Supramolecular columnar liquid crystals formed by hydrogen bonding between a clicked star-shaped s-triazine and benzoic acids

Article abstract of DOI:10.1002/chem.201500477

A star-shaped tris(triazolyl)triazine is shown to establish hydrogen-bond interactions with polycatenar benzoic acids. The formation of hydrogen-bonded triazine/acid complexes has been demonstrated both in solution and in bulk by different techniques. The

Full text of DOI:10.1002/chem.201500477

DOI: 10.1002/chem.201500477
Full Paper
&
Liquid Crystals
Supramolecular Columnar Liquid Crystals Formed by Hydrogen
Bonding between a Clicked Star-Shaped s-Triazine and Benzoic
Acids
Beatriz Feringµn,[a] Pilar Romero,[a] JosØ Luis Serrano,[b] Raquel GimØnez,*[a] and
Teresa Sierra*[a]
Abstract: A star-shaped tris(triazolyl)triazine is shown to es-
tablish hydrogen-bond interactions with polycatenar benzoic
acids. The formation of hydrogen-bonded triazine/acid com-
plexes has been demonstrated both in solution and in bulk
by different techniques. The complexes, mainly formed by
nonmesogenic components, all show enantiotropic hexago-
nal columnar mesomorphism, which relies on the formation
of hydrogen-bond complexes in a triazine/acid ratio of 1:3.
This approach combines the straightforward synthesis of
a nonmesomorphic triazine core by click chemistry, and the
preparation of a supramolecular complex, providing a much
more convenient route than covalent synthesis to modify
the periphery of triazine discotics and thus to modulate
their functionality.
Introduction
ic building blocks that are able to interact through hydrogen
bonding are necessary. In this context, we studied the hydro-
gen-bonding ability of a novel p-functional core bearing
triazine and triazole rings. The results of our investigation are
reported herein.
Since the earliest reported examples,[1] supramolecular liquid
crystals, and, in particular, those showing columnar arrange-
ments, have been considered to be useful for the development
of strategies that increase both structural complexity and mo-
lecular order in functional materials.[2] These systems can form
the same liquid-crystal phases seen in covalent liquid crystals
but, generally, they are much easier to prepare because the
smaller building blocks self-assemble spontaneously to give
the columnar arrangements. In addition, these systems are
dynamic, which allows a rapid response to external stimuli.
One approach to obtain supramolecular columnar liquid
crystals is by hydrogen-bonding of a heterocyclic core that
does not show the columnar phase with complementary pe-
ripheral units, such as some readily accessible promesogenic
carboxylic acids.[3] There are only a few examples of systems
that have been produced by following this method,[4] although
it is an interesting and useful strategy to access novel function-
al columnar nanostructures when a p-conjugated core is
used.[5] Therefore, to obtain novel supramolecular columnar
liquid crystals by interaction with carboxylic acids, novel discot-
The 1,3,5-triazine ring (s-triazine) is a star-shaped p-function-
al unit[6] that has been used to obtain columnar liquid crys-
tals.[7] In addition, triazine-based cores, such as triarylaminotria-
zine[8] or N-alkylmelamine derivatives,[9] can also form hydrogen
bonds with carboxylic acids, providing columnar mesomorphic
behavior. Additionally, the 1,2,3-triazole ring, which is easily ac-
cessible through click chemistry,[10] is very attractive for func-
tional materials,[11] and also for considering supramolecular in-
teractions.[12] The formation of hydrogen bonds between 1,2,3-
triazole and anions is well-known,[13] but this ring can also
form hydrogen bonds with carboxylic acids.[4d,e,14] For example,
hydrogen-bonded complexes consisting of a C3-symmetric
tris(triazolyl)benzene unit with benzoic acids in a 1:3 stoichi-
ometry, has been shown to exhibit columnar mesomor-
phism.[4d] Moreover, click chemistry allows the appropriate
modification of the periphery of the molecules to obtain dis-
cotic liquid crystals.[15] Indeed, the versatile 1,3,5-triazine scaf-
fold can be triply functionalized with triazole rings by click
chemistry to give the star-shaped 2,4,6-tris(triazolyl)-1,3,5-
triazine.[16] This core can be readily prepared and gives rise
to derivatives with columnar mesomorphism.[16,17]
[a] B. Feringµn, Dr. P. Romero, Dr. R. GimØnez, Dr. T. Sierra
Departamento de Química Orgµnica
Instituto de Ciencia de Materiales de Aragón
Universidad de Zaragoza-CSIC, 50009 Zaragoza (Spain)
E-mail: rgimenez@unizar.es
In the present work, we go a step further and explore the
ability of the core described above to form hydrogen bonds
because of the presence of triazine and triazole rings. In partic-
ular, we study the formation of supramolecular hydrogen-
bonded complexes between a tris(triazolyl)triazine derivative
and several benzoic acids to obtain columnar liquid crystals
that could be, in this way, easily modulated in their periphery.
tsierra@unizar.es
[b] Prof. J. L. Serrano
Departamento de Química Orgµnica
Instituto de Nanociencia de Aragón
Universidad de Zaragoza, 50018, Zaragoza (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500477.
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The tris(triazolyl)triazine derivative used as a core (T3C4) had
a p-butoxyphenyl substituent in each triazole ring (Scheme 1)
and was not liquid crystalline. The butoxy terminal chains aid
solubility in organic solvents, which is important for the
formation of the supramolecular complexes by dissolution of
a mixture of the components.
Figure 1. Molecular structure of the target supramolecular complexes.
Results and Discussion
Synthesis
The
2,4,6-tris[1’-(4’’-butoxyphenyl)-1’,2’,3’-triazol-4’-yl]-1,3,5-
triazine core (T3C4) was synthesized by a copper-catalyzed
alkyne-azide cycloaddition (CuAAC) “click reaction”. Given that
the required alkyne 2,4,6-tris(ethynyl)-1,3,5-triazine is unstable,
a previously described one-pot procedure,[16] which uses the
protected precursor 2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-tria-
zine,[18] was used. In this one-pot reaction, the alkyne groups
of 2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-triazine were depro-
tected and allowed to react in situ with the aromatic azide in
the presence of catalytic copper(I) at room temperature
(Scheme 1). The reaction gave the desired product in a moder-
ate yield (33%). The required aromatic azide, 1-azido-4-butoxy-
benzene, was synthesized by diazotization of 4-aminophenol
followed by reaction with sodium azide[16] (Scheme 1). The
acids A1–A3 were synthesized by using known procedures.[8c]
The hydrogen-bonded complexes were prepared by dissolv-
ing a mixture of the tris(triazolyl)triazine core and the corre-
sponding benzoic acid derivative in a 1:3 proportion, respec-
tively, in dichloromethane. After evaporation of the solvent by
continuous stirring at room temperature, the obtained mix-
tures were subjected to a thermal treatment consisting of
heating to the isotropic liquid and then cooling to room
temperature. The thermally treated mixtures were observed
by polarizing optical microscopy (POM), and they appeared
as homogeneous materials with liquid crystalline behavior,
which suggested that the complexes had been formed.
Scheme 1. Synthetic procedure for the preparation of the tris(triazolyl)-
triazine derivative T3C4.
To study the formation of the supramolecular complexes
and the induction of columnar mesomorphism, three 3,4,5-tri-
substituted benzoic acids were used. Two of these bear three
alkoxy chains of ten carbon atoms (A1) or twelve carbon
atoms (A2), which allows the influence of the length of the
alkoxy chains on the stability of the complex to be compared.
The third acid is a benzoic acid with three alkoxy terminal
chains of twelve carbon atoms in a dendron-like aromatic core
(A3). By using the T3C4 core and these carboxylic acids, mix-
tures of 1:3 stoichiometry were prepared. The structures of the
target supramolecular complexes are shown in Figure 1. In ad-
dition to the hydrogen bond between the hydroxyl group of
the benzoic acids and the nitrogen atoms in the triazine and
triazole rings, we observed the formation of another hydrogen
bond between the carbonyl group of the acid and the hydro-
gen atom situated at the C4 position of the 1,2,3-triazole ring,
which is allowed by the electropositive character of this
triazole hydrogen.[4d]
Formation of hydrogen-bonded complexes in solution
To study the interaction between the T3C4 and the different
1
benzoic acids in solution when mixed in a 1:3 ratio, H NMR ex-
periments in [D2]dichloromethane were performed. Assuming
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that there is fast exchange between the complex and its com-
ponents in solution, strong evidence of hydrogen-bonding
interactions between the T3C4 core and acid molecules was
found in the solution 1H and 13C NMR spectra of the
complexes.
The 1H NMR spectra of the T3C4 core, the acid A1, and a mix-
ture of both components in a 1:3 ratio are compiled in
Figure 2. It can be observed that the signal corresponding to
Figure 3. 1H NMR spectra of the core T3C4 in [D2]dichloromethane at
different concentrations (0.1, 0.025, and 0.002m).
The 1H NMR spectra of the mixture T3C4 and A1 in a 1:3
ratio at different concentrations in [D2]dichloromethane were
also recorded (Figure 4). On increasing the concentration, all
the signals corresponding to T3C4 shifted upfield, which is
consistent with p-stacking aggregation of the complex, as ob-
served for the core itself.
Figure 2. 1H NMR spectra of the core T3C4 (0.1m), the acid A1 (0.3m), and
their mixture in [D2]dichloromethane, maintaining these concentrations.
Figure 4. 1H NMR spectra of the mixture T3C4 and A1 in a 1:3 ratio in
[D2]dichloromethane at different concentrations (0.1 and 0.002m, referred to
T3C4).
the hydrogen of triazole (Hc) undergoes a shift from d=
8.90 ppm in pure T3C4 to d=8.98 ppm when the core and the
acid are mixed, which indicates that this proton interacts
through hydrogen bonding with the carbonyl group of the
acid. Simultaneously, the aromatic protons of T3C4 (He and Hf)
show significant shielding. This upfield shift suggests
aggregation of the complex due to p-stacking.
Similar results to those discussed above, were observed for
mixtures of the core T3C4 and acids A2 (Figures S7, S8) or A3
(Figures S12, S13), the spectra for which are included in the
Supporting Information.
To determine the stoichiometry of the complexes in solution,
Job’s plot analysis was applied to 1H NMR experiments in
[D2]dichloromethane solutions. The Job’s plot for the hydrogen
of the triazole group shows a maximum at cA =0,5, which indi-
cates that core–acid association occurs in a 1:1 stoichiometry
in [D2]dichloromethane solution (see the Supporting Informa-
tion, Figure S14). This result is similar to those obtained for
previously reported complexes between melamines and
carboxylic acids, and does not preclude the formation of 1:3
complexes in the mesophase.[9]
To check the possibility of self-aggregation of the core by p-
1
stacking, H NMR spectra of pure T3C4 at different concentra-
tions were recorded (Figure 3). On increasing the concentra-
tion, all the signals, including the triazole proton Hc, are clearly
shifted upfield, which is consistent with p-stacking aggrega-
tion.
By comparing Figure 2 and Figure 3 at the same concentra-
tion of T3C4 core (0.1m), it can be seen that the presence of
the acid in a 1:3 ratio causes further upfield shift of the aro-
matic protons He and Hf, which can indicate that aggregation
by p-stacking is even more favored upon formation of a hydro-
gen-bonded complex. Interestingly, the triazole proton Hc does
not show an upfield shift but displays a downfield shift for the
complex, which confirms that this proton is involved in a
hydrogen-bonding interaction.
Thermal properties and mesomorphic behavior
The thermal properties of the hydrogen-bonded complexes
and their individual components were studied by using polar-
izing optical microscopy (POM) and differential scanning
calorimetry (DSC).
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The tris(triazolyl)triazine T3C4 is not a liquid crystal. It was
obtained as a crystalline solid that melted at 1738C and then,
on cooling, an amorphous solid with a glass transition about
608C was obtained. Acids A1 and A2 (with three alkoxylic
chains, ÀOC10H21 and ÀOC12H25, respectively) do not display
mesomorphic properties.[19] However, acid A3 exhibits hexago-
nal columnar mesomorphism (Cr 65 Colh 144 I), in which the
stacking units consist of H-bonded dimers.[8c,20]
tion processes in the corresponding DSC heating scans (see
the Supporting Information, Figures S15 and S16).
The textures observed by POM are characteristic of hexago-
nal columnar mesophases, although the assignment of the
type of mesophase and the determination of the lattice param-
eters were achieved by X-ray diffraction studies (Table 1). X-ray
experiments were carried out on samples that were cooled
from the isotropic liquid. For complexT3C4-A3, which can
retain the mesophase, the experiments were performed at
room temperature (Figure 6). For complexes T3C4-A1 and T3C4-
A2, the mesophases of which are metastable at room tempera-
ture, the experiments were carried out at higher temperatures
(see the Supporting Information, Figures S18 and S19), where
the mesophases are known to be stable according to the DSC
thermograms.
As mentioned above, all the complexes were obtained as
homogeneous materials with mesomorphic behavior within
broad temperature intervals. Furthermore, mesomorphic prop-
erties were observed from nonmesogenic components in the
case of T3C4-A1 and T3C4-A2, which also provides evidence for
the formation of the hydrogen-bonded complexes. Characteris-
tic textures of columnar mesophases were observed by POM
for all the complexes (Figure 5). The thermal properties (transi-
tion temperatures and enthalpies) of the hydrogen-bonded
complexes are given in Table 1.
The complex T3C4-A3 can retain the mesophase at room
temperature, as deduced from the DSC cooling scan (see the
Supporting Information, Figure S17). However, for T3C4-A1 and
T3C4-A2, the DSC thermograms are not fully clear. For these
complexes, partial crystallization or partial separation of the
components can be deduced from the corresponding DSC
cooling scans. Nevertheless, the observations with POM clearly
show that phase separation does not occur, and homogeneous
materials that uniformly melt to the isotropic liquid are ob-
tained. The mesophases of complexes T3C4-A1 and T3C4-A2
are metastable at room temperature and they crystallize on
heating, as deduced from the appearance of cold crystalliza-
Figure 6. X-ray diffraction patterns of T3C4-A3: a) Room temperature;
b) room temperature, partially aligned sample. The reinforced reflections
(arrows) and the rubbing direction (dashed line) are indicated.
The X-ray patterns of all the three complexes contain
a single diffraction maximum at small angles. In the absence of
other reflections, it is difficult to unambiguously assign the
type of mesophase on the basis of X-ray diffraction data alone.
Nevertheless, similar behavior has been found in other struc-
tures that were previously described as hexagonal columnar
phases, and it is due to a minimum in the form factor, which
precludes the observation of peaks in this small-angle re-
gion.[8c,16,21] The observation of only one diffraction maximum
could also be consistent with lamellar phases, as described for
polycatenar disc-like ionic complexes with peripheral long
tails.[22] Unlike these ionic complexes, the peripheral long tails
in the present complexes are contained in benzoic acid deriva-
tives. These acids bring additional aromatic rings radiating
from the tris(triazolyl)triazine core that contribute to the planar
disk-like core, making it prone to p-stacking. Given the com-
plex structure and considering the textures observed by POM,
we suggest that these complexes show hexagonal columnar
mesomorphism, Colh. All the complexes show a diffuse halo at
4.5 Š in the wide-angle region due to the liquid-like order be-
tween the disordered aliphatic chains. Additionally, an outer
diffuse halo at 3.4 Š is observed for complex T3C4-A3, which is
indicative of a periodic stacking distance along the columns
(parameter c). The value of the parameter c for this complex is
consistent with the stacking distance found in columnar meso-
phases of tris(triazolyl)triazines.[16] Moreover, in a partially ori-
ented diagram, which was measured for a mechanically treated
sample of T3C4-A3 (Figure 6b), this latter reflection is rein-
forced along the alignment direction, whereas the low-angle
reflection is reinforced in the perpendicular direction. This pat-
Figure 5. Microphotographs of the textures observed by POM for a) T3C4-
A1, Colh, 508C (cooling); b) T3C4-A2, Colh, 458C (cooling); and c) T3C4-A3,
Colh, 1228C (cooling).
Table 1. Thermal properties of the hydrogen-bonded complexes and
structural parameters measured by X-ray diffraction of the respective col-
umnar mesophases.
Complex
Thermal properties (T [8C], (DH [kJmolÀ1]))[a]
Lattice para-
meters [Š]
T3C4-A1
T3C4-A2
T3C4-A3
I 71 (2.8) Colh 9 (0.7)[b] Colh +Cr
Colh +Cr 30 (À1.4) Cr 37 (5.6) Colh 85 (2.1) I
I 67 (2.5) Colh 20 (19.1)[b] Colh +Cr
Colh +Cr 30 (À19.3) Cr 55 (47.9) Colh 82 (1.5) I
I 136 (12.1) Colh
a=33.5
a=35.6
a=42.3
c=3.4
Colh 142 (12.4) I
[a] DSC data of the first cooling process and the second heating process
at a rate of 108CminÀ1. Temperatures at the maxima of the peaks are
given. I: isotropic liquid; Colh: hexagonal columnar phase; Cr: crystalline
phase; [b] partial crystallization.
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tern is consistent with a columnar mesophase in which col-
umns are oriented along the rubbing direction.
Assuming that the low-angle maxima correspond to the
(100) reflection of the corresponding hexagonal columnar mes-
ophases, the lattice parameters (a) of the complexes were de-
termined from these maxima. As can be expected, the hexago-
nal lattice constant a increases as the length of the aliphatic
chain of the benzoic acid derivative is increased. Thus, for com-
plex T3C4-A2, with twelve carbon atoms in each chain, the hex-
agonal parameter a=35.6 Š is larger than the value for com-
plex T3C4-A1 (a=33.5 Š), with decyloxy chains. Furthermore,
the hexagonal lattice constant measured for complex T3C4-A3
is significantly larger (a=42.3 Š) because of the extended size
of acid A3.
Figure 7. IR spectra of T3C4, A3, and complex T3C4-A3 (room temperature)
recorded on KBr pellets. On the right, an expansion of the 1850–1400 cmÀ1
region is shown.
In addition, the density of the complexes was calculated on
the basis of the parameters measured by X-ray diffraction. The
relationship between the density (1) of the complexes in the appear, and only the broad band related to hydrogen-bonding
mesophase and the measured lattice parameters is given by interactions is observed. Additionally, bands that correspond to
the equation 1=(M · Z)/(NA · V), where M is the molar mass of the heterocyclic rings shift to slightly higher wavenumber
the 1:3 complex, Z is the number of molecules in the unit cell, (e.g., from 1566 to 1571 cmÀ1) when the complex is formed.
NA is Avogadro’s number, and V is the unit cell volume (V=a”
The 13C cross-polarization magic-angle spinning (13C CPMAS)
p
a
3/2”c”10À24). The density of the complexes (on the as- NMR spectrum could be recorded at room temperature in the
sumption that Z=1) was calculated to be close to 1 gcmÀ3
,
Colh mesophase for complex T3C4-A3, providing strong
which is reasonable for organic compounds, and thus is in evidence for the formation of the complex according to a 1:3
agreement with the formation of discotic 1:3 complexes with stoichiometry. Figure 8 shows the spectrum of this complex
one complex per stacking unit.
compared with those of its corresponding components, acid
A3 and core T3C4.
The spectrum of acid A3 shows two signals corresponding
to the carbon atom of the carbonyl group, at 172.2 ppm and
Formation of the complexes in the mesophase
Further confirmation of the formation of 1:3 complexes in the 166.0 ppm, which correspond to the dimeric form and the free
mesophase was attained from studies performed in the meso- form of A3, respectively. These signals are not visible in the
phase, which were based on infrared spectroscopy (IR), solid- 13C NMR spectrum of the complex but, instead, a new signal
state NMR and fluorescence spectroscopies.
appears overlapped with the carbon atom of the triazine ring.
As described for the X-ray experiments, the IR spectra of the This signal can be interpreted as corresponding to the carbon
complexes were recorded at temperatures at which the meso- atom of the carbonyl group forming hydrogen-bonding inter-
phases are known to be stable on cooling from the isotropic actions with the core. Other signals of both the core and the
states: for complex T3C4-A3, the experiment was carried out at acid are also shifted when the complex is formed in the bulk,
room temperature, and for complexes T3C4-A1 and T3C4-A2, and this can be related to differences in the stacking order of
the spectra were recorded at 608C (see the Supporting the complex with respect to the core or the acid, separately. In
Information, Figures S21 and S22).
general, the signals corresponding to the core are shifted up-
As a representative example that demonstrates the forma- field upon complexation. Triazole carbon atoms b and c, aro-
tion of the hydrogen-bonding associations, the infrared spectra matic carbon atoms e and f, and the carbon atoms linked to
of T3C4, A3, and the corresponding 1:3 complex T3C4-A3 are the oxygen atom in the terminal chains, h, are shifted upfield
very revealing (Figure 7). A significant difference between the upon complexation. With respect to the acids, some of their
C=O stretching bands in the acid and in the complex spectra aromatic signals shift slightly to lower fields upon formation of
can be observed. In the acid spectrum, two C=O stretching the complex. Finally, the signals of the aliphatic chains of both
bands appear at 1667 and 1733 cmÀ1, which correspond to the the acid and the core undergo significant changes upon
dimeric form and the free form of A3, respectively. In the IR complexation, likely due to conformational changes.
spectrum of complex T3C4-A3, however, a single band is ob-
A comparative study of the photoluminescence of both the
served at an intermediate position, at 1709 cmÀ1. Moreover, complexes and their corresponding pure components also re-
significant changes in the band corresponding to the hydroxyl vealed differences in their emission behavior (Table 2). The
group can be observed when the complex is formed. In the fluorescence spectra were recorded on films, which were pre-
acid spectrum, two OÀH stretching bands appear: the typical pared by cooling the compounds from their isotropic liquid
broad band between 3600 and 2400 cmÀ1, due to the dimeric state between quartz plates. The spectra of the individual com-
form of the acid, and an additional band between about 3550 ponents, core and acids, were recorded at room temperature
and 3150 cmÀ1, which corresponds to the free form. In the in their solid phases obtained by cooling the isotropic liquid.
spectrum of complex T3C4-A3, this latter band does not As was performed for X-ray and IR experiments, the fluores-
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Figure 9. Emission spectra of T3C4, A3, and complex T3C4-A3 in thin films at
room temperature.
sion at lower wavelengths, in the UV region (344 or 380 nm).
In contrast, the fluorescence spectra of the complexes show
a band at 405 nm. The disappearance of the emission band of
the acid and the observed redshift support the conclusion that
hydrogen-bonded complexes are formed.[4d] This is interesting
in terms of having an additional technique to monitor the for-
mation and stability of these supramolecular complexes by
simply recording the emission spectra of the film.
Figure 8. 13C CPMAS NMR spectra of T3C4, A3, and their corresponding 1:3
complex, T3C4-A3. The most representative signals that shift upon complex-
ation are indicated. All the spectra were recorded at room temperature.
Conclusion
New supramolecular complexes based on hydrogen-bonding
interactions between a tris(triazolyl)triazine derivative (T3C4)
and three molecules of polycatenar benzoic acids (A1, A2, A3)
have been prepared. When these components are mixed in
[D2]dichloromethane in a 1:3 ratio, respectively, NMR experi-
ments have demonstrated the interaction between the hydro-
gen of the triazole ring of the tris(triazolyl)triazine core and the
carboxylic acids. This interaction leads to the formation of a
hydrogen-bonded complex, with the 1:1 stoichiometry of the
species being stable in solution, which additionally show
significant tendency to self-aggregate by p-stacking.
Table 2. Fluorescent data (excitation and emission wavelengths) of thin
films.
Compound
lexc [nm]
lem [nm]
A1
A2
A3
T3C4
T3C4-A1
T3C4-A2
T3C4-A3
275
275
280
286
290
290
290
344
344
380
423
405
405
405
In the bulk, all three complexes, mainly formed by nonmeso-
genic components, show hexagonal columnar mesomorphism
at different ranges of temperatures, as demonstrated by POM,
DSC, and X-ray diffraction experiments. This behavior is consis-
tent with the formation of T3C4-A1, T3C4-A2, T3C4-A3 com-
plexes with a 1:3 stoichiometry. Further evidence for the for-
mation of the 1:3 complexes has been obtained from the
study of the mesomorphic materials by different techniques
such as infrared spectra, solid-state NMR (13C CPMAS) and
fluorescence spectroscopy. These results suggest a decisive
role of mesogenic driving forces (mainly p-stacking) to help
the formation of the 1:3 complex in the bulk.
cence and excitation spectra of the complexes were recorded
at temperatures at which the mesophases are known to be
stable: for complex T3C4-A3 the experiments were carried out
in the columnar mesophase at room temperature (Figure 9
and S27), and for complexes T3C4-A1 and T3C4-A2 the spectra
were recorded at 608C (see the Supporting Information, Figur-
es S23–S26). The absorption spectrum of complex T3C4-A3
could be measured in the mesophase at room temperature
and was similar to its excitation spectrum (see the Supporting
Information, Figure S28).
Finally, the preparation and study of these complexes pro-
vides a versatile approach to ordered materials that can be
readily prepared and easily modified in their periphery, to
modulate their functionality.
The core T3C4 shows an emission band in the blue region of
the visible spectrum (423 nm), whereas the acids show emis-
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115.9, 68.2, 31.4, 19.4, 14.0 ppm; IR (NaCl): n˜ =2959 (Csp3ÀH), 2109
(N3), 1504 (Ar), 1245 cmÀ1 (CÀO).
Experimental Section
General remarks
2,4,6-Tris[1’-(4’’-butoxyphenyl)-1’,2’,3’-triazol-4’-yl]-1,3,5-triazine
(T3C4): 2,4,6-Tris[(trimethylsilyl)ethynyl]-1,3,5-triazine[18] (1.00 g,
All reagents were purchased from Aldrich and used without further
purification. Anhydrous dichloromethane and THF were purchased
from Scharlab and dried by using a solvent purification system. H
2.7 mmol),
1 (1.61 g, 8.4 mmol), sodium ascorbate (0.16 g,
1
0.8 mmol), and CuSO4·5H2O (0.10 g, 0.4 mmol) were dissolved in
a mixture of THF/H2O (1:1, 35 mL) under an argon atmosphere. The
mixture was stirred at RT for 3 min and then tetrabutylammonium
fluoride (TBAF; 1m in THF, 8.4 mL, 8.4 mmol) was added. The reac-
tion was stirred at RT in the dark for 16 h. Water (20 mL) was then
added and the mixture was extracted with CH2Cl2 (3”60 mL). The
combined organic layers were dried over MgSO4 and the solvent
was evaporated. The residue was purified by column chromatogra-
phy on silica gel, gradually increasing the polarity of the eluent
from CH2Cl2 to CH2Cl2/ethyl acetate 9.5:1 and further recrystalliza-
tion from ethanol, giving the product as a yellowish solid. Yield:
and 13C NMR spectra were acquired with a Bruker AV400 spectrom-
eter. The experiments were performed at RT in deuterated solvents
(CDCl3 or [D2]dichloromethane). Chemical shifts are given in ppm
relative to TMS and the solvent residual peak was used as the in-
ternal standard. Solid-state NMR experiments were performed by
using a double resonance (1H-X) probe with a rotor of 2.5 mm di-
ameter, and the spinning frequency was set to 15 kHz. Data were
acquired at 298 K and chemical shifts are referenced to TMS. The
1H and 13C pulse length were 8 and 5.7 ms, respectively, and the CP
contact time was 1.5 ms. The recycle delay was 5 s. The pulse se-
quence employed consisted of ramped cross-polarization with
spinal-64 decoupling. IR spectra were recorded with a Bruker
Vertex 70 FTIR spectrometer. The samples were prepared on KBr
pellets with a concentration of the product of 1–2% (w/w). A tem-
perature controller was adapted to measure the spectra at differ-
ent temperatures. Mass spectra were obtained with a MICROFLEX
Bruker (MALDI+) spectrometer with a dithranol matrix. Elemental
analyses were performed with a PerkinElmer 240C microanalyzer.
The mesophases were examined by polarizing optical microscopy
with a polarizing optical microscope Olympus BX51 equipped with
an Olympus DP12 digital camera and connected to a Linkam
THMS600 hot-stage and a Linkam TMS94 controller. Transition tem-
peratures and enthalpies were obtained by differential scanning
calorimetry with DSC TA instruments Q-20 and Q-2000 at heating
and cooling rates of 108CminÀ1. The apparatus were previously
calibrated with indium (156.68C, 28.44 JgÀ1). Powder X-ray experi-
ments were performed with a pinhole diffractometer (Anton Paar)
operating with a point focused Ni-filtered Cu-Ka beam. The sam-
ples were held in Lindemann glass capillaries (0.7 and 0.9 mm di-
ameter) and heated with a variable-temperature attachment. The
diffraction patterns were collected on photographic films. Fluores-
cence spectra were recorded with a PerkinElmer LS50B spectropho-
tometer. Films were prepared between two quartz plates by
heating the compounds to their isotropic liquid and cooling to
the experiment temperatures and they were measured by
front-detection.
1
33% (0.66 g); H NMR (400 MHz, [D2]dichloromethane): d=9.03 (s,
3H; H triazole), 7.86–7.76 (m, 6H; ArH), 7.17–7.05 (m, 6H; ArH),
4.06 (t, J=6.5 Hz, 6H; OCH2), 1.86–1.77 (m, 6H; CH2), 1.59–1.48 (m,
6H; CH2), 1.01 ppm (t, J=7.4 Hz, 9H; CH3); 13C NMR (100 MHz,
[D2]dichloromethane): d=167.3, 160.5, 146.4, 130.3, 126.2, 122.7,
115.9, 68.8, 31.8, 19.8, 14.2 ppm; IR (KBr): n˜ =2958 (Csp3ÀH), 2932
(Csp3ÀH), 2872 (Csp3ÀH), 1611, 1566, 1513 (arCÀC, triazole), 1249
(CÀO), 1169 cmÀ1 (CÀO); MS (MALDI+, dithranol): m/z: 749.4
[M+Na]+, 1476.1 [M2+Na]+; elemental analysis calcd (%) for
C39H42N12O3·H2O: C 62.89, H 5.95, N 22.57; found: C 62.76, H 5.99, N
22.23.
Synthesis and characterization of A1, A2, and A3
The acids A1–A3 were synthesized by following procedures
described elsewhere.[8c,19,20]
Preparation of the hydrogen-bonded complexes
Before the preparation of the mixtures, the pure components were
dissolved in CH2Cl2, filtered, and the solvent was evaporated. The
hydrogen-bonded complexes were prepared from a CH2Cl2 solu-
tion of a mixture of T3C4 and the corresponding carboxylic acid in
a 1:3 proportion. The solvent was evaporated by stirring at RT, and
the mixtures were heated to their isotropic states and then cooled
to RT before being used for further experiments.
Acknowledgements
Synthesis and characterization of T3C4
This work was financially supported by the MINECO-FEDER
funds (projects MAT2012-38538-CO3-01, CTQ2012-35692, and
MAT2011-27978-CO2-01), and the Gobierno de Aragón-FSE
(E04 research group). B.F. acknowledges the DGA for the EPIF
grant. The authors would like to acknowledge the use of
“Servicios Científico-TØcnicos” of CEQMA (UZ-CSIC).
1-Azido-4-butoxybenzene (1): 4-Butoxyaniline (2.50 g, 15.1 mmol)
was dissolved in an aqueous mixture consisting of H2O (40 mL),
glacial acetic acid (90 mL), and HCl conc. (8 mL). NaNO2 (1.22 g,
17.7 mmol) dissolved in H2O (10 mL) was added slowly to the mix-
ture at 08C. The mixture was stirred for 5 min and then NaN3
(1.10 g, 16.9 mmol) dissolved in H2O (10 mL) was slowly added.
The reaction was stirred at 08C for 90 min and the mixture was
then extracted with CH2Cl2. The organic layers were combined and
washed with an aqueous solution of NaOH (10%). The organic
layer was dried over MgSO4 and the solvent was evaporated. The
crude oil was dried at 508C under vacuum for 4 h and the product
was purified by column chromatography with silica gel stationary
phase and hexane as eluent to give the product as an orange oil.
Yield: 93% (2.71 g); 1H NMR (400 MHz, CDCl3): d=6.97–6.92 (m,
2H; ArH), 6.91–6.86 (m, 2H; ArH), 3.94 (t, J=6.5 Hz, 2H; OCH2),
1.83–1.70 (m, 2H; CH2), 1.55–1.43 (m, 2H; CH2), 0.98 ppm (t, J=
7.4 Hz, 3H; CH3); 13C NMR (100 MHz, CDCl3): d=156.7, 132.2, 120.1,
Keywords: liquid crystals · luminescence · mesophases ·
noncovalent interactions · self-assembly
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Received: February 5, 2015
Published online on May 11, 2015
Chem. Eur. J. 2015, 21, 8859 – 8866
www.chemeurj.org
8866
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim