Synthesis and Characterization of Dibenzothieno[,]pentalenes Enabling Large Antiaromaticity and Moderate Open-Shell Character through a Small Energy Barrier for Bond-Shift Valence Tautomerization.

Experimental and theoretical rationalization of bond-shift valence tautomerization, characterized by double-well potential surfaces, is one of the most challenging topics of study among the rich electronic properties of antiaromatic molecules. Although the pseudo-Jahn-Teller effect (PJTE) is an essential effect to provide attractive characteristics of 4π systems, an understanding of the structure-property relationship derived from the PJTE for planar 4π electron systems is still in its infancy. Herein, we describe the synthesis and characterization of two regioisomers of the thiophene-fused diareno[,]pentalenes and . The magnetic and optoelectronic properties characterize these sulfur-doped diareno[,]pentalenes as open-shell antiaromatic molecules, in sharp contrast to the closed-shell antiaromatic systems of and , in which these main cores consist of the same number of π electrons as and . Notably, thiophene-fused and showed pronounced antiaromaticity, the strongest among the previous systems, as well as moderate open-shell characteristics. Our experimental and theoretical investigations concluded that these properties of and are derived from the small energy barrier for the bond-shift valence tautomerization. The energy profile of the single crystal of showed the temperature-dependent structural variations assigned to the dynamic mutual exchange between the two -symmetric structures, which was also supported by changes in the chemical shifts of variable-temperature H NMR spectra in the solution phase. Both experimental and computational results revealed the importance of introducing heteroaromatic rings into 4π systems for controlling the PJTE and manifesting the antiaromatic and open-shell natures originating from the high-symmetric structure. The findings of this study advance the understanding of antiaromaticity characterized by the PJTE by controlling the energy barrier for bond-shift valence tautomerizations, potentially leading to the rational design of optoelectronic devices based on novel antiaromatic molecules possessing the strong contributions of their high-symmetric geometries.