Highly efficient orange electrophosphorescence from a trifunctional organoboron-Pt(ii) complex

Article abstract of DOI:10.1039/c0cc04014b

High efficiency orange OLEDs have been achieved using a trifunctional Pt(ii) complex that contains an electron-transporting triarylborane and a hole-transporting triarylamine. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0cc04014b

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Highly efficient orange electrophosphorescence from a trifunctional
organoboron–Pt(II) complexw
Zachary M. Hudson,a Michael G. Helander,b Zheng-Hong Lub and Suning Wang*a
Received 21st September 2010, Accepted 22nd October 2010
DOI: 10.1039/c0cc04014b
High efficiency orange OLEDs have been achieved using a
trifunctional Pt(II) complex that contains an electron-transporting
triarylborane and a hole-transporting triarylamine.
thereby providing large increases in FP and emission brightness
in some cases. As a result, we have shown that functionali-
zation with triarylboron can be used to fabricate OLEDs
based on Pt(II) emitters with greatly improved efficiencies.5b
Although several examples of highly efficient green electro-
phosphorescent devices based on Pt(II) compounds have been
reported recently,5b,6 high performance orange or red Pt(II)
based EL devices remain rare. Herein we report the synthesis,
Triarylboron compounds have been the subject of considerable
research due to their ability to act as powerful electron
acceptors, facilitated by the empty pp orbital on the boron
centre. When the boron atom is protected by appropriate
bulky substituents, highly stable materials can be synthesized
with impressive charge-transporting properties.1 In addition,
the electron-accepting boron centre readily facilitates intra-
molecular charge-transfer, providing highly luminescent
materials in the presence of an electron donor. We and others
have previously demonstrated that by combining an electron-
transporting triarylborane with a hole transporting triaryl-
amine, trifunctional molecules can be prepared that show
strong luminescence as well as the ability to effectively transport
charge, making them attractive for using in OLEDs.2 One
example of such molecules is BNPB, (Scheme 1) which has
been used to fabricate highly efficient blue electroluminescent
devices.2d,e
photophysical properties and device performance of
a
trifunctional Pt(II) phosphor, incorporating electron-transport,
hole-transport and high-efficiency phosphorescence into a
single material. This compound has been used to fabricate a
series of orange electrophosphorescent devices with performance
among the highest observed for Pt(II)-based OLEDs.6
This compound, Pt-BNPB2, was designed as a successor to
the highly fluorescent BNPB, able to take advantage of the
much higher efficiencies achievable using triplet emitters in
OLEDs due to spin statistics. Replacement of the phenyl
spacer adjacent to the boron centre with an electron-deficient
pyridine moiety not only enhances donor–acceptor charge-
transfer, but also allows this compound to act as a cyclo-
metalating ligand to Pt(II). The strong ligand field provided by
this rigid N–C chelate raises the energy of deactivating d–d
excited states, reducing their interaction with the emissive state
and increasing FP.7 Acetylacetonate (acac) is a good ancillary
ligand, providing excellent solution and solid-state stability. In
addition, its highly rigid structure and high triplet level help to
ensure efficient phosphorescence from the N–C chelate. The
advantages of the boron moiety are threefold: in addition
to supporting reversible reduction and promoting MLCT
phosphorescence, this bulky group also aids in the formation
of amorphous films, greatly reducing exciplex emission that is
often problematic from square-planar Pt(II) compounds. The
donor moiety was incorporated from the well-known hole-
transport material N,N0-di-[(1-naphthalenyl)-N,N0-diphenyl]-
(1,10-biphenyl)-4,40-diamine (NPB), and readily facilitates
oxidation and hole transport. In addition, this moiety promotes
charge-transfer to triarylboron with a high quantum efficiency
that is also preserved in the metal complex despite the large
red shift in the emission spectrum. This complex is similar
to an earlier trifunctional complex reported by our group, but
displays improved quantum efficiency and is the first to be
evaluated in OLEDs.
More recently, our group and others have shown that the
triarylboron moiety is capable of enhancing charge-transfer
from metal centres, including Ir(III),3 Re(I),4 and Pt(II).5 In the
case of Pt(II), the low-lying CT state afforded by the boron
functionality greatly increases the energy separation between
the emissive state and radiatively quenching d–d states,
Scheme 1
a Department of Chemistry, Queen’s University, 90 Bader Lane,
Kingston, Ontario, Canada K7L 3N6.
The ligand BNPB2 was prepared by selective Suzuki
coupling of 2,5-dibromopyridine with p-(1-naphthylphenyl-
amino)phenyl pinacolborane, which can be synthesized in
two steps from 1-naphthylphenylamine.
E-mail: suning.wang@chem.queensu.ca
b Department of Materials Science and Engineering,
University of Toronto, 184 College Street, Toronto, Ontario,
Canada M5S 3E4. E-mail: zhenghong.lu@utoronto.ca
w Electronic supplementary information (ESI) available: Complete
experimental and EL performance details and spectral data. See
DOI: 10.1039/c0cc04014b
Subsequent metal–halogen exchange and reaction with
FBMes2 at low temperature affords BNPB2 in good yield.
Conventional methods for cyclometalation to form the Pt acac
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 755–757 755

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Table 1 Photophysical properties of BNPB2 and its Pt complex
Compound
Absa, lmax/nm, ea/cmꢀ1 Mꢀ1
lEMa/nm
Fa,b
E1/2red c/V
E1/2ox c/V
BNPB2
Pt-BNPB2
396 (42 700)
456 (37 100)
509
590
1.0
0.91
ꢀ2.19
ꢀ1.97
0.56
0.49
Measured in CH2Cl2 at 1 ꢁ 10ꢀ5 M at 298 K. Fluorescence QY measured relative to anthracene (FF = 0.36),10 phosphorescent QY measured
a
b
relative to Ir(ppy)3 (FP = 0.97).11 All QYs are ꢂ5%. In DMF relative to FeCp2
.
c
0/+
complex,8 which typically involve heating the ligand at high
temperatures ( Z 80 1C) are unsuitable for the synthesis of the
complex in this case, as the pyridine–boron bond is unstable in
solution at these temperatures. Using a recently developed
strategy based on the method of Crespo, reaction of the ligand
with 1 equivalent of PtCl(DMSO)(acac) and NaOAc in refluxing
methanol affords the desired complex which can then be isolated
in high purity by column chromatography.5b,9
polar charge-transfer excited state, suggesting that this is
cancelled by the large ground state dipole typical of Pt(II)
complexes.12
This compound shows impressive performance when
doped into a host material and used as the emissive layer in
phosphorescent OLEDs. We have recently shown that a
transition metal oxide hole injection layer, such as MoO3,
raises the work function of the ITO anode sufficiently so as to
allow direct charge injection into the host material. This
eliminates the need for a discrete hole-transport layer (HTL)
entirely, instead requiring simply a layer of un-doped host
material.13 Using CBP (4,40-N,N0-dicarbazolebiphenyl) as
HTL and host, and 1,3,5-tris(N-phenylbenzimidazole-2-yl)-
benzene (TPBI) as the electron-transport layer (ETL), we first
evaluated the EL performance of Pt-BNPB2 by fabricating a
series of devices in which the position of the doped emission
zone between the electrodes was varied (Fig. 2).
Both the ligand and the complex are capable of reduction
and oxidation by cyclic voltammetry (Table 1, also see ESIw),
highlighting the electron- and hole-transport capabilities of the
triarylboron and triarylamine moieties. In addition, these
compounds show strong, broad absorption bands in their
UV-visible spectra characteristic of B ’ N donor–acceptor
charge-transfer (Fig. 1). This is consistent with TD-DFT
calculations carried out on the complex at the B3LYP level
of theory, which indicate that the lowest energy electronic
transition is primarily between the electron-rich amino group
(HOMO) and the electron-deficient boron centre (LUMO),
with high oscillator strength and some contribution from the
Pt d orbitals (see ESIw).
The BNPB2 molecule shows bright fluorescent emission in
the solid state and in solution (FF E 1.0 in CH2Cl2). This
emission is highly dependent on solvent polarity, with the
emission maximum shifting from 431 nm in hexanes to 531 nm
in MeCN owing to the highly polarized nature of the charge-
transfer excited state. The emission spectra of BNPB2 are
red-shifted relative to BNPB (lmax = 418 nm in hexanes,
513 in MeCN), due to the increased planarity afforded by the
phenylpyridine linker.2a Upon cyclometalation, the complex
shows bright orange phosphorescence with remarkably high
quantum efficiencies in both solid state (FP = 0.46) and
solution (FP = 0.91, tP = 40.1 ms in CH2Cl2). Its emission
maximum shows almost no solvent dependence despite the
We evaluated a 20 nm double emission zone of 10%
Pt-BNPB2 doped into both CBP and TPBI (device A) as well
as devices in which the emitter was doped into TPBI (device B)
and CBP (device C) individually. Devices with Pt-BNPB2
doped into TPBI had lower efficiencies, most likely due to
poor carrier balance. Furthermore, a series of devices we
fabricated incorporating an additional hole-blocking layer of
BCP, CBP or BAlq (see ESIw) also showed reduced efficiency,
likely due to exciton quenching at the EML/HBL interface due
to accumulation of charge.
The highest efficiency devices were thus obtained using CBP
as host in the absence of any HBL (device C). With this
structure, orange electrophosphorescence was achieved with
maximum current and power efficiencies of 35.0 cd Aꢀ1 and
36.6 lm Wꢀ1 and a maximum external quantum efficiency of
10.6%, the highest reported for a device using a triarylboron-
based phosphorescent EML and among the highest reported
using Pt(II) in OLEDs. Furthermore, these devices show
Fig.
2 Device structure and energy level diagram of OLEDs
Fig. 1 Absorption (---) and emission (—) spectra at 1.0 ꢁ 10ꢀ5 M in
CH2Cl2. lEX = 365 nm for BNPB2 and 470 nm for Pt-BNPB2.
fabricated using Pt-BNPB2 as the emitter at a doping concentration
of 10 wt%.
c
756 Chem. Commun., 2011, 47, 755–757
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Table 2 EL device performance
Device
V
on/V
Luminancea/cd mꢀ2
Zext,max [%]
ZLa/cd Aꢀ1
ZPa/lm Wꢀ1
l
max/nm (CIE)b
A
B
C
3.0
3.0
3.0
4966 (10.2)
2134 (10.0)
5590 (10.4)
10.1
7.5
10.6
35.0 (3.0)
24.8 (3.0)
33.2 (3.0)
36.6 (3.0)
26.0 (3.0)
34.8 (3.0)
581 (0.52, 0.47)
529 (0.38, 0.50)
580 (0.51, 0.48)
a
b
Voltages at which the values were obtained are shown in brackets. CIE chromaticity coordinates.
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In conclusion, a trifunctional donor–acceptor Pt(II) com-
pound that displays bright phosphorescence from a mixed
LC/MLCT excited state promoted by the triarylboron group
has been achieved. When incorporated into the emissive layer
of organic light-emitting diodes, high-efficiency orange electro-
luminescence is obtained that ranks among the highest obtained
using a Pt(II) complex as emissive material. This high performance
may be attributed in part to the presence of both electron and
hole transport functional groups in the molecule.
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support.
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 755–757 757