Energy transfer between organic fluorescent CBP host and blue phosphorescent FIrpic and FIrN4 guests

We study the absorption and photoluminescence spectra of iridium(III) bis(4,6-difluorophenylpyridinato-N,C2′)picolinate (FIrpic) and iridium(III) bis(4,6-difluorophenylpyridinato-N,C2′)[5-(2-pyridyl)terazolate] (FIrN4) which were doped in 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) at 10–296 K. It is confirmed, from the temperature dependence, that the intensities of the blue phosphorescence are weak at low temperatures by the exothermic energy transfer from the guest to the CBP host because the T1 state of CBP locates at lower-energy than those of the guest molecules but they are recovered with increasing temperature from 10 to 200 K by the endothermic energy transfer from the host to the guests. The intensity decrease observed above 200 K is attributed to the energy diffusion from the excited CBP to the neighboring unexcited CBP which is more dominant than the endothermic energy transfer. It is observed that the T1 emission of CBP is enhanced much more in the doped CBP films than in the non-doped CBP film because of exothermic energy transfer from FIrpic and FIrN4. Discussion is given on the difference between FIrpic and FIrN4 regarding the energy transfer from CBP host.