Light-driven supramolecular chirality in propeller-like hydrogen-bonded complexes that show columnar mesomorphism

Article abstract of DOI:10.1002/anie.200603796

(Graph Presented) Shine a light! Columnar mesomorphic assembles of propeller-like hydrogen-bonded complexes show a chiral photo-response upon irradiation with circularly polarized light (CPL) of a given handedness (see picture). The materials can behave as a supramolecular switch that undergoes helix inversion when irradiated with the oppositely polarized light.

Full text of DOI:10.1002/anie.200603796

Angewandte
Chemie
DOI: 10.1002/anie.200603796
Chiral Photoinduction
Light-Driven Supramolecular Chirality in Propeller-Like Hydrogen-
Bonded Complexes That Show Columnar Mesomorphism**
Francisco Vera, Rosa M. Tejedor, Pilar Romero, Joaquín Barberµ, M. Blanca Ros,
JosØ Luis Serrano,* and Teresa Sierra*
Nature offers many beautiful examples of biological systems
whose unique functionalities arise from a well-defined helical
organization as the expression of supramolecular chirality.[1]
Research on artificial helical architectures[2] that can endow a
material with unique properties has found that columnar
liquid crystals[3] are an excellent synthetic building block.[4,5]
These systems allow helical organizations to be built with
amplified supramolecular chirality through different types of
noncovalent interactions, such as p stacking, hydrogen bond-
ing, and solvophobic and ionic interactions, which permit
molecular chirality to be transferred to the supramolecular
organization in the mesophase.[6] However, the transfer of
chirality by an external stimulus such as light remains
unexplored in these systems.
that this reaction offers for the control of self-assembly in
liquid crystals have been recognized.[8] The use of circularly
polarized light (CPL) as the irradiation source has led to
enantiomeric excess values in two chiral domains in the Bx
phase of bent-core molecules.[9] Moreover, supramolecular
chirality photoinduced by CPL has been reported in azopol-
ymers[10] and in main-chain polymeric liquid-crystal systems
doped with azobenzene-containing W-shaped molecules.[11]
However, attempts to induce supramolecular chirality in
columnar mesophases formed by azobenzene-containing
mesogens have not been described to date. We demonstrate
herein the possibility of controlling the supramolecular
chirality of a helical stack of propeller-like complexes using
an external chiral stimulus such as CPL. This control must
involve not only the modification of an existing chirality,
which is transferred from the molecular chirality, but also the
induction of supramolecular chirality within the columnar
mesophase consisting of achiral mesogens.
Hydrogen-bonded complexes that incorporate azoben-
zene groups in each of the arms of the V-shaped acids were
prepared following an approach developed in our previous
work (see Scheme 1 and the Supporting Information). The
presence of the photoaddressable azobenzene group should
allow CPL to be used as the external stimulus to induce or
switch the supramolecular chirality of the columnar liquid
crystalline organization.
In a recent publication,[7a] we described supramolecular
complexes in which nonmesogenic V-shaped acids show an
ability to form mesogenic phases upon formation of a
tetrameric complex around a melamine derivative.[7] This
unusual mesogenic core, which was interpreted as a propeller-
like structure, promotes the appearance of well-defined
rectangular columnar mesophases by combining the p-stack-
ing tendency of the melamine derivative and the lateral
interaction between V-shaped molecules. Moreover, on the
basis of X-ray diffraction and circular dichroism (CD)
measurements on chiral complexes, we proposed a model
that is compatible with the existence of supramolecular
chirality arising from an inherent helical stacking of propeller-
like supramolecules. Given the helical stacking of these
hydrogen-bonded complexes, our next challenge is to control
and induce a supramolecular chiral response of the supra-
molecular organization by using light as an external stimulus.
The isomerization of azobenzene upon irradiation with
light is a well-documented phenomenon and the possibilities
The formation of the tetrameric complex was studied by
IR and NMR spectroscopy (see the Supporting Information).
Assuming that there must be fast exchange between the
complex and its components in solution, strong evidence of
hydrogen-bonding interactions between the triazine and acid
molecules was found in the solution 1H and 13C NMR spectra
of the complexes.
The 13C NMR spectra of solid samples were particularly
revealing and helped to confirm the complex stoichiometry.
The 13C cross polarization (CP) magic-angle spinning (MAS)
NMR spectra of several mixtures of melamine T and the acid
A(S)10* in different proportions are shown in Figure 1. It is
especially significant that the peak corresponding to the
carbonyl group of the acid itself (A(S)10*) does not appear in
the spectrum of the mixture T-A(S)10* (1:3), thus indicating
that all the acid present must be complexed to the melamine
derivative. In order to provide further evidence to support
this conclusion, we compared the 13C NMR spectrum of the
complex T-A(S)10* (1:3) with the spectra of mixtures
containing an excess of acid, a situation that ensures the
presence of some acid that is not complexed with T
(T:A(S)10* proportions of 1:3.5 and 1:4). It is evident from
Figure 1 that as soon as free acid is present the original
[*] F. Vera, Dr. R. M. Tejedor, Dr. P. Romero, Dr. J. Barberµ, Dr. M. B. Ros,
Prof. Dr. J. L. Serrano, Dr. T. Sierra
Insitituto de Ciencia de Materiales de Aragón–CSIC
Departamento de Química Orgµnica, Facultad de Ciencias
Universidad de Zaragoza
50009 Zaragoza (Spain)
Fax: (+34)97-676-1209
E-mail: joseluis@unizar.es
tsierra@unizar.es
[**] This work was supported by the CICYT (projects MAT2003-07806-
CO2-01 and MAT2005-06373-CO2-01), FEDER (EU), and the DGA.
F.V. thanks the MEC (Spain) for a grant. We warmly thank Prof. R.
Alcalµ for allowing us to use his laser facilities and for helpful
discussions. We thank F. Sedeæo for producing the model drawings.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
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acid and the melamine units are significantly
shifted upon complexation. The upfield shift
of the signals for the carbon atoms within the
N-alkyl tail of Tupon complexation is worthy
of note. This shift is also visible in solution
experiments (see the Supporting Informa-
tion), and must be associated with the strong
influence that the aromatic rings of the acid
units have on the carbon atoms of the tail.
These NMR experiments in the solid state
support the idea that a mixture of one part of
T and three parts of the V-shaped acid
provides mostly the tetrameric hydrogen-
bonded complex T-A(S)10* (1:3), which is
stable in the solid state at room temperature.
The thermal behavior of the materials
was studied by the commonly used techni-
ques polarizing optical microscopy (POM)
and differential scanning calorimetry (DSC).
The melamine derivative T is mesomorphic
over a very small temperature range. It shows
a smectic A mesophase on cooling from the
isotropic liquid to a temperature (388C)
below the melting point of the crystal
(1038C).[7a] The acids used to prepare the
complexes are not mesogenic (melting
points: 1918C for A(S)10* and A(R)10*;
1708C for A14). In contrast, both chiral and
achiral complexes show textures that indicate
mesomorphic behavior, which could be sug-
gestive of columnar arrangements (see the
photomicrographs in the Supporting Infor-
mation). DSC revealed only one peak in the
cooling process, which corresponds to the
transition from the isotropic liquid to the mesophase. Addi-
tionally, a glass transition was detected at 518C for the chiral
complexes (see the thermograms in the Supporting Informa-
tion). The thermal data are collected in Table 1.
Scheme 1. Synthesis of hydrogen-bonded complexes that incorporate azobenzene groups
in each of the arms of the V-shaped acid.
carbonyl carbon peak of the acid is visible in the spectrum.
Furthermore, the signals for some carbon atoms of both the
Table 1: Phase transition temperatures (T) and enthalpies (DH) of the
complexes.[a]
Compound
Thermal properties
Phase T [8C] DH [kJmolÀ1
Phase T [8C] Phase
]
T-A(S)10*
T-A(R)10*
T-A14
I
I
I
123
123
134
6.8
6.8
6.1
Colh
Colh
Colr
51
51
g
g
[a] Transition temperatures are given for the cooling process. I=iso-
tropic liquid, Colh =hexagonal columnar mesophase, Colr =rectangular
columnar mesophase, g=glass.
The most outstanding result concerning the thermal
behavior found in the complexes is in the fact that the
combination of nonmesogenic molecular components leads to
mesogenic complexes that show mesomorphism over broad
temperature ranges. Moreover, the mesophase order ach-
ieved on cooling from the isotropic liquid is stable at room
temperature for long periods of time in all cases, as
determined by POM and DSC.
Figure 1. 13C CPMAS NMR spectra of T, A(S)10*, and mixtures of T and
A(S)10* with different proportions of acid. Arrows indicate the most
meaningful shift differences indicating the presence or absence of free
acid.
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Confirmation of the nature of both types of mesophase
(see Table 1) was achieved by X-ray diffraction experiments
at room temperature, which also confirmed the stability of the
mesophase order at this temperature. The X-ray patterns of
one of the enantiomers of the chiral complex T-A(S)10* are
shown in Figure 2a. The small-angle X-ray scattering (SAXS)
required. Despite this fact, on the basis of the inherent helical
arrangement proposed for this type of complex in our
previous study[7a] we could envisage a supercoiled structure
consisting of a double helix.[13]
On the basis of the helical stacking proposed for these
complexes, we assessed the optical activity of the mesophases
of the chiral complexes by CD.[14] These materials do not show
any CD signal indicative of optical activity in the isotropic
liquid. However, a strong signal, which was identified as an
exciton splitting, appeared on cooling to room temperature
(solid line in Figure 3). This optical activity must come from a
Figure 2. SAXS patterns with indexing of a) the Colh mesophase of
complex T-A(S)10*, where a is the lattice parameter of the hexagonal
arrangement of the chiral complex, and b) the Colr mesophase of
complex T-A14, where a and b are the lattice parameters of the
rectangular arrangement of the achiral complex. Both patterns were
recorded at roomtemperature after cooling fromthe isotropic liquid.
Figure 3. CD spectra corresponding to successive irradiation of a cast
filmof complex T-(R)A10* with l-CPL and r-CPL at roomtemperature.
The irradiation times are given with respect to the previous irradiation
step.
pattern shows a number of maxima that are consistent with a
hexagonal lattice. The lattice parameter of the hexagonal
columnar (Colh) phase was calculated from these maxima.
The wide-angle X-ray scattering (WAXS) pattern (see the
Supporting Information) shows two diffuse maxima. The
inner maximum corresponds to the “molten” aliphatic chains
and is usually observed in liquid-crystalline systems, whereas
the outer maximum corresponds to the average stacking
distance, for which a value of 3.3 Š was calculated. This
distance allowed us to estimate the density of the material. In
the hexagonal mesophase, the number of tetrameric com-
plexes per unit cell (Z) is 2, which is not usual in such a
mesophase.
The X-ray patterns of complex T-A14, whose mesophase
was proposed to be rectangular columnar (Colr) by POM, are
shown in Figure 2b and in the Supporting Information. The
lattice parameters a and b of the rectangular arrangement
were calculated from the diffraction maxima observed in the
SAXS pattern. Classically, a Colr phase has two columns per
unit cell, one of which is in the center and the other in the
corner of the unit cell. However, in this case, an unusual value
of Z = 4, that is, four tetrameric complexes per unit cell, was
calculated from density estimations.[12] These unusual Z
values (Z = 2 for the hexagonal mesophase and Z = 4 for
the rectangular one) are consistent with a bidimensional
arrangement in which two columns must be located at each
node of the unit cell. Further information on how two
columns can be accommodated in each node is therefore
chiral environment that corresponds to the mesophase
arrangement, as confirmed by X-ray studies at room temper-
ature. The appearance of an exciton coupling at the maximum
absorption wavelength of the chromophore (azobenzene
group) can be assigned to a helical disposition of at least
two chromophores,[15] which is consistent with a helical
stacking. Moreover, the handedness of this helical arrange-
ment is biased towards a given chiral sense that is determined
by the configuration of the stereogenic center. Thus, the CD
spectrum of the complex formed by the enantiomeric acid
presents an opposite sign (see the Supporting Information),
which indicates an opposite helical handedness.
In an effort to fulfill our objective, we undertook experi-
ments aimed at controlling and tuning the chirality of the
mesophase by irradiating the columnar-ordered material with
CPL. The CD spectra corresponding to different irradiation
processes of the same chiral material (T-(R)A10*) are shown
in Figure 3. The sample was irradiated with left-handed CPL
(l-CPL) for 30 min at room temperature. The resulting CD
spectrum has the same shape as the original one but with a
considerably higher intensity. This means that an amplifica-
tion of the chirality occurs, and this is likely to arise from an
increase in the population of azobenzene chromophores
absorbing light within a helical disposition. Subsequent
irradiation with the orthogonal (i.e., right-handed) CPL for
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30 min caused a decrease in the CD intensity almost to the
In summary, chiral columnar mesophases have been
obtained from hydrogen-bonded tetrameric complexes of a
melamine derivative and nonmesomorphic V-shaped acids.
Chirality transfer from the molecule to the mesophase occurs
when stereogenic centers are present in the acid counterparts.
Furthermore, the chiral information of CPL is transferred to
the columnar organization through photochromic azoben-
zene groups. This process enables reversible switching of the
supramolecular chirality of the columnar arrangement upon
illumination with CPL of the opposite handedness. Further-
more, achiral columnar systems can be biased towards a chiral
supramolecular organization by illumination with the corre-
sponding CPL. The chiral photoresponse achieved upon
illumination is stable for long periods of time. The results
described herein make these systems promising as supra-
molecular chiroptical switches[16,17] for optical information
storage. Control of the chiroptical response of materials with
irradiation time is the subject of current research. Further
work should be devoted to gaining a deeper understanding of
the relationship between the chemical composition of the
mesogenic entities and the chiral photoinduction.
original one. This observation can be interpreted in terms of
the chiral information achieved by the supramolecular
organization during the first irradiation being erased and
returning to the initial situation. Irradiation with r-CPL for an
additional 30 min afforded an induced optical activity of
similar magnitude to that observed after irradiation with l-
CPL, but with the opposite sign. Finally, the chiral informa-
tion attained in any irradiation process could be thermally
erased by heating at 908C for a few seconds. It was not
necessary to heat the material above the clearing point. These
experiments demonstrate that it is possible to control and
tune the supramolecular chirality of a columnar mesophase as
if the proposed helical stacking were able to wind and unwind
following the handedness of the incident CPL through the
photoisomerization of the azobenzene groups.
The next challenge was to assess whether the chiral
information in the CPL could be transferred to the columnar
arrangement of achiral mesogens. We therefore performed
irradiation experiments with the Colr mesophase of the
achiral complex T-A14. This complex clearly does not show
optical activity in the “as-prepared” cast film (Figure 4).
Received: September 15, 2006
Published online: January 24, 2007
Keywords: chirality · hydrogen bonds · liquid crystals ·
.
supramolecular chemistry · X-ray diffraction
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