Transient electronic processes of phosphorescent Ir(ppy)3 under endothermic energy transfer from fluorescent TPD host material

Temperature-sensitive photoluminescence (PL) of Ir(ppy)3 is observed in a spin-coated film of TPD containing 5 wt% Ir(ppy)3. Numerical calculation is made for the emission intensity using rate equations for three zero-field splitting substates of the triplet T1 state. Good agreement is obtained for the temperature dependence between the calculated and measured PLs. It is concluded that (1) the increase of PL intensity observed at 10–100 K is due to the spin-phonon transitions among the three substates and the exothermic energy transfer from Ir(ppy)3 to TPD, (2) the endothermic energy transfer from TPD to Ir(ppy)3 is responsible for the increase above 100 K, and (3) the decrease of PL intensity observed above 200 K is caused by the energy diffusion from TPD to neighboring unexcited TPS.