Efficient green coumarin dopants for organic light-emitting devices

Article abstract of DOI:10.1021/ol049903d

We have synthesized a new green fluorescent dopant C-545P having incorporated five strategically placed "methyl" steric spacers on the julolidyl ring system. C-545P has good thermal properties and photostability, and when fabricated as a dopant in an Alq₃-hosted OLED device, it shows notable improvement in luminance efficiency and is more resistant to concentration quenching than C-545T, particularly in the doping concentration range between 1 and 2% v/v, while achieving comparable device stability.

Full text of DOI:10.1021/ol049903d

ORGANIC
LETTERS
2004
Vol. 6, No. 8
1241-1244
Efficient Green Coumarin Dopants for
Organic Light-Emitting Devices
Meng-Ting Lee,† Chia-Kuo Yen,† Wen-Ping Yang,† Hsian-Hung Chen,†
Chi-Hung Liao,† Chih-Hung Tsai,† and Chin H. Chen*
,‡
Department of Applied Chemistry & Display Institute, Microelectronics and
Information Systems Research Center, National Chiao Tung UniVersity,
Hsinchu, Taiwan 300, R.O.C
fredchen@mail.nctu.edu.tw
Received January 15, 2004
ABSTRACT
We have synthesized a new green fluorescent dopant C-545P having incorporated five strategically placed “methyl” steric spacers on the
julolidyl ring system. C-545P has good thermal properties and photostability, and when fabricated as a dopant in an Alq3-hosted OLED device,
it shows notable improvement in luminance efficiency and is more resistant to concentration quenching than C-545T, particularly in the
doping concentration range between 1 and 2% v/v, while achieving comparable device stability.
One of the key developments in the advancement of organic
light-emitting device (OLED) technology for flat panel
display applications can be attributed to the discovery of the
guest-host doped emitter system.1 This is because a single
host with optimized transport and luminescent properties such
as tris(8-hydroquinolinato)aluminum (Alq3) may be used
together with a variety of highly fluorescent or phosphores-
cent guest dopants leading to electroluminesence of desirable
hues with very high efficiencies. Another advantage of the
doped emitter system in OLED is the enhancement of its
operational stability by transferring the electrogenerated
exciton to the highly emissive and stable dopant site thus
minimizing its possibility for nonradiative decay.2 In fluo-
rescent dopants, one of the best green dopants used in many
of today’s OLED prototypical products is 10-(2-benzo-
thiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-
benzo[l]-pyrano[6,7,8-ij]quinolizin-11-one (C-545T).3 These
include, for example, monchrome and area color passive dot-
matrix displays first introduced to the market by Pioneer,4
prototype of passive-matrix full-color OLEDs, in 1998
followed by Kodak/Sanyo’s 2.4” QVGA AMOLED exhib-
ited at JES’995 and a 5.5 LTPS QVGA active-matrix full-
color OLED display shown at SID 2000.6
However, upon further examination of the relationship
between electroluminescent (EL) efficiency (cd/A) and
doping concentration, it is revealed that the luminance
efficiency of C-545T tends to drop rather quickly after
peaking at around 1%. This narrow window of optimal
doping concentration and the lack of robustness in process
control for fabrication of the doped emitter can be dis-
advantageous in production where high display quality
assurance is required.
(3) Fox, J. L.; Chen, C. H. U.S. Patent 4 736 032, 1988. Inoe, T.;
Nakatani, K. Japan Patent 6 009 952, 1994. Ito, J. Japan Patent 7 166 160,
1995.
(4) Nakada, H.; Tohma, T. Display DeVices ‘98, 1998, 29.
(5) Miyaguchi, S.; Ishizuka, S.; Wakimoto, T.; Funaki, J.; Fukuda, Y.;
Kubota, H.; Yoshida, K.; Watanabe, T.; Ochi, H.; Sakamoto, T.; Tsuchida,
M.; Ohshita, I.; Tohma, T. J. SID 1997, 7 (3), 221.
† Department of Applied Chemistry.
‡ Display Institute.
(1) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65,
3610.
(2) VanSlyke, S. A.; Bryan, P. S.; Tang, C. W. The 8th International
Workshop on EL’96, Inorganic & Organic EL’96, Berlin; Manch, R. H.,
Gumlich, H. E., Eds.; pp 195-199.
(6) Rajeswaran, G.; Itoh, M.; Boroson, M.; Barry, S.; Hatwar, T. K.;
Kahen, K. B.; Yoneda, K.; Yokoyama, R.; Yamada, T.; Komiya, N.; Kanno,
H.; Takahashi, H. SID ‘00 Digest 2000, 40, 1.
10.1021/ol049903d CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/16/2004

Recently, a sterically bulky di(tert-butyl)-substituted de-
rivative, 10-[2-(5,7-di-tert-butyl)-benzothiazolyl-1,1,7,7-tetra-
methyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano-[6,7,8-ij]-
quinolizin-11-one (C-545TB) was found to be superior to
C-545T in terms of thermal properties, EL performance, and,
in particular, its resistance to concentration quenching.7
However, as reported in the cited patent literature,8 the
synthesis of C-545TB was rather tedious and expensive.
In this letter, we report an alternative derivative, 10-(2-
benzothiazolyl)-1,3,3,7,7-pentamethyl-2,3,6,7-tetrahydro-
1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]-quinolizin-11-one (ab-
breviated as C-545P), which can be successfully synthesized
by a simple procedure. Having five strategically placed
“methyl” steric spacers, the molecular design feature of
C-545P is quite different from that of C-545T. Notably, there
are now two gem-methyl groups substituted R- to the lone
pair p-orbital of the nitrogen of the julolidyl ring system in
C-545P. From the perspective of stereochemistry, the lack
of molecular symmetry in the pattern of methyl substitution
on julolidyl ring is expected to prevent molecular aggregation
and thus could further delay the onset of concentration
quenching at a high doping concentration. When fabricated
as a doped green emitter in OLEDs, this new green dopant
C-545P notably improves the EL performance over the
corresponding C-545T as well as its thermal properties,
photostabiltiy, and device stability without affecting its
emissive color.
by “super base” with powder KOH in DMSO at low
temperature with i-propyl bromide. Although a similar
reaction has been reported for m-methoxyaniline (1b), the
cyclization was found to produce a 5-methoxy-substituted
isomer (35%) in addition to the desired 7-methoxy-isomer
(2b) (46%). This lack of regioselectivity is attributed to the
electron-donating properties of both alkoxyl and amino
groups, which direct the annulation toward both their ortho
and para positions.10 The excess contamination of the
5-isomer in the reaction mixture caused considerable prob-
lems in separation (by column chromatography), purification,
and scale-up of the reaction.
To alleviate the regioisomer problem, a bulky i-propyl
group is substituted in place of the methyl, as in m-(i-
propyloxy)aniline (1a) in the present scheme, as the Friedel-
Crafts cyclization is known to be controlled more by steric
than electronic factors. As expected, the desired 7-(i-
propyloxy)-2,2,24-trimethyl-1,2-dihydro-quinoline (2a) can
be produced predominantly over the 5-isomer with a ratio
of more than (93:7) as estimated by the 1H NMR integration
of their corresponding C-4 methyl singlets at δ 1.84 and 2.18,
respectively. The lack of isomeric contamination of the
acetone-anil reaction allows the desired 7-(i-propyloxy)-
substituted isomer to be separated readily by recrystallization
in 85% yield without needing column chromatography.
Catalytic hydrogenation proceeds smoothly to give the 7-(i-
propyloxy)-2,2,4-trimethyl-1,2,3,4-tetra-hydroquioline (3),
which can be alkylated with 1-chloro-3-methyl-2-butene
followed by acid-catalyzed cyclization according to a
patented procedure11 to give the key intermediate 8-hydro-
1,3,3,7,7-penta-methyl-julolidine (4) directly with concomi-
tant loss of the protective i-propyloxy group. Vilsmeir
formylation of 4 then gives rise to 9-formyl-8-hydroxy-
1,3,3,7,7-pentamethyljulolidine (5), which is condensed with
ethyl (2-benzthiazolyl)acetate12 under Knoevenagel condi-
tions to give the desired C-545P in 83% yield. The molecular
structure of C-545P was confirmed by 1H and 13C NMR and
elemental analysis. Its purity was checked by TLC as well
as HPLC (>99%), and its mass spectrum revealed no
The synthesis of C-545P is outlined in Scheme 1 in which
one of the key steps is the acetone-anil reaction9 for the
Scheme 1
preparation of 7-(i-propyloxy)-2,2,4-trimethyl-1,2-dihydro-
quinoline (2a). This is accomplished from m-(i-propyloxy)-
aniline (1a) by heating with acetone in the presence of a
catalytic amount of iodine at elevated temperature. Selective
protection of the hydroxy group in m-aminophenol is readily
achieved by alkylation of the phenolate that can be generated
(7) Chen, C. H.; Tang, C. W. Appl. Phys. Lett. 2001, 79, 3711.
(8) Chen, C. H.; Tang, C. W.; Shi, J.; Klubek, K. P. U.S. Patent 6 020
078, 2000.
(9) Weaver, M. A.; Wallace, D. J.; Straley, J. M. U.S. Patent 3 247
211, 1996.
Figure 1. Optical absorption and photoluminescence (PL) spectra
of C-545P and C-545T.
1242
Org. Lett., Vol. 6, No. 8, 2004

extraneous peak other than the desired parent m/e ) 444
(M+ for C27H28N2O2S).
Figure 1 shows the optical absorption and photolumines-
cent (PL) spectra of a dilute solution of C-545P in 1,2-
dichloroethane in comparison with those of C-545T, and their
spectral data and quantum efficiency are given in Table 1.
the hole transport layer, dopant +Alq3 as the emitter, Alq3
as electron transport layer, and LiF as the electron-injection
layer. The layer structure of the doped device and the
corresponding HOMO-LUMO energy levels are depicted
in Figure 2.
Table 1. Spectral, Thermal, and Electrochemical Data of
C-545T and C-545P
abs
PL
λmax
λmax
Tg
Tm
Td
Eord
(nm)
(nm)
Φf
(°C)
(°C)
(°C)
(V)
C-545T
C-545P
480
480
512
514
1
0.99
100
102
230
252
318
343
0.47
0.47
Both of their UV-vis absorption and PL spectra are
essentially superimposable with the fluorescence λmax of
C-545P at 514 nm, which is only 2 nm longer than that of
C-545T. Their emission bands with full width at half-
maximum (fwhm) of 48 nm are also identical, and so are
their respective quantum yields, with that of C-545P being
only 1% less than that of C-545T.
The thermal property of C-545P was evaluated by means
of thermogravimetric analysis (TGA) in a nitrogen atmo-
sphere. The weight loss of C-545P is less than 5% upon
heating to 343 °C, which is 25 °C more thermally stable
than that of C-545T. The glass transition temperature (Tg)
of C-545P was determined by differential calorimetry (DSC)
to be 102 °C, which is similar to that of C-545T.
Figure 2. Doping concentration vs luminance efficiency of C-545P
and C-545T and energy diagram of doped green OLEDs (inset).
All EL devices were fabricated according to a protocol
previously established at Kodak.14 The active area of the EL
device, defined by the overlap of the ITO and the cathode
electrodes, was 9 mm2. The EL device was completed with
encapsulation in a dry nitrogen glovebox. The EL emission
spectra and current-voltage-luminance characteristics of the
devices were measured with a diode array rapid scan system
using a Photo Research PR650 spectrophotometer and a
computer-controlled dc source.
The plots of doping concentration (v/v %) in Alq3 vs
luminance efficiency (cd/A) of C-545P and C-545T are
compared in Figure 2. Both dopants have their peak
efficiencies at around 1% with C-545P showing better
luminance efficiency of 11.3 cd/A and power efficiency of
4.6 lm/W as compared to those of C-545T of 10.4 cd/A and
3.9 lm/W, respectively. At optimal concentration, the emis-
sive colors of both devices expressed in terms of 1931 CIE
coordinates are essentially identical at around (0.31, 0.65).
From the profile of the curves, the luminance efficiency of
C-545P in the doping concentration range of 1-2% has
dropped only 6%, but that of C-545T has dropped more than
20%. It is notable that C-545P is more resistant to concen-
tration quenching than C-545T, particularly in the doping
concentration range between 1 and 2%. In addition, the
descending trend of both curves is similar over 2%; the
absolute luminance efficiency of C-545P is, however, higher
than that of C-545T in that doping range.
By cyclic voltammetric (CV) measurements, samples (C-
545T and C-545P) were dissolved in deoxygened acetonitrile
with tetra-n-butylammonium tetrafluoroborate as the elec-
trolyte. A platinum working electrode and a saturated Ag/
AgCl reference electrode were used. Ferrocene was used for
potential calibration (all reported potentials are referenced
to ferrocene/ferrocenium). The HOMO and LUMO levels
were calculated by the following equation13 (HOMO ) Eoxd
oxd
onset
(or E ) + 4.8; LUMO ) HOMO - Eg) and the band gap
energy (Eg) was calculated from the edge of UV-vis
absorption peak. The band gap energies of both of these
green dopants are estimated to be 2.4 eV with the HOMO
at -5.3 eV and the LUMO at -2.9 eV and are most
conductive for energy-transfer from the Alq3 host with the
HOMO and the LUMO at -5.5 and -2.7 eV, respectively.
The EL properties of the new dopant C-545P are evaluated
in the device structure of ITO/CuPc (15 nm)/NPB (60 nm)/
v% dopant + Alq3 (37.5 nm)/Alq3 (37.5 nm)/LiF (1 nm)/Al
(200 nm), where the ITO on glass with a SiO2 barrier layer
was used as the substrate with a sheet resistance of 20 ohm/
sq, CuPc (copper phthalocyanine) as the hole-injection layer,
NPB (1,4-bis[N-(1-naphthyl)-N′-phenylamino]biphenyl) as
(10) Lin, W.-C.; Huang, S.-T.; Lin, S.-T. J. Chinese Chem. Soc. 1996,
43, 497.
(11) Fox, J. L.; Chen, C. H. U.S. Patent 4 736 032, 1988.
(12) Trapani, G.; Franco, M.; Latrofa, A.; Genchi, G.; Lisa, G. Eur. J.
Med. Chem. Chim. Ther. 1992, 27, 39.
Figure 3 shows the comparison of both devices doped at
the optimal concentration of 1%. Like C-545T, C-545P
(13) Gosser, D. K., Jr. Cyclic Voltammetry Sumulation and Analysis of
Reaction Mechanisms, New York, 1993; p 27.
(14) VanSlyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett. 1996,
69, 2160.
Org. Lett., Vol. 6, No. 8, 2004
1243

Figure 3. Luminance efficiency vs current density and doping
concentration vs 1931 CIExy coordination (inset) of C-545P and
C-545T.
displays a nearly flat response of luminance efficiency (cd/
A) with respect to a wide range of drive current conditions
from 10 mA/cm2 and an earlier plateau to 100 mA/cm2 at
all doping concentrations. C-545P also shows a much steeper
rise and reaches the plateau in efficiency earlier than C-545T
in the low-drive current region. This is particularly desirable
for the passive-matrix displays, where the system would need
to be capable of high luminance at low voltage and have a
“flat” cd/A respond with respect to drive voltage for the IC
drive circuit design. In the insert, the emissive color of both
C-545P and C-545T expressed in 1931 CIE coordinates are
plotted against their respective doping concentration. The
robust nature of the green C-545P doped emitter in terms
of color variation is also comparable with that of C-545T.
Figure 4a shows the photostability of C-545P vs C-545T.
In a photostability study, samples were irradiated with a 400
W high-pressure mercury lamp (λ ) 254∼600 nm) in
acetonitrile under ambient conditions to study their fluores-
cent decay with time of radiation. It was found that the half-
life (t1/2) of C-545P was about 5 h, while that of C-545T
was much shorter (3 h). The device operational stability test
is shown in Figure 4b. L0, measured at the green device of
1% C-545P and C-545T doped in Alq3, was 2847 and 2779
cd/m2, respectively. From an extrapolation of the curve to
half the initial luminance of C-545P and C-545T, t1/2 ) 425
and 425 h, respectively. Assuming the scalable Coulombic
degradation,14 for driving at L0 of 100 cd/m2, t1/2 of C-545P
and C-545T are projected to be 12 100 and 11 810 h,
respectively.
Figure 4. (a) Photostability of C-545T and C-545P. (b) Device
operational test of C-545T and C-545P.
new green dopant C-545P has good thermal properties and
photostability, and when fabricated as a dopant in an Alq3-
hosted OLED device, it shows notable improvement in
luminance efficiency and more resistance to concentration
quenching than C-545T, particularly in the doping concentra-
tion range between 1 and 2% v/v, while achieving compa-
rable device stability.
Acknowledgment. This work was supported by the
Ministry of Education of Taiwan, Republic of China, under
a Grant from PPAEU (No. 91-E-FA04-2-4-B). The generous
supply of OLED materials provided by e-Ray Optoelectron-
ics Technology Co., Ltd., is gratefully acknowledge.
Supporting Information Available: Synthetic proce-
dures, spectral data, and EL-devices of C-545P and C-545T.
This material is available free of charge via the Internet at
http://pubs.as.org.
In conclusion, we have successfully synthesized a new
green fluorescent dopant C-545P by a simple procedure. The
OL049903D
1244
Org. Lett., Vol. 6, No. 8, 2004