Design and synthesis of alkynylplatinum(II) terpyridine complexes and their polymer derivatives : from photophysics and energy transfer sensitization to supramolecular assembly

Abstract

A series of amphiphilic alkynylplatinum(II) terpyridine-containing polystyrene complexes with different functionalites on the terpyridine ligand has been successfully synthesized and characterized. Their UV-vis absorption and emission properties have been investigated. The self-assembly behaviors of the platinum(II) polystyrene complexes in different solvent systems including water-dioxane mixtures and hexane-dioxane mixtures have been systemically explored. The complexes were found to undergo a self-assembly process into various superstructures upon addition of water or hexane, along with remarkable changes in color, luminescence properties and morphologies, as revealed by UV-vis absorption and emission spectroscopy, electron microscopy and dynamic light scattering (DLS). One of the complexes was found to exhibit an interesting morphological transformation from a spherical multilayer structure to a compact spherical micelle on further increasing the hexane content. With the aid of different experiments, Pt···Pt, π-π stacking and hydrophobic-hydrophobic interactions, as well as amphiphilicity and steric bulkiness of the complexes were found to play substantial roles in the formation of superstructures and the turning-on/off of triplet metal-metal-to-ligand charge transfer (3MMLCT) emission or exciermic emission of the complexes. In addition, a series of alkynylplatinum(II) terpyridine complexes with tetraphenylethylene (TPE) moiety has been synthesized and characterized. The corresponding UV-vis absorption and emission properties have been explored. The introduction of the TPE moiety has given rise to the typical aggregation-induced emission (AIE) behavior upon self-assembly, which was shown to sensitize the 3MMLCT emission arising from the Pt···Pt and/or π-π stacking interactions of the platinum(II) terpyridine moieties. Interestingly, dual-emissive properties have been observed for the complexes with a lengthening of the spacers between the TPE unit and the platinum(II) terpyridine unit. Furthermore, the AIE behaviors could be hindered or restored via the modification of the molecular structures. Apart from the spectroscopic changes, this class of platinum(II) complexes also displayed different superstructures (including nanorods, nanospheres, nanoleaves and nanowires) upon self-assembly process. The amphiphilicity and the steric bulkiness were found to play crucial roles in the formation of the superstructures. The importance of the directional Pt···Pt and/or π-π stacking interactions on the spectroscopic properties and the construction of distinctive superstructures has been examined by UV-vis absorption and emission spectroscopy and electron microscopy. Apart from polystyrene, conjugated polymers have also been explored for their incorporation into the platinum(II) terpyridine system. A series of alkynylplatinum(II) terpyridine-containing conjugated polymers and two reference alkynylplatinum(II) terpyridine complexes have been synthesized and characterized. The platinum(II)-containing conjugated polymers were prepared through “click” reaction. The photophysical and luminescence properties of the platinum(II)-containing polymers, platinum(II) complexes and the corresponding organic polymers have been investigated. Dual-emissive behaviors have been observed for all of the platinum(II)-containing conjugated polymers, which gave singlet intraligand (1IL) fluorescence originated from the polymer backbone and 3MMLCT emission arising from the significant intramolecular Pt···Pt and/or π-π stacking interactions between the platinum(II) pendants. Different Fӧrster resonance energy transfer (FRET) efficiencies and thermo-responsive emission changes have been observed from the platinum(II)-containing conjugated polymers.