Bring back order in the polymethine dye medley: Classification, structure and spectra

Abstract

The term polymethine dyes was introduced by Walter König in 1922 and four years later he created a general classification of polymethine dyes. In current terms polymethine dyes are characterized by a conjugated chain of (2n + 1) sp2-hybridized carbon atoms that connects a terminal π-electron accepting group and a terminal π-electron donating group. The second feature is an odd number 2n + 3 of π-centres and an even number 2n + 4 of π-electrons, where n is equal to the number of –C(R) = C(R′)– groups (usually vinylene groups –CHCH–). These features confer special structure-absorption relationships on the class owing to their strong influence over the nature of possible transitions from the electronic ground state and thus dye absorption spectra. Over the years, the general definition has been divided into sub-classes - in some instances, not systematically - with a great diversity of complete different chemical structures, belonging to the polymethine dyes. Some of these aspects have been forgotten or overlooked or even misunderstood and such mistakes have been propagated in the literature. In this review an attempt is made to bring clarity to the nomenclature of dye sub-classes, the history of the respective terms and to highlight common structural features.