Asymmetrically substituted benzene-1,3,5-tricarboxamides: self-assembly and odd-even effects in the solid state and in dilute solution

Asym. benzene-1,3,5-tricarboxamides (aBTAs) comprising two n-octyl and one chiral methyl-alkyl side chain were synthesized and characterised. The influence of the position and the configuration of the chiral Me group (Me at the alpha , beta or gamma position) in the aliph. side chains on the liq.-cryst. properties and the aggregation behavior of the aBTAs was systematically studied and compared to sym. benzene-1,3,5-tricarboxamides (sBTAs). Solid-state characterization (polarised optical microscopy, IR spectroscopy, X-ray diffraction and differential scanning calorimetry) revealed that all aBTAs show threefold, alpha -helical-type intermol. hydrogen bonding between neighboring mols. and exhibit a columnar hexagonal organization from room temp. to well above 200 DegC. Moving the chiral Me group closer to the amide group stabilizes the liq.-cryst. state, as evidenced by a higher clearing temp. and corresponding enthalpy. The self-assembly of dil. solns. of the aBTAs in methylcyclohexane (~10-5 molL-1) was investigated with CD (CD) spectroscopy. The sign of the Cotton effect demonstrated a pronounced odd-even effect, whereas the value of the molar ellipticity, Delta e, in the aBTAs was independent of the position of the Me group. Subsequent temp.-dependent CD measurements showed that the aggregation of all aBTAs can quant. be described by the nucleation-growth model and that the stability of the aggregates increases when the chiral Me group is closer to the amide moiety. The results presented herein illustrate that even small changes in the mol. structure of substituted benzene-1,3,5-tricarboxamides affect their solid-state properties and their self-assembly behavior in dil. solns. [on SciFinder (R)