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

Abstract

\n\nA new series of organic liquid crystal tetramers containing two symmetrical mesogenic units of 4-(4’-phenoxylimino)methyl)phenol joined by methylene –(CH2)m- spacer wherein m varied from 5 to 10 has been synthesized. 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) while the texture and the phase transition temperatures were studied using polarizing optical microscopy (POM) along with differential scanning calorimetry (DSC). All the homologues in this series are enantiotropic mesogens exhibiting oily streak texture or fan shaped texture characteristic of 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. The computational study via constraint molecular 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.\n