1. Benzothiazole as structural components in liquid crystals

Transworld Research Network 37/661 (2), Fort P.O. Trivandrum-695 023 Kerala, India

Liquid Crystalline Organic Compounds and Polymers as Materials of the XXI Century: From Synthesis to Applications, 2011: 1-17 ISBN: 978-81-7895-523-0

Editors: Agnieszka Iwan and Ewa Schab-Balcerzak

1. Benzothiazole as structural components in liquid crystals

Sie-Tiong Ha1, Teck-Ming Koh2, Siew-Teng Ong1, Yip-Foo Win1

and Yasodha Sivasothy3 1Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti

Bandar Barat, 31900 Kampar, Perak, Malaysia; 2Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore; 3Chemistry Department, Faculty of

Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia

Abstract. Four new homologous series of liquid crystals comprising a benzothiazole core were prepared and studied. These homologous series are 4-[(1,3-benzothiazol-2-ylimino)methyl]phenyl alkanoates, 4-[(1,3-benzothiazol-2-ylimino)methyl]-3-hydroxyphenyl alkanoates, 4-{[(6-methoxy-1,3-benzothiazol-2-yl)imino]methyl}phenyl alkanoates, and 3-hydroxy-4-{[(6-methoxy-1,3-benzothiazol-2-yl)-imino]methyl}phenyl alkanoates. The members in each series differed in the length of their alkanoyloxy chain. The mesomorphic properties were determined by differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction techniques. The effect of the introduction of the benzothiazole moiety into the molecular structure of the liquid crystals on the mesomorphic properties are discussed and compared with that of other well-known molecular fragments. The influences of the change in the substituents attached to the benzothiazole core on the mesomorphic properties are also reported.

Correspondence/Reprint request: Dr. Sie-Tiong Ha, Department of Chemical Science, Faculty of Science Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia E-mail: [email protected], [email protected]

Sie-Tiong Ha et al. 2

1. Heterocyclic liquid crystals Molecular order in liquid crystal phases depends mainly on the mesogenic core structure, its geometry, polarizability, molecular conformation, length-to-breadth ratio as well as the number and the position of permanent dipole moments in the core [1]. Interest in the study of mesomorphic heterocycles has dramatically increased in the recent years due to their wider range of structural templates as well as their optical and photochemical properties [2]. The significance of the heterocyclic core in determining the properties of liquid crystals have been reported in a series of review papers [3]. Heterocyclic liquid crystals can be synthesized into having high dielectric biaxiality built into the compact core unit which is essential in technological devices. These materials possess great potential application in spatial light modulation, all-optical signal processing, optical information storage, organic thin-film transistors, fast switching ferroelectric materials, fluorescent probes for the detection and analysis of biomolecules [4]. Heterocyclic mesogens are usually incorporated with heteroatoms, such as N, O and S, resulting in a reduced symmetry in the overall molecule as well as the generation of a stronger polar induction. The inclusion of the heteroatom can considerably change the polarity, polarizability and to a certain extent the geometry of a molecule, thus influencing the type of mesophase, the phase transition temperatures, dielectric constants and other properties of the mesogens [5]. Examples of liquid crystals with incorporated heterocyclic rings are pyridine [1], thiophene [6], oxadiazole [7] and benzoxazole [8]. Benzothiazole liquid crystals Although many compounds having a heterocyclic core exhibit mesomorphic properties, mesogenic examples derived from benzothiazole are relatively rare. Therefore, the benzothiazole ring is chosen as the mesogenic core in this study. Conventionally, liquid crystalline organic compounds have been widely used as materials in liquid crystal displays. Recently, a few series of smectic reactive benzothiazole mesogens having non-conjugated diene end groups were reported owing to their potential light-emitting and charge-transporting behavior in organic light-emitting devices (OLEDs) [9]. Besides that, assembling in smectic liquid crystalline phase can also induce the overlapping of aromatic cores, hence facilitating the hopping of charge carriers between the molecules. Owing to this phenomenon, Hanna and

Benzothiazole as structural components in liquid crystals 3

co-workers also studied carrier transport properties of some smectic benzothiazole liquid crystalline derivatives [10-13]. Thus, based on the above mentioned interesting results, this has prompted us to study benzothiazole-based liquid crystals. Hence, we report on four new homologous series of liquid crystals consisting of a benzothiazole core and their liquid crystalline properties were also investigated. 2. Liquid crystalline compounds containing benzothiazole core 4-[(1,3-Benzothiazol-2-ylimino)methyl]phenyl alkanoates, 4-[(1,3-benzo-thiazol-2-ylimino)methyl]-3-hydroxyphenyl alkanoates, 4-{[(6-methoxy-1,3-benzothiazol-2-yl)imino]methyl}phenyl alkanoates, and 3-hydroxy-4-{[(6-methoxy-1,3-benzothiazol-2-yl)imino]methyl}phenyl alkanoates were synthesized and their liquid crystalline behaviors are discussed. The members in each of the series differed in the length of their alkanoyloxy chain. The synthetic protocol mainly involved the condensation between benzothiazole amino and aromatic aldehydes followed by the Steglich esterification of the intermediate compound (Schiff base) with fatty acids. The synthetic scheme is depicted in Scheme 1.

N

SNH2

R1

+C2H5OH

1. DCM, DMF

3. Cn-1H2n-1COOH

R2

OH

O

H

R2

OH

S

NN

R1

R2S

NN

R1

OOCn-1H2n-1

2. DCC, DMAP

Compound R1 R2 n nBSP H H 2 to 6, 8, 10, 12, 14, 16, 18

nBSP-OH H OH 8, 10, 12, 16, 18 CH3O-nBSP OCH3 H 2 to 6, 8, 10, 12, 14, 16, 18

CH3O-nBSP-OH OCH3 OH 2 to 6, 8, 10, 12, 14, 16, 18 Scheme 1. Synthetic routes towards four homologues series of benzothiazole derivatives.


Series nBSP and nBSP-OH

The main structural difference between both series is the presence of the lateral hydroxyl group in nBSP-OH and the absence of it in nBSP. The liquid crystalline behaviour of the compounds was first examined through differential scanning calorimetry. The graph of the phase transition temperatures against the number of carbons in the alkanoyloxy chain of nBSP and nBSP-OH were illustrated in Figures 1a and 1b, respectively.

(a)

90

100

110

120

130

6 8 10 12 14 16 18Number of carbon atoms (n) in alkanoyloxychain

Tran

sitio

n te

mpe

ratu

re o C

Cr-I

(b) Figure 1. Graph of transition temperatures against the number of carbon atoms (n) in the alkanoyloxy chain of (a) series nBSP and series nBSP-OH. Cr, SmA and I denote the crystalline, smectic A, and isotropic phases, respectively.

I


Liquid crystal phase was only appeared in series nBSP (higher members). The presence of the liquid crystal phase was further characterized via optical polarizing microscopy technique. Observation under the polarizing microscope revealed that the higher members (n = 10 to 18) in series nBSP exhibited a SmA phase. Optical photomicrographs of 10BSP and 16BSP are shown in Figure 2. As for series nBSP-OH, no liquid crystal phase was detected. For 10BSP, upon cooling from its isotropic liquid state, the SmA phase emerged as bâtonnet (Figure 2a) and then coalesced to form a fan-shaped focal conic texture. As for compound 16BSP, the co-existence of the fan-shaped and homeotropic (dark region) textures (Figure 2b) were observed. In the homeotropic region, the director of the phase is orthogonal to the layer planes. Consequently, the observed phase is assigned as a SmA phase. Varying geometric anisotropy (ratio between length and breadth of a molecule) by increasing the alkyl chain length is an important factor for the diverse properties among the members. From the graph (Figure 1a), it is clearly noticed that no liquid crystal phase was observed for those short chain derivatives of nBSP (n = 2, 3, 4, 5, 6, 8). In general, a rigid molecular structure is not favoured in generating a liquid crystal phase. However, the liquid crystal phase starts to emerge from the n-decanoyloxy derivative onwards as a monotropic (metastable) SmA phase. Thus, it was confirmed that a certain length of the flexible chain (C10) is a prerequisite in promoting liquid crystal phase formation for this homologous series. The length of the alkanoyloxy chain also plays an important role in determining the stability of the mesophase. As can be seen from the graph, when the alkanoyloxy chain length was increased by two methylene groups, from the C10 to the C12 member, it caused the compound to exhibit an enantiotropic (stable) SmA phase. Instead of showing a SmA phase during the cooling scan alone (monotropic), the SmA phase was observed during both the heating and cooling scans (enantiotropic). Further lengthening of the carbon chain by another two methylene groups stabilized the SmA phase where the SmA phase range increased from 4.8 oC for the C12 member to 8.4 oC for the C14 member. The tetradecanoyloxy chain was found to generate the widest SmA phase range in this series as further lengthening of the chain to the C16 member resulted in the SmA phase range decreasing to 5.0 oC. The decrease of the phase range was noticed as the chain length kept increasing. This was proven by the monotropic SmA phase which was observed in the C18 member. Certain extent of flexibility is essential for promoting liquid crystal phase, however, the continuous increasing of the carbon chain length of a compound will cause the molecule to be too flexible hence reducing the stability of its mesophase (in term of phase range) or even completely diminishing the mesophase [14].


(a) (b)

(c) (d)

(e) (f) Figure 2. Optical photomicographs of benzothiazole liquid crystals obtained from microscopy technique. (a) 10BSP exhibiting smectic A phase, emerging as bâtonnet upon cooling from isotropic liquid at 69 oC; (b) 16BSP exhibiting smectic A phase with fan-shaped and homeotropic textures at 74 oC; (c) CH3O-10BSP exhibiting nematic phase with disclination lines at 89 oC; (d) CH3O-10BSP exhibiting transition from nematic phase (top right corner) to SmC (bottom left) phase upon further cooling at 55 oC; (e) CH3O-16BSP-OH exhibiting nematic droplets at 122 oC; (f) CH3O-16BSP-OH exhibiting nematic phase with marble texture at 97 oC.


Series CH3O-nBSP and CH3O-nBSP-OH In order to vary the mesomorphic behaviours of the previous series, a terminal methoxyl group was introduced into the molecular fragment. Here, two homologues series of benzothiazole derivatives comprising a terminal 6-methoxyl substituent were synthesized. A plot of the transition temperatures against the number of carbons in the alkanoyloxy chain for series, CH3O-nBSP and CH3O-nBSP-OH, are given in Figures 3a and 3b, respectively.

(a)

(b)

Figure 3. Graph of transition temperatures against the number of carbon atoms (n) in the alkanoyloxy chain of (a) series CH3O-nBSP and (b) series CH3O-nBSP-OH. Cr, SmC, N and I denote the crystalline, smectic C, nematic and isotropic phases, respectively.


In the case of series nBSP and CH3O-nBSP, liquid crystal phase was observed beginning from the shorter chain derivatives (n ≥ 4) in CH3O-nBSP as compared to series nBSP (whereby the liquid crystal phase was only observed for the longer chain derivatives, n ≥ 10). As for series nBSP-OH and CH3O-nBSP-OH, the effect of introducing a terminal methoxyl group was rather obvious. In the previous section, it has been mentioned that series nBSP-OH was non-mesogenic. However, liquid crystal phase was successfully generated in CH3O-nBSP-OH by incorporating a methoxyl group into the parent molecular fragment. For series CH3O-nBSP, short chain members (n = 2 and 3) were non-mesogenic compounds while the medium chain members (n = 4 to 8) exhibited a nematic phase. Both nematic and SmC phases were observed for the long chain members (n = 10, 12, 14, 16, 18). For series CH3O-nBSP-OH, no liquid crystal phase was observed for the C2 and C3 members, however, a nematic phase was observed in the remaining members of this series. Optical photomicrographs of CH3O-10BSP taken during the cooling cycle are depicted in Figures 2c-d. Meanwhile, optical photomicrographs of CH3O-16BSP-OH showing nematic phase with marble texture are given in Figures 2e-f. According to Figure 3a, the odd-even effect on the mesomorphic properties of CH3O-nBSP was not obvious in this series. The short chain derivatives (n = 2 and 3) were non-mesogenic compounds owing to the excessive rigidity in their molecules thus unfavouring mesophase formation. As for the C4, C5, C6, C7 and C8 members, their terminal chain lengths are long enough for promoting mesophase (nematic phase) formation. As the alkyl chain length increased by another two methylene groups to the C10 member, a monotropic (metastable) SmC phase was observed. Similarly, once the carbon chain reaches a certain length (n ≥ 12), enantiotropic (stable) SmC phase which accompanied by nematic phase at a higher temperature was induced. The melting points exhibited a descending trend from the C2 to the C12 member, and then increased from the C12 to the C18 members. As for the clearing temperature, it demonstrated a descending trend owing to the mesogenic core dilution resulting from the flexibility provided by the increased terminal alkanoyloxy chain [15]. On the other hand, the nematic phase range (ΔN) apparently decreased as the alkyl chain length increased. This resulted from the long carbon chain being attracted and intertwined which in turn facilitated the lamellar packing causing a decrease in the nematic phase range. However, by increasing the length of the carbon chain, the SmC phase range (ΔSmC) did not exhibit the usual trend whereby ΔSmC should increase. The ΔSmC was found to be 7.6 oC for the C12 member and it decreased to 0.9 oC


for the C14 member, and increased again to 4.6 oC and 9.2 oC for the C16 and C18 members, respectively. The increase in the Van der Waals forces led to the increase of the melting temperature from the C12 to the C18 members which in turn caused an unusual smectogenic phase range in CH3O-nBSP. Nevertheless, it still can be deduced that the smectic phase stability was enhanced where the SmC-to-N transition temperature increased from the C10 to the C18 derivative. Accordingly (Figure 3b), the melting temperature (TCr-N) of CH3O-nSBP-OH was considerably reduced with ascending chain length owing to the increase in its structural flexibility. On the other hand, the clearing temperatures (TN-I) descended as the number of carbon atoms increased, resulting from the dilution of the core system. A noticeable reduction in the nematic phase range was observed as the alkyl chain length increased. The nematic phase was generally exhibited by compounds possessing short to medium length chains. As the chain length increased, the nematogenic properties decreased and were accompanied by reduced nematic phase stability, therefore leading to a decrease in the phase range. The nematic phase will diminish if the length of the chain kept increasing. 3. XRD studies of benzothiazole liquid crystals The presence of SmA phase in series nBSP and CH3O-nBSP was further studied through power X-ray diffraction (XRD) analysis. The XRD pattern of the representative compounds 14BSP and CH3O-12BSP are shown in Figure 4 while their XRD data are given in Table 1.

Table 1. Power X-ray diffraction data of 14BSP and CH3O-12BSP.

14BSP CH3O-12BSP 2 theta (°) 2.35 2.48 d-spacing 35.52 Ǻ 30.81 Ǻ

L 31.57 Ǻ 31.48 Ǻ d/L 1.13 0.98

Phase SmA SmC In Figure 4a, the XRD patterns of 14BSP showed a sharp diffraction peak at 2.35 o, implying the formation of a layered structure which is typically characteristic of a layer structure observed for a smectic phase [16]. Generally, a sharp and strong peak at a low angle (1o < 2θ < 6o) in a small angle X-ray scattering curve is observed for smectic structures, but not seen in nematic and cholesteric structures.


It is common that when the reflection within the SmA layer corresponds to d~L (d is layer spacing and L is molecular length), the smectic A layer is called the monolayer [17]. However, when the d-layer spacing is an intermediate between L and 2L, then it is called a partial bilayer phase. The d-layer spacing upon cooling of 14BSP from its isotropic liquid is 35.52 Ǻ whereas the molecular length (L) obtained by MM2 molecular calculation is 31.57 Ǻ. This provides the evidence for a partial bilayer structure of the smectic layers (L < d < 2L). The d/L ratio of 14BSP (1.13) falls within the range between 1.12 to 1.20 for partial bilayer arrangement [18]. The smectic phases of compounds CH3O-nBSP were further studied by temperature-dependent XRD analysis to confirm their SmC nature as well as 11

(a)

(b)

Figure 4. XRD diffractograms of (a) 14BSP and (a) CH3O-12BSP.


to determine the layer spacing at the operating time upon cooling. In Figure 4b, the presence of a sharp diffraction peak at a low angle (2.48o) confirmed that the layer periodicity is present in the mesophase structure whilst the broadly diffuse signal at the wide angle region indicated the short range order typical of the nematic phase [19]. However, for a nematic phase, no peak appears at a small angle and a broad peak at 2θ ≈ 20o can be observed in the XRD diffractogram. Upon combining the results from the polarized optical microscopy and XRD analysis, the presence of the SmA and SmC phases in both the series can been confirmed. 4. Structure-liquid crystal property relationships Here, comparisons between the title compounds with other structurally related compounds are presented and the structure-property relationships were discussed. The phase transition temperatures of benzothiazole derivatives and reference compounds are presented in Tables 2-5, where Cr, SmA, SmC, N, I, denote the crystalline, smectic A, smectic C, nematic and isotropic phases, respectively. Values given in parentheses refer to monotropic phase transitions.

Table 2. Effect of linking group on mesomorphic properties.

Compounds Structures and transition temperatures (oC)

12BSP N

S

N

O

O

C11H23

Cr 80.8 SmA 85.6 I

A [20]

N

S

OC12H25

Cr 86.9 (SmA 81.2) I

CH3O-12BSP N

S

N

O

O

C11H23

H3CO

Cr 65.6 SmC 73.1 N 114.3 I

B [21] OC12H25N

S

N

NH3CO

Cr 113.0 (SmA 108.0) N 135.0 I


Table 3. Effect of lateral hydroxyl group on mesomorphic properties.

Compounds

Average mesophase range (oC)

Average thermal stability (oC)

N SmA SmC TM TC

N

S

N

O

O

R

nBSP

- 6.0 - 97.1 89.5

N

S

N

O

O

R

OH nBSP-OH

- - - - 86.7

N

S

N

O

O

R

H3CO

CH3O-nBSP

35.0 - 5.6 89.0 119.3

N

S

N

O

O

R

OH

H3CO

CH3O-nBSP-OH

11.1 - - 136.8 142.2

Effects of linking group on mesomorphic properties A summary of the structures and transition temperatures of compounds 12BSP, A, CH3O-12BSP and B are tabulated in Table 2. Upon comparison between the structures of 12BSP and compound A, differences are in the Schiff base linkage between the mesogenic cores and the ester linkage in the terminal chain. The stepped core structure in 12BSP caused by the Schiff base linkage resulted in a more stable SmA phase whereby an enantiotropic SmA was observed in 12BSP while monotropic SmA was found in compound A. Another example can be seen from the comparison between CH3O-12BSP and compound B. Although both compounds have the stepped core structure due to either the Schiff base or azo linkage, an enantiotropic smectic phase was only observed in CH3O-12BSP. This was probably due to the presence of the ester linkage in the terminal chain. The existence of the ester linkage instead of the ether linkage increases the length of the core unit which in turn favors the lamellar packing in the liquid crystal phase, hence stabilizing the smectic phase.


Table 4. Effect of terminal substituent group on mesomorphic properties.

N

S

N

O

O

C11H23

12BSP

N

S

N

OH3CO

O

C11H23

CH3O-12BSP

N

S

N

N OC12H25Cl

compound C

N

S

N

N OC12H25O2N

compound D

Compounds Transition temperatures (oC) Mesophase range (oC)

Sm N

12BSP Cr 80.8 SmA 85.6 I 4.8 -

CH3O-12BSP Cr 65.6 SmC 73.1 N 114.3 I 7.5 41.2

C [22] Cr 80 SmA 172 I 38.0 -

D [23] Cr 156 SmA 194 I 92.0 - Effects of lateral hydroxy group on mesomorphic properties In order to understand the effect of the lateral hydroxyl group on the mesomorphic properties, the molecular structures and transition temperatures of series nBSP, nBSP-OH, CH3O-nBSP and CH3O-nBSP-OH are listed in Table 3. It can be noticed that series nBSP exhibited a SmA phase while series nBSP-OH is a non-mesogenic series. The presence of the lateral hydroxy group in series nBSP-OH prohibited the formation of mesophase by increasing the molecular broadness. It is known that the length-to-breadth ration is crucial in generating a liquid crystal phase. The presence of the lateral hydroxyl group has broadened the molecule, hence decreasing the molecular length-to-breadth ratio.


Table 5. Effect of mesogenic core on mesomorphic properties.

Compounds Structures and transition temperatures (oC)

14BSP N

SN

O

O

C13H27

Cr 82.3 SmA 90.7 I

E [24] N

OC14H29

Cr 97 I

CH3O-16BSP-OH N

SN

O

O

C15H31

H3CO

OH Cr 123.2 N 129.7 I

F [25]

H3CO N

O

O

C15H31

OH Cr 98 SmA 111 I

Sometimes, the presence of a lateral hydroxyl group may also inhibit the formation of smectic phase while favoring the formation of other phases. By comparing series CH3O-nBSP and CH3O-nBSP-OH, we found that the analogues bearing the lateral hydroxy group exhibited only a nematic phase. A broader core unit (CH3O-nBSP-OH) is more favourable in nematic phase formation. Compounds with a lateral hydroxyl group normally exhibited higher melting and clearing temperatures. From the comparison between series CH3O-nBSP and CH3O-nBSP-OH, it can be seen that both melting (TM) and clearing (TC) temperatures are higher in series CH3O-nBSP-OH. The Schiff base linkage may form intramolecular hydrogen bonding with the o-hydroxyl group, hence, increases the melting and clearing temperatures [26]. Effects of terminal substituent group on mesomorphic properties A terminal group can be introduced into the molecules to alter their mesomorphic properties. A summary of the molecular structures and transition temperatures of compound 12BSP, CH3O-12BSP, C and D are tabulated in Table 4.


The unsubstituted benzothiazole derivative, 12BSP, possessed the shortest mesophase range (4.8 oC) among these compounds. The introduction of a polar methoxyl terminal group into the benzothiazole derivative (CH3O-12BSP), increased the mesophase range drastically from 4.8 oC to 48.7 oC (total for smectic and nematic phases). Incorporating a smaller and more polar chlorine atom into compound C further increased the melting (80 oC) and clearing (172 oC) temperatures of the compound to a certain extent. The highest melting and clearing temperatures were observed for the nitro-substituted benzothiazole derivative (compound D), exhibiting 156 oC and 194 oC as melting and clearing temperatures, respectively. Effects of mesogenic core on mesomorphic properties Table 5 summarizes the molecular structures and transition temperatures of compounds 14BSP, E, CH3O-16BSP-OH and F. Comparison between these compounds enables us to understand the effects of the benzothiazole core on their mesomorphic properties. According to Table 5, 14BSP, bearing a benzothiazole core unit, exhibits an enantiotropic SmA phase while compound E with a naphthalene core is a non-mesogenic compound. The benzothiazole ring structure, incorporated with electronegative heteroatoms N and S, results in a reduced symmetry in the overall molecule, thus generating a stronger polarity. The induction of polarity by the N and S atoms on these heterocyclic cores may be responsible for the formation and enhancement of mesophases [3]. The comparison between CH3O-16BSP-OH and compound F has revealed that the benzothiazole core was also responsible in the formation of the nematic phase. It can be seen that CH3O-16BSP-OH exhibited only a nematic phase whereas compound F exhibited a SmA phase. Compared to the single benzene ring, the fused-ring structure of the benzothiazole core enhanced the molecular polarizability in turn increasing the intermolecular cohesive forces which induced the formation of the nematic phase [23]. Besides, the melting and clearing temperatures of CH3O-16BSP-OH were found to be slightly higher than that of compound F. As a result of its enhanced polarizability and increased intermolecular cohesive forces, the thermal stability of CH3O-16BSP-OH was increased, resulting in higher melting and clearing temperatures. 4. Summary Four homologues series of compounds were successfully synthesized. Only one series did not exhibit liquid crystal phase. The liquid crystalline


properties of the remaining three series were thoroughly investigated where by nematic, SmA and SmC phases were observed in those compounds. Through a series of comparisons, structure properties relationship was established in which the effects of the linking group, lateral hydroxyl group, terminal group and mesogenic core were discussed. The information presented here may lead to a better understanding of the structure-properties relationship, making it possible to fine tune the desired properties of the compounds. Acknowledgements The authors would like to thank the Universiti Tunku Abdul Rahman, Malaysia Toray Science Foundation, MOHE and MOSTI for the financial supports. References 1. Collings, P.J., Hird, M. 1998, Introduction to Liquid Crystals: Chemistry and

Physics, Taylor & Francis Ltd., London, UK; Khoo, I.C. 2007, Liquid Crystals, John Wiley & Sons Inc., New Jersey, USA.

2. Srividhya, D., Manjunathan, S., Thirumaran, S. 2009, E-Journal of Chem., 6, 928; Srividhya, D., Manjunathan, S., Thirumaran, S., Saravanan, C., Senthil, S. 2009, J. Mol. Struct., 2009, 927, 7; Tsai, H.-H. G., Chou, L.C., Lin, S.C., Sheu, H.S., Lai, C.K. 2009, Tetrahedron Lett., 50, 1906; Huang, R.T., Wang, W.C., Yang, R.Y., Lu, J.T., Lin, I.J. 2009, Dalton Trans., 35, 7121; Kozhevnikov, V.N., Cowling, S.J., Karadakov, P.B. , Bruce, D.W. 2008, J. Mater. Chem., 18, 1703; He, C.F., Richards, G., Kelly, S., Contoret, A., O’Neill, M. 2007, Liq. Cryst., 34, 1249; Kozhevnikov, V.N., Whitwood, A.C., Bruce, D.W. 2007, Chem. Commun., 3826; Fan, R., Malliaras, G., Sukhomlinova, L., Gu, S., Twieg, R.J. 2000, American Physical Society, Annual March Meeting, Minneapolis, abstract #G21.009;

3. Petrov, V.F. 2006, Mol. Cryst. Liq. Cryst., 457, 121; Petrov, V.F. 2005, Mol. Cryst. Liq. Cryst., 442, 51; Petrov, V.F., Pavluchenko, A.I., 2003, Mol. Cryst. Liq. Cryst., 393, 1; Petrov, V.F., Pavluchenko, A.I., 2003, Mol. Cryst. Liq. Cryst., 393, 15; Petrov, V.F. 2001, Liq. Cryst., 28, 217; Bezborodov, V.S., Petrov, V.F., Lapanik, V.I. 1996, Mol. Cryst. Liq. Cryst., 20, 785; Titov, V.V., Pavlyuchenko, A.I. 1980, Chemistry of Heterocyclic Compounds, 16, 1.

4. Seed, A. 2007, Chem. Soc. Rev. 36, 2046; Lai, C.K., Liu, H.C., Li, F.J., Cheng, K.L., Sheu, H.S. 2005, Liq. Cryst. 32, 85.

5. Zhang, B.Y., Jia, Y.G., Yao, D.S., Dong, X.W. 2004, Liq. Cryst., 31, 339. 6. Majumdar, K.C., Mondal, S., Ghosh, T., 2010, Mol. Cryst. Liq. Cryst., 524, 17;

Gipson, R.M., Sampson, P., Seed, A.J., 2010, Liq, Cryst., 37, 101; Hird, M., Toyne, K.J., Goodby, J.W., Gray, G.W., Minter,V., Tuffin, R.P., McDonnell


D.G. 2004, J. Mater. Chem., 14, 1731; Eichhorn, S.H., Paraskos, A.J., Kishikawa, K., Swager, T.M., 2002, J. Am. Chem. Soc., 124, 12742; Campbell, N.L., Duffy, W.L., Thomas, G.I., Wild, J.H., Kelly, S.M., Bartle, K., O'Neill, M., Minter, V., Tuffin, R.P. 2002, J. Mater. Chem., 12, 2706.

7. Prajapati, A.K., Modi, V. 2010, Liq. Cryst., 37, 407; Han, J., Zhang, Y.F., Wang, J.Y., 2010, Key Eng. Mater., 428, 52; Parra, M.L., Elgueta, E.Y., Jimenez, V., Hidalgo, P.I., 2009, Liq. Cryst., 36, 301; Zhang, P., Bai, B., Wang, H., Qu, S., Yu, Z., Ran, X., Li, M., 2009, Liq. Cryst., 36, 7; Parra, M.L., Hidalgo, P.I., Elgueta, E.Y., 2008, Liq. Cryst., 35, 823; Sparavigna, A., Mello, A., Montrucchio, B., 2008, Phase Trans., 81, 471; Karamysheva, L.A., S.I. Torgova, Agafonova, I.F., Petrov, V.F. 2000, Liq. Cryst. 27, 393.

8. Liao, C.C., Wang, C.S., Sheu, H.S., Lai, C.K., 2008, Liq. Cryst., 64, 7977; Wang, H.C., Wang, Y.J., Hu, H.M., Lee, G.H., Lai, C.K., 2008, Tetrahedron, 64, 4939; Lai, C.K., Liu, H.C., Li, F.J., Cheng, K.L., Sheu, H.S. 2005, Liq. Cryst., 32, 85; Wang, C.S., Wang, I.W., Cheng, K.L., Lai, C.K., 2006, Tetrahedron, 62, 9383.

9. Aldred, M.P., Vlachos, P., Dong, D., Kitney, S.P., Tsoi, W.C., O’Neill, M., Kelly, S.M. 2005, Liq. Cryst., 32, 951.

10. Funahashi, M., Hanna, J. 1997 Phys. Rev. Lett., 78, 2184. 11. Tokunaga, K., Takayashiki, Y., Iino, H., Hanna, J. 2009, Phys. Rev. B, 033201, 1. 12. Tokunaga, K., Iino, H., Hanna, J. 2009, Mol. Cryst. Liq. Cryst., 510, 241. 13. Tokunaga, K., Takayashiki, Y., Iino, H., Hanna, J. 2009, Mol. Cryst. Liq. Cryst.,

510, 250. 14. Koden, M., Yagyu, T., Takenaka, S., Kusabayashi, S. 1983, J. Phys. Chem., 87,

4730. 15. Berdague, P., Bayle, J.P., Ho, M.S., Fung, B.M. 1993, Liq. Cryst., 14, 667. 16. Wang, Y., Zhang, B.Y., He, X.Z. 2007, Colloid Polym. Sci., 285, 1077. 17. Liao, C.C., Wang, C.S., Sheu, H.S., Lai, C.K. 2008, Tetrahedron, 64, 7977. 18. Reddy, R.A, Sadashiva, B.K. 2004, J. Mater. Chem., 14, 310. 19. Pal, S.K., Raghunathan, V.A., Kumar, S. 2007, Liq. Cryst., 34, 135. 20. Ha, S.T., Koh, T.M., Yeap, G.Y., Lin, H.C., Boey, P.L., Win, Y.F., Ong, S.T.,

Ong, L.K. 2009, Mol. Cryst. Liq.Cryst., 506, 56. 21. Prajapati, A.K., Bonde, N.L. 2006, J. Chem. Sci., 118, 203. 22. Prajapati, A.K., Bonde, N.L. 2005, Proc. of 11th National Conference on Liquid

Crystals, Allahabad, 114. 23. Prajapati, A.K., Bonde, N.L. 2009, Mol. Cryst. Liq.Cryst., 501, 72. 24. Vora, R.A., Prajapati, A.K. 1998, Liq. Cryst., 25, 567. 25. Yeap, G.Y., Ha, S.T., Boey, P.L., Mahmood, W.A.K., Ito, M.M., Youhei, Y.

2006, Mol. Cryst. Liq.Cryst., 452, 73. 26. Yeap, G.Y., Ha, S.T., Ishizawa, N., Suda, K., Boey, P.L., Mahmood, W.A.K.

2003, J. Mol. Struct., 658, 87.

Documents

1. Benzothiazole as structural components in liquid crystals