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The mechanism of the internal heavy atom effect is discussed from the viewpoints of the dynamic and spectral properties of the triplet spin sublevels of 1-halonaphthalenes. The heavy atom effect on the radiative rate constant for phosphorescence emission in the O–O band is most prominent in the Tx emission, somewhat less in the Ty emission, and is small in the Tz emission. (The x and y axes correspond to the long and short axes of the naphthalene molecule, and the z axis is perpendicular to the molecular plane.) The heavy atom effect on the nonradiative decay rate constant is most prominent in the decay of the Tx and Ty sublevels and is small in the decay of the Tz sublevel. These experimental results are interpreted in terms of a mechanism in which spin-orbit coupling involving the halogen atomic orbital plays an essential role. The heavy atom enhancement in the radiative and nonradiative decay of the Tx sublevel is mainly governed by the spin–orbit coupling between the (ππ*) triplet state and (σπ*) or (πσ*) singlet states. The heavy atom enhancement in the radiative and nonradiative decay of the Ty sublevel is, on the other hand, mainly due to the spin–orbit coupling with (nπ*) singlet states, where n stands for the halogen nonbonding orbital lying along the x axis. The radiative and nonradiative decay of the Tz sublevel, however, is not influenced by the heavy atom because the spin–orbit coupling involving the halogen atomic orbital does not possess one center terms.
Saigusa et al. (Wed,) studied this question.