Heavy Atom Effect on the Phosphorescence of Aromatic Hydrocarbons. II. Quenching of Perdeuterated Naphthalene by Alkali Halide

Abstract

The effects of varying concentrations of alkali halide, external heavy atom perturber, on the phosphorescent characteristics of naphthalene-d8 were investigated in rigid solutions of ethyl alcohol at 77°K. It was found that the total phosphorescence could be considered as being composed of two phosphorescent species existing in equilibrium with each other: an excited, free naphthalene and an excited, 1:1 complex of perturber and naphthalene. The phosphorescent decay rates of the latter were in the range of 0.3–0.4 sec−1 when complexed with alkali iodides. The phosphorescent characteristics did not depend significantly upon the metal ion associated with the halide, and CsF up to 0.9M did not quench the phosphorescence measurably. The phosphorescent intensities and mean durations were calculated based on the proposed charge-transfer mechanism. The experimental data were correlated very well using this mechanism. The direct spin–orbit interaction mechanism proposed by Lin and Tweed was found to be unimportant in these systems. The phosphorescent state was discussed theoretically from the charge-transfer viewpoint. The triplet state in the donor was considered to be perturbed by spin–orbit and charge-transfer interactions which gave rise to two terms: one involving only charge-transfer interaction and a second, a cross term involving both spin–orbit and charge-transfer interactions. Thus, it is predicted (and observed) that the mixing of the donor triplet arises only when a charge-transfer complex is formed and this mixing will give rise to the heavy atom effect if a heavy atomic species is present in the acceptor. The nonradiative processes were also discussed in the light of this mechanism.