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 mixtu res 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, polarizab ility, polarity and proticity. Our data show that O-1(2) properties ar e independent of both solvent polarity and solvent proticity, and conf irm 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 solv ent medium; in 1,4-dioxane mixtures, this increase is almost Linear. H owever, for other solvents the correlation with polarizability is more solvent specific. For example, polarizability remains virtually uncha nged 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, polariza bility decreases with increasing D2O concentration while the value of k(tau) increases twofold. This divergence suggests that, besides colli sion complexes, the interaction of O-1(2) with these solvents may invo lve a more specific interaction such as exciplex formation. Such speci fic 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 mea sured values of k(tau) and tau in a variety of mixed solvents; these v alues can be helpful in calculating O-1(2) yields from measurements of steady state phosphorescence. (C) 1997 Elsevier Science S.A.