Insight into the supramolecular organization of columnar assemblies with phototunable chirality

Article abstract of DOI:10.1039/c2jm33331g

New supramolecular associations with photoaddressable azobenzene units have been synthesized in order to better understand the structure of these complexes and their mesophases. The complexes are formed by a melamine core and three V-shaped acids and they exhibit columnar mesomorphism. The acids consist of two different arms, one derived from azobenzene and the other derived from phenyl benzoate. This design has allowed decreasing T_g values and also avoiding the tendency to crystallize, favouring regular and reproducible structures, together with the possibility of photoresponse. The mesomorphic properties of the complexes have been studied by POM, DSC and X-ray diffraction, showing Col_h or Col_r mesophases. CD activity of these materials in the mesophase (that reveals chiral structures) has been controlled by irradiation with CPL. XRD and AFM studies have been carried out to check the molecular organization in the films before and after CPL exposure, which confirmed that the mesophase retains its structure after switching the corresponding supramolecular chirality.

Full text of DOI:10.1039/c2jm33331g

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PAPER
Insight into the supramolecular organization of columnar assemblies with
phototunable chirality†
a
Francisco Vera, Jose Luis Serrano, Maria Penelope De Santo, Riccardo Barberi, Marıa Blanca Ros
b
c
c
a
ꢀ
ꢀ
a
*
and Teresa Sierra
Received 24th May 2012, Accepted 9th July 2012
DOI: 10.1039/c2jm33331g
New supramolecular associations with photoaddressable azobenzene units have been synthesized in
order to better understand the structure of these complexes and their mesophases. The complexes are
formed by a melamine core and three V-shaped acids and they exhibit columnar mesomorphism. The
acids consist of two different arms, one derived from azobenzene and the other derived from phenyl
benzoate. This design has allowed decreasing Tg values and also avoiding the tendency to crystallize,
favouring regular and reproducible structures, together with the possibility of photoresponse. The
mesomorphic properties of the complexes have been studied by POM, DSC and X-ray diffraction,
showing Colh or Colr mesophases. CD activity of these materials in the mesophase (that reveals chiral
structures) has been controlled by irradiation with CPL. XRD and AFM studies have been carried out
to check the molecular organization in the films before and after CPL exposure, which confirmed that
the mesophase retains its structure after switching the corresponding supramolecular chirality.
porphyrin aggregates, a process determined by rotational and
magnetic forces during the nucleation step.26
Introduction
In recent years, a great deal of attention has been focused on the
incorporation of chirality into self-organized materials, ranging
from bulk materials to solvent-assisted aggregates.1–3 The chiral
properties of self-organized materials not only rely on the
molecular structure but also on the supramolecular interactions
between building blocks.4,5 Accordingly, a challenge in this area
is the design of appropriate building blocks that can self-
assemble into helical architectures with a given handedness.
Helical handedness often comes from the chiral information
encoded in the constituent molecules (i.e. chiral building
blocks).6–12 Other origins for chirality have been observed in self-
organized systems. These include spontaneous resolution into
chiral domains due to particular molecular structures and
phases13–18 and the induction of chirality through the interaction
of appropriately designed achiral building blocks with external
chiral sources, such as light,19–21 chiral templates,22,23 solvents,24,25
etc. Very recently, chiral selection has been reported for
Particularly, the efficient transfer of chirality to the supra-
molecular architecture of self-organized bulk materials has been
demonstrated for columnar liquid crystals.27 Columnar meso-
phases allow the construction of functional materials with well-
defined 1D organizations.28–31 In this respect, the structural
control of mesogens that provide columnar assemblies, and of
the resulting columnar mesomorphic order, has proven to be a
valuable tool to construct self-organized chiral materials with
well-defined 1D helical architectures.27 In this type of system,
supramolecular chirality has also been reported to be generated
from different origins such as chiral mesogens,32–34 spontaneous
resolution35 or light-induced chirality.36,37 Moreover, the use of
the supramolecular chemistry strategy for the design of columnar
LCs brings additional features to these systems such as dynamic
responses and further possibilities for molecular design.38,39
Triazine derivatives, such as diamino-1,3,5-triazine40–42 and
triamino-1,3,5-triazine,43–48 constitute a valuable class of supra-
molecular synthons for the design of liquid crystals with well-
defined mesomorphic organizations. In previous papers we
described the remarkable self-organizing ability of propeller-like
H-bonded complexes built from 2-alkylamino-4,6-diamino-
1,3,5-triazine to give columnar mesophases.36,49 This ability was
ascribed to the synergistic action of the p-stacking tendency of
the melamine-derived core and the strong lateral interactions
between V-shaped molecules. This ability is general for this type
of complex, regardless of the chemical composition of the acid,
and this allowed functionality to be imparted to these systems by
tuning the molecular structure of the acid. Thus, transfer of
aDepartamento de Quꢀımica Orgaꢀnica, Facultad de Ciencias – Instituto de
Ciencia de Materiales de Aragoꢀn (ICMA), CSIC – Universidad de
Zaragoza, 50009 Zaragoza, Spain. E-mail: tsierra@unizar.es; Fax: +34
976761209; Tel: +34 976762276
b
ꢀ
Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza,
50009 Zaragoza, Spain
cCNR-IPCF Licryl Cosenza, Department of Physics – University of
Calabria, 87036 Rende, Italy
† Electronic supplementary information (ESI) available: Detailed
experimental procedures and characterization data, CD, UV-Vis
spectroscopy, and calculations from XRD data. See DOI:
10.1039/c2jm33331g
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molecular chirality to the columnar organization,49 luminescent
properties50 and photoresponsive features established by the
ability of azobenzene-derived systems to follow the helicity of
circularly polarized light36 has been verified in these materials.
In this paper we present a study of these systems with partic-
ular emphasis on their columnar organization and whether it is
affected by the dynamic process involved in response to CPL.
For this purpose, we designed the new complexes represented in
Chart 1. The novelty of the complexes lies in the chemical
structure of the V-shaped acid, which combines two different
arms – one derived from phenyl benzoate and the other arm
containing the azobenzene photoactive group. These complexes
show good processing features and are good film-forming
materials. The incorporation of asymmetric V-shaped acid
structures allowed us to decrease the contents of photoactive
units to 50% with respect to the previously described materials in
a controlled manner. Furthermore, a general study was carried
out on the structure of the mesophase shown by these complexes
and the switching and chirality induction behaviour. The films
prepared from these complexes were studied by X-ray diffraction
(XRD) and atomic force microscopy (AFM) before and after
irradiation in order to determine whether the columnar organi-
zation of the complexes is affected upon the CPL irradiation
process.
2,4-diamino-6-chloro-1,3,5-triazine.51 The synthetic pathway
followed to prepare the V-shaped acids is outlined in Scheme 1.
Acid 1 was prepared by azoic coupling of ethyl 4-aminobenzoate
and phenol followed by Williamson etherification of the hydroxy
group and hydrolysis of the ethyl ester, as described previ-
ously.36,37 The phenyl benzoate arm, acid 5, was synthesized by
esterification of benzyl 4-hydroxybenzoate with the corre-
sponding 4-alkoxybenzoic acid using DCC/DMAP in dichloro-
methane, and subsequent cleavage of the benzyl ester by
hydrogenation.49 The V-shaped acid, 7, was synthesized in two
steps. First, the central ring, 3,52 was reacted with acid 1, via the
acid chloride 2,36,37 in a 1 : 1 molar ratio. The intermediate 4 was
separated by flash chromatography. The phenol intermediate 4
was then coupled with acid 5 by the Steglich reaction. Cleavage
of the TIPS group of compound 6 was carried out with tetra-n-
butylammonium fluoride (TBAF) in THF and this afforded the
final acids, 7, after acidification with acetic acid. All of the
products were fully characterized by 1H-NMR, IR and their
purity was confirmed by elemental analysis (see ESI†).
Characterization of the complexes
The supramolecular associations were studied in deuterated
chloroform solution by 1H-NMR spectroscopy, which evidenced
the formation of supramolecular species given the significant
changes observed in the chemical shifts of some protons in both
supramolecular synthons, as previously described for analogous
Results and discussion
Synthesis
The supramolecular complexes T–AR–ER0 were prepared by
slow evaporation of a dichloromethane solution of the two
supramolecular synthons, triazine and V-shaped acid, in a 1 : 3
ratio, respectively. The mixture was then heated to the isotropic
state, and the sample was used for subsequent experiments. The
melamine derivative 2,4-diamino-6-dodecylamino-1,3,5-triazine,
T, was synthesized by reaction of n-dodecylamine with
Chart 1
Scheme 1
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complexes. Furthermore, DOSY experiments indicated that a
fast exchange between associated and isolated species occurs in
solution, since only one apparent diffusion coefficient was
observed (see ESI†). The formation of the complexes T–AR–ER0
[1 : 3] was also confirmed by solid state 13C-CPMAS NMR
spectroscopy (Fig. 1). It is especially significant that the peak
corresponding to the carbonyl group of the non-complexed acid
(174 ppm) is broadened and shifted in the spectrum of the
mixture. Furthermore, the signals for some carbon atoms, of
both the acid and the melamine synthons, are significantly shifted
upon complexation. The upfield shift of the signals for the
carbon atoms within the N-alkyl tail of the melamine is worth
noting, and must be due to the strong influence that the aromatic
rings of the acid partners have on the carbon atoms of the tail.
Fig. 2 POM pictures taken at room temperature for associations
(a) T–A12–E(S)10*, Colh, and (b) T–A(S)10*–E12, Colr.
Table 1 Thermal and thermodynamic properties of the complexesa
Transition temperatures, ꢀ
C
Complex
(enthalpy values, kJ molꢁ1
)
Mesomorphic properties
T–A12–E12
I 108 (6.1) Colr
I 103 (8.5) Colr
I 92 (2.9) Colh
I 85 (4.3) Colh
T–A(S)10*–E12
T–A12–E(S)10*
T–A(S)10*–E(S)10*
All of the supramolecular complexes prepared from the mela-
mine derivative, T, and the V-shaped acids, AR–ER0, combining
azo and ester arms, appear as uniform materials by optical
microscopy and show textures, between crossed polarizers, that
reveal liquid crystalline behaviour over broad temperature
ranges. Only the complex T–A(S)10*–E(S)10* had to be
annealed for 1 hour, at a temperature a few degrees below the
clearing point measured by DSC, in order to allow the meso-
phase to develop a texture that was visible through the crossed-
polarizers. None of the acids used to form these complexes are
liquid crystalline (see ESI†). The melamine derivative (T) shows
metastable SmA mesomorphic behaviour on cooling from the
a
I: isotropic liquid; Colr: rectangular columnar mesophase; Colh:
hexagonal columnar mesophase.
cooling rate led to the segregation of the supramolecular syn-
thons and prevented the formation of the mesogenic complex.49
However, once formed, the mesophase was stable at room
temperature for all the complexes studied.
To confirm the nature and structural details of each meso-
phase, powder X-ray diffraction experiments were carried out on
all materials at room temperature. Measurements were per-
formed on samples that were previously treated in the capillary,
i.e. the material was heated to the isotropic liquid and then
cooled down to room temperature at a rate of 10 ꢀC minꢁ1. X-ray
diffractograms were, in general, rich in reflections, as shown in
Fig. 3. The structural parameters of the mesophase for each
complex are given in Table 2. The type of columnar mesophase
was confirmed as Colh for complexes T–A12–E(S)10* and
T–A(S)10*–E(S)10*. In contrast, the complexes that have the
same arm E12 [T–A12–E12 and T–A(S)10*–E12] show a Colr
mesophase. It is difficult to establish the reason for this behav-
iour. What can be deduced is that there is a relationship between
the number of chiral tails and the mesophase symmetry, which
also depends on the position of the chiral tail. Indeed, the achiral
complex T–A12–E12 shows rectangular columnar behaviour,
isotropic liquid; C 103 C (SmA 38 ꢀC) I.51
ꢀ
All of the complexes showed textures between crossed-polar-
izers that are indicative of columnar mesophases (Fig. 2). The
type of two-dimensional arrangement was further confirmed by
X-ray diffraction (see below). Transition temperatures and
enthalpy values, determined by differential scanning calorimetry,
are gathered iꢁn1Table 1. Thermograms recorded at a cooling rate
ꢀ
of 10 C min (see ESI†) showed a single peak in the cooling
process and this corresponds to the transition from the isotropic
liquid to the mesophase. It was found that the cooling rate was
crucial for the appearance of the mesophases, since a rapid
Fig. 1 13C-CPMAS spectra of T, T–A(S)10*–E12, and A(S)10*–E12.
Vertical lines indicate the chemical shifts of the C atoms of the carbonyl
group of the acid and those of the alkyl tail of the melamine core.
Fig. 3 SAXS patterns of left: the Colh mesophase of the complex T–
A12–E(S)10*, and right the Colr mesophase of complex T–A(S)10*–E12
(the reflection 31 was too weak to be seen in the digitalized pattern).
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Table 2 Lattice parameters for the associations measured by powder
X-ray diffraction at room temperature
every complex, six CD spectra were recorded at six different
orientations measured by rotating the sample cells by 60ꢀ around
the light beam. The spectra were almost identical for all orien-
tations of the sample, and this rules out the prevalence in the
observed signals of possible linear dichroism effects due to the
macroscopic orientation.53 The supramolecular chirality of
the mesophase was expected to originate from a helical disposi-
tion of the chromophores along the column. Indeed, all of the
complexes bearing chiral tails gave spectra with bands corre-
sponding to circular dichroism (Fig. 4), which can be accounted
for by the transmission of the molecular chirality to the supra-
molecular organization of the mesophase. It was found that the
presence of a single chiral tail in the V-shaped acid [i.e. T–A12–
E(S)10* and T–A(S)10*–E12] is sufficient to induce CD-active
columnar organizations, and this occurs regardless of the
chemical nature of the rigid arm in which the chiral tail is located,
either the azobenzene (A) group or the phenyl benzoate (E)
group. In addition, the transmission of the molecular chirality to
the supramolecular organization occurs regardless of the type of
two-dimensional organization of the columnar mesophase
(hexagonal or rectangular), indicating that the supramolecular
chirality appears because of the intrinsic chirality of the complex,
and does not depend on the arrangement of the columns.
ꢁ ꢁ ꢁ
Phase Parameters (A) dobs (A) dcalc (A) hk
Complex
T–A12–E12
Colr
Colr
Colh
a ¼ 94; b ¼ 72 57.2
57.2
47.0
36.0
28.6
28.7
23.5
19.7
51.8
34.0
24.8
20.0
17.0
14.3
12.1
59.2
34.2
22.4
17.1
56.6
32.7
21.4
11
20
02
22
31
40
42
11
02
31
40
04
34
35
10
11
21
22
10
11
21
47.0
36.0
28.6
23.5
19.5
T–A(S)10*–E12
T–A12–E(S)10*
a ¼ 80; b ¼ 68 51.7
33.9
24.7
19.7
16.8
14.0
11.8
a ¼ 68.4
a ¼ 65.4
59.2
34.4
23.0
17.1
56.9
33.5
20.6
T–A(S)10*–E(S)10* Colh
whereas the complex with two chiral tails T–A(S)10*–E(S)10*
forms a hexagonal arrangement.
It is worth noting the differences observed in the spectrum
profiles of the different complexes. These differences are ascribed
to the presence of two different chromophores, phenyl benzoate
(E) and azobenzene (A) groups, with either chiral or achiral tails,
randomly stacked along the columns.50 The absorption maxima
of these two units are around 280 nm and 360 nm, respectively.
The CD spectra can show bands corresponding to the helical
disposition of either identical chromophores or different chro-
mophores, and this must increase the complexity of the corre-
sponding CD curves. In any case, the optical activity of all the
materials was maintained over time. The material T–A12–E12
was not CD active under the same experimental conditions.
Examination of the WAXS patterns showed that a high-angle
halo was absent in all of the materials, apart from the diffuse
maximum corresponding to the disordered aliphatic tails. This
means that the correlations along the column in the mesophase are
only short-range and there is no regular stacking distance.
Nevertheless, on the basis of our previous studies,36,49 the stacking
ꢁ
distance in the complexes can be estimated as 3.3–4.0 A, and this
allowed us to estimate the density of the materials. It was found
from these estimations that, as in previous studies, the number of
molecules per unit cell, Z, is 2 for the hexagonal mesophases and 4
for the rectangular mesophases. This situation is consistent with
the findings for previous complexes of this type.36,49,50 The
differences in the rectangular lattice constants between the non-
chiral complex T–A12–E12 and the chiral complex T–A(S)10*–
E12 might well be due to the existence of molecular tilting in the
latter. Indeed, if the 1 : 3 complex tilts as a whole with respect to
the column axis, the cross-section of the column must decrease as
the cosine of the tilt angle. The consequence of the tilt is a larger
value for the mean stacking distance, with an estimated value that
Chiral switching and chiral induction with CPL
The presence of azobenzene units in the V-shaped acids confers
on the complexes the ability to respond to light as an external
ꢁ
ꢁ
evolves from about 3.5 A for T–A12–E12 to about 4 A for the
chiral counterparts. The hexagonal lattice constant of T–A12–
E(S)10* and T–A(S)10*–E(S)10* is in agreement with the
parameter found for the previously described complexes derived
from all-azo V-shaped acids, and is consistent with an estimated
34
ꢁ
value for the mean stacking distance close to 3.3 A.
Chiroptical properties
The optical activity of the mesophase was studied for all of the
chiral complexes by Circular Dichroism (CD) experiments on
thin films (ca. 300 nm to 1mm thickness measured by profilom-
etry) prepared by casting a dichloromethane solution of the
corresponding complex onto an amorphous quartz plate and
subsequent thermal treatment as for X-ray experiments. For
Fig. 4 CD spectra recorded at r.t. for thin films of (a) T–A(S)10*–E12,
(b) T–A12–E(S)10* and (c) T–A(S)10*–E(S)10*.
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stimulus. Indeed, the photoinduced E/Z isomerisation of azo-
benzene is a well documented phenomenon, which proceeds
reversibly upon irradiation with UV and visible light. The
isomerization of azobenzene units proceeds with a large struc-
tural change that affects the dipole moment and geometry and
gives rise to the Weigert effect,54 responsible for the photo-
orientation of azobenzene materials.55 This effect can trigger
macroscopic effects such as induction of linear birefringence and
dichroism56 or photomechanical responses57 in azopolymers.
Additionally, liquid crystals are especially interesting for
these effects due to the intrinsic anisotropic organization of
azobenzene groups.58
Fig. 6 CD spectra recorded at r.t. for a thin film of T–A12–E12 irra-
diated for 30 min with left-CPL (dotted line) and with right-CPL (dashed
line).
When light used is circularly polarized (CPL), chiral photo-
induction has been described for azobenzene-containing liquid
crystals.56,59,60 In general, the transfer of the angular momentum
from CPL to the liquid crystalline medium by means of the
asymmetric photoisomerization of azobenzene groups is
proposed.
necessary to reach the isotropic state and destroy the columnar
organization to eliminate the photoinduced effect.
The transfer of chiral information from light to the materials
was studied in all cases after 30 minutes irradiation in order to
ensure that each material reached a photostationary situation
(see ESI†). However, it was considered of interest to determine
the time needed to achieve transmission of chiral information
and thus examine the ability of the material to respond to the
stimulus. The experiment was carried out on the achiral material
T–A12–E12 and it was observed that after only 30 seconds
irradiation the material was optically active, as shown by the CD
spectra (Fig. 7). Nevertheless, irradiation for 30 minutes gave rise
to a signal that did not increase upon longer irradiation times.
An additional band at around 490 nm, which does not corre-
spond to an absorption band in the UV spectra (see ESI†), was
observed for films of T–A12–E(S)10* upon irradiation for 1 hour
(Fig. 5c). Under these conditions, the intensity of the CD curves
did not increase except the band at 490 nm. This type of band has
been detected previously and reported as a Bragg band reflection
in nematic azobenzene polymers61,62 or nematic azodendrimers.63
However, this band was not seen for smectic azobenzene poly-
mers.64 The origin of this band is unclear59 but it could corre-
spond to a higher order supramolecular organization, as in chiral
nematic liquid crystals65 or chiral nematic columnar liquid
crystals,66 with a periodic pitch approximately equivalent to the
wavelength of the incident light. Nevertheless, it was not possible
to demonstrate such type of organization in the present case.
In the particular case of columnar liquid crystals we previously
described the possibility of inducing chirality into the columnar
organizations for H-bonded complexes containing V-shaped
acids consisting of two azobenzene arms.36 The complexes
reported herein contain only one azobenzene group per V-shaped
acid.
Thin films of all the three chiral complexes were irradiated at
room temperature with CPL (488 nm, Ar+ laser line, 20 mW
cmꢁ2) and the chirality of the material followed the handedness
of the light, either increasing the intensity of their natural CD
signal or displaying the opposite sign (Fig. 5). Likewise, irradi-
ation of the achiral material T–A12–A12 with CPL gives rise to
materials that show optical activity by CD spectroscopy, with
signs corresponding to the handedness of the light used (Fig. 6),
showing that the chirality of light is transmitted to the CD silent
materials. Chiral photoswitching occurs in a reproducible
manner for all the complexes, which give rise to CD signals of
opposite sign for left and right-CPL irradiation. Furthermore,
heating the materials at 80 ꢀC for a relatively short time (20–60 s)
led to the original situation (see ESI†), showing that it is not
Structural study of the films
The chirality induction and switching phenomena described
above are based on columnar systems and it is proposed that
Fig. 5 CD spectra recorded for thin films of (a) T–A(S)10*–E12,
(b) T–A(S)10*–E(S)10* after irradiation for 30 min; (c) T–A12–E(S)10*
after irradiation for 60 min. Dotted line: after left-CPL irradiation;
dashed line: after right-CPL irradiation.
Fig. 7 CD spectra recorded at r.t. for a thin film of T–A12–E12 before
irradiation (solid line) and irradiated for 30 seconds, 1 min and 30 min
(the film is different from that used for experiment in Fig. 6).
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these are related to the changes in the handedness of the helical
organization along the column. One of the objectives of the
present study was to gain a deeper insight into the supramolec-
ular organization in the thin films before and after the irradiation
experiments. The surface of the film and the mesomorphic
organization within the film were investigated by AFM and
X-ray diffraction techniques, respectively.
AFM images were taken for cast films on amorphous quartz
plates at room temperature, after thermal treatment, in tapping
mode (Nanoscope IIIa, Veeco). The pictures on the left refer to
topography, while the ones on the right show the amplitude
signal (Fig. 8). The latter images are shown to emphasize steep
changes in height. The AFM images recorded for the three
complexes T–A12–E12, T–A12–E(S)10* and T–A(S)10*–E(S)
10* show line patterns in which the gaps between lines are within
the range of the cell parameters (i.e. intercolumnar distance)
measured by X-ray diffraction (Table 2). The topographic relief
observed can thus be related to either columns lying on the
surface of the plate or, most probably, columns tilted with
respect to the plate. Both situations can explain the observed
AFM images. On comparing the surface images of all three
complexes, it is worth noting the different organization of line-
domains, which seems to be related to the chirality of the system.
For the achiral complex, T–A12–E12, the columns seem to orient
in large domains. In contrast, smaller domains with different
orientations in the organization of the columns are seen when all
tails in the complex are chiral, T–A(S)10*–E(S)10*. In the case of
T–A12–E(S)10*, there are some zones that could be interpreted
as being homeotropic regions in which columns lie probably
perpendicular to the quartz plate.
Fig. 9 AFM images of different zones of thin films of complex T–A12–
E(S)10* deposited on quartz, before irradiation (a) and after 60 min
irradiation (b).
irradiation (Fig. 9b). It can be seen that the line pattern visualized
on the surface is similar for both samples.
Further information about the supramolecular organization
within the film was obtained by X-ray diffraction experiments.
X-ray diffractograms for thin films of the complexes T–A12–E(S)
10* and T–A(S)10*–E12 are shown in Fig. 10. These samples
were chosen as representative examples of Colh and Colr orga-
nizations, respectively. The diffractograms are compared with
the patterns registered for powder samples in capillaries, from
which lattice parameters were measured (Table 2). As deduced by
the coincidence of the two most intense maxima, it is clear that in
AFM observations after CPL irradiation proved that the
surface morphology of the thin films is not altered. AFM images
of a thin film of T–A12–E(S)10* on a quartz plate are shown in
Fig. 9 before CPL irradiation (Fig. 9a) and after 1 hour
Fig. 8 AFM images of (a) T–A12–E12, (b) T–A12–E(S)10*, and its 3D
visualization; the homeotropic region is visible in the bottom right
corner, and (c) T–A(S)10*–E(S)10*. These films were deposited on a
quartz plate.
Fig. 10 XRD diffractograms (transmission and reflection) taken at
room temperature before CPL irradiation and after irradiation of
T–A12–E(S)10* and T–A(S)10*–E12.
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Fig. 11 SEM images (both on a quartz plate) of a cut region of T–A12–
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Conclusions
Rectangular and hexagonal columnar mesophases have been
obtained from supramolecular assemblies consisting of H-
bonded synthons (a melamine derivative and three V-shaped acid
molecules) that self-organize into columns. When stereogenic
centres are present in the V-shaped molecules, chiral transfer into
the columns occurs. The presence of photoaddressable azo-
benzene units in the arms of the acids makes it possible to induce
chirality in chiral and non-chiral mesophases by CPL irradiation.
It is deduced that decreasing the concentration of azobenzene
groups in the columnar architecture to 50%, with respect to
previous studies, did not hamper the photoresponse of the
materials and not only photocontrol but also photoinduction of
supramolecular chirality was achieved. In addition, the induced
chiral information is stable for a long time. The presence of ester
bonds in the arms of the V-shaped acids makes these materials
easier to process and more difficult to crystallize. A structural
study of these compounds (CD, XRD in powder and film and
AFM) reveals that after irradiation there are no changes in the
mesophase structures, meaning that changes in chirality after
CPL irradiation may be due to a small change in the disposition
of the tilted arms of the V-shaped acids attached to the melamine
centres rather than to modifications of the corresponding
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Acknowledgements
This work was supported by the MICINN-FEDER projects,
MAT2009-14636-C03-01 and MAT2011-27978-C02-01, and
Aragon Government-FSE. F.V. thanks the MEC of Spain for
the grant. The authors wish to acknowledge Dr Giovanni
€
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