Energy-transfer and light-emission mechanism of blue phosphorescent molecules in guest-host systems

We investigated the energy-dissipative processes of blue phosphorescent molecules of bis[(4,6-difluorophenyl)-pyridinato-N,C2′](picolinate) iridium(III) (FIrpic) doped into two host molecules, 4,4′-N,N′-dicarbazol-biphenyl (CBP) and 4,4′-bis(9-carbazolyl)-2,2′-dimethyl-biphenyl (CDBP), which have different triplet energies. The phosphorescence spectra were measured over a wide range of temperatures from 8 to 295 K. For FIrpic in a CBP host, we observed both forward and backward energy transfer from the FIrpic triplet states to the CBP triplet states, resulting in unique temperature dependence of the phosphorescence intensity. However, the phosphorescence intensity of FIrpic in a CDBP host did not show any temperature dependence, indicating that the triplet energy was efficiently confined on the FIrpic molecules.