Class of chemical compounds; intensely colorful dyes
The formazans are compounds of the general formula [R-N=N-C(R')=N-NH-R"], formally derivatives of formazan [H2NN=CHN=NH], unknown in free form.[1]
Formazan dyes are artificial
reductases
. They have a variety of colors from dark blue to deep red to orange, depending on the original tetrazolium salt used as the substrate for the reaction.
Structure and reactivity
Formazans are intensely colorful compounds characterized by the following structure: [-N=N-C(R)=N-NH-],
azo (−N=N−) dyes. Their structure was first defined in 1892, by von Pechmann and by Bamberger and Wheelwright independently.[3][4] Their deep colour and redox chemistry derive from their nitrogen-rich backbone.[5]
Formazans have a high tautomeric and conformational flexibility.[5] Due to the two alternating double bonds in the backbone, formazans can exist in four possible isomeric forms: syn, s-cis (closed form); syn, s-trans (open form); anti, s-cis; and anti, s-trans (linear form).[6]
1,5-disubstituted formazans can exist as two tautomers (1 and 2 in the image below). Upon