Inversion of supramolecular helicity in oligo-p-phenylene-based supramolecular polymers: Influence of molecular atropisomerism

Article abstract of DOI:10.1002/anie.201309172

The helical organization of oligo-p-phenylene-based organogelators has been investigated by atomic force microscopy, circular and vibrational circular dichroism, and Raman techniques. Whilst OPPs with more than two phenyl rings in the core self-assemble into left-handed helices, that with a biphenyl core shows an inversion of the supramolecular helicity depending on the formation conditions through the atropisomerism of the biphenyl central unit. The results presented herein outline a new example of kinetically controlled modulation of supramolecular helicity.

Full text of DOI:10.1002/anie.201309172

Angewandte
Chemie
DOI: 10.1002/anie.201309172
Helical Structures
Inversion of Supramolecular Helicity in Oligo-p-phenylene-Based
Supramolecular Polymers: Influence of Molecular Atropisomerism**
Fꢀtima Aparicio, Belꢁn Nieto-Ortega, Francisco Nꢀjera, Francisco J. Ramꢂrez,
Juan T. Lꢃpez Navarrete,* Juan Casado,* and Luis Sꢀnchez*
Abstract: The helical organization of oligo-p-phenylene-based
organogelators has been investigated by atomic force micros-
copy, circular and vibrational circular dichroism, and Raman
techniques. Whilst OPPs with more than two phenyl rings in
the core self-assemble into left-handed helices, that with
a biphenyl core shows an inversion of the supramolecular
helicity depending on the formation conditions through the
atropisomerism of the biphenyl central unit. The results
presented herein outline a new example of kinetically con-
trolled modulation of supramolecular helicity.
bles has important mechanistic implications in supramolec-
ular chemistry and is intimately connected with the topic of
chirality transmission, which is of high significance in life
sciences.
Herein, we report on the influence of the molecular
atropisomerism of oligo-p-phenylene-based (OPP) organo-
gelators (1–4; Figure 1) on the helical chirality resulting from
their supramolecular polymerization.[6] This helical chirality
in 2–4 was investigated on surfaces, in solution, and in a gel
state by utilizing circular dichroism (CD), vibrational circular
dichroism (VCD), and Raman optical activity (ROA) com-
bined with quantum model studies.
Chirality has been the inspiration for many scientists
investigating the processes that yield asymmetric products
from originally achiral racemates.[1] Asymmetric chemical
synthesis and catalysis,[2] liquid crystals,[3] and conductive
materials[4] are examples of the application of chiral structures
to different research areas. The term chirality describes the
structural property of an object that is non-superimposable on
its mirror image and can be achieved by the presence in
a molecule of: a) stereogenic centers; b) chiral planes;
c) chiral axes, or d) helical chirality. The last is often achieved
by the supramolecular polymerization of self-assembling
molecules decorated with peripheral side chains containing
stereogenic centers that act as the sole inductor element of
helical chirality.[5] Besides stereogenic seeds, the influence of
other alternative sources of chirality for the generation of, or
the interconversion between, left-handed (M-type) and right-
handed (P-type) supramolecular helices remains unprece-
dented. The control of the helicity in supramolecular ensem-
Compounds 3 and 4 form robust and stable chiral helical
aggregates, while 2 exhibits an exchangeable helicity depend-
ing on the reaction time, concentration, and temperature and
reveals a new example of kinetic-versus-thermodynamic
[*] F. Aparicio, L. Sꢀnchez
Departamento de Quꢁmica Orgꢀnica
Facultad de Ciencias Quꢁmicas
Ciudad Universitaria s/n, 28040 Madrid (Spain)
E-mail: lusamar@quim.ucm.es
B. Nieto-Ortega, F. J. Ramꢁrez, J. T. Lꢂpez Navarrete, J. Casado
Departamento de Quꢁmica Fꢁsica, Facultad de Ciencias
Universidad de Mꢀlaga
Campus de Teatinos s/n, 29071 Mꢀlaga (Spain)
E-mail: teodomiro@uma.es
casado@uma.es
F. Nꢀjera
Departamento de Quꢁmica Orgꢀnica, Facultad de Ciencias
Universidad de Mꢀlaga
Campus de Teatinos s/n, 29071 Mꢀlaga (Spain)
[**] Financial support from the MINECO of Spain (CTQ2011-22581 and
CTQ2012-33733) is acknowledged. F.A and B.N.O. are indebted to
MEC for their FPU studentship.
Figure 1. a) Chemical structures of the self-assembling OPP-based
organogelators 1–4. b) Photograph of the organogels formed from
compounds 1–4 in toluene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309172.
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competitive modulation of supramolecular helicity.[7] At low
temperature, low concentration, and short polymerization
times (kinetic control) compound 2 self-assembles into right-
handed metastable supramolecular helices, as dictated by the
axial chirality of the biphenyl core. In contrast, at higher
temperature, higher concentration, and long reaction times
(thermodynamic control), the axial chirality in 2 is cancelled
out and only the external S stereocenters impose a left-
handed durable supramolecular helix. In contrast, 3 and 4
self-assemble into left-handed aggregates, with no inversion
of their helicity. These findings imply the lack of influence of
the oligophenyl atropisomerism in the helical organization of
3 and 4 into their supramolecular aggregates, which is
exclusively dictated by the external stereocenters, such as of
2 under thermodynamic conditions. The studies presented
here contribute to the understanding of the chemical and
topological control in the generation of helical supramolec-
ular structures and the impact of the synergy between
different chiral elements.
The preparation of the OPP derivatives 2–4 starts with the
synthesis of the previously reported N-(2-aminoethyl)-3,4,5-
tri-((S)-3’,7’-dimethyloctyl)oxybenzamide (5 in Scheme S1 in
the Supporting Information),[8] which is coupled with
biphenyl-4,4’-dicarboxylic acid to yield 2 or with 4-iodoben-
zoic acid to give the corresponding iodobisamide. A double
Figure 2. AFM phase images of the diluted gels of 2 (a and b), 3 (c),
and 4 (d) on HOPG (1ꢃ10ꢀ5 m, toluene, 298 K).
ꢀ
Suzuki C C cross-coupling reaction with benzenediboronic
tures, in which the links are clearly visible but the helicity
cannot be assigned (Figure 2b and see Figure S1a–c in the
Supporting Information).
acid bis(pinacol) ester or with 4,4’-biphenyldiboronic acid
bis(pinacol) ester in the presence of a palladium catalyst[9]
affords compounds 3 and 4, respectively (see Scheme S1 in
the Supporting Information). All the newly reported com-
pounds have been fully characterized by NMR and FTIR
spectroscopy as well as by high-resolution mass spectrometry
(see the Supporting Information).
The helicity of the aggregates formed by OPPs 2–4 in
solution was first investigated by CD spectroscopy at room
temperature (Figure 3a). The CD spectra of compounds 2–4
as well as the previously reported 1[6] exhibit a positive
dichroic signal at around 220 nm, which was assigned to an
excitation within the peripheral trialkoxybenzamide units.
The increasing number of conjugated phenyl groups in 3 and 4
results in a more intense, red-shifted and (+ /ꢀ) bisignated
Cotton UV/Vis band (see Figure S4a in the Supporting
Information), which is diagnostic of the presence of M-type
helical structures in solution (Figure 3a). These experimental
The ability of compounds 2–4 to self-assemble was first
tested by the formation of organogels in apolar solvents such
as methylcyclohexane (MCH) and toluene. OPPs 2–4, as
reported for 1,[6] form transparent organogels in toluene
(Figure 1b). The formation of organogels is indicative of the
generation of columnar supramolecular structures that fur-
ther aggregate to form bundles
of fibers that support the sol-
vent upon inversion of the
vial.[10] AFM images of diluted
samples of the gel on highly
oriented pyrolytic graphite
(HOPG) show enantiomeri-
cally enriched, helical supra-
molecular structures (Figure 2,
see also Figures S1–S3 in the
Supporting Information). The
three organogels are organized
into ropelike, broad fibers with
height profiles of around 3–
5 nm. These ropelike fibers
exhibit a left-handed (M-type)
helicity. However,
a
closer
inspection of the AFM images
Figure 3. a) Electronic CD spectra (MCH, 1ꢃ10ꢀ5 m, 258C); b) ROA spectra (MCH, 3ꢃ10ꢀ3 m, 258C); and
of organogelator 2 shows the
presence of chainlike struc- c) VCD spectra (MCH, 3ꢃ10ꢀ3 m, ꢀ258C) of compounds 1–4.
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data are in good correlation with the helical structures
visualized in the AFM images. However, compound 2 shows
no clear bisignated CD response, and has a weaker negative
band, which suggests the same M-type helical organization as
in its larger congeners 3 and 4 (Figure 3a). Variable-temper-
ature CD (VT-CD) measurements show nonsigmoidal shapes
for the cooling curves of 2, 3, and 4,[12] thus demonstrating the
cooperative character of their supramolecular polymerization
by self-assembling into M-type supramolecular polymers (see
Figure S5 in the Supporting Information).[5,13] The presence of
more-planar backbones in 3 and 4 induces a more-efficient p-
p stacking that stabilizes the helical aggregates, as is suggested
by the higher critical temperature at which the nucleation
regime changes into an elongation one (see Figure S5 in the
Supporting Information).[5]
atures, and times (these are the three parameters that can
condition the switch of a kinetic product to the corresponding
thermodynamic one). The VCD spectrum of compound 2,
with a (+ /ꢀ) amide I pattern at 3 ꢀ 10ꢀ3 m and ꢀ258C,
exhibits the same (+ /ꢀ) VCD pattern at the same concen-
tration and at 08C, while the VCD signal completely vanishes
at 258C (Table 1 and see Figure S6 in the Supporting
Table 1: Sign of the VCD amide I band of 2–4 at different concentrations,
temperatures, and times in MCH.
Concentration
T
2
3
4
+258C
08C
ꢀ258C
no signal
+/ꢀ
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
3ꢃ10ꢀ3
8ꢃ10ꢀ3
m
m
+/ꢀ
+/ꢀ (t0 =0 h)
ꢀ/+ (tf =24 h)
+/ꢀ(t0=0 h)
[a]
Raman and ROA spectroscopic studies (298 K, 3 ꢀ 10ꢀ3 m)
were also used to investigate the chiral features of 1–4
(Figure 3b and see Figure S4c in the Supporting Informa-
tion). The Raman spectra of compounds 2–4 feature a clear
Raman band around 1610 cmꢀ1 that slightly shifts to lower
wavenumbers on going from 2 to 4 (see Figure S4c in the
Supporting Information) as a result of the more-efficient
conjugation in the cores with more phenyl rings. This
conjugation restricts the conformational flexibility of the
oligophenyl chains and facilitates intermolecular p-p stack-
ing. This Raman (and ROA) band is not detected in
compound 1 because of the absence of conjugation. The
spectra of compounds 3 and 4 show strong ROA positive
bands at about 1600 cmꢀ1. Again, the ROA spectrum of 2
does not follow the trend observed for 3 and 4, but shows
a negative and a positive signal at 1627 and 1611 cmꢀ1,
respectively, thus highlighting its distinctive behavior com-
pared its two larger congeners (Figure 3b).
+258C
08C
+/ꢀ(t0=0 h)
ꢀ258C
[a]
+258C
08C
ꢀ258C
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
ꢀ/+
1.2ꢃ10ꢀ2
m
ꢀ/+
ꢀ/+
[a] No changes within the 24 h timescale. Possibly longer times are
required.
Information). Increasing the concentration of 2 to 1.2 ꢀ
10ꢀ2 m results in an inverted (ꢀ/ +) pattern for the amide I
band at ꢀ25 and 08C, which persists at 258C (Table 1 and see
Figure S7 in the Supporting Information). This pattern for 2 at
1.2 ꢀ 10ꢀ2 m is the same as those of 3 and 4 at all temperatures
and concentrations (see Figure S7 in the Supporting Infor-
mation). These data already indicate that 2 can form different
enantiomerically enriched helical supramolecular structures
depending on the concentration of the sample, and under
these conditions these helices are stable over time.
To corroborate the presence of the kinetic-versus-ther-
modynamic competition in the self-assembly of 2, VCD
experiments were conducted at an intermediate concentra-
tion of 8 ꢀ 10ꢀ3 m as a function of time (Figure 4). At + 258C
and 8 ꢀ 10ꢀ3 m, compound 2 shows the same (+ /ꢀ) pattern as
that observed for the more dilute sample. Interestingly,
keeping this gel at this temperature for 24h results in the
inversion of the amide I band to a (ꢀ/ +) pattern, a time
period that unambiguously demonstrates that the supra-
molecular polymerization of 2 can follow either a kinetic or
a thermodynamic pathway depending on the conditions. We
also performed the VCD analysis of 3 and 4 by modifying the
concentration and temperature. Unlike compound 2, the sign
of the amide I band in 3 and 4 does not change, with a (ꢀ/ +)
pattern observed for the VCD amide I band, regardless of the
experimental conditions used (Table 1 and see Figures S9 and
S10 in the Supporting Information).
FTIR and VCD measurements provide valuable informa-
tion about the molecular-level helicity of the supramolecular
aggregates.[13] The vibrational measurements for 1–4 needed
to be carried out at a higher concentration (3 ꢀ 10ꢀ3 m, MCH)
and lower temperature (248 K) than those routinely used for
the preparation of the corresponding gels. A strong and
=
ꢀ
extensive C O···H N hydrogen-bonding network, which
significantly stabilizes the supramolecular polymers of 1–4,
is inferred from the vibrational amide I stretching band that
appears at around 1631 cmꢀ1 (see Figure S4b in the Support-
ing Information).[5d,e] Whereas this amide I VCD band in 1, 3,
and 4 has the same (+ /ꢀ) pattern, that of 2 is the opposite,
namely it has a (ꢀ/ +) signal. This VCD pattern implies the
inversion of the chirality from 2 to 1/3/4 at this concentration
and temperature (Figure 3c).
Assuming that the four samples have the same S-type of
stereocenter, the dissimilar VCD behavior found for com-
pounds 2 and 3/4 suggest that their supramolecular polymer-
ization could result in the formation of distinct aggregates of
opposite handedness, which was recently reported for the
supramolecular polymerization of an oligo(p-phenyleneviny-
lene)triazine (S-OPV) by either kinetic or thermodynamic
pathways.[14] Since the kinetic-versus-thermodynamic compe-
tition strongly depends on the experimental conditions, we
have investigated the supramolecular polymerization of
compounds 2–4 by VCD at different concentrations, temper-
It must be taken into account when rationalizing these
phenomena that the structural difference between 2 and 3/4
stems from the number of phenyl rings present in the central
aromatic moiety. In biphenyls, the steric hindrance exerted by
the ortho protons of adjacent phenyl rings prevails over the
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Figure 4. VCD spectra of 2. a) At high and low concentration (same
formation time and temperature). b) As formed (t0 =0 h) and after
24 h (tf =24 h) at the same concentration and temperature.
Scheme 1. Schematic illustration of the aggregation pathways of 2–4.
At low concentrations, low temperatures, and short times, the helical
organization of 2 is dominated by the atropisomerism of the central
biphenyl unit, and metastable P-type helices are formed. 3 and 4, and
also 2 at higher concentrations, higher temperatures, and longer
times, self-assemble into supramolecular structures of the opposite
helicity (M-type).
coplanarity induced by p conjugation, and this results in
a nonplanar conformation of the two inner aromatic units. As
a result, the two atropisomers P and M can exist (see
Figure S11 in the Supporting Information).[15] DFT quantum
chemical calculations at the B3LYP/6-31G** level were used
to predict the VCD spectra of these two atropisomers of 2
(the peripheral S stereocenters have been omitted in the
models in Figure S11 in the Supporting Information). The
calculated amide I bisignated VCD band for the M enan-
tiomer is (ꢀ/ +), which is opposite to that of the P enantiomer
(+ /ꢀ). Moreover, no VCD signal for the planar biphenyl
conformation was predicted (Figure S11 in the Supporting
Information). In the next step of the simulation, we consid-
ered the presence of the six peripheral S stereocenters, thus
generating the corresponding pair of M,S and P,S diasterer-
oisomers. The calculated VCD spectrum of the P,S diaster-
eomer coincides with the experimental VCD spectrum of 2
recorded at low temperature (ꢀ258C) and low concentration
(3 ꢀ 10ꢀ3 m) (Figure 3c). Additionally, these calculations
reveal that the rotational barrier required for the intercon-
version between the M,S and P,S diasteroisomers of 2 is
2.09 kcalmolꢀ1 (see Figures S12 and S13 in the Supporting
Information).
With all this data together, two different scenarios for the
supramolecular polymerization of the reported organogela-
tors are possible. The first concerns the helical organization of
3 and 4, which is only governed by the peripheral S stero-
centers, and results in M-type helical structures. No exchange-
able helicity is observed for 3 and 4 due to the rather planar
conformation of the oligophenyl core. The second scenario is
exclusive for 2, which discloses a large dihedral angle between
the two innermost phenyl rings, thus allowing two possible
atropisomers to enter into play. At low concentration and low
temperatures, atropisomerism through the P,S diastereomer
is effective and generates a kinetically controlled situation
where P-handed nuclei are initially formed (t0 in Table 1).
These further elongate to yield metastable P-type supra-
molecular polymers with a typical (+ /ꢀ) VCD amide I band
(Scheme 1). At higher concentrations (p-p stacking would
help to planarize the biphenyl) and higher temperatures (the
activation energy or rotational barrier of the biphenyl system
is easily overcome) the molecular atropisomerism of 2 is
cancelled out, and only the chiral, peripheral S stereocenters
dictate the helicity of the supramolecular aggregates, which
corresponds to the thermodynamically controlled M-type
helical supramolecular polymerization. Very interestingly, at
intermediate concentrations (8 ꢀ 10ꢀ3 m) and 258C, we find
a situation where at short times (t0 = 0h, as formed) the
polymerization reaction yields the kinetic P-type product,
which reverses its helicity after 24h (tf = 24h) to a thermody-
namically M-type final product. This P-type!M-type helical
inversion corroborates the kinetic-versus-thermodynamic
modulation of the supramolecular helicity of 2.
In summary, we have reported on the atropisomerization
of helical supramolecular polymers formed by the coopera-
tive self-assembly of oligo-p-phenylene-based organogelators
2–4. This helical atropisomerism is driven by the axial
chirality of the oligophenyl moieties. In this regard, biphenyl
atropisomerism and biphenyl axial chirality are synonymous.
The balance between orthogonality and p conjugation of the
oligophenyl units induces the molecular atropisomerism,
which plays a fundamental part in the expression of the
chirality of the supramolecular helices by means of kinetically
or thermodynamically controlled polymerization mecha-
nisms. Different techniques (CD, AFM, VCD, ROA, and
theoretical calculations) unambiguously demonstrate the left-
handed helical organization of polymers of 3 and 4 under all
the conditions investigated. Supramolecular polymerization
of 2 is highly sensitive to the initial conditions, which drive
either a kinetic or thermodynamic pathway to produce helical
structures of opposite handedness. Under particular condi-
tions, the kinetic product converts into the thermodynamic
one over time. All these data contribute to an increased
knowledge and control over the generation of helical
supramolecular structures by the synergy of different chiral
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Published online: December 18, 2013
Keywords: atropisomerism · chirality · helical structures ·
.
self-assembly · supramolecular polymers
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