External Heavy-Atom Spin-Orbital Coupling Effect. I. The Nature of the Interaction

The effect of external heavy-atom spin-orbital coupling perturbation has been studied in the lowest energy triplet←singlet absorption spectrum of various 1-halonaphthalenes in 1:1 molar 1-halonaphthalene: alkyl halide binary solutions. It is shown that the interaction prerequisite to the charge transfer which must occur in these systems is probably of a donor—acceptor nature, the alkyl halide functioning as the acceptor and the 1-halonaphthalene functioning as the donor. Steric effects are estimated using molar refraction data and are undoubtedly of importance, particularly when the alkyl halide contains more than one halogen atom. The external heavy-atom effect is spin-orbital coupling in nature as may readily be seen by comparison of the enhancements induced in the T←S absorption of the 1-halonaphthalene by various alkyl halides; the order of enhancement is alkyl iodide>alkyl bromide>alkyl chloride and follows roughly the order of the atomic spin-orbital coupling factors squared. The external heavy-atom effect is greater the larger the internal heavy-atom effect, as is shown both experimentally and theoretically. In fact, an external bromine is more effective than an internal fluorine, while an external iodine is more effective than an internal chlorine, and roughly comparable in enhancement ability to an internal bromine. Simple theoretical considerations enable one to deduce the singlet—triplet mixing coefficients for both the external and internal effects, and to validate the spin-orbital coupling nature of the effect.