Facile iterative synthesis of biphenyl dendrons with a functionalized focus

Article abstract of DOI:10.1021/ol101717c

Figure Presented. An iterative procedure gives 1,3,5-phenyl-linked dendrons of up to the fourth generation and enables the formation of different generations of iridium(III) complex-cored dendrimers. The convergent synthesis uses N,N′-1,8-napthyl-3,5-dibromophenylboronamide as the key building block. The iterative synthesis cycle involves deprotection of the boronamide-focused dendron to form a boronic acid and subsequent Suzuki coupling either with the N,N′-1,8-napthyl-3,5-dibromophenylboronamide to give the next dendron generation or with an activated core to form a dendrimer.

Full text of DOI:10.1021/ol101717c

ORGANIC
LETTERS
2010
Vol. 12, No. 19
4338-4340
Facile Iterative Synthesis of Biphenyl
Dendrons with a Functionalized Focus
Ellen J. Wren, Xin Wang, Anthony Farlow, Shih-Chun Lo, Paul L. Burn,* and
Paul Meredith
Centre for Organic Photonics & Electronics, The UniVersity of Queensland, School of
Chemistry & Molecular Biosciences, Chemistry Building, Queensland, Australia, 4072
p.burn2@uq.edu.au
Received July 28, 2010
ABSTRACT
An iterative procedure gives 1,3,5-phenyl-linked dendrons of up to the fourth generation and enables the formation of different generations
of iridium(III) complex-cored dendrimers. The convergent synthesis uses N,N′-1,8-napthyl-3,5-dibromophenylboronamide as the key building
block. The iterative synthesis cycle involves deprotection of the boronamide-focused dendron to form a boronic acid and subsequent Suzuki
coupling either with the N,N′-1,8-napthyl-3,5-dibromophenylboronamide to give the next dendron generation or with an activated core to form
a dendrimer.
Optoelectronic materials fall into three main families, small
molecules, conjugated polymers,1 and dendrimers.2 For
dendrimer-based optoelectronic applications such as organic
light-emitting diodes and solar cells, dendrimers with
conjugated dendrons have proved to be the most successful.2
Optoelectronic dendrimers are generally comprised of a core,
dendrons, and surface groups. In the case of luminescent
dendrimers that have been used in organic light-emitting
diodes, the core is generally the emissive component with
the dendrons acting as rigid spacers to control the intermo-
lecular interactions that can lead to quenching of the
luminescence.3 The surface groups provide solubility and
solution processability. A key feature of light-emitting
dendrimers is that their properties are strongly dependent
on dendrimer generation. Iterative procedures for the forma-
tionofhigher-generationphenylacetylenyl,4stilbenyl,5polyphe-
nyl,6 and carbazolyl7 dendrons are now well established, but
the formation of different generations of simple 1,3,5-linked
phenylene dendrons and their subsequent transformation to
dendrimers has been less well studied.8 Dendrimers with first-
and second-generation 1,3,5-linked phenylene dendrons and
iridium(III) complex cores have been used successfully to
give OLEDs with external quantum efficiencies of up to 16%
at a usable brightness.9 The two generations of dendrimers
were formed by complexation of dendronized ligands.10 The
synthesis of the second-generation dendrimer was hampered
by the fact that the second-generation dendron could not be
easily formed. Hence to achieve the synthesis of the second-
generation dendrimer it was necessary to elaborate the ligand
(5) (a) Meier, H.; Lehmann, M. Angew. Chem., Int. Ed. 1998, 37, 643.
(b) Pillow, J. N. G.; Halim, M.; Lupton, J. M.; Burn, P. L.; Samuel, I. D. W.
Macromolecules 1999, 32, 5985.
(6) Qin, T.; Ding, J.; Wang, L.; Baumgarten, M.; Zhou, G.; Mu¨llen, K.
J. Am. Chem. Soc. 2009, 131, 14329.
(7) (a) Promarak, V.; Ichikawa, M.; Sudyoadsuk, T.; Saengsuwan, S.;
Jungsuttiwong, S.; Keawin, T. Thin Solid Films 2008, 516, 2881. (b)
Gambino, S.; Stevenson, S. G.; Knights, K. A.; Burn, P. L.; Samuel, I. D. W.
AdV. Funct. Mater. 2009, 19, 317.
(1) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes,
A. B. Chem. ReV. 2009, 109, 897.
(2) (a) Li, J.; Liu, D. J. Mater. Chem 2009, 19, 7584. (b) Lo, S.-C.;
Burn, P. L. Chem. ReV. 2007, 107, 1097.
(8) Finke, A. D.; Moore, J. S. Org. Lett. 2008, 10, 4851.
(9) Lo, S.-C.; Male, N. A. H.; Markham, J. P. J.; Magennis, S. W.;
Burn, P. L.; Salata, O. V.; Samuel, I. D. W. AdV. Mater. 2002, 14, 975.
(3) Burn, P. L.; Lo, S.-C.; Samuel, I. D. W. AdV. Mater. 2007, 19, 1675.
(4) Xu, Z.; Moore, J. S. Angew. Chem., Int. Ed. 1993, 32, 246.
10.1021/ol101717c  2010 American Chemical Society
Published on Web 09/08/2010

to enable the use of the first-generation dendron.10 A simpler
and more convergent method of preparing light-emitting
dendrimers of higher generations is to have dendrons with
reactive groups at their foci that can react with complemen-
tary functionality on the core. Herein we describe a two-
step iterative method for the preparation of 1,3,5-linked
phenylene dendrons up to the fourth generation with a
protected boronic acid at their focus and their subsequent
conversion to different dendrimer generations.
Scheme 1. Iterative Cycle for Dendron Synthesis
The basis of the strategy was the report that haloarylboronic
acids protected with 1,8-diaminonapthalene, giving the corre-
sponding boronic acid amides (boronamides), could undergo
Suzuki cross-coupling reactions without polymerization. Depro-
tection of the boronamide then allowed further palladium-
catalyzed cross couplings to build up more complex asymmetric
aromatic oligomers.11 Elaboration of this chemistry to form the
1,3,5-linked dendrons required the formation of N,N′-1,8-
napthyl-3,5-dibromophenylboronamide 2.
Boronamide 2 was formed in four consecutive steps. First,
1,3,5-tribromobenzene was metalated with n-butyllithium,
and the carbanion was reacted with trimethylborate before
subsequent hydrolysis with dilute hydrochloric acid to the
corresponding boronic acid. Simple boronic acids are known
to dimerize and trimerize, and hence in this work the
corresponding acids were used immediately in the next step.
The 3,5-dibromophenylboronic acid was reacted with 1,8-
diaminonaphthalene in toluene heated at reflux for 5 h under
Dean-Stark conditions to give 2 in a 59% overall yield for
the four steps.
In previous work, it was found that 2-ethylhexyloxy surface
groups gave the required solubility and processability for the
1,3,5-phenylene linked dendrons and dendrimers, and hence
they were used in the current study.10 The first step in the
iterative procedure for forming the dendrons was the coupling
of 2-[4-(2-ethylhexyloxy)phenyl]-4,4,5,5-tetramethyl-1,3,2-di-
oxaborolane 1 with boronamide 2 to give the first-generation
boronamide focused dendron 3. This was achieved in a 79%
yield using palladium(0) catalysis after 24 h. Each step of the
iterative procedure then required the deprotection of the bo-
ronamide to the corresponding boronic acid and subsequent
reaction with 2. For example, the first-generation boronamide
3 was deprotected by reaction with aqueous acid to give the
dendron with a boronic acid at its focus. The first-generation
boronic acid was then coupled under palladium(0)-catalyzed
conditions to give 4, the second-generation dendron with a
boronamide focus, in 65% yield. The cycle was repeated for
the third-, 5, and fourth-generation, 6, dendrons to give yields
of 77% and 36%, respectively (Scheme 1). It is important to
note that increasing the reaction times caused a decrease in the
yields.
The reported method for forming the iridium(III) complex-
cored dendrimers with 1,3,5-linked phenylene dendrons
involved the formation of the dendronized ligand and
subsequent complexation to the metal cation.10 The disad-
vantages of this route were that the method was only efficient
if a first-generation dendron was used, and the yield of the
complexation step was often low leading to waste of
advanced intermediates. The alternative route is to couple
the dendrons directly to the core complex, and we illustrate
this process for green emissive phosphorescent iridium(III)
complex cored dendrimers.
The synthesis of the first-generation fac-tris(2-phenylpyrid-
yl)iridium(III) cored dendrimer is shown in Scheme 2, and the
higher generations were formed in a similar manner. The
boronamides were again deprotected to give the correspond-
ing boronic acid, which was immediately used in the Suzuki
coupling reaction with fac-tris[2-(5-bromophenyl)pyridyl]-
iridium(III). For the first-generation dendrimer 7, dendron 3
was deprotected with aqueous acid, and the boronic acid was
then coupled using palladium(0) catalysis to the iridium(III)
complex. The overall yield of 7 for the deprotection and three
Suzuki couplings was 46%. This corresponds to an overall
yield of 36% for the linear sequence from boronate ester 1,
which is higher than the 17% overall yield of 7 from 1
prepared via the shortest pathway using the dendronised
ligand route.10a The second- and third-generation dendrons
4 and 5 were deprotected and reacted with the same core to
(10) (a) Lo, S.-C.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W.
Macromolecules 2003, 36, 9721. (b) Frampton, M. J.; Namdas, E. B.; Lo,
S.-C.; Burn, P. L.; Samuel, I. D. W. J. Mater. Chem. 2004, 14, 2881. (c)
Lo, S.-C.; Richards, G. J.; Markham, J. P. J.; Namdas, E. B.; Sharma, S.;
Burn, P. L.; Samuel, I. D. W. AdV. Funct. Mater. 2005, 15, 1451. (d) Lo,
S.-C.; Harding, R. E.; Shipley, C. P.; Stevenson, S. G.; Burn, P. L.; Samuel,
I. D. W. J. Am. Chem. Soc. 2009, 131, 16681.
(11) Noguchi, H.; Hojo, K.; Suginome, M. J. Am. Chem. Soc. 2007,
129, 758.
Org. Lett., Vol. 12, No. 19, 2010
4339

1H NMR and thin-layer chromatography showed that
dendrimers 7, 8, and 9 were pure, but careful analysis by
gel permeation chromatography indicated that the first- and
second-generation dendrimers contained small amounts
(0.5-1.6%) of compounds that we believe are partially
dendronized materials (dendrimers with one or two den-
drons). In contrast, the third-generation dendrimer was pure
by all criteria and had a polydispersity of 1.0. The first- and
second-generation dendrimers have been previously reported
to have photoluminescence quantum yields (PLQYs) in solution
of 70% and 69%, respectively.10a In this work, the third-
generation dendrimer 9 was found to have a PLQY of 89%.
Interestingly, for 9 excitation in the dendron absorption region
(310 nm) gave emission almost exclusively from the fac-tris(2-
phenylpyridyl)iridium(III) core indicating that even for high
generations intramolecular energy transfer can be efficient.
Scheme 2
.
First-Generation Iridium(III)-Cored Dendrimer
Synthesis
In conclusion, we have developed a simple iterative route
to 1,3,5-linked phenylene dendrons that has been used to
form up to the fourth generation. The strategy uses a masked
boronic acid in the form of a boronamide. Successive Suzuki
couplings and deprotections allow the dendrons to be formed
in a convergent route, and the unmasked boronic acids can
be used to couple to activated cores to give dendrimers of
different generations.
Acknowledgment. This work was supported by the
Australian Research Council. Professor Paul L. Burn is a
recipient of an Australian Research Council Federation
Fellowship (project number FF0668728). Professor Paul
Meredith is a Queensland Smart State Senior Fellow. We
also thank Dr Z. Lui for his contribution to this work.
Supporting Information Available: Experimental pro-
cedures and characterization for compounds 1-9. This
material is available free of charge via the Internet at
http://pubs.acs.org.
give the equivalent second- and third-generation dendrimers
(8 and 9) in 36% and 14% yield, respectively. The lower
yield for the third-generation dendrimer, which is the highest
generation of this type of dendrimer formed thus far, suggests
that steric compression is becoming an important factor.
OL101717C
4340
Org. Lett., Vol. 12, No. 19, 2010