Multifaceted Regioregular Oligo(thieno[3,4‑b]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap

Thiophene-based materials have occupied a crucial position in the development of organic electronics. However, the energy band gaps of oligo- and polythiophenes are difficult to modulate without resorting to push–pull electronic effects. We describe herein a new series of monodisperse oligo­(thieno­[3,4-b]­thiophene) derivatives with well-defined regioregular structures synthesized efficiently by direct C–H arylation. These compounds show a unique palette of colors and amphoteric redox properties with widely tunable energy band gaps. The capacity to stabilize both cations and anions results in both anodic and cathodic electrochromism. Under excitation, these compounds can produce photoionized states able to interconvert into neutral triplet or form these through singlet exciton fission or intersystem crossing. These features arise from a progressive increase in quinoidization on a fully planar platform making the largest effective conjugation length among hetero-oligomers. Oligo­(thieno­[3,4-b]­thiophene)­s might represent the more distinctive family of oligothiophenes of this decade.