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

DOI: http://dx.doi.org/10.1016/j.ica.2013.08.005

Reference: ICA 15607

To appear in: Inorganica Chimica Acta

Received Date: 30 October 2012

Revised Date: 13 June 2013

Accepted Date: 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*

*aDDE, bDepartment 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.

aPresent address: aDDE, 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

performed on a Carlorerba-1106 microanalyzer. 1H 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 = 1.47 ns & 2 = 5.16 ns in the nanosecond time scales,

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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NH

N

Br

+ N

N

N

NIr

ClCl

Ir

2

N

N2

Cu, Na2CO3Nitrobenzene

Refluxed2-ethoxyethanol

cpy [(cpy)2Irμ-Cl]2

IrCl3.nH2OH2O

N

NIr

Cl

ClIr

2

N

N

2

2-ethoxyethanol+ N

NIr

2

[Ir(cpy)2(prz)]

Na2CO3

NN

OHO

NN

OO

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

Figure 1

N N

R

n

NN

N

N

R

R

a b cd

N

e

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)

Chemical formula

Formula weight (amu)

Crystal description

Crystal size (mm)

Crystal system

Space group

T (K)

Wavelength (Å)

a (A˚) & α (˚)

b (A˚) & β (˚)

c (A˚) & γ (˚)

Volume ( Å3) & Z

Dcalc (Mg/m3)

μ (mm-1)

θ Range(˚)

Tmax & Tmin

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)

C41H29Cl4IrN6O2

971.70

red

0.30 x 0.20 x 0.20

Monoclinic

P21/n

293(2) K

0.71073

14.6070(6) & 90.00

18.2082(7) & 111.465(2)

15.0343(6) & 90.00

3721.3(3) & 4

1.734

3.922

2.00 to 25.00

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

1.124

1.480 and -0.753

19

Table 2 The photophysical and electrochemical behavior of [Ir(cpy)2(prz)] complex.

Complex Absorption λabs/nm (ε, 104M-1cm-1)

λem (nm)

Eaox

(V) Eb

red (V)

HOMOc/LUMOd

(eV) Ee

g (eV)

[Ir(cpy)2(prz)] 252 (30), 342 (6.8), 498(1.2)

418, 646, 698 0.44, 1.04 -1.00 -5.25/-2.92 2.33

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

A new iridium(III) complex [Ir(cpy)2(prz)] has been synthesized.

The [Ir(cpy)2(prz)] complex has been structurally characterized.

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.

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.

The photophysical, electrochemical properties and thermal stability of the complex

showed green luminescence and good electrochemical stability with high lying HOMO

energy levels.