Syntheses and Phosphorescent Properties of Blue Emissive Iridium Complexes with Tridentate Pyrazolyl Ligands

Abstract

Novel neutral mixed-ligand Ir(N=C=N)(N=C)X complexes (N=C=N = 1,3-bis(3-methylpyrazolyl)benzene (bpzb), 1,5-dimethyl-2,4-bis(3-methylpyrazolyl)benzene (dmbpzb), and 1,5-difluoro-2,4-bis(3-methylpyrazolyl)benzene (dfbpzb); N=C = 2-phenyl pyridine (ppy); and X = Cl or CN) have been synthesized and characterized. An X-ray single-crystal structure of the complex Ir(dmbpzb)(ppy)Cl shows that the nitrogen atom in the ppy ligand occupied the trans position to the carbon atom in the tridentate N=C=N ligand of dmbpzb with the Ir-C bond length of 1.94(1) A, whereas the coordinating carbon atom occupied the trans position of chlorine. Electrochemical data show that the complexes exhibit an oxidation Ir(III/IV) process in the potential range of +0.5 approximately 0.9 V and two irreversible reductions at approximately -2.6 and -3.0 V against Fc (0)/Fc (+), respectively. All of the Ir(III) complexes do not emit phosphorescence at room temperature, although strong phosphorescence is exhibited at 77 K with the 0-0 transition centered at around 450 nm and lifetimes of 3-14 mus. DFT calculations indicate that the HOMOs are mainly localized on iridium 5dpi and chlorine ppi*, whereas the LUMOs are mainly from the ppy ligand pi* orbitals. The phosphorescence originates from a (3)LC state mixed with the (3)MLCT and (3)XLCT ones. Temperature-dependent lifetime measurements of Ir(dfbpzb)(ppy)Cl reveal the existence of a thermal deactivation process with a low activation energy (1720 cm (-1)) and very high frequency factor (2.3 x 10 (13) s (-1)). An unrestricted density functional theory indicates that the dd state, in which both the Ir-N (pyrazolyl) bond lengths increase considerably, exists almost at the same energy as that for the phosphorescent state. A thorough analysis based on the potential energy surfaces for the T 1 and S 0 states allows us to determine the reaction pathway responsible for this thermal deactivation. The calculated activation energies of 1600 approximately 1800 cm (-1) are in excellent agreement with the observed values.

Chirality in the Photochemical mer→fac Geometrical Isomerization of Tris(1‐phenylpyrazolato,N,C2′)iridium(III)

“…Nonradiative deactivation via a thermally accessible dd state might be an important route. [20] We also tried to rationalize the mechanism of the present isomerization. Route A and B in Figure 4 proceed by Ir-N 1 and Ir-N 3 bond dissociations of the axial ligands, respectively, and these reactions proceed through rehybridization from square pyramidal (SP) to trigonal bipyramidal (TB) intermediates.…”

“…These HSOMO must have antibonding interactions between the Ir and axial N atoms, which is similar to the recent study of a decay process involving the tridentate ligand of an Ir complex. [20] The α-122 HSOMO orbital has metal-to-ligand charge-transfer character for the fully optimized structure as shown in Figure 6 (left-hand side), and the structure has Ir-N 1 and Ir-N 3 bond lengths of 2.05 and 2.04 Å, respectively, which match the values obtained by X-ray singlecrystal analysis (Ir-N 1 2.205 and Ir-N 3 2.013 Å). [6] In this structure, the lowest MO having an antibonding Ir-N bond is observed at α-132, and this is far above the HSOMO.…”

Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices

“…28) [187]. The emissive triplet states (T 1 ) of these complexes are described as admixtures of states with MLCT (Ir → ppy) XLCT (Cl → ppy), and LC (ppy → *) character.…”

Ag/Tio2 Nano Thin Films Catalyzed Efficient Synthesis of 6-Amino-4-Aryl-3-Methyl-1,4-Dihydropyrano[2,3-C] Pyrazole-5-Carbonitriles At Green Conditions

“…Due to these properties, they are widely utilized in the pharmaceutical industry, medicinal chemistry and they are also important core in natural products 18,19 . In addition pyrazoles have been used for treatment of type 2 diabetes, obesity, thrombopiotinmimetics and antiangiogenesis because of their kinase inhibitory effects 20 . Furthermore, some pyrazoles are employed in polymer and supramolecular chemistry, in the food industry, and as UV stabilizers and cosmetic colorings and in some cases it was observed that these compounds also have liquid crystal properties [21][22][23][24][25] .…”