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Abstract
This paper reviews the results of a series of efforts to develop QSAR models for aromatic chemicals whose toxicity is enhanced by the ultraviolet radiation present in sunlight. Photoinduced toxicity of polycyclic aromatic hydrocarbons (PAHs) was found to be a result of competing factors: structural (such as molecular stability and light absorbance) and external (irradiation energy and intensity). These two factors interact, producing a complex, multilinear relationship between toxicity and electronic structure. The HOMO-LUMO gap provided a useful ground-state index to explain the persistence, light absorption, and eventually, the photoinduced toxicity of PAHs. The derived QSAR clearly distinguished phototoxic differences between pairs of structurally similar PAHs, such as phenanthrene and anthracene, benzo [a] anthracene and tetracene, et cetera. Those PAHs exhibiting photoinduced toxicity were consistently within a specific range of the electronic parameter. Further modeling revealed a significant correlation between molecular electronic structure of excited-state PAHs and toxicity. The effect of substituents on photoinduced acute toxicity of PAHs also was investigated. Some substituents such as alkyl and hydroxy moieties do not significantly reduce the HOMO-LUMO gap of parent PAHs. Nitro- and chloro- moieties cause more significant variations of the HOMO-LUMO gap. It is concluded that photoinduced toxicity of PAHs is mainly dictated by the electronic structure of the parent chemicals. Evaluation of the phototoxicity of flexible aromatic molecules (α-terthienyls), generally supported the PAH models.