A comparison of the absorption spectra of some typical unsymmetrical cyanine dyes

Abstract
A study has recently been made of the absorption spectra of eleven series of typical symmetrical cyanine dyes; each series comprised, in general, four members, in which two similar heterocyclic nuclei were linked by one, three, five, and seven methenyl groups (Fisher and Hamer 1936). The correlation of frequencies of maximum absorption with changes in chemical constitution proved so interesting that it seemed desirable to extend our work to other series of cyanine dyes, having two dissimilar, instead of two similar, nuclei. The object, with which we set out, was to combine each of the nuclei already studied with every other one, in order to ascertain how exactly the absorption maximum of each unsymmetrical cyanine thus obtained is the mean of those of the two related symmetrical dyes. That it is approximately the mean has been often accepted as axiomatic. For instance, Mills and Odams (1924) considered the structure of their new 2 : 4´-carbocyanine to be confirmed by the fact that its absorption maximum was intermediate in position between those of the 2 : 2´- and 4 : 4´-carbocyanines. As before, ethiodides were selected for the study, and our aim was to examine purified specimens of the compounds, under standard conditions, in methyl alcoholic solution.