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How does oxidation of luciferin lead to the emission of a photon?

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time-lapse photo of fireflies — Bioluminescence Time-lapse image of fireflies in the Catskill Mountains, New York. Firefly light is a cold light, with approximately 100 percent of the energy given off as light and only a minute amount as heat. Only light in the visible spectrum is emitted.

In bioluminescent organisms, the emission of a photon occurs through an enzyme-catalyzed oxidation process. While specific mechanisms vary by species, the general principle involves the enzyme luciferase facilitating the oxidation of the substrate luciferin by molecular oxygen.12

Taking the well-studied firefly as an example, the process begins when luciferin reacts with ATP and luciferase to form a complex known as luciferase-luciferyl-adenylate. When this complex reacts with oxygen, it releases sufficient energy to shift the electronic configuration of the complex from a stable, low-energy ground state to a high-energy excited state.12

As this high-energy complex is unstable, it rapidly loses its excess energy by radiating a photon of visible light, allowing the molecule to return to its ground state. In other organisms, such as bioluminescent bacteria, the process involves the oxidation of reduced flavin mononucleotide (FMNH2) to form an intermediate complex, which subsequently reacts with an aldehyde to trigger light emission.12