Deuterium Isotope Effects on the Lifetime of the Phosphorescent Triplet State of Naphthalene

The effect of the number and position of deuterium substituents on the phosphorescence lifetime has been studied for eight different isotopic naphthalenes at 76°K. These studies reveal a significant dependence of the rate of radiationless decay to the ground state on the positions of the protons in the molecule. This conclusion is contrary to the conclusion of Lin and Bersohn that the radiationless decay is independent of the positions of the protons. It is found that protons in the $\alpha$ positions are about 1.6 times as effective as protons in the $\beta$ positions in promoting radiationless transitions. A simple empirical relation is found to give a reasonable fit to the data. A small intermolecular isotope effect on the triplet‐state lifetime of naphthalene‐d₈ in ethanol is also found upon deuteration of the solvent. The experimental results are discussed in terms of existing theories of radiationless processes.