Mechanism of Photochemical Phase Transition of Single-Component Phototropic Liquid Crystals Studied by Means of Holographic Grating Recording

Phototropic liquid crystals (PtLC) are a new class of materials possessing number of potential applications in photonics devices. However, so far a significant majority of PtLC materials has been realized by the doping a classical liquid crystal with a photochromic dye. The photochemical phase transition in such systems was investigated mainly by the monitoring of the changes in the transmittance. In this study, the photochemical phase transition of single-component phototropic liquid crystals was investigated using a holographic grating recording in combination with a polarized optical microscope. The cis-trans photoisomerization of compounds causes the isotropic-to-nematic (I-N) phase transition and so that the interference pattern can be mapped as a diffraction grating. The process of the grating build up was monitored by the first-order light diffraction, and simultaneously the area of the material exposed to the light was observed directly under a polarized microscope. The combination of the holographic technique with polarized optical microscopy has allowed to propose the mechanism of the I-N phase transition of LC compounds. It assumes three processes responsible for the grating formation. The results have a crucial importance in understanding the mechanism of photochemical phase transition of PtLCs, and thus they can be useful in construction of new optical devices.