Synthesis and structures of unsymmetrical bull's horn-shaped heptathienoacenes with isomeric location of sulfur atoms

Controlling and balancing the isomeric location of sulfur atoms in thienoacenes are important strategies to tune the functions of intramolecular pi-electronic conjugation and intermolecular interactions. In this work, three kinds of unsymmetrical bull's horn-shaped heptathienoacenes (1-3) and one kind of symmetrical bull's horn-shaped heptathienoacene (4) with different isomeric locations of sulfur atoms in the two terminal thiophene rings have been successfully designed and synthesized using dithieno[3,2-b:3 ',4 '-d]thiophene (st-DTT), dithieno[2,3-b:2 ',3 '-d]thiophene (bt-DTT), and dithieno[3,2-b:2 ',3 '-d]thiophene (tt-DTT) as building blocks through the Suzuki cross-coupling and intramolecular cyclization reactions. The results of single-crystal analysis, electrostatic potential measurement, UV-vis absorption spectroscopy, fluorescence spectroscopy and cyclic voltammetry show remarkable structure-property relationships. On the one hand, the isomeric location of sulfur atoms in the two terminal thiophene rings in 1-4 leads to different intermolecular interactions, resulting in different crystal packing patterns. On the other hand, the isomeric location of sulfur atoms plays a key role in intramolecular pi-electronic conjugation and exhibits rich absorption behaviors.