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How was Tyrian purple dye made using a photochemical reaction?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

Tyrian purple was produced using a photochemical reaction involving secretions from specific predatory sea snails, such as Bolinus brandaris (formerly Murex brandaris) and Stramonita haemastoma.1

The process relied on a pus-like substance contained in a small cyst near the animal's head. When this secretion was applied to textile materials like wool or linen and exposed to sunlight, a photochemical reaction occurred that developed the characteristic purple-red color.1

Chemically, this reaction results in the formation of 6,6'-dibromoindigo. The dye is closely related to indigo, and its production represents one of the earliest human applications of photochemistry, dating back to the late Bronze Age. The intensity and permanence of the color made it a highly valued commodity in antiquity, often associated with imperial or royal status.123