Proton Spin Relaxation of a Liquid Crystal with a Glassy Cholesteric State

Field-cycling and standard pulsed N M R techniques have been used to study the frequency dependence of the longitudinal proton spin relaxation time Tx in the crystalline estradiol compound ( + )3,l,7-/?-bis-(4n-butoxybenzoyloxy)-estra-l,3,5-(10)-trien or BET, which is a mesogenic material with a chiral molecular structure. From the measured Larmor frequency and temperature dependences we conclude that, at low NMR frequencies in the cholesteric phase, T, reflects in addition to the relaxation process familiar from nematic liquid crystals (director fluctuation modes) another slow mechanism theoretically predicted for cholesteric systems, namely diffusion induced rotational molecular reorientation. These relaxation processes are not or much less effective in the crystalline and glassy state, where they are frozen. Also the high NMR frequency relaxation dispersion strongly differs between the cholesteric mesophase and the not liquid crystalline samples. This is interpreted by a change from essentially translational self-diffusion to rotational diffusion controlled proton relaxation.