Synthesis of π-conjugated dendrimers based on azaborines

Article abstract of DOI:10.1021/ol901035d

Image Persented π-Conjugated dendrons and dendrimers based on dibenzoazaborine were synthesized. The azaborine dendrons exhibited strong light absorption and photoluminescence, reflecting the optical properties of the azaborine. The fluorescence from the

Full text of DOI:10.1021/ol901035d

ORGANIC
LETTERS
2009
Vol. 11, No. 16
3534-3537
Synthesis of π-Conjugated Dendrimers
Based on Azaborines
Tomohiro Agou, Tatsuo Kojima, Junji Kobayashi, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
takayuki@chem.s.u-tokyo.ac.jp
Received May 12, 2009
ABSTRACT
π-Conjugated dendrons and dendrimers based on dibenzoazaborine were synthesized. The azaborine dendrons exhibited strong light absorption
and photoluminescence, reflecting the optical properties of the azaborine. The fluorescence from the azaborine dendrimers bearing a
benzothiadiazole core was strongly red-shifted or quenched, indicating photoinduced electron transfer from the azaborine dendrons to the
core unit.
The spherical shape of dendrimers, as well as the core-
branch-shell-type hierarchical structures, often provides
useful features that are not accessible by ordinary small
molecules or macromolecules (dendritic effects).1 Therefore,
the construction of new dendrimers featuring unique struc-
tures is an attractive strategy for the development of novel
functions, such as self-assembly,2 catalyst,3 drug-delivery,4
electrooptical devices,5 and so on.
and amorphousness can be improved by their rigid and bulky
three-dimensional structure. (3) Because of the dense packing
of conjugated units, such dendrimers are expected to show
efficient intramolecular energy or charge transfer ability.
These potentials of the π-conjugated dendrimers are highly
prized from the viewpoint of organic electrooptical materials,
and thus π-conjugated dendrimers bearing various π-systems
have been synthesized recently. For example, π-conjugated
dendrimers based on electron-donating units, such as
thiophene,6 carbazole,7 and phenothiazine,8 exhibited inter-
esting optical and electronic properties. In contrast, electron-
accepting units have not been widely utilized as a component
unit of π-conjugated dendrimers,9,10 and π-conjugated den-
drimers bearing both electron donors and acceptors are still
π-Conjugated dendrimers have several advantages com-
pared with a flexible one: (1) because of their rigidity, internal
molecular motions are suppressed, and the spatial arrange-
ment of the functional units in the dendritic structure can be
precisely controlled, which is important for the design of
the electronic interaction between the units. (2) The solubility
¨
(6) Harpham, M. R.; Su¨zer, O.; Ma, C.-Q.; Ba¨uerle, P.; Goodson, T.,
(1) (a) Vo¨gtle, F.; Richardt, G.; Werner, N. Dendrimer Chemistry; Wiley-
VCH: Weinheim, 2009. (b) Fre´chet, J. M. J., Tomalia, D. A., Eds.
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10.1021/ol901035d CCC: $40.75
Published on Web 07/21/2009
 2009 American Chemical Society

unknown. Such dendrimers are expected to show n-type or
ambipolar charge transfer ability, both of which have recently
become important in the field of highly functionalized
organic field effect transistors.11
electron-accepting benzothiadiazole unit as the core.15 The
structures of azaborine dendrons and dendrimers are shown
in Figure 1. These dendrons could be synthesized by
palladium-catalyzed coupling reactions between branching
unit 1 and terminal unit 2 or D1-H.
The synthesis of terminal unit 2 is shown in Scheme 1.
Ortho-selective bromine-lithium exchange of MOM-
protected diarylamine 3 followed by the reaction with
MesB(OMe)2 gave azaborine 4 in good yield.12c The bromine
atoms of 4 were removed, and deprotection of the MOM
group of 5 under acidic conditions produced terminal unit
2.
Scheme 1. Synthesis of Terminal Unit 2
Figure 1. Azaborine-based dendrons D1, D2, and D2′ and den-
drimers G1, G2, and G2′. Synthetic strategy of the dendrons is
also shown.
Branching unit 1 and first-generation dendron D1-TMS
were synthesized from triarylamine 6 (Scheme 2). Triaz-
enylphenyl azaborine 7 was synthesized from 6, and the
triazenyl group was quantitatively substituted with an iodine
atom by treatment with I2 in CH2I2.16 Sonogashira coupling
of 8 with TMSCCH gave 1 in good yield, and the bromo
substituents of 1 were removed to give D1-TMS.
Recently, we have been investigating the properties of
dibenzoheteraborines and their π-extended derivatives.12,13
In particular, dibenzoazaborines and their π-extended deriva-
tives exhibited strong light absorption and emission because
of the intramolecular charge-transfer interaction between
electron-donating nitrogen atoms and electron-withdrawing
boron atoms in the acene-like rigid structures. These results
implied to us that an accumulation of dibenzoazaborine units
in the dendritic architecture would provide dendrimers with
beneficial characteristics, including light-harvesting antenna,
ambipolar charge transfer, and charge separation upon
photoexcitation of the dibenzoazaborine units.14 Here, we
report the syntheses and optical properties of azaborine-based
conjugated dendrons and their dendrimers bearing a strongly
Scheme 2
.
Syntheses of Branching Unit 1 and Dendron
D1-TMS
(9) (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
122, 6335. (b) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc.
2001, 123, 11372.
(10) Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H.
Nature 2002, 415, 509.
(11) Nishida, J.-i.; Murakami, S.; Tada, H.; Yamashita, Y. Chem. Lett.
2006, 35, 1236, and references therein. .
(12) (a) Agou, T.; Kobayashi, J.; Kawashima, T. Org. Lett. 2006, 8,
2241. (b) Agou, T.; Kobayashi, J.; Kawashima, T. Chem.sEur. J. 2007,
13, 8051. (c) Agou, T.; Kobayashi, J.; Kawashima, T. Chem. Commun.
2007, 3204. (d) Kobayashi, J.; Kato, K.; Agou, T.; Kawashima, T. Chem.
Asian J. 2009, 4, 42. (e) Agou, T.; Kobayashi, J.; Kawashima, T. Org.
Lett. 2005, 7, 4373. (f) Agou, T.; Sekine, M.; Kobayashi, J.; Kawashima,
Second-generation dendrons D2-TMS and D2′-TMS were
obtained by Buchwald-Hartwig coupling and Sonogashira
coupling of 1 and the corresponding terminal unit, respec-
T. Chem. Commun. 2009, 1894
.
(13) (a) Agou, T.; Kobayashi, J.; Kawashima, T. Inorg. Chem. 2006,
45, 9137. (b) Lee, M. H.; Agou, T.; Kobayashi, J.; Kawashima, T.; Gabba¨ı,
F. P. Chem. Commun. 2007, 1133. (c) Agou, T.; Kobayashi, J.; Kim, Y.;
Gabba¨ı, F. P.; Kawashima, T Chem. Lett. 2007, 36, 976. (d) Agou, T.;
(15) (a) Neto, B. A. D.; Lopes, A. S.; Eveling, G.; Gonc¸alves, R. S.;
Costa, V. E. U.; Quina, F. H.; Dupont, J. Tetrahedron 2005, 61, 10975. (b)
Chen, C.; Lin, J. T.; Yeh, M. P. Org. Lett. 2006, 8, 2233. (c) Kato, S.;
Matsumoto, T.; Ishii, T.; Thiemann, T.; Shigeiwa, M.; Gorohmaru, H.;
Maeda, S.; Yamashita, Y.; Mataka, S. Chem. Commun. 2004, 2342.
(16) Wu, Z.; Moore, J. S. Tetrahedron Lett. 1994, 35, 5539.
Sekine, M.; Kobayashi, J.; Kawashima, T. Chem.sEur. J. 2009, 15, 5056
.
(14) (a) Xu, T.; Lu, R.; Liu, X.; Zheng, X.; Qiu, X.; Zhao, Y. Org.
Lett. 2007, 9, 797. (b) Kozaki, M.; Akita, K.; Okada, K. Org. Lett. 2007,
9, 1509.
Org. Lett., Vol. 11, No. 16, 2009
3535

tively (Scheme 3).17 The use of t-Bu-XPhos is critical to
facilitate the coupling between 1 and 2.
at 391 nm and a new band at 415 nm. The new absorption
band is assigned to the localized excitation on the branching
unit by comparison with the optical data of 9 (λmax 417 nm).
The second longest maximum is due to the absorption of
the terminal units, but the absorption coefficient increased
substantially compared with that of D2-TMS. Such a strong
light-absorbing property is useful for various applications,
such as light-harvesting antennas.18
Scheme 3. Synthesis of Dendrons D2-TMS and D2′-TMS
D1-TMS exhibited strong fluorescence with a small Stokes
shift. The photoluminescence of D2-TMS was somewhat red-
shifted and broadened, and the fluorescence quantum yield
decreased, probably because of a deactivation process such
as internal rotations. D2′-TMS also showed red-shifted
emission at 426 nm, which is assigned to fluorescence from
the branching unit, because 9 exhibits strong fluorescence
at nearly the same wavelength. In addition, excitation of the
terminal units also gave the same emission, indicating an
efficient energy transfer from the terminal units to the
branching unit.19
The longest absorption maximum of D2-TMS hardly
changed from that of D1-TMS, and the absorption coefficient
of D2-TMS was almost three times as great as that of D1-
TMS (Figure 2, Table 1). These results indicate that the three
The TMS group of azaborine-based dendrons D1-TMS,
D2-TMS, and D2′-TMS was removed by basic alcoholysis.
The resulting terminal acetylene was coupled with 4,7-
dibromobenzo[c][1,2,5]thiadiazole to give dendrimers G1,
G2, and G2′, respectively (Table 2).
Table 2. Synthesis of the Azaborine-Based Dendrimers
Figure 2. Optical spectra of the dendrons in CH2Cl2.
TMS-protected dendron
dendrimer
yield (%)
D1-TMS
D2-TMS
D2′-TMS
G1
G2
G2′
52
62
13
Table 1. Optical Data of the Dendrons and Dendrimers in
CH2Cl2
The absorption profiles of G1, G2, and G2′ are the
superposition of those of the core unit and the corresponding
dendrons (Figure 3, Table 1). This means that the dendrons
and the core do not interact with each other in their ground
states.
Irradiation of G1 resulted in a broad and red-shifted
emission, and the fluorescence from the dendrons was
completely quenched. An excitation spectrum of G1 revealed
that the excitation of the dendrons contributes to the
emission.19 The large bathochromic shift of the emission
maximum from that of 10 indicates that the emissive state
is not a local excited state of the core but an intramolecular
charge-transfer (ICT) state formed between the core and the
dendrons.20
Density functional theory (DFT) calculations on G1 show
that its LUMO is a π* orbital of the core unit, and its
azaborine units in D2-TMS are electronically independent
and behave as isolated chromophores.7 D2′-TMS showed a
more intense light-absorption band for the azaborine units
(18) Zhu, Z.; Moore, J. S. J. Org. Chem. 2000, 65, 116.
(19) See the Supporting Information for detail.
(17) Anderson, K. W.; Tundel, R. E.; Ikawa, T.; Altman, R. A.;
Buchwald, S. L. Angew. Chem., Int. Ed. 2006, 45, 6523.
(20) In benzonitrile, the emission maximum of G1 was shifted to 660
nm, suggesting the highly polar nature of the emissive state.
3536
Org. Lett., Vol. 11, No. 16, 2009

Table 3. Parameters of Rehm-Weller Equation and the
Estimated Free Energy Change on the Electron Transfer (∆GET
a
)
Eoxb/V
Eredb/V
E00c/eV
∆GETd/eV
D1-TMS
D2-TMS
D2′-TMS
10
1.0e
1.0e
1.1e
3.4
3.3
3.2
-1.7f
G1
G2
G2′
-0.7
-0.6
-0.4
a In CH2Cl2. b The redox potentials (vs Fc/Fc+) were measured by cyclic
voltammetry in CH2Cl2 containing 0.1 M TBAP with a scan rate of 0.1
V·s-1
.
c E00 was estimated by the absorption edge of the UV-vis spectrum.
d ∆GET ) Eox - Ered - E00 - Ze0 /εR. e Irreversible, peak potential.
2
f Reversible, peak potential.
Figure 3. Optical spectra of the dendrimers in CH2Cl2.
cases, ∆GET values are negative, and thus, the ET from the
dendrons to the core is a thermodynamically favorable
process, indicating the fast formation of the ICT states soon
after the photoexcitation.
The difference in the fate of the ICT states between these
denrimers is quite intriguing. While the ICT state of G1 and
G2′ resulted in a long-wavelength emission, G2 exhibited
only weak dendron- and core-based fluorescence, indicating
that its ICT state is not emissive and participates in the
fluorescence quenching, unlike G1 and G2′. Further inves-
tigation to clarify the origin of such a difference is now
underway in our laboratory.
In summary, the first donor-acceptor-type π-conjugated
dendrons and dendrimers based on dibenzoazaborines have
been synthesized. The optical properties of the dendrons are
similar to those of the parent azaborines, indicating that the
azaborine units are aligned perpendicularly to each other.
Dendrimers containing the azaborine dendrons and the
benzothiadiazole core form ICT excited states, but the fate
of these excited states strongly depends on the generation
of the dendrons.
degenerate HOMO and HOMO-1 are π orbitals of the two
dendrons, suggesting that electron transfer (ET) from the
dendrons to the core unit to give the ICT state is a favorable
process. TD-DFT calculations also revealed that the lowest
excited state of G1 is an ICT state between the dendrons
and the core, and local excited states of the dendrons or the
core are located at higher energy levels. Therefore, a
Franck-Condon state of G1 can be transformed to the ICT
state, which may give the observed fluorescence emission.21
G2 did not exhibit a long-wavelength emission similar to
that of G1. Instead, it showed a very weak and broad
photoluminescence band that is ascribed to the emission from
the dendrons (λem 429 nm) and the core (around 500 nm).
The depression in its fluorescence intensity may be due to
the fluorescence quenching by the ET from the dendrons to
the core. On the other hand, the fluorescence of G2′
contained not only a short-wavelength emission (λem 450 nm)
but also a long-wavelength one (λem 635 nm), and the
fluorescence quantum yield was moderate. The short-
wavelength emission is ascribed to the dendron- and core-
based fluorescence, and the long-wavelength one may be
from the ICT state of G2′. Therefore, the fluorescence
property of G2′ can be described as a mixture of those of
G1 and G2.
Acknowledgment. We thank Tosoh Finechem Corp. and
Shin-etsu Chemical Co. Ltd. for the generous gifts of
alkyllithiums and silicon reagents, respectively. This work
is partially supported by the Global COE Program for
Chemistry Innovation and the Scientific Researches from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan and the Japan Society for the Promotion of
Science. We acknowledge Prof. Kazuhiko Mizuno and Prof.
Hiroshi Ikeda (Osaka Prefacture University) for transient
absorption measurements.
The feasibility of such ET processes was analyzed on the
basis of the Rehm-Weller equations (Table 3).22,23 In all
(21) In CH2Cl2, pulsed laser irradiation (355 nm) of G1 gave a
development of transient absorption maxima at 473, 525, and 560 nm, and
their lifetime was ca. 27 µs. Almost the same transient absorption profiles
were observed in benzonitrile, indicating little solvent effect on the excited
states. In addition, an air-saturated CH2Cl2 or benzonitrile solution of G1
did not exhibit clear transient absorptions. These results suggest that the
intermediate observed in these measurements is not an ICT state but a triplet
state. A pathway from the Franck-Condon state of G1 to the triplet state
is not elucidated yet, but the ICT state is assumed to be formed as a primary
intermediate that gives the fluorescence and is transformed into the triplet
state to some extent.
Supporting Information Available: Experimental and
spectral data for all compounds and a description of
theoretical calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
(22) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259
.
2
(23) The Coulomb interaction term (Ze0 /εR) was neglected here because
both the electron donor and acceptor are neutral molecules, and hence, the
Coulomb interaction between them should be quite small.
OL901035D
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