Impact of Photoisomerization on the One-Dimensional Fluid and Three-Dimensional Abrikosov-like Photonic Structures of Liquid Crystals

We report the first investigations of the effect of photoisomerization on the structure and light-driven transition in chiral nematic (Ch) and twist grain boundary smectic C* (TGBC*) phases. Both the phases exhibit photonic bandgap, with the former phase having it in one dimension and the latter in three dimensions in an anisotropic fashion as well. The TGBC* phase is the tilted analogue of the Abrikosov phase in superconductors besides having a helical structure in all three dimensions. The shift in both the TGBC*-Ch and Ch-isotropic transition temperatures as well as the associated dynamics of this isothermal photoisomerization caused changes brought about by UV light with a wavelength of 365 nm, and they are mapped through polarizing optical microscopy and laser transmission measurements. The grain boundary structure of the TGBC* certainly makes a difference for these parameters. Detailed time as well as temperature-dependent measurements of the photonic characteristics in the TGBC* and Ch phases, in terms of the unique spacing of the square grid pattern and the selective reflection wavelength, respectively, reveal several interesting features. For example, upon irradiation, the grid spacing shows a vertical divergence and completely reverses to the equilibrium situation when the UV light is turned OFF. The associated dynamics are seen to be well described by using the exponential as well as sigmoid functions. Layer spacing measurement by using X-ray diffraction measurement points to the role of nanophase segregation of the employed azobenzene molecule.