Mesogenic naphthyl derivatives

Article abstract of DOI:10.1080/15421406.2016.1262712

Two new mesogenic homologous series of liquid crystalline naphthalene derivatives with different central linkages: 2- Naphthyl-4-n-alkoxybenzoates (I) and trans-2-Naphthyl -4-n –alkoxycinnamates(II), have been synthesized and characterized by a combination of elemental analysis and standard spectroscopic methods. Twelve compounds have been synthesized in both the series I and II. In series I, methoxy to n-pentyloxy derivatives are non mesogenic, n-hexyloxy to n-hexadecyloxy derivatives exhibit monotropic nematic mesophase. Smectic A mesophase commences from n-decyloxy derivatives as a monotropy and persist up to the last member synthesized. All the 12 compounds of series II exhibit mesomorphism. Methoxy to n-propyloxy derivatives exhibit monotropic nematic mesophse, and n-butyloxy to n-tetradecyloxy derivatives exhibit enantiotropic nematic mesophase. n-Hexayloxy to n-hexadecyloxy derivatives also exhibit enantiotropic smectic A mesophase. The mesomorphic properties of present series were compared with each other and with a structurally related mesogenic homologous series to evaluate the effects of central linkage on mesomorphism.

Full text of DOI:10.1080/15421406.2016.1262712

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesogenic naphthyl derivatives
H. N. Patel & A. K. Prajapati
To cite this article: H. N. Patel & A. K. Prajapati (2017) Mesogenic naphthyl derivatives, Molecular
Crystals and Liquid Crystals, 643:1, 106-115, DOI: 10.1080/15421406.2016.1262712
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1262712
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Date: 23 February 2017, At: 21:32

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Mesogenic naphthyl derivatives
H. N. Patela and A. K. Prajapatib
aShree Maneklal M. Patel Institute of Sciences & Research, Gandhinagar, Kadi Sarva Vishwavidhyalaya,
Gandhinagar, India; bChemistry Department, Faculty of Science, The M.S. University of Baroda, Vadodara, India
ABSTRACT
KEYWORDS
Liquid crystal; Smectic phase;
Nematic phase;
Mesomorphic; Enantiotropic
Two new mesogenic homologous series of liquid crystalline naph-
thalene derivatives with different central linkages: 2- Naphthyl-4-n-
alkoxybenzoates (I) and trans-2-Naphthyl -4-n –alkoxycinnamates(II),
have been synthesized and characterized by a combination of ele-
mental analysis and standard spectroscopic methods. Twelve com-
pounds have been synthesized in both the series I and II. In series
I, methoxy to n-pentyloxy derivatives are non mesogenic, n-hexyloxy
to n-hexadecyloxy derivatives exhibit monotropic nematic mesophase.
Smectic A mesophase commences from n-decyloxy derivatives as a
monotropy and persist up to the last member synthesized. All the 12
compounds of series II exhibit mesomorphism. Methoxy to n-propyloxy
derivatives exhibit monotropic nematic mesophse, and n-butyloxy to
n-tetradecyloxy derivatives exhibit enantiotropic nematic mesophase.
n-Hexayloxy to n-hexadecyloxy derivatives also exhibit enantiotropic
smectic A mesophase. The mesomorphic properties of present series
were compared with each other and with a structurally related meso-
genic homologous series to evaluate the effects of central linkage on
mesomorphism.
Introduction
Controlling mesophase transition temperatures has always been an important issue in liq-
uid crystal materials research. Varying the length of alkyl chain has often been used as an
easy and systematic way by which to understand trends in phase transition temperatures [1].
However, many other structural factors can also be manipulated in order to adjust the melting
and clearing temperatures as well as the mesophase temperature ranges. Examples include the
introduction of terminal [2] or lateral [3] fluorine-containing units, the introduction of dou-
ble bonds into the terminal chains [4, 5] and the introduction of a cyano group [6]; each of
these approaches has been shown to exert a great impact on transition temperatures as well as
on mesogenic properties. Symmetry lowering has also played an important role in controlling
the transition temperatures and temperature ranges of liquid crystals [7, 8]. A vast number
of mesogenic naphthalene derivatives are reported in the literature [9, 10], as naphthalene
derivatives exhibit rich mesomorphism if properly designed. Weygand [11] reported several
mesogenic Schiff bases of 2, 6-, 1, 5- and 1, 4-diaminonaphthalenes. Gray and Jones [12]
investigated liquid crystalline properties of different alkoxynaphthoic acids. Coates and Gray
CONTACT H. N. Patel
hemant_patel@yahoo.co.in
/, Gandhinagar, Kadi Sarva Vishwavidhyalaya, Gandhinagar, India.
Shree Maneklal M. Patel Institute of Sciences & Research, Sector
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
107
[13] synthesized 4-n-alkyl/alkoxy phenyl esters of 6-n-alkyl/alkoxy-naphthalene-2-carboxylic
acids. Dave and others studied a variety of liquid crystalline naphthalene derivatives such as n-
alkoxybenzoates of 1,5- and 1,4-dihydroxynaphthalene [14], esters of cholesterol [15] and n-
alkoxynaphthyliedene Schiff bases [16–18], exhibiting smectic, nematic and cholesteric meso-
morphism. Malthete et al. [19] synthesized tetra acylated 1, 4, 5, 8-teterahydroxy-naphthalene
derivatives, which may be looked upon as “conjoined twin” mesogens. A significant number
of research papers on naphthalene LC cores appeared in the literature [20–30]. Hsu et al. [31]
reported the liquid crystalline materials, the 1, 4-bis (6-alkoxyl-2-naphthyl)-1, 3-butadiynes,
to probe the effects of the terminal chains on the mesophase transition temperatures when
compared with those of the analogous compound without chains. The naphthyl and ethynyl
groups have been reported to show high optical anisotropy [32–36], which is an important
characteristic for additives in mixtures of liquid crystal materials for displays. The synthe-
sis and mesomorphic properties of banana-shaped compounds derived from 2, 7-dihydroxy
naphthalene have also been reported [37–42].
Earlier we have reported mesogenic homologous series of Schiff base esters containing
the naphthalene moiety and studied the effect of lateral thiol [43] and methoxy [44] sub-
stituents on mesomorphism. We have reported the synthesis of mesogenic homologous series
of azomesogens without lateral substituent [45], with lateral methyl [46] and chloro [47]
groups, and evaluated the effect of lateral methyl as well as chloro groups on mesomorphism.
We prepared a mesogenic homologous series of azoesters containing two naphthalene nuclei
[48] and studied the effect of the naphthalene moiety on mesomorphism. We have also syn-
thesized a mesogenic homologous series of Schiff base cinnamates [49] comprising a naph-
thalene moiety, and investigated the effect of an ethylene linking group (cinnamoyl linkage)
and naphthalene moiety on the mesomorphic properties of such compounds. In 2006, we
[50] have reported two new mesogenic homologous series of salicylaldimines with the naph-
thalene moiety, viz. The 4-n-alkoxy-2-hydroxybenzy lidene-2-naphthylamines and 4-(4ꢀ-n-
alkoxybenzoy-loxy)-2-hydroxybenzylidene-2-naphthyl amines, as well as metallomesogens
of both series of ligands containing the Cu (II) atom, and have examined the phase transition
behavior of these compounds.
Naphthalene ring is an interesting aromatic core for the synthesis of liquid crystalline
compounds. It increases the breadth of the molecules, which can decrease the meso-
genic properties. However, the naphthyl derivative may exhibit rich mesomorphism, if the
molecule is properly designed. In continuation of our work on liquid crystalline naphthalene
derivatives, we have synthesize two new mesogenic homologous series viz. 2-Naphthyl-4-n-
alkoxybenzoates (Series I) and trans-2-Naphthyl-4-n-alkoxycinnamates (Series II) to study
the effect of naphthalene moiety and ester and cinnamoyl central linkages on mesomorphism.
Experimental
4-Hydroxybenzoic acid, 4-hydroxybenzaldehyde, 1-bromoalkane, potassium hydroxide
(KOH), potassium carbonate (K2CO3), malonic acid, pyridine, pipyridine, 2-naphthol, DCC,
DMAP, and THF were used as received. Solvents were dried and distilled before use. Micro-
analysis of the compounds was performed on a Coleman carbon–hydrogen analyzer, and the
values obtained are in close agreement with those calculated. IR spectra were determined for
1
KBr pellets, using a Shimadzu IR-408 spectrophotometer. H NMR spectra were obtained
with a Perkin-Elmer R-32 spectrometer using tetramethylsilane (TMS) as internal reference
standard. The chemical shifts are quoted as δ (ppm) downfield from the reference; CDCl3

108
H. N. PATEL AND A. K. PRAJAPATI
Scheme . Synthetic route to series I and II compounds
was used as solvent for all the compounds. The phase assignments and transition tempera-
tures were determined by thermal polarizing optical microscopy using a polarizing micro-
scope (Leitz Laborlux 12 POL) equipped with a heating stage. The enthalpies of transitions,
reported in J g−1, were measured by cryo-DSC model DSC 822e, Mettler Toledo, Switzerland,
having stare software system, at a scanning rate of 5°C min−1. The instrument was calibrated
using pure indium as a standard.
The synthetic route to compounds of series I and II is illustrated in Scheme 1.
4-n-alkoxybenzoic acids [A] were synthesized from 4-hydroxybenzoic acid by employing a
Williamson’s ether synthesis protocol[51]. 4-n-Alkoxybenzaldehydes [B] were prepared using
the method of Vyas and Shah [52]. trans-4-n-Alkoxycinnamic acids [C] were synthesized
from the corresponding 4-n-alkoxybenzaldehydes and malonic acid following the method
reported by Gray and Jones [53]. 0.1 Mole of 4-n-Alkoxybenzoic acids (for series I)/0.1 mole
of trans-4-n-Alkoxycinnamic acids (for series II) (for series II), 0.1 mole of 2-naphthol, 0.1
mole of DCC [54], 0.01 mole of DMAP were dissolved in dry THF and stirred at room tem-
perature for overnight. The insoluble solid was removed through filtration. The solution was
chromatographed on silica gel using petroleum ether (60–80°C)–ethyl acetate mixture (97:3)
as eluant. Removal of solvent from the eluate afforded a solid material which was crystallized
repeatedly from methanol till constant transition temperatures were obtained. The purities of
all these compounds were checked by thin layer chromatography (Merck kiesel gel 60F254 pre-
coated plates). Yield in general is 70–75%. The elemental analysis of all the compounds was
found to be satisfactory. The UV, IR, and 1H NMR spectral data of representative compounds

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
109
(n-tetradecyloxy derivative of series I and II) were found to be consistent with the proposed
structure.
Series I: 2ꢀ-Naphthyl-4- n –tetradecyloxybenzoate
UV Specturm (MDC) nm: 262 and 286.
IR spectrum (KBr) νmax/cm−1: 2918, 1732 ( COO-), 1601, 1515, 1464, 1286, 1249, 1160,
–
816.
1
–
–
–
H NMR spectrum (400 MHz): δ 0.90 (t, 3H, CH3), 1.0–1.57 (m, 20H, 10 × CH2-),
– –
1.85 (quant., 2H, Ar-O C CH2-), 4.00 (t, 2H, -O CH2-), 6.95 (d, 2H, Ar-H at C-3 and C-
5), 7.35(d, 2H of naphthalene ring system at C-5ꢀand C-8ꢀ), 7.50–7.60 (m, 2H of naphthalene
ring system at C-6ꢀ and C-7ꢀ), 7.70 (s, 1H of naphthalene ring system at C-1ꢀ), 7.80–7.95(m,
2H of naphthalene ring system at C-3ꢀ and C-4ꢀ), 8.20 (d, 2H, Ar–H at C-2 and C-6).
Series II: trans- 2ꢀ-Naphthyl-4- n-tetradecyloxycinnamates
UV Spectrum (MDC) nm: 275 and 325.
−1
–
–
–
=
–
IR spectrum (KBr) νmax/cm : 2916, 2872, 1745 ( COO ), 1603 ( CH CH ), 1600, 1512,
1
–
1464, 1286, 1249, 1160, 817. H NMR spectrum (400 MHz): δ 0.90 (t, 3H, CH3), 1.20–1.70
–
– – –
– –
(m, 20H, 10 x CH2-), 1.85 (quant., 2H, Ar O C CH2-), 4.00 (t, 2H at O CH2-), 6.45 (d,
–
=
–
1H, Ar CH ), 6.95 (d, 2H, Ar H at C-3 and C-5), 7.20–7.30 (m, 2H of naphthalene ring
system at C-6ꢀ and C-7ꢀ), 7.40 (d, 2H of naphthalene ring system at C-5ꢀ and C-8ꢀ), 7.55 (d, 1H,
ꢀ
–
–
= CH COO ), 7.65 (s, 1H of naphthalene ring system at C-1 ), 7.90 (d, 2H of naphthalene
ꢀ
ꢀ
–
ring system at C-3 and C-4 ), 8.11 (d, 2H, Ar H at C-2 and C-6).
Results and discussion
Optical microscopy studies
As a preliminary investigation, the mesophases exhibited by compounds of series I and II
were concluded by the optical microscopic studies.
On cooling, isotropic liquids of series I, n-hexyloxy to n-hexadecyloxy derivatives formed
small droplets that coalesced to the classical schileren texture of the nematic phase. On
further cooling n-decyloxy to n-hexadecyloxy derivatives also gave focal conic texture
characteristic of smectic A mesophase. On cooling, isotropic liquids of series II, methoxy to
n-tetradecyloxy derivatives exhibit schileren texture of the nematic phase which on further
cooling exhibited focal conic texture characteristic of smectic A mesophase for the n-hexyloxy
to n-tetradecyloxy derivatives (Fig. 1). On cooling, isotropic liquids of the last homologues
of series II (n = 16) gave only focal conic texture characteristic of smectic A mesophase.
Differential scanning calorimetry studies
Calorimetry is a valuable method for the detection of the liquid crystalline phase transition.
It yields quantitative results; therefore, conclusion may be drawn concerning the nature of the
phases which participate in the transition. In the present study, enthalpies of n-hexyloxy and
n-octyloxy derivatives of series I as well as n-tetradeccloxy derivative of series II as represen-
tative members were measured by DSC (cryo-DSC model DSC 822e, Mettler Toledo, Switzer-
land, having stare software). Data are recorded in Table 1 and the enthalpy values of the phase
transitions agree well with the existing literature value [55] which has helped in further con-
firmation of the mesophase type. The phase transition behavior were studied from ambient

110
H. N. PATEL AND A. K. PRAJAPATI
Figure . Microphotograph of (a) schlieren texture of nematic mesophase (Series II; n = ) at °C and (b)
focal conic texture of SmA mesophase (Series II; n = ) at °C on cooling.
temperature to 120°C with a rate of 5°C per minute and cooling from 120°C to 25°C with a
similar scanning rate (i.e., 5°C /min). The differential calorimetry thermograms shows one
endotherm for the crystal to isotropic phase transition (i.e., isotropisation) peak on heating
scan, and at the time of cooling it shows first exotherm for the isotropic to nematic mesophase
transitions and the second endotherm represents mesophase to crystallization phase for both
the compounds of series I. When the compounds melt to its isotropic temperature its calori-
metric values are very high as compared from its isotropisation to crystal growth or any-other
phase transition occurs. Here, series I and series II compounds have higher enthalpy values
at the isotropic temperature while heating cycle. But when the compounds shows some phase
Table . DSC data for series I and II compounds.
Phase sequence
Series
Compound No
Heating
Cooling
I
I
II
6
8
23
Cr . [.]a Iso
Cr . [.]a Iso
Iso . [.] N . [.]a Cr
Iso . [.] N . [.]a Cr
Iso . [.] N . [.]
SmA . [.] Cr
Cr . [.]a SmA .
[.] N . [.] Iso
a
–
Total enthalpy including any other Cr Cr transition

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
111
transition before crystallization from the isotropic temperature, its enthalpy values are compa-
rable less as expected. For series I, on heating cycle, the compounds 6 and 8 showed one peak
at 87.96°C (࢞H = 96.55 J/g) and 88.32°C (࢞H = 142.26 J/g) which corresponds to the Cr–I
transitions, respectively. On cooling, the compounds 6 and 8 showed the two peaks at 61.87°C
(࢞H = 1.36 J/g) and 58.77°C (࢞H = 57.03 J/g); and 67.88°C (࢞H = 1.51 J/g) and 61.77°C
(࢞H = 87.03 J/g) for the I–N and N–Cr transitions, respectively. These values of the enthalpy
changes for the N–I for 6 and 8 if converted to entropy changes then it should be noted that
they are very small and what would be the expected for nematic–isotropic transitions [55]. The
differential calorimetry thermograms of compound 23 show three endotherm for the crys-
tal to smectic A mesophase transition, smectic A to the nematic mesophase and nematic to
isotropic phase transition peak on heating scan, and at the time of cooling also it shows three
exotherm for the isotropic to nematic mesophase transitions, nematic to smectic A mesophase
transition and the third endotherm represents smectic A mesophase to crystallization phase.
For series II, on heating cycle, the compounds 23 showed three peak at 73.51°C (࢞H =
68.31 J/g), 93.72°C (࢞H = 2.23 J/g), and 111.02°C (࢞H = 1.02 J/g) which corresponds to
the Cr–SmA, SmA–N, and N–I transitions, respectively. On cooling also, the compound 23
showed three peaks at 110.54°C (࢞H = 0.96 J/g), 93.43°C (࢞H = 2.67 J/g), and 68.32°C
(࢞H = 35.60 J/g) for the I–N, N–SmA, and SmA–Cr transitions, respectively.
Mesomorphic behavior
Series I: 2ꢀ-Naphthyl-4-n-alkoxybenzoates
Twelve compounds have been synthesized in series I. Methoxy to n-pentyloxy derivatives
are non mesogenic, n-hexyloxy to n-hexadecyloxy derivatives exhibit monotropic nematic
mesophase. Smectic A mesophase commences from n-decyloxy derivatives as a monotropy
and persist up to the last member synthesized. The transition temperatures are recorded in
Table 2. The plot of transition temperatures against the number of carbon atoms in the alkyl
chain for the series II is given in Fig. 2. It can be noticed that crystal-isotropic transition
temperatures decrease sharply with an initial increase in the length of terminal alkoxy tail
except for the last two homologues. The SmA–N and N–I transition temperatures exhibited
rising tendency.
Series II: trans-2-Naphthyl -4-n–alkoxycinnamates
All the 12 compounds exhibit mesomorphism. Methoxy to n-propyloxy derivatives exhibit
monotropic nematic mesophase, and n-butyloxy to n-tetradecyloxy derivatives exhibit enan-
tiotropic nematic mesophase. n-Hexayloxy to n-hexadecyloxy derivatives exhibit enan-
tiotropic smectic A mesophase. The transition temperatures are recorded in Table 2. The plot
of transition temperatures against the number of carbon atoms in the alkoxy chain (Fig. 3),
from which it can be noticed that the crystal-mesophase/isotropic transition temperatures
decrease sharply with the increase in the length of terminal alkoxy tail and covers the larger
area, which is responsible for enantiotropic nature of two series from n-butyloxy onwards
derivatives. The N–I transition temperatures decrease with increase in terminal alkoxy chain
and exhibit pronounced odd even effect. SmA–N transition temperatures exhibited falling
tendency up to n-decyloxy derivatives and then rises up to n-tetradecyloxy derivatives.
It is now well established that the thermotropic liquid crystals are highly sensitive to their
molecular constitution. To determine the effects of alterations in the molecular core on the

112
H. N. PATEL AND A. K. PRAJAPATI
Table . Transition temperatures (°C) of the series I and II compounds.
=–
Compound No.
R
CnHn+ n=
Cr
Sm A
N
I
Series I
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
1
2
























—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
)
)
)
)
—
—
—
—
—
—
—
—
—
—
)
)
)
)
)
)
)
3
4
5
r
6
(
(
(
(
(
(
(
r
7
r
8
r
r
9
(
(
(
(
r
r
10
11
r
r
r
r
12
Series II
13
14
15
16
17
18
19
20
21
22
23
24
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
r
























—
—
—
—
—
—
—
—
—







(
(
(
)
)
)






r
r
r
r
r
r
r
r
r
r
—
r
r
r
r
r
r
r


—
—
r=
() = monotropic value; Cr = crystalline solid; SmA = smectic A phase; N = nematic phase; I = isotropic liquid phase;
phase exists.
mesogenic properties of a compound, the thermal stability and mesophase length as a mea-
sure of mesomorphism can be correlated with the molecular constitution of the compounds.
Table 3 summarizes the mesophase length (width of mesophase), thermal stability (SmA–N
and N–I transition temperatures) and molecular structure of the n-dodecyloxy derivative of
present series I and II (compounds 10 and 22) and structurally related compounds A [43]
and B [45]. Table 3 indicates that series I and A are mesogenic, whereas series B is non-
mesogenic. The molecules of series I, A and series B differ in central linkages. Series I have
Figure . The phase behavior for series I compounds.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
113
Figure . The phase behavior for series II compounds.
–
–
= –
ester ( COO-) central linkage, Series A have central azomethine ( CH N ) linkage and the
molecules of series B have azo central linkage. It seems that ester and azomethine linkage is
more conductive to mesomorphism as compared to azo central linkage. Reference to Table 3
shows that the smectic mesophase length and thermal stabilities of series II is higher by
19°C and 33°C, respectively, and the nematic mesophase length as well as thermal stability of
series II is higher by 9°C and 42°C than series I. Reference to the molecular structure of both
these compounds shows that both compounds differ only at the central linkage. The series II
–
=
–
have central cinnamoyloxy ( CH CH–COO ) linkage, whereas series I have central ester
( COO ) linkage. Series II is longer and more polarizable compared to the series I due to the
presence of central cinnamoyloxy ( CH CH COO ) linkage. Gray [56] has explained that
the addition of double bond to the system increase the length and polarizability of rod-like
molecules. Therefore, the greater smectic and nematic mesophase range and thermal stability
of series II can be explained in terms of the greater molecular length and polarizability of the
–
–
–
=
–
–
–
=
–
molecule resulting from additional ( CH CH ) units in the central linkages.
Table 3 shows that smectic and nematic mesophase length as well as thermal stabilities of
series II are higher as compared to series A and B. This is understandable as the molecule
of series II is little longer and more polarizable as compared to series A and B due to the
–
=
–
additional CH CH unit in the central linkage.
Table . The mesophase length, phase transition and comparative molecular structure of n-dodecyloxy
derivative of Series I, II and A, B.
Mesophase Length
Thermal stabilities
Commencement of
Smectic phase
Series No.
Sm
N
SmA-N
N-I
I















II
A
B
-Non Mesogenic-
—
- - -

114
H. N. PATEL AND A. K. PRAJAPATI
Conclusion
Two new mesogenic homologous series containing naphthalene moiety were synthesized. The
–
–
–
–
presence of cinnamoyloxy ( CH CH COO ) central linkage enhances the mesophase length
–
– – – – – = –
and polarizability of the molecule as compared to COO / CH N / N N central linkages
–
=
–
due to additional CH CH units in the central linkages.
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