Synthesis, characterization, photophysics and electrophosphorescent applications of phosphorescent platinum cyclometalated complexes with 9-arylcarbazole moieties

Article abstract of DOI:10.1016/j.jorganchem.2009.03.002

A series of cyclometalating platinum(II) complexes with substituted 9-arylcarbazolyl chromophores have been synthesized and characterized. These complexes are thermally stable and most of them have been characterized by X-ray crystallography. The phosphorescence emissions of the complexes are dominated by ³MLCT excited states. The excited state properties of these complexes can be modulated by varying the electronic characteristics of the cyclometalating ligands via substituent effects, thus allowing the emission to be tuned from bright green to yellow, orange and red light. The correlation between the functional properties of these metallophosphors and the results of density functional theory calculations was made. Because of the propensity of the electron-rich carbazolyl group to facilitate hole injection/transport, the presence of such moiety can increase the highest occupied molecular orbital levels and improve the charge balance in the resulting complexes relative to the parent platinum(II) phosphor with 2-phenylpyridine ligand. The solution-processed electrophosphorescent organic light-emitting diodes doped with these platinum-based phosphors have been fabricated which showed a maximum external quantum efficiency of 2.77% for the best device, corresponding to a power efficiency of 3.48 lm/W and a luminance efficiency of 8.49 cd/A. The present work enables the rational design of platinum-carbazolyl electrophosphors by synthetically tailoring the structure of carbazolylpyridine ring that can permit good color-tuning versatility suitable for multi-color display technology.

Full text of DOI:10.1016/j.jorganchem.2009.03.002

Journal of Organometallic Chemistry 694 (2009) 2735–2749
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, photophysics and electrophosphorescent
applications of phosphorescent platinum cyclometalated complexes
with 9-arylcarbazole moieties
a,
b
b,
b
c
Cheuk-Lam Ho a, , Wai-Yeung Wong , Bing Yao , Zhiyuen Xie , Lixiang Wang , Zhenyang Lin
*
*
*
a Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, PR China
b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
c Department of Chemistry, The Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of cyclometalating platinum(II) complexes with substituted 9-arylcarbazolyl chromophores have
been synthesized and characterized. These complexes are thermally stable and most of them have been
characterized by X-ray crystallography. The phosphorescence emissions of the complexes are dominated
by 3MLCT excited states. The excited state properties of these complexes can be modulated by varying the
electronic characteristics of the cyclometalating ligands via substituent effects, thus allowing the emis-
sion to be tuned from bright green to yellow, orange and red light. The correlation between the functional
properties of these metallophosphors and the results of density functional theory calculations was made.
Because of the propensity of the electron-rich carbazolyl group to facilitate hole injection/transport, the
presence of such moiety can increase the highest occupied molecular orbital levels and improve the
charge balance in the resulting complexes relative to the parent platinum(II) phosphor with 2-phenyl-
pyridine ligand. The solution-processed electrophosphorescent organic light-emitting diodes doped with
these platinum-based phosphors have been fabricated which showed a maximum external quantum effi-
ciency of 2.77% for the best device, corresponding to a power efficiency of 3.48 lm/W and a luminance
efficiency of 8.49 cd/A. The present work enables the rational design of platinum-carbazolyl electrophos-
phors by synthetically tailoring the structure of carbazolylpyridine ring that can permit good color-tun-
ing versatility suitable for multi-color display technology.
Received 5 February 2009
Received in revised form 27 February 2009
Accepted 2 March 2009
Available online 9 March 2009
Keywords:
Carbazole
Platinum complexes
Optical properties
Organic light-emitting diodes
Phosphorescence
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
metal ion results in strong spin–orbit coupling in these complexes,
which promotes efficient mixing of singlet and triplet states, thus
Luminescent platinum(II) compounds with organic conjugated
ligands have attracted much attention since they may potentially
be used as new materials for applications, including organic
light-emitting devices (OLEDs) [1,2], dye-sensitized solar cells [3],
photoreceptors in biological molecules [4], etc. In particular, re-
search on OLEDs has made significant advances by many research
groups over the past several years, demonstrating them as candi-
dates for full-color display applications. For OLED applications,
successful materials used should be facile with balanced charge
transport as well as high conversion efficiency of excitons to light
[5]. Although relatively fewer d8 metal complexes are known to be
emissive in fluid solutions at room temperature compared to other
metal complexes, the phosphorescence of cyclometalated plati-
num(II) complexes has been well documented. The heavy Pt(II)
enhancing phosphorescence emission and shortening emission
lifetimes [6,7]. Room temperature phosphorescence emission from
simple Pt(II) acetylides is rarely detected due to the presence of
low-lying metal-centered excited states, which provide facile radi-
ationless deactivation pathways via molecular distortion [8–10].
Thus, in order to obtain strong luminescence at room temperature
it is necessary to utilize ligands with low-lying excited state orbi-
tals or strong electron-donating ability. The strong ligand field
associated with such ligands raises the energy of the metal d–d
states, withdrawing their deactivating effect. Among them, Pt(II)
complexes incorporating pyridine ligands such as bipyridine [11–
23] and terpyridine [24] derivatives have attracted a great deal of
interest as luminescent dyes for OLED applications since they often
exhibit high phosphorescence quantum yields. On the other hand,
several cyclometalated homo- or heteroleptic Pt(II) complexes
based on 2-phenylpyridine (ppy) and analogues have been re-
ported in the literature [25–27].
* Corresponding authors. Tel.: +852 34117074; fax: +852 34117348 (W.-Y.
Wong).
E-mail addresses: lamlam_ho2004@yahoo.com.hk (C.-L. Ho), rwywong@hkbu.
edu.hk (W.-Y. Wong), xiezy_n@ciac.jl.cn (Z. Xie).
In order to better meet the requirements of OLEDs, a proper
choice of the electron and hole-transport materials is important
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.03.002

2736
C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
in order to achieve a better confinement of charge recombination
region and, therefore, a better efficiency, can be achieved in a mul-
tilayer device. One simple and useful method for the construction
of OLEDs employs a bilayer structure comprising a hole-transport
X
N
X
N
X
X
I
H
1,10-phenanthroline
N
NBS (1 equiv)
CH2Cl2
N
SnBu3
+
CuI, KOH
Pd(PPh3)4, toluene
110 oC, 48 h
N
p-xylene, 140 oC
Br
N
X= H
F
L1
L2
OMe L3
X1
X2
NBS (1 equiv)
CH2Cl2
(i) BuLi, THF
N
Cl
N
N
N
N
(ii) B(OMe)3
(iii) HCl
Pd(PPh3)4, toluene, 2 M Na2CO3
110 oC, 48 h
Br
B(OH)2
X1
X2
X1 = H, X2 = CF3 L4
N
X1 = H, X2 = F
X1 = Me, X2 = H
L5
L6
N
Br
Br
N
N
I
L7
H
1,10-phenanthroline
CuI, KOH
N
SnBu3
N
Pd(PPh3)4, toluene
110 oC, 48 h
p-xylene, 140 oC
Br
N
I
Br
N
N
L8
X
X
N
X
Cl
(i) BuLi, THF
N
N
N
(ii) B(OMe)3
(iii) HCl
Pd(PPh3)4, toluene, 2 M Na2CO3
110 oC, 48 h
Br
B(OH)2
N
X = H
L9
X = OMe L10
Scheme 1. Synthetic routes to cyclometalating ligands L1–L10
.

C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
2737
layer and an electron-transport layer, with one or both layers being
luminescent [28]. The durability, i.e. thermal and morphological
stability of deposited films, is another important factor that has a
dramatic influence on the physical performance of OLEDs [29]. It
is well recognized that the thermal stability or glassy-state dura-
bility of organic compounds can be greatly improved upon incor-
poration of a carbazole moiety in the core structure [30–32].
Therefore, thermally and morphologically stable dyes with hole-
transporting (HT) and light-emitting carbazole chromophore are
a preferred choice for the fabrication of OLED devices. Due to the
fact that the nature of cyclometalating ligands can influence dras-
tically the lowest energy emitting excited state [33–36], adding the
electron-donating or -withdrawing groups and different degree of
[37–42]. L1–L3 can be obtained from N-arylation of carbazole by
the modified Ullmann condensation between carbazole and the
appropriate p-iodoarene using CuI/phen/KOH as the catalyst com-
bination. The desired N-arylated 3-bromocarbazole precursors can
be readily synthesized from the reaction of the corresponding carb-
azoles with one stoichiometric amount of N-bromosuccimide
(NBS). These compounds were then converted into their cyclo-
metalating partners L1ꢀL3 by using Pd-catalyzed Stille-coupling
reaction with 2-(tributylstannyl)pyridine [43]. The X group spans
from H to F and OMe. Pyridyl-substituted 9-phenyl-3-(pyridin-2-
yl)carbazoles L4–L6 were synthesized from palladium-catalyzed
Suzuki cross coupling [44–47] of commercially available 2-
chloro-5-trifluoromethylpyridine, 2-chloro-5-fluoropyridine or 2-
chloro-4-methylpyridine with 9-phenylcarbazole-3-boronic acid,
respectively [48,49]. For L7–L8, the N-arylation of carbazole was
carried out by the Ullmann condensation of carbazole with 1-bro-
mo-4-iodobenzene and 2-bromo-7-iodo-9,9-diethylfluorene [50]
to give 9-(4-bromophenyl)carbazole and 9-(7-bromo-9,9-diethyl-
fluoren-2-yl)carbazole which can then undergo Stille-coupling
protocol with 2-(tri-n-butylstannyl)pyridine to afford L7 and L8
p-electron conjugation into different positions of the ligands can
increase or decrease the amount of electron density at the metal
center. The amount of ground state electron density on the metal
will subsequently affect the contribution of MLCT character in
the lowest energy transition, thus altering the color of emission
and radiative lifetime of the excited state of the resulting com-
plexes. Therefore, the purpose of this study is to design a new ser-
ies of electroactive multi-component Pt(II) cyclometalated
complexes with hole-transporting carbazole in which the electro-
luminescent and HT components are integrated into a single mol-
ecule. The electrophosphorescence properties of these metal
complexes were also investigated in solution-processed OLEDs.
which carries
a cyclometalating 2-phenylpyridine site for
coordination with metal groups. For the isoquinoline-based
cyclometalating ligands L9–L10, 3-bromo-9-phenylcarbazole and
3-bromo-9-(4-methoxyphenyl)carbazole were first converted to
their 3-boronic acid derivatives, which then underwent subse-
quent Suzuki coupling reactions with 1-chloroisoquinoline to give
2. Results and discussion
the desired ligands L9–L10
.
The chelating cyclometalating ligands L1–L10 were then used to
synthesize a series of new Pt(II) complexes. The targeted Pt(II)
emitting materials (Scheme 2) were obtained via two distinctive
pathways as previously reported [6]. The first one involves heating
the K2PtCl4 salt with 2.5 equivalents of the cyclometalating ligands
2.1. Preparation and characterization
A series of carbazole-based cyclometalating ligands L1–L10 were
synthesized according to the reaction steps shown in Scheme 1
N
C
N
Cl
Cl
K2PtCl4
Pt
Pt
C
N
2-ethoxyethanol/water
C
CH3
H3C
O
O
Acetylacetone/Na2CO3
2-ethoxyethanol
Pt
N
C
X
N
N
=
C
N
X = X1 = X2 = H
X = F, X1 = X2 = H
X = OMe, X1 = X2 = H
1
2
3
7
X = H, X1 = CF3, X2 = H
X = H, X1 = F, X2 = H
X = H, X1 = H, X2 = Me
4
5
6
N
N
X1
X2
X
N
N
X = H
9
8
OMe 10
N
N
Scheme 2. Synthetic routes to new platinum(II) complexes 1ꢀ10.

2738
C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
Table 1
in a 3:1 mixture of 2-ethoxyethanol and water to 80 °C. The dimers
Selected bond lengths (Å) and angles (°) around the metal core in 1ꢀ4 and 8ꢀ10.
were isolated without characterization and used directly in the
subsequent reactions. The generated Pt(II)
l-dichloro-bridged di-
PtꢀN
PtꢀC
PtꢀO
CꢀPtꢀN
OꢀPtꢀO
mers were then cleaved with acetylacetone (Hacac) in the presence
of a base (Na2CO3) in 2-ethoxyethanol at an elevated temperature.
Purification of the mixture by silica chromatography furnished the
corresponding mononuclear complexes 1–10 in different solid-
state colors as air-stable powders in high purity.
1
1.996(5)
1.953(6)
2.106(4)
1.999(4)
2.108(5)
2.003(4)
2.094(3)
2.004(3)
2.099(3), 1.993(3)
[2.099(3), 1.990(3)]
2.086(3)
1.995(3)
2.094(3)
2.006(3)
2.104(8), 2.019(7)
[2.105(8), 2.012(7)]
81.8(2)
91.4(2)
2
1.988(5)
1.999(4)
1.963(6)
1.961(4)
81.4(2)
81.8(2)
91.5(2)
91.4(1)
3
4a
8
1.986(4)
[1.997(4)]
1.986(4)
1.963(5)
[1.966(5)]
1.972(4)
81.5(2)
[81.4(2)]
81.2(2)
91.9(1)
[91.5(1)]
91.8(1)
All of the complexes are soluble in chlorinated solvents and can
be stored without any special precautions. The identities of the
products were accomplished through NMR spectroscopic and mass
spectrometric methods from which they were shown to consist of
well-defined structures. In all cases, 1H NMR resonances arising
from the protons of the organic moieties were observed. For in-
stance, the ortho proton on the pyridyl ring shows a characteristic
doublet at the most downfield position in the proton NMR spec-
trum in each case. The CH resonance of the acac ligand is located
typically at around 5.40 ppm. Two distinct methyl protons of acac
fragment were observed as a singlet at around 2 ppm. Some of the
carbon NMR spectra were not recorded due to the limited solubil-
ity of these complexes in deuterated solvents. For the measured 13C
NMR spectra, two sets of non-equivalent methyl carbons of acac
groups were observed due to the asymmetrical chemical environ-
ments of two sets of carbons restricted by the square-planar geom-
etry of Pt complexes. The formulae of 1ꢀ10 were also successfully
established by appearance of the intense molecular ion peaks in
their positive FAB mass spectra.
9
1.986(3)
1.953(4)
80.3(2)
91.3(1)
10
2.000(9)
[1.976(8)]
1.96(1)
[1.96(1)]
80.3(2)
[80.8(4)]
91.4(3)
[90.4(3)]
a
The values for the second independent molecule per asymmetric unit are
shown in parentheses.
2.2. Crystal structures
To further establish their molecular structures, a majority of
these Pt(II) complexes (1–4 and 8–10) were subjected to X-ray sin-
gle-crystal diffraction studies. Most of them can rapidly form
microcrystalline solids from solvent mixtures of chlorinated
hydrocarbons and hexane. Perspective drawings of complexes
1–4 and 8–10 are shown in Fig. 1 and key bonding parameters
Fig. 1. ORTEP drawings of 1–4 and 8–10, with thermal ellipsoids shown at the 25% probability levels. Labels on carbon atoms (except for those bonded to Pt) and all hydrogen
atoms are omitted for clarity.

C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
2739
are given in Table 1. Due to the bulkiness of carbazolyl chromoph-
ores, all of the structures do not show the stacking interac-
[2.086(2)–2.108(5) Å] than with nitrogen atom [1.990(3)–
2.019(7) Å], a feature commonly observed for other chelating dike-
tonato complexes containing bidentate cyclometalating ligands
[1,51–56] and the bite N–Pt–C angles are similar to those found
for related (C^N)Pt-type complexes [6]. For 4 and 10, two unique
independent molecules were observed in the asymmetric unit of
the unit cells in each case.
p–p
tions between molecules. In all cases, the platinum center
exhibits the expected distorted square-planar coordination and
the phenyl–pyridyl metallacycle is essentially coplanar. There is a
very little distortion away from the square plane, and the C–Pt–N
and O–Pt–O chelate planes are only slightly bowed. The bond
length of Pt–N [1.975(3)–2.000(9) 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 two Pt–O bond
lengths are different, the longer of the two consistently corre-
sponding to that disposed trans to the cyclometalating carbon. This
2.3. Photophysical and thermal properties
Since good thermal stabilities of the compounds are very impor-
tant for the OLED applications, the onset decomposition tempera-
tures (Tdecomp) of the new compounds were determined from
is
a reflection of the stronger trans influence with carbon
Table 2
Photophysical and thermal data for 1ꢀ4.
Absorption (293 K)
Emission (293 K)
Emission (77 K)
Tdec (°C)
kabs (nm)a CH2Cl2
kem (nm) CH2Cl2
UP
sP
[ls]
kem (nm) CH2Cl2
sP (l
s)c
b
c
1
2
259 (1.99)
300 (2.66)
327 (2.63)
366 0.72)
373 (0.67)
408 (0.04)
259 (2.02)
299 (2.65)
327 (2.63)
366 (0.70)
374 (0.58)
408 (0.18)
261 (4.19)
300 (4.81)
327 (4.69)
373 (1.48)
409 (0.75)
264 (4.64)
304 (7.32)
336 (4.07)
393 (2.53)
432 (0.17)
263 (9.65)
301 (13.60)
333 (8.71)
377 (2.64)
422 (0.79)
256 (4.71)
303 (5.52)
323 (6.11)
363 (1.97)
406 (0.93)
261 (11.65)
293 (5.70)
329 (4.07)
345 (4.31)
401 (1.45)
259 (11.86)
294 (9.76)
344 (10.19)
405 (3.47)
422 (3.18)
262 (3.87)
321 (3.34)
334 (2.71)
384 (1.25)
413 (1.34)
469 (0.61)
258 (3.07)
322 (2.62)
385 (0.49)
414 (1.02)
467 (0.87)
493
525sh
0.11
0.13
486
27.9
324
493sh
513sh
524
557sh
493
524sh
0.12
0.12
491
501sh
529
21.7
312
561sh
3
4
5
6
7
8
9
495
525sh
0.08
0.16
0.1
0.14
0.13
0.09
0.14
0.35
494
25.2
9.7
289
312
234
350
318
320
297
503sh
524sh
532
563sh
499
519sh
537sh
613sh
520
551sh
504
533sh
493
502sh
533
23.5
18.7
14.7
16.5
22.7
572sh
488
519sh
565sh
0.1
484
501sh
511sh
522
560sh
497
502
536sh
0.23
0.1
540
575sh
547
589sh
0.12
511
558
608sh
d
606
650sh
0.06
564
613
675sh
d
10
607
650sh
0.05
595
647
706sh
14.5
294
a
e
values (104 Mꢀ1 cmꢀ1) are shown in parentheses.
b
c
Measured in CH2Cl2 relative to fac-[Ir(ppy)3] (UP = 0.40), kex = 400 nm.
In CH2Cl2 at 293 K. sh = shoulder.
Not determined due to the inherent instrumental limitation. sh = shoulder or broad peak.
d

2740
C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
thermogravimetric analysis (TGA) under a nitrogen stream. From
the onset of the TGA curves, all the complexes show very good
thermal stabilities with their Tdecomp ranging from 234 to 350 °C.
The first weight loss of the compounds was mostly due to dissoci-
ation of the ancillary acac ligand.
vy metal center, which are not observed in the free ligands. The
substituents on the 9-phenyl ring of 2-[3-(N-arylcarbazolyl)]pyri-
dine have no significant influence on the absorption properties of
the complexes 1–3 as well as their isoquinoline counterparts 9–
10. The lower-lying band is shifted upon the attachment of differ-
ent substituents on the pyridyl ring in the order: 4 (X2 = CF3,
The absorption spectra of all the metallized complexes were
measured in CH2Cl2 at room temperature and the pertinent data
are presented in Table 2. In common with most cyclometalated
Pt(II) complexes the absorption spectra can be divided into two re-
gions: the strong absorptions in the UV region (<350 nm) are de-
kmax = 432 nm) > 5 (X2 = F, kmax = 422 nm) > 1 (X1 = X2 = H, kmax
=
408 nm) > 6 (X1 = Me, kmax = 406 nm). These results agree with
the literature data [59] that introduction of electron-donating
groups to the pyridine ring can cause a decrease of the HOMO level
and a larger band gap than the unsubstituted one and vice versa.
Fig. 2 shows the absorption spectra of 1ꢀ10.
rived from a typical ligand-centered
corresponding transitions were also observed in the free ligands.
For example, the lowest energy
p* absorption of complex 1
p–
p* transition because the
p–
While all of the ligands fluoresce in the blue, their Pt complexes
emit strong photoluminescence (PL) originated from a triplet man-
ifold at room temperature in fluid solution accompanied by large
Stokes shifts (>100 nm). No fluorescence could be detected for each
of the complexes. The room temperature phosphorescence spectra
of 1ꢀ10 are depicted in Fig. 3. The colors of the complexes span
from green to yellow, orange and red. Fig. 4 shows the color-tuning
strategy achieved by the carbazolyl ligands in our Pt complexes.
The emission spectra possess distinctive vibronic like progression
which we can expect a significant amount of 3MLCT state mixing
was found at 327 nm while the corresponding free ligand L1 ap-
peared at 322 nm. Although these transitions are slightly red-
shifted from their corresponding free ligands, they are not solvato-
chromic. These are only attributed to the perturbation from the
metal. Metal-centered, d–d transitions are not observed for these
complexes. It is believed that the strong ligand field of the C^N li-
gands shifts the d–d transitions to high energy, putting them under
the more intense ligand-centered and metal-to-ligand charge
transfer (MLCT) transitions [57,58]. Therefore, the second regions
that extend to the visible region are conventionally assigned as
mixing among singlet and triplet metal-to-ligand charge transfer
transitions (1MLCT and 3MLCT) and to a certain extent with intral-
with the ligand-centered 3p
–p transitions [60,61]. As for the
absorption features, the change of substituents on the 9-phenyl
ring of carbazole has no profound effect on the emission maxi-
mum. The emission energies are, however, strongly sensitive to
igand 3
p–p
transitions through the spin–orbit coupling by the hea-
7
8
1
2
3
4
5
6
9
10
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 2. Absorption spectra of 1ꢀ10 in CH2Cl2 at 293 K.
1
7
2
8
3
4
5
9
10
6
450
500
550
600
650
700
750
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
Fig. 3. Photoluminescence (PL) spectra of 1ꢀ10 in CH2Cl2 at 293 K.

C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
2741
X
N
N
N
X
N
N
N
N
N
CF
3
N
N
N
N
N
N
F
Me
Fig. 4. Color-tuning strategy in cyclometalated Pt(II) complexes.
the electronic properties of substituents on the pyridine ring. A
weakly -donating methyl group in the 4-position on the pyridyl
ring (with respect to the IrꢀN bond) results in a slight blue shift
of 9 nm for 6. The electron-withdrawing F group in the 5-position
causes a bathochromic shift on the emission (complex 5), and the
tem in the heterocycle and they all emit red phosphorescence with
a maximum at 606 and 607 nm for 9 and 10, respectively.
All of the Pt complexes are intensely emissive at 77 K in a rigid
matrix. Most of the complexes show some extent of rigidochromic
blue shifts compared to their room temperature emission features.
Complexes 1–7 show well-defined spectra with many separated
vibronic attenuations. For example, the spectrum of complex 1
consists of two main sets of bands with well-defined structure,
one with kem = 486 nm and the other one with kem = 524 nm. How-
ever, different behavior was observed in 8–10 which displayed less
vibronic attenuations. Fig. 5 illustrates the representative emission
spectra of 1, 4, 8 and 9 at 77 K. Substituents effects on pyridyl ring
are less pronounced for altering the phosphorescence emission
energies at low temperature.
We further probed the photophysical properties of 1–10 by
measuring their PL quantum yields (UP) in dilute degassed CH2Cl2
solutions. Among all the green/yellow compounds 1–7, complex 7
has the highest room temperature quantum efficiency of 23%,
which is higher than that for its isomeric compound 1 (11%). An in-
crease in UP (16%) was observed when a trifluoromethyl group was
attached to the pyridine ring. As the energy of emission decreases,
a corresponding general decrease in UP at room temperature is ob-
served, except for the fluorene-containing complex 8 which has UP
of 10%. This may be attributed to the fact that fluorene is a well-
known functional core possessing high quantum efficiency among
all the organic species. The lifetimes of the complexes at room tem-
perature and 77 K were also measured at infinite dilution under
nitrogen atmosphere. They generally feature long-lived excited
states in the sub-microsecond region at room temperature, signal-
ling that the emissions originate from a mixed 3LC/3MLCT state.
The phosphorescence lifetimes at 77 K are much longer than those
at room temperature since the non-radiative decay rates usually
decrease with lowering temperature [36].
r
stronger CF3
r-electron acceptor imparts a more substantial red
shift for 4. Therefore, kem decreases in the order: 4 > 5 > 1 > 6. Since
the LUMO level is principally located on the electron deficient pyr-
idyl ring [59,62], substitution of an electron-withdrawing group
would decrease the HOMO–LUMO energy gap, thus a red shift of
the emission was observed. This sensitivity of transition energy
to the substitution effect on the pyridyl ring has been previously
observed in related tris(cyclometalated) iridium complexes
[59,63–69]. For the two pairs of geometrical isomers (1 versus 7
and 2 versus 5), we note an obvious ligand or substituent effect
on the emission position. There is a red shift of ꢁ10 nm in kem upon
going from 1 (493 nm) to 7 (502 nm) or 2 (493 nm) to 5 (504 nm).
On the other hand, incorporating a more conjugative cyclometalat-
ing ligand on the phenyl ring also enables the tuning of the energy
of the emissive state for this kind of Pt complexes. When fluorene
was inserted between the pyridine ring and the carbazole unit
(complex 8), a pronounced red shift in kem was observed, i.e. 8
(547 nm) > 7 (502 nm). An increase in the conjugation of the phe-
nyl ring would decrease the LUMO level, leading to a red shift in
wavelength. For the isoquinoline derivatives, large red shifts can
be found by extending the size of the
p
-conjugated fused ring sys-
1.0
0.8
0.6
0.4
0.2
0.0
1
4
8
9
2.4. Density functional theory (DFT) calculations
To study the electronic structures and understand the nature of
excited states of our new Pt(II) phosphors, density functional the-
ory (DFT) and time-dependent DFT (TDDFT) calculations were car-
ried out using B3LYP hybrid functional theory for selected
molecules. Our TDDFT calculations show that the S0 ? S1 transi-
tions correspond to the HOMO ? LUMO transitions with non-zero
oscillator strengths (Table 3). Therefore, we will place our special
emphasis on the HOMO–LUMO analyses in the following discus-
sion. The calculations show that HOMOs consist of the metal d
500
600
700
Wavelength (nm)
orbital mixing with the
p orbitals of the phenyl ring that is directly
Fig. 5. PL spectra of 1, 4, 8 and 9 in CH2Cl2 at 77 K.
bonded to the metal center. The LUMOs correspond to the lowest

2742
C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
Table 3
Percentage contribution of the metal d orbitals to HOMO and LUMO together with the TDDFT calculation results.
p
Compound
Contribution of d orbitals
to HOMO (%)
Contribution of d orbitals
to LUMO (%)
The largest coefficient in the CI
expansion of the S1 statea
The oscillator strength (f) of the
0 ? S1 transition
S
1
2
3
4
8
9
10
21.4
20.6
16.0
19.1
3.9
23.5
15.4
23.8
2.9
3.4
3.1
3.2
5.6
2.8
3.3
5.8
H ? L: 0.65
H ? L: 0.63
H ? L: 0.53
H ? L: 0.61
H ? L: 0.53
H ? L: 0.68
H ? L: 0.60
H ? L: 0.65
0.012
0.010
0.014
0.016
0.172
0.031
0.015
0.056
[Pt(Fl-py)acac]
a
H ? L represents the HOMO to LUMO transition. CI stands for configuration interaction.
p
* orbital from the Pt-bonded pyridine ring. The HOMO and LUMO
levels have
p symmetry, with opposite phases above and below the
molecular plane. That is, the HOMO consists of a mixture of phenyl,
Pt, and acac orbitals while the LUMO is predominantly 2-phenyl-
pyridyl in character. From Fig. 6, it is clear that the HOMO–LUMO
gaps calculated for 1ꢀ3 are close to each other, revealing that the
HOMO and LUMO levels are not influenced by the F and OMe sub-
stituents on the aryl ring of carbazole. In contrast, adding an elec-
tron-withdrawing CF3 group on the pyridine ring affects the LUMO
level significantly, resulting in a marked decrease in energy gap.
Much smaller calculated band gap values were observed in 9 and
10, consistent with the optical studies. The HOMO stability and
emission energy are almost invariant of the identity of X on the
aryl ring of carbazole. For 8, the HOMO showing a greater delocal-
ization over the aryl ligand leads to a less extent of metal d orbital
mixing. The greater delocalization due to the participation of orbi-
tals from the carbazolyl substituent gives a smaller HOMO–LUMO
gap in 8 when compared with that in [Pt(Fl-py)acac] (Fig. 7) [70].
2.5. Electrochemical and electronic characterization
Fig. 7. Contour plots of dominant excitation orbitals of fluorene-containing
platinum(II) complex 8 and its non-carbazole parent species [Pt(Fl-py)acac].
The electrochemical properties of 1ꢀ10 were examined using
cyclic voltammetry, and the redox data are given in Table 4. All
of the electrochemical potentials reported here were measured rel-
ative to an internal ferrocene standard (Cp2Fe/Cp2Fe+) [71]. All of
the Pt complexes show reversible reduction and irreversible oxida-
tion waves. This electrochemical behavior is similar to that re-
ported for related (C^N)Pt-type complexes [6]. As pointed out
above, the HOMO of cyclometalated Pt(II) complexes typically con-
sists of a mixture of phenyl, Pt, and acac orbitals while the LUMO is
predominantly pyridyl in character [6]. Unfortunately, electro-
chemical oxidation of Pt(II) complexes is typically an irreversible
Fig. 6. Contour plots of dominant excitation orbitals of some of the platinum(II) complexes.

C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
2743
Table 4
(ꢀ3.0 eV). The close match of the HOMO levels of our Pt dyes
and the Fermi level of PEDOT likely results in a good HI contact
and an improvement in the hole carrier balance in the OLEDs even
without the need for an additional HT layer (vide infra).
Electrochemical properties and frontier orbital energy levels of 1ꢀ10.
ox
a
red
a
Complex
E
ðVÞ
HOMO (eV)b
E
ðVÞ
LUMO (eV)c
1=2
1=2
1
2
3
4
5
6
7
8
9
10
0.28
0.30
0.29
0.24, 0.44
0.34
0.29
0.32
0.38
0.28
ꢀ5.08
ꢀ5.10
ꢀ5.09
ꢀ5.03
ꢀ5.14
ꢀ5.09
ꢀ5.11
ꢀ5.18
ꢀ5.08
ꢀ5.08
ꢀ2.52
ꢀ2.52
ꢀ2.54
ꢀ2.28
ꢀ2.28
ꢀ2.26
ꢀ2.59
ꢀ2.40
ꢀ2.18
ꢀ2.40
ꢀ2.44
ꢀ2.68
ꢀ2.67
2.6. Electrophosphorescent OLED characterization
ꢀ2.21, ꢀ2.61
ꢀ2.40
Today, OLED is believed to be one of the most promising tech-
nologies for flexible passive- and active-matrix flat panel displays
because OLEDs can be easily fabricated over large-area plastic sub-
strates at room temperature and low cost. Therefore, materials that
can be spun into uniform and thin films are industrially demand-
ing. While we expect our Pt compounds here are sublimable for
the fabrication of doped electrophosphorescent multilayered
OLEDs, our ultimate goal, however, is to make some solution-pro-
cessed devices for the realization of large-area displays. The emit-
ting layers for all of the devices were prepared by using the
relatively cheaper solution-processed spin-coating technique. It
should be noted that CBP itself normally cannot be spin-coated
from solution to form good-quality thin films. When the dopant
concentration is very low (e.g. <3 wt.-%), the CBP host is prone to
crystallization in the as-prepared films and the devices short eas-
ily. This illustrates that the good film-forming properties of our
bulky metal phosphors can ensure a good chemical compatibility
with the CBP host, resulting in a homogeneous distribution of each
Pt dopant in CBP [75]. In this way, various OLEDs were fabricated
by spin-coating the selected Pt(II) materials onto indium tin oxide
(ITO) coated glass, which is used as an anode. The configuration of
Pt-based devices and the molecular structures of the compounds
used in these devices are depicted in Fig. 8. A layer of poly(3,4-eth-
ylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was
adopted which can also be used as the HI layer to smooth the
ITO surface [76] and reduce interfacial surface area and lower the
leakage current. A layer of conductive host material 4,40-N,N0-dicar-
bazolebiphenyl (CBP) doped with different concentrations (3–
12 wt.-%) of Pt dyes was employed to optimize the device effi-
ciency. We have used CBP as the host because of its proven perfor-
mance as host for Pt complexes and theoretical confirmation of
favorable triplet energy [77–79]. But the conventional HT layer of
NPB was not necessary in fabricating our devices. This can, to a cer-
tain extent, corroborate with the improved HI/HT properties of our
materials from the PEDOT:PSS layer. Then the emitting layer com-
posed of the triplet dopant and CBP host was deposited. The de-
vices consist of a thin layer of 2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline (BCP) for hole and exciton blocking to confine the
recombination zone inside the doped CBP layer; a layer of elec-
tron-transporting tris[8-hydroxyquinoline] (Alq3) for electron-
transport [80], and LiF as an electron injection layer. This configu-
ration was adopted from those reported previously [81]. Important
device performance characteristics are collected in Table 5.
ꢀ2.62
ꢀ2.40
ꢀ2.36, ꢀ2.79
ꢀ2.12, ꢀ2.76
ꢀ2.13, ꢀ2.76
0.28
a
0.1 M [Bu4 N]PF6 in THF, scan rate 100 mV sꢀ1, versus Fc/Fc+ couple.
Irreversible wave.
Reversible wave.
b
c
process caused by solvent coordination to the open coordination
site on the metal center (i.e. susceptible to nucleophilic attack by
solvents) and subsequent ligand rearrangement [33,35,36]. There-
fore, the Eox values do not necessarily represent the true thermody-
namic potential for removing an electron from the complexes. In
order to get a more meaningful interpretation, we look at the de-
tails of reduction processes of the complexes. All of the complexes
show a reduction peak in the range of ꢀ2.02 to ꢀ2.62 V. Complex 1
has the Ered value (ꢀ2.52 V) more negative than [Pt(ppy)(acac)]
(ꢀ2.39 V) [6], suggesting that the LUMO level (ca. ꢀ2.28 and
ꢀ2.41 eV for 1 and [Pt(ppy)(acac)], respectively) is destabilized
by the inclusion of carbazole unit. This is mainly attributed to
the poorer electron-accepting property of carbazole as compared
to the phenyl ring. From these results, we observe that the reduc-
tion potentials of these complexes are strongly sensitive to the nat-
ure of substituents on the pyridyl ring. When electron-donating
groups (such as 4-methyl-substituted group in 6) were attached
to the pyridyl ring, destabilized LUMO level was observed relative
to the unsubsituted one in 1. On the other hand, addition of elec-
tron-withdrawing substituents, such as fluorine atom (in 5) and
trifluoromethyl group (in 4) stabilized the LUMO levels. Therefore,
the LUMO level decreases in the order of
6
(ꢀ2.18 eV) > 1
(ꢀ2.28 eV) > 5 (ꢀ2.40 eV) > 4 (ꢀ2.58 eV). This is in agreement with
some of the literature data [59,62] and a red shift in the emission
wavelength often occurs owing to a lowering of the LUMO energy
level.
However, substituents on the 9-phenyl ring of carbazole have
no marked influence on the redox properties of the complexes.
Similar Ered was recorded for 1–3. Complex 7 has a reduction wave
at ꢀ2.40 V, making it 110 mV easier to reduce than that for 1. There
is a gentle decrease in the reduction potential along the sequence 7
(ꢀ2.40 V) > 8 (ꢀ2.36 V) which is mainly caused by the increase of
p-conjugation in the cyclometalating ligands that leads to a greater
stabilization of the negative charge on the more delocalized
p
-
OLEDs using phosphors 2 and 3 exhibited bright green emission
when a positive bias was applied between the electrodes. Fig. 9
shows the voltage-independent electroluminescence (EL) spectra
at different doping concentrations. The EL spectra are nearly coin-
cident with the PL spectra of the corresponding complexes in the
polymer matrix at room temperature when the doping concentra-
tion is low (e.g. at 3 wt.-%), indicating an effective energy transfer
from the host exciton to the phosphor upon electrical excitation
and the effective hole-blocking effect of BCP. The blue emission
from the host and HT layers is negligible. However, emission from
aggregate states is observed from both devices and the extent of
aggregation is increased with increasing doping concentrations
so that broad emission bands appear at the lower energy shoulder.
For device B4 with dopant 3 at 12 wt-%, the EL shows three peaks,
the first two being vibronic transitions from the parent dopant
(496 and 536 nm), while a new aggregate emission band appears
orbital system. This means that increasing the electron extension
p
in the C^N ligands renders the complexes easier to reduce.
Replacement of the pyridyl ring by isoquinoline also shows stabi-
lized LUMO levels: 9 (ꢀ2.68 eV) ꢂ 10 (ꢀ2.67 eV).
As compared to [Pt(ppy)acac] (HOMO: ꢀ5.34 eV) [6,36], the
HOMO levels of 1ꢀ10 are all elevated when the electron-rich and
-donating carbazolyl units are attached to the pyridyl rings, signal-
ling that they have a lower ionization potential than their ppy con-
gener, leading to a better hole injection (HI) and/or HT ability and
thus our complexes can serve as dual functional triplet emitters.
Energetics governing the HOMO and LUMO levels of the materials
are usually crucial for efficient OLED architecture. The HOMO en-
ergy levels of our carbazole-containing compounds lie within those
of ITO (HOMO: ꢀ4.70 eV) [72], PEDOT:PSS (ꢀ5.0 eV) [73] and Alq3
(ꢀ6.00 eV) [74], while the LUMO energy levels lie above Alq3

2744
C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
N
Al (100 nm)
O
LiF (1 nm)
O
Al
N
Alq (40 nm)
3
N
BCP (15 nm)
x% Pt:CBP (80 nm)
PEDOT:PSS (40 nm)
ITO
O
N
N
Alq
3
BCP
Glass
O
O
O
O
S
+
S
S
S
+
n
O
O
O
O
N
N
_
_
SO H
3
SO
3
SO H
3
SO
SO H
3
3
CBP
n
PEDOT:PSS
Fig. 8. The general structure for Pt-doped OLED devices and the molecular structures of the relevant compounds used.
Table 5
Performance of Pt-doped electrophosphorescent OLEDs.
Device
Phosphor (wt.-%)
Vturn-on (V)
Luminance (cd/m2)
gext (%)a
gL (cd/A)a
gp (lm/W)a
kem (nm) (x,y)c
A1
A2
A3
A4
B1
B2
B3
B4
2 (3)
2 (6)
2 (9)
2 (12)
3 (3)
3 (6)
3 (9)
3 (12)
3 (3)
3 (6)
3 (9)
3 (12)
4 (3)
4 (6)
4 (9)
4 (12)
4 (3)
4 (6)
4 (9)
4 (12)
8 (3)
8 (6)
8 (9)
8 (12)
7.0
8.6
9.7
9.8
7.5
8.3
7.7
9.3
5.7
5.7
5.7
4.1
6.5
8.6
7.8
7.9
4.6
4.2
3.3
4.3
12.6
11.9
12.4
12.5
1173 (12.5)
1363 (14)
1147 (16)
1286 (16.5)
1403 (13.5)
2059 (15)
2271 (14.5)
2376 (16)
724 (10)
2135 (10.5)
1527 (10)
3223 (10.5)
16550 (16)
11440 (18)
8982 (19)
0.22 (11.0)
0.39 (11.0)
0.28 (14.0)
0.36 (14.0)
0.43 (12.0)
0.50 (12.5)
1.23 (14)
0.90 (13.0)
0.17 (8.0)
0.44 (8.0)
0.50 (8.5)
0.68 (6.5)
1.75 (15.5)
1.53 (17.5)
1.46 (18.0)
1.32 (19.0)
2.36 (7.5)
2.65 (7.0)
2.77 (8.5)
2.10 (9.5)
0.40 (19.0)
0.35 (20.5)
0.32 (20.0)
0.34 (20.0)
0.64 (11.0)
1.10 (11.0)
0.77 (14.0)
0.99 (14.0)
1.26 (12.0)
1.40(12.5)
2.80 (12.5)
2.39 (16.0)
0.44 (8.0)
1.25 (8.0)
1.45 (8.5)
2.28 (9.0)
5.99 (15.5)
4.98 (17.5)
4.21 (18.0)
3.73 (19.0)
7.50 (7.5)
8.33 (7.0)
8.49(8.5)
0.18 (11.0)
0.33 (10.0)
0.18 (13.0)
0.23 (13.0)
0.59 (10.5)
0.39 (11.0)
0.77 (14.0)
0.44 (13.0)
0.18 (7.0)
0.49 (8.0)
0.55 (7.0)
0.96 (6.5)
1.30 (13.0)
0.89 (17.5)
0.81 (16.0)
0.68 (16.5)
3.29 (7.0)
3.89 (6.5)
3.48 (7.5)
2.51 (6.5)
0.19 (17.5)
0.15 (17.0)
0.12 (19.5)
0.14 (14.0)
496sh, 527 (0.34, 0.55)
494, 523sh (0.35, 0.54)
496sh, 527 (0.38, 0.54)
496sh, 527 (0.38, 0.54)
495sh, 526 (0.36, 0.56)
495sh, 526, 563sh (0.39, 0.54)
495sh, 531, 564sh (0.41, 0.53)
499sh, 535sh, 572 (0.43, 0.52)
491, 520sh (0.25, 0.45)
496, 523sh (0.28, 0.51)
496, 524sh (0.30, 0.53)
496, 528sh (0.31, 0.54)
515, 543sh (0.32, 0.62)
515, 543sh (0.34, 0.61)
515, 544sh (0.36, 0.59)
515, 548sh (0.38, 0.57)
512, 540sh (0.34, 0.59)
512, 540sh (0.34, 0.59)
512, 543sh (0.36, 0.59)
512, 543sh (0.36, 0.58)
547, 586sh, 647sh (0.48, 0.51)
547, 590sh, 643sh (0.49, 0.51)
547, 592sh, 647sh (0.51, 0.49)
547, 591sh, 647sh (0.51, 0.49)
B5b
B6b
B7b
B8b
C1
C2
C3
C4
7627 (20)
C5b
C6b
C7b
C8b
D1
D2
D3
D4
14010 (11.5)
15970 (12.5)
16960 (13)
11910 (13)
1592 (19.5)
1601 (22)
6.41 (9.5)
1.13 (19.0)
0.92 (20.5)
0.73 (20.0)
0.74 (20.0)
1359 (22.5)
1246 (22)
a
Maximum values of the devices. Values in parentheses are the voltages at which the maximum values were obtained.
Spin-coated emissive layer was obtained by using toluene during OLED fabrication.
CIE coordinates shown in parentheses.
b
c
near 570 nm. This emission band diminishes as the doping concen-
of the molecule easily accommodates
p-stacking interactions in
tration is low. The emission properties of luminescent Pt com-
pounds typically undergo large changes upon going from a dilute
solution to the neat solid-state. Intermolecular interactions often
lead to emission from a neat solid that is red-shifted in comparison
to what is observed from the isolated molecule. This phenomenon
is particularly pronounced for Pt(II) complexes coordinated to li-
the solid-state [82–85]. Indeed, generating white light from phos-
phor–excimer or aggregate emission has been proven effective and
leads to simpler device structures than those requiring separate
red, green and blue emitters. It is true that a solvent used for film
formation could affect the solid molecular packing of the active
material. The low-energy emission is becoming more apparent in
chloroform-processed devices with increase of dopant concentra-
gands with conjugated
p-systems as the square-planar geometry

C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
2745
1.0
0.8
0.6
0.4
0.2
0.0
1.0
A1
A2
A3
A4
(b)
(a)
B1
B2
B3
B4
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 9. EL spectra of (a) 2 and (b) 3-doped OLEDs at different dopant concentrations.
tions. Such characteristics have been used in the literature to fab-
ricate white OLEDs by using excimer or aggregate emission from a
phosphor dopant coupled with the blue emission from the host
due to the incomplete energy transfer [83,85–88].
concentration (device A3), it shows a poorer performance with gext
of 0.28%, gP of 0.18 lm/W and gL of 0.77 cd/A. We notice a rather
slow drop of the EL efficiency with increasing voltage. This may
be attributed to the relatively short lifetimes of these complexes
that can help to reduce the roll-off resulting from electric field-in-
duced exciton quenching effects, exciton–exciton annihilation, and
charge–carrier–exciton interactions on long-lived excited states, as
found for other electrophosphorescent devices [89,90].
To illustrate the difference of substituents on the pyridyl ring in
the device efficiencies, compound 4 was also fabricated in the same
configuration. The device displays an intense green-yellow emis-
sion with a peak maximum and a shoulder at around 516 nm
and 544 nm, respectively, in the EL spectrum with CIE coordinates
(0.34, 0.61) for device C2, which closely resembles the PL spectrum
in dichloromethane solution, indicating that the EL emission of the
device originates from the triplet excited states of the phosphor.
Fig. 11 displays the EL spectra of 4-doped OLEDs. The EL spectra
changed slightly with the doping concentration due to excimer for-
mation. Relative to the non-trifluoromethylated complexes 2 and
Although all of the devices show monomer and red-shifted exci-
mer or aggregate emissions, the intensity of the peaks clearly vary.
Whether the occurrence of this low-energy emission is due to exci-
mer or aggregation formation certainly needs more investigation.
Device B5 shows the Commission Internationale de L’Eclairage
(CIE) value x = 0.25, y = 0.45 which appears near white to the naked
eyes. The green OLEDs utilizing compounds 2 and 3 have CIE coor-
dinates of (0.34, 0.55) and (0.36, 0.56), respectively at 3 wt.-%. The
electrical characteristics of these devices are displayed in the cur-
rent density–voltage–luminance (J–V–L) curves (Fig. 10). From the
data, the current density decreases as the doping concentration in-
creases at the same driving potential. This is mainly attributed to
the increased amount of hole trapping when the platinum mole-
cules tend to aggregate upon device operation at a higher doping
concentration. The overall efficiencies of the device B3 appear
the highest among the green-emitting devices in this study. It
has a turn-on voltage of 7.7 V, maximum luminance of 2271 cd/
3, the emission caused by aggregation or
p–p stacking is signifi-
cantly weaker for 4 especially at high doping concentrations. This
may be attributed to the introduction of sterically bulky CF3 group
which can alter the molecular packing and minimize the self-
quenching behavior [69,91–93].
m2 at 13.5 V, external quantum efficiency (
efficiency ( P) of 0.77 lm/W and current efficiency (
g
ext) of 1.23%, power
L) of 2.80 cd/
g
g
A, respectively. On the contrary, for complex 2 at the same doping
800
600
400
200
0
2500
2000
1500
1000
500
1000
1400
(a)
(b)
800
600
400
200
0
1200
1000
800
600
400
200
0
B1
B2
B3
B4
A1
A2
A3
A4
B1
B2
B3
B4
A1
A2
A3
A4
0
0
2
4
6
8
10
12
14
16
18
0
2
4
6
8
10
12
14
16
18
Voltage (V)
Voltage (V)
Fig. 10. Current density-voltage-luminance (JꢀL–V) characteristics of the electrophosphorescent OLED devices with different dopant levels of (a) 2 and (b) 3.

2746
C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
(b)
(a)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
C5
C6
C7
C8
C1
C2
0.8
C3
C4
0.6
0.4
0.2
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 11. EL spectra of 4-doped OLEDs prepared from (a) CHCl3 and (b) toluene at different dopant concentrations.
5
4
5
4
3
(a)
(b)
8
2
η
η
6
3
3
2
1
0
2
1
η
η
4
η
η
1
0
2
0
0
-1
-1
0
100
200
300
0
100
200
300
400
500
Current Density (mA/cm2 )
Current Density (mA/cm2 )
Fig. 12. gL (j), gP (d) and gext (N) as a function of current density for OLED devices (a) C3 and (b) C7.
Significantly better performance and lower turn-on voltage
were achieved for 4 under the same doping level, especially in
the case of using toluene as solvent for the spin-coating process.
This may be due to the higher solubility effect by the CF3 group
which can facilitate the homogeneous film formation as compared
to the unsubstituted one. The lower turn-on voltage is presumably
caused by the reduced electron injection barrier after introduction
of the trifluoromethyl substituents, that is in agreement with the
lower LUMO level estimated from the results of cyclic voltammetry
and literature reports in which fluorination is expected to enhance
the electron mobility, reduce triplet–triplet annihilation processes
[59,94] and result in a better balance of charge injection. The mark-
edly higher luminance efficiency and brightness were achieved at
the 6 and 9 wt.-% doping concentrations (devices C6 and C7). For
device C7, the maximum luminance is 16960 cd/m2 at 13 V. The
The observation of different EL performance by using CHCl3 and
toluene as the processing solvents suggest that the relative inten-
1.0
D1
D2
D3
0.8
D4
0.6
0.4
0.2
0.0
maximum gL, gext and gP are 8.49 cd/A, 2.77% and 3.48 lm/W,
respectively. For device C6, the EL data are characterized by a max-
imum brightness of 16150 cd/m2 at 12.5 V, and peak efficiencies of
8.33 cd/A, 2.65% and 3.89 lm/W, respectively. We note that the gL
,
g
P and ext decrease by a larger extent for devices spin-coated with
g
toluene but such decrease is not apparent for devices spin-coated
400
500
600
700
800
with chloroform up to 200 mA/cm2 at the same doping concentra-
Wavelength (nm)
tions. Fig. 12 shows the gL
, gext and gP data as a function of current
density for OLED devices C3 and C7.
Fig. 13. EL spectra of 8-doped OLEDs at different dopant concentrations.

C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
2747
sities of monomer and excimer emission are dependent of the nat-
ure of solvent used as well as the doping level. At the same doping
concentration, degree of aggregation by dopant packing was dis-
similar in the two solvent systems. Diminished aggregation arising
from the excimer formation was observed by using toluene and
accordingly, toluene-coated devices gave a higher luminance at
the same voltage.
The devices derived from 8 were also fabricated. Fig. 13 shows
the normalized EL spectra of devices doped with 8 in different dop-
ing concentrations. Similar to other complexes, a concentration
dependent EL spectra was observed. Each of them emits an orange
color centered at 548 nm with a broad shoulder at around 640 nm.
As the concentration was increased, it revealed a clear aggregation
phenomenon. The CIE coordinates of 8 are (0.49, 0.51) at 6 wt.-%
doping concentration. A maximum gext of 0.40%, gP of 0.19 lm/W
and gL of 1.13 cd/A was achieved for complex 8 at 3 wt.-% doping.
The device performance only slightly decreases with increasing
All of the cyclometalated platinum(II) complexes were synthe-
sized by refluxing a mixture of 2.5 molar equivalents of the corre-
sponding ligand and K2PtCl4 in a mixture of 2-ethoxyethanol and
water (3:1, v/v) under a nitrogen atmosphere to 80 °C for 16 h.
After cooling to room temperature, the mixture was added to
water (20 mL), and the precipitate was washed with water
(20 mL ꢃ 4) and dried at 50 °C under vacuum. Without further
purification, the precipitate was then treated with 3 molar equiva-
lents of acetylacetone and 10 equivalents of Na2CO3 in 2-ethoxy-
ethanol (8 mL) at 100 °C under nitrogen for 16 h. After the
removal of all volatile components, the compound was purified
by column chromatography on silica using CH2Cl2/hexane mixture
as the eluent to afford the desired products.
1: A pale yellow powder (yield: 16%). MS (FAB): m/z 613 (M+).
1H NMR (CDCl3): d (ppm) 8.95 (d, 1H, J = 5.4 Hz, Ar), 8.22 (s, 1H,
Ar), 8.10 (d, 1H, J = 8.1 Hz, Ar), 7.78 (m, 2H, Ar), 7.66ꢀ7.55 (m,
6H, Ar), 7.45ꢀ7.25 (m, 3H, Ar), 7.05 (m, 1H, Ar), 5.43 (s, 1H, acac),
2.04 (s, 3H, CH3), 1.88 (s, 3H, CH3). Anal. Calc. for C28H22N2O2Pt: C,
54.81; H, 3.61; N, 4.57. Found: C, 54.65; H, 3.44; N, 4.67%.
2: A pale yellow powder (yield: 18%). MS (FAB): m/z 631 (M+).
1H NMR (CDCl3): d (ppm) 8.96 (m, 1H, Ar), 8.22 (s, 1H, Ar), 8.10
(d, 1H, J = 8.1 Hz, Ar), 7.78 (m, 2H, Ar), 7.63ꢀ7.58 (m, 2H, Ar),
7.37ꢀ7.25 (m, 6H, Ar), 7.04 (m, 1H, Ar), 5.44 (s, 1H, acac),
1.99ꢀ1.89 (m, 6H, CH3). Anal. Calc. for C28H21N2FO2Pt: C, 53.25;
H, 3.35; N, 4.44. Found: C, 53.43; H, 3.53; N, 4.55%.
current density and remains as
gext = 0.39%, gP = 0.19 lm/W and
g
L = 1.12 cd/A at a
luminance of 1000 cd/m2. The luminance
reaches a maximum of 1592 cd/m2 at 19.5 V at this concentration.
3. Concluding remarks
In summary, a series of luminescent Pt(II) complexes featuring
functionalized hole-transporting carbazole ligands were synthe-
sized. The structure–property–function relationships of the result-
ing Pt(II) complexes were studied as a function of ligand and
substituent effects. A variety of cyclometalating ligands can be
used to tune the energy of the emissive state over a wide spectral
color range. Strong spin–orbit coupling of the heavy Pt atom allows
3: A pale yellow powder (yield: 31%). MS (FAB): m/z 643 (M+).
1H NMR (CDCl3): d (ppm) 8.94 (d, 1H, J = 2.7 Hz, Ar), 8.20 (s, 1H,
Ar), 8.09 (d, 1H, J = 8.1 Hz, Ar), 7.76 (m, 2H, Ar), 7.55ꢀ7.07 (m,
9H, Ar), 5.43 (s, 1H, acac), 3.90 (s, 3H, OMe), 1.98 (s, 3H, CH3),
1.88 (s, 3H, CH3). 13C NMR (CDCl3): d (ppm) 185.49, 183.76 (CO),
183.76, 168.52, 158.49, 146.85, 142.11, 141.01, 137.80, 136.85,
136.73, 130.30, 128.48, 124.85, 124.15, 119.81, 119.56, 119.15,
117.73, 115.65, 114.60, 110.32, 109.86 (Ar), 102.36 (CH), 55.64
(OMe), 28.35, 27.22 (CH3). Anal. Calc. for C29H24N2O3Pt: C, 54.12;
H, 3.76; N, 4.35. Found: C, 53.89; H, 3.82; N, 4.49%.
for mixing of 1MLCT with ligand-based 3
p–p states, making the for-
mally forbidden radiative relaxation of these states an efficient
process, therefore all of these complexes exhibit intense phospho-
rescence emissions at room temperature. These neutral and air-
stable complexes are easily prepared, rendering them good poten-
tial as candidates for use in optoelectronics devices. We have also
incorporated some of these Pt complexes into OLEDs based on
solution-processing method. While the device performance is yet
to be optimized, extension of these carbazole derivatives to other
device configurations or by vacuum deposition would warrant fur-
ther investigation.
4: A yellow powder (yield: 28%). MS (FAB): m/z 681 (M+). 1H
NMR (CDCl3): d (ppm) 9.14 (s, 1H, Ar), 8.15 (s, 1H, Ar), 8.01 (d,
1H, J = 5.4 Hz, Ar), 7.86 (d, 1H, J = 5.4 Hz, Ar), 7.73 (d, 1H,
J = 5.4 Hz, Ar), 7.54ꢀ7.18 (m, 9H, Ar), 5.38 (s, 1H, acac), 1.94 (s,
3H, CH3), 1.82 (s, 3H, CH3). 13C NMR (CDCl3): d (ppm) 186.13,
184.00 (CO), 144.30, 142.67, 140.85, 138.58, 137.35, 135.19,
134.67, 129.54, 127.48, 127.19, 125.41, 124.28, 122.85, 122.50,
120.49, 120.16, 119.38, 117.48, 117.35, 110.56, 110.21 (Ar),
102.55 (CH), 28.28, 26.98 (CH3), 14.12 (CF3). Anal. Calc. for
C29H21N2F3O2Pt: C, 51.11; H, 3.11; N, 4.11. Found: C, 51.32; H,
3.02; N, 4.13%.
4. Experimental
5: A pale yellow powder (yield: 69%). MS (FAB): m/z 631 (M+).
1H NMR (CDCl3): d (ppm) 8.90 (m, 1H, Ar), 8.15ꢀ8.08 (m, 2H, Ar),
7.78ꢀ7.36 (m, 11H, Ar), 5.44 (s, 1H, acac), 2.00 (s, 3H, Me), 1.88
(s, 3H, CH3). Anal. Calc. for C28H21N2FO2Pt: C, 53.25; H, 3.35; N,
4.44. Found: C, 53.10; H, 3.30; N, 4.57%.
4.1. General information
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. All reactions were monitored
by thin-layer chromatography (TLC) with Merck pre-coated glass
plates. Flash column chromatography and preparative TLC were
carried out using silica gel from Merck (230ꢀ400 mesh). Fast atom
bombardment (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 spectrometer; chemical shifts were
quoted relative to the internal standard tetramethylsilane for 1H
and 13C{1H} NMR data.
6: A yellow powder (yield: 29%). MS (FAB): m/z 627 (M+). 1H
NMR (CDCl3): d (ppm) 8.70ꢀ8.69 (d, 1H, J = 2.7 Hz, Ar), 8.17 (s,
1H, Ar), 8.08 (d, 1H, J = 5.4 Hz, Ar), 7.65ꢀ7.54 (m, 6H, Ar),
7.43ꢀ7.34 (m, 3H, Ar), 6.81 (m, 1H, Ar), 6.80 (m, 1H, Ar), 5.40 (s,
1H, acac), 2.41 (s, 3H, CH3), 1.95 (s, 3H, CH3), 1.85 (s, 3H, CH3).
13C NMR (CDCl3): d (ppm) 185.56, 183.79 (CO), 167.83, 149.65,
146.20, 141.62, 140.59, 137.72, 137.21, 137.00, 129.42, 127.17,
127.13, 124.90, 121.15, 120.02, 119.73, 119.25, 118.50, 115.37,
110.33, 109.94 (Ar), 102.35 (CH), 28.25, 27.04 (CH3), 21.57 (CH3).
Anal. Calc. for C29H24N2O2Pt: C, 55.50; H, 3.85; N, 4.46. Found: C,
55.43; H, 3.76; N, 4.48%.
4.2. General procedures for the synthesis of cyclometalated
platinum(II) complexes
7: A yellow powder (yield: 54%). MS (FAB): m/z 613 (M+). 1H
NMR (CDCl3): d (ppm) 9.04ꢀ9.02 (d, 1H, J = 5.4 Hz, Ar), 8.13 (d,
2H, J = 2.7 Hz, Ar), 7.87ꢀ7.68 (m, 2H, Ar), 7.68ꢀ7.62 (m, 2H, Ar),
7.56 (m, 2H, Ar), 7.41ꢀ7.31 (m, 2H, Ar), 7.28 (m, 3H, Ar), 7.16 (m,
The ligands L1ꢀL10 were prepared following the published pro-
cedures [37–42].

2748
C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
Table 6
Summary of crystal data.
1
2
3
4
8
9
10
CCDC no.
Formula
717476
C28H22N2O2Pt
717477
C28H21N2FO2Pt
717478
C29H24N2O3Pt
717479
C29H21N2F3O2Pt
717480
717481
717482
C33H26N2O3Pt
C39H34N2O2Ptꢄ2H2
C
32H24N2O2Pt
O
Mr
613.57
631.56
643.59
681.57
793.80
663.62
693.65
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
0.28 ꢃ 0.25 ꢃ 0.15 0.30 ꢃ 0.13 ꢃ 0.11 0.23 ꢃ 0.21 ꢃ 0.20 0.28 ꢃ 0.26 ꢃ 0.26 0.30 ꢃ 0.25 ꢃ 0.22 0.29 ꢃ 0.23 ꢃ 0.15 0.31 ꢃ 0.24 ꢃ 0.14
Triclinic
Triclinic
Triclinic
Monoclinic
P21/c
Monoclinic
P21/n
Monoclinic
P21/c
Triclinic
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
P1
7.1749(5)
11.4511(9)
13.557(1)
94.376(1)
98.620(1)
94.054(1)
1094.2(1)
2
7.1661(8)
11.537(1)
13.693(2)
95.622(2)
98.124(2)
94.740(2)
1110.0(2)
2
7.0782(4)
11.6085(7)
14.9566(9)
101.067(1)
96.455(1)
95.303(1)
1190.2(1)
2
13.1924(7)
11.9666(6)
32.939(2)
90
99.006(1)
90
15.1757(7)
12.8628(6)
17.5829(8)
90
94.843(1)
90
11.6935(6)
8.6213(4)
25.188(1)
90
94.267(1)
90
10.895(1)
13.886(2)
20.094(2)
72.831(2)
80.262(2)
69.524(2)
2713.6(5)
4
a
(°)
b (°)
c
(°)
V (Å3)
5135.9(5)
8
3420.0(3)
4
2532.3(2)
4
Z
Dcalcd (g cmꢀ3
)
1.862
1.890
1.796
1.763
1.542
1.741
1.698
l
(mmꢀ1
)
6.441
6.359
5.929
5.515
4.145
5.574
5.208
F(000)
596
612
628
2640
1584
1296
1360
h range (°)
No. of reflections
collected
1.79ꢀ25.00
2.21ꢀ25.00
1.80ꢀ28.27
1.81–25.00
25175
1.96–25.00
16621
2.29–25.00
12065
2.00ꢀ25.00
5438
5567
7063
13293
No. of unique reflections 3779
3852
5179
9039
5994
4448
9345
[R(int)
No. of reflections with
I > 2.0 (I)
No. of parameters
]
0.0244
3488
0.0286
3252
0.0262
4794
0.0313
6614
0.0284
4620
0.0349
3757
0.0400
5766
r
298
307
316
667
415
334
703
R1 [I > 2.0
r
(I)]a
0.0324
0.0844
1.060
0.0345
0.0773
1.003
0.0291
0.0766
1.023
0.0294
0.0748
1.028
0.0271
0.0719
1.045
0.0254
0.0639
1.001
0.0523
0.1323
0.983
wR2 (all data)a
GOF on F2b
P
P
P
P
2
2
1=2
kFoj ꢀ jFck= jFoj. wR2 ¼ f ½wðF2o ꢀ Fc2Þ ꢅ= ½wðFo2Þ ꢅg
.
a
R1
¼
2
1=2
GOF ¼ fð½wðF20 ꢀ F2c Þ ꢅ=ðNobs ꢀ NparamÞÞg
.
b
1H, Ar), 5.44 (s, 1H, acac), 2.00 (s, 3H, CH3), 1.84 (s, 3H, CH3). 13C
NMR (CDCl3): d (ppm) 185.85, 184.44 (CO), 167.45, 147.48,
143.76, 140.89, 138.36, 137.80, 128.61, 125.79, 124.09, 123.33,
122.05, 121.44, 120.08, 119.65, 118.65, 110.46 (Ar), 102.58 (CH),
28.20, 27.05 (CH3). Anal. Calc. for C28H22N2O2Pt: C, 54.81; H,
3.61; N, 4.57. Found: C, 54.76; H, 3.54; N, 4.75%.
137.49, 131.02, 130.22, 128.45, 128.16, 127.44, 126.70, 125.60,
124.96, 124.68, 121.56, 120.11, 119.47, 119.16, 118.15, 114.67,
110.46, 110.09 (Ar), 102.44 (CH), 55.61 (OMe), 28.35, 27.17
(CH3). Anal. Calc. for C33H26N2O3Pt: C, 57.14; H, 3.78; N, 4.04.
Found: C, 57.01; H, 3.93; N, 3.94%.
8: An orange powder (yield: 37%). MS (FAB): m/z 757 (M+). 1H
NMR (CDCl3): d (ppm) 9.02ꢀ9.01 (d, 1H, J = 2.7 Hz, Ar), 8.18ꢀ7.99
(m, 4H, Ar), 7.99ꢀ7.70 (m, 2H, Ar), 7.54ꢀ7.29 (m, 9H, Ar), 7.08
(m, 1H, Ar), 5.51 (s, 1H, acac), 2.09ꢀ2.03 (m, 10H, C2H5 + CH3),
0.46 (t, 6H, J = 13.5 Hz, C2H5). 13C NMR (CDCl3): d (ppm) 185.85,
184.10 (CO), 184.10, 168.17, 152.45, 147.27, 145.59, 143.71,
142.06, 141.08, 141.04, 138.00, 137.80, 136.35, 125.90, 125.71,
123.26, 121.51, 121.45, 120.83, 120.28, 119.74, 118.39, 117.49,
109.83 (Ar), 102.61 (CH), 56.01 (quat. C), 28.31, 27.23 (CH3),
32.98, 8.65 (C2H5). Anal. Calc. for C39H34N2O2Pt: C, 61.82; H,
4.52; N, 3.70. Found: C, 61.70; H, 4.34; N, 3.91%.
5. X-ray crystallography
Crystals suitable for X-ray diffraction studies were grown by
slow evaporation of each of the respective solutions in CH2Cl2/hex-
ane at room temperature. Geometric and intensity data were col-
lected at 293 K using graphite-monochromated Mo K
a radiation
(k = 0.71073 Å) on a Bruker Axs SMART 1000 CCD diffractometer.
The collected frames were processed with the software SAINT [95]
and an absorption correction (SADABS) [96] was applied to the col-
lected reflections. The structure was solved by the Direct or Patter-
son methods (SHELXTL) [97] 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 assigned
with anisotropic displacement parameters. Pertinent crystal data
for the structures are collected in Table 6.
9: A red powder (yield: 12%). MS (FAB): m/z 663 (M+). 1H NMR
(CDCl3): d (ppm) 9.13 (m, 1H, Ar), 8.97ꢀ8.91 (m, 2H, Ar), 8.26 (d,
1H, J = 8.1 Hz, Ar), 7.90ꢀ7.65 (m, 9H, Ar), 7.55ꢀ7.33 (m, 4H, Ar),
5.52 (s, 1H, acac), 2.09 (s, 3H, CH3), 1.97 (s, 3H, CH3). 13C NMR
(CDCl3): d (ppm) 185.75, 183.81 (CO), 171.89, 141.47, 140.54,
139.60, 138.99, 138.23, 137.44, 137.38, 130.90, 129.39, 128.06,
127.36, 127.17, 127.00, 126.55, 125.55, 124.94, 124.84, 121.42,
120.26, 119.59, 119.14, 118.14, 110.47, 109.11 (Ar), 102.38 (CH),
55.61 (OMe), 28.43 (CH3). Anal. Calc. for C32H24N2O2Pt: C, 57.92;
H, 3.65; N, 4.22. Found: C, 57.87; H, 3.55; N, 4.12%.
Acknowledgements
This work was supported by a CERG Grant from the Hong Kong
Research Grants Council (HKBU202005) and a Faculty Research
Grant from the Hong Kong Baptist University (FRG/08-09/I-20).
10: A red powder (yield: 11%). MS (FAB): m/z 693 (M+). 1H NMR
(CDCl3): d (ppm) 9.02ꢀ9.01 (d, 1H, J = 2.7 Hz, Ar), 8.83ꢀ8.76 (m,
2H, Ar), 8.11 (d, 1H, J = 8.1 Hz, Ar), 7.77ꢀ7.68 (m, 3H, Ar),
7.58ꢀ7.48 (m, 3H, Ar), 7.31ꢀ7.28 (m, 4H, Ar), 7.05 (d, 2H,
J = 8.1 Hz, Ar), 5.38 (s, 1H, acac), 3.85 (s, 3H, OMe), 1.95 (s, 3H,
CH3), 1.83 (s, 3H, CH3). 13C NMR (CDCl3): d (ppm) 185.92, 184.04
(CO), 169.61, 158.66, 141.99, 141.09, 139.54, 139.07, 138.09,
Appendix A. Supplementary material
CCDC 717476, 717477, 717478, 717479, 717480, 717481 and
717482 contain the supplementary crystallographic data for this

C.-L. Ho et al. / Journal of Organometallic Chemistry 694 (2009) 2735–2749
2749
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