Luminescence Properties Of Lanthanide-Titanium Oxo Clusters Constructed By Chelating Ligands

As a novel type of heterogeneous cluster,lanthanide-titanium oxo clusters have a uniform scale and precise and modifiable atomic structure,which are conducive to understand the relationships between structures and activities.They not only have a lower bandgap and a variety of catalytic sites,but also have excellent luminescence properties.However,the research on lanthanide-titanium oxo clusters has just started,and it is not yet mature and perfect in terms of syntheses,property expansions and mechanism explanations.The main manifestations are:the current syntheses of lanthanide-titanium oxo clusters mainly adopt alcohols or carboxylic acids,and the applications of chelating ligands(such as β-diketones,1,10-phenanthroline,bipyridines,etc.)are rarely reported.Besides,the relationships between structures and properties remain to be further studied.In addition,the syntheses of chiral lanthanide-titanium oxo clusters and the study of their circularly polarized luminescence properties are still lacking.Based on the above background,around the title "Luminescence Properties of Lanthanide-Titanium Oxo Clusters Constructed by Chelating Ligands",a total of nine types of lanthanide-titanium oxo clusters were synthesized with the protection of β-diketones,S-heterocyclic carboxylic acids,1,10-phenanthroline and chiral bipyridine ligands.Furthermore,their(circularly polarized)luminescence properties and the structure-activity relationships were studied and discussed preliminarily.The detailed findings are following:Ⅰ:Based on β-diketones(acetylacetone and trifluoroacetylacetone),La6Ti-acac,La6Ti-tfac and Ln9Ti2(Ln=Tb,Dy)clusters based on cubane units were obtained by low-temperature crystallization.The room temperature and temperature-dependent photoluminescent properties of Tb9Ti2 were explored.The results show that Tb9Ti2 exhibits intense green luminescence under ultraviolet irradiation both in the solid and the solution with dichloromethane.The lifetimes are 635 μs and 899 μs and the absolute quantum yields are 49.9%and 60.5%,respectively.At the same time,the ACQ effect was observed in the solution luminescence investigation.Between 170-300 K,the intensity of the emission peak of Tb9Ti2 is negatively correlated with temperature,and the sensitivity is 0.62%/K,indicating that it has potential application in molecular luminescent thermometers.Ⅱ:Based on thiophene-2-carboxylic acid,the lanthanide-titanium oxo clusters Ln2Ti4(Ln=Eu,Tb,Sm),EuTi9,Eu2Ti8 and[Ln2Ti2][Ln2Ti8](Ln=Tb,Gd)containing the same structural unit {LnTi4} are obtained by doping 1,10-phenanthroline as the second ligand and adjusting different kinds of rare earth salts,organic solvents and feed ratios.By exploring their luminescence properties,we found that Ln2Ti4,EuTi9 and Eu2Ti8 all emit luminescence under ultraviolet irradiation.Among them,Eu2Ti4 has excellent luminescence performance,and the quantum yield is as high as 89.5%.In addition,the influence of antenna effect,charge transfer,coordination site between ligands and metals,coordination number and average coordination bond length on the luminescence of lanthanide-titanium oxo clusters is preliminarily discussed.Ⅲ:Based on the self-synthesized chiral bipyridine ligands,supplemented by thiophene-2-carboxylic acid and benzoic acid,two pairs of chiral lanthanide-titanium oxo clusters R/S-Eu2Ti4 were obtained for the first time.Single crystal diffraction,UV-Vis spectrum and CD spectrum verified the chiral structure of the clusters.Photoluminescence spectroscopy shows that both types of R/S-Eu2Ti4 are good photoluminescent materials with high quantum yields.Circular polarized luminescence spectroscopy shows that the R/S-Eu2Ti4 coordinated with benzoic acid possess strong CPL signals in dichloromethane,and the maximum luminescence asymmetry factor is 0.004.