Emission properties of Ir(ppy)3 and of Ir(ppy)2(CO)(Cl) (ppy- = 2-phenylpyridinate) are investigated between 1.2K and 300K. Distinct differences are found for the emission spectra, decay behavior, and relaxation dynamics. For both compounds, the emission spectra are broad or only moderately resolved, nevertheless, from time-resolved investigations individual properties of the triplet substates can be deduced. For Ir(ppy)3, the emission stems from three well separated triplet substates I, II, and III with ΔEII,I = 13.5 cm-1 and ΔEIII,I = 83cm-1.
The decay times are τI = 145 μs, τII = 11 μs, and τIII = 750 ns. At ambient temperature, all three substates contribute to the emission process, while at 1.5K, the emission results only from substate I. This is due to fast relaxation processes. For Ir(ppy)2(CO)(Cl), also three substates are identified, but they are only separated by less than 1 cm-1. Therefore at T = 1.2K, all three substates emit independently with three decay times (τI = 330 μs, τII = 100 μs, τIII = 9 μs) due to long spin-lattice relaxation (SLR) times. With increasing temperature to T ≥ 30K and thus growing SLR rates the emission decay becomes monoexponential. In particular, from the amount of splitting (zero-field splitting, ZFS) of the emissive triplet, it is concluded that the emission of Ir(ppy)3 stems from 3MLCT substates, which result from metal-to-ligand charge transfer (Ir5dppyπ*) states, while Ir(ppy)2(CO)(Cl) emits from ligand centered triplet substates (3LC) of ppyππ* character. Cyclovoltammetric data are also given and discussed in relation to the spectroscopic data.