Design, synthesis and photophysics of alkynylplatinum(II) terpyridine complexes : from organic/inorganic hybrids to supramolecular assemblies and nanostructures

Abstract

Several series of organic-inorganic hybrid-derived alkynylplatinum(II) terpyridine complexes have been synthesized and characterized. The supramolecular and photophysical properties of the complexes have been systematically investigated. Through the rational design and delicate balance of molecular interactions, some of the alkynylplatinum(II) terpyridine complexes functionalized with polyhedral oligomeric silsesquioxane (POSS) were found to exhibit molecular association properties with possible morphological transformation, ranging from spheres to plates and nano-rings to rods, in response to solvent polarities. The photophysical properties associated with the morphological transformation processes have been investigated, revealing the development of metal–metal-to-ligand charge transfer (MMLCT) transitions, indicative of the stabilization of nanostructures via the directional Pt···Pt interactions. The manipulation of self-assembly properties of the system has been demonstrated via the structural modification of the alkynyl or pincer ligands, in which various molecular interactions, ranging from π–π stacking to hydrophobic–hydropobic interactions, were found to affect the molecular packing, with the correlation between the molecular structures and aggregation properties being systematically investigated. Furthermore, the mechanisms and thermodynamics of the molecular association processes were further explored via another class of cyclotetrasiloxane-appended alkynylplatinum(II) terpyridine complexes. This series of complexes were demonstrated to self-associate into various supramolecular architectures, ranging from twisted rods to plates, depending on the solvent polarities. Such molecular association processes were further investigated via a nucleation-elongation model, revealing the isodesmic growth mechanism in non-polar media, with the underlying thermodynamic parameters determined. In addition, the influence of electrostatic interactions on the aggregation processes has also been explored, in which a series of siloxane-derived complexes has been modulated with various charged alkynyl domains. Particularly, the zwitterionic complex was demonstrated to exhibit remarkable color changes and distinctive nanostructures with interesting multi-stage morphological transformation from spheres to nanoplates in response to solvent conditions. Such assembly mechanism was found to be associated with an intriguing aggregation-partial deaggregation-aggregation process via the interplay and fine balance of various intermolecular interactions. In contrast to the common strategies in constructing nanomaterials, stereoisomeric domains has been exploited to further cast diversity on the supramolecular assembly behaviors. A series of stereospecific dinuclear alkynylplatinum(II) terpyridine complexes have been synthesized and characterized, with the underlying structural correlation among the isomers and the assembly properties investigated. In particular, the dinuclear platinum(II) complexes with bi-functionalized POSS backbone were shown to exhibit cis and trans isomerism. Corresponding microscopy studies revealed significantly different morphologies, despite their structural similarity, indicative of the influence of isomerism in regulating the molecular association processes. The effects of molecular configuration in governing the self-association behaviors have been further explored via the substitution of the stereoisomeric domain with the closo–carborane isomers. The associated photophysical and self-assembly properties have generally provided further insights and in-depth understanding towards the supramolecular chemistry of the isomeric complexes.