Molecular design and synthesis of artificial double helices.

This account describes novel artificial double helices recently developed by our group. We have designed and synthesized the double helices consisting of two complementary, m-terphenyl-based strands that are intertwined through chiral amidinium-carboxylate salt bridges. Due to the chiral substituents on the amidine groups, the double helices adopted an excess one-handed helical conformation in solution as well as in the solid state. By extending the modular strategy, we have synthesized double helices bearing Pt(II) linkers, which underwent the double helix-to-double helix transformations through the chemical reactions of the Pt(II) complex moieties. In addition, artificial double-stranded metallosupramolecular helical polymers were constructed by combining the salt bridges and metal coordination. In contrast to the design-oriented double helices based on salt bridges, we have serendipitously developed a spiroborate-based double helicate bearing oligophenol strands. The optical resolution of the helicate was successfully attained by a diastereomeric salt formation. We have also unexpectedly found that oligoresorcinols consisting of a very simple repeating unit self-assemble into double helices with the aid of aromatic interactions in water. Furthermore, a bias in the twist sense of the double helices can be achieved by incorporating chiral substituents at both ends of the strands.