Synthesis And Properties Of Silver Clusters Protected By Dipyridinium Ligands

Since the appearance of structurally precise metal nanoclusters,this class of novel nanomaterials has attracted great interest due to their precise structure and unique physicochemical properties.Owing to their quantum size effects and discrete electronic energy levels,metal nanoclusters exhibit atomic structure tunable properties.That is to say,slight tuning of nanocluster structures can trigger significant differences in their properties.Thus,metal nanocluster is an ideal platform to study the correlation of structural properties.As an emerging class of programmable nanomaterials,metal nanoclusters could be used in a variety of promising applications,such as catalysis,drug delivery,energy storage,and biological applications.Among them,silver nanoclusters have received extensive attention from researchers due to their excellent optical properties.However,their poor stability and low luminous efficiency limit their practical applications.The preparation of silver nanoclusters with stable structure and excellent luminescence performance remains challenging.As the outermost layer of nanoclusters,organic ligands play a crucial role.They not only affect the size,stability,and solubility of metal nanoclusters,but also affecttheir physical and chemical properties.Different from other ligand systems(thiolate,phosphine,alkynyl,etc.),nitrogen-containing ligands have strong coordination ability and diverse coordination modes,which can effectively protect silver nanoclusters and enhance their stability.In addition,the increased electron transfer pathway inside the cluster could improve the luminescence intensity of silver nanoclusters.In this paper,we selected 2,2’dipyridinamine(Hdpa),a nitrogen-like ligand containing multiple ligands,as the protective ligand and successfully constructed Ag(I)clusters with stable chemical structure and unique optical properties.The specific work content was as follows:Firstly,the silver nanoclusters with yellow photoluminescence using Hdpa ligand with AgStBu and AgPF6 were obtained in methanol,Ag9(StSu)6(Hdpa)6(PF6)3(Ag9-Y,Y represents yellow emission).Interestingly,the fresh Ag9-Y crystal surface was gradually changed from colorless to red,accompanying the color of photoluminescence changed to red(555 nm to 610 nm)under UV irradiation.The core-shell structure Ag9YR was obtained by controlling the irradiation time(Ag9-YR,YR represents that the inside of the crystal has yellow emission,and the outside has red emission).Combined with the characterization and control experiments,the above change was proved to be caused by the partial conversion of Ag9 to Ag62 in the crystal.Even more exciting,although the physical properties of the crystal surface change significantly after irradiation,the crystal lattice has not been destroyed.Thus,combined with the epitaxial growth method in the mother liquid environment the multi-layer core-shell structure(Ag9-YRY)can be continuously constructed.This convenient strategy further opens up a new avenue for the construction of diverse functional cluster-based core-shell crystal materials with broad promising applications.Secondly,we used silver trifluoroacetate(CF3COOAg),silver isopropyl sulfide(AgSiPr)and Hdpa ligands in methanol to obtain dodecaucleated silver nanoclusters[(Ag12(SiPr)6(CF3COO)6(Hdpa)4](Ag12L4)co-protected by polydentate nitrogen and thiolate groups.The Ag12L4 has orange photoluminescence at room temperature.Meanwhile,Ag12L4 not only displays the optical properties of thermochromic fluorescence,but also exhibits mechanical grinding stimuli-responsive luminescence.This dual stimulus-responsive material has great application potential in data encryption and anti-counterfeiting.Subsequently,we used chiral L/D-C10H15OSO3Ag to replace silver trifluoroacetate in acetonitrile to obtain a pair of dodecameric enantiomeric silver nanoclusters,[(Ag12(SiPr)6(L/DC10H15OSO3)6(Hdpa)2(CH3CN)2](L/D-Ag12L2),co-protected by thiolate,carboxylic acid and polydentate nitrogen ligands.The L/D-Ag12L2 exhibits good circular dichroism(CD)in both solution and solid-state.By analyzing the CD spectrum and structure of L/D-Ag12L2,we found that chirality was induced by introducing chiral ligands to twist the metal motif.Therefore,this work is of great significance for the study of the structure and optical properties of silver nanoclusters,and further proves the feasibility of using chiral ligands to construct chiral metal nanoclusters.