Helical Rosette Nanotubes with Tunable Chiroptical Properties

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Abstract
On the basis of transmission electron microscopy (TEM), dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and circular dichroism (CD) studies, compound 1 was shown to exist mainly in two states: (a) At high concentration (≥1 mM, in methanol), 1 undergoes hierarchical self-assembly to generate rosette nanotubes with ∼4 nm diameter and a concentration-dependent hydrodynamic radius in the range 10−100 nm. Under these conditions, addition of a chiral amino acid promoter (l-Ala), that binds to the crown ether moiety of 1 via electrostatic interactions, promotes a rapid transition (k0 ≈ 0.48 s-1, for [1] = 0.046 mM, [l-Ala] = 2.8 mM) from racemic to chiral rosette nanotubes with predefined helicities as indicated by the resulting induced circular dichroism (ICD). (b) At low concentration (≤0.04 mM, in methanol), 1 exists mainly in a nonassembled state as shown by TEM and DLS. Addition of l-Ala in this case triggers a relatively slow (k0 ≈ 0.07 s-1 for [1] = 0.04 mM, [l-Ala] = 2.4 mM) sequence of supramolecular reactions leading to the hierarchical self-assembly of rosette nanotubes with predefined helicities. Under both conditions a and b, the kinetic data unveiled the intrinsic ability of the rosette nanotubes to promote their own formation (autocatalysis). The degree of chiral induction was found to depend dramatically upon the chemical structure of the promoter. This process appears also to follow an all-or-none response, as the vast majority of the crown ether sites must be occupied with a promoter for a complete transition to chiral nanotubes to take place. Finally, both supramolecular pathways a and b offer an efficient approach for the preparation of helical rosette nanotubes with tunable chiroptical properties and may also be viewed as a process by which a predefined set of physical and chemical properties that characterizes a molecular promoter is expressed at the macromolecular level.