Mesoionic compounds with a bridging nitrogen atom

Mesoionic thiazoloazinium oxides -have been synthesized previously [i], and their spectral characteristics have been investigated [2]. Since these compounds absorb in the visible region of the spectrum (440-600 nm), and since the effective length of the heterocyclic residues in these dyes [3] is quite large (with respect to their effect on color, the heterocyclic residues are equivalent to about 6-7 vinylene groups [4, 5]), it was of interest to us to prepare a vinylogous series of symmetrical polymethine dyes based on these derivatives, and to study their spectral characteristics.