Experimental and computational studies of laterally ethoxy Schiff base-ester liquid crystalline magnets

ABSTRACT

A series of liquid crystals 4-X-(3’-ethoxy-4’-alkyoxyphenyimino)anilines with elongated molecular axis through the introduction of alkoxy chains containing different substituents (X = CH₃, OCH₃ and Cl) were synthesised. The thermal–optical behaviours of these organic magnets were characterised by differential scanning calorimetry and polarised optical microscopy. Elemental analysis, FT-IR and ¹H-NMR were employed to elucidate the molecular structures of the target compounds. Experimental data show that the nematic phase temperature range increases in the order of CH₃ < Cl < OCH₃. The laterally ethoxy Schiff base-ester liquid crystal with the most electronegative Cl atom is presumed to outweigh both analogues containing CH₃ and OCH₃, leading to the highest clearing temperature (T_c) value, but the present work proved that the presence of the O atom from OCH₃ is inclined to orientate in a parallel arrangement that increases the length-to-breadth ratio and the polarisability of the molecule. Computational studies on the different substituents against its kinetic stability were carried out based on the variation in the energy value of HOMO–LUMO gap (ΔE_gap). A relationship between the energy gap and the substituents under the present study shows that when the ΔE_gap increases from OCH₃ < Cl < CH₃, the chemical reactivity decreases, leading to the lowering of the clearing temperature.

Graphical abstract

KEYWORDS:

• Liquid crystalline magnets
• substituent effect
• thermal–optical
• DFT calculation
• HOMO–LUMO gap

Acknowledgments

The main author (G.-Y. Yeap) would like to thank the Ministry of Higher Education Malaysia for Fundamental Research Grant Scheme (FRGS 2020-1) with Project Code: FRGS/1/2020/STG04/USM/01/2. All researchers are also grateful for the research funding under International Collaborative Research by Soka University (Japan).

Disclosure statement

No potential conflict of interest was reported by the author(s).

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Funding

This work was supported by the Ministry of Higher Education Malaysia [FRGS/1/2020/STG04/USM/01/2]; International Collaborative Research Soka University [ICR].