Molecular design approach of increasing the triplet energy of host materials using pyrrole as a core structure

Article abstract of DOI:10.1039/c5ra17469d

We developed and synthesized 9,9′-(1-phenyl-1H-pyrrole-2,5-diyl)bis(9H-carbazole) (PPyCz2) as a hole transport type high triplet energy host material for blue phosphorescent organic light emitting diodes. We approached the molecular design by inserting a pyrrole moiety to prevent extension of conjugation of molecule and achieved a high triplet energy of 2.99 eV in the PPyCz2 host. Furthermore, the PPyCz2 host showed a high external quantum efficiency of 14.8% in blue phosphorescent organic light emitting diodes.

Full text of DOI:10.1039/c5ra17469d

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Molecular design approach of increasing the triplet
energy of host materials using pyrrole as a core
structure†
Cite this: RSC Adv., 2015, 5, 100378
a
b
b
*
Mounggon Kim, Sang Kyu Jeon and Jun Yeob Lee
We developed and synthesized 9,90-(1-phenyl-1H-pyrrole-2,5-diyl)bis(9H-carbazole) (PPyCz2) as a hole
transport type high triplet energy host material for blue phosphorescent organic light emitting diodes.
We approached the molecular design by inserting a pyrrole moiety to prevent extension of conjugation
of molecule and achieved a high triplet energy of 2.99 eV in the PPyCz2 host. Furthermore, the PPyCz2
host showed a high external quantum efficiency of 14.8% in blue phosphorescent organic light emitting
diodes.
Received 28th August 2015
Accepted 15th November 2015
DOI: 10.1039/c5ra17469d
www.rsc.org/advances
of conjugation have been conducted, which were introduction of
tetrahedral geometry based silane,23,24 phosphine oxide25,26 or sp3
carbon,27,28 control of connecting position in aromatic rings,29
and distortion of the backbone structure by steric hindrance.30
These methods could maintain the triplet energy of the core
structures, but the tetrahedral geometry weakens the chemical
bond between the cores and substituents. Therefore, a molecular
design approach to increase the triplet energy of the molecular
structure without tetrahedral linkage would be desirable to
develop the high triplet energy host materials.
Herein, we approached the design of high triplet energy host
material by introducing a pyrrole core between carbazoles and
investigated blue phosphorescent OLEDs fabrication using the
new hole transport type pyrrole based host material. We
demonstrated a high triplet energy of 2.99 eV and EQE of 14.8%
in blue phosphorescent OLEDs using a 9,90-(1-phenyl-1H-
pyrrole-2,5-diyl)bis(9H-carbazole) (PPyCz2) host material
without any tetrahedral geometry based linkage.
Introduction
Phosphorescent organic light emitting diodes (OLEDs) have
emerged as high performance OLEDs because of their high
external quantum efficiency (EQE) in recent years.1–6 Phospho-
rescent emitting materials are key components of the high
efficiency phosphorescent OLEDs and they were generally
added to the host material, which called a host–guest system.7–10
In this system, the host material can prevent the concentration
quenching of dopant material, efficiently transfer energy to
dopant, and eventually improve device performances. For these
reasons, a main focus of OLED research is to develop a host
material having high triplet energy to archive high perfor-
mances in OLEDs.
High triplet energy host materials have been developed for
their distinct purpose to prevent back energy transfer from host
to dopant for high efficiency in OLEDs. Especially, it is important
to apply the high triplet energy host material in blue phospho-
rescent devices compared to red or green devices because blue
phosphorescent dopants have a high triplet energy.
To design the high triplet host materials, it is necessary to use
a high triplet energy core structure. There are several hole
transport type cores having the high triplet energy such as
carbazole11–17 acridine,18,19 and dibenzothiophene.20–22 Although
all of them have high triplet energy, the host materials derived
from the cores possessed lower triplet energy than the cores
because of extension of conjugation by the substituents of the
cores. Therefore, molecular design studies limiting the extension
Experimental
General information
1-Phenylpyrrole, n-bromosuccinimide, carbazole, (ꢀ)-trans-1,2-
diaminocyclohexane, and copper iodide (CuI) were purchased
from Sigma Aldrich. Co., and potassium phosphate (K3PO4),
1,4-dioxane, and tetrahydro furan (THF) were obtained from
Duksan Sci. Co.
Synthesis
aDepartment of Polymer Science and Engineering, Dankook University, 152, Jukjeon-ro,
Suji-gu, Yongin-si, Gyeonggi-do, 448-701, Korea. Fax: +82-31-299-4716
bSchool of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-
gu, Suwon-si, Gyeonggi-do, 440-746, Korea. E-mail: leej17@skku.edu
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra17469d
2,5-Dibromo-1-phenyl-1H-pyrrole. 1-Phenylpyrrole (0.50 g,
3.49 mmol) was dissolved in THF (20 ml) and the solution was
cooled at ꢁ78 ꢂC. Aer 30 min, n-bromosuccinimide (1.24 g, 6.98
mmol) was added to the solution and the solution was stirred for
30 min. The color of the solution was turned into green and the
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ꢂ
temperature of the solution was raised up to ꢁ10 C. Aer stir-
ring for 10 h, the solution was extracted with methylene chloride
and distilled water. The methylene chloride was evaporated, and
a residual solid was puried by column chromatography with
hexane. The product was obtained as a solid (0.95 g, yield 91%).
1H NMR (400 MHz, DMSO): d 7.58–7.53 (m, 3H), 7.31–7.27 (m,
2H), 6.44 (s, 2H). MS m/z 302 [(M + H)+].
9,90-(1-Phenyl-1H-pyrrole-2,5-diyl)bis(9H-carbazole) (PPyCz2).
2,5-Dibromo-1-phenyl-1H-pyrrole (0.80 g, 2.66 mmol), 9H-carba-
zole (1.02 g, 6.11 mmol), K3PO4 (2.82 g, 13.29 mmol), and CuI
(0.30 g, 1.56 mmol) were dissolved in 1,4-dioxane (30 ml). And
(ꢀ)-trans-1,2-diaminocyclohexane (0.18 g, 1.56 mmol) was added
in the solution, and the solution was reuxed overnight. The
solution was ltered, extracted with ethyl acetate, and puried by
column chromatography. The nal product was further puried
by sublimation and was obtained as a white powder (0.24 g, 19%).
1H NMR (400 MHz, DMSO): d 8.10 (d, 2H, J ¼ 7.2 Hz), 7.49–7.42
(m, 8H), 7.24 (t, 4H, J ¼ 16 Hz), 6.91 (d, 2H, J ¼ 7.2 Hz), 6.78–7.72
(m, 5H). 13C NMR (100 MHz, DMSO): d 142.0, 134.6, 128.4, 128.2,
127.6, 126.4, 125.8, 124.2, 122.4, 120.3, 110.2, 107.5. MS m/z 474
[(M + H)+].
Device fabrication
Hole and electron only device structures of PPyCz2 were indium
tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene
sulfonate (PEDOT:PSS, 60 nm)/1,1-bis[(di-4-tolylamino)phenyl]
cyclohexane (TAPC, 20 nm)/N,N0-dicarbazolyl-3,5-benzene (mCP,
10 nm)/PPyCz2 (25 nm)/TAPC (5 nm)/Al (200 nm) and ITO/
PEDOT:PSS (60 nm)/diphenyl-4-triphenylsilylphenyl-phosphine
oxide (TSPO1, 10 nm)/PPyCz2 (25 nm)/TSPO1 (5 nm)/1,3,5-
tris(N-phenylbenzimidizol-2-yl)benzene (TPBi, 30 nm)/LiF (1
nm)/Al (200 nm), respectively. ITO/PEDOT:PSS/TAPC/mCP/
PPyCz2:FIrpic (bis[2-(4,6-diuorophenyl)pyridinato-C2,N](picoli-
nato)iridium(III))/TSPO1/TPBi/LIF/Al was the blue device
conguration.
Fig. 1 The calculated Gaussian data of phenyl pyrrole and phenyl
carbazole.
than 3.18 eV of PCz, proposing that the PPy core is appropriate
as the core of the high triplet energy material. As can be easily
predicted from the chemical structure, the absence of two
phenyl units dispersing the aromatic character of carbazole
increased the triplet energy of PPy.
The carbazole modied target molecule, PPyCz2, was also
compared with carbazole modied carbazole compounds, 90-
phenyl-90H-9,20:70,900-tercarbazole (27PTCz), and 90-phenyl-90H-
9,30:60,900-tercarbazole (36PTCz), by molecular calculation
Measurements
The ultraviolet-visible (UV-vis) spectra were measured by SCINCO
S-3100 UV-vis spectrophotometer and photoluminescence (PL)
spectra were analyzed by HITACHI F-7000 uorescence spec-
trometer. The cyclic voltammetry (CV) data were measured by
IVIUM B09054. The 1H and 13C nuclear magnetic resonance
(NMR) spectra were recorded by JEOL JNM-LA400 spectrometer.
Fabricated devices were electrically analyzed by Keithley 2400
source measurement unit and light emission performances were
measured by CS2000 spectroradiometer.
Results and discussion
The molecular design of the high triplet energy material was
carried out using a pyrrole moiety. The feasibility of the pyrrole
moiety as the core of the high triplet energy material was esti-
mated by calculated Gaussian data of phenylpyrrole (PPy) and
phenylcarbazole (PCz) in Fig. 1. A basic set of B3LYP/G-31G* of
Gaussian 09 soware was used for the calculation. The calcu-
lated triplet energy of PPy was 3.58 eV and was much higher Fig. 2 The calculated Gaussian data of PPyCz2, 27PTCz, and 36PTCz.
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summarized in Fig. 2. The calculated triplet energy of PPyCz2
(3.18 eV) was much higher than that of 27PTCz (2.89 eV) and
36PTCz (3.06 eV). The replacement of the PCz core of 27PTCz
and 36PTCz with the PPy core increased the triplet energy by
more than 0.1 eV by limiting the expansion of the conjugated
structure using the PPy core. Although two carbazole moieties
were directly attached to the PPy core without any tetrahedral
linkage or intentional distortion, calculated triplet energy of
PPyCz2 was higher than that of 27PTCz and 36PTCz.
Scheme 1 Synthetic scheme of PPyCz2.
The molecular orbital of PPyCz2 was further studied for deep
understanding of the triplet energy calculation results in Fig. 2.
Geometrical structure and the highest occupied molecular
orbital (HOMO)/the lowest unoccupied molecular orbital (LUMO)
distribution in Fig. 3 can give hints about the origin of the high
triplet energy of PPyCz2. One main reason is the dihedral angle
(79ꢂ) between pyrrole and carbazole suppressing the conjugation
between the two moieties. The substitution of two carbazole
moieties at 2,5-positions of PPy renders the carbazole moieties
and phenyl unit of PPy to repel each other because of steric
hindrance by hydrogen of carbazole and phenyl unit of PPy.
Therefore, the triplet energy of PPyCz2 was increased due to the
limitation of conjugation. The little HOMO and LUMO disper-
sion on the PPy is also an output of the disconnected conjuga-
tion. Both the HOMO and LUMO were mostly located on
carbazole due to the good electron donating and accepting
nature of the carbazole moiety by aromatic character of carba-
zole. From the molecular calculation results, it can be predicted
that the HOMO and LUMO levels of the PPyCz2 would be similar
to those of common carbazole compounds. Therefore, the
PPyCz2 may have higher triplet energy than common carbazole
Fig. 4 UV and PL spectra of PPyCz2.
type host materials while keeping the HOMO and LUMO levels of
the carbazole compounds.
Scheme 1 shows synthetic scheme of PPyCz2. The compound
was easily synthesized by electrophilic halogenation and Ull-
mann coupling reaction as the high selectivity of electrophilic
bromination in PPy at 2,5-positions caused high yield of 91%.
Chemical identication results by NMR and mass analysis are
presented in Experimental section.
Material characterization results of PPyCz2 by UV-vis
absorption and PL emission are presented in Fig. 4. The p–p*
transition of PPyCz2 contributes to UV-vis absorption peaks
below 300 nm and n–p* transition of carbazole is involved in
the absorption peaks above 300 nm. The calculated edge energy
of the UV-vis spectra is 3.51 eV which can be an estimated UV-
gap of PPyCz2. Triplet energy of PPyCz2 from phosphorescent
emission peak by low temperature PL spectra is 2.99 eV, which
is higher than that of other carbazole type host materials
without any tetrahedral linkage or substituents for steric
hindrance. Peak position of the solution PL spectrum in toluene
was 364 nm.
Fig. 3 Molecular orbital distribution and dihedral angle of PPyCz2.
100380 | RSC Adv., 2015, 5, 100378–100383
Fig. 5 CV measurement data of PPyCz2.
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Fig. 8 Current density, voltage, and luminance curves of PPyCz2
devices.
Fig. 6 Hole and electron only device performance of PPyCz2.
CV was used to calculate the ionization potential (IP) and
electron affinity (EA) of PPyCz2 through onset point of oxidation
and reduction scans (Fig. 5). Oxidation and reduction potentials
of PPyCz2 were 1.28 V and ꢁ2.42 V through the CV scan. The
calculated IP and EA of PPyCz2 were ꢁ6.08 eV and ꢁ2.38 eV,
which were similar to those of common carbazole type host
materials. The CV results agree with the HOMO and LUMO
calculation results presented in Fig. 3. In the case of thermal
properties, differential scanning calorimeter scan of PPyCz2
was carried out, but glass transition temperature was not
observed as is presented in ESI.†
Hole and electron only device performances of PPyCz2 are
shown in Fig. 6. The hole current density of PPyCz2 was higher
than electron current density due to hole transport type char-
acteristic of PPyCz2. As the PPyCz2 host is composed of PPy and
carbazole which have hole carrying property, hole current
density of PPyCz2 was higher than electron current density.
The blue phosphorescent OLED fabrication was performed
using PPyCz2 as a host and FIrpic as a dopant with different
doing concentrations of 3%, 5%, and 10%. The device structure
can be schematized as shown in Fig. 7. The ITO as electrode,
PEDOT:PSS as a hole injection layer (60 nm), TAPC as a hole
transport layer (20 nm), FIrpic doped PPyCz2 as an emission
layer (30 nm), mCP (10 nm) and TSPO1 (5 nm) as exciton
blocking layers, TPBi as an electron transport layer (30 nm), LIF
as an electron injection layer (1 nm), and Al as an electrode
(200 nm) were used for the blue device conguration. The hole
injection might be efficient because of similar HOMO level of
Fig. 9 EQE-luminance data of PPyCz2:FIrpic device.
PPyCz2 and mCP and hole transport type character of PPyCz2.
Electron injection might also be efficient as there is an energy
barrier of only 0.12 eV for electron injection from TSPO1 to
PPyCz2.
Fig. 8 shows current density, voltage, and luminance curves
of the PPyCz2 blue devices. At high doping concentration, the
current density was increased because of electron hopping
behavior by FIrpic dopant as proved in other work. The shallow
LUMO levels of PPyCz2 induced electron trapping by the FIrpic
dopant, and the electron transport is assisted by electron
hopping phenomenon at high FIrpic concentration, resulting in
high current density at high doping concentration.
The current density data can be correlated with EQE-
luminance data in Fig. 9. The maximum EQE of the PPyCz2
devices was 13.9, 14.8, and 12.9% at 3%, 5% and 10% doping
concentrations, respectively. The high doping of FIrpic caused
concentration quenching of dopant and decreased the EQE at
10% doping. The moderate 5% doping was better than 10%
doping in terms of concentration quenching effect and was
better than 3% doping in terms of charge balance by better
electron transport. In particular, the high triplet energy of
PPyCz2 (2.99 eV) could be matched with that of FIrpic (2.65 eV)
for energy transfer and contributed to the EQE. However, rela-
tively low EQE compared to other EQE of FIrpic devices was
caused by low quantum yield of FIrpic doped PPyCz2. The
measured PL quantum yields of the FIrpic doped PPyCz2 lms
were 0.21, 0.27 and 0.19 at 3%, 5% and 10% doping
Fig. 7 Blue device structure of PPyCz2 device.
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Table 1 Summarized device performance data of PPyCz2
EQEa (%)
CEb (cd Aꢁ1
)
PEc (lm Wꢁ1
)
Doping concentration
[1000 cd]
Max
[1000 cd]
Max
[1000 cd]
Max
Color coordinate
3%
5%
10%
8.9
11.1
10.6
13.9
14.8
12.9
16.3
20.8
20.5
25.1
27.6
24.8
5.1
7.2
7.0
16.4
17.0
17.1
(0.15, 0.28)
(0.15, 0.29)
(0.15, 0.31)
a
EQE, external quantum efficiency. b CE, current efficiency. c PE, power efficiency.
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Fig. 10 Electroluminescence spectrum of PPyCz2.
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concentrations, respectively, which can explain the doping
concentration dependency of EQE. The current efficiency and
power efficiency data of the PPyCz2 devices were presented in
ESI.† The current efficiency and power efficiency data were also
optimized at 5% doping concentration. Detailed device perfor-
mance data are summarized in Table 1.
Electroluminescence (EL) spectra of blue devices are pre-
sented in Fig. 10. The color coordinates of PPyCz2 devices were
(0.15, 0.28), (0.15, 0.29), and (0.15, 0.31) at 3, 5, and 10% doping
concentrations, respectively. Furthermore, the energy transfer
from host to dopant might be efficient because other peaks are
not observed in EL spectra.
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Conclusions
A PPy based high triplet host material, PPyCz2, was designed
and synthesized using high triplet energy pyrrole core and
carbazole substituents. The high triplet energy of the PPy core
and distortion of the carbazole unit in PPyCz2 allowed high
triplet energy of 2.99 eV and high external quantum efficiency of
14.8% in blue phosphorescent OLEDs. Therefore, proper
modication of the pyrrole core can increase triplet energy of
the host material for blue phosphorescent OLEDs.
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