Effect of skew cybotactic structure on the optical properties of a nematogen with a lateral cyano substituent

Abstract

We report an unusual thermal variation of the ordinary refractive index (n_o) of the nematogenic compound 2-cyano-4-heptyl-phenyl-4'-pentyl-4-biphenyl carboxylate, which has a cyano group making a large angle (~60°) with the molecular axis. As the sample is cooled from the NI transition point, n_o initially decreases as usual, but on further cooling it shows an increasing trend after reaching a broad minimum. On the other hand, the extraordinary index n_e shows the normal temperature variation. X-ray investigations of a monodomain sample show that the nematic has a skewed cybotactic type of short-range order, the tilt angle increasing considerably as the temperature is lowered from the NI transition point. We have made a model calculation to explain the temperature variation of n_o in terms of the variation of the tilt angle.