Phosphorescence quenching of neutral and cationic iridium(III) complexes by molecular oxygen and aromatic electron acceptors

• Oxygen quenching of Ir(III) complexes proceeds via competing nonCT and CT channels.
• Contribution of the CT channel can be predicted by ΔGel for electron transfer.
• The rate constants of the non CT channels are evaluated by kinetic analyses.
• The O2 quenching mechanism involving spin mixing in the CT complexes is revealed.

Phosphorescence quenching of neutral and cationic Ir(III) complexes by molecular oxygen and aromatic electron acceptors (AEAs) have been systematically investigated in acetonitrile. The phosphorescence quenching rate constant (kq) by AEAs increased with decreasing Gibbs energy change (ΔGel) of electron transfer and gave a diffusion-controlled rate constant (kd) of 2.0 × 1010 M−1s−1. The occurrence of electron transfer was substantiated by transient absorption measurements, although the separated ion yields were as low as 0.10–0.17. The oxygen quenching of the Ir(III) complexes originated mainly from two competing pathways through singlet and triplet encounter complexes: a noncharge-transfer (nCT) channel, which is regarded as internal conversion to produce singlet oxygen (1O2), and a charge-transfer (CT) channel, whose rate is governed by ΔGel. The kd value for the Ir(III) complex/O2 systems was estimated to be 4.1 × 1010 M−1s−1. For the systems with ΔGel ≲ −0.8 eV, the limiting kq value was 2.5 × 1010 M−1s−1 and the efficiencies (fΔ) of 1O2 production from triplet states quenched by oxygen were 0.40. The limiting kq value being greater than (4/9)kd and the fΔ value being larger than 0.25 suggested the involvement of 1O2 formation through intersystem crossing between singlet, triplet, and quintet CT complexes.

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