Phosphorescent platinum(II) complexes derived from multifunctional chromophores: Synthesis, structures, photophysics, and electroluminescence

Article abstract of DOI:10.1021/ic061566c

The synthesis and structural, photophysical, electrochemical, and electroluminescent properties of a novel class of trifunctional Pt(II) cyclometalated complexes are reported in which the hole-transporting triarylamine, electron-transporting oxadiazole, and electroluminescent metal components are integrated into a single molecule. These neutral metal chelates display good thermal stability (>250°C under N₂) and morphological stability. All of them exhibit intense ligand-centered fluorescence and phosphorescence in fluid solutions at room temperature, but the emission spectra become largely dominated by triplet emission bands in CH ₂Cl₂ glass at 77 K. Substituents with different electronic properties were introduced into the bipolar cyclometalating ligands to fine-tune the absorption and emissive characteristics of the compounds, and the results were correlated with theoretical calculations using density functional theory. A comparison of the photophysics and electrochemistry of our multifunctional systems to those only derived from each of the constituent components was also made and discussed. These Pt complexes can be vacuum-sublimed and applied as emissive dopants for the fabrication of vapor-deposited electrophosphorescent organic light-emitting devices (OLEDs), which generally exhibit good device performance with efficiencies up to 3.6%, 11.0 cd A^-1, and 5.8 Im W^-1. While the electroluminescence energy resembles that recorded in fluid solutions for these Pt emitters, these monochromatic OLEDs can emit tunable colors by varying the aryl ring substituents and the level of doping. Saliently, single dopant white-light electroluminescence, triggered by the simultaneous fluorescence/phosphorescence emission of the metal complexes and a variation of applied driving voltages, has also been realized based on some of these multifunctional complexes with peak electrophosphorescence efficiencies of 6.8 cd A^-1 and 2.6%.

Full text of DOI:10.1021/ic061566c

Inorg. Chem. 2006, 45, 10922−10937
Phosphorescent Platinum(II) Complexes Derived from Multifunctional
Chromophores: Synthesis, Structures, Photophysics, and
Electroluminescence
Ze He,† Wai-Yeung Wong,*,† Xiaoming Yu,‡ Hoi-Sing Kwok,‡ and Zhenyang Lin§
Department of Chemistry and Centre for AdVanced Luminescence Materials, Hong Kong Baptist
UniVersity, Waterloo Road, Kowloon Tong, Hong Kong, People’s Republic of China, Departments
of Electronic and Electrical Engineering and Centre for Display Research and of Chemistry, Hong
Kong UniVersity of Science and Technology, Clearwater Bay, Hong Kong, People’s Republic of
China
Received August 18, 2006
The synthesis and structural, photophysical, electrochemical, and electroluminescent properties of a novel class of
trifunctional Pt(II) cyclometalated complexes are reported in which the hole-transporting triarylamine, electron-
transporting oxadiazole, and electroluminescent metal components are integrated into a single molecule. These
neutral metal chelates display good thermal stability (>250 °C under N2) and morphological stability. All of them
exhibit intense ligand-centered fluorescence and phosphorescence in fluid solutions at room temperature, but the
emission spectra become largely dominated by triplet emission bands in CH2Cl2 glass at 77 K. Substituents with
different electronic properties were introduced into the bipolar cyclometalating ligands to fine-tune the absorption
and emissive characteristics of the compounds, and the results were correlated with theoretical calculations using
density functional theory. A comparison of the photophysics and electrochemistry of our multifunctional systems to
those only derived from each of the constituent components was also made and discussed. These Pt complexes
can be vacuum-sublimed and applied as emissive dopants for the fabrication of vapor-deposited electrophosphorescent
organic light-emitting devices (OLEDs), which generally exhibit good device performance with efficiencies up to
3.6%, 11.0 cd A-1, and 5.8 lm W-1. While the electroluminescence energy resembles that recorded in fluid solutions
for these Pt emitters, these monochromatic OLEDs can emit tunable colors by varying the aryl ring substituents
and the level of doping. Saliently, single dopant white-light electroluminescence, triggered by the simultaneous
fluorescence/phosphorescence emission of the metal complexes and a variation of applied driving voltages, has
also been realized based on some of these multifunctional complexes with peak electrophosphorescence efficiencies
1
of 6.8 cd A- and 2.6%.
Introduction
market share in next-generation flat-panel display technol-
ogy.2 A successful OLED requires facile and balanced charge
transport as well as a high conversion efficiency of excitons
to light.3 To fulfill these requirements, OLEDs often contain
a number of layers that individually perform the roles of
Since the pioneering work by Tang et al. on a multilayer
device configuration for small-molecule organic light-emit-
ting diodes (OLEDs),1 this research field has stimulated
tremendous impetus to develop OLEDs using organic or
metal-organic compounds as emitters because of its huge
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* To whom correspondence should be addressed. E-mail: rwywong@
hkbu.edu.hk.
† Hong Kong Baptist University.
‡ Department of Electronic and Electrical Engineering and Centre for
Display Research, Hong Kong University of Science and Technology.
§ Department of Chemistry, Hong Kong University of Science and
Technology.
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10922 Inorganic Chemistry, Vol. 45, No. 26, 2006
10.1021/ic061566c CCC: $33.50
© 2006 American Chemical Society
Published on Web 12/07/2006

Phosphorescent Platinum(II) Complexes
charge transport and light emission.4 Each layer requires
materials with the correct highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
energies to either promote or disrupt the flow of charge in
the device.5 The sophisticated multilayer configuration
consisting of the hole-transporting (HT), the electron-
transporting (ET), and the electroluminescent (EL) layers is
frequently used to achieve a balanced injection and transport
of holes and electrons. A current challenge for high-efficiency
OLEDs is focused on the optimization of device structures6
and the use of electrophosphorescent OLEDs.7 Phosphores-
cent small molecules are promising for achieving highly
efficient OLEDs by generation of light emission from both
singlet and triplet excitons.8 Such an impressive approach
is to efficiently harvest triplet excitons through incorporation
of heavy-metal centers, which would increase spin-orbit
coupling to facilitate intersystem crossing into the triplet state.
The scope and diversity of studies on metal-organic
phosphors in the realm of optoelectronics have continued to
expand. Apart from the emission efficiency issue, an
exploration of possible methods to bring about a variation
in the emission color would also be important. Most of the
common approaches involve the use of different light-
emitting materials or multicomponent blended mixtures of
light emitters with different emission characteristics for color
tuning.9 Remarkably, recent reports have shown that varying
emission colors of OLEDs could be generated using metal
phosphors as dopants or nondoping emitters through a change
in the dopant concentration or a variation of bias voltages.10
Although there is a large body of literature information
about electrophosphorescent heavy-metal complexes, utiliza-
tion of their derivatives playing multifunctional roles for
OLEDs was not well elucidated and remains to be largely
studied.11 To our knowledge, only a few trifunctional
polymers that rival those from multilayer devices in perfor-
mance have been prepared.12 To advance the development
of novel metal-based phosphors to fill this gap, we take
advantage of the characteristic of Pt d8 groups coupled with
the bipolar character of new charge-balance ligand systems
featuring both HT and ET modules. These new neutral
complexes can be used not only as emitters but also as
multifunctional chromophores for improved charge trans-
portation. Metal-based phosphors composed of a hole-
conducting block and an ET unit can combine the properties
of both functional components to produce new properties
that are hardly possible with either one separately.
Herein, we describe our endeavors to design some elec-
troactive materials composed of simple modular units, each
of which performs a specific functional role, namely, electron
transfer, hole transfer, and light emission. Furthermore, we
chose chromophoric units that may be easily derivatized,
thereby giving greater flexibility to this design strategy. This
class of Pt complexes is stable with respect to sublimation
and thus is suitable for vacuum deposition in small-molecule
OLED fabrication. On the other hand, white OLEDs
(WOLEDs) have also drawn much attention in the academic
and industrial R&D sectors because of their potential use in
display backlights, in full color applications, and in solid-
state lighting purposes.13 Interestingly, these new trifunctional
Pt(II) cyclometalated complexes, which are capable of
emitting strong blue and orange colors in both steady-state
emission and electroluminescence, can be exploited in the
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Inorganic Chemistry, Vol. 45, No. 26, 2006 10923

He et al.
110 °C for 50 h. Upon cooling to room temperature, the solvent
was removed by rotary evaporation and the residue was purified
by column chromatography using CH2Cl2 as the eluent to afford
realization of WOLEDs. We also report in this contribution
the achievement of well-balanced white emission from an
emissive layer with only a single luminescent dopant, which
emits simultaneously from singlet and triplet excited states.
1
the title compound as a yellow powder (0.28 g, 61%). H NMR
(CDCl3): δ 8.74-8.72 (m, 1H, Ar), 8.23-8.15 (m, 4H, Ar), 7.97-
7.93 (m, 2H, Ar), 7.82-7.80 (m, 2H, Ar), 7.32-7.27 (m, 1H, Ar),
7.16-7.11 (m, 4H, Ar), 7.07-7.00 (m, 6H, Ar). 13C NMR
(CDCl3): δ 164.58, 163.81, 160.93, 158.49, 155.97, 150.93, 149.88,
142.46, 142.43, 142.11, 136.94, 128.15, 127.46, 127.42, 127.38,
127.15, 124.27, 122.84, 120.77, 119.87, 116.70, 116.48, 115.80
(Ar). FABMS: m/z 503 (M+). Anal. Calcd for C31H20N4F2O: C,
74.09; H, 4.01; N, 11.15. Found: C, 73,98; H, 4.08; N, 11.02.
6-H. Yield: yellow powder, 48% [CH2Cl2/ethyl acetate (5:1,
Experimental Section
General Comments. All reactions were carried out under a N2
atmosphere with the use of standard Schlenk techniques, but no
special precautions were taken to exclude O2 during the workup.
Solvents were predried and distilled from appropriate drying agents.
All chemicals, unless otherwise stated, were obtained from com-
mercial sources and used as received. Preparative thin layer
chromatography (TLC) was performed on 0.7-mm silica plates
1
v/v)]. H NMR (CDCl3): δ 8.75-8.72 (m, 1H, Ar), 8.24-8.15
(Merck Kieselgel 60 GF254) prepared in our laboratory. 1H and 13
C
(m, 4H, Ar), 7.99-7.94 (m, 2H, Ar), 7.82-7.79 (m, 2H, Ar), 7.36-
7.27 (m, 5H, Ar), 7.19-7.09 (m, 8H, Ar). 13C NMR (CDCl3): δ
164.54, 163.64, 155.87, 150.85, 149.76, 146.49, 141.98, 136.82,
129.49, 127.95, 127.34, 127.07, 125.59, 124.31, 124.26, 122.73,
120.97, 120.69, 115.88 (Ar). FAB-MS: m/z 467 (M+). Anal. Calcd
for C31H22N4O: C, 79.81; H, 4.75; N, 12.01. Found: C, 79.87; H,
4.56; N, 11.89.
6-Me. Yield: yellow solid, 85% [CH2Cl2/ethyl acetate (5:1, v/v)].
1H NMR (CDCl3): δ 8.70-8.69 (m, 1H, Ar), 8.20-8.11 (m, 4H,
Ar), 7.91-7.88 (m, 2H, Ar), 7.76-7.73 (m, 2H, Ar), 7.26-7.21
(m, 1H, Ar), 7.13-7.01 (m, 10H, Ar), 2.33 (s, 6H, CH3). 13C NMR
(CDCl3): δ 164.48, 163.32, 155.62, 150.98, 149.56, 143.72, 141.70,
136.65, 133.98, 129.99, 127.72, 127.14, 126.84, 125.62, 124.14,
122.57, 120.48, 119.50, 114.72 (Ar), 20.89 (CH3). FABMS: m/z
495 (M+). Anal. Calcd for C33H26N4O: C, 80.14; H, 5.30; N, 11.33.
Found: C, 80.20; H, 5.15; N, 11.16.
NMR spectra were measured in appropriate solvents on a JEOL
EX270 or a Varian INOVA 400-MHz Fourier transform NMR
spectrometer, with the chemical shifts quoted relative to SiMe4.
Fast atom bombardment mass spectrometry (FABMS) spectra were
recorded on a Finnigan MAT SSQ710 mass spectrometer. Electronic
absorption spectra were obtained with a Hewlett-Packard 8453
UV-visible spectrometer. For solid-state thin-film emission mea-
surement, the 325-nm line of a He-Cd laser was used as an
excitation source. The luminescence spectra were analyzed by a
0.25-m-focal-length double monochromator with a Peltier cooled
photomultiplier tube and processed with a lock-in amplifier. The
solution emission spectra and lifetimes were measured on a Photon
Technology International Fluorescence Master Series QM1 spec-
trophotometer using an Xe lamp and a N2 laser as the excitation
source, respectively. The fluorescence quantum yields were deter-
mined in CH2Cl2 solutions at 298 K against the anthracene standard
(Φ ) 0.27).14 Thermal analyses were performed with the Perkin-
6-OMe. Yield: yellow solid, 67% [CH2Cl2/ethyl acetate (10:1,
1
v/v)]. H NMR (CDCl3): δ 8.74-8.72 (m, 1H, Ar), 8.23-8.14
Elmer TGA6 thermal analyzer at a heating rate of 20 °C min-1
.
(m, 4H, Ar), 7.91-7.88 (m, 2H, Ar), 7.81-7.79 (m, 2H, Ar), 7.31-
7.26 (m, 1H, Ar), 7.15-7.12 (m, 4H, Ar), 6.96-6.87 (m, 6H, Ar),
3.82 (s, 6H, OCH3). 13C NMR (CDCl3): δ 164.76, 163.42, 156.71,
155.92, 151.58, 149.74, 141.87, 139.32, 136.79, 127.90, 127.53,
127.30, 127.01, 124.38, 122.69, 120.66, 118.06, 114.89, 114.05
(Ar), 55.52 (OCH3). FABMS: m/z 527 (M+). Anal. Calcd for
C33H26N4O3: C, 75.27; H, 4.98; N, 10.64. Found: C, 75.11; H,
5.08; N, 10.44.
7-F. A mixture of 6-F (0.10 g, 0.20 mmol) and K2PtCl4 (0.04 g,
0.10 mmol) was stirred in 2-ethoxyethanol (6 mL) and water
(2 mL), and the suspension was heated to 80 °C for 16 h. After
cooling to room temperature, it was added to water (20 mL), and
the precipitate collected was washed with water (20 mL × 4) and
Theoretical calculations based on the density functional theory
(DFT) approach at the B3LYP level were performed with the use
of the Gaussian03 program.15 Experimental structures, whenever
available, were used in the calculations. The geometry of 6-OMe
was derived from the X-ray structure of 7-OMe, whereas that of
10 was derived from the X-ray structure of 11. The basis set used
for C, N, O, and H atoms was 6-31G, while effective core potentials
with a LanL2DZ basis set were employed for the Pt atom.16
Polarization functions were added for the P atom [úd(P) ) 0.34].
All of the molecular orbital plots were made with the use of Molden,
version 3.5.17 The syntheses of 1-R-5-R (R ) F, H, Me, OMe)
are shown in the Supporting Information.
6-F. A mixture of 2-(tri-n-butylstannyl)pyridine (0.53 g, 1.43
mmol), Pd(PPh3)4 (0.10 g, 0.09 mmol), and 5-F (0.46 g, 0.92 mmol)
was stirred in toluene (20 mL), and the mixture was heated to
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N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
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Heeger, A. J. Solid State Commun. 1998, 107, 673-679. (k) Shih,
P.-I.; Shu, C.-F.; Tung, Y.-L.; Chi, Y. Appl. Phys. Lett. 2006, 88,
251110-1-251110-3.
(16) (a) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (b)
Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(17) Schaftenaar, G. Molden, version 3.5; CAOS/CAMM Center Nijmegen,
Toernooiveld: Nijmegen, The Netherlands, 1999.
(14) Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251-
3260.
10924 Inorganic Chemistry, Vol. 45, No. 26, 2006

Phosphorescent Platinum(II) Complexes
vacuum dried. The precipitate then reacted with acetylacetone
(Hacac; 0.03 mL, 0.30 mmol) and Na2CO3 (0.11 g, 1.00 mmol) in
2-ethoxyethanol (8 mL) at 100 °C for 16 h. The solvent was
removed under reduced pressure, and the residue was purified by
preparative TLC plates on silica using CH2Cl2/ethyl acetate (10:1,
v/v) as the eluent to afford a yellow product with an isolated yield
(Ar). FABMS: m/z 322 (M+). Anal. Calcd for C23H18N2: C, 85.68;
H, 5.63; N, 8.69. Found: C, 85.70; H, 5.48; N, 8.55.
9. A mixture of 8 (0.20 g, 0.63 mmol) and K2PtCl4 (0.10 g,
0.25 mmol) in 2-ethoxyethanol (6 mL) and water (2 mL) was heated
to 80 °C for 16 h. After cooling to room temperature, it was added
to water (20 mL) and the precipitate was washed with water
(20 mL × 4) and dried under vacuum. Reaction of the precipitate
with Hacac (0.036 g, 0.36 mmol), in the presence of Na2CO3 (0.13
g, 1.20 mmol), in 2-ethoxyethanol (6 mL) at 100 °C for 20 h gave
a residue that was purified by preparative TLC plates on silica,
eluting with CH2Cl2 to afford the product as a yellow solid in 24%
yield (0.037 g). 1H NMR (CDCl3): δ 8.88-8.87 (m, 1H, Ar), 7.73-
7.69 (m, 1H, Ar), 7.47-7.45 (d, 1H, Ar), 7.29-7.24 (m, 6H, Ar),
7.22-7.17 (m, 4H, Ar), 7.06-6.97 (m, 3H, Ar), 6.78-6.75 (m,
1H, Ar), 5.30 (s, 1H, CH), 1.96 (s, 3H, CH3), 1.72 (s, 3H, CH3).
13C NMR (CDCl3): δ 185.61, 184.01, 167.76, 148.34, 147.45,
147.09, 139.80, 137.94, 137.84, 129.08, 125.41, 123.90, 123.13,
122.95, 119.79, 117.64, 117.48 (Ar), 102.29 (CH), 28.24, 26.82
(CH3). FABMS: m/z 616 (M+). Anal. Calcd for C28H24N2O2Pt:
C, 54.63; H, 3.93; N, 4.55. Found: C, 54.34; H, 3.61; N, 4.27.
10. A mixture of 2-(tri-n-butylstannyl)pyridine (0.75 g, 2.50
mmol), Pd(PPh3)4 (0.09 g, 0.08 mmol), and (2-phenyl)-5-[1-(2-
bromo)-4-phenyl]-1,3,4-oxadiazole19 (0.92 g, 2.50 mmol) in toluene
(20 mL) was heated to 110 °C for 48 h. Upon cooling to room
temperature, the solvent was removed by rotary evaporation and
the residue was purified by column chromatography using CH2-
Cl2/hexane (6:1, v/v) as the eluent to afford a yellow powder of 10
(0.66 g, 88%). 1H NMR (CDCl3): δ 8.75-8.73 (d, 1H, Ar), 8.27-
8.15 (m, 6H, Ar), 7.82-7.80 (d, 2H, Ar), 7.56-7.52 (m, 3H, Ar),
7.32-7.26 (m, 1H, Ar). 13C NMR (CDCl3): δ 164.48, 164.20,
155.76, 149.75, 142.20, 136.82, 131.64, 128.96, 127.35, 127.19,
126.84, 124.00, 123.75, 122.79, 120.71 (Ar). FABMS: m/z 300
(M+). Anal. Calcd for C19H13N3O: C, 76.24; H, 4.38; N, 14.04.
Found: C, 76.10; H, 4.12; N, 13.86.
11: A mixture of 10 (0.09 g, 0.24 mmol) and K2PtCl4
(0.05 mg, 0.12 mmol) in 2-ethoxyethanol (6 mL) and water (2 mL)
was allowed to react at 80 °C for 16 h. After cooling to room
temperature, it was added to water (20 mL), and the precipitate
obtained was washed with water (20 mL × 4) and dried over
anhydrous MgSO4. Then, the precipitate reacted with Hacac
(0.03 g, 0.30 mmol) and Na2CO3 (0.106 g, 1.00 mmol) in
2-ethoxyethanol (8 mL) at 100 °C for 20 h. The solvent was
removed under reduced pressure, and purification of the residue
was achieved by preparative TLC plates on silica using CH2Cl2 as
the eluent to afford the desired product as a yellow solid in 90%
yield (0.06 g). 1H NMR (CDCl3): δ 9.06-9.04 (d, 1H, Ar), 8.34-
8.33 (d, 1H, Ar), 8.17-8.14 (m, 2H, Ar), 7.94-7.84 (m, 2H, Ar),
7.71-7.68 (d, 1H, Ar), 7.58-7.53 (m, 4H, Ar), 7.22-7.18 (m, 1H,
Ar), 5.53 (s, 1H, CH), 2.11 (s, 3H, CH3), 2.04 (s, 3H, CH3). 13C
NMR (CDCl3): δ 185.74, 184.23, 166.85, 164.90, 164.31, 147.45,
139.32, 138.26, 131.43, 128.94, 128.37, 127.41, 126.81, 124.14,
123.38, 123.03, 122.44, 122.07, 119.08 (Ar), 102.60 (CH), 28.30,
27.31 (CH3). FABMS: m/z 593 (M+). Anal. Calcd for C24H19N3O3-
Pt: C, 48.65; H, 3.23; N, 7.09. Found: C, 48.43; H, 3.01; N, 6.78.
Electrochemical Measurements. Electrochemical measurements
were made using a BAS CV-50W model potentiostat. A conven-
tional three-electrode configuration consisting of a Pt working
electrode, a Pt-wire counter electrode, and an Ag/AgCl reference
electrode was used. The solvent in all measurements was CH2Cl2,
and the supporting electrolyte was 0.1 M [Bu4N]PF6. Ferrocene
was added as a calibrant after each set of measurements, and all
1
of 50% (0.04 g, 0.05 mmol). H NMR (CDCl3): δ 9.02-9.00 (d,
1H, Ar), 8.25-8.25 (d, 1H, Ar), 7.95-7.92 (m, 2H, Ar), 7.88-
7.81 (m, 2H, Ar), 7.66-7.64 (d, 1H, Ar), 7.53-7.50 (d, 1H, Ar),
7.18-7.11 (m, 5H, Ar), 7.07-7.00 (m, 6H, Ar), 5.49 (s, 1H, CH),
2.06 (s, 3H, CH3), 2.01 (s, 3H, CH3). 13C NMR (CDCl3): δ 185.84,
184.26, 166.92, 164.48, 164.35, 160.87, 158.43, 150.74, 147.81,
147.48, 142.57, 142.55, 139.39, 138.30, 128.15, 128.04, 127.41,
127.33, 123.52, 123.09, 122.33, 122.06, 120.03, 119.10, 116.68,
116.46, 116.25 (Ar), 102.61 (CH), 28.21, 27.24 (CH3). FABMS:
m/z 796 (M+). Anal. Calcd for C36H26N4F2O3Pt: C, 54.34; H, 3.29;
N, 7.04. Found: C, 54.10; H, 3.53; N, 6.87.
7-H. Yield: yellow solid, 58% (eluent: CH2Cl2). 1H NMR
(CDCl3): δ 9.03-9.01 (d, 1H, Ar), 8.27-8.26 (d, 1H, Ar), 7.97-
7.80 (m, 4H, Ar), 7.67-7.64 (d, 1H, Ar), 7.54-7.51 (d, 1H, Ar),
7.36-7.26 (m, 4H, Ar), 7.19-7.10 (m, 9H, Ar), 5.50 (s, 1H, CH),
2.06-2.02 (d, 6H, CH3). 13C NMR (CDCl3): δ 185.64, 184.12,
166.81, 164.30, 150.65, 147.64, 147.37, 146.60, 139.28, 138.19,
129.48, 128.09, 127.85, 126.33, 125.54, 124.18, 123.51, 123.01,
122.27, 121.96, 121.16, 119.01, 116.36 (Ar), 102.56 (CH), 28.29,
27.35 (CH3). FABMS: m/z 760 (M+). Anal. Calcd for C36H28N4O3-
Pt: C, 56.91; H, 3.71; N, 7.37. Found: C, 56.76; H, 3.55; N, 7.16.
1
7-Me. Yield: yellow solid, 36% (eluent: CH2Cl2). H NMR
(CDCl3): δ 9.02-9.00 (d, 1H, Ar), 8.25-8.24 (d, 1H, Ar), 7.92-
7.80 (m, 4H, Ar), 7.66-7.63 (d, 1H, Ar), 7.53-7.50 (d, 1H, Ar),
7.15-7.03 (m, 11H, Ar), 5.49 (s, 1H, CH), 2.35 (s, 6H, CH3), 2.06-
2.01 (d, 6H, CH3). 13C NMR (CDCl3): δ 185.62, 184.12, 166.84,
164.44, 164.19, 150.97, 147.58, 147.35, 144.01, 139.27, 138.16,
134.01, 130.09, 128.05, 127.76, 125.72, 123.57, 122.99, 122.24,
121.92, 119.90, 118.99, 115.37 (Ar), 102.53 (CH), 28.28, 27.34,
21.00 (CH3). FABMS: m/z 788 (M+). Anal. Calcd for C38H32N4O3-
Pt: C, 57.94; H, 4.09; N, 7.11. Found: C, 57.68; H, 3.96; N, 7.05.
1
7-OMe. Yield: yellow solid, 44% (eluent: CH2Cl2). H NMR
(CDCl3): δ 9.05-9.03 (d, 1H, Ar), 8.27-8.26 (d, 1H, Ar), 7.91-
7.83 (m, 4H, Ar), 7.69-7.66 (d, 1H, Ar), 7.56-7.53 (d, 1H, Ar),
7.21-7.10 (m, 5H, Ar), 6.96-6.87 (m, 6H, Ar), 5.51 (s, 1H, CH),
3.82 (s, 6H, OCH3), 2.07-2.03 (d, 6H, CH3). 13C NMR (CDCl3):
δ 185.67, 184.19, 166.90, 164.54, 164.10, 156.62, 151.40, 147.53,
147.40, 139.45, 139.19, 138.22, 128.07, 127.81, 127.52, 123.69,
123.02, 122.29, 121.94, 119.00, 118.20, 114.86, 114.50 (Ar), 102.56
(CH), 55.53 (OCH3), 28.29, 27.36 (CH3). FABMS: m/z 820 (M+).
Anal. Calcd for C38H32N4O5Pt: C, 55.68; H, 3.93; N, 6.83. Found:
C, 55.50; H, 3.68; N, 6.55.
8. A mixture of 2-(tri-n-butylstannyl)pyridine (0.75 g, 2.04 mmol),
Pd(PPh3)4 (0.05 g, 0.08 mmol), and (4-iodophenyl)diphenylamine18
(0.50 g, 1.35 mmol) was combined in toluene (30 mL), and the
mixture was heated to 110 °C for 48 h. Upon cooling to room
temperature and removal of the solvent, the residue was purified
by column chromatography using CH2Cl2 as the eluent to afford a
yellow solid of 8 in 58% yield (0.25 g). 1H NMR (CDCl3): δ 8.65-
8.63 (m, 1H, Ar), 7.87-7.84 (m, 2H, Ar), 7.73-7.65 (m, 2H, Ar),
7.29-7.21 (m, 4H, Ar), 7.18-7.16 (m, 7H, Ar), 7.09-7.02 (m,
2H, Ar). 13C NMR (CDCl3): δ 157.02, 149.55, 148.62, 147.42,
136.62, 133.07, 129.27, 127.68, 124.67, 123.17, 121.43, 119.84
(18) Kato, S.-I.; Matsumoto, T.; Ishi-I, T.; Thiemann, T.; Shigeiwa, M.;
Gorohmaru, H.; Maeda, S.; Yamashita, Y.; Mataka, S. Chem. Commun.
2004, 2342-2343.
(19) Hou, S.; Chan, W. K. Macromolecules 2002, 35, 850-856.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10925

He et al.
Table 1. Summary of the Crystal Structure Data
6-F
7-F‚CH2Cl2
7-H‚H2O
7-Me‚CH2Cl2
7-OMe‚CH2Cl2
9
11
formula
C31H20N4F2O
C37H28N4Cl2-
F2O3Pt
C36H30N4O4Pt
C39H34N4Cl2-
O3Pt
C39H34N4Cl2-
O5Pt
C28H24N2O2Pt
C24H19N3O3Pt
Mr
cryst size
[mm]
cryst syst
space group
a (Å)
502.51
880.62
777.73
0.30 × 0.26
× 0.12
monoclinic
P21/c
19.827(9)
13.692(6)
11.835(6)
90
97.041(10)
90
3189(3)
4
872.69
904.69
615.58
592.51
0.28 × 0.20
× 0.10
orthorhombic
Pca21
24.5142(17)
4.9526(3)
34.174(2)
90
90
90
4149.0(5)
8
1.897
0.30 × 0.25
0.30 × 0.26
0.34 × 0.26
0.30 × 0.25
0.24 × 0.20
× 0.23
× 0.12
× 0.24
× 0.20
× 0.14
triclinic
P1h
triclinic
triclinic
P1h
triclinic
monoclinic
P21/c
P1h
P1h
9.3204(7)
11.9915(9)
12.1553(9)
73.676(1)
83.694(1)
77.613(1)
1271.7(2)
2
12.5305(7)
16.5828(9)
18.2259(10)
68.0830(10)
87.4360(10)
81.3220(10)
3473.0(3)
4
9.9455(6)
13.5126(8)
14.3504(8)
105.2070(10)
103.1460(10)
90.1550(10)
1808.3(2)
2
10.8475(4)
18.5133(7)
19.3000(8)
102.6460(10)
100.2840(10)
93.1570(10)
3703.2(2)
4
13.4384(7)
9.5829(5)
18.3541(10)
90
103.2660(10)
90
2300.5(2)
4
1.777
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
calcd (g cm-3
)
1.312
0.092
520
1.684
4.247
1728
1.620
4.446
1536
1.603
4.070
864
1.623
3.982
1792
µ (mm-1
F(000)
)
6.127
1200
6.796
2288
θ range (deg)
no. of reflns
collected
no. of unique
reflns
1.80-28.25
1.43-25.00
1.81-25.00
1.86-25.00
1.38-25.00
2.28-28.29
1.66-25.00
7611
17447
15455
9030
18499
13298
19085
5523
12046
5589
6239
12783
5345
7064
Rint
0.0142
3078
0.0414
7292
0.1077
2932
0.0154
5708
0.0179
10327
0.0361
3853
0.0625
5064
no. of reflns with
I > 2.0σ(I)
no. of params
R1 [I > 2.0σ(I)]a
wR2 (all data)a
GOF on F 2 b
343
883
407
442
914
298
559
0.0480
0.1586
0.996
0.0469
0.1131
0.953
0.0603
0.1651
0.925
0.0227
0.0590
1.031
0.0340
0.0969
1.012
0.0304
0.0647
1.039
0.0453
0.1105
0.990
2
2
2
a R1 ) ∑||Fo| - |Fc|| /∑|Fo|. wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
b GOF ) [(∑w|Fo| - |Fc|)2/(Nobs - Nparam)]1/2
.
potentials reported were quoted with reference to the ferrocene/
out by the modified Ullmann condensation with the corre-
sponding p-iodoarene under the CuCl/phen/KOH catalytic
system to give the substituted triarylamines 1-R.23 Formyl-
ation of 1-R with POCl3 in N,N-dimethylformamide affords
2-R, which can be derivatized to 3-R using a hydroxylamine
hydrochloride/acetic acid/pyridine mixture and then to 4-R
via the use of NaN3/NH4Cl. The key bipolar ligands 6-R in
this work can be obtained from 4-tetrazoyltriphenylamine
4-R6a by a two-step reaction. Compounds 4-R react with
4-bromobenzoyl chloride to give 5-R, which can then
undergo Stille coupling with 2-(tri-n-butylstannyl)pyridine
to afford 6-R, carrying a cyclometalating 2-phenylpyridine
site for coordination with the Pt(II) ion. Reaction of K2PtCl4
with 6 in a mixture of 2-ethoxyethanol and water results in
the dinuclear chloro-bridged Pt(II) complex, which is cleaved
with Hacac in the presence of a base to give the correspond-
ing mononuclear complex 7-R.24 The design rationale of the
trifunctional molecule 7-R lies in the notion that [Pt(ppy)-
(acac)]-type complexes (ppy ) 2-phenylpyridyl anion) are
known to be highly emissive and have been used as triplet-
emitting dopants in a variety of OLED structures.10,12b,24b
1,3,4-Oxadiazole heterocycles are widely used as ET groups
owing to their electron deficiency and good thermal
stability.2c,25 The triarylamine subunit can act as a HT group
and can also be easily functionalized by chemical means.26
ferrocenium (Fc/Fc+) couple at a scan rate of 100 mV s-1
.
Crystallography. Single crystals suitable for X-ray crystal-
lographic analyses were grown by slow evaporation of their
respective solutions in CH2Cl2/n-hexane at room temperature.
Important crystal data pertinent to individual compounds and the
structure refinement results are assemblied in Table 1. The
diffraction experiments were carried out at 293 K on a Bruker Axs
SMART 1000 CCD area detector diffractometer using graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å). The raw
intensity data frames were integrated with the SAINT+ program
using a narrow-frame integration algorithm.20 Corrections for
Lorentz and polarization effects were also applied by SAINT. For
each analysis, an empirical absorption correction based on the
multiple measurement of equivalent reflections was applied by using
the program SADABS.21 The structures were solved by direct
methods and expanded by difference Fourier syntheses using the
software SHELTXL.22 Structure refinements were made on F 2 by
the full-matrix least-squares technique. All non-H atoms were
refined with anisotropic displacement parameters. The H atoms were
placed in their ideal positions but not refined.
Results and Discussion
Synthesis and Characterization. Scheme 1 shows the
chemical structure and synthetic strategies to the integrated
Pt(II) complexes containing all three functionalities in the
present study. First, the N-arylation of aniline was carried
(23) (a) Hassan, J.; Se´vignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem.
ReV. 2002, 102, 1359-1470. (b) Mart´ınez-Palau, M.; Perea, E.; Lo´pez-
Calahorra, F.; Velasco, D. Lett. Org. Chem. 2004, 1, 231-237.
(24) (a) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba,
I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055-3066.
(b) Laskar, I. R.; Hsu, S.-F.; Chen, T.-M. Polyhedron 2005, 24, 881-
888.
(20) SAINT+, version 6.02a; Bruker Analytical X-ray System, Inc.:
Madison, WI, 1998.
(21) Sheldrick, G. M. SADABS, Empirical Absorption Correction Program;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(22) Sheldrick, G. M. SHELXTLTM, Reference Manual, version 5.1;
Madison, WI, 1997.
10926 Inorganic Chemistry, Vol. 45, No. 26, 2006

Phosphorescent Platinum(II) Complexes
Scheme 1. Synthesis of Multifunctional Pt(II) Complexes and Their Ligand Precursors
These modular units were covalently linked to form trifunc-
tional molecules as outlined in Scheme 1. For comparison,
complexes 9 and 11 were also prepared as model compounds
to examine the effects of each of the individual oxadiazole
and aromatic amine moieties. They differ from 7-H in that
complex 9 does not contain an oxadiazole ring whereas the
arylamine unit is absent in 11. Coupling of the 2-pyridyl
unit with a suitable halo-substituted triphenylamine or 2,5-
diphenyloxadiazole chromophore can furnish 8 and 10, which
can be employed to give 9 and 10 following the classical
cyclometalation synthetic protocol (Scheme 2).24 The crude
products from these reaction mixtures were purified by
column chromatography or preparative TLC on silica and
recrystallized where necessary.
spectroscopic data shown in the Experimental Section are
in line with their assigned structures.
Crystallography. The three-dimensional molecular struc-
tures of 6-F, 7-F, 7-H, 7-Me, 7-OMe, 9, and 11 were
confirmed by X-ray crystallography, and Figure 1 illustrates
perspective views of all of the Pt complexes. Pertinent bond
distances and angles are given in Table 2. The crystal
structures of these cycloplatinates reveal a typical four-
coordinate Pt core with a square-planar geometry, and there
is very little distortion away from the square plane for Pt.
The oxadiazole ring and the Pt square plane differ from
coplanarity by 6.0-18.6°. Intermolecular interactions in the
crystal lattices of Pt(II) complexes are known to play a crucial
role in their solid-state emission properties.10 Except for 7-Me
and 11, the lattice is characterized by the presence of Pt‚‚‚Pt
intermolecular interactions (ca. 3.240-3.741 Å) between
adjacent molecules, and crystal packing effects may, among
other not yet realized factors, contribute to this structural
difference. This metal-metal interaction can be compared
to those observed for other monocyclometalated π-π-
stacked Pt(II) complexes containing a simpler 2-phenylpy-
ridine moiety in the literature,24,27 where the shortest Pt‚‚‚Pt
length reported is 3.528 Å.27b As noted below, these
interactions may be invoked to rationalize the lower-energy
All of the new metal complexes are air-stable orange
crystalline solids and can be stored without any special
precautions. They are found to be soluble in chlorinated
solvents such as CH2Cl2 but are generally insoluble in
hydrocarbons. They all gave satisfactory analytical data and
were characterized by FABMS and NMR spectroscopy. The
(25) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater.
2004, 16, 4556-4573.
(26) (a) Strohriegl, P.; Grazulevicius, J. V. AdV. Mater. 2002, 14, 1439-
1452. (b) Hreha, R. D.; George, C. P.; Haldi, A.; Domercq, B.;
Malagoli, M.; Barlow, S.; Bre´das, J.-L.; Kippelen, B.; Marder, S. R.
AdV. Funct. Mater. 2003, 13, 967-973. (c) Kunda, P.; Thomas, K.
R. J.; Lin, J. T.; Tao, Y.-T.; Chien, C.-H. AdV. Funct. Mater. 2003,
13, 445-447.
(27) (a) Ma, B.; Li, J.; Djurovich, P. I.; Yousufuddin, M.; Bau, R.;
Thompson, M. E. J. Am. Chem. Soc. 2005, 127, 28-29. (b) Chassot,
L.; Muller, E.; von Zelewsky, A. Inorg. Chem. 1984, 23, 4249-4253.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10927

He et al.
Scheme 2. Synthesis of Some Model Pt(II) Complexes 9 and 10
solid-state emissions displayed by complexes 7-R, where
appropriate. The bond length of Pt-N [1.955(11)-1.996(3)
Å] is slightly longer compared to that of common Pt-N
bonds because this N is opposite to the acac O atom having
a weak trans influence. The bond distance of the Pt-O edge
trans to C [2.064(11)-2.094(3) Å] is larger compared to that
of the other Pt-O bond [1.991(9)-2.003(3) Å] because the
O atom trans to C has a strong trans influence. All other
bond lengths around the Pt core are typical for cyclometalates
and â-diketonate derivatives of Pt(II).24,27
features at 316 and 388 nm.11c However, we cannot rule out
completely the possibility of a minor component for ligand-
to-ligand charge transfer in these bands. For 7-R, the lower
energy absorption bands around 370-385 nm are notably
red-shifted from the free ligand absorption because of
perturbation from the metal but are not solvatochromic.
Metal-centered d-d transitions are not observed for 7-R, and
it is likely that the strong ligand field of the cyclometalating
ligands shifts the d-d transitions to high energy, putting them
under the more intense ligand-centered (LC) transitions.
While the higher energy absorption peak does not change
with the R groups, the lower-lying band is slightly blue-
shifted by electron-withdrawing units (λmax ) 364 nm) and
red-shifted by electron-donating moieties (λmax ) 375 nm).
Attachment of the Pt core into the ligand was shown to enrich
the photoluminescent (PL) properties. The absorption spectra
of 7-R in CH2Cl2 also feature two intense bands at 293-
295 and 370-385 nm in each case due to the LC transitions
from the π orbitals of arylamine to the π* orbitals of
oxadiazole-based 2-arylpyridine (Figure 2b). The ligand
substituent effect is also operative for the lowest energy
emission bands for 7-R, in line with the absorption studies.
These assignments were also supported by theoretical
calculations for 6-R and 7-R, which reveal that the HOMO
is basically triphenylamine-based and the LUMO predomi-
nantly corresponds to the π* orbitals of the 2-(2-arylpyri-
dine)-1,3,4-oxadiazole group. The HOMO-1 level also
contains substantial contribution from the Pt’s d orbital
Thermal and Photophysical Properties. The thermal
properties of the compounds were examined by thermo-
gravimetric analysis (TGA). All of the pyridyl derivatives
and their Pt(II) complexes generally exhibit high thermal
stability, and their onset decomposition temperatures (Tdecomp
)
range from 341 to 466 °C for the cyclometalating ligands
and from 255 to 370 °C for the Pt complexes (Table 3). In
addition, both series of 6-R and 7-R can be easily sublimed
at low pressures (∼10-6 Pa) without decomposition. Hence,
complexes 7-R are sufficiently stable to be incorporated into
multilayer OLEDs by the vacuum deposition method.
However, complex 11 tends to degrade readily upon subli-
mation at reduced pressures and does not readily form a good
solid thin film.
The absorption and emission data of the new metallized
compounds 7-R and their ligand precursors 6-R are shown
in Table 3. For 6-R, two distinct absorption bands are
observed at 293-296 and 364-375 nm, which may be
attributed to the π-π* transitions associated with the N-Ar
and N-C6H4-X fragments (Figure 2a). Similar data were
observed by Low et al. in the spectra of Ar2N-C6H4-X
species near 300 and 350 nm.28 An analogous ligand system
containing a 2,2′-bipyridyl unit also shows absorption
(28) (a) Low, P. J.; Paterson, M. A. J.; Goeta, A. E.; Yufit, D. S.; Howard,
J. A. K.; Cherryman, J. C.; Tackley, D. R.; Brown, B. J. Mater. Chem.
2004, 14, 2516-2523. (b) Low, P. J. M.; Paterson, A. J.; Puschmann,
H.; Goeta, A. E.; Howard, J. A. K.; Lambert, C.; Cherryman, J. C.;
Tackley, D. R.; Leeming, S.; Brown, B. Chem.sEur. J. 2004, 10,
83-91.
10928 Inorganic Chemistry, Vol. 45, No. 26, 2006

Phosphorescent Platinum(II) Complexes
Figure 1. Perspective drawings of (a) 7-F, (b) 7-H, (c) 7-Me, (d) 7-OMe, (e) 9, and (f) 11. In each case, thermal ellipsoids are drawn at the 25% probability
level. H atoms and all labels for the C atoms are omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) around the
Metal Core in 7-F, 7-H, 7-Me, 7-OMe, 9, and 11
to increase the donor-acceptor (D-A) character of the
ligand, leading to a larger π-conjugation space and/or strong
Pt-N
Pt-C
Pt-O
C-Pt-N O-Pt-O Pt‚‚‚Pt
intramolecular D-A interaction. For 11, a shoulder band at
421 nm with an extinction coefficient of 3000 M-1 cm-1 is
also observed as a result of the concomitant observation of
the metal-to-ligand CT (MLCT) transition. This can be
corroborated with the computational results in which the
valence orbital picture consists of combined LC and MLCT
components in the lowest energy absorption (vide infra). In
contrast, the other Pt(II) complexes possess predominantly
3LC character in the excited state.
7-F
1.991(7) 1.980(8) 2.002(6) 81.3(3)
92.0(2)
3.741
2.077(6)
1.955(11) 1.921(13) 1.991(9) 80.9(5)
2.088(9)
1.996(3) 1.969(3) 2.002(2) 81.65(12) 92.10(10) 4.605
2.079(2)
7-H
7-Me
91.7(3)
3.350
7-OMe 1.991(3) 1.968(3) 2.003(3) 81.57(15) 91.61(12) 3.240
2.082(3)
9
1.995(4) 1.957(4) 2.001(3) 81.56(17) 92.04(13) 3.394
2.094(3)
11
1.987(13) 2.014(15) 1.998(10) 80.5(6)
2.064(11)
92.0(4)
4.953
Both 6-R and 7-R are good light emitters in fluid solutions
and in the solid state at 298 K. Each of the luminescence
spectra of 6-R is dominated by LC (π-π*) transitions in
the visible region, and the emissions vary from 462 to
537 nm (Figure 3a). For 6-R, the emissions are sensitive to
character for 7-R (vide infra). Complex 9 absorbs at a longer
wavelength than that for [Pt(ppy)(acac)], and the introduction
of an electron-donating diphenylamino group into the
electron-deficient pyridine moiety of the ligand is expected
1
Inorganic Chemistry, Vol. 45, No. 26, 2006 10929

He et al.
Table 3. Photophysical Data for the New Pt Complexes and Their Ligands
λ
em (nm)b
compound
6-F
6-H
6-Me
6-OMe
8
10
ppy
7-F
7-H
7-Me
7-OMe
9
11
Tdecomp (°C)
λ
max (nm)a
CH2Cl2 (298 K)
Φ (%)
frozen CH2Cl2 (77 K)
466
456
466
421
341
358
293 (5.4), 364 (6.7)
294 (6.5), 368 (7.1)
296 (5.0), 375 (5.2)
294 (2.2), 375 (2.2)
462 (3.8)c
81
70
57
40
89
98
2.0
3.0
2.2
1.7
0.7
0.5
0.5
2.1e
470 (4.2)c
499 (4.6)c
537 (2.7)c
347 (2.2)
307 (1.8)
429 (3.1)c
341*, 357 (6.4),c 373*
333*, 349 (0.13)c
222 (1.0), 248 (1.2), 276 (0.9)
293 (5.1), 370 (4.9)
294 (8.0), 372 (7.5)
295 (5.3), 381 (4.5)
294 (1.5), 385 (1.3)
307 (1.7), 337 (1.5), 421 (1.6)
322 (3.2), 341 (2.9), 421* (0.3)
250 (3.2), 277 (2.2), 313 (1.1),
325 (1.0), 360 (0.6), 392* (0.4)
304
370
341
255
316
350
294
429* (3.0),c 533 (3.1),d 573*
471 (2.8),c 533 (7.0),d 570*
496* (3.1),c 534 (5.4),d 571*
468* (2.2),c 534 (3.1),d 572*
479 (2.7),c 535 (4.1)d
422* (2.1),c 533 (4.9),d 573*
481 (2.0)d,e
537 (6.2),d 582*, 625*
538 (7.2),d 582*, 625*
536 (6.4),d 583*, 625*
537 (3.1),d 583*, 626*
537 (4.7),d 579*
538 (5.0),d 579
[Pt(ppy)(acac)]
478 (5.3), 495*, 505*,
516*, 548*e
a Extinction coefficients (104 M-1 cm-1) are shown in parentheses. b Asterisks indicate emission peaks appearing as shoulders or weak bands. c Fluorescence
lifetimes (ns) shown in parentheses are measured in CH2Cl2. d Phosphorescence lifetimes (µs) shown in parentheses are measured in CH2Cl2. e Data measured
in 2-methyltetrahydrofuran can be taken from ref 24a.
Figure 2. Absorption spectra in CH2Cl2 at 298 K for (a) 6-R and (b) 7-R.
the para substituents and the emission wavelength is affected
by the electronic properties of R in the order OMe > Me >
H > F. In other words, compound 6-F, which contains
electron-withdrawing fluoro-substituted triphenylamine, shows
an ipsochromic-shifted emission (∆λ ) 8 nm), while those
containing electron-releasing substituents (Me and OMe)
undergo a large bathochromic-shifted emission (∆λ ) 29
and 67 nm for the Me and OMe cases) with respect to the
unsubstituted ligand 6-H. Hence, it can be inferred that the
position occupied by the substituents in the amine group is
dominated by HOMO character. These results also indicate
that the electron-donating and -accepting moieties can raise
and lower the HOMO levels, respectively. For 7-R, each of
them shows dual emission peaks (viz., fluorescence and
phosphorescence) at 298 K (Figure 3b). Upon introduction
of both triarylamino and oxadiazole functional groups into
the phenylpyridine ligands, the Pt(II) center should show a
reduced spin-orbit interaction. As a result, both LC fluo-
rescence and phosphorescence can be observed.9f The large
Stokes shift, the structured luminescence, and microsecond
radiative decay lifetimes for the low-lying bands are in line
of the intraligand π-π* transitions in the emission, and these
vibronic fine structures preclude the assignment of MLCT
3
states, which are usually broad and featureless. Consistent
with the X-ray structural data, examination of the emission
behavior of 7-R (except for 7-Me) in CH2Cl2 and in the solid
state suggests the presence of solid-state aggregates in thin
neat films.29 Figure 3c depicts the solid-state PL spectra for
three of them, which indicate the existence of an aggregate
emission band at 650 and 653 nm for 7-H and 7-OMe,
respectively, but it is absent for 7-Me. As is evident from
the absence of Pt‚‚‚Pt short contacts in the crystal state,
complex 7-Me is less prone to aggregation and the emission
solely arises from the triplet exciton in the solid phase. While
the higher-lying fluorescent emission was found to show a
red shift in the wavelength along the series 7-F f 7-H f
7-Me f 7-OMe, attachment of different R groups does not
seem to alter the phosphorescent emission energies of these
complexes. Presumably, the singlet state may be described
as a zwitterion, and so the inductive effect is consistent with
the shift observed. For the phosphorescence, the triplet state
is better described as a diradical, and so the inductive effect
is much less felt and not much shifting would be antici-
pated.30 Also, by looking at the fluorescence band, we see
3
with emission arising from a LC (π-π*) excited state in
7-R.24 The vibronic splitting at ca. 1407-1506 cm-1 (ν0-1
)
in the emission profile corresponds to the aromatic stretching
modes of the ligands, which is diagnostic of the involvement
(29) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605-1644 and references
cited therein.
10930 Inorganic Chemistry, Vol. 45, No. 26, 2006

Phosphorescent Platinum(II) Complexes
Figure 3. Solution PL spectra of (a) 6-R and (b) 7-R in CH2Cl2 at 298 K. (c) Neat thin-film PL spectra of 7-R (R ) H, Me, OMe). (d) EL spectra of OLED
devices I-IV at the 5 wt % dopant level of 7 at 8 V.
no vibronic structure, while phosphorescence has some. This
tells us that there may be a twist CT phenomenon, where a
twist angle changes very much in the singlet state.31 Low-
temperature emission spectra were also recorded for the new
Pt(II) complexes at 77 K, and they essentially become
dominated by the intense phosphorescence bands in frozen
CH2Cl2 at the expense of the fluorescence peaks. The
phosphorescence lifetimes at 77 K are longer than those at
room temperature. The small quantum yields of these
complexes indicate that the emission lifetimes of the Pt(II)
complexes are determined by nonradiative decay rates, which
usually decrease with lowering temperature. Thus, a length-
ening of the lifetime at low temperature would be anticipated.
DFT Calculations. Molecular orbital calculations at the
B3LYP level of DFT were performed on the two series of
compounds 6-R and 7-R to study their electronic structures.
For 6-R, the HOMO mainly consists of an out-of-phase
combination between the π orbitals of the three N(amine)-
bonded phenyl rings and the amine-N pπ orbital, while the
LUMO is derived from the π* orbitals of the oxadiazole
ring and the phenylpyridyl group. For 7-R, HOMO and
LUMO are similar to the corresponding 6-R except that the
orbitals are slightly more localized. In addition, contribution
of the metal orbitals can be found in the HOMO-1 and
LUMO of 7-R. The DFT results indicate that the feature of
the frontier molecular orbitals does not change significantly
with different substituents. As an example, Figure 4 shows
four frontier molecular orbitals for 6-F and 7-F. The
calculated HOMO-LUMO gaps for 6-R follow the order
6-OMe (3.27 eV) < 6-Me (3.37 eV) < 6-H (3.55 eV) <
6-F (3.82 eV). The trend is expected because the highly
electronegative F substituents tend to stabilize the HOMO
significantly, giving the largest energy gap, while the strongly
electron-donating OMe groups give the smallest gap. As
expected, the HOMO-LUMO gaps become smaller with the
presence of a metal fragment because metalation of 6-R
makes 7-R “electron-richer”. Like 6-R, the calculated order
of HOMO-LUMO gaps for 7-R is 7-OMe (2.99 eV) <
7-Me (3.22 eV) < 7-H (3.46 eV) < 7-F (3.47 eV).
For 6-R and 7-R, it was shown that the NAr2 groups
govern the feature of the HOMOs. We also examined the
molecular orbitals for the model compounds 10 and 11
without the NAr2 units. Figure 5 contains the relevant frontier
molecular orbitals for 7-H and 11. The LUMO for 11 is
similar to that for 10, corresponding to the lowest π* orbital
extensively delocalized on the oxadiazole-containing ligand.
The HOMO for 10 is derived from the orbitals of the
phenylpyridine unit and the oxadiazole ring. For 11, however,
the HOMO is derived mainly from the orbitals of the inner
phenyl ring of the organic ligand and the metal-acac
fragment with a substantial amount of metal character. The
theoretical HOMO-LUMO gaps of 4.51 eV for 10 and 3.79
eV for 11 are clearly greater than those calculated for 6-H
and 7-H, respectively, indicating that these non-amine
compounds are relatively “electron-poor”. Again, we see that
(30) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Mill Valley, CA, 1991.
(31) (a) Harvey, P. D.; Durocher, G. J. Photochem. Photobiol. 1984, 27,
29-39. (b) Harvey, P. D.; Durocher, G. Can. J. Spectrosc. 1984, 29,
84-86.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10931

He et al.
Figure 4. Contour plots of the frontier molecular orbitals for 6-F and 7-F.
the presence of the metal fragment reduces the HOMO-
LUMO gap in 11 relative to 10.
0.37-0.76 V (for 6-R) and 0.25-0.70 V (for 7-R) that can
be assigned to the oxidation of the triphenylamine moiety
were observed. It is noteworthy that both series of complexes
6-R and 7-R reveal a gentle decrease in the oxidative
potentials in the sequence F > H > Me > OMe, which is a
manifestation of the inductive effect induced by the R
substituents (viz., negative effect for F and positive effect
for both Me and OMe). When 7-H and 9 are compared, it is
clear that the NPh2 moiety in 9 tends to oxidize more easily
than the one in 7-H with the electron-withdrawing oxadiazole
ring. This is also the case for the pure organic species 6-H
and 8. However, the purely oxadiazole-containing counterpart
11 reveals a high oxidation potential at 0.89 V that appears
harder to be oxidized compared to 7-H having the electron-
rich Ph2N unit anchored to the oxadiazole ring. Notably,
addition of electron-pushing NAr2 groups to the ligand core
in 11 caused a negative shift in the anodic half-wave potential
by 0.19-0.64 V for 7-R, with the largest shift observed for
R ) OMe. On the basis of the redox data, the HOMO levels
can be estimated by taking -4.8 eV as the HOMO level for
the Fc/Fc+ couple with respect to the vacuum level.32 The
HOMO energies, together with the absorption edge data,
were then used to compute the LUMO energy levels, and
the results are also summarized in Table 4. The HOMO
energies (from -5.05 to -5.50 eV) can be tuned easily by
For 8 and 9, which do not have an oxadiazole unit, the
HOMOs of the two compounds are again mainly related to
an out-of-phase combination between the π orbitals of the
three N(amine)-bonded phenyl rings and the amine-N p
orbital while the LUMOs are derived mainly from π* orbitals
of the pyridyl ring. The metal d orbitals contribute slightly
to the HOMO-1 and LUMO of 9. The calculated HOMO-
LUMO gap for 8 (3.97 eV) is smaller than that for 10, as a
result of the fact that the oxadiazole unit is electron-
withdrawing in nature. The gap calculated for 9 is less than
that for 7-H, suggesting that the neat NPh2 entity can render
9 more electron-rich relative to the latter one containing an
electron-deficient oxadiazole bridge. The gap calculated for
9 is smaller than that calculated for 8, showing a similar
metalation effect.
Redox Behavior. To investigate the electronic effects
caused by the addition of R groups, cyclic voltammetry
experiments were carried out for all complexes, including
the ligand precursors. Because of the limited range available
in CH2Cl2 and the inability of our instrument to measure
the reduction potentials in the range from -2.7 to -3.5 V,
we obtained only the oxidation potentials for our compounds
(Table 4). The ligand redox properties are affected by
cyclometalation, and complex 7-R is easier to oxidize than
6-R for each R. Reversible oxidation waves in the ranges of
(32) Thelakkat, M.; Schmidt, H.-W. AdV. Mater. 1998, 10, 219-223.
10932 Inorganic Chemistry, Vol. 45, No. 26, 2006

Phosphorescent Platinum(II) Complexes
Figure 5. Contour plots of the frontier molecular orbitals for 7-H, 9, and 11. The inset shows the DFT-calculated HOMO and LUMO levels for comparison,
and the data for [Pt(ppy)(acac)] were taken from ref 24a.
Table 4. Electrochemical Properties and Frontier Orbital Energy Levels
for 6-11
HOMO level of -5.29 eV is expected to facilitate transport
of holes efficiently for achieving the best device performance
ox
Eg
(eV)
E
HOMO LUMO
T1
among the four complexes (vide infra). When the diaryl-
amino end groups are attached to the oxadiazole rings in
11, the HOMO values of 7-R are raised by as much as
0.64 eV for 7-OMe and the HOMO of 7-OMe is close to
that for 9. The elevated HOMO energy levels for 7-R as
compared to those for 11 (-5.69 eV) indicate that 7-R
compounds are more electropositive (or have lower ionization
potentials) than the non-NPh2-capped analogue 11, and a
better HT character in 7-R can be expected. The LUMO
levels of 7-R (from -2.48 to -2.77 eV) are lower than that
of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD; -2.4 eV),2c one of the most widely used hole-
blocking/ET materials, showing the good ET ability for 7-R.
Our data can be compared to the poorer ET property for the
prototypical [Pt(ppy)(acac)] with a higher LUMO level of
-2.41 eV.24 Complexes 7-R are thus anticipated to display
intriguing bipolar character, akin to some organic EL
1/2
(V)a
complex
(eV)b
(eV)c
(eV)d
6-F
3.00 +0.76
2.99 +0.66
2.95 +0.54
-5.56
-5.47
-5.34
-2.56
-2.48
-2.39
-2.27
-2.01
6-H
6-Me
6-OMe
8
2.90 +0.37, +0.98 (i) -5.17
3.25 +0.46
3.59
-5.26
10
7-F
2.73 +0.70
2.70 +0.66
2.65 +0.49
-5.50
-5.42
-5.29
-2.77
-2.72
-2.64
-2.48
-2.45
-2.74
-2.41f
2.31
2.30
2.31
2.31
2.31
2.31
2.59
7-H
7-Me
7-OMe
9
2.57 +0.25, +0.85 (i) -5.05
2.64 +0.29, +0.83
2.95 +0.89
-5.09
-5.69
e
11
[Pt(ppy)(acac)] 2.93
e
a 0.1 M [Bu4N]PF6 in CH2Cl2 vs a Fc/Fc+ couple. b The HOMO level
was calculated by using the Fc value of 4.8 eV below the vacuum level.
c Estimated from the HOMO and band-gap energies (LUMO ) Eg
+
HOMO). d Determined from the PL spectrum in CH2Cl2 glass at 77 K.e Not
reported. f The value was taken from ref 24. i ) irreversible.
attaching various R groups to the aryl rings. The HOMO
levels for 7-R match closely with the energy levels for 4,4′-
bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB; HOMO
) -5.20 eV)33 and especially for 7-Me; its promising
(33) (a) 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. (b) Lee, S. T.; Wang, Y.
M.; Hou, X. Y.; Tang, C. W. Appl. Phys. Lett. 1999, 74, 670-672.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10933

He et al.
Figure 6. General structure for OLED devices and the molecular structures of the compounds used in these devices. TPBI: 2,2′,2′′-(1,3,5-phenylene)-
tris(1-phenyl-1H-benzimidazole). CBP: 4,4′-N,N′-dicarbazolebiphenyl. NPB: 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl. ITO: indium-tin oxide.
materials.6a The LUMO levels are also compatible with that
of 2,2′,2′′-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole)
(TPBI; LUMO ) -2.70 eV).34 The electrochemical behavior
here is in good agreement with a description of ligand-
localized HOMO and LUMO states, as seen in our DFT
calculations.
triplet emitter to prevent reverse energy transfer from the
guest back to the host and to effectively confine triplet
excitons on guest molecules. Clearly, complexes 7-R show
triplet levels of 2.30-2.31 eV, well below the level of the
CBP host (2.56 eV). The Pt(II) complexes were doped into
the host material in the emitting zone with various doping
contents for each sample. It is worth noting that, even at
high current densities, emission from CBP is negligible in
all cases, indicating complete energy and/or charge transfer
from the host exciton to the phosphor molecule upon
electrical excitation. Comparisons of the performances for
the devices are summarized in Table 5. Figure 3d illustrates
the EL spectra of the devices at the 5 wt % dopant level of
7-R at a driving voltage of 8 V, and the impact of the aryl
ring substituents with regards to the emission properties of
this class of emitters is obvious. Generally, we observe both
the electrogenerated fluorescence and phosphorescence bands.
As expected, the electrofluorescence energy depends on the
nature of R groups, but the electrophosphorescence energy
is rather invariant of R, which is consistent with that observed
for the PL in solution. There is no sign of metal complex
aggregation in the fabricated devices I-IV. At the 5% doping
level, the excited-state properties of 7-R have been adjusted
to yield blue-green (for 7-F), green-yellow (for 7-H and
7-Me), and greenish-white (for 7-OMe). For devices II-
IV, the EL spectra exhibited no significant change with
variation of the operating bias voltages from 6 to 12 V but
the EL spectral pattern is dependent on the dopant concentra-
tion. In each case, the EL peak resembles that of its
corresponding PL spectrum from a thin film, indicating that
the same optical transition is responsible for light emission.
As indicated below, the color of the EL can be tuned by a
variation in the applied dc voltage as well as the dopant
concentration.
EL Devices and Substituent Effects. Organometallic
phosphors 7-R have very good film-forming properties for
accessing their electrophosphorescent ability in evaporated
devices. However, complexes 9 and 11 do not form good-
quality films for electronic characterization in device struc-
tures. The good sublimability and short phosphorescence
lifetimes of 7-R [cf. >50 µs for Pt(OEP)]7b render them
suitable candidates as efficient electrophosphors. These
phosphors are better used as a phosphorescent dopant rather
than a single emission layer in OLEDs to prevent the severe
self-quenching and formation of aggregates expected in the
solid film. Taking the advantage that these cyclometalated
Pt(II) complexes are neutral and are thermally stable,
fabrication of OLEDs can be achieved in a guest-host
system by vacuum deposition of such luminescent com-
pounds. Figure 6 depicts the general four-layer structures
for the electrophosphorescent devices and the molecular
structures of the compounds employed. 4,4′-N,N′-Dicarba-
zolebiphenyl (CBP) acts as a small-molecule host material
for the electrophosphor, NPB as a hole-transport layer, TBPI
as both a hole-blocker and an electron-transporter, and LiF
as an electron-injection layer. Here, TBPI, instead of the
commonly used 2,9-dimethyl-4,7-diphenyl-1,10-phenanthro-
line or tris(8-hydroxyquinolinato)aluminum, was adopted for
the devices to confine excitons within the emissive zone
because it shows a higher electron mobility.35 It is essential
that the triplet level of the host be larger than that of the
(34) Tao, Y. T.; Chuen, C. H.; Ko, C. W.; Peng, J. W. Chem. Mater. 2002,
14, 4256-4261.
(35) (a) Duan, J.-P.; Sun, P.-P.; Cheng, C.-H. AdV. Mater. 2003, 15, 224-
228. (b) Chen, C. H.; Shi, J.; Tang, C. W. Macromol. Symp. 1997,
125, 1.
Figure 7a shows the current density-voltage-luminance
(I-V-L) curves of 7-Me-doped OLEDs at four different
doping concentrations. Essentially, each of the devices IIIa-
10934 Inorganic Chemistry, Vol. 45, No. 26, 2006

Phosphorescent Platinum(II) Complexes
Table 5. Performance of Ir-Doped Electrophosphorescent OLEDs Recorded at 8 V
device
phosphor dopant
Vturn-on (V)
L (cd m-2
)
η
ext (%)
ηL (cd A-1
)
ηp (lm W-1
)
λ
max (nm)
CIE
Ia
5% 7-F
3.5
4239
1.93 (1.8)a
1.00b
3.58 (4.1)a
1.92b
2.81 (1.7)a
0.86b
443, 537, 578
444, 537, 579
445, 541, 582
445, 538, 575
445, 538, 578
445, 538, 578
542, 583
(0.22, 0.20)
(13.0)a
3256
Ib
8% 7-F
3.6
3.7
6.3
6.0
5.4
4.7
4.7
4.2
4.3
5.1
5.6
9.4
1.73 (2.8)a
0.88b
3.27 (4.1)a
1.92b
2.29 (2.7)a
0.87b
(0.25, 0.27)
(0.36, 0.42)
(0.30, 0.43)
(0.36, 0.54)
(0.37, 0.55)
(0.44, 0.53)
(0.38, 0.44)
(0.33, 0.39)
(0.30, 0.36)
(0.25, 0.35)
(0.27, 0.39)
(0.22, 0.36)
(13.0)a
1066
Ic
10% 7-F
1.01 (2.1)a
0.51b
1.11 (5.4)a
0.66b
0.83 (3.3)a
0.31b
(13.0)a
2250
IIa
5% 7-H
2.41 (1.6)a
1.32b
3.23 (1.7)a
2.09b
1.48 (0.5)a
0.73b
(13.5)a
3364
IIb
IIc
10% 7-H
15% 7-H
5% 7-Me
7% 7-Me
10% 7-Me
12% 7-Me
2.5% 7-OMe
5% 7-OMe
7% 7-OMe
1.93 (2.1)a
1.11b
4.89 (0.9)a
2.80b
2.56 (0.2)a
1.02b
(13.2)a
4055
1.25 (2.4)a
0.74b
4.24 (1.2)a
2.81b
2.42 (0.2)a
1.11b
(12.8)a
2039
IIIa
IIIb
IIIc
IIId
IVa
IVb
IVc
3.59 (1.8)a
1.80b
10.95 (1.2)a
5.50b
5.73 (1.6)a
1.68b
(11.0)a
1876
2.72 (2.7)a
1.51b
8.20 (0.3)a
4.16b
4.87 (0.4)a
1.29b
450, 540, 584
460, 540, 584
460, 540, 582
468, 542
(11.0)a
3269
2.63 (1.4)a
1.17b
6.79 (1.6)a
3.00b
4.07 (2.0)a
1.08b
(11.0)a
2495
2.27 (3.0)a
1.28b
5.48 (3.4)a
3.07b
4.13 (0.5)a
1.20b
(11.0)a
8462
2.30 (0.6)a
1.60b
5.50 (0.6)a
3.82b
2.50 (0.3)a
1.07b
(20.1)a
4649
1.10 (1.8)a
0.94b
2.90 (1.7)a
2.42b
1.30 (0.1)a
0.64b
487, 540
(18.1)a
2327
0.61 (6.9)a
0.54b
1.50 (6.8)a
1.30b
0.30 (6.8)a
0.25b
490
(20.1)a
a Maximum values of the devices. Values in parentheses are the voltages at which they were obtained. b At 20 mA cm-2
.
mance was realized in the green-yellow device IIIa, where
a maximum external quantum efficiency (ηext) of 3.59%, a
luminance efficiency (ηL) of 10.95 cd A-1, and a power
efficiency (ηp) of 5.73 lm W-1 were obtained. For devices
IIa and IVb, the EL spectra are dominated by two major
emission bands and the corresponding peak efficiencies are
ηext ) 2.41 and 1.10%, respectively. At a 2.5% ratio of
7-OMe in device IVa, improved luminance (∼8460 cd m-2)
and efficiency (∼2.30%) are observed relative to device IVb.
The higher efficiency and superior performance of 7-Me over
the others may be attributed to the better match of its HOMO
level with NPB, which leads to excellent balancing of holes
and electrons. At high concentrations beyond 5 wt %, ηext
tends to decrease slightly, apparently as a consequence of
aggregate-induced quenching, and the maximum ηext was
achieved at 5 wt % concentration in most cases except for
device IV, where the highest ηext at the 2.5 wt % level was
obtained. In common with most phosphorescent devices,
there was a gradual decrease in efficiency with increasing
current density, which has been attributed to a combination
of triplet-triplet annihilation36 and field-induced quenching
effects.37
WOLEDs. To achieve the goal of white light emission,
two strategies have been employed in this study using the
multifunctional compounds 7-F and 7-Me. For device Ic with
10% 7-F, a strong voltage dependence of the EL spectrum
is observed, with the blue color intensity increasing relative
to the orange component at increasing voltage, owing to the
requirement for high-energy excitation of the blue-light
emitter. Figure 8a shows the EL spectra of device Ic at
Figure 7. (a) I-V-L characteristics of the electrophosphorescent devices
IIIa-IIId with different dopant concentrations of 7-Me. (b) Luminance
efficiency as a function of the current density curve for devices IIIa-IIId.
IIId exhibits a prominent electrophosphorescence peak at
about 540 nm with low turn-on voltages (Vturn-on) for light
emission at 1 cd m-2 of 4.2-4.3 V. The luminance efficiency
of the 5%-doped device IIIa-IIId as a function of the
current density is presented in Figure 7b. The best perfor-
(36) Baldo, M. A.; Adachi, C.; Forrest, S. R. Phys. ReV. B 2000, 62, 10967-
10977.
(37) Kalinowski, J.; Stampor, W.; Mezyk, J.; Cocchi, M.; Virgili, D.; Fattori,
V.; Di Marco, P. Phys. ReV. B 2002, 66, 235321-1-235321-15.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10935

He et al.
Table 6. Performance Parameters of Various WOLEDs
device
voltage (V)
max ηext (%)a
CIE (x, y)
Ic
Ic
Ic
Ic
IIIc
IIId
10
12
14
16
8
1.0 (2.1)
1.0 (2.0)
1.0 (1.9)
1.0 (1.9)
2.6 (1.4)
2.3 (3.0)
(0.33, 0.36)
(0.31, 0.33)
(0.29, 0.31)
(0.29, 0.30)
(0.33, 0.39)
(0.30, 0.36)
8
a Values in parentheses are the voltages at which they were obtained.
to almost white by applying different dc voltages from 10
to 16 V (Figure 8a and Table 6).
Additionally, to realize the generation of white EL that is
not driven by varying the bias voltage, devices IIIc and IIId
comprising 10 and 12 wt % of the dopant 7-Me were
prepared. Both devices begin to glow at ∼4.2 V and exhibit
two emission bands at 460 and 540 nm in the EL spectra.
The WOLED device IIIc gives peak ηext, ηL, and ηP of
2.63%, 6.79 cd A-1, and 4.07 lm W-1, respectively. The
highest achieved luminance is 3270 cd m-2 at 11 V. Saliently,
the performances of both WOLED devices are slightly better
than the previously reported results for Pt(II) Schiff base
emitters (ηext ) 1.1% and ηP ) 0.79 lm W-1).38 The
Commission Internationale de L’Eclairage (CIE) coordinates
of devices IIIc and IIId are (0.33, 0.39) and (0.30, 0.36),
respectively (Figure 8c), which closely approach that of white
light [defined as (0.33, 0.33)]; the near-white emission is
produced by the simultaneous EL of both singlet and triplet
excitons of the trifunctional molecules in the device. Upon
an increase in the concentration of 7-Me, it seems that the
EL efficiencies are lowered gradually but the singlet emission
of 7-Me at ∼460 nm becomes more important in the EL
spectra (Figure 8b); this is consistent with more efficient
intermolecular quenching of triplet excitons at high dopant
concentrations. As a consequence, the concentration-depend-
ent color tuning can be accomplished by making use of the
relationship between the singlet-triplet intensity ratio of the
dual emission spectrum and the dopant concentration.
Although the dependence of the EL color on the dopant level
of other square-planar metal complexes has been reported,
in which the EL spectra displayed both monomeric and
excimeric emissions with different relative intensities upon
variation of the dopant concentration,10 the present work
confers another platform for the tuning of OLED colors.
While most examples of white phosphorescent OLEDs
require dual or multiple dopants in multilayer devices, we
note that a near-white EL has been achieved in this work
using a single dopant configuration.
Figure 8. (a) EL spectra of 7-F-doped device Ic at different dc voltages
applied (from front to back: 6, 8, 10, 12, 14, and 16 V). (b) Devices IIIa-
IIId with different concentrations of 7-Me (5, 7, 10, and 12 wt %) as the
dopant, upon applying 8 V of dc voltage. (c) CIE coordinates of white
light sources.
Conclusions
In conclusion, the present work reports the synthesis and
characterization of some new light-emitting multifunctional
cyclometalated complexes of Pt(II) incorporating both HT
triarylamine and ET oxadiazole moieties, the study of their
photophysical properties, single-crystal X-ray structural
analyses, and their applications in monochromatic OLEDs.
different dc voltages applied. At a lower dc voltage (<6 V),
the EL spectrum is essentially characterized by an orange
vibronic-structured band at ∼541 nm. The EL spectra exhibit
a blue broad band at ∼445 nm as the dc voltage is increased
from 6 V. Upon increasing the bias voltage, the relative
triplet-singlet intensity ratio of 7-F decreases gradually at
the expense of the orange triplet emission. Accordingly, the
emission color of device I can be tuned from green-yellow
(38) Che, C.-M.; Chan, S.-C.; Xiang, H.-F.; Chan, M. C. W.; Liu, Y.; Wang,
Y. Chem. Commun. 2004, 1484-1485.
10936 Inorganic Chemistry, Vol. 45, No. 26, 2006

Phosphorescent Platinum(II) Complexes
Modification of the para substituents on the NAr2 groups
has been undertaken in order to facilitate the fine-tuning of
the emission energy for the complexes and the HOMO-
LUMO levels. End-capping of the [Pt(ppy)(acac)]-type
molecules with a NAr2 chromophore in the phosphor has
been shown to offer advantages in terms of lowering the
first ionization potential, which improves the HT properties.
The color of the EL can be modulated by a variation in the
applied dc voltage as well as by a change in the dopant
concentration. These multicomponent systems have been
demonstrated to be useful in accomplishing white light
sources through variation of the dopant concentration and
the driving voltage. Management of the relative EL intensities
of the singlet and triplet excitons has been shown to be a
good tool for making efficient single-dopant WOLEDs.
These new triplet emitters have the potential for further
optimization for tuning the emission closer to pure white by
a judicious change in the composition.
Acknowledgment. Financial support from a CERG grant
from the Research Grants Council of the Hong Kong SAR,
People’s Republic of China (Project HKBU 2024/04P) and
a FRG grant from the Hong Kong Baptist University (Grant
FRG/04-05/II-59) is gratefully acknowledged for this work.
Supporting Information Available: CIF data, synthetic pro-
cedures and spectroscopic data for 1-R-5-R, EL spectra and
I-V-L curves for devices IIa-c and IVa-c, and absorption and
PL spectra for 9 and 11. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC061566C
Inorganic Chemistry, Vol. 45, No. 26, 2006 10937