Quenching of the first excited singlet state of substituted benzenes by molecular oxygen

Stern–Volmer plots for the quenching of the first excited singlet states of a variety of substituted benzenes by molecular oxygen in the vapour phase are presented and fluorescence decay times measured, yielding quenching rate constants. These values are shown to vary only slightly from compound to compound when the ionization potential of the aromatic molecule is less than 9.0 eV, but above this value, the quenching rate constants fall markedly with increase in ionization potential. The results can be understood in terms of an encounter complex which is stabilized by charge-transfer from excited aromatic to oxygen.