The temperature dependence of the triplet lifetimes of some aryl iodides and bromides

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Abstract
A variable temperature pulse radiolysis study of the triplet state of iodonaphthalene and iodobiphenyl indicates that an intramolecular energy relocation process is inhibited as the temperature is lowered from 20 °C, resulting in a lengthening of the lifetime of the triplet state. It is postulated that the triplet state energy of these aryl iodides is channeled into the C–I bond, causing bond dissociation. The activation energies for this process, in toluene, have been measured to be 5.9, 4.8, and 3.4 kcal/mole for 1‐iodonaphthalene, 2‐iodonaphthalene and 4‐iodobiphenyl, respectively. Similar experiments with aryl bromides in toluene, at temperatures greater than 20 °C, are also interpreted as indicating the dissociation of the C–Br bond from the triplet state. For 4‐bromobiphenyl and 4,4′‐dibromobiphenyl the activation energy is in the range of 10–13 kcal/mole. The results are discussed in relation to the work of Bersohn and co‐workers.