Tuning the photophysical properties of heteroleptic Ir(III) complexes through ancillary ligand substitution: Experimental and theoretical investigation

Article abstract of DOI:10.1016/j.jphotochem.2017.09.054

A series of new phosphorescent heteroleptic Iridium(III) complexes with 2-phenylpyridine, acetylacetone and 2-picolinic acid as ancillary ligands have been designed and synthesized. The photophysical and electrochemical properties have been investigated e

Full text of DOI:10.1016/j.jphotochem.2017.09.054

Accepted Manuscript
Title: Tuning the photophysical properties of heteroleptic
Ir(III) complexes through ancillary ligand substitution:
Experimental and theoretical investigation
Authors: Aravind Babu Kajjam, V. Sivakumar
PII:
S1010-6030(17)31208-X
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2017.09.054
JPC 10901
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Journal of Photochemistry and Photobiology A: Chemistry
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Accepted date:
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21-9-2017
23-9-2017
Please cite this article as: Aravind Babu Kajjam, V.Sivakumar, Tuning the
photophysical properties of heteroleptic Ir(III) complexes through ancillary ligand
substitution: Experimental and theoretical investigation, Journal of Photochemistry and
Photobiology A: Chemistry https://doi.org/10.1016/j.jphotochem.2017.09.054
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Tuning the photophysical properties of heteroleptic Ir(III) complexes
through ancillary ligand substitution: Experimental and theoretical
investigation.
Aravind Babu Kajjam, and V. Sivakumar*
Optoelectronics laboratory, Department of Chemistry, National Institute of Technology,
Rourkela, India.
* To whom correspondence should be addressed. Email: vsiva@nitrkl.ac.in (V. Sivakumar)
Tel: +91-661-2462654;
Graphical abstract
Highlights:
 Successfully synthesised three phosphorescent heteroleptic Ir(III) complexes with
three different auxiliary ligands.
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 Photophysical and electrochemical properties of Ir(III) complexes were studied and
verified by DFT analysis.
 Among all the Ir(III) complexes Ben-Ir-acac complex shows yellow emission with
high PLQY (0.29).
 From the DFT analysis, compare to all Ir(III) complexes Ben-Ir-acac acts a potential
emitter for OLED applications.
Abstract:
A series of new phosphorescent heteroleptic Iridium(III) complexes with 2-phenylpyridine,
acetylacetone and 2-picolinic acid as ancillary ligands have been designed and synthesized.
The photophysical and electrochemical properties have been investigated experimentally and
theoretically. The UV-Visible absorption and photoluminescence (PL) emission spectra of
the complexes were carried out in solution as well as thin film. The PL study indicates that
the Iridium complexes show broadband emission ranging from green to yellowish-green in
solution and greenish-yellow to yellow emission in PMMA films with appropriate CIE color
gamut. The PLQY of the complexes were found to be in the range of 0.12 – 0.29. Singlet and
triplet energy levels were calculated by using time dependent DFT (TD-DFT) calculations.
The excited triplet energy was estimated and correlated to structural and energetic
characteristics of the reported host materials. Further, investigations have been performed to
explore the optical, electronic, charge transport, and stability properties of Ir(III) complexes
as charge transport and emissive materials for organic light emitting devices (OLEDs).
Furthermore, all the complexes are promising luminescent and its hole transporting
performance is more favorable than electron transport performances. According to theoretical
2

calculations, it is clearly indicating that, Ben-Ir-acac complex is a more potential emitter for
OLED applications.
Keywords: Benzil/ ppy/ acac/ pic/ Ir(III) metal ion/ DFT.
Introduction:
Luminescent metal complexes have great attention in the field of different application like
optoelectronics, photochemistry, biological applications and chemo sensors, etc. [1]
The
phosphorescent transition metal complexes have revolutionized the field of optoelectronics
due to the advantages they possess over the other alternative emissive materials [2]. In
particular, the second and third row transition metals with d6 configuration have been
extensively studied due to the spectroscopic and photophysical properties they exhibit that
can be conveniently modified according to the desired objective [3]. Due to the mixing of
singlet and triplet energy levels in these complexes, theoretically it is predicted that they
provide 100% internal quantum efficiencies unlike the case in fluorescent metal complexes
[4-5]. Among them, the cyclometalated Iridium (III) complexes show high phosphorescent
efficiencies and relatively short lifetimes and are thus considered one of the most promising
candidates as emissive materials for phosphorescent OLEDs [6-9]. Hence in recent years, a
significant amount of work is being carried out on the development of cyclometalated (CΛN)
Ir(III) complexes. The efficiency, brightness and emission wavelengths of Iridium(III)
complexes depend strongly on the structure of the cyclometalated ligand [10]. The emission
3

wavelength of the Ir(III) complexes can be normally tuned by changing the electronic nature
and the position of the substituents on the ligands [11]. Several groups have demonstrated
the tuning of photoluminescence emission wavelength from blue to red by appropriate
functionalization of the electron withdrawing and electron donating substituents in the ligand
structure [12–14].
Previous works have shown that the highest occupied molecular orbitals (HOMOs) are
located on the phenyl groups of the cyclometalating ligands and the 5d orbitals of the Ir(III)
ion, whereas the lowest unoccupied molecular orbitals (LUMOs) are found on the ancillary
ligands [15-20]. Hence an emission shift from blue to greenish-yellow can be obtained by
stabilizing the HOMO or destabilizing the LUMO levels [21]. A large number of Iridium
complexes were found conducive to this desired target which are mostly based on the
cyclometalating ligand 2-phenylpyridine (ppy) with ancillary ligands such as acetylacetonate
(acac) or picolinate (pic) [22 - 29].
T. Fei et al., reported Phenylpyrazole based iridium(III) complexes, Ir(ppz)3, that show deep
blue emission at 77 K in CH2Cl2 but a quite poor emission at room temperature [12a,30a].
Their picolinate complex (Ir(ppz)2(pic)) and acetylacetonate complex (Ir(ppz)2(acac)), have
shown a relatively stronger green emission (507 and 514 nm) at room temperature [30b]. The
advances in technology simulataneously led to raised concern over the environmental issues
which perturbed the energy conservation and the utilization of depleting natural resources.
This demanded energy efficient lighting systems (solid state lighting (SSL), which include
white OLEDs), that can help to conserve energy and reduce the overall lighting costs. In this
scenario, white OLEDs are considered as potential candidates for future solid-state lighting
applications and backplane illumination in large-area smart displays. One of the general
approaches in obtaining white light involves the combination of blue and orange-red or
yellow emitters in the active layer [31]. Till date a lot of blue emitting Ir(III) complexes were
4

explored for white OLEDs [32] however, the complimentary yellow emitting Ir(III)
complexes are scarcely reported. The design and synthesis of novel yellow emitting Ir(III)
complexes is considered as an active research area in the field of white OLEDs. Our group
has perpetuated the work by developing a series of heteroleptic Ir(III) complexes with
benzilimidazole as cyclometalating ligand and three commonly used ancillary ligands
(acetylacetonate (OΛO), picolinate (OΛN) and phenyl pyridine (CΛN) (Fig. 1). We found that
the alterations in the ancillary ligands in the complex had a note-worthy impact on the
electronic and phosphorescent properties (as well as emission color tuning). These results
provide a well-defined pathway to use the ancillary ligand modification in controlling the
phosphorescence emission in Ir-based emitters and offer a powerful tool in the design of the
emitter architecture. Herein, we report the systematic design, synthesis, and characterization
of green/yellow-emitting Ir(III) benzilimidazole complexes through the variation of ancillary
ligands. The emission peak wavelength can be fine-tuned in greenish-yellow range (over the
540-555 nm range in solution and 530-560 nm range in PMMA films). All the complexes
exhibited high photoluminescence quantum efficiency (PLQE). The HOMO and LUMO
energy levels are calculated by using cyclic voltammetry analysis and the values are verified
by using B3LYP method, 6–31G(d) basis set for ligands and LANL2DZ basis set for an
Iridium atom. In addition, we have explored the possibility of using the currently synthesized
Ir(III) complexes for OLED as an emitters or host materials. We have calculated the other
electronic properties, such as ionization potentials (IPs), electronic affinities (EAs) and
reorganization energy (λhole/electron) to shed light on the photophysical properties. And
also, we report the investigation of both host–guest interaction and signaling properties from
theoretical point of view for these Ir(III) complexes. Further in-depth interpretations of the
available reported electron and hole transporting materials electronic characteristics have
been discussed by the investigation of the optical and electronic properties of this system.
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Furthermore, quantum-chemical studies of the auxiliary ligand effect on the electronic and
optical properties of these systems are also studied.
Experimental:
1H NMR and 13C NMRspectra were measured on a BRUKER AV 400 Avance-III (400MHz)
in CDCl3 using tetramethylsilane as an internal reference. UV–Vis absorption spectra were
recorded using a Shimadzu UV-2450 spectrophotometer. Emission spectra were measured on
a Horiba Jobin Yvon, Fluoromax 4P fluorescence spectrophotometer. Lifetime measurements
are measured in DCM solution at 298K with Edinburgh instruments FLS 980 and pulsed
xenon lamp was used as excitation source. Cyclic voltammetry studies were performed with a
computer-controlled EG&G potentiostat/galvanostat Metrohm model in a one-compartment
electrolysis cell consisting of a platinum bottom working electrode, a platinum wire counter
electrode, and an Ag/AgCl reference electrode. Cyclic voltammograms were monitored at a
scan rate of 100 mV/ s and recorded in dimethylformamide. The concentration of the
complex was maintained at 1.0 mM and each solution contained 0.1 M of
tetrabutylammonium perchlorate (TBAP) as the electrolyte. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy level were
calculated by assuming the energy level of ferrocene/ferrocenium to be 4.4 eV.
Computational Methods:
Molecular orbital calculations: The ground and low-lying excited electronic states of Ir(III)
complexes were calculated using the Gaussian 09 and 03 program package. For the calculated
ground state geometries, the electronic structure is examined in terms of the highest occupied
molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). Based
on Becke-LeeYoung-Parr composite exchange-correlation functional (B3LYP) method with
the 6–31G(d) basis set were used for the geometry optimization and the energy level
6

calculation of the ground state of those ligands, respectively [33]. Calculations on the
electronic ground states of the Ir(III) complexes have been carried out using the same
calculation method except LANL2DZ basis set for an Ir atom. The electronic populations on
the central atom were calculated to show the significant admixture of ligand π character with
the amount of iridium 5d character in the occupied molecular orbitals related to those MLCT
transitions. The UV-visible spectra, singlet and triplet energy levels were calculated by using
TD-DFT calculations [34]. In addition to these, other electronic properties, such as ionization
potentials (IPs), electronic affinities (EAs) and reorganization energy (λhole/electron) were
also calculated to shed light on the photophysical properties of these Ir(III) complexes. All
calculations were carried out using Gaussian09 and Gauss View 05 suite of programs.
Synthesis: General procedure for the synthesis of benzillimidazole ligand: [35] A
mixture of benzil (1gm, 4.75mmol, 1 eq), benzaldehyde (0.555 gm, 5.23 mmol, 1.1 eq),
ammonium acetate (3.65 gm, 47.5 mmol, 10 eq) and aniline (0.442 gm, 4.756 mmol, 1eq) in
0
the presence of glacial acetic acid (20 mL) were heated up to 120 – 130 C for 12 hrs under
N2 atmosphere in a round bottomed flask equipped with a reflux condenser. After cooling to
room temperature, the reaction mixture was poured into distilled water with continuous
stirring. The separated solid was filtered off and washed with water. The obtained solid was
dried to give expected product with good yield 95%). 1H – NMR data (CDCl3, 400 MHz): 
= (7.48 - 7.50 (d, 2H), 7.202 – 7.359 (m, 18 H). 13C-NMR Data (CDCl3, 100MHz):  =
126.6, 127.42, 127.95, 128.10, 128.25, 128.35, 128.97, 129.06.6, 130.53, 130.65, 130.86,
131.13, 134.44, 137.11, 138.29, 146.94 ppm.
General procedure for the synthesis of the iridium complexes: All synthetic procedures
involving IrCl3·3H2O and other Ir(III) species were carried out in inert gas atmosphere
despite the air stability of the compounds, the main concern being the oxidative stability of
7

intermediate complexes at the high temperatures used in the reactions [36a]. Cyclometalated
Ir(III)-chloro-bridged dimers of general formula (CΛN)2Ir(µ-Cl)2Ir(CΛN)2, where CΛN
represents the cyclometalated ligand, were synthesized by the same method reported by
Nonoyama [36]. The crude products of these dimers were used for the subsequent preparation
of (CΛN)2Ir(CΛX), CΛX represents the ancillary ligand.
Cyclometalated Ir(III)-chloro-bridged dimers [(Ben)2IrCl]2 were synthesized according to the
Nonoyama route [36a], A mixture of benzilimidazole ligand (100 mg, 0.27 mmol), Iridium
chloride trihydrate (47 mg, 0.135 mmol), 2 – ethoxyethanol (6 mL), water (2mL) were
refluxed under nitrogen for 24 hrs. The precipitate was collected by filtration, washed with
water and methanol, and dried. A mixture of chloro-bridged dimer, 0.125 mL (1.2 mmol) of
2,4-pentanedione, and 0.322 g (3.0 mmol) of sodium carbonate was refluxed under a nitrogen
atmosphere in 10 mL of 2-ethoxyethanol for 12 h. After the reaction mixture was cooled to
room temperature, the mixture was poured into water for extraction with dichloromethane.
The organic extracts were collected by using column chromatography (yield 35%). The NMR
spectra of the synthesized Ir(III) complexes were mentioned in supplementary information.
Ben-Ir-ppy: 1H – NMR data (CDCl3, 400 MHz): δ = 8.32 (d, 2H), 8.30 (d, 2H), 7.89 – 7.99
(d, 4H) 7.79 – 7.88 (m, 10H), 7.69 (d, 6H), 7.66 - 7.67 (m, 8H), 7.50 – 7.58 (m, 8H), 7.33 –
7.34 (m, 6H). 13C NMR data (100MHz, CDCl3): δ= 149.18, 146.45, 137.77, 136.57, 136.31,
133.92, 130.62, 130.36, 130.11, 129.99, 129.18, 128.58, 128.48, 128.27, 127.93, 127.85,
127.77, 127.68, 127.68, 127.62, 127.47, 126.93, 126.44, 126.13, 121.64, 120.14. (CHNS):
Anal. calcd for C68H46IrN8:C, 69.96; H, 3.97; N, 9.60 Found: C, 69.51; H, 3.89; N, 9.54.
Ben-Ir-pic: 1H-NMR Data (CdCl3, 400MHz):  = 8.02 (d, 4H), 7.72 – 7.76 (m, 5H), 7.61 (d,
7H), 7.50 – 7.51 (m, 5H), 7.30 – 7.48 (m, 8H), 7.13 – 7.21 (d, 6H), 7.05 – 7.10 (d, 2H), 6.82
– 6.92 (m, 5H). 13C-NMR Data (CdCl3, 100MHz):  = 169.45, 146.45, 137.77, 136.59,
8

133.92, 130.84, 130.62, 130.36, 130.13, 130.01, 129.72, 129.48, 128.56, 128.48, 128.56,
128.48, 127.94, 127.85, 127.76, 127.66, 127.60, 127.46, 127.22, 126.93, 126.11. (CHNS):
Anal. calcd for C61H42IrN8: C, 65.93; H, 3.81; N, 10.08. Found: C, 65.99; H, 3.79; N, 10.18.
Ben-Ir-acac: 1H – NMR data (CDCl3, 400 MHz): δ = 7.98 (d, 4H), 7.87 (d, 3H), 7.74 – 7.73
(m, 2H), 7.58-7.68 (m, 5H), 7.65 (d, 3H), 7.54 (t, 3H), 7.50 – 7.52 (m, 6H), 7.41-7.49 (m,
13
6H), 7.25-7.27 (d, 2H), 6.85 – 6.92 (m, 5H), 5.24 (s, 1H), 2.33 (s, 6H). C NMR data
(100MHz, CDCl3-d6): δ= 150.00, 147.45, 146.76, 137.71, 137.66, 131.12, 130.15, 129.35,
128.99, 128.93, 128.83, 128.75, 128.25, 128.19, 127.82, 127.34, 126.61, 126.42, 126.34,
125.98, 125.69, 124.92, 124.45, 123.52, 123.35, 122.91, 121.58, 120.20, 118.06, 60.85,
28.25. (CHNS): Anal. calcd for C61H48IrN5O3: C, 67.14; H, 4.43; N, 6.42; Found: C, 67.03;
H, 4.29; N, 6.12.
Results and discussion:
Photophysical properties:
The UV–Vis absorption spectra of the heteroleptic Ir-complexes are shown in Fig. 2. The
major absorptions observed at 220-320 were appear to be π–π* absorptions of aromatic
moiety present in the ligand and the absorption at 330-475 nm can be assigned to singlet
metal-to-ligand charge transfer (1MLCT) band and triplet metal-to-ligand charge transfer
(3MLCT) band, respectively. In general, the absorption and excitation spectrum should be
similar. However, in the presently studied complexes, we have observed the intensity
difference in the organic ligand absorption as well as metal ligand charge transfer spectral
range. In the excitation spectra it is clearly showing that, the π-π* excited states are non-
emissive, whereas the MLCT is emissive. From the DRS spectra of the PMMA films showed
(Fig. S10) that, the band gap from the DRS spectra of the Ir(III) complexes are 2.50 eV for
Ben-Ir-acac, 2.52 eV for Ben-Ir-pic and 2.88 eV for Ben-Ir-ppy. Fig. 3 shows the
9

photoluminescence (PL) spectrum of the Ir-complex in 10-4 M CH2Cl2 solution and PMMA
films. The Ir- complexes shows strong photoluminescence at 533- 561 nm, which is much red
shifted from that of the free ligand (at 462 nm). As the change in ancillary ligand from ppy to
pic the luminescence is shifted towards higher spectral region (red-shift), which could be
attributed due to the alteration in the electronic structure of the ancillary ligand and formation
of aggregates in the PMMA matrix and strong intermolecular interactions are present in the
PMMA matrix [37]. The changes in the PL spectrum of the complexes (spectral data’s) are
tabulated in Table 1. The low-energy MLCT band in Ben-It-acac complex (542 nm) is
significantly red-shifted compared to that of Ben-Ir-ppy and Ben-Ir-pic complexes (Fig. 3).
The Ben-Ir-ppy emits green light with a maximum peak at 523 nm, whereas the Ben-Ir-pic
emits greenish yellow light with a maximum peak at 534 nm (Fig. 3) which is in good
consistency with their absorption spectra. The small Stokes shift between emission signal and
the lowest energy absorption band in the Iridium complexes, in combination with a structure
3
less spectral feature, suggest that the phosphorescence originates primarily from the MLCT
3
state, together perhaps with a lesser contribution from the π–π∗ excited states. Similarly,
compared with Ben-Ir-acac, Ben-Ir-pic also shows a 10 nm blue-shift of the emission band.
The results imply that the orientation of π conjugation would exert a large effect on the
photophysical properties of the phosphorescent Ir (III) complexes. Compared to the
complexes Ir(ppz)2(pic) and Ir(ppz)2(acac), the synthesized complexes show emission
towards red and have high PLQY [33a, b]. It clearly indicates that the modification of an
ancillary ligand is certainly followed by the change in emission spectra. Compared to
emission in the solution, emissions with a small red shift were observed (in PMMA films) in
the case of Ben-Ir-ppy to Ben-Ir-pic. By changing the auxiliary ligand from pic (Ben-Ir-pic)
complex to acac (Ben-Ir-acac) there is a bathochromic shift (20 nm) in PMMA films.
Similar observations were also been reported by Yafei Wang et al., in Ir(III) complexes with
10

pic and acac as auxiliary ligands [33c]. The emission values of the PMMA films were shown
in table 1.
Photoluminescence Quantum yield calculations:
Quantum yields of the Ir(III) complexes were calculated by using Ir(ppy)3 (PLQY=0.97) as
standard according to equation 1 [38].
Φ = Φ’. (푰푰 ) . (푨푨 ) . (풏풏 ) ………………. 1
′
′
′
I (sample) and I' (standard) are the integrated emission peak areas, A (sample) and A'
(standard) are the absorbance at the excitation wavelength, and n (sample) and n' (standard)
are the refractive indices of the solvents. Based on the above equation, the PLQY of the
heteroleptic Ir(III) complexes were calculated and corresponding values are mentioned in
Table. 1. The observed quantum yields for the synthesized Ir complexes in solution and
PMMA films in the range of 0.12 – 0.29 and 0.15-0.28, respectively.
Lifetime measurements:
Lifetime decay curve analysis of Iridium(III) complexes are shown in the Figure 4. The
measured data was fitted with the bi-exponential function given by below equation. [39a,b]
(τ) = (A1흉ퟐퟏ + A2흉ퟐퟐ)/(A1τ1+A2τ2)
Where, (τ) is the mean lifetimes, τ1, τ2 are short- and long decay components, and A1, A2 are
weight factors, respectively. Compared to the well know Ir complex (FIrpic, 1.69 μs) [39c],
all the novel Ir(III) complexes have shown slightly higher decay times. The luminescent
decay time values (τ) of Ben-Ir-ppy, Ben-Ir-pic and Ben-Ir-acac were found to be 1.48, 1.76
and 1.82 µs, respectively.
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Electrochemical properties: The HOMO and LUMO energy levels were calculated by using
equation reported by de Leeuw et al [40].
EHOMO= −(퐸표푂푛푥푠푖푒푡→푆퐶퐸 +4.4) eV
ELUMO= −(퐸표푅푛푒푠푑푒푡→푆퐶퐸 +4.4) eV
Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior of the
iridium complexes. The highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels of the materials were estimated according to the
electrochemical performance. As shown in Fig.5, the oxidation potential was measured
relative to a ferrocenium/ferrocene reference (Fc+/Fc). The HOMO energy values for these
Iridium complexes were calculated based on the value of −4.4 eV for Fc with respect to zero
vacuum level. The electrochemical data and energy levels of the complexes are summarized
in Table 2. The oxidation of the three substituted heteroleptic Ir(III) complexes (ben-Ir-ppy,
Ben-Ir-pic and Ben-Ir-acac) are generally chemically irreversible and appear at more positive
potentials (1.25, 1.30 and 1.45 V). The irreversibility in such cases is generally attributed to
the susceptibility of the electrogenerated Ir(III) species to nucleophilic attack by the solvent.
Thus, the chemical reversibility in the present instance, as well as the substantially lower
potential of the oxidation, suggests that the resulting charge may be more delocalized over
the cyclo-metallating ligand than in the auxiliary ligands. The reduction irreversibility of
Ir(III) complexes might be attributed to the ancillary ligand environment [41]. The
HOMO−LUMO level of these Ir (III) complexes is also affected by the ancillary ligand. This
was revealed by the fact that substitution of the acac ligand by the ppy ligand markedly raised
the LUMO energy from −2.87 eV of Ben-Ir-ppy to −3.04 eV of Ben-Ir-pic, while the
replacement of ppy ligand with pic led to a shift from −2.87 eV to −3.06 eV was observed. It
should be noted that major changes at the HOMO were also observed i.e, when replacement
of ppy ligand with pic ligand the shifting was observed from −5.85 eV to −5.70 eV, while in
12

the case of ppy to acac the shifting was observed from -5.85 eV to -5.65 eV. It is concluded
that, by changing the auxiliary ligand there is a affect in the HOMO and LUMO levels were
absorbed.
DFT analysis:
The optimized structures of the synthesized Ir(III) complexes (Ben-Ir-acac, Ben-Ir-ppy and
Ben-Ir-pic) are depicted in Fig.6. All the Ir(III) complexes are coordinated with two primary
ligands (Benzilimidazole) and one ancillary ligand (acac/ppy/pic). The representation
structures of the Ir(III) complexes are shown in Fig. S9 along with the numbering of some
key atoms. The lowest lying ground state (S0) energy levels were calculated to observe the
changes in the geometry structures upon excitation, and the corresponding values are
enumerated in Table 4. The results indicating that, there is no much deviation on bond
distances around Ir(III) atoms by introduction of different substitutions on the primary
ligands. In addition, for all the three complexes, the bond lengths of Ir–N1/N2 are about 0.02–
0.06 Å longer than that of Ir–C1/C2, which is due to the different features of the N and C
atoms. There is a possibility to form a Ir–N coordination bond through a pair of electrons
from N (lone-pair of electrons for the hybrid N atom) entering the unoccupied orbit of the
Ir(III) ion, whereas the Ir–C bond is formed by sharing electrons because of the absence of
the lone-pair of electrons for the hybrid C atom [42]. The shorter distances indicate a stronger
interaction between the benzilimidazole rings and the metal center. Moreover, both the
benzilimidazole ligands are almost perpendicular to the ancillary ligand with the bond angles
of N1–Ir–N1 and N2–Ir–N1 being 78°–79° and 79°–80°, respectively. To explore the influence
of different ancillary ligands on the electronic structures especially for the FMOs (Frontier
Molecular Orbitals), the contour plots of the FMOs and energy levels for all complexes are
shown in Table 5. The HOMOs of all complexes present similar features which are
principally localized on the Ir(III) d-orbital and and the relatively electron rich phenyl group
13

of the cyclometalated ligand. In addition, the LUMOs primarily distribute over the the N-
heterocyclic part of the cyclometalated ligand and the ancillary ligand. Based on the HOMO,
LUMO energy level band gap was calculated and tabulated in table 3.
Absorption spectra:
To provide a suitable inference on the electronic transition properties of the Ir(III) complexes
has been done by using TD-DFT. The absorption process were performed by TD-DFT
calculations with PCM in gas phase with the basis of the optimized ground-state geometry
(Fig. 8). The absorption spectra of the theoretical DFT functional with the same basis set 6-
31G(d)-LANL2DZ, i.e., TD-B3LYP were employed to perform the calculations for the three
heteroleptic Ir(III) complexes, whose corresponding experimental values are shown in
Table.1. For all the three complexes, the experimental absorption spectra show intense
absorption bands around 320 - 400 nm and an obscure long absorption band at 400 - 475 nm.
Neither the band shape nor the position obtained at the TD-B3LYP level agrees well with the
experimental results. According to Kasha's rule (photoluminescence (PL) occurs from the
lowest relaxed excited state) and due to spin orbit coupling (heavy metal ion effect), all the
excited energies in the higher excited levels of the heavy metal complexes tend to reach T1
state through internal conversion (IC) or intersystem crossing (ISC), thus, it is reasonable to
investigate only the T1 excited state here. The obtained corresponding singlet and triplet
energy levels are tabulated in Table. 3. By changing the ppy ligand with pic ligand there is a
red shift was observed from 379 to 392 nm. Simultaneously, compared with Ben-Ir-pic
complex with Ben-Ir-acac, there is a red shift observed from 392 – 399 nm which suggests
that, the substitution of the ancillary ligand can influence the absorption properties of
synthesized Ir(III) complexes. The vertical transitions, their oscillator strengths and
configurations of the heteroleptic Ir(III) complexes are tabulated in table 6.
14

aOrbital contributions below 10% are omitted.
Charge transport properties:
The balance between charge injection and transfer character of molecules is very important in
deciding the electronic properties of OLEDs [43]. Thus, it is necessary to evaluate the energy
barrier for the hole and electron injection and transfer mobility. Ionization potential (IP) and
electron affinity (EA) can be used to evaluate the energy barrier for hole and electron
injections [44]. In general lower IP (large EA) value suggests an easy injection of holes
(electrons).
To evaluate the charge transfer rate and balance properties, the reorganization energy (λ) is
estimated for all the synthesized Ir(III) complexes. A small λ value is essential for an efficient
charge transport process. Generally, the λ for hole and electron transfers can be simply
defined by the following eqs 1 and 2 and is schematically presented in Scheme 3.
λhole = [E+(M)] – E(M)] + [E+(M+) – E(M+)] or [λ1 + λ2] = IPv – HEP …………. (1)
λelectron = [E(M–)] – E–(M–)] + [E(M) – E–(M)] or [λ3 + λ4] = EEP – EAv …….....(2)
where E, E+, and E− represent the energies of the neutral, cation, and anion species,
respectively, and M, M+, and M− denote the optimized geometries of neutral, cation, and
anion, respectively. HEP and EEP are hole and electron extraction potentials, respectively.
Here the HEP and EEP are the expressions of IP and EA, respectively. HEP is the energy
15

difference from M+(cationic) to M (neutral molecule), using M+ geometric structure in the
calculation and EEP is the energy difference from M−(anionic) to M, using M− geometric
structure in calculation. E(M) and E+(M+) represent the energies of the neutral and cation
species in their lowest energy geometries respectively, while E(M+) and E+(M) represent the
energies of the cation and neutral species, respectively. In the table 7, the calculated IPv
values increase in the following order: Ir-Ben-ppy<Ir-Ben-acac<Ir-Ben-pic. For all the
complexes, the reorganization energies of hole transport (λhole) are lower than those for
electron transport (λelectron), which reveals that the hole transport performance of these
complexes is slightly better than the electron transport ability. Moreover, when the acac
group introduced is in the complex, the imbalances between λhole and λelectron for Ben-Ir-acac
are smaller than the remaining complexes, demonstrating that the performance of the OLED
device is more affected on substituted ligand. In addition, the Δλ of Ben-Ir-acac is the
smallest one among the other Ir(III) complexes, revealing that Ben-Ir-acac is a more potential
emitter for OLED applications.
Comparison of performance in OLEDs:
The investigation on the matching relationship between the guest and host materials is an
important subject for research on the efficiency of OLED devices. We have taken CPB,
TCTA and TPBI host materials to compare the efficiency of our studied complexes [45].
TCTA is a typical hole transport (HT) host material and TPBI is mostly used as an electron
transport (ET) type host in a mixed host system or a double emissive layer. As seen from Fig.
5, the triplet energies of the three complexes acting as guest materials (phosphorescent
dopant) in the OLED devices are smaller than those of the host materials. To achieve efficient
electrophosphorescence, the host should also have the high triplet energies than those of the
16

guest emitters, so that the triplet excitons confine in the emissive layer. It will also prevent
the reverse energy transfer from the guest back to the host. Based on the results obtained
from the calculations, the triplet energy levels of the complexes are in the range of 2.37-2.62
eV. Thus CBP can be proposed as the best host material for the synthesized Ir(III) complexes.
According to the Marcus theory, Dexter energy transfer plays a important role in obtaining
triplet excitons for the guest materials, and the consistent transfer rate is usually associated
with the changes of Gibbs free energy (ΔG0) [45]. When the values of ΔG0 approach 0, the
corresponding electron-transfer rates were reaches to their highest values, whereas when the
values approaches far from 0, then Dexter energy transfer velocities will be very slow. Here,
ΔG0 can be considered as the triplet energy difference between the host and guest materials
[46]. Furthermore, large free energy changes can help the exciton confinement. If ΔG0«0, the
triplets can be strongly confined on the guest material, whereas if ΔG0»0, triplet excitons will
be strongly confined on the host material. And in the latter case, the studied systems need to
have a long host lifetime in order to achieve high electroluminescent quantum efficiency [46].
Based on the theoretical results, the triplet energy levels of the host and guest are calculated
and shown in Fig. 9.
Fig. 9, shows that all the synthesized complexes acting as guest materials in the OLED
devices having lower triplet energies than CBP host material. It clearly demonstrates that,
Dexter energy transfer from the host to the guest has taken place for the synthesized Ir(III)
complexes. On comparing with FIrpic (ET=2.82 eV) [47], all the Ir(III) complexes increases
the values of ΔG0, which indicates that the triplet excitons can be more confined on the guest
materials.
Stability properties:
17

Stability is a useful criterion to evaluate the nature of devices for charge transport and
luminescent materials. To predict the stability of the Ir(III) complexes from a viewpoint of
molecular orbital theory, the absolute hardness η of Ir(III) complexes were calculated using
the operational definitions [48] given by:
2
1 δμ
δμ 훿 휇
IP−EA
η= [ ] = [ ] [ 2] =
2 훿푁
훿푁 훿푁
2
Where μ is the chemical potential and N is the total electron number. In this work, the values
for IP (ionization potential) and EA (electron affinity) were determined according to the
equation 1 and 2. The absolute hardness η is the resistance of the chemical potential to
change in the number of electrons. The results displayed in Table 7 reveals that the η values
of all Ir(III) complexes in the range of Ir(III) complexes are 2.64-3.12.
CIE color coordinates:
CIE chromaticity coordinates were calculated for Ir(III) complexes from emission spectral
data of solution as well as PMMA films. The calculated the CIE coordinates for the
complexes are shown in Fig.10 and the corresponding x, y values are tabulated in Table 8.
Based on this data, the Ben-Ir-ppy and Ben-Ir-pic complexes shows greenish emission and
the Ben-Ir-acac shows greenish-yellow emission (0.370, 0.574) in solution but in the case of
PMMA films the emission of Ben-Ir-ppy and Ben-Ir-pic was in greenish-yellow, Ben-Ir-acac
shows yellow emission (0.450, 0.540). Based on the CIE values we can concludes that, the
complexes are showing green to greenish-yellow emission in the CIE gamut color space in
three phases and close to NTSC (National Television System Committee) standards.
18

Conclusion:
In summary, we have reported the synthesis, photophysical and electrochemical properties of
three novel heteroleptic Ir(III) complexes with benzilimidazole as cyclometalated ligands and
ppy, acac and pic as ancillary ligands. All the complexes have shown broad emission with
PLQYs in the range of 0.125-0.291 in CH2Cl2 and PMMA films at 298 K. We have
calculated the HOMO-LUMO energy levels by using cyclic voltammetry and further they
were correlated with DFT analysis. The electron densities of the frontier molecular orbital
distributions are also influenced by different auxiliary ligand substitutions of these complexes
and hence their effect on the phosphorescent properties and OLED performance was studied
through theoretical calculations. The obtained results suggest the effect of different auxiliary
ligands on the electron density distributions, the phosphorescent properties and the OLED
performance. Besides, Ben-Ir-acac is superior to other Ir(III) complexes in view of the charge
balance and its excellent hole trapping and hole injection capability through Dexter energy
transfer rate. The results from theoretical calculations reveal that, there is a well
interpretation on structure property relationships and matching rules between host and guest
materials in OLED devices. Hence, these Ir(III) complexes can be used as hole transport
materials for OLEDs. And further, it clearly indicates that, Ben-Ir-acac complex is a more
potential emitter for OLED applications.
Acknowledgements:
This work is funded by the Department of Science and Technology (DST), Government of
India (SERB/F/4895/2012-13 and INSPIRE award no IFA12-CH-48).
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26

Figures
Fig.1. Chemical structure of heteroleptic Ir(III) complexes.
Fig.2. UV-Visible (left) and excitation (right) spectra of heteroleptic Ir(III) complexes in 10-4
M CH2Cl2 solution.
27

Fig. 3. Emission spectra of heteroleptic Ir(III) complexes in solution (left) in 10-4 M CH2Cl2
solution and PMMA films (right).
Fig. 4. Lifetime of heteroleptic Ir(III) complexes.
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Fig.5. Cyclic voltammetry curves of the three Ir(III) complexes (Epa and Epc are the cathodic
and anodic potential, respectively).
Fig.6. Optimized structures of heteroleptic Ir(III) complexes.
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