2, 2′-Bi [-3 (2H)-thiophenon] ylidene with two bicyclo [2.2. 2]-octadiene moieties was quantitatively converted to thioindigo by the retro-Diels–Alder reaction. The thioindigo solid obtained from the precursor had the P21/c structure which was different from the commercial thioindigo samples (P21/n).