Readily synthesised arylamino fumaronitrile for non-doped red organic light-emitting diodes

Article abstract of DOI:10.1039/b309780c

Bright (maximum 10034 cd m^-2, 455 cd m^-2 at 20 mA cm^-2) and efficient (maximum 2.4% at 4 mA cm^-2) red (λ_max^el 634-636 nm) organic light-emitting diodes employ arylamino-substituted fumaronitrile as the novel host emitter, which is readily prepared and easily purified.

Full text of DOI:10.1039/b309780c

Readily synthesised arylamino fumaronitrile for non-doped red organic
light-emitting diodes
Hsiu-Chih Yeh, Shi-Jay Yeh and Chin-Ti Chen*
Institute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, R. O. C. E-mail: cchen@chem.sinica.edu.tw;
Fax: +886-2-2783-1237; Tel: +886-2-2789-8542
Received (in Cambridge, UK) 14th August 2003, Accepted 5th September 2003
First published as an Advance Article on the web 18th September 2003
Bright (maximum 10034 cd m22, 455 cd m22 at 20 mA
cm22) and efficient (maximum 2.4% at 4 mA cm22) red
filtration of the reaction solution. The second step in the
preparation of NPAFN involved the Pd-catalysed amination of
Br2FN. The reaction conditions were modified from those
reported by Yamamoto et al. in the high-yield synthesis of
triarylamine from diarylamine.8 However, the common base,
sodium or potassium tert-butoxide, used in the reaction caused
undesired reaction of the cyano group of NPAFN. Other bases,
such as sodium hydroxide and sodium hydride, also failed to
yield reasonable amounts of NPAFN. After several attempts,
we found Cs2CO3 could accomplish the amination of Br2FN in
forming NPAFN with the most satisfactory result. In the
isolation of NPAFN, we encountered purification problems if
column chromatography was used. NPAFN seems to isomerise
easily into its Z-isomer upon contact with silica gel or
aluminium oxide. The purification problem was solved by
subjecting the product mixture to train sublimation, the process
we used for the purification of other OLED materials, before the
device fabrication. The desired NPAFN was isolated cleanly
from other reaction side products by sublimation (at 1025 Torr,
with the zone temperatures set at 310, 190, and 150 °C).†
NPAFN is unusual because it is brightly fluorescent but only
in the solid state. The fluorescence of NPAFN is virtually
invisible in common organic solvents. The solution fluores-
cence of NPAFN was barely observed in nonpolar solvents
el
(lmax 634–636 nm) organic light-emitting diodes employ
arylamino-substituted fumaronitrile as the novel host emit-
ter, which is readily prepared and easily purified.
Organic light-emitting diode (OLED)-based displays have
started entering the market of consumer electronics, which has
been dominated for the past three decades by the liquid-crystal
display (LCD).1 Small OLED displays can be found in some
name-brand car stereos, mobile phones, electric shavers, and
digital cameras. Red OLEDs are an indispensable component in
full colour displays. However, due to their chemical structure,
red light-emitting materials have to be used as dopants in OLED
fabrication.2–4 This is because fluorophores showing red
emission either have extended p-conjugation or bear donor–
acceptor polar substituents. Both types of red fluorophore are
prone to crystallization in the solid state and hence are highly
concentration quenching. Realistically, OLEDs based on do-
pant emitters are more difficult to adapt for mass production
processes than those based on non-doped host emitters,
considering the reproducibility of the optimum doping level that
requires careful manufacture control.4
Satisfactory non-doped pure red OLEDs are extremely
rare.3–5 Herein, we report a new red fluorophore, NPAFN
(bis(4-(N-(1-naphthyl)phenylamino)phenyl)fumaronitrile),
suitable for high performance non-doped red OLEDs. Sig-
nificantly, NPAFN can be easily prepared in a simple two-step
procedure (Scheme 1) with overall isolated yield > 45%.
Moreover, the whole preparation process can be performed
without chromatography, which is an advantage for mass
production. This is probably one of the first red fluorophores,
either as a red dopant or a non-doped host emitter, having such
convenient accessibility. Synthetically, the starting material
4-bromophenylacetonitrile used in the synthesis of NPAMLMe
(N-methyl-3,4-bis(4-(N-(1-naphthyl)phenylamino)phenyl)ma-
leimide), one of the first novel non-doped red OLED materials
discovered by us, is also used in the two-step preparation of
NPAFN (Scheme 1).4 Slight but critical modification of the
synthetic procedure of NPAMLMe can lead to the easy
preparation of dibromo-substituted fumaronitrile Br2FN (bis(4-
bromophenyl)fumaronitrile). By controlling the concentration
of 4-bromophenylacetonitrile to ca. 0.25 M (near the saturation
point) and 2 equiv. of the base (NaOCH3) in the reaction, the
product Br2FN was obtained in almost quantitative yield
( > 90%).6,7 The beauty of the Br2FN synthesis was that the
analytically pure product could be isolated by simple suction
em
em
such as benzene (lmax
= 586 nm) and hexane (lmax
=
545–560 nm). This may be due to the easily exchangeable
multiple conformations of the nonplanar benzene and naph-
thalene rings of the molecule. The twin dipolar nature also
contributes to the non-radiative relaxation of NPAFN in the
excited state. However, a full understanding of the unusual
fluorescence properties remains to be developed.9 In the solid
state, bright orange-red fluorescence with lmaxem at 616 nm was
detected for NPAFN (see Fig. 1 for the photoluminescence, PL,
spectrum).
Nevertheless, the strong solid-state fluorescence observed for
NPAFN is reflected in the thermal analysis data. In differential
scanning calorimetry (DSC) thermograms (Fig. 2), the glass
transition temperature (Tg) around 109 °C was detected for
NPAFN in either heating or cooling cycles (10 °C/min). It is
notable that the melting temperature (260 °C) occurs only in the
first heating cycle and no exothermic crystallization signal
appears regardless of repeated heating and cooling of NPAFN.
Scheme 1 Reagents and conditions: i, I2 (1 equiv.) in diethyl ether, NaOCH3
(2.1 equiv.) in CH3OH, 30 min at 278 °C, 4 h at 0 °C, then 3% HCl(aq) (90%
by filtration); ii, N-phenyl-1-naphthylamine (2.2 equiv.) in toluene,
Pd(OAC)2 (3%), P(t-Bu)3 (9%), Cs2CO3 (3 equiv.), 110 °C, 24 h (50% by
sublimation).
Fig. 1 EL spectra of OLED device, with NPB (thin solid line) and without
NPB (dotted line). The PL spectrum of NAPFN solid film (thick solid line),
which was prepared by vacuum deposition, was taken by using 420 nm as
the excitation energy.
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This journal is © The Royal Society of Chemistry 2003

Performance-wise, the non-doped red OLED reported here is
comparable with a previously reported non-doped red OLED
based on NPAMLMe.4 However, the synthesis and purification
of the red material NPAFN is even more convenient than those
of NPAMLMe. This makes NPAFN more commercially
significant than NPAMLMe to be employed in the fabrication
of non-doped red OLEDs.
In summary, we have developed a new red fluorophore
NPAFN as a rare host emitter in non-doped red OLEDs.
NPAFN is weakly emissive in solution but highly fluorescent in
the solid state and its photophysics are worth further explora-
tion. The easy synthesis and purification of NPAFN make non-
doped red OLEDs more feasible than ever. The performance of
the NPAFN-based red OLED is comparable with or better than
most fluorophore-based, dopant or non-doped red OLEDs,
although there is plenty of room for improvement. Conceivable
tests including the thickness of the NPAFN and BCP layers,
different hole-blocking and electron-transporting materials
other than BCP and TPBI, respectively, are underway. For the
material itself, the diarylamino moieties of NPAFN can be
easily replaced with various readily available substituents
through synthesis and the fluorescence and amorphous proper-
ties can thus be optimised.
Fig. 2 DSC thermograms of NPAFN with sequential heating and cooling.
The signal of the first heating thermogram is reduced two-fold to be
accommodated in the figure and the cooling thermograms are enlarged two-
fold for clarity.
The DSC characteristics imply there is no strong contact of
NPAFN molecules in the solid state and NPAFN shows limited
tendency to crystallize.
The bright solid-state fluorescence renders NPAFN quite
possible to be used as the non-doped host emitter in the
fabrication of red OLEDs. Two OLED devices with configura-
tions ITO/NPB(40 nm)/NPAFN(30 nm)/BCP(10 nm)/TPBI (30
nm)/MgAg and ITO/NPAFN(50 nm)/BCP(10 nm)/TPBI (30
nm)/MgAg were fabricated by sequential vacuum deposition of
organic materials on pre-treated ITO (indium tin oxide)-coated
glass.10 Despite the difference of the NPB layer, red electro-
luminescence (EL) of the both OLED devices became visible at
the about the same driving voltage between 3 and 4 V. The EL
of the OLED had emission maxima (lmaxel) around 634 and 636
nm (Fig. 1), although the spectra were broadened and red-
shifted (+18–20 nm) compared with the PL of the solid film.
Usually, such a difference between PL and EL indicates there is
a certain degree of aggregation formation of NPAFN providing
charge-trapped sites with lower energy.11 Nevertheless, both EL
spectra corresponded to coordinates (x = 0.64, y = 0.36) of the
1931 CIE (Commission Internationale de l’Eclairage) chroma-
ticity diagram, indicative of a red EL comparable with (x =
0.64, y = 0.33) of the National Television System Committee
(NTSC) standard red colour. The intensity of EL reached 10034
cd m22 and 9359 cd m22 for the devices with and without NPB,
respectively, as the maximum values recorded at 14.5 V and
over 1000 mA cm22 (Fig. 3). These are the two brightest non-
doped red OLEDs reported so far. More practical, at a low
current density of 20 mA cm22, the device without NPB still
emitted red EL with intensity as high as 392 cd m22, which is
bright enough for active-matrix-driven OLED displays. Moreo-
ver, at a current density of 20 mA cm22 (about 6.9 V), the
device reached 1.8% external quantum efficiency (or 2.0 cd
A21 photometric efficiency or 0.9 lm W21 power efficiency).
The performance of the device without NPB is even better at
low current density. The external quantum efficiency was as
high as 2.4% (or 2.5 cd A21 photometric efficiency or 1.7 lm
W21f power efficiency) at 4.5 V and 4 mA cm22. At 20 mA
cm22 (about 5.7 V), the device emitted red EL with intensity
455 cd m22, brighter than the device containing the NPB
layer.
Financial support from the National Science Council and
Academia Sinica is gratefully acknowledged.
Notes and references
† Characterization data of NPAFN: An orange red solid with red
fluorescence purified from train sublimation once. 1H NMR (400 MHz,
CD2Cl2): d(ppm) 7.94 (d, 2H, J = 8.1Hz), 7.91 (d, 2H, J = 8.5Hz), 7.87 (d,
2H. J = 8.3Hz), 7.61 (d, 4H, J = 9.1Hz), 7.56–7.47 (m, 4H), 7.45–7.39 (m,
4H), 7.33–7.27 (m, 4H), 7.25–7.21 (m, 4H), 7.10 (t, 2H, J = 7.3Hz), 6.92
(d, 4H, J = 9.1Hz). 13C NMR (100 MHz, CD2Cl2): d(ppm) 151.5, 147.0,
142.6, 140.0, 131.6, 130.4, 130.1, 129.2, 128.2, 128.1, 127.4, 127.0, 124.8,
124.7, 124.6, 124.2, 121.5, 119.0, 118.2. FAB-MS: calcd MW 664; m/z =
664. Anal. Found (calcd) for C48H32N4: C 86.61(86.72), H 4.80(4.85), N
8.23(8.43)%.
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9 Recently there have been two reports about the aggregation-induced
emission, although both are short wavelength of blue-green colour. J.
Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X.
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10 TPBI, NPB, and BCP here denote 2,2A,2B-(1,3,5-phenylene)tris(1-
phenyl-1H-benzimidazole),
1,4-bis(1-naphylphenylamino)biphenyl,
and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, respectively. De-
tails of the device fabrication and the method of EL characterization,
including luminescence (cd m22), current density (mA cm22), driving
voltage (V), and external quantum efficiency (%), follow a similar
procedure to that reported by Chan et al.: L.-H. Chan, R.-H. Lee, C.-F.
Hsieh, H.-C. Yeh and C.-T. Chen, J. Am. Chem. Soc., 2002, 124,
6469.
Fig. 3 EL intensity and external quantum efficiency of OLED, ITC/NPB(40
nm)/NPAFN(30 nm)/BCP(10 nm)/TPBI(30 nm)/MgAg (solid line) and
ITC/NPAFN(50 nm)/BCP(10 nm)/TPBI(30 nm)/MgAg (dotted line).
11 J. Kallinowski, in Organic Electroluminescent Materials and Devices,
ed. S. Miyata and H. S. Nalwa, Gordon and Breach, 1997, pp. 1–72.
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