linkages with a biphenyl moiety compounds involving...

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Studies of Calamitic Liquid CrystallineCompounds Involving Ester-Azo CentralLinkages with a Biphenyl MoietyB. T. Thaker a , Y. T. Dhimmar a , B. S. Patel a , D. B. Solanki a , N. B.Patel a , N. J. Chothani a & J. B. Kanojiya aa Department of Chemistry, Veer Narmad South Gujarat University,Surat, Gujarat, India

Available online: 07 Oct 2011

To cite this article: B. T. Thaker, Y. T. Dhimmar, B. S. Patel, D. B. Solanki, N. B. Patel, N. J. Chothani& J. B. Kanojiya (2011): Studies of Calamitic Liquid Crystalline Compounds Involving Ester-Azo CentralLinkages with a Biphenyl Moiety, Molecular Crystals and Liquid Crystals, 548:1, 172-191

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Mol. Cryst. Liq. Cryst., Vol. 548: pp. 172–191, 2011Copyright © Taylor & Francis Group, LLCISSN: 1542-1406 print/1563-5287 onlineDOI: 10.1080/15421406.2011.591677

Studies of Calamitic Liquid CrystallineCompounds Involving Ester-Azo Central Linkages

with a Biphenyl Moiety

B. T. THAKER,∗ Y. T. DHIMMAR, B. S. PATEL,D. B. SOLANKI, N. B. PATEL, N. J. CHOTHANI,AND J. B. KANOJIYA

Department of Chemistry, Veer Narmad South Gujarat University, Surat,Gujarat, India

Two mesogenic homologous series involving ester-azo central linkages with a biphenylmoiety have been synthesized, such as 4′-[(4-n-alkoxyphenyl)diazenyl]biphenyl-4-ol(series I) and 4′-[(4-n-alkoxyphenyl) diazenyl]-4-butoxy phenyl biphenyl-4-carboxylate(series II). Azobiphenyl of series I having a free hydroxyl group with strong hydrogenbonding exhibits a high-temperature enantiotropic smectic phase. Whereas in series II,compounds containing C1–C8 carbon atoms exhibit only a monotropic smectic phaseand compounds with C10, C12, C14, and C16 atoms show an enantiotropic smecticphase. These compounds were characterized by elemental analysis, FT-IR, 1H-NMR,and mass spectral studies. The phase transition and mesogenicity of these substanceswere studied by polarizing optical microscopic and differential scanning calorimetrictechniques. Their thermal stabilities and other characteristics are discussed.

Keywords Biphenyl; ester-azo; nematic and mesophase; smectic

Introduction

While designing new liquid crystal molecules with definite properties, it is necessary tokeep in mind that their mesogenic behavior is strongly influenced by the structure of therigid molecular core and by the lateral substitution on the aromatic rings, with the positionof the substituted ring in the molecular core being important [1]. Liquid crystal oligomersconsist of molecules containing two or more mesogenic units interconnected via flexiblespacers, most commonly alkyl chains [2–5]. There is substantial literature on the studies ofbiphenyl and its derivatives. 2,3,4-monosubstituted biphenyls have been studied extensivelyby various workers [6–13] for their molecular geometry, crystallization behavior, crystalpacking, and thermal motion, while the literature on growth and structural aspects of linearlychained biphenyls (liquid crystals) is quite insufficient.

Biphenyl esters are typical mesogens with various mesophases having different de-grees of order according to substituents [14]. There are many examples of rigid, ex-tended chemical subunits in mesogens. The most common subunit used in synthesizing

∗Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad South GujaratUniversity, Surat, Gujarat, India+91-261-226-7957. E-mail: [email protected]

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Liquid Crystals with Ester-Azo Linking Groups 173

Scheme 1. Series I: synthesis of 4-n-alkoxy anilines.

calamitic liquid crystals is the linearly para-substituted phenyl ring [15]. Functionalizedazobenzenes were among the first successful nematic liquid crystals used in the dis-play industry [16–20], which constitute an important class of materials for informationprocessing and storage and are being explored as molecular photoswitches. Recently,Michael Hird et al. studied the mesomorphic properties of ortho difluoroterphenyls witha bulky terminal chain [21]. Johnson et al. [22] have synthesized a homologous series of

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174 B. T. Thaker et al.

Scheme 2. Series II: synthesis of 4-n-alkoxy benzoyl chlorides.

4-(4-alkylphenylazo)phenols. An attempt has been made to synthesize two series of com-pounds by using 4-hydroxy biphenyl instead of phenol.

Experimental Methods

Reagents and Techniques

4-hydroxy benzoic acid, alkyl bromide (Lancaster, England), and 4-hydroxy biphenyl wereused without further purification. Acetone, ethanol, methanol, hydrochloric acid (HCl),

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Figure 1. FT-IR spectra of compound A10 (series I).

KOH, NaNO2, NaOH, and thionyl chloride were supplied by Polypharm Mumbai, India;certain solvents and reagents were used after distillation and purification by using the stan-dard methods described in the literature [23]. Other auxiliary chemicals were of laboratorygrade. Elemental analyses (C, H, N) were performed at the Central Drug Research Institute(CDRI), Lucknow, India. Infrared spectra were recorded by a Perkin-Elmer 2000 FT-IRspectrophotometer in the frequency range 4000–400 cm−1 with samples embedded in KBr

Figure 2. FT-IR spectra of compound A14 (series I).

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Figure 3. FT-IR spectra of compound B10 (series II).

discs. 1H-NMR spectra of the compounds were recorded by a Jeol-GSX-400 instrumentusing CDCl3 as a solvent and tetramethylsilane as an internal reference at SAIF, PanjabUniversity, Chandigarh, India. Also, mass spectra of the compounds were recorded at SAIF.Thin layer chromatography (TLC) analyses were performed using aluminum-backed silica-gel plates (Merck60 F524) and examined under shortwave UV light. The phase transitiontemperatures were measured using Shimadzu DSC-50 at heating and cooling rates of 10◦Cmin−1, respectively. The textures of the mesophase were studied using a Leitz Labourluxpolarizing microscope provided with a Kofter heating stage at the Applied ChemistryDepartment, M. S. University of Baroda, Vadodara, Gujarat, India.

Figure 4. FT-IR spectra of compound B14 (series II).

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Figure 5. 1H-NMR spectra of compound A10 (series I).

OHRO N

N

4′-[(4-n-alkoxyphenyl)diazenyl] biphenyl-4-ol (where R = C10H21 series 1)

Synthesis

Series I compounds synthesized as per Scheme 1

Synthesis of 4-n-Alkoxy Anilines. 4-n-Alkoxy Acetanilides. Paracetamol (0.1 mol), anhy-drous potassium carbonate (0.15 mol), respectively, n-alkyl bromide (0.15 mol), and dryacetone (60 ml) were taken in a round-bottom flask (RBF) provided with a condenser anda guard tube. The reaction mixture was refluxed in a water bath for 8–10 hr. The wholemass was then added to water and extracted with ether. The ether was evaporated and theresidual solids were obtained as alkoxy acetanilides.

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Figure 6. 1H-NMR spectra of compound A14 (series I).

OHRO N

N

4′-[(4-n-alkoxyphenyl)diazenyl] biphenyl-4-ol (where R = C14H29 series I)

4-n-Alkoxy Anilines. A mixture of 4-n-alkoxy acetanilide (0.146 mol), water (70 ml),and concentrated HCl (45 ml) was stirred for 10–12 hr at 90◦C–95◦C and then cooledto room temperature. The mixture was made alkaline with 50% NaOH at 20◦C. The oilyproduct (for the lower members C1–C8) was extracted with ether. The ether extract wasdried and concentrated at reduced pressure to give oil, which was purified by distillation.The higher members (C10–C16) were separated as solid and filtered directly without etherextraction. The boiling points and melting points of all the alkoxy anilines agree well withthe values reported in the literature [24–26].

Diazotization of Alkoxy Aniline [27]. Alkoxy aniline (0.005 mol, 1.15 g) was taken in50 ml of water in a beaker. It was then cooled to 0◦C–5◦C with ice in an ice bath. Later on,concentrated HCl (0.03 mol, 3.6 ml) was added and the reaction mixture was stirred for 1hr. A solution of NaNO2 (0.005 mol, 0.35 g) in water (5 ml) previously cooled to 0◦C wasthen added over a period of 5 min with stirring. The solution was further stirred for another1 hr. At this stage, Congo red paper turns blue and starch iodide paper also turns blue. Itshowed the positive test (i.e., the presence of nitrous acid). Then, sulfamic acid was addedto remove excess of nitrous acid. At this stage, Congo red paper turns blue (positive test)and starch iodide paper had no effect (negative test). The diazonium salt was obtained as aclear solution, which was used for subsequent coupling reaction.

Synthesis of 4′-[(4-n-Alkoxyphenyl)Diazenyl]Biphenyl-4-ol. To a well-stirred solution of4-hydroxy biphenyl (0.005 mol, 0.85 g) in water (50 ml) was added a diazonium chloride

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Figure 7. 1H-NMR spectra of compound B10 (series II).

C

O

O OC4H9

RO NN

4′-[(4-n-alkoxyphenyl)diazenyl]4-butoxy phenyl biphenyl-4-carboxylate (where R = C10H21 seriesII).

of alkoxy aniline gradually for 1 hr at 0◦C–5◦C. The pH (7.0) was maintained by theaddition of NaOH solution (10% w/v). The mixture was stirred for another 3–4 hr forcomplete separation and the dye was isolated by filtration, washed with water, dried, andcrystallized from ethyl acetate to get orange-colored crystals. The yield was about 75%. Allthe compounds had been purified by column chromatography on silica gel (80–120 mesh)using a mixture of ethyl acetate/petroleum ether (7/3) as an eluent.

Data. Compound A10 (Series I). Yield: 73%, MP 119◦C; elemental analysis for calculatedC 78.10%, H 7.96%, N 6.51%; found C 78.44%, H 8.20%, N 6.02% for C28H34N2O2; FT-IR(KBr pallete) 3061 cm−1 (–C–H aromatic stretching), 2850–2920 cm−1 (–CH3 aliphaticstretching), 1598 cm−1 (–N N–), 1418–1481 cm−1 (–CH bending of CH2), 1367–1372cm−1 (–C–H bending of CH3), 817–758 cm−1 (–CH bending out of plane), 723 cm−1 (–CH2

rocking); 1H-NMR (CDCl3): 0.90 ppm (t, 3H, CH3 of aliphatic chain), 1.28–1.85 ppm (m,16H, –CH2 of alkyl chain), 3.86–3.89 ppm (d, 2H, –OCH2 of alkoxy chain), 6.09 ppm (s,phenolic –OH free), 6.81–7.29 ppm (m, 12H, Ar–H); mass (GC-MS): molecular weight ofcompound 486 g/mol and molecular ion peak as (M)+ at 485.4 (m/z).

Series II compounds synthesized as per Scheme II.

Synthesis of 4-n-Alkoxy Anilines. 4-n-Alkoxy anilines were prepared as described inSeries I.

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Figure 8. 1H-NMR spectra of compound B14 (series II).

C

O

O OC4H9

RO NN

4′-[(4-n-alkoxyphenyl)diazenyl]4-butoxy phenyl biphenyl-4-carboxylate (where R = C14H29

series II).

Diazotization of Alkoxy Aniline. Diazotization of alkoxy aniline was carried out as de-scribed in Series I.

Synthesis of 4′-[(4-n-Alkoxyphenyl)Diazenyl]Biphenyl-4-ol. Synthesis of 4′-[(4-n-alkoxyphenyl)diazenyl]biphenyl-4-ol was carried out as per the procedure described inSeries I.

Preparation of 4-n-Alkoxy Benzoic Acid [28,29]. 4-hydroxy benzoic acid (0.1 mol, 13.8 g),corresponding n-alkyl bromide (0.12 mol, 23.20 ml), and KOH (0.25 mol, 14.0 g) weredissolved in methanol (lower member)/ethanol (higher member) (100 ml) in an RBF fittedwith a reflux condenser and refluxed the mixture in a water bath for 8 hr. Then 10% aqueousKOH solution (25.0 ml) was added to the flask and reflux continued for 2–3 hr to hydrolyzeany ester if formed. The solution was cooled to room temperature, and the reaction mixturewas acidified by pouring 1:1 ice-cooled dilute HCl and water; the precipitated mass wasfiltered and washed by water. Then, the isolated mass was dried in a vacuum oven. Thealkoxy acids were crystallized in methanol until a constant transition temperature wasobtained. The transition temperature thus obtained is in good agreement with the valuesreported in the literature.

Preparation of 4-n-Alkoxy Benzoyl Chlorides [30]. 4-n-alkoxy benzoic acid (0.01 mol, 2.5g) and freshly distilled thionyl chloride (0.03 mol, 2.19 ml) were taken in an RBF attached

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Figure 9. Mass spectra of compound A14 (series I).

to a reflux condenser fitted with a calcium chloride guard tube. The mixture was refluxedin a water bath till the evolution of HCl gas ceased. Excess of thionyl chloride was distilledoff under pressure using a vacuum pump and the 4-n-alkoxy benzoyl chloride left behindwas directly treated for the next reaction without further purification.

Synthesis of 4′-[(4-n-alkoxyphenyl)Diazenyl]4-Butoxy Phenyl Biphenyl Carboxylate. 4′-[(4-n-alkoxyphenyl)diazenyl]biphenyl-4-ol (0.002 mol, 0.86 g) was dissolved in dry pyri-dine (10 ml) and was added dropwise with occasionally stirring in to ice-cold 4-butoxybenzoyl chloride (0.002 mol, 0.39 g) in an RFB. Then the mixture was refluxed in a hotwater bath for 2 hr and was allowed to stand for overnight. The mixture was acidified usingcold 1:1 diluted HCl to precipitate the product. The solid obtained was filtered, washedsuccessively with saturated NaHCO3 solution, dilute NaOH solution, and two to threetimes with water; the crude solid thus obtained was purified a number of times using hotwater until a constant melting temperature was obtained. The purity of all of these com-pounds was checked by TLC, yield in general 65%–75%. All the compounds have beenpurified by column chromatography on silica gel (80–120 mesh) using a mixture of ethylacetate/petroleum ether (7/3).

Data. Compound B10 (Series II). Yield: 66%, MP 85◦C; elemental analysis for calculatedC 77.20%, H 7.64%, N 4.62%; found C 77.57%, H 7.95%, N 4.91% for C39H46N2O4;FT-IR (KBr pallete) 3037 cm−1 (–C–H aromatic stretching), 2852–2918 cm−1 (–CH3

aliphatic stretching), 1687 cm−1 (C O stretching), 1606 cm−1 (–N N–), 1409–1469 cm−1

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Figure 10. Mass spectra of compound B10 (series II).

(–CH bending of CH2), 1370 cm−1 (–C–H bending of CH3), 1255–1064 cm−1 (–C–O–C–stretching of alkoxy chain), 819–758 cm−1 (–CH bending out of plane), 717 cm−1 (–CH2

rocking); 1H-NMR (CDCl3): 0.90 ppm (m, 6H, CH3 of aliphatic chain), 1.29–1.83 ppm (m,28H, –CH2), 3.88–3.95 ppm (m, 4H, –OCH2 of alkoxy chain), 6.87–6.95 ppm (m, 16H,Ar–H); FAB mass spectra: molecular weight of compound 606 g/mol and molecular ionpeak as (M+1)+ at 607.2 (m/z).

Results and Discussion

The compounds of both series are subjected to elemental analysis. The elemental analysisdata agreed with theoretical values as per the expected structure. The FT-IR spectra ofrepresentative compounds are shown in Figs. 1–4. The 1H-NMR spectra of representativecompounds are shown in Figs. 5–8. The mass spectra of these compounds are shown inFigs. 9 and 10. The m/z ratios obtained from the spectra of representative samples arematched with their molecular ion peak. The purity of the compounds is checked by TLC. Itshows one spot, indicating single compound. All the compounds were purified by columnchromatography using silica gel (100–200 mesh) and ethyl acetate/petroleum ether (7:3)solvent system.

In earlier report [22], the alkoxy was prepared from azophenols using a Pd-catalyzedcoupling reaction of a suitable azobenzene precursor with alkyl zinc chlorides.

In the present case, we have used alkoxy aniline containing C1–C8, C10, C12, C14, andC16 carbon atoms in the alkoxy chain followed by diazotization with 4-hydroxy biphenylusing a classical method [31] and the mesogenic properties were investigated for thesecompounds.

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Table 1. Transition temperature data of series I

Transition temperature (◦C)

Code no. R = n-alkyl Sm I

A1 Methyl 156 182A2 Ethyl 150 177A3 Propyl 144 173A4 Butyl 148 160A5 Pentyl 130 158A6 Hexyl 129 155A7 Heptyl 130 140A8 Octyl 104 136A10 Decyl 72 119A12 Dodecyl 89 113A14 Tetradecyl 87 102A16 Hexadecyl 86 104

OHRO N

N

Series I

In another case, the –OH group of a biphenyl moiety is esterified by the (C4) alkoxyacid. On the one end, the number of carbon atoms in the alkoxy chain was fixed (C4H9)and on the other end it was varied (C1–C8, C10, C12, C14, and C16); for these compounds,the mesogenic properties were also investigated.

Mesomorphic properties and thermal stability for the two new homologous series I andII were determined by a hot-stage polarizing microscope. Transition temperatures of bothseries are given in Tables 1 and 2.

In series I, all compounds exhibit enantiotropic smectic-A mesophase. The plot oftransition temperatures versus the number of carbon atoms in the alkoxy chain (Fig. 11)exhibits no usual odd–even effect but as the series is ascended the curve shows a fallingtendency.

In series II, compounds containing C1–C8 carbon atoms in the alkoxy chain aremonotropic smectic-A liquid crystalline compounds, whereas C10, C12, C14, and C16 areenantiotropic smectic-A compounds. The plot showing transition temperatures versus thenumber of carbon atoms in the alkoxy chain (Fig. 12) exhibits odd–even effect up to C5

carbon atom.The transition temperatures data obtained from a polarizing microscope are compared

with differential scanning calorimetry (DSC) data and the data of some representativecompounds are given in Table 3. Both the data are almost comparable. The DSC curves ofcompounds of series I and II are shown in Figs. 13–16.

The above two series made it possible to observe the effects of structural changes onmesomorphic behavior in a system that was studied previously.

The texture of the liquid crystalline compounds is given as microphotographs inFig. 17.

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0

20

40

60

80

100

120

140

160

180

200

1614121086420Number of carbon atoms

Tra

nsi

tio

n t

emp

erat

ure

(̊C

)

Sm

I

Figure 11. Transition temperature graph of series I.

It has been observed that series I compounds have high transition temperature thanthat of series II compounds, in spite of high molecularity of compounds. This is becauseof the presence of hydrogen bonding in compounds of series I as reported earlier [32]. The

Table 2. Transition temperature data of series II

Transition temperature(in ◦C)

Code no. R = n-alkyl Sm II

B1 Methyl 142a 162B2 Ethyl 120a 155B3 Propyl 136a 149B4 Butyl 96a 142B5 Pentyl 110a 136B6 Hexyl 118a 130B7 Heptyl 85a 128B8 Octyl 84a 116B10 Decyl 74 94B12 Dodecyl 80 92B14 Tetradecyl 100 114B16 Hexadecyl 82 95

aMonotropic phase.

C

O

O OC4H9

RO NN

Series II

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0

20

40

60

80

100

120

140

160

180

1614121086420

Number of carbon atoms

Tra

ns

itio

n t

em

pe

ratu

re (˚C

)Sm

I

Figure 12. Transition temperature graph of series II.

hydrogen bonding in compounds of series I is confirmed by IR spectra. The IR spectra ofcompounds show that the broad band centered between 3430 cm−1 and 3414 cm−1. Thisband disappeared after esterification in series II. Both the series compounds show smectic-A mesophase because of the presence of a biphenyl ring due to which the lateral cohesionforce is more compared with the terminal cohesion force. That is why the molecules remainin the form of lamellar bunch.

OHRO N

N

Series I

C

O

O OC4H9

RO NN

Series II

R N

N OH

Series A

CnH2n+1 NN O

C

O

C5H11

Series B

A comparison of the reported 4(4-alkyl phenylazo)phenols shows it is not mesomorphic(series A), whereas 4′-[(4-n-alkoxyphenyl)diazenyl]biphenyl-4-ol (series I) is mesomorphic(Table 4).

The diazotization reactions are run in aqueous solutions, and there is no difficultyfor higher analogues. The present compound is smectogenic having a higher clearingtemperature than that of phenol analogues.

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Figure 13. DSC thermogram for compound A10 on heating and cooling (series I).

Compounds with alkyl and alkoxy terminal groups on both sides and phenyl andbiphenyl moieties in the central core and having central linkages are identical in both typesof series compounds.

It has been observed that when an alkyl group is present at the terminal phenyl group,the clearing temperature is always lower than that of the alkoxy terminal group as indicatedby Gray et al. [33].

Figure 14. DSC thermogram for compound A14 on heating and cooling (series I).

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Figure 15. DSC thermogram for compound B10 on heating and cooling (series II).

Figure 16. DSC thermogram for compound B10 on heating and cooling (series II).

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Figure 17. Microphotographs of the compounds on heating.

The comparison of the reported 4(4-alkyl phenylazo)phenyl-4-pentylbenzoate(series B) with 4′[(4-alkoxy phenyl)diazenyl]4-n-butoxy phenyl biphenyl carboxylate (se-ries II) reveals that the clearing temperatures of series B compounds are greater than thoseof the present series II (Tables 5 and 6). Another interesting observation seen in the aboveseries is that when the hydrocarbon chains are connected directly to the benzene ring, themesophase starts at lower temperatures. In series B, a nematic mesophase starts from very

Table 3. Transition temperature and DSC data of series I and II

Peak temperatureCode no. Transition (Microscopic temperature) (in ◦C) �H (J g−1) �S (J g−1 k−1)

A10 Cr-Sm 71.26 (72) 15.5776 0.0926Sm-I 119.15 (119) 27.7671 0.0406

A14 Cr-Sm 90.46 (87) 27.5514Sm-I 99.15 (102) 27.7671 0.2070

B10 Cr-Sm 74.73 (74) 25.3786 −0.2367Sm-I 92.50 (94) 47.2723 −0.3153

B14 Cr-Sm 100.24 (100) 10.1864 0.0780Sm-I 114.12 (114) 33.4346 0.1313

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Table 4. Transition temperature data of series A and I

Series A Series I

n (number of n (number ofcarbon atoms) Isotropic (◦C) carbon atoms) Isotropic (◦C)

8 76 1 18210 77 2 17712 87 3 17314 91 4 16016 98 5 15818 100 6 15520 104 7 14022 – 8 136– – 10 119– – 12 113– – 14 102– – 16 104

first member of the carbon chain, while smectic-C mesophase starts from higher homo-logues say C14. In the present series II, the clearing temperatures are lower than those ofseries B. For the first carbon atom up to C16 carbon atom in the alkyl chain, smectic-Amesophase was obtained because both ends of the series have alkoxy groups and the middlecore of the series has a biphenyl moiety, which increases the polarizability of the molecule.As a result of this, the molecule becomes more lamellar, stratified, and highly arranged,thus showing smectic-A phase.

Conclusion

All the compounds of series I exhibit mesomorphic behavior showing enantiotropic smectic-A phase. From the comparison of series I and II, it has been observed that the transitiontemperature of series I compounds is higher than that of series II, which is due to hydrogen

Table 5. Transition temperature data of series B and series II

n (number of carbon atoms) Smectic-C Nematic Isotropic

2 – 120 207.414 – 84 200.16 – 76 187.78 – 80 177.6

10 – 86 169.112 – 79 159.114 80.0 (Sm-C) 84.7 153.116 84.0 (Sm-C) 89.2 146.418 86.0 (Sm-C) 93.3 140.720 90.0 (Sm-C) 96.5 135.522 86.0 (Sm-C) 100.9 130.5

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Table 6. Transition temperature data of series II

n (number of carbon atoms) Smectic A Isotropic

1 142 1622 120 1553 136 1494 96 1425 110 1366 118 1307 85 1288 84 116

10 74 9412 80 9214 100 11416 82 95

bonding present in series I. In series II, mesomorphic compounds show smectic-A phasefrom C1 to C8 and hence are monotropic, whereas C10, C12, C14, and C16 are enantiotropic.Both series are smectogenic because of the presence of a biphenyl moiety in the centralcore.

Acknowledgments

We are grateful to Prof. R. A. Vora, retired Prof. and Head, Department of Applied Chem-istry, Faculty of Technology and Engineering, Kalabhavan, M. S. University of Baroda,Vadodara, India, for his valuable suggestions. We are also thankful to CDRI, Lucknow, AtulIndustries Ltd., and SAIF Chandigarh for providing facilities such as elemental analysis,FT-IR, 1H-NMR, mass spectral, and thermal analyses.

References

[1] Demus, D., Goodby, J., Gray, G. W., Spiess, H. W., and Vill, V. (1998). Handbook of LiquidCrystals: Fundamentals, Wiley-VCH: Weinheim.

[2] Imrie, C. T., and Luckhurst, G. R. (1998). Liquid crystal dimers and oligomers. In: D. Demus, J.Goodby, G. W. Gray, H. W. Spies, & V. Vill (Eds.), Handbook of Liquid Crystals, Wiley-VCH:Weinheim, pp. 801–833.

[3] Imrie, C. T. (1999). Struct. Bonding, 95, 149–192.[4] Imrie, C. T. and Henderson, P. A. (2002). Curr. Opin. Colloid Interface Sci., 7, 298–311.[5] Imrie, C. T. and Henderson, P. A. (2007). Chem. Soc. Rev., 36, 2096–2124.[6] Brock, C. P. and Haller, K. L. (1984). J. Phys. Chem., C18, 3570.[7] Brock, C. P. and Haller, K. L. (1984). Acta Cryst., C40, 1387.[8] Brock, C. P. and Morelan, G. L. (1986). J. Phys. Chem., 90, 5631.[9] Brock, C. P. and Minton, R. P. (1989). J. Am. Chem. Soc., 111, 4586.

[10] Rajnikant, Watkin, D. J., and Tranter, G. (1995). Acta Cryst., CSI, 2388.[11] Rajnikant, Watkin, D. J., and Tranter, G. (1995). Acta Cryst., C51, 1452.[12] Rajnikant, Watkin, D. J., and Tranter, G. (1995). Acta Cryst., CSI, 2071.[13] Rajnikant, Watkin, D. J., and Tranter, G. (1995). Acta Cryst., C51, 2161.[14] (a) Goodby, J. W. (1991). Ferroelectric Liquid Crystals, Gordon & Breach: Newark, NJ, p. 131

(b) Goodby, J. W., Gray, G. W., and McDonnel, D. G. (1977). Mol. Cryst. Liq. Cryst., 34, 183.

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[15] Peter, J. C., and Michael, H. (1997). Introduction to Liquid Crystals, Taylor & Francis: London,79–92.

[16] Steinstrasser, R., and Pohl, L. (1973). Angew. Chem. Int. Ed. Engl., 12, 617–630.[17] Castellano, J. A. (1972). RCA Rev., 33, 296–316.[18] Elliott, G. (1973). Chem. Br., 9, 213–220.[19] Pohl, L., and Steinstrasser, R. (1971). German Patent No. 2024269.[20] Yamazaki, Y. (1973). Japanese Patent No. 48020787.[21] Michael, H. and Ibrahim, A. R. (2009). Liq. Cryst., 36(12), 1417–1430.[22] Johnson, L., Ringstrand, B., and Kaszynski, P. (2009). Liq. Cryst., 36(2), 179–185.[23] Saleh, A. A., Pleune, B., Fetting, J. C., and Poli, R. (1997). Polyhedron, 16, 1391.[24] Vyas, G. N. and Shah, N. M. (1963). Org. Syn. Coll., IV, 836.[25] Criswell, T. R., Klanderman, B. H., and Batesky, B. C. (1973). Mol. Cryst. Liq. Cryst., 22, 211.[26] Vora, R. A. and Dixit, N. (1979). In: Presented at Annual Convention of Chemists, Kurukshetra,

India.[27] Vogel, A. I. (1989). Text Book of Practical Organic Chemistry, 5th ed, ELBS and Longmann

Group Ltd: London, p. 946.[28] Gray, G. W. and Jones, B. (1952). J. Chem. Soc., 4179.[29] Jones, B. (1935). J. Chem. Soc., 1874.[30] Dave, J. S. and Vora, R. A. (1970). Liquid Crystals and Ordered Fluids: Johnson, J. F., & Porter,

R. S. Eds.; Plenum Press: New York, p. 477.[31] Meltzer, V., Rau, G., Iacobescu, G., and Pincu, E. (2004). Analele Universitatii din Bucuresti-

Chimie, Annal XIII, I–II, Analele Universitatii din Bucuresti, Romania, 233–238.[32] Nishikawa, E., Yamamoto, J., and Yokoyama, H. (2003). Liq. Cryst., 30(7), 785–798.[33] Gray, G. W., and Goodby, J. W. (1984). Smectic Liquid Crystals: Textures and Structures,

Leonard Hill, London.

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Synthesis, characterisation and liquid crystallineproperties of some Schiff base-ester central linkageinvolving 2, 6- disubstituted naphthalene ring systemB.T. Thaker a , N.J. Chothani a , Y.T. Dhimmar a , B.S. Patel a , D.B. Solanki a , N.B. Patel a ,J.B. Kanojiya a & R.S. Tandel aa Department of Chemistry, Veer Narmad South Gujart University, Surat-, 395007, IndiaVersion of record first published: 02 Mar 2012.

To cite this article: B.T. Thaker , N.J. Chothani , Y.T. Dhimmar , B.S. Patel , D.B. Solanki , N.B. Patel , J.B. Kanojiya & R.S.Tandel (2012): Synthesis, characterisation and liquid crystalline properties of some Schiff base-ester central linkage involving2, 6- disubstituted naphthalene ring system, Liquid Crystals, 39:5, 551-569




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http://dx.doi.org/10.1080/02678292.2012.664175


Liquid Crystals,Vol. 39, No. 5, May 2012, 551–569

Synthesis, characterisation and liquid crystalline properties of some Schiff base-ester centrallinkage involving 2, 6- disubstituted naphthalene ring system

B.T. Thaker*, N.J. Chothani, Y.T. Dhimmar, B.S. Patel, D.B. Solanki, N.B. Patel, J.B. Kanojiya and R.S. Tandel

Department of Chemistry, Veer Narmad South Gujart University, Surat-395007, India

(Received 13 December 2011; final version received 2 February 2012)

Four new mesogenic homologous series, each containing a 6-alkoxy 2-naphthoic acid and Schiff base-esteras central linkage, have been synthesised by esterification of 4-{[(4-hydroxyphenyl) imino] methyl} phenyl4-propoxy benzoate, 4-{[(4-hydroxyphenyl) imino] methyl} phenyl 4- (pentyloxy) benzoate, 4-{[(4-hydroxy-phenyl)imino]methyl}-2-methoxyphenyl 4- nitrobenzoate and 4-{[(4-hydroxyphenyl)imino]methyl}-2-methoxyphe-nyl 4- chlorobenzoate with different 6-alkoxy 2-naphthoic acid to give Series-A, -B, -C and -D, respectively. Thesecompounds were characterised by elemental analysis, Fourier transform infrared, 1H nuclear magnetic resonance,ultraviolet-visible and mass spectral studies. Their mesomorphic behaviour was studied by polarising opticalmicroscope (POM) with a heating stage. POM data were compared with differential scanning calorimetry ther-mograms. In Series-A and -B all compounds exhibit mesomorphism. Series-A compounds exhibit a enantiotropicnematic mesophase, while a smectic A mesophase is observed from the butoxy derivative and persists up to the lastmember of the homologou series. Series-B compounds also exhibit the enantiotropic nematic mesophase, while thesmectic A mesophase is observed from the ethoxy derivative and persists up to the last member of the homologouseries. The mesomorphic properties of both series are compared with each other and the other structurally relatedSeries-C and –D compounds. In Series-C and -D all compounds exhibit the only nematic mesophase; no smecticmesophase is observed even for higher members of the homologous.

The aim of the research was to synthesise and characterise novel liquid crystalline compounds containing2,6-disubstituted naphthalene and to study their mesomorphic properties.

Keywords: Schiff base; ester; smectic; nematic; naphthalene

1. Introduction

The molecules of liquid crystalline compounds areelongated; they are rod or lath-shaped, thin and oftenflat, possessing middle and terminal polar groups.Molecules which form liquid crystals have dipoles intheir structure, with often a strong dipole towardsthe centre and a weak dipole towards the end of themolecules. When more than two benzene rings arelinked through more than one central group, the liquidcrystalline properties are enhanced the most.

Although many mesomorphic compounds havebeen reported, little is known about the effect ofchanges in molecular constitution and shape on thedegree of anisotropy in the melt. Inspection of theformula of mesomorphic compounds makes it clearthat all the molecules are characterised by their pre-dominant length. The molecules have a rod shape,which favours the linear molecular arrangement pro-posed by Friedel [1] for the smectic and the nematicstates. Such molecules will have a strong tendencyto lie with their long axes parallel, and this will beaccentuated by any dipoles in the molecules. Onlymolecules broader than benzene have been found to bemesomorphic [2].

*Corresponding author. Email: [email protected]

Thermotropic liquid crystals have greattechnological importance [3]. A vast number of meso-genic compounds containing naphthalene moiety asa core system showing nematic or other mesophaseshave been reported [4–6]. Dave and coworkers studieda variety of liquid crystalline compounds exhibit-ing smectic, nematic and choleseric mesomorphismcontaining naphthalene moiety, such as alkoxyben-zoates of 1,5- and 1,4- dihydroxynaphthalene [7];Vora and Prajapati also reported the mesogenichomologous series of Schiff’s base-esters containingnaphthalene moiety and studied the effect of lateralthiol and methoxy substituent on mesomorphism[8, 9]. Malthete et al. [10] synthesised tetra-acylated1,4,5,8-tetrahydroxynaphthalene derivatives. In thelast decade a significant number of research papers onnaphthalene liquid crystal cores have appeared in theliterature [11–27].

Yang and Lin [28] synthesised and characterisedthree analogous series of symmetric banana-shapedliquid crystalline molecules containing bisnaphthylunits. Lin et al. [29] synthesised two fused-ringstructures, 6-n-decyloxy-2-naphthoic acid and6-dodecyloxyisoquinoline, and used as a proton

ISSN 0267-8292 print/ISSN 1366-5855 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/02678292.2012.664175http://www.tandfonline.com

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552 B.T. Thaker et al.

donor and acceptor moieties to construct a series ofsimple mesogenic supramolecules. The other comple-mentary hydrogen-bonded (H-bonded) moieties arebenzoic acids, thiophenecarboxylic acid and pyridinescontaining different alkyl chain lengths connected byether and ester linkages. Kolhe et al. [30] reportedthe synthesis, characterisation and device studiesof poly(benzobisoxazole imide) containing peryleneor naphthalene units in an alternating fashion withthe oxazole unit. Sandhya et al. [31] reported pho-toconductivity measurements in a binary system ofnaphthalene-based liquid crystals. Chia et al. [32]synthesised and characterised two homologous seriesof pyridine-containing liquid crystalline compounds,2-(4-alkoxyphenyl)-5-phenylpyridines and 2-(6-alkoxynaphthalen-2-yl)-5-phenylpyridines, and theirthermotropic behaviours were studied. Seed et al. [33]synthesised compounds based on 2,6-disubstitutednaphthalenes or related 1-benzothiophene moietieswith butyl sulfanyl and cyno or isothiocyanato termi-nal group. The compounds with naphthyl and phenylgroups are solely nematogenic; for these compoundsthe naphthyl unit gives an average increase in theNematic-Isotropic (N-I) value and melting pointcompared to the values for the compounds with aphenyl in place of the naphthyl unit. Wu and Lin [34]synthesised two series of ferroelectric liquid crystalsderived from (S)-2-(6-methoxy-2-naphthyl)propionicacid, with non-fluorinated or semi-perfluorinatedalkanes positioned at a chiral terminal chain and

thermal properties studied by differential scanningcalorimetry, polarising optical microscopy andelectro-optical measurements. Mohammady et al.[35] synthesised four homologous series belonging tothe family of 4-(4-substituted phenylazo)-1-naphthyl4- alkoxybenzoates in which the 4- substituent (X)was varied between CH3O, CH3, Cl and NO2; withineach homologous series, the number of carbonatoms was varied between eight and 14. The resultswere discussed in terms of mesomeric, polarisabilityand steric effects. Kohout et al. and Novotna et al.synthesised and studied liquid crystals based on lat-erally substituted 7-hydroxynaphthalene-2-carboxylic[36–38]. Recently many researchers synthesised andstudied liquid crystals involving naphthalene moiety[39–49]. The majority of naphthalene-based mesogenshave the 2,6-disubstitution pattern as this is the mostlinear substitution pattern for naphthalene. Many ofthese 2,6-disubstituted mesogens incorporate alkoxy,alkynyl, alkyl and cyano terminal groups to give widemesophase ranges but often with associated highmelting points [50–57].

In our studies on naphthalene-based materials[58], in order to investigate the influence of termi-nal groups, lateral substitution and central linkageon the mesomorphic properties of liquid crystallinecompounds an attempt has been made to synthe-sise four homologous series having Schiff base-estercentral linkage with different terminal alkoxy chainand the general structural formula as follows:

Series-A

OOR

O

OC3H7

O

O

N

where R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16;

Series-B

OOR

O

OC5H11

O

O

N

where R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16;

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Liquid Crystals 553

Series-C

OOR

O

O

O

N

OCH3

NO2

where R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16;

Series-D

OOR

O

Cl

O

O

N

OCH3

where R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16.

2. Expermental

2.1 GeneralFor the synthesis of the compounds of the homol-ogous series, the following materials were used:4-hydroxy benzoic acid, alkyl bromide (Lancaster,England), 4-chloro benzoic acid, 4-nitro benzoicacid, 4-hydroxy benzaldehyde, vanillin (Rankem,India), 4-amino phenol, 6-hydroxy-2-naphthoic acid(H. L. Chemicals and Engineering Pvt. Ltd, Maroli).N,N-dimethylaminopyridine (DMAP) was purchasedfrom Mark (Germany) and dicyclohexylcarbodi-imide (DCC) was purchased from Fluka Chemie(Switzerland). The solvents were used after purifica-tion using the standard methods prior to use.

Elemental analysis (C, H, N) was performed onThermo Scientific FLASH 2000 at G.N.F.C. (GujaratNarmada Valley Fertilizer Company Ltd., Bharuch).Infrared spectra was recorded with a ThermoScientific Nicolet iS10 FT-IR Spectrophotometerat the Department of Chemistry, Veer NarmadSouth Gujarat University in the frequency range4000–400 cm−1 with samples embedded in KBrdiscs. 1H nuclear magnetic resonance (NMR) spec-tra of the compounds were recorded with a BrukerAvance II 400 NMR spectrometer using CDCl3and DMSO d6 as a solvent and Tetramethylsilane(TMS) as an internal reference at SAIF (SophisticatedAnalytical Instrument Facilities), Chandigarh; massspectra Electron Ionization (EI) of the compound wererecorded with a Firmegan MAT-8230 mass spectrom-eter also at SAIF, Chandigarh. Merk 60 F524 thin

layer plate were used for Thin Layer Chromatography(TLC) and examined under short-wave UV light. UV-visible spectra of a 10−5 M solution of the samplesusing CHCl3 as solvent were recorded with a ThermoScientific Evolution 300 UV-VIS spectrometer in therange 200–800 nm within our department.

Thermal analyses and differential scanningcalorimetry (DSC) of the liquid crystalline com-pounds were carried out at the Metteler ToledoIndia Pvt. Ltd, Powai, Mumbai. DSC analysis wasperformed on a Metteler Toledo DSC-1 with heatingrate of 10◦C/min in a N2 atmosphere. The opticalmicroscopy studies were carried out with a NiconEclipse 50i POL (Japan) microscope equipped with aLinkam Analysa-LTS 420 hot stage at the Departmentof Chemistry, Veer Narmad South Gujarat University.The textures of the compounds were observed usingpolarised light with crossed polariser with the samplein a thin film sandwiched between a glass slide andcover slip.

2.2 Synthesis of series-A, -B, -C and -D compounds2.2.1 Synthesis of 4-n-alkoxy benzoic acid (1)

Compound 1 was prepared using the method reportedby Dave and Vora [59].

2.2.2 Synthesis of 6-alkoxy-2-naphthoic acid

6-hydroxy-2-naphthoic acid (2.44 g, 13 mmol)and KOH (1.46 g, 26 mmol) were dissolved inethanol/water (100 mL, 9/1) and the solution was

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stirred for 20 min; corresponding n-alkyl bromide(32.50 mmol) was then added and the mixture heatedunder reflux for 24 h. When the reaction was com-plete, KOH (0.73 g, 13 mmol) was added and themixture heated under reflux for a further 4 h. Theethanol was evaporated, and the mixture poured intowater and acidified to approximately pH = 2∼3 withacetic acid. The precipitate was filtered and washedwith water and ether, and then recrystallised twicefrom glacial acetic acid and then from ethanol. Thesynthetic route has also been described in the literature[28, 60].

Yield 71–80%, infrared (IR) (KBr): υmax/cm−1:3061, 2930, 2828, 1680, 1603, 1311, 1472, 1450, 1383,1216, 1060,721.

2.2.3 Synthesis of 4-formylphenyl4-alkoxy benzoate

DCC (10.31 g, 50 mmol) and DMAP (0.61 g, 5 mmol)were added to a solution of 1 (50 mmol) and 4-hydroxy benzaldehyde (6.10 g, 50 mmol) in 120 mLof dichloromethane (DCM). The mixture was stirredfor 12 h. The dicyclohexylurea was filtered off andwashed with DCM and then the filtrate was suc-cessively washed with 5% aqueous acetic acid, 5%aqueous sodium hydroxide and water; the solventfrom the filtrate was evaporated. The crude productwas purified by column chromatography (silica gel,DCM/hexane 1/1) [61].

Yield 68%, clearing point (C.P.) 128◦C IR (KBr):υmax/cm−1: 3073, 2827, 2745, 1896, 1736, 1604, 1511,1474, 1450, 1385, 1270, 1210, 1060, 721.

2.2.4 Synthesis of 4-formyl-2-methoxyphenyl4-subsituted benzoate

A mixture of 6 (7.60 g, 50 mmol), 7 (8.35 g, 50 mmol),DCC (10.31 g, 50 mmol) and DMAP (0.61 g, 5 mmol)in tetrahydrofuran (30 mL) was stirred at room tem-perature for 24 h. After the reaction mixture wasfiltered, the solution was evaporated. The resultingsolid was then recrystallised from CH2Cl2/ethanol 1:1.The crude product was purified by column chromatog-raphy (silica gel, DCM/hexane 1/1) [62].

Yield 76%, C.P. 116◦C, IR (KBr): υmax/cm−1:3069, 2824, 2745, 1896, 1736, 1604, 1529, 1511, 1311,1270, 1089, 750.

2.2.5 Synthesis of 4-{-[(4-hydroxyphenyl)imino]methyl}phenyl 4-alkoxy benzoate

A mixture 4-amino phenol (5.45 g, 50 mmol) and 3(14.21 g, 50 mmol) in 150 mL of ethanol was heated atreflux for 3 h with stirring. The solid product obtainedon cooling was filtered off and recrystallised fromethanol. The crude product was purified by column

chromatography (silica gel, petroleum ether/ethylacetate 7/1) [63, 64].

Yield 61%, C.P. 162◦C, IR (KBr): υmax/cm−1:3450, 2929, 2851, 1774, 1736, 1626, 1606, 1509, 1449,1385, 1270, 1210, 1060, 721.

2.2.6 Synthesis of 4-[(-{4-[(4-alkoxybenzoyl)oxy]phenyl}methylidene)amino]phenyl6-alkoxy-2-naphthoate

DCC (2.06 g, 10 mmol) and DMAP (0.122 g, 1 mmol)were added to a solution of 2 (10 mmol) and 4(3.75 g, 10 mmol) in 120 mL of DCM. The mixturewas stirred for 24 h. The dicyclohexylurea was fil-tered off and washed with DCM and then the filtratewas successively washed with 5% aqueous acetic acid,5% aqueous sodium hydroxide and water; the solventfrom the filtrate was evaporated. The crude productwas purified by column chromatography (silica gel,CHCl3/EtOAc 9/1) [27, 61, 62, 65].

Data:

A7: Yield 76%, C.P. 221◦C, UV/visible λmax:258 nm, elemental analysis for C41H41NO6: calcdC, 76.49; H, 6.42; N, 2.18; found: C, 76.36;H, 6.28; N, 2.06%, MS m/z (rel.int %): 644(M+1)+ IR (KBr): υmax/cm−1 2929–2851 cm−1

(C–H Stretching (Str.) of aliphatic), 1736 cm−1(C=OStr. of ester), 1626 cm−1 (–C=N– Str. of azome-thine linkage), 1604, 1511 cm−1 (C=C Str. ofaromatic), 1210,1060 cm−1 (C–O–C Str. of alkoxy),1270 cm−1 (C–O Str. of ester), 1H NMR (CDCl3):δ 0.85–0.88 ppm (t, 3H, CH3), 1.25–1.86 ppm (m,16H, 8×CH2), 4.10–4.12 ppm (t, 2H, OCH2 attachedwith naphthalene ring), 4.02–4.06 ppm (t, 2H, OCH2

attached with phenyl ring), 6.85–8.71 ppm (m,18H, Ar–H), 8.27 ppm (s, 1H, aldehydic proton ofazomethine linkage –HC=N–).

A16: Yield 72%, C.P. 213◦C, UV/visible λmax: 256 nm,elemental analysis for C50H59NO6: calcd C, 77.99;H, 7.72; N, 1.82; found: C, 77.86; H, 7.65; N,1.73%: MS m/z (rel. int. %): 769 (M)+ IR (KBr):υmax/cm−1 2927–2851 cm−1(C–H Str. of aliphatic),1736 cm−1(C=O Str. of ester), 1626 cm−1 (–C=N–Str. of azomethine linkage), 1604, 1511 cm−1 (C=CStr. of aromatic), 1209,1060 cm−1 (C–O–C Str.of alkoxy), 1271 cm−1 (C–O Str. of ester), 1HNMR (DMSO–d6): δ 0.87–0.90 ppm (t, 3H, CH3),1.25–1.86 ppm (m, 34H, 17×CH2), 4.10–4.12 ppm(t, 2H, OCH2 attached with naphthalene ring),4.03–4.08 ppm (t, 2H, OCH2 attached with phenylring), 6.86–8.71 ppm (m, 18H, Ar–H), 8.27 ppm(s, 1H, aldehydic proton of azomethine linkage–HC=N–).

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Liquid Crystals 555

B7: Yield 75%, C.P. 209◦C, UV/visible λmax: 254 nm,elemental analysis for C43H45NO6: calcd C, 76.87;H, 6.75; N, 2.08; found: C, 76.75; H, 6.62; N,1.96%: MS m/z (rel. int. %): 671 (M)+ IR (KBr):υmax/cm−1 2931–2851 cm−1 (C–H Str. of aliphatic),1735 cm−1 (C=O Str. of ester), 1626 cm−1 (–C=N– Str. of azomethine linkage), 1605, 1511 cm−1

(C=C Str. of aromatic), 1211, 1057 cm−1 (C–O–CStr. of alkoxy), 1271 cm−1 (C–O Str. of ester), 1HNMR (DMSO–d6): δ 0.88–0.91 ppm (t, 3H, CH3),1.26–1.83 ppm (m, 20H, 10×CH2), 4.09–4.11 ppm(t, 2H, OCH2 attached with naphthalene ring),4.05–4.08 ppm (t, 2H, OCH2 attached with phenylring), 6.83–8.69 ppm (m, 18H, Ar–H), 8.25 ppm(s, 1H, aldehydic proton of azomethine linkage–HC=N).

B16: Yield 75%, C.P. 213◦C, UV/visible λmax: 254 nm,elemental analysis for C52H63NO6: calcd C, 78.26; H,7.96; N, 1.71; found: C, 78.16; H, 7.87; N, 1.65%:MS m/z (rel. int. %): 797 (M)+ IR (KBr): υmax/cm−1

2928–2951 cm−1(C–H Str. of aliphatic), 1736 cm−1

(C=O Str. of ester), 1626 cm−1 (–C=N– Str. of azome-thine linkage), 1605, 1511 cm−1 (C=C Str. of aro-matic), 1211, 1056 cm−1 (C–O–C Str. of alkoxy),1271 cm−1 (C–O Str. of ester), 1H NMR (DMSO–d6): δ 0.86–0.89 ppm (t, 3H, CH3), 1.25–1.83 ppm(m, 38H, 19×CH2), 4.10–4.13 ppm (t, 2H, OCH2

attached with naphthalene ring), 4.06–4.09 ppm (t,2H, OCH2 attached with phenyl ring), 6.82–8.69 ppm(m, 18H, Ar–H), 8.26 ppm (s, 1H, aldehydic proton ofazomethine linkage –HC=N–).

C7: Yield 72%, C.P. 226◦C, UV/visible λmax: 254 nmand 302 nm, elemental analysis for C39H36N2O8:calcd C, 70.90; H, 5.49; N, 4.24; found: C, 70.76; H,5.33; N, 4.10%: MS m/z (rel. int. %): 661 (M+1)+IR (KBr): υmax/cm−1 2928–2851 cm−1(C–H Str. ofaliphatic), 1737 cm−1(C=O Str. of ester), 1627 cm−1

(–C=N– Str. of azomethine linkage), 1593, 1502 cm−1

(C=C Str. of aromatic), 1210, 1054 cm−1 (C–O–C Str. of alkoxy), 1272 cm−1 (C–O Str. of ester),1529, 1311 cm−1 (–NO2) 1H NMR (DMSO–d6): δ

0.85–0.89 ppm (t, 3H, CH3), 1.25–1.89 ppm (m, 12H,6×CH2), 4.09–4.11 ppm (t, 2H, OCH2 attached withnaphthalene ring), 7.10–8.70 ppm (m, 17H, Ar–H),3.90 ppm (s, 3H, OCH3), 8.25 (s, 1H, aldehydic protonof azomethine linkage).

C16: Yield 70%, C.P. 195 ◦C, UV/visible λmax: 254 nmand 304 nm, elemental analysis for C48H54N2O8:calcd C, 73.26; H, 6.92; N, 3.56; found: C, 73.16;H, 6.80; N, 3.42%: MS m/z (rel. int. %): 786 (M)+IR (KBr): υmax/cm−1 2927–2850 cm−1(C–H Str. ofaliphatic), 1736 cm−1(C=O Str. of ester), 1627 cm−1


(C=C Str. of aromatic), 1211, 1060 cm−1 (C–O–C Str. of alkoxy), 1271 cm−1 (C–O Str. of ester),1529, 1311 cm−1 (–NO2) 1H NMR (DMSO–d6): δ


D7:Yield 62%, C.P. 185◦C, UV/visible λmax: 254 nmand 302 nm, elemental analysis for C39H36ClNO6:calcd C, 72.05; H, 5.58; N, 2.15; found C, 71.89;H, 5.46; N, 2.00%: MS m/z (rel. int. %): 650 (M)+IR (KBr): υmax/cm−1 2928–2851 cm−1 (C–H Str. ofaliphatic), 1738 cm−1 (C=O Str. of ester), 1627 cm−1


(C=C Str. of aromatic), 1208, 1065 cm−1 (C–O–C Str. of alkoxy), 1271 cm−1 (C–O Str. of ester),1089, 750 cm−1 (C–Cl) 1H NMR (DMSO–d6): δ


D16: Yield 72%, C.P. 179◦C, UV/visible λmax: 252 nmand 304 nm, elemental analysis for C48H54ClNO6:calcd C, 74.25; H, 7.01; N, 1.80; found C, 74.10;H, 6.87; N, 1.69%: MS m/z (rel. int. %): 776 (M)+IR (KBr): υmax/ cm−1 2928–2851 cm−1(C–H Str. ofaliphatic), 1736 cm−1 (C=O Str. of ester), 1626 cm−1


(C=C Str. of aromatic), 1208, 1067 cm−1 (C–O–C Str. of alkoxy), 1270 cm−1 (C–O Str. of ester),1089, 751 cm−1 (C–Cl) 1H NMR (DMSO–d6): δ

0.85–0.88 ppm (t, 3H, CH3), 1.24–1.88 ppm (m, 30H,15×CH2), 4.11–4.13 ppm (t, 2H, OCH2 attached withnaphthalene ring), 7.09–8.70 ppm (m, 17H, Ar–H),3.88 ppm (s, 3H, OCH3), 8.24 (s, 1H, aldehydic protonof azomethine linkage) (see Scheme 1).

3. Result and discussion

The elemental analysis data and other physical param-eters of the four series are in agreement with thetheoretical values as per the expected structure. Thepurity of the compounds has been checked by thinlayer chromatography, which shows a single spot, indi-cating a single compound. The representative com-pounds have been characterised by UV-visible, Fouriertransform infrared, 1H NMR and mass spectralstudies.

The mesomorphic properties and thermal stabili-ties of all the compounds of Series-A, -B, -C and -Dwere determined by a polarising optical microscopeattached with a Linkam heating stage and DSC.

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OHHOOC OR1HOOCi

1

OH

COOH RO

COOH

ii

2

R1O

O

O CHO

OHOHC+iii

1

3

OHNH2+3 iv R1O

O

O

N OH

4

OOR

O

OR1

O

O

N

V

4 + 2

5

OHOHC

OCH3

+ XHOOC X

O

O CHO

H3CO

vi

6 7 8

OHNH2+8 iv X

O

O

N OH

H3CO

9

OOR

O

X

O

O

N

OCH3

V

9 + 2

10

Where, R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16 R1 = -C3H7, -C5H11

X = -NO2, -Cl

Scheme 1. Synthetic route of the compounds of Series-A, -B, -C and -D. Reagents and conditions: (i) R-Br, KOH, methanol,reflux 8–14 h; (ii) R-Br, KOH, methanol/ethanol, reflux 24–28 h; (iii) DCC, DMAP, dry CH2Cl2, stirred at room temperature,12 h; (iv) ethanol, reflux 3 h with stirring; (v) DCC, DMAP, dry CH2Cl2, stirred at room temperature, 24 h. (vi) DCC, DMAP,dry tetrahydrofuran (THF), stirred at room temperature.

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Liquid Crystals 557

Table 1. Transition temperature data of the 4-[(-{4-[(4- propoxybenzoyl)oxy]phenyl}methylidene)amino]phenyl6-alkoxy-2-naphthoate (Series-A).


CompoundsR= nalkoxy Cr SmA N I

A1 Methyl • 225 – – • 237 •A2 Ethyl • 230 – – • 239 •A3 Propyl • 216 – – • 232 •A4 Butyl • 91 • 224 • 234 •A5 Pentyl • 85 • 210 • 223 •A6 Hexyl • 77 • 190 • 229 •A7 Heptyl • 73 • 190 • 221 •A8 Octyl • 72 • 206 • 226 •A10 Decyl • 72 • 204 • 221 •A12 Dodecyl • 68 • 196 • 218 •A14 Tetradecyl • 66 • 184 • 214 •A16 Hexadecyl • 65 • 175 • 213 •

3.1. Mesomorphic propertiesIn Series-A, methoxy to n-hexadecyloxy derivativesexhibit an enantiotropic nematic mesophase (thread-like textures). The smectic A (SmA) mesophase com-mences from the n-butyloxy derivative and persistsup to the last member of the homologous series.The transition temperatures of Series-A are given inTable 1.

The plot of transition temperatures versus thenumber of carbon atoms in the alkoxy chain (Figure 1)exhibits the usual odd–even effect for the solid toisotropic transition and as the series is ascended thecurve shows a tendency to fall for the mesomorphic–isotropic transition temperature throughout the series.

Series-A also exhibits a usual odd-even effect for thecrystal to nematic or smectic A to nematic (TCr-N orT SmA-N) transition temperatures for lower membersand a tendency to fall for the compounds A-8 to A-16. From the figure it can also be seen that the crystalto smectic A (TCr–SmA) transition temperature rises asthe chain length decreases.

In Series-B, methoxy to n-hexadecyloxy derivativesexhibit an enantiotropic nematic mesophase (thread-like textures) as in Series-A, while the smectic Amesophase commences from the n-ethyloxy derivativeand persists up to the last member of the homolo-gou series. The transition temperatures of Series-B aregiven in Table 2.

The plot of transition temperatures versus thenumber of carbon atoms in the alkoxy chain (Figure 2)exhibits the usual odd–even effect and as the seriesis ascended the curve shows a tendency to fallfor the mesomorphic–isotropic transition temperaturethroughout the series. Series-B exhibits the usual odd–even effect for TCr–N or TSmA–N and a tendency tofall for TCr–SmA. The alternation of nematic–isotropictransition temperatures is less readily dealt with onthe basis of a zig-zag alkyl chain conformation. If, forshorter alkyl chains, this chain extends strictly alongits own axis then the terminal methyl groups presentdifferent faces to one another or to other end groupsin the molecule depending on whether the chain iseven or odd. The different attractive forces resultingcould affect the energy of the system and account forthe alternation of the transition temperatures. Withthe higher homologous, the alkyl chain may be forcedinto line with the main axis defined by the more rigidaromatic parts.

0 2 4 6 8 10 12 14 16 1860

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Tm (Cr to Sm)Tm (Sm to N)Tc (N to I)

Figure 1. Transition temperature versus number of carbon atoms (n) in the terminal alkoxy chain for Series-A.

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Table 2. Transition temperature data of the 4-{[(4-{[4-(pentyloxy)benzoyl]oxy}phenyl)methylidene]amino}phenyl6-alkoxy-2-naphthoate (Series-B).


CompoundsR = nalkoxy Cr SmA N I

B1 Methyl • 212 – – • 230 •B2 Ethyl • 102 • 224 • 235 •B3 Propyl • 98 • 212 • 224 •B4 Butyl • 92 • 217 • 229 •B5 Pentyl • 96 • 210 • 219 •B6 Hexyl • 84 • 195 • 225 •B7 Heptyl • 93 • 177 • 209 •B8 Octyl • 83 • 203 • 223 •B10 Decyl • 76 • 205 • 219 •B12 Dodecyl • 72 • 188 • 216 •B14 Tetradecyl • 69 • 187 • 215 •B16 Hexadecyl • 65 • 184 • 213 •

DSC is a valuable method for the detectionof phase transitions. It yields quantitative results;therefore, we may draw conclusions concerning thenature of the phases that occur during the transition.In the present study, the enthalpy of two derivativesof Series-A and -B were measured by DSC. DSCdata of Series-A and -B are recorded in Table 3which helps to further confirm the mesophase. Table 3shows the phase transition temperatures, the associ-ated enthalpy (�H) and the molar entropy (�S) forcompounds of Series-A (A7 and A16) and Series-B(B7 and B16). Enthalpy values of the various tran-sitions agree well with the existing related literaturevalues [66]. The DSC curves of the representative com-pounds of Series-A and -B are shown in Figures 3 to 6.Microscopic transition temperature values are almost

Table 3. Transition temperature and DSC data of Series-Aand -B.

Compound Transition

Microscopictemperature (peaktemperature) (◦C)

�H(J g−1)

�S(J g−1

K−1)

A7 Cr–SmA 72.92 (73) 4.06 0.0117SmA–N 190.32 (190) 30.94 0.0667N–I 220.88 (221) 8.11 0.0164

A16 Cr–SmA 65.24 (65) 15.54 0.0459SmA–N 174.65 (175) 7.84 0.0175N–I 212.95 (213) 4.37 0.0089

B7 Cr–SmA 93.05 (93) 9.62 0.0262SmA–N 177.15 (177) 11.46 0.0254N–I 209.06 (209) 8.33 0.0172

B16 Cr–SmA 65.26 (65) 8.37 0.0247SmA–N 183.86 (184) 10.65 0.0233N–I 212.85 (213) 8.33 0.0171

similar to DSC data. At one end the alkoxy group isfixed, i.e. for Series-A having –OC3H7, Series-B having–OC5H11, and the alkoxy chain length of 6-alkoxy-2-naphthoic acid is varied. It has been observed thatin Series-A compounds the transition temperature ishigher than that of the Series-B compounds. This maybe attributed to the shorter alkoxy chain giving highertemperatures than the longer alkoxy chain.

Table 4 shows the difference in the averagenematic–isotropic and smectic–nematic thermal sta-bilities and mesophase ranges of Series-A and -B.In Series-A, the nematic–isotropic thermal stabili-ties are 4.3◦C higher than those of Series-B and thesmectic–nematic thermal stabilities are 3.7◦C lowerthan those of Series-B. From the table it can be seenthat the average nematic mesophase range of Series-A is 1.4◦C higher compared with that of Series-B

0 2 4 6 8 10 12 14 16 1860

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Tm (Cr to Sm)Tm (Sm to N)Tc (N to I)

Figure 2. Transition temperature versus number of carbon atoms (n) in the terminal alkoxy chain for Series-B.

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Liquid Crystals 559

Figure 3. DSC thermogram for compound A7 (Series-A).


Figure 5. DSC thermogram for compound B7 (Series-B).

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Figure 6. DSC thermogram for compound B16 (Series-B).

Table 4. Average thermal and mesophase stabilities ofSeries-A and -B.

Series A B

N–I 225.5◦C 221.2◦CSmA–N 197.4◦C 201.1◦CNematic mesophase range (◦C) 21.5◦C 20.1◦CSmectic mesophase range(◦C) 32.7◦C 34.0◦CCommencement of Smectic phase C4 C2

and the average smectic mesophase range of Series-Ais 1.3◦C lower compared with that of Series-B. Thecommencement of the smectic mesophase in Series-Ais from the n-butyloxy derivatives while it appears fromthe n-ethyloxy derivatives in Series-B.

The upper transition points for 6-n-alkoxy-2-naphthoic acids are higher than those for p-n-alkoxybenzoic acids with the same alkyl groups, byan average of 47◦C; therefore, 6-n-alkoxy-2-naphthoicacids are more mesomorphic. Thus, despite theirsmaller molecular breadths (6.8 Å) the benzoic acidsare less mesomorphic than the naphthoic acids (7.9 Å).Moreover, the molecules of the naphthoic acids arelonger by 2.2 Å, but unlike the increases in a homol-ogous series, this greater length results from the pres-ence of the second aromatic ring of the naphthalenenucleus. This second ring will contribute more to theintermolecular cohesion than a single benzene ringand so enhances the thermal stability. Due to the abovereason the compounds of Series-A and -B have highernematic and smectic thermal stability. The azome-thine central linkage is more coplanar and providessuch packing for the molecules that the smectic phasethermal stability increases. It is also known that theliquid crystalline properties are enhanced most whenall the rings are conjugated, i.e. the liquid crystal

transition temperatures are highest when the entiresystem is linked through central linking groups involv-ing multiple bonds (e.g., −CH=N− or −CH=CH−).However, the central ester linkage does not link thesystem through a multiple bond and hence the meso-genic thermal stability of a system connected viaazomethine linkage is higher. So the azomethine link-age increases the smectic thermal stability of thecompounds of Series-A and -B.

Since the dipolar (alkoxy) terminals and polaris-able (azomethine) centres of the molecule have becomefurther separated from one another as a result of thelengthened alkyl chain, the terminal intermolecularattractions have decreased while the residual lateralattractions are essentially unchanged. This increase inthe ratio of lateral to terminal cohesive forces makesthe probability greater that the layer arrangement,which is characteristic of the smectic mesophase, willpersist after melting occurs. Changes in this ratioare therefore quite important in determining the typeof mesomorphism exhibited by certain molecule ina series of liquid crystalline compounds as well asthe temperature at which the mesomorphic transitionoccur. Strong lateral and weak terminal intermolec-ular cohesions will give rise to a smectic mesophase,which, if the lateral cohesions are high enough, maypersist until an isotropic liquid is formed. However,the addition of a methylene group increases the over-all polarisability of the molecule [67]; consequently, thelateral intermolecular attractions increase, as the chainlength grows. Each methylene unit forces apart thepolarisable centres in the molecule and decreases theresidual terminal attraction. There should be a relativedecrease in the strength of the terminal intermolec-ular cohesive interactions [68]. This will decrease thenematic–isotropic transition temperature. So Series-Bhas a lower nematic–isotropic transition temperature

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Liquid Crystals 561

than Series-A. In Series-A the terminal intermolecularcohesive interactions are more due to the shorteralkoxy chain at one terminal compared to Series-B.So Series-A compounds have a higher nematic ther-mal stability and nematic mesophase range (nematiccharacter), while in Series-B the lateral intermolecu-lar attractions increase due to the longer alkoxy chainat the terminal which increases the smectic thermalstability and smectic mesophase range (smectic char-acter). The lateral attraction increased as increasedpolarisability of the molecules this is due to lengthen-ing of the alkyl chain.

In Series-A fewer increments in the smectic–nematic transition temperature are found when theaverage transition temperatures are high. It may beconcluded that when the lateral attractions betweenthe molecules are low, the increased lateral attractionsarising from the lengthening of the alkyl chain are rel-atively small in their effect, and vice versa. So Series-Bcompounds have more smectic character than Series-Acompounds.

The transition temperatures of Series-C and -D aregiven in Tables 5 and 6. In Series-C and -D methoxyto n-hexadecyloxy derivatives show only a nematicmesophase. No smectic mesophase is observed evenfor higher members of the homologous.

Figures 7 and 8 show plots of transition temper-atures against the number of carbon atoms in thealkoxy chain for Series-C and series-D, from whichit can be noticed that transition temperatures exhibitan odd–even effect of the crystal–nematic state. Thesolid–isotropic transition temperatures also exhibit anodd–even effect. This is probably due to relative dif-ferences between the terminal and lateral cohesions.Lateral substitution results in a large decrease in the

Table 5. Transition temperature data of the 4-[({3-methoxy-4-[(4- nitrobenzoyl) oxy]phenyl}methylidene)amino]phenyl6-methoxy-2-naphthoate (Series-C).


CompoundsR = nalkoxy Cr Sm N I

C1 Methyl • 232 – – • 239 •C2 Ethyl • 229 – – • 241 •C3 Propyl • 207 – – • 227 •C4 Butyl • 214 – – • 237 •C5 Pentyl • 206 – – • 231 •C6 Hexyl • 188 – – • 234 •C7 Heptyl • 188 – – • 226 •C8 Octyl • 194 – – • 229 •C10 Decyl • 193 – – • 222 •C12 Dodecyl • 176 – – • 214 •C14 Tetradecyl • 158 – – • 200 •C16 Hexadecyl • 123 – – • 195 •

Table 6. Transition temperature data of the 4-[({4-[(4-chlorobenzoyl)oxy]-methoxyphenyl} methylidene)amino]phenyl 6-alkoxy-2-naphthoate (Series-D).


CompoundsR = nalkoxy Cr Sm N I

D1 Methyl • 202 – – • 207 •D2 Ethyl • 200 – – • 209 •D3 Propyl • 188 – – • 200 •D4 Butyl • 192 – – • 203 •D5 Pentyl • 175 – – • 192 •D6 Hexyl • 170 – – • 198 •D7 Heptyl • 132 – – • 185 •D8 Octyl • 174 – – • 192 •D10 Decyl • 167 – – • 187 •D12 Dodecyl • 146 – – • 186 •D14 Tetradecyl • 147 – – • 180 •D16 Hexadecyl • 139 – – • 179 •

nematic–isotropic transition temperatures. In contrast,the reduction in the entropy change associated withthe nematic–isotropic transition is only slight [69].Enthalpies of the derivatives of Series-C and -D weremeasured by DSC. DSC data of the two series arerecorded in Table 7, which shows the phase transitiontemperatures, the associated enthalpy (�H) and themolar entropy (�S) for compounds of Series-C (C7

and C16) and Series-D (D7 and D16). The DSCcurves of the representative compounds are shown inFigures 9 to 12. Microscopic transition temperaturevalues are almost similar to DSC data.

Table 8 shows the difference in the averagenematic–isotropic thermal stabilities and mesophaseranges of Series-C and -D. In Series-C, the nematic–isotropic thermal stability is 31.4◦C higher than that ofSeries-D. From the table it can be seen that the averagenematic mesophase range of Series-C is 9.6◦C highercompared with that of Series-D. Since the nitro groupis the highest polar group under investigation, onewould expect it to enhance the stability. Substitutionwith the electron withdrawing chlorine atom enhancesthe nematic phase stability.

The terminal substituents affected the polarisabil-ity of the aromatic rings to which they are attachedto varying extents. As the polarity of the substituentincreases, the clearing point (TC) increases also.In Series-C, containing the nitro group which is morepolar, the clearing point increases, whereas Series-D,containing the chloro group at the terminal which ismoderately dipolar, shows a lower clearing point thanSeries-C.

Generally, in the phenyl benzoate system, liquidcrystallinity is more persistent as mutual conjugationbetween the substituent and the ester C–O group isincreased. A change in the degree of conjugation will

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0 2 4 6 8 10 12 14 16 18

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Tra

nsiti

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°C)


Tm (Cr to N)Tc (N to I)

Figure 7. Transition temperature versus number of carbon atoms (n) in the terminal alkoxy chain for Series-C.

0 2 4 6 8 10 12 14 16 18

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200

210

Tra

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Tm (Cr to N)Tc (N to I)

Figure 8. Transition temperature versus number of carbon atoms (n) in the terminal alkoxy chain for Series-D.

Table 7. Transition temperature and DSC data of Series-C and -D.

Compound TransitionMicroscopic temperature(peak temperature) (◦C) �H(J g−1) �S(J g−1 K−1)

C7 Cr–N 188.06 (188) 43.32 0.0939N–I 226.12 (226) 55.37 0.0110

C16 Cr–N 122.56 (123) 12.18 0.0307N–I 194.80 (195) 56.48 0.1207

D7 Cr–N 131.95 (132) 4.08 0.0100N–I 185.20 (185) 14.88 0.0324

D16 Cr–N 138.60 (139) 3.13 0.0076N–I 178.53 (179) 35.45 0.0785

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Liquid Crystals 563

Figure 9. DSC thermogram for compound C7 (Series-C).

(a) (b)

Figure 10. DSC thermogram for compound C16 (Series-C).

Figure 11. DSC thermogram for compound D7 (Series-D).

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(a) (b)

Figure 12. DSC thermogram for compound D16 (Series-D).

Table 8. Average thermal and mesophase stabilities ofSeries-C and -D.

Series C D

N–I 224.5◦C 193.1◦CNematic mesophase range (◦C) 32.4◦C 22.7◦C

alter both the polarisability and the resultant dipolemoment of the molecule, the latter due to an effecton the mesomeric moment. A decrease in the polar-isability will lead to a decrease in the dispersion forces,and consequently to a decrease in the thermal stabilityof the mesophase. When X = -NO2 or halogen, astrong dipole operates straight out from the p-positionof the benzal grouping. Such a dipole may act eitherby attracting the ester groupings of neighbouring ter-minally situated molecules or by repelling like dipolesin neighbouring molecules lying in a layer arrange-ment of a smectic kind. Indeed, this would suggestthat certain orientations of dipoles can have a disad-vantageous effect upon the smectic thermal stability.Thus, the dipole moments associated with the terminalnitro group are directed along the axis of the molecule.Moreover, these dipoles lie in line in the smectic stateand a net repulsive force may operate, reducing lateralattractions. In this case, a smectic arrangement will beless likely than a nematic arrangement, i.e. the ratioof the lateral to the terminal attractions will be low,also leading to enhanced terminal attractions. So thecompounds of Series-C and -D are only nematic.No smectic mesophase is observed due to the low ratioof the lateral to the terminal attractions. Polar groupssuch as -CN, -OMe and -NO2, increase the length ofthe molecule and the extent of the polarisable parts

of the system, and from both points of view wouldbe expected to enhance the nematic thermal stability.This effect is moderate in moderately polarisable halo-gen substituents (-C1 and -Br). However, the chlorosubstituent is often a source of instability and largerin size causing a high viscosity [70]. The effect of thechlorine atom (or for that matter any other halogenatom) in this situation is that the electron withdrawinginductive effect is very strong and is not overwhelmedby the electron releasing, resonance effect of the chlo-rine atom. Thus, the chloro substituent will have arelatively high charge density, identical in sign to thatcarried by the oxygen atom of the carbonyl moiety ofthe ester linking group. Thus, repulsion between thesetwo will cause a reduction in the coplanarity betweenthe carbonyl moiety and the phenyl ring to which it isattached. Due to this reason the nematic thermal sta-bility of the Series-D compounds will be partially lostthrough the loss of conjugation of the molecule.

The oxygen of the lateral methoxy (-OCH3) group,being in conjugation with the aromatic core, in addi-tion to extending the length of the rigid core, enhancesthe polarisibility. The presence of a side methoxygroup larger in size creates an additional dipolemoment at an angle to the long axis of the molecule,leading to destabilisation of the mesomorphic state.The introduction of lateral methoxy groups thereforeleads to a reduction in the phase transition tempera-tures in Series-C and -D compare to Series-A and -B.

Due to the lower lateral attraction and the greaterterminal attraction (low lateral to terminal attractionratio), the Series-C and -D compounds are nemato-genic.

Compare Series-A and -B with Series-C and –D (Scheme 2), which have different terminal groups

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Liquid Crystals 565

OOR

O

OC3H7

O

O

N

OOR

O

OC5H11

O

O

N

OOR

O

O

O

N

OCH3

NO2

OOR

O

Cl

O

O

N

OCH3

Series-A

Series-B

Series-C

Series-D

Scheme 2. Comparative geometry of Series-A, B, C and D.

and lateral groups. In Series-A and -B the terminalgroups are the alkoxy chain whereas there is no lat-eral substitution. Series-A and -B show nematic as wellas smectic character while Series-C and -D show onlynematic character.

The overall thermal stability of the compounds ofSeries-C is lowered by 1◦C than that of the compoundsof Series-A and 3.3◦C higher than that of the com-pounds of Series-B. The mesophase length of Series-Ccompounds is higher by 10.8◦C than that of Series-Acompounds and higher by 12.2◦C than that of Series-B compounds. The lateral methoxy group increasedlateral separation between the molecules as a resultdecreased lateral attractions between molecules.

The overall nematic mesophase thermal stabilityof the compounds of Series-D is lowered by 32.4◦Cthan that of the compounds of Series-A and 28.0 ◦Chigher than that of the compounds of Series-B. Thenematic mesophase length of Series-D compounds ishigher by 1.1◦C than that of Series-A compounds and2.59 ◦C than that of Series-B compounds. Due tolower lateral attraction there is no smectic mesophaseobserved in the compounds of Series-C and -D. Thelateral methoxy group decreases the thermal stabilitiesof the smectic and nematic states by broadening the

molecule, as a result of the increased lateral separationand the decreased lateral attractions. This effectivelyincreases the aspect ratio of the materials so that thelength-to-breadth ratio is much larger for Series-A and-B compounds, thereby effectively extending the com-bined molecular lengths. Systems with larger aspectratios will have higher clearing points, just as a three-ring compound has a higher clearing point thana two-ring material. The aspect ratios will also bedependent on the amount of time that the individualmolecules spend in paired associations in fluctuatingsystems, which in turn will be dependent on the rel-ative strengths of the interactions. This model pointsto the stabilisation of the liquid crystal phases and themagnitude of their relative transition temperatures bygross shape and strength of interactions, and not bysegregative effects or the local steric structure [71].

4. Texture study

The textures of the compounds were observed usingpolarised light with crossed polarisers with samplesin a thin film sandwiched between a glass slide andcover slip. The textures of the compounds of Series-Aand -B are shown in Figure 13. Compound A16 shows

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Nematic (thread-like) phase at 208.5ºC(on heating) A7

Typical phase at 90.8ºC(on cooling) B8

Smectic A phase at 85.3ºC(on cooling) A16

Nematic (thread-like) phase at 190.1ºC(on cooling) B16

Typical phase at 79.8ºC (on cooling) A14

Smectic A phase at 78.3ºC(on cooling) B16

Figure 13. Micro photograph of liquid crystalline compound.

a texture of the SmA phase on cooling from theisotropic phase at 85.3◦C and A7 shows a thread-like texture of the nematic phase on heating of thesolid at 208.5◦C. Compound B16 shows a texture ofthe SmA phase on cooling from the isotropic phaseat 78.3◦C and also shows a thread-like texture of thenematic phase on cooling from the isotropic phase at190.1◦C. Compounds B8 and A14 show A typical tex-ture on cooling from the isotropic phase at 90.8◦Cand 79.8◦C, respectively. The textures of the com-pounds of Series-C and -D are shown in Figure 13.Compound C16 shows a schlieren-like texture of thenematic phase on cooling from the isotropic phaseat 140.2◦C and compound C7 shows that nematicdroplets start on cooling from the isotropic phase at

189.2◦C, which coalesce to form the thread-like tex-ture of the nematic phase at 196.1◦C. Compound D16

shows a schlieren-like texture of the nematic phaseon cooling at 150.8◦C and compound D7 shows aschlieren-like texture of the nematic phase on heatingof the solid at 149.4◦C. Compound D8 shows a typicaltexture on heating of the crystals at 175.8◦C.

5. Conclusion

In this paper we have presented the synthesis, char-acterisation and mesomorphic properties of newliquid crystalline compounds involving 6-alkoxy-2-naphthoic acid and the Schiff base as central linkage.Series-A and -B exhibit the nematic as well as the SmA

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Liquid Crystals 567

Nematic (schlieren-like) phase at 140.2ºC(on cooling) C16

Nematic (thread-like) phase at 196.1ºCon cooling) C7

Nematic (schlieren-like) phase at 150.8ºC(on cooling) D16

Nematic droplets starts to coalesce to form thenematic texture at 189.2ºC (on cooling) C7

Nematic (schlieren -like) phase at 149.4ºC(on heating) D7

Typical phase at 175.8ºC(on heating) D8

Figure 13. (Continued).

phase. The compounds of Series-A exhibit higher ther-mal nematic stability and good nematic mesophaserange, whereas Series-B exhibit higher thermal smecticstability and good smectic mesophase range due to theadditional methylene group at one terminal. In Series-C and -D the nematic phase was observed from themethoxy to n-hexadecyloxy derivatives. No smecticphase was observed even in higher homologous. Thestudy also revealed that the significant influence on themesomorphic properties of variation of the terminalgroup in two series made it possible to observethe effects of structural changes on mesomorphicbehaviour in a system. The thermal stability andmesophase range of the Series-C (-NO2) terminalgroup is higher than that of the Series-D (-Cl) terminal

group. lateral methoxy group, which is large in size,causes the boarding of the molecule by increasingbreath of the molecule, which, decrease thermal sta-bilities of the smectic and nematic states by decreasingthe ratio of length/breath (l/b) of the molecule.

Acknowledgements

We are grateful to Dr R.A. Vora for giving valuable sug-gestions. We are also grateful to Mettler Todelo, IndiaPvt. Ltd, Powai, Mumbai for their support with the DSCanalyses and H. L. Chemicals and Engineering Pvt. Ltd,Maroli for providing 6-hydroxy-2-naphthoic acid. We alsothank Gujarat Narmada Valley Fertilizer Company Ltd.(G.N.F.C.), Bharuch, and SAIF Chandigarh for providingfacilities such as elemental analysis, 1H NMR and massspectra.

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Molecular Crystals and Liquid CrystalsPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/gmcl20

Synthesis, Characterization andMesomorphic Properties of New Rod-likeThiophene Based Liquid CrystalsB. T. Thaker a , B. S. Patel a , Y. T. Dhimmar a , D. B. Solnki a , N. J.Chothani a , N. B. Patel a , K. B. Patel a & U. Makavana aa Department of Chemistry, Veer Narmad South Gujarat University,Surat, Gujarat, IndiaVersion of record first published: 30 Jul 2012.

To cite this article: B. T. Thaker , B. S. Patel , Y. T. Dhimmar , D. B. Solnki , N. J. Chothani , N. B.Patel , K. B. Patel & U. Makavana (2012): Synthesis, Characterization and Mesomorphic Properties ofNew Rod-like Thiophene Based Liquid Crystals, Molecular Crystals and Liquid Crystals, 562:1, 98-113




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http://dx.doi.org/10.1080/10426507.2012.673943


Mol. Cryst. Liq. Cryst., Vol. 562: pp. 98–113, 2012Copyright © Taylor & Francis Group, LLCISSN: 1542-1406 print/1563-5287 onlineDOI: 10.1080/10426507.2012.673943

Synthesis, Characterization and MesomorphicProperties of New Rod-like Thiophene Based

Liquid Crystals

B. T. THAKER,∗ B. S. PATEL, Y. T. DHIMMAR, D. B. SOLNKI,N. J. CHOTHANI, N. B. PATEL, K. B. PATEL,AND U. MAKAVANA

Department of Chemistry, Veer Narmad South Gujarat University, Surat,Gujarat, India

Two new mesogenic homologous series of Schiff base esters, 2-[4-(4′-n-Alkoxy benzoy-loxy) benzylidenamino] 3-cyno thiophine (Series-A) and Schiff base cinnamates, 2-[4-(4′-n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), compris-ing a thiophene moiety were synthesized. Structural elucidation was carried out usingelemental analysis and spectroscopic techniques such as FT-IR, 1H-NMR and 13C-NMR,and mass spectrometry. The mesomorphic properties and thermal stabilities of the titlecompounds were studied by using differential scanning calorimetry and optical polar-izing microscopy. All the derivatives are mesomorphic in nature showing the nematicphase, and the higher members of Series-A show a smectic C phase whereas Series-Bexhibits only the nematic mesophase. The mesomorphic properties of the present seriesare compared with other structurally related compounds.

Keywords 3-cyno thiophene; cinnamates; ester; nematic; schiff base; smectic C

1. Introduction

The field of liquid crystals (LCs) has incorporated numerous different organic systemsin both low and high molecular weight materials [1–3]. Although classical thermotropicliquid crystals are commonly composed of rod-like molecules, many other types of lowmolecular mass compounds with unconventional molecular structures have been shown toexhibit liquid crystalline properties [4].

Many series of liquid crystalline compounds containing heterocyclic groups have beensynthesized due to their potentially wide range of applications, such as in the optical,electrical, biological, and medical fields [5–9]. During the last decades a large numberof mesomorphic compounds containing heterocyclic units were synthesized and evaluated[10,11]. Interest in these compounds arises because the inclusion of heteroatoms can cause

∗Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad SouthGujarat University, Surat-395007,, Gujarat, India. Tel.: (+91) 9228377618. E-mail: [email protected]

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Thiophene Liquid Crystals 99

large changes in the type of mesophase present and/or in the physical properties of thematerials.

Heterocycles are of great importance as core units in thermotropic liquid crystals dueto their ability to impart lateral and/or longitudinal dipoles combined with changes inthe molecular shape. These materials hold great potential for use in spatial light modula-tion [12], all-optical signal processing, optical information storage [13], organic thin-filmtransistors [14,15], fast switching ferroelectric materials [16], fluorescent probes for thedetection and analysis of biomolecules etc. [17]. Thiophene in particular has emerged asa core unit that is receiving increasing attention. Sulfur-based heterocycles are also beingused to elucidate the structures of complex mesophases. Thiophenes played a major rolein the synthesis of systems displaying supramolecular chirality when dissolved in solventswhere dissolution is not strongly favored [18]. Five-membered heterocycles have potentialfor flexoelectric applications such as found in bistable nematic displays. A number of thio-phenes [19] have already been evaluated for such applications, and other bent heterocyclesmay have equal promise.

There are relatively less examples of LC materials incorporating the thiophene ring.This is despite the fact that thiophene-based LC materials (a) have lower melting pointsthan the 1,4-phenylene analogues, (b) promote negative dielectric anisotropy, and (c) have atendency to generate a range of different liquid crystalline phases [20–26]. Five-memberedrings provide materials of low melting point and viscosity, large optical anisotropy, and fastswitching times [27].

There has been a continuing interest in the study of heterocyclic-based liquid crys-tal compounds owing to the great variety of their structures. Thiophene-based calamiticliquid crystals are currently the subject of intensive study [28–34]. Their applications asferroelectric materials as well as potential materials for molecular electronic devices, suchas organic field effect transistors, are of special interest. Heterocyclic compounds suchas five-membered thiadiazole or thiophene rings can be incorporated into the principalstructure of calamitic mesogens [35–39]. Sulfur containing heterocycles are important syn-thetic intermediates and have found a variety of applications in medicinal, agricultural,and materials chemistry [40–41]. LCs containing heterocyclic cores, such as thiophene, areof particular interest due to their slightly bent structure, which leads to features includinga reduced packing ability, a medium to strong lateral dipole and high anisotropy of thepolarizability.

The mesomorphic properties of aromatic Schiff base esters arising from substituentsvarying in their polarities have been reported by Ha et al. [42]. Many mesogenic ho-mologous series contain two central linkages, one of which may be ester and the otherazomethine [43,44]. Vora and Rajput [45] reported binary mixtures of cinnamate esterexhibit wide rang of smectic and nematic mesophase. Previously we have reported twomesogenic homologous series of cinnamate-azomethine [46] containing thiophene and fu-ran heterocycles. The ethylene linking group is very useful structural unit connecting onepart of a rigid core with another in calamitic mesogene molecules. This fully conjugativegroup enhances the longitudinal polarizability and extends the molecular length maintaininglinearity of the molecule. Recently, there has been a continuing interest in study the effectof an ethylene linking group and thiophene moiety on the mesomorphic properties of suchmolecules.

The present investigation concerns the synthesis, characterization, and mesomorphicproperties of two new liquid crystalline homologous series with a common central linkage(azomethine) with a differing central linkage (cinnamate-ester) with a terminal heterocyclicmoiety such as 2-amino-3-cyanothiophene.

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CRO

O

O CH N S

NC

2-[4-(4´-n-alkoxy benzoyloxy)benzylidenamino] 3-cyano thiophine

(Series-A)

RO CH CH C

O

O CH N S

NC

2-[4-(4´- n-alkoxy cinnamoyloxy)benzylidenamino] 3-cyano thiophine

(Series-B).

Where, R=CnH2n+1, n=1 to 8,10,12,14,16,18.

2. Experimental Details

2.1 Materials

4-Hydroxy benzoic acid and 4-hydroxy benzaldehyde were obtained from Merck (Ger-many). Alkyl bromide (Lancaster, England). 2-amino 3-cyano thiophine and malonic acidwere purchased from Fluka Chemie (Switzerland). N,N′-dicyclohexylcarbodiimide (DCC)were purchased from Acros Organics (USA). DMAP (N,N-dimethylaminopyridine) waspurchased from Merck (Germany). Pyridine, piperidine, anhydrous potassium carbon-ate, acetone, ethanol, methanol, acetic acid, ethyl acetate, HCl, KOH, NaOH etc. wereused as received. Column chromatography was performed using Acme’s Silica Gel (100–200 mesh). Solvents were dried and distilled prior to use.

2.2 Measurements

The C, H, and N contents of selected mesogenic samples was estimated by G.N.F.C. (GujaratNarmda Valley Fertilizer Company Ltd., Bharuch). Infrared spectra were recorded witha THERMO SCIENTIFIC NICOLET iSO-10 spectrophotometer in the frequency range4000–400 cm−1 with samples embedded in KBr discs at our department. High resolution(400 MHz) NMR spectra of the mesogenic compounds were recorded at room temperatureas 15%–20% solution in CDCl3 using TMS as internal standard on a BRUKER AVANCEII 400 NMR spectrometer at SAIF (Sophisticated Analytical Instrument Facilities), PanjabUniversity, Chandigarh. Mass spectra (TOF MS ES+) of the compounds were recorded usingFinnegan MAT-8230 Mass Spectrometer at SAIF (Sophisticated Analytical InstrumentFacilities), Panjab University, Chandigarh. Thin-layer chromatography (TLC) analyseswere performed using aluminium-backed silica-gel plates (Merck60 F524) and examinedunder shortwave UV light. Thermal (DSC) analyses of the liquid crystalline compoundswere carried out from Atul Industries Ltd. P-P site Atul. DSC analyses were performed onMETTELER M-3 thermo balance (Switzerland) with microprocessor TA-300 instrumentat a heating rate of 10◦C/min in N2 atmosphere. The optical microscopy studies were

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determined by using polarizing microscope NICON ECLIPSE 50i POL (Japan) equippedwith Linkam Analysa-LTS420 hot stage (London) at our department. The textures of thecompounds were observed using polarized light with crossed polarizers with the sample ina thin film sandwiched between a glass slide and cover slip.

2.3 Synthesis of Series-A and Series-B Compounds

2.3.1 4-n-alkoxy benzoic acid. Number of methods is known for alkylation of 4-hydroxybenzoic acid. However, in the present study, the method devloped by Dave and Vora [47]was followed. The clearing point of these compounds was compared with the reported oneand they are almost similar to reported values [48,49].

2.3.2 4-(4′-n-alkoxy benzoyloxy) benzaldehydes. The compound has been prepared byetherification of the appropriate 4′-n-alkoxy acid (2.02 mmol) and 4-hydroxy benzaldehy-des (0.246 g, 2.02 mmol), dicyclohexylcarbodiimide (0.457 g, 2.22 mmol) and dimethy-laminopyridine (0.002 g, 0.2 mmol) in dry CH2Cl2 (20 mL) was stirred at room temperaturefor 24 h. The ensuing white precipitate was isolated by Buchner filtration and discarded,while the filtrate was evaporated to dryness in vacuo. The resultant crude residue was pu-rified by column chromatography on silica gel eluting with dichloromethane, followed byrepeated recrystallization from ethanol until constant transition temperatures were achieved[50–52].

2.3.3 2-[4-(4′-n-alkoxy benzoyloxy) benzylidenamino] 3-cyano thiophene. A mixture of4-(4′-n-alkoxy benzoyloxy) benzaldehydes (10 mmol) and 2-amino 3-cyano thiophene(1.241g, 10 mmol) and three drops of acetic acid in absolute ethanol (10 mL) were refluxedfor 4 h. The reaction mixture was allowed to cool and was stirred at room temperatureovernight. The residue obtained on removal of solvent was chromatographed on silicagel (100–200 mesh) using petroleum ether (60◦C–80◦C) ethyl acetate mixture (80:20) aseluant. Removal of solvent from the eluant afforded a solid material, which was crystallizedrepeatedly from ethanol until constant transition temperatures were obtained. The purityof these compounds was checked by thin layer chromatography (Merck silica gel 60 F254precoated plates).

Data:A6: Yield 82%. Clearing Point (C.P.) 94◦C, UV-Visible (CHCl3) λmax: 354 nm, 278

nm, Found C, 70.55; H, 6.16; N, 6.08; Calc. for C29H32N2SO3 (488 gm/mole); C, 70.43;H, 6.08; N, 6.08;%. IR (KBr) υmax cm−1 3079 (C H Str. aromatic), 2932, 2858 (C HStr. aliphatic), 1731 (C O Str. ester), 1641 (CH N, Str. azomethine), 2228 ( N C). 1HNMR (400 MHz, CDCl3): �/ppm 0.86–0.89 (t, CH3), 1.25–1.84 (m, CH2), 4.03–4.06(t,OCH2), 6.63–8.12 (m, Ar-H), 8.54 (s, CH N). 13C NMR (CDCl3): �/ppm 14.15 (CH3),23.71–31.94 (CH2) 68.04 (OCH2), 114.45–163.15 (Ar-C), 159.95( CH = N), 164.41( C O ), 115.63 ( N C) TOF MS ES+ m/z (rel.int%): 488.5 (M)+m/z.

2.3.4 4-n-alkoxy benzaldehydes. These were synthesized by alkylation of 4-hydroxy ben-zaldehyde using the reported method of Vyas and Shah [53]. The clearing points of thesecompounds were compared with the reported one and they are almost similar to reportedvalues.

2.3.5 4-n-alkoxy cinnamic acid. 4-n-alkoxy cinnamic acid were prepared by the methodof Gray and Jones [54].

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2.3.6 4-(4′-n-alkoxy cinnamoyloxy) benzaldehydes. The compound has been prepared byesterification of a mixture of the appropriate 4′-n-alkoxy acid and the appropriate 4-hydroxy benzaldehydes (0.246 g, 2.02 mmol), dicyclohexylcarbodiimide (0.457g, 2.22mmol), dimethylaminopyridine (0.002g, 0.2 mmol) and dry CH2Cl2 (20 mL) was stirred atroom temperature overnight. The ensuing white precipitate was isolated by Buchner filtra-tion and discarded, while the filtrate was evaporated to dryness in vacuo. The resultant cruderesidue was purified by column chromatography on silica gel eluting with dichloromethane,followed by repeated recrystallization from ethanol until constant transition temperatureswere achieved [50–52].

2.3.7 2-[4-(4′-n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene. A mixture of4-(4′-n-alkoxy cinnamoyloxy) benzaldehydes (10 mmol) and 2-amino 3-cyano thiophene(1.241g, 10 mmol) and three drops of acetic acid in absolute ethanol (10 mL) was refluxedfor 4 h. The reaction mixture was allowed to cool and was stirred at room temperatureovernight. The residue obtained on removal of solvent was chromatographed on silicagel (100–200 mesh) using petroleum ether (60◦C–80◦C) ethyl acetate mixture (80:20) aseluant. Removal of solvent from the eluant afforded a solid material, which was crystallizedrepeatedly from ethanol until constant transition temperatures were obtained. The purityof these compounds was checked by thin layer chromatography (Merck silica gel 60 F254precoated plates).

Data:B6: Yield 80%. Clearing Point (C.P.) 90◦C, UV-Visible (CHCl3) λmax: 322 nm,

Found C, 72.44; H, 6.59; N, 5.41; Calc. for C31H34N2SO3 (514 gm/mol); C72.37; H,6. 61; N, 5.44;%. IR (KBr) υmax cm−13096 (C H Str. aromatic), 2970, 2884 (C HStr. aliphatic), 1725 (C O Str. ester), 1630 (CH N, Str. azomethine), 2232 ( N C).1HNMR (400 MHz, CDCl3): �/ppm 0.86–0.89(t, CH3), 1.25–1.78 (m, CH2), 4.03–4.07(t, OCH2), 6.63–8.12 (m, Ar-H), 8.53 (s, CH N). 13C NMR (CDCl3): �/ppm 14.21(CH3), 22.77–32.00 (CH2) 68.47 (OCH2), 114.52–163.22 (Ar-C), 160.02 ( CH N),164.48 ( C O ), 114.49, 147.21 ( CH CH ), 114.70 ( N C) TOF MS ES+ m/z(rel.int%): 514.4 (M)+ m/z.

3. Results and Discussion

The synthetic route used for the preparation of Series-A and B is shown in Scheme 1.All compounds were characterised by elemental analysis, 1H-NMR, 13C-NMR, FT-IRspectroscopy. The mesomorphic properties of all the synthesized compounds have beeninvestigated by differential scanning calorimetry (DSC) and polarizing optical microscope(PMO) attached with a Linkam hot stage.

3.1 The phase Behavior of Series A and B

All the thirteen members of Series-A exhibit an enantiotropic nematic phase. The SmCmesophase commences from the n-dodecanoyloxy derivative along with the nematic phase.The transition temperatures are recorded in Table 1 and a plot of transition temperaturesagainst the number of carbon atoms in the alkoxy chain is given in Fig. 1. It can be noticedthat the nematic-isotropic transition temperature shows a smooth falling tendency anddoes not exhibit an odd–even effect. It also exhibits a tendency for rising smectic-nematictransition temperatures in the ascending Series-A.

All the compounds synthesized in Series-B exhibit enantiotropic nematic phase. Thetransition temperatures are recorded in Table 2 and a plot of transition temperatures against

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COOHOH COOHRO CRO

O

O CHO

CRO

O

O CH N S

NC

(A)(B)

(i) (ii)

(iii)

OH CHO RO CHO RO CH CH COOH

RO CH CH C

O

O CHO

RO CH CH C

O

O CH N S

NC

(D) (E)

(F)

(iv) (v)

(vii)

(vi)

Where, R=C nH2n+1, n=1 to 8,10,12,14,16,18.

Scheme 1. Synthetic route to Series-A and B. Reagents and conditions: (i) R-Br, KOH, Ethanol; (ii)DCC, DMAP, CH2Cl2, 4-Hydroxy benzaldehyde stirred at 0◦C for 1 h, stirred at room temperaturefor 24 h; (iii) Ethanol, 2 to 3 drop AcOH reflux for 4 h; (iv) RBr, K2CO3, Dry acetone; (v) Malonicacid, Piperidine reflux for 6–8 h; (vi) DCC, DMAP, CH2Cl2, 4-Hydroxy benzaldehyde stirred at 0◦Cfor 1 h, stirred at room temperature for 24 h; (vii) Ethanol, 2 to 3 drop AcOH reflux for 4 h.

the number of carbon atoms in the alkoxy chain is given in Fig. 2. It can be noticed thatthe crystal to mesophase transition temperatures increase with the usual old-even effectfor lower members. The nematic-isotropic transition temperatures also show no odd–eveneffect.

All the compounds of Series-A and Series-B exhibit mesomorphism. On cooling theisotropic liquid of Series-A the compounds form small droplets that coalesce to classicalSchlieren textures characteristic of the nematic phase. On further cooling, higher membersshow the focal-conic texture characteristic of the SmC mesophase. For Series-B on coolingthe isotropic liquid, all the members exhibit the Schlieren texture of the nematic phase,and no smectic mesophase is observed even in higher homologues. It is consistent with theassignment of each mesophase type using the classification system reported by Sackmannand Demus [55] and Gray and Goodby [56].

DSC is a valuable method for the detection of phase transition. It yields quantitativeresults; therefore, we may draw concerning the nature of the phase that occurs duringthe transition. The phase transition temperatures and corresponding enthalpy changes of

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Table 1. Transition temperatures (◦C) data of 2-[4-(4′-n-alkoxy benzoyloxy) benzylide-namino] 3-cyano thiophene (Series-A)

Transition temperatures ◦C

Compounds R = n alkoxy Cr SmC N I

A1 Methyl • – – 126 • 165 •A2 Ethyl • – – 118 • 162 •A3 Propyl • – – 121 • 160 •A4 Butyl • – – 114 • 156 •A5 Pentyl • – – 110 • 153 •A6 Hexyl • – – 105 • 150 •A7 Heptyl • – – 94 • 127 •A8 Octyl • – – 78 • 110 •A10 Decyl • – – 69 • 94 •A12 Dodecyl • (49)∗ • 72 • 91 •A14 Tetradecyl • 47 • 74 • 88 •A16 Hexadecyl • 52 • 67 • 86 •A18 Hexadodecyl • 54 • 64 • 83 •

()∗ monotropic.

compounds A6, A10, B6, B10 were determined using a DSC. The data obtained from theDSC analysis and from POM are summarized in Table 3, which helps to further confirm themesophase type. Table 3 shows the phase transition temperatures, associated enthalpy (�H)and molar entropy �S for compounds of Series-A (A6, A10) and Series-B (B6, B10). TheDSC curves of representative compounds are shown in Figs. 3–6. Microscopic transitiontemperature values are almost similar to DSC data. 7

0

20

40

60

80

100

120

140

160

180

0 2 4 6 8 10 12 14 16 18 20

Number of carbon atom

Tra

nsi

tio

n t

emp

erat

ure

Cr-SmC

SmC-N

N-I

Figure 1. Transition temperature curve of Series-A.

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Table 2. Transition temperatures (◦C) data of 2-[4-(4′-n-alkoxy cinnamoyloxy) benzylide-namino] 3-cyano thiophene (Series-B)

Transition temperatures ◦C

Compounds R = n alkoxy Cr Sm N I

B1 Methyl • – – 148 • 197 •B2 Ethyl • – – 151 • 202 •B3 Propyl • – – 154 • 191 •B4 Butyl • – – 143 • 180 •B5 Pentyl • – – 147 • 185 •B6 Hexyl • – – 139 • 171 •B7 Heptyl • – – 116 • 153 •B8 Octyl • – – 95 • 134 •B10 Decyl • – – 81 • 99 •B12 Dodecyl • – – 77 • 96 •B14 Tetradecyl • – – 74 • 95 •B16 Hexadecyl • – – 72 • 92 •B18 Hexadodecyl • – – 69 • 88 •

3.2 Mesomoxrphic Behavior

In Series-A, as the length of the carbon chain increased, an enantiotropic smectic C phasewas observed from the A14 derivative. In fact, the smectic phase observed as monotropicon cooling for the compound A12. The crystal to nematic mesophase transition temperaturegradually decreased from the C4 members. Clearing points descended with the increase in

0

20

40

60

80

100

120

140

160

180

0 2 4 6 8 10 12 14 16 18 20Number of carbon atom

Tra

nsi

tio

n t

emp

erat

ure

Cr-SmC

SmC-N

N-I

Figure 2. Transition temperature curve of Series-B.

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Table 3. DSC data of the Series-C and D

Compound Peak/POM �H �SSeries No. Transition Temp. ◦C J/g J/g◦K

A A6 Cr-N 104.67(105) 32.05 0.3062N-I 150.67(150) 11.87 0.0787

A10 Cr-N 69.31(69) 12.70 0.1832N-I 94.17(94) 4.93 0.0523

B B6 Cr-N 139.18(139) 36.29 0.2607N-I 170.80(171) 15.47 0.0905

B10 Cr-N 80.81(81) 10.37 0.1283N-I 99.29(99) 24.93 0.2510

length of the carbon chain due to the dilution of the mesogenic core resulting from the flex-ibility provided by the terminal alkanoyloxy chain. Generally, short-chain members favornematic formation, whereas the smectic phase is more favorable in long-chain members[57]. This general trend was obeyed by the Series-A depicted in Figure 1 in which thenematic phase range reduced as the length of the terminal chain increased.

In Series-B, all synthesized compounds show only the nematic mesophase, the nematictransition temperature does not show the odd–even effect in the short-chain members (n =1 to 6). Then Clearing points descended with the increase in length of the carbon chain dueto the dilution of the mesogenic core resulting from the flexibility provided by the terminalalkanoyloxy chain [58].


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3.3 Chemical Structure-Mesomorphic Property Relationship

There is close relation between mesomorphism and molecular constitution of organiccompounds. Hence, transition temperatures and mesophase range as measures of meso-morphism can be correlated with the molecular constitution of the compounds. Table 4summarizes the average thermal stabilities, mesophase range, and comparative geometry

Figure 5. DSC thermogram for compound no B6 (Series-B).

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Figure 6. DSC thermogram for compound no B10 (Series-B).

of the present Series-A, B, and structurally related Series-I [59], II [60], III, and IV [61]reported in the literature. The average nematic mesophase range of Series-B is higherby 0.32◦C and the N-I mesophase thermal stability is higher by 19.84◦C compared tothe respective mesophase ranges of Series-A. This is understandable, as the molecules ofSeries-B are longer and more polarizable compared to the molecules of Series-A due to thepresence of additional cinnamoyloxy ( CH CH COO ) central linkage. The moleculesof Series-A and B differ only at the central linkages. The molecules of Series- B havecinnamoyloxy ( CH CH COO ) central linkage, while Series-A have ester ( COO )central linkage.

Gray [62] has explained that the addition of double bond in the system increasesthe polarizability and length of the rod-like molecules. Therefore, the greater mesophasethermal stability of the present Series-B must be explained in terms of the greater molecular

Table 4. The mesophase range and thermal stabilities of Series-A, B, and structurallyrelated series-I to IV

Mesophase range (◦C)Thermal stabilities

(◦C)Commencement

Series Smectic Nematic Sm-N N-I of Smectic phase

A 18.75 31.76 93.23 125.00 C12

I 89.0 39.58 181.33 221.00 C12

II NonmesogenicB — 32.08 — 144.84 —III 16.0 69.0 133.69 202.23 C10

IV — 82.28 — 212.08 —

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Fig

ure

7.M

icro

phot

ogra

phof

liqui

dcr

ysta

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com

poun

ds.

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length and polarizability of the molecule resulting from additional CH CH units inthe central linkage.

Series-A exhibits both smectic as well as nematic mesophases (texture of smectic andnematic phases shown in Figure-7), where as Series-B exhibits only the nematic mesophase(nematogenic). There is only one difference between Series-A and B. Series-A containingester-azomethine central likage while Series-B having cinnamate-azomathine linkage. Theester group is more conducive for mesophase then cinnamate group. Therefore, Series-Aexhibit both mesophase, i.e., smectic at higher homologus and nematic mesophase frommethoxy group until the end. In case of length to breath ratio is higher, which shows nematicphase only. It can be seen that Series-B having higher length to breath ratio than Series-A.As a result of this, Series B exhibit only nematic phase (textures of compound B6 and B7are shown in Figure 7) where as Series A exhibit both smetic as well nematic mesophase.

Comparison of Series-A with Series-I and Series-II, respectively, gives insight on therole played by the terminal ring. The structural difference between the series is one of the

CRO

O

O C

H

SN

NC

(Series-A)

CRO

O

O C

H

N NN

N

(Series-I)

CRO

O

O C

H

N

(Series-II)

RO CH CH C

O

O C

H

SN

NC

(Series-B)

C

O

OCHCHRO NC

H

N

S F

(Series-III)

C

H

RO N O CHCHC

O

(Series-IV)

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terminal rings. The compounds of Series-A and Series-I are mesogenic, whereas Series-IIis nonmesogenic in nature because of the presence of the cyclohexane ring.

Series-A is compared with related Series-I. All the members of Series-A exhibit anenantiotropic nematic phase. The SmC mesophase commences from the n-dodecyloxyderivative as a monotropic phase. n-Tetradecyloxy to n-octadecyloxy members exhibitan enantiotropic SmC phase, whereas in Series-I the n-dodecyloxy to n-hexadecyloxyderivatives exhibit the SmC phase along with an enantiotropic nematic phase. The lonepairs of electrons on the nitrogen atoms act to broaden the molecule and also introduceattractive forces, which aid smectic formation. Reference to Table 4 indicates that thenematic mesophase length and N-I phase thermal stability of Series-I are higher by 7.82◦Cand 96.0◦C, respectively, than that of present Series-A. Both the series differ only at oneterminus. Series-A has a 3-cyano thiophene ring at the terminus instead of the 1,2,4-triazole ring of Series-II. Owing to the inherent nature of thiophene, it nonlinearly reducesthe efficiency of packing and thus lowers the mesophase thermal stability of members ofSeries-A than Series-I.

Reference to Table 4 indicates that the nematic mesophase length and N-I phase thermalstability of Series-B are lower by 36.92◦C and 57.39◦C, respectively, than that of presentSeries-III, similarly Series-B are lower by 50.2◦C and 67.24◦C, respectively, than thatof present Series-IV. Both the series differ only at one terminus. Series-A has a 3-cyanothiophene ring at the terminus instead of the 6-fluro benzothiazole ring of Series-III andbenzene ring of Series-IV. The fact that thermal stabilities of Series-III and IV are higherthan that of Series-B (of present work) suggests that even though Series-B contains a five-membered thiophene ring, which normally imparts nonlinearity, the essential attractingforces are similar to one present in Series-IV benzene analogues. Oh [63] has reportedthat all the transition temperatures of pyridine analogues were lower compared to thebenzene analogues. In the present study also shows that the thiophene derivatives havelower transition temperatures. The hetero atom has high electro negativity and, therefore,withdraws electron from the other atoms of the ring system, rendering the ring deactivatedrelated to benzene.

4. Conclusion

In this paper, we have presented the synthesis and characterization of new mesogenic ho-mologous series viz 2-[4-(4′-n-alkoxy benzoyloxy) benzylidenamino] 3-cyano thiophene(Series-A) which containing ester-azomethine central likage and 2-[4-(4′-n-alkoxy cin-namoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), which contain a cinnamate-azomethine central linkage. Series-A with an ester-azomethine central linkage has lowerthermal stabilites compared with Series-B with a cinnamate-azomethine central linkage.The ester central linkage is more conducive to conferring mesophases in the materials thanthe cinnamate central linkage. Series-A exhibits both smectic and nematic mesophaseswhile Series-B exhibits only the nematic mesophase. The mesophase range of the Series-Banalogues is higher than those of Series-A, which is attributed to the high polarizabilityof the molecules. Members of series-B with a cinnamoyloxy central linkage are more sta-ble compared with the Series-A members containing ester-azomethine central linkage dueto greater molecular length and polarizability of the molecule resulting from additional

CH=CH units in the central linkage. Moreover, the terminal thiophene derivatives havelower transition temperatures due to high electro negative “S” atom. There is no mucheffect have been observed on the transition temperature by the presence of CN group atlateral position in heterocyclic ring.

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Acknowledgment

The authors are thankful to Gujarat Narmada Valley Fertilizer Company Ltd. (G.N.F.C.),Bharuch, for providing facilities of elemental analysis, to Atul Industries Ltd. Atul, for DSCanalysis, and also to SAIF Chandigarh for providing facilities of FT-IR, 1H-NMR, 13C-NMR, and Mass spectral analysis.

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[31] Kobmehl, G., & Hoppe, F. D. (1994). Mol. Cryst. Liq. Cryst., 257, 169.[32] Kobmehl, G., & Hoppe, F. D. (1997). Liq. Cryst., 22, 137.[33] Gray, G. W., Hird, M., Lacey, D., & Toyne, K. J. (1989). Mol. Cryst. Liq. Cryst., 172, 165.[34] Kiryanov, A. A., Sampson, P., & Seed, A. J. (2001). J. Mater. Chem., 11, 3068.[35] Parra, M., Alderete, J., Zuniga, C., Gallardo, H., Hidalgo, P., Vergara, J., & Hernandez S. (2001).

Liq. Cryst., 28, 1659.[36] Parra, M., Hernandez, S., Alderete, J., & Zuniga, C. (2000). Liq. Cryst., 27, 995.[37] Parra, M., Alderete, J., Zuniga, C., & Hernandez, S. (2002). Liq. Cryst., 29, 647.[38] Parra, M., Alderete, J., Zuniga, C., Jimenez, V., & Hidalgo, P. (2003). Liq. Cryst., 30, 297.[39] Gallardo, H., & Favarin, I. (1993). Liq. Cryst., 13(1), 115.[40] Press, J. B. (1984). In: Gronowitz, S. (Ed.), The Chemistry of Heterocyclic Derivetves. Thiophene

and its Derivatives; Wiley: New York, Vol. 44 (Part-1), pp. 353–456.[41] Press, J. B. (1990). In: Gronowitz, S. (Ed.) The Chemistry of Heterocyclic Derivetves. Thiophene

and its Derivatives; Wiley: New York, Vol. 44 (Part-4), pp. 397–502.[42] Ha, S. T., Ong, L. K., Wong, J. P. W., Yeap, G. Y., Lin, H. C., Ong S. T., & Koh, T. M. (2009).

Phase Transitions, 82, 387.[43] Yeap, G. W., Ha, S. T., Lim, P. L., Ito, M. M., & Sanehisa, S. (2004). Mol. Cryst. Liq. Cryst.,

423, 73.[44] Dave, J. S., & Kurian, G. (1997). Mol. Cryst. Liq. Cryst., 175, 42.[45] Vora, R. A., & Rajaput, S. J. (1991). Mol. Cryst. Liq. Cryst., 209, 265.[46] Thaker, B. T., Patel, B. S., Solanki, D. B., Dhimmer, Y. T., & Dave, J. S. (2010). Mol. Cryst.

Liq. Cryst., 517, 63.[47] Dave, J. S., & Vora, R. A. (1970). In: J. F. Johnson & R. S. Porter (Eds.), Liquid Crystals and

Ordered Fluids; Plenum Press: New York, p. 477.[48] Gray, G. W., & Jones, B. (1955). J. Chem. Soc., 236 (1954), J. Chem. Soc., 236.[49] Jones, B. (1935). J. Chem. Soc., 1874.[50] Pinol, R., Ros, M. B., Serrano, J. L., & Sierra, T. (2004). Liq. Cryst., 31, 1293.[51] Guillevic, M. A., & Dvancanw, B. (2000). Liq. Cryst., 27(1), 153.[52] Yeap, G. Y., Ha, S. T., Lim, P. L., & Boeg, P. L. (2006). Mol. Cryst. Liq. Cryst., 452, 63.[53] Vyas, G. N., & Shah, N. M. (1963). Org. Syn. Coll. Vol. IV, (Revised-edition of annual volume

30–39), Jhon Wiley and Sons Inc.: New York, p. 836.[54] Gray, G. W., & Jones, B. (1954). J. Chem. Soc., 1467.[55] Sackmann, H., & Demus, D. (1966). Mol. Cryst. Liq. Cryst., 2, 81.[56] Gray, G. W., & Goodby, J. W. (1984). Smectic liquid Crystals: Textures and Structures, Leonard

Hill: Glesgow, London.[57] Collings, P. J., & Hird, M. (1998). Introduction to Liquid Crystals: Chemistry and Physics,

Taylor & Francis Ltd.: London, UK.[58] Berdague, P., Bayle, J. P., Ho, M. S., & Fung, B. M. (1993). Liq. Cryst., 14, 667.[59] Thaker, B. T., & Patel, P. (2008). Mol. Cryst. Liq. Cryst., 482, 3.[60] Vora, R. A., Mahajan, R., & Nandedkar, H. (1997). Liq. Cryst., 304, 495.[61] Matharu, A. S., & Chambers, A. D. (2007). Liq. Cryst., 34, 1317.[62] Gray, G. W. (1962). Molecular Structure and the Properties of Liquid Crystals, Academic Press:

London and New York.[63] Oh, C. S. (1972). Mol. Cryst. Liq. Cryst., 19, 95.

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This article was downloaded by: [Veer Narmad South Gujarat University]On: 16 January 2013, At: 02:24Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

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Mesomorphic studies of novel azomesogens having abenzothiazole core: Synthesis and characterisationB.T. Thaker a , B.S. Patel a , Y.T. Dhimmar a , N.J. Chothani a , D.B. Solanki a , Neeraj Patel a

, K.B. Patel a & U. Makawana aa Department of Chemistry, Veer Narmad South Gujarat University, Surat, Gujarat, IndiaVersion of record first published: 30 Oct 2012.

To cite this article: B.T. Thaker , B.S. Patel , Y.T. Dhimmar , N.J. Chothani , D.B. Solanki , Neeraj Patel , K.B. Patel & U.Makawana (2013): Mesomorphic studies of novel azomesogens having a benzothiazole core: Synthesis and characterisation,Liquid Crystals, 40:2, 237-248




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http://dx.doi.org/10.1080/02678292.2012.737478


Liquid Crystals, 2013Vol. 40, No. 2, 237–248, http://dx.doi.org/10.1080/02678292.2012.737478

Mesomorphic studies of novel azomesogens having a benzothiazole core: Synthesis andcharacterisation

B.T. Thaker*, B.S. Patel, Y.T. Dhimmar, N.J. Chothani, D.B. Solanki, Neeraj Patel, K.B. Patel and U. Makawana

Department of Chemistry, Veer Narmad South Gujarat University, Surat, Gujarat, India

(Received 25 January 2012; final version received 3 October 2012)

Novel liquid crystals, including 2-[4-(4′-n-alkoxybenzoyloxy)phenylazo]-6-fluorobenzothiazoles (Series E) and2-[4-(4′-n-alkoxybenzoyloxy)naphtha-1-ylazo]-6-fluorobenzothiazoles (Series F), have been synthesised and char-acterised. Each series contains 13 homologous members differing by the length of the alkoxy chain. In series Ethe derivative with C1 to C7 chain length were found to exhibit enantiotropic smectic C (SmC) and nematic (N)mesophases. The C8 homologue possessed enantiotropic SmC, smectic A (SmA) and N mesophases, while thelonger chain homologues (C10 to C14) showed enantiotropic SmC and SmA mesophases. The C16 and C18 homo-logues possessed only SmA mesophases. In Series F all the compounds (C1 to C18) exhibited only the enantiotropicnematic mesophase.

Keywords: 6-fluorobenzothiazole; azomesogens; SmA; nematic; enantiotropic

1. Introduction

Thermotropic liquid crystals possess a number ofunique properties that have received considerableattention. Significant efforts have been focused onthe synthesis of new compounds [1,2], and it hasbeen found that molecules having an extended rod-likeshape often exhibit a thermotropic liquid crystallinephase [3,4].

As part of this programme a large numberof mesomorphic compounds containing heterocyclicunits have been synthesised, and interest in suchstructures remains strong [5–7], not only because ofthe possibilities presented by heterocyclic moietiesin the design of new mesogenic molecules, but alsobecause the insertion of hetero-atoms strongly influ-ences the formation of mesomorphic phases. Theseheterocyclic structures generally incorporate unsatu-rated atoms such as O, N or S, and their electronegativ-ity often results in reduced symmetry in the moleculesand a stronger polar induction. The use of uniqueheterocyclic moieties to produce materials of low sym-metry or/and non-planar structures can be technolog-ically important in liquid crystal applications [8–12].

Fluoro-substituted liquid crystals are of particularinterest [13–17], since they generally exhibit excellentproperties compared with the corresponding unsubsti-tuted compounds such as lower viscosity, high volt-age mean retention and high specific resistance [18].The fluorine atom often produces interesting effects

*Corresponding author. Email: [email protected] paper was initially presented at the 18th Indian Liquid Crystal Conference, held at the Department of Physics, NERIST,Itanagar, India, 15–17 November, 2011

[19]. Fluoro-substitution in the terminal position pro-vides nematic compounds with positive dielectricanisotropy, analogous to that provided by a terminalcyano-substituent. However, fluoro-substituted com-pounds have the added advantage of high resistivityand are suitable for use in state-of-the-art active matrixdisplays, for example in thin-film transistors [20,21].Matsui et al. [22] have reported fluorine-containingbenzothiazolyl bisazo dyes and their application inguest–host liquid crystal displays has been examined.

The introduction of heterocyclic rings into corestructures to generate mesogenic materials has beenwell demonstrated [23–27], in particular ring struc-tures such as thiophene [28,29], pyridine [30] or1,3,4-thiadiazole [31]. A number of publications haverecently described the incorporation of benzothiazoleas a core unit in liquid crystalline compounds [32–41], for example Prajapati and Bonde [42] havereported the use of two mesogenic homologous seriescomprising 6-substituted benzothiazole ring systemswith a central azo linkage. Their study has revealedthat a methoxy-substituent in the sixth positionof the benzothiazole ring favours the formation ofthe nematic phase. A benzothiazole ring, contain-ing the electron-rich sulphur atom, can contribute toa low ionisation potential and can also induce thesmectic phase. Benzothiazole derivatives are addition-ally important due to their anti-tumour and antibac-terial activity [43] and as photoconductive materials

© 2013 Taylor & Francis

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[44–46]. These derivatives have also been investigatedfor use in thin-film organic field-effect transistors [47].Funahashi and Hanna [48,49] have reported the fasthole transportation properties of a photoconductivecalamatic liquid crystal.

The naphthalene ring is another interesting aro-matic core in the synthesis of liquid crystalline com-pounds. A significant variety of naphthalene-basedmaterials have been synthesised as highly birefringentmaterials for electro-optical devices [50], and naphtha-lene derivatives exhibiting liquid crystalline propertieshave also been well documented [51–58]. Gray andJones [59–61] have investigated the liquid crystallineproperties of a range of alkoxy naphthoic acids. Daveand Prajapati [62,63] synthesised a number of Schiff’sbase homologous series based on the naphthalenemoiety in order to establish the relationship betweenthe width of the aromatic core and mesomorphicbehaviour.

The growing scientific interest in the synthesis,analysis and mesomorphic behaviour of heterocyclic-based liquid crystals led us to synthesise two series of6-fluoro-benzothiazole-based liquid crystals, each con-taining a central naphthalene or benzene core with twoazo and ester-linked arms.

2. Experimental

2.1 Materials4-Hydroxybenzoic acid was obtained from Merck,Germany. Alkyl bromides were purchased fromLancaster (UK), and malonic acid from Fluka Chemie(Switzerland). N,N′-Dicyclohexylcarbodiimide (DCC)and dimethylaminopyridine (DMAP) were purchasedfrom Acros Organics (USA). 4-Fluoroaniline was pro-vided by Aarti Chemicals, Vapi (India), and usedwithout further purification. Other starting materials,including bromine, potassium thiocyanate, phenol, α-naphthol, acetone, ethanol, methanol, acetic acid, andethyl acetate, were used as received. Column chro-matography was performed using Acme Silica Gel(100–200 mesh). Solvents were dried and distilled priorto use.

2.2 MeasurementsElemental analysis (C,H,N) was performed using athermo Scientific FLASH 2000 at Gujarat NarmadaValley Fertilizer Company Ltd., Bharuch. Infraredspectra were recorded on a Thermo Scientific NicoletiS–10 spectrophotometer in the frequency range4000–400 cm−1 with samples embedded in KBrdiscs in our department. High resolution (400 MHz)NMR spectra were recorded at room temperature

as a 15–20% solution in CDCl3 using TMS asinternal standard, on a Bruker Avance II 400 NMRspectrometer, and mass spectra (TOF MS ES+)were recorded using a Finnigan MAT–8230 massspectrometer, both at Sophisticated AnalyticalInstrument Facilities, Panjab University, Chandigarh.Thin-layer chromatography (TLC) was performedusing aluminium-backed silica gel plates (Merck60 F524) and examined under short-wave ultravioletlight. Thermal (DSC) analysis of the liquid crystallinecompounds was carried out by Atul Industries Ltdusing a Mettler M–3 thermo balance (Switzerland)with microprocessor TA–300, at a heating rate of10◦C min−1 under a nitrogen atmosphere. The opticalmicroscopy studies were determined using a polarisingmicroscope, Nikon Eclipse 50i POL (Japan), equippedwith Linkam Analysa–LTS420 hot-stage (London),using a standard procedure in our department.

2.3 Synthesis of compounds in Series E and FThe synthesis of compounds in Series E and F was car-ried out as shown in Scheme 1, and their structures aresummarised in Scheme 2.

2.3.1 4-n-Alkoxy benzoic acid

A number of methods [59–61,64] are available for thealkylation of 4-hydroxybenzoic acid. In the presentstudy, however, the method developed by Dave andVora [65] was followed.

2.3.2 2-Amino-6-fluorobenzothiazole

2-Amino-6-fluorobenzothiazole was prepared from 4-fluoroaniline using the method reported in [66]. Theresulting compound was crystallised from 1:1 aque-ous ethanol, giving pale yellow needles. Yield: 87%;clearing point: 184◦C.

2.3.3 2-(4-Hydroxyphenylazo)-6-fluorobenzothiazole [B]

A well stirred mixture of 2-amino-6-fluorobenzo-thiazole (1.68 g, 0.01 mol) and (40 mL) of a 1:1 dilu-tion of concentrated H2SO4 was cooled to below 5◦Cand a solution of NaNO2 (0.76 g, 0.011 mol) in water(20 mL) added dropwise so that the temperature of themixture remained within the range 0–5◦C. The colddark yellow solution was added dropwise to a coldmixture of phenol (0.94 g, 0.01 mol), NaOH (5 g,10%) and water (50 mL), during which the tempera-ture of the mixture was kept below 8◦C. The diazo-tised solution was stirred for about 30 min and then

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Liquid Crystals 239

N N OH

N

SF

N N OH

N

SF

+

+

OHHOOC ORHOOC

AN

SF

A

NH2NH2F

A

(i)

(ii) (iii)

(iv)

(v)

Series E

Series F

B

(iv)

N N ORC

O

O

N

SF1

2

345

6

78

91'

2' 3'

4'1"

2" 3"

4"

5"6"5'6'

N N ORC

O

O

N

SF1

2

345

6

78

91'

2' 3'

4'

5'

6'

8'

9' 10'

1"

2" 3"

4"

7'

5"6"

Scheme 1. (i) R-Br anhydrous K2CO3 CH3OH/C2H5OH, (ii) KSCN, Br2, AcOH, (iii) NaNO2 + H2SO4, 0 to 8◦C, phenol inNaOH, (iv) DCC, DMAP, dry CH2Cl2 stirring 24 h, (v) NaNO2 + H2SO4, 0 to 8◦C, 1-naphthol in NaOH.

acidified with (1:1) aqueous HCl. The crude productwas filtered off and dried in air before recrystallising anumber of times from alcohol.

2.3.4 2-[4-(4′-n-Alkoxybenzoyloxy) phenylazo]-6-fluorobenzothiazole

This series of compounds was prepared by esteri-fication of a mixture of the appropriate carboxylicacid A (2.02 mmol) and phenol B (2.02 mmol), inthe presence of dicyclohexylcarbodiimide (2.02 mmol),dimethylaminopyridine (0.2 mmol) and dry methylenechloride (CH2Cl2; 20 mL), stirring at room tempera-ture overnight under an argon atmosphere. The ensu-ing white precipitate was filtered off and discarded,and the filtrate was evaporated to dryness in vacuo.The resulting residue was purified by chromatogra-phy on silica gel (100–200 mesh) using a petroleumether (60–80◦C): ethyl acetate mixture (7:3) as eluent.

Removal of the solvent gave a solid material, whichwas crystallised repeatedly from ethanol until constanttransition temperatures were obtained.

The purity of this series of compounds was checkedby thin layer chromatography (Merck Kiesel gel60 F254 per-coated plates).

2.3.5 2-[4-(4′-n-Octyloxybenzoyloxy)phenylazo]-6-fluorobenzothiazole

Yield: 80%; clearing point: 312◦CUV (CHCl3). λmax: 382 and 266 nm.Elemental analysis (%). Found: C, 66.87; H, 5.79;

N, 8.54. Calc. for C28H28N3SO3F (505 g mol−1): C,66.53; H, 5.54; N, 8.31.

IR (KBr). υmax cm−1: 3058 (C–H Str. aro-matic), 2927, 2854 (C–H Str. aliphatic), 1740 (C=OStr. ester), 1606 (N=N, Str. azo) 1471, 1365, 1078(benzothiazole), 874, 847, 641 (C–S–C).

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RON C

O

N O

N

SF

Compound E12

N N ORC

O

O

N

SF

Compound F12

RON C

O

N O

N

SO2N

Compound I

RON C

O

N O

N

SCl

Compound II

RON C

O

N O

N

SH3CO

Compound III

RON C

O

N O

Compound IV

RON C

O

N O

Compound V

R = CnH2n+1 and n = 12

Scheme 2. Comparision of existing E12 and F12 Compounds with structurally related Series I to V compound (R = C12H25)As shown in Table-4.

1H NMR (400 MHz, CDCl3): δ ppm 0.88–0.91 (t,3H(a)–CH3 of alkyl chain; J H–H = 7 Hz), 1.25–1.50(m, 10H(b) 5XCH2, –CH2 of alkyl chain), 1.80–1.87(quin 2H(c), Ar–O–C–CH2 of alkyl chain; J H–H =4 Hz), 4.04–4.07 (t, 2H (d), Ar–O–CH2 alkyl chain;

J H–H = 5 Hz), 6.97–6.98 and 7.00–7.01 (tt, 2H, C-2′′ and C-6′′due to C-3′′ and C-5′′; J 2′′6′′ = 2.4 Hzand J 3′′5′′ = 6 Hz), 7.26–7.30 (m, 2H, C-7 and C-5 due to C-4 and long range 19F coupling at C-6),7.43–7.44 and 7.45–7.47 (tt, 2H, C-3′′ and C-5′′ due

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Liquid Crystals 241

to Ar–H C-2′′and C-6′′; J 3′′5′′ = 2.6 Hz and J 2′′6′′= 3.7 Hz), 7.56–7.57 and 7.58–7.59 (dd, 1H, C-5 dueto Ar–H at C-7 and C-4), 8.13–8.17 (m, 4H, Ar–H atC-2′, C-3′, C-5′ and C-6′). These assignments are inagreement with data reported previously [40].

13C NMR (CDCl3): δ ppm 14.12 (CH3), 24.31(CH2) 68.42 (OCH2), 115.82, 119.45, 121.46, 123.76,127.04, 128.80, 130.60, 141.10, 146.3, 150.4, 152.50,158.20, 160.30, 161.80, 162.90 (Ar–C), 171.31(–C=O–).

TOF MS ES+ m/z (rel. int. %): 506.4 (M+1)+m/z.

2.3.6 2-(4-Hydroxynaphth-1-ylazo)-6-fluorobenzothiazole

A well stirred mixture of 2-amino-6-fluorobenzo-thiazole (1.68 g, 0.01 mol) and 40 mL of a 1:1 dilu-tion of conc. H2SO4 was cooled to below 5◦C anda solution of NaNO2 (0.76 g, 0.011 mol) in water(20 mL) added dropwise so that the temperature of themixture remained within the range 0–5◦C. The colddark yellow solution was added dropwise to a coldmixture of (1.44 g, 0.01 mol), NaOH (5 g, 10%) andwater (50 mL), during which the temperature of themixture was maintained below 8◦C. After diazotisa-tion, the solution was stirred for about 30 min andthen acidified with 1:1 aqueous HCl, giving the crudeproduct, which was filtered off, dried in air and recrys-tallised a number of times form alcohol (compound Cin Scheme 1).

2.3.7 2-[4-(4′-n-Alkoxybenzoyloxy)naphth-1-ylazo]-6-fluorobenzothiazole

The synthetic procedure was identical to that reportedfor phenyl-based compounds in 2.3.4.

2.3.8 2-[4-(4′-n-Octyloxybenzoyloxy)naphth-1-ylazo]-6-fluorobenzothiazole

F8: Yield: 79%; clearing point: 163◦C.UV (CHCl3) λmax: 278 nm, 290 nm and 306 nm.Elemental analysis. Found: C, 67.66; H, 5.72; N,

8.05. Calc. for C30H30N3SO3F (531 g mol−1): C, 67.79;H, 5.64; N, 7.90.

IR (KBr) υmax cm−1: 3058 (C–H Str. aromatic),2921, 2848 (C–H Str. aliphatic), 1732 (C=O Str.ester), 1606 (N=N, Str. azo) 1477, 1393, 1071(benzothiazole), 868, 849, 649 (C–S–C).

1H NMR (400 MHz, CDCl3): δ ppm 0.87–0.89 (t,3H(a), –CH3 of alkyl chain; J H–H = 8 Hz), 1.17–1.68(m, 10H(b) 5XCH2, –CH2 of alkyl chain), 1.74–1.81(quin, 2H(c), Ar–O–C–CH2 of alkyl chain; J H–H

= 3.5 Hz), 3.99–4.03 (t, 2H (d) Ar–O–CH2 of alkylchain; J H–H = 4 Hz), 6.89–6.93 (t, 2H, C-2′′ and C-6′′ due to C-3′′ and C-5′′, J 6′′5′′ = J 2′′3′′ = 6 Hz),7.00–7.23 (m (quin + octate), 6H due to C-2′, C-3′,C-5′, C-6′, C-7′ and C-′8), 7.60–7.69 (ddd, 3H, C-4, C-5 and C-7 split due to 19F at C-6 long-range coupling),7.70–7.73 (quart 1H, C-5 split due to C-4 J5.4 = 8 Hzand C-7 J5.7 = 3 Hz ), 7.90–7.93 (1t, 2H, C-3′′ and C-5′′ due to C-2′′ and C-6′′, J 3′′2′′ = J 5′′6′′ = 6 Hz),8.03–8.05 and 8.14–8.16 (d, d, 2H, C-7 splits due toC-5, J7.5 = 4 Hz and C-4, J7.4 = 2Hz).

13C NMR (CDCl3): δ ppm 14.04 (CH3),24.90–33.78 (CH2) 68.07 (OCH2), 114.38, 119.82,121.03, 123.99, 127.04, 128.12, 130.24, 132.30, 138.83,140.31, 141.80, 150.66, 152.50, 157.89, 160.52, 162.01,162.34 (Ar–C), 171.86 (–C=O–).

TOF MS ES+ m/z (rel. int. %): 531.6 (M+1)+m/z.

3. Results and discussion

3.1 Microscopic behaviourThe melting points and transition temperatures of thecompounds in Series E are given in Table 1. Thesecompounds exhibited enantiotropic mesomorphism.The SmC phase appeared in compounds E1 to E14, andthe transition temperature of the SmC–SmA phasedecreased as the series was ascended. In the sameseries, the higher homologues (C8–C18) displayed bothSmA and SmC mesophases, but in the C16 and C18

homologues only the SmA mesophase was observed.This was due to the fact that an increase in the num-ber of –CH2– groups in the alkoxy straight chainalso caused an overall increase in polarisability. While

Table 1. Transition temperatures (◦C) of 2-[4-(4′-n-alkoxy-benzoyloxy)phenylazo]-6-fluorobenzothiazoles (Series E).

ORN C

O

N O

N

SF

Transition temperature, ◦C

Compound Cr SmC SmA N IR = n-alkoxy

E1 Methyl • 116 • − − 227 • 337 •E2 Ethyl • 108 • − − 217 • 334 •E 3 Propyl • 104 • − − 216 • 332 •E 4 Butyl • 100 • − − 214 • 332 •E 5 Pentyl • 102 • − − 210 • 321 •E6 Hexyl • 105 • − − 207 • 320 •E7 Heptyl • 93 • − − 205 • 319 •E8 Octyl • 96 • 153 • 204 • 312 •E10 Decyl • 90 • 142 • − − 299 •E12 Dodecyl • 85 • 134 • − − 275 •E14 Tetradecyl • 73 • 130 • − − 272 •E16 Hexadecyl • − − 86 • − − 266 •E18 Octadecyl • − − 94 • − − 263 •

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04080

120160200240280320360

0 2 4 6 8 10 12 14 16 18 20Number of carbon atom

Tra

nsiti

on te

mpe

ratu

re

Cr - SmC/SmA

SmC - N

SmC - SmA

SmA - N

SmA - I

N - I

Figure 1. Transition temperatures versus number of carbonatoms (n) in the terminal alkoxy chain for Series E.

this is true for an individual molecule, the dilutioneffect of the increasing number of –CH2– groupsreduces the bulk polarisability, as reported by Hirdet al. [67].

The melting points and clearing points of Series Ecompounds containing an n-alkoxy group are plottedin Figure 1 and show a falling tendency. On the otherhand the SmA–N transition temperature exhibits arising tendency as the series is ascended.

It is seen that the SmC mesophase can be observedfrom the first member of the homologous Series E.This can be explained by the fact that the lengthto breadth ratio is sufficient to provide lateral forcesof cohesion, which hold together the compounds ina more ordered layer structure even after the firstmelting. The compounds in Series E have an asymmet-rical terminal benzothiazole ring containing a fluoro-group at the sixth position, and this is electronegative.Fluoro-aromatic rings are a key structural unit of anumber of functional molecules, including liquid crys-tals [68]. Terminal attraction is therefore relativelyhigh for compounds E1 to E8. The clearing temper-ature of Series E compounds are very high, due tothe presence of a heterocyclic ring and F substituted(electronegative element) benzene ring compared withdifferent substituted electronegative elements with dif-ferent electronegative elements and the fact that themolecule is itself asymmetrical.

The compounds in Series F exhibit an enan-tiotropic nematic mesophase. Smectic phases were notobserved of any type, even in the higher homologues.This means that Series F was entirely nematogenic incharacter. Looking at the compounds of Series F, thesecontain a naphthalene moiety in the central core of along molecule, and this increases the breadth of themolecule. The length to breadth ratio is therefore quitelow [69], and the lateral attraction between moleculesis not high enough to form a layer; as a result nosmectic phase was observed in Series F compounds.The transition temperatures of Series F compoundsare listed in Table 2. The melting and clearing points

Table 2. Transition temperatures (◦C) of 2-[4-(4′-n-alkoxybenzoyloxy)naphtha-1-ylazo]-6-fluorobenzothiazoles(Series F).

N N ORC

O

O

N

SF

Transition temperature, ◦C

Compound Cr Sm N IR = n-alkoxy

F1 Methyl • − − 194 • 224 •F2 Ethyl • − − 178 • 211 •F3 Propyl • − − 184 • 205 •F4 Butyl • − − 170 • 199 •F5 Pentyl • − − 159 • 192 •F6 Hexyl • − − 145 • 187 •F7 Heptyl • − − 136 • 174 •F8 Octyl • − − 121 • 163 •F10 Decyl • − − 113 • 155 •F12 Dodecyl • − − 102 • 149 •F14 Tetradecyl • − − 77 • 134 •F16 Hexadecyl • − − 76 • 132 •F18 Octadecyl • − − 74 • 129 •

240

200

160

120

Tran

sitio

n te

mpe

ratu

re

80

40

00 2 4 6 8

Cr-NN-I

10 12Number of carbon atom

14 16 18 20

Figure 2. Transition temperatures versus number of carbonatoms (n) in the terminal alkoxy chain for Series F

on heating Series F compounds containing a n-alkoxygroup are plotted in Figure 2, and show that the N–Itransition temperature exhibits a falling tendency, andthere was no noticeable odd–even effect observed forthe lower members of the series.

3.2 Texture and DSC studiesThe mesophases exhibited by the compounds in SeriesE and Series F were identified according to their opti-cal textures, observed by polarising optical microscopyusing the classification system reported by Sackmannand Demus [70] and Gray and Goodby [71].

The DSC thermogram of compound E8 (Figure 3)showed four endothermic peaks. The first of threecorresponded to the crystal to mesophase transition

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Liquid Crystals 243

Figure 3. DSC thermogram of Compound E8 (Series E)

Figure 4. Schlieren texture of SmC of Compound E-8 observed at 88◦C

whereas the second and third peaks are mesophase tomesophase (SmC–SmA and SmA–N) and the fourthpeak represents the mesophase to isotropic (N–I) tran-sition. Typical textures were observed for compoundE8 at various temperatures and these are illustrated inFigures 4 to 6.

Similarly, the DSC thermogram of compound F8

showed two endothermic peaks during the heatingcycle, as shown in Figure 7, the first of these cor-responding to the crystal to mesophase (Cr–N) andthe second to the nematic to isotropic (N–I) phasetransition, respectively. During the cooling cycle oneexothermic peak was observed, indicating that onlyone mesophase was exhibited by compound F8. A typ-ical Schlieren (threaded) texture with two and fourbrushes was observed, which was identified as thenematic phase, and shown in Figure 8. The transition

Figure 5. Focal conic texture of SmA of Compound E-8 observed at 146◦C

temperatures obtained from DSC thermograms andby POM, and also the values of �H and �S, are givenin Table 3.

3.3 Mesogenic properties and molecular constitutionThere is a close relationship between mesomorphismand molecular constitution in organic compounds.This means that transition temperatures and the widthof the mesophase can be correlated with the molec-ular composition of the compounds. Table 4 sum-marises the transition temperatures, the width ofthe mesophase, the thermal stability and molecularstructure of representative compounds, E12 and F12,of Series E and F, and structurally related compounds

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Figure 6. Schlieren (thread-like) texture of Compound E-8 observed at 200◦C

I [34], II [72], III [42], IV [73] and V reported in theliterature [74].

For E12 the SmC–SmA and SmA–I mesophaseranges were 49◦C and 141◦C, respectively. The com-pound F12 was purely nematogenic and its N toI mesophase range was 47◦C. The clearing pointsof compounds E12 and F12 were 275◦C and 149◦C,respectively. The absence of the nematic mesophasein compound E12 was the result of the phenylgroup in the central core, which gives greater lin-earity to the structure of the molecule (E12) thanthe naphthyl group. Overall, compound E12 showedgreater mesophase thermal stability and width of themesophase than did F12.

In terms of molecular structure, compound E12

differed from compound F12 only in its central aro-matic core. The presence of the naphthalene nucleus

Figure 7. DSC thermogram of Compound F8 (Series F)

Figure 8. Schlieren (thread like) texture of Compound F-8observed at 121◦C

at the centre increases the breath of the molecule.Gray [69] has suggested that an increase in the breathof a molecules reduces both nematic and smecticmesophase stability. The presence of a naphthalenenucleus at the centre not only increases the breathof the molecule but also weakens lateral attractionbetween them. Both of these factors would tend toeliminate smectogenic tendencies as well as decreasingthe mesophase range.

Table 4 shows also that the SmC mesophase widthand clearing point of compound E12 are higher by28◦C and 74◦C, respectively, than those of compoundIV. Compounds E12 and IV differ only at the termi-nals. However, the higher mesophase thermal stability

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Liquid Crystals 245

Table 3. DSC and POM data for representative compounds in Series E and F.

Series Compound Transition Peak/(POM)Temp. (◦C) �H J g−1 �S J g−1 (◦K)

E E8 Cr–SmC 87.67 (96) 8.31 0.0948SmC–SmA 145.8 (153) 11.67 0.0800SmA–N 200.00 (204) 2.51 0.0125N–I 311.05 (312) 59.14 0.1901

E14 Cr1–Cr 66.80 (67) 1.75 0.0262Cr–SmC 71.97 (73) 2.01 0.0279SmC–SmA 127.64 (130) 10.07 0.0789SmA –I 273.86 (272) 77.25 0.2800

F F8 Cr–N 120.70 (121) 14.34 0.1188N–I 162.89 (163) 11.21 0.0688

F14 Cr–N 76.95 (77) 3.24 0.0421N–I 134.62 (134) 5.03 0.0374

Table 4. Comparison of mesophase width and thermal stability (◦C) of E12, F12 and structurally relatedcompounds I to V.

Transition temperature (◦C) Mesophase width

Compound Cr Sm C Sm A N I Sm NCommencement

of Sm phase

E12 • 85 134 − 275 49/141 − C1

F12 • − − 102 149 − 47 −I • − 151 − 277 126 − C1

II • − 115 264 268 149 4 C1

III • − − 131 254 − 123 −IV • 112 − 133 201 21 68 C10

V • − − 111 144 − 33 −

of compound E12 compared with compound IV is dueto the terminal fluoro-containing benzothiazole ringsystem, which increases the overall polarisability [75]of the molecule and reduces its symmetry comparedwith the phenyl or naphthyl derivatives, giving a highertransition temperature.

Comparing compound F12 with V, the width ofthe mesophase and clearing point of compound F12

were higher by 14◦C and 5◦C, respectively, than thoseof compound V; this is due to the fact that com-pound F12 had only one naphthalene ring in thecentral core, whereas compound V had two naphtha-lene rings, increasing its overall molecular breadth.Similarly, compound E12 exhibited an enantiotropicSmC mesophase, together with SmA, whereas com-pound I exhibited only enantiotropic SmA.

The clearing point of compound I was higher by2◦C than that of compound E12. This may be the resultof the more polar –NO2 group at the sixth position incompound I, compared with the less polar –F in E12.The more polar –NO2 group increased the polarisabil-ity of the molecule, and possibly also the moleculardipolarity of compound I compared with compoundE12. It has been observed in Table 4 that compound

E12 was smectogenic only, showing SmC and SmAmesophases, whereas compound II exhibited enan-tiotropic SmA and nematic mesophases. The smecticmesophase width and overall clearing point of com-pound E12 were higher by 41◦C and 7◦C, respectively,than those of compound II. These may have been dueto the more polar F-group in compound E12, com-pared to the less polar Cl-group at a similar position incompound II. This is also reflected in compound E12

in comparison with compound III. The order of thepolarity of the substituted group at the 6-position ofthe benzothiazole ring was: NO2 > F > Cl > OCH3.

4. Conclusions

In this article we have presented the synthesisand characterisation of two series of 6-fluorobenzo-thiazole-based liquid crystals differing at the centrallinkage (an azo or ester group) and in the core unit(naphthalene or benzene). The mesomorphic proper-ties exhibited by compounds of Series E and SeriesF show that the former had higher mesophase ther-mal stability due to the presence of a naphthyl in place

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of a phenyl group. Compounds of Series E exhibitednematic mesophases along with SmC and SmAmesophases, whereas Series F was purely nematogenic.By comparing the present series with other structurallyrelated series it was found that the benzothiazole ringgreatly affected the thermal stability of the mesophase.The study also showed that the naphthalene analogueof the benzothiazole ring favoured the formation ofthe nematic mesophase. It was also observed that thefluoro-substituent was more conducive to the genera-tion of the smectic mesophase than was the chloro- orthe methoxy-substituent, and was less conducive thanthe nitro-substituent. The order of polarity of the sub-stituted group at 6-position of benzothiazole ring wasNO2 > F > Cl > OCH3.

Acknowledgments

We are grateful to Gujarat Narmada Valley FertilizerCompany Ltd, Bharuch, for providing facilities for the ele-mental analyses, to Atul Industries Ltd, Atul, for DSCanalyses, and to SAIF Chandigarh for FT–IR, 1H–NMR,13C–NMR and mass spectral analyses. We also wish to thankProfessor D.W. Bruce, University of York, UK, for providingliterature.

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Molecular Crystals and LiquidCrystalsVolume 575, Issue 1, 2013

Synthesis and Mesomorphic Investigations of Liquid Crystalline Compounds Having aBenzothiazole Ring

B. T. Thakera, N. J. Chothania, B. S. Patela, Y. T. Dhimmara, D. B.Solankia, Neeraj Patela, K. B. Patela & U. Makawanaa

pages 64-76

AbstractTwo homologous series of calamitic liquid crystals containing a benzothiazole ring and twodifferent linkages have been prepared, and their liquid crystalline properties are studied andcompared with each other and those of similar structure. The mesogens with only the cinnamatelinking group showed better thermal properties than those with an ester. Nematic and smecticphases were observed. All the compounds of both the series were characterized by elementalanalysis, FT-IR, mass spectrometry, 1H-NMR, and 13C-NMR. Phase transition temperatures andthe thermal parameters were obtained from differential scanning calorimetery (DSC). Thetextural observations were performed using hot-stage Polarizing Optical Microscopy (POM).

Synthesis, characterization and liquid crystalline properties of someSchiff base and cinnamate central linkages involving 1,3,5-trisubstitutedpyrazolone ring system and their Cu(II) complexes

B.T. Thaker*, D.B. Solanki, B.S. Patel, and Neeraj B. Patel

Department of Chemistry, Veer Narmad South Gujarat University, Surat-395007, India

*E-mail: [email protected] new mesogenic homologous series, each containing 1,3,5-trisubstituted pyrazolone

derivatives, 4-n-alkoxyphenyl and Schiff base-cinnamate central linkages, have been synthesizedto give 4-[(5-hydroxy-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl) methyleneamino] phenyl3-(4-n-alkoxyphenyl)acrylate [Series-A] and 4-[(5-hydroxy-3-methyl-1-p-tolyl-4,5-dihydro-1Hpyrazol-4-yl)methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-B] and theirCu(II)complexes have also been synthesized. These compounds were characterised by elementalanalysis, Fourier transform infrared, 1H nuclear magnetic resonance, 13C-NMR, ultravioletvisibleand mass spectral studies. Their mesomorphic behaviour was studied by polarising opticalmicroscope (POM) with a heating stage. POM data were compared with differential scanningcalorimetry thermograms. In Series-A and -B all compounds exhibit mesomorphism. Series-Acompounds exhibit an enantiotropic nematic mesophase except propyl derivative, while asmectic A mesophase is observed from the heptyl derivative and persists up to the last memberof the homologous series. n-Heptyloxy derivative is monotropic for smectic A phase. Series-Bcompounds also exhibit the enantiotropic nematic mesophase, while the smectic A mesophase isobserved from the heptyl derivative and persists up to the last member of the homologous series.n-Dodecyloxy derivative exhibits monotropic smectic A and nematic mesophases. Themesomorphic properties of both series are compared with each other and the other structurallyrelated compounds. The study reveals that cinnamate linkage containing liquid crystals havehigher thermal stability compared to structurally related series containing chalcone linkage. Incase of complex series, only one compound from each series gives nematic mesophase.Keywords: Schiff base, cinnamate, liquid crystal, pyrazoloneThis work has been presented at19thNational Conference on Liquid Crystal at ThaperUniversity, Patiala (India) during 21-23, November, 2012.URL: http://mc.manuscriptcentral.com/tlct Email: [email protected]

Dear Prof. ThakerYour manuscript entitled "Synthesis, characterization and liquid crystalline properties of someSchiff base and cinnamate central linkages involving 1,3,5-trisubstitutedpyrazolone ring system and their Cu(II) complexes" which you submitted to Liquid Crystals, hasbeen reviewed. The reviewer's comments are included at the bottom of this letter.

The review is favourable and suggests that, subject to minor revisions, your paper is suitable forpublication. Please consider these suggestions, and I look forward to receiving your revision.

Yours sincerelyProf. Corrie ImrieEditor, Liquid [email protected]

Publications:

Published a paper “Studies of Calamitic Liquid Crystalline Compounds

Involving Ester-Azo Central Linkages with a Biphenyl Moiety” in journal of

Mol. Cryst. Liq. Cryst., Vol. 548: pp. 172–191, 2011.

Published a paper “Synthesis, characterisation and liquid crystalline

properties of some Schiff base-ester central linkage involving 2, 6-

disubstituted naphthalene ring system” in journal of Liquid Crystals, Vol.

39, No. 5, 551–569, 2012.

Published a paper “Synthesis, Characterization and Mesomorphic Properties

of New Rod-like Thiophene Based Liquid Crystals” in journal of Mol.

Cryst. Liq. Cryst., Vol. 562: pp. 98–113, 2012.

Published a paper “Mesomorphic studies of novel azomesogens having a

benzothiazole core: Synthesis and characterization” in journal of Liquid

Crystals, Vol. 40, No. 2, 237–248, 2013.

Published a paper “Synthesis and Mesomorphic Investigation of Liquid

Crystalline Compounds Having a Benzothiazole” in journal of Mol. Cryst.

Liq. Cryst., Vol. 575: pp. 64–763, 2013.

Accepted a paper “"Synthesis, characterization and liquid crystalline

properties of some Schiff base and cinnamate central linkages involving 1,

3, 5-trisubstituted pyrazolone ring system and their Cu(II) complexes"” in

journal of Liquid Crystals, 2013.

Workshop:

Attended two day state level workshop on “ Symmetry, Group Theory and

Spectroscopy” organized by the Department of Chemistry, Navyug Science

College, Surat held on 26th-27th September, 2009.

Conferences:

Participated 2nd National conference on Thermodynamics of Chemical and

Biological Systems organized by the Department of Chemistry, Veer Narmad

South Gujarat University, Surat held on 30th-1st November, 2006.

Presented a paper “Synthesis and characterization of novel pyrazole

containing mesogens” at National Conference on Green Chemistry organized

by the Department of Chemistry, Veer Narmad South Gujarat University,

Surat held on 6th-8th February, 2009.

Presented a paper in International conference on Polymer Science and

Technology organized by Asian Polymer Association at Delhi IIT, Delhi held

on 17th-20th December, 2009.

Presented a paper “ Synthesis, Characterization and Mesomorphic Properties

of New Liquid Crystalline compounds containing pyrazolone moiety” in 17th

National Conference on Liquid Crystals organized by the Department of

Chemistry, Veer Narmad South Gujarat University, Surat held on 15th-17th

November, 2010.

Presented a paper “Design, synthesis, characterization and mesomorphic

properties of symmetrical binary dimmers and their metal-complexes

involving 2-n-alkoxy-6-naphthoicacid” in 19th National Conference on Liquid

Crystals organized by Thaper University, Patiala held on 21st-23rd November,

2012.

linkages with a biphenyl moiety compounds involving...

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