Charge transfer interactions in the generation of singlet oxygen O2(1Δg) by strong electron donors

The quenching rate constant of the triplet state by molecular oxygen and the efficiency of singlet oxygen generation have been measured for 12 amines in cyclohexane and benzene. For the best electron donors, the average rate constant for quenching by energy transfer is 6.0 x 10(9) M-1 s(-1). For the same compounds, the rate constant of quenching via enchancement of internal conversion is 1.2 x 10(10) M-1 s(-1). The energy transfer component of the total quenching rate constant is almost twice as fast as the maximum from the standard Porter model. The mechanisms of quenching via quintet, singlet, and triplet channels are discussed for amines and aromatic hydrocarbons, and intersystem crossing out of the quintet manifold is proposed.