Highly efficient blue-emitting cyclometalated platinum(II) complexes by judicious molecular design

Article abstract of DOI:10.1002/anie.201302541

Deep-blue emitters based on cyclometalated platinum(II) complexes were synthesized, characterized, and used in organic light-emitting devices. The complexes with tetradentate ligands exhibited improved photophysical properties over iridium analogues, and one such compound achieved a peak external quantum efficiency (EQE) of 23.7 %. Copyright

Full text of DOI:10.1002/anie.201302541

Angewandte
Chemie
DOI: 10.1002/anie.201302541
Blue Organic LEDs
Highly Efficient Blue-Emitting Cyclometalated Platinum(II)
Complexes by Judicious Molecular Design**
Xiao-Chun Hang, Tyler Fleetham, Eric Turner, Jason Brooks, and Jian Li*
Luminescent properties of cyclometalated Ir and Pt com-
plexes have been the focus of considerable research, driven in
large part by their potential use as emitters in organic light-
emitting diodes (OLEDs).[1] This class of phosphorescent
emitters has demonstrated the ability to harvest both electro-
generated singlet and triplet excitons, resulting in a theoretical
100% electron-to-photon conversion efficiency.[2] Driven by
the technological need for full-color displays and solid-state
lighting applications, the development of stable and efficient
Ir and Pt complexes that emit in the range of 400–460 nm
(blue region) is vital.[3] Thus far, the approach to achieve
efficient blue-phosphorescent OLEDs has focused on Ir-
based complexes with either high triplet energy cyclometa-
lated ligands, such as 4,6-difluorophenylpyridine, or electron-
withdrawing ancillary ligands, such as picolinate and tetra-
kis(1-pyrazolyl)borate.[4] There are comparatively few reports
on deep blue phosphorescent emitters with fluorine-free
cyclometalating ligands,[5–7] despite potential for improved
optoelectronic stability compared to fluorinated derivatives.[6]
An example of such a class of materials is metal
complexes cyclometalated with the methyl-2-phenylimida-
zole (pmi) ligand and related analogues that are coordinated
to the metal through a neutral carbene.[7] Several Ir complexes
have been reported to have efficient deep blue phosphor-
escent emission at room temperature, including mer-tris(N-
decay rate (knr) and low radiative decay rate (kr), which are
dictated by the intrinsic properties of the selected metal
complex system.[8b] Thus, it will be highly desirable to identify
rational design motifs that can improve the luminescent
properties of deep blue phosphorescent emitters.
Compared to Ir analogues, there are relatively few reports
on platinum complexes cyclometalated with phenylimidazole
carbene ligands.[9] However, one such compound, platinu-
m(II) bis(methylimidazolyl)benzene chloride (Pt-16),[9b] has
demonstrated impressive device performance with a maxi-
mum external quantum efficiency (EQE) of 15.7% and
Commission Internationale de Lꢀꢁclairage (CIE) coordinates
of (0.16, 0.13). Moreover, PtII complexes can provide addi-
tional structural variation owing to the square-planar config-
uration allowing ligands to be designed that are bidentate,
tridentate and tetradentate.[10] These variations can signifi-
cantly alter the ground and excited state properties of Pt
complexes. Herein, we report (pmi)Pt-based complexes that
demonstrate a higher luminescent quantum yield and faster
radiative decay process than published Ir carbene analogues.
A new class of Pt complexes with tetradentate ligands have
been synthesized. The complexes have a conventional cyclo-
metalated fragment bridged with oxygen to an LLꢀ chelating
group, where LLꢀ is an ancillary chelate, such as, phenoxyl
pyridine (POPy) or carbazolyl pyridine (CbPy). The struc-
tures of Pt[pmi-O-POPy], Pt[pmi-O-CbPy], and Pt[ppz-O-
CbPy] are shown in Scheme 1, and are denoted as PtOO7,
dibenzofuranyl-N-methylimidazole)
(dbfmi)],[7a]
tris(1-cyanophenyl-3-methylimidazolin-2-yli-
dene-C,C2’) iridium(III) [Ir(cnpmic)],[7b] and mer-tri-
s(phenyl-methyl-benzimidazolyl) iridium(III) [m-Ir-
iridium(III)
[Ir-
(pmb)3].[7c] However, these complexes suffer from either
long luminescent decay or relatively low quantum efficiency
compared to Ir complexes based on the cyclometalated 2-
phenylpyridine ligand that have quantum efficiency F of 0.8–
1 and a luminescent lifetime t of 1–5 ms.[8] This difference can
be attributed to the combined effects of a high non-radiative
[*] X. Hang,[+] T. Fleetham,[+] E. Turner, Prof. J. Li
Material Science and Engineering, Arizona State University
Tempe, AZ 85287 (USA)
E-mail: Jian.Li.1@asu.edu
Dr. J. Brooks
Universal Display Corporation
375 Phillips Blvd, Ewing, NJ 08618 (USA)
Scheme 1. Chemical structures of cyclometalated iridium and platinum
complexes discussed herein.
[+] These authors contributed equally to this work.
[**] The authors thank the NSF CHE-0748867, DOE-EE0005075, the
Universal Display Corporation, and the Advanced Photovoltaics
Center for partial support of this work. E.T. wants to acknowledge
the support from NSF GK-12 Fellowship.
PtON7, and PtON1. Along with the photophysical results,
a PtON7-based OLED is reported to have a maximum EQE
of 23.7% with CIE coordinates of (0.14, 0.15).
The absorption spectra for Pt-16, PtOO7, and PtON7 are
shown in Figure 1a. All of the complexes exhibit very strong
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302541.
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Table 1: Photophysical properties of PtON7 and their analogues in
a doped PMMA film.
Complex
lmax [nm]
F [%] t [ms] kr
[ꢀ104 sꢀ1
knr
]
[ꢀ104 sꢀ1
]
PtON7
PtOO7
Pt-16[9b]
452
442
450
89
58
32
20
4.1
2.5
5.1
21
23
6.3
0.8
4
3.6
2.6
17
13.3
3.2
1.1
1.5
3.3
Pt(Mepmic)[9a] 419
25
[7b]
Ir(cnpmic)3
Ir(dbfmi)[7a]
PtON1
425(sh), 450 78
445
449
19.5
19.6
4.5
70[a]
85
19
[a] PO9 film was used instead of PMMA film.
previously reported bidentate Pt(Mepmi) and tridentate Pt-
16, which have structured luminescent spectra with resolved
vibronic progressions,[9] both PtOO7 and PtON7 have a less
vibronically structured room-temperature emission spectra,
resembling their Ir analogues, for example, tris-cyclometa-
lated Ir(dpfmi) and Ir(cnpmic).[7] Moreover, both PtOO7 and
PtON7 have much greater radiative decay rates (> 20 ꢂ
104 sꢀ1) than Pt and Ir analogues with similar emitting ligands
(Table 1). The less-resolved T1 absorption band, supressed
vibronic structure in the room temperature solution emission
spectra, and faster radiative process indicate tetradentate Pt
complexes like PtON7 may have more 1MLCT/3MLCT
character in their lowest excited states.[4b] This can be
attributed to the structural changes of metal complexes by
adding the ancillary POPy and CbPy chelates. Between the
two structures described here, PtON7 has much lower non-
radiative decay rate than PtOO7, resulting in a higher
quantum efficiency (0.89). Therefore PtON7 is considered
a desirable candidate as a blue emissive material for OLED
applications.
Devices employing Pt-16, PtOO7, and PtON7 as emitters
were fabricated in the following device structure (type I):
ITO/PEDOT:PSS/NPD (30 nm)/TAPC (10 nm)/2% emit-
ter:26 mCPy (25 nm)/PO15 (40 nm)/LiF/Al (NPD = N,N’-
diphenyl-N,N’-bis(1-naphthyl)-1,1’-biphenyl-4,4’’-diamine,
TAPC = di-[4-(N,N-ditolyl-amino)phenyl]cyclohexane,
26 mCPy = 2,6-bis(N-carbazolyl)pyridine, and PO15 = 2,8-
bis(diphenylphosphoryl) dibenzothiophene.[11] The corre-
sponding EL spectra and CIE coordinates for each device
are displayed in Figure 2a,b, and EQE versus current density
plots are shown in Figure 2d. A summary of device perfor-
mance at the brightness of 100 cdmꢀ2 and 1000 cdmꢀ2 are
presented in Table 2. Unlike tridentate Pt-16, PtON7 and
PtOO7 do not show evidence of excimer formation. Both
PtON7 and PtOO7 devices demonstrate EL spectra similar to
their room temperature solution emission spectra, confirming
that the EL spectra are generated exclusively from the
emitters themselves. The CIE coordinates of the PtON7
device are (0.14, 0.19) and its peak device efficiency exceeds
16%.
Figure 1. a) Comparison of room-temperature absorption spectra of
~
*
Pt-16 ( ), PtOO7 (&), and PtON7 ( ) in CH2Cl2. The T1 absorption
transitions are shown in the inset of (a). b)–e) The emission spectra of
b) Pt-16, c) PtOO7, d) PtON7, and e) PtON1 in solution at room
temperature (c) and at 77 K (a). Redox potentials for each
compound are given in the legends.
absorption bands below 300 nm assigned to 1p–p* transitions
localized on the cyclometalating ligands (1LC). The intense
bands in the 300–420 nm region are attributed to metal-to-
ligand charge-transfer (1MLCT) transitions. Weaker absorp-
tion bands between 420–450 nm can be identified as the
triplet absorption on the basis of the small energy shift
between absorption and emission at room temperature.
Compared to the tridentate structure, Pt-16, both PtOO7
and PtON7 demonstrate stronger MLCT transitions at longer
wavelengths (between 380–410 nm). Moreover, both PtOO7
and PtON7 possess a less-resolved triplet absorption band
(Figure 1a inset). Among the three Pt complexes, PtON7
demonstrates the most intense absorption for all of three
1
types of transitions (1LC, MLCT andS0!T1).
The room-temperature and low-temperature (77 K) solu-
tion emission spectra and solid-state emission spectra at
5 wt% in an optically inert polymethylmethatcrylate
(PMMA) matrix were recorded for both PtOO7 and PtON7
and their analogues (Table 1 and Supporting Information).
The emission spectra for Pt-16, PtOO7, PtON7, and PtON1 at
room temperature and 77 K are shown in Figure 1b–e. Unlike
PtON7 device performance was further improved by
increasing the dopant concentration and using a different hole
injection material, 1,4,5,8,9,11-hexaazatriphenylenehexacar-
bonitrile (HATCN) and a different electron-transporting
material, diphenylbis[4-(pyridin-3-yl)phenyl]silane (DPPS).
2
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and Table 2) reaching a maximum forward viewing EQE of
hext = 25.2%. The color quality and device efficiency of the
PtON7 and PtON1 devices are better than the best reported
deep blue phosphorescent OLEDs with fluorine-free Ir-based
complexes (a maximum EQE of 18.6% and CIE coordinates
of (0.15, 0.19) for the Ir(dbfmi) device) in a similar device
setting.[7a] Furthermore, PtON7 and PtON1 device perform-
ances are also comparable or superior to the best reported
deep blue phosphorescent OLEDs, which employed the
fluorinated complex, iridium(III) bis(3’,5’-difluoro-4’-cyano-
phenylpyridinato-N,C2’) picolinate (FCNIrpic) as an emissive
material (a maximum EQE of 24.2% and CIE coordinates of
(0.14, 0.20).[13] It is also notable that 26 mCPy, a carbazole-
based host materials with an estimated triplet energy of
2.9 eV, can be used for deep blue phosphorescent OLEDs.
The best devices of PtON7 and PtON1 (type II) were also
tested for device lifetime at 2 mAcmꢀ2, which corresponds to
a normal operational brightness in the range of 400–
500 cdmꢀ2. For comparison, devices were fabricated with
the stable and efficient green emitter, fac-Ir(ppy)3, in the
same structure as well as the known stable structure: ITO/
Figure 2. a–c) Plots of electroluminescent spectra for devices based on
Pt-16, PtOO7, PtON7, and PtON1 with corresponding CIE coordinates.
d) Plots of external quantum efficiency (EQE) versus luminance for
devices of the various emitters. Device type I: PEDOT:PSS/NPD/
TAPC/26mCPy:emitter(2%)/PO15/LiF/Al; device type II: ITO/HATCN/
NPD/TAPC/26mCPy:emitter(6%)/DPPS/LiF/Al. The EL spectra were
HATCN(10 nm)/NPD(40 nm)/6%
Ir(ppy)3:CBP(25 nm)/
measured at 100 cdmꢀ2
.
BAlq(10 nm)/Alq(40 nm)/LiF/Al (CBP = 4,4’-bis(N-carbazo-
lyl)biphenyl, BAlq = bis(2-methyl-8-quinolinolato)(biphenyl-
4-olato)aluminum, and Alq = tris-(8-hydroxyquinoline)alu-
minum).[14] In Figure 3, the relative luminance versus time is
plotted for 10 h of constant operation. It is encouraging that
the devices of both PtON7 and PtON1 retained a significant
percentage of their initial luminance for over an hour, and in
particular that PtON1 emitted a non-negligible relative
luminance up to 10 h. Furthermore, it is evident from the
comparison of Ir(ppy)3 devices between the two structures
that the reduced device lifetimes in the type II devices can be
greatly attributed to the degradation of either the electron or
hole blockers, TAPC and DPPS respectively, or the 26 mCPy
host. Therefore, it is challenging to create any significant
conclusions about the stability of these complexes until stable
Table 2: A summary of device characteristics at 100 cdmꢀ2 and
1000 cdmꢀ2 for the devices with two different structures.[a]
Device Emitter
type
@100 cdmꢀ2
@1000
cdmꢀ2
CIE x CIE y EQE P.E.
EQE P.E.
[%] [Lm/
W]
[%]
[Lm/
W]
I
I
Pt-16
PtOO7 0.15 0.10
0.16 0.15 10.1 9.0
3.8
0.5
2.4
0.25
4.1 3.7
I
II
II
PtON7 0.14 0.19 13.5 14.3
PtON7 0.15 0.14 20.4 16.8
PtON1 0.15 0.13 23.3 20.6
8.35 6.6
15.4 10.2
16.8 11.7
[a] Device type I: PEDOT:PSS/NPD/TAPC/26mCPy: emitter(2%)/
PO15/LiF/Al. Device type II: ITO/HATCN/NPD/TAPC/26mCPy:
emitter(6%)/DPPS/LiF/Al.
Despite the weak electron-transporting capabilities of DPPS,
its high bandgap and deep HOMO level has allowed
a demonstrated performance of nearly 100% IQE for deep
blue emitting devices.[12] Through the device structure
(type II), ITO/HATCN(10 nm)/NPD(40 nm)/TAPC(10 nm)/
6% PtON7:26 mCPy(25 nm)/DPPS(40 nm)/LiF/Al, a maxi-
mum forward viewing EQE of hext = 23.7% was achieved at
a luminance of 2 cdmꢀ2 and this only decreases to 20.4% and
15.4% at 100 and 1000 cdmꢀ2, respectively. This device also
gives
a maximum forward power efficiency of hp =
26.9 LmWꢀ1 but this drops to hp = 16.8 LmWꢀ1 at
100 cdmꢀ2, which is attributed to the weak electron-trans-
porting capabilities of DPPS. Moreover, a narrower EL
spectrum was observed, yielding highly desirable CIE coor-
dinates of (0.14, 0.15) for the PtON7 devices. A similar
improvement was observed for phenylpyraole-based Pt com-
plexes, such as PtON1 (Scheme 1, Figure 1, Figure 2, Table 1,
Figure 3. Plot of relative luminance against time for devices based on
Ir(ppy)3 (a), PtON7 (g), and PtON1 (d) with the structure
ITO/HATCN/NPD/TAPC/6% emitter:26mCPy/DPPS/LiF/Al. Also plot-
ted are the lifetime data for the device ITO/HATCN/NPD/6%Ir-
(ppy)3:CBP/BAlq/Alq/LiF/Al (c). The devices were run at a constant
current of 2 mAcmꢀ2
.
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Angew. Chem. 2011, 123, 3240; Angew. Chem. Int. Ed. 2011, 50,
3182.
host and charge blocking materials are developed for deep
blue phosphorescent devices. Efforts towards using these
phosphorescent emitters with stable hosts and blockers are
currently under significant study, and an improvement of
more than two orders of magnitude in device operational
lifetime can be expected, which will be presented in future
publications.
In conclusion, three deep blue platinum complexes are
reported based on a tetradentate ligand design. The com-
pounds PtON1 and PtON7 show excellent photophysical
properties and a high PLQY and radiative rate. An OLED
employing PtON7 is reported to have a maximum device
efficiency of 23.7% with CIE coordinates of (0.14, 0.15). The
photophysical properties and device performance of PtON7
are much improved compared to its Pt and Ir analogues with
similar emitting ligands reported previously. The device
operational lifetimes of these complexes were also studied.
Continued characterization and development of this class of
materials should provide a viable route to develop stable and
efficient deep blue phosphorescent emitters for display and
lighting applications.
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Keywords: blue emitters · coordination modes ·
cyclometalation · OLEDs · platinum
.
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These are not the final page numbers!

Angewandte
Chemie
Communications
Blue Organic LEDs
Deep-blue emitters based on cyclometa-
lated platinum(II) complexes were syn-
thesized, characterized, and used in
organic light-emitting devices. The com-
plexes with tetradentate ligands exhibited
improved photophysical properties over
iridium analogues, and one such com-
pound achieved a peak external quantum
efficiency (EQE) of 23.7%.
X. Hang, T. Fleetham, E. Turner, J. Brooks,
J. Li*
&&&& — &&&&
Highly Efficient Blue-Emitting
Cyclometalated Platinum(II) Complexes
by Judicious Molecular Design
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!