Bridged cyanine dyes. Part 1. 1-(N-methyl-4-pyridinio)-3-(N-methylpyridylene)cyclopenta-1,4-dienes

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 10⁵.