DEPARTMENT OF CHEMISTRY

Nitro-polycyclic aromatic hydrocarbons, also referred to as nitro aromatic pollutants, pose a significant health concern. Human exposure to nitro aromatic pollutants has the potential to cause lung cancer. Moreover, many nitro aromatics are acutely toxic, mutagenic, or carcinogenic on laboratory mammals and in in vitro test systems. They originate primarily from incomplete combustion processes and are present in food, ambient air, aquatic systems, soils and sediments. Despite its potential negative impact on human health, nitro aromatic compounds continue to be emitted into ambient air from municipal incinerators, motor vehicles, industrial power plants, and other sources.

The prevalent hypothesis is that degradation by light is the main route of natural removal of nitro aromatic pollutants from the environment. However, studies focused on understanding the electronic energy relaxation pathways in nitro aromatic pollutants are lacking and might hold the key for predicting the fate of these pollutants in the environment and for designing effective pollution control strategies. Recently, Assistant Professor Crespo-Hernández and collaborators have investigated how the excess electronic energy evolves in 1-nitropyrene after light absorption. A schematic representation of their findings is shown in the figure. 1-nitropyrene is a nitro aromatic compound of environmental concern implicated to the direct-acting mutagenicity of diesel exhaust particles. Crespo-Hernández et al. have shown that when 1-nitropyrene absorbs visible light, the excess electronic energy in the lowest-excited singlet state is redistributed to nuclear degrees of freedom in less than 100 femtoseconds (1 fs = 10^-15 seconds). Then, intersystem crossing to a high-energy triplet state occurs in seven picoseconds (1 ps = 10^-12 seconds). The high-energy triplet state decays in an ultrafast timescale to the triplet state of lower energy. Once in the lower-energy triplet state, 1-nitropyrene returns to the ground state thirty millionths of a second later. The results by Dr. Crespo-Hernández et al. show that 1-nitropyrene absorbs visible light and then dissipates most of it in ultrafast time scales.  This effectively allows the molecule to avoid chemical transformation.  This resistance to light degradation might explain the persistence of 1-nitropyrene in the atmosphere and its relative abundance in urban air compared to other nitro aromatic pollutants.