Robust phosphorescent platinum(ii) complexes with tetradentate O ∧N∧C∧N ligands: High efficiency OLEDs with excellent efficiency stability

Article abstract of DOI:10.1039/c2cc37862k

The Pt(ii) complexes (1-3) bearing tetradentate O^∧N ^∧C^∧N ligands display high emission quantum yields (0.76-0.90) and good thermal stability (T_d > 400 °C). Complex 3 is an excellent green phosphorescence dopant for OLEDs with excellent efficiency and low efficiency roll-off (η_L, η _Ext(max) = 66.7 cd A^-1, 18.2%; η_L, η_Ext (1000 cd m^-2) = 65.1 cd A^-1, 17.7%). The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cc37862k

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Robust phosphorescent platinum(II) complexes with
tetradentate O4N4C4N ligands: high efficiency
OLEDs with excellent efficiency stability†
Cite this: Chem. Commun., 2013,
49, 1497
Received 30th October 2012,
Accepted 19th December 2012
Steven C. F. Kui, Pui Keong Chow, Gang Cheng, Chi-Chung Kwok,
Chun Lam Kwong, Kam-Hung Low and Chi-Ming Che*
DOI: 10.1039/c2cc37862k
www.rsc.org/chemcomm
The Pt(II) complexes (1–3) bearing tetradentate O4N4C4N ligands
display high emission quantum yields (0.76–0.90) and good thermal
stability (Td > 400 8C). Complex 3 is an excellent green phosphores-
cence dopant for OLEDs with excellent efficiency and low efficiency
roll-off (gL, gExt(max) = 66.7 cd Aꢀ1, 18.2%; gL, gExt (1000 cd mꢀ2) =
65.1 cd Aꢀ1, 17.7%).
Square planar platinum(II) complexes containing N-donor and/or
cyclometalated ligands have emerged to be important classes of
phosphorescent materials1–6 with useful practical applications in
molecular and bio-molecular sensing2 and in organic electronics.3–6
Our interest in this area is to develop robust phosphorescent Pt(II)
complexes supported by tetradentate ligands. Platinum(II) complexes
bearing symmetric bis(20-phenol)bipyridines (N2O2),7 Schiff base
Chart 1 The literature electrophosphorescent Pt(II) complexes with tetraden-
tate ligands and chemical structure of complexes 1–3.
(Salphen),8 bis(pyrrole)diimine (Prtmen),9 N,N-di(2-phenylpyrid-6-yl)- complexes supported by the rigid tetradentate O4N4C4N ligands
aniline (C4N*N4C)10 and tetradentate bis(N-heterocyclic carbene) (O4N4C4N = 2-(4-(3,5-di-tert-butylphenyl)-6-(3-(pyridin-2-yl)phenyl)-
(tetra-NHC)11 ligands have been reported with their emission energy pyridin-2-yl)phenolate and its derivatives; Chart 1). Complexes 1–3
ranging from blue to red color (Chart 1). We have also reported display high emission quantum yields (f) up to 90%, are thermally
that the OLED fabricated with [Pt(salphen)] (salphen = N,N0- stable (Td > 400 1C), and could be obtained in high purity by sub-
bis(salycilidene)-1,2-phenylene-diamine) achieved a long device limation at B290 1C under 4 ꢁ 10ꢀ5 torr. Importantly, 3 shows
lifetime.8 This may be attributed to the stability of Pt(II) emitter minimal intermolecular aggregation or excimer formation thereby
endowed by the dianionic tetradentate ligand so that the OLED minimizing the device efficiency roll-off at high luminance.
device lifetime could be greatly improved.
The preparation of 1–3 involves a one-step reaction with high
Recently, we reported a series of cyclometalated platinum(II) product yields (up to 80%). They were prepared by the reaction of
complexes supported by a new tetradentate IndO4N4C4N ligand H–O4N4C4N ligands with K2PtCl4 in a refluxing CH3COOH and
1
(
IndO4N4C4N = 5,5-dibutyl-2-(3-(pyridin-2-yl)-4,6-difluoro-phenyl)- CHCl3 mixture (9 : 1). The H-NMR spectra of 1–3 and molecular
5H-indeno[1,2-b]pyridin-9-olate; Chart 1).12 This Pt(II) complex shows packing diagrams and crystallographic data of 2 and 3 are given in
both high energy monomer emission (B480–520 nm) and low energy the ESI.† Perspective views of 2 and 3 are depicted in Fig. 1. The
excimer emission (>600 nm) enabling the fabrication of high- [(O4N4C4N)Pt] motif in each case is virtually planar with the torsion
efficiency WOLEDs by using it as a single emissive dopant. However, angles O1–N1–C13–N2 in the range of 0.111 to 2.551. Two molecules
excimer emission is not desirable in achieving high performance red, of 2 or 3 are aligned in a head-to-head orientation with a PtꢂꢂꢂPt
green and blue OLEDs. Herein, we report a new class of Pt(II) distance of 3.218 Å and 3.396 Å, respectively. There are intermolecular
pꢂꢂꢂp interactions (B3.5 Å) between each pair of molecules of 2 or 3.
State Key Laboratory of Synthetic Chemistry, HKU-CAS Joint Laboratory on New
Materials, Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, China. E-mail: cmche@hku.hk
The spectroscopic and photophysical data of 1–3 are presented
in Table 1. As depicted in Fig. 2, these complexes in CH2Cl2
display intense absorption bands at wavelengths below 300 nm
(e > 3.8–4.5 ꢁ 104 molꢀ1 dm3 cmꢀ1), which are assigned to 1pꢀp*
transitions of the O4N4C4N ligands, and moderate intense absorp-
tion bands at 421–438 nm (e E 6580–9440 molꢀ1 dm3 cmꢀ1) with
† Electronic supplementary information (ESI) available: Experimental proce-
dures, X-ray structures of 2 and 3, device performance. CCDC 830685 and
907730. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc37862k
c
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Fig. 1 Perspective view of 2 (left) and 3 (right) (all hydrogen atoms are omitted
Fig. 2 UV absorption spectra and emission spectra of 1–3 in CH2Cl2
(2.0 ꢁ 10ꢀ5 mol dmꢀ3).
for clarity).
weak absorption tails at >460 nm (e E 1600–2600 molꢀ1 dm3 cmꢀ1). the intermolecular interactions can be minimized even at high
The absorption bands of all complexes follow Beer’s law with the dopant concentration.13
complex concentrations ranging from 10ꢀ4 to 10ꢀ5 mol dmꢀ3
.
Multi-layer devices by thermal deposition have been fabri-
Solvatochromic effect has been observed. With 1 as an example, cated using 1 or 3 as emitting material and with a device
the absorptions at l > 400 nm red shift in energy in low-polarity structure of ITO/di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane
solvents (e.g. complex 1, the broad absorption at lmax 420 nm in (TaPc, 70 nm)/mCP: 1 or 3 (x%, 30 nm)/1,3,5-tri[(3-pyridyl)-
MeCN is red-shifted to 444 nm in C6H6). We tentatively assign phen-3-yl]benzene (TmPyPB, 40 nm)/LiF (0.5 nm)/Al (100 nm).
the absorptions at > 400 nm to charge transfer transition with TaPc and TmPyPB were chosen as hole-transporting material
mixed phenoxide lone pair (l) to p* orbital of N4C4N, i.e. 3[l - and electron-transporting material respectively; this combi-
1
p*(N4C4N)] and [Pt(5d) - p*(O4N4C4N)] MLCT characters. nation has been known to give a good device efficiency for
In degassed CH2Cl2, complexes 1–3 are strongly emissive phosphorescent green emitters. On the other hand, in the case
with emission quantum yields in the range of 0.76–0.90 of the commonly used host material, 4,40-bis(carbazol-9-yl)-
and emission lifetimes (t) in the microsecond time regime biphenyl (CBP), the LUMO (ꢀ3.0 eV) cannot cover the LUMOs of 1
(B4–5 ms) (Table 1). Each of the complexes shows a structure- and 3, therefore a wider band-gap derivative, 1,3-bis(carbazol-9-yl)-
less emission band with lmax at 503, 518 and 522 nm for 1, 2, benzene (mCP), was used as host material. The device performance
and 3, respectively. Similar to the absorption data, the emission was optimized by varying the doping concentration of the complex.
of 1 also displays a solvatochromic effect (e.g. lmax of 1 in MeOH A maximum current efficiency of 30.2 cd Aꢀ1 was obtained at
at 500 nm is red-shifted to 512 nm in C6H6). We tentatively assign 1% dopant concentration of 1 (device A) while a maximum
the emissions of 1–3 to mixed 3MLCT and 3[l - p*(N4C4N)] (l = current efficiency of 66.7 cd Aꢀ1 was achieved at 13% of 3
lone pair of phenoxide) excited states. Notably the self-quenching (device B); the performance data are depicted in Table 2. As
rate constant kq of 3 (8.82 ꢁ 107 molꢀ1 dm3 sꢀ1) is smaller than depicted in Fig. 3, the EL spectra of devices A and B have not
that of 1 and 2 (1.78–2.08 ꢁ 109 molꢀ1 dm3 sꢀ1) and is relatively been affected by the operation voltage. As the position and shape
smaller than the reported values (B108 to 109 molꢀ1 dm3 sꢀ1) of of the EL spectra are similar to the solution PL spectra, the
the other reported luminescent Pt(II) complexes. This reveals EL of both devices are assigned to come from the platinum(II)
that the bulky norbornane group on the pyridine moiety can complexes. A blue emitting component at around 450 nm,
block intermolecular interactions thus disfavouring excimer which is attributed to the emission of the host material, has
formation in solutions.
been observed in the EL spectra of device A. This is a sign
As complex 3 has a high phosphorescence quantum yield, a of incomplete energy transfer from the host material to 1.
reasonably short radiative lifetime (tr = 5.4 ms) and ineffective Presumably this is due to the low dopant concentration of 1%.
self-quenching at high complex concentrations, this complex The blue emitting component vanished when the dopant
is a good candidate for PHOLED since the triplet–triplet concentration increased to 2% (Fig. S15, ESI†). Nevertheless,
annihilation and concentration quenching effect arising from at the same time, another emitting component at >600 nm
Table 1 Physical, spectroscopic and photophysical data of 1–3
Emissiona
UV-vis absorptiona
l
)
max/nm
Solutiona lmax/nm Quantumc kq
/
HOMOe LUMOe Electrochemical Td
d
Complex (e/molꢀ1 dm3 cmꢀ1
(tp; tr/ms)b
yield (fp)
molꢀ1dm3 sꢀ1 [eV]
[eV]
bandgap [eV]
[1C]
1
2
284 (38 070), 371 (13 240), 426 (6580)
285 (42 520), 374 (15 720), 404 (10 090), 518 (3.7; 4.5)
438 (7170)
503 (4.7; 6.4)
0.73
0.82
2.08 ꢁ 109
ꢀ4.93
ꢀ2.69
ꢀ2.64
2.24
2.29
518
430
1.79 ꢁ 109
ꢀ4.93
ꢀ5.16
3
284 (44 730), 371 (20 950), 421 (9440)
522 (4.9; 5.4)
0.90
8.82 ꢁ 107
ꢀ2.65
2.51
405
a
b
Determined in a degassed dichloromethane solution (2 ꢁ 10ꢀ5 mol dmꢀ3).
tp = room temperature lifetime; tr = radiative lifetime of the triplet
state deduced from tr = tp/fp. Emission quantum yield was measured in a degassed dichloromethane solution (2 ꢁ 10ꢀ5 mol dmꢀ3) by the optical
c
d
dilute method with 9,10-bis(phenylethynyl)anthracene (BPEA) in degassed benzene as the standard (fr = 0.85). Self-quenching constant.
e
The HOMO and LUMO levels were estimated from onset potentials using Cp2Fe0/+ values of 4.8 eV below the vacuum level.
c
1498 Chem. Commun., 2013, 49, 1497--1499
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Table 2 Device performance for device A and device B
Efficiency (ZLb/cd Aꢀ1; Zpc/lm Wꢀ1; ZExtd/%)
CIE coordinates (x, y)e
Devicea
Conc.
Max.
@ 1000 cd mꢀ2
@ 10 000 cd mꢀ2
Max.
@ 1000 cd mꢀ2
@ 10 000 cd mꢀ2
A
B
1%
13%
30.2; 14.6; 8.6
66.7; 34.1; 18.2
29.8; 13.8; 8.5
65.1; 30.7; 17.7
17.1; 5.0; 4.9
48.5; 17.2; 13.2
(0.234, 0.650)
(0.282, 0.657)
(0.234, 0.650)
(0.304, 0.647)
(0.238, 0.639)
(0.296, 0.561)
a
b
c
d
ITO/TaPc (70 nm)/mCP: 1 or 3 (x%, 30 nm)/TmPyPB (40 nm)/LiF (0.5 nm)/Al (100 nm). Current efficiency. Power efficiency. External
e
quantum efficiency. Commission Internationale d’Eclairage chromaticity coordinates.
O4N4C4N ligand. To the best of our knowledge, device B is the best
green-emitting OLED using platinum(II) emitting material, taking
into account the maximum device efficiency (ZL: 66.7 cd Aꢀ1
;
Z
Ext: 18.2%), colour purity (CIE: 0.282, 0.657) and efficiency
stability (2.4% roll-off@1000 cd mꢀ2).
This work was partially supported by the Innovation and Tech-
nology Commission of the HKSAR Government (GHP/043/10),
Research Grants Council of Hong Kong SAR (HKU 7008/09P) and
Theme-Based Research Scheme (T23-713/11), National Natural
Science Foundation of China/Research Grants Council Joint
Research Scheme [N_HKU 752/08], CAS-Croucher Funding Scheme
for Joint Laboratories. 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.
Fig. 3 Comparisons between PL and EL spectra (left – device A; right – device B).
developed and the CIEx increased. This is attributed to excimer
emission from 1. As a result, the color of the devices gradually
shifted from green to greenish-yellow upon an increase in the
dopant concentration of 1 and the maximum device efficiency
dropped to 26.9 cd Aꢀ1 at 4% dopant concentration.
Notes and references
On the other hand, the maximum efficiency of the devices
fabricated with 3 increased with dopant concentration up to a high
dopant level of 13% (Fig. 4). No extra emitting component or
significant shift in CIE was observed. These data show that excimer
formation is successfully suppressed by the norbornane group. Green
emission with CIE coordinates of (0.304, 0.647) has been obtained at
1000 cd mꢀ2. Besides device efficiency and color, the devices fabri-
cated with 3 showed excellent performance in efficiency roll-off. Only
2.4% roll-off (65.1 cd Aꢀ1) was observed at 1000 for the device B.
Phosphorescent platinum(II) complexes have been reported
for white OLED (WOLED) fabricated with one single emitter.
Excimer emission from Pt(II) complexes can be obtained even
at a low dopant concentration of 3%.4,12,14 Nevertheless, this
characteristic feature of Pt(II) complexes could be detrimental
to RGB panel application as the color purity, device efficiency
and efficiency stability can also be affected at high dopant
concentration. However, low dopant concentration is not desirable
for industry/practical application. In this work, this dilemma has
been resolved by incorporation of the norbornane group to the
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Fig. 4 Efficiency of the device fabricated with 3.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1497--1499 1499