Highly efficient blue electrophosphorescent polymers with fluorinated poly(arylene ether phosphine oxide) as backbone

Article abstract of DOI:10.1021/ja305634j

In view of the tolerance of F atoms in FIrpic to the nucleophilic aromatic substitution polymerization, an activated fluorinated poly(arylene ether phosphine oxide) backbone is used to construct novel blue electrophosphorescent polymers containing FIrpic as the blue emitter, because they can be synthesized under a milder temperature of 120 °C. Compared with the counterparts prepared at high temperature (165 °C), unexpected bathochromic shift is successfully avoided, and a state-of-art luminous efficiency as high as 19.4 cd A^-1 is achieved. The efficiency is comparable to the corresponding physical blend system, which indicates that the fluorinated poly(arylene ether phosphine oxide) has the potential to be used as the platform for the development of high-performance all-phosphorescent white polymer based on single polymer system.

Full text of DOI:10.1021/ja305634j

Communication
pubs.acs.org/JACS
Highly Efficient Blue Electrophosphorescent Polymers with
Fluorinated Poly(arylene ether phosphine oxide) as Backbone
Shiyang Shao,†,‡ Junqiao Ding,*,† Lixiang Wang,*,† Xiabin Jing,† and Fosong Wang†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P. R. China
‡Graduate School, Chinese Academy of Sciences, Beijing 100039, P. R. China
S
* Supporting Information
primary importance is to increase the triplet energy of the
polymeric backbone to be higher than 2.75 eV,5 so that triplet
ABSTRACT: In view of the tolerance of F atoms in
FIrpic to the nucleophilic aromatic substitution polymer-
excitons can be effectively confined on the phosphor.6
ization, an activated fluorinated poly(arylene ether
phosphine oxide) backbone is used to construct novel
blue electrophosphorescent polymers containing FIrpic as
the blue emitter, because they can be synthesized under a
milder temperature of 120 °C. Compared with the
counterparts prepared at high temperature (165 °C),
unexpected bathochromic shift is successfully avoided, and
a state-of-art luminous efficiency as high as 19.4 cd A−1 is
achieved. The efficiency is comparable to the correspond-
ing physical blend system, which indicates that the
fluorinated poly(arylene ether phosphine oxide) has the
potential to be used as the platform for the development of
high-performance all-phosphorescent white polymer based
on single polymer system.
Simultaneously, appropriate HOMO and LUMO levels should
match well with the Fermi levels of the electrodes to facilitate
charge injection. However, it is very difficult to meet these two
requirements at the same time, because the polymer with high
triplet energy will usually suffer from charge injection
problems.7 For example, poly(N-vinylcarbazole) (PVK) with
a high triplet energy of 3.0 eV is often used as the backbone for
blue electrophosphorescent polymers. However, it has a deep
HOMO level of −5.9 eV and a shallow LUMO level of −2.1
eV, which leads to large charge injection barriers for air stable
electrodes, and thus poor device performance (5.5 cd A−1).3c
In our previous work, a trade-off between HOMO/LUMO
levels and triplet energy has been realized on a novel bipolar
polymer host, PCzPO, by adopting a poly(arylene ether
phosphine oxide) scaffold.4a Compared with PVK, its HOMO
and LUMO levels are tuned to −5.7 and −2.3 eV, respectively,
to improve the charge injection capability while the triplet
energy remains as high as 2.96 eV. With PCzPO as the host of a
typical blue phosphor, iridium(III)[bis(4,6-difluorophenyl)-
pyridinato-N,C2]-picolinate (FIrpic), a luminous efficiency of
23.3 cd A−1 is attained, which indicates that PCzPO is a
promising polymer host for FIrpic.
To realize efficient blue emission using electrophosphor-
escent polymer rather than physical blend system, we attempt
to directly introduce FIrpic to the side chain of PCzPO through
a covalent linkage for the construction of blue electro-
phosphorescent polymers (Scheme 1a). Unfortunately, the
resultant polymer PB′-0.05 exhibits a bathochromic shift of
11−18 nm relative to the corresponding physical blend that is
composed of 5 mol % FIrpic mixed with PCzPO (Figure S1).
This observed abnormal emission of PB′-0.05 may be ascribed
to the displacement of the F atoms in FIrpic by nucleophilic
attack under the harsh polycondensation condition that
involves the adoption of a base in a polar aprotic solvent at a
high temperature (165 °C).
lectrophosphorescent polymers,1 where the phosphores-
Ecent dyes are incorporated into the polymeric main chain
or side chain via covalent bonds, have attracted considerable
interest since they can not only harvest both singlet and triplet
excitons to achieve nearly 100% internal quantum efficiency,
but also be fabricated by low-cost wet processes, such as spin-
coating and inkjet printing. Most importantly, compared with
the physical blends of polymer hosts and phosphors, the risk of
phase segregation2 can be effectively prevented for electro-
phosphorescent polymers. To date, much effort has been paid
to the development of red and green electrophosphorescent
polymers, and their luminous efficiencies have reached up to
26.71f and 50.5 cd A−1,1k respectively. By contrast, the highest
reported efficiency of blue electrophosphorescent polymers has
remained 5.5 cd A−1 since the discovery in 2003,1b,e,3 which is
far inferior to that of the corresponding physical blend system.4
On the other hand, as one of the three primary colors, blue
electrophosphorescent polymers are indispensable components
for low-cost full-color displays as well as white light sources. In
view of these two points, it is highly desirable to design efficient
blue electrophosphorescent polymers. Nevertheless, this is a
challenging task due to the lack of the polymer hosts with
suitable triplet energy level and HOMO/LUMO levels used as
their backbones.
In this communication, we report the usage of a different
fluorinated poly(arylene ether phosphine oxide) as the
backbone and its attachment to the same Ir complex, FIrpic
(Scheme 1b). This change allows the successful preparation of
high-efficiency blue electrophosphorescent polymers with
To achieve efficient blue electrophosphorescence, in
principle, some critical criteria have to be taken into account
for the backbone of blue electrophosphorescent polymers. Of
Received: June 11, 2012
Published: September 6, 2012
© 2012 American Chemical Society
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Communication
Scheme 1. Comparison of the Synthetic Routes for Blue Electrophosphorescent Polymers
normal triplet emission from FIrpic. Here several issues should
be noted as follows: (1) Owing to the activation of the
monomer 6F-Cz-MON induced by the incorporated four
electron-withdrawing F atoms adjacent to the C−F bonds para
to P atom, the nucleophilic aromatic substitution polymer-
ization can be carried out smoothly at a moderate temperature
of 120 °C. Consequently, FIrpic is compatible with this milder
condition, and unwanted color alteration of blue electro-
phosphorescent polymers can be avoided. (2) Although six C−
F bonds exist, the nucleophilic reaction occurs merely on the
two activated C−F bonds para to P atom. This is in accordance
with the literature,8 and supported by the fact that only one
strong singlet peak appears at −8.02 ppm (vs fluorobenzene) in
the 19F NMR spectrum of FPCzPO (Figure S2). If the C−F
bonds meta to P atom take part in the polymerization, there
would be at least two sets of signals. (3) The incorporation of
additional F atoms almost does not affect the original electronic
properties of the main chain. As presented in Figure S3, the
triplet energy and HOMO/LUMO levels of FPCzPO are
estimated to be 2.96 and −5.7/−2.3 eV, respectively. These
values are comparable to those of PCzPO, implying that
FPCzPO can act as a suitable host for FIrpic as PCzPO does.
On the basis of the above-mentioned modified backbone and
milder polymerization condition, a series of blue electro-
phosphorescent polymers (PB-0.025 ∼ PB-0.10) have been
synthesized by tailoring the feed ratio between 6F-Cz-MON
and Blue-MON. Workup of simply washing by water, and then
precipitating in methanol affords these polymers pure enough
to be used in PLEDs without any residual catalyst
contamination, which is always a big trouble in transition-
metal-catalyzed conjugated polymers.9 These polymers possess
good solubility in common organic solvents together with high
decomposition temperatures (Td) above 370 °C. Meanwhile,
the actual content of FIrpic in the polymer is quantified by
comparing the integration of the 1H NMR signals at 3.95−4.10
ppm from -CH2O- groups in the alkyl spacer with the singlet
peak at 8.10 ppm related to the protons of carbazole units
(Figure S4). As listed in Table S1, the Ir loading is found to be
slightly lower than the feed ratio, consistent with the
literature.10
Figure 1 shows the absorption spectra in dichloromethane
(DCM) solutions of PB-0.025 ∼ PB-0.10 and their photo-
Figure 1. The absorption and PL spectra of PB-0.025 ∼ PB-0.10 in
DCM solutions (a), and their PL spectra in films (b).
luminescent (PL) spectra both in DCM solutions and solid
states. All the polymers display strong π−π* absorption peaks
at 297/329/344 nm, similar to those of FPCzPO. And the low
energy absorption bands at 375−450 nm corresponding to the
metal-to-ligand charge transfer (MLCT) transition of FIrpic are
too weak to be discernible due to the low Ir loading. As can be
clearly seen in Figure 1a, there exist two distinct PL peaks at
about 402 and 472 nm, which are assigned to the emissions of
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Journal of the American Chemical Society
Communication
FPCzPO and FIrpic, respectively. It is worthy noting that,
unlike PB′-0.05, PB-0.025 ∼ PB-0.10 have normal phosphor-
escence from FIrpic, indicative of the successful introduction of
FIrpic into the FPCzPO side chain without destroying the
structure of FIrpic. Furthermore, with the increasing Ir loading,
the relative emission intensity ratio between FIrpic and
FPCzPO increases noticeably. This demonstrates that intra-
process. This hypothesis can be further confirmed by the Ir
complex concentration dependence of the current density (J)
versus voltage (V) characteristics shown in Figure S6. Except
for PB-0.025, the J−V curves shift toward a lower driving
voltage with the increasing Ir loading for PB-0.05, PB-0.075 and
PB-0.10.
As depicted in Figure 2b and Table 1, the influence of Ir
loading on device performance in this case is not as strong as
molecular Forster energy transfer can happen from FPCzPO to
̈
FIrpic, because there is a good overlap between the absorption
spectrum of FIrpic and PL spectrum of FPCzPO (Figure S5).11
On going from solutions to solid states, the emission of
FIrpic further dominates the whole PL profiles (Figure 1b),
suggesting the contribution from both intra- and intermolecular
Table 1. Device Performance of the Blue Polymers
a
b
c
d
e
f
Von
ηl,max
ηp,max
EQE
Lmax
CIE
polymer
[V] [cd A−1
]
[lm W−1
]
(%) [cd m−2
]
(x, y)
PB-0.025
PB-0.05
PB-0.075
PB-0.10
5.1
5.3
5.3
5.3
17.1
19.4
17.3
16.5
9.0
10.0
8.9
8.0
9.0
8.0
7.6
3927
5508
5531
6367
Forster energy transfer. Even in PB-0.025, noticeably, the
̈
emission of FPCzPO is quenched to a large extent. It differs
from PCzSiIr2.5,3b where the emission of the polymeric host,
3,6-carbazole-alt-tetraphenylsilane copolymer (P36HCTPSi), is
much more intense albeit the same incorporated Ir complex
content. This difference can be reasonably explained by the
higher triplet energy of FPCzPO than P36HCTPSi, which
prevents the loss of triplet excitons.
(0.18, 0.33)
8.8
a
b
Turn-on voltage at a brightness of 1 cd/m2. Maximum luminous
c
d
efficiency. Maximum power efficiency. Maximum external quantum
efficiency. Maximum brightness. CIE coordinates at a voltage of 8 V.
e
f
To evaluate their electroluminescent (EL) performance, PB-
0.025 ∼ PB-0.10 are used as the emitting layer (EML) to
fabricate electrophosphorescent devices with a configuration of
ITO/PEDOT:PSS (40 nm)/EML (40 nm)/TPCz (50 nm)/
LiF (1 nm)/Al (100 nm), where TPCz12 acts as an electron
transporting and hole blocking material. Figure 2a presents the
that observed in red electrophosphorescent polymers. The
luminous efficiency first rises from 17.1 cd A−1 of PB-0.025 to
19.4 cd A−1 of PB-0.05, and then decreases to 16.5 cd A−1 of
PB-0.10. And a variance of about 15% is observed when the Ir
loading changes in the range of 2.5−10 mol %. Among these
blue electrophosphorescent polymers, PB-0.05 gives the best
performance with a peak luminous efficiency of 19.4 cd A−1, a
peak power efficiency of 10.0 lm W−1, a peak external quantum
efficiency (EQE) of 9.0% and a maximum brightness of 5508 cd
m−2. Even at a brightness level of 1000 cd m−2, the luminous
efficiency still remains as high as 14.5 cd A−1 (EQE 6.7%),
indicating a gentle roll-off of device efficiency at high current
density. To our best knowledge, this is the highest efficiency for
blue electrophosphorescent polymers ever reported, which
nearly approaches that of the physical blend of PCzPO and
FIrpic. In addition, an about 3.5-fold improvement of the
luminous efficiency has been realized by comparison to the blue
electrophosphorescent polymers with PVK as the main chain.1b
Considering that the grafted guest is the same in these studies
(FIrpic), we propose that the state-of-art performance of the
resulting polymers can be attributed to the use of a fluorinated
poly(arylene ether phosphine oxide) backbone.
In conclusion, novel blue electrophosphorescent polymers
with a fluorinated poly(arylene ether phosphine oxide) as their
backbone have been successfully synthesized under a
compatible condition with FIrpic because of the activation of
the monomer resulting from the fluorine substituents. Most
importantly, the resulting polymers emit normal phosphor-
escence from FIrpic, and show a promising luminous efficiency
as high as 19.4 cd A−1. We believe this work will pave the way
for the development of high-performance blue electro-
phosphorescent polymers as well as all-phosphorescent single
white polymers14 in the future.
Figure 2. EL spectra of the polymers at a driving voltage of 8 V (a),
and the luminous efficiency versus current density curves of PB-0.025
∼ PB-0.10 (b).
EL spectra of PB-0.025 ∼ PB-0.10 at a driving voltage of 8 V.
They all reveal similar EL feature with a Commission
Internationale de L’Eclairage (CIE) coordinates of (0.18,
0.33). Moreover, the EL spectra are very close to their PL
counterparts, and no additional emissions from FPCzPO or
TPCz are observed. It is believed that, different from red
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures; structural characterization
data, photophysical and electroluminescent properties of the
polymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
electrophosphorescent polymers,13 Forster energy transfer
̈
rather than charge trapping is mainly responsible for the EL
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Journal of the American Chemical Society
Communication
(12) Ding, J.; Wang, Q.; Zhao, L.; Ma, D.; Wang, L.; Jing, X.; Wang,
F. J. Mater. Chem. 2010, 20, 8126−8133.
(13) Ma, Z.; Ding, J.; Zhang, B.; Mei, C.; Cheng, Y.; Xie, Z.; Wang,
L.; Jing, X.; Wang, F. Adv. Funct. Mater. 2010, 20, 138−146.
(14) (a) Wang, L.; Jing, X.; Wang, F. Acta Polym. Sin. 2009, 1, 980−
991. (b) Bian, C.; Jiang, G.; Cheng, Y.; Xie, Z.; Wang, L. Acta Polym.
Sin. 2012, 0, 334−346.
AUTHOR INFORMATION
Corresponding Author
junqiaod@ciac.jl.cn; lixiang@ciac.jl.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to the 973 Project (2009CB623601
and 2009CB930603), National Natural Science Foundation of
China (Nos. 20923003, 21174144 and 21074130), and Science
Fund for Creative Research Groups (No.20921061) for
financial support of this research.
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