New Azochromophores as Dopants to Blend Compositions for Control of Photoinduced Phase Transition

Abstract

New azochromophores (ACh) with terminal nitrile group have been synthesized. All compounds are crystalline at room temperature. Their melting points were measured by differential scanning calorimetry. The AСh containing carboxyl group have very high melting points. In this respect, carboxyl-free derivatives were preferred as azochromophoric additives to liquid crystal (LC) matrices. Blend compositions of the synthesized ACh with low molecular and polymer liquid crystals have been prepared and studied. From the point of view of homogeneity, the optimal content of ACh in low-molecular LC-matrices is 10 wt. %, whereas in polymer matrices, the ACh content does not exceed 5 wt. %. The thermal properties of the LC blend compositions, which were capable to the melt into isotropic phase due to LC order disruption caused by photoisomerization, have been studied. The ability of some blend compositions to undergo a UV-photoinduced isotropic phase transition at temperatures below the melting point of nematic matrix has been demonstrated. The times of UV-irradiation required for the implementation of trans-cis isomerisation and the times of visible light irradiation needed for recombination process have been assessed. The irradiation times were estimated by polarized optical microscopy.