M / P Helicity Switching and Chiral Amplification in Double-Helical Monometallofoldamers.

Short-stranded double-helical monometallofoldamers capable of M / P -switching were constructed by the complexation of two strands, each with two L-shaped units linked by a 2,2'-bipyridine, with a Zn(II) cation. The helix terminals of the "double-helical form" folded by π-π interactions can unfold in solution to equilibrate with the "open forms" that are favored at higher temperatures. Interestingly, the helical chirality of the monometallofoldamers with chiral side chains induced a single-handed helix sense and controlled M / P -switching depending on achiral solvent stimuli. For instance, the ( M )-helicity was favored in nonpolarized solvents (toluene, hexane, Et 2 O), whereas the ( P )-helicity was favored in Lewis basic solvents (acetone, DMSO). Circular dichroism (CD) and rotating-frame overhauser enhancement spectroscopy (ROESY) measurements revealed that the conformational change of the chiral side chains due to interaction of Lewis basic solvents with the double helices induced helicity bias. These novel double-helical monometallofoldamers possessed a stable helical structure and exhibited switchable chiroptical properties ( g abs ∼ 10 -3 -10 -2 ). In addition, the chiral strand exhibited chiral transfer and amplification abilities through the formation of chiral heteroleptic double-helical monometallofoldamers when mixed with an achiral strand.