Effect of intra-cyclohexane rings in H-shaped reactive molecules on the negative dispersion of optical retardation†

Artificial materials which show negative dispersion of retardation are of great importance and interest for applications to geometric phase retarders, compensation films, and augmented reality. Nevertheless, few negative dispersion materials have been reported and the exact correlation between the dispersion and the molecular structure has not been clearly elucidated yet. Here, new H-shaped reactive molecules with different chemical structures were synthesized and an early conversion from positive to negative dispersion of retardation was observed. The effect of the molecular structure on the dispersion of retardation was investigated experimentally as well as theoretically from the view point of molecular orientation and intrinsic molecular refractive index dispersion. The cyclohexane-inserted H-shaped reactive molecule represented a better planar orientation where the central linkage groups are aligned parallel to the surface plane. The relative UV absorption intensity along the long molecular axis was reduced compared to that of the molecule without a cyclohexane ring, while the absorption peak wavelength was not changed. By these two effects, the conversion from positive to negative dispersion could be shown at a lower concentration of the H-shaped molecules.