Steric effect of di-lateral methyl substituent on the mesophase behavior of four-ring azo/ester/azo homologues

ABSTRACT

Four series of azo/ester/azo four-rings homologues, namely, 4-substituted phenylazo-3-methyl phenyl-4′-n-alkoxy 3-methyl phenylazo benzoates (In_a-d), bearing two terminals substituents, the first is a compact polar group (X = methoxy, methyl, H and bromo) and the other terminal is an alkoxy group of varying chain length (n = 6 to 16), were prepared and their molecular structures confirmed. Their thermotropic behaviours were analysed with differential scanning calorimetry (DSC) and polarising optical microscopy (POM). TGA analysis revealed that the prepared materials are thermally stable within their mesophase temperature range. Irrespective of the polarity of X and/or the alkoxy chain length all compounds showed to display a nematic phase with high thermal stability and wide temperature range. The methoxy homologues showed to exhibit higher thermal stabilities. Comparing the nematic stability of the di-laterally dimethyl substituted (In_c) derivatives with X = CH₃, with their previously reported mono-laterally methyl-substituted analogues (IIn_c) revealed that In_c homologues have thermal stabilities and mesophase temperature ranges lower than their corresponding analogues IIn_c. The effect of introducing the two lateral methyl groups on the extent of anisotropic polarisability of the whole molecule was discussed and their effect on the mesophase behaviour investigated. The photo-physical property was studied via UV-vis spectroscopy.

KEYWORDS:

• Azo/ester/azo four-ring liquid crystals
• two lateral methyl substituents
• thermal stability
• mesomorphic properties
• molecular polarisability

Acknowledgments

The authors would like to gratefully acknowledge Prof. Dr. Magdi M. Naoum (Prof. of Physical Chemistry, Department of Chemistry, Cairo University) for his help in the discussion.

Disclosure statement

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