# Barriers and Chemistry in a Bottle: Mechanisms in Today's Oxygen Barriers for Tomorrow's Materials
## 2 Background
### Oxidative Degradation Mechanisms
#### 2.1.1 Chemistry of Oxygen
The sensitizing effect is also very intense in milk, where vitamins A and B\({}_{2}\) are very prone to oxidation [28; 29; 30]. Diary products are therefore usually packaged in a completely opaque packaging, but often without any extra oxygen barrier added. Figure 3: Electronic conversions in the highest occupied molecular orbital (HOMO) (\(\pi\)) and lowest occupied molecular orbital (LUMO) (\(\pi^{\bullet}\)) of an unsaturated carbon-carbon bond after light absorption. Molecular orbitals of the 1s and 2s atomic orbitals are left out of the figure. Figure 2: Molecular orbitals of triplet oxygen [19]. In organic solar cells, a photosensitizer is present as the so-called p-type semi-conductor phase, which is also able to undergo intersystem crossing and subsequent oxygen activation. Additionally, this phase is in contact with an electron acceptor (also noted as the n-type semi-conductor phase) with a LUMO that has a lower energy than the p-type semi-conductor. The electron-donating p-type can consist of a wide variety of molecular structures (thiophene-based, carbazole-based, etc. ), but the electron accepting n-type rather consistently contains fullerenes [23]. The mechanism of the solar cell relies on the absorption of light by the photosensitizing p-type semi-conductor. At the interface between the two semi-conductor phases, the excited electron can be donated to the fullerene structure, creating two radical species. As both the positively charged donor and the negatively charged acceptor are connected to electrodes by semi-conductive polymer chains, an electric potential is created and the radicalar charges move to their respective electrode. Both of these radicalar species are prone to the activation of triplet oxygen, similar to type I photosensitization. In OLED structures, this approach is similar but reversed to produce light from electricity [23,24]. Following the initiation, the propagation step will further break down the relatively unstable organic peroxyl groups to reactive oxygen species (ROS), which is a collective term for oxygen radicals such as peroxyl-, superoxide- or hydroxyl-radicals [31]. The decay of peroxyl groups may involve many different pathways, leading to different products such as carbonyl groups, alcohols, unsaturated carbon bonds or epoxides. An exemplary pathway is the reaction of two peroxyl radicals through a short-lived tetroxide intermediate, given in Figure 5[32]. The differentiation of the reaction products shows the complexity of the possible reactions in an oxidative environment. The first reaction is a radical termination reaction, but the others will propagate the radical chain. As such, the oxidation reactions are autocatalytic and therefore referred to as the autoxidation of organic compounds. The effect is increased in the presence of middle to late transition metal ions (Fe, Cu, Mn, Co, etc.). These metals can be converted from one stable oxidation state to another with the exchange of one electron. They are therefore very susceptible to radical species in reactions called the Fenton-like reactions. The name is a reference to [PERSON] who was the first to report the activation of hydrogen peroxide by iron in 1894 [33]. Fenton chemistry involves the activation of relatively stable peroxides by transition metals to highly reactive radicals, thereby catalyzing radical oxidation in aqueous environments. These reactions are utilized in applications such as water treatment or paint drying, but are mostly known for their negative impact as it promotes degradation of food and induces oxidative stress in cell structures [34, 35, 36, 37]. The reactivity of a Fenton-catalyst includes a list of many different one-electron exchanges between the metal ions, ROS and organic substrates. The Haber-Weiss  
Figure 4: Sensitization of triplet oxygen to reactive oxygen species. The initiating component is the excited triplet state of a sensitiizer (e.g., dyes or pigments) after light absorption and subsequent intersystem crossing.