Chirality Transition From Inorganic Cluster To Organic Ligands: Towards The Fabrication Of Dynamic Nanoflask With The Ability To Selectively Trap Chiral Guest

With the diffusion of the nanotechnology and chiral science, chiral nano-materials have captured imagination of researchers worldwide in the fields of chemistry, biology and materials science. Chemists provide particularly important contribution in the preparation of chiral nano-materials and the exploration of the functions of the resultant materials towards realizing ambitious applications. Up to now, a large amount of chiral nano-complexes have been synthesized through sophisticated molecular design. Among them, one of the most convenient methods for the creation of chiral nano-complexes is anchoring chiral organic ligands on the surface of nano-objects via covalent or non-covalent bonds. Correspondingly, the chiral transition phenomenon in this system has been of immense interest. This interest stems from the fact that how the chirality can be transfer from organic ligands to the nano-sized objects. Although, the development of a theory to depict and explain the physical phenomenon of these chiral systems is still at early stage, accepted point has been suggested that the densely adsorption of chiral ligands can create a local handedness, which endows the electronic structure of the nano-objectives with chirality. It is believed that new insight or understanding will be provided with deep development of this system. In great contrast, the chirality transition from nano-object to organic ligands has not attached intense attention. This inverse transition process will be a new area in the field of chiral science. A challenge task for chemistry in this field is the rational design and option of nano-objects with inherent chiral feature.Polyoxometalates(POMs), carrying early transition metal ions, is a kind of inorganic nano-objects with well-defined topology, mono-dispersity and versatile properties. The synthetic chemistry of polyoxometalates had over 100 year revolution. In this periods, chemists designed and synthesized a large amount of polyoxometalates with different topologies, components, size and properties. Recently, chiral POM frameworks have been synthesized by structure distortion, bond length alteration, substitution with other metals or ligands, the removal of one metal atom of POMs, and the introduction of chiral point with the framework. POMs can be considered as an ideal model system to investigate the chirality transition from POM to organic ligands because of their good water-solubility, high stability, and polyanionic feature.In the present work, we prepared a new kind of nano-complexes by grafting azobenzene-containing organic ligands on the surface of chiral POMs by ionic self-assembly strategy. We investigated the chirality transition from POMs to the azobenzene units of organic ligands and evaluated the chiroptical activity of the resultant complexes based on the reversible cis-trans isomerization of azobenzene units.First, we prepared two homochiral POM clusters(L- and D-POM). Meanwhile, we also designed and synthesized two kind of azobenzene-containing organic ligands. Ligand I is azobenzene-containing quaternary ammonium molecules with short alkyl tails, and Ligand II is azobenzene-containing quaternary ammonium molecules without tails. With these components in hand, we prepared a series of nano-complexes by grafting organic ligands on the surface of chiral POMs via electrostatic interactions. The as-prepared complexes were characterized by 1H NMR and MS spectra, elementary analysis and TGA.Second, we investigated the chirality transition from POMs to the ligands by comparing the CD signals of the two chiral complexes with same ligand but different chiral POMs. We also fabricated a kind of chiroptical nano-complexes based on the reversible cis-trans isomerization of azobenzene units, and detected the relationship between aggregation effect of the complexes and the chirality transition.Third, we evaluated the chiral recognition of the complexes on ?-cyclodextrin. The NMR data revealed clear host-guest interaction between D-complex and ?-cyclodextrin, however, similar interaction was not observed in D-complex. Apart from the chiral recognition, we also explored the selectively chiral trapping ability of the nano-complexes taking advantage of the assembly and disassembly of the complexes triggered by UV-irradiation.In summary, the present work provides a new model system which is favorable for the investigation of the chirality transition from nano-objective to organic ligands. The effect of the aggregation of the complexes and the distance between azobenzene unit and the POM cluster on the chirality transition were evaluated. More importantly, the kind of chiral nano-complexes show the ability to selectively recognize and capture chiral guests. We believe that the present study may provide new insight in the extension of the library of chiral nano-complexes and the development of new functions beyond the conventional work.