Blue phosphorescent ir(III) complex with high color purity: Fac-tris(2′,6′-difluoro-2,3′-bipyridinato-N,C 4′)iridium(III)

Article abstract of DOI:10.1021/ic801643p

A blue phosphorescent iridium(III) complex (1) bearing fluorine-substituted bipyridine (dfpypy) has been synthesized and characterized to investigate the effect of the substitution and replacement of the phenyl ring in ppy (phenylpyridine) with pyridine o

Full text of DOI:10.1021/ic801643p

Inorg. Chem. 2009, 48, 1030-1037
Blue Phosphorescent Ir(III) Complex with High Color Purity:
fac-Tris(2′,6′-difluoro-2,3′-bipyridinato-N,C4′)iridium(III)
Seok Jong Lee,† Ki-Min Park,‡ Kiyull Yang,‡ and Youngjin Kang*,§
LG Display Co., Ltd. 642-3, Jinpyung-dong, Gumi Gyungbuk, 730-726, Republic of Korea,
Research Institute of Natural Science & Department of Chemistry Education, Gyeongsang
National UniVersity, Jinju 660-701, Republic of Korea, and DiVision of Science Education,
Kangwon National UniVersity, Chuncheon 200-701, Republic of Korea
Received August 28, 2008
A blue phosphorescent iridium(III) complex (1) bearing fluorine-substituted bipyridine (dfpypy) has been synthesized
and characterized to investigate the effect of the substitution and replacement of the phenyl ring in ppy (phenylpyridine)
with pyridine on the solid state structure and its photoluminescence. The optical properties and electrochemical
behaviors of 1 have also been systematically evaluated. The structure of 1 has also been determined by a single-
crystal X-ray diffraction analysis. There are varied intermolecular interactions caused by the pyridine and fluorine
substituents, such as C-H· · ·N, C-H · · · F, and π · · ·π interactions of either face-to-face type or edge-to-face
C-H· · ·π and halogen · · ·π in crystal packing. In electrochemistry, the remarkably higher oxidation potential than
that of FIrpic was observed. The emission λmax of 1 at room temperature is at 438 nm with a higher PL quantum
efficiency. Complex 1 exhibits intense blue emission with high color purity (CIE x ) 0.14, y ) 0.12), which has
been attributed to metal-to-ligand charge-transfer triplet emission based on DFT calculations.
Introduction
(C∧N)3Ir(III) complexes with facial geometry have shown
high thermal stability and high phosphorescent efficiency,
and hence their derivatives are promising phosphorescent
metal complexes in OLEDs.3 Luminescent Ir(ppy)3 (ppy )
2-phenylpyridine) derivatives have been widely investigated
because the emission wavelength of Ir(ppy)3 can be readily
controlled either by the introduction of electron-donating and/
or electron-withdrawing groups into the ppy ligand or by
derivatization of the C∧N chelating modes using ben-
zothiophene and quinoline.4 For example, Thompson and
Forrest et al. reported blue and red emission energies by the
introduction of a fluorine group into the phenyl ring for the
Luminescent complexes containing third-row transition
metals have attracted much attention owing to their varied
applications, such as photonics and electronics.1 In particular,
cyclometallated iridium(III) complexes are regarded as
excellent phosphorescent materials and have been widely
studied because of their ability to achieve maximum internal
quantum efficiency, nearly 100%, as well as high external
quantum efficiency in OLEDs.2 Cyclometallated
* To whom correspondence should be addressed. Phone: +82-33-250-
6737. Fax: +82-33-242-9598. E-mail: kangy@kangwon.ac.kr.
†
LG Display Co., Ltd 642-3.
Gyeongsang National University.
Kangwon National University.
‡
(3) (a) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.;
Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc.
2003, 125, 7377–7387. (b) Ostrowski, J. C.; Robinson, M. R.; Heeger,
A. J.; Bazan, G. C. Chem.Commun. 2002, 784–785. (c) Adamovich,
V.; Brooks, J.; Tamayo, A.; Djurovich, P.; Alexander, A.; Thompson,
M. E. New J. Chem. 2002, 1171–1178. (d) Xie, H. Z.; Liu, M. W.;
Wang, O. Y.; Zhang, X. H.; Lee, C. S.; Hung, L. S.; Lee, S. T.; Teng,
P. F.; Kwong, H. L.; Zheng, H.; Che, C. M. AdV. Mater. 2001, 13,
1245–1248. (e) Kang, D. M.; Kang, J.-W.; Park, J. W.; Jung, S. O.;
Lee, S.-H.; Park, H.-D.; Kim, Y.-H.; Shin, S. C.; Kim, J.-J.; Kwon,
S.-K. AdV. Mater. 2008, 20, 2003–2007.
(4) (a) Grushin, V. V.; Herron, N.; LeCloux, D. D.; Marshall, W. J.; Petrov,
V. A.; Wang, Y. Chem. Commun. 2001, 1494–1495. (b) Sajoto, T.;
Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson,
M. E.; Holmes, R. J.; Forrest, S. R. Inorg. Chem. 2005, 44, 7992–
8003.
§
(1) (a) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Chem. ReV. 2007,
107, 1233–1271, and references cited therein. (b) Segal, M.; Singh,
M.; Rivoire, K.; Difley, S.; Voorhis, T. V.; Baldo, M. A. Nature 2007,
6, 374–378. (c) Chou, P.-T.; Chi, Y. Chem. Eur. J. 2007, 13, 380–
395. (d) Kalyanasundaram, K.; Gra¨tzel, M. Coord. Chem. ReV. 1998,
177, 347–414.
(2) (a) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature 2000, 403,
750–753. (b) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson,
M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4–6. (c) Adachi, C.;
Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000,
77, 904–906. (d) Yuichiro Kawamura, Y.; Goushi, K.; Brooks, J.;
Brown, J. J.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 071104.
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5048–5051.
1030 Inorganic Chemistry, Vol. 48, No. 3, 2009
10.1021/ic801643p CCC: $40.75  2009 American Chemical Society
Published on Web 12/22/2008

Blue Phosphorescent Iridium(III) Complex
voltammetry was performed with an Autolab potentiostat by
Echochemie under a nitrogen atmosphere in a one-compartment
electrolysis cell consisting of a platinum wire working electrode, a
platinum wire counter electrode, and a quasi Ag/AgCl reference
electrode. Cyclic voltammograms were monitored at scan rates of
either 100 or 50 mV s-1 and recorded in distilled dichloromethane/
acetonitrile. The concentration of the complex was maintained at
0.5 mM or less, and each solution contained 0.1 M tetrabutylam-
monium hexafluorophosphate (TBAP) as the electrolyte. The
thermogravimetric spectrum was recorded on a Perkin-Elmer
TGA-7 under nitrogen environment at a heating rate of 10 °C/min
over a range of 25-700 °C.
Theoretical Calculations. Computations on the electronic
ground state of 1 were performed using Becke’s three-parameter
density functional in combination with the nonlocal correlation
functional of Lee, Yang, and Parr (B3LYP).8 6-31G(d) basis sets
were employed for the ligand and the relativistic effective core
potential of Los Alamos and double-ꢀ basis sets were employed
for the Ir (LANL2DZ).9 The ground-state geometries were fully
optimized at the B3LYP level, and time-dependent DFT
(TDDFT)10a calculations were preformed to obtain the vertical
singlet and triplet excitation energies. All computations were
performed using the Gaussian-98 package.10b
X-ray Crystallographic Analysis. Suitable crystals of 1 were
obtained from slow vapor diffusion of hexane into a solution of 1
in dichloromethane/benzene. The single crystal of 1 was attached
to glass fibers and mounted on a Bruker SMART diffractometer
equipped with graphite monochromated Mo KR (λ ) 0.710 73 Å)
radiation, operating at 50 kV and 30 mA with a CCD detector; 45
frames of two-dimensional diffraction images were collected and
processed to obtain the cell parameters and orientation matrix. All
data collections were performed at 173(2) K. The data collection
2θ range was 3.7-53.0°. No significant decay was observed during
the data collection. The raw data were processed to give structure
factors using the SAINT-plus program.11a Empirical absorption
corrections were applied to the data sets using the SADABS.11b
The structure was solved by direction methods and refined by full-
matrix least-squares against F2 for all data using SHELXTL
software.11c All non-hydrogen atoms in compound 1 were aniso-
tropically refined. All hydrogen atoms were included in the
calculated positions and refined using a riding model with isotropic
thermal parameters 1.2 times those of the parent atoms. The
compound 1 cocrystallizes with benzene/0.7 dichloromethane, and
former, and by using a quinoline unit instead of a pyridine
ring for the latter. In addition, the emission energy can be
fine-tuned by the combination of auxiliary (two o-chelating
(C∧N) ligands) and LX type of ancillary ligands (e.g.,
monoanionic chelating ligands, acac ) acetylacetone, pic
) picolinate, sal ) salicylimine, iq ) isoquinolinecarboxy-
late, bpz ) pyrazolylborate).5
To achieve full color display, three primary colors, such
as blue, green, and red, are necessary. However, homoleptic
Ir(III) complexes with both blue emission at room temper-
ature and facial geometry are very rare, compared to those
of phosphorescent green and red.4b,6 The following consid-
erations should be needed for the development of blue
phosphorescent Ir(III) complexes: (i) the adoption of a ligand
with large triplet energy, since the emission energies of
cyclometallated Ir(III) complexes are mainly determined by
the triplet energy of the C∧N chelating ligand, and (ii) the
inducement of a larger band gap by stabilizing the HOMO
and destabilizing the LUMO level.
Recently, the Tamao group reported that pyridine has
lower HOMO and LUMO energy levels than those of some
nitrogen-containing heterocyclic compounds. Moreover, the
pyridine ring is more electronegative than a nonsubstituted
phenyl ring.7 These facts prompted us to develop a new C∧N
chelating bipyridine (bpy or pypy) ligand, as opposed to the
phenylpyridine (ppy) ligand system.
Herein, we describe the results of our investigation on the
preparation, structural characterization, electrochemical be-
havior, and photophysical properties of the fac-Ir(dfpypy)3
(dfpypy ) 2′,4′-difluoro-2,3′-bipyridinato-N,C4′) complex.
Experimental Section
General Considerations. All experiments were performed under
a dry N2 atmosphere using standard Schlenk techniques. All solvents
were freshly distilled over appropriate drying reagents prior to use.
All starting materials were purchased from either Aldrich or Strem
and used without further purification.
1
Measurement. H NMR and mass spectra were recorded on a
Bruker DRX 400 MHz spectrometer and JEOL-JMS 700 instru-
ment, respectively. UV/vis and photoluminescent spectra for all
samples with concentrations in the range of 10-50 µM were
obtained from the UV/vis spectrometer Lambda 900 and a Perkin-
Elmer luminescence spectrometer LS 50B, respectively. All solu-
tions for photophysical experiments were degassed with more than
three repeated freeze-pump-thaw cycles in a vacuum line. Cyclic
(8) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(9) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634–1641.
(10) (a) Jarmorski, C.; Casida, M. E.; Salahub, D. R. J. Chem. Phys. 1996,
104, 5134–5147. (b) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.;
Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.;
Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.;
Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas,
O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.;
Pomelli, C.; Adame, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.;
Ayala, P. Y.; Cui, Q.; Morokuma, K.; Salvador, P.; Dannenberg, J. J.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong,
M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.;
Pople, J. A. Gaussian 98, Revision A.7; Gaussian, Inc.: Pittsburgh,
PA, 1998.
(5) (a) You, Y.; Park, S. Y. J. Am. Chem. Soc. 2005, 127, 12438–12439.
(b) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Tsyba, I.; Ho, N. N.; Bau,
R.; Thompson, M. E. Polyhedron 2004, 23, 419–428. (c) Lamansky,
S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba,
I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem. 2001,
40, 1704–1711. (d) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-
Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.;
Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304–4312. (e) Kwon,
T.-H.; Cho, H. S.; Kim, M. K.; Kim, J.-W.; Kim, J.-J.; Lee, K. H.;
Park, S. J.; Shin, I.-S.; Kim, H.; Shin, D. M.; Chung, Y. K.; Hong,
J.-I. Organometallics 2005, 24, 1578–1585.
(6) (a) Yeh, Y.-S.; Cheng, Y.-M.; Chou, P.-T.; Lee, G.-H.; Yang, C.-H.;
Chi, Y.; Shu, C.-F.; Wang, C.-H. Chem. Phys. Chem. 2006, 7, 2294–
2297, and references cited therein. (b) Chew, S.; Lee, C. S.; Lee, S.-
T.; Wang, P.; He, J.; Li, W.; Pan, J.; Zhang, X.; Kwong, H. Appl.
Phys. Lett. 2006, 88, 93510.
(11) (a) Bruker, SMART (ver. 5.625) and SAINT-plus (ver. 6.22). Area
Detector Control and Integration Software; Bruker AXS Inc.: Madison,
WI, 2000. (b) Bruker, SADABS (ver. 2.03). Empirical absorption and
correction software; Bruker AXS Inc.: Madison, WI, 1999. (c) Bruker,
SHELXTL (ver. 6.10). Program for Solution and Refinement of Crystal
Structures; Bruker AXS Inc.: Madison, WI, 2000.
(7) Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi,
S. J. Am. Chem. Soc. 1996, 118, 11974–11975.
Inorganic Chemistry, Vol. 48, No. 3, 2009 1031

Lee et al.
(s, 2H). Anal. Calcd for C5H4BF2NO2: C, 37.79; H, 2.54; N, 8.81.
Found: C, 37.75; H, 2.58; N, 8.79.
Ligand dfpypy (L1). 2-Bromopyridine (0.31 mL, 3.99 mmol),
2,6-difluoropyridyl-3-boronic acid (0.76 g, 4.78 mmol), and Pd-
(PPh3)4 (0.28 g, 0.24 mmol) were dissolved in THF (25 mL). A
solution of 5% K2CO3 (10 mL) was added, and the mixture was
refluxed with stirring for 18 h under a nitrogen atmosphere. After
being cooled, the mixture was poured into water and extracted with
ethyl acetate. The organic layer was dried over MgSO4. The solvent
was removed under reduced pressure to give a crude residue. The
crude product was purified by column chromatography on silica
gel (EtOAc/hexane, 1/3, v/v) to obtain a colorless solid (65%). 1H
NMR (CDCl3): δ 8.57 (m, 2H), 7.69 (m, 2H), 7.18 (m, 1H), 6.87
(m, 1H). Anal. Calcd for C10H6F2N2: C, 62.50; H, 3.15; N, 14.58.
Found: C, 62.48; H, 3.14; N, 14.60.
Complex Ir(dfpypy)3 (1). Ligand dfpypy (1.92 g, 10.0 mmol)
and Ir(acac)3 (1.22 g, 2.50 mmol) were dissolved in ethylene glycol
(50 mL), and the mixture was heated to reflux under nitrogen for
25 h. The reaction mixture was then cooled to room temperature,
1 N HCl was added, and the mixture was filtered to give a crude
product, which was flash chromatographed on a silica column using
dichloromethane to yield 0.31 g (20%) of material. The compound
was then further purified by sublimation. 1H NMR (CD2Cl2): δ 8.36
(d, J ) 8 Hz, 3H), 7.89 (dt, J ) 8 Hz, 2 Hz, 3H), 7.52 (dd, J ) 5
Hz, 1 Hz, 3H), 7.13 (m, 3H), 6.25 (t, J ) 2 Hz, 3H). 13C NMR
(CD2Cl2): δ 180.7, 162.8, 160.8, 147.8, 139.0, 124.3, 124.1, 123.9,
113.0, 112.7 (C-F resonance not located). MS (FAB): m/z ) 767
[M+]. Anal. Calcd for C30H15F6N6Ir: C, 47.06; H, 1.97; N, 10.98.
Found: C, 47.10; H, 1.95; N, 10.95.
Figure 1. Coordination environment of the Ir(III) ion in complex 1 with
selected numbering scheme. Selected bond lengths (Å) and angles (deg):
Ir1-C10 2.001(5), Ir1-C20 1.997(5), Ir1-C30 2.005(5), Ir1-N1 2.115(4),
Ir1-N3 2.136(4), Ir1-N5 2.1278(4), C10-Ir1-C20 93.9(2), C10-Ir1-C30
92.9(2), C10-Ir1-N1 79.7(2), C10-Ir1-N3 93.72(18), C10-Ir1-N5
171.57(19), C20-Ir1-C30 97.0(2), C20-Ir1-N1 171.26(19), C20-Ir1-N3
79.5(2), C20-Ir1-N5 90.74(19), C30-Ir1-N1 89.24(19), C30-Ir1-N3
172.72(19), C30-Ir1-N5 79.52(19), N1-Ir1-N3 94.93(17), N1-Ir1-N5
96.40(17), N5-Ir1-N3 94.06(16).
Table 1. Crystal Data and Structure Refinement for 1
empirical formula
formula weight
crystal system
space group
a (Å)
[IrC30H15N6F6](C6H6)(CH2Cl2)0.7
903.24
monoclinic
P21/n
15.3684(8)
10.3143(5)
22.4069(12)
104.8030(10)
3433.9(3)
4
b (Å)
c (Å)
ꢁ (deg)
V (Å3)
Z
D(calcd) (g cm-3
)
1.747
4.067
1758
Result and Discussion
µ (mm-1
F(000)
)
The fluorinated bipyridine ligand was synthesized using
a Suzuki coupling by the reaction of 2-bromopyridine with
2,6-difluoropyridinyl-3-boronic acid in the presence of
K2CO3 and Pd(PPh3)4 as a catalyst, as shown in Scheme 1.
This ligand was obtained in good yields (∼85%). Complex
1 was synthesized by a slight modification of a previous
synthetic methodology reported by Watt et al.12 By the
reaction of Ir(acac)3 with the ligand at high temperature
(>200 °C), complex 1 was obtained in a moderate yield (ca.
20∼40%).
scan type
ꢂ-ω
no. of reflections collected
no. of independent reflections
absorption correction
goodness of fit
19 755
7077 (Rint ) 0.0267)
empirical (SADABS)
1.092
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0393, 0.1008
0.0460, 0.1041
the crystal system is the monoclinic space group P21/n. The solvent
molecules were modeled successfully. Crystal data for 1 is
summarized in Table 1. Selected bond lengths and angles for 1 are
given in Figure 1. The refined atomic coordinates and anisotropic
thermal parameters are deposited in the Supporting Information.
Crystallographic data (cif files for 1) for the structure reported here
have been deposited with the Cambridge Crystallographic Data
Center (Deposition No. CCDC-698914). The data can be obtained
free of charge via http://www.ccdc.cam.ac.uk. /perl/catreq/catreq.cgi
(or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK;
fax, +44 1223 336033; e-mail, deposit@ccdc.cam.ac.uk).
Syntheses. 2,6-Difluoropyridyl-3-boronic Acid. A stirred solu-
tion of diisopropylamine (1.7 mL, 12 mmol) in diethyl ether (20
mL) was cooled to -78 °C. Addition of n-BuLi (22.5 mL, 36 mmol,
1.6 M in hexane) over 20 min resulted in a pale yellow solution.
After stirring for 1 h at this temperature, 2,6-difluoropyridine (0.91
mL, 10 mmol) was added. The solution was stirred for an additional
hour, and then trimethyl borate (1.4 mL, 12.5 mmol) was added at
-78 °C. The mixture was allowed to warm to room temperature
and to react for 1 h. The mixture was quenched by the slow addition
of 5% aqueous NaOH solution (20 mL). After 10 min, the mixture
was acidified by the dropwise addition of 3 N HCl. Extraction with
ethyl acetate and evaporation of the organic layer furnished the
pure title compound, which was obtained as a white solid in 90%
yield. 1H NMR (CDCl3): δ 8.4 (q, J ) 8 Hz, 1H), 6.9 (m, 1H), 5.3
Complex 1 is very stable under air and was fully
characterized by NMR, mass spectrometry, and elemental
analysis (see Supporting Information). In the 1H NMR
spectrum, the complex exhibits five well-resolved peaks in
the region of 6.0∼8.5 ppm due to the pyridine rings,
indicating that the complex has facial geometry around the
Ir atom. Two peaks at -69 and -73 ppm in the 19F-NMR
spectrum were observed as doublets due to the coupling of
the neighboring F atom. The coupling constant (4JF-F) was
9.87 Hz, which is agreement with a previous report.13 To
investigate the thermal stability of 1, thermogravimetric
analysis (TGA) was conducted. No loss of weight was
observed in the range below 250 °C, and the decomposition
temperature, which is defined as a 5% loss of weight,
(12) (a) Dedeian, K.; Djurovich, P. I.; Garces, F. O.; Carlson, G.; Watt,
R. J. Inorg. Chem. 1991, 30, 1687–1688. (b) Jung, S.; Kang, Y.; Kim,
H.-S.; Kim, Y.-H.; Lee, C.-L; Kim, J.-J; Lee, S.-K; Kwon, S.-K. Eur.
J. Inorg. Chem. 2004, 17, 3415–3423.
(13) Dedeian, K.; Shi, J.; Shepherd, N.; Forsythe, E.; Morton, D. C. Inorg.
Chem. 2005, 44, 4445–4447.
1032 Inorganic Chemistry, Vol. 48, No. 3, 2009

Blue Phosphorescent Iridium(III) Complex
Scheme 1. Synthetic Routes and Structures of L1 and 1 Used in This Study
appeared at 452 °C. This result indicates that complex 1 has
high thermal stability and is much more stable than fac-
[Ir(ppy)3](423 °C).14 The observed high thermal stability
seems to be caused by the F atom introduced onto the
pyridine backbone, in that, by the general rule of thumb,
the F atom induces sublimity of a compound and increases
the thermal stability by strong C-F bond.
The structure of 1 was unambiguously established by
single-crystal X-ray diffraction analysis. The crystal structure
and packing diagram of 1 are presented in Figure 1 and
Figure 2, respectively. Complex 1 in the crystal structure
exhibits only facial configuration with distorted octahedral
geometry around the Ir atom, as shown in Figure 1.
The range of dihedral angles between two pyridine rings
of each ligand in 1 is 3.8(4)-5.6(3)°, indicating that the two
pyridine rings are almost coplanar. The Ir-C bond length,
ranging from 1.997(5) to 2.005(5) Å, is slightly shorter than
those of other mononuclear complexes with blue emission
(Ir-C ) 2.015(7)∼2.027(6) Å for fac-Ir(ppz)3; Ir-C )
2.019(7)∼2.024(8) Å for fac-Ir(flz)3).4b Furthermore, the
C-C and C-N bond lengths and angles are within normal
ranges and are in agreement with corresponding parameters
described for other similarly constituted complexes.15 All
σ-donor C atoms (C10, C20, and C30) in metallated
difluoronated pyridine ligands arrange a trans placement to
dative N atoms (N5, N6, and N3) in the adjacent pyridine
ring. The Ir-N bond lengths (2.116(4)∼2.135(4) Å) observed
in 1 are in agreement with those of fac-Ir(flz)3 (flz ) 1-[9,9,-
dimethyl-2fluorenyl])pyrazolyl) (2.095(7)∼2.121(6) Å), and
fac-Ir(ppz)3 (1-phenylpyrazoly-N,C2′) (2.117(5)∼2.135(5)
Å).4b The crystal packing of 1, as shown in Figure 2, is
mainly stabilized by two kinds of the intermolecular interac-
tions. One is weak hydrogen bonding such as the C-H · · ·N
and C-H · · · F type. The distances of H · · · N and H · · ·F are
in the region of 2.41-2.67 Å (Table 2). The other is π · · ·π
interactions, such as the face-to-face type, with the average
distance between pridyl ring planes being 3.45 Å. In addition,
weak intermolecular interactions, such as edge-to-face
C-H· · ·π(py)16 and halogen · · ·π(py) interactions,17 are also
observed in the crystal packing structure. The structural
parameters for the intermolecular interactions are summarized
(14) Wong, W.-Y.; Ho, C.-L; Gao, Z.-X.; Mi, B.-X.; Chen, C.-H.; Cheah,
K.-W.; Lin, A. Angew. Chem., Int. Ed. 2006, 45, 7800–7803.
(15) Bo¨ttcher, H.-C.; Graf, M.; Kru¨ger, H.; Wagner, C. Inorg. Chem.
Commun. 2005, 5, 278–280.
(16) (a) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem.
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D. L. J. J. Am. Chem. Soc. 1990, 112, 4768–4774.
(17) (a) Schollmeyer, D.; Shishkin, O. V.; Ru¨hl, T.; Vysotsky, M. O. Cryst.
Eng. Commun. 2008, 10, 715–723. (b) Swierczynski, D.; Luboradzki,
R.; Dolgonos, G.; Lipkowski, J; Schneider, H.-J. Eur. J. Org. Chem.
2005, 6, 1172–1177. (c) Desiraju, G. R.; Steiner, T. The Weak
Hydrogen Bond in Structural Chemistry and Biology; Oxford Uni-
versity Press: Oxford, 1999. (d) Jetti, R. K. R.; Nangia, A.; Xue, F.;
Mak, T. C. W. Chem. Commun. 2001, 919–920.
Figure 2. Crystal packing structure of 1. Dotted lines represent intermo-
lecular interactions: face-to-face π· · ·π, brown; halogen · · ·π, blue. The
C-H· · ·X (X ) N or F) hydrogen bonds are not shown for clarity.
Inorganic Chemistry, Vol. 48, No. 3, 2009 1033

Lee et al.
Table 2. Structural Parameters of Hydrogen Bonds and Intermolecular
Interactions for 1 (Å and deg)a
D-H· · ·A
d(D-H) d(H· · ·A) d(D· · ·A) ∠(DHA)
C1-H1· · ·F5b
0.95
0.95
0.95
0.95
0.95
0.95
0.99
2.46
2.56
2.67
2.67
2.41
2.67
2.54
3.096(6)
3.337(8)
3.322(7)
3.598(8)
3.087(7)
124.8
138.9
126.6
166.2
127.9
C1-H1· · ·N6b
C2-H2· · ·F3c
C13-H13· · ·N2d
C21-H21· · ·F6d
C36-H36· · ·N4e
C37-H37A· · ·F3f
π· · ·π interactions
A· · ·Ae
3.419(13) 135.9
3.142(14) 119.3
3.34(1)
3.55(1)
0.0(3)
35.1(4)
Bg · · ·C
C-H· · ·π interactions
C23-H23· · ·Cg1b
halogen· · ·π interactions
Cl2· · ·Cg2
0.95
2.90
3.73
145.8
3.40
2.99
F4· · ·Cg3h
a A, B, and C define each pyridyl ring plane including N2, N5, and N6,
respectively. Cg1 and Cg2 define the centroids of each pyridyl ring including
N1 and N4, respectively. Cg3 defines the centroid of C26 and C27.
Symmetry codes given in footnotes b-h. b -x + 0.5, y + 0.5, -z + 0.5.
c x + 0.5, -y + 1.5, z + 0.5. d x, y + 1, z. e -x + 1, -y + 1, -z. f -x, -y
+ 2, -z. g -x + 0.5, y - 0.5, -z + 0.5. h -x, 1 - y, -z.
Figure 4. Emission spectra of 1 and FIrpic under the same conditions.
(λmax > 350 nm) are metal-to-ligand charge-transfer (MLCT)
transitions. The next long tail absorption in the region of
420-450 nm can reasonably be assigned to a mixed state
3
involving both spin-orbit coupling enhanced π-π* and
3a
1
3MLCT transitions. The blue-shifted MLCT band (λmax
)
ca. 360 nm, ε ) 8200 M-1 cm-1) for complex 1 compared to
that of FIrpic (λmax ) ca. 375 nm, ε ) 7500 M-1 cm-1) was
observed under the same experimental conditions. The intensi-
ties of 1MLCT and 3MLCT in 1 are stronger than those of the
reference (FIrpic). This observation implies the presence of
remarkable singlet-triplet coupling due to spin-orbit coupling
in 1. This result can be attributed to the introduction of the
electron-withdrawing pyridine ring, as well as the fluorine
substituents. Therefore, fluorine substituents and the replacement
of phenyl with pyridine not only cause a spectral blue-shift for
1
the π-π* and MLCT absorptions but also increase the
transition moment.
As shown in Figure 4, the emission spectrum of 1 is well
structured, while the spectrum in FIrpic is less structured
and broader than that of 1 (full-width at half-maximum
(fwhm): 63nm for 1, 56 nm for FIrpic). The emission
maximum of 1 appears at 438 nm, and an additional intense
peak appears at 463 nm. The intensity of each wavelength
is almost the same, but the intensity at 463 nm is slightly
higher than at 438 nm. In general, the emission band from
MLCT states are broad and featureless, while the highly
structured emission band mainly originates from the 3π-π*
state.19 Accordingly, the emission observed in 1 is attribut-
able to the 3π-π* with contribution of the MLCT transition.
The Commission Internationale de L’Eclairage (CIE) coor-
dinates x and y for 1 were calculated to be 0.14 and 0.12,
respectively, which is a high color purity and a purer blue
than in FIrpic (CIE x ) 0.18, y ) 0.34).18 The photolumi-
nescence (PL) quantum yield in degassed CH2Cl2 solution
at room temperature was determined to be 0.71((10%) by
using quinine sulfate in 0.1 N H2SO4 as the standard (ΦPL
) 0.54). This value is slightly higher than that of FIrpic and
comparable to that of Ir(dfppy)3 (dfppy ) 4,6-difluorophe-
nylpyridine, ΦPL ) 0.77). The rigidity of the molecular
structure has an influence on the fluorescence quantum
Figure 3. UV/vis absorption spectra of 1 and FIrpic at room temperature
under the same conditions (concentration, 2.5 × 10-5 M; solvent, CH2Cl2).
Inset: MLCT band of 1 in UV/vis.
in Table 2. The thermal stability of 1 may originate from
these intermolecular interactions.
To examine the photophysical properties of complex 1
synthesized by using ligand L1, UV/vis and photoluminescent
spectra were measured in a dilute CH2Cl2 solution. As expected,
when being irradiated by UV light, complex 1 showed a bright
blue emission. Therefore, the absorption and emission spectra
of 1 were compared with those of FIrpic (bis(4,6-difluorophe-
nylpyridinato-N,C2)picolinatoiridium(III)) as a reference, which
has well-known blue phosphorescence.18 Figure 3 shows the
UV-visible absorption spectrum for 1 in CH2Cl2 at room
temperature. The strong absorption band observed at 230-300
nm closely resembles the spectrum of the free ligand (see
Supporting Information), which is assigned to a ligand-centered
1π-π* transition. The broad absorption bands at lower energy
(18) The CIE coordinates x and y was measured to be 0.18 and 0.22, based
on our preliminary investigation on OLEDs performance using 1 as
emitting material. (a) Adachi, C.; Kwong, R. C.; Djurovich, P.;
Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl.
Phys. Lett. 2001, 79, 2082. (b) Mulder, C. L.; Celebi, K.; Milaninia,
K. M.; Baldo, M. A. Appl. Phys. Lett. 2007, 90, 211109.
(19) Hong, H.-W.; Chen, T.-M. Mater. Chem. Phys. 2007, 101, 170–176.
1034 Inorganic Chemistry, Vol. 48, No. 3, 2009

Blue Phosphorescent Iridium(III) Complex
contribution (ca. 35∼40%) from a π orbital localized on the
fluorine-substituted pyridine ring. However, the LUMO is
largely dominated by the π* orbitals of the pyridine rings
with small contributions of the π* orbitals of the fluorine-
substituted pyridine ring. To investigate the effect of the
fluorine substituents, the comparison of energy levels of both
Ir(ppy)3 and 1 was conducted in Figure 7. For 1, the energies
of the HOMO and LUMO are lowered by about 1.60 and
1.04 eV, respectively, as compared to those of Ir(ppy)3. The
decrease of the HOMO level is a little deeper, indicating
that the fluorine substituents and the replacement of pyridine
have significant influence on the HOMO energy levels. Since
the lowering of the HOMO in general induces a narrowing
energy barrier between the dopant and the host or electron-
transporting (ET) materials in OLEDs, this observation
(HOMO lowered) makes complex 1 suitable for the easy
transfer of electrons from the host or ET materials. The
donation of d oritals of the Ir atom for 1 is significantly
higher than in Ir(ppy)3 or even FIrpic. The calculated electron
populations of the three HOMOs for 1 are from 58% to 60%,
while the d orbital populations of both Ir(ppy)3 and FIrpic
are below 50% based on the results of previous reports.9,21
This implies that complex 1 shows strong spin-orbit mixing
between the π and d orbitals of the Ir atom. It is noteworthy
that significant contributions of the Ir metal (d orbital) in
the LUMO level were not observed. The energy gap between
the HOMO (-6.39 eV) and LUMO (-2.26 eV) is 4.13 eV.
As shown in Table 3, complex 1 exhibits a larger energy
gap relative to other Ir(III) derivatives (energy gap: 3.66 eV
for Ir(ppy)3, 3.75 eV for FIrpic, respectively).22 The excita-
tion energies for the low-lying singlet and triplet states of 1
were obtained from time-dependent DFT (TDDFT) calcula-
tions. Excitation energies for a singlet (S1) and a triplet (T1)
state are 3.33 eV (372 nm) and 2.97 eV (417 nm),
Figure 5. Cyclic voltammograms of 1 in 0.1 M n-Bu4NPF6 at a scan rate
of 100 mV/s.
yields.20 The replacement of C-H bonds with C-F bonds
and the pyridine moiety introduced into the ligand bring
about a lower vibrational frequency through the strong
intermolecular interaction, leading to a reduced nonradiative
decay rate and improved PL efficiency, as supported by the
crystal structure.
To investigate the electronic effects caused by fluorine
substituents on the bipyridine rings, cyclic voltammetry
experiments including FIrpic were carried out (see Figure 5
and Supporting Information). Due to the limited range of
CH2Cl2 and the limitations of our instrument to measure the
reduction potentials in the range of -2.7∼-3.5 V, we
obtained only reliable oxidation potentials. The reversible
oxidation wave is observed for 1 at 1.78 V (Epa). It is
noteworthy that the complex shows higher oxidation poten-
tials than that of FIrpic (Epa ) 1.45 V), indicating that a
relative ease of oxidation does not exist. This can be ascribed
to the greater electron-withdrawing ability of the fluorine
atoms and pyridine ring. This observation implies that the
HOMO energy level has been lowered by the introduction
of the fluorine atoms into the 2′,4′-position of the 2,3′-
bipyridine ligand. Furthermore, the replacement of the
picolate ligand in FIrpic with difluorobipyridine resulted in
an additional lowered HOMO level.
1
respectively. Experimentally, we observed that the MLCT
and 3MLCT bands of 1 in the UV/vis spectrum were at 361
and 428 nm, respectively. Therefore, these results are in
agreement with experimental values despite some deviations.
The small deviation of the current theoretical approach from
the experimental results is probably due to experimental
difficulties in determining the absorption peak frequency.22
According to these results, both calculated S1 and T1 excited
states could be assigned as MLCT states based on the strong
5d component of the occupied orbitals and the mostly ligand
π*(py) virtual orbitals. These DFT calculations provide
evidence that the luminescence observed in complex 1
originates from the Ir-perturbed ligand-centered transitions.
In summary, a new type of fac-Ir(III) complex containing
fluorinated bipyridine as a ligand has been synthesized to
investigation the effects of the substituents on the nature of
its solid state, photoluminescence, and electrochemical
behaviors. In addition, to understand the origin of the
luminescence, DFT calculations were also conducted. The
To understand the nature of the luminescence exhibited
by 1, DFT calculations were conducted employing the
Gaussian-98 package. The initial geometric parameters in
the calculations were employed from crystal structure data
for geometry optimization. The experimental values of bond
lengths, which were obtained from the X-ray structure, are
comparable to the calculated values. The contour plots of
the three lowest unoccupied (LUMO, LUMO + 1, and
LUMO + 2) and the three highest occupied molecular
orbitals (HOMO, HOMO - 1, and HOMO - 2) for 1 mainly
involved in the transitions are presented in Figure 6. The
calculated energy levels of the lowest singlet and triplet states
are depicted in Table 3. As shown Figure 6, the HOMO of
2
1 involves the contribution of Ir dz orbital (60%) with the
(21) Park, N. G.; Choi, G. C.; Lee, Y. H.; Kim, Y. S. Curr. Appl. Phys.
2006, 6, 620–626.
(22) Tung, Y.-L.; Chen, L.-S.; Chi, Y.; Chou, P.-T.; Cheng, Y.-M.; Li,
E. Y.; Lee, G.-H.; Shu, C.-F.; Wu, F.-I.; Carty, A. J. AdV. Funct. Mater.
2006, 16, 1615–1626.
(20) (a) Morrison, D. J.; Trefz, T. K.; Piers, W. E.; McDonald, R.; Parvez,
M. J. Org. Chem. 2005, 70, 5309–5312. (b) Endo, A.; Suzuki, K.;
Yoshihara, T.; Tobita, S.; Yahiro, M.; Adachi, C. Chem. Phys. Lett.
2008, 460, 155–157.
Inorganic Chemistry, Vol. 48, No. 3, 2009 1035

Lee et al.
Figure 6. Diagrams showing the surfaces and energies of selected frontier orbitals for 1.
Table 3. Calculated Energy Levels of the Low-Lying Singlet and
Triplet States
spin state
S1
(nm)
assignment (%)
f
372 (3.33 eV)
369 (3.36 eV)
369 (3.36 eV)
HOMO f LUMO (96%)
HOMO f LUMO + 1 (95%)
HOMO f LUMO + 2 (95%)
HOMO-1 f LUMO (4%)
HOMO-1 f LUMO + 2 (45%) 0.0042
HOMO-2 f LUMO (42%)
HOMO-5 f LUMO (5%)
0.0043
0.0015
0.0015
S2
S3
347 (3.58 eV)
HOMO-2 f LUMO + 1 (10%)
eV) HOMO f LUMO + 1 (12%)
HOMO-4 f LUMO + 1 (15%)
HOMO f LUMO (38%)
417 (2.97
HOMO f LUMO (5%)
HOMO-3 f LUMO + 2 (7%)
HOMO-1 f LUMO + 2 (7%)
HOMO-4 f LUMO (10%)
HOMO f LUMO+2 (20%)
HOMO f LUMO+1 (25%)
HOMO-3 f LUMO (9%)
T1
417 (2.97 eV)
417 (2.97 eV)
HOMO-3 f LUMO+1 (4%)
HOMO-3 f LUMO+2 (7%)
HOMO f LUMO (9%)
HOMO f LUMO+1 (12%)
HOMO f LUMO (28%)
Figure 7. Schematic drawing of orbital energies of highest occupied and lowest
unoccupied MOs of Ir(dfpypy)3 from B3LYP calculations. The orbital energies
of Ir(ppy)3 were cited from ref 9 for the purpose of comparison. HOMO, HOMO
- 1, and HOMO - 2 were denoted d2, d1a, and d1b. LUMO, LUMO + 1, and
LUMO + 2 were denoted π1*, π2a, and π2b*. (1 au ) 27.212 eV)
Ir(III) complex provided bright blue phosphorescence with
high color purity (x ) 0.14, y ) 0.12), which is most likely
caused by MLCT transitions. The replacement of the phenyl
ring in ppy with pyridine and the introduction of fluorine
substituents into the pyridine ring significantly increased the
thermal stability and molecular rigidity due to the increased
intermolecular interactions in the solid state. Based on DFT
calculations, the substituents have significant effect on
lowering the HOMO and LUMO energy levels, where the
diminution of the HOMO energy is much larger than in the
LUMO. It suggests that the complex 1 has disadvantages
for hole injection. However, the lower LUMO level makes
this complex suitable for the easy transfer of electrons from
the electron-transporting layer. Therefore, further studies of
1036 Inorganic Chemistry, Vol. 48, No. 3, 2009

Blue Phosphorescent Iridium(III) Complex
Supporting Information Available: X-ray crystallographic data,
spectroscopic data (NMR, mass, etc.), and UV/vis and PL spectra.
X-ray crystallographic files are available free of charge via the
Internet at http://pubs.acs.org.
this complex may lead to a new class of emitting materials
for phosphorescence OLEDs.
Acknowledgment. This study was supported by a 2007
Research Grant from Kangwon National University.
IC801643P
Inorganic Chemistry, Vol. 48, No. 3, 2009 1037