ORGANIC
LETTERS
2005
Vol. 7, No. 14
2809-2812
Effect of Guest Molecule Flexibility in
Access to Dendritic Interiors
Sivakumar V. Aathimanikandan,† Britto S. Sandanaraj,† Christopher G. Arges,‡
Christopher J. Bardeen,‡,§ and S. Thayumanavan*,†
Department of Chemistry, UniVersity of Massachusetts, Amherst, Massachusetts 01003,
and Department of Chemistry, UniVersity of Illinois, Urbana, Illinois 61801
Received March 16, 2005
ABSTRACT
Dendrimers are attractive scaffolds for catalysis, since catalytic sites can be isolated and the catalysts are recoverable and reusable. Herein,
we show that conformationally constrained molecules have better access to dendritic cores compared to the more flexible counterparts. The
results reported here should have implications in utilizing dendrimers as scaffolds for artificial selectivity in catalysis.
Encapsulation of catalytically active functionalities using
dendrimers has attracted much attention in recent years.1
While the properties of an encapsulated electroactive species
could be elucidated from studying electron-transfer kinetics
and thermodynamics,2 catalytic activity is dependent on the
bimolecular collision efficiency between the encapsulated
species and the substrate. One could reasonably expect that
at higher generations, when the species at the cores of
dendrimers is encapsulated, the dendritic backbones could
act as gatekeepers for substrate access to the catalytic centers.
In fact, this is one of the ways that enzymes maintain high
substrate specificity in nature. While there have been reports
on selectivity in access to dendritic interiors,3 a systematic
investigation on structural factors in substrates that govern
their access to dendritic cores has been lacking. Conventional
wisdom about dendrimers would suggest that the access
would mainly depend on the size of the guest molecules.
However, here we report that conformational flexibility of
the guest molecules plays a significant role in accessibility
to the cores of dendrimers.
We used distance-dependent excited-state quenching through
photoinduced electron transfer as the probe for this study.4
For this purpose, we synthesized benzyl ether dendrons 1-4
and dendrimers 5-8 up to the fourth generation with
anthracene as the fluorescent unit at the focal point and core,
respectively (Figure 1).5 Benzyl ether dendrons were as-
† University of Massachusetts.
‡ University of Illinois.
§ Current address: Department of Chemistry, University of California,
Riverside, CA 92521.
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10.1021/ol050579b CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/03/2005