Highly efficient and stable deep-blue emitting anthracene-derived molecular glass for versatile types of non-doped OLED applications

Article abstract of DOI:10.1039/c1jm14482k

New anthracene-based deep-blue emitting molecular glass, 9-(9-phenylcarbazole-3-yl)-10-(naphthalene-1-yl)anthracene (PCAN), which is asymmetrically functionalized with N-phenylcarbazole and naphthalene, has been designed, synthesized, and characterized. The deep-blue emitting PCAN is efficiently secured for color purity, has high glass transition temperature (T_g = 151 °C), and has excellent solubility (>100 mg mL ^-1 in toluene), due to its highly tilted asymmetric molecular conformation. Using processing versatility of PCAN, vacuum-deposited and solution-processed non-doped deep-blue fluorescent organic light-emitting diodes (OLEDs) were prepared, which employ PCAN as an emitter. The vacuum deposited, non-doped EL device exhibited not only the excellent luminance efficiency and external quantum efficiency (as high as 3.64 cd A^-1 and 4.61%, respectively) for the saturated deep-blue CIE chromaticity coordinates of (0.151, 0.086), but also stable performance and a good device lifetime. Furthermore, the solution processed EL device also exhibited an encouraging level of performance (1.24%, 1.15 cd A^-1) and deep-blue emission (0.159, 0.105).

Full text of DOI:10.1039/c1jm14482k

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PAPER
Highly efficient and stable deep-blue emitting anthracene-derived molecular
glass for versatile types of non-doped OLED applications†
*
Illhun Cho,‡ Se Hun Kim,‡ Jong H. Kim, Sanghyuk Park and Soo Young Park
Received 9th September 2011, Accepted 19th September 2011
DOI: 10.1039/c1jm14482k
New anthracene-based deep-blue emitting molecular glass, 9-(9-phenylcarbazole-3-yl)-10-
(naphthalene-1-yl)anthracene (PCAN), which is asymmetrically functionalized with N-
phenylcarbazole and naphthalene, has been designed, synthesized, and characterized. The deep-blue
emitting PCAN is efficiently secured for color purity, has high glass transition temperature (Tg ¼ 151
ꢀC), and has excellent solubility (>100 mg mLꢁ1 in toluene), due to its highly tilted asymmetric
molecular conformation. Using processing versatility of PCAN, vacuum-deposited and solution-
processed non-doped deep-blue fluorescent organic light-emitting diodes (OLEDs) were prepared,
which employ PCAN as an emitter. The vacuum deposited, non-doped EL device exhibited not only the
excellent luminance efficiency and external quantum efficiency (as high as 3.64 cd Aꢁ1 and 4.61%,
respectively) for the saturated deep-blue CIE chromaticity coordinates of (0.151, 0.086), but also stable
performance and a good device lifetime. Furthermore, the solution processed EL device also exhibited
an encouraging level of performance (1.24%, 1.15 cd Aꢁ1) and deep-blue emission (0.159, 0.105).
Since the first reports on the efficient emission properties and
Introduction
excellent stability of 9,10-di-(2-naphthyl)anthracene (ADN),5
there has been intense interest in the development of anthracene
derivatives as promising blue emitters for EL devices.6 Recent
reports by Shu et al. and Park et al. showed luminance efficien-
cies of 5.5 cd Aꢁ1 and 3.1 cd Aꢁ1 for anthracene derivatives
applied as non-doped blue EL devices, but insufficient deep-blue
emission was shown, with CIE coordinates of (0.15, 0.12) and
(0.16, 0.13), respectively.7,8 Lee et al. reported a new blue emitter
with saturated color coordinates of (0.149, 0.086), but
a maximum luminance efficiency of only 2.63 cd Aꢁ1.9 Lai et al.
and Park et al. reported efficient saturated blue-emitting mate-
rials ((0.15, 0.07), 3.2 cd Aꢁ1, (0.156, 0.088), 3.64 cd Aꢁ1).10,11
However, these data cannot be fully interpreted, because no
mention was made of device lifetime. Despite the fact that device
lifetime is one of the most important requirements for EL
devices, reports of highly efficient deep-blue emitting materials
possessing sufficiently long device lifetime are still rare. In
addition, small molecular deep-blue anthracene OLEDs that can
be wet-processed have not yet been reported. To realize highly
efficient and stable deep-blue emitters with wet processibility,
these materials must possess highly efficient emitting centers and
amorphous glass characteristics, with not only thermal and
morphological stability and a high glass transition temperature
(Tg), but also high solubility through a bulky backbone structure.
Here we describe our discovery, in the course of investigating
anthracene-derived deep-blue emitting materials, of 9-(9-phe-
Organic light-emitting diodes (OLEDs) have emerged as the
most promising next-generation flat-panel display technology in
the decades since the pioneering work of Tang et al.1 Although
OLEDs have recently been developed that can be employed in
small display panels, there remain several obstacles to be over-
come before they can be applied in the mainstream display panel
market. A critical drawback is the poor performance of saturated
deep-blue OLEDs compared with green and red OLEDs.
Because of the intrinsic wide-band-gap nature of deep-blue
emitters, balancing the various requirements is a significant
challenge in the development of new OLEDs.2 For proper non-
doped (ND) blue electroluminescence (EL) devices, the require-
ments of high efficiency, thermal stability, sufficiently long device
lifetime, and saturated deep-blue emission with a Commission
Internationale de l’Eclairage (CIE) coordinate of y < 0.10 that
can be matched to the National Television System Committee
(NTSC) (0.14, 0.08) must be concurrently satisfied.3 In addition,
amenability to wet processing, which is an inevitable prerequisite
for large area, low cost mass production, must be taken into
consideration.4
Creative Research Initiative Center for Supramolecular Optoelectronic
Materials and WCU Hybrid Materials Program, Department of
Materials Science and Engineering, Seoul National University,
1
Gwanak-ro, Gwanak-gu, Seoul, 151-744, Korea. E-mail: parksy@snu.ac.
kr; Fax: +82-2-886-8331; Tel: +82-2-880-8327
nylcarbazole-3-yl)-10-(naphthalene-1-yl)anthracene
(PCAN),
† Electronic supplementary information (ESI) available: DSC and TGA
thermograms of PCAN. See DOI: 10.1039/c1jm14482k
‡ Both of these authors contributed equally to this work.
which is asymmetrically functionalized with N-phenylcarbazole
and naphthalene, and which showed saturated deep-blue
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J. Mater. Chem., 2012, 22, 123–129 | 123

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emission (0.151, 0.086) and highly efficient device performance
(3.64 cd Aꢁ1, 4.61%), with long device lifetime. Furthermore,
using the high solubility and amorphous-film-formation prop-
erties of PCAN, we demonstrated a wet-processed EL device
with saturated blue emission (0.159, 0.105) and high efficiency
(1.15 cd Aꢁ1, 1.24%). Herein, we report the synthesis, charac-
terization and electroluminescent device results of PCAN.
Fig. 1 Density functional theory (DFT) calculation. (a) Three-dimen-
sional optimized geometry of PCAN, and (b) the calculated HOMO and
LUMO electron density map of PCAN.
Result and discussion
Synthesis
ꢀC. The onset-of-decomposition temperature (Td), defined as the
The molecular structure and synthesis route for PCAN are
depicted in Scheme 1. Briefly, compound 1 was synthesized by
reaction of 3-iodo-9-phenylcarbazole with n-BuLi at ꢁ78 ꢀC,
followed by lithium exchange with 2-isopropoxy-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane. Compound 2 was synthesized by
the Suzuki cross-coupling reaction of 9-bromoanthracene with
1-naphthyl boronic acid, in the presence of a catalytic amount of
Pd(PPh3)4, 2 N NaOH aqueous solution, and a toluene/ethanol
4 : 1 mixed solution. Compound 3 was synthesized by the NBS
bromination of compound 2 in CHCl3. The final product,
PCAN, was synthesized by the Suzuki cross-coupling reaction of
compound 3 with compound 1. PCAN was obtained by using the
same reaction conditions with compound 2. The molecular
temperature at which 5% mass loss occurs, as measured by TGA,
ꢀ
was determined to be 374 C (see ESI†).
Because of its highly bulky backbone structure and the
included carbazole moiety, PCAN exhibits a molecular glass
character (i.e., it is a thermally stable, glass-like solid that does
not undergo a first-order melting transition).13 We probed these
molecular glass features, and the birefringence, using a polarized
optical microscope (POM). In the glass state, PCAN showed
a fine surface, transparency, and emitted in the deep blue under
irradiation with UV light. Birefringence was not shown at all
directions (see Fig. 2).
To confirm the thermal stability and morphological durability
against thermal stress, we also carried out an atomic force
microscopy (AFM) study of a vacuum-deposited (VD) PCAN
film (see Fig. 3). Samples were prepared using the thermal vapor
deposition technique on a silicon wafer substrate of about 40 nm
structure was characterized by H NMR, 13C NMR, elemental
1
analysis (EA), and mass spectrometry (MS).
Theoretical calculations
To understand the optimized geometry and the frontier molec-
ular orbital energy levels, we also carried out density functional
theory (DFT) calculations at the B3LYP/6-31G* using Gaussian
03.12 As shown in Fig. 1, the calculated highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) were found to be mainly localized on the
anthracene moiety. Because of large torsional stresses, the 9,10-
substituents are tilted almost 90ꢀ to the anthracene emitting
center. Consequently, PCAN has a bulky, non-coplanar struc-
ture that can efficiently prevent molecular recrystallization
through intermolecular interactions, resulting in an emitting
center that is totally isolated from neighboring molecules that
can induce excimer or exciplex formation.
Thermal and morphological properties
The thermal properties of PCAN were characterized with
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) in an N2 atmosphere. Although PCAN ꢀis a low
molecular weight material, it showed a high Tg of 151 C, and
a melting temperature (Tm) was not observed from 50 ꢀC to 300
Fig. 2 Polarized optical microscope (POM) images of PCAN molecular
glass ((a) PCAN molecular glass images and (b) polarized angular posi-
tion images for the birefringence test).
Scheme 1 Synthesis of PCAN.
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Fig. 3 Atomic force microscope (AFM) topographic images of PCAN and ADN with thermal treated samples ((a) no treatment, (b) 100 ꢀC 12 h, and
(c) 100 ꢀC 24 h). RMS roughness ¼ Rrms
.
thickness to match that used in the actual electroluminescent
device. For a direct comparison, we used ADN as a control
was ꢁ2.51 eV (Table 1). The Eg calculated from the threshold of
optical absorption was 2.98 eV.
ꢀ
material. We prepared thermally treated sample films at 100 C
with times of 0 h, 12 h, and 24 h under an N2 atmosphere. The
untreated ADN VD film had a root mean square roughness
(Rrms) of 15.4 nm, and the PCAN VD film had an Rrms of 5.5 nm.
In the ADN case, the Rrms significantly increased (48.2 nm at 12
h, 55.6 nm at 24 h) during the thermal treatment, due to
recrystallization. In contrast, the PCAN VD film showed
a negligible change in its Rrms (ꢂ1 nm from 0 h to 24 h). Due to
the Joule heat generated during device operation, high thermal
and morphological stability against heat in the organic material
is critically important. Consequently, we expect that PCAN
materials having high thermal and morphological stability will
maintain a uniform and stable emitting layer interface during
device operation, which is a basic factor contributing to long
device operation lifetime.
Electroluminescent properties
To explore the OLED characteristics, we fabricated ND EL
devices with a device structure of ITO (150 nm)/2-TNATA (60
nm)/NPB (20 nm)/blue emitting layer (PCAN-40 nm and ADN-
40 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (100 nm), where 2-TNATA
¼
4,40,400-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine,
NPB ¼ N,N0-bis(naphthalene-1-yl)-N,N0-bis(phenyl)-benzidine,
Alq3 ¼ tris(8-hydroxy-quinolinato)aluminium. To evaluate the
efficiencies and device lifetime of PCAN objectively, we used
ADN that we synthesized (under exactly the same environmental
conditions as were used for the synthesis of PCAN) as a control
material. Fig. 4(b) shows the EL spectra of PCAN and ADN at
10 mA cmꢁ2. The emission lmax features of PCAN and ADN
were located at 447 nm and 452 nm, and the full width at half
maximum (FWHM) values were 54 nm and 60 nm, respectively.
As a direct consequence of the emission properties, the CIE
chromaticity coordinates (x,y) of PCAN and ADN EL emissions
were (0.151, 0.086) and (0.157, 0.111) at 10 mA cmꢁ2. As
expected, PCAN showed much more pure deep-blue emission
than ADN, due to the former molecule’s bulky and asymmetric
Photophysical and electrochemical properties
The photophysical properties of PCAN were explored using
diluted THF solutions, and in solid thin films VD on quartz
plates. The absorption spectra in the diluted solution expressed
the archetypal vibronic structure of isolated anthracene (358, 375
and 396 nm). In the case of PL, although the emission lmax of the
VD film was slightly red-shifted compared with the diluted
solution (430 nm for the solution and 455 nm for the VD film),
the total emission range was mainly located in the deep-blue
region, and the two emission spectra showed almost the same
emission band edge (see Fig. 4(a)). The resulting emission spectra
revealed that, through the isolation of the anthracene emitting
core from the neighboring molecules, excimer formation,
significantly red-shifted emission and concentration quenching
occurring due to p–p interaction were effectively inhibited, as
predicted by the DFT calculations.
The energy levels of PCAN were also measured by cyclic
voltammetry (CV) and optical band-gap measurements. The
HOMO energy level measured using CV was ꢁ5.49 eV, and the
LUMO energy level calculated from the optical band gap (Eg)
Fig. 4 (a) The absorption and PL spectra of PCAN in dilute THF
solution, and in vacuum-deposited film. (b) EL spectra of PCAN and
ADN.
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Table 1 Optical, electrochemical, and thermal properties of PCAN
Compound
labsa/nm
lemb/nm
lemc/nm
Tg/Tm/Td/ꢀC
HOMO/LUMOd/eV
PCAN
358, 375, 396
430
455
151/N.D./374
ꢁ5.49/ꢁ2.51
a
b
c
Maximum absorption wavelength, measured in THF (10ꢁ5). Maximum emission wavelength, measured in THF (10ꢁ5). Maximum emission
wavelength, measured in film state (films prepared on the quartz plate by the thermal evaporation method). The HOMO energy level was
measured by CV, and the LUMO energy level was determined from the HOMO level and optical band-gap (N.D.: not detected).
d
molecular structure. As shown in Fig. 5, the luminance efficiency,
power efficiency, and external quantum efficiency values for
the lifetime of PCAN and ADN EL devices was measured as the
operational half lifetime (t1/2) under an initial luminance of 400
cd mꢁ2 at a constant current (see Fig. 6(b)). The PCAN EL device
showed a lifetime (610 h) that was approximately 5 times longer
than that of the ADN device (133 h), as expected by the AFM
thermal properties study.
PCAN were 3.64 cd Aꢁ1, 1.41 lm Wꢁ1, and 4.61% at 10 mA cmꢁ2
respectively. The values for ADN were 3.09 cd Aꢁ1, 1.24 lm Wꢁ1
,
,
and 3.22%, respectively, at 10 mA cmꢁ2 (Table 2). Notably,
PCAN not only had deeper blue emission than ADN, it also
showed higher performance for all measures of efficiency.
Furthermore, the CIE value deviation of the PCAN EL spectra
in response to the current change was negligible (x ¼ 0.000, y ¼
0.001) from 10 mA cmꢁ2 to 100 mA cmꢁ2 (see Fig. 6(a)). Through
the excellent color stability of the PCAN EL device, we
confirmed that the electronic energy level matching with neigh-
boring layers was finely tailored, and charge carrier injection and
recombination occurred efficiently at the PCAN emitting layer at
all current values. To investigate the device operation stability,
These findings suggest that the greatly improved efficiency
and lifetime characteristics arise from a combination of the
highly efficient emitting nature of PCAN and its high color
purity, the finely tailored electronic energy levels in the EL
device, and the fact that the layer interface remains stable during
device operation due to PCAN’s high thermal and morpholog-
ical stability.
Although PCAN is comprised of aromatic rings without
aliphatic chains, it is readily soluble in common organic solvents.
Based on the excellent solubility of PCAN (>100 mg mLꢁ1 in
toluene at room temperature), we investigated the possibility of
solution-processed deep-blue EL devices with a device structure
of ITO (150 nm)/PEDOT:PSS (40 nm)/PCAN (70 nm)/Bphen (30
nm)/LiF (1 nm)/Al (100 nm), where PEDOT:PSS ¼ poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate), Bphen ¼ 4,7-
diphenyl-1,10-phenanthroline. This preliminary solution-pro-
cessed device also showed deep-blue EL emission with CIE
chromaticity coordinates (x,y) of (0.159, 0.105). The maximum
device performance was recorded at 1.15 cd Aꢁ1 luminance effi-
ciency, 0.71 lm Wꢁ1 power efficiency, and 1.24% external
quantum efficiency at 20 mA cmꢁ2 (see Fig. 7). We believe that
further improvements in the device performance can be achieved
through energy level adjustment between neighboring layers,
charge mobility matching, and so on. We also expect PCAN can
be a suitable emitter for the fully solution-processed device if an
appropriate processing method is developed in near future.
Although the results shown here are not from an optimized
device, it is notable that we successfully fabricated a solution-
processed EL device using the small molecule PCAN, which
shows excellent solubility without dendritic structure and/or
aliphatic chains. Furthermore, the most important point is that
Table 2 Electroluminescent characteristics of blue non-doped OLEDs
b c/V
hc
3.64
hp
1.41
hex
4.61
CIExy
T1/2
610
133
c
d
e
f
Vona/V
V
,
EML
PCAN 4.6
8.10
0.15, 0.08
(11.15) (3.04) (0.86) (3.90) (0.15, 0.08)
7.85 3.09 1.24 3.22 0.15, 0.11
(11.12) (2.71) (0.77) (3.05) (0.15, 0.10)
ADN
4.6
a
b
c
Turn-on voltage, at 1 cd mꢁ2 (V). At 10 mA cmꢁ2 (V). Luminance
d
e
efficiency (cd Aꢁ1). Power efficiency (lm Wꢁ1). External quantum
Fig. 5 (a) Luminance efficiency (LE) and power efficiency (PE) of the
blue non-doped device with PCAN and ADN. (b) External quantum
efficiency (EQE) of the blue non-doped device with PCAN and ADN.
efficiency (%). f Device half-lifetime at 400 cd mꢁ2 (h).
126 | J. Mater. Chem., 2012, 22, 123–129
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Fig. 7 (a) Luminance efficiency and power efficiency of the solution-
processed ND blue device with PCAN. (b) EQE of the solution-processed
ND blue device (inset—EL spectra of the solution processed device).
Fig. 6 (a) CIE coordinates deviation of EL spectra in response to the
current change of PCAN and ADN EL devices (from 10 mA cmꢁ2 to 100
mA cmꢁ2). (b) Operational half lifetime of EL devices (under an initial
luminance of 400 cd mꢁ2).
Experimental
Synthesis
both the VD device and the solution-processed small molecule
device showed the best performance for each device type.
PCAN was synthesized according to the procedure shown in
Scheme 1.
Conclusions
Synthesis of N-phenyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane-2-yl)-6H-carbazole (1). The degassed solution of 3-iodo-N-
phenylcarbazole (6 g, 16.25 mmol) in anhydrous THF (100 mL)
was added to 20.31 mL (32.50 mmol) of n-butyllithium (1.6 M in
hexane) at ꢁ78 ꢀC. After 2 h stirring at ꢁ78 ꢀC, 6.65 mL (32.50
mmol) of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
was rapidly added to the solution. The resulting mixture was
warmed slowly to room temperature and stirred overnight. After
completion of the reaction, the mixture was poured into 300 mL
of brine and extracted with dichloromethane. The organic
extracts were washed with brine three times and dried over
anhydrous MgSO4. The organic solvent in the crude product was
removed by rotary evaporation, and the crude product was
purified by column chromatography (chloroform/hexane, 2 : 1
v/v). Yield: 71% (4.3 g). 1H NMR (300 MHz, CDCl3, d): 8.64 (s,
1H), 8.19 (d, J ¼ 7.7, 1H), 7.87 (d, J ¼ 8.3, 1H), 7.63–7.53 (m,
4H), 7.47 (t, J ¼ 7.1, 1H), 7.40–7.35 (m, 3H), 7.29 (t, J ¼ 7.8, 1H),
1.40 (s, 12H).
In summary, we have developed a multi-processable, deep-blue
emitting molecular glass PCAN, which exhibits excellent deep-
blue emission, along with high thermal and morphological
stability. The highly tilted asymmetric conformation of PCAN
efficiently suppresses the close packing of molecules in the solid
state, and consequently secures color purity and elevates solu-
bility. The VD ND EL device exhibited not only excellent device
performance (4.61% and 3.64 cd Aꢁ1) with saturated deep-blue
emission (0.151, 0.086), but also a highly improved device life-
time. Furthermore, we successfully demonstrated a solution-
processed deep-blue EL device (0.159, 0.105) with high perfor-
mance (1.24%, 1.15 cd Aꢁ1). To the best of our knowledge, this is
the first report of a processing-versatile, small molecular deep-
blue emitter that shows excellent efficiency in both VD and wet
processed devices. We believe that PCAN has strong advantages
for the OLED industry owing to its capability to be used in
various fabrication processes.
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Synthesis of 9-(naphthalene-1-yl)anthracene (2). A mixed
solution of 9-bromo-anthracene (3.56 g, 13.845 mmol), naph-
thalene-1-boronic acid (2.5 g, 14.537 mmol), tetrakis(triphenyl-
phosphine)palladium(0) (0.48 g, 0.415 mmol), toluene (40 mL),
ethanol (8 mL), and 2 N NaOH aqueous solution (20 mL) was
heated at 100 ꢀC with stirring under an argon atmosphere. After
12 h, the mixture was cooled to room temperature. The reaction
mixture was then quenched with water (300 mL), neutralized (1
N HCl), and extracted with CH2Cl2 (3 ꢃ 100 mL). The combined
organic phase was dried (anhydrous MgSO4) and concentrated
under reduced pressure. The crude product was purified using
flash column chromatography (CHCl3) and recrystallization
JMS-700), and elemental analysis (EA1110, CE Instrument). The
glass transition temperatures and melting temperatures of the
compounds were determined using DSC under an N2 atmo-
sphere, using a PERKIN ELMER DSC7, TAC7/DX. The
decomposition temperatures of the compounds were obtained
using TGA under an N2 atmosphere, using a TA instruments
Q50 model. UV-Vis spectra were recorded on a SHIMADZU
UV-1650PC, while PL spectra were taken using a Varian Cary
Eclipse fluorescence spectrophotometer. HOMO levels of
organic materials were obtained from the cyclic voltammetry
measurements. Cyclic voltammetric measurements were per-
formed using a 273A (Princeton Applied Research) with a one-
compartment electrolysis cell consisting of a platinum working
electrode, a platinum wire counter-electrode, and a quasi Ag+/Ag
electrode as reference. Measurements were performed in a 0.5
mM CH2Cl2 solution with tetrabutylammonium tetra-
fluoroborate as the supporting electrolyte, at a scan rate of 100
mV sꢁ1. Each oxidation potential was calibrated using ferrocene
as a reference. LUMO levels were calculated from the HOMO
level and the energy band-gap, which was obtained from the edge
of the absorption spectra. Atomic force microscopy (AFM,
PSIA, XE-150) was used to characterize the vacuum-deposited
films surface morphology. AFM images were obtained in non-
contact mode (scan rate and scan size were 0.5 Hz and 5 ꢃ 5 mm2,
respectively).
1
(CHCl3) to afford compound 2 (2.75 g, 65%). H NMR (300
MHz, CDCl3, d): 8.58 (s, 1H), 8.15 (d, J ¼ 8.7, 2H), 8.06 (d, J ¼
8.7, 1H), 8.01 (d, J ¼ 8.4, 1H), 7.70 (t, J ¼ 8.1, 1H), 7.53 (d, J ¼
6.9, 1H), 7.5–7.3 (m, 5H), 7.2–7.1 (m, 3H), 7.07 (d, J ¼ 8.7, 1H).
Synthesis of 9-bromo-10-(naphthanen-1-yl)-anthracene (3). A
mixed solution of 2 (2.75 g, 9.034 mmol), NBS (1.77 g, 9.938
ꢀ
mmol), and CHCl3 (100 mL) was heated at 60 C with stirring
under an argon atmosphere. After 3 h, the mixture was cooled to
room temperature. The reaction mixture was then quenched with
water (300 mL), and extracted with CHCl3 (3 ꢃ 50 mL). The
combined organic phase was dried (anhydrous MgSO4) and
concentrated under reduced pressure. The crude product was
purified through flash column chromatography (CHCl3) and
reprecipitation (CH2Cl2/methanol) to afford compound 3 (3.4 g,
98%). 1H NMR (300 MHz, CDCl3, d): 8.69 (d, J ¼ 8.9, 2H), 8.08
(d, J ¼ 8.3, 1H), 8.02 (d, J ¼ 8.2, 1H), 7.68 (t, J ¼ 8.0, 1H), 7.60–
7.55 (m, 2H), 7.51–7.45 (m, 2H), 7.41 (d, J ¼ 8.7, 2H), 7.29–7.24
(m, 2H), 7.19 (t, J ¼ 7.9, 1H), 7.04 (d, J ¼ 8.5, 1H).
Device fabrication and evaluation
Multi-layer device (vacuum deposited device). Blue OLEDs
were fabricated by thermal evaporation onto a cleaned glass
substrate pre-coated with ITO, without breaking the vacuum.
Prior to organic layer deposition, the ITO substrates were
exposed to UV-ozone flux for 20 min followed by degreasing in
isopropyl alcohol. All layers were grown by thermal evaporation
at a base pressure of <5 ꢃ 10ꢁ7 Torr. Layers were deposited in
the following order: HIL/HTL/blue EML/ETL/cathode. A 60
nm thick 2-TNATA layer was used as the HIL, 20 nm thick NPB
was used as the HTL, 40 nm thick PCAN (40 nm thick ADN)
was used as the blue EML, and 20 nm thick Alq3 was used as the
ETL. Finally, the cathode, consisting of a 1 nm thick LiF layer
and a 100 nm thick layer of Al, was deposited on the sample
surface. All the materials were purified using train sublimation
before use.
Synthesis of PCAN. A mixed solution of 1 (1.92 g, 5.199
mmol), 3 (1.993 g, 5.199 mmol), tetrakis(triphenylphosphine)
palladium(0) (0.36 g, 0.312 mmol), toluene (40 mL), ethanol (8
mL), and 2 N NaOH aqueous solution (20 mL) was heated at 100
ꢀC with stirring under an argon atmosphere. After 12 h, the
mixture was cooled to room temperature. The reaction mixture
was then quenched with water (300 mL), neutralized (1 N HCl),
and extracted with CH2Cl2 (3 ꢃ 100 mL). The combined organic
phase was dried (anhydrous MgSO4) and concentrated under
reduced pressure. The crude product was purified through
column chromatography (CHCl3/n-hexane, 3 : 7 v/v) and repre-
1
cipitation (CH2Cl2/methanol) to give PCAN (1.7 g, 60%). H
NMR (300 MHz, CDCl3, d): 8.35 (d, J ¼ 15.5, 1H), 8.13 (t, J ¼
7.02, 1H), 8.09 (d, J ¼ 8.2, 1H), 8.04 (d, J ¼ 8.2, 1H), 7.87 (d, J ¼
8.6, 2H) 7.74–7.45 (m, 14H), 7.33–7.20 (m, 7H). 13C NMR (500
MHz, CDCl3, d): 141.6, 140.6, 138.5, 138.0, 137.1, 135.0, 134.0,
133.9, 131.0, 130.8, 130.7, 130.2, 129.6, 129.5, 129.4, 128.4, 128.3,
127.8, 127.7, 127.4, 127.2, 127.0, 126.5, 126.2, 125.8, 125.4, 125.2,
123.7, 123.5, 123.4, 120.7, 120.4, 110.2, 109.9. MS(FAB, m/z):
calcd for C42H27N, 545.21; found, 545.21. Anal calcd for
C42H27N: C: 92.44, H: 4.99, N: 2.57; found: C: 92.44, H: 4.98, N:
2.58.
Double-layer device (solution processed device). Blue OLEDs
were fabricated by spin-coating onto a cleaned glass substrate
pre-coated with ITO in an N2 atmosphere (PEDOT:PSS and
EML), and thermal evaporation, without breaking the vacuum
(Bphen, LiF and Al). The last three layers were grown by thermal
evaporation at a base pressure of <5 ꢃ 10ꢁ7 Torr. Prior to
organic layer deposition, the ITO substrates were exposed to
UV-ozone flux for 20 min, followed by degreasing in isopropyl
alcohol. Layers were deposited in the following order: HIL/blue
EML/ETL/cathode. 40 nm thick PEDOT:PSS (Clevious P VP Al
4083) (used as the HIL) was spin-coated (1500 rpm, 40s), then
baked at 150 ꢀC for 10 min to remove any residual solvent. The
70 nm thick PCAN solution used as the EML (in toluene at 2.5
weight%) was spin-coated (2000 rpm, 40s) onto the PEDOT:PSS
layer, then baked at 150 ꢀC for 1 h to remove any remaining
Characterization
1
Chemical structures were fully identified by H NMR (Bruker,
Avance-300), 13C NMR (Bruker, Avance 500), GC-Mass (JEOL,
128 | J. Mater. Chem., 2012, 22, 123–129
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Acknowledgements
This research was supported by Basic Science Research Program
(CRI; RIAMIAM0209(0417-20090011)) and WCU (World
Class University) project (R31-2008-00010075-0) through
National Research Foundation of Korea funded by the Ministry
of Education, Science and Technology.
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