Study On The Optical Properties Of Silver Chalcogenolate Cluster-Based Metal-Organic Framework Materials Based On Pyridine Ligands Construction

Silver chalcogenolate clusters with precise atomic structure are an important branch of coinage metal clusters,which have abundant structures and excellent physical and chemical properties,and have received extensive attention in recent years.However,the instability and low luminescence quantum efficiency of silver chalcogenolate clusters greatly limit their development.Therefore,how to improve its stability and luminescence intensity is a hot research topic for scientists.Assembling discrete silver chalcogenolate clusters into framework materials with appropriate rigid organic bridging ligands can not only improve the stability,but also reduce the non-radiative transition,thereby increasing the luminescence intensity.Silver chalcogenolate cluster-based metal-organic frameworks（SCC-MOFs）,as a new member of the MOFs family,exhibit excellent photoluminescence properties due to various charge transfer pathways such as ligand-to-metal（LMCT）and metal-to-ligand charge transfer（MLCT）,and their optical properties can be modulated by changing the structure of the clusters and the type of bridging ligands.Circularly polarized luminescence（CPL）has received a lot of attention from researchers because of its potential applications in three-dimensional display,information encryption,asymmetric synthesis,and chiral optical sensing.Combining chiral components with luminescent components is an effective way to obtain materials with CPL activity.Therefore,a new class of functional materials with CPL activity is expected to be obtained by introducing chiral components into the non-chiral luminescent SCC-MOFs system.In addition,the precise atomic structure helps to analyze the intrinsic connection between structure-chiral optical properties at the atomic level.SCC-MOFs materials not only have the properties of individual metal clusters and MOFs,but the combination of the two can also achieve bi-directional performance enhancement,providing a broader platform for the systematic study of the relationship between structure and properties.The research content of this paper is mainly divided into the following two parts:Ⅰ.An example of SCC-MOFs,[Ag₁₂（S^tBu）₈（CF₃COO）₄（bpy-2NH₂）₄]_n（SCC-MOFs-2）,was successfully constructed using bipyridyl organic bridging ligands with blue luminescence:2-amino-4,4’-bipyridine（bpy-2NH₂）,self-assembled with silver chalcogenolate clusters.By comparing the structure of[Ag₁₂（S^tBu）₈（CF₃COO）₄（bpy-3NH₂）₄]_n（SCC-MOFs-1）constructed using 3-amino-4,4’-bipyridine（bpy-3NH₂）previously reported by our group,we found that the introduction of amino groups containing lone pairs of electrons at the 3-and 2-positions of the organic bridging ligand4,4’-bipyridine（bpy）,respectively,led to the same structural compounds with only changes in the synthetic strategy.Interestingly,SCC-MOFs-1 and SCC-MOFs-2exhibited completely different photoluminescence phenomena at room temperature and low temperature,with SCC-MOFs-1 exhibiting double-emission behavior at 293 K and83 K,with emission peaks at 456 nm and 556 nm,respectively,while SCC-MOFs-2exhibited single-emission behavior at 293 K,with emission peaks at 471 nm,and multi-emission behavior at 83 K,with emission peaks at 469 nm,522 nm and 600 nm,respectively.This suggests that the photoluminescence properties of these materials can be regulated by adjusting the positions of the bridging ligand substituents,which provides new ideas for the design and synthesis of SCC-MOFs materials with specific emission behaviors.2.Based on two strategies of chiral ligand modification and host-guest chemistry,two pairs of chiral SCC-MOFs materials with CPL activity were constructed on the basis of Ag₁₂bpy previously reported by our group.（1）A pair of chiral SCC-MOFs with circularly polarized luminescence,[(Ag₁₂（S^tBu）₈（R/S-TFL）₄（bpy）₄)]_n(R/S-Ag₁₂bpy),were successfully prepared by using a chiral R/S-trifluorolactic acid（R/S-TFL）ligand by modifying the surface of the cluster nodes,the asymmetry factor(g_lum)reaches±8.5×10^-4.（2）A pair of chiral SCC-MOFs,[(Ag₁₂（S^tBu）₈（CF₃COO）₄（bpy）₄（R-PO）_1.5]_n and[((Ag₁₂（S^tBu）₈（CF₃COO）₄（bpy）₄（S-PO）₂]_n(R/S-Ag₁₂bpy-PO),were prepared by host-guest interactions using the chiral solvent（R）-（+）-propylene oxide/（S）-（-）-propylene oxide（R/S-PO）as the guest molecule,and successfully turned on the circularly polarized luminescence of the host[(Ag₁₂（S^tBu）₈（CF₃COO）₄（bpy）₄]_n(Ag₁₂bpy)with a g_lum of±1.16×10^-3.This induction of chirality in framework materials by encapsulating chiral guest molecules improves the asymmetry factor by an order of magnitude over that induced by chiral ligand modifications.Due to the very low boiling point of the chiral guest R/S-PO,this CPL signal can disappear and reappear during alternate chiral guest treatments,thus achieving CPL switching properties.The entire chiral transfer process can be monitored and captured by single crystal to single crystal（SC-SC）conversion,thus revealing the intrinsic structure-chirality relationship at the atomic level.