Stabilization and induction of discotic mesophases by trifluoroacetic acid

It is shown that trifluoroacetic acid (TFA) can form 1:1 molar complexes with alkyloxy discotic compounds, thereby enhancing their mesomorphic properties. In particular, when hexa-alkyloxytriphenylenes (THEn) (n being the number of carbons per side chain) are crystallized from, or mixed with, TFA, complexes are formed which exhibit the D-ho(Col(ho)) mesophase, as do the neat THEn's, but with a broader range of thermal stability and a higher degree of order. The mesophases of these complexes are completely miscible with those of the neat compounds. Likewise, 1:1 mixtures of hexa-alkyloxytribenzocyclononene (TBCEn) and TFA yield complexes showing enhanced mesomorphic properties for the mesogenic homologues of this series and induced mesomorphism for the lower homologues, which in the neat form are not mesogenic. The mesomorphic properties of these TFA complexes with THEn (n=3-6) and with TBCEn (n=4-6) are investigated by means of optical microscopy, differential scanning calorimetry, X-ray diffraction and NMR spectroscopy. It is suggested that the enhancement of the mesomorphic properties is due to formation of oxonium ion complexes.