Orthogonal action of noncovalent interactions for photoresponsive chiral columnar assemblies

Article abstract of DOI:10.1002/anie.201000580

(Figure Presented) Making a stack: Helical columnar architectures are prepared by a hierarchical self-assembly process involving H-bonding, π-π, and ion-dipole interactions (see picture). The strategic combination of these supramolecular interactions within mesomorphic systems yields materials in which photoinduced chirality and the incorporation of ions can be used to design multifunctional liquid crystals.

Full text of DOI:10.1002/anie.201000580

Communications
DOI: 10.1002/anie.201000580
Supramolecular Chirality
Orthogonal Action of Noncovalent Interactions for Photoresponsive
Chiral Columnar Assemblies**
Francisco Vera, Joaquꢀn Barberꢁ, Pilar Romero, Josꢂ Luis Serrano, M. Blanca Ros, and
Teresa Sierra*
Chiral supramolecular aggregation to give helical architec-
tures appears to be a valuable strategy to attain functional
materials under structural control.[1,2] An attractive aspect
regarding helically structured materials is the possibility of
implementing dynamic responses to external stimuli, includ-
ing the amplification of chirality,[3] photodriven supramolec-
ular chirality,[4] and ion sensing.[5]
endowed with a rigid V shape (A in Figure 1a) formed H-
bonded complexes with a melamine derivative (T in Fig-
ure 1a) in a 3:1 proportion. These complexes self-organized
into helical columnar mesomorphic systems, the chirality of
which was controlled by chiral building blocks and/or
circularly polarized light (CPL).[13]
Liquid crystals have been shown as useful materials for
the creation and stabilization of dynamic helical architectures
based on noncovalent interactions. Indeed, the exploitation of
the selectivity and directionality of hydrogen bonds and
p stacking within columnar mesophases is particularly useful
for this purpose, since it allows the spontaneous assembly of
mesogens to give well-defined helical architectures within
two-dimensional mesomorphic arrangements.[6] The key issue
is the design of the appropriate molecular building blocks that
give rise to such mesomorphic arrangements.
In this respect, supramolecular macrocycles[7] have
received growing interest because of the possibility of
achieving functional complex structures, such as tubular
aggregates in solvents[8] or self-organized columnar meso-
morphic arrangements.[9] Some of these assemblies have been
reported to adopt chiral helical architectures originating from
chiral building blocks.[5a,10] Since the first report on “rosette”
systems,[11] many examples of these supramolecular macro-
cycles have been described based on the use of self-
complementary species, such as melamine and cyanuric acid
derivatives.[12]
Herein, we combine the two systems mentioned above,
namely columnar liquid crystals and supramolecular macro-
cycles, to attain helical supramolecular architectures imple-
mented with dynamic functionality. Previous results of our
research showed that nonmesogenic carboxylic acids
Figure 1. a) Chemical structures of the melamine derivative T and
V-shaped acids A12 and A(S)10*, used to build the supramolecular
complexes. b) 1H NMR spectra obtained for different concentrations of
the acid A12, while maintaining constant the concentration of T
(9.2 mm in CD2Cl2). Peaks a–c represent the three types of NH proton
(H-bonded or non-H-bonded) that can be distinguished in the spectra.
[*] Dr. F. Vera, Dr. J. Barberꢀ, Dr. P. Romero, Dr. M. B. Ros, Dr. T. Sierra
Instituto de Ciencia de Materiales de Aragꢁn, Quꢂmica Orgꢀnica
Facultad de Ciencias, Universidad de Zaragoza-C.S.I.C.
50009 Zaragoza (Spain)
Fax: (+34)976-762-276
E-mail: tsierra@unizar.es
In the present study, our attention is focused on the
versatility shown by these two supramolecular units to build
functional columnar systems, and on how we can make use of
it to attain novel dynamic helical architectures based on
supramolecular macrocycles through an easy preparation
process. The idea is based on the ability of melamine
derivatives to self-associate into rosettes, which in turn can
self-organize into columnar systems. The challenge of our
melamine/V-shaped acid binary system is to achieve control
of the competition between the establishment of two different
Prof. Dr. J. L. Serrano
Instituto de Nanociencia de Aragꢁn
Universidad de Zaragoza (Spain)
[**] This work was supported by the CICYT projects MAT2009-14636-
C03-01 and CTQ2009-09030, FEDER funding (EU), and the Aragon
Government. F.V. thanks the MEC of Spain for a grant. We thank
Prof. R. Alcalꢀ (Univ. Zaragoza) for the laser facilities and Dr. R.
Tejedor (Univ. Zaragoza) for help with VCD spectroscopy.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000580.
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H-bonding interactions, that is, melamine–melamine or
melamine–acid.
consisting of a molecule of melamine T, H-bonded to a
molecule of acid A (Figure 2a), was taken as the simplest
The strategy pursued is the formation of complexes of the
V-shaped acids and the melamine in a 1:1 ratio, which should
lead the rigid acid counterpart to play the role of the bulky
peripheral group that favors the formation of rosettes versus
ribbonlike supramolecular structures.[14] Moreover, by taking
advantage of the strong lateral self-interactions between
V-shaped rigid mesogens,[15] these rosettes can yield twisted
columnar assemblies with a tubular shape, the chirality of
which can be controlled by chiral building blocks and/or CPL.
Furthermore, ion–dipole interactions can be established
along the helical architecture, which should open new
possibilities for these systems.
building block in the mesomorphic organization.
Complexes between T and A in a 1:1 ratio were prepared
from CH2Cl2 solution by slowly removing the solvent under
mechanical stirring. The equilibrium established in solution
between these compounds and their supramolecular associ-
ations was studied by 1H NMR spectroscopy. Titration experi-
ments in CD2Cl2 showed that all NH proton signals of the
melamine derivative T are shifted downfield upon increasing
the proportion of the V-shaped acid in the solution (Fig-
ure 1b). The so-called continuous variation method yielded
Jobꢀs plots with a maximum at cA = 0.5 for these NH protons,
a finding that clearly indicates a 1:1 stoichiometry for the T–A
association in this solvent. Accordingly, a significant binding
constant of 335 Lmolꢀ1 was calculated by nonlinear regres-
sion analysis (see the Supporting Information).[16]
On removing the solvent, the materials prepared (T-
A(S)10* and T-A12) appeared homogeneous to the eye and
showed textures consistent with columnar mesomorphism by
polarized optical microscopy (POM; see the Supporting
Information). The transition temperatures and associated
enthalpy values as well as mesophase lattice parameters are
presented in Table 1.
According to these results, columnar mesomorphic mate-
rials consisting of T-A complexes in a 1:1 ratio were obtained
from nonmesogenic supramolecular units. Nevertheless, the
intriguing question about these results concerns the structure
of the supramolecular entity responsible for the formation of
the columnar mesomorphic arrangement. In line with the
stoichiometry found in the NMR experiments, the complex
Table 1: Phase transition temperatures (T) and enthalpies (DH) for the
complexes.[a] Lattice parameters were measured in the mesophase by
X-ray diffraction at room temperature.
Lattice
Compound
Thermal properties
parameters
[ꢃ]
Phase T [8C] DH [kJmolꢀ1
]
Phase
T-A(S)10*
T-A12
I
I
98.2
114.7
3.1
3.2
Colh
Colh
a=74.5
a=79.4
h=3.35
a=70.7
h=3.5
Figure 2. Hierarchical self-assembly process proposed for the forma-
tion of the mesophase in these systems. a) Structure of the T-A [1:1]
complex. b) Rosette structure of the macrocycle formed by six T-A [1:1]
complexes. The two types of H bond, that is, melamine–melamine and
melamine–acid, are shown in different colors. c) Hexagonal two-
dimensional packing (green hexagon) in the mesophase. Each node of
the lattice is formed by a rosette. Three acids of contiguous rosettes
coincide in the corners of a hexagonal sublattice (orange hexagon).
-OR represents the azobenzene arms of the V-shaped acids.
T-A(S)10*-LiTf
I
I
151.9
7.9
Colh
Colh
T-A12-LiTf
147.1 10.1
a=97.6
h=3.35
[a] Transition temperatures are given for the cooling process at
108Cminꢀ1 scans. I=isotropic liquid, Colh =hexagonal columnar mes-
ophase, LiTf=lithium triflate.
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pffiffi
The values of the lattice parameters, estimated from XRD
electronic density with a period 3 times shorter than the
studies, can be used to deduce the number Z of these [1:1]
complexes per two-dimensional unit cell in the Colh meso-
phase on the basis of the packing density (see the Supporting
Information). For both T-A12 and T-A(S)10*, reasonable
density values of about 0.8 gcmꢀ3 were obtained on the
assumption that there are six T-A [1:1] complexes per unit cell
(Z = 6).
The value Z = 6 means that, for both materials, there are
six melamine derivatives and six V-shaped acids in each node
of the hexagonal lattice. In analogy to other similar structures
described in the literature,[9b] it can be proposed that these
T-A [1:1] complexes are able to self-associate through the free
H-bonding sites remaining in their corresponding melamine
component (Figure 2a). These interactions generate rosette-
like supramolecular structures (Figure 2b), each of which
contains six melamine molecules and six V-shaped acid
molecules assembled in such a way that two sets of H-
bonding interactions are formed.[17] In this organization, an
inner set of melamine–melamine interactions is surrounded
by a set of melamine–acid H-bonding interactions. In the
proposed structure, the resulting columns consist of a central
core containing the stacked hexameric melamine macro-
cycles, each surrounded by six V-shaped acids, organized in a
hexagonal lattice (Figure 2c).
(10) spacing. The latter is consistent with the existence of a
hexagonal sublattice within the hexagonal lattice. From
Figure 2c it is apparent that the electronic density distribution
deduced from the X-ray patterns corresponds to groups of
three acids located at the corners of the hexagon defined by
the sublattice, with a sublattice constant of a= 3.
pffiffiffi
In line with this model, it can be proposed that the
assembly process that leads to the formation of the columnar
mesophase is hierarchical in nature, and involves the for-
mation of H-bonded acid–melamine [1:1] complexes that self-
associate into rosettes. These rosettes stack to give well-
organized columnar mesophases, in which the interior of the
columns has electron-rich and electron-poor atoms that make
the assembly susceptible to hosting small ions, stabilized by
ion–dipole interactions. Indeed, the incorporation of lithium
triflate (LiTf) in the ratio T/A/LiTf 1:1:1/6 causes changes in
the mesomorphic behavior. POM observations showed tex-
tures (see the Supporting Information) similar to those
observed for the material without LiTf. However, the I–Colh
temperatures and their corresponding enthalpy values are
higher than those of the corresponding salt-free systems
(Table 1), a finding that indicates stabilization of the colum-
nar architecture. The existence of ion–dipole interactions that
reinforce the p stacking of the rosette complexes could
account for this behavior.
Experimental confirmation of this proposal was provided
by the X-ray diffraction pattern of T-A(S)10*-LiTf. The
pattern showed an additional large-angle scattering halo (see
the Supporting Information), which corresponds to a distance
of 3.5 ꢁ (Table 1) and which is assigned to a regular stacking
of the disks. The fact that the hexagonal lattice constant, a,
decreases after the incorporation of the salt may probably be
related to some tilt of the melamine hexamers, a change that
would reduce the cross section of the column. The density
deduced for this LiTf-containing material, assuming Z = 6, is
0.9 gcmꢀ3. Thus, the columnar arrangement is the same as in
the salt-free material, although the packing is slightly denser.
In addition, the innermost ring, that is, the (10) reflection
(Figure 3c), is more intense than that of the salt-free complex.
This result is consistent with a density increase in the core of
the columns as a consequence of the tilt of the rosettes (the
same mass is accommodated in a narrower cross section).
In contrast, the supramolecular organization of the
columnar mesophase is probably different in T-A12-LiTf, as
deduced from the noticeable increase in the hexagonal lattice
constant (a = 97.6 ꢁ). The outermost halo is still observed and
corresponds to the same value of 3.35 ꢁ. However, the lattice
parameter a of 97.6 ꢁ is not consistent with the rosette model
proposed above. In fact, the Z value deduced for this material
is 10. A possible structural model to account for this situation
is a ribbonlike supramolecular structure of H-bonded mela-
mines (each of which is complexed to one acid molecule),
which may combine regular stacking between melamine units
and a significantly larger lattice parameter. This ribbonlike
unit would adopt a helical arrangement, which accommodates
the ions.
Additionally, it can be seen in the X-ray patterns
(Figure 3a,b) that the reflection (11) is unusually strong.
The most intense ring is usually the fundamental reflection
(10). This difference can be interpreted as being the result of a
combination of two effects: 1) there is a hole in the center of
the rosette, the diameter of which is estimated to be between
4 and 5 ꢁ; and 2) there is a strong modulation of the
Figure 3. Small-angle X-ray scattering (SAXS) patterns of the meso-
phase of the complexes T-A(S)10* (a), T-A12 (b), T-A(S)10*-LiTf (c),
and T-A12-LiTf (d) at room temperature after cooling from the
isotropic liquid.
As for the salt-free complexes, the X-ray pattern in
Figure 3d shows a very weak maximum for the (10) reflection,
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thus suggesting a low electronic density at the nodes of the
two-dimensional hexagonal lattice, and a strong (11) max-
imum, which can be accounted for by the disposition of the
acid counterparts in the outer part of the column. The
formation of a continuous ribbonlike structure in this com-
plex upon the incorporation of LiTf is supported by the fact
that it forms gels in dichloromethane (see the Supporting
Information).
In both salt-free and salt-containing supramolecular
organizations, efficient columnar assembly arises from p in-
teractions between the stacked melamine units self-associated
either in rosettes (T-A12, T-A(S)10*, and T-A(S)10*-LiTf) or
in helical ribbons (T-A12-LiTf). In this situation, the sur-
rounding V-shaped molecules can be tilted with respect to the
plane perpendicular to the column axis, to avoid steric
hindrance between their long rodlike arms and to optimize
space filling. This disposition favors an inherent helical
arrangement along the column. For the chiral complexes
T-A(S)10* and T-A(S)10*-LiTf, this kind of stacking gener-
ates respective systems that show superstructural chirality, as
revealed by CD (Figure 4a and b, solid lines)[18] and vibra-
tional CD (VCD; see the Supporting Information) spectra,
which suggest the formation of a helical stacking that strongly
involves the V-shaped acids. For the complexes with nonchiral
acids, the two helical senses should coexist in the same
proportion, thus yielding CD-silent materials (Figure 4c and
d, solid lines).
the architecture adopted by T-A(S)10*-LiTf. The sign of the
induced CD, which corresponds to absorption bands in the
UV/Vis spectra (see the Supporting Information), is depen-
dent on the CPL sign, and this indicates the possibility of
external modulation of the supramolecular chirality. Further-
more, the original CD spectra of both systems can be
recovered by heating at 908C for 5 seconds. This means that
it is not necessary to destroy the columnar organization (see
Table 1) to erase the chiral information recorded by irradi-
ation.
Finally, it was possible to transfer the chirality of CPL to
the achiral systems. Both systems, the rosette-type associa-
tion, T-A12, and the proposed helical ribbonlike H-bonded
organization, T-A12-LiTf, show intense CD bands (Figure 4c
and d) upon irradiation with CPL, thus indicating the
induction of chirality into the supramolecular systems. On
irradiation with light of the opposite handedness, the CD
shows the opposite sign, which indicates that the supramolec-
ular chirality of the mesophase can be inverted by the external
chiral radiation. The chiral photoresponse achieved upon
illumination is stable for long periods of time.
In summary, we have demonstrated the hierarchical self-
assembly of simple nonmesogenic building blocks into
hexagonal columnar mesophases. During the hierarchical
process, the orthogonal action of different noncovalent
interactions takes place. Indeed, two types of H-bonding
interaction, melamine–melamine and melamine–acid, oper-
ate in the plane of the macrocycle to form a rosettelike
stacking unit, whereas p–p interactions are mainly active in
the direction perpendicular to the rosette plane. These
interactions account for the formation of the columns that
organize within the Colh mesophase. Furthermore, it is shown
that these columns can accommodate the ions of a salt such as
lithium triflate after small architectural modifications, which
involve the formation of columns with long-range stacking
order.
Our interpretation of the observed structural changes
relies on the influence of the smallest Li+ ions. Ion–dipole
interactions between the N atoms of the triazine ring and Li+
are proposed to occur, and these allow the incorporation of
Li+ ions most likely sandwiched between rosettes. For the
chiral complex T-A(S)10*, which does not show a regular
stacking distance, the inclusion of Li+ cations in the proposed
way compels the rosettes to get closer along the column.
Complex T-A12, which shows a regular stacking distance, also
accommodates the Li+ cations without disrupting the colum-
nar mesomorphic order.
With regard to our proposal for the formation of inherent
helical structures along the column, it is shown that this self-
assembly process leads to functional materials, from simple
building blocks, which are capable of showing dynamic
supramolecular chirality and working as chirooptical switches.
In fact, during the light-induced reorientation process of
azobenzene groups, it is possible to tune the supramolecular
chirality at will by using CPL of different handedness.
Figure 4. CD spectra of cast films of T-A12, T-A12-LiTf, T-A(S)10*, and
T-A(S)10*-LiTf recorded at room temperature as fresh samples (c),
and after irradiation with a 488 nm Ar+ laser with left-handed CPL
(g) and right-handed CPL (a). The ellipticity measured strongly
depended on the cell thickness, which was smaller for the complex
T-A(S)10*-LiTf.
Subsequent experiments on the photomodulation and
photoinduction of supramolecular chirality were based on this
proposed helical model. Accordingly, illumination of the
chiral system T-A(S)10* with CPL led to either an increased
CD signal or the opposite sign depending on the handedness
of the CPL used (Figure 4a,b). Similar behavior was found for
Received: January 31, 2010
Revised: April 14, 2010
Published online: June 9, 2010
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Communications
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Keywords: chirality · helical structures ·
noncovalent interactions · self-assembly ·
supramolecular chemistry
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