Synthesis and OLED characteristics of isomeric phosphine oxides containing naphthoxazole moiety

Article abstract of DOI:10.1166/jnn.2017.14070

2-(2-(Diphenylphosphoryl)phenyl)naphtho[2,3-d]oxazole (2-PPN), 2-(3-(diphenylphosphoryl)phenyl) naphtho[2,3-d]oxazole (3-PPN), and 2-(4-(diphenylphosphoryl)phenyl)naphtho[2,3-d]oxazole (4-PPN) were synthesized as new light-emitting materials based on the phosphine oxide-naphthoxazole structure. The one-pot synthesis of the phosphine-naphthoxazole compound was achieved using PPA as a solvent and as a catalyst for the cyclization reaction. The phosphine structure was oxidized to phosphine oxide using aq. H₂O₂, and the chemical structures were characterized via ¹H-NMR, ¹³C-NMR, FT-IR, UV-Vis, elemental analysis (EA) and X-ray photoelectron spectroscopy (XPS). TGA under an N₂ flow shows that the PPN derivatives were thermally stable at up to 400°C. The photoluminescence (PL) spectra of the PPN derivatives in chloroform exhibited maximum wavelengths at around 439 nm for 2-PPN, 447 nm for 3-PPN, and 436 nm for 4-PPN. Electroluminescence (EL) devices with different configurations (1-4) were fabricated via vacuum deposition, and devices 1-4 emitted greenish-blue light with a maximum emission at around 509 (2-PPN), 498 (2-PPN), 528 (3-PPN) and 501 (4-PPN) nm.

Full text of DOI:10.1166/jnn.2017.14070

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Nanoscience and Nanotechnology
Vol. 17, 3298–3303, 2017
Copyright © 2017 American Scientific Publishers
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Synthesis and OLED Characteristics of Isomeric
Phosphine Oxides Containing Naphthoxazole Moiety
Ik-Hwan Kim1, Kyeong Hyeon Kim1, Hyunhee Jo1, Burm-Jong Lee1ꢀ∗,
Dong-Eun Kim2, and Hoon-Kyu Shin2
1Department of Chemistry, Inje University, Gimhae 50834, Korea
2National Center for Nanomaterials Technology, POSTECH, Pohang 37673, Korea
2-(2-(Diphenylphosphoryl)phenyl)naphtho[2,3-d]oxazole (2-PPN), 2-(3-(diphenylphosphoryl)phenyl)
naphtho[2,3-d]oxazole (3-PPN), and 2-(4-(diphenylphosphoryl)phenyl)naphtho[2,3-d]oxazole (4-PPN)
were synthesized as new light-emitting materials based on the phosphine oxide-naphthoxazole
structure. The one-pot synthesis of the phosphine-naphthoxazole compound was achieved using
PPA as a solvent and as a catalyst for the cyclization reaction. The phosphine structure was
oxidized to phosphine oxide using aq. H2O2, and the chemical structures were characterized via
1H-NMR, 13C-NMR, FT-IR, UV-Vis, elemental analysis (EA) and X-ray photoelectron spectroscopy
(XPS). TGA under an N2 flow shows that the PPN derivatives were thermally stable at up to
400 ꢀC. The photoluminescence (PL) spectra of the PPN derivatives in chloroform exhibited
maximum wavelengths at around 439 nm for 2-PPN, 447 nm for 3-PPN, and 436 nm for 4-PPN.
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Electroluminescence (EL) devices with different configurations (1–4) were fabricated via vacuum
IP: 185.89.101.52 On: Tue, 07 Mar 2017 18:26:50
Copyright: American Scientific Publishers
deposition, and devices 1–4 emitted greenish-blue light with a maximum emission at around 509
(2-PPN), 498 (2-PPN), 528 (3-PPN) and 501 (4-PPN) nm.
Keywords: OLED,
Phosphine
Electroluminescence.
Oxide-Naphthoxazoles,
Photoluminescence,
1. INTRODUCTION
electron-trapping properties.8ꢀ9 Recently, carbazolyl and
aryl phosphine oxide (APO) based ambipolar host materi-
als for blue electrophosphorescence were investigated via
a density functional theory.10
To this end, we have extended the scope of OLED
materials by newly synthesizing isomeric phosphine oxide-
naphthoxazole derivatives of 2-, 3-, 4-PPN, and we
characterized them in terms of their structures and physic-
ochemical properties. As is typical, their isomerically-
featured OLED properties were also investigated.
Organic light-emitting diodes (OLEDs) have drawn a sig-
nificant amount of interest due to their potential use in full-
color flat-panel displays and in lighting sources. OLEDs
offer the advantages of a wide viewing angle, high-
response velocity, low-power consumption, etc.1–6 How-
ever, OLEDs still need to be improved in terms of the
emission efficiency in the blue color.
An efficient blue OLED can be produced if a blue-
emitting material with a high electron trap capacity can be
fabricated. In such a case, the injected hole would be effi-
ciently recombined with trapped electrons in the emitting
material. As such, an ambipolar phosphine oxide material
has been previously shown to have a high-power efficiency
in its blue phosphorescence.7
2. EXPERIMENTAL DETAILS
2.1. Materials
3-Amino-2-naphthol, nickel(II)bromide, 2-(diphenyl phos-
phino) benzoic acid (>97%), 4-(diphenyl phosphino)
benzoic acid (>97%), chlorodiphenyl-phosphine, poly-
phosphoric acid (PPA), and N-methyl-2-pyrrolidone
(NMP) were purchased from Sigma-Aldrich Chemical Co.
Sodium hydroxide was obtained from SAMCHUN Chemi-
cal; ethyl 3-bromobenzoate from TCI, 2,2ꢁ-bipyridine from
Accordingly, we have designed new phosphine oxide-
oxazole compounds where the oxygen, nitrogen, and phos-
phorus atoms are combined with naphthalene and three
phenyls to produce a blue emission with the desired
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2017.14070

Kim et al.
Synthesis and OLED Characteristics of Isomeric Phosphine Oxides Containing Naphthoxazole Moiety
Alfa Aesar; hydrogen peroxide 30% in H2O from Jun-
sei Chemical; and p-toluenesulfonic acid from KANTO
Chemical. The solvents were purified using normal proce-
dures and were handled under a moisture-free atmosphere.
via TLC. After cooling to ambient temperature, the mix-
ture was extracted with toluene, and the organic solution
was collected and evaporated. Then the crude intermediate
was stirred in 10 mL CH2Cl2, and then H2O2 (1.2 mL,
10 mmol, 30%) was added drop-wise while stirring at
room temperature. The organic solution was separated
and washed with deionized water, and the extract was
evaporated to dryness, affording to a pale yellow solid.
This residue was dissolved in CHCl3 and was added
to methanol in order to obtain a solid precipitate. The
crude product was chromatographed on silica gel with
CHCl3:ethyl acetate, 5:1 to obtain pure 2-PPN.
2.2. Instrumentation
1H-NMR and 13C-NMR spectra were obtained using a
LC-NMR spectrometer (VNMRS 500, Varian). The opti-
cal absorption spectra were measured using a Shimadzu
UV-2501 PC Spectrometer. The PL spectrum was taken
using a Perkin-Elmer LS45 luminescence spectrometer.
The current density–voltage–luminance (I–V –L) charac-
teristics and the efficiency were measured using an IVL
300 series device (JBS Inc.).
Anal.Calc.(%) for C29H20NO2P, C:78.19, H:4.53,
N:3.14, O:7.18. Found (%, XPS) C:77.93, H:4.69, N:2.95,
1
O:6.46. H-NMR (500 MHz, CDCl3, ppm) ꢁ 8.18–8.21
(m, 2H), 7.99 (s, 1H), 7.90 (q, ꢁ = 11.5 Hz, 1H), 7.78–
7.82 (m, 4H), 7.73–7.76 (m, 4H), 7.62 (q, ꢁ = 7.5 Hz, 1H),
7.42–7.47 (m, 6H), 7.43 (s, 1H). 13C-NMR (125 MHz,
CDCl3, ppm) ꢁ 164.02, 149.22, 141.18, 135.37, 135.29,
133.66, 133.25, 132.89, 132.40, 132.04, 131.71, 131.67,
131.60, 131.59, 131.48, 131.46, 131.26, 131.03, 130.98,
130.93, 130.84, 128.26, 128.17, 125.41, 124.55, 117.61,
106.57. FT-IR (KBr, cm−1ꢂ 1549 (C N), 1456 (P–C),
1435 (aromatic C C), 1190 (stretch P O), 1113 (C–O).
2.3. Synthesis of PPN Derivatives
2.3.1. 2-(2-(diphenylphosphoryl)phenyl)naphtho
[2,3-d]oxazole (2-PPN)
Figure 1 shows the synthetic scheme for 2-PPN. The
synthesis of the PPN derivatives followed closely-
related procedures.11ꢀ12 After 3-amino-2-naphthol (0.159 g,
1.0 mmol), 2-(diphenylphosphino)benzoic acid (0.306 g,
1.0 mmol) and p-toluenesulfonic acid (0.190 g, 1.0 mmol)
were dissolved in 30 ml toluene, the mixture was stirred
for 15 hours at 150 ꢀC under an N2 atmosphere in a
round bottom flask. The reaction progress was monitored
2.3.2. Ethyl-3-(diphenylphosphoryl)benzoate (1)
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NiBr2(bpy) was prepared according to a previously-
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reported method.13 A mixture of ethyl 3-bromobenzoate
Copyright: American Scientific Publishers
(1.601 ml, 10 mmol), chlorodiphenylphosphine (1.85 ml,
10 mmol) and NiBr2(bpy) were dissolved in 5 ml NMP
(N-methyl-2-pyrrolidone). Zinc dust (1.1 g, 16 mmol) was
added portion-wise in a few minutes. The solution was
ꢀ
heated under nitrogen to 110 C, and after 3 hours, the
reaction was finished. The solution was then allowed to
cool at room temperature and was subsequently filtered.
The filtered solution was evaporated and dried under
a vacuum, and the dried precipitate was dissolved in
10 mL CH2Cl2. Then, H2O2 (2 mL, 30%) was added drop-
wise while stirring at room temperature. The solution was
extracted 3 times with water, and then the organic layer
was collected and evaporated.
FT–IR (KBr, cm−1) 3057 (stretch, –CH3ꢂ, 2991
(stretch, –CH2–), 1717 (C O), 1436 (bend, –CH2–),
1113 (bend, –CH3).
2.3.3. Sodium-3-(diphenylphosphoryl)benzoate (2)
Sodium 3-(diphenylphosphoryl)benzoate was synthe-
sized via saponification of ethyl 3-(diphenyl phospho-
ryl)benzoate. A methanolic solution of NaOH (2–3 N, 1–4
equiv.) was added to a solution of ester (1 equiv.) in a
CH2Cl2/CH3OH (9:1) mixture, with the final concentration
of alkali being about 0.3 N. After stirring for 10 min, the
solvents were then removed using an evaporator. FT-IR
(KBr, cm−1ꢂ 1721 (C O).
Figure 1. Synthetic scheme of 2-PPN.
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Synthesis and OLED Characteristics of Isomeric Phosphine Oxides Containing Naphthoxazole Moiety
Kim et al.
2.3.4. 2-(3-(diphenylphosphoryl)phenyl)
naphtho[2,3,-d]oxazole
(3-PPN)
Figure
2 shows the synthetic scheme of 3-PPN.
3-Amino-2-naphthol (0.159 g, 1.0 mmol) and sodium
3-(diphenylphosphoryl)benzoate (0.344 g, 1.0 mmol) were
dissolved in 30 mL polyphosphoric acid (PPA), and the
ꢀ
mixture was stirred for 15 hours at 150 C under a nitro-
gen atmosphere. The progress of the reaction was moni-
tored via TLC. The crude solution was stirred with water
3 times, filtered, and extracted with CHCl3 to collect and
evaporate the organic layer. The crude product was chro-
matographed on silica gel with CHCl3:ethanol, 50:1 to
obtain pure 3-PPN.
Figure 3. Synthetic scheme of 4-PPN.
2.3.5. Synthesis of 2-(4-(diphenylphosphoryl)phenyl)
naphtho[2,3-d]oxazole (4-PPN)
Figure
3 shows the synthetic scheme of 4-PPN.
Anal. Calc.(%) for C29H20NO2P, C:78.19, H:4.53,
N:3.14, O:7.18. Found(%) C:76.42, H:4.69, N:4.65,
3-Amino-2-naphthol (0.159 g, 1.0 mmol) and 4-(diphenyl-
phosphino)benzoic acid (0.306 g, 1.0 mmol) were ꢀadded
to PPA (30 mL). The mixture was heated at 150 C for
15 hours under an N2 atmosphere, and the progress of
the reaction was confirmed via TLC. After the reaction
mixture had cooled, the reaction solution was extracted
with CHCl3. The organic layer was dried using MgSO4
and was then filtered. The crude intermediate was stirred
in 10 mL CH2Cl2 in a round bottom flask, and H2O2
(1.2 mL, 10 mmol, 30%) was then slowly added into the
mixture while stirring at room temperature. The organic
layer was separated and washed with water, and the
1
O:7.69. H-NMR (500 MHz, CDCl3, ppm) ꢁ 8.66–8.68
(m, 2H), 8.53 (d, J = 12 Hz, 1H), 8.19 (s, 1H), 7.89–8.00
(m, 4H), 7.73–7.77 (m, 4H), 7.68 (t, J = 6ꢃ7 Hz, 1H),
7.60 (t, J = 7ꢃ2 Hz, 1H), 7.46–7.54 (m, 6H). 13C-NMR
(125 MHz, CDCl3, ppm) ꢁ 163.80, 149.56, 141.72, 135.34,
134.55, 133.73, 132.30, 132.12, 132.08, 132.04, 131.95,
131.93, 131.86, 131.51, 131.49, 131.39, 131.37, 129.30,
129.20, 128.78, 128.69, 128.54, 128.45, 127.92, 127.45,
125.67, 124.81, 117.61, 106.54. FT-IR (KBr, cm−1ꢂ: 1549
(C N), 1456 (P–C), 1435 (aromatic C C), 1190 (stretch
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P
O), 1113 (C–O).
extract was evaporated to dryness, affording a pale yel-
low solid. The residue was dissolved in CHCl3 and was
IP: 185.89.101.52 On: Tue, 07 Mar 2017 18:26:50
Copyright: American Scientific Publishers
added to methanol in order to obtain a solid precipitate.
The crude product was chromatographed on silica gel with
CHCl3:ethyl acetate, 5:1 to obtain pure 4-PPN.
Anal.Calc.(%) for C29H20NO2P, C:78.19, H:4.53,
N:3.14, O:7.18. Found(%) C:78.18, H:4.52, N:2.80,
1
O:7.22. H-NMR (500 MHz, CDCl3, ppm) ꢁ 8.41–8.43
(m, 2H), 8.23 (s, 2H), 7.96–8.02 (m, 2H), 7.86–7.90 (m,
4H), 7.70–7.74 (m, 4H), 7.47–7.61 (m, 6H). 13C-NMR
(125 MHz, CDCl3, ppm) ꢁ 163.77, 149.60, 141.71, 136.98
(3C), 136.17 (2C), 132.68, 132.60, 132.28, 132.27, 132.12,
132.04, 131.98, 131.57, 131.45, 130.05, 130.02, 128.75,
128.65, 128.59, 127.94, 127.92, 127.84, 125.78, 124.88,
117.84, 106.59. FT-IR (KBr, cm−1ꢂ: 1543 (C N), 1486
(P–C), 1437 (aromatic C C), 1179 (stretch P O), 1112
(C–O).
2.4. OLED Device Fabrication
The OLED devices were fabricated on glass substrates.
First, the organic layers were vacuum-deposited using
thermal evaporation at a vacuum base pressure of 2 ×
10−7 torr. The multilayer structure consists of the follow-
ing layers (as shown in Table I): Indium tin oxide (ITO)
as the anode; 1,3-bis(1-naphthylphenylamino) biphenyl
(NPB) as a transport layer (40 nm); 2-, 3-, or 4-PPN as an
emitting layer (50 nm); 1,3,5-tris(N-phenylbenzimidizol-2-
yl)benzene (TPBi) as an electron transport layer (30 nm);
Figure 2. Synthetic scheme of 3-PPN.
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Kim et al.
Synthesis and OLED Characteristics of Isomeric Phosphine Oxides Containing Naphthoxazole Moiety
Table I. Configuration of the OLED devices.
Device
Structure
1
2
ITO/NPB (50 nm)/2-PPN (50 nm)/Liq (0.5 nm)/Al (120 nm)
ITO/NPB (50 nm)/2-PPN (50 nm)/Alq3 (15 nm)/
Liq (0.5 nm)/Al (120 nm)
3
4
ITO/NPB (40 nm)/3-PPN (50 nm)/TPBi (30 nm)/
Liq (0.5 nm)/Al (120 nm)
ITO/NPB (40 nm)/4-PPN (50 nm)/TPBi (30 nm)/
Liq (0.5 nm)/Al (120 nm)
8-hydroxyquinolate lithium (Liq) as an electron injection
layer (0.5 nm); and Al as a cathode (120 nm).
Figure 5. UV-vis spectra of PPN derivatives.
3. RESULTS AND DISCUSSION
were observed for the C atom in the C N bond at 164.02
(2-PPN), 163.80 (3-PPN), and 163.77 (4-PPN) ppm.
Figure 5 shows the UV-absorption spectra of the PPN
derivatives, which was measured in CHCl3, exhibiting a
maximum absorption of around 266 (2-PPN), 272 (3-PPN)
and 276 (4-PPN) nm. The photoluminescence (PL) spectra
(Fig. 6) were obtained from the excitation by the max-
imum absorption wavelength. The PPN derivatives simi-
larly show blue PL emissions around 439, 447 and 436 nm
in CHCl3.
3.1. Structure Determination and PL Properties
The chemical structures of 2-PPN, 3-PPN and 4-PPN were
determined via 1H-NMR, 13C-NMR, FT-IR, and elemental
analysis. The FT-IR spectra (Fig. 4) of the PPN derivatives
confirmed the newly-formed C N bond (1549 cm−1ꢂ of
the oxazole ring.
The 1H-NMR spectra showed aromatic peaks only from
around 7 to 9 ppm. The formation of the oxazole ring
was also confirmed by the 13C-NMR spectra. 13C peaks
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3.2. Thermal Properties by TGA and DSC
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The thermal properties of the PPN derivatives were
Copyright: American Scientific Publishers
investigated via thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) under a nitrogen
atmosphere. The TGA and DSC spectra are shown in
Figure 7. The decomꢀposition ꢀof 2-PPN, 3-PPN and 4-PPN
ꢀ
begins at about 427 C, 426 C and 393 C, respectively.
The results of the TGA indicate that the PPN deriva-
tives possess a high thermal stability, which may be an
advantage in OLED fabrication. The melting points (Tmꢂ
were observed at 217.5 ꢀC (2-PPN), 259.6 ꢀC (3-PPN) and
291.8 ꢀC (4-PPN).
Figure 4. FT-IR spectra of 2-PPN (top), 3-PPN (middle) and
4-PPN (bottom).
Figure 6. PL spectra of PPN derivative.
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Synthesis and OLED Characteristics of Isomeric Phosphine Oxides Containing Naphthoxazole Moiety
Kim et al.
Figure 8. EL spectra of OLED devices.
of 5.0 V, current density of 160 mA/cm2 and maximum
brightness of 440 cd/m2 at 13.0 V.
Although the OLED devices using the PPN deriva-
tives had a relatively low luminance efficiency, we
expect that the EL efficiency could be further improved
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Figure 7.
TGA (a) and DSC (b) curvesIoPf:th1e8P5P.N89d.e1riv0a1tiv.5es2. On: Tue, 07 Mar 2017 18:26:50
Copyright: American Scientific Publishers
3.3. Electroluminescent (EL) Properties
To illustrate the EL properties of the PPN derivatives, typi-
cal multilayer OLED devices were fabricated using 2-PPN,
3-PPN and 4-PPN as the emitting layer (Table I). Figure 8
shows the EL spectra of devices 1–4. The EL devices emit
green light with an emission maximum at 509, 498, 528
and 501 nm, respectively. The EL maximum shows a red-
shift when compared to the PL maximum. Thus, the blue
PL emission changed to a greenish-blue EL emission as
the difference between EL and PL is common for this kind
of chelate complex. All compounds also showed different
maximum values for the PL and EL due to differences in
the conjugation length of the PPN derivatives. Figure 9
displays the current density–voltage–luminance (I–V –L)
characteristics of the devices.
Device 1 uses 2-PPN as the emitter, and it exhibits a
turn-on voltage of 5.0 V, current density of 180 mA/cm2
and maximum brightness of 340 cd/m2 at 7.0 V. Device 2
uses 2-PPN as the emitter together with Alq3, and it shows
a turn-on voltage of 6.0 V, current density of 240 mA/cm2
and maximum brightness of 480 cd/m2 at 9.0 V. Similarly,
Device 3 uses 3-PPN as the emitter and shows a turn-on
voltage of 10.0 V, current density of 44.65 mA/cm2 and
maximum brightness of 62.11 cd/m2 at 20.0 V. Device 4
uses 4-PPN as the emitter and shows a turn-on voltage
Figure 9. Current density versus applied voltage (a) and luminance ver-
sus applied voltage (b) of the EL devices.
3302
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Kim et al.
Synthesis and OLED Characteristics of Isomeric Phosphine Oxides Containing Naphthoxazole Moiety
by searching for other buffer layers with the appro-
priate compounds in order to obtain high-efficiency
devices.
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We investigated the properties of OLED devices using
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Acknowledgment: This paper was supported by a
research grant from Inje University in 2016.
Received: 5 January 2016. Accepted: 26 August 2016.
Delivered by Ingenta to: State University of New York at Binghamton
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Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 17, 3298–3303, 2017
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