1,4,5,8-Tetraazafulvalene – Darstellung schwefelhaltiger Derivate und Zuordnung des Chromophors / 1,4,5,8-Tetraazafulvalenes – Synthesis of Sulfur-Containing Derivatives and Classification of the Chromophor

Abstract

Abstract In order to obtain sulfur-containing tetraazafulvalenes the derivatives 1 - 3 were cross-coupled with different types of acetylenes via palladium-catalyzed reactions. Starting from the tetrabromoaryl derivative 3a, four bromine atoms could be replaced by 2-ethynylthiophene. Under analogous conditions, the methylsulfanyl esters 7 -11 could be obtained by employing 4-ethynylbenzoic acid 4-(methylsulfanyl)butylester 6. Lipoic acid could be integrated into tetraazafulvalenes successfully in a two step reaction. First, the Sonogashira coupling method yielded the compound 12 possessing two (4-anilino)ethynyl residues which were then condensed with lipoic acid to give derivative 13. Whereas the NMR data suggest the predominance of prototropic form A, UV/vis spectra of the deeply colored tetraazafulvalenes are in favor with structure B rather than with structure A. DFT calculations at the B3LYP/6-31G(d) level showed that tautomeric form B is about 60 kJ/mol more stable than A. In addition, time-dependent density functional theory calculations support the substructure of two crossed diazaheptamethinemerocyanines.