Spin Label Studies of Chain Organization in a Smectic Liquid Crystal

ESR studies of chain motion in 4,4'-dioctadecyloxyazoxybenzene are reported. The specially synthesised spin labels, closely resembling the host molecule, were marked at different positions along the chain. The results show that the solid* SB transition is accompanied by a melting of the chains. A consideration of the calorimetric data for a number of similar compounds suggests that chain melting may often be the dominant feature of this transition. In the smectic phases an odd-even dependence of the order parameter on label position is observed and considered in the light of the discrepancy between ESR and NMR order parameter data in long chain smectic systems. The tilt angle has been measured to be 1 2 O in the SB and 230 in the Sc phase, and the discontinuity related to the density data at the transition. An apparent odd-even alternation in the tilt angle is believed to be due to the interaction of the alternating form of the label molecules with the non-axial ordering potential.