Self-assembled optical gratings with banana-shaped liquid crystals

We demonstrated that a homologous series of banana-shaped liquid crystals, 1,3-phenylene bis(4-alkyloxybenzylideneamine), could assemble themselves into various kinds of groove-free diffraction gratings when their isotropic melts were slowly cooled into mesophases between two pieces of glass substrates. The groove-free diffraction gratings included one-dimensional parallel gratings, two-dimensional crossed gratings, two-dimensional fan-shaped gratings and two-dimensional circular gratings. Characterization by means of polarized optical microscopy showed that a pattern of periodic modulation of the refractive index was developed in the thin films formed by the banana-shaped compound. Our laser light diffraction experiments confirmed that these groove-free gratings could effectively diffract the incident red light from a helium-neon laser. On the basis of the diffraction equations derived for the self-assembled groove-free optical gratings, the diffraction patterns were simulated for the parallel gratings, orthogonally crossed gratings, fan-shaped gratings and circular gratings, respectively, and good agreement was achieved. The mechanisms on the self-assembly of the banana-shaped molecules were discussed in terms of intermolecular interactions. Our work provides an alternative method for manufacturing diffraction gratings by harnessing the self-assembly of banana-shaped molecules.