Synthesis and characterization of aza-bridged thiophene-pyrrole dyads and their isomers

Aza-bridged thiophene-pyrrole dyads were synthesized and characterized. Thiophenes substituted with electron-withdrawing groups (EWGs) were used as building blocks for the framework. However, an unprecedented cyclization occurred, and a thieno[2,3-d]pyrimidine derivative (4) was obtained in the reaction between 2,5-diamino-3,4-dicyanothiophene (1) and 3,5-diiminoisoindoline (3), whereas the reaction between 2,5-diamino-3,4-dicarboxylic acid ethyl ester (2) and 3 produced the desired dyad (5). The structures of 4 and 5 were unambiguously characterized by X-ray diffraction. Compound 5 is the first example of a core-modified azadipyrromethene, and the electronic interaction between the thiophene and pyrrole moieties was estimated with several spectroscopies. Several kinds of aryl groups can be attached at the terminal amino moiety of 4, and their unique absorption properties were also revealed.