Question

for azo dyes, why does an instantaneous and dramatic color change occur when the two coupling reagents are mixed together?

Answer 1

Azo dyes are prepared by azo coupling reaction involving the coupling of a diazonium compound on another aromatic system where the former acts as the electrophile for a substitution reaction.

Azo dyes exhibit vivd coloration primarily due to the extensive pi-electron delocalization that flows between the two aromatic coupling molecules through the diazo bond in the dye. The extensive delocalization results in the molecule being stabilized greatly which in turn results in the pi-electrons absorbing ambient light and transmitting in the visible region of the electromagnetic spectrum.

In general, all compounds, aromatic or otherwise but with significant delocalization fluoresce in the ultraviolet region. In case of azo dyes, since the system is stabilized even more due to more resonance, the delocalized electrons are excited more easily from amibent light thus reflecting light of a lower energy. When such a transmittance occurs in the visible region, the compound appears brightly coloured. Also, since the starting materials do not possess such extensive delocalization, the formation of the azo dye through such a fast reaction is observed by a marked appearance of colour.