Rethinking aromaticity of reduced thienoquinoids: insights from an S-Pechmann dye dianion

A dianionic species of a thienoquinoid derivative bearing a cross-conjugated γ-thiolactone, the so-called S-Pechmann dye, has been synthesized and characterized. Comparative analysis with a pentafulvalene dianion with an isoelectronic structure revealed that the S-Pechmann dye dianion exhibits significantly reduced aromatic character due to preferential charge delocalization over the C₄O units. This work provides an in-depth understanding of the fundamental electronic nature of thienoquinoid-based compounds.