A novel phenothiazine derivative for application in high performance red emitting electroluminescent device

Article abstract of DOI:10.1080/15421400500365086

The new red emitting materials based on phenothiazine and DCM were synthesized by Knoevenagel condensation following Vilsmeyer-Haack formulation. The obtained materials showed good thermal stability and amorphous property. The ITO/CuPc/NPB/Alq 3 :KSR-1 (0

Full text of DOI:10.1080/15421400500365086

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A Novel Phenothiazine
Derivative for Application in
High Performance Red Emitting
Electroluminescent Device
Yun-Hi Kim a , Hyung-Sun Kim a , Kwang-Hoi Lee a
Soon-Ki Kwon a & Sung-Han Kim b
,
a Department of Polymer and Engineering &
Engineering Research Institute , Gyeongsang
National University , Chinju, Korea
b Display Lab. Technology Division, Samsung Display
Devices Co., LTD
Published online: 20 Aug 2006.
To cite this article: Yun-Hi Kim , Hyung-Sun Kim , Kwang-Hoi Lee , Soon-Ki Kwon
& Sung-Han Kim (2006) A Novel Phenothiazine Derivative for Application in High
Performance Red Emitting Electroluminescent Device, Molecular Crystals and Liquid
Crystals, 444:1, 257-263, DOI: 10.1080/15421400500365086
To link to this article: http://dx.doi.org/10.1080/15421400500365086
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Mol. Cryst. Liq. Cryst., Vol. 444, pp. 257–263, 2006
Copyright # Taylor & Francis LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500365086
A Novel Phenothiazine Derivative for Application in High
Performance Red Emitting Electroluminescent Device
Yun-Hi Kim
Hyung-Sun Kim
Kwang-Hoi Lee
Soon-Ki Kwon
Department of Polymer and Engineering & Engineering Research
Institute, Gyeongsang National University, Chinju, Korea
Sung-Han Kim
Display Lab. Technology Division, Samsung Display Devices Co., LTD
The new red emitting materials based on phenothiazine and DCM were synthe-
sized by Knoevenagel condensation following Vilsmeyer-Haack formulation. The
obtained materials showed good thermal stability and amorphous property. The
ITO=CuPc=NPB=Alq3:KSR-1 (0.2%)=Alq3=LiF=Al device showed 3.5 cd=A of
efficiency and CIE color coordinate of (0.46, 0.47) at 10 mA=cm2 and 29000 cd=m2
of maximum brightness at 11 V and 512 mA=cm2. In the device using the
Alq3:KSR-1 (0.5%) as emitting material device, 1.5 cd=A of efficiency and CIE
color coordinate of (0.55, 0.42) at 10 mA=cm2 and 19200 cd=m2 of maximum
brightness at 12 V and 702 mA=cm2 were observed.
Keywords: amorphous; phenothiazine; red emitting
INTRODUCTION
Organic light emitting devices (OLEDs) have received much attention
because of their potential applications in flat panel displays [1,2].
Since the initial works on high efficiency OLEDs, many research
efforts have focused on the development of full color diaplays. For full
This work was supported by Korea Research Foundation Grant (KRF-2000-005-
D00251) and Ministry of Commerce, Industry and Energy.
Address correspondence to Soon-Ki Kwon, Department of Polymer and Engineering
& Engineering Research Institute, Gyeongsang National University, Chinju, 660-701,
Korea. E-mail: skwon@gsnu.ac.kr
257

258
Y.-H. Kim et al.
color applications, it is essential to have RGB (red-green-blue) materi-
als with good color purity and high efficiency. After intensive studies
in the past decade, efficient blue and green materials have been
developed to meet the requirement of commercial OLED applications.
However, there is still a lack of red materials with good color purity,
high efficiency, and good stability. Many known and new red fluor-
escent dyes have been tried and more recently some red phosphor-
escence dyes have also been introduced [3,4]. Among these, one
prominent example is the development of 4-(dicyanomethylene)-2-
tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyrane (DCJTB),
which is arguably the state of the art red dopant, by modifying the well
known laser dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminos-
tryl)-4H-pyrane (DCM). However, for OLEDs doped with DCJTB,
device efficiency and color purity are usually traded and compromised.
For rare-earth complexes, the color purity is excellent, but the
efficiency and chemical stability is much below the requirement of
commercial applications. Recently, triplet emitters have been
developed to obtain very efficient red devices. However, due to the long
lifetime of the triplet state, the density of the triplet state can be more
easily saturated for high brightness applications [5]. As a result, the
efficiency of triplet red emitter usually decreases very quickly as
brightness increases. This will pose a potential problem for applica-
tions requiring high excitation density such as in passive dot matrix
displays. Thus, new high performance red dyes are still much in
demand.
In the present work, we designed and synthesized the new red emit-
ting phenothiazine derivatives, which are expected to have inhibited
intermolecular reaction due to bulky t-butyl group and to have high
efficient red emitting due to introduction of sulfur having low band
gap [6]. We also investigated the spectral and photo physical behaviors
of novel phenothiazine derivatives and the electroluminescent (EL)
performance of the compound as the red dopant in OLED.
EXPERIMENTAL
N-(4-tert-butylphenyl)-3-Formylphenothiazine
The Vilsmeyer-Haack formulation was used to synthesize C. 0.4 mol of
phosphoryl chloride was added dropwise over a period of 0.5 h to a mix-
ture solution of 0.04 mol of N-(4-tert-butylphenyl)-phenothiazine
which is obtained from the phenothiazine and 4-tert-butyl-iodobenzene,
and 0.4 mol of dimethylformamide in 15 mL of 1,2-dichloroethane
at À15ꢀC. After refluxing at 83ꢀC for 72 h, the solution was cooled to

A Novel Phenothiazine Derivative
259
room temperature. The solution was poured into a sodium acetate
aqueous solution and then stirred for 4 h. to complete the hydrolysis.
The resulting mixture was concentrated under reduced pressure
and poured into acetone. Insoluble salts in acetone were filtered off,
and the filtrate was concentrated. The residue was extracted with
water and dichloromethane, and organic layer was concentrated.
The purification was carried out by silica gel column with hexane as
eluent.
4-(Dicyanomethylene)-2-Tert-Butyl-6-[N-(4-tert-butylphenyl)-
phenothiazylene] (KSR-1)
The synthesis is based on Knoevenagel condensation of 2.1 mol of N-(4-
tert-butylphenyl)-3-formylphenothiazine and 2-methyl-6-tert-butyl-
4H-pyrane-4-ylidine)propanedinitryl. The Knoevenagel condensation
was carried out in the 1-propanol. After reaction was finished, the
crude product was purified by column chromatography using methyl-
1
ene chloride as eluent. H-NMR (CDCl3) d 7.6 (d, 2H), 7.3 (d, 2H), 7.5
(m, 3H), 7.0 (d, 1H), 6.8 (m, 1H), 6.6 (m, 3H), 6.2 (n, 2H), 1.4 (s, 9H), 1.3
(s, 9H). FT-IR (KBr pellet, cmÀ1): 3050 (aromatic & vinylic CÀH), 2964
(aliphatic CÀH), 2207 (CꢁN).
4-(Dicyanomethylene)-2-Methyl-6-[N-(4-tert-butylphenyl)-
phenothiazylene] (KSR-2)
The synthesis was similarly carried by above procedure. 1H-NMR
(CDCl3) d 7.6 (d, 2H), 7.3 (d, 2H), 7.5 (m, 3H), 7.0 (d, 1H), 6.8 (m,
1H), 6.6 (m, 3H), 6.2 (n, 2H), 2.1 (s, 3H), 1.3 (s, 9H), FT-IR (KBr pellet,
cmÀ1): 3050 (aromatic & vinylic CÀH), 2964 (aliphatic CÀH), 2207
(CꢁN).
RESULTS AND DISCUSSION
The synthesis of new red emitting materials are showed in the Scheme 1.
The new red emitting materials are obtained by Knoevenagel con-
densation following Vilsmeyer-Haack formulation. The obtained mate-
rials are confirmed by various spectroscopic methods. The thermal
property of synthesized materials were evaluated by the means of
TGA under nitrogen atmosphere. TGA curve shows that the materials
showed 5% weight loss at 300ꢀC for KSR-2 and 360ꢀC for KRS-1,
respectively. The DSC measurement showed the glass transition at
around 215ꢀC for KSR-2. However, for KSR-1, Tm and Tg were not
observed even heating up 250ꢀC. From the results, the obtained new

260
Y.-H. Kim et al.
SCHEME 1 Synthetic routes of KSR-1 and KSR-2.
red emitting materials had amorphous property, which can increase
the device longevity.
Figure 1 shows the UV-vis absorption and photoluminescence spec-
tra of the new red emitting materials in CHCl3 solution. The absorp-
tion spectrum of solution of KSR-2 showed the absorption maximum
at 487 nm. For KSR-1, the absorption maximum was observed at
488 nm. And absorption cutoff wavelength was similarly observed at
580 nm in both KSR-1 and KSR-2 regardless of substituents. However,

A Novel Phenothiazine Derivative
261
FIGURE 1 UV-vis absorption and photoluminescence spectra in CHCl3.
in the photoluminescence, the emission maximum of KSR-1 and KSR-2
was observed at 656 nm and 660 nm, respectively. It can be explained
that tert-butyl group for KSR-1 inhibits the planarity, and the photo-
luminescence spectrum was slightly blue shifted.
The LED devices with configuration ITO=CuPc (200)=NPB (500)=
Alq3:KSR=Alq3=LiF (10)=Al (1500) were fabricated, where CuPc as
hole injection, NPB as hole transporting, Alq3 as host and electron
FIGURE 2 Electroluminescence (EL) spectra of the devices.

262

A Novel Phenothiazine Derivative
263
transporting, LiF as electron injection and Al as cathode were used.
Since the color of LEDs with red dopants was expected to be dependent
on the concentration dye, LEDs with different dye concentration were
fabricated. The electroluminescence (EL) spectra of the devices, which
were fabricated with different dye concentrations, are shown in
Figure 2. The results and brightness are summarized in Table 1.
The results showed that the color purity was increased as doping level
of red dopant, however, the efficiency and maximum brightness
was decreased.
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[2] Shirota, Y. (2000). J. Mater. Chem., 10, 1.
[3] Adachi, C., Baldo, M. A., & Forrest, S. R. (2000). J. Appl. Phys., 87, 8049.
[4] Adachi, C., Baldo, M. A., Forrest, S. R., Lamansky, S., Thompson, M. E., & Kwong, R.
C. (2001). Appl. Phys. Lett., 78, 1622.
[5] Kido, J., Nagai, K., Okamoto, Y., & Skothheim, T. (1991). Chem. Lett., 1267.
[6] Kim, S. H., Yu, H. S., Kwon, S. K., Kim, Y. H., Shin, D. C., An, D.C. B.G. (2003).
Patent 10-0377573-0000.