Symmetrical trimeric star-shaped mesogens based on 1,3,5-trisubstituted benzene incorporating Schiff base and azobenzene fragments as the peripheral units: Synthesis and mesomorphic properties

Article abstract of DOI:10.1007/s12039-014-0612-1

Two series of symmetrical three-armed star-shaped mesogens based on 1,3,5-trihydroxybenzene as a core unit, interconnecting three Schiff base or azobenzene moieties via oxymethylene spacers have been synthesized and characterized by spectroscopic techniqu

Full text of DOI:10.1007/s12039-014-0612-1

c
J. Chem. Sci. Vol. 126, No. 3, May 2014, pp. 847–859. ꢀ Indian Academy of Sciences.
Symmetrical trimeric star-shaped mesogens based
on 1,3,5-trisubstituted benzene incorporating Schiff base
and azobenzene fragments as the peripheral units: Synthesis
and mesomorphic properties
GUAN-YEOW YEAPa,∗, YEW-HONG OOIa, NOZOMI UCHIDAb and MASATO M ITOb
aLiquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia,
11800 Minden, Penang, Malaysia
bDepartment of Environmental Engineering for Symbiosis, Faculty of Engineering, Soka University,
Hachioji, Tokyo 192-8577, Japan
e-mail: gyyeap@usm.my
MS received 27 July 2013; revised 2 December 2013; accepted 30 December 2013
Abstract. Two series of symmetrical three-armed star-shaped mesogens based on 1,3,5-trihydroxybenzene
as a core unit, interconnecting three Schiff base or azobenzene moieties via oxymethylene spacers have been
synthesized and characterized by spectroscopic techniques. Every member in these series possesses either chlo-
rine (Cl) or bromine (Br) terminal atom, with different alkyl spacer length (CnH2n whereby n ranging from 3
to 6). Their thermal stability and mesomorphic properties are investigated by employing DSC and POM. The
dependence of phase transition in relation to the alkyl spacer length is shown by both series. These star-shaped
mesogens exhibit only nematic and smectic phases. The difference between the two series lies on the structure
of linking group in the peripheral units (-CH=N- for series PSB-X-n and -N=N- for series PAZ-X-n). There-
fore, a comparison study of the mesomorphic properties between these two series of star-shaped mesogens is
discussed whereby the azobenzene-based mesogens are thermally more stable than the Schiff base counterpart.
In addition, soft crystalline phase is observed for the azobenzene-based star-shaped mesogens possessing hexyl
alkyl spacer.
Keywords. Star-shaped liquid crystals; symmetrical; terminal substituent; Schiff base and azobenzene
moieties; comparative study.
1. Introduction
components (tails and spacers) and several anisomet-
ric (mesogenic) segments which are chemically incom-
patible with each other.1,4,5 On account of their aston-
ishing self-assembly into complex soft materials, non-
conventional LCs are of massive interest at present. To
date, there are various types of non-conventional LCs
documented in the literature (example, such as multi-
arm mesogens,6–8 oligomers,9–13 bent-core or V-shaped
mesogens,14–18 metallomesogens19–22 and hydrogen-
bonded mesogens).23–25 Thus, the interesting properties
of these materials have prompted researchers to further
study and investigate novel non-conventional materi-
als which can eventually lead to new findings on novel
mesophase morphologies and deeper insights in the
driving force of their self-organization.
In the last two decades, the development of non-con-
ventional liquid crystals has received overwhelming
interest from researchers due to its great potential to
serve as novel medium in electro-optical applications,
sensing applications and electron/ion/molecular trans-
porting system.1,2 Generally, the prototype of con-
ventional liquid crystal molecule consists of a rod-
like rigid core or a relatively flat disc-shaped core
substituted by terminal flexible chain and small late-
ral substituent. In contrast, non-conventional LCs are
molecules with an anisotropic shape that deviates from
the classical rod- and disc-shaped molecules.3 They are
made up of complex self-organising molecules com-
prising discrete molecular fragment namely flexible
Among the various types of non-conventional LCs,
star-shaped mesogens is considered as one of the most
popular studied materials in recent year. The sim-
plest star-shaped mesogen is termed as Hekates,26,27
which consists of only three identical mesogenic/
non-mesogenic groups symmetrically attached to a
∗For correspondence
847

848
Guan-Yeow Yeap et al.
disc-shaped core unit, hence representing yet another
important class of low molecular mass liquid crystals.
Star-shaped LCs are attractive because of their ability
to exhibit unique optical behaviour and a broad range
of mesophases such as soft crystalline phases, nematic,
smectic, cholesteric, as well as columnar phases.28–33
The driving force for the type of mesophase formed is
presumably dominated by the peripheral units. Never-
theless, the length of spacer also plays a crucial role
in the mesophase stability. For instance, star-shaped
mesogens possessing short spacer hardly show any
mesophase as the short chain may prevent the periphe-
ral units from the appropriate arrangement in the liq-
uid crystalline phase. However, when the flexible spacer
length is long enough, the peripheral units are capa-
ble of folding an anisotropic shape to give rise to
the appearance of liquid crystalline state. A related
example can be shown by the phloroglucinol-based
star-shaped mesogens whereby homologues with butyl
spacer or higher (n ≥ 4) exhibit cholesteric phase and
the mesophase interval of the cholesteric phase widens
significantly.28 Besides, star-shaped LCs comprised of
a benzene core unit with three extended peripheral unit
connected via flexible spacers can form low tempera-
ture glassy state.29 Hence, they own great potential to be
utilized as soft functional and glass-forming liquid crys-
tals materials. This type of low molar mass compound is
most preferred over polymeric glass-forming LCs due
to their lower viscosity which can eventually provide
fast and better orientation.34 In addition, glass-forming
LCs also possess many special optical, mechanical and
thermal stable properties.29 Thus, star-shaped LCs are
especially important not only for the theoretical under-
standing of liquid crystalline phenomenon, but also
attractive in the view of preparing novel functional
materials owing to their unique physical and optical
properties.
Our present interest focuses on the investigation of
phloroglucinol-based star-shaped mesogens incorporat-
ing Schiff base and azobenzene moieties as the periphe-
ral units. The intermediary Schiff base or azobenzene
compounds possessing chloro (Cl) or bromo (Br) as the
terminal substituent with varying spacer length (n) are
joined to the 1,3,5-position of phloroglucinol to yield
the symmetrical three-armed star-shaped molecules.
The acronyms used to refer the two series of star-shaped
mesogens in this article are PSB-X-n and PAZ-X-n, in
which PSB-X-n is referred to the star-shaped mesogen
incorporating Schiff base moieties while PAZ-X-n is
referred to the mesogen comprising azobenzene frag-
ments as the peripheral units. The symbol X denotes the
terminal chloro or bromo substituent, while n denotes
the number of methylene units linking the peripheral
Figure 1. Molecular structure of the symmetrical trimeric
star-shaped mesogens (a) PSB-X-n and (b) PAZ-X-n (where
X = Cl, Br and CnH2n where n = 3,4,5,6).
unit and the benzene core. The molecular structure of
present star-shaped mesogens are depicted in figure 1.
Since the molecular structure of the star-shaped com-
pounds and their liquid crystalline properties are closely
related, therefore, the structure-mesomorphic property
relationship in terms of the linking group, terminal sub-
stituent and spacer length towards the mesomorphic
properties of the present star-shaped liquid crystals are
described.
2. Experimental
2.1 Chemicals and reagents
4-Chloroaniline, 4-bromoaniline, phenol, 1,3-dibro-
mopropane, 1,4-dibromobutane, 1,5-dibromopentane
and 1,6-dibromohexane were obtained from Merck
(Germany). Phloroglucinol anhydrous and 4-hydro-
xybenzaldehyde were purchased from Acros (Bel-
gium). Potassium carbonate anhydrous and sodium

Symmetrical trimeric star-shaped mesogens based on 1,3,5-trisubstituted benzene
849
nitrate were purchased from R&M Chemicals (UK) were measured through differential scanning calorime-
while potassium iodide was obtained from Fisher Sci- try (DSC) using a Seiko DSC120 Model 5500 differ-
entific (USA). All chemicals and solvents were used ential scanning calorimeter. The scanning profile in all
directly without any purification.
cases was heated and cooled at the rate of 5◦C. Tex-
ture characterization was performed using a Carl Zeiss
Axioskop 40 Polarizing Optical Microscope equipped
with a Linkam LTS350 hot stage. The samples were
prepared as a thin film sandwiched between a glass slide
and a cover.
2.2 Methods and characterization
The molecular structure of all the star-shaped com-
pounds and their intermediates were confirmed and sub-
stantiated by 1H and 13C-NMR spectroscopy along with
two-dimensional NMR experiments (1H-1H COSY,
2.3 Synthesis
1
1H-13C HMQC and H-13C HMBC), using a Bruker
Avance 500 MHz spectrometer. Deuterated chloroform The synthetic pathway towards the formation of the
(CDCl3) was used as the solvent with tetramethylsi- star-shaped mesogens is outlined in scheme 1. The
lane (TMS) as internal standard. Elemental analysis intermediates SB-X-n were prepared by using the pro-
of the final products in terms of carbon, hydrogen cedure as reported by Yeap et al.,35 while AZ-X-n were
and nitrogen were measured using Perkin Elmer 2400 prepared according to the method described by García
LS series CHSN/O analyzer. While the FT-IR spec- et al.36 The preparation of the star-shaped compounds,
tra were recorded using Perkin Elmer 2000-FTIR spec- PSB-X-n and PAZ-X-n will be described in the fol-
trophotometer in a frequency range of 4000–400 cm−1 lowing section. Overall, the representative codes for
in which the samples were embedded in KBr discs. PSB-X-n and PAZ-X-n with respective terminal atom,
The thermal behaviour of the star-shaped mesogens linking group and spacer were summarized in table 1.
Scheme 1. Synthetic route towards the preparation of three-armed star-shaped mesogens PSB-X-n and PAZ-X-n.

850
Guan-Yeow Yeap et al.
Table 1. The linking group (Z), terminal substituent (X)
and the spacer length (n) of the star-shaped mesogen,
PSB-X-n and PAZ-X-n.
2.6 Synthesis of 1,3,5-tris{4-[(4-substituted-
phenylimino)methyl]phenoxy}-n-alkyloxy benzene,
PSB-X-n (X = Cl, Br ; n = 3,4,5,6)
Terminal
Compound Linking group, Z substituent, X Spacer, n
Phloroglucinol anhydrous (0.31 g, 2.5 mmol) was first
dissolved in 10 mL of acetone. Potassium carbon-
ate anhydrous (7.00 g, 50 mmol) was added to the
solution and allowed to stir at reflux for 1 h. Excess
amount of compound SB-X-n (10 mmol) was dissolved
in 50 mL of hot N,Nꢁ-dimethylformamide (DMF) and
was added to the reaction mixture followed by a cata-
lytic amount of potassium iodide. The reaction mix-
ture was stirred at 80◦C for 48 h (2 days). Then, it
was allowed to cool down and left to evaporate at room
temperature until the total volume of the mixture was
40 mL. Cold water was added and the resulting pre-
cipitate was filtered off, dried and purified by column
chromatography whereby dichloromethane was used as
the mobile phase. The precipitate thus obtained was
recrystallized from acetone to obtain the star-shaped
PSB-CL-3
PSB-CL-4
PSB-CL-5
PSB-CL-6
PSB-BR-3
PSB-BR-4
PSB-BR-5
PSB-BR-6
PAZ-CL-3
PAZ-CL-4
PAZ-CL-5
PAZ-CL-6
PAZ-BR-3
PAZ-BR-4
PAZ-BR-5
PAZ-BR-6
CH=N
CH=N
CH=N
CH=N
CH=N
CH=N
CH=N
CH=N
N=N
N=N
N=N
N=N
N=N
Cl
Cl
Cl
Cl
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Br
Br
Br
Br
3
4
5
6
3
4
5
6
3
4
5
6
3
4
5
6
N=N
N=N
N=N
1
compounds, PSB-X-n. The physical data, IR, H- and
13C-NMR along with elemental results were listed as
follows:
2.4 Synthesis of 4-[(4-substituted-
phenylimino)methyl]phenoxy-n-bromoalkane,
SB-X-n
PSB-CL-3: Yield: 43% White precipitate. Elemental
analysis: found, C 69.02, H 5.30, N 4.51; calculated
(C54H48N3O6Cl3), C 68.90, H 5.14, N 4.46. IR (KBr)
v/cm−1: 2946, 2870 (C-H aliphatic), 1619 (C=N), 1571,
1602 (C=C), 1250 (C-O ether). 1H-NMR (CDCl3)
δ/ppm: 2.24–2.31 (m, 6H, CH2), 4.12 (t, 6H, J = 6.0 Hz,
OCH2), 4.20 (t, 6H, J = 6.0 Hz, OCH2), 6.12 (s, 3H,
Ar), 6.97 (d, 6H, J = 9.0 Hz, Ar), 7.11 (d, 6H, J =
8.5 Hz, Ar), 7.33 (d, 6H, J = 8.5 Hz, Ar), 7.81 (d,
2H, J = 8.5 Hz, Ar), 8.33 (s 3H, CH=N). 13C-NMR
4-[(4-Chlorophenylimino)methyl]phenol or 4-[(4-bro-
mophenylimino)methyl]phenol was synthesized via
condensation reaction in methanol between 4-hydro-
xybenzaldehyde and 4-chloro- or 4-bromoaniline. The
Schiff base compounds were further reacted with va-
rious α,ω-dibromoalkanes ranging from C3H6Br2 to
C6H12Br2 through Williamson etherification to afford
the ω-brominated intermediary Schiff base side-chain,
SB-X-n.
(CDCl3) δ/ppm: 29.20 (Caliphatic), 64.39, 64.65 (Carom
-
OCH2), 94.23, 114.77, 122.19, 129.09, 129.19, 130.62
(Carom), 131.06 (Carom-Cl), 150.80 (Carom-N), 159.93
(C=N), 160.75, 161.73 (Carom-O).
PSB-CL-4: Yield: 45% White precipitate. Elemental
analysis: found, C 69.77, H 5.64, N 4.21; calculated
(C57H54N3O6Cl3), C 69.62, H 5.53, N 4.27. IR (KBr)
v/cm−1: 2947, 2874 (C-H aliphatic), 1620 (C=N), 1573,
1604 (C=C), 1250 (C-O ether). 1H-NMR (CDCl3)
δ/ppm: 1.96–2.02 (m, 12H, CH2), 3.98 (t, 6H, J =
5.7 Hz, OCH2), 4.07 (t, 6H, J = 5.7 Hz, OCH2), 6.06
(s, 3H, Ar), 6.95 (d, 6H, J = 8.7 Hz, Ar), 7.12 (d, 6H,
J = 8.7 Hz, Ar), 7.31 (d, 6H, J = 8.7 Hz, Ar), 7.82 (d,
6H, J = 8.4 Hz, Ar), 8.32 (s, 3H, CH=N). 13C-NMR
(CDCl3) δ/ppm: 26.28, 26.36 (Caliphatic), 68.07, 68.24
(Carom-OCH2), 94.31, 115.15, 122.60, 129.51, 129.62,
131.23 (Carom), 132.40 (Carom-Cl), 145.32 (Carom-N),
160.41 (C=N), 161.23, 162.44 (Carom-O).
2.5 Synthesis of 4-(4-substituted-
phenyldiazenyl)phenoxy-n-bromoalkane,
AZ-X-n
Azobenzene compounds 4-[(4-chlorophenyl)diazenyl]
phenol or 4-[(4-bromophenyl)diazenyl]phenol were
prepared through diazonium coupling between the
diazonium ion (obtained from treating the substituted-
aniline with diluted hydrochloric acid and sodium
nitrate in 0–5◦C) and phenol. The azobenzene com-
pounds were further alkylated via Williamson etherifi-
cation with various α,ω-dibromoalkanes ranging from
C3H6Br2 to C6H12Br2 in order to yield the ω-brominated
intermediary azo side-chain, AZ-X-n.

Symmetrical trimeric star-shaped mesogens based on 1,3,5-trisubstituted benzene
851
PSB-CL-5: Yield: 42% White precipitate. Elemental J = 8.5 Hz, Ar), 7.47 (d, 6H, J = 8.5 Hz, Ar), 7.81 (d,
analysis: found, C 70.44, H 5.87, N 4.05; calculated 6H, J = 8.5 Hz, Ar), 8.33 (s, 3H, CH=N). 13C-NMR
(C60H60N3O6Cl3), C 70.27, H 5.90, N 4.10. IR (KBr) (CDCl3) δ/ppm: 25.90, 25.98 (Caliphatic), 67.47, 67.67
v/cm−1: 2946, 2869 (C-H aliphatic), 1619 (C=N), 1571, (Carom-OCH2), 94.02, 114.76, 122.60, 128.95, 130.65,
1603 (C=C), 1250 (C-O ether). 1H-NMR (CDCl3) 132.14 (Carom), 118.86 (Carom-Br), 151.33 (Carom-N),
δ/ppm: 1.65–1.72 (m, 6H, CH2), 1.83–1.92 (m, 12H, 160.00 (C=N), 160.85, 161.91 (Carom-O).
CH2), 3.97 (t, 6H, J = 6.0 Hz, OCH2), 4.07 (t, 6H, J
= 6.0 Hz, OCH2), 6.10 (s, 3H, Ar), 6.90 (d, 6H, J =
8.7 Hz, Ar), 7.17 (d, 6H, J = 8.4 Hz, Ar), 7.36 (d, 6H, J
= 8.7 Hz, Ar), 7.87 (d, 6H, J = 8.4 Hz, Ar), 8.37 (s, 3H,
CH=N). 13C-NMR (CDCl3) δ/ppm: 22.72, 28.82, 28.98
(Caliphatic), 67.70, 68.18 (Carom-OCH2), 93.23, 114.76,
PSB-BR-5: Yield: 41% Beige precipitate. Elemental
analysis: found, C 62.10, H 5.31, N 3.66; calculated
(C60H60N3O6Br3), C 62.19, H 5.22, N 3.63. IR (KBr)
v/cm−1: 2941, 2868 (C-H aliphatic), 1621 (C=N), 1572,
1603 (C=C), 1251 (C-O ether). 1H-NMR (CDCl3)
δ/ppm: 1.61–1.67 (m, 6H, CH2), 1.82–1.91 (m, 12H,
CH2), 3.95 (t, 6H, J = 6.3 Hz, OCH2), 4.05 (t, 6H, J
= 6.3 Hz, OCH2), 6.08 (s, 3H, Ar), 6.96 (d, 6H, J =
8.7 Hz, Ar), 7.06 (d, 6H, J = 8.7 Hz, Ar), 7.48 (d, 6H, J
122.21, 129.12, 129.27, 131.20 (Carom), 131.99 (Carom
-
Cl), 144.92 (Carom-N), 160.08 (C=N), 160.90, 161.14
(Carom-O).
PSB-CL-6: Yield: 41% Beige precipitate. Elemental = 8.7 Hz, Ar), 7.81 (d, 6H, J = 8.7 Hz, Ar), 8.33 (s, 3H,
analysis: found, C 71.05, H 6.58, N 4.02; calculated CH=N). 13C-NMR (CDCl3) δ/ppm: 22.75, 28.91, 28.98
(C63H66N3O6Cl3), C 70.88, H 6.23, N 3.94. IR (KBr) (Caliphatic), 67.76, 67.99 (Carom-OCH2), 93.99, 114.76,
v/cm−1: 2939, 2858 (C-H aliphatic), 1620 (C=N), 1573, 122.58, 128.89, 130.63, 132.13 (Carom), 151.35 (Carom
-
1604 (C=C), 1251 (C-O ether). 1H-NMR (CDCl3) N), 118.83 (Carom-Br) 160.02 (C=N), 160.92, 161.99
δ/ppm: 1.55–1.59 (m, 12H, CH2), 1.81–1.88 (m, 12H, (Carom-O).
CH2), 3.95 (t, 6H, J = 6.3 Hz, OCH2), 4.06 (t, 6H, J
= 6.3 Hz, OCH2), 6.10 (s, 3H, Ar), 6.99 (d, 6H, J =
8.7 Hz, Ar), 7.15 (d, 6H, J = 8.7 Hz, Ar), 7.35 (d, 6H, J
= 8.7 Hz, Ar), 7.85 (d, 6H, J = 8.7 Hz, Ar), 8.36 (s, 3H,
CH=N). 13C-NMR (CDCl3) δ/ppm: 26.22, 26.29, 29.50,
29.57 (Caliphatic), 68.21, 68.46 (Carom-OCH2), 94.26,
PSB-BR-6: Yield: 38% Beige precipitate. Elemental
analysis: found, C 63.45, H 5.57, N 3.61; calculated
(C63H66N3O6Br3), C 63.01, H 5.54, N 3.50. IR (KBr)
v/cm−1: 2940, 2866 (C-H aliphatic), 1620 (C=N), 1572,
1605 (C=C), 1250 (C-O ether). 1H-NMR (CDCl3)
δ/ppm: 1.56–1.60 (m, 12H, CH2), 1.82–1.91 (m, 12H,
CH2), 3.95 (t, 6H, J = 6.3 Hz, OCH2), 4.05 (t, 6H, J
= 6.3 Hz, OCH2), 6.09 (s, 3H, Ar), 6.98 (d, 6H, J =
8.7 Hz, Ar), 7.08 (d, 6H, J = 8.7 Hz, Ar), 7.50 (d, 6H, J
115.15, 123.53, 129.58, 131.07 (Carom), 131.42 (Carom
-
Cl), 151.14 (Carom-N), 160.40 (C=N), 161.35, 162.46
(Carom-O).
PSB-BR-3: Yield: 38% Beige precipitate. Elemental = 8.7 Hz, Ar), 7.83 (d, 6H, J = 9.0 Hz, Ar), 8.36 (s, 3H,
analysis: found, C 60.52, H 4.61, N 3.95; calculated CH=N). 13C-NMR (CDCl3) δ/ppm: 29.57, 29.50, 26.29,
(C54H48N3O6Br3), C 60.35, H 4.50, N 3.91. IR (KBr) 26.22 (Caliphatic), 68.21, 68.46 (Carom-OCH2), 94.16,
v/cm−1: 2942, 2868 (C-H aliphatic), 1620 (C=N), 1571, 115.16, 123.01, 130.22, 131.09, 132.53, (Carom), 119.25
1603 (C=C), 1250 (C-O ether). 1H-NMR (CDCl3) (Carom-Br), 151.64 (Carom-N), 160.45 (C=N), 161.35,
δ/ppm: 2.23–2.31 (m, 6H, CH2), 4.12 (t, 6H, J = 6.0 Hz, 162.47 (Carom-O).
OCH2), 4.21 (t, 6H, J = 6.0 Hz, OCH2), 6.12 (s, 3H,
Ar), 6.97 (d, 6H, J = 8.7 Hz, Ar), 7.06 (d, 6H, J =
8.7 Hz, Ar), 7.48 (d, 6H, J = 8.4 Hz, Ar), 7.81 (d,
2.7 Synthesis of 1,3,5-tris{4-[(4-substituted-
phenyl)diazenyl]phenoxy}-n-alkyloxy benzene,
PAZ-X-n (X = Cl, Br ; n = 3,4,5,6)
6H, J = 8.7 Hz, Ar), 8.33 (s, 3H, CH=N). 13C-NMR
(CDCl3) δ/ppm: 29.20 (Caliphatic), 64.40, 64.66 (Carom
-
OCH2), 94.26, 114.78, 122.58, 129.10, 130.64, 132.14
(Carom), 118.04 (Carom-Br), 151.31 (Carom-N), 159.92
(C=N), 160.76, 161.75 (Carom-O).
The star-shaped mesogens PAZ-X-n were synthesized
by employing the similar procedure as mentioned in
PSB-BR-4: Yield: 43% Beige precipitate. Elemental Section 2.6.
analysis: found, C 61.40, H 4.85, N 3.80; calculated PAZ-CL-3: Yield: 37% Yellow precipitate. Elemen-
(C57H54N3O6Br3), C 61.30, H 4.87, N 3.76. IR (KBr)
v/cm−1: 2944, 2873 (C-H aliphatic), 1623 (C=N), 1572,
1603 (C=C), 1252 (C-O ether). 1H-NMR (CDCl3)
δ/ppm: 1.98–2.03 (m, 12H, CH2), 3.99 (t, 6H, J =
6.0 Hz, OCH2), 4.09 (t, 6H, J = 6.0 Hz, OCH2), 6.08
(s, 3H, Ar), 6.96 (d, 6H, J = 8.5 Hz, Ar), 7.06 (d, 6H,
tal analysis: found, C 64.96, H 4.82, N 8.91; cal-
culated (C51H45N6O6Cl3), C 64.87, H 4.80, N 8.90.
IR (KBr) v/cm−1: 2952, 2873 (C-H aliphatic), 1588,
1600 (C=C), 1473 (N=N), 1250 (C-O ether). H-NMR
(CDCl3) δ/ppm: 2.24–2.31 (m, 6H, CH2), 4.13 (t, 6H,
J = 6.0 Hz, OCH2), 4.22 (t, 6H, J = 6.0 Hz, OCH2),
1

852
Guan-Yeow Yeap et al.
6.13 (s, 3H, Ar), 7.00 (d, 6H, J = 9.0 Hz, Ar), 7.45 (C51H45N6O6Br3), C 56.84, H 4.21, N 7.80. IR (KBr)
(d, 6H, J = 9.0 Hz, Ar), 7.81 (d, 6H, J = 8.5 Hz, v/cm−1: 2942, 2870 (C-H aliphatic), 1584–1600 (C=C),
Ar), 7.88 (d, 6H, J = 9.0 Hz, Ar). 13C-NMR (CDCl3) 1475 (N=N), 1250 (C-O ether). 1H-NMR (CDCl3)
δ/ppm: 29.22 (Caliphatic), 64.40, 64.82 (Carom-OCH2), δ/ppm: 2.24–2.30 (m, 6H, CH2), 4.12 (t, 6H, J =
94.29, 114.80, 123.82, 124.87, 129.25 (Carom), 136.17 6.0 Hz, OCH2), 4.21 (t, 6H, J = 6.0 Hz, OCH2),
(Carom-Cl), 146.91, 151.14 (Carom-N), 160.76, 161.59 6.12 (s, 3H, Ar), 6.99 (d, 6H, J = 9.0 Hz, Ar), 7.60
(Carom-O).
(d, 6H, J = 8.5 Hz, Ar), 7.73 (d, 6H, J = 8.5 Hz,
PAZ-CL-4: Yield: 35% Yellow precipitate. Elemen- Ar), 7.88 (d, 6H, J = 9.0 Hz, Ar). 13C-NMR (CDCl3)
tal analysis: found, C 65.73, H 5.25, N 8.50; cal- δ/ppm: 29.12 (Caliphatic), 64.38, 64.80 (Carom-OCH2),
culated (C54H51N6O6Cl3), C 65.75, H 5.21, N 8.52. 94.25, 114.80, 124.08, 124.91, 132.24 (Carom), 124.58
IR (KBr) v/cm−1: 2951, 2872 (C-H aliphatic), 1586, (Carom-Br), 146.90, 151.32 (Carom-N), 160.76, 161.62
1
1600 (C=C), 1477 (N=N), 1250 (C-O ether). H-NMR (Carom-O).
(CDCl3) δ/ppm: 1.94–2.00 (m, 12H, CH2), 3.97 (t, 6H, PAZ-BR-4: Yield: 38% Yellow precipitate. Elemen-
J = 5.7 Hz, OCH2), 4.08 (t, 6H, J = 5.7 Hz, OCH2), tal analysis: found, C 57.94, H 4.55, N 7.59; cal-
6.05 (s, 3H, Ar), 6.97 (d, 6H, J = 9.0 Hz, Ar), 7.43 culated (C54H51N6O6Br3), C 57.92, H 4.59, N 7.51.
(d, 6H, J = 8.7 Hz, Ar), 7.80 (d, 6H, J = 8.7 Hz, IR (KBr) v/cm−1: 2948, 2872 (C-H aliphatic), 1582,
Ar), 7.88 (d, 6H, J = 9.0 Hz, Ar). 13C-NMR (CDCl3) 1599 (C=C), 1475 (N=N), 1251 (C-O ether). H-NMR
1
δ/ppm: 25.89, 26.02 (Caliphatic), 67.48, 67.84 (Carom
-
(CDCl3) δ/ppm: 1.95–2.00 (m, 12H, CH2), 3.99 (t, 6H,
OCH2), 94.05, 114.79, 123.82, 124.88, 129.25 (Carom), J = 6.0 Hz, OCH2), 4.09 (t, 6H, J = 6.0 Hz, OCH2),
136.15 (Carom-Cl), 146.83, 151.17 (Carom-N), 160.86, 6.07 (s, 3H, Ar), 6.98 (d, 6H, J = 9.0 Hz, Ar), 7.60
161.77 (Carom-O).
PAZ-CL-5: Yield: 35% Yellow precipitate. Elemen- Ar), 7.89 (d, 6H, J = 9.0 Hz, Ar). 13C-NMR (CDCl3)
tal analysis: found, C 66.70, H 5.67, N 8.21; cal- δ/ppm: 25.88, 26.02 (Caliphatic), 67.47, 67.83 (Carom
(d, 6H, J = 8.5 Hz, Ar), 7.74 (d, 6H, J = 9.0 Hz,
-
culated (C57H57N6O6Cl3), C 66.57, H 5.59, N 8.17. OCH2), 94.04, 114.79, 124.07, 124.91, 132.23 (Carom),
IR (KBr) v/cm−1: 2944, 2870 (C-H aliphatic), 1582, 124.53 (Carom-Br), 146.82, 151.52 (Carom-N), 160.85,
1
1600 (C=C), 1477 (N=N), 1250 (C-O ether). H-NMR 161.80 (Carom-O).
(CDCl3) δ/ppm: 1.61–1.67 (m, 6H, CH2), 1.81–1.90 PAZ-BR-5: Yield: 38% Yellow precipitate. Elemen-
(m, 12H, CH2), 3.94 (t, 6H, J = 6.5 Hz, OCH2), 4.04 tal analysis: found, C 59.08, H 4.98, N 7.20; cal-
(t, 6H, J = 6.5 Hz, OCH2), 6.06 (s, 3H, Ar), 6.97 culated (C57H57N6O6Br3), C 58.93, H 4.95, N 7.23.
(d, 6H, J = 8.5 Hz, Ar), 7.43 (d, 6H, J = 8.5 Hz, IR (KBr) v/cm−1: 2940, 2868 (C-H aliphatic), 1583,
1
Ar), 7.80 (d, 6H, J = 8.5 Hz, Ar), 7.87 (d, 6H, 1599 (C=C), 1475 (N=N), 1251 (C-O ether). H-NMR
J = 9.0 Hz, Ar). 13C-NMR (CDCl3) δ/ppm: 22.75, (CDCl3) δ/ppm: 1.61–1.67 (m, 6H, CH2), 1.81–1.90
28.93, 28.98 (Caliphatic), 67.75, 68.15 (Carom-OCH2), (m, 12H, CH2), 3.93 (t, 6H, J = 6.0 Hz, OCH2), 4.04
93.97, 114.76, 123.81, 124.88, 129.15 (Carom), 136.12 (t, 6H, J = 6.5 Hz, OCH2), 6.06 (s, 3H, Ar), 6.97 (d,
(Carom-Cl), 146.77, 151.16 (Carom-N), 160.91, 161.84 6H, J = 8.5 Hz, Ar), 7.59 (d, 6H, J = 8.5 Hz, Ar),
(Carom-O).
7.72 (d, 6H, J = 9.0 Hz, Ar), 7.87 (d, 6H, J = 9.0 Hz,
PAZ-CL-6: Yield: 32% Yellow precipitate. Elemen- Ar). 13C-NMR (CDCl3) δ/ppm: 22.75, 28.93, 28.98
tal analysis: found, C 67.43, H 5.84, N 7.89; cal- (Caliphatic), 67.75, 68.15 (Carom-OCH2), 93.96, 114.77,
culated (C60H63N6O6Cl3), C 67.32, H 5.93, N 7.85. 124.07, 124.91, 132.24 (Carom), 124.52 (Carom-Br),
IR (KBr) v/cm−1: 2940, 2867 (C-H aliphatic), 1583, 146.77, 151.52 (Carom-N), 160.92, 161.88 (Carom-O).
1
1600 (C=C), 1472 (N=N), 1250 (C-O ether). H-NMR PAZ-BR-6: Yield: 38% Yellow precipitate. Elemen-
(CDCl3) δ/ppm: 1.54–1.60 (m, 12H, CH2), 1.80–1.88 tal analysis: found, C 59.95, H 5.31, N 7.03; cal-
(m, 12H, CH2), 3.95 (t, 6H, J = 6.6 Hz, OCH2), 4.06 culated (C60H63N6O6Br3), C 59.86, H 5.27, N 6.98.
(t, 6H, J = 6.6 Hz, OCH2), 6.09 (s, 3H, Ar), 7.01 IR (KBr) v/cm−1: 2941, 2868 (C-H aliphatic), 1583,
1
(d, 6H, J = 9.0 Hz, Ar), 7.47 (d, 6H, J = 9.0 Hz, 1600 (C=C), 1473 (N=N), 1247 (C-O ether). H-NMR
Ar), 7.84 (d, 6H, J = 9.0 Hz, Ar), 7.91 (d, 6H, J = (CDCl3) δ/ppm: 1.51–1.58 (m, 12H, CH2), 1.79–1.86
9.0 Hz, Ar). 13C-NMR (CDCl3) δ/ppm: 25.82, 25.88, (m, 12H, CH2), 3.93 (t, 6H, J = 6.0 Hz, OCH2), 4.04
29.12,29.17 (Caliphatic), 67.84, 68.19 (Carom-OCH2), (t, 6H, J = 6.5 Hz, OCH2), 6.07 (s, 3H, Ar), 6.99
93.94, 114.78, 123.81, 124.88, 129.24 (Carom), 136.10 (d, 6H, J = 8.5 Hz, Ar), 7.61 (d, 6H, J = 8.5 Hz,
(Carom-Cl), 146.76, 151.18 (Carom-N), 160.97, 161.90 Ar), 7.75 (d, 6H, J = 8.5 Hz, Ar), 7.89 (d, 6H, J =
(Carom-O).
8.5 Hz, Ar). 13C-NMR (CDCl3) δ/ppm: 25.81, 25.87,
PAZ-BR-3: Yield: 41% Yellow precipitate. Elemental 29.10, 29.15 (Caliphatic), 67.79, 68.20 (Carom-OCH2),
analysis: found, C 56.93, H 4.37, N 7.86; calculated 93.80, 114.75, 124.05, 124.90, 132.22 (Carom), 124.49

Symmetrical trimeric star-shaped mesogens based on 1,3,5-trisubstituted benzene
853
(Carom-Br), 146.70, 151.48 (Carom-N), 160.92, 161.90 polarizable bromine atom at the terminal which can
(Carom-O).
enhance the overall intermolecular attraction.38 In short,
series PSB-X-n reveals that star-shaped mesogens pos-
sessing butyl (n = 4) and pentyl (n = 5) spacers show
only monotropic behaviour while the member possess-
ing hexyl spacer (n = 6) exhibit enantiotropic prop-
erties. In addition, across the series of PSB-X-n, the
clearing temperatures show a regular dependence on the
number of methylene units, n in the spacer whereby
the temperature decreases with the spacer length. For
instance, the highest clearing point is recorded for PSB-
CL-3, whilst PSB-CL-6 possesses the lowest clear-
ing temperature. This phenomenon can be explained
by the dilution effect of the peripheral mesogenic units
owing to the longer aliphatic chain of the spacer which
eventually increases the flexibility of the star-shaped
molecule.39
In order to establish the effect of spacer length
towards the mesomorphic properties, the dependence
of phase transition temperatures of these homologous
series PSB-X-n on the number of methylene units,
n in the flexible alkyl spacers is plotted (figure 6).
The commonly observed zigzag pattern or also known
as the odd–even effect is hardly seen on the isotropic-
mesophase temperature for series PSB-X-n. Instead,
the plot clearly shows that in PSB-X-n, homologues
with even-parity spacer exhibit higher mesophase sta-
bility compare to the odd-member star-shaped meso-
gens. Besides, one distinct difference between both
series is that in order for PSB-CL-n to show
mesophase, the number of methylene unit (n) must be at
least 4, but for the bromo-derivatives PSB-BR-n, the n
must be at least 5. By comparing PSB-CL-4 and PSB-
BR-4, the chloro derivative is found to be mesogenic
but PSB-BR-4 does not show liquid crystalline state.
This observation can be resulted from the influence of
different terminal atom in terms of their size. Thus, the
effect of terminal end group is presumably to cause such
alternation as the radius for bromine atom is considered
larger than chlorine and thus may tend to disrupt the
lamellar packing of the molecule.40 As a result, PSB-
BR-4 does not exhibit liquid crystalline properties due
to the increased substituent size.
3. Results and discussion
3.1 Thermal stability and optical properties
for PSB-X-n and PAZ-X-n
The phase sequences, transition temperatures and asso-
ciated enthalpies of the star-shaped compounds PSB-
X-n and PAZ-X-n upon heating and cooling are tabu-
lated in table 2. All the star-shaped mesogens exhibit
liquid crystalline behaviour except for the homologues
possessing the shortest alkyl spacer (n = 3). These
compounds undergo direct isotropization on heating
and crystallization on cooling, thus indicating the non-
mesogenic properties. In addition, the non-mesogenic
properties can also be inferred from the DSC ther-
mograms of these homologues which display only
one endotherm and exotherm upon heating-cooling
cycle. The absence of mesophase is presumably due
to the short flexible spacer as the short chain may
tend to hinder the peripheral units from the appro-
priate anisotropic arrangement in forming liquid crys-
talline properties.26 Furthermore, shorter alkyl spacer
also does not increase the polarity and polarizability of
the molecules.37 Therefore, phase generation is usually
less apparent especially in molecule having short alkyl
chain.
From the POM investigation on series PSB-X-n,
the chloro derivatives exhibit both N and SmC phases
upon cooling. The N phase is identified based on
the formation of nematic droplets at transition point
which eventually coalesce to give a Schlieran tex-
ture (figure 2(a)). On further cooling, a transition
from the N phase to SmC phase is detected prior
to crystallization (figure 2(b)). The phase sequence
is also supported by the DSC thermograms of PSB-
CL-5 as depicted in figure 3. Interestingly, PSB-
CL-6 is found to be enantiotropic but for PSB-CL-
4 and PSB-CL-5, they only show monotropic char-
acteristic where mesophase is only observed during
the cooling cycle. As for PSB-BR-n which con-
tain para-substituted bromo atom, focal conic fan-
shaped texture of SmA phase is observed when cooling
PSB-BR-5 and PSB-BR-6 from the isotropic state
except for member with butyl spacer (n = 4).
Figure 4 shows the optical photomicrograph of PSB-
BR-5 exhibiting the SmA fan-shaped texture. Among
the bromo-substituted derivatives, PSB-BR-5 is found
to be monotropic (figure 5) while PSB-BR-6 is enan-
tiotropic. Only smectic phase is observed for PSB-BR-
n and this can be attributed to the presence of highly
For the azo series PAZ-X-n, star-shaped mesogens
with the chloro substituent show both N and SmA phase
in their liquid crystalline state when viewed under a
polarized optical microscope. For instance, homologues
possessing the even-parity spacers PAZ-CL-4 and
PAZ-CL-6 exhibit both N and SmA phases on cool-
ing. However, the butyl analogue PAZ-CL-4 is found
to be monotropic while PAZ-CL-6 is enantiotropic in
nature. While for PAZ-CL-5 which is comprised of
odd-parity spacer, it exhibits enantiotropic SmA

854
Guan-Yeow Yeap et al.
Table 2. Phase transition temperatures (◦C) and associated enthalpy
changes (kJ mol−1) for compounds PSB-X-n and PAZ-X-n upon
heating and cooling cycles.
Compound
PSB-CL-3
PSB-CL-4
PSB-CL-5
PSB-CL-6
PSB-BR-3
PSB-BR-4
PSB-BR-5
PSB-BR-6
PAZ-CL-3
PAZ-CL-4
PAZ-CL-5
PAZ-CL-6
PAZ-BR-3
PAZ-BR-4
PAZ-BR-5
PAZ-BR-6
Transition temperatures, ◦C (ꢀ H, kJ mol−1
)
Cr 139.4 (65.1) I
I 70.6 (7.9) Cr
Cr 129.8 (59.8) I
I 103.7 (1.9) N 78.9 (1.7) SmC 72.2 (4.6) Cr
Cr1 93.7 (9.1) Cr2 120.3 (70.7) I
I 98.2 (1.4) N 78.8 (0.8) SmC 66.5 (3.1) Cr
Cr1 82.3 (20.5) Cr2 101.2 (6.8) N 117.6 (2.5) I
I 101.4 (1.9) N 81.3 (0.6) SmC 58.4 (2.7) Cr
Cr1 153.2 (46.9) Cr2 168.8 (34.1) I
I 113.6 (7.4) Cr1 97.5 (13.8) Cr2
Cr 153.7 (14.5) I
I 109.4 (7.8) Cr
Cr 145.0 (71.4) I
I 115.3 (9.3) SmA 106.8 (7.0) Cr
Cr1 67.2 (16.8) Cr2 96.8 (5.9) SmA 108.1 (3.0) I
I 106.7 (3.9) SmA 90.0 (6.2) Cr
Cr1 174.5 (56.2) I
I 98.1 (41.7) Cr
Cr 145.7 (95.2) I
I 110.4 (1.3) N 107.2 (10.7) SmA 94.8 (34.9) Cr
Cr 110.0 (3.7) SmA 142.4 (75.9) I
I 120.2 (12.9) SmA 75.3 (29.4) Cr
Cr 76.8 (1.2) SmA 104.3 (40.0) 112.5 (32.1) I
I 111.2(4.3) N 108.7 (4.8) SmA 55.3 (1.4) Cr B
Cr1 166.8 (15.8) Cr2 181.7 (54.5) I
I 116.3 (48.3) Cr
Cr 154.0 (71.44) I
I 122.3 (13.6) SmA 97.6 (15.0) Cr
Cr1 101.2 (23.1) Cr2 137.4 (11.2) SmA 152.2 (76.8) I
I 135.1 (17.9) SmA 98.6 (3.6) SmB 94.3 (6.6) Cr
Cr 125.1 (58.3) SmA 133.7 (33.7) I
I 130.2 (15.8) SmA 88.0 (1.7) Cr B
Cr B, Soft crystal B phase; Cr, Crystal; SmA, smectic A; SmB, smectic
B; SmC, smectic C; N, nematic; I, isotropic
*Italic denotes cooling scan
Figure 2. Optical photomicrographsof PSB-CL-5 taken during the cooling cycle. Upon
cooling from the isotropic, (a) Schlieran texture two- and four-fold brushes of N phase
appeared followed by the (b) SmC phase.

Symmetrical trimeric star-shaped mesogens based on 1,3,5-trisubstituted benzene
855
Figure 3. DSC traces of PSB-CL-5 on heating and cooling Figure 5. DSC trace of PSB-BR-5 on heating and cooling
cycles at the rate of 5◦C.
cycles at the rate of 5◦C.
in PAZ-BR-5 is further substantiated from the DSC
thermograms (figure 9) whereby three phase transitions,
I-SmA, SmA-SmB and SmB-Cr are detected in the
cooling run.
phase only. A representative optical photomicrograph
of PAZ-CL-4 showing the Schlieran texture of N phase
and the fan-shaped texture of SmA phase are depicted
in figure 7. In the series of PAZ-BR-n, the forma-
tion of mesophase is unfavourable for the short spacer,
but when the number of methylene units of the spacer
reaches n = 4, mesophase is detected. The formation
of batonets that coalesce to form the fan-shaped tex-
ture is the main feature of SmA phase. DSC ther-
mograms also confirm the monotropic feature of this
mesogen. POM observation on PAZ-BR-5 reveals an
additional transition at lower temperature identified as
SmB phase, which is assigned based on the appearance
of transient transition bars across the phase boundary
(figure 8).41 The transformation of textural feature from
SmA to SmB phase only involve very subtle changes in
the texture, thus this often makes the transition between
these two phases difficult to be detected by POM. Thus,
the existence of SmB phase following the SmA phase
Apart from that, star-shaped azo mesogens with
hexyl spacer (PAZ-CL-6 and PAZ-BR-6) are found
to exhibit an interesting feature as compared to oth-
ers whereby full recrystallization from the SmA phase
upon cooling to room temperature is not observed.
However, from the DSC analysis, a small transition
accompanied by low enthalpy value is detected at lower
temperature. This phenomenon can be attributed to the
emergence of additional phase transition upon cool-
ing compounds PAZ-CL-6 and PAZ-BR-6. Hence, it
is presumed that the compounds do not crystallize
but transform into a soft crystal state which can be
identified as soft crystal B phase. In general, soft crys-
talline phase cannot be distinguished unambiguously
by polarizing microscope alone but require additional
DSC technique to substantiate the phase transition. At
room temperature, the non-crystalline texture which
closely resembles the fan-shaped texture stays firm and
unchanged even when mechanical stress is induced.
For example, PAZ-BR-6 shows a fan-shaped texture
of SmA phase at high temperature; however, this fea-
ture retained upon cooling into the soft crystal phase.
From the observation through microscope, a change
in birefringence is observed during the transition to
soft crystal B phase.42 The DSC thermograms of PAZ-
BR-6 showing the transition to soft crystalline state is
depicted in figure 10. Therefore, PAZ-CL-6 and PAZ-
BR-6 do not crystallize but tend to solidify into a soft
crystal B phase. By referring to figure 11, the smec-
tic phase range for PAZ-CL-n and PAZ-BR-n appa-
rently increase upon elongation of spacer length. This
is because the long alkyl chain resulting from the addi-
tional methylene units enables more flexibility of the
peripheral units being attracted and aligned in parallel
Figure 4. Optical photomicrograph of PSB-BR-5 exhibit-
ing the focal conic fan-shaped texture of SmA phase.

856
Guan-Yeow Yeap et al.
(a)
(b)
Figure 6. Mesomorphic behaviour as a function of the number of methylene unit (n) in
the spacer for (a) PSB-CL-n and (b) PSB-BR-n.
order. This in turn facilitates the lamellar packing of importance in the field of liquid crystals study. Hence,
the star-shaped molecules and eventually causes an the phase transition temperatures and the nature of
increment in the smectic phase stability.43 Regardless mesophases exhibited by PSB-X-n and PAZ-X-n are
of which terminal substituent, PAZ-X-n exhibit simi- compared in order to see the relative effect of –CH=N–
lar trend in the isotropic-mesophase temperature. As and –N=N– linkages toward the mesomorphism of the
the spacer chain increases from n = 3 to 5, both homo- present star-shaped mesogens. From the comparative
logues show a gradual increment in the I-SmA tran- study, both linkages are found to exhibit similar fashion
sition temperature. However, as the spacer reaches a as they are inclined to mesomorphism. This can be due
certain length (n = 6), the I-SmA transition tempera- to the even number of atoms in their structure which can
ture become lower. Furthermore, the odd–even effect eventually enhance molecular linearity, hence promot-
on the mesomorphic properties is also not noticeable in ing the formation of liquid crystalline properties.43,44
the azobenzene star–shaped series. Overall, the results By comparing the Schiff base and azobenzene group
obtained for both PSB-X-n and PAZ-X-n indicate that to those having odd number of atoms such as chalcone
the length of the flexible spacer greatly affect the for- (-CH=CH-CO-), the latter is known to be less con-
mation of thermotropic mesophases in this star-shaped ducive to mesomorphism based on the literature. This
system.
chalcone linkage hardly exhibit any liquid crystalline
behaviour because of their non-linearity and angu-
lar strain of such group resulting from the angle of
keto group.37 In short, linking units which have even
number of atoms like Schiff base and azo groups
own great ability in altering the linearity and rigid-
ity of the molecule since they restrict the freedom
of rotation.45 Thus, they are recognized as a very
useful structural component in increasing molecular
3.2 Structure-mesomorphic property relationships:
Comparative study and influence of linking group
between star-shaped mesogens PSB-X-n and PAZ-X-n
Relationship between the molecular architecture and
the liquid crystalline behaviour is of fundamental
Figure 7. Optical photomicrographs of PAZ-CL-4: (a) The formation of Schlieran
texture of N phase upon cooling and followed by the (b) fan-shaped texture characteristic
of SmA phase.

Symmetrical trimeric star-shaped mesogens based on 1,3,5-trisubstituted benzene
857
substituent reported by Gray.46 One significant advan-
tage of the azobenzene-based star-shaped liquid crys-
tals over the Schiff base counterparts is that the for-
mer are thermally more stable than the latter. For exam-
ple, the Cr-I transition temperature for PSB-CL-4 is
129.8◦C but the azo counterpart PAZ-CL-4 is higher
which is 145.7◦C. This behaviour is also observed in
the unconventional V-shaped mesogens documented by
Prasad.47 Furthermore, higher smecitc phase stability
of the star-shaped mesogens can be achieved in the
series which incorporates azobenzene as the periph-
eral units. The ꢀSmA is found to be 8.5◦C for PSB-
BR-5, but for PAZ-BR-5, the ꢀSmA is higher (36.5◦C).
Therefore, PAZ-X-n tend to favour the smectic order
over the nematic which is fully compatible with the
molecular arrangement resulting from the side-by-side
organization of the azobenzene peripheral units which
lies parallel with each other, thus stabilize the layer-
ing of the smectic phase. Through self-assembly, the
symmetrical star-shaped mesogens is capable of fold-
ing an anisotropic shape to give rise to the appearance
of calamitic liquid crystalline order (nematic and smec-
tic) instead of discotic phase because the alkyl spacer
between the linearly aromatic peripherals and the ben-
zene core is flexible enough to allow the peripherals
to rotate freely.48 A similar observation has also been
reported for three-armed star-shaped mesogens which
Figure 8. Optical photomicrograph of PAZ-BR-5 upon
cooling showing the appearance of transition bars across the
fan-shaped during the SmA to SmB phase transition.
anisotropy leading to the formation of liquid crystalline
properties.
Besides, there is a common feature observed in
both series. Since the introduction of halogen atom as
the terminal substituent will provide the star-shaped
molecules with high polarity and terminal attraction, the
star-shaped compounds bearing either terminal chloro
or bromo substituent are found to exhibit mesophase.
By referring to table 2, all the chloro-substituted star-
shaped mesogens regardless of linking group and spacer
chain show both nematic and smectic phases while
for the bromo-substituted star-shaped mesogens, only
smectic phase is detected. Therefore, the presence of
4-bromo derivatives is capable of enhancing higher
tendency of smectic order relative to the 4-chloro-
substituted compounds. This finding is also in agree-
ment with the order of decreasing smectic thermal
stability of the compounds in terms of the terminal
Figure 9. DSC trace of PAZ-BR-5 on heating and cooling Figure 10. DSC traces of PAZ-BR-6 upon heating and
cycles at the rate of 5◦C.
cooling in the first and second cycle.

858
Guan-Yeow Yeap et al.
Figure 11. Mesomorphic behaviour as a function of the number of methylene unit (n)
in the spacer for (a) PAZ-CL-n and (b) PAZ-BR-n.
incorporate three rod-shaped azobenzene molecular absorption bands associated with the selected functional
sub-units joining to the 2,4,6-positions of the triazine groups are consistent in terms of frequencies.
nucleus.30,49 Therefore, variation of the peripheral units
in both series made it possible to observe the effects
of structural changes on the star-shaped mesomorphic
4. Conclusion
system which has not been studied previously.
In this article, we have presented the synthesis and
characterization as well as the mesomorphic behaviour
of two star-shaped homologues series, 1,3,5-tris{4-[(4-
3.3 Spectroscopic analysis
substituted-phenylimino)methyl]phenoxy}-n-alkyloxy
3.3a FT-IR spectroscopy: FT-IR discussion on the
benzene (PSB-X-n) which comprised of Schiff base
three-armed star-shaped compounds will be based on
peripheral units and 1,3,5-tris{4-[(4-substituted-
the representative homologues of PSB-CL-3 and PAZ-
phenyl)diazenyl]phenoxy}-n-alkyloxy benzene (PAZ-
BR-5. The absence of the -OH stretching signal in both
X-n) which comprised of azobenzene moieties as
spectra indicates that the alkylation reaction between
the mesogenic side-chain. Star-shaped compounds
the intermediate and phloroglucinol has taken place. In
with short spacer are non-mesogenic. N, SmA and
the FT-IR spectrum of PSB-CL-3, two distinct sharp
SmC phases are detected in PSB-X-n while PAZ-X-n
absorption bands located at the frequency range of
exhibit both N, SmA and SmB phases. An additional
2870–2916 cm−1 can be attributed to the C-H aliphatic
feature found in the series of PAZ-X-n is that the
stretching. As for the azo derivative PAZ-BR-5, same
hexyl homologues tend to form soft crystalline phase
type of stretching frequency attributed to the aliphatic
on cooling. The molecular structures of peripheral
methylene chain is observed at the region of 2868–
rod-shaped Schiff base and azobenzene fragment
2940 cm−1. On the other hand, the existence of strong
accompanied with the non-polar aliphatic spacer deter-
absorption bands at the frequency range of 1571–
mine the thermal stability as well as the structure of
1602 cm−1 in both spectra of PSB-CL-3 and PAZ-BR-
mesophase formed by the star-shaped mesogens. In
5 can be ascribed to the C=C of phloroglucinol ben-
short, the liquid crystalline properties should be driven
zene core and the aromatic fragments of the periphe-
by the rod-like units and the benzene core act only as a
ral units. In addition, the presence of C–O ether link-
linking unit interconnecting the rods. The Schiff base
age in both star-shaped compounds gives rise to an
and azobenzene groups are able to increase the linea-
intense absorption band at 1250–1251 cm−1 which indi-
rity and polarizability of the molecule, thus making it
cates the occurrence of alkylation. The medium absorp-
possible in the generation of mesomorphism.
tion band observed in the FT-IR spectrum of PSB-
CL-3 at 1619 cm−1can be assigned to the stretching
of azomethine group which connects the two aromatic
rings of 4-chloroaniline and 4-hydroxybenzaldehyde.
Acknowledgements
While for PAZ-BR-5, the stretching of N=N absorp- The author G-YY would like to thank Universiti
tion band can be found at 1475 cm−1. This value is Sains Malaysia for providing the research funding
also in agreement with the absorption bands of azo through RU Grant No 203/PKIMIA/6711265. Y-HO
stretching for 2,2ꢁ-(4-(alkyloxy)-1,3-phenylene)bis(1- would like to thank the Ministry of Higher Education
(4-substituted phenyl)diazene) as highlighted in the Malaysia (MOHE) for the scholarship under the scheme
literature.50 Overall, it can be concluded that the MyBrain15.

Symmetrical trimeric star-shaped mesogens based on 1,3,5-trisubstituted benzene
859
28. Yao D S, Zhang B Y, Zhang W W and Tian M 2008 J.
Mol. Struct. 881 83
References
29. Zhang B Y, Yao D S, Meng F B and Li Y H 2005 J. Mol.
Struct. 741 135
30. Salisu A A and Kogo A A 2010 Bayero J. Pure Appl.
Sci. 3 54
31. Meier H, Lehmann M, Holst H C and Schwöppe D 2004
Tetrahedron 60 6881
32. Lehmann M, Gearba R I, Ivanov D A and Koch M H J
2004 Mol. Cryst. Liq. Cryst. 411 397
1. Achalkumar A S, Hiremath U S, Shankar Rao D S and
Yelamaggad C V 2011 Liq. Cryst. 38 1563
2. Gemming S, Popov I and Lehmann M 2007 Philos.
Mag. Lett. 87 883
3. Tschierske C 1998 J. Mater. Chem. 8 1485
4. Yoshizawa A 2008 J. Mater. Chem. 18 2877
5. Tschierske C 2001 Annu. Rep. Prog. Chem. Sect. C 97
191
33. Stackhouse P J, Wilson A, Lacey D and Hird M 2010
Liq. Cryst. 37 1191
34. Pfeuffer T, Hanft D and Strohriegl P 2002 Liq. Cryst. 29
1555
6. Tian M, Zhang B Y, Cong Y H, He X Z, Chu H S and
Zhang X Y 2010 Liq. Cryst. 37 1373
7. Tian M, Zhang B Y, Cong Y H, Zhang N and Yao D S
2009 J. Mol. Struct. 923 39
35. Yeap G Y, Hng T C, Yeap S Y, Gorecka E, Ito M M,
Ueno K, Okamoto M, Mahmood W A K and Imrie C T
2009 Liq. Cryst. 36 1431
36. García T, Larios-López L, Rodîguez-González R J,
Martínez-Ponce G, Solano C and Navarro-Radîhuez D
2012 Polymer 53 2049
8. Yao D S, Zhang B Y, Li Y H and Xiao X Q 2004
Tetrahedron Lett. 45 8953
9. Donaldson T, Henderson P A, Achard M F and Imrie C
T 2011 Liq. Cryst. 38 1331
10. Henderson P A and Imrie C T 2005 Liq. Cryst. 32
1531
37. Thaker B T and Kanojiya J B 2011 Mol. Cryst. Liq.
Cryst. 542 84/[606].
11. Yeap G Y, Hng T C, Mahmood W A K, Gorecka E,
Takeuchi D and Osakada K 2009 Mol. Cryst. Liq. Cryst.
506 109
12. Imrie C T, Henderson P A and Yeap G Y 2009 Liq.
Cryst. 36 755
13. Nishiyama I, Yamamoto J, Goodby J W and Yokoyama
H 2003 J. Mater. Chem. 13 2429
14. Umadevi S and Sadashiva B K 2005 Liq. Cryst. 32 1233
15. Yelamaggad C V, Shashikala I, Shankar Rao D S and
Prasad S K 2004 Liq. Cryst. 31 1027
38. Yeap G Y, Lee H C, Mahmood W A K, Imrie C T,
Takeuchi D and Osakada K 2011 Ph. Trans. 84 29
39. Yeap G Y, Al-Taifi E A, Ong C H, Mahmood W A K,
Takeuchi D and Ito M M 2012 Ph. Trans. 85 483
40. Filippov S K, Kolomiets I P, Sokolova O S, Antonov E
A, Zorin I M and Bilibin A Y 1998 Liq. Cryst. 24 787
41. Goodby J W and Gray G W 1979 J. Phys. 4 363
42. Dierking I 2003 Textures of liquid crystals (Weinheim,
Germany: Wiley-VCH) p. 141.
16. Prasad V 2001 Liq. Cryst. 28 145
17. Wang H, Zheng Z and Shen D 2012 Liq. Cryst. 39 99
18. Balamurugan S and Kannan P 2009 J. Mol. Struct. 934
44
43. Collings P J and Hird M 1998 Introduction to liquid
crystals: Chemistry and physics (London, UK: Taylor
& Francis Ltd)
44. Singh S and Dunmur D A 2002 Liquid crystals: Funda-
mentals (London, UK: World Scientific Publishing Co
Pvt Ltd) p. 103.
45. Demus D, Goodby J W, Gray G W, Spiess H W and Vill
V 1998 Handbook of liquid crystals Vol 3: High molec-
ular liquid crystals (Weinheim, Germany: Wiley-VCH)
p. 40.
46. Gray G W 1962 Molecular structure and the properties
of liquid crystals (London, UK: Academic)
47. Prasad V 2001 Liq. Cryst. 28 145
48. Attard G S, Douglass A G, Imrie C T and Taylor L 1992
Liq. Cryst. 11 779
19. Jung B M, Huang Y D and Chang J Y 2010 Liq. Cryst.
37 85
20. Abe Y, Nakazima N, Tanase T, Katano S, Mukai H and
Ohta K 2007 Mol. Cryst. Liq. Cryst. 466 129
21. Chae H W, Kadkin O N and Choi M G 2009 Liq. Cryst.
36 53
22. Rezvani Z, Divband B, Abbasi A R and Nejati K 2006
Polyhedron 25 1915
23. Lu H, Wang J and Song X 2011 Mol. Cryst. Liq. Cryst.
537 93
24. Lee J H, Han M J, Hwang S H, Jang I, Lee S J, Yoo
S H, Jho J Y and Park S Y 2005 Tetrahedron Lett. 46
7143
49. Goldmann D, Janietz D, Schmidt C and Wendorff J H
1998 Liq. Cryst. 25 711
25. Lee S J, You M K, Lee S W, Lee J, Lee J H and Jho J Y
2011 Liq. Cryst. 38 1289
26. Lehmann M 2012 Top. Curr. Chem. 318 193
27. Lehmann M 2009 Chem. Eur. J 15 3638
50. Yeap G Y, Ong C H, Takeuchi D, Kakeya M, Osakada
K, Mahmood W A K, Atsuko O and Vill V 2008 J. Mol.
Struct. 882 1