Synthesis, characterization, crystal structure and liquid crystal studies of some symmetric naphthalene derivative molecules

Article abstract of DOI:10.1016/j.molstruc.2018.07.036

The synthesis of a series of symmetrical liquid crystals having naphthalene as the central rigid core and attached to long chain flexible 4′-alkoxybenzoate moiety at positions 2,6 - of the aromatic ring are reported. The mesophase behavior of the molecules was investigated using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction studies. The studies reveal that the synthesized compounds exhibit stable enantiotropic mesophase of Smectic A (SmA) and Nematic (N) phase. The mesophase appearance is independent of the length of the alkyl chain. The molecular structure was determined by single crystal X-ray diffraction technique. The derivative with n-heptyloxy- flexible chain crystallizes under triclinic, P_1ˉspace group with unit cell dimension a = 5.569(3) ? b = 10.540(5) ? c = 15.254(9) ? α = 73.434(18)^o, β = 80.807(19)^o and γ = 82.02(2)^o, V = 843.1(8) ?³. The dihedral angle between the naphthalene ring system and the benzoate moiety is 63.63(2)^o.

Full text of DOI:10.1016/j.molstruc.2018.07.036

Accepted Manuscript
Synthesis, characterization, crystal structure and liquid crystal studies of some
symmetric naphthalene derivative molecules
H.T. Srinivasa, B.S. Palakshamurthy, H.C. Devarajegowda, S. Hariprasad
PII:
S0022-2860(18)30855-X
10.1016/j.molstruc.2018.07.036
MOLSTR 25441
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Accepted Date:
20 April 2018
10 July 2018
Please cite this article as: H.T. Srinivasa, B.S. Palakshamurthy, H.C. Devarajegowda, S.
Hariprasad, Synthesis, characterization, crystal structure and liquid crystal studies of some
symmetric naphthalene derivative molecules, Journal of Molecular Structure (2018), doi: 10.1016/j.
molstruc.2018.07.036
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Graphical abstract
O
C O
O
O C
RO
OR
Liquid crystals
R = C6H13 - C18H37
Nematic phase
Smectic phase
Single crystal

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Synthesis, characterization, crystal structure and liquid crystal studies of some symmetric
naphthalene derivative molecules
H. T. Srinivasa1, B. S. Palakshamurthy2, H. C. Devarajegowda3 and S. Hariprasad4*
1Raman Research Institute, Soft Condensed Matter Group, Sadashivanagar, Bengaluru-560 080
2 Department of PG Studies and Research in Physics, University College of Science,
Tumkur University, Tumakuru-572 103
3Department of Physics, Yuvaraja’s College, Mysore University, Mysuru-570005
4Department of Chemistry, Central College Campus, Bangalore University, Palace Road,
Bengaluru-560 001, KARNATAKA, INDIA
*Corresponding author E-mail: hariprasad@bub.ernet.in
Abstract: The synthesis of a series of symmetrical liquid crystals having naphthalene as the
central rigid core and attached to long chain flexible 4′-alkoxybenzoate moiety at positions 2,6 -
of the aromatic ring are reported. The mesophase behavior of the molecules was investigated
using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray
diffraction studies. The studies reveal that the synthesized compounds exhibit stable
enantiotropic mesophase of Smectic A (SmA) and Nematic (N) phase. The mesophase
appearance is independent of the length of the alkyl chain. The molecular structure was
determined by single crystal X-ray diffraction technique. The derivative with n-heptyloxy-
flexible chain crystallizes under triclinic, P space group with unit cell dimension a = 5.569(3) Å,
1
b = 10.540(5) Å, c = 15.254(9) Å, α = 73.434(18)o, β = 80.807(19)o and γ = 82.02(2)o, V =
843.1(8) Å3. The dihedral angle between the naphthalene ring system and the benzoate moiety is
63.63(2)o.

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Key words: Liquid crystal, naphthalene, smectic, nematic, schleiren texture, symmetric.
1. Introduction
Conventional rod-like liquid crystals are composed of a central rigid core with one/two flexible
hydrocarbon tails at terminal ends. Over the last three decades, a variety of such motifs have
been studied which highlights their significance [1-4]. Many conventional liquid crystal
materials have been synthesized which exhibit a variety of mesophases [5]. In order to
determine the correlation between molecular structure and physical properties, various molecular
designs have been utilized to achieve target molecules with expected and desired properties [6-
8]. From these studies, it is generally acknowledged that a simple change in the length of the
achiral aliphatic terminal chains can significantly affect the mesomorphic properties of liquid
crystals [9, 10].
Naphthalene is a molecule with central rigid core. Due to its aromaticity, it is planar. Its
derivatives have been extensively investigated for their fascinating liquid crystal properties such
as electromagnetic and photophysical properties. Their unusual characteristics have been applied
to use in LCD devices.
In this regard, we expected that their further study and development would attract the
interest of the vast scientific community by-and-large [11-13]. Recent studies by other scientists
too have focused on naphthalene cores with various differing functionalities. A variety of
derivatives have been examined for their liquid crystalline mesophases by taking into
consideration the geometric shapes of the molecular structures. The studies have inferred that
the acute angle at central naphthalene core does not favor mesomorphism. However, it is also
found that moderate and strain free molecular structures do favor the LC property [14, 15].

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The modification of naphthalene core with straight molecular shapes by slightly differing
the positions of the alkyl chains have resulted in the strong formation of smectic phase [16, 17].
Other examples for this observation include single benzene ring compounds with a semi-
fluorinated chain, which unexpectedly exhibited stable smectic phase [18, 19].
In practice, with naphthalene core, only small alkyl chain derivatives can be used to get
low temperature nematic phase which is useful for the liquid crystal applications [20, 21].
There also exists a report on naphthalene molecules showing an intense bluish purple
luminescence in the crystal state as a photophysical property [22]. Thermotropic polyesters
based on 2,6-disubstituted naphthalene are also reported to possess unique thermotropic
properties and find applicability as high strength plastics and fiber material [23].
We were also of the opinion and reasoned that some insight into their solid-state
structures may widen their applicability further since the thermotropic polyesters are utilized as
solids. In this regard, it may be noted that the 2,6-disubstituted naphthalene units give structures
parallel to a polymer backbone. Thus, it was of interest to study the effects of incorporation of a
2,6-disubstituted naphthalene unit on mesomorphic properties and solid-state structures [24].
To our knowledge, the liquid crystal and single crystal formation in symmetric
naphthalene rod-like molecules has been not explored [25]. We presumed that naphthalene core
will promote the single crystal characters, which is the main aim of our present studies under
liquid- and single- crystal studies.
In this article, we report the preparation and investigation of the mesophasic properties of
a series of homologous rod-like molecules: the 2,6-bis-(4ʹ-alkoxybenzoate)naphthalene
derivatives which possess the 2,6-etheric-naphthalene core, in conjunction with straight alkyl
chains attached to the external side of the naphthalene core.

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2. Experimental
All chemicals used were of analytical grade. Purity of the compounds were checked by TLC
silica gel 60 F254 using pure dichloromethane as mobile phase and purified by column
chromatography on silica gel (230-400) mesh with dichloromethane as eluent. Re-crystallisation
of the solids was performed using appropriate protic alcoholic solvents like ethanol. The
structures of newly synthesized compounds were determined using infrared spectroscopy
(Shimadzu FTIR-8400 spectrophotometer), 1H NMR, 13C NMR, (Bruker Biospin 500
spectrometer), elemental analysis (Carlo-Erba1106 analyser). UV-Vis absorption spectra were
recorded using Perkin Elmer λ 35 double-beam spectrophotometer. The transition temperature
and the associated enthalpy value for all compounds were determined using differential scanning
calorimetry (DSC): Perkin-Elmer, Model Pyris 1. The textures of mesophase were observed
using a polarising optical microscope (POM): Olympus BX50 equipped with a heating hot
stage: Mettler FP82HT and a central processor Mettler FP90. The thermogravimetric analysis
(TGA) was measured under nitrogen atmosphere at a heating rate of 20 °C/min using a Perkin
Elmer TGA-4000 thermal analyzer. X-Ray diffraction (XRD) patterns were obtained at liquid
crystalline state using a DY 1042-Empyrean X-ray diffractometer with a pixel 3D detector and
Cu-Kα radiation.
Single crystal of compound 5b of the size 0.28 × 0.2 × 0.16 mm3, was selected under a
polarizing microscope and affixed to Hampton Research Cryoloops using paratone-N oil for
single crystal data collection. The X-ray diffraction data sets were collected on a Bruker
SMART APEX II CCD diffractometer using Mo Kα radiation. The crystal-to-detector distance
was fixed at 40 mm for all the crystals. The scan width per frame was Δω = 0.5°. The cell

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refinement and data reduction were carried out using the SAINT [26]. The crystal structure was
solved by direct methods using SHELXS97 [27] and refined in the spherical-atom approximation
(based on F2) by using SHELXL97 [28] included in the WinGX package suite [29]. The ORTEP
and packing diagrams (Figures 7 and 8) were generated using the MERCURY package [30]. The
experimental and all crystallographic information are listed in Table 2.
2.1 General synthetic procedure for the preparation of 2,6-bis-(4ʹ-
alkoxybenzoate)naphthalene derivatives:
To
a
suspension of 2,6-dihydroxynaphthalene (4) (0.1 g, 6.2 mmol),
dicyclohexylcarbodiimide (DCC) (0.146 g, 7.0 mmol) and dimethylaminopyridine (DMAP)
(0.04 g) catalyst in dry CH2Cl2 (20 mL), the 4-alkoxybenzoic acids 3a-i (12.42 mmol) were
added individually. The reaction mixture was stirred for 1 h at room temperature and progress
of the reaction was monitored by thin layer chromatography using dichloromethane as mobile
phase. After completion of the reaction, as indicated by the disappearance of the reactant spots,
the reaction mass was diluted with water and extracted with CH2Cl2 (25 mL × 2). The organic
layer was washed with water and dried over anhydrous sodium sulphate. The crude product thus
obtained was concentrated on a rotary evaporator and purified by column chromatography using
silica gel as stationary phase and pure CH2Cl2 as mobile phase. The eluted product was
concentrated under vacuum to isolate off-white solids. Further re-crystallization from ethanol
gave the pure products 5a-i with isolated yields in the range 85-90%.
2.2 Spectroscopic data:
2,6-Bis-(4ʹ-n-hexyloxybenzoate)naphthalene 5a.

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1
IR (KBr): v = 2924, 2854, 1720, 1604, 1464, 1259, 1066 cm-1. H NMR (500 MHz, CDCl3): δ =
8.20 (m, 4H, Ar-H), 7.88 (m, 2H, Ar-H), 7.70 (s, 2H), 7.38 (m, 2H), 6.99 (m, 4H), 4.06 (t, J =
6.5 Hz, 4H), 1.92-1.80 (m, 4H), 1.56-1.35 (m, 12H), 0.93 (t, J = 6.65 Hz, 6H) ppm; 13C NMR
(125 MHz, CDCl3): 165.1, 163.5, 148.4, 132.2, 131.7, 129, 122.2, 121.6, 118.8, 114.4, 68.4,
31.8, 29.2, 29.0, 25.8, 22.7, 14.2 ppm. Elemental analysis calculated for C36H40O6; C, 76.03; H,
7.09; found C, 76.10; H, 7.17%.
2,6-Bis-(4´-n-heptyloxybenzoate)naphthalene 5b. IR (KBr): v = 2924, 2852, 1722, 1602, 1458,
1
1257, 937 cm-1. H NMR (500 MHz, CDCl3): δ = 8.23 (m, 4H, Ar-H), 7.90 (m, 2H, Ar-H), 7.74
(s, 2H), 7.42 (m, 2H), 7.0 (m, 4H), 4.09 (t, J = 6.5 Hz, 4H), 1.88-1.84 (m, 4H), 1.59-1.36 (m,
16H), 0.95 (t, J = 6.65 Hz, 6H) ppm; 13C NMR (125 MHz, CDCl3): 165.1, 163.6, 148.6, 132.3,
131.8, 129, 122.2, 121.5, 118.7, 114.3, 68.3, 31.8, 29.1, 29.0, 25.9, 22.6, 14.1 ppm. Elemental
analysis calculated for C38H44O6; C, 76.48; H, 7.43; found C, 76.53; H, 7.49%.
2,6-Bis-(4ʹ-n-octyloxybenzoate)naphthalene 5c. IR (KBr): v = 2922, 2854, 1724, 1604, 1456,
1
1257, 844 cm-1. H NMR (500 MHz, CDCl3): δ = 8.22 (m, 4H, Ar-H), 7.87 (m, 2H, Ar-H), 7.69
(s, 2H), 7.36 (m, 2H), 6.97 (m, 4H), 4.02 (t, J = 6.5 Hz, 4H), 1.91-1.80 (m, 4H), 1.54-1.34 (m,
13
20H), 0.92 (t, J = 6.65 Hz, 6H) ppm; C NMR (125 MHz, CDCl3): 165.2, 163.7, 148.5, 132.4,
131.9, 129, 122.3, 121.4, 118.8, 114.4, 68.4, 31.9, 29.2, 29.1, 26, 22.5, 14.2 ppm. Elemental
analysis calculated for C40H48O6; C, 76.89; H, 7.74; found C, 76. 97; H, 7.83%.
2,6-Bis-(4ʹ-n-nonyloxybenzoate)naphthalene 5d. IR (KBr): v = 2924, 2852, 1730, 1602, 1456,
1377, 1257, 1064, 844 cm-1. 1H NMR (500 MHz, CDCl3): δ = 8.20 (m, 4H, Ar-H), 7.88 (m, 2H,

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Ar-H), 7.70 (s, 2H), 7.38 (m, 2H), 6.99 (m, 4H), 4.06 (t, J = 6.5 Hz, 4H), 1.92-1.80 (m, 4H),
1.56-1.35 (m, 24H), 0.93 (t, J = 6.65 Hz, 6H) ppm; 13C NMR (125 MHz, CDCl3): 165.1, 163.5,
148.5, 132.2, 131.7, 129, 122.1, 121.4, 118.6, 114.2, 68.2, 31.7, 29.1, 29.0, 25.8, 22.7, 14.2 ppm.
Elemental analysis calculated for C42H52O6; C, 77.27; H, 8.03; found C, 77.33; H, 8.10%.
2,6-Bis-(4ʹ-n-decyloxybenzoate)naphthalene 5e. IR (KBr): v = 2924, 2853, 1723, 1605, 1454,
1256, 845 cm-1. 1H NMR (500 MHz, CDCl3): δ = 8.22 (m, 4H, Ar-H), 7.86 (m, 2H, Ar-H), 7.72
(s, 2H), 7.39 (m, 2H), 7.00 (m, 4H), 4.05 (t, J = 6.5 Hz, 4H), 1.93-1.81 (m, 4H), 1.57-1.36 (m,
13
28H), 0.94 (t, J = 6.65 Hz, 6H) ppm; C NMR (125 MHz, CDCl3): 165.3, 163.5, 148.7, 132.2,
131.7, 129, 122.3, 121.6, 118.8, 114.4, 68.4, 31.9, 29.2, 29.0, 26, 22.5, 14.2 ppm. Elemental
analysis calculated for C44H56O6; C, 77.61; H, 8.29; found C, 77.67; H, 8.35%.
2,6-Bis-(4ʹ-n-dodecyloxybenzoate)naphthalene 5f. IR (KBr): v = 2924, 2855, 1729, 1605,
1
1458, 1259, 1056, 840 cm-1. H NMR (500 MHz, CDCl3): δ = 8.23 (m, 4H, Ar-H), 7.90 (m, 2H,
Ar-H), 7.72 (s, 2H), 7.40 (m, 2H), 7.01 (m, 4H), 4.08 (t, J = 6.5 Hz, 4H), 1.93-1.80 (m, 4H),
1.57-1.36 (m, 36H), 0.96 (t, J = 6.65 Hz, 6H) ppm; 13C NMR (125 MHz, CDCl3): 165.2, 163.7,
148.7, 132.4, 131.9, 129, 122.3, 121.6, 118.8, 114.4, 68.4, 31.9, 29.2, 29.0, 25.9, 22.7, 14.1 ppm.
Elemental analysis calculated for C48H64O6; C, 78.22; H, 8.75; found C, 78.29; H, 8.83%.
2,6-Bis-(4ʹ-n-tetradecyloxybenzoate)naphthalene 5g. IR (KBr): v = 2924, 2858, 1726, 1606,
1
1458, 1256, 1060, 786 cm-1. H NMR (500 MHz, CDCl3): δ = 8.20 (m, 4H, Ar-H), 7.88 (m, 2H,
Ar-H), 7.71 (s, 2H), 7.38 (m, 2H), 6.98 (m, 4H), 4.06 (t, J = 6.5 Hz, 4H), 1.93-1.80 (m, 4H),
1.54-1.35 (m, 44H), 0.93 (t, J = 6.65 Hz, 6H) ppm; 13C NMR (125 MHz, CDCl3): 165.1, 163.5,

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148.6, 132.3, 131.7, 129, 122.2, 121.7, 118.7, 114.5, 68.3, 31.9, 29.1, 29.0, 25.9, 22.7, 14.1 ppm.
Elemental analysis calculated for C52H72O6; C, 78.75; H, 9.15; found C, 78.85; H, 9.24 %.
2,6-Bis-(4ʹ-n-hexadecyloxybenzoate)naphthalene 5h. IR (KBr): v = 2928, 2850, 1730, 1608,
1
1454, 1253, 842 cm-1. H NMR (500 MHz, CDCl3): δ = 8.21 (m, 4H, Ar-H), 7.88 (m, 2H, Ar-
H), 7.70 (s, 2H), 7.38 (m, 2H), 6.99 (m, 4H), 4.06 (t, J = 6.5 Hz, 4H), 1.92-1.80 (m, 4H), 1.56-
1.35 (m, 52H), 0.93 (t, J = 6.65 Hz, 6H) ppm; 13C NMR (125 MHz, CDCl3): 165.2, 163.7, 148.6,
132.4, 131.8, 129, 122.3, 121.6, 118.7, 114.6, 68.3, 31.9, 29.1, 29.1, 25.9, 22.8, 14.2 ppm.
Elemental analysis calculated for C56H80O6; C, 79.20; H, 9.50; found C, 79.27; H, 9.59 %.
2,6-Bis-(4ʹ-n-octadecyloxybenzoate)naphthalene 5i. IR (KBr): v = 2920, 2852, 1720, 1602,
1
1454, 1257, 1165, 937 cm-1. H NMR (500 MHz, CDCl3): δ = 8.23 (m, 4H, Ar-H), 7.89 (m, 2H,
Ar-H), 7.71 (s, 2H), 7.39 (m, 2H), 7.0 (m, 4H), 4.06 (t, J = 6.5 Hz, 4H), 1.94-1.82 (m, 4H), 1.56-
1.37 (m, 60H), 0.95 (t, J = 6.65 Hz, 6H) ppm; 13C NMR (125 MHz, CDCl3): 165.1, 163.8, 148.8,
132.4, 131.7, 129, 122.5, 121.7, 118.6, 114.5, 68.5, 31.6, 29.3, 29.1, 25.9, 22.5, 14.2 ppm.
Elemental analysis calculated for C60H88O6; C, 79.60; H, 9.80; found C, 79.67; H, 9.88 %.
3. Results and discussion
Low and high molecular mass mesogens of naphthalene derivatives which exhibit
attractive thermosetting and photophysical properties have been reported in literature [10-22].
Due to these interesting properties of the naphthalene core unit, the present work is mainly
focused on the synthesis of a library of naphthalene derivatives which may exhibit liquid crystal
and single crystal properties. We now report the synthesis of the naphthalene derivatives 5a-i,
and their liquid crystalline property studies.

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All 4-alkoxybenzoic acids 3a-i were prepared according to reported literature procedures
[31, 32]. The esterification of 2,6-dihydroxynaphthalene with the 4-alkoxybenzoic acids 3a-i
using DCC as coupling reagent gave the products 5a-i. The general broad outline for the
preparation of nine products of 2,6-bis-(4ʹ-alkoxybenzoate)naphthalene derivatives 5a-i is
highlighted in Scheme 1.
OR
OR
OH
K2CO3, R-Br
10% Aqueous KOH
EtOH, Reflux, 5hrs
HO
OH
OR
MEK, 90 0C, 24hrs
COOH
COOCH3
COOCH3
4
DCC, DMAP
CH2Cl2, rt, 1 hrs
3a-i
2
1
O
O
RO
C O
O C
5a. R = n-C6H13 5d. R = n-C9H19 5g. R = n-C14H29
5b. R = n-C7H15 5e. R = n-C10H21 5h. R = n-C16H33
5c. R = n-C8H17 5f. R = n-C12H25 5i. R = n-C18H37
5a-i
Scheme 1. Synthetic scheme for the series of compounds of 5a-i
The chemical structures of new compounds 5a-i were characterized by IR, 1H-NMR, 13C-
NMR spectral studies and elemental analysis techniques. The general observations for the
spectral data are highlighted below for some individual compounds:
In the IR spectrum of compound 5a, the alkyl (C-H) stretching frequency was observed at
2924 cm-1, where as the carboxyl group (C=O) stretching frequency showed a strong absorption
band at 1720 cm-1. The ether (C-O) stretching frequency exhibited a strong absorption band at
1
1259 cm-1. The H NMR spectrum of compound 5b exhibited multiplet peaks in the region δ =

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8.23-7.0 for fourteen aromatic (Ar-H) protons. This range was found to be standard throughout
for the aromatic proton signals for the entire series of compounds. A triplet was observed at δ =
4.09 due to the etheric methylene protons (-OCH2-) of alkoxy chain. The terminal methyl (-CH3)
group protons of alkyl chain were found to resonate as a triplet at δ = 0.95 cm-1. The 13C NMR
spectral data of compound 5c shows singlets for the carboxylic carbon (C=O) of the ester
functionality and the aryloxy- carbon at δ = 165.2 and 163.7 cm-1 respectively, where as the
etheric methylene (C-O) signal peak appeared at δ = 68.3 cm-1. These values confirmed the
molecular structures of target compounds.
Thermotropic and LC texture behaviour of synthesized compounds were studied and
confirmed by the combination of DSC, POM and X-ray diffraction measurements at mesophase
stages, respectively. The DSC curves were obtained during two successive heating-cooling
scans at a heating/cooling rate of 5 C min-1. As can be seen from the curves, all compounds
exhibit enantiotropic liquid crystal properties. The spectral results are presented in experimental
section while transition temperatures along with associated enthalpies and mesophase types are
presented in Table 1.
Table 1. Transition temperatures and mesophase types of compounds 5a-i
Sl No
5a
n
6
Heating Scan
Cr1 133.02 (15.54) Cr2 145.34 (62.70) N
Cooling Scan
Iso 262.01 (3.72) N 120.74
263.37 (3.76) Iso
(59.06) Cr
5b
5c
5d
5e
7
8
Cr1 154.61 (85.73) N 248.46 (3.10) Iso
Iso 247.19 (2.90) N 120.20
(76.21) Cr
Cr 137.93 (13.29) SmA 142.84 (2.38) N
229.99 (2.44) Iso
Iso 228.97 (2.66) N 141.94 (2.34)
SmA 100.95 (53.58) Cr
Iso 227.46 (1.86) N 159.0 (3.55)
SmA 114.82 (67.79) Cr
Iso 220.66 (2.36) N 169.93 (4.00)
9
Cr 143.89 (70.55) SmA 160.24 (3.71) N
228.15 (2.31) Iso
10
Cr 135.84 (67.12) SmA 170.84 (4.17) N

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221.27 (2.44) Iso
SmA 112.83 (63.84) Cr
5f
5g
5h
5i
12
14
16
18
Cr 127.10 (67.86) SmA 181.33 (5.39) N
206.96 (2.10) Iso
Iso 206.35 (1.98) N 180.65 (5.11)
SmA 108.50 (44.18) Cr
Cr1 104.28 (18.46) Cr2 125.01 (67.49) SmA Iso 194.62 (2.10) N 182.80 (5.70)
184.40 (5.82) N 195.72 (2.32) Iso SmA 105.0 (65.77) Cr
Cr1 117.15 (22.32) Cr2 126.03 (75.52) SmA Iso 186.54 (1.95) N 183.12 (9.01)
183.83 (9.23) N 187.0 (2.08) Iso
SmA 106.28 (73.40) Cr
Iso 179.46 (13.64) SmA 108.55
(74.59) Cr
Cr 125.98 (73.77) SmA 180.54 (13.55) Iso
Cr = Crystal; SmA = Smectic A phase; N = Nematic phase; n = number of carbon atoms in the
terminal alkyl chain
3.1 Liquid crystal property
All the compounds 5a-i was found to possess rich mesomorphism, as smectic and nematic liquid
crystalline phases over a wide temperature range. Compound 5a showed enantiotropic phase
sequences of Cr-Cr-N-I transitions which were confirmed by DSC and POM textural
observation. On cooling, the isotropic-nematic transition appeared and it crystallized at 120.74
C (Table 1). The compound 5b also showed similar transitions with similar mesophase
behavior as that of 5a. DSC thermogram of compound 5b is given in Figure 1, where heating
and cooling transitions can be clearly seen. Compound 5c exhibited enantiotropic phase
sequences of Cr-Cr-SmA-N-I transitions. On cooling, the isotropic-nematic-smectic transitions
appeared and crystallized at 100.95 C. An additional smectic A phase was introduced in the
compound 5c. Under the polarizing optical microscope, focal-conic texture observed for smectic
A phase was observed for compound 5c as shown in Figure 2 (a). A typical mosaic texture for
nematic phase Fig 2 (b) of compound 5c was also observed. The compounds 5d-h showed
resemblance of 5c transition peaks on heating and cooling cycles, which are attributed to the Cr-

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smectic-N-Iso. On cooling, similar reverse peaks corresponding to and I-N-smectic-Cr
transitions were observed. The POM observations revealed that the other type of texture for
nematic phase is Schlieren texture. Figure 2 (c) texture was also observed for compound 5h.
Compound 5i on heating melted at about 125.98 °C to the SmA mesophase (ΔH = 73.77 kJ mol-
1) and finally cleared (isotropic) at 180.54 °C (ΔH = 13.55kJ mol-1). On cooling, it showed a
transition in DSC centered at around 179.46 °C (ΔH = 13.64kJ mol-1) followed by crystallization
at 108.55 °C (ΔH = 74.59 kJ mol-1). Under POM, it exhibited a well-defined smectic batonnets
texture for smectic A mesophase as shown in Figure 2 (d). Compound 5i exhibited only smectic
A phase, which is good agreement with the theory of the homologous series of liquid crystals.
Thus, the mesomorphic range of the smectic phase is shorter than that of the nematic phase.
From these observations, we conclude that the series 5a-i under present discussion here is
predominantly smectogenic and partly nematogenic. The statement that liquid crystallinity
involving the naphthyl- unit with lower chain length substitutions yields nematic phase is re-
iterated. Moderate chain length gives both nematic and smectic phase; and higher chain length
affords predominantly only smectogenic character in the homologues and is found in the present
investigation.
Further, the crystallizing tendency arising in lower members is probably due to their
relatively high level of forces of attraction. The lack of flexibility also causes for nematic
mesophase formation in the lower members 5a and 5b of the series.

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Compd 5b
100
50
0
Crystal
Nematic phase
Iso
Crystal
Nematic Phase
100
150
200
250
Temperature in (0C)
Figure 1. Differential scanning calorimetric trace of 5b
The thermal behavior of all compounds 5a-i is in accordance with literature. Hence,
increasing the terminal chain length (5a vs. 5i), as expected, decreases the transition temperature
(Table 1). In addition, the odd-even effect on the melting temperature was seen to compound 5a-
d, where compounds 5a-d has a higher isotropic transition temperature compared to 5e-i. The
enthalpy changes determined by DSC are in agreement with the values usually observed for
typical crystalline-nematic, nematic-isotropic, crystalline-smectic, smectic-nematic, nematic-
isotropic, smectic-isotropic and smectic-crystalline transitions which confirm the presence of a
smectic and a nematic mesophases. The solid-liquid crystal or liquid crystal-solid transitions
involve much more energy than the liquid crystal-isotropic liquid or liquid crystal-liquid crystal
transitions [33-35]. It is also found that increasing terminal chain length decreases the transition
temperature as expected. A number of reports are found in literature to support such studies in
calamatic liquid crystalline materials [36, 37].

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Fig. 2 Polarizing optical microscope images. (a) Focal-conic texture of SmA phase of 5c, (b)
Mosaic texture of nematic phase for 5c, (c) Schlieren nematic texure of 5h, (d) Batonnets texture
of SmA phase for 5h.
A plot of heating cycle transition temperatures as a function of the number of carbon atoms in
the terminal n-alkoxy chain is shown in Figure 3. The plot shows fairly dropping smooth curves
in relation to both melting and isotropic transition temperatures of compounds 5a-i. This study
shows that lower homologous members’ exhibit only nematic phase, middle homologous
members show both SmA and N phases and finally higher homologues members’ exhibit only
SmA phase.

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Crystal - SmA
SmA - N
N - Iso
240
200
160
120
Isotropic
Nematic phase
SmA Phase
Crystal
6
8
10
12
14
16
18
20
No of crabon atoms in alkyl chain
Fig. 3 Plot of heating cycle melting and clearing points (T/°C) with number of carbon atoms in
the terminal alkyl chains for the homologues series 5a-i.
The UV-Visible absorption spectra were recorded over the range of wavelengths (λ = 200-500
nm) in dichloromethane solution at concentrations of 1mg/mL for compounds 5a, 5e and 5h.
The respective UV-visible absorption spectra are shown in Figure 4. In the absorption spectra of
5a, 5e and 5f, a strong absorption band at λ = 270 nm is attributed to the E band of aromatics for
the π-π* electronic transitions. The low-intensity n-π* transitions observed at λ = 370-380 nm
are due to the non-bonding n-electrons of the ester oxygens. All ultra violet spectra were found
to be similar in shape because of their structural similarities [38]. Interestingly, all of the
compounds were found to be luminescent with very weak blue light emission in solid state in the
visible light region. This observed blue light emission may be attributed to extended conjugation
in association with the mesogenic core.

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2.5
2.0
1.5
1.0
0.5
0.0
-0.5
5a
5e
5h
300
400
500
Wavelength(nm)
Figure 4. UV/visible absorption spectra of 5a, 5e and 5h in dichloromethane.
Thermogravimetric analysis was performed for the compound 5b, 5f and 5i under a nitrogen
atmosphere at a heating rate of 20 °C min-1. The thermogravimetric traces are shown in Figure 5.
The temperatures correspond to 100 % weight loss which occurs between 350 °C-500 °C. The
compounds are stable up to 350 °C and start degrading thereafter in nitrogen. The degradation
occurs in a single step between 350 °C-500 °C. The data revealed that in general all the
compounds in a homologues series are stable up to 325 °C.

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5b
5f
5i
100
80
60
40
20
0
100
200
300
400
500
600
700
800
Temperature (0C)
Figure 5. Thermogravimetric analysis traces of 5b, 5f and 5i.
The representative liquid crystal state X-ray analysis carried out to confirm the mesophases
obtained for the compound 5e are shown in Figure 6. In the X-ray pattern of non-oriented
sample of 5e at 190 °C for nematic phase, only one reflection at wide angle region with d-space
value 4.60 Å indicates the nematic phase. Two reflections were found at 130 °C for SmA phase,
the d-space value 29.33 Å in the small angle region and d-space value 4.53 Å in wide angle
region confirming the SmA phase. The typical X-ray patterns established the presence of both
nematic and smectic mesophase. Further, in this regard, the practically obtained d-spacing value
(29.33 Å) is smaller than theoretically obtained d-spacing value (42.97 Å) of compound 5e.
These values give the insight that the molecules are inter-digitized and showing layered SmA
mesophase.

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1300C
1400C
1600C
1900C
100000
10000
1000
100
0
5
10
15
20
25
30
2Theta
Figure 6. X-ray diffraction studies of 5e at nematic phase and Smectic phase.
The rigidity of the central naphthalene with benzene core connected via ester bond accomplished
the growth of single crystal. Interestingly, we found that lower member with heptyloxy- chain
length has a perfect crystal structure at room temperature and hence it helped us to study the
crystal structure as a representative compound 5b. The single crystal study parameters are
tabulated in Table 2. The atoms were joined together by using reflections obtained from single
crystal X-ray studies and which provides ORTEP diagram (Figure 7) of actual structure of the
chemical structure. The H atoms of 5b was positioned with idealized geometry using a riding
model with C—H = 0.93-0.97 Å. All H-atoms were refined with isotropic displacement
parameters (set to 1.2-1.5 times of the Ueq of the parent atom).
3.2 Single crystal X-ray diffraction studies

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Table 2. Crystallographic data, experimental details of the compound (5b)
Details
Compound (5b)
C19H22O3 (C38H44O6)
298.37
300.15
0.71073
Empirical formula
Formula weight
Temperature/K
Wavelength/ Å
Triclinic, P
Crystal system, Space group
1
a/Å
5.569(3)
b/Å
c/Å
α/°
β/°
10.540(5)
15.254(9)
73.434(18)
80.807(19)
82.02(2)
γ/°
Volume/Å3 , Z
Calculated density
Absorption coefficient/mm-1
F(000), F’(000)
Crystal size/mm3
843.1(8), 2
1.175
0.078
320.0, 320.26
0.28 × 0.2 × 0.16
Theta range for data collection/° 2.8 to 50
Limiting indices
-6 ≤ h ≤ 6, -12 ≤ k ≤ 12, -18 ≤ l ≤ 18
Reflection collected/unique
Data/restraints/parameters
Goodness-of-fit on F2
16846 /2963[R(int) = 0.0691]
2963/54/234
1.062
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
R1 = 0.0612, wR2 = 0.1750
R1 = 0.0931, wR2 = 0.2022
0.32/-0.23
Bruker SMART APEX II
diffractometer
Measurement
Program system
Structure determination
CCDC
SAINT
Direct method (SHELX97,
SHELXS97)
985487

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Figure 7. ORTEP diagram obtained by single crystal X-ray diffraction for 5b.
Figure 8. The crystal packing of the compound 5b. Cg1 is the centroid of the ring C7/C8/C9/
C10/C11/C12.
The molecular structure of the compound 5b shows that the asymmetric unit contains half of the
molecule, the planes of the naphthalene ring C1/C2/C3/C4/C5/C1A/C2A/C3A/C4A/C5AC6 and
C7/C8/C9/C10/C11/C12 and C7A/C8A/C9A/C10A/C11A/C12A benzene rings form a dihedral
angle of 63.62(2)° and 63.60 (5)° respectively, and there is no significant difference in the
dihedral angles. The crystal structure is stabilized by C4—H4. . . Cg1 interaction, where Cg1 is
the centroid of C7/C8/C9/C10/C11/C12 benzene ring shown in Figure 8. Further, the hydrogen
bond was established and the data obtained tabulated in Table 3 with centroid and carbon
numbers which are involved in hydrogen bond formation.
Table 3. Hydrogen-bond exhibited by compound 5b, in (Å, o); where Cg1 is the centroid
for the ring C7/C8/C9/ C10/C11 /C12
D—H. . . A
D—H
H. . . A
D. . . A
D—H. . . A
C4—H4. . . Cg1ii
0.93
2.80
3.5227
135
Symmetry code (ii): −x+1, −y+1, −z.

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4. Conclusions
In the present study, a library of nine compounds as a homologous series of new symmetric
naphthalene derivatives have been prepared and characterized to determine their liquid crystal
properties. All the molecular structures were confirmed by different spectroscopic techniques
and elemental analysis. The POM and DSC analysis reveals the presence of smectic and nematic
phase. The thermal behavior of all the compounds and their mesophase thermal stabilities are
comparable with each other and it is in good agreement with the practical values. The ORTEP
diagram generated by single crystal structure analysis also supports and confirms the molecular
structure with necessary chemical compositions.
Acknowledgement
The authors are thankful to SIF for NMR spectral analysis, Indian Institute of Science,
Bengaluru. BSPM thank the VGST (Grant No: VGST/CISEE/GRD319) Karnataka. Thanks to
K. N. Vasudha for DSC and X-ray analysis.
Additional Information
Crystallographic data for 5b reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the

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data can be obtained free of charge on quoting the depository numbers CCDC-985487. (Fax:
+44-1223-336-033; E-Mail: deposit-@ccdc.cam.ac.uk).

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 We have synthesized symmetric naphthalene centered molecules
 All the molecules were characterized by UV, IR, NMR analysis
 Liquid crystal property was evaluated by DSC and POM techniques
 Single crystal structure was confirmed by X-ray analysis