Novel hydroxy- and methyl-terminated triaromatic schiff base compounds: Synthesis and mesogenic properties

Article abstract of DOI:10.1071/CH09405

A series of Schiff base hydroxy-mesogens and their model compounds have been synthesized. The mesogen is constructed from three-ring-containing mesogens linked through ester and azomethine groups with a terminal hydroxy group. The related model compounds were also synthesized with varying chain length with a terminal methyl group and their properties were compared with that of hydroxy mesogens. Extensive characterization of all mesogenic compounds was carried out by Fourier-transform IR, ¹H and ¹³C NMR and mass spectroscopy. The phase characteristics such as nature of phase, melting and clearing temperatures and phase range were evaluated using a hot-stage optical polarizing microscope and differential scanning calorimetry. The appearance of enantiotropic smectic phases is due to the high molecular polarizability of the triaromatic core with azomethine and ester linkages. CSIRO 2010.

Full text of DOI:10.1071/CH09405

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 276–285
www.publish.csiro.au/journals/ajc
Novel Hydroxy- and Methyl-Terminated Triaromatic Schiff
Base Compounds: Synthesis and Mesogenic Properties
Natarajan Senthilkumar,A,C Tanneru Narasimhaswamy,B
and Aravamudhan RaghavanA,C
ADepartment of Polymer Science, University of Madras, Guindy campus, Chennai 600 025, India.
BPolymer Laboratory, Central Leather Research Institute, Chennai 600 020, India.
CCorresponding authors. Email: nskumar77@yahoo.com; sreeraghu44@yahoo.com
A series of Schiff base hydroxy-mesogens and their model compounds have been synthesized. The mesogen is constructed
from three-ring-containing mesogens linked through ester and azomethine groups with a terminal hydroxy group. The
related model compounds were also synthesized with varying chain length with a terminal methyl group and their properties
were compared with that of hydroxy mesogens. Extensive characterization of all mesogenic compounds was carried out by
Fourier-transform IR, 1H and 13C NMR and mass spectroscopy. The phase characteristics such as nature of phase, melting
and clearing temperatures and phase range were evaluated using a hot-stage optical polarizing microscope and differential
scanning calorimetry. The appearance of enantiotropic smectic phases is due to the high molecular polarizability of the
triaromatic core with azomethine and ester linkages.
Manuscript received: 25 July 2009.
Manuscript accepted: 25 August 2009.
Introduction
used flexible structural units for the design of liquid crystalline
materials.
Design and synthesis of thermotropic liquid crystals have been
one of the most exciting frontiers of current chemical research.[1]
Small molecules with geometrical anisotropy and high polariz-
ability may exhibit one or more liquid crystalline phases besides
thewell-knowncrystallineandisotropicphases.[2] Incorporation
of such moieties into the main chain or side group of a polymer
can result in the formation of liquid crystalline polymers that
form mesophases at moderate temperatures. The formation of
the liquid crystalline phase, as well as its type, is closely related
to molecular structure.[3,4] In general, it has been found that liq-
uid crystalline compounds have a rigid core that serves as the
mesogenic unit and flexible alkyl chains at the terminal ends.[5]
This feature is of most importance in the design and synthesis of
new liquid crystalline compounds. The observation of thermo-
tropic liquid crystalline behaviour in the main chain polymers
comprising a rigid core and flexible chain units arranged alter-
natively along the main chain has prompted much attention and
led to the synthesis of new liquid crystalline materials with high
versatility.[6,7]
In the present work, the hydroxy-terminated mesogens were
designed and synthesized from two simple dihydroxybenzenes
(resorcinolandhydroquinone), whichwerelinkedthroughmulti-
functional groups such as ether, ester, and azomethane. These
mesogens exhibit liquid crystalline behaviour as a result of their
high aspect ratio (length to breadth ratio). The related model
mesogenic compounds were also synthesized with varying chain
length with a terminal methyl group and their properties are
compared with that of hydroxy mesogens. A three-ring-based
core was used in the present investigation as its aspect ratio
would be expected to be high in contrast to two-ring systems. In
calamitic mesogens, in addition to molecular polarizability and
linearity, the aspect ratio of the mesogen is a crucial factor.[1,2]
Moreover, the introduction of a hydroxy group adjacent to the
azomethine link, which is essential for complexing a metal ion,
enhances the breadth of the molecule.[13] The introduction of a
reactive terminal hydroxy group in mesogens enables them to
undergo radical or condensation polymerization to form liquid
crystalline polymers.[14]
The pioneering work of Lenz and others in the field of
thermotropic liquid crystalline polyesters established the advan-
tage of triaromatic-ring mesogenic units when used as core
molecules.[8,9] Imine compounds are the earliest and most
extensively studied class of liquid crystalline materials. The
advantages of incorporating imine functionalities are versatility,
structural variety, simple preparation and its derivatization.[10]
Some of these imines possess the ability to coordinate with
metals to form Schiff base metallomesogens. As a constituent
of metallomesogens and banana-shaped molecules, the imine
moiety gives rise to a large variety of different new liquid crys-
tal (LC) phases.[10–12] Alkyl chains represent the most widely
Experimental
Measurements and Materials
Infrared (IR) spectra were recorded by the KBr pellet method in a
Thermo Mattson Satellite model Fourier-transform (FT)IR spec-
trophotometer. H NMR and 13C NMR spectra were recorded
1
using a JEOL GSX 400 MHz NMR instrument. The elec-
tron impact (EI) mass spectra were recorded using a JEOL
DX-303 mass spectrometer. Elemental analyses were per-
formed on a Perkin–Elmer 240B elemental analyzer. Differential
© CSIRO 2010
10.1071/CH09405
0004-9425/10/020276

Novel Hydroxy- and Methyl-Terminated Triaromatic Schiff Base Compounds
277
scanning calorimetry (DSC) was performed using a NETZSCH-
Gerätebau Thermal Analyzer from 25 to 250◦C, cooled rapidly,
and reheated at a heating rate of 10◦C min−1 under a nitro-
gen atmosphere with a purge rate of 60 mL min−1. The sample
size used was 8–18 mg in a sealed aluminium pan. LC phase
transitions were studied with an Olympus BX 50 polarizing
microscope equipped with a Linkam THMS 600 heating stage.
Resorcinol (Aldrich), hydroquinone (Aldrich), 2-bromo-
ethanol (Lancaster), 1,6-hexanediol (Lancaster), 1,8-octanediol
(Lancaster), 1,10-decanediol (Lancaster), sodium hydroxide
pellets (Loba Chemie), 4-nitrobenzoic acid (Lancaster),
N,Nꢀ-dicyclohexyl carbodiimide (DCC) (Fluka), 4-dimethyl-
aminopyridine (DMAP) (Fluka), p-toluenesulfonic acid (PTSA)
(Aldrich), palladium on carbon (10% Pd/C) (Aldrich) were used
as received. The solvent N,Nꢀ-dimethyl formamide (DMF) was
stirred with calcium hydride and distilled under reduced pressure
before use. Ethanol was purified by refluxing with lime fol-
CH2), 1.77–1.84 (m, 2H, CH2), 3.99 (t, 2H, J 6.5, CH2), 6.39 (d,
1H, Ar–H), 6.51 (d, 1H, J 6.81, Ar–H), 7.39 (d, 1H, J 3.4, Ar–H),
9.69 (s, 1H, CHO), 11.46 (s, 1H,Ar–OH). δC (100 MHz, CDCl3)
14.14, 19.43, 22.74, 28.25, 28.85, 29.42, 29.52, 29.64, 29.85,
32.92, 68.60, 70.66, 101.08, 108.75, 115.06, 135.15, 164.60,
166.48, 194.20. m/z (EI) 306.2 (M+).
4-(6-Hydroxyhexyloxy)-2-hydroxybenzaldehyde 3d
Yield: 2.72 g (38%). Mp 73◦C. νmax(KBr)/cm−1 3364
(aliphatic OH), 3284 (phenolic OH), 2944, and 2858 (C–H),
2752 (CHO), 1627 (C=O), 1228 (C–O–C). δH (400 MHz,
CDCl3) 1.44 (sbr, 1H, OH), 1.45–1.63 (m, 6H, CH2), 1.79–1.83
(m, 2H, CH2), 3.66 (t, 2H, J 6.6, CH2), 4.01 (t, 2H, J 6.6, CH2),
6.40 (d, 1H, J 2.4, Ar–H), 6.52 (dd, 1H, J 8.8, Ar–H), 7.41 (d,
1H, J 8.8, Ar–H), 9.70 (s, 1H, CHO), 11.47 (s, 1H, Ar–OH).
δC (100 MHz, CDCl3) 24.81, 25.10, 28.23, 31.93, 62.10, 67.79,
100.49, 108.05, 114.44, 134.55, 163.86, 165.76, 193.61. m/z (EI)
238.7 (M+).
lowed by distillation. n-Butanol (Qualigens), toluene (Merck),
THF (Merck), dichloromethane (DCM) (Merck), ethyl acetate
(Qualigens), hexane (Qualigens), and petroleum ether (SD Fine
Chemicals) were used as received.
4-(8-Hydroxyoctyloxy)-2-hydroxybenzaldehyde 3e
Yield: 2.80 g (35%). Mp 63◦C. νmax(KBr)/cm−1 3458
(aliphatic OH), 3384 (phenolic OH), 2915, and 2846 (C–H),
2749 (CHO), 1644 (C=O), 1224 (C–O–C). δH (400 MHz,
CDCl3) 1.36 (sbr, 1H, OH), 1.45–1.58 (m, 10H, CH2), 1.75–
1.80 (m, 2H, CH2), 3.63 (t, 2H, J 6.6, CH2), 3.99 (t, 2H, J 6.6,
CH2), 6.40 (d, 1H, J 4.4, Ar–H), 6.52 (dd, 1H, J 8.4, Ar–H), 7.41
(d, 1H, J 8.4, Ar–H), 9.70 (s, 1H, CHO), 11.47 (s, 1H, Ar–OH).
δC (100 MHz, CDCl3) 25.68, 25.84, 28.87, 29.25, 29.30, 32.67,
62.74, 68.55, 101.04, 108.73, 114.97, 135.26, 164.42, 166.44,
194.37. m/z (EI) 266.1 (M+).
Synthesis of O-Alkylated Benzaldehyde 3a–f
2,4-Dihydroxybenzaldehyde 2 (30 mmol) (which was prepared
from 2,4-dihydroxybenzene 1 as reported in the literature[15]
)
and sodium hydroxide (30 mmol) were dissolved in n-butanol
(50 mL), stirred under a nitrogen atmosphere at room tempera-
ture for 30 min, and then the temperature was raised to reflux
temperature. The corresponding bromoalkane or bromoalkanol
(30 mmol) dissolved in n-butanol (25 mL) was slowly added
into the reaction mixture and refluxing was continued for 60 h.
Evaporation of solvent from the n-butanol solution yielded crude
compound. Column chromatography (SiO2; hexane/EtOAc, 7:2
v/v) gave the corresponding O-alkylated aldehyde 3a–f as a
liquid or solid in 31–43% yield.
4-(10-Hydroxydecyloxy)-2-hydroxybenzaldehyde 3f
Yield: 2.74 g (31%). Mp 60◦C. νmax(KBr)/cm−1 3437
(aliphatic OH), 3278 (phenolic OH), 2940, and 2850 (C–H),
2749 (CHO), 1644 (C=O), 1211 (C–O–C). δH (400 MHz,
CDCl3) 1.31 (sbr, 1H, OH), 1.41–1.58 (m, 14H, CH2), 1.75–
1.82 (m, 2H, CH2), 3.63 (t, 2H, J 6.6, CH2), 4.00 (t, 2H, J
6.5, CH2), 6.41 (d, 1H, J 2, Ar–H), 6.52 (dd, 1H, J 8.68, Ar–
H), 7.41 (d, 1H, J 8.64, Ar–H), 9.69 (s, 1H, CHO), 11.48 (s,
1H, Ar–OH). δC (100 MHz, CDCl3) 25.73, 25.89, 28.89, 29.27,
29.39, 29.44, 29.50, 32.75, 62.94, 68.58, 101.04, 108.78, 114.98,
135.23, 164.48, 166.46, 194.34. m/z (EI) 294.1 (M+).
4-(Hexyloxy)-2-hydroxybenzaldehyde 3a
Yield: 2.87 g, 43%, liquid. νmax(KBr)/cm−1 2952, 2932, and
2858 (C–H), 2744 (CHO), 1648 (C=O), 1224 (C–O–C). δH
(400 MHz, CDCl3) 0.91 (t, 3H, J 3.26, CH3), 1.31–1.47 (m, 6H,
CH2), 1.77–1.80 (m, 2H, CH2), 3.99 (t, 2H, J 6.96, CH2), 6.40
(d, 1H, J 1.71, Ar–H), 6.52 (d, 1H, J 3.48, Ar–H), 7.39 (d, 1H,
J 8.68, Ar–H), 9.69 (s, 1H, CHO), 11.48 (s, 1H, Ar–OH). δC
(100 MHz, CDCl3) 14.02, 22.58, 25.61, 28.99, 31.51, 68.61,
101.09, 108.76, 118.92, 135.22, 164.54, 166.49, 194.30. m/z (EI)
222 (M+).
Synthesis of O-Alkylated Hydroquinones 5a–c
A mixture of hydroquinone 4 (33.03 g, 300 mmol) and sodium
hydroxide (12 g, 300 mmol) were dissolved in n-butanol
(150 mL), stirred under a nitrogen atmosphere at room temper-
ature for 30 min, and then the temperature was raised to 100◦C.
The corresponding bromoalkane (300 mmol) dissolved in
n-butanol (70 mL) was slowly added into the reaction mix-
ture and the reaction was continued for 60 h. Evaporation
of solvent from the butanol solution with a rotary evapora-
tor yielded crude compound. Column chromatography (SiO2;
petroleum ether/EtOAc, 93:7 v/v) gave the corresponding mono
O-alkylated product 5a–c as a pale yellow solid.
4-(Decyloxy)-2-hydroxybenzaldehyde 3b
Yield: 3.17 g, 38%, liquid. νmax(KBr)/cm−1 2924, 2952, and
2850 (C–H), 2746 (CHO), 1644 (C=O), 1219 (C–O–C). δH
(400 MHz, CDCl3) 0.88 (t, 3H, J 6.35, CH3), 1.27–1.43 (m, 14H,
CH2), 1.73–1.80 (m, 2H, CH2), 3.96 (t, 2H, J 6.53, CH2), 6.37
(d, 1H, J 1.72, Ar–H), 6.49 (d, 1H, J 6.96, Ar–H), 7.35 (d, 1H,
J 8.64, Ar–H), 9.65 (s, 1H, CHO), 11.46 (s, 1H, Ar–OH). δC
(100 MHz, CDCl3) 15.49, 24.11, 27.36, 30.37, 30.55, 30.76,
30.99, 31.07, 69.75, 71.98, 102.46, 110.32, 116.45, 136.51,
165.92, 167.82, 195.53. m/z (EI) 278.2 (M+).
4-(Heptyloxy)phenol 5a
Yield: 22.50 g, 36%. Mp 81◦C. νmax(KBr)/cm−1 3433 (OH),
2952, 2932, and 2858 (Ali–C–H), 1513 (Ar–C–H), 1244
(C–O–C). δH (400 MHz, CDCl3) 0.88 (t, 3H, J 6.81, CH3), 1.29–
1.44 (m, 8H, CH2), 1.71–1.76 (m, 2H, CH2), 3.89 (t, 2H, J 6.71,
CH2), 5.43 (sbr, 1H, OH), 6.7–6.8 (m, 4H, Ar–H). δC (100 MHz,
4-(Dodecyloxy)-2-hydroxybenzaldehyde 3c
Yield: 3.59 g, 39%, liquid. νmax(KBr)/cm−1 2956, 2924, and
2850 (C–H), 2744 (CHO), 1644 (C=O), 1215 (C–O–C). δH
(400 MHz, CDCl3) 0.88 (t, 3H, J 6.2, CH3), 1.26–1.55 (m, 18H,

278
N. Senthilkumar, T. Narasimhaswamy, and A. Raghavan
CDCl3) 13.80, 22.36, 25.77, 28.83, 29.15, 31.56, 68.76, 115.6,
1529 and 1346 (NO2), 1272 and 1191 (C–O). δH (400 MHz,
CDCl3) 0.90 (t, 3H, J 6.7, CH3), 1.27–1.62 (m, 18H, CH2),
1.73–1.83 (m, 2H, CH2), 3.96 (t, 2H, J 6.58, CH2), 6.95 (d, 2H,
J 6.88, Ar–H), 7.14 (d, 2H, J 6.76, Ar–H), 8.36 (d, 4H, J 2.04,
Ar–H). δC (100 MHz, CDCl3) 14.13, 22.71, 26.05, 29.27, 29.37,
29.41, 29.60, 29.65, 29.68, 29.80, 31.94, 68.49, 115.22, 122.11,
123.70, 131.25, 135.13, 143.80, 150.86, 157.28, 163.70. m/z (EI)
427.2 (M+).
115.87, 149.28, 153.03. m/z (EI) 208.1 (M+).
4-(Decyloxy)phenol 5b
Yield: 25.54 g, 34%. Mp 80◦C. νmax(KBr)/cm−1 3429 (OH),
2952, 2932, and 2858 (Ali–C–H), 1517 (Ar–C–H), 1240
(C–O–C). δH (400 MHz, CDCl3) 0.88 (t, 3H, J 6.84, CH3),
1.27–1.45 (m, 14H, CH2), 1.70–1.79 (m, 2H, CH2), 3.89 (t,
2H, J 6.71, CH2), 4.9 (sbr, 1H, OH), 6.73–6.79 (m, 4H, Ar–H).
δC (100 MHz, CDCl3) 14.04, 22.64, 26.03, 29.19, 29.28, 29.37,
29.52, 29.55, 31.87, 68.87, 115.73, 116.01, 149.43, 153.34. m/z
(EI) 250.2 (M+).
Synthesis of 4-(Alkyloxy)phenyl 4-Aminobenzoate 7a–c
A mixture of nitro compound 6a–c (10 g), 10% palladium on
carbon (0.36 g) and a mixture of THF/ethanol (120 mL, 1:1
by volume) was vigorously stirred under hydrogen pressure at
room temperature for 24 h. After filtration through Celite 540,
palladium/carbon was removed.The filtrate was concentrated by
rotary evaporator and poured into hexane to precipitate the prod-
uct. The precipitate was collected by filtration, recrystallized
from acetone at 0◦C, and was filtered and dried under vacuum.
The amino products 7a–c were obtained as white solids.
4-(Dodecyloxy)phenol 5c
Yield: 32.58 g, 34%. Mp 78◦C. νmax(KBr)/cm−1 3429 (OH),
2952, 2911, and 2846 (Ali–C–H), 1517 (Ar–C–H), 1240
(C–O–C). δH (400 MHz, CDCl3) 0.88 (t, 3H, J 6.67, CH3), 1.26–
1.44 (m, 18H, CH2), 1.71–1.76 (m, 2H, CH2), 3.89 (t, 2H, J 6.58,
CH2), 4.9(sbr, 1H, OH), 6.73–6.82(m, 4H,Ar–H). δC (100 MHz,
CDCl3) 14.02, 22.67, 26.07, 29.22, 29.26, 29.33, 29.47, 29.53,
29.56, 31.81, 31.89, 68.86, 115.73, 116.03, 149.41, 153.37. m/z
(EI) 278.2 (M+).
4-(Heptyloxy)phenyl 4-Aminobenzoate 7a
Yield: 8.52 g, 93%. Mp 149◦C. νmax(KBr)/cm−1 3409, 3340,
and 3234 (NH2), 2952, 2932, and 2854 (C–H), 1701 (C=O),
1281 and 1171 (C–O). δH (400 MHz, CDCl3) 0.89 (t, 3H, J 6.26,
CH3), 1.31–1.43 (m, 8H, CH2), 1.72–1.75 (m, 2H, CH2), 3.89–
3.93 (m, 2H, CH2), 4.16 (sbr, 2H, NH2), 6.65 (d, 2H, J 8.39,
Ar–H), 6.82 (s, 2H, Ar–H), 6.91 (d, 1H, J 8.83, Ar–H), 7.07
(d, 1H, J 8.74, Ar–H), 7.99 (d, 2H, J 8.39, Ar–H). δC (100 MHz,
CDCl3) 14.10, 22.62, 26.03, 29.29, 29.41, 31.80, 68.64, 113.80,
115.01, 115.38, 118.73, 122.55, 132.26, 144.50, 151.48, 153.17,
156.65, 165.65. m/z (EI) 327.3 (M+).
Synthesis of 4-(Alkyloxy)phenyl 4-Nitrobenzoate 6a–c
A mixture of corresponding 4-alkoxyphenol 5a–c (20 mmol),
4-nitrobenzoic acid (5 g, 20 mmol), and DMAP (0.24 g, 2 mmol)
was dissolved in dry DMF/DCM mixture (25 mL, 1:1) and
stirred using a mechanical stirrer at 0◦C under an N2 atmo-
sphere. To this solution, DCC (4.07 g, 20 mmol) dissolved in
DCM (20 mL) was added dropwise and stirred at 0◦C for 1 h and
then at room temperature for 12 h. Finally, the reaction mixture
was filtered and DCM removed from the filtrate by evapora-
tion. The solution was then poured into water to precipitate
the product. The crude product was purified by column chro-
matography (SiO2; petroleum ether/EtOAc, 97:3 v/v) to give the
corresponding esterified product 6a–c as a yellow solid.
4-(Decyloxy)phenyl 4-Aminobenzoate 7b
Yield: 8.32 g, 90%. Mp 145◦C. νmax(KBr)/cm−1 3409, 3340,
and 3234 (NH2), 2952, 2920, and 2850 (C–H), 1705 (C=O),
1280 and 1171 (C–O). δH (400 MHz, CDCl3) 0.88 (t, 3H, J
6.25, CH3), 1.27–1.43 (m, 14H, CH2), 1.72–1.75 (m, 2H, CH2),
3.86–3.94 (m, 2H, CH2), 4.15 (sbr, 2H, NH2), 6.63 (d, 2H, J 8.37,
Ar–H), 6.81 (s, 2H, Ar–H), 6.89 (d, 1H, J 8.82, Ar–H), 7.07 (d,
1H, J 8.74, Ar–H), 7.97 (d, 2H, J 8.37, Ar–H). δC (100 MHz,
CDCl3) 14.10, 22.70, 26.06, 29.30, 29.34, 29.42, 29.58, 29.61,
31.92, 68.64, 113.79, 115.00, 115.37, 118.65, 122.56, 132.25,
144.51, 151.55, 153.18, 156.66, 165.67. m/z (EI) 369 (M+).
4-(Heptyloxy)phenyl 4-Nitrobenzoate 6a
Yield: 5.10 g, 71%. Mp 74◦C. νmax(KBr)/cm−1 3107, 3079,
and 3047 (NO2), 2952, 2931, and 2850 (C–H), 1741 (C=O),
1525 and 1341 (NO2), 1244 and 1191 (C–O). δH (400 MHz,
CDCl3) 0.90 (t, 3H, J 6.70, CH3), 1.32–1.48 (m, 8H, CH2),
1.77–1.81 (m, 2H, CH2), 3.96 (t, 2H, J 6.54, CH2), 6.94 (d, 2H,
J 8.98, Ar–H), 7.13 (d, 2H, J 8.97, Ar–H), 8.34 (m, 4H, Ar–H).
δC (100 MHz, CDCl3) 14.11, 22.63, 26.01, 29.07, 29.26, 31.80,
68.45, 115.18, 122.18, 123.68, 131.24, 135.09, 143.78, 150.82,
157.26, 163.68. m/z (EI) 357.4 (M+).
4-(Dodecyloxy)phenyl 4-Aminobenzoate 7c
Yield: 8.46 g, 91%. Mp 139◦C. νmax(KBr)/cm−1 3413, 3340,
and 3234 (NH2), 2956, 2920, and 2850 (C–H), 1701 (C=O),
1280 and 1171 (C–O). δH (400 MHz, CDCl3) 0.89 (t, 3H, J 6.25,
CH3), 1.26–1.45 (m, 18H, CH2), 1.72–1.77 (m, 2H, CH2), 3.87–
3.95 (m, 2H, CH2), 4.16 (sbr, 2H, NH2), 6.65 (d, 2H, J 8.58, Ar–
H), 6.81 (s, 2H, Ar–H), 6.90 (d, 1H, J 8.92, Ar–H), 7.07 (d, 1H, J
8.91, Ar–H), 7.98 (d, 2H, J 8.49, Ar–H). δC (100 MHz, CDCl3)
14.14, 22.70, 26.05, 29.29, 29.37, 29.42, 29.60, 29.62, 29.65,
29.68, 31.93, 68.65, 113.81, 115.01, 115.37, 118.74, 122.55,
132.26, 144.49, 151.48, 153.18, 156.66, 165.65. m/z (EI) 397.1
(M+).
4-(Decyloxy)phenyl 4-Nitrobenzoate 6b
Yield: 5.83 g, 73%. Mp 73◦C. νmax(KBr)/cm−1 3096, 3065,
and 3042 (NO2), 2940, 2915, and 2850 (C–H), 1749 (C=O),
1533 and 1350 (NO2), 1278 and 1194 (C–O). δH (400 MHz,
CDCl3) 0.89 (t, 3H, J 6.84, CH3), 1.28–1.61 (m, 14H, CH2),
1.76–1.81 (m, 2H, CH2), 3.95 (t, 2H, J 6.56, CH2), 6.94 (d, 2H,
J 8.96, Ar–H), 7.13 (d, 2H, J 9.08, Ar–H), 8.36 (d, 4H, J
3.16, Ar–H). δC (100 MHz, CDCl3) 14.13, 22.69, 26.05, 29.26,
29.33, 29.40, 29.57, 29.59, 31.91, 68.49, 115.23, 122.11, 123.70,
131.26, 135.12, 143.80, 150.86, 157.28, 163.70. m/z (EI) 399
(M+).
Synthesis of Model Mesogens 8a–c
A mixture of corresponding aldehyde 3a–c (10 mmol), amine
7a–c (10 mmol), andPTSAindryethanol(120 mL)wasrefluxed
for 5 h. The product precipitated out as a yellow solid when
cooled to room temperature.The precipitate was filtered, washed
4-(Dodecyloxy)phenyl 4-Nitrobenzoate 6c
Yield: 5.90 g, 69%. Mp 71◦C. νmax(KBr)/cm−1 3111, 3079,
and 3051 (NO2), 2944, 2924, and 2850 (C–H), 1733 (C=O),

Novel Hydroxy- and Methyl-Terminated Triaromatic Schiff Base Compounds
279
with methanol and dried under vacuum. The solid product was
recrystallized from chloroform to afford pure yellow solid.
(E)-4-(Heptyloxy)phenyl 4-(4-(6-Hydroxyhexyloxy)-
2-hydroxybenzylideneamino)benzoate 9a
Yield: 4.22 g, 77%. Mp 129◦C. νmax(KBr)/cm−1 3417
(aliphatic OH), 3285 (phenolic OH), 2952, 2936, and 2858
(C–H), 1733 (C=O), 1631 (C=N), 1285 and 1195 (C–O). δH
(400 MHz, CDCl3) 0.90 (t, 3H, J 6.64, CH3), 1.26 (sbr, 1H,
OH), 1.32–1.62 (m, 14H, CH2), 1.77–1.83 (m, 4H, CH2), 3.67
(t, 2H, J 6.38, CH2), 3.96 (t, 2H, J 6.52, CH2), 4.02 (t, 2H,
J 6.46, CH2), 6.51 (s, 2H, Ar–H), 6.93 (d, 2H, J 8.96, Ar–H),
7.12 (d, 2H, J 8.84, Ar–H), 7.29–7.35 (m, 3H, Ar–H), 8.23 (d,
2H, J 8.44, Ar–H), 8.57 (s, 1H, imine C–H), 13.39 (s, 1H, Ar–
OH). δC (100 MHz, CDCl3, 50◦C) 11.80, 20.34, 23.24, 23.56,
23.73, 26.75, 26.79, 27.01, 29.52, 30.38, 60.61, 65.90, 66.18,
99.29, 105.78, 110.55, 112.85, 118.88, 120.09, 124.97, 126.03,
129.35, 131.72, 141.97, 150.83, 154.65, 160.72, 161.82, 161.90.
m/z (EI) 548.8 (M+). (Calc. for C33H41NO6 (547.69): C 72.36,
H 7.56, N 2.56. Found: C 72.72, H 7.93, N 2.44%.)
(E)-4-(Heptyloxy)phenyl 4-(4-(Hexyloxy)-
2-hydroxybenzylideneamino)benzoate 8a
Yield: 4.14 g, 78%. Mp 96◦C. νmax(KBr)/cm−1 3441
(OH), 2956, 2919, and 2854 (C–H), 1741 (C=O), 1631
(C=N), 1284and1195(C–O). δH (400 MHz, CDCl3)0.90(t, 6H,
J6.34, CH3), 1.25–1.46(m, 14H, CH2), 1.77–1.82(m, 4H, CH2),
3.95 (t, 2H, J 6.58, CH2), 4.00 (t, 2H, J 6.6, CH2), 6.50–6.52
(m, 2H, Ar–H), 6.93 (d, 2H, J 8.96, Ar–H), 7.12 (d, 2H, J 8.96,
Ar–H), 7.25–7.34 (m, 3H, Ar–H), 8.22 (d, 2H, J 8.52, Ar–H),
8.55 (s, 1H, imine C–H), 13.39 (s, 1H, Ar–OH). δC (100 MHz,
CDCl3, 50◦C) 13.34, 13.41, 21.93, 21.97, 25.02, 25.38, 28.42,
28.68, 29.06, 30.90, 31.16, 67.76, 67.89, 101.02, 107.44, 112.23,
114.55, 120.51, 121.88, 126.67, 130.97, 133.36, 143.71, 152.53,
156.33, 162.36, 163.52, 163.69, 164.43. m/z (EI) 530.7 (M+).
(Calc. for C33H41NO5 (531.69): C 74.54, H 7.79, N 2.63. Found:
C 74.72, H 7.83, N 2.49%.)
(E)-4-(Decyloxy)phenyl 4-(4-(6-Hydroxyhexyloxy)-
2-hydroxybenzylideneamino)benzoate 9b
(E)-4-(Decyloxy)phenyl 4-(4-(Decyloxy)-
2-hydroxybenzylideneamino)benzoate 8b
Yield: 4.36 g, 74%. Mp 129◦C. νmax(KBr)/cm−1 3384
(aliphatic OH), 3283 (phenolic OH), 2956, 2916, and 2850
(C–H), 1733 (C=O), 1627 (C=N), 1281 and 1195 (C–O). δH
(400 MHz, CDCl3) 0.89 (t, 3H, J 6.80, CH3), 1.28 (sbr, 1H,
OH), 1.31–1.62 (m, 20H, CH2), 1.79–1.83 (m, 4H, CH2), 3.68
(t, 2H, J 6.38, CH2), 3.96 (t, 2H, J 6.54, CH2), 4.03 (t, 2H,
J 6.48, CH2), 6.51–6.53 (m, 2H, Ar–H), 6.93 (d, 2H, J 9.04,
Ar–H), 7.12 (d, 2H, J 9.04, Ar–H), 7.26–7.35 (m, 3H, Ar–H),
8.23 (d, 2H, J 8.56, Ar–H), 8.57 (s, 1H, imine C–H), 13.45 (s,
1H, Ar–OH). δC (100 MHz, CDCl3, 50◦C) 13.99, 22.64, 25.57,
25.62, 25.88, 26.09, 26.17, 29.09, 29.29, 29.40, 29.58, 31.90,
32.74, 62.85, 68.32, 68.37, 101.91, 108.07, 113.10, 115.37,
121.14, 122.36, 127.55, 131.61, 132.57, 134.03, 144.59, 153.29,
157.10, 162.99, 164.37, 164.99. m/z (EI) 589 (M+). (Calc. for
C36H47NO6 (589.78): C 73.31, H 8.05, N 2.38. Found: C 72.96,
H 8.01, N 2.41%.)
Yield: 4.98 g, 79%. Mp 95◦C. νmax(KBr)/cm−1 3441 (OH),
2956, 2919, and 2850 (C–H), 1737 (C=O), 1631 (C=N), 1281
and 1199 (C–O). δH (400 MHz, CDCl3) 0.81(t, 6H, J 6.34, CH3),
1.20–1.40 (m, 28H, CH2), 1.67–1.74 (m, 4H, CH2), 3.86–3.94
(m, 4H, CH2), 6.43 (d, 2H, J 6.56, Ar–H), 6.85 (d, 2H, J 9.04,
Ar–H), 7.04 (d, 2H, J 8.00, Ar–H), 7.17–7.26 (m, 3H, Ar–H),
8.14 (d, 2H, J 8.40, Ar–H), 8.47 (s, 1H, imine C–H), 13.31 (s,
1H, Ar–OH). δC (100 MHz, CDCl3, 50◦C) 14.06, 14.09, 22.67,
22.69, 25.63, 25.98, 26.05, 26.31, 29.06, 29.11, 29.19, 29.31,
29.35, 29.41, 29.56, 29.58, 31.56, 31.89, 68.46, 68.68, 101.59,
108.08, 115.12, 115.41, 121.14, 122.36, 127.23, 131.60, 133.99,
144.27, 153.11, 153.22, 156.93, 162.98, 164.27, 165.08. m/z (EI)
629.5 (M+). (Calc. for C40H55NO5 (629.88): C 76.27, H 8.82,
N 2.22. Found: C 75.84, H 8.96, N 2.31%.)
(E)-4-(Dodecyloxy)phenyl 4-(4-(Dodecyloxy)-
2-hydroxybenzylideneamino)benzoate 8c
(E)-4-(Dodecyloxy)phenyl 4-(4-(6-Hydroxyhexyloxy)-
2-hydroxybenzylideneamino)benzoate 9c
Yield: 5.10 g, 74%. Mp 94◦C. νmax(KBr)/cm−1 3417 (OH),
2956, 2915, and 2846 (C–H), 1737 (C=O), 1623 (C=N), 1281
and 1195 (C–O). δH (400 MHz, CDCl3) 0.88 (t, 6H, J 6.48,
CH3), 1.27–1.56 (m, 36H, CH2), 1.76–1.84 (m, 4H, CH2), 3.94
(t, 2H, J 6.28, CH2), 4.01 (t, 2H J 6.42, CH2), 6.50 (m, 2H,
Ar–H), 6.93 (d, 2H, J 8.68, Ar–H), 7.12 (d, 2H, J 8.56, Ar–H),
7.28–7.34 (m, 3H, Ar–H), 8.23 (d, 2H, J 8.08, Ar–H), 8.56 (s,
1H, imine C–H), 13.38 (s, 1H, Ar–OH). δC (100 MHz, CDCl3,
50◦C) 14.05, 14.08, 22.63, 22.68, 25.59, 25.89, 26.11, 26.23,
26.29, 26.34, 26.41, 29.02, 29.14, 29.20, 29.37, 29.44, 29.51,
29.59, 29.63, 29.72, 31.59, 31.96, 68.52, 68.72, 101.66, 108.11,
115.17, 115.47, 131.16, 122.41, 127.33, 131.56, 133.87, 144.31,
153.07, 153.27, 156.95, 162.96, 164.30, 164.98. m/z (EI) 685
(M+). (Calc. for C44H63NO5 (685.99): C 77.03, H 9.28, N 2.04.
Found: C 77.31, H 9.07, N 2.11%.)
Yield: 4.51 g, 73%. Mp 128◦C. νmax(KBr)/cm−1 3417
(aliphatic OH), 3267 (phenolic OH), 2956, 2920, and 2850
(C–H), 1733 (C=O), 1627 (C=N), 1281 and 1195 (C–O). δH
(400 MHz, CDCl3) 0.88 (t, 3H, J 6.52, CH3), 1.26 (sbr, 1H,
OH), 1.27–1.62 (m, 24H, CH2), 1.79–1.83 (m, 4H, CH2), 3.67
(t, 2H, J 6.34, CH2), 3.96 (t, 2H, J 6.48, CH2), 4.03 (t, 2H,
J 6.44, CH2), 6.50–6.52 (m, 2H, Ar–H), 6.93 (d, 2H, J 8.96,
Ar–H), 7.12 (d, 2H, J 8.92, Ar–H), 7.29–7.35 (m, 3H, Ar–H),
8.23 (d, 2H, J 8.32, Ar–H), 8.57 (s, 1H, imine C–H), 13.39 (s,
1H, Ar–OH). δC (100 MHz, CDCl3, 50◦C) 14.06, 22.71, 25.61,
25.73, 25.93, 26.14, 29.13, 29.37, 29.42, 29.45, 29.65, 29.68,
29.81, 29.97, 32.77, 62.89, 68.34, 68.73, 101.91, 108.12, 113.15,
115.38, 121.19, 122.40, 127.55, 131.66, 132.63, 134.08, 144.60,
153.29, 157.12, 163.02, 164.39, 165.06. m/z (EI) 617.4 (M+).
(Calc. for C38H51NO6 (617.83): C 73.87, H 8.34, N 2.27. Found:
C 73.98, H 8.46, N 2.24%.)
Synthesis of Hydroxy-Terminated Mesogens 9a–e
(E)-4-(Dodecyloxy)phenyl 4-(4-(8-Hydroxyoctyloxy)-
2-hydroxybenzylideneamino)benzoate 9d
Yield: 4.78 g, 74%. Mp 128◦C. νmax(KBr)/cm−1 3419
(aliphatic OH), 3281 (phenolic OH), 2956, 2936, and 2855
(C–H), 1733 (C=O), 1627 (C=N), 1281 and 1195 (C–O). δH
(400 MHz, CDCl3) 0.88 (t, 3H, J 6.58, CH3), 1.25 (sbr, 1H, OH),
A mixture of corresponding aldehyde 3d–f (10 mmol) and amine
7a–c(10 mmol)indryethanol(120 mL)wasrefluxedfor6h.The
product precipitated out as a yellow solid when cooled to room
temperature.The precipitate was filtered, washed with methanol,
and dried under vacuum. The solid product was recrystallized
from chloroform.

280
N. Senthilkumar, T. Narasimhaswamy, and A. Raghavan
OH
OH
OH
NaOH
Br
CH3
( )mꢁ1
ꢀ
n-Butanol
OH
OH
4
O
CH3
( )mꢁ1
1
5a–c
POCl3
DMF
DCC/DMAP
COOH
O2N
CHO
DCM:DMF
OH
ꢀ
Br-(CH2)nꢁ1CH3
CH3
( )mꢁ1
O
O2N
COO
6a–c
OH
NaOH
n-Butanol
2
Pd/C, H2
THF:EtOH
CHO
O
OH
CH3
O
H2N
COO
ꢀ
(
)
mꢁ1
7a–c
CH3
( )nꢁ1
EtOH
Reflux
PTSA
3a–c
H
H3C
O
C
8a n ꢂ 6, m ꢂ 7
8b n ꢂ 10, m ꢂ 10
8c n ꢂ 12, m ꢂ 12
(
)
nꢁ1
CH3
( )mꢁ1
N
O
COO
OH
8a–c
m ꢂ 7, 10, 12
n ꢂ 6, 10, 12
Scheme 1. Synthesis of model mesogens.
1.26–1.58 (m, 28H, CH2), 1.79–1.82 (m, 4H, CH2), 3.64 (t, 2H,
J 6.6, CH2), 3.96 (t, 2H, J 6.52, CH2), 4.02 (t, 2H, J 6.5, CH2),
6.50–6.52 (m, 2H, Ar–H), 6.93 (d, 2H, J 9.04, Ar–H), 7.12 (d,
2H, J 9.03, Ar–H), 7.29–7.35 (m, 3H, Ar–H), 8.23 (d, 2H, J
8.48, Ar–H), 8.57 (s, 1H, imine C–H), 13.41 (s, 1H, Ar–OH).
δC (100 MHz, CDCl3, 50◦C) 14.17, 22.84, 25.63, 25.73, 25.89,
26.12, 26.16, 29.24, 29.42, 29.50, 29.64, 29.68, 29.81, 29.95,
31.85, 32.10, 33.02, 63.20, 68.59, 68.86, 101.87, 108.28, 113.07,
115.51, 121.32, 122.52, 127.68, 131.80, 132.63, 134.19, 144.71,
153.47, 157.25, 163.17, 164.41, 165.12. m/z (EI) 645.4 (M+).
(Calc. for C40H55NO6 (645.88): C 74.38, H 8.60, N 2.17. Found:
C 74.12, H 8.73, N 2.19%.)
674 (M+). (Calc. for C42H59NO6 (673.94): C 74.85, H 8.84, N
2.08. Found: C 74.69, H 8.97, N 2.03%.)
Results and Discussion
Synthesis of Model Mesogens and Hydroxy-Terminated
Mesogens
The model mesogenic compounds were synthesized from a
triaromatic ring core linked through ester, azomethine, and
alkoxy groups of different chain length. Scheme 1 shows the
overall synthetic route for the preparation of model meso-
gens. 2,4-Dihydroxybenzaldehyde was synthesized using resor-
cinol 1 based on the Vilsmeier–Haack reaction.[15] The O-
alkylation of 2,4-dihydroxybenzaldehyde 2 and hydroquinone 4
were carried out with various bromoalkanes (n-bromoheptane,
n-bromodecane, n-bromododecane) in the presence of sodium
hydroxide as described in the preceding sections. The alkylated
products 3a–c were obtained as colourless liquids after purifi-
cation by column chromatography. The alkylated products 5a–c
were obtained as pale yellow solids, which were treated with
4-nitrobenzoic acid in the presence of DCC and DMAP. The
resulting esters (yellow solids 6a–c) were selectively reduced to
amines in the presence of Pd/C as a catalyst in a THF/ethanol
mixture. The amines 7a–c were further treated with substituted
2,4-dihydroxybenzaldehyde 3a–c in the presence of a catalytic
amount of PTSA to get the Schiff base ligands 8a–c as yellow
solids in high yield.
(E)-4-(Dodecyloxy)phenyl 4-(4-(10-Hydroxydecyloxy)-
2-hydroxybenzylideneamino)benzoate 9e
Yield: 4.85 g, 72%. Mp 128◦C. νmax(KBr)/cm−1 3433
(aliphatic OH), 3287 (phenolic OH), 2956, 2920, and 2850
(C–H), 1737 (C=O), 1631 (C=N), 1281 and 1199 (C–O). δH
(400 MHz, CDCl3) 0.88 (t, 3H, J 6.56, CH3), 1.26 (sbr, 1H,
OH), 1.27–1.61 (m, 32H, CH2), 1.79–1.83 (m, 4H, CH2), 3.66
(t, 2H, J 6.34, CH2), 3.96 (t, 2H, J 6.48, CH2), 4.03 (t, 2H, J 6.5,
CH2), 6.50–6.52 (m, 2H,Ar–H), 6.93 (d, 2H, J 8.92,Ar–H), 7.12
(d, 2H, J 8.92, Ar–H), 7.29–7.35 (m, 3H, Ar–H), 8.23 (d, 2H, J
8.48, Ar–H), 8.57 (s, 1H, imine C–H), 13.37 (s, 1H, Ar–OH).
δC (100 MHz, CDCl3, 50◦C) 14.07, 22.74, 25.61, 25.72, 25.78,
26.07, 26.16, 26.37, 29.34, 29.42, 29.48, 29.54, 29.67, 29.76,
29.83, 29.97, 31.83, 32.14, 33.07, 62.98, 68.39, 68.76, 101.90,
108.12, 113.13, 115.46, 121.24, 122.43, 127.39, 131.77, 132.56,
134.22, 144.69, 153.31, 157.16, 163.12, 164.39, 165.18. m/z (EI)
The core mesogen was also constructed from three phenyl
rings, which were linked through ester and azomethine
groups at 1,4-positions. The terminal groups were designed

Novel Hydroxy- and Methyl-Terminated Triaromatic Schiff Base Compounds
281
CHO
CHO
OH
OH
NaOH
ꢀ
Br-(CH2)xOH
CH3
ꢀ
O
H2N
COO
(
)
mꢁ1
n-Butanol
7a–c
OH
O
OH
( )x
EtOH
Reflux
2
3d–f
H
N
O
C
HO
(
)
x
CH3
( )mꢁ1
O
COO
OH
9a x ꢂ 6, m ꢂ 7
9b x ꢂ 6, m ꢂ 10
9c x ꢂ 6, m ꢂ 12
9d x ꢂ 8, m ꢂ 12
9e x ꢂ 10, m ꢂ 12
9a–e
m ꢂ 7, 10, 12
x ꢂ 6, 8, 10
Scheme 2. Synthesis of hydroxy-terminated mesogens.
Table 1. Synthesis and thermal properties of model mesogens
Chain length Differential scanning calorimetry
S. no.
Model mesogen
6CH37CH3Lig
Microscope [◦C]
n
m
Transition
Temp. [◦C]
ꢀH [kJ mol−1
]
ꢀT [◦C]
Tm
97
Ti
ꢀT
8a
6
7
Cr → SmC
SmC → SmA
SmA → N
N → I
95.6
195.0
225.3
230.4
95.4
182.8
94.1
100.1
179.2
35.77
–
134.80
233
136
0.29
0.63
46.32
0.84
61.67
–
8b
8c
10CH310CH3Lig
12CH312CH3Lig
10
12
10
12
Cr → SmA
SmA → I
87.41
85.14
96.2
95
202
172
105.8
77
Cr → SmC
SmC → SmA
SmA → I
3.47
in such a way that one end was functionalized with
alkoxy groups, while the other end was functionalized with
hydroxyalkoxy group as shown in Scheme 2. Based on
the mesophase characteristics of the model compounds, the
terminal hydroxy groups were selectively introduced. The
O-alkylation of 2,4-dihydroxybenzaldehyde was achieved by
treating with bromoalkanols (6-bromohexanol, 8-bromooctanol,
10-bromodecanol) in the presence of sodium hydroxide. The
relevant bromoalkanols were also prepared from the correspond-
ing alkyl diols, followed by purification with column chro-
matography and were well characterized according to literature
procedures.[16] The alkylated products 3d–f were obtained as
whitesolids, afterpurificationbycolumnchromatography.These
molecules subsequently reacted with amino compounds 7a–c to
afford Schiff base ligands 9a–e as yellow solids in high yield.
are alkoxy groups.[17] The length of the terminal alkoxy (n, m)
units was varied with a view to study their role on mesophase
transition temperatures.
The mesophase characteristics such as melting tempera-
ture, clearing temperature, thermal stability, phase stability, and
nature of the phase, along with transition enthalpy values are
shown inTables 1 and 2. The DSC thermograms of model meso-
gens were measured at a heating rate of 10◦C min−1 and shown
in Figs 1 and 2. The obtained transition temperatures from DSC
are in agreement with the HOPM results. Fig. 3 shows HOPM
photographs of model mesogen 8a and hydroxyl mesogen 9a
from which the identification of texture was carried out.
Table1liststhemesophasecharacteristicsofmodelmesogens
as synthesized following Scheme 1. The compound 8a showed
polymesomorphism as confirmed both by HOPM and DSC. The
compound, when viewed under the microscope (while cooling),
showed birefringent droplets characteristic of a nematic phase
at 229.4◦C. This, on cooling, changed to the focal conic fan tex-
ture of smectic A (SmA) (the transition from nematic to SmA is
clear from Fig. 3). Further cooling of the compound showed
a broken-focal conic texture typical of smectic C (SmC).[18]
As this phase was formed from SmA, it retained some of
its features and hence non-schlieren SmC was formed. The
crystallization of the compound was observed at 69◦C.The DSC
shows multiple signals with varying transitional enthalpy val-
ues. However, while heating, three signals at 95.6◦C, 225.3◦C,
and 230.4◦C were observed that were assigned based on the
HOPM observations as TC-SmA, TSmA-N, and TN-I transitions.
The change in enthalpy of these transitions also supports the
Liquid Crystalline Properties
The liquid crystalline properties of model mesogens and
terminal-hydroxy mesogens were investigated by hot-stage opti-
cal polarizing microscope (HOPM) and DSC. The HOPM was
used to observe the characteristic textures for identification
of phases, whereas DSC was employed for confirming the
phase observed in the microscope by determining the transition
enthalpy values. Thin samples of the compounds sandwiched
between cover-slips were placed under the microscope and
heated in a programmed manner. In this work, the core is
designed to have three phenyl rings linked through ester and
azomethine groups at 1,4-positions while the terminal groups

282
N. Senthilkumar, T. Narasimhaswamy, and A. Raghavan
Table 2. Synthesis and thermal properties of hydroxy terminated mesogens
Chain length Differential scanning calorimetry
S. no.
9a
Mesogen
Microscope [◦C]
x
m
Transition
Temp. [◦C]
ꢀH [kJ mol−1
]
ꢀT [◦C]
Tm
Ti
ꢀT
6OH7CH3Lig
6OH10CH3Lig
6
6
7
Cr → SmA
SmA → N
N → I
128.9
207.1
226.0
129.8
144.3
213.3
217.0
129.2
202.4
128.1
161.0
194.3
127.8
177.3
190.3
60.96
1.14
1.35
67.53
3.05
0.79
0.54
84.64
6.02
87.65
2.34
2.47
90.75
–
97.1
128
128
228
100
9b
10
Cr → SmC
SmC → SmA
SmA → N
N → I
87.2
218
90
9c
6OH12CH3Lig
8OH12CH3Lig
6
8
12
12
Cr → SmC
N → I
73.2
66.2
128
128
210
202
82
74
9d
Cr → SmA
SmA → SmC
SmC → I
Cr → SmC
SmC → N
N → I
9e
10OH12CH3Lig
10
12
62.5
128
191
63
5.98
c
TSmA-I
TC-SmC
b
Cooling
TSmA-I
Heating
TC-SmA
a
TSmA-N
TN-I
TC-SmC
50
100
150
200
250
50
100
150
Temperature [°C]
200
250
Temperature [°C]
Fig. 2. Differential scanning calorimetry thermograms of model mesogen,
8a (both heating and cooling curves).
Fig. 1. Differential scanning calorimetry thermograms of model mesogens
(while heating), 8a–c.
microscope observations. For instance, the ꢀH of the signal
appearing at 95.6◦C is 35.77 kJ mol−1, while for the other two
signals, the values were 0.29 and 0.63 kJ mol−1. As HOPM
shows the SmC phase while cooling and DSC does not show
the associated signal either heating or cooling, the phase was
characterized as monotropic. It is well established in the litera-
ture pertaining to calamitic mesogens that the SmA to SmC is
a second-order transition and the associated enthalpy change
is small and often not observed.[2] Fig. 2 shows the DSC
heating and cooling thermogram of model mesogen 8a. On
the cooling process from the isothermal (I), three exother-
mic peaks appeared at ∼222.5◦C, 216.3◦C, and 56.2◦C. These
three peaks were more clearly observed at faster cooling rates
because the peak temperature of the lower-temperature side peak
was significantly decreased with the increase of the cooling
rate.
was observed. The appearance of batonnets while cooling the
isotropic liquid and the formation of a fan texture characteris-
tic of SmA on further cooling was supported by the DSC data
where TC-SmA and TSmA-I were noted at 95.4◦C and 182.8◦C.
Compound 8c, which consists of 12 carbons in the terminal
chains (n = m = 12), also showed the SmA phase, which was
confirmed by noticing the batonnets, and on further cooling of
the sample, the broken-focal conic texture typical of SmC was
observed. Here again, in DSC, the transition from SmA to SmC
was not seen. The ꢀH values calculated from the DSC data were
61.67 and 3.47 kJ mol−1 for smectic A phase and its transition
to isotropic liquid respectively.
Figs 4–6 show DSC thermograms and HOPM pictures of
hydroxy-terminated mesogens 9a–e. The compound 9a (x = 6;
m = 7) shows nematic and SmA phases. While cooling this
compound, the isotropic liquid showed the formation of a
threaded nematic texture (nematic schlieren) and immediately
transformed to homeotropy. On further cooling, the formation
Interestingly, on increasing the length of the terminal alkoxy
chains (8b) to 10 carbons on both sides, only SmA phase

Novel Hydroxy- and Methyl-Terminated Triaromatic Schiff Base Compounds
283
(a)
(b)
(c)
(d)
Fig. 3. The hot-stage optical polarizing microscope photographs of model mesogen, 8a (while cooling) showing: (a) droplet texture of nematic phase at
229.4◦C; (b) nematic to smectic A phase transition at 223.8◦C; (c) focal conic fan-like texture of smectic A phase at 203.4◦C; (d) broken focal conic texture
of smectic A to smectic C phase transition at 167.8◦C.
e
TN-I
TC-SmC
d
TSmA-SmC
TSmC-I
TC-SmA
c
Cooling
Heating
TN-I
TC-SmC
b
TSmC-SmA
TN-I
TSmA-N
TC-SmC
a
TSmA-N
TN-I
TC-SmA
50
100
150
200
250
50
100
150
200
250
Temperature [°C]
Temperature [°C]
Fig. 4. Differential scanning calorimetry thermograms of hydroxy meso-
gens (while heating), 9a–e.
Fig. 5. Differential scanning calorimetry thermograms of hydroxy meso-
gen, 9a (both heating and cooling curves).

284
N. Senthilkumar, T. Narasimhaswamy, and A. Raghavan
9c, with m = 12, showed SmC and nematic phases. Interestingly,
this compound exhibited a nematic phase on heating, while SmC
was observed only on cooling (monotropic). In DSC, only two
(a)
signals were noted corresponding to TC-N and TN-I
.
Thecompound 9d consistsofterminalalkoxyunitswithx = 8
and m = 12, and HOPM observations indicate the appearance
of SmA phase. This phase was ascertained by noticing focal
conic fan domains among homeotropy. The sample on cooling
transformed into broken-focal conics (SmC) phase. For mesogen
9e, where one terminal consists of 10 carbons while the other
has 12 carbons, HOPM showed nematic phase (threads) and
homeotropy, and on cooling, it transformed to SmC phase (smec-
tic schlieren). The corresponding temperatures found in DSC
were TC-SmC 127.8◦C, TSmC-N 177.3◦C, TN-I 190.3◦C, respec-
tively, and the enthalpy changes for these transitions are listed
in the Table 2.
(b)
Conclusions
With judicious modifications of ligand design, mesogenic moi-
eties as hydroxy-terminated compounds have been synthesized.
The mesogens were constructed from a three-ring structure
linked through ester and azomethine groups with a terminal
hydroxy group. The model mesogenic compounds (without
hydroxy) were also synthesized with varying chain length,
and mesophase properties with hydroxy-terminated mesogens
were compared. Spectroscopic studies confirmed the structure
of the respective compounds. All the compounds synthesized
in the present investigation exhibit an enantiotropic nematic,
smectic A and smectic C phases with excellent phase stability,
and the appearance of enantiotropic smectic phases indicates
high molecular polarizability of the mesogenic core.
(c)
Accessory Publication
Spectra of compounds are available on the Journal’s website.
Acknowledgements
Part of this work was supported by Tamil Nadu-Junior Research Fellowship,
University of Madras.
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