Experimental and DFT Studies on Well-Defined Odd-Even Effect in Organic Liquid Crystal Tetramers Incorporating Two Chiral Centers

Abstract

Introduction: A new series of organic liquid crystal tetramers containing two symmetrical mesogenic units of 4-(4’-phenoxylimino)methyl)phenol joined by methylene –(CH2)mspacer wherein m varied from 5 to 10 have been synthesized. Methods: The length for each of the mesogenic units was extended further via connection with two chiral entities of (S)-2- methylbutyl-4-(4’-bromobutyloxyphenyl)benzoate situated at both terminal ends. The structures of these tetramers were elucidated using elemental analysis and spectroscopic techniques (FT-IR and 1H-NMR) whilst the texture, and the phase transition temperatures were studied using polarizing optical microscopy (POM) along with differential scanning calorimetry (DSC). Results: All the homologues in this series are enantiotropic mesogens exhibiting oily streak texture or fan-shaped texture characteristic of the chiral nematic phase. The odd-even effect can be observed upon increasing the methylene spacer length. The structure-properties connectivity has demonstrated that the phase transition temperature for tetramers with even-numbered methylene units in the spacer is relatively higher in comparison with the odd-numbered members. Conclusion: The computational study via molecular constraint dynamics performed at the DFTB level of theory has further supported that more energy is required for the even-numbered tetramer than the odd-numbered member to undergo the structural transition from folded to extended geometries.