Investigation of molecular order and dynamics in liquid crystals confined in porous media using the dipolar-correlation effect on the stimulated echo

A new application of the stimulated echo pulse sequence is presented that permits the elucidation of molecular order and dynamics in a time scale between about 100 μs and the spin-lattice relaxation time. The technique exploits the influence of dipolar coupling on the quotient of the amplitudes of the stimulated and primary echoes produced by the standard three 90°-pulse sequence. Results obtained for a nematic liquid crystal in bulk and confined in porous glass (mean pore diameter 4 nm) are compared. In both cases the echo amplitude quotient oscillates as a function of the pulse spacing. In a bulk nematic crystal these oscillations originate from strong unaveraged dipolar interactions and directly reflect the molecular order in the material. In porous glass a real nematic order is absent. In this case, the oscillations can be attributed to spin exchange between inequivalent protons. Exchange rates are estimated.