Aggregation Induced Emission Of Metal Nanoclusters And Their Applications In Light Emitting Diodes

As a kind of solid-state lighting,light-emitting diodes（LEDs）are drawing more and more attention in laboratories and industries because of their attractive luminous efficiency,high-speed response time and wide range of applications.In order to obtain highly-efficient,environmental-friendly and stable LEDs,researchers have been committed to developing new light-emitting materials for a long time.In recent years,metal nanoclusters（MNCs）have attracted a great deal of interest in preparing new LEDs owing to their facile synthesis,tunable emitting color,good biocompatibility and good stability.Compared with traditional light-emitting materials,the biggest shortcoming of MNCs is their low photoluminescent quantum yield（PLQY）.Therefore,the performance of LEDs based on MNCs is still at the initial developmental stage.Researchers have developed many approaches to improve the PLQY of MNCs,the aggregation induced emission（AIE）is the most attractive one.In this paper,we designed different AIE systems to enhance the PL performance of MNCs according to their structure and requirements,and synthesized a series of MNCs with high PLQY.Based on these MNCs,we fabricated highly-efficient photoluminescent LEDs（PL-LEDs）,electroluminescent LEDs（EL-LEDs）and electroluminescent circularly polarized LEDs（CPLEDs）,which exhibited the great potential of metal nanoclusters in LED fields as emitters.In the second chapter,we synthesized sliver nanoclusters（NAC-Ag NCs）with PLQY of 0.02%using N-acetyl-L-cysteine（NAC）as the capping ligand.The hydrogen bond and hydrophobic interactions between ligands were used to achieve self-assembly induced emission.Based on this,we synthesized NAC-Ag NCs self-assemblies with PLQY of 26.8%.Further researches indicated that the mechanism of self-assembly induced emission was attributed to two aspects:（1）The self-assembly inhibited the vibration and rotation of the ligands and promoted the radiative relaxation in the excited state;（2）The self-assembly reduced the energy barrier between the triplet state and singlet state of NAC-Ag NCs,which promoted the reverse intersystem crossing of triplet excitons and realized the thermally activated delayed fluorescent（TADF）.The self-assemblies were separated from solution and applied as the color conversion in PL-LEDs.Such self-assembly induced TADF emission demonstrated the great potential of the AIE effect in enhancing the PL properties of MNCs and provided a new strategy for developing new environmental-friendly color conversions based on metal nanoclusters self-assemblies.In the third chapter,with the combination of AIE and surface ligand engineering,we employed a triplet-layered ligand coordination of gold nanoclusters,from the Au kernel out,the ligands:6-aza-2-thiothymine（ATT）,arginine（Arg）and tetraoctyl ammonium bromide（TOAB）,through hydrogen bonds and electrostatic interactions（TOAB/Arg/ATT-Au NCs）.The triplet-layered ligand coordination of Au NCs achieved AIE at single-cluster level,which suppressed the nonradiative transition in excited state caused by the vibration of the ligands and gold core.The PLQY of gold nanoclusters was improved from 2.1%to 73.4%while the mono-dispersity was maintained.In addition,the good solubility of TOAB/Arg/ATT-Au NCs in ethanol satisfied the solvent requirements of dispersibility,orthogonality and wettability in device fabrication and improved the thin film quality of emitting layer.The EL-LEDs based on TOAB/Arg/ATT-Au NCs（Au NCs-LEDs）exhibited a maximum brightness of 1104cd/m² and a peak external quantum efficiency（EQE）of 5.1%,which was the highest performance among the Au NCs-LEDs.Primarily,the Au NCs-LEDs reported here met the color purity and brightness requirements for display applications,and such a triple ligand engineering strategy broadened the horizons in future design of high-performance metal NC-based EL-LEDs.In the fourth chapter,we synthesized enantiomeric（R/S）-4-phenylthiazolidine-2-thione capped silver（R/S-Ag₆（PTLT）₆）NCs with both AIE and circularly polarized luminescence（CPL）by recrystallization.Further studies indicated that ordered and dense packing arrangements were essential for the R-Ag₆（PTLT）₆ NCs aggregates to fulfill efficient AIE and achieve high PLQY.On this basis,1,3-bis（carbazol-9-yl）benzene（m CP）was introduced as a host molecule and further enhanced the photoluminescence（PL）emission by constructing ordered and dense packing arrangements of the Ag₆（PTLT）₆ NCs in solid films viaπ-πinteractions.The PLQY of solid thin film was increased from 40.3%to 71.0%.As the hybrid solid thin films were employed for CPLEDs,m CP also suppressed the triplet-triplet annihilation and balanced the charge transport.The as-fabricated CPLEDs exhibited a maximum brightness of 3906 cd/m²,a peak EQE of 10.0%and dissymmetry factors of-5.3×10^-3and 4.7×10^-3.As our efforts achieved the MNC-based CPLEDs with best performance,the host molecule enhanced AIE of MNCs provided a promising strategy for fabricating high-performanced MNC-based CPLEDs and EL-LEDs.