Its molecular structure, definitively established in 1966, consists of a xanthone core with four chlorine atoms and three hydroxyl groups, giving it the chemical formula C14H6Cl4O5.

The compound serves an important biological role in lichens as a UV-protective agent, absorbing harmful UVA radiation while allowing visible light to pass through for photosynthesis.

[2] Some confusion arose in 1937 when researchers examining an Irish specimen of L. sordida isolated a different compound with a melting point of 258–260 °C (496–500 °F), which they mistakenly identified as thiophanic acid.

In 1966, Siegfried Huneck definitively established thiophanic acid's structure as 2,4,5,7-tetrachloro-1,3,6-trihydroxy-8-methylxanthone, placing it firmly within the xanthone class of natural products.

This discovery helped explain why thiophanic acid represents a unique category of chlorinated xanthones – a class of compounds that are almost exclusively produced by lichens through specialised biosynthetic pathways.

The resulting yellow crystalline product had a melting point of 243–244°C and was confirmed to be identical to natural thiophanic acid isolated from the lichen Lecanora rupicola through matching physical and spectral properties.

The resulting compound was then chlorinated using sulphuryl chloride and finally demethylated (removal of a methyl group) using pyridine hydrochloride to yield thiophanic acid.

[11] This synthesis successfully replicated the distinctive substitution pattern that makes thiophanic acid an important chemotaxonomic marker – part of the characteristic set of secondary metabolites used to identify and classify lichen species.