The Triplet State of fac-Ir(ppy)3

Abstract

The emitting triplet state of fac-Ir(ppy)₃ (fac-tris(2-phenylpyridine)iridium) is studied for the first time on the basis of highly resolved optical spectra in the range of the electronic 0−0 transitions. For the compound dissolved in CH₂Cl₂ and cooled to cryogenic temperatures, three 0−0 transitions corresponding to the triplet substates I, II, and III are identified. They lie at 19 693 cm⁻¹ (507.79 nm, I → 0), 19 712 cm⁻¹ (507.31 nm, II → 0), and 19 863 cm⁻¹ (503.45 nm, III → 0). From the large total zero-field splitting (ZFS) of 170 cm⁻¹, the assignment of the emitting triplet term as a ³MLCT state (metal-to-ligand charge transfer state) is substantiated, and it is seen that spin−orbit couplings to higher lying ^1,3MLCT states are very effective. Moreover, the studies provide emission decay times for the three individual substates of τ_I = 116 μs, τ_II = 6.4 μs, and τ_III = 200 ns. Further, group-theoretical considerations and investigations under application of high magnetic fields up to B = 12 T allow us to conclude that all three substates are nondegenerate and that the symmetry of the complex in the CH₂Cl₂ matrix cage is lower than C₃. It follows that the triplet parent term is of ³A character. Studies of the emission decay time and photoluminescence quantum yield, Φ_PL, of Ir(ppy)₃ in poly(methylmethacrylate) (PMMA) in the temperature range of 1.5 ≤ T ≤ 370 K reveal average and individual radiative and nonradiative decay rates and quantum yields of the substates. In the range 80 ≤ T ≤ 370 K, Φ_PL is as high as almost 100%. The quantum yield Φ_PL drops to ∼88% when cooled to T = 1.5 K. The investigations show further that the emission properties of Ir(ppy)₃ depend distinctly on the complex’s environment or the matrix cage according to distinct changes of spin−orbit coupling effectiveness. These issues also have consequences for optimizations of the material’s properties if applied as an organic light-emitting diode (OLED) emitter.