Synthesis and Determination of Thermotropic Liquid Crystalline Behavior of Cinnamaldehyde-Based Molecules with Two Schiff Base Linking Units.

Zuhair Jamain,Melati Khairuddean,Nor Faizawani Omar

doi:10.3390/molecules25173780

Zuhair Jamain, Melati Khairuddean + Show 1 more

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https://doi.org/10.3390/molecules25173780

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Abstract

A series of liquid crystal molecules with two Schiff base linking units and a cinnamaldehyde core with different terminal groups were synthesized and characterized. The intermediates of 4-heptyloxybenzaldehyde (1a) and 4-dodeyloxybenzaldehyde (1b) were synthesized through the alkylation of 4-hydroxybenzaldehyde with a series of bromoalkane. A condensation reaction of cinnamaldehyde, 1,4-phenylenediamine and a series of substituted benzaldehydes with different terminal groups such as bromo, chloro, hydroxy, cinnamaldehyde, hydrogen, methoxy, heptyloxy and dodecyloxy produced a series of new cinnamaldehyde-based compounds, 2–9, respectively. All these compounds were characterized using Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and CHN elemental analysis. The liquid crystal properties of these compounds were determined using polarized optical microscopy (POM), and their transitions were further confirmed using differential scanning calorimetry (DSC). Compounds with chloro, bromo, methoxy, heptyloxy, and dodecyloxy substituents are mesogenic compounds with nematic phase behavior. However, the other compounds were found to be non-mesogenic without any mesophase transitions. The structure–property relationship was investigated in order to study the effect of different terminal groups and Schiff base linking units on the liquid crystalline behavior of these compounds.

Highlights

Nowadays, materials forming liquid crystal (LC) phases have found wide application in the manufacturing of displays [1], spatial light modulators [2], optical connectors [3], switches [4], and molecular sensors and detectors [5,6]
The strong demand for new LC applications has led to the synthesis of a wide range of molecules—in particular, thermotropic LC molecules [7,8]
The structures of produced eight Schiff base compounds (2–9) with different substituents (Scheme 2). The structures these intermediates and compounds were successfully characterized using Fourier transform infrared of these intermediates and compounds were successfully characterized using Fourier transform (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and CHN elemental analysis

Summary

Introduction

Materials forming liquid crystal (LC) phases have found wide application in the manufacturing of displays [1], spatial light modulators [2], optical connectors [3], switches [4], and molecular sensors and detectors [5,6]. The strong demand for new LC applications has led to the synthesis of a wide range of molecules—in particular, thermotropic LC molecules [7,8]. Thermotropic LC can be characterized by the various phase transitions that occur during heating, displaying temperature transitions between the crystal and the liquid (isotropic) phases. The thermal motion destroys the delicate ordering of the liquid crystal phase at high temperatures and causes the material to change into the isotropic phase

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