An infrared technique for the alignment control of highly viscous mesophases: The formation of homeotropic and planar domains for hexagonal columnar mesophase of Triphenylene mesogens

A new phenomenon was found for the alignment change of hexagonal columnar (Col_h) mesophase of a typical discotic liquid crystal of triphenylene derivatives, which is caused by a polarizing infrared laser irradiation with a wavelength to excite the vibrational mode of the selected chemical bond in the mesogen. A series of experiments has revealed that the polarization direction of incident laser is correlated to the molecular alignment generated by the irradiation, where the molecules are reoriented in a way that the molecules no longer absorb the incidence. The resultant new domain has a certain level of uniformity for the alignment from an optical point of view and remains for several hours to minutes depending on the temperature against the isotoropization. The manual scanning of the beam successfully gives uniform domains with a shape and one can generate a character "H", for example, with lath-like shaped uniform domains in the mesophase film. It was found that such new domains could be freezed into the polymer film by photopolymerization when the mesogens have polymerizable groups in the molecular flamework. This strongly indicate that the combination of the infrared control of the molecular alignment of Col_h mesophase and the following photopolymerization could realize highly functional polymer films where the structured domains with a variety of shapes and alignments are assembled in the film. Recently, it was also demonstrated that a homeotropic domain could be generated using a circularly polarized incidence at 6.18 μm which corresponds to the wavelength to excite the aromatic C-C stretching band, being coincident with the relation of the alignment to the polarization direction of incidence.