Coupled Polymethine Dyes: Six Decades of Discoveries

This review provides a comprehensive examination of the applications of the seminal coupling principle introduced by Siegfried Dähne and Dieter Leupold in 1966. Their heuristic and groundbreaking work proposed that combining multiple polymethine subunits within a single chromophore enables orbital coupling, consequently narrowing the HOMO-LUMO gap, and yielding redshifted optical properties. These outcomes are particularly valuable for developing organic dyes tailored for visible-to-near-infrared applications. Despite their potential, coupled polymethines remain relatively underexplored, with most reported instances being serendipitous discoveries over the last six decades. In light of this, our review compiles and discusses the reported coupled polymethine structures, covering synthetic, spectroscopic, theoretical and applicative aspects, offering insights into the structure-property relationships of this unique class of dyes and perspectives for their future applications.

6-Chloro-3H-benzo[d][1,2,3]dithiazol-2-ium Chloride

This short note describes the synthesis of an amorphous benzo[1,2,3]dithiazole chloride salt (commonly known as a ‘Herz salt’) by use of the Herz reaction. Hetero- and homolytic transformations of this species to a variety of useful adducts in medicinal and materials chemistry are well established, although there are limited examples of isolation in the literature, and characterisation data is even harder find. While several studies have confirmed the structure of the benzodithiazole ring beyond doubt, (having generated suitably crystalline salts with large counterions for XRD-analysis), there remains value in understanding and optimising the synthesis of the simple, amorphous polymorphs. For the first time, MS data is provided for this compound and a new mechanism of its formation is proposed based upon new experimental observations and data.

New 3,1,2,4-benzothiaselenadiazines, related π-heterocycles including Herz cations, radicals and molecular complexes, and Bunte salts

3,1,2,4-Benzothiaselenadiazines, 1,3,2,4-benzodithiadiazines and 1,2,4,3,5-benzotrithiadiazepines are synthesized from Ar–NSN–SiMe3 and chalcogen chlorides, and converted into Herz salts, radicals and molecular complexes, and S- and Se-Bunte salts.