Precisely Confined AIEgens in Giant Imidazolium–Terpyridinyl Cuboctahedra with Enhanced Fluorescence for Single Supramolecule

Abstract

hindrance, [7] covalent bonds restriction, [8] tuning solvents polarity, [9] concentrations, covalent interactions [10] and external stimuli. [11] Among these, controlling the intermolecular distances of AIE molecules is a good way to lock the molecules from each other through cultivating single crystals or ordered solids to study the solid-phase properties, [12] tuning the concentration of solution by mixing solvents with different solvency and use density functional theory (DFT) calculation to get the average intermolecular distances. [6c] Recently, confining AIE molecules in limited space such as supramolecular hosts, [13] metal-organic frameworks (MOFs), [11d,14] covalent organic frameworks (COFs), [15] and micelles [16] have been intensively reported. These host-like spaces especially discrete cage structures offer space to encapsulate one or more AIE molecules to push them together, but there is still a challenge for the exploration of anchoring quantitative fluorescent molecules in a giant cavity which could mimic the biomolecules with rigid skeleton and flexible luminescent moieties.Coordination-driven self-assembly provides a powerful tool to access cage-like supramolecules, due in part, to ligand−metal directivity and shape persistent frameworks. [17] Recently, based on the hollow interiors of these artificial 3D cages, the focus of the research in this area has shifted to the functions and applications in confined catalysis, [18] guest encapsulation, [19] and enzyme mimics. [20] Particularly, Prof. Makoto Fujita has systematically developed a class of fascinating spherical cages combining 12 Pd 2+ or 12 Pt 2+ ions with 24 pyridinyl ligands that can be exo-and endo-functionalized. [21] Utilizing this type of cage, Prof. Peter Stang reported that 24 tetraphenylethene (TPE) molecules were placed in the cages to enhance the fluorescence emission, which provided an excellent strategy for the investigation of the effect of intermolecular distances on the AIE phenomenon. [22] However, the nanospheres have low quantum yields due to fluorescence quenching by Pd and Pt. Meanwhile, owing to the cavity size of the cage is small and there are so many branched groups, it is too crowded to adjust the intermolecular distances. The investigation of the relationship between the intermolecular distances and the fluorescence emission efficiency of AIE molecules necessitates a supramolecular cage with a larger cavity and fewer ligands.We have previously reported the quantitative self-assembly of imidazolium-based, cuboctahedron-shaped cage with a 6.5 nm inner diameter (7.3 nm outer diameter) that is consist To study the fluorescent behavior of quantitatively confined aggregationinduced emission (AIE) molecules in a giant cavity, herein a new type of coordination-driven cuboctahedron cage is presented, with 12 AIE molecules anchored endohedrally linked with different-length alkyl chains. The cage with confined tetraphenylethylene molecules shows strong fluorescence in highly diluted conditions (4 × 10 −7 m). The fluor...