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

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 (2 n + 1) sp 2 -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 2 n + 3 of π -centres and an even number 2 n + 4 of π -electrons, where n is equal to the number of –(R) = C(R′)– groups (usually vinylene groups –CH CH–). 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. The first synthesized polymethine dyes were colorants of beautiful cyan color. Since many different new dye classes were prepared and used in a wide variety of industrial applications. A lot of them belong to the polymethine dye class due to their special π -electronic features. 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. • Many different dye classes were prepared and used in a wide variety of industrial applications since the synthesis of the first three dyes of beautiful cyan color in 1857. A lot of them belong to the polymethine dye class due to their special π -electronic features. However, due to long history there are sometimes different names for the same dye class. In addition, some of these names and their origin have been forgotten. This article attempts to bring clarity to the names of the individual polymethine dye classes, the background of the naming, to highlight common π -electronic and structural features and to discuss the associated typical properties. • Furthermore, it was shown that the fine structure in electronic absorption spectra arises from the symmetric carbon–carbon valence vibration of the polymethine chain in S 1 electronic state. In symmetrical polymethine dyes the differences in equilibrium bond lengths [ R e ( S 1 ) – R e ( S 0 )] of S 0 and S 1 electronic states is low and λ max corresponds to the 0–0 vibronic transition. With progressive deviation from c 2 = 0.5 the π -electronic structure in S 0 becomes increasingly unsymmetrical and thus the bond-length alternation grows. As a result, the differences R e ( S 1 ) – R e ( S 0 ) grows and λ max corresponds to the 0–1, 0–2 or any other vibronic transition. This effect should always be taken into account in all discussions on relationships between π -electronic structure and λ max values.