Abstract 1, 3-Bis (N-methyl-4-pyridinio) propane is condensed with a series of α-diketones to form deep red monocyanine dyes 8. In strongly acidic solutions C-protonation occurs at the central carbon atom. A similar condensation with α-ketoesters yields zwitterionic cyanines 12 with extinction coefficients exceeding 10 5.
![1-methyl-4-[3-(1-methylpyridin-1-ium-4-yl)propyl]pyridin-1-ium,diiodide Structure](https://image.chemsrc.com/caspic/314/61736-46-7.png)