STUDY OF GEOMETRY-EFFECTS ON HEAVY-ATOM PERTURBATION OF THE ELECTRONIC-PROPERTIES OF DERIVATIVES OF THE NONALTERNANT POLYCYCLIC AROMATIC-HYDROCARBONS FLUORANTHENE AND ACENAPHTHO[1,2-K]FLUORANTHENE

A detailed study of the electronic behavior of a series of even nonalternant polycyclic aromatic hydrocarbons derived from fluoranthene is presented. Compounds were synthesized that contained substantial barriers to rotation of substituents into conjugation with the parent hydrocarbon. The heavy atom bromine was incorporated into several compounds to probe its ability to quench fluorescence by spin-orbital coupling. External heavy atom solvent effects on the quenching of fluorescence were also investigated. Fluorescence spectra are presented that clearly indicate that intramolecular heavy atom effects (HAEs) can be transmitted through phenyl substituents conjugated with a polycyclic aromatic hydrocarbon. The magnitude of the transmitted HAE is shown to vary as a function of the ability of the substituent to rotate into planarity with the polycyclic core. The steric strain present in the various fluoranthene derivatives is modeled using molecular mechanics and semiempirical molecular orbital calculations. Semiempirical molecular orbital calculations of the HOMO and LUMO energies are compared with data obtained experimentally by cyclic voltammetry.