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
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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,
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