Influence of Molecular Core Structure and Chain Length on the Physical Properties of Nematogenic Fluorobenzene Derivatives

Abstract

The influence of molecular core structure and chain length on various physical properties of nine nematogenic fluorobenzene derivatives obtained by optical polarizing microscopy, differential scanning calorimetry, X-ray diffraction, and optical birefringence techniques have been studied. It is observed that the temperature range of the nematic phase decreases when a cyclohexyl ring is replaced by a phenyl ring and when the number of fluorine atoms in the benzene group is increased. A change of some properties was noticed in difluoro (ff) derivatives compared to monofluoro (f) and trifluoro (fff) derivatives. The average intermolecular distance is found to increase systematically from mono to trifluoro derivatives. Effective molecular length (l) is found to be higher than the most extended length of the molecules, suggesting the presence of anti-parallel-type dimers. l is also found to be less in ff derivatives than in f and fff derivatives, indicating that the overlap between dimerized molecules in ff derivatives is more than in f and fff derivatives. The optical anisotropy (Δn) is found to be highest in an ff system compared to f and fff systems in 5ccp and 5cpp series, but the opposite behavior is observed in 3ccp series. 5ccp-f, having the lowest Δn and nematic phase over a large temperature, is expected to be useful for high-information-content display devices. The density of fff derivatives is found to be more than in ff derivatives in all systems, which is also true in the crystalline state of a 3ccp system. The orientational order parameters <P ₂> obtained from polarizability data are found to be slightly more, whereas the X-ray order parameters are found to be close to the mean field values in all the systems. A decreasing trend of <P ₂> is observed as one moves from f ⇒ fff ⇒ ff system.

Keywords:

• Density and polarizability
• nematogenic fluorobenze derivatives
• optical birefringence
• orientational order parameters
• X-ray diffraction study

Acknowledgment

The work was supported by Indo-German Joint Research Project No. IND 00/007. Financial assistance under Indo-Polish collaboration is also gratefully acknowledged.