Shell-by-shell inside-out complexation of organic anions in flexible and rigid pyridinium dendrimers

Article abstract of DOI:10.1021/ma201615w

Polycationic flexible pyridinium and rigid bipyridinium (=viologen) dendrimers were prepared. The cationic charges are persistent and equally distributed all through the dendrimers. The dendrimers are filled with monoanionic (benzenesulfonate (BS)), dianionic (anthraquinone disulfonate (AQDS), naphthalene disulfonate (NDS)), and trianionic (pyranine (Pyr)) guest molecules in a stepwise inside-out, shell-by-shell fashion. The total cationic charge per dendrimer subshell divided by the charge of a guest anion defines the maximum guest capacity of the corresponding shell. These numbers appear as "magic numbers" in ¹H NMR titrations. Thus, the proof for the sequential inside-out complexation scenario is based on the sequential appearance of the innermost, then the mid and finally the outermost dendrimer shell. Anions with matching size show the sharpest ¹H δ-transitions at the subshell equivalence points indicating that beside simple charge interaction molecular recognition is playing an important role. The end points of the generation 0 dendrimers were further probed by electrochemical techniques, yielding K > 10 ×10³ M ^-1 for the last guest molecule. Two reasons for the inside-out filling were identified from simple MM+ based MD calculations, i.e., (i) backfolding of outer branches leading to multiple complexation of anionic sites in all except the outermost shell, and (ii) the reduced mobility of the core region as compared to the outer branches leading to an enhancement of the cationic attractor over time and space in the central dendrimer region. The sequential inside-out filling of guest counterions within the dendrimers is in agreement with a wrapping process in combination with the observed decreasing hydrodynamic radius.

Full text of DOI:10.1021/ma201615w

ARTICLE
pubs.acs.org/Macromolecules
Shell-by-Shell Inside-Out Complexation of Organic Anions in Flexible
and Rigid Pyridinium Dendrimers
Murugavel Kathiresan and Lorenz Walder*
Institute of Chemistry OCII, University of Osnabrueck, Barbarastrasse 7, Osnabrueck, D 49069, Germany
S Supporting Information
b
ABSTRACT:
Polycationic flexible pyridinium and rigid bipyridinium (=viologen) dendrimers were prepared. The cationic charges are persistent
and equally distributed all through the dendrimers. The dendrimers are filled with monoanionic (benzenesulfonate (BS)), dianionic
(anthraquinone disulfonate (AQDS), naphthalene disulfonate (NDS)), and trianionic (pyranine (Pyr)) guest molecules in a
stepwise inside-out, shell-by-shell fashion. The total cationic charge per dendrimer subshell divided by the charge of a guest anion
defines the maximum guest capacity of the corresponding shell. These numbers appear as “magic numbers” in 1H NMR titrations.
Thus, the proof for the sequential inside-out complexation scenario is based on the sequential appearance of the innermost, then the
1
mid and finally the outermost dendrimer shell. Anions with matching size show the sharpest H δ-transitions at the subshell
equivalence points indicating that beside simple charge interaction molecular recognition is playing an important role. The end
points of the generation 0 dendrimers were further probed by electrochemical techniques, yielding K > 10 ꢀ 103 Mꢁ1 for the last
guest molecule. Two reasons for the inside-out filling were identified from simple MM+ based MD calculations, i.e., (i) backfolding
of outer branches leading to multiple complexation of anionic sites in all except the outermost shell, and (ii) the reduced mobility of
the core region as compared to the outer branches leading to an enhancement of the cationic attractor over time and space in the
central dendrimer region. The sequential inside-out filling of guest counterions within the dendrimers is in agreement with a
wrapping process in combination with the observed decreasing hydrodynamic radius.
exists a preference for a substoichiometric amount of guest
molecules to coordinate first in the center, at the branches or
at the periphery of the dendrimer. So far there exist only two
principal types of dendrimers with identical repeating sub-
units from the core into the periphery on which this question
has been studied. These are (i) the phenylazomethine den-
drimers with phenylimine based basic coordination sites for
1. INTRODUCTION
Dendrimers can act as hosts1 for smaller molecular guests, a
fact that has been used for fundamental studies2,3 as well as for
drug delivery applications.4ꢁ8 Depending on the functional-
ities present on the dendrimer - ionic,9ꢁ15 neutral,16
hydrophobic,2,6 and H-bonding,2,17ꢁ21 guest molecules can
be complexed and transported.22 The usual divergent syn-
thetic procedures allow producing such functionalities exclu-
sively at the core, along the branches or at the periphery,
opening the design of localized complexation sites within a
dendrimer.23ꢁ26 Beside active complexation, steric phenom-
ena can determine the localization of molecular guests in a
dendrimer, i.e., the voids in the innermost molecular shell
of highly branched dendrimers appearing close to their limit-
ing generation.1 On the other hand there exist dendrimers
with complexation sites (e.g., charges, Lewis base coordina-
tion sites) distributed all through the dendrimer (center,
branches and periphery) equally. The PPI (polypropylen-
eimine), PAMAM (polyamidoamine), and poly(phenylazo-
methine), as well as pyridinium and viologen dendrimers are
examples of such structures, and the question arises, if there
Sn2+,27ꢁ29 Fe3+,30 ferrocenium,31 triphenylmethylium,31
2ꢁ 31,32
PtCl4
,
and Ti4+33 all described by Yamamoto and co-
workers, and (ii) a pyridinium dendrimer with equally dis-
tributed cationic sites binding anthraquinone disulfonate
electrostatically.34 Interestingly, in both types of hosts it was
found that the dendrimers are filled with the corresponding
guest molecules following a stepwise shell-by-shell mecha-
nism starting with the innermost molecular shell and ending
with the outermost shell. In the case of the phenylazomethine
dendrimers, it was argued that a radial Lewis basicity gradient
Received: July 14, 2011
Revised:
September 21, 2011
Published: October 11, 2011
r
2011 American Chemical Society
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Scheme 1. Structure of Host and Guest Moleculesa
Scheme 2. Divergent Synthesis of Viologen Dendrimers with
4-tert-Butylbenzyl End Groupa
a Key: (a) 4-t-Bu-BnBr/CH3CN/80 °C; (b) NH4PF6/H2O; (c) V1/
CH3CN/80 °C; (d) HBr/HOAc/RT; (e) E/CH3CN/80 °C, (f) V2/
CH3CN/80 °C; dotted circles represent subshells rh: hydrodynamic
radii from DOSY
a (i) Cationic dendritic hosts: (a) TMDPy0ꢁ2 dendrimers; (b)
DPy0ꢁ2 dendrimers. (ii) Anionic guest molecules; rh: hydrody-
namic radii from DOSY.
dendrimers was not addressed.38 It has been shown that viologen
dendrimers complexes DNA (polyanions with phosphate
residues).39 In the present work, we check the generality of the
inside-out filling mechanisms with emphasis on the mutual fit of
guest anions and host coordination sites. We include guest
anions with one to three negative charges and we study the
influence of the rigidity of the dendrimer structure. Finally, we try
to rationalize the results by simulation of the coordination
process using MD (molecular dynamics) calculations.
within the dendrimer is responsible for this selectivity, and it
was also shown that an electron withdrawing dendrimer core
can invert the gradient leading to an outside-in filling
mechanism.28 Notably, there are several examples of dendri-
mers with tailored gradients consisting of branches with a
continuously changing property, but most concern radial
electron and photon flow rather than guest molecules. These
dendrimers have a build-in redox gradient,10,32,35 or a built-in
energy gradient.36,37
2. RESULTS AND DISCUSSION
A radial guest distribution can be synthetically provoked by a
corresponding functionality gradient on the dendrimer branches.
However, the inside-out scenario has also been observed without
tailoring, using equally distributed identical functionalities on the
dendrimer skeleton. Yamamoto has described this situation for a
series of guests (cationic Lewis acids) together with phenylazo-
methine based dendrimers. We have found it for anthraquinone
disulfonate and a dendrimer with equally distributed cationic
pyridinium coordination sites. The reasons for this “dendritic
effect” remain unclear.
Hence, we extended our studies using two classes of dendri-
mers one built from relatively rigid dicationic viologen subunits
(DPy0ꢁ2) and the other containing more flexible trimethylene-
dipyridinium subunits (TMDPy0ꢁ2). They were probed with a
series of anionic guest molecules (Scheme 1). For the viologen
dendrimers efficient anion complexation and guest release has
been reported by Balzani et al. but the mechanism of filling these
a. Synthesis. The dendrimer structures and the guest anions
used in the study are presented in Scheme 1.
The synthesis of the TMDPy0ꢁ2 series has been reported
recently.34 For the current study we have prepared the viologen
dendrimers (DPy0ꢁ2) with the same 4-t-BuBn peripheral groups
rendering them sufficiently soluble in DMSO (dimethyl
sulfoxide) (Scheme 2). The synthesis follows the general proce-
dure as reported for viologen dendrimers with peripheral ethyl
end groups.9 The intermediates V1, V2.PF6 have been prepared
as described,40 synthesis and characterization of P0 3PF6,
3
P1 6PF6 and P2 18PF6 is presented in the Experimental Section
and in the Supporting Information.
3
3
The three peripheral nitrogen’s in P0 3PF6 react quantitatively
3
with excess of 4-t-Bu-BnBr. Its reaction with P0 3PF6 followed
3
by ion exchange yielded DPy0 6PF6 in 75%. The dendrimers
DPy1 18PF6 and DPy2 42PF6 were available from the reaction
of the corresponding precursors P1 6PF6 and P2 18PF6 with the
3
3
3
3
3
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end group E PF6, yielding 74 and 67%, respectively, after ion
3
exchange. The intermediates and products were characterized by 1H
NMR, 13C NMR, and DEPT measurements. The purity of the
compounds was further checked by elemental analysis (samples
were pure apart from variable water contents). The completeness of
N-alkylation was judged from the integration of the 1HNMRsignals
for the peripheral group as compared to the core resonances (For
detailed synthetic procedures and characterization, see Supporting
Information). The hydrodynamic radii of the dendrimers were
measured by DOSY (diffusion ordered spectroscopy).
In the current paper, the complexation behavior of both series of
dendrimers (DPy0ꢁ2 and TMDPy0ꢁ2) was studied with mono-
anionic (benzenesulfonate (BS)), dianionic (anthraquinone disul-
fonate (AQDS) and naphthalene disulfonate (NDS)) and
trianionic (pyranine (Pyr)) guest molecules. Both dendrimer series
carry more or less equally distributed cationic charges which are
arranged in molecular shells, i.e., 6, 12, and 24 pyridinium units for
the second generation dendrimers. The interactions were moni-
1
tored by H NMR technique. MM+ and MD simulations were
performed to understand the radial affinity gradient.
b. HostꢁGuest Interactions—1H NMR titrations. 1H NMR
technique has been often used to evaluate the intermolecular
interactions between host and guest in supramolecular assem-
blies, as even small differences in the chemical environment
occurring upon complex formation can be detected.41ꢁ43 Guest
interaction on dendrimers monitored by NMR techniques have
been reported, e.g. the protonation of DAB (diaminobutane) and
PAMAM (polyamidoamine) dendrimers by different acids
(vitamin C, B3, and B6) by Astruc et al.,17 or hydrogen bonding
interactions in adamantyl-urea functionalized PPI dendrimers by
Meijer et al.44,45 In the current case, hostꢁguest complexation
studies were carried out with DPy0ꢁ2 and TMDPy0ꢁ2 dendri-
mers, and with benzenesulfonate (BS), naphthalene-2,6-disulfo-
nate (NDS), anthraquinone-2,6-disulfonate (AQDS) and pyra-
nine (Pyr) as guest molecules. The complexation progress was
followed on host protons shifts.
Figure 1. 1H NMR spectrum of (a) TMDPy1 and (b) DPy1 with
chemical shift assignment. Dotted pink circles indicate the protons
monitored during titration; the inset table shows the host protons
monitored for different guests.
1H NMR spectra of the TMDPy1 and DPy1 dendrimers
dissolved as PF6ꢁ salts in DMSO-d6 are presented in Figure 1.
The corresponding spectra of TMDPy0, DPy0, TMDPy2, and
DPy2 are similar (see Supporting Information). The complete
structural assignment of the protons within the repetitive sub-
units is possible, but no subshell specific shifts are observed. The
H1 resonances on the pyridinium (TMDPy and DPy) shows
large splitting, whereas H3 on the phenyl branching unit shows
only minor splitting. This points to a preferential conformation
of the methylene bearing H5 rendering the two H1 magnetically
nonequivalent, but the three H3 magnetically almost equivalent.
The resonances due to meta-H2 (TMDPy and DPy) at pyridi-
nium, as well as the protons at the trimethylene bridge [TMDPy
(H6 and H7)] show almost no splitting indicating reasonable
rotational freedom of the bridge methylene groups before
complexation. Since the region from δ 0.9 to 4 (trimethylene
(in the case of TMDPy) and t-Bu end groups) is dominated by
solvent peaks and tetrabutyl ammonium (counterion of the
anionic guests), we focused mainly on the changes in the region
δ 5.7ꢁ9.5.
consistently smaller shifts (0.01ꢁ0.04 ppm) for H2 and H4, and
larger splitting on the host protons was observed for increasing size
and charge on the guest molecules. Proton H5 shows beside the
splitting phenomenon peak broadening, possibly resulting from a
short spinꢁspin relaxation time T2.46,47 The similarity of the
splitting of the host proton resonances for different guest molecules
(even though all flat and anionic) is astonishing. It points to similar
locations of the anions within the host and to a similar freezing of the
host conformational freedom. In spite of the obvious hostꢁguest
1
interaction, no hostꢁguest H,1H COSY (correlation spectro-
scopy) signals could be detected.
The following section concerns 1H NMR studies of the flexible
TMDPy0ꢁ2 and the rigid DPy0ꢁ2 dendrimers titrated with the
different anions from the substoichiometric up to slight guest
excess. It is organized according to the guest molecules, each of
them interacting with generation 0ꢁ2 of the two dendrimer
types. Assuming charge interaction as the main driving force,
stoichiometric addition is defined as the point where equal
amounts of charges from host and guest are present in the
solution (charge ratio =1). For example, a dendrimer of genera-
tion 2 has 42 persistent positive charges, and thus 42 equiv of a
guest with 1 negative charge, or 21 guest equivalents with 2
negative charges and so on are necessary for total charge
compensation. Of importance for the following discussion is
Upon addition of guest anions the host protons H1, H2, H3, H4,
and H5 (pink circles in Figure 1) were splitting, indicating sub-
stantial interactions between host and guest. Because of interfering
guest proton resonances it was not possible to monitor the whole set
of indicator protons on the host for each guest molecule. Maximum
splitting (0.1ꢁ0.4 ppm) were observed for H1, H3, and H5, and
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Table 1. Theoretical Maximum Sub-Shell Capacities for
Mono-, Di-, and Trianions
spectral changes, it is obvious that “magic numbers” can even-
tually be interpreted within the theory of subshell occupancies
(eq 1-2).
gen. 0
ss 0
gen. 1
ss 1
gen. 2
The experimental recognition of a discontinuity and its
discriminative value with respect to the different filling scenarios
is not straightforward. For the following we define discontinuities
as the start of 1H NMR peak splitting or a prominent change in
the slope of host peak shifts upon guest addition. We use a
triangle to indicate such a discontinuity in case of an end-point
and circles in case of a filled subshell. Full lines are used if the
discontinuity is well developed, dotted circles and triangles, if
their identification is difficult. We stretch these symbols horizon-
tally in order to include the exp. value and its interpretation
(additive subshell capacity, end point) under the surface of the
symbol. Thus, the weaker and broader a symbol appears, the
weaker is its interpretative power. The interpretation of magic
numbers as additive subshell capacities relies obviously on
complete complex formation (no free guest), and on significant
differences in subshell-specific association constants.
Σ
ss 0
Σ
ss 0
ss 1
ss 2
Σ
Aꢁ
A2ꢁ
A3ꢁ
6
3
2
6
3
2
6
3
2
12
6
18
9
6
3
2
12
6
24
12
8
42
21
14
4
6
4
the concept of subshells. A subshell (ss) is defined as the space
occupied by bipyridinium moieties with identical number of
bonds from the dendrimer center, i.e. a generation 0 dendrimer
has a single ss, whereas generations 1 and 2 have 2 and 3
subshells, respectively. Theoretical maximum subshell capacities
for mono-, di-, and trianionic guests (Aꢁ, A2‑, A3‑) in our
dendrimers are as listed in Table 1.
The above-mentioned stoichiometric addition for total charge
compensation corresponds to the sum of subshell capacities (∑).
In the framework of subshells, the inside-out filling of guest
molecules (A) into a second generation dendrimer means that
upon substoichiometric equivalent additions first subshell 0, then
subshell 1 and finally subshell 2 (identical with end point
titration) are filled. For an outside-in scenario the subshell filling
sequence is reversed. The number of added equivalents for a
change in subshell filling is represented in the following equa-
tions for dendrimers of generation 2 and guests with charge ꢁ1
to ꢁ3:
i. Interaction with AQDS. Selected 1H NMR regions showing
the host protons H1, H2/H3 and H5 for all 6 dendrimers
TMDPy0ꢁ2 and DPy0ꢁ2 as a function of added dianionic AQDS
equivalents are shown in Figure 2 (notably, the left panel in
Figure 2 has been published earlier34 and is shown here solely for
the purpose of comparison). No precipitation is observed during
the titration except above the point of total charge compensation,
where haziness starts. For TMDPy2, changes become small
above full charge compensation, indicating an end point situa-
tion, whereas in case of DPy0ꢁ2 spectral changes tend to
continue after the theoretical end point. This may be related to
a better fit of AQDS onto the host site, i.e., trimethylene-spaced
bipyridinium in TMDPy0ꢁ2 as compared to 4,40-bipyridinium in
DPy0ꢁ2. Prominent changes occur at the same number of added
equivalents AQDS independent of the dendrimer type and its
generation, e.g. Three equiv AQDS added leads to splitting of H5
in TMDPy1 and TMDPy2, or after 9 equiv added, collapse of the
splitting of H1 in TMDPy1 and TMDPy2 is observed. Thus, the
“magic numbers” 3, 9, and 21 are read out for both dendrimer
types with AQDS as a guest, which is consistent with an inside-
out filling scenario (eq ꢁ1). Furthermore, addition of 3 equiv of
AQDS to TMDPy0 (end point) has a similar effect on H1 and H5
when added to TMDPy1 and TMDPy2 (subshell filling) or on
H5 for DPy0ꢁ2(black arrows). This behavior is a typical “inner
most shell localized” response of protons.
inside-out for Aꢁ 6 f 18 f 42
outside-in for Aꢁ 24 f 36 f 42
inside-out for A2ꢁ 3 f 9 f 21
outside-in for A2ꢁ 12 f 18 f 21
inside-out for A3ꢁ 2 f 6 f 14
outside-in for A3ꢁ 8 f 12 f 14
ð1Þ
ðꢁ1Þ
ð2Þ
∑
∑
∑
∑
∑
∑
ðꢁ2Þ
ð3Þ
ðꢁ3Þ
Generation 1 dendrimers shows the same pattern except that
the last column is missing, and generation 0 dendrimers can
obviously only give an end point. If there would be no preference
for any subshell, i.e., assuming no gradient, the filling would be
governed purely by probability, i.e., by the sites available in the
different subshells as compared to the total amount of coordina-
tion sites. Intermediate cases governed by statistics and a gradient
are also conceivable. Thus, the experimental problem is to find a
spectroscopic method which discriminates filled and empty
subshells during a titration experiment. This is done by looking
at discontinuities in a plot of the spectroscopic observable vs
added number of guest aliquots. In the studies of Yamamoto
et al.,29 it is the appearance of a new isosbestic point in UV/vis
titration, and in our study, it is the appearance of a new 1H NMR
splitting distance (jump) or in some cases, a change of slope (δ vs
guest conc.). In both studies completion of a subshell filling is
related to the start/end and not the midpoint of the discontinuity
in the titration.
ii. Interaction with NDS. NDS is dianionic as AQDS, but
slightly smaller. A detailed study of the NMR shifts of the host
protons H1 and H5 for all dendrimers TMDPy0ꢁ2 and DPy0ꢁ2
as a function of added dianionic NDS equivalents is shown in
Figure 3. No precipitation is observed during the titration.
Changes observed above full charge compensation are more
pronounced as compared to AQDS, but still indicating an end
point situation at least for TMDPy0ꢁ2. H5 discontinuities at
subshell fillings are better structured in DPy as compared to the
TMDPy dendrimers. Beside the above-discussed “inner most
sub-shell response” a localized response from the second shell
can be observed (magenta arrows). The observed “magic num-
ber” sequence 3, 9, 21 is read out and interpreted again as an
inside-out filling mechanism, according to eq ꢁ1.
iii. Interaction with Monoanions. Trifluoromethanesulfonate
(TFMS) and benzenesulfonate (BS) were chosen as monoanion-
ic guest molecules and checked for their interaction with the
dendrimers using the same technique (Figure 4). According to
We have called such discontinuities on the aliquot axis “magic
numbers”. Even without knowing the physical background of the
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1
Figure 2. Plot of H NMR peaks H1, H3 and H5 of TMDPy0ꢁ2 vs AQDS equivalent additions: [TMDPy0] = 3.2 mM; [TMDPy1] = 1.1 mM;
[TMDPy2] = 0.48 mM (left 3 columns) adapted from the literature;34 and DPy0ꢁ2 vs AQDS equivalent additions; [DPy0] = 3.3 mM; [DPy1] =
1.2 mM; [DPy2] = 0.5 mM (right 3 columns) in DMSO-d6. Key: dotted lines, dendrimer subshell capacities; circles, subshell appearance; triangles, end
point appearance; black arrows, innermost shell localized response.
eq 1 the magic numbers 6, 18, 42 are expected for the inside-out
filling. TFMS additions do not affect the NMR spectrum of the
dendrimers at all. The PF6ꢁ ion, which is present as a counterion
in all dendrimers from the synthesis and the added TFMS do not
interact closely with the dendrimer. Benzenesulfonate addition,
on the other hand reveals that its interaction with the dendrimer
is important (Figure 4). Less structured titration curves and
smaller splitting distances are observed as compared to the two
dianionic guests. In the case of TMDPy0ꢁ2, prominent changes
of the H1 occur at or near the theoretical end points, whereas in
the case of DPy0ꢁ2 end points are not easily identified (triangles,
Figure 4). The innermost subshell appears as “magic number”
6 (mostly offset toward smaller aliquot concentrations) on most
indicator protons in both dendrimer series (H1, H2 and H5,
circles in Figure 4). The second subshell changes are again subshell
specific (magenta arrows) and manifest themselves mainly on the
H5 resonance. This finding points to the same mechanism, i.e. the
general inside-out filling.
iv. Interaction with Pyr. The titration curves of TMDPy and
DPydendrimerswiththe trianion pyranineareshown inFigure 5.
According to eq 2 the “magic numbers” 2, 6, and 14 are expected
for an inside-out filling mechanism. Only H5 could be monitored
because of interfering absorptions of the guest molecule, and
unfortunately generation 2 precipitated above 8 equiv of guest
addition. Splitting of H5 is twice the amount observed for
dianions. This is probably related to the increased conforma-
tional freezing of the dendrimer structure, as a single bipyridi-
nium recognition site cannot account for total charge com-
pensation of a trianionic guest requiring partial assistance of a
second subunit. The end points in generation 0 and 1 are
recognizable (the generation 2 end point was not measurable).
Subshell filling is nicely reflected for the innermost shell in
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Figure 3. Plot of H1 and H5 dendrimer peaks of TMDPy0ꢁ2 vs NDS equivalent additions: [TMDPy0] = 3.2 mM; [TMDPy1] = 1.1 mM; [TMDPy2] =
0.48 mM; DPy0ꢁ2 vs NDS equivalent additions; [DPy0] = 3.3 mM; [DPy1] = 1.2 mM; [DPy2] = 0.5 mM. Key: dotted lines, dendrimer shell capacities;
circles, subshell appearance; triangles, end-point appearance; magenta arrows, second shell localized response.
generations 1 and 2, but the second subshell in the generation 2
dendrimers does not show up. The observed “magic number” 2 is
still indicative for an inside-out filling mechanism (eq 2). How-
ever, as one of the reviewer mentions, the mutual fit of host and
guest charges is less clear, and other filling scenarios can be
envisaged.
The results of the titration experiments are summarized in
Table 2. Notably, in the Table we represent the experimental
subshell capacities in contrast to the additive subshell capa-
cities (magic numbers).
In conclusion, the monoanion BS, the dianions AQDS and NDS,
as well as the trianion Pyr show all more or less well-defined titration
end points, except for some extension into the excess guest region.
This happens mainly for guests which do not fit well onto the
pyridinium coordination sites. An excellent mutual fit is observed
between AQDS and the dicationic subunits in TMDPy0ꢁ2. This is
similar as the results of Balzani et al. using eosin2‑ and DPy0ꢁ2 with
different peripheral groups.12 In contrast to their work based on
fluorescence quenching, we are able to observe additionally the
subshell filling. Subshell filling can be as sharp as the end point
titration, and from the sequence of subshell capacities appearing in
the titration we have established a general shell-by-shell inside-out
filling mechanism for pyridinium based dendrimers and the anionic
organic guests presented here.
NMR titration have further been probed by electrochemical
methods. In Figure 6, the cyclic voltammograms (CVs) of the
nonelectroactive TMDPy0 (0.5 mM) in DMSO/0.1 M TBA.PF6
with 1ꢁ6 equiv electroactive AQDS (1 equiv = 0.17 mM,
corresponding to 1 of 3 available coordination sites in TMDPy0)
are represented. Free and complexed AQDS deliver different
oxidation peaks, the complexed AQDS showing a retarded
oxidation (slow electron transfer). The anodic peak currents of
complexed and free AQDS are representative for their respective
concentrations. If the peak currents are plotted against the
equivalents of added AQDS (Figure 7.II), a well-defined end
point of 3 can be read. The slight deviations from the extra-
polated dotted and dashed lines indicate ca. 90% complexation
for the third guest molecule (eq ꢁ2).
½TMDPy ꢁ AQ DS3ꢂ
K30=0
¼
½TMDPy ꢁ AQ DS2ꢂ ꢀ ½AQ DSꢂ
0:45 ꢀ 10ꢁ3
¼
¼ 18 ꢀ 103 Mꢁ1
0:05 ꢀ 10ꢁ3 ꢀ 0:05 ꢀ 10ꢁ3
Using the concentrations defined in Figure 6 and assuming
90% complexation a value of K30/0 = 18 ꢀ 103 Mꢁ1 results (for
further information on K values see Supporting Information).
Notably, K30/0 is the association constant for the third and last
guest molecule pick-up of the generation 0 dendrimer and the
Coulombic interaction is only 2 plus/2 minus. The association
c. Electrochemical End Point Titrations. Different hostꢁ
1
guest interactions observed in the preceding paragraph by H
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Figure 4. Plot of H1, H2, H3/H4, and H5 1H NMR peaks (TMDPy0ꢁ2 and DPy0ꢁ2) vs benzenesulfonate (BS) addition: [TMDPy0] = 3.2 mM;
[TMDPy1] = 1.1 mM; [TMDPy2] = 0.48 mM; [DPy0] = 3.3 mM; [DPy1] = 1.2 mM; [DPy2] = 0.5 mM. Key: dotted lines, dendrimer subshell
capacities; triangle, end point; circles, subshell fillings.
constants for the first and second AQDS (K10/0 and K20/0) are
therefore supposed to be larger (see peak current growth for the
first and second addition indicating stoichiometric complex-
ation). From the same reasoning the higher generation den-
drimers are expected to have even larger association constants.
Changing the electrolyte concentration (tetrabutylammonium
hexafluorophosphate) in the range from 0.025 to 0.2 M has
only a minor effect on the association constants (not shown),
on the second reduction of the dendrimer (not shown). The
phenomenon could be related to the expulsion of NDS upon
reduction of the host.
If the amount of NDS is further increased, E0 decreases
0
steadily (ca. 10 mV per decade, its origin is not clear). As the
fourth addition is sitting on the dotted line in Figure 7, we assume
again complete association with three guest molecules present.
An accurate quantification of the K30/0 value is not possible, but
its minimum value is >10 ꢀ 103 Mꢁ1
.
ꢁ
indicating that PF6 has an association constant orders of
magnitude smaller.
The electroactive dendrimer DPy0 was titrated with the
d. Molecular Modeling. The observed inside-out guest filling
scenario is astonishing. Molecular dynamics calculations (using
the MM+ force field) were carried out to shine light on the
background of this mechanism.
nonelectroactive NDS yielding a minor but reproducible jump
0
0
of the first formal reduction potential (E ) for the first, second
and third guest molecules, i.e., up to the stoichiometric
amount is observed (Figure 7). The same behavior is observed
Force field modeling MM+ implemented on Hyperchem 8.0848
was used to judge the structure and size of the dendrimers as a
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Figure 5. Plot of H5 1H NMR-peak of TMDPy (a)) and DPy (b)) dendrimers vs added Pyr equivalents: [TMDPy0] = 3.2 mM; [TMDPy1] = 1.1 mM;
[TMDPy2] = 0.48 mM; [DPy0] = 3.3 mM; [DPy1] = 1.2 mM; [DPy2] = 0.5 mM in DMSO-d6. Key: dotted lines, dendrimer subshell capacities; black
circles, appearance of ss(0) in generation 2.
Table 2. Observed and Theoretical Sub-Shell Capacities
no. of guest anions in the 1st |2nd | 3rd dendrimer subshella
monoanion
BS
dianions
trianion
Pyr
NDS
exp. observedd
AQDS
exp. observedd
hosts
Dpy0
exp. observedd
model
model
model
exp. observedd
model
∼6ꢁ9c
6
2ꢁ3c
3
3c
3|6c
3
2
2
Dpy1
∼4ꢁ6|∼12
∼5|∼12|ꢁ
4ꢁ6c
6 |12
6|12|24
6
3|6
3|6
3|6
2|3ꢁ5
2|-b
2|4
2|4|8
2
Dpy2
3|6|∼12c
3|6|12
3
∼2|∼6|∼12c
3|6|12
3
TMDPy0
TMDPy1
TMDPy2
2ꢁ3
3c
2ꢁ3
2|4ꢁ5
2|-b
∼6|∼12c
∼6|1ꢁ2|24
6|12
2ꢁ3|∼6
2ꢁ3|6|12
3|6
∼3|6ꢁ8c
3|6ꢁ8|∼12
3|6
2|4
6||12|24
3|6|12
3|6|12
2|4|8
a Individual subshell capacities, in contrast to eq 1ꢁ2, where additive subshell capacities are used. b Precipitation. c Guest excess. d Interpretation of results
in Figure 2-5
function of the generation before and after guest complexation.
MD calculations were used with the same force field to visualize
the pick-up of the anionic guest molecules and thefinal localization
of the guest molecules within the subshells. Notably, these are gas
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Figure 6. (I) Cyclic voltammetry (CV) of nonelectroactive TMDPy0
(2.5 ꢀ 10ꢁ6 mol, 0.5 mM) in DMSO/0.1 M TBA.PF6 with 1ꢁ6 equiv
electroactive AQDS (1 equiv = 0.83 ꢀ 10ꢁ6 mol = 2.5 ꢀ 10ꢁ6/3 mol =
1 site of 3 available in TMDPy0); CVs showing oxidation of free and
TMDPy0-complexed AQDS at different potentials. (II) Analysis of the
peak currents of free (circles) and complexed AQDS (squares) vs
AQDS-additions yielding ca. 90% complexation for 3 AQDS added.
Figure 8. Equilibrated situations from MM+ MD calculations on
TMDPy1ꢁ2 and DPy1ꢁ2 in the presence of selected guest anions.
cations and reduce this effect under experimental conditions, but
an open structure is still ensured.50
The generation 0 dendrimers adopt a flat disk structure, whereas
generation 1 and especially 2 have dumbbell structure, all of them
displaying an open architecture ensuring free access for a guest
molecule at any pyridinium site (in contrast to higher generation
PAMAM dendrimers with only partial protonation51). The ap-
proximate radii of the long axis of the ellipsoids together with those
calculated from the exp. diffusion coefficients from DOSY experi-
ments are in the Supporting Information. We have shown earlier
that TMDPy0ꢁ2 contract upon addition of AQDS using DOSY.
Unfortunately with the new systems described here, such measure-
ments were not possible, because of large fluctuation in the
diffusion coefficient values leading to large error margins (not
reproducible). Upon addition of guest anions outside of the
dendrimer periphery “in silico” the anions move toward the
dendrimer obviously driven by the long-range electrostatic field
established by the many point charges on the pyridinium ions. The
first contact with a pyridinium charge is not necessarily their final
destination. If the bathing temperature is high enough, they start
generally to walk along the side chains toward the center of the
dendrimer until they find their final place usually at the innermost
free site. Upon stepwise addition of aliquots and waiting for the
thermalized situation allows us to simulate the titration curve. The
mono- or dianions are generally first filling the innermost coordi-
nation sphere, and then the second and third. It is astonishing to
observe that in many cases our experimental results are 1:1
mimicked by the calculations. This is represented in Figure 8 by
the end situations of MD calculations with different dendrimer
types, and different generations in combination with different
anions. The detailed analyses, as well as the 12 movies showing
the complete approach and final complexation are available in the
Supporting Information.
0
0
Figure 7. Formal reduction potential (E ) from CV’s (not shown) of
electroactive DPy0 (2.5 ꢀ 10ꢁ6 mol, 0.5 mM) in DMSO/0.1 M TBA.
PF6 with 1ꢁ24 equiv nonelectroactive NDS (1 equiv = 0.83 ꢀ 10ꢁ6 mol
= 2.5 ꢀ 10ꢁ6/3 mol =1 site of 3 available in DPy0.
phase calculation neglectingsolvent and counterions and therefore
the charge interaction (each pyridinium site carries +1, and each
sulfonate oxygen ꢁ0.33), a fact that was counterbalanced by
relatively high bath temperatures during MD equilibration. More
sophisticated MD calculations on the guest location within a
dendrimer are known, but they do not concern electrostatic
binding.49 In the absence of counterions our dendrimers extend
their branches as far as possible from each other because of the
repulsive charge interaction of the naked pyridinium sites. Non-
dissociated counterions and a polar solvent would shield the
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Complexation in the outermost coordination shell is probably
less efficient (i) because there is no additional positive charge in a
next outer shell that can fold back to complex a negatively
charged guest in the next inner shell,34 (ii) because of steric
hindrance induced by the peripheral t-Bu group, and (iii) the
electrostatic attractor in the center is stronger than in the
branches or in the periphery. The latter can be explained by
the larger motional freedom of the dendrimer subunits in the
periphery as compared to those close to the center. Identical
dendrimer subunits in the center move slower than in the
periphery because of the different inertial masses involved. The
corresponding pyridinium charges are therefore well localized
over time in the center of the dendrimer, whereas they are
smeared over space and time in the periphery. This phenomenon
results in a stronger electrostatic attractor in the center of the
dendrimer. Definitely, this is only fulfilled at generations far
below the limiting dendrimer generation. Back-folding of outer
branches onto an inner guest, and thereby favoring the inside-out
mechanism, is also mainly in case of an open structure con-
ceivable. Backfolding was observed during simulation and per-
sists in the equilibrated situation; and it was detected experi-
mentally from an increase of the diffusion coefficient upon small
counterion addition. Further anion additions reopen the back
folded branches as observed in silico and experimentally.34
25 °C using CD3CN or DMSO-d6 as a solvent and internal reference. All
chemical shifts are reported in parts per million (δ, ppm) with respect to the
internal standard. DOSY spectra were recorded on a Bruker Avance III 500
MHz spectrometer at 25 °C using DMSO-d6 as a solvent. Diffusion
1
measurements were performed using a H NMR pulsed-gradient experi-
ment: the simulated spinꢁecho sequence which leads to the measurement
of the diffusion coefficient D, where D is the slope of the straight line
obtained when ln(I) is displayed against the gradient-pulse power’s square
according to the following equation: ln(I) = ꢁγ2G2Dδ2(Δꢁδ/3), where
Iis the relative intensity of a chosen resonance, γis the proton gyromagnetic
ratio, Δ is the intergradient delay (60 ms), δ is the gradient pulse duration
(varied between 1.5 to 5 ms), and G is the gradient intensity.
Elemental analyses were performed on Elementar Vario Micro cube
instrument.
Electrochemical Titrations. Cyclic voltammetry was carried out
at room temperature in a standard three-electrode cell using DMSO/0.1
M tetrabuyl ammonium hexafluorophosphate as solvent/electrolyte.
The working electrodes were a glassy carbon disk (area = 0.066 cm2,
Metrohm 6.0804.010) and the counter electrode was a Pt wire. The
reference electrode was a Ag/AgCl/KCl (3 M) electrode (Metrohm,
6.0724.140) separated by a salt bridge containing DMSO/0.1 M
tetrabuyl ammonium hexafluorophosphate. Prior to each experiment,
the working electrode was carefully polished with alumina powder and
rinsed with distilled water. Potentiostatic control was provided by a
PGSTAT 20 potentiostat from AUTOLAB connected to the cell and
controlled by a PC running under GPES for Windows, Version 4.2
0
0
(ECO Chemie 1995). Formal potentials (E ) were calculated from
3. CONCLUSIONS
cathodic and anodic peak potentials in CV’s according to E0 = (Epc
+
0
Flexible and rigid pyridinium dendrimers are sequentially
complexed with 4 different types of mono-, di- and trianionic
guest molecules in a stepwise inside-out, shell-by-shell fashion.
The proof is based on the sequential appearance of the subshell
capacities in NMR titrations. Anions with matching symmetry
show more or less sharp transitions (δ, ppm) of the 1H chemical
shift at the end point and at subshell fillings. The same finding
was reported earlier by Yamamoto for a very different dendrimer
but also equally distributed coordination sites.29
The same “inside-out” filling scenario was also found by gas
phase MM+ MD calculations, pointing to a simple reason behind
the phenomenon. Backfolding of outer branches and electro-
static attractors pointing toward the center of the dendrimer were
visually identified. The central attractors develop because the
charges in the periphery are in fast motion and therefore smeared
as compared to the charges in the center.
Epa)/2. The scan rate was 100 mV/s.
1H NMR Titration. The titration was carried out on a Bruker 250/
500 MHz spectrometer in DMSO-d6 by using a solution of dendrimer
([TMDPy/DPy] = 0.4ꢁ3.3 mM) at 298 K and adding aliquots of the
anion which was more concentrated than the host solution. The
concentration of the dendrimer remained constant throughout the
experiment.
Synthetic Procedures. Synthesis of 1-(4-tert-Butylbenzyl)-4-
(pyridin-4-yl)pyridinium Hexafluorophosphate (E PF6). 4,4-Bipyridine
(1.72 g, 11 mmol) and 4-tert-butylbenzyl bromide (1 g, 4.4 mmol) were
ꢀ
3
dissolved in 50 mL DCM, refluxed for 2 h. The obtained yellow
precipitate was filtered, washed with DCM and dried. The yellow solid
thus obtained is dissolved in minimal quantity of water, precipitated with
3 M NH4PF6, filtered, washed with water and dried to yield 1-(4-tert-
butylbenzyl)-4-(pyridin-4-yl)pyridinium hexafluorophosphate as a pale
white powder (1.65 g, 84%). 1H NMR (250 MHz, CD3CN): δ,
ppm 8.87 (s, 4H), 8.33 (s, 2H), 7.79 (s, 2H), 7.50 (dd, 4H), 5.74 (s,
2H), 1.34 (s, 9H). 13C NMR (63 MHz, CD3CN): δ, ppm 154.5, 153.3,
151.1, 144.8, 141.2, 130.1, 128.9, 126.5, 126.3, 121.9, 63.9, 34.4, 30.4.
Anal. Calcd for C21H23F6N2P: C, 56.25; H, 5.17; N, 6.25. Found C,
55.89; H, 5.16; N, 6.21.
Besides their theoretical impact, our findings open a way to fill
a dendrimer shell-by-shell inside-out with different guests. The-
oretically, one could sequentially complex different guest mol-
ecules and try to irreversibly fix them in a second step, creating a
radial chemical gradient within a dendrimer that was originally
seemingly without any gradient.
DPy0 6PF6. P0 3PF6 (0.2 g, 195 μmol) and 4-tert-butylbenzyl
3
3
bromide (0.27 g, 1.2 mmol) were dissolved in 20 mL of CH3CN, and
the mixture was stirred at 80 °C for 2 d. The solution was cooled,
filtered, washed with CH3CN and dried. It was then dissolved in
MeOH:H2O (1:1) mixture, precipitated with 3 M NH4PF6, filtered,
’ EXPERIMENTAL SECTION
Materials. All starting materials and solvents were purchased from
Sigma-Aldrich and used without further purification. All reactions were
performed under dry conditions. For the hostꢁguest interaction studies,
commercially available Benzenesulfonic acid sodium salt (BS), Anthra-
quinone-2,6-disulfonic acid disodium salt (AQDS), Naphthalene-2,6-
disulfonic acid disodium salt (NDS) and pyranine trisodium salt (Pyr),
washed and dried to yield DPy0 6PF6 as a pale yellow powder (0.28
3
g, 75%). 1H NMR (500 MHz, DMSO-d6): δ, ppm 9.50 (d, J(H,H) =
6.5 Hz, 6H), 9.35 (d, J(H,H) = 6 Hz, 6H), 8.75 (d, J(H,H) = 7 Hz,
6H), 8.71 (d, J(H,H) = 6.5 Hz, 6H), 7.78 (s, 3H), 7.53 (dd, 12H), 5.92
(s, 12H), 1.28 (s, 27H). 13C NMR (125 MHz, DMSO-d6): δ,
ppm 152.8, 150.1, 149.3, 146.4, 146.2, 135.9, 131.6, 131.2, 129.2,
127.6, 127.5, 126.6, 63.9, 63.2, 34.9, 31.4. Anal. Calcd for
DMSO-d6 were ion-exchanged using TBA Br in DCM/H2O and the
3
obtained tetrabutylammonium derivatives were NMR pure [slight
excess of TBA Br was noticed].
3
Characterization and Measurements. 1HNMR, 13CNMR, and
C72H78F36N6P6 3H2O: C, 44.32; H, 4.34; N, 4.31. Found C,
3
DEPT spectra were recorded on Bruker 250/500 Avance spectrometer at
44.35; H, 4.36; N, 4.39.
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DPy1 18PF6. P1 6PF69 (0.15 g, 65 μmol) and E PF6 (0.22 g, 492 μmol)
were dissolved in 50 mL of CH3CN, and th emixture was stirred at 80 °C
for 4 d. The solution was cooled, filtered, washed with CH3CN and dried. It
was then dissolved in MeOH:H2O (1:1) mixture, precipitated with 3 M
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Aranzaes, J.; Astruc, D. Chem. Commun. 2007, 5093.
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3
3
3
NH4PF6, filtered, washed and dried to yield DPy1 18PF6 as a pale yellow
3
powder (0.26 g, 74%). 1H NMR (500 MHz, DMSO-d6) δ ppm 9.45 (m,
36H), 8.73 (m, 36H), 7.79 (m, 12H), 7.53 (dd, 24H), 5.93 (s, 36H), 1.28
(s, 54H). 13C NMR (125 MHz, DMSO-d6) δ ppm 152.8, 150.1, 149.4,
146.4, 146.2, 135.8, 131.6, 131.2, 129.2, 127.6, 127.5, 126.6, 63.9, 63.2, 34.9,
31.4. Anal. Calcd for C192H198F108N18P18 9H2O: C, 41.71; H, 3.93; N,
3
4.56. Found C, 41.64; H, 3.80; N, 4.49.
9
DPy2 42PF6. P2 18PF6 (0.2 g, 32 μmol) and E PF6 (0.20 g, 455
3
3
3
μmol) were dissolved in 50 mL CH3CN, stirred at 80 °C for 4 d. The
solution was cooled, filtered, washed with CH3CN and dried. It was then
dissolved in MeOH:H2O (1:1) mixture, precipitated with 3 M NH4PF6,
filtered, washed and dried to yield DPy2 42PF6 as a pale yellow powder
3
(0.27 g, 67%). 1H NMR (500 MHz, DMSO-d6) δ ppm 9.44 (m, 84H),
8.74 (m, 84H), 7.80 (m, 30H), 7.53 (dd, 48H), 5.93 (s, 84H), 1.28
(s, 108H); 13C NMR (125 MHz, DMSO-d6) δ ppm 152.8, 150.1, 149.7,
149.4, 146.6, 146.4, 146.2, 135.8, 131.6, 131.2, 129.2, 127.6, 127.5, 126.6,
63.9, 63.3, 34.9, 31.4; Anal. Calcd for C432H438F252N42P42.21H2O: C,
40.90; H, 3.81; N, 4.64. Found C, 40.77; H, 3.91; N, 4.53.
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’ ASSOCIATED CONTENT
(25) Hecht, S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1047.
(26) Posocco, P.; Ferrone, M.; Fermeglia, M.; Pricl, S. Macromole-
cules 2007, 40, 2257.
(27) Imaoka, T.; Tanaka, R.; Arimoto, S.; Sakai, M.; Fujii, M.;
Yamamoto, K. J. Am. Chem. Soc. 2005, 127, 13896.
(28) Higuchi, M.; Tsuruta, M.; Chiba, H.; Shiki, S.; Yamamoto, K.
J. Am. Chem. Soc. 2003, 125, 9988.
S
Supporting Information. Text giving synthetic proce-
b
dures of the intermediates, spectroscopic and analytical data for
the intermediates, a table summarizing the physical parameters,
MM+ simulations with start and end situations, 12 movies
showing simulated complexations, and figures showing 1H, 13
C
and DEPT spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
(29) Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H.
Nature 2002, 415, 509.
(30) Nakajima, R.; Tsuruta, M.; Higuchi, M.; Yamamoto, K. J. Am.
Chem. Soc. 2004, 126, 1630.
’ AUTHOR INFORMATION
(31) Ochi, Y.; Fujii, A.; Nakajima, R.; Yamamoto, K. Macromolecules
2010, 43, 6570.
(32) Albrecht, K.; Yamamoto, K. J. Am. Chem. Soc. 2009, 131, 2244.
(33) Nakashima, T.; Satoh, N.; Albrecht, K.; Yamamoto, K. Chem.
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Corresponding Author
*E-mail: Lowalder@uos.de.
’ ACKNOWLEDGMENT
M.K thanks Graduate College 612 funded by the DFG and
University of Osnabrueck for financial assistance.
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