Metal Ion-modulated Luminescence Properties Of Novel Benzimidazole Derivatives

It is of great significance to develop photo-and electro-luminescent functionalmaterials. Recently, compounds based on benzimidazole derivatives have attractedconsiderable attention for their good light-emitting and charge transport properties.In this thesis, a series of new benzimidazole derivatives are designed and synthesized,which display different emission mechanism when they are coordinated to differentmetal ions. The phenomena are rationalized by supporting DFT computationalresults, which reveal the correlation between luminescent properties and electronicfeatures of the complexes. Our works are summarized as follows.1. Nine mononuclear Cu(I) complexes of general formula Cu(YPBI)(PPh3)X havebeen synthesized by the reaction between CuX (X=Cl, Br, I), PPh3and2-(2'-diphenylphosphinophenyl)-1-phenylbenzimidazole derives (YPBI: Y=H, MO,PO) in a molar ratio of1/1/1in CH2Cl2solution. X-Ray crystal structure analysisreveals that the central Cu(I) ion is in a distorted tetrahedral coordination environment.The nine complexes in solid state exhibit strong phosphorescence covering the visiblespectrum from green to red (557nm-612nm). The luminescence quantum yield (?) ofcomplexes Cu(YPBI)(PPh3)I are higher than25%at room temperature, especially forCu(HPBI)(PPh3)I (?=37.53%), indicating that these novel complexes are promisingphosphorescent materials.2. With the bulky benzoimidazole phosphine oxide (HL) as a new ancillary ligand,two new ppy-type phosphorescent complexes (Pt-1and Ir-1) have been synthesized.X-ray crystallographic studies reveal that the platinum(II) binds to L through aN^O-coordinating mode and forms a square-planar coordination geometry. However,the HL coordinates to iridium(III) with N^C-coordinating mode, resulting in theformation of meridinoal isomer with weak trans-effect. The crystal packings of Pt-1and Ir-1show that no apparent stacking interactions can be detected due to the stericeffect of the ancillary ligand. Both complexes exhibit characteristic phosphorescencein solution with highly quantum efficiencies (Pt-1:31%, Ir-1:25%) at roomtemperature, showing the first successful example of using benzoimidazole phosphine oxide as an ancillary ligand. Notably, Pt-1exhibits efficient green phosphorescencewith monomeric character in powder (?=43.94%). The OLED using Pt-1asphosphor exhibits a maximum brightness of12020cd m2at11V. Of particularsignificance, the device is able to achieve a maximum current efficiency (?c) of25.86cd/A, external quantum efficiency (?ext) of9.23%and power efficiency (?p) of25.81lm/W, which makes it a promising candidate as luminescent materials in OLEDs.3. Six novel2-(2'-hydroxyphenyl)-4-hydroxybenzimidazole derivatives have beensynthesized and characterized by elemental analysis, MS and NMR. The compoundsexhibit bright fluorescence, via excited-state intramolecular proton transfer (ESIPT),revealing their potential applications as sensors, probes and optical materials. SixZn(II) complexes based on this kind of ligands have been prepared. X-raycrystallographic study shows that Zn2+ion has a usual five-coordinate environmentand adopts a distorted square pyramidal geometry. It's worthy noting that there arestrong intramolecular hydrogen-bonds in the complexes, especially for the ZnE1.The photophysical studies show that, when the ligands are coordinated to Zn(II),most of the complexes exhibit normal intra-ligand fluorescence. However, ZnE1exhibits emission based on excited-state intramolecular proton transfer (ESIPT).4. The reaction of2-hydroxybenzophenone derivatives with europium ions hasafforded a new family of luminescent nonanuclear Eu(III) clusters. Crystal structureanalysis of the clusters reveals that the metal core comprises two vertex-sharingsquare pyramidal units. Most of these complexes show emissions typical of Eu3+ionunder visible light excitation (400–420nm) at room temperature. Photophysicalcharacterization and DFT study reveal a correlation between luminescent efficienciesof Eu(III) complexes and the electronic features of the ligands, which in true can betuned by the nature of substituents in the4-position of the ligands. The ligands withfluorine substituent possess more suitable triplet energy levels, resulting in moreintensive luminescence.