High-triplet-energy host materials derived from directly-coupled carbazole-pyridoindole moieties

Article abstract of DOI:10.1016/j.dyepig.2016.03.004

Directly-coupled carbazole-pyridoindole moieties containing host materials for green and blue phosphorescent organic light-emitting diodes were synthesized by linking the two moieties via two different positions on the carbazole-pyridoindole core structur

Full text of DOI:10.1016/j.dyepig.2016.03.004

Accepted Manuscript
High-triplet-energy host materials derived from directly-coupled carbazole-
pyridoindole moieties
Sung Yong Byeon, Jun Yeob Lee
PII:
S0143-7208(16)30063-8
10.1016/j.dyepig.2016.03.004
DYPI 5129
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 January 2016
Revised Date: 1 March 2016
Accepted Date: 3 March 2016
Please cite this article as: Byeon SY, Lee JY, High-triplet-energy host materials derived from directly-
coupled carbazole-pyridoindole moieties, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.03.004.
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High-triplet-energy host materials derived from directly-
coupled carbazole-pyridoindole moieties
Sung Yong Byeon and Jun Yeob Lee *
School of Chemical Engineering, Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea
E-mail: leej17@skku.edu
Tel : 82-31-299-4716
ABSTRACT: Directly-coupled carbazole-pyridoindole moieties containing host materials for green and
blue phosphorescent organic light-emitting diodes were synthesized by linking the two moieties via two
different positions on the carbazole-pyridoindole core structure. The carbazole-pyridoindole moieties
were functionalized using benzonitrile and phenyl substituent groups to introduce controlled-charge
transport properties. The host materials synthesized using the carbazole-pyridoindole moieties showed a
maximum quantum efficiency of 19.1% in green phosphorescent devices, and 18.2% in blue
phosphorescent devices.
Key words : carbazole, pyridoindole, host material, phosphorescent device, quantum efficiency,
substitution position
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Introduction
There have been tremendous efforts to obtain high efficiency in organic light-emitting diodes (OLEDs).
Among these, phosphorescent organic light-emitting diodes (PHOLEDs) have emerged as a key
technology in obtaining high efficiency[1-3]. PHOLEDs can approach approximately 100% internal
maximum quantum efficiency (QE) through intersystem crossing, while fluorescent organic light-
emitting diodes (FLOLEDs) can reach only 25% internal QE[4-5].
Although PHOLEDs are regarded as a promising technology in obtaining high efficiency, there are
several challenges that need to be overcome. The long exciton lifetime of PHOLEDs in comparison with
that of FLOLEDs may cause triplet-triplet annihilation (TTA) and result in a QE decrease at high
doping concentrations[6]. Therefore, it is necessary for emitting materials to be dispersed throughout the
host matrix in order to suppress the TTA process[7]. Further, especially for PHOLEDs, the triplet-
energy of the host materials is significant since the phosphorescence emission originates from triplet
excitons, and triplet exciton confinement can improve device performance[8-14].
The host materials for blue PHOLEDs are required to have higher triplet-energy than those for green
PHOLEDs. Therefore, the molecular design of the host materials for blue PHOLEDs proceeded using
high-triplet-energy moieties. Carbazole is a widely used hole-transporting moiety with a high-triplet-
energy and good thermal stability. Despite the fact that the carbazole moiety has excellent expandability,
the most common modification strategy is to use the 9-position via N-amination, and the 3-position via
halogenation[12-18]. Our group recently reported the 4-position of carbazole-based host materials to
increase the triplet-energy and dihedral angle in comparison with the 3-position[19].
Based on the results, we applied 2- and 4-position-modified carbazole in our work and developed
carbazole-pyridoindole derivatives[20]. The 2-position linking of the carbazole-pyridoindole derivatives
led to a relatively low triplet-energy of 2.63 eV compared with that of the 4-position connection of
carbazole-pyridoindole, and a maximum quantum efficiency of 19.1% in green PHOLEDs. The 4-
position of carbazole-pyridoindole-based host materials showed a high triplet-energy greater than 2.88
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eV, and a maximum quantum efficiency of 18.2% in blue PHOLEDs. It was demonstrated that the
carbazole-pyridoindole moiety can be diversely applied as the core structure of triplet host materials for
green and blue PHOLEDs by changing the linking position between carbazole and pyridoindole.
.
Experimental
General information
Potassium iodide, potassium iodate, copper iodide, (±)-trans-1,2-diaminocyclohexane, 3-
bromobenzonitrile, triethylborate, n-butyllithium, and bromobenezene were purchased from Aldrich
Chem. Co. Tetrakis(triphenylphosphane) palladium(0), 9H-pyrido[2,3-b]indole, and (9-phenyl-9H-
carbazol-2-yl)boronic acid were provided by P&H tech Co. Acetic acid, sodium bicarbonate, potassium
carbonate, and potassium phosphate tribasic from Duksan Sci. Co. were used without further
purification. Tetrahydrofuran (THF, Duksan Sci. Co.) was distilled with calcium hydride and sodium.
Chemical, electrochemical, and photophysical analysis were carried out using the instruments described
in our previous work[19].
Synthesis
The synthetic schemes of the 2CzCbCN, 2CzCbPh, 4CzCbCN, and 4CzCbPh compounds are described
in Scheme 1.
6-iodo-9H-pyrido[2,3-b]indole (1)
The synthetic procedure of 6-iodo-9H-pyrido[2,3-b]indole was the same as that reported in the
literature[20].
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4-bromo-9-phenyl-9H-carbazole
The synthetic procedure of 4-bromo-9-phenyl-9H-carbazole was described in recent literature[21].
(9-phenyl-9H-carbazol-4-yl)boronic acid
4-bromo-9-phenyl-9H-carbazole (0.50 g, 0.16 mmol) was vacuum dried for 3 h, dissolved in THF under
a nitrogen atmosphere, and stirred for 30 min at -30°C. n-buthyl lithium (0.12 g, 0.19 mmol) was slowly
injected into the solution, which was subsequently stirred for 1 h. Triethylborate (0.27 g, 0.19 mmol)
was added into the reaction mixture, and the solution was stirred for a further 12 h at room temperature.
2 M aqueous hydrochloric acid was poured into the solution, which was then stirred for 30 min. The
solution was neutralized with potassium carbonate and extracted using methylene chloride and distilled
water. The organic layer was dehydrated with magnesium sulfate and reprecipitated using n-hexane.
The mixture was filtered, and the precipitate was obtained as a white solid.
Yield 90% (0.40 g), 1H NMR (400 MHz, DMSO-d6): δ 8.55 (s, 2H), 8.50 (d, 1H, J=8.0 HZ), 7.70 (t, 2H,
J=7.8 HZ), 7.57 (t, 3H, 7.0 HZ), 7.38 (d, 2H, J=8.4 HZ), 7.34-7.31 (m, 2H), 7.26-7.22 (m, 2H) MS (ESI)
m/z 211.2 [(M + H)+].
6-(9-phenyl-9H-carbazol-2-yl)-9H-pyrido[2,3-b]indole
The synthetic procedure of 6-(9-phenyl-9H-carbazol-2-yl)-9H-pyrido[2,3-b]indole was the same as the
method listed in our previous work[20]. (9-Phenyl-9H-carbazol-2-yl)boronic acid was used instead of
(9-phenyl-9H-carbazol-3-yl)boronic acid.
1
Yield 56%, H NMR (400 MHz, DMSO): δ 11.81 (s, 1H), 8.58 (s, 1H, J=7.6 HZ), 8.47 (s, 1H), 8.39 (d,
1H, J=6.4 HZ), 8.31 (s, 1H, J=7.6 HZ), 8.24 (d, 1H, 7.2 HZ), 7.75 (d, 1H, J=8.8 HZ), 7.70-7.49 (m, 8H),
7.42-7.34 (m, 2H), 7.27 (t, 1H, J=7.4 HZ), 7.17 (t, 1H, J=6.4 HZ) MS (ESI) m/z 410.2 [(M + H)+].
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6-(9-phenyl-9H-carbazol-4-yl)-9H-pyrido[2,3-b]indole
The synthetic procedure of 6-(9-phenyl-9H-carbazol-4-yl)-9H-pyrido[2,3-b]indole was the same as the
method listed in our previous work[20]. (9-phenyl-9H-carbazol-4-yl)boronic acid was used instead of
(9-phenyl-9H-carbazol-3-yl)boronic acid.
1
Yield 80%, H NMR (400 MHz, DMSO): δ 11.99 (s, 1H), 8.56 (d, 1H, J=8.0 HZ), 8.47 (d, 1H, J=6.4
HZ), 8.41 (s, 1H), 7.75-7.65 (m, 6H), 7.59 (t, 1H, J=8.0 HZ), 7.51 (t, 1H, J=7.8 Hz), 7.42-7.31 (m, 4H),
7.24-7.17 (m, 2H), 6.99-6.94 (m, 1H) MS (ESI) m/z 410.2 [(M + H)+].
3-(6-(9-phenyl-9H-carbazol-2-yl)-9H-pyrido[2,3-b]indol-9-yl)benzonitrile (2CzCbCN)
The synthetic procedure of 3-(6-(9-phenyl-9H-carbazol-2-yl)-9H-pyrido[2,3-b]indol-9-yl)benzonitrile
was the same as that described in previous work[20]. 6-(9-phenyl-9H-carbazol-2-yl)-9H-pyrido[2,3-
b]indole was used instead of 6-(9-phenyl-9H-carbazol-3-yl)-9H-pyrido[2,3-b]indole.
Yield 72%, Tg 121oC, 1H NMR (500 MHz, DMSO): δ 8.79 (d, 1H, J=7.5 HZ), 8.65 (s, 1H), 8.46 (d, 1H,
J=6.0 HZ), 8.37 (s, 1H, J=8.0 HZ), 8.27 (t, 2H, J=7.8 HZ), 8.01 (d, 1H, J=8.5 Hz), 8.00 (d, 1H, J=8.0 Hz),
7.85 (t, 2H, J=7.8 HZ), 7.73-7.70 (m, 5H), 7.66-7.61 (m, 2H), 7.56 (t, 1H, J=8.5 HZ), 7.44 (t, 1H, J=7.8
HZ), 7.40-7.38 (m, 2H), 7.31 (t, 1H, 7.5 HZ) 13C NMR (100 MHz, DMSO): δ 151.3, 146.5, 140.8, 140.7,
139.0, 138.1, 136.8, 136.6, 134.6, 132.0, 131.1, 130.9, 130.5, 130.3, 129.6, 127.8, 126.8, 126.2, 122.5,
121.7, 121.3, 121.0, 120.6, 120.2, 120.1, 119.7, 118.2, 117.2, 116.1, 112.5, 110.7, 109.6, 107.6 MS
(ESI) m/z 511.1 [(M + H)+].
9-phenyl-6-(9-phenyl-9H-carbazol-2-yl)-9H-pyrido[2,3-b]indole (2CzCbPh)
The synthetic procedure of 2CzCbPh was the same as that described in previous work[20]. 6-(9-phenyl-
9H-carbazol-2-yl)-9H-pyrido[2,3-b]indole was used instead of 6-(9-phenyl-9H-carbazol-3-yl)-9H-
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pyrido[2,3-b]indole.
Yield 81%, Tg 111oC, 1H NMR (500 MHZ, DMSO): δ 8.77 (d, 1H, J=7.5 HZ), 8.65 (s, 1H), 8.45 (d, 1H,
J=6.5 HZ), 8.37 (d, 1H, J=8.5 HZ), 8.29 (d, 1H, J=7.5 HZ), 7.84 (d, 1H, J=8.5 HZ), 7.74-7.65 (m, 10H),
7.59-7.51 (m, 3H), 7.45 (t, 1H, J=7.5 HZ), 7.40-7.31 (m, 3H) 13C NMR (100 MHZ, DMSO): δ 151.6,
146.5, 140.8, 140.7, 139.2, 138.6, 136.8, 135.8, 134.1, 130.3, 129.5, 129.4, 127.8, 127.5, 127.1, 126.8,
126.7, 126.2, 122.6, 121.7, 121.0, 120.6, 120.2, 120.0, 119.7, 116.6, 115.8, 110.6, 109.6, 107.6 MS
(ESI) m/z 485.7 [(M + H)+].
3-(6-(9-phenyl-9H-carbazol-4-yl)-9H-pyrido[2,3-b]indol-9-yl)benzonitrile (4CzCbCN)
The synthetic procedure of 4CzCbCN was the same as described in previous work[20]. 6-(9-phenyl-9H-
carbazol-4-yl)-9H-pyrido[2,3-b]indole was used instead of 6-(9-phenyl-9H-carbazol-3-yl)-9H-
pyrido[2,3-b]indole.
Yield 80%, Tg 119oC, 1H NMR (400 MHz, DMSO): δ 8.72 (d, 1H, J=8.0 HZ), 8.57 (s, 1H), 8.51 (d, 1H,
J=6.4 HZ), 8.36 (s, 1H), 8.20 (d, 1H, J=8.0 HZ), 8.01 (d, 1H, J=8.4 Hz), 7.90 (t, 1H, J=8.0 Hz), 7.80-7.71
(m, 4H), 7.65 (d, 2H, J=8.4 Hz), 7.59 (t, 1H, J=7.8 Hz), 7.52 (t, 1H, J=7.8 HZ), 7.44-7.31 (m, 5H), 7.23
(d, 1H, J=7.6 HZ), 6.97 (t, 1H, J=8.0 HZ) 13C NMR (100 MHz, DMSO): δ 151.3, 146.6, 140.8, 140.6,
138.3, 137.3, 136.7, 136.7, 136.6, 133.8, 132.1, 131.0, 130.8, 130.6, 130.2, 129.3, 128.3, 127.9, 127.1,
125.9, 122.2, 121.9, 121.7, 121.6, 120.9, 120.1, 119.5, 118.1, 117.1, 116.0, 112.5, 110.2, 109.5, 108.5
MS (ESI) m/z 511.4 [(M + H)+].
9-phenyl-6-(9-phenyl-9H-carbazol-4-yl)-9H-pyrido[2,3-b]indole (4CzCbPh)
The synthetic procedure of 4CzCbPh was the same as that described in previous work[20]. 6-(9-phenyl-
9H-carbazol-4-yl)-9H-pyrido[2,3-b]indole was used instead of 6-(9-phenyl-9H-carbazol-3-yl)-9H-
pyrido[2,3-b]indole.
Yield 70%, Tg 106oC, 1H NMR (400 MHZ, DMSO): δ 8.68 (d, 1H, J=8.0 HZ), 8.55 (s, 1H), 8.48 (d, 1H,
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J=6.4 HZ), 7.79-7.62 (m, 10H), 7.60-7.48 (m, 3H), 7.43 (d, 1H, J=8.0 HZ), 7.36-7.30 (m, 4H), 7.22 (d,
1H, J=8.0 HZ), 6.96 (t, 1H, J=8.2 HZ) 13C NMR (100 MHZ, DMSO): δ 151.6, 146.5, 140.8, 140.6, 138.8,
137.4, 136.7, 135.8, 133.3, 130.1, 129.4, 129.1, 128.1, 127.9, 127.4, 127.2, 127.1, 125.9, 122.2, 121.9,
121.6, 120.6, 120.1, 119.5, 116.5, 115.7, 110.1, 109.4, 108.4 MS (ESI) m/z 485.8 [(M + H)+].
Device fabrication and measurements
The device structures of the hole-only and electron-only devices were ITO (50 nm)/PEDOT:PSS (60
nm)/TAPC (20 nm)/mCP (10 nm)/2CzCbCN or 2CzCbPh or 4CzCbCN or 4CzCbPh (25 nm)/TAPC (5
nm)/Al (200 nm) and ITO (50 nm)/PEDOT:PSS (60 nm)/TSPO1 (10 nm)/2CzCbCN or 2CzCbPh or
4CzCbCN or 4CzCbPh (25 nm)/TSPO1 (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (100 nm), where ITO,
PEDOT:PSS, TAPC, mCP, TSPO1, and TPBi are indium tin oxide, poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate),
methylphenyl)aniline], 1,3-bis(N-carbazolyl)benzene,
4,4’-cyclohexylidenebis[N,N-bis(4-
diphenylphosphine oxide-4-
(triphenylsilyl)phenyl, and 2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole). The device
structure of green phosphorescent devices was ITO (50 nm)/PEDOT:PSS (60 nm)/ TAPC (20 nm)/mCP
(10 nm)/2CzCbCN or 2CzCbPh:Ir(ppy)3 (25 nm, 3 %)/TSPO1 (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (200
nm), where Ir(ppy)3 is tris[2-phenylpyridinato-C2,N]iridium(III). The blue phosphorescent device
structure was ITO (50 nm)/PEDOT:PSS (60 nm)/ TAPC (20 nm)/mCP (10 nm)/4CzCbCN or 4CzCbPh
or 4CzCbTh:FIrpic (25 nm, 10 %)/TSPO1 (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (200 nm), where Firpic
is bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III).
Results and Discussion
We recently studied carbazole-pyridoindole-based host materials for blue phosphorescent OLEDs,
which consisted of 3-position-modified carbazole and 6-position-modified pyridoindole[20]. In that
report, carbazole-pyridoindole-based host materials showed a high triplet-energy and excellent device
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performance. Based on our previous study, we developed two different types of isomerized carbazole-
pyridoindole core moieties by interconnecting the 2-position of carbazole and the 6-position of
pyridoindole (2CzCb), and the 4-position of carbazole and the 6-position of pyridoindole (4CzCb). A
Suzuki coupling reaction was used to synthesize the newly developed carbazole-pyridoindole core
moieties, and each core moiety was modified with two different substituent groups, benzonitrile and
phenyl, to control the charge-transport properties and assign bipolar characteristics.
The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) distribution of the host materials were estimated using the Gaussian 09 program[22]. The
molecular structures of the host materials were optimized by the density functional theory using B3LYP
exchange functional with 6-31G* basis sets[23]. The HOMO and LUMO distribution of the host
materials is described in Figure 1. According to the calculation data, the HOMO of both 2CzCb and
4CzCb was located in the carbazole-pyridoindole core moiety. However, the HOMO of the 4CzCb
derivatives was mostly located in the phenylcarbazole moiety, with weak location in the pyridoindole
moiety, while the HOMO of the 2CzCb derivatives was strongly located in carbazole-pyridoindole core
moiety. This phenomenon may be due to the dihederal angle difference between carbazole and
pyridoindole, originating from the position of the carbazole moiety. Since the backbone structure of the
4CzCb core moiety was connected via the 4-position of carbazole, conjugation was broken by twisting
of the backbone structure, which resulted in electronic isolation of the carbazole and pyridoindole
moieties. Therefore, the HOMO was dominantly dispersed over the electron-rich carbazole moiety,
giving rise to a different HOMO distribution in the 4CzCb derivatives compared with the carbazole-
pyridoindole derivatives using 2-position-modified carbazole. In the case of LUMO, the LUMO of
2CzCbCN and 4CzCbCN was strongly located in the benzonitrile group due to its strong electron
deficiency, and the LUMO of 2CzCbPh and 4CzCbPh was located in the relatively weak electron-
deficient group of the pyridoindole moiety. From the calculated HOMO and LUMO distributions, it was
found that the HOMO of carbazole-pyridoindole derivatives was dominantly located in the carbazole-
pyridoindole core moiety, while the LUMO distribution varied as the electron-deficient groups changed.
Figure 2 shows the dihedral angle depending on the position of the carbazole moiety. As mentioned
above, HOMO and LUMO are closely related to the dihedral angle between carbazole and pyridoindole
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of the core moiety. The dihederal angle of 2CzCbPh and 4CzCbPh was compared using molecular
simulation. The results show that the dihedral angles of 2CzCbPh and 4CzCbPh were 38.3 and 57.2°,
respectively. 4CzCbPh showed a large dihedral angle due to the sterically-hindered linkage via the 4-
position of carbazole.
Cyclic voltammetry (CV) was used in order to estimate the HOMO and LUMO energy levels of the
host materials. Figure 3 describes the CV curves of the host materials. The HOMO and LUMO energy
levels of 2CzCbCN, 2CzCbPh, 4CzCbCN, and 4CzCbPh host materials were -5.96/-2.83, -5.92/-2.73, -
6.06/-2.89, and -6.04/-2.78, respectively. The HOMO levels of the host materials containing 2CzCb and
4CzCb derivatives were similar, showing little difference. The HOMO levels of 2CzCbCN and
2CzCbPh were approximately -5.95 eV, and the HOMO levels of 4CzCbCN and 4CzCbPh were
approximately -6.05 eV. From these results, the HOMO levels of the 4CzCb derivatives were somewhat
deeper than those of the 2CzCb derivatives. This can be explained by the fact that the dihedral angle
difference between the core moieties of 2CzCb and 4CzCb derivatives caused a difference in electron-
donating characteristics. The HOMO levels of the 2CzCb derivatives were approximately 0.1 eV deeper
than those of the 4CzCb derivatives, since the 2CzCb core moiety had a small dihedral angle and acted
as a strong donating moiety. However, due to the large dihedral angle, the carbazole and pyridoindole
moieties of the 4CzCb core structure could not act as a strong donating moiety, and instead act
separately. In the case of the LUMO, relatively deep LUMO levels were observed in both 2CzCbCN
and 4CzCbCN, due to the strongly electron-deficient benzonitrile moiety. The LUMO levels of
2CzCbPh and 4CzCbPh were similar to those predicted by the LUMO distribution. Overall tendencies
of the HOMO and LUMO were quite similar to that of our previous work[20]. The calculated and
measured HOMO/LUMO values are summarized in Table 1.
The photophysical properties were measured using ultraviolet-visible (UV-vis) and photoluminescence
(PL) spectrometers. Figure 4 shows the UV-vis absorption and PL spectra of the host materials. The
absorption energy gap of the materials was measured from the UV-vis edge, and 2CzCbCN, 2CzCbPh,
4CzCbCN, and 4CzCbPh showed almost the same energy gap of 3.40 eV. The triplet-energies of
2CzCbCN, 2CzCbPh, 4CzCbCN, and 4CzCbPh were 2.63, 2.64, 2.88, and 2.89 eV, respectively. The
host materials containing the 2CzCb core structure showed relatively small triplet-energies compared
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with those of host materials containing the 4CzCb core structure. This can be interpreted as the fact that
the small dihedral angle of the 2CzCb core moiety could not effectively confine the decrease in triplet-
energy. From the photophysical data, it was found that the triplet-energy of the host materials was
highly influenced by the core moiety, and this phenomenon can be interpreted as the contribution from
the linkage of the carbazole-pyridoindole core moiety. Although different substituents were adopted in
each host material, they did not have a great effect on the photophysical characteristics.
Figure 5 shows the thermograms of the host materials analyzed by differential scanning calorimetry in
order to detect glass transition temperatures (Tg). The measured Tgs of 2CzCbCN, 2CzCbPh, 4CzCbCN,
and 4CzCbPh were 121.0, 111.1, 119.1, and 106.6oC, respectively. A relatively high Tg was observed
for 2CzCbCN and 4CzCbCN, due to the strong intermolecular interaction by the highly polar CN units.
Even the 2CzCbPh and 4CzCbPh host materials, having the simplest structure in this report, exhibited a
high Tg, above 100oC, as a result of the rigidity of the carbazole-pyridoindole core.
Single carrier devices were fabricated to elucidate the charge-transport performance of the materials.
The data of the single carrier device are described in Figure 6. In the case of the hole-only devices,
hole-current densities of the 2CzCbPh and 4CzCbPh devices were similar, while the hole-current
densities of the 2CzCbCN and 4CzCbCN devices showed relatively lower hole-current densities. The
tendency of the hole-current densities were laid inversely in order of the electron deficiency of the
substituent moieties. In the case of the electron-only devices, the electron-current densities of 2CzCbCN,
2CzCbPh, and 4CzCbPh devices were similar, while the electron-current density of the 4CzCbCN
device was higher than that of the other electron-only devices. The high electron-current density of the
4CzCbCN device is thought to be the result of the strong electron-deficiency of the benzonitrile
substituent group. It was shown that the overall tendency of the single carrier densities was affected by
the electron-deficiency of the substituent moieties.
To elucidate the device performance of 2CzCbCN, 2CzCbPh, 4CzCbCN, and 4CzCbPh, blue
PHOLEDs were fabricated. Due to the different triplet-energies of the host materials, 2CzCbCN and
2CzCbPh were doped with the green dopant Ir(ppy)3, while 4CzCbCN and 4CzCbPh were doped with
the blue dopant FIrpic.
As shown in Figure 7(a), 2CzCbCN and 2CzCbPh were doped with Ir(ppy)3 at a doping concentration
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of 5 wt%. Figure 7(b) shows the current density and luminance against voltage plots of the blue
PHOLEDs. In the case of current densities of the green PHOLEDs, the tendency was very similar to the
results of the single carrier devices. 2CzCbCN showed a better current density than the 2CzCbPh-based
green phosphorescent device, which was due to the improved electron density as a result of the strong
electron-deficient substituent group of benzonitrile. Figure 7(c) shows the quantum efficiency and
power efficiency against luminance plots. The 2CzCbCN and 2CzCbPh devices showed maximum
quantum efficiencies of 19.4 and 17.8%, respectively. From the efficiency plots, it was noticed that, not
only current densities are affected by electron deficiency of the substituent groups, but the efficiencies
are also highly affected. Figure 7(d) shows the EL spectra of the green PHOLEDs. The color
coordinates of the 2CzCbCN and 2CzCbPh devices were (0.30, 0.62) and (0.31, 0.61), respectively.
Clear Ir(ppy)3 spectra of 2CzCbCN and 2CzCbPh indicates effective energy transfer from the host to
green dopant material.
Figure 8(a) shows the structure of the devices using 4CzCbCN and 4CzCbPh host materials. The host
materials were doped with the blue phosphorescent-emitting material FIrpic at a doping concentration of
10 wt%. Figure 8(b) shows the current density and luminance against voltage plots of the blue
PHOLEDs. The tendency of the current densities and luminance against voltage of the host materials
was similar to the order of the current density in the single carrier devices. Figure 8(c) shows the
quantum efficiency and power efficiency against luminance plots. According to the quantum efficiency
curves, 4CzCbCN showed a higher maximum quantum efficiency of 18.2% than that of another host
material, 4CzCbPh (16.7%). The quantum efficiency of 4CzCb derivatives was mostly affected by the
electron-carrying capability of the substituent groups, since the 4CzCb core is a hole transport-type core.
The color coordinates of the 4CzCbCN and 4CzCbPh devices were (0.15, 0.33) and (0.15, 0.32). The
host materials showed clear FIrpic emission without host- or hole-transport layer emission, which
indicates that energy transfer effectively occurred from the host to the FIrpic dopant. The EL spectra of
4CzCbCN and 4CzCbPh are described in Figure 8(d).
Conclusions
Carbazole-pyridoindole derivatives, using the 2- and 4-position of carbazole, and the 6-position of
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pyridoindole, were newly developed as host materials for green and blue PHOLEDs. The host materials
showed different characteristics with respect to dihederal angle and triplet-energy. Carbazole-
pyridoindole core-based moieties showed that the triplet-energies of the host materials are easily tunable
using different interconnections of carbazole and pyridoindole, while maintaining a high quantum
efficiency of nearly 20%.
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correction. J. Comput. Chem., 2006, 27, 1787-1799.
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Table TITLE
Table 1. Calculated and measured HOMO and LUMO levels of the host materials.
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Table 1. Calculated and measured HOMO and LUMO levels of the host materials.
2CzCbCN
2CzCbPh
4CzCbCN
4CzCbPh
calculated measured calculated measured calculated measured calculated measured
HOMO
(eV)
-5.46
-1.45
-5.96
-2.83
-5.26
-1.07
-5.92
-2.73
-5.35
-1.43
-6.06
-2.89
-5.21
-1.05
-6.04
-2.78
LUMO
(eV)
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SCHEME TITLES
Scheme 1. Synthetic scheme of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh
CHART TITLES
Figure 1. HOMO and LUMO distribution of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh
Figure 2. Dihedral angle comparison between 2CzCbPh and 4CzCbPh
Figure 3. CV curves for the oxidation and reduction of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh
Figure 4. UV-vis, solution PL and low temperature PL spectra of tetrahydrofuran solution of 2CzCbCN,
2CzCbPh, 4CzCbCN and 4CzCbPh
Figure 5. DSC thermograms of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh at a heating rate of 10
oC/min in nitrogen atmosphere.
Figure 6. Single carrier device data of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh
Figure 7. Device properties of 4CzCbCN and 4CzCbPh based blue PHOLEDs (a) Device structure of
the green PHOLEDs, (b) current density-luminance against voltage plots, (c) quantum efficiency-power
efficiency against luminance plots, (d) EL spectra of blue PHOLEDs
Figure 8. Device properties of 4CzCbCN and 4CzCbPh based blue PHOLEDs (a) Device structure of
the green PHOLEDs, (b) current density-luminance against voltage plots, (c) quantum efficiency-power
efficiency against luminance plots, (d) EL spectra of blue PHOLEDs
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Scheme 1. Synthetic scheme of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh
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HOMO
LUMO
2CzCbCN
2CzCbPh
4CzCbCN
4CzCbPh
Figure 1. HOMO and LUMO distribution of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh
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2CzCbPh
4CzCbPh
Figure 2. Dihedral angle comparison between 2CzCbPh and 4CzCbPh
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2CzCbCN
2CzCbPh
4CzCbCN
4CzCbPh
-4
-3
-2
-1
0
1
2
3
Voltage (V)
Figure 3. CV curves for the oxidation and reduction of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh
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(a)
(b)
2CzCbPh
2CzCbCN
UV-Vis
Solution PL
LTPL
UV-Vis
Solution PL
LTPL
250
350
450
550
650
250
350
450
550
650
Wavelength (nm)
Wavelength (nm)
(d)
(c)
4CzCbPh
4CzCbCN
UV-Vis
Solution PL
LTPL
UV-Vis
Solution PL
LTPL
250
350
450
550
650
250
350
450
550
650
Wavelength (nm)
Wavelength (nm)
Figure 4. UV-vis, solution PL and low temperature PL spectra of tetrahydrofuran solution of 2CzCbCN,
2CzCbPh, 4CzCbCN and 4CzCbPh
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2CzCbCN
2CzCbPh
4CzCbCN
4CzCbPh
50
100
150
200
Temperature(oC)
Figure 5. DSC thermograms of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh at a heating rate of 10
oC/min in nitrogen atmosphere.
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6
4
2
0
6
2CzCbCN (Hole)
2CzCbPh (Hole)
4CzCbCN (Hole)
4CzCbPh (Hole)
2CzCbCN (Electron)
2CzCbPh (Electron)
4CzCbCN (Electron)
4CzCbPh (Electron)
4
2
0
0
2
4
6
8
0
2
4
6
8
Voltage (V)
Voltage (V)
Figure 6. Single carrier device data of 2CzCbCN, 2CzCbPh, 4CzCbCN and 4CzCbPh
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(b) 40
100000
10000
1000
100
(a)
2CzCbCN
2CzCbPh
35
30
25
20
15
10
5
10
1
0.1
0
0.01
0
2
4
6
8
10
Voltage (V)
80
70
60
50
40
30
20
10
0
60
50
40
30
20
10
0
(d)
(c)
2CzCbCN
2CzCbPh
2CzCbCN
2CzCbPh
380
480
580
680
780
1
10
100
1000
10000
Luminance (cd/m2)
Wavelength (nm)
Figure 7. Device properties of 2CzCbCN and 2CzCbPh based green PHOLEDs (a) Device structure of
the green PHOLEDs, (b) current density-luminance against voltage plots, (c) quantum efficiency-power
efficiency against luminance plots, (d) EL spectra of green PHOLEDs
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(b) 40
100000
10000
1000
100
(a)
-2.0
4CzCbCN
4CzCbPh
35
30
25
20
15
10
5
-2.5
-2.5
-2.8
-2.9 -2.8
-2.9
-2.9
LiF/Al
10
ITO
1
-4.7
-5.1
0.1
-5.5
-5.8
0
0.01
-6.1
-6.1 -6.0
-6.2
0
2
4
6
8
10
-6.79
Voltage (V)
60
50
40
30
20
10
0
40
35
30
25
20
15
10
5
(d)
(c)
4CzCbCN
4CzCbPh
4CzCbCN
4CzCbPh
0
380
480
580
680
780
1
10
100
1000
10000
Luminance (cd/m2)
Wavelength (nm)
Figure 8. Device properties of 4CzCbCN and 4CzCbPh based blue PHOLEDs (a) Device structure of
the green PHOLEDs, (b) current density-luminance against voltage plots, (c) quantum efficiency-power
efficiency against luminance plots, (d) EL spectra of blue PHOLEDs
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- Directly linked carbazole-pyridoindole core structure for host material
- High triplet energy host materials with thermal stability
- Benzonitrile and phenyl modification to manage charge transport properties