Assignments of Lowest Triplet State in Ir Complexes by Observation of Phosphorescence Excitation Spectra at 6 K

We tried the assignment of the origin of phosphorescent bands in Ir complexes. It is important to elucidate the luminescent mechanism in order to design organic light-emitting devices (OLEDs) besed on new materials. The Stokes shift between the phosphorescence and phosphorescence excitation spectra of Ir complexes such as fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3], fac-tris(2-(2-thienyl)pyridine) iridium(III) [Ir(thpy)3], bis(2-phenylpyridine) iridium(III)benzo[h]quinoline [Ir(ppy)2bzq], fac-tris(benzo[h]quinoline) iridium(III) [Ir(bzq)3] and bis[(4,6-difluorophenyl)pyridinato](picolinato) iridium(III) [FIrpic] was measured in a solution and (phenyl)4Sn at 6 K. The amount of Stokes shift corresponds to the nature of the lowest triplet state. We discovered that the amount of Stokes shift clearly differs depending on whether the lowest triplet state of each Ir complex is triplet metal-to-ligand charge transfer (3MLCT) or 3π–π*. Namely, the case of 3MLCT shows a large shift, while the case of 3π–π* shows a small shift. We also present the resolved phosphorescence and phosphorescence excitation spectra of Ir complexes in (phenyl)4Sn. The sharp bands were assigned to the 3π–π* transition, and the broad bands were assigned to the 3MLCT state. The nature of the lowest triplet state is also discussed on the basis of resolved spectra.