New phosphorescent platinum(ii) Schiff base complexes for PHOLED applications

Article abstract of DOI:10.1039/c2jm32266h

Some new symmetric and asymmetric platinum(ii) Schiff base complexes with bulky substituents such as tert-butyl and triphenylamino groups have been synthesized which effectively reduced the aggregation or excimer formation. Using selected complexes as phosphorescent emitting materials, yellow light-emitting devices were fabricated with improved efficiency compared with the previously reported analogues. In addition, the phosphorescent white organic light-emitting device (WOLED) was fabricated using a single emissive layer composed of yellow- and blue-emitting materials. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm32266h

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PAPER
New phosphorescent platinum(II) Schiff base complexes for PHOLED
applications†
a
Jie Zhang, Fangchao Zhao, Xunjin Zhu, Wai-Kwok Wong, Dongge Ma and Wai-Yeung Wong
b
a
a
b
a
*
*
*
*
Received 11th April 2012, Accepted 6th June 2012
DOI: 10.1039/c2jm32266h
Some new symmetric and asymmetric platinum(II) Schiff base complexes with bulky substituents such
as tert-butyl and triphenylamino groups have been synthesized which effectively reduced the
aggregation or excimer formation. Using selected complexes as phosphorescent emitting materials,
yellow light-emitting devices were fabricated with improved efficiency compared with the previously
reported analogues. In addition, the phosphorescent white organic light-emitting device (WOLED) was
fabricated using a single emissive layer composed of yellow- and blue-emitting materials.
triplet excitons can be harnessed, theoretically approaching
100% internal quantum efficiency. Until now, cyclometalated
iridium(III) complex is recognized as one of the best candidates of
phosphorescent dyes with high quantum yields due to the short
lifetime of triplet excited states.4i Apart from the Ir(III) systems,
platinum(II) complexes have also been extensively studied to
construct PHOLEDs. Cyclometalated platinum(II) C^N^N
complexes of 6-phenyl-2,20-bipyridine and N^C^N complexes of
1,3-dipyridyl-benzene have been reported to display strong
phosphorescence and electrophosphorescence.5e Recently, Che
et al. developed highly blue phosphorescent Pt(II) complexes
based on tetradentate ligands with a bis(NHC) dianionic feature
(NHC ¼ N-heterocyclic carbene), which demonstrated that
incorporation of NHC, a strong s-donor but poor p-acceptor,
into the cyclometalated Pt(II) complexes can shorten the ligand
p-conjugation lengths and consequently broaden the energy
gaps.7 Che and co-workers have also demonstrated that plati-
num(II) Schiff base complexes could be used for high-perfor-
mance OLEDs.8 At low dopant concentrations (<5.0 wt%) of
platinum(II) Schiff base complexes, the device efficiency was
increased. However, at higher dopant concentrations (>5.0
wt%), aggregate/excimer formation was found to reduce the
device efficiency and affect the color purity.
Introduction
In 1987, a semiconducting organic small molecule was found by
Tang and Van Slyke to be emissive with layered films under the
application of an electric field.1 In 1990, semiconducting conju-
gated polymers were also shown to emit green-yellow light under
an electric field.2 Since then, many efforts have been made to
develop organic light-emitting devices (OLEDs). Although
organic light-emitting devices have been commercialized as flat
panel displays since 1997, only singlet excitons were utilized in
this context. In order to harvest both singlet and triplet excitons
for emission, heavy metal atoms have been incorporated into the
emissive layer (EML) of OLEDs to increase the spin–orbit
coupling and thereby facilitate the intersystem crossing from the
singlet to the triplet excited states. After the premier work by
Forrest, Thompson, and co-workers,3 some of the best materials
arising from phosphorescent metal complexes, typically those
composed of heavy metal ions, e.g. Ir(III),4 Pt(II),5 and Os(II),6
have been developed. In phosphorescent organic light-emitting
devices (PHOLEDs) based on these materials, both singlet and
aInstitute of Molecular Functional Materials (Areas of Excellence Scheme,
University Grants Committee, Hong Kong) and Department of Chemistry
and Institute of Advanced Materials, Hong Kong Baptist University,
Waterloo Road, Hong Kong, P. R. China. E-mail: xjzhu@hkbu.edu.hk;
wkwong@hkbu.edu.hk; rwywong@hkbu.edu.hk; Fax: +86 852 3411
7348; Tel: +86 852 3411 5157
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, P. R. China. E-mail: mdg1014@ciac.jl.cn; Fax: +86 431
8526 2873; Tel: +86 431 8526 2357
In this report, we designed and synthesized some new
symmetric and asymmetric platinum(II) Schiff base complexes
with bulky substituents such as tert-butyl and triphenylamino
groups, which effectively reduced the aggregation or excimer
formation and subsequently improved the performance of
PHOLEDs. In addition, the phosphorescent white organic light-
emitting device (WOLED) was fabricated using a single EML
composed of yellow and blue emitting materials. Though the
efficiency of PHOLEDs based on these platinum(II) Schiff base
complexes cannot compete with Ir(III) systems, they are still
attractive because of their preparative accessibility and thermal
stability, and would provide access to new classes of phospho-
rescent materials of practical interest.
† Electronic supplementary information (ESI) available. Crystal data for
2b. C28H34Br2N2O2Pt, monoclinic, space group C2/c, a ¼ 29.187(4), b ¼
ꢀ
3
ꢀ
ꢀ
13.372(2), c ¼ 15.048(2) A, b ¼ 109.333(2) , V ¼ 5542(1) A , Z ¼ 8, Dc ¼
1.883 g cmꢁ3, m(Mo-Ka) ¼ 7.975 mmꢁ1, F(000) ¼ 3040, T ¼ 173(2) K. R1
¼ 0.0461, wR2 ¼ 0.1076 for 4743 independent reflections with a
goodness-of-fit of 1.069. The data were collected on a Bruker Smart
1000 CCD diffractometer and the structure was solved using the
program SHELXLTL. CCDC 875500. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2jm32266h
16448 | J. Mater. Chem., 2012, 22, 16448–16457
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5-bromo-3-tert-butyl-2-hydroxybenzaldehyde
in
dichloro-
Results and discussion
methane was added to the former reaction systems, followed by
the slow addition of an excess amount of anhydrous triethyl-
amine as a deprotection base. The target asymmetric salen ligand
1f was isolated in 60–85% yield as a light yellow solid by means of
column chromatography on a silica gel pretreated with methanol
or methanol–triethylamine. Also, treating 1f with K2PtCl4 in
DMF solution afforded the corresponding platinum(II) Schiff
base complex 2f in 36% yield. Based on 2f, the other unsym-
metrical platinum(II) Schiff base complexes 2g–2j were obtained
in moderate yields (58–74%) via Suzuki–Miyaura reactions
between 2f and different types of boronic acid with bulky
substituents.
Synthetic procedures and chemical characterization
The Schiff base ligands (1a–1e) were easily prepared in good
yields (62–85%) via condensation of the substituted 2-hydroxy-
benzaldehydes with diamines (Scheme 1).9 The Schiff base
ligands were purified by recrystallization and then reacted with
ꢀ
K2PtCl4 and KOH in DMF at 70 C overnight. A yellow solid
was precipitated from the solution when water was added. The
crude solid was filtered and purified on a silica gel column using
CH2Cl2 and hexane as the eluent to give orange (salen
complexes) or red (salophen complexes) target complexes.8,10 On
the other hand, the synthesis of asymmetric salens has proven to
be challenging. Based on a straightforward one-pot protocol
reported recently by Jones et al., asymmetric salen ligands can be
prepared efficiently on gram scales.11 The first condensation
between trans-1,2-diamino-cyclohexane mono-(hydrogen chlo-
ride) and 3,5-di-tert-butyl-2-hydroxy-benzaldehyde was carried
out in a 1 : 1 (v/v) mixture of anhydrous methanol and ethanol at
All the new symmetric and asymmetric Schiff base ligands and
their corresponding platinum(II) complexes were characterized
by 1H NMR, IR, MS, UV-Vis spectrum and elemental analysis.
1
The H NMR spectrum of the symmetric Schiff base ligands
1a, 1b and 1e in CDCl3 exhibits a broad singlet at d 13.72, 13.86
and 13.66 ppm for the O–H protons, respectively. The chemical
shift is typical for the resonance-assisted hydrogen bonded
(RAHB) proton of O–H/N]C. The other characteristic imino
protons of the ligands 1a, 1b and 1e appear as sharp singlet at d
ꢀ
ambient temperature. Activated 4 A molecular sieves were added
to remove the water formed during the reaction. After the first
condensation was completed within about 4 h, a solution of
Scheme 1 Synthesis of symmetrical and unsymmetrical platinum(II) Schiff base complexes 2a–2j.
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atoms in a square planar geometry. The average Pt–N and Pt–O
ꢀ
distances are 1.944 and 1.992 A, respectively. Thanks to the
bulky tert-butyl substituents, the closest Pt(II)/Pt(II) distance is
ꢀ
ꢀ
4.976 A, which is outside the range of 2.7–3.5 A for any metal-
lophilic interaction. Moreover, the presence of the bulky groups
effectively inhibits intermolecular p–p stacking interactions of
the aryl rings.
Photophysical and thermal properties
The UV-Vis absorption spectra of 1a–1j are very similar and are
characteristic of intraligand transitions of Schiff base ligands
(Fig. S1† in the ESI).9 The UV-Vis absorption spectral data of
platinum(II) complexes 2a–2j are listed in Table 1. In CH2Cl2, the
absorption spectra of 2a, 2b, and 2f–2j, having the aliphatic R1
motif, are similar to each other (Fig. 2a and b). Each of them
exhibits intense high-energy absorption bands at lmax ¼ 253–
352 nm and moderately intense absorption bands at lmax ¼ 425–
441 nm, which can be assigned to intraligand p–p* (200–400 nm)
and metal-to-ligand charge-transfer (MLCT, 1[Pt(5d) / p*(L)])
ꢀ
Fig. 1 A perspective view of 2b. Selected bond lengths (A): Pt(1)–N(1)
1.944(6), Pt(1)–N(2) 1.943(6), Pt(1)–O(1) 1.988(5), Pt(1)–O(2) 1.997(4).
Selected bond angles (ꢀ): N(1)–Pt(1)–N(2) 83.7(2), N(1)–Pt(1)–O(1)
95.0(2), N(2)–Pt(1)–O(2) 94.9(2), O(1)–Pt(1)–O(2) 86.5(19), N(2)–Pt(1)–
O(1) 177.4(2), N(1)–Pt(1)–O(2) 177.8(2).
8.30, 8.17 and 8.63 ppm, respectively. On the other hand, the 1H
NMR spectrum of the asymmetric Schiff base ligand 1c shows
two broad singlets at d 13.81 and 13.45 ppm for the O-H protons
and two sharp singlets at d 9.52 and 8.70 ppm for the imino
protons. The 1H NMR spectrum of symmetric platinum(II) Schiff
base complexes 2a, 2b and 2e in CDCl3 exhibits a singlet at d 8.07,
1
mixed with intraligand [lone pair (phenoxide) / p* (imine)]
transitions (400–600 nm), respectively.8 Complexes with the
phenylene R1 motif (2c–2e) show two high-energy absorptions
with peak maxima at lmax ¼ 368–393 nm, moderately intense
absorption bands at lmax ¼ 476–555 nm, which are red-shifted
compared with the aliphatic R1 bridged platinum(II) complexes
(2a, 2b, 2f–2j) due to the expansion of p-conjugation (Fig. 2a).
The absorptions of these complexes are similar, which indicate
that the substituents on the Schiff base ligands only have a little
effect on the energy of absorption.
1
7.96 and 8.61 ppm for the imino protons, respectively. The H
NMR spectrum of the asymmetric platinum(II) Schiff base
complex 2c in CDCl3 exhibits a singlet for each of the two
different imino protons in the range of d 7.93–9.64 ppm.
The IR absorption peak between 1627 and 1641 cmꢁ1 can be
assigned to the C]N stretching vibration, and the absorption
peak at 2224 cmꢁ1 of 2h to the C^N stretching vibration. The
MALDI-TOF-MS spectra of platinum(II) complexes exhibit the
[M]+ or [M + H]+ peaks at m/z values which are deviated less than
9 ppm from the theoretical values.
The emission spectra of 2a–2j at 298 K are depicted in Fig. 3
and the photophysical data are summarized in Table 1. Intense
emission bands with lmax at 557–568 nm (Fem ¼ 0.11–0.20; s ¼
2.60–12.80 ms) are observed upon excitation of the platinum(II)
complexes with an aliphatic R1 motif (2a, 2b, 2f, 2g, 2h, 2j) at
400 nm in CH2Cl2 at 298 K, while 2i shows very weak lumines-
cence (Fem ¼ 0.0071) with lmax at 556 nm due to the electron
Complex 2b was also characterized by X-ray crystallography
(Fig. 1). The Pt(II) ion coordinates to two N atoms and two O
Table 1 Summary of UV-visible absorption and luminescent data of complexes 2a–2j at room temperature in CH2Cl2 (1.0 ꢂ 10ꢁ5 M)
Absorption, lmax/nm
[log(3 dmꢁ3 molꢁ1 cmꢁ1)]a
Luminescence,
lmax (nm) at 298 K
Luminescence,
lmax (nm) at 77 Kb
s (ms),
298 K
s (ms),
77 K
c
Compound
Fem
2a
2b
2c
2d
253 (4.59), 316 (4.09), 353 (4.15),
437 (3.74)
261 (4.62), 322 (4.08), 351 (4.10),
441 (3.74)
258 (4.47), 373 (4.37), 389 (4.47),
486 (3.78), 561 (3.78)
255 (4.33), 311 (3.90), 321 (3.94),
359 (4.18) 379 (4.22), 454 (3.14),
526 (3.30)
560, 602 (sh)
559, 603 (sh)
650
551, 599
563, 609
663, 736 (sh)
632
0.15
0.18
0.12
0.17
3.36
4.89
2.60
7.72
9.65
11.56
13.73
11.86
612, 673 (sh)
2e
2f
262 (4.51), 330 (4.17), 373 (4.54),
392 (4.61) 497 (3.95), 556 (3.87)
260 (4.69), 319 (4.16), 352 (4.17),
438 (3.77)
590 (sh), 649
568, 612 (sh)
651, 724
554, 602
0.11
0.15
2.85
3.94
8.98
10.33
2g
2h
2i
259 (4.27), 337 (4.43), 437 (3.48)
248 (4.33), 357 (4.37), 425 (3.59)
266 (3.61), 357 (4.30), 431 (4.32)
287 (4.28), 342 (4.50), 432 (3.67)
568
555, 603
555, 604
546, 559
551, 592
0.17
0.16
0.007
0.20
6.34
8.39
12.80
5.37
11.18
9.25
19.69
10.23
557, 596 (sh)
556
565, 608 (sh)
2j
a
Photophysical measurements were made with a concentration of 1 ꢂ 10ꢁ5 M in CH2Cl2 at room temperature. b Emission at 77 K was made in frozen
c
CH3CN at a concentration of 1 ꢂ 10ꢁ5 M. Luminescence quantum yields (Fem) in degassed dichloromethane solution were determined with
[Ru(bpy)3](PF6)2 (bpy ¼ 2,20-bipyridine) in acetonitrile as a standard reference solution (Fem ¼ 0.062).
16450 | J. Mater. Chem., 2012, 22, 16448–16457
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Fig. 2 UV-Vis absorption spectra of the Schiff base platinum(II) complexes 2a–2e (a) and 2f–2j (b) in CH2Cl2 (1.0 ꢂ 10ꢁ5 M).
deficient nitro group. Those platinum(II) complexes (2c, 2e)
Electrochemical studies
bridged by the phenylene R1 motif show red emission with bands
Cyclic voltammetry behaviour of platinum(II) Schiff base
complexes was recorded in degassed CH2Cl2 at a scan rate of
100 mV sꢁ1 with 0.1 M [n-Bu4N][PF6] as the supporting electro-
lyte. All potentials are referenced to the ferrocenium–ferrocene
(Fc–Fc+) couple. A glassy carbon electrode was used as the
working electrode, a platinum wire as the counter electrode and a
Ag/AgCl as the reference electrode. Then the oxidation potentials
of 2a–2j were estimated via cyclic voltammetry to be in the range of
0.29–0.60 V. The HOMO was deduced by the equation EHOMO
¼ ꢁ(Eox + 4.80) eV. Estimated from the UV-visible absorption
spectrum, the energy band gap of the platinum(II) complexes was
obtained. Thus the LUMO was calculated to be the sum of the
energy band gap and HOMO. The HOMO and LUMO levels of
these complexes are in the range of ꢁ5.09 to ꢁ5.40 eV and ꢁ2.56
to ꢁ3.05 eV, respectively. The oxidation potentials and the
HOMO–LUMO energy gap are summarized in Table 2.
at lmax ¼ 650, 649 nm and quantum yields and lifetimes of Fem
¼
0.12, 0.11 and s ¼ 2.60, 2.85 ms, respectively. The platinum(II)
complex (2d) bridged by the pyridyl R1 motif shows an orange
emission at lmax ¼ 612 nm (Fem ¼ 0.17; s ¼ 7.72 ms). The decay
time in the microsecond scale indicates that the emission band
originates from the triplet emissive excited state.8b It is note-
worthy that when 2g and 2j are excited at 300 and 380 nm,
respectively, only emissions corresponding to the platinum(II)
complexes are observed. The singlet excited emission of triphe-
nylamine or fluorene segment is quenched, which indicates an
efficient intramolecular energy transfer from triphenylamine or
fluorene to the Schiff base platinum(II) complexes.12 The plati-
num(II) complexes 2a, 2b, 2f, 2g, 2h and 2j, based on aliphatic R1
motif bridged symmetric and asymmetric Schiff base ligands,
show very similar emission wavelengths, while those based on
electron-withdrawing group substituted Schiff base ligands (2d,
2h, 2i) show longer lifetimes. At 77 K, the emission spectra of the
platinum(II) Schiff base complexes also show peaks in the visible
range, which are consistent with the room temperature emission
spectra despite the blue or red shift of the wavelengths.
Furthermore, a longer lifetime was observed at 77 K (Fig. S2† in
the ESI).
Electrophosphorescent OLED characterization
All of the new platinum(II) Schiff base complexes can be readily
sublimed under vacuum which should be amenable to thermally
evaporated OLED fabrication. Herein, to evaluate the perfor-
mance of selected complexes as phosphorescent emitters, devices
with the structure ITO/MoO3 (8 nm)/1,4-bis(1-naph-
thylphenylamino)-biphenyl (NPB) (80 nm)/4,40,400-tri(9-carba-
zoyl)-triphenylamine (TCTA) (5 nm)/TCTA : x wt % Pt(II)
The thermal stabilities of 2a–2j were investigated by the ther-
mogravimetric analysis (TGA) method. All complexes are ther-
mally stable with decomposition temperature in the range of
ꢀ
279–404 C (Table 2).
Fig. 3 Emission spectra of the platinum(II) Schiff base complexes 2a–2e (a) and 2f–2j (b) in CH2Cl2 (1.0 ꢂ 10ꢁ5 M) at room temperature.
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Table 2 Summary of cyclic voltammetry data and TGA of 2a–2j
that of the dopant and host, thus preventing electron leakage
from the emissive layer. MoO3 and LiF function as the hole- and
electron-injecting layers, respectively. The indium tin oxide
(ITO) coated glass substrate with 20 U per square sheet resistivity
was used as the transparent anode.
a
d
Eg
Compound
Eox
HOMOb
LUMOc
Td (ꢀC)
2a
2b
2c
2d
2e
2f
2g
2h
2i
0.38
0.29
0.33
0.30
0.58
0.60
0.39
0.32
0.43
0.33
ꢁ5.18
ꢁ5.09
ꢁ5.13
ꢁ5.10
ꢁ5.38
ꢁ5.40
ꢁ5.19
ꢁ5.12
ꢁ5.23
ꢁ5.13
ꢁ2.63
ꢁ2.56
ꢁ3.05
ꢁ2.89
ꢁ3.27
ꢁ2.88
ꢁ2.68
ꢁ2.58
ꢁ2.72
ꢁ2.61
2.55
2.53
2.08
2.21
2.11
2.52
2.51
2.54
2.51
2.52
380
330
279
388
372
336
401
341
341
404
The performances of the devices using 2a, 2b, 2c and 2f as
electrophosphorescent dopant materials are summarized in
Table 3. The EL spectra in the range of 500–800 nm well match
the photoluminescence (PL) spectra of the corresponding
complexes, revealing that EL emissions in this region come
directly from the triplet excited state. The devices fabricated with
complexes 2a, 2b and 2f showed yellow emission with CIE
coordinates at around (0.49, 0.51), while the devices with
complex 2c showed red emissions with CIE coordinates at
around (0.68, 0.32). To optimize the device efficiency, concen-
tration dependence experiments were carried out in the range of
6 to 12 wt%. At low platinum(II) dopant concentrations (#8.0
wt%), the device efficiency was increased. It should be noted that
aggregation/excimer formation was not obvious in our study
even at high platinum(II) dopant concentrations of up to 12 wt%,
and the slightly reduced device efficiency might arise from the
triplet–triplet annihilation. As TCTA has a relatively higher
triplet energy level,13 an efficient exothermic energy transfer from
the TCTA triplet state to the triplet state of platinum(II) Schiff
base complexes would occur given the perfect alignment of triplet
state energy, leading to excellent triplet exciton confinement
within the emissive layer. However, it is worth noting that
incomplete electron trapping by the platinum(II) Schiff base
complexes is evidenced by the observation of an EL band at
416 nm at low dopant concentrations, which primarily comes
from the fluorescence of the host material TCTA.
2j
a
Versus ferrocene–ferrocenium couples. b Estimated from the oxidation
potentials HOMO ¼ ꢁ(Eox + 4.80) eV. c Deduced from the HOMO and
Eg. d Estimated from the UV-vis absorption spectra.
The yellow OLED doped with 8 wt% 2a shows an impressive
performance with a turn-on voltage (Von) of 3.5 V, a maximum
brightness (Lmax) of 10 826 cd mꢁ2 at 13.5 V, a peak external
quantum efficiency (hext,max) of 7.0%, a peak luminance efficiency
(hL,max) of 21 cd Aꢁ1 and a peak power efficiency (hp,max) of 17 lm
Wꢁ1 (Table 3). Another yellow light-emitting device with 8 wt%
2g is also attractive in performance showing a better EL effi-
ciency of 8.3%, 23 cd Aꢁ1 and 17 lm Wꢁ1. This device turns on at
3.5 V and its light output can reach 11 106 cd mꢁ2 at 13.7 V
(Fig. 5a and b). Unlike most other platinum(II) systems,7 in which
the EL was found to show significant spectral shifts upon dopant
aggregation to give a broad red-shifted excimer emission, the EL
spectra using 2a or 2g as dopant are found to be independent of
the driving voltage from 4.5 to 15 V (Fig. 5c). The voltage-
independent EL may be attributed to a lower tendency of
complex 2a or 2g to form aggregates through Pt/Pt and/or p–p
Fig. 4 The device structure and energy level diagram of a OLED using
2f as the emitting material.
complex (20 nm)/1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)
phenyl (TPBI) (40 nm)/LiF (1 nm)/Al were fabricated as shown
in Fig. 4. In the devices, the layers of NPB and TPBI were used as
the hole- and electron-transporting layer, respectively. And
TCTA was chosen as the host material because its emission
spectrum overlaps with the absorption spectra of the platinum(II)
complexes in this work. The selected platinum(II) Schiff base
complexes with different dopant concentrations (4–12 wt%) in
the host material TCTA served as the emissive layer. Another
thin layer of TCTA was inserted between the emissive layer and
the hole transporting layer, with LUMO of at least 0.3 eV above
Table 3 Summary of device performances at best dopant concentrations
b
c
d
Dopant
Dopant/wt%
Vona/V
Lmax /cd mꢁ2
hL,max /cd Aꢁ1
hP,max /lm Wꢁ1
hext,maxe/%
ELmaxf/nm
CIE, x, y
2a
2b
2c
2g
8
10
8
8
10 and 1
3.5
3.7
3.9
3.5
3.1
10 826
1236
2468
11 106
20 905
21
17
7.0
2.2
2.4
8.3
7.7
560
560
656
564
0.49, 0.51
0.48, 0.51
0.68, 0.32
0.50, 0.50
0.34, 0.44
6.0
1.3
23
5.0
0.9
17
FIrpic and 2g
21
17
474, 500, 560
a
b
c
d
e
Turn-on voltage. Maximum luminance. Maximum current efficiency. Maximum power efficiency. Maximum external quantum efficiency.
Peak emission wavelength.
f
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Fig. 5 (a) Performance of the OLED with 2g as dopant: power efficiency–luminance curves; (b) current–voltage and luminance–voltage characteristics
of the devices; (c) representative EL spectra of the OLED with 8 wt% dopant 2g at the driving voltage from 5 to 13 V.
stacking interactions in the thin films. To the best of our
knowledge, using 2a or 2g as the single EML, the device
performance is remarkable and comparable to the ever reported
analogous platinum(II) complexes as a single EML.14
0.46) at 4 V and CIE (0.34, 0.44) at 9 V. Our primary results
indicate that these platinum(II) Schiff bases are competitive
candidates as phosphorescent dopant as yellow- or white-emit-
ting OLED.
White organic light-emitting devices (WOLEDs), especially
fabricated with triplet emitters, show great promise for applica-
tions in flat-panel displays and solid-state lighting, because of
their low operating voltage and the high brightness obtained.15
Inspired by the good device performance of the new platinum(II)
Schiff base complex 2g, we anticipate its great potential to obtain
a white light emission by combining blue and yellow dopants in a
single EML. Herein, a preliminary evaluation is given for
WOLED that uses complex 2g and bis(4,6-difluorophenylpyr-
idine)(picolinate) iridium(III) (FIrpic) in the device structure of
ITO/MoO3 (8 nm)/NPB (80 nm)/TCTA (5 nm)/TCTA : x%
FIrpic : y% 2g (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al. The base
device configuration is similar to the one shown in Fig. 4 with
TCTA, NPB and TPBI as the host, hole- and electron-trans-
porting materials, respectively. The device differs only in the
composition of the EML, using a single EML with 2g and FIrpic
co-deposited from two separate sublimation sources. We deter-
mined through systematic variation that a device using 11 wt%
dopants having a FIrpic/2g (blue/yellow) ratio of 10 : 1 gave the
white emission as depicted in Fig. 6. The turn-on voltage for this
device is 3.1 V and the light output can reach up to 20 905 cd mꢁ2
at 13.1 V. The hext,max of 7.7%, hL,max efficiency of 21 cd Aꢁ1 and
hP,max of 17 lm Wꢁ1 were obtained. The EL peak is voltage
dependent, and increasing the applied voltage strengthens the
proportion of blue emission with the corresponding CIE (0.40,
Conclusions
We have developed a series of symmetrical and asymmetric
phosphorescent platinum(II) Schiff base complexes with good
thermal stability. These complexes with bulky substituents
showed moderate emission quantum yields without obvious
aggregation/excimer formation in solution or solid state. Using
complexes 2a, 2b and 2g as phosphorescent emitting materials,
yellow light-emitting devices were fabricated with improved
efficiency compared with other previously reported analogues.
The maximum current efficiency, power efficiency, and bright-
ness for the 8.0 wt% 2g doped OLEDs are 23 cd Aꢁ1, 17 lm Wꢁ1
and 11 106 cd mꢁ2, respectively. Aggregation was successfully
inhibited and the emission did not change even at high concen-
trations in the OLEDs. Given the synthetic simplicity and ease of
structural variation, the preliminary results here indicate that
these platinum(II) Schiff base complexes are good candidates for
phosphorescent OLEDs. In addition, a single EML white-emit-
ting device with a maximum efficiency of 7.7% and CIE coor-
dinates of (0.34, 0.44) is also reported. The performance of
yellow and white OLEDs using these complexes can be further
enhanced by employing new matrix and carrier-transporting
materials, and further investigation of the lifetime of the devices
based on these platinum(II) Schiff bases is currently underway.
Fig. 6 Performance of WOLED using a single EML doped with 2g and FIrpic: (a) EL spectrum; (b) power efficiency-luminance, luminescence
efficiency-luminescence and external quantum efficiency-luminescence; (c) current–voltage and luminance–voltage characteristics of devices.
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J. Mater. Chem., 2012, 22, 16448–16457 | 16453

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min. Then, the sample was transferred to the deposition system.
8 nm of MoO3 was firstly deposited to the ITO substrate,
followed by 80 nm NPB, 5 nm TCTA, 20 nm emissive and 40 nm
TPBI layers. Finally, a cathode composed of 1 nm of lithium
fluoride and 100 nm of aluminum was sequentially deposited
onto the substrate under vacuum at 10ꢁ6 Torr to construct
the device. The EL data were measured with a Keithley 2400
Source meter and a Keithley 2000 Source multimeter equipped
with a calibrated silicon photodiode. The EL spectra were
measured by a JY SPEX CCD3000 spectrometer. All measure-
ments were carried out at room temperature under ambient
conditions.
Experimental section
General information
The chemicals were purchased from Sigma-Aldrich Company
and used as received. NMR spectra were recorded on a Bruker
Ultrashield 400 Plus NMR spectrometer. The intermediates
5-bromo-3-tert-butyl-2-hydroxy-benzaldehyde16 and 4,5-bis(do-
decyloxy)benzene-1,2-diamine17 were prepared according to the
literature methods. High-resolution matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass
spectra were recorded with a Bruker Autoflex MALDI-TOF
mass spectrometer and low-resolution FAB mass spectra with a
Finnigan TSQ710 mass spectrometer. The C, H and N micro-
analyses were performed with a Carlo Erba 1106 elemental
analyzer. The IR spectra (KBr pellets) were recorded with a
Nicolet Magna 550 FTIR spectrometer. Electronic absorption
spectra in the UV-Vis region were recorded on a Varian Cary 100
UV-Vis spectrophotometer, steady-state visible fluorescence and
PL excitation spectra on a Photon Technology International
(PTI) Alphascan spectrofluorimeter and visible decay spectra on
a pico-N2 laser system (PTI TimeMaster) with lexc ¼ 337 nm.
Luminescence quantum yields (Fem) in degassed dichloro-
methane solution were determined with [Ru(bpy)3](PF6)2 (bpy ¼
2,20-bipyridine) in acetonitrile as a standard reference solution
(Fem ¼ 0.062) and calculated by Fs ¼ Fr(Br/Bs)(ns/nr)2(Ds/Dr),
where the subscripts r and s denote the reference standard and
sample solution, respectively, and n, D, and F are the refractive
index of the solvents, the integrated intensity, and the lumines-
cence quantum yield, respectively. The quantity B is calculated
by B ¼ 1 ꢁ 10ꢁAL, where A is the absorbance at the excitation
wavelength and L is the optical path length. Electrochemical
measurements were made using a Princeton Applied Research
model 273A potentiostat at a scan rate of 100 mV sꢁ1. A
conventional three-electrode configuration consisting of a glassy
carbon working electrode, a platinum wire counter electrode,
and a Ag/AgCl reference electrode was used. The supporting
electrolyte was 0.1 M [n-Bu4N]PF6 in THF. Ferrocene was added
as a calibrant after each set of measurements, and all potentials
reported were quoted with reference to the ferrocene–ferroce-
nium (Fc–Fc+) couple (taken as E1/2 ¼ +0.27 V relative to the
reference electrode). The HOMO was deduced by the equation
EHOMO ¼ ꢁ(Eox + 4.80) eV, assuming that the energy level of
ferrocene–ferrocenium to be ꢁ4.80 eV. Estimated from the UV-
visible absorption spectrum, the energy band gap of the plati-
num(II) complexes was obtained. Thus the LUMO was calcu-
lated to be the sum of the energy band gap and HOMO.18
Thermal analyses were performed with Perkin-Elmer Pyris
Diamond DSC and Perkin-Elmer TGA6 thermal analyzers.
General procedure for the preparation of Schiff base ligands 1a–1e
To a stirred solution of one equivalent aliphatic or aromatic
diamine in absolute ethanol, two equivalents of salicylaldehyde
were added. The resulting mixture was refluxed overnight. After
cooling to room temperature, the yellow product was precipi-
tated out, and the product was filtered and washed with cold
ꢀ
ethanol and petroleum ether (b.p. 30–40 C).
1a: cyclohexane-1,2-diamine (443 mg, 3.89 mmol) and 3,5-di-
tert-butyl-2-hydroxybenzaldehyde (2000 mg, 8.55 mmol) were
1
used. Yield: 1807 mg (85%). H NMR (400 MHz, CDCl3): d ¼
13.72 (s, 2H, OH), 8.30 (s, 2H, C]NH), 7.30 (d, J ¼ 2.4 Hz, 2H,
Ar-H), 6.98 (d, J ¼ 2.4 Hz, 2H, Ar-H), 3.38–3.26 (m, 2H,
cyclohexane-H), 1.98–1.66 (m, 8H, cyclohexane-H), 1.41 (s, 18H,
tert-butyl), 1.24 (d, J ¼ 6.7 Hz, 18H, tert-butyl) ppm. IR (KBr):
n ¼ 3435, 2952, 2865, 1630, 1595, 1469, 1439, 1361, 1270, 1174,
772 and 644 cmꢁ1. FAB MS (positive mode): m/z ¼ 547 [M + H]+.
1b: cyclohexane-1,2-diamine (40 mg, 0.35 mmol) and tert-
butyl-bromobenzaldehyde (200 mg, 0.78 mmol) were used. Yield:
1
160 mg (77%). H NMR (400 MHz, CDCl3): d ¼ 13.86 (s, 2H,
OH), 8.17 (s, 2H, C]NH), 7.31 (d, J ¼ 2.4 Hz, 2H, Ar-H), 7.08
(d, J ¼ 2.4 Hz, 2H, Ar-H), 3.38–3.27 (m, 2H, cyclohexane-H),
1.89–2.01 (m, 4H, cyclohexane-H), 1.71–1.79 (m, 2H, cyclo-
hexane-H), 1.45–1.50 (m, 2H, cyclohexane-H), 1.38 (s, 18H, tert-
butyl) ppm. IR (KBr): n ¼ 3412, 2957, 2932, 2859, 1634, 1599,
1448, 1427, 1391, 1269, 1174, 772, 558 cmꢁ1. HRMS (MALDI-
TOF, positive mode, CH2Cl2): m/z ¼ 593.1196 [M + H]+
(C28H36Br2N2O2 + H: calcd 593.1198, Dm ¼ ꢁ0.34 ppm).
1c: pyridine-3,4-diamine (212 mg, 1.94 mmol) and 3,5-di-tert-
butyl-2-hydroxybenzaldehyde (1000 mg, 4.27 mmol) were used.
Yield: 758 mg (72%). 1H NMR (400 MHz, CDCl3): d ¼ 13.81 (s,
1H, OH), 13.45 (s, 1H, OH), 9.52 (s, 1H, C]NH), 8.70 (s, 1H,
C]NH), 8.39 (dd, J ¼ 4.7, 1.6 Hz, 1H, Ar-H), 7.58 (dd, J ¼ 7.8,
1.6 Hz, 1H, Ar-H), 7.49 (d, J ¼ 2.4 Hz, 1H, Ar-H), 7.46 (d, J ¼
2.4 Hz, 1H, Ar-H), 7.36 (d, J ¼ 2.4 Hz, 1H, Ar-H), 7.30–7.26 (m,
1H, Ar-H), 7.23 (d, J ¼ 2.4 Hz, 1H), 1.50 (s, 9H, tert-butyl), 1.42
(s, 9H, tert-butyl), 1.33 (s, 9H, tert-butyl), 1.32 (s, 9H, tert-butyl)
ppm. IR (KBr): n ¼ 3435, 2955, 2907, 1614, 1589, 1556, 1454,
1433, 1359, 1272, 1172, 773, 675 cmꢁ1. HRMS (MALDI-TOF,
positive mode, CH2Cl2): m/z ¼ 542.3751 [M + H]+ (C35H47N3O2
+ H: calcd 542.3741, Dm ¼ 1.84 ppm).
OLED fabrication and measurement
The hole-injection MoO3, hole-transporting material NPB (1,4-
bis(1-naphthylphenylamino)-biphenyl) and TCTA (4,40,400-tri(9-
carbazolyl)-triphenylamine), and electron-transporting material
TPBI (1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl) were
commercially available. Commercial indium tin oxide (ITO)
coated glass with a sheet resistance of 10 U sqꢁ1 was used as the
starting substrate. Before device fabrication, the ITO glass
substrates were pre-cleaned carefully and treated by UV/O3 for 2
1d: 3,4-diaminobenzoic acid (566 mg, 3.73 mmol) and 2-
hydroxybenzaldehyde (1000 mg, 8.20 mmol) were used. Yield:
1049 mg (78%). 1H NMR (400 MHz, [D6]DMSO): d ¼ 13.13 (s,
1H, COOH), 12.80 (s, 1H, OH), 12.57 (s, 1H, OH), 9.03 (s, 1H,
C]NH), 8.97 (s, 1H, C]NH), 7.98 (d, J ¼ 1.8 Hz, 1H, Ar-H),
16454 | J. Mater. Chem., 2012, 22, 16448–16457
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7.94 (dd, J ¼ 8.2, 1.8 Hz, 1H, Ar-H), 7.71 (td, J ¼ 7.8, 1.4 Hz, 2H,
Ar-H), 7.52 (d, J ¼ 8.2 Hz, 1H, Ar-H), 7.43 (qd, J ¼ 8.2, 1.6 Hz,
2H, Ar-H), 7.01–6.91 (m, 4H, Ar-H) ppm. IR (KBr): n ¼ 3435,
2959, 2857, 1683, 1615, 1592, 1559, 1453, 1432, 1361, 1278, 1193,
761, 679 cmꢁ1. HRMS (MALDI-TOF, positive mode, CH2Cl2):
m/z ¼ 361.1176 [M]+ (C21H16N2O4: calcd 361.1183, Dm ¼ ꢁ1.94
ppm).
cyclohexane-H), 1.92–1.94 (m, 2H, cyclohexane-H), 1.52–1.53
(m, 20H, cyclohexane-H and tert-butyl), 1.40–1.41 (m, 2H,
cyclohexane-H), 1.29 (s, 18H, tert-butyl) ppm. IR (KBr): n ¼
3435, 2952, 2866, 1610, 1528, 1433, 1319, 1254, 1168, 781 and 545
cmꢁ1. HRMS (MALDI-TOF, positive mode, CH2Cl2): m/z ¼
739.3687 [M]+ (C36H52N2O2Pt: calcd 739.3674, Dm ¼ ꢁ1.76
ppm). UV-Vis (CH2Cl2, 20 ꢀC): lmax [log(3 dmꢁ3 molꢁ1 cmꢁ1)] ¼
253 (4.59), 316 (4.09), 353 (4.15), 437 (3.74) nm.
C36H52N2O2Pt$0.5H2O (748.9): calcd C 57.74, H 7.13, N 3.74;
found: C 57.44, H 7.08, N 3.79%.
1e:
0.97 mmol) and 3,5-di-tert-butyl-2-hydroxybenzaldehyde
4,5-bis(dodecyloxy)benzene-1,2-diamine
(463
mg,
1
(500 mg, 2.14 mmol) were used. Yield: 547 mg (62%). H NMR
(400 MHz, CDCl3): d ¼ 13.66 (s, 2H, OH), 8.63 (s, 2H, C]NH),
7.41 (d, 2H, J ¼ 2.4 Hz, Ar-H), 7.20 (d, 2H, J ¼ 2.4 Hz, Ar-H),
6.81 (s, 2H, Ar-H), 4.07 (m, 4H, -OCH2-), 1.84–1.88 (m, 4H,
-OCH2CH2-), 1.49–1.56 (m, 6H), 1.43 (s, 18H, tert-butyl), 1.35
(m, 4H), 1.32 (s, 18H, tert-butyl), 1.27 (m, 26H), 0.88 (s, 6H,
-CH3) ppm. IR (KBr): n ¼ 3435, 2955, 2924, 1615, 1579, 1513,
1472, 1439, 1361, 1263, 1173, 773, 686 cmꢁ1. HRMS (MALDI-
TOF, positive mode, CH2Cl2): m/z ¼ 909.7403 [M + H]+
(C21H16N2O4: calcd 361.1183, Dm ¼ ꢁ4.51 ppm).
2b: Schiff base ligand 1b (100 mg, 0.17 mmol), KOH (19 mg,
0.34 mmol) and K2PtCl4 (84 mg, 0.20 mmol) were used. Yield: 34
1
mg (26%). H NMR (400 MHz, CDCl3): d ¼ 7.96 (s, 2H, C]
NH), 7.50 (d, 2H, J ¼ 2.8 Hz, Ar-H), 7.28 (d, 2H, J ¼ 2.6 Hz, Ar-
H), 3.50–3.53 (m, 2H, cyclohexane-H), 2.59–2.62 (m, 2H,
cyclohexane-H), 1.88–1.91 (m, 2H, cyclohexane-H), 1.51–1.52
(m, 2H, cyclohexane-H), 1.48 (m, 18H, tert-butyl), 1.30–1.36 (m,
2H, cyclohexane-H) ppm. IR (KBr): n ¼ 3436, 2944, 2867, 1623,
1526, 1429, 1313, 1257, 1164, 778 and 582 cmꢁ1. HRMS
(MALDI-TOF, positive mode, CH2Cl2): m/z ¼ 786.0629 [M]+
(C28H34Br2N2O2Pt: calcd 786.0609, Dm ¼ 2.54 ppm). UV-Vis
(CH2Cl2, 20 ꢀC): lmax [log(3 dmꢁ3 molꢁ1 cmꢁ1)] ¼ 261 (4.62), 322
(4.08), 351 (4.10), 441 (3.74) nm. C28H34Br2N2O2Pt (785.47):
calcd C 42.82, H 4.36, N 3.57; found: C 42.69, H 4.55, N 3.52%.
2c: Schiff base ligand 1c (100 mg, 0.18 mmol), KOH (21 mg,
0.37 mmol) and K2PtCl4 (92 mg, 0.22 mmol) were used. Yield: 96
Preparation of asymmetric Schiff base ligand 1f
A solution of 3,5-di-tert-butyl-2-hydroxybenzaldehyde (1000 mg,
4.27 mmol), trans-diaminocyclohexane mono(hydrogen chlo-
ꢀ
ride) (645 mg, 4.27 mmol) and 4 A molecular sieves (500 mg) in
anhydrous methanol (15 mL) were stirred overnight at room
temperature under nitrogen. Then, 5-bromo-3-tert-butyl-2-
hydroxybenzaldehyde (1098 mg, 4.27 mmol) in anhydrous
CH2Cl2 (20 mL) and anhydrous NEt3 (863 mg, 8.54 mmol) were
added. The mixture was stirred at room temperature for an
additional 10 h. The residue after solvent evaporation was
purified by silica-gel column chromatography using a solvent
mixture of hexane and CH2Cl2 as the eluent to give a yellow solid
1
mg (71%). H NMR (400 MHz, CDCl3): d ¼ 9.64 (s, 1H, C]
NH), 8.87 (s, 1H, C]NH), 8.37 (dd, 1H, J ¼ 4.4, 0.8 Hz, Ar-H),
8.29 (dd, 1H, J ¼ 8.8, 0.8 Hz, Ar-H), 7.70 (td, 2H, J ¼ 1.6, 0.8 Hz,
Ar-H), 7.47 (d, 1H, J ¼ 2.0 Hz, Ar-H), 7.31 (d, 1H, J ¼ 2.0 Hz,
Ar-H), 7.17 (dd, 1H, J ¼ 8.0, 4.8 Hz, Ar-H), 1.59 (s, 18H, tert-
butyl), 1.35 (s, 18H, tert-butyl) ppm. IR (KBr): n ¼ 3435, 2953,
2867, 1598, 1519, 1441, 1360, 1268, 1170, 782 and 542 cmꢁ1
.
1
of 1f. Yield: 1632 mg (67%). H NMR (400 MHz, CDCl3): d ¼
HRMS (MALDI-TOF, positive mode, CH2Cl2): m/z ¼ 735.3263
[M + H]+ (C35H45N3O2Pt: calcd 735.3235, Dm ¼ 3.81 ppm). UV-
Vis (CH2Cl2, 20 ꢀC): lmax [log(3 dmꢁ3 molꢁ1 cmꢁ1)] ¼ 258 (4.47),
373 (4.37), 389 (4.47), 486 (3.78), 561 (3.78) nm.
C35H45N3O2Pt$1/3CH2Cl2 (763.14): calcd C 55.61, H 6.03, N
5.51; found: C 55.44, H 5.89, N 5.57%.
13.95 (s, 1H, OH), 13.59 (s, 1H, OH), 8.26 (s, 1H, C]NH), 8.18
(s, 1H, C]NH), 7.31 (s, 1H, Ar-H), 7.28 (s, 1H, Ar-H), 7.08 (s,
1H, Ar-H), 6.94 (s, 1H, Ar-H), 3.30 (m, 2H, cyclohexane-H),
1.73–1.95 (m, 8H, cyclohexane-H), 1.42 (s, 9H, tert-butyl), 1.37
(s, 9H, tert-butyl), 1.23 (s, 9H, tert-butyl) ppm. IR (KBr): n ¼
3429, 2956, 2862, 1629, 1469, 1439, 1361, 1272, 1174, 772,
644 cmꢁ1. FAB MS (positive mode): m/z ¼ 570 [M]+.
2d: Schiff base ligand 1d (100 mg, 0.28 mmol), KOH (31 mg,
0.55 mmol) and K2PtCl4 (138 mg, 0.33 mmol) were used. Yield:
112 mg (68%). 1H NMR (400 MHz, [D6]DMSO): d ¼ 9.60 (s, 1H,
C]NH), 9.57 (s, 1H, C]NH), 8.40 (s, 1H, Ar-H), 8.37 (d, 1H,
J ¼ 8.8 Hz, Ar-H), 8.10 (dd, 1H, J ¼ 8.4, 0.8 Hz, Ar-H), 7.94 (td,
2H, J ¼ 7.8, 2.0 Hz, Ar-H), 7.58–7.64 (m, 2H, Ar-H), 7.16 (d, 2H,
J ¼ 4.0 Hz, Ar-H), 6.83 (qd, 2H, J ¼ 8.0, 0.6 Hz, Ar-H) ppm. IR
(KBr): n ¼ 3429, 2953, 1612, 1524, 1385, 1366, 1253, 1183, 779
and 545 cmꢁ1. HRMS (MALDI-TOF, positive mode, CH2Cl2):
General procedure for the preparation of platinum(II) Schiff base
complexes 2a–2f
A solution of one equivalent of the Schiff base ligand 1 and two
equivalents of ꢀKOH was stirred about 0.5 h in DMF under
nitrogen at 70 C. Then, excess K2PtCl4 in DMSO (5 mL) was
added and continuously stirred for 24 h under nitrogen at 70 ꢀC.
The reaction mixture was extracted with CH2Cl2 and washed
with water several times. The residue after solvent evaporation
was purified by silica-gel column chromatography using a
solvent mixture of hexane and CH2Cl2 as eluent to give an orange
solid.
m/z ¼ 591.9498 [M]+ (C21H13KN2O4Pt: calcd 591.5152, Dm
¼
7.34 ppm). UV-Vis (DMSO, 20 ꢀC): lmax [log(3 dmꢁ3 molꢁ1
cmꢁ1)] ¼ 255 (4.33), 311 (3.90), 321 (3.94), 359 (4.18), 379 (4.22),
454 (3.14), 526 (3.30) nm. C21H13KN2O4Pt (591.52): calcd C
42.64, H 2.22, N 4.74; found: C 42.53, H 2.32, N 4.89%.
2e: Schiff base ligand 1e (200 mg, 0.22 mmol), KOH (25 mg,
0.44 mmol) and K2PtCl4 (110 mg, 0.26 mmol) were used. Yield:
75 mg (31%). 1H NMR (400 MHz, CDCl3): d ¼ 8.61 (s, 2H, C]
NH), 7.63 (s, 2H, Ar-H), 7.36 (s, 2H, Ar-H), 7.31 (s, 2H, Ar-H),
4.14 (m, 4H, -OCH2-), 1.91 (m, 4H, -OCH2CH2-), 1.57 (s, 18H,
tert-butyl), 1.27–1.41 (m, 54H), 0.88 (s, 6H, -CH3) ppm. IR
2a: Schiff base ligand 1a (100 mg, 0.18 mmol), KOH (21 mg,
0.37 mmol) and K2PtCl4 (91 mg, 0.22 mmol) were used. Yield:
51 mg (38%). 1H NMR (400 MHz, CDCl3): d ¼ 8.07 (s, 2H, C]
NH), 7.56 (d, 2H, J ¼ 2.4 Hz, Ar-H), 7.08 (d, 2H, J ¼ 2.4 Hz, Ar-
H), 3.43–3.45 (m, 2H, cyclohexane-H), 2.65–2.68 (m, 2H,
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(KBr): n ¼ 3435, 2952, 2852, 1606, 1585, 1515, 1463, 1358, 1275,
1.53–1.55 (m, 18H, tert-butyl), 1.27–1.30 (m, 11H, tert-butyl and
cyclohexane-H), 0.88 (m, 2H, cyclohexane-H) ppm. IR (KBr):
n ¼ 3435, 2948, 2867, 2224, 1597, 1531, 1435, 1321, 1255, 1167,
780 and 543 cmꢁ1. HRMS (MALDI-TOF, positive mode,
CH2Cl2): m/z ¼ 785.3370 [M + H]+ (calcd for C39H47N3O2Pt +
1167, 781 and 545 cmꢁ1. HRMS (MALDI-TOF, positive mode,
CH2Cl2): m/z
¼
1102.7044 [M]+ (C60H94N2O4Pt: calcd
1102.6938, Dm ¼ 9.52 ppm). UV-Vis (CH2Cl2, 20 ꢀC): lmax [log(3
dmꢁ3 molꢁ1 cmꢁ1)] ¼ 262 (4.51), 330 (4.17), 373 (4.54), 392 (4.61),
497 (3.95), 556 (3.87) nm. C60H94N2O4Pt (1102.48): calcd C
65.37, H 8.59, N 2.54; found: C 65.40, H 8.63, N 2.71%.
ꢀ
H: 785.3392, Dm ¼ ꢁ2.82 ppm). UV-Vis (CH2Cl2, 20 C): lmax
[log(3 dmꢁ3 molꢁ1 cmꢁ1)] ¼ 248 (4.33), 357 (4.37), 425 (3.59) nm.
C39H47N3O2Pt (784.89): calcd C 59.68, H 6.04, N 5.35; found: C
59.65, H 6.12, N 5.28%.
2f: Schiff base ligand 1f (2000 mg, 3.51 mmol), KOH (393 mg,
7.02 mmol) and K2PtCl4 (1747 mg, 4.21 mmol) were used. Yield:
964 mg (36%). 1H NMR (400 MHz, CDCl3): d ¼ 8.01 (s, 1H, C]
NH), 7.93 (s, 1H, C]NH), 7.58 (d, J ¼ 2.5 Hz, 1H, Ar-H), 7.48
(d, J ¼ 2.5 Hz, 1H, Ar-H), 7.21 (d, J ¼ 2.5 Hz, 1H, Ar-H), 7.09
(d, J ¼ 2.5 Hz, 1H, Ar-H), 3.49–3.60 (m, 2H, cyclohexane-H),
2.53–2.62 (m, 2H, cyclohexane-H), 1.82–1.85 (m, 2H, cyclo-
hexane-H), 1.51 (s, 9H, tert-butyl), 1.48–1.49 (m, 11H, cyclo-
hexane-H and tert-butyl), 1.28–1.31 (m, 11H, cyclohexane-H and
tert-butyl) ppm. IR (KBr): n ¼ 3436, 2950, 2866, 1613, 1528,
1431, 1315, 1254, 1166, 779 and 582 cmꢁ1. HRMS (MALDI-
TOF, positive mode, CH2Cl2): m/z ¼ 763.2203 [M + H]+
(C32H43BrN2O2Pt + H: calcd 763.2217, Dm ¼ ꢁ1.83 ppm). UV-
Vis (CH2Cl2, 20 ꢀC): lmax [log(3 dmꢁ3 molꢁ1 cmꢁ1)] ¼ 260 (4.69),
319 (4.16), 352 (4.17), 438 (3.77) nm. C32H43BrN2O2Pt (762.68):
calcd C 50.39, H 5.68, N 3.67; found: C 50.42, H 5.73, N 3.49%.
2i: complex 2f (100 mg, 0.13 mmol), 4-nitrophenylboronic acid
(26 mg, 0.16 mmol), 3 mol% Pd(PPh3)4 (5 mg, 0.004 mmol) and
1
Na2CO3 (139 mg, 1.31 mol) were used. Yield: 79 mg (75%). H
NMR (400 MHz, CDCl3): d ¼ 8.24 (d, J ¼ 8.8 Hz, 2H, C]NH),
8.19 (s, 1H, C]NH), 8.08 (s, 1H, Ar-H), 7.76 (d, J ¼ 2.3 Hz, 1H,
Ar-H), 7.65 (d, J ¼ 8.8 Hz, 2H, Ar-H), 7.59 (d, J ¼ 2.3 Hz,
1H, Ar-H), 7.48 (d, J ¼ 2.3 Hz, 1H, Ar-H), 7.12 (d, J ¼ 2.4 Hz,
1H, Ar-H), 3.50–3.55 (m, 2H, cyclohexane-H), 2.67–2.73 (m, 2H,
cyclohexane-H), 1.93–1.95 (m, 2H, cyclohexane-H), 1.56 (s, 11H,
tert-butyl and cyclohexane-H), 1.53 (s, 9H, tert-butyl), 1.39 (m,
2H, cyclohexane-H), 1.31 (s, 9H, tert-butyl) ppm. IR (KBr): n ¼
3435, 2946, 2867, 1609, 1587, 1509, 1435, 1337, 1255, 1166, 781
and 544 cmꢁ1. HRMS (MALDI-TOF, positive mode, CH2Cl2):
m/z ¼ 804.1544 [M]+ (C38H47N3O4Pt: calcd 804.8755, Dm ¼ 8.97
ppm). UV-Vis (CH2Cl2, 20 ꢀC): lmax [log(3 dmꢁ3 molꢁ1 cmꢁ1)] ¼
266 (3.61), 357 (4.30), 431 (4.32) nm. C38H47N3O4Pt (804.88):
calcd C 56.71, H 5.89, N 5.22; found: C 56.47, H 5.69, N 5.13%.
2j: complex 2f (100 mg, 0.13 mmol), 9,9-dihexyl-9H-fluoren-2-
ylboronic acid (59 mg, 0.16 mmol), 3 mol% Pd(PPh3)4 (5 mg,
0.004 mmol) and Na2CO3 (139 mg, 1.31 mol) were used. Yield:
57 mg (43%). 1H NMR (400 MHz, CDCl3): d ¼ 8.17 (s, 1H, C]
NH), 8.06 (s, 1H, C]NH), 7.82 (s, 1H, Ar-H), 7.64–7.65 (m, 2H,
Ar-H), 7.58 (s, 1H, Ar-H), 7.45–7.48 (m, 3H, Ar-H), 7.28–7.33
(m, 3H, Ar-H), 7.09 (s, 1H, Ar-H), 3.57–3.60 (m, 2H, cyclo-
hexane-H), 2.56 (m, 2H, cyclohexane-H), 1.89–1.98 (m, 8H),
1.54–1.59 (m, 18H, tert-butyl), 1.25–1.39 (m, 15H), 1.05 (m,
12H), 0.76 (s, 6H, -CH3) ppm. IR (KBr): n ¼ 3435, 2952, 2856,
1605, 1527, 1434, 1318, 1254, 1167, 780 and 582 cmꢁ1. HRMS
(MALDI-TOF, positive mode, CH2Cl2): m/z ¼ 1015.5505 [M]+
(C57H76N2O2Pt: calcd 1015.5553, Dm ¼ ꢁ4.73 ppm). UV-Vis
(CH2Cl2, 20 ꢀC): lmax [log(3 dmꢁ3 molꢁ1 cmꢁ1)] ¼ 287 (4.28), 342
(4.50), 432 (3.67) nm. C57H76N2O2Pt (1016.3): calcd C 67.36, H
7.54, N 2.76; found: C 67.41, H 7.52, N 2.79%.
General procedure for the preparation of platinum(II) asymmetric
Schiff base complexes 2g–2j
A suspension of 2f, 1.2 equivalent of boronic acid functionalized
reagent, 3 mol% Pd(PPh3)4 and 10 equivalents of Na2CO3 were
refluxed in toluene for 12 h under nitrogen. The reaction mixture
was evaporated to dryness and the crude product was purified on
a silica-gel column using a solvent mixture of hexane and CH2Cl2
as eluent to give an orange target compound.
2g: complex 2f (100 mg, 0.13 mmol), 4-(diphenylamino)phe-
nylboronic acid (45 mg, 0.16 mmol), 3 mol% Pd(PPh3)4 (5 mg,
0.004 mmol) and Na2CO3 (139 mg, 1.31 mol) were used. Yield:
86 mg (71%). 1H NMR (400 MHz, CDCl3): d ¼ 8.12 (s, 1H, C]
NH), 8.06 (s, 1H, C]NH), 7.71 (dd, 1H, J ¼ 7.4, 2.0 Hz, Ar-H),
7.57 (d, 1H, J ¼ 2.4 Hz, Ar-H), 7.39–7.42 (m, 2H, Ar-H), 7.35
(dd, 1H, J ¼ 7.4, 2.0 Hz, Ar-H), 7.23–7.25 (m, 4H, Ar-H), 7.09–
7.13 (m, 7H, Ar-H), 7.00 (t, 2H, J ¼ 7.2 Hz, Ar-H), 3.52–3.58 (m,
2H, cyclohexane-H), 2.62–2.65 (m, 2H, cyclohexane-H), 2.04 (s,
1H, cyclohexane-H), 1.87–1.90 (m, 2H, cyclohexane-H), 1.68 (s,
1H, cyclohexane-H), 1.55 (s, 9H, tert-butyl), 1.53 (s, 9H, tert-
butyl), 1.35–1.36 (m, 2H, cyclohexane-H), 1.30 (s, 9H, tert-butyl)
ppm. IR (KBr): n ¼ 3435, 2947, 2866, 1607, 1529, 1434, 1319,
1254, 1166, 780 and 541 cmꢁ1. HRMS (MALDI-TOF, positive
Acknowledgements
We thank Hong Kong Baptist University (FRG2/11-12/0071)
and Hong Kong Research Grants Council (HKBU 202308) for
the financial support. W.-K.W. and W.-Y.W. also thank a grant
from Areas of Excellence Scheme, University Grants Committee,
Hong Kong (Project No. [AoE/P-03/08]). W.-Y.W. also
acknowledges the financial support from Hong Kong Research
Grants Council (HKUST2/CRF/10).
mode, CH2Cl2): m/z
¼
926.4130 [M]+ (C50H57N3O2Pt:
calcd 926.4097, Dm ¼ 3.56 ppm). UV-Vis (CH2Cl2, 20 ꢀC): lmax
[log(3 dmꢁ3 molꢁ1 cmꢁ1)] ¼ 259 (4.27), 337 (4.43), 437 (3.48) nm.
C50H57N3O2Pt (927.08): calcd C 64.78, H 6.20, N 4.53; found: C
64.56, H 6.17, N 4.32%.
2h: complex 2f (100 mg, 0.13 mmol), 4-cyanophenylboronic
acid (23 mg, 0.16 mmol), 3 mol% Pd(PPh3)4 (5 mg, 0.004 mmol)
and Na2CO3 (139 mg, 1.31 mol) were used. Yield: 66 mg (64%).
1H NMR (400 MHz, CDCl3): d ¼ 8.09 (s, 1H, C]NH), 8.03 (s,
1H, C]NH), 7.54–7.67 (m, 6H, Ar-H), 7.35 (s, 1H, Ar-H), 7.09
(d, 1H, J ¼ 2.0 Hz, Ar-H), 3.54–3.65 (m, 2H, cyclohexane-H),
2.61 (m, 2H, cyclohexane-H), 1.81–1.83 (m, 2H, cyclohexane-H),
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