Synthesis of diaryl squaraines and diaryl maleimides

The 3,4-diaryl-1,2-squaraines (3,4-diaryl cyclobut-3-ene-1,2-diones) and 3,4-diaryl maleimides (3,4-diaryl-1H-pyrrole-2,5-diones) are strong electron acceptors that can be paired with electron rich aromatics to create simple but powerful donor-acceptor monomeric units to be further developed toward effective conjugated polymers. A series of conjugated 3,4-dithienyl derivatives of 1,2-squaraines were obtained from Friedel-Crafts-like substitution protocol in good to excellent yields (up to 96 %). The simple one-step conversion of squaraine core to the analogous electron-accepting maleimides were just realized, which led to efficient access of diaryl maleimides from squaraines. The possible electrocyclic ring opening/re-closing followed by oxidation mechanism was proposed for the transformation.