Highly Phosphorescent Bis-Cyclometalated Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light Emitting Diodes

The photophysics of octahedral 4d6 and 5d6 complexes has been studied extensively. These complexes, particularly those prepared with Ru and Os, have been used in a variety of photonic applications, including photocatalysis and photoelectrochemistry. These complexes are attractive in photochemical applications, because they generally have long-lived excited states and high luminescence efficiencies, increasing the likelihood of either energy or electron transfer occurring prior to radiative or nonradiative relaxation. Strong spin–orbit coupling of the 4d or 5d ion leads to efficient intersystem crossing of the singlet excited states to the triplet manifold. The long lifetimes of these excited states are due to the triplet character of this state. Mixing of the singlet and triplet excited states, via spin–orbit coupling, removes the spin-forbidden nature of the radiative relaxation of the triplet state, leading to high phosphorescence efficiencies.