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Studies of Calamitic Liquid Crystalline Compounds Involving Ester-Azo Central Linkages with a Biphenyl Moiety a

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B. T. Thaker , Y. T. Dhimmar , B. S. Patel , D. B. Solanki , N. B. a

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Patel , N. J. Chothani & J. B. Kanojiya

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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 Central Linkages with a Biphenyl Moiety, Molecular Crystals and Liquid Crystals, 548:1, 172-191 To link to this article: http://dx.doi.org/10.1080/15421406.2011.591677

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

Downloaded by [Veer Narmad South Gujarat University] at 03:26 18 April 2012

Studies of Calamitic Liquid Crystalline Compounds 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 biphenyl moiety 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 hydrogen bonding exhibits a high-temperature enantiotropic smectic phase. Whereas in series II, compounds containing C1 –C8 carbon atoms exhibit only a monotropic smectic phase and compounds with C10 , C12 , C14 , and C16 atoms show an enantiotropic smectic phase. These compounds were characterized by elemental analysis, FT-IR, 1H-NMR, and mass spectral studies. The phase transition and mesogenicity of these substances were studied by polarizing optical microscopic and differential scanning calorimetric techniques. 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 to keep in mind that their mesogenic behavior is strongly influenced by the structure of the rigid molecular core and by the lateral substitution on the aromatic rings, with the position of the substituted ring in the molecular core being important [1]. Liquid crystal oligomers consist of molecules containing two or more mesogenic units interconnected via flexible spacers, most commonly alkyl chains [2–5]. There is substantial literature on the studies of biphenyl and its derivatives. 2,3,4-monosubstituted biphenyls have been studied extensively by various workers [6–13] for their molecular geometry, crystallization behavior, crystal packing, and thermal motion, while the literature on growth and structural aspects of linearly chained biphenyls (liquid crystals) is quite insufficient. Biphenyl esters are typical mesogens with various mesophases having different degrees of order according to substituents [14]. There are many examples of rigid, extended chemical subunits in mesogens. The most common subunit used in synthesizing ∗ Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad South Gujarat University, Surat, Gujarat, India+91-261-226-7957. E-mail: [email protected]

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Liquid Crystals with Ester-Azo Linking Groups

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

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

B. T. Thaker et al.


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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 compounds 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 were used 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 standard methods described in the literature [23]. Other auxiliary chemicals were of laboratory grade. 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-IR spectrophotometer 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 instrument using CDCl3 as a solvent and tetramethylsilane as an internal reference at SAIF, Panjab University, Chandigarh, India. Also, mass spectra of the compounds were recorded at SAIF. Thin layer chromatography (TLC) analyses were performed using aluminum-backed silicagel plates (Merck60 F524) and examined under shortwave UV light. The phase transition temperatures were measured using Shimadzu DSC-50 at heating and cooling rates of 10◦ C min−1, respectively. The textures of the mesophase were studied using a Leitz Labourlux polarizing microscope provided with a Kofter heating stage at the Applied Chemistry Department, 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).

RO

N N

OH

4 -[(4-n-alkoxyphenyl)diazenyl] biphenyl-4-ol (where R = C10 H21 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), anhydrous potassium carbonate (0.15 mol), respectively, n-alkyl bromide (0.15 mol), and dry acetone (60 ml) were taken in a round-bottom flask (RBF) provided with a condenser and a guard tube. The reaction mixture was refluxed in a water bath for 8–10 hr. The whole mass was then added to water and extracted with ether. The ether was evaporated and the residual solids were obtained as alkoxy acetanilides.


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

RO

N N

OH

4 -[(4-n-alkoxyphenyl)diazenyl] biphenyl-4-ol (where R = C14 H29 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 cooled to room temperature. The mixture was made alkaline with 50% NaOH at 20◦ C. The oily product (for the lower members C1 –C8 ) was extracted with ether. The ether extract was dried 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 ether extraction. The boiling points and melting points of all the alkoxy anilines agree well with the values reported in the literature [24–26]. Diazotization of Alkoxy Aniline [27]. Alkoxy aniline (0.005 mol, 1.15 g) was taken in 50 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 1 hr. A solution of NaNO2 (0.005 mol, 0.35 g) in water (5 ml) previously cooled to 0◦ C was then added over a period of 5 min with stirring. The solution was further stirred for another 1 hr. At this stage, Congo red paper turns blue and starch iodide paper also turns blue. It showed the positive test (i.e., the presence of nitrous acid). Then, sulfamic acid was added to 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 a clear solution, which was used for subsequent coupling reaction. Synthesis of 4 -[(4-n-Alkoxyphenyl)Diazenyl]Biphenyl-4-ol. To a well-stirred solution of 4-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).

RO

O

N N

C O

OC 4H 9

4 -[(4-n-alkoxyphenyl)diazenyl]4-butoxy phenyl biphenyl-4-carboxylate (where R = C10 H21 series II).

of alkoxy aniline gradually for 1 hr at 0◦ C–5◦ C. The pH (7.0) was maintained by the addition of NaOH solution (10% w/v). The mixture was stirred for another 3–4 hr for complete separation and the dye was isolated by filtration, washed with water, dried, and crystallized from ethyl acetate to get orange-colored crystals. The yield was about 75%. All the 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 calculated C 78.10%, H 7.96%, N 6.51%; found C 78.44%, H 8.20%, N 6.02% for C28 H34 N2 O2 ; FT-IR (KBr pallete) 3061 cm−1 (–C–H aromatic stretching), 2850–2920 cm−1 (–CH3 aliphatic stretching), 1598 cm−1 (–N N–), 1418–1481 cm−1 (–CH bending of CH2 ), 1367–1372 cm−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 of compound 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 in Series I.


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

RO

O

N N

C O

OC 4H 9

4 -[(4-n-alkoxyphenyl)diazenyl]4-butoxy phenyl biphenyl-4-carboxylate (where R = C14 H29 series II).

Diazotization of Alkoxy Aniline. Diazotization of alkoxy aniline was carried out as described in Series I. Synthesis of 4 -[(4-n-Alkoxyphenyl)Diazenyl]Biphenyl-4-ol. Synthesis of 4 -[(4-nalkoxyphenyl)diazenyl]biphenyl-4-ol was carried out as per the procedure described in Series 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) were dissolved in methanol (lower member)/ethanol (higher member) (100 ml) in an RBF fitted with a reflux condenser and refluxed the mixture in a water bath for 8 hr. Then 10% aqueous KOH solution (25.0 ml) was added to the flask and reflux continued for 2–3 hr to hydrolyze any ester if formed. The solution was cooled to room temperature, and the reaction mixture was acidified by pouring 1:1 ice-cooled dilute HCl and water; the precipitated mass was filtered and washed by water. Then, the isolated mass was dried in a vacuum oven. The alkoxy acids were crystallized in methanol until a constant transition temperature was obtained. The transition temperature thus obtained is in good agreement with the values reported in the literature. Preparation of 4-n-Alkoxy Benzoyl Chlorides [30]. 4-n-alkoxy benzoic acid (0.01 mol, 2.5 g) 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 refluxed in a water bath till the evolution of HCl gas ceased. Excess of thionyl chloride was distilled off under pressure using a vacuum pump and the 4-n-alkoxy benzoyl chloride left behind was 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 pyridine (10 ml) and was added dropwise with occasionally stirring in to ice-cold 4-butoxy benzoyl chloride (0.002 mol, 0.39 g) in an RFB. Then the mixture was refluxed in a hot water bath for 2 hr and was allowed to stand for overnight. The mixture was acidified using cold 1:1 diluted HCl to precipitate the product. The solid obtained was filtered, washed successively with saturated NaHCO3 solution, dilute NaOH solution, and two to three times with water; the crude solid thus obtained was purified a number of times using hot water until a constant melting temperature was obtained. The purity of all of these compounds was checked by TLC, yield in general 65%–75%. All the compounds have been purified by column chromatography on silica gel (80–120 mesh) using a mixture of ethyl acetate/petroleum ether (7/3). Data. Compound B10 (Series II). Yield: 66%, MP 85◦ C; elemental analysis for calculated C 77.20%, H 7.64%, N 4.62%; found C 77.57%, H 7.95%, N 4.91% for C39 H46 N2 O4 ; 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 ion peak as (M+1)+ at 607.2 (m/z).

Results and Discussion The compounds of both series are subjected to elemental analysis. The elemental analysis data agreed with theoretical values as per the expected structure. The FT-IR spectra of representative compounds are shown in Figs. 1–4. The 1H-NMR spectra of representative compounds are shown in Figs. 5–8. The mass spectra of these compounds are shown in Figs. 9 and 10. The m/z ratios obtained from the spectra of representative samples are matched with their molecular ion peak. The purity of the compounds is checked by TLC. It shows one spot, indicating single compound. All the compounds were purified by column chromatography 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-catalyzed coupling 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 , and C16 carbon atoms in the alkoxy chain followed by diazotization with 4-hydroxy biphenyl using a classical method [31] and the mesogenic properties were investigated for these compounds.


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Table 1. Transition temperature data of series I


Transition temperature (◦ C) Code no.

R = n-alkyl

Sm

I

A1 A2 A3 A4 A5 A6 A7 A8 A10 A12 A14 A16

Methyl Ethyl Propyl Butyl Pentyl Hexyl Heptyl Octyl Decyl Dodecyl Tetradecyl Hexadecyl

156 150 144 148 130 129 130 104 72 89 87 86

182 177 173 160 158 155 140 136 119 113 102 104

RO

N N

OH Series I

In another case, the –OH group of a biphenyl moiety is esterified by the (C4 ) alkoxy acid. On the one end, the number of carbon atoms in the alkoxy chain was fixed (C4 H9 ) 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 and II were determined by a hot-stage polarizing microscope. Transition temperatures of both series are given in Tables 1 and 2. In series I, all compounds exhibit enantiotropic smectic-A mesophase. The plot of transition 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 falling tendency. In series II, compounds containing C1 –C8 carbon atoms in the alkoxy chain are monotropic smectic-A liquid crystalline compounds, whereas C10 , C12 , C14 , and C16 are enantiotropic smectic-A compounds. The plot showing transition temperatures versus the number 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 representative compounds are given in Table 3. Both the data are almost comparable. The DSC curves of compounds 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 on mesomorphic behavior in a system that was studied previously. The texture of the liquid crystalline compounds is given as microphotographs in Fig. 17.

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Transition temperature (˚C)


200

Sm

180

I

160 140 120 100 80 60 40 20 0 0

2

4

6

8

10

12

14

16

Number of carbon atoms

Figure 11. Transition temperature graph of series I.

It has been observed that series I compounds have high transition temperature than that of series II compounds, in spite of high molecularity of compounds. This is because of 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 B2 B3 B4 B5 B6 B7 B8 B10 B12 B14 B16


142a 120a 136a 96a 110a 118a 85a 84a 74 80 100 82

162 155 149 142 136 130 128 116 94 92 114 95

a

Monotropic phase.

RO

O

N N

C O Series II

OC 4H 9


185

Transition temperature (˚C)


180 160 140

Sm

120

I

100 80 60 40 20 0

0

2

4

6

8

10

12

14

16

Number of carbon atoms

Figure 12. Transition temperature graph of series II.

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

N N

OH

Series I RO

O

N

C

N

O

OC 4H 9

Series II R

N N

OH

Series A

C n H 2n+1

N N

O C O

C 5 H 11

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 difficulty for higher analogues. The present compound is smectogenic having a higher clearing temperature 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 and biphenyl moieties in the central core and having central linkages are identical in both types of 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 indicated by Gray et al. [33].

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



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 (series II) reveals that the clearing temperatures of series B compounds are greater than those of the present series II (Tables 5 and 6). Another interesting observation seen in the above series is that when the hydrocarbon chains are connected directly to the benzene ring, the mesophase 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 Code no.

Transition

A10

Cr-Sm Sm-I Cr-Sm Sm-I Cr-Sm Sm-I Cr-Sm Sm-I

A14 B10 B14

Peak temperature (Microscopic temperature) (in ◦ C)

H (J g−1)

71.26 (72) 119.15 (119) 90.46 (87) 99.15 (102) 74.73 (74) 92.50 (94) 100.24 (100) 114.12 (114)

15.5776 27.7671 27.5514 27.7671 25.3786 47.2723 10.1864 33.4346

S (J g−1 k−1) 0.0926 0.0406 0.2070 −0.2367 −0.3153 0.0780 0.1313


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Table 4. Transition temperature data of series A and I Series A


n (number of carbon atoms)

Series I Isotropic (◦ C)

n (number of carbon atoms)

Isotropic (◦ C)

76 77 87 91 98 100 104 – – – – –

1 2 3 4 5 6 7 8 10 12 14 16

182 177 173 160 158 155 140 136 119 113 102 104

8 10 12 14 16 18 20 22 – – – –

first member of the carbon chain, while smectic-C mesophase starts from higher homologues say C14 . In the present series II, the clearing temperatures are lower than those of series B. For the first carbon atom up to C16 carbon atom in the alkyl chain, smectic-A mesophase was obtained because both ends of the series have alkoxy groups and the middle core 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 smecticA phase. From the comparison of series I and II, it has been observed that the transition temperature 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) 2 4 6 8 10 12 14 16 18 20 22

Smectic-C

Nematic

Isotropic

– – – – – – 80.0 (Sm-C) 84.0 (Sm-C) 86.0 (Sm-C) 90.0 (Sm-C) 86.0 (Sm-C)

120 84 76 80 86 79 84.7 89.2 93.3 96.5 100.9

207.41 200.1 187.7 177.6 169.1 159.1 153.1 146.4 140.7 135.5 130.5

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n (number of carbon atoms) 1 2 3 4 5 6 7 8 10 12 14 16

Smectic A

Isotropic

142 120 136 96 110 118 85 84 74 80 100 82

162 155 149 142 136 130 128 116 94 92 114 95

bonding present in series I. In series II, mesomorphic compounds show smectic-A phase from 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 central core.

Acknowledgments We are grateful to Prof. R. A. Vora, retired Prof. and Head, Department of Applied Chemistry, Faculty of Technology and Engineering, Kalabhavan, M. S. University of Baroda, Vadodara, India, for his valuable suggestions. We are also thankful to CDRI, Lucknow, Atul Industries 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 Liquid Crystals: 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 BucurestiChimie, 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.


Liquid Crystals Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tlct20

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

a

a

a

a

a

B.T. Thaker , N.J. Chothani , Y.T. Dhimmar , B.S. Patel , D.B. Solanki , N.B. Patel , a

J.B. Kanojiya & R.S. Tandel

a

a

Department of Chemistry, Veer Narmad South Gujart University, Surat-, 395007, India Version 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 involving 2, 6- disubstituted naphthalene ring system, Liquid Crystals, 39:5, 551-569 To link to this article: 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 central linkage 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-ester as central linkage, have been synthesised by esterification of 4-{[(4-hydroxyphenyl) imino] methyl} phenyl 4-propoxy benzoate, 4-{[(4-hydroxyphenyl) imino] methyl} phenyl 4- (pentyloxy) benzoate, 4-{[(4-hydroxyphenyl)imino]methyl}-2-methoxyphenyl 4- nitrobenzoate and 4-{[(4-hydroxyphenyl)imino]methyl}-2-methoxyphenyl 4- chlorobenzoate with different 6-alkoxy 2-naphthoic acid to give Series-A, -B, -C and -D, respectively. These compounds were characterised by elemental analysis, Fourier transform infrared, 1 H nuclear magnetic resonance, ultraviolet-visible and mass spectral studies. Their mesomorphic behaviour was studied by polarising optical microscope (POM) with a heating stage. POM data were compared with differential scanning calorimetry thermograms. In Series-A and -B all compounds exhibit mesomorphism. Series-A compounds exhibit a enantiotropic nematic mesophase, while a smectic A mesophase is observed from the butoxy derivative and persists up to the last member of the homologou series. Series-B compounds also exhibit the enantiotropic nematic mesophase, while the smectic A mesophase is observed from the ethoxy derivative and persists up to the last member of the homologou series. The mesomorphic properties of both series are compared with each other and the other structurally related Series-C and –D compounds. In Series-C and -D all compounds exhibit the only nematic mesophase; no smectic mesophase is observed even for higher members of the homologous. The aim of the research was to synthesise and characterise novel liquid crystalline compounds containing 2,6-disubstituted naphthalene and to study their mesomorphic properties. Keywords: Schiff base; ester; smectic; nematic; naphthalene

1.

Introduction

The molecules of liquid crystalline compounds are elongated; they are rod or lath-shaped, thin and often flat, possessing middle and terminal polar groups. Molecules which form liquid crystals have dipoles in their structure, with often a strong dipole towards the centre and a weak dipole towards the end of the molecules. When more than two benzene rings are linked through more than one central group, the liquid crystalline properties are enhanced the most. Although many mesomorphic compounds have been reported, little is known about the effect of changes in molecular constitution and shape on the degree of anisotropy in the melt. Inspection of the formula of mesomorphic compounds makes it clear that all the molecules are characterised by their predominant length. The molecules have a rod shape, which favours the linear molecular arrangement proposed by Friedel [1] for the smectic and the nematic states. Such molecules will have a strong tendency to lie with their long axes parallel, and this will be accentuated by any dipoles in the molecules. Only molecules broader than benzene have been found to be mesomorphic [2].

*Corresponding author. Email: [email protected] ISSN 0267-8292 print/ISSN 1366-5855 online © 2012 Taylor & Francis http://dx.doi.org/10.1080/02678292.2012.664175 http://www.tandfonline.com

Thermotropic liquid crystals have great technological importance [3]. A vast number of mesogenic compounds containing naphthalene moiety as a core system showing nematic or other mesophases have been reported [4–6]. Dave and coworkers studied a variety of liquid crystalline compounds exhibiting smectic, nematic and choleseric mesomorphism containing naphthalene moiety, such as alkoxybenzoates of 1,5- and 1,4- dihydroxynaphthalene [7]; Vora and Prajapati also reported the mesogenic homologous series of Schiff’s base-esters containing naphthalene moiety and studied the effect of lateral thiol and methoxy substituent on mesomorphism [8, 9]. Malthete et al. [10] synthesised tetra-acylated 1,4,5,8-tetrahydroxynaphthalene derivatives. In the last decade a significant number of research papers on naphthalene liquid crystal cores have appeared in the literature [11–27]. Yang and Lin [28] synthesised and characterised three analogous series of symmetric banana-shaped liquid crystalline molecules containing bisnaphthyl units. Lin et al. [29] synthesised two fused-ring structures, 6-n-decyloxy-2-naphthoic acid and 6-dodecyloxyisoquinoline, and used as a proton


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donor and acceptor moieties to construct a series of simple mesogenic supramolecules. The other complementary hydrogen-bonded (H-bonded) moieties are benzoic acids, thiophenecarboxylic acid and pyridines containing different alkyl chain lengths connected by ether and ester linkages. Kolhe et al. [30] reported the synthesis, characterisation and device studies of poly(benzobisoxazole imide) containing perylene or naphthalene units in an alternating fashion with the oxazole unit. Sandhya et al. [31] reported photoconductivity measurements in a binary system of naphthalene-based liquid crystals. Chia et al. [32] synthesised and characterised two homologous series of pyridine-containing liquid crystalline compounds, 2-(4-alkoxyphenyl)-5-phenylpyridines and 2-(6alkoxynaphthalen-2-yl)-5-phenylpyridines, and their thermotropic behaviours were studied. Seed et al. [33] synthesised compounds based on 2,6-disubstituted naphthalenes or related 1-benzothiophene moieties with butyl sulfanyl and cyno or isothiocyanato terminal group. The compounds with naphthyl and phenyl groups are solely nematogenic; for these compounds the naphthyl unit gives an average increase in the Nematic-Isotropic (N-I) value and melting point compared to the values for the compounds with a phenyl in place of the naphthyl unit. Wu and Lin [34] synthesised two series of ferroelectric liquid crystals derived from (S)-2-(6-methoxy-2-naphthyl)propionic acid, with non-fluorinated or semi-perfluorinated alkanes positioned at a chiral terminal chain and

thermal properties studied by differential scanning calorimetry, polarising optical microscopy and electro-optical measurements. Mohammady et al. [35] synthesised four homologous series belonging to the family of 4-(4-substituted phenylazo)-1-naphthyl 4- alkoxybenzoates in which the 4- substituent (X) was varied between CH3 O, CH3 , Cl and NO2 ; within each homologous series, the number of carbon atoms was varied between eight and 14. The results were discussed in terms of mesomeric, polarisability and steric effects. Kohout et al. and Novotna et al. synthesised and studied liquid crystals based on laterally substituted 7-hydroxynaphthalene-2-carboxylic [36–38]. Recently many researchers synthesised and studied liquid crystals involving naphthalene moiety [39–49]. The majority of naphthalene-based mesogens have the 2,6-disubstitution pattern as this is the most linear substitution pattern for naphthalene. Many of these 2,6-disubstituted mesogens incorporate alkoxy, alkynyl, alkyl and cyano terminal groups to give wide mesophase ranges but often with associated high melting points [50–57]. In our studies on naphthalene-based materials [58], in order to investigate the influence of terminal groups, lateral substitution and central linkage on the mesomorphic properties of liquid crystalline compounds an attempt has been made to synthesise four homologous series having Schiff base-ester central linkage with different terminal alkoxy chain and the general structural formula as follows:

Series-A RO

O

O

N O OC3H7 O

where R = C nH2n+1, n = 1 to 8, 10, 12, 14, 16; Series-B

RO

O

O

N O OC5H11 O

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

Liquid Crystals

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Series-C

RO

O OCH3 O

N O NO2

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

O

Series-D


RO

O OCH3 O

N O Cl

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

2.

Expermental

2.1 General For the synthesis of the compounds of the homologous series, the following materials were used: 4-hydroxy benzoic acid, alkyl bromide (Lancaster, England), 4-chloro benzoic acid, 4-nitro benzoic acid, 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 purchased from Mark (Germany) and dicyclohexylcarbodiimide (DCC) was purchased from Fluka Chemie (Switzerland). The solvents were used after purification using the standard methods prior to use. Elemental analysis (C, H, N) was performed on Thermo Scientific FLASH 2000 at G.N.F.C. (Gujarat Narmada Valley Fertilizer Company Ltd., Bharuch). Infrared spectra was recorded with a Thermo Scientific Nicolet iS10 FT-IR Spectrophotometer at the Department of Chemistry, Veer Narmad South Gujarat University in the frequency range 4000–400 cm−1 with samples embedded in KBr discs. 1 H nuclear magnetic resonance (NMR) spectra of the compounds were recorded with a Bruker Avance II 400 NMR spectrometer using CDCl3 and DMSO d6 as a solvent and Tetramethylsilane (TMS) as an internal reference at SAIF (Sophisticated Analytical Instrument Facilities), Chandigarh; mass spectra Electron Ionization (EI) of the compound were recorded with a Firmegan MAT-8230 mass spectrometer also at SAIF, Chandigarh. Merk 60 F524 thin

O

layer plate were used for Thin Layer Chromatography (TLC) and examined under short-wave UV light. UVvisible spectra of a 10−5 M solution of the samples using CHCl3 as solvent were recorded with a Thermo Scientific Evolution 300 UV-VIS spectrometer in the range 200–800 nm within our department. Thermal analyses and differential scanning calorimetry (DSC) of the liquid crystalline compounds were carried out at the Metteler Toledo India Pvt. Ltd, Powai, Mumbai. DSC analysis was performed on a Metteler Toledo DSC-1 with heating rate of 10◦ C/min in a N2 atmosphere. The optical microscopy studies were carried out with a Nicon Eclipse 50i POL (Japan) microscope equipped with a Linkam Analysa-LTS 420 hot stage at the Department of Chemistry, Veer Narmad South Gujarat University. The textures of the compounds were observed using polarised light with crossed polariser with the sample in a thin film sandwiched between a glass slide and cover slip. 2.2 Synthesis of series-A, -B, -C and -D compounds 2.2.1 Synthesis of 4-n-alkoxy benzoic acid (1) Compound 1 was prepared using the method reported by 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 in ethanol/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 heated under reflux for 24 h. When the reaction was complete, KOH (0.73 g, 13 mmol) was added and the mixture heated under reflux for a further 4 h. The ethanol was evaporated, and the mixture poured into water and acidified to approximately pH = 2∼3 with acetic acid. The precipitate was filtered and washed with water and ether, and then recrystallised twice from glacial acetic acid and then from ethanol. The synthetic route has also been described in the literature [28, 60]. Yield 71–80%, infrared (IR) (KBr): υ max /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 4hydroxy benzaldehyde (6.10 g, 50 mmol) in 120 mL of dichloromethane (DCM). The mixture was stirred for 12 h. The dicyclohexylurea was filtered off and washed with DCM and then the filtrate was successively washed with 5% aqueous acetic acid, 5% aqueous sodium hydroxide and water; the solvent from the filtrate was evaporated. The crude product was purified by column chromatography (silica gel, DCM/hexane 1/1) [61]. Yield 68%, clearing point (C.P.) 128◦ C IR (KBr): υ max /cm−1 : 3073, 2827, 2745, 1896, 1736, 1604, 1511, 1474, 1450, 1385, 1270, 1210, 1060, 721. 2.2.4 Synthesis of 4-formyl-2-methoxyphenyl 4-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 temperature for 24 h. After the reaction mixture was filtered, the solution was evaporated. The resulting solid was then recrystallised from CH2 Cl2 /ethanol 1:1. The crude product was purified by column chromatography (silica gel, DCM/hexane 1/1) [62]. Yield 76%, C.P. 116◦ C, IR (KBr): υ max /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 at reflux for 3 h with stirring. The solid product obtained on cooling was filtered off and recrystallised from ethanol. The crude product was purified by column

chromatography (silica gel, petroleum ether/ethyl acetate 7/1) [63, 64]. Yield 61%, C.P. 162◦ C, IR (KBr): υ max /cm−1 : 3450, 2929, 2851, 1774, 1736, 1626, 1606, 1509, 1449, 1385, 1270, 1210, 1060, 721. 2.2.6 Synthesis of 4-[(-{4-[(4-alkoxy benzoyl)oxy]phenyl}methylidene)amino]phenyl 6-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 mixture was stirred for 24 h. The dicyclohexylurea was filtered off and washed with DCM and then the filtrate was successively washed with 5% aqueous acetic acid, 5% aqueous sodium hydroxide and water; the solvent from the filtrate was evaporated. The crude product was 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 C41 H41 NO6 : calcd C, 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): υ max /cm−1 2929–2851 cm−1 (C–H Stretching (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=C Str. of aromatic), 1210,1060 cm−1 (C–O–C Str. of alkoxy), 1270 cm−1 (C–O Str. of ester), 1 H 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 attached with 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 of azomethine linkage –HC=N–). A16 : Yield 72%, C.P. 213◦ C, UV/visible λmax : 256 nm, elemental analysis for C50 H59 NO6 : 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): υ max /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=C Str. of aromatic), 1209,1060 cm−1 (C–O–C Str. of alkoxy), 1271 cm−1 (C–O Str. of ester), 1 H NMR (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 phenyl ring), 6.86–8.71 ppm (m, 18H, Ar–H), 8.27 ppm (s, 1H, aldehydic proton of azomethine linkage –HC=N–).


Liquid Crystals B7 : Yield 75%, C.P. 209◦ C, UV/visible λmax : 254 nm, elemental analysis for C43 H45 NO6 : 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): υ max /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–C Str. of alkoxy), 1271 cm−1 (C–O Str. of ester), 1 H NMR (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 phenyl ring), 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 C52 H63 NO6 : 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): υ max /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 azomethine linkage), 1605, 1511 cm−1 (C=C Str. of aromatic), 1211, 1056 cm−1 (C–O–C Str. of alkoxy), 1271 cm−1 (C–O Str. of ester), 1 H 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 of azomethine linkage –HC=N–). C7 : Yield 72%, C.P. 226◦ C, UV/visible λmax : 254 nm and 302 nm, elemental analysis for C39 H36 N2 O8 : 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): υ max /cm−1 2928–2851 cm−1 (C–H Str. of aliphatic), 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 ) 1 H 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 with naphthalene ring), 7.10–8.70 ppm (m, 17H, Ar–H), 3.90 ppm (s, 3H, OCH3 ), 8.25 (s, 1H, aldehydic proton of azomethine linkage). C16 : Yield 70%, C.P. 195 ◦ C, UV/visible λmax : 254 nm and 304 nm, elemental analysis for C48 H54 N2 O8 : 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): υ max /cm−1 2927–2850 cm−1 (C–H Str. of aliphatic), 1736 cm−1 (C=O Str. of ester), 1627 cm−1 (–C=N– Str. of azomethine linkage), 1593, 1503 cm−1

555

(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 ) 1 H NMR (DMSO–d6 ): δ 0.85–0.88 ppm (t, 3H, CH3 ), 1.25–1.87 ppm (m, 30H, 15×CH2 ), 4.10–4.12 ppm (t, 2H, OCH2 attached with naphthalene ring), 7.12–8.72 ppm (m, 17H, Ar–H), 3.90 ppm (s, 3H, OCH3 ), 8.28 (s, 1H, aldehydic proton of azomethine linkage). D7 :Yield 62%, C.P. 185◦ C, UV/visible λmax : 254 nm and 302 nm, elemental analysis for C39 H36 ClNO6 : 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): υ max /cm−1 2928–2851 cm−1 (C–H Str. of aliphatic), 1738 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), 1208, 1065 cm−1 (C–O– C Str. of alkoxy), 1271 cm−1 (C–O Str. of ester), 1089, 750 cm−1 (C–Cl) 1 H NMR (DMSO–d6 ): δ 0.86–0.90 ppm (t, 3H, CH3 ), 1.25–1.88 ppm (m, 12H, 6×CH2 ), 4.10–4.12 ppm (t, 2H, OCH2 attached with naphthalene ring), 7.10–8.72 ppm (m, 17H, Ar–H), 3.87 ppm (s, 3H, OCH3 ), 8.25 (s, 1H, aldehydic proton of azomethine linkage). D16 : Yield 72%, C.P. 179◦ C, UV/visible λmax : 252 nm and 304 nm, elemental analysis for C48 H54 ClNO6 : 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): υ max / cm−1 2928–2851 cm−1 (C–H Str. of aliphatic), 1736 cm−1 (C=O Str. of ester), 1626 cm−1 (–C=N– Str. of azomethine linkage), 1593, 1502 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) 1 H 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 with naphthalene ring), 7.09–8.70 ppm (m, 17H, Ar–H), 3.88 ppm (s, 3H, OCH3 ), 8.24 (s, 1H, aldehydic proton of azomethine linkage) (see Scheme 1). 3.

Result and discussion

The elemental analysis data and other physical parameters of the four series are in agreement with the theoretical values as per the expected structure. The purity of the compounds has been checked by thin layer chromatography, which shows a single spot, indicating a single compound. The representative compounds have been characterised by UV-visible, Fourier transform infrared, 1 H NMR and mass spectral studies. The mesomorphic properties and thermal stabilities of all the compounds of Series-A, -B, -C and -D were determined by a polarising optical microscope attached with a Linkam heating stage and DSC.

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B.T. Thaker et al. HOOC

i

OH

HOOC

OR1

1 HO ii

COOH

RO COOH

2 O

1

+ OHC

iii

OH

R 1O O

CHO


3 O

3

+

H2N

iv

OH

R1O O N

OH

4

4+2 V RO

O

O

N O OR1

5

OHC

OH

+

HOOC

O

X

vi

O X O

CHO

OCH3

7

6

8

H3CO

O

8

+

H2N

OH

iv

X O N

OH

H3CO

9+2

9

V RO

O OCH3

O

N O X

10 Where, R = CnH2n+1, n = 1 to 8, 10, 12, 14, 16 R1 = -C3H7, -C5H11 X = -NO2, -Cl

O

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 CH2 Cl2 , stirred at room temperature, 12 h; (iv) ethanol, reflux 3 h with stirring; (v) DCC, DMAP, dry CH2 Cl2 , stirred at room temperature, 24 h. (vi) DCC, DMAP, dry tetrahydrofuran (THF), stirred at room temperature.

Liquid Crystals Table 1. Transition temperature data of the 4-[(-{4[(4- propoxybenzoyl)oxy]phenyl}methylidene)amino]phenyl 6-alkoxy-2-naphthoate (Series-A). Transition temperature (◦ C)

A1 A2 A3 A4 A5 A6 A7 A8 A10 A12 A14 A16

R= n alkoxy

Cr


• • • • • • • • • • • •

SmA 225 230 216 91 85 77 73 72 72 68 66 65

– – – • • • • • • • • •

N – – – 224 210 190 190 206 204 196 184 175

• • • • • • • • • • • •

I 237 239 232 234 223 229 221 226 221 218 214 213

• • • • • • • • • • • •

3.1. Mesomorphic properties In Series-A, methoxy to n-hexadecyloxy derivatives exhibit an enantiotropic nematic mesophase (threadlike textures). The smectic A (SmA) mesophase commences from the n-butyloxy derivative and persists up to the last member of the homologous series. The transition temperatures of Series-A are given in Table 1. The plot of transition temperatures versus the number of carbon atoms in the alkoxy chain (Figure 1) exhibits the usual odd–even effect for the solid to isotropic transition and as the series is ascended the curve shows a tendency to fall for the mesomorphic– isotropic transition temperature throughout the series.

Series-A also exhibits a usual odd-even effect for the crystal to nematic or smectic A to nematic (T Cr-N or T SmA-N ) transition temperatures for lower members and a tendency to fall for the compounds A-8 to A16. From the figure it can also be seen that the crystal to smectic A (T Cr–SmA ) transition temperature rises as the chain length decreases. In Series-B, methoxy to n-hexadecyloxy derivatives exhibit an enantiotropic nematic mesophase (threadlike textures) as in Series-A, while the smectic A mesophase commences from the n-ethyloxy derivative and persists up to the last member of the homologou series. The transition temperatures of Series-B are given in Table 2. The plot of transition temperatures versus the number of carbon atoms in the alkoxy chain (Figure 2) exhibits the usual odd–even effect and as the series is ascended the curve shows a tendency to fall for the mesomorphic–isotropic transition temperature throughout the series. Series-B exhibits the usual odd– even effect for T Cr–N or T SmA–N and a tendency to fall for T Cr–SmA . The alternation of nematic–isotropic transition temperatures is less readily dealt with on the basis of a zig-zag alkyl chain conformation. If, for shorter alkyl chains, this chain extends strictly along its own axis then the terminal methyl groups present different faces to one another or to other end groups in the molecule depending on whether the chain is even or odd. The different attractive forces resulting could affect the energy of the system and account for the alternation of the transition temperatures. With the higher homologous, the alkyl chain may be forced into line with the main axis defined by the more rigid aromatic parts.

240 220 Transition temperature (°C)


Compounds

200 180 160

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

140 120 100 80 60

0

2

4

557

6 8 10 12 Number of carbon atoms

14

16

18

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}phenyl 6-alkoxy-2-naphthoate (Series-B).

Microscopic temperature (peak Compound Transition temperature) (◦ C)

Transition temperature (◦ C)

B1 B2 B3 B4 B5 B6 B7 B8 B10 B12 B14 B16

R=n alkoxy

Cr


• • • • • • • • • • • •

SmA 212 102 98 92 96 84 93 83 76 72 69 65

– • • • • • • • • • • •

N – 224 212 217 210 195 177 203 205 188 187 184

• • • • • • • • • • • •

I 230 235 224 229 219 225 209 223 219 216 215 213

• • • • • • • • • • • •

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

A7

A16

B7

B16

Cr–SmA SmA–N N–I Cr–SmA SmA–N N–I Cr–SmA SmA–N N–I Cr–SmA SmA–N N–I

72.92 (73) 190.32 (190) 220.88 (221) 65.24 (65) 174.65 (175) 212.95 (213) 93.05 (93) 177.15 (177) 209.06 (209) 65.26 (65) 183.86 (184) 212.85 (213)

H (J g−1 )

S(J g−1 K−1 )

4.06 30.94 8.11 15.54 7.84 4.37 9.62 11.46 8.33 8.37 10.65 8.33

0.0117 0.0667 0.0164 0.0459 0.0175 0.0089 0.0262 0.0254 0.0172 0.0247 0.0233 0.0171

similar to DSC data. At one end the alkoxy group is fixed, i.e. for Series-A having –OC3 H7 , Series-B having –OC5 H11 , and the alkoxy chain length of 6-alkoxy2-naphthoic acid is varied. It has been observed that in Series-A compounds the transition temperature is higher than that of the Series-B compounds. This may be attributed to the shorter alkoxy chain giving higher temperatures than the longer alkoxy chain. Table 4 shows the difference in the average nematic–isotropic and smectic–nematic thermal stabilities and mesophase ranges of Series-A and -B. In Series-A, the nematic–isotropic thermal stabilities are 4.3◦ C higher than those of Series-B and the smectic–nematic thermal stabilities are 3.7◦ C lower than those of Series-B. From the table it can be seen that the average nematic mesophase range of SeriesA is 1.4◦ C higher compared with that of Series-B



Compounds

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

200 180 160

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

140 120 100 80 60

0

2

4


14

16

18

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


Liquid Crystals

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 of Series-A and -B. Series N–I SmA–N Nematic mesophase range (◦ C) Smectic mesophase range(◦ C) Commencement of Smectic phase

A

B

225.5◦ C

221.2◦ C 201.1◦ C 20.1◦ C 34.0◦ C C2

197.4◦ C 21.5◦ C 32.7◦ C C4

and the average smectic mesophase range of Series-A is 1.3◦ C lower compared with that of Series-B. The commencement of the smectic mesophase in Series-A is from the n-butyloxy derivatives while it appears from the n-ethyloxy derivatives in Series-B. The upper transition points for 6-n-alkoxy-2naphthoic acids are higher than those for p-nalkoxybenzoic acids with the same alkyl groups, by an average of 47◦ C; therefore, 6-n-alkoxy-2-naphthoic acids are more mesomorphic. Thus, despite their smaller molecular breadths (6.8 Å) the benzoic acids are less mesomorphic than the naphthoic acids (7.9 Å). Moreover, the molecules of the naphthoic acids are longer by 2.2 Å, but unlike the increases in a homologous series, this greater length results from the presence of the second aromatic ring of the naphthalene nucleus. This second ring will contribute more to the intermolecular cohesion than a single benzene ring and so enhances the thermal stability. Due to the above reason the compounds of Series-A and -B have higher nematic and smectic thermal stability. The azomethine central linkage is more coplanar and provides such packing for the molecules that the smectic phase thermal stability increases. It is also known that the liquid crystalline properties are enhanced most when all the rings are conjugated, i.e. the liquid crystal

transition temperatures are highest when the entire system is linked through central linking groups involving multiple bonds (e.g., −CH=N− or −CH=CH−). However, the central ester linkage does not link the system through a multiple bond and hence the mesogenic thermal stability of a system connected via azomethine linkage is higher. So the azomethine linkage increases the smectic thermal stability of the compounds of Series-A and -B. Since the dipolar (alkoxy) terminals and polarisable (azomethine) centres of the molecule have become further separated from one another as a result of the lengthened alkyl chain, the terminal intermolecular attractions have decreased while the residual lateral attractions are essentially unchanged. This increase in the ratio of lateral to terminal cohesive forces makes the probability greater that the layer arrangement, which is characteristic of the smectic mesophase, will persist after melting occurs. Changes in this ratio are therefore quite important in determining the type of mesomorphism exhibited by certain molecule in a series of liquid crystalline compounds as well as the temperature at which the mesomorphic transition occur. Strong lateral and weak terminal intermolecular cohesions will give rise to a smectic mesophase, which, if the lateral cohesions are high enough, may persist until an isotropic liquid is formed. However, the addition of a methylene group increases the overall polarisability of the molecule [67]; consequently, the lateral intermolecular attractions increase, as the chain length grows. Each methylene unit forces apart the polarisable centres in the molecule and decreases the residual terminal attraction. There should be a relative decrease in the strength of the terminal intermolecular cohesive interactions [68]. This will decrease the nematic–isotropic transition temperature. So Series-B has a lower nematic–isotropic transition temperature


Liquid Crystals than Series-A. In Series-A the terminal intermolecular cohesive interactions are more due to the shorter alkoxy chain at one terminal compared to Series-B. So Series-A compounds have a higher nematic thermal stability and nematic mesophase range (nematic character), while in Series-B the lateral intermolecular attractions increase due to the longer alkoxy chain at the terminal which increases the smectic thermal stability and smectic mesophase range (smectic character). The lateral attraction increased as increased polarisability of the molecules this is due to lengthening of the alkyl chain. In Series-A fewer increments in the smectic– nematic transition temperature are found when the average transition temperatures are high. It may be concluded that when the lateral attractions between the molecules are low, the increased lateral attractions arising from the lengthening of the alkyl chain are relatively small in their effect, and vice versa. So Series-B compounds have more smectic character than Series-A compounds. The transition temperatures of Series-C and -D are given in Tables 5 and 6. In Series-C and -D methoxy to n-hexadecyloxy derivatives show only a nematic mesophase. No smectic mesophase is observed even for higher members of the homologous. Figures 7 and 8 show plots of transition temperatures against the number of carbon atoms in the alkoxy chain for Series-C and series-D, from which it can be noticed that transition temperatures exhibit an odd–even effect of the crystal–nematic state. The solid–isotropic transition temperatures also exhibit an odd–even effect. This is probably due to relative differences between the terminal and lateral cohesions. Lateral substitution results in a large decrease in the

Table 5. Transition temperature data of the 4-[({3-methoxy4-[(4nitrobenzoyl) oxy]phenyl}methylidene)amino] phenyl6-methoxy-2-naphthoate (Series-C). Transition temperature (◦ C) Compounds C1 C2 C3 C4 C5 C6 C7 C8 C10 C12 C14 C16

R=n alkoxy

Cr


• • • • • • • • • • • •

Sm 232 229 207 214 206 188 188 194 193 176 158 123

– – – – – – – – – – – –

N – – – – – – – – – – – –

• • • • • • • • • • • •

I 239 241 227 237 231 234 226 229 222 214 200 195

• • • • • • • • • • • •

561

Table 6. Transition temperature data of the 4-[({4[(4-chlorobenzoyl)oxy]-methoxyphenyl} methylidene) amino]phenyl 6-alkoxy-2-naphthoate (Series-D). Transition temperature (◦ C) Compounds D1 D2 D3 D4 D5 D6 D7 D8 D10 D12 D14 D16

R=n alkoxy

Cr


• • • • • • • • • • • •

Sm 202 200 188 192 175 170 132 174 167 146 147 139

– – – – – – – – – – – –

N – – – – – – – – – – – –

• • • • • • • • • • • •

I 207 209 200 203 192 198 185 192 187 186 180 179

• • • • • • • • • • • •

nematic–isotropic transition temperatures. In contrast, the reduction in the entropy change associated with the nematic–isotropic transition is only slight [69]. Enthalpies of the derivatives of Series-C and -D were measured by DSC. DSC data of the two series are recorded in Table 7, which shows the phase transition temperatures, the associated enthalpy (H) and the molar entropy (S) for compounds of Series-C (C7 and C16 ) and Series-D (D7 and D16 ). The DSC curves of the representative compounds are shown in Figures 9 to 12. Microscopic transition temperature values are almost similar to DSC data. Table 8 shows the difference in the average nematic–isotropic thermal stabilities and mesophase ranges of Series-C and -D. In Series-C, the nematic– isotropic thermal stability is 31.4◦ C higher than that of Series-D. From the table it can be seen that the average nematic mesophase range of Series-C is 9.6◦ C higher compared with that of Series-D. Since the nitro group is the highest polar group under investigation, one would expect it to enhance the stability. Substitution with the electron withdrawing chlorine atom enhances the nematic phase stability. The terminal substituents affected the polarisability of the aromatic rings to which they are attached to varying extents. As the polarity of the substituent increases, the clearing point (TC ) increases also. In Series-C, containing the nitro group which is more polar, the clearing point increases, whereas Series-D, containing the chloro group at the terminal which is moderately dipolar, shows a lower clearing point than Series-C. Generally, in the phenyl benzoate system, liquid crystallinity is more persistent as mutual conjugation between the substituent and the ester C–O group is increased. A change in the degree of conjugation will

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Transition temperature (°C)

240

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

220 200 180 160 140

0

2

4

6 8 10 12 N um ber of carbon atoms

14

16

18

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

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



120

190 180 170 160 150 140 130 0

2

4


14

16

18

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 C7 C16 D7 D16

Transition

Microscopic temperature (peak temperature) (◦ C)

H(J g−1 )

S(J g−1 K−1 )

Cr–N N–I Cr–N N–I Cr–N N–I Cr–N N–I

188.06 (188) 226.12 (226) 122.56 (123) 194.80 (195) 131.95 (132) 185.20 (185) 138.60 (139) 178.53 (179)

43.32 55.37 12.18 56.48 4.08 14.88 3.13 35.45

0.0939 0.0110 0.0307 0.1207 0.0100 0.0324 0.0076 0.0785


Liquid Crystals

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

(a)

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

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

(b)

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B.T. Thaker et al.

(a)

(b)

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

Table 8. Average thermal and mesophase stabilities of Series-C and -D. Series N–I Nematic mesophase range (◦ C)

C

D

224.5◦ C 32.4◦ C

193.1◦ C 22.7◦ C

alter both the polarisability and the resultant dipole moment of the molecule, the latter due to an effect on the mesomeric moment. A decrease in the polarisability will lead to a decrease in the dispersion forces, and consequently to a decrease in the thermal stability of the mesophase. When X = -NO2 or halogen, a strong dipole operates straight out from the p-position of the benzal grouping. Such a dipole may act either by attracting the ester groupings of neighbouring terminally situated molecules or by repelling like dipoles in neighbouring molecules lying in a layer arrangement of a smectic kind. Indeed, this would suggest that certain orientations of dipoles can have a disadvantageous effect upon the smectic thermal stability. Thus, the dipole moments associated with the terminal nitro group are directed along the axis of the molecule. Moreover, these dipoles lie in line in the smectic state and a net repulsive force may operate, reducing lateral attractions. In this case, a smectic arrangement will be less likely than a nematic arrangement, i.e. the ratio of the lateral to the terminal attractions will be low, also leading to enhanced terminal attractions. So the compounds of Series-C and -D are only nematic. No smectic mesophase is observed due to the low ratio of the lateral to the terminal attractions. Polar groups such as -CN, -OMe and -NO2 , increase the length of the molecule and the extent of the polarisable parts

of the system, and from both points of view would be expected to enhance the nematic thermal stability. This effect is moderate in moderately polarisable halogen substituents (-C1 and -Br). However, the chloro substituent is often a source of instability and larger in size causing a high viscosity [70]. The effect of the chlorine atom (or for that matter any other halogen atom) in this situation is that the electron withdrawing inductive effect is very strong and is not overwhelmed by the electron releasing, resonance effect of the chlorine atom. Thus, the chloro substituent will have a relatively high charge density, identical in sign to that carried by the oxygen atom of the carbonyl moiety of the ester linking group. Thus, repulsion between these two will cause a reduction in the coplanarity between the carbonyl moiety and the phenyl ring to which it is attached. Due to this reason the nematic thermal stability of the Series-D compounds will be partially lost through the loss of conjugation of the molecule. The oxygen of the lateral methoxy (-OCH3 ) group, being in conjugation with the aromatic core, in addition to extending the length of the rigid core, enhances the polarisibility. The presence of a side methoxy group larger in size creates an additional dipole moment at an angle to the long axis of the molecule, leading to destabilisation of the mesomorphic state. The introduction of lateral methoxy groups therefore leads to a reduction in the phase transition temperatures in Series-C and -D compare to Series-A and -B. Due to the lower lateral attraction and the greater terminal attraction (low lateral to terminal attraction ratio), the Series-C and -D compounds are nematogenic. Compare Series-A and -B with Series-C and – D (Scheme 2), which have different terminal groups

Liquid Crystals RO

565

O

O

N O OC3H7

Series-A RO

O

O

O

N O OC5H11


Series-B RO

O

O

OCH3 O

N O NO2

Series-C RO

O

O

OCH3

O

N O Cl

Series-D

O

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

and lateral groups. In Series-A and -B the terminal groups are the alkoxy chain whereas there is no lateral substitution. Series-A and -B show nematic as well as smectic character while Series-C and -D show only nematic character. The overall thermal stability of the compounds of Series-C is lowered by 1◦ C than that of the compounds of Series-A and 3.3◦ C higher than that of the compounds of Series-B. The mesophase length of Series-C compounds is higher by 10.8◦ C than that of Series-A compounds and higher by 12.2◦ C than that of SeriesB compounds. The lateral methoxy group increased lateral separation between the molecules as a result decreased lateral attractions between molecules. The overall nematic mesophase thermal stability of the compounds of Series-D is lowered by 32.4◦ C than that of the compounds of Series-A and 28.0 ◦ C higher than that of the compounds of Series-B. The nematic mesophase length of Series-D compounds is higher by 1.1◦ C than that of Series-A compounds and 2.59 ◦ C than that of Series-B compounds. Due to lower lateral attraction there is no smectic mesophase observed in the compounds of Series-C and -D. The lateral methoxy group decreases the thermal stabilities of the smectic and nematic states by broadening the

molecule, as a result of the increased lateral separation and the decreased lateral attractions. This effectively increases the aspect ratio of the materials so that the length-to-breadth ratio is much larger for Series-A and -B compounds, thereby effectively extending the combined molecular lengths. Systems with larger aspect ratios will have higher clearing points, just as a threering compound has a higher clearing point than a two-ring material. The aspect ratios will also be dependent on the amount of time that the individual molecules spend in paired associations in fluctuating systems, which in turn will be dependent on the relative strengths of the interactions. This model points to the stabilisation of the liquid crystal phases and the magnitude of their relative transition temperatures by gross shape and strength of interactions, and not by segregative effects or the local steric structure [71].

4. Texture study The textures of the compounds were observed using polarised light with crossed polarisers with samples in a thin film sandwiched between a glass slide and cover slip. The textures of the compounds of Series-A and -B are shown in Figure 13. Compound A16 shows


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B.T. Thaker et al.

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 the isotropic phase at 85.3◦ C and A7 shows a threadlike texture of the nematic phase on heating of the solid at 208.5◦ C. Compound B16 shows a texture of the SmA phase on cooling from the isotropic phase at 78.3◦ C and also shows a thread-like texture of the nematic phase on cooling from the isotropic phase at 190.1◦ C. Compounds B8 and A14 show A typical texture on cooling from the isotropic phase at 90.8◦ C and 79.8◦ C, respectively. The textures of the compounds of Series-C and -D are shown in Figure 13. Compound C16 shows a schlieren-like texture of the nematic phase on cooling from the isotropic phase at 140.2◦ C and compound C7 shows that nematic droplets start on cooling from the isotropic phase at

189.2◦ C, which coalesce to form the thread-like texture of the nematic phase at 196.1◦ C. Compound D16 shows a schlieren-like texture of the nematic phase on cooling at 150.8◦ C and compound D7 shows a schlieren-like texture of the nematic phase on heating of the solid at 149.4◦ C. Compound D8 shows a typical texture on heating of the crystals at 175.8◦ C. 5.

Conclusion

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


Liquid Crystals

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

Nematic (thread-like) phase at 196.1ºC on cooling) C7

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

Nematic droplets starts to coalesce to form the nematic 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

567

Figure 13. (Continued).

phase. The compounds of Series-A exhibit higher thermal nematic stability and good nematic mesophase range, whereas Series-B exhibit higher thermal smectic stability and good smectic mesophase range due to the additional methylene group at one terminal. In SeriesC and -D the nematic phase was observed from the methoxy to n-hexadecyloxy derivatives. No smectic phase was observed even in higher homologous. The study also revealed that the significant influence on the mesomorphic properties of variation of the terminal group in two series made it possible to observe the effects of structural changes on mesomorphic behaviour in a system. The thermal stability and mesophase range of the Series-C (-NO2 ) terminal group 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 increasing breath of the molecule, which, decrease thermal stabilities of the smectic and nematic states by decreasing the ratio of length/breath (l/b) of the molecule. Acknowledgements We are grateful to Dr R.A. Vora for giving valuable suggestions. We are also grateful to Mettler Todelo, India Pvt. Ltd, Powai, Mumbai for their support with the DSC analyses and H. L. Chemicals and Engineering Pvt. Ltd, Maroli for providing 6-hydroxy-2-naphthoic acid. We also thank Gujarat Narmada Valley Fertilizer Company Ltd. (G.N.F.C.), Bharuch, and SAIF Chandigarh for providing facilities such as elemental analysis, 1 H NMR and mass spectra.

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

Synthesis, Characterization and Mesomorphic Properties of New Rod-like Thiophene Based Liquid Crystals a

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B. T. Thaker , B. S. Patel , Y. T. Dhimmar , D. B. Solnki , N. J. a

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Chothani , N. B. Patel , K. B. Patel & U. Makavana

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Department of Chemistry, Veer Narmad South Gujarat University, Surat, Gujarat, India Version 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 of New Rod-like Thiophene Based Liquid Crystals, Molecular Crystals and Liquid Crystals, 562:1, 98-113 To link to this article: http://dx.doi.org/10.1080/10426507.2012.673943


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


Synthesis, Characterization and Mesomorphic Properties 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 benzoyloxy) benzylidenamino] 3-cyno thiophine (Series-A) and Schiff base cinnamates, 2-[4(4 -n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), comprising a thiophene moiety were synthesized. Structural elucidation was carried out using elemental analysis and spectroscopic techniques such as FT-IR, 1H-NMR and 13C-NMR, and mass spectrometry. The mesomorphic properties and thermal stabilities of the title compounds were studied by using differential scanning calorimetry and optical polarizing microscopy. All the derivatives are mesomorphic in nature showing the nematic phase, and the higher members of Series-A show a smectic C phase whereas Series-B exhibits only the nematic mesophase. The mesomorphic properties of the present series are 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 systems in both low and high molecular weight materials [1–3]. Although classical thermotropic liquid crystals are commonly composed of rod-like molecules, many other types of low molecular mass compounds with unconventional molecular structures have been shown to exhibit liquid crystalline properties [4]. Many series of liquid crystalline compounds containing heterocyclic groups have been synthesized 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 number of 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 South Gujarat University, Surat-395007,, Gujarat, India. Tel.: (+91) 9228377618. E-mail: btthaker1@ yahoo.co.in

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large changes in the type of mesophase present and/or in the physical properties of the materials. Heterocycles are of great importance as core units in thermotropic liquid crystals due to their ability to impart lateral and/or longitudinal dipoles combined with changes in the molecular shape. These materials hold great potential for use in spatial light modulation [12], all-optical signal processing, optical information storage [13], organic thin-film transistors [14,15], fast switching ferroelectric materials [16], fluorescent probes for the detection and analysis of biomolecules etc. [17]. Thiophene in particular has emerged as a core unit that is receiving increasing attention. Sulfur-based heterocycles are also being used to elucidate the structures of complex mesophases. Thiophenes played a major role in the synthesis of systems displaying supramolecular chirality when dissolved in solvents where dissolution is not strongly favored [18]. Five-membered heterocycles have potential for flexoelectric applications such as found in bistable nematic displays. A number of thiophenes [19] have already been evaluated for such applications, and other bent heterocycles may 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 points than the 1,4-phenylene analogues, (b) promote negative dielectric anisotropy, and (c) have a tendency to generate a range of different liquid crystalline phases [20–26]. Five-membered rings provide materials of low melting point and viscosity, large optical anisotropy, and fast switching times [27]. There has been a continuing interest in the study of heterocyclic-based liquid crystal compounds owing to the great variety of their structures. Thiophene-based calamitic liquid crystals are currently the subject of intensive study [28–34]. Their applications as ferroelectric materials as well as potential materials for molecular electronic devices, such as organic field effect transistors, are of special interest. Heterocyclic compounds such as five-membered thiadiazole or thiophene rings can be incorporated into the principal structure of calamitic mesogens [35–39]. Sulfur containing heterocycles are important synthetic intermediates and have found a variety of applications in medicinal, agricultural, and materials chemistry [40–41]. LCs containing heterocyclic cores, such as thiophene, are of particular interest due to their slightly bent structure, which leads to features including a reduced packing ability, a medium to strong lateral dipole and high anisotropy of the polarizability. The mesomorphic properties of aromatic Schiff base esters arising from substituents varying in their polarities have been reported by Ha et al. [42]. Many mesogenic homologous series contain two central linkages, one of which may be ester and the other azomethine [43,44]. Vora and Rajput [45] reported binary mixtures of cinnamate ester exhibit wide rang of smectic and nematic mesophase. Previously we have reported two mesogenic homologous series of cinnamate-azomethine [46] containing thiophene and furan heterocycles. The ethylene linking group is very useful structural unit connecting one part of a rigid core with another in calamitic mesogene molecules. This fully conjugative group enhances the longitudinal polarizability and extends the molecular length maintaining linearity of the molecule. Recently, there has been a continuing interest in study the effect of an ethylene linking group and thiophene moiety on the mesomorphic properties of such molecules. The present investigation concerns the synthesis, characterization, and mesomorphic properties of two new liquid crystalline homologous series with a common central linkage (azomethine) with a differing central linkage (cinnamate-ester) with a terminal heterocyclic moiety such as 2-amino-3-cyanothiophene.

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2. Experimental Details 2.1 Materials 4-Hydroxy benzoic acid and 4-hydroxy benzaldehyde were obtained from Merck (Germany). Alkyl bromide (Lancaster, England). 2-amino 3-cyano thiophine and malonic acid were purchased from Fluka Chemie (Switzerland). N,N -dicyclohexylcarbodiimide (DCC) were purchased from Acros Organics (USA). DMAP (N,N-dimethylaminopyridine) was purchased from Merck (Germany). Pyridine, piperidine, anhydrous potassium carbonate, acetone, ethanol, methanol, acetic acid, ethyl acetate, HCl, KOH, NaOH etc. were used 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. (Gujarat Narmda Valley Fertilizer Company Ltd., Bharuch). Infrared spectra were recorded with a THERMO SCIENTIFIC NICOLET iSO-10 spectrophotometer in the frequency range 4000–400 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 temperature as 15%–20% solution in CDCl3 using TMS as internal standard on a BRUKER AVANCE II 400 NMR spectrometer at SAIF (Sophisticated Analytical Instrument Facilities), Panjab University, Chandigarh. Mass spectra (TOF MS ES+) of the compounds were recorded using Finnegan MAT-8230 Mass Spectrometer at SAIF (Sophisticated Analytical Instrument Facilities), Panjab University, Chandigarh. Thin-layer chromatography (TLC) analyses were performed using aluminium-backed silica-gel plates (Merck60 F524) and examined under shortwave UV light. Thermal (DSC) analyses of the liquid crystalline compounds were carried out from Atul Industries Ltd. P-P site Atul. DSC analyses were performed on METTELER M-3 thermo balance (Switzerland) with microprocessor TA-300 instrument at 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) equipped with Linkam Analysa-LTS420 hot stage (London) at our department. The textures of the compounds were observed using polarized light with crossed polarizers with the sample in a 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-hydroxy benzoic 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 one and they are almost similar to reported values [48,49]. 2.3.2 4-(4 -n-alkoxy benzoyloxy) benzaldehydes. The compound has been prepared by etherification of the appropriate 4 -n-alkoxy acid (2.02 mmol) and 4-hydroxy benzaldehydes (0.246 g, 2.02 mmol), dicyclohexylcarbodiimide (0.457 g, 2.22 mmol) and dimethylaminopyridine (0.002 g, 0.2 mmol) in dry CH2 Cl2 (20 mL) was stirred at room temperature for 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 purified by column chromatography on silica gel eluting with dichloromethane, followed by repeated 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 of 4-(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 refluxed for 4 h. The reaction mixture was allowed to cool and was stirred at room temperature overnight. The residue obtained on removal of solvent was chromatographed on silica gel (100–200 mesh) using petroleum ether (60◦ C–80◦ C) ethyl acetate mixture (80:20) as eluant. Removal of solvent from the eluant afforded a solid material, which was crystallized repeatedly from ethanol until constant transition temperatures were obtained. The purity of these compounds was checked by thin layer chromatography (Merck silica gel 60 F254 precoated 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 C29 H32 N2 SO3 (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 H Str. aliphatic), 1731 (C O Str. ester), 1641 (CH N, Str. azomethine), 2228 ( N C). 1H NMR (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 benzaldehyde using the reported method of Vyas and Shah [53]. The clearing points of these compounds were compared with the reported one and they are almost similar to reported values. 2.3.5 4-n-alkoxy cinnamic acid. 4-n-alkoxy cinnamic acid were prepared by the method of Gray and Jones [54].

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2.3.6 4-(4 -n-alkoxy cinnamoyloxy) benzaldehydes. The compound has been prepared by esterification of a mixture of the appropriate 4 -n-alkoxy acid and the appropriate 4hydroxy benzaldehydes (0.246 g, 2.02 mmol), dicyclohexylcarbodiimide (0.457g, 2.22 mmol), dimethylaminopyridine (0.002g, 0.2 mmol) and dry CH2 Cl2 (20 mL) was stirred at room temperature overnight. 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 purified by column chromatography on silica gel eluting with dichloromethane, followed by repeated recrystallization from ethanol until constant transition temperatures were achieved [50–52]. 2.3.7 2-[4-(4 -n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene. A mixture of 4-(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 refluxed for 4 h. The reaction mixture was allowed to cool and was stirred at room temperature overnight. The residue obtained on removal of solvent was chromatographed on silica gel (100–200 mesh) using petroleum ether (60◦ C–80◦ C) ethyl acetate mixture (80:20) as eluant. Removal of solvent from the eluant afforded a solid material, which was crystallized repeatedly from ethanol until constant transition temperatures were obtained. The purity of these compounds was checked by thin layer chromatography (Merck silica gel 60 F254 precoated 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 C31 H34 N2 SO3 (514 gm/mol); C72.37; H, 6. 61; N, 5.44;%. IR (KBr) υmax cm−13096 (C H Str. aromatic), 2970, 2884 (C H Str. aliphatic), 1725 (C O Str. ester), 1630 (CH N, Str. azomethine), 2232 ( N C).1H NMR (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-IR spectroscopy. The mesomorphic properties of all the synthesized compounds have been investigated 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 SmC mesophase commences from the n-dodecanoyloxy derivative along with the nematic phase. The transition temperatures are recorded in Table 1 and a plot of transition temperatures against the number of carbon atoms in the alkoxy chain is given in Fig. 1. It can be noticed that the nematic-isotropic transition temperature shows a smooth falling tendency and does not exhibit an odd–even effect. It also exhibits a tendency for rising smectic-nematic transition temperatures in the ascending Series-A. All the compounds synthesized in Series-B exhibit enantiotropic nematic phase. The transition temperatures are recorded in Table 2 and a plot of transition temperatures against

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O

Where, R=C n H 2n+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, CH2 Cl2, 4-Hydroxy benzaldehyde stirred at 0◦ C for 1 h, stirred at room temperature for 24 h; (iii) Ethanol, 2 to 3 drop AcOH reflux for 4 h; (iv) RBr, K2 CO3 , Dry acetone; (v) Malonic acid, Piperidine reflux for 6–8 h; (vi) DCC, DMAP, CH2 Cl2 , 4-Hydroxy benzaldehyde stirred at 0◦ C for 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 that the crystal to mesophase transition temperatures increase with the usual old-even effect for lower members. The nematic-isotropic transition temperatures also show no odd–even effect. All the compounds of Series-A and Series-B exhibit mesomorphism. On cooling the isotropic liquid of Series-A the compounds form small droplets that coalesce to classical Schlieren textures characteristic of the nematic phase. On further cooling, higher members show the focal-conic texture characteristic of the SmC mesophase. For Series-B on cooling the 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 the assignment of each mesophase type using the classification system reported by Sackmann and Demus [55] and Gray and Goodby [56]. DSC is a valuable method for the detection of phase transition. It yields quantitative results; therefore, we may draw concerning the nature of the phase that occurs during the 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) benzylidenamino] 3-cyano thiophene (Series-A)

Compounds

R = n alkoxy

Cr

A1 A2 A3 A4 A5 A6 A7 A8 A10 A12 A14 A16 A18

Methyl Ethyl Propyl Butyl Pentyl Hexyl Heptyl Octyl Decyl Dodecyl Tetradecyl Hexadecyl Hexadodecyl

• • • • • • • • • • • • •

SmC – – – – – – – – – (49)∗ 47 52 54

N

– – – – – – – – – • • • •

126 118 121 114 110 105 94 78 69 72 74 67 64

• • • • • • • • • • • • •

I 165 162 160 156 153 150 127 110 94 91 88 86 83

• • • • • • • • • • • • •

()∗ monotropic.

compounds A6, A10 , B6, B10 were determined using a DSC. The data obtained from the DSC analysis and from POM are summarized in Table 3, which helps to further confirm the mesophase 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 ). The DSC curves of representative compounds are shown in Figs. 3–6. Microscopic transition temperature values are almost similar to DSC data. 7

180 160 Transition temperature


Transition temperatures ◦ C

140 120 Cr-SmC

100

SmC-N

80

N-I

60 40 20 0 0

2

4

6

8

10

12

14

16

18

Number of carbon atom

Figure 1. Transition temperature curve of Series-A.

20


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

Compounds

R = n alkoxy

Cr

B1 B2 B3 B4 B5 B6 B7 B8 B10 B12 B14 B16 B18

Methyl Ethyl Propyl Butyl Pentyl Hexyl Heptyl Octyl Decyl Dodecyl Tetradecyl Hexadecyl Hexadodecyl

• • • • • • • • • • • • •

Sm – – – – – – – – – – – – –

– – – – – – – – – – – – –

N • • • • • • • • • • • • •

148 151 154 143 147 139 116 95 81 77 74 72 69

I 197 202 191 180 185 171 153 134 99 96 95 92 88

• • • • • • • • • • • • •

3.2 Mesomoxrphic Behavior In Series-A, as the length of the carbon chain increased, an enantiotropic smectic C phase was observed from the A14 derivative. In fact, the smectic phase observed as monotropic on cooling for the compound A12. The crystal to nematic mesophase transition temperature gradually decreased from the C4 members. Clearing points descended with the increase in

180 160 Transition temperature


Transition temperatures ◦ C

140 120 Cr-SmC

100

SmC-N

80

N-I

60 40 20 0 0

2

4

6 8 10 12 14 16 Number of carbon atom

18

20

Figure 2. Transition temperature curve of Series-B.

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Series A

Compound No. A6 A10


B

B6 B10

Transition

Peak/POM Temp. ◦ C

H J/g

S J/g◦ K

Cr-N N-I Cr-N N-I

104.67(105) 150.67(150) 69.31(69) 94.17(94)

32.05 11.87 12.70 4.93

0.3062 0.0787 0.1832 0.0523

Cr-N N-I Cr-N N-I

139.18(139) 170.80(171) 80.81(81) 99.29(99)

36.29 15.47 10.37 24.93

0.2607 0.0905 0.1283 0.2510

length of the carbon chain due to the dilution of the mesogenic core resulting from the flexibility provided by the terminal alkanoyloxy chain. Generally, short-chain members favor nematic 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 the nematic phase range reduced as the length of the terminal chain increased. In Series-B, all synthesized compounds show only the nematic mesophase, the nematic transition 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 due to the dilution of the mesogenic core resulting from the flexibility provided by the terminal alkanoyloxy chain [58].




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3.3 Chemical Structure-Mesomorphic Property Relationship There is close relation between mesomorphism and molecular constitution of organic compounds. Hence, transition temperatures and mesophase range as measures of mesomorphism can be correlated with the molecular constitution of the compounds. Table 4 summarizes 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 higher by 0.32◦ C and the N-I mesophase thermal stability is higher by 19.84◦ C compared to the respective mesophase ranges of Series-A. This is understandable, as the molecules of Series-B are longer and more polarizable compared to the molecules of Series-A due to the presence of additional cinnamoyloxy ( CH CH COO ) central linkage. The molecules of Series-A and B differ only at the central linkages. The molecules of Series- B have cinnamoyloxy ( 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 increases the polarizability and length of the rod-like molecules. Therefore, the greater mesophase thermal 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 structurally related series-I to IV

Series A I II B III IV

Mesophase range (◦ C)

Thermal stabilities (◦ C)

Smectic

Nematic

Sm-N

18.75 89.0

31.76 39.58

— 16.0 —

32.08 69.0 82.28

N-I

93.23 125.00 181.33 221.00 Nonmesogenic — 144.84 133.69 202.23 — 212.08

Commencement of Smectic phase C12 C12 — C10 —

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Figure 7. Micro photograph of liquid crystalline compounds.



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length and polarizability of the molecule resulting from additional CH CH units in the central linkage. Series-A exhibits both smectic as well as nematic mesophases (texture of smectic and nematic 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 containing ester-azomethine central likage while Series-B having cinnamate-azomathine linkage. The ester group is more conducive for mesophase then cinnamate group. Therefore, Series-A exhibit both mesophase, i.e., smectic at higher homologus and nematic mesophase from methoxy group until the end. In case of length to breath ratio is higher, which shows nematic phase 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 B7 are 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 the role played by the terminal ring. The structural difference between the series is one of the

NC

RO

C

O

C

O

N

S

H

(Series-A) N RO

C

O

C

O

N

N

N

H

(Series-I) RO

C

O

C

O

N

H

(Series-II) NC

RO

CH

CH

C

O

C

N

S

H

O

(Series-B) N CH

RO

CH

C

C

O

O

N S

H

(Series-III) RO

N

C

O

C O

H

(Series-IV)

CH

CH

F



111

terminal rings. The compounds of Series-A and Series-I are mesogenic, whereas Series-II is 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 an enantiotropic nematic phase. The SmC mesophase commences from the n-dodecyloxy derivative as a monotropic phase. n-Tetradecyloxy to n-octadecyloxy members exhibit an enantiotropic SmC phase, whereas in Series-I the n-dodecyloxy to n-hexadecyloxy derivatives exhibit the SmC phase along with an enantiotropic nematic phase. The lone pairs of electrons on the nitrogen atoms act to broaden the molecule and also introduce attractive forces, which aid smectic formation. Reference to Table 4 indicates that the nematic mesophase length and N-I phase thermal stability of Series-I are higher by 7.82◦ C and 96.0◦ C, respectively, than that of present Series-A. Both the series differ only at one terminus. Series-A has a 3-cyano thiophene ring at the terminus instead of the 1,2,4triazole ring of Series-II. Owing to the inherent nature of thiophene, it nonlinearly reduces the efficiency of packing and thus lowers the mesophase thermal stability of members of Series-A than Series-I. Reference to Table 4 indicates that the nematic mesophase length and N-I phase thermal stability of Series-B are lower by 36.92◦ C and 57.39◦ C, respectively, than that of present Series-III, similarly Series-B are lower by 50.2◦ C and 67.24◦ C, respectively, than that of present Series-IV. Both the series differ only at one terminus. Series-A has a 3-cyano thiophene ring at the terminus instead of the 6-fluro benzothiazole ring of Series-III and benzene ring of Series-IV. The fact that thermal stabilities of Series-III and IV are higher than that of Series-B (of present work) suggests that even though Series-B contains a fivemembered thiophene ring, which normally imparts nonlinearity, the essential attracting forces are similar to one present in Series-IV benzene analogues. Oh [63] has reported that all the transition temperatures of pyridine analogues were lower compared to the benzene analogues. In the present study also shows that the thiophene derivatives have lower transition temperatures. The hetero atom has high electro negativity and, therefore, withdraws electron from the other atoms of the ring system, rendering the ring deactivated related to benzene.

4. Conclusion In this paper, we have presented the synthesis and characterization of new mesogenic homologous 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 cinnamoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), which contain a cinnamateazomethine central linkage. Series-A with an ester-azomethine central linkage has lower thermal stabilites compared with Series-B with a cinnamate-azomethine central linkage. The ester central linkage is more conducive to conferring mesophases in the materials than the cinnamate central linkage. Series-A exhibits both smectic and nematic mesophases while Series-B exhibits only the nematic mesophase. The mesophase range of the Series-B analogues is higher than those of Series-A, which is attributed to the high polarizability of the molecules. Members of series-B with a cinnamoyloxy central linkage are more stable compared with the Series-A members containing ester-azomethine central linkage due to greater molecular length and polarizability of the molecule resulting from additional CH=CH units in the central linkage. Moreover, the terminal thiophene derivatives have lower transition temperatures due to high electro negative “S” atom. There is no much effect have been observed on the transition temperature by the presence of CN group at lateral 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 DSC analysis, and also to SAIF Chandigarh for providing facilities of FT-IR, 1H-NMR, 13CNMR, and Mass spectral analysis.


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Mesomorphic studies of novel azomesogens having a benzothiazole core: Synthesis and characterisation a

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a

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B.T. Thaker , B.S. Patel , Y.T. Dhimmar , N.J. Chothani , D.B. Solanki , Neeraj Patel a

, K.B. Patel & U. Makawana

a

a

Department of Chemistry, Veer Narmad South Gujarat University, Surat, Gujarat, India Version 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 To link to this article: http://dx.doi.org/10.1080/02678292.2012.737478


a

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

Mesomorphic studies of novel azomesogens having a benzothiazole core: Synthesis and characterisation 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

Downloaded by [Veer Narmad South Gujarat University] at 02:24 16 January 2013

(Received 25 January 2012; final version received 3 October 2012) Novel liquid crystals, including 2-[4-(4 -n-alkoxybenzoyloxy)phenylazo]-6-fluorobenzothiazoles (Series E) and 2-[4-(4 -n-alkoxybenzoyloxy)naphtha-1-ylazo]-6-fluorobenzothiazoles (Series F), have been synthesised and characterised. Each series contains 13 homologous members differing by the length of the alkoxy chain. In series E the 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 the longer chain homologues (C10 to C14 ) showed enantiotropic SmC and SmA mesophases. The C16 and C18 homologues possessed only SmA mesophases. In Series F all the compounds (C1 to C18 ) exhibited only the enantiotropic nematic mesophase. Keywords: 6-fluorobenzothiazole; azomesogens; SmA; nematic; enantiotropic

1. Introduction Thermotropic liquid crystals possess a number of unique properties that have received considerable attention. Significant efforts have been focused on the synthesis of new compounds [1,2], and it has been found that molecules having an extended rod-like shape often exhibit a thermotropic liquid crystalline phase [3,4]. As part of this programme a large number of mesomorphic compounds containing heterocyclic units have been synthesised, and interest in such structures remains strong [5–7], not only because of the possibilities presented by heterocyclic moieties in the design of new mesogenic molecules, but also because the insertion of hetero-atoms strongly influences the formation of mesomorphic phases. These heterocyclic structures generally incorporate unsaturated atoms such as O, N or S, and their electronegativity often results in reduced symmetry in the molecules and a stronger polar induction. The use of unique heterocyclic moieties to produce materials of low symmetry or/and non-planar structures can be technologically important in liquid crystal applications [8–12]. Fluoro-substituted liquid crystals are of particular interest [13–17], since they generally exhibit excellent properties compared with the corresponding unsubstituted compounds such as lower viscosity, high voltage mean retention and high specific resistance [18]. The fluorine atom often produces interesting effects

[19]. Fluoro-substitution in the terminal position provides nematic compounds with positive dielectric anisotropy, analogous to that provided by a terminal cyano-substituent. However, fluoro-substituted compounds have the added advantage of high resistivity and are suitable for use in state-of-the-art active matrix displays, for example in thin-film transistors [20,21]. Matsui et al. [22] have reported fluorine-containing benzothiazolyl bisazo dyes and their application in guest–host liquid crystal displays has been examined. The introduction of heterocyclic rings into core structures to generate mesogenic materials has been well demonstrated [23–27], in particular ring structures such as thiophene [28,29], pyridine [30] or 1,3,4-thiadiazole [31]. A number of publications have recently described the incorporation of benzothiazole as a core unit in liquid crystalline compounds [32– 41], for example Prajapati and Bonde [42] have reported the use of two mesogenic homologous series comprising 6-substituted benzothiazole ring systems with a central azo linkage. Their study has revealed that a methoxy-substituent in the sixth position of the benzothiazole ring favours the formation of the nematic phase. A benzothiazole ring, containing the electron-rich sulphur atom, can contribute to a low ionisation potential and can also induce the smectic phase. Benzothiazole derivatives are additionally important due to their anti-tumour and antibacterial activity [43] and as photoconductive materials

*Corresponding author. Email: [email protected] This paper was initially presented at the 18th Indian Liquid Crystal Conference, held at the Department of Physics, NERIST, Itanagar, India, 15–17 November, 2011 © 2013 Taylor & Francis


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[44–46]. These derivatives have also been investigated for use in thin-film organic field-effect transistors [47]. Funahashi and Hanna [48,49] have reported the fast hole transportation properties of a photoconductive calamatic liquid crystal. The naphthalene ring is another interesting aromatic core in the synthesis of liquid crystalline compounds. A significant variety of naphthalene-based materials have been synthesised as highly birefringent materials for electro-optical devices [50], and naphthalene derivatives exhibiting liquid crystalline properties have also been well documented [51–58]. Gray and Jones [59–61] have investigated the liquid crystalline properties of a range of alkoxy naphthoic acids. Dave and Prajapati [62,63] synthesised a number of Schiff’s base homologous series based on the naphthalene moiety in order to establish the relationship between the width of the aromatic core and mesomorphic behaviour. The growing scientific interest in the synthesis, analysis and mesomorphic behaviour of heterocyclicbased liquid crystals led us to synthesise two series of 6-fluoro-benzothiazole-based liquid crystals, each containing a central naphthalene or benzene core with two azo and ester-linked arms.

as a 15–20% solution in CDCl3 using TMS as internal standard, on a Bruker Avance II 400 NMR spectrometer, and mass spectra (TOF MS ES+) were recorded using a Finnigan MAT–8230 mass spectrometer, both at Sophisticated Analytical Instrument Facilities, Panjab University, Chandigarh. Thin-layer chromatography (TLC) was performed using aluminium-backed silica gel plates (Merck 60 F524) and examined under short-wave ultraviolet light. Thermal (DSC) analysis of the liquid crystalline compounds was carried out by Atul Industries Ltd using a Mettler M–3 thermo balance (Switzerland) with microprocessor TA–300, at a heating rate of 10◦ C min−1 under a nitrogen atmosphere. The optical microscopy studies were determined using a polarising microscope, Nikon Eclipse 50i POL (Japan), equipped with Linkam Analysa–LTS420 hot-stage (London), using a standard procedure in our department.

2.3 Synthesis of compounds in Series E and F The synthesis of compounds in Series E and F was carried out as shown in Scheme 1, and their structures are summarised in Scheme 2.

2.3.1 4-n-Alkoxy benzoic acid 2. Experimental 2.1 Materials 4-Hydroxybenzoic acid was obtained from Merck, Germany. Alkyl bromides were purchased from Lancaster (UK), and malonic acid from Fluka Chemie (Switzerland). N,N -Dicyclohexylcarbodiimide (DCC) and dimethylaminopyridine (DMAP) were purchased from Acros Organics (USA). 4-Fluoroaniline was provided by Aarti Chemicals, Vapi (India), and used without further purification. Other starting materials, including bromine, potassium thiocyanate, phenol, αnaphthol, acetone, ethanol, methanol, acetic acid, and ethyl acetate, were used as received. Column chromatography was performed using Acme Silica Gel (100–200 mesh). Solvents were dried and distilled prior to use.

2.2 Measurements Elemental analysis (C,H,N) was performed using a thermo Scientific FLASH 2000 at Gujarat Narmada Valley Fertilizer Company Ltd., Bharuch. Infrared spectra were recorded on a Thermo Scientific Nicolet iS–10 spectrophotometer in the frequency range 4000–400 cm−1 with samples embedded in KBr discs in our department. High resolution (400 MHz) NMR spectra were recorded at room temperature

A number of methods [59–61,64] are available for the alkylation of 4-hydroxybenzoic acid. In the present study, however, the method developed by Dave and Vora [65] was followed.

2.3.2 2-Amino-6-fluorobenzothiazole 2-Amino-6-fluorobenzothiazole was prepared from 4fluoroaniline using the method reported in [66]. The resulting compound was crystallised from 1:1 aqueous ethanol, giving pale yellow needles. Yield: 87%; clearing point: 184◦ C.

2.3.3 2-(4-Hydroxyphenylazo)-6fluorobenzothiazole [B] A well stirred mixture of 2-amino-6-fluorobenzothiazole (1.68 g, 0.01 mol) and (40 mL) of a 1:1 dilution of concentrated H2 SO4 was cooled to below 5◦ C and a solution of NaNO2 (0.76 g, 0.011 mol) in water (20 mL) added dropwise so that the temperature of the mixture remained within the range 0–5◦ C. The cold dark yellow solution was added dropwise to a cold mixture of phenol (0.94 g, 0.01 mol), NaOH (5 g, 10%) and water (50 mL), during which the temperature of the mixture was kept below 8◦ C. The diazotised solution was stirred for about 30 min and then

Liquid Crystals HOOC

(i)

OH

HOOC

239

OR A

F

NH2

N

(ii) F

N

(iii)

NH2 S

N

N S

F

OH +

A

B

(iv) 4


5

9

6

F

8

7

3 N

2' 2

S 1

N

3'

1'

N

O

4'

C O

5' 6' Series E

2"

3"

6"

4" 5"

1"

OR

N

(v)

N

N

OH +

A

S

F

(iv) 5 F

4

6 7

9

3 N

8

S 1

2' 2

N

N

1' 9' 8'

2"

3' 4' 10' 5'

O

C O

3"

1" 4" 6"

OR

5"

6' 7' Series F

Scheme 1. (i) R-Br anhydrous K2 CO3 CH3 OH/C2 H5 OH, (ii) KSCN, Br2 , AcOH, (iii) NaNO2 + H2 SO4 , 0 to 8◦ C, phenol in NaOH, (iv) DCC, DMAP, dry CH2 Cl2 stirring 24 h, (v) NaNO2 + H2 SO4 , 0 to 8◦ C, 1-naphthol in NaOH.

acidified with (1:1) aqueous HCl. The crude product was filtered off and dried in air before recrystallising a number of times from alcohol. 2.3.4 2-[4-(4 -n-Alkoxybenzoyloxy) phenylazo]-6fluorobenzothiazole This series of compounds was prepared by esterification of a mixture of the appropriate carboxylic acid A (2.02 mmol) and phenol B (2.02 mmol), in the presence of dicyclohexylcarbodiimide (2.02 mmol), dimethylaminopyridine (0.2 mmol) and dry methylene chloride (CH2 Cl2 ; 20 mL), stirring at room temperature overnight under an argon atmosphere. The ensuing white precipitate was filtered off and discarded, and the filtrate was evaporated to dryness in vacuo. The resulting residue was purified by chromatography on silica gel (100–200 mesh) using a petroleum ether (60–80◦ C): ethyl acetate mixture (7:3) as eluent.

Removal of the solvent gave a solid material, which was crystallised repeatedly from ethanol until constant transition temperatures were obtained. The purity of this series of compounds was checked by thin layer chromatography (Merck Kiesel gel 60 F254 per-coated plates). 2.3.5 2-[4-(4 -n-Octyloxybenzoyloxy)phenylazo]-6fluorobenzothiazole Yield: 80%; clearing point: 312◦ C UV (CHCl3 ). λmax : 382 and 266 nm. Elemental analysis (%). Found: C, 66.87; H, 5.79; N, 8.54. Calc. for C28 H28 N3 SO3 F (505 g mol−1 ): C, 66.53; H, 5.54; N, 8.31. IR (KBr). υ max cm−1 : 3058 (C–H Str. aromatic), 2927, 2854 (C–H Str. aliphatic), 1740 (C=O Str. ester), 1606 (N=N, Str. azo) 1471, 1365, 1078 (benzothiazole), 874, 847, 641 (C–S–C).

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N F

O

C

S

OR

O

Compound E12 N N

N

O

C

S

F

OR

O

Compound F12 Downloaded by [Veer Narmad South Gujarat University] at 02:24 16 January 2013

N N

N O2N

O

C

S

OR

O

Compound I N N

N Cl

O

C

S

OR

O

Compound II N N

N H3CO

O

C

S

OR

O

Compound III N

N

O

C

OR

O

Compound IV N

N

O

C

OR

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 = C12 H25 ) As shown in Table-4.

H 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; 1

J H–H = 5 Hz), 6.97–6.98 and 7.00–7.01 (tt, 2H, C2 and C-6 due to C-3 and C-5 ; J 2 6 = 2.4 Hz and J 3 5 = 6 Hz), 7.26–7.30 (m, 2H, C-7 and C5 due to C-4 and long range 19 F 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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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 due to Ar–H at C-7 and C-4), 8.13–8.17 (m, 4H, Ar–H at C-2 , C-3 , C-5 and C-6 ). These assignments are in agreement with data reported previously [40]. 13 C 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)-6fluorobenzothiazole A well stirred mixture of 2-amino-6-fluorobenzothiazole (1.68 g, 0.01 mol) and 40 mL of a 1:1 dilution of conc. H2 SO4 was cooled to below 5◦ C and a solution of NaNO2 (0.76 g, 0.011 mol) in water (20 mL) added dropwise so that the temperature of the mixture remained within the range 0–5◦ C. The cold dark yellow solution was added dropwise to a cold mixture of (1.44 g, 0.01 mol), NaOH (5 g, 10%) and water (50 mL), during which the temperature of the mixture was maintained below 8◦ C. After diazotisation, the solution was stirred for about 30 min and then acidified with 1:1 aqueous HCl, giving the crude product, which was filtered off, dried in air and recrystallised a number of times form alcohol (compound C in Scheme 1). 2.3.7 2-[4-(4 -n-Alkoxybenzoyloxy)naphth-1-ylazo]6-fluorobenzothiazole The synthetic procedure was identical to that reported for phenyl-based compounds in 2.3.4.

= 3.5 Hz), 3.99–4.03 (t, 2H (d) Ar–O–CH2 of alkyl chain; J H–H = 4 Hz), 6.89–6.93 (t, 2H, C-2 and C6 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, C5 and C-7 split due to 19 F at C-6 long-range coupling), 7.70–7.73 (quart 1H, C-5 split due to C-4 J 5.4 = 8 Hz and C-7 J 5.7 = 3 Hz ), 7.90–7.93 (1t, 2H, C-3 and C5 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 to C-5, J 7.5 = 4 Hz and C-4, J 7.4 = 2Hz). 13 C 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 behaviour The melting points and transition temperatures of the compounds in Series E are given in Table 1. These compounds exhibited enantiotropic mesomorphism. The SmC phase appeared in compounds E1 to E14 , and the transition temperature of the SmC–SmA phase decreased as the series was ascended. In the same series, the higher homologues (C8 –C18 ) displayed both SmA 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 number of –CH2 – groups in the alkoxy straight chain also caused an overall increase in polarisability. While Table 1. Transition temperatures (◦ C) of 2-[4-(4 -n-alkoxybenzoyloxy)phenylazo]-6-fluorobenzothiazoles (Series E). N N

2.3.8 2-[4-(4 -n-Octyloxybenzoyloxy)naphth-1ylazo]-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 C30 H30 N3 SO3 F (531 g mol−1 ): C, 67.79; H, 5.64; N, 7.90. IR (KBr) υ max 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). 1 H 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

241

F

Compound E1 E2 E3 E4 E5 E6 E7 E8 E10 E12 E14 E16 E18

N

O

S

C

OR

O

Transition temperature, ◦ C

R = nalkoxy

Cr

Methyl Ethyl Propyl Butyl Pentyl Hexyl Heptyl Octyl Decyl Dodecyl Tetradecyl Hexadecyl Octadecyl

• • • • • • • • • • • • •

SmC 116 108 104 100 102 105 93 96 90 85 73 − −

• • • • • • • • • • • − −

SmA − − − − − − − 153 142 134 130 86 94

− − − − − − − • • • • • •

N 227 217 216 214 210 207 205 204 − − − − −

• • • • • • • • − − − − −

I 337 334 332 332 321 320 319 312 299 275 272 266 263

• • • • • • • • • • • • •

Transition temperature

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B.T. Thaker et al. 360 320 280 240 200 160 120 80 40 0

Cr - SmC/SmA SmC - N

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

SmC - SmA

N

SmA - N N-I

0

2

Transition temperature


F1 F2 F3 F4 F5 F6 F7 F8 F10 F12 F14 F16 F18

C

OR

O

Transition temperature, ◦ C

R = nalkoxy

Cr

Methyl Ethyl Propyl Butyl Pentyl Hexyl Heptyl Octyl Decyl Dodecyl Tetradecyl Hexadecyl Octadecyl

• • • • • • • • • • • • •

Compound

this is true for an individual molecule, the dilution effect of the increasing number of –CH2 – groups reduces the bulk polarisability, as reported by Hird et al. [67]. The melting points and clearing points of Series E compounds containing an n-alkoxy group are plotted in Figure 1 and show a falling tendency. On the other hand the SmA–N transition temperature exhibits a rising tendency as the series is ascended. It is seen that the SmC mesophase can be observed from the first member of the homologous Series E. This can be explained by the fact that the length to breadth ratio is sufficient to provide lateral forces of cohesion, which hold together the compounds in a more ordered layer structure even after the first melting. The compounds in Series E have an asymmetrical terminal benzothiazole ring containing a fluorogroup at the sixth position, and this is electronegative. Fluoro-aromatic rings are a key structural unit of a number of functional molecules, including liquid crystals [68]. Terminal attraction is therefore relatively high for compounds E1 to E8 . The clearing temperature of Series E compounds are very high, due to the presence of a heterocyclic ring and F substituted (electronegative element) benzene ring compared with different substituted electronegative elements with different electronegative elements and the fact that the molecule is itself asymmetrical. The compounds in Series F exhibit an enantiotropic nematic mesophase. Smectic phases were not observed of any type, even in the higher homologues. This means that Series F was entirely nematogenic in character. Looking at the compounds of Series F, these contain a naphthalene moiety in the central core of a long molecule, and this increases the breadth of the molecule. The length to breadth ratio is therefore quite low [69], and the lateral attraction between molecules is not high enough to form a layer; as a result no smectic phase was observed in Series F compounds. The transition temperatures of Series F compounds are listed in Table 2. The melting and clearing points

O

S

F

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

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

N

N

SmA - I

Sm − − − − − − − − − − − − −

− − − − − − − − − − − − −

N 194 178 184 170 159 145 136 121 113 102 77 76 74

• • • • • • • • • • • • •

I 224 211 205 199 192 187 174 163 155 149 134 132 129

• • • • • • • • • • • • •

240 200 160

Cr-N N-I

120 80 40 0

0

2

4

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

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

on heating Series F compounds containing a n-alkoxy group are plotted in Figure 2, and show that the N–I transition temperature exhibits a falling tendency, and there was no noticeable odd–even effect observed for the lower members of the series.

3.2 Texture and DSC studies The mesophases exhibited by the compounds in Series E and Series F were identified according to their optical textures, observed by polarising optical microscopy using the classification system reported by Sackmann and Demus [70] and Gray and Goodby [71]. The DSC thermogram of compound E8 (Figure 3) showed four endothermic peaks. The first of three corresponded to the crystal to mesophase transition


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243

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

Figure 4. Schlieren texture of SmC of Compound E8 observed at 88◦ C

whereas the second and third peaks are mesophase to mesophase (SmC–SmA and SmA–N) and the fourth peak represents the mesophase to isotropic (N–I) transition. Typical textures were observed for compound E8 at various temperatures and these are illustrated in Figures 4 to 6. Similarly, the DSC thermogram of compound F8 showed two endothermic peaks during the heating cycle, as shown in Figure 7, the first of these corresponding to the crystal to mesophase (Cr–N) and the second to the nematic to isotropic (N–I) phase transition, respectively. During the cooling cycle one exothermic peak was observed, indicating that only one mesophase was exhibited by compound F8 . A typical Schlieren (threaded) texture with two and four brushes was observed, which was identified as the nematic phase, and shown in Figure 8. The transition

Figure 5. Focal conic texture of SmA of Compound E8 observed at 146◦ C

temperatures obtained from DSC thermograms and by POM, and also the values of H and S, are given in Table 3. 3.3 Mesogenic properties and molecular constitution There is a close relationship between mesomorphism and molecular constitution in organic compounds. This means that transition temperatures and the width of the mesophase can be correlated with the molecular composition of the compounds. Table 4 summarises the transition temperatures, the width of the mesophase, the thermal stability and molecular structure 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 E8 observed at 200◦ C

I [34], II [72], III [42], IV [73] and V reported in the literature [74]. For E12 the SmC–SmA and SmA–I mesophase ranges were 49◦ C and 141◦ C, respectively. The compound F12 was purely nematogenic and its N to I mesophase range was 47◦ C. The clearing points of compounds E12 and F12 were 275◦ C and 149◦ C, respectively. The absence of the nematic mesophase in compound E12 was the result of the phenyl group in the central core, which gives greater linearity to the structure of the molecule (E12) than the naphthyl group. Overall, compound E12 showed greater mesophase thermal stability and width of the mesophase than did F12. In terms of molecular structure, compound E12 differed from compound F12 only in its central aromatic 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-8 observed at 121◦ C

at the centre increases the breath of the molecule. Gray [69] has suggested that an increase in the breath of a molecules reduces both nematic and smectic mesophase stability. The presence of a naphthalene nucleus at the centre not only increases the breath of the molecule but also weakens lateral attraction between them. Both of these factors would tend to eliminate smectogenic tendencies as well as decreasing the mesophase range. Table 4 shows also that the SmC mesophase width and clearing point of compound E12 are higher by 28◦ C and 74◦ C, respectively, than those of compound IV. Compounds E12 and IV differ only at the terminals. However, the higher mesophase thermal stability

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245

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

Compound

Transition

Peak/(POM)Temp. (◦ C)

H J g−1

S J g−1 (◦ K)

E8

Cr–SmC SmC–SmA SmA–N N–I Cr1 –Cr Cr–SmC SmC–SmA SmA –I Cr–N N–I Cr–N N–I

87.67 (96) 145.8 (153) 200.00 (204) 311.05 (312) 66.80 (67) 71.97 (73) 127.64 (130) 273.86 (272) 120.70 (121) 162.89 (163) 76.95 (77) 134.62 (134)

8.31 11.67 2.51 59.14 1.75 2.01 10.07 77.25 14.34 11.21 3.24 5.03

0.0948 0.0800 0.0125 0.1901 0.0262 0.0279 0.0789 0.2800 0.1188 0.0688 0.0421 0.0374

E14

F

F8


F14

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

Mesophase width

Compound

Cr

Sm C

Sm A

N

I

Sm

N

Commencement of Sm phase

E12 F12 I II III IV V

• • • • • • •

85 − − − − 112 −

134 − 151 115 − − −

− 102 − 264 131 133 111

275 149 277 268 254 201 144

49/141 − 126 149 − 21 −

− 47 − 4 123 68 33

C1 − C1 C1 − C10 −

of compound E12 compared with compound IV is due to the terminal fluoro-containing benzothiazole ring system, which increases the overall polarisability [75] of the molecule and reduces its symmetry compared with the phenyl or naphthyl derivatives, giving a higher transition temperature. Comparing compound F12 with V, the width of the mesophase and clearing point of compound F12 were higher by 14◦ C and 5◦ C, respectively, than those of compound V; this is due to the fact that compound F12 had only one naphthalene ring in the central core, whereas compound V had two naphthalene rings, increasing its overall molecular breadth. Similarly, compound E12 exhibited an enantiotropic SmC mesophase, together with SmA, whereas compound I exhibited only enantiotropic SmA. The clearing point of compound I was higher by 2◦ C than that of compound E12 . This may be the result of the more polar –NO2 group at the sixth position in compound I, compared with the less polar –F in E12 . The more polar –NO2 group increased the polarisability of the molecule, and possibly also the molecular dipolarity of compound I compared with compound E12 . It has been observed in Table 4 that compound

E12 was smectogenic only, showing SmC and SmA mesophases, whereas compound II exhibited enantiotropic SmA and nematic mesophases. The smectic mesophase width and overall clearing point of compound E12 were higher by 41◦ C and 7◦ C, respectively, than those of compound II. These may have been due to the more polar F-group in compound E12 , compared to the less polar Cl-group at a similar position in compound II. This is also reflected in compound E12 in comparison with compound III. The order of the polarity of the substituted group at the 6-position of the benzothiazole ring was: NO2 > F > Cl > OCH3 .

4. Conclusions In this article we have presented the synthesis and characterisation of two series of 6-fluorobenzothiazole-based liquid crystals differing at the central linkage (an azo or ester group) and in the core unit (naphthalene or benzene). The mesomorphic properties exhibited by compounds of Series E and Series F show that the former had higher mesophase thermal stability due to the presence of a naphthyl in place


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of a phenyl group. Compounds of Series E exhibited nematic mesophases along with SmC and SmA mesophases, whereas Series F was purely nematogenic. By comparing the present series with other structurally related series it was found that the benzothiazole ring greatly affected the thermal stability of the mesophase. The study also showed that the naphthalene analogue of the benzothiazole ring favoured the formation of the nematic mesophase. It was also observed that the fluoro-substituent was more conducive to the generation of the smectic mesophase than was the chloro- or the methoxy-substituent, and was less conducive than the nitro-substituent. The order of polarity of the substituted group at 6-position of benzothiazole ring was NO2 > F > Cl > OCH3 .

Acknowledgments We are grateful to Gujarat Narmada Valley Fertilizer Company Ltd, Bharuch, for providing facilities for the elemental analyses, to Atul Industries Ltd, Atul, for DSC analyses, and to SAIF Chandigarh for FT–IR, 1 H–NMR, 13 C–NMR and mass spectral analyses. We also wish to thank Professor D.W. Bruce, University of York, UK, for providing literature.

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Molecular Crystals and Liquid Crystals Volume 575, Issue 1, 2013

Synthesis and Mesomorphic Investigations of Liquid Crystalline Compounds Having a Benzothiazole 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

Abstract Two homologous series of calamitic liquid crystals containing a benzothiazole ring and two different linkages have been prepared, and their liquid crystalline properties are studied and compared with each other and those of similar structure. The mesogens with only the cinnamate linking group showed better thermal properties than those with an ester. Nematic and smectic phases were observed. All the compounds of both the series were characterized by elemental analysis, FT-IR, mass spectrometry, 1H-NMR, and 13C-NMR. Phase transition temperatures and the thermal parameters were obtained from differential scanning calorimetery (DSC). The textural observations were performed using hot-stage Polarizing Optical Microscopy (POM).

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 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]

Two new mesogenic homologous series, each containing 1,3,5-trisubstituted pyrazolone derivatives, 4-n-alkoxyphenyl and Schiff base-cinnamate central linkages, have been synthesized to give 4-[(5-hydroxy-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl) methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-A] and 4-[(5-hydroxy-3-methyl-1-p-tolyl-4,5-dihydro1Hpyrazol-4-yl)methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-B] and their Cu(II)complexes have also been synthesized. These compounds were characterised by elemental analysis, Fourier transform infrared, 1H nuclear magnetic resonance, 13C-NMR, ultravioletvisible and mass spectral studies. Their mesomorphic behaviour was studied by polarising optical microscope (POM) with a heating stage. POM data were compared with differential scanning calorimetry thermograms. In Series-A and -B all compounds exhibit mesomorphism. Series-A compounds exhibit an enantiotropic nematic mesophase except propyl derivative, while a smectic A mesophase is observed from the heptyl derivative and persists up to the last member of the homologous series. n-Heptyloxy derivative is monotropic for smectic A phase. Series-B compounds also exhibit the enantiotropic nematic mesophase, while the smectic A mesophase is observed 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. The mesomorphic properties of both series are compared with each other and the other structurally related compounds. The study reveals that cinnamate linkage containing liquid crystals have higher thermal stability compared to structurally related series containing chalcone linkage. In case of complex series, only one compound from each series gives nematic mesophase. Keywords: Schiff base, cinnamate, liquid crystal, pyrazolone This work has been presented at 19thNational Conference on Liquid Crystal at Thaper University, Patiala (India) during 21-23, November, 2012. URL: http://mc.manuscriptcentral.com/tlct Email: [email protected]

Dear Prof. Thaker Your manuscript entitled "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" which you submitted to Liquid Crystals, has been 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 for publication. Please consider these suggestions, and I look forward to receiving your revision. Yours sincerely Prof. Corrie Imrie Editor, Liquid Crystals [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, 6disubstituted 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.