Luminescent platinum(II) complexes as building blocks for supramolecular assembly : from design and synthesis to photocontrolled self-assembly and organic resistive memory

Abstract

The interplay and balance of non-covalent interactions to construct supramolecular assemblies have been a fascinating and challenging research focus in the field of supramolecular chemistry. Square-planar platinum(II) complexes with high tendency to form metal-metal interactions represent an outstanding candidate to construct well-defined nanostructures. With the intriguing spectroscopic properties, platinum(II) complexes could serve as probes for monitoring the formation of molecular assemblies. In this thesis, several series of alkynylplatinum(II) complexes with tridentate 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine (bzimpy) ligand and cyclometalated platinum(II) complexes with bidentate functionalized thienylpyridine C^N ligands have been synthesized and characterized. With the proper choices of coordinating scaffolds, interesting self-assembly behaviors of the complexes have been explored by UV-vis absorption, emission and NMR spectroscopy, electron microscopy and dynamic light scattering.

A series of phosphole oxide-based alkynylplatinum(II) bzimpy complexes has been synthesized and characterized. Their self-assembly properties have been demonstrated in DMSO-water mixtures, and found to be driven by Pt-Pt, π-π stacking and hydrophobic-hydrophobic interactions upon increasing the content of the non-solvents. The alkyl chain length on bzimpy moiety and the size of the π-surface on alkynyl ligand have significant influences on the control of the overall self-assembly process. A morphological transformation from sheet-like to fibre-like, and to sphere-like aggregates was observed upon increasing the hydrophobic segments on the bzimpy moiety, suggesting the importance of rational molecular design in controlling the morphologies of aggregates.

A series of phosphole oxide-based cyclometalated platinum(II) b-diketonate complexes has also been synthesized and characterized. The existence of Pt-Pt interactions in the solid-state packing was revealed by X-ray crystallographic studies. Solvent-induced self-assembly of the complexes has been explored and a cooperative growth mechanism has been found to operate in the self-assembly process. Upon extending the aromatic π-surface of the phosphole oxide-based cyclometalating ligand, the complexes were found to exhibit tunable self-assembly properties in THF-water mixtures and different morphologies of aggregates, suggesting the importance of both π-π stacking and/or hydrophobic-hydrophobic interactions in controlling the self-assembly process. Binary resistance-switching behavior has been observed for this series of complexes in the organic resistive memory study. Ultra-long retention times of over 45,000 s for both OFF and ON states have been observed, indicating the low-misreading possibility of the devices.

A series of photoresponsive ortho-nitrobenzyl (o-NB) ester-based alkynylplatinum(II) bzimpy complexes has been synthesized. These complexes were found to show an enhancement of amphiphilicity by forming the hydrophilic benzoic acid-based moieties upon UV irradiation. The changes in the hydrophobicity/hydrophilicity balance and the solubility of the complexes have led to an enhancement of molecular associations, giving rise to the formation of sophisticated nanoaggregates.

A series of photoresponsive malachite green (MG)-based alkynylplatinum(II) bzimpy complexes has also been synthesized. The photoremoval of cyanide group has led to a geometry change at the alkynyl ligand and the generation of an extra positive charge, leading to the formation of larger and higher-ordered nanoaggregates. These molecular responses to the external stimulus, light, with distinct spectroscopic properties could provide deeper insights into the molecular engineering of stimuli-responsive materials in the application of material sciences. (491 words)