Synthesis of Pyrazinopyrazine-Fused Azaacenes through Direct Condensation Reactions between Quinoxalinediamine and Diketones

We report the synthesis of a new
type of pyrazinopyrazine-fused
azaacene molecules by a simple and versatile procedure. 6,9-Dihexyldithieno­[3,2-f:2′,3′-h]­quinoxaline-2,3-diamine
was synthesized through the condensation between 2,7-dihexylbenzo­[1,2-b:6,5-b′]­dithiophene-4,5-diamine
and bis­(2,2,2-trifluoroethyl) oximidate. A series of derivatized molecules
with extended two-dimensional aromatic fused-ring structures could
be obtained by simple condensation reactions between the quinoxalinediamine
intermediate and various diketones. The reaction was proved to be
effective for the construction of tetrazaacene derivatives with extended
heterocyclic aromatic ring systems. The molecules obtained exhibit
low-lying LUMO levels that can be fine-tuned by modifying the molecular
structure. Crystallographic results showed that in a solid state,
the molecules form “brick wall” structures with a close
π–π stacking mode. The stacking between the π-ring
systems in the molecules could be further enhanced by expanding the
large 2D planar-conjugated structure.