Study of line shape and angular variation of ESR spectra in two smectic a liquid crystals

Abstract

Electron spin-resonance (ESR) measurement of the androstane nitroxide spin-probe is reported in isotropic, nematic and smectic A phases of the liquid crystals N(p-Octyloxy-benzylidine)-p-toludine [OBT] and p-nitrophenyl-p-n-octyloxy benzoate [NPOB]. Mono-domain samples in flat quartz tube were formed for the study of the angular variation of the spectra in smectic A phase. The analysis of the spectra using the theory of Polnaszek, Bruno and Freed (PBF) has resulted in the determination of the order parameter S, the rotational correlation time TR, and the anisotropic diffusion parameter N at different temperatures. The comparison between the two samples shows that the molecular geometry of the liquid crystals play important role in determining the value of the order parameter. The entire smectic A phase in OBT is characteristic of slow tumbling region (TR > 10_-9 sec) and the anisotropic parameter N shows an anomalous increase in this phase. This indicates that the slowly relaxing local structure (SRLS) mechanism plays important role in the slow tumbling region. In NPOB the incipient slow tumbling region is almost at the end of smectic A phase and the SRLS mechanism does not seem to play as important role as in OBT.