Photochemistry of perylene adsorbed on unactivated silica gel and alumina used as model substrates of the atmospheric particulate matter

Perylene is a member of a large and diverse class of organic pollutants known as polycyclic aromatic hydrocarbons (PAHs) which are ubiquitous environmental contaminants released into the atmosphere either by anthropogenic (man-made) or by natural sources. They are derived from the combustion of organic matter (e.g., fossil fuels, forest fires). Once in the atmosphere, PAHs are associated with atmospheric particulate matter where they undergo chemical and photochemical transformations that lead either to their degradation or to their transformation to products which could be more dangerous than the parent compound.; Perylene is a phototoxic PAR Although not definitively established, its phototoxicity could suggest possible carcinogenicity. Since perylene could be transformed into more toxic compounds, such as other PAHs, the study of its phototransformations when adsorbed on surfaces, and the characterization and identification of the products is relevant due to the health risk effects to human beings. In addition, it would provide information of the effect of the nature of the surface or adsorbent on the physical and chemical properties of this PAR. All this information could be significant in order to reach a better understanding of the mechanisms of phototransformations and its fate in the environment.; This dissertation work was dedicated to the study of the photochemistry and photophysics of perylene adsorbed on silica gel and alumina in order to understand its environmental fate. These surfaces are used as models of the atmospheric particulate matter. The photophysical and photochemical properties of perylene adsorbed on the different surfaces were studied using UV-Vis absorption and fluorescence spectroscopy. The intermediate species were characterized using diffuse reflectance laser flash photolysis and electron spin resonace techniques.; Our results show that the chemical and physical properties of the adsorbent induced changes in the absorption and emission properties of perylene. Coadsorbed species (O₂, Ar, H₂O) also induced changes in the photochemistry and photophysics of perylene adsorbed on silica gel and alumina. Coadsorbed O₂ enhanced the rate of photodegradation suggesting its participation in this process. Coadsorbed H₂O induces the formation of a new species, probably a Perylene-H₂O complex or a perylene aggregate formed by its displacement from the adsorption sites by H₂O. The experimental data show that both the radical cation and singlet oxygen participate in the route of photodegradation of perylene, thus the photodegradation mechanism occurs via a mixed pathway (Type I and Type II mechanisms). The separation, characterization and identification of the photoproducts was carried out using HPLC equipped with UV-Vis diode array and MS detection. Two of the products were identified as 1,12-perylenedione and 3,10-perylenedione. Three products were characterized as a perylenedione and two perylenediols based on their m/z.; This thesis work provides information on the phototransformations of perylene adsorbed on models of the atmospheric particulate matter under controlled laboratory conditions. This information helps in the understanding of the probable processes undergone by perylene and other PAHs in the environment. The contribution of this work is of relevance on environmental issues since knowing the nature of the photoproducts and their mechanisms of phototransformations, the fate of perylene in the atmosphere can be established, and the the human health risks of Perylene and other PAHs can be evaluated.