Thiophen-2-yl and bithienyl substituted pyrazine-2,3-dicarbonitriles as precursors for tetrasubstituted zinc azaphthalocyanines

Peripheral thiophen-2-yl substituents extend the macrocyclic conjugation of zinc(II)azaphthalocyanines, (ZnAzaPc), due to coplanarity between the two ring systems, and red-shifted UV–Vis Q-bands are observed. This lowering in energy is favourable for various applications. Pyrazine-2,3-dicarbonitriles substituted in the 5-position with alkyl-thiophen-2-yl groups or bithienyl groups, were synthesized as precursors for the parent tetrasubstituted ZnAzaPcs, expected to show further red-shifted Q-bands. The reagent Zn(quinoline)2Cl2 was used for one-step cyclotetramerizations. None of the bithienyl substituted precursors gave clean reactions, and mainly polymeric material resulted. Two ZnAzaPcs tetrasubstituted with respectively 5-ethyl-thiophen-2-yl and 3,4-dimethyl-thiophen-2-yl groups, were obtained in fair yields, 40–50%. UV–Vis Q-bands at 660 nm and molar extinction coefficients 111 000 and 89 000 M−1 cm−1 were observed for the blue-green DMF solutions of the two tetramers. 2D NMR methods were applied in analyses of DMF-d7 solutions. Broad and partly overlapping 1H NMR signals for both compounds indicate aggregation. In addition, the extensive number and distribution of sharp peaks in the spectrum reflect structural isomerism. Molecular ions were determined by mass spectrometry (MALDI-TOF). Four pyrazino[2,3-b]pyrazine-2,3-dicarbonitriles, substituted with phenyl or alkylated thiophen-2-yl groups, were synthesized from 5,6-diamino-pyrazine-2,3-dicarbonitrile and aryl glyoxylaldehydes. These precursors were not sufficiently stable for cyclotetramerization.Structure analyses of 5-(5-ethylthiophen-2-yl)pyrazine-2,3-dicarbonitrile, and 5-(5′-ethyl-2,2′-bithiophen-5-yl)pyrazine-2,3-dicarbonitrile, reveal extended π-electron systems in both structures. The high degree of electron delocalization in the three-ring system of the latter compound presumably makes self-association through π-π interactions the greatly preferred mechanism for condensation.

Graphical abstractTwo tetrasubstituted zinc azaphthalocyanines with peripheral alkyl thiophen-2-yl groups have been synthesized and characterized. Three bithienyl substituted pyrazine-2,3-dicarbonitriles and four pyrazino[2,3-b]pyrazine-2,3-dicarbonitriles were synthesized as well. Those precursors were not sufficiently stable for cyclotetramerization due to the extended chromophoric systems imposed by the bithienyl substituents or the pyrazinopyrazine core.Figure optionsDownload full-size imageDownload as PowerPoint slide