Ultrafast Intersystem Crossing in a Red Phosphorescent Iridium Complex

Femtosecond photoluminescence (PL) and transient absorption (TA) studies have been carried out on the red phosphorescent metal complex tris(1-phenylisoquinoline)iridium(III) [Ir(piq)(3)] following excitation of the metal-ligand charge transfer singlet state. Rapid decay of the PL observed at 270 meV above the phosphorescence peak and TA dynamics are indicative of intersystem crossing, which occurs with a time constant of 70 fs. PL decays at 140 meV above the phosphorescence peak are biexponential with time constants of 95 fs and 3 ps, attributed to intramolecular vibrational redistribution (IVR) and vibrational cooling. The larger Ir(piq)(3) ligands facilitate faster dissipation of excess energy by IVR than the smaller Ir(ppy)(3) core.