Photochemistry of the singlet oxygen [O-2((1)Delta(g))] sensitizer perinaphthenone (phenalenone) in N,N '-dimethylacetamide and 1,4-dioxane

1H-Phenalen-1-one (perinaphthenone), an efficient singlet oxygen [O₂(¹Δ(g))] sensitizer with a quantum yield of singlet oxygen production (Φ(Δ)) close to unity, has been proposed as a universal standard for O₂(¹Δ(g)) studies. However, data about the photochemical stability of phenalenone are limited to a few protic solvents. This parameter is of crucial interest for singlet oxygen studies under continuous irradiation over long periods of time. In this contribution, we present experimental results demonstrating that, despite low quantum yields, significant photochemical decomposition of phenalenone occurs under steady-state irradiation in air equilibrated 1,4-dioxane and N,N'-dimethylacetamide (DMA), mainly due to hydrogen abstraction from the solvent by its triplet excited state. Furthermore, singlet oxygen lifetime measurements as a function of irradiation time and comparison of the rates of decrease of the steady-state O₂(¹Δ(g)) near-infrared (1270 nm) phosphorescence with those of phenalenone disappearance have led to the conclusion that at least one of the products formed during phenalenone irradiation contributes to singlet oxygen sensitization. Quantum yields of singlet oxygen production by phenalenone, determined using laser-induced optoacoustic calorimetry (L1OAC) and time- resolved near-infrared phosphorescence (TRPD), remain high in 1,4-dioxane and in DMA (0.99 ± 0.05 and 0.87 ± 0.05, respectively), confirming that this compound is a very efficient ¹O₂ sensitizer in a large variety of solvents (the Φ(Δ) value in DMA is the lowest measured so far).