Sky-blue phosphorescent iridium(III) complexes with two substituted 2-phenylpyridine derivatives and one picolinic acid for organic light-emitting diodes

Article abstract of DOI:10.1016/j.jorganchem.2012.11.016

Two sky-blue phosphorescent heteroleptic cyclometalated iridium(III) complexes, bis(2-(2′,4′-difluoro-3′-trifluoromethylphenyl)-4- methylpyridinato-N,C^2′)iridium(picolinate) [(dfpmpyCF ₃)₂Ir(pic)] (1) and bis(2-(2′,4′-difl

Full text of DOI:10.1016/j.jorganchem.2012.11.016

Accepted Manuscript
Sky-blue phosphorescent iridium(III) complexes with two substituted 2-phenylpyridine
derivatives and one picolinic acid for organic light-emitting diodes
Seong-Jae Yun, Hoe-Joo Seo, Myungkwan Song, Sung-Ho Jin, Sung Kwon Kang,
Young-Inn Kim
PII:
S0022-328X(12)00665-1
10.1016/j.jorganchem.2012.11.016
JOM 17781
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 12 May 2012
Revised Date: 8 November 2012
Accepted Date: 13 November 2012
Please cite this article as: S.-J. Yun, H.-J. Seo, M. Song, S.-H. Jin, S.K. Kang, Y.-I. Kim, Sky-blue
phosphorescent iridium(III) complexes with two substituted 2-phenylpyridine derivatives and one
picolinic acid for organic light-emitting diodes, Journal of Organometallic Chemistry (2012), doi: 10.1016/
j.jorganchem.2012.11.016.
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ACCEPTED MANUSCRIPT
Sky-blue phosphorescent iridium(III) complexes with two substituted 2-phenylpyridine
derivatives and one picolinic acid for organic light-emitting diodes
Seong-Jae Yun, Hoe-Joo Seo, Myungkwan Song, Sung-Ho Jin, Sung Kwon Kang, and Young-Inn Kim*
Graphical abstract
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ACCEPTED MANUSCRIPT
Seong-Jae Yun, Hoe-Joo Seo, Myungkwan Song, Sung-Ho Jin, Sung Kwon Kang,
Young-Inn Kim
Sky-blue phosphorescent iridium(III) complexes with two substituted 2-phenylpyridine derivatives and one
picolinic acid for organic light-emitting diodes
Two sky-blue phosphorescent iridium(III) complexes, [(dfpmpyCF3)2Ir(pic)] (1) and
[(dfpmpyhCz)2Ir(pic)] (2), were synthesized for, and their electrochemical and photopysical properties
were characterized along with a single crystal structure of complex 1 for the fabrication of OLEDs.
The –CF3 substituted on the phenylpyridine-based ligand caused a hypsochromic shift in complex 1,
and there is an energy transfer from the carbazolyl moieties to the iridium(III) core resulting in high
external quantum efficiency in complex 2.

ACCEPTED MANUSCRIPT
Sky-blue phosphorescent iridium(III) complexes with
two substituted 2-phenylpyridine derivatives and
one picolinic acid for organic light-emitting diodes
Seong-Jae Yun,a Hoe-Joo Seo,b Myungkwan Song,a Sung-Ho Jin,a Sung Kwon Kang,c
and Young-Inn Kima*
a
Department of Chemistry Education and Interdisciplinary Program of Advancde Materials
Information and Display, Pusan National University, Jangjeon-dong Kumjeong-gu, Busan
609-735, Republic of Korea.
b Department of Chemistry, Pusan National University, Jangjeon-dong Kumjeong-gu, Busan
609-735, Republic of Korea.
C Department of Chemistry, Chungnam National University, Daejeon 305-764, Korea
* Corresponding author. Tel: +82 51 510 2695; fax: +82 51 581 2348; E-mail address:
yikim@pusan.ac.kr
1

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Abstract
Two sky-blue phosphorescent heteroleptic cyclometalated iridium(III) complexes, bis(2-
(2',4'-difluoro-3'-trifluoromethylphenyl)-4-methylpyridinato-N,C2’)iridium(picolinate)
[(dfpmpyCF3)2Ir(pic)]
(1)
and
bis(2-(2',4'-difluoro-3'-hexylcarbazolephenyl)-4-
methylpyridinato-N,C2’)iridium(picolinate) [(dfpmpyhCz)2Ir(pic)] (2), were synthesized and
characterized. Energy transfer from the carbazol moiety to the iridium core in compound 2
resulted in higher quantum efficiency than with compound 1. The electronegative effect of
the -CF3 group on the phenyl ring in the phenylpyridine main ligand resulted in a
hypsochromic shift of the emitting spectra in compound 1. In contrast, the sky-blue emission
without a red shift in compound 2 indicated a negligible inductive effect of the long chain
hexylcarbazole unit on the electronic state of the phenyl ring. The single crystal X-ray
structural analysis showed that compound 1 adopts a distorted octahedral geometry around
the iridium metal exhibiting cis-C,C’ and trans-N,N’ chelate disposition. An
electroluminescent devices using the iridium complexes as a dopant in the emitting layer has
been fabricated. The best device performance based on compound 1 demonstrated a
maximum luminance (L), external quantum efficiency (EQE), luminance efficiency (LE) and
power efficiency (PE) of 5750 cd/m2, 7.53 %, 16.86 cd/A and 5.88 lm/W, respectively with
Commission Internationale de L’Eclairage (CIE) coordinates of (0.17,0.40).
Keywords: Phosphorescent iridium(III) complex; Sky-blue phosphorescent emitting materials,
Organic light-emitting diodes (OLEDs)
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1. Introduction
Electroluminescent heavy-metal materials, such as iridium, platinum, ruthenium, and
osmium complexes, have attracted considerable attention due to their potential applications in
full-color flat panel displays and solid-state lighting [1-6]. Among phosphorescent emitters,
the best performing phosphorescent materials were those based on iridium(III) complexes
because of the relatively short triple lifetime, potential high device efficiency and emission
wavelengths from blue to deep red. Iridium(III)-based phosphorescent cyclometalated
complexes are used as efficient dopants for applications in the area of phosphorescent organic
light-emitting diodes (PhOLEDs) [7-12], light-emitting electrochemical cells (LECs) [13-16]
as well as chemosensors [17-18]. The properties of iridium(III) complexes, such as efficiency,
brightness and emission wavelength, depend strongly on the structure of the cyclometalated
ligand. The emission wavelength of the iridium(III) complexes can normally be tuned by
changing the electronic nature or position of the substituents on the ligands. For example, the
emission wavelength of the iridium(III) complexes covered the entire visible region by a
modification or variation of cyclometalated 2-arylpyridine ligands [19]. Therefore, the design
of cyclometalated ligands for iridium(III) complexes is important for achieving high
efficiency and color purity for PhOLEDs.
In PhOLEDs, carbazole derivatives are used as host materials for both small-molecule
PhOLEDs, such as 4,4'-bis(9-carbazolyl)-2,2'-biphenyl (CBP), and polymer OLEDs, such as
poly(N-vinylcarbazole) (PVK), because of their high triplet energy and good hole-
transporting ability [20-22]. Many iridium(III) complexes with phenylpyridine modified by
introducing a carbazole unit have been reported. (2-CzPy)2Ir(acac) [2-CzPy: 2-[3-(N-
phenylcarbazolyl)]pyridine] and (3-PyC)2Ir(acac) [3-PyC: 9-decyl-3-(pyridin-3-yl)-9H-
carbazole] are examples exhibiting high luminance efficiency of 7.7 and 18.77 cd/A,
respectively, and the emission colors of these complexes ranged from orange (515 nm) to red
(594 nm) because of the large electron density of iridium (III) complexes by introducing a
carbazole unit in the main ligand [23-25]. Therefore, iridium(III) complexes containing a
carbazole unit that emits a blue color without a red shift are of interest.
Recently, an iridium(III) complex containing a long-chain carbazole moiety in a
phenylpyridine-based ligand was designed to prevent the interaction between carbazole and
the main ligand. Blue iridium(III) complexes containing two 2-(2',4'-difluoro-3'-
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hexylcarbazolephenyl)-4-methylpyridine (dfpmpyhCz) and one ancillary ligand, such as
acetylacetone, picolinic acid N-oxide and biimidazole, and exhibiting energy transfer from
the carbazole moieties to the iridium(III) cores were reported [26]. The reported complexes
showed emission in the blue region with enhanced luminescent efficiency without a red shift.
Furthermore, in a trial involving the synthesis of blue emitting iridium(III) materials, we
reported the (dfpmpyCF3)2Ir(picolinic-N-oxide) complex, where dfpmpyCF3 is 2-(2',4'-
difluoro-3'-trifluoromethylphenyl)-4-methylpyridine, which showed bright deep-blue
emission with the CIE coordinates of (0.147,0.210) and high external quantum efficiency of
23.3 % [27]. In this compound, the –CF3 group in the 3-position of the phenyl ring played an
important role in tuning the electronic state of the phenylpyridine ligand leading a blue shift
in emission.
As a part of an ongoing study on blue emitting materials, this paper reports the synthesis
and characterization of two iridium(III) complexes (dfpmpyCF3)2Ir(pic) (1) and
(dfpmpyhCz)2Ir(pic) (2), where pic is picolinic acid. Picolinic acid is a commonly used
ancillary ligand that exhibits stable blue emission color. The emission properties of the
synthesized iridium(III) complexes were studied and the device performance was examined
for applications to OLEDs based on these complexes .
2. Experimental
2.1. Synthesis
The chemicals were purchased from Aldrich Chemical Co. and used as received unless
otherwise noted. IrCl3·3H2O was obtained commercially from Alfa Aesar. All solvents were
purified and distilled freshly prior to use according to the literature procedures. Scheme 1
shows the synthetic procedures.
2.1.1. Synthesis of 2-(2',4'-difluoro-3'-hexylcarbazolephenyl)-4-methylpyridine (dfpmpyhCz)
(4)
2-(2’,4’-Difluorophenyl)-4-methylpyridine (3) was prepared using a Suzuki coupling
reaction between 2,4-difluorophenylboronic acid and 2-bromo-4-methylpyridine. 2-(2',4'-
Difluoro-3'-hexylcarbazolephenyl)-4-methylpyridine (4) was prepared employing a method
reported elsewhere [26]. 2-(2',4'-Difluorophenyl)-4-methylpyridine (4.1 g, 20 mmol) was
4

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dissolved in 50 mL of THF and cooled to -78 °C. Lithium diisopropylamide (2.0 g, 20 mmol)
was added to the mixture. The mixed solution was stirred for 1h, and 9-(6-
bromohexyl)carbazole (6.6 g, 20 mmol) was added to the solution. After stirring the solution
for 1h at room temperature, the mixture was extracted with ethyl acetate (80 mL × 3 times)
and dried over anhydrous MgSO4. The crude product was subjected to flash column
chromatography (ethyl acetate : n-hexane = 1 : 8) to afford compound 4 as a yellow oil (3.45
1
g, 38 %). H NMR (300 MHz, CDCl3) δ 8.56 (d, 1H, J = 5.1Hz), 8.11 (d, 2H, J = 6.6Hz),
7.77-7.69 (m, 1H), 7.52-7.38 (m, 4H), 7.23 (t, 2H, J = 7.8Hz), 7.08 (d, 1H, J = 4.5Hz), 6.95 (t,
1H, J = 7.2Hz), 4.31 (t, 2H, J = 7.2Hz), 2.72 (t, 2H, J = 7.5Hz), 2.41 (s, 3H), 1.89 (t, 2H, J =
7.2Hz), 1.61 (t, 2H, J = 6.9Hz), 1.44 (t, 4H, J = 3.9Hz). 13C NMR (CDCl3) δ 172.70, 165.48,
163.98, 163.28, 161.23, 160.02, 159.69, 157.72, 156.67, 151.75, 149.73, 148.11, 147.95,
147.22, 146.41, 145.71, 140.32, 137.85, 128.28, 128.04, 125.52, 123.16, 122.71, 120.26,
118.71, 114.22, 113.38, 110. 47, 108.49, 42.90, 29.31, 29.01, 28.79, 26.93, 21.47. MALDI-
TOF MS: m/z 454.32.
2.1.2. Synthesis of 2-(2',4'-difluoro-3'-trifluoromethylphenyl)-4-methylpyridine (dfpmpyCF3)
(5)
2-(2',4'-Difluoro-3'-trifluoromethylphenyl)-4-methylpyridine (5) was prepared using a
previously published procedure [27]. A mixture of copper(I) iodide (6.00 g, 31.5 mmol) and
anhydrous potassium fluoride (1.10 g,19.0 mmol) was heated under reduced pressure until
the color changed to yellow. 2-(2’,4’-Difluoro-3’-iodophenyl)-4-methylpyridine (4.18 g, 12.6
mmol), 1-methyl-2-pyrrolidone (30 mL) and (trifluoromethyl)trimethylsilane (4 mL) were
then added to the mixture. The resulting mixture was then stirred vigorously for 24 h at room
temperature. The mixture was poured into an aqueous ammonia solution (14%, 100 mL) and
extracted with dichloromethane (50 mL × 3 times) to give an organic phase. This phase was
washed with water (50 mL) and brine (50 mL), dried over anhydrous MgSO4 and
concentrated under vacuum. The crude product was subjected to flash column
1
chromatography (ethyl acetate : n-hexane = 1 : 8) to give compound 5 (1.39 g, 39%). H
NMR (300 MHz, CDCl3, ppm): 8.59 (d, 1H, J = 5.1 Hz), 7.98 (td, 1H, J = 6.3, 9 Hz), 7.60 (s,
5

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1H), 7.17 (d, 1H, J = 4.8 Hz), 7.01–7.07 (m, 1H), 2.46 (s, 3H). 13C NMR (CDCl3) δ 161.42,
159.29, 157.99, 155.78, 150.97, 149.57, 147.84, 146.98, 135.42, 125.26, 123.94, 112.94,
20.93.
2.1.3. Synthesis of (dfpmpyCF3)2Ir(pic) (1)
The cyclometalated iridium(III) chloride-bridged dimer, [(dfpmpyCF3)2Ir(µ-Cl)]2 (6) was
prepared using a modification of the method reported by Nonoyama [28]. The iridium(III)
trichloride hydrate (2.1 g, 6 mmol) and dfpmpyCF3 (5) (9.54 g, 21 mmol) were dissolved in a
solution of 2-ethoxyethanol/water (20 mL; 3:1 v/v). After heating the mixture to 130 °C for
24 h, the reaction mixture was cooled to room temperature, and the precipitate was collected.
The dimer complex 6 was washed with ethanol and hexane and dried under vacuum (2.96 g,
64%). The dimer complex 6 (1.06 g, 0.5 mmol), sodium carbonate (0.5 g, 5 mmol) and
picolinic acid (0.18 g, 1.5 mmol) was dissolved in 2-ethoxyethanol (30 mL). The mixture was
heated under reflux for 24 h. The mixture was extracted with dichloromethane (50 mL × 3
times) and dried over anhydrous MgSO4. The crude product was subjected to flash column
chromatography (methanol : dichloromethane = 1 : 20) to give compound 1. Yellow solid.
Yield: 0.24 g (66%). 1H NMR (300 MHz, CDCl3) δ 8.57 (d, 2H, J = 6.0Hz), 8.34 (d, 1H, J =
6.9Hz), 8.15 (s, 2H), 7.99-7.83 (m, 2H), 7.48 (t, 1H, J = 6.5Hz), 7.25 (s, 1H), 7.10 (d, 1H, J =
4.2Hz), 6.89 (d, 1H, J = 6.0Hz), 5.97 (d, 1H, J = 10.5Hz), 5.97 (d, 1H, J = 10.5Hz), 5.69 (d,
1H, J = 10.5Hz), 2.48 (s, 6H). 13C NMR (CDCl3) δ 177.63, 167.91, 154.61, 153.27, 151.38,
150.62, 142.53, 133.70, 131.86, 130.57, 128.29, 127.73, 126.82, 119.92, 108.04, 34.62, 25.69,
24.38, 17.15. Elementaal analyses were performed at the Korean Basic Science Center. Anal.
calcd. for C32H18F10IrN3O2: C, 44.76; H, 2.11; N, 4.89. Found: C, 44.53; H, 2.38; N, 4.71.
2.1.4. Synthesis of (dfpmpyhCz)2Ir(pic) (2)
The iridium(III) complex (2) was prepared using a similar method to that employed for
complex (1) using 2-(2',4'-difluoro-3'-hexylcarbazolephenyl)-4-methylpyridine (4). Yellow
1
solid. Yield : 0.46 g (76 %). H NMR (300 MHz, CDCl3) δ 8.54 (d, 1H, J = 6.0Hz), 8.30 (d,
1H, J = 7.2Hz), 8.10 (d, 4H, J = 6.6Hz), 8.04 (s, 2H), 7.87 (t, 2H, J = 7.8Hz), 7.71 (d, 2H, J =
4.5Hz), 7.46-7.37 (m, 4H), 7.28-7.22 (m, 4H), 6.96 (d, 1H, J = 6.0Hz), 6.74 (d, 1H, J =
6.0Hz), 5.82 (d, 1H, J = 9.3Hz), 5.57 (d, 1H, J = 9.3Hz), 4.27 (t, 4H, J = 6.9Hz), 2.53 (s, 6H),
1.86-1.84 (m, 6H), 1.49-1.35 (m, 14H). 13C NMR (CDCl3) δ 172.97, 168.38, 165.76, 164.16,
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163.28, 161.67, 160.25, 158.31, 157.72, 156.67, 154.41, 152.05, 150.04, 148.45, 148.23,
147.49, 146.78, 140.61, 138.12, 128.56, 128.11, 125.80, 124.09, 123.82, 123.02, 120.58,
118.93, 114.39, 13.38, 111.39, 111.20, 108.92, 107.09, 42.95, 32.78, 31.39, 29.62, 29.29,
29.06, 27.24, 22.49, 21.79. MALDI-TOF MS: m/z 1222.08. Anal. calcd. for C32H18F10IrN3O3:
C, 43.94; H, 2.07; N, 4.80. Found: C, 44.18; H, 2.35; N, 4.57.
2.2. Instruments and device fabrications
2.2.1. Instruments
1H and 13C nuclear magnetic resonance (NMR, Varian Mercury) spectra were recorded at
300 MHz using CDCl3 as a solvent. The chemical shifts are reported in parts per million
(ppm) relative to the residual CDCl3 at 7.26 ppm (for 1H NMR) and 77.0 ppm (for 13C NMR).
The UV-visible spectra were recorded on a Jasco V-570 spectrophotometer. The
photoluminescence spectra were measured using a Hitachi F-4500 fluorescence
spectrophotometer. The equation, Φs = Φr(ηs2ArIs/ ηr2AsIr), was used to calculate the quantum
yields, where the symbols are the usual meanings [29]. Thermal analyses were carried out on
a Metter Toledo TGA/SDTA 851e analyzer under a N2 atmosphere at a heating rate of 10 °C
min-1. Cyclic voltammetry (CV, Bioanalytical Systems CV-50W) was carried out at a
potential scan rate of 50-100 mV s-1 in a 0.1 M solution of tetra(n-butyl)ammonium
tetrafluoroborate in dichloromethane under a N2 atmosphere at room temperature. An
Ag/AgNO3 (0.1 M) electrode and Pt wire was used as the reference and counter electrode,
respectively. The working electrode was a Pt disc electrode (0.2 cm2). The potential was
reported relative to a ferrocene/ferrocenium (Cp2Fe/Cp2Fe+) redox couple used as an internal
reference (0.45 V vs SCE) [30].
X-ray intensity data was collected at room temperature on a Bruker SMART APEX-II CCD
diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The
structure of complex 1 was solved by applying the direct method using a SHELXS-97 and
refined by a full-matrix least-squares calculation on F2 using SHELXL-97. All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were placed in the ideal positions on
their respective carbon atoms (Biso = 1.2 Beq and 1.5 Beq). Crystal data for compound 1:
C30H10Cl6F10IrN3O2, Mw = 1097.43, Triclinic, P-1, a = 11.280(2) Å, b = 12.010(2) Å, c =
16.030(3) Å, α = 93.95(3) °, β = 105.94(3) °, γ = 108.92(3) °, V = 1945.3(8) Å3, Z = 2, Dcalc
=
1.874 Mg m-3, ꢀ = 3.928 mm-1, θmax = 27.50 °, 34894 reflections (Rint = 0.0716), 8918 unique
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reflections, 505 parameters refined. GooF = 1.050, Final R indices [I > 2σ(I)]: R1 = 0.0560,
ωR2 = 0.1416, all data: R1 = 0.0786, ωR2 = 0.1575.
2.2.2. Device fabrication
Organic light-emitting devices were fabricated based on iridium(III) complex 1 and 2 as
blue dopants in the emitting layers. The ITO (indium tin oxide)-coated glass substrates were
precleaned and UV-ozone treated for 20 min. The surface of the ITO substrate was modified
by spin-coating a conducting PEDOT:PSS layer (CLEVIOS PH500) to a thickness of 40 nm
followed by baking at 120 °C for 10 min under a N2 atmosphere. A 50 nm-thick emitting
layer was prepared by spin-coating in a chlorobenzene solution containing 12 wt%
iridium(III) complexes doped into the host (PVK:OXD-7:UGH3), where PVK is poly(N-
vinylcarbazole), OXD-7 is 1,3-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzene and
UGH3 is 1,3-bis(triphenylsilyl)benzene. A 20 nm-thick layer of TmPyPB [1,3,5-tri(m-pyrid-
3-yl-phenyl)benzene)] was spin-coated on top of the emitting layer and annealed at 80 °C for
60 min. LiF and Al were then evaporated sequentially at a pressure of 5 × 10-8 Torr. The film
thickness was determined using an α-Step IQ surface profiler (KLA Tencor, San Jose, CA,
USA). The EL spectra and CIE coordinates were measured using a PR650 spectra colorimeter.
The current density-voltage-luminescence (J-V-L) changes were determined using a
current/voltage source meter (Keithley 238) and optical power meter (CS-1000, LS-100). All
processes and measurements described above were carried out in air at room temperature.
3. Results and discussion
3.1. Synthesis and structural characterization
Scheme 1 shows the synthetic method. The reaction of 2, 4-difluorophenylboronic acid with
2-bromo-4-methylpyridine yielded 2-(2,4-difluorophenyl)-4-methylpyridine (3). 2-(2', 4'-
Difluoro-3'-hexylcarbazolephenyl)-4-methylpyridine (4) was obtained from a reaction of 9-
(6-bromohexyl)carbazole with compound 3.
A
reaction of compound
3
with
(trifluoromethyl)trimethylsilane in the presence of CuI and KF gave 2-(2,4-difluoro-3-
trifluoromethylphenyl)-4-methylpyridine (5). The cyclometalated iridium(III) ꢀ-chloride-
bridged dimers 6 and 7 were synthesized from IrCl3·3H2O with an excess of compounds 4
and 5, respectively. The dimer compounds 6 and 7 could be converted easily to the target
8

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iridium(III) complex (dfpmpyCF3)Ir(pic) (1) and (dfpmpyhCz)Ir(pic) (2) by replacing the
bridged chlorides by bidentate monoanionic 2-picolinic acid. The final iridium(III)
1
complexes were characterized by H and 13C NMR, elemental analysis and single crystal X-
ray analysis. Compounds 1 and 2 showed good solubility in chlorinated solvents as well as
high thermal stability with 5 % weight loss at 346 and 398 °C, respectively, as determined by
thermogravimetric analysis (TGA) under a nitrogen atmosphere.
The single crystal of compound 1 was obtained from a mixed solvent system
(CHCl3/hexane) and the structure was characterized by single crystal X-ray crystallography.
Figure 1 shows the ORTEP diagram of compound 1 as well as the selected bond lengths and
bond angles. As depicted in Figure 1, the iridium(III) ion adopts a distorted octahedral
geometry being coordinated with three N atoms, two C atoms and one O atom of three
bidentate ligands. The coordinated three ligands split the d-orbitals of the central iridium
metal unequally and prefers the cis-C,C’ and trans- N,N’ chelate disposition [31]. The angles
around the iridium atoms were in the range, 77.0(2) ~ 99.3(3)°. The Ir-O46 bond distance of
2.216(5) Å was longer than the mean Ir-O values of 2.088 Å reported in the Cambridge
Crystallographic Database. This value can be understood by the large trans influence of the
phenyl ring in 2-(2,4-difluoro-3-trifluoromethylphenyl)-4-methylpyridine. The Ir-N bond
lengths were 2.075 and 2.039 Å, which are close to the reported values [32-33].
3.2. Optical and electrochemical properties
Figure 2 shows the absorption spectra of compounds 1 and 2 in a dichloromethane solution
(~10-5 M) and the results are summarized in Table 1. By comparing the reported iridium(III)
complexes [34-35], the intense peaks in the short wavelength region below 360 nm were
assigned tentatively to the spin-allowed 1π-π* transition (1LC) and the weaker absorption tails
that appeared above 360 nm were attributed to the metal-to-ligand charge transfer (MLCT)
3
transitions [36]. The absorption spectra revealed substantial mixing of ligand-based π-π*
1
states, spin forbidden metal to ligand charge transfer (3MLCT) and a higher-lying MLCT
transition induced by the spin-orbit coupling effect [37-38]. The photoluminescence (PL)
spectra of compounds 1 and 2 excited at 300 nm were examined both in a dichloromethane
solution (~10-5 M) and in film form. The film was fabricated by doping iridium(III)
complexes in poly(methylmethacrylate) (PMMA) with a concentration of 5 wt %. The PL
spectra are shown in Figure 3 and the results are listed in Table 1. The emissions of the
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complexes showed vibronic fine structures with maximum PL peaks at 464 and 490 nm for
compound 1, and 475 and 501 nm for compound 2 in the solution state. Based on these
observed vibronic fine structures, it is likely that the lowest exited triplet states are dominated
by ligand-based 3π-π* states.
The maximum peak of compound 1 was blue-shifted 6 nm from that of FIrpic (470 nm) [39].
The PL of compound 1 in the film state was red-shifted 11 nm compared to that in the
solution state, whereas compound 2 showed a 9 nm blue shift compared to the solution state.
Compound 2 in solution showed emission at approximately 355 nm, which is believed to be
from a carbazole unit [27] and exhibited higher PL quantum efficiency of 18.4 % than that of
compound 1 (10.2 %). This suggests that efficient energy transfer from the carbazole unit to
iridium(III) cores in compound 2 was achieved. The emission peak at 355 nm was quenched
in the films, suggesting that intermolecular energy transfer was also achieved due to
aggregation resulting from the decreased distance between the molecules in films.
The optical band gaps (ꢁEg) of compounds 1 and 2, which were deduced from the
absorption edge of the absorption spectra, were 2.65 and 2.63 eV, respectively. The triplet
energy levels (T1), which were obtained from the PL spectra at 77K, was 2.67 eV (464 nm)
for compound 1, and 2.69 and 2.48 eV (461 and 500 nm) for compound 2. The HOMO
energy levels were obtained by cyclic voltammetry (CV) using ferrocene as a standard to
evaluate the charge injection and device performance. Cyclic voltammetry exhibited a quasi-
reversible oxidation potential (Eox) of 1.25 V for compound 1 and two quasi-
reversible/reversible oxidation potentials of 0.74 and 1.27 V vs. Ag/AgNO3 in a CH2Cl2
solution for compound 2, as shown in Figure 4. The second oxidation potential of compound
2 was assigned to the carbazole unit of the main ligand [40-41], indicating that the carbazole
unit did not affect the electrochemical properties of the cored-iridium(III) complexes
significantly. The LUMO levels were estimated from the HOMO levels and optical band gaps
using the formula, ELUMO = EHOMO + ꢁEg. The resulting data is summarized in Table 1.
3.3. Elecrtroluminescent properties
The light emitting devices based on compounds 1 and 2 were prepared and the OLED
emission characteristics were examined. The device configuration was ITO/PEDOT:PSS (40
nm)/Host-Ir (50 nm)/TmPyPB (20 nm)/LiF (1 nm)/Al (100 nm), and Figure 5 shows their
energy levels. The PVK:OXD-7:UGH3 was used as a host and 12 wt % of the iridium(III)
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complexes were used as a dopant. The PVK:OXD-7 ratio was 1:1, and the UGH3 content in
the host was varied to examine energy transfer from the host to guest because UGH3 has a
higher triplet energy and wide band gap, which was used for both electron transfer and as a
hole blocking layer. Devices I, II and III corresponding to 29, 22 and 17 wt % UGH3 in the
host, respectively, were fabricated using compounds 1 and 2.
The EL spectra (Figure 6) showed two emission peaks, which resembled the PL spectra of
the compounds. For compound 1, the emission peaks were red-shifted by ~10 nm compared
to the PL spectra of the corresponding compounds, and the intensity of the second peak
increased with increasing UGH3 content in the host. For example, the intensity of device I
was stronger than that of device III, showing that the CIE coordinates (x,y) changed from
(0.17, 0.40) for device 1-I to (0.16, 0.39) for device 1-III using compound 1. On the other
hand, the devices using compound 2 gave almost completely superposed EL spectra. This
suggests that the EL peaks and their CIE coordinates (x, y) were similarly independent of the
UGH3 concentration in a host. This means that the carbazole unit in compound 2 might be
attributed to the supplement of energy transfer between the host and guest.
Table 2 summarizes the device performance. Figure 7 shows the current density-voltage-
luminance (I-V-L) curves for all devices. Device I of compound 1 (device 1-I) demonstrated a
maximum luminance (L), external quantum efficiency (EQE), luminance efficiency (LE) and
power efficiency (PE) of 5750 cd/m2, 7.53 %, 16.86 cd/A and 5.88 lm/W, respectively.
Device I showed the best performance compared to devices II and III using compounds 1 and
2. The appropriate concentration of UGH3 in the host of device I may enhance energy
transfer from the host to guest resulting in higher external quantum efficiency.
4. Conclusions
Two sky-blue phosphorescent iridium(III) complexes (dfpmpyCF3)Ir(pic) (1) and
(dfpmpyhCz)Ir(pic) (2) were synthesized using two electron density modulated
phenylpyridine main ligands and one picolinic acid ancillary ligand for the fabrication of
OLEDs. These complexes were thermally and electrochemically stable, and showed good
photoluminescence efficiency (> 15%). The –CF3 substituted on the phenylpyridine-based
ligand caused a hypsochromic shift in the emitting spectra in compound 1, and
hexylcarbazole substituted iridium(III) complex 2 exhibited energy transfer from the
carbazolyl moieties to the iridium(III) core with high external quantum efficiency.
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Supplementary Materials.
The crystallographic data for the structure of compound 1 was deposited in the Cambridge
Crystallographic Data Center (Deposition No. CCDC-459401). The data can be obtained
free of charge at www.ccdc.cam.ac.uk/deposit (or from the CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: +44-01223 336033; E-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
This work was supported by the basic research fund of Samsung Display, Korea.
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Scheme 1. Synthetic procedure of main ligands and iridium(III) compounds 1 and 2.
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Figure Captions
Figure 1. ORTEP diagram of compound 1 (ellipsoids with 30% probability). Selected bond
lengths (Å) and bond angles (°) for compound 1; Ir-C(8) 1.992(8), Ir-C(27) 1.984(7), Ir-N(1)
2.075(6), Ir-N(20) 2.039(6), Ir-N(39) 2.134(6), Ir-O(46) 2.126(5), C(27)-Ir-C(8) 89.6(3),
C(27)-Ir-N(20) 80.5(3), C(8)-Ir-N(20) 94.2(3), C(27)-Ir-N(1) 97.0(3), C(8)-Ir-N(1) 80.5(3),
N(20)-Ir-N(1) 174.2(2), C(27)-Ir-O(46) 175.1(2), C(8)-Ir-O(46) 94.2(2), N(20)-Ir-O(46)
96.1(2), N(1)-Ir-O(46) 86.7(2), C(27)-Ir-N(39) 99.3(3), C(8)-Ir-N(39) 170.9(3), N(20)-Ir-
N(39) 89.0(2), N(1)-Ir-N(39) 96.6(2), O(46)-Ir-N(39) 77.0(2).
Figure 2. Absorption spectra of compounds 1 and 2 in solution (1.0 × 10-5 M in CH2Cl2).
Figure 3. Photoluminescence spectra of compounds 1 and 2 in solution (1.0 × 10-5 M in
CH2Cl2) and 5 wt % doped in PMMA film.
Figure 4. Cyclic voltammograms of compounds 1 and 2 at scan rate of 100 mV/S.
Figure 5. Energy levels and triplet energy of materials in an EL study.
Figure 6. Electroluminescence and photoluminescence spectra of compounds 1 (a) and 2 (b)
at current density of 10 mA/cm2.
Figure 7. Electroluminescence characteristics: J-V-L curves of compounds 1 (a) and 2 (b);
Luminance efficiency and power efficiency versus current density of compounds 1 (c) and 2
(d).
16

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Fig. 1.
17

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1.2
1.0
0.8
0.6
0.4
0.2
0.0
1
2
4
1500
4 (PL)
1000
500
0
300
350
400
450
500
Wavelength (nm)
Fig. 2
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1 (Solution)
2 (Solution)
1 (Film)
1.0
0.8
0.6
0.4
0.2
0.0
2 (Film)
350 400 450 500 550 600 650 700
Wavelength (nm)
Fig. 3
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1
2
2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
Potential (eV)
Fig. 4
20

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Fig. 5
21

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(a)
1 (PL)
1 - I
1 - II
1 - III
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
Wavelength(nm)
(b)
2 (PL)
2 - I
2 - II
2 - III
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 6
22

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(a)
(b)
104
104
103
102
101
100
80
60
40
20
0
80
,
,
,
1 - I
1 - II
1 - III
,
,
,
2 - I
2 - II
2 - III
103
102
101
100
60
40
20
0
0
2
4
6
8
10
12
14
16
0
2
4
6
8
10
12
14
16
Bias (V)
Bias (V)
(d)
(c)
35
30
25
20
15
10
5
35
30
25
20
15
10
5
101
100
10-1
10-2
101
100
10-1
10-2
,
,
,
1 - I
1 - II
1 - III
,
,
,
2 - I
2 - II
2 - III
0
0
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
Current density (mA/cm2)
Current density (mA/cm2)
Fig. 7
Table 1. Photophysical and electrochemical properties for iridium(III) compounds 1, 2 and
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ligand 4a
f
d
Φc
Eox
(V)
HOMOe LUMO
PL
Eg
Compound
UVa
MCa filmb
(%)
(eV)
(eV)
(eV)
258(0.74),283(0.62),
304(0.48),329(0.31)
,375(0.18),407(0.11),
442(0.03)
464,
490
475,
494
15.1
1.25
-5.75
-3.10
2.64
1
263(0.54),282(0.78),
294(0.62),314(0.38),
334(0.31),347(0.26),
337(0.17),419(0.04)
285(0.20),297(0.25),
316(0.04),336(0.08),
349(0.07)
475,
501
466,
489
0.74,
1.27
-5.24,
-5.77
-2.61,
-3.14
18.4
2.63
3.44
2
4
355,
369
-
-
1.36
-6.08
-2.63
aAll data were measured in a CH2Cl2 solution (concentration = 1×10-5 M).
bPMMA film doped with 5 wt% of Ir(III) complex.
cExcitation wavelengths were 380 nm using Ir(ppy)3 (0.40) as an external reference.
dPotential values were reported vs. Fc/Fc+.
eDetermined from the onset oxidation potential.
fCollected in CH2Cl2 solution using a UV-vis. spectrophotometer.
Table 2. Device performance based on iridium(III) compounds 1 and 2
24

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Turn-on
EQE
LE
PE
L
EL
CIE
Compound Device
I
voltage(V)a (%)a (cd/A)a (lm/W)a (cd/m2)a (nm)b (x,y)b
5750
at 12 V
3682
472,
502
473,
504
469,
493
478,
502
478,
505
478,
502
0.17,
0.40
0.16,
0.41
0.16,
0.39
0.19,
0.46
0.18,
0.44
0.18,
0.44
5
5
7.53
6.03
4.80
7.44
5.99
4.12
16.86
14.38
10.96
19.48
15.00
10.53
5.88
5.64
3.82
8.74
5.89
3.32
1
II
III
I
at 13 V
2240
5
at 15 V
5007
5.5
5.5
5.5
at 13 V
4353
2
II
at 13 V
2363
III
at 17 V
aMaximum efficiency.
bValues collected at a current density of 10 mA/cm2.
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Highlights
▶ Two sky-blue phosphorescent iridium(III) complexes were developed.
▶ The energy transfer from carbazolyl moiety to iridium(III) core was observed.
▶ The devices showed that EQE increased with increasing UGH3 in the host.