Anomalous Phosphorescence from an Organometallic White-Light Phosphor

We report theoretical results on the possible violation of Kasha's rule in the phosphorescence process of (acetylacetonato)bis(1-methyl-2-phenylimidazole)iridium(III) and show that the anomalous emission from both the T₁ and T₂ states is key to its white-light phosphorescence. This analysis is supported by the calculated Boltzmann-averaged phosphorescence lifetime of 2.21 μs, estimated including both radiative and nonradiative processes and in excellent agreement with the experimentally reported value of 1.96 ± 0.1 μs. The T₂ state is found to be of metal-to-ligand charge-transfer character (dπ → nπ), and the d orbital contribution comes from 5d_z and 5d_x-y, whereas the S₁ and T₁ states both have dπ-pπ character with significant 5d_xz orbital contribution, allowing for efficient intersystem crossing from the S₁ to the T₂ state and, in turn, phosphorescence from the T₂ state. Our results open new opportunities for tailoring the phosphorescence wavelength and thus the design of molecules with improved photovoltaic properties.