Design and synthesis of alkynylplatinum(II) complexes with tridentate N-donor ligands as building blocks for supramolecular assemblies and metallofoldamers : from photophysics to the control of self-assembly properties and nanostructures

Abstract

The construction of well-defined and high-ordered supramolecular hierarchies remains fascinating and challenging in the research on supramolecular chemistry. Rational design of the molecular structures and a delicate balance between multiple non-covalent interactions in the microenvironment are crucial to the self-assemblies and their morphologies. In addition, square-planar platinum(II) system represents an attractive candidate for the construction of well-defined nanostructures due to their propensity to exhibit directional non-covalent metal-metal interactions. Not only could the molecules be aligned by these non-covalent interactions, the intriguing spectroscopic responses attributed to the metal-metal interactions could also serve as a spectroscopic probe for the assemblies. In this thesis, several series of platinum(II) complexes with terpyridine and 2,6-bis(N-alkylbenzimidazol-2′-yl)pyridine (bzimpy) as the tridentate N-donor ligands have been synthesized and characterized. Through the judicious choice of the ligand scaffolds including the shape-persistent oligo(p-phenylene ethynylene) and the foldable oligo(m-phenylene ethynylene) derivatives, the distinctive self-assembly behaviors resulting from these classes of complexes have been explored by NMR, electronic absorption and emission spectroscopy, electron microscopy and dynamic light scattering analysis. More importantly, the non-covalent Pt···Pt interactions have been found to play a crucial role in governing the supramolecular assemblies.

The oligo(p-phenylene ethynylene)-containing alkynylplatinum(II) bzimpy complexes were found to exhibit intriguing self-assembly properties and morphological transformation from plates to fibers and to spherical aggregates upon increasing solvent polarity, which were attributed to the switching-on of the Pt···Pt interaction. Moreover, the hydrophobicity and hydrophilicity of the complexes have been systematically modified to study the effect on the self-assembly mechanisms. A delicate balance between non-covalent interactions has been found to significantly influence the cooperativity of the self-assemblies, allowing the present studies to provide a correlation between molecular structures and self-assembly mechanisms, which have been rarely explored and reported in the literature.

A series of oligo(m-phenylene ethynylene)-containing alkynylplatinum(II) bzimpy complexes was found to exhibit folding processes directed by non-covalent Pt···Pt interactions. The conformations of the complexes have been explored, which were found to result in distinctive molecular association modes, leading to the formation of various kinds of morphologies. The optimal chain length of five repeating units has been determined for the formation of a single-turn helix. The hydrophobicity and hydrophilicity of the complexes, together with the optimal chain length and the involvement of Pt···Pt interactions, were found to govern the supramolecular assemblies of this class of complexes, leading to the formation of high-ordered hierarchy such as metallogel.

A series of oligo(2,6-pyridylene ethynylene)-containing alkynylplatinum(II) terpyridine/bzimpy metallofoldamers has also been synthesized in order to further investigate the potential applications of metallofoldamers that were stabilized by Pt···Pt interaction. Apart from the control of the folding/unfolding processes by solvent compositions and temperatures, this class of metallofoldamers was also found to exhibit reversible folding/unfolding behaviors mediated by the addition of acids/bases due to the incorporation of the acid-sensitive oligo(2,6-pyridylene ethynylene) derivatives. Such systematic responses to external stimuli with distinctive spectroscopic characteristics could lead to the multi-dimensional control of the folding/unfolding processes, which could provide further insights into the molecular engineering of stimuli-responsive materials for potential applications in material sciences.