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

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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) Å, 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∼0.9 V and two irreversible reductions at approximately −2.6 and −3.0 V against Fc0/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 μs. DFT calculations indicate that the HOMOs are mainly localized on iridium 5dπ and chlorine pπ*, whereas the LUMOs are mainly from the ppy ligand π* orbitals. The phosphorescence originates from a 3LC state mixed with the 3MLCT and 3XLCT 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 × 1013 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 T1 and S0 states allows us to determine the reaction pathway responsible for this thermal deactivation. The calculated activation energies of 1600∼1800 cm−1 are in excellent agreement with the observed values.

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