Naphthalene core-based mesogens with terminal halogen substituents: Synthesis and mesomorphic behavior

Article abstract of DOI:10.1080/15421400600580154

We report three series of naphthalene core-based mesogens obtained by varying two different terminal substituents. The flexibility in these systems is provided by attaching a straight alkoxy chain at the one end, while a halogen terminal substituent accounts for the polarity. To understand the structure-property relation, alkoxy tails (C 1 to C 8, C 10, C 12, C 14, C 16 , and C 18 ) as well as halogen atoms (F, Cl, and Br) have been varied. The molecular structures of these mesogens have been confirmed by spectroscopic and elemental analyses, whereas their thermal behavior was evaluated mainly by polarizing optical microscopic observations. The study shows that, in general, all the compounds display nematic and/or smectic A mesophases. In all three series of compounds, the lower and middle homologues exhibit the nematic phase only and higher homologues also display the smectic A phase. These compounds follow the general trend that with the increase in the length of alkoxy tail, the nematic to isotropic phase transition temperature decreases, which is reminiscent of a large number of reported homologues series. The transition temperatures of the compounds with fluro substituents are significantly lower when compared to the materials with chloro or bromo substitutions.

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Naphthalene Core-Based
Mesogens with Terminal
Halogen Substituents: Synthesis
and Mesomorphic Behavior
B. T. Thaker a & P. K. Tandel a
a Department of Chemistry , South Gujarat
University , Surat, India
Published online: 17 Oct 2011.
To cite this article: B. T. Thaker & P. K. Tandel (2006) Naphthalene Core-Based
Mesogens with Terminal Halogen Substituents: Synthesis and Mesomorphic
Behavior, Molecular Crystals and Liquid Crystals, 451:1, 127-138, DOI:
10.1080/15421400600580154
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Mol. Cryst. Liq. Cryst., Vol. 451, pp. 127–138, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600580154
Naphthalene Core-Based Mesogens with Terminal
Halogen Substituents: Synthesis and
Mesomorphic Behavior
B. T. Thaker
P. K. Tandel
Department of Chemistry, South Gujarat University, Surat, India
We report three series of naphthalene core-based mesogens obtained by varying two
different terminal substituents. The flexibility in these systems is provided by
attaching a straight alkoxy chain at the one end, while a halogen terminal sub-
stituent accounts for the polarity. To understand the structure–property relation,
alkoxy tails (C1 to C8, C10, C12, C14, C16, and C18) as well as halogen atoms
(F, Cl, and Br) have been varied. The molecular structures of these mesogens have
been confirmed by spectroscopic and elemental analyses, whereas their thermal
behavior was evaluated mainly by polarizing optical microscopic observations.
The study shows that, in general, all the compounds display nematic and=or smec-
tic A mesophases. In all three series of compounds, the lower and middle homolo-
gues exhibit the nematic phase only and higher homologues also display the
smectic A phase. These compounds follow the general trend that with the increase
in the length of alkoxy tail, the nematic to isotropic phase transition temperature
decreases, which is reminiscent of a large number of reported homologues series.
The transition temperatures of the compounds with fluro substituents are signifi-
cantly lower when compared to the materials with chloro or bromo substitutions.
Keywords: azomethine; mesogens; naphthalene; nematic and smectic
1. INTRODUCTION
Thermotropic liquid crystals are of great technological importance [1].
Schiff base compounds are synthesized in large numbers as these com-
pounds exhibit nematic and smectic mesopheses [2,3]. The interest in
synthesis and structural investigations of liquid crystalline Schiff
bases with azo-groups in the mesogenic units has considerably
increased because of their potential application in the field of
Address correspondence to B. T. Thaker, Department of Chemistry, South Gujarat
University, Surat 395 007, India. E-mail: btthaker1@yahoo.co.in
127

128
B. T. Thaker and P. K. Tandel
nonlinear optics, in optical storage devices, and in electrooptical dis-
plays. The photoresponsive azobenzene group appears particularly
interesting because irradiation can induce the trans-cis isomerization
of the photo chromic group, including a local phase transition.
Mesomorphic properties of some homologous series of Schiff bases
SCHEME 1

Naphthalene Core-Based Mesogens
129
containing a naphthalene core are well known. Wiegand prepared and
studied liquid crystalline properties of anisilidene amino naphtha-
lenes [4,5]. Dave et al. reported the mesomorphic behavior of a homolo-
gous series obtained by condensing 4-n-alkoxy-1-napthaldehydes with
benzidine [6]; p-phenylene diamine [7], and p-amino benzoic acid [8].
Arora and Fergason studied a benzylidene Schiff base series contain-
ing an azo group [9].
In continuation of our investigations on naphthalene based rod-
like liquid crystals, it was thought that the introduction of an azo
group in a naphthyldene series with halogens and alkoxy tails as
terminal substituents could help in further understanding the under-
lying structure–property relation. Here we report three series of
mesogens, namely, 4-[40-n-alkoxybenzylidene amino]naphthalene-1-
(400-halo-1-azobenzene) in which alkoxy tails (C1 to C8, C10, C12,
C14, C16, and C18) as well as halogen atoms (F, Cl, and Br) have been
varied. The molecular structure of these compounds is shown in
Scheme 1.
2. EXPERIMENTAL
2.1. General Information
All the solvents (acetone, methanol, ethanol, ethylacetate, and acetic
acid) were obtained from local sources and purified as per the standard
procedures. 4-Hydroxy-benzaldehyde, a-naphthylamine, 4-fluoroani-
line, 4-chloroaniline, 4-bromoaniline, and alkyl bromides were
obtained from British Drug House, Poole, England and used without
further purification. IR spectra were recorded using Elmer FTIR
between 400–4000cmꢀ1 spectral range. The UV=visible absorption
spectra for the mesomorphic compounds were recoded in chloroform
using a Shimadzu UV=PC 160-A spectrophotometer in the 200–800-
1
nm spectral range. H NMR spectra were recorded using a Jeol FX
300-MHz spectrometer, and the chemical shifts are reported in parts
per million (ppm) relative to tetramethylsilane (TMS) as an internal
standard. The purity of the compounds was checked by thin-layer
chromatography and further confirmed by elemental analysis using
a Colemen USA C, H, N analyser. The identification of the mesophases
and the transition temperatures of the dimers were determined
mainly by using a polarizing microscope (Leitz Labourlux) in conjunc-
tion with a programmable Kofter hot stage. For some representative
compounds, transition temperatures and associated enthalpies were
determined by differential scanning calorimetry using a DSC Mettler
TA 4000 calorimeter at a rate of 10ꢁC minꢀ1
.

130
B. T. Thaker and P. K. Tandel
2.2. Synthesis
2.2.1. 4-aminonaphthalene-1-haloazobenzene (4a–c)
These dyes were prepared [10–12] by condensing a-naphthylamine
with diazonium salts 5a–c of substituted aniline (6a–c). First the sub-
stituted anilines 6a–c were diazotised with dilute HCl and NaNO2 at
0–5ꢁC and coupled with a-naphthylamine at pH 4–4.5. Then the
reaction mixture was neutralized with dil. NaOH, and the obtained
precipitate was collected by filtration and then air dried. The crude
compounds were purified by repeated recrystallizations using abs.
ethanol as solvent.
4a: A deep red crystalline solid; yield: 80%; mp: 151–152ꢁC; elemen-
tal analysis calc. for C16H12N3F: C, 72.45; H, 4.52; N, 15.84; found: C,
72.43; H, 4.51; N, 15.82%.
4b: A deep red crystalline solid; yield: 85%; mp: 181–182ꢁC; elemen-
tal analysis calc. for C16H12N3Cl: C, 68.2; H, 4.26; N, 14.92; found: C,
68.18; H, 4.24; N, 14.9%.
4c: A deep red crystalline solid; yield: 82%; mp: 176–177ꢁC; elemen-
tal analysis calc. for C16H12N3Br: C, 58.89; H, 3.68; N, 12.88; found: C,
58.87; H, 3.66; N, 12.85%.
2.2.2. 4-n-alkoxybenzaldehydes (7a–m)
These were prepared by the reported method of Gray and Jones
[13]. A mixture of 4-hydroxybenzaldehyde (1mol) and K2CO3
(1.5 mol) in 250 ml of anhyd. acetone was stirred and heated at 40ꢁC
while 1-bromoalkane (1.02 mol) was added slowly (1–2 h), after which
mixture was refluxed for 16h. The reaction mixture was cooled to
room temperature and poured into water. The product obtained was
extracted with ethyl ether several times, and the combine extracts
were dried over MgSO4 and concentrated at reduced pressure to
obtain the crude product, which was purified as reported. The molecu-
lar structure of these aldehydes were confirmed by spectroscopic
analyses and found to be in agreement with the reported data.
2.2.3. 4-[40-n-alkoxybenzylideneamino]naphthalene-1-
(400-halo-1-azobenzene): (1a–m, 2a–m, and 3a–m)
To a stirred solution of the 4-aminonaphthalene-1-haloazobenzene
(4a–c) (0.1 mol) in ethanol was added an appropriate alkoxyaldehyde
(0.1 mol) and 2–3 drops of acetic acid [14]. The reaction mixture was
reflux for 2–3 h. The orange precipitate was collected by filtration
and washed with hot abs. ethanol. The crude products were recrystal-
lized several times from ethyl acetate until constant transition tem-
peratures were obtained. All the compounds were realized by this

Naphthalene Core-Based Mesogens
131
method and exhibited spectral data consistent with their molecular
structure. As representative cases we provide the molecular structural
characterization data for 1d, 2g, and 3g.
1d: An orange–red crystalline solid; mp 186–187ꢁC; yield: 76%;
UV-vis: kmax ¼ 415.5nm (2 ¼ 1.3 ꢂ 104Lmolꢀ1cmꢀ1), 268.4 nm
(2 ¼ 1.8ꢂ 104L molꢀ1cmꢀ1); IR (KBr pellet): nmax 2881, 1630, 1602,
1
1417, 1366, 1261, 1170, 1070, 831, and 720 cmꢀ1; H NMR (CDCl3,
300 MHz): d 8.97 (s, 1H, 1ꢂ ꢀCH¼Nꢀ), 8.4–7.1 (m, 14H, Ar), 4.05 (t,
2H, 1ꢂ ꢀOCH2ꢀ), 1.86–1.46 (m, 4H, 2ꢂ ꢀCH2ꢀ), and 1.03 (t, 3H,
1 ꢂ ꢀCH3); elemental analysis calc. for C27H24N3OF: C, 76.23; H,
5.64; N, 9.88; found: C, 76.25; H, 5.68; N, 9.87%.
2g: An orange–red crystalline solid; mp 182–183ꢁC; yield: 85%;
UV-vis: kmax ¼ 423 nm (2 ¼ 0.65ꢂ 104 Lmolꢀ1cmꢀ1), 273.8 nm
(2 ¼ 1.35 ꢂ 104L molꢀ1cmꢀ1); IR (KBr pellet): nmax 2868, 1629,
1602, 1430, 1378, 1240, 1163, 1056, 834, and 722 cmꢀ1 1H NMR
;
(CDCl3, 300MHz): d 8.97 (s, 1H, 1ꢂ ꢀCH¼Nꢀ), 8.43–7.1 (m, 14H,
Ar), 4.05 (t, 2H, 1ꢂ ꢀOCH2ꢀ), 1.86–1.46 (m, 10H, 5ꢂ ꢀCH2ꢀ), and
0.91 (t, 3H, 1 ꢂ ꢀCH3); elemental analysis calc. for C30H30N3OCl: C,
74.45; H, 6.2; N, 8.68: found: C, 74.5; H, 6.15; N, 8.66%.
3g: An orange–red crystalline solid; mp 178–179ꢁC; yield: 82%;
UV-vis:
kmax ¼ 424 nm
(2 ¼ 1.6 ꢂ 104 L molꢀ1cmꢀ1),
269.4 nm
(2 ¼ 1.3ꢂ 104 L molꢀ1cmꢀ1); IR (KBr pellet): nmax 2868, 1630, 1602,
1423, 1373, 1252, 1163, 1064, 834, and 720 cmꢀ1; H NMR (CDCl3,
1
300 MHz): d 8.97 (s, 1H, 1 ꢂ ꢀCH¼Nꢀ), 8.43–7 (m, 14H, Ar), 4.05
(t, 2H, 1 ꢂ ꢀOCH2ꢀ), 1.86–1.44 (m, 10H, 5 ꢂ ꢀCH2ꢀ) and 0.91 (t, 3H,
1 ꢂ ꢀCH3); elemental analysis calc. for C30H30N3OBr: C, 68.18; H,
5.68; N, 7.95; found: C, 68.15; H, 5.66; N, 7.93%.
3. RESULTS AND DISCUSSION
3.1. Synthesis
The target molecules of the three series 1a–m, 2a–m, and 3a–m, and
their intermediates 4a–c, 5a–c, and 7a–m were synthesized as out-
lined in Scheme 1. The requisite 4-n-alkoxybenzaldehydes (7a–m)
were prepared by refluxing 4-hydroxybenzaldehyde with 1-n-
bromoalkane under mild basic reaction conditions in a solvent medium
[13]. The key intermediates, azonaphthlynene amines 4a–c, were rea-
lized by coupling a-naphthylamine with diazonium salts 5a–c, which
in turn were prepared by the diazotization of 4-haloanilines (6a–c).
Finally, the aldehydes 7a–m were condensed with azonaphthlynene
amines 4a–c under mild acidic reaction conditions in ethanol to
obtain the target molecules in reasonably good yields. The molecular

132
B. T. Thaker and P. K. Tandel
structures of all the compounds were confirmed by spectroscopic
analysis (see Experimental section).
3.2. Thermal Behavior
As mentioned earlier, in an effort to understand the structure–pro-
perty relation, the chain length of the terminal alkoxy tail as well as
the terminal halogen substitutions have been varied, which resulted
in three series of compounds, series 1 (1a–m), series 2 (2a–m), and ser-
ies 3 (3a–m). The transition temperatures and the phase sequence of
the compounds synthesized in the present investigation are presented
in Tables 1, 2, and 3. The mesomorphic behavior of the target com-
pounds have been evaluated by optical studies. However, for some rep-
resentative compounds, the phase transition temperatures have been
confirmed by calorimetric studies, which are presented in Table 4. All
the members of the three series exhibit nematic (N) and=or smectic
A (SmA) phases. The nematic phase was identified based on the obser-
vation of a characteristic schlieren (Fig. 1) or marble texture when
unrubbed slides treated for planar orientation were used. In a home-
otropically aligned sample, the N phase shows a pseudoisotropic tex-
ture, which flashed when subjected to shearing stress. The presence
TABLE 1 Transition Temperatures (ꢁC) of Compoundsa of Series 1
Compounds
No.
Transition temperatures
R
Cr
Heating
SmA
Heating
N
Heating
I
1a
1b
1c
1d
1e
1f
1g
1h
1i
CH3
.
.
.
.
.
.
.
.
.
.
.
.
.
150
152
140
146
133
119
112
99
112
89
95
91
90
-
-
-
-
-
-
-
-
-
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
194
203
189
186
172
169
162
156
147
132
135
128
123
.
.
.
.
.
.
.
.
.
.
.
.
.
C2H5
C3H7
C4H9
C5H11
C6H13
C7H15
C8H17
C
10H21
1j
1k
1l
C12H25
(57)b
101
109
112
C
C
C
14H29
16H33
18H37
1m
aTemperatures ascertained by optical microscopic observation at a rate of 3ꢁC= min.
bMonotropic transition.
. ¼ phase exists; - ¼ phase does not exist.
Cr ¼ crystal; SmA ¼ smectic A; N ¼ nematic phase; I ¼ isotropic phase.

Naphthalene Core-Based Mesogens
133
TABLE 2 Transition Temperatures (ꢁC) of Compoundsa of Series 2
Compounds
No.
Transition temperatures
R
Cr
Heating
SmA
Heating
N
Heating
I
2a
2b
2c
2d
2e
2f
2g
2h
2i
CH3
.
.
.
.
.
.
.
.
.
.
.
.
.
170
155
139
132
120
127
114
122
112
101
90
-
-
.
.
.
.
.
.
.
.
.
.
.
.
.
234
238
224
209
195
185
182
179
168
160
151
149
135
.
.
.
.
.
.
.
.
.
.
.
.
.
C2H5
C3H7
C4H9
C5H11
C6H13
C7H15
C8H17
C
10H21
C12H25
14H29
C16H33
18H37
.
.
.
.
.
(83)b
109
120
128
122
2j
2k
2l
C
82
87
2m
C
aTemperatures ascertained by optical microscopic observation at a rate of 3ꢁC=min.
bMonotropic transition.
of the SmA phase has been confirmed based on the microscopic obser-
vation of the characteristic focal-conic texture (Fig. 2) in slides treated
for planar orientation and a dark field of view in slides treated for
TABLE 3 Transition Temperatures (ꢁC) of Compoundsa of Series 3
Compounds
No.
Transition temperatures
R
Cr
Heating
SmA
Heating
N
Heating
I
3a
3b
3c
3d
3e
3f
3g
3h
3i
CH3
.
.
.
.
.
.
.
.
.
.
.
.
.
183
168
142
129
130
122
125
130
115
102
97
-
-
.
.
.
.
.
.
.
.
.
.
.
.
.
236
248
229
212
192
186
179
175
166
161
153
150
137
.
.
.
.
.
.
.
.
.
.
.
.
.
C2H5
C3H7
C4H9
C5H11
C6H13
C7H15
C8H17
C
C
C
10H21
12H25
14H29
.
.
.
.
.
(85)b
109
119
131
120
3j
3k
3l
C16H33
18H37
80
88
3m
C
aTtemperatures ascertained by optical microscopic observation at a rate of 3ꢁC=min.
bMonotropic transition.

134
B. T. Thaker and P. K. Tandel
TABLE 4 DSC Data for Some Representative Compounds of Series 1, 2, and 3
Total
Peak temp ꢁC
(microscopic
temp.)
entropy
change
DS=Jgꢀ1kꢀ1
Compound
Transition
DH=Jgꢀ1
DS=Jgꢀ1kꢀ1
1d
Cr! N
N! I
143.4 (146)
185.8 (186)
90.7 (91)
212.3
3.5
112.1
—
1.48
0.019
1.2358
—
0.013
1.48
0.0091
4.26
—
0.035
5.3482
0.039
2.0705
—
1.5
1m
K! SmA
SmA! N
N! I
— (109)
1.25
1.42
127.8 (128)
115.4 (114)
181.3 (182)
101.44 (101)
— (109)
159.6 (160)
116.3 (115)
164.53 (166)
91.33 (88)
— (120)
1.18
162.4
1.65
431.9
—
5.508
622.1
6.429
189.1
—
2g
2j
K! N
N! I
K! SmA
SmA! N
N! I
4.29
5.39
2.11
3i
K! N
N! I
3m
K! SmA
SmA! N
N! I
137.0 (137)
5.856
0.043
FIGURE 1 Photomicrograph of a characteristic schlieren texture of the
nematic phase observed for the compound 3i at 159ꢁC while cooling the sample
from the isotropic phase.

Naphthalene Core-Based Mesogens
135
FIGURE 2 Photomicrograph of the characteristic focal-conic texture seen in
slides treated for planar orientation of the compound 11 at 105ꢁC while cooling
from the isotropic phase.
homeotropic orientation. When untreated slides were employed, both
the focal-conic and psuedoisotropic textures have been observed. As
can be seen in Table 1, fluoro-substituted compounds with C1 to C10
tail display an enantiotropic N phase, whereas the compound with
the C12 tail also shows the monotropic SmA phase. The higher homo-
logues with C14, C16, and C18 tails exhibit both enantiotropic SmA and
N phases. The dependence of the transition temperatures on the num-
ber of carbon atoms in the alkoxy chain of this series has been shown
in Fig. 3. As the chain length increases, the terminal cohesive forces,
which account for nematic–isotropic change, decrease, as result of
which the nematic–isotropic curve obtained for even members in these
series is falling one. A similar curve should be obtained for odd mem-
bers, but in practice a rising curve is obtained, which shows a falling
tendency from the middle member. This may probably due to relative
differences between the terminal and lateral cohesions. As the chain
length increases, lateral cohesion increases and it may support the ter-
minal forces; this, to a certain degree, helps in explaining the rise in
the curve, which afterward shows the tendency of becoming a normal
falling curve. Probably these differences may be due to differences in
packing and cohesion between the terminal methyl groups of alkoxy
chains containing odd and even numbers of carbon atoms.

136
B. T. Thaker and P. K. Tandel
FIGURE 3 Plot of transition temperature against the number of carbon
atoms in thne-alkoxy chains for the compounds of series 1.
Thermal behavior of the compounds with chloro (Table 2) and
bromo (Table 3) substitutions (series 2 and series 3) is almost similar
to that of compounds of series 1. The dependence of the transition tem-
peratures on the number of carbon atoms in the alkoxy chain of series
FIGURE 4 Plot showing the dependence of transition temperature against
the number of carbon atoms in the n-alkoxy chains for the chloro compounds
of series 2.

Naphthalene Core-Based Mesogens
137
FIGURE 5 Plot of transition temperature against the number of carbon atoms
in thne-alkoxy chains for the compounds of series 3.
2 and series 3 shown in Figs. 4 and 5 respectively. The smectic phase
begins to appear from C10 derivatives in both the cases, which perhaps
could be attributed to the fact that the overall polarizability of the
chloro- and bromo-substituents is higher than that of the fluoro-
substituent. Further, the transition temperatures, in particular the
N–Iso, is lower for compounds with fluoro-substitution, which is
attributed to low polarity and low intermolecular attractive forces of
these molecules.
4. SUMMARY
In conclusion we have synthesized as many as 39 new naphthalene
core-based mesogens with azo linkage and evaluated their meso-
morphic behavior. These compounds form three series of compounds
resulting from the variations in two terminal substitutions. To under-
stand structure–property relations alkoxy tails (C1 to C8, C10, C12, C14,
C16, and C18) and halogen atoms (F, Cl, and Br) as terminal substitu-
ents have been varied. Our investigations reveal that, in general, all
the compounds display nematic and or smectic A mesophases. The
lower and middle homologues exhibit the nematic phase only whereas
higher homologues also display the smectic A phase. Thus, the ther-
mal behavior of these compounds is reminiscent of a large number of
homologues series reported in the literature.

138
B. T. Thaker and P. K. Tandel
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