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Dynamics of the Excited States of [Ir(ppy)2bpy]+ with Triple Phosphorescence

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journal contribution
posted on 30.09.2010, 00:00 by Shih-Hsiang Wu, Jung-Wei Ling, Szu-Hsueh Lai, Min-Jie Huang, Chien Hong Cheng, I-Chia Chen
We investigated the relaxation dynamics of bis(2-phenylpyridinato-)(2,2′-bipyridine)iridium(III), [Ir(ppy)2bpy]+ using the technique of time-resolved spectroscopy. In the visible emission spectra this molecule exhibits triple phosphorescence: displaying blue, green, and orange bands. From the dependence of spectral shifts with polarity of solvent, decay lifetimes, and the results of calculations using time-dependent density functional theory, we assigned these three emitting states to be triplet interligand charge-transfer (3LLCT), metal-to-ligand ppy charge transfer (3MLCTppy), and metal-to-ligand bpy charge transfer (3MLCTbpy) states. The blue states were formed promptly after excitation at wavelength 355 nm; the one lying at higher energy decaying with a time coefficient 0.79−2.56 ns is assigned to be a triplet MLCT, and the other at lower energy decaying in 1.5−2.8 μs is assigned to 3LLCT(A), A symmetry. This decay time coefficient of 3LLCT(A) decreases with increasing dielectric constant of the solvent indicating this state mixing of some MLCT character. The green state 3MLCTppy decays in 0.13−4.8 ns to a nearby intermediate state either 3MLCTppy or 3MLCTbpy. The orange state 3MLCTbpy is coupled to the intermediate state to have a rise time about 0.36−0.84 ns and decays in 425−617 ns. Although many triplet states exist in a small energy range, they couple weakly to display triple emission. All 3LLCT and 3MCLT states are coupled to the singlet 1LLCT manifold directly and/or indirectly and contribute to the emission in the visible range.