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Homochiral Columns Constructed by Chiral Self-Sorting During Supramolecular Helical Organization of Hat-Shaped Molecules

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journal contribution
posted on 17.12.2015, 02:05 by Cécile Roche, Hao-Jan Sun, Margaret E. Prendergast, Pawaret Leowanawat, Benjamin E. Partridge, Paul A. Heiney, Fumito Araoka, Robert Graf, Hans W. Spiess, Xianbingon Zeng, Goran Ungar, Virgil Percec
A library of dendronized cyclotriveratrylene (CTV) crowns substituted with chiral, racemic, or achiral peripheral alkyl chains, including enantiopure R and S branched alkyls, “racemic by mixture”, “racemic by synthesis”, n-octyl, and n-dodecyl groups was synthesized. In solvophobic solvents and in bulk they self-assemble in helical columns. Their solution and bulk shape-persistent supramolecular structures were determined by a complementary combination of circular dichroism (CD) and UV in solution and thin film, microspot CD in thin film, differential scanning calorimetry combined with fiber X-ray diffraction, computer simulation, and molecular models. In solution, self-assembly via a cooperative mechanism generates single-handed columns from enantiopure CTVs and mixtures of right- and left-handed columns from racemic by mixture, racemic by synthesis, other combinations of R and S, and even from achiral compounds. In bulk state all supramolecular columns form a 3D hexagonal crystalline phase, Φhk (P63 symmetry), that can be obtained only from single-handed columns and a columnar hexagonal 2D liquid crystal, Φh. The highest order Φhk consists of enantiopure single-handed columns that are slightly distorted 12-fold triple helices. The “hat-shaped” dendronized CTV assembles in bent-branch pine-tree columns that allow interdigitation of alkyl groups in adjacent columns regardless of their direction. Enantiomerically rich, racemic, and achiral compositions undergo deracemization in the crystal state by transfer of the transient disc-like conformer of dendronized CTV from column to column during crown inversion. Solid state NMR experiments identified motional processes that allow such transfer. This unprecedented supramolecular chiral self-sorting will impact the creation of functions in complex systems.