Quantitative analysis of 2H NMRT1Q, T1Z and T2 relaxation times in the SmA phase of a liquid crystal dendrimer

Abstract

In this work the ²H NMR spin–lattice relaxation times, T_1Z and T_1Q, and the spin–spin relaxation times measured by means of the quadrupolar echo sequence, T₂, have been analyzed in order to study the dynamic processes of a liquid crystalline dendrimer labelled on the aromatic core of the lateral mesogenic units in the SmA phase. Several theoretical models describing both internal and overall molecular diffusion motions have been used to reproduce the temperature dependence of the spectral densities J₀(0), J₁(ω₀) and J₂(2ω₀) experimentally determined at the Larmor frequency ω₀ of 61.3 MHz. This analysis identifies in the internal rotation of the deuterated phenyl ring around its para axis the main contribution to the longitudinal relaxation. Moreover, the best fitting of the minimum in the trend of both T_1Z and T_1Q was obtained by introducing a distribution of activation energies for such internal motion, thus confirming previous hypothesis of dynamic heterogeneity. A much slower motion, affecting the sole transverse relaxation and ascribable to the reorientation of the whole macromolecular dendrimers, was modelled through the so called “slowly relaxing local structure” approach.