Luminescent platinum(II) complexes with tridentate N-donor ligands as building blocks for supramolecular-assembled 1D materials and metallofoldamers

Abstract

Nature inspires us with the construction of fascinating, complicated and well-organized supramolecular architectures which are of tremendous importance in our biological systems. Scientists, on one hand, would like to discover the mysteries behind the building of biomacromolecules, while on the other hand, they also aim at the developing of artificial systems that could mimic what nature does and even go beyond that. However, the construction of high-ordered supramolecular hierarchies still remains challenging with much room to be explored. A rational design of molecular structures and a wise interplay of a range of non-covalent interactions are crucial in determining the self-assembly properties and the morphologies formed. In parallel, square-planar platinum(II) complexes have demonstrated their potential in the construction of nanostructures based on their strong propensity to form directional metalmetal interactions. In particular, intriguing spectroscopic properties of platinum(II) polypyridine complexes could serve as spectroscopic probes to inform the degree of aggregation, allowing in-depth studies on the aggregation properties of a supramolecular system. In this thesis, several series of platinum(II) complexes with terpyridine and 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) as the tridentate N-donor ligands have been synthesized and characterized. On top of their basic photophysical properties, their distinctive self-assembly behaviors have been explored by UV-vis absorption, emission, NMR, X-ray diffraction spectroscopies, electron and confocal fluorescence microscopies and mass spectrometry.

A series of double complex salts (DCSs) based on the platinum(II) bzimpy system has been synthesized. They were found to show aggregation behaviors in high water content and the opposite charged complex ions were found to exist in alternate stacking fashion to form 1D infinite columns in their lattice packing. The ancillary ligands have been systematically varied to study the steric effect on the assembled structures. Chiral alkyl chains were also incorporated to investigate supramolecular chirality. Modification on the charge combinations of the complex ions has demonstrated the possibility in preparation of DCSs of different charges. This represented a rare example of water-soluble DCS system with platinum(II) complexes containing tridentate N-donor ligands that has demonstrated its potential application in molecular electronics.

A series of platinum(II) complexes containing hexameric phenylene-ethynylene macrocycles has been synthesized. They were found to exhibit intriguing aggregation behaviors and distinctive morphological structures in both polar and non-polar media, ascribed to the formation of Pt(II)···Pt(II) interactions. The steric effect and hydrophobicity of the platinum(II) moieties to the overall self-association properties have been explored. In addition to the solution-state aggregation, a detailed investigation of the solid-state lattice packing has been carried out and the metalmetal interaction has been found to play a crucial role in governing the 1D supramolecular assemblies.

A series of oligo(m-phenylene ethynylene) derivatives containing trinuclear platinum(II) terpyridine complexes has been synthesized. The effect of solvation and temperature on the folding processes, stabilized by Pt(II)···Pt(II) interactions, has been examined. The steric bulkiness and hydrophobicity of the platinum(II) terpyridine moieties has been modified to study their influence on the folding processes. The metallofoldamer was found to exhibit interesting conformation-dependent host-guest interactions with charge-neutral platinum(II) guest molecules.