Ultrafast Excited-State Dynamics in Cyclometalated Ir(III) Complexes Coordinated with Perylenebisimide and Its π‑Radical Anion Ligands

Cyclometalated Ir­(III) complexes showing strong absorption of visible light and a long-lived triplet state are of particular interest as photocatalysts and photosensitizers. Steady-state and femtosecond transient absorption spectroscopy, complemented with TD-DFT computations, are used to study the ultrafast intersystem crossing (ISC) dynamics of two perylenebisimide (PBI)-containing cyclometalated Ir­(III) complexes (Ir–PBI and Ir–NPBI). The S₁ → T_n ISC dynamics of these complexes is ultrafast, similar to the conventional Ir­(III) complexes bearing a ¹MLCT → ³MLCT transition. Much longer triplet decay lifetimes are observed in Ir–PBI (τ_T = 11.4 μs) and Ir–NPBI (τ_T = 7.9 μs) compared to conventional Ir­(III) complexes (τ_T < 5 μs). This can be rationalized by the character of the lowest-energy triplet excited state (T₁). In the case of the PBI-containing complexes, the T₁ is of intraligand character (³IL), and the T₁ → S₀ spin–orbital coupling (SOC) is 3 cm^–1. This is in contrast with conventional Ir­(III) complexes, where the ³MLCT → S₀ has SOCs of 490 cm^–1. In the presence of a typical sacrificial electron donor, triethanolamine (TEOA), the coordinated PBI ligands are transformed into their radical form under an inert atmosphere, either with or without photoexcitation. We found that the triplet-state population is decreased with formation of the radical. The doublet excited state has a lifetime in the range of 4–7 ps.