Electron Paramagnetic Resonance Studies of a Viscous Nematic Liquid Crystal

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Abstract
We have studied the order as a function of temperature in the liquid crystal bis(4′‐n‐octyloxybenzal)‐2‐chloro‐1,4‐phenylenediamine using vanadyl acetylacetonate as a paramagnetic probe. The order found in the upper part of the mesophase (105–179°C) was normal. However, in the lower part of the mesophase (40–105°C) the order found was much higher than ordinarily observed, approaching complete alignment near the melting point. In addition, the EPR lines assumed an unusual shape in this lower temperature region. The observations were accounted for by assuming the liquid crystal was too viscous to allow free tumbling of the probe molecules, and by developing a new model in which a spatial rather than a temporal average was performed. Supporting evidence for the validity of the assumption was obtained experimentally by demonstrating that the liquid crystal had an unusually long response time for reorientation in the magnetic field.