A multifunctional phosphine oxide-diphenylamine hybrid compound as a high performance deep-blue fluorescent emitter and green phosphorescent host

Article abstract of DOI:10.1039/c3cc48531e

A novel phosphine oxide-diphenylamine hybrid compound POA was designed and synthesized with the aim of developing new multifunctional blue fluorophores. POA is the first kind of compound that can be used as a high-efficiency deep-blue emitter (5.4% EQE) and a host to fabricate high-performance green phosphorescent OLEDs (18.1% EQE).

Full text of DOI:10.1039/c3cc48531e

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A multifunctional phosphine oxide–diphenylamine
hybrid compound as a high performance
deep-blue fluorescent emitter and green
phosphorescent host†
Cite this: Chem. Commun., 2014,
50, 2027
Received 8th November 2013,
Accepted 17th December 2013
Xiao-Ke Liu,ab Cai-Jun Zheng,*a Ming-Fai Lo,c Jing Xiao,d Chun-Sing Lee,*c
DOI: 10.1039/c3cc48531e
Man-Keung Fungc and Xiao-Hong Zhang*ae
www.rsc.org/chemcomm
A novel phosphine oxide–diphenylamine hybrid compound POA was host materials for phosphorescent dopants due to their unbalanced
designed and synthesized with the aim of developing new multi- charge-transporting properties and low triplet energy level (T1).
functional blue fluorophores. POA is the first kind of compound that Therefore, much recent efforts have been made on exploration of
can be used as a high-efficiency deep-blue emitter (5.4% EQE) and a host new types of blue fluorophores with good performance as blue-
to fabricate high-performance green phosphorescent OLEDs (18.1% EQE). emitters as well as hosts for phosphors.11,20–22 Nevertheless, there is
still much room for improvement in terms of the efficiency of blue
Organic light-emitting diodes (OLEDs) have attracted great interest fluorescence and the capability of sensitizing green phosphors. For
for their applications in the next-generation flat-panel displays and example, we recently reported a blue OLED with a high external
solid-state lighting sources.1–3 For full-color displays and high- quantum efficiency (EQE) of 4.9% by using a new fluorophore
quality white light emission, efficient emitters of three primary (DPMC). However, when DPMC is used as a host for sensitizing
colors (red, green, and blue) with high color purity and stability are green phosphors, the EQE is limited to 12.3% only.20 On the other
important. Compared with the phosphorescent complexes, blue hand, Ma and his coworkers achieved efficient green phosphorescent
emitters based on fluorescent molecules show good potential for OLEDs with improved maximum EQEs of 15.3 and 15.9%, respec-
stable deep-blue emission.4–8 Moreover, due to their wide energy tively, by using two blue-emitting metal complexes Be(PPI)2 and
gaps, blue fluorophores can serve as host materials for fluorescent Zn(PPI)2.21 However, Be(PPI)2 and Zn(PPI)2 show low efficiencies as
¨
and phosphorescent dopants of green and red emission via Forster blue emitters with maximum EQEs of 2.82 and 2.08%, respectively.
or Dexter energy transfer.9,10 The use of such multifunctional blue Therefore, it is still challenging to develop multifunctional blue
fluorophores can simplify the manufacturing process and produc- fluorophores that can exhibit high-efficiency blue emission and be
tion cost of monochromatic and white OLEDs.10–13 However, employed as high performance hosts.
common efficient blue fluorophores based on large p-conjugated
In this work, we designed and synthesized a new compound
aromatic structures, such as anthracene,14–16 oligomerfluorene,17 40-(diphenylphosphoryl)-N,N-diphenylbiphenyl-4-amine (POA) with
di(styryl)arylene,18 and pyrene19 derivatives, are often not suitable the aim of developing efficient blue fluorescent emitters that can be
employed as high performance hosts for green phosphors. As shown
in Scheme 1, POA consists of a diphenylphosphine oxide moiety and a
diphenylamine moiety linked by a biphenyl-p-conjugated bridge. The
a Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, Technical Institute of Physics and
electron-accepting phosphine oxide moiety was selected considering
Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.
its good electron-transporting ability and relatively high T1.23–27 While
E-mail: xhzhang@mail.ipc.ac.cn, zhengcaijun@mail.ipc.ac.cn;
Fax: +86-10-64879375; Tel: +86-10-82543510
phosphine oxide–carbazole hybrid compounds have been used
as hosts for sensitizing blue and green phosphors, they usually
emit in the near-ultraviolet region and are not suitable for use
b University of Chinese Academy of Sciences, Beijing 100039, P. R. China
c Center of Super-Diamond and Advanced Films (COSDAF) and Department of
Physics and Materials Science, City University of Hong Kong, Hong Kong SAR,
P. R. China. E-mail: apcslee@cityu.edu.hk
d College of Physics and Electronic Engineering, Taishan University, Taian,
Shandong 271021, P. R. China
e Functional Nano & Soft Materials Laboratory (FUNSOM) and Jiangsu Key
Laboratory for Carbon-Based Functional Materials & Devices, Soochow University,
Suzhou, Jiangsu 215123, P. R. China
† Electronic supplementary information (ESI) available: Synthetic procedures,
cyclic voltammogram, DFT calculations and device characteristics. See DOI:
10.1039/c3cc48531e
Scheme 1 Synthetic route and molecular structure of POA.
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as blue emitters.25,28,29 To shift the emission maximum to the to that of ferrocene (Fig. S2, ESI†),34 the HOMO energy level was
visible region, the electron-donating diphenylamine moiety was determined to be ꢀ5.22 eV. As no clear reduction curve was
adopted in POA. The diphenylamine group shows a relatively observed, the LUMO energy level was calculated to be ꢀ2.15 eV
strong electron-donating ability when compared to that of the from the HOMO energy level and Eg. To gain insight into the
carbazole group, which is beneficial for generating long- structure–property relationship of POA at the molecular level,
wavelength emission. Moreover, the electron donor and acceptor density functional theory (DFT) calculations were performed at
are linked via a biphenyl-p-conjugation bridge for providing high the B3LYP/6-31G (d) level. Molecular orbital distributions in POA
fluorescent quantum yield (Ff) by an effective radiative decay are shown in Fig. S3 (ESI†). The LUMO is mainly localized at the
from intramolecular charge-transfer (CT) excitons.6,30 Indeed, biphenyl p bridge, P, O and N atoms, while the HOMO resides
the carefully designed POA shows excellent optical and electrical primarily at the diphenylamine moiety and the N-phenyl ring. The
properties as both an emitter for deep-blue emission and a host separation between these two orbitals maintains the individual
material for green phosphor.
electronic properties of the functional groups and benefits the
POA was synthesized through a coupling reaction of chloro- molecule’s efficient hole- and electron-transporting properties.21,35
diphenylphosphine with 40-bromo-N,N-diphenylbiphenyl-4-amine31 On the other hand, there is still enough LUMO–HOMO overlap to
followed by oxidation (Scheme 1). Room-temperature UV-Vis absorp- give a highly emissive intramolecular CT excited state as demon-
tion and photoluminescence (PL) spectra of POA in ethyl acetate and strated by its high PL quantum yield.30
thin films are shown in Fig. 1. In ethyl acetate, the first absorption
We first characterized the electroluminance performance of
band at around 305 nm can be assigned to the triphenylamine- POA as a blue emitter by fabricating a non-doped OLED with a
centered n–p* transition, while the strong absorption band peaking configuration of ITO/NPB (30 nm)/TCTA (10 nm)/POA (30 nm)/
at 345 nm can be attributed to the delocalized transition from TPBI (30 nm)/LiF (1.5 nm)/Al. In this device, ITO (indium tin
the electron-rich diphenylamine moiety to the electron-deficient oxide) and LiF/Al are the anode and the cathode, respectively;
diphenylphosphine oxide group. From the onset of the solid-state NPB (4,40-bis[N-(1-naphthyl)-N-phenyl amino] biphenyl) is the hole-
absorption spectrum, the energy gap (Eg) of POA is estimated to be transporting layer (HTL); TCTA (4,40,400-tris(N-carbazolyl)-
3.07 eV. POA shows deep-blue emissions peaked at around 433 nm triphenylamine) is the exciton-blocking layer; and TPBI serves
in both ethyl acetate and the thin film. Moreover, the full width at as the electron-transporting layer (ETL), the hole-blocking layer
half maximum (FWHM) of the solid-state emission spectrum of POA and the exciton-blocking layer. The non-doped device exhibits
is 58 nm. The Ff of POA was determined to be 0.80 in ethyl acetate deep-blue emission with CIE coordinates of (0.15, 0.06), and
by using 9,10-diphenylanthracence (Ff = 0.90 in cyclohexane)32 as a remains almost unchanged over a wide range of luminance
standard and the value in the deposited film was calculated to be (Fig. S4, ESI†). The device has a low turn-on voltage (at a brightness
0.48. Fig. 1 also shows a phosphorescence spectrum of POA in of 1 cd mꢀ2) of 3.0 V and maximum current and external quantum
2-methyltetrahydrofuran at 77 K. The characteristic vibrational efficiencies of 2.0 cd Aꢀ1 and 5.4%, respectively. Such high
structure for triphenylamine around 495 and 528 nm indicates that efficiencies may result from the high Ff and bipolar carrier-
the T1 state is a 3pp* state located mostly on the electron donating transporting property27,35 of POA. To the best of our knowledge,
moiety.33 The T1 energy of POA is estimated to be 2.50 eV from the these performance parameters are comparable with the highest
highest-energy peak of its phosphorescence spectrum, which is values reported for non-doped blue OLEDs with CIEy below 0.06.6
sufficiently high to excite green phosphorescent dopants.
Moreover, the current efficiency (CE) and EQE of the device is
Thermal properties of POA were determined by thermo- maintained at 1.2 cd Aꢀ1 and 3.0%, respectively, at a high bright-
gravimetric analysis (TGA) and differential scanning calorimetry ness of 1000 cd mꢀ2, indicating relatively mild efficiency roll-offs.
(DSC) (Fig. S1, ESI†). The decomposition temperature (corre-
POA was then employed as the host material for a green
sponding to 5% weight loss) of POA is high at 310 1C and the phosphorescent OLED. The device structure is ITO/NPB (30 nm)/
glass transition temperature is determined to be 75 1C. Cyclic TCTA (10 nm)/POA:Ir(ppy)3 (30 nm)/TPBI (30 nm)/LiF (1.5 nm)/Al.
voltammetry was used to investigate the electrochemical proper- In this device, the optimized doping concentration of Ir(ppy)3
ties of POA. From the onset of its oxidation potential with respect ( fac-tris(2-phenylpyridine)iridium(III)) is 4 wt%. The CE–luminance–
EQE plots of the non-doped and the green phosphor doped devices
are shown in Fig. 2, while their luminance–voltage–current density
characteristics are shown in Fig. S5 (ESI†). The doped device
Fig. 1 Room-temperature UV-Vis absorption spectra, PL spectra of POA
in ethyl acetate and thin films, as well as its phosphorescence spectrum in
2-methyltetrahydrofuran at 77 K.
Fig. 2 CE–luminance–EQE plots of (a) the non-doped blue fluorescent
device and (b) the green phosphorescent device doped with Ir(ppy)3.
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Table 1 Electroluminescence properties of compounds used as blue
emitters as well as hosts for green phosphor
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We thank the financial support by the National High-tech R&D
Program of China (863 Program) (Grant No. 2011AA03A110), the
National Natural Science Foundation of China (Grant No. 51373190,
51033007, 51103169, and 51128301), the Beijing Natural Science
Foundation (No. 2111002), the Instrument Developing Project of the
Chinese Academy of Sciences (Grant No. YE201133), Foshan city the
introduction of innovative R&D team, P. R. China and the Research
Grants Council of the Hong Kong Special Administrative Region,
China (Project No. T23-713/11).
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