Energy-transfer dynamics of blue-phosphorescent iridium and rhodium complexes doped in fluorescent molecules

— The temperature-dependent photoluminescence features of polycarbonate thin films doped with blue-phosphorescent molecules, either bis[(4,6-difluorophenyl)-pyridinato-N,C2′] (picolinate) iridium (Flrpic) or bis(2-phenylpyridinato-N,C2′) (acetylacetonate) rhodium [(ppy)2Rh(acac)], which have an equivalent triplet energy of 2.64 eV, have been studied. The photoluminescence intensity of the Flrpic-doped polycarbonate thin film did not show any dependence on temperature. On the other hand, as for the (ppy)2Rh(acac)-doped polycarbonate thin film, decreasing photoluminescence intensity with increasing temperature (especially above 100K) was clearly visible. These results reflect that the internal heavy-atom effect of (ppy)2Rh(acac) is weaker than that of Flrpic. Furthermore, the steady-state and time-resolved photoluminescence spectra of tris(8-hydroxyquinoline) aluminum (Alq3) thin films heavily doped with Flrpic or (ppy)2Rh(acac) (50 wt.%) at 8K was studied. It was found that the enhanced phosphorescence from Alq3 is mainly due not to the external heavy-atom effect by doping with the phosphorescent molecule but to the exothermic triplet energy transfer from the phosphorescent molecule to Alq3.