Charge-Transfer Quenching of Singlet Oxygen O2(1Δg) by Amines and Aromatic Hydrocarbons

The quenching rate constants of singlet oxygen O 2 ( 1 Δ g ) luminescence at 1.27 μm by 29 amines and aromatic hydrocarbons were measured in acetonitrile and benzene. Quenching occurs via reversible charge transfer with the formation of an exciplex with a partial (δ ≈ 0.2 esu) electron transfer. An estimate of the intersystem-crossing rate constant for the exciplex of ca. 10 10 s -1 is obtained, and a modest decrease with increasing exciplex energy is noted. The data predict and it is confirmed that charge transfer plays a significant role in the nonradiative decay of singlet oxygen in solvents as difficult to oxidize as toluene- d 8 and mesitylene. It is also confirmed that N, N, N ‘, N ‘-tetramethyl- p -phenylene diamine quenches O 2 ( 1 Δ g ) nearly exclusively by full electron transfer in D 2 O, but electron transfer is not observed in other solvents.