Organic and Organometallic Nanofibers Formed by Supramolecular Assembly of Diamond-Shaped Macrocyclic Ligands and Pd~(II) Complexes

摘要

With recent advances in supramolecular synthetic methods for nano- to microsized aggregates, design guidelines for component molecules have been developed for constructing various functional aggregates such as vesicles, micelles, membranes, fibers, and tubes. Among the various types of molecular building blocks, rigid and flat macrocyclic compounds show promise for one-dimensional aggregates such as fibers and tubes through face-to-face association of their macrocyclic skeletons. Furthermore, the macrocyclic cavities can be designed so as to recognize molecules and ions depending on their size and shape, and therefore their aggregates potentially provide platforms for guest assembly. In particular, stackable macrocyclic ligands with inward metal binding sites would serve as a template for the formation of metal arrays within the tubular nano-space. We previously reported that a diamond-shaped macrocycle 2 with two inward phenanthroline ligands and two outward hexyloxy side-chains provides an excellent platform for metal arrangement to form homodinuclear and heterodinuclear metallomacrocycles. Herein, aiming at the construction of functional nanofibers capable of metal arrangement, we synthesized macrocycle 1 with the same macrocyclic skeleton as 2 and six long alkyl side-chains (Figure 1 a). In this study, we found that the macrocycle 1 and its dinuclear Pd~(II) complex form organic and organometallic nanofibers, respectively, through face-to-face self-assembly in low polar solvents (Figure 1b). Their fibrous structures were microscopically observed, and the component of the Pd~(II)-containing aggregate was determined by energy dispersive X-ray spectroscopy (EDS) mapping on a scanning transmission electron microscope (STEM). So far, only a few examples of metal arrangement within macrocycle-based nanofibers and nanotubes have been reported. Furthermore, the aggregation behavior of metal complexes of 1 is highly dependent on the coordination geometries of metals. The square-planar Pd~(II) complexes of 1 orderly stack to form metallo-nanofibers, whereas the tetrahedral Zn~(II) complexes of 1 form spherical aggregates.