Study of Orientational Order of in Nematic and Smectic Phases by means of Optical Spectroscopy Methods

Abstract

The long-range orientational order of 4-n-pentylphenylthio-4′-n-decyloxybenzoate () doped with a dichroic dye was studied as a function of temperature in the nematic, smectic A, smectic C and smectic J phases by means of two optical spectroscopy methods: dichroism of absorption in the visible spectral region and fluorescence depolarization. On the basis of absorption and fluorescence spectra the order parameters < P ₂ > and < P ₄ > as well as the orientational distribution function were determined. The discontinuous change of both < P ₂ > and < P ₄ > parameters at the smectic A – nematic phase transition for dye/ mixtures was observed, in an agreement with the microscopic theory proposed by McMillan. An influence of the dye molecular structure on the orientational order of the dye/ mixture was observed. The correlation between the order parameter values and the shift of the phase transitions temperatures due to the dye addition was confirmed.

Keywords:

• dichroic dye
• light absorption and emission
• orientational order
• phase transition
• thermotropic liquid crystal

Acknowledgments

This work was supported by Poznań University of Technology Research Project no TB 64-001/2007-DS.