Cholesteric liquid Crystals: Physical Properties and Molecular-Statistical Theories

It seems certain now that cholesteric liquid crystals (cholesterics, or CLC, for short) have been the first examples of mesomorphic state of matter known to humankind. Apart from the well-known discovery of the two-stage melting of cholesteryl benzoate made by Reinitzer in 1888¹ and cited in almost all the textbooks, one may recall some other papers where the authors, quite unaware of the real significance of their observations, presented undeniable evidence of the liquid crystalline state of matter. E.g., Professor Planer from the Lvov University as early as in 1861 did observe, putting it in the presentday terms, selective reflection from the planar texture of cholesteryl chloride forming a monotropic cholesteric mesophase on cooling.² One should also note that the works of Fergason^3,4 outlining the prospects of practical applications for CLC, did, in fact, precede similar endeavors by Heilmeier in the field of nematics. From the viewpoint of organic chemistry, a number of mesogenic cholesterol derivatives had been already synthesized and thoroughly studied⁵ by the time the name of MBBA (p-methoxybenzylidene-p′-n-butylaniline), to say nothing about cyanobiphenyls, was not yet known to anybody. The mere listing of CLC prospective applications^6,7. seems so impressive that, keeping in mind the above-described historical background, one should expect to find cholesterics in a key position in the field of liquid crystal science.