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

Pomarico, E.; Silatani, M.; Messina, F.; Braem, O.; Cannizzo, Andrea; Barranoff, E.; Klein, J. H.; Lambert, C.; Chergui, M. (2016). Dual Luminescence, Interligand Decay, and Nonradiative Electronic Relaxation of Cyclometalated Iridium Complexes in Solution. Journal of physical chemistry. C, 120(30), pp. 16459-16469. American Chemical Society 10.1021/acs.jpcc.6b04896

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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 population of the lowest excited state, which then undergoes vibrational relaxation in ∼0.5 ps. Finally, at short times, Ir5 exhibits fluorescence from the lowest 1LC state, which decays in ∼100 fs to the manifold of 3LC states. Overall, this study shows that, although the ultrafast relaxation to the lowest electronic states is quite similar to that of other transition-metal complexes, most of the differences occur at the lowest emissive states, with effects such as time-delayed dual fluorescence, interligand decay, and nonradiative relaxation to the ground or lower-lying metal-centered states. Understanding these effects is crucial for obtaining optimal performances of iridium complexes, calling for further iterations between chemical synthesis and photophysical studies to optimize these complexes.

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