Synthesis And Properties Of Near-infrared Emitting Iridium Complexes

Near-infrared （NIR） emitting materials have recently spurred increasing interestbecause of their potential applications in night-vision displays, optical communication,photodynamic diagnoses and therapies, etc. Up to now, NIR dyes under investigationmainly focus on organic small molecular dyes, semiconductor quantum dots, lanthanidecomplexes and transition metal complexes. Among them, iridium complexes are apromising kind of phosphors due to their phosphorescent nature and rich photophysicalproperties. However, in contrast with their brilliant success in the visible light region,iridium complexes are underperforming in NIR region. In this dissertation, four novelnear-infrared emitting iridium complexes were designed, synthesized and characterized.The photophysical and electrochemical properties of those complexes were studied bycomparative method, and the relationship between molecular structures andcharacteristics were studied with the aid of quantum chemical calculation. Moreover,the novel complexes were successfully used in near-infrared light-emitting devices andliving cell imaging. The main contents are shown as follows:（1） Sp2-hybrid N was adopted opposite to the chelating N atom in thecyclometallated ligand in order to expanding the emission of Iridium complexes to thetruly NIR region of the spectrum. Bulky and rigid structures were also introduced tomaintain high luminescent efficiency and stability of the NIR-emitting complexes. Fouriridium complexes were designed and synthesized:[Ir（pbq-g）₂（bpy）]⁺PF₆^-（PR1, pbq-g=phenyl-benzo[g]quinoline, bpy=bipyridine）,[Ir（pbq-g）₂（bphen）]⁺PF₆^-（PR2, bphen=4,7-diphenyl-1,10-phenanthroline）,[Ir（mpbqx-g）₂（bphen）]⁺PF₆^-（PR3, mpbqx-g=2-methyl-3-phenylbenzo[g]quinoxaline） and Ir（mpbqx-g）₂acac （PR4, acac=2,4-pentanedione）.（2） The photophysical and electrochemical properties of those complexes werestudied by comparative method. Quantum chemical calculations were carried out toreveal the relationship between molecular structures and characteristics. The resultsconfirm that the adoption of sp²-hybrid N in the cyclometallated ligand can significantlylower the LUMOs of corresponding iridium complexes. Truly NIR emission wasobtained, spanning from700nm to1000nm. （3） Based on PR2, PR3and PR4, NIR-emitting organic light-emitting devices（OLEDs） have been fabricated with the emission covering690-900nm. Compared withcommon OLEDs, these NIR-emitting devices demonstrate exclusive small efficiencyroll-off with increasing current density. Through the optimization of device structure,the device based on PR4exhibited the maximum external quantum efficiency （EQE） ashigh as1.38%（at a current density of44mA/cm²） and a maximum radiant emittanceexceeding1.74mW/cm². Such unique characteristics is desirable for the practicalapplications of OLEDs in terms of energy saving. It can be ascribed to the bulkyaromatic cyclometallated and ancillary ligands together with octahedral configuration ofIr（III） complexes, thereby hindering the triplet-triplet annihilation under highpopulations of triplet excitons.（4） The cationic complexes PR1and PR2were used in living cell imaging. PR1exhibits exclusive staining in the cytoplasm with good cell membrane permeability,while PR2causes the break-up of living cells. The results imply that though thevariation of diimine ancillary ligands has little influence on the photophysical propertiesof corresponding complexes, they are fundamental to cellular uptake in terms oflipophilicity and cytotoxicity. These observations indicate that the ancillary ligands ofbis-cyclometalated iridium complexes can be modified to expand the applications ofthese iridium complexes in bioimaging.