Improved Mesophase Behavior by Lateral Fluorination

Some of the reasons for the preference for fluorinated liquid crystals date back to the beginnings of LCD technology, others have gained paramount importance since the introduction of active matrix LCD in 1989. 

The strong effect on the mesophase sequence of lateral mono- and difluorination at different sites of an aromatic mesogenic core structure is illustrated by the data in Table 4.5 [40]. The relatively moderate mesogenic properties of material 14 were dramatically improved by introduction of two lateral fluorine substituents in the liquid crystal 22. Unfortunately, this target was achieved only by a purely empirical trial-and-error process requiring a tremendous amount of synthetic work. 

The decrease of the clearing point induced by lateral fluorination can be understood in terms of reduction of the length-to-breadth ratio of the liquid crystal molecule caused by any lateral substituent. As rule of thumb, for most compounds a decrease of the clearing temperature of 30-40 K is observed for each lateral fluorine atom. 

Another reason to make use of fluorinated substructures in the design of liquid crystals is the strong polarity of the carbon-fluorine bond, as a result of the large difference between the electronegativities of carbon (2.5) and fluorine (4.0). 

By analogy with nitrile-based liquid crystals, the most simple SFM (super-fluori-nated materials) carry one terminal fluorine substituent on an aromatic moiety. 

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