Helical nanostructures of an optically active metal-free porphyrin with four optically active binaphthyl moieties: Effect of metal-ligand coordination on the morphology

摘要

An optically active metal-free porphyrin (1) with four chiral binaphthyl units attached at the meso-phenyl substituents through crown ether moieties has been designed, synthesized, and characterized. Its self-assembly behavior in the absence and presence of K~+ was comparatively investigated by electronic absorption and circular dichroism (CD) spectra, transmission electron microscopy (TEM), scanning electron microscopy (SEM), atom force microscopy (AFM), and energy-dispersive X-ray (EDX) spectroscopy. In the absence of K ~+, metal-free porphyrin self-assembles into nanoparticles depending mainly on the van der Waals interaction among neighboring metal-free molecules. In the presence of K~+, additionally formed metal-ligand K-O _(crown) coordination bonds between K+ and crown units of (R)-and (S)-1 molecules, together with chiral discrimination of chiral side chains and intermolecular van der Waals interactions, induce a righthanded and left-handed helical arrangement in a stack of (R)-and (S)-1 molecules, respectively, with an ordered "head-to-tail" internal molecular arrangement. This then further hierarchically self-assembles into highly ordered fibrous nanostructures with a helicity opposite to that of the original porphyrin stack. This clearly reveals the effect of the metal-ligand coordination bonding interaction on the morphology and handedness of self-assembled nanostructures.