The effect of deuterium and chlorine substitution on triplet → singlet transition probabilities in naphthalene

The effect of substitution in aromatic molecules on the radiative transition probability from the triplet phosphores cent state (T) to the ground (S/sup 0/) state, was measured by irradiating a glassy solution of the phosphorescent substance, at 77 deg K, in a microwave cavity. The relative concentration of molecules in their triplet state was determined from the (integrated) intensity of the magnetic resonance signal. After the exciting light was switched off the corresponding phosphorescence intensity and decay time were measured by means of a photomultiplier and a recording instrument. Preliminary results on naphthalene, deuteronaphthalene, and beta - chloronaphthalene are tabulated. (L.N.N.)