Molecular structure and the twist-bend nematic phase: the role of terminal chains

The synthesis and characterisation of two homologous series of non-symmetric dimers are reported, the 1-(4-methoxybiphenyl-4MODIFIER LETTER PRIME-yl)-6-(4-alkylanilinebenzylidene-4MODIFIER LETTER PRIME-oxy)hexanes (MeOB6O.m, m = 1-10) and 1-(4-methoxybiphenyl-4MODIFIER LETTER PRIME-yl)-6-(4-alkyloxyanilinebenzylidene-4MODIFIER LETTER PRIME-oxy)hexanes (MeOB6O.Om, m = 1-9). All 10 members of the MeOB6O.mseries exhibit the conventional nematic phase. At lower temperatures, the members withm = 1-7 formed the twist-bend nematic phase, NTB, whereas form = 8-10 smectic behaviour replaced the NTBphase. All nine members of the MeOB6O.Omseries also show the conventional nematic phase and form = 1-3, a strongly monotropic NTBphase is also observed. The alkyloxy terminated dimers show the higher values of TNIand TNTBN. For both series, the values of TNIand TNTBNshow a modest alternation and in the same sense asmis increased. These observations suggest that the spatial uniformity of molecular curvature is important in driving the formation of the NTBphase. The observation of smectic behaviour is attributed to the molecular inhomogeneity arising from the long terminal alkyl chain driving microphase separation. The transitional behaviour of these series is compared to those of the corresponding cyanobiphenyl-based series and overarching observations discussed.