figure 6. (a) TG-DTA curves of 1–4; (b) DSC curves of 1–4
Figure 7. General structure of device
Table 3. Performances of electroluminescence devices I–IV
Device
Vd (V)
Lmax (cd/m2)
ɳ
ext (%)
ɳc cd/A
ɳp (lm/W)
ELmax (nm)
I
II
III
IV
3.2
5.0
3.5
3.2
11 430, 11 V
15 260, 12 V
19 803, 11 V
24 356, 11 V
8.3, 6 V
6.0, 10 V
13.0, 8 V
14.2, 8 V
32.6, 6 V
21.6, 9.5 V
41.3, 8 V
40.8, 8 V
25.0, 5 V
8.7, 10 V
25.3, 6 V
20.6, 7 V
559
575
570
580
Vd: driving voltage; Lmax: luminous efficiency; ɳ C: current efficiency; EL max: electroluminescence maxima; ɳ p: power efficiency;
ext: external quantum yield
ɳ
The melting point (Tm) of compounds 1, 2, 3 and 4 measured
by DSC examinations is 380, 381, 370 and 385 °C, respectively.
The high Tm and Td5 values indicate that the compounds 1–4
are thermally stable and are able to undergo the vacuum
thermal sublimation process. Therefore, these derivatives could
be used in EL devices since the high Tm and Tg values improve
the lifetime of the devices.
Electroluminescent properties
To demonstrate the electroluminescent properties of iridium com-
plexes, typical OLED devices using the prepared iridium complexes
as dopants have been fabricated (Fig. 7). The devices had a multi-
layer configuration ITO/NPB (30 nm)/iridium complex: CBP (7%)
(30 nm)/BCP (10 nm)/Alq (40 nm)/Mg:Ag, in which ITO was used
Figure 8. Electroluminescence spectra of 1–4 in CH2Cl2
J. Phys. Org. Chem. 2014, 27 504–511
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J. JAYABHARATHI, R. SATHISHKUMAR AND V. THANIKACHALAM
Figure 9. (a) Plot of brightness vs voltage; (b) plot of external quantum yield vs current density
Figure 10. (a) Plot of current efficiency vs current density; (b) plot of power efficiency vs current density
as the anode, NPB (4,40-bis[N-(1-naphthyl)-N-phenylamino]biphe-
nyl) was used as the hole-transporting material, CBP (4,40-N,N0-
dicarbozole biphenyl) as the host, the iridium complexes as
the dopant, BCP (2,9-dimethyl-4,7-dipheny-1,10-phenanthroline)
as the hole blocker, Alq (tris(8-hydroxyquinolinato)aluminium)
as the electron transporter and Mg:Ag as the cathode. Key
characteristics of these devices are listed in Table 3. The devices
1–4 emitted strong green light with an emission maximum at
559, 575, 570 and 580 nm, respectively (Fig. 8). Figures 9a and
9b show the brightness–voltage and the external quantum
yield–current density characteristics of the devices, respectively.
All devices show quite appreciable efficiencies and brightness.
Devices III and IV show the better performance in terms of bright-
ness and power efficiency, with brightness of 19803 cd/m2 at 11 V
and 24 356 cd/m2 at 11 V, respectively. Device IV with of Ir(fdmppi)2
(pic) as dopant shows an extremely high power (Fig. 10a) and
current (Fig. 10b) efficiencies of 25.3 Im/w and 7.0 V, respectively
(Table 3).
3π–π* states of the excited states varied. While the performance
of the devices described herein are limited, we have illustrated that
these iridium(III) complexes warrant further investigations as OLED
materials due to their tunable emission properties and their ability
to mediate efficient energy transfer.
Acknowledgements
One of the authors Prof. J. Jayabharathi is thankful to DST [No.
SR/S1/IC-73/2010], DRDO (NRB-213/MAT/10-11) and CSIR (NO
3732/NS-EMRII) for providing funds to this research study.
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