Design, Synthesis, and Self-Assembly of Supramolecular Fractals Based on Terpyridine with Different Transition Metal Ions

Fractals have existed in a variety of natural forms, and the concept of fractal geometry has been developed in many areas by humankind, including art, economics, mathematics, physics, chemistry, material science, and etc. In the field of chemistry, researchers have endeavored great efforts to express the concept using molecules either by theoretical prediction or using synthetic methods in the past few decades, and one successful example is the “Sierpiński triangle”. Challenge still remains to achieve other fractal geometries as well as high generation of fractals with precisely controlled shape and size. Coordination-driven self-assembly has witnessed a wide array of well-defined metallo-supramolecular architectures constructed from organic ligands with metal ions. Among the diverse library of organic ligands, 2,2\u27:6\u27,2 -terpyridine (tpy) has gained extensive attention as a tridentate motif because of its excellent complexing ability with different metal ions. The metal ions adopted to construct large discrete structures through direct self-assembly, however, were limited to three metal ions with highly reversible coordination, viz., Cd(II), Zn(II), and Fe(II). In this dissertation, a series of metallo-supramolecules (G1-G5) with fractal features that is different from Sierpiński triangle has been successfully synthesized using tpy-based ligands under the principle of coordination-driven self-assembly. Different synthetic approaches to ligands have been explored and demonstrated in depth. Additionally, many divalent transition metal ions (Mn, Fe, Co, Ni, Cu, Zn, and Cd) have been successfully introduced into the fractal G4 to significantly broaden the spectrum of connectivity in this field. Moreover, three different connectivities have been incorporated into the fractal G5 using a step-wise strategy of combining self-recognition and self-assembly. In particular, the effect of metal ions on the assemblies generated has been extensively explored and two structures have been achieved using different metal ions while without changing the ligand in the self-assembly. Characterization of those giant and discrete architectures includes 1D and 2D NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), traveling-wave ion mobility mass spectrometry (TWIM-MS), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM). Photophysical, electrochemical properties, and dynamic feature of the ligand exchange process have been investigated as well. In addition, several supramolecules displayed interesting hierarchical self-assembly behaviors either at solid/liquid interface or in solution based on their well-defined scaffolds, which may provide more opportunities for developing novel materials at supramolecular level