Intermolecular organisation of triphenylene-based discotic mesogens by interdigitation of alkyl chains

Two series of hexakis(alkyloxy)triphenylenes have been synthesised and characterised. All materials contain two different lengths of n-alkyloxy chains. Series I (CmCn) materials contain three -OC5H11 chains and three -OCnH2n + 1 chains. Series II (CmCn) materials contain three -OCmH2m + 1 chains and three -OCnH2n + 1 chains, such that m + n = 10. Hexagonal columnar mesophases are observed in Series I for n greater than or equal to 3 and, in Series II, for C5C5 and C6C4. Investigation by XRD shows that, within Series I, increasing the C-n chain length increases the intercolumnar spacing. Furthermore, the intercolumnar spacings for constitutional isomers are identical. The difference in the lengths of the two types of alkyl chain at the periphery of the molecules is defined as the Interdigitation Length (IL) and is a measure of the maximum extent of interdigitation that is possible between neighbouring molecules. Series I and II materials have been studied to probe the effect of IL on the change in enthalpy (DeltaH(Col-I)) and entropy (DeltaS(Col-I)) for the mesophase to isotropic liquid phase transition. In contrast to CxCx materials, which show a decrease in DeltaH(Col-I) and DeltaS(Col-I) with increasing chain length, Series I and II mesogens all exhibit DeltaH(Col-I) and DeltaS(Col-I) values similar to those of long chain CxCx materials, irrespective of chain length.