Synthesis, mesomorphic properties and structural studies on 1,3,5-trisubstituted benzene-based star-shaped derivatives containing Schiff base ester as the peripheral arm

Article abstract of DOI:10.1016/j.molstruc.2013.08.015

Six new symmetrical three-armed star-shaped liquid crystals comprising phloroglucinol (1,3,5-trihydroxybenzene) as a core center and three Schiff base ester as the mesogenic arms have been synthesized. Their molecular structures were supported by spectros

Full text of DOI:10.1016/j.molstruc.2013.08.015

Journal of Molecular Structure 1051 (2013) 361–375
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, mesomorphic properties and structural studies
on 1,3,5-trisubstituted benzene-based star-shaped derivatives
containing Schiff base ester as the peripheral arm
b
Yew-Hong Ooi a, Guan-Yeow Yeap a, , Daisuke Takeuchi
⇑
a Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
b Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohoma, Japan
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2013
Received in revised form 28 July 2013
Accepted 12 August 2013
Available online 19 August 2013
Six new symmetrical three-armed star-shaped liquid crystals comprising phloroglucinol
(1,3,5-trihydroxybenzene) as a core center and three Schiff base ester as the mesogenic arms have been
synthesized. Their molecular structures were supported by spectroscopic techniques (FT-IR, 1H NMR, 13
C
NMR and two dimensional COSY, HMQC and HMBC). Each of the member differs in the length of the ter-
minal alkyl chain (RACOOA where R = C8H17, C10H21, C12H25, C14H29, C16H33, C18H37). All the star-shaped
mesogens are found to be monotropic smectogens with the short chain derivative (R = C8H17) exhibits
only smectic A phase. While both the smectic A and smectic C phases are common for the derivatives
Keywords:
Phloroglucinol
Three-armed star-shaped liquid crystals
Schiff base esters
Smectic phase
with C10H21AC16H33 alkyl chain, the member with C18H37 shows only smectic C phase. Further investiga-
tion by X-ray diffraction confirms the smectic phase of the materials whereby two sharp reflections at
28.8 Å and 57.3 Å recorded via small angle in a ratio of 1:2 indicate unambiguously the presence of lay-
ered structure and a diffused scattering via the wide angle (4.5 Å) indicates a liquid-like order within the
smectic layer. Increasing the length of terminal flexible chain has influenced remarkably the thermal and
phase stability of the titled compounds. Besides, the liquid crystalline properties of all intermediary
XRD
2D NMR spectroscopy
x-brominated Schiff base ester prior joining to the benzene core were also evaluated.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
[1,2]. This class of liquid crystals is a type of molecule with an
anisotropic shape that deviates from the classical rod- and disc-
shaped molecules [3]. One of the requirements in the design of
novel non-conventional mesogens is that it must allow for the pos-
sibility of segregating the incompatible molecular segments. The
core group of the mesogen utilized can be any shape as long as it
is sufficiently flexible to provide nano-segregation as it plays a vital
The investigation on non-conventional liquid crystals (LCs) has
become an interesting research area within the last two decades
⇑
Corresponding author. Tel.: +60 4 6533568; fax: +60 4 6574854.
E-mail address: gyyeap@usm.my (G.-Y. Yeap).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.08.015

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Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
role in the self-assembly or self-organization of liquid crystal struc-
ture [4]. Non-conventional star-shaped LCs, is considered as one of
the simplest multi-arm mesogens consisting of a disc-like core as a
central unit (mesogenic or non-mesogenic) with at least three
mesogenic peripheral arms. This type of mesogen has attracted
increasing interest owing to their symmetrical structures and
interesting properties. Various mesophases can be exhibited by
the star-shaped mesogens, including the discotic columnar phase
as well as the nematic, smectic and cholesteric phase [5–10]. This
is because this class of liquid crystals has both disc-like units with
high substitution symmetry which is favourable for the formation
of discotic mesophase and structural elements (rod-like mesogenic
units) that promote calamitic mesomorphism [11]. They are also
termed as Hekates in order to easily differentiate them from other
mesogenic trimer such as linear trimers or irregular tripodes [12].
The numerous applications of star-shaped LCs include non-linear
optical filters, flat panel displays and fast switching materials
[13–16]. In addition, some of the star-shaped LCs tend to exhibit
glassy properties [17,18] upon cooling from isotropic state and this
phenomenon is imperative to show uncommon optical, mechani-
cal and thermal stable properties [19–22] and preferred to be uti-
lized as soft functional materials like anisotropic electron/hole-,
ion-, proton conducting and electro-optical application [23].
A great number of star-shaped mesogens comprising of a ben-
zene unit as a core group have been synthesized and their proper-
ties investigated [17,24–27]. Liquid crystalline behaviour of a
star-shaped mesogen is presumably dependent on its molecular
architecture in which the molecular geometry of the peripheral
unit may bring about substantial changes in its mesomorphic
properties. From the literature studies, star-shaped mesogens can
exhibit both nematic and smectic phase characteristic of calamitic
LCs and also columnar phase, which is commonly found in discotic
LCs. For instance, Kumar and Manickam had prepared two series of
star-shaped trimers whereby three triphenylene-based or nitro-
functionalized triphenylene-based units were synthesized as the
peripheral mesogenic unit and linked to the 1-,3-,5-position of
the benzene central core via an alkyl spacer and ester linkage
[28]. From their findings, the nitro-functionalized compound
exhibited a monotropic columnar phase at 158.7 °C on cooling
while the unfunctionalized compound was found to be non-meso-
genic, which crystallized at 166 °C on cooling. Yao and co-workers
had also synthesized a series of cholesteric star-shaped mesogens
Fig. 1. A schematic illustration of the structure of Hekates in which three rod-
shaped molecular subunits attached to a benzene central linking unit via a flexible
spacer to give the three-armed star-shaped compound.
benzene) with Schiff base ester derivatives as the peripheral units.
Moreover, morphologies of star-shaped mesogens can be further
regulated by carefully adjusting the molecular topology. As the con-
tinuation of our effort to understand the structure–property in such
star-shaped system, this paper describes the synthesis and meso-
morphic properties of a homologue series of trimeric star-shaped
LCs, 1,3,5-tris[{alkyl-(4-phenylimino)methyl}benzoate-40-oxy]pro-
pyloxy benzene. It is a star-shaped system functionalized with a
disc-shaped benzene ring as a core unit and three linearly extended
rod-like Schiff base ester fragments in its periphery (Fig. 1). Each of
the members differs in the even-parity terminal alkyl chain length
(R = CnH2n+1) of which the n ranges from 8 to 18. Besides, the influ-
ence of the terminal alkyl chain length on the phase behaviour of
the star-shaped compounds is discussed in the following
discussion.
based on phloroglucinol with three
x-cholesteric alkyl diacid
monoester fragments as the peripheral units. Cholesteric focal-
conic texture was detected in their liquid crystalline state. In
addition, as the diacid alkyl spacer lengthened, the melting tem-
perature descended but the mesomorphic temperature range in-
creased [7]. Therefore, it has been demonstrated that benzene
ring can be utilized as a suitable structural component in synthe-
sizing the star-shaped LCs.
Due to the growing scientific interest in the synthesis of star-
shaped LCs, this has prompted us to further investigate the aniso-
tropic properties of phloroglucinol-based star-shaped LCs.
Recently, the mesomorphic behaviour of phloroglucinol-based
symmetrical star-shaped mesogens containing three Schiff base
ether peripheral units has been documented [29]. From our study,
all the analogues exhibited predominately SmC phase except for
the member possessing the shortest terminal alkyl chain. The for-
mation of smectic phase by the star-shaped mesogen was found
to be generated by the side-by-side organization of the peripheral
arms which lie parallel to each other. Thus, the mesophase forma-
tion of the star-shaped mesogens is mainly attributed to the rod-
like calamitic Schiff base units whereas the benzene core acts only
as a linking group interconnecting the rods. To the best of our
knowledge, there were relatively few studies on the star-shaped
LCs incorporating a core unit of phloroglucinol (1,3,5-trihydroxy-
2. Experimental
2.1. Reagents and chemicals
The starting materials such as phloroglucinol anhydrous, 4-
carboxybenzaldehyde, 4-aminophenol were obtained from Acros
Organics (Geel, Belgium). 1-Bromooctance, 1-bromodecane, 1-bro-
mododecane, 1-bromotetradecane, 1-bromohexadecane, 1-bro-
moactadecane and 1,3-dibromopropane, TLC Silica gel 60 F254
aluminium sheets and silica gel 60 (0.040–0.063 mm) for column
chromatography were purchased from Merck (Darmstadt, Ger-
many). Potassium carbonate anhydrous was purchased from QREc
(Auckland, New Zealand) while potassium iodide was purchased
from Fisher Scientific (Waltham, MA, USA). All the chemicals and
solvents were used directly without further purification.
2.2. Characterization
2.2.1. Physical measurements
The CHN analytical data were acquired on Perkin Elmer 2400 LS
series CHNS/O analyzer. Melting point of the compounds were
measured using Gallenkamp melting point apparatus.

Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
363
2.2.2. FT-IR measurement
1b (R = C10H21): Yield: 76%. Brown viscous liquid. IR (salt plate
AgCl) v/cmꢀ1: 2954, 2917, 2850 (CAH aliphatic), 1716 (C@O ester),
1698 (C@O aldehyde), 1285 (CAO ester).
1c (R = C12H25): Yield: 80%. Brown viscous liquid. IR (salt plate
AgCl) v/cmꢀ1: 2952, 2916, 2849 (CAH aliphatic), 1717 (C@O ester),
1698 (C@O aldehyde), 1285 (CAO ester).
Fourier transform infrared (FT-IR) spectra for all the intermedi-
ate and target compounds were recorded using a Perkin Elmer
2000-FTIR spectrophotometer in the frequency range 4000–
400 cmꢀ1 with samples embedded in KBr discs.
1d (R = C14H29): Yield: 84%. White precipitate, m.p. 44–45 °C.
Elemental analysis: found, C 76.34, H 9.90, N 13.69; calculated
(C22H34O3), C 76.26, H 9.89, N 13.85. IR (KBr) v/cmꢀ1: 2953,
2916, 2851 (CAH aliphatic), 1718 (C@O ester), 1696 (C@O alde-
hyde), 1284 (CAO ester).
1e (R = C16H33): Yield: 81%. White precipitate, m.p. 55–56 °C.
Elemental analysis: found, C 76.44, H 10.11, N 13.40; calculated
(C23H36O3), C 76.62, H 10.06, N 13.31. IR (KBr) v/cmꢀ1: 2960,
2917, 2851 (CAH aliphatic), 1718 (C@O ester), 1697 (C@O alde-
hyde), 1285 (CAO ester).
2.2.3. NMR measurement
NMR spectra were recorded in CDCl3 at 298 K on a Bruker
Avance 500 MHz Ultrashield FT-NMR spectrometer equipped with
a 5 mm BBI inverse gradient probe, using tetramethylsilane (TMS)
as an internal standard. Complete 1H and 13C NMR assignment of
representative compound were obtained and substantiated by
means of 1HA1H correlation spectroscopy (COSY), 1HA13C hetero-
nuclear multiple quantum correlation (HMQC) and 1HA13C heter-
onuclear multiple bond correlation (HMBC) spectroscopic
measurements.
1f (R = C18H37): Yield: 88%. White precipitate, m.p. 58–59 °C.
Elemental analysis: found, C 76.79, H 10.21, N 12.83; calculated
(C24H38O3), C 76.96, H 10.23, N 12.81. IR (KBr) v/cmꢀ1: 2955,
2917, 2851 (CAH aliphatic), 1718 (C@O ester), 1698 (C@O alde-
hyde), 1285 (CAO ester).
2.2.4. Phase transition temperature and enthalpy values
The phase transitions temperature along with its associated en-
thalpy change were determined from thermograms obtained using
a Seiko DSC 6200R calorimeter, with both heating and cooling rate
of 2 °C in Tokyo Institute of Technology, Japan.
2.3.2. Synthesis of compounds 2a–2f
A solution of 4-aminophenol (26 mmol, 2.8 g) in absolute ethanol
was added to a stirring 50 mL ethanolic solution of compounds
1a–1f (25 mmol) in a round bottom flask. Subsequently, 3 drops of
glacial acetic acid was added as catalyst and the reaction mixture
was refluxed at 78 °C for 4 h. The reaction mixture was left to cool
down to room temperature and the resulting precipitate thus
formed was isolated and recrystallized from absolute ethanol to
yield the desired Schiff base ester intermediates, 2a–2f. The analyt-
ical and FT-IR data for compounds 2a–2f are summarized as follows:
2a (R = C8H17): Yield: 75%. Beige precipitate, m.p. 125–126 °C.
Elemental analysis: found, C 74.88, H 7.76, N 3.98; calculated
(C22H27NO3), C 74.76, H 7.70, N 3.96. IR (KBr) v/cmꢀ1: 3411 (OH
phenolic), 2954, 2920, 2854 (CAH aliphatic), 1716 (C@O ester),
1622 (C@N azomethine), 1289 (CAO ester).
2b (R = C10H21): Yield: 81%. Beige precipitate, m.p. 90–91 °C.
Elemental analysis: found, C 75.46, H 8.23, N 3.68; calculated
(C24H31NO3), C 75.56, H 8.19, N 3.67. IR (KBr) v/cmꢀ1: 3410 (OH
phenolic), 2954, 2916, 2849 (CAH aliphatic), 1715 (C@O ester),
1622 (C@N azomethine), 1284 (CAO ester).
2c (R = C12H25): Yield: 79%. Beige precipitate, m.p. 95–96 °C.
Elemental analysis: found, C 76.43, H 8.80, N 3.39; calculated
(C26H35NO3), C 76.25, H 8.61, N 3.42. IR (KBr) v/cmꢀ1: 3409 (OH
phenolic), 2954, 2916, 2849 (CAH aliphatic), 1716 (C@O ester),
1622 (C@N azomethine), 1286 (CAO ester).
2.2.5. Liquid crystalline texture observation
The optical microscopy studies were carried out with a Carls
Zeiss Axioskop 40 polarizing microscope equipped with a Linkam
TMS94 temperature controller and an LTS350 hot stage in the
School of Chemical Sciences, USM. The textures of the compounds
were observed using polarized light with a cross polarizer whereby
the sample studied was prepared in a thin film sandwiched be-
tween a glass slide and coverslip.
2.2.6. X-ray diffraction measurement
The powder X-ray diffraction measurements were performed
using a wide-angle Bruker D8 diffractometer (Cu K
Vantec detector. A glass slide containing a thin layer of sample
was placed on a heating stage and orientated homeotropically.
The X-ray beam was directed almost parallel to the substrate sur-
face and the temperature of the sample was controlled by an accu-
racy of 0.1°.
a line) with a
2.3. Synthesis
The synthesis of the intermediates 1a–1f, 2a–2f, 3a–3f and the
final star-shaped compounds, 1,3,5-tris[{alkyl-(4-phenylimi-
no)methyl}benzoate-40-oxy]propyloxy benzene, 4a–4f were car-
ried out using the experimental procedure as illustrated in
Scheme 1. The codes for compounds 3a–3f and 4a–4f were tabu-
lated in Table 1.
2d (R = C14H29): Yield: 83%. Beige precipitate, m.p. 99–100 °C.
Elemental analysis: found, C 76.84, H 8.91, N 3.25; calculated
(C28H39NO3), C 76.85, H 8.98, N 3.20. IR (KBr) v/cmꢀ1: 3411 (OH
phenolic), 2954, 2917, 2850 (CAH aliphatic), 1715 (C@O ester),
1622 (C@N azomethine), 1289 (CAO ester).
2.3.1. Synthesis of compounds 1a–1f
The preparation of alkyl 4-formylbenzoate was carried out
according to the literature with some modification [30]. Commer-
cially available 4-carboxybenzaldehyde (30 mmol, 4.5 g) was trea-
ted with various even-parity bromoalkane (32 mmol) ranging from
8 to 18 in DMF with the aid of potassium carbonate and stirred at
90 °C for 24 h. The reaction mixture was then poured into 250 mL
water and extracted with dichloromethane twice. Sodium sulphate
anhydrous was added as drying agent and the solvent was re-
moved by distillation. A viscous brown liquid was obtained. For
homologues with 14, 16 and 18 carbon chain, white precipitate
was obtained and recrystallized from absolute ethanol.
2e (R = C16H33): Yield: 85%. Beige precipitate, m.p. 103–104 °C.
Elemental analysis: found, C 77.45, H 9.44, N 3.10; calculated
(C30H43NO3), C 77.38, H 9.31, N 3.01. IR (KBr) v/cmꢀ1: 3409 (OH
phenolic), 2954, 2916, 2849 (CAH aliphatic), 1716 (C@O ester),
1622 (C@N azomethine), 1287 (CAO ester).
2f (R = C18H37): Yield: 84%. Beige precipitate, m.p. 108–109 °C.
Elemental analysis: found, C 77.99, H 9.69, N 2.88; calculated
(C32H47NO3), C 77.85, H 9.60, N 2.84. IR (KBr) v/cmꢀ1: 3410 (OH
phenolic), 2955, 2916, 2850 (CAH aliphatic), 1716 (C@O ester),
1622 (C@N azomethine), 1286 (CAO ester).
1a (R = C8H17): Yield: 78%. Brown viscous liquid. IR (salt plate
AgCl) v/cmꢀ1: 2953, 2916, 2851 (CAH aliphatic), 1718 (C@O ester),
1696 (C@O aldehyde), 1284 (CAO ester).
2.3.3. Synthesis of compounds 3a–3f
In a round bottom flask, a mixture containing compounds 2a–2f
(20 mmol) and potassium carbonate anhydrous (60 mmol, 8.3 g)

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Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
Scheme 1. Synthetic route used to prepare the intermediates 1a–1f, 2a–2f, 3a–3f and the three-armed star-shaped mesogens 4a–4f.
recrystallized from acetone. The analytical and FT-IR data for com-
Table 1
pounds 3a–3f are summarized as follows:
The intermediary Schiff base ester 3a–3f and the title three-armed star-shaped
mesogens 4a–4f.
3a (R = C8H17): Yield: 69%. Light beige precipitate. Elemental
analysis: found, C 63.33, H 6.95, N 2.96; calculated (C25H32NO3Br),
C 63.29, H 6.80, N 2.95. IR (KBr) v/cmꢀ1: 2953, 2920, 2849 (CAH
aliphatic), 1713 (C@O ester), 1623 (C@N azomethine), 1285 (CAO
ester), 1245 (CAO ether).
3b (R = C10H21): Yield: 68%. Light beige precipitate. Elemental
analysis: found, C 64.50, H 7.30, N 2.85; calculated (C27H36NO3Br),
C 64.54, H 7.22, N 2.79. IR (KBr) v/cmꢀ1: 2953, 2919, 2850 (CAH
aliphatic), 1714 (C@O ester), 1623 (C@N azomethine), 1287 (CAO
ester), 1246 (CAO ether).
R
Intermediate
Compound
C8H17
3a
3b
3c
3d
3e
3f
4a
4b
4c
4d
4e
4f
C
C
C
C
C
10H21
12H25
14H29
16H33
18H37
was heated in 50 mL acetone. Excess of 1,3-dibromopropane
(80 mmol, 16.2 g) was then added dropwise into the reaction
mixture whereby it was subjected to reflux for 16 h. Upon cooling
to room temperature, 100 mL cold water was poured into the reac-
tion mixture. The precipitate thus obtained was filtered out and
3c (R = C12H25): Yield: 74%. Light beige precipitate. Elemental
analysis: found, C 65.76, H 7.71, N 2.77; calculated (C29H40NO3Br),
C 65.65, H 7.60, N 2.64. IR (KBr) v/cmꢀ1: 2952, 2917, 2850 (CAH
aliphatic), 1713 (C@O ester), 1622 (C@N azomethine), 1283 (CAO
ester), 1246 (CAO ether).

Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
365
Table 2
3d (R = C14H29): Yield: 77%. Light beige precipitate. Elemental
analysis: found, C 66.82, H 7.96, N 2.49; calculated (C31H44NO3Br),
C 66.66, H 7.94, N 2.51. IR (KBr) v/cmꢀ1: 2952, 2917, 2849 (CAH
aliphatic), 1713 (C@O ester), 1623 (C@N azomethine), 1285 (CAO
ester), 1245 (CAO ether).
3e (R = C16H33): Yield: 75%. Light beige precipitate. Elemental
analysis: found, C 67.51, H 8.27, N 2.41; calculated (C33H48NO3Br),
C 67.56, H 8.25, N 2.39. IR (KBr) v/cmꢀ1: 2952, 2917, 2850 (CAH
aliphatic), 1714 (C@O ester), 1623 (C@N azomethine), 1287 (CAO
ester), 1246 (CAO ether).
3f (R = C18H37): Yield: 78%. Light beige precipitate. Elemental
analysis: found, C 68.44, H 8.64, N 2.26; calculated (C35H52NO3Br),
C 68.39, H 8.53, N 2.28. IR (KBr) v/cmꢀ1: 2952, 2916, 2849 (CAH
aliphatic), 1714 (C@O ester), 1622 (C@N azomethine), 1286 (CAO
ester), 1245 (CAO ether).
Phase transition temperatures (°C) and the corresponding enthalpy changes for
compounds 3a-3f upon heating and cooling. The values in Italics denote the cooling
cycle.
Compound
Phase transition temperature, °C
(enthalpy change, kJ molꢀ1
)
3a (R = C8H17
)
Cr 57.9 (32.6) SmA 78.9 (4.9) I
I 74.7 (4.9) SmA 46.7 (1.7) SmC 28.6 (6.7) Cr
Cr 61.2 (29.1) SmA 75.7 (4.5) I
I 70.6 (5.0) SmA 43.1 (0.9) SmC 33.5 (23.6) Cr
Cr 71.7 (39.9) SmA 78.1 (5.0) I
I 74.0 (5.1) SmA 53.2 (39.4) Cr
Cr 78.4 (45.6) I
3b (R = C10H21
)
3c (R = C12H25
)
3d (R = C14H29
)
)
I 74.4 (4.6) SmA 62.6 (39.1) Cr
Cr 81.8 (61.7) I
3d (R = C16H33
I 70.1* SmA 68.3 (59.2) Cr
3f (R = C18H37
)
Cr 84.1 (61.1) I
I 73.6* SmA 72.7 (59.9) Cr
Note: Cr, crystal; SmA, smectic A; SmC, smectic C; I, isotropic.
*
Transition temperature determined by POM, undetected via DSC.
2.3.4. Synthesis of compounds 4a–4f
The present star-shaped compounds 4a–4f are synthesized
based on the following method with some modification from that
reported in the literature [31]. Phloroglucinol anhydrous
(2.5 mmol, 0.31 g) was dissolved in 10 mL N,N0-dimethylformam-
ide (DMF). Potassium carbonate anhydrous (50 mmol, 7.0 g) was
added to the reaction mixture and allowed to stir at 80 °C for 1 h.
Excess amount of compound 3a–3f (10 mmol) was dissolved in
50 mL hot DMF and was added to the reaction mixture followed
by a catalytic amount of potassium iodide. The reaction mixture
was stirred at 80 °C for 48 h (2 days). Once completed, it was al-
lowed to cool down and left to evaporate at room temperature un-
til the total volume of the mixture was 40 mL. Cold water was
added and the resulting precipitate was filtered off, dried and puri-
fied by column chromatography whereby dichloromethane was
used as the mobile phase. The precipitate thus obtained was
recrystallized from DMF to obtain the star-shaped compounds,
4a–4f. The elemental analytical and FT-IR data for compounds
4a–4f are summarized as follows:
4a (R = C8H17): Yield: 26%. Beige precipitate. Elemental analysis:
found, C 74.78, H 7.70, N 3.27; calculated (C81H99N3O12), C 74.45, H
7.64, N 3.22. IR (KBr) v/cmꢀ1: 2955, 2920, 2851 (CAH aliphatic),
1714 (C@O ester), 1624 (C@N azomethine), 1284 (CAO ester),
1250 (CAO ether).
4b (R = C10H21): Yield: 25%. Beige precipitate. Elemental analy-
sis: found, C 75.20, H 8.02, N 3.06; calculated (C87H111N3O12), C
75.13, H 8.04, N 3.02. IR (KBr) v/cmꢀ1: 2956, 2921, 2853 (CAH
aliphatic), 1713 (C@O ester), 1624 (C@N azomethine), 1281 (CAO
ester), 1250 (CAO ether).
4c (R = C12H25): Yield: 32%. Beige precipitate. Elemental analy-
sis: found, C 75.88, H 8.55, N 2.94; calculated (C93H123N3O12), C
75.73, H 8.41, N 2.85. IR (KBr) v/cmꢀ1: 2956, 2919, 2851 (CAH ali-
phatic), 1714 (C@O ester), 1623 (C@N azomethine), 1281 (CAO
ester), 1249 (CAO ether).
4d (R = C14H29): Yield: 30%. Beige precipitate. Elemental analy-
sis: found, C 76.35, H 8.65, N 2.87; calculated (C99H135N3O12), C
76.26, H 8.73, N 2.70. IR (KBr) v/cmꢀ1: 2956, 2920, 2850 (CAH
aliphatic), 1713 (C@O ester), 1623 (C@N azomethine), 1282 (CAO
ester), 1250 (CAO ether).
4e (R = C16H33): Yield: 36%. Beige precipitate. Elemental analy-
sis: found, C 76.83, H 8.95, N 2.63; calculated (C105H147N3O12), C
76.74, H 9.02, N 2.56. IR (KBr) v/cmꢀ1: 2955, 2918, 2850 (CAH
aliphatic), 1713 (C@O ester), 1625 (C@N azomethine), 1285 (CAO
ester), 1251 (CAO ether).
4f (R = C18H37): Yield: 34%. Beige precipitate. Elemental analy-
sis: found, C 77.29, H 9.25, N 2.44; calculated (C111H159N3O12), C
77.18, H 9.28, N 2.43. IR (KBr) v/cmꢀ1: 2956, 2921, 2852 (CAH
aliphatic), 1714 (C@O ester), 1624 (C@N azomethine), 1282 (CAO
ester), 1250 (CAO ether).
3. Results and discussion
3.1. Liquid crystalline behaviour of the intermediary compounds
3a–3f
The phase sequences for compounds 3a–3f with well-defined
transition temperatures are shown in Table 2. All the observed li-
quid crystalline textures are in accordance to those reported in
the literatures [32].
As seen in Table 2, all the intermediary 3a–3f are found to be
smectogenic. Compounds 3a–3c exhibit enantiotropic properties
whereby the phase sequences of crystal–mesophase–isotropic are
recorded during heating and cooling cycles. Such transitions have
also been supported by the enthalpy values of the respective com-
pounds. Hence, they are enantiotropic smectogens exhibiting the
SmA phase during heating and both SmA and SmC phases during
cooling except for 3c, which does not show the tilted SmC phase
on cooling. The SmA phase of 3a–3c is identified based on the for-
mation of batonnets which eventually coalesce to form the focal
conic fan-shaped texture when viewed under POM. On further
cooling, SmC phase is observed only in 3a and 3b whereby the focal
conic fan-shaped changes into the broken fan-shaped texture be-
fore undergoing crystallization process at lower temperature.
Fig. 2 shows the optical photomicrographs of 3a exhibiting the
focal conic fan-shaped texture of SmA phase and the broken fan-
shaped texture of SmC phase upon cooling. Whilst the DSC traces
for 3a upon heating–cooling cycle is shown in Fig. 4. Compounds
3d–3f exhibit solely monotropic SmA phase. From the DSC thermo-
grams of 3e and 3f, the I-SmA transition upon cooling is undetect-
able but under the polarized light, the SmA phase is apparent prior
to crystallization when a slower cooling rate of 1 °C minꢀ1 is
employed instead of 5 °C minꢀ1. Under this condition, the forma-
tion of tiny circular focal-conic domain (Fig. 3) is observed fol-
lowed by immediate crystallization. A noteworthy feature in
relation to the I-SmA is that it occurs very rapidly and hence is
not detectable by the DSC. Therefore, it can be suggested that the
I-SmA phase transition range has narrowed and the IACr transition
become dominant as the series ascends towards homologue with
much longer terminal alkyl chains. By referring to Table 2, the
tilted SmC phase apparently disappear starting from 3c onwards
which possess R = C12H25. In short, the SmC phase becomes not vis-
ible upon elongation of carbon chain. This can be due to the differ-
ence in the dipole distribution between the alkyl groups at both
end of the molecule which eventually reduces its ability to sustain
the tilting arrangement and attraction between the molecules.
Across the series, the melting temperatures increase which show

366
Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
Fig. 2. Optical photomicrograph showing (a) the SmA fan-shaped texture of compound 3a (R = C8H17) at 74.7 °C and (b) the SmC broken fan-shaped texture at 46.7 °C upon
cooling.
in melting temperature on increasing the alkyl chain length sug-
gests that the intermolecular interactions between the alkyl chains
play
a vital role in determining the melting point of the
compounds.
3.2. Thermal behaviour and texture observation of star-shaped
mesogens 4a–4f
The observed mesophases for the star-shaped compounds 4a–
4f with well-defined phase transition temperatures, associated
enthalpy and entropy changes obtained from DSC upon heating
and cooling are summarized in Table 3.
All the star-shaped compounds are monotropic smectogens
with 4a exhibiting only SmA phase and 4f exhibiting SmC phase,
while the other homologues display both SmA and tilted SmC
phase with the phase sequence of Iso–SmA–SmC–Cr. On heating,
all the compounds show direct isotropization from crystals but
on cooling, the exotherm characteristic of isotropic–mesophase–
crystal phase sequence is observed thus indicating the monotropic
behaviour. Observation under crossed polarizer for 4a–4e shows
the formation of the focal conic fan-shaped texture characteristics
of SmA phase. However, for 4b–4e, a change from the fan-shaped
texture into the broken fan-shaped texture on further cooling is
observed indicating the presence of SmC phase prior to crystalliza-
tion at lower temperature (Fig. 5). Whilst for 4f which possesses
the longest terminal alkyl chain displays only SmC phase as it
exhibits a gradual appearance of batonnets within the matrix
which has eventually led to the formation of the broken fan-
shaped texture on cooling from the isotropic state. In short, as
the series ascends towards homologues with much longer terminal
alkyl chain, the emergence of SmC phase after SmA phase become
apparent (see Table 4).
Fig. 3. Optical photomicrograph showing the formation or growth of tiny circular
focal-conic domain of compound 3e (R = C16H37) along with the crystallization
process.
It is worthwhile to mention that, in the DSC thermograms of
4b–4e, the exotherm associated with the I-SmA transition is de-
tected but the exotherm involving the SmA–SmC transition appear
to be unresolved. However, the phase transition from SmA to SmC
is clearly visible under the observation of POM whereby a change
of fan-shaped texture into broken fan-shaped texture is observed.
This kind of phenomenon had been observed previously in chiral
liquid crystal trimers in which the SmAꢁ–SmCꢁ phase transition
was not accompanied by any enthalpy change but was visible un-
der the POM observation [34]. Investigation on the smectic
arrangement of the present star-shaped mesogens by X-ray diffrac-
tion method will be discussed in the following section. Hence, it is
suggested that the enthalpy change for the SmA–SmC transition is
very small and thus could not be detected by the DSC. The DSC
traces upon heating–cooling for 4e is shown in Fig. 6.
Fig. 4. DSC thermograms of compound 3a (R = C8H17) with heating and cooling
cycles at the rate of 2 °C.
a regular dependence on the length of terminal alkyl group. For in-
stance, the lowest melting point is recorded for 3a which possess
the shortest alkyl chain while the melting point for 3f is the high-
est. Therefore, as the series ascends towards longer members, extra
energy is required to overcome the strength of the Van der Waals
interaction exert by the additional methylene groups (CH2) upon
lengthening the terminal chain [33]. The essentially linear increase
The length of the terminal alkyl chain plays an important role in
determining the stability of the mesophase. From the plot of

Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
367
Table 3
Phase transition temperatures (°C), associated enthalpy (kJ molꢀ1) and entropy changes for the star-shaped compounds 4a–4f upon heating
and cooling. The values in Italics denote the cooling cycle.
Compound
Phase transition temperature, °C (enthalpy change, kJ molꢀ1
)
DSSmA-I/R
4a (R = C8H17
)
Cr 125.7 (72.3) I
I 102.1 (8.6) SmA 89.3 (53.2) Cr
Cr 121.6 (83.2) I
2.7
3.4
4.0
4.7
4.3
–
4b (R = C10H21
4c (R = C12H25
4d (R = C14H29
4e (R = C16H33
4f (R = C18H37
)
I 106.1 (10.8) SmA 102.3* SmC 90.7 (61.5) Cr
Cr 120.5 (35.5) Cr2 131.6 (34.9) I
I 116.6 (12.9) SmA 113.6* SmC 90.3 (57.0) Cr
Cr 124.2 (39.0) Cr2 134.4 (52.0) I
I 122.3 (15.5) SmA 116.2* SmC 82.3 (62.2) Cr
Cr 126.5 (83.8) I
)
)
)
I 118.6 (13.9) SmA 117.5* SmC 95.1 (79.0) Cr
Cr 126.9 (132.3) I
)
I 115.2 (16.6) SmC 103.4 (121.6) Cr
Note: Cr, crystal; SmA, smectic A; SmC, smectic C; I, isotropic.
*
Transition temperature determined by POM, undetected via DSC.
Fig. 5. Optical photomicrograph showing (a) the SmA fan-shaped texture of compound 4e (R = C16H33) at 118.6 °C and (b) the SmC broken fan-shaped texture at 117.5 °C
upon cooling.
Table 4
Data for layer spacing vs. R-alkyl chain length for
selected homologues.
R
d (Å)
C
C
C
12H25
14H29
16H33
55.7
57.3
58.2
transition temperatures against the number of carbon atoms of the
terminal alkyl chain (Fig. 7), a smooth increment of the isotropic–
mesophase transition temperature is observed throughout the ser-
ies from the member with octyl chain (R = C8H17) until the ana-
logue with tetradecyl moiety (R = C14H29) before the transition
temperature begin to descend upon further elongation of alkyl
chain. Members having medium chain length are found to exhibit
larger smectic phase range. For example, 4d shows the widest
smectic mesophase region (40 °C) followed by 4c and 4e, which
are 26.3 °C and 23.5 °C, respectively. This phenomenon can be as-
cribed to the influence of terminal polarity group which plays a
vital role in the mesophase formation as the long alkyl chain is
capable of stabilizing the molecular orientation through the acting
forces of molecular induction and polarization. With the extension
of terminal chain (from R = C12H25AC16H33), the molecular cooper-
ative packing is enhanced and thus resulting in an increment of the
structural anisotropy [17]. As seen in Fig. 7, the longest chain
homologue has the smallest mesophase range. This implies that
as the alkyl chain length become too long, the rigidity of the molec-
ular axis is furthered weakened and thus the linearity of the mol-
ecule affected. Consequently, the molecules do not fit readily into
the layered molecular arrangement of the smectic phase due to
Fig. 6. DSC thermograms of compound 4e (R = C16H33) with heating and cooling
cycles at the rate of 2 °C. The SmA–SmC phase transition appears to be unresolved
in the thermograms.
the more significant bending conformation. As a result, the phase
stability is reduced in 4f. Overall, the smectic phase stability is en-
hanced in between the decyl to the hexadecyl derivatives while the
decrease of isotropic–mesophase transition temperature for the
member with the longest terminal chain causes the SmA phase
to become insignificant. This trend is also similar with the observa-
tion reported for a series of cyclic trimers derived from benzene-
1,3,5-tricarboxylic acid [11].
Apart from that, the enthalpy changes associated with the
isotropic–smectic of 4a–4f upon cooling are found to be in the
range of 8.6–16.6 kJ molꢀ1. The enthalpy values tend to ascend

368
Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
Fig. 7. Plot of phase transition temperature (°C) against the number of carbon in the
alkyl chain (R) of compounds 4a–4f upon cooling.
with increasing chain length as seen from 4a to 4d and this phe-
nomenon would be expected given the increased conformational
component arising from the longer alkyl chains. It is noteworthy
to mention that the clearing enthalpies for 4a–4f are much higher
than those of 3a–3f implying the higher orientational and transla-
tional ordering within the trimers. In addition, the transitional en-
tropy changes associated with the I-SmA transition of these star-
shaped mesogens 4a–4e expressed in dimensionless quantity
Fig. 8. The low angle X-ray diffraction pattern for the SmA phase for compound 4d
(R = C14H29) at 120 °C during the cooling process (2-D image). The two sharp signals
indicate a lamellar arrangement of molecules.
phase, tilt must be very small as the SmA–SmC phase transition
is not visible in layer thickness temperature evolution for homo-
logues 4c and 4e and hardly visible for 4d. The result of XRD mea-
surement for 4d is depicted in Fig. 9, in which the molecular layer
spacing is plotted against temperature. Since the molecular tilt an-
gle of the star-shaped mesogen is found to be very small as inferred
from the XRD result, the absence of SmA–SmC transition in the DSC
thermograms as mentioned in the earlier section might be attrib-
uted to the small enthalpy change of the phase transition from
SmA to SmC. Usually extending the molecular tail length by
ACH2 group will gives the increase in the layer thickness of
1.25 Å. However, for the present compounds, the increase is smal-
ler by 0.625 Å and this suggests that molecules intercalate their
tails slightly between the layers as the layer distance is found to
be half of the molecular tail length. Thus, it is presumed that the
formation of intercalation is attributed to a specific interaction be-
tween the terminal non-polar alkyl groups. This could be expected
as these molecules have broad mesogenic cores, and when rotate
they do not fill space very well as the cross section of core is much
larger than cross-section of tail.
D
SSmA-I/R, have been calculated and listed in Table 3. The DSSmA-
I/R values fall in the range 2.7–4.7, which is somewhat lower than
those earlier reported star-shaped mesogens [11,29]. The reduced
values of the
DSSmA-I/R in the present series is presumably a reflec-
tion of the lower shape of anisotropy of the molecule as well as
changes in the molecular biaxiality which can be due to the addi-
tional carbonyl group (C@O) linking the terminal alkyl chain
(RACOO). This finding is not uncommon as reduced entropy values
have also been observed for other non-conventional liquid crystals
dimers containing branched terminal chains [35] and additional
disulphide link [36] or dimers/trimers in which the biaxiality of
the mesogenic groups have been increased [37–39]. As expected,
it has been reported that the entropy changes of the isotropic to
smectic transition is much larger than those observed for dimeric
mesogen [40]. This could be a direct consequence of the high molar
mass of present trimeric star-shaped compounds 4a–4f.
3.3. XRD analysis
Fig. 10 illustrates a possible intercalated arrangement adopted
by the trimeric star-shaped mesogen. In this system, the driving
force for the formation of intercalated phases can be attributed
to a specific interaction between the terminal non-polar alkyl
groups. This arrangement also explain why only smectic phase is
present instead of a discotic phase for 4a–4f because the linearly
extended mesogenic arms which attached to a benzene core via
a flexible alkyl spacer are said to be comparable to the calamitic
mesogen. As a rule, mesogenic units which arrange in a parallel
direction are prone to generate liquid crystals [42]. This kind of
star-shaped system is more favourable for the arm-to-arm interac-
tion to occur between neighbouring or within molecules compared
to the molecular stacking arrangement as seen in columnar phase.
Consequently, their ability to undergo self-assembly eventually led
to the formation of smectic phase in the star-shaped liquid crystals
4a–4f. The type of mesophase formed is solely determined by the
mesogenic Schiff base ester fragment whereby the smectic phase
should be driven by the rod-like units and the benzene core group
act only as a linking unit interconnecting the rods while the length
In an effort to investigate the mesophase and molecular
arrangement of the star-shaped mesogens more conclusively, pow-
der X-ray diffraction measurement was conducted for compounds
4a, 4c, 4d and 4e in order to provide more detail information on
the liquid crystalline phase structures.
For 4a which possesses the shortest alkyl chain among all, the
SmA mesophase range is found to be small. Hence on cooling the
formation of SmA phase is immediately accompanied by the crys-
tal phase and thus, the layer thickness in SmA phase for 4a could
not be distinguished and evaluated. For other homologues possess-
ing longer alkyl group at the terminal, X-ray measurement for
these selected compounds 4c, 4d, and 4e show typical patterns
for smectic phase with Bragg reflection at small angle coming from
layered structure and diffused signal at high angle corresponding
to 4.5 Å indicating a liquid-like order within the smectic layer.
Hence, the presence of two sharp reflections at 28.8 Å and 57.3 Å
(small angle) in a ratio of 1:2 indicates unambiguously the pres-
ence of layer structure [41]. A representative X-ray pattern ob-
tained for an oriented sample 4d is shown in Fig. 8. In SmC

Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
369
units made it possible to observe the effects of structural changes
on the star-shaped mesomorphic system which has not been stud-
ied previously.
3.4. Comparison studies between the present star-shaped compounds
4a–4f and previously reported star-shaped mesogens 5a–5f
In order to study the influence of different terminal functional
group on the liquid crystalline behaviour, comparison is made be-
tween the properties of 4a–4f and those of our earlier reported
analogous star-shaped mesogens [29]. The star-shaped mesogens,
with even-parity terminal chain ranging from octyl to octadecyl,
are labeled as 5a–5f and the general molecular structure is de-
picted in Fig. 11. The difference between the two series lies on
the functional group linking the terminal tails. The terminal alkyl
chains in 5a–5f are connected to the aromatic system via ether
bond, whereas in the present paper it is an ester bond.
From our findings, both 4a–4f and 5a–5f exhibit liquid crystal-
line properties and are found to be solely smectogenic. One of the
distinctive differences that can be noticed between the two series
of symmetrical star-shaped mesogens is that 5a–5e exhibit enan-
tiotropic smectic phases, while 4a–4f do not. For instance, 5d
shows the Schlieran texture of SmC phase on both heating and
cooling whereas 4d only show smectic phase upon cooling cycle.
However, it is worthwhile to mention that both SmA and SmC
phases are found in 4a–4f whereas for 5a–5f, they exhibit only
SmC phase.
The smectogenic properties of 4a–4f indicate that the molecular
breadth and shape anisotropy play a vital role in governing the li-
quid crystalline behaviour of the star-shaped mesogens. This point
is further supported by the fact that the clearing points and ther-
mal stability of 4a–4f are significantly lower in comparison to
those of 5a–5f. For example, the clearing temperature for 4b is
121.6 °C, while the clearing temperature for 5b with similar decyl
terminal chain is 134.8 °C, which is 13.2 °C higher than that re-
corded for 4b. Besides, the isotropic–smectic transition tempera-
Fig. 9. Dependence of molecular layer thickness on temperature for compound 4d.
of the terminal chain plays a crucial role in determining the meso-
phase stability. The present star-shaped mesogens in which the
benzene core and peripheral Schiff base ester are linked by a flex-
ible propylene spacer favours a much easier conformational
change. As a result, the rod-like fragment can easily align parallel
to form a smectic layered structure. This is in agreement with
the observation reported for such star-shaped homologues series
whereby a parallel arrangement of the rod-like sub-units is most
preferable for liquid crystalline behaviour [8,11,29,43]. Through
self-assembly, the star-shaped mesogens is capable of folding an
anisotropic shape to give rise to the appearance of calamitic liquid
crystalline phase (nematic and smectic) instead of discotic phase
because the alkyl spacer linked between the linearly aromatic
peripherals and the benzene core is flexible enough to allow the
peripherals to rotate freely. Therefore, variation of the peripheral
Fig. 10. Schematic drawing of the proposed intercalated molecular arrangement in smectic phase of the star-shaped compound 4d.

370
Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
Fig. 11. Molecular structure of the symmetrical star-shaped mesogens 5a–5f derived from three rod-shaped (p-alkyloxybenzylidene) anilines with different length of even-
parity terminal alkyl chain (R) ranging from 8 to 10 attached to a disc-shaped benzene ring via a propylene spacer and ether linkages.
Table 5
1H NMR chemical shifts (d/ppm) of the final star-shaped compounds 4a–4f.
Compound
Chemical shift (d/ppm)
H2, H20, H200, H2000
H1
H3
H4
H7/H11
H8/H10
H12
H14/H18
4a
4b
4c
4d
4e
4f
0.90 (t)
0.88 (t)
0.88 (t)
0.88 (t)
0.89 (t)
0.89 (t)
1.30–1.49 (m)
1.27–1.48 (m)
1.26–1.48 (m)
1.26–1.48 (m)
1.27–1.50 (m)
1.25–1.48 (m)
1.74–1.80 (m)
1.75–1.81 (m)
1.75–1.81 (m)
1.76–1.81 (m)
1.76–1.81 (m)
1.74–1.80 (m)
4.33 (t)
4.33 (t)
4.33 (t)
4.33 (t)
4.34 (t)
4.33 (t)
8.13 (d)
8.11 (d)
8.11 (d)
8.11 (d)
8.13 (d)
8.14 (d)
7.96 (d)
7.94 (d)
7.94 (d)
7.94 (d)
7.96 (d)
7.98 (d)
8.54 (s)
8.52 (s)
8.51 (s)
8.52 (s)
8.55 (s)
8.55 (s)
7.28 (d)
7.25 (d)
7.25 (d)
7.25 (d)
7.28 (d)
7.27 (d)
H15/H17
H19
H20
H21
H23
4a
4b
4c
4d
4e
4f
6.96 (d)
6.94 (d)
6.94 (d)
6.94 (d)
6.96 (d)
6.95 (d)
4.17 (t)
4.16 (t)
4.16 (t)
4.17 (t)
4.17 (t)
4.17 (t)
2.23–2.27 (m)
2.23–2.28 (m)
2.23–2.28 (m)
2.23–2.28 (m)
2.23–2.28 (m)
2.22–2.28 (m)
4.13 (t)
4.13 (t)
4.13 (t)
4.13 (t)
4.13 (t)
4.13 (t)
6.15 (s)
6.13 (s)
6.13 (s)
6.13 (s)
6.15 (s)
6.13 (s)
TMS as internal standard.
s, Singlet; d, doublet; t, triplet; m, multiplets.
tures are also higher for 5a–5f than for the title compounds. This
phenomenon can be substantiated by the molecular structure of
4a–4f. As seen in the title compounds, the larger size of ester group
due to the presence of additional carbonyl (C@O) unit as compared
to the ether group which possess only oxygen atom reduces the
shape anisotropy of 4a–4f to an extent that the resulting effect is
observed in the thermal properties of these compounds. The ester
group is presumably responsible for the increase of molecular
breadth as the ester linkage may tend to lengthen the peripheral
units due to its larger size, which consequently results in lower
clearing temperature.
the stretching frequency of CAO (ester) while another absorption
occurring at 1250 cmꢀ1 can be attributed to the stretching of
CAO (ether).
In order to elucidate the molecular structure of the star-shaped
compound, one of the representative homologue, 4b was analyzed
with both one-dimensional and two-dimensional NMR spectros-
copy. The 1H and 13C NMR data for 4a–4f are tabulated in
Table 5 and Table 6, respectively. The molecular structure of 4b
along with its complete atomic numbering scheme is depicted in
Fig. 12. Thus, the following NMR discussion is solely based on
the representative compound 4b.
From the 1H NMR spectrum of 4b (Fig. 13), the singlet observed
at downfield d = 8.52 ppm is attributed to the resonance by
the azomethine proton H12 and the down-field shift of the azome-
3.5. Spectroscopic characterization
Since all the homologues exhibit similar absorption frequency,
thus, FT-IR discussion based on a representative compound 4b is
described herein. The absorption bands assignable to the stretch-
ing frequency of aliphatic CAH from the terminal decyl chain and
spacer can be found within the frequency range of 2853–
2956 cmꢀ1. A strong intensity band due to the presence of carbonyl
C@O stretching (ester) is recorded at 1713 cmꢀ1. The stretching of
C@N (azomethine) can be assigned to the medium intensity band
appears at 1624 cmꢀ1. Band appearing at 1284 cmꢀ1 represent
thine proton absorption is due to the circulation of p electrons in
the double bond [44]. The aromatic protons H15/H17, H14/H18,
H8/H10 and H7/H11 give rise to four sets of doublets at the low-
field region at d = 6.94, 7.25, 7.94 and 8.11 ppm, respectively. An-
other singlet located at d = 6.13 ppm is assigned to the aromatic
proton H23 from the benzene core group. Signal attributable to
H4 is observed as a triplet at d = 4.33 ppm. Two closely positioned
triplets at d = 4.13 and 4.16 ppm are assignable to H19 and H21,
which is the oxymethylene protons. H20 appears as a multiplet

Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
371
Table 6
13C NMR chemical shifts (d/ppm) of the final star-shaped compounds 4a-4f.
Compound
Chemical shift (d/ppm)
C2000
C1
C2
C20
C200, C3 and C20
C4
C5
C6
C7/C11
C8/C10
4a
4b
4c
4d
4e
4f
14.14
14.11
14.11
14.14
14.11
14.10
22.71
22.68
22.69
22.71
22.69
22.68
31.94
31.89
31.92
31.94
32.04
31.92
26.06
26.04
26.05
26.05
26.05
26.04
28.70–29.55
28.72–29.54
28.72–29.65
28.72–29.71
28.72–29.78
28.74–29.85
65.45
65.43
65.42
65.45
65.42
65.41
166.23
166.23
166.22
166.25
166.23
166.21
132.40
132.41
132.42
132.39
132.39
132.39
129.91
129.91
129.91
129.92
129.90
129.90
128.35
128.36
128.36
128.37
128.34
128.34
C9
C12
C13
C14/C18
C15/C17
C16
C19
C21
C22
C23
4a
4b
4c
4d
4e
4f
140.26
140.21
140.22
140.19
140.26
140.27
156.77
156.88
156.85
156.91
156.72
156.70
144.29
144.39
144.40
144.35
144.23
144.24
122.41
122.42
122.41
122.43
122.40
122.40
115.08
115.08
115.09
115.06
115.08
115.07
158.21
158.04
158.05
158.03
158.17
158.27
64.69
64.73
64.74
64.69
64.72
64.69
64.50
64.52
64.53
64.49
64.53
64.50
160.89
160.85
160.79
160.76
160.74
160.84
94.14
94.21
94.23
94.14
93.99
93.99
TMS as internal standard.
Fig. 12. Molecular structure of compound 4b along with its complete atomic numbering scheme. Only one Schiff base ester with terminal decyl chain is shown in the
chemical structure while X represents the other two Schiff base ester moieties.
Fig. 13. 1H NMR spectrum of compound 4b in CDCl3.
at d = 2.23–2.28 ppm while H3 also gives rise to a multiplet at fur-
ther high field d = 1.75–1.81 ppm. As for the terminal aliphatic
methylene protons H2, H20, H200 and H2000, they resonate in the
chemical shift of d = 1.27–1.48 ppm due to the similar bonding nat-
ure and environment of the proton. Hence, strong intensity of mul-
tiplet can be observed in the spectrum. For the triplet found at
d = 0.88 ppm, it is assignable to the terminal methyl proton H1.
The chemical shifts observed in the 13C NMR spectra of these
homologues are in consistent with those detected for 4b. The azo-
methine carbon C12 absorbs at d = 156.88 ppm. Subsequently, four
strong peaks attributed to the protonated aromatic carbons C7/
C11, C8/C10, C14/C18 and C15/C17 of 4b can be found at
d = 129.91, 128.36, 122.42 and 115.06 ppm, respectively. In addi-
tion, another signal detected at d = 94.21 ppm is ascribed to the
protonated aromatic carbon C23 of the benzene core group. Three
closely positioned signals assignable to C4, C19 and C21 are found
in the range of d = 64.51–65.43 ppm which is attributed to the oxy-
methylene carbons. At the high field, the aliphatic carbon of the
flexible spacer together with those of the terminal decyl chain
resonate in the range of d = 22.68–31.89 ppm. As the terminal

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Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
Fig. 14. 13C NMR spectrum of compound 4b in CDCl3.
chain length increases, the number of peaks corresponding to the
Table 7
1HA1H correlations as inferred from 2D NMR COSY
spectrum of compound 4b.
carbon atoms of the chain length also increases. Apart from the
protonated carbons, the ester carbonyl carbon C5 resonates at
the lowest field at d = 166.23 ppm. The quaternary carbons C16
and C22 which are directly bonded to the oxygen atom exhibit a
signal at d = 158.04 and 160.85 ppm, respectively. Whilst the qua-
ternary carbon C13 which bonded directly to a nitrogen atom
absorbs at d = 144.39 ppm. Others quaternary carbons such as C6
and C9 are observed at respective chemical shift of d = 132.41
and 140.21 ppm. All the quaternary carbons are assigned based
on the results as inferred from HMQC and HMBC spectra of 4b
(Fig. 14).
Atom
1HA1H COSY correlation
H1
H20
H3
H4
H7/H11
H14/H18
H19
H20
H21
H2
H2 and H200
H2000 and H4
H3
H8/H10
H15/H17
H20
H19 and H21
H20
Fig. 15. 1HA1H COSY NMR spectrum of compound 4b.

Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
373
Fig. 16. 1HA13C HMQC NMR spectrum of compound 4b.
Fig. 17. 1HA13C HMBC NMR spectrum of compound 4b.
The correlations of 1HA1H COSY measurement for the represen-
tative compound 4b are listed in Table 7. According to the 1HA1H
COSY spectrum (Fig. 15), the aromatic protons H7/H11 which res-
onates at d = 8.11 ppm as a doublet is correlated with H8/H10
while another doublet at d = 7.25 ppm assignable to H14/H18 is
couple with protons H15/H17. The azomethine proton H12 and
the aromatic proton of benzene ring H23 do not couple with any
proton, hence, both appear as a singlet. H4 which appears as a trip-
let at d = 4.33 ppm correlates with H3 as inferred from the spec-
trum. In addition, both protons H19 and H21 exhibit the same
correlation with only H20. At high field, another triplet evident
at d = 0.88 ppm is attributed to the protons H1 of the terminal
methyl group and the adjacent neighboring protons is found to
be H2.

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Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
Table 8
small enthalpy change as the tilt angle of the star-shaped mesogen
in the SmC phase is found to be very small and hardly visible as in-
ferred from the layer thickness temperature evolution of XRD anal-
ysis. However, the appearance of broken fan-shaped texture from
the POM has further ascertained the presence of SmC phase in
the titled compounds. Besides, the XRD study also confirms that
only the smectic phase is present upon cooling cycle. In addition,
a prerequisite for the observation of an intercalated smectic phase
in star-shaped liquid crystals appears to be the existence of a spe-
cific anisotropic interaction of the linearly peripheral units with
neighboring molecule, hence produces an orientation which is con-
ducive for molecular packing within the smectic layers.
1HA13C correlations as inferred from two-dimensional HMQC and HMBC experiments
for compound 4b.
Compound
Atom
HMQC
1J
HMBC
2J
3J
H1
C1
C2
C20
–
–
–
–
C200
H2
C2
C20
C200
C20
C3
H20
C20
4b
H200
C200
H2000
C2000
C2000
H3
C3
C2000, C5
C5, C9
C6, C12
C8/C10, C13
C16
C13
C16, C21
–
H4
C4
C3
H7/H11
H8/H10
H12
H14/H18
H15/H17
H19
H20
H21
H23
C7/C11
C8/C10
C12
C14/C18
C15/C17
C19
C20
C21
C23
C8/C10
C7/C11
C9
C13
C16
C20
C19, C21
C20
Acknowledgements
The author, Prof. G.-Y. Yeap gratefully acknowledges the fund-
ing of this research under the Fundamental Research Grant Scheme
(No. 203/PKIMIA/6711265). Y.-H. Ooi also wishes to thank the
Ministry of Higher Education of Malaysia (MOHE) for the scholar-
ship assistantship under the MyBrain15 scheme. Authors also
thank Ms Anna Zep and her supervisor Prof. E. Gorecka from War-
saw University for the powder X-ray diffraction measurement.
C19, C22
–
C22
According to the HMQC spectrum (Fig. 16), the peak at
d = 156.88 ppm can be assigned to C12, which is the Schiff base car-
bon, owing to its correlation with H12 at d = 8.52 ppm. At
d = 65.43 ppm, this particular carbon signal is found to correlate
with H4, thus it is identified as the resonance by C4. The two clo-
sely positioned signals at d = 64.52 and 64.73 ppm can be assigned
to C21 and C19 based on their correlation with H21 and H19 which
appear as a triplet at d = 4.13 ppm and 4.16 ppm. Whilst for the
methylene carbons C20 and C3, their exact carbon signal could
not be conclusively allocated due to unresolved overlapping peaks
of other carbon signals. However, they are found to appear within
d = 28.72–29.54 ppm. At the high field, C1 of the terminal decyl
chain is detected at d = 14.11 ppm because of its correlation with
H1.
In contrary to the protonated aromatic carbons, the quaternary
carbons are assigned based on the long-range coupling between
the carbons and other protons which are located two to three
bonds away. As seen in the HMBC spectrum (Fig. 17), carbon C5
of the ester group at d = 166.23 ppm correlates through 3J with
H4 and H7/H11. The long-range HMBC cross-peak between the
quaternary carbon C9 and H12 through 2J and H7/H11 through 3J
confirm that the peak at d = 140.21 ppm is assigned to C9. The cor-
relations of C13 positioned at d = 144.39 ppm show cross-peak
with H12 and H15/H17 through 3J, thus confirm the correct assign-
ment of C13. C22 at d = 160.85 ppm is assigned based on its corre-
lation with H23 and H21 while C16 is associated with the peak at
d = 158.04 ppm due to its correlation with H19, H14/H18 and H15/
H17. The overall 1HA13C HMQC and HMBC correlations for 4b are
collated in Table 8.
References
[1] D. Demus, Liq. Cryst. 5 (1989) 75–110.
[2] C. Tschierske, Annu. Rep. Prog. Chem. Sect. C: Phys. Chem. 97 (2001) 191–267.
[3] C. Tschierske, J. Mater. Chem. 8 (1998) 1485–1508.
[4] S. Gemming, I. Popov, M. Lehmann, Philos. Mag. Lett. 87 (2007) 881–891.
[5] M. Lehmann, R.I. Gearba, D.A. Ivanov, M.H.J. Koch, Mol. Cryst. Liq. Cryst. 411
(2004). 397/[1439]–406/[1448].
[6] H. Meier, M. Lehmann, H.C. Holst, D. Schwöppe, Tetrahedron 60 (2004) 6881–
6888.
[7] D.S. Yao, B.Y. Zhang, W.W. Zhang, M. Tian, J. Mol. Struct. 881 (2008) 83–89.
[8] D. Goldmann, D. Janietz, C. Schmidt, J.H. Wendorff, Liq. Cryst. 25 (1998) 711–
719.
[9] K.C. Majumdar, N. De, B. Roy, A. Bhaumik, Liq. Cryst. 37 (2010) 1459–1464.
[10] G.Y. Yeap, Y.H. Ooi, K. Kenji, M.M. Ito, Phios. Trans. 86 (2013) 365–379.
[11] G.S. Attard, A.G. Douglass, C.T. Imrie, L. Taylor, Liq. Cryst. 11 (1992) 779–784.
[12] M. Lehmann, Top. Curr. Chem. 318 (2012) 193–224.
[13] X.H. Cheng, S. Diele, C. Tschierske, Angew. Chem. Int. Ed. 39 (2000) 592–595.
[14] L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R.H. Friend, J.D.
MacKenzie, Science 293 (2001) 1119–1122.
[15] A. Miniewicz, A. Gniewek, J. Parka, Opt. Mater. 21 (2003) 605–610.
[16] A.M. Van de Craats, N. Stutzmann, O. Bunk, M.M. Nielsen, M. Watson, K.
Mullen, H.D. Chanzy, H. Sirringhaus, R.H. Friend, Adv. Mater. 15 (2003) 495–
499.
[17] B.Y. Zhang, D.S. Yao, F.B. Meng, Y.H. Li, J. Mol. Struct. 741 (2005) 135–140.
[18] D.S. Yao, B.Y. Zhang, Y.H. Li, W.Q. Xiao, Tetrahedron Lett. 45 (2004) 8953–8956.
[19] T. Pfeuffer, D. Hanft, P. Strohriegl, Liq. Cryst. 29 (2002) 1555–1564.
[20] H.P. Chen, D. Katsis, J.C. Mastrangelo, S.H. Chen, S.D. Jacobs, P.J. Hood, Adv.
Mater. 12 (2000) 1283–1286.
[21] D.E. Feldman, Int. J. Mol. Phys. B 15 (2001) 2945–2976.
[22] P. Vande witte, J. Lub, Liq. Cryst. 26 (1999) 1039–1046.
[23] M. Lehmann, M. Jahr, B. Donnio, R. Graf, S. Gemming, I. Popov, J. Chem. Eur. 14
(2008) 3562–3576.
[24] S.J. Lee, M.K. You, S.W. Lee, J. Lee, J.H. Lee, J.Y. Jho, Liq. Cryst. 38 (2011) 1289–
1299.
[25] X.Z. He, L. Han, F.B. Meng, M. Tian, B.Y. Zhang, Liq. Cryst. 39 (2012) 779–787.
[26] A. El Mansoury, R.J. Bushby, N. Karodia, Liq. Cryst. 39 (2012) 1222–1230.
[27] J.H. Lee, H.J. Lee, S.J. Lee, J. Jang, S.H. Yoo, J.Y. Jho, Liq. Cryst. 40 (2013) 112–119.
[28] S. Kumar, M. Manickam, Liq. Cryst. 26 (1999) 939–941.
[29] Y.H. Ooi, G.Y. Yeap, C.C. Han, H.C. Lin, K. Kubo, M.M. Ito, Liq. Cryst. 40 (2013)
516–527.
4. Conclusions
A series of symmetrical three-armed star-shaped liquid crystals
based on phloroglucinol has been prepared and studied. All the six
homologues exhibited predominantly monotropic smectic behav-
iour. The study revealed that the length of terminal alkyl chain
gives a significant effect upon the mesomorphic properties where-
by the mid-chain derivatives (R = C12H25, C14H29, C16H33) exhibit
greater phase stability. Besides, the isotropic–smectic enthalpy
and entropy changes of the homologues are considerably lower
than those of the previously reported trimeric star-shaped liquid
crystals. It is worthwhile to mention that the absence of SmA–
SmC transition in the DSC thermograms can be attributed to the
[30] C. Klein, E. Baranoff, M.K. Nazeeruddin, M. Gratzel, Tetrahedron Lett. 51 (2010)
6161–6165.
[31] D.W. Yoon, S.D. Jeong, M.Y. Song, C.H. Lee, Supramol. Chem. 19 (2007) 265–
270.
[32] I. Dierking, Textures of Liquid Crystals, Wiley-VCH, Weinheim, 2003.
[33] A.G. Cook, J.L. Wardell, C.T. Imrie, Chem. Phys. Lipids 164 (2011) 118–124.
[34] D. Tsuji, Y. Takanishi, J. Tamamoto, A. Yoshizawa, J. Phys. Chem. C 116 (2012)
8678–8687.
[35] G.Y. Yeap, T.C. Hng, S.Y. Yeap, E. Gorecka, M.M. Ito, K. Ueno, M. Okamoto,
W.A.K. Mahmood, C.T. Imrie, Liq. Cryst. 36 (2009) 1431–1441.
[36] H.C. Lee, Z.B. Lu, P.A. Henderson, M.F. Achard, W.A.K. Mahmood, G.Y. Yeap, C.T.
Imrie, Liq. Cryst. 39 (2012) 259–268.
[37] T. Donaldson, H. Staesche, Z.B. Lu, P.A. Henderson, M.F. Achard, C.T. Imrie, Liq.
Cryst. 37 (2010) 1097–1110.

Y.-H. Ooi et al. / Journal of Molecular Structure 1051 (2013) 361–375
375
[38] T.N. Chan, Z.B. Lu, W.S. Yam, G.Y. Yeap, C.T. Imrie, Liq. Cryst. 39 (2012) 393–
402.
[43] M.R. Lutfor, M. Yusoff, C. Tshierske, K. Pelz, U. Baumeister, S. Sidik, Nematic
and smectic mesophase formation by novel triphenylene-azobenzene
a
[39] T. Donaldson, P.A. Henderson, M.F. Achard, C.T. Imrie, Liq. Cryst. 38 (2011)
1331–1339.
hybride molecule. Electronic liquid crystal communication. <http://
www.e.lc.org/docs/2005_06_26_21_47_19>, 2005.
[40] A.T.M. Marcelis, A. Koudijs, E.J.R. Sudhölter, Liq. Cryst. 27 (2000) 1515–1523.
[41] A.A. Salisu, Int. J. Pure Appl. Sci. 2 (2008) 119–124.
[42] M. Marcos, A. Omenat, J.L. Serrano, C. R. Chim. 6 (2003) 947–957.
[44] P. Berdague, J. Courtieu, P.M. Maitlis, J. Chem. Soc. Chem. Commun. 11 (1994)
1313–1314.