Supramolecular assembly induced chiral interface for electrochemical recognition of tryptophan enantiomers

The β-CD@PEI-Fc chiral interface was prepared based on the supramolecular host-guest interaction between ferrocene (Fc) grafted polyethyleneimine (PEI-Fc) and chiral β-cyclodextrin (β-CD). SEM results show that β-CD@PEI-Fc interface has a regular spatial structure, which can effectively distinguish tryptophan (Trp) enantiomers. Under the optimal conditions, differential pulse voltammetry shows that the peak current ratio (Id/Il) of Trp enantiomers can reach 2.84 at 15 °C. More interestingly, the β-CD@PEI-Fc/GCE exhibited chiral recognition of d-Trp and l-Trp via water contact angle measurements. There was a good linear relationship between the peak current and the concentration of Trp enantiomers in the range from 0.005 mM to 0.10 mM. Finally, the chiral interface can be applied for quick detection of the proportion of isomers in Trp racemic solution, which is very important for chiral recognition in racemic mixture of chiral compounds. Meanwhile, the β-CD@PEI-Fc/GCE showed good stability and reproducibility.

Enhanced Surface Ligands Reactivity of Metal Clusters by Bulky Ligands for Controlling Optical and Chiral Properties

Surface ligands play critical roles in determining the surface properties of metal clusters. However, modulating the properties and controlling the surface structure of clusters through surface-capping-agent displacement is challenging. Herein, [Ag₁₄ (SPh(CF₃ )₂ )₁₂ (PPh₃ )₄ (DMF)₄ ] (Ag₁₄ -DMF; DMF=N,N-dimethylformamide), with weakly coordinated DMF ligands on surface silver sites, was synthesized by a mixed-ligands strategy. Owing to the high surface reactivity of Ag₁₄ -DMF, the surface ligands are labile, easily dissociated or exchanged by other ligands. Based on the enhanced surface reactivity, easy modulation of the optical properties of Ag₁₄ by reversible "on-off" DMF ligation was realized. When chiral amines were introduced to as-prepared products, all eight surface ligands were replaced by amines and the racemic Ag₁₄ clusters were converted to optically pure homochiral Ag₁₄ clusters as evidenced by circular dichroism (CD) activity and single-crystal X-ray diffraction (SCXRD). This work provides a new insight into modulation of the optical properties of metal clusters and atomically precise homochiral clusters for specific applications are obtained.

Cations Controlling the Chiral Assembly of Luminescent Atomically Precise Copper(I) Clusters

Chiral assembly and asymmetric synthesis are critically important for the generation of chiral metal clusters with chiroptical activities. Here, a racemic mixture of [K(CH₃ OH)₂ (18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (1⋅CH₃ OH) in the chiral space group was prepared, in which the chiral red-emissive anionic [Cu₅ (S^t Bu)₆ ]^- cluster was arranged along a twofold screw axis. Interestingly, the release of the coordinated CH₃ OH of the cationic units turned the chiral 1⋅CH₃ OH crystal into a mesomeric crystal [K(18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (1), which has a centrosymmetric space group, by adding symmetry elements of glide and mirror planes through both disordered [Cu₅ (S^t Bu)₆ ]^- units. The switchable chiral/achiral rearrangement of [Cu₅ (S^t Bu)₆ ]^- clusters along with the capture/release of CH₃ OH were concomitant with an intense increase/decrease in luminescence. We also used cationic chiral amino alcohols to induce the chiral assembly of a pair of enantiomers, [d/l-valinol(18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (d/l-Cu_5V ), which display impressive circularly polarized luminescence (CPL) in contrast to the CPL-silent racemic mixture of 1⋅CH₃ OH and mesomeric 1.

Kernel Homology in Gold Nanoclusters

Homology is well known in organic chemistry; however, it has not yet been reported in nanochemistry. Herein, we introduce the concept of kernel homology to describe the phenomenon of metal nanoclusters sharing the same "functional group" in kernels with some similar properties. To illustrate this point, we synthesized two novel gold nanoclusters, Au₄₄ (TBBT)₂₆ and Au₄₈ (TBBT)₂₈ (TBBTH=4-tert-butylbenzenethiol), and solved their total structures by X-ray crystallography, which reveals that they have the same Au₂₃ bi-icosahedron capped with a similar bottom cap (Au₆ and Au₈ , respectively) in the kernels. The two novel gold nanoclusters, together with the existing Au₃₈ (PET)₂₄ nanocluster (PETH=phenylethanethiol), have the same "functional group"-Au₂₃ -in their kernels and have some similar properties (e.g., electrochemical properties); therefore, they are comparable to the homologues in organic chemistry.