Abstract
A heterocycle, 4-[(1,3-benzothiazol-2-ylimino)methyl]phenyl dodecanoate, was synthesized and its IR, 1H NMR, 13C NMR, elemental analysis and MS spectroscopic data are presented. This new compound exhibited smectic A phase.
Schiff bases have attracted much attention ever since the discovery of the first room temperature liquid crystal, 4-methoxybenzylidene-4’-butylaniline [1]. Many kinds of heterocyclic structures, such as pyridine [2], furan [3], thiophene [4] and benzothiazole [5,6,7] have been introduced as core centre in liquid crystalline compounds. In this paper, we report the synthesis of a new Schiff base comprising the benzothiazole moiety: 4-[(1,3-benzothiazol-2-ylimino)methyl]phenyl dodecanoate. This new compound exhibits enantiotropic smectic A phase, as indicated by thermal (DSC) and polarizing optical microscopy studies.
In analogy to a recently published procedure [8], a solution of 2-aminobenzothiazole (6.01 g, 40 mmol) and 4-hydroxybenzaldehyde (4.88 g, 40 mmol) in absolute ethanol (60 mL) was heated under reflux for 3 h. The solvent was removed by slow evaporation and Schiff base 1 thus obtained was recrystallized from absolute ethanol. Then, Schiff base 1 (5.09 g, 20 mmol) in dimethylformamide (10 mL), was added to a solution of dodecanoic acid (4.01 g, 20 mmol) and 4-dimethylaminopyridine (1.22 g, 10 mmol) in dichloromethane (70 mL). The resulting mixture was stirred in an ice bath. To this solution, N,N’-dicyclohexylcarbodiimide (4.13 g, 20 mmol) in 10 mL of dichloromethane was added dropwise while stirring in the ice bath for 1 h. The resulting mixture was subsequently stirred at room temperature for another 3 h. Then, the reaction mixture was filtered and the excess solvent was removed from the filtrate by evaporation. Recrystallization from absolute ethanol gave the Schiff base 2 as yellow solid (44%).
Thermal data obtained from DSC analysis (enthalpy changes, kJ mol-1 in bracket):
Heating: Crystal 80.8 oC (45.07) Smectic A 85.6 oC (7.44) Isotropic.
Cooling: Crystal 52.7 oC (37.81) Smectic A 81.7 oC (8.17) Isotropic.
Optical photomicrograph showing fan-shaped texture of smectic A phase observed under polarizing optical microscope:
MS (EI): m/z = 436 (M+, 9.2%), 254 (100), 225 (8.1), 57 (4.9), 43 (6.4).
IR (KBr, cm-1): 3064, 3033 (C-H aromatic), 2922, 2851 (C-H aliphatic), 1747 (C=O ester), 1618 (C=N, imine), 1600 (C=N, thiazole), 1509 (C=C aromatic).
1H NMR (400 MHz, CDCl3): δ/ppm 0.88 (t, 3H, J = 7.0 Hz, CH3-), 1.27-1.43 (m, 16H, CH3-(CH2)8-CH2-), 1.70-1.80 (q, 2H, J = 7.3 Hz, -CH2-CH2-COO-), 2.59 (t, 2H, J = 7.6 Hz, -CH2-COO-), 7.25 (d, 2H, J = 6.8 Hz, Ar-H), 7.37 (t, 1H, J = 8.3 Hz, Ar-H), 7.48 (t, 1H, J = 8.3 Hz, Ar-H), 7.84 (d, 1H, J = 8.1 Hz, Ar-H), 7.99 (d, 1H, J = 8.1 Hz, Ar-H), 8.06 (d, 2H, J = 6.8 Hz, Ar-H), 9.05 (s, 1H, -N=CH-).
13C NMR (100 MHz, CDCl3): δ/ppm 171.7 (-COO-), 164.8 (C=N), 154.6, 151.6, 134.63, 134.6, 132.2, 131.5, 126.4, 125.1, 123.0, 122.3, 121.6 for aromatic carbons, 34.4, 31.9, 29.5, 29.4, 29.3, 29.2, 29.0, 24.8, 22.6 for methylene carbons [-COO-(CH2)10-CH3], 14.1 (-CH3).
Elemental analysis: Calculated for C26H32N2O2S: C, 71.52%, H, 7.39%, N, 6.42%; Found: C, 71.65%, H, 7.50%, N, 6.53%.
Supplementary materials
Supplementary File 1Supplementary File 2Supplementary File 3Acknowledgements
The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman (UTAR) for the research facilities and financial support through UTAR Research Fund (Vote No. 6200/H002). T.M. Koh would like to acknowledge UTAR for the award of the research and teaching assistantships.
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