Comprehensive Summary S‐fused heterocycles have become popular building blocks to construct functional polycyclic compounds. In contrast to the abundant synthetic methodologies for thiophene‐fused aromatics, the synthesis of S‐heterocycles containing six‐membered thiopyran and seven‐membered thiepine rings is much less reported owing to their unfavorable synthetic protocols and the thermal instabilities. Herein, a series of thiepine‐containing polycyclic S‐heterocycles have been successfully synthesized via different synthetic routes which involve initial construction of sulfur bridges and final ring‐closure reactions. Therefore, the dilithium intermediates are excluded, which facilitates the fusion on the thiepine ring with different S‐heterocycles, including thiophene and thiopyran derivatives. Typically, a S‐fused multi‐membered polycyclic compound simultaneously involving thiophen, thiopyran, and thiepine rings has been successfully prepared. Interestingly, nucleus‐independent chemical shift calculations reveal that the incorporated thiopyran and thiepine rings demonstrate aromatic and nonaromatic characteristics, respectively. Moreover, the thermal stabilities of the thiepine derivatives have been tremendously improved after the fusion on the three vinyl groups in the thiepine unit, which is attributed to the enhancements of the activation energies for the S‐extrusion reactions, as revealed by density functional theory calculations. Therefore, our findings not only provide a facile synthetic methodology for S‐fused multi‐membered polycyclic heterocycles, but also furnish a novel construction strategy towards thermally stable thiepine derivatives.

Three S‐fused polycyclic aromatic hydrocarbons (PAHs) bearing cyclopenta[b]thiopyran moieties have been designed and successfully synthesized. With the conjugation extension, the absorption onset of the longest PAH reaches 1110 nm. All the three S‐fused PAHs exhibit significant halochromic properties in both solution and solid states. Upon protonation, the proton is incorporated on the cyclopentadiene ring while the positive charge is localized on the thiopyrylium ring. Moreover, no significant difference can be found for the two shorter PAHs upon the protonation by different organic acids, such as trifluoroacetic acid (TFA) and trifluoromethanesulfonic acid (TfOH), while the longest PAH can be only mono‐protonated by TFA but di‐protonated by stronger TfOH. Furthermore, after protonation, the non‐emissive S‐fused PAHs exhibit strong fluorescence and can be regenerated by simply neutralization with triethylamine. The enhanced emission of mono‐protonated products stem from S2→S0 transitions, which disobey the Kasha's rule.

Chemoselective 6-endo-dig and ring-expansion cycloisomerizations have been discovered for ortho-disubstituted thiophene derivatives bearing 1-en-3-yne moieties.