Soluble dipolar dendrimers with peripheral sulfone groups

Article abstract of DOI:10.1016/S0040-4039(02)01902-0

A dipolar dendron based on meta and para linked poly(phenyl acetylene)s with eight electron donor-acceptor pairs in direct conjugations is synthesized through a convergent approach.

Full text of DOI:10.1016/S0040-4039(02)01902-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 7903–7906
Soluble dipolar dendrimers with peripheral sulfone groups
Meng Lu, Yongchun Pan and Zhonghua Peng*
Department of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, USA
Received 5 August 2002; revised 30 August 2002; accepted 5 September 2002
Abstract—A dipolar dendron based on meta and para linked poly(phenyl acetylene)s with eight electron donor–acceptor pairs in
direct conjugations is synthesized through a convergent approach. © 2002 Published by Elsevier Science Ltd.
Organic conjugated molecules and polymers are electri-
cally active materials that have found increasing appli-
cations as active components in a number of electronic
devices,1 such as light emitting diodes (LEDs),2 field-
effect transistors (FETs),3 solid state lasers,4 and other
electro-optic devices.5 As a fundamentally new macro-
molecular architecture, dendritic conjugated macro-
molecules have also been developed in recent years.6
The p-conjugated segments in these dendrimers impart
interesting photophysical properties7 and applications
in, for example, LEDs have been demonstrated.8 How-
ever, application of conjugated dendimers as nonlinear
optical (NLO) materials has received less attention.9
This is not surprising as the p-conjugations of the
majority of the conjugated dendrimers, due to the
interruption by the commonly applied meta-branching
scheme, are rather limited. It is also synthetically chal-
lenging to introduce multiple electron donor–acceptor
pairs into the dendritic structure in a way so that they
all contribute to the overall nonlinearity. In this paper,
we report the synthesis of a third generation dendron,
shown as G3, which is based on asymmetrically
branched phenyl acetylene.10 Sulfone, as the electron-
withdrawing group, is introduced to the para position
of each peripheral phenyl ring. To improve the solubil-
ity, a long alkyl chain is attached to the sulfone group.
There are eight methoxy groups as the electron donors,
distributed at the core and the center-layer phenyl
rings. Each methoxy group forms a conjugated pair
with one sulfone group, which constitutes a traditional
linear organic NLO active chromophore.11 The G3
dendron thus contains eight chromophores in one
molecule. Due to steric interactions, the dendron likely
adopts a three-dimensional (3D) conformation, thus
representing a unique 3D chromophore where both the
diagonal and off-diagonal b-components (b is the first
hyperpolarizability) will contribute to the overall
nonlinearity.12
Keywords: dendrimers; electronic spectra; dipole moments.
* Corresponding author. Tel.: (816) 235-2288; fax: (816) 235-5502; e-mail: pengz@umkc.edu
0040-4039/02/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0040-4039(02)01902-0

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M. Lu et al. / Tetrahedron Letters 43 (2002) 7903–7906
Scheme 1.
Except for the peripheral phenyl rings, all other phenyl
groups in G3 have a 1,2,4,5 substitution pattern. Five
compounds (4–8) with such a substitution scheme were
synthesized as the building blocks for the construction
of the dendron series (G1, G2, G3). The syntheses for
such compounds are shown in Scheme 2.
G3 was synthesized through a convergent approach,
which starts from the exterior of the eventual dendron.6
The peripheral unit (compound 3) was synthesized
according to Scheme 1. Starting from 4-bromobenz-
enethiol, alkylation of thiol with 1-bromododecane
gives compound 1 in 89% yield. Subsequent oxidation
of sulfide to sulfone yields 2. Palladium-catalyzed cou-
pling between aryl bromide with trimethylsilylacetylene,
followed by desilylation gives compound 3 in 88%
overall yield (from 2).
The syntheses of 4, 5 and 6 have previously been
reported.10 Compound 7 was prepared from 4 by the
Sonogashira coupling reaction.13 Compound 8 was syn-
thesized from 5 in excellent yields.
The iterative synthesis to G3 as well as the synthesis to
lower-generation dendrons G1 and G2, is shown in
Scheme 3. Direct coupling between 3 (2 equiv.) and 4
gives G1 in 67% yield. To prepare higher-generation
dendrons, a first generation dendron with a triflate
group at the core, G1-OTf, was synthesized by the
coupling of 3 with 8, taking advantage of the higher
reactivity of iodo over triflate in the Sonogashira cou-
pling reaction.14 The cross-coupling of triflate with
acetylene could be carried out between G1-OTf and 7
to give G2 in 74% yield or between G1-OTf and 6 to
give G2-OH in 67% yield. Conversion of G2-OH to
G2-OTf is trivial and efficient. Similar to the synthesis
of G2, G3 was prepared from the coupling of triflate
with acetylene.15 The yield is 58%. It should be noted
that the procedures from G1-OTf to G3, namely G1-
OTf“G2-OH“G2-OTf“G3, could be repeated on
Scheme 2. (i) (a) Trimethylsilylacetylene, Pd(PPh3)2Cl2, CuI,
NEt3, DMF; (b) Bu4NF, THF; (ii) (CF3SO2)2O/pyridine.
Figure 1. UV–vis absorption and fluorescence emission spectra of G1–G3.

M. Lu et al. / Tetrahedron Letters 43 (2002) 7903–7906
7905
Scheme 3. (i) Pd(PPh3)2Cl2, CuI, NEt3, DMF; (ii) (CF3SO2)2O/pyridine.
G2-OTf (“G3OH“G3-OTf“G4) to prepare even
higher generation dendrons.
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