Probing every layer in dendrons

Article abstract of DOI:10.1021/ja043356+

Dendrons with a fluorescent probe in a specific location have been synthesized and characterized. Accessibility of guest molecules to each of these layers was then analyzed using an intermolecular photoinduced electron-transfer process. Comparisons of the

Full text of DOI:10.1021/ja043356+

Published on Web 01/27/2005
Probing Every Layer in Dendrons
Kulandaivelu Sivanandan,† Sivakumar V. Aathimanikandan,† 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 November 3, 2004; E-mail: thai@chem.umass.edu
Scheme 1. Synthesis of the Monomer 8 and G0-Dendron 5
Dendrimers have been a molecular structure of intense research
in recent years.1 These molecules are being pursued for a wide
variety of applications, including targeted drug delivery, controlled
drug release, and catalysis.2 In many cases, the interest in these
molecules is motivated by the fact that the globular shape of the
dendrimers afford a different microenvironment at the core.
Properties of a dendritic core have been studied quite extensively
through encapsulations of electro-, photo-, and catalytically active
units.3 However, little if anything is known regarding the properties
of functionalities incorporated in the intermediate layers of the
dendrimer. In this contribution, we investigate the properties of
every layer of dendron by incorporating a single fluorescent probe
unit in a specific location. The extent of guest molecule accessibility
to each location within a dendrimer is then analyzed using an
intermolecular photoinduced electron transfer (PIET)-based fluo-
rescence quenching process. For the first time, we show here that
the steepest change in accessibility occurs in the middle layers.
We also show that there is a significant difference in generation-
dependent accessibility to even the peripheral layers of dendrimer.
Anthracene is used as the fluorescent probe in this study, and
benzyl ether dendrimers were used as the scaffold. To incorporate
anthracene in specific locations of a dendrimer, we first synthesized
an anthracene containing monomer unit 8 (Scheme 1). The
combination of the monomer unit 8 and 3,5-dihydroxybenzyl
alcohol was used to synthesize 14 different G1-G4 dendrons 1-4
(Chart 1) and the control G0 molecule 5 (Scheme 1). Synthetic
methodologies, developed by us to sequence dendrimers, were used
here to achieve the site-specific incorporation of these fluorescent
probes.4,5 All dendrimers were characterized by 1H, 13C, and mass
spectrometry. Size exclusion chromatography was used as an
additional test of purity.5
Chart 1. Structures of Dendrimer with Anthracene as the
Fluorescent Probe in Specific Locations
Fluorescence intensity of the anthracene moiety decreases with
increasing concentration of the quencher, N,N,N′,N′,N′′,N′′-hexa-
methyltris(2-aminoethyl)amine (tren-Me6). This intermolecular
PIET-based fluorescence quenching is used as the measure of
accessibility of tren-Me6 to the anthracene moiety in each dendrimer.
Substituents, excited-state energy, and the redox potential of the
anthracene derivatives and tren-Me6 suggest that fluorescence
quenching due to energy transfer or a heavy atom effect is unlikely;
these are, however, appropriate for PIET. Moreover, fluorescence
quenching is much less in apolar toluene and more in polar DMF
compared to that in the current solvent THF:CH3CN (6:4), which
is a hallmark of PIET.
Extent of this fluorescence quenching is related to the concentra-
tion of the quencher through the Stern-Volmer equation: I0/I ) 1
+ KSV[Q], where I0 and I are the fluorescence intensities in the
absence and presence of quencher, [Q] is the concentration of the
quencher, and KSV is the Stern-Volmer quenching constant. The
constant KSV is a product of the bimolecular quenching rate constant
kq and fluorescence lifetime of anthracene dendrimer in the absence
of the quencher (τ0), i.e., KSV ) kq × τ0.5 The kq values could be
considered as a measure of the accessibility of a quencher to a
fluorophore.
When using kq as a measure for estimating the relative acces-
sibility of dendrimers 1-4, two factors need to be taken in to
account: (i) Is the fluorescence quenching static or dynamic? and
(ii) Is the excited-state energy for the anthracene species in all
dendrimers the same to allow a comparison? We have addressed
both these issues. Stern-Volmer quenching constants of anthracene-
based dendrimers were measured through both steady-state and
† University of Massachusetts.
‡ University of Illinois.
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J. AM. CHEM. SOC. 2005, 127, 2020-2021
10.1021/ja043356+ CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
and third layer, where there is about 42% change in the kq value
(compare 4b and 4c). From the third to fourth to the focal point
(4c-4e), there is only about 15% change with each layer. It is
interesting that the first significant change in accessibility occurs
at the third layer from the periphery in these dendrons. In the case
of the fourth generation, the third layer exhibits the steepest change
in accessibility (compare 4a-4e). In fact, it is in this layer that
there is maximum difference between the generations (compare 2c-
4c). Therefore, it is the difference at this layer that is translated to
varied levels of encapsulation at the focal point.
In summary, we have incorporated a probe in each layer of
dendron and investigated the accessibility of these locations using
an intermolecular PIET process. The trends reported here should
have implications in areas such as catalysis and drug delivery. The
study is relevant to catalysis because PIET is considered to be based
on a bimolecular collision process, an event that is necessary
between a catalytic site and the substrate. In drug delivery,
approaches using cleavable dendrimer-drug conjugates have been
suggested.7 It is necessary in these cases that the linkers are not
cleaved until they reach the target and therefore should be
encapsulated. It is also advantageous to load several drug molecules
in a single dendrimer. Note, however, that loadability decreases as
one moves toward the core and the encapsulation increases toward
the core. The current study suggests that the intermediate layers
with significant loading capacity and encapsulation could be useful
(e.g., third layer of the G4 in this case). While these observations
clearly have implications, further understanding of the reasons for
the observed behavior is still needed, which is the focus of current
work in our laboratories.
Figure 1. Plot of kq for different layers in dendrons.
time-resolved spectroscopy. The obtained KSV’s were similar in both
techniques for a most encapsulated anthracene (fourth generation)
as well as a least encapsulated anthracene (first generation). These
results confirm that the observed fluorescence quenching is the
result of a dynamic process. As an additional evidence, we did not
observe any saturation in fluorescence quenching or change in
absorption or fluorescence spectra upon increasing the concentration
of quencher. We are also confident that the excited-state energy of
the anthracene moiety for all the compounds 1-5 is the same,
because neither the absorption nor the emission spectra exhibited
any shift. Note that the intersection of absorption and emission
spectra is considered to be an estimate of ∆E0-0. This confirms
that the change in kq values observed in various compounds is not
due to the inherent change in the electronic property of the
anthracene moiety in different dendrimers.
The observed bimolecular quenching rate constants and the trends
for the compounds 1-5 are shown in Figure 1. Three interesting
trends are noteworthy: (i) Comparison of the peripheral moieties
in 1-4, (ii) comparison of a position in one generation with another
position in a different generation, and (iii) a trend within different
layers of a dendrimer.
Acknowledgment. We are grateful to the NIGMS of the
National Institutes of Health for support.
Supporting Information Available: Synthetic and other experi-
mental details. This material is available free of charge via the Internet
at http://pubs.acs.org.
Encapsulation of the anthracene moiety in the periphery varies
significantly with generation (compare 1a-4a). There is essentially
no difference in accessibility between 1a and the control molecule
5, while there is about a 26% change in accessibility between 1a
and 2a. The difference between 1a and 4a is about 48%. In fact,
the accessibility of the peripheral anthracene in 4a is less than that
in the core anthracene in 2c and is about the same as that in the
anthracene moiety in 3c. The fact that there is a generation
dependence in accessibility to periphery is not that surprising, since
the backfolding of these dendrimers will result in burying of some
of the branches.6 Since only one of the branches has the anthracene
unit, the kq value is a true measure of the average accessibility of
the peripheral moieties. It is surprising to us, however, that the
difference is significant even at the second generation.
Another interesting observation in this study is that beyond the
second layer of a dendron, the accessibility of a layer within a
dendron seems to be similar to the next layer of the previous
generation. For example, the second-layer compound 3b exhibits
a similar kq value as the third-layer compound 2c. Similar trends
can also be noted by comparing 3c with 4b and 3d with 4c. The
reasons for this trend are not obvious to us at this time and are
under investigation.
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(5) See Supporting Information for details.
(6) An alternate explanation is the possible differences caused by aggregation
of these dendrons. The size of G2-G4 dendrons (measured by DLS)
ranged from 2 to 4 nm depending on the generation in the solvent under
study (THF:CH3CN, 6:4), indicating a monomeric state. Therefore, this
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The difference between the periphery and the next layer of the
dendrons is negligible in all dendrons. From the second to third
layer, there is a significant difference in accessibility. In the third
generation, a gradual decrease in accessibility from the second to
the fourth layer (3b-3d) is observed. However, in the fourth-
generation 4, the steepest difference seems to be between the second
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