Synthetic dyes with the general formula $\ce{R2N[CH=CH]_{n}CH=N^{+}R2 <->}$ $\ce{R2N^{+}=CH[CH=CH]_{n}NR2}$ ($n$ is a small number) in which the nitrogen and part of the conjugated chain usually form part of a heterocyclic system, such as imidazole, pyridine, pyrrole, quinoline and thiazole, e.g.