Dual Luminescence, Interligand Decay, and Nonradiative Electronic Relaxation of Cyclometalated Iridium Complexes in Solution

Femtosecond broadband photoluminescence studies are presented for Ir(ppy)3 (Ir1), Ir(ppy)2(pic) (Ir2), Ir(ppy)2(bpy)(PF6) (Ir3), Ir(ppz)3 (Ir4), and Ir(ppz)2dipy (Ir5) (where ppy = 2-phenylpyridine, pic = picolinate, bpy = 2,2′-bipyridine, ppz = 1-phenylpyrazole, and dipy = 5-phenyldipyrrinato) in solution. Upon 400-nm excitation of Ir1–Ir3, we observed a prompt population of the lowest MLCT states. The higher states decay on an ultrafast time scale (<100 fs), whereas the lowest 3MLCT state undergoes further vibrational relaxation on a 1-ps time scale. In Ir3, this relaxation is accompanied by an interligand decay from the ppy to the bpy ligand in ∼1.5 ps. For the ppy-containing complexes (Ir1 and Ir2), we found that, at 100 ps, the luminescence is red-shifted with respect to the steady-state emission. This is explained in terms of a time-delayed dual luminescence, which we attribute to a double-well minimum configuration of the lowest emitting triplet states involving the ppy moiety. Ir4 shows a prompt p...