New mesogenic Schiff base esters comprising benzothiazole moiety: Synthesis and mesomorphic properties

New mesogenic Schiff base esters comprising benzothiazole moiety:

Synthesis and mesomorphic properties

Sie Tiong Ha

*

, Teck Ming Koh

, Guan Yeow Yeap

, Hong Cheu Lin

,

Jun Kit Beh

, Yip Foo Win

, Peng Lim Boey

a

Faculty of Engineering & Science, Universiti Tunku Abdul Rahman, Jln Genting Klang, Setapak, 53300 Kuala Lumpur, Malaysia

b

Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia

c

Department of Materials Science & Engineering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 300, Taiwan

d

Faculty of Science, Engineering & Technology, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia Received 12 February 2009

Abstract

A homologous series of Schiff base esters, 6-methoxy-2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles,

compris-ing a benzothiazole moiety as the core was synthesized. All the members of this series exhibited an enantiotropic nematic phase.

The azomethine linkage along with the lateral hydroxyl and terminal methoxyl groups were found to exert an effect on the

mesomorphic properties.

# 2009 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Schiff bases; Benzothiazoles; Mesomorphic; Nematic; Structure–liquid crystal property relationship

Liquid crystalline behaviour of an organic compound is essentially dependent on its molecular architecture, in

which a slight change in its molecular geometry brings about considerable changes in its mesomorphic properties.

Detailed studies by liquid crystal researchers have led to empirical rules, one of which includes the effect of the

chemical constitution in the formation of nematogenic and smectogenic mesophases

[1]

. Most of these studies focus

mainly on Schiff’s bases since the discovery of 4-methoxybenzylidene-4

-butylaniline which exhibited a nematic

phase at room temperature

[2]

.

The mesomorphic properties of aromatic Schiff base esters arising from substituents varying in their polarities

have been reported in our previous studies

[3–7]

. In order to further explore the factors which govern the thermal

stability of liquid crystals with a Schiff base core, and the relationship with its molecular structures, we have

introduced thiazole, a heterocylic unit, into the aniline fragment of benzylideneaniline. The novel compounds

reported here are 6-methoxy-2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles and their synthetic

route is illustrated in

Scheme 1

. 2-Amino-6-methoxybenzothiazole and 2,4-dihydroxybenzaldehyde were coupled

by reflux in ethanol for 3 h, following which the Schiff base intermediate was subjected to Steglich esterification

with the appropriate fatty acids in the presence of DCC and DMAP

[8–10]

. The crude products were purified upon

Available online at www.sciencedirect.com

Chinese Chemical Letters 20 (2009) 1081–1084

* Corresponding author.

E-mail addresses:[email protected],[email protected](S. Tiong Ha).

1001-8417/$ – see front matter # 2009 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2009.03.048

repeated recrystallization using ethanol and their structures were elucidated via elemental analysis, FT-IR, NMR

and EI-MS spectroscopic techniques

[11]

.

The liquid crystalline textures of the products were observed under a polarizing optical microscope equipped with a

hotstage and temperature regulator. Phase identification was made by comparing the observed textures with those

reported in the literature

[12,13]

. Transition temperatures and associated enthalpy changes were determined using a

differential scanning calorimeter. The results are summarized in

Table 1

.

The target compounds are all nematogens. Upon cooling of the isotropic liquid, small droplets (

Fig. 1

a) appeared

and coalesced to form the classical schlieren or marble texture (

Fig. 1

b) which is characteristic of a nematic phase.

From the DSC results (

Table 1

), it is clearly noticed that the target compounds showing enantiotropic properties as the

mesophase were observed during heating and cooling cycles

[14]

.

The linking group is one of the factors influencing the target compounds in exhibiting the nematic phase instead of a

smectic phase. The azomethine (CH N) linking unit confers a stepped structure that disrupts the lamellar packing

S. Tiong Ha et al. / Chinese Chemical Letters 20 (2009) 1081–1084 1082

Scheme 1. Synthetic route for the target compounds. (i) C2H5OH and (ii) Cn1H2n1COOH, DCC, DMAP, CH2Cl2, DMF.

Table 1

Phase transition and transition enthalpy changes for n-MHBABTH upon heating and cooling.

Compound Phase transition (8C) (corresponding enthalpy changes (kJ mol1))

6-MHBABTH Heating Cr 131.1 (38.74) N 155.6 (0.63) I Cooling Cr 91.8 (32.89) N 141.8 (0.81) I 8-MHBABTH Heating Cr 133.2 (37.66) N 145.8 (0.60) I Cooling Cr 100.7 (33.72) N 132.5 (0.84) I 10-MHBABTH Heating Cr 127.5 (90.04) N 139.8 (1.81) I Cooling Cr 98.9 (89.07) N 135.9 (2.55) I 12-MHBABTH Heating Cr 124.9 (43.25) N 136.8 (0.98) I Cooling Cr 107.7 (43.28) N 133.8 (0.53) I 14-MHBABTH Heating Cr 123.6 (54.67) N 133.6 (0.84) I Cooling Cr 103.7 (53.75) N 130.7 (0.88) I 16-MHBABTH Heating Cr 123.2 (61.86) N 129.7 (1.51) I Cooling Cr 95.7 (61.22) N 126.5 (1.51) I 18-MHBABTH Heating Cr 122.3 (61.27) N 125.2 (1.23) I Cooling Cr 110.8 (58.31) N 117.0 (0.89) I

which is unfavourable for the formation of a smectic phase

[15]

. The terminal methoxyl group at the sixth position of

the benzothiazole core diminishes the formation of the smectic mesophase

[16]

. The position of the lateral substituent

is also crucial in determining the phase morphology. Since the hydroxyl group is situated at the inner-core position, it

does not occupy any vacant space at the edge of the core, hence causing the broadening of the molecule, in turn

favouring the formation of the nematic phase

[15]

.

A plot of the transition temperatures against the number of carbons in the alkanoyloxy chain during the heating

scan is shown in

Fig. 2

. The melting temperatures (T

), were considerably reduced by the increase in the length

of the chain owing to the increase in its flexibility

[15]

. On the other hand, the clearing temperatures (T

),

dropped as the number of carbon atoms increased, resulting from the dilution of the core induced by the increase

in the length of the terminal chain

[6]

. The nematic phase range is noticed to have decreased as the length of the

chain increased. The nematic phase was generally exhibited by compounds possessing short to medium-chain

lengths. As the length of the chain increases, the nematic phase stability decreases, therefore leading to a decrease

in the phase range

[14]

.

In conclusion, all the target compounds exhibited enantiotropic nematic phase. The length of the terminal

alkanoyloxy chain affects the melting, clearing temperatures and phase ranges. The azomethine linking group,

lateral hydroxyl group at the inner-core position and the terminal methoxyl group at the sixth position of the

benzothiazole moiety were among the factors influencing the formation of the nematic phase in the target

compounds.

S. Tiong Ha et al. / Chinese Chemical Letters 20 (2009) 1081–1084 1083

Fig. 1. Liquid crystal textures of 16-MHBABTH upon cooling. The small droplets (a) coalesced to form the marble texture of the nematic phase in (b).

Acknowledgments

The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman (UTAR) for the research facilities and

financial support (No. 6202/K06), and the Malaysian Toray Science Foundation (No. 4359/000) for funding this

project. T.M. Koh would like to acknowledge UTAR for the award of the research assistantship.

References

[1] G.W. Gray, Molecular Structure and the Properties of Liquid Crystals, Academic Press, London, 1962. [2] H. Kelker, B. Scheurle, Angew. Chem. Int. Ed. 81 (1996) 903.

[3] G.Y. Yeap, S.T. Ha, P.L. Lim, P.L. Boey, W.A.K. Mahmood, M.M. Ito, S. Sanehisa, Mol. Cryst. Liq. Cryst. 423 (2004) 73. [4] G.Y. Yeap, S.T. Ha, P.L. Lim, P.L. Boey, M.M. Ito, S. Sanehisa, V. Vill, Mol. Cryst. Liq. Cryst. 452 (2006) 63.

[5] G.Y. Yeap, S.T. Ha, P.L. Boey, W.A.K. Mahmood, M.M. Ito, Y. Youhei, Mol. Cryst. Liq. Cryst. 452 (2006) 73. [6] G.Y. Yeap, S.T. Ha, P.L. Lim, P.L. Boey, M.M. Ito, S. Sanehisa, Y. Youhei, Liq. Cryst. 33 (2006) 205.

[7] S.T. Ha, L.K. Ong, S.T. Ong, G.Y. Yeap, J.P.W. Wong, T.M. Koh, H.C. Lin, Chin. Chem. Lett. 20 (7) (2009) 767. [8] S.T. Ha, L.K. Ong, Y.F. Win, T.M. Koh, G.Y. Yeap, Molbank 3 (2008) M582.

[9] S.T. Ha, L.K. Ong, Y.F. Win, T.M. Koh, G.Y. Yeap, Molbank 1 (2009) M584. [10] S.T. Ha, L.K. Ong, Y.F. Win, T.M. Koh, G.Y. Yeap, Molbank 1 (2009) M585.

[11] Analytical and spectroscopic data for the representative compound 16-MHBABTH: Yield 63%, EI-MS m/z (rel. int. %): 538 (9) [M+], 300 (100), IR nmax(KBr, cm1): 3447 (O–H); 3095, 3069 (C–H aromatic); 2920, 2851 (C–H aliphatic); 1757 (C O ester); 1611 (C N thiazole);

1460 (C C aromatic),1_{H NMR (400 MHz, CDCl3, d ppm): 0.8 (t, 3H, CH}

3–), 1.2–1.4 (m, 24H, CH3–(CH2)12–), 1.7 (m, 2H, –CH2–CH2–

COO–), 2.5 (t, 2H, –CH2–COO–), 3.8 (s, 3H, CH3–O–), 6.7 (d, 1H, Ar–H), 6.8 (s, 1H, Ar–H), 7.0 (d, 1H, Ar–H), 7.2 (s, 1H, Ar–H), 7.4 (d, 1H,

Ar–H), 7.8 (d, 1H, Ar–H), 9.1 (s, 1H, –N CH–),13_{C NMR (100 MHz, CDCl}

3, d ppm): 14.06 (CH3), 22.64, 29.00, 29.18, 29.31, 29.40, 29.55,

29.60, 29.62, 29.64, 31.87 for methylene carbons (CH3(CH2)12–), 24.77 (–CH2CH2COO–), 34.36 (–CH2COO–), 55.68 (OCH3), 104.24,

110.57, 113.68, 115.94, 116.24, 123.60, 134.70, 135.86, 145.74, 155.99, 157.73, 163.03, 165.15 for aromatic carbons, 166.25 (CH N), 171.33 (COO), Anal. calcd. for C31H42N2O4S: C, 69.11%, H, 7.86%, N, 5.20%; Found: C, 69.19%, H, 7.79%, N, 5.22%.

[12] D. Demus, L. Richter, Textures of Liquid Crystals, Verlag Chemie, New York, 1978. [13] I. Dierking, Textures of Liquid Crystals, Wiley-VCH, Weinheim, 2003.

[14] S. Kumar, Liquid Crystals Experimental Study of Physical Properties and Phase Transitions, Cambridge University Press, 2001. [15] P.J. Collings, M. Hird, Introduction to Liquid Crystals: Chemistry and Physics, Taylor & Francis Ltd., UK, 1998.

[16] A.K. Prajapati, N.L. Bonde, J. Chem. Sci. 118 (2) (2006) 203.

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