Novel ambipolar orthogonal donor-acceptor host for blue organic light emitting diodes

Article abstract of DOI:10.1021/ol402001v

Ambipolar triplet hosts comprising 1,2,4-triazole and carbazole in ortho-positions have been developed. The blue PHOLED has a high current efficiency of 47.1 cd A^-1, power efficiency of 41.2 lm W ^-1, and low efficiency roll-off. The high efficiency was attributed to the successful control of π-conjugation through orthogonal arrangement of the substituents so that a wide T₁-S₀ gap could be maintained.

Full text of DOI:10.1021/ol402001v

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Novel Ambipolar Orthogonal
DonorꢀAcceptor Host for Blue Organic
Light Emitting Diodes
Man-kit Leung,*,†,‡ Yu-Hsuan Hsieh,† Ting-Yi Kuo,† Pi-Tai Chou,† Jiun-Haw Lee,§
Tien-Lung Chiu,*, and Hsin-Jen Chen
Department of Chemistry, Institute of Polymer Science and Engineering, Department of
Electrical Engineering and Graduate Institute of Photonics and Optoelectronics,
National Taiwan University, 1 Roosevelt Road Section 4, Taipei 106, Taiwan, R.O.C.,
and Department of Photonics Engineering, Yuan Ze University, 135 Yuan-Tung Road,
Chung-Li 32003, Taiwan, R.O.C.
mkleung@ntu.edu.tw; tlchiu@saturn.yzu.edu.tw
Received July 17, 2013
ABSTRACT
Ambipolar triplet hosts comprising 1,2,4-triazole and carbazole in ortho-positions have been developed. The blue PHOLED has a high current
efficiency of 47.1 cd Aꢀ1, power efficiency of 41.2 lm Wꢀ1, and low efficiency roll-off. The high efficiency was attributed to the successful control of
π-conjugation through orthogonal arrangement of the substituents so that a wide T1ꢀS0 gap could be maintained.
Organic light-emitting diodes (OLEDs) have high
potential in full-color flat-panel displays and lighting
applications.1 Phosphorescent organic light-emitting
diodes (PHOLEDs) are particularly attractive because
their theoretical quantum efficiency can be, in principle,
four times higher than that of the fluorescent based
OLEDs.2 However, high efficiency blue-light-emitting
PHOLEDs are difficult to achieve due to the high-lying
triplet state requirement that usually leads to low emis-
sion efficiency and a short device lifetime. In addi-
tion, host matrices are required so that phosphorescent
emitters can be dispersed homogeneously into a suitable
organic host, and with this strategy, a detrimental effect
such as aggregation quenching and tripletꢀtriplet anni-
hilation of phosphors can be suppressed. Good host
materials should meet some requirements: (i) higher
triplet T1 energy to prevent exothermic reverse energy
transfer (ET) from the emitters and hence confine triplet
excitons within the emitting layer,3 (ii) suitable HOMO/
LUMO levels to lower the interfacial energy barriers
between its neighboring active layers,4 and (iii) balanced
charge transport properties to restrict the electronꢀhole
recombination within the emitting layer and thus reduce
the efficiency roll-off.5 Recent research emphasizes the
use of ambipolar hosts to keep the charge-transport
balanced.6 The challenge is to incorporate electron-donating
† Department of Chemistry, National Taiwan University.
‡ Institute of Polymer Science and Engineering, National Taiwan University.
§ Department of Electrical Engineering and Graduate Institute of Photo-
nics and Optoelectronics, National Taiwan University.
(3) (a) Padmaperuma, A. B.; Sapochak, L. S.; Burrows, P. E. Chem.
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Chem. Mater. 2008, 20, 169.
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R. C.; Hack, M.; Brown, J. J. Appl. Phys. Lett. 2002, 81, 3921.
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Duan, L.; Qiao, J.; Sun, Y.; Qiu, Y. Adv. Mater. 2011, 23, 1137.
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r
10.1021/ol402001v
XXXX American Chemical Society

Scheme 1
Figure 1
and -withdrawing moieties into one molecule without
creating strong donorꢀacceptor interactions so that the low
triplet energy (ET) states caused by intramolecular charge
transfer (CT) can be avoided. Many successful designs7 with
typical linking groups such as diphenylsilane8 and diphe-
nylphosphine oxide9 have been used.
1,2,4-Triazoles (Tazs) and carbazoles (Cbzs) are com-
mon electron-transporting and donating high triplet host
materials for PHOLEDs. In the present work, ambipolar
hosts 1ꢀ3 (Figure 1) comprising Taz and Cbz moieties
and bridged with an ortho-substituted benzene ring have
been developed and studied.
Crystallographic Analysis. X-ray crystallography is
useful for understanding material properties.12 Figure 2
shows the ORTEPs of 1, 2, and 3. Due to steric repulsions
between the Taz and the Cbz moieties, these substituents
possess noncoplanar alignment to the central benzene
ring with the dihedral angles (DAs) mainly larger than 45°
so that the π-conjugations are interrupted. However, the
close distances between the Taz and Cbz groups still allow
effective through-space π-interactions.
Synthesis. Nucleophilic aromatic substitution of 4 with
9H-carbazole led to 5 (87%) (Scheme 1).10 Condensation
of 5 with aniline, using AlCl3 as a Lewis acid catalyst as well
as a dehydrating agent, gave 1 (58%).11 Under similar
conditions, 6 and 7 were converted to the corresponding
N-phenyltriazole intermediates, followed by nucleophilic aro-
matic substitution to give 2 (79%) and 3 (70%), respectively.
(7) (a) Hung, W. Y.; Tu, G. M.; Chen, S. W.; Chi, Y. J. Mater. Chem.
2012, 22, 5410. (b) Rothmann, M. M.; Fuchs, E.; Schildknecht, C.;
Langer, N.; Lennartz, C.; Munster, I.; Strohriegl, P. Org. Electron.
2011, 12, 1192. (c) Su, S. J.; Cai, C.; Takamatsu, J.; Kido, J. Org. Electron.
2012, 13, 1937. (d) Zhuang, J.; Su, W.; Li, W.; Zhou, Y.; Shen, Q.; Zhou,
M. Org. Electron. 2012, 13, 2210. (e) Chen, H. F.; Chi, L. C.; Hung, W. Y.;
Chen, W. J.; Hwua, T. Y.; Chen, Y. H.; Chou, S. H.; Mondal, E.; Liu,
Y. H.; Wong, K. T. Org. Electron. 2012, 13, 2671. (f) Lee, C. W.; Lee, J. Y.
Org. Electron. 2013, 14, 1602. (g) Lee, C. W.; Kim, J.-K.; Joo, S. H.; Lee,
J. Y. ACS Appl. Mater. Interfaces 2013, 5, 2169. (h) Lee, C. W.; Lee, J. Y.
Chem. Commun. 2013, 49, 1446. (i) Swensen, J. S.; Polikarpov, E.; Von
Ruden, A.; Wang, L.; Sapochak, L. S.; Padmaperuma, A. B. Adv. Funct.
Mater. 2011, 21, 3250. (j)Hua, D. h.;Cheng, G.;Li, H.;Lv, Y.;Lu, P.;Ma,
Y. g. Org. Electron. 2012, 13, 2825. (k) Peng, T.; Yang, Y.; Bi, H.; Liu, Y.;
Hou, Z. m.; Wang, Y. J. Mater. Chem. 2011, 21, 3551. (l) Zhao, X.-H.;
Zhang, Z.-S.; Qian, Y.; Yi, M.-D.; Xie, L.-H.; Hu, C.-P.; Xie, G.-H.; Xu,
H.; Han, C.-M.; Zhao, Y.; Huang, W. J. Mater. Chem. C 2013, 1, 3482.
(m) Wada, A.; Yasuda, T.; Zhang, Q. s.; Yang, Y. S.; Takasu, I.;
Enomotob, S.; Adachi, C. J. Mater. Chem. C 2013, 1, 2404. (n) Xia,
Z.-Y.; Zhang, Z.-Y.; Su, J.-H.; Zhang, Q.; Fung, K.-M.; Lam, M.-K.; Li,
K.-F.; Wong, W.-Y.; Cheah, K.-W.; Tian, H.; Chen, C. H. J. Mater.
Chem. 2010, 20, 3768.
(8) (a) Leung, M. k.; Yang, W. H.; Chuang, C. N.; Lee, J. H.; Lin,
C. F.; Wei, M. K.; Liu, Y. H Org. Lett. 2012, 14, 19. (b) Holmes, R. J.;
D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.; Thompson, M. E.
Appl. Phys. Lett. 2003, 83, 3818. (c) Ren, X.; Li, J.; Holmes, R. J.;
Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 2004,
16, 4743.
(9) (a) Chou, H. H.; Cheng, C. H. Adv. Mater. 2010, 22, 2468. (b)
Chang, H. H.; Tsa, W. S.; Chang, C. P.; Chen, N. P.; Wong, K. T.; Hung,
W. Y.; Chen, S. W. Org. Electron. 2011, 12, 2025. (c) Jiang, W.; Duan, L.;
Qiao, J.; Dong, G. f.; Wang, L.d.; Qiu, Y. Org. Lett. 2011, 13, 3146.
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Org. Lett. 2009, 11, 3072.
Figure 2. ORTEPs of 1ꢀ3.
A single crystal of 1 clearly shows that the N-phenyl
group on the Taz segment and the Cbz ring are face-to-
face aligned. Both groups are rotated with respect to the
central benzene ring, and the DAs 53.5° for C1ꢀN1ꢀ
C13ꢀC18 and 51.9° for N2ꢀC19ꢀC18ꢀC17 were re-
˚
corded. The short estimated centroid distance of 3.67 A
strongly suggests through-space electronic πꢀπ interac-
tions. Similar conformational preferences were found in 2
and 3. In 2, the DAs 43.1° and 67.4° for the carbazole
rings were found. The sandwiched Taz unit is kept at a
(12) (a) Zhou, G.-j.; Wang, Q.; Ho, C.-L.; Wong, W.-Y.; Ma, D.-g;
Wang, L.-x.; Lin, Z.-y Chem. Asian J. 2008, 3, 1830. (b) Zhou, G.-j.; Ho,
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55, 175.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Physical Properties of 1ꢀ3
onset
onset
UVꢀvis
FL
Ph
ET
Eg
Eox
(V)
Ered
(V)
HOMO/
Tg
Td
(λmax, nm)
(λ, nm)
(λ, nm)
(eV)
(eV)
LUMO (eV)
(°C)
(°C)
1
2
3
286
286
290
395, 341/358
403
451
442
443
3.09
3.07
3.09
3.44
3.34
3.40
ꢀ2.22
ꢀ2.15
ꢀ2.21
1.22
1.19
1.20
ꢀ5.7/ꢀ2.3
ꢀ5.7/ꢀ2.3
ꢀ5.7/ꢀ2.3
87
ꢀ
349
365
353
395, 342/358
ꢀ
large DA of 60.1° to the benzene linker group. In 3, the N-
phenyl group on the Taz segment is parallel displaced
with the Cbz rings and with centroid short distances of
˚
3.74 and 3.84 A being measured. The DAs 54.2° for
C(7)ꢀC(8)ꢀN(5)-C(21), 47.4° for C(19)ꢀC(20)ꢀN(4)ꢀ
C(33), 60.4° for N(3)ꢀC(2)ꢀC(3)ꢀC(4), 52.6° for N(2)ꢀ
C(1)ꢀC(15)ꢀC(16), and 55.9° for C(1)ꢀN(1)ꢀC(9)ꢀ
C(10) were recorded. Again, all the DAs were found to
be >45°, indicating orthogonal alignments of the aro-
matic units. This will be beneficial for maintaining the T1
states (the lowest triplet state) at high energy levels.
Photophysical Properties. Compounds 1ꢀ3 show strong
absorptions below 300 nm that are attributed to the πꢀπ*
transitions of the Taz and Cbz chromophores (Table 1 and
Figure 3). The absorptions that ranged 300ꢀ350 nm with
ample vibronic patterns are characteristic for the Cbz chro-
mophore that is nearly identical to that of N-phenylcarbazole
(NPC).13 The presence of the Taz group in 1ꢀ3 does not
introduce any significant electronic perturbations or red shift
to the absorption spectra of the carbazole moiety. This may
be due to the nonconjugative orthogonal alignment of the
aromatic units, which were confirmed in the crystallographic
analysis, that allows the substituents to function as indepen-
dent chromophores.
Room-Temperature Photoluminescence (PL). Com-
pounds 1 and 3 strongly fluoresce to give dual emissions
at 325ꢀ535 nm that are well overlapped with the MLCT
absorption of FIrpic, allowing ET from 1ꢀ3 to FIrpic.
The first emission of 1 and 3 shows the vibronic fine
pattern peaked at 341 and 358 nm, with a reasonably
small Stokes shift of 8 nm. The emission pattern is nearly
identical to that of NPC, suggesting that this emission is
governed by the Cbz fluorophore. The second emission
peaking at 395 nm shows typical CT character; the
emissions are broad and structureless with the intensity
solvent polarity dependent. On the other hand, formation
of the CT state is extremely effective for 2 so that the CT
emission dominates in its photoemission. The dual-emission
phenomena were further confirmed by time-dependent fluor-
escence-decay experiments. For example, the transient spec-
tra of 1(Figure 3d) show a decay lifetime of 1.19 ns at 340 nm
and a rise time of 0.94 ns followed by a decay lifetime of 2.23
ns at 450 nm. Furthermore, the excitation spectra of 1 about
λ = 360 and 450 nm superimpose with each other perfectly,
supporting the hypothesis of dual-emission properties from
the same species; the short lifetime and the superimposition in
Figure 3. UVꢀvisible, fluorescence, low temperature fluores-
cence, and low temperature phosphorescence spectra of (a) 1; (b)
2; (c) 3, and (black) UV; (red) FL; (green) Ph; (blue) LTFL. (d)
Transient spectra of 1 at λ = 340 (blue) and 450 nm (green) in the
time-dependent fluorescence decay experiments witn excitation at
310 nm (orange: instrument response function; blue: emission at
340 nm, τ=(þ) 1.19; green: emission at 450 nm, τ=(þ) 2.23(ꢀ) 0.94.
excitation spectrum indicate that the CT emission may be an
intramolecular process with the same ground state.
Low Temperature PL Behavior. The second CT emis-
sion band disappears in the PL spectra in THF organic
glass at 77 K, and only emission from the carbazole unit
could be detected. This may probably due to two reasons:
(1) Formation of the CT excited state requires conforma-
tional change from the initial S1 state that could be
inhibited by restricted rotation in the organic glass. (2)
Stabilization of the CT excited state through dipoleꢀdipole
interactions requires solvent orientation relaxation that is
prohibited at low temperature in the organic glassy matrix.
The low temperature phosphorescence spectra of 1ꢀ3 and
NPC are almost identical with similar ET levels of 3.00ꢀ3.09 eV
that were recorded but significantly different from that
of 3,4,5-triphenyl-1,2,4-triazole (TPT). This observation
suggested that the triplet state is confined at the carbazole
(13) Lee, C. C.; Leung, M.-k.; Lee, P.-Y.; Chiu, T.-L.; Lee, J.-H.; Liu,
C.; Chou, P.-T. Macromolecules 2012, 45, 751.
Org. Lett., Vol. XX, No. XX, XXXX
C

Table 2. Performances of PHOLED Devices 1ꢀ3
b
c
d
Lmax
(cd/m2)
ηc.max
ηp.max
CIE shift
devicea
(cd/A)
(lm/W)
EQEmax
Δx, Δy
1, 15%
2, 12%
3, 9%
17 770
17 510
15 730
47.1
43.3
40.2
41.2
38.6
36.1
20.2%
17.9%
17.1%
0.016, 0.043
0.016, 0.039
0.013, 0.031
a Host-FIrpic doping level. b At 12 V. c Eapplied for 1, 2, and 3 are 4, 4,
and 3.5 V. d At 3.5 V.
unit. The ET is obviously higher than that of FIrpic
(2.65 eV),3b implying that 1ꢀ3 are potential host materials
for blue-light triplet emitters.
Figure 4. PHOLED device properties of 1: (a) Voltageꢀ
currentꢀbrightness plots; (b) Efficiency plots.
Electrochemical Properties. The ambipolar behaviors
of 1ꢀ3 (10ꢀ3 M) were evaluated by cyclic voltammetry
(See Table 1 and SI). Regardless of the structural differences,
1ꢀ3 show similar redox behavior. In the anodic scans, 1ꢀ3
show irreversible oxidation on the Cbz ring with nearly the
same Eonset at 1.2 V. In the cathodic scans, the irreversible
reduction on the Taz unit was observed at Eonset = ꢀ2.2 V
(Table 1). These results again indicated that electronic cou-
plings between the Cbz units and the Taz units are minimal
and can be considered more or less as nonconjugated ones.
Electroluminescence Performances. The electronic
properties of 1ꢀ3 were examined by the hole- and elec-
tron-only methods14 (see SI). The discrepancy of the hole
and electron currents is within 1 order of magnitude,
strongly suggesting that 1ꢀ3 are ambipolar materials.
The PHOLED performance was evaluated on a device
of ITO/NPB/mCP/FIrpic in 1, 2, or 3/TAZ/LiF/Al
(Table 2), in which TAZ is 3-(4-biphenyl)-4-phenyl-5-
tert-butylphenyl-1,2,4-triazole and NPB is N,N0-diphenyl-
N,N0-bis(1-naphthyl)(1,10-biphenyl)-4,40diamine. The high
HOMOꢀLUMO gap of 3.34ꢀ3.44 eV and ET of 3.07ꢀ
3.09 eV for 1ꢀ3 enable the intermolecular ET from 1ꢀ3 to
FIrpic (2.65 eV). Thus, all the PHOLED exhibited a main
emission peak at 472 nm, which is typical for FIrpic-
containing devices withhosts. The PHOLED performance
was FIrpic doping-level dependent. Maximum lumines-
cence (Lmax) of 19160, 17510, and 16720 cd/m2 was
achieved at 12 V for 1, 2, and 3, with FIrpic doping levels
of 18, 12, and 15 wt % being adopted respectively. Since the
tripletꢀtriplet annihilation effect may occur at high concen-
tration, we optimized the FIrpic doping levels so that 1 (15
wt %), 2 (12 wt %), and 3 (9 wt %) are found for obtaining
the highest current efficiency (ηc) of 47.1 (4 V), 43.3 (4 V),
and 40.2 cd/A (3.5 V); power efficiency (ηp) of 41.2, 38.6,
and 36.1 lm/W at 3.5 V; and external quantum efficiency
(EQEmax) of 20.2%, 17.9%, and 17.1%.
Figure 4 shows the voltageꢀcurrentꢀbrightness and the
current densityꢀefficiency plots of the resultant device for
1. Although rolling-off of the efficiency for PHOLED
is commonly observed at high driving currents due to
tripletꢀtriplet annihilation, the current efficiency (ηc) at
100 and 1000 cd/m2 remains high in the present cases. The
comparably high ηc of 46.3, 42.3, and 38.8 cd/A for the
resultant devices of 1, 2, and 3 at 100 cd/m2 and 41.1, 39.5,
and 35.7 cd/A at 1000 cd/m2 were recorded respectively. The
blue color stability is outstanding. The CIE (Commission
International de L’Eclairage) of (0.16, 0.38) at 6 V for all the
devices, with only small CIE coordinate shifts, were recorded
upon variation of the applied electrical voltage in the range
3.5ꢀ12 V.
In conclusion, we report a simple design of ambipolar
hosts 1ꢀ3 for FIrpic that leads to high luminescence ηc,
low efficiency roll-off, and high color stability. Without
adopting any high performance carrier-blocking materi-
als such as 1,1-bis-(4-bis(4-methylphenyl)aminophenyl)-
cyclohexane (TAPC), hole-blocking material such as
1,4-bis(triphenylsilyl)benzene (UGH-2),8b or electron-
transport materials such as tris[3-(3-pyridyl)mesityl]borane
(3TPYMB),15 or by any special technique such as microlens
for light extraction,16 significantly high efficiency for the
resultant devices of 1, 2, and 3 were obtained. The high
efficiency was attributed to the successful control of the
degree of conjugation in the host molecules through or-
thogonal arrangement of the substituents so that a wide
T1ꢀS0 gap could be maintained.
Acknowledgment. We are thankful for financial support
from NSC-101-2113-M-002-010-MY3, NSC-102-2622-
E-155-008-CC3, 102-EC-17-A-08-S1-183, and to Mr. Jau-Jiun
Huang for the energy transfer measurements.
Supporting Information Available. NMR, DSC, TGA,
CV, CIF, HOMO/LUMO, and OLED data. This material
is available free of charge via the Internet at http://pubs.
acs.org.
(14) Chu, T. Y.; Song, O. K. Appl. Phys. Lett. 2007, 90, 203512.
(15) Eom, S.-H.; Zheng, Y.; Wrzesniewski, E.; Lee, J.; Chopra, N.;
So, F.; Xue, J. Org. Electron. 2009, 10, 686.
(16) Wei, M.-K.; Lin, H.-Y.; Lee, J.-H.; Chen, K.-Y.; Ho, Y.-H.; Lin,
C.-C.; Wub, C.-F.; Lin, H.-Y.; Tsai, J.-H.; Wu, T.-C. Opt. Commun.
2008, 281 (2008), 5625.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX