On the Importance of Ligand-Centered Excited States in the Emission of Cyclometalated Ir(III) Complexes.

The photophysical behavior of the cyclometalating Ir(III) complexes [Ir(ppy) ₂ (bpy)] ⁺ , where Hppy is 2-phenylpyridine and bpy is 2,2'-bipyridine (complex 1 ), and [Ir(diFppy) ₂ (dtb-bpy)] ⁺ , where diFppy is 2-(2,4-difluorophenyl)pyridine and dtb-bpy is 4,4'-di- tert -butyl-2,2'-bipyridine (complex 2 ), has been theoretically investigated by performing density functional theory calculations. The two complexes share the same molecular skeleton, complex 2 being derived from complex 1 through the addition of fluoro and tert -butyl substituents, but present notable differences in their photophysical properties. The remarkable difference in their emission quantum yields (0.196 for complex 1 in dichloromethane and 0.71 for complex 2 in acetonitrile) has been evaluated by characterizing both radiative and nonradiative decay paths. It has emerged that the probability of decaying through the nonradiative triplet metal-centered state, normally associated with the loss of the emission quantum yield, does not appear to be the reason behind the reported substantially different emission efficiency. A more critical factor appears to be the ability of complex 2 to emit from both the usual metal-to-ligand charge-transfer state and from two additional ligand-centered states, as supported by the fact that the respective minima belong to the potential energy surface of the lowest triplet T ₁ state and that their phosphorescence lifetimes are in the same order of magnitude. In contrast, the emission of complex 1 can be originated only from the metal-to-ligand charge-transfer state, being the only emissive T ₁ minimum. The results constitute a significant case in which the emission from ligand-centered states is the key for determining the high emission quantum yield of a complex.