Synthesis of asymmetrically arranged dendrimers with a carbazole dendron and a phenylazomethine dendron

Article abstract of DOI:10.1021/ma0499674

Asymmetrically arranged dendrimers with a carbazole dendron and a phenylazomethine dendron were synthesized by the combination of Ullmann reaction and a dehydration reaction in the presence of titanium tetrachloride. Stepwise complexation in the phenylazo

Full text of DOI:10.1021/ma0499674

Macromolecules 2004, 37, 5531-5537
5531
Synthesis of Asymmetrically Arranged Dendrimers with a Carbazole
Dendron and a Phenylazomethine Dendron
Atsu sh i Kim oto, J u n -Sa n g Ch o, Ma sa yosh i Higu ch i, a n d Kim ih isa Ya m a m oto*
Department of Chemistry, Faculty of Science and Technology, Keio University,
Yokohama 223-8522, J apan
Received J anuary 5, 2004; Revised Manuscript Received May 19, 2004
ABSTRACT: Asymmetrically arranged dendrimers with a carbazole dendron and a phenylazomethine
dendron were synthesized by the combination of Ullmann reaction and a dehydration reaction in the
presence of titanium tetrachloride. Stepwise complexation in the phenylazomethine dendron unit within
these dendrimers and SnCl2 suggests a gradient in the electron density associated with the imine groups.
The complexation of the dendrimer changes the HOMO/LUMO energy gap of the dendrimer. We show
the dendrimers with higher generations have the larger HOMO values. The most electron-rich molecule,
Cz3-DP A3, has the highest HOMO value of 5.35 eV and, accordingly, is expected to have the lowest
barrier for the hole injection from the ITO electrode (4.6 eV) in OLEDs. However, for the HOMO energy
levels of the carbazole dendrimer complex with SnCl2, the energy levels of the carbazoles did not change
based on almost the same redox potentials as those of the dendrimers themselves. Using Cz3-DP A3 as
a hole-transport layer (HTL), only complexation with metal ions results in the enhanced maximum
luminescence from 4041 to 10 640 cd/m2 by only complexing with SnCl2 under the nonoptimized conditions.
A complexation leads to a high EL efficiency because of the p-type-doped structure of the dendrimers as
a hole-transport layer.
In tr od u ction
Above all, a number of dendrimers have been applied
in OLEDs19-21 designed by their characteristic synthetic
procedure, the convergent method. The advantages of
adopting monodispersed and well-defined dendrimers
as active components in OLEDs are that they can be
easily prepared in high purity and have a better
amorphous property and high solubility due to their
geometry without close packing, resulting in the easier
fabrication of thin films by the spin-coating method, a
promising approach for large area display applications
as well as polymeric materials.
Branches of the dendrimers1,2 with a monodispersed
and well-defined structure growing up symmetrically by
modular synthesis3 have developed with a regular
architecture, producing materials of intriguing proper-
ties, such as light harvesting,4 catalysis,5 and so on, with
a better amorphous property and high solubility due to
the geometry of these molecules without close packing.
We should note that dendrimers, especially rigid ones,
can possibly be regularly assembled by packing on a
plate without deformation of the molecule6 and are
expected to expand the field of nanomaterials.7,8 The
synthetic control to obtain designed dendrimers
arranged asymmetrically, such as layered,9-11 seg-
mented,9,12,13 or tailored ones,14,15 is difficult, requiring
stepwise synthetic methodology. Generally, asymmetri-
cally substituted dendrimer building blocks are pre-
pared by interrupting after the first of two possible
reactions using a large excess of monomer. Indeed, when
preparing asymmetric soft dendrimers with σ bonds, the
developing process, which includes many types of reac-
tions with a lower reactivity, is easily applied to asym-
metric building blocks. However, asymmetric building
blocks with rigid branches consisting of π-conjugated
bonds are difficult to prepare because of a few varieties
of growth reactions and reacting substituents with high
reactivity. Thus, few examples of preparing and devel-
oping asymmetrical dendritic copolymers are reported,
compared with those of symmetrically substituted ones,
and above all, to the best of our knowledge, few
examples of rigid asymmetric dendrimers have been
reported.16
Efforts have been ongoing to develop a novel hole-
transport polymer for advanced electronic devices. This
is a key material for improving the turn-on voltage,
luminescence intensity, operation lifetime, full color
display capability, durability, reasonable power ef-
ficiency, and so on. Generally, such high-performance
devices are obtained by developing sequential HOMO/
LUMO energy gradients in the device by introducing
the multilayered structure fabricated by repeatedly
making thin films. For example, introducing another
layer on the ITO electrode, with a HOMO energy level
between that of the hole-transporting layer and of the
electrode, that is a hole-injecting layer, lowers the
energy barrier for hole injection. This approach involves
placing the injecting layer between the ITO and
the transport layer, such as copper phthalocyanine
(CuPc)22 and 4,4′,4′′-tris(3-methylphenylphenylamino)-
triphenylamine (m-MTDATA),19 thus resulting in an
enhanced EL efficiency. Another approach to develop
the characters of the OLEDs has recently been raised
by the insertion of thick doped materials such as doped
triarylamine,23,24 poly(vinylcarbazole),25 polythio-
phene,26-29 and polyaniline,30,31 resulting in the en-
hancement of a carrier injection and transport with a
lower driving voltage. However, these two approaches
require highly layered structures because of their mo-
lecular structures and syntheses and the indefinite
Organic materials for various electrooptical applica-
tions, for example, organic light-emitting diodes (OLE-
Ds)17 and photocells,18 generally consist of rigid π-con-
jugated structures with a narrow HOMO-LUMO gap.
* Corresponding author: e-mail yamamoto@chem.keio.ac.jp.
10.1021/ma0499674 CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/01/2004

5532 Kimoto et al.
Macromolecules, Vol. 37, No. 15, 2004
Ch a r t 1. Str u ctu r es of Den d r im er s
added a solution of SnCl2 (1 equiv vs the dendrimer) in
acetonitrile. The yellow solution changed to light orange based
on the complexation and evaporated to dryness to give the
dendrimer complex.
functional separation of the buffer and hole transport
in one component. Moreover, the use of polymeric
materials is restricted by the fabrication of such more
complex structures because of the erosion of the fabri-
cated film in advance followed by the method of making
thin films, the spin-coating method.
We now report the syntheses of asymmetrically ar-
ranged dendrimers with a carbazole dendron (hole
transporter) and a phenylazomethine dendron (metal
collector) with a definite functional chemical structure.
Stepwise complexation in the phenylazomethine den-
dron unit within these dendrimers and SnCl2 suggests
a gradient in the electron density associated with the
imine groups, and the complexation of the dendrimer
changes the HOMO/LUMO energy gap of the den-
drimer. Only complexation with metal ions results in a
high EL efficiency because of the p-type-doped structure
of the dendrimers as a hole-transport layer.
Cz2 Den d r on . To a solution of carbazole (8.35 g, 50.0 mmol)
in acetic acid (140 mL) was added potassium iodide (11 g, 66.3
mmol). With stirring, potassium iodate (16 g, 74.8 mmol) was
slowly added to the mixture and refluxed for 10 min. The
reaction mixture was cooled to room temperature, and then
the mixture of iodinated compounds (7.0 g) was obtained by
filtration. The mixture was dissolved in acetic anhydride (42
mL) and added boron trifluoride diethyl etherate (0.15 mL)
with refluxing for 20 min to yield 9-acetyl-3,6-diiodocarbazole
(3.94 g, 39%, two steps) as a residue. A mixture of carbazole
(6.38 g, 38.2 mmol), 9-acetyl-3,6-diiodocarbazole (8.0 g, 17.4
mmol), and copper(I) oxide (7.45 g, 52.1 mmol) in N,N-
dimethylacetamide (DMAc) (150 mL) was heated in an oil bath
at 160 °C for 36 h. The reaction mixture was cooled to room
temperature and then filtered through Celite. The filtrate was
poured into a large amount of methanol (2 L), and the mixture
of acetylated Cz2 dendron was obtained by filtration. The
mixture was dissolved in THF (350 mL), DMSO (150 mL), H2O
(10 mL), and then KOH (9.72 g, 0.173 mol) was added, and
the mixture was refluxed for 2 h. The reaction mixture was
cooled to room temperature, neutralized by HCl, and then
poured into water to give the final mixture. Cz2 dendron (4.73
g, 55%, two steps) was isolated by silica gel column chroma-
tography using 4:1 hexane/ethyl acetate with 2% Et3N from
the mixture.1H NMR (400 MHz, CDCl3, TMS standard, 20 °C,
ppm): δ 8.48 (s, 1H), 8.20 (d, J ) 2.0 Hz, 2H), 8.16 (d, J ) 8.0
Hz, 4H), 7.69 (d, J ) 8.8 Hz, 2H), 7.61 (dd, J ) 8.4, 2.0 Hz,
2H), 7.42-7.35 (m, 8H), 7.30-7.25 (m, 4H). 13C NMR (100
MHz, CDCl3, TMS standard, 20 °C, ppm): δ 141.68, 139.15,
129.85, 126.14, 125.79, 124.00, 123.03, 120.20, 119.74, 119.58,
111.95, 109.64. MALDI-TOF-MS: 497.2 ([M]+ calcd for
Exp er im en ta l Section
All chemicals were purchased from Aldrich, Tokyo Kasei Co.,
Ltd., and Kantoh Kagaku Co., Inc. (reagent grade), and used
without further purification. UV-vis spectra were obtained
using a Shimadzu UV-2400PC spectrometer. 1H NMR and 13C
NMR spectra were measured on a J EOL 400 MHz ET-NMR
(J MN 400). Cyclic voltammetry experiments were performed
with a BAS-100 electrochemistry analyzer. All measurements
were carried out at room temperature with a conventional
three-electrode configuration consisting of a platinum working
electrode, an auxiliary platinum electrode, and a nonaqueous
Ag/AgNO3 reference electrode. The solvent in all experiments
was 1,2-dichloroethane, and the supporting electrolyte was 0.1
M tetrabutylammonium hexafluorophosphate. The E1/2 values
were determined as 1/2(Epa + Epc), where Epa and Epc are the
anodic and cathodic peak potentials, respectively. All poten-
tials reported are referenced to Fc/Fc+ external and are not
corrected for the junction potential.
Conventional OLED devices having the ITO/dendrimer/Alq/
CsF/Al structure were fabricated by spin-coating the den-
drimer solutions in chlorobenzene on an ITO-coated glass
anode. Alq (50 nm), CsF (2 nm), and Al (100 nm) were
successively vacuum-deposited on top of the hole-transporting
layer. The emitting area was 9 mm2. The current-voltage
characteristics were measured using an Advantest R6243
current/voltage unit. Luminance was measured with a Minolta
LS-100 luminance meter under air at room temperature.
The dendrimer complex was prepared by the following
method. To a solution of the dendrimer in chloroform was
C
36H23N3: 497.19).
Gen er a l P r oced u r e for th e Ullm a n n Rea ction . A mix-
ture of p-iodoacetanilide and the corresponding carbazole
dendron and an excess amount of copper(I) oxide in N,N-
dimethylacetamide (DMAc) was heated in an oil bath at 160
°C for 24 h under a N2 atmosphere. The reaction mixture was
cooled to room temperature and then filtered through Celite
to remove excess copper complex. The filtrate was evaporated
to dryness. The pure product was isolated by silica gel column
chromatography.
Cz1-NHAc. The previous procedure was followed using 1.31
g (5.00 mmol) of p-iodoacetanilide, 0.930 g (5.56 mmol) of
carbazole, and 1.07 g (7.50 mmol) of copper oxide. The product
was isolated by silica gel column chromatography using 2:1
hexane/ethyl acetate with 2% Et3N as eluent, yielding 1.15 g

Macromolecules, Vol. 37, No. 15, 2004
Asymmetrically Arranged Dendrimers 5533
(77%). 1H NMR (400 MHz, CDCl3, TMS standard, 20 °C,
ppm): δ 8.14 (d, 7.6 Hz, 2H), 7.74 (d, J ) 8.8 Hz, 2H), 7.51 (d,
J ) 8.8 Hz, 2H), 7.43-7.35 (m, 5H), 7.28 (dd, J ) 8.8, 7.6 Hz,
2H), 2.25 (s, 3H). 13C NMR (100 MHz, CDCl3, TMS standard,
20 °C, ppm): δ 168.31, 140.83, 136.92, 133.47, 127.70, 125.85,
123.17, 121.01, 120.21, 119.81, 109.60, 24.72. MALDI-TOF-
MS: 299.3 ([M]+ calcd for C20H16N2O: 300.13). Anal. Calcd
for C20H16N2O: C, 79.98; H, 5.37; N, 9.33. Found: C, 79.96;
H, 5.52; N, 9.23.
Cz3-NH2. The previous procedure of Ullmann reaction was
followed using 0.822 g (1.49 mmol) of Cz1-I2-NHAc, 1.64 g
(3.30 mmol) of Cz2 dendron, and 0.644 g (4.50 mmol) of copper
oxide. The product was purified by silica gel column chroma-
tography using chloroform as eluent. The above procedure of
the hydrolysis/deprotection was followed using 1.4 mL of water
and 1.6 mL (30 mmol) of H2SO4. The product was isolated by
silica gel column chromatography using 2:2:1 hexane/ethyl
acetate/chloroform with 2% Et3N as eluent, yielding 0.685 g
1
Cz2-NHAc. The above procedure was followed using 0.521
g (2.00 mmol) of p-iodoacetanilide, 1.10 g (2.20 mmol) of Cz2
dendron, and 0.429 g (3.00 mmol) of copper oxide. The product
was isolated by silica gel column chromatography using 2:1
hexane/ethyl acetate with 2% Et3N as eluent, yielding 1.24 g
(99%). 1H NMR (400 MHz, CDCl3, TMS standard, 20 °C,
ppm): δ 8.26 (s, 2H), 8.16 (d, J ) 7.6 Hz, 4H), 7.83 (d, J ) 8.8
Hz, 2H), 7.66 (d, J ) 8.8 Hz, 2H), 7.60 (t, J ) 8.4 Hz, 4H),
7.41-7.37 (m, 9H), 7.29-7.25 (m, 4H), 2.27 (s, 3H). 13C NMR
(100 MHz, CDCl3, TMS standard, 20 °C, ppm): δ 168.39,
141.64, 140.66, 137.63, 132.74, 130.23, 127.82, 126.18, 125.80,
123.77, 123.05, 121.23, 120.21, 119.61, 111.15, 109.63, 24.86.
MALDI-TOF-MS: 630.6 ([M]+ calcd for C44H30N4O: 630.24).
Gen er a l P r oced u r e for th e Hyd r olysis/Dep r otection
of th e Am id e Bon d . To a solution of amide in THF and
methanol were slowly added water and an excess amount of
H2SO4. After refluxing for 2 h, the reaction mixture was cooled
in an ice bath, neutralized by NaOH(aq), and then extracted
with CHCl3. The organic layer was washed with Na2CO3
solution, dried over anhydrous Na2SO4, and evaporated to
dryness. The pure product was isolated by silica gel column
chromatography.
(37%, two steps). H NMR (400 MHz, CDCl3, TMS standard,
20 °C, ppm): δ 8.50-8.27 (m, 8H), 8.16-8.13 (m, 6H), 7.80-
7.15 (m, 34H), 7.05-6.78 (m, 4H), 3.97-3.83 (m, 2H). 13C NMR
(100 MHz, CDCl3, TMS standard, 20 °C, ppm): δ 146.54,
141.69, 141.50, 140.84, 140.54, 134.84, 134.55, 130.00, 129.29,
129.06, 128.53, 128.33, 128.16, 126.84, 126.46, 126.16, 125.79,
124.80, 123.95, 123.59, 123.02, 120.19, 119.65, 119.57, 116.04,
115.92, 115.80, 112.31, 111.95, 111.37, 111.26, 109.68. MALDI-
TOF-MS: 1250.4 ([M]+ calcd for C90H56N8: 1248.46).
Gen er a l P r oced u r e for th e Syn th esis of th e Asym -
m etr ica lly Ar r a n ged Den d r im er s w ith a Ca r ba zole Den -
d r on a n d a P h en yla zom eth in e Den d r on . To a mixture of
Czm -NH2 (m ) 1-3), the corresponding phenylazomethine
dendron, DP An (n ) 1-3), and DABCO in chlorobenzene was
added TiCl4 dropwise. The addition funnel was rinsed with
chlorobenzene. The reaction mixture was heated in an oil bath
at 125 °C for 4 h. The precipitate was removed by filtration.
The filtrate was concentrated, and the pure product was
isolated by silica gel column chromatography.
Cz1-DP A1. The previous procedure was followed using
0.240 g (0.931 mmol) of Cz1-NH2, 0.254 g (1.40 mmol) of
benzophenone (DP A1), 0.157 g (1.40 mmol) of DABCO, and
0.132 g (6.98 mmol) of TiCl4. The product was isolated by silica
gel column chromatography using 9:1:1 hexane/ethyl acetate/
chloroform with 2% Et3N as eluent, yielding 0.245 g (62%).
1H NMR (400 MHz, CDCl3, TMS standard, 20 °C, ppm): δ 8.11
(d, 7.2 Hz, 2H), 7.82 (d, J ) 7.2 Hz, 2H), 7.53-7.20 (m, 16H),
6.93 (d, J ) 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3, TMS
standard, 20 °C, ppm): δ 169.03, 150.59, 140.98, 139.29,
135.97, 132.51, 130.92, 129.52, 129.36, 128.82, 128.22, 127.97,
127.32, 125.71, 123.08, 122.27, 120.14, 119.59, 109.63. MALDI-
TOF-Mass: 420.6 ([M]+ calcd for C31H22N2: 422.2). Anal. Calcd
for C31H22N2: C, 88.12; H, 5.25; N, 6.63. Found: C, 87.72; H,
5.48; N, 6.47.
Cz2-DP A2. The previous procedure was followed using
0.0993 g (0.169 mmol) of Cz2-NH2, 0.184 g (0.340 mmol) of
DP A2, 0.246 g (2.19 mmol) of DABCO, and 0.104 g (0.547
mmol) of TiCl4. The product was isolated by silica gel column
chromatography using chloroform with 2% Et3N as eluent,
yielding 0.163 g (87%). 1H NMR (400 MHz, CDCl3, TMS
standard, 20 °C, ppm): δ 8.27 (s, 2H), 8.16 (d, J ) 7.6 Hz,
4H), 7.77 (d, J ) 7.2 Hz, 2H), 7.70 (d, J ) 7.2 Hz, 21H), 7.63-
6.94 (m, 40H), 6.78 (d, J ) 8.4 Hz, 2H), 6.68 (d, J ) 8.4 Hz,
2H). 13C NMR (100 MHz, CDCl3, TMS standard, 20 °C, ppm):
δ 168.93, 168.68, 168.45, 153.85, 151.99, 151.58, 141.67,
139.20, 138.90, 135.75, 134.23, 131.41, 130.95, 130.49, 130.35,
130.12, 130.04, 129.38, 129.25, 128.82, 128.67, 128.18, 128.02,
127.69, 127.36, 126.07, 125.80, 123.63, 123.04, 122.88, 120.55,
120.34, 120.19, 119.57, 111.30, 109.64. MALDI-TOF-Mass:
1110.4 ([M]+ calcd for C31H22N2: 1109.0). Anal. Calcd for
Cz1-NH2. The previous procedure was followed using 0.303
g (1.01 mmol) of Cz1-NHAc, 0.9 mL of water, and 1.0 mL (20
mmol) of H2SO4. The product was isolated by silica gel column
chromatography using 2:1 hexane/ethyl acetate with 2% Et3N
1
as eluent, yielding 0.258 g (99%). H NMR (400 MHz, CDCl3,
TMS standard, 20 °C, ppm): δ 8.13 (d, J ) 8.0 Hz, 2H), 7.39
(t, J ) 7.6 Hz, 2H), 7.33-7.24 (m, 6H), 6.86 (d, J ) 6.6 Hz,
2H), 3.85 (s, 2H). 13C NMR (100 MHz, CDCl3, TMS standard,
20 °C, ppm): δ 145.82, 141.38, 128.42, 128.07, 125.63, 122.87,
120.10, 119.33, 115.83, 109.70. MALDI-TOF-MS: 257.2 ([M]+
calcd for C18H14N2: 258.12). Anal. Calcd for C18H14N2: C, 83.69;
H, 5.46; N, 10.84. Found: C, 83.42; H, 5.86; N, 10.33.
Cz2-NH2. The above procedure was followed using 0.296 g
(0.469 mmol) of Cz2-NHAc, 0.42 mL of water, and 0.25 mL
(4.7 mmol) of H2SO4. The product was isolated by silica gel
column chromatography using 1:2 hexane/ethyl acetate with
2% Et3N as eluent, yielding 0.168 g (61%). 1H NMR (400 MHz,
CDCl3, TMS standard, 20 °C, ppm): δ 8.24 (s, 2H), 8.15 (d, J
) 8.0 Hz, 4H), 7.56 (t, 9.6 Hz, 4H), 7.43-7.36 (m, 10H), 7.30-
7.23 (m, 4H), 6.90 (d, J ) 8.4 Hz, 2H), 3.89 (s, 2H). 13C NMR
(100 MHz, CDCl3, TMS standard, 20 °C, ppm): δ 146.47,
141.71, 141.21, 135.64, 129.77, 128.44, 127.23, 125.99, 125.77,
123.42, 123.01, 120.19, 119.53, 115.95, 111.25, 109.67. MALDI-
TOF-MS: 587.3 ([M]+ calcd for C42H28N4: 588.23).
P r oced u r e for th e Syn th esis of Cz3-NH2. The aniline
derivative substituted by the Cz3 dendron (Cz3-NH2) was
prepared according to the following procedures as well as the
reported ones:
Cz1-I2-NHAc. To a solution of Cz1-NHAc (0.600 g, 2.00
mmol) in acetic acid (25 mL) was added potassium iodide
(0.442 g, 2.66 mmol). With stirring, potassium iodate (0.642
g, 3.00 mmol) was slowly added to the mixture and refluxed
for 20 min. The reaction mixture was cooled to room tem-
perature, and then the pure product (0.967 g, 88%) was
obtained by filtration and washed with NaHCO3(aq) and
C
81H54N6: C, 87.54; H, 4.90; N, 7.56. Found: C, 87.39; H, 5.06;
N, 7.46.
Cz3-DP A3. The previous procedure was followed using
0.425 g (0.340 mmol) of Cz3-NH2, 0.427 g (0.340 mmol) of
DP A3, 0.246 g (2.19 mmol) of DABCO, and 0.104 g (0.547
mmol) of TiCl4. The product was isolated by silica gel column
chromatography using chloroform with 2% Et3N as eluent,
yielding 0.746 g (88%). 1H NMR (400 MHz, CDCl3, TMS
standard, 20 °C, ppm): δ 8.51 (s, 2H), 8.30 (s, 4H), 8.15 (d, J
) 7.2 Hz, 8H), 7.80-6.95 (m, 88H), 6.89 (t, J ) 8.8 Hz, 4H),
6.76-6.55 (m, 12H). 13C NMR (100 MHz, CDCl3, TMS stan-
dard, 20 °C, ppm): δ 168.78, 168.54, 168.41, 168.26, 168.12,
154.37, 153.79, 153.71, 153.37, 151.90, 151.78, 151.30, 141.59,
141.39, 139.10, 138.91, 138.67, 135.55, 133.99, 133.82, 133.60,
130.82, 130.48, 130.36, 130.20, 129.98, 129.27, 128.60, 128.06,
127.91, 127.74, 127.50, 126.37, 126.09, 125.70, 125.34, 123.68,
1
water. H NMR (400 MHz, DMSO-d6, TMS standard, 20 °C,
ppm): δ 10.26 (s, 1H), 8.71 (s, 2H), 7.87 (d, J ) 8.6 Hz, 2H),
7.71 (d, J ) 8.6 Hz, 2H), 7.51 (d, J ) 7.8 Hz, 2H), 7.17 (d, J )
7.8 Hz, 2H), 2.12 (s, 3H). 13C NMR (100 MHz, DMSO-d6, TMS
standard, 20 °C, ppm): δ 168.41, 139.44, 139.00, 134.67,
130.25, 129.37, 127.03, 123.72, 120.15, 112.09, 83.35, 24.10.
MALDI-TOF-MS: 551.0 ([M]+ calcd for C20H14I2N2O: 551.92).
Anal. Calcd for C20H14I2N2O: C, 43.51; H, 2.56; N, 5.07.
Found: C, 43.57; H, 2.82; N, 4.67.

5534 Kimoto et al.
Sch em e 1. Syn th esis of Cz2 Den d r on a
Macromolecules, Vol. 37, No. 15, 2004
the same reactions.35 The convergent approach was
employed to synthesize the first-generation (G1) and
second-generation (G2) dendrimers. The G1 and G2
substituted aniline derivatives were simply synthesized
from p-iodoacetanilide, and carbazole or Cz2 dendron,
via the Ullmann condensation to yield Cz1-NHAc and
Cz2-NHAc, respectively (Scheme 2), following the hy-
drolysis reaction of the amide-protecting unit using
sulfuric acid to obtain Cz1-NH2 and Cz2-NH2, respec-
tively. Generally, the hydrolysis of the amide can easily
proceed in the presence of a strong base such as NaOH,
KOH, and so on.35 However, in this case, this hydrolysis
reaction cannot proceed when using them.
The synthesis of Cz3-NH2 was carried out with a
route like a double-stage convergent method as shown
in Scheme 3. This is due to the decreased reactivity of
the amine site at the Cz3 dendron, which can be
synthesized by the same procedures as the Cz2 dendron.
We then adopted the growth procedure stacking the Cz2
dendron on the carbazole unit. First, the iodination of
Cz1-NHAc was pursued to give Cz1-I2-NHAc, and then
Cz1-I2-NHAc was allowed to react with the Cz2 den-
dron under the Ullmann reaction conditions successfully
yielding Cz3-NH2.36
As shown in Scheme 4, a series of dendrimers
substituted asymmetrically (Czm -DP An ) were synthe-
sized by the dehydration reaction using TiCl4 from the
corresponding 4-amino-N-phenylcarbazoles (Czm -NH2)
and phenylazomethine dendron (DP An ), which were
obtained by a previously reported procedure.6,37 These
dendrimers were characterized by 1H and 13C NMR
spectroscopies, elemental analysis, and matrix-assisted
laser desorption/ionization time-of-flight mass spectros-
copy (MALDI-TOF-MS).
The addition of SnCl2 to a chloroform/acetonitrile
solution of asymmetrically substituted dendrimers (Czm -
DP An ) resulted in a complexation with a stepwise
spectral change, similar to that for the previously
reported dendrimers.38-41 During addition of SnCl2, the
solution color of Cz3-DP A3 changed from light to deeper
yellow. We observed that the complexation was complete
in 10 min by the spectral change after the addition of
SnCl2; that is, the complexation equilibrium is reached
within at least several minutes. Using UV-vis spec-
troscopy to profile the complexation until an equimolar
a
Reagents and conditions: (i) KI, KIO3, AcOH, 20 min; (ii)
Ac2O, BF3-Et2O, 10 min; (iii) carbazole, Cu2O, DMAc, 36 h;
(iv) KOH, H2O, THF, DMSO, 2 h.
123.54, 122.94, 120.83, 120.74, 120.40, 120.10, 119.93, 119.60,
119.49, 111.69, 111.16, 109.55. MALDI-TOF-Mass: 2487.0
([M]+ calcd for C31H22N2: 2486.3).
Resu lts a n d Discu ssion
The reactivity and generality of the substrate con-
structing the phenylazomethine backbone are much
higher than those of an arylamine backbone. Although
the triarylamine can easily be formed using palladium-
catalyzed reactions, various reaction conditions, such as
palladium complex, ligands, and so on, are required,
resulting in lower generalities. On the other hand, the
formation of the phenylazomethine backbone affords
great generality by the dehydration reaction of aromatic
amines with aromatic ketones in the presence of tita-
nium(IV) tetrachloride and 1,4-diazabicyclo[2.2.2]octane
(DABCO). For these reasons, we considered the strategy
to prepare the asymmetrically substituted dendrimers
by first introducing a dendritic carbazole unit and then
the dendritic phenylazomethines. As shown in Scheme
1, the Cz2 dendron,32 that is, the carbazole trimer, was
obtained by the combination of the iodination,33 acety-
lation, and the modified Ullmann condensation34 (Scheme
1). We adopted the acetyl group for the protection of the
amines. This is due to the high thermal stability of the
amide bond compared with that of the Boc (t-butoxy-
carbonyl) group that is a traditional protecting group
of amines.
The aniline derivatives with a carbazole dendron
(Czm -NH2) were obtained from p-iodoacetanilide using
a
Sch em e 2. Syn th eses of Cz1-NH2 a n d Cz2-NH2
a
Reagents and conditions: (i) carbazole, Cu2O, DMAc, 24 h; (ii) H2SO4, H2O, THF, MeOH, 2 h; (iii) CzG2 dendron, Cu2O,
DMAc, 24 h.

Macromolecules, Vol. 37, No. 15, 2004
Asymmetrically Arranged Dendrimers 5535
a
Sch em e 3. Syn th esis of Cz3-NH2
a
Reagents and conditions: (i) KI, KIO3, AcOH, 20 min; (ii) Cz2 dendron, Cu2O, DMAc, 24 h; (iii) H2SO4, H2O, THF, MeOH, 2 h.
Sch em e 4. Gen er a l Syn th esis of Czm -DP An Den d r im er sa
a
Reagents and conditions: (i) TiCl4, DABCO, PhCl.
Ta ble 1. P h ysica l Da ta of th e Den d r im er s (Czm -DP An )
a n d Th eir Com p lex w ith 1 Sn Cl2
oxidation potential/
V vs Fc/Fc+a
calcd HOMO/
eV
Cz1-DPA1
Cz1-DPA1-SnCl2
Cz2-DPA2
Cz2-DPA2-SnCl2
Cz3-DPA3
Cz3-DPA3-SnCl2
0.687, 0.882
0.616, 0.753
0.616, 0.753
0.594, 0.739
0.555, 0.735
0.538, 0.734
-5.49
-5.42
-5.41
-5.39
-5.35
-5.34
a
Measured in 1,2-dichloroethane.
with an isosbestic point at 374 nm up to the addition of
1 equiv of SnCl2 (Figure 1). The isosbestic point then
shifted upon the further addition of SnCl2 and appeared
at 372 nm between 1 and 3 equiv, moving to 368 nm
when adding between 3 and 7 equiv. Overall, the
number of added equivalents of SnCl2 required to induce
a shift was in agreement with the number of imine sites
present in the different layers of Cz3-DP A3. The
titration results suggest that the complexation proceeds
in a stepwise fashion from the core imines to the
terminal imines of Cz3-DP A3 (Scheme 5).42 A similar
stepwise complexation was also observed with Cz2-
DP A2. From the theoretical calculations, the stepwise
complexation requires the larger equilibrium constants
with at least 10 times larger of the inner imine sites
than the outer ones.39 In other words, the basicity for
F igu r e 1. UV-vis spectra changes of Cz3-DP A3 on stepwise
addition of equimolar SnCl2 in CH3CN/CHCl3 ) 1/4. (Inset)
Enlargements focusing isosbestic points.
amount of SnCl2 had been added, three changes in the
position of the isosbestic points were observed, indicat-
ing that the complexation proceeds, not randomly, but
stepwise. This result suggests that three different
complexes are successively formed upon the SnCl2
addition.
The absorption band around 400 nm attributed to the
complex increases with a decrease in the absorption
bands around 320 nm, attributed to the phenylazome-
thine unit. The spectra of Cz3-DP A3 gradually changed,

5536 Kimoto et al.
Macromolecules, Vol. 37, No. 15, 2004
Ta ble 2. Electr olu m in escen ce P r op er ties for th e Devices
turn-on
voltage/V
max brightness/
cd/m2
current densitya/
mA/cm2
driving
current densityb/
mA/m2
EL effb/
lm/W
quantum
effb/%
HTL
voltageb/V
Cz3-DPA3
Cz3-DPA3 + SnCl2
5.6
3.0
4041
10640
18.4
227.3
9.2
6.1
7.86
7.04
0.43
0.73
0.42
0.46
a
b
Taken at 9 V. Taken at 100 cd/m2.
Sch em e 5. Sch em a tic Rep r esen ta tion of Step w ise
a
Com p lexa tion of Cz3-DP A3 w ith Sn Cl2
F igu r e 2. Luminance-voltage characteristics of double-layer
OLEDs with CsF-Al cathodes using Cz3-DP A3 (diamond)
and its complex (square) with 1 SnCl2 as HTL.
a
This complexation thermodynamically proceeds in a step-
ation and evaporated to dryness to give the dendrimer
complex. The films were obtained by the spin-coating
method. As shown in Figure 2 and Table 2, the low
driving voltage and enhanced efficiency were followed
by simply assembling the metal ions. The turn-on
voltage was reduced from 5.6 to 3.0 V, and the maxi-
mum luminescence was enhanced from 4041 to 10640
cd/m2 by only complexing with SnCl2 under the nonop-
timized conditions. Also, the current performance of the
G3-Sn complex was over 10 times greater (227.3 mA/
cm2) for the same forward driving voltage (9 V) com-
pared to that of the uncomplexed device (18.4 mA/cm2).
The threshold voltages for obtaining a luminance of 100
cd/m2 were 9.2 and 6.1 V for the cells, respectively.
For the HOMO energy levels of the carbazole-
dendrimer complex, the energy levels of the carbazoles
did not change on the basis of the same redox potentials
when complexing. When considering the influence of
doping on the turn-on voltage and the performance at
a lower driving voltage, the reduction in the bulk
resistance followed by the p-type doping facilitates
efficient hole injection into HTL.40 The reduction of
space charge layers at the interface of the ITO and the
dendrimer complex leads to efficient carrier injection
due to tunneling.
In summary, we have synthesized asymmetrically
arranged dendrimers with a carbazole dendron and a
phenylazomethine dendron. Stepwise complexation in
the phenylazomethine dendron unit within these den-
drimers and SnCl2 suggests a gradient in the electron
density associated with the imine groups, and the
complexation of the dendrimer changes the HOMO/
LUMO energy gap of the dendrimer. Only complexation
with metal ions results in a high EL efficiency because
of the p-type-doped structure of the dendrimers as a
hole-transport layer as our past research. A develop-
ment study about the mechanism for the development
of hole-transport in OLEDs is now in progress.
wise fashion from the core imines to the terminal ones,
according to the electron gradients of Cz3-DP A3.
the inner phenylazomethine is higher with increasing
the generation number, and the stability of the pheny-
lazomethine-SnCl2 complex intensified. In another
aspect, the higher coordination constant of inner imines
conducts phenylazomethine dendron to the highly elec-
tron-donating substituent.
The electrochemical properties of the dendrimers were
studied by cyclic voltammetry. The HOMO values are
determined from the first oxidation potential values
with respect to ferrocene (Fc), as shown in Table 1.43
All the dendrimers displayed quasi-reversible oxidation
waves attributed to the oxidation of triarylamine in the
region between 0.2 and 1.4 V vs Fc/Fc+. We have found
that comparing the generation of the asymmetrically
arranged dendrimers, the reduced oxidation potential
of the Czm unit was observed on the basis of the
electron richness and, therefore, the HOMO values with
higher generations. Indeed, the most electron-rich mol-
ecule, Cz3-DP A3, has the highest HOMO value of
-5.35 eV and, accordingly, expected to have the lowest
barrier to hole injection from the ITO electrode (-4.6
eV) in OLEDs. Complexing with 1 equiv of SnCl2, redox
waves resulted with almost the same potential (-5.34
eV) as previously observed in the oxidation wave of the
carbazole dendron. However, an additional stable redox
wave attributed to the reduction of the azomethine-
metal complex unit was not observed within the poten-
tial range.
A bright green emission was observed for all the cells
when a positive dc voltage was applied to the ITO
electrode. The electroluminescence spectrum was in
accord with the photoluminescence spectrum of Alq.
This indicates that the electroluminescence originates
from the recombination of holes and electrons injected
into Alq. For the two-layer OLEDs, Cz3-DP A3 and its
complex with the SnCl2 films were then employed as
the hole-transporting layer. To a solution of the den-
drimer in chloroform was added a solution of SnCl2 (1
equiv vs the dendrimer) in acetonitrile. The yellow
solution changed to light orange based on the complex-
Ack n ow led gm en t. This work was supported by a
Kanagawa Academy Science and Technology Research
Grant (Project No. 23) and Grant-in-Aid for the 21st
Century COE program “KEIO Life Conjugate Chemis-

Macromolecules, Vol. 37, No. 15, 2004
Asymmetrically Arranged Dendrimers 5537
(20) Wang, P.-W.; Liu, Y.-J .; Devadoss, C.; Bharathi, P.; Moore,
J . S. Adv. Mater. 1996, 8, 237.
(21) Freeman, A. W.; Koene, S. C.; Malenfant, P. R. L.; Thompson,
M. E.; Fre´chet, J . M. J . J . Am. Chem. Soc. 2000, 122, 12385.
(22) VanSlyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett.
1996, 69, 2160.
try” from the Ministry of Education, Culture, Sports,
Science, and Technology (MEXT), J apan.
Su p p or tin g In for m a tion Ava ila ble: Cyclic voltammo-
grams of Cz1-DP A1, Cz2-DP A2, and Cz3-DP A3 in 1,2-
dichloroethane. This material is available free of charge via
the Internet at http://pubs.acs.org.
(23) Endo, J .; Matsumoto, T.; Kido, J . J pn. J . Appl. Phys., Part 2
2002, 41, L358.
(24) Zhou, X.; Pfeiffer, M.; Blochwitz, J .; Werner, A.; Nollau, A.;
Fritz, T.; Leo, K. Appl. Phys. Lett. 2001, 78, 410.
(25) Partridge, R. H. Polymer 1983, 24, 733.
(26) Hayashi, S.; Etoh, H.; Saito, S. J pn. J . Appl. Phys., Part 2
1986, 25, L773.
(27) Romero, D. B.; Schaer, M.; Zuppiroli, L.; Cesar, B.; Francois,
B. Appl. Phys. Lett. 1995, 67, 1659.
(28) Carter, S. A.; Angelopoulos, M.; Karg, S.; Brock, P. J .; Scott,
J . C. Appl. Phys. Lett. 1997, 70, 2067.
(29) Gross, M.; Mu¨ller, D. C.; Nothofer, H.-G.; Scherf, U.; Neher,
D.; Bra¨uchle, C.; Meerholz, K. Nature (London) 2000, 405,
661.
(30) Yang, Y.; Heeger, A. J . Appl. Phys. Lett. 1994, 64, 1245.
(31) Sheats, J . R.; Antoniadis, H.; Hueschen, M.; Leonard, W.;
Miller, J .; Moon, R.; Roitman, D.; Stocking, A. Science 1996,
273, 884.
Refer en ces a n d Notes
(1) Fre´chet, J . M. J .; Tomalia, D. A. Dendrimers and Other
Dendritic Polymers; J . Wiley and Sons: New York, 2001.
(2) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules: Concept, Synthesis, Perspectives; VCH: Weinheim,
Germany, 1996.
(3) Grayson, S. M.; Fre´chet, J . M. J . Chem. Rev. 2001, 101, 3819.
(4) Adronov, A.; Fre´chet, J . M. J . Chem. Commun. 2000, 1701.
(5) van Heerbeek, R.; Kamer, P. C. J .; van Leeuwen, P. W. N.
M.; Reek, J . N. H. Chem. Rev. 2002, 102, 3717.
(6) Higuchi, M.; Shiki, S.; Ariga, K.; Yamamoto, K. J . Am. Chem.
Soc. 2001, 123, 4414.
(7) Hofkens, J .; Maus, M.; Gensch, T.; Vosch, T.; Cotlet, M.; Ko¨hn,
F.; Herrmann, A.; Mu¨llen, K.; De Schryver, F. J . Am. Chem.
Soc. 2000, 122, 9278.
(8) Schlupp, M.; Weil, T.; Berresheim, A. J .; Wiesler, U. M.;
Bargon, J .; Mu¨llen, K. Angew. Chem., Int. Ed. 2001, 40, 4011.
(9) Hawker, C. J .; Fre´chet, J . M. J . J . Am. Chem. Soc. 1992, 114,
8405.
(10) Devadoss, C.; Bharathi, P.; Moore, J . S. J . Am. Chem. Soc.
1996, 118, 9635.
(11) J unge, D. M.; Wu, M.-J .; McElhanon, J . R.; McGrath, D. V.
J . Org. Chem. 2000, 65, 5306.
(12) Grayson, S. M.; Fre´chet, J . M. J . J . Am. Chem. Soc. 2000,
122, 10335.
(13) Wooley, K. L.; Hawker, C. J .; Fre´chet, J . M. J . J . Am. Chem.
Soc. 1993, 115, 11496.
(14) Hawker, C. J .; Fre´chet, J . M. J . Macromolecules 1990, 23,
4726.
(15) Bo, Z.; Scha¨fer, A.; Franke, P.; Schlu¨ter, A. D. Org. Lett. 2000,
2, 1645.
(32) McClenaghan, N. D.; Passalacqua, R.; Loiseau, F.; Campagna,
S.; Verheyde, B.; Hameurlaine, A.; Dehaen, W. J . Am. Chem.
Soc. 2003, 125, 5356.
(33) Tucker, S. H. J . Chem. Soc. 1926, 546.
(34) Bacon, R. G. R.; Maitland, D. J . J . Chem. Soc. C 1970, 1973.
(35) Kimoto, A.; Cho, J .-S.; Higuchi, M.; Yamamoto, K. Chem. Lett.
2003, 32, 674.
(36) We identified the purity of intermediates by SEC.
(37) Higuchi, M.; Shiki, S.; Yamamoto, K. Org. Lett. 2000, 2, 3079.
(38) Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba,
H. Nature (London) 2002, 415, 509.
(39) Imaoka, T.; Horiguchi, H.; Yamamoto, K. J . Am. Chem. Soc.
2003, 125, 340.
(40) Sato, N.; Cho, J .-S.; Higuchi, M.; Yamamoto, K. J . Am. Chem.
Soc. 2003, 125, 8104.
(16) Weil, T.; Wiesler, U. M.; Herrmann, A.; Bauer, R.; Hofkens,
J .; De Schryver, F. C.; Mu¨llen, K. J . Am. Chem. Soc. 2001,
123, 8101.
(17) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(18) Bach, U.; Lupo, D.; Comte, P.; Moser, J . E.; Weisso¨rtel, F.;
Salbeck, J .; Spreitzer, H.; Gra¨tzel, M. Nature (London) 1998,
395, 583.
(19) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada,
H.; Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994,
65, 807.
(41) Higuchi, M.; Tsuruta, M.; Chiba, H.; Shiki, S.; Yamamoto,
K. J . Am. Chem. Soc. 2003, 125, 9988.
(42) The absorption band around 600 nm attributed to the
complexation of SnCl2 with carbazole unit has not been
observed during the complexation with imine units because
of the larger equilibrium constants of imine units.
(43) Pommerehne, J .; Vestweber, H.; Guss, W.; Mahrt, R. F.;
Ba¨ssler, H.; Porsch, M.; Daub, J . Adv. Mater. 1995, 7, 551.
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