Ferroelectric liquid crystals derived from isoleucine II. Orientational ordering by carbon-13 separated local field spectroscopy

The orientational ordering of a series of ferroelectric liquid crystals 4'-[(2S,3S)-3-methyl-2-halopentanoyloxy]-4-hexyloxybiphenyls (3M2XPHOB, X = F for fluorine, C for chlorine, B for bromine) and their racemates have been studied by two-dimensional carbon-13 separated local field spectroscopy combined with off-magic-angle spinning. All carbons are well-resolved in the carbon-13 NMR spectrum so that carbon-proton dipolar coupling constants for all carbons in the liquid crystal molecule can be determined, giving detailed segmental orientational ordering information. (The order parameters of the biphenyl core segments and each carbon-proton bond in the aliphatic chains were measured as a function of temperature.) The results show that the substitution of different halogens on the chiral chain affects the carbon-proton bond orientational ordering of the entire chiral chain, while the orientational ordering of the rest of the liquid crystal molecule (core and achiral chain) remains unchanged. For the fluoro-substituted liquid crystals, carbon-fluorine dipolar couplings are also observed. At the SmA-to-SmC* transition, there is a discontinuous change in the magnitudes of all C-H bond order parameters. However, the aliphatic ordering relative to the core ordering is unchanged, suggesting that there is a negligible change in the mesogen conformation and the environment of the aliphatic chain at this transition, i.e. there is no evidence for substantial 'chiral interactions' in the SmC* phase.