ESR line shape study of two nematic liquid crystals

Abstract

Electron spin-resonance (ESR) measurements of the spin-probe17β-hydroxy-4,4'-dimethyl spiro[5α-androstane-3,2-oxazolidine]-3'-yloxyl in the liquid crystals 4-n-Amylacetophenon O-(4-n-heptyl benzoyl)-oxime [AAHBO] and 4-n-Butoxyphenyl hexyl benzoate [BPHB] are reported in isotropic and nematic phases. On the basis of the temperature variation of ESR line width, an extra, solid to solid, phase transition is observed in BPHB. The theory of Polnaszek, Bruno and Freed (PBF) appropriate for anisotropic viscosity diffusion is used to analyze the ESR spectra and thereby to determine the values of the order parameter, the rotational diffusion tensor and the correlation time at different temperatures for the two liquid crystals. The analysis shows that the slowly relaxing local structure mechanisms (SRLS) is active in slow tumbling region specially in AAHBO.