Quantitative determination of 1Σg+ and 1Δg singlet oxygen in solvents of very different polarity.: General energy gap law for rate constants of electronic energy transfer to and from O2 in the absence of charge transfer interactions

The quenching of excited triplet states of sufficient energy by O-2 leads to O-2((1)Sigma(+)(g)) and O-2((1)Delta(g)) singlet oxygen and O-2((3)Sigma(-)(g)) ground-state oxygen as well. The present work investigates the question whether in the absence of charge transfer (CT) interactions between triplet sensitizer and O-2 the rate constants of formation of the three different O-2 product states follow a generally valid energy gap law. For that purpose, lifetimes of the upper excited O-2((1)Sigma(+)(g)) have been determined in a mixture of 7 vol % benzene in carbon tetrachloride, in chloroform, and in perdeuterated acetonitrile. They amount to 1.86, 1.40, and 0.58 ns, respectively. Furthermore, rate constants of O-2(1 Sigma(+)(g)), O-2((1)Delta(g)), and O-2(3 Sigma(-)(g)) formation have been measured in these three solvents for five pi pi* triplet sensitizers with negligible CT interactions. The rate constants are independent of solvent polarity. After normalization for the multiplicity of the respective O-2 product state, the rate constants follow a common dependence on the excess energies of the respective product channels. This empirical energy gap relation describes also quantitatively the rate constants of quenching of O-2((1)Delta(g)) by 28 carotenoids. Therefore, it represents in the absence of CT interactions a generally valid energy gap law for the rate constants of electronic energy transfer to and from O-2.