Rapid intersystem crossing in highly phosphorescent iridium complexes

Femtosecond time-resolved absorption measurements on Ir complexes are performed. On excitation at wavelength 400 nm, singlet metal-to-ligand charge-transfer (MLCT) states are excited. On probing at 580 nm, a transient absorption appears with time constant 70–100 fs. This transient state rising rapidly but decaying very slowly is assigned to be the lowest triplet MLCT state. Accordingly intersystem crossing in these Ir complexes is rapid. Measured with a nanosecond laser, emission from the lowest triplet MLCT state decays with lifetimes 1.55, 1.65 and 1.7 μs for Ir(ppy)3, Ir(DBQ)2(acac) and Ir(MDQ)2(acac), respectively. On 266-nm excitation with a femtosecond laser, high-energy states with mostly π–π* character (ligand-centered states) are accessed; they relax to the lowest triplet MLCT state with time constant 100–350 fs. The efficient flow of energy from high-energy electronic states to 3MLCT results in high quantum yield in electroluminescent phosphorescence in organic light-emitting diodes.