Making Dioxazine Violet — The Staggeringly Powerful Purple That Replaced Cobalt Violet
Dioxazine Violet (Pigment Violet 23, CI 51319) is a synthetic organic pigment based on the triphenodioxazine chromophore — one of the most powerful colourants ever synthesised. A single grain of PV23 tints as much paint as fifty grains of cobalt violet. This extraordinary tinting strength, combined with perfect lightfastness and chemical resistance, made it the dominant violet pigment of the twentieth century.
The molecule was first synthesised in 1928 during research into carbazole chemistry at the Farbwerke Hoechst laboratories in Germany, but it was not commercialised as a pigment until the 1950s. The chromophore consists of two N-ethylcarbazole units linked by a central dioxazine ring system — two fused oxazine rings containing both nitrogen and oxygen bridges. This extensive conjugated system absorbs strongly in the yellow-green region, producing an intense, deep blue-violet.
The synthesis involves three distinct stages: first, carbazole (a tricyclic aromatic from coal tar) is N-ethylated and then nitrated and reduced to give 3-amino-N-ethylcarbazole. Second, two molecules of this amine are condensed with chloranil (tetrachloro-1,4-benzoquinone), displacing two chlorine atoms and forming two C–N bonds. Third, heating the intermediate at high temperature causes oxidative ring closure — the quinone oxygen atoms bridge to the carbazole rings, forming the two oxazine rings that complete the dioxazine chromophore.
PV23 rapidly displaced both cobalt violet (PV14, expensive and weak) and manganese violet (PV16, weak tinting strength) for virtually all applications — artists' paints, printing inks, automotive coatings, and plastics.
SAFETY WARNING: This synthesis uses chloranil (toxic irritant), nitrobenzene (highly toxic — absorbed through skin, causes methaemoglobinaemia), ethyl bromide (toxic lachrymator), and concentrated nitric acid (corrosive oxidiser). Perform the entire synthesis in a fume hood with full respiratory protection. Nitrobenzene has a deceptively pleasant almond odour — do not rely on smell as a warning.
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