Synthesis and mesomorphic evaluation of new calamitic liquid crystals containing benzothiazole core

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Synthesis and mesomorphic evaluation of new calamitic liquid

crystals containing benzothiazole core

Teck Ming Koh

a,b

, Sie Tiong Ha

c,

*

, Teck Leong Lee

c

, Siew Ling Lee

d

,

Guan Yeow Yeap

e

, Hong Cheu Lin

f

, Ramesh T. Subramaniam

g

a

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

b

Department of Chemistry, National University of Singapore, Singapore 117543, Singapore

c

Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia

d

Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

e_{Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia} f_{Department of Materials Science & Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, ROC}

g_{Center for Ionics University Malaya, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia}

Received 10 September 2010 Available online 1 March 2011

Abstract

The design and synthesis of new calamitic benzothiazole-based liquid crystals, 2-[4-(4-alkyloxybenzoyloxy)-phenyl]ben-zothiazoles are presented. The target compound was characterized using spectroscopic techniques, such as FT-IR, NMR (1H and

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C), microanalysis and EI-MS. The liquid crystalline behaviours of these compounds were thoroughly examined by differential scanning calorimetry and polarizing optical microscope techniques. These materials exhibited enantiotropic nematic phase with high thermal stability (>168 8C). Smectic A phase starts to emerge as monotropic (metastable) phase from C10 member and changes into enantiotropic (stable) phase from C12 and persists up to C16 members.

Keywords: Benzothiazole; Alkoxy chain; Nematic; Smectic A; Liquid crystal

Liquid crystals (LCs) have gained additional attention as new type of organic semiconductor exhibiting self-organization, liquid crystal displayer and separation membrane materials [1]. In the 1970s, much attention has been focused on the electrical properties of LCs, as a result, the electrical properties of different calamitic LCs have been examined [2,3]. Research focus shifted from calamitic to discotic LCs since the discovery of discotic LCs in 1977 [4]. High hole mobility found in the hexagonal columnar phase of hexapenthyloxy-triphenylene becomes an important milestone in the electronic studies of LCs[1]. An important interest, however, still remains to be explored in calamitic mesophase due to their different degrees of molecular order and arrangement. As for calamitic LCs, the electronic conduction was first established in the SmA phase of 2-phenylbenzothiazole

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Chinese Chemical Letters 22 (2011) 619–622

* Corresponding author.

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

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derivatives[5,6]. In fact, smectic phases exhibit stronger intermolecular interaction than that of the nematic phase, resulting in a higher order in terms of the molecular alignment and in a higher viscosity[5]. Furthermore, the layer structure present in all smectic LCs induces p–p stacking interactions which facilitate hopping of charge carrier mobility.

In our previous studies on LCs[7–10], we found that rod-like molecules incorporated with benzothiazole core ring enabled them to exhibit mesophase easily. Here, we designed and synthesized a homologues series of compounds according toScheme 1. The heterocyclic benzothiazole ring containing electron-rich sulfur atom was used to induce a smectic phase. Three core unit (one benzothiazole and two phenyl rings) was coupled together in order to enhance the p stacking within the smectic layer structures. An ester linkage that connected two phenyl rings increases the anisotropy polarizability which in turn strengthen the mesomorphic properties.

The synthetic route toward the title compounds, 2-[4-(4-alkyloxybenzoyloxy)phenyl]benzothiazoles (nBPEP, where n = 8, 10, 12, 14, 16) is illustrated in Scheme 1. 2-Aminothiophenol was condensed with 4-hydroxy-benzaldehyde upon refluxing in ethanol for 5 h and yield intermediate A[7]. Ethyl-4-hydroxybenzoate undergone Williamson ether synthesis in acetone with appropriate 1-bromoalkane and followed by acidification by concentrated hydrochloric acid to produce intermediate B[11]. Intermediate A and B were then subjected to Steglich esterification with the appropriate fatty acid in the presence of DCC and DMAP according to previously reported procedure[12]. All the crude products were purified upon repeated recrystallization using ethanol until constant melting points were obtained. Structural elucidation of the title compound was carried out via elemental analysis, FT-IR, NMR and EI-MS spectroscopic techniques[13].

The liquid crystalline textures of the title compounds were observed under polarizing optical microscope (POM) and phase identification was made by comparing the observed textures with those reported in the literature[14,15]. Transition temperatures and corresponding enthalpy changes were determined using a differential scanning calorimeter (DSC) and the data was tabulated inTable 1.

FromTable 1, it can be noted that all synthesized compounds are mesogenic. Nematic phase was observed in all the

compounds, however, only 8BPEP is pure nematogenic compound. SmA phase emerged as monotropic phase (metastable) in 10BPEP. From C12 derivative onwards, SmA phase exists as enantiotropic phase (stable) and persists to C16 derivative.

The occurrence of nematic phases in 8BPEP was evidenced by observation of Schlieren and marble textures of nematic phase. As a representative case of nematic phase, the optical photomicrograph of 8BPEP was illustrated in

Fig. 1(a). Upon cooling the isotropic liquid, the appearance of colorful birefringence domains was noted.

Representative optical photomicrographs of 16BPEP were depicted inFig. 1. By cooling the isotropic liquid phase, Schlieren texture showing a network of black brushed connecting centers of point and line defects,Fig. 1(b), was observed. On further cooling the nematic phase, more ordered SmA phase was observed at lower temperature,

Fig. 1(c). The co-existence of the homogeneous (fan-shaped texture) and homeotropic texture (dark area) was

observed for the presence of SmA. The appearance of the homeotropic area is a diagnostic feature of SmA phase[16].

T.M. Koh et al. / Chinese Chemical Letters 22 (2011) 619–622 620

[()TD$FIG]

NH2 SH

+

OH O H nBPEP N S OH O H5C2O OH O H5C2O OCnH2n+1 O O H OCnH2n+1 S N O O OCnH2n+1

+

[A] [B] [A] [B] (i) (ii) (iii) (iv)

Scheme 1. Synthetic route for the target compounds. (i) EtOH, reflux 6 h, (ii) CnH2n+1Br, K2CO3, CH3COCH3, reflux 5 h, (iii) H2O:EtOH (1:1)

reflux 5 h, KOH, conc. HCl, (iv) DCC, DMAP, DCM, DMF, reflux 5 h. Yield of 8BPEP (48%), 10BPEP (51%), 12BPEP (56%), 14BPEP (66%), and 16BPEP (74%).

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A plot of transition temperatures against the number of carbons in the alkoxy chain during the heating cycle is shown inFig. 2. Based on the plot, both melting (Cr-SmA/N) and clearing (N-I) points showed a descending trend as the length of the carbon chain increased. The flexible terminal alkoxy chain acts as a diluent to the mesogenic core ring system, hence, depressed both melting and clearing temperatures of compounds nBPEP[17]. As can be seen from the graph, the length of alkoxy chain also influenced the types of mesophase formed. All the compounds exhibited enantiotropic nematic phase and smectogenic properties commenced only as the chain length increased. Furthermore, the nematic phase range (DN) is reduced and the SmA phase range (DSmA) is increased as the alkyl chain length

ascended. The increasing van der Waals forces tend to stabilize the SmA phase by favouring the lamellar packing, on the other hand, suppressed the nematic phase range.

In conclusion, all the synthesized compounds exhibited mesomorphic properties whereby nematic phase with high thermal stability exists throughout the whole series and SmA phase emerged from the C10 derivatives onwards. The presence of the ordered smectic structure in the title compounds becomes potential interest in electrical studies for device application.

T.M. Koh et al. / Chinese Chemical Letters 22 (2011) 619–622 621 Table 1

Phase transition and transition enthalpy changes for nBPEP upon heating and cooling. Compound Phase transition, 8C

(corresponding enthalpy changes, kJ mol 1₎

Heating Cooling 8BPEP Cr 137.3 (30.6) N 187.9 (0.6) I Cr 109.2 (33.6) N 184.4 (1.2) I 10BPEP Cr 134.3 (29.0) N 186.2 (0.9) I Cr 100.0 (23.8) SmA 132.3 (0.8) N 183.8 (1.0) I 12BPEP Cr 127.7 (33.3) SmA 142.5 (1.0) N 176.1 (0.9) I Cr 92.3 (27.7) SmA 140.4 (1.2) N 173.7 (1.0) I 14BPEP Cr1103.6 (16.8) Cr2121.4 (31.2) SmA 156.0 (1.3) N 174.5 (1.2) I Cr187.0 (29.9) Cr295.6 (4.23) SmA 154.5 (1.4) N 172.8 (1.5) I 16BPEP Cr 110.3 (49.5.6) SmA 158.8 (1.4) N 168.8 (1.5) I Cr181.7 (37.6) Cr295.5 (6.4) SmA 157.4 (1.6) N 167.4 (1.6) I

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

[()TD$FIG]

Fig. 1. Polarizing optical photomicrographs (100) of (a) nematic phase in 8BPEP, (b) nematic phase in 16BPEP and (c) SmA phase in 16BPEP.

[()TD$FIG]

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Acknowledgments

The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman (UTAR) for the UTAR Research Fund (No. 6200/H02) and the Malaysia Toray Science Foundation (No. 4359/000) for funding this project. T.M. Koh would like to acknowledge UTAR for the award of the research and teaching assistantships.

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[13] Analytical and spectroscopic data for the representative compound 12BPEP: Yield 56%; EI-MS m/z (rel. int.%): 515 (4) [M+], 289 (100); IR (KBr, cm 1: 3053 (C–H aromatic), 2921, 2850 (C–H aliphatic), 1733 (C O ester), 1608 (C N thiazole), 1260 (C–O, aromatic ether);1H NMR (400 MHz, CDCl3): d 0.9 (t, 3H, J = 6.6 Hz, CH3–), 1.3 (m, 16H, CH3–(CH2)8–(CH2)3–O–), 1.5 (p, 2H, J = 7.1 Hz, –CH2–CH2–CH2–O–

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(t, 1H, J = 8.1 Hz, Ar–H), 7.5 (t, 1H, J = 7.3 Hz, Ar–H), 7.9 (d, 1H, J = 7.8 Hz, Ar–H), 8.1 (d, 1H, J = 8.3 Hz, Ar–H), 8.2 (d, 2H, J = 7.1 Hz, Ar–H), 8.2 (d, 2H, J = 7.1 Hz, Ar–H);13C NMR (100 MHz, CDCl3): d 14.10 (CH3–), 22.67, 25.96, 29.07, 29.32, 29.34, 29.54, 29.56, 29.61,

29.64, 31.90 for methylene carbons (CH3–(CH2)10–), 68.35 (–CH2O–), 114.36, 121.11, 121.60, 122.47, 123.21, 125.20, 126.35, 128.73,

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7.23%, N, 2.72%; Found: C, 74.60%, H, 7.18%, N, 2.67%.

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