Design, Synthesis And Properties Of Near-infrared Phosphorescent Iridium Complexes

In recent years, near-infrared-(NIR-) emittingcomplexes have aroused rapidly growing attention due to their prospect in information-secured displays, biological image and medical therapies, optical fiber communications, etc. To date, platinum complexes have demonstrated the record maximum external quantum efficiencies(EQEs) for NIR-OLEDs. However,those devices generally suffer from severe efficiency roll-off with increasing current density, which has been mainly ascribed to the easy aggregation of the planar-square configuration and the intrinsic long phosphorescence lifetime of the complexes. In comparison, the transition-metal complex Ir(III) with octahedral configurations would help to alleviate the efficiency roll-off in the corresponding NIR-OLEDs. For these reasons, seven novel cyclometalated iridium complexes were rationally designed and synthesized in this work. We systematically study the relationship between the structure and near-infrared-(NIR-) emitting performance of these complexes from photophysical properties, electrochemical properties and theory calculations. Moreover, we successfully used these complexes in organic light emitting devices and near infrared bioimaging. The details are as follows:(1) We successfully designed and synthesized seven kinds of new iridium metal complexes based benzo[g]phthalazine 、 quinazoline and quinoline ligand: Ir[(dpbpa)2Bphen]+PF6-、Ir[(dtbpa)2Bphen]+PF6-、Ir(dtbpa)3、Ir[(dtq)2bpy]+PF6-、(dtq)2Iracac 、(dbtq)2Iracac Ir[(pbq-g)2phen]+PF6-. The crystal structure of Ir[(dtq)2bpy]+PF6- was obtained successfully.(2) We systematically studied the photophysical properties, electrochemical properties and theory calculations of these complexes. And the relationship between molecular structure and their complex properties were analyzed by quantum chemical calculations. The results show that the introduction of benzophthalazine and the quinazoline groups make the luminous red-shift. These complexes achieve near-infrared emitting except for complex Ir[(dtq)2bpy]+PF6-, spanning from 700-900 nm with the preferable PLQY. For example, complex(dtq)2Iracac exhibited a NIR emission peaked at 710 nm with a PL quantum yield of 9.2% in PMMA solid. Complex Ir(dtbpa)3 exhibited a NIR emission peaked at 820 nm with a unexpectedly PL quantum yield of 5.2% in PMMA solid. The newly cyclometalated benzo[g]phthalazine and quinazoline ligands have several areas of superiority over the previous benzo[g]quinoline and benzo[g]quinoxaline ligands in views of stronger Ir–N bonds, smaller chelate congestion,higher electron-accepting ability, thus improving the overall phosphorescence of the corresponding iridium complexes in the NIR region.(3) Employing these complexes Ir[(dpbpa)2Bphen]+PF6-、Ir[(dtbpa)2Bphen]+PF6-、 Ir(dtbpa)3, the NIR-OLEDs have been fabricated and the intense NIR emission was detected at 715 nm, 791 nm and 820 nm for the devices, respectively. It is worth noting that all the devices exhibited nearly flat efficiency-current density characteristics over a wide range of current density of 1-100 m A/cm2. This work further confirmed that iridium complexes are competent alternatives for NIR-emitting OLEDs.(4) The complex Ir[(pbq-g)2phen]+PF6- was used in cell imaging and compared with Ir[(pbq-g)2Bphen]+PF6-. This comparative study suggested that the structure change of auxiliary ligand almost does not affect the photophysical properties, but it could fine-tune the amphiphilicity and cytotoxicity of the cyclometalated iridium(III) complexes and thus might play a key role in the design of NIR-emitting iridium(III) complexes for practical applications in bioimaging.