Synthesis and mesomorphic properties of new Schiff base esters with different alkyl chains

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Synthesis and mesomorphic properties of new Schiff base

esters with different alkyl chains

Sie Tiong Ha

a,

*

, Lay Khoon Ong

a

, Siew Teng Ong

a

, Guan Yeow Yeap

b

,

Joanna Pik Wan Wong

a,1

, Teck Ming Koh

a

, Hong Cheu Lin

c 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 Received 3 December 2008

Abstract

A new series of Schiff base esters, 4-(dimethylamino)benzylidene-4

0

-alkanoyloxyanilines containing even number of carbons at

the end group of the molecules (C

n1

H

2n1

COO, n = 6, 8, 10, 12, 14, 16, 18) were synthesized. The present compounds were

monotropic liquid crystals. It was also found that the end groups of the molecules had effect on the mesomorphic properties.

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

Keywords: Liquid crystals; Schiff base esters; Monotropic compounds; Mesomorphic properties

The discovery of thermotropic liquid crystal 4-methoxybenzylidene-4

0

-butylaniline (MBBA) and the application of

its room temperature nematic phase in displays sparked a renewed interest liquid crystals and in research to establish

structure–property relationships

[1,2]

. The understanding of structure–property correlations is elemental to selecting

molecular modifications for the synthesis of new mesogens with desirable properties and future applications

[3]

.

In our earlier studies, the results revealed that ester and azomethine linking units are the useful structural

components for generating mesomorphism in two and three aromatic rings thermotropic liquid crystals

[4–7]

.

Therefore, a continuing effort has been carried out which focused on the synthesis and characterization of a new series

of Schiff base esters, 4-(dimethylamino)benzylidene-4

0

-alkanoyloxyanilines. The synthesis of the target molecules is

outlined in

Scheme 1

, where 4-dimethylaminobenzaldehyde was coupled with 4-aminophenol by heating under reflux

for about 3 h in ethanol solution. Then, the Schiff base intermediate was subjected to Steglich esterification with

suitable fatty acids in the presence of DCC and DMAP

[8]

. The crude products were purified by repeated

crystallization from ethanol. The structure of the products was confirmed using elemental analysis, IR, NMR and

EI-MS spectroscopic techniques

[9]

.

The liquid crystalline texture studies and phase transition temperatures were determined by means of polarizing

microscope equipped with hotstage and temperature regulator. Phase identification was made by comparing the

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Chinese Chemical Letters 20 (2009) 767–770

* Corresponding author.

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

1_{Present address: Department of Chemistry, University of York, Heslington, York YO10 5DD, United Kingdom.}

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observed textures with those reported in literatures

[10,11]

. Transition temperatures and associated enthalpy changes

were measured using differential scanning calorimeter (DSC). The results obtained were listed in

Table 1

.

All of the compounds exhibited an endotherm corresponding to the direct melting of the crystal phase to the

isotropic liquid phase during heating cycle (

Fig. 1

). The subsequent cooling of these compounds demonstrated

exotherms characteristic of the isotropic–mesophase and mesophase–crystal transitions which is indicative of the

monotropic properties.

Under the polarizing microscope, n-hexanoyloxy, n-octanoyloxy, n-decanoyloxy and n-dodecanoyloxy derivatives

exhibited monotropic nematic phase. In the monotropic mesogens, the melting points were always equal to or higher

than the clearing points, hence exhibiting supercooling properties

[12]

. The mesophase was identified by the nematic

droplets texture. Brownian flashes, a characteristic of the nematic phase were also observed before recrystallization

[13]

. An interesting phenomenon was observed for n-dodecanoyloxy derivative where an additional phase with

S.T. Ha et al. / Chinese Chemical Letters 20 (2009) 767–770 768

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

Table 1

Phase transitions and transition enthalpy changes for nDMABAA upon heating and cooling.

Compound Phase transitions, 8C (heating/cooling) (corresponding enthalpy changes, kJ mol1)

6DMABAA Cr 108.6 (36.5) I/I 97.2 (1.5) N 91.3 (28.0) Cr

8DMABAA Cr 109.8 (37.0) I/I 94.4 (1.2) N 87.6 (32.4) Cr

10DMABAA Cr 110.0 (51.0) I/I 94.5 (1.6) N 87.4 (44.4) Cr

12DMABAA Cr 104.8 (46.8) I/I 93.2 (1.7) N 91.5 (0.26) SmA 88.3 (42.5) Cr

14DMABAA Cr 105.7 (38.9) I/I 94.0 (2.7) SmA 89.6 (34.6) Cr

16DMABAA Cr 108.1 (42.6) I/I 96.6aSmA 94.3 (43.1) Cr

18DMABAA Cr 109.3 (59.8)/I 99.4aSmA 94.6 (60.3) Cr

Cr, crystal; N, nematic; SmA, smectic A; I, isotropic.

a

Polarizing optical microscopy data.

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fan-shaped texture assigned as smectic A (SmA) phase was observed after the formation of the nematic phase on the

cooling run.

Further increase in the molecular length of these compounds from C14 to C18 did not show co-existence of nematic

and smectic phases. These compounds exhibited only fan-shaped textures. This feature is a characteristic of the SmA

phase owing to the formation of battonets (

Fig. 2

a) that coalesce to form the fan-shaped texture (

Fig. 2

b). This

indicates that with increasing in the length of the terminal chain, the nematic properties decreased and led to the

emergence of the smectic phase. This is due to attraction between the long alkyl chains leading to their intertwining,

which facilitates the lamellar packing and is essential for the smectic phase

[14]

. It can therefore be proposed that in

order to generate the smectic phase in the analogous substituted C

6

H

5

CH NC

6

H

5

compounds, the number of carbons

in the alkyl chain must be at least 12 (n

12).

In conclusion, all the target compounds exhibited monotropic liquid crystal properties. The early members of the

series (n = 6, 8, 10) had only nematic phases, medium member (n = 12) displayed co-existence of both nematic and

smectic phases and higher members (n = 14, 16, 18) exhibited only SmA phase. The tendency of liquid crystal

formation has been strongly enhanced by dimethylamino terminal group as compared to the unsubstituted compound,

benzylidene-4-octyloxyaniline which did not exhibit liquid crystal phase

[15]

. Owing to the presence of

dimethylamino as donor group, these compounds may also serve as mesogenic charge-transfer donors in our future

studies by interacting with acceptor molecules to form binary liquid crystals

[16]

.

Acknowledgments

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

financial support through UTAR Vote No. 6202/O05, and Universiti Sains Malaysia for the use of its research facilities.

L.K. Ong would like to acknowledge UTAR for the award of the research assistantship.

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S.T. Ha et al. / Chinese Chemical Letters 20 (2009) 767–770 769

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[9] Analytical and spectroscopic data for the representative compound 14DMABAA: Yield 21%, EI-MS m/z (rel. int. %): 451 (6) [M+1]+_{, 450 (20)}

[M]+_{, 240 (100), IR (KBr, cm}1_{): 2953, 2918, 2850 (C–H aliphatic); 1752 (C O ester); 1608 (C N); 1590, 1498 (C C aromatic),}1_{H NMR}

(300 MHz, CDCl3, d ppm): 0.90 (t, 3H, J = 6.5 Hz, CH3), 1.28–1.42 {m, 20H, CH3(CH2)10–}, 1.77 (qt, 2H, J = 7.3 Hz, –CH2CH2COO–), 2.58

(t, 2H, J = 7.4 Hz, –CH2COO–), 6.74 (d, 2H, J = 8.9 Hz, Ar-H), 7.07 (d, 2H, J = 8.8 Hz, Ar-H), 7.20 (d, 1H, J = 8.8 Hz, Ar-H), 7.76 (d, 1H,

J = 8.9, Ar-H), 8.33 (s, 1H, CH N),13_{C NMR (75 MHz, CDCl}

3dppm): 14.63 (CH3), 23.17, 29.57, 29.74, 29.83, 29.93, 30.06, 30.11, 30.14,

32.38 for methylene carbons (CH3(CH2)10–), 25.41 (–CH2CH2COO–), 34.85 (–CH2COO–), 40.63 {N(CH3)2}, 111.92, 122.18, 122.48, 124.53,

130.91, 148.48, 150.90, 152.89 for aromatic carbons, 160.93 (CH N), 173.09 (COO), Anal. Calcd. For C29H42N2O2: C, 77.29%, H, 9.39%, N,

6.22%; found: C, 77.37%, H, 9.47%, N, 6.16%.

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