Bipyridyl-substituted benzo[1,2,3]triazoles as a thermally stable electron transporting material for organic light-emitting devices

Article abstract of DOI:10.1039/c1jm10901d

We developed new electron-transporting materials (ETMs) for organic light-emitting devices (OLEDs) based on benzo[1,2,3]triazole and two bipyridines. Four derivatives based on the same skeleton were synthesized with four different substituents: phenyl (BpyBTAZ-Ph), biphenyl (-BP), m-terphenyl (-mTP), and o-terphenyl (-oTP). These BpyBTAZ compounds have good thermal stabilities, and their decomposition temperatures were greater than 410 °C, which is significantly higher than that of tris(8-quinolinolato)aluminium (Alq), the conventional OLED material. BpyBTAZ compounds show preferable amorphous nature, and moreover, the glass transition temperatures (T_gs) of both BpyBTAZ-TP compounds exceed 100 °C. Furthermore, BpyBTAZ-BP exhibits no melting point and is fully amorphous. The electron affinities of the materials are as large as 3.3 eV and their electron mobility is sufficiently high. These characteristics accounted for a reduction in the operational voltage of OLEDs with BpyBTAZ compounds compared with the reference device with Alq as an ETM. Specifically, the electron mobility of all the BpyBTAZ compounds exceeds 1 × 10^-4 cm² V^-1 s^-1 at an electric field of 1 MV cm^-1. In addition, it was revealed that both BpyBTAZ-TP-based devices showed longer luminous lifetimes and smaller voltage increases during continuous operation at 50 mA cm^-2, compared with the Alq reference device. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1jm10901d

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PAPER
Bipyridyl-substituted benzo[1,2,3]triazoles as a thermally stable electron
transporting material for organic light-emitting devices†
ab
Shunji Mochizuki,a Hyeon-Gu Jeon,a Shuichi Hayashi,c Norimasa Yokoyamac
*
Musubu Ichikawa,
and Yoshio Taniguchid
Received 2nd March 2011, Accepted 25th May 2011
DOI: 10.1039/c1jm10901d
We developed new electron-transporting materials (ETMs) for organic light-emitting devices (OLEDs)
based on benzo[1,2,3]triazole and two bipyridines. Four derivatives based on the same skeleton were
synthesized with four different substituents: phenyl (BpyBTAZ-Ph), biphenyl (-BP), m-terphenyl
(-mTP), and o-terphenyl (-oTP). These BpyBTAZ compounds have good thermal stabilities, and their
decomposition temperatures were greater than 410 ꢀC, which is significantly higher than that of tris(8-
quinolinolato)aluminium (Alq), the conventional OLED material. BpyBTAZ compounds show
preferable amorphous nature, and moreover, the glass transition temperatures (Tgs) of both
BpyBTAZ-TP compounds exceed 100 ꢀC. Furthermore, BpyBTAZ-BP exhibits no melting point and is
fully amorphous. The electron affinities of the materials are as large as 3.3 eV and their electron
mobility is sufficiently high. These characteristics accounted for a reduction in the operational voltage
of OLEDs with BpyBTAZ compounds compared with the reference device with Alq as an ETM.
Specifically, the electron mobility of all the BpyBTAZ compounds exceeds 1 ꢁ 10ꢂ4 cm2 Vꢂ1 sꢂ1 at an
electric field of 1 MV cmꢂ1. In addition, it was revealed that both BpyBTAZ-TP-based devices showed
longer luminous lifetimes and smaller voltage increases during continuous operation at 50 mA cmꢂ2
compared with the Alq reference device.
,
transport of electrons, easy injection of electrons from the
cathode, and good operational durability.20 Tris (8-hydroxy-
quinolinato) aluminium(III) (Alq) was used in the first bright
OLED reported by Tang and VanSlyke in 1987.5 Until recently,
owing to its good operational durability, Alq was industrially
used as an ETM despite its low electron mobility.28 However, the
speed of electron feeding from the cathode into an emissive layer
is one of the crucial device characteristics. Thus, the development
of efficient ET organic amorphous semiconducting materials is
an important objective.
1. Introduction
Organic light-emitting devices (OLEDs) are of interest for
potential applications in various fields such as flat-screen televi-
sions, mobile displays,1,2 lights,3 and optical communication light
sources.4 OLEDs are usually constructed with functionally
divided organic multilayers—for example, hole-transporting
(HT), emissive, and electron-transporting (ET) layers.5,6 Many
kinds of amorphous molecular semiconductor materials,7,8 HT
materials,9–16 and ET materials (ETMs)17–27 have been proposed
during the last 10 years. Among these substances, HT molecular
semiconducting materials have become practical due to their
high charge-carrier mobility and excellent operational durability,
but few reports exist to date on ET organic semiconducting
amorphous materials that perform well in terms of high-speed
1,2,4-Triazole derivatives are frequently utilized as ETMs,
hole-blocking materials, host materials, and as ligands for light-
emitting phosphorescent materials for OLEDs.29–35 1,2,4-triazole
has other isomeric forms, namely, 1H-1,2,3-triazole and 2H-
1,2,3-triazole as shown in Fig. 1. Based on computational
chemistry, the energies of the lowest unoccupied molecular
orbital (LUMO) are ꢂ0.65 eV for 1H-1,2,3-triazole, ꢂ0.71 eV for
2H-1,2,3-triazole, and ꢂ0.48 eV for 1,2,4-triazole. Therefore,
ETMs based on 1,2,3-triazoles appear to be a good potential for
OLEDs because of their high electron affinity. Nevertheless, the
reports of OLEDs based on 1,2,3-triazole derivatives are
scarce—for example, phosphorescent Ir complexes containing
1H-1,2,3-triazolyl derivatives as a ligand36,37 and a bipolar host
material consisting of carbazolyl and 1H-1,2,3-triazolyl groups
for a blue phosphorescent light-emitter.38 As far as we know,
aFunctional Polymer Science Course, Division of Chemistry and Materials,
Faculty of Textile Science and Technology, Shinshu University, 3-15-1
Tokida, Ueda, 386-8567, Japan
bPresto, Japan Science and Technology Agency (JST), 4-8-1 Honcho,
Kawaguchi, Saitama, 332-0012, Japan
cHodogaya Chemical Co., Ltd., 45 Miyukigaoka, Tsukuba, Ibaraki, 305-
0841, Japan
dShinshu University, 3-15-1 Tokida, Ueda, 386-8567, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm10901d
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J. Mater. Chem., 2011, 21, 11791–11799 | 11791

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materials were synthesized in the same manner except for one of
the starting reactants. The first intermediate in the scheme, AA-
Ar, was synthesized from o-phenylenediamine and a nitroben-
zene derivative based on the literature.42 The resulting AA-Ar
was brominated directly in chloroform, and then Br-BTAZ-Ar
was obtained by an oxidative cyclization with diacetoxy-
iodobenzene (PhI(OAc)2) in toluene, as reported in the litera-
ture.43 Br-BTAZ-Ar was changed into PinB-BTAZ-Ar by
Suzuki–Miyaura coupling reaction with bis(pinacolato)diboron
(Pin2B2), and then BpyBTAZ-Ar was obtained by Suzuki–
Miyaura coupling reaction with 6-bromo[2,20]bipyridine, the
synthesis of which was described in detail elsewhere.44 Before
evaluation and measurement, the resulting BpyBTAZ
compounds were purified by temperature-gradient sublimation
in a flow stream of pure Ar gas. Other OLED materials were
obtained from industrial sources as sublimation grades and used
without further purification.
Fig. 1 LUMO and HOMO energies of the three isomers of triazole.
there are no reports of 1H- and 2H-1,2,3-triazole derivatives used
as an ETM, and also no reports about OLED materials based on
2H-1,2,3-triazole. However, 2H-benzo[1,2,3]triazole (BTAZ)-
based materials must have a good potential to be used as ETMs
since 2H-1,2,3-triazole has the deepest LUMO and, moreover,
a derivative group of 2H-1,2,3-triazole, 2-(20-hydroxyphenyl)
benzo[1,2,3]triazoles, is widely utilized in polymers and plastics
as an ultra-violet (UV) light absorber with a high thermal
stability. Hence, we have tried to develop ETMs based on the
BTAZ skeleton. We introduce bipyridyl groups to BTAZ
because we have successfully produced several good ETMs based
on bipyridyl-substituted compounds.39–41 In this paper we
demonstrate bis(bipyridyl)-substituted 2H-benzo[1,2,3]triazole
derivatives (BpyBTAZ compounds) as ETMs with high thermal
stability.
2.2 Synthesis
4,6-Dibromo-2-phenyl-benzo[1,2,3]triazole
(Br-BTAZ-Ph).
o-Phenylenediamine (8 g, 74.0 mmol), nitrobenzene (7.56 ml,
74.0 mmol), and sodium hydroxide (3.0 g, 74.0 mmol) were
mixed and triturated, followed by heating at 70 ꢀC with constant
scratching for 5 h. The dark pasty mass thus obtained was cooled
to room temperature and extracted with toluene. The toluene-
soluble part was subjected to column chromatography on a silica
gel (NH silica) column. The third band (Rf ¼ 0.7) that was eluted
by a hexane–toluene mixture (1/1 v/v) was collected, and upon
evaporation of the solvent, this procedure yielded 2.4 g (16.4%)
of the pure 2-(phenylazo)aniline (AA-Ph). dH (600 MHz,
CDCl3): 7.84 (3H, d, J ¼ 7.8 Hz), 7.49 (2H, dd, J ¼ 7.8 Hz), 7.42
(1H, dd, J ¼ 7.8 Hz), 7.22 (1H, dd, J ¼ 7.8 Hz), 6.82 (1H, dd, J ¼
7.2 Hz), 6.77 (1H, d, J ¼ 8.2 Hz), 5.9 (2H, s). AA-Ph (5.7 g, 28.9
mmol) was dissolved in dry chloroform (100 ml), bromine (3.0
2. Experimental
2.1 Materials
Scheme 1 shows chemical structures of the newly developed
BpyBTAZ compounds and their synthetic routes. These
Scheme 1 Synthetic route of BpyBTAZ compounds.
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ml, 58.4 mmol) in chloroform (60 ml) was added dropwise for 30
min, and stirring was continued 1 h. The pH of the reaction
solution was adjusted to 7.0 by adding drops of aqueous solution
of sodium hydrogen carbonate, and the chloroform layer was
then dried (MgSO4) and evaporated. The product Br-AA-Ph was
obtained as orange flakes (9.75 g, 95% yield) at 88% purity by
high-performance liquid chromatography (HPLC). dH (600
MHz, CDCl3): 7.93 (1H, s), 7.84 (2H, d, J ¼ 7.8 Hz), 7.62 (1H, s),
7.52 (2H, dd, J ¼ 7.2 Hz), 7.47 (1H, dd, J ¼ 7.2 Hz), 6.25 (2H, s).
Br-AA-Ph (7.5 g, 21.1 mmol) and PhI(OAc)2 (7.5 g, 23.3 mmol)
were dissolved in toluene (375 ml) and stirred continuously for 23
h at 40 ꢀC. Water (150 ml) was added and the pH of the mixture
was again adjusted to 7.0 by using an aqueous solution of satu-
rated sodium sulfite. Then, the chloroform layer was dried
(MgSO4) and evaporated. The residue was solved in toluene and
purified by adsorbing with NH silica gel. The final recrystalli-
zation from ethyl acetate gave ivory white crystal of Br-BTAZ-
Ph at 5.96 g (63%) yield and 99% purity by HPLC. dH (600 MHz,
CDCl3): 8.36 (2H, d, J ¼ 8.2 Hz), 8.07 (1H, d, J ¼ 1.4 Hz), 7.73
(1H, d, J ¼ 1.4 Hz), 7.57 (2H, dd, J ¼ 7.6 Hz), 7.51–7.49 (1H, m).
refluxing with constant scratching for 17 h. The reaction mixture
was cooled to room temperature and poured into a saturated
NaCl aqueous solution. The obtained precipitation was filtered
and extracted with toluene. The toluene-soluble part was sub-
jected to column chromatography on a silica gel column. The
third band (Rf ¼ 0.3) that was eluted by a hexane–toluene
mixture (3/4 v/v) was collected, and upon evaporation of the
solvent yielded 2-(20-biphenylazo)aniline (AA-BP) (6.94 g, 31%
yield) at 97% purity by HPLC. dH (600 MHz, CDCl3): 8.30 (1H,
d, J ¼ 7.8 Hz), 8.11 (1H, d, J ¼ 6.0 Hz), 7.5–7.4 (5H, m), 7.41
(2H, t, J ¼ 7.2 Hz), 6 (1H, d, J ¼ 7.2 Hz), 7.13 (1H, dd, J ¼ 8.3
Hz), 6.78 (1H, dd, 8.4 Hz), 6.60 (1H, d, J ¼ 8.4 Hz), 5.80 (2H, s).
AA-BP (15.0 g, 54.9 mmol) was dissolved in dry chloroform (200
ml), and bromine (6.8 ml, 131.8 mmol) in chloroform (100 ml)
was added dropwise for 30 min, and the mixture was stirred
continuously for 5 h. Water (100 ml) was added, and the chlo-
roform layer was then dried (MgSO4) and evaporated. The
residue (dark pasty mass) was subjected to column chromatog-
raphy on a silica gel column. The second band (Rf ¼ 0.5) that was
eluted by a chloroform–hexane mixture (1/3 v/v) was collected,
and upon evaporation of the solvent yielded 2-(20-biphenylazo)
4,6-dibromoaniline (Br-AA-BP) (9.90 g, 42.6% yield) at a purity
of 96% by HPLC. dH (600 MHz, CDCl3): 7.93 (1H, s), 7.82 (1H,
d, J ¼ 7.2 Hz), 7.55 (1H, s), 7.52 (2H, d, J ¼ 4.2 Hz), 7.5–7.4 (6H,
m), 6.33 (2H, s). Br-AA-BP (11.7 g, 27.1 mmol) and PhI(OAc)2
(8.7 g, 27.1 mmol) were dissolved in toluene (300 ml) and stirred
continuously for 7 h at 60 ꢀC. Water (200 ml) was added and the
pH of the mixture was adjusted to 7.5 by adding soda ash. Then,
the toluene layer was dried (MgSO4) and evaporated. The residue
was subjected to column chromatography on a silica gel column.
The fourth band (Rf ¼ 0.3) that was eluted by a chloroform–
hexane mixture (1/3 v/v) was collected, and upon evaporation of
the solvent yielded 2-(20-biphenylazo)-4,6-dibromo-2H-benzo-
triazole (Br-BTAZ-BP) (10.5 g, 91% yield) at 95% purity by
HPLC. dH (600 MHz, CDCl3): 7.91 (1H, s), 7.76 (1H, d, J ¼ 7.2
Hz), 7.68 (1H, s), 7.63 (1H, t, J ¼ 7.2 Hz), 7.58 (1H, d, J ¼ 6.6
Hz), 7.55 (1H, t, J ¼ 8.4 Hz), 7.23–7.19 (3H, m), 7.06(2H, d, J ¼
7.2 Hz). Br-BTAZ-Ph (10.5 g, 25.4 mmol), Pin2B2 (15.5 g, 61.0
mmol), potassium acetate (15.0 g, 152.4 mmol), and the catalyst
PdCl2(dppf)2 were dissolved in 1,4-dioxane (250 ml). The reac-
tion mixture was refluxed for 7 h at 90 ꢀC and then cooled to
room temperature. Pure water (500 ml) was added, and the
mixture was stirred for 30 min and filtered to remove precipitates.
A black crude pasty mass was obtained by extraction using
chloroform and evaporation. The crude mass was dissolved in
ethyl acetate and then purified by adsorption on silica gel.
Evaporation and hexane washing produced PinB-BTAZ-BP as
a greenish white crystalline solid (8.6 g, 66% yield) at 93% purity
by HPLC. dH (600 MHz, CDCl3): 8.42 (1H, s), 8.25 (1H, s), 7.86
(1H, d, J ¼ 7.2 Hz), (3H, m), 7.14(3H, m), 7.05 (2H, d, J ¼ 7.2
Hz), 1.36 (26H, d, J ¼ 5Hz). BpyBTAZ-BP was synthesized from
PinB-BTAZ-BP (4.0 g, 6.90 mmol) and Br-Bpy (3.8 g, 17.3
mmol) by the Suzuki–Miyaura coupling reaction using 2 M
K2CO3 aqueous solution (10.4 ml, 20.7 mmol) as base, tetrakis
(trisphenylphosphine)palladium(0) (0.40 g, 0.35 mmol) as cata-
lyst, and 200 ml of a 1/4 ethanol/toluene mixture as solvent. The
reaction mixture was refluxed with stirring for 6 h and then
cooled to room temperature. Water (1000 ml) was poured onto
the mixture and the toluene layer was dried (MgSO4) and
BpyBTAZ-Ph. Br-BTAZ-Ph (7.62 g, 21.6 mmol), Pin2B2
(13.7 g, 53.8 mmol), potassium acetate (13.2 g, 134.4 mmol),
and [1,10-bis(diphenylphosphino)ferrocene] dichloropalladium
(II) (PdCl2(dppf)2) as catalyst were dissolved in 1,4-diozane (160
ml). The reaction mixture was refluxed for 72 h at 90 ꢀC, and then
cooled to room temperature. Pure water (500 ml) was added, and
the mixture was stirred for 30 min and filtered to remove
precipitates. Black crude crystalline powder was obtained by
extraction using ethyl acetate and evaporation. The crude was
dissolved in toluene–ethyl acetate mixed solvent (1/1 v/v) and
then purified by adsorbing with silica gel. Evaporation and
hexane washing gave a gray crystalline solid of PinB-BTAZ-Ph
at 7.6 g (70%) yield and 92% purity by HPLC. dH (600 MHz,
CDCl3): 8.56 (1H, s), 8.42 (2H, d, J ¼ 7.8 Hz), 8.30 (1H, s), 7.55
(2H, dd, J ¼ 8.4 Hz), 7.44 (1H, dd, J ¼ 7.2 Hz), 1.45 (13H, s),
1.39 (13H, s). BpyBTAZ-Ph was synthesized from PinB-BTAZ-
Ph (4.0 g, 8.95 mmol) and Br-Bpy (5.0 g, 22.4 mmol) by Suzuki–
Miyaura coupling reaction using 2 M K2CO3 aqueous solution
(13.4 ml, 26.8 mmol) as base, tetrakis(trisphenylphosphine)
palladium(0) (0.52 g, 0.45 mmol) as catalyst, and 200 ml of a 1/4
ethanol/toluene mixture as solvent. The reaction mixture was
refluxed with stirring for 5 h and then cooled to room tempera-
ture. Precipitations appeared and were collected by filtration,
and then washed with pure water and toluene. A final wash with
acetonitrile under reflux produced 4.7 g (85%) of a greenish white
crystal of BpyBTAZ-Ph at 99% purity by HPLC. dH (600 MHz,
CDCl3): 9.60 (1H, d, J ¼ 2.1 Hz), 8.98 (1H, d, J ¼ 7.6 Hz), 8.83–
8.79 (3H, m), 8.74 (2H, t, J ¼ 4.1 Hz), 8.51–8.48 (4H, m), 8.08–
8.04 (2H, m), 8.00 (1H, dd, J ¼ 7.6 Hz), 7.87 (2H, td, J ¼ 7.6, 2.1
Hz), 7.62 (2H, dd, J ¼ 7.9 Hz), 7.51 (1H, dd, J ¼ 7.2 Hz), 7.38–
7.36 (2H, m).
BpyBTAZ-BP. BpyBTAZ-BP was synthesized by the same
route as BpyBTAZ-Ph except for using 2-nitrobiphenyl instead
of nitrobenzene to obtain a corresponding azo intermediate. o-
Phenylenediamine (9.1 g, 83.8 mmol), 2-nitrobiphenyl (16.7 g,
83.8 mmol), sodium hydroxide (5.57 g, 117.5 mmol), and toluene
(35 ml) were mixed and triturated, followed by heating under
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evaporated. The residue was subjected to column chromatog-
raphy on a silica gel column. The third band (Rf ¼ 0.5) that was
eluted by chloroform was collected, and upon evaporation of the
solvent yielded BpyBTAZ-BP (3.2 g, 73% yield) at 98% purity by
HPLC. dH (600 MHz, DMSO-d6): 9.60 (1H, s), 8.85 (1H, s), 8.75
(2H, d, J ¼ 3.4 Hz), 8.68 (2H, dd, J ¼ 8.6 Hz), 8.42–8.41 (2H, m),
8.31 (1H, d, J ¼ 6.9 Hz), 8.16 (1H, d, J ¼ 8.9 Hz), 8.10 (1H, dd,
J ¼ 7.9 Hz), 8.06 (1H, d, J ¼ 4.5 Hz), 7.99–7.98 (3H, m), 7.80
(1H, dd, J ¼ 7.2 Hz), 7.75–7.72 (2H, m), 7.53–7.52 (2H, m), 7.30–
7.27 (3H, m), 7.15 (2H, d, J ¼ 6.9 Hz). Mass spectrum: 580.44
and isotopes (100%) ([M + H]+; 579.22 for M: C36H25N7) after
sublimation purification.
AA-oTP. dH (600 MHz, CDCl3): 7.51(1H, dd, J ¼ 6.6 Hz),
7.45 (2H, d, J ¼ 7.8 Hz), 7.34 (4H, m), 7.27 (6H, m) 7.05 (1H, dd,
J ¼ 7.2 Hz), 6.91(1H, d, J ¼ 7.8 Hz), 6.63(1H, d, J ¼ 8.4 Hz), 6.42
(1H, dd, J ¼ 7.2 Hz), 5.75(2H, s).
Br-AA-oTP. dH (600 MHz, CDCl3): 7.66 (1H, s), 7.58 (1H, dd,
J ¼ 7.8 Hz), 7.50 (2H, d, J ¼ 7.8 Hz), 7.37 (4H, dd, J ¼ 7.2 Hz),
7.25 (6H, d, J ¼ 7.8 Hz), 7.02 (1H, s).
Br-BTAZ-oTP. dH (600 MHz, CDCl3): 7.82 (1H, s) 7.69 (1H,
dd, J ¼ 7.8 Hz), 7.57 (1H, s), 7.55 (2H, d, J ¼ 7.2 Hz), 7.10–7.26
(10H, m).
BpyBTAZ-mTP. BpyBTAZ-mTP was synthesized by almost
the same procedure as BpyBTAZ-BP except for the following: 50-
nitro[1,10;30,100]terphenyl was used instead of nitrobenzene to
obtain a corresponding intermediate, toluene was removed from
the reaction mixture using the Dean–Stark apparatus at the
initial stage of the reaction, and the reaction conditions were 140
ꢀC for 8 h.45 The resulting BpyBTAZ-mTP was pale yellow with
the following characteristics: dH (600 MHz, CDCl3): 9.56 (1H, s),
8.92 (1H, d, J ¼ 7.2 Hz), 8.80 (3H, d, J ¼ 8.4 Hz), 8.74–8.73 (2H,
m), 8.67 (2H, s), 8.49 (2H, dd, J ¼ 7.8 Hz, 9.0 Hz), 8.06 (1H, dd, J
¼ 7.8 Hz), 8.01 (1H, d, J ¼ 7.8 Hz), 7.97 (1H, dd, J ¼ 7.8 Hz),
7.91 (1H, s), 7.84–7.81 (6H, m), 7.54 (4H, dd, J ¼ 7.8 Hz), 7.48
(2H, dd, J ¼ 7.8 Hz), 7.37 (2H, dd, J ¼ 5.4 Hz). Mass spectrum:
655.96 and isotopes (100%) ([M + H]+; 655.25 for M: C44H29N7)
after sublimation purification.
PinB-BTAZ-oTP. dH (600 MHz, CDCl3): 8.15(1H, s), 7.97
(1H, s), 7.86 (1H, t, J ¼ 8.4 Hz), 7.66 (2H, d, J ¼ 7.8 Hz) 7.13–
7.06 (10H, m), 1.34 (12H, m), 1.30(12H, m).
2.3 Device fabrication and measurements
All OLEDs were fabricated on 150 nm-thick layers of indium-tin
oxide (ITO) that were commercially pre-coated onto glass
substrates with a sheet resistance of 14 U sqꢂ1. The solvent-
cleaned ITO surface was treated with a UV ozone cleaner for 15
min and then the substrate was immediately loaded into a high-
vacuum chamber (base pressure below 2 ꢁ 10ꢂ4 Pa) where
organic layers, 0.5 nm thick LiF, and 200 nm thick aluminium
cathode layers were deposited by thermal evaporation. Deposi-
ꢂ1
tion rates were 0.6 A s for organic materials, 0.1 A sꢂ1 for LiF,
ꢀ
ꢀ
and 6 A sꢂ1 for Al. The structure of the devices fabricated in this
ꢀ
AA-mTP. dH (600 MHz, CDCl3): 8.05(2H, s), 7.88 (1H, d, J ¼
10.2 Hz), 7.87 (1H, s), 7.73 (4H, d, J ¼ 7.8 Hz), 7.50 (4H, dd,
J ¼ 7.8 Hz), 7.41 (2H, dd, J ¼ 7.8 Hz), 7.24 (2H, m) 6.84 (1H, dd,
J ¼ 7.8 Hz), 6.79 (1H, d, J ¼ 7.8 Hz), 5.98 (2H, s).
study is shown in Fig. 2. The current density–applied voltage–
luminance (J–V–L) characteristics of the OLEDs were measured
with a commercial OLED-characteristics measurement system
(Precise Gauge EL1003) and a source meter (Keithley 2400).
Br-AA-mTP. dH (600 MHz, CDCl3): 8.03 (2H, s), 7.98 (1H, s),
7.91(1H, s), 7.71 (4H, d, J ¼ 7.2 Hz), 7.63 (1H, s), 7.50 (4H, dd,
J ¼ 7.8 Hz), 7.42 (2H, dd, J ¼ 6 Hz), 6.34 (2H, s).
Br-BTAZ-mTP. dH (600 MHz, CDCl3): 8.56 (2H, s), 8.08 (1H,
s), 7.92 (1H, s), 7.76 (4H, d, J ¼ 7.2 Hz), 7.74(1H, s), 7.52 (4H, d,
J ¼ 7.2 Hz), 7.45(2H,d, J ¼ 7.8 Hz).
PinB-BTAZ-mTP. dH (600 MHz, CDCl3): 8.63 (2H, s), 8.59
(1H, s), 8.33 (1H, s), 7.88 (1H, s), 7.78(4H, d, J ¼ 7.2 Hz), 7.54
(4H, dd, J ¼ 8.4 Hz), 7.44 (2H, d, J ¼ 7.8 Hz), 1.46 (12H,s), 1.40
(12H, s).
BpyBTAZ-oTP. BpyBTAZ-oTP was synthesized by the same
route as BpyBTAZ-mTP except for using 20-nitro[1,10;30,100]ter-
phenyl instead of nitrobenzene to obtain a corresponding inter-
mediate. The resulting BpyBTAZ-oTP was white with the
following characteristics: dH (600 MHz, CDCl3): 9.31 (1H, s),
8.74–8.73 (4H, m), 8.57 (1H, s), 8.44–8.41 (2H, m), 7.96–7.96
(2H, m), 7.92 (1H, d, J ¼ 7.2 Hz), 7.85 (1H, d, J ¼ 7.2 Hz), 7.83
(2H, dd, J ¼ 7.8 Hz), 7.73 (1H, dd, J ¼ 7.2 Hz), 7.64 (2H, d, J ¼
7.8 Hz), 7.35 (2H, dd, J ¼ 7.8 Hz), 7.14–7.14 (10H, m). Mass
spectrum: 655.94 and isotopes (100%) ([M + H]+; 655.25 for M:
C44H29N7) after sublimation purification.
Fig. 2 Structures of devices and chemicals used in this study.
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Electroluminescence (EL) spectra were recorded simultaneously
with the EL1003 during the J–V–L measurements.
Decomposition temperatures (Td) with 5% weight loss were 418
ꢀC for BpyBTAZ-Ph, 422 ꢀC for -BP, 479 ꢀC for -mTP, and 440
ꢀC for -oTP, respectively. These Td values indicate that BpyB-
TAZ compounds can be continuously evaporated for a long time
with less byproduct due to decomposition during vacuum
evaporations. In fact, as shown in the figure, the Td values of
BpyBTAZ compounds were higher than that of Alq (366 ꢀC),
one of the most conventional OLED materials. The evaporation
temperatures of the BpyBTAZ compounds in vacuum evapora-
tors are important when discussing thermal stability. However,
instead of the evaporation temperature, we list the heater
temperature of each BpyBTAZ compound in the sublimation
purification process: 200 ꢀC for BpyBTAZ-P, 230 ꢀC for -BP, 300
1H and 13C NMR spectra were recorded on a Fourier trans-
form (FT)-NMR spectrometer (JEOL JNM-ECA600). Mass
spectroscopic analysis was carried out with a matrix-assisted
laser desorption ionization (MALDI) time-of-flight (TOF) mass
spectrometer (Bruker Daltonics Autoflex) with no internal
standard. Thermal analyses were performed on a differential
scanning calorimeter (Seiko Instruments DSC-6200) and a ther-
mogravimeter/differential thermal analyzer (Bruker AXS TG/
DTA 2000S) at a heating rate of 10 ꢀC minꢂ1 under N2 gas. UV
and visible absorption spectra were recorded with a spectropho-
tometer (Shimadzu UV-3150); photoluminescence (fluorescence
and phosphorescence) spectra were recorded with a spectrofluo-
rometer (Horiba FluoroMax4P). Ionization potential (Ip) was
determined with a photoelectron emission yield spectrometer
(Riken Keiki AC-3). Optical band gaps were determined by the
spectral onset of each UV-visible absorption spectrum; electron
affinity (Ea) was then estimated by subtracting the band gap from
Ip. Spectroscopic measurements were conducted with thin films
prepared by thermal evaporation on quartz substrates. We also
determined the densities of the thin films prepared on Si wafers
by thermal evaporation by using X-ray reflectometry with
a Rigaku SmartLab.
ꢀ
ꢀC for -mTP, and 230 C for -oTP. These ꢀvalues are smaller or
almost the same compared with that (305 C) obtained for Alq
using the same experimental setup. In addition, the figure also
shows the TG curve of bipyridyl-substituted 4H-1,2,4-triazole,41
whose IUPAC name is 2-(1-naphthyl)-6-[4-phenyl-5-[6-(2-pyr-
idyl)-2-pyridyl]-1,2,4-triazol-3-yl]pyridine (BpyTAZ-03). We
consider this 1,2,4-triazole to be a good ET and hole-blocking
material with high electron mobility for both fluorescent and
phosphorescent OLEDs.41 The Td of this 1,2,4-triazole was 394.0
ꢀC. The improved Td values of BpyBTAZ compounds compared
with the 1,2,4-triazole are due to the high thermal stability of
benzo[1,2,3]trizole.
Electron mobility was measured by conventional TOF tech-
niques. The excitation light was a 500-ps-duration optical pulse
from a N2 gas laser (l ¼ 337 nm, Lasertechnik Berlin MNL 200).
Test samples were prepared by thermal evaporation in vacuum
and encapsulated with fresh desiccant under a highly inert
atmosphere of N2 at the dew point of almost ꢂ60 ꢀC and an O2
concentration of <5 ppm. A 100 nm-thick fullerene C60 layer was
used as a charge-generation layer for optical excitation.40 The
structure of TOF samples is shown in Fig. 2.
DSC curves of BpyBTAZ compounds in the figure indicate
that all BpyBTAZ compounds can form amorphous glassy thin
films because glass transition behaviors could be observed in the
second heating after quick cooling at the rate of ꢂ50 C minꢂ1
.
ꢀ
That is, we can find the baseline shifts at 78 ꢀC for BpyBTAZ-Ph,
91 ꢀC for BpyBTAZ-BP, 107 ꢀC for BpyBTAZ-mTP, and 117 ꢀC
for BpyBTAZ-oTP, respectively. BpyBTAZ-oTP exhibited the
highest glass transition temperature (Tg) of 117 ꢀC among all
BpyBTAZ compounds, probably, because of the larger molec-
ular weight and bulkiness of the substituent at the 2nd position of
BTAZ. Surprisingly, BpyBTAZ, whose conformation can be
assumed to be almost planar as shown in Fig. 4(a), has an
amorphous character. Usually, planar aromatic molecules are
crystalline and rarely form amorphous glassy thin films. We
believe that the asymmetric substitutions of bipyridines to BTAZ
might prevent crystallization and cause the amorphous char-
acter. In addition, the geometry of BpyBTAZ-Ph shown in Fig. 4
(b) is the second lowest, and the energy difference is only 0.07 kJ
molꢂ1. This small energy difference can enable mixing of the two
conformations in quenched (rapid cooling) solids and also in
evaporated thin films. This mixing probably also results in the
amorphous nature as reported in the literature.40,41 Note that
BpyBTAZ-BP showed no Tm but exhibited Tg even in the first
heating. This means that BpyBTAZ-BP is a perfectly amorphous
material. The thermal properties of BpyBTAZ compounds are
summarized in Table 1. Note that the densities of BpyBTAZ
compounds in thin-film state increased with increase in molec-
ular weight. This straightforward result probably implies that the
molecular geometries of BpyBTAZ compounds are independent
of the introduced substituents.
Computational chemistry was conducted using commercial
software (Wavefunction Spartan 08W) on a personal computer
with a dual-core processor. Density function theory (DFT) was
used to determine conformations, with a B3LYP hybrid func-
tional and a 6-311 + G** basis set for determining optimized
geometries and their energies.
3. Results and discussion
3.1 Thermal and electronic properties
Fig. 3 shows thermogravimetry (TG) and differential scanning
calorimetry (DSC) curves of BpyBTAZ compounds.
Fig. 5 shows the UV-visible absorption and fluorescence
spectra of BpyBTAZ compounds in their thin-film state. The
figure also shows the phosphorescence spectrum of BpyBTAZ-
oTP in 2-methyltetrahydrofuran at 77 K. All BpyBTAZ
Fig. 3 (a) TG and (b) DSC curves of BpyBTAZ compounds. Dotted
lines in panel (b) show vertically 5-times-magnified curves around the
temperature range of each Tg. The DSC curves were measured in the
second heating at a heating rate of 10 ꢀC minꢂ1 after rapid cooling.
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Fig. 4 Optimized geometries of BpyBTAZ-Ph with the lowest (a) and
the second-lowest (b) energies. The bipyridine on the left of the BTAZ
ring in the lowest geometry (a) was almost coplanar with the BTAZ ring,
whose dihedral angle was 11.0ꢀ, because there was no steric hindrance of
the hydrogen atoms at each ortho-position. However, the dihedral angle
between the BTAZ ring and the other bipyridine was 24.9ꢀ due to the
steric hindrance of the pair of the hydrogen atoms at the other ortho-
positions. The energy difference between the two geometries was 0.07 kJ
Fig. 5 UV-visible absorption and PL spectra of BpyBTAZ compounds
in their thin-film states. The figure also shows the phosphorescence
spectrum of BpyBTAZ-BP in 2-methyltetrahydrofuran at a temperature
of 77 K.
molꢂ1
.
Fig. 5 also shows the phosphorescence spectrum of BpyBTAZ-
oTP, which has the largest triplet energy of the materials. The
phosphorescence was observed in the red region while the fluo-
rescence occurs in the blue region even in solid state, and the
energy difference between the first excited singlet (S1) and triplet
(T1) states was large. In general, the energy difference between S1
and T1 is caused by exchange interaction of the two unpaired
electrons, either in the HOMO or the LUMO, and then each
distribution of the HOMO and LUMO in a molecule. In other
words, spatial overlap of both orbitals strongly affects the energy
of interaction. The large energy differences between S1 and T1 of
all the BTAZ compounds imply that the HOMOs and LUMOs
of the materials largely overlap each other, resulting in consis-
tency with the abovementioned discussion of distribution of their
HOMOs and LUMOs.
compounds show the optical absorption onset at a wavelength of
about 400 nm and pure blue to sky blue fluorescence that peaked
at 410 nm to 480 nm. Band gaps (Eg) of the materials were
estimated from the onsets of the optical absorption spectra and
then summarized in Table 1. The Eg values were as large as 3 eV,
and the Ip values that were determined using photoemission
electron yield spectroscopy varied from 6.2 to 6.5 eV. The
resulting Ea values, estimated by subtracting each Eg from each
Ip, were almost the same (about 3.3 eV). These findings suggest
that the lowest unoccupied molecular orbital (LUMO) of each
BpyBTAZ probably is localized in the bis(bipyridyl)benzo[1,2,3]
triazole unit, but each of the highest occupied molecular orbitals
(HOMOs) appear to be in the BTAZ ring while each substituent
is joined at the second nitrogen atom. This is because the phenyl
ring occupies the same plane as the BTAZ ring with respect to the
absence of steric hindrance between the phenyl ring and the tri-
azole. On the other hand, the blue-shifted fluorescence spectra
and absorption onsets of BpyBTAZ-BP and -oTP (resulting in
the wider Eg and larger Ip shown in Table 1) implies that its
molecular geometry should twist at the single bond connecting
the BTAZ ring with biphenyl or terphenyl due to strong steric
hindrance of another ortho-linked phenyl ring. The large Ea
values of the materials can be effective for electron injection from
the cathode, resulting in reduction of operational voltage. The
relatively deep Ip values probably affect hole-blocking but we did
not investigate this relationship.
3.2 OLED performance
Fig. 6(a) presents J–V curves of the devices with each BpyBTAZ.
The devices with BpyBTAZ compounds showed lower opera-
tional voltage than the reference device without BpyBTAZ.
Specifically, the devices with BpyBTAZ-mTP and -oTP exhibited
a very low operational voltage and a current density of 7.5 mA
cmꢂ2 at a voltage of 3.0 V. In addition, the current density of the
devices with both BpyBTAZ-TP rapidly increased with a small
boost of applied voltage, and reached 100 mA cmꢂ2 at just 4.2 V
Table 1 Basic properties of BpyBTAZ compounds. Tm, Tg, and Td were measured in their solid state. All other properties except for triplet energy (ET)
were measured in their thin-film state. ET was estimated from its methyltetrahydrofuran solution at 77 Ka
Material
Tm/ꢀC
Tg/ꢀC
Td/ꢀC
Density/g cmꢂ3
Ip/eV
Ea/eV
Eg/eV
ET/eV
BpyBTAZ-Ph
BpyBTAZ-BP
BpyBTAZ-mTP
BpyBTAZ-oTP
224
ND
249
231
78
91
107
117
418
422
479
440
1.20
1.22
1.24
ND
6.23
6.50
6.35
6.51
3.25
3.36
3.33
3.27
2.98
3.14
3.02
3.24
2.03
2.08
2.03
2.14
a
ND: not determined.
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Fig. 6 (a) Current density–voltage (J–V) and (b) luminous efficiency–current density (h–J) curves of devices with each BpyBTAZ. The inset in panel (b)
shows the EL spectrum of the BpyBTAZ-mTP device and the dotted line is a ꢁ10 magnification.
for -mTP and 4.3 V for -oTP. This low operational voltage
strongly implies that BpyBTAZ-mTP and -oTP probably possess
both a good electron injection property from the cathode and
high electron mobility. The relatively large Eas of BpyBTAZ
compounds probably caused this good electron injection from
the cathode. The variation of the operational voltages of the
devices with BpyBTAZ compounds indicates that the electron
mobilities of BpyBTAZ-Ph and -BP would be smaller than those
of BpyBTAZ-mTP and -oTP.
voltage. This means that BpyBTAZ compounds have high elec-
tron mobility.
3.3 Electron mobility
Finally, we would like to discuss briefly the electron mobility in
the new materials. Fig. 7 shows, on a double logarithmic scale,
the transient electron photocurrent waveform obtained by TOF
measurements for the BpyBTAZ-mTP film whose thickness is 5.2
mm. Although each photocurrent waveform looks non-disper-
sive, we could clearly see that the transits of photogenerated
charge carriers—electrons in this case—become faster with
increased applied voltages because the photocurrents decayed
more rapidly. In accordance with the conventional method of
transit time (tT) determination for dispersive transportations, we
drew two auxiliary straight lines as shown in the figure and
determined the tT at the crossing point of these auxiliary lines.
Note that the slopes of the auxiliary lines for each photocurrent
are similar, and this also supports the assumption that the
observed photocurrents are due to drift motions in the film. If we
take electron mobility m to be
Fig. 6(b) shows luminous efficiency–current density (h–J)
characteristics of the devices with each BpyBTAZ. The luminous
efficiencies of the devices with BpyBTAZ compounds were
slightly lower than the reference. The EL spectrum of the
BpyBTAZ-mTP device shown in the inset of the figure indicates
that recombination of both charge carriers (electron and hole)
should occur in the Alq layer; this is also true for the devices with
other BpyBTAZ compounds (see Fig. S2 in the ESI†). Note that
no other EL peak was presented even when observed at high
magnification (see inset of Fig. 6(b)). However, the device
showed a lower luminous efficiency. This decrease is probably
caused by interactions at the heterointerfaces of NPB/Alq and/or
Alq/BpyBTAZ. In accordance with Matsumoto and Adachi,46
HTLs with Ip < 5.7 eV cause exciplex-like interactions that
reduce the luminescence yield of Alq in its excited state. Fig. 6(b)
also shows the h–J curve of another device in which a bilayer
combination of 1,1-bis(N-biphenyl-N-phenylaminophenyl)
cyclohexane (TPD-012) having a deeper Ip of 5.7 eV and 4,40-bis
(diphenylanilino-400-yl)N,N,N0,N0-tetraphenylbenzidine (TPT-1)
was used as an HTM instead of NPB.47 The luminous efficiency
for a current of 4.1 cd Aꢂ1 was almost the same as that for
standard Alq-emissive layer devices while its onset voltage (VON
)
obtained at a current density of 5 mA cmꢂ2 was 3.1 V. This is
higher than the VON (2.8 V) of the original device using NPB as
the HTM (see Fig. S3 in the ESI†) and lower than the VON (3.9
V) of the reference device with Alq as the ETM and TPD-012/
TPT-1 as the HTM. We infer that the small increase of VON in
the TPD-012/TPT-1 device compared with the original device
(difference of 0.3 V) arises due to the small HOMO gap of 0.3 eV
at the interface between TPD-012 and TPT-1. Therefore, it can
be assumed that a recombination zone at the HTL/Alq interface
caused the lower luminous efficiencies of the devices with
BpyBTAZ compounds, which could achieve low operational
Fig. 7 Transient photocurrent curves of TOF measurement of BpyB-
TAZ-mTP at the applied voltages of 450 and 540 V. Red straight lines are
drawn to help determine the transit time.
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Dd
m ¼
(1)
tTV
where D is the distance between the two electrodes (5.3 mm), d is
the thickness of the prepared BpyBTAZ-mTP layer (5.2 mm), and
V is the applied voltage, we can determine, for example, that m ¼
3.28 ꢁ 10ꢂ4 cm2 Vꢂ1 sꢂ1 at an electric field E ¼ V/D ¼ 1.02 ꢁ 106
V cmꢂ1. This mobility is two orders of magnitude higher than
that for Alq (3.3 ꢁ 10ꢂ6 cm2 Vꢂ1 sꢂ1) at approximately the same
electric field.48 This high mobility probably accounts for a better
J–V curve for the OLED with BpyBTAZ-mTP compared to that
for the OLED with the corresponding reference. Fig. 8 shows
electron mobility as a function of the square root of the applied
electric field for several materials, including BpyBTAZ
compounds. Both BpyBTAZ-Ph, -mTP, and -oTP show mobility
that is at least two orders of magnitude higher than that of Alq.
This high mobility is comparable to that of a representative
ET material with high electron mobility, such as a silole
derivative, 2,5-bis(60-(20,200-bipyridine))-1,1-dimethyl-3,4-diphe-
nylsilole (PyPySiPyPy).48,49 Almost the same mobility of BpyB-
TAZ-Ph, -mTP, and -oTP indicate that the BpyBTAZ unit
transport electron. On the other hand, while only BpyBTAZ-BP
showed inferior mobility compared with the other BpyBTAZ
compounds, we have no sufficient physical and/or chemical
insight of it at present. The full amorphous nature of BpyBTAZ-
BP might affect it.
Fig. 9 Luminance and voltage changes under constant current density
operation at 50 mA cmꢂ2 for devices with different ETMs. The device
structure is the same as the test OLED shown in Fig. 2. The devices were
encapsulated by a glass cap with fresh desiccant using UV glue in an inert
glove box (O2 < 5 ppm, dew point < ꢂ50 ꢀC).
lifetimes of the test devices but it is clear that the two BpyBTAZ
compounds improve the durability of OLEDs. The lower oper-
ation voltages of the BpyBTAZ-based devices might offer better
operation durability; and the smaller voltage changes in these
devices compared with Alq devices support this speculation. We
infer that the smaller electron injection barrier due to the deeper
LUMO and/or high electron mobility of BpyBTAZ compounds
gives rise to smaller voltage increases. As the decomposition
temperatures of the BpyBTAZ compounds are high, the BpyB-
TAZ-based devices have less impurities and this could also
contribute to their durability.
3.4 Durability
Fig. 9 shows the normalized luminance and voltage changes
under constant current density operation at 50 mA cmꢂ2 (an
accelerated condition) of the three devices with different ETMs:
BpyBTAZ-mTP, -oTP, and Alq. The luminance of the devices
with BpyBTAZ compounds hardly decrease compared to that of
the Alq reference device. The devices for this test were fabricated
at the university, and it is well known that operation durability of
OLEDs strongly depend on the quality of device fabrication
including substrate cleaning processes and encapsulation of the
devices. Consequently, it is difficult to determine the exact
4. Summary
We have developed new ETM compounds for OLEDs based on
benzo[1,2,3]triazole and two asymmetrically substituted bipyr-
idines. The materials, BpyBTAZ compounds, have good thermal
stabilities and their decomposition temperatures are greater than
ꢀ
ꢀ
410 C, which is higher than that (366 C) of Alq, the conven-
tional OLED material. Since benzo[1,2,3]triazole derivatives are
well known as UV absorbers with high thermal stability,
BpyBTAZ compounds also probably possess high thermal
stabilities. BpyBTAZ compounds exhibit a preferable amor-
phous nature, and, specifically, the Tgs of both BpyBTAZ-TP
compounds exceed 100 ꢀC. Furthermore, BpyBTAZ-BP exhibits
no melting point and is fully amorphous. The electron affinities
of the materials are as large as 3.3 eV, and the electron mobility
of these materials is sufficiently high. These characteristics
accounted for a reduction in the operational voltage of OLEDs
with BpyBTAZ compounds compared with the reference device
with Alq as ETM. The mobility of all the BpyBTAZ compounds
exceeded 1 ꢁ 10ꢂ4 cm2 Vꢂ1 sꢂ1 at an electric field of 1 MV cmꢂ1
.
Fig. 8 Electron mobilities of BpyBTAZ compounds as a function of
square root of electric field. Data for Alq were referred from the literature
by Murata (see text).
The devices with both BpyBTAZ-TP compounds require just 4.2
or 4.3 V for 100 mA cmꢂ2, a sufficiently large current density. In
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24 Y.-J. Pu, M. Yoshizaki, T. Akiniwa, K.-i. Nakayama and J. Kido,
Org. Electron., 2009, 10, 877–882.
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T. Takeda, Y.-J. Pu, K.-i. Nakayama and J. Kido, Chem. Mater.,
2008, 20, 5951–5953.
addition, the two BpyBTAZ-TP-based devices showed longer
luminous lifetimes and smaller voltage increases than the Alq
reference device. This study is the first report of 2H-1,2,3-triazole
as an OLED material, and we clearly demonstrated that bipyr-
idine-substituted benzo[1,2,3]triazoles, which are derivatives of
2H-1,2,3-triazole, must be useful as ETMs for OLEDs.
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Acknowledgements
The authors would like to thank Prof. Dr M. Kimura for mass
spectroscopic measurements. The authors would like to thank
Riken Keiki and Rigaku for Ip and density measurements.
31 Z. H. Li, M. S. Wong, H. Fukutani and Y. Tao, Org. Lett., 2006, 8,
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