Vanishing First- and Second-Order Intramolecular Heavy-Atom Effects on the (π*→n) Phosphorescence in Carbonyls

The coupling schemes and matrix elements resulting from the first-order spin—orbit perturbation of the lowest n,π* triplet state in halogenated carbonyl compounds are examined. Because of the one-electron nature of Hs·o·, only seven of the 24 states resulting from one-electron transitions in X2CO, are found to perturb the 3A2(n,π*) state. Of these seven, only three states give rise to interactions involving the halogen atom: 1B2(n,σ2*) through two-center terms and small contribution of one-center term, 1A1(n,σ1*) through a small contribution of two-center term, and 1B1(σ1,π*) through three-center terms. Qualitative calculation indicates that of these three only the first interaction, when X=iodine, can be an order of magnitude larger than the one-center term on oxygen in H2CO. The absence of one- and two-center terms in the moment of the 1B2(n,σ2*)↔1A1 transition, however, reduces the importance of this state in inducing intramolecular heavy-atom effects. Because of the one-electron property of the spin—orbit and vibronic operators, only two states, the 1B1(σ1,π*) and 1B2(n,σ2*), can be used for second-order (s.o. and vibronic) intramolecular heavy-atom perturbations. It is found that there are no intermediate states through which these states can couple with the emitting triplet state. These conclusions indicate that intramolecular heavy-atom effects on the π*→n phosphorescence radiative lifetime are expected to be small in X2CO or in any other carbonyl. It is pointed out that the large intramolecular heavy-atom effects observed for the π*→π phosphorescence is a result of the relatively small spin—orbit perturbation present in the parent heavy-atom-free molecule.