Synthesis Of Chiral Gold Clusters And CdSe Quantum Dots And Manipulation Of Their Optical Properties

Chiral gold clusters and quantum dots(QDs)possess excellent optical properties in visible light wavelength region,and have possible application in the field of polarization control,negative refractive index,chiral sensing and advanced display devices based on CPL.However,there are still some problems with them,such as low optical activity,unclear mechanism and lacking of method for controlling their optical properties,shortage of samples with CPL properties,and so on.Therefore,it is of great importance to find effective method to manipulate their optical properties and push the boundaries of chiral inorganic nanomaterials in real applications.The optical properties of chiral gold clusters and QDs are determined by not only the surface chiral ligands but also the inorganic cores.In this thesis,two new optical active chiral gold clusters have been successfully synthesized,and then used as the building blocks.In order to investigate the influence of the interaction between surface chiral ligands on the optical properties of chiral gold clusters,the intermolecular interaction were controlled through self-assembly method.In order to investigate the influence of inorganic core composition on the optical properties of chiral CdSe QDs,the inorganic cores were overovercoated with various shells.Two novel chiral gold clusters have been synthesized by direct synthesis method and their optical properties have been well studied.Methyl groups are introduced into the para-position of the phenyl rings in(R)-BINAP and(S)-BINAP to form(R)/(S)-Tol-BINAP and increase the steric hindrance of ligand,and used as the liga nd during the synthesis of gold clusters.The growth kinetics of gold clusters have been influenced and followed by the change of gold cluster size,and two new kind of chiral gold clusters have been sucessfully obtained.The single crystal XRD analysis reveal that the formula of these chiral gold clusters to be Au3[(R)-Tol-BINAP]3Cl and Au3[(S)-Tol-BINAP]3Cl.Moreover,the as prepared chiral gold clusters possess giant optical activity in 220 ~ 450 nm region.The absorption anisotropy factor reaches 7 × 10-3 at 366 nm,which is the highest value among the reported chiral gold clusters.By analyzing the single crystal structure of the clusters,it clearly shows that the chirality of the clusters is mainly due to the coupling of electron states between the ligands and Au atoms.TDDFT theoretical calculation reveals that the excited state transition corresponding to CD signal is mainly metal-to-ligand charge transfer.In order to investigate the influence of the interaction between surface chiral ligands on the optical properties of chiral gold clusters,the intermolecular interaction were controlled through self-assembly method.The as synthesized chiral Au3[(R)-Tol-BINAP]3Cl and Au3[(S)-Tol-BINAP]3Cl clusters have been used as building blocks in self-aseembly by adding poor solvent.Impressively,along with the fraction of poor solvent increase,the CD signal of the gold clusters show gradually redshift and enhancement,and the non-emissive gold clusters become shining.Through fine control of the self-assemble process,Au3[(R)-Tol-BINAP]3Cl clusters and Au3[(S)-Tol-BINAP]3Cl clusters have been successfully assembled into highly ordered,bodycentered cubic packing nanocubes.Once the ordered structure is formed,the CD intensity is significantly enhanced with absorption anisotropy factor up to 8.6 × 10-3 and a remarkable CPL response with emissive anisotropy factor up to 7 × 10-3.The detailed structure analysis disclose that the ordered nanocube possess the same packing structure with the signle crystal of gold clusters.Both the enhancement and redshift of CD signal and the generation of CPL originate from restricted intramolecular rotation and the ordered stacking of the chiral stabilizers,which are fastened in the crystalline lattices throuh strong CH/? interaction.In order to investigate the influence of inorganic core composition on the optical properties of chiral CdSe QDs,the inorganic cores were overcoated with various shells.First,zinc-blende CdSe quantum dots with various diameter have been successfully synthesized through heat-up method and overcoated with CdS or ZnS shells with various thickness.Thus,the quasi-type ? CdSe@CdS and type ? CdSe@ZnS core-shell quantum dots have been successfully prepared.The ligand exchange procedure have been done with chiral L/D-cysteine to replace the original achiral organic ligand,which results in the formation of optical active QDs.When CdS shell were overcoated on the surface of chiral CdSe QDs,the delocalization distribution of the electron wave function in the CdS shell gradually decreases with the increase of the thickness of the CdS shell.The coupling effect between the electron wave function and the surface chiral molecular energy level gradually weakened,as a result,the optical activity of the quantum dot gradually decreased.When ZnS shell were overcoated on the surface of chiral CdSe quantum dots,the electron wave function and the hole wave function were strongly confined in CdSe.The coupling between the exciton wave function and the surface chir al molecule energy level were very weak,which result in the fast decay of optical activity.Along with the increasement of the diameter of the chiral CdSe QDs,the CdS or ZnS shell has a stronger quantum confinement effect on the excitonic wave function,which finally accelerated the decay rate of optical activity in chiral QDs.