How does a supramolecular polymeric nanowire form in solution?

Abstract

Supramolecular polymerization follows a nucleation-elongation mechanism; however, after the supramolecular polymerization, what happens while forming nanowires? In order to answer this question, we systematically investigated the molecular packing and growth mechanism of a series of supramolecular polymeric nanowires. We carefully analyzed the molecular packing in the nanowires and proposed a packing model through X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). We used dynamic light scattering (DLS) to investigate in situ the growth process of these nanowires. The DLS results showed that after nucleation growth, a mesoscale assembly existed as an intermediate, which then formed the nanowires. This is the first example to directly monitor the growth of organic nanowires in solution. We also investigated the solvent effect on the self-assembly process of the side-chain functionalized monomers. Our investigation demonstrates that the weaker the interactions between the lateral groups, and the stronger the interactions of the lateral groups with the solvent are, the more obvious the tendency to 1D growth. Based on these results, we proposed that an “oriented-attachment” growth mechanism existed in this system after the supramolecular polymerization. Furthermore, single molecular nanowires and side-by-side attached single molecular nanowires were also observed through atomic force microscopy (AFM), which further supported the “oriented-attachment” mechanism. Accordingly, we demonstrate that after the supramolecular polymerization, “oriented attachment” growth mechanism is another critical process for the construction of large anisotropic organic assemblies, such as organic nanowires.