Synthesis and Characterization of Kinetically Stabilized Octadehydrobisthieno[12]annulenes

This study presents the synthesis and characterization of kinetically stabilized b- and c-isomers of octadehydrobisthieno[12]annulene (ODTA) derivatives. The synthesis of c- and b-ODTA derivatives was attempted with four 3,5-di-tert-butylphenyl groups on the thiophene rings. A tetraaryl-substituted c-ODTA derivative was successfully isolated and characterized, whereas tetraaryl-substituted b-ODTA derivatives could not be synthesized despite utilizing the same reaction conditions. By replacing the aryl groups at the β-position of the thiophene rings with smaller n-butyl groups, anti-b-ODTA derivative was successfully isolated and characterized, whereas syn-b-ODTA derivative could not be detected. Intramolecular coupling of the linear precursor produced syn-b-ODTA which decomposed during solvent evaporation. The different reactivities of anti- and syn-isomers of b-ODTA derivatives under ambient conditions are attributed to the different degrees of steric protection afforded by substituent groups around the strained butadiynylene units. Ultraviolet-visible absorption spectroscopy and cyclic voltammetry of c- and anti-b-ODTA derivatives confirmed the narrower HOMO-LUMO energy gap and higher HOMO energy level of anti-b-ODTA compared to those of c-ODTA, which is attributed to the stronger antiaromatic character of the 12-membered ring of anti-b-ODTA.