Conversely, replacement of nitro with nitrile suppresses the formation of the NF phase, as does the replacement of the 4-nitrophenyl unit with the 3-nitro isomer. Related materials, comprising of the 4-(4-nitrophenyl)benzoyl)benzoate core but lacking the lateral unit, exhibit the re-entrant N-SmA-N phase sequence rather than the NF phase. Of all of the variants of RM230 encountered so far, RM734, which features a 2,4-dimethoxybenzoate head group, has the 'best' transition temperatures. A number of alternate polar groups were explored by Goodby et al; 5 the 3-fluoro-4-nitrophenyl unit gives a small enhancement in TN F -N and notable reduction in TN-Iso. As will be shown later, fluorination is a viable strategy for enhancing TN F -N in this class of materials, presumably via increased dipole-dipole interactions. Other polar units, shown in Figure , were found to eliminate the NF phase. Aya et al reported a number of further variations to the RM734 core structure. Elongation of the lateral methoxy unit reduces TN F -N relative to RM734, however melting points and clearing points are also reduced. Imrie et al also reported a number of RM734 derivatives with alternate lateral groups, including the (S)-2-methylbutoxy group which leads to the chiral NF phase, with this material also being reported independently by others. Again, the increased steric bulk of these lateral groups depresses TN F -N when compared to the parent material. Aya et al synthesised materials in which the lateral unit is positioned to the central phenyl ring, all of which generate the NF phase with a modest reduction in TN F -N. Use of azo or nmethylamide linking units eliminates the NF phase entirely, and also the nematic phase in the case of the amide. 28 As already shown for RM230 and RM734 (Fig. and Fig. , respectively), fluorination ortho to the nitro unit gives notable increases in the onset temperature of the NF phase while also depressing the clearing point slightly. Jakli et al reported a fluorinated variant of RM734 (known as RT11001, Figure ) in which a fluorine atom is positioned in the 3-position of the central ring. The increase in molecular dipole moment leads to a large increase in spontaneous polarisation, with the reported value of 6.9 µC/cm2being the largest ever reported for a liquid crystalline system. Imrie et al. provided further examples of fluorinated RM734 derivatives, finding one material that exhibits a direct isotropic to NF phase transition (Figure ).