Properties of singlet molecular oxygen O-2((1)Delta(g)) in binary solvent mixtures of different polarity and proticity

We have measured O-1(2) lifetimes (tau) and radiative rate constants (k(tau)) for O-1(2) phosphorescence in a number of binary solvent mixtures with components of water, deuterium oxide, acetonitrile (AN), 1,4-dioxane (D), propylene carbonate (PC), or ethylene carbonate (EC). We have correlated k(r) and tau with the mixtures' composition, polarizability, polarity and proticity. Our data show that O-1(2) properties are independent of both solvent polarity and solvent proticity, and confirm that the radiative rate constant k(tau) for O-1(2) phosphorescence best correlates with solvent polarizability. The prevailing trend is that k(tau) values increase with the increasing polarizability of solvent medium; in 1,4-dioxane mixtures, this increase is almost Linear. However, for other solvents the correlation with polarizability is more solvent specific. For example, polarizability remains virtually unchanged over a wide composition range in AN-H2O mixtures, but the k(tau) value increases by almost a factor of 5. In H2O-D2O mixtures, polarizability decreases with increasing D2O concentration while the value of k(tau) increases twofold. This divergence suggests that, besides collision complexes, the interaction of O-1(2) with these solvents may involve a more specific interaction such as exciplex formation. Such specific interactions are probably responsible for considerable scattering that is usually observed in plots that relate k(r) values measured in chemically different solvents to macroscopic polarizability. The best way to obtain accurate k(tau) and k(d)(tau) values for O-1(2) in mixed solvents is still to measure them experimentally. We present here measured values of k(tau) and tau in a variety of mixed solvents; these values can be helpful in calculating O-1(2) yields from measurements of steady state phosphorescence. (C) 1997 Elsevier Science S.A.