Design and synthesis of oligophenylene ethynylene (OPE)-derived alkynylplatinum(II) terpyridine complexes : from photophysics to supramolecular tessellation, metallogels and supramolecular nanostructures

Abstract

The construction of complex supramolecular architectures from the self-assembly of geometrically simple molecules has been of particular interest in the field of supramolecular chemistry. Square-planar d8 platinum(II) polypyridine systems represent one of the ideal candidates for the formation of nanostructures due to their strong propensity to exhibit Pt···Pt interactions accompanied by intriguing spectroscopic properties. Despite this, the formation of morphologies with higher complexity has remained a challenge as it would require a delicate balance and control in the microenvironment and a judicious molecular design. With a delicate balance of different non-covalent interactions through rational molecular design, or with the incorporation of different self-assembling moieties, a wide variety of nanostructures could be constructed in a more controlled and predictable manner.

A series of symmetrical dinuclear alkynylplatinum(II) terpyridine complexes with varying OPE-derived backbones was synthesised and studied. Structural analysis has been performed on the complexes and one of the them showed supramolecular tessellation properties. Through the delicate interplay between the metal···metal and π–π stacking interactions and the immiscibility and subsequent phase separation of the tailored hydrophobic and hydrophilic side chains, an unusual geometry analogous to a rhombitrihexagonal Archimedean tiling was observed. A possible key to the sophisticated packing is the topological transition into circular trimers, which are noncovalently interlocked by Pt···Pt and π–π interactions, thereby allowing for cofacial stacking in a prismatic assembly. Furthermore, the co-assembly properties of two different platinum(II) amphiphiles were systematically studied, which resulted in a symmetry lowering of the regular motifs with an orthorhombic tessellating lattice under the molar ratio of 2:1, illustrating the potentials of supramolecular engineering in creating complex self-assembled architectures of soft materials.

Apart from exploring the symmetrical dinuclear platinum(II) system, a series of mononuclear complexes and unsymmetrical dinuclear alkynylplatinum(II) terpyridine complexes with OPE-derived backbone with a variation of the lengths of alkoxy chains and OPE backbones was synthesised. The morphological properties, packing lattice and gelation behaviours of the mononuclear complexes were systematically studied and the thermodynamic parameters and packing lattices of the dinuclear complexes were also investigated, providing further insights into the criteria, delicate balance and roles of the non-covalent interactions in constructing sophisticated supramolecular architectures. Metallogel and nanotube formation could be observed upon a delicate balance of the intermolecular interactions by altering the lengths and strucutural dimensions accordingly.

Fascinated by the self-assembly properties of C60 and metalloporphyrin molecules, a series of C60-containing dinuclear alkynylplatinum(II) terpyridine complexes with OPE-derived backbone was synthesised. The photophysical properties, morphological properties and packing lattices were rationally studied to provide insights into the effect of the incorporation of the fullerene moieties. Another series of porphyrin-containing dinuclear and tetranuclear alkynylplatinum(II) terpyridine complexes was synthesised and their photophysical properties and morphological properties were systematically studied, accompanied by computational modelling to understand the dimer conformation. The axial coordination properties of selected complexes were also investigated with a variety of N-donor ligands, resulting in morphological changes. The interesting spectroscopic properties and intriguing self-assembly behaviours illustrate the versatility and tunability of the incorporation of self-assembling moieties into the structural design.