Blue-emitting Iridium Research Articles

Synthesis and optophysical properties of blue-emitting iridium (III) complex bearing oxadiazole-based picolinic acid derivative

An iridium (III) bis[(4,6-difluorophenyl)pyridinato-N, C2][6-(6′-(4″-(5‴-phenyl-1‴, 3‴, 4‴-oxadiazole- 2‴-yl) phenoxy) hexyloxy picolinate) was synthesized and characterized by 1H NMR and elementary analysis in order to study the effect of ancillary ligand of the oxadiazole-based picolinic acid derivative on optophysical properties of its iridium complex, and further to obtain an iridium complex with highly-efficient blue emission. The thermal stability, UV absorption and photoluminescent properties of this iridium complex were investigated. Compared with iridium (III) bis[(4,6-difluorophenyl)pyridinato-N, C2](picolinate) reported as a highly-efficient blue electroluminescent material, this iridium complex bearing an oxadiazole-based picolinic acid derivative presents higher thermal stability, more intense UV absorption at 291 nm and similar photoluminescent spectrum peaked at 469 nm. This indicates that tuning ancillary ligand of picolinic acid with an oxadiazole unit can improve the optophysical properties of its iridium complex.

Preparation of Blue-Emitting Phosphorescent Iridium(III) Complex Under Ultrasound Reaction

F2MeIrpic (Iridium(III)bis[2-(2′,4′-difluorophenyl)-4-methyl pyridinato-N,C2′] picolinate) was synthesized to develop a blue-emitting phosphorescent Ir(III) complex. The synthesized F2MeIrpic showed sky blue light with the maximum PL peak at 470 nm. Generally the reaction time and maximum yield of F2MeIrpic were known to be 78 hours and 28%, respectively. The reaction is composed of three steps which are the synthesis of ligand (F2Meppy), Ir-dimer and F2MeIrpic with Ir-dimer and picolinic acid. The synthesis of F2MeIrpic is a rate determining step. The solubility and dispersibility of Ir-dimer in solvent are important reaction parameters which determine reaction time and yield, because Ir-dimer is not soluble in almost all solvent. Among various solvents Tetrahydrofuran(THF) was the best solvent to solve Ir-dimer. Ir-dimer was dispersed well by an ultra-sonication in the solvent during reaction. The reaction time was reduced to 36 hours and yield was enhanced to 45% by ultra sound in THF.

Theoretical Studies of Blue-Emitting Iridium Complexes with Different Ancillary Ligands

The structural and electronic properties of two heteroleptic iridium complexes Ir(dfppy)2(pic) (FIrpic) and Ir(dfppy)2(acac) (FIracac) have been investigated theoretically, where dfppy = 2-(2,4-difluorophenyl) pyridine, pic = picolinic acid, and acac = acetoylacetonate. The geometries of ground and excited states are optimized at PBE0/LANL2DZ and CIS/LANL2DZ levels, respectively. Time-dependent density functional theory (TDDFT) method is employed to explore the absorption and emission properties. In the ground state, the highest-occupied molecular orbital has a significant mixture of metal Ir(d) and dfppy(pi), the lowest-unoccupied orbital locates primarily on pi* of pic for FIrpic and pi* of dfppy for FIracac. The luminescence of each complex originates from the lowest triplet excited state, which is assigned to the mixing of metal-to-ligand charge transfer and intraligand charge transfer characters. The effects of ancillary ligands pic and acac on absorption and emission spectra are observed by analysis of TDDFT results. The connection between the nature of excited states and the behavior of the complexes with different ancillary ligands is elucidated.

Blue-Emitting Iridium Complexes with Substituted 1,2,4-Triazole Ligands: Synthesis, Photophysics, and Devices

Neutral heteroleptic mononuclear iridium(III) complexes with (2,4-difluoro)phenylpyridine and different pyridine-1,2,4-triazole ligands were synthesized and fully characterized. We investigated the effects of substituents in the 5-position of the triazole ring on the photophysical and electrochemical behavior. Increasing the electron-withdrawing capabilities generally leads to a lowering of the HOMO level with a consequent slight widening of the HOMO-LUMO gap and a blue shift in emission. The complexes reported exhibit high emission quantum yields and long luminescent lifetimes, typical of iridium(III) complexes, and most of them show reversible redox processes in solution. Also, many of the complexes reported here have been obtained as single crystals suitable for X-ray crystallography. Two of the complexes were further tested as phosphorescent dyes in OLED devices and showed high external quantum efficiencies (~7%) and color points better than the "standard" for blue iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2']picolinate (FIrpic). We also report the full electrochemical investigation of FIrpic in different solvents.

Efficient and bright blue-emitting phosphorescent materials

A new type of ancillary ligand for blue-emitting heteroleptic iridium complexes has been successfully developed. New ligands, 3-(trifluoromethyl)-5-(pyridin-2-yl)-1,2,4-triazolate and 5- (pyridin-2-yl)-tetrazolate, show stronger blue-shifting power than that of the picolate of FIrpic (iridium (III) bis(4,6-difluorophenylpyridinato)picolate). Organic light-emitting diodes (OLEDs) fabricated with a new complex, FIrtaz (iridium (III) bis(4,6-difluorophenylpyridinato)(5-(pyridine-2-yl)-1,2,4-triazo- late) or FIrN4 ((iridium (III) bis(4,6-difluorophenylpyridinato)(5-(pyridin-2-yl)-tetrazolate), as the blue dopant in the host of mCP (1,3-bis(9-carbazolyl)benzene), exhibit near-saturated blue electrophos- phorescence with Commision Internale de l'Eclairage (CIEx,y) coordinates of (0.14, 0.18) and (0.15, 0.24), respectively. Organic phosphorophores have recently achieved one of the major breakthroughs in the area of organic light-emitting diodes (OLEDs). These materials can enhance the effi- ciency of electroluminescence (EL) by three-fold, for which the external quantum efficiency (EQE) is usually limited to ~5% for devices based on fluorescence emitters. Organic materials are mostly hydrocarbons that do not usually emit phosphorescent light from the triplet state, which is often a forbidden state. Heavy atoms, such as Ir, Os, and Pt, provide the effective spin-orbital coupling that partially relieves the forbidden transition. Among these metal complexes, Ir(ppy)3, ppy = 2-phenylpyridinato, is probably one of the most useful materials for OLED applications. Ir(ppy)3 gives green emission with high efficiency, and several other structural modifications have been attempted in the last decade for the required emitting material suitable in the fabrication of full-color OLED displays. 1 Although many green- and red-emitting iridium complexes have been suc- cessfully synthesized, 2 authentic blue-emitting iridium complexes for phosphorescent OLEDs are notably sparse (Scheme 1).