Crystal violet lactone

The central carbon in the leuco form is in a tetrahedral configuration, with four covalent bonds.

In an acidic environment, the lactone ring is broken, with the oxygen detaching from the central carbon.

This now-trivalent position is a planar carbocation that is resonance stabilized, interconnecting the π systems of the aromatic rings and the amino functional groups.

This single large conjugated system is a chromophore with strong absorption in visible spectrum, giving this compound its distinctive color.

Crystal violet lactone as a fuel marker was covered by the BASF patent DE4422336A1 until June 2014.