Emission properties of Ir(ppy)_3 and Ir(ppy)_2(CO)(Cl). Compounds with different transition types

摘要

Emission properties of Ir(ppy)_3 and of Ir(ppy)_2(CO)(Cl) (ppy~- = 2-phenylpyridinate) are investigated between 1.2 K and 300 K. Distinct differences are found for the emission spectra, decay behavior, and relaxacion 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 ΔE_(Ⅱ, Ⅰ) = 13.5 cm~(-1) and ΔE_(Ⅲ, Ⅰ) = 83 cm~(-1). The decay times are τ_Ⅰ = 145 μs, τ_Ⅱ = 11 μs, and τ_(Ⅲ) = 750 ns. At ambient temperature, all three substates contribute to the emission process, while at 1.5 K, 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 (τ_I = 330 μs, τ_II = 100 μs, τ_(Ⅲ) = 9 μs) due to long spin-lattice relaxation (SLR) times. With increasing temperature to T ≥ 30 K 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. Cyclovoltarnmetric data are also given and discussed in relation to the spectroscopic data.