Figure 1.

Mechanism of the formation of electronically excited species by oxidative metabolic processes. One-electron reduction of molecular oxygen by highly reducing species forms O₂^•- (reaction 1). The dismutation of O₂^•- generates H₂O₂ (reaction 2), whereas the subsequent one-electron reduction of H₂O₂ leads to the formation of HO^• (reaction 3). The triplet-triplet energy transfer from triplet chromophore to molecular oxygen results in the formation of ¹O₂ (reaction 4). Formation of ROOH from R and ¹O₂ via ene reaction (reaction 5). The hydrogen abstraction from biomolecules (lipids, proteins and nucleic acids) (RH) by radical ROS (HO^•, HO₂^•) generates R^• (reaction 6). The subsequent one-electron oxidation of R^• brings about the formation ROO^• (reaction 7). The consequent hydrogen abstraction from another R by ROO^• forms ROOH (reaction 8). The one-electron reduction of ROOH results in the formation of RO^• and OH⁻ (reaction 9). ROOR is formed by either the cycloaddition of ¹O₂ (reaction 10) or the cyclization of ROO^• (reaction 11). ROOOOR is formed by the recombination of two ROO^• (reaction 12). The decomposition of ROOR (reaction 14) or ROOOOR (reaction 15) results in the formation of ³R=O^*. Alternatively, the decomposition of ROOOOR can lead to the formation of two RO^• and O₂ (reaction 13). The electronic transition from ³R=O^* to R=O is accompanied by the photon emission (16). The energy transfer from ³R=O^* to chromophores results in the formation of ¹C^* (reaction 17) and ³C^* (reaction 18) chromophores. The electronic transition from ¹C^* and ³C^* to the ground state of chromophores is accompanied by the photon emission. (reaction 19, 20). The triplet-triplet energy transfer from ³R=O^* (reaction 21) and ³C^* (reaction 22) to molecular oxygen forms (¹O₂). Alternatively, the decomposition of ROOOOR via the Russell-type mechanism, results in ¹O₂ (reaction 23) with dimol photon emission (reaction 24) and monomol photon emission (reaction 25).