Acridinone/amine(carbazole)-based bipolar molecules: Efficient hosts for fluorescent and phosphorescent emitters

Article abstract of DOI:10.1021/ol901584g

Three acrldinone-based molecules ADBP, ACBP, and DABP were synthesized, and their application to the OLED devices was Investigated. When used as the host for either the deep blue singlet or the green triplet emitter in OLED devices, the bipolar molecules

Full text of DOI:10.1021/ol901584g

ORGANIC
LETTERS
2009
Vol. 11, No. 19
4310-4313
Acridinone/Amine(carbazole)-Based
Bipolar Molecules: Efficient Hosts for
Fluorescent and Phosphorescent
Emitters
Dileep A. K. Vezzu,† Joseph C. Deaton,‡ Mohammad Shayeghi,† Yumin Li,† and
Shouquan Huo*,†
Department of Chemistry, East Carolina UniVersity, GreenVille, North Carolina 27858,
and Research Laboratories, Eastman Kodak Company, 1999 Lake AVenue,
Rochester, New York 14650
huos@ecu.edu
Received July 14, 2009
ABSTRACT
Three acridinone-based molecules ADBP, ACBP, and DABP were synthesized, and their application to the OLED devices was investigated.
When used as the host for either the deep blue singlet or the green triplet emitter in OLED devices, the bipolar molecules ADBP and ACBP
demonstrated superior performance compared to either DABP or commonly used host CBP, remarkably lowering the drive voltage and improving
efficiencies.
Since the ground-breaking demonstration of organic light-
emitting diodes (OLEDs) by Tang and VanSlyke,1 there has
been increasing interest in developing highly efficient OLED
devices because of their great potential in display, solid-
state lighting, and other applications.2 The OLEDs are
current-driven devices; therefore, power efficiency is one of
the major issues associated with OLED technology, which
has attracted much research interest, particularly under the
current environment of an energy crisis. There are two ways
of improving power efficiency; one is to increase the
quantum efficiency and the other is to reduce the drive
voltage. Quantum efficiency can be maximized by using a
phosphorescent emitter doped into the emissive layer of an
OLED device since both singlet and triplet excited states
(3) (a) Adachi, C.; Baldo, M. A.; Forrest, S. R. Appl. Phys. Lett. 2000,
77, 904. (b) Wang, Y.; Herron, N.; Grushin, V. V.; LeCloux, D. D.; Petrov,
V. A. Appl. Phys. Lett. 2001, 79, 449. (c) D’Andrade, B. W.; Thompson,
M. E.; Forrest, S. R. AdV. Mater. 2002, 14, 147. (d) Tsuboyama, A.;
Iwawaki, H.; Furugari, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama,
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Chem. Soc. 2003, 125, 12971. (e) Adachi, C.; Baldo, M. A.; Thompson,
M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048. (f) He, G.; Schneider,
O.; Qin, D.; Zhou, X.; Pfeiffer, M.; Leo, K. J. Appl. Phys. 2004, 95, 6773.
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7992.
† East Carolina University.
‡ Eastman Kodak Company.
(1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(2) For recent reviews see: (a) So, Franky; Shi, J. Opt. Sci. Eng 2008,
133, 351. (b) Wong, T. K. S. In Handbook of Organic Electronics and
Photonics; Nalwa, H. S., Eds.; American Scientific Publishers: Stevenson
Ranch, CA, 2008; Vol. 2, pp 413-472. (d) Shinar, J.; Shinar, R. J. Phys.
D: Appl. Phys. 2008, 41, 133001.
10.1021/ol901584g CCC: $40.75
Published on Web 08/27/2009
 2009 American Chemical Society

generated by the charge recombination (electroexcitation) can
be harnessed with the use of a phosphorescent dopant (triplet
dopant).3 However, in principle, phosphorescent OLEDs
require higher drive voltage because the host used in the
emissive layer needs to have a larger band gap than that of
a host used in a fluorescent device that emits the same color
of light. Therefore, reduction in drive voltage becomes
critical to the development of highly efficient phosphorescent
devices in terms of energy consumption.
has been widely used as the host in phosphorescent OLEDs,
acridinone should possess better electron injecting/transport-
ing ability because of the presence of the electron-withdraw-
ing carbonyl group. Third, steric hindrance would lead to a
twisted geometry in these molecules and make extension of
conjugation between the acridinone moiety and the biphenyl
moiety unlikely; therefore, the required energy gap is
maintained while both electron-transporting and hole-
transporting properties are being introduced into one mol-
ecule.
The synthesis of ADBP and ACBP is shown in Scheme
1. A procedure reported by Buchwald et al.8 was used for
the C-N cross coupling of the intermediate 4′-bromo-N,N-
diphenylbiphenyl-4-amine (1) and 9-(4′-bromobiphenyl-4-
yl)-9H-carbazole (2) with acridinone to give ADBP and
ACBP, respectively. The coupling reaction proceeded in
refluxing DMF to produce the desired compounds in high
yields. The diketone ligand is critical to the success of the
reaction. When replacing the diketone ligand with a diamine
ligand such as trans-N,N′-dimethylcyclohexanediamine, only
a small amount of the desired product (<5%) was detected
under otherwise the same reaction conditions. No reaction
was detected without using a ligand in the reaction. The
choice of the solvent was also important. For example,
although the use of DMA gave similar results, the reaction
did not proceed when dioxane or toluene was used as the
solvent under refluxing conditions. DABP9 was also syn-
thesized similarly by cross coupling of 4,4′-dibromobiphenyl
with acridinone.
Several approaches can be employed to reduce drive
voltage and improve the performance of OLED devices,
which include doping transporting layers,4 designing better
transporting materials,5 and using mixed host materials6 in
the emissive layer. Instead of the mixed host, bipolar
molecules capable of transporting both charges offer clear
advantage because of the simplification of the device
fabrication. Several bipolar molecules have been reported,7
such as borane/amine-,7a quinoxaline/fluorene-,7b triazole/
amine-,7c bathophenanthroline/amine-,7d benzimidazole/
amine-,7e and oxadiazole/carbazole-based7f compounds. They
have been used either as the host for phosphorescent emitters
or used in single-layer fluorescence OLEDs. Herein reported
are novel bipolar molecules ACBP and ADBP (Scheme 1)
Scheme 1. Synthesis of ADBP, ACBP, and DABP
Photophysical properties of the compounds were investi-
gated, and the results are summarized in Table 1. The
Table 1. Physical and Chemical Properties of ADBP, ACBP,
and DABP
ADBP
ACBP
DABP
a
λabs (nm)
375, 394
408/1.8
1.07
375, 394
404/28
1.37
376, 394
405/31
1.56
λem[nm]/Φfb(%)
c
Eox (eV)
c
Ered (eV)
-1.84
-1.82
-1.82
HOMO/LUMOd(eV) -5.41/-1.66 -5.76/-1.75 -5.92/-1.93
∆Ee (eV)
3.75
2.65 (2.73)
4.00
2.66 (2.74)
3.99
2.67 (2.74)
f
ET (eV)
designed by incorporating an acridinone moiety and a
triarylamine or carbazole moiety into one molecule, which
can be used as the host for both fluorescent and phospho-
rescent OLEDs with remarkable reduction in drive voltage
and improvement in efficiencies.
a Lowest absorptions in dichloromethane. b Emission maximum in
dichloromethane. Quantum yields were measured using quinine sulfate
monohydrate in 0.1 M aqueous sulfuric acid solution as reference.
c Determined vs SCE in toluene-acetonitrile (v/v 1:1). d Estimated from
the DFT calculations. e HOMO-LUMO gap. f Triplet energy estimated from
the phosphorescent spectra. Calculated values are shown in parentheses.
The design of ACBP and ADBP is based on the following
considerations. First, triarylamines are well-known hole-
transporting materials. Second, compared to CBP (4,4′-di(9H-
carbazol-9-yl)biphenyl) that is considered to be bipolar and
comparison of the absorption spectra of ADBP, ACBP, and
DABP with those of intermediates 1, 2, and acridinone
suggests that the conjugation is essentially interrupted
between acridinone and biphenyl moieties, since the lowest
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C. H. Appl. Phys. Lett. 2006, 89, 253504. (b) Toshinori, T.; Adachi, C.
J. Appl. Phys. 2008, 103, 034501. (c) He, G.; Schneider, O.; Din, D.; Zhou,
X.; Pfeiffer, M.; Leo, K. J. Appl. Phys. 2004, 95, 5773.
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Phys. Lett. 2005, 86, 103506. (b) Kondakova, M. E.; Deaton, J. C.;
Kondakov, D. Y.; Pawlik, T. D.; Young, R. H.; Brown, C. T.; Giesen, D. J.
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G. Org. Electronics. 2008, 9, 285.
Org. Lett., Vol. 11, No. 19, 2009
4311

energy absorptions for all three compounds are almost the
same and very similar in both shape and energy to those of
acridinone (371, 389 nm). The lowest energy absorption can
be assigned as πfπ* transition.10 All three compounds
emitted in the deep blue region with the emission maximum
between 404 and 408 nm in dichloromethane. The quantum
yield measured in solution for ADBP is significantly lower
than those of the other two compounds. Triplet energies were
estimated to be 2.65-2.67 eV from the phosphorescent
spectra of the compounds measured in a frozen matrix in
toluene at 77 K, which indicate that all three compounds
can be used to sensitize triplet green emitters or even blue
green emitters. The oxidation and reduction potentials were
measured from their cyclic voltammograms. All three
compounds exhibited similar reduction potentials ranging
from 1.82 to 1.84 eV (vs SCE), indicating that they are
potentially electron-transporting materials. The oxidation
potential increases in the order of ADBP, ACBP, and DABP,
indicating the decreasing hole injection/transporting ability
of the molecules.
Further examination of the electronic properties by DFT
calculations was carried out on ADBP, ACBP, and DABP.
Optimized ground-state geometries revealed twisted struc-
tures for these molecules. The acridinone moiety and its
N-phenyl ring are nearly perpendicular to each other with
the torsional angle between 89.8° and 90.8° for the three
compounds. For ACBP, the torsional angle between the
carbazole and its N-phenyl ring is 57.1°. The two phenyl
rings of the biphenyl unit are twisted by 38.2°, 38.3°, and
41.0° in ADBP, ACBP, and DABP, respectively. These
structural features offer further explanation for the disruption
of the conjugation in the molecules. Calculated HOMO and
LUMO energies and triplet energies of the compounds are
listed in Table 1. The HOMO and LUMO characters are
shown in Figure 1. ADBP has clearly localized frontier
10(9H)-yl group as an electron-withdrawing group, since the
nitrogen would provide its lone pair to form an extended
conjugation within the acridinone moiety. Therefore, the
biphenyl in DABP becomes more electron-deficient. The
same argument is also applied to ACBP.
To evaluate new compounds as a host for triplet emitters
in phosphorescent OLED devices, we have chosen the
commonly used triplet green emitter Ir(ppy)3 for our device
fabrications. The OLED devices were fabricated by multi-
layer vapor deposition with the structure of indium tin
oxide(ITO)/NPB(75 nm)/TCTA(10 nm)/host + Ir(ppy)3 (6%)
(20 nm)/Bphen (50 nm)/LiF/Al. TCTA is used as the exciton-
blocking layer to prevent excitons generated by electroex-
citation from diffusing into the hole-transporting NPB layer.
LiF is used as the electron injection layer and Bphen (4,7-
diphenyl-1,10-phenanthroline) as the electron-transporting
layer. The performance of the devices at 1 and 5 mA/cm2 is
summarized in Table 2 with the data obtained at 5 mA/cm2
listed on the right-hand column. All four devices emit
essentially the same color of green light that is from the
triplet dopant Ir(ppy)3. Several points are noteworthy by
comparing the performance of these devcies, for example,
at 1 mA/cm2. First, the drive voltage for devices with new
bipolar host ADBP (2.79 V) or ACBP (3.29 V) is substan-
tially lower than that for the device with CBP (4.16 V) or
DABP (3.99 V) as the host. Second, the efficiencies achieved
with new bipolar hosts ADBP (49.1 cd/A; 14.0% EQE) and
ACBP (58.9 cd/A; 16.9% EQE) are higher than those
obtained with CBP (40.5 cd/A; 11.6% EQE) or DABP (47
cd/A; 13.4% EQE) as the host. Third, as can be seen from
Table 2, the combination of both the improvement in
quantum efficiency and the reduction in the drive voltage
results in remarkably higher power efficiency. Both ADBP
(55.3 lm/W) and ACBP (56.2 lm/W) demonstrated around
80% and 50% increases in power efficiency when compared
to CBP and DABP, respectively. The performance at 5 mA/
cm2 shows the same trend but with a slight decrease in
efficiencies for all hosts.
The use of the bipolar materials as the host for singlet
emitters in OLED devices was also explored. By using
recently developed deep blue dopant, difluoro[6-mesityl-N-
(2(1H)-quinolinylidene-κN)-(6-mesityl-2-quinolinaminato-
κN1)]boron (BFD),11 the devices were fabricated with the
structure of ITO/NPB(75 nm)/TCTA(10 nm)/host + BFD
(1%) (20 nm)/Bphen (40 nm)/LiF/Al. The performances of
the devices with different hosts are shown in Table 2. The
(7) (a) Doi, H.; Kinoshita, M.; Okumoto, K.; Shirota, Y. Chem. Mater.
2003, 15, 10804. (b) Chen, C.-T.; Wei, Y.; Lin, J.-S.; Moturu, M. V. R. K.;
Chao, W.-S.; Tao, Y.-T.; Chien, C.-H. J. Am. Chem. Soc. 2006, 128, 10992.
(c) Li, Z. H.; Wong, M. S.; Fukutani, H.; Tao, Y. Org. Lett. 2006, 8, 427.
(d) Ge, Z.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.; Miyamoto, H.;
Kajita, T.; Kakimoyo, M. Org. Lett. 2008, 10, 421. (e) Lai, M.-Y.; Chen,
C.-H.; Huang, W.-S.; Lin, J.-T.; Ke, T.-H.; Chen, L.-Y.; Tsai, M.-H.; Wu,
C.-C. Angew. Chem., Int. Ed. 2008, 47, 581. (f) Tao, Y.; Wang, Q.; Yang,
C.; Zhang, Z.; Zou, T.; Qin, J.; Ma, D. Angew. Chem., Int. Ed. 2008, 47,
810.
Figure 1. HOMO and LUMO surfaces from DFT calculations.
orbitals with HOMO localized in triphenylamine moiety and
LUMO in acridinone moiety. The HOMO and LUMO in
ACBP are less localized with HOMO predominately in
carbazolyl and the LUMO in the biphenyl moiety. Interest-
ingly, the HOMO in DABP are localized in the acridinone
moiety and the LUMO is localized in the biphenyl moiety.
This might be explained by considering the 9-oxoacridin-
(8) Buchwald, S. L.; Shafir, A. J. Am. Chem. Soc. 2006, 128, 8742.
(9) This compound was reported in a patent literature, but no charac-
terization was provided . Onikubo, S.; Enokida, T.; Suda, Y.; Toba, Y.;
Kimura, Y.; Kaneko, T. Jpn. Kokai Tokkyo Koho, JP2005089544A, 2005.
(10) Nikolov, P.; Petkova, I.; Ko¨hler, G.; Stojanov, S. J. Mol. Struct.
1998, 448, 247.
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Org. Lett., Vol. 11, No. 19, 2009

Table 2. Performance of OLED Devices at Different Current Densities (1 mA/cm2 and 5 mA/cm2)a
e
g
host
dopant
Vb (V)
Lc (cd/m2)
ηcd (cd/A)
ηp (lm/W)
EQEf (%)
λmax (nm)
CIE (x,y)h
ADBP Ir(ppy)3 2.79 3.38 491 2361 49.1 47.2 55.3 44.0 14.0 13.5
ACBP Ir(ppy)3 3.29 3.94 589 2793 58.9 55.9 56.2 44.5 16.9 16.1
DABP Ir(ppy)3 3.99 4.83 472 2290 47.0 45.8 37.1 29.8 13.4 13.0
513
514
519
515
453
454
456
453
513
514
518
514
453
454
456
453
0.29, 0.63 0.29,0.63
0.29, 0.63 0.29,0.63
0.32, 0.62 0.31,0.62
0.29, 0.63 0.29,0.63
0.15, 0.17 0.15, 0.17
0.14. 0.13 0.14, 0.13
0.15, 0.16 0.15,0.15
0.14, 0.11 0.14, 0.11
CBP
Ir(ppy)3 4.16 5.05 405 1849 40.5 37.0 30.6 23.0 11.6 10.6
ADBP BFD
ACBP BFD
DABP BFD
2.91 3.50
3.43 4.06
3.87 4.52
4.10 5.29
57
56
26
9
278
266
150
62
5.7
5.6
2.6
1.0
5.6
5.3
3.0
1.3
6.1
5.1
2.1
0.7
5.0
4.1
2.1
0.7
4.1
5.3
2.1
1.0
4.1
5.1
2.5
1.3
CBP
BFD
a The values at 5 mA/cm2 are listed on the right-hand column. b Drive voltage. c Luminance. d Current efficiency. e Power efficiency. f External quantum
efficiency. g Electroluminescent emission maximum. h Commission Internationale de I’Eclairage chromaticity coordinates.
same trends as those observed in the performance of the
triplet devices were also found for the corresponding singlet
devices. For example, ADBP demonstrated 190% and 770%
increase in power efficiency compared to DABP and CBP,
respectively. The lowest drive voltage (2.91 V) was observed
for the devices with ADBP as the host while the highest
quantum efficicency (5.3% EQE) was obtained when ACBP
was used as the host.
In summary, new bipolar molecules capable of transporting
both electrons and holes have been constructed by incorpo-
rating both acridinone and triarylamine/carbazolyl moieties
into the molecule while maintaining a desired energy gap
for using as the host for both singlet and triplet emitters.
The photophysical properties of the compounds have been
investigated. DFT calculations were performed to further
elucidate the electronic structures and geometries of the
molecules. Bipolar ADBP and ACBP have demonstrated
superior performance as the host for the deep blue fluorescent
dopant and the green phosphorescent dopant in the OLED
devices, showing significant reductions in drive voltage and
improvements in efficiencies. The new host materials capable
of sensitizing both deep blue fluorescent and green phos-
phorescent dopants may have potential for use as the host
material in hybrid white OLED devices where the quantum
efficiency of the devices could be maximized.12
Acknowledgment. We thank Dustin Comfort and Rebacca
Winters at Kodak for device febrications and Marcel Madaras
at Kodak for electrochemical studies. S.H. is thankful to the
startup funds provided by ECU. D.A.K.V. is a recipient of
the Burroughs Wellcome Fellowship at ECU.
Supporting Information Available: Synthesis informa-
tion, experimental details; 1H and 13C NMR chemical shifts;
photophysical spectra of ADBP, ACBP, and DABP. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL901584G
(11) Deaton, J. C.; Place, D. W.; Brown, C. T.; Rajeswaran, M.;
Kondakova, M. E. Inorg. Chim. Acta 2008, 361, 1020.
(12) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.;
Forrest, S. R. Nature 2006, 440, 908.
Org. Lett., Vol. 11, No. 19, 2009
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