Effect of substituents on the photoluminescent and electroluminescent properties of substituted cyclometalated iridium(III) complexes

Effect of substituents on the photoluminescent and electroluminescent

properties of substituted cyclometalated iridium(III) complexes

Hsiao-Wen Hong, Teng-Ming Chen

∗

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30055, Taiwan

Received 20 October 2005; received in revised form 10 February 2006; accepted 22 March 2006

Abstract

The photophysics of octahedral 4d

_{and 5d}

_{complexes has been studied extensively. The d}

_{iridium(III) complexes show intense}

phospho-rescence at room temperature. Our goal of this research is to study the synthesis, characterization and applications of the light emitting dopants

based on iridium(III) complexes. We have prepared and characterized a series of substituted (4-CF

, 4-Me, 4-OMe, 4-F, 3-F) 2-phenylbenzoxazole

ligand. The intermediate di-irrido and the six-coordinated mononuclear iridium(III) dopants of the above ligands have been synthesized and

characterized. These complexes are thermally stable between 280–320

C depending upon the substituents and sublimable. They emit bright

yellow to green light. The peak emission wavelengths of the dopants can be finely tuned depending upon the electronic properties of the

substituents as well as their positions in the ring. The emission spectral diagrams show that the emissive states of the complexes have major

contribution of MLCT state. In the absorption spectra, the

_{MLCT and}

_{MLCT transitions have been resolved in the range, 385–500 nm.}

The physical parameters of the electroluminescent devices for the substituted and the unsubstituted complexes has been compared and

dis-cussed.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Photoluminescence; Electroluminescence; OLED; Ir complexes

1. Introduction

A significant research effort has been focused on the

syn-thesis and photophysical characterization of octahedral 4d

and

5d

metal complexes. Luminescent d

metal complexes of Re(I),

Ru(II), Os(II) and Ir(III) have attracted considerable attention

due to their intriguing photophysical, photochemical and excited

state redox properties

[1]

and potential applications in photonic

and photoelectronic devices

[2]

. Strong spin–orbit coupling of

the 4d or 5d ion leads to efficient intersystem crossing of the

singlet excited states to the triplet manifold. More recently, a

number of groups have investigated the Rh

and Ir

metal

complexes and there also have been growing interests in

electro-luminescent devices (EL) with phosphor complex dopants of the

above metals as emitting layers

[3–6]

. The research group lead

by Mark Thompson et al. have synthesized and reported a series

of neutral emissive cyclometalated complexes of iridium(III)

[7]

∗_{Corresponding author. Fax: +886 35723764.}

E-mail address:[email protected](T.-M. Chen).

and platinum(II)

[8]

and used them in the fabrication of organic

light emitting diode (OLEDs) as a phosphorescent dopant

suc-cessively. Researchers from Dupont de Nemours and Co. have

also synthesized and characterized a series of iridium(III)

com-plexes with fluorinated 2-arylpyridines and showed that the

emissive colors of the materials can be finely tuned by systematic

control of the nature and position of the substituents of the

lig-ands

[9]

. These research activities inspired us to initiate a

system-atic study to investigate the color tuning of the iridium(III)

com-plexes by choosing a particular cyclometalated ligand with

sub-stituents exhibiting different electronic properties. Of the above

phosphor emitters that have been reported for (OLED) devices,

iridium(III)-based materials have displayed the most promising

due to their high stability, high photoluminescence (PL)

effi-ciency and relatively short excited state lifetime. So, we choose

2-phenylbenzoxazole (bo) as the cyclometalated ligand with

substituents (i.e., -CF

, -F, -Me, -OMe) showing different

elec-tronic properties to synthesize mononuclear emissive complex

with iridium(III). We present the syntheses and photophysical

studies of (x/ybo)

Ir(acac) (x = 4-CF

, 4-F, 4-Me, 4-OMe; y =

3-F; acac = acetylacetonate) and their applications in OLEDs.

0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.03.011

2. Experimental section

2.1. Materials

All of the preparative work involving iridium(III) trichloride hydrate (IrCl3·3H2O, Alfa Aesar Company Ltd.), 2-ethoxyethanol (H5C2OC2H4OH, Fluka) and all other reagents (Tokyo Chemical Industry, Japan) were carried out in inert atmosphere and used without further purification.

2.2. Optical measurements and compositions analysis

The ultraviolet–visible (UV–vis) spectra of the phosphorescent Ir(III) com-plexes were measured on an UV–vis spectrophotometer (Agilent model 8453) and corrected for background due to solvent absorption. Photoluminescence (PL) spectra were carried out with a spectrophotometer (Jobin-Yvon Spex, model Fluorolog-3). Solution photoluminescence quantum efficiency were measured by a relative method[22]using Ir(ppy)3[21]as standard in dichloromethane. NMR spectra were recorded on Varian 300 MHz and MS spectra (both EI and FAB) were taken on an Micromass TRIO-2000. Cyclic voltammetry (CV) anal-yses were performed by using CHI 2.05, the TG–DTA analysis was carried out by using a thermal analyzer (SEIKO 1TG/DTA 200).

2.2.1. Synthesis of x/y(bo) (x = 4-CF3, 4-F, 4-Me, 4-OMe and y = 3F; bo = 2-phenylbenzoxazole) and complexes with unsubstituted ligands

Synthesis of x/y(bo) (x = 4-CF3, 4-F, 4-Me, 4-OMe and y = 3-F; bo = (2-phenyl)benzoxazole): these were prepared by a general method[10]where 10 mmol o-aminophenol was dissolved into 20 ml of 1-methyl-pyrrolidinone under inert atmosphere; then the stoichiometric amount of the corresponding acid chloride was added slowly at room temperature. The mixture was heated at 100◦C for 1 h. After cooling, the solution was poured into cold water and the mixture was adjusted to pH 8–9 with 7N aqueous ammonia. A white colored solid compound was separated out. The crude material was filtered, washed with water for several times and then purified by column chromatography.

bo: 2-phenylbenzoxazole.1_{H NMR (300 MHz, CDCl} 3): 7.86 (m, 3H), 7.64 (d, 8.1, 1H), 7.25 (m, 4H), 7.13 (m, 1H), EIMS: m/z 195, Calcd. 195. (4-MeO)bo: (4-methoxy)2-phenylbenzoxazole. 1_{H NMR (300 MHz,} CDCl3): 8.22 (d, 2H, J 1.5 Hz), 7.74 (m, 1H), 7.56 (m, 1H), 7.31 (m, 2H), 3.90 (s, 3H), EIMS: m/z 225, Calcd. 225. (4-CF3)bo: (4-trifluoromethyl)2-phenylbenzoxazole.1H NMR (300 MHz, CDCl3):δ, ppm 8.38 (d, 2H, J 8.1 Hz), 8.11 (d, 3H, J 3.3 Hz), 7.61 (m, 1H), 7.44 (m, 2H), EIMS: m/z: 263, Calcd. 263. (4-Me)bo: (4-methyl)2-phenylbenzoxazole.1_{H NMR (300 MHz, CDCl} 3):δ, ppm 8.05 (d, 1H, J 8.4 Hz), 7.98 (d, 2H, J 8.4 Hz), 7.88 (dd, 1H, J 0.6, 8.1 Hz), 7.47 (t, 1H, J 7.5 Hz), 7.36 (t, 1H, J 7.2 Hz), 7.27 (d, 2H, J 7.8 Hz), EIMS: m/z: 209, Calcd. 209. (4-F)bo: (4-fluoro)2-phenylbenzoxazole.1H NMR (300 MHz, CDCl3):δ, ppm 8.35 (s, 1H), 7.93 (m, 1H), 7.90 (m, 2H), 7.14 (m, 2H), 7.05 (d, 1H, J 8.1 Hz), 6.91 (t, 1H, J 6.6, 6.9 Hz), EIMS: m/z: 213, Calcd. 213. (3-F)bo: (3-fluoro)2-phenylbenzoxazole.1_{H NMR (300 MHz, CDCl} 3):δ, ppm 8.21 (s, 1H), 7.66 (m, 2H), 7.52 (m, 2H), 7.17 (d, 1H, J 8.7), 7.07 (d, 1H, J 8.1 Hz), 6.95 (t, 1H, J 6.3, 6.9 Hz), EIMS: m/z: 213, Calcd. 213. 2.2.2. Synthesis of dinuclear iridium(III) complexes,

x/y(bo)2Ir(μ-Cl)2x/y(bo)2

Synthesis of the dichoro-bridged iridium(III) complexes, x/ybo2Ir( ␮-Cl)2x/ybo2: these were prepared by refluxing the mixture of IrCl3 (1 mmol) and the ligands (x/ybo) (2.4 mmol) in 2-ethoxyethanol for 24–25 h. The orange-yellow mixture was cooled to room temperature and 20 ml 1 M HCl was added to precipitate the product. The mixture was filtered and washed with 100 ml 1 M HCl followed by 50 ml methanol solution for several times then dried.

[Ir(bo)2Cl2]2:1H NMR (300 MHz, CDCl3):δ, ppm 8.23 (d, 4H, J 7.8 Hz), 7.56 (d, 4H, J 7.5 Hz), 7.16 (d, 4H, J 7.2 Hz), 6.96 (d, 4H, J 7.5 Hz), 6.72 (d, 4H, J 7.2 Hz), 6.05 (s, 4H), 1.98 (s, 12H), FABMS: m/z 1232, Calcd. 1232. [Ir(4-MeObo)2Cl2]2:1H NMR (300 MHz, CDCl3):δ, ppm 8.14 (d, 4H, J 7.8 Hz), 7.55 (d, 4H, J 8.4, 2.4 Hz), 7.25 (d, 4H, J 9.0 Hz), 7.00 (t, 4H, J 7.5, 8.4 Hz), 6.86 (t, 4H, J 8.1, 7.2 Hz), 6.44 (d, 4H, J 8.7 Hz), 5.71 (s, 4H), 3.25 (s, 12H), FABMS: m/z 1351, Calcd. 1351. [Ir(4-CF3bo)2Cl2]2:1H NMR (300 MHz, CDCl3):δ, ppm 8.12 (d, 4H, J 7.2 Hz), 7.76 (d, 4H, J 8.1 Hz), 7.41 (d, 4H, J 8.4 Hz), 7.29 (d, 4H, J 0.06 Hz), 7.16 (d, 4H, J 7.8 Hz), 7.01 (t, 4H, J 8.4, 8.1 Hz), 6.38 (s, 4H), FABMS: m/z 1504, Calcd. 1504. [Ir(4-Mebo)2Cl2]2:1H NMR (300 MHz, CDCl3):δ, ppm 8.23 (d, 4H, J 7.8 Hz), 7.56 (d, 4H, J 7.5 Hz), 7.16 (d, 4H, J 7.2 Hz), 6.96 (d, 4H, J 7.5 Hz), 6.72 (d, 4H, J 7.2 Hz), 6.05 (s, 4H), 1.98 (s, 12H), FABMS: m/z 1287, Calcd. 1287. [Ir(4-Fbo)2Cl2]2:1H NMR (300 MHz, CDCl3):δ, ppm 8.16 (d, 4H, J 8.1 Hz), 7.69 (dd, 4H, J 5.7, 7.8 Hz), 7.35 (d, 4H, J 8.1 Hz), 7.17 (t, 4H, J 8.4, 7.8 Hz), 6.96 (t, 4H, J 8.1, 7.5 Hz), 6.67 (t, 4H, J 8.7, 8.6 Hz), 5.82 (dd, 4H, J 9.9, 1.2 Hz), FABMS: m/z 1303, Calcd. 1303. [Ir(3-Fbo)2Cl2]2:1H NMR (300 MHz, CDCl3):δ, ppm 8.33 (d, 4H, J 8.1 Hz), 7.63 (d, 4H, J 6.6 Hz), 7.20 (m, 8H), 7.02 (m, 8H), 6.37 (s, 4H), FABMS: m/z 1303, Calcd. 1303.

2.2.3. Syntheses of mononuclear iridium(III) complex dopants (x/ybo)2Ir(acac)

The chloride bridged dinuclear iridium(III) complex (x/ybo)2Ir( ␮-Cl)2Ir(x/ybo)2 (0.1 mmol), acetylacetone (0.3 mmol) and sodium carbonate (1 mmol) were mixed in 10 ml of 2-ethoxyethanol (30 ml). The mixture was refluxed under nitrogen for 11–12 h. The reaction mixture was then cooled and the resulted precipitate was collected through filtration. The product was puri-fied by recrystallization from a solution of the mixture of dichloromethane and methanol (2:1). Ir(bo)2(acac):1H NMR (300 MHz, CDCl3):δ, ppm 8.25 (d, 2H), 7.93 (m, 2H), 7.74 (dd, 2H, J 8.1, 1.9 Hz), 7.56 (m, 4H), 6.86 (td, 2H, J 7.8, 1.1 Hz), 6.59 (td, 2H, J 7.5, 1.0 Hz), 6.2 (d, 2H, J 7.5 Hz), 5.12 (s, 1H), 1.71 (s, 6H), FABMS: m/z 680, Calcd. 680. Ir(4-MeObo)2(acac):1H NMR (300 MHz, CDCl3):δ, ppm 7.59 (d, 4H, J 8.4 Hz), 7.47 (m, 2H), 7.34 (m, 4H), 6.45 (dd, 2H, J 8.4, 2.4 Hz), 5.98 (s, 2H), 5.22 (s, 2H), 3.47 (s, 6H), 1.82 (s, 6H), FABMS: m/z 741, Calcd. 741.

Ir(4-CF3bo)2(acac):1H NMR (300 MHz, CDCl3):δ, ppm 7.74 (t, 4H, J 8.1, 7.8 Hz), 7.51 (t, 4H, J 6.9, 7.2 Hz), 7.43 (d, 2H, J 8.1 Hz), 7.13 (d, 2H, J 8.7 Hz), 6.64 (s, 2H), 5.24 (s, 1H), 1.83 (s, 6H), FABMS: m/z 816, Calcd. 816. Ir(4-Mebo)2(acac):1H NMR (300 MHz, CDCl3):δ, ppm 7.64 (d, 2H, J 6.9 Hz), 7.51 (m, 4H), 7.36 (m, 4H), 6.67 (d, 2H, J 7.2 Hz), 6.29 (s, 2H), 1.95 (s, 6H), 1.81 (s, 6H), FABMS: m/z 708, Calcd. 708. Ir(4-Fbo)2(acac):1H NMR (300 MHz, CDCl3):δ, ppm 7.65 (t, 4H, J 8.7, 5.7 Hz), 7.43 (m, 6H), 6.60 (m, 2H), 6.09 (d, 2H, J 9.9 Hz), 5.26 (s, 1H), 1.83 (s, 6H), FABMS: m/z 716, Calcd. 716. Ir(3-Fbo)2(acac):1H NMR (300 MHz, CDCl3):δ, ppm 7.65 (d, 2H, J 7.8 Hz), 7.54 (m, 4H), 7.41 (m, 4H), 6.91 (m, 2H), 6.39 (t, 2H, J 8.4, 8.4 Hz), 5.30 (s, 2H), 1.86 (s, 6H), FABMS: m/z 716, Calcd. 716.

2.3. Crystallography

Diffraction data for (CF3bo)2Ir(acac) single crystals were collected on a Bruker CCD diffractometer with Mo K␣ (λ = 0.71073 ˚A). Data collection in the 2θ scan mode, cell refinement and data reduction were carried out using program Bruker SHELXTL. The structure was solved by direct methods using the SHELXS-97 package of computer programs. The structure was refined by full-matrix least-squares methods based on F2 _{using SHELXL-97. The} non-hydrogen atom positions were refined anisotropically whereas the non-hydrogen positions were not refined.

2.4. OLED fabrication and testing

Organic layers were fabricated by high-vacuum thermal evaporation onto a glass substrate precoated with an indium–tin–oxide (ITO) layer with a sheet resistance of 20. Prior to use, the ITO surface was ultrasonicated in a deter-gent solution followed by rinsing with deionized (DI) water, dipped into ace-tone, trichloroethylene, and 2-propanol, and then degreased with a vapor of 2-propanol. After degreasing, the substrate was oxidized and cleaned in a UV-ozone chamber before it was loaded into an evaporator. In a vacuum chamber at pressure of 10−6Torr, 500 ˚A of NPB as the hole transporting layer; 200 ˚A the complex doped (7%) CBP as the emitting layer; 100 ˚A of

2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP) as a hole and exciton blocking layer (HBL); 650 ˚A of Alq3as electron transport layer; and a cathode composed of 10 ˚A lithium fluoride and 2000 ˚A aluminum were sequentially deposited onto the substrate to give the device structure. The current–voltage (I–V) profiles and light intensity characteristics for the above-fabricated devices were measured in a vacuum chamber of 10−6Torr at ambient temperature using a Keithley 2400 Source Meter/2000 Multimeter coupled to a PR 650 Optical Meter.

3. Results and discussion

3.1. Synthesis and characterization of (x/ybo)

Ir(acac)

complexes

Iridium complexes were prepared according to the

proce-dure reported previously

[11]

. The synthesis of the ligands

and iridium complexes is depicted in Eq.

(1)

. Complexes of

(x/ybo)

Ir(acac) have been prepared

[7]

with all the substituents

in the cyclometalated ligand bo. The synthetic method used to

prepare these complexes involves two steps. In the first step,

IrCl

·3H

O is allowed to react with an excess of the

cyclomet-alated ligand (2.5 times) to give a chloro-bridged dinuclear

complex, i.e. (x/ybo)

Ir(␮-Cl)

Ir(x/ybo)

.

2IrCl

·3H

O

+ 4 (x/ybo)

→ (x/ybo)

Ir(

␮-Cl)

Ir(x

/ybo)

+ 4HCl + 3H

O(g)

(1)

(x

/ybo)

Ir(␮-Cl)

Ir(x

/ybo)

+ 2L

XH

→ 2(x/ybo)

Ir(L

X)

+ 2HCl

(2)

The chloro-bridged dinuclear complexes can be readily

con-verted to emissive, mononuclear complexes (x/ybo)

Ir(acac) by

replacing the two bridging chlorides with bidentate

acetylacet-onate. Thus, the iridium(III) ion is octahedrally coordinated by

the three chelating ligands. The coordination geometry of the

“(x/ybo)

Ir” fragment in the mononuclear complex is the same

as that for the dinuclear complexes. All the mononuclear

com-plex dopants with different substitutions are thermally stable

up to 280–320

C. The mononuclear Ir(III) complexes can be

sublimed easily at reduced pressure.

As indicated by the reported NMR data, the resonance

spec-tra of ligands show poorly resolved multiplets, whereas the

well-resolved multiplets are observed for the complexes. The

maximum high-field chemical shift is observed for the proton

to the ortho-metalated carbon atom that experiences the largest

shielding of any of the ligand protons. Again the chemical shift of

this particular proton varies with the electronic properties of the

substituents present in the ligand. The complex containing-CF

substituted ligand shows the lowest field chemical shift for the

proton to the ortho-metalated carbon atom due to the less

shield-ing power arisshield-ing from the strong electron withdrawshield-ing property

of the substituent, whereas the strongest electron-releasing

sub-stituent (i.e., –OCH

group) shows the highest field chemical

shift for the same proton. The (x/ybo)

Ir(acac) complexes are

stable in air and can be sublimed in a vacuum without

decom-position during device fabrication.

Fig. 1. Comparison of the UV–vis absorption spectra of five cyclometalled irid-ium complexes in CH2Cl2.

3.2. UV–vis absorption spectra

Fig. 1

shows the UV–visible absorption spectra for six

dif-ferent ligand substituted (x/ybo)

Ir(acac) complexes in CH

Cl

solution at room temperature. The strong absorption bands

observed at 250–300 nm are assigned to ligand centered

␲–␲

transitions, whilst the broad absorption bands at lower energy

(

λ

> 350 nm) are typical metal to ligand charge-transfer

(MLCT) transitions

[12,13]

. In other cyclometalated complexes

these bands have been assigned to singlet and triplet MLCT

tran-sitions, and the same assignment is likely here

[14]

. The long

tail toward lower energy is assigned to

MLCT transitions that

gains intensity by mixing with the higher lying

MLCT

transi-tion through the spin–orbit coupling of iridium(III). This mixing

is strong enough in these complexes that the formally spin

for-bidden

MLCT has an extinction coefficient that is almost equal

to the spin-allowed

MLCT transition (

Table 1

). The presence of

another transition observed at around 385 nm for the complexes

is also well pronounced, which corresponds to an admixture of

MLCT and ligand

␲–␲

states.

3.3. Photoluminescence (PL) spectra

Fig. 2

shows the PL emission spectra for (x/y/bo)

Ir(acac)

complexes in CH

Cl

solution at room temperature. All of these

complexes show strong luminescence from the triplet states in

dichloromethane solution. The emission spectra shows that all

of these complexes emit in the range of 500–600 nm, which

corresponds to green-yellow light. It is known that the

emis-sion bands from MLCT states are generally broad and

fea-tureless, while

(␲–␲

) states typically give highly structured

emission. It is observed that on changing the substituents in

(x/y/bo)

Ir(acac) typically have effect on the maximum emission

wavelength in the spectrum. Amongst all of the substituents,

tri-fluoromethyl is the strongest electron-withdrawing group

show-ing the emission wavelength red shift than the

correspond-ing unsubstituted complex, (bo)

Ir(acac), and the other groups

Table 1

Spectroscopic, redox and photophysical properties for all Ir-complexes synthesized in this research Complex (x-subs) Absorbance,

λ (nm)

Emission,λmax (nm) in CH2Cl2

Oxidation potential versus ferrocene (volt)

HOMO (eV)a LUMO (eV) Energy gap (eV)b Absolute PL efficiencyc Unsubstituted 386, 400 529, 562 1.036 5.317 2.837 2.48 0.4 4CF3 361, 437 543, 578 1.263 5.544 3.297 2.25 0.6 4F 370, 408 511, 543 1.209 5.490 2.920 2.57 1.6 4Me 377, 430 522, 558 0.995 5.276 2.726 2.55 0.8 4OMe 362, 410 510, 543 1.013 5.294 2.754 2.54 0.1 3F 374, 418 520, 552 1.208 5.489 2.989 2.50 0.4

a_{Electrochemical properties were obtained by cyclic voltammetry using CHI 604A.} b _{The energy gap can be calculated from the edge of UV–vis absorption peak.} c_{The standard material is Ir(ppy)}

3in CH2Cl2(φ = 0.4)[21].

(F/OMe/Me) has electron-donating effects, so exhibiting the

emission wavelength blue shift than the corresponding

unsubsti-tuted complex, (bo)

Ir(acac). When the position of the

substitu-tion (4F

→ 3F) is changed, the maximum emission wavelength

(

λ

) of the complex shows an apparent red shifted emission

[15]

.

The HOMO/LUMO energies have also been calculated for

all the complexes and reported in

Table 1

based on the

exper-imental redox potentials and the absorption wavelengths. It is

observed that the energy gap (HOMO–LUMO) for the

trifluo-romethyl substituted complex is the least, whereas 4-OMe and

3-F substituted complexes show the maximum energy gap and

the rest exhibit intermediate values among all the investigated

and these energy gap data can well be correlated with the

cor-responding maximum emission wavelengths of the respective

complexes.

3.4. Redox chemistry

Analytical results from cyclic voltammetry indicate that

all of the (x/ybo)

Ir(acac) complexes undergo a reversible

one-electron oxidation; however, no reduction processes

were observed in dichloromethane. The increasing order

of oxidation potential of the complexes is as followings:

Fig. 2. Comparison of solution PL spectra for six triplet Ir(III)-based complexes in CH2Cl2.

–CF

> –F > –Me > –OCH

and the results are summarized in

Table 1

, which show the order of the strength of

electron-withdrawing nature of the substituents in the complexes and,

conversely, it is the increasing order of oxidation potentials.

3.5. Structure of (CF

bo)

Ir(acac) complex

The ORTEP drawing of the (CF

bo)

Ir(acac) complex is

shown in

Fig. 3

. Relevant crystallographic data are given in

Table 2

, and selected bond lengths ( ˚

A) are presented in

Table 3

.

These complexes have an octahedral coordination geometry

around Ir and has cis-C,C trans-N,N chelate disposition. The

Ir–C bonds of these complexes (Ir–C

= 2.007 ˚

A) are shorter

than the Ir–N bonds (Ir–N

= 2.039 ˚

A). But the Ir–C bond length

is similar to the analogues complexes reported

[16]

. The Ir–N

bond lengths also fall within the range of values for the similar

type of reported complexes. The Ir–O bond lengths of 2.110(10)

and 2.124(9) ˚

A are longer than the mean Ir–O value of 2.088 ˚

A

reported in the Cambridge Crystallographic Database

[17]

and

reflect the large trans influence of the phenyl groups. All other

Fig. 3. ORTEP drawing of (CF3-bo)2Ir(acac): the thermal ellipsoids represent 50% probability limit.

Table 2

Crystal data and structure refinement for (CF3bo)2Ir(acac)

Identification code ot40 m

Empirical formula C33H27F6IrN2O7

Formula weight 869.77

Temperature 294(2) K

Wavelength 0.71073 ˚A

Crystal system Monoclinic

Space group P2(1)/c

Unit cell dimensions a = 12.211(2) ˚A (90◦)

b = 20.967(4) ˚A (112.872(4)◦) c = 14.774(3) ˚A (90◦) Volume 3485.1(11) ˚A3 Z 4 Density (calculated) 1.658 Mg/m3 Absorption coefficient 3.911 mm−1 F(0 0 0) 1704 Crystal size 0.20 mm× 0.08 mm × 0.05 mm

Theta range for data collection

1.78–24.73◦

Index ranges −14 ≤ h ≤ 12, −24 ≤ k ≤ 24, −17 ≤ l ≤ 17 Reflections collected 19584

Independent reflections 5958 (R(int) = 0.1328) Completeness toθ = 24.73◦ 99.8%

Absorption correction Empirical Maximum and minimum

transmission

0.82563 and 0.45062

Refinement method Full-matrix least-squares on F2 Data/restraints/parameters 5958/0/442

Goodness-of-fit on F2 _0.982

Final R indices [I > 2σ(I)] R1 = 0.0587, wR2 = 0.1604 R indices (all data) R1 = 0.1178, wR2 = 0.1768

Largest diffraction peak and hole

1.604 and−1.132 e ˚A-3

Table 3

Selected bond lengths ( ˚A) for (CF3-bo)2Ir(acac)

Atom(1)–atom(2) Distance ( ˚A) Ir(1)–C(23) 2.003(15) Ir(1)–C(9) 2.012(14) Ir(1)–N(2) 2.029(10) Ir(1)–N(1) 2.049(10) Ir(1)–O(3) 2.110(10) Ir(1)–O(4) 2.124(9)

Fig. 4. EL device structures with (a) CF3-, (b) Me-substituted (x/y-bo)2Ir(acac), and (c) parent (bo)2Ir(acac) complexes as a dopant.

bond lengths within the chelate ligands are analogues to the

similar type of complexes reported

[18–20]

.

3.6. Description of OLED devices fabricated with

(CF

bo)

Ir(acac), (CH

bo)

Ir(acac) and (bo)

Ir(acac)

dopants in the emissive layer

We have also fabricated three electroluminescent (EL)

devices

using

three

different

iridium(III)

complexes

[(CF

bo)

Ir(acac), (CH

bo)

Ir(acac) and (bo)

Ir(acac)] as

dopants in the emitting layers. In order to compare the relative

EL properties, we have selected two ligand substituted complex

dopants [–CF

(device a) and –Me (device b)] with opposite

electronic properties and the other is the unsubstituted bo

complex (device c). These complexes were doped into the

emissive layer of the OLED devices at a concentration of

mole 7%. The device structure and their thickness of the

layers (i.e., ITO/NPB (500 ˚

A)/CBP + 7% dopant (200 ˚

A)/BCP

(100 ˚

A)/Alq

(650 ˚

A)/LiF (10 ˚

A)/Al (2000 ˚

A)) have been

kept constant (

Fig. 4

). The comparative variation of quantum

efficiency as a function of current density for these devices is

shown in the

Fig. 4

. Furthermore,

Table 4

also summarizes and

compares the EL performances of devices a, b and c. It has been

observed that the quantum efficiency of the devices a, b and

c was found to increase with increasing current density and it

Table 4

Comparative study of the electroluminescent properties of three EL devices based on (bo)2Ir(acac), (bo-Me)2Ir(acac) an(bo-CF3)2Ir(acac) as a dopant

(bo)2Ir(acac) (Me-bo)2Ir(acac) (CF3-bo)2Ir(acac)

Turn-on voltage (V) at 0.5 mA/cm2 4.62 4.64 5.33

EL color Green Green Yellow

Peak wavelength (nm) 532 528 544

Luminance (cd/m2_{) at 100 mA/cm}2 _{7546 8100 9200}

Yield (cd/A) at 20 mA/cm2 _{8.94 10.5 9.95}

C.I.E. coordinate (x, y) 0.38, 0.56 0.37, 0.58 0.44, 0.54

Power efficiency (lm/W) 4.03 (at 4.62 V) 6.20 (at 4.64 V) 4.53 (at 5.33 V)

shows a maximum at 9.95, 10.5 and 8.94 cd/A at 20 mA cm

,

respectively. The turn-on voltage was found to be 5.33, 4.64

and 4.62 for the devices a, b and c, respectively. The luminance

efficiency was found to decrease with increasing current density

for all the devices as indicated in

Fig. 5

which may be attributed

due to the triplet–triplet annihilation. The power efficiencies

were found to be 4.53, 6.20 and 4.03 lm/W for the devices,

a, b and c, respectively. At a current density 100 cd m

, the

brightness of the EL device a is 9189 cd m

, whereas that for

the device b and c are 8018 and 7546 cd m

, respectively. The

C.I.E. was measured to be (0.44, 0.54), (0.37, 0.58) and (0.38,

0.56) for the devices, a, b and c, respectively.

The lower efficiency of the (bo)

Ir(acac) based OLED relative

to that of the (CF

bo)

Ir(acac)-based device may be attributed

to a lower phosphorescence efficiency of the former as

com-pared to the latter. On comparison of these devices with the

corresponding devices a, b and c, power efficiency is always

higher throughout the current densities and voltages applied, as

indicated in

Table 4

, whereas the luminance and the quantum

yield (described in

Fig. 5

) are also found to increase at lower

current densities. Furthermore, no emission from CBP or Alq

was observed, indicating a complete energy transfer from the

host exciton to the Ir-dopant. Meanwhile, there is no exciton

decay in the Alq

layer due to the hole blocking action of the

BCP layer. As shown in the insets of

Fig. 5

(a)–(c), the device

a having trifluoromethyl-substituted dopant shows the expected

red-shift and stronger intensity in the EL spectra as compared

to that observed in the device c with unsubstituted dopant.

Sim-ilarly, the device b having methyl-substituted dopant shows the

expected blue-shift and stronger intensity in the EL spectra as

compared to that observed in the device c with unsubstituted

dopant. The EL spectra of a, b and c devices are similar to the

PL spectra of same phosphors in a dilute solution. Thus, the

EL emission is confirmed to originate from the triplet excited

states of the phosphors. It is expected that the devices with the

rest of the dopants synthesized and investigated in our work will

exhibit more or less similar EL properties with those of devices a,

b and c.

4. Conclusions

Four new cyclometallated iridium complex dopants using

various types of substituted (2-phenyl)benzoxazole ligands have

been synthesized and shown to exhibit high phosphorescence

efficiency which made them ideal for OLED applications.

These complexes show different quantum efficiencies in

solu-tion depending upon the nature of the substituents. All the

complexes show one-electron oxidation in solution. The

wave-length can be tuned by ca. 25 nm depending upon the electronic

properties of the substituents present in the ligand. We have

fabricated and investigated three EL devices and observed that

the device dopants with substituted ligands show the higher

luminance yield compared to the dopant with unsubstituted

ligands.

Acknowledgements

This research is supported by Program for Promoting

Univer-sity Academic Excellence from Ministry of Education, Taiwan,

Republic of China under the Contract 91-E-FA04-2-4-(B).

Pro-fessor Chin Hsin Chen and OLED Laboratory of National Chiao

Tung University are gratefully acknowledged for assistance with

the fabrication of OLED devices and providing helpful

com-ments on this work.

References

[1] (a) V. Balzani, A. Credi, F. Scandola, in: L. Fabbrizzi, A. Poggi (Eds.), Transition Metals in Supramolecular Chemistry, Kluwer, Dordrecht, The Netherlands, 1994, p. 1;

(b) J.-M. Lehn, Supramolecular Chemistry—Concepts and Properties, VCH, Weinheim, Germany, 1995;

(c) C.A. Bignozzi, J.R. Schoonover, F. Scandola, Prog. Inorg. Chem. 44 (1997) 1.

[2] (a) L. Tan-Sien-Hee, A.K.-D. Mesmaeker, J. Chem. Soc. Dalton Trans. 24 (1994) 3651;

(b) K.-F. Chin, K.-K. Cheung, H.-K. Yip, T.C.W. Mak, C.M. Che, J. Chem. Soc. Dalton Trans. 4 (1995) 657;

(c) K. Kalyanasundaram, M. Gratzel, Coord. Chem. Rev. 177 (1998) 347. [3] (a) J.K. Lee, D. Yoo, M.F. Rubner, Chem. Mater. 9 (1997) 1710;

(b) F.G. Gao, A. Bard, J. Am. Chem. Soc. 122 (2000) 7426.

[4] (a) Y. Li, Y. Liu, J. Guo, F. Wu, W. Tian, B. Li, Y. Wang, Synth. Met. 118 (2001) 175;

(b) K. Wang, L. Huang, L. Gao, L. Jin, C. Huang, Inorg. Chem. 41 (2002) 3353.

[5] F.G. Gao, A. Bard, J. Chem. Mater. 14 (2002) 3465.

[6] (a) C.-L. Lee, K.B. Lee, J.-J. Kim, Appl. Phys. Lett. 77 (2000) 2280; (b) R.R. Das, C.-L. Lee, J.-J. Kim, Mater. Res. Soc. Symp. Proc. 708 (2002), BB3.39.1;

(c) J.P.J. Markham, S.-C. Lo, S.W. Magennis, P.L. Burn, I.D.W. Samuel, Appl. Phys. Lett. 80 (2002) 2645.

[7] (a) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-F. Lee, C. Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304;

(b) M.E. Thompson, S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong, S.R. Forrest, M.A. Baldo, P.E. Burrows, US Patent 20020034656A1.

[8] J. Brooks, Y. Babayan, S. Lamansky, P.I. Djurovich, I. Tsyba, R. Bau, M.E. Thompson, Inorg. Chem. 41 (2002) 3055.

[9] V.V. Grushin, N. Herron, D.D. LeCloux, W.J. Marshall, V.A. Petrov, Y. Wang, Chem. Commun. (2001) 1494.

[10] A. Brembilla, D. Roizard, P. Lochon, Syn. Commun. 20 (1990) 3379. [11] Y. Zhang, C.D. Baer, C.C. Neto, P. O’Brien, D.A. Sweigart, Inorg. Chem.

30 (1991) 1685.

[12] M. Maestri, D. Sandrini, V. Blazani, U. Maeder, A. vonZelewsky, Inorg. Chem. 26 (1987) 1323.

[13] M.G. Colombo, A. Hauser, H.U. G¨udel, Inorg. Chem. 32 (1993) 3088.

[14] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, M.E. Thompson, Inorg. Chem. 40 (2001) 1704.

[15] I.R. Laskar, T.-M. Chen, Chem. Mater. 16 (2004) 111.

[16] J. Vicente, A. Arcas, D. Bautista, M.C.R. de Arellano, J. Organomet. Chem. 663 (2002) 164.

[17] F.H. Allen, J.E. Davies, J.J. Galloy, O. Johnson, O. Kennard, C.F. Macrae, E.M. Mitchell, G.F. Mitchell, J.M. Smith, D.G. Watson, J. Chem. Inf. Com-put. Sci. 31 (1991) 187.

[18] F.O. Garces, K. Dedian, N.L. Keder, R. Watts, J. Acta Crystallogr. C 49 (1993) 1117.

[19] R. Urban, R. Kr¨amer, S. Mihan, K. Polborn, B. Wagner, W. Beck, J. Organomet. Chem. (2000) 1039.

[20] F. Neve, A. Crispini, Eur. J. Inorg. Chem. (2000) 1039.

[21] K.A. King, P.J. Spellane, R.J. Watts, J. Am. Chem. Soc. (1985) 1431. [22] J.N. Demas, G.A. Crosby, J. Phys. Chem. (1971) 991.