Fig. 7. A schematic of phototoxic cascade reactions in the presence of BL and thymol.

Endogenous proporphyrins-like compounds absorb BL to attain an excited triplet-state from an electronic ground state and react with a ground state molecular oxygen (³O₂) to generate singlet oxygen (¹O₂) (I). The ¹O₂ oxidizes thymol to form TQ and/or THQ via an endoperoxide intermediate (II). The resultant TQ and THQ, particually, TQ act as a photosensitizer and generates more O₂•⁻ and ¹O₂ that can in turn oxidize thymol, continuously replenishing the TQ pool, which forms the first autoxidation cycle (blue-dashed outline). The second autoxidation cycle comprises THQ oxidation into TQ that can be then photo-hydrolyzed into THQ (green dashed outline). O₂•⁻ undergoes dismutation and forms H₂O₂ that is converted into the most detrimental •HO via a Fenton reaction or photolysis (III). The deleterious •HO initiates a chain reaction that oxidatively damages the bacteria.