Hydrogen-bond-directed formation of supramolecular polymers incorporating head-to-tail oriented dipolar merocyanine dyes

Article abstract of DOI:10.1021/ma2004184

A self-complementary Hamilton-receptor-functionalized merocyanine dye has been synthesized by incorporating a hydrogen bonding receptor site to the donor moiety of a merocyanine chromophore that bears a barbituric acid acceptor unit. The optical and elect

Full text of DOI:10.1021/ma2004184

ARTICLE
pubs.acs.org/Macromolecules
Hydrogen-Bond-Directed Formation of Supramolecular Polymers
Incorporating Head-to-Tail Oriented Dipolar Merocyanine Dyes
Ralf Schmidt, Matthias Stolte, Matthias Gr€une, and Frank W€urthner*
Institut f€ur Organische Chemie and R€ontgen Research Center for Complex Material Systems, Universit€at W€urzburg,
Am Hubland, 97074 W€urzburg, Germany
S Supporting Information
b
ABSTRACT: A self-complementary Hamilton-receptor-func-
tionalized merocyanine dye has been synthesized by incorpor-
ating a hydrogen bonding receptor site to the donor moiety of a
merocyanine chromophore that bears a barbituric acid acceptor
unit. The optical and electro-optical properties of monomeric
dyes have been studied by UVꢀvis, fluorescence, and electro-
optical absorption spectroscopy in chloroform, 1,4-dioxane,
tetrahydrofuran, and dimethyl sulfoxide under dilute condition.
The self-assembly properties of this self-complementary mero-
cyanine in solution have been investigated by concentration-
and temperature-dependent UVꢀvis and one- and two-dimensional NMR spectroscopy. These studies revealed that the present
merocyanine dyes self-assemble in a head-to-tail fashion by forming six hydrogen bonds. The size of the assemblies in solution was
determined by dynamic light scattering and diffusion-ordered spectroscopy NMR investigations. In nonpolar solvent mixtures at
high concentrations, this merocyanine dye forms appreciably fluorescent gels (Φfl = 0.17) while the monomeric dye and self-
assemblies in solution are only weakly fluorescent (Φfl = 0.07).
’ INTRODUCTION
dye enforce an almost perpendicular orientation of the two planes of
the Hamilton receptor and the chromophore. Thereby, the antipar-
allel aggregation of the even more dipolar bimolecular complex is
impeded.14 On the basis of this concept, it appears feasible that self-
complementary, hydrogen-bonding receptor containing merocya-
nines would lead to supramolecular polymers by head-to-tail orienta-
tion of the dipolar dyes. Therefore, we have constructed the self-
complementary merocyanine dye 1 that consists of a Hamilton
receptor and a merocyanine chromophore bearing a barbituric acid
acceptor group, and studied in detail the self-assembly properties of
dye 1 (Scheme 1).
Highly directional noncovalent interactions such as hydrogen
bonding are known to facilitate the formation of organogels.15
Moreover, it has been shown previously that organogels can be
prepared from bis(merocyanine) dyes.2 However, such gels are
nonemissive because their supramolecular main chain is based on
antiparallel dimer aggregates of these dipolar chromophores
which exhibit H-type excitonic coupling of their transition
dipole moments. Strong quenching of the fluorescence is well-
established for such H-type dimer aggregates,16 and theoretical
interpretations for such quenching have been discussed in the
literature.17 Nevertheless, some exceptions of weakly emissive
H-aggregates are known in the literature that are rationalized
by rotational offsets of the transition dipole moments or coupling
For anisotropic dipolar molecules such as merocyanines,
usually the mutual antiparallel orientation in dimeric aggregates
is favored by electrostatic interactions.1 This characteristic
property of merocyanine dyes has been used to create supra-
molecular polymers by tethering two dyes via a spacer unit.2,3
Hydrogen-bonding interactions, in turn, have been extensively
used for creating supramolecular polymers and several reviews
have been published on this topic.4 Hamilton receptors have
been employed in various systems for molecular recognition,5
complexes enabling photoinduced electron-transfer studies,6 supra-
molecular polymers,7 supramolecular dendrimers,6b,8 foldamers,9
noncovalently cross-linked block copolymers,10 and binding of
barbiturates to surfaces.11 Structurally related systems based on
melamines were used to arrange merocyanine dyes via hydrogen
bonds and afforded a variety of complex architectures.12 Formation
of supramolecular sheet aggregates from self-complementary bis-
(acylaminopyridines) was also reported.13
The question arises whether antiparallel orientation of mero-
cyanine dyes can be circumvented and supramolecular polymers
of such dipolar dyes can be achieved by their head-to-tail orienta-
tion through hydrogen bonding. Recently, we have shown that
the antiparallel arrangement of merocyanines can be overcome in
hydrogen-bonded bimolecular complexes of Hamilton-receptor-
functionalized merocyanines containing a N-alkyl-substituted
barbiturate acceptor moiety and complementary merocyanines
bearing a barbituric acid unit.14 Two methyl substituents at the ortho
positions of the dihydropyridone heterocycles of the merocyanine
Received: February 22, 2011
Revised:
April 11, 2011
Published: May 02, 2011
r
2011 American Chemical Society
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Scheme 1. Concept for the Hydrogen-Bond-Directed Formation of Supramolecular Polymers Consisting of Self-Complementary
Merocyanine Dyes 1 Arranged in a Head-to-Tail Fashion and Schematic Representation of the Orientation of Dipole Moments
upon Self-Assembly
of the (forbidden) lowest energy excitonic transition to molecular
vibrations.12b,18 Organogels based on hitherto unknown hydrogen-
bond-mediated merocyanine dye assemblies with head-to-tail
arrangement of the dipolar chromophores are good candidates
for achieving more favorable emission properties owing to a
more suitable orientation of their transition dipole moments.
Therefore, we have also investigated the gelation properties of
Hamilton-receptor-functionalized merocyanine dye 1 and eluci-
dated their fluorescence properties at the different hierarchies of
self-assembly.
CH-acidic barbituric acid 9 yielded the desired Hamilton-
receptor-functionalized merocyanine dye 1 in 4%. The low yield
of the last step can partially be attributed to the demanding
purification process, comprising successive column chromatog-
raphy, gel permeation chromatography (GPC), and recrystalliza-
tion. Merocyanine dye guest molecule 10 equipped with a
barbituric acid acceptor group, reference Hamilton receptor 11
and Hamilton-receptor-functionalized merocyanine 12 with n-
butyl substituents in imide positions were prepared according to
the literature (structures are shown in Chart 1).14,19 Merocya-
nine 12 is a valuable reference system as it is, in contrast to newly
synthesized merocyanine 1, not self-complementary. The precursor 8
and the target compound merocyanine 1were characterized by NMR
spectroscopy (see the Supporting Information, Figures S1, S2)
and high-resolution mass spectrometry (HRMS, see Supporting
Information, Figures S3, S4), and for 1 MALDI-TOF mass
spectrum has also been measured (Supporting Information,
Figure S5). For the detailed structural assignment of monomeric
dye 1 by 1-D and 2-D NMR spectroscopy, see the section
Structural Elucidation by NMR Spectroscopy and the Support-
ing Information (Table S1).
’ RESULTS AND DISCUSSION
Synthesis. The Hamilton-receptor-functionalized merocya-
nine dye 1 was synthesized according to the route outlined in
Scheme 2. Diamide 7 was prepared according to the literature,
starting with the reaction of 5-aminoisophthalic acid 2 and
dehydroacetic acid 3 to pyridone hydrochloride 4.14 Esterifica-
tion of the latter resulted in diethyl ester derivative 5, which
could be transformed to diamide 7 with monolithiated 2,6-
diaminopyridine.14 Subsequent acylation of 7 with dodecanoyl
chloride afforded the precursor 8 for the target compound 1 in
42% yield. The Knoevenagel condensation reaction of 8 with
Spectroscopic Studies of Merocyanine 1 Monomers. In
order to investigate the optical and electro-optical properties of
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Scheme 2. Synthesis of Self-Complementary Hamilton-Receptor-Functionalized Merocyanine Dye 1
Chart 1. Structures of Merocyanine Dye Guest Molecule 10 Bearing a Barbituric Acid Acceptor Group, Reference Hamilton
Receptor 11 and Hamilton-Receptor-Functionalized Merocyanine 12
monomers of merocyanine dye 1, solvents of high polarity like
dimethyl sulfoxide (DMSO, εr = 46.5) and solvents of strong
hydrogen bond acceptor capability like 1,4-dioxane (εr = 2.25)
were used. The investigation of monomers of 1 in nonpolar
solvents is hampered by self-assembly of this self-complementary
system. The presence of monomers in deuterated DMSO was
λmax(DMSO) = 389 nm) and the absorption coefficient εis decreased
in more polar solvent (Figure 1). This solvatochromism may be
attributed to differences in the stabilization of the ground and excited
states by the solvent, and hence to the change of the dipole moment
difference Δμ = μe ꢀ μg (μe and μg denote the excited and ground
state dipole moments, respectively) upon optical excitation in solvents
of different polarity.21,22
1
confirmed by H NMR spectroscopy (for details, see NMR
section). Highly dilute solutions were employed to ensure the
presence of monomers in 1,4-dioxane (c = 5.3 ꢁ 10ꢀ7 M)20 and
CHCl3 (c = 1.4 ꢁ 10ꢀ7 M, T = 328 K). Merocyanine 1 is existent
as monomer in highly diluted CHCl3 solution at elevated
temperature as was attested by temperature-dependent fluores-
cence experiments (for details, see fluorescence section). The
UVꢀvis spectra of 1in 1,4-dioxane, CHCl3, and DMSO exhibit typical
absorption properties of monomeric merocyanine dyes. The intense
charge transfer (CT) absorption band shows negative solvatochro-
mism as this band is hypsochromically shifted with increasing solvent
polarity (λmax(1,4-dioxane) = 396 nm, λmax(CHCl3) = 392 nm,
Dipole moments of the ground state μg and the dipole moment
differences Δμ of merocyanine dye 1, and the reference compounds
10 and 12 in dilute 1,4-dioxane solution were determined by electro-
optical absorption (EOA) spectroscopy.23,24 A quantitative evalua-
tion of the optical and electro-optical data of these chromophores
provides the dipole moments μg, transition dipole moment μeg, and
Δμ (Table 1). Only minor differences of the CT absorption bands
of merocyanine dyes 1, 10, and 12 could be observed (see also
Figure S6 in the Supporting Information). The data shown in
Table 1 reveal that the covalent attachment of a Hamilton receptor
to the donor moiety of the merocyanine chromophore leads to a
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Figure 1. UVꢀvis absorption spectra of monomeric 1 in 1,4-dioxane
(black solid line, 298 K), CHCl3 (gray solid line, 328 K), and DMSO
(black dotted line, 298 K) and normalized fluorescence spectrum in
CHCl3 (gray dashed line, 328 K, λex = 370 nm).
Table 1. Dipole Moments and Optical Data of 1, 10, and 12
Determined by UVꢀvis and EOA Spectroscopy in 1,4-Diox-
ane at 298 Ka
λmax (nm) εmax (Mꢀ1 cmꢀ1
)
μeg (D)
μgb (D)
Δμb (D)
10
12
1
379
391
396
50600
56000
50800
6.5
7.2
6.9
10.1 ( 0.1 ꢀ3.9 ( 0.1
7.7 ( 0.1 ꢀ3.7 ( 0.1
7.9 ( 0.2 ꢀ4.3 ( 0.4
Figure 2. (a) Concentration-dependent UVꢀvis spectra of 1 in THF at
298 K at the concentration range from c = 4.7 ꢁ 10ꢀ4 M to 3.4 ꢁ 10ꢀ6 M.
Arrows indicate spectral changes upon dilution. (b) Fraction of associated
species Rass in THF (9, calculated from the apparent absorption
coefficient ε at 370 nm) and CHCl3 (b, calculated from the apparent
absorption coefficient at 375 nm) plotted against cT and the fitting curves
obtained by the nonlinear regression analysis based on the isodesmic
model.
a Values for compounds 10 and 12 are taken from ref 14a. b “Gas phase”
dipole moments were calculated by a solvent correction within the
approximations of Onsager’s continuum model.24 (1 D = 3.336 ꢁ
10ꢀ30 C m).
bathochromic shift of the absorption maximum (λmax) and an
increase of the transition dipole moment (μeg). The dipolar
character of the merocyanine decreases upon attachment of the
Hamilton receptor which is attributed to the partial dipole moments
of the Hamilton receptor subunit.25 However, the dipole difference
Δμ remains almost unaffected. The negative Δμ values of these
merocyanines are in accordance with the observation of nega-
tive solvatochromism and are in accordance with recent EOA
investigations of bimolecular complex 10:12 which have revealed
the formation of hydrogen-bond-mediated highly dipolar struc-
tures.14 Thus, self-complementary Hamilton-receptor-functiona-
lized merocyanine dye 1 constitutes a building block for the
creation of self-assembled linear polymers bearing merocyanine
dyes in the main chain as suggested in Scheme 1.
Self-Assembly Studies. First indication for the self-assembly
of Hamilton-receptor-functionalized merocyanine dyes 1 in the
gas phase was obtained by mass spectrometry. Mass peaks
observed in MALDI-TOF spectra of dye 1 correspond to
monomeric (m/z 944.2 ([M]þ); calcd. 943.5) and dimeric
(m/z 1888.1 ([M2]þ); calcd. 1888.3) species (see Supporting
Information, Figure S5). To gain insight into the self-assembly
properties of self-complementary merocyanine dye 1 in solution,
concentration- and temperature-dependent UVꢀvis spectro-
scopic studies were performed. Owing to the low solubility of
1 in 1,4-dioxane, no solutions of high concentration could be
prepared. Therefore, tetrahydrofuran (THF) was chosen for self-
assembly studies since for the formation of the structurally
related bimolecular complex 10:12 similar binding constants
were found in 1,4-dioxane and THF.14 The concentration-
dependent measurements in THF at 298 K reveal a hypsochro-
mic shift of the absorption maximum upon increasing the
concentration (Figure 2a). The observation of an isosbestic
point indicates an equilibrium between free and bound dye
molecules 1. Monomeric and self-assembled species may be
distinguished on the basis of their absorption maxima at λmax
=
396 and 382 nm, respectively. The absorption maxima of free and
bound species were extrapolated using eqs 1 and 2 (vide infra).
A hypsochromic shift was observed upon self-assembly at high
concentrations that can be ascribed to an effect exerted by the
receptor which is similar to that of solvents of different polarity
on a solvatochromic dye.14b,21,22 Similar absorption spectral
changes were observed for the formation of bimolecular com-
plexes between reference compounds 10 and 11.14
The isodesmic model26 could be applied to the formation of
supramolecular polymers from self-complementary dyes 1. Ac-
cording to this model, the molar fraction of associated species
R
ass can be expressed as a function of the total concentration cT
and the equilibrium constant K26
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2KcT þ 1 ꢀ 4KcT þ 1
2K2c2T
Rass ¼ 1 ꢀ
ð1Þ
R
ass can be calculated from the apparent absorption coefficient of
the solution ε(cT) and the absorption coefficients of monomer
εmon and assembly εass
εðcTÞ ꢀ εmon
Rass
ꢂ
ð2Þ
εass ꢀ εmon
The equilibrium constant K is defined according to:
½1n þ 1
ꢃ
1n þ 1 h 1n þ 1
;
K ¼
ð3Þ
½1nꢃ½1ꢃ
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Nonlinear regression analysis of the data obtained from the
concentration-dependent UVꢀvis measurements in THF at
certain wavelengths by employing eq 1 yielded an equilibrium
constant of K = (1.6 ( 0.3) ꢁ 104 Mꢀ1 (Figure 2b). The
equilibrium constant was averaged over the values deduced
at four different wavelengths (see Supporting Information,
Figure S7).
where Tm denotes the melting temperature of the assembly
and ΔH the molar enthalpy of assembly formation. Rass(T)
can be obtained from absorption spectra using the following
equation
εðTÞ ꢀ εmon
RassðTÞ ꢂ
ð5Þ
εass ꢀ εmon
Similar concentration-dependent UVꢀvis experiments in
CHCl3 at 298 K revealed a more pronounced driving force of
1 toward self-assembly (Figure 2b, for more details see Support-
ing Information, Figures S8 and S9). Hence, it was not possible to
cover the whole binding isotherm with data points. Nevertheless,
the equilibrium constant in CHCl3 could be roughly estimated to
6.2 ꢁ 106 Mꢀ1. This value is in good agreement with that for the
formation of bimolecular complex 10:12 from reference mer-
ocyanines 10 and 12 in CHCl3. For this complex, an equilibrium
constant of Kdim > 106 Mꢀ1 was obtained from 1H NMR titration
experiments at a constant host concentration and 298 K.14
Temperature-dependent UVꢀvis measurements in THF
revealed spectral changes that are in good accordance with
the concentration-dependent series. The self-assembly of 1 was
probed upon variation of temperature from 10 to 60 °C in steps
of 5 °C at a concentration of c = 3.5 ꢁ 10ꢀ5 M (see Supporting
Information, Figure S10a). A decrease of the temperature
resulted in a hypsochromic shift of the absorption maximum,
suggesting self-assembly of 1. The equilibrium between free and
bound species is indicated by an isosbestic point. Since, in
comparison to the concentration-dependent series, only a
smaller part of the transition region from monomeric to
polymeric species can be covered within the accessible tem-
perature range (Supporting Information, Figure S10b), the
extrapolation of the absorption maxima of free and bound
species according to eqs 4 and 5 (vide infra) is less precise.
However, the maximum values for monomeric and self-as-
sembled compound at λmax = 396 and 385 nm, respectively,
are in reasonable agreement.
Application of this model to the apparent absorption coeffi-
cients ε(T) at 395 nm provided Tm = 316 K, the molar Gibbs free
energy ΔG° = ꢀ26.6 kJ molꢀ1, and the molar enthalpy of
Hamilton complex formation ΔH° = ꢀ46.2 kJ molꢀ1 as well
as the change of entropy ΔS° = ꢀ65.8 J molꢀ1 Kꢀ1 (see
Supporting Information, Table S2). Comparison of the enthalpy
and entropy contributions to the total Gibbs free energy changes
for association reveals that the self-assembly process is entropi-
cally disfavored but enthalpy driven. The equilibrium constant as
a function of the temperature was calculated to K = 4.5 ꢁ 104 Mꢀ1
at 298 K (see Supporting Information, Figure S10d). It is note-
worthy that, in order to investigate the hydrogen-bond-directed
association process by UVꢀvis spectroscopy, conditions were
applied where only monomers up to small oligomers are present.
The average of the two equilibrium constants determined
from concentration- and temperature-dependent UVꢀvis ex-
periments in THF was calculated to K = 3.1 ꢁ 104 Mꢀ1, and this
value is in very good agreement with the dimerization constant of
reference merocyanines 10 and 12 (Kdim = 3.6 ꢁ 104 Mꢀ1).14b
Linear free energy relationships (LFERs) were previously ap-
plied to estimate the equilibrium constant in solvents of low
polarity.21,28 With such LFER analysis, a K value in the order of
1015 Mꢀ1 was estimated for the self-assembly of dye 1 in pure n-
hexane (see Supporting Information, Figure S11 and Table S3).
Thus, while under dilute conditions applied in our UVꢀvis
studies in polar solvents only small oligomers are formed, the
thermodynamic data suggest the formation of extended supra-
molecular polymers at higher concentration and in less polar
environments.
The isodesmic self-assembly process was confirmed by an
excellent fit of the temperature-dependent isodesmic model27
(see Supporting Information, Figure S10b). This model ex-
presses the molar fraction of associaated species Rass(T) as a
function of temperature27
Structural Elucidation of Monomeric and Self-Assembled
Dye 1 by NMR Spectroscopy. The structural binding motif
of assemblies of 1 was studied in detail by NMR spectroscopy
at a concentration of 0.5 mmol Lꢀ1 (Figure 3). In the 1H NMR
spectrum of 1 in deuterated dimethylsulfoxide (DMSO-d6), a
simple pattern of sharp signals was observed, indicating that
dye 1 is monomerically dissolved in this polar solvent. Upon
changing the solvent to CDCl3, significant changes compared
1
RassðTÞ ¼
ð4Þ
mÞ=RTm2 ꢃ
1 þ eꢀ0:908ΔH½ðT ꢀ T
Figure 3. Sections of the 1H NMR spectra of 1 in (a) DMSO-d6 (c = 5.0 ꢁ 10ꢀ4 M, 400 MHz, 300 K) and (b) CDCl3 (c = 5.0 ꢁ 10ꢀ4 M, 600 MHz, 293
K). The colored dots highlight the splitting of selected monomer signals upon self-assembly in CDCl3.
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Figure 4. Sections of the 600 MHz ROESY NMR spectra of dyes 1 (a) and 12 (b) in CDCl3 (c = 5.0 ꢁ 10ꢀ4 M) at 293 K and 250 ms mixing time.
Positive and negative signals are represented by black and red lines, respectively. Red circles highlight selected intermolecular cross-coupling peaks.
(c) ROESY cross-coupling signals (highlighted by red circles in a) that indicate spatial proximities between the respective protons were implemented in a
structural model for a dimer of 1 obtained by molecular modeling on semiempirical level (AM1, HyperChem 8.0530). The long alkyl chains in the
Hamilton receptor are replaced by butyl groups for simplicity.
to the spectrum in DMSO-d6 were observed, apparently due
to self-assembly. Structural features of the self-assembled species
were elucidated by (1H,1H)-COSY, (1H,13C)-HSQC, and
(1H,13C)-HMBC NMR experiments (Figure 4). For comparison,
similar NMR data were acquired for the reference Hamilton-receptor-
functionalized merocyanine 12 in CDCl3 which cannot form
self-assemblies owing to N-alkyl substituents of the barbiturate
receptor in solution are usually not observable due to fast
interconversion on the NMR time scale.9c Apart from solvent-
induced shifts of signals, a similar pattern was obtained for
monomeric reference compound 12 in CDCl3 (see Supporting
Information, Table S1).
A considerably more complex spectrum was obtained for
supramolecular polymers of 1 in CDCl3. The signals are broader
and split into intricate patterns compared to those of monomeric
1 in DMSO-d6. The splitting for selected proton signals in CDCl3
is highlighted in Figure 3 by dots of the same color. Upon
complex formation with a barbituric acid the Hamilton receptor
becomes fixed in a specific conformation that deviates from
planarity by about 45°.25 Self-assembly into extended oligomers
and the loss of symmetry due to deviation from planarity lead to
1
acceptor moiety. The complete assignment of the H NMR
signals is given in Table S1 in the Supporting Information.
Monomers of 1 in DMSO-d6 showed a single set of nicely
resolved 1H NMR signals (Figure 3). The protons marked with
and without prime (see the structures in Figure 3) are chemically
equivalent, which is in accordance with an overall C2v-symmetry
of monomeric 1. The different conformers of the Hamilton
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Figure 6. Geometry-optimized (AM1, HyperChem 8.0530) structure of
complex 10:13:11. Long alkyl chains were replaced by methyl groups.
compound 12 for which signals between protons 2 and 3, 3 and 4,
4 and 6, 5 and 6, 7 to 9, and the protons within the respective alkyl
chain can be found (Figure 4b).
Size of the Assemblies in Solution. Dynamic light scattering
(DLS) experiments were performed to determine the size of
head-to-tail self-assembled merocyanine dyes 1 in solution. For
these studies, we applied a concentration of 4.5 ꢁ 10ꢀ4 M of 1
in CHCl3 that according to the thermodynamic parameters
obtained by UV/vis spectroscopy should afford a degree of
polymerization (DPN) of about 53. DLS measurements revealed
average particle sizes from 53 to 516 nm, depending on the
scattering angle (Figure 5a and for autocorrelation functions, see
Supporting Information, Figure S12). The dependence of the
particle size on the scattering angle provides strong evidence for
the nonspherical nature of the assemblies. The relatively broad
distribution of particle sizes from ∼13 to 1606 nm supports the
formation of supramolecular polymers. Further evidence for
the formation of extended hydrogen-bonded supramolecular
polymers was obtained by diffusion-ordered spectroscopy
(DOSY) NMR experiments with DP (degree of polymerization)
values of ∼21 at a concentration of 5.0 ꢁ 10ꢀ4 M at 293 K (for
further details see the Supporting Information, Figure S13).
Addition of merocyanine dye 10 and receptor 11 to a solution
of 1 in CHCl3 should result in oligomeric self-assemblies of
smaller size because these molecules act as chain stoppers.31
Indeed, DLS of a mixture of 10, 1, and 11 in a ratio 1:3:1 in
CHCl3 (c(1) = 1.0 ꢁ 10ꢀ4 M) at 293 K revealed a significantly
decreased average particle size of only 4.3 nm (Figure 5b).
Molecular modeling of complex 10:13:11 on semiempirical level
(AM1, HyperChem 8.0530) revealed a bended structure with a
length of 5.3 nm (Figure 6), which is in good accordance with the
value estimated by DLS investigations. The observed narrow size
distribution is indicative for the formation of a well-defined
oligomeric complex instead of supramolecular polymers.
Gelation and Morphology Study. Gelation tests for dye 1
were conducted in different organic solvents and solvent mixtures. No
gel formation could be observed in polar solvents with hydrogen
bond acceptor ability like DMSO and THF. In nonpolar aromatic
solvent like toluene, precipitation was observed upon cooling the
solution to room temperature, while this dye is not soluble in aliphatic
solvents like n-hexane, even at elevated temperature. In CHCl3/n-
hexane mixtures with more than 70 vol % of CHCl3 the compound
remained dissolved and in solvent mixtures with less than 40 vol %
CHCl3 precipitation was observed. Only in a very narrow range, that
is, CHCl3/n-hexane (40:60 vol %) at 88 mM concentration, a yellow
opaque gel was formed at room temperature within 30 min after
addition of appropriate amounts of n-hexane to a stock solution of 1
in CHCl3 (see Figure 10, inset). The critical gel concentration was
found to be 8 wt %.
Figure 5. (a) Size distribution of self-assembled 1 at 293 K in CHCl3
(c = 4.5 ꢁ 10ꢀ4 M) obtained from DLS at different scattering angles
indicated in the inset (no evaluable autocorrelation function was
obtained for 90°). (b) Size distribution for a mixture of 10, 1, and 11
in the ratio of 1:3:1 (c(1) = 9.5 ꢁ 10ꢀ4 M, 293 K) measured at 90°.
signal splitting and broadening. In addition, changes of the
chemical shifts for protons at positions 5 (0.39 ppm) and 11
(0.23 ppm), and even more for the amide protons 6 (3.60 ppm)
and 10 (3.22 ppm), upon self-assembly of 1 could be observed by
comparing the 1H NMR data of 1 and 12 in CDCl3 (see Supporting
Information, Table S1). Similar shifts have recently been reported
for bimolecular complexes of merocyanine 10 with reference
Hamilton receptor 11 and merocyanine 12, respectively,14 which
indicates the formation of self-assemblies of 1 in CDCl3.29
The close proximity of protons of two molecules should result
in through-space coupling, which can be investigated by rotating-
frame Overhauser enhancement spectroscopy (ROESY) NMR.
Thus, ROESY experiments were performed with merocyanine 1
and reference compound 12 in CDCl3 at a concentration of
5.0 ꢁ 10ꢀ4 M. Through-space couplings between the protons of
1 at positions 1 and 6, 1 and 5, 2 and 11, as well as between 3 and
11, 12, 13, and 14 are observed and highlighted by red circles in
Figure 4a. All these couplings are in accordance with the
geometry-optimized dimer structure (AM1, HyperChem
8.0530) of 1 shown in Figure 4c, and thus provide evidence for
the Hamilton-receptor-mediated self-assembly of 1 in CDCl3.
On the basis of the calculated dimer of 1, the intermolecular
distances between the pairs of protons that show cross-peaks in
the ROESY spectrum are well within the range of such experi-
ments, while the intramolecular distances between the respective
protons are far beyond the sensitivity of this technique. According
to the structure shown in Figure 4c the distance between protons 1
and 10 is short enough to expect through-space couplings in the
ROESY spectrum of 1 in CDCl3. However, due to signal splitting
1
and broadening in the assembly, the H peaks of these protons
partially overlap. Therefore, the region with possible cross peaks
between 1 and 10 is excluded from the analysis.
In contrast to the observation of intermolecular cross peaks for
1, only intramolecular couplings are observed for reference
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Figure 7. Transmission electron micrograph of self-assembled 1 (c =
2.3 ꢁ 10ꢀ4 M, CHCl3) after evaporation of the solvent on a carbon-
coated copper grid.
Figure 9. (a) Temperature-dependent fluorescence spectra of 1 in
CHCl3 (c = 1.4 ꢁ 10ꢀ7 M, λex = 370 nm) from 10 to 55 °C in steps of
5 °C. The arrow indicates changes upon decreasing temperature.
(b) Plot of the temperature-dependent molar fractions of self-assembled
species Rass calculated from the integrated fluorescence intensity and
fitting curve obtained by nonlinear regression analysis based on the
isodesmic model.
twisting mechanism and constitutes a common feature of
merocyanine and cyanine dyes in solution.33
Figure 8. UVꢀvis absorption and fluorescence spectra (λex = 370 nm)
of assemblies of merocyanine dye 1 in CHCl3 (c = 0.7 ꢁ 10ꢀ4 M, 298 K).
The emission property of 1 is markedly affected by the
formation of head-to-tail supramolecular polymers. Concen-
trated solutions of 1 in CHCl3 (c = 0.7 ꢁ 10ꢀ4 M, Rass = 1)
showed increased emission (Φfl = 0.07) at λem = 489 nm
(Figure 8) in comparison to that of monomers (Φfl = 0.03).
Excitation spectra ensured that the emission originates from the
self-assembled species (see Supporting Information, Figure S15).
The increase of the emission might be ascribed to the loss of
radiationless deactivation pathways upon self-assembly owing to
the rigidification of the molecules upon self-assembly. Several
polymethine dyes have been reported to exhibit enhanced emis-
sion intensities after being physically or chemically rigidified.34
Temperature-dependent fluorescence spectra of merocyanine 1 at
low concentration (c = 1.4 ꢁ 10ꢀ7 M, Rass ≈ 0.5 at room
temperature) in CHCl3 revealed an increase of the emission with
decreasing temperature (Figure 9a). The fluorescence quantum yield
increases from Φfl = 0.03 to 0.07 upon decreasing the temperature
from 55 to 10 °C. This increase of fluorescence should be attributed
to a transformation of monomers at high temperatures to self-
assembled oligomers at low temperatures. The temperature-depen-
dent isodesmic model (vide supra) was applied to a plot of the molar
fractions of self-assembled species Rass against the temperature
(Figure 9b). From this plot the thermodynamic parameters Tm, K,
ΔG°, ΔH°, and ΔS° for the self-assembly of merocyanine 1 could be
obtained (Table 2). Rass was calculated with eq 5 using the integrated
fluorescence intensity instead of absorption. With a value of 7.3 ꢁ 106
Once the self-assembly of Hamilton-receptor-functionalized
merocyanine dye 1 in solution and its gelation in nonpolar
solvent mixtures at high concentrations have been confirmed,
the morphology of the assemblies was investigated by transmis-
sion electron microscopy (TEM). TEM micrographs show long
strands which extend over several micrometers (Figure 7). Their
average diameter being ∼14ꢀ18 nm is much larger than
expected for single fibrils from molecular modeling (see Support-
ing Information, Figure S14), indicating that individual supra-
molecular polymers of 1 merge to strands, which further form
entangled networks.
Fluorescence Properties. To get information on the impact
of supramolecular organization on the fluorescence properties of
merocyanine dye 1, we have carried out fluorescence studies at
different concentrations and temperatures. To avoid any influ-
ence of the solvent on the emission properties (wavelength,
quantum yield), which can be substantial, all investigations were
carried out in CHCl3. Like for other common merocyanine
dyes,32 only a very weak fluorescence could be observed for
solutions of monomeric dye 1 in CHCl3. To ensure the presence
of monomers, these measurements were carried out at low
concentrations and elevated temperature (c = 1.4 ꢁ 10ꢀ7 M,
328 K). The maximum of the emission band λem was observed at
408 nm (see Figure 1) and the fluorescence quantum yield was
Φfl = 0.03. The low-fluorescence quantum yield can be attributed
to rapid nonradiative deactivation pathways through a bond-
M
ꢀ1, the equilibrium constant is in very good agreement with the
value obtained from concentration-dependent UVꢀvis experiments
(K = 6.2 ꢁ 106 Mꢀ1).
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Table 2. Thermodynamic Parameters Tm (K), ΔG° (kJ
molꢀ1), ΔH° (kJ molꢀ1), ΔS° (J molꢀ1 Kꢀ1), and K (Mꢀ1
chromophores, in particular merocyanines, in supramolecular
polymer networks can afford interesting fluorescent materials
as desired for sensory and photonic applications if the design of
the supramolecular polymer impedes quenching sites such as
πꢀπ-aggregates with H-type excitonic interaction. Future direc-
tions of research on such supramolecular polymers may be in the
area of electro-optical and nonlinear optical applications where
desired polarizability anisotropies and second order nonlinear
optical effects arise from the orientation of dipolar molecules
and/or assemblies in external electric fields.35 Other applications
may give rise to light emitting devices or sensory systems. The
enhanced emission properties of the supramolecular polymers
and gel phase in comparison to the nearly nonemissive monomer
are particularly promising in this regard.
)
Obtained From the Temperature-Dependent Self-Assembly
of 1 (c = 1.4 ꢁ 10ꢀ7 M, CHCl3) Based on the Isodesmic Model
Tm
ΔG°
ΔH°
ΔS°
K a
302
ꢀ39.2
ꢀ137
ꢀ131
7.3 ꢁ 106
a Value determined at 298 K.
’ EXPERIMENTAL SECTION
Materials and Methods. Solvents and reagents were acquired from
commercial suppliers and used without further purification, unless otherwise
stated. THF was dried according to literature.36 Column chromatography was
performed using silica gel (Merck Silica 60, particle size 0.04ꢀ0.063 nm).
Thin layer chromatography (TLC) was performed on silica gel plates (60
F254 Merck, Darmstadt). Gel permeation chromatography (GPC) was
carried out on a recycling preparative LC-9105 system (Japan Analytical
Industries Co., Ltd.) with a UVꢀvis (UV-3702, 380 nm) and a refractive
index detector (RI-7s) using a preparative JAIGEL-1Hþ2H column and
solvents of purity grad “pa”. Determination of the melting points
(uncorrected) was conducted on a B€uchi Melting Point B545 heating stage.
Solvents for UVꢀvis absorption and AFM studies were of spectroscopic
grade and used as received. Unless otherwise stated, 1H NMR spectra were
recorded on a Bruker Avance 400 spectrometer at 300 K with TMS or
residual undeuterated solvent as internal standard. MALDI-TOF mass
spectrometry was performed on an autoflex II instrument (Bruker
Daltronik GmbH) in positive mode with a DCTB matrix. A microTOF
focus instrument (Bruker Daltronik GmbH) was used to perform high
resolution ESI-TOF mass spectrometry in positive mode with CH3CN
and CHCl3 or THF and CHCl3 as solvent.
UVꢀvis Experiments. UVꢀvis absorption spectra were recorded
on a Perkin-Elmer Lambda 950 UVꢀvis spectrophotometer with a
spectral bandwidth of 0.20 nm and a scan rate of 141 nm/min.
Conventional quartz cells of 0.1ꢀ50 mm path length were used to
cover a suitable concentration range. Temperature control was achieved
with a PTP-1 Peltier element (Perkin-Elmer).
Fluorescence Measurements. The steady-state fluorescence
spectra in solution were recorded under ambient conditions on a PTI
QM4/2003 spectrofluorometer. A front-face setup was used for the self-
assembled solution in CHCl3 owing to the high optical density of the
sample. All fluorescence spectra were corrected against photomultiplier
and lamp intensity. Because of the overlap of the excitation peak with the
emission, the excitation peak was removed by subtraction of a spectrum
of the pure solvent. The fluorescence quantum yields were determined
using the optical dilution method with diphenylanthracene (Φfl = 0.67
in benzene37) as standard.38 The given quantum yields are averaged over
the values obtained at three different excitation wavelengths. Absolute
quantum yields of the gel were measured under ambient conditions on
an absolute photoluminescence quantum yield measurement system
C9920-02 (Hamamatsu).
Figure 10. Fluorescence spectrum of a gel prepared from merocyanine
1 in CHCl3/n-hexane (40:60 vol %, c = 88 mM, λex = 370 nm, solid line)
and of monomeric (c = 1.4 ꢁ 10ꢀ7 M, 328 K, λex = 370 nm, dotted line)
and self-assembled (c = 0.7 ꢁ 10ꢀ4 M, λex = 370 nm, dashed line) dye 1
in CHCl3. The inset shows a photograph of the gel at ambient conditions
and the fluorescence upon excitation with UV light (λex = 366 nm).
The gel phase of merocyanine dyes 1 showed a remarkably
intense bluish-white fluorescence emission (Figure 10). In
comparison to the monomeric (Φfl = 0.03) and self-assembled
species (Φfl = 0.07), the fluorescence of the gel is significantly
increased (Φfl = 0.17) and shifted to slightly longer wavelengths
(λem = 496 nm). The modest shift corroborates our earlier
conclusion that the individual dyes remain isolated by means of
the sterical shielding of the bulky and orthogonally arranged
Hamilton receptors (see Scheme 1). No indication for the
formation of excimers as observed for antiparallel dimer aggre-
gates of related dipolar merocyanine dyes18 is obtained from the
emission spectra of the self-assembled species of merocyanine 1
shown in Figure 10. These results demonstrate that even such
highly dipolar dyes like 1 can be self-assembled by strong 6-fold
hydrogen-bonding receptors in a head-to-tail supramolecular
polymer without forming quenching sites by close πꢀπ-contacts
through dipolar aggregation.
’ CONCLUSIONS
In this work, the first example of a head-to-tail supramolecular
polymer with incorporated merocyanine dyes is reported. Self-
complementary Hamilton-receptor-functionalized merocyanine
dyes 1 enabled to outwit the most common antiparallel aggrega-
tion of the dipolar merocyanine dyes, and hence to achieve the
highly desirable head-to-tail orientation of such dyes by 6-fold
hydrogen bonding, leading to the formation of supramolecular
polymers with appreciable solubility in common organic solvents
such as CHCl3 and THF. Evaluation of concentration- and
temperature-dependent UVꢀvis experiments of merocyanine
dye 1 by employing the isodesmic model revealed high equilib-
rium constants for the self-assembly of 1 that gave rise to
high degrees of polymerization even in dilute solutions. Our
results further revealed that the rigidification of weakly emissive
Dynamic Light Scattering (DLS). DLS experiments were carried
out using a commercial N5 Submicrometer Particle Size Analyzer (Beckman
Coulter, Inc.) laser light scattering spectrometer equipped with a 25 mW
heliumꢀneon laser operating at 632.8 nm. Sample solutions (c(1) = 1.0 ꢁ
10ꢀ4 M) were filtrated into a dust-free vial through a 0.45 μm Teflon filter.
The size distribution profile deconvolution algorithm is based on the
CONTIN program.
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Gelation Tests. The measured amount of organogelator (merocyanine 1)
was dissolved in the respective amount of CHCl3 in a screw-capped
sample vial. Gels were formed upon addition of appropriate amounts of
n-hexane to the stock solution. The formation of the gel was tested by
the “stable-to-inversion of a vial” method39 after leaving the sample for
1 h at ambient conditions.
’ ACKNOWLEDGMENT
We gratefully thank Dr. Xin Zhang for performing TEM
measurements and the Volkswagen Foundation for financial
support of this work within the priority program “Complex
Materials: Cooperative Projects on the Natural, Engineering and
Biosciences”.
Synthesis and Characterization of Precursor 8 and the
Target Compound 1. 1-{3,5-Bis[(6-dodecyrylamino)pyridine-2-
yl]carbamoyl}phenyl-2,6-dimethyl-4-pyridone (8). Diamine 7 (2.22 g,
4.72 mmol) was dissolved in dry THF (140 mL) under an argon atmo-
sphere and cooled to 0 °C. At this temperature, dodecanoyl chloride (2.38 g,
2.59 mL, 10.9 mmol) was added dropwise. Afterward, the reaction mixture
was stirred for 16 h at room temperature. The resultant suspension was
poured into a saturated aqueous solution of NaHCO3 (200 mL, pH 8ꢀ10).
After extracting the aqueous layer with CHCl3 (4 ꢁ 50 mL), the combined
organic layers were dried over MgSO4. The solvent was removed under
reduced pressure and the residue was purified by column chromatography
using silica gel with EtOAc/EtOH = 10:1 vol % as eluent to give pure 8
(1.65 g, 1.98 mmol, 42%). 1H NMR (400 MHz, CDCl3): δ 9.92 (br s, 4 H,
NH), 8.86 (s, 1 H, ArH), 8.25 (s, 2 H, ArH), 8.06 (d, 3J = 7.2 Hz, 2 H,
CHpyridine), 8.01 (d, 3J = 5.9 Hz, 2 H, CHpyridine), 7.77 (t, 3J = 7.4 Hz, 2 H,
CHpyridine), 2.33 (t, 3J = 7.2 Hz, 4 H, COCH2), 1.77 (s, 6 H, CH3), 1.67
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’ AUTHOR INFORMATION
Corresponding Author
*Fax: (þ49) 931-31-84756. E-mail: wuerthner@chemie.
uni-wuerzburg.de.
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