Molecular design of modifying 4-position of dibenzofuran for high temperature stability and high efficiency

Article abstract of DOI:10.1016/j.dyepig.2016.01.018

A molecular design modifying 4- position of dibenzofuran with a pyridoindole moiety for high temperature stability and high efficiency was examined as an approach to develop host materials for blue phosphorescent organic light-emitting diodes. The simple pyridoindole modification of 4- position of carbazole substituted dibenzofuran lead to high glass transition temperature above 130°C in addition to high quantum efficiency in the blue phosphorescent device.

Full text of DOI:10.1016/j.dyepig.2016.01.018

Accepted Manuscript
Molecular design of modifying 4- position of dibenzofuran for high temperature
stability and high efficiency
Yirang Im, Jun Yeob Lee
PII:
S0143-7208(16)00029-2
10.1016/j.dyepig.2016.01.018
DYPI 5071
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 14 December 2015
Revised Date: 8 January 2016
Accepted Date: 18 January 2016
Please cite this article as: Im Y, Lee JY, Molecular design of modifying 4- position of dibenzofuran
for high temperature stability and high efficiency, Dyes and Pigments (2016), doi: 10.1016/
j.dyepig.2016.01.018.
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ACCEPTED MANUSCRIPT
Molecular design of modifying 4- position of dibenzofuran for high
temperature stability and high efficiency
Yirang Im, Jun Yeob Leea)
School of chemical and Engineering, Sunkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 16419, Korea
Abstract
A molecular design modifying 4- position of dibenzofuran with a pyridoindole
moiety for high temperature stability and high efficiency was examined as an approach
to develop host materials for blue phosphorescent organic light-emitting diodes. The
simple pyridoindole modification of 4- position of carbazole substituted dibenzofuran
o
lead to high glass transition temperature above 130 C in addition to high quantum
efficiency in the blue phosphorescent device.
Key words : dibenzofuran, thermal stability, pyridoindole, blue device
a) To whom correspondence should be addressed
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E-mail : leej17@skku.edu
Tel : 82-31-299-4716 Fax:82-31-299-4716
2

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Introduction
The development of blue phosphorescent organic light-emitting diodes (PHOLEDs) is
a critical issue in the field of organic light-emitting diodes (OLEDs) because blue
PHOLEDs can solve the problem of high power consumption of OLED panels[1-3]. It
was proven that the blue PHOLEDs would increase the current efficiency of blue device
experimentally by more than three times, which motivated researchers to search and
develop better blue PHOLEDs by material and device design[4-8].
Although it is essential to find and apply an optimized device structure, optimized host
and dopant materials are even more important to reach the device performance level
satisfying the specification of blue PHOLEDs. One of key parameters to be considered
is thermal stability of the devices which is related with glass transition temperature (Tg)
o
of host materials. High Tg above 120 C is needed to satisfy the high temperature
criteria of the OLED devices. Therefore, thermally stable host materials should be
employed in the emitting layer of blue PHOLEDs[8-11]. Other than the temperature
stability, the host material should have a design to reach high efficiency by balancing
carriers[12-15].
In this work, a molecular design to increase the Tg of the host materials was
approached by modification of 4- position of dibenzofuran with a pyridoindole moiety.
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The 4- position modification approach using the pyridoindole moiety increased the Tg
o
of the carbazole substituted dibenzofuran host materials above 130 C and allowed
bipolar charge transport characteristics. Moreover, the quantum efficiency (QE) values
of the blue PHOLEDs having the high Tg host material were as high as 19.1% and
22.7% by doping phenylpyridine type and phenylimidazole type blue triplet emitters.
Experimental Section
General information
General analysis method of the synthesized compounds is described in previous
work[16,17]. 9H-Carbazole (Cz), periodic acid, copper iodide (CuI), trans-1,2-
diaminocyclohexane (1,2-TCH), and n-butyl lithium (n-BuLi) (Aldrich Chem. Co.),
dibenzofuran and diiodoethane (TCI. Co.), 9H-pyrido[2,3-b]indole (α-carboline, α-Cb)
(P&H tech Co.), sulfuric acid (H2SO4), acetic acid, potassium phosphate (K3PO4), and
1,4-dioxane (Duksan Sci. Co.), iodine (I2) (Samchun Chem. Co.) were also used as
received.
Synthesis
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2-Iododibenzofuran
(1),
9-(dibenzofuran-2-yl)-9H-carbazole
(2),
9-(4-
iododibenzofuran-2-yl)-9H-carbazole (3), 2,8-diiododibenzo[b,d]furan (4), 2,8-di(9H-
carbazol-9-yl)dibenzo[b,d]furan (5), 9-(2-(9H-carbazol-9-yl)dibenzo[b,d]furan-4-yl)-
9H-pyrido[2,3-b]indole (CzDBFCb), 9,9'-(4-iododibenzo[b,d]furan-2,8-diyl)bis(9H-
carbazole) (6), and 9-(2,8-di(9H-carbazol-9-yl)dibenzo[b,d]furan-4-yl)-9H-pyrido[2,3-
b]indole (BCzDBFCb) are synthesized according to previous literature[16,17].
9-(2-(9H-carbazol-9-yl)dibenzo[b,d]furan-4-yl)-9H-pyrido[2,3-b]indole (CzDBFCb)
Compound 3 (0.72g, 1.57mmol), α-carboline (0.37 g, 2.19 mmol), K3PO4 (0.83 g, 3.91
mmol) and CuI (0.21 g, 1.09 mmol) were dissolved in 1,4-dioxane. After nitrogen
bubbling, 1,2-TCH (0.13 ml, 1.09 mmol) was added to the solution. The mixture was
refluxed overnight under nitrogen atmosphere and then cooled down to room
temperature. The mixture was filtered and the solution was extracted with methylene
chloride and distilled water. The organic layer was dried over anhydrous MgSO4 and
evaporated using a rotary evaporator. The product was purified by column
chromatography using methylene chloride/n-hexane as an eluent. A white powder was
obtained by vacuum train sublimation (0.28 g, yield: 36%).
1H NMR (400 MHz, CDCl3): δ 8.52 (d, 1H, J = 6.4 Hz), 8.46 (d, 1H, J = 9.2 Hz), 8.31
(d, 1H, J = 2.0 Hz), 8.21-8.18 (m, 3H), 8.04 (d, 1H, J = 8.0 Hz), 8.00 (d, 1H, J = 2.0 Hz),
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7.70 (s, 2H), 7.55-7.29 (m, 11H) 13C NMR (100 MHz, CDCl3): δ 157.08, 152.17,
150.42, 146.77, 141.35, 139.99, 133.39, 128.62, 128.49, 127.89, 127.30, 126.26, 126.14,
126.63, 124.00, 123.63, 123.59, 121.89, 121.44, 121.33, 121.23, 120.52, 120.29, 119.05,
116.98, 116.79, 112.48, 111.08, 110.05. MS (FAB) m/z 500 [(M+H)+]. Elemental
Analysis (calculated for C35H21N3O) : C, 84.15; H, 4.24; N, 8.41; O, 3.20. Found : C,
84.22; H, 4.24; N, 8.46; O, 3.27.
9-(2,8-di(9H-carbazol-9-yl)dibenzo[b,d]furan-4-yl)-9H-pyrido[2,3-b]indole
(BCzDBFCb)
Compound 6 (1g, 1.60 mmol), α-carboline (0.32 g, 1.92 mmol), K3PO4 (0.68 g, 3.20
mmol) and CuI (0.15 g, 0.80 mmol) were dissolved in 1,4-dioxane. After nitrogen
bubbling, 1,2-TCH (0.10 ml, 0.80 mmol) was added to the solution. The mixture was
refluxed overnight under nitrogen atmosphere and then cooled down to room
temperature. The mixture was filtered, diluted with methylene chloride, and washed
whith distilled water. The organic layer was dried over anhydrous MgSO4 and
evaporated using a rotary evaporator. The product was purified by column
chromatography using methylene chloride/n-hexane as an eluent. A white powder was
obtained by vacuum train sublimation (0.31 g, yield: 29%).
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1H NMR (400 MHz, CDCl3): δ 8.54 (d, 1H, J = 6.4 Hz), 8.48 (d, 1H, J = 9.2 Hz), 8.30
(d, 1H, J = 2.4 Hz), 8.23-8.15 (m, 7H), 8.07 (d, 1H, J = 2.0 Hz), 7.70 (m, 4H), 7.56 (t,
13
1H, J = 7.8 Hz), 7.50-7.29 (m, 12H) C NMR (100 MHz, CDCl3): δ 155.84, 152.10,
151.11, 146.78, 141.55, 141.20, 139.87, 133.82, 133.56, 128.69, 127.93, 127.37, 126.79,
126.30, 126.20, 125.52, 123.62, 123.45, 121.46, 121.32, 120.52, 120.39, 120.30, 120.17,
119.19, 116.99, 116.91, 113.75, 110.94, 109.90, 109.64. MS (FAB) m/z 665 [(M+H)+].
Elemental Analysis (calculated for C47H28N4O) : C, 84.92; H, 4.25; N, 8.43; O, 2.41.
Found : C, 84.93; H, 4.24; N, 8.43; O, 2.42.
Device fabrication and measurements
The device structure of blue PHOLEDs was indium tin oxide (ITO, 50
nm)/polyethylene-3,4-dioxythiophene:poly-styrenesulfonate (PEDOT:PSS) (60 nm)/1,
1′-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) (20 nm)/N,N-dicarbazolyl-3,5-
benzene (mCP) (10 nm)/ CzDBFCb or BCzDBFCb : iridium (III) bis[2-(4,6-
difluorophenyl)-pyridinato-N,C2]picolinate (FIrpic) (25 nm, 5%)/diphenylphosphine
oxide-4-(triphenylsilyl)phenyl (TSP01) (35 nm)/LiF (1 nm)/Al (100 nm). Single carrier
device fabrication and device characterization were performed according to the method
reported in the literature[18].
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Results and discussion
A dibenzofuran core has been known to be activated at 2- position by halogenation
and 4- position by lithiation[19-22]. The 2- position or 4- position activation followed
by substitution with hole or electron transport moieties produced hole or electron or
bipolar type compounds. Hole transport type substituent was mostly carbazole and
electron transport type substituent was typically diphenylphosphine oxide. However, the
diphenylphosphine oxide derived host materials have an issue of poor stability because
of low dissociation energy of C-P bond[23-24]. Therefore, a molecular design route to
stabilize the molecular structure, to obtain bipolar charge carrying property, and to
secure high temperature stability is needed. The molecular stability and bipolar charge
transport character requirements can be met by using a pyridoindole moiety and the
high temperature stability can be satisfied by substituting the pyridoindole moiety at 4-
position of dibenzofuran as reported in other articles[22,25-27]. Based on this concept,
two compounds, CzDBFCb and BCzDBFCb, were designed, simulated, synthesized,
characterized, and evaluated as the host materials of blue PHOLEDs.
Reaction scheme of CzDBFCb and BCzDBFCb is shown in Scheme 1. Carbazole
substituted dibenzofuran intermediates were prepared and they were lithiated, iodinated,
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and coupled with a pyridoindole moiety via 9- position of pyridoindole. Wet and dry
purification by column chromatography and sublimation provided highly pure
CzDBFCb and BCzDBFCb.
In the case of the host materials, photophysical properties such as photoluminescence
(PL) emission spectrum and triplet energy need to be characterized as they are directly
related with light emission of dopant materials. Fluorescence and phosphorescence
spectra of the host materials are presented in Figure 1. Fluorescent emission peaks of
CzDBFCb and BCzDBFCb were positioned at 380 nm and 394 nm, and
phosphorescent emission peaks were detected at 428 nm and 429 nm, respectively. The
conversion of the emission wavelength into energy offered triplet energy of 2.89 eV in
the two compounds. The triplet energy was similar in the two compounds because the
degree of conjugation was not increased by the additional carbazole unit of BCzDBFCb.
The long wavelength shift of the fluorescent emission peak in the BCzDBFCb host is
caused by weak charge transfer character of the BCzDBFCb by two electron rich
carbazole units and relatively electron poor dibenzofuran unit. Although there was long
wavelength shift of the emission peak, the PL emission wavelength range of CzDBFCb
and BCzDBFCb was well overlapped with absorption wavelength range of blue
emitting FIrpic dopant. Ultraviolet-visible (UV-Vis) spectra were similar in the two host
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materials because the building units of the two host materials were carbazole,
dibenzofuran, and pyridoindole except for the slight red-shift of absorption edge by two
carbazole units.
Electrochemical characterization of the host materials was conducted using cyclic
voltammetry (CV). Figure 2 represents electrochemical oxidation and reduction
behavior of CzDBFCb and BCzDBFCb. The oxidation behavior is dominated by
electron rich group of the host materials and the reduction behavior is dictated by
electron poor group of the host materials. The electron rich unit is carbazole and the
electron poor unit is dibenzofuran both in the CzDBFCb and BCzDBFCb as presented
in molecular orbital calculation results in Figure 3. Therefore, the oxidation and
reduction of the CzDBFCb and BCzDBFCb were similar, which lead to similar
ionization potential (IP) and electron affinity (EA) in the two host materials. The IP/EAs
of CzDBFCb and BCzDBFCb were -5.78/-3.16 eV and -5.63/-3.27 eV, respectively.
The HOMO was destabilized by the two carbazole units in the BCzDBFCb and the
HOMO was stabilized by small UV-Vis gap. The IP-EA gaps of CzDBFCb and
BCzDBFCb were 2.62 eV and 2.36 eV, respectively.
One of objectives of designing the CzDBFCb and BCzDBFCb host materials was to
increase the Tg by substitution of pyridoindole at 4- position of dibenzofuran. The Tg
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data obtained by differential scanning calorimeter scan of host materials are shown in
Figure 4. The Tgs of CzDBFCb and BCzDBFCb determined from second heating scan
o
of the host materials were 130 and 182 C, respectively. The substitution of the
pyridoindole moiety at 4- position of dibenzofuran played a crucial role of increasing
the Tg of CzDBFCb and BCzDBFCb through restricted segmental motion of the
pyridoindole moiety. In other works, the 4- position substitution of dibenzofuran
increased the Tg and thermal stability of the host materials although the substituent was
different[26,27]. The 4- position substitution was superior to 2- position substitution in
terms of thermal stability. Similar results were observed in the CzDBFCb and
BCzDBFCb.
Fundamentally, the CzDBFCb and BCzDBFCb host materials showed high triplet
o
energy of 2.89 eV, high Tg above 130 C, and fluorescence emission between 350 nm
and 450 nm for energy transfer to blue emitting FIrpic, which satisfied basic
requirements of host materials of blue PHOLEDs. Before device development, single
carrier device performances of CzDBFCb and BCzDBFCb were tested using hole only
and electron only devices. Hole and electron current densities of the CzDBFCb and
BCzDBFCb devices are displayed in Figure 5. Although the hole current density of
CzDBFCb was slightly higher than that of BCzDBFCb, both CzDBFCb and
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BCzDBFCb exhibited similar hole and electron current densities in the single carrier
devices, which demonstrates that they have similar hole and electron carrying capability.
The carbazole was a main hole carrying moiety and dibenzofuran assisted by
pyridoindole was a main electron carrying moiety in the CzDBFCb and BCzDBFCb.
Blue PHOLEDs were grown using the CzDBFCb:FIrpic and BCzDBFCb:FIrpic
emitting layers and device data of the blue PHOLEDs were plotted in Figure 6. The
CzDBFCb and BCzDBFCb devices behaved similarly in terms of current density (J),
luminance (L) and voltage (V) of the devices. As demonstrated in the single carrier
device data, the similarity of the hole and electron current densities of the CzDBFCb
and BCzDBFCb devices lead to the similar J, V and L performances. However, the QEs
of the CzDBFCb and BCzDBFCb blue PHOLEDs were quite different (Figure 7). The
CzDBFCb blue PHOLEDs performed much better than the BCzDBFCb and achieved
high QE of 19.1% relative to 12.4% of the BCzDBFCb device. In addition to the high
QE, the CzDBFCb host also stabilized the high QE even at high luminance, resulting in
high QE of 17.4% at 1,000 cd/m2. The higher QE of the CzDBFCb device than that of
the BCzDBFCb device is due to improved carrier balance associated with slightly
higher hole current density and lower electron current density of CzDBFCb than those
of BCzDBFCb.
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Emission spectra of the CzDBFCb and BCzDBFCb devices in Figure 8 were quite
similar and the color coordinates of (0.15,0.30) and (0.15,0.31) were observed. Effective
energy transfer and charge confinement produced the typical FIrpic emission spectra
from the CzDBFCb and BCzDBFCb devices.
The
two
host
materials
were
further
doped
with
tris[1-(2,4-
diisopropyldibenzo[b,d]furan-3-yl)-2-phenylimidazole]
(Ir(dbi)3)
phosphorescent
emitter and QE-L data are presented in Figure 9. QEs of the CzDBFCb:Ir(dbi)3 and
BCzDBFCb:Ir(dbi)3 devices were 22.7% and 19.2%, respectively. The CzDBFCb host
still performed better than the BCzDBFCb host in the Ir(dbi)3 doped blue devices, but
the QE gap between the two host materials was not as large as that of the FIrpic doped
devices by hole trapping effect of Ir(dbi)3.
Conclusions
Development of two pyridoindole modified dibenzofuran derivatives at 4-position,
CzDBFCb and BCzDBFCb, as thermally stable host materials for blue PHOLEDs
proposed that CzDBFCb would be suitable as the host materials of blue PHOLEDs by
providing high QE of 19.1% in the FIrpic device, 22.7% in the Ir(dbi)3 device, and high
Tg of 130 oC. Therefore, the approach of developing blue host materials by modifying 4-
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position of dibenzofuran was confirmed to be successful to guarantee good thermal
stability and high QE.
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List of figures
Scheme 1. Synthetic scheme of CzDBFCb and BCzDBFCb.
Figure 1. UV-vis absorption, solution PL and low temperature PL spectra of CzDBFCb
and BCzDBFCb.
Figure 2. Oxidation and reduction curves of CzDBFCb and BCzDBFCb.
Figure 3. HOMO and LUMO distributions of CzDBFCb and BCzDBFCb.
Figure 4. DSC curves CzDBFCb and BCzDBFCb.
Figure 5. Current density-voltage curves of the hole only and electron only devices of
CzDBFCb and BCzDBFCb.
Figure 6. Current density-voltage-luminance characteristics of CzDBFCb and
BCzDBFCb devices doped with FIrpic.
Figure 7. External quantum efficiency-luminance plots of CzDBFCb and BCzDBFCb
devices doped with FIrpic.
Figure 8. Electroluminescence spectra of CzDBFCb and BCzDBFCb devices doped
with FIrpic.
Figure 9. External quantum efficiency-luminance plots of CzDBFCb and BCzDBFCb
devices doped with Ir(dbi)3.
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Scheme 1
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1.2
1
CzDBFCb (UV-Vis)
BCzDBFCb (UV-Vis)
CzDBFCb (Solution PL)
BCzDBFCb (Solution PL)
CzDBFCb (Low Temp PL)
BCzDBFCb (Low Temp PL)
0.8
0.6
0.4
0.2
0
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Figure 1
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CzDBFCb
BCzDBFCb
-4
-3
-2
-1
0
1
2
3
Voltage (V)
Figure 2
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CzDBFCb
BCzDBFCb
LUMO
HOMO
Figure 3
24

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CzDBFCb
BCzDBFCb
50
100
150
200
250
300
Temperature (oC)
Figure 4
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12
10
8
Hole only (CzDBFCb)
Hole only (BCzDBFCb)
Electron only (CzDBFCb)
Electron only (BCzDBFCb)
6
4
2
0
0
2
4
6
8
10
Voltage (V)
Figure 5
26

ACCEPTED MANUSCRIPT
40
35
30
25
20
15
10
5
10000
1000
100
10
CzDBFCb
BCzDBFCb
0
1
0
2
4
6
8
10
Voltage (V)
Figure 6
27

ACCEPTED MANUSCRIPT
25
20
15
10
5
CzDBFCb
BCzDBFCb
0
1
10
100
1000
10000
Luminance (cd/m2)
Figure 7
28

ACCEPTED MANUSCRIPT
CzDBFCb
BCzDBFCb
380
480
580
680
780
Wavelength (nm)
Figure 8
29