Synthesis, photophysical and electroluminescent properties of green organic light emitting devices based on novel iridium complexes containing benzimidazole ligands

Article abstract of DOI:10.1016/j.jorganchem.2014.03.002

Synthesis, photophysical and electroluminescent analysis of cyclometalated iridium (III) complexes Ir(bpb)₂(pic) are reported. The strongly allowed phosphorescence is the result of significant spin-orbit coupling of the Ir center. The lowest en

Full text of DOI:10.1016/j.jorganchem.2014.03.002

Journal of Organometallic Chemistry 761 (2014) 74e83
Contents lists available at ScienceDirect
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Review
Synthesis, photophysical and electroluminescent properties of green
organic light emitting devices based on novel iridium complexes
containing benzimidazole ligands
*
Jayaraman Jayabharathi , Karunamoorthy Jayamoorthy, Venugopal Thanikachalam
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis, photophysical and electroluminescent analysis of cyclometalated iridium (III) complexes
Ir(bpb)2(pic) are reported. The strongly allowed phosphorescence is the result of significant spin-orbit
coupling of the Ir center. The lowest energy excited sate is a mixture of metal to ligand charge trans-
Received 30 December 2013
Received in revised form
25 February 2014
fer (MLCT) and
pep
* states. Weak bands at longer wavelength were assigned to 1MLCT ) S0 and
Accepted 1 March 2014
3MLCT ) S0 transitions. These complexes were doped into emissive region of multilayer organic light
emitting diode (OLED) by vapor-deposition emit green electroluminescence with similar currentevolt-
age characteristics. These Ir(bpb)2(pic) doped OLEDs show appreciable efficiencies as compared with
other iridium complex based OLEDs in the literature, which result from efficient trapping and radiative
relaxation of singlet and triplet excitons formed in electroluminescent process. Devices with Ir(bpb)2(pic)
shows the better performance in terms of brightness and power efficiency with brightness of 130301 cd/
m2 at 14.5 V and 26.2 lm/W at 5.5 V, respectively.
Keywords:
OLED
Electroluminescence
Green emitter
Iridium complex
Ó 2014 Elsevier B.V. All rights reserved.
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Optical measurements and compositions analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
General procedure for the synthesis of iridium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Absorption and emission spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Mixing of excited states (LC and MLCT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Electronic transition theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Effect of substituents on tuning wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
0
Description of the structure of Iridium(III)bis(1-benzyl-2-phenyl-1H-benzimidazolato-N,C2 )(picolinate), [Ir(bpb)2 (pic)] (1) . . . . . . . . . . . . . . . . . . . 79
Thermal studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Electrochemical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Electroluminescent properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
* Corresponding author. Tel.: þ91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
http://dx.doi.org/10.1016/j.jorganchem.2014.03.002
0022-328X/Ó 2014 Elsevier B.V. All rights reserved.

J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83
75
Introduction
as the supporting electrolyte, at scan rate of 0.1 VSꢀ1. The potentials
were measured against an Ag/Agþ (0.01 M AgNO3) reference elec-
trode using ferrocene/ferrocenium (CP2Fe/CP2Feþ) as the internal
standard. The onset potentials were determined from the inter-
section of two tangents drawn at the rising current and background
current of the cyclic voltammogram. All calculations were per-
formed using density functional theory (DFT) as implemented in
the with Gaussian-03 program using the Becke3-Lee-Yang-Parr
(B3LYP) functional supplemented with the standard 6-31G (d, p)
basis set [33].
Third-row transition-metal complexes are well-known for their
ability to achieve high-efficiency phosphorescence at room tem-
perature [1,2] and has 100% internal quantum efficiency, because
they can effectively harvest both singlet and triplet excitons [3]. As
a result, Ru (II)-, Os (II)-, Ir (III)- and Pt (II)- based cyclometalated
complexes were significantly developed which exhibited wide
application in organic light-emitting devices (OLEDs) [4e7]. Among
these, iridium (III) complexes display best electrophosphorescence
with an external quantum efficiency as high as 27% in OLEDs and
are considered to be a class of promising electrophosphorescent
materials due to their non-planar configuration and short phos-
phorescent lifetime [8e14].
Device fabrication
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
0.1 nm sꢀ1. LiF and Alq3 were thermally evaporated onto the surface
of organic layer. The thickness of the organic materials and the
cathode layers were controlled using a quartz crystal thickness
monitor. A series of devices (I, II, III and IV) with the multilayer
configuration ITO/NPB (30 nm)/iridium complex: CBP (7%) (30 nm)/
BCP (10 nm)/Alq (40 nm)/Mg:Ag was fabricated. Measurements of
current, voltage and light intensity of these devices (IeIV) 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.
However, there is a crucial issue of phase separation between
iridium (III) complexes and host materials that influence the doped
device performance. This is overcome by increasing the bulkiness
of the iridium (III) complexes which improved the dispersibility
and high-efficiency emission results. Attempts were made to
enhance the efficiencies of OLEDs by increasing the solubility and
dispersibility of iridium (III) complexes [15e20]. Emission color of
Ir (III) complexes is strongly governed by the nature of cyclo-
metalating ligand and also by ancillary ligands [21e23]. The
emission colors of Ir(dfppy)2(LX) complexes are tuned from blue to
red by changing ancillary ligands [24,25]. The non-radiative decay
of Ir(ppz)3 at ambient temperature are possible through higher
thermally activated metal-to-ligand charge transfer (MLCT) states
or MLCT to metal dd state conversion [26,27]. The picolinate
complex Ir(ppz)2(pic) shows relatively stronger green emission
(526 nm) at room temperature whereas the emission colors change
from blue (422 nm) to orange-red (587 nm) at low temperature
(77 K) [28]. Despite elegant research on green phosphores [29e32],
there are only few reports on room temperature green phosphores.
The energy gap has been tuned by incorporating the substituents in
the ligand to obtain the desired emission. The purpose of the
present study is the molecular design of a highly efficient green-
General procedure for the synthesis of iridium complexes
A mixture of corresponding aldehyde (2 mmol), o-phenylenedi-
amine (1 mmol) and ammonium acetate (2.5 mmol) has been
refluxed at 80 ꢂC in ethanol which yields the benzimidazole
derivatives. The benzimidazole based cyclometalated iridium com-
plexes have been synthesized via Nonoyama route [34a] (Scheme 1).
phosphorescent complexes viz., iridium (III) bis (1-benzyl-2-
0
phenyl-1H-benzimidazolato-N,C2 )(picolinate), [Ir(bpb)2(pic)] (1),
iridium (III) bis(1-(4-fluorobenzyl)-2-(4-fluorophenyl)-1H-benzi-
0
midazolato-N,C2 ) (picolinate) [Ir(fbfpb)2(pic)] (2), iridium (III)
bis(1-(4-methybenzyl)-2-p-tolyl-1H-benzimidazolato-
Iridium(III)bis(1-benzyl-2-phenyl-1H-benzimidazolato-
0
N,C2 )(picolinate) Ir(mbtb)2(pic)] (3) and iridium (III) bis(1-(4-
0
N,C2 )(picolinate), [Ir(bpb)2 (pic)], (1)
Yield: 88%. 1H NMR (400 MHz, CDCl3):
d
5.72 (d, 1H, J ¼ 8.0 Hz),
methoxybenzyl)-2-(4-methoxyphenyl)-1H-benzimidazolato-
0
N,C2 )(picolinate) [Ir(mbmpb)2(pic)] (4) suitable for green OLED
5.81e5.97 (m, 3H), 6.35 (d, 1H, J ¼ 7.2 Hz), 6.32e6.73 (m, 3H), 6.78e
6.88 (m, 2H), 7.16e7.54 (m, 16H), 7.86e7.95 (m, 2H), 8.14 (s, 1H),
devices. In addition to high phosphorescence efficiency, the com-
plexes should possess high thermal stability for device fabrication
and stable device performance. We particularly focused our
attention on designing metal complexes that provide green emis-
sion from the MLCT excited state.
8.25 (d, 1H, J ¼ 7.6 Hz). 13C NMR (100 MHz, CDCl3):
d 48.12, 48.37,
109.58, 110.29, 114.45, 117.68, 120.67, 121.39, 122.88, 123.51, 123.74,
124.62, 124.78, 124.86, 125.94, 126.01, 127.33, 127.68, 128.09, 128.02,
129.27,129.33,129.49,129.83,134.26,134.36,134.60,135.06,135.23,
135.28, 135.69, 137.28, 139.80, 149.17, 149.94, 153.57, 163.28, 164.68,
173.55. Anal. calcd. for C46H34IrN5O2: C, 62.71; H, 3.89; N, 7.95.
Found: C, 62.67; H, 3.90; N, 7.98. MS: m/z 882.1, calcd. 881.23.
Experimental
Optical measurements and compositions analysis
The ultravioletevisible (UVevis) spectra of the phosphorescent
iridium complexes were measured in an UVevis spectrophotom-
eter (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. Cyclic voltammetry (CV) analysis were
performed by using CHI 630A potentiostat electrochemical
analyzer. Measurements of oxidation and reduction were under-
taken using 0.1 M tetra(n-butyl)ammonium- hexafluorophosphate
Iridium(III)bis(1-(4-fluorobenzyl)-2-(4-fluorophenyl)-1H-
0
benzimidazolato-N,C2 ) (picolinate), [Ir(fbfpb)2(pic)], (2)
Yield: 90%. 1H NMR (400 MHz, CDCl3):
d
5.65 (d, 1H, J ¼ 8.4 Hz),
5.83e5.92 (q, 5H), 6.23 (d, 1H, J ¼ 10.4 Hz), 6.46e6.60 (m, 2H),
6.84e7.49 (m, 22H), 7.93 (d, 2H, J ¼ 8.4 Hz), 8.09 (s, 1H), 8.28 (d, 1H,
J ¼ 7.2 Hz). 13C NMR (100 MHz, DMSO):
d 46.36, 46.59, 108.34,
111.58, 111.93, 113.21, 115.62, 115.68, 115.84, 115.90, 119.17, 123.77,
124.18, 124.27, 126.88, 128.04, 128.85, 130.94, 132.30, 135.11, 135.34,
138.52, 139.03, 149.08, 152.07, 152.82, 160.28, 161.65, 162.48, 162.68,
172.17. Anal. calcd. for C46H30F4IrN5O2: C, 57.95; H, 3.18; N, 7.37.
Found: C, 57.98; H, 3.17; N, 7.35. MS: m/z 954.1, calcd. 953.20.

76
J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83
Scheme 1. Synthetic route of iridium (III) complexes 1e4.
Iridium(III)bis(1-(4-methybenzyl)-2-p-tolyl-1H-benzimidazolato-
absorption bands of complexes (1e4) show three kinds of bands.
The intense band around 302 nm in the ultraviolet part of the
spectrum can be assigned to the allowed ligand centered (pep*)
0
N,C2 ) (picolinate), [Ir(mbtb)2(pic)], (3)
Yield: 86%. 1H NMR (400 MHz, CDCl3):
d
1.95 (s, 3H), 2.04 (s, 3H),
2.30 (s, 6H), 5.66e5.82 (m, 3H), 5.93 (d, 2H, J ¼ 17.2 Hz), 6.15 (s, 1H),
6.42 (s, 1H), 6.57 (dd, 2H, J ¼ 8 Hz, 8 Hz), 6.80e6.84 (t, 1H,
J ¼ 15.6 Hz), 7.02e7.44 (m, 17H), 7.85e7.88 (t, 1H, J ¼ 15.2 Hz), 7.98
(d, 1H, J ¼ 5.2 Hz), 8.08e8.10 (t, 1H, J ¼ 8 Hz), 8.24 (d, 1H, J ¼ 7.6 Hz).
transitions [34b,35] and somewhat weaker bands also observed in
the lower part of energy. The band position, size and extinction
coefficient of the bands in the range 425e482 nm suggest that
these are MLCT transitions (1MLCT and 3MLCT) [36,37]. Similar to
our earlier reports [38e44], weak bands located at longer wave-
length were assigned to the 1MLCT ) S0 and 3MLCT ) S0 transi-
tions of iridium complexes. Thus the broad absorption shoulders at
448 and 481 nm observed for complex 1 are likely to be ascribed to
the 1MLCT ) S0 and 3MLCT ) S0 transitions, respectively. The
intensity of the 3MLCT ) S0 transition is close to that of
1MLCT ) S0 transition, suggesting that 3MLCT ) S0 transition is
strongly allowed by SeT mixing of spin-orbit coupling [45] and
similar observations are made for complexes 2e4. Absorption
around 440 nm for complexes 1e4 corresponds to the transition of
13C NMR (100 MHz, CDCl3):
d 21.76, 21.71, 21.76, 47.70, 47.94,109.38,
110.09, 114.29, 117.44, 121.82, 122.52, 123.10, 123.46, 124.40, 124.55,
125.85, 125.95, 127.23, 127.58, 129.87, 129.89, 131.54, 131.93, 132.27,
132.37, 135.12, 135.57, 135.63, 137.12, 137.74, 137.89, 139.29, 139.75,
139.80, 139.86, 149.32, 149.99, 151.59, 153.59, 163.23, 164.63, 173.54.
Anal. calcd. for C50H42IrN5O2: C, 64.08; H, 4.52; N, 7.47. Found: C,
64.07; H, 4.51; N, 7.49. MS: m/z 938.1, calcd. 937.30.
Iridium(III)bis(1-(4-methoxybenzyl)-2-(4-methoxyphenyl)-1H-
0
benzimidazolato-N,C2 ) (picolinate), [Ir(mbmpb)2(pic)], (4)
Yield: 91%. 1H NMR (400 MHz, DMSO):
d 3.37 (s, 3H), 3.41 (s, 3H),
3.68 (s, 6H), 5.55 (d,1H, J ¼ 8 Hz), 5.64 (s,1H), 5.76 (s,1H), 5.95e6.11
(m, 4H), 6.39e6.46 (m, 2H), 6.82e6.95 (m, 5H), 7.04e7.12 (dd, 4H,
J ¼ 8 Hz, J ¼ 8 Hz), 7.25e7.33 (m, 3H), 7.68e7.81 (m, 6H), 7.90 (d,1H),
8.02e8.12 (m, 2H). 13C NMR (100 MHz, DMSO):
d 46.33, 54.09, 54.16,
54.99, 105.54, 105.72, 111.04, 111.36, 112.85, 114.15, 114.20, 115.14,
119.14, 119.41, 122.59, 122.85, 123.51, 123.61, 126.56, 126.72, 126.81,
126.91, 127.23, 128.00, 128.06, 128.51, 135.07, 135.32, 138.46, 138.87,
138.92, 148.93, 151.14, 152.39, 154.33, 158.57, 159.47, 159.65, 162.49,
163.37, 172.21. Anal. calcd. for C50H42IrN5O6: C, 59.99; H, 4.23; N,
7.80. Found: C, 59.97; H, 4.24; N, 7.81. MS: m/z 1001.9, calcd.1001.12.
Results and discussion
Absorption and emission spectra
Fig. 1 depicts the absorption and emission spectra of iridium (III)
complexes 1e4 in dichloromethane at room temperature. The
Fig. 1. Absorption and emission spectra of iridium complexes 1e4 in CH2Cl2.

J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83
77
the 1MLCT state as evident from its extinction coefficient of the
order 103. The absorption like long tail toward lower energy and
higher wavelength around 485 nm is assigned to 3MLCT transition
and gains intensity by mixing with the higher lying 1MLCT transi-
tion through the spin-orbit coupling of iridium (III) [39]. Both
singlet MLCT (1MLCT) and triplet MLCT (3MLCT) bands are typically
observed for these complexes in all solvents. In order for these
iridium (III) complexes to be useful as phosphors 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.
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
Fig. 2. Lifetime decay spectra of iridium complexes 1e4 in CH2Cl2.
complexes, the wave function of the excited triplet state FT
,
responsible for the phosphorescence is expressed as,
ꢀ
ꢁ
at the emission wavelength of individual complexes. The absolute
PL quantum yields were measured by comparing phosphorescence
intensities (integrated areas) of a standard sample (Coumarin 46)
FT ¼ aFT
p
ꢀ
p* þ bFTðMLCTÞ
(1)
and the unknown sample [42] using the formula Funk
¼
Fstd(Iunk/
where ‘a’ and ‘b’ are the normalized co-efficient, FT
(MLCT) are the wave function of
(p
ep
*) and FT
hstd)2where, Funk is the phosphorescence
3
(pep
*) and 3(MLCT) excited
Istd)(Astd/Aunk)(hunk
/
quantum yield of the sample, Fstd 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 ab-
sorbances of the sample and the standard at the excitation wave-
length, respectively. hunk and hstd are the indexes of refraction of the
sample and standard solutions [44]. The radiative and non-
radiative decays of the excited state of iridium complexes were
obtained using the quantum yields and lifetimes and are listed in
Table 1. Moreover, radiative lifetime of these complexes fall in the
states, respectively. For these iridium complexes, the wave function
of the triplet state (FT) responsible for the phosphorescence and
equation (1) implies that the excited triplet state of these iridium
complexes are mixture of FT (pep*) and FT (MLCT) [36]. The triplet
state is attributed to dominantly 3p
ep* excited state when a > b
and dominantly 3MLCT excited state when b > a.
The phosphorescence spectra of these complexes 1e4 obtained
at room temperature show significant broad shape. According to
our previous studies [40e42], phosphorescence spectra from
ligand centered 3p
ep* state display vibronic progressions, while
range of 1.60e1.79
radiative (kr) and non-radiative (knr) rate constants is Fp
); )
m
s. The formula employed to calculate the
FISC {kT/
ꢀ1, where,
those from the 3MLCT state are broad in shape. Complexes 1, 3 and
4 were excited state with large contribution of 3MLCT whereas
¼
(kr þ knr)}; kr ¼ Fp
/s
; knr ¼ (1/
s
) ꢀ (Fp
/s
s
¼ (kr þ knr
complex 2 was excited state with large contribution of 3p
ep*. All
Here, FISC is the intersystem-crossing yield. For the iridium com-
plexes FISC is safely assumed to be 1.0 because of the strong spin-
orbit interaction caused by heavy atom effects of iridium [45].
these complexes show emission at 526, 512, 521 and 515 nm in
dichloromethane, respectively (Table 1). The emission spectra of
complexes 2e4 are blue shifted when compared with complex 1
and this may be due to the electronic effect of the substituents. The
emission spectrum of complex 2 is more blue shifted in comparison
with that of complex 1 which is the result of the introduction of
electron withdrawing fluoro substituent into the phenyl ring
attached to C(2) carbon of the imidazole ring.
The kr and knr are the radiative and non-radiative deactivation; sf is
the lifetime of the excited state. Perusal of the radiative and non-
radiative rate constants shows that the nonradiative emission is
predominant over radiative transitions.
Experimentally observed result indicates that the kr values of
complexes increases with an increase of lemi. According to the
theory of electronic transition, the kr is proportional to square of
electric dipole transition moment (MTeS). First-order perturbation
theory gives an approximate expression for MTeS [46]
Electronic transition theory
The time correlated single photon counting (TCSPC) results fit to
single exponentials decay shown in Fig. 2, DAS6 software was used
for the fit and the c2 values are around 1. The phosphorescence
lifetime of all iridium(III) complexes were measured in degassed
CH2Cl2 solution at room temperature and the signal was measured
X
MTꢀS
¼
bn
h
1FnjMj1F0
i
(2)
where 1Fn and 1F0 are the wave functions of Sn and So states,
respectively and M is the electric dipole vector with the use of the
Table 1
Absorption (labs), emission (lemi), fluorescence quantum yield (V), lifetime (
iridium complexes 1e4.
s), radiative rate constant (kr), nonradiative rate constant (knr) and electrochemical behavior of
Complex
labs (nm)
lemi (nm)
F
s
(
m
s)
kr (sꢀ1
)
knr (sꢀ1
)
lonset (nm)
E1/2oxi (V)
HOMO (eV)
LUMO (eV)
Eg (eV)
1
2
3
4
481.7, 448.8, 310.6
526
512
521
515
0.48
0.32
0.42
0.38
1.68
1.60
1.79
1.71
0.29
0.20
0.23
0.22
0.31
0.43
0.30
0.33
467
462
471
476
0.45
0.58
0.35
0.30
5.25
5.38
5.15
5.10
2.60
2.70
2.52
2.50
2.65
2.68
2.63
2.60
470.91, 425.57, 307.2
482.24, 450.50, 309.4
470.91, 430.10, 302.16

78
J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83
spin-orbit coupling operator (Hso) and the wave function of the
lowest excited triplet state (3F1), bn is formulated as
1
3
bn ¼ h1FnjHSO
j
3F1i=ð En ꢀ E1Þ
(3)
where 1En and 3E1 are the energies of Sn and lowest excited triplet
states, respectively. Now, equation (2) is simply expressed as
1
3
MTꢀS ¼ fh1FnjHSO
j
3F1ih1F1jMj1F0ig=ð E1 ꢀ E1Þ
=ð E1 ꢀ E1Þ
1
3
¼
a
(4)
Equation
(4)
predicts
that,
when
a
¼ h1Fn
H
SOj
3F1ih1F1 M 1F0i is approximately constant, kr in-
j
j j
creases with a decrease in the energy difference (1E1 ꢀ 3E1). In the
present study, kr increases with an increase in Eemi. This fact is
explained by assuming that the S1 energy does not differ signifi-
cantly among the complexes and the energy difference (1E1 ꢀ 3E1)
increases with decrease of Eemi. Actually in these complexes the
S1 ) S0 absorption bands are located at 302e310 nm but phos-
phorescence S0 ) T1 exhibits large bathochromic shift ranging
from 515 to 526 nm. These data provide the evidence that the en-
ergy difference (1E1 ꢀ 3E1) increases with the decrease of Eemi and
therefore, kr increases with an increase in lemi [46]. The non-
radiative transition (knr) is explained in terms of coupling of
vibronic states on an initial potential energy surface to isoenergetic
levels of the final state (Eg) [47aef], [47g]. The relation between knr
and the energy gap (E0) is given by
Fig. 3. Optimized structure of Iridium(III)bis(1-benzyl-2-phenyl-1H-benzimidazolato-
0
N,C2 )(picolinate).
occupied molecular orbital (HOMO) stability and emission energy
gap are controlled by the nature and number of substituent and its
inductive influence on aromatic ring. The photophysical study of
these complexes demonstrates that the electron withdrawing flu-
oro substituent increases the absorption and emission energies of
complexes by stabilizing the HOMO level. Besides increasing the
emission energy, the lower HOMO energies decrease the energy
separation between the 1MLCT and 3LC states, which in turn
modified the excited state properties of the iridium complexes.
ꢂ
ꢃ
ꢂ
ꢃ
ꢀ
g
E0
E0
SMZuM
knrfexp
;
g
¼ ln
ꢀ 1
(5)
Z
uM
where, the energy gap (E0) is related to the zero-point energy (ZPE)
separations of two coupled surfaces. The -uM describes, in the limit
of a single configuration coordinates model, the average energy of
the vibrational mode(s) that couples the final state to the initial
state. The degree to which the two surfaces are vibronically coupled
is given by the Huang-Rhys factor (SM),
ꢂ
ꢃ
1
2
M
uM
SM
¼
ð
D
QeÞ2
(6)
Z
where, M is the reduced mass of the oscillator, uM is the funda-
mental frequency and Qe is the difference between the ground
D
and excited-state equilibrium geometries with respect to the
specified nuclear (i.e., vibrational) coordinate. Decrease in energy
gap (E0) at constant
DQe results in an increase in the vibrational
overlap between the lowest energy vibrational state of the excited
state and isoenergetic vibrational states of the ground state surface.
Effect of substituents 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
Table 2
ꢂ
ꢂ
ꢀ
Selected bond lengths (A), bond angles ( ) and torsional angles ( ) of [Ir(bpb)2 (pic)],
(1) by DFT/B3LYP/6-31 (d,p).
Bond length DFT/B3LYP/6-31(d,p) Bond angles (ꢂ)
DFT/B3LYP/6-31(d,p)
Ir(1)eO(1)
Ir(1)eC(1)
Ir(1)eN(1))
Ir(1)eN(2)
Ir(1)eC(26)
Ir(1)eC(46)
2.14 (4)
2.14 (5)
2.05 (5)
2.08 (4)
2.00 (6)
2.07 (5)
O(1)eIr(1)eN(1)
O(1)eIr(1)eN(2)
O(1)eIr(1)e(26)
O(1)eIr(1)e(46)
N(1)eIr(1)eN(2)
N(1)eIr(1)e(26)
86.5(15)
96.9(15)
177.5(2)
93.3(19)
87.6(19)
95.5(2)
Fig. 4. (a) TGeDTA curves of the iridium complexes 1e4; (b) DSC curves of the iridium
complexes 1e4.

J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83
79
Fig. 5. (a) Cyclic voltammogram curves of the iridium complexes 1e4; (b) HOMO LUMO energy levels of the iridium complexes 1e4; (c) the HOMO-LUMO orbital picture of
complexes 1 and 2.
The redox potentials of the cyclometalated iridium complexes
were measured relative to an internal ferrocene reference (Cp2Fe/
Cp2Feþ ¼ 0.45 V versus SCE in CH2Cl2 solvent) [48e50]. The
reduction occurs primarily on the more electron accepting het-
erocyclic portion of the cyclometalated benzimidazole ligands
(lowest unoccupied molecular orbital (LUMO) contribution)
whereas the oxidation process is to largely involve in the Ir-phenyl
center (HOMO contribution). The calculated energies of HOMO and
LUMO along with energy gap are given in Table 1. The iridium
complexes show reversible oxidation behavior, HOMO and LUMO
energies were calculated based on the HOMO energies and lowest-
energy absorption edges of the absorption spectra [48]. From the
energy gap values it was concluded that all the reported dopants
(1e4) are green emitters.
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” [49]. The IreC bonds of the
ꢀ
complex 1, i.e IreCav ¼ 2.038 A is shorter than IreN bonds i.e., Ire
ꢀ
ꢀ
Nav ¼ 2.060 A. The IreO bond length [2.138 A] is longer than the
ꢀ
mean IreO bond length (2.088A) reported [51] and these obser-
vations reflect the trans influence of the phenyl groups. All other
bond lengths and bond angles are analogous to the similar type of
complexes (Fig. 3).
Thermal studies
The thermal properties of the iridium complexes were investi-
gated by thermal gravimetric analyses (TGA) under nitrogen at-
mosphere. The iridium complexes are amorphous in nature possess
high thermal stability. Decomposition of Ir(bpb)2 (pic) and
(Ir(mbtb)2(pic) begins at about 382 ꢂC and 363 ꢂC respectively and
proceeds in three stages. whereas Ir(fbfpb)2(pic) decomposition
begins at about 390 ꢂC and proceeds in four stages. Decomposition
of Ir(mbmpb)2(pic) begins at about 368 ꢂC as shown in Fig. 4a.
These complexes exhibits a very complicated decomposition
Description of the structure of Iridium(III)bis(1-benzyl-2-phenyl-
0
1H-benzimidazolato-N,C2 )(picolinate), [Ir(bpb)2 (pic)] (1)
The selected bond lengths and bond angles of Ir(bpb)2 (pic) (1)
are presented in Table 2 and the optimization has been obtained
using density functional theory (DFT/B3LYP/6-31 (d,p)). This com-
plex exhibits an octahedral geometry around metal iridium and
prefers cis eC,C and trans-N,N chelate disposition instead of trans-

80
J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83
Fig. 6. (a) General structure of multilayer OLED device; (b) schematic energy diagram of LUMO/HOMO for devices IeIV; (c) color of the green OLED device of Ir(bpb)2(pic) 1.
Table 3
Performances of electroluminescence devices 1e4.
Device
Vd (V)
Lmax (cd/m2)
hext (%)
hc cd/A
hp (lm/W)
ELmax (nm)
C.I.E@8V (x,y)
I
II
III
IV
3.0
4.6
3.0
3.0
130301, 14.5 V
38156, 16 V
103156, 18 V
83882, 18 V
10.1, 6 V
6.2, 9.5 V
12.0, 8 V
13.8, 8 V
37.3, 6 V
20.8, 9.5 V
46.2, 8 V
47.8, 8 V
26.2, 5.5 V
7.2, 9.5 V
21.2, 7 V
22.9, 7 V
519
505
518
509
(0.28, 0.62)
(0.23, 0.60)
(0.30, 0.62)
(0.29, 0.61)
Vd: driving voltage; Lmax: luminous efficiency; h: current efficiency; ELmax: electroluminescence maxima.

J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83
81
Electroluminescent properties
To illustrate the electroluminescent properties of iridium com-
plexes, typical OLED devices using the iridium complexes (1e4) as
dopants were fabricated (Fig. 6a). The devices (IeIV) had a multi-
layer configuration, ITO/NPB (30 nm)/iridium complex: CBP (7%)
(30 nm)/BCP (10 nm)/Alq3 (40 nm)/Mg:Ag, in which ITO (indium tin
oxide) was used as the anode, NPB (4,40-bis[N-(1-naphthyl)-N-
phenylamino]biphenyl) was used as the hole-transporting material,
CBP (4,40-N,N0-dicarbozolebiphenyl) as the host, the iridium
complexes as dopant, BCP (2,9-dimethyl-4,7-dipheny-1,10-
phenanthroline)
as
the
hole
blocker,
Alq
(tris(8-
hydroxyquinolinato)aluminum) as the electron transporter and
Mg:Ag as the cathode. The electroluminescent characteristics are
displayed in Table 3. The relative energies of these materials are
depicted in Fig. 6b. The devices IeIV emit strong green light (Fig. 6c)
with an emission maximum at 519, 505, 518 and 509 nm, respec-
tively (Fig. 7), which are well matched with those of the PL spectra in
solution (512e526 nm). There are no characteristic emission peaks
from CBP or Alq3, indicating that the emission originates mostly
from the dopant. These results imply that effective energy transfer
from the host (CBP) to the dopant (Ir complex) occurs in the emissive
layer. The turn-on voltage of devices IeIV, are 3.0, 4.6, 3.0 and 3.0 eV,
Fig. 7. Electroluminescence spectra of complexes 1e4 in CH2Cl2.
process and mass loss calculations are not in agreement with an
intermediate of definite composition. However, the total mass loss
between 363 and 1000 ꢂC corresponds to the presence of metallic
iridium as the final decomposition product as identified by chem-
ical analysis and also from its lustrous appearance. We have
investigated the thermal properties of the complexes by differential
scanning calorimetry (DSC). Fig. 4b indicates that these iridium
complexes undergo a glass transition around 92 ꢂC, followed by
crystallization around 154 ꢂC and crystalline melting at 381, 395,
392 and 378 ꢂC for complexes, respectively and there was no phase
transition signal observed from 30 to 300 ꢂC.
respectively. The intermolecular
pep stacking interactions in the
solid state and are considered as the important factors in deter-
mining the operating voltage. The strong electronic coupling and
intermolecular interactions contribute to effective injection and
transport of charges may lead to lower driving voltages [54,55]. The
dopant molecules in green devices can be deep trap sites from both
of electrons and holes. Electron and hole movements may follow the
deeply trapped charge transport mechanism. Usually, carrier
mobility in deeply trapped organic layers is very slow. Deeply
trapped dopants could delay carrier mobility by several orders [56].
However, owing to HOMO and/or LUMO levels of dopants, easier
electron injection contributes some voltage reduction. When a guest
with a narrow Eg is doped into a host with awide Eg [Eg of dopants 1e
4 is about 0.85 eV lower than that of CBP], the difference in HOMO
and/or LUMO levels between the guest and the host significantly
increases. Then, the guest becomes a deep trap for hole and/or
electron transport in the emitting layer and reduction of the driving
voltage is the results. Upon reduction of the driving voltage the
power efficiency of the OLED was greatly improved [57]. Fig. 8a and
b shows the brightnessevoltage and the external quantum yielde
current density characteristics of the devices, respectively. All de-
vices show quite appreciable efficiencies and brightness when
compared to device with benzimidazole based acetylacetanat0o
complex, iridium(III)bis(1,2-diphenyl-1H-benzimidazolato-N,C2 )
(acetylacetanato) [(pbi)2Ir(acac)], as dopant show brightness of
Electrochemical analysis
In order to investigate the HOMO and LUMO energy levels, cyclic
voltammetry studies were carried out. On the basis of the onset
potential of oxidation, HOMO energies of the iridium complexes
can be estimated with regard to the energy level of the ferroce-
nium/ferrocene redox couple (Fig. 5a) [52]. The HOMO energies
were calculated to be ꢀ5.25, ꢀ5.38, ꢀ5.15 and ꢀ5.10 eV for com-
plexes 1e4 respectively. This is consistent with the theoretical
calculations 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 [53]. The HOMO and
LUMO energy levels are shown in Fig. 5b. The 3D orbitals of HOMO
and LUMO of complexes 1 and 2 are shown in Fig. 5c.
Fig. 8. (a) Plot of brightness ns voltage; (b) plot of external quantum yield ns current density.

82
J. Jayabharathi et al. / Journal of Organometallic Chemistry 761 (2014) 74e83
Fig. 9. (a) Plot of current efficiency ns current density; (b) plot of power efficiency ns current density.
46393 cd/m2, external quantum efficiency 12.7%, current efficiency
46 cd/A and power efficiency of 12 lm/W [58]. Devices I and II show
the better performance than others in terms of brightness of
130301 cd/m2 at 14.5 V,103156 cd/m2 at 18 V, respectively. Device (I)
with of Ir(bpb)2(pic) as dopant, shows high power (Fig. 9a) as well as
current (Fig. 9b) efficiencies, 26.2 Im/w at 5.5 V and 37.3 cd/A at 6 V,
respectively (Table 3).
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We have synthesized a series of iridium(III) complex dopants
using various substituted benzimidazole ligands. These complexes
exhibit different quantum efficiencies in solution depending upon
the nature of substituents. The wavelength can be tuned by
adjusting the electronic properties of the substituents in the ligand.
Some of the complexes discussed here showed 3MLCT predominant
mixing states for their lowest excited triplet states. But the degree
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ep* states of the excited states
varied. We have fabricated the OLED that can be driven with a low
voltage by using the phosphorescent iridium complexes as dopants.
All devices show quite appreciable efficiencies and brightness
when compared to device with benzimidazole based acetylaceta-
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0
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narrower HOMOeLUMO energy gap.
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One of the authors Prof. J. Jayabharathi is thankful to Depart-
ment of Science and Technology, Ministry of Science and Technol-
ogy, India [No. SR/S1/IC-73/2010], Defence Research and
Development Organisation, India (NRB-213/MAT/10-11), University
Grants Commission, India (F. No. 36-21/2008 (SR)) and Council of
Scientific and Industrial Research, India (NO. 3732/NS-EMRII) for
providing funds to this research study.
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