Effect of External Heavy Atoms on Decay Processes from the Triplet State

An investigation was made to determine the relative sensitivities to the external heavy-atom perturbation of the radiative (kp) and the nonradiative (kqp) decay constants for the return of aromatic molecules in their first triplet state to the ground state. The investigation was carried out in glasses at 77°K using alkyl halide and ethanol mixtures as solvents. A combination of EPR and optical emission spectroscopy was used as the diagnostic technique. The results of the study show that kp is more sensitive than kqp to the nature of the heavy atom in the solvent. It is also found that kIS (the intersystem crossing rate constant) is less sensitive than kp to the heavy-atom perturbation. Finally, analysis of the data indicates not only that kqp is not totally quenched upon perdeuteration of naphthalene but also that kqp>kp in naphthalene-d8 in ethanol.