Fan-shaped and toric textures of mesomorphic oxadiazoles
Fan-shaped and toric textures of mesomorphic oxadiazoles
A. Sparavigna
, A. Mello
,  and B. Montrucchio
 Dipartimento di Fisica, Politecnico di Torino
 Dipartimento di Automatica ed Informatica, Politecnico di Torino
C.so Duca degli Abruzzi 24, Torino, Italy
Keywords: Liquid crystals, phase transitions
Author for correspondence: A. Sparavigna, amelia.sparavigna@polito.it
Abstract
When a family of non symmetrical heterocycled compounds is investigated, a variety of
mesophases can be observed with rather different features. Here we report the  behaviour of seven
different members among a family of such materials, that consists of  mesomorphic oxadiazole
compounds. In two of these compounds, the optical microscope investigation shows very
interesting behaviours. In their smectic phases, fan-shaped and toric textures, sometimes with
periodic instability, are observed. Moreover,  the nematic phase displays a texture transition.
Texture transitions have been previously observed only inside the nematic phase of some
compounds belonging to the families of the oxybenzoic and cyclohexane acids. In these two
oxadiazole compounds  we can observe what we  define as a "toric nematic phase", heating the
samples from the smectic phase. The toric nematic texture disappears as the sample is further
heated, changing into a smooth texture.
Introduction
The correlation between the existence of mesophases and their features and the chemical structure
of molecules is the core of the research in liquid crystals. As we have seen studying some
alkyloxybenzoic acids,  an increased length of the alkyl chain produces a strong increase of the
nematic mesophase range [1,2]. These acids are materials where the mesogenic unit structure is a
symmetrical hydrogen-bonded dimer. A simple binary mixture of two members of the same group
of acids is able to enlarge the smectic range [3]. When a family of non symmetrical structures is
under investigation, the variety of results is strongly enhanced. Here we discuss the mesomorphic
behaviour of some members among a family of non symmetrical compounds, consisting of
mesomorphic oxadiazole compounds [4-6]. These materials  have been developed because their
molecular structure with heterocycles as substituents gives smectic and nematic mesophases. It was
observed in particular, that not only the chemical structure of the substituents, but also their
position with respect to the oxadiazolic ring of the molecule is relevant for the mesophases.
The optical microscope investigations, which we are discussing in this paper, show very interesting
behaviour of the smectic and the nematic phase of some of these oxadiazole compounds. In the
smectic phases, fan-shaped and toric textures, with periodic instability in the toric structure, are
observed. Moreover, it is also remarkable the behaviour of the nematic phase, displaying a texture
transition. These transitions have been previously observed inside the nematic phase of some
mesomorphic thermotropic materials belonging to the families of the alkyloxybenzoic and
cyclohexane acids [7-13]. It is  a more or less abrupt change, driven by the temperature, in the
nematic texture seen by means of the polarised light microscope. In some cases, it is also possible
to observe the transition in the calorimetric investigations and dielectric spectroscopy [8,14]. In
other cases, it is necessary an image processing aided procedure to enhance the observation of the
transition [1,9].
Alkyloxybenzoic and cyclohexane acids are mesogenic because the mesogenic units are dimers.
The more common explanation for the presence of a texture transition in the nematic range is based
on the existence of cybotactic clusters of dimers, favouring a local smectic order in the nematic
range [15,16]. Our observation of a texture transition inside the nematic phase of two oxadiazole
compounds suggests a possible existence of cybotactic clusters in these materials too. We start with
a short description of the oxadiazole family used for investigations and then the more deep
discussion of the smectic phases and of the texture transitions.
The oxadiazole compounds.
The members of the oxadiazole family we are now discussing are those whose structures are
reported in Fig.1. These compounds have a non symmetrical heterocycled structure. They have
been previously studied with the DSC differential scanning calorimetry [5].  As it is possible  to see
in Table 1, which reports the transition temperatures, the molecular structure is strongly influencing
the mesophase ranges. Not only the presence of benzene or cyclohexane rings, but also their
position, with respect to the thiadiazolic or imidazolic part of the molecule, is important for the
existence of mesophases.
All the samples were inserted in the  cell when the material was in the isotropic phase. The walls of
liquid crystal cells are untreated clean glass surfaces. No treaments were done to favor planar or
homeotropic alignments. The liquid crystal cells are heated and cooled in a thermostage and
textures observed with a polarized light microscope. Let us start the discussion from compound A
of Figure 1: it has only the smectic phase.
Compound A
It is an interesting material, that displays at room temperature the texture shown in the upper part of
Figure 2. In Table I, we guess the phase at room temperature  to be a crystal phase, or  a rigid
smectic phase. On heating, the sample changes into a smectic phase with a fan-shaped texture at the
temperature  of 140 °C. The  material becomes an isotropic liquid at the temperature of 210°C. The
material does not show a nematic phase.  On cooling the sample has a more complex behaviour.
First of all, beautiful bâtonnets appears cooling from the isotropic liquid, with periodic instabilities
insides. Then larger domains grow, and the texture changes very fast in the image frame as in a
scene played by moving fans. The smectic phase has a strong hysteresis on cooling, because the
fan-shaped texture is observed till down 110°C. Then the sample probably becomes a crystal or a
rigid smectic F.  The transition is shown in Fig.3. A further reduction of the temperature gives the
texture in Figure 2. In this rigid phase observed with crossed polarisers, the crystal shows thick
dark lines  among the lamellar structure. These lines are cracks as it is possible to see when they
arise as the sample is  cooled down from the smectic phase.
Compound B
On heating, we  observed the following temperature transitions: 156°C from crystal to smectic,
172°C from smectic to nematic and 200°C from the nematic to the isotropic phase. On cooling, we
have the transition from the isotropic liquid to the nematic phase at 198°C and the transition into
the smectic phase at 168°C. The crystal phase appears at 145°C. This compound possesses a
nematic phase too. The smectic phase has not only the fan-shaped texture  but also regions with
macroscopic  toric arrangement of the  smectic planes. The toric  domains are  focal conic domains
where the ellipse is degenerated in a circle and the hyperbola is degenerated in a straight line [17].
If we look at the sequence in Figure 4, the nematic, the smectic and the crystal phases have textures
where the same overall  arrangement of defects is unchanged on cooling. It could be a memory
effect of the surface layers of the material, close to the  cell walls.
Compound C
The sample has a crystal phase till 94°C. Then a smectic phase till 172°C. We observed the clearing
point at 233°C. On cooling, the nematic phase appears at 232°C, the smectic phase at 163°C, and
the crystal at 65°C. What is remarkable in this compound is  a texture transition inside the nematic
phase. We can observe what we  define as a "toric nematic phase" on heating till 195-200°C, and
then a smooth nematic texture till the clearing point. On cooling, the toric phase appears around
200°C. This  behaviour of the nematic phase is observed also in the compound D, that we will
describe in the next section.
The smectic phase of sample C has a fan shaped and toric smectic phase. The Figure 5 shows in its
upper part, the arrangement of the toric defects in the smectic phase. Sometimes on cooling the
smectic phase, it happens that the toric texture displays inside an undulation instability (lower part
of Fig.5). A recent theory of the lamellar phases could be invoked to explain the appearance of
undulations in the planes of the lamellar phases [18,19].
Let us discuss more deeply the nematic phase. On heating, the nematic phase substitutes the
smectic phase, the transition is shown in Fig.6.  The nematic phase in the figure has a texture that
shows a "toric" arrangement of the defects, and this is why we call this nematic phase a "toric
nematic phase" . The texture remembers the shape of  domains in the smectic phase. On heating,
this nematic texture disappears between 195-200°C, depending on the position in the cell observed
at microscope, and a smooth texture is displayed by the nematic.  This is  the texture that is
observed on cooling from the isotropic phase. On cooling, around 200°C, the toric texture starts to
grow in the cell as shown in the Figure 7.
It is remarkable that a compound, rather different from the dimeric acids [1,2], has a texture
transition in the nematic phase. If we assume as a reasonable explanation for the existence of a
texture transition,  the presence in the nematic melt of cybotactic clusters with local smectic order,
we can invoke the presence of such clusters also in this material. The local smectic order is
responsible for the toric nematic texture.
Compound D
The sample has a crystal phase that changes in another crystal phase at 82°C. The crystal becomes
to be smectic at 103°C. The smectic phase persists till 110°C. We observed the clearing point at
232°C. On cooling, the nematic phase appears at 230°C, the smectic phase at 97°C, and the crystal
at 71°C. This compound, as the previous one, has  a texture transition inside the nematic phase. We
can observe the toric nematic phase on heating till 150°C, and then a nematic texture till the
clearing point. On cooling, the toric phase appears around 150°C. The Figure 8 shows a beautiful
toric nematic phase, with the bent structure of the defects. The bent contours between the nematic
domains are clearly visible in this figure.
In fact, samples C and D are similar in the behaviour of the smectic and nematic phases. They have
a very slight change in the structure of the molecule, as we can see in the Figure 1, with respect of
the oxadiazolic group.  The different position changes the transition temperatures.
Compounds E,F and G
These three compounds have only the nematic mesophase. In the nematic range there is no
evidence of a texture transition. Sample E has a transition from a crystal phase in the nematic phase
at 46°C, on heating. The clearing point is at 109°C. On cooling, the nematic phase appears at
107°C. The nematic phase remains till the room temperature and just after a very long relaxation
time, the crystal is formed. The nematic phase has always a visible thermal flickering of the texture.
The phase sequence for sample F is: a crystal phase till 72°C that changes into a nematic phase till
99°C. On cooling, the nematic starts from 98°C with a very fast transition. The sample remains
nematic till 50°C. For sample G, we observed a transition from crystal to nematic at 80°C, and a
short nematic range till 87°C. On cooling, the nematic range extends till 51°C. This behaviour, a
wide nematic range on cooling, is common to these three materials and confirmed by previous DSC
measurements.
Discussion.
As already observed in some members of the alkyloxybenzoic acid family (6-,7-,8-,9OBAC), in the
binary mixtures of 6OBAC with other members of this family and in a cyclohexane acid (C6), the
nematic phase exhibits a different order at low and at high temperatures. This different order is
observed with the optical microscopy because the cell has two different textures in the nematic
range. For the previously mentioned compounds, the low temperature nematic texture is composed
of very small domains and looks like a granular structure. If the sample has a smectic phase, the
texture of the smectic phase is also showing a granular texture.
In the case of the oxadiazole compounds C and D, we observe a texture transition in the nematic
phase driven by the temperature, with a low temperature subphase textured with bent-shaped
contours between domains. We defined this texture  as a toric texture, because a further cooling of
the sample gives a smectic phase with toric domains. This is consistent with the hypothesis that a
concentration of cybotactic clusters is at the origin of the texture transition in the nematic phase.
Let us remember that the cybotactic clusters  are groups of molecules with a local smectic phase;
they can be imagined as smectic seeds in the nematic fluid.
In a previous paper, we proposed a model to explain the origin of  different textures in the nematic
phase [1]. Let us imagine a thin liquid crystal cell in the smectic phase, with defects in the smectic
structure. Rising the temperature, the material phase changes into a nematic phase, the smectic
planes disappear but, in some places of the cell, these planes  remain as little clusters, anchored at
the cell walls. We have then these cybotactic clusters preserving a local smectic order. Moreover,
defects on the walls can contribute to the persistence of these clusters. When the temperature
further increases, the smectic order in cybotactic clusters is suppressed and the texture changes,
more or less abruptly, in a smooth texture. The persistence of a local smectic order at the walls
explains the memory effect observed in the cells of compounds C and D, if we suppose that the
mechanism in the formation of the textures in these compounds is the same as in the
alkyloxybenzoic materials.
The memory effect is also displayed in the cells prepared with compounds A and B, the materials
with a more "smectic" behaviour. Samples E,F and G have  only the nematic phase that possesses,
on cooling, a wider range compared with that observed on heating.  No memory effect is seen. The
presence of cybotactic clusters can be excluded in  E,F and G compounds.   Or, it is better to tell in
other words, cybotactic clusters do not grow with the decrease in the temperature, consistent with
the fact that the medium does not support a smectic phase. This can be due to a kind of frustration
developed in the medium, wherein  a phase transition into layered structure is not possible due to
the molecular structure. As it is shown in the molecular scheme of Fig.1, the responsible can be  the
position of the Cl atom.
All the results reported in this paper, on the smectic and nematic phases, confirm that the family of
oxadiazoles is very promising for further researches in the investigation of  texture transitions, and
the relationship between the molecular structure and the features of the mesophases.  Let us
remember once more that  two of these compounds shows a texture transition in the nematic range.
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Compound Transition temperatures on heating
A K-140°C-Sm-210°C-I
B K-156°C-Sm-172°C-N-200°C-I
C K-94°C-Sm-172°C-N-233°C-I
D K-103°C-Sm-108°C-N-232°C-I
E K-46°C-N-109°C-I
F K-72°C-N-99°C-I
G K-80°C-N-87°C-I
TABLE I: Transition temperatures of the compounds on heating. The first compound (A) has only the
smectic mesophase. E,F and G possess only a nematic mesophase.
Figure captions
Figure 1: Molecular structures of the seven oxadiazole compounds under investigation.
Figure 2: The crystal texture of  compound A (upper part). Note the thick dark lines  among the
lamellar structure. These are cracks appearing on cooling from the smectic phase. In its lower part,
the figure  shows the bâtonnets  growing in the isotropic melt on cooling (image dimensions
0.38mm x 0.5mm).
Figure 3: The sequence shows the cooling of  compound A from the smectic fan-shaped texture in
the lamellar texture of the crystal phase (image dimensions 0.38mm x 0.5 mm).
Figure 4: The sequence shows the cooling of compound B. In the upper part the nematic, in the
middle the smectic and, in the lower part of the figure, the crystal phase .
Figure 5: The smectic phase of compound C. Note the toric focal conic domains. In the lower part
of the figure, domains with undulations inside.
Figure 6: The transition from the smectic into the nematic phase of compound C: note the toric
texture of the nematic phase.
Figure 7:  The texture transition for sample C, on cooling. Note the growth of the low temperature
nematic texture with the bent-shaped lines.
Figure 8: The toric nematic phase as it appears when heating the smectic phase of compound D.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
