Benzo[4,5]thieno[2,3-b]pyridine derivatives as host materials for high efficiency green and blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c2cc38049h

Benzo[4,5]thieno[2,3-b]pyridine (BTP) was newly developed as an electron deficient moiety for high triplet energy materials and two BTP derivatives with the BTP and carbazole groups were synthesized as high triplet energy bipolar host materials. The BTP derivatives were effective as the host materials for green and blue phosphorescent organic light-emitting diodes and a high quantum efficiency above 20% was achieved both in green and blue devices.

Full text of DOI:10.1039/c2cc38049h

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Benzo[4,5]thieno[2,3-b]pyridine derivatives as host
materials for high efficiency green and blue
phosphorescent organic light-emitting diodes†
Cite this: Chem. Commun., 2013,
49, 1446
Received 7th November 2012,
Accepted 25th December 2012
Chil Won Lee and Jun Yeob Lee*
DOI: 10.1039/c2cc38049h
www.rsc.org/chemcomm
Benzo[4,5]thieno[2,3-b]pyridine (BTP) was newly developed as an
electron deficient moiety for high triplet energy materials and two
BTP derivatives with the BTP and carbazole groups were syn-
thesized as high triplet energy bipolar host materials. The BTP
derivatives were effective as the host materials for green and blue
phosphorescent organic light-emitting diodes and a high quantum
efficiency above 20% was achieved both in green and blue devices.
The external quantum efficiency of phosphorescent organic
light-emitting diodes (PHOLEDs) has been greatly improving
for the last 10 years since the first development of PHOLEDs
using Ir based dopant materials.1 A high external quantum
efficiency above 20% has been achieved in red, green and blue
PHOLEDs through the development of device architecture and
organic materials for PHOLEDs.2–4 In particular, high triplet
energy host and dopant materials mainly increased the external
quantum efficiency of PHOLEDs.
Scheme 1 Synthetic scheme of BTP1 and BTP2. (a) Pd(PPh3)4 (0), K2CO3,
toluene–water, reflux, 24 h, N2; (b) tert-butyl nitrite, AcOH–THF, 0 1C (12 h),
and (c) Pd2(dba)3, S-Phos, K3PO4, toluene–water, reflux, 36 h, N2.
Many high triplet energy materials have been synthesized as electron transport moieties is required to improve the device
host materials for PHOLEDs. In general, high triplet moieties performance of PHOLEDs.
such as carbazole,5–10 arylsilane,11–13 fluorene,14 dibenzofuran15–17
In this work, a novel electron deficient high triplet energy
and dibenzothiophene18,19 were used in the molecular struc- unit based on benzothienopyridine, benzo[4,5]thieno[2,3-b]-
ture as core structures to obtain high triplet energy. Hole pyridine (BTP), was synthesized as the core structure of high
transport type cores were combined with electron transport triplet energy host materials and two high triplet energy bipolar
groups, while electron transport type cores were modified with host materials, BTP1 and BTP2, were developed by modifying
hole transport units for bipolar charge transport properties. BTP with carbazole. It was demonstrated that a high external
Carbazole is the most widely used hole transport core for triplet quantum efficiency above 20% was achieved in green and blue
host materials because of a high triplet energy of 3.02 eV and PHOLEDs using the BTP derivatives as the host materials.
was substituted with various electron transport units such
as diphenylphosphine oxide,8 oxadiazole9 and triazole10 to Scheme 1. Detailed synthesis is described in ESI.† BTP was
balance hole and electron transport properties. high designed as a high triplet energy electron deficient unit for high
The synthetic scheme of the BTP derivatives is described in
A
external quantum efficiency above 20% was achieved using the triplet energy materials. Although dibenzothiophene has been
carbazole based bipolar host materials. However, the number of used as the high triplet energy unit, it was not effective as an
electron transport units is limited and further development of electron deficient unit.19 Therefore, pyridine was introduced
in the molecular structure to improve the electron deficiency
of dibenzothiophene. BTP was synthesized by ring closing
reaction of a methylsulfanyl unit with a pyridinylamine unit
prepared from 2-(methylthio)phenylboronic acid and 5-chloro-
Department of Polymer Science and Engineering, Dankook University, 126,
Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448-701, Republic of Korea.
E-mail: leej17@dankook.ac.kr; Fax: +82 31 8005 3585; Tel: +82 31 8005 3585
3-iodopyridin-2-amine by Suzuki coupling. The synthetic yield
of the chlorinated BTP moiety was 61%.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2cc38049h
c
1446 Chem. Commun., 2013, 49, 1446--1448
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absorption peaks at 243 nm and 298 nm which are assigned to
p–p* transition of carbazole and BTP. UV-Vis absorption peaks
of BTP2 were shifted to a longer wavelength compared to those
of BTP1. The weak absorption of carbazole was observed
between 330 nm and 370 nm and strong p–p* absorption of
the directly linked carbazole–BTP moiety appeared at 248 and
303 nm. The extended conjugation between carbazole and BTP
induced the red shift of the UV-Vis absorption peaks. Optical
bandgaps calculated from the absorption edge of the UV-Vis
spectra of BTP1 and BTP2 were 3.40 eV and 3.24 eV, respec-
tively. PL emissions of BTP1 and BTP2 were observed at 387 nm
and 405 nm, respectively, and the PL emission of BTP2 was red-
shifted by 18 nm compared with that of BTP1 due to extended
conjugation of BTP2. The HOMO of BTP2 was extended to the
BTP unit due to extended conjugation, which induced the red-
shift of PL emission. The extended conjugation also affected
the triplet energy of BTP1 and BTP2. The triplet energies of
BTP1 and BTP2 were 2.73 eV and 2.65 eV, respectively, from the
first phosphorescent emission peak of low temperature PL
spectra measured at 77 K. The triplet energy of BTP2 was also
reduced by 0.08 eV compared with that of BTP1 because of the
extended conjugation. In the case of BTP1, separation of the
HOMO and LUMO resulted in high triplet energy.
The HOMO levels of BTP1 and BTP2 were measured by cyclic
voltammetry and were À6.11 eV and À6.04 eV, respectively. The
LUMO levels of BTP1 and BTP2 were À2.71 eV and À2.80 eV,
respectively, from the HOMO and optical bandgaps. Considering
the HOMO and LUMO levels, BTP2 may be better than BTP1
because of the deep LUMO level for better electron injection and
the shallow HOMO level for better hole injection.
Hole and electron only devices of BTP1 and BTP2 were
fabricated to compare hole and electron density in the emitting
layer. Fig. 2 shows current density–voltage curves of hole and
electron only devices of BTP1 and BTP2. Hole and electron
current densities of BTP2 were much higher than those of
BTP1. The high hole and electron current densities of BTP2 can
be explained by the HOMO and LUMO levels for hole and
electron injection, and better charge transport properties. The
HOMO and LUMO levels of BTP2 were suitable for hole and
electron injection (Fig. S2, ESI†), which contributed to the high
hole and electron current densities. The wide dispersion of
HOMO of BTP2 is also responsible for the high hole current
density of BTP2 because of better hole hopping between
Fig. 1 HOMO and LUMO distribution of BTP1 and BTP2.
Two BTP derivatives, BTP1 and BTP2, were designed to show
high triplet energy by modifying the BTP moiety with a high
triplet energy carbazole unit. The carbazole unit was connected
to the BTP unit through a phenyl linkage in BTP1, while it was
directly attached to the BTP unit in BTP2. Two host materials
were synthesized by Suzuki coupling reaction of chlorinated
BTP with the corresponding boronic acid of phenylcarbazole.
The synthetic yields of BTP1 and BTP2 were 61% and 54%,
respectively.
Molecular simulation of BTP1 and BTP2 was carried out to
compare the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) distribution.
Fig. 1 shows the HOMO and LUMO distribution of BTP1 and
BTP2. The HOMO of BTP1 was dispersed over the phenyl-
carbazole unit, while the LUMO was localized on the BTP unit.
The HOMO and LUMO were separated from each other in the
molecular structure because of electron withdrawing properties
of BTP and electron donating properties of carbazole. The
HOMO distribution of BTP2 was different from that of BTP1
and the HOMO was extended from phenylcarbazole to the BTP
unit. As carbazole was directly connected to the BTP unit, the
HOMO of BTP2 was widely dispersed over the molecule. In the
case of BTP1, the phenyl linkage isolated the carbazole and BTP
units. The LUMO distribution of BTP2 was similar to that of
BTP1 due to electron deficiency of BTP.
The photophysical properties of BTP1 and BTP2 were analyzed
using ultraviolet-visible (UV-Vis) and photoluminescence (PL)
spectrometers. UV-Vis absorption, PL and low temperature PL
emission spectra of BTP1 and BTP2 host materials are shown in
Fig. S1 in ESI† and data are summarized in Table 1. Solid films
of BTP1 and BTP2 prepared by thermal vacuum deposition were
used for the UV-Vis and PL measurements. BTP1 exhibited
weak UV-Vis absorption peaks between 300 nm and 350 nm
which are assigned to n–p* transition of carbazole8 and strong
Table 1 Photophysical properties and energy levels of host materials
UV-Vis
Material (nm)
PL
(nm) (eV)
HOMOa LUMOb Band
Triplet
(eV)
gapc (eV) energyd (eV)
BTP1
BTP2
a
243, 298, 387 À6.11
330, 344
À2.71
3.40
3.24
2.73
2.65
248, 303, 405 À6.04
À2.80
339, 354
b
HOMO was measured by cyclic voltammetry. LUMO was calculated
c
from the difference of HOMO and bandgap. Bandgap was calculated
from the absorption edge of UV-Vis spectra. Triplet energy was
d
Fig. 2 Current density–voltage curves of hole and electron only devices of BTP1
and BTP2.
calculated from the first emission peak of low temperature PL spectra.
c
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Therefore, the low quantum efficiency of the BTP2 device is
related to the triplet energy of BTP2. The triplet energy of BTP2
was 2.65 eV, which was similar to that of the FIrpic dopant
(2.65 eV). The triplet energy of BTP2, similar to that of FIrpic,
induces reverse energy transfer from the FIrpic dopant to the
BTP2 host material, decreasing the quantum efficiency of the
BTP2 device in spite of good charge balance in the emitting
layer. This can be confirmed by the electroluminescence (EL)
spectra of the BTP1 and BTP2 green and blue PHOLEDs in
Fig. S6 (ESI†). The reduced main emission peak of the BTP2
device at 471 nm indicates back energy transfer from FIrpic to
BTP2. Although the quantum efficiency of BTP1 and BTP2
devices was lower than the state-of-the-art quantum efficiency
of PHOLEDs,21 a high quantum efficiency above 20% has been
Fig. 3 Quantum efficiency–luminance curves of green and blue PHOLEDs with
BTP1 and BTP2.
adjacent molecules. The molecular structure of BTP2 hinders achieved using the host materials with a BTP core, which
hole and electron transport less and improves these properties corresponds to close to a 100% internal quantum efficiency
in BTP2. Compared with BTP1 with the carbazole and BTP units value.
separated by phenyl linkage, the carbazole and BTP units were
In conclusion, a new electron deficient unit, BTP, was
directly connected in BTP2, reducing steric hindrance of carba- developed as a high triplet energy moiety of host materials
zole and BTP. Therefore, hole and electron transport properties and two BTP derivatives were synthesized as the host materials
of BTP2 were enhanced, resulting in high hole and electron for green and blue PHOLEDs. The BTP unit was effective as a
current densities in the hole and electron only devices. building block of high triplet energy host materials and a high
Although the electron current density was lower than the hole quantum efficiency above 20% was achieved in green and blue
current density in the hole and electron only devices, there was PHOLEDs.
less than one order difference, indicating bipolar charge trans-
port properties of BTP1 and BTP2.
Notes and references
As the triplet energy of BTP1 and BTP2 was higher than that of
1 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
green emitting tris(2-phenylpyridine)iridium and blue emitting
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
2 L. Xiao, S. J. Su, Y. Agata, H. Lan and J. Kido, Adv. Mater., 2009,
bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate iridium (FIrpic),
green and blue PHOLEDs were fabricated using BTP1 and
BTP2. Device structure of PHOLEDs is described in ESI.†
External quantum efficiency–luminance curves of the BTP1
and BTP2 devices are shown in Fig. 3. Current density–
voltage–luminance curves are shown in Fig. S3 and S4 in ESI.†
High current density was observed in the BTP2 device due to
high hole and electron current densities.20 The maximum
quantum efficiencies of the BTP1 and BTP2 green devices
were 25.4% and 24.7%, while the quantum efficiencies at
1000 cd mÀ2 of the BTP1 and BTP2 devices were 23.5% and
24.1%, respectively. Both devices showed high external quan-
tum efficiency at low and high luminances. The high quantum
21, 1271.
3 N. Chopra, J. Lee, Y. Zheng, S. H. Eom, J. Xue and F. So, Appl. Phys.
Lett., 2008, 93, 143307.
4 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl.
Phys., 2001, 90, 5048.
5 A. Saeed, M. Irfan and S. A. Samra, J. Org. Chem., 2011, 7, 638.
6 K. H. Lee, H. J. Kang, H. M. Kim, J. H. Seo, Y. K. Kim and S. S. Yoon,
J. Nanosci. Nanotechnol., 2011, 11, 1499.
7 Z. Jiang, X. Xu, Z. Zhang, C. Yang, Z. Liu, Y. Tao, J. Qin and D. Ma,
J. Mater. Chem., 2009, 19, 7661.
8 S. O. Jeon, S. E. Jang, H. S. Son and J. Y. Lee, Adv. Mater., 2011,
23, 1436.
9 Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and
D. Ma, Angew. Chem., Int. Ed., 2008, 47, 8104.
10 J. H. Kim, D. Y. Yoon, J. W. Kim and J. J. Kim, Synth. Met., 2007,
157, 743.
efficiency of two green PHOLEDs is related to the charge 11 X. Ren, J. Li, R. J. Holmes, P. I. Djurovich, S. R. Forrest and
M. E. Thompson, Chem. Mater., 2004, 16, 4743.
12 S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma,
balance in the emitting layer. Although the hole and electron
current densities of BTP2 were higher than those of BTP1, the
Adv. Funct. Mater., 2011, 21, 1168.
relative ratio of the hole current density to the electron current 13 J. Lin, W. Liao, H. Huang, F. Wu and C. Cheng, Adv. Funct. Mater.,
2008, 18, 485.
density was similar in the two host materials. Therefore,
a similar, high quantum efficiency was obtained in the two
14 C. H. Chien, L. R. Kung, C. H. Wu, C. F. Shu, S. Y. Chang and Y. Chi,
J. Mater. Chem., 2008, 18, 3461.
host materials. The high quantum efficiency of the two host 15 S. H. Jeong, C. W. Seo, J. Y. Lee, N. S. Cho, J. K. Kim and J. H. Yang,
Chem.–Asian J., 2011, 6, 2895.
16 P. A. Vecchi, A. B. Padmaperuma, H. Qiao, L. S. Sapochak and
materials is due to the balanced charge density in the emitting
layer. The current efficiency was similar in the two devices, but
P. E. Burrows, Org. Lett., 2006, 8, 4211.
the power efficiency was high in the BTP2 device due to low 17 C. Hanm, G. Xiw, H. Xu, Z. Zhang, L. Xie, Y. Zhao, S. Liu and
W. Huang, Adv. Mater., 2011, 23, 2491.
18 X. Cai, A. B. Padmaperuma, L. S. Sapochak, P. A. Vecchi and
driving voltage of the BTP2 device (Fig. S5, ESI†).
The maximum quantum efficiency and quantum efficiency at
P. E. Burrows, Appl. Phys. Lett., 2008, 92, 083308.
1000 cd mÀ2 of the BTP1 blue PHOLEDs were 24.3% and 20.3%, 19 S. H. Jeong and J. Y. Lee, J. Mater. Chem., 2011, 21, 14604.
20 D. Kabra, L. P. Lu and R. H. Friend, Adv. Funct. Mater., 2012,
respectively, which were much higher than those of the BTP2 blue
PHOLEDs. Considering the charge balance in the emitting layer,
22, 4165.
21 S.-J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater., 2008,
the two materials should show similar quantum efficiencies.
20, 4189.
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1448 Chem. Commun., 2013, 49, 1446--1448