Phosphorescence dynamics and spin-lattice relaxation of the OLED emitter Ir(btp)2(acac)

The lowest triplet state T1 of Ir(btp)2(acac) doped into CH2Cl2 is studied by site-selective and time-resolving emission and by excitation spectroscopy at cryogenic temperatures. Different sites due to different environments of Ir(btp)2(acac) show dissimilar intensity distributions of the substate emissions, zero-field splittings, emission decay times, and spin-lattice relaxation (SLR) times. Temperature dependent investigations of the SLR processes reveal the importance of the direct process and the Orbach process. Moreover, the emitting T1 state is assigned as 3LC (ligand-centered) state with significant 1,3MLCT (metal-to-ligand charge transfer) perturbation. This classification is related to the usability of phosphorescent emitters in OLEDs.