Oxidative polymerization of hydroxylated naphthalene derivatives: Modeling free radical coupling pathways to PAH-derived melanins of biological and astrochemical relevance

The oxidative polymerization of hydroxylated naphthalene derivatives, including chiefly 1,8-dihydroxynaphthalene (1,8-DHN), is an important process of broad biological, environmental and astrochemical relevance, being involved for example in the biosynthesis of black melanin pigments in certain pathogenic fungi (Aspergillus fumigatus) and halophilic ascomycetous black yeasts, in the combustion of organic matter, in the metabolic evolution of xenobiotics in water and soil pollution, and in the complex transformations of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). The latter processes, especially, attract growing interest in relation to the abiogenetic theory of the origin of life. PAHs are widely diffuse in the ISM and more than 20% of the carbon in the universe may be associated with PAHs, thus serving as precursors to life molecules. Dust grain chemistry in particular could be central to PAHs reprocessing, since icy matrices can trap several astrochemically relevant CHON-bearing molecules, and mineral catalysis could have played a significant role in prebiotic chemistry. Studies on PAHs reactivity under astromimetic conditions have resulted in the identification of several hydroxylated products, which may represent the initial products of reprocessing. In addition, growing interest is currently focusing on the possible role of PAHs and their oxygenated derivatives as the main determinants of infrared emission features seen in different astrophysical environments. Early experiments showed that exposure of naphthalene to ultraviolet radiation in ice under astrophysical conditions leads to the generation of phenolic and quinone derivatives, allowing specific prediction of the existence and relative abundances of various oxidized naphthalenes in meteorites. However, apart from those studies, the mechanisms of conversion of PAHs under conditions mimicking those occurring in the ISM are largely uncharted, which makes it difficult to probe the actual role of this chemistry in the reprocessing of carbon compounds. Aim of this thesis is to investigate the oxidative pathways of hydroxylated naphthalenes, including 1-naphthol, 2-naphthol, 1,8-DHN, 1,6-DHN and 2,6-DHN, as representative in order to gain a first insight into the mode of coupling of these compounds, the nature and properties of the resulting polymers and their actual formation under astrochemically relevant conditions. To this aim, the thesis developed along three main phases: a) the isolation and characterization of the main oligomer intermediates in the oxidative conversion of the selected hydroxynaphthalenes; b) the synthesis and characterization of the resulting melanin-like polymers; c) the characterization of the species formed by UV-induced oxidation of hydroxynaphthalenes following adsorption on astrochemically-relevant minerals, by comparison with chemically-produced model polymers. The main outcome of these studies was: a) the elucidation of the reaction pathways and mode of coupling of hydroxylated naphthalenes; b) the definition of basic structure-property relationships in DHN-derived polymers; c) the characterization of melanin-type materials generated via solid state polymerization on mineral surface. Overall, these results fill a gap in the chemical literature on the oxidative polymerization of hydroxylated naphthalenes and provide an improved background to inquire into the mechanisms of PAH reprocessing in the ISM and other astrochemically relevant environments