Antiaromaticity of Planar Bisdehydro[12]- and Tetrakisdehydro[16]annulenes Fused with Dithieno[3,4- b:3',4'- d]thiophenes.

Thermally stable bisdehydro[12]- and tetrakisdehydro[16]annulenes with planar structures were successfully synthesized by fusion with two dithieno[3,4- b:3',4'- d]thiophene units. The planarized [4n]annulene cores induced substantial antiaromaticity. However, the HOMO-LUMO gap slightly increased as the π-core expanded from the planar cyclooctatetraene. This phenomenon, in contrast to that of typical π-conjugated systems, was attributed to the decrease in antiaromaticity of the [4n]annulene cores. Both TD-HF and GIAO-HF calculations supported this conclusion.