QSAR Estimates of Excited States and Photoinduced Acute Toxicity of Polycyclic Aromatic Hydrocarbons

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Abstract
Direct calculation of the energy of excited states for polycyclic aromatic hydrocarbons using semi-empirical methods on a supercomputer were inadequate in explaning spectrosopic data or measured phototoxicity. The energy difference between frontier orbitals HOMO-LUMO gap of “average” excited state structures of the PAHs correlated with the measured excited state energies and their observed photoinduced toxicity. The multi-linear relationship between phototoxicity and hypothetical triplet state HOMO-LUMO gap is similar to that based on ground state structures. This molecular descriptor discriminated phototoxic PAHs into a narrow range of approximately 6.2+ mn;0.4 eV. Chemicals with a HOMO-LUMO gap in the triplet state greater than 7.1 eV were not phototoxic in simulated sunlight. Chemicals with gaps less than 5.8 eV are likely to be unstable in water and degrade too rapidly to enable determination of photoinduced potency to aquatic organisms. Several preliminary structure-phototoxicity relationships for PAHs are presented, together with predictions of toxicity for eight untested aromatic chemicals suggested as suitable test chemicals for evaluating the reliability of the QSARs. Practical issues associated with computing ground state and excited state indices for use in QSAR of phototoxicity are discussed.

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