Synthesis of Novel Carbazole Dendrimers Having a Metal Coordination Site

Article abstract of DOI:10.1246/cl.2003.674

We have synthesized novel carbazole dendrimers via cyclo-trimerization. This synthetic procedure to prepare the dendrimers, especially with a higher generation, via the cyclization reaction was found to be an extremely effective method. These dendrimers have the ability to assemble metal ions such as Sn ²⁺ and Eu³⁺, resulting in a change in its fluorescence property.

Full text of DOI:10.1246/cl.2003.674

674
Chemistry Letters Vol.32, No.8 (2003)
Synthesis of Novel Carbazole Dendrimers Having a Metal
Coordination Site
Atsushi Kimoto, Jun-Sang Cho, Masayoshi Higuchi, and Kimihisa Yamamotoꢀ
Kanagawa Academy of Science & Technology (KAST), and Department of Chemistry, Faculty of Science & Technology,
Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522
(Received April 11, 2003; CL-030313)
We have synthesized novel carbazole dendrimers via cyclo-
I
I
I
trimerization. This synthetic procedure to prepare the dendri-
mers, especially with a higher generation, via the cyclization re-
action was found to be an extremely effective method. These
dendrimers have the ability to assemble metal ions such as
Sn2þ and Eu3þ, resulting in a change in its fluorescence proper-
ty.
N
N
Carbazole, Cu2O
DMAc
KI, KIO3
AcOH
O
O
O
NHAc
NHAc
2
NHAc
N
N
N
Discrete triphenylamine arrays based on linear,1 cyclic,2
and dendritic3–5 systems with a conjugated backbone have at-
tractive magnetic and electronic properties. Also, acting as
hole-transport materials in various electro-optical applications,
for example, electroluminescent devices6 and photocells,7
branched and dendritic structures exhibit a better amorphous
property and high solubility because of the geometry of these
molecules without close packing. From a synthetic viewpoint,
carbazole analogous to the triphenylamine backbone can be
easily functionalized at its 3-, 6-, and 9-positions and covalently
linked to other molecular moieties.8 As a result of its special
photo, electrical, and chemical properties, carbazole has been
used as a functional building block in the fabrication of organic
photoconductors, nonlinear optical materials, and photorefrac-
tive materials. Azomethines, imines, or Schiff bases are well-
known to be a strong coordination site for various metal assem-
bling which results in electrochemical and/or photochemical
properties as well as polyaniline. We now report the synthesis
of novel carbazole dendrimers via the cyclotrimerization. These
dendrimers have a phenylazomethine core as the metal coordi-
nation site, and the spectral change results in change in the de-
gree of energy transfer.
i) 1, Cu2O, DMAc
N
N
N
ii) KOH, H2O, MeOH, THF
N
O
NH2
G3 Monomer
Scheme 2. Synthesis of the G3 monomer.
ing group of amines. The cyclization monomers were obtained
from 4-acetylamino-40-iodobenzophenone 2 using the same re-
actions. The G1 and G2monomers were simply synthesized
from 2, and carbazole or 1 via the Ullmann condensation. How-
ever, the synthesis of the G3 monomer was carried out with a
route like a double stage convergent method as shown in
Scheme 2. This is due to the decreased reactivity of the amine
site at the G3 dendron, which can be synthesized by the same
reactions as the G2dendron.
The G1, G2, and G3 carbazole dendrimers were synthesized
via the cyclotrimerization of the corresponding monomers in the
presence of titanium (IV) tetrachloride (0.75 equiv.) and 1,4-di-
azabicyclo[2.2.2]octane (DABCO) (1.5 equiv.) in 30, 56, and
95% yields, respectively (Scheme 3).11 As a conventional syn-
thetic technique to prepare the dendrimers, the convergent
The G2dendron, that is, the carbazole trimer was obtained
by the combination of the iodination,9 acetylation, and the
modified Ullmann condensation10 (Scheme 1). We adopted
the acetyl group for protection of the amines. This is due to
the high thermal stability of the amide bond compared with
the Boc (tert-butoxycarbonyl) group that is a traditional protect-
N
I
I
N
N
i ) KI, KIO3, AcOH
ii) Ac2O, BF3Et2O
N
N
N
n-1
N
N
N
Ac
H
N
N
TiCl , DABCO
4
n-1
N
N
n-1
N
PhCl
O
n-1
125 °C, 16 h
NH
2
N
i ) Carbazole, Cu2O, DMAc
ii) KOH, H2O, DMSO, THF
N
N
n= 1-3
N
Gn monomer
N
N
H
Gn dendrimer
1, G2 Dendron
Scheme 3. Synthesis of the dendrimers via the cyclotrimeriza-
tion.
Scheme 1. Synthesis of the G2dendron.
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.8 (2003)
675
method results in a lower conversion yield with increasing the
generation due to their steric hindrance of the bulky dendrons
with a small core.12 We developed a useful synthetic method
for preparative dendrimer formation in which the core is gener-
ated from a dendritic precursor by a cyclization reaction.13 We
have already reported that aminophenylketones such as 4-ami-
noacetophenone produce only the corresponding linear poly-
mer; however, the reaction of the 4-aminobenzophenone deriv-
atives provides the formation of cyclic phenylazomethines in
high yield owing to the steric hindrance of the bulky a-phenyl
ring in the monomer.14 Thus, this synthetic procedure to prepare
dendrimers, especially with a higher generation, via the cycliza-
tion reaction was found to be an extremely effective method.
The UV–vis spectra of the dendrimers show the absorption
bands at 300–425 nm (ꢀmax; 343 nm) attributed to the p-pꢀ tran-
sition of the cyclic phenylazomethine unit at the core and at
225–350 nm (ꢀmax; 294 nm) of the dendritic polycarbazole moi-
ety. No significant red shifts in the absorption were observed.
This means that the degree of p delocalization is limited.15
The absorption attributed to the carbazole unit proportionally
intensifies as the number of carbazole units with the growth
of their generation number, even though the absorption attribut-
ed to their core unit does not change. This localization is caused
by the loss of coplanarity of the aromatic systems due to the
twists in the polycarbazole dendrons at the 3 and 6 positions.
The MM2calculation also supported the fact that they are twist-
ed by 36.3 degrees between the carbazole units. These dendri-
mers are expected to come close to the sphere-like structure
with higher generations.
Eu(OTf)3, SnCl2
N
N
orange
yellow
Scheme 4. Formation of the imine-metal complex.
The photoluminescence spectra showed emission from the
carbazole moieties that were quenched by the Forster type ener-
¨
gy transfer to the imine unit. When complexing with metals, a
lower fluorescence intensity was observed on the basis of the
change of the UV–vis spectra. The larger spectral overlap be-
tween fluorescence from the dendrimer and the absorption at-
tributed to the p-pꢀ transition at 400–500 nm (ꢀmax; 425 nm)
of the phenylazomethine–metal unit resulted in a higher
quenching and less fluorescence at 370–500 nm (ꢀem; 408 nm)
(Figure 1, inset). This phenomenon suggests that a spectral
overlap and the degree of energy transfer can be controlled by
the complexation.
In summary, we have synthesized novel carbazole dendri-
mers via cyclotrimerization. These dendrimers have the ability
to trap metal ions such as Sn2þ and Eu3þ and their complexa-
tion results in a spectral overlap. The degree of energy transfer
can be controlled by the complexation. A development study
about hole-transport in organic electroluminescence devices is
now in progress.
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
Chemistry’’ from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
The dendrimers have 3 C=N units as a core. With the ad-
dition of SnCl2 or Eu(OTf)3 to the dichloromethane–acetonitrile
(4:1) solution of the dendrimer (G3), the yellow solution (ꢀmax
;
343 nm) changed to light orange (ꢀmax; 425 nm) based on the
complexation. The UV spectrum changed with an isosbestic
point at 371 nm (Figure 1, Scheme 4). The complexation pro-
ceeds at random and the imine groups act as an excellent coor-
dination site.16 The equilibrium constants of the formation of
1:1 complexes with SnCl2 on the imine sites were estimated
to be K ¼ 5 ꢁ 105 (Mꢂ1) which was not influenced by the gen-
erations. Similar complexations with Eu(OTf)3 were confirmed.
These results indicate that these dendrimers have the ability to
assemble metal ions with no steric effects among the carbazole
dendrons and the HOMO and LUMO energy levels change by
the complexation, which results in the red shift of the spectra.
References and Notes
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Figure 1. UV-visible spectra of G3 dendrimer during
complexation with SnCl2 (4:1 dichloromethane–aceto-
nitrile, at room temperature). The inset shows the
change of the fluorescence spectra by the complexation.
Published on the web (Advance View) July 7, 2003; DOI 10.1246/cl.2003.674