Asymmetrically Substituted Benzene-1,3,5-tricarboxamides: Self-Assembly and Odd-Even Effects in the Solid State and in Dilute Solution

Asymmetric benzene-1,3,5-tricarboxamides (aBTAs) comprising two n-octyl and one chiral methylalkyl side chain were synthesised and characterised. The influence of the position and the configuration of the chiral methyl group (methyl at the alpha, beta or gamma position) in the aliphatic side chains on the liquid-crystal line properties and the aggregation behaviour of the aBTAs was systematically studied and compared to symmetrical benzene-1,3,5-tricarboxamides (sBTAs). Solid-state characterisation (polarised optical microscopy IR spectroscopy, X-ray diffraction and differential scanning calorimetry) revealed that all aBTAs show threefold, alpha-helical-type intermolecular hydrogen bonding between neighbouring molecules and exhibit a columnar hexagonal organisation from room temperature to well above 200 degrees C. Moving the chiral methyl group closer to the amide group stabilises the liquid-crystalline state, as evidenced by a higher clearing temperature and corresponding enthalpy. The self-assembly of dilute solutions of the aBTAs in methylcyclohexane (approximate to 10(-5) mol L-1) was investigated with circular dichroism (CD) spectroscopy. The sign of the Cotton effect demonstrated a pronounced odd-even effect, whereas the value of the molar ellipticity, Delta epsilon, in the aBTAs was independent of the position of the methyl group. Subsequent temperature-dependent CD measurements showed that the aggregation of all aBTAs can quantitatively be described by the nucleation-growth model and that the stability of the aggregates increases when the chiral methyl group is closer to the amide moiety. The results presented herein illustrate that even small changes in the molecular structure of substituted benzene-1,3,5-tricarboxamides affect their solid-state properties and their self-assembly behaviour in dilute solutions.