A more complex chromophoric group is the triple ethylenic grouping: C > C =, the introduction of which was rendered necessary by the discovery of certain coloured hydrocarbons.
The chromophoric groups are rarely strongly acid or basic; on the other hand, the auxochromes are strongly acid or basic and form salts very readily.
Notable differences attend the neutralization of the chromophoric and auxochromic groups.
With basic substances, the chromophoric combination with a colourless acid is generally attended by a deepening in colour; auxochromic combination, on the other hand, with a lessening.
Metals producing colourless salts with acids, is attended by colour changes contrary to those given above, auxochromic combination being accompanied by a deepening, and chromophoric by a lessening of the tint.
Mention may be made of the phenomenon of halochromism, the name given to the power of colourless or faintly-coloured substances of combining with acids to form highly-coloured substances without the necessary production of a chromophoric group. The researches of Adolf von Baeyer and Villiger, Kehrmann, Kauffmann and others, show that this property is possessed by very many and varied substances.
(2) The chromophore-auxochrome theory (Kauffmann) regards colour as due to the entry of an " auxochrome " into a " chromophoric " molecule.