Research On The Synthesis Of High-efficiency Metal Nanoclusters And Their Photophysics

Metal nanoclusters（NCs）are a class of aggregates of a few to several hundred metal atoms protected by a monolayer of organic ligands.The core diameter of metal NCs is around 1-3 nm,which is intermediate in size between organic molecules and metal nanoparticles,and is a bridge between molecular and nanoparticle science.Metal NCs have atomically precise structures which make them ideal models for exploring the connection between microscopic material structures and macroscopic physical and chemical properties.In addition,ultra-small-sized metal NCs exhibit unique molecule-like properties such as intrinsic chirality,tautomerization,total synthesis,molecular-like absorption and photoluminescence（PL）,and hierarchical assembly.They have shown great application prospects in optoelectronics,biomedicine,catalysis,and energy conversion,and have become a hot research topic in recent years.However,in the research field of PL performance of metal NCs,there are still research difficulties such as low PL efficiency and difficulty in regulating the PL color,which make them unable to be comparable with other mainstream nanoluminescent materials,further limiting the practical applications of metal NCs.To address these problems,this paper regulates the structure of metal NCs from the inside out,improves the PL efficiency（PLQY）of metal NCs from the inhibition of coherent acoustic oscillations of metal cores in the clusters,regulates the electron transfer process from the interface motif to the defective state,and the activation of multiple luminescent centers of the clusters by the self-assembly of the clusters induced by Zn²⁺,which reveal the effects of different modification strategies on the photophysical properties of metal NCs and their PL colors.The mechanism of different modification strategies on the photophysical properties of metal NCs was revealed.The specific work is as follows:（1）To address the low PLQY of metal cores in solution-state metal NCs,we successfully anchored three organic ligands,6-aza-2-thiothymidine（ATT）,L-arginine（ARG）and tetra-n-octylammonium（TOA）,on the surface of AuNCs by utilizing supramolecular interactions such as hydrogen bonding and electrostatic attraction.The layer-by-layer protection of the three ligands can effectively improve the rigidity of the chemical environment on the surface of AuNCs but has little effect on the electronic properties and local structure of the metal core.More importantly,the confinement effect of the three-layer ligand can be transmitted into the metal core to modulate the low-frequency acoustic vibration of the metal core,which reduces the amplitude of the coherent oscillations of the metal core from 1.82×10^-2 to 4.00×10^-3,and further suppresses the non-radiative structural relaxation process dominated by the metal core in the excited-state electronic dynamics.Under this effect,the quantum efficiency（PLQY）of AuNCs was significantly increased from<1%for ATT monolayer ligand protection to 59.6%for ATT and ARG bilayer ligand co-protection and 90.3%for ATT,ARG,and TOA trilayer ligand co-protection.In addition,the PLQY of water-soluble AuNCs prepared using water-soluble trimethylphenylammonium（TMPA）ligand instead of organic phase TOA ligand was also increased to 86.7%.（2）Although the suppression of coherent oscillations in the metal core can greatly improve the PL intensity of the metal core of the NCs,the interfacial motif of the metal NCs still contains defective state luminescence centers,which prevents the PL of the metal core of the metal NCs from reaching the desired PLQY.To address this problem,we introduced simple water molecules into AuAg NCs,and simultaneously prepared AuAg NCs before water absorption,after 56%ambient humidity treatment,and in the state of aqueous solution.The emission spectra showed that the introduction of water molecules caused the PL of the AuAg NCs to be continuously blue-shifted from yellow-green light at 534 nm to blue light at 482 nm.Further,a series of steady-state spectra and transient fluorescence lifetimes as well as femtosecond transient absorption and fluorescence spectra reveal that the source of the PL of the defective state at 534 nm in the AuAg NCs is the transfer of electrons from the luminescent centers of the cluster intrinsic state to the centers of the defective state.The introduction of water molecules can modulate the electron transfer process,which slows down the electron transfer process from 29.1 ps before water absorption to 268.3 ps after water absorption.The electron transfer process in aqueous AuAg NCs completely disappears,and the clusters fully exhibit fluorescence emission dominated by the metal core at 486 nm.The external water molecules mainly act on the interfacial staple patterns in the AuAg NCs,effectively reducing the electron-optical phonon coupling strength and energy,and thus increasing the radiative excursion rate and simultaneously decreasing the nonradiative excursion rate in the AuAg NCs.Ultimately,the PLQY of AuAg NCs is increased from 6.6%of the PL in the defective state before water absorption to 67.7%of the PL in the intrinsic state after water absorption and 76.1%in the aqueous solution.（3）Modulating the coherent oscillations of metal cores in metal NCs and the defective state PL of interface motifs can enhance the PL intensity of metal cores to a certain extent.However,it is still not possible to realize the multi-peak emission of metal NCs to improve the PLQY.This is mainly due to the difficulty in balancing the interactions between multiple luminescent centers in metal NCs.To address this problem,we induced the self-assembly of AuNCs using the strong electrostatic interaction between Zn²⁺cations and deprotonated COO^-in the ligands on the cluster surface.The pre-assembly AuNCs only exhibit aggregation-induced effect（AIE）single-peak yellow emission at 580 nm,whereas sustained self-assembly can activate the dual cluster eigenstate PL complexes at 475 nm and 580 nm simultaneously to produce white light emission.The PLQY of AuNCs in aqueous and powdered states was increased from 18.2%and 23.5%to 42.1%and 53.6%,respectively,by self-assembly.Variable-temperature PL studies showed that the 475 nm blue light in the Aunanocluster assemblies was attributed to the single-linear-state fluorescence of the metal cores in the clusters,whereas the 580 nm yellow light was attributed to the trilinear-state phosphorescence of the interfacial staple patterns in the clusters.More importantly,the electron transfer rate from the cluster staple pattern to the metal core can be regulated by precisely adjusting the relative content of the Au（0）/Au（I）component in the Aunanocluster assemblies,and the wide color temperature white light modulation of the Aunanocluster assemblies from the warm white light of 3426K to the cool white light of 24973 K is finally realized.The main innovations of this paper are as follows:（1）Aiming at the low PLQY of the metal core in solution-state metal NCs,a modification strategy of three-layer ligand capping is proposed to conduct the confinement effect on the surface of the NCs into the metal core,modulate the low-frequency acoustic vibration of the metal core,reduce the amplitude of the coherent oscillations of the metal core,and inhibit the non-radiative structural relaxation process dominated by the metal core in the excited-state electronic dynamics.The improvement of PLQY from<1%to 90.3%for solution-state AuNCs was finally realized.（2）Aiming at the problem of low PLQY of the metal core due to the luminescent centers of the defective states in the interface motif of metal NCs,a strategy of water molecules passivating the PL of the defective states was proposed,which revealed the PL source of the defective states of the NCs,modulated the electron transfer process from the cluster intrinsic states to the defective states,and suppressed the strength and energy of the electron-optical phonon couplings in the motifs.The regulation of the PL color of AuAg NCs from 534 nm to 482 nm and the corresponding PLQY from6.6%to 76.1%were finally achieved.（3）Aiming at the inability of metal NCs to realize the co-expression of multiple luminescent centers,we proposed a strategy to induce the self-assembly of AuNCs by using the strong electrostatic interactions between Zn²⁺and deprotonated COO^-in the ligands on the surface of the clusters to achieve the double emission of 475 nm blue light and 580 nm yellow light,clarifying the structural origin of the double-peak emission,and regulating the electron transfer rate of the cluster motifs to the metal The electron transfer rate from the cluster motif to the metal core was regulated.Finally,we realized the modulation of Aunanocluster assemblies in a wide range of color temperatures for cold and warm white light.