J.-A Seo et al. / Organic Electronics 15 (2014) 3773–3779
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1H NMR (400 MHz, CDCl3): d 7.06–7.13 (m, 5H), 7.18–
7.39 (m, 13H), 7.47–7.54 (m, 4H), 7.70 (d, 1H, J = 8.4 Hz),
7.85 (d, 1H, J = 1.2 Hz), 7.92 (d, 1H, J = 3.6 Hz), 8.05ꢀ8.13
(m, 5H). 13C NMR (100 MHz, CDCl3): 57.3, 109.7, 111.3,
113.0, 113.6, 114.6, 118.5, 119.9, 120.3, 120.4, 121.3,
121.4, 122.2, 122.4, 122.6, 123.0, 123.3, 125.9, 126.0,
126.4, 126.9, 127.9, 128.0, 128.2, 130.3, 130.7, 132.0,
132.3, 132.6, 137.1, 137.7, 138.8, 141.7, 141.8, 146.2,
155.8. MS (FAB) m/z 663 [(M+H)+]. Anal. Calcd for
hinder the molecular packing of the indoloacridine core
simultaneously. The indoloacridine core was linked to
dibenzofuran based aromatic moieties via sp3 carbon for
high triplet energy and carbazole or carboline modified
dibenzofuran was introduced to prevent the molecular
packing of the planar indoloacridine core. The carbazole
or carboline modified dibenzofuran moiety can also
increase the glass transition temperature of AcHCz and
AcHCb host materials.
C
49H30N2O: C, 88.80; H, 4.56; N, 4.23. Found: C, 88.36; H,
Synthetic scheme of AcHCz and AcHCb host materials is
shown in Scheme 1. The indoloacridine core was synthe-
sized by ring closing reaction of 9-(2-bromophenyl)-9H-
carbazole with (8-iododibenzo[b,d]furan-2-yl)(phenyl)
methanone using catalytic sulfuric acid. Iodinated indolo-
acridine core (3) was reacted with carbazole or carboline
to produce AcHCz and AcHCb as final products. Column
chromatography was used as a wet purification method
and vacuum train sublimation was used as a final dry puri-
fication process to obtain high purity above 99%.
Molecular orbital of AcHCz and AcHCb was calculated
using Gaussian 09 program to investigate the electron dis-
tribution in the host materials. The highest occupied
molecular orbital (HOMO) and lowest unoccupied molecu-
lar orbital (LUMO) distribution of the host materials calcu-
lated using B3LYP 6-31G⁄ basis sets is shown in Fig. 1. The
HOMO of AcHCz was centered on the carbazole unit
attached to the dibenzofuran moiety due to high electron
density of the carbazole unit and the LUMO of AcHCz
was localized on the dibenzofuran moiety because of rela-
tively low electron density of dibenzofuran by high elec-
tronegativity of oxygen. The HOMO was shifted to the
indoloacridine core in the AcHCb host owing to high elec-
tron density of indoloacridine compared to carboline. The
LUMO was mostly dispersed over the dibenzofuran moiety
because of electron-withdrawing character of oxygen of
dibenzofuran.
4.61; N, 4.28.
2.2.5. 8-(8-(9H-pyrido[2,3-b]indol-9-yl)dibenzo[b,d]furan-2-
yl)-8-phenyl-8H-indolo[3,2,1-de]acridine(AcHCb)
AcHCb was synthesized according to the same proce-
dure as the synthesis of AcHCz except that
was used instead of 9H-carbazole.
a-carboline
(AcHCb) yield 70%. 1H NMR (400 MHz, CDCl3):d 7.06–7.12
(m, 5H), 7.19–7.24 (m, 4H), 7.28–7.53 (m, 9H), 7.57–7.61 (m,
2H), 7.74 (d, 1H, J = 4.4 Hz), 7.92–7.94 (m, 2H), 8.07–8.14 (m,
4H) 8.38 (d, 1H, J = 3.6 Hz), 8.45 (d, 1H, J = 2.4 Hz). 13C NMR
(100 MHz, CDCl3): 57.3, 110.4, 111.1, 112.8, 113.6, 114.6,
116.1, 116.3, 118.4, 120.6, 120.7, 120.8, 121.0, 121.2, 121.3,
122.1, 122.4, 122.9, 123.5, 125.8, 126.3, 126.4, 126.8, 126.9,
127.0, 127.1, 127.8, 128.0, 128.2, 128.4, 130.2, 130.5, 131.2,
132.0, 132.4, 137.1, 137.7, 138.8, 140.9, 141.6, 146.1, 146.7,
152.6, 155.7, 156.0. MS (FAB) m/z 664 [(M+H)+]. Anal. Calcd
for C48H29N3O: C, 86.86; H, 4.40; N, 6.33. Found: C, 86.36; H,
4.41; N, 6.34.
2.3. Device fabrication and measurements
The blue PHOLEDs were prepared by thermal deposi-
tion of 4,40-cyclohexylidenebis[N,N-bis(4-methylphenyl)
aniline] (TAPC, 20 nm), 1,3-bis(N-carbazolyl)benzene
(mCP, 10 nm), AcHCz: iridium(III) bis[(4,6-difluorophenyl)
Photophysical properties of AcHCz and AcHCb host
materials were characterized with ultraviolet–visible
(UV–vis) and photoluminescence (PL) measurements.
Fig. 2 shows UV–vis absorption, solution PL and low tem-
perature PL emission spectra of AcHCz and AcHCb in anhy-
drous tetrahydrofuran solvent. The AcHCz and AcHCb
showed similar UV–vis absorption spectra and weak UV–
pyridinato-N,C2]picolinate
(25 nm), diphenylphosphine
(FIrpic)
or
AcHCb:FIrpic
oxide-4-(triphenylsilyl)
phenyl (TSPO1, 35 nm), LiF (1 nm) and Al (200 nm) on
a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS, 60 nm) coated indium tin oxide (ITO, 120 nm)
substrate layer by layer. FIrpic was doped in the AcHCz
and AcHCb hosts at a doping concentration of 10%. Single
carrier devices with device structures of ITO (50 nm)/
PEDOT:PSS (10 nm)/TAPC (20 nm)/mCP (10 nm)/AcHCz or
AcHCb (25 nm)/TAPC (5 nm)/Al (200 nm) (hole only device)
and ITO (50 nm)/PEDOT:PSS (10 nm)/TSPO1 (10 nm)/AcHCz
or AcHCb (25 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm)
(electron only device) were also prepared to compare hole
and electron density in the emitting layer.
Electrical measurements of the blue PHOLED and single
carrier devices were performed using Keithley 2400 source
measurement unit and luminance measurements were
carried out using CS 2000 spectroradiometer in ambient
condition after encapsulation of the devices.
vis absorption peaks assigned to n-
indoloacridine, carbazole, carboline and dibenzofuran moi-
p
⁄ transition of the
eties were observed between 320 nm and 360 nm in addi-
tion to the strong UV–vis absorption peaks by
p–
p⁄
transition of the conjugated backbone structure. UV–vis
absorption edges of AcHCz and AcHCb were 364 nm and
365 nm, which corresponded to an energy gap of 3.40 eV.
PL emission peaks of the AcHCz and AcHCb were observed
at 371 nm and 372 nm and the first phosphorescent peak
positions of low temperature PL emission were 426 nm
and 433 nm, respectively. Triplet energies calculated from
the low-temperature PL spectra were 2.91 and 2.86 eV for
AcHCz and AcHCb. The separation of conjugation by sp3
carbon linkage between indoloacridine and dibenzofuran
allowed the high triplet energy of AcHCz and AcHCb.
Electrochemical oxidation and reduction behavior of
AcHCz and AcHCb was studied by cyclic voltammetry
(CV) to measure ionization potential (IP) and electron
3. Results and discussion
The design of the two indoloacridine based host materi-
als was intended to increase the triplet energy and to