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

Abstract

The quenching rate constants of singlet oxygen O2(1Δg) 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. 1010 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-d8 and mesitylene. It is also confirmed that N,N,N‘,N‘-tetramethyl-p-phenylene diamine quenches O2(1Δg) nearly exclusively by full electron transfer in D2O, but electron transfer is not observed in other solvents.