Bis(carbazolyl)benzodifuran has a high triplet energy level for application in blue phosphorescent OLED

Article abstract of DOI:10.1002/asia.201100326

Ambipolar semiconducting material o-MeCZBDF, which has a high excited triplet energy level (E_T), has been synthesized. This compound has high carrier mobilities for both holes and electrons in the amorphous state. Such characteristics are attributed to the electronegative character of the oxygen atom incorporated in the π-electron system. Copyright

Full text of DOI:10.1002/asia.201100326

COMMUNICATION
DOI: 10.1002/asia.201100326
Bis(carbazolyl)benzodifuran has a High Triplet Energy Level for Application
in Blue Phosphorescent OLED
Chikahiko Mitsui, Hideyuki Tanaka, Hayato Tsuji,* and Eiichi Nakamura*[a]
Organic semiconductors occupy an important position in
chemistry and electronics because of their widely acknowl-
edged utility in organic electronics, such as organic light-
emitting diodes (OLEDs)[1] and solar cells.[2,3] Among them,
the host materials in blue phosphorescent OLED
(PHOLED) devices have received particular attention[4,5]
because of the challenge of building in three different prop-
erties in a molecule: a wide HOMO/LUMO gap (for carrier
confinement), a high excited triplet-state energy level (ET;
to prevent back energy transfer from the dopant) and a high
carrier transport ability. However, these properties often re-
quire contradictory molecular designs. The wide HOMO/
LUMO gap requires the reduction of p-conjugation,[6] whilst
the high carrier mobility tends to require an expanded
p-electron system for the efficient intermolecular overlap of
wavefunctions. Herein, we report a new framework for a
blue PHOLED, a benzo[1,2-b:6,5-b’]difuran (o-BDF), which
shows a high ET level and high carrier mobilities in the
order of 10À3 cm2 VÀ1 sÀ1 for both holes and electrons in
the amorphous state. One of these compounds, 2,6-dimeth-
yl-3,7-(p-N-carbazolylphenyl)benzo[1,2-b:6,5-b’]difuran (o-
MeCZBDF) showed an ET value of 2.77 eV, which is high
enough to illuminate a typical blue phosphorescent dopant,
FIrpic (ET =2.65 eV),[7] and thus allowed us to fabricate a
full-color PHOLED.
for both holes and electrons in the amorphous state.[9] The
relatively wide HOMO/LUMO energy gap (ca. 3.3 eV) ena-
bled us to fabricate blue and green fluorescent hetero- and
homojunction OLEDs, yet its low ET value (ca. 2.1 eV) al-
lowed only a red phosphorescent OLED to be driven. We
needed to raise the ET level, and hence designed and syn-
thesized o-MeCZBDF.
We synthesized the o-BDF by the zinc-mediated cycliza-
tion procedure shown in Scheme 1. This method has the ad-
vantage of versatility over several literature preparations of
this class of compounds.[10,11] Thus, 3,6-dialkynylcatechol (3),
which was prepared from 1,4-diiodo-2,3-dimethoxybenzene
(1) in two steps, was deprotonated with butyllithium, trans-
metalated to zinc, and heated to obtain a dizincio cyclization
product (4). The Negishi coupling with N-(para-bromophe-
nyl)carbazole afforded the isolated o-MeCZBDF in 61%
yield (from 3). p-MeCZBDF was also prepared as a refer-
ence compound according to a published procedure.[8]
Our design achieved the high ET character of
o-MeCZBDF without sacrificing the charge-carrier mobility
(see below). The highest-energy 0–0 phosphorescence spec-
tra, taken in a frozen 2-methyltetrahydrofuran matrix at
77 K, afforded ET values of 2.77 and 2.64 eV for
o-MeCZBDF and p-MeCZBDF, respectively (Figure 1a and
1b, black line). This ET value of 2.77 eV for o-MeCZBDF is
high enough to emit from FIrpic (ET =2.65 eV; see below).
In contrast, the 2.64 eV ET value of p-MeCZBDF is high
enough only for a green phosphorescent dopant, such as
The molecular design of o-BDF stems from the assets and
drawbacks of benzo[1,2-b:4,5-b’]difurans (p-BDF), which
serve as thermally stable p-type semiconductors[8] upon the
installation of a carbazolyl group (p-PhCZBDF). This com-
pound was found to be an excellent ambipolar material that
shows high carrier mobilities greater than 10À3 cm2 VÀ1 sÀ1
[IrACHTNUTGRNEUNG
(ppy)3] (ET =2.42 eV).[12]
The optical gap (Eg) was estimated from the onset wave-
length of the UV/Vis absorption spectra to be as large as
3.49 eV (lonset =355 nm for both compounds, Figure 1a and
1b) in solution. The fluorescence spectra of these
MeCZBDFs were almost identical, showing an intense peak
at 351 nm and a shoulder at 365 nm. The fluorescent quan-
tum yields at room temperature were 0.25 and 0.19 for o-
MeCZBDF and p-MeCZBDF, respectively, as determined
by the absolute method.
[a] Dr. C. Mitsui, Dr. H. Tanaka, Dr. H. Tsuji, Prof. Dr. E. Nakamura
Department of Chemistry, School of Science
The University of Tokyo
7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5800-6889
E-mail: nakamura@chem.s.u-tokyo.ac.jp
tsuji@chem.s.u-tokyo.ac.jp
Absorption and fluorescence spectra for a vacuum-depos-
ited thin film (see the Supporting Information, Figure S2)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100326.
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Chem. Asian J. 2011, 6, 2296 – 2300

p-MeCZBDF has mobilities of
1.9ꢁ10À3 cm2 VÀ1 sÀ1 and 2.4ꢁ
10À3 cm2 VÀ1 sÀ1 for holes and
electrons, respectively, at an
electric
field
of
2.5ꢁ
105 VcmÀ1.
o-MeCZBDF
showed almost the same order
of mobility for both carriers as
p-MeCZBDF: 1.8ꢁ10À3 and
2.0ꢁ10À3 cm2 VÀ1 s, for holes
and electrons, respectively (for
transient current and electric-
field dependence of mobilities,
see the Supporting Informa-
tion).
The MeCZBDFs also show
high thermal stability, which is
also an important characteristic
for a semiconducting material.
Glass-transition temperatures
(Tg) measured using dif
ferential scanning calorimetry
(DSC) are 1538C and 1548C
for p-MeCZBDF and o-
MeCZBDF, respectively. Ther-
mogravimetric analysis (TGA)
showed decomposition temper-
atures (Td) as high as 4718C
and 4588C for p-MeCZBDF
and o-MeCZBDF, respectively.
Single-crystal X-ray analysis
of o-MeCZBDF and p-
MeCZBDF (Figure 2) showed
that the BDF and the carba-
zole units are electronically
disconnected from each other
because of the twisting of the
phenylene linker (e.g., a dihe-
dral angle of 39.9–48.08 against
the BDF plane). This twisting
accounts for the wide HOMO/
LUMO gap in these com-
pounds.
Scheme 1. Synthesis of o-MeCZBDF. THF=tetrahydrofuran, Ts=para-toluenesulfonyl, TMSI=trimethylsilyl
iodide, dba=(1E,4E)-1,5-diphenylpenta-1,4-dien-3-one, S-Phos=dicyclohexyl(2’,6’-dimethoxybiphenyl-2-yl)-
phosphine.
À
were virtually identical to that for the solution, thus indicat-
ing that there is little intermolecular interaction in the amor-
phous state. Accordingly, we noted a red-shift of 2–8 nm and
a difference in the optical gap (Eg =3.42 and 3.43 eV for
p-MeCZBDF and o-MeCZBDF, respectively) to that in so-
lution. The ionization potential (IP) values obtained by pho-
toemission yield spectroscopy (PYS) measurements in the
thin film were 6.04 and 6.08 eV for p-MeCZBDF and
o-MeCZBDF, respectively, and hence gave us estimated
electron affinity values in the thin film of 2.62 and 2.65 eV,
respectively.
The analysis of the C C bond-lengths revealed an intrigu-
ing structural similarity between MeCZBDFs and isoelec-
tronic all-carbon compounds (phenanthrene and anthracene;
Figure 2). In o-MeCZBDF, r2 (1.391 ꢂ) is shorter than r3
(1.408 ꢂ), thus suggesting that the furan rings conform to
the standard canonical representation, giving the central
benzene ring a 1,3,5-cyclohexatriene-like structure. Such a
structural feature is known for phenanthrene. This feature
of o-BDF can be considered to be the origin of the higher
ET level. In contrast, in p-MeCZBDF, r2 (1.402 ꢂ) is longer
than r3 (1.390 ꢂ), thereby indicating that the furan structure
is disrupted and the central six-membered ring takes a 3p/
3p structure. This feature is known for anthracene.
Carrier-drift mobility measurements by the time-of-flight
(TOF) method revealed that MeCZBDFs have a well-bal-
anced ambipolar character and a high carrier mobility.
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H. Tsuji, E. Nakamura et al.
COMMUNICATION
With these data in hand, we fabricated heterojunction
PHOLED devices using p-MeCZBDF and o-MeCZBDF as
host materials (for details of the device fabrication, see the
Supporting Information). p-MeCZBDF (ET =2.64 eV)
served as the host material for phosphorescent red dopant
[IrACHTUNRTGENNUG CAHTUNGTERN(NUNG ppy)3] (ET =
(piq)3] (ET =2.0 eV)[14] and green dopant [Ir
2.42 eV)[12] (devices A and C, respectively, Table 1), but not
Table 1. Summary of PHOLED device performances.
[a]
[b]
[c]
Device
Host
Dopant Color V1000
h1000
[lmWÀ1
EQEmax
[%]
[V]
]
A
p-
MeCZBDF
CBP
[Ir
G
8.5
2.8
8.1
B
C
[Ir
T
1.8
10.5
7.1
8.9
p-
[Ir
[Ir
U
MeCZBDF
CBP
D
E
9.1
3.2
7.8
5.4
o-
A
7.3
MeCZBDF
mCP
F
G
blue
9.1
2.1
5.5
[a] Driving voltage at 1000 cdmÀ2. [b] Power efficiency at 1000 cdmÀ2
.
[c] Maximum external quantum efficiency. piq=1-phenylisoquinoline,
ppy=2-phenylpyridine.
for blue dopants such as FIrpic (ET =2.65 eV) because of
their low ET value. The p-MeCZDF-based-devices A and C
exhibited superior performance over the corresponding
CBP-based device B and D for the driving voltage and the
maximum external quantum efficiency (EQE). The lower
driving voltage and higher efficiency are due to higher carri-
er mobilities for p-MeCZBDF than CBP as well as more-ef-
fective carrier confinement within the dopant, [IrACHTUNGTRENNUNG(ppy)3],
from the p-MeCZBDF host (Figure 3a).[15]
On the other hand, o-MeCZBDF (ET =2.77 eV) has a suf-
ficiently high ET value to allow FIrpic to exhibit blue phos-
phorescent electroluminescence (device E). Compared to
Figure 1. Photophysical properties of MeCZBDFs. a) o-MeCZBDF, b) p-
MeCZBDF. UV/Vis absorption and fluorescence spectra obtained at am-
bient temperature in CH2Cl2, and
phosphorescence spectra obtained at
77 K in 2-methyltetrahydrofuran.
Thus, o-BDF and p-BDF are
structurally similar to phenan-
threne and anthracene, respec-
tively, but differ in their
HOMO/LUMO gap and trip-
let energy levels (see above),
which are much higher than
those of their carbon counter-
parts
(anthracene:
ET =
1.86 eV, phenanthrene: ET =
2.61 eV).[13] We ascribe this dif-
ference to the high electrone-
gativity of the oxygen atom,
which makes the oxygen lone-
pair less-effectively conjugated
with the p-electrons of the re-
maining carbon framework.
Figure 2. Structural features of MeCZBDFs. ORTEP (thermal ellipsoids set at 30% probability) and some se-
À
lected C C bond-lengths for a) o-MeCZBDF, and b) p-MeCZBDF.
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Chem. Asian J. 2011, 6, 2296 – 2300

Application of Bis(carbazolyl)benzodifuran in a Blue Phosphorescent OLED
12 h. After cooling to ambient temperature, the resulting mixture was
passed through a short pad of Florisil column with dichloromethane, and
the crude material was purified by flash column chromatography on
silica gel (n-hexane/toluene (90:10 to 70:30) to afford the title compound
as a white solid (736 mg, 1.10 mmol, 61%). M.p.: 3328C (TG-DTA).
1H NMR (500 MHz, CDCl3): d=2.74 (s, 6H, CH3), 7.32 (dd, J=8.1 Hz,
8.1 Hz, 4H, carbazole), 7.46 (dd, J=8.1 Hz, 8.1 Hz, 4H, carbazole), 7.55
(d, J=8.0 Hz, 4H, carbazole), 7.58 (s, 2H, benzodifuran), 7.73 (d, J=
8.6 Hz, 4H, phenylene), 7.81 (d, J=8.0 Hz, 4H, phenylene), 8.18 ppm (d,
J=8.0 Hz, 4H, carbazole); 13C NMR (125 MHz, CDCl3): d=13.1, 109.8,
114.3, 117.2, 120.0, 120.4, 123.4, 126.0, 126.3, 127.4, 130.3, 132.0, 136.6,
138.6, 140.8, 151.0 ppm. Anal. Calcd (%) for C48H32N2O2: C 86.20,
H 4.82, N 4.19. Found: C 86.03, H 5.00, N 4.00. MS (APCI+): 669 [M+1].
Acknowledgements
We thank Dr. Yoshiharu Sato for helpful discussion on PHOLED, and
Rigaku Corporation for data collection on VariMax with RAPID and the
structure analysis of MeCZBDFs. We are grateful to the MEXT for fi-
nancial support (KAKENHI for E.N., No. 22000008, H.T. No. 20685005)
and the Global COE Program for Chemistry Innovation. This work was
partly supported by the Strategic Promotion of Innovative R&D, JST.
C.M. thanks the JSPS for the Research Fellowship for Young Scientists
(No. 21·9262).
Figure 3. Energy diagram of the materials (in eV). The emissive layer is
surrounded by a broken line, and the dopant and two different hosts are
shown inside. a) Green-emitting devices C and D, and b) blue-emitting
devices E and F.
device F, which utilized mCP as a host, device E showed a
lower driving voltage (7.3 V, c.f. 9.1 V for device F). This
smaller driving voltage can be largely attributed to the
higher carrier mobility of o-MeCZBDF than that of mCP[16]
because the carrier confinement within the dopant is effec-
tive for both host materials (Figure 3b). The power efficien-
cy of device E (3.2 lmWÀ1) was higher than device F
(2.1 lmWÀ1), at a luminance of 1000 cdmÀ2, whilst the
EQEmax value was comparable (5.4% for device E and
5.5% for device F).
Keywords: benzodifuran
electroluminescence · excited triplet state · semiconductors
·
cyclization
·
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A
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Chem. Asian J. 2011, 6, 2296 – 2300
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H. Tsuji, E. Nakamura et al.
COMMUNICATION
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Received: March 31, 2011
Published online: July 11, 2011
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Chem. Asian J. 2011, 6, 2296 – 2300