  Long-lived singlet states (LLS) of nuclear spin pairs have been extensively
studied and utilized in the isotropic phase via liquid state NMR. However,
there are hardly any reports of LLS in the anisotropic phase that allows
contribution from the dipolar coupling in addition to the scalar coupling,
thereby opening many exciting possibilities. Here we report observing LLS in a
pair of nuclear spins partially oriented in the nematic phase of a liquid
crystal solvent. The spins are strongly interacting via the residual
dipole-dipole coupling. We observe LLS in the oriented phase living up to three
times longer than the usual spin-lattice relaxation time constant ($T_1$). Upon
heating, the system undergoes a phase transition from nematic into isotropic
phase, wherein the LLS is up to five times longer lived than the corresponding
$T_1$. Interestingly, the LLS prepared in the oriented phase can survive the
transition from the nematic to the isotropic phase. As an application of LLS in
the oriented phase, we utilize its longer life to measure the small
translational diffusion coefficient of solute molecules in the liquid crystal
solvent. Finally, we propose utilizing the phase transition to lock or unlock
access to LLS.
