CHAPTER VI Salicylaldimine-based banana-shaped liquid …shodhganga.inflibnet.ac.in/bitstream/10603/8217/12/12... · 2015-12-04 · compound (1a). Reduction of this compound by 10%

CHAPTER VI

Salicylaldimine-based banana-shaped liquid crystals

VI. 1. Scope of the work

In a way to realize stable banana-shaped compounds several mesogenic

segments such as tolane, biphenyls, and phenylbenzoates have been employed. As

expected these bent cores show good thermal stability with a remarkable

mesomorphic behavior. Synthesis of bent-core compounds with different chemical

architecture is of topical interest for understanding the relationship between the

chemical structure and mesophase behaviour. Most of the BC compounds

exhibiting mesophases that have been reported in the literature are symmetrical

about the central unit and are derived from 1, 3-dihydroxybenzene [1-6]. 2, 7-

dihydroxynaphthalene [7, 8] and benzene-1, 3-dicarboxylic acid [9]. Introduction

of lateral substituents on the arms of bent core molecules for modifying the

mesomorphic properties has been carried out [10-15] on a number of different

systems. With respect to the bent-core molecular architectures, the traditional

banana-shaped liquid crystals were generally formed by two bent-substituted rigid

arms connected to a central cyclic ring (through polar or nonpolar functional

groups) with a suitable bent angle and linking, where appropriate lengths of

flexible chains were attached [17]. A variety of interesting phenomena, especially

related to chirality and polarity, has been found in these materials, and the topic

has been object of several reviews [17-20]. For example salicylaldimine core

based classical ferroelectric liquid crystals are known to be stable [16b, 16c](Fig.

VI.1.a)

NO

O HCnH2n+1

MORA-n Series

NO

O HCnH2n+1

MBRA-n Series

NO

O HCnH2n+1OC

OH2C

(a)

(b)

Figure. VI. 1. (a) Molecular structure of salicylaldimine-based FLCs, (b) salicyaldimine-

based achiral side-chain polymer exhibiting antiferroelectric switching characteristics.

Alternatively, stable bent cores can be designed by incorporating

salicylaldimine mesogenic segment which has close chemical resemblance to

the Schiffs base moiety and is known to be stable due to the presence of

intramolecular hydrogen bonding between H-atom of hydroxyl group and N-

atom of bonding between H-atom of hydroxyl group and N-atom of imine

functionality [16a]. Salicylaldimine core has played a vital role in the first

discovery of Soto-Bustamante / Blinov anticlinic bilayer antiferroelectrics

formed by achiral side chain polymer in which it is a pendant segment [16d].

The Soto-Bustamante / Blinov results reinforce the concept that such structural

features may prove interesting when incorporated into bent core molecules.

Owing to these attractive features of salicylaldimine mesogenic segment, we

intended to explore this modification to the classical banana shaped molecule.

VI. 2. Molecular design

In the present work we propose to study a new series of symmetric banana

shaped liquid crystals consisting of Isophthalic acid bis-(4-amino-phenyl) ester

core connected to 4-Decyloxy-2-hydroxy-benzaldehyde via ester linkage. In the

following sectiion we present the synthesis and characterisation of the compounds

in detail. A general molecular structure of the target banana molecule is shown

below.

OO

O O

N

N

RO OH

HO ORR=CnH2n+1

n = 4, 5, 6, 7, 8, 9, 10, 11, 16

VI. 3. Synthesis and molecular structural characterization

The intermediate (1b) were synthesized in two steps. Firstly,the

esterification of isophthalic acid with 4-nitro phenol, dicyclohexylcarbodiimide,

N, N-dimethylaminopyridine and tetrahydrofuran as the solvent to obtain

compound (1a). Reduction of this compound by 10% Pd/C using tetrahydrofuran

as the solvent gives the intermediate (1b). Both intermediates were performed

according to literature procedures [21]. As a representative case the 1H NMR

spectra of the banana molecule is shown in Fig.VI. 2.

Figure. VI. 2. 1H NMR (200MHz) spectra of D-10 in CDCl3

OO

HO OH

OO

O O

O2N NO2

OO

O O

H2N NH2

HO CHO

OH

RO CHO

OH

OO

O O

N

N

RO OH

HO OR

(i)

(ii) (iii)

R=CnH2n+1

n = 4, 5, 6, 7, 8, 9, 10, 11, 16

(iv)

Reagents and condition; (i) 4-nitrophenol, DCC, DMAP, tetrahtdrofuran; (ii) 10%

Pd/C, tetrahydrofuran; (iii) 1-bromoalkane, KHCO3, MEK; (iv) absol.ethanol,

gla.CH3COOH.

(1b)(2a)

(1a)

series D-n

VI. 4. Results and discussion

The liquid crystalline behaviour of the compounds was preliminarily

investigated with the help of POM and DSC. Transition temperatures and

associated enthalpies obtained from DSC thermograms are shown in Table. 1. A

study of large number of compounds has indicated that the occurrence of B1 and

B2 phases is dependent on terminal chain length. However the nature of the

mesophase has varied form lower to higher homologues. The transition

temperatures are higher for salicylaldimines due to the presence of intramolecular

H-bonding between H-atom of hydroxyl group and N-atom of imine functionality.

All the compounds were found to be mesomorphic and the obtained enthalpy

values of about 19 to 24 J g-1

for mesophase to isotropic phase transition are in

agreement with the values reported for banana-shaped compounds. As a

representative case the DSC scans obtained at a rate of 10oC/min for the D-10

sample is shown in Fig. I. 3.

Fig. VI. 3. Differential scanning calorimetric thermogram of compound D-10 obtained at

a rate of 10oC min

-1: (a) heating cycle (b) cooling cycle.

Table 1. Transition temperatures (oC) and enthalpies of transitions (KJ molˉ

1) for the compounds

Compound n Cr Heating

Cooling

B1 Heating

Cooling

B2 Heating

Cooling

D-4 4 • 164.0[33.9]

179.3[32.6]

• 221.4[21.9]

224.8[22.4]

- -

D-5 5 • 157.2[39.0]

162.3[36.8]

• 218.6[20.5]

217.6[21.0]

- -

D-6 6 • 141.5[29.0]

153.9[30.1]

• 220.3[19.2]

219.0[18.1]

- -

D-7 7 • 149.2[28.8]

158.2[32.0]

• 214.8[23.7]

215.0[23.1]

- -

D-8 8 • 120.1[31.0]

127.3[30.9]

• 213.8[19.2]

214.5[18.5]

- -

D-9 9 • 118.4[27.5]

125.8[29.0]

• 213.9[15.7]

213.1[17.6]

- -

D-10 10 • 116.5[33.6]

123.7[30.4]

- - • 209.5[22.9]

210.0[23.4]

D-11 11 • 113.4[28.7]

121..0[29.]

- • 211.7[21.8]

209.3[21.5]

D-16* 16

*This compound exhibit Cr-Cr transition.

Cr-crystal; B1-banana phase; B2-banana phase; I-isotropic.

All the compounds except D-16 exhibit enantiotropic liquid crystalline phases, its

type depending on the length of the terminal alkoxy chain. The longer

homolouges D-10 and D-11 exhibit B2 mesophase, but D-16 does not show any

phase. It exhibit only crystal to crystal transition. An example of the B2 textures

observed for D-10 is given in plate VI. 4. 1. The bent cores D-6 and D-7 show an

identical mesophase over 219oC and 215

oC temperature range respectively.

plate VI. 4. 1 Photomicrograph of the texture observed on cooling from the isotropic state for D-

10 at 210oC.

plate VI. 4. 2. Photomicrograph of the texture observed on cooling from the isotropic state for D-8

at 214. 5oC.

For the short tailed members, the B1 phase was observed on cooling

from the isotropic state by the formation of small battonets. PlateVI. 4.2 show the

texture seen under the polarized microscope for the compound D-8 at 214.5oC.

The texural patterns matches with textures obtained for the B1 phase of the lower

homologues of the parent series of banana compounds.

VI. 5. Conclusion

All nine salicylaldimine-based compounds that have been synthesized

exhibit stable liquid crystalline phases. As in most banana–shaped liquid crystals,

a phenyl or biphenyl central part promotes the formation of B-phases. In our

studies B1 phase was observed for the lower homologues and the higher

homologues exhibit B2 phase.

VI. 6. Experimental

In this section the detailed synthetic procedures and the molecular

structural characterization data has been presented for the intermediates and target

compounds

General procedure for the synthesis of Isophthalic acid bis (4-nitro-phenyl)

ester (1)

Isophthalic acid (1g, 0. 006 mol) and 4-nitro phenol (1. 46g, 0. 012 mol)

were dissolved in dry CH2Cl2 (30 ml). To this DCC (2. 68g, 0. 013 mol) and a

catalytic amount of DMAP were added and the mixture was stirred at room

temperature for about 3 hours. The precipitated dicyclohexylurea was filtered off

and washed thoroughly with CH2Cl2. The combined filtrate was washed with

water and dried over Na2SO4. The crude product was purified by column

chromatography using silica gel (100-200 mesh) and 10% EtOAc-hexane elution

afforded the product.

Rf = 0.4 in 30% EtOAc-Hexane; Yield=42%; A white solid; IR (KBr pellet): 1985,

1769, 1635, 1192, 1025, 875 cm-1

; ¹H NMR (CDCl3, 200MHz) δ: 9.03 (s, 1H, Ar),

8.35 (d, J=8.6 Hz, 2H, Ar), 8.27 (d, J=8.6 Hz, 2H, Ar), 7.59 (s, 1H, Ar), 7.40 (d,

J=8.6 Hz, 2H, Ar)

General procedure for the synthesis of Isophthalic acid bis - (4-amino-phenyl)

ester

Isophthalic acid bis (4-nitro-phenyl) ester (1g, 1equiv. ) was dissolved in

dry THF and 10% Pd-C (10% weight of the nitro compound) was added. The

reaction mixture was degassed and stirred under H2 gas (1 atmospheric pressure)

for 4 h at rt. The reaction mixture was filtered over celite bed, concentrated and

the solid obtained was recrystallized from hexanes to afford a brown solid.

Rf = 0.23 in 50% EtOAc-Hexane; Yield=35%; A brown solid; IR (KBr pellet):

1975, 1769, 1625, 1192, 1025, 875 cm-1

; ¹H NMR (CDCl3,200MHz) δ: 9.02 (s,

1H, Ar), 8.38 (d, J=8.6 Hz, 2H, Ar), 6.92 (d, 2H, Ar), 6.51 (d, J=8.6 Hz, 2H, Ar).

General procedure for the synthesis of 2-hydroxy-4-n-alkoxybenzaldehydes

A mixture of 2,4-dihydroxybenzaldehyde (1equiv.), 1-bromoalkane (0. equiv.

), KHCO3 (dry) (3.0 equiv. ) and dry acetone (~20 ml) was refluxed under N2

atmosphere for 12 h. Acetone was evaporated off and to the residue about 200ml of

water was added. A dark brown coloured suspension thus obtained was extracted with

CH2Cl2 (20 mlX2). The combined organic extracts were washed with brine and dried

over Na2SO4. Evaporation of solvent furnished a brown coloured mass, which was

purified by column chromatography using silica gel (100-200-mesh). Elution with a

mixture of 10% EtOAc-hexane furnished pure product.

2-hydroxy-4-n-pentyloxybenzaldehyde

Rf = 0.41 in 10% EtOAc-hexane; Yield: 41.2%; A pale yellow liquid; IR (Neat):

2960, 2929, 1629, 1574, 1508cm-1

; 1H NMR (CDCl3,200MHz)δ: 11.48 (s, 1H,

1XOH), 9.7 (s, 1H, 1XCHO), 7.41 (d, J=8.6 Hz, 1H, Ar), 6.52 (d, J=8.6 Hz, 1H,

Ar), 6.41 (d, J=2.3 Hz, 1H, Ar), 4.01 (t, J=6.5 Hz, 2H, 1XOCH2), 1.83-1.25 (m, 6H,

3XCH2), 0.99 (t, J=7.1, 3H, 1XCH3).

2-hydroxy-4-n-hexyloxybenzaldehyde


2930, 2929, 1654, 1574, 1508cm-1

; 1H NMR (CDCl3,200MHz)δ: 11.48 (s, 1H,


Ar), 6.41 (d, J=2.3 Hz, 1H, Ar), 4.01 (t, J=6.5 Hz, 2H, 1XOCH2), 1.83-1.25 (m,

8H, 4XCH2), 0.99 (t, J=7.1, 3H, 1XCH3).

2-hydroxy-4-n-heptyloxybenzaldehyde


2960, 2855, 1629, 1575, 1507cm-1

; 1H NMR (CDCl3,200MHz) δ: 11.47 (s, 1H,



10H, 5XCH2), 0.89 (t, J=7.1, 3H, 1XCH3).

2-hydroxy-4-n-octyloxybenzaldehyde


2926, 2855, 1629, 1575, 1507cm-1

; 1H NMR (CDCl3,200MHz) δ: 11.47 (s, 1H,



12H, 6XCH2), 0.89 (t, J=7.1, 3H, 1XCH3).

2-hydroxy-4-n-nonyloxybenzaldehyde


2960, 2854, 1629, 1575, 1508cm-1

; 1H NMR (CDCl3,200MHz) δ: 11.47(s, 1H,

1XOH), 9.70(s, 1H, 1XCHO), 7.41(d, J=8.6 Hz, 1H, Ar), 6.52(d, J=8.6 Hz, 1H,

Ar), 6.41(d, J=2.3 Hz, 1H, Ar), 4.01(t, J=6.5 Hz, 2H, 1XOCH2), 1.83-1.30 (m,

14H, 7XCH2), 0.89 (t, J=7.1, 3H, 1XCH3).

2-hydroxy-4-n-decyloxybenzaldehyde

Rf = 0.45 in 10% EtOAc-hexane;Yield: 39%; A white solid; m.p: 29oC; IR (Neat):

2926, 2855, 1630, 1575, 1507cm-1

; 1H NMR (CDCl3,200MHz) δ: 11.48 (s, 1H,



16H, 8XCH2), 0.89 (t, J=7.1, 3H, 1XCH3).

2-hydroxy-4-n-undecyloxybenzaldehyde

Rf = 0.45 in 10% EtOAc-hexane; Yield: 31%; A white solid; m.p: 44oC; IR

(Neat): 2960, 2854, 1629, 1575, 1507cm-1

; 1H NMR (CDCl3,200MHz) δ: 11.47 (s,

1H, 1XOH), 9.70 (s, 1H, 1XCHO), 7.41 (d, J=8.6 Hz, 1H, Ar), 6.52 (d, J=8.6 Hz,

1H, Ar), 6.41 (d, J=2.3 Hz, 1H, Ar), 4.01 (t, J=6.5 Hz, 2H, 1XOCH2), 1.83-1.30

(m, 18H, 9XCH2), 0.88 (t, J=7.1, 3H, 1XCH3).

2-hydroxy-4-n-hexadecyloxybenzaldehyde

Rf = 0.46 in 10% EtOAc-hexane; Yield: 42%; A white solid; m.p: 55oC; IR

(Neat): 2960, 2854, 1677, 1629, 1575, 1507cm-1

; 1H NMR (CDCl3,200MHz) δ:

11.47 (s, 1H, 1XOH), 9.70 (s, 1H, 1XCHO), 7.41 (d, J=8.6 Hz, 1H, Ar), 6.52 (d,

J=8.6 Hz, 1H, Ar), 6.41 (d, J=2.3 Hz, 1H, Ar), 4.01 (t, J=6.5 Hz, 2H, 1XOCH2),

1.83-1.30 (m, 28H, 14XCH2), 0.88 (t, J=7.1, 3H, 1XCH3).

General procedure for the synthesis of Isophthalic acid bis-{4-[(4-butyloxy-2-

hydroxy-benzylidine)-amino]-phenyl} ester

A mixture of Isophthalic acid bis - (4-amino-phenyl) ester (3) (1 equiv. ), 2-

hydroxy-4-n-alkyloxybenzaldehyde (2. 1 equiv. ), absolute ethanol (20 ml) and a

few traces of acetic acid was refluxed until the yellow solid compound

precipitated out (2 hours). The crude product obtained was collected by filtration

and repeatedly washed with hot absolute ethanol. It was purified by repeated

recrystallization with a mixture of ethanol-CH2Cl2 (9:1).

D-4

Yield: 47%: a yellow solid [found C, 72.78; H, 5.35; N, 4.6.C42H40N2O8 requires

C, 71.98; H, 5.75; N, 4.00]; IR (KBr pellet): 3219, 2938, 1759, 1764, 1619, 1267,

839, 714 cm-1

.¹H NMR (CDCl3,200MHz) δ: 8.91 (t, 1H, Ar), 8.42 (s, 2H, 2X CH-

N), 8.39 (t, 2H, Ar), 7.51 (t, 1H, Ar), 7.37 (d, 2H, Ar), 7.34 (d, 4H, Ar), 7.24 (d,

4H, Ar), 6.36-6.27 (m, 4H, Ar), 4.01 (t, 4H, 2X OCH2), 1.80-1.42 (m, 8H,

4XCH2), 0.89 (t, 6H, 2XCH3)

D-5


C, 72.51; H, 6.09; N, 3.84]; IR (KBr pellet): 3219, 2938, 1759, 1764, 1619, 1267,

839, 714 cm-1



4H, Ar), 6.36-6.27 (m, 4H, Ar), 4.01(t, 4H, 2X OCH2), 180-1.42 (m,12H,

6XCH2), 0.89 (t, 6H, 2XCH3).

D-6


C, 73.00; H, 6.39; N, 3.70]; IR (KBr pellet): 3219, 2938, 1759, 1764, 1609, 1267,

839, 724 cm-1




8XCH2), 0.88 (t, 6H, 2XCH3).

D-7

Yield: 55%: a yellow solid [found C, 74.05; H, 6.88; N, 3.27.C48 H52N2O8 requires

C, 73.45; H, 6.68; N, 3.57]; IR (KBr pellet): 3219, 2937, 1759, 1764, 1609, 1269,

839, 724 cm-1




10XCH2), 0.88 (t, 6H, 2XCH3).

D-8


C, 73.87;H,6.94; N,3.45]; IR (KBr pellet): 3219, 2937, 1759, 1764, 1609, 1269,

839, 727 cm-1

.¹H NMR (CDCl3,200MHz) δ: 8.92 (t, 1H, Ar), 8.42 (s, 2H, 2XCH-

N), 8.38 (t, 2H, Ar), 7.50(t, 1H, Ar), 7.38(d, 2H, Ar), 7.34(d, 4H, Ar), 7.24(d, 4H,

Ar), 6.36-6.27 (m, 4H, Ar), 4.01(t, 4H, 2X OCH2), 1.80-1.42(m, 24H, 12XCH2),

0.88 (t, 6H, 2XCH3).

D-9


C, 74.26; H, 7.19; N, 3.33]; IR (KBr pellet): 3212, 2937, 1759, 1764, 1609, 1269,

836, 727 cm-1



4H, Ar), 6.36-6.27 (m, 4H, Ar), 4.01(t, 4H, 2X OCH2), 1.80-1.42 (m, 28H,

14XCH2), 0.88 (t, 6H, 2XCH3).

D-10

Yield: 60%: a yellow solid [found C,74.33;H,7.72; N,3.22.C54 H64 N2 O8 requires

C, 74.63; H, 7.42; N, 3.22]; IR (KBr pellet): 3212, 2941, 1759, 1764, 1609, 1269,

836, 727 cm-1

.¹H NMR (CDCl3,200MHz) δ:.8.91 (t,1H,Ar), 8.42 (s, 2H, 2X CH-

N), 8.38 (t, 2H, Ar), 7.50 (t,1H, Ar), 7.38 (d, 2H, Ar), 7.34 (d, 4H, Ar), 7.24 (d,

4H, Ar), 6.36-6.29 (m, 4H, Ar), 4.01(t, 4H,2X OCH2), 1.80-1.42

(m,32H,16XCH2), 0.88 (t,6H, 2XCH3).

D-11


C, 74.97; H, 7.64; N, 3.12]; IR (KBr pellet): 3212, 2941, 1759, 1764, 1609, 1269,

836, 727 cm-1

.¹H NMR (CDCl3,200MHz) δ: 8.91 (t,1H,Ar), 8.42 (s,2H,2XCH-N),

8.38 (t,2H,Ar), 7.50 (t,1H,Ar), 7.38 (d,2H,Ar), 7.34 (d,4H,Ar), 7.24 (d,4H,Ar),

6.36-6.30 (m,4H,Ar), 4.01(t,4H,2XOCH2), 1.80-1.42 (m,36H,18XCH2), 0.88

(t,6H, 2XCH3).

D-16

Yield: 71%: a yellow solid [found C,77.31; H,9.15; N,2.52.C66H88N2O8 requires

C,76.41; H,8.55; N,2.70]; IR (KBr pellet): 3212, 2951,1759, 1765,1609,1269,

839, 723 cm-1

.¹H NMR(CDCl3,200MHz) δ: 8.91(t,1H,Ar), 8.42 (s,2H,2X CH-N),

8.38 (t,2H,Ar), 7.50 (t,1H,Ar), 7.38 (d, 2H, Ar), 7.34 (d,4H,Ar), 7.24 (d, 4H, Ar),

6.36-6.30 (m, 4H, Ar), 4.02 (t,4H,2XOCH2), 1.80-1.42 (m,56H, 28XCH2), 0.89 (t,

6H, 2XCH3).

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CHAPTER VI Salicylaldimine-based banana-shaped liquid …shodhganga.inflibnet.ac.in/bitstream/10603/8217/12/12... · 2015-12-04 · compound (1a). Reduction of this compound by 10%

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