Isothianaphthene Quinoids: Pyrazine-Annelated Structures for Tuning Electronic Properties

The development of quinoidal systems with extended pi-conjugation has elucidated the influence that diradical characteristics exert on structure-property relationships, which is significant because it broadens the possibilities for the use of organic semiconducting materials in organic electronics. However, the chain-length elongation of such quinoidal molecules has resulted in low chemical stability due to the large contribution of diradical characteristics and to the high level of the highest occupied molecular orbital (HOMO), both of which limit the synthesis of pi-extended quinoidal molecules. Here, we describe solving this problem via aromatic stabilization. To accomplish this, we designed a system that utilizes electron-accepting pyrazine-fused thieno[3,4-b]pyrazine following the development of the isothianaphthene quinoids of thiophene 3-mer and 6-mer. Theoretical calculations indicate that the introduction of a pyrazine-annelated structure suppresses the diradical characteristics and stabilizes the HOMO energy level of quinoidal oligothiophenes. The thermal, photophysical, and physicochemical properties of newly synthesized quinoidal molecules with full annelation of the benzene and pyrazine rings were investigated. Quinoidal thiophene 3-mer functioned as an acceptor in organic solar cells with a power conversion efficiency of 1.04%. This study demonstrates that the introduction of pyrazine-fused rings is an effective molecular design to extend the chain length of quinoidal oligothiophenes. Graphical Abstract Isothianaphthene-based quinoidal oligothiophenes containing pyrazine-annelated structures were designed and synthesized to develop highly extended quinoidal conjugated systems with suppressed diradical character and a stabilized highest occupied molecular orbital energy level. Physical measurements indicated that the electronic structures were controlled precisely by the pyrazine annelations. The quinoidal terthiophene functioned as an organic semiconductor of a nonfullerene acceptor in organic solar cells.