Luminescent columnar liquid crystals based on tristriazolotriazine

Article abstract of DOI:10.1021/la3020139

Five discotic molecules comprising a tris[1,2,4]triazolo[1,3,5] triazine core were designed and synthesized to obtain luminescent and charge-transporting columnar liquid crystalline materials. With the exception of one compound containing terminal hydroxyl groups all compounds presented a wide thermal range and stable columnar liquid crystalline phase, characterized by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction (XRD) techniques. The phase formation appeared to be associated to some extent with interdigitation of the alkoxy and benzylalkoxy portion, as suggested by the XRD results. All compounds have a strong blue luminescence in solution and solid phase. At the temperature at which the compounds enter in the mesophase the luminescence decreases significantly. This result suggests that entrance into the Col_h phase is accompanied by a better π-stacking of the peripheral phenyl rings compared to the solid phase, consistent with the intramolecular distances (3.5 A) observed in the XRD analysis. These compounds based on tristriazolotriazine are quite robust with good optical and thermal properties for application as solid state emitters, and we anticipate that they may provide an interesting alternative to other discotic molecules based on N-heterocycles, which generally present a high-temperature Col _h phase.

Full text of DOI:10.1021/la3020139

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Article
Luminescent Columnar Liquid Crystals Based on Tristriazolotriazine
Hugo Gallardo, Rodrigo Cristiano, Juliana Eccher, Ivan Helmuth
Bechtold, Catarina N Tironi, André A Vieira, and Fernando Molin
Langmuir, Just Accepted Manuscript • DOI: 10.1021/la3020139 • Publication Date (Web): 02 Jul 2012
Downloaded from http://pubs.acs.org on July 4, 2012
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Graphical Abstract
41x21mm (300 x 300 DPI)
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Luminescent Columnar Liquid Crystals Based on
Tristriazolotriazine
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Rodrigo Cristiano†, Juliana Eccher‡, Ivan H. Bechtold‡, Catarina N. Tironi , André A. Vieira ,
Fernando Molin and Hugo Gallardo*,
†Departamento de Química, Universidade Federal da Paraíba, Campus I, João PessoaꢀPB, 58051ꢀ
900, Brazil
‡Departamento de Física, Universidade Federal de Santa Catarina, FlorianópolisꢀSC, Brazil
Departamento de Química, Universidade Federal de Santa Catarina, FlorianópolisꢀSC, 88040ꢀ
900, Brazil
ABSTRACT: Five discotic molecules comprised of a tris[1,2,4]triazolo[1,3,5] triazine core were
designed and synthesized to obtain luminescent and chargeꢀtransporting columnar liquid
crystalline materials. With the exception of one compound, containing terminal hydroxyl groups,
all compounds presented a wideꢀthermalꢀrange and stable columnar liquid crystalline phase,
characterized by differential scanning calorimetry, polarized optical microscopy and Xꢀray
diffraction (XRD) techniques. The phase formation appeared to be associated to some extent
with interdigitation of the alkoxy and benzylalkoxy portion, as suggested by the XRD results. All
compounds have a strong blue luminescence in solution and solid phase. At the temperature at
which the compounds enter in the mesophase the luminescence decreases significantly. This
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result suggests that entrance into the Colh phase is accompanied by a better πꢀstacking of the
peripheral phenyl rings compared to the solid phase, consistent with the intramolecular distances
(3.5 Å) observed in the XRD analysis. These compounds based on tristriazolotriazine are quite
robust with good optical and thermal properties for application as solid state emitters and we
anticipate that they may provide an interesting alternative to other discotic molecules based on
Nꢀheterocycle, which generally present a high temperature Colh phase.
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INTRODUCTION
The search for new cores able to exhibit a high thermal stability, columnar mesomorphism at
close to room temperature, and a high tendency to yield oriented monodomains is a subject of
great interest in the design of smart materials.1ꢀ3 The interest in discotic liquid crystals (DLCs) is
due to the advantageous properties of these materials, such as longꢀrange selfꢀassembling (order),
selfꢀhealing structural defects, ease of processing, high solubility in organic solvents, and chargeꢀ
carrier mobility.4,5 These properties are the key feature of new smart materials for electronic and
optoelectronic devices based on organic compounds.
Among the DLCs, πꢀconjugated Nꢀheterocycles are strong candidates to be considered in the
synthesis of these materials. They can form efficient πꢀπ columnar stacks which result in oneꢀ
dimensional (1D) high chargeꢀcarrier mobilities (up to 0.1–1.3 cm2 V−1 s−1).6ꢀ8 The magnitude of
the mobilities is fundamentally determined by the degree of order and πꢀπ molecular orbital
overlap within the columnar stacks.9,10 Moreover, discꢀlike molecules with a πꢀconjugated Nꢀ
heterocycle are easily functionalized to present columnar mesomorphism. The insertion of
simple peripheral aliphatic chains may lead to columnar phases and highly ordered structures
with exciting properties from both the scientific and technological points of view.11 Over recent
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decades new discotic liquid crystals have been reported with a large number of πꢀconjugated Nꢀ
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heterocycle cores such as triazine,12ꢀ14 hexaazatriphenylene,15 cyclotriphosphacene,16 triindole,17
tricycloquinazoline,18,19
phthalocyanine,20ꢀ22
hexaazatrinaphthylene,23,24
porphyrin,25,26
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perylenediimide,27 and dodecaazatrianthracene.28 These cores have been exploited in ion
transport29ꢀ31 and charges,32ꢀ35 catalysis,36 and electroꢀoptical applications.37ꢀ40 All of the
properties of the DLCs can be controlled by their molecular selfꢀorganization processes. Thus, it
is important to understand the structureꢀproperty relationships on the molecular level. This can
be studied through slightly modifying the molecule (e.g. varying the length, volume and
functionality of the aliphatic chains) and observing the effect on the DLC properties. In this
context, we previously reported the synthesis and chemical characterization of new
tris[1,2,4]triazolo[1,3,5] triazines, of which compound 1 exhibited a large stable hexagonal
columnar mesophase.41 Herein, we report the results of our further investigation42 of this
heterocycle as a core in discotic liquid crystal compounds and the study of the mesomorphic
behavior, luminescence and conductive capabilities in the solid phase and mesophase range of
these compounds. Tristriazolotriazine43 was found to represent an attractive and interesting
alternative to other Nꢀheterocycle cores reported thus far. It was observed to be quite efficient as
a core for wideꢀthermalꢀrange Colh phase, blue luminescence and stable thin films.
EXPERIMENTAL SECTION
Materials. All the chemicals were purchased from Aldrich, Acros or Vetec. Toluene,
tetrahydrofuran (THF) and Et2O were distilled over Na/benzophenone. CHCl3 was purified by
refluxing over CaH2 followed by distillation. If not specified otherwise, the other solvents were
deoxygenated by bubbling with N2 for 1 h prior to use.
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Measurements and Characterization. H and 13C NMR spectra were obtained with a Varian
Mercury Plus 400ꢀMHz instrument using tetramethylsilane (TMS) as the internal standard.
Infrared spectra were recorded on a PerkinꢀElmer model 283 spectrometer in KBr discs or films.
Mass spectra were recorded on Bruker Autoflex III Smartbean with MALDIꢀTOF techniques,
using αꢀcyanoꢀ4ꢀhydroxycinnamic acid as matrix and high resolution mass spectra were
recorded on Bruker micrOTOFꢀQ II APCIꢀQqꢀTOF mass spectrometer. The textures of the
mesophases were determined using an Olympus BX50 microscope equipped with a Mettler
Toledo FPꢀ82 hot stage and a PMꢀ30 exposure control unit. Thermal transitions and enthalpies
were determined by DSC measurements carried out using TA equipment Q2000 module. TGA
measurements were carried out using Shimadzu equipment with a TGAꢀ50 module. Elemental
analysis was carried out using a Carlo Erba model Eꢀ1110 instrument.
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X-ray Diffraction (XRD) Experiments. We used the X’PERTꢀPRO (PANalytical)
diffractometer with CuKα radiation (λ = 1.5418 Å), the applied power was 1.2 kVA and the
scans performed in continuous mode from 2° to 30° (2θ angle). The samples consisted of an
amount of powder on a glass plate placed in the diffractometer chamber on the TCU2000
temperature control unit (Anton Paar), allowing the temperature control during the measurement.
The samples were first heated until the isotropic phase and the diffraction patterns collected
during cooling back through the mesophases. As a result, we obtained films of approximately 1
mm thickness.
Absorbance and Fluorescence Measurements. An HP UVꢀVis model 8453
spectrophotometer was used to record the absorption spectra. Fluorescence spectra were recorded
on a HitachiꢀFꢀ4500. The relative quantum yields of fluorescence (ΦFl) were determined
according to a published method.44 The absorption and fluorescence measurements in solid state
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(thin films) were carried out with the OceanOptics USB4000 spectrophotometer. The absorption
was performed on spinꢀcoated films of the compounds on glass substrates (further details
available in Ref. 45). To determine the fluorescence as a function of the temperature, the same
procedure applied to the XRD samples was adopted. The samples were placed on a hot stage,
illuminated with a UV lamp and the emission captured with an optic fiber close to the film. The
angular position was chosen so as to avoid the direct incidence and the specular reflection (from
the glass plate) of the lamp.
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Circular Dichroism (CD). CD spectra were recorded in a Jasco Jꢀ810. Neat samples were
prepared by casting a tetrahydrofuran solution of the material, at 2 wt %, onto a quartz plate and
subsequent melting above the clearing point. As the heating stage, a Mettler FP82, with a central
processor Mettler FP80, was used appropriately modified for fixing within the sample holder of
the CD spectrometer.
Atomic Force Microscopy (AFM). AFM was performed on the same spinꢀcoated films used
for the absorption characterization. We used the model EasyScan2 (Nanosurf) in tapping mode
with a scanning rate of 1.0 Hz and 512 x 512 lines. To obtain the film thickness, part of the film
was removed with a needle in order to create a step between the film and the substrate.46
Conductivity Measurements. Conductivity was obtained from a.c. impedance measurements
using the an Autolab Electrochemical Instruments conductivity meter, over the frequency range
of 0.1ꢀ106 Hz. Cells were prepared using ITOꢀcoated glass plates separated by spacers of 9.0
ꢀm. The compounds were inserted by capillarity action using a hot stage, by heating the material
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until the isotropic phase was achieved, followed by cooling at a rate of 5 C/min until the
mesophase was obtained for the measurement. This procedure is commonly used to generate
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homeotropic alignment and thus the conductivity is measured in the direction of the column
organization.
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Synthesis. The tristriazolotriazines 1ꢀ5 were prepared according to previously published
procedures.41 For more detail see support information.
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RESULTS AND DISCUSSION
Synthesis and Characterization of the Materials. The discꢀlike molecules 1-5 based on a
tristriazolotriazine core were synthesized according to our previous methodology41 by reacting
three equivalents of the respective aryltetrazole with cyanuric chloride in the presence of
potassium carbonate (Scheme 1). Our design strategy was to modify the peripheral region of the
discꢀlike molecule in order to investigate the influence of a larger volume and different
functionality (chirality and the presence of OH groups) on the columnar arrangement. The
chemical structures were fully characterized by 1H and 13C NMR spectroscopy and MALDIꢀMS.
In our previous work we have entirely elucidated the regiochemistry of the three triazole ring
obtained from this reaction, including the Xꢀray structure and 15N NMR of the
tris[1,2,4]triazolo[1,3,5]triazine ring.
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Scheme I. Synthesis of the discotic liquid crystals based on tris[1,2,4]triazolo[1,3,5]triazines.
Compound 3 is a diastereomeric mixture
Liquid Crystalline Properties. The thermotropic liquid crystalline properties of compounds
1-5 are summarized in Table 1. With regard to the thermal characteristics, all molecules
presented good stability, measured by TGA under a nitrogen atmosphere, with decomposition
temperatures above 400 °C. An exception was compound 5, with more labile benzyl groups, for
which the onset of decomposition was at 315 °C. With the exception of compound 4, containing
the terminal hydroxyl groups, all other compounds presented columnar liquid crystalline phases,
characterized by Differential Scanning Calorimetry (DSC), Polarized Optical Microscopy (POM)
and Xꢀray diffraction (XRD).
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Table 1 Thermal properties and XRD data for compounds 1-5.
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XRD data
Compound. Phase transition profile a
a b (Å)
T (°C)
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Cr 92.2 (19.3) Colh 207.6 (6.8) I
150
150
150
ꢀꢀꢀ
30.4
1
I 203.3 (5.8) Colh 91.1 (19.6) Cr
Cr 41.7 (8.6) Colh 166.6 (4.1) I
26.3
24.1
ꢀꢀꢀ
2
I 164.0 Colc 159.2 Colh (4.4) 31.7 (5.4) Cr
Colh 158.3 (2.8) I
3
d
I 157.4 (2.4) Colh
Cr 111.3 (46.5 ) I
4
I 100.9 (52.9) Cr
Cr 174.4 (10.8) Colh 264.1e (6.4) I
200
38.1
5
I 252.2 (3.0) Colh 153.8 (6.8) Cr
aTransition temperatures (°C) and in parentheses the transition enthalpies (kJ/mol), determined
by DSC during heating (first line) and cooling (second line) cycles. Cr = crystalline; Colh =
b
hexagonal columnar; I = isotropic liquid. Lattice parameters for the hexagonal lattice.
cOverlapped with the ColꢀColh phase transition; dNo crystallization observed at room
e
temperature (20 °C). Some extent of thermal decomposition observed, especially when paused
for a long time at this temperature
For all DLC compounds observed under a polarizing optical microscope, on cooling from the
isotropic phase a fanꢀlike texture appears which is characteristic of a hexagonal columnar phase
(Colh) (Figure 1). On slower cooling (1 °C/min) the dendritic growth typical of a Colh phase can
be easily observed. Compound 2 contains a chiral center in each terminal chain that is known to
cause a helical stacking in the LC columnar assemblies.47
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Figure 1 Polarized optical photomicrographs (100x) of fanꢀlike textures of Colh obtained on
cooling of: a) compound 2 at 100 °C; b) compound 3 at 140 °C; c) compound 5 at 170 °C; and d)
compound 5 at 142 °C.
On cooling compound 2 from an isotropic liquid, the DSC measurements showed two
overlapped transitions, indicating that it passes through a different columnar arrangement before
entering the Colh phase (Table 1). Interestingly, there was no apparent difference in the optical
texture even on bringing the compound very slowly from the isotropic liquid. Due to the very
narrow temperature ranges within which these transitions occur, they could not be properly
characterized by XRD.
Compared to compound 1, which contains straight 12 C alkyl chains, compounds 2 and 3 with
branched and shorter peripheral units showed lower isotropization temperatures, and exhibited
lower melting points. Compound 2, for instance, exhibited a columnar LC phase upon heating at
above 41 °C and compound 3 showed the Colh phase at room temperature. The lower values for
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the transition enthalpies (ꢁH = 2 – 4 kJ/mol) for the transition from the Colh to the isotropic
phase compared to the other studied compounds (ꢁH = 6 – 7 kJ/mol) provide evidence of
weakened intermolecular π−π interactions associated with the core in these molecules, which
may result in an isotropic liquid phase at lower temperatures. Due to the presence of the benzyl
groups in 5, this compound has a higher melting point (174 °C) along with a higher
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isotropization temperature (264 °C). On cooling compound 5, as observed by POM, the
transition from the Colh to the crystalline phase is accompanied by a sudden color change from
green to reddish, with no other visible changes in the LC texture (see Figures 1c and 1d). Also,
according to the POM analysis, some decomposition of compound 5 occurs when the
isotropization temperature is reached. The DSC measurements showed no reversibility in the
transition temperatures after the second scan, providing further evidence for decomposition of
the material.
To confirm the columnar LC structures, XRD measurements were performed for all
compounds (Table 1). As in the case of compound 1, reported in our previous communication,
the other compounds also exhibited diffractograms with profiles typical of a hexagonal columnar
structure. A representative XRD pattern for 5 in the Colh phase at 200 °C is shown in Figure 2.
This shows an intense peak in the smallꢀangle region at 33.0 Å, and two very weak peaks at 19.5
and 17.0 with a reciprocal dꢀspacing ratio of 1 : 31/2 : 2, being indexed as (10), (11), and (20)
reflections of a twoꢀdimensional hexagonal lattice. In the wide angle region, a diffuse peak at 4.6
Å relative to the peripheral alkoxybenzyl groups is observed. The intercolumnar distances
obtained from the XRD analysis are in agreement with the relative sizes of the molecules,
diminishing from 30.4 Å in compound 1, containing the straight alkyl chains with twelve C
atoms, to 26.3 Å, 24.1 Å and 38.1 Å in compounds 2, 3 and 5 respectively. Although the relative
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sizes are in agreement with the values found for intercolumnar distances, the measured values
are lower than the estimated molecular diameter for all compounds (43, 36, 29, and 50 Å for 1, 2,
3 and 5, respectively). This could be related to many factors as the interdigitation of the outer
chains between neighboring columns, nonlinear conformations of the chains and accommodation
of the disordered chains inside the intracolumnar spaces.48 On cooling from Colh, the
crystallization occurs with retention of the phase texture observed by POM. The XRD pattern for
5 at 100 °C gave a profile similar to that obtained in the Colh, which differ from each other in
terms of the dimension of the lattice parameter, this being larger for the lower temperature. To
confirm whether interdigitation of the outer chains between neighboring columns is crucial for
this tristriazolotriazine core to achieve a columnar LC phase, we designed a molecule containing
a hydrogen bonding group (OH group) in the end of the aliphatic chains, compound 4. Indeed,
compound 4 did not show an LC phase, although a glassy state with a fanꢀshaped texture after
two heating/cooling cycles could be observed by POM. Although the POM analysis alone was
not sufficient to fully discard the presence of a columnar phase, the DSC measurements clearly
revealed only a crystalline to isotropic phase transition, with a large transition enthalpy (around
50 kJ/mol). In addition, the XRD analysis of compound 4 showed a crystalline profile, although
some similarities with columnar packing were noted, as also seen by POM. The results for the
POM and XRD measurements imply a much more organized columnar structure in compound 4,
and we believe that this is due to the hydrogen bonding between the terminal OH groups, which
prevents interdigitation of the chains. This process may destroy the Colh formation, in
comparison with compound 1 which is of similar length and volume.
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Figure 2 XRD pattern of compound 5 in the Colh phase at 200 °C.
Optical properties. UV absorption and photoluminescence in CH2Cl2 solution, in films and in
the Colh phase, were performed and the results are summarized in the Table 2. The lowest energy
band of the tristriazolotriazine core presented maximum absorption wavelengths for compounds
1-5 in solution at around 308ꢀ311 nm. For the spinꢀcoated films the maximum wavelengths had
small blueꢀshifts compared to those in solution (305ꢀ309 nm). Very thin films (spin coated or
vacuum deposited) may shift the emission toward the blue region49.
These bands showed high molar absorption coefficients (ε ~ 20000 L molꢀ1 cmꢀ1), attributed to
the π−π* transitions in the heteroaromatic portion of the molecules. Compounds 1-5 exhibited
blue fluorescence in solution peaking at around 386 and 389 nm with good quantum yields (41ꢀ
49%). These compounds are good emitters in solid state. Spinꢀcoated thin films showed intense
blue photoluminescence, their spectra become broader compared to that in solution, which
indicates the occurrence of intermolecular electronic interactions. The appearance of shoulders in
the emission spectra at different intensities led to migration of the maximum wavelength, in most
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cases, bathochromically compared to those in solution. An exception in this regard was
compound 2 which presented a blue shift in the emission maximum wavelength of around 10
nm. Compound 5 had the largest redꢀshift in the photoluminescence maximum wavelength
(around 30 nm) from 388 nm in solution to 416 nm in solid state, with a much broader spectrum.
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Table 2 Optical data for compounds 1-5
UV/vis absorption
max/nm
photoluminescence
λ
λ
max/nm
[b]
[c]
Comp.
Solution[a]
Film
307
305
308
307
309
Solution[a]
387
Film
Colh
ΦFL
Eg/eV[h]
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1
311
391
378[d]
396
393
381[e]
396
ꢀ
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0.47
0.41
0.44
0.49
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3
4
5
311
388
3.52
310
389
3.51
311
389
397
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308
386
416f
382g
3.46
aCH2Cl2 solution (10ꢀ5 M). bTaken at a temperature just above the CrꢀColh transition. cRelative to
PBD (ΦFl=0.83).54 dShoulder appeared at 402 nm. Shoulders ahppeared at 403 and 416 nm.
e
fShoulders appeared at 377 and 478 nm. Very weak emission. Determined from absorption
g
spectra of the films.
Photoluminescence was measured in the condensed LC state. On heating the samples from
room temperature, several spectra were taken for each of the LC compounds in the crystal, liquid
crystal and isotropic phases (Figure 3a). In general, the maximum wavelength did not vary
significantly from that obtained at room temperature. We found that the emission intensity
decreased with increasing temperature for all compounds. This same behavior was found by Liao
et. al.50 and Yelamaggad et al.49 in the study of the temperatureꢀdependent emission spectra of
DLCs derived from platinum(II) complexes of pyridyl pyrazolate chelates and tris(Nꢀ
salicylideneaniline)s respectively. In fact there are only few works treating the temperatureꢀ
dependent emission of DLCs in literature.49ꢀ52 A graph of the emission intensity of the maximum
wavelength versus the temperature is shown in Figure 3b. It is known that semiconductors
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behave in such a manner, with a decrease in fluorescence intensity due to a larger quantity of
selfꢀquenching aggregates and also nonꢀradiative decay processes that take place with increasing
temperature.53 In our previous studies on the theoretical and Xꢀray structure of the
tristriazolotriazine core41 we found that the three peripheral phenyl rings are twisted with respect
to the triazine core, and thus the πꢀstacking is sterically inhibited in the solid phase. The coreꢀ
core mean distance observed in the XRD analysis in the Colh phase (around 3.5 Å) is suitable for
πꢀstacking. Thus, we believe that the transition from the solid to LC phase is accompanied by a
reorganization of the molecule to acquire a more planar conformation, in which the
intermolecular πꢀstacking is likely to quench the emission. The quenching of fluorescence by
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intermolecular πꢀstacking may be occurring together with nonꢀradiative decay processes, due to
an increase at a higher temperature in segment and molecular mobility and consequently the
increased possibility of the excited state to relax via internal conversion or transfer of the energy
by collision.
For 1 and 2, a significant decrease in the emission intensity is evident when the samples pass
from the solid to Colh phase. Also, there were small changes in the spectra profiles before and
after the CrꢀColh transition. For compound 2 redꢀshifted shoulders at 403 and 415 nm became
evident above 45 °C, whose can be related to the chirality present in this compound and and
specific molecular interactions that leads aggregation of molecules when the compound enters
the mesophase. Compound 3 is in a Colh phase at room temperature and, interestingly, heating up
to 60 °C did not lead to any significant loss in the emission intensity, which started to decrease
linearly with heating above this temperature. The branched alkyl chains in 3, prevented the loss
of the emission intensity even up to relatively high temperatures. This may be due to a more
hindered packing of these molecules, avoiding the presence of a larger quantity of selfꢀquenching
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aggregates at a certain temperature. Compound 3 presented a less structured spectra with a 10 nm
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Figure 3 a) Emission spectra of compounds 1-3 recorded during heating from solid to Colh and
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to isotropic phase. b) Plot of the emission intensity of the maximum wavelength versus the
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temperature for samples of 1-3. Results for compound 5 can be found in the SI.
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At higher temperatures, with the isotropization of the columns, the drop in the emission
intensities became less intense and showed a tendency to stabilize at a minimum value. With the
exception of compound 5, which demonstrated thermal decomposition in this study, all
compounds showed the same profile in the cooling process from the isotropic to the solid phase
(the emission increasing with decreasing temperature).
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On the basis of the helical model proposed for the columnar liquid crystals,44 we measured the
circular dichroism (CD) in the mesophase for compounds 1 and 2 (Figure 4).The aim was to
determine whether the chirality of the chiral alkyl chains is transmitted to the supramolecular
organization of the mesophase and whether this may be related to a helical disposition of the
molecules along the column.
The CD measurements were performed on films prepared on untreated quartz slides, heated
until the isotropic phase and slowly cooled. The spectra are the result of the averaging of six CD
spectra recorded at different film orientations obtained by rotating (inꢀplane) the sample cells by
60° relative to the light beam. The spectra found were almost identical for all orientations of the
sample, which excludes the possibility of linear dichroism effects due to macroscopic
orientation.55 As expected, compound 1 was not active in terms of CD because of the absence of
chirality in the structure. Compounds 3, 4 and 5 also showed no signals on the CD spectra.
The CD spectra for compound 2 displayed an intense signal (for tristriazolotriazine λmax
=
327nm), which is in agreement with the helical disposition of the chromophores56 in the liquid
crystalline phase. Moreover, compound 2 presented significant CD even before the thermal
treatment, due to the wide range of low temperatures associated with the mesophase. The
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relationship between the appearance of CD bands and the formation of chiral superstructures
whose sign is related by the configuration of the stereogenic centers in the alkoxy chain is clearly
confirmed by the CD spectra profile.
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The optical value for the energy gaps (Eg), between HOMO and LUMO, were calculated from
film absorption spectra and found to be as high as 3.5 eV for the studied compounds (Table 2).
Cyclic voltammetry of the DLC compounds showed only a nonꢀreversible oxidation wave,
peaking at between 1.6 and 1.9 V (vs. Ag/AgCl). The results of the electrochemical study
suggested that the tristriazolotriazine core potentially has electronꢀtransporting characteristics.
Conductivity Measurements in the Colh Phase. The electrical characterization of columnar
liquid crystals is essential to evaluate their applications as charge or ion conductors. The a.c.
impedance spectroscopy is a useful technique that provides such information by applying an
alternate voltage with a frequency that ranges from MHz to mHz. The results are usually
interpreted in terms of equivalent circuits consisting of electrical components as resistors,
capacitors and inductors57. Therefore, impedance measurements were performed in order to
obtain information on the conductivity of these compounds in the LC state. Figure 5 shows the
complex impedance graphs (Z’’ vs. Z’) and the POM textures of compounds 1 and 3 in the Colh
phase. The plots show the wellꢀdefined semiꢀcircle which is characteristic of a circuit consisting
of a resistor and a capacitor in parallel, where R is interpreted as the bulk resistance, and the
inclined lines at lower frequencies indicate the existence of interfacial effects at the electrodes.
All the investigated compounds showed the same behavior. From the R values the conductivity
could be calculated using σ = d/(R.A), where d is given by the thickness of the spacers, which
provides the distance between the electrodes, and A is their surface area (in all cells this was
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around 2.4 cm2). The values obtained for the calculated conductivity σ in the Colh phase are
5.8x10ꢀ8, 1.4x10ꢀ8, 2.1x10ꢀ8 and 1.0x10ꢀ9 S.mꢀ1 for compounds 1, 2, 3 and 5, respectively.
A careful analysis of the capacitance curves as a function of the frequency, where we observed
a huge increasing of the real capacitance (about three orders of magnitude, see SI) at the low
frequency regime, suggested that the obtained conductivity is mostly ionic dominated. In this
case, the straight line after the semicircle is related to the double layer capacity due to the ions at
the interface of the electrodes.58 The low values obtained here are probably related to relatively
high radii of the ionic species. Chemical ionic impurities, resulting from the synthesis
procedures, are usually present in liquid crystalline semiconductors, where the ionic transport
can be dominant for a concentration of ions over a few ppm.59 Our values are about four orders
of magnitude lower than values in the literature for columnar ionic liquids29, where the
incorporation of lithium salts in those columnar materials enhanced their ionic conductivity
measured along the columnar direction. The POM textures (Figure 5) show that the investigated
cells of 1 and 3 after annealing presented planar domains, indicating that the homeotropic
alignment was not so efficient. This means that our compounds may find applications as ionic
conductors since the conducting properties can be optimized by the addition of proper salts and
alignment of the columns.
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Figure 4 (a) CD spectra recorded on thin films of compounds 1 (dashed line) at 110°C and 2
(solid line) at 60°C, and respective (b) absorbance spectra.
Morphology aspects of thin films. We used Atomic Force Microscopy (AFM) to investigate
the morphology aspects of spinꢀcoated films of the LC compounds (1, 2, 3 and 5). From the
measurements we obtained the mean surface roughness and the film thickness. Electronic
applications require homogeneous films with low surface roughness. Figure 6 shows
representative images of compound 5, where a homogenous covering of the surface was
observed, even for images as large as 50 ꢀm x 50 ꢀm. The same behavior was observed for the
other compounds investigated. The values obtained for the mean surface roughness (RMS) and
the thickness are presented in Table 3 for all compounds, where it is clear that, under the same
experimental conditions, 1 displays the highest roughness and 3 the highest thickness. The higher
roughness of 1 may be related to a more crystalline profile, due to its straight and less bulky
peripheral alkyl chain. Compounds 2 and 3, with branched alkyl groups, and 5 with bulky
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alkoxybenzyl peripheral groups are more amorphous materials and present smoother surfaces,
particularly 3 and 5. Compound 3, a room temperature LC, displays the greatest thickness. It is
worth to noting that the roughness values indicate that these surfaces are quite smooth and
appropriate for electronic purposes. In addition, the thickness can be reduced by increasing the
spinning velocity or by decreasing the concentration of the deposited species.
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Figure. 5 Left side: (a) Impedance Z’’ vs. Z’ plots of compounds 1 and 3 captured in their Colh
phases. R indicates the resistance values used for the conductivity calculations. Right side: POM
images with 100x amplification of the cells used for impedance measurements, containing
compounds 1 (b) and 3 (c) in the Colh phase after annealing. The images indicate the presence of
planar domains.
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Figure 6 AFM images of compound 5. Image size of (a) 50 ꢀm x 50 ꢀm and (b) 15 ꢀm x 15 ꢀm
in a 3D perspective.
Table 3. Mean surface roughness (RMS) and thickness of the spinꢀcoated films.
Compounds
RMS (nm)
1
2
3
5
4.3
1.5
0.6
0.7
Thickness (nm)
168±10
150±10
240±10
165±10
CONCLUSIONS
We have synthesized and characterized a series of luminescent columnar liquid crystalline
compounds based on a tris[1,2,4]triazolo[1,3,5]triazine core. On the basis of the results obtained,
these materials offer an easily synthesized alternative to other Nꢀheterocycle cores based
columnar LC compounds with the advantage of possessing a low temperature stable Colh phase
and the capacity to form a helical architecture, when containing chiral peripheral chains. They
also present an intense blue luminescence, being characterized as high energy gap (~3.5 eV) blue
emitter materials. The electronꢀaccepting capability of the tristriazolotriazine core, confirmed by
voltammetry analysis, along with the formation of an efficient πꢀbonding interaction of the
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heteroaromatic and aromatic portions in the columnar phase, detected by optical and XRD
analysis, makes these materials good candidates for application in optoelectronic devices, such as
organic lightꢀemitting diodes, field effect transistors, and photovoltaic cells.
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ACKNOWLEDGMENT
We thank the following institutions for financial support: CAPES/PNPD,
CNPq/PRONEX/FAPESC, INCT/INEO and INCTꢀCatálise. The XRD experiments were carried
out in the Laboratório de Difração de RaiosꢀX (LDRXꢀCFM/UFSC). We also thank Dr.
Fernando Ely (CTI, CampinasꢀSP, Brazil) for the cells used in the conductivity experiments, Dr.
José Luis Serrano and Dr. Teresa Sierra (Universidad de ZaragozaꢀCSIC) for the free access to
the CD instrument, and Dr. Hernán Terenzi for free access to MALDIꢀTOF and micrOTOFꢀQ II
APCIꢀQqꢀTOF instruments at CEBIMEꢀUFSC.
AUTHOR INFORMATION
*Email: hugo.gallardo@ufsc.br; fax: +55 48 37216850; tel: +55 48 37219544
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