Luminescent columnar liquid crystals generated by self-assembly of 1,3,4-oxadiazole derivatives

Article abstract of DOI:10.1039/c0jm04570e

Five new V-shaped acids derived from 1,3,4-oxadiazole are described. These compounds were used to prepare supramolecular complexes via hydrogen bonding interactions with 2,4-diamino-6-dodecylamino-1,3,5-triazine in a 3:1 ratio. The formation of the comple

Full text of DOI:10.1039/c0jm04570e

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PAPER
Luminescent columnar liquid crystals generated by self-assembly of 1,3,4-
oxadiazole derivatives†
a
a
b
b
Andre A. Vieira, Hugo Gallardo, Joaquın Barbera, Pilar Romero, Jose Luis Serrano and Teresa Sierra
b
ꢀ
ꢀ
ꢀ b
ꢀ
*
*
Received 29th December 2010, Accepted 22nd February 2011
DOI: 10.1039/c0jm04570e
Five new V-shaped acids derived from 1,3,4-oxadiazole are described. These compounds were used to
prepare supramolecular complexes via hydrogen bonding interactions with 2,4-diamino-6-
dodecylamino-1,3,5-triazine in a 3 : 1 ratio. The formation of the complexes was evidenced by infrared
and NMR techniques. All the complexes were studied by polarizing optical microscopy, differential
scanning calorimetry and small-angle X-ray diffraction. Rectangular and hexagonal columnar
mesophases were observed for the complexes at room temperature, without signs of crystallization.
Circular dichroism studies demonstrated that, in the liquid crystalline state, these materials show
supramolecular optical activity. It is proposed that this phenomenon arises due to a helical columnar
organization. Furthermore, the complexes display strong blue light emission in solution, with good
photoluminescence quantum yields, and in the mesophase. These materials could therefore be
promising candidates for optoelectronic applications.
The 1,3,4-oxadiazole heterocycle is well known in the literature
for exhibiting high photoluminescence and electron-accepting
properties as well as for its high thermal and hydrolytic stability
and resistance to oxidative degradation.13 The introduction of
a 1,3,4-oxadiazole ring into an aromatic core may also lead to
fluorescence properties in the molecule, generate a significant
lateral dipole moment and also give a bent rigid core.
Introduction
Since the discovery of mesomorphism promoted by hydrogen
bonding in organic compounds by Bennett and Jones1 in 1939,
many liquid crystals (LCs) based on self-organized H-bonded
systemshavebeenstudied.2 Thegreatinterestinthisresearchareais
stimulated by several examples in nature that are capable of
forming highly organized structures.3 Among self-organized LC
systems based on H-bonding interactions are the columnar orga-
nizations.4 Columnar liquid crystals exhibit a molecular organi-
zation that allows the transport of energy or charge.5 These
materials have applications such as semiconductors, photo-
conductors, organic light emitting diodes (OLED) and photovol-
taic cells.6 Moreover, columnar mesophases are an interesting
startingpointfortheachievementofhelicalarchitecturesthatallow
the expression of supramolecular chirality.7 The reproduction8 and
control9 ofthesehelicalarchitectureshavebeenthesubjectofrecent
works because this can endow a material with unique properties.10
Within this context, helical organization has been studied to eval-
uate the impact of light emission from columnar liquid crystals,
especially for metallomesogens.11 The possibility of obtaining
polarized light emission through supramolecular structures
appears to be a major challenge in this field.12
We describe here the synthesis and characterization of a novel
series of asymmetric V-shaped acids derived from 1,3,4-oxadia-
zole.
A mixture of the 2,4-diamino-6-dodecylamino-1,3,5-
triazine and the oxadiazole acids in a 1 : 3 ratio gives rise to
supramolecular complexes through H-bonds. These complexes
have an H-bonded core designed to generate mesomorphism,
fluorescence and helical superstructures.
Synthesis
The melamine derivative 2,4-diamino-6-dodecylamino-1,3,5-
triazine (M) was prepared by the reaction of dodecylamine with
2,4-diamino-6-chloro-1,3,5-triazine using sodium hydrogen
carbonate as a base.14 The V-shaped acids X12E12, X(S)10*E12,
X(R)10*E12, X(S)10*E(S)10* and X12E(S)10* were prepared
according to the synthetic pathway outlined in Schemes 1 and 2.
The synthetic route for the acids begins with the 4-alkox-
ybenzonitriles 1, which were used to obtain the respective tetra-
zole heterocycles 2 by Huisgen 1,3-dipolar cycloaddition using
sodium azide and ammonium chloride in DMF.15 The tetrazole
compounds 2 were reacted with freshly prepared methyl
4-(chlorocarbonyl)benzoate in pyridine to afford the 2,5-disub-
stituted-1,3,4-oxadiazoles16 4 in good yields (68–93%). We also
tested a different methodology to obtain the 1,3,4-oxadiazole
a
ꢀ
ꢀ
Departamento de Quımica, INCT-Catalise, Universidade Federal de
Santa Catarina—UFSC, 88040-900 Florianopolis-SC, Brazil. E-mail:
ꢀ
hugo@qmc.ufsc.br; Tel: +55 48 37219544
b
ꢀ
ꢀ
ꢀ
Instituto de Ciencia de Materiales de Aragon, Quımica Organica, Facultad
de Ciencias, Universidad de Zaragoza—CSIC, 50009 Zaragoza, Spain.
E-mail: tsierra@unizar.es; Fax: +34 976 762286; Tel: +34 976 762278
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0jm04570e
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Scheme 1 Synthetic route for the intermediate acids 5.
Scheme 3 General preparation of the complexes M-XE from 3 eq. of the
acids (XE) and 1 eq. of the triazine (M).
The formation of the complexes M-XE was studied by IR and
NMR spectroscopy (see ESI†). The IR spectra of the acids XE
differed from those of the complexes M-XE, especially in the
regions corresponding to carbonyl, –COOH and N–H groups,
which are responsible for the interactions between molecules. An
effective shift of the carbonyl group stretching band from 1744 to
1735 cmꢀ1 was observed after formation of the complex
M-X12E12.
The 1H NMR spectra clearly show the formation of
complexes, assuming that there is a rapid equilibrium between
the complex and its components.8a Large displacements were also
observed for the five NH groups of the melamine complexes as
these are responsible for the hydrogen bonds. The downfield
shifts of the five N–H hydrogens of melamine (M) are shown in
Fig. 1.
Scheme 2 Synthesis of the final acids 8 (XE).
The proton of the amino group with the dodecyl chain is
shifted from 5.20 to 6.84 ppm on complexation. The four protons
of the primary amino groups of the melamine are shifted
downfield, from 5.32 to 6.48 ppm, and a broad peak is observed
at 5–7 ppm after interaction with the acid M-X(S)10*E(S)10* in
CDCl3. Likewise, the protons of the N-methylene group of the
melamine alkyl chain and the hydrogens of the central aromatic
intermediate, which used benzyl 4-(2H-tetrazol-5-yl)benzoate
and 4-alkoxybenzoyl chloride, but the yields were very poor
(1–3%). The carbonyl group in the 4-position with respect to the
tetrazole ring probably inhibits the progress of this reaction. The
acid intermediates 5 were obtained in quantitative yield by simple
hydrolysis of the ester function of compounds 4 with potassium
hydroxide in ethanol/water. Compounds 7 were obtained by
esterification of phenol17 6 with the respective acid 5 using
N,N0-dicyclohexylcarbodiimide (DCC) and (N,N-dimethy-
lamino)pyridinium 4-toluenesulfonate (DPTS) in dichloro-
methane (Scheme 2).
The desired final acids 8 (XE) were obtained in good yields
(75–95%) by deprotection of the triisopropylsilyl intermediate
with tetra-n-butylammonium fluoride in dichloromethane (see
ESI†). The structures and purities of the XE compounds were
1
verified by H and 13C NMR, elemental analysis, and MALDI-
TOF (see ESI†).
Results and discussion
Having synthesized and characterized the final acids (XE) and
the melamine (M), the complexes M-XE were prepared (Scheme
3). The complexes were prepared by dissolving both compounds
in dichloromethane, mixing them in the ratio 1 mole of melamine
to 3 moles of acid XE and allowing the solvent to evaporate by
stirring the solution at room temperature.
Fig. 1 1H NMR spectrum of the acid X(S)10*E(S)10*, complex M-
X(S)10*E(S)10* and melamine (M) in CDCl3 solution at 25 ꢁC. Signals
due to protons belonging to the amino groups of M are shifted upon
formation of the complex.
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ring of the acid X(S)10*E(S)10* are shifted with respect to those
in the non-complexed melamine and acid, respectively.
optical microscopy, on cooling from the isotropic liquid, are
consistent with the presence of columnar mesomorphism. A
microphotograph of the natural texture at 25 ꢁC observed for the
complex M-X12E12 is shown in Fig. 2 and this phase was further
characterized as Colr. This type of texture was common to all
complexes except for M-X(S)10*E(S)10*, which showed no
birefringence between crossed polarizers but a change in fluidity
on cooling to the temperature indicated by DSC (see Table 1).
The measured enthalpies associated with the corresponding
transitions to the isotropic state had high values, a finding
consistent with those reported for previous complexes of this
type, which were described as Colr.8a The only exception was
complex M-X(S)10*E(S)10*, which gave a smaller enthalpy
value that is more consistent with the transition from Colh to I, as
described for previously reported chiral complexes.9b
Titration NMR experiments in CD2Cl2 showed that all NH
proton signals, as well as the methylene hydrogen atoms of the
alkyl chain of the melamine M and the hydrogens of the central
aromatic ring of the acid X12E(S)10*, were displaced upon
increasing the proportion of acid while keeping the melamine
concentration constant (see Fig. S8 and S9 in the ESI†). These
experiments allow the estimation of the binding constant (488 ꢂ
27 Mꢀ1 in CD2Cl2 at 25 ꢁC) for the complexation of the melamine
derivative and the acids. This value was calculated by nonlinear
curve fitting of the chemical shifts. These titration experiments
also demonstrated that these complexes have a 1 : 1 stoichiom-
etry in solution, as previously described in similar systems.9b The
stoichiometry, however, becomes 3 : 1 in the bulk material when
the molar amount of the acid is three times that of the melamine,
as also demonstrated previously.8a,9a
Moreover, all of the complexes show glass transitions in their
DSC thermograms (see ESI†) and these correspond to the
transition between a glassy state and the mesophase in the
heating process. Accordingly, these complexes retain the orga-
nization of the mesophase in a glassy state, and this is stable at
room temperature without crystallization. This fact provides
added value to these materials since they have columnar order
frozen at room temperature and this would help processing, such
as the formation of thin films for the evaluation of properties
(optical activity and luminescence). The complex M-X12E(S)10*
is the only one that shows some tendency to crystallize. Indeed,
during the heating process, the mesophase undergoes a cold
transition. This was interpreted as a partial crystallization on
heating at the same time as the transition from the mesophase to
the isotropic state. This is accounted for by the broad peak
observed in the DSC heating scan (see ESI†), corresponding to
the transition to the isotropic liquid, that has a higher enthalpy
than the corresponding peak in the cooling scan.
Previous studies showed that DOSY experiments can also help
to determine whether hydrogen bonding interactions are estab-
lished between the melamine derivative M and carboxylic acids.
This technique allows the diffusion coefficients to be correlated
with the molecular composition by observing the chemical shifts.
Self-diffusion of chemical species in a solvent depends on its
molecular size and hydrodynamic volume. According to this
principle, the association can promote changes in the molecular
diffusion coefficient itself and this can be used to detect the
presence of a complex formed by hydrogen bonds in solution.18
DOSY experiments were performed in CD2Cl2 for the pure acid
X12E(S)10* and pure melamine (M) as well as the complex
M-X12E(S)10* (see ESI†). The signals corresponding to protons
within the complex M-X12E(S)10* have the same diffusion
coefficient (5.9 ꢃ 10ꢀ10 ꢂ 0.03 m2 sꢀ1). This value corresponds to
the apparent diffusion coefficient of the complex because there is
rapid exchange between the complex and the components on the
NMR time scale. More importantly, this coefficient measured for
the complex is much smaller than that of the melamine (12.6 ꢃ
10ꢀ10 ꢂ 0.03 m2 sꢀ1) and this can be accounted for by the fact that
the melamine and the acid X12E(S)10* diffuse within the same
supramolecular species in solution. In the case of weak interac-
tion between melamine M and acid, the diffusion coefficients of
both components would have remained unchanged in the DOSY
spectrum of the mixture.
With respect to clearing temperatures and mesophase ranges,
it is important to note the broad mesophase ranges shown by
these complexes. In addition, all of them display moderate
clearing temperatures, which enables easy processing for the
evaluation of properties and avoids the high temperatures at
which the melamine can decompose. Depending on the periph-
eral tails, reasonable dependence between the clearing tempera-
ture and the presence of branches in the tail (i.e. citronellyl
derivatives) is observed. Accordingly, the complexes bearing the
chiral tail, M-X(S)10*E12, M-X(R)10*E12, M-X12E(S)10* and
M-X(S)10*E(S)10*, have lower clearing temperatures than the
achiral complex, M-X12E12, with the greatest difference
observed for the complex with all six chiral tails.
Thermal properties
The acids XE do not exhibit mesomorphism and have melting
ꢁ
points in the range 141–165 C (see ESI†). Investigation of the
thermal stability of these new V-shaped acids by thermogravi-
metric analysis (TGA) indicated that they have good stability
with decomposition under nitrogen at temperatures above
272 ꢁC.
Structural characterization of the mesophase
ꢁ
For all complexes, X-ray experiments were carried out at 25 C
with the samples slowly cooled from the isotropic liquid so that
the mesophase could develop completely. The reflections
obtained for the complexes M-XE are consistent with rectan-
gular (Colr) and hexagonal columnar (Colh) mesophases.
The thermal behavior of the final complexes M-XE was
investigated by polarizing optical microscopy (POM) and
differential scanning calorimetry (DSC). Transition tempera-
tures and enthalpy values are given in Table 1.
The achiral complex M-X12E12 and the chiral complexes
M-X(S)10*E12, M-X(R)10*E12, and M-X12E(S)10* displayed
a rectangular columnar mesomorphic order (Colr). The X-ray
patterns of complex M-XE, whose mesophase was proposed to
be rectangular columnar (Colr) by POM, are shown in the ESI†.
All of the complexes showed liquid crystalline behavior over
broad temperature ranges, which is indicative of the strength of
hydrogen bonding between acid and melamine, in agreement
with previous results.8,9 The textures observed by polarizing
5918 | J. Mater. Chem., 2011, 21, 5916–5922
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Table 1 Thermal properties and lattice parameters of the complexes M-XEa
ꢁ
Parameters lattice/A
Complexes
Phase
Temp./ꢁC
111.6
100.3
100.9
78.0
DH/kJ molꢀ1
16.7
Phasec
Colr
Temp.b/ꢁC
Phase
ꢁ
M-X12E12
a ¼ 81.6 A
I
I
I
I
I
70.0
60.8
63.1
64.4
73.4
g
g
g
g
g
ꢁ
b ¼ 77.0 A
ꢁ
M-X(S)10*E12
M-X(R)10*E12
M-X(S)10*E(S)10*
M-X12E(S)10*
a ¼ 77.6 A
14.2
Colr
ꢁ
b ¼ 79.2 A
ꢁ
a ¼ 77.0 A
13.2
Colr
ꢁ
b ¼ 77.8 A
ꢁ
a ¼ 48.1 A
6.2
Colh
Colr
ꢁ
h ¼ 3.4 A
ꢁ
a ¼ 77.0 A
97.1
11.6
ꢁ
b ¼ 77.8 A
All X-ray diffraction experiments were carried out at room temperature. b Transition temperatures were determined by DSC on cooling at 10 ꢁC minꢀ1
.
a
c
Crystallization was not observed until ꢀ20 ꢁC. I ¼ isotropic liquid, Colh ¼ hexagonal columnar mesophase, and Colr ¼ rectangular columnar
mesophase. g ¼ glassy state that maintains the mesophase organization.
Z value for the complex M-X(S)10*E(S)10*, is 1. This value is
reasonable compared to those generally observed for hexagonal
mesomorphism.19 The wide- and small-angle X-ray scattering
(SAXS) patterns of the Colh mesophase for this complex are
shown in Fig. 3. A value Z ¼ 4 is deduced for the Colr mesophase
exhibited by the remaining complexes. This is reasonable
considering the large values obtained for the rectangular lattice
constants compared to the hexagonal lattice constant and it is
also consistent with our previous results found for similar
systems.8a
Fig. 2 A polarized optical microphotograph of M-X12E12, texture at 25 ꢁC.
Circular dichroism
The lattice parameters a and b of the rectangular arrangement
were calculated from the diffraction maxima observed in the
SAXS pattern.
The possibility of forming chiral helical architectures in
columnar liquid crystals has already been mentioned.7 Taking
into account the propeller-like conformation proposed for this
type of complex,8a we carried out circular dichroism (CD)
measurements in the mesophase for the chiral complexes
M-X(S)10*E12, M-X(R)10*E12, M-X(S)10*E(S)10* and
M-X12E(S)10* (Fig. 4). The objective was to determine whether
the chirality of the chiral tails is transmitted to the supramolec-
ular organization of the mesophase and whether this can be
related with a helical disposition of the complexes along the
column.
In general, the parameters of the rectangular mesophase
depend on the size of the tails in the V-shaped acid but, as the
alkyl chains used are similar for all the complexes M-XE, large
differences were not observed between these parameters. Thus,
ꢁ
M-X12E12 showed the biggest a and b parameters (a ¼ 81.6 A;
ꢁ
77.0 A), while the complexes M-X(S)10*E12,
b
¼
M-X(R)10*E12, and M-X12E(S)10* presented parameters
ꢁ
around 77 A for a and b.
In contrast, at 25 ꢁC the complex M-X(S)10*E(S)10*, with six
chiral chains, gave X-ray patterns that are unambiguously
characteristic of hexagonal columnar mesomorphic order (Colh).
This is revealed by the presence of a set of four low-angle sharp
maxima with a reciprocal spacing ratio 1 : O3 : O4 : O7. These
four maxima can be assigned to the (100), (110), (200) and (210)
reflections of
a two-dimensional hexagonal lattice. This
arrangement is probably the result of the presence of six
branched chains in the final complex, which reduces the inter-
action between the aromatic cores and leads to the formation of
8
ꢁ
a hexagonal phase. A lattice parameter a ¼ 48.1 A was calcu-
lated from the diffraction pattern. Furthermore, two diffuse
maxima in the wide-angle X-ray scattering (WAXS) region were
observed. The inner maximum corresponds to the aliphatic
chains and this is usually observed in liquid-crystalline materials.
The outer maximum is related to the average stacking parameter
ꢁ
(h), which was deduced to be 3.4 A. With the values of the
stacking parameter (h) it was possible to determine the number of
complexes per unit cell (Z). Considering a density close to
1 g cmꢀ3, the average value for these tetrameric complexes, the
Fig. 3 X-Ray diffraction pattern of complex M-X(S)10*E(S)10*. (a)
WAXS diagram and (b) SAXS diagram.
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films, at room temperature. The UV-vis absorption and fluo-
rescence spectroscopy data for solutions and thin films are
summarized in Table 2.
As shown in Fig. 5, all materials showed similar absorption
patterns with intense absorption bands with maxima at 264 nm
and 318 nm.
The absorption band at 318 nm is assigned to the p–p*
transition and this is characteristic of the 1,3,4-oxadiazole
heterocycle due to the high molar absorption coefficients
measured (3 z 10 000 molꢀ1 cmꢀ1). All of these compounds
displayed strong blue emission in solution (lFL 370–540 nm),
with intense emission maxima at 423 nm and 429 nm. It is
important to note that the absorption and emission spectra in
solution are not significantly different to those of the
carboxylic acid derivatives alone or the supramolecular
complexes M-XE. This observation indicates that the inter-
molecular interaction between the carboxylic acid and the
melamine molecules does not affect the electronic structure of
the chromophores. The complexes showed good photo-
luminescence quantum yields (fFL ¼ 0.65–0.70) in dichloro-
methane solution in comparison to the standard quinine
sulfate. There were small changes in the absorbance and
fluorescence spectra of the films of complexes M-XE compared
with the solution spectra.
Fig. 4 CD spectra recorded on thin films (25 ꢁC) of complexes (a) M-
X(S)10*E12 (solid line), M-X(R)10*E12 (dashed line) and respective (b)
absorbance spectra, (c) M-X(S)10*E(S)10* and (d) M-X12E(S)10*. For
the absorbance spectra of (c) and (d) see ESI†.
The CD measurements were performed on films prepared on
untreated quartz slides, heated until the isotropic phase and
slowly cooled to room temperature. All of the spectra are aver-
ages of several CD spectra recorded at different film positions
relative to the light beam. Indeed, the origin of these CD peaks in
the mesophase of all M-XE chiral complexes was authenticated
by recording the spectra at six different orientations obtained by
rotating (in-plane) the sample cells by 60ꢁ. The spectra obtained
were almost identical for all sample orientations (see ESI†). This
rules out the possible linear dichroism effects due to macroscopic
orientation.20 The complex M-X12E12 was CD silent because of
the absence of chirality in its chemical structure.
Optical absorption and emission were also measured in thin
films of all these complexes. Good quality films were obtained
by casting the corresponding dichloromethane solution onto
a quartz plate (Fig. 6). There were no significant changes in
the fluorescence and absorption spectra of films of the
complexes M-XE compared with the solution. Spectra were
recorded on as-prepared cast films and films heated and
slowly cooled down to room temperature. The absorption and
fluorescence spectra of the films are shown in Fig. 6 (spectra
normalized for ease of comparison). The two types of
absorption spectra were similar. In contrast, a red-shift of the
emission maximum was observed on heating–cooling the
sample (Table 2). The origin of the band shift observed in the
excitation and emission spectra of the films is known to
originate from the cooperative effects of energy transfer
existing in the solid state.22 We consider here that there is
greater organization in the mesophase (after slow heating–
cooling) than in the as-prepared film.
All the CD spectra show signals that are accounted for by
a helical disposition of at least two chromophores21 in the mes-
ophase, and this is consistent with the proposed helical stacking.
Analysis of the CD spectra measured for all the chiral complexes
showed that they differ in shape, and this can be related to the
presence of two types of chromophores, i.e. 2,5-diphenyl-1,3,4-
oxadiazole and phenyl benzoate (lmax 323 and 274 nm, respec-
tively), randomly stacked along the column and with the
different positions of the chiral tails. The optical activity of all
complexes was maintained with time.
The results are in agreement with preliminary X-ray observa-
tions, which showed that the samples did not show significant
diffraction maxima unless they were heated in the capillary to the
clearing point and then allowed to cool down to room temper-
ature. These observations could be related with the degree of
order achieved in the glassy state depending on the cooling rate.
The slowly cooled sample has a higher chance of developing
a more organized mesophase, which on further cooling freezes
below the Tg.
The relationship between the appearance of CD bands and the
formation of chiral superstructures, the sign of which is
addressed by the configuration of the stereogenic centers in the
tails, is clearly confirmed by the CD spectra of the pair of
enantiomers M-X(R)10*E12 and M-X(S)10*E12. Both spectra
show opposite signs and this arises from an inversion of the
configuration of the chiral center from the M-X(S)10*E12
complex to M-X(R)10*E12 (Fig. 4a). In addition, both spectra
present a profile that can be interpreted as two overlapped
exciton coupling signals corresponding to the absorption bands
of both types of chromophores (as mentioned above).
In any case, the results demonstrate that this class of
complex M-XE is capable of maintaining the intrinsic
fluorescence from the 1,3,4-oxadiazole in solution to the
glassy solid. The optical band gaps (Eg) of these compounds
were determined by their corresponding absorption in thin
Photophysical properties
films, using
a
method reported in the literature.23 The
The photophysical properties of the complexes M-XE were
optical band gap for these compounds (Eg) is around
investigated in dilute dichloromethane solutions and in thin
3.15 eV.
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Table 2 Optical properties of the complexes M-XE
Abs. lmax/nm
Fl. lmax/nm
c
Compound
Sol.a
Filmb
Sol.a
Filmb
fFL
Egd/eV
M-X12E12
318
318
318
318
318
314
314
314
314
314
423
423
429
429
423
421
421
421
421
421
0.67
0.67
0.65
0.70
0.67
3.15
3.15
3.15
3.15
3.15
M-X(S)10*E12
M-X(R)10*E12
M-X(S)10*E(S)10*
M-X12E(S)10*
a
b
c
CH2Cl2 solution (10ꢀ5 mol Lꢀ1). Measurements in mesophase. Fluorescence quantum yield relative to quinine sulfate (fFL ¼ 0.546) in CH2Cl2.
d
Optical band gap determined from absorption spectra of the films.
complexes through hydrogen bonding with three acid molecules
around the 2,4-diamino-6-dodecylamino-1,3,5-triazine (M). The
formation of the complexes was characterized by IR, NMR and
DOSY. None of the V-shaped acids presented mesomorphism;
on the other hand, all of the complexes M-XE exhibited liquid
crystalline profiles. The thermal properties were studied by
POM, DSC and the mesophases were characterized by wide- and
small-angle X-ray scattering. These materials showed a tendency
to rectangular columnar mesomorphism, but a hexagonal
columnar phase was also obtained in one case. All of the meso-
phases showed freezing below the glass transition and crystalli-
zation did not occur. The complexes M-X(S)10*E12,
M-X(R)10*E12, M-X(S)10*E(S)10* and M-X12E(S)10*—
prepared from chiral acids derived from citronellyl—transfer the
chirality to the supramolecular structures, as evidenced by the
appearance of CD signals, which is consistent with the formation
of helical architectures for these materials. Furthermore, all of
Fig. 5 Absorbance (dashed line) and emission (solid line) normalized
spectra of the complexes M-XE in solution in CH2Cl2.
the complexes display strong photoluminescence in solution,
solid phase and mesophase with good quantum yields. Thus, the
complexes were characterized as highly organized, strongly
luminescent systems with potential interest for electrooptical
applications.
Acknowledgements
We thank the following institutions for financial support: CNPq,
FAPESC, and INCT-Catalise, MICINN projects MAT2009-
14636-C03-01 and CTQ2009-09030, FEDER funding and
ꢀ
Gobierno de Aragon.
Notes and references
1 G. M. Bennett and B. Jones, J. Chem. Soc., 1939, 420–425.
2 (a) T. Kato, T. Yasuda, Y. Kamikawa and M. Yoshio, Chem.
Commun., 2009, 729–739; (b) S. Laschat, A. Baro, N. Steinke,
€
F. Giesselmann, C. Hagele, G. Scalia, R. Judele, E. Kapatsina,
S. Sauer, A. Schreivogel and M. Tosoni, Angew. Chem., Int. Ed.,
2007, 46, 4832–4887.
3 (a) G. N. Ramachandran and G. Karkha, Nature, 1954, 174, 269–270;
(b) J. D. Watson and F. Crick, Nature, 1953, 171, 737–738; (c)
A. Klug, Angew. Chem., Int. Ed., 1983, 22, 565–582.
Fig. 6 Normalized absorbance (solid line) and emission (dashed line)
spectra of the complex M-X12E12 in the as-prepared film (black) and in
the film heated and slowly cooled down to rt (red).
4 (a) U. Beginn, Prog. Polym. Sci., 2003, 28, 1049–1105; (b) S. Jin,
Y. Ma, S. C. Zimmerman and S. D. Zheng, Chem. Mater., 2004,
16, 2975–2977; (c) A. R. A. Palmans, J. A. J. M. Vekemans,
R. A. Hikmet, H. Fischer and E. W. Meijer, Adv. Mater., 1998, 10,
873–876; (d) I. Paraschiv, K. De Lange, M. Giesbers, B. Van
Lagen, F. C. Grozema, R. D. Abellon, L. D. A. Siebbeles,
E. J. R. Sudholter, H. Zuilhof and A. T. M. Marcelis, J. Mater.
Chem., 2008, 18, 5475–5481.
Conclusions
In summary, fluorescent V-shaped acid derivatives of heterocy-
clic 1,3,4-oxadiazoles were synthesized with different alkyl
chains. These acids XE were used to form supramolecular
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5 (a) M. O’Neill and S. M. Kelly, Adv. Mater., 2003, 15, 1135–1146; (b)
J. W. Goodby, V. Gortz, S. J. Cowling, G. Mackenzie, P. Martin,
D. Plusquellec, T. Benvegnu, P. Boullanger, D. Lafont, Y. Queneau,
S. Chambert and J. Fitremann, Chem. Soc. Rev., 2007, 36, 1971–
2032; (c) S. Kumar, Chem. Soc. Rev., 2006, 35, 83–109; (d)
R. Cristiano, H. Gallardo, A. J. Bortoluzzi, I. H. Bechtold,
C. E. M. Campos and R. L. Longo, Chem. Commun., 2008, 5134–5136.
6 (a) S. Sergeyev, W. Pisula and Y. H. Geerts, Chem. Soc. Rev., 2007,
36, 1902–1929; (b) Y. Shirota and H. Kageyama, Chem. Rev., 2007,
107, 953–1010; (c) M. Sawamura, K. Kawai, Y. Matusuo, K. Kanie
and T. Kato, Nature, 2002, 419, 702–705; (d) I. Seguy, P. Jolinat,
P. Destruel and R. Mamy, J. Appl. Phys., 2001, 89, 5442–5448; (e)
A. M. Van de Craats, N. Stutzmann, M. M. Nielsen and
M. Watson, Adv. Mater., 2003, 15, 495–499; (f) S. Laschat,
I. K. Spiliopoulos, T. S. Kasimis, A. P. Kulkarni and
S. A. Jenekhe, Macromolecules, 2003, 36, 9295–9302; (c)
C. S. Wang, G. Y. Jung, Y. L. Hua, C. Pearson, M. R. Bryce,
M. C. Petty, A. S. Batsanov, A. E. Goeta and J. A. K. Howard,
Chem. Mater., 2001, 13, 1167–1173.
14 Y. Cao, X. Chai, S. Chen, Y. Jiang, W. Yang, R. Lu, Y. Ren,
M. Blanchard-Desce, T. Li and J. Lehn, Synth. Met., 1995, 71,
1733–1734.
15 (a) E. Meyer, C. Zucco and H. Gallardo, J. Mater. Chem., 1998, 8,
1351–1354; (b) H. Gallardo, R. Magnago and A. J. Bortoluzzi, Liq.
Cryst., 2001, 28, 1343–1352.
16 (a) R. Cristiano, A. A. Vieira, F. Ely and H. Gallardo, Liq. Cryst.,
2006, 33, 381–390; (b) R. M. Srivastava, R. A. W. Neves,
R. Schneider, A. A. Vieira and H. Gallardo, Liq. Cryst., 2008, 35,
737–742; (c) H. Gallardo, R. Cristiano, A. A. Vieira,
R. A. W. Neves and R. M. Srivastava, Synthesis, 2008, 605–609; (d)
R. Cristiano, D. M. P. D. Santos, G. Conte and H. Gallardo,
Liq.Cryst., 2006, 33, 997–1003.
€
A. Baro, N. Steinke, F. Giesselmann, C. Hagele, G. Scalia,
R. Judele, E. Kapatsina, S. Sauer, A. Schreivogel and M. Tosoni,
Angew. Chem., Int. Ed., 2007, 46, 4832–4887.
7 F. Vera, J. L. Serrano and T. Sierra, Chem. Soc. Rev., 2009, 38, 781–
796.
17 See ESI† for the preparation of the monophenol 7.
ꢀ
8 (a) J. Barbera, L. Puig, P. Romero, J. L. Serrano and T. Sierra, J. Am.
Chem. Soc., 2006, 128, 4487–4492; (b) J. Barbera, L. Puig, P. Romero,
18 (a) Y. Cohen, L. Avram and L. Frish, Angew. Chem., Int. Ed., 2005,
44, 520–554; (b) C. S. Johnson, Jr, Prog. Nucl. Magn. Reson.
Spectrosc., 1999, 34, 203–256; (c) E. J. Cabrita and S. Berger,
Magn. Reson. Chem., 2001, 39, S142–S148.
ꢀ
J. L. Serrano and T. Sierra, Chem. Mater., 2005, 17, 3763–3771; (c)
ꢀ
ꢀ
E. Beltran, E. Cavero, J. Barbera, J. L. Serrano, A. Elduque and
R. Gimenez, Chem.–Eur. J., 2009, 15, 9017–9023.
ꢀ
ꢀ
19 (a) J. Barbera, L. Puig, J. L. Serrano and T. Sierra, Chem. Mater.,
ꢀ
ꢀ
ꢀ
9 (a) F. Vera, R. M. Tejedor, P. Romero, J. Barbera, M. B. Ros,
J. L. Serrano and T. Sierra, Angew. Chem., Int. Ed., 2007, 46, 1873–
2004, 16, 3308–3317; (b) L. Alvarez, J. Barbera, L. Puig,
P. Romero, J. L. Serrano and T. Sierra, J. Mater. Chem., 2006, 16,
3768–3773.
ꢀ
1877; (b) F. Vera, P. Romero, J. Barbera, M. B. Ros, J. L. Serrano
and T. Sierra, Angew. Chem., Int. Ed., 2010, 49, 1–6; (c)
R. M. Tejedor, L. Oriol, J. L. Serrano and T. Sierra, J. Mater.
Chem., 2008, 18, 2899–2908.
20 G. Gottarelli, S. Lena, S. Masiero, S. Pieraccini and G. P. Spada,
Chirality, 2008, 20, 471–485.
21 A. Painelli, F. Terenziani, L. Angiolini, T. Benelli and L. Giorgini,
Chem.–Eur. J., 2005, 11, 6053–6063.
22 (a) J. R. Lackowicz, Principles of Fluorescence Spectroscopy, Kluwer
Academic Publishers, New York, 2nd edn, 1999; (b) N. F. Marcelo,
A. A. Vieira, R. Cristiano, H. Gallardo and I. H. Bechtold, Synth.
Met., 2009, 159, 675–680.
23 (a) A. Joshi, M. O. Manasreh, E. A. Davis and B. D. Weaver, Appl.
Phys. Lett., 2006, 89, 111907–111910; (b) R. Cristiano, E. Westphal,
I. H. Bechtold, A. J. Bortoluzzi and H. Gallardo, Tetrahedron,
2007, 63, 2851–2858.
10 S. Chandrasekar, Handbook of Liquid Crystals, Wiley-VCH,
Weinheim, 1998, vol. 2B.
11 (a) D. Pucci, G. Barberio, A. Bellusci, A. Crispini, B. Donnio,
L. Giorgini, M. Ghedini, M. La Deda and E. I. Szerb, Chem.–Eur. J.,
2006, 12, 6738; (b) K. Binnemans, J. Mater. Chem., 2009, 19, 448–453.
12 (a) D. B. Amabilino and J. Veciana, Top. Curr. Chem., 2006, 265, 253–
302; (b) M. Grell and D. C. Bradley, Adv. Mater., 1999, 11, 895–905.
13 (a) J. Bettenhausen, M. Greczmiel, M. Jandke and P. Strohriegl,
Synth. Met., 1997, 91, 223–228; (b) J. A. Mikroyannidis,
5922 | J. Mater. Chem., 2011, 21, 5916–5922
This journal is ª The Royal Society of Chemistry 2011