MO‐LCAO‐Berechnungen an schwefelhaltigen π‐Elektronensystemen. XLIX. Tetramethinverbrückte Oxalsäurederivate – neue farbige Pigmente von indigoider Struktur

MO‐LCAO Calculations on Sulfur‐containing π‐Electron Systems. XLIX. Tetramethine Bridged Derivatives of the Oxalic Acid – Novel Coloured Pigments of Indigoid StructureWhereas thioamides react with the oxalylchloride to Δ2‐thiazoline‐4,5‐diones 1, which are precursors of the versatile thioacylisocyanate 2 synthons, the analogous reaction between thioamides and oxalyl‐bis(arylimidchlorides) completely fails. In the case of aliphatic thioamides the intermediate 2‐methyl‐Δ2‐thiazoline‐4,5‐diimides 3 undergo oxydative dimerization. The poorly soluble products are tetramethine bridged heteroanalogous oxalates 4 or isomers. The u.v./vis spectra of the blue compounds are in favour with structure 4 rather than with structure 6. This conclusion has been drawn from the results of Pariser‐Parr‐Pople(PPP) type calculations. According to the calculated position of the colour band of the pigments and of different molecular subunits, and the correspondence of their electronic states involved in the excitation, the dye molecules can be thought as consisting of two symmetrically crossed polymethine chains. This structural feature is inherent to the H indigo chromophore.