# 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  
Thermodynamically, molecular oxygen is a highly oxidative species, as evidenced by highly exothermic reactions such as burning. However, the reaction of oxygen with organic compounds is hindered kinetically. Initiation of an oxidative reaction with molecular oxygen requires the crossing of a large energy barrier. Oxygen molecules can be excited to a singlet state with the absorption of 95 kJ mol\({}^{-1}\) of energy. This might be attained from heat during burning processes, but at room temperature the major pathway of oxygen activation in food and beverages is the excitation of photosensizers [19]. Visible and UV-light provide photon energies above 150 kJ mol\({}^{-1}\) and can therefore act as a source to cross the kinetic barrier. In order for this energy to get transferred to oxygen, the light first needs to be absorbed by a chromophore, which can be provided by organic molecules called photosensizers. Such compounds encompass dyes and pigments (e.g., methylene blue, chlorophyll, etc. ), riboflavin or sometimes other aromatic hydrocarbons [20]. Active compounds in OLEDs and organic solar cells get similarly excited before emitting visible light or after absorbing visible light, providing the same energy levels. They therefore possess a photosensitizing ability as well and are present in a much more concentrated form, proving their high need for protection from oxygen [21; 22; 23; 24]. Oxygen in its most abundant form is a diradical with a quantum spin number of 3, noted as a triplet. This is the most stable form, following [PERSON]'s rule [25]. It is visualized in Figure 2. Triplet oxygen prefers to react through one-electron oxidations. However, this is unfavorable for most organic molecules that are usually in a fully paired state with quantum spin number 1 (singlet state). An exception to this rule is the presence of organic radical species, providing unpaired electrons that triplet oxygen is able to readily attack [19]. Photosensitizing compounds absorb light in order to form an excited singlet state. In other words, an electron in the \(\pi\)-orbital, also noted as the highest occupied molecular orbital (HOMO) will jump to the anti-bonding \(\pi^{\bullet}\)-orbital, which is the lowest unoccupied molecular orbital (LUMO). This excited electron will fall back to a lower energy state. This can be either to the ground state, emitting heat or light (fluorescence) or it can be subjected to intersystem crossing. In the latter case, the now separated \(\pi\)-electron pair will align its spin, thus creating a triplet state. Figure 3 shows these conversions in the molecular orbitals of a photosensitizer [19]. When at any time intersystem crossing does occur, the excited triplet state sensitizer can again return to the ground state by emitting heat or light (phosphorescence) or it can activate triplet oxygen via two reaction types, depending on the sensitizer and the substrate (Figure 4) [26]. Type I reactions require molecules with an easily abstractable electron or hydrogen atom. It is therefore most common for substrates with a phenolic structure and quinones. Type I reactions are driven by a radicalmechanism. Alternatively, the sensitizer may donate its own electrons to triplet oxygen in the formation of a superoxide radical. Type II photosensitization occurs by converting the triplet oxygen molecules to singlet oxygen. Singlet oxygen then readily reacts with double bonds in a non-radical mechanism [19; 26]. Each sensitization type ultimately leads to the formation of peroxides, which can further propagate radically. Beers and wines, especially red wines, contain many pigments and highly oxidizable compounds. Protection from light is required here to limit the oxidation processes. Usually, the packaging is adapted to this by adding green or amber colorants to the bottles. The green bottles will block UV-light, limiting photosensitization in this area, while amber colorants also block a wider range of the visible light [27].