Iridium complexes refer to metal complexes with six-coordinated structure and distorted octahedral configuration, consisting of metal ion centers and polydentate ligands. Phosphorescent materials composed of transition-metal complexes have attracted extensive in recent years because of their ability to harvest both singlet and triplet exciton in organic light-emitting diodes. Iridium complexes are currently the most studied and promising phosphorescent materials due to their short triplet lifetime, high luminous efficiency and good wavelength tunability.
According to the different molecular structures, phosphorescent iridium complexes can be divided into the following two categories.
The main emission mechanism of homoleptic iridium complexes is ligand-based metal-ligand charge transfer (MLCT) emission. By changing the type and structure of the ligand (CˆN), and the energy level of the anti-bonding orbital (LUMO) of the molecule, the emission wavelength of the phosphorescent material can be adjusted. Homoleptic iridium complexes with the structure Ir(CˆN)3 exhibit two steric configurations at different reaction temperatures, namely meridional (mer) and facial (fac). The meridional isomers are kinetically controlled and usually form at lower temperatures while the facial isomers are controlled by thermodynamic factors. Under the conditions of heat and light, the configuration of the iridium complex with the same ligand will change to facial. 
Fig.1 Fac and mer configurations and cyclometalated ligands
Heteroleptic iridium complexes have two cyclometalated (CˆN) ligands and a single bidentate auxiliary ligand (LX), and the dominate emission mechanism is auxiliary ligand intermediated MLCT emission. The regulation of the light-emission wavelength can be achieved by adjusting the structure of the main ligands (CˆN) or auxiliary ligands. Acetylacetonate (acac) is often used as an (LX) ligand, which can further react with the ligand (HCˆN) to form homoleptic iridium complexes.
The emission spectrum of phosphorescent iridium complexes can basically cover the entire visible light region and a series of phosphorescent materials based on iridium complexes have been synthesized and applied. According to the different emission colors, they are usually divided into red, green and blue iridium complex phosphorescent materials.
The π-conjugated system of red phosphorescent complex is relatively large, so it is easy to have a strong π-π interaction, which aggravates the aggregation between the complex molecules and easily leads to the quenching phenomenon. By introducing various chemical groups into piq(1-phenylisoquinoline), btp(2-(benzo[b]thiophen-2-yl)pyridine) and other ligands, a series of red iridium complex phosphorescent materials are synthesized.
The research on green phosphorescent materials is the earliest among three materials. The ligands of green iridium complex phosphorescent materials are mostly 2-phenylpyridine (ppy) and its derivatives. A series of multifunctional phosphorescent iridium complex materials were synthesized by functionalizing or modifying the ligand ppy. Cyclometalated iridium complex Ir(ppy3) is the earliest green electrophosphorescent material used in devices.
The energy of blue iridium complex phosphorescent materials is the highest, so these complexes have a relatively wide energy-level gap. Blue iridium complexes phosphorescent materials have a certain degree of conjugate structure, but at the same time, the dipole moment of the molecule should not be too large, otherwise red shift will occur. Most blue phosphorescent materials emit blue light by introducing highly electronegative fluorine atoms to increase the triplet energy level of the molecule. The most widely commercialized blue iridium complexes is iridium(III) bis[(4,6-diﬂuorophenyl)-pyridinato-N,C2']picolinate (FIrpic).
Fig.2 Chemical structure and ORTEP drawing of FIrpic
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