Helicene-Grafted Silica Nanoparticles Capture Hetero-Double-Helix Intermediates during Self-Assembly Gelation.

A mixture of a pseudoenantiomeric ethynylhelicene (M)-tetramer and a (P)-pentamer forms a hetero-double-helix in a solution, which self-assembles and gelates solvents. When gelation was conducted in the presence of chiral silica (P)-nanoparticles grafted with (P)-helicene, the resulting hetero-double-helix intermediate was adsorbed on the (P)-nanoparticles, and was removed from the solution by aggregation and precipitation. The resulting precipitates contained only the hetero-double-helix, not random coil or clusters of the hetero-double-helix. (P)-Nanoparticles did not extract the hetero-double-helix from the self-assembly gels. The hetero-double-helix was then isolated by liberating it from the precipitates in 2-bromopropionic acid, and was crystallized from the solution. The crystalline hetero-double-helices were isolated for several other combinations of pseudoenantiomeric ethynylhelicene oligomers.