Structural rules for the chiral supramolecular organization of OPE-based discotics: Induction of helicity and amplification of chirality

Article abstract of DOI:10.1021/ja210443m

A systematic study on the structural rules that regulate the chiral supramolecular organization of oligo(phenylene ethynylene) (OPE)-based discotics is presented. This study is based on the chirooptical properties of two different series of triangular sha

Full text of DOI:10.1021/ja210443m

Article
pubs.acs.org/JACS
Structural Rules for the Chiral Supramolecular Organization of OPE-
based Discotics: Induction of Helicity and Amplification of Chirality
Fatima García and Luis Sanchez*
́
́
Departamento de Química Organ
́
ica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
S
* Supporting Information
ABSTRACT: A systematic study on the structural rules that regulate the chiral supramolecular organization of oligo(phenylene
ethynylene) (OPE)-based discotics is presented. This study is based on the chirooptical properties of two different series of
triangular shape OPEs. The first of them is composed by OPE-based trisamides with a variable number of chiral side chains
(compounds 1) that self-assemble following a cooperative mechanism. The CD experiments carried out with these
desymmetrized trisamides demonstrate that only one stereogenic center is sufficient to achieve a helical organization with a
preferred handedness. However, the ability to amplify the chirality decreases upon decreasing the number of stereocenters at the
peripheral side chains. The second series is constituted by triangular shape OPEs with a variable number of ether and amide
functional groups and constant absolute configuration of the stereogenic centers at all of the peripheral chains (compounds 2).
These compounds do not self-assemble into helical aggregates as demonstrated by the corresponding CD studies. The
amplification of chirality observed in the mixtures of some of the components of both series has been investigated. The
combination of chiral trisamide 1d with chiral but nonhelical 2b or 2c does not produce an amplification of chirality most
probably due to the mismatch between the stereogenic centers of both components. However, the combination of achiral
trisamide 1a with chiral but nonhelical bisamide 2c generates, in a cooperative manner, helical structures with a preferred
handedness in a process involving the transfer of helicity from 1a to 2c and the transfer of chirality from 2c to 1a. The structural
features of the OPE discotics also exert a strong influence on the columnar aggregates. Thus, while achiral 1a bundles into thick
filaments to form an organogel, the gelation ability of these triangular OPEs decreases upon increasing the number of stereogenic
centers, being totally canceled for compounds 2 in which the amide functionalities are replaced by ether linkages. Finally, we have
also registered AFM images of the helical aggregates obtained from the mixture of 1a+2c, which implies an efficient transfer of
the chiral objects from solution to surfaces. The study presented herein increases the understanding of the structural rules that
regulate the chiral supramolecular organization of discrete molecules in general and, more specifically, those based on π-
conjugated oligomers.
homochirality.4,5 More recently, chemical self-assembly of
INTRODUCTION
■
relative small and simple molecules has yielded hierarchically
organized, complex structures in a process known as supra-
molecular polymerization.6,7 Many of these supramolecular
structures exhibit a preferred handedness due to the
incorporation of stereocenters at the peripheral substituents.8
Most of the investigations dealing with amplification of
chirality are based on the combination of two species that have
identical chromophores but differ in the chiral or achiral nature
The sophistication and functionality reached by biological
helices is inspiring scientists to utilize helical structures in
chirality sensing,1 enantioselective catalysis,2 and especially to
unravel the origin of homochirality.3 The generation of chirality
is directly related with an efficient transfer of chiral information
from chiral units to complex supramolecular structures through
noncovalent interactions. These supramolecular architectures
are generally helices that are inherently chiral since right- and
left-handed helices are mirror images that cannot be super-
imposed. Big efforts have been devoted to crystalline
architectures and helical polymers to unravel the origin of
Received: November 7, 2011
Published: December 5, 2011
© 2011 American Chemical Society
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of the side chains.9 Two main experiments have been
developed to control and study the amplification of the
chirality phenomenon in polymers and supramolecular
polymers: (a) “sergeants-and-soldiers” in which the helicity of
a racemic mixture of both enantiomers (the soldiers) is biased
by the addition of minute amounts of chiral units (the
sargeants)10 and (b) “majority rules”, where a slight excess of
one of the two possible enantiomers shifts the whole helicity to
that of this enantiomer.11 More scarce are the examples in
which the strategy to achieve amplification of chirality consists
of the preparation of mixtures with species of different nature.
The solvation of (supramolecular) polymers with chiral
solvents or the formation of complexes between achiral
(supramolecular) polymers and chiral guests exemplifies this
strategy based on using mixed systems.12,13 A key issue in the
generation of helical structures for both synthetic and
supramolecular polymers is the synthesis of appropriate chiral
building blocks. The total number of stereogenic centers at the
peripheral chains and their relative position, the chemical
nature of the central core supporting these side chains, the
connectivity between these two units and, very importantly, the
nature of the noncovalent interactions that induce the
hierarchical helical organization are pivotal factors in the
rational design of the chiral momoners that interact to yield the
helical structure.
chiral molecule produces disordered aggregates with unpre-
ferred helicity.15
To establish the structural rules that regulate the chiral
supramolecular organization of OPE discotics, we have
synthesized OPE-based trisamides with a variable number of
chiral side chains (compounds 1 in Figure 1) and new
triangular shape alkoxy-substituted OPEs with a variable
number of ether and amide functional groups keeping the
absolute configuration of the stereogenic centers constant at all
peripheral chains (compounds 2 in Figure 1). The correspond-
ing CD studies reveal that the presence of the three amide
functionalities but only one stereogenic center is sufficient to
attain helical supramolecular structures although it substantially
decreases the ability to amplify the chirality of the racemic
mixture of 1a. However, replacing the amide functionalities by
ether linkages (compounds 2) impedes the α-helical columnar
organization and no dichroic response is observed. We have
demonstrated that the combination of the racemic helical
mixture formed by the achiral trisamide 1a and the chiral but
nonhelical bisamide 2c gives rise to helical structures with a
preferred handedness in a cooperative fashion. This process
involves the transfer of helicity from 1a to 2c and the transfer of
chirality from 2c to 1a and represents one of the very scarce
examples in which chirality is obtained from the formation of
host−guest complexes between structurally analogous species.
The columnar aggregates formed by the self-assembly of 1a
bundle into thick filaments that finally form an organogel in
toluene as solvent. The gelation ability of this class of triangular
OPE decreases upon increasing the number of stereogenic
centers being totally canceled for the alkoxy OPEs 2. The
fibrillar supramolecular structures formed from trisamides 1b
and 1c has been visualized by atomic force microscopy (AFM)
imaging. Finally, we have also registered AFM images of the
helical aggregates obtained from the mixture of 1a+2c, which
implies an efficient transfer of the chiral objects from solution
to a surface.
Very recently, we have described that oligo(phenylene
ethynylene) (OPE) based trisamides 1a and 1d (Figure 1)
RESULTS AND DISCUSSION
■
Synthesis. Symmetric trisamides 1a and 1d as well as
trialkoxy-OPE 2a have been prepared by a 3-fold Sonogashira
C−C cross coupling reaction catalyzed by palladium.14,15
However, the synthesis of the asymmetrical trisamides 1b and
1c and also the OPE decorated with a variable number of amide
and alkoxy functional groups (2b−c) requires the desymmet-
rization of the central aromatic ring. To accomplish this
purpose, we have followed two general procedures starting
from 1,3,5-tribromobenzene. For the case of trisamides 1b and
1c, a stoichiometrically controlled Sonogashira reaction yields
1-bromo-3,5-bis(2-(trimethylsilyl)ethynyl)benzene (3),16
which allows controlling the attachment of the achiral or chiral
chains. A further Sonogashira cross-coupling between 3 and the
chiral 4-ethynyl-N-((S)-3,7-dimethyloctyloxy))benzamide
(4c*) or achiral 4-ethynyl-N-decyllbenzamide (5c) yields
compounds 6a* or 7a, respectively. The deprotection of the
trimethylsilyl group and subsequent Sonogashira cross-coupling
of the resulting alkyne derivatives 6b* or 7b with 5c or 4c*,
respectively, afford asymmetrical trisamides 1b and 1c (Scheme
1).
Figure 1. Chemical structure of the triangular-shape OPE-based
trisamides 1 and alkoxy-amides 2.
self-assemble into helical columnar aggregates by the operation
of a 3-fold α-helical-type intermolecular amide hydrogen
bonding and π−π stacking of the aromatic central units.
Pristine chiral trisamide 1d and the mixture of achiral 1a and
chiral 1d exhibit strong Cotton effects demonstrating the
helical nature of the aggregates formed from these OPEs even
at high diluted conditions.14 However, the attachment of the
chiral side chains to the aromatic C3-symmetric core by an ether
linkage (compound 2a in Figure 1) results in a complete lack of
dichroic response, which implies that the self-assembly of these
The asymmetrical compounds 2b and 2c have been readily
obtained by following a one-pot Sonogashira cross-coupling
protocol in which one or two equivalents of 4a*14 units are first
attached to the central aromatic fragment.17 The reaction was
monitored by TLC until all starting materials 8b18 and 4a*
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Scheme 1. Synthesis of the Asymmetric Trisamides and Alkoxy-OPEs 1 and 2
disappeared. After that, an excess of 1-((S)-3,7-dimethylocty-
loxy)-4-iodobenzene (9*)19 was added to the reaction mixture
to yield compounds 2b and 2c (Scheme 1).
All the compounds and intermediates were characterized by
means of analytical and spectroscopic techniques (see
1
Supporting Information). The H NMR spectra of 1b and 1c
show only three aromatic resonances: two doublets at δ ∼7.7
and 7.5, and a singlet at δ ∼7.6. The presence of the stereogenic
centers is unambiguously probed by the apparition of two sharp
doublets at δ ∼0.9 and 0.8 adscribable to the methyl groups of
the chiral (S)-3,7-dimethylocyl side chains. However, the
asymmetry of 2b and 2c induces the anisochrony of the two
different para-substituted aromatic fragments and the corre-
sponding 1H NMR spectra show two different sets of
resonances ascribable to an AA′BB′ spin system typical of
para-substituted aromatic compounds, the first one at δ ∼7.7
and 7.5, and the second one at δ ∼7.4 and 6.8. Additionally, the
central aromatic protons appear as a singlet at δ ∼7.6.
IR Spectroscopy. A first indication of the characteristic 3-
fold intermolecular hydrogen-bonding motif that yields helical
columnar stacks in supramolecular polymers endowed with
amide functional groups is IR spectroscopy. More specifically,
Figure 2. Partial FTIR spectra of trisamides 1a−c and alkoxy-amides
2b,c.
the α-helix arrangement can be inferred from the N−H, and
Amide I (CO) stretching bands as well as the Amide II (C−
N) bending band.20 The FTIR spectra of asymmetric trisamides
1b and 1c show well-defined bands at ∼3290, 1636, and 1547
cm−1 diagnostic of a helical columnar stack, in good agreement
with the previously data reported for the symmetric trisamides
1a and 1d (Table S1, Supporting Information, and Figure 2).14
Compounds 2b and 2c, endowed with one or two amide
functional groups, respectively, present different IR features. In
these triangular OPEs, the N−H stretching appears at higher
wavenumber (∼3314 cm−1) being the Amide I and Amide II
vibrations at 1638 and 1547 cm−1, respectively (Table S1,
Supporting Information, and Figure 2). These values
correspond to H-bonded amide functionalities but are deviated
from those values reported for α-helical columnar stacks.20
Therefore, the presence of lateral H-bonds between the amide
groups of molecules of different columnar stacks could justify
these experimental findings for compounds 2b and 2c.
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In addition, the FTIR of all the triangular OPEs studied
herein exhibit well-defined bands at ν ∼2925, 2860, and 1465
cm−1 corresponding to the stretching of the paraffinic −CH2−
groups that suggest the interdigitation of the paraffinic side
chains (Table S1, Supporting Information, and Figure 1).21
Self-assembly in Solution. The formation of stacked
organized columns in solution from the compounds reported
herein has been investigated by circular dichroism (CD). This
technique is very sensitive to detect chiral supramolecular
structures. The CD spectra of trisamides 1b and 1c show a
bisignated Cotton effect identical to that reported for the
temperature (Te), the enthalpy release in the elongation
regimen (he) and the degree of cooperativity expressed by the
activation constant Ka. These thermodynamic parameters for
1b and 1c are collected in Table 1 together with that reported
Table 1. Thermodynamic Parameters Corresponding to the
Supramolecular Polymerization of Trisamides 1a−d and the
Mixture of 1a+2c (MCH, 1−1.5 × 10−5 M)
compound
he (kJ mol−1)
Te (K)
Ka
1a
−61.0
−114.3
−100.1
−97.1
348
328
332
344
332
2.1 × 10−3
4.9 × 10−4
1.2 × 10−4
5.0 × 10−4
4.4 × 10−4
1b
symmeric trisamide 1d, that is, negative at high energy (λmax
=
1c
280 nm), positive at low energy (λmax = 303 nm) and with a
zero crossing at 291 nm.14 These chirooptical characteristics
can be assigned to right-handed helically organized structures
and demonstrate that only one stereogenic center at the
peripheral side chains is able to regulate the helical sense of the
aggregates formed by the self-assembly of C3-symmetric OPE
trisamides (Figure 3).22 These results are in good correlation
with those reported for benzene-1,3,5-trisamides (BTAs).23
1d
1a+2c
−107.0
previously for symmetric 1a and 1d. The stability of the
aggregates, determined by the Te values, decreases for the
asymmetric trisamides 1b and 1c in comparison to the
symmetric 1a and 1d. The lower stability of the aggregates
could be accounted for by considering a less efficient
interdigitation of the achiral and the chiral side chains that
decorate the desymmetrised trisamides. The low value
calculated for Ka expresses a high degree of cooperativity.
The melting curves of 1b and 1c also show that the CD
response diminishes at low temperatures. This phenomenon
can be reasonably assigned to the scarce solubility of these
compounds in MCH.
The cooperative mechanism that governs the supramolecular
polymerization of trisamides 1b and 1c has been confirmed by
the nonsigmoidal curves obtained in the corresponding
temperature dependent UV−vis experiments in MCH.
Similarly to compounds 1a and 1d, asymmetric compounds
1b and 1c present two absorption maxima at around 300 and
314 nm at high temperature (363 K) whose intensity decreases
upon cooling appearing a new shoulder at 325 nm (Figure S2,
Supporting Information).
The replacement of the amide functional groups by alkoxy
linkages strongly conditions the helical organization of the
triangular OPEs. Similarly to the symmetrical, chiral trialkoxy
derivative 2a, asymmetric 2b and 2c endowed with one or two
amide functionalities, respectively, showed no CD response in
the same experimental conditions as trisamides 1b−d. The lack
of dichroic signal confirms that these triangular OPEs are not
able to self-assemble into helical columnar stacks, as already
suggested by the corresponding FTIR data. Therefore, the
operation of the three highly directional H-bonds between the
amide functional groups plays a pivotal role in the hierarchical
self-assembly of triangular shape OPEs.
Amplification of Chirality in Mixed Systems. The
systematic variation of the chemical structure of the reported
OPE discotics allows investigating of the influence of the
chemical structure on the amplification of chirality. Considering
the above rationalized structural requirements necessary for
triangular OPEs to self-assemble into helical aggregates, we
have investigated the chiral behavior of mixed systems by
following “sergeants-and-soldiers” experiments. We have
reported that the chirality of self-assembled 1a is amplified
upon the addition of chiral 1d and the maximum handedness
value is reached when a 20% of the chiral sergeant 1d is added
(Figure 4, bottom).14 Taking into account the chirooptical
features of asymmetric trisamides 1b and 1c, we have also
performed “sergeants-and-soldiers” experiments by mixing
Figure 3. CD spectra of 1b (red line), 1c (blue line), 2b (black line)
and 2c (green line) (MCH, 1.5 × 10−5 M). (Inset) Nonsigmoidal
melting curves of 1b (red) and 1c (blue) from 363 to 288 K at
intervals of 0.5 K min−1.
We have previously demonstrated that achiral 1a and chiral
1d form supramolecular polymers following a nucleation-
elongation or cooperative mechanism.14,24 Analogously, the
cooperative supramolecular polymerization of 1b and 1c is
inferred from the nonsigmoidal shape of the curves obtained by
variable temperature CD experiments performed in methyl-
cyclohexane (MCH). These experiments were carried out at
diluted conditions and applying a slow cooling rate of 0.5 K
min−1 to suppress possible kinetic effects during the polymer-
ization process (inset in Figure 3). The melting curves also
present an abrupt change between the nucleation regimen, in
which the corresponding trisamide is molecularly dissolved and
no CD signal is observed, and the elongation regimen, in which
the supramolecular polymer grows rapidly (Figures 3 and S1,
Supporting Information).
To extract the thermodynamic parameters that define the
cooperative supramolecular polymerization of 1b and 1c, we
have fitted the temperature dependent CD response at 303 nm
to the nucleation-elongation model proposed by Van der
Schoot.25 This model allows calculating the elongation
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amounts of symmetric 1d as chiral sergeant thus forming chiral
host−guest complexes (Figures 5 and S4, Supporting
Figure 4. Amplification of chirality experiments for achiral 1a upon
mixing with chiral 1b (top), 1c (middle), and 1d (bottom) (298 K,
MCH, total concentration 1 × 10−5 M, 298 K).
Figure 5. CD spectra of 2b upon addition of increasing amounts of 1d
(MCH, total concentration 1 × 10−5 M, 298 K). (Inset) Variation of
the CD signal upon increasing the amount of 1d.
these chiral OPEs with racemic 1a. The addition of increasing
amounts of chiral 1cendowed with two sterogenic centers
into a solution of achiral 1a keeping the total concentration
constant and further heating above the Te value leads to the
appearance of a chirooptical response that increases nonlinearly
(Figure 4, middle and Figure S3, Supporting Information). In
this case, the maximum handedness is obtained upon adding a
larger amount of the chiral sergeant 1c (around 40%), which
suggests the strong dependence of the amplification of chirality
with the number of stereogenic centers present in the chiral
sergeant.
This dependence is confirmed in the sergeants-and-soldiers
experiments carried out with trisamides 1a and 1b, which
possess only one stereogenic center and the peripheral side
chains. The chirooptical response obtained for the mixture 1a
+1b increases almost linearly which indicates that the
amplification of chirality in this mixture is negligible. These
results demonstrate the increasing the number of stereocenters
per monomeric unit increases the ability to transfer the chiral
information reaching a fully amplified state. A plausible
explanation on the influence of the number of sterogenic
centers at the peripheral positions of the OPE-based trisamides
on the amplification of chirality can be extracted by utilizing the
concepts of helix reversal penalty (HRP) and mismatch penalty
(MMP), recently investigated for BTAs.26 HRP is directly
related to the strength of the intermolecular hydrogen bonds
and penalizes a helix reversal within a columnar stack, while
MMP penalizes the mismatch induced when a chiral monomer
is introduced into a stack of its unpreferred helicity. Since the
four trisamides 1a−d exhibit equally strong intermolecular H-
bonding arrays, as suggested by the FTIR features, remarkable
HRP deviations can be ruled out and the weaker sergeants-and-
soldiers effect could be justified by a lower MMP.
Information). The CD response of the mixtures formed by
2b and 1d as well as 2c and 1d increases linearly upon the ratio
of the chiral counterpart increases (insets in Figures 5 and S4,
Supporting Information) which implies that there is no
amplification of chirality in these mixtures. On the contrary,
the CD intensity of 2b+1d and 2c+1d is smaller than the CD
signal observed for pristine chiral 1d at the same range of
concentration (Figure S5, Supporting Information). These data
suggest that both alkoxy-OPEs 2b and 2c interact with 1d
diminishing the helicity of the whole sample most probably due
to a mismatch between the stereogenic centers at the side
chains in both types of triangular OPEs.
The interaction between alkoxy-OPEs 2b−c and symmetric
trisamide 1d prompted us to evaluate the induction of helicity
and amplification of chirality in mixtures formed by trisamide
1a and triangular OPEs 2b and 2c. The former self-assembles
into an equimolar mixture of right- and left-handed helices
whereas the latter are inherently chiral but do not self-assemble
into helical aggregates in diluted conditions. The complexation
of these compounds, structurally close but not identical, could
produce the transfer of helicity from 1a to 2b or 2c, and the
transfer of chirality from chiral 2b or 2c to achiral 1a thus
generating helical supramolecular polymers. The interaction
between 1a and 2b or 2c has been monitored by CD
experiments. The addition of increasing amounts of 2b to a
diluted solution of 1a, keeping the total concentration constant
of the mixture (1 × 10−5 M) and heating the mixture above the
Te did not show any dichroic signal (Figure S6, Supporting
Information). The lack of chirooptical properties in the mixture
of 1a and 2b demonstrates that the interaction of the only
amide functionality of 2b with the amide functional groups of
1a is not efficient and the α-helix H-bonding array between
these groups, responsible of the helical organization of
triangular OPEs, is not operating in these diluted conditions.
A sharp contrast is observed in the experiments carried out in
the mixture formed by the achiral but helical trisamide 1a and
the chiral but nonhelical bisamide-alkoxy OPE 2c. The direct
mixture of both components at room temperature and at total
After these sergeants-and-soldiers experiments in which we
only changed the (a)chiral nature of the peripheral side chains
of the two components of the mixture, we have also
investigated the chirooptical behavior experienced by systems
formed by mixing both OPE-based discotics 1 and 2.
Considering that compounds 2b and 2c are inherently chiral
but do not form helical supramolecular structures, we have
utilized them as achiral soldiers and we have added increasing
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constant concentration of 1 × 10−5 M did not result in any
dichroic response, due to the kinetic inertness of the aggregates
formed by 1a at these experimental conditions that impedes the
intercalation of the chiral guest 2c. However, heating the
mixture above the Te value leads the solution to a molecularly
dissolved state that allows the coassembly of both components
of the mixture upon cooling. In fact, the slow cooling of the
mixture results in the apparition of a clear bisignated Cotton
effect with a negative and a positive maxima at 280 and 303 nm,
respectively, and a zero crossing at 290 nm identical to that
exhibited by chiral trisamides 1b−d (Figure 6a). Unlike the
also presents a bell shape with inflections in the region close to
mole fractions of 0 and 1. These features of the Job plot are
consistent with the interaction of n molecules of 1a with n
molecules of 2c to form the complex 1an2cn with an estimated
constant of around 106 M−1.27 The generation of the complex
1an2cn would start with the formation of chiral dimeric species
that further elongate to give rise to helical supramolecular
polymers (Figure 7).
Figure 7. Schematic illustration of the transfer of helicity and
amplification of chirality in the mixture of trisamide 1a and bisamide-
alkoxy-OPE 2c.
The stability of the helical aggregates formed by the mixture
1a+2c has been explored by temperature dependent CD
experiments at different concentrations and compared with the
results previously reported for pristine achiral 1a.14 The CD
spectra of the mixture 1a+2c shows the typical bisignated
Cotton effect with the negative and the positive maxima at 280
and 303 nm, respectively, even at a concentration as low as 1 ×
10−6 M (Figure 8a). The normalized CD cooling curves of the
mixture 1a+2c at different concentrations are not sigmoidal in
shape with an abrupt drop in the dichroic response at around
320 K for the more concentrated samples, due to the scarce
solubility of the resulting mixture (Figure 8b). Despite the
different supramolecular processes taking place in the
aggregation of the mixture 1a+2c, it is possible to simplify
the mechanism of this aggregation considering a cooperative
supramolecular polymerization, analogously to pristine com-
pounds 1. Similar thermodynamic parameters to those
calculated for 1a−d (Te =332 K, he = −107 kJ mol−1, and Ka
= 4.4 × 10−4; see Table 1) has been determined for the mixture
1a+2c at 1 × 10−5 M applying the above-mentioned
cooperative model proposed by Van der Schoot.25 The lower
Te value calculated for the mixture 1a+2c (Te = 332 K) in
comparison to pristine 1a (Te = 348 K) is indicative of the
weaker character of the aggregates formed by the mixture.
Additionally, eq 1 allows determining the average number of
molecules required to form the active chiral nucleus (⟨Nn(Te)⟩)
that further elongates giving rise to the final supramolecular
polymer.25 For a Ka value of 4.4 × 10−4, the parameter ⟨Nn(Te)⟩
for the mixture 1a+2c is of around ten molecules, that is,
around five pairs of 1a and 2c.
Figure 6. (a) CD spectra of 1a upon addition of increasing amounts of
2c (MCH, total concentration 1 × 10−5 M, 298 K). (b) Job plot of the
mole fraction of 2c versus CD intensity at 303 nm (MCH, 298 K, and
1 × 10−5 M as total concentration).
mixture 1a+2b, the interaction between the three amides of 1a
and the two amides of 2c is very efficient and the α-helix H-
bonding array between them together with the π−π
interactions between the aromatic units generates stable chiral
and helical complexes even at diluted conditions. Additionally,
the lack of stereogenic centers in 1a cancels the possible
mismatch effect between the two triangular OPEs as occurs in
the mixture 1d+2c.
The stoichiometry of the complex formed by the interaction
of 1a and 2c has been determined by a Job plot experiment
monitoring the CD intensity at 303 nm. The maximum
intensity is achieved for a molar fraction of 0.5 but the Job plot
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Figure 8. (a) CD spectra of the 1:1 mixture 1a+2c at different concentrations (MCH, 298 K). (b) Melting curves of the 1:1 mixture 1a+2c at
different concentrations from 363 to 288 K at intervals of 0.5 K min−1.
which the chirality is obtained from the formation of host−
guest complexes between structurally analogous species.
1
K
⟨N (T )⟩ =
n
e
3
(1)
(2)
a
Morphology of the Supramolecular Polymers onto
Surfaces. The columnar organization of symmetric trisamides
1a and 1d has been visualized previously by scanning electron
microscopy (SEM).14 This columnar aggregation can favor the
entangling of fibrils to give rise to organogels upon capturing
molecules of solvent.29 Achiral 1a is able to form an organogel
at a concentration relatively high (∼10 wt %) by using toluene
as solvent. The formation of the organogel is envisioned by
inversion of the glass vial and experience a gel-to-sol transition
upon heating (inset in Figure 9). This organogel is constituted
K = K /K
a
2
A complete determination of the thermodynamic parameters
governing the supramolecular polymerization of the mixture 1a
+2c can be also calculated from the data obtained in the Van
der Schoot analysis at different concentrations. The degree of
cooperativity in a supramolecular polymerization process is
determined by Ka that, in turn, is the quotient between the
nucleation (K2) and the elongation (K) constants (eq 2). A
modified version of the Van’t Hoff analysis is obtained by
plotting the natural logarithm of the concentration (ln(C))
versus the inverse of Te (1/Te). The critical concentration at
room temperature can be calculated by simple extrapolation in
the resulting straight line (Figure S7, Supporting Information).
This critical concentration value is equated to the inverse of the
elongation constant K, and a value of K = 8.40 × 107 M−1 is
extracted.28 The high calculated value of K for the mixture 1a
+2c is in good correlation with the value estimated from the
Job plot analysis for this constant. Upon calculating the
elongation constant K and by utilizing eq 2, it is possible to
determine that the value of the nucleation constant K2 is 3.06 ×
105 M−1. The high values of K and K2 are accounted for by
considering the synergy between the H-bonding interactions
established between the amide functional groups and the π−π
stacking between the aromatic OPEs moieties. A similar
calculation has been carried out for pristine achiral 1a utilizing
the parameters deduced form temperature dependent UV−vis
experiments previously reported.14 The supramolecular poly-
merization process of trisamide 1a is defined by an elongation
and a nucleation constants of K = 3.49 × 107 M−1 and K2 is 1.04
× 105 M−1, respectively. These values for K and K2 are in very
good correlation with those calculated for the mixture 1a+2c.
The CD investigations presented herein for 1a+2c mixture
confirm that the combination of the racemic helical mixture
formed by the achiral trisamide 1a and the chiral but nonhelical
bisamide 2c gives rise to helical structures with a preferred
handedness in a cooperative manner. This process implies the
transfer of helicity from 1a to 2c and the transfer of chirality
from 2c to 1a and represents one of the very few examples in
Figure 9. SEM image of the organogel formed by 1a in toluene (glass
substrate, ∼10 wt %). (Inset) Picture of the gel−sol transition of the
organogel formed by 1a in toluene.
by a dense network of thick filaments of around 1 μm thick as it
can be observed by SEM imaging (Figures 9 and S8,
Supporting Information). The introduction of stereogenic
centers at the peripheral side chains of trisamides 1 diminishes
the ability of this class of compounds to form organogels. Thus,
compounds 1b, 1c and 1d are not able to gelify in toluene at
very high concentration (>15 wt %). In fact, the formation of
intertwined fibers diminishes with the increasing number of
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Journal of the American Chemical Society
Article
sterogenic centers, most probably due to a poorer interdigita-
tion between the peripheral chains.
CONCLUSIONS
■
The self-assembly of two different series of OPE-based discotics
has been extensively studied. The first of these series consists of
OPE-based trisamides with a variable number of chiral side
chains (compounds 1) that self-assemble cooperatively. The
CD experiments carried out with these desymmetrized discotics
demonstrate that only one stereogenic center at the peripheral
side chains is sufficient to achieve a helical organization with a
preferred handedness. However, the ability of the chiral
trisamides 1b−d to bias the chirality of the racemic mixture
formed by the self-assembly of achiral 1a decreases upon
decreasing the number of stereocenters at the peripheral side
chains. In the second series, we have investigated triangular
shape OPEs with a variable number of ether and amide
functional groups keeping the absolute configuration of the
stereogenic centers at all peripheral chains constant (com-
pounds 2). The CD studies performed with these mixed
triangular OPEs indicate that there is no helical organization.
We have also investigated the amplification of chirality
experienced by mixing some of the components of the two
series. Thus, while the mixture of chiral trisamide 1d with chiral
but non helical 2b or 2c does not result in an amplification of
chirality, the combination of achiral trisamide 1a with chiral but
nonhelical bisamide 2c generates helical structures with a
preferred handedness in a cooperative fashion in a process
involving the transfer of helicity from 1a to 2c and the
amplification of chirality from 2c to 1a. These studies represent
one of the very scarce examples in which chirality is obtained
from the formation of host−guest complexes between
structurally analogous species that separately are CD-silent.
The gelation ability of this class of triangular OPE decreases
upon increasing the number of stereogenic centers being totally
canceled for compounds 2 in which the amide functionalities
are replaced by ether linkages.
AFM images of the aggregates formed in toluene at 1 × 10−4
M solutions are shown in Figure S9 (Supporting Information).
Trisamide 1b, endowed with only one stereogenic center at the
peripheral chains, self-assemble into a dense network of fibrils
that resembles to an organogel (Figure S9a, Supporting
Information). However, the introduction of a second stereo-
genic center in trisamide 1c reduces the efficient interdigation
of the peripheral side chains and smaller fibrils of ∼3 nm height
are observed in the corresponding AFM images (Figure S9b,
Supporting Information).
Unlike trialkoxy 2a which forms isolated chiral rods,15
compounds 2b and 2c are not able to form fibrillar structures
and only amorphous material is visualized by AFM imaging.
These findings are in good correlation with that observed in the
corresponding FTIR spectra in which the amide functional
groups are not forming an α-helix H-bonding array. However,
the morphology of the supramolecular structures formed by the
combination of 2c with racemic 1a can be visualized by AFM
imaging (Figures 10 and S10, Supporting Information).
The systematic study on the effect of the number of
stereogenic centers at the side chains and the connectivity
between those side chains and the central aromatic moiety in
OPE-based discotics presented herein increases the under-
standing of the structural rules that regulate the chiral
supramolecular organization of discrete molecules in general
and, more specifically, those based on π-conjugated oligomers.
Figure 10. AFM height images (a and c) of the helical columnar
aggregates obtained by the mixture 1a+2c onto HOPG (z scale = 35
nm for (a) and 15 nm for (c); toluene, 1 × 10−5 M). (b) Histogram of
the height distribution of the columnar aggregates shown in (a).
ASSOCIATED CONTENT
AFM images of a drop-casted toluene solution at 1 × 10−4 M
of the mixture show long fibers that bundled to form thicker
filaments (Figure S10, Supporting Information). The fibrils
formed by this mixture are stable even at diluted conditions and
stacks of right-handed filaments are observed in the AFM
images after depositing a 1 × 10−5 M solution of the mixture in
toluene. The histogram determined for the stacks height shows
two distributions of aggregates centered the most populated at
3.2 nm, and the less populated at 5.2 nm (Figure 10b). The
smaller effective height fits with the calculated size for a single
trisamide,14 which clearly suggests that the columnar π-stacked
aggregates are deposited perpendicular to the substrate and can
bundle to form thicker deposits. These images confirm that it is
possible to efficiently transfer the supramolecular polymers
formed by the mixture of 1a+2c onto surfaces, which is a great
advantage in the potential application of these aggregates.
■
S
* Supporting Information
FTIR data, UV−vis and CD melting curves, sergeants and
soldiers experiments, SEM and AFM images, and experimental
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
lusamar@quim.ucm.es
ACKNOWLEDGMENTS
■
Financial support by the MICINN of Spain (CTQ2011-22581)
and UCM (GR42/10-962027) is acknowledged. F.G. is
indebted to MEC for a FPU studentship. We gratefully
́
acknowledge to Prof. N. Martın, Prof. G. Orellana, Prof. F.
Gavilanes, and Dr. B. Yelamos from UCM.
́
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