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Molecular Dynamics in a Chiral Smectic Liquid Crystal Studied by 13C-Nuclear Magnetic Resonance Spin-Lattice Relaxation Time Observation

In order to study molecular rotation around its long axis in chiral smectic phases, the temperature dependence of the spin-lattice relaxation time T1 is investigated in an antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxy-carbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate (S-MHPOBC) by means of solid-state 13C-NMR spectroscopy. The spin-lattice relaxation time T1 of the aromatic core part has a minimum at about 95°C in its temperature dependent curve. Since T1 minimum occurs under a condition of ω0τc∼1, the correlation time τc of molecular motion should be of the order of 2π/ω0∼10-8 s here, with the Larmor frequency ω0 of 100 MHz in the present measurements. The temperature dependence of T1 reveals that the activation energy in the SmCA* phase is larger than that in SmA for most of the carbon nuclei of the aromatic core part. It suggests that the molecular rotation is hindered in the SmCA* phase, in which the large spontaneous polarization exists in a layer.