mesomorphic properties of liquid crystalline compounds with ......liquid crystalline property (lc)...

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WSN 54 (2016) 202-216 EISSN 2392-2192

Mesomorphic properties of liquid crystalline

compounds with central linkage chalconyl ester and laterally substituted bromo group

Priya K. Patela, R. B. Patelb and R. R. Shahc

Chemistry Department, K. K. Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad, Gujarat, India

a-cE-mail address: [email protected] , [email protected] , [email protected]

ABSTRACT

To investigate the mesomorphic property of the substitution on mesomorphism, the new series

of liquid crystals having the following structures have been synthesized. Presently synthesized series

contain thirteen homologue (C1 to C8, C10, C12, C14, C16, C18) in which eleven homologue (C3 to C18)

shows both smectic as well as nematic phase during heating and cooling condition in enantiotropic

manner, while C2 homologue shows only nematic phase. All the homologues were characterised by

elemental analysis and spectroscopic techniques like 1H-NMR and IR analysis. Their liquid crystalline

behaviours were measured by polarised optical microscope (POM) and Differencial Scanning

Calorimetry (DSC).

Keywords: Smectic, Nematic; Liquid crystalline; Mesomorphic; Enantiotropic

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World Scientific News 54 (2016) 202-216

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Graphical Structure

1. INTRODUCTION

From the last many years, the field of liquid crystal, is becoming very innovative and

more useful to mankind. Liquid Crystal is a very delicate, beautiful and unique state in

between Liquid and Crystal. Liquid Crystals have different molecular geometry, different

phases and phase transition [1-2]. Liquid crystalline property (LC) of a substance is an unique

property, which flows on the surface like liquid and possesses optical properties like crystals.

Therefore such substances of thermotropic or lyotropic varieties are neither fully crystalline

nor fully liquidous [3-8]. A little change in geometry of molecule brings big difference in

molecule’s mesomorphic property [9-12]. Now a days liquid crystalline compound are used in

many fields. Many compounds of liquid crystal are based on chalcones, are products of

simple or substituted aromatic aldehydes with simple or substituted acetophenone in presence

of base and alcohol. A number of chalcone having reported to exhibit a broad spectrum of

anti-bacterial, antifungal, antiulcer, antimalarial, antitumor, anticancer, anti-inflammatory,

antitubercular. The presence of α, β-unsaturated functional group in chalcone (-CH=CH-CO-)

is responsible for anti-microbial activity, which can be altered depending upon the type of

substituent present on the aromatic rings [13-17]. Chalcone possess biological activity like

antitubercular, antifungal, antiviral, anticancer, antibacterial, etc. [18-20]. In this present

article, we have demonstrated the rod like structure of present newly synthesized homologous

series makes them more suitable to establish or invent the liquid crystalline characteristics and

to establish the effect or relation between chalconyl ester and lateral halogenated (-Br) group

and three phenyl ring through two central linkage –CH=CH-COO- and –CH=CH-CO- which

has terminal end groups –OR at left side chain. Doshi et al. reported benzoates and cinnamate

based linking group in-built with chalconyl linkage group to exhibit LCs property [21-26].

Recently, our research group studied chalconyl ester based linear and nonlinear homologues

series and observed the effect of flexibility by increasing alkyl spacer at terminal group and

effect of lateral side group on mesomorphism [27-33]. Consequently, an attempt to made

liquid crystalline compound that display LCs property with good thermal stability and


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commencement of mesophase from lower homologue to higher homologue respectively. Rod

like structure of series indicates that rigidity and flexibility of molecule vary from homologue

to homologue with changing alkyl group at left alkoxy terminal.

2. EXPERIMENTAL

2. 1. Reaction Scheme

Scheme 1. Synthetic route of the series


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2. 2. Synthesis

4-hydroxy benzaldehyde was alkylated by using suitable alkyl halides (R-X) to convert

it into 4-n-alkoxy benzaldehyde which reacts with malonic acid in presence of pyridine and

piperidine as catalyst to form trans 4-n-alkoxy Cinnamic acid [A] [34].

4-hydroxy benzaldehyde and o-bromo acetophenone reacts with each other to produce

α-1-bromo benzoyl-β-4'-hydroxy phenyl ethylene [B] [35]. Then 4-n-alkoxy cinnamic acid

and α-1-bromo benzoyl-β-4'-hydroxy phenyl ethylene were condensed in 1,3-di-cyclohexyl

carbodiimide (DCC), 4-dimethyl amino pyridine ( DMAP ) and dichloro methane ( DCM ) to

produce α-4-[4’-n-alkoxy cinnamoyloxy]phenyl-β-1”-bromo benzoyl ethylenes [C] [36].

The final product were filtered, washed with water and NaHCO3 solution then dried and

purified till constant transition temperatures obtained using an optical polarising microscope

equipped with a heating stage. Alkyl halides, EtOH, KOH, 4-Hydroxy 3-Bromo

acetophenone, 4-Hydroxy benzaldehyde etc., required for synthesis were used as received

except solvents which were dried and distilled prior to use. The synthetic route to the series is

mentioned below as Scheme 1.

2. 3. Characterization

The homologous of present series were characterised by IR and 1H-NMR. IR analysis

were recorded on a Perkin-Elmer spectrum GX and 1H-NMR were recorded on a Bruker

instrument using CDCl3 as a solvent. Elemental analysis was performed by Perkin Elmer PE

2400 CHN analyser (Table 1). Texture image of some homologues for smectic and nematic

mesophases were determined by miscibility method (Table 2). The textures of smectic and

nematic mesophase were determined by a miscibility method. The mesomorphic properties

and transition temperature (Table 3) were determined by optical polarising microscope.

The compound is sandwiched between glass slide and cover slip and the heating and

cooling rate is 2 C. Decomposition temperatures were determined using of Shimadzu DSC

60 differential Scanning Calorimeter with a heating rate of 5 to 10.0 °C min-1

.

2. 4. Analytical data

Table 1. Elemental Analysis for (1) Ethyloxy (2) Pentyloxy (3) Hexyloxy (4) Heptyloxy

(5) Octyloxy (6) Decyloxy.

Sr.

No

Molecular

formula

Elements % found Elements % Calculated

C H O Br C H O Br

1 C26H21O4Br 64.24 4.00 12.11 15.65 65.40 4.40 13.41 16.77

2 C29H28O4Br 64.87 4.74 11.56 15.06 66.92 5.38 12.30 15.38

3 C30H30O4Br 65.62 5.21 11.04 14.21 67.41 5.61 11.98 14.98

4 C31H32O4Br 65.86 5.34 10.89 13.18 67.88 5.83 11.67 14.59


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Table 2. Texture of Nematic Phase of C7, C12, C14, C18 by miscibility method.

Sr. No. Homologue Texture

1 C7 Threaded type

2 C12 Nematic droplets

3 C14 Schlieren

4 C18 Rod like broken fan

IR Spectra in cm-1

for propyloxy and tetradecyloxy derivatives:

Propyloxy derivative (C3): 655.80 (-CH-Br stretching), 823.60 (polymethylene (-CH2)n of

–OC3H7), 891.11 (-C-H def. of o, p-disubstituted), 977.91 (-C-H def. of hydrocarbon),

1068.56, 1165.00 (-C-O stretching), 1381.03 (-C-O stretching in (-CH2)n chain), 1508.33

(-C=C- stretching), 1571.99 (-C-H def. in –CH2), 1645.28 ( C=O group), 2856.79, 2929.87

(-CH stretching in –CH3). IR data confirms the molecular structure.

Tetradecyloxy derivative(C14): 648.08 (-CH-Br stretching), 815.89 (polymethylene (-CH2)n

of –OC14H29), 891.11 (-C-H def. of o, p-disubstituted), 979.84 (-C-H def. of hydrocarbon),

1006.84, 1070.49, 1170.79 (-C-O stretching), 1247.94 (-C-O stretching in (-CH2)n chain),

1508.33 (-C=C- stretching), 1573.91 (-C-H def. in –CH2), 1625.99 ( C=O group), 2850.79,

2920.23 (-CH stretching in –CH3). IR data confirms the molecular structure.

1H-NMR:

1H

NMR spectra in CDCl3 in δ ppm for butyloxy and hexadecyloxy Derivative:

Butyloxy (C4): 0.83 (t, 6H, -CH3 of –C4H9), 1.0 (-CH3 of –C4H9), 1.2-1.3 (m, n-

polymethylene groups of –C4H9), 1.32 (q, m, n-polymethylene groups of –C4H9), 1.54 (m, n-

polymethylene groups of –OC4H9), 2.22-2.29 (-CH=CH-), 3-3.4 (s, -O-CH2-CH2- of –

OC4H9). NMR confirms the structure.

Hexadecyloxy (C16): 0.82-0.94 (t, 6H, CH3 of –C16H33), 1.0 (-CH3 of –C16H33), 1.2-1.3 (m, n-

polymethylene groups of –C16H33), 1.56 (m, n-polymethylene groups of –OC16H33), 1.63 (m,

n-polymethylene groups of –OC16H33), 2.70-2.78 (-CH=CH-), 3.09 (s, -O-CH2-CH2- of –

OC16H33), 5.06 (-OCH2 of –OC16H33), 5.54 (=CH-Ar). NMR confirms the structure. C4 and

C16 are as shown below.

5 C32H34O4Br 66.10 5.88 10.60 13.02 68.32 6.04 11.38 14.23

6 C34H38O4Br 67.92 6.12 9.78 12.69 69.15 6.44 10.84 13.55


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C4:


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C16 :


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Table 3. Transition Temperature in °C.

Sr.no R= n-alkyl group

Transition temperatures in 0C

Cr Smectic Nematic Isotropic

1 C1 · - - - - 176.0 ·

2 C2 · - - 142.0 - 172.0 ·

3 C3 · 112.0 - 136.0 - 160.0 ·

4 C4 · 110.0 - 140.0 - 162.0 ·

5 C5 · 98.0 - 158.0 · 171.0 ·

6 C6 · 92.0 · 151.0 · 170.0 ·

7 C7 · 116.0 · 142.0 · 154.0 ·

8 C8 · 96.0 · 132.0 · 148.0 ·

9 C10 · 112.0 · 160.0 · 177.0 ·

10 C12 · 110.0 · 162.0 · 176.0 ·

11 C14 · 82.0 · 144.0 · 168.0 ·

12 C16 · 77.0 · 114.0 · 142.0 ·

13 C18 · 84.0 · 112.0 · 140.0 ·

3. RESULT AND DISCUSSION

3. 1. POM analysis

α-4-[4'-n-alkoxy cinnamoyloxy] phenyl-β-1-bromo benzoyl ethylenes was produced by

condensation of α-1-bromo benzoyl-β-4'-hydroxy phenyl ethylene and trans 4-n-alkoxy

cinnamic acid which both possess non mesomorphic properties. Present series contain thirteen

homologous (C1 to C8, C10, C12, C14, C16, C18) in which C1 is non-mesomorphs while C2 to C18

possess mesomorphic properties in enantiotropic manner. Transition temperature (Table 3)

determined by polarizing optical microscope and the phase diagram (Figure 1) is plotted

against the transition temperature versus the number of carbon atoms present in n-alkyl chain

of the left n-alkoxy terminal end group (-OR). In present series, we have discussed the effect

of mesomorphic property by geometrical shape, rigidity and flexibility due to bromo group (-

ortho) at right side and varying alkoxy side chain at left side of series. Cr-I/M curves

increased and then decreased as series is increased, -CH2 (methylene group) linked in it. Odd-

even effect is present in Cr/M curves Cr-I/M curves ascended at C7 homologue because of

presence of odd-even parity and then abnormally descended at C8 and then ascended at C10 to


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C12 homologue and again descended at C14 to C18 homologue. The mesomorphic properties

are varying from homologue to homologue because of addition of –CH2 group at left side of

n-alkoxy side chain. As we increased the number of methylene group at left n-alkoxy side

chain moving of sliding arrangement causes smectic (broken fan type / needle type) from C3

to C18 homologues. Sm-N transition curve falling from C5 to C8 homologue and then rising up

to C10 and again falling at C12 upto C18 respectively.

Figure 1. Phase diagram of Series-1.


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The presence of liquid crystalline property from C2 to C18 homologue, indicates the

presence of lamellar packing in molecule while applying heating. As a result of favourable

molecular rigidity and flexibility which facilitated statistically parallel orientational manner of

molecules in floating condition on the surface causes nematic phases. N-I/M transition curve

descended at C7 to C8 and ascended at C10 while again descended at C12 up to C18 homologue

respectively due to anisotropic forces of intermolecular attractions due to fittest magnitudes of

molecular polarity and polarizability and permanent dipole moment etc.

Absence of LCs property due to short alkyl spacer of molecule causes high crystallising

tendency and directly converted to isotropic phase without passing any liquid crystalline

mesophase and again on cooling condition its directly converting isotropic to solid crystal.

Some mesomorphic characteristics were obtained from thermotropic analysis of present

series-1 which compared with series-X [26] as shown below (Figure 2). In Sereis-1, three

phenyl rings bonded through –CH=CH-COO- and -CH=CH-C=O- group while in series-X,

three phenyl rings were bonded via –COO- and –CO-CH=CH- group. They also differ from

each other to right end terminal group. Therefore, degree and characteristics of

mesomorphism depends upon changing group from series to series.

Figure 2. Structurally similar Series.

The variation in mesomorphic property in to the both Series-1 and X is due to molecular

length, length and breadth ratio, permanent dipole moment, suitable or unsuitable magnitudes

of intermolecular anisotropic forces or dispersion forces of cohesion and closeness, molecular

flexibility and rigidity etc.

Table 4. Thermal stabilities in °C.

Series→ Series-1 Series-X

Smectic-Isotropic or

smectic- Nematic

Commencement of

Smectic phase

141.0

( C3 – C18 )

C3

-


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Nematic-Isotropic

Commencement of

Nematic phase

161.6

( C2 – C18 )

C2

110.25

( C6 – C18 )

C6

Total mesophase length

Range in °C

ti to tj

Ci to Cj

30.0 – 86.0

C2 C14

10.0 – 39.0

C18 C10

Table 4 indicates, Series-1 shows smectic phase in presence of nematic phase while

Series-X shows only nematic mesophase. Smectic phase commences from C3 homologue in

Series-1. Nematic mesophase commences from C2 homologue in Series-1 while from C6

homologue in series-X. Total mesophase lengths vary from minimum to maximum between

C2 to C14 for all the series under comparison. Difference in mesogenic and physical properties

of both series is attributed to changing flexibility due to sequentially added –CH2 group at n-

alkyl chain of left side terminal group. Molecular rigidity is also differing from homologue to

homologue and series to series due to different two central bridges. Thermal stability of

Series-1 is higher than series-X due to presence of halogenated bromo group which increase

the polarity and polarizability of molecule to induce mesophase earlier from lower homologue

to higher homologue. While in series-X, the molecule become more flexible due to presence

of terminal dodecyloxy group as results mesophase appeared at lower transition temperature.

3. 2. DSC analysis

Figure 3. DSC thermogram of Compound C10


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DSC is a valuable method for detecting phase transitions. The thermal behaviour of

novel homologous Series-1 was confirmed by using DSC measurement shown in Figure 3 &

4. Thermogram is traces in both heating and cooling condition. For compound C10, first

endothermic peak is observed at 106.64 °C on heating condition, this indicates the presence of

SmC phase, while second endothermic peak traced at 148.56 °C, which indicates the presence

of nematic phase, on cooled condition again two endothermic were traces at 105.92 C and

149.87 C. For compound C4, first endothermic peak is observed at 118.15 °C on heating

condition, this indicates the presence of SmC phase, while second endothermic peak traced at

142.23 °C, which indicates the presence of nematic phase, on cooled condition again two

endothermic were traces at 119.23 C and 144.10 C. Smectic mesophase is observed at 110

C by POM analysis, while in DSC thermogram it traced at 118.15 C on heating and 119.23

C on cooling condition.

Figure 4. DSC thermogram of Compound C10

Series-1 consisted three phenyl rings, and presence of –CH=CH-CO- linkage group

while in Series-X contain –CO-CH=CH- linkage group. In Series-2, -CO- group is directly

linked with phenyl ring causes variation of molecular rigidity and flexibility which affects to

the physical properties of a substance to exhibits a nematic mesophase with low thermal

stability and late commencement of mesophase. In Series-1, presence of vinyl carboxy (-

CH=CH-COO-) group which increases the length of molecule and –CH=CH- linked with

second phenyl ring to maintain its linearity. The non-coplanirity of molecule due to twist

received on account of the bumping of the oxygen and hydrogen atoms of the adjacent

benzene core [37]. The commencement of smectic as well as nematic phase in series-1 early

as compared to series-X due to the lateral interaction to terminal attractions maintain the

proportionate molecular force [38].


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4. CONCLUSION

Present novel linear series having chalconyl-ester central linkage group with three

phenyl rings and different terminal end group. C3 to C18 homologue shows smectic

mesophase. The observed smectic phase is fan type and needle type. From C2 to C18

homologue shows nematic mesophase and is threaded and schlieren. Tail ended n-alkoxy

chain can induce mesomorphic property in chalconyl-ester homologous series. The

mesomorphic property was confirmed by POM and DSC. Group efficiency order derived on

the basis of (i) thermal stability (ii) early commencement of mesophase and (iii)

mesophaselength for smectic and nematic are as under.

Acknowledgement

Authors thanks to the K. K. Shah Jarodwala Maninagar Science College to providing facilities. Authors also

thank to NFDD Centre, Rajkot for providing analytical and spectral services like 1H-NMR and IR study.

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( Received 14 August 2016; accepted 31 August 2016 )

mesomorphic properties of liquid crystalline compounds with ......liquid crystalline property (lc)...

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