A First Synthesis of Thiophene Dendrimers

Article abstract of DOI:10.1021/ol025943a

(Matrix Presented) Thiophene dendrons and dendrimers were designed and synthesized using a convergent approach. Metal-mediated coupling reactions were used in the synthesis. A rational approach allowed the formation of αα, ββ, and αβ linkages between the dendrons and thiophene units.

Full text of DOI:10.1021/ol025943a

ORGANIC
LETTERS
2002
Vol. 4, No. 12
2067-2070
A First Synthesis of Thiophene
Dendrimers
,†
Chuanjun Xia, Xiaowu Fan, Jason Locklin, and Rigoberto C. Advincula*
Department of Chemistry, UniVersity of Alabama at Birmingham,
Birmingham, Alabama 35294
gobet@uab.edu
Received March 28, 2002
ABSTRACT
Thiophene dendrons and dendrimers were designed and synthesized using a convergent approach. Metal-mediated coupling reactions were
used in the synthesis. A rational approach allowed the formation of rr, ââ, and râ linkages between the dendrons and thiophene units.
During the past several decades, oligothiophenes and poly-
thiophenes have been extensively studied not only for
fundamental interest but also for their promising applications
in the organic semiconductor field.1 A large number of
substituted derivatives have been designed and synthesized,
and their electronic properties have been investigated.
Organic electronic devices, such as organic light-emitting
diodes (OLEDs)2 and Field-effect transistors (FETs),3 based
on these derivatives have been fabricated and tested. The
optical and electronic properties of oligo- and polythiophene
derivatives can be easily tuned by introducing functional
substituents to different position on the backbone.1
Dendrimers, which are perfect monodispersed macromol-
ecules with a regular and highly branched three-dimensional
architecture, represent a key stage in the ongoing evolution
of polymer chemistry.4 Along with flexible backbone den-
drimers, conjugated dendrimers with rigid structures, such
as phenylacetylene,5 phenylene vinylene,6 and polyphen-
ylene7 dendrimers have also been developed in recent years.
† Current address: Department of Chemistry, University of Houston,
Houston, Texas, 77204.
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10.1021/ol025943a CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/23/2002

These shape-persistent objects with nanometer sizes exhibit
interesting aggregation properties.8 Some of these compounds
can self-assemble into nanowires and other supramolecular
structures as a result of π-π stacking interactions. On the
other hand, generational dendrimers containing all-thiophene
units have not been reported in the literature even though
their macrocyclic,9 tubular,10 and linear analogues have been
extensively studied. However, the optical, electronic, and
self-assembly properties of thiophene dendrimers may be
intriguing because of their spatial arrangement and micro-
structure. The intrinsic asymmetric structure can result in
different conjugation lengths, resulting in broad absorption,
which has been recently observed for another asymmetric
polyphenyl acetylene dendrimer.11
Scheme 1. Synthesis of Thiophene Dendrons
In this paper, we present our design and synthesis strategy
for a series of thiophene dendrons and dendrimers, using
mainly a convergent synthesis methodology. The purpose
of this macromolecule architecture is to investigate photo-
chemical and physical properties resulting in (1) unusual
broad absorption, rendering it a good candidate for an
antenna molecule;12 (2) formation of regular 2-D supramo-
lecular structures on graphite or gold surfaces and confor-
mational change induced through different bias;13 and (3)
formation of self-stacked long-ranged ordered structures.
Scheme 1 shows the synthetic route for the thiophene
dendrons. The key starting material for the synthesis is 2,3-
dibromothiophene, which is commercially available. In a
convergent synthetic sequence, 2,3-dibromothiophene acts
as an AB2 type monomer. A C-C bond can form through
the C-Br group at the 2,3 positions using metal-mediated
coupling. The fifth position can undergo other transforma-
tions for further coupling reactions, in our case, Stille
coupling. 2-Bromo-5-hexylthiophene was synthesized from
thiophene by treating with butyllithium and 1-bromohexane,
followed by bromination with NBS/DMF. The pure product
was obtained by distillation under reduced pressure; 3T was
made with an excellent yield (90%) using the Kumada
method.14 Although the Grignard coupling could be used to
continue the synthesis, the ease of the Stille coupling15
reaction attracted our attention. As has been well docu-
mented, Stille coupling offers efficient C-C bond formation
for heterocycles such as thiophenes. The tolerance of a
variety of functional groups makes it very useful for modern
synthesis. In addition, the reaction is easy to perform.
Harmful oxygen can be easily removed by simply doing
several freeze-thaw cycles.
Compound 3T was readily lithiated using butyllithium and
reacted with tributyltin chloride to give the stannyl compound
for the following Stille coupling. There was no 3T left after
this reaction, and hence, this compound, which is liable to
acids, was used without further purification. An excess of
the stannyl compound was used to ensure completion of the
coupling reaction. No monocoupled product was detected
from the NMR. The efficiency of the coupling decreases as
the product becomes more crowded around the focal point
thiophene, i.e., 7T was synthesized with 90% yield and 15T
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2068
Org. Lett., Vol. 4, No. 12, 2002

with a lesser 77% yield. Prolonged reaction time did not
increase the yield significantly. Attempts on lithiation of 15T
with butyllithium were unsuccessful probably as a result of
steric hindrance, which makes the central thiophene inac-
cessible. In addition, only trace bromination products were
isolated when treated with bromine or NBS. Once again, we
attribute the inert reactivity mainly to the inaccessibility of
the central thiophene.
Scheme 3. Synthesis of Thiophene Dendrimers
Direct coupling of these dendrons afforded the dendrimer
structures (Scheme 2). Compounds 6T and 14T-1 were
Scheme 2. BuLi/CuCl2 Coupling to Thiophene Dendrimers
directly coupled from 3T and 7T, respectively, through a
BuLi/CuCl2 method, which is widely applied for oligo-
thiophene synthesis.16 However, this coupling is not as
efficient as those of linear oligomers and gave only moderate
yields, e.g., 54% for 6T and 40% for 14T-1. Unfortunately,
the coupling of 15T was not practical because of its poor
reactivity (despite several attempts).
as chloroform-d, THF-d8, and methylene chloride-d2, in both
peak positions and split patterns. This phenomenon indicates
the association of the molecules in different solvents. Figure
1 shows the aromatic region of the 1H NMR spectra of 6T,
For dendrimer synthesis, assembly of dendrons to a core
is widely used in order to form functional dendrimers or to
simply enlarge the compound. We adopted this method to
synthesize 14T-2 and 30T as shown in Scheme 3. Treating
2,3-dibromothiophene with LDA, followed by CuCl2, gave
compound 11, 4,5,4′,5′-tetrabromo-[3,3′]bithiophene. Instead
of abstracting the proton at the fifth position, LDA removed
the proton mainly at the fourth position. On the other hand,
the lithiation mainly occurred at the fifth position for
2-bromothiophene. Compounds 14T-2 and 30T were thus
made by coupling the tetrabromothiophene with compounds
4 and 6. The unexpected lithiation allows us to compare the
properties of 14T-1 and 14T-2 because they were linked
through the R,R- and â,â-position in the central thiophene,
respectively. Therefore, they should have different π-con-
jugation characteristics and also give different NMR spectra.
All of these thiophene dendrons and dendrimers are highly
soluble in common solvents such as chloroform, THF, and
even hexane. Polar solvents such as DMF and DMSO cannot
dissolve compounds bigger than 7T at room temperature.
Only 14T-1 and 30T solidified after standing for a long time
at room temperature; the others are viscous liquids at room
temperature. Interestingly, the NMR spectra of compounds
bigger than 7T differed when using different solvents, such
Figure 1. Aromatic region of the 1H NMR spectra of the dendrons
and dendrimers.
Org. Lett., Vol. 4, No. 12, 2002
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7T, 15T, 14T-1, 14T-2, and 30T in THF-d8 at 290 K. For
6T, 14T-1, 14T-2, and 30T the interior protons on the
thiophene ring show singlets downfield and the exterior
protons show doublets upfield. For 7T and 15T, two doublets
were found for the two protons at the focal thiophene, one
at the most downfield and the other hidden in the singlets.
Temperature control experiments were also performed for
14T-1, 14T-2, and 30T in THF-d8. Upon cooling, the
resonance of those singlet protons shifted markedly down-
field. Only small shifts were observed for the external
protons. The association of the dendrimers in solution is
under investigation. The MALDI-TOF analysis verified
monodispersed masses for 14T-1, 14T-2, 15T, and 30T,
giving 1824.0, 1823.6, 1905.3, and 3809.5, respectively.
These are consistent with the calculated values: 1823.6,
1823.6, 1905.6, and 3809.2.
reactions through a convergent approach. All the character-
ization results, including NMR, elemental analysis, and
MALDI-TOF-MS, confirmed the structure proposed in the
schemes. The methodology allows for a rational design of
generational dendrimers with different conjugation lengths
and microstructures. These materials are currently being
systematically investigated for their optical properties, redox
behavior under potential cycling, aggregation behavior in
solution, self-assembly in ultrathin film and bulk state, and
possible applications in organic electronics.
Acknowledgment. We thank the NSF-CAREER award
DMR 99-82010 for funding of this work.
Supporting Information Available: Experimental details
and characterization data for all the compounds reported. This
material is available free of charge via the Internet at
http://pubs.acs.org.
In conclusion, thiophene dendrons and dendrimers have
been successfully synthesized by metal-mediated coupling
(16) (a) Kauffmann, T. Angew. Chem. 1974, 86, 321. (b) Kagan, J.; Arora,
S. K. Hetreocycles 1983, 20, 1937. (c) Garnier, F.; Horowitz, G.; Fichou,
D.; Synth. Met. 1989, 28, C705.
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