New platinum(II) complexes as triplet emitters for high-efficiency monochromatic pure orange electroluminescent devices

Article abstract of DOI:10.1016/j.jorganchem.2007.04.013

New multi-component orange phosphorescent platinum complexes [Pt(L)(acac)] (Hacac = acetylacetone, HL = (9,9-diethyl-7-pyridin-2-ylfluoren-2-yl)diphenylamine 1, (9,9-diethyl-7-pyridin-2-ylfluoren-2-yl)di(p-tolyl)amine 2) were prepared and characterized by

Full text of DOI:10.1016/j.jorganchem.2007.04.013

Journal of Organometallic Chemistry 692 (2007) 3461–3473
www.elsevier.com/locate/jorganchem
New platinum(II) complexes as triplet emitters for
high-efficiency monochromatic pure orange electroluminescent devices
a
a,
c
Gui-Jiang Zhou , Xing-Zhu Wang a,b, Wai-Yeung Wong *, Xiao-Ming Yu ,
c
d
Hoi-Sing Kwok , Zhenyang Lin
a
Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, PR China
b
Institute of Polymer Science, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China
Department of Electronic and Electrical Engineering, Centre for Display Research, Hong Kong University of Science and Technology,
c
Clearwater Bay, Hong Kong, PR China
Department of Chemistry, Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong, PR China
d
Received 19 March 2007; received in revised form 11 April 2007; accepted 11 April 2007
Available online 21 April 2007
Abstract
New multi-component orange phosphorescent platinum complexes [Pt(L)(acac)] (Hacac = acetylacetone, HL = (9,9-diethyl-7-pyri-
din-2-ylfluoren-2-yl)diphenylamine 1, (9,9-diethyl-7-pyridin-2-ylfluoren-2-yl)di(p-tolyl)amine 2) were prepared and characterized by
spectroscopic and X-ray crystallographic methods. We report the redox and photophysical properties of 1 and 2 and compare these
results with the unsubstituted analogue [Pt(L)(acac)] (HL = 9,9-diethyl-2-pyridin-2-ylfluorene 3). Efficient pure orange-emitting organic
light-emitting devices (OLEDs) based on 1 were fabricated. The device performance with 3,5-dicarbazolylbenzene (mCP) as the host can
furnish maximium external quantum, current and power efficiencies of 4.65%, 11.75 cd/A and 5.27 lm/W at 7 V, respectively. The device
with 4,40-N,N0-dicarbazolebiphenyl (CBP) as the host can perform better with peak external quantum and current efficiencies of 6.64%
and 15.41 cd/A at 7.5 V and a power efficiency of 7.07 lm/W at 6.5 V. Unlike the OLEDs made from other cyclometalated Pt(b-diketo-
nato) complexes in which the electroluminescence spectra generally displayed both the monomeric and excimeric emissions with different
relative intensities upon variation of dopant concentration, our devices emit a strong pure orange light with stable CIE color coordinates.
From a steric point of view, no evidence of low-energy aggregate emission is observed for a doping level up to 12 wt.%. The present work
confers a good platform for the realization of robust triplet emitters in the fabrication of highly efficient monochromatic OLEDs through
the design of multifunctional chelating ligands.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Arylamine; Platinum; Crystal structures; Fluorene; Phosphorescence
1. Introduction
for studying their unique photophysical properties which
would find vital applications in the aforesaid frontiers.
Recently, substantial research was focused on the use of
iridium(III) complexes in organic light-emitting diodes
(OLEDs) [6,7]. Similarly, the application of platinum(II)
complexes chelated with aromatic ligands such as bipyri-
dine, phenanthroline, 2-phenylpyridine, or similar deriva-
tives as electrophosphorescent emitters has recently been
demonstrated to impact the development of high-perfor-
mance monochromatic OLEDs and white OLEDs (WOL-
EDs) [8,9]. These Pt(II) phosphors are attractive by virtue
of their large energy gap between the MLCT or 3LC
The design and synthesis of transition metal complexes
has been recognized as the research focus in the develop-
ment of organometallic-based optoelectronics, such as
light-emitting diodes, chemosensors and photovoltaic dye-
sensitized devices [1]. Enormous research work on cyclo-
metalated or oligopyridyl complexes of Ru(II) [2], Os(II)
[3], Re(I) [4] and Ir(III) [5] has been pursued over the years
*
Corresponding author. Tel.: +852 34117074; fax: +852 34117348.
E-mail address: rwywong@hkbu.edu.hk (W.-Y. Wong).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.04.013

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G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
emissive states (MLCT = metal-to-ligand charge transfer,
LC = ligand centered) and high-energy metal-centered
(MC) states [10,11]. However, many of these Pt(II) emitters
still suffer from many intrinsic obstacles. Among these, for
example, the Pt(OEP) (H2OEP = octaethylporphyrin) dis-
plays a ligand-based p–p* phosphorescence with a long life-
time up to 30–50 ls. Accordingly, saturation of the emission
signals and a rapid drop in device efficiency at high driving
currents are anticipated. There are also undesirable occa-
sions where their MLCT/3LC and MC states lie too close
in energy so that they can thermally equilibrate rapidly,
thereby quenching the emission via the fast radiationless
decay of the MC states [12]. Furthermore, the open coordi-
nation spheres of the square-planar Pt(II) core can allow for
other deactivation pathways to occur through metal interac-
tions with the environment [13]. So, they usually display a
organic ligands HL1, HL2 and HL3 were prepared via
Stille coupling of (7-bromo-9,9-diethylfluoren-2-yl)diphe-
nylamine, (7-bromo-9,9-diethylfluoren-2-yl)di(p-tolyl)amine
or 7-bromo-9,9-diethylfluorene with 2-(tributylstannyl)pyr-
idine in the presence of a catalytic quantity of Pd(PPh3)4.
All the Pt(II) complexes were synthesized in two steps from
the cyclometalation of K2PtCl4 with the organic ligands to
form initially the chloro-bridged dimers, followed by chlo-
ride cleavage with acetylacetone using Na2CO3 [18]. Purifi-
cation of the mixture by silica chromatography furnished
1–3 as air-stable orange to yellow powders in high purity.
They were each thoroughly characterized by NMR spectros-
copy and FAB-MS to confirm their well-defined chemical
structures.
2.2. Crystal structure analyses
3
mixed LC/MLCT excited state as well as an aggregate
emission band caused by the strong intermolecular interac-
tions, which would broaden the electroluminescence (EL)
spectra to result in a poor color purity. Instead these com-
plexes are more frequently employed in the fabrication of
white organic light-emitting diodes (WOLEDs) [14,15]. All
of these factors essentially contribute to the inferior effi-
ciency of OLEDs and render these Pt(II) complexes less
attractive as efficient monochromatic light sources. There-
fore, a rational design aimed at the reduction of the phos-
phorescence radiative lifetime and suppressing the self-
aggregation of Pt(II) complexes is needed to exploit the full
potential of Pt(II) electrophosphors.
Recently, the issues on carrier-injection in OLEDs arouse
much concern, because some of the promising phosphores-
cent emitters such as those functionalized with fluorene-
based chromophores suffer from a large hole-injection (HI)
barrier [16,17]. We are interested in looking for some new
metal phosphors exhibiting improved HI and hole-trans-
porting (HT) features. In this contribution, we show that
the luminescent platinum cores can be incorporated into
the multifunctional conjugated organic architectures. The
synthesis, redox, photophysical and electroluminescent
(EL) properties of these cyclometalated Pt(II) complexes
with diarylaminofluorene moiety will be discussed in which
HI/HT and EL functional groups are integrated into one
molecular unit catering for more efficient charge transport
in the EL process. It has been demonstrated that the diaryla-
minofluorenyl ligand can offer a good resort in facilitating
the injection of holes, displaying a pure emission color,
enhancing the thermal and morphological stability and
weakening the intermolecular interaction in Pt-based triplet
emitters that are essential for efficient monochromatic
OLEDs.
Good-quality crystals of 1–3 were grown from their
respective CHCl3/hexane solutions at room temperature
(rt) and characterized using X-ray crystallography (Fig. 1
and Table 1). Intermolecular interactions in the crystal lat-
tices of Pt(II) complexes are known to play a key role in
their solid-state emission properties [11,12]. Unlike other
relevant Pt complexes in which two molecules exist as a
dimer packed in a head-to-tail pattern in the asymmetric
˚
unit (3.15–3.76 A) [9,19], there is only one unique discrete
molecule per asymmetric unit among 1–3. The Ptꢀ ꢀ ꢀPt sep-
˚
aration is calculated to be 5.1–7.3 A, which is too long for
any significant Pt–Pt contacts. So, there should be negligi-
ble interaction among the molecules of 1–3 in the solid
state. The structures of 1 and 2 differ from 3 by the pres-
ence of a diarylamino core attached to the fluorene ring.
All of them have a distorted square planar geometry at
the Pt center. The Pt–C bond lengths for 1, 2 and 3 are
˚
1.969(4), 1.958(3) and 1.973(3) A, respectively, which are
close to the mean value reported [20]. The bond length of
˚
Pt–N (1.985(3)–1.997(3) A) is slightly longer compared to
that of common Pt–N bonds, as this N is opposite to the
acac O atom having a weak trans influence [21]. The Pt–
˚
O distances are within the range of 1.994(3)–2.087(3) A
as for other cyclometalated Pt(b-diketonato) derivatives
and it is clear that the Pt–O edge trans to carbon
˚
(2.080(2)–2.087(3) A) is longer compared to that of the
˚
other Pt–O bond (1.995(3)–2.018(3) A), as the O atom trans
to carbon has a strong trans influence [19,22,23].
2.3. DFT calculations
Density functional theory (DFT) calculations at the
B3LYP level were performed on the basis of their experi-
mental geometries from the X-ray data. The HOMO ꢁ 1,
HOMO and LUMO orbitals for 1–3 are shown in Fig. 2.
It can be seen clearly that the HOMO and LUMO corre-
spond to the p and p* molecular orbitals of the conjugated
system, respectively. Despite the subordinate contribution
from the Pt orbitals, the HOMO is mainly delocalized over
the fluorenyl moiety for 1–3 and it also extends through the
2. Results and discussion
2.1. Preparation and characterization
The synthetic protocol for the platinum complexes 1–3 is
shown in Scheme 1. As the key synthons, the fluorene-based

G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
3463
R
R
R
Br
N
N
N
R = H (HL1)
R
R = Me (HL2)
K2PtCl4
Pd(PPh3)4
2-Ethoxyethanol/water
N
SnBu3
Toluene
Br
N
(HL3)
R
R
R
R
R
R
N
N
N
Cl
O
Pt
Pt
Pt
O
N
N
Cl
N
Acetylacetone
Na2CO3
R = H (1)
R = Me (2)
Cl
Cl
Pt
Pt
O
N
N
Pt
O
N
(3)
Scheme 1. The synthetic sketches for the platinum complexes 1–3.
diarylamino group in 1 and 2. In the LUMO, the pp orbi-
tals from atoms of the pyridyl group make the major con-
tribution although mixing of the pp orbitals from the
fluorenyl moiety can also be clearly seen. On the basis of
these computational results, we consider the HOMO–
LUMO transitions for 1–3 to the LC p–p* band of the con-
jugated organic moiety. For 1 and 2, the transition band
also contains a substantial intraligand charge transfer
(ILCT) character from the diarylaminofluorenyl unit to
the pyridyl ring whereas a MLCT component may be pres-
ent for 3 because of the metal contribution in the HOMO.
In all cases, there is an involvement of Pt orbitals in the
HOMO ꢁ 1. Our results also show that the NAr2 groups
(Ar = Ph, tol) in 1 and 2, which are electron-donating,
reduce the HOMO–LUMO gap much with respect to 3
(3.13 eV, 3.11 eV and 3.68 eV for 1–3, respectively). How-
ever, 1 and 2 display similar energy gaps, signalling that
the effect of Me group substitution on the gap lowering is
quite small.
metal chromophore lowers the HOMO–LUMO gap signif-
icantly. The metal unit pushes up the orbital energy of the
HOMO slightly because the metal-bonded fluorenyl moiety
in 1, which makes significant contribution in the HOMO,
becomes more electron-rich relative to HL1. To the
LUMO, the metal lowers down its orbital energy because
the pyridyl group, which makes the major contribution in
the LUMO, becomes less electron-rich after metalation.
2.4. Photophysical and thermal properties
The UV–Vis and photoluminescence (PL) spectra of 1–
3 are depicted in Fig. 3 and the corresponding data are
listed in Table 2. The intense absorption bands below
400 nm appear to be LC transitions since they closely
resemble the spectra of the free ligand (see Supporting
1
Information), and are assigned to the spin-allowed p–p*
transitions associated with the arylamino and aminofluor-
enyl fragments. For 1 and 2, the absorption bands around
364–387 nm are red-shifted from the free ligand absorp-
tions because of perturbation from the metal but are not
solvatochromic. Similar absorption features were also
Comparing HL1 and 1, DFT data shows that the metal
coordination does not have a notable change in the orbital
characteristics in the frontier orbitals but the presence of

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G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
Fig. 1. Perspective drawings of (a) 1, (b) 2 and (c) 3.
Table 1
(vide supra). While the higher energy absorption peak does
not change with the R groups, the lower-lying band is
slightly red-shifted by the electron-pushing units
(kabs = 435 nm for 1 and 440 nm for 2). For 3, the intense
1p–p* transition at 326 nm is accompanied by weaker,
lower energy features with extinction coefficients of
6300–6600 Mꢁ1 cmꢁ1 that probably corresponds to a com-
bination of 1MLCT, 3MLCT, and 3p–p* excited states [19].
The triplet character arises from the strong spin–orbital
coupling between the singlet and triplet manifolds in such
heavy metal complexes, which are commonplace for other
similar Pt cyclometalates. This can be corroborated with
the DFT results for 3 in which the valence orbital picture
consists of concomitant LC and MLCT components in the
lowest energy absorption (vide supra). The lowest energy
absorption peak of 1 and 2 is located at wavelength
(kabs = 434 nm for 1 and kabs = 439 nm for 2) longer than
that of 3 without the diarylamino units (kabs = 422 nm).
The introduction of an electron-donating diarylamino
group into the electron-deficient pyridine moiety is
expected to increase the intramolecular donor–acceptor
(D–A) character of the ligand, causing a red-shift of the
absorption features. All the Pt complexes show strong
emission bands at rt. For 1, there are two distinguishable
˚
Selected bond lengths (A) and angles (ꢁ) around the metal core in 1–3
Pt–N
Pt–C
Pt–O
C–Pt–N
O–Pt–O
1
2
3
1.990(4)
1.997(3)
1.985(3)
1.969(4) 2.085(3), 2.018(3) 81.07(17) 92.05(13)
1.958(3) 2.087(3), 1.994(3) 81.88(13) 91.95(11)
1.973(3) 2.080(2), 1.995(3) 81.49(13) 91.89(11)
observed in the spectra of Ar2N–C6H4–X compounds near
300 and 350 nm [24]. Also, the lowest-lying absorption fea-
tures observed in the visible light region beyond 400 nm
for 1 and 2 are solvatochromic (kabs = 429, 435 and
447 nm for 1 and 434, 440 and 453 nm for 2 as we decrease
the solvent polarity from DMSO to CH2Cl2 and CCl4),
indicative of a typical charge-transfer character, and are
thus tentatively attributed to an admixture of ILCT and
3p–p* excited states. The assignment was supported by the-
oretical calculations for 1 and 2, which reveal that the
HOMO is basically triarylamine-based and the LUMO
predominantly corresponds to the p* orbitals of the fluore-
nylpyridine group. However, we cannot totally rule out the
possibility of a minor MLCT-type component in the
absorption band since the HOMO ꢁ 1 level also contains
substantial contribution from the Pt’s d orbital character

G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
3465
Fig. 2. Contour plots of the frontier molecular orbitals for (a) 1, (b) 2 and (c) 3. (1 au = 27.2114 eV).
emission bands in CH2Cl2 solution at rt comprising essen-
3
tially the mixed LC fluorescence and p–p* phosphores-
cence. The broad and featureless band at ca. 498 nm
should be assigned to the ILCT emission which exhibits
a small Stokes shift of ca. 65 nm, a singlet lifetime of
2.52 ns and a usual rigidochromic blue-shift of 30 nm on
cooling of the solution sample to 77 K (Fig. 3) [19,25].
However, the band at ca. 569 nm should emanate from
3
the LC p–p* state for its vibrational fine structure
observed and the intensity enhancement at low tempera-
ture. Examination of the PL behavior of 1 in the solid-
state (kem = 568 nm) suggests no fluorescence peak and
the absence of aggregate states in the thin neat film (vide
infra), consistent with the X-ray structural data where
there is no apparent Ptꢀ ꢀ ꢀPt short contacts. Excitation
spectra were also taken for 1 in solution to confirm the
Fig. 3. UV–Vis and photoluminescence spectra of 1–3 in CH2Cl2.
Table 2
Photophysical and thermal data for 1–3
Compound
Absorption (298 K) kabs (nm)a
Emission kem (nm) 298 K/77 K
Up (%)b
sp (ls)c
DT5%/Tg (ꢁC)d
1
2
3
a
303 (4.36), 364 (4.23), 382 (4.26), 415 (4.18) 435 (4.21)
305 (4.49), 368 (4.29), 387 (4.34), 423 (4.36) 440 (4.40)
311 (4.36), 326 (4.43), 363e (3.92), 405 (3.82) 422 (3.80)
498, 569, 628e/468, 573, 625e
516, 570e, 632e/475, 575, 623e
538, 582e, 624e/541, 588e, 619e
3.9
8.6
16.8
58.3
54.6
25.0
305/176
303/175
301/136
Measured in CH2Cl2 at a concentration of 10ꢁ5 M and loge values are shown in parentheses.
In degassed CH2Cl2 relative to fac-[Ir(ppy)3] (Up = 0.40), kex = 400 nm.
b
c
Measured at 77 K in CH2Cl2 at a concentration of 10ꢁ5 M for the solution samples, and the excitation wavelength was set at 337 nm.
DT5% is the 5% weight-reduction temperature and Tg is the glass-transition temperature.
Shoulder band.
d
e

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G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
Table 3
Redox properties of 1–3
two emission bands to arise from the same species (see
Supporting Information). Complex 2 shows a similar PL
behavior to that of 1 at both rt and 77 K. At rt, the emis-
sion band fluoresces at 516 nm with a lifetime of 2.46 ns
E1/2 (V)a
E1/2 (V)a
red
HOMO (eV)
LUMO (eV)
ox
Complex
1
2
3
a
0.32, 0.48
0.26, 0.39
0.49b
ꢁ2.44
ꢁ2.42
ꢁ2.46
ꢁ5.12
ꢁ5.06
ꢁ5.29
ꢁ2.36
ꢁ2.38
ꢁ2.34
+
accompanied by
a phosphorescence shoulder peak.
Because of the much closer energy level between the ILCT
and the LC 3p–p* states, the triplet emission appeared only
as an unconspicuous shoulder on the ILCT emission. The
structured luminescence profile of 3 corresponds to the
typical stretching modes of the aromatic ligand which is
0.1 M [Bu4N]PF6 in CH2Cl2, scan rate 100 mV sꢁ1, vs. Fc/Fc couple.
Irreversible wave.
b
the LUMOs of these complexes (Fig. 2). Complexes 1 and
2 show two quasi-reversible oxidation waves (0.32 and
0.48 V for 1, 0.26 and 0.39 V for 2), which can be assigned
to the oxidation of the electron-rich diarylamino ligand
rather than the metal-localized oxidation since the Pt(II)
redox process is usually irreversible [19]. While complex 3
exhibits only one irreversible oxidation wave at ca. 0.49 V,
this event is probably caused by the oxidation of metal ion
[19], because the cyclometalating ligand in 3 lacks the diaryl-
amine moiety and as a result is more difficult to be oxidized as
compared to 1 and 2. When the diarylamino end group is
attached to the fluorene ring, each of 1 and 2 can show an ele-
vated HOMO energy level (ꢁ5.12 eV for 1 and ꢁ5.06 eV for
2) relative to 3 (ꢁ5.29 eV), indicating that compounds 1 and
2 are more electropositive (or have a lower ionization poten-
tial) than the non-arylamine capped analogue, and a better
HT ability in 1 and 2 can be expected. In other words, the dia-
rylamino capping group has the function of facilitating the
HI/HT characteristics of these bifunctional triplet emitters.
3
diagnostic of the involvement of the LC p–p* transition
in the emission, and its vibronic pattern likely precludes
the assignment of MLCT states that are usually broad
and featureless. When the size of the cyclometalating
organic group increases by attaching the diarylamino func-
tional group to the fluorenylpyridine ligand, the Pt(II) cen-
ter would be anticipated to show a lessened spin–orbital
coupling interaction in 1 and 2 relative to 3, presumably
because of extended delocalization of the HOMO and lack
of Pt participation in the transition state. As a result, both
LC fluorescence and phosphorescence can be observed in
the PL spectrum of 1 and 2 at rt [26]. Low-temperature
PL spectra were also recorded for 1–3 at 77 K and they
are all essentially dominated by the intense phosphores-
cence bands in frozen CH2Cl2 at the expense of the
higher-lying singlet emissions. Furthermore, the relatively
long lifetimes for 1–3 at 77 K (sp = 58.3 ls for 1, 54.6 ls
for 2 and 25.0 ls for 3) confirm the LC 3p–p* states instead
3
of MLCT states. Attachment of different R groups does
2.6. Electrophosphorescent OLED characterization
not seem to alter the phosphorescent emission energies
of 1 and 2. Presumably, the singlet state may be described
as an zwitterionic species and so the inductive effect is con-
sistent 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 anticipated [27]. Also, by looking at the fluores-
cence band, we see no vibronic structure while the phos-
phorescence has some. This suggests that there may be a
twist charge transfer phenomenon where a twist angle
changes much in the singlet state [28].
The thermal properties of 1–3 were characterized by
thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC). All the Pt complexes are thermo-
stable up to ca. 300 ꢁC. Complexes 1 and 2 show higher
glass-transition temperatures (Tg ꢂ 175 ꢁC) than that of 3
(136 ꢁC). These data are highly desirable for OLEDs of
high stability and efficiency. We contend that the 9-substi-
tuted fluorene moiety will be mainly responsible for confer-
ring the amorphous properties to the complexes while the
diarylamino unit plays a crucial role in improving Tg.
To illustrate the electrophosphorescent ability and light-
emitting performance in optoelectronic applications, the
OLEDs with triplet emitter 1 doped in different organic
hosts were fabricated and characterized. Firstly, 3,5-dicar-
bazolylbenzene (mCP) was chosen as the host material for
the excellent overlap between the UV–Vis absorption of 1
with the PL spectrum of mCP and the devices were made
in the configuration of ITO/NPB/x% Pt:mCP/TPBI/LiF/
Al (Fig. 4). 4,40-Bis[N-(1-naphthyl)-N-phenylamino]biphe-
nyl (NPB) layer is the HT layer. 2,20,200-(1,3,5-Pheny-
lene)tris(1-phenyl-1H-benzimidazole) (TPBI) behaves as
both a hole-blocker (HBL) and an electron-transporter
(ETL). LiF acts as an electron-injection layer and alumi-
num as the cathode. TPBI, which has a higher electron
mobility than the common materials such as 2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and
tris(8-hydroxyquinoline) (Alq3), was used to confine exci-
tons within the emissive zone. The HOMO (ꢁ5.12 eV)
and LUMO (ꢁ2.36 eV) levels in 1 closely match the energy
levels for NPB (HOMO: ꢁ5.2 eV) and mCP (HOMO:
ꢁ5.8 eV, LUMO: ꢁ2.3 eV). To optimize the device effi-
ciency, concentration dependence experiment was carried
out in the doping range between 1 and 12 wt.% (devices
A–D). In the present work, no emission from mCP was
observed for all doping levels of complex 1 even at high
current density, indicating an effective energy transfer from
2.5. Electrochemical and electronic characterization
The electrochemical properties of 1–3 were examined
using cyclic voltammetry (Table 3). All of them show a
quasi-reversible reduction couple between ꢁ2.42 and
ꢁ2.46 V, which is consistent with the pyridyl character for

G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
3467
N
N
Al (60 nm)
LiF (1 nm)
Device Dopant
HBL/ETL
mCP
A
B
C
D
E
1 wt.-% 1 in mCP
TPBI (45 nm)
TPBI (45 nm)
TPBI (45 nm)
TPBI (45 nm)
HBL/ETL (45 nm)
x% Pt:Host (20 nm)
NPB (75 nm)
ITO
5 wt.-% 1 in mCP
10 wt.-% 1 in mCP
12 wt.-% 1 in mCP
10 wt.-% 1 in CBP
BCP (20 nm)/Alq3 (2 nm)
N
N
Glass
CBP
N
N
N
O
N
N
O
Al
N
N
N
N
O
N
N
N
N
BCP
Alq3
NPB
TPBI
Fig. 4. The general structure of OLEDs and the chemical structure of the compounds used in these devices.
the host exciton to the platinum phosphor upon electrical
excitation and the effective hole-blocking of TPBI. There
is no sign of metal complex aggregation in the fabricated
devices. Fig. 5 shows the EL spectra of these devices at a
driving voltage of 8 V. From the vibronic progressions of
the spectra, the EL emission should mainly come from
might be due to the effective energy transfer between the
two emitting states. These results suggest that a new avenue
can be developed to ensure a good purity of emission color
of Pt phosphors by eliminating such drawbacks as strong
intramolecular interactions and poor emitting color purity
of the cyclometalated Pt(b-diketonato) complexes.
3
the LC p–p* states. Gratifyingly, we note that the maxi-
Basically, each of the devices A–D exhibits a strong
orange EL peak at about 570 nm with turn-on voltages
(Vturn-on) of 5.6–6.7 V for brightness at 1 cd/m2. The key
performance characteristics of the devices A–D are summa-
rized in Table 4. Fig. 6 presents current density–voltage–
luminance (J–V–L) curves of the Pt-doped devices A–D.
The luminance of the devices A–D reached 3158–4834 cd/
m2 at 15 V. The best device performance is achieved at
the doping level of 5 wt.% (device B). At 7 V, device B
achieved the external quantum efficiency (gext) of 4.65%
photons/electron (ph/el), current efficiency (gL) of
11.75 cd/A and power efficiency (gp) of 5.27 lm/W.
Although the dependence of EL color on the dopant level
of other square-planar Pt complexes has been reported,
in which the spectra displayed both the monomeric and
aggregated EL peaks with different relative intensities upon
variation of the dopant concentration, our results show
that complex 1 is suitable for the realization of single-color
EL devices by virtue of the bulkiness of the complex. Plots
of the device efficiencies as a function of current density for
A–D are depicted in Fig. 7. As is the case for other elec-
trophosphorescent devices, the device efficiencies witnessed
decay with increasing driving voltage and current density.
The decrease has been ascribed to a combination of trip-
let–triplet annihilation of the phosphor-bound excitons
[29] and field-induced quenching effects [30].
mum EL peak, spectral profile and the Commission Inter-
nationale de L’Eclairage (CIE) color coordinates are
almost invariant of the guest concentration in each case,
and no additional excimeric band was observed. This is
in contrast to many other devices based on similar plati-
num emitters which often show monomeric and excimeric
dual emissions even at low doping concentration due to
the exhibition of p–p stacking and metal–metal interactions
between adjacent molecules [11,19]. Furthermore, apart
3
from the p–p* phosphorescence, there is no emission for
the high-lying LC fluorescence in its EL spectra, which
1.0
0.8
0.6
0.4
0.2
0.0
Device D
Device C
increasing
concentration
of 1
Device B
Device A
400 450 500 550 600 650 700 750
Wavelength (nm)
In order to optimize the device performance further,
CBP was also employed as the triplet host for fabricating
Fig. 5. Concentration dependence of the EL spectra of 1-doped OLEDs
A–D at 8 V under different dopant concentrations with mCP as the host.

3468
G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
Table 4
Performance of 1-doped electrophosphorescent OLEDs
Device
Phosphor dopant
Vturn-on (V)
Luminance L (cd/m2)a
gext (%)
gL (cd/A)
gp (lm/W)
kmax (nm)d
A
1 (1 wt.%)
5.6
3158 (15)
4.39 (7)a
4.19b
11.40 (7)
10.89
3.75
11.75 (7)
10.13
6.08
10.04 (8.5)
9.39
4.15
8.98 (7)
8.76
4.23
4.89 (7)
4.42
1.10
5.27 (7)
3.92
1.80
3.71 (8.5)
3.17
1.17
4.03 (7)
2.84
1.17
568
(0.53, 0.46)
1.44c
B
C
D
E
a
1 (5 wt.%)
1 (10 wt.%)
1 (12 wt.%)
1 (10 wt.%)
6.2
6.7
6.5
5.3
4834 (15)
3371 (15)
3540 (15)
4195 (15)
4.65 (7)a
4.10b
572
(0.54, 0.46)
2.40c
4.18 (8.5)a
3.92b
572
(0.55, 0.45)
1.88c
3.55 (7)a
3.11b
572
(0.55, 0.45)
1.67c
6.64 (7.5)a
6.60b
15.41 (7.5)
15.30
10.31
7.07 (7.5)
6.03
2.87
572
(0.55, 0.45)
4.45c
Maximum values of the devices. Values in parentheses are the voltages at which they were obtained.
Values collected at 100 cd/m2.
Values collected at 1000 cd/m2.
b
c
d
CIE coordinates [x, y] in parentheses.
Alq3 serves as the electron-transporter. Our results show
that the optimum device performance is achieved at the
doping level of 10 wt.% (device E) with a turn-on voltage
of 5.3 V, a maximum luminance of 4195 cd/m2 at 15 V, gext
of 6.64% at 7.5 V, gL of 15.41 cd/A at 7.5 V, and gp of
7.07 lm/W at 6.5 V (Table 5). The J–V–L characteristics,
EL spectrum and the efficiency curves of device E are
shown in Figs. 8–10, respectively. Device E shows a similar
EL spectrum with that of device B, the best device with
mCP host. We observe that device E outperforms that of
device B and its EL spectrum is also dominated by the
5000
4000
3000
2000
1000
0
Device A
Device B
Device C
Device D
100
10
1
0.1
0.01
3
6
8
10
12
14
16
LC p–p* emission at kmax = 570 nm. The lower triplet
Voltage (V)
energy of CBP (kmax ꢂ 460 nm) [31] compared with that
of mCP (kmax ꢂ 410 nm) [11,32] ensures a more efficient
energy transfer for the triplet of CBP to the LC 3p–p* states
of 1.
Fig. 6. The current density–voltage–luminance (J–V–L) characteristics of
devices A–D.
On the other hand, WOLEDs have also drawn much
recent attention in the academic and industrial R&D sec-
tors because of their potential use in display backlights, full
color applications, as well as in solid-state lighting pur-
poses [11,12,33]. But most WOLEDs utilize emission from
several different colored emitters in separate layers to avoid
the OLEDs with the configuration ITO/NPB/x% Pt:CPB/
BCP/Alq3/LiF/Al using a combination of BCP/Alq3 as
the alternative layer instead of TPBI for the best results,
typical for Ir(ppy)3-type compounds (Hppy = 2-phenyl-
pyridine). BCP is used as the hole-blocking material and
6
5
4
3
12
10
8
5
4
3
2
1
6
η
2
η
4
η
Device A
Device B
Device C
Device D
Device A
Device A
Device B
Device C
Device D
Device B
Device C
Device D
1
2
0
0
0
0.01
0.1
1
10
100
0.01
0.1
1
10
100
0.01
0.1
1
10
100
Current density (mA/cm2)
Current density (mA/cm2)
Current density (mA/cm2)
Fig. 7. The efficiencies of the devices A–D as a function of current density.

G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
3469
Table 5
Summary of crystal structure data for 1–3
1 Æ CHCl3
40H37Cl3N2O2Pt
2
3
Formula
Mr
C
C41H40N2O2Pt
787.84
C27H27NO2Pt
592.59
879.16
crystal size (mm)
Crystal system
Space group
0.30 · 0.19 · 0.17
Monoclinic
P21/c
24.1106(15)
13.6691(9)
13.1241(8)
90
90.2780(10)
90
4325.3(5)
4
0.32 · 0.24 · 0.22
Monoclinic
P21/c
9.7385(5)
11.2842(5)
31.6544(15)
90
94.9410(10)
90
3465.6(3)
4
0.28 · 0.25 · 0.20
Monoclinic
P21/c
8.1679(5)
26.7111(15)
10.9458(6)
90
107.2510(10)
90
2280.7(2)
4
˚
a (A)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (g cmꢁ3
)
1.350
3.461
1744
1.510
4.086
1576
1.726
6.176
1160
l (mmꢁ1
F(000)
)
2h Range (ꢁ)
2.25–25.00
20919
7563
2.10–28.27
20129
7979
2.47–28.30
13355
5267
Number of reflections collected
Number of unique reflections
Rint
0.0344
0.0206
0.0232
Number of reflections with I > 2.0r(I)
Number of parameters
R1, wR2 [I > 2.0r(I)]a
5598
469
0.0422, 0.1168
0.0648, 0.1322
1.070
6760
415
0.0307, 0.0710
0.0394, 0.0742
1.082
4382
280
0.0272, 0.0616
0.0309, 0.0628
1.176
R1, wR2 (all data)a
Goodness-of-fit on F2b
P
P
P
P
a
2
R1 = iFoj ꢁ jFci/ jFoj. wR2 = { [w(Fo ꢁ Fc2)2]/ [w(Fo2)2]}1/2
.
P
b
GoF = [( wjFoj ꢁ jFcj)2/(Nobs ꢁ Nparam)]1/2
.
140
120
100
80
4000
3000
2000
1000
60
40
20
0
0
4
6
8
10
12
14
16
Voltage (V)
Fig. 9. The current density–voltage–luminance (J–V–L) curves of device
E.
Fig. 8. The EL spectra of device E with CBP as the host (the EL of device
B and solid-state PL of 1 are included in the inset).
the ready energy transfer from the higher energy dye
to the lower energy dye and achieve a well-balanced
emission color. Therefore, this will complicate the device
structure and fabrication process. Additionally, in order to
realize the generation of white EL that is not driven by
varying the bias voltage, we obtained here a simple
WOLED through a control of the extent of energy transfer
from the blue host 9,10-di(2-naphthyl)anthracene (ADN)
[34] to the orange dopant 1 in a configuration similar to
that of devices A–E by replacing mCP or CBP with
ADN. A near-white emission is produced by the simulta-
neous EL of both the blue host and triplet excitons of
the dopant (Fig. 11). The CIE coordinates of this device
is (0.34, 0.39), which is very close to that of ideal white
point of (0.33, 0.33). While preliminary results indicate that
the EL efficiency is not yet attractive (<0.2% ph/el or
0.4 cd/A) because of the low triplet energy of ADN and
hence less efficient energy transfer to the dopant, such a
strategy would be useful for obtaining WOLEDs in a single
dopant configuration, in contrast to most examples of
white phosphorescent OLEDs that require dual or multiple
dopants in multilayer devices. More detailed work on this

3470
G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
8
6
4
2
pure orange-emitting OLEDs of high performance. After
16
14
12
10
8
optimization of the fabrication conditions, all the devices
emit an intense orange light at about 570 nm with very sta-
ble CIE color coordinates and the best device shows an
external quantum efficiency of 6.64% corresponding to cur-
rent and power efficiencies of 15.41 cd/A and 7.07 lm/W.
Additionally, these bifunctional platinum phosphors
should have the potential to excel in the development of
WOLEDs through a wise combination with an appropriate
blue host or emitter. Management of the relative EL inten-
sities of the blue host/emitter and the dopant can be a valu-
able tool for making single-dopant WOLEDs.
η
η
ηext
ηp
ηL
6
η
4
2
0.01
0.1
1
10
100
2
Current density (mA/cm )
4. Experimental
Fig. 10. The dependence of EL efficiencies of device E on current density.
4.1. General procedures
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
All reactions were performed under nitrogen. Solvents
were carefully dried and distilled from appropriate drying
agents prior to use. Commercially available reagents were
used without further purification unless otherwise stated.
The procedure for the synthesis of (7-bromo-9,9-diethylflu-
oren-2-yl)diphenylamine followed the methods reported in
the literature [35]. All reactions were monitored by thin-
layer chromatography (TLC) with Merck pre-coated glass
plates. Compounds were visualized with UV light irradia-
tion at 254 and 365 nm. Flash column chromatography
and preparative TLC were carried out using silica gel from
Merck (230–400 mesh) at rt. UV–Vis spectra were obtained
on a HP-8453 spectrophotometer. The photoluminescent
properties and lifetimes of the compounds were probed on
the Photon Technology International (PTI) Fluorescence
Master Series QM1 system. The phosphorescence quantum
yields were determined in CH2Cl2 solutions at 298 K against
fac-[Ir(ppy)3] as a reference (U = 0.40) [36]. Fast-atom bom-
bardment (FAB) mass spectra were recorded on a Finnigan
MAT SSQ710 system. NMR spectra were measured in
CDCl3 on a Varian Inova 400 MHz FT-NMR spectrome-
ter; chemical shifts were quoted relative to the internal stan-
400
500
600
700
800
Wavelength (nm)
Fig. 11. The EL spectrum of WOLED prepared using complex 1 in the
ADN host.
aspect is still ongoing for the eventual realization of white
light sources.
3. Conclusions
1
In summary, the present work reports the synthesis and
characterization of some new light-emitting cyclometalated
complexes of platinum(II) incorporating both hole-trans-
porting arylamine and light-emitting fluorene moieties,
study of their photophysical properties, single-crystal X-
ray structural analyses and their applications in monochro-
matic and white OLEDs. Saliently, end-capping of the
fluorene-based metal phosphors with the sterically hin-
dered NAr2 chromophore can offer advantages in terms
of lowering the first ionization potential, which can
improve the hole-transporting properties as well as sup-
pressing excimer formation. Due to the multi-component
ligand structure, we can successfully circumvent the draw-
backs of aggregate formation among the phosphor mole-
cules as revealed from X-ray diffraction studies and
suppress the unwanted intermolecular interactions of the
dopant that would otherwise cause concentration-depen-
dent spectral changes. In this way, it enables us to prepare
dard tetramethylsilane for H and 13C{1H} NMR data.
Thermal analyses were performed with the Perkin–Elmer
Pyris Diamond DSC and Perkin–Elmer TGA6 thermal ana-
lyzers. Cyclic voltammetry was performed using an EG&G
potentiostat model 283. Anhydrous THF was used as the
solvent under a nitrogen atmosphere, and 0.1 M tetra(n-
butyl)ammonium hexafluorophosphate was used as the
supporting electrolyte. A Pt wire was used as the pseudo-ref-
erence electrode, and a Pt sheet was used as the counter elec-
trode. The working electrode was glassy carbon. The redox
potentials were reported relative to a ferrocene/ferrocenium
(Cp2Fe/Cp2Fe+) redox couple used as an internal standard
[37]. The oxidation (Eox) and reduction (Ered) potentials
were used to determine the HOMO and LUMO energy
levels using the equations EHOMO = ꢁ(Eox + 4.8) eV and
ELUMO = ꢁ(Ered + 4.8) eV which were calculated using
the internal standard ferrocene value of ꢁ4.8 eV with
respect to the vacuum level [38]. DFT calculations at the

G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
3471
B3LYP level were performed based on their experimental
geometries from the X-ray data. The basis set used for C,
O, N and H atoms was 6-31G while effective core potentials
with a LanL2DZ basis set were employed for Pt [39]. All the
calculations were performed with the GAUSSIAN 03 package.
For the OLED fabrication and measurements, commercial
indium tin oxide (ITO)-coated glass with sheet resistance
of 20–30 X/square was used as the starting substrate. Before
device fabrication, the ITO glass substrates were cleaned by
ultrasonic baths in organic solvents followed by ozone treat-
ment for 10 min. Each of the orange-emitting devices was
assembled in the following sequence: ITO on glass substrate
(anode), 75 nm of NPB, 20 nm of the emitting layer made of
the organic host (mCP or CBP) and phosphorescent Pt dop-
ant (x%), 45 nm of TPBI or 20 nm of BCP followed by 2 nm
of Alq3, 1 nm of LiF, and 60 nm of Al (cathode). The
organic layers were evaporated and laminated in the above
sequence under 4 · 10ꢁ4 Pa without breaking vacuum
between each vacuum deposition process. The emissive layer
was formed by co-deposition of the dopant and the host.
The layer thickness was monitored in situ using a quartz
crystal oscillator. The active area of the device was 5 mm2
as defined by the shadow mask. The electrical and optical
characteristics of these devices were measured using R6145
DC voltage–current source, FLUKE 45 dual display mul-
timeter and Spectrascan PR650 spectrophotometer in a
dark room under ambient air condition.
129.12, 128.27, 123.86, 123.44, 122.40, 121.10, 120.81,
120.39, 118.99, 118.51, 117.35 (Ar), 102.51 (acac CH),
55.65 (quat. C), 32.88, 32.45, 28.30, 8.57 (Et + Me).
FAB-MS (m/z): 759 [M+]. Anal. Calc. for C39H36N2O2Pt:
C, 61.65; H, 4.78; N, 3.69. Found: C, 61.56; H, 4.55; N,
3.40%.
4.2.2. Synthesis of (2)
The complex was synthesized from HL2 by a similar
protocol as above for 1, and it was isolated as an orange
1
powder in an overall yield of 15%. H NMR (CDCl3): d
(ppm) 8.97 (d, J = 4.8 Hz, 1H, Ar), 7.84 (s, 1H, Ar),
7.77–7.74 (m, 1H, Ar), 7.65–7.60 (m, 2H, Ar), 7.33 (s,
1H, Ar), 7.07–6.94 (m, 10H, Ar), 6.95 (dd, J = 8.0,
2.0 Hz, 1H, Ar), 5.49 (s, 1H, acac), 2.32 (s, 6H, Me), 2.05
(s, 3H, acac), 2.01 (s, 3H, acac), 1.98–1.86 (m, 4H, Et),
0.36 (t, J = 7.6 Hz, 6H, Et). 13C NMR (CDCl3): d (ppm)
185.71, 184.08 (acac CO), 168.50, 151.87, 147.68, 147.17,
145.64, 145.21, 143.09, 142.18, 137.83, 137.57, 136.04,
131.94, 129.72, 124.09, 122.29, 120.94, 120.59, 120.29,
118.10, 117.77, 117.32 (Ar), 102.51 (acac CH), 55.59 (quat.
C), 32.89, (Et), 28.31, 27.21 (acac Me), 20.80 (Me), 8.57
(Et). FAB-MS (m/z): 787 [M+]. Anal. Calc. for
C41H40N2O2Pt: C, 62.50; H, 5.12; N, 3.56. Found: C,
62.34; H, 5.02; N, 3.40%.
4.2.3. Synthesis of (3)
This compound was prepared similarly from HL3 and it
was isolated as a yellow–orange solid in 35% yield. 1H
NMR (CDCl3): d 9.00 (d, J = 5.6 Hz, 1H, Ar), 7.97
(s, 1H, Ar), 7.82–7.77 (m, 2H, Ar), 7.69 (d, J = 8.0 Hz,
1H, Ar), 7.39 (s, 1H, Ar), 7.35–7.30 (m, 3H, Ar), 7.10–
7.06 (m, 1H, Ar), 5.51 (s, 1H, acac), 2.08–2.02 (m, 10H,
Et + Me), 0.33 (t, J = 7.4 Hz, 6H, Et). 13C{1H} NMR
(CDCl3): d 185.72, 184.10, 168.39, 150.59, 147.23, 145.33,
143.27, 143.03, 141.74, 137.92, 137.41, 127.09, 126.65,
122.59, 121.29, 120.63, 120.41, 118.29, 117.46 (Ar),
102.55 (acac CH), 55.67 (quat. C), 32.68, 28.32, 27.21,
8.55 (Et + Me). MS (FAB) m/z: 592 [M+]. Anal. Calc.
for C27H27NO2Pt: C, 54.72; H, 4.59; N, 2.36. Found: C,
54.59; H, 4.75; N, 2.12%.
4.2. Preparations of complexes
The syntheses of the cyclometalating ligands are given in
the Supporting Information.
4.2.1. Synthesis of (1)
Under a N2 atmosphere, HL1 (0.50 g, 1.07 mmol) and
K2PtCl4 (0.28 g, 0.67 mmol) were added to a 2-ethoxyetha-
nol/water mixture (15 mL, 3:1, v/v). The reaction mixture
was stirred at 80 ꢁC for 16 h. After it was cooled to rt,
the reaction mixture was poured into water (200 mL). A
yellow precipitate appeared which was collected and dried
under vacuum to give the chloro-bridged dimer as a yellow
solid that was used for the next step without further puri-
fication. The Pt dimer (0.40 g, 0.29 mmol), Na2CO3
(0.60 g, 5.66 mmol) and acetylacetone (0.2 mL) were added
to 2-ethoxyethanol (10 mL) under a N2 atmosphere. The
reaction temperature was kept at 100 ꢁC for 16 h. After
reaction, it was cooled to rt and water (50 mL) was added.
The yellow precipitate was collected and dried. It was then
purified by column chromatography with CH2Cl2/hexane
(1:1, v/v) as eluent. The product was obtained as a bright
5. X-ray crystallography
Diffraction data were collected at 293 K using graphite
˚
monochromated Mo Ka radiation (k = 0.71073 A) on a
Bruker Axs SMART 1000 CCD diffractometer. The col-
lected frames were processed with the software SAINT+
[40] and an absorption correction (SADABS) [41] was applied
to the collected reflections. The structure was solved by the
Direct methods (SHELXTL) [42] in conjunction with standard
difference Fourier techniques and subsequently refined by
full-matrix least-squares analyses on F2. Hydrogen atoms
were generated in their idealized positions and all non-
hydrogen atoms were refined anisotropically. Table 5 con-
tains some pertinent crystal data for 1–3.
1
orange solid (0.05 g, 11%). H NMR (CDCl3): d 8.97 (d,
J = 5.1 Hz, 1H, Ar), 7.87–7.62 (m, 3H, Ar), 7.33–6.97 (m,
15H, Ar), 5.49 (s, 1H, acac), 2.05 (s, 3H, Me), 2.01 (s,
3H, Me), 1.97–1.87 (m, 4H, Et), 0.37 (t, J = 7.3 Hz, 6H,
Et). 13C NMR (CDCl3): d 185.74, 184.10, 168.49, 152.04,
148.05, 147.24, 145.33, 142.88, 142.49, 137.86, 136.95,

3472
G.-J. Zhou et al. / Journal of Organometallic Chemistry 692 (2007) 3461–3473
[6] (a) I.M. Dixon, J.P. Collin, J.P. Sauvage, L. Flamigni, S. Encinas, F.
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Council (Grant No.: HKBU202106) and Hong Kong Bap-
tist University (FRG/04-05/II-59) for the financial support
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Appendix A. Supplementary material
CCDC 639790, 639791 and 639792 contain the supple-
mentary crystallographic data for this paper. These data
can be obtained free of charge via http://www.ccdc.cam.a-
c.uk/conts/retrieving.html, or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:
10.1016/j.jorganchem.2007.04.013.
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