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Effects of Different Terminal Substituents on the Mesomorphic Behavior ofSome Azo-Schiff Base and Azo-Ester-Based Liquid CrystalsB. T. Thakera; J. B. Kanojiyaa; R. S. Tandela

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

First published on: 20 October 2010

To cite this Article Thaker, B. T. , Kanojiya, J. B. and Tandel, R. S.(2010) 'Effects of Different Terminal Substituents on theMesomorphic Behavior of Some Azo-Schiff Base and Azo-Ester-Based Liquid Crystals', Molecular Crystals and LiquidCrystals, 528: 1, 120 — 137To link to this Article: DOI: 10.1080/15421406.2010.504632URL: http://dx.doi.org/10.1080/15421406.2010.504632

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Effects of Different Terminal Substituents on theMesomorphic Behavior of Some Azo-Schiff Base

and Azo-Ester-Based Liquid Crystals

B. T. THAKER, J. B. KANOJIYA, ANDR. S. TANDEL

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

In order to investigate the influence of the terminal substitution on mesomorphism,two new series of azo-Schiff base and azo-ester liquid crystals having the followingstructures have been synthesized. All the compounds possess mesomorphic proper-ties. In series A compounds A1 and A3 exhibit only a nematic mesophase, whereascompounds A2, A4, A5, A6, A7, A8, A9, and A10 exhibit a smectic as well as anematic phase. In series B compounds B1 to B9 exhibit only a nematic mesophase,and compounds B10 and B11 exhibit a smectic as well as a nematic phase, but com-pound B12 exhibits only a smectic phase. All these compounds were characterizedby elemental analyses and spectroscopic techniques (Fourier transform infrared[FTIR], 1H nuclear magnetic resonance [NMR], and mass spectroscopy). Theirmesomorphic properties were measured by optical polarized light microscopy anddifferential scanning calorimetry (DSC).

Keywords Azo; azo benzene; ester; mesophase; nematic; smectic

Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad SouthGujarat University, Udhana-Magdalla Road, Surat 395 007, India. E-mail: [email protected]

Mol. Cryst. Liq. Cryst., Vol. 528: pp. 120–137, 2010

Copyright # Taylor & Francis Group, LLC

ISSN: 1542-1406 print=1563-5287 online

DOI: 10.1080/15421406.2010.504632

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Introduction

Liquid-crystalline materials, either low molar mass or polymeric in nature, contain-ing an azo group in the mesogenic core, are often studied from the point of view oftheir interesting optical properties, which enable application in, for example, opticalswitching, holography, and optical storage devices [1–6]. Azo dyes are also beingused in liquid-crystal display devices for the guest–host interaction [7].

Aromatic azo-containing compounds (Ar�N=N�Ar0) can undergo a mucheasier photoinduced trans-cis isomerization than the C=C bond and thus can gener-ate more interesting photoactive liquid crystals (LCs). Among these photoactivemesogenic units, azobenzene derivatives are most extensively investigated. The rigidrod-like structure of azobenzene molecules makes them suitable candidates for exhi-biting liquid crystallinity [8,9]. On the other hand, the unique characteristics ofazobenzene molecules provide the possibility of molecular motion in response tolight or heat and thus offer many opportunities in photonic applications. In suchazo-containing liquid-crystalline materials, the azo-containing moiety is usuallyincorporated into the side chain of the polymer backbone to induce the formationof LCs [10–15].

Azo compounds have the following advantages over substances with other lin-kages such as ester, tolane, or Schiff’s base. Azo compounds are thermally verystable and are attractive from the point of view of studying photoinduced effects [16].

For potential commercial applications, the existence of mesophases at lowertemperatures is of very high importance. Lateral substitution by a methyl groupwas used for some of these azo compounds in order to decrease the phase transitiontemperatures [17].

Most of the studies have been on Schiff bases or esters containing benzene orbiphenyl units; comparatively few studies have been done on the influence of anaphthalene core on mesomorphism. Gray [18] has reported that 6-n-alkoxy-2-naphthanoic acids are mesomorphic, whereas 1,4- and 1,5-alkoxy naphthoic acidsare non-mesomorphic. Dave and coworkers [19–21] have synthesized a number of4-n-alkoxy-1-naphthylidene Schiff bases and cholesteryl naphthoates and studiedtheir mesomorphism. Interest in naphthalene LC cores has revived in the last decade,as indicated by a significant number of research papers [22–30]. Recently, Prajapatiet al. have reported few mesogenic homologous series of Schiff’s base ester [31–33]and azo-esters [34–36] containing naphthalene moiety.

Demus and Sackmann [37] have reported mesomorphic azo ester dyes having abiphenyl core. Arora and Fergason [38] and Dave and Menon [39] have reported theazo ester dyes with different terminal substitutions. Vora and Dixit [40] havereported a homologous series containing an azo ester linking group and lateral sub-stitution. In recent years Bubnov et al. [41] have reported azo-ester-based new ferro-electric liquid-crystalline compounds derived from chiral S-lactic acid as a terminalmoiety. Prasad and Jakli [42] have synthesized azo-ester-based liquid crystals ofachiral bent core and observed photoinduced effects in antiferroelectric tilted smec-tic mesophase. Long and Lin [43] also have synthesized and evaluated azo dye com-pounds and show variation of mesogenic behavior with the length of alkyl chains.Liquid crystal polymers that contain an azo-ester linkage in the main chain havealso been synthesized [44].

Therefore, in the present work an attempt has been made to study the thermalstabilities of azomethine-azo and ester-azo central linkages. This led us to prepare

Azo-Schiff Base and Azo-Ester Liquid Crystals 121


more such compounds that would help further in understanding the effect ofsubstitution of lateral and terminal groups on mesomorphism. In the present workwe have selected two alkoxy chains such as octyl and hexadecyl, viz. 4-(40-n-alkoxybenzylidene amino)-naphthalene-1-azo-benzene (series A) and 4-(40-n-alkoxy ben-zoyloxy)-3-methyl-1-400 substituted-1-azobenzene (series B) comprising azomethine-(-CH=N)-azo(-N=N) and ester (-COO)-azo(-N=N) linkages for synthesis. Thesetwo types of compounds also contain a lateral methyl (-CH3) group at the 20-positionand 20,30-position fused ring system and they also consist of terminal chloro (-Cl),bromo (-Br), methyl(-CH3), methoxy (-O CH3), and nitro (-NO2) groups. We havesynthesized these compounds to study the influence of the lateral, as well as terminalgroup and also central linkages on mesomorphism and thermal stabilities of thesecompounds.

Experimental

Reagents and Technique

4-Hydroxy benzaldehyde, and a-naphthylamine were obtained from Merck,Germany. Benzoic acid, alkyl bromide (Lancaster, England). 4-Hydroxy o-Cresol,aniline, p-toludine, p-anisidine, p-chloroaniline, p-bromoaniline, and p-nitroanilinewere provided by Nova Dyes Stuff [P] Ltd., Surat and used without further purifi-cation. The solvents were used after purification using the standard methodsdescribed in the literature [45].

Elemental analyses (C, H, N) were performed at Central Drugs Research Insti-tute (CDRI) Lucknow. Infrared spectra were recorded with a Perkin-Elmer 2000Fourier transform infrared (FTIR) spectrophotometer (S.A.I.F), Punjab University,Chandigarh in the frequency range 4000–400 cm�1 with samples embedded in KBrdiscs. 1H nuclear magnetic resonance (NMR) spectra of the compounds wererecorded with a JEOL-GSX-400 using CDCl3 as a solvent and TMS as an internalreference, and mass spectra (EI) of the compounds at Sofisticated Analytical Instru-ment Facilities (SAIF), IIT Madras, Chennai. Thin-layer chromatography analyseswere performed by using aluminum-backed silica-gel plates (Merck 60 F524) andexamined under short-wave ultraviolet (UV) light.

The phase-transition temperatures were measured using a Shimadzu DSC-50 atheating and cooling rates of 5�Cmin�1. The optical microscopy studies were carriedout with a Leitz Loborlux 12 POL (Wetzler, Germany)polarizing microscopeequipped with a Mettler FP52 hot stage. The textures of the compounds wereobserved using polarized light with crossed polarizers with the sample in a thin filmsandwiched between a glass slide and coverslip.

Synthesis

Synthesis of 4-alkoxy benzaldehydes. 4-Alkoxy benzaldehydes were prepared by areported method [46–49]. The m.p. of these compounds was compared withreported values and they are very similar.

Synthesis of 4-amino-naphthalene-400-substituted-1-azobenzene. These dyes wereprepared by condensing a-naphthylamine with diazonium salts of substitutedaniline. First the substituted anilines were diazotized with dilute HCl and NaNO2

122 B. T. Thaker et al.


at 0–5�C and coupled with a-naphthylamine. Then the reaction mixture wasneutralized with diluted NaOH, and the obtained precipitate was collected byfiltration and then air dried. The crude compounds were purified by repeatedrecrystallizations using ethanol as solvent [50–51].

Synthesis of 4-(40-n-alkoxy benzylidene amino)-naphthalene-400-substituted-1-azo-benzene (series A). A mixture of 1mmol 4-alkoxy benzaldehyde and 4-amino-naphthalene-400-substituted-1-azobenzene and three drops of acetic acid in absoluteethanol were heated at reflux for 4 h. The reaction mixture was allowed to cool andwas stirred at room temperature overnight. The solid was collected and recrystallizedfrom technical ethanol. The product was purified by column chromatography usinga mixture of hexanes=ethyl acetate (7=1) as eluent. The above procedure wasadopted for the synthesis of mesogenic dyes having terminal substitutions such asmethyl, methoxy, chloro, and flouro azobenzene [52–54].

A5: Molecular formula: C31H32ON3Br. Elemental analysis, calculated forC 68.63% H 5.90%, and N 7.75%; found: C 68.95%, H 5.78%, and N 7.94%,EI-MS m=z (rel.int%): 541 (M)þ FTIR (in KBr), 2864, 2925 cm�1 (C-H aliphatic),1602 cm�1 (-C=N of azomethine), 1575–1515 cm�1 (C=C aromatic ring stretching),1470 cm�1 (C�H bending of CH2), 1166 cm

�1 (C�O�C); 1025 cm�1 (C�O stretch-ing); 1H-NMR (CDCl3, d, ppm): 0.87–0.92 (3H, t, CH3, alkyl chain), 1.24–1.87 (m,-(CH2)n, alkyl chain), 4.03–4.07 (2H, t, -CH2O, alkoxy chain), 7.03–7.98 (m, phenylprotons), 8.49 (s, CH=N, aldehydic), 8.40–8.43 (d), and 8.95–8.98 (d) aromatic orthoproton of first ring).

A8: Molecular formula: C40H51O2N3. Elemental analysis, calculated for C79.33%, H 8.40%, and N 6.94%; found: C 79.32%, H 8.40%, and N 6.91%. EI-MSm=z (rel.int%): 606 (Mþ 1)þ FT-IR (in KBr), 2858, 2911 cm�1 (C�H aliphatic),1602 cm�1(-C=N of azomethine), 1575–1514 cm�1 (C=C aromatic ring stretching),1480 cm�1 (C�H bending of CH2), 1166 cm

�1 (C-O-C), 1030 cm�1 (C�O stretching);1H-NMR (CDCl3, d, ppm): 0.87–0.92 (3H, t, CH3, alkyl chain), 1.24–1.87 (m,-(CH2)n, alkyl chain), 3.91 (s, terminal �OCH3 proton attached to the aromaticring), 4.03–4.07 (2H, t, -CH2O, alkoxy chain), 7.03–7.98 (m, phenyl protons), 8.49(s, CH=N azomethine), 8.40–8.43 (d), and 8.95–8.98 (d) (aromatic ortho protonof first ring).

Synthesis of 4-n-Alkoxy Benzoic Acids. 4-n-Alkoxy benzoic acids were prepared asreported by Dave and Vora [55]. The m.p. of these compounds was comparedwith the reported values and they are very similar.

Synthesis of 4-n-Alkoxy Benzoyl Chlorides. 4-n-alkoxy benzoyl chlorides wereprepared by the reported method [55]. The m.p. of these compounds wascompared with the reported values and they are very similar.

Synthesis of 4-hydroxy-3-methyl-1-azobenzene. A well-stirred mixture of aniline(0.01mol, 0.93 g) and concentrated HCl (0.03mol, 3.6mL) was cooled below 0–5�Cand a solution of NaNO2 (0.01mol, 0.7 g) in water (5mL) was added dropwise insuch a way that the temperature of the mixture was in the range 0–5�C. The cold,dark solution was added dropwise to a cold mixture of o-cresol (0.01mol, 1.08g),NaOH (20% w=v) during which the temperature of the mixture was maintainedbelow 0–5�C. Acidification with aqueous HCl furnished the crude product, whichwas collected by filtration, dried in air, and crystallized several times from ethanol [56].



Synthesis of 4-(40-n-alkoxy benzoyloxy)-3-methyl-1-azobenzene (series B).4-Hydroxy-3-methyl-1-azobenzene (0.01mol, 1.98 g) was dissolved in dry pyridine(10mL) and was added dropwise with occasional stirring into ice-cold 4-n-alkoxybenzoyl chloride (0.01mol, 2.69 g) in a round-bottom flask. Then the mixture wasrefluxed in a hot water bath for 2 h and allowed to stand overnight. The mixturewas acidified with cold 1:1 diluted hydrochloric acid to precipitate the product.The solid obtained was filtered, washed with water, saturated sodium bicarbonatesolution, and again water and thus the crude azo dye obtained. The solid esterazo dye was recrystallized from acetic acid until constant transition temperaturewas obtained [55]. The above procedure was adopted for the synthesis ofmesogenic dyes having terminal substituents such as methyl, methoxy, chloro,flouro, and nitro azobenzene.

B1: Molecular formula: C28H32O3N2. Elemental analysis: calculated for C75.68%, H 7.10%, and N 6.31%; found: C 75.96%, H 7.45%, and N 6.58%. EI-MSm=z (rel.int%): 443 (M-1)þ FTIR (in KBr), 2864, 2925 cm�1 (C�H aliphatic),1737 cm�1(-C=O of ester), 1575–1515 cm�1 (C=C aromatic ring stretching),1470 cm�1 (C�H bending of CH2); 1273 cm

�1 (C�O stretching of ester), 1172 cm�1

(C�O�C); 1H-NMR (CDCl3, d, ppm): 0.87–0.91 (3H, t, CH3, alkyl chain), 1.26–1.87 (m, -(CH2)n, alkyl chain), 2.33 (s, lateral �CH3 proton attached to the aromaticring), 4.03–4.07 (2H, t, -CH2O, alkoxy chain), 6.97–8.20 (m, phenyl protons).

B7: Molecular formula: C36H48O3N2. Elemental analysis: calculated for C77.70%, H 8.63%, and N 5.04%; found: C 77.95%, H 8.85%, and N 5.33%. EI-MSm=z (rel.int%): 556 (M)þ FTIR (in KBr), 2864, 2932 cm�1 (C�H aliphatic),1730 cm�1(-C=O of ester), 1575–1515 cm�1 (C=C aromatic ring stretching),1480 cm�1 (C�H bending of CH2); 1273 cm

�1 (C�O stretching of ester), 1179 cm�1

(C�O�C); 1H-NMR (CDCl3, d, ppm): 0.87–0.91 (3H, t, CH3, alkyl chain),1.26–1.87 (m, -(CH2)n, alkyl chain), 2.33 (s, lateral �CH3 proton attached to the aro-matic ring), 4.03–4.07 (2H, t, �CH2O, alkoxy chain), 6.97–8.20 (m, phenyl protons).

Synthesis

SCHEME 1

Preparation of 4-alkoxy benzaldehydes

Diazotization of aniline



Synthesis of 4-amino-naphthalene-1-azobenzene

Synthesis of 4-(40-n-alkoxy benzylidene amino)-naphthalene-1-azo-benzene(Series A)

SCHEME 2

Synthesis of 4-n-alkoxy benzoic acids

Synthesis of 4-n-alkoxy benzoyl chlorides



Diazotization of Aniline

Synthesis of 4-hydroxy-3-methyl-1-azobenzene

Synthesis of 4-(40-n-alkoxy benzoyloxy)-3-methyl-1-azobenzene (Series B)

Results and Discussion

In order to investigate the influence of the terminal substitution on mesomorphismtwo new series, viz. 4-(40-n-alkoxy benzylidene amino)-naphthalene-400-substituted-1-azo-benzene (series A) containing azo-schiff base as a central linkage and 4-(40-n-alkoxy benzoyloxy)-3-methyl-1-400 substituted-1-azobenzene (series B) containingazo-ester as a central linkage and different terminal substitutions, were synthesizedand their mesomorphic properties were studied. The common structural featuresof the compounds are that they consist of the same terminal substitutent at oneend. The mesomorphic properties of all the synthesized compounds have been char-acterized by differential scanning calorimetry (DSC) and polarizing opticalmicroscopy (POM) attached with a Mettler hot stage.

The transition temperatures of both series are given in Tables 1 and 2. In series Acompounds A1 and A3 exhibit only a nematic mesophase, whereas compounds A2,



A4, A5, A6, A7, A8, A9, and A10 exhibit a smectic as well as a nematic phase. In seriesA compounds A1 is monotropic, whereas the rest of the compounds are enantiotro-pic in nature. In series B compounds B1 to B9 exhibit only a nematic mesophase, andcompounds B10 and B11 exhibit a smectic as well as a nematic phase, but compoundB12 exhibits only a smectic phase.

DSC is a valuable method for the detection of phase transitions. It yields quan-titative results; therefore, we may draw conclusions concerning the nature of thephases that occur during the transition. In the present study, enthalpies of two deri-vatives of series A and series B were measured by DSC. DSC data for series A andseries B are recorded in Tables 3 and 4, which helps to further confirm the mesophasetype. Table 3 shows the phase transition temperatures, associated enthalpy (DH),and molar entropy (DS) for compounds of series A (A2, A4, A5, and A7) and

Table 1. Transition temperatures of Series A

Transition temperature (�C)

Compounds R¼ n-Alkoxy X S N I

A1 Octyloxy -H — 103a 108A2 Octyloxy -CH3 111 126 175A3 Octyloxy -OCH3 — 107 205A4 Octyloxy -Cl 115 122 179A5 Octyloxy -Br 108 130 174A6 Hexadecyloxy -H 65 74 94A7 Hexadecyloxy -CH3 68 82 140A8 Hexadecyloxy -OCH3 92 112 163A9 Hexadecyloxy -Cl 82 128 149A10 Hexadecyloxy -Br 80 131 150

aMonotropic mesophase.

Table 2. Transition temperatures of Series B

Transition temperature (�C)

Compounds R¼ n-Alkoxy X S N I

B1 Octyloxy -H — 67 91B2 Octyloxy -CH3 — 85 161B3 Octyloxy -OCH3 — 96 198B4 Octyloxy -Cl — 77 170B5 Octyloxy -Br — 81 160B6 Octyloxy -NO2 — 111 201B7 Hexadecyloxy -H — 63 83B8 Hexadecyloxy -CH3 — 76 134B9 Hexadecyloxy -OCH3 — 81 154B10 Hexadecyloxy -Cl 78 117 140B11 Hexadecyloxy -Br 91 106 136B12 Hexadecyloxy -NO2 86 — 161



Table 4 shows the phase transition temperatures, associated enthalpy (DH), andmolar entropy (DS) for compounds of series B (B2, B4, B5, B8, and B10). Enthalpyvalues of the various transitions agree well with the existing related literature values[57]. The DSC curves of representative compounds are shown in Figs. 1 to 4. Micro-scopic transition temperature values are very similar to the DSC data.

A number of homologues were synthesized constituting a p-phenylene unit tostudy the effect of chemical constitution on liquid-crystalline properties. There is avery delicate balance between the chemical constitution and liquid-crystalline

Table 3. DSC data for Series A compounds

Series Compound TransitionPeak temp. (microscopic

temp.) (�C)DH

(Jg�1)DS

(Jg�1K�1)

A A2 Cr-S 112 (111) 97.80 0.2547S-N 124.29 (126) 14.57 0.0367N-I 175 (175) 2.10 0.0047

A4 Cr-S 115 (115) — —S-N 125.54 (122) 70.66 0.1773N-I 179 (179) 1.94 0.0043

A5 Cr-S 108 (108) 7.69 0.0202S-N 131 (130) 64.36 0.1593N-I 174 (174) 1.35 0.0030

A7 Cr-S 72 (68) 6.28 0.0182S-N 81.37 (82) 32.81 0.0926N-I 137.59 (140) 1.55 0.0038

A9 Cr-S 81.96 (82) 47.55 0.1339S-N 128 (128) — —N-I 145 (149) 1.45 0.0034

A10 Cr-S 85 (80) 92.98 0.2597S-N 132 (131) — —N-I 144.57 (150) 1.40 0.0034

Table 4. DSC data for Series B compounds

Series Compound TransitionPeak temp. (microscopic

temp.) (�C)DH

(Jg�1)DS

(Jg�1K�1)

B B2 Cr-N 83.51 (85) 49.76 0.1396N-I 162.6 (161) 3.17 0.0073

B4 Cr-N 82 (77) 78.47 0.2210N-I 165 (170) 1.94 0.0044

B5 Cr-N 78.5 (81) 78.64 0.2237N-I 158 (160) 3.17 0.0074

B8 Cr-N 78.2 (76) 35.11 0.1000N-I 130.7 (134) 2.11 0.0052

B10 Cr-S 81 (78) 33.63 0.1000S-N 118 (117) — —N-I 136.66 (140) 1.67 0.0041



properties. Generally the introduction of a lateral substituent is different for all typesof mesophases. In the quest of obtaining low melting liquid crystals with broadmesophase range, it was noted that a lateral substituent like chloro (-Cl) or methyl(-CH3) is quite effective in reducing crystal mesophase temperature to give lowmelting liquid crystals [58–60].

In light of this, it was of interest to introduce lateral methyl and terminal hal-ogens (-Cl, -Br), methyl, methoxy, and nitro substituents on the benzene ring at

Figure 1. DSC curve of compound A4.

Figure 2. DSC curve of compound A9.



one end of the p-phenylene system to evaluate the effect of substituents onmesomrphic behavior.

In series A 10 compounds were synthesized, out of which A1-A5 have an octy-loxy (OC8H17) chain and A6-A10 have a hexadecyloxy (-OC16H33) chain, in additionto different terminal subsituents like �H, -CH3, -OCH3, -Cl, and -Br. In series B 12compounds were synthesized, of which B1-B6 have an octyloxy (OC8H17) chain andB7-B12 have a hexadecyloxy (-OC16H33) chain; in addition to different terminal sub-situents like �H, -CH3, -OCH3, -Cl, -Br, and -NO2.

Figure 3. DSC curve of compound B2.

Figure 4. DSC curve of compound B10.



Thermal Stability

The average thermal stability of the compounds of series A, that is, azomethine-basedazo dyes, is higher than compounds of series B, that is, ester-based azo dyes. Becauseboth series have the same terminal subsituents at one end and of the other end an-alkoxy chain. The two types of compounds differ only in the central linkage andlateral substitution. Compounds of series A have an azomethine (-CH=N)-azo(-N=N) central linkage and a lateral substituted benzene ring that is closed atthe 20,30-position of the central unit, whereas compounds of series B have an ester(-COO)-azo(-N=N) central linkage and a lateral substituted methyl group at the30-position of the central unit.

The presence of a naphthalene unit as the central unit in series A shows that thenaphthalene compounds give rise to higher melting points, more stable mesophases,and wider liquid-crystalline phase ranges in comparison with their phenyl analogues.These properties are a direct consequence of the molecular arrangement in both thesolid and the liquid-crystal phases. As mentioned earlier, the phenyl and naphthylcentral core units differ mainly in their length and extension of the electronic conju-gation. The increased transition temperatures can clearly be attributed to theincreased length and higher polarizability of the naphthalene structure that allowsboth stronger and wider-ranging molecular interaction.

The introduction of the lateral methyl group in series B compounds changes themolecular conformation and broadens the molecules, which decreases, the transitiontemperature of all phases. This factor is a direct result of an increase in the breath orthickness of the molecule [61]. Thus, the introduction of the lateral methyl groupresults in decreased transition temperature of all phases in the series B compounds.This has been attributed to the broadening of the molecules as well as the change inmolecular conformation. The increased dissymmetry resulting from the lateralmethyl group leads to less effective packing in the crystal lattice and therefore lowersthe crystal to mesophase transition temperature as shown in Tables 1 and 2.

As mentioned earlier, series A has the central azomethine (-CH=N-) group,whereas series B has the central carboxy (-COO-) group. The oxygen atom of thiscentral carboxy group in the molecules of series B will bump into the nonbonded sideof the adjacent �H of the aromatic ring, thereby causing considerable strain on themolecule. This will cause some twist around the C-O bond and force the benzene ringout of the plane of the molecule and broaden the molecule [58] as shown below.

The H atom of the central azomethine (-CH=N-) group will also behave in thesame way and bring about the twist in the molecules of series A, but the twist in thiscase will be much less than in the molecules of series B. Hence, the molecules of seriesB will be more non-coplanar than those of series A. This effect is observed in thepresent case where the thermal stabilities of series B compounds are less than thoseof series A compounds.



The N-I average thermal stability order with respect to the terminal group inseries A compounds is observed as

OCH3 >ð184�CÞ

�Cl >ð164�CÞ

�Br >ð162�CÞ

�CH3 >ð158�CÞ

�Hð101�CÞ

In the above order, it shows that N-I average thermal stability of the �OCH3 groupis higher than that of other terminal groups. This could be explained that in �O-CH3

group, the lone pair of electrons of oxygen is shielded by an insulator-like methylgroup. The repulsive forces involving the oxygen lone pairs are thereby substantiallyreduced and allow a close approach of the neighboring molecules, increasing bondingforces. This leads to an increase in the N-I transition temperature. The N-I thermalstability of �CH3 is less than that of �Cl and �Br groups. This is because the �CH3

group is less polar, whereas the �Cl and �Br groups will endow the molecule withhigher polarity, high terminal attraction, and hence higher N-I average thermalstability. The same trend was observed by Gray et al. [18,62] and Coates [63].

In series A mesogenic compounds, the S-N average thermal stability of terminalgroup is observed only in both the chain as it shows that is series-A compoundsA1-A5 having octyloxy chain and compounds A6-A10 having hexadecyloxy chain.In both octyloxy and hexadecyloxy chain the terminal substituents are �Br, �Cl,& CH3 in which average smectic-nematic thermal stabilities is given in order.

�Br >ð130:5�CÞ

�Cl >ð125�CÞ

�CH3ð104�CÞ

The above order shows that terminal -CH3 substituted compounds are lessmesomorphic than those with �Cl and �Br units. This is because -CH3 less polar,whereas the �Cl and �Br groups endow the molecules with high polarity, greaterterminal attraction, and hence higher S-N thermal stability. Enhanced smectic ten-dencies have been found by Gray et al. [18] for the 40-bromo and 40-chloro deriva-tives relative to the unsubstituted compound.

Gray [18] has given the order of decreasing smectic thermal stability of thecompounds in terms of the terminal substituents �X- as

This order indicates no simple relationship between the strength of the dipole andsmectic thermal stability. Thus, the nitro group with its strong dipole acting along thelong axis of the molecule yields a smectic mesophase that is less thermally stable thanthat of X=Cl. Also, with no terminal dipole X=H, the smectic mesophase is thermallymore stable than for X¼-OCH3, for which the dipole acts at an angle across the longaxis of the molecule. This fact suggests that terminal dipoles contribute to the ter-minal intermolecular attractions, either by polarization or dipole–dipole interactionsas well as the lateral attractions. If such terminal attraction occurs, the contribution ofthe dipole to the lateral attraction will be decreased and the balance of the lateral and



terminal cohesion will be affected. The determination of the smectic characteristics ofthe molecules is therefore complicated by the possible dual role of these dipoles inaffecting both terminal and lateral attractions. The relative span of temperature ofvarious p-substituted groups of series A and series B compounds is shown in Figs. 5and 6. The mesophase textures observed are shown in Fig. 7.

Commencement of Smectic Mesophase

In series B compounds, the smectic phase commences late at the hexadecyl deriva-tives of alkyl chain, whereas it commences at the octyl and hexadecyl derivative

Figure 5. Plot of the mesomorphic behavior of compounds A1 to A10.

Figure 6. Plot of the mesomorphic behavior of compounds B1 to B12.



respectively in series A. It is rather difficult to postulate the exact commencement ofthe smectic mesophase in the compounds of series A because it depends on a numberof factors like polarizability, breath (thickness), and geometry of the molecules.

The difference between the two types of compounds is in the central groups. Inseries A compounds have an azomethine (-CH=N-) group and naphthalene nucleus,which make the molecules longer and more polarized. Thus, early commencement ofa smectic phase in series A can be understood. Series B has a central ester (-COO)group and lateral substituted �CH3 group. In series B the lateral �CH3 group,which is ortho to �COO- linkage, will have twofold effects: (i) increasing the breath(thickness) of the molecules and (ii) reducing the coplanarity of the molecules. Boththese effects explain the late commencement of the smectic mesophase and lowerthermal stability of the compounds of series B.

From the above results it can be seen that not only the geometry of the moleculesbut also length, breath (thickness), and polarizability of the molecules play an impor-tant role in the mesomorphic behavior of compounds.

Conclusion

In this article we have presented the synthesis and characterization of two newmesogenic series, viz. 4-(40-n-alkoxy benzylidene amino)-naphthalene-400-substituted-1-azo-benzene (series A) containing azo-Schiff base as a central linkage and 4-(40-n-alkoxy benzoyloxy)-3-methyl-1-400 substituted-1-azobenzene (series B) containing

Figure 7. (a) Texture of nematic phase of A4 of series A at 122�C on cooling. (b) Thread-liketexture of the nematic phase of A10 of series A at 131�C on cooling. (c) Marble texture of thenematic phase of B1 of series B at 67�C on cooling. (d) Marble texture of the nematic phase ofB9 of series B at 81�C.



azo-ester as a central linkage and different terminal substitutions. The presence of anaphthalene nucleus as a central unit in series A shows that the naphthalenecompounds give rise to higher melting points, higher mesophase stability, and widerliquid-crystalline phase ranges in comparison with their phenyl analogues. Theseproperties are a direct consequence of the molecular arrangement in both the solidand the liquid-crystal phases. The phenyl and naphthyl central core units differmainly in their length and extension of the electronic conjugation. The increasedtransition temperature can clearly be attributed to the increased length and higherpolarizability of the naphthalene structure that allows both stronger andwider-ranging molecular interactions. The introduction of the lateral methyl groupin series B compounds changes the molecular confirmation and broadening of themolecules, which decreases the transition temperatures of all phases. The increaseddissymmetry resulting from the lateral methyl group leads to less effective packingin the crystal lattice and therefore lowers the crystal mesomorphic transitiontemperature.

Acknowledgment

We are thankful to Solvay Chemicals Ltd., Ankleshwar (Gujarat), for providingthermal studies (DSC) and the Department of Applied Chemistry, Faculty of Tech-nology and Engineering, M. S. University of Baroda, Vadodara, for providing uswith an optical polarizing microscope for mesophase studies.

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Substitution Effects on the Liquid Crystalline Properties of ThermotropicLiquid Crystals Containing Schiff Base Chalcone LinkagesB. T. Thakera; P. H. Patela; A. D. Vansadiyaa; J. B. Kanojiyaa

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

First published on: 17 December 2009

To cite this Article Thaker, B. T. , Patel, P. H. , Vansadiya, A. D. and Kanojiya, J. B.(2009) 'Substitution Effects on theLiquid Crystalline Properties of Thermotropic Liquid Crystals Containing Schiff Base Chalcone Linkages', MolecularCrystals and Liquid Crystals, 515: 1, 135 — 147To link to this Article: DOI: 10.1080/15421400903291533URL: http://dx.doi.org/10.1080/15421400903291533

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Substitution Effects on the Liquid CrystallineProperties of Thermotropic Liquid CrystalsContaining Schiff Base Chalcone Linkages

B. T. Thaker, P. H. Patel, A. D. Vansadiya, andJ. B. KanojiyaDepartment of Chemistry, Veer Narmad South Gujarat University,Surat, India

Two homologous series of Schiff base Chalcones, Series-A: 1-{4-[(2,4-dihydroxybenzylidene) amino] phenyl}-3-(4-alkoxy phenyl) prop-2-en-1-one and series-B:1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]amino}phenyl)-3-(4-methoxyphenyl)prop-2-en-1-one were synthesized and the physical characterization was carried outalong with spectroscopic techniques (FT-IR, 1H NMR,13C NMR, and MS). Thethermotropic properties were investigated by optical polarized light microscopyand differential scanning calorimetry and evaluated as a function of chain lengthand linking group. In series-A compounds C3 to C8 exhibit the nematic mesophasewhile compounds C10, C12, C14, and C16 exhibit a smectic phase as well as thenematic mesophase. Compounds C1 and C2 do not exhibit any mesophase. Inseries-B, all the compounds (C1 to C8, C10, C12, C14, and C16) exhibit only thenematic mesophase.

Keywords: chalcone; mesomorphic; nematic mesophase; Schiff base; smectic meso-phase

INTRODUCTION

Mesogens with a chalcone central linkage are relatively rare. Ithas been observed that �CO�CH=CH� linkage is less conducive tomesomorphism compared to �CH=N�(azomethine), �COO� (ester),�N=N�(azo) linkages due to the non linearity and angle strain arisingfrom the keto group [1]. But when �CO�CH=CH� linkage is present

This paper was presented at the 15th National conference on Liquid Crystals atIndian Institute of Science (I.I.Sc.), Banglore, Dec. 13th–15th, 2008.

Address correspondence to B. T. Thaker, Department of Chemistry, Veer NarmadSouth Gujarat University, Udhana-Magdalla Road, Surat, 395 007, India. Tel: þ91261 2227141-46; Fax: þ91 261 2227312. E-mail: [email protected]

Mol. Cryst. Liq. Cryst., Vol. 515, pp. 135–147, 2009



DOI: 10.1080/15421400903291533

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with other central linkages it becomes condusive to mesomorphism. Inthe literature there are several reports of mesogenic compoundshaving chalcone linkage. Many years ago Vora et al. [2] reported ahomologous series of polymers containing a chalcone linkage. Soonafter words Chudghar et al. [3] reported a homologous series contain-ing on ester-chalcone linkages. Recently Yeap et al. [4] have alsosynthesized mesomorphic compounds containing on ester-chalconelinkage. In our previous work we have reported the homologous seriescontaining a Schiff base-chalcone linkage [5].

Chalcone is an important class of chemical compound and is beingstudied extensively because of its significant use or application invarious sectors. In the fields of biology and biochemistry, chalconehas been claimed to be one of the compounds that plays a vital role inanti-tumor [6,7], anti-inflammatory [8,9], and anti-malaria [10] activ-ities. It has also been documented that the chalcone possesses aremarkable nonlinear optical (NLO) property, which is an essential ele-ment for optical communications devices. The other importance of thiscompound is its high photosensitivity and thermal stability, which areused in developing various crystalline electro-optical devices [11–13].

In view of the outstanding behavior of chalcone compounds, asystematic study focusing on the synthesis and characterization of thisclass of compound has been carried out in our present laboratory overpast few years [5,14]. In addition, terminal and lateral subtituentsalso play a vital role in imparting liquid crystallinity to potentiallymesogenic compounds [15–19]. The correlation between chemicalstructure and mesomorphic properties is one of the most importantproblems in liquid crystals science [20]. An understanding of theinfluence of different structural elements of the molecules on thephysico-chemical characteristics of mesomorphic organic compoundsallows chemists to synthesize liquid crystals with required properties.

Laterally substituted mesogens are of considerable interest becausethese compounds deviate from the classical rod-like shape. Howeverlateral hydroxyl groups are little exploited as it was reported thatthe phenolic ‘hydroxy’ group may destroy mesomorphism. Gray [21]has proposed that this rarity is associated with intermolecular hydro-gen bonding raising the melting point above the mesophase-isotropicliquid transition temperature, and perhaps also encouraging a non-linear molecular arrangement that is incompatible with mesophaseformation. After an initial investigation by Weissflog and Demus[22], a number of homologues series with a trisubstituted benzenenucleus have been reported [23–30]. Schroeder and Schroeder [31]reported a few terminally hydroxy substituted mesogens. Vora andGupta [32] for the first time reported homologous series with terminal



and lateral hydroxy groups. Subsequently a few more mesogenic serieswith a lateral hydroxy group have been reported [33–35]. In general, arigid lateral substituent disrupts the ordering of the liquid crystallinephase [36–38], causing a significant depression in the clearing pointand liquid crystal phase transition.

In our previous work [5] we have synthesized two homologous seriescontaining Schiff base-chalcone linkages having substituted pyrazo-lone ring as a terminal group. An attempt has been made to synthesizetwo new homologous series containing the same central linkages, withthree aromatic rings in the main core and substituted by a lateralaromatic branch on the terminal ring to study the influence of theterminal and lateral hydroxy group on mesomorphic and thermalstability of these compounds.

EXPERIMENTAL

Reagents and Techniques

For the synthesis of compounds of the homologous series, followingmaterials were used. 4-Hydroxy benzaldehyde, alkyl bromides(Lancaster, England), 2-4 dihydroxy acetophenone, p-amino acetophe-none (Lancaster, England) were used without further purification. Allthe solvents were used after purification using the standard methodsdescribed in literature [39].

Elemental analyses (C, H, N) were performed at CDRI, Lucknow.Infrared spectra were recordedwith a Perkin-Elmer 2000 FT-IR spectro-photometer in the frequency range 4000–400cm�1 with samplesembedded in KBr discs. 1H-NMR spectra of the compoundwere recordedwith JEOL-GSX-400 using CDCl3 as a solvent and TMS as an internalreference at SAIF, IIT Madras, Chennai. 13C NMR spectra of the comp-oundwere recordedwith BRUKERAVANCE II 400NMRSpectrometer,SAIF, Chandigarh. Mass spectra (EI) of the compounds were recorded atSAIF, IIT Madras, Chennai. Thin-layer chromatography analyses wereperformed by using aluminium-backed silica-gel plates (Merck 60 F524)and examined under short-wave UV light.

The phase-transition temperatures were measured using aShimadzu DSC-50 at heating and cooling rates of 5�Cmin�1. TheDSC data are shown in Table-III. The optical microscopy studieswere carried out with a ‘‘Leitz Loborlux 12’’ POL (Wetzler, Germany)polarizing microscope equipped with a Mettler FP52 hot stage andtemperature controller. The textures of the compounds were observedusing polarized light with crossed polarizers with sample in a thin filmbetween a glass slide and a cover slip.

Properties of Thermotropic Liquid Crystals 137


SYNTHESIS

Synthesis of n-Alkoxy Benzaldehydes

n-Alkoxy benzaldehydes were prepared by a reported method [40–43].

Synthesis of 1-{4-[(20,40-Dihydroxybenzylidene) Amino]Phenyl} Ethanone

A mixture of 2,4-dihydroxy benzaldehyde (1mmol) and 4-aminoaceto-phenone(1mmol) and three drops of acetic acid in absolute ethanol(10ml) was heated at reflux for 4 hr. The reaction mixture was allowedto cool and stirred at room temperature overnight. The solid wascollected and recrystallized from acetone.

Synthesis of 1-(4-{[(1-(2,4-Dihydroxyphenyl)ethylidene]amino}phenyl)ethanone

A mixture of 2,4-dihydroxy acetophenone (1mmol) and 4-aminoaceto-phenone(1mmol) and three drops of acetic acid in absolute ethanol(10ml) was heated at reflux for 4 hr. The reaction mixture was allowedto cool and stirred at room temperature overnight. The solid wascollected and recrystallized from acetone.

Synthesis of 1-{4-[(2,4-Dihydroxybenzylidene) Amino] Phenyl}-3-(4-alkoxyphenyl) Prop-2-en-1-One (Series-A)

A mixture of 1-{4-[(20,40-dihydroxybenzylidene) amino] phenyl} etha-none (1mmol) and n-alkoxy benzaldehyde (1mmol) was dissolved inthe alcoholic sodium hydroxide solution (80ml ethanol and 10% NaOHsolution). The reaction mixture was heated at 80�C for 7 hr. The mix-ture was left at room temperature overnight. An HCl aqueous solutionwas added to the mixture, and then yellow precipitate was obtained.The precipitate was washed with water until a neutral aqueous solu-tion was obtained. Then, the solid was washed withmethanol and driedunder vacuum at 60�C. The obtained yellow solid was purified byrecrystallization from acetone. The product was obtained in 45% yield.The physical data of the series A compounds are given in Table 1.

Data

A12, Molecular Formula: C34H41O4NElemental analysis, calculated for C 71.70%; H 7.20% and N 9.84%,

found: C 71.78%; H 7.12% and N 9.93%, FT-IR (in KBr), 3432 cm�1



(OH-phenolic); 2879, 2968 cm�1 (C-H aliophatic) 1573 cm�1 (C-Hphenyl ring); 1681 cm�1 (C=O, chalcone group); 1641 cm�1 (-C=N of azo-methine); 1597 cm�1 (-C=C-, vinyl group of chalcone); 1120–1029 cm�1

(C-O-C); 1H-NMR (CDCl3, d, ppm): 0.96–1.33 (3H, t, CH3, alkyl chain),1.29–1.71 (m, -(CH2)n, alkyl chain), 3.94–4.03 (2H, t, -CH2O, alkoxychain), 7.56 & 7.90 (2H, d, olefinic H), 6.32–7.80 (m, phenyl protons),11.09 (s, phenolic OH), Mass (FAB): Molecular ion peak: A12 {m=z�528 (Mþ 1)þ}.

1-(4-{[1-(2,4-Dihydroxyphenyl)ethylidene]amino}phenyl)-3-(4- methoxyphenyl) Prop-2-en-1-one (Series-B)

A mixture of 1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]amino}phenyl)ethanone (1mmol) and n-alkoxy benzaldehyde (1mmol) was dissolvedin the alcoholic sodium hydroxide solution (80ml ethanol and 10%NaOH solution). The reaction mixture was heated at 80�C for 7hr.The mixture was left at room temperature overnight. An HCl aqueoussolution was added to the mixture, and then yellow precipitate wasobtained. The precipitate was washed with swater until a neutralaqueous solution was obtained. Then, the solid was washed withmethanol and dried under vacuum at 60�C. The obtained yellow solidwas purified by recrystallization from acetone. The product wasobtained in 45% yield. The physical data of the series-B compoundsare given in Table 2.

TABLE 1 Transition Temperature Data of the Series (Series-A)

Transition temperature in �C

Code No. R¼n-alkyl S N I

A2 Ethyl – – 183A3 Propyl – – 167A4 Butyl – 146 159A5 Pentyl – 131 154A6 Hexyl – 129 146A7 Heptyl – 112 141A8 Octyl – 106 135A10 Decyl – 103 121A12 Dodecyl 81 99 123A14 Tetradecyl 85 94 121A16 Hexadecyl 66 79 109A18 Octadecyl 69 82 98



Data

B12, Molecular Formula: C35H43O4NElemental analysis, calculated for C 77.63%; H 7.94% and

N 2.58%, found: C 77.58%; H 7.87% and N 2.54%, FT-IR(in KBr), 3439 cm�1 (OH-phenolic); 2000–1667 cm�1 (C-H phenylring, out of plane bending); 1684 cm�1 (C=O, chalcone group);1632 cm�1 (-C=N); 1606 cm�1 (-C=C-, vinyl group of chalcone);1120–1030 cm�1 (C-O-C); 1H-NMR (CDCl3, d, ppm): 0.89–0.93(3H, t, CH3, alkyl chain), 0.94 (3H, s, CH3 of acetophenone)1.44–1.86 (m, -(CH2)n, alkyl chain), 4.03–4.06 (2H, t, -CH2O, alkoxychain), 6.68 and 7.73 (2H, d, olefinic H), 7.0–7.97 (m, phenyl pro-tons), 9.90 (s, phenolic OH), Mass (FAB): Molecular ion peak: B12

{m=z� 540 (M�1)þ}.

Scheme: Synthetic Route for Series-A & Series-B

Synthesis of -n-alkoxy benzaldehydes

TABLE 2 Transition Temperature Data of the Series (Series-B)

Transition temperature in �C

Code No. R¼n-alkyl S N I

B2 Ethyl – 124 132B3 Propyl – 111 120B4 Butyl – 116 126B5 Pentyl – 110 118B6 Hexyl – 113 124B7 Heptyl – 97 115B8 Octyl – 102 121B10 Decyl – 95 108B12 Dodecyl – 98 112B14 Tetradecyl – 99 110Bl6 Hexadecyl – 112 120Bl8 Octadecyl – 78 119



Synthesis of 1-{4-[(20,40-dihydroxybenzylidene) amino] phenyl}ethanone

Synthesis of 1-(4-{[(1-(2,4-dihydroxyphenyl)ethylidene]amino}phenyl)ethanone

Synthesis of 1-{4-[(2,4-dihydroxybenzylidene) amino] phenyl}-3-(4-alkoxyphenyl) prop-2-en-1- one (Series-A)



Synthesis of 1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]amino}-phenyl)-3-(4-methoxyphenyl)prop-2-en-1-one (Series-B)

RESULT AND DISCUSSION

In the present study, 12 homologues from each of the two series,1-{4-[(2,4-dihydroxybenzylidene) amino] phenyl}-3-(4-alkoxyphenyl)prop-2-en-1-one (Series-A) and 1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]amino}phenyl)-3-(4-methoxyphenyl)prop-2-en-1-one (Series-B) weresynthesized and their mesomorphic properties studied. The meso-morphic properties of all the synthesized compounds have been charac-terized by differential scanning calorimetry (DSC) and polarizingoptical microscope (POM) attached with Mettler hot stage. Phaseidentification was based on the optical textures, and the magnitude ofthe isotropization on enthalpies is consistent with the assignment ofeach mesophase type, using the classification systems reported by SackMann and Demus [44], and Gray and Goodby [45].

In series-A, out of twelve compounds, only ten compounds exhibitthe enantiotropic nematic mesophase along with the Smectic-C phase.In series-A, the first two compounds C1 & C2 do not exhibitingmesomorphism. The terminal benzene ring is disubstituted by –OHgroup. One –OH group is ortho to the imine linkage and meta toanother –OH group which does not show linearity in the molecule.The two ends of the molecule possess –OH and either –OCH3 or–OC2H5 groups which do not increase polarity and polarizability ofthe molecule, and hence the first two compounds do not show meso-morphism. As the alkoxy chain length increases, the linearity andpolarizability of the molecules increase which increases the cohesiveforces, resulting in showing nematic mesomorphism.



The transition temperature ranges Dt (S�N) and Dt (N� I) arebetween 9–18�Cand 13–30�C respectively. No odd-even effect is observedfor N-I transition temperature in series-A. The smectic mesophaseappears for the n-decyloxy to n-hexadecyloxy derivatives. The transitiontemperature are recorded in Table 1. The plot of transition temperatureagainst the number of carbon atoms in the alkoxy chain (Figure 1)shows a smooth falling tendency for the nematic-isotropic transitiontemperature throughout the series. Series-A also exhibits a fallingtendency of the SC-N transition temperature for higher homologous.

By incorporating on electron donating group (–CH3) on the iminelinkage in series-B, leading to a disruption of SC phase and it’s onlyappearance of the nematic phase. The transition temperature arerecorded in Table 2. All phase transition temperatures for the methylsubstituted compounds are significantly lower compared with thenon-substituted series-A. The plot of transition temperature againstthe no. of carbon atoms in the alkoxy chain shown in Figure 2. Thiseffect is well known and can be explained in terms of steric influenceof the methyl group on molecular packing [46].

Comparison of series-A and B indicates that the smectic phase wasdisappeared in series-B, which was observed in series-A. The methyllateral group on the imine linkage decreases the SC phase stabilityand this also affects the stability of nematic phase, which results ina reduced N–I range compared to series-B [47].

The increase in the SC-N transition temperature with increasingthe alkoxy chain length in compounds of series-A (n¼ 10, 12, . . . .etc.), can be explained by the increasing polarisability of the molecule.

FIGURE 1 Mesomorphic behavior as a function of the number of carbonatoms (n) in the terminal alkoxy chain for series-A.



This will increase the cohesive forces, acting between the sides andplane of the molecules, increasing the tendency for forming the smec-tic layers. An increase in the nematic and smectic mesophase range isobserved with increasing number of methylene units in the terminalgroup [36]. The enthalpies of the n-tetradecyloxy derivatives of eachof the series were measured by DSC. The data is recorded in Table 3.

Both the series are structurally similar, consisting of the threearomatic cores, imine and chalcone linkages, and n-alkoxy as one ofthe terminal group. Molecules of series-A & B differ only in the lateralgroup on imine linkage. Series-A has a simple imine linkage andseries-B has a lateral methyl group on imine central linkage. It shouldbe remembered that the one central group in both the series is chalconylgroup and its effect has been taken to be similar for both series [48,49].

FIGURE 2 Mesomorphic behavior as a function of the number of carbonatoms (n) in the terminal alkoxy chain for series-B.

TABLE 3 Transition Temperature and DSC Data of the Series-A & B

Compound Transition Peak Temp. (Microscopic temp.) �C DH (Jg�1) DS (jg-lk�1)

A12 Cr–S 81 (81) 16.07 0.0453S–N 99 (99) 18.01 0.0484N–I 123 (124) 20.67 0.0520

A14 Cr–S 85 (86) 11.09 0.0308S–N 94 (95) 6.63 0.0180N–I 121 (121) 13.06 0.0331

B12 Cr–N 97.89 (98) 7.90 0.0213N–I 111.34 (112) 10.78 0.0280

B14 Cr–N 98.79 (99) 9.89 0.0265N–I 109.69 (110) 14.56 0.1324



There is a decrease in thermal stabilities of the liquid crystal phasesin the series-B compounds, this may be attributed to the broadeningeffect caused by the lateral methyl group in an otherwise relativelylinear molecule [50]. An increase in the molecular breadth forces thelong axes of the molecules apart, as a result of which the interactionsare decreased and consequently the liquid crystal temperatures arelowered. A change in the degree of conjugation between the alkeneand the carbonyl group in the chalcone linkage will alter the polarisa-bility. This effect of the change in resultant moments is subtle; how-ever, consequentially, the decrease in the polarisability will cause adecrease in the thermal stability of liquid crystal phase in series-B.Variation of –CH3 group in series-B made it possible to observe theeffects of structural changes on mesomorphic behavior in a systemwhich had not been studied previously.

CONCLUSION

Systematic studies on a homologous series of compounds have allowedfor better understanding of the relationship between the molecularstructures and mesomorphic properties. Mesomorphic data obtainedin this work suggests that the smectic mesophases might be thermody-namically disrupted due to the presence of a lateral methyl group onimine central linkage. Strong molecular interactions in the mesophasecould be partially overcome with surrounding by a more or a lesspolarisable terminal group and mesophase stabilities of chalcone link-age is less compare to ester linkage liquid crystals. The present studycompletes our objective of analyzing and establishing the effect ofdifferent structural modifications on mesomorphism.

ACKNOWLEDGMENT

We are thankful to I.I.T. Bombay, CDRI Lucknow and Garda chemicalsLtd., Ankleshwar (Gujarat) to providing an elemental analysis, FT-IR,1H-NMR, 13C NMR Mass, and Thermal studies and Department ofApplied Chemistry, Faculty of Technology and Engineering, M. S.University of Baroda, Vadodara for providing us Optical polarizingmicroscope for mesophase study.

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New Liquid Crystalline Compounds InvolvingEster-Chalcone Linkages Having 1,3,5-TrisubstitutedPyrazole as a Terminal Group

B. T. Thaker, D. M. Patel, and J. B. KanojiyaDepartment of Chemistry, Veer Narmad South Gujarat University,Surat, India

Mesogens with chalcone central linkage are rare. It has been observed that–CO–CH=CH– linkage is less conducive to mesomorphism compared to –CH=N–(azomethine), –COO– (ester), –N=N–(azo) linkages due to the non linearity andangle strain arising from the keto group. But when –CO–CH=CH– linkage ispresent with other central linkages it becomes condusive to mesomorphism. Inthe present investigation two homologous series were synthesized having chalconeas a one of the central linkage. The homologous series have been derived from1,3,5-trisubstituted pyrazole, p-hydroxy acetophenone, and alkoxy acid. Viz.4(40-n-alkoxybenzoloxy) phenyl-propane-3-one(1-phenyl-3-methyl-2-pyrazoline-5-one) [series-I] and 4(40-n-alkoxybenzoloxy) phenyl-propane-3-one(1-phenyl(400-methyl)-3-methyl-2-pyrazoline-5-one) [series-II]. The compounds of the both serieshave been characterized by elemental analyses, FT-IR, 1H-NMR, and Mass spec-trometry methods. Their liquid crystalline properties have been investigated byoptical polarizing microscopy and DSC studies. All the derivatives are meso-morphic in nature. C1 to C5 of both the series exhibit only nematic phase. C6 toC8 in series-I and C7 & C8 in series-II exhibit smectic as well as nematic phase.Whereas C10–C16 in series-I and C8–C16 in series-II showing only smectic phase.

Keywords: chalcone; ester; mesomorphic; nematic phase; smectic phase; trisubstitudepyrazole

We are thankful to I.I.T. Bombay, CDRI Lucknow and Garda chemicals Ltd.,Ankleshwar (Gujarat) to providing an elemental analysis, FT-IR, 1H-NMR, 13C NMR,Mass and Thermal studies and Department of Applied Chemistry, Faculty of Technologyand Engineering, M. S. University of Baroda, Vadodara for providing us Optical polariz-ing microscope for mesophase study.

Address correspondence to B. T. Thaker, Department of Chemistry, Veer NarmadSouth Gujarat University, Udhana-Magdalla Road, Surat, Surat-395007, India. E-mail:[email protected]

Mol. Cryst. Liq. Cryst., Vol. 509, pp. 145=[887]–164=[906], 2009



DOI: 10.1080/15421400903065549

145=[887]

INTRODUCTION

The mesomorphic behaviour of an organic compound is basicallydependent on its molecular architecture in which a slight change in themolecular geometry brings about considerable change in itsmesomorphic properties. A number of mesogenic homologous serieshave been reported that have –COO–, –CH¼N–, –N¼N–, –CH¼CH–,–CH¼CHCOO–, –C¼C–, –C�C– etc., groups as their central linkages.Many mesogenic homologous series contain two central linkages, bothof which may either be ester [1–3] or azomethine groups [4,5] or one ofwhich may be ester and the other azomethine [6,7]. In the literaturethere are few reports of mesogenic compounds having chalconelinkages. However, many years ago Vora et al. [8] have reportedhomologous series of polymer containing chalcone linkage. Soon afterthat Chudghar et al. [9] reported homologous series containingester-chalcone linkages. Recently Yeap et al. [10] have also synthesizedmesomorphic compound containing ester-chalcone linkage. In our pre-vious work we have reported the homologous series containing Schiffbase-chalcone linkage [11].

Chalcone is one of the important chemical compounds and isbeing studied extensively because of its significant application invarious sectors. In the fields of biology and biochemistry, chalconehas been claimed to be one of the compounds that plays a vitalrole in antitumor [12,13], antiinflammatory [14,15] and antima-laria [16] activities. It has also been documented that the chalconepossesses a remarkable nonlinear optical (NLO) property, which isan essential element for optical communications devices. The otherimportance of this compound is its high photosensitivity andthermal stability, which are used in developing various crystal-line electro-optical devices [17–19] and also having fluorescentproperties [9].

However, heterocyclic compounds provide a great synthetic andstructural versatility due to presence of number of potential substitu-tion positions. Furthermore heteroatoms offer the possibility of severalmodes of co-ordination. In particular, pyrazoles derivatives allowstructural design to tune the molecular shape for the appearance ofmesomorphic properties. The mesogenic series with hetero atoms havecreated wide interest in liquid crystal field. The introduction ofheteroatom causes considerable changes in chemical and physicalproperties and influences the type of liquid crystal phase, also phasetransition temperatures and other properties of the mesogens [20].Mesogenic heterocyclic homologous series containing nitrogen, oxygenetc. as heteroatom are reported. Schubert et al. [21] have reported

146=[888] B. T. Thaker et al.

mesogenic pyron derivatives. Demus et al. [22,23] have reportednumber of dioxygen derivatives exhibiting mesomorphic behaviour.Pavluchenko et al. [24] have reported mesomorphic homologous seriescontaining benzoxazole and benzthiazoles hetero atoms. Nash and Gray[25] have also reported some heterocyclic mesogens and have tried toexplain the mesogenic behaviors of these heterocyclic moieties. Indeed,some 3, 5-disubstituted pyrazoles and 4-substituted pyrazoles havedemonstrated their ability to show liquid crystalline behavior [26–31].

In our previous work [32] we have synthesized two homologous seriescontaining ester-chalcone linkages having substituted benzene ring as aterminal group. An attempt have been made to synthesized two newhomologous series containing same central linkages but different term-inal heterocyclic moiety to study the influence of the terminal group onmesomorphic and thermal stability of these compounds.

In this article, we report series of some newly analogues derived from1,3,5-trisubstituted pyrazole as terminal groups and synthesized twohomologues series, Viz. 4(40-n-alkoxybenzoloxy) phenyl-propane-3-one(1-phenyl-3-methyl-2-pyrazoline-5-one) [series-I] and 4(40-n-alkoxyben-zoloxy) phenyl-propane-3-one(1-phenyl(400-methyl)-3-methyl-2-pyrazoline-5-one) [series-II] in which the ester and chalcone are essential centrallinkages.

All the compound of both the series has been characterized by ele-mental analysis, FT-IR, 1H-NMR, 13C-NMR, and Mass spectrometry.The liquid crystalline behaviors of these compounds were observedby DSC study and polarizing microscope.

EXPERIMENTAL

Reagents and Technique

For the synthesis of compounds of the homologous series, followingmaterials were used. 4-Hydroxy benzoic acid, 4-hydroxy acetophe-none, alkyl bromide (Lancaster, England). The solvents wereused after purification using the standard methods described in theliterature [33].

Elemental analyses (C, H, N) were performed at CDRI(CentralDrugs Research Institute), Lucknow. Infrared spectra were recordedwith a Perkin-Elmer2000 FT-IR spectrophotometer in the frequencyrange 4000-400 cm�1 with samples embedded in KBr discs. 1H-NMRspectra of the compound were recorded with JEOL-GSX-400 usingCDCl3 as a solvent and TMS as an internal reference and Mass spectra(EI) of the compounds at SAIF(Sofisticated Analytical InstrumentFacilities), IIT Madras, Chennai. 13C NMR spectra of the compound

New Liquid Crystalline Compounds 147/[889]

were recorded with BRUKER AVANCE II 400 NMR Spectrometer,SAIF, Chandigarh. Thin-layer chromatography analyses wereperformed by using aluminium-backed silica-gel plates (Merck 60F524) and examined under short-wave UV light.

The phase-transition temperatures were measured using a shi-madzu DSC-50 at heating and cooling rates of 5�C min�1, respectively.The DSC data are shown in Table 3. The optical microscopy studieswere carried out with a ‘‘Leitz Loborlux 12’’ POL(Wetzler,Gerrmany)polarizing microscope equipped with a Mettler FP52 hot stage. Thetextures of the compounds were observed using polarized light withcrossed polarizers with sample in thin film sandwiched between aglass slide and coverslip.

Synthesis

Synthesis of 4-n-alkoxy benzoic acid4-n-Alkoxy benzoic acid were prepared as reported by Dave and

Vora method [34]. The m.p. of these compounds were compared withthe reported one and they are almost similar to reported values.

Synthesis of 4-n-alkoxy benzoyl chlorides4-n-alkoxy benzoyl chlorides were prepared by reported method

[34]. The m.p. of these compounds were compared with the reportedone and they are almost similar to reported values.

Synthesis of 4-(40-n-alkoxybenzoloxy)-acetophenone4-hydroxy aetophenone (0.01mole, 1.22 gm) was dissolved in dry

pyridine (10.0ml) and was added drop wise with occasionally stirringinto ice-cold 4-n-alkoxy benzoyl chloride (0.01mole, 2.41 gm) in a roundbottom flask. Then mixture was refluxed on hot water bath for twohours and was allowed standing overnight. The mixture was acidified

148/[890] B. T. Thaker et al.

with cold 1:1 dilute hydrochloric acid to precipitate the product. Thesolid obtained was filtered, washed successive with saturatedNaHCO3 solution, dilute NaOH solution and two to three times withwater thus crude solid was obtained which has number of timespurified by hot water until the constant transition temperature wereobtained. The transition temperatures are in good accordance withthe literature [35].

Synthesis of Compound Series-I and Series-II [36–39]

Series-I Synthesis of 4(40-n-alkoxybenzoloxy) phenyl-propen-3-one(1-phenyl-3-methyl-2-pyrazolin-5-one)

Take 4-(40-n-alkoxybenzoloxy) acetophenone (0.005mole 1.6 gm)and 1-phenyl-3-methyl-4-formyl-2-pyrazolin-5-one in flat bottom flaskcontaining 25ml absolute alcohol and stirred it on a stirrer, till boththe reactants were dissolved completely. Then cold 10% aq. KOH solu-tion added dropwise and continue the stirring further for four hours.The reaction mixture was allowed to stand for overnight. The mixturewas neutralized by cold dilute HCl up to pH (6.5–7.0). Then solidobtained was filtered, washed with water and dried. The crude pro-ducts were purified by using column chromatography and crystallizedfrom methanol. The melting points and transition temperatures forthis homologous series are given in Table 1.


Data

Series-I Compound-C12

Yield 70%. M.P.113�C. Found: C, 75.05; H, 7.21; N, 4.63 Calc. forC38H44O5N2; C, 75.01; H, 7.24; N, 4.61%. EI-MS m=z (rel.int %): 607(M-1)þ IR (KBr): Vmax=cm�1 2850–2935 cm�1 (C–H aliphatic),1776 cm�1 (C¼O of ester), 1677 cm�1 (C¼O of chalcone), 1606 cm�1

(C¼C of vinyl gr. of chalcone), 1582 cm�1 (C¼C of aromatic), 1255 cm�1

(C–O–C), 1056 cm�1 (C–O), 1H NMR (CDCl3): d 0.89–0.93 ppm (CH3),1.28–1.82 ppm (CH2), 2.38 ppm (–CH3 of the pyrazolone ring) 4.01–4.05 ppm (OCH2), 6.91–7.26 ppm (Ar–H), a¼ 8.06 ppm, b¼ 6.94 ppm(R–CO–CaH¼CbH–R).

Series-II Synthesis of 4(40-n-alkoxybenzoloxy) phenyl-propen-3-one(1-phenyl(400-methyl)-3-methyl-2-pyrazolin-5-one)

Take 4-(40-n-alkoxybenzoloxy) acetophenone (0.005mole 1.6 gm)and 1-phenyl(400-methyl)-3-methyl-4-formyl-2-pyrazolin-5-one in flatbottom flask containing 25ml absolute alcohol and stirred it on astirrer, till both the reactants were dissolved completely. Then cold10% aq. KOH solution added dropwise and continues the stirringfurther for four hours. The reaction mixture was allowed to stand forovernight. The mixture was neutralized by cold dilute HCl up to pH(6.5–7.0). Then solid obtained was filtered, washed with water anddried. The crude products were purified by using column chromatogra-phy and crystallized from methanol. The melting points and transitiontemperatures for this homologous series are given in Table 2.

TABLE 1 Transition Temperature of Series-I

Transition temperature �C

Compounds R¼n alkoxy Sm N I

C1 Methyl – 126 223C2 Ethyl – 116 220C3 Propyl – 131 231C4 Butyl – 110 212C5 Pentyl – 146 197C6 Hexyl 63 104 192C7 Heptyl 87 101 174C8 Octyl 78 93 140C10 Decyl 89 – 125C12 Dodecyl 55 – 113C14 Tetradecyl 64 – 107C16 Hexadecyl 51 – 121

Note: Sm: Smectic; N: Nematic; I: Isotropic.


Data

Series-II Compound-C12

Yield 86%. M.P.107�C. Found: C, 75.21; H, 7.35; N, 4.47 Calc. forC39H46O5N2; C, 75.24; H, 7.39; N, 4.50%. EI-MS m=z (rel.int %): 621(M-1)þ IR (KBr): Vmax=cm�1 2850–2953 cm�1(C-H aliphatic),1708 cm�1 (C¼O of ester), 1642 cm�1 (C¼O of chalcone), 1607 cm�1

(C¼C of vinyl gr. of chalcone), 1580 cm�1 (C¼C of aromatic),1260 cm�1 (C–O–C), 1063 cm�1 (C–O), 1H NMR (CDCl3): d 1.01–1.06 ppm (CH3), 1.41–1.87 ppm (CH2), 2.45 ppm (CH3 of the pyrazolone

TABLE 2 Transition Temperature of Series-II

Transition temperature �C

Compounds R¼n alkoxy Sm N I

C1 Methyl – 124 224C2 Ethyl – 119 218C3 Propyl – 128 216C4 Butyl – 115 204C5 Pentyl – 136 207C6 Hexyl 73 143 195C7 Heptyl 81 105 168C8 Octyl 69 – 147C10 Decyl 86 – 113C12 Dodecyl 69 – 107C14 Tetradecyl 57 – 105C16 Hexadecyl 59 – 115

Note: Sm: Smectic; N: Nematic; I: Isotropic.


ring) 4.01–4.16 ppm (OCH2), 7.01–7.39 ppm (Ar–H), a¼ 7.04 ppm,b¼ 8.19 ppm (R–CO–CaH¼CbH–R).

Results and Discussion

In our recent work, 12 homologous from each of the two series,4(40-n-alkoxybenzoloxy) phenyl-propane-3-one(1-phenyl-3-methyl-2-pyrazoline-5-one) [series-I] and 4(40-n-alkoxybenzoloxy) phenyl-propane-3-one(1-phenyl(400-methyl)-3-methyl-2-pyrazoline-5-one) [series-II] havebeen synthesized having ester-chalcone central linkages containingterminal heterocyclic ring and their mesomorphic properties havestudied. The mesomorphic properties of all the synthesized compoundshave been characterized by differential scanning calorimetry(DSC) and polarizing optical microscope (PMO) attached with Mettlerhot stage.

The transition temperature of both series are given in Tables 1 and 2.In series-I the compounds (C1–C5) exhibit enatiotropic nematicmesophase and (C6–C8) exhibit enantiotropic smectic and nematicmesophases. While compound C10, C12, C14, and C16 exhibit only

FIGURE 1 (a) Texture of SmC phase of C7 of series-I at 87�C, (b) Schlierentexture of the Nematic phase of C3 of series-I at 130�C on cooling, (c)Schlieren texture of the Nematic phase of C5 of series-II at 136�C on cooling,and (d) Schlieren texture of the Nematic phase of C6 of series-II at 143�C.


enantiotropic smectic mesophase. In second homologous series-II, thecompound (C1 to C5) exhibit enatiotropic nematic mesophase C6 andC7 exhibit enantiotropic smectic and nematic phases. While compoundC8, C10, C12, C14, and C16 exhibit only enantiotropic smectic mesophase.The texture of the series I & II are given in Figure 1. On cooling theisotropic liquid, small droplets appear, which coalesce to a classicalschlieren (threaded) texture characteristic of the nematic phase. It isconsistent with the assignment of each mesophase type using the clas-sification systems reported by Gray and Goodby [40]. The plot of transi-tion temperature against the number of carbon atom in the alkoxychain are shown in Figures 2 and 3. In which Cr-M transition tempera-ture, a regular alternation of the temperature occurring betweenhomologous containing odd and even number carbon atoms in alkylchain. In the present work N-I transition temperature for the series Iand II of compounds do not behave in this way. This is because of theterminal 1,3,5-trisubstituted pyrazole ring plays some role either in

FIGURE 2 Mesomorphic behaviour as a function of the number of carbonatoms (n) in the terminal alkoxy chain for series-I.

FIGURE 3 Mesomorphic behaviour as a function of the number of carbonatoms (n) in the terminal alkoxy chain for series-II.


packing of the molecule in crystal lattice or due to the stereochemistryof the molecule is not symmetrically linear.

The series also exhibits smectic properties, the Sm-N transition tem-perature fond to fall a number of carbon increase in alkyl chain. Inseries-I and II only two or three compound exhibit smectic and nematicbothmesophases. Therefore, we could not give the trend for Sm-N curve.

In these both series Sm-I transition usually begins at about thedecyl, dodecyl, hexadecyl and octadecyl ethers together with the N-Itransition temperatures for the lower homologous contains an evennumber of carbon atoms in alkyl chain, constitute one smoothly fallingmesorphic – isotropic transition temperatures. From the plot of transi-tion temperatures against the number of carbon atoms (Figures 2and 3), it can be noticed that Cr-M transition temperatures decreasedwith increase in the length of terminal alkoxy chain. This is in agree-ment with the observation reported for such homologous series [41].

Cr-M transition shows some what large transition temperaturechanges for n¼ 5 to n¼ 6. This could be explained as, a common pat-tern of behavior is that the lower homologues are nematic, the middlemembers exhibit a smectic mesophase followed by nematic mesophaseand the long chain members, i.e., C10, C12, C14, C16 are purely smecticas observed in Tables 2 and 3.

The Cr-M for n¼ 5 in series-I is observed at 146�C. The increment ofeach methylene unit brings about regular changes in the transitiontemperature of the series as reported by Gray [41]. For shorter chaincompounds C5, the separation of the aromatic nuclei is at a minimumand terminal cohesive forces are strongest and predominant. Asnematic phase is highly disordered which required high energy toovercome such forces, therefore, C5 compound exhibit nematic phaseat high temperature.

TABLE 3 DSC Data for Series-I and II Compounds

Series Compound TransitionPeak temp.

(Microscopic temp.) �C DH Jg�1 DS Jg�1K�1

I C6 Cr-Sm 61.66 (63) 11.31 0.033Sm-N 101.91 (104) 29.87 0.079N-I 190.28 (192) 35.31 0.076

C12 Cr-Sm 53.72 (55) 148.70 0.455Sm-I 111.60 (113) 0.833 0.002

II C6 Cr-Sm 72.80 (73) 48.51 0.140Sm-N 146.73 (143) 2.767 0.006N-I 197.01 (195) 2.368 0.005

C10 Cr-Sm 87.47 (86) 80.34 0.222Sm-I 112.35 (113) 7.618 0.019


However, in the middle members e.g., n¼ 6 (C6), the smectic proper-ties appear, because the alkyl chain is increasing the lateral cohesiveforces and the molecules may maintain themselves in the layerarrangement. The smectic phase is highly ordered–one compared tonematic mesophase, which requires lower energy to observe smecticbehavior. Therefore, smectic mesophase observed in compound n¼ 6(C6), at 63�C. Therefore, such large differences in Cr-N and Cr-Smtemperature have been observed in present two series.

DSC is a valuable method for the detection of phase transition. Ityields quantitative results; therefore we may draw conclusions con-cerning the nature of the phases that occur during the transition. Inthe present study, enthalpies of two derivatives of series I and seriesII were measured by DSC. Data are recorded in Table 3. Which helpsthe further confirm the mesophase type. Table 3 shows the phase tran-sition temperatures, associated enthalpy (DH) and molar entropy (DS)for compound of series-I (C6, C12) and series-II (C6, C10). Enthalpyvalues of the various transitions agree well with the existing relatedliterature values [42]. The DSC curves of representative compoundsare shown in Figures 4 to 7. Microscopic transition temperature valuesare almost similar to DSC data.

Table 4 shows the comparison of Sm-N and N-I transition tempera-ture of compound n¼ 6, series I, and structurally related compoundsn¼ 6, series II. The Sm-N mesophase range of compound n¼ 6(series-I) is lower by 29�C, respectively, when compared with

FIGURE 4 DSC Curves of the compound C6 of series-I.


compound n¼ 6 (series-II). The N-I transition temperature ofcompound, series-I, is higher by 36�C, respectively, when comparedwith compound 6 (series-II).

Although the mesomorphic phase stability is greater in series-IIcompounds than that of series I, this is because of presence of –CH3

FIGURE 6 DSC Curves of the compound C6 of series-II.



group at pera- position on 1-phyenyl ring (which is attached to pyrazo-line ring) produce a steric hindrance. The order of group efficiencyderived by Dave and Dewar [43,44] based on the magnitude of thegroups slope value. The decreasing order of the group efficiency is inthe decreasing order of group polarizability. The increase in N-Itransition temperature with increasing alkoxy chain in compoundsof series-I can be explained by increasing overall polarizability of themolecule.

The FT-IR spectra of the mesomorphic ester-chalcone central link-age exhibit different vibration modes corresponding to the stretchingand bending mode of different functional groups present in the mole-cule. The FT-IR spectra of representative compound of homologousseries-I and Series-II are shown in Figures 8 and 9. IR spectra ofester-chalcone central linkage show strong or medium bands around1706 cm�1and 1620 cm�1, 1686 cm�1 attributed to t(C¼O) of ester

TABLE 4 Different Transition Temperatures and Range of MesophasesObserved in Series I and II

Series Compound SmC N ISm-N mesophase

range (�C)N-I mesophase

range (�C)

I n¼6 63 104 192 41 88II n¼6 73 143 195 70 52



group chalcone and pyrazoline ring respecting strong band appearedat around 1606 cm�1 is attributed to t(C¼C) vinyl group of chalcone.

In the present case all the spectra of mesomorphic compound showtwo sharp bands at around 2919 cm�1 and 2849 cm�1 are due to

FIGURE 8 IR Spectra of C12 of series-I.

FIGURE 9 IR Spectra of C12 of series-II.


aliphatic t(C–H) symmetrical and asymmetrical stretching vibrationof –CH3 and –CH2 groups of n-alkyl chain. Another two bands are alsoobserved 1465 cm�1 and 1391 cm�1 are due to aliphatic –CH3 and–CH2 deformation vibrations. The sharp and medium band observedat around 1578–1513 cm�1 region due to aromatic n(C¼C) stretching.The weak band observed at around 631–763 cm�1 is due to rockingvibration of –CH2 groups of alkyl chain. Out of plane deformation ofring hydrogen band is establishing the position of substituent onaromatic nuclei is well known. Although correlation can often beuseful in analysis of spectra of the mesomorphic compounds but theirperformance is variable. Infrared spectra of the present class of thecompounds often show long wavelength regions crowded with a largenumber of bands of similar intensities, and it is difficult to know whichbands belongs to this class. Further more compounds often containmore than one aromatic nucleus and are often substituted withstrongly electronegative groups, such as alkoxy, which tend to upsetsthe correlations.

1H-NMR spectra of representative compounds are shown inFigures 10 and 11 and the chemical shifts (d, ppm) are noted. The1H NMR spectra of ester-chalcone central linkage type of compoundsexhibited six signals present in spectra, indicating six type of differentenviroment proton present in these type of compound. The first signal

FIGURE 10 1H NMR spectra of C6 of series-I.


observed between d 0.91–0.95 ppm, which is sharp and triplet corre-sponding terminal methyl proton of straight chain of n- alkoxy group.The multiplate signals observed between d 1.28–1.82 ppm are due tomethylene (–CH2) proton of straight alkyl chain of n-alkoxy groupand d 2.38 ppm –CH3 proton of the pyrazolone ring. The triplet signalobserved between d 4.09–4.15 ppm is attributed to –OCH2 proton ofn-alkoxy group like to phenyl ring. The multiplate signal observedbetween d 6.88–8.13 ppm are corresponding to substituted phenylrings. The ‘a’ and ‘b’ types of proton in the central linkage[COCaH¼CbH] sharp singlet at d 8.14 ppm and d 6.91 ppmrespectively.

The mass spectra of series-I and series-II are shown in Figures 12and 13. The m=z ratios obtained from the spectra of each samples arematched with route for mass fragmentation and abundances in themass spectrum. The molecular wt. obtained from the mass spectraare almost equivalent to the calculated value of the compounds.

In series-I and II there are two linkages, one is ester and other ischalcone. Mesogens with different central linkages are known [41].However most of these central linkages have an even number of linkinggroups. The chalcone linkage has an odd number of atoms. Moreover,very few lowmolecular mesogens with the chalcone linkage are known.–CH¼CH–CO– linkage is less conducive to mesomorphism comparedto –CH¼N–, –COO– linkages due to the non-linearity and angle strainarising from the keto group [9]. Nguyen et al. [45] have reported that a

FIGURE 11 1H NMR spectra of C7 of series-II.


ketonic group linking two phenyl rings (benzophonene derivatives) isnon-conducive to mesomorphism due to the angular shape of suchmolecules resulting from the angle of keto group. But when moleculehaving two linkage in which one is chalcone and other is –CH¼N or–COO– it becomes conducive to the mesomorphism [9,11,32].

FIGURE 12 Mass Spectra of C12 of Series-I.

FIGURE 13 Mass Spectra of C12 of Series-II.


INFLUENCE OF THE TERMINAL GROUP

In this paper we have just presented compounds with the generalstructure

Where R0 ¼ 1,3,5-trisubstituted pyrazole ring.In our previous paper [32] we presented two homologous series A &

B as following structure,

In order to have better understanding we have synthesized the pre-sent two new homologous series containing substituted pyrazolonering as a terminal group.

On comparision of the present pyrazole ring with previous benzenering we have observed that the transition temperature range of thepresent homologous series containing pyrazole as a terminal groupbecomes higher than the previous one and also mesophase stabilityof these series becomes higher than simple benzene analogous dueto the introduction of pyrazole ring as a terminal group. Because pyr-azole ring having highly delocalized system as a result of it shows veryhigh thermal stability compare to benzene analogous.

CONCLUSION

In this article we have presented the synthesis and characterization ofmesogenic two homologous series of 1,3,5-trisustituted pyrazole hav-ing ester-chalcone linkage. All the compounds of the series exhibitenantiotropic mesomorphism. The mesophase range of present series-Iis higher than those of structurally related compounds of series-II thathas been attributed to the high polarizability of the molecule becausethe bulky group reduced the polarity of molecule and decreases themesophase stability of compound. Both series show nematic mesophase


and higher homologues show smectic mesophase. The above studies ona limited number of heterocyclic mesogens strongly suggests thatdominant effect of the hetero atom is to produce change in conjugativeinteractions within the molecule which effect factors such as polariz-ability and dipolarity. Intermolecular effects produced by the lone pairof electrons are apparently, in certain case.

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