Tuning the supramolecular chirality of one-and two-dimensional aggregates with the number of stereogenic centers in the component porphyrins

Article abstract of DOI:10.1021/ja101533j

A synthetic strategy was developed for the preparation of porphyrins containing between one and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups which give the chiral forms. The low-dimensional nanoscale a

Full text of DOI:10.1021/ja101533j

Published on Web 06/18/2010
Tuning the Supramolecular Chirality of One- and
Two-Dimensional Aggregates with the Number of Stereogenic
Centers in the Component Porphyrins
Patrizia Iavicoli,† Hong Xu,‡ Lise N. Feldborg,† Mathieu Linares,⊥,§,|
Markos Paradinas,† Sven Stafstro¨m,| Carmen Ocal,† Belen Nieto-Ortega,#
Juan Casado,# Juan T. López Navarrete,# Roberto Lazzaroni,*,§
Steven De Feyter,*,‡ and David B. Amabilino*,†
Institut de Cie`ncia de Materials de Barcelona (CSIC), Campus UniVersitari de Bellaterra,
08193 Cerdanyola del Valle`s, Catalonia, Spain, Katholieke UniVersiteit LeuVen, Laboratory of
Photochemistry and Spectroscopy, and INPAC-Institute for Nanoscale Physics and Chemistry,
Celestijnenlaan 200-F, 3001 HeVerlee, Belgium, SerVice de Chimie des Mate´riaux NouVeaux,
UniVersite´ de Mons, 20, Place du Parc, B-7000 Mons, Belgium, Department of Physics,
Chemistry and Biology, Linko¨ping UniVersity, SE-581 83 Linko¨ping, Sweden, and Departamento
de Qu´ımica F´ısica, Facultad de Ciencias, UniVersidad de Ma´laga, Ma´laga 29071, Spain
Received February 22, 2010; E-mail: amabilino@icmab.es; steven.defeyter@chem.kuleuven.be;
roberto.lazzaroni@umons.ac.be
Abstract: A synthetic strategy was developed for the preparation of porphyrins containing between one
and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups
which give the chiral forms. The low-dimensional nanoscale aggregates of these compounds reveal the
profound effects of this varying molecular chirality on their supramolecular structure and optical activity.
The number of stereogenic centers influences significantly the self-assembly and chiral structure of the
aggregates of porphyrin molecules described here. A scanning tunneling microscopy study of monolayers
on graphite shows that the degree of structural chirality with respect to the surface increases almost linearly
with the number of stereogenic centers, and only one handedness is formed in the monolayers, whereas
the achiral compound forms a mixture of mirror-image domains at the surface. In solution, four hydrogen
bonds induce the formation of an H-aggregate, and circular dichroism measurements and theoretical studies
indicate that the compounds self-assemble into helical structures. Both the chirality and stability of the
aggregates depend critically on the number of stereocenters. The chiral porphyrin derivatives gelate
methylcyclohexane at concentrations dependent on the number and position of chiral groups at the periphery
of the aromatic core, reflecting the different aggregation forces of the molecules in solution. Increasing the
number of stereogenic centers requires more material to immobilize the solvent, in all likelihood because
of the greater solubility of the porphyrins. The vibrational circular dichroism spectra of the gels show that
all compounds have a chiral environment around the amide bonds, confirming the helical model proposed
by calculations. The morphologies of the xerogels (studied by scanning electron microscopy and scanning
force microscopy) are similar, although more fibrous features are present in the molecules with fewer
stereogenic centers. Importantly, the presence of only one stereogenic center, bearing a methyl group as
the desymmetrizing ligand, in a molecule of considerable molecular weight is enough to induce single-
handed chirality in both the one- and two-dimensional supramolecular self-assembled structures.
or noncovalent bonds,2 the effect that the number of stereogenic
centers in molecules has on the amplification of chirality in the
aggregates that they form is (as far as we are aware) a largely
unknown and unexplored area,3 except from a very nice recent
study4 carried out in parallel to the work reported here, which
explores this somewhat barren quarter.
Introduction
Chiral structures are interesting from a number of viewpoints,1
and for this reason, the induction of chirality and its comprehen-
sion are worthy pursuits. While it has been established that the
position of stereogenic centers clearly influences the sign and
magnitude of optical activity in molecules united by covalent
† ICMAB-CSIC.
‡ Katholieke Universiteit Leuven.
(1) (a) Crego-Calama, M., Reinhoudt, D. N., Eds. Supramolecular
Chirality. In Top. Curr. Chem. 2006, 26. (b) Soai, K., Ed. Amplification
of Chirality. In Top. Curr. Chem. 2008, 284. (c) Amabilino, D. B.,
Ed. Chirality at the Nanoscale; Wiley-VCH: Weinheim, 2009. (d)
Raval, R. Chem. Soc. ReV. 2009, 38, 707–721. (e) Crassous, J. Chem.
Soc. ReV. 2009, 38, 830–845.
§ Universite´ de Mons.
| Linko¨ping University.
⊥ Present address: Department of Theoretical Chemistry, Royal Institute
of Technology, Roslagstullsbacken 15, S-106 91, Stockholm, Sweden.
# Universidad de Ma´laga.
9
9350 J. AM. CHEM. SOC. 2010, 132, 9350–9362
10.1021/ja101533j  2010 American Chemical Society

Supramolecular Chirality of One- and Two-Dimensional Aggregates
A R T I C L E S
The question of the influence of the number of stereogenic
centers on chiral structure and optical activity is important
because, while sergeants and soldiers5 and majority rules6 effects
can drive chirality induction in supramolecular systems, the
cases are relatively scarce, and it seems more habitual to find a
linear dependence of optical activity on chiral content in achiral
materials. The potential implications of understanding chiral
induction are important because the relative disposition of
molecules in condensates has profound effects on their proper-
ties, and the twist between chromophores is one key feature. In
this regard, functional self-assembled molecular materials with
well-defined shapes and dimensions are of great current interest,7
especially for applications in electronics, photonics, light-energy
conversion, and catalysis. In biological systems, tetrapyrrolic
pigments are often self-organized into nanoscale superstructures
that perform many of the essential light-harvesting and energy-
and electron-transfer functions. An example is the light-
harvesting rods of the chlorosomes of green-sulfur bacteria,
which are composed entirely of aggregated bacteriochlorophyll.8
Some synthetic porphyrins are known to form aggregates with
interesting optical and electronic properties,9 and these ag-
gregates sometimes occur in the form of useful nanostructures
including fibers, nanorods, or thin stripes on surfaces.10 There-
fore, because of their desirable functional properties, porphyrins
and other tetrapyrroles are attractive building blocks for
functional nanostructures,11 as well as interesting chromophores
for studies related to induction of chirality,12 and are the core
structures we have used in the studies reported here.
Bearing these considerations in mind, we designed and
synthesized porphyrin derivatives 1-6 which contain amide
groups as a simple hydrogen-bonding functional unit (Figure
1) and alkyl chains to provide solubility for the systems in
solution as well as adhesion to each other and surfaces through
van der Waals interactions. The compounds differ from each
other in the number and position of the stereogenic centers they
contain, and our aim was to investigate the role of these chiral
centers in the formation of supramolecular 1-D and 2-D
aggregate nanostructures. This study was done in solution using
circular dichroism (CD) spectroscopy and at an interface by
scanning tunneling microscopy (STM). The interpretation of the
results was assisted by molecular mechanics (MM) and molec-
ular dynamics (MD) simulations of the supramolecular as-
sembly.13 In addition, the ability of each one of these compounds
to gelate organic solvents was investigated, and the character-
istics of the gels and xerogels were explored by vibrational
circular dichroism (VCD), scanning electron microscopy (SEM)
and scanning force microscopy (SFM), respectively.
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Result and Discussion
Synthesis and Characterization. The porphyrin derivatives
1-6 were prepared by the synthetic route described in Scheme
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A R T I C L E S
Iavicoli et al.
Scheme 1. Synthesis of the Porphyrin Derivatives 1-6
Figure 1. Chemical structures of the compounds prepared and studied in
this work. Both the R and S enantiomers of the chiral compounds were
synthesized, but only the R enantiomers are discussed here.
1. All of the discussion here refers to the R enantiomers, but
we also prepared the S enantiomers, which give equal and
opposite chirostructural and chiroptical effects.
This strategy was carefully chosen so as to be able to separate
the different compounds with one, two, and three stereogenic
centers and the constitutional isomers of the compounds with
two stereogenic compounds from each other after the one-pot
reaction used to make all the intermediate porphyrins 9-13.
The long alkyl chain in one of the aldehydes in this condensation
reaction with pyrrole makes the products sufficiently different
so as to allow their isolation by silica gel column chromatog-
raphy. Thus, (R)-methyl 2-(4-formylphenoxy)propionate ((R)-
7)14 and 2-(4-formylphenoxy)-N-octadecylacetamide (8), pre-
pared by the reaction of 4-hydroxybenzaldehyde and N-octadecyl-
2-chloroacetamide15 (see the Experimental Section in the
Supporting Information) were condensed with pyrrole in re-
fluxing propionic acid in air to give the corresponding chiral
porphyrin derivatives 9-13 and the achiral porphyrin 6. These
compounds were separated from one another by extensive
column chromatography and were characterized by different
spectroscopic techniques in order to distinguish them from one
another (see the Experimental Section in the Supporting
Information).
the molecule and so with the amount of specifically aggregated
molecules in the solid state. With an increasing number of amide
groups there is a general shift to lower frequency of the peak
of the amide group in the solids (1679 cm-1 for (R,R,R)-10,
1672 for (R,R)-11, 1666 for (R,R)-12, 1672 for (R)-13, and 1655
cm-1 for 6).
It was challenging to distinguish between the constitutional
isomers (R,R)-11 and (R,R)-12. The first eluted sample from
the column was expected to be the less polar 5,15 porphyrin
derivative (R,R)-11, but the 1H NMR spectra measured at room
temperature were virtually identical to those of the next eluted
porphyrin, the two having identical mass spectra. While the 1H
NMR spectra measured at room temperature show a double
doublet resonance for the hydrogen atoms of the pyrrole groups
for both fractions, when the spectrum was run at 60 °C (when
the rate of interconversion of the hydrogen atoms attached to
the pyrrole nitrogen atoms is apparently sufficiently faster
than the NMR time scale so that a well-resolved averaged
spectrum results) compound (R,R)-11 gave a clear double
doublet and compound (R,R)-12 gave a double doublet with
two singlets almost overlapping the inner peaks of the doublets
In particular, the IR spectra of solids 10-13 showed two
signals from the carbonyl moiety of the ester group at 1756
and 1739 cm-1 and one peak from the equivalent bond in the
amide group. The relative intensity of the peaks corresponding
to the two different groups changes with their number in each
molecule, and the peak from the carbonyl moiety in the amide
group changes position with the number of amide groups in
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Supramolecular Chirality of One- and Two-Dimensional Aggregates
A R T I C L E S
Figure 2. STM images of porphyrin derivatives 2-6 physisorbed at the HOPG-1-heptanol interface (Iset ) 0.6 nA; Vset ) -0.2 V): (A) (R,R,R)-2, (B)
(R,R)-3, (C) (R,R)-4, (D) (S,S)-4, (E) (R)-5, (F) 6. The insets show STM images of HOPG (not to scale) corresponding to sites underneath the monolayer
(Iset )0.6 nA; Vset ) -0.001 V). The solid white lines in the insets indicate the direction of the main symmetry axis of HOPG. The dashed red lines in all
insets and main images are the selected HOPG reference axes <-1 1 0 0>. Unit cells are indicated in black. The solid black line is the propagation axis of
the porphyrin rows. ꢀ is the angle between the reference axis and the propagation axis. Each inset relates to the area underneath the monolayer.
(see the Supporting Information). Even at room temperature,
the exchange is sufficiently slow so as to broaden the spectrum
so that definitive assignment cannot be achieved.
unit cell.18 The monolayers that are formed are not chiral
globally because of the presence of enantiomeric domains.
For the present system, a better approach to probe molecule-
based two-dimensional chirality is by evaluating the monolayer
chirality by comparing its orientation versus graphite; i.e., the
angle (ꢀ) between the normal of a main symmetry axis of
HOPG, i.e., the HOPG reference axis <-1 1 0 0> and unit cell
vector a. In a previous STM study, we showed that the porphyrin
rows of compound 1 self-assembled at the HOPG-1-heptanol
interface do not run parallel to the reference axis of the substrate
but exhibit an angle of approximately +13 ( 2° with respect
to that direction.19 The same orientation is observed in all
domains probed: The porphyrin rows are always rotated
clockwise with respect to the reference axis for the R enantiomer.
This result indicates clearly that the stereogenic centers influence
strongly the molecule-surface interaction in the monolayers
of 1, an assertion backed up by modeling.19 Therefore, the
degree of chiral induction might not be optimum. For this reason,
a key question remaining from that study was how the amount
of chiral information at the molecular level is transferred and
expressed structurally at the monolayer level. In order to try to
answer to this question, we analyzed the self-assembly of
compounds 2-6 (together with the (S) enantiomers) at the same
interface to discover how the angle formed by the monolayer
with respect to the substrate is affected by the number and
location of stereogenic centers in the molecule (Figure 2). The
solvent was always 1-heptanol, as the solvent plays a crucial
The ester groups adjacent to the stereogenic centers in the
porphyrin derivatives 9-13 were subsequently amidated by
mixing the compounds with octadecyl amine and heating to 80
°C using the latter as solvent. This procedure afforded the chiral
amides 1-5 in quite respectable yields (around 85%) after
thorough purification by column chromatography. The absorp-
tion spectra were the typical ones of free base porphyrins, and
1
the H NMR spectra showed all the resonances characteristic
of the desired compounds. Mass spectra and elemental analysis
were also in accord with the target compounds.
It is perhaps worthy of attention that the difference in
molecular mass of the compounds with between one and four
stereogenic centers is only 55 Da (the molecular masses range
between 1917 and 1974 Da, respectively), so there is a less
than 3% mass difference.16 This value is interesting to be borne
in mind given the quite dramatic effects that are seen in their
assembly characteristics.
Investigation on the Structural Chirality of the Monolayers
Formed at the Solid-Liquid Interface. The assembly of
porphyrins 1-6 was studied at the highly oriented pyrolytic
graphite (HOPG)-1-heptanol interface by dissolving the com-
pounds in the solvent, applying a drop of the solution to a
graphite slab, and performing the STM imaging of the interface
with the tip of the microscope inserted into the solution.17
Inspection of the resulting STM images permits the chiral nature
of the domains to be recognized by the oblique shape of the
(18) (a) Qiu, X.; Wang, C.; Zeng, Q.; Xu, B.; Yin, S.; Wang, H.; Xu, S.;
Bai, C. J. Am. Chem. Soc. 2000, 122, 5550–5556. (b) Zhou, Y.; Wang,
B.; Zhu, M.; Hou, J. G. Chem. Phys. Lett. 2005, 403, 140–145. (c)
Otsuki, J.; Nagamine, E.; Kondo, T.; Iwasaki, K.; Asakawa, M.;
Miyake, K. J. Am. Chem. Soc. 2005, 127, 10400–10405.
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(16) To make the masses identical, additional carbon atoms would have to
be added at some location in the compounds, perhaps most logically
in the alkyl chains. However, this modification would give rise to odd-
even effects in the assembly.
(17) Elemans, J. A. A. W.; De Cat, I.; Xu, H.; De Feyter, S. Chem. Soc.
ReV. 2009, 38, 722–736.
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A R T I C L E S
Iavicoli et al.
Table 1. Unit Cell Parameters (a, b, γ), Angles of Direction of Unit
Cell Vector a with Respect to the HOPG Reference Axis <-1 1 0
0 > (ꢀ), and Numbers of Domains Investigated of 1-6 at the
HOPG-1-Heptanol Interface (n)a
n
a (nm)
b (nm)
γ (deg)
ꢀ (deg)
ꢀMD (deg)
(R,R,R,R)-1 29 1.9 ( 0.1 4.0 ( 0.1 80 ( 2 +13 ( 4 12.2 ( 0.4
(R,R,R)-2
(R,R)-3
(R,R)-4
(R)-5
13 2.0 ( 0.1 4.2 ( 0.3 80 ( 5 +10 ( 3 10.2 ( 0.2
21 1.9 ( 0.1 4.1 ( 0.1 80 ( 2 +7 ( 2
7.5 ( 0.2
8
1.9 ( 0.1 4.0 ( 0.1 81 ( 4 +9 ( 4 10.1 ( 0.3
19 1.9 ( 0.1 4.1 ( 0.1 79 ( 4 +7 ( 4
8.0 ( 0.3
6.9 ( 0.4
6
6
5
9
2.0 ( 0.2 4.1 ( 0.2 79 ( 4 +6 ( 4
1.9 ( 0.1 4.3 ( 0.3 75 ( 3 -8 ( 2
1.9 ( 0.1 4.1 ( 0.1 82 ( 3 -8 ( 2
(S,S)-4
a As achiral molecule 6 forms two types of domains with ꢀ > 0 and
ꢀ < 0, values for these domains are treated separately. ꢀMD is the result
of a molecular dynamics simulation.
role in the formation of this type of physisorbed self-assembled
monolayers.20
The monolayers revealed some dynamics immediately after
all of the porphyrin solutions were deposited on HOPG.
Therefore, all images were captured at least 1 h after deposition.
As shown in Figure 2, each compound self-assembles at the
HOPG-1-heptanol interface forming nice patterns of ordered
rows of molecules. The organization of molecules and the unit
cell parameters are the same (within error) as those measured
for 1 (Table 1). Importantly, the monolayers are essentially
isostructural, with the porphyrin units adopting a coparallel
orientation with respect to the surface and the alkyl chains
extending away from the core and interdigitated with those
coming from adjacent rows. This fact allows us to compare
directly the influence of the number of stereogenic centers on
the angle formed by the superstructure with respect to the
surface.
The only discernible difference in the monolayers of com-
pounds 1-6 is the angle that the lamellae form with respect to
the graphite reference axis. Although all the layers present
lamellae with positive angles for the R enantiomers, the average
value decreases with the diminishing of the number of chiral
centers in the molecules (Table 1).
The achiral molecule 6 has an average angle ꢀ of about 0°
over all domains, but for a given domain the angle of the rows
with respect to the graphite reference axis is approximately 7°.
Therefore, the achiral chain does not favor an achiral arrange-
ment on the surface, but spontaneous resolution of conformers
is observed.21
Though the positions of the chiral centers in compounds 3
and 4 are different, no obvious packing differences are observed.
STM images of (S,S)-4 (Figure 2) show that the molecular
packing appears as mirror image of the enantiomer (R,R)-4.
All these data seem to indicate that the chiral centers have
no major influence on the overall monolayer structure, except
for the fact they select only one of the equivalent mirror-image
patterns. A molecular modeling study was undertaken in order
to put this hypothesis to test. The molecular rows of porphyrin
1 self-assembled at the HOPG-1-heptanol interface present an
angle of +13 ( 2° (observed with STM) with respect to a
reference axis of the substrate.19 Calculations showed an angle
Figure 3. Snapshot from an MD simulation illustrating the deviation of
rows of molecules (R,R,R,R)-1 (green line) with respect to the reference
axis of the graphite substrate (blue line). The reference axis runs
perpendicular to one main axis of the graphite (red line).
of deviation of +12.2 ( 0.4°, in excellent agreement with
experiment (see Figure 3).
This deviation originates from a specific adsorption of the
porphyrin molecules on the surface, with the methyl group of
the stereogenic centers pointing toward the surface and thus
forcing a lateral shift of the porphyrin core of the adjacent
molecule.22 Starting from the self-assembly here obtained for
porphyrin 1, the methyl group on one chiral center was replaced
by a hydrogen atom to study the deviation for the molecular
rows of porphyrin 2. We then performed MD simulations in
the NVT canonical ensemble (constant number of particles,
volume and temperature) at 300 K. The system was first
equilibrated by running a 100 ps MD simulation, and the
deviation of the porphyrin rows with respect to the graphite
reference axis was then investigated in a 100 ps long MD run
(with frames recorded every 0.1 ps). The same strategy was
applied to study the deviation with respect to the reference axis
for compounds 3-6 by replacing sequentially all the methyl
groups. The calculated values are reported in Table 1, together
with the experimental data.
A very good agreement exists between the calculated and
the experimental values for the deviation of porphyrin rows with
respect to the reference axis. This clearly confirms that the
number of chiral centers on these porphyrin compounds
increases the deviation with respect to the reference axis, which
is a measurement of the chirality at the nanoscale. The origin
(22) Computationally, the determination of the deviation of rows of
porphyrins with respect to the reference axis is a delicate task because
of commensurability issues between the layer and the substrate. One
can use simulations under periodic boundary conditions (PBC) to
determine the self-assembly cell parameters, but PBC must be removed
to study the deviation with respect to a reference axis. See: Linares,
M.; Minoia, A.; Brocorens, P.; Beljonne, D.; Lazzaroni, R. Chem.
Soc. ReV. 2009, 38, 806–816.
(20) Yang, Y.; Wang, C. Curr. Opin. Colloid Interface Sci. 2009, 14, 135–
147.
(21) This situation is common for this type of monolayer; see: (a) Pe´rez-
Garc´ıa, L.; Amabilino, D. B. Chem. Soc. ReV. 2002, 31, 342–356. (b)
Pe´rez-Garc´ıa, L.; Amabilino, D. B. Chem. Soc. ReV. 2007, 36, 941–
967.
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Supramolecular Chirality of One- and Two-Dimensional Aggregates
A R T I C L E S
of the increased angle appears to be the space required by the
methyl group on the surface, which forces a displacement of
one porphyrin core with respect to the adjacent ones. This effect
is apparently averaged out over the monolayers, as in the
molecules with a low number of stereogenic centers the lamellae
still have regular packing on the surface.
Solution-State Studies of Porphyrins and Their Aggregates.
The porphyrins reported here were also designed to be able to
self-assemble into one-dimensional aggregates in solution
through hydrogen bonds. Absorption spectroscopy is ideally
suited for the study of the aggregates because the spectra of
porphyrins are dominated by the electronic π-π* transitions
associated with the aromatic chromophore.23 The most intense
band in the UV-visible range (the Soret band or B band)
appears at around 420 nm with an extinction coefficient over
104 mol L-1 cm-1 for related compounds,24 and it is associated
with two quasidegenerate transitions oriented perpendicular to
each other. This absorption band proved to be the most
informative for the investigation of these compounds. UV-vis
absorption measurements of all compounds in chloroform
showed a Soret band at 421 nm, indicating that these molecules
were behaving as isolated chromophores in the solution. The
absorption spectra of the same compounds in methylcyclohexane
showed a slightly blue-shifted Soret band at 419 nm (the shift
is most likely caused by solvent polarity difference when
compared with chloroform) and a shoulder at 400 nm (not
present in the chloroform spectra) which is caused by exciton
coupling between porphyrin units (see the Supporting Informa-
tion). Exciton theory25 suggests that a blue shift of the Soret
band implies that the porphyrins are overlapped and parallel to
each other26 (the bathochromic and hypsochromic shifts cor-
respond to the edge-to-edge (J aggregates) and face-to-face (H
aggregates) aggregations, respectively). At room temperature,
the shoulder at approximately 400 nm corresponding to the
aggregated porphyrins is much more evident for compounds 5
and 6, which assemble most strongly in solution while still
displaying the Soret band at 419 nm from the solvated
porphyrins that are isolated from one another (Figure 4). The
remaining compounds show only the absorption bands corre-
sponding to the nonaggregated porphyrins. The spectra in
chloroform and methylcyclohexane for compounds 1-4 are
virtually identical at room temperature, 5 × 10-6 M.
Figure 4. UV-vis absorption spectrum of compounds 1-6 (the spectra
of 3 and 4 overlap perfectly) at room temperature in methylcyclohexane (5
× 10-6 M) showing the increase of the shoulder at 400 nm with decreasing
number of stereogenic centers. The absorption band at 420 nm corresponds
to nonaggregated porphyrins and the exciton band that appears as a shoulder
at 400 nm corresponds to their H-aggregates.
strongly absorbing (identical) chromophores are oriented chirally
with respect to each other, an exciton spectrum is observed and
the wavelength at which the maximum in absorption occurs
(λmax) corresponds closely to zero in CD intensity. The latter
case is rather frequent during the formation of helical aggregates
which are a not infrequent basis of gel formation. During the
self-assembly processes leading to gel formation, the CD
spectrum undergoes dramatic modifications.28 If two or more
porphyrins are located nearby in space, the spectral region
around 400 nm shows the coupling between the B transitions.
The extent of any kind of interaction between electronic
transitions is directly proportional to their intensity and inversely
proportional to the energy difference.
Solutions of compounds 1-6 in methylcyclohexane were
prepared (at a concentration of 1 × 10-5 M), and the CD spectra
were measured at different temperatures between +25 and -10
°C (see the Supporting Information for all data). It is important
to note that at least 20 min was necessary for the CD signal to
stabilize, indicating a clear kinetic effect on the formation of
the aggregates.
The enantiomers of compound 1 gave mirror image CD
curves with a very weak Cotton effect at the position of the
Soret band for all the temperatures studied, which accompanied
by the lack of any change in the absorption spectra indicates
that it does not associate under these conditions. At the other
extreme, compound 6 could only be studied at 25 °C (below
this temperature it precipitates) and shows a flat CD spectrum,
as expected because of its achiral nature, and only shows noise
in the region around the wavelength of the Soret band. Neither
spontaneous symmetry breaking nor linear dichroic effects were
ever observed.
CD spectroscopy was used to obtain more information about
the self-assembly of the molecules and especially the twisted
orientation adopted between them.27 In general, there is a direct
correlation between the regions of absorption and the Cotton
effects observed in CD. In the case of noncoupled chro-
mophores, the shapes of the two spectra are similar, although
the vibrational fine structure can be different. If two or more
(23) Ghosh, A. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: New York, 1978; Vol. III, pp 1-
165.
(24) Iavicoli, P.; Simo´n-Sorbed, M.; Amabilino, D. B. New J. Chem. 2009,
33, 358–365.
(25) (a) McRae, G. M.; Kasha, M. J. Chem. Phys. 1958, 28, 721–722. (b)
Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Pure Appl. Chem.
1965, 11, 371–392. (c) Kasha, M. In Spectroscopy of the Excited State;
Bartolo, B. D., Ed.; Plenum: New York, 1976; pp 337-363.
(26) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525–
5534.
While the solutions of compounds 2-5 were cooled, absorp-
tion spectra displayed a decrease in the intensity of the Soret
band and a blue-shifted peak related to the aggregated form
growth, along with distinctly bisignate CD signals with negative
and positive Cotton effects measured at -10 °C (Table 2). This
(27) (a) Mammana, A.; D’Urso, A.; Lauceri, R.; Purrello, R. J. Am. Chem.
Soc. 2007, 129 (26), 8062–8063. (b) Wei, Y.; Zhanshuang, L.; Tianyu,
W.; Minghua, L. J. Colloid Interface Sci. 2008, 326, 460–464. (c)
Hoeben, F. J. M.; Wolffs, M.; Zhang, J.; De Feyter, S.; Lecle`re, P.;
Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2007, 129,
9819–9828.
(28) (a) Hirst, A. R.; Huang, B.; Castelletto, V.; Hamley, I. W.; Smith,
D. K. Chem.sEur. J. 2007, 13, 2180–2188. (b) RyongNam, S.; Lee,
H. Y.; Hong, J. I. Chem.sEur. J. 2008, 14, 6040–6043. (c) Smith,
D. K. Chem. Soc. ReV. 2009, 38, 684–694.
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A R T I C L E S
Iavicoli et al.
Figure 5. CD spectra (top) and corresponding absorption signal from the CD spectrometer (bottom) of compounds 2-5 in methylcyclohexane (1 × 10-5
M) at (A) -5 °C and (B) -10 °C.
Table 2. Positive and Negative Cotton Effects of Compounds 2-5
in the Spectral Region of Their B-Bands in Methylcyclohexane at
-10°C
The relative intensity of the aggregated and isolated absorp-
tion signals at each temperature is different for each one of the
compounds. If we consider the absorption of 2-5 at -5 °C, it
can be observed that while 2 only starts to self-assemble into
helix, 3 and 4 already have approximately half of their
monomers (with a slight difference between them) in the
aggregated form and 5 has almost all of its monomers in the
aggregated form (Figure 5). From these observations, we can
say that the free energy of interaction between the molecules
diminishes as follows: 5, 4-3 (with approximately the same
value), and 2. These results are supported by theoretical study
(vide infra).
The sum of the maximum intensities of each Cotton effect
of a split CD curve is defined as the “amplitude” (A), and either
a positive or a negative sign is assigned to it depending on
whether the highest wavelength Cotton effect is positive or
negative.31 The A value is proportional to ε2 and inversely
proportional to the square of the interchromophore distance (R2)
Cotton effects, λmax
compd
negative
positive
2
3
4
5
413
409
408
403
396
397
396
389
finding indicates that at room temperature the molecules are
essentially isolated in the solvent, while at lower temperatures
they self-assemble into helical structures giving rise to an
exciton-type CD signal.29 Because the CD spectra exhibit a
negative sign for the first Cotton effect, it means that the dipole
moments (in the porphyrin chromophores) orient in an anti-
clockwise direction in the aggregate of the R enantiomers (left-
handed helical structure).30 As seen in Table 2 the optical
activity of 5 is different from that of the other compounds: The
Cotton effect of 5 is blue-shifted compared with the other
aggregates, and this effect can be attributed to a different
organization in the superstructure.
(30) (a) Jung, J. H.; Ono, Y.; Shinkai, S. Chem.sEur. J. 2000, 6, 4552–
4557. (b) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.;
Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785–8789. (c) Sugiyasu,
K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 1229–
1233. (d) S Kawano, S.; Fujita, N.; Shinkai, S. Chem.sEur. J. 2005,
11, 4735–4742.
(29) Jonkheijm, P.; van der Schoot, P.; Schenning, A. P. H. J.; Meijer,
(31) Matile, S.; Berova, N.; Nakanishi, K.; Fleischhauer, J.; Woody, R. W.
J. Am. Chem. Soc. 1996, 118, 5198–5206.
E. W. Science 2006, 313, 80–83.
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Supramolecular Chirality of One- and Two-Dimensional Aggregates
A R T I C L E S
Figure 6. CD spectra (top) and corresponding absorption signal from the CD spectrometer (bottom) of compounds 3-5 in methylcyclohexane (1 × 10-5
M). The temperature is indicated in the figure.
Table 3. Critical Gel Concentrations for the R Enantiomers of
Porphyrins 1-6 in Methylcyclohexane (Identical Values Are
Obtained for the S Enantiomers)
critical gel concentration
(in mg/mL) of porphyrins
in methylcyclohexane
compd
(R,R,R,R)-1
(R,R,R)-2
(R,R)-3
(R,R)-4
(R)-5
16 (not transparent)
22
8
6
2
6
2
solvents was studied in methylcyclohexane, which was the best
solvent found for these compounds. They also gelate hexane
and toluene at a similar critical gel concentration of methylcy-
clohexane, with the exception of compound 6 which precipitates
in hexane. Methylcyclohexane is also a convenient solvent for
studying the optical properties of the aggregates because it is
not very volatile and is transparent in the visible and near-
ultraviolet parts of the electromagnetic spectrum. The porphyrins
formed clear pink gels as shown in Figure 7. The critical gel
concentration of each compound is showed in Table 3.
The enantiomers of compound 1 form a very thick and
nontransparent gel-like material at 1.6 wt % in methylcyclo-
hexane, 2 gelates the same solvent at even higher concentration
than 1 (although for 2 the gel is transparent), while porphyrins
3 and 4 need three times less material than 2. The compound
Figure 7. Photograph of the gel formed by 5 in methylcyclohexane.
and has a parabolic dependence on the dihedral angle between
the coupling transitions.32 For a compound containing two or
more identical chromophores, A can be estimated by the
summation of each interacting pair. Thus, in a system containing
three identical chromophores I/II/III, the observed amplitude
can be estimated by the sum of the three interacting chro-
33
mophore pairs (basis pairs): Atotal ) AI,II + AII,III + AI,III
.
At
-10 °C, the value of A is -6 mdeg for 2, -26 mdeg for 3,
-56 mdeg for 4, and -27 mdeg for 5. The different value of
A for each compound could be related to a different length of
the helical assembly where a different number of pairs interact
between themselves or/and to a different structure of these
helices.
Under the same conditions of concentration of the aggregated
form (same absorption intensity), a symmetric exciton coupling
band is observed for 3 and 4 and a dissymmetric one for 5
(Figure 6). This observation, together with the different Cotton
effect values seen for 5 (see above), can be explained by a
different aggregate structure of 5 when compared with the other
porphyrins, where the distance and/or the angle between the
chromophores is different.
Gelation Properties of the Porphyrin Derivatives. The ability
of each compound with four amide groups to gelate organic
(32) Harada, N.; Nakanishi, K. Circular Dichroism Spectroscopy-Exciton
Coupling in Organic Stereochemistry; University Science Books: Mill
Valley, CA, 1983.
(33) Nakanishi, K.; Berova, N. In Circular Dichroism; Nakanishi, K.,
Berova, N., Woody, R. W., Eds.; VCH Publishers Inc.: Weinheim,
1994; pp 361-395.
Figure 8. UV-vis absorption spectra of compounds 1-6 in their gel state
in methylcyclohexane. The relative absorbance cannot be compared because
of the different path lengths in the samples (see text).
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A R T I C L E S
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Figure 9. IR (left) and VCD (right) spectra of all studied compounds.
with only one stereogenic center (5) has the same critical gel
concentration as the achiral compound 6, which shares the
lowest critical gel concentrations of those reported here, 0.2 wt
%. It seems clear that the fewer number of stereogenic centers
present in the molecule the lower is the critical gel concentration.
The different critical gel concentrations of compounds 1-6
probably result from disturbed interactions between molecules
by the methyl group attached to the stereogenic center. In fact,
there is a small difference between the critical gel concentration
of constitutional isomers 3 and 4 uniquely because of the
position of the stereogenic center. This argument is supported
by the observation of different conformations that chiral chains
of a related type adopt compared with their achiral counter-
parts.34 It does not take into account that, by and large,
stereogenic centers can lead to increased solubility in this kind
of compounds, a factor which can be important in the formation
of gels35,36 and which could also play a role in the phenomenon
observed.
The gel state was not convenient for the study of the optical
activity of the compounds because for some of them very high
concentrations were required, leading to experimental noise as
well as the possibility of birefringence effects.37 The porphyrin
concentrations in the gel were so high that the absorption spectra
could not be obtained using a conventional optical cell. Thus,
the gel-phase absorption spectra were measured with a gel
sample sandwiched between two glass plates so that the ordinate
axis is expressed with an arbitrary unit (Figure 8). The gel of
enantiomerically pure compounds 2-6 in methylcyclohexane
showed an absorption peak at about 400 nm, indicative of
exciton coupling, and the formation of an H-aggregate.38-40
Because of the less favorable conditions for compound 1 to form
strong interactions between molecules, its gel in methylcyclo-
hexane shows the Soret band and a very broad weak band shifted
to the red region.
The observation of an exciton band at around 400 nm in the
absorption spectra of the gels implies that the mode of
aggregation is very similar to that seen in the CD experiments
in solution. Indeed, it is probable that the aggregates observed
at low temperature in solution are precursors in the formation
of the gels.41
We were able to probe the structural chirality of the gels using
VCD, which has emerged as a powerful chiroptical method for
studying the structure of supramolecular assemblies.42 This
technique combines the advantage of vibrational spectroscopy
(rich structural fingerprint region of IR absorption spectra) with
conformational sensitivity. The identification of specific parts
of a molecule’s configuration which is influenced by chiral
supramolecular aggregation is the advantage of VCD. In
particular, it has been successfully applied previously to the case
of the gelation process of a porphyrin.41
The IR absorption and VCD spectra of the gels under study
here are shown in Figure 9. The IR spectra are dominated by a
strong signal at 1656 cm-1 assigned to the CdO stretching mode
of the amide groups (amide I) and is equally observed in the
chiral and nonchiral porphyrins.
The VCD spectra show that all the chiral compounds with
between one and three stereocenters show a signal at the position
of the amide I band. The achiral molecule is CD silent, and the
gel containing the compound with four stereocenters is too
concentrated to be studied. The signal in the region of the amide
I band is a result of chiral induction caused by aggregation,
since when a nonaggregated form is studied by VCD a negative
band centered on 1721 cm-1 is observed (which corresponds
to a band in the IR spectrum).
(34) De Feyter, S.; Gesquiere, A.; Wurst, K.; Amabilino, D. B.; Veciana,
J.; De Schryver, F. C. Angew. Chem., Int. Ed. 2001, 40, 3217–3220.
(35) Hirst, A. R.; Coates, I. A.; Boucheteau, T. R.; Miravet, J. F.; Escuder,
B.; Castelletto, V.; Hamley, I. W.; Smith, D. K. J. Am. Chem. Soc.
2008, 130, 9113–9121.
(36) Pham, Q. N.; Brosse, N.; Frochot, C.; Dumas, D.; Hocqueta, A.;
Jamart-Gregoire, B. New J. Chem. 2008, 32, 1131–1139.
(37) (a) Shindo, Y.; Ohmi, Y. J. Am. Chem. Soc. 1985, 107, 91–97. (b)
Livolant, F.; Maestre, M. F. Biochemistry 1988, 27, 3056–3068. (c)
Kuroda, R.; Harada, T.; Shindo, Y. ReV. Sci. Instrum. 2001, 72, 3802–
3810. (d) Yao, H.; Isohashi, T.; Kimura, K. Chem. Phys. Lett. 2004,
396, 316–322.
(39) Choi, M. S. Tetrahedron Lett. 2008, 49, 7050–7053.
(40) De Luca, G.; Romeo, A.; Villari, V.; Micali, N.; Filtran, I.; Foresti,
E.; Lesci, I. G.; Roveri, N.; Zuccheri, T.; Scolaro, M. S. J. Am. Chem.
Soc. 2009, 131, 6920–6921.
(41) Kra´l, V.; Pataridis, S.; Setnicˇka, V.; Za´raba, K.; Urbanova´, M.; Volka,
K. Tetrahedron 2005, 61, 5499–5506.
(38) Shirakawa, M.; Kawano, S.; Fujita, N.; Sada, K.; Shinkai, S. J. Org.
Chem. 2003, 68, 5037–5044.
(42) Smulders, M. M. J.; Buffeteau, T.; Cavagnat, D.; Wolffs, M.;
Schenning, A. P. H. J.; Meijer, E. W. Chirality 2008, 20, 1016–1022.
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Supramolecular Chirality of One- and Two-Dimensional Aggregates
A R T I C L E S
Figure 10. SEM images of xerogels of (R,R,R)-2, (R,R)-3, (R,R)-4, (R)-5, and 6 on glass.
Linear dichroism is a potential issue,43 but this has been ruled
out in our case by repeating the spectra on different gel samples
of the same compound and by recording the spectra at different
angles. The quality of the data is judged to be good because
the achiral compound shows no VCD signal and the enantiomers
show mirror image spectra.
For the chiral compounds, we observe that the VCD spectra
display more variations in the series than the IR absorptions.
On the one hand, only the gel of (R,R,R)-2 shows a well-defined
bisignate band at (-)1662/(+)1649 cm-1 that indicates a twisted
secondary structure of the aggregate. On the other hand, the
other samples display a strong negative signal. The frequencies
of the bands for compounds (R,R,R)-2 and (R,R)-4 show the
negative component at 1662 cm-1, while (R)-5 and (R,R)-3 show
this signal at 1656 cm-1. There is a connection between the
frequency position and the intermolecular interactions in the
aggregated gels which produces these two-to-two similarities
and differences. Moreover, only in (R,R)-4, which is paired in
frequency with (R,R,R)-2, is a positive signal related with the
strong negative one observed corroborating that aggregation in
both samples might follow a similar pattern.
A weak positive chiral signal in the four samples in the
1600-1610 cm-1 range is observed showing a noticeable frequency
downshift from (R)-5 (1609 cm-1), (R,R)-4 (1608 cm-1), and
(R,R)-3 (1607 cm-1) to (R,R,R)-2 (1600 cm-1). Considering that
this vibrational mode corresponds to the CdC stretching bands of
the tetrapyrrole core, the stacking and helicoidal-like aggregation
might be the reasons for its VCD signal. In conclusion, the different
ways of inducing chirality in these porphyrin systems are nicely
detected in the VCD spectra of the gels.
Morphology of Xerogels. The morphology of the dried gels
(xerogels) of all compounds was investigated by optical microscopy
and SEM on a glass slide and by SFM on HOPG. Compound 1
does not form a transparent gel, and an irregular porous morphology
formed when it is deposited onto glass, dried, and observed through
an optical microscope (see the Supporting Information). The gel
from 2 forms a porous structure of disordered fibers. The gels from
3-6 present essentially the same morphology, which is constituted
of fibers with relatively long-range uniform orientation. A bigger
difference in the morphology of the xerogels can be observed from
SEM images (Figure 10). The achiral porphyrin 6 forms well
separated and oriented fibers, while 3-5 give bundles of oriented
fibers and 2 gives disordered bundles of fibers as in the optical
(43) Brizard, A.; Berthier, D.; Aime, C.; Buffeteau, T.; Cavagnat, D.;
Ducasse, L.; Huc, I.; Oda, R. Chirality 2009, 21, S153–S162.
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microscope, which seem to form a porous material. In some cases,
in areas of higher xerogel concentration, 5 gives more disordered
fibers.
In order to obtain a thin film easy to study by SFM (deposition
of lumps of the gels gives an extremely rough surface), the
porphyrin layer was formed after casting a drop of the hot solution
of the corresponding compound in methylcyclohexane onto graphite
and leaving it to cool to room temperature. The dry gels on graphite
appear as randomly distributed overlapped fibers whose width goes
from 20 to 200 nm, and the roughness of the SFM images is in
the range 3.5-6.0 nm. (Figure 11). It is not possible to ascertain
the height of the fibers from these images because of the density
of the structures which touch and overlap. A qualitative difference
can be observed in the images of xerogels 2-6. Compounds 5
and 6 appear as bundles of long fibers, whereas 2-4 form more
random distributed shorter fibers. No relevant images could be
recorded for 1, probably because of the tendency of the compound
to form large irregular aggregates and not self-assemble into
superstructures with long-range order.
To put the morphology of the gels of these porphyrins into
context, in materials formed by other organogelators much better
defined fibers are usually observed, and these objects are not as
bundled as the ones seen here. The compound with the most
classical morphological features is 6, but even this compound shows
mainly bundled fibers. Two features distinguish the organogelators
discussed here from more habitual systems. First, their molecular
weight is relatively large. Most compounds described in the
literature have molecular weights below 1000 Da. Second, and
perhaps more importantly, the symmetry of the porphyrins is
different from most organogelators: the porphyrins are (pseudo)
C4-symmetric, a shape which often promotes discotic assemblies,
while other gels are comprised of calamitic (rod-shaped) molecules.
The latter can form smectic (layer-like) assemblies, which might
encourage film formation. On the other hand, discotic-type
molecules can, in principle, interact uniformly in all directions
perpendicular to the disk-stacking direction. This effect might also
explain the lack of any observation of helices in the images obtained
of these chiral fiber containing materials. At the very least, the
domain sizes are much smaller than most gels reported in the
literature, and the small coherence length will cause the systems
to require a high concentration of gelator before immobilization
of the solvent takes place.
Molecular Modeling Study of Porphyrin Aggregation. The
self-assembly into helical stacks of the fully chiral and fully achiral
porphyrin (derivatives 1 and 6, respectively) were modeled using
periodic boundary conditions in order to avoid edge effects.44
Because of the C2 symmetry of the molecules (caused by the
localization of the pyrrolic protons), it is only necessary to rotate
them by 180° to retrieve a situation identical to the initial one. For
the achiral porphyrin (6), it is possible to build two different helices:
a clockwise (CW) helix and a counterclockwise (CCW) helix
(Figure 12). For each of these two helices, the pitch angle is about
2.4° resulting in 74 porphyrin molecules in the periodic box. The
intermolecular distances are about 4.6 Å, resulting in a box with a
length of 34.5 nm.
Figure 11. SFM images of xerogels from methylcyclohexane of (A)
(R,R,R)-2; (B) (R,R)-3; (C) (R,R)-4; (D) (R)-5; (E) 6.
As illustrated in Figure 12 (bottom), three main interactions
drive the assembly between the porphyrin molecules: the π-π
interactions between the porphyrin cores, the hydrogen bonds
between the amide groups and the van der Waals interactions
between the alkyl chains. The combination of these three
interactions provides a strong cohesion to the structures, and
the stabilization in these stacks is found to be 56.9 kcal/mol
per porphyrin molecule for both CW and CCW assemblies.
Starting from the two helices (CW and CCW) obtained with
the achiral porphyrin 6, the two corresponding stacks of 1 were
built and the structures were optimized. The methyl group on
the stereogenic center has a major influence on the packing.
Upon optimization, two different situations appear: a well
organized structure is maintained in the counterclockwise helix
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Supramolecular Chirality of One- and Two-Dimensional Aggregates
A R T I C L E S
the stabilization energy is 59.5 kcal/mol per molecule in the
CCW arrangement but only 49.3 kcal/mol per molecule in the
CW arrangement. This preference is a direct result of the steric
hindrance caused by the methyl group on the chiral center, which
disturbs the formation of hydrogen bonds between adjacent
porphyrin molecules in the CW helix. We also optimized and
computed the stabilization energies for both CW and CCW
helices for compounds 2-5 (to build these helices, we use a
random distribution of all the possible situations: chiral centers
on top of each other or alternated). The results are summarized
in Figure 14, which presents the stabilization energy per
molecule for the CW and CCW helices for compounds 1-6.
In the CW helix, the insertion of methyl group(s) on the chiral
center(s) creates an increasing steric hindrance and breaks
hydrogen bonds; this is the reason why the stabilization energy
becomes smaller when one increases the number of chiral
centers (from 56.9 to 49.3 kcal/mol per molecule). For the CCW,
the introduction of the methyl group induces a slight change in
the position of the amide group, toward a more stable
conformation. This movement explains why the stabilization
energy increases slightly with increasing number of chiral
centers (from 56.9 to 59.5 kcal/mol.molecule). This observation
is in perfect agreement with the CD measurements which show
the formation of a left-handed (CCW) helix for the R enanti-
omers of compounds 2-5 dissolved in methylcyclohexane.
Moreover, the calculations indicate that the pitch of the helix
does not change with the introduction of a chiral center. The
observed lower aggregation propensity of the molecules with
more stereocenters is therefore ascribed to the greater solubility
of the compounds in methylcyclohexane, in line with other work
in organogels.
Conclusions
The synthetic strategy developed for the preparation of
porphyrins containing between one and four stereogenic centers
provides compounds which are extremely useful for the study
of chiral induction phenomena in a range of systems. In this
paper, we have reported the study of low dimensional systems,
in the form of fibrous aggregates and monolayers, and have
found that the number of stereogenic centers has a profound
effect on the optical activity, self-assembly and chiral structure
of the porphyrin molecules described here.
Figure 12. Illustrations of the model of the helix formed by 6. Top: Top and
side views of the CW and CCW helices. Bottom: A view of the π-π stacking
between the porphyrin cores and the hydrogen bonds between the amide groups.
(Figure 13A), whereas a more disordered structure is observed
for the initially clockwise helix (Figure 13B,C).
This result indicates that for compound 1 the CCW arrange-
ment is much more stable than the CW arrangement; indeed,
In the self-assembled fibers, the four hydrogen bonds induce
the formation of a hydrogen bonded aggregate, as witnessed
by the blue-shifted exciton band, and CD measurements indicate
Figure 13. Model of 1 self-assembled into the helix: side view of the CCW helix (A), side view of the CW helix (B), close-up on the CW helix (C) showing
that the methyl group on the chiral center disturbs the formation of hydrogen bonds.
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A R T I C L E S
Iavicoli et al.
Figure 14. Stabilization energy per molecule (kcal/mol) for the CW and CCW helices for the R enantiomers of compounds 1-5 and achiral compound 6.
that these compounds self-assemble in solution into helical
structures. The nature and stability of the aggregates depend
critically on the number of stereocenters.
the number of stereogenic centers, and only one handedness is
formed in the monolayers, whereas the achiral compound forms a
mixture of mirror-image domains at the surface.
At sufficiently high concentrations the chiral porphyrin
derivatives are able to gelate methylcyclohexane, and this
property depends on the number and position of chiral groups
at the periphery of the aromatic core, reflecting the different
aggregation forces of the molecules in solution. The single
methyl group allows the compound to form a gel at a relatively
low concentration (1.18 × 10-3 M). We found the conditions
needed for forming a chiral gel at the same concentration as
the corresponding achiral derivative gelates were the same, while
an increased number of stereogenic centers required more
material, in all likelihood because of their greater solubility.35
Another possible contribution to the reduced ability of the
compound with four stereogenic centers to form aggregates
could be steric interactions resulting in a mismatch penalty,45
although our calculations on the H-aggregates formed here do
not point to such an effect.
The presence of only one stereogenic center is enough to
induce single-handed chirality in the self-assembled structures
in both the one and two-dimensional supramolecular structures,
in which the relative orientation of the molecules is completely
different, pointing therefore to a common effect.
Acknowledgment. This work was supported by the European
Union Marie Curie Research Training Network CHEXTAN (MRTN-
CT-2004-512161) and NoE MAGMANet (515767-2) and NMP4-CT-
2006-032109 (STREP “SURMOF”), the Direccio´n General de Inves-
tigacio´n, Ciencia y Tecnolog´ıa (Spain, Project Nos. MAT2007-62732,
and CTQ2009-10098), the Junta de Andalucia (Grant No. FQM-0159,
Project No. P09-4708), and the DGR, Catalonia (Project No. 2009
SGR 158). The collaboration between Leuven and Mons is supported
by the Science Policy Office of the Belgian Federal Government (PAI
6/27). Research in Mons is also supported by FRS-FNRS. We warmly
thank Amable Bernabe´ at the ICMAB for recording IR and LDI-TOF
spectra. We also thank the reviewers for constructive criticism which
helped improve the quality of the article.
The SFM and SEM analyses of the gels showed the aggregates
to have very similar morphology. We believe that this absence of
morphological chirality is a result of the long pitch and symmetry
of these discotic-type systems, an unusual one in organogel systems
which are usually based on calamitic molecules.
Supporting Information Available: Full experimental section
including a description of the synthesis and characterization of
all new materials and all the techniques employed in the research
reported here and figures showing variable-temperature CD and
UV-vis absorption spectra, optical micrographs of the gels, and
NMR spectra of the isomers 11 and 12. This material is available
free of charge via the Internet at http://pubs.acs.org.
In the two-dimensional systems, the degree of structural chirality
with respect to reference surface axes increases almost linearly with
(44) Iavicoli, P.; Linares, M.; Pe´rez del Pino, A.; Lazzaroni, R.; Amabilino,
D. B. Superlattices Microstruct. 2008, 44, 556–562.
(45) Smulders, M. M. J.; Filot, I. A. W.; Leenders, J. M. A.; van der Schoot,
P.; Palmans, A. R. A.; Schenning, A. P. H. J.; Meijer, E. W. J. Am.
Chem. Soc. 2010, 132, 611–619.
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