Novel nonsymmetric trimeric liquid crystals exhibiting glassy nematic state at low temperatures

Article abstract of DOI:10.1080/15421400802198599

Series of nonsymmetric 4-ethyl- and 4-fluoroanilinebenzylidene-2',4'-oxy- bis(4''-halogenoanilinebenzylidene-4'''oxy)alkanes trimers have been synthesized and characterized. Every member in this series possesses either Cl or Br terminal atoms. The molecul

Full text of DOI:10.1080/15421400802198599

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Novel Nonsymmetric Trimeric
Liquid Crystals Exhibiting
Glassy Nematic State at Low
Temperatures
Guan-Yeow Yeap a , Tiang-Chuan Hng a , Wan Ahmad
Kamil Mahmood a , Ewa Gorecka b , Daisuke Takeuchi
c & Kohtaro Osakada c
a Liquid Crystal Research Laboratory, School of
Chemical Sciences, Universiti Sains Malaysia ,
Penang , Malaysia
b Department of Chemistry , Warsaw , Poland
c Chemical Resources Laboratory, Tokyo Institute of
Technology , Midori-ku , Yokohama , Japan
Published online: 31 Aug 2012.
To cite this article: Guan-Yeow Yeap , Tiang-Chuan Hng , Wan Ahmad Kamil
Mahmood , Ewa Gorecka , Daisuke Takeuchi & Kohtaro Osakada (2008) Novel
Nonsymmetric Trimeric Liquid Crystals Exhibiting Glassy Nematic State at Low
Temperatures, Molecular Crystals and Liquid Crystals, 487:1, 135-152, DOI:
10.1080/15421400802198599
To link to this article: http://dx.doi.org/10.1080/15421400802198599
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Mol. Cryst. Liq. Cryst., Vol. 487, pp. 135–152, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802198599
Novel Nonsymmetric Trimeric Liquid Crystals Exhibiting
Glassy Nematic State at Low Temperatures
Guan-Yeow Yeap1, Tiang-Chuan Hng1, Wan Ahmad Kamil
Mahmood1, Ewa Gorecka2, Daisuke Takeuchi3, and
Kohtaro Osakada3
1Liquid Crystal Research Laboratory, School of Chemical Sciences,
Universiti Sains Malaysia, Penang, Malaysia
2Department of Chemistry, Warsaw, Poland
3Chemical Resources Laboratory, Tokyo Institute of Technology,
Midori-ku, Yokohama, Japan
Series of nonsymmetric 4-ethyl- and 4-fluoroanilinebenzylidene-20,40-oxy-bis(400-
halogenoanilinebenzylidene-4000oxy)alkanes trimers have been synthesized and
characterized. Every member in this series possesses either Cl or Br terminal atoms.
The molecular structure of these novel trimers is composed of six aromatic rings
interconnected via two methylene spacers and three azomethine linking groups.
In each molecule, the two identical methylene spacers vary from either C4H8 or
C6H12. The trimers exhibit relatively low Tm and Tc (<160ꢀC). Three of the homo-
logues with ethyl group are glassy nematic liquid crystals with Tg < 15ꢀC. The
low transition temperatures are attributed to the branched molecular architecture
which leads to steric hindrance and lesser degree of lateral intermolecular attrac-
tion. The liquid crystallinity of the intermediary aldehydes is also investigated.
Keywords: glassy nematic liquid crystals, halogen, low transition, nonsymmetric,
trimer
INTRODUCTION
A liquid crystalline trimeric molecule consists of three mesogenic frag-
ments which are linked to each other via two flexible spacers [1,2].
Many of these liquid crystal trimers or trimesogens are symmetrical
in terms of molecular structure [3]. However, the nonsymmetric tri-
mers have also been reported in which the cholesteryl ester, biphenyl,
Address correspondence to Guan-Yeow Yeap, Liquid Crystal Research Laboratory,
School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia. E-mail:
gyyeap@usm.my or gyyeap_liqcryst_usm@yahoo.com
135

136
G.-Y. Yeap et al.
and azobenzene served as three mesogenic fragments [4]. The investi-
gation on liquid crystal dimers and trimers is essential as these mat-
erials represent the shorter oligomeric members to model for the
technologically important semi-flexible main chain liquid crystal poly-
mers [5,6]. The liquid crystal trimers are also different in comparison to
the dimeric systems in terms of physical properties, e.g., exhibiting
greater magnitude of TN1 alternation among odd and even members [1].
Generally, rod-like molecules are highly favourable to induce meso-
morphic behavior due to the high shape anisotropy [7–9]. The presence
of lateral substituents is known to perturb the orderings in the liquid
crystalline phase and in some cases the mesomorphic properties are
completely diminished [10]. In order to further explore the effect of
molecular branching on the liquid crystalline properties, several
researchers have introduced lateral aromatic fragments to the existing
calamitic structures. The results thus obtained show that the influence
of a lateral aromatic fragment on liquid crystallinity depends on
whether the fragment is attached to the inner or outer rings.
In this paper, we report a series of liquid crystal trimers in which
the mesogenic cores comprise 4-ethyl- and 4-fluoroanilinebenzyli-
dene-20,40-oxy-bis(400-halogenoaniline benzylidene-400-oxy)alkanes with
three interconnected benzylideneanilines. The chlorinated and bromi-
nated benzylideneaniline moieties are linked to the 2,4-dihydroxyben-
zaldehyde molecule at ortho and para positions via the flexible
spacers. Unlike banana-shaped liquid crystals, these trimers do not
have a rigid bent mesogenic core. The trimeric molecules possess
higher degree of flexibility at the centre of the structure and not neces-
sarily adopt the bent geometry like the banana-shaped liquid crystals.
The asymmetry of the trimers can be ascribed to the third benzylide-
neaniline core which branched out from the substituted 2,4-dihydrox-
ybenzaldehyde moiety at the centre. The general structure of the
trimers is given as follows:
The trimers vary with different combinations of CnH2n (where n ¼ 4
or 6) methylene spacers, terminal atoms X of Cl or Br and Y of a F
atom or C2H5, group. The liquid crystalline properties of the homolo-
gous compounds are rationalized based upon the molecular geometry,
the effect of different halogen atoms and flexibility contributed by the

Novel Nonsymmetric Trimeric Liquid Crystals
137
spacers. The mesomorphic properties of the intermediary aldehydes
are also investigated and compared with the title compounds.
EXPERIMENTAL
Synthesis
The synthetic routes are given in Scheme 1. Compounds 1a and 1b
were synthesized via condensation reaction between 4-hydroxybenzal-
dehyde and corresponding anilines. Compounds 2a–2d were obtained
via Williamson ether synthesis of 1a and 1b with a series of a x-dibro-
moalkanes. Compounds 2a–2d were subsequently reacted with
2,4-dihydroxybenzaldehyde to form the aldehyde-based asymmetric
trimers. The intermediary trimers 3a–3d were then reacted with
4-ethylaniline which also served as a reflux co-medium to give the
title compounds 4a–4d. In a similar manner to 4-ethylaniline, 4-
fluoroaniline was also used as co-medium to dissolve 3a–3d leading
to 5a–5d, respectively. Compounds 3a–3d, 4a–4d, and 5a–5d with
respective terminal substituents and spacers are listed in Table 1.
SCHEME 1 The synthetic routes leading to the compounds 4a–4d and 5a–5d.

138
G.-Y. Yeap et al.
TABLE 1 List of Intermediary Aldehydes (3a–3d) and 4-ethyl- and 4-fluoro
Anilinebenzylidene-20 ,40-oxy-bis(400-Halogenoanilinebenzylidene-4000-oxy)-
alkanes Trimers (4a–4d and 5a–5d)
Compound
X
Y
n
3a
3b
3c
3d
CHO
CHO
CHO
CHO
Cl
Cl
Br
Br
4
6
4
6
4a
4b
4c
4d
5a
Cl
Cl
Br
Br
Cl
4
6
4
6
4
5b
5c
Cl
6
4
Br
5d
Br
6

Novel Nonsymmetric Trimeric Liquid Crystals
139
Synthesis of Compound 1a
In a round-bottom flask, 4-chloroaniline (3.83 g, 30.0 mmole)
in 40 mL absolute ethanol was added to a 50 mL ethanolic solution
of 4-hydroxybenzaldehyde (3.66 g, 30.0 mmole). The mixture was
heated at 70ꢀC overnight. The resulting solution was left to evaporate
off at room temperature whereupon the precipitate was recrystall-
ized from chloroform to yield the pure intermediate. Yield 82%.
Elemental analysis: found, C 67.40, H 4.37, N 6.05; calculated
(C13H10NOCI), C 67.39, H 4.35, N 6.05. IR (KBr) n=cmꢁ1, 3413 (OH),
1637–1601 (C N). 1H-NMR (CDCl3) d=ppm, 5.15 (s, 1H, OH), 6.92
=
(d, 2H, Ar), 7.14 (d, 2H, Ar), 7.37 (d, 2H, Ar), 7.81 (d, 2H, Ar), 8.37
=
(s, 1H, CH N).
Synthesis of Compound 1b
Yield 86%. Elemental analysis: found, C 56.57, H 3.67, N 5.06;
calculated (C13H10NOBr), C 56.55, H 3.65, N 5.07. IR (KBr) n=cmꢁ1
,
1
=
3415 (OH), 1638–1615 (C N). H-NMR (CDC13) d=ppm, 5.10 (s, 1H,
OH), 6.94 (d, 2H, Ar), 7.09 (d, 2H, Ar), 7.50 (d, 2H, Ar), 7.81 (d, 2H,
=
Ar), 8.36 (s, 1H, CH N).
Synthesis of Compound 2a
Compound 1a (0.93 g, 4.0 mmole) was dissolved in 50 mL acetone in
a round-bottom flask with the presence of potassium carbonate anhy-
drous (2.21 g, 16.0 mmole). 1,4-Dibromobutane (6.05 g, 28.0 mmole)
was then added dropwise, and the mixture was subsequently refluxed
for 18 h. The acetone solution was then left at room temperature to
evaporate off. Fifty mL of water was subsequently added to the pre-
cipitate and the desired solid was filtered and dried. The product
was recrystallized from chloroform. Yield 78%. Elemental analysis:
found, C 55.70, H 4.68, N 3.83; calculated (C17H17NOClBr), C 55.68,
H 4.67, N 3.82. IR (KBr) n=cmꢁ1, 2940–2853 (C-H alkyl), 1608
(C N), 1253 (O-CH2). 1H-NMR (CDCl3) d=ppm, 1.59–2.11 (m, 4H,
=
CH2), 3.54 (t, 2H, CH2Br), 4.05 (t, 2H, OCH2) 7.00 (d, 2H, Ar), 7.16
=
(d, 2H, Ar), 7.35 (d, 2H, Ar), 7.85 (d, 2H, Ar), 8.38 (s, 1H, CH N).
Synthesis of Compound 2b
Yield 70%. Elemental analysis: found, C 57.82, H 5.36, N 3.56; cal-
culated (C19H21NOClBr), C 57.84, H 5.36, N 3.55. IR (KBr) n=cmꢁ1
,
1
=
2935–2863 (C-H alkyl), 1607 (C N), 1254 (O-CH2). H-NMR (CDCL3)
d=ppm, 1.53–1.95 (m, 8H, CH2), 3.45 (t, 2H, CH2Br), 4.05 (t, 2H,
OCH2), 7.00 (d, 2H, Ar), 7.16 (t, 2H, Ar), 7.34 (m, 2H, Ar), 7.83 (d,
=
2H, Ar), 8.37 (s, 1H, CH N).

140
G.-Y. Yeap et al.
Synthesis of Compound 2c
Yield 80%. Elemental analysis: found, C 49.67, H 4.19, N 3.42;
calculated (C17Hl7NOBr2), C 49.66, H 4.17, N 3.41. IR (KBr) n=cmꢁ1
,
2943–2848 (C-H alkyl), 1622–1606 (C N), 1252 (O-CH2). 1H-NMR
(CDC13) d=ppm, 1.59–2.13 (m, 4H, CH2), 3.53 (t, 2H, CH2Br), 4.09 (t,
2H, OCH2), 6.98 (d, 2H, Ar), 7.08 (d, 2H, Ar), 7.52 (d, 2H, Ar), 7.86
=
=
(d, 2H, Ar), 8.37 (s, 1H,CH N).
Synthesis of Compound 2d
Yield 65%. E1emental analysis: found, C 51.98, H 4.84, N 3.20;
calculated. (C19H21NOBr2), C 51.96, H 4.82, N 3.19. IR (KBr) n=cmꢁ1
,
2940–2855 (C-H alkyl), 1618–1604 (C N), 1248 (O-CH2). 1H-NMR
(CDCl3) d=ppm, 1.53–1.95 (m, 8H, CH2), 3.46 (t, 2H, CH2Br), 4.05 (t,
2H, OCH2), 7.00 (d, 2H, Ar), 7.10 (d, 2H, Ar), 7.52 (d, 2H, Ar), 7.83
=
=
(d, 2H, Ar), 8.36 (s, 1H, CH N).
Synthesis of Compound 3a
In a round-bottom flask, compound 2a (0.62 g, 1.7 mmole) and 2,4-
dihydroxybenzaldehyde (0.11 g, 0.8 mmole) were dissolved in 50 mL
of N,N0-dimethylformamide (DMF). Upon the addition of potassium
carbonate anhydrous (0.44 g, 3.2 mmole), the mixture was heated at
70ꢀC for 18 h and left to evaporate until 40 mL was left. The mixture
was then cooled in ice bath for 0.5 h prior to addition of 50 mL of water.
The precipitate was filtered off, dried, and recrystallized with hexane
and chloroform mixture to yield the desired intermediate. Yield 61%.
Elemental analysis: found, C 69,42, H 5.41, N 3.94; calculated
(C14H38N2O,Cl2), C 69.39, H 5.40, N 3.95. IR (KBr) n=cmꢁ1, 2949–
=
=
2854 (C-H alkyl), 1670 (C O), 1620–1605 (C N), 1258 (O-CH2).
1H-NMR (CDCl3) d=ppm, 2.06 (m, 8H, CH2), 4.14 (m, 8H, OCH2),
6.46 (d, 1H, Ar), 6.58 (dd, 1H, Ar), 7.00 (dd, 4H, Ar), 7.16 (d, 4H,
=
Ar), 7.35 (d, 4H, Ar), 7.84 (2d, 5H, Ar), 8.37 (s, 2H, CH N), 10.36 (s,
=
1H, CH O).
Synthesis of Compound 3b
Yield 63%. Elemental analysis: found, C 70.59, H 6.08, N 3.67; cal-
culated (C45H46N2O5Cl2), C 70.58, H 6.06, N 3.66. IR (KBr) n=cmꢁ1
,
=
=
2939–2853 (C-H alkyl), 1678 (C O), 1620–1603 (C N), 1254 (O-CH2).
1H-NMR (CDC13) d=ppm, 1.57–1.89 (m, 16H, CH2), 4.06 (m, 8H,
OCH2), 6.45 (d, 1H, Ar), 6.58 (dd, 1H, Ar), 7.00 (d, 4H, Ar), 7.16 (d,
=
4H, Ar), 7.35 (d, 4H, Ar), 7.83 (2d, 5H, Ar), 8.37 (s, 2H, CH N),
=
10.35 (s, 1H, CH O).

Novel Nonsymmetric Trimeric Liquid Crystals
141
Synthesis of Compound 3c
Yield 56%. Elemental analysis: found, C 61.69, H 4.82, N 3.51; cal-
culated (C41H38N2O5Br2), C 61.67, H 4.80, N 3.51. IR (KBr) n=cmꢁ1
,
=
=
2951–2848 (C-H alkyl), 1670 (C O) 1620–1606 (C N), 1259 (O-CH2).
1H-NMR (CDC13) d=ppm, 2.05 (m, 8H, CH2), 4.14 (m, 8H, OCH2),
6.45 (d, 1H, Ar), 6.58 (dd, 1H, Ar), 7.00 (dd, 4H, Ar), 7.10 (d, 4H,
=
Ar), 7.49 (d, 4H, Ar), 7.84 (2d, 5H, Ar), 8.37 (s, 2H, CH N), 10.36
=
(s, 1H, CH O).
Synthesis of Compound 3d
Yield 58%. Elemental analysis: found, C 63.25, H 5.44, N 3.30; cal-
culated (C45H46N2O5Br2), C 63.24, H 5.42, N 3.28. IR (KBr) n=cmꢁ1
,
=
=
2940–2856 (C-H alkyl), 1676 (C O), 1620–1603 (C N), 1252 (O-CH2).
1H-NMR (CDC13) d=ppm, 1.55–1.89 (m, 16H, CH2), 4.06 (m, 8H,
OCH2), 6.45 (d, 1H, Ar), 6.56 (dd, 1H, Ar), 7.00 (d, 4H, Ar), 7.10 (d,
=
4H, Ar), 7.50 (d, 4H, Ar), 7.85 (2d, 5H, Ar), 8.36 (s, 2H, CH N),
=
10.35 (s, 1H, CH O).
Synthesis of Compound 4a
In a round-bottom flask, compound 3a (0.28 g, 0.4 mmole) was
dissolved in 7 mL of 4-ethylaniline. Forty mL absolute ethanol was
then added to the stirring mixture and subsequently refluxed for
8 h. The mixture was then cooled in ice bath for 0.5 h. The resulting
precipitate was filtered off and washed with hexane to remove the resi-
due of 4-ethylaniline. The precipitate was re crystallized with hexane
and chloroform mixture to yield the desired compound 4a. Yield 61%.
Elemental analysis: found, C 72.41, H 5.85, N 5.18; calculated
(C49H47N3O4Cl2), C 72.40, H 5.83, N 5.17. IR (KBr) n=cmꢁ1, 2959–
1
=
2872 (C-H alkyl), 1607–1594 (C N), 1254 (O-CH2). H-NMR (CDCl3)
d=ppm, 1.28 (t, 3H, CH3), 1.61–2.05 (m, 8H, CH2), 2.68 (q, 2H, CH2),
4.13 (m, 8H, OCH2), 6.48 (d, 1H, Ar), 6.61 (dd, 1H, Ar), 6.98 (2d, 4H,
Ar), 7.16 (2d, 6H, Ar), 7.22 (2d, 6H, Ar), 7.85 (2d, 4H, Ar), 8.11 (d,
=
=
1H, Ar), 8.42 (2s, 2H, CH N), 8.85 (s, 1H, CH N).
Synthesis of Compound 4b
Yield 53%. Elemental analysis: found, C 73.28, H 6.40, N 4.85; cal-
culated (C53H55N3O4Cl2), C 73.26, H 6.38, N 4.84. IR (KBr) n=cmꢁ1
,
2941–2869 (C-H alkyl), 1623–1592 (C N), 1249 (O-CH2). 1H-NMR
(CDC13) d=ppm, 1.28 (t, 3H, CH3), 1.55–1.88 (m, 16H, CH2), 2.70 (q,
2H, CH2), 4.05 (m, 8H, OCH2), 6.48 (d, 1H, Ar), 6.59 (dd, 1H, Ar),
6.98 (2d, 4H, Ar), 7.17 (2d, 6H, Ar), 7.22 (2d, 6H, Ar), 7.86 (2d, 4H,
=
=
=
Ar), 8.12 (d, 1H, Ar), 8.41 (2s, 2H, CH N), 8.85 (s, 1H, CH N).

142
G.-Y. Yeap et al.
Synthesis of Compound 4c
Yield 50%. Elemental analysis: found, C 65.29, H 5.26, N 4.65; cal-
culated (C49H47N3O4Br2), C 65.27, H 5.25, N 4.66. IR (KBr) n=cmꢁ1
,
2959–2872 (C-H alkyl), 1607–1593 (C N), 1254 (O-CH2). 1H-NMR
(CDC13) d=ppm, 1.28 (t, 3H, CH3), 1.59–2.05 (m, 8H, CH2), 2.68 (q,
2H, CH2), 4.13 (m, 8H, OCH2), 6.48 (d, 1H, Ar), 6.59 (dd, 1H, Ar),
6.98 (2d, 4H, Ar), 7.17 (2d, 6H, Ar), 7.22 (2d, 6H, Ar), 7.84 (2d, 4H,
=
=
=
Ar), 8.11 (d, 1H, Ar), 8.40 (2s, 2H, CH N), 8.85 (s, 1H, CH N).
Synthesis of Compound 4d
Yield 56%. Elemental analysis: found, C 66.47, H 5.81, N 4.41; cal-
culated (C53H55N3O4Br2), C 66.46, H 5.79, N 4.39. IR (KBr) n=cmꢁ1
,
2941–2870 (C-H alkyl), 1624–1592 (C N), 1251 (O-CH2). 1H-NMR
(CDCl3) d=ppm, 1.27 (t, 3H, CH3), 1.56–1.88 (m, 16H, CH2), 2.68 (q,
2H, CH2), 4.05 (m, 8H, OCH2), 6.48 (d, 1H, Ar), 6.59 (dd, 1H, Ar),
6.98 (2d, 4H, Ar), 7.17 (2d, 6H, Ar), 7.22 (2d, 6H, Ar), 7.84 (2d, 4H,
=
=
=
Ar), 8.09 (d, 1H, Ar), 8.41 (2s, 2H, CH N), 8.85 (s, 1H, CH N).
Synthesis of Compound 5a
The method to synthesize compound 5a and the rest of the fluori-
nated homologs is similar to the synthesis of 4a of which 4-ethylaniline
was substituted by 4-fluoroaniline. Yield 57%. Elemental analysis:
found, C 70.34, H 5.28, N 5.25; calculated (C47H42N3O4FCl2), C
70.32, H 5.27, N 5.23. IR (KBr) n=cmꢁ1, 2957–2877 (C-H alkyl),
1
=
1626–1594 (C N), 1251 (O-CH2). H-NMR (CDCL3) d=ppm, 2.05 (m,
8H, CH2), 4.13 (m, 8H, OCH2), 6.49 (d, 1H, Ar), 6.59 (dd, 1H, Ar),
6.99 (2d, 4H, Ar), 7.09 (m, 6H, Ar), 7.19 (m, 6H, Ar), 7.81 (2d, 4H,
=
=
Ar), 8.10 (d, 1H, Ar), 8.38 (2s, 2H, CH N), 8.81 (s, 1H, CH N).
Synthesis of Compound 5b
‘‘Yield 60%. Elemental analysis: found, C 71.33, H 5.87, N 4.90; cal-
culated (C5lH50N3O4FCl2), C 71.32, H 5.87, N 4.89. IR (KBr) n=cmꢁ1
,
2944–2870 (C-H alkyl), 1623–1593 (C N), 1251 (O-CH2). 1H-NMR
(CDCl3) d=ppm, 1.58–1.89 (m, 16H, CH2), 4.06 (m, 8H, OCH2), 6.48
(d, 1H, Ar), 6.59 (dd, 1H, Ar), 6.98 (2d, 4H, Ar), 7.08 (m, 6H, Ar),
7.17 (m, 6H, Ar), 7.83 (2d, 4H, Ar), 8.08 (d, 1H, Ar), 8.38 (2s, 2H,
=
=
=
CH N), 8.81 (s, 1H, CH N).
Synthesis of Compound 5c
Yield 50%. Elemental analysis: found, C 63.33, H 4.76, N 4.72; cal-
culated (C47H42N3O4FBr2), C 63.31, H 4.75, N 4.71. IR (KBr) n=cmꢁ1
,
2958–2877 (C-H alkyl), 1626–1594 (C N), 1251 (O-CH2). 1H-NMR
=

Novel Nonsymmetric Trimeric Liquid Crystals
143
(CDC13) d/ppm, 2.06 (m, 8H, CH2), 4.13 (m, 8H, OCH2), 6.49 (d, 1H,
Ar), 6.59 (dd, 1H, Ar), 7.01 (2d, 4H, Ar), 7.09 (m, 6H Ar), 7.19 (m,
=
6H, Ar), 7.84 (2d, 4H, Ar), 8.09 (d, 1H, Ar), 8.39 (2s, 2H, CH N),
=
8.81 (s, 1H, CH N).
Synthesis of Compound 5d
Yield 67%. Elemental analysis: found, C 64.65, H 5.34, N 4.43; cal-
culated (C51H50O4FBr2), C 64.63, H 5.32, N3 N 4.43. IR (KBr) n=cmꢁ1
,
2946–2873 (C-H alkyl), 1623–1594 (C N), 1252 (O-CH2). 1H-NMR
(CDCl3) d=ppm, 1.55–1.89 (m, 16H, CH2), 4.06 (m, 8H, OCH2), 6.48
(d, 1H, Ar), 6.59 (dd, 1H, Ar), 6.98 (2d, 4H, Ar), 7.09 (m, 6H, Ar),
7.18 (m, 6H, Ar), 7.83 (2d, 4H, Ar), 8.08 (d, 1H, Ar), 8.38 (2s, 2H,
=
=
=
CH N), 8.81 (s, 1H, CH N).
Characterization
Elucidation of molecular structure of intermediary and title com-
pounds was obtained by FT-IR and 1H-NMR spectroscopy. The
FT-IR spectroscopy was performed on a Perkin Elmer 2000 FTIR spec-
trometer. The samples were prepared in KBr pellets and the spectra
1
were recorded within 4000-400 cmꢁ1. The H-NMR spectroscopy was
carried out on a Bruker 400 MHz UltrashieldTM spectrometer.
Deuterated chloroform (CDCl3) with TMS as internal standard was
used to dissolve 20 mg of samples in the NMR tube. For compounds
1a and 1b, the temperature was maintained at 50ꢀC using the spec-
trometer internal heater throughout the analyses. CHN microanalyses
were conducted via Perkin Elmer 2400 LS Series CHNS=O analyzer.
The optical properties of the compounds were investigated using a Carl
Zeiss Axioskop 40 polarizing microscope linked to a Linkam TMS94
temperature controller and LTS350 hot-stage. Differential scanning
calorimetry was performed on a Seiko DSC6200R calorimeter with
the heating and cooling rate of ꢂ5ꢀCminꢁ1 at Tokyo Institute of Tech-
nology, Japan. Powder X-ray diffraction analysis on a representative
compound 4c was conducted using a Bruker D8 diffractometer at
Warsaw University, Poland. On the same sample, further conforma-
tional studies were carried out based on MM2 calculations with mini-
mum energetic parameters. The software used for the calculations is
CS Chem3D Pro produced by CambridgeSoft Corporation.
RESULTS AND DISCUSSION
Thermal and Liquid Crystalline Properties
The phase sequences, transition temperatures, and associated enthal-
pies for compounds 3a–3d, 4a–4d, and 5a–5d are collated in Table 2.

144
G.-Y. Yeap et al.
TABLE 2 Phase Transition Temperatures (ꢀC) and Associated Enthalpies (kJ
molꢁ1) of Compounds 3a–3d, 4a–4d, and 5a–5d (. ¼ enantiotropic phase,
ꢀ ¼ monotropic phase, only the Highest Melting Point of Crystal is Reported.
Tg ¼ glass transition temperature, Cr ¼ crystal, N ¼ nematic, I ¼ isotropic)
Compound
Tg
Cr
N
I
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
5d
.
.
.
.
.
.
.
.
.
.
.
.
132.7 (30.6)
100.1 (25.2)
125.3 (19.0)
111.4 (41.3)
117.6 (54.5)
138.0 (81.9)
119.8 (29.5)
133.1 (95.1)
155.2 (73.5)
130.1 (77.6)
156.5 (86.7)
137.3 (29.3)
ꢀ
[122.2]a
106.2 (1.4)
.
.
.
.
.
.
.
.
.
.
.
.
.
ꢀ
.
ꢀ
.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
[98.5] (4.1)
144.3 (4.7)
[117.7] (3.9)
146.5 (5.3)
[119.0] (5.9)
[141.6] (4.0)
[124.5] (5.1)
[143.2] (5.0)
[117.2] (4.0)
13.4 (0.4)b
5.2 (0.3)b
[] Denotes transition temperature in formation of monotropic N phase.
a
Denotes transition temperature by polarizing microscopy, undetectable by DSC.
Denotes heat capacity change in kJ (molꢃꢀC) ꢁ 1 accompanying glass transition
directly to N phase from 2nd heating run onwards; pure Cr phase is only observed for
the 1st heating run.
b
Representative DSC traces of compounds 4a and 4b are depicted
in Fig. 1a and 1b, respectively. Compound 3a exhibits monotropic
nematic phase. By increasing the spacer length from C4H8 to
C6H12,the enantiotropic nematic phase is observed for compound 3b.
On the other hand, compound 3c is not mesogenic. However, the
monotropic nematogenic properties of the Br homolog are induced by
replacing C4H8 with C6H12 spacers for compound 3d. The overall
phase transitions for compounds 3a–3d as depicted in Fig. 2 indicate
that the compounds 3b and 3d with C6H12 members possess greater
tendency in comparison to C4H8 members (3a and 3c) in exhibiting
nematic phase. This observation could possibly be due to the increase
in molecular flexibility in compounds 3b and 3d leading to the
reduction of strong intermolecular forces induced at the aldehyde core.
With the presence of 4-ethylaniline fragment, both chlorinated and
brominated trimers exhibit nematogenic properties. However, the
mesomorphic properties of compounds 4a–4d are in contrast to the
results we obtained for 3a–3d as discussed earlier. The compounds
4a and 4c consisting C4H8 members exhibit enantiotropic nematic
phase. By increasing the spacer length to C6H12, the monotropic
nematic phase was detected for compounds 4b and 4d. The presence

Novel Nonsymmetric Trimeric Liquid Crystals
145
FIGURE 1 DSC tꢀraces of (a) compound 4a with heat capacity change of 0.4 kJ
(molꢃꢀC) ꢁ 1 at 13.4 C and (b) compound 4b with heat capacity change of 0.3 kJ
(molꢃꢀC) ꢁ 1 at 5.2ꢀC. G indicates the glassy state. The heat capacity change is
observed at G-N phase transition upon second heating.
FIGURE 2 The plot of phase temperature (ꢀC) range upon cooling for
compounds 3a–3d (ꢄ denotes mp).

146
G.-Y. Yeap et al.
of enantiotropic nematic phase for the butyl members suggests the
higher phase stability in comparison to the hexyl members. Although
compounds 4a and 4c possess relatively short flexible spacers, the
flexibility of the molecule as a whole is sufficient to stabilize the
nematic phase due to the presence of an additional ethyl group at
the central core. The additional two methylene units in 4b and 4d
results in excess of flexibility which hinder the formation of enantio-
tropic nematic phase.
By introducing a polar fluorine atom to the central mesogenic core,
all trimers exhibit monotropic nematic phase. The monotropic nematic
temperature ranges of compounds 5a–5d are shown in Fig. 3 based on
DSC result with heating and cooling rate of ꢂ5ꢀCminꢁ1. As previously
reported on fluorinated derivatives with various mesogenie cores, the
fluorine atom is known to alter the thermal and chemical stability and
viscosity of liquid crystals [11]. For compounds 5a–5d, introduction of
a polar fluorine atom increases the polarity at the middle segment of
the trimeric structure. As such, the terminal attractions of the trimers
will be affected causing the decrease in the stability of the nematic
phase. This observation contradicts with those compounds having
higher degree of terminal intermolecular attractions which favors
the formation of nematic phase [12]. For compounds 5a–5d, even with
FIGURE 3 The plot of phase temperature (ꢀC) range upon cooling for
fluorinated compounds 5a–5d (ꢄ denotes mp).

Novel Nonsymmetric Trimeric Liquid Crystals
147
a polar fluorine atom the smectic phase could not be induced. The
layered arrangement in the smectic phase is not possible because of
the branched structure of the trimers and lower degree of shape ani-
sotropy as reported for a series of azobenzene derivatives with lateral
alkoxy group [13].
Among the twelve homologous members, full recrystallization from
the nematic phase upon cooling is not observed for compounds 4a–4c.
Instead, the microscopic samples undergo the glass transition at low
temperatures like a polymer and therefore exhibit the glassy state.
This characteristic is evident by the observation of noncrystalline
texture which closely resembles the nematic schlieren texture on
supercooling below room temperature. In such a condition, the orien-
tational order characteristic of the nematogenic fluid is frozen in the
solid state [14]. The melting point of compound 4a (117.6ꢀC) can only
be detected on first heating in which the latent heat of fusion is 54.5 kJ
molꢁ1. Under polarized light, it is observed that the cooling of the sam-
ple increases the viscosity but no Cr phase is visually detected. Our
observation is, however, consistent with the DSC result as N-Cr tran-
sition is absent even when sample is cooled to ꢁ40ꢀC (Fig. 1a). On
second heating, instead of the melting point, the glass transition is
detected by DSC at 13.4ꢀC with heat capacity change of 0.4 kJ
(mol ꢃ ꢀCꢁ1).
Compound 4b exhibits similar glassy texture of compound 4a.
However, there are two important differences between the two homo-
logs. On first heating, in contrast to compound 4a, the nematic phase
is not observed for compound 4b. The second notable difference is the
presence of a broad exothermic curve between 67.0ꢀC and 118.0ꢀC on
heating of compound 4b as shown in the DSC trace (Fig. 1b). This sig-
nal indicates the occurrence of some recrystallization at the same ꢀtime
heat is consistently applied to the sample at 5ꢀCminꢁ1. At 118.8 C, a
small endothermic peak (0.8 kJ molꢁ1) is assigned to slight melting
transition. Upon further heating, the sample clears into isotropic at
137.4ꢀC with an enthalpy value of 51.5 kJ molꢁ1. This high isotropiza-
tion enthalpy indicates that the melting involves crystalline structure
and certainly not only the nematic phase despite that schlieren tex-
ture is clearly observed.
Similar to compound 4a, compound 4c is an enantiotropic nemato-
gen. On second heating, a step change which represents heat capacity
change is not observed in the DSC thermogram as expected. On the
other hand, an endothermic peak with 1.0 kJ molꢁ1 is detected at
14.6ꢀC. This peak indicates some recrystallization over the glassy state
prior to this transition. At 119.9ꢀC, a peak measuring 0.9 kJ molꢁ1
is detected by DSC. This temperature is consistent with the melting

148
G.-Y. Yeap et al.
temperature (119.8ꢀC) upon first heating and, therefore, suggests that
minor recrystallization occurs in the nematic matrix before 119.9ꢀC
upon second heating. The low enthalpy (0.9 kJ molꢁ1) indicates that
the major portion of the sample is in the nematic phase despite the
occurrence of some recrystallization. In terms of textural observation,
both nematic and crystal phases are not detectable under polarized
light as nematic schlieren texture covers the whole microscopic
domain.
Compound 4d exhibits monotropic nematic phase. Unlike com-
pounds 4a–4c, the glassy state is not observed for compound 4d.
Recrystallization of the sample upon cooling is evident under the
polarized light for both first and second cycles and the observation
is in agreement with the DSC results as N-Cr transition is detected
thermodynamically.
In order to further investigate the nematic phase and glassy state,
powder X-ray diffraction was conducted on representative compound
4c. The X-ray diffraction intensity profile of the nematic phase is given
˚
in Fig. 4. The peak position corresponds to 32 A in terms of average
distances among molecules and the peak broadness indicates the short
range positional order in the nematic phase. It is also observed that
the shape of the signal fits the Lorentzian function as typically
obtained for phases with short range order. From MM2 calculations,
the proposed most stable conformation of 4c trimer is approximately
˚
40 A measuring from one Br-substituted terminal to another end as
FIGURE 4 The powder X-ray diffraction intensity profile for compound 4c in
the nematic phase (120.0ꢀC) The red line indicates that the signal fits the
Lorentzian function.

Novel Nonsymmetric Trimeric Liquid Crystals
149
FIGURE 5 Proposed conformation of 4c based on MM2 calculations with
minimum energetic parameters. The conformation measures approximately
˚
40 A.
shown in Fig. 5. The d=l value of 0.8 suggests some intercalation
among the trimers in the nematic phase which is not uncommon. On
the other hand, the glassy state of compound 4c could not be analyzed
via powder X-ray diffraction as the state only exists below 14.6ꢀC.
However, because the glassy state and the nematic phase are similar
in terms of orders, the positional order in the glassy state is extrapo-
lated from the powder X-ray diffraction results obtained for the
nematic phase.
The low glass transition temperatures of compounds 4a–4c are
attributable to the flexibility induced by the mefhylene spacers and
most importantly, the ethyl group at the central core [15]. This expla-
nation is further supported by the ability of compounds 5a–5d which
do not have an ethyl group, to recrystallize despite possessing recrys-
tallization temperature more than 40ꢀC below the melting point.
Although most substituents at lateral position are known to create
some hindrance, the small size of a fluorine atom minimizes such
effect in comparison to bulkier groups such as the ethyl chain. From
DSC and polarizing microscopy, we found that the microscopic sam-
ples of 5a–5d are able to recrystallize to the original crystal phase
upon cooling of the nematic phase. This characteristic suggests that
the fluorinated trimers have less difficulty to pack into the crystal lat-
tices from the nematic phase.
From the microscopic observation, all mesogenic trimers in this ser-
ies exhibit schlieren texture with twofold (s ¼ ꢂ 1=2) and fourfold
(s ¼ ꢂ1) brushes as depicted in Fig. 6. The schlieren texture indicates
that the director is parallel to the untreated glass supports and. varies
slowly in the planes [16]. On the other hand, the thin preparation of
compound 4c between the substrates gives rise to marble texture
instead of the schlieren texture.
The TNI of compound 4a is higher than the Tm of compound 4b
which exhibits monotropic nematic phase. Similar characteristics

150
G.-Y. Yeap et al.
FIGURE 6 The texture of schileren defect with twofold and fourfold brushes
of compound 4d 117.8ꢀC on cooling.
are also observed for compounds 4c and 4d. For compounds 5a–5d, the
trimers with C4H8 spacers possess higher Tm than the corresoponding
homologues with C6H12 spacers. The lowering of Tm or Tc with the
increase of spacer length indicates dilution of the mesogenic cores as
reported for other series of trimeric liquid crystals such as 4,40-bis[x-
(4-cyanobiphenyl-40-yloxy)alkoxy]biphenyls [17].
The transition temperatures (Tm and Tc) recorded for the twelve tri-
mers are lower than 160ꢀC. Generally, for trimeric structure, the pres-
ence of three mesogenic cores often gives rise to higher transition
temperatures. In this series of nonsymmetric trimers, the relatively
low transition temperatures can be attributed to the branched molecu-
lar architecture at the central core.
The exclusive nematogenic characteristics of the trimers can also be
explained from the viewpoint of trimeric structure. The lower shape
anisotropy of the trimers due to branching at the 2,4-dihydroxybenzal-
dehyde aromatic ring results in less ordered mesophase. The addition
of another aromatic ring to the central core increases the overall
transition temperatures of the trimers as reported in the homologous
series of 4-(trans-40-n-pentylcyclohexyl) benzoates [18]. Compounds
3c, 4c, and 5c, n particular, exhibit three different types of thermal
properties with respect to the changes in the central mesogenic frag-
ment. From being non-mesogenic for compound 3c, enantiotropic
nematic phase is induced for 4c with addition of 4-ethylaniline frag-
ment. By substituting the ethyl group with a fluorine atom, the mono-
tropic nematic phase is observed for 5c.

Novel Nonsymmetric Trimeric Liquid Crystals
151
CONCLUSIONS
Series of nonsymmetric 4-ethyl- and 4-fluoroanilinebenzylidene-20,40-
oxy-bis(400-halogenoanilinebenzylidene-4000-oxy)alkanes trimers have
been synthesized and characterized, for their liquid crystalline proper-
ties. The trimers differ from each other in terms of C4H8 or C6H12 flex-
ible spacers, terminal X of Cl or Br atom, and terminal Y of F atom or
C2H5 group. Every member in this series was nematogenic. Enantio-
tropic nematic phase was observed only for the trimers with C2H5
group and C4H8 spacer. For compounds 4a–4d, lengthening the
C44H8 spacer to C6H12 led to the monotropic nature of the nematic
phase. Compounds 4a–4c exhibited the glassy nematic state as the
molecular motion in the nematic phase was frozen on cooling. The
Tg for compounds 4a and 4b were detected below 15ꢀC by DSC. For
compound 4c, instead of a step change in heat capacity, a peak which
indicates an enthalpy was observed in the DSC thermogram. The
glassy state of the compounds 4a–4c can be attributed to the presence
of the C2H5 group because such property was not observed for the
fluorinated trimers. In contrast to C2H5 homologues, all F-substituted
trimers 5a–5d recrystallized on cooling the nematic phase. The
fluorinated homologues exhibited monotropic nematic phase regard-
less of spacers and terminal halogen atoms.
ACKNOWLEDGMENTS
The main author (G.-Y. Yeap) would like to thank the Malaysian
Ministry of Science, Technology, and Innovation (MOSTI) and Minis-
try of Higher Education for providing financial support via Science
Fund (03-01-05-SF0062 or 305=PKIMIA=613315) and Fundamental
Research Grant Scheme (304=PKIMIA=671025), respectively. Authors
are also grateful to the Research and Innovation Division in Universiti
Sains Malaysia for the support and assistance.
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