Crystallization-induced Emission Enhancement(CIEE):A Novel Fluorescent Au-Ag Bimetallic Nanocluster With Precise Atomic Structure

In recent years,thiol-protected precious metal nanoclusters as a new discipline have received much attention from nanoscientists.In the past few decades,nanoscientists have been devoted to improving the synthesis of nanoclusters to get more new types of clusters.More than 100 molecules of this type have been reported since the advent of the first thiol-protected Au102 cluster in 2008,including Au18,Au25,Au38,Au44,Au102,Au246,Au279,Agi4,Ag25,Ag29,Ag44 as well as alloy nanoclusters AuAg18,AuAg25,AuAg38,AuAg44,and PtAg25.Unlike organic molecules,changing the size of precious metal nanoclusters can bring about quite different changes in nature.It would be bring unexpected surprises by adding or subtracting one atom.This is why the nanoclusters are wonderful.Precious metal nanoclusters are attracting much attention due to their unique physicochemical properties(such as chirality,magnetism,catalysis and photochemistry)from nanoscientists.Among these properties,the fluorescence of noble metal nanoclusters is generally in the visible-near-infrared region,nanoclusters can also be applied to the field of biological science.In particular,the large Stokes shift of the nanoclusters in the protein-protected visible region can be used for various sensing applications.Therefore,the design of synthetic fluorescence nanoclusters is of great significance.In order to further explore the relationship between fluorescence and crystal structure,this paper designs and synthesizes noble metal nanoclusters with special fluorescence phenomenon on the basis of the predecessors.The specific research contents are as follows:In this paper,a one-pot method was used to synthesize AuAg bimetallic nanoclusters protected by thiolate and bisphosphine ligands.The structure of Au4Ag13(DPPM)3(SR)9 was determined by x-ray crystallography.It exhibits a special Crystallization-induced emission enhancement(CIEE),which has been frequently observed in small molecules and macromolecules,has not been reported for MNCs.The Au4Ag13 shows strong red fluorescence at 695 nm,in contrast,there is no fluorescence in organic solution.After the analysis of the structure,we found that Au4Ag13 is a nanocluster with an axis of symmetry of C3 and there is a pair of enantiomers.With 4 pairs enantiomers in each unit cell,both clockwise and counterclockwise around the C3 axis.The distance between each pair of enantiomers is very narrow,both are maintained by non-covalent bonds(C-H...?).In the crystalline state,due to the tight arrangement between the cluster molecules,there are a large number of C-H...? but by grinding or solvent dissolution,this will cause the weak interaction to be destroyed.Combine the theoretical calculations show the compact C-H...? interaction between molecules limits the intramolecular rotation and vibration,thus significantly enhancing the crystal fluorescence enhancement caused by the radiation transition in the crystalline state.