J. Am. Chem. Soc. 2001, 123, 8101-8108
8101
Polyphenylene Dendrimers with Different Fluorescent Chromophores
Asymmetrically Distributed at the Periphery
Tanja Weil, Uwe M. Wiesler, Andreas Herrmann, Roland Bauer, Johan Hofkens,‡
Frans C. De Schryver,‡ and Klaus Mu1llen*
Contribution from the Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10,
55128 Mainz, Germany, and Department of Chemistry, Katholieke UniVersiteit LeuVen,
Celestijnenlaan 200F, 3001 HeVerlee, Belgium
ReceiVed March 5, 2001
Abstract: A new synthetic approach leading to asymmetrically substituted polyphenylene dendrimers is
presented. Following this method, polyphenylene dendrimers decorated with an increasing number of
chromophores at the periphery have been obtained up to the second generation. Especially the synthesis of a
polyphenylene dendrimer bearing three donor chromophores and one acceptor chromophore has been realized.
Intramolecular energy transfer within this molecule is demonstrated by applying absorption and fluorescence
measurements.
Introduction
or just one functional group15,16 at the periphery. However, the
proposed use of dendrimers as nanosupports often requires that
they provide a predefined spatial nanoenvironment, which in
turn determines the necessary position of functional groups on
their surfaces. The flexibility of dendrimers built up from
aliphatic chains often precludes such a perfect spatial separation
of the functional groups, as backfolding of the peripheral
substituents can occur.17,18 Especially the study of chro-
mophore-chromophore through-space interactions in such a
system requires a rigid support since increased flexibility leads
to undesired interactions such as aggregation, excimer formation,
and dye self-quenching.19,20 Recently, the shape persistence of
polyphenylene dendrimers has successfully been proven by
applying advanced NMR studies21 and AFM measurements.22
In particular, solid-state NMR characterization has clearly
proven that, due to the sterical hindrance imposed on a given
phenyl group by its neighbors which are bound to the same
core, only collective reorientations of all rings around one central
core can take place. As a result, only limited dynamics which
were identified as a well-localized, restricted reorientation of
single terminal phenyl substituents around fixed axes could be
observed. Furthermore, the stiffness and the size of these
In recent years there has been an increasing interest in the
design of macromolecules on the nanometer scale.1 Among these
nanoobjects, dendrimers play an important role, which is due
to their distinct size, monodispersity, and the ease of introduction
of functional groups.2,3 Some effort has been made toward the
synthesis of dendrimers bearing more than one type of functional
group at the periphery.4 Only the convergent approach allows
an accurate control over the number and placement of func-
tionalities and therefore fulfils the requirement for the buildup
of asymmetric surface-functionalized dendrimers. Fre´chet and
co-workers were able to introduce two different functions via
the convergent attachment of two differently substituted poly-
(aryl ether) dendrons to a bifunctional core.5 In this way
fascinating new molecules such as amphiphilic dendrimers
coated with carboxylic acid groups on one-half and alkyl chains
on the other6-9 or molecular dipoles10 have been synthesized.
Recently, there have been a few examples of dendrimers
carrying multiple functions in close proximity to each other11-14
* Address correspondence to this author: Max-Planck-Institut fu¨r
Polymerforschung; (Phone/Fax) ++49-6131-379151; (e-mail) muellen@mpip-
mainz.mpg.de.
‡ Katholieke Universiteit Leuven.
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VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1996.
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10.1021/ja010579g CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/15/2001