Synthesis and hole-injection property of phenothiazinyl derivative containing triphenylamine moiety in OLED device

Article abstract of DOI:10.1080/15421400701495690

We report the synthesis of a new conjugated compounds, tris-{4-[10-(4-tert- butylphenyl)-10H-phenothiazin-3-yl]phenyl}amine[TPPPA]. TPPPA showed UV-visible absorption maximum value of 352 nm and photoluminescence (PL) maximum value of 455 nm. It was found

Full text of DOI:10.1080/15421400701495690

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Synthesis and Hole-Injection
Property of Phenothiazinyl
Derivative Containing
Triphenylamine Moiety in OLED
Device
Soo-Kang Kim a , Chang-Jun Lee a , Jong-Wook Park a
& Se-Young Oh b
a Department of Chemistry/Display Research
Center , The Catholic University of Korea , Pucheon,
Korea
b Department of Chemical Engineering , Sogang
University , Mapo, Seoul, Korea
Published online: 22 Sep 2010.
To cite this article: Soo-Kang Kim , Chang-Jun Lee , Jong-Wook Park & Se-Young Oh
(2007) Synthesis and Hole-Injection Property of Phenothiazinyl Derivative Containing
Triphenylamine Moiety in OLED Device, Molecular Crystals and Liquid Crystals, 470:1,
231-239, DOI: 10.1080/15421400701495690
To link to this article: http://dx.doi.org/10.1080/15421400701495690
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Mol. Cryst. Liq. Cryst., Vol. 470, pp. 231–239, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701495690
Synthesis and Hole-Injection Property of
Phenothiazinyl Derivative Containing Triphenylamine
Moiety in OLED Device
Soo-Kang Kim
Chang-Jun Lee
Jong-Wook Park
Department of Chemistry=Display Research Center, The Catholic
University of Korea, Pucheon, Korea
Se-Young Oh
Department of Chemical Engineering, Sogang University, Mapo,
Seoul, Korea
We report the synthesis of a new conjugated compounds, tris-{4-[10-(4-tert-
butylphenyl)-10H-phenothiazin-3-yl]phenyl}amine[TPPPA].
TPPPA
showed
UV-visible absorption maximum value of 352 nm and photoluminescence (PL)
maximum value of 455 nm. It was found that TPPPA could be applied to HIL
material by HOMO level calculation from cyclic voltammetry data. Organic
light-emitting diodes (OLEDs) device with configuration of ITO=TPPPA(60 nm)=
NPB(15 nm)=Alq3 (70 nm)=LiF(1 nm)=Al(200 nm) exhibited a current efficiency
2.85cd=A and power efficiency of 1.18 lm=W at 10 mA=cm2 current density.
Keywords: electroluminescent; Hole Injection Layer (HIL); OLEDs; phenothiazine;
Star-burst
INTRODUCTION
Organic light-emitting diodes (OLEDs) have been the subject of inten-
sive investigation in the past few years because of their potential
application as emissive elements for flat panel displays [1,2].
This Study was supported by the department specialization Fund, 2007 of Catholic
University of Korea. This work was supported by grant No. R01-2006-00010196-0 from
the Basic Research Program of the Korea Science & Engineering Foundation.
Address correspondence to Jong-Wook Park, Department of Chemistry=Display
Research Center, The Catholic University of Korea, Pucheon, 420-743, Korea. E-mail:
hahapark@catholic.ac.kr
231

232
S.-K. Kim et al.
Recently, a star-burst amine 4,40,400-tris{N,(3-methylphenyl)-N-
phenylamino}-triphenylamine (m-MTDATA), 4,4’,4’’-tris{N,-(2-naphthyl)-
N-phenylamino}-triphenylamine(2-TNATA) [3] and copper phthalocya-
nine (CuPc) [4] compound acting the role of the hole injection layer
(HIL) in OLED device, have been inserted between the hole transport
layer(HTL) and the transparent anode[typically an indium tin oxide
(ITO)] to improve device performance. Unfortunately CuPc absorbs
slightly blue and red light, it can’t be used in full color display. m-
MTDATA has disadvantage of low Tg and recrystallization or melting
of amorphous organic materials causes to device degradation [5]. In
principle, all organic layers forming the EL devices had better have
glass-transition temperature (Tg) as high as they can. The individual
layer that has low Tg is likely to limit the thermal stability of the OLED.
In this article, we synthesized a new HIL material by using phe-
nothiazinly and triphenylamine moieties, tris-{4-[10-(4-tert-butyl-
phenyl)-10H-phenothiazin-3-yl]phenyl}amine [TPPPA].
Electro-optical properties of this material were characterized by
cyclic voltammetry (CV), UV-visible and photoluminescence (PL) spec-
tra. Moreover, multilayer EL device was fabricated using this material
as a hole injection material.
EXPERIMENTAL
Synthesis
10-(4-t-Butyl-phenyl)-10H-phenothiazine (2)
Phenothiazine (1)(15 g, 75 mmol), t-BuONa (21.35 g, 225 mmol),
Pd2(dba)3 (0.2 g, 2.25 mmol) were added to dried toluene solvent
(500 ml), then (t-Bu)3P and 1-bromo-4-t-butyl-benzen (14.5 ml,
82.5 mmol) were added into the reaction mixture. The mixture was
heated to 110ꢀC for 1 hr under nitrogen. After the reaction mixture
cooled, it was then acidified with 1N HCl solution, and extracted with
CHCl3. The organic layer was dried by anhydrous MgSO4 and filtered.
The solution was evaporated. The residue was dissolved in CHCl3 and
added to methanol. The precipitate was filtered and washed with meth-
anol. The dark blue residue was purified by column chromatography
with CHCl3: n-hexane (1:1) eluent to afford white solid(2) (23 g, 93%).
1H NMR(300 MHz, CDCl3): d(ppm) 1.41(s, 9H), 6.20(d, 2H), 6.78(t, 2H),
6.84(t, 2H), 6.99(d, 2H), 7.27(d, 2H), 7.60(d, 2H), Fabþ -Mass m=e:330.
3-Bromo-10-(4-t-butyl-phenyl)-10H-phenothiazine (3)
10-(4-t-Butyl-phenyl)-10H-phenothiazine(2) (15g, 45mmol), NBS
(7.9 g, 44 mmol) were added to 300 ml of CHCl3 solution. The mixture

Synthesis Phenothiazinyl Derivative in OLED Device
233
was stirred at room temperature for 1 hr. The product was isolated by
silicagel column chromatography using n-hexane as the eluent to afford
white solid(3) (10 g, 54%).
1H NMR(300 MHz, CDCl3): d(ppm) 1.40(s, 9H), 6.01(d, 1H), 6.16(d,
1H), 6.77 ꢁ 6.85(m, 2H), 6.90(d, 1H), 6.97(d, 1H), 7.09(s, 1H), 7.27(d,
2H), 7.60(d, 2H), Fabþ -Mass m=e:410.
10-(4-t-Butyl-phenyl)-3-(4,4,5,5-tetramethyl)-
1,3,2dioxaborolan-2-yl)-10Hphenothiazine(4)
3-Bromo-10-(4-t-butyl-phenyl)-10H-phenothiazine (6 g, 14.6 mmol)
was dissolved in 200 ml of dry THF solution and stirred at ꢂ78ꢀC,
then 1.6 M n-BuLi 15 ml was added. And then isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (6 ml, 30 mmol) was added to the
reaction mixture after 30 min. After the reaction finished, the reaction
mixture was extracted with diethyl ether and water. The organic layer
was dried by anhydrous MgSO4 and filtered. The product was isolated
by silicagel column chromatography using ethyl acetate:n-hexane
(1:19) as the eluent to afford yellowish green solid(4) (4 g, 60%).
1H NMR(300 MHz, CDCl3): d(ppm) 1.10(s, 12H), 1.30(s, 9H), 6.13(d,
2H), 6.79(m, 2H), 6.94(d, 1H), 7.22 ꢁ 7.28(t, 3H), 7.40 (s, 1H), 7.60(d,
2H), Fabþ-Mass m=e:457.
Tris-{4-[10-(4-tert-butyl-phenyl)-10H-phenothiazin-
3-yl]phenyl}amine[TPPPA]
10-(4-t-Butyl-phenyl)-3-(4,4,5,5-tetramethyl)-1,3,2dioxaborolan-2-yl)-
10H-phenothiazine(4) (4 g, 8.75 mmol), tris(4-bromophenyl)-amine(5)
(1.1 g, 2.23 mmol), Pd(OAc)2 (0.5 g, 2.1 mmol) and (tris-o-tolyl)3P
(0.7 g, 2.2 mmol) were added to DME (300 ml) solution, then the pre-
pared K2CO3 solution, which was dissolved in DME and H2O (1:1,
100 ml) solvent, was added to reaction mixture. The organic layer
was concentrated and then poured into acetone to give crude solid.
The obtained solid was purified by column chromatography with
CHCl3: n-hexane (1:1) eluent to afford yellow solid (TPPPA) (2 g, 72%).
1H NMR(500 MHz, Acetone): d(ppm) 1.42(s, 27H), 6.21(d, 3H0,
6.25(d, 3H), 6.85(t, 3H), 6.91(t, 3H), 7.05(d, 3H), 7.13(d, 6H), 7.18(d,
3H), 7.32(s, 3H), 7.38(d, 6H), 7.53(d, 6H), 7.75(d, 6H), Fabþ-Mass
m=e:1233.
Characterization
1H NMR spectra were recorded on a Brucker, Advance DPX-300.
Fast atom bombardment (FAB) mass spectra were recorded on a

234
S.-K. Kim et al.
JEOL, JMS-AX505WA, HP5890 series II. The optical absorption
spectra were measured by a HP 8453 UV-VIS-NIR spectrometer.
Perkin Elmer luminescence spectrometer LS50(Xenon flash tube)
was used for photo- and electro-luminescence spectroscopy. Melting
point (Tm), grass-transition temperatures (Tg), and crystallization
temperatures (Tc) of respective compounds were measured by differ-
ential scanning calorimetry (DSC) under nitrogen atmosphere using
a DSC2910 (TA instrument) and thermogravimetric analysis (TGA)
using SDP-TGA2960 (TA instrument). Redox potential of the com-
pounds were determined by cyclic voltammetry (CV) using an
EG&G 362 electrochemical workstation with a scanning rate of
50 ꢁ 200 mV=s. The compound of interest was dissolved in N,N-
Dimethylformamide (DMF) with 0.1 M tetrabutylammonium tetra-
fluoroborate as the electrolyte. We used a platinum working
electrode and a saturated Ag=AgNO3 referenced electrode. Ferro-
cene was used for potential calibration (all reported potentials are
referenced against ferrocene=ferrocenium, FOC) and for reversibility
criteria.
EL devices were fabricated using TPPPA as a hole injection layer,
NPB as a hole transporting layer, Alq3 as electron transporting and
˚
emitting layer, LiF as electron injection layer, ITO(1200 A, 30 ohm)
as the anode and Al as the cathode; ITO=TPPPA (60 nm)=NPB
(15 nm)=Alq3 (70 nm)=LiF (1 nm)=Al (200 nm). The organic layer were
vacuum-deposited using thermal evaporation at a vacuum base press-
ure of 10ꢂ6 torr, the rate of deposition being 1 A=s to give an emitting
˚
area of 9 mm2 and aluminum layer was continuously deposited with
same vacuum condition.
Current-voltage (I-V) characteristics of the fabricated EL devices
were measured using Keithley 2400 electrometer. Light intensity
was obtained by Minolta CS-1000.
RESULTS AND DISCUSSION
In material synthesis (Scheme 1), we have placed triphenylamine moi-
ety, which is very useful for hole injection property, into core site of
molecule and have attached tri-molecules of phenothiazine group to
triphenylamine as star-burst type. We also used tertiary butyl group
in order to decrease packing effect and light absorption at visible light
region. Phenothiazine group has been previously reported as hole
injection layer by our group [6].
TPPPA was finally purified with column method to have highly
pure powder and were identified NMR, Fabþ -MS.

235

236
S.-K. Kim et al.
FIGURE 1 UV-Visible(&) and PL(ꢀ) spectra of TPPPA.
Figure 1 shows UV-visible absorption and PL spectra of TPPPA.
UV-visible absorption maximum value was 352 nm and edge value
was 401 nm, therefore this compound could be applied to HIL layer
due to no absorption at visible light region. PL maximum values were
shown at 455 nm and 475 nm.
Thermal property of TPPPA was determined by DSC and TGA
experiment. As shown in Table 1, TPPPA exhibited glass transition
temperatures of 130ꢀC, which is relatively higher than conventional
commercialized hole injection materials such as m-MTDATA
(Tg ¼ 75ꢀC) and 2-TNATA (Tg ¼ 110ꢀC) [3]. We believe that TPPPA
has better property for retaining the film morphology during device
operation due to its high glass transition and recrystallization
temperature [5]. TPPPA showed its own melting temperature as
Tm ¼ 212ꢀC and initial thermal degradation temperature of
Td ¼ 402ꢀC under the nitrogen atmosphere.
To investigate electrochemical kinetic behavior, the measured cyclic vol-
tammograms of TPPPA with the various scan rate (50 mV=s ꢁ 200 mV=s)
TABLE 1 Optical, Electrochemical and Thermal Property of TPPPA
UVonset
(nm)
UVmax
(nm)
PLmax
(nm)
Tg
(ꢀC)
Tm
(ꢀC)
Td
(ꢀC)
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
401
352
455
130
212
402
5.08
1.99
3.09

Synthesis Phenothiazinyl Derivative in OLED Device
237
are shown Figure 2. The peak potentials are gradually shift to higher
potentials as the scan rate is increased.
The redox current value gradually increased as the scan rate
increased. This result suggest that the electrochemical process of
TPPPA is reproducible in the potential range of ꢂ0.7 ꢁ 0.7 V vs.
Ag=Agþ.
It has been reported that the relationship between the redox peak cur-
rent and the scan rate can be expressed as a power law type as follows: [7]
ip;a ¼ k vx
ð1Þ
ð2Þ
log ip;a ¼ log k þ x log v
where ip,a ¼ oxidation peak current density, v ¼ scan rate,
k ¼ proportional constant, and x ¼ exponent of scan rate.
Assuming that electrode kinetics satisfies Eq. (1), the electrochemi-
cal redox reaction on the electrode is controlled by either the electron
transfer process, where x ¼ 1, or the reactant diffusion process, where
x ¼ 0.5 [7].
The oxidation current density of TPPPA versus the scan rate is
approximately linear relationship in the range of 50 ꢁ 200 mV=sec.
The exponent of scan rate, x, value of TPPPA is found to be 0.40.
FIGURE 2 Cyclic voltammograms of TPPPA with various scan rate (0.1 M
(n-Bu)4NBF4=DMF). (a) 50 mV=sec, (b) 80 mV=sec, (c) 100 mV=sec, (d) 150 mV=sec,
sec, (e) 200 mV=sec.

238
S.-K. Kim et al.
FIGURE 3 Energy diagram and current density-voltage characteristic of
ITO=TPPPA=NPB=Alq3=LiF=Al device.
HOMO, LUMO level and energy band gap were summarized
in Table 1 and Figure 3. This electronic energy level showed that
HOMO level of TPPPA is located between ITO work function and
NPB HOMO level and is well matched with ITO and NPB layer.
TPPPA LUMO level was located at high position of 1.99 eV and could
be expected to block electron movement from EML layer to anode.
EL devices were fabricated in ITO=TPPPA (60 nm)=NPB (15 nm)=
Alq3(70 nm)=LiF(1 nm)=Al(200 nm) configuration.
Figure 3 showed I-V characteristics of ITO=TPPPA (60 nm)=PB
(15 nm)=Alq3 (70 nm)=LiF (1 nm)=Al (200 nm) device and turn-on
voltage of 5.5 V.
TABLE 2 Electroluminescent Properties of Multi-layered Device: ITO=
TPPPA (60 nm)=NPB (15 nm)=Alq3 (70 nm)=LiF (1 nm)=Al (200 nm)
Current
density
(mA=cm2)
Voltage ELmax
Brightness
(cd=m2)
CIE
(x, y)
Efficiency Efficiency
(V)
(nm)
(cd=A)
(lm=W)
10
25
50
7.6
8.7
9.8
515
515
515
515
285
740
1500
2930
(0.313, 0.524)
(0.313, 0.524)
(0.312, 0.523)
(0.313, 0.522)
2.85
2.96
3.00
2.93
1.18
1.07
0.96
0.85
100
10.8

Synthesis Phenothiazinyl Derivative in OLED Device
239
We summarized EL performance in Table 2. The device showed
high luminance efficiency of about 3 cd=A and power efficiency of
about 1 lm=W. It also exhibited typical Alq3 green emission of
515 nm and x ¼ 0.31, y ¼ 0.52 CIE value.
CONCLUSIONS
We synthesized new phenothiazyl derivatives containing triphenyla-
mine moiety and applied this compound to OLED device as HIL
material.
UV-visible absorption maximum value of TPPPA film was 352 nm
and edge value was 401 nm, this compound could be applied to HIL
layer due to proper HOMO level and no absorption at visible light
region. PL maximum values were shown at 455 nm and 475 nm.
TPPPA exhibited glass transition temperatures of 130ꢀC, which is
relatively higher than conventional commercialized hole injection
materials such as m-MTDATA (Tg ¼ 75ꢀC) and 2-TNATA(Tg ¼ 110ꢀC).
The oxidation current density of TPPPA versus the scan rate is
approximately linear relationship in the range of 50ꢁ200 mV=sec.
The exponent of scan rate, x, value of TPPPA is found to be 0.40.
EL devices were fabricated in ITO=TPPPA (60 nm)=NPB(15 nm)=
Alq3(70 nm)=LiF(1 nm)=Al(200 nm) configurations. The device showed
high luminance efficiency of about 3 cd=A and power efficiency of
about 1 lm=W. It also exhibited typical Alq3 green emission of
515 nm and x ¼ 0.31, y ¼ 0.52 CIE value.
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