"Click chemistry" as a versatile route to synthesize and modulate bent-core liquid crystalline materials

Article abstract of DOI:10.1039/c2jm33612j

Three series of bent-shaped compounds containing the 1,2,3-triazole ring in the central core of the molecule have been prepared by the most extended "click chemistry" reaction, the copper-catalyzed azide-alkyne cycloaddition (CuAAC). This research demonstrates the versatility of this synthetic approach with the aim of achieving innovative compact supramolecular organizations. The appropriate combination of the 1,2,3-triazole synthon linked either to a methylene unit (series M) or to a methylenoxycarbonyl block (series MC) has allowed the induction of a variety of non-classical bent-core liquid crystal phases versus the classic mesophases promoted by 1,4-diphenyl-1,2,3- triazole derivatives (series T). Through a suitable selection of common lateral structures connected by "click chemistry" both the transition temperatures and mesomorphism, ranging from lamellar to columnar or B4-like supramolecular liquid crystalline organizations, can be tuned. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm33612j

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PAPER
‘‘Click chemistry’’ as a versatile route to synthesize and modulate bent-core
liquid crystalline materials†
a
b
c
Nelida Gimeno, Rafael Martın-Rapun, Sofıa Rodrıguez-Conde, Jose Luis Serrano, Cesar L. Folcia,
d
c
ꢀ
Miquel A. Pericas and M. Blanca Ros
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
b
a
*
*
ꢀ
Received 5th June 2012, Accepted 3rd July 2012
DOI: 10.1039/c2jm33612j
Three series of bent-shaped compounds containing the 1,2,3-triazole ring in the central core of the
molecule have been prepared by the most extended ‘‘click chemistry’’ reaction, the copper-catalyzed
azide-alkyne cycloaddition (CuAAC). This research demonstrates the versatility of this synthetic
approach with the aim of achieving innovative compact supramolecular organizations. The
appropriate combination of the 1,2,3-triazole synthon linked either to a methylene unit (series M) or to
a methylenoxycarbonyl block (series MC) has allowed the induction of a variety of non-classical bent-
core liquid crystal phases versus the classic mesophases promoted by 1,4-diphenyl-1,2,3-triazole
derivatives (series T). Through a suitable selection of common lateral structures connected by ‘‘click
chemistry’’ both the transition temperatures and mesomorphism, ranging from lamellar to columnar or
B4-like supramolecular liquid crystalline organizations, can be tuned.
crystals (BCLC). This name comes from the bent shape of the
molecules involved in generating these new phases (Fig. 1).
Through a compact arrangement that restricts their rotational
freedom, these molecules offer types of mesophases and
phenomena different from those observed in traditional liquid
crystals.2 A wide variety of soft lamellar and columnar molecular
packings (Fig. 2), some of which exhibit supramolecular chirality
and other properties, such as ferro- and antiferroelectricity,
nonlinear optical properties,3 piezoelectricity,4 optical biaxiality
or high flexoelectricity,5 has been described for these materials
since the pioneering reports.1,6
Introduction
In 1996, Niori et al.1 reported for the first time on a new type of
mesogenic material generated by bent-shaped molecules. Since
then, research into such materials has focused the interest of
many experts on liquid crystals. Actually, they can be considered
as a new field in mesogenic materials named as: bent-core liquid
The main structural difference for this type of material with
respect to classical calamitic or discotic liquid crystals is the
presence of a noticeable bent angle in the central part of the
molecule. Indeed, the central core is a key factor when generating
bent-core mesomorphism, joined to other structural features
such as long terminal chains (C6–C22) or rigid lateral structures.2
Modifications of the central cores have been mainly concerned
with the rigidity and polarity of this moiety. Among these
structures, typical mesomorphism of bent-shaped compounds
has been observed in molecules where the central core consists of
1,3-disubstituted benzene, 2,6-disubstituted pyridine, 2,7-disub-
stituted naphthalene or 3,40-disubstituted biphenyl systems,
which are by far the most frequently used.2
In these cases, an expected bent-core angle of about 120ꢀ is
generated, which is considered to be the optimal angle to
promote the compact packing affording polar mesophases.
Alternatively, through appropriate molecular changes, the
bending angle has been varied in the range between 105ꢀ and
140ꢀ, also leading to mesomorphism.
Fig. 1 Schematic representation of the molecular structures of a rod-like
mesogen (left), and a bent-shaped mesogen (right) where the bent-core
(blue), the lateral structures (green) and terminal substituent (red) are
differentiated.
aDepartamento de Quꢀımica Orgaꢀnica, Facultad de Ciencias, Instituto de
ꢀ
Ciencia de Materiales de Aragon, Universidad de Zaragoza-CSIC,
50009-Zaragoza, Spain. E-mail: bros@unizar.es
bInstitute of Chemical Research of Catalonia, Avgda. Paisos Catalans, 16,
43007-Tarragona, Spain. E-mail: mapericas@iciq.es
cDepartamento de Fꢀısica de la Materia Condensada, Facultad de Ciencia y
ꢀ
Tecnologıa, UPV/EHU, Apdo. 644, 48080 Bilbao, Spain
dDepartamento de Quꢀımica Orgaꢀnica, Facultad de Ciencias, Instituto de
Nanociencia de Aragoꢀn, Universidad de Zaragoza, 50009-Zaragoza, Spain
† Electronic supplementary information (ESI) available: Experimental
details about the synthesis and structural characterization of the
compounds. Summary of the representative POM textures and
differential scanning
10.1039/c2jm33612j
calorimetry
thermograms.
See
DOI:
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interactions between molecules. The compounds containing five
membered 1,2,3-triazole rings have attracted recent attention
owing to the following advantageous properties: (i) a high
chemical stability (in general, being inert to severe hydrolytic,
oxidizing and reducing conditions, even at high temperature), (ii)
a strong dipole moment (5.2–5.6 D), (iii) an aromatic character
and (iv) a good hydrogen-bond-accepting ability.13 There exist
also in the materials science literature interesting results on
further attractive possibilities for incorporating the 1,2,3-triazole
moiety. Thus, it acts as an electron donor and lowers the band
gap of aromatic systems, likewise it does not hinder the packing
of the conjugated backbones and it can be considered as a good
candidate in organic photovoltaic devices.14
Despite these advantages, CuAAC has only been used
marginally for the synthesis of calamitic15 or discotic16 liquid
crystalline materials and the triazole ring has never succeeded in
BCLCs. With the aim of going deeper into this synthetic
approach and to investigate the supramolecular packing of new
heterocyclic bent-core structures so as to generate compact
BCLCs, we focussed our interest on innovative triazole-based
bent-core structures.
In our methodology the bent-core angle is generated in the last
reaction step through a CuAAC. The synthesis of different
alkyne and azide functionalized lateral structures has allowed the
preparation of a broad number of materials, in most cases
asymmetric in nature, in an easy and straightforward method.
The compounds selected for this study have been divided into
three series (Chart 1); Series T joining structures with a triazole
Fig. 2 Some of the structural models proposed for bent-core liquid
crystal mesophases apart from strictly smectic ones. Colob: columnar
oblique, USmCP: undulated smectic polar. Different periodicities of the
polar and tilt directions are possible. A schematic representation of the
bent-core molecule (P is the polarization direction) is also displayed.
This is the case of using the incorporation of different lateral
groups (such as halogen, methyl, cyano or nitro groups)2 on the
6-membered central aromatic ring as they cause a distortion of
the original angle, or by the introduction of 5-membered ring
heterocycles.7 Likewise, recent interest focused on very acute
central angles, such as for 1,7-dihydroxynaphtalene derivatives
should be also mentioned.8
ring (1,2,3-C2HN3) as central core; Series M gathering
compounds with a triazole-methylene moiety (1,2,3-C2HN3–
CH2–) and finally, Series MC focussed on molecules with the
triazole-methyleneoxycarbonyl unit (1,2,3-C2HN3–CH2–OOC–).
4-Substitued-benzoyloxybenzoates or benzoates have been
selected as lateral structures, due to their well-known tendency to
induce wide bent-core mesophase ranges, low transition
temperatures, as well as a high chemical stability.2 As terminal
chains, a linear n-tetradecyloxy chain has been used in most
cases. In series M, the incorporation of an oxyethylenic chain in
order to decrease the transition temperatures,9d has also been
checked. Finally, in some cases a chlorine atom has been also
added into the lateral structure with the purpose of disturbing the
compact packing of these molecules.
Nevertheless, singular examples where a small and bent linking
unit,9 such as –CH2–, –CO–, –S–, –O– or –CH2OOC– or flexible
odd numbered bent acyclic10 or cyclic11 aliphatic spacers have
been also checked to promote this kind of supramolecular
packing. Besides its main role in mesomorphism induction, the
central core is also a useful tool to introduce asymmetry in the
whole bent-core structure which has attracted a lot of attention
in recent years due to the excellent and varied properties2,12 even
if in most cases the synthesis is not straightforward.
Experimental
In this approach, a good opportunity to advance bent-core
designs would be opened if convergent synthetic strategies
through a restricted number of building blocks could be devel-
oped. Then a variety of fragments could be diversely combined
with a final orthogonal reaction, if possible high yielding and
with easy work up. ‘‘Click chemistry’’ is a concept typically used
to describe this set of characteristics. The most representative
‘‘click chemistry’’ reaction, the copper catalyzed azide-alkyne
cycloaddition (CuAAC) has been widely applied in fields in
which the easy and fast access to a great diversity of structures is
desirable, such as in pharmaceutical or materials targets.
The presence of a heterocyclic system such as 1,2,3-triazole
leads to an increase in molecular dipole and dielectric anisotropy,
thereby producing molecular level polar organisations by
generating electrostatic and molecular shape dependent
Synthesis
The general synthetic procedure to prepare all the bent-shaped
compounds is outlined in Schemes 1 and 2. All the final
compounds were prepared by the copper-catalyzed azide-alkyne
cycloaddition using catalyst I in a THF/DMF (1/1) mixture
under microwave conditions. The strong binding nature of the
tris-triazolyl ligand in I and its high activity, i.e. low catalyst
loading, precludes any copper contamination in the product.17
Azido compound 1 was synthesized from 4-aminophenol by
formation of the diazonium salt and subsequent substitution
with sodium azide. The Appel reaction on 4-(hydroxymethyl)
phenol and in situ nucleophilic substitution with sodium azide
yielded 4-(azidomethyl)phenol 2. 4-Ethynylphenol
3 was
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Chart 1 Molecular structure and notation of the compounds studied
joining three different series: series T, series M and series MC.
synthesized through the Corey–Fuchs reaction from 4-
hydroxybenzaldehyde, whereas the nucleophilic substitution of
propargyl bromide with sodium 4-hydroxybenzoate resulted in
propargyl 4-hydroxybenzoate 4. Azido- and alkyne-functional-
ized 9–18 as lateral structures could be easily obtained from the
phenols 1–4 by esterification with benzoic acids 5–8 using N,N0-
dicyclohexylcarbodiimide and N,N-dimethylaminopyridine in
dichloromethane.
Results and discussion
1. Liquid crystalline properties of compounds
The mesomorphic properties of the compounds were investigated
by polarized light optical microscopy (POM) on a hot stage,
differential scanning calorimetry (DSC) and X-ray diffraction
(XRD) studies at variable temperature. The transition temper-
atures and corresponding enthalpy values of the products are
summarized in Tables 1 and 2.
Scheme
1
Synthetic routes followed to prepare the intermediate
compounds of Series T, Series M and Series MC. (a) (1) NaNO2/HCl,
ꢀC; (2) NaN3, 0 ꢀC. (b) CCl4, PPh3, NaN3, DMF. (c) CBr4, PPh3,
0
CH2Cl2. (d) (1) BuLi, THF, ꢁ78 ꢀC; (2) H2O, ꢁ78 ꢀC. (e) (1) NaHCO3,
Intermediate compounds. The thermal properties of all the
azide and alkyne derivates used to build up the final bent-core
compounds were also studied. Interestingly, eight out of ten
intermediate compounds form a mesophase.
DMF, 80 ꢀC; (2) propargyl bromide. (f) DCC, DMAP, CH2Cl2.
rings, incorporation of a chlorine atom, polar/apolar nature of
the terminal chain or presence of azide/methylene azide
terminal unit). Compounds 9–14, bearing a terminal azide unit,
The mesophase ranges depend strongly on the modifications
introduced in the general structure (i.e. number of aromatic
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behaviour. Compounds 15 and 16 (analogous to 9 and 10) form
a nematic phase above the SmA as droplets appearing on
cooling from the isotropic liquid could be detected by POM. In
general, it can be stated that these calamitic structures favour
orthogonal smectic mesomorphism, while the transition
temperatures can be modulated by the structural changes
detailed here.
Series T. The presence of a 1,2,3-triazole ring as the central
core is expected to provide the molecules with the most rigid
and open angular structure. As
a consequence of this,
compounds T1 and T2, with a central triazole ring, show broad
ranges of mesophase (>80 ꢀC) and very high transition
temperatures (above 270 ꢀC), that in the case of T1 causes its
thermal decomposition. For T2, on cooling from the isotropic
liquid, a broken fan-type texture is observed while a schlieren
pattern upon shearing is observed for T1 (Fig. 3a). X-Ray
diffraction studies reveal reflections consistent with a liquid
crystalline nature and a sole halo at small angles from which
ꢁ
layer spacings of 43 A (T1) and 36 A (T2) could be estimated.
ꢁ
These two values are to a great extent smaller than those
ꢁ
ꢁ
theoretically calculated using ChemSketch, 77 A and 63 A,
respectively. All these data point to a lamellar packing and
both a molecular tilting within the layers as well as an inter-
digitation of the terminal alkoxylic chains could be proposed.
5 mm Linkam commercial cells could be filled with T2 in the
isotropic state. Interestingly, when applying an electric field to
this sandwiched sample a colour texture change of the meso-
phase was observed. This has been attributed to a reorganiza-
tion of the molecules within the mesophase promoted by the
electric field, rather than to a switching phenomenon, as no
peak was observed in the polarization curve. In light of these
Scheme
2 Synthetic routes followed to prepare the bent-core
compounds of Series T, Series M and Series MC. (g) catalyst I (1 mol%),
THF/DMF (1/1), 80 ꢀC (microwave).
show in most cases a smectic A phase which could be assigned
by the fan-shaped textures observed by POM (see ESI†). Only
two of these compounds, 12 and 14, bearing methylene azide
moieties do not form a mesophase. However, all the products
containing a terminal alkyne unit show liquid crystalline
Table 1 Transition temperatures and enthalpies for intermediates 9–18
R/n/A/X
Phase transitions (ꢀC) [kJ molꢁ1 a,b
]
9
10
R: –OC14H29, n: 1, A: H, X: –N3
R: –OC14H29, n: 0, A: H, X: –N3
Cr 101.0 [60.4] SmA 250 decomp.
Cr 63.8 [50.7] SmA 84.5 [5.3] I
I 84.5 [5.3] SmA 39.6 [42.8] Cr
Cr 87.4 [50.2] SmA 145.7 N 149.8 [5.2]c I
I 150.6 [4.0]d N 111.3 [0.7] SmA 74.3[46.9] Cr
Cr 59.9 [70.0] Id
Cr 102.8 [34.5] SmA 133.5 [4.0] I
I 132.0 [4.1] SmA 82.9 [34.4] Cr
Cr 47 [9.1] Id
Cr 117.0 [64.7] SmA 182.5 [1.6] N 194.2 [0.7] I
I 186.8 [0.5] N 177.0 [0.8] SmA 56.9 [27.3] Cr
Cr 84.0 [65.4] I
I 80.1[1.6] N 73.1 [0.5] SmA 49.7 [46.0] Cr
Cr 86.4 [38.2] C0 92.1 [12.2] SmA 170.7 [6.8] I
I 169.4 [6.9] SmA 68.6 [45.0] Cr
Cr 65.9 [67.8] I
11
R: –OC14H29, n: 1, A: H, X: –CH2N3
12
13
R: –OC14H29, n: 0, A: H, X: –CH2N3
R: –OC14H29, n: 1, A: Cl, X: –CH2N3
14
15
R: –O(CH2CH2O)4CH3, n: 1, A: Cl, X: –CH2N3
R: –OC14H29, n: 1, A: H, X: –C^CH
16
17
18
R: –OC14H29, n: 0, A: H, X: –C^CH
R: –OC14H29, n: 1, A: H, X: –COOCH2C^CH
R: –OC14H29, n: 0, A: H, X: –COOCH2C^CH
I 58.5 [6.1] SmA 49.0 [53.6] Cr
a
b
Data determined by DSC, from second scans at a scanning rate of 10ꢀC minꢁ1
.
Cr: crystal, SmA: smectic A mesophase, N: nematic mesophase, I:
isotropic liquid. c Joint enthalpy of the transition. Data determined from the 1st scan.
d
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Table 2 Transition temperatures (T, ꢀC) and enthalpies [DH, kJ molꢁ1] for compounds of Series T, Series M and Series MC
Compound
R
x
y
A
Phase transitions (ꢀC) [kJ molꢁ1 a,b
]
T1
T2
–OC14H29
–OC14H29
1
0
1
0
H
H
Cr 149.2 [19.2] Cr0 223.5 [34.3] SmC 370c Id
Cr 194.2 [36.4] SmC 273.0 [6.0] Ie
I 250.6e [4.0] SmC 183.3 [20.6] Cr
Cr 64.7 [12.8] B4-like 226.2 [63.6]f I
I 221.2 [64.6] B4-like 73.5 [8.8] Cr
Cr 191.4 [74.7]f I
M1
–OC14H29
1
1
H
M2
M3
–OC14H29
–OC14H29
0
1
1
0
H
H
Cr 85.9 [21.6] B4-like 192.9 [70.6] I
I 189.9 [70.3] B4-like 79.9 [22.5] Cr
Cr 161.3 [74.4] I
M4
M5
–OC14H29
–OC14H29
0
1
0
1
H
Cl
Cr 91.8 [44.3] Cr0 220.7 [16.1] USmCP
243.4 [34.5] I
I 242.6 [38.1] USmCP 194.1 [14.0] Crd
Cr 182.9 [27.0] USmCP 207.1 [29.9] I
I 204.0 [35.2] USmCP 179.0 [19.0] Cr
Cr 217.0 [57.8]f I
M6
M7
–OC14H29
1
1
0
1
Cl
Cl
–O(CH2CH2O)4CH3
I 211.6 [55.6]f SmCP 202 Cr
Cr 172.6 [52.3] I
M8
MC1
MC2
–O(CH2CH2O)4CH3
–OC14H29
–OC14H29
1
1
0
0
1
1
Cl
H
H
Cr 131.1 [12.8] SmCP 219 decomp.
Cr 154.6 SmCP 162.0 [31.5]f I
I 160.4 [16.0] SmCP 130.5 [20.4] Cr
Cr 119.9 [23.0] Colob 166.4 [15.2] I
I 164.5 [18.3] Colob 95.9 [16.0] Cr
Cr 110.4 [16.4] Colob-USmCP 122.3 [19.5]
I
MC3
MC4
–OC14H29
–OC14H29
1
0
0
0
H
H
I 121.9 [21.9] USmCP-Colob 99.8 [19.0] Cr
a
b
. Cr: crystal, SmC: smectic C mesophase, B4-like: B4-like mesophase,
Data determined by DSC, from second scans at a scanning rate of 10ꢀC minꢁ1
Colob: columnar oblique mesophase, SmCP: polar smectic C mesophase, I: isotropic liquid. c Data obtained by POM. d Data from the 1st heating scan.
e
A partial decomposition of the material at the isotropic liquid was detected by POM. f Two peaks joint enthalpy.
data the formation of a classic smectic C mesophase has been
arrows in Fig. 4). The pattern is similar to the typical diffracto-
grams of the B4 phase reported for some bent-core mate-
rials.2,12,18,19 In particular, the wide-angle reflections are not
harmonics of the small angle ones, indicating that other peri-
odicities are present apart from the smectic one. Like in Perdi-
guero et al.19 the wide-angle peaks could be indexed in the form
(hk0) with the strongest ones in this case being also the reflections
proposed for T1 and T2.
Series M. In this series the triazine system created inserts a
methylene unit (–CH2–) in the central core, leading to changes of
both the central angle (a closer and flexible bending angle) and
the symmetry of the molecule.
ꢂ
indexed as (110) and (110) (numbered as 8 and 9 in Fig. 4).
In the first stage, four compounds showing this bent-unit were
planned: M1 and M4, analogous to T1 and T2 respectively, as
well as two molecules with different lateral parts, closely related
to the former ones, M2 and M3. The versatility of our approach
was then checked by means of the appropriate combination of
two azides (11 and 12) and two alkyne-compounds (15 and 16)
through CuAAC (Scheme 2). Furthermore, following a similar
methodology by using the same alkynes but new azides (13 and
14), four new chlorinated-compounds, structurally related and
bearing either an n-tetradecyloxy or an oxyethylenic chain were
also synthesized: M5 and M7 (analogous to M1), and M6 and
M8 (analogous to M3).
However, we also observe in our material the reflection (020)
(number 7 in Fig. 4). On the other hand, the positions corre-
sponding to the rather undefined reflections indexed as (120) and
ꢂ
(130) are indicated. The indexing was performed on the basis of a
ꢁ
ꢁ
monoclinic lattice with parameters a ¼ 4.85 A, b ¼ 10.85 A, c ¼
ꢀ
ꢁ
63.72 A and g ¼ 98.3 . The value obtained for the c parameter
implies a tilted structure with a tilt angle of about 24ꢀ, taking into
account the calculated molecular length in the most extended
ꢁ
configuration (L ¼ 70 A). This is in agreement with the model
proposed by Hough et al.20 and differs from other proposed B4
structures where molecules are not tilted.21
The majority of the compounds of this new series form mes-
ophases, with the exception of M2, M4 and M8. As a general
trend, viscous soft materials were observed by POM. On cooling
from the isotropic liquid, compounds M1 and M3 slowly form a
mesophase in agreement with the broad peaks observed by DSC,
showing defects such as lancets and Maltese crosses that ended
up in schlieren textures (Fig. 3c and d).
Therefore, although on the basis of the above discussion, this
phase has been tentatively assigned as a B4-like phase, bulk
optical activity, a characteristic of most B4 phases reported so
far, is not observed for M1 and M3, pointing to distinct supra-
molecular packing. Recent studies report that the B4 phase is
neither a traditional crystalline solid, nor a conventional liquid
crystal phase. The phase is composed of single layers which
spontaneously twist into a helical structure. The detailed nature
of these layers is still under active investigation, but solid state
NMR studies suggest the layers are crystalline21 with potential
applications of the B4 phase in molecular electronics and
photovoltaics.22 Interestingly, it seemed that the B4 phase was
Interestingly, the X-ray diffraction pattern of compound M1
shows up to six sharp reflections at periodic distances in the
small-angle region, pointing to a lamellar order (Fig. 4).
Furthermore, at wide angles the broad diffuse maximum char-
acteristic of the SmCP phases splits into different peaks (blue
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Fig. 4 X-Ray diffractogram of compound M1. The intensity is plotted
against 2q, where q is the Bragg angle. Reflections indicated by red arrows
are (00l) reflections with l of 1 to 6. The peaks in the wide-angle region of
ꢂ
the diagram (blue arrows) were indexed as 7: (020), 8: (110), 9: (110), 10:
ꢂ
(120), 11: (130).
analogues without the chlorine atom (M1 and M3 respectively),
but a clear change concerning the supramolecular packing can be
noticed. In both cases the mesomorphic character is checked by
the presence of a broad reflection at wide angles corresponding to
ꢁ
a liquid disorder of 4.6 A. The X-ray diffractogram of M5 shows
ꢁ
clear smectic character with a lamellar spacing of 71.3 A.
However, two very weak reflections, not commensurable with the
strong smectic peak, were observed, indicating some small
amplitude layer undulation. On the other hand, the X-ray
diagram obtained for compound M6 (Fig. 5a) is representative of
a 2D structure and was indexed on the basis of an oblique lattice
ꢀ
ꢁ
ꢁ
with parameters a ¼ 79.8 A, c ¼ 51.9 A and b ¼ 81.2 . Fig. 5b
shows the most feasible charge density map calculated from the
ꢂ
Fig. 3 Microphotographs of the textures of: (a) the SmC mesophase of
compound T1 at 277 ꢀC; (b) the SmC mesophase of compound T2 at
270 ꢀC; (c) the B4-like phase of M1 at 220 ꢀC; (d) the B4-like phase of M3
at 180 ꢀC; (e) the isotropic liquid–mesophase transition of M5; (f) the
USmCP mesophase of M6 at 196 ꢀC in a coating cell; (g) the SmCP
monotropic mesophase of M7 at 208 ꢀC; (h) the SmCP mesophase of
MC2 at 140 ꢀC; (i) texture of the Colob mesophase of compound MC3 at
150 ꢀC; (j) the Colob mesophase of MC4 at 120 ꢀC.
intensities of the reflections (10), (01), (11), (11), (21) and (02)
following the method described by Folcia et al.23 Briefly, due to
the existence of two-fold symmetry axes along the b-direction in
the structure, the electron density r(x, z) satisfies the condition
r(x, z) ¼ r(ꢁx, z) for some particular points in the unit cell. As a
consequence, the structure factor F(hl) for each reflection is a
(positive or negative) real quantity and different r(x, z) map
results from the different sign combinations. Furthermore, we
can select a given cell origin by arbitrarily fixing the signs of two
reflections with different parity (for example (01) and (11)) so
that the number of sign combinations reduces to eight. The
correct sign combination is then decided by the physical merits of
the obtained density map, together with complementary
knowledge such as molecular size, packing conditions, optimi-
zation of steric interactions, etc. The electron density map in
Fig. 5b indicates a 2D undulated smectic structure with an
oblique unit cell. From the relationship of the smectic spacing
limited to traditional Schiff-base bent-core mesogen structures.
Then a new B4-like mesogenic system, to join to the simple recent
biphenyl derivative reported by Walba et al. is offered.22
Regarding M5 and M6 (A: Cl, R: OC14H29), both compounds
form liquid crystal phases. On cooling slowly from the isotropic
liquid, a highly birefringent mesophase with Maltese crosses and
stripes appear for compound M6. Similar defects but less bire-
fringent, were also observed for compound M5. Upon further
cooling, both materials develop a birefringent schlieren texture
or a focal-conic one in the coating cells (Fig. 3e and f). M5 and
M6 show slightly higher transition temperatures than their
ꢁ
with respect to the calculated molecular length (L ¼ 64.2 A), a
molecular tilt of 37ꢀ results.
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heterocyclic moiety seems to lead to strong intermolecular
interactions and hence to high transition temperatures. In order
to hinder this compact packing and to decrease the transition
temperatures, a further step in our versatile approach was
assayed. Thus, four compounds showing a triazole-methyl-
eneoxycarbonyl unit (1,2,3-C2HN3–CH2–OOC–) were prepared
and characterized: MC1, MC2, MC3 and MC4, all of them
structurally related with M1–4 (1,2,3-C2HN3–CH2-based
compounds). All these materials form liquid crystalline
arrangements and with the exception of MC1, the longest
molecule which decomposed around 217 ꢀC before reaching the
isotropic liꢀquid state, these compounds show clearing points
below 170 C. Interestingly, for MC4, the shortest structure, it
ꢀ
drops to around 120 C.
X-Ray patterns for compounds MC1 and MC2 at mesophase
temperature show two reflections at periodic distances in the
small-angle region. On the other hand, the diffuse scattering in
the wide-angle region of MC2 presents a certain structure when
the sample is heated, indicating some degree of crystallinity that
was not observed on cooling from the isotropic phase. Besides,
ꢁ
layer spacings of 54 A and 80 A for compounds MC1 and MC2,
ꢁ
respectively can be estimated. For MC1, this distance is signifi-
ꢁ
cantly smaller than the theoretical molecular length (72 A) but
within the ꢀrange of a tilted arrangement of the molecules (tilt
ꢁ
angle of 42 ). On the contrary, a higher layer spacing (80 A) than
ꢁ
the theoretical molecule length (67 A) was measured for MC2.
Upon cooling MC2 from the isotropic liquid, the texture
appeared as Maltese crosses and lancets that coalesced to yield a
typical texture (Fig. 3h). These results point to a SmCP meso-
phase, but in the case of MC2, interdigitated tilted layers formed
by two molecules should be proposed.
Fig. 5 (a) Small angle region of the X-ray diffraction pattern of
compound M6. The intensity is plotted against 2q, where q is the Bragg
angle. The diagram was indexed on the basis of an oblique translation
lattice (see text). (b) 2D electron density map of compound M6 calculated
from the data in (a). The brighter regions represent the areas of higher
electron density. The unit cell parameters are also sketched.
In contrast, MC3 and MC4 present different structural char-
acteristics. Upon cooling from the isotropic liquid, compounds
MC3 and MC4 develop mosaic textures typical of columnar
mesophases (Fig. 3i and j). Fig. 6 represents the X-ray pattern of
MC3. It was indexed on the basis of a 2D oblique lattice,
although it is clear that the structure presents a strong lamellar
ꢁ
With regards to the effect of incorporating an oxyethylenic
terminal chain in this sort of bent-core molecule (M7 and M8), it
can be said that this structural change significantly decreases the
transition temperatures (an isotropization temperature drop of
around 30 ꢀC) but seriously affects the formation of a meso-
phase. Thus, M8 does not form any mesophase and for M7 a very
short range of monotropic mesomorphism was detected. Crys-
tallization of the sample prevented the appropriate XRD char-
acterization of this compound, nevertheless based on the POM
observation; a SmCP phase has also been proposed for this
material (Fig. 3g). In order to check the polar character of the
mesophase attributed to these compounds (M5, M6 and M7),
electro-optic and polarization studies under an electric field were
performed in Linkam and EHC commercial cells. In all cases, the
electro-optical responses of these materials were observed when
applying a high electric field (up to 150 Vpp), despite the small
size of the defects observed. No neat current peak in the polar-
ization current curve was measured for these liquid crystalline
materials due to the dielectric breaking.
character. The obtained lattice parameters are a ¼ 97.04 A, c ¼
ꢀ
ꢁ
48.6 A and b ¼ 87.3 . The obtained electron density map is
similar to that appearing in Fig. 5b for M6. It is in agreement
with the nearly smectic structure proposed for this material. A
molecular tilt of about 43ꢀ can be estimated from the layer
ꢁ
spacing and the calculated molecular length (L ¼ 67 A).
Compound MC4, on the other hand, shows different behav-
iour. For the temperature range between the isotropic transition
temperature TI and TI ꢁ 10 ꢀC the diffraction diagram presents a
clear lamellar character, although a broad plateau appears at
higher angles beside the smectic peak (see Fig. 7a). This could
indicate a sort of layer undulation similar to that shown in
Fig. 5b. However, at the temperature TI ꢁ 10 ꢀC the X-ray
pattern changes as indicated by the orange points in Fig. 7a.
The so-called smectic peak remains but, instead of the plateau,
two other strong reflections appear which are not commensu-
rable with the lamellar one. There are no further reflections and
the diagram cannot be indexed on the basis of a rectangular cell.
In view of this, we propose the index scheme indicated in Fig. 7a
that corresponds to a 2D oblique lattice with parameters a ¼ 76.3
ꢀ
ꢁ
ꢁ
Series MC. Series M succeeded in inducing bent-core meso-
A, c ¼ 66.0 A and b ¼ 76.5 . Fig. 7b represents the Fourier map
morphism through 1,2,3-triazole based cores; however this
obtained on the basis of the above indexing and the measured
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Fig. 6 Small angle X-ray diffraction pattern of compound MC3. The
intensity is plotted against 2q, where q is the Bragg angle. The diagram
was indexed on the basis of an oblique translation lattice (see text). The
region in the blue circle is represented in the inset, where full circles and
squares correspond to different temperatures within the mesophase.
diffracted intensities. The projection of the c lattice parameter
ꢁ
along the reciprocal direction (001) equals 64.2 A which is
practically the same as the calculated molecular length (L ¼ 61.9
ꢁ
A). This is indicative of a structure where the molecules are
oriented as indicated in Fig. 7b. According to the present scheme,
we assume that at the temperature TI ꢁ 10 ꢀC there is a transition
from a non-tilted nearly lamellar structure (with some undula-
tion, like the one in Fig. 5b) to a different one where the smectic
layers frustrate, resulting in the structure represented in Fig. 7b.
Furthermore, compounds MC2, MC3 and MC4 were studied
under an electric field in 5 mm Linkam commercial cells. When
applying an electric field of 150 Vpp, for MC2 a clear molecular
switching could be observed by POM. However, once again no
polarization peak can be detected in the current polarization
curve. On the contrary, no response under the electric field (up to
300 Vpp) was detected for MC3 and MC4. Besides, the behavior
of MC4 in surface treated cell was remarkably different from that
observed for two untreated glasses. Upon the heating process,
when the sample reached the mesophase, the texture became
almost homeotropic and some defects and birefringent straight
stripes appeared, probably due to surface coating of the cells.
Upon the cooling process after reaching the isotropic liquid, no
changes were observed, indicating that the homeotropic texture
was kept until 100 ꢀC when the sample crystallized in birefringent
patterns. This behaviour could be attributed to the alignment of
the columns of the mesophase, perpendicular to the glasses.
Fig. 7 (a) Small angle region of the X-ray diffraction patterns of
compound MC4. The intensity is plotted against 2q, where q is the Bragg
angle. Black points represent the data obtained for a temperature in the
range TI–(TI ꢁ 10 ꢀC). Orange points represent the data obtained below
the transition at TI ꢁ 10 ꢀC. It was indexed on the basis of an oblique
translation lattice (see text). (b) 2D electron density map calculated from
the data in (a) (orange points). The brighter regions represent the areas of
higher electron density. The expected molecular orientation, as deduced
from the lattice parameters and the calculated molecular length are
schematized. The unit cell parameters are also sketched.
cases with compact packing consistent with bent-core
mesophases.
In good agreement with further 5-member heterocyclic
compounds previously studied, long lateral structures are needed
to promote liquid crystals, also leading to calamitic arrange-
ments. However, by linking it to either a methylene (–CH2–) or a
methyleneoxycarbonyl (–CH2–OOC–) group, available by the
proposed synthetic approach, an attractive sort of compact
supramolecular organization can be tuned through the appro-
priate selection of the structural moieties.
2. Chemical structure-liquid crystal properties relationship in
1,2,3-triazole-based bent-core compounds
The results reported here prove that the 1,2,3-triazole as part of a
variety of bent-core structures, easily created by ‘‘click chem-
istry’’, is a suitable moiety to induce mesomorphism, in most
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(a) Influence of the bent-core. From these series of
compounds, the methylene-oxycarbonyl-triazole moiety (MC)
can be considered as the most versatile and convenient bent-core
unit from those studied here as all the materials form liquid
crystals and lead to a rich mesomorphism. On the other hand, the
methylene-triazole core (M) shows a strong tendency to promote
undulated lamellar molecular arrangements. It is also worth
mentioning that the central triazole-based bent-core emerges as a
key tool to modulate transition temperatures. Indeed, when
comparing compounds with the same lateral structures and
terminal chains, but different central cores (see T1 vs. M1 and
MC1 or T2 vs. M4 and MC4), it can be seen that there is a
pronounced drop in the clearing tꢀemperatures (in particular for
new bent-core structures. Through the combination of
different azide and alkyne ended building blocks, a ‘‘library’’
of symmetrical and non-symmetrical bent-shaped cores can be
achieved. The presence of a 1,2,3-triazole ring promotes well-
built intermolecular interactions. Thus, a variety of supramo-
lecular organizations can be targeted and tuned through
appropriate structural adjustments. 1,4-Disubstituted 1,2,3-tri-
azole structures induce highly stable classic calamitic meso-
phases. However, if a methylene or methylenoxycarbonyl
block is attached to the 1,2,3-triazole ring, bent-core liquid
crystalline packing is favored. The appropriate combination of
a 1,2,3-triazole synthon, aromatic rings and the presence of a
lateral chlorine atom have allowed us to tune both the tran-
sition temperatures and a variety of the non-classic bent-core
phases, ranging from the lamellar to columnar or B4-like
supramolecular liquid crystalline organizations. These results
offer a versatile approach to new bent-core structures with
unexplored but promising possibilities, not only from a liquid
ꢀ
T2 vs. MC4, from 273 C to 122 C). The incorporation of the
methylene or the methylenoxycabonyl units certainly allows the
conformational freedom of the central core disturbing enough
compact molecular arrangement as to decrease the transition
temperatures.
crystals point of view and the incorporation of
a new
(b) Effect of the number and position of aromatic rings. In
good agreement with the trends reported for other bent-core
structures,2 the lower the number of aromatic rings, the weaker
the intermolecular interactions and hence, a decrease of the
transition temperatures (see MC1 vs. MC2 or MC3) or even a
loss of the mesomorphic properties (see M1 vs M4) could be
expected. Interestingly in the asymmetric structures, for a
similar number of aromatic rings, no remarkable effect on the
transition temperatures is observed, but alternatively, signifi-
cant changes in the molecular arrangement within the meso-
phase could be induced. Thus, compound M3 (with three rings
in the azide derivative part) showed a B4-like phase, whereas
the analogous compound M2 (with two rings in the same part)
does not form any mesophase. The same tendency is observed
for the equivalent materials from Series MC; MC3 is able to
show a columnar mesophase whereas MC2 exhibited lamellar
order.
heterocyclic moiety, but also for those materials where
compact and controllable supramolecular packing are pursued.
Acknowledgements
The authors thank the financial support from the Spanish
Government (MICINN-FEDER projects MAT2009-14636-C03,
ꢀ
Aragon-FSE (project E04) and Basque (project GI/IT-449-10)
governments, and the Juan de la Cierva-MICINN (N. G. and R.
M-R) and JAE-DOC-CSIC (N.G.) fellowship programs are
greatly appreciated. SRC thanks the Spanish MEC for a grant.
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