Design, synthesis and functionalization of alkynylplatinum(II) complexes with tridentate N-donor ligands : from photophysics to the control of supramolecular self-assembly properties for photochemical reactions

Abstract

The square-planar alkynylplatinum(II) complexes with tridentate N-donor pincer ligands have aroused tremendous research interests owing to their enriched and tunable spectroscopic properties as well as the pronounced tendency to form supramolecular assemblies with the directional Pt∙∙∙Pt interactions serving as additional driving forces for the stabilization of the assemblies. More interestingly, the preorganization of this class of complexes upon self-assembly could be utilized to assist the photoinduced topochemical reactions which require a close proximity of the reactive moieties. By judicious molecular design, several series of alkynylplatinum(II) polypyridine complexes with the incorporation of various photoresponsive moieties have been successfully synthesized and structurally characterized. Their photophysical, self-assembly and photochemical properties have been investigated by the UV–vis absorption and emission spectroscopies, electron microscopies and dynamic light scattering analysis. In particular, it has been demonstrated that the directional intermolecular Pt∙∙∙Pt interactions played an essential role in governing the self-assembly behaviors with the preorganization of the complexes for ready photochemical reactions. Moreover, the self-assembly behaviors of the complexes have been found to be perturbed by the in situ photochemical reactions, providing important insights into the judicious design of molecular structures with desirable self-assembly properties that could be modulated by the photochemical activities. A series of diacetylene-containing alkynylplatinum(II) complexes with terpyridine and bzimpy pincer ligands was found to form supramolecular self-assemblies in tetrahydrofuran solutions at high concentrations, displaying intriguing spectroscopic features with the formation of different high-ordered nanostructures. Facilitated by the interplay of multiple non-covalent interactions, particularly the directional Pt∙∙∙Pt interactions, the preorganization of the molecules upon self-assembly has allowed the ready photopolymerization of the diacetylene moiety. Such topochemical photopolymerization was found to perturb the self-assembly behaviors of the complexes as revealed by electron microscopy. Moreover, a series of mononuclear and dinuclear anthracene-containing alkynylplatinum(II) terpyridine complexes was found to display intriguing self- assembly and photochemical properties. With more rigid structures and extended π-surfaces, the dinuclear complexes had more pronounced tendency to self-assemble in solutions with drastic color changes from yellow to green upon cooling, indicating the presence of Pt∙∙∙Pt interactions. In addition, the photooxygenation of anthracene moiety upon photoirradiation has been shown to alter the self-assembly behaviors of the complexes, leading to morphological changes from well-defined twisted fiber-like structures to disordered aggregates. By taking advantage of the intermolecular Pt∙∙∙Pt interactions, the changes in the spectroscopic properties and self-assembly behaviors of these complexes upon photooxygenation could be readily visualized. The self-assembly properties of a series of dinuclear coumarin-containing alkynylplatinum(II) terpyridine complexes with p-phenyleneethynylene backbone have been investigated upon changing the solvent compositions, in which the complexes exhibited drastic color changes from yellow to orange and to red as well as luminescence enhancements owing to the formation of intermolecular Pt∙∙∙Pt interactions. The photodimerization of coumarin moiety was found to only occur when the complexes were preorganized with the formation of intermolecular Pt∙∙∙Pt interactions, demonstrating the essential role of the Pt∙∙∙Pt interactions in governing the supramolecular aggregates for ready photodimerization. Furthermore, the photodimerization was shown to perturb the self-assembly behaviors of the complexes with changes in their aggregate morphologies.