Bent-core luminescent and electroactive bis(triazolyl)triazines with compact columnar mesomorphism

Article abstract of DOI:10.1039/c4ra02926g

The bulk self-assembly of bent-core molecules based on the novel structure 2-methoxy-4,6-bis(1′,2′,3′-triazol-4′-yl)-1,3, 5-triazine is reported together with their luminescent and electrochemical properties. Two families of compounds with lateral branches of different lengths have been investigated. Columnar mesomorphism with short stacking distances and periodic twisted structures were found. A compound exhibiting two hexagonal columnar mesophases that show different emission spectra and a transition from a bimolecular assembly to a unimolecular assembly is described.

Full text of DOI:10.1039/c4ra02926g

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Bent-core luminescent and electroactive
bis(triazolyl)triazines with compact columnar
Cite this: RSC Adv., 2014, 4, 23554
mesomorphism†
Eduardo Beltran,a Beatriz Robles-Hernandez,b Nerea Sebastian,b Jose Luis Serrano,c
´
´
a
´
´
a
*
*
Raquel Gimenez and Teresa Sierra
´
The bulk self-assembly of bent-core molecules based on the novel structure 2-methoxy-4,6-bis(10,20,30-
triazol-40-yl)-1,3,5-triazine is reported together with their luminescent and electrochemical properties.
Two families of compounds with lateral branches of different lengths have been investigated. Columnar
mesomorphism with short stacking distances and periodic twisted structures were found. A compound
exhibiting two hexagonal columnar mesophases that show different emission spectra and a transition
from a bimolecular assembly to a unimolecular assembly is described.
Received 2nd April 2014
Accepted 12th May 2014
DOI: 10.1039/c4ra02926g
www.rsc.org/advances
the micro- or nanoscale. Bent-core liquid crystals are achiral
organic molecules that generally contain a 120ꢀ disubstituted
Introduction
aromatic ring and they are able to pack in a compact way to
form lamellar or columnar phases.36–38 Some of the mesophases
are endowed with chirality or polarity, and excellent properties
that include ferroelectric or antiferroelectric behaviour, piezo-
electricity, or nonlinear optical properties have been repor-
ted.39–41 The columnar arrangements found with these
compounds are different to regular stacks of molecules and are
actually dened as modulated phases formed by frustrated
layers.42 On the other hand, the formation of properly stacked
molecules in columnar mesophases of bent-core molecules has
also been observed in some cases,43–45 and the interest in these
systems lies in the possibility of obtaining a macroscopic
polarization along the columnar axis and polar switching.46–52
Heterocyclic derivatives have been widely explored in the
molecular design of columnar liquid crystals, as the dipole
originating from the heteroatoms plays an important role in the
columnar stabilization and may give rise to additional properties
such as increased electron affinity, as occurs for nitrogen-rich
aromatics (azaaromatics).53 In this regard, we recently described
the synthesis and properties of a new class of compounds
based on the nitrogen rich star-shaped core 2,4,6-tris(10,20,30-tri-
azol-40-yl)-1,3,5-triazine, which show columnar mesomorphism,
luminescence and electron-acceptor characteristics.54,55 The tri-
azole rings joined to the triazine ring confer exibility to the core
and this enabled these molecules to adopt unusual assemblies
with different symmetry conformations in the columns.55
Columnar liquid crystals have the ability to spontaneously
aggregate into stacks and this represents an interesting
approach to one-dimensional so anisotropic systems for the
transport of electrons, ions, energy, photons or molecules.1–4
Properly designed columnar mesogens have demonstrated
good performance in optoelectronic devices.5–11
Columnar mesophases are mainly based on discotic mole-
cules12,13 but molecules with different shape anisotropies,14,15
such as star-shaped molecules,16 T-shaped molecules,17 phas-
midics,18,19 dendrimers,20–23 bowl-shaped,24–29 half-disk,30–32
taper-shaped33–35 or bent-core36 molecules as examples, can also
provide columnar mesophases. A delicate balance between non-
covalent interactions such as H-bonding, p-stacking, van der
Waals forces, microsegregation and steric effects provides the
driving force for the effective lling of the free volume in a
columnar arrangement.
The bent-core geometry is, of the unconventional shapes, an
example of the tight relationship between molecular structure,
supramolecular arrangement and properties of the material.
Furthermore, this system highlights the importance of the
assembly in transferring the molecular properties to the
macroscopic level or in nding new and exciting properties on
a
´
´
´
Departamento de Quımica Organica, Instituto de Ciencia de Materiales de Aragon
(ICMA) – Facultad de Ciencias, CSIC – Universidad de Zaragoza, 50009 Zaragoza,
Spain. E-mail: rgimenez@unizar.es; tsierra@unizar.es; Tel: +34 976 762276
b
´
´
Departamento de Fısica Aplicada II, Facultad de Ciencia y Tecnologıa, Universidad
The aim of the work described here was to study the properties
of the triazine–triazole group in a bent-core structure, based on the
special self-assembly that this geometry provides. Thus, the
synthesis and characterisation of novel compounds derived from
2-methoxy-4,6-bis(10,20,30-triazol-40-yl)-1,3,5-triazine and their bulk
self-assembly in mesomorphic columnar structures are reported.
´
del Paıs Vasco UPV/EHU, 48080 Bilbao, Spain
c
´
Instituto de Nanociencia de Aragon (INA), Edicio I+D, Universidad de Zaragoza,
50018 Zaragoza, Spain
† Electronic supplementary information (ESI) available: Synthetic procedures and
characterisation data, DSC thermograms, UV-Vis and emission spectra and
electrochemical data. See DOI: 10.1039/c4ra02926g
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Scheme 1 Synthetic pathway followed for the preparation of BTs and BTBs.
The stacking found along the columns is compact with short mol of cyanuric chloride by a Negishi coupling.56,57 In a second
intermolecular distances. The molecules are uorescent and synthetic step, an excess of methanol was added and the reac-
electroactive, thus endowing the assemblies with these properties. tion mixture was heated under reux in order to introduce the
Two families of compounds (BTs and BTBs) with lateral branches methoxy group into the triazine ring, thus affording C in 57%
of different lengths have been investigated (Scheme 1).
yield. According to the expected molecular structure, the 1H-
NMR spectrum shows two singlets, which correspond to the
methoxy group (4.07 ppm) and the trimethylsilyl group (0.28
ppm).
The target BTs and BTBs were prepared by direct reaction of
C with the aromatic azides A or B by adapting a synthetic
procedure previously described for tris(triazolyl)triazines.54 The
reaction gave the products in 60–80% yield at room temperature
and in one-pot. The process involves two orthogonal reactions,
Results and discussion
Synthesis
The synthetic route followed is depicted in Scheme 1. In order to
obtain the compounds with the bent shape, the core 2-methoxy-
4,6-bis(trimethylsilyl)-1,3,5-triazine, C, was synthesised by
reaction of two moles of chlorotrimethylsilylethynylzinc(II) per
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Table 2 XRD data for the liquid crystalline phases
the double alkyne deprotection and the double 1,3-dipolar
cycloaddition of the aromatic azides to the alkyne groups
catalyzed by copper(I) (CuAAC click reaction58).
Compound
T/ꢀC
Phase
Colhp
d/A
hk
Parametersa
˚
1
The H-NMR spectra display a singlet corresponding to the
˚
BT4
20
29.6
16.8
10.9
10
11
21
a ¼ 33.7 A
C5 proton of the triazolyl ring at 8.9–9.0 ppm, indicating that
the 1,4-disubstituted triazole regioisomer is formed. In addi-
tion, both series show a singlet corresponding to the methoxy
group at 4.30 ppm. The results indicate the compatibility of this
methodology with the methoxy substitution of the triazine ring.
4.4 (br)
3.8
3.4
30.8
4.4 (br)
31.8
20b
20
Colh
ColAh
a ¼ 35.6 A
˚
˚
BT6
10
11
a ¼ 36.9 A
˚
Liquid crystalline behaviour and self-assembly models
18.6
c ¼ 3.3 A
15.5c
4.3 (br)
3.3c
The thermal properties and the liquid crystalline behaviour of
BTs and BTBs were investigated by thermogravimetric analysis
(TGA), polarising optical microscopy (POM), differential scan-
ning calorimetry (DSC) and X-ray diffraction (XRD). The tran-
sition temperatures and enthalpy values are collected in Table
1. The mesophase parameters obtained by XRD are summarised
in Table 2. Compounds of the BT series with four and six
terminal chains, BT4 and BT6, and the compound of the BTB
series with six terminal chains, BTB6, are columnar liquid
crystals. The other compounds are crystalline solids that melt
directly to the isotropic liquid. For BTB2 a decomposition
process takes place along with the transition to the isotropic
liquid and, prior to this, an unknown phase with a lamellar
01
10
ColBh
Colh
26.0
a ¼ 30.0 A
˚
100
20
12.7c
4.5 (br)
37.4
˚
BTB6
10
a ¼ 43.2 A
15.1c
4.4 (br)
3.4c
c ¼ 3.4 A
˚
01
a
a ¼ (2/O3) ((d10) + (d11)O3 + (d20)O4 + (d21)O7 +./nreections). b Sample
c
heated at 100 ꢀC and rapidly cooled down to 20 ꢀC. Reinforced along
the meridian in an oriented pattern. (br) ¼ broad halo.
arrangement was observed in some experiments. However, the 94 ꢀC (Fig. 1a). The XRD pattern obtained at room temperature
lack of reproducibility between DSC cycles (ESI, Fig. S1†) did not for compound BT4 shows three reections in the small angle
allow us to draw rm conclusions.
region in a ratio 1 : 1/O3 : 1/O7, which is consistent with a
Compound BT4 displayed a columnar phase from room hexagonal symmetry for the columnar mesophase (Fig. 2a). In
ꢀ
˚
temperature up to 104 C, at which point the transition to the the high angle region, the typical diffuse halo at 4.4 A, related to
isotropic liquid occurred. In the cooling process, two transitions the uid character of the decyloxy tails, and two additional
˚
˚
were observed due to the appearance of an additional columnar reections located at 3.8 A and 3.4 A, which are related to
mesophase over a small temperature range (monotropic mes- stacking, are observed. These latter two reections are indica-
ophase). The DSC thermogram shows that the two transitions tive of a certain 3D order of the phase and are compatible with a
are partially overlapped (ESI, Fig. S2†). By POM, a pseudofocal- hexagonal plastic columnar phase13 (Colhp) with a cell param-
˚
conic texture of a columnar mesophase started to appear at eter of 33.7 A. A diffractogram of the monotropic mesophase of
100 ꢀC on cooling the isotropic liquid (Fig. 1a, inset) and a BT4 was obtained by rapid cooling at room temperature and
decrease in birefringence was observed on further cooling at this shows only a sharp maximum in the small angle region and
˚
a diffuse halo at 4.3 A (Fig. 2b). This kind of pattern does not
unambiguously conrm a hexagonal symmetry but rules out
Table 1 Phase behavioura of BTs and BTBs
other columnar symmetries and it has been observed for
hexagonal columnar phases of several non-conventional
mesogens.43,59–63 The diffractogram, together with the observed
POM texture, led us to propose that the small-angle maximum
corresponds to the (10) reection of a hexagonal columnar
Compound
BT2
T/ꢀC (DH/kJ molꢁ1 b
)
Cr 13 (3.8) [Cr0 + Cr00] 169c (37.6) I
I 164 (22.4) Cr00 141 (9.0) Cr0 12 (4.1) Cr
Colhp 104 (29.9) I
˚
BT4
mesophase with a cell parameter (a) of 35.6 A. Taking into
I 100 Colh 96 (27.5) Colhp
ColAh 59 (13.2) ColBh 121c (7.6) I
I 116 (9.2) ColBh 42 (13.0) ColAh
Cr 85 (12.9) P 243 I + Dec
Cr 115 (37.8) Cr0 194 (48.4) I
I 189 (47.3) Cr0 86 (51.2) Cr
Colh 172 (27.5) I
account the cell parameter of these two columnar phases, the
BT6
unit cell should contain about two molecules (Z ¼ 2) on the
reasonable assumption that the density is around 1 g cmꢁ3
.
64
BTB2
BTB4
On the basis of these data, a model is proposed for the molec-
ular arrangement in a columnar stratum in which two bent-core
molecules are disposed with their molecular dipoles in an
antiparallel manner to form a disk like unit (Fig. 2c). This
arrangement allows efficient space lling and a partial inter-
digitation of the polar methoxytriazine groups belonging to two
antiparallel molecules.
BTB6
I 166 (29.4) Colh
a
Colh ¼ hexagonal columnar mesophase, Colhp ¼ hexagonal plastic
columnar phase, Cr ¼ crystalline phase, I ¼ isotropic liquid, Dec ¼
b
decomposition process, P ¼ unknown phase. Onset temperatures
c
from DSC heating and cooling cycle at 10 ꢀC minꢁ1
corresponding to the maximum of a broad peak.
.
Temperature
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Fig. 1 Textures observed by POM in the cooling process between crossed polarisers (a) BT4 in the Colhp, (94 ꢀC), inset corresponds to the Colh
(100 ꢀC), (b) BT6 in the ColBh (112 ꢀC), (c) BT6 in the ColhA (20 ꢀC) and (d) BTB6 in the Colh (161 ꢀC).
unimolecular assembly. A similar decrease in the cell parameter
has been reported for derivatives of tris(triazolyl)triazine when
they show bimolecular or unimolecular arrangements in the
columnar mesophase.8 It is remarkable that in this case, the
triazine–triazole group is in a different molecular geometry, a
bent-shape and not a star-shape, and the two different
columnar arrangements with hexagonal symmetry are found in
the same compound with a rst-order transition between them.
For both mesophases, it was also possible to prepare
partially aligned samples (Fig. 3c and d). It was observed that
some maxima were reinforced in the meridian region (direction
parallel to the columnar axis), indicating that there are addi-
tional periodic modulations of the electronic density along the
columns. In the room temperature mesophase (ColAh) these
˚
˚
maxima are found at 15.5 A and 3.3 A. To account for this we
propose a model in which two molecules are arranged in an
antiparallel manner in a column slice (Fig. 4a), and these
˚
dimers are stacked at 3.3 A and arranged periodically along the
columnar axis, in such a way that each slice is rotated along the
˚
column in order to give a periodicity of 15.5 A. This situation
can be explained by considering that there is a repetitive situ-
ation every 15.5/3.3 slices, i.e., approximately 4.5 dimers. Since
Fig. 2 Diffractograms corresponding to BT4 in (a) Colhp and (b) Colh.
(c) Proposed model for the arrangement of two molecules of BT4 in a
columnar slice.
Compound BT6 displays two enantiotropic hexagonal
columnar mesophases (ESI, Fig. S3†). In the cooling process, a
dendritic texture typical of a columnar mesophase developed at
116 ꢀC (Fig. 1b). This texture became blurred at 42 ꢀC, i.e., at the
transition to another columnar mesophase, and remained
stable at room temperature (Fig. 1c). The diffractogram
observed for the room temperature mesophase (ColAh) is
consistent with a hexagonal columnar mesophase with a cell
˚
parameter (a) of 36.9 A and intracolumnar order with a stacking
˚
periodicity (c) of 3.3 A (Fig. 3a). For the high temperature mes-
B
˚
ophase (Colh) a cell parameter of 30.0 A, which is much smaller
than the low temperature phase, was measured and there is a
loss of the regular molecular stacking along the columns
(Fig. 3b and Table 2). Based on density calculations for the unit
cell similar to those performed for BT4, the low temperature
mesophase should contain about two molecules per unit cell (Z
˚
¼ 2), whereas the high temperature mesophase, with a 6.9 A
decrease in the (a) parameter, should contain only one (Z ¼ 1).
This means that in the ColhA-to-ColBh transition there is a change
in the mode of arrangement, from a bimolecular assembly to a
Fig. 3 Diffractograms corresponding to compound BT6 in (a) ColAh at
20 ꢀC and (b) ColBh at 100 ꢀC, (c) partially oriented pattern at 20 ꢀC, (d)
partially oriented pattern at 100 ꢀC.
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Fig. 5 Change in the dielectric permittivity of BT6 on cooling and
heating at 1 ꢀC minꢁ1
.
Compound BTB6 exhibits a stable enantiotropic columnar
mesophase from room temperature up to the clearing point at
172 ꢀC. Curled birefringent laments were observed by POM on
cooling from the isotropic liquid into the mesophase (Fig. 1d).
The enthalpy value, 29.4 kJ molꢁ1, is consistent with a highly
ordered mesophase and the uidity found at room temperature
demonstrates the mesomorphic nature (ESI, Fig. S4 and S5†).
The XRD pattern only displays a reection in the low angle
region but, together with two additional reections that are
reinforced in an orthogonal direction, this allows us to assign
the mesophase as hexagonal columnar with a cell parameter of
Fig. 4 (a) Proposed model for the arrangement of two molecules of
BT6 in the ColAh mesophase. (b) Top view of the arrangement in the
ColAh and (b) ColBh mesophases. Columnar stacking is represented by
different colours. (c) Side view of the ColBh.
each dimer has a binary symmetry axis, 4.5 dimers or 9 mole-
cules mutually rotated by 40ꢀ will constitute a 180ꢀ turn
(Fig. 4b). Therefore, in a 360ꢀ full turn there will be 9 dimers (18
molecules) and the twisted periodic structure will have a pitch
˚
˚
43.2 A and a periodic interdisk distance of 3.4 A. Density esti-
mations indicate that the number of molecules per unit cell is
two (Z ¼ 2). Analysis of a partially aligned sample of this
compound revealed that, as before, there are two reections
˚
distance of 15.5 ꢂ 2 ¼ 31 A.
In the case of the high temperature mesophase (ColBh), with Z
¼ 1, the interdisk distance is not observed but there is a diffuse
maximum that is reinforced in the meridian at a distance of
˚
that are reinforced in the meridian region, one at 3.4 A (inter-
˚
disk distance) and an additional one at 15.1 A (Fig. 6). In
˚
12.7 A. This can be assigned to a stacking correlation of about 4
contrast to BT6 the reections are unusually strong and are very
well dened, which indicates that the intracolumnar order has a
longer range.62 These results are also in agreement with the type
of growth of the mesophase texture from the isotropic liquid. As
discussed for the ColAh phase of BT6, the intramolecular periodic
molecules, for which we propose an arrangement of the four
molecules as two interdigitated dimers mutually rotated by 90ꢀ
(Fig. 4c). Thus, the transition ColAh-to-ColhB on heating can be
visualised as follows, the two molecules of the dimer situated in
one columnar slice in the low temperature phase start to
interdigitate and overlap, resulting in stacking of interdigitated
antiparallel dimers (Fig. 4b), thus leading to a decrease in the
diameter of the columns.
˚
distance of 15.1 A led us to propose a similar arrangement for
BTB6, in which there is a twisted periodic structure formed by
In view of these arrangements and the possible orientation
of the polar molecules along a column to give rise to a polar
response, electric elds up to 60 V mmꢁ1 were applied by
sandwiching BT6 in a Linkam cell (5 mm thick). The sample was
very viscous and reorientation effects or polar switching was not
observed in either of the two mesophases. The dielectric
permittivity at 1 kHz was measured using an HP4192 imped-
ance analyzer. The transition between the two columnar phases,
as well as the transition to the isotropic phase, was clearly
visible as a change in the dielectric permittivity (Fig. 5).
However, despite the fact that both columnar phases have a
rather low permittivity value, this value is lower in the high
temperature ColBh and this could be due to the interdigitated
antiparallel arrangement of the dimers.
Fig. 6 Partially oriented pattern for BTB6 at 20 ꢀC.
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Table 3 UV-Vis absorption and emission data for BTs and BTBs
Compound
lTabHsF/nm (log 3)
lcaybcslohexane/nm (log 3)
lTemHF/nm
lceymclohexane/nm
lablms /nm
lemlm/nm
F
a
a
BT2
BT4
BT6
BTB2
BTB4
BTB6
283 (4.63), 289sh (4.61)
277 (4.73)
255 (4.50), 294 (4.61)
275 (4.97)
273 (4.92), 291 (4.85)
284 (4.87)
—
366
446
464
378
376
375
—
270, 286sh
254, 302
266, 305
282
405
459
473
441
425
406
0.10b
0.64b
0.13b
283 (3.67), 300 (3.50)
253 (4.49), 293 (4.50)
380
397
a
a
—
—
#0.01c
#0.01c
#0.01c
276 (3.84), 295 (3.81)
283 (4.93)
393
347
273, 295, 303sh
284
Low solubility. b Quantum yields in THF relative to 9,10-diphenylanthracene (F ¼ 0.9 in cyclohexane).69 Quantum yields in THF relative to 2-(4-
a
c
biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD) (F ¼ 0.8 in benzene).70
4.5 stacked dimers (9 molecules) mutually rotated by 40ꢀ along columnar phases were also measured (Fig. 7). The spectra of the
the columnar axis. Therefore, for BTB6 a similar model to the two phases show a broad band consistent with compact or
one represented for the ColAh of BT6 would apply (Fig. 4b).
homogeneous packing, but at slightly different wavelengths,
473 nm for ColAh and 464 nm for ColBh. The longer emission
wavelength of the low temperature mesophase is consistent
with the observed longer degree of intracolumnar order.67
Absorption and emission properties
UV-Vis and uorescence spectra were recorded on dilute solu-
tions in tetrahydrofuran (THF) or cyclohexane and in thin lms
at room temperature (Table 3).
Electrochemical properties
In solution, for both BTs and BTBs the maximum absorption
is located in the UV spectrum below 300 nm and, due to their
absorption coefficient values, is consistent with p–p* transi-
tions. In the condensed phase the prole is similar to that
obtained for the solutions but with small red-shis.
Compounds of the BT series are uorescent in THF solution in
the blue-green region of the visible spectrum and they have low
or moderate quantum yield values. The emission maxima are
dependent on the number of peripheral long tails, with a clear
red-shi observed when the number increases from two to six.
The Stokes shis therefore increase, with values of 82 nm, 169
nm and 170 nm for BT2, BT4 and BT6, respectively. These
values are consistent with a higher charge transfer and/or
conformational relaxation in the excited state on increasing the
number of peripheral chains, as described for star-shaped
compounds with a similar triazole–triazine group54 or with
other nitrogenated mesogens.63,65 The emission maxima are
also red-shied on increasing the polarity of the solvent from
cyclohexane to THF. This effect is found in molecular systems
with electron donor groups (alkoxy) linked to an electron
acceptor group (triazine) by a similar conjugated system55 and it
is known as positive solvatouorochromism.66
The electrochemical properties of the compounds were studied
by cyclic voltammetry (CV) using a glassy carbon working elec-
trode, a Pt counter electrode, and an Ag/AgCl reference elec-
trode in a three-electrode cell. The experiments were carried
out under argon in CH2Cl2, with Bu4NPF6 as a supporting
electrolyte (0.1 M) at a scan rate of 100 mV sꢁ1 (ESI, Table S1†).
None of the compounds, except for BT6, showed any oxidation
process between 0 V and +2.2 V. In general, compounds of the
BT series show two reduction waves located at ꢁ1.78 V and
ꢁ0.92 V (vs. Ag/AgCl), for which a LUMO value of ꢁ3.40 eV can
be calculated. In addition, BT6 shows an oxidation wave at
+1.49 V (vs. Ag/AgCl) and this could be due to the higher number
of donor chains that make the oxidation of the methoxy group
more favourable. For the BTB series there is only a reduction
wave located at ꢁ1.79 eV and this corresponds to a LUMO value
of ꢁ2.53 eV. Comparison of the LUMO values (ꢁ3.40 eV for BTs
and ꢁ2.53 eV for BTBs) with the value of a reference compound
such as 2,4,6-triphenyl-1,3,5-triazine (ꢁ2.71 eV)68 shows that, as
For the BTB series the emission bands in solution appear in
the near-UV region, the emission maxima in THF are almost
insensitive to the number of long tails of the compounds and
the Stokes shi values are around 100 nm. The main difference
between BTs and BTBs is that the introduction of the benzoy-
loxy groups increased the possibility of non-radiative deactiva-
tion pathways and much lower quantum yield values than for
BT series are found.
All compounds show luminescence as thin lms at room
temperature in the blue-green region of the visible spectrum. BT
derivatives show, as in solution, a red-shi on increasing the
number of chains at the periphery. The opposite is found for the
BTBs, indicating that in this case the polarity of the excited
states is different. The emission spectra of BT6 in both Fig. 7 Emission spectra for BT6 in a thin film.
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observed in the BT series, the triazole rings together with the
alkoxyphenyl peripheral groups lead to an increase in the
electron affinity. When the peripheral groups are alkoxy-
benzoyloxyphenyl groups, i.e., BTBs, the electron affinity
decreases in all cases with respect to the parent 2,4,6-triphenyl-
8 M. Mathew and Q. Li, in Self-organized Organic
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¨
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