Polycyclic aromatic hydrocarbons containing antiaromatic chalcogenopyrano[3,2-b]-chalcogenopyrans

Bicyclic chalcogenopyrano[3,2-b]chalcogenopyran with two six-membered chalcogenopyran rings fused with each other contains 12 π-electrons, and thus is antiaromatic and unstable. Polycyclic compounds containing such an antiaromatic subunit have been rarely reported due to the difficult synthesis and poor stability. Herein, two series of chalcogenopyran-fused polycyclic compounds containing chalcogenopyrano[3,2-b]chalcogenopyran subunits: i.e., thiopyrano[3,2-b]thiopyran, selenopyrano[3,2-b]thiopyran, and selenopyrano[3,2-b]selenopyran, have been successfully synthesized via ring-expansion reactions from functionalized chalcogenopheno[3,2-b]chalcogenophene precursors. Interestingly, upon fusing two cyclopentadiene rings, the antiaromatic chalcogenopyrano[3,2-b]chalcogenopyrans switched to stable aromatic subunits in these polycyclic aromatic hydrocarbons (PAHs), which was verified by the calculated nucleus-independent chemical shift (NICS) values, anisotropy of the induced current density (AICD) plots, and two-dimensional isochemical shielding surface (2D-ICSS) maps. Single-crystal X-ray structural analysis revealed that the dioctyl substituted PAHs SS2, SSe2, and SeSe2 exhibited slipped π-stacking structures with short chalcogen–chalcogen contacts, while the tetraoctyl functionalized PAHs SS4, SSe4, and SeSe4 exhibited much looser honeycomb packing motifs. Consequently, the charge carrier mobilities of the OFET devices based on the dioctyl PAH series were higher than those for the tetraoctyl PAH series. Moreover, a clear exchange effect of the chalcogen atoms can be found and the highest hole mobility was achieved for an SSe2 based OFET due to the higher charge density of the Se atom and the moderate interplanar distance. Overall, this work provides not only synthetic protocols for chalcogen-embedded polycyclic compounds, but also a construction strategy towards stable organic semiconductors containing antiaromatic subunits.