Monotropic or enantiotropic mesophases? Liquid-crystalline and solid state polymorphism 4-Chloro-1,3-phenylene bis-[4-(4-alkyloxyphenylazo)benzoates

Article abstract of DOI:10.1016/j.tca.2014.04.024

This article presents a homologous series of bent-core molecules consisting of a 4-chlororesorcinol as a central unit and 4-[(4-alkyloxyphenyl)diazenyl] benzoic acids as wings. With the use of polarizing optical microscopy (POM), thermal analysis (TOA) and differential scanning calorimetry (DSC) the mesogenic properties were detected. Furthermore, the X-ray diffraction (XRD) measurements were also performed for selected samples. Compounds of the above mentioned series form nematic, B₆, and smectic C mesophases. An interesting phenomenon in the solid phase polymorphism was observed in some cases. In our experiment one crystalline form (CrI) melts to nematic phase while the second form (CrII) melts at higher temperature directly to isotropic phase. The solid state modification determines whether a monotropic and an enantiotropic mesophase is observed.

Full text of DOI:10.1016/j.tca.2014.04.024

Thermochimica Acta 587 (2014) 59–66
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Monotropic or enantiotropic mesophases? Liquid-crystalline and
solid state polymorphism 4-Chloro-1,3-phenylene
bis-[4-(4-alkyloxyphenylazo)benzoates
Izabela Niezgodaa, Damian Pociechab, Zbigniew Galewskia,∗
a Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14, Wroclaw 50-383, Poland
b Faculty of Chemistry, Warsaw University, Zwirki i Wigury 101, Warsaw 02-089, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
This article presents a homologous series of bent-core molecules consisting of a 4-chlororesorcinol
as a central unit and 4-[(4-alkyloxyphenyl)diazenyl]benzoic acids as wings. With the use of polariz-
ing optical microscopy (POM), thermal analysis (TOA) and differential scanning calorimetry (DSC) the
mesogenic properties were detected. Furthermore, the X-ray diffraction (XRD) measurements were also
performed for selected samples. Compounds of the above mentioned series form nematic, B6, and smec-
tic C mesophases. An interesting phenomenon in the solid phase polymorphism was observed in some
cases. In our experiment one crystalline form (CrI) melts to nematic phase while the second form (CrII)
melts at higher temperature directly to isotropic phase. The solid state modification determines whether
a monotropic and an enantiotropic mesophase is observed.
Received 17 February 2014
Received in revised form 8 April 2014
Accepted 22 April 2014
Available online 2 May 2014
Keywords:
Bent-core liquid crystals
Nematic phase
Azobenzene
Smectic C
© 2014 Elsevier B.V. All rights reserved.
B6 phase
1. Introduction
New bent-core molecules are created in order to achieve a rich
liquid-crystalline polymorphism as well as interesting optical and
Azo compounds constitute a large branch of the liquid crystals
sciences. In addition to a variety of possible applications, azo dyes
have very interesting physical, chemical, and liquid-crystalline
properties, which were investigated for many years, [1,2] also at our
university [3–9]. Further studies and the ability to control isomer-
ization process make this group a very interesting research object
[10]. The sensitivity to light allows azo compounds to be used as
components of advanced polymers that form a broad group of
photo-sensitive smart materials [11,12]; as well as they can be
applied as a holographic grid in modern optoelectronics [13,14].
Furthermore, the above described compounds can also provide a
good linking unit in novel bent-core molecules.
The first banana-shaped compounds were synthesized in 1929
[15,16]. However, the real development of research on bent-core
liquid crystals occurred only in 1997 after Niori and his co-
workers pioneered publication on the polar properties of such type
molecules [17,18]. Interesting features and possible applications
[19,20] allowed banana-shaped compounds to be synthesized and
studied extensively in many research centers.
physicochemical properties. As the central unit of banana-shaped
mesogens many substances were applied including resorcinol and
its mono- and polysubstituted derivatives [21]. Schematic structure
composed of a core and wings allows the synthesis of a large num-
ber of compounds using a variety of linking and lateral groups. The
most common linking group is an ester linkage [22–24]. Moreover,
commonly used are also Schiff bases [25–28] and in recent years
azobenzenes [29,30]. Also the effects of substituent in the aromatic
rings of the wins were also extensively researched [31,32].
The goal of our study, was the synthesis and investigation of
mesogenic properties of five ring bent-core compounds based on
the 4-chlororesorcinol and the azo group included in the 4-[(4-
alkyloxyphenyl)diazenyl]benzoic acids. In addition, we wanted to
determine the influence of the length of alkyloxy chain on the
liquid-crystalline properties of our substances. Five members of
this series (Res-8, Res-10, Res-12, Res-14 and Res-16) were recently
described by Tschierske’s group [33].
2. Experimental
2.1. Measurements setup
Chemical structure of synthesized compounds was checked by
two methods: elemental analysis (EA) and nuclear magnetic res-
onance spectroscopy (NMR). EA measurements were carried out
∗
Corresponding author.
E-mail address: zbigniew.galewski@chem.uni.wroc.pl (Z. Galewski).
http://dx.doi.org/10.1016/j.tca.2014.04.024
0040-6031/© 2014 Elsevier B.V. All rights reserved.

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I. Niezgoda et al. / Thermochimica Acta 587 (2014) 59–66
with the Vario EL III elemental analyzer. All NMR spectra were
measured with the Bruker Avance III 500 MHz high resolution spec-
trometer with the use of deuterated chloroform (CDCl3) as the
solvent.
For mesogenic properties investigation we used polarized opti-
cal microscopy (POM), thermo-optical analysis (TOA) [34] and
differential scanning microscopy (DSC). All calorimetric measure-
ments were made using the Perkin Elmer 8500 calorimeter in
nitrogen atmosphere. Scan rate amounted to 10 K min −1
.
Textures and liquid-crystalline phase transitions were observed
by the Olympus polarized light microscope BX61-P (TRF). Addi-
tional elements of the thermo-optical analysis (TOA) are: the
Linkam heating stage and the Motic digital camera. For selected
samples XRD studies were conducted in reflection mode. XRD wide
angle measurements of the samples, which were prepared as drops
on heating plate, were performed with the Bruker D8 GADDS sys-
tem with an additional LINKAM heating stage.
2.2. Synthesis
Main root of synthesis of 4-Chloro-1,3-phenylene bis-[4-
(4-alkyloxyphenylazo)benzoates] is shown in Fig. 1. The most
important of the above mentioned synthesis is diazotization reac-
tion, which we also performed, this reaction is quite popular
and frequently used to create a double chemical bond between
the two nitrogen atoms [33–41]. Ethyl 4-aminobenzoate, 4-
chlororesorcinol and other inorganic substrates were obtained
from commercial sources and used as obtained. Progress of the
reaction was checked using thin layer chromatography (TLC) with
aluminum plates covered with silica gel and dichloromethane as
the eluent.
2.2.1. Synthesis of ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate
(2):
Ethyl 4-aminobenzoate (1) (50 g, 0.303 mol) was dissolved in
Fig.
1. Main
root
of
synthesis
4-Chloro-1,3-phenylene
bis-[4-(4-
1 M aqueous HCl and kept in the ice bath at 0–5 ◦C. NaNO2 (20.9 g,
alkyloxyphenylazo)benzoate].
Fig. 2. Textures of Res-2 seen by polarizing microscopy. (A) Nematic texture. (B) B6 mesophase texture. (C) Crystallization of CrI from B6 phase during cooling. (D)
Crystallization process of CrII from nematic phase during heating.

I. Niezgoda et al. / Thermochimica Acta 587 (2014) 59–66
61
carbonate was filtered off and dichloromethane was evaporated.
The crude product was purified using flash chromatography (silica
gel Fluka 60 mash and dichloromethane) and recrystallized from
hexane. Yield: 54%. 1H NMR (500 MHz, CDCl3) ␦ 8.20 (d, J = 8.7 Hz,
2H), 7.95 (d, J = 8.9 Hz, 4H), 7.04 (d, J = 9.0 Hz, 2H), 4.44 (q, J = 7.1 Hz,
2H), 4.16 (q, J = 7.0 Hz, 2H), 1.47 (t, J 7.1 Hz, 6H). Other 4-[(4-
alkyloxyphenyl)diazenyl]benzoates were synthesized according to
procedure described above.
2.2.3. Synthesis of 4-[(4-ethoxyphenyl)diazenyl]benzoic acid (4)
Compound (3) was dissolved in EtOH and NaOH (1 g, 0.025 mol)
was added. Mixture was refluxed for 24 h and after cooling to room
temperature was acidified with diluted HCl. After vacuum filtration
the crude product was twice recrystallized from acetic acid. Yield:
82%. 1H NMR (500 MHz, CDCl3) ␦ 8.19 (d, J = 8.6 Hz, 2H), 7.97 (d, J
=8.9 Hz, 4H), 7.04 (d, J = 9.0 Hz, 2H), 4.17 (q, J = 7.0 Hz, 2H), 1.50 (t,
J = 7.0 Hz, 3H). Other 4-[(4-alkyloxyphenyl)diazenyl]benzoic acids
were synthesized according to procedure described above.
Fig. 3. Schematic diagram of G(T) for Res-2.
0.303 mol), dissolved in a minimum amount of water, which was
added dropwise to the ethyl 4-aminobenzoate and HCl mixture
to produce a diazonium salt solution. Sodium carbonate (53.0 g
0.500 mol) and phenol (28.5 g 0.303 mol) were dissolved in 200 ml
water cooled to less than 5 ◦C and then earlier prepared, cooled dia-
zonium salt solution was added dropwise. Resulting mixture was
stirred for 3 h at 0–5 ◦C and next neutralized with diluted HCl result-
ing with yellow crude product. Then it was filtered off and air-dried.
Yield: 70.9 g (88%). 1H NMR (500 MHz, CDCl3) ␦ 8.20 (d, 2H), 7.93
(d, 4H), 7.00 (d, 2H), 4.45 (q, 2H), 1.45 (t, 3H).
2.2.4. Synthesis of 4-Chloro-1,3-phenylene
bis-[4-(4-alkyloxyphenylazo)benzoates] (5)
The 4-chlororesorcinol (0.08 g, 0.59 mmol), appropriate 4-
[(4-alkyloxyphenyl)diazenyl]benzoic
acid
(4)
(1.18 mmol),
N,Nꢀ-Dicyclohexylcarbodiimide (DCC) (0.24 g, 1.18 mmol) and,
a pinch of 4-Dimethylaminopyridine (DMAP) were dissolved in
dichloromethane and stirred at room temperature for about 24 h.
After evaporation of solvent the orange precipitate was purified
with the use of gel column chromatography with dichloromethane
as the eluent. Solvent was removed using evaporator and recrys-
tallized from hexane. Yield: 27%–91%. Res-2: 1H NMR (500 MHz,
CDCl3) ␦ 8.38 (d, J = 8.7 Hz, 2H), 8.33 (d, J = 8.7 Hz, 2H), 8.03–7.97
(m, 8H), 7.58 (d, J = 8.8 Hz, 1H), 7.36 (d, J = 2.6 Hz, 1H), 7.22 (dd,
J = 8.8, 2.7 Hz, 1H), 7.04 (dd, J = 9.0, 1.2 Hz, 4H), 4.16 (q, J = 7.0 Hz,
4H), 1.48 (dt, J = 6.9, 3.4 Hz, 6H). Elemental analysis: calc.: N 8.63%,
C 66.61%, H 4.50%, Cl 5.46%, O 14.79%. Found: N 8.52% (ꢀ 0.11), C
66.44% (ꢀ 0.17), H 4.48% (ꢀ 0.02).
2.2.2. Synthesis of ethyl 4-[(4-ethoxyphenyl)diazenyl]benzoate
(3)
Compound (2) (10 g, 0.037 mol), ethyl bromide (16.2 g,
0.09 mol), K2CO3 (2.5 g) were all dissolved in 200 ml of acetone.
The mixture was refluxed for 24 h. After evaporation of acetone
the achieved mixture was dissolved in dichloromethane. Potassium
Fig. 4. Textures of 4-Chloro-1,3-phenylene bis-[4-(4-propyloxyphenylazo)benzoate]. (A) Texture of B6 mesophase. (B) Crystals CrI. (C) Crystallization of CrII from isotropic
phase seen under polarizing microscope during heating. (D) Crystals of CrII.

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I. Niezgoda et al. / Thermochimica Acta 587 (2014) 59–66
Fig. 5. Various textures of banana mesophase B6 observed in 4-chlororesorcinol derivatives. (A) Focal-conic fan shaped texture of Res-2. (B) Fan shaped texture of Res-4. (C)
The appearing of battones and coalescales from the isotropic of compound Res-8.
All NMR and elemental analysis data of other Res-n compounds
are included in the supplementary data.
3. Results and discussion
The
4-Chloro-1,3-phenylene
have
bis-[4-(4-
interesting
alkyloxyphenylazo)benzoates]
very
liquid-crystalline properties. During our investigation of these
compounds we found 4 mesophases: nematic, smectic C, banana
mesophase B6 and unknown phase X.
Even the shortest homologue of this series, Res-2, is meso-
morphic. Despite the relatively high melting point (179.5 ◦C) we
observed both enantiotropic nematic (Fig. 2A) and monotropic B6
phase, which forms characteristic focal-conic fan-shaped texture.
In the cooling mode we observed under a polarizing microscope
phase transition from the isotropic phase to a nematic, and then
to the banana phase and then crystallization results. This phe-
nomenon is a very complex process.
Obtained crystalline form (Fig. 2C) named CrI was formed during
cooling with scan rate of 10 K min−1. This sample was next heated
at the same rate. Then we saw the sample melt to the nematic phase
and next changed to the isotropic phase. While keeping the system
for some time in temperature range of a nematic phase we observed
crystallization process of other crystalline form, CrII (Fig. 2B), which
is characterized by higher melting point (193.5 ◦C) than tempera-
ture of Iso-N phase transition. As a consequence, CrII during heating
does not show liquid-crystalline properties. We also have cooled
our sample using scan rate of 5 K min−1. Then we noticed nematic
and B6 mesophases, crystallization of CrI form and then slow grow-
ing of crystals CrII. To this behaviour we can give thermodynamic
explanation proposed by Gibbs’ function depending upon the tem-
perature (shown in Fig. 3). Supercooling of the system during the
measurements at low speed causes that form CrII is thermodynami-
cally more stable above a certain temperature Tꢀ. Slow cooling of
the sample favors the formation of crystals CrII (above Tꢀ). In fast
cooling mode we can cool the sample below Tꢀ temperature with-
out the transformation to CrII. So CrI in this temperature range is
more stable. It means that the chemical potential of CrI (ꢁCrI) is less
than the chemical potential of CrII (ꢁCrII) ꢁCrI < ꢁCrII
Fig. 6. The XRD measurements of Res-6.
Compounds with an average alkyloxy chain length (from n = 4
to n = 8) manifest the liquid-crystalline properties in form of one
monotropic banana type B6 phase. This mesophase can form both
focal-conic (as for n = 2, Fig. 5A) as well as fan-shaped texture (for
n = 4, Fig. 5B), which appears from isotropic as battones and coa-
lescales (as for n = 8, Fig. 5C). To confirm phase assignment XRD
Next
homolog,
4-Chloro-1,3-phenylene
bis-[4-(4-
propyloxyphenylazo)benzoate], has only monotropic B6 phase
and significantly lower melting point in relation to the Res-2.
Despite the absence of a nematic phase as in the previous case, we
observed a polymorphism of the solid phase. For this compound
we can use the same explanation as for the Res-2. Quick cooling
(50 K min−1) promotes the formation of a form CrI having a lower
clearing point, possessing texture as shown in Fig. 4A. Slow rate of
cooling allows creation of the second crystalline form CrII (Fig. 4B)
from B6. In addition, this form (CrII) crystallizes from the isotropic
phase during fast heating (Fig. 4C).
Fig. 7. The UV–Vis spectra obtained for Res-5 for freshly prepared sample and after
irradiation.

I. Niezgoda et al. / Thermochimica Acta 587 (2014) 59–66
63
Table 1
Temperatures of phase transitions of 4-Chloro-1,3-phenylene bis-[4-(4-alkyloxyphenylazo)benzoate] obtained during cooling mode.
n
Melting CrI
Melting CrII
Crystallization
(to CrI)
B6
X
SmC
N
Iso
2
179.5c
159.2c
193.5c
162.9c
–
142.4
[34.32]
131.1
[36.00]
131.5
[31.55]
91.6
•
•
•
•
•
•
•
•
•
(167.5)
[3.54]
(158.7)
[8.40]
(156.5)
[8.94]
(137.5)
[9.79]
(130.5)
[10.95]
(115.6)
[10.76]
(105.8)
[9.56]
•
186.7
[0.56]
•
•
•
•
•
•
•
•
•
•
•
•
•
3
4
163.3
[45.21]
143.1
[42.14]
137.2
5
–
[24.74]
85.5
6
–
[47.65]
[17.90]
7
125.3b
–
75.6b
8
115.0
–
84.5
[14.24]
82.1
[53.22]
9
104.4c
110.0c
105.0c
102.7c
104.0c
103.8c
104.0c
(91.3)
[6.97]
•
•
•
•
•
•
95.3
[0.64]
96.2
[0.22]
96.9
[0.63]
96.2
[0.83]
100.6
[1.18]
101.3
[4.35]a
[21.66]
78.0
10
12
14
16
18
96.8c
89.1c
93.1c
96.0c
97.9c
(79.6)
[32.80]a
79.8
[32.80]a
[32.80]
82.9
[42.85]
87.2
•
•
89.4
[47.25]
89.0
[0.50]
92.9
•
(98.4)
[49.71]a
[49.71]a
[4.35]a
a
Enthalpy is given together for two phase transitions due to overlapping of peaks.
Enthalpy could not be measured because of slow crystallization process.
Quantitative calorimetric measurements are not possible because of the presence of both crystalline form at the same time.
b
c
studies were performed for the selected compound (n = 6, 4-Chloro-
1,3-phenylene bis-[4-(4-hexyloxyphenylazo)benzoates]). In B6
mesophase single sharp diffraction peak was observed at low
angle region and a broad diffused signal in high angle range
phase transitions temperatures for this compound presented in
Table 1 which are fully consistent with Halle’s [33]. For 4-Chloro-
1,3-phenylene bis-[4-(4-pentyloxyphenylazo)benzoate] we also
measured UV–Vis spectrum. We prepared a 10−5 M solution of Res-
5 in dichloromethane. First, we measured the spectrum of a freshly
prepared solution, which was then exposed for 20 min irradiation
(ꢂ = 365 nm). Then we measured spectrum directly after irradia-
tion. Collected spectra (shown in Fig. 7) demonstrate that the light
˚
(Fig. 6). The smectic layer thickness, 20.3 A, evaluated from low
˚
angle signal corresponds to half of the molecular length (43 A)
evidencing strong intercalation typical for B6 phase. Res-8 has
also been recently investigated by Halle’s group (Germany). Our
Fig. 8. Polarizing microscopy textures of Res-16. (A) Mesophase X. (B) Nematic texture. (C) Texture of CrI. (D) Crystals of CrII.

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I. Niezgoda et al. / Thermochimica Acta 587 (2014) 59–66
Fig. 10. The XRD measurements of Res-18, which confirm SmC mesophase.
during cooling just prior to crystallization. Most likely it is a crys-
talline or smectic phase. However, after the texture analysis we
were not able to identify phase X. XRD measurements were unsuc-
cessful because of crystallization process. In addition, a nematic
phase (Fig. 8B) and two crystalline forms (Fig. 8C and D) are present.
Microscopic observations are also consistent with the TOA and the
DSC measurements (Fig. 9). In the cooling mode both methods used
(DSC and TOA) detected three phase transitions. The first is the Iso-
N. The TOA scan shows increase of the signal which next decreases
because of the homeotropisation. Next nearly at 89.5 ◦C an anomaly
is observed and it corresponds with smectic A phase appearance.
The DSC scan presents it as a complex crystallization process. In the
heating mode both methods used identified melting to the nematic
phase and five 5 ◦C above isotropisation process. The part C of Fig. 9
shows the TOA curve after creation of CrII in the heating mode. No
mesophase was observed.
Fig. 9. The DSC and the TOA diagram of Res-16.
influences on azo-bond isomerization (from trans to cis form). This
process is confirmed by the appearance of a broad band at about
480 nm corresponding to the n–␲* transition. There is also a visible
change in the band intensity at 370 nm, which corresponds to the
transition ␲–␲*. As in [33], we left overnight the irradiated sample
in the dark. Obtained spectrum was the same as before irradiation.
Sample returned to a more stable trans isomer. Comparing the spec-
trum of the Res-5 with Res-16 [33], we may conclude that the alkyl
chain length does not affect the position of the bands.
Chain elongation favors the formation of the nematic phase and
but it has negative effect on more ordered smectic B6 phase. For
nonyl and decyl derivatives TOA and DSC studies showed the pres-
ence of both mesophases: nematic and B6. However, for Res-10
banana-type phase temperature range is very narrow, (1.6 ◦C). This
homolog is a compound with the longest alkyloxy chain, within
the B6 mesophase is present. It is also worth mentioning that the
appearance of the nematic phase from the Res-9 induces solid-state
polymorphism, as in the case of Res-2. Also in the case of Res-10 we
obtained the phase transition temperatures comparable with the
results of Alasaar et al. [33].
In the next two homologous series of 4- chlororesorcinol Res-12
and Res-14 nematic phase and solid phase polymorphisms were
observed. As in the previous compounds Res-2 and Res-10 the
liquid-crystalline phase is relative, enantiotropic or monotropic.
For this series of compounds the nature of the nematic mesophase
depends on the crystalline type. For CrI we observe the character-
istic nematic texture during both heating and cooling processes.
However, for the form CrII mesophase is a monotropic, therefore it
is only observed at cooling. Phase transition temperatures of Res-
12, Res-14 and Res-16, that we present (Table 1) are consistent with
those given by Alasaar et al. [33] and are within the experimental
error.
Also the investigation of mesogenic properties of homologues
with the longest chain (n = 16, n = 18) showed some interesting
phenomena. Firstly, the solid phase polymorphism is observed. Sec-
ondly, for Res-16 and Res-18 there is visible under the polarizing
microscope a very narrow phase X (Fig. 8A). This mesophase occurs
Fig. 11. The TOA and the DSC measurements of 4-Chloro-1,3-phenylene bis-[4-(4-
octadecyloxyphenylazo)benzoate].

I. Niezgoda et al. / Thermochimica Acta 587 (2014) 59–66
65
Fig. 12. Textures of Res-18. (A) Nematic phase, (B) SmC mesophase, (C) texture of CrI, (D) crystals of CrII, (E) CrII and nematic form, which was formed from CrI, (F) crystals
of CrII and isotropic phase (from CrI).
Fig. 13. Phase behaviour of 4-Chloro-1,3-phenylene bis-[4-(4-octadecyloxyphenylazo)benzoates].
More interesting liquid-crystalline polymorphism is shown in
Res-18 possessing the longest alkyloxy chain in this experiment.
According to the DSC and the TOA measurements Res-18 forms
nematic (Fig. 12A) and smectic C phases (Fig. 12B). Smectic C phase
was confirmed by XRD studies (Fig. 10). Also very narrow phase
X is visible during the DSC and the TOA measurements as well
as crystallization process (Fig. 11). As in the previous homologues
solid state polymorphism exists. Both crystalline forms are char-
acterized by different melting temperatures and textures, which
are shown in Fig. 12C and D. In addition, their liquid-crystalline
properties are diverse. CrI form melts at a lower temperature and
has enantiotropic nematic phase (Fig. 12E), while the CrII crystals
shown in Fig. 12F melt to isotropic phase. This behaviour is shown
in Fig. 11. In the cooling mode both methods used identified four
phase transitions and three types of mesophases: nematic, smec-
tic C and X mesophase. The last phase is rather soft phase because
small value of the enthalpy smectic C- X, compared to the crys-
tallization process. In the heating mode the melting from CrI to
nematic was observed by both methods used. However, extraor-
dinary phase transition CrI–CrII and melting CrII–Iso is observed.
Because of the presence of both crystalline forms in the sample
quantitative calorimetric measurements are not possible. The pres-
ence of SmC is confirmed with XRD method (Fig. 10). Narrow low
angle peak identifies lamellar character of the mesophase, and diff-
used wide angle ring is typical for smectic C mesophase.
4. Conclusions
To conclude, we synthesized 13 bent-core compounds, which
form a homologous series of 4-chlororesorcinol derivatives. Also
mesogenic studies have been performed with use of POM (and
TOA), DSC and XRD. We found liquid-crystalline mesophases in
each investigated substance (Fig. 13). Also solid-state polymor-
phism which can change character of the presented mesophases
has been noted. Moreover we observed a very rare phenomenon

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I. Niezgoda et al. / Thermochimica Acta 587 (2014) 59–66
of the gap without nematic phase among derivatives with middle
alkyl chain length.
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AID-MACP1556>3.0.CO;2-L.
In
the
group
of
4-Chloro-1,3-phenylene
bis-[4-(4-
alkyloxyphenylazo)benzoate] only five homologues have been
recently described [33]. Therefore it was very interesting to note
in our data the polymorphism of the solid states. Depending on
the type of the crystal modification monotropic or enantiotropic
mesophase can be obtained.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tca.2014.04.024.
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