Spectral behaviour of linked heterocyclic systems and related dyes.

A wide series of electronic spectra of assembled heterocyclic systems (pyridine or quinoline, benzo- or pyrido-X-azole), of their quaternary salts and of the related polymethine dyes has been considered in the light of general relationships obtained. The position and the intensity of the main bands depend on electronic and steric factors according to the relative positions of the linked rings, the nature of heteroatoms, the number and the position of positive charges and the length of the chromogens. A self-consistent set of simple rules was obtained, which explained and justified peculiar behaviours and/or apparent exceptions.