Solvent- and guest-responsive self-assembly of hamilton receptor tethered bis(merocyanine) dyes

Article abstract of DOI:10.1002/chem.201002159

A novel supramolecular building block (8) that consists of a Hamilton receptor and two merocyanine dyes has been synthesized, and the self-assembly based on orthogonal hydrogen bonding and dipolar interactions has been studied in detail. Different self-as

Full text of DOI:10.1002/chem.201002159

DOI: 10.1002/chem.201002159
Solvent- and Guest-Responsive Self-Assembly of Hamilton Receptor
Tethered Bis(merocyanine) Dyes
Ralf Schmidt, Shinobu Uemura, and Frank Wꢀrthner*[a]
Abstract:
A
novel supramolecular
tration. Moreover, this system is highly
responsive toward molecular stimuli
such as merocyanine molecules with
the barbituric acid motif that are
bound by the Hamilton receptors. De-
tailed UV/Vis absorption studies pro-
vided insight into isodesmic or cooper-
ative steps during the self-assembly of
8 into different species. The size of the
aggregates in solution and the mor-
phology on substrates have been ex-
plored by a combination of dynamic
light scattering (DLS), atomic force mi-
croscopy (AFM), and TEM investiga-
tions.
building block (8) that consists of a
Hamilton receptor and two merocya-
nine dyes has been synthesized, and
the self-assembly based on orthogonal
hydrogen bonding and dipolar interac-
tions has been studied in detail. Differ-
ent self-assembled species, including
oligomers, polymers, and inverted mi-
celles could be observed upon variation
of the solvent polarity and the concen-
Keywords: cooperative effects · di-
polar aggregation · dyes/pigments ·
hydrogen bonds · self-assembly
Introduction
To create artificial supramolecular systems that emulate
the formation of defined complex architectures and have
the possibility of undergoing structural changes as found in
nature, directed noncovalent interactions are needed. The
most important directional supramolecular synthons found
in nature are based on hydrogen bonds.[10] Accordingly, it is
not surprising that hydrogen bonds have been extensively
utilized for the construction of finite and infinite supra-
molecular architectures.[11] Other noncovalent forces are far
less directional. However, our recent work on the aggrega-
tion of dipolar merocyanine dyes suggests directional
dipole–dipole interactions that lead to defined antiparallel
dimers,[12] which constitute a predictable supramolecular
unit.[13] As a consequence, the orthogonal character of hy-
drogen bonds and dipolar interactions might enable structur-
ally well-defined assemblies and responsive materials. Al-
though examples for well-defined architectures based on
such interplay of hydrogen-bonding sites and dipolar mero-
cyanine dyes have already been demonstrated,[14–16] novel
functionalities such as responsiveness towards environmen-
tal changes remained unexplored. For this purpose, we have
designed a system (8, Scheme 1) that contains two dipolar
merocyanine dyes tethered by a Hamilton receptor[17] and
investigated the solvent- and guest-responsive self-assembly
of this novel molecular system.
Nature consummately employs noncovalent interactions be-
tween a limited set of supramolecular building blocks for
precise structural control of sophisticated macromolecular
architectures that provide unrivaled functionality.[1] Beside
the formation of a certain topology, several biological poly-
mers are capable of undergoing structural alterations be-
tween defined species as a result of environmental changes
or the presence of molecular stimuli.[1,2] Subtle variations in
their structure decisively influence the functions of these
molecules. Examples include helix-coil,[3] b-sheet-to-coil
transitions in peptides,[4] and the denaturation of proteins[5]
and RNA[6]. The folding of biopolymers—as, for example,
proteins—is known to be highly cooperative.[5,7] Structural
changes were also found for the folding of synthetic poly-
mers or their collapse into globular conformations due to
solvophobic effects,[8] and for block copolymers that sponta-
neously self-assemble into well-defined micelles.[9]
[a] R. Schmidt, Dr. S. Uemura, Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg, Institut fꢀr Organische Chemie
and Rçntgen Research Center for Complex Material Systems
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax : (+49)931-31-84756
E-mail: wuerthner@chemie.uni-wuerzburg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002159.
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FULL PAPER
Scheme 1. Synthesis of Hamilton receptor tethered bis(merocyanine) 8 and chemical structure of guest 9.
Results and Discussion
8 in tetrahydrofuran/methylcyclohexane (10:90 vol%, THF/
MCH, c=1.0ꢃ10ꢀ4 m) were measured by MALDI-TOF and
peaks that corresponded to the protonated cationic mono-
mer ([M+H]+) and sodium adduct ([M+Na]+) as well as
dimer ([M2+H]+), trimer ([M3+H]+), and traces of tetramer
([M4+H]+) were observed (see Figure S3 in the Supporting
Information). The self-assembly of Hamilton receptor teth-
ered bis(merocyanine) 8 was further studied by UV/Vis
spectroscopy in THF/MCH mixtures of different composi-
tion. The solvent polarity strongly affects the thermodynam-
ic equilibrium between different self-assembled species ob-
served for these dyes as shown previously by solvent-depen-
dent UV/Vis studies for a variety of bis(merocyanine) dyes
in THF/MCH mixtures.[13] Due to the relatively high polarity
of pure THF (er =7.52), Hamilton receptor tethered bis-
(merocyanine) dye 8 is present predominantly in its mono-
meric form (further denoted as A) in this solvent with a
Synthesis: Hamilton receptor tethered bis(merocyanine) dye
8 was synthesized according to the route depicted in
Scheme 1. Etherification of commercially available 3-hy-
droxyisophthalic acid methyl ester 1 with trialkoxybenzyl
chloride 2, which was prepared from gallic acid methyl ester
in three steps according to literature procedures,[18] afforded
diester 3 that bears a trialkoxybenzyl group in 96% yield.
Amidation of this diester with monolithiated 2,6-diamino-
pyridine afforded diamine 4 in 44% yield. Carboxylic acid
functionalized merocyanine 7 was synthesized by a Knoeve-
nagel-type condensation reaction of pyridone 6, which was
obtained according to literature procedures,[19] with the re-
spective aldehyde 5[20] in 64% yield. The coupling of mero-
cyanine 7 with diamine 4 by amidation using 2-(1H-7-aza-
benzotriazol-1-yl)-1,1,3,3-tetramethyl uronium hexafluoro-
phosphate (HATU) and diisopropylethylamine (DIPEA) as
coupling reagents in DMF afforded the desired Hamilton re-
ceptor ligated bis(merocyanine) 8 in 42% yield. The de-
tailed synthetic procedures and structural characterization
data for the unknown compounds 3, 4, 7, and 8 are given in
the Experimental Section. Merocyanine 9 with a barbituric
acid acceptor group for the formation of sixfold hydrogen
bonding to the Hamilton receptor was synthesized according
to literature procedures and was used in molecular stimuli
experiments with bis(merocyanine) 8.[15]
characteristic narrow cyanine-type absorption band at lmax
=
538 nm (Figure 1).[21] Upon increasing the amount of unpo-
lar MCH (er =2.02), this chromophore aggregates on ac-
count of the increased Coulomb forces between highly dipo-
lar dye molecules (mg =14 D).[12] Two distinct transitions can
be recognized by quasi-isosbestic points at 517 and 496 nm,
respectively (see Figure S4 in the Supporting Information).
The first transition to a species with a hypsochromically
shifted band at 510 nm (further denoted as B) occurred
upon increasing the ratio of MCH and was most pronounced
in THF/MCHꢁ40:60 vol% at a concentration of 1.0ꢃ
10ꢀ4 m. This band could be assigned to H-type aggregated
dimers of excitonically coupled chromophores,[12] which indi-
cates the formation of a supramolecular polymer (or oligo-
Solvent-induced self-assembly: The first indication of the
self-aggregation of 8 was obtained by mass spectrometry.
Samples prepared by solvent evaporation from solutions of
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F. Wꢀrthner et al.
Figure 1. UV/Vis absorption spectra of monomers
100:0 vol%, solid line), and self-assembled species
A
B
(THF/MCH=
(THF/MCH=
40:60 vol%, dashed line) and C (THF/MCH=10:90 vol%, dotted line)
emerging from dye 8 in various THF/MCH mixtures (c(8)=1.0ꢃ10ꢀ4 m)
at 298 K. The inset shows the reduced viscositiy hred of 8 in THF/MCH=
40:60 vol% at different concentrations.
mer) on account of the bifunctionality of dye 8. Further in-
crease of the MCH content in solution led to the formation
of a second species (further denoted as C) with a broader
absorption band that is indicative of a high heterogeneity of
the local dye–dye contacts (“amorphous” state). Similarly,
broad spectra have been previously observed for the applied
aminothiophene oxopyridine (ATOP) chromophores in
amorphous photorefractive films.[21] The corresponding a50
values for the different aggregated species are summarized
in the Supporting Information (Table S2). Notably, the sol-
vent ratio for the formation of different species is dependent
on the total monomer concentration of 8 (see Figure S6 in
the Supporting Information).
An initial screening of the aggregate sizes by means of ca-
pillary viscosity measurements revealed an unexpected be-
havior (Figure 1, inset). Thus, upon increasing the concen-
tration, the viscosity hred first increases as expected for a
growing chain of a supramolecular polymer, but then de-
creases once a concentration of 0.2 mgmLꢀ1 (which corre-
sponds to 1.1ꢃ10ꢀ4 m) is exceeded. This unexpected behav-
ior is indicative of the formation of a new species C with a
homogeneous size, which is smaller than that of B.
Figure 2. a) Concentration-dependent UV/Vis spectra of
8 in THF/
MCH=30:70 vol% at 298 K from c=2.5 ꢃ10ꢀ8 to 1.0ꢃ10ꢀ5 m. Arrows in-
dicate changes upon increasing concentration. b) Apparent absorption
coefficient at 533 nm plotted against cT and the result of the nonlinear re-
gression analysis based on the isodesmic model for the formation of B.
nied by the emergence of a weak, new hypsochromically
shifted transition centered at around 510 nm, which can be
ascribed to H-type supramolecular aggregates (Figure 2).
The isodesmic model (also called equal K model)[22] could
successfully be applied to the transition from A to B. Ac-
cording to this model, the molar fraction of the self-assem-
bled species can be expressed by Equation (1):[22]
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
cmon
cT
2KcT þ 1 ꢀ 4KcT þ 1
¼ 1 ꢀ
2K2c2T
ð1Þ
aagg ¼ 1 ꢀ amon ¼ 1 ꢀ
Self-assembly of B: The self-assembly of the aggregated spe-
cies B was initially investigated by H NMR spectroscopic
in which amon and aagg are the molar fraction of the mono-
meric and self-assembled species, respectively, cmon and cT
are the concentration of the monomeric species in solution
and the total concentration of all dyes in the system, respec-
tively, and K is the equilibrium constant. This mathematical
expression can be related to the UV/Vis absorption spectra.
By applying Equation (1) in Equation (2), Equation (3) can
be obtained:
1
experiments in deuterated THF/MCH (50:50 vol%) mix-
tures (see Figure S7 in the Supporting Information). The
spectra of B showed broad signals in the aromatic region,
thereby indicating aggregation of the merocyanine dyes and
conformational flexibility of the diacyl diaminopyridine moi-
eties.
Concentration- and temperature-dependent UV/Vis ex-
periments were performed to gain insight into the self-
assembly processes in solution. Figure 2 shows the concen-
tration-dependent spectral changes of bis(merocyanine) 8 at
concentrations between 2.5ꢃ10ꢀ8 and 1.0ꢃ10ꢀ5 m in THF/
MCH (30:70 vol%) mixtures at ambient temperature. De-
pletion of the absorption maximum at 538 nm is accompa-
eðcTÞ ꢀ eagg
ð2Þ
ð3Þ
aagg ¼ 1 ꢀ
emon ꢀ eagg
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2KcT þ 1 ꢀ 4KcT þ 1
2K2c2T
eðcTÞ ¼
ðemon ꢀ eaggÞ þ eagg
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Bis(merocyanine) Dyes
FULL PAPER
in which emon, eagg, and e(cT) denote the molar absorption co-
efficients of free monomers A, self-assembled dyes, and the
apparent absorption at the concentration cT, respectively. By
fitting the data obtained from the concentration-dependent
series in THF/MCH (30:70 vol%) to Equation (3), an equi-
librium constant of K=(3.9ꢂ0.5)ꢃ105 mꢀ1 was obtained
(Figure 2b). This equilibrium constant compares well to di-
merization constants reported for simple merocyanine dyes
that bear similar donor and acceptor subunits.[12b] The
number-averaged degree of polymerization DPN and the
weight-averaged degree of polymerization DPW according to
the concentration-dependent isodesmic model could be cal-
culated as 7 and 13, respectively, for a 1.0ꢃ10ꢀ4 m concentra-
tion of 8 (see Table S3 in the Supporting Information).
investigations. A solution of 8 (1.0ꢃ10ꢀ5 m) in THF/MCH
(30:70 vol%) was drop-cast on highly ordered pyrolytic
graphite (HOPG) and tapping-mode AFM images were re-
corded at room temperature under ambient conditions (Fig-
ure 4a). The images disclosed the formation of long fibrils
Temperature-dependent UV/Vis studies were subsequent-
ly performed to gain further insight into the self-assembly
processes. A more sophisticated analysis[23] of temperature-
dependent UV/Vis absorption spectra confirms the isodes-
mic self-assembly mechanism (Figures S9 and S11 in the
Supporting Information) and provides the melting tempera-
ture of the aggregate TM =301 K and the thermodynamic
parameters of molar enthalpy of aggregate formation DH=
Figure 4. a) AFM height image of spin-coated samples of B (4000 rpm,
c(8)=1.0ꢃ10ꢀ5 m, THF/MCH=30:70 vol%, 298 K) onto HOPG.
b) Transmission electron micrograph of B after evaporation of the sol-
vent (c(8)=1.0ꢃ10ꢀ4 m, THF/MCH=40:60 vol%) on a carbon-coated
copper grid.
ꢀ88.6 kJmolꢀ1
and
change
in
entropy
DS=
ꢀ203 Jmolꢀ1 Kꢀ1.[24] Comparison of the enthalpy and entro-
py contributions to the Gibbs free-energy changes for aggre-
gation leads to the conclusion that the self-assembly process
is enthalpy-driven, whereas it is entropically disfavored.
The size of supramolecular species B was determined by
dynamic light scattering (DLS) experiments. A relatively
broad distribution of particle sizes that ranged from around
80 to 350 nm with a maximum at 147 nm was found
(Figure 3), which is in accordance with our structural assign-
ment of supramolecular polymers. This comparatively high
degree of polydispersity can be expected for supramolecular
polymers in which a rapid dynamic equilibrium between
free and associated species takes place.
with lengths between 80 and 300 nm with a maximum at
143 nm (see Figure S22a in the Supporting Information).
These dimensions are in accordance with those determined
by DLS experiments. Cross-section analysis revealed an
average distance between two fibrils of (5.5ꢂ0.5) nm,
whereas the rather small height of (0.29ꢂ0.04) nm of these
structures can be attributed to the presence of simple supra-
molecular chains. Transmission electron microscopy images
of B (Figure 4b) performed onto carbon-coated copper grids
showed larger network-like structures with strands up to
several micrometers in length. Apparently, further aggrega-
tion takes place during the evaporation process, thus leading
to merging individual fibrils. Structural models that explain
the organization of these supramolecular polymers on
HOPG by the peripheral alkyl chains were derived by mo-
lecular modeling (Figure S23a). Dye aggregation leads to
the formation of polymeric chains, whereas the alkyl moiet-
ies are directed perpendicular to the propagation direction.
The distance between two chains suggests interpenetration
of the alkyl chains, thus leading to adhesive linear struc-
tures.
Convincing evidence for the formation of linear supra-
molecular aggregates was obtained by atomic force micros-
copy (AFM) and transmission electron microscopy (TEM)
The above-described results indicate that species B con-
sists of supramolecular polymers based on antiparallel ag-
gregation of the merocyanine dye units of ditopic monomer
8 with high equilibrium constants in solution.
Self-assembly of C: Similarly to the polymeric structure B,
the next level of structural hierarchy, that is, the self-assem-
bly of species C was thoroughly investigated in solution and
on substrates by a number of techniques. Upon increasing
1
the MCH ratio, the peak broadening of the H NMR spectra
Figure 3. Hydrodynamic radius Rh distributions obtained from DLS ex-
was even more pronounced for C in comparison with poly-
meric species B, thereby suggesting that a distinctly new spe-
cies is forming (see Figure S7 in the Supporting Informa-
ꢀ3
&
*
)
periments on B ( ) (c(8)=1.0ꢃ10 m, THF/MCH=50:50 vol%), C (
(c(8)=1.0ꢃ10ꢀ3 m, THF/MCH=10:90 vol%), and
10ꢀ3 m, THF/MCH=20:80 vol%).
D
(
(c(8)=1.0ꢃ
~
)
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F. Wꢀrthner et al.
tion). Concentration-dependent UV/Vis experiments were
initially performed to evaluate in detail the self-assembly
processes. Experiments in THF/MCH=20:80 vol% in a
concentration range of c=1.3 ꢃ10ꢀ6 to 1.3ꢃ10ꢀ4 m revealed
Scheme 2. Self-assembly of complementary oligomers, of which one can
form a closed structure.[7]
The total concentration cT may be expressed as a function
of the monomer concentration c1, the equilibrium constant
K of the linear chain formation, the effective molarity (EM)
as defined in Equation (4), the parameter for allosteric co-
operativity s, and the number of molecules N in the oligo-
mer that forms the closed structure [Eq. (5)]:[7]
Kc
K
ð4Þ
ð5Þ
EM ¼ N
c1½sꢀ1ð1 ꢀ Kc1Þ þ Kc1ð2 ꢀ Kc1Þꢃ
2
cT ¼
þ KNðEMÞcn1
2
sꢀ1ð1 ꢀ Kc1Þ
The molar fraction of aggregated species aagg can subse-
quently be described by Equation (6):[7]
2
Nꢀ1
sꢀ1ð1 ꢀ Kc1Þ KðEMÞðKc1Þ
þ Kc1
aagg
¼
2
Nꢀ1
sꢀ1ð1 ꢀ Kc1Þ ½1 þ KðEMÞðKc1Þ ꢃ þ Kc1ð2 ꢀ Kc1Þ
ð6Þ
Since it is exhaustive to solve Equation (5) for c1 and insert
the result into Equation (6), curves for different combina-
tions of K, EM, s, and N were calculated and plotted against
the normalized concentration scale logK’cT for better com-
parison. Here, K’=1/c50 and c50 corresponds to the concen-
tration at which aagg =0.5. A relation of the experimental
data points to the ordinate can be established by taking into
account the Equation (7):
Figure 5. a) Concentration-dependent UV/Vis spectra of
8 in THF/
MCH=20:80 vol% at 298 K from c=1.3 ꢃ10ꢀ6 to 1.3ꢃ10ꢀ4 m. b) Frac-
tion of aggregated molecules aagg plotted as a function of logK’cT accord-
ing to the isodesmic model (dashed curve) and to the self-assembly of a
closed oligomer (solid curve) and plot of the experimental data at
507 nm for the formation of C after manual fit of the line shape. The
exact parameter values used to calculate the solid curve in Figure 5b are
given in the Supporting Information (see Figure S13).
eðcTÞ ꢀ eaggC
ð7Þ
aagg ¼ 1 ꢀ
eaggB ꢀ eaggC
in which eaggB, eaggC, and e(cT) denote the molar absorption
coefficients of species B, species C, and the apparent absorp-
tion at the concentration cT, respectively.
a transition from B to C (Figure 5a). The isodesmic model
was not applicable to this transition as evidenced by a sharp
decrease of the apparent absorption at 507 nm upon reach-
ing a certain concentration (Figure 5b, dashed curve). Such
abrupt changes indicate a cooperative process. Application
of the K2–K model[22] and the more general model of Gold-
stein and Stryer[25] yielded good fits. However, it seems sci-
entifically more appropriate to use a model that explicitly
comprises the formation of a closed system from an oligo-
meric species since further investigations by DLS, NMR
spectroscopy, and AFM suggest the formation of a discrete
species (vide infra). Therefore, a model was used that in-
cludes the possibility to describe the formation of closed sys-
tems.[7] The self-assembly of complementary oligomers, of
which one can form a closed structure, is schematically de-
picted in Scheme 2.
The calculated curves were fitted manually for the best
match to a plot of the experimental data points for aagg
against log K’cT (Figure 5b). The abrupt changes in the ab-
sorption spectra suggest high values for EM and N. Because
of the large number of parameters and manual fitting of the
data, it would not make much sense to give the exact values
for K, EM, s, and N. Instead, a qualitative discussion of the
results based on the order of magnitude of the respective
parameter is provided. The best fit was obtained for K
values on the order of 105 mꢀ1, which is in accordance with
the equilibrium constant observed for the formation of B
from monomers A. High values for EM, likewise on the
order of 105 m, could be estimated. Such high effective mo-
larities strongly favor the formation of the closed species
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Bis(merocyanine) Dyes
FULL PAPER
rather than further growth of the chain. Almost no allosteric
cooperativity seems to be present (sꢁ1). The number of
molecules in the oligomer that tends to form the closed
structure was found to be on the order of 20. Thus, taking
into account Equation (4), the equilibrium constant Kc for
the formation of C can be roughly estimated to be 5ꢃ108.
The aggregate size of species C in solution could be deter-
mined by DLS performed on 1.0ꢃ10ꢀ3 m solutions of 8 in
THF/MCH=10:90 vol%. Surprisingly, a remarkably narrow
distribution of particles that ranged from 2 nm (monomer)
to around 10 nm with a maximum at 4.2 nm was observed
(Figure 3), which clearly supports the formation of aggre-
gates with discrete size. Similar particle sizes were found by
diffusion-ordered spectroscopy (DOSY) NMR experiments
at the same concentration and solvent composition (see Fig-
ure S8 in the Supporting Information).
The morphology of aggregates C formed on substrates
was investigated by AFM imaging on HOPG. The images
revealed discrete globular objects with diameters of (9.0ꢂ
2.0) nm and heights of (2.8ꢂ0.3) nm (Figure 6), which are in
Figure 7. a) UV/Vis titration of barbiturate dye 9 (as host at a constant
concentration of c=1.0ꢃ10ꢀ5 m) with increasing amounts of bis(merocya-
nine) 8 as guest in chloroform at 298 K. b) Titration isotherm for a 1:1
complex model at a wavelength of 381 nm.
gain insight into the receptor properties of 8, thus revealing
a binding constant of Kass =5.2ꢃ106 mꢀ1 in CHCl3 (Figure 7).
For practical reasons (detection of spectral changes in the
visible region), the role of the host and guest were reversed
in these experiments. From this Kass value, a binding con-
stant of approximately 1.2ꢃ1013 mꢀ1 can be estimated in pure
MCH by assuming a linear free-energy relationship with re-
lated Hamilton receptor host–guest systems.[15] Therefore,
almost complete complex formation can be safely assumed
in mixtures with high MCH ratios even under dilute condi-
tions.
Subsequent to the formation of a stoichiometric host–
guest complex between 8 and 9, the aggregation of AG (AG
denotes the monomeric complex 8:9) could again be studied
by concentration-dependent UV/Vis spectroscopy, which re-
vealed a clear transition between two species with a defined
isosbestic point at 510 nm (Figure 8). Upon increasing the
concentration, the band of the monomeric dye at 538 nm de-
creased at the expense of a hypsochromically shifted aggre-
gate band at lmax =500 nm that is characteristic of an anti-
parallel p-stacked arrangement of ATOP chromophores in a
dimeric unit as reported previously.[12] Accordingly, due to
the bifunctionality of AG, a supramolecular polymer (de-
noted as D) formed. It is noteworthy that barely any spec-
tral changes could be observed within the band at 365 nm,
which corresponds to the guest dye, thereby indicating that
the self-assembly is driven by the dipolar aggregation of 8.
Figure 6. a) AFM height image of spin-coated samples of C (4000 rpm,
c(8)=1.0ꢃ10ꢀ5 m, THF/MCH=5:95 vol%, 298 K) onto HOPG. b) Cross-
section along the yellow line in the previous panel.
accordance with the values determined by DLS and DOSY
experiments. Notably, the size of the aggregates did not
change upon changing the substrate, in particular HOPG
versus Si wafer (Figure S20 in the Supporting Information).
The overall results suggest that aggregate C is formed as a
result of a solvent-induced collapse of B as soon as the olig-
omers reach a certain size and the polarity of the applied
solvent is low. The cooperative self-assembly, the size in so-
lution, and the morphology on substrates suggest the forma-
tion of inverted micelles in which all polar dyes are encapsu-
lated and shielded from the nonpolar solvent by the aliphat-
ic tails of the molecules.
Guest-induced self-assembly: The capability of the Hamil-
ton receptor tethered bis(merocyanine) dye 8 to host-appro-
priate barbiturate guest molecules through a pattern of six
hydrogen bonds has been explored to evaluate the influence
of guest binding in the stability of the above-described
supramolecular structures. Constant host UV/Vis titration
experiments[26] with guest molecule 9 were performed to
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F. Wꢀrthner et al.
Figure 9. a) AFM height image of spin-coated samples of D (4000 rpm,
c(8)=1.0ꢃ10ꢀ5 m with 1 equiv of guest 9, THF/MCH=5:95 vol%, 298 K)
onto HOPG. b) Transmission electron micrograph of D after evaporation
of the solvent (THF/MCH=20:80 vol%, c(8)=4.0ꢃ10ꢀ4 m) on a carbon-
coated copper grid.
height of (0.36ꢂ0.05) nm, cross-section analysis revealed
almost identical results for the two polymeric aggregates.
The chain lengths of D were found to be smaller than those
observed for B. In accordance with the DLS studies, fibrils
with average lengths between 10 and 130 nm were obtained
with a maximum at 40 nm (see Figure S22b in the Support-
ing Information). Kinks in supramolecular structures indi-
cate the formation of polymers. TEM images of D also re-
vealed network-like structures with lengths of the strands up
to micrometers but with the width being much smaller than
that for B (Figure 9b). A structural model that explains the
organization of D on HOPG was obtained by molecular
modeling (Figure S23b in the Supporting Information) and
suggests a similar binding motif as for B. Chain propagation
is a result of the aggregation of the dyes of ditopic host 8,
whereas interaction between the chains on HOPG may be
ascribed to interdigitation of alkyl chains.
Species D could also be obtained from inverted micelles
C, thereby demonstrating the responsiveness of these supra-
molecular systems to the presence of guest molecules. Addi-
tion of one equivalent of 9 to aggregates C (relating to the
total concentration of 8) in THF/MCH=5:95 vol% afforded
D after a period of several hours as revealed by UV/Vis
spectroscopy and AFM studies (see Figures S15 and S16 in
the Supporting Information).
Figure 8. a) Concentration-dependent UV/Vis spectra of AG in THF/
MCH=20:80 vol% at 298 K from c=1.2 ꢃ10ꢀ6 to 2.3ꢃ10ꢀ3 m. Arrows in-
dicate changes upon increasing concentration. b) Apparent absorption
coefficient at 533 nm plotted against cT and the result of the nonlinear re-
gression analysis based on the isodesmic model for the formation of D.
The isodesmic model could be applied with very high accu-
racy to the formation of this supramolecular polymer D
from complex AG (Figure 8b). By fitting the data obtained
from the concentration-dependent series, an equilibrium
constant of (2.8ꢂ0.1)ꢃ103 mꢀ1 was obtained, which is signifi-
cantly lower than that for the formation of B. This might be
ascribed to steric hindrance for the dipolar interaction
evoked by the binding of a guest molecule and small attrac-
tive dipole–dipole interactions between the dipolar guest
and the dyes covalently linked to the Hamilton receptor.
Temperature-dependent measurements revealed similar
changes as observed for the concentration-dependent series,
and almost identical equilibrium constants were calculated
(see Figures S10 and S12, and Table S4 in the Supporting In-
formation).
It is worth noting that the addition of guest 9 to polymers
B entailed the formation of mainly monomeric complex AG
as could be shown by UV/Vis and NMR spectroscopy (see
Figure S17 and S18 in the Supporting Information). Such be-
havior confirms the considerably lower equilibrium constant
for the formation D compared to B.
The size of D in solution for a 1.0ꢃ10ꢀ3 m solution of com-
plex AG in THF/MCH=20:80 vol% was determined by
DLS. Larger average particle sizes of 12 nm were obtained
for species D and they show a broad size distribution
(Figure 3). Particles with sizes that range from 2 nm (mono-
mer) up to 200 nm were found. Such dissemination is indica-
tive of the formation of supramolecular oligomers or poly-
mers, rather than discrete objects.
Similarly to species B, AFM images of D on HOPG
under air showed long fibrils with kinks (Figure 9a). With an
average distance between two fibrils of (5.0ꢂ0.4) nm and a
Summary of the self-assembly processes: The incorporation
of a hydrogen-bonding site as spacer unit between two dipo-
lar merocyanine dyes offered a new tool to switch between
different supramolecular architectures. The self-assembly
processes of Hamilton receptor tethered bis(merocyanine)
dye 8 explored by different methods as described above are
summarized in Figure 10. Starting from monomeric bis(mer-
ocyanine) dye in pure THF (A), the linear, polymeric spe-
cies B was reversibly formed upon decreasing the solvent
polarity, using 30–50 vol% MCH, depending on the total
13712
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Bis(merocyanine) Dyes
FULL PAPER
monomer concentration. The
formation of B was studied by
concentration- and tempera-
ture-dependent UV/Vis meas-
urements and properly agreed
with the isodesmic model. Fur-
ther decreasing the solvent po-
larity by increasing the ratio of
MCH to 90–95 vol%, again de-
pending on the concentration,
led to the solvent-induced col-
lapse of B to inverted micelles
C.
Concentration-dependent
UV/Vis studies revealed a co-
operative self-assembly mecha-
nism from species B to C.
Upon offering a complemen-
tary guest 9 to A, complex AG
was formed. Analogously to the
formation of B from A, the
linear oligomeric species
D
could be formed from AG upon
decreasing the solvent polarity.
Investigations on the self-
assembly of this complex re-
vealed an isodesmic self-assem-
bly mechanism, which ended up
with supramolecular oligomers.
Addition of guest molecules to
inverted micelles C led to the
transformation to supramolec-
ular polymers D, thus empha-
Figure 10. Schematic representation of the solvent- and guest-dependent self-assembly of Hamilton receptor
tethered bis(merocyanine) dyes 8. Starting with the monomer A in pure THF, supramolecular polymers B
were formed upon addition of MCH, which collapse into globular inverted micelles C in a highly unpolar envi-
ronment. Addition of guest 9 switched the inverted micelles back to a supramolecular polymer D, which was
also formed by self-assembly of complex AG.
sizing the responsiveness of the present system.
bly model. This unique combination of two orthogonal di-
rectional noncovalent interactions allowed adjustable organ-
ization upon environmental changes and molecular stimuli.
It is noteworthy that the observed self-encapsulation of the
dyes in inverted micelles C enables high dye concentrations
in unpolar media and, at the same time, low viscosities,
which are both highly attractive features for the coloration
of low-polarity commodity plastics such as polyethylene and
polypropylene.[27]
Conclusion
Hamilton receptor tethered bis(merocyanine) dye 8 is a re-
markably adaptive supramolecular building block. Depend-
ing on assessable parameters such as solvent polarity, con-
centration, and the presence of guest molecules, the forma-
tion of different supramolecular species can be triggered.
Lowering the solvent polarity yielded in the isodesmic for-
mation of supramolecular polymers composed of well-de-
fined dimeric dye entities. Upon further increase of the ratio
of nonpolar MCH, these polymers “collapsed” into inverted
micelles, the absorption spectra of which resembled those of
amorphous films of these chromophores.[21] This reorganiza-
tion process was proven to be cooperative, which is similar
to that observed for protein folding.[5,7] A cooperative
model that describes the formation of a closed species from
a particular oligomer was successfully applied to concentra-
tion-dependent UV/Vis spectra. More intricate supramolec-
ular polymers were formed by the addition of guest 9 to the
inverted micelles. These polymers, which incorporated mer-
ocyanine chromophore guest molecules, could also be built
up from complex 8:9 by following the isodesmic self-assem-
Experimental Section
Materials and methods: Solvents and reagents were obtained from com-
mercial suppliers and used without further purification, unless otherwise
stated. Trialkoxybenzyl chloride 2, thiophene 5, and pyridone 6 were syn-
thesized according to literature procedures.[18–20] Column chromatography
was performed on silica gel (Merck Silica 60, particle size 0.04–0.063 nm)
and thin-layer chromatography (TLC) was conducted on silica gel plates
(60 F254 Merck, Darmstadt). High-performance liquid chromatography
(HPLC) was carried out using a PU 2080 PLUS system (JASCO) with a
UV/Vis detector (UV 2077 PLUS) using a semipreparative SP 250/21
Nucleodur 100-7 C18 ec column (Macherey–Nagel) and purity-grade
“pa” solvents. Melting points were determined using a Bꢀchi Melting
Point B545 heating stage and are uncorrected. Solvents for UV/Vis ab-
sorption and AFM studies were of spectroscopic grade and used as pur-
chased. 1H NMR spectra were recorded using a Bruker Advance 400
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13713

F. Wꢀrthner et al.
spectrometer at 298 K, unless otherwise stated, with TMS or residual
nondeuterated solvent as internal standard. High-resolution ESI-TOF
mass spectrometry was carried out using a microTOF focus instrument
(Bruker Daltronik GmbH) in positive mode with CH3CN or CHCl3 as
solvent, unless otherwise stated.
tone/dry ice). Then tert-butyllithium (1.95 mL of a 1.5m solution in pen-
tane, 2.93 mmol) followed by diester 3 (500 mg, 0.586 mmol) in dry THF
(7 mL) were added dropwise. The resulting solution was stirred for 4 h at
ꢀ788C and an additional 16 h at room temperature. After the addition of
aqueous NaHCO3 (8 mL) and CHCl3 (6 mL), the aqueous phase was ex-
tracted with CHCl3 (3ꢃ5 mL). The combined organic layers were dried
over MgSO4, and the solvent was removed under reduced pressure. The
crude product was purified by column chromatography using silica and
CH2Cl2/MeOH=80:1 vol% as eluent to give pure 4 as a beige solid.
Yield: 257 mg (0.256 mmol, 44%); m.p. 1148C; 1H NMR (400 MHz,
CDCl3): d=8.79 (s, 4H; NH2), 8.07 (s, 1H; ArH), 7.73 (s, 2H; ArH), 7.67
(d, 3J=7.6 Hz, 2H; Hpyridine), 7.53 (t, 3J=7.9 Hz, 2H; Hpyridine), 6.65 (s,
UV/Vis experiments: UV/Vis absorption spectra were measured using a
Perkin–Elmer Lambda 950 UV/Vis spectrophotometer with a spectral
bandwidth of 0.20 nm and a scan rate of 141 nmminꢀ1 with conventional
quartz cells of 0.1–50 mm path length to cover a suitable concentration
range. Temperature was controlled using a PTP-1 Peltier element of
Perkin–Elmer.
Viscosity measurements: Solution viscosities were recorded in purity-
grade “pa” solvents at 258C with a capillary viscosimeter using the auto-
mated viscosity measuring device AVS 360 (Schott Gerꢁte GmbH) to
obtain reproducible run times. The effective capillary diameter was
0.64 mm. The setup was mounted in a thermostat, which was controlled
by circulating water. The measurements were performed from concen-
trated to diluted solutions. Dilution was achieved by using an automated
Titronic universal unit (Schott Gerꢁte GmbH). The reduced viscositiy
hred was obtained from Equation (8):
3
2H; ArH), 6.32 (d, J=8.1 Hz, 2H; Hpyridine), 5.07 (s, 2H; OCH2Ar), 3.98
3
3
(t, J=7.3 Hz, 4H; OCH2), 3.96 (t, J=7.4 Hz, 2H; OCH2), 1.84–1.71 (m,
6H; CH2), 1.52–1.42 (m, 6H; CH2), 1.40–1.17 (m, 52H; CH2), 0.88 ppm
(t, 3J=6.9 Hz, 9H; CH3); HRMS (ESI)+ (CH3CN/CHCl3 =1:1 vol%):
m/z: calcd for C61H95N6O6 [M+H]+: 1006.4915; found: 1006.4904.
Compound 7: Carboxylic acid functionalized pyridone
6 (1.11 g,
5.0 mmol) and 2-dibutylamino-5-formylthiophene 5 (1.20 g, 5.0 mmol) in
Ac2O (5 mL) were stirred at 908C for 2 h. After cooling down to room
temperature the solvent was removed under reduced pressure. The re-
sulting solid was recrystallized two times from MeOH to give pure 7 as a
red crystalline solid. Yield: 1.41 g (3.18 mmol, 64%); m.p.>3508C;
1H NMR ([D6]DMSO): d=8.06 (d, 3J=5.4 Hz, 1H; Hthiophene), 7.88 (s,
1H; Hmethine), 6.89 (d, 3J=5.2 Hz, 1H; Hthiophene), 4.05 (t, 3J=7.8 Hz, 2H;
NCH2CH2COOH), 3.62 (t, 3J=7.7 Hz, 4H; NCH2), 2.46 (s, 3H; CH3),
2.43 (t, 3J=7.8 Hz, 2H; NCH2CH2COOH), 1.75–1.61 (m, 4H; CH2),
1.43–1.20 (m, 4H; CH2), 0.94 ppm (t, 3J=7.5 Hz, 6H; CH3); HRMS
(ESI)+ (CH3CN/CHCl3 =1:1 vol%): m/z: calcd for C23H29N3O4S [M]+:
443.18733; found: 443.18728.
t ꢀ to 1
ð8Þ
hred
¼
to
c
in which t and t0 represent the run times of the dye solution and the pure
solvent, respectively, whereas c corresponds to the concentration of the
dye solution in gLꢀ1
.
Dynamic light scattering (DLS): DLS measurements were carried out at
q=908 and 258C using an ALV CGS-3 goniometer with an HeNe laser
(l=632.8 nm) and an ALV LSE-5004 correlator. Sample solutions
(c(8)=1.0ꢃ10ꢀ3 m for B, C, or D) were filtered into a dust-free vial
through a 0.45 mm hydrophobic polytetrafluoroethylene (PTFE) filter.
The correlation curve was fitted in a data point interval of 10–150 by
using the default DLS exponential g2(t) fit function with a target PROB1
parameter of 0.5.
Compound 8: Carboxylic acid functionalized merocyanine 7 (433 mg,
0.976 mmol), HATU (407 mg, 1.07 mmol), and DIPEA (202 mg, 265 mL,
1.56 mmol) in dry DMF (2 mL) under an argon atmosphere were stirred
for 20 min at room temperature. Diamin 4 (49.1 mg, 0.0488 mmol) in dry
DMF (0.5 mL) was added dropwise, and the resulting mixture was stirred
for 15 h at room temperature. The solvent was removed under reduced
pressure, and the residue was purified by column chromatography using
silica gel with CH2Cl2/MeOH=98.5:1.5 vol% as eluent to give pure 8.
Yield: 38.0 mg (20.5 mmol, 42%) and 11.9 mg (8.30 mmol, 17%) of mono-
acylated product. The purity (>99%) of product 8 was confirmed by
HPLC (reversed phase, MeOH/CH2Cl2 =7:3 vol%). M.p. 2068C;
1H NMR (400 MHz, CD2Cl2/[D4]MeOH=4:1 vol%): d=8.10 (s, 1H;
ArH), 7.94–7.92 (m, 2H; Hpyridine), 7.75 (s, 2H; ArH), 7.67–7.63 (m, 4H;
Hpyridine), 7.61 (d, 3J=5.6 Hz, 2H; Hthiophene), 7.54 (s, 2H; ArH), 6.63 (s,
Transmission electron microscopy (TEM): TEM measurements were per-
formed using a Siemens Elmiskop 101 electron microscope operating at
an acceleration voltage of 80 kV. For the observation of aggregates, a
drop of sample solutions (1.0ꢃ10ꢀ4 m solution of 8 in THF/MCH=
40:60 vol% and 4.0ꢃ10ꢀ4 m solution of 8 in THF/MCH=20:80 vol%
with one equivalent of 9) was placed on 100-mesh Formvar copper grids
coated with carbon.
Atomic force microscopy (AFM): AFM measurements were carried out
under ambient conditions by using a Veeco MultiMode Nanoscope IV
system operating in tapping mode under air. Silicon cantilevers (Olym-
pus, OMCLAC160TS) with a resonance frequency of around 300 kHz
were used. The 512ꢃ512 pixel images were collected at a rate of 2 scan
lines per second. Large-scale images were recorded at a scan rate of
1 Hz. Solutions of bis(merocyanine) dyes 8 in the respective THF/MCH
mixture and with and without barbiturate guest 9 were spin-coated onto
highly oriented pyrolytic graphite (HOPG, NanoTechnology Instruments,
Netherlands).
3
2H; Hmethine), 6.45 (d, J=5.2 Hz, 2H; Hthiophene), 5.06 (s, 2H; OCH2), 4.23
(t, 3J=7.6 Hz, 4H; NCH2CH2CON), 3.91 (t, 3J=6.44, 4H; OCH2), 3.86
(t, 3J=6.6 Hz, 2H; OCH2), 3.45 (t, 3J=7.7 Hz, 8H; NCH2), 2.67 (t, 3J=
7.3 Hz, 4H; NCH2CH2CON), 2.37 (s, 6H; CH3), 1.74–1.58 (m, 14H;
CH2), 1.57–1.34 (m, 6H; CH2), 1.33–1.12 (m, 64H; CH2), 0.87 (t, 3J=
7.3 Hz, 12H; CH3), 0.78 ppm (t, 3J=9.8 Hz, 9H; CH3); UV/Vis (THF):
lmax (emax, Lmolꢀ1 cmꢀ1)=538 nm (272ꢃ103); HRMS (ESI)+ (THF): m/z:
calcd for C107H149N12O12S2Na [M+Na]+: 1857.07486; found: 1857.07811.
Compound 3: 5-Hydroxyisophthalic acid dimethyl ester (130 mg,
0.619 mmol) and K2CO3 (102 mg, 0.736 mmol) were suspended in dry
DMF (1.5 mL) under an argon atmosphere and 3,4,5-tris(dodecyloxy)-
benzyl chloride 2 (0.50 g, 0.736 mmol) was added. The resulting mixture
was heated to 908C for 14 h. After removal of the solvent under reduced
pressure, the crude product was purified by column chromatography
using silica and CH2Cl2/MeOH=70:1 vol% as eluent to give pure 3 as a
white solid. Yield: 507 mg (0.594 mmol, 96%); m.p. 418C; 1H NMR
(400 MHz, CDCl3): d=8.29 (s, 1H; Hisophthalic acid ester), 7.84 (s, 2H; Hisophthalic
Acknowledgements
We gratefully acknowledge financial support for this work by the Volks-
wagen Foundation within the priority program “Complex Materials.”
ester), 6.63 (s, 2H; ArH), 5.03 (s, 2H; OCH2Ar), 4.01–3.94 (m, 6H;
acid
[1] a) K. A. Dill, Biochemistry 1990, 29, 7133–7155; b) J. M. Berg, J. L.
Tymoczko, L. Stryer, Biochemistry, 6th ed., W. H. Freeman, New
York, 2006.
[2] a) M. Beato, S. Chꢄvez, M. Truss, Steroids 1996, 61, 240–251;
b) S. R. Hammes, Proc. Natl. Acad. Sci. USA 2003, 100, 2168–2170.
[3] A. Chakrabartty, R. L. Baldwin, Adv. Protein Chem. 1995, 46, 141–
176.
OCH2), 3.94 (s, 6H; OCH3), 1.83–1.71 (m, 6H; CH2), 1.52–1.39 (m, 6H;
CH2), 1.38–1.19 (m, 52H; CH2), 0.88 ppm (t, 3J=6.9 Hz, 9H; CH3);
HRMS (ESI)+ (CH3CN/CHCl3 =1:1 vol%): m/z: calcd for C53H89O8
[M+H]+: 854.2894; found: 854.2887.
Compound 4: 2,6-Diaminopyridine (320 mg, 2.93 mmol) was dissolved in
dry THF (5 mL) under an argon atmosphere and cooled to ꢀ788C (ace-
13714
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Chem. Eur. J. 2010, 16, 13706 – 13715

Bis(merocyanine) Dyes
FULL PAPER
[4] a) H. L. Schenck, S. H. Gellman, J. Am. Chem. Soc. 1998, 120,
4869–4870; b) T. Kortemme, M. Ramirez-Alvarado, L. Serrano, Sci-
ence 1998, 281, 253–256.
[5] H. S. Chan, S. Bromberg, K. A. Dill, Philos. Trans. R. Soc. London
B 1995, 348, 61–70.
[6] D. E. Draper, Trends Biochem. Sci. 1996, 21, 145–149.
[7] C. A. Hunter, H. L. Anderson, Angew. Chem. 2009, 121, 7624–7636;
Angew. Chem. Int. Ed. 2009, 48, 7488–7499.
[8] a) C. Williams, F. Brochard, H. L. Frisch, Annu. Rev. Phys. Chem.
1981, 32, 433–451; b) S. Hecht, Mater. Today 2005, 8, 48–55; c) E. J.
Foster, E. B. Berda, E. W. Meijer, J. Am. Chem. Soc. 2009, 131,
6964–6966.
[9] M. Lee, B. K. Cho, W. C. Zin, Chem. Rev. 2001, 101, 3869–3892.
[10] R. Taylor, O. Kennard, Acc. Chem. Res. 1984, 17, 320–326.
[11] a) J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995;
b) J. H. K. K. Hirschberg, L. Brunsveld, A. Ramzi, J. A. J. M. Veke-
mans, R. P. Sijbesma, E. W. Meijer, Nature 2000, 407, 167–170; c) L.
Brunsveld, B. J. B. Folmer, E. W. Meijer, R. P. Sijbesma, Chem. Rev.
2001, 101, 4071–4097; d) L. J. Prins, D. N. Reinhoudt, P. Timmer-
man, Angew. Chem. 2001, 113, 2446–2492; Angew. Chem. Int. Ed.
2001, 40, 2382–2426; e) I. Huc, Eur. J. Org. Chem. 2004, 17–29;
f) L. Sꢄnchez, N. Martꢅn, D. M. Guldi, Angew. Chem. 2005, 117,
5508–5516; Angew. Chem. Int. Ed. 2005, 44, 5374–5382.
[12] a) F. Wꢀrthner, S. Yao, Angew. Chem. 2000, 112, 2054–2057; Angew.
Chem. Int. Ed. 2000, 39, 1978–1981; b) F. Wꢀrthner, S. Yao, T. De-
baerdemaker, R. Wortmann, J. Am. Chem. Soc. 2002, 124, 9431–
9447.
b) J. Schmidt, R. Schmidt, F. Wꢀrthner, J. Org. Chem. 2008, 73,
6355–6362.
[16] a) S. Yagai, M. Higashi, T. Karatsu, A. Kitamura, Chem. Mater.
2005, 17, 4392–4398; b) S. Yagai, M. Higashi, T. Karatsu, A. Kita-
mura, Chem. Commun. 2006, 1500–1502; c) S. Yagai, T. Kinoshita,
M. Higashi, K. Kishikawa, T. Nakanishi, T. Karatsu, A. Kitamura, J.
Am. Chem. Soc. 2007, 129, 13277–13287.
[17] S.-K. Chang, A. D. Hamilton, J. Am. Chem. Soc. 1988, 110, 1318–
1319.
[18] V. S. K. Balagurusamy, G. Ungar, V. Percec, G. Johansson, J. Am.
Chem. Soc. 1997, 119, 1539–1555.
[19] M. Segal (Hoechst AG), PCT EP 0169457(A1), 2000.
[20] D. Prim, G. Kirsch, J. F. Nicoud, Synlett 1998, 383–384.
[21] F. Wꢀrthner, S. Yao, J. Schilling, R. Wortmann, M. Redi-Abshiro, E.
Mecher, F. Gallego-Gomez, K. Meerholz, J. Am. Chem. Soc. 2001,
123, 2810–2824.
[22] a) R. B. Martin, Chem. Rev. 1996, 96, 3043–3064; b) D. Zhao, J. S.
Moore, Org. Biomol. Chem. 2003, 1, 3471–3491; c) Z. Chen, A.
Lohr, C. R. Saha-Mçller, F. Wꢀrthner, Chem. Soc. Rev. 2009, 38,
564–584; d) T. F. A. De Greef, M. M. J. Smulders, M. Wolffs,
A. P. H. J. Schenning, R. P. Sijbesma, E. W. Meijer, Chem. Rev. 2009,
109, 5687–5754.
[23] a) M. M. J. Smulders, M. M. L. Nieuwenhuizen, T. F. A. de Greef, P.
van der Schoot, A. P. H. J. Schenning, E. W. Meijer, Chem. Eur. J.
2010, 16, 362–367; b) P. van der Schoot, Supramolecular Polymers,
2nd ed. (Ed.: A. Ciferri), Taylor & Francis, New York, 2005, pp. 77–
106.
[13] a) F. Wꢀrthner, S. Yao, U. Beginn, Angew. Chem. 2003, 115, 3368–
3371; Angew. Chem. Int. Ed. 2003, 42, 3247–3250; b) A. Lohr, S.
Uemura, F. Wꢀrthner, Angew. Chem. 2009, 121, 6281–6284; Angew.
Chem. Int. Ed. 2009, 48, 6165–6168.
[14] a) F. Wꢀrthner, S. Yao, B. Heise, C. Tschierske, Chem. Commun.
2001, 2260–2261; b) L. J. Prins, C. Thalacker, F. Wꢀrthner, P. Tim-
mermann, D. N. Reinhoudt, Proc. Natl. Acad. Sci. USA 2001, 98,
10042–10045.
[24] For details concerning data evaluation, see the Supporting Informa-
tion.
[25] R. F. Goldstein, L. Stryer, Biophys. J. 1986, 50, 583–599.
[26] a) K. A. Connors, Binding Constants, Wiley, New York, 1987; b) H.-
J. Schneider, A. Yatsimirsky, Principles and Methods in Supramolec-
ular Chemistry, Wiley, New York, 2000.
[27] D. Horn, J. Rieger, Angew. Chem. 2001, 113, 4460–4492; Angew.
Chem. Int. Ed. 2001, 40, 4330–4361.
[15] a) F. Wꢀrthner, J. Schmidt, M. Stolte, R. Wortmann, Angew. Chem.
2006, 118, 3926–3930; Angew. Chem. Int. Ed. 2006, 45, 3842–3846;
Received: July 28, 2010
Published online: November 4, 2010
Chem. Eur. J. 2010, 16, 13706 – 13715
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13715