Helical self-assembly and co-assembly of fluorinated, preorganized discotics

Abstract

The synthesis and self-assembly properties of a fluorinated C₃-symmetrical 3,3′-bis(acylamino)-2,2′-bipyridine discotic (1) in the mesophase and in solution are described. First, 3,4,5-tris-(1H,1H,2H,2H,3H,3H-perfluoroundecyl-1-oxy)benzoyl chloride was coupled to mono-t-BOC protected 2,2′-bipyridine-3,3′-diamine to afford after deprotection the corresponding fluorinated aromatic amine on a multigram scale. Then, three-fold reaction of this amine with trimesyl chloride yielded the target C₃-symmetrical fluorinated disc. The latter displayed columnar liquid crystallinity over a temperature range of more than 350 K in which helical rectangular and hexagonal columnar mesophases were detected by X-ray diffraction measurements. ¹H-NMR spectroscopy showed a preorganized structure due to strong intramolecular hydrogen bonding between the amide N–H's and bipyridine nitrogen atoms, even in the presence of a large excess of hexafluoroisopropanol. This preorganized structure allows the formation of helical self-assemblies in fluorinated solvents, as was established using UV-Vis spectroscopy. The fluorinated disc and two chiral hydrocarbon analogues (a C₃-symmetrical and a desymmetrized disc) were mixed in a 1 : 10 v : v mixture of methoxynonafluorobutane (MNFB) and 1,1,2-trichloro-1,2,2-trifluoroethane (Freon 113). Importantly, the C₃-symmetrical hydrocarbon disc dissolves only in the presence of fluorinated disc in the latter solvent mixture, proving a mutual interaction. CD spectroscopy performed on these mixtures points to a preference for alternating self-assemblies of fluorinated and chiral hydrocarbon discotics.