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

Abstract

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 → Tn 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 (τT = 11.4 μs) and Ir–NPBI (τT = 7.9 μs) compared to conventional Ir(III) complexes (τT < 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) comp...