Robust phosphorescent platinum(II) complexes containing tetradentate O^N^C^N ligands: Excimeric excited state and application in organic white-light-emitting diodes

Article abstract of DOI:10.1002/chem.201203687

The bright white lights: A series of highly robust platinum(II) complexes supported by tetradentate O N C N ligands with high emission quantum yields (0.72-0.93) and high T_d (>400 °C) have been synthesized. Among the complexes, that shown in th

Full text of DOI:10.1002/chem.201203687

COMMUNICATION
DOI: 10.1002/chem.201203687
Robust Phosphorescent Platinum(II) Complexes Containing Tetradentate
O^N^C^N Ligands: Excimeric Excited State and Application in Organic
White-Light-Emitting Diodes
Steven C. F. Kui, Pui Keong Chow, Glenna So Ming Tong, Shiu-Lun Lai, Gang Cheng,
Chi-Chung Kwok, Kam-Hung Low, Man Ying Ko, and Chi-Ming Che*[a]
Intra- and intermolecular
metal–metal and ligand–ligand
interactions are important
characteristic features of plati-
num(II) complexes that have
chelating
C and/or N atom
donor ligands;[1–3]such com-
plexes could be harnessed for
self-assembly of nanostruc-
tured materials[4] and used for
Figure 1. Chemical structures of selected PtII complexes showing low-energy excimeric emissions.
developing luminescent sensors of biological targets.[5] These
intramolecular/intermolecular interactions give rise to low-
energy metal–metal-to-ligand charge transfer (MMLCT)
tween aromatic ligands of interplanar separation <3.5 ꢀ.[6]
Brꢁdas and Kim lately reported DFT/TDDFT calculations
on
FPt1
(FPt1=[2-(4’,6’-difluorophenyl)pyridinato-
and/or p(ligand)···p
N
N,C2’](2,4-pentanedionato)platinum(II)) and their conclu-
sion is in concordance with the proposal of Miskowski and
co-workers, namely, an excimer is formed through the coop-
to red-shifts in both absorption and emission energies of
oligomers from the corresponding monomeric counter-
parts.[1–3] In this area, cyclometalated platinum(II) com-
plexes have been extensively studied as they exhibit dual
phosphorescence in the high-energy (bluish green) and low-
energy (reddish orange) spectral regions. These two types of
emission bands are often attributed to monomeric and exci-
meric emissions, respectively. The electronic structure of the
excimeric excited states of PtII complexes (Figure 1), howev-
er, remains elusive. More than 20 years ago, Miskowski
et al. suggested that the low-energy excimer emission is as-
cribed to 1) metal–metal-to-ligand charge transfer
(3MMLCT) for which upon excitation, there is an enhanced
Pt···Pt interaction with concomitant shortening of the Pt···Pt
distance or 2) transitions involving p···p interactions be-
erative effect of both Pt···Pt and interACTHNUTRGENNGUliACHTUGTNRENNUGgand p···p interac-
tions.[7] On the other hand, Forrest and DꢂAndrade, based
on the photoluminescent and electroluminescent studies on
neat films of FPt1, proposed that the triplet excimer is
formed through interactions between a monomer triplet ex-
citon and a monomer ground state; the stabilization of the
excimer is accounted for by a configuration interaction be-
tween exciton resonance and charge-transfer resonance
states.[8] Recently, Kalinowski et al. made a similar conclu-
sion on the formation of excimers from [PtACTHNURGTNE(NUG N^C^N)Cl]
(N^C^N=1,3-di(2-pyridyl)benzene and itꢂs derivatives).[9]
Herein, we report a new series of cyclometalated plati-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
1–5 Scheme 1). The substitution pattern of the O^N^C^N
ligand was found to significantly affect the properties of the
excimeric emissive excited states. High-efficiency white or-
ganic light-emitting diodes (WOLEDs) [peak current effi-
[a] Dr. S. C. F. Kui,+ P. K. Chow,+ Dr. G. S. M. Tong,+ Dr. S.-L. Lai,
Dr. G. Cheng, Dr. C.-C. Kwok, Dr. K.-H. Low, M. Y. Ko,
Prof. C.-M. Che
State Key Laboratory of Synthetic Chemistry
Department of Chemistry
HKU-CAS Joint Laboratory on New Materials and
The University of Hong Kong, Pokfulam Road
Hong Kong (P. R. China)
Fax: (+852)2915-5176
E-mail: cmche@hku.hk
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203687.
Scheme 1. Chemical structure of complexes 1–5.
Chem. Eur. J. 2013, 19, 69 – 73
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
69

ciency (hL(max))=71.0 cdAÀ1, power efficiency (hp(max))=
55.8 lmWÀ1, external quantum efficiency (hExt(max))=16.5%,
CIE=0.33, 0.42, CRI=77] and white polymer light-emitting
diodes (WPLEDs) (hL(max) =17.0 cdAÀ1, hp(max) =9.1 lmWÀ1
,
hExt(max) =9.7%, CIE=0.43, 0.45, CRI=78) have been fabri-
cated using a simple device architecture and with 5 as a
single emissive dopant.
Platinum(II) complexes bearing symmetric tetradentate
chelating ligands, such as octaethylporphyrin (OEP),[10]
bis(2’-phenol)bipyridine (N2O2),[11] Schiff base (Salphen),[12]
bisACHTUNGTRENNUNG
(pyrrole)diimine (Prtmen),[13] N,N-di(2-phenylpyrid-6-
yl)aniline (C^N^N^C)[14] and bis(N-heterocyclic carbene)
(tetra-NHC),[15] are good phosphorescent emitters. The un-
symmetric PtII complexes of O^N^C^N ligands in this work
are highly emissive (f up to 0.93), thermally stable
(Td >4008C), and could be obtained in high purity by subli-
mation at about 2908C under 4ꢄ10À5 Torr. They were pre-
pared by a procedure that gave high product yields (up to
80%). The characterization data of 1–5 and X-ray crystallo-
graphic data of 1, 3, and 5 are given in the Supporting Infor-
mation along with the synthetic procedures.[16] A perspective
view of 5 is depicted in Figure 2.
The [(O^N^C^N)Pt] motifs of 1, 3, and 5 are virtually
planar with the O1-N2-C11-N1 torsion angles being 0.13,
0.69, and 0.178, respectively. Orthogonal packing is observed
in the unit cells of 1, 3, and 5 with short intermolecular
Figure 2. Perspective view (top) and molecular packing (bottom) of com-
plex 5 (all hydrogen atoms are omitted for clarity).
C H···p distances in the range of 2.67–2.81 ꢀ between the
gand 1p–p* transitions of the
are assigned to intraACTHNGURTENNUlG iACHUTNGTRENNUNG
À
H atoms of the n-butyl chain and the O^N^C^N moiety. In
the crystal structures of 1, 3, and 5, the molecules in each
case are orientated in pairs with a head-to-tail arrangement.
Extensive intermolecular p···p interactions (p···p distances=
3.48–3.51 ꢀ) are observed, but the intermolecular Pt···Pt dis-
tances are greater than 4.5 ꢀ, revealing no intermolecular
Pt···Pt interactions.
O^N^C^N ligands and mixed 1 MLCT/1p–p* transitions
(MLCT=metal-to-ligand charge transfer), respectively.
Complexes 1–5 show vibronic structured emission bands
at lmax in the range 480–520 nm with emission quantum
yields of 0.72–0.93 and emission lifetimes (t) in the micro-
second time regime in degassed CH2Cl2 (Table 1). As the vi-
brational spacings of 1300–1400 cmÀ1 correspond to the
C=N/C=C stretching frequencies of the O^N^C^N ligands,
the emissions are attributed to come from triplet excited
states with predominant ligand character.
The absorption spectra of 1–5 in CH2Cl2 (Table 1) display
intense bands at wavelengths below 300 nm (e in the range
2.1–4.8ꢄ104 dm3 molÀ1 cmÀ1) and moderate intense absorp-
tion bands at 400–435 nm (e in the range 5300–
9600 dm3 molÀ1 cmÀ1) with weak absorption tails at >460 nm
(e in the range 400–600 dm3 molÀ1 cmÀ1). These absorptions
Upon increase in complex concentration, a low-energy
emission is observed with 2, 4, or 5 in CH2Cl2. As depicted
in Figure 3, the high-energy vibronic structured emission at
Table 1. Physical data of complexes 1–5.
UV/Vis absorption[a]
Emission
Quantum Kq
HOMO[d] LUMO[d] Electro-
Td
[a]
[c]
lmax [nm] (e [ꢄ104 molÀ1 dm3 cmÀ1])
Solution
lmax[nm] (t [ms])
[eV]
[eV]
chemical
[oC]
yield[b]
[molÀ1dm3 sÀ1
5.4ꢄ108
]
bandgap [eV][d]
1
2
3
4
5
254 (4.59), 280 (3.16), 354 (1.77), 390 (1.46),
426 (0.91)
254 (4.24), 261 (4.18), 290 (2.51), 352 (1.71),
388 (1.42), 429 (0.57)
247 (3.72), 261 (3.43), 279 (2.67), 356 (1.52),
395 (1.15), 439 (0.70)
251 (4.82), 261 (4.55), 294 (2.07), 350 (1.83),
381ACHTUNGTRENNUNG(1.40), 424 (0.86)
245 (4.58), 259 (4.45), 289 (2.65), 3.01 (1.86), 482, 512 (17.7)
349 (1.78), 376 (1.35), 424 (0.66)
485, 517, 557 (12.0) 0.72
À5.11
À5.17
À5.12
À5.15
À5.24
À2.62
À2.66
À2.70
À2.72
À2.71
2.49
2.51
2.42
2.43
2.53
414
418
411
406
432
488, 522 (28.0)
0.80
3.3ꢄ108
8.5ꢄ108
1.3ꢄ108
1.2ꢄ109
508, 543, 594 (11.0) 0.89
488, 518 (13.2)
0.93
0.75
[a] Determined in degassed CH2Cl2 (2ꢄ10À5 moldmÀ3). [b] Emission quantum yield was measured in degassed CH2Cl2 (2ꢄ10À5 moldmÀ3) by the optical
dilute method with [Ru(bpy)3](PF6)2 (bpy=2,2’-bipyridine) in degassed CH3CN as standard (fr =0.062). [c] Self-quenching constant. [d] The HOMO and
LUMO levels are estimated from onset potentials using Cp2Fe0/+ value of 4.8 eV below the vacuum level.
A
ACHTUNGTRENNUNG
70
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ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 69 – 73

Robust Phosphorescent Platinum(II) Complexes
COMMUNICATION
Figure 4. Difference density plots of 3MLCTA (left) and 3 MLCTB (right)
for complex 5. Black: decrease in electron density; gray: increase in elec-
tron density.
were found for complex 3, see the Supporting Information).
In the case of 5, these two triplet excited states are separat-
ed by about 0.18–0.24 eV in CH2Cl2, with 3MLCTB being
lower in energy at their optimized geometries. The calculat-
3
3
ed emission energies for MLCTA and MLCTB are 564 and
593–642 nm, respectively.[17] On the other hand, for 3, the
two triplet excited states are quasi-degenerate in CH2Cl2,
3
3
with MLCTA being 0.01–0.07 eV more stable than MLCTB.
3
3
The calculated emission energies for MLCTA and MLCTB
are 586 and 555 nm, respectively. Thus, for 5, two emission
peaks are observed, while for 3, the two emission peaks may
coincide. We thus assign the high-energy emission peak of 5
3
at 480–520 nm to be derived from MLCTA and the accom-
panying low-energy emission peak at 620 nm to be coming
3
from MLCTB, whereas for 3, the band at about 510 nm may
3
3
possibly arise from both MLCTA and MLCTB.
Figure 3. Emission spectra of 1–5 in CH2Cl2 (top: 2.0ꢄ10À5 moldmÀ3
;
Conventionally, the low-energy emission that grows as the
concentration increases is attributed to excimer formation.
Here, we propose that the low-energy “excimer”-like emis-
sion of 5 at about 620 nm could be mainly attributed to
bottom: 1.0ꢄ10À4 moldmÀ3).
3
480–520 nm decreases in intensity, while a low-energy emis-
sion band at about 620 nm develops as the complex concen-
tration increases from 2ꢄ10À5 to 1ꢄ10À4 moldmÀ3 (see
Figure 3 and Figure S15 in the Supporting Information). As
the excitation spectra for both the high- and low-energy
emissions are the same and the absorption spectra of these
three PtII complexes obey the Beer–Lambert law in the con-
centration range of 2ꢄ10À5 to 1ꢄ10À4 moldmÀ3, the low-
energy emission in each of the three cases should not come
from a ground-state dimer of the PtII complex with a close
Pt···Pt contact. Instead, we attribute this finding to excimer
emission. In contrast, both 1 and 3 display high-energy vi-
bronic structured emission even at concentrations of 1ꢄ
10À4 moldmÀ3 (Figure 3). The details are given in Figure S14
in the Supporting Information.
emission from the MLCTB excited state localized on one
[Pt(O^N^C^N)] motif. We have also performed calculations
of the dimer of complex 5 to see if there is a triplet excited
state that delocalizes over the two monomers and has simi-
lar emission energy as the low-energy emission at about
620 nm. It was found that the optimized triplet excited state
of the dimer of 5 is localized on only one monomer and the
3
nature is similar to MLCTB with emission energy calculated
to be at approximately 640 nm (see Figure 5).
The strong excimer emission of 5 with high emission
quantum yield even in dilute solution (ꢄ10À5 moldmÀ3) sug-
gests the potential of using this complex as a single emitter
for WOLEDs. Devices A–F were fabricated with a simple
configuration of ITO/NPB (40 nm)/mCP:complex 5 (6–
16%, 30 nm)/BAlq (40 nm)/LiF (0.5 nm)/Al (80 nm) [ITO=
indium tin oxide; NPB=N,N’-diphenyl-N,N’-bis(1-naph-
thyl)-(1,1’-biphenyl)-4,4’-diamine; mCP=1,3-bis(N-carbazo-
DFT/TDDFT calculations were performed on complexes
3
3 and 5. Two triplet excited states, labeled here as MLCTA
and 3MLCTB, have been located for both complexes. For
3MLCTA, the transition is mainly localized on the phenyl
and pyridyl rings of the O^N^C^N ligand, whereas for
3MLCTB, the transition is localized mainly on the phenyl
and the indenyl rings (see Figure 4 for the difference density
plots for the two triplet excited states of 5; similar plots
lyl)benzene; BAlq=aluminumACTHNGUTERNNU(G III) bis(2-methyl-8-quinoli-
nate)-4-phenylphenolate]. The electroluminescence (EL)
spectra and CIE coordinates of devices A–F are depicted in
Figures 6 and 7, respectively. Device A (6%, complex 5)
shows a warm white emission with CIE coordinates of (0.32,
0.43), hL(max) of 36.6 cdAÀ1, hp(max) of 25.5 lmWÀ1, and hExt of
Chem. Eur. J. 2013, 19, 69 – 73
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
71

C.-M. Che et al.
Figure 5. Difference density plots of the optimized triplet excited state of
complex 5 dimer. Black: decrease in electron density; gray: increase in
electron density.
Figure 7. CIE 1931 chromaticity coordinates of devices A–F, H, and K;
the inset shows a photograph of device H at 9 V.
G–J were kept to be the same as device A at around (0.32,
0.43) with CRI about 77. The turn-on voltages of devices G–
J were found to be below 4 V. Values for the parameters
hL(max) of 71.0 cdAÀ1, hp(max) of 55.8 lmWÀ1, and hExt of
16.5% were obtained with device H (Table S16 in the Sup-
porting Information). The performance of device H is com-
parable with the WOLED fabricated by Jabbour et al. and
ACTHNUTRGNEUNG
Williams et al. by using FPt1[18] and [Pt(N^C^N)Cl][19–20]
(Figure 1), respectively, as single emissive dopant, which
have the an hExt of about 18% and hp(max) of about
30 lmWÀ1.
To demonstrate further application of these complexes,
white polymer light-emitting devices (WPLEDs) based on
the emissions of complex 5 were fabricated and character-
ized. The optimized device (device K) had
a struc-
Figure 6. Top: EL spectra of devices A–F based on complex 5 with 6–
16% dopant concentration at 100 cdmÀ2. Bottom: EL spectra of device
A at different luminance (inset: a plot of CIE against luminance).
ture of ITO/PEDOT:PSS/PVK:OXD-7:complex 5/TmPyPb
(40 nm)/LiF (0.8 nm)/Al (100 nm) [PEDOT:PSS=poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonate); PVK=
polyvinylcarbazole,
OXD-7=1,3-bis[(4-tert-butylphenyl)-
1,3,4-oxadiazolyl]phenylene, TmPyPb=1,3,5-tri(m-pyrid-3-
ylphenyl)benzene]. In the active layer, the weight ratio of
PVK/OXD-7/complex 5 was 100:5:20. A value of 17.0 cdAÀ1
for hL(max) was obtained at 100 cdmÀ2. In addition, the EL
spectrum was stable, CIE coordinates were only slightly
shifted from (0.43, 0.45) with CRI=78 at 100 cdmÀ2 to
(0.44, 0.44) with CRI=81 at 10000 cdmÀ2 (Figure 7 and
Table S16 in the Supporting Information). To the best of our
knowledge, the efficiencies of device K with 5 as single
emissive dopant are the best among the reported blend-type
WPLEDs.[21]
17.7% (Table S16 in the Supporting Information). The EL
spectra and CIE coordinates of device A are insensitive to
luminance (1–1000 cdmÀ2) (Figure 6).
To further improve the device efficiency, an electron
blocking layer (EBL) was inserted between the hole-trans-
port layer and the emissive layer to give devices G–J (ITO/
NPB (40 nm)/EBL (10 nm; G: mCP or H: TCTA or I: [Ir-
A
5
(6%, 20 nm)/BAlq
(40 nm)/LiF (0.5 nm)/Al (80 nm) [TCTA=4,4’,4’’-tris-(N-
carbazolyl)triphenlyamine]. The CIE coordinates of devices
72
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ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 69 – 73

Robust Phosphorescent Platinum(II) Complexes
COMMUNICATION
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In conclusion, a series of highly robust platinum(II) com-
plexes 1–5 supported by tetradentate O^N^C^N ligands
with high emission quantum yields (0.72–0.93) and high Td
(> 4008C) have been synthesized. And high-efficiency
WOLED (hL(max) =71.0 cdAÀ1, hp(max) =55.8 lmWÀ1, hExt
=
16.5%, CIE=0.33, 0.42, CRI=77) and WPLED (hL =
17.0 cdAÀ1, hp =9.1 lmWÀ1, hExt =9.7%, CIE=0.43, 0.45,
CRI=78) have been achieved with single emissive dopant 5.
DFT/TDDFT calculations revealed that the low-energy
emission of 5 comes from excimeric excited state with a lo-
calized structure. We thus conceive that it may be possible
to obtain both high- and low-energy emissions with high
quantum yields by varying the substitution pattern of the
tetradentate ligands without recourse to 3MMLCT excited
state, which is governed by Pt···Pt contacts, a factor difficult
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Acknowledgements
[12] a) C.-M. Che, S.-C. Chan, H.-F. Xiang, M. C. W. Chan, Y. Liu, Y.
Wang, Chem. Commun. 2004, 1484–1485; b) C.-M. Che, C.-C.
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The work described in this paper was supported by the Innovation and
Technology Commission of the HKSAR Government (GHP/043/10), Re-
search Grants Council of Hong Kong SAR (HKU 7008/09P) and Theme-
Based Research Scheme (T23-713/11), National Natural Science Founda-
tion of China/Research Grants Council Joint Research Scheme [N HKU
752/08]. This work was also supported by Guangdong Special Project of
the Introduction of Innovative R&D Teams, China and Guangdong
Aglaia Optoelectronic Materials Co., Ltd. The calculations in this work
are supported by the Hong Kong UGC Special Equipment Grant (SEG
HKU09).
[16] CCDC-830686 (1), 854346 (3), and 885383 (5) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Keywords: charge transfer · chelates · cyclometalation ·
organic light-emitting diodes · phosphoresence · platinum
[17] We have used two different strategies to optimize the lowest triplet
excited state: DFT and TDDFT. Both methods locate the lowest
3
triplet excited state of 5 with similar character, namely, MLCTB, but
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À
the latter method predicts a shorter C10 C12 distance (1.41784 ꢀ
with DFT and 1.39676 ꢀ with TDDFT). Frequency calculations
were performed on both DFT and TDDFT optimized geometries
and both were found to have no imaginary frequency. This shows
that the potential energy surface of 3MLCTB is rather flat while the
ground state of 5 is not. Thus, we report here a range of energies for
the low-energy emission for 5.
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Received: October 16, 2012
Published online: December 13, 2012
Chem. Eur. J. 2013, 19, 69 – 73
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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