Synthesis and properties of dumbbell-shaped dendrimers containing 9-phenylcarbazole dendrons

Article abstract of DOI:10.1021/ol702060u

(Chemical Equation Presented) We report the synthesis and structural characterization of two dumbbell-shaped dendrimers incorporating 9-phenylcarbazole units as dendrons, as well as their thermal, morphological, photophysical, and electrochemical properti

Full text of DOI:10.1021/ol702060u

ORGANIC
LETTERS
2007
Vol. 9, No. 22
4531-4534
Synthesis and Properties of
Dumbbell-Shaped Dendrimers
Containing 9-Phenylcarbazole Dendrons
Ken-Tsung Wong,* Yu-Hsien Lin, Hsu-Hsuan Wu, and Fernando Fungo
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, and
Departamento de Qu´ımica, UniVersidad Nacional de R´ıo Cuarto, Agencia Postal 3
(5800), R´ıo Cuarto, Argentina
kenwong@ntu.edu.tw
Received August 21, 2007
ABSTRACT
We report the synthesis and structural characterization of two dumbbell-shaped dendrimers incorporating 9-phenylcarbazole units as dendrons,
as well as their thermal, morphological, photophysical, and electrochemical properties.
Dendrimers are monodispersed macromolecules possessing
well-defined branched structures that can be precisely tailored
with discrete and designated functionality to create multi-
functional materials. Their unique structures and properties
make dendrimers suitable subjects for a wide range of
biomedical and industrial applications, such as drug delivery,1
multivalent bioconjugates2 and multivalent diagnostics for
magnetic resonance imaging (MRI),3 extremely efficient
light-harvesting antennae,4 and homogeneous catalysis.5
Recently, dendrimers have also found promising applications
as materials for organic electronic devices, such as organic
light-emitting diodes (OLEDs).6 Among the various types
of dendrimers, carbazole-based dendrimers have emerged as
a new family of intriguing materials that possess several
attractive properties. For example, the fully conjugated
9-phenylcarbazole monodendrons synthesized by Moore and
co-workers exhibit fluorescence quenching effects via through-
space interactions.7 Carbazole-based dendrons that have been
incorporated at antennal positions surrounding porphyrin
* Address correspondence to this author at National Taiwan University.
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10.1021/ol702060u CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/28/2007

Scheme 1
centers8 and Ru(II)-complex cores9 mediate highly efficient
carbazole13 to afford the G1-N-G1 system 1 in good yield.
Using the same synthetic protocol, we coupled compound 1
with 3,6-dibromo-9-(4-nitrophenyl)carbazole, which we had
synthesized in moderate yield through the bromination of
9-(4-nitrophenyl)carbazole with Br2, to give the nitro-
substituted G2 dendron 2 in good yield. Reduction of the
nitro group of 2 with SnCl2 gave a good yield of the amino-
G2 dendron 3, which we further coupled with 1-bromo-4-
tert-butylbenzene to afford the unsymmetrical dendron 4 in
moderate yield. Linking this dendron to 4,4′-didromobiphenyl
provided a moderate yield of the unsymmetrical dumbbell-
shaped dendrimer 5. For comparison, we synthesized the G1-
derived counterpart 6 in a similar two-step synthetic route
with a total yield of 70% (see the Supporting Information).
Although we were unable to convert the primary amino
group of 3 into an iodo moiety, we did prepare the triflate-
substituted G2 derivative 7 (Scheme 2) as an electrophilic
variant of that dendron. Coupling of the G1-N-G1 species 1
with 3,6-dibromo-9-(4-methoxyphenyl)carbazole gave a high
yield of the methoxy-substituted dendron 8, the methyl group
of which we subsequently removed through treatment with
BBr3 to afford a high yield of the hydroxy-substituted G2
dendron 9. Treatment of 9 with triflic anhydride gave the
desired pseudohalogen-substituted G2 dendron 7 in good
yield. C-N coupling of the amino- and triflate-substituted
G2 dendrons 3 and 7 in the presence of Pd2(dba)3 as catalyst
and 2-(di-tert-butylphosphine)biphenyl as cocatalyst provided
a moderate yield of the G2-N-G2 dendron 10.
energy transfer; they have also been employed as effective
charge transporting moieties for iridium-based phosphores-
cent emitters.10 Dendrimers equipped with carbazole-based
dendrons are promising materials for use as efficient hole-
transporting materials in OLEDs.11 Most carbazole-based
dendrimers, except for those reported by Moore7 and
Dehaen,12 have been prepared through formation of C-N
linkages, coupling the nitrogen atoms of an external carbazole
unit to the active C3 and C6 sites of an inner carbazole
moiety. Here we report the convergent synthesis (through
efficient C-N bond coupling reactions) and physical char-
acterization of two novel dumbbell-shaped carbazole-
containing dendrimers incorporating 9-phenylcarbazole den-
drons. Carbazole derivatives exhibit high triplet energy and
are capable of transporting hole, thus, these new carbazole-
containing dendrimers may present interesting applications
as efficient hole-transporters as well as host materials in
electrophosphorescence devices.
Scheme 1 outlines our synthesis of the 9-phenylcabazole-
based dendrons. The generational growth began with con-
nection of the phenylene rings of two 9-phenylcabazole
molecules through Pd-catalyzed C-N bond formation be-
tween 9-(4-aminophenyl)carbazole and 9-(4-iodophenyl)-
(8) (a) Loiseau, F.; Campagna, S.; Hameurlaine, A.; Dehaen, W. J. Am.
Chem. Soc. 2005, 127, 11352. (b) Xu, T. H.; Lu, R.; Qiu, X. P.; Liu, X. L.;
Xue, P. C.; Tan, C. H.; Bao, C. Y.; Zhao, Y. Y. Eur. J. Org. Chem. 2006,
4041.
(9) McClenaghan, N. D.; Passalacqua, R.; Loiseau, F.; Campagna, S.;
Verheyde, B.; Hameurlaine, A.; Dehaen, W. J. Am. Chem. Soc. 2003, 125,
5356.
(10) (a) Lo, S.-C.; Namdas, E. B.; Shipley, C. P.; Markham, J. P. J.;
Anthopolous, T. D.; Burn, P. L.; Samuel, I. D. W. Org. Electron. 2006, 7,
85. (b) Zhou, G.; Wong, W.-Y.; Yao, B.; Xie, Z.; Wang, L. Angew. Chem.,
Int. Ed. 2007, 46, 1149.
(11) (a) Kimoto, A.; Cho, J.-S.; Higuchi, M.; Yamamoto, K. Macromol.
Symp. 2004, 209, 51. (b) Kimoto, A.; Cho, J.-S.; Ito, K.; Aoki, D.; Miyake,
T.; Yamamoto, K. Macromol. Rapid Commun. 2005, 26, 597.
(12) Hameurlaine, A.; Dehaen, W. Tetrahedron Lett. 2003, 44, 957.
Our attempts at coupling this G2-N-G2 dendron with 4,4′-
dibromobiphenyl or the more reactive 4,4′-diiodobiphenyl
under the conditions of Pd-catalyzed C-N bond formation
produced complicated mixtures of products. Gratifyingly,
conventional Ullmann conditions (Cu, K2CO3, dichloroben-
(13) Chen, Y.-C.; Huang, G.-S.; Hsiao, C.-C.; Chen, S.-A. J. Am. Chem.
Soc. 2006, 128, 8549.
4532
Org. Lett., Vol. 9, No. 22, 2007

Scheme 2
zene) allowed us to react the G2-N-G2 dendron 10 with 4,4′-
of their phenylcarbazole moieties, with the absorbance
intensity increasing along with an increase in the number of
phenylcarbazole units per molecule. We ascribe the absorp-
tions at wavelengths exceeding 350 nm to the presence of
the tetraphenylbenzidine (TPB) cores.14 The photolumines-
cence spectra of 6 and 12 display the typical emission
characteristics of TPB moieties. The corresponding spectra
of the dendrimers 5 and 11 exhibit slightly red-shifted
emission wavelengths, centered at 433 and 438 nm, respec-
tively, relative to those of their counterparts 6 and 12.
Because the G2-N-G2 dendron 10, which lacks a TPB
chromophore, displays an emission maximum centered at
diiodobiphenyl to form the symmetrical dumbbell-shaped
dendrimer 11 in good yield. As compare to the unsym-
metrical 5, the symmetric 11 could provide different mor-
phology in solid films, which is an important parameter
governing the charge mobility in organic electronic devices.
For comparison, we synthesized the G1-derived counterpart
12 in high yield through Pd-catalyzed C-N bond formation
reactions between the G1-N-G1 system 1 and 4,4′-dibromo-
biphenyl (see the Supporting Information).
We obtained satisfactory spectroscopic and MALDI-TOF
mass spectrometric (Figure S-1, Supporting Information) data
that were consistent with the structural identity of the high
molecular weight dendron 10, the unsymmetrical dumbbell-
shaped dendrimer 5, and the symmetrical dumbbell-shaped
dendrimer 11. Table 1 summarizes the results of thermo-
gravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) measurements of the various compounds. We
attribute the observed increased thermal and morphological
stabilities of 5 and 11srelative to those of their low
molecular weight counterparts 6 and 12, respectivelysto their
greater molecular weights.
Table 1. Physical Properties of Dendrimers 5 and 11 and Their
Low Molecular Weight Counterparts 6 and 12
oxd
Tg/Td
(°C)a
UV-vis λmax
PL λmax
(nm)c
E1/2
(nm)b
(V)d
5
6
11
12
269/559
158/459
296/579
195/546
294, 330, 345
294, 328, 345
294, 330, 345
294, 328, 345
433
412
438
416
0.71, 0.92
0.70, 0.90
0.72, 0.96
0.73, 0.86e
Figure 1 provides a comparison of the electronic absorption
and fluorescence spectra of the unsymmetrical dumbbell-
shaped dendrimer 5, the symmetrical dendrimer 11, and their
respective counterparts 6 and 12.
The UV-vis spectra of these compounds exhibit the
absorption characteristics (peaks at 294, 328, and 345 nm)
a Td ) 5% weight loss temperature. b Measured from a 1.0 × 10-6
M
solution in CH2Cl2. c Measured from a 1.0 × 10-6 M solution in CH2Cl2,
excited at 330 nm. d Measured from a 1.0 × 10-3 M solution in CH2Cl2
containing 0.1 M Bu4NPF6 as the supporting electrolyte and with use of a
glassy carbon electrode. e Measured with a Pt electrode and 0.1 M Bu4NClO4
as the supporting electrolyte.
Org. Lett., Vol. 9, No. 22, 2007
4533

potentials, over multiple CV scans (0-1.6 V) for dendrimers
5, 6, and 11 are good indicators that electropolymerization
occurred at the peripheral carbazole groups (Figure S-2,
Supporting Information). Accordingly, oxidation of the
carbazole groups can be excluded from the contributions to
the first two oxidation potentials of dendrimers 5 and 11,
which contain four oxidizable sites (the four different
nitrogen atom centers). Furthermore, the CV trace of dendron
10 revealed two reversible oxidation potentials, at 0.71 and
0.89 V, respectively (Figure S-3, Supporting Information),
which we suggest may be ascribed to the oxidations of the
more-electron-rich 3,6-diaminocarbazole moiety. Because the
oxidation potentials of 10 are similar to those of the TPB-
like core of dendrimer 6, it is difficult to accurately determine
the origins of the first two oxidation potentials detected for
dendrimers 5 and 11. Nevertheless, our results indicate that
the inner chromophores (i.e., TPB or 3,6-diaminocarbazole)
have lower oxidation potentials, whereas the outer carbazole
moieties oxidize at higher potentials; thus an intriguing redox
gradient of electroactive dendrimers can be achieved,16 which
may facilitate the hole migration from the core to the
peripheral carbazoles
Figure 1. UV-vis absorption and photoluminescence spectra of
the high molecular weight dendrimers 5 and 11 and their respective
low molecular weight counterparts 6 and 12.
436 nm, the long-wavelength fluorescence observed for 5
and 11 can be unambiguously attributed as arising mainly
from the lowest π-π* transition of the 3,6-diamino-9-
phenylcarbazole chromophore.
In summary, using efficient C-N bond formation reactions
and a convergent strategy, we have synthesized two dumb-
bell-shaped dendrimers (5 and 11) containing 9-phenylcar-
bazole units as dendrons and have characterized their
structures and photophysical and electrochemical properties.
The application of these dendrimers as hole-transporting and
host material in organic optoelectronic devices is currently
under investigation.
We used cyclic voltammetry (CV) to characterize the
electrochemical properties of the high molecular weight
dendrimers 5 and 11 and their low molecular weight
counterparts 6 and 12. When the CV scanning range was
between 0 and 1.2 V (vs Ag/AgCl)susing a glassy carbon
electrode as the working electrode in CH2Cl2 and 0.1 M Bu4-
NPF6 as the supporting electrolyteswe observed two revers-
ible oxidation potentials (at ca. 0.70 and 0.90 V) for the
dendrimers 5, 6, and 11. In contrast, the CV trace of 12 under
the same conditions displayed a peak typical of a product
adsorbed weakly onto the electrode surface (Figure S-2,
Supporting Information). The two reversible oxidations of
12 were recovered when using a Pt working electrode (Figure
S-3, Supporting Information). It appears quite reasonable to
assign these potentials of the dendrimers 6 and 12 to the
successive oxidations of the TPB-like cores. Because car-
bazoles lacking substituents at the C3 and C6 positions
normally undergo oxidation at higher potentials, the presence
of oxidized carbazole moieties usually leads to polymeric
materials.15 Indeed, the consecutive increases in the response
currents, together with the oxidation peaks shifting to lower
Acknowledgment. This study was supported financially
by the National Science Council and Ministry of Education
of Taiwan.
Supporting Information Available: Detailed experi-
mental procedures, spectroscopic characterization of new
compounds, MALDI-TOF mass spectra of compounds 5, 10,
and 11, and cyclic voltammograms of compounds 5, 6, 10,
11, and 12. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL702060U
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