A SEMIEMPIRICAL MODEL FOR LIQUID-CRYSTAL REFRACTIVE-INDEX DISPERSIONS

A single-band model and a three-band model which consists of one sigma --> sigma* and two major pi --> pi* electronic transitions were developed for understanding the refractive index dispersions of uniaxial liquid crystals. This information is particularly important for liquid-crystal-display applications. By comparing the three-band model with experimental results of 5CB and 7CB (two cyanobiphenyl homologs), the contribution of each band to both refractive indices and birefringence is evaluated quantitatively. It is found that the sigma --> sigma* band makes the primary contribution to refractive indices, but its contribution to birefringence is not as significant. Conversely, the pi --> pi* bands make the primary contribution to birefringence, but their contribution to refractive indices is not comparable to the sigma --> sigma* band. These characteristics originate from the differences in transition wavelength, intensity, and absorption anisotropy of the sigma --> sigma* and pi --> pi* bands.