Synthesis of phosphine oxides with tris(benzoxazole/benzothiazole) moieties and their OLED characteristics

Article abstract of DOI:10.1080/15421406.2016.1200944

We have developed a series of phosphine oxides with heterocycles for the light-emitting materials of various colors. In this study, we report novel phosphine oxides with tris(benzoxazole/benzothiazole) moieties, such as (4-(benzo[d]thiazol-2-yl)phenyl)pho

Full text of DOI:10.1080/15421406.2016.1200944

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis of phosphine oxides with
tris(benzoxazole/benzothiazole) moieties and their
OLED characteristics
Hyeonhee Jo, Kyeonghyeon Kim, Dong-Eun Kim, Hoon-Kyu Shin & Burm-Jong
Lee
To cite this article: Hyeonhee Jo, Kyeonghyeon Kim, Dong-Eun Kim, Hoon-Kyu Shin & Burm-
Jong Lee (2016) Synthesis of phosphine oxides with tris(benzoxazole/benzothiazole) moieties
and their OLED characteristics, Molecular Crystals and Liquid Crystals, 636:1, 60-66, DOI:
10.1080/15421406.2016.1200944
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1200944
Published online: 01 Nov 2016.
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Date: 05 February 2017, At: 22:38

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Synthesis of phosphine oxides with
tris(benzoxazole/benzothiazole) moieties
and their OLED characteristics
a
a
b
b
Hyeonhee Jo , Kyeonghyeon Kim , Dong-Eun Kim , Hoon-Kyu Shin ,
a
and Burm-Jong Lee
a
b
Department of Chemistry, Inje University, Gimhae, Korea; National Center for Nanomaterials Technology,
Pohang University of Science and Technology, Pohang, Korea
ABSTRACT
KEYWORDS
We have developed a series of phosphine oxides with heterocycles for
the light-emitting materials of various colors. In this study, we report
novel phosphine oxides with tris(benzoxazole/benzothiazole) moieties,
such as (4-(benzo[d]thiazol-2-yl)phenyl)phosphine oxide (4-TBTPO)
and tris(4-(benzo[d]oxazol-2-yl)phenyl)phosphine oxide (4-TBOPO). The
phosphine oxide;
benzoxazole; benzothiazole;
OLED; yellow-green emission
ꢀ
ꢀꢀ
cyclization reactions were carried out from 4,4 ,4 -phosphoryltribenzoic
acid and 2-aminobenzenethiol or 2-aminophenol with tetrabutylammo-
nium bromide and triphenyl phosphite as catalysts. The structures and
1
13
thermal properties were investigated by H-NMR, C-NMR, FT-IR, UV-Vis,
TGA and DSC. The OLED devices of ITO/MoOx(30 nm)/NPB(600 nm)/4-
TBTPO or 4-TBOPO(200 nm)/Alq (300 nm)/Liq(10 nm)/ Al(120 nm)
3
showed yellow-green emissions at (0.429∼0.529) and (0.406∼0.563) of
CIE color coordinates, respectively.
Introduction
Organic electrophosphorescence devices have attracted intense interests because their intrin-
sic efficiencies may be up to four times higher as the fluorescence ones [1]. In the design of
highly efficient phosphorescent devices, the selection of a proper material for each layer is
also of great importance. In accordance with the efficient phosphorescent materials, bipolar
structures of phosphine oxides with heterocycles have caught great attention [2,3]. A notice-
able advantage of using a bipolar host is the improvement of carrier mobility and balance
of electron and hole fluxes in the emitting layer [4]. On the other hand, two intrinsic crite-
ria have restricted the development of blue host materials: (1) their triplet energy (ET) level
must be higher than blue dopant to prevent exothermic reverse energy transfer from guest to
host and confine the triplet exciton within the emitting layer, and (2) in order to extend the
operational lifetime of the device, the blue host must possess good morphological and chem-
ical stabilities to form stable amorphous films [5–7]. On these reasons the new structures
of phosphine oxides with heterocycles are expected to expand the scope of OLED materi-
als. We have also reported a series of isomeric triphenylphosphine oxides with heterocycles
and found the characteristic EL properties [8,9]. Particularly, the synthesis of heterocycles
CONTACT Burm-Jong Lee
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
 Taylor & Francis Group, LLC
chemlbj@inje.ac.kr
Department of Chemistry, Inje University, Gimhae, Korea.
©

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[251]/61
with benzoxazole and benzothiazole was efficiently carried out by one-pot cyclization [10].
In this study, we synthesized novel phosphine oxide materials with three arms of heterocycle
moieties such as tris(4-(benzo[d]thiazol-2-yl)phenyl)phosphine oxide (4-TBTPO) and tris(4-
(
benzo[d]oxazol-2-yl)phenyl)phosphine oxide (4-TBOPO). The three-armed structures are
expected to show enhanced bipolarity and amorphous aggregate morphology in films. The
characterization of the materials and their OLED properties are presented.
Experimental
1
. Material
-Aminophenol and 2-aminothiophenol were purchased from Aldrich and used with-
2
out further purification. Triphenyl phosphite, tetrabutylammonium bromide and tri(p-
tolyl)phosphine were purchased from TCI, potassium permanganate from DAEJUNG and
sodium carbonate from SAMCHUN Chemical. All the purchased chemicals were also used
without further purification.
2. Synthesis

4
.. Synthesis of tris(-(benzo[d]thiazol--yl)phenyl)phosphine oxide (-TBTPO)
,4 ,4 -Phosphoryltribenzoic acid (TCPO) was firstly prepared from oxidation of tri(p-
ꢀ
ꢀꢀ
tolyl)phosphine by potassium permanganate. Tri(p-tolyl)phosphine (0.3 g, 1.0 mmol),
sodium carbonate (0.24 g, 3.0 mmol) and potassium permanganate (0.47 g, 3.0 mmol) were
added to water (30mL). The mixture was heated at 120°C for 5 h with reflux condenser. After
filtering the reaction mixture, dilute aq. HCl was slowly added to the filtrate. The precipitates
were recrystallized from methanol. The structure of TCPO could be simply confirmed by
spectroscopic methods. Then TCPO (0.42 g, 1 mmol), triphenyl phosphite(0.78 mL, 3 mmol),
tetrabutylammonium bromide (0.96 g, 3 mmol), 2-aminothiophenol (0.32 mL, 3 mmol) were
added to flask with reflux condenser and heated at 120°C for 3h. The progress of reaction was
monitored by TLC. After the completion of reaction, the product was precipitated from the
solution by adding methanol and filtered. Then the product was dissolved in CHCl , added
3
anhydrous MgSO , and stirred at room temperature for 4h. Filtered and evaporated the sol-
4
vents to get a yellow solid. 4-TBTPO; Yield 39%; EA Found (%) C 68.20, N 6.18, O 2.20,
1
H 3.32, S 13.86; Calcd. (%) C 69.11, N 6.20, O 2.36, H 3.57, S 14.19; H-NMR (500 MHz,
CDCl ): δ (ppm) 8.23∼8.24(m, 6H), 8.10∼8.11(d, 2H), 7.94∼7.92(d, 1H), 7.90∼7.86(m, 6H),
3
13
7
1
1
.54∼7.51(m, 4H), 7.45∼7.42(m, 4H), 7.26(s, 1H); C-NMR (125 MHz, CDCl ) δ (ppm)
3
66.24, 154.11, 137.16, 135.26, 134.63, 133.74, 132.81 132.73, 127.72, 127.68, 126.56, 123.64,
21.77.
.. Synthesis of tris(-(benzo[d]oxazol--yl)phenyl)phosphine oxide (-TBOPO)
The synthesis of tris(4-(benzo[d]oxazol-2-yl)phenyl)phosphine oxide was progressed simi-
larly to the synthesis of 4-TBTPO. The synthesized TCPO (0.42 g, 1 mmol), triphenyl phos-
phite(0.78 mL, 3 mmol), tetrabutylammonium bromide (0.96 g, 3 mmol), 2-aminotriphenol
(
0.42 g, 3 mmol) were added to flask with reflux condenser and heated at 120°C for 3h. The
progress of reaction was monitored by TLC. After the completion of reaction, the product was
precipitated from the solution by adding methanol and filtered. Then the product was dis-
solved in CHCl , added anhydrous MgSO , and stirred at room temperature for 4h. Filtered
3
4
and evaporated the solvents to get a yellow solid. 4-TBOPO; Yield 36%; EA Found (%) C 73.01,

6
2/[252]
H. JO ET AL.
1
N 3.90, O 10.71, H 6.30; Calcd. (%) C 74.40, N 3.84, O 10.16, H 6.67; H-NMR (500 MHz,
CDCl ): δ (ppm) 8.43∼8.40(m, 6H), 7.94∼7.90(m, 6H), 7.82∼7.79(m, 3H), 7.64∼7.60(m,
3
13
3
1
1
H), 7.43∼7.37(m, 5H), 7.26(s, 1H); C-NMR (125 MHz, CDCl ) δ (ppm) 161.59, 150.86,
3
41.95, 135.12, 134.30, 132.67, 132.59, 130.91, 130.87, 127.90, 127.67, 125.83, 124.98, 120.39,
10.76.
3. Instrumentation
Infrared spectra were recorded on a Scimitar 1000 (Varian). H-NMR and 13C-NMR spec-
tra were obtained with a VNMRS500 NMR spectrometer. UV-Vis absorption spectra were
measured by a UV-Vis Spectrometer (ShimadzuUV-3100S). The characteristics of the cur-
rent density-voltage-luminance (I-V-L) were measured by IVL 300 series (JBS Inc.).
1
4
. Device fabrication
The OLED devices were fabricated on indium tin oxide (ITO) glass substrates. The
organic layers were vacuum-deposited using thermal evaporation at a vacuum pressure
−7
of 2×10 torr. Molybdenum trioxide (MoOx) as a charge generation layer, 1,4-bis(1-
naphthylphenylamino)biphenyl (NPB) as a hole transport layer, 4-TBTPO or 4-TBOPO
as an emitting layer, 8-hydroxyquinolate aluminium (Alq ) as electron transport layers, 8-
3
hydroxyquinolate lithium (Liq) as an electron injection layer, and Al as a cathode were respec-
tively employed. Device 1: ITO / MoOx [30] / NPB [600] / 4-TBOPO [200] / Alq [300] / Liq
3
[
/
10] /Al [120nm] and Device 2: ITO / MoOx [30] / NPB [600] / 4-TBTPO [200] / Alq [300]
Liq [10] /Al [120nm].
3
Results and discussion
1
. Synthesis and structure determination
The ring structures of benzoxazole and benzothiazole were synthesized from cyclization reac-
ꢀ
ꢀꢀ
tion of 4,4 ,4 -phosphoryltribenzoic acid (TCPO) with corresponding 2-aminophenol and
-aminothiophenol. TCPO was firstly prepared from oxidation of tri(p-tolyl)phosphine by
2
KMnO . The synthetic scheme is shown in Figure 1. Although a few cyclization methods
4
were attempted for higher yield and easy purification, the catalytic system using triphenyl
phosphite and tetrabutylammonium bromide was most efficient in this case [10].
The structures of 4-TBTPO and 4-TBOPO were confirmed by FT-IR, H-NMR, 13C-
NMR, and EA. The FT-IR spectra of 4-TBTPO and 4-TBOPO show the evanishment of car-
1
−
1
−1
boxyl group (1695 cm ). The bands from P=O bonds were respectively found at 1164 cm
−1
−1
(
TCPO), 1189 cm (4-TBTPO) and 1199 cm (4-TBOPO). The formation of C=N bonds
1
−
−1
was confirmed from the bands at 1598 cm (4-TBTPO) and 1545 cm (4-TBOPO). The
−1
−1
presence of C-O bond was proved by the bands at 1117 cm (4-TBTPO) and 1118 cm
4-TBOPO). H-NMR spectra showed aromatic peaks ranged from 7.26 to 8.23 ppm in 4-
1
(
TBTPO and from 7.26 to 8.43 ppm in 4-TBOPO. The chemical shifts of the peaks were
presented in Experimental section. The ring formation of benzoxazole and benzothiazole
13
was also confirmed by C-NMR spectra from 166.24 ppm (4-TBTPO) and 161.59 ppm (4-
TBOPO), which originates from C of C=N bonds. Elemental analysis showed that the ele-
mental compositions of 4-TBTPO and 4-TBOPO were agreed with calculated values as pre-
sented in Experimental section.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[253]/63
Figure . Synthetic scheme of -TBTPO and -TBOPO.
2. Thermal properties by DSC and TGA
The thermal stability of OLED materials is very important, because the durability of multilayer
devices significantly depends on the thermal stability of organic materials. We confirmed the
thermal properties of 4-TBTPO and 4-TBOPO by differential scanning calorimetry (DSC)
and thermogravimetric analysis (TGA) as summarized in Table 1. The melting points of 4-
TBTPO and 4-TBOPO were respectively determined as 278 and 283°C by DSC. Thermal
degradation of 4-TBTPO began around 400°C, while 4-TBOPO started the degradation from
around 220°C. The temperature as of 50% weight loss was found as 565°C (4-TBTPO) and
4
75°C (4-TBOPO), respectively. The TGA curves are shown in Figure 2. The high melting
points and thermal stability of 4-TBTPO and 4-TBOPO indicate enough to fabricate stable
OLED devices.
3
. Electroluminescence (EL) properties
The UV absorption spectra of 4-TBTPO and 4-TBOPO are shown in Figure 3. 4-TBTPO
shows somewhat broader band and longer maximum absorption wavelength (λmax) than 4-
TBOPO. The structural difference between 4-TBTPO and 4-TBOPO is only the O and S in
heterocycles. Therefore, the different electronic contributions to the aromatic heterocycles are
mainly thought to attribute to the absorption patterns.
Table . Thermal properties of -TBTPO and -TBOPO.
TGA
Compound
DSC T a (°C)
Tdb(°C)
Residue (wt. %)
m


-TBTPO
-TBOPO




.
.
a
Melting temperature determined by DSC (heat at °C/min and N atmosphere).
Decomposition temperature determined by TGA (% weight loss at °C/min and N atmosphere).

b


6
4/[254]
H. JO ET AL.
Figure . TGA curves of -TBTPO and -TBOPO.
Figure . UV-Vis spectra of -TBTPO and -TBOPO in CHCl.
Figure . EL spectra of -TBTPO and -TBOPO devices.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[255]/65
Figure . Current density-voltage-luminance (I-V-L) characteristics of -TBTPO and -TBOPO.
The EL device 1 was fabricated with the structure of ITO / MoOx[30] / NPB[600] / 4-
TBOPO[200] / Alq [300] / Liq[10] /Al[120nm]. Device 2 also has same structure as device
3
1
with only differently using 4-TBTPO instead of 4-TBOPO. Figure 4 shows the EL spectra
of device 1 (4-TBOPO) and device 2 (4-TBTPO). The maximum emissions of the devices
are measured on yellow-green regions of λmax 550 nm (4-TBTPO) and 530 nm (4-TBOPO),
respectively. In case of 4-TBTPO, the EL spectra are found red-shifted about 20 nm compared
with 4-TBOPO as also found in the UV absorption spectra. Another difference is the pattern
of EL peak, that is, while device 1 (4-TBOPO) shows split peaks, device 2 (4-TBTPO) does
one broad peak. Although the difference of peak pattern is thought to originate from different
intermolecular aggregates of the compounds in films, the detail clarification on the EL patterns
is not clear at this point.
The current density-voltage-luminance (I-V-L) characteristics of the device are shown in
2
Figure 5. The device 1 showed a maximum current density of 150 mA/cm . The maximum
2
luminance exhibited about 65 cd/m at 9V. The two devices showed very similar I-V-L curves,
though device 2 started EL emission at somewhat low potential. A large difference between
the two devices was found in luminance-current density curve. Although device 1 shows a
linear increase of luminance with the current density, the device 2 shows early saturation of
luminance at low current density. In case of device 2, the efficiency also rapidly decreased with
the increase of current density.
The CIE value of device 1 (4-TBOPO) changed from (0.406, 0.563) at 5 V to (0.410, 0.553)
at 8V. And device 2 (4-TBTPO) also showed the change of CIE value from (0.429, 0.529) at
6
V to (0.424, 0.527) at 9V. However, the difference of CIE values along with applied voltages
is a very little, the devices are thought to show stable EL emissions. The EL properties above
are summarized in Table 2.

6
6/[256]
H. JO ET AL.
Table . EL properties of -TBTPO and -TBOPO.
Max. luminance (cd m )
(applied voltage, V)
−
a
Compound
EL color
λ
(nm)
CIE coordinate
(., .) at
max

-TBOPO
-TBTPO
yellow-green
yellow-green


 ()
 ()
V

(., .) at

V
a
EL device structure: ITO / MoOx[] / NPB[] / -TBOPO[] / Alq [] / Liq[] /Al[nm]

Conclusion
We have synthesized novel OLED materials, 4-TBTPO and 4-TBOPO, based on three–armed
1
phosphine oxide with aromatic heterocycles. The chemical structure was proved by H-NMR,
13
C-NMR, FT-IR, UV-Vis and EA. The materials showed high melting points and good ther-
mal stability. It means that all of compounds have thermal stability enough to make the device
under vacuum evaporation. The EL devices employed 4-TBTPO and 4-TBOPO as emitting
2
layers were fabricated, and both showed the maximum luminance of 65 cd/m at the current
2
density of 150 mA/cm .
Acknowledgment
This paper was supported by the 2015 Inje University research grant.
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