Green-emitting phosphorescent iridium(III) complex: Structural, photophysical and electrochemical properties

Article abstract of DOI:10.1016/j.ica.2013.08.005

A new iridium(III) complex [Ir(cpy)₂(prz)], (cpy = 9-(pyridin-2-yl)-9H-carbazole; prz = pyrazine-2-carboxylic acid) has been synthesized and characterized. The molecular structure of [Ir(cpy) ₂(prz)] was confirmed by a single-crystal X-ray diffraction. The iridium metal center adopts a distorted octahedral structure coordinated to two cpy and one prz ligand, showing cis C-C and trans N-N chelate dispositions. The photophysical, electrochemical properties and thermal stability of the complex were investigated. The complex showed green luminescence and good electrochemical stability with high lying HOMO energy levels.

Full text of DOI:10.1016/j.ica.2013.08.005

Accepted Manuscript
Note
Green-emitting phosphorescent Iridium(III) complex: Structural, photophysical
and electrochemical properties
V. Thamilarasan, A. Jayamani, P. Manisankar, Young-Inn Kim, N.
Sengottuvelan
PII:
S0020-1693(13)00444-1
http://dx.doi.org/10.1016/j.ica.2013.08.005
ICA 15607
DOI:
Reference:
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Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
30 October 2012
13 June 2013
3 August 2013
Please cite this article as: V. Thamilarasan, A. Jayamani, P. Manisankar, Y-I. Kim, N. Sengottuvelan, Green-emitting
phosphorescent Iridium(III) complex: Structural, photophysical and electrochemical properties, Inorganica Chimica
Acta (2013), doi: http://dx.doi.org/10.1016/j.ica.2013.08.005
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1
Green-emitting phosphorescent Iridium(III) complex: Structural,
photophysical and electrochemical properties.
V. Thamilarasana, A. Jayamania, P. Manisankarb, Young-Inn Kimc, N. Sengottuvelana*
b
*aDDE, Department of Industrial Chemsitry, Alagappa University, Karaikudi -3, Tamilnadu,
India
cDepartment of Chemistry Education and Interdisciplinary Program of Advanced Information
and Display Materials, Pusan National University, Pusan 609-735, Korea
ABSTRACT
A new iridium (III) complex [Ir(cpy)2(prz)], (cpy = 9-(pyridin-2-yl)-9H-carbazole; prz =
pyrazine-2-carboxylic acid) has been synthesized and characterized. The molecular structure of
[Ir(cpy)2(prz)] was confirmed by a single-crystal X-ray diffraction. The iridium metal center
adopts a distorted octahedral structure coordinated to two cpy and one prz ligand, showing cis C-
C and trans N-N chelate dispositions. The photophysical, electrochemical properties and thermal
stability of the complex were investigated. The complex showed green luminescence and good
electrochemical stability with high lying HOMO energy levels.
Keywords: Iridium(III) complex, Phosphorescence, Carbazole, Photoluminescence,
Electrochemical studies.
* Corresponding authors. Tel./fax: +91 9488260744 (N. Sengottuvelan).
E-mail address: nsvelan1975@yahoo.com.
a
aPresent address: DDE, Department of Chemsitry, Alagappa University, Karaikudi -630003,
Tamilnadu, India

2
1. Introduction
Iridium(III)-based phosphorescent complexes as emitters in organic light-emitting diodes
(OLEDs) have attracted great attention because, they can fully utilize both singlet and triplet
exciton by the strong spin-orbital coupling of heavy-metal ions [1,2]. In OLEDs carbazole
derivatives are usually used as host materials for both small molecule OLEDs such as 4,4’-bis(9-
carbazolyl)-2,2’-biphenyl (CBP) and polymer OLEDs such as poly-9-vinylcarbozole due to their
high triplet energy and good hole-transporting ability [3-5]. Carbazole based iridium complexes
which exhibit red and green emission, having unique charge carrier mobility, good thermal
stability, hole transport ability and play a very important role in the electroactive and photoactive
materials [6-11]. Moreover, a carbazole-modified cyclometalated ligand could provide steric
protection around the metal center, which could restrain triplet–triplet annihilation and facilitate
charge transport across the bulk for high-performance OLEDs [12-14]. Thus manipulation of the
skeletal arrangement as well as the substituent group of the cyclometalating ligand may represent
a promising venue for the development of highly phosphorescent Ir(III) complexes. The 2-
phenylpyridine (ppy) (Figure 1a) structures are the most popular and recently cyclic
phenylvinylpyridine, (Figure 1b) or 1-(2,4-difluorophenyl)pyrazole (Figure 1c) as organic
ligands for phosphorescent iridium complexes were reported. [15,16] Further, all these iridium
complexes were prepared in a five-membered chelated framework. Iridium complexes with six-
membered chelated framework were rarely reported.[17,18] It was reported that 8-
phenylquinoline based six membered chelated iridium complexes are deep red emitting OLED
(Figure 1d). Along this line, we have synthesized a iridium(III) complex which containing 9-
(pyridin-2-yl)-9H-carbazole (Figure 1e) as a cyclometalated ligands and pyrazine-2-carboxylic
acid as ancillary ligand. The pyridine and carbazole units in a cyclometalted ligand are connected

3
through a nitrogen atom and this molecule act as π-type conjugation with aryl groups. The X-ray
structural analysis, photophysical, electrochemical properties and thermal stability of the
complex have been investigated. The introduction of carbazole units might influence the HOMO
energy level of the cyclometalated complex, solubility in common organic solvents and tune the
hole-transporting ability of the material.
2. Experimental
All reagents and solvents were commercially obtained from Alfa acsar chemicals and used
without further purifications. Elemental analyses of carbon, hydrogen and nitrogen were
1
performed on a Carlorerba-1106 microanalyzer. H NMR spectra were measured on a
MERCURY-300BB spectrometer and chemical shift were referenced to CDCl3 as an internal
standard. UV–Vis absorption spectra were recorded on Shimadzu 160A recording
spectrophotometer. PL spectra were recorded on Perkin–Elmer LS55 luminescence
spectrophotometer. Timeresolved fluorescence lifetimes were measured with a Horiba Jobin
Vyon (IBH) (TCSPC) instrument with time correlated single photon counting techniques. Cyclic
voltammetry (CV) was carried out in nitrogen purged dichloromethane at room temperature with
CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1 M) was
used as supporting electrolyte. The conventional three-electrode configuration consists of Glassy
Carbon working electrode, a platinum wire counter electrode, and an Ag/AgCl reference
electrode.
2.1. Synthesis of ligand 9-(pyridin-2-yl)-9H-Carbazole
A mixture of carbazole (1 g, 6.09 mmol), potassium carbonate (1.16 g, 8.41 mmol), 2-
bromopyridine (0.97 ml, 10 mmol) and Cu-bronze (0.15 g) in nitrobenzene (20 ml) were
refluxed for 10 h. After cooling to room temperature the solvent was removed in vacuum from

4
the mixture, ammonia solution (50 ml) was added and the mixture was left to stand for 2h.
CH2Cl2 (150 ml) and water were added. The organic phase was separated washed with water
(100 ml x 2), brine solution (100 ml), dried over anhydrous Na2SO4, filtered, and the solvent
removed to dryness. Purification by Column Chromatography using silica gel eluting with a
mixture of CH2Cl2 and hexane followed by recrystallization with hexane afforded yellow solid.
Yield: 69 %. (1.01 g). H1 NMR (300 MHz, CDCl3) δ 7.288 (t, 4H), 7.636 (t, 3H), 7.63 (d, 1H),
7.843 (d, 2H), 7.87 (s, 2H), 8.123 (s, 3H), 8.149 (s, 3H), 8.734 (d, 1H). Anal. Calc. (%) for
C17H12N2: C, 83.58; H, 4.95; N, 11.47. Found (%): C, 83.58; H, 4.98; N, 11.47.
2.1.1. Synthesis of iridium(III) complex
The mixture of organic ligand 9-(pyridin-2-yl)-9H-Carbazole (0.41 g, 6.7 mmol),
IrCl3.nH2O (0.2 g 0.67 mmol) in a mixed solvent of 2-ethoxyethanol (12 ml) and water (4 ml)
was stirred under argon at 120 °C for 24 h according to the similar method reported by
Nonoyama [19]. Cooled to room temperature, the precipitate was collected by filtration and
washed with water, ethanol and hexane successively, and then dried in vacuum to give a
cyclometallated Ir(III) μ-chloro-bridged dimer. The dimer (0.2 g, 0.19 mmol), pyrazine-2-
carboxylic acid (0.07 g, 0.58 mmol) and Na2CO3 (0.20 g, 1.9 mmol) were dissolved in 2-
ethoxyethanol (10 ml) and the mixture was then stirred under argon at 100 °C for 16 h. After
cooling to room temperature, the precipitate was filtered off and washed with water, ethanol and
hexane. The crude product was flash chromatographed on silica gel using CH2Cl2 as eluent to
afford the desired Ir(III) complex [(cpy)2Ir(prz)] as red Solid. Yield: 52%. (0.14 g). Anal. Calc.
(%) for C41H29Cl4IrN6O2: C, 50.68; H, 3.01; N, 8.65. Found (%): C, 50.42; H, 3.15; N, 8.42. 1H
NMR (300 MHz, CDCl3) δ (ppm)9.31(1H, s), 8.42 (1H, d), 8.30(1H, d), 8.12 (2H, m), 7.85(2H,

5
ddd), 7.67 to 7.34 (4H, m), 7.25 (2H, s),6.93 (2H, t) 6.78 (2H, m), 6.66 (2H, t) 6.46 (2H, d), 6.31
(2H, dd), 5.89 (2H, d).
2.2. X-ray crystallography
X-ray intensity data [(cpy)2Ir(prz)] was collected at room temperature (T=296 K) on a
Bruker X8 KAPPA APEX-II CCD diffractometer equipped with graphite monochromated Mo
Kα radiation. Initial unit cell parameters were obtained from SMART software [20]. Data
integration, correction for Lorentz and polarization effects and final cell refinement were
performed by SAINTPLUS [21]. An empirical absorption correction based on the multiple
measurement of equivalent reflections was applied using SADABS program [22]. Structure was
obtained by a combination of the direct methods and difference Fourier syntheses and refined by
fullmatrix least-squares on F2 using the SHELXTL [23]. All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were replaced in ideal positions and refined as riding atoms
with relative isotropic displacement parameters.
3. Results and discussion
3.1. X-ray crystallography
A single crystal of [Ir(cpy)2(prz)] was grown by diffusion of hexane into concentrated
dichloromethane solution and its structure was unambiguously confirmed by X-ray
crystallography. The ORTEP diagram of complex is shown in Fig. 2. The details of the crystal
data and refinement for [Ir(cpy)2(prz)] are listed in Table 1. The crystal structure of
[Ir(cpy)2(prz)] consists of [Ir(cpy)2(prz)] and two lattice CH2Cl2 molecules. The complex
crystallizes in the monoclinic space group P21/n. As depicted in Fig. 2, complex 1 reveals a
distorted octahedral geometry around iridium(III) ion, consisting of two cyclometalated cpy
ligands and a pyrazine carboxylic acid ligand. The prz ligand arrangement is planar. The bite

6
angles at iridium(III) ion are 90.12(19)o and 88.73(19)o for the cyclometalating ligands and
76.53(15)o for Ir-prz. The cpy ligands adopt a mutual eclipse configuration with their
coordinated N(2) and N(4) atoms and C(2) and C(18) atoms being in a trans and cis orientation,
respectively, whereas the prz is located at a unique position opposite to the carbon atoms of the
cpy ligands. This ligand arrangement is similar to those of the parent 2-phenylpyridine ligands in
chloride-bridged dimer complex [(ppy)2Ir(μ-Cl)]2 [24] and diketonate complex (ppy)2Ir(acac)
[25,26] suggesting that the prz ligand in our case is attached to the metal via a simple
replacement of both chloride ligands. The bond angles in the iridium octahedron vary from
76.53(15) to 99.85(18)o and from 172.88(19) to 176.34(17)o. The Ir-C distance (Ir-C(18) = 2.006
A°) are shorter than the Ir-N bond length (Ir-N(5) = 2.148 A°), which implies that a stronger
trans influence of the phenyl ring in comparison to the pyridyl groups show a trans N-N'
configuration. It is clearly shown by the rather short π–π interaction (3.361A°) occurs between
two neighboring phenyl moieties and each two adjacent molecules stack closely through short
C–H4… π (C27)interaction (2.799 A˚) between the carbazole-carbazole ring moiety. The
carbazole ring is approximately coplanar with the phenyl ring. As observed from the crystal
packing structure, the introduction of carbazole moieties, as steric hindrance units induces an
increase in the closest Ir-to-Ir distance of 9.661 Å in a unit cell of the complex [27].
3.2. Thermostability
The thermal durability of complex is analyzed (Figure 1s) by the thermo gravimetric data
under an N2 atmosphere and the decomposition temperature of Ir(cpy)2(prz) is measured to be as
high as Td5 = 240 oC, suggesting that the complex is thermally stable enough to be vaporized for
OLED fabrication.

7
3.3. Photophysical properties
The absorption (UV-vis) and photoluminescence (PL) spectrum of [Ir(cpy)2(prz)] in CH2Cl2
solution at room temperature are shown in Fig. 3. The photophysical properties of the
iridium(III) complex are listed in Table 2. The absorption bands below 252 nm (ε = 30.2 x 104
M-1cm-1) were tentatively assigned as 1(π–π*) transition of carbazole pyridyl-based ligands while
the absorption at 342 nm (ε = 6.8 x 104 M-1cm-1) is due to the carbazole moiety [28]. In a lower
energy region at 498 nm (ε = 1.2 x 104 M-1cm-1) we could observe a weak shoulder, which can
be attributed to spin allowed and spin-forbidden metal-to-ligand charge transfer MLCT transition
of Ir(III) complex [29]. The MLCT absorption bands from 370 nm to 520 nm, exhibit good
spectral overlap with the PL emission band [30]. This good spectral overlap indicated that an
efficient Forster or Dexter energy transfer from the host (poly(vinylcarbazole) (PVK), to the
iridium complex guests would be expected in the electroluminescent device. The PL spectrum of
the complex showed green emission bands at 418, 646, 698 nm excited at 340 nm (Fig. 2S),
exhibiting vibronic progressions with three maximum peaks. [31,32]. Phosphorescence relative
quantum yield of [Ir(cpy)2(prz)] in dichloromethane solution was measured to be 0.22 at room
temperature using typical phosphorescent Ir(ppy)3 as a standard (Φpl = 0.4). Fig. 3S shows the
lifetime decay of the complex in dichloromethane solution at room temperature. It was found
that the observed emission lifetimes = 1.47 ns & = 5.16 ns in the nanosecond time scales,
1
2
indicating the phosphorescent nature of the emission (Fig. S3).
3.4. Electrochemical properties
The electrochemical behavior of [Ir(cpy)2(prz)] complex was investigated by cyclic
voltammetry (Fig.4) in CH2Cl2 solutions with ferrocene/ferrocenium (Fc/Fc+) redox couple as an
internal reference and the results are given in Table 2. The complex reveals a quasireversible

8
oxidation peak at 1.04 V. Interestingly, an additional irreversible oxidation wave at 0.44 V was
also observed and according to the previous report [28], it was assign to the oxidation of
carbazole. The reduction peak at -1.0 V can be assigned to the reduction of the pyridine
heterocyclic portion of the ligands. As revealed previously by electrochemistry and theoretical
calculations [33-36] of cyclometalated iridium(III) complexes, the reduction occurs primarily on
the more electron accepting heterocyclic portion of the cyclometalated cpy ligands (LUMO
contribution) whereas the oxidation process is to largely involve in the Ir-cpy center (HOMO
contribution). Consistent with this conclusion, our complex have similar LUMO energy levels
due to the same prz and pyridyl based frame, which makes the reduction potential of these
complexes stay in a narrow range. The HOMO and LUMO level of were calculated to be -5.25
eV and - 2.92 eV respectively. From the energy gap value it was concluded that the reported
dopant is green emitters. [37, 38]
4. Conclusion
In conclusion, we have developed a new carbazole based cyclometalated iridium(III)
complex by incorporating hole transporting carbazole moieties into pyridine moiety which
exhibits green emission. Its molecular structure is confirmed by single crystal analysis, which
suggests that the Ir(III) center occupies a octahedral coordination center. Photoluminescence,
thermal stability and electrochemical characteristics of [Ir(cpy)2(prz)] are investigated and
[Ir(cpy)2(prz)] is found to be a promising good phosphorescent material for OLEDs with high
photoluminescence, good thermal stability and proper HOMO/LUMO energy levels. Further
investigation on material design (pure red-, green- and blue-light full-color applications) and
electroluminescent properties of the series of complexes used for the fabrication of OLEDs is
currently in progress.

9
Acknowledgement
The work was funded (project No. 40-46/2011 (SR)) by a grant from University Grant
Commission, New Delhi. X-ray data were collected at the center for Research Facilities in IIT-
Madras, Chennai-600036. We are also grateful to the National centre for Ultrafast process
Chennai for PL decay measurements.
Supplementary material
CCDC-891999 contains the supplementary crystallographic data for the complex
[Ir(cpy)2(prz)]. These data can be obtained free of charge from The Cambridge Crystallographic
Data Center via http://www.ccdc.cam.ac.uk/data_request/cif.
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IrCl3.nH2O
H2O
Cu, Na2CO3
Nitrobenzene
Refluxed
Cl
Cl
N
N
+
Ir
Ir
2
N
2-ethoxyethanol
N
N
N
N
N
H
Br
2
[(cpy)2Irμ-Cl]2
cpy
N
N
N
O
N
2-ethoxyethanol
Na2CO3
Cl
N
N
N
N
Ir
Ir
+
Ir
Cl
O
N
N
HO
O
2
2
2
[Ir(cpy)2(prz)]
Scheme. General synthetic route of [Ir(cpy)2(prz)].

13
Figure Captions:
Fig. 1. Ligands for the iridium complexes, a) 2-phenylpyridine ligand, b) phenylvinylpyridine
ligand, c) phenylpyrazole ligand, d) phenylquinoline ligand, e) 9-(pyridin-2-yl)-9H-carbazole
ligand.
Fig. 2. a) ORTEP drawing of [Ir(cpy)2(prz)] showing the atom numbering scheme: Selected
bond lengths (A˚ ) and angles (˚):C(2)-Ir = 1.987(5), C(18)-Ir = 2.006(5) N(2)-Ir = 2.054(4),
N(4)-Ir = 2.049, N(5)-Ir = 2.148(4), O(1)-Ir = 2.159(4), C(2)-Ir-C(18) = 87.2(2), C(2)-Ir-N(4) =
93.54(19), C(18)-Ir-N(4) = 88.73(19), C(18)-Ir-N(2) = 91.43(19), C(2)-Ir-N(2) = 90.12(19),
N(4)-Ir-N(2) = 176.34(17), C(2)-Ir-N(5) = 99.85(18), N(4)-Ir-N(5) = 91.79(16), N(2)-Ir-N(5) =
87.61(16), C(2)-Ir-O(1) = 176.16(17), C(18)-Ir-O(1) = 96.44(18), N(4)-Ir-O(1) = 85.35(16),
N(2)-Ir-O(1) = 91.01(16), N(5)-Ir-O(1) = 76.53(15).
b) Crystal packing diagram between two adjacent molecules showing an intermolecular π-π
stacking interaction. The phenyl…phenyl contact distance is 3.361A°.
Fig. 3. UV-vis and PL spectrum of [Ir(cpy)2(prz)] in CH2Cl2 solution at 298 K. The inset shows
original color (a) and emission color (b) variations observed in CH2Cl2 solution of [Ir(cpy)2(prz)]
(1x10-4 M).
Fig. 4. Cyclic voltammogram of the [Ir(cpy)2(prz)] in CH2Cl2 solution.

14
N
N
N
N
N
R
N
R
N
e
n
R
d
c
a
b
Figure 1

15

16
Figure 2
Figure 3

17
Figure 4

18
Table -1: Crystallographic data and structure refinement parameters for [Ir(cpy)2(prz)].
Complex
[Ir(cpy)2(prz)]. 2CH2Cl2 (1)
C41H29Cl4IrN6O2
971.70
Chemical formula
Formula weight (amu)
Crystal description
Crystal size (mm)
Crystal system
Space group
red
0.30 x 0.20 x 0.20
Monoclinic
P21/n
T (K)
293(2) K
Wavelength (Å)
a (A˚) & α (˚)
b (A˚) & β (˚)
c (A˚) & γ (˚)
0.71073
14.6070(6) & 90.00
18.2082(7) & 111.465(2)
15.0343(6) & 90.00
3721.3(3) & 4
Volume ( Å3) & Z
Dcalc (
3)
1.734
Mg/m
μ (mm-1)
3.922
2.00 to 25.00
θ Range(˚)
Tmax & Tmin
0.5753 & 0.3245
-17 ≤ h ≤ 17; -21 ≤ k ≤ 21; -17 ≤ l ≤ 17
61903 / 6548 [R(int) = 0.0362]
R1 = 0.0274, wR2 = 0.0717
R1 = 0.0370, wR2 = 0.0814
6548/40/487
Index ranges
Reflections collected/unique
Final R indices [I > 2 (I)]
R indices (all data)
Data/Restraints/parameters
Goodness-of-fit on F2
∆ρmax and ∆ρmin (e Å-3)
1.124
1.480 and -0.753

19
Table 2 The photophysical and electrochemical behavior of [Ir(cpy)2(prz)] complex.
Complex
Absorption
λem
Eaox
(V)
Eb
(V)
HOMOc/LUMOd Eeg
(eV) (eV)
2.33
red
λabs/nm (ε, 104M-1cm-1) (nm)
[Ir(cpy)2(prz)] 252 (30), 342 (6.8), 418, 646, 698 0.44, 1.04 -1.00 -5.25/-2.92
498(1.2)
aOxidation potential measured by cyclic voltammetry.
bReduction potential measured by cyclic voltammetry.
cCalculated using empirical equation: HOMO = (Eonset + 4.4)
dLUMO = HOMO + Eg
eEstimated from onset of absorption (Eg = 1240/ λonset
)

20
Highlights
¾
¾
A new iridium(III) complex [Ir(cpy)2(prz)] has been synthesized.
The [Ir(cpy)2(prz)] complex has been structurally characterized.
¾
The photophysical and electrochemical properties of the complex are described.
[Ir(cpy)2(prz)] is found to be a good green phosphorescent material for OLEDs.
¾

21
Green-emitting phosphorescent iridium(III) complex: Structural,
photophysical and electrochemical properties.
V. Thamilarasana, A. Jayamania, P. Manisankarb, Young-Inn Kimc, N. Sengottuvelana*

Green-emitting phosphorescent Iridium(III) complex: Structural,
photophysical and electrochemical properties.
V. Thamilarasana, A. Jayamania, P. Manisankarb, Young-Inn Kimc, N. Sengottuvelana*
A new iridium(III) complex [Ir(cpy)2(prz)], (cpy = 9-(pyridin-2-yl)-9H-carbazole; prz =
pyrazine-2-carboxylic acid) has been synthesized and characterized.
The molecular structure of [Ir(cpy)2(prz)] was confirmed by a single-crystal X-ray
diffraction.
¾
¾
¾
The photophysical, electrochemical properties and thermal stability of the complex
showed green luminescence and good electrochemical stability with high lying HOMO
energy levels.