HEAVY ATOM SUBSTITUENT EFFECTS ON THE PHOSPHORESCENCE OF NAPHTHALENE AND PHENANTHRENE

This work was supported by a grant of the National Institutes of Health.Author Institution: Department of Chemistry, University of ColoradoHeavy atom effects on triplet decay rates have been investigated for the eight monohalonaphthalenes and ten monohalophenanthrenes. The consequent decrease in the lifetime with substitution is shown to vary with the square of the atomic spin-orbit coupling factor for the attached halogen. The position dependence is qualitatively related to the unpaired spin density distribution in the molecule. An anomolously short life time measured for 4-bromophenanthrene is attributed to deviation from planarity induced by steric hindrance. Investigation of the lifetimes of several polyhalogenated naphthalenes and phenanthrenes indicate both positive and negative deviations from an additive model for heavy atom effects. Negative deviations occur whenever two substituents are ortho to each other and these deviations increase with decreasing carbon-carbon distance and with increasing substituent size. Steric considerations are thus implicated as responsible for negative deviations, Polyhalogenated molecules exhibiting positive deviation from additivity are discussed in terms of resonance effects. The phosphorescence emission spectra of all of the halogenated compounds have been examined and the halogen induced shifts of the 0,0 band are tabulated and attributed to conjugative effects