Mirror helices and helicity switch at surfaces based on chiral triangular-shape oligo(phenylene ethynylenes)

Article abstract of DOI:10.1002/chem.201001596

The self-assembly of triangular-shaped oligo(phenylene ethynylenes) (OPEs), peripherally decorated with chiral and linear paraffinic chains, is investigated in bulk, onto surfaces and in solution. Whilst the X-ray diffraction data for the chiral studied s

Full text of DOI:10.1002/chem.201001596

FULL PAPER
DOI: 10.1002/chem.201001596
Mirror Helices and Helicity Switch at Surfaces Based on Chiral Triangular-
Shape Oligo(phenylene ethynylenes)
Fꢀtima Aparicio,[a] Fꢀtima Garcꢁa,[a] Gustavo Fernꢀndez,[a] Emilio Matesanz,[b] and
Luis Sꢀnchez*[a]
Abstract: The self-assembly of triangu-
lar-shaped oligo(phenylene ethyny-
lenes) (OPEs), peripherally decorated
with chiral and linear paraffinic chains,
is investigated in bulk, onto surfaces
and in solution. Whilst the X-ray dif-
fraction data for the chiral studied sys-
tems display a broad reflection cen-
tered at 2q ~208 (l=CuKa), the higher
crystallinity of OPE 3, endowed with
three linear decyl chains, results in a
diffractrogram with a number of well-
resolved reflections that can be accu-
rately indexed as a columnar packing
arranged in 2D oblique cells. Com-
pounds (S)-1a and (R)-1b—endowed
with (S)- and (R)-3,7-dimethyloctyloxy
chains—transfer their chirality to the
supramolecular structures formed upon
their self-assembly, and give rise to hel-
ical nanostructures of opposite handed-
ness. A helicity switch is noticeable for
the case of chiral (S)-2 decorated with
(S)-2-methylnonyloxy chains which
forms right-handed helices despite it
possesses the same stereoconfiguration
for their stereogenic carbons as (S)-1a
that self-assembles into left-handed
helices. The stability and the mecha-
nism of the supramolecular polymeri-
zation in solution have been investigat-
ed by UV/Vis experiments in methylcy-
clohexane. These studies demonstrate
that the larger the distance between
the stereogenic carbon and the aromat-
ic framework is, the more stable the ag-
gregate is. Additionally, the self-assem-
bly mechanism is conditioned by the
peripheral substituents: whereas com-
pounds (S)-1a and (R)-1b self-assem-
ble in a cooperative manner with a low
degree of cooperativity, the aggrega-
tion of (S)-2 and 3 is well described by
an isodesmic model. Therefore, the in-
teraction between the chiral coil chains
conditions the handedness of the heli-
cal pitch, the stability of the supra-
molecular structure and the supra-
molecular polymerization mechanism
of the studied OPEs.
Keywords: chirality
·
helical
structures · nucleation-elongation ·
pi interactions · self-assembly
Introduction
(mechanical, optical, electrical) exhibited by the aggregates
could be directly related with the mechanism followed by
the building blocks to self-assemble.[2–4] This self-assembly
process strongly depends on thermodynamic parameters
such as concentration or temperature and it is possible to es-
tablish three different mechanisms: a) isodesmic, b) ring-
chain, and iii) cooperative or nucleation-elongation.[3] An
isodesmic aggregation presents a single binding constant (K)
for each assembly step. For a ring–chain mechanism, the
connection of two reversibly associating units by a flexible
spacer is a must. Concentration is crucial in this mechanism
to switch rings into linear chains. The cooperative mecha-
nism can be simplified as a two-state model in which an un-
favored nucleus needs to be formed prior to the fast and en-
ergetically favored elongation of the polymeric chain. This
circumstance implies that two different binding constants
are necessary to describe the nucleation-elongation mecha-
nism.[3,5]
DNA, collagen or tobacco mosaic virus are paradigmatic
natural molecules in which weak supramolecular interac-
tions operate to create helical structures and condition their
specific biological function. The presence of stereogenic cen-
ters at the molecular building blocks is responsible for the
helicity of such natural compounds. Mimicking the natural
process of creating helical supramolecular structures has
prompted researchers to synthesize artificial helical ensem-
bles with the primary goal of contributing to unravel the
mechanisms followed by Nature in the generation of such
chiral superstructures. Interestingly, the chiral organization
of unnatural molecules holds potential applicability in supra-
molecular electronics and chirotechnology.[1] The properties
[a] F. Aparicio, F. Garcꢀa, Dr. G. Fernꢁndez, Prof. Dr. L. Sꢁnchez
Departamento de Quꢀmica Orgꢁnica, Facultad de Ciencias Quꢀmicas
Ciudad Universitaria s/n, 28040 Madrid (Spain)
Fax : (+34)9139-44-103
p-Conjugated molecules—many of them exhibiting a
strong tendency to aggregate—are playing a prominent role
in the integration of organic materials into optoelectronic
devices.[4a,6] Regarding the self-assembly of p-conjugated
systems, two end groups are electronically coupled and p–p
stacking interactions are the main driving force for these
systems to self-assemble. If an isodesmic mechanism oper-
E-mail: lusamar@quim.ucm.es
[b] Dr. E. Matesanz
C.A.I. Difraccion de Rayos X, Facultad de Ciencias Quꢀmicas
Ciudad Universitaria s/n, 28040 Madrid (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001596.
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ates in the self-assembly of such p-conjugated systems, the
strength of p–p interactions is unaffected upon aggregation.
Nonetheless, if some additional contribution—conformation-
al changes or other secondary attractive interaction—takes
place upon, or participates in the self-assembly, a nuclea-
tion–elongation mechanism will describe the corresponding
aggregation process. Different classes of achiral p-conjugat-
ed molecules, like phenyl–acetylene macrocycles,[7] hexakis-
(phenylethynyl)triindoles,[8] tris(phenylisoxazolyl)benzenes,[9]
hexa-peri-hexabenzocoronenes (HBCs)[10] or perylene bisi-
mide derivatives,[11] have been reported to form electronical-
ly coupled, linear supramolecular structures following an
isodesmic or equal-K mechanism. More interestingly, the in-
corporation of chiral peripheral substituents to some of
these aromatic platforms—specially phenylenevinylenes,
phenyleneethynylenes or phenylenes—gives rise to helical
structures in which a simple stereogenic center bias the
overall stacking in a hierarchical manner.[11–15]
We have recently reported on the self-assembly of a varie-
ty of amphiphilic radial oligo(phenylene ethynylenes)
(OPEs) that self-assemble to form hollow vesicles or toroids,
nanosheets or wires.[16] Concentration dependent UV/Vis
studies carried out with all these aromatic systems demon-
strate an isodesmic mechanism for the corresponding supra-
molecular aggregation process in solution. Herein we report
on the formation of mirror helices from chiral triangular
OPEs—compounds (S)-1a, (R)-1b and (S)-2, see Scheme 1
in analogy to previous amphiphilic OPEs reported by our
research group and is depicted in Scheme 1.[16] The first syn-
thetic step to obtain triangular-shape OPEs (S)-1a and (R)-
1b, endowed with peripheral (S)- or (R)-3,7-dimethylocty-
loxy chains, respectively, requires the catalytic hydrogena-
tion of commercially available (S)- and (R)-citronellol.[17]
The peripheral aromatic units decorated with the corre-
sponding chiral chains ((S)-6a, (R)-6b and (S)-7) were pre-
pared by utilizing a bimolecular nucleophilic substitution
(SN2) Mitsunobuꢃs reaction[18] between the corresponding
chiral alcohols (S)-4a, (R)-4b or (R)-decan-2-ol ((R)-5) and
commercial 4-iodophenol. The SN2 mechanism of the Mitsu-
nobuꢃs reaction completely inverts the R absolute configura-
tion of the stereogenic center of commercial (R)-decan-2-ol
((R)-5) and ensures the same stereoconfiguration for the
chiral peripheral chains of both compounds (S)-6a and (S)-
7. Finally, compounds (S)-6a, (R)-6b and (S)-7 were reacted
with 1,3,5-triethynylbenzene (8)[19] by following a C C
ꢀ
cross-coupling Sonogashira-type reaction catalyzed by palla-
dium[20] to yield the target OPEs (S)-1a, (R)-1b and (S)-2 in
82, 68 and 55%, respectively. All new compounds reported
in this study ((R)-6b, (S)-7, (S)-1a, (R)-1b and (S)-2) have
been fully characterized by using NMR and FTIR spectros-
copy, and mass spectrometry (see the Supporting Informa-
tion). The C3 symmetry of the synthesized OPEs (S)-1a,
(R)-1b and (S)-2 results in simple 1H NMR spectra, in
which only three aromatic resonances appear (d ~7.6, 7.4,
and 6.8). In the aliphatic region, the signals at d ~4 ppm cor-
responding to the methylene group next to the oxygen atom
and at d ~0.9 ppm ascribable to the methyl group present at
the stereogenic center, confirm the structure of the final
compounds. As expected, the enantiomeric character of (S)-
1a and (R)-1b yields identical spectroscopic features for
both of them.
Besides the structural information provided by MALDI-
TOF (matrix assisted laser desorption ionization-time-of-
flight) spectrometry, these studies are a powerful tool to
evaluate the capability of organic compounds to interact
non-covalently.[21] Thus, compound (S)-1a shows an intense
peak at m/z at 846 corresponding to the molecular peak to-
gether with less intense peaks at m/z 1693, 2539 and 3386 di-
agnostic of the aggregation of two, three or four units
(Figure 1). The closer proximity of the stereogenic center to
the aromatic backbone in (S)-2 could reduce the stability of
the supramolecular polymer and only the peaks correspond-
Scheme 1. Synthesis of chiral OPEs (S)-1a, (R)-1b and (S)-2.
and formula below—and compare their self-assembly mech-
anism with the previously reported triangular OPE endowed
with three linear decyl peripheral chains (compound 3).[9]
The nature of the peripheral chiral chains conditions the
helicity of the aggregates and also the association mecha-
nism of the studied OPEs.
Results and Discussion
Synthesis: The synthesis of chiral OPEs (S)-1a, (R)-1b and
(S)-2 has been carried out by following a multistep protocol,
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Chiral Triangular-Shape Oligo(phenylene ethynylenes)
FULL PAPER
ing to the molecular ion (m/z 846) and the dimer (m/z
1693) appear in its MALDI-TOF spectrum (Figure S1).[22]
Figure 1. MALDI-TOF mass spectra of OPE (S)-1a in a ditranol matrix.
Organization in bulk: The organization of triangular-shape
OPEs (S)-1a, (S)-2 and 3 in the bulk-state has been ana-
lyzed by X-ray diffraction experiments (l=CuKa). Both
chiral (S)-1a and (S)-2 display a diffuse halo in the wide-
angle region at 2q ~208 (Figure 2a,b). In addition, (S)-2
presents also some crystalline peaks at 7.296, 4.220 and
3.656 ꢄ, being the result of some degree of order. To inves-
tigate a potential increasing in the crystallinity of (S)-1a,
we have carried out a temperature-dependent X-ray scat-
tering analysis (Figure S2). Unfortunately, no changes
appear in the range of studied temperatures (263–298 K)
and only the above mentioned diffuse halo at 2q ~208 is
discernible. Similarly to (S)-1a and (S)-2, the X-ray diffrac-
togram of OPE 3 shows the diffuse halo centered at 2q
~208 together with a high number of well-resolved reflec-
tions (Figure 2c) which can be accurately indexed as a two-
dimensional oblique unit cell with typical parameters of a=
2.8, b=2.7 nm, and g=71.48 (Table S1 and Figure 2c).
These X-ray scattering data for 3 suggest a columnar pack-
ing of 2D oblique cells with the decyl chains interdigitated
and interacting by van der Waals forces (Figure 2d).
Figure 2. X-ray diffraction patterns (298 K) plotted against the angle 2q
for OPEs a) (S)-1a, b) (S)-2, and c) 3. The inset in c) shows the expanded
region of large angles. d) Schematic representation of the 2D oblique
unit cell.
Self-assembly on surfaces: The self-assembling features of
the triangular OPEs (S)-1a, (R)-1b, (S)-2 and 3 on surfaces
have been firstly investigated by atomic force microscopy
(AFM). Tapping-mode AFM images from a spin-coated so-
lution of chiral OPE (S)-1a in methylcyclohexane (MCH,
~10ꢀ4 m) on highly ordered pyrolytic graphite (HOPG) ex-
hibit isolated rod aggregates of several micrometers long
(Figure 3a and S3). These rods clearly present a rope-like
structure with ~3.2 nm in height in good agreement with the
molecular dimensions calculated for all these triangular-
Figure 3. Tapping-mode AFM images (air, 298 K) of a spin-cast MCH so-
lution (~10ꢀ4 m, 2000 r.p.m., HOPG) of a) (S)-1a showing a M-type helix
(Z scale=10 nm), and b) (R)-1b showing a P-type helix (Z scale=
10 nm). The insets depict the corresponding height profile and the helical
pitch along the black line in a) and b).
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L. Sꢁnchez et al.
shape OPEs (Figure S4).[23] Additionally, the rope-like struc-
tures present a left-handed helicity (Figure 3a) due to the
transfer of the chiral information embedded in the alkyl pe-
ripheral chains of (S)-1a to the columnar stacks. The M-type
helical wound arrangement of (S)-1a results in an average
pitch of 35 nm, in good correlation with many other helices
formed upon the self-assembly of a variety of organic scaf-
folds.[24] An optimal packing of molecules of triangular (S)-
1a involves a face-to-face p-stacking with all the peripheral
wedges rotated in the same direction. This columnar packing
gives rise to preferred stable chiral objects (Figure 4a and
S5a).[25]
larly to (S)-1a and (R)-1b. However, two sharp differences
are observed when both OPEs (S)-1a and (S)-2 are com-
pared: a) the helicity of the arrangement of (S)-2 is right-
handed (P-type), and b) the pitch of the P helix is of
~28 nm, a smaller value than that found for (S)-1a. The hel-
icity switch between (S)-1a and (S)-2 is consequence of the
change in the position of the chiral substituent along the
paraffinic chain. A plausible explanation of this behaviour
can be extracted from the calculated molecular geometry of
OPEs, (R)-1b and (S)-2 (Figure S4). As expected, com-
pounds (S)-1a and (R)-1b appear as non-superimposable
mirror images due to the opposite absolute configuration of
the stereogenic centers (to
guide the eye we have placed
the methyl substituent of the
stereogenic center in green and
pointing to the reader in Fig-
ure S4a, S4c, and S4e). Chiral
(S)-2 presents an opposite dis-
tribution of the methyl groups
of the stereogenic center than
(S)-1a which finally results in
contrary rotation turns when
stacks on top of each other to
attain stable helices (Figure 4c
and S5c).
The difference in the magni-
tude of helical pitches is ascrib-
able to the closer localization of
the chiral center in (S)-2 in
comparison with (S)-1a and
(R)-1b (Figure 4c and S5c).
The closer proximity of the ste-
reogenic center in (S)-2 exerts a
bigger steric demand in the op-
timal packing and induces a
Figure 4. Schematic illustration of the self-assembly of a) (S)-1a, b) (R)-1b and c) (S)-2. The yellow line de-
picts the helicity of the aggregates. The ellipsoids indicate the steric hindrance between the peripheral substitu-
ents or the lack of such steric interaction.
AFM images of (R)-1b (MCH, ~10ꢀ4 m, HOPG) also
reveal rope-like aggregates with ~3 nm in height and
~32 nm of pitch, in good correlation with the calculated mo-
lecular dimensions and the above mentioned pitch observed
for (S)-1a. Interestingly, the nanostructures formed upon
the self-assembly of (R)-1b are mirror images of those ob-
tained form (S)-1a (Figure 3b and S6) and right-handed
helices can be visualized as a consequence of the change in
the stereoconfiguration of the chiral coiled chains from S to
R. The change in the helicity, scarcely reported for small or-
ganic compounds,[24d] implies that to achieve an optimal p-
stacking between the aromatic backbones, all the peripheral
chiral wedges are oppositely rotated in (R)-1b than for com-
pound (S)-1a (Figure 4a, b and S5).
Figure 5. a) Tapping-mode and b) 3D AFM images (air, 298 K, HOPG)
of a spin-cast solution (MCH, ~10ꢀ4 m, 2000 r.p.m.) of (S)-2 showing a
P-type helix (Z scale=10 nm).
more pronounced rotation of the peripheral aromatic units
in the stacks, giving rise to helical structures with lower
pitches.
We have also investigated the morphology of the supra-
molecular structures formed from achiral 3. The AFM
images obtained upon spin-casting a ~10ꢀ4 m MCH solution
Spin-casting (2000 r.p.m.) a ~10ꢀ4 m solution of chiral
OPE (S)-2 in MCH onto HOPG also results in the appear-
ance of linear rods with lengths of several micrometers and
height of ~3 nm (Figure 5). Expanded AFM images of the
wires formed from (S)-2 also show rope-like structures, simi-
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Chiral Triangular-Shape Oligo(phenylene ethynylenes)
FULL PAPER
of OPE 3 clearly show long monodimensional filaments
with opposite handedness (Figure S7). Despite the achiral
character of compound 3, AFM images show the presence
of left- and right-handed columnar stacks, which suggests
that 3 behaves as an enantiomeric mixture that upon self-as-
sembly gives rise to helices of opposite helicity. Taking into
account that compound 3 lacks any additional attractive sec-
ondary interaction or chiral substituent to induce the helici-
ty, its helical organization could be justified by the local
ꢀ
dipole moments present between the C O bonds of the pe-
ripheral wedges.[26] To the best of our knowledge, this is the
first time that a mixture of M and P columnar stacks is vi-
sualized by AFM from the deposition of an achiral molecule
onto a surface.
The surface and also the deposition technique play a cru-
cial role in the morphology of the aggregates and, especially,
in the visualization of chirality. Thus, scanning (SEM) and
transmission-electron microscopy (TEM) images demon-
strate the formation of long fibrils (Figures 6, S8, and S9).
Figure 7. Normalized UV/Vis absorption spectra of (R)-1b (MCH, 298 K,
2ꢅ10ꢀ4 to 1.6ꢅ10ꢀ7 m). Arrows indicate the direction of change with in-
creasing concentration. The inset depicts the experimental aaggr values of
chiral (R)-1b (MCH, 298 K, 2ꢅ10ꢀ4 to 1.6ꢅ10ꢀ7 m) at 318 nm plotted as a
function of KcT with different s values according to cooperative nuclea-
tion-elongation model (lines from left to right: s=1, 0.2, 0.1, and 0.01).
the increase of the two lower energetic bands at ~300 and
~314 nm (Figure 7).[16] Considering the previous results ob-
tained for radial OPEs, we firstly considered that the aggre-
gate formation in solution could be described by an isodes-
mic model. If this is the case, the molar fraction of aggre-
gates (aaggr) or the molar fraction of monomeric species
(amon) are related with the binding constant K by Equation
(1):[3,5,28]
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2KcT þ ꢀ 2KcT þ 1
ð1Þ
aaggr ¼ 1 ꢀ amon ¼ 1 ꢀ
2
2ðKcTÞ
Plotting the variation of aaggr against the concentration of
chiral (S)-1a and (R)-1b fails in fitting with the isodesmic
model (Figures 7, S10, and S11). This non-isodesmic aggre-
gation process implies the operation of a nucleation–elonga-
tion mechanism in the self-assembly of these two chiral
OPEs. The simplest cooperative model for self-assembly
considers that the nucleation step consists in the formation
of a dimer, defined by K2, followed by the isodesmic elonga-
tion step expressed by K. The degree of cooperativity is
given by the parameter s that is described as K2/K. There-
fore, the smaller the s value is, the higher the cooperativity
of the supramolecular polymerization is. The mathematical
relation among the different binding constants (K2 and K),
s, and amon is given by the cubic Equation (2). The impossi-
bility to solve Equation (2) requires the utilization of Equa-
tions (3) and (4) to estimate s, K2 and K:[5,28]
Figure 6. SEM image of a long fiber formed by the aggregation of (S)-2
(MCH, ~10ꢀ4 m). The inset depicts the diameter of the fiber.
The diameter of the fibers seen in SEM images is in the
range of ~120 nm that corresponds to several intertwined
wires. To our surprise, the rope-like structures seen in the
AFM images are not observed in these images which is indi-
cative of the strong influence of the substrate and also of
the deposition technique into the morphology of the aggre-
gates.[27]
Self-assembly in solution: The mechanism followed by the
chiral OPEs (S)-1a, (R)-1b and (S)-2 to self-assemble in so-
lution has been investigated by concentration dependent
UV/Vis experiments. As in our previous reported radial
OPEs, these studies disclose the interaction of compounds
(S)-1a, (R)-1b and (S)-2 by p-stacking since the reduction
of the high energetic band at ~284 nm is concomitant with
a3monðKcTÞ ðs ꢀ 1Þ þ am2 onKcTðKcT ꢀ 2ðs ꢀ 1ÞÞ
ꢀamonð2KcT þ 1Þ þ 1 ¼ 0
2
ð2Þ
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L. Sꢁnchez et al.
sKcmon
(CD) (Figure S14). CD studies of chiral (S)-1a, (R)-1b, and
(S)-2 resulted in a very weak response in all the range of
concentrations utilized (~5ꢅ10ꢀ4– ~10ꢀ5 m). The planarity of
the aromatic framework[29]—that results in no conformation-
al changes upon self-assembly—and the lack of any secon-
dary non-covalent interaction—that efficiently direct the ro-
tation in only one sense during the aggregation—could form
disordered columnar stacks of nonpreferred helicity and
would justify the lack of a clear dichroic response.[14b]
KcT ¼ ð1 ꢀ sÞKcmon
þ
ð3Þ
ð4Þ
2
ð1 ꢀ Kcmon
Þ
Kcmon
KcT
aaggr ¼ 1 ꢀ amon ¼ 1 ꢀ
Plotting Kcmon vs KcT according to the nucleation–elongation
model for a number of s values [Eq. (3)] allows calculating
the theoretical aaggr values for cooperative processes by ap-
plying Equation (4) (Figure S12). The experimental aaggr
values found for (R)-1b fit quite well with the calculated
curve for a s value of 0.2, and binding constants of K2 =
2.5ꢅ104 mꢀ1 and K=1.25ꢅ105 mꢀ1 can be extracted. Similar
results have been obtained for chiral (S)-1a being the pa-
rameters corresponding to a nucleation–elongation mecha-
nism of s=0.2, and binding constants of K2 =2.6ꢅ104 mꢀ1
and K=1.3ꢅ105 mꢀ1 (Figure S11).
Unexpectedly, the variation of the molar extinction coeffi-
cient with increasing concentration observed for chiral OPE
(S)-2 results in a sigmoidal curve that fits with Equation (1)
with an R2 value of 0.99687 (Figure S13) indicating that the
supramolecular polymerization of this chiral system is well
described by an isodesmic model, that is, with a s value of 1,
and a binding constant of K=4.8ꢅ104 mꢀ1. These results are
in clear contrast with those obtained for (S)-1a and (R)-1b
that self-assemble cooperatively. All these data suggest the
influence exerted by the peripheral substituents on the
supramolecular polymerization mechanism (Figure 4). The
closer proximity of the stereogenic center in (S)-2 in com-
parison with compounds (S)-1a and (R)-1b, induces a more
pronounced rotation of the peripheral aromatic units in the
stacks that generates helical structures with low pitches (Fig-
ure 4c and S5c) and also a less efficient overlap of the aro-
matic frameworks when the molecules of (S)-2 stack on top
of each other. The smaller contact surface results in a lower
binding constant.
The different degree of rotation of the p-stacked mole-
cules induced for the stereogenic center in the studied chiral
OPEs could also justify the dissimilar supramolecular poly-
merization mechanism observed for chiral (S)-1a and (R)-
1b in comparison with chiral (S)-2 and achiral 3.[16b] Consid-
ering that, first, all the studied OPEs present an identical ar-
omatic skeleton and, second, there are no additional secon-
dary interactions operating in the aggregation of any of
these systems, the cooperative character of the supramolec-
ular mechanism determined for (S)-1a and (R)-1b can only
be justified by considering the branched nature of their pe-
ripheral coil segments. We postulate that the steric con-
straints exerted by the 3,7-dimethyloctyloxy chains difficult
the aggregation of (S)-1a and (R)-1b. A conformational
change should operate in these coil segments that releases
the steric hindrance between them and induces the forma-
tion of a nucleus of two molecules that interact by p–p
stacking forces, as clearly suggests the calculated value of
the nucleation K2. Once this nucleus is formed the system
elongates to form the helical supramolecular polymer.
Finally, we tried to corroborate the mechanism of the self-
assembly of (S)-1a, (R)-1b, and (S)-2 by circular dichroism
Conclusion
In conclusion, we report herein the helical aggregation of
chiral C3-symmetric OPEs. Chiral (S)-1a and (R)-1b self-as-
semble into supramolecular polymers of mirror helicity,
being left-handed for (S)-1a and right-handed for (R)-1b, to
avoid the steric constraints exerted by the stereocenter pres-
ent in the peripheral coiled chains. Interestingly, a helical
switch is observed for chiral (S)-2 despite it possesses the
same stereoconfiguration as (S)-1a. In addition, the closer
proximity of the chiral center to the aromatic framework in
(S)-2 results in a smaller value for the pitch of the P-helix.
C-dependent UV/Vis investigations demonstrate that the
supramolecular polymerization mechanism depends upon
the chemical nature of the peripheral, paraffinic chains. An
isodesmic aggregation mechanism applies for the case of
chiral (S)-2 and achiral 3. Contrary to that, chiral (S)-1a and
(R)-1b self-assemble by following a cooperative nucleation-
elongation mechanism with values of s=0.2, K2 ~2.5ꢅ
104 mꢀ1, and K ~1.5ꢅ105 mꢀ1. The dissimilar aggregation
mechanism followed by the studied triangular-shape OPEs
could be explained by considering the steric demand exerted
by the 3,7-dimethyloctyloxy groups that decorate the periph-
ery of (S)-1a and (R)-1b. Our overall results bring to light
that minute variations in the molecular structure of the su-
pramolecularly interacting species—as it occurs in nature—
can decisively alter the outcome of a self-assembly process.
The comprehension of the variables that control the self-or-
ganization processes in artificial systems would certainly
help to create supramolecular systems that exhibit the com-
plexity of their natural counterparts.
Experimental Section
1,3,5-Tris(2-(4-((S)-3,7-dimethyloctyloxy)phenyl)ethynyl)benzene
[(S)-
1a]: Compound (S)-6a (1.18 g, 3.3 mmol), bis(triphenylphosphine)palla-
dium(II) chloride (0.03 g, 0.05 mmol) and copper(I) iodide (0.011 mg,
0.06 mmol) were dissolved in triethylamine (16 mL). The mixture was
subjected to several vacuum/argon cycles and compound
8 (0.15 g,
1 mmol) was added. The mixture was heated at 608C overnight. After
evaporation of the solvent under reduced pressure, the residue was puri-
fied by column chromatography (silica gel, hexane/dichlorometane 4:1)
affording (S)-1a as
300 MHz, TMS): d=7.58 (s, 3H; Ha), 7.46 (d, J
a
yellow oil (0.69 g, 82%). 1H NMR (CDCl3,
3
(H,H)=8.8 Hz, 6H; Hb),
6.88 (d, 3J
1.75–1.48 (br, 9H; Hk+e), 1.40–1.12 (br, 18H; Hh+i+j), 0.96 (d, 3J
6.4 Hz, 9H; Hg), 0.89 ppm (d, 3J(H,H)=6.6 Hz, 18H; Hl+m); 13C NMR
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
2774
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Chiral Triangular-Shape Oligo(phenylene ethynylenes)
FULL PAPER
(CDCl3, 75 MHz, TMS): d=159.5, 133.4, 133.2, 124.3, 114.8, 114.6, 90.5,
86.7, 66.5, 39.3, 37.3, 36.2, 29.9, 28.0, 24.7, 22.7, 22.6, 19.7 ppm; FTIR
(neat): n˜ = 683, 832, 878, 1020, 1051, 1109, 1174, 1249, 1292, 1385, 1471,
1510, 1580, 1608, 2211, 2870, 2928, 2955 cmꢀ1; MALDI-TOF-MS: m/z:
864.4; HRMS: m/z: calcd for C60H78O3 [M+]: 846.595; found: 846.594.
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1,3,5-Tris(2-(4-((R)-3,7-dimethyloctyloxy)phenyl)ethynyl)benzene [(R)-
1b]: Compound (R)-6b (0.20 g, 0.56 mmol), bis(triphenylphosphine)pal-
ladium(II) chloride (0.006 g, 0.008 mmol) and copper(I) iodide (0.002 mg,
0.01 mmol) were dissolved in triethylamine (3 mL). The mixture was sub-
jected to several vacuum/argon cycles and compound
8 (0.02 g,
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0.17 mmol) was added. The mixture was heated at 608C overnight. After
evaporation of the solvent under reduced pressure, the residue was puri-
fied by column chromatography (silica gel, hexane/dichlorometane 8:2)
affording (R)-1b as
300 MHz, TMS): d=7.58 (s, 3H; Ha), 7.46 (d, J
a
yellow oil (0.098 g, 68%); 1H NMR (CDCl3,
3
(H,H)=8.8 Hz, 6H; Hb),
6.88 (d, 3J
1.75–1.48 (br, 9H; Hk+e), 1.40–1.12 (br, 18H; Hh+i+j), 0.96 (d, 3J
6.4 Hz, 9H; Hg), 0.89 ppm (d, 3J(H,H)=6.6 Hz, 18H; Hl); 13C NMR
ACHTUNGTRENNUNG
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AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(CDCl3, 75 MHz, TMS): d=159.5, 133.4, 133.2, 124.3, 114.8, 114.6, 90.5,
86.7, 66.5, 39.3, 37.3, 36.2, 29.9, 28.0, 24.7, 22.7, 22.6, 19.7 ppm; FTIR
(neat): n˜ = 683, 832, 878, 1020, 1051, 1109, 1174, 1249, 1292, 1385, 1471,
1510, 1580, 1608, 2211, 2870, 2928, 2955 cmꢀ1; HRMS: m/z: calcd for
C60H78O3 [M+]: 846.595; found: 846.594.
1,3,5-Tris(2-(4-((S)-decan-2-yloxy)phenyl)ethynyl)benzene [(S)-2]: Com-
pound (S)-7 (0.87 g, 2.42 mmol), bis(triphenylphosphine)palladium(II)
chloride (0.03 g, 0.05 mmol) and copper(I) iodide (0.007 mg, 0.04 mmol)
were dissolved in triethylamine (20 mL). The mixture was subjected to
several vacuum/argon cycles and compound 8 (0.12 g, 0.78 mmol) was
added. The mixture was heated at 608C for 12 h and then was allowed to
reach room temperature. After evaporation of the solvent under reduced
pressure, the residue was purified by column chromatography (silica gel,
hexane/chloroform 2:1) affording (S)-2 as a brown oil (0.36 g, 55%);
1H NMR (CDCl3, 300 MHz, TMS): d=7.60 (s, 3H; Ha), 7.47 (d,
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3J
C
ACHTUNGRTEN(NGNU H,H)=8.8 Hz; 6H; Hc), 4.40 (sxt,
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U
TMS): d = 159.0, 133.7, 133.6, 124.7, 116.1, 114.9, 90.9, 87.1, 74.4, 36.8,
32.3, 30.0, 29.9, 29.7, 25.9, 23.1, 20.1, 14.5 ppm; FTIR (neat): n˜ = 684,
745, 832, 878, 915, 1011, 1063, 1123, 1173, 1246, 1287, 1377, 1461, 1506,
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This work has been supported by UCM (UCM-SCH-PR34/07-15826).
The MICINN of Spain is thanked by F.A. and F.G. for a F.P.U. research
grant. The authors are gratefully acknowledged to Prof. N. Martꢀn
(UCM, Spain) for the generous help.
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Received: June 7, 2010
Revised: August 23, 2010
Published online: January 26, 2011
2776
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Chem. Eur. J. 2011, 17, 2769 – 2776